Magnetic hyperthermia (MH) shows great potential in clinical applications because of its very localized action and minimal side effects. Because of their high saturation magnetization values, reduced forms of iron are promising candidates for MH. However, they must be protected in order to overcome their toxicity and instability  under biological conditions. In this work, a novel methodology for the protection of iron nanoparticles through confinement within graphitic carbon layers after thermal treatment of preformed nanoparticles supported on carbon is reported. We demonstrate that the size and composition of the nascent confined iron nanoparticles, as well as the thickness of their protective carbon layer can be controlled by selecting the nature of the carbon support. Our findings reveal that a higher nanoparticle–carbon interaction, mediated by the presence of oxygen‐containing groups, induces the formation of small and well‐protected α‐Fe‐based nanoparticles that exhibit promising results towards MH based on their enhanced specific absorption rate values. The control of both the thickness of the protective carbon layer, and the size and composition of the nascent confined iron nanoparticles is driven by carbon support interactions. The iron hybrid materials (Fe3C@C/CNFAC) obtained from acid‐treated carbon supports exhibit enhanced specific absorption rate values in biocompatible media. Nanomaterials are widely used in medicine because of their small size (1–100 nm), tunable properties, and ability to conjugate with therapeutic and diagnostic molecules.Because of its localized action and minimal side effects, magnetic hyperthermia (MH) is a potential tool for cancer treatment. It consists of an induced local heating by applying an external alternating magnetic field to magnetic nanomaterials delivered into a tumour tissue, leaving in turn the surrounding tissue intact.13, 143O4 and Fe2O3) are the most widely used magnetic nanomaterials in MH.3O4 particles of a wide range of sizes (from 7.5 to 416 nm) and found the highest magnetic coercivity and SAR values for samples of a mean diameter of 46 nm. On the other hand, an increase in SAR values can be achieved by cubic nanoparticles instead of the classical spherical ones due to the beneficial role of surface anisotropy,3C, are very promising candidates for hyperthermia applications due to their higher saturation magnetization.Due to their biocompatibility and saturation magnetization, as well as their abundance, iron oxide‐based nanoparticles  upon thermal treatment of preformed Fe3O4 nanoparticles supported on modified carbon nanofibers (CNF). By selecting the nature of the CNF support, both the protective carbon thickness, and the size and composition of the nascent confined iron nanoparticles can be controlled. We show that CNF supports with a high number of defects limit the growth of iron nanoparticles during thermal treatment, inducing the formation of small and well‐protected α‐Fe‐based nanoparticles. The obtained iron‐based hybrid nanostructures show enhanced SAR values demonstrating a great potential for alternating current (AC) magnetic hyperthermia applications.Here, we report a novel methodology for controlling the protection of reduced forms of iron nanoparticles for their application in MH by confinement within graphitic carbon layers  were first prepared using iron(III) acetylacetonate (Fe(acac)3) as precursor following a procedure reported by Sun et al.3O4NP were supported on CNF and heated at high temperature (1000 °C) in vacuum to promote the simultaneous reduction of iron oxide to iron, and the consequently dissolution of carbon in iron. To investigate whether the structure and composition of the thermally annealed iron nanoparticles can be controlled by the interaction with the carbon support, two different types of carbon nanofibers (before (CNF) and after acid treatment (CNFAC), Figures S5–S8) were employed, enabling different nanoparticle–carbon interactions through functional groups and defects in the CNF support.The two‐step procedure synthesis of Fe3O4NP dispersed on CNF before thermal treatment  with no noticeable change on the nanoparticle size with respect to the free‐standing preformed nanoparticles . It is observed that the synthetic protocol using iron(III) acetylacetonate as precursor is suitable for preparing pseudo‐spherical iron oxide nanoparticles with a very narrow size distribution  images of Fet Figure a, b reve Figure Sb. It is 3O4NP/CNF after thermal treatment  show the formation of larger nanoparticles. SEM images of annealed hybrids  further confirm the dramatic increase of the nanoparticle size with respect to the hybrid before thermal treatment . Furthermore, there is a big difference in chemical composition, as well as in the nanoparticle size between Fe3C@C/CNF and Fe3C@C/CNFAC materials, being 346±321 and 64±39 nm, respectively . Inner core of all types of nanoparticles confirmed the interplanar distances of Fe3C, measured as 0.198 and 0.210 nm, corresponding to the characteristic crystalline planes (112) (2θ=45.9°) and (211) (2θ=60.8°) of Fe3C, respectively  in scanning transmission electron microscopy (STEM) was used to confirm the presence of iron, carbon and oxygen in the NPs .For example, based on the degree of carbon protection three different types of nanoparticles can be found: big nanoparticles with an oxide shell , nanoparticles with a very thin carbon shell (3 nm thickness corresponding to 8 graphitic layers with a d‐spacing of 0.35 nm), and relatively small nanoparticles coated with very thick carbon layers  is transformed to α‐Fe and Fe3C phases after thermal treatment . In contrast, for Fe3O4NP supported on acid‐treated CNF , a mixture of phases (Fe3C and α‐Fe) after thermal treatment can be noticed by the increase in the intensity of the peak at 45.0° (2θ) and the presence of two new 2θ peaks (65.3 and 82.4) that are not observed for Fe3C@C/CNF  reveals that the thermal treatment significantly changes the crystalline phases of iron nanoparticles. Hence, it is observed that the initial crystalline structure of Fe−1, along with the characteristic D, G and 2D carbon bands. In contrast, it is observed that the Fe−O band disappears in both composites after thermal treatment . The increase in this ratio could be due to the defects produced by nanoparticles damaging the carbon structure during their diffusion and growth at high temperature. The formation of Fe3C after the thermal treatment was confirmed by the presence of two bands  by using a 785 nm laser  and iron carbide core (in Fe3C@C/CNF). As expected, the higher weight increment in Fe3C@C/CNFAC correlates well with TEM analysis and further confirms that larger number of protected iron nanoparticles  was observed for both thermally annealed samples.Thermogravimetric analysis (TGA) in air of the hybrid materials before thermal treatment  in harsh oxidative conditions  in a three‐electrode set‐up. As shown in Figure 3C@C/CNFAC, as it has much smaller nanoparticles than Fe3C@C/CNF based on TEM analysis. Moreover, both materials exhibit a similar redox behavior in a potential window from 0 to 1.2 V, which is related to the reversible Fe2+/Fe3+ redox process (between 0.7 and 1 V) and the complex oxidation of Fe3C to Fe2+ (between 0.2 and 0.5 V). TOverall, the introduction of acid groups on the carbon support precursors leads to a higher interaction with the preformed nanoparticles, and hence their diffusion is avoided, and a higher dissolution of carbon is promoted, producing materials with controlled particle size and composition. It is important to highlight that the iron content has an effect on the final morphology of the nanoparticle, but it is minimal in comparison to the effect of the nanoparticles‐support interaction. For example, it can be observed in Figure 3C@C/CNF and Fe3C@C/CNFAC) were prepared in water containing sodium dodecyl sulfate that is a biocompatible surfactant. By dynamic light scattering (DLS), the colloidal solution used for SAR measurements shows a monodisperse hydrodynamic radius (HR) of 2276 nm that remains practically unchanged after hours , highlighting its stability and suitability for practical applications.To study the suitability of the produced hybrid materials for magnetic hyperthermia applications, magnetic SAR values, which correspond to the amount of energy that is transformed into heat per mass of magnetic material, were obtained using AC magnetometry. In comparison to conventional calorimetric methods, in which the increase in temperature of the sample is directly measured,f × H), stated by Hergt as 5×109 Am−1s−1.Figure 3C@C/CNFAC , since the magnetization of bulk α‐Fe (220 emu g−1) is higher than Fe3C (140 emu g−1).The higher SAR values of Fe3C@C/CNFAC was purified by acid treatment in a HCl aqueous solution to dissolve the unprotected nanoparticles. The iron content of the resultant material (Fe3C@C/CNFAC_HCl) decreased from 69.3 to 5.3 % as observed in TGA measurements , while CV and TEM measurements reveal that only small and protected particles remain in the material . Statistical analysis of nanoparticle size shows very narrow distribution with a mean size of 54±21 nm . Magnetic SAR of purified Fe3C@C/CNFAC  reveals a significant increase  in comparison to free‐standing, as a consequence of the large area and high heat conductivity provided by the carbon nanofibers .3O4NP exhibit, both as free‐standing and supported on carbon nanofibers, a transition from superparamagnetic to ferromagnetic by decreasing temperature below the blocking temperature as indicated by the in‐phase and out‐of‐phase signal of AC susceptibility at 1 Hz under zero DC field . However, it is observed that the blocking temperature shifts to a significantly lower value from 61 to 16 K when Fe3O4 nanoparticles are supported on carbon nanofibers, being in agreement with the zero‐field‐cooled (ZFC)‐field cooled (FC) measurements with the field of 100 Oe in the temperature range 2–300 K .3O4  and Fe3O4@CNF , thus indicating a typical behavior for interacting  and non‐interacting systems , respectively.3O4 NPs are combined with CNF. Hence, magnetic hysteresis loops for Fe3O4 and Fe3O4NP/CNF further confirm that the nanoparticles are ferromagnetic below the blocking temperature  and superparamagnetic above the blocking temperature , as revealed by the absence of coercivity and remanence in the hysteresis loop at 300 K.In comparison with other Fe/Fees Table , which c Figure S7.3O4NP e3O4 NP precursors with the carbon support. Hence, it has been demonstrated that a high number of defects on the CNF support limits the growth of reduced iron nanoparticles and facilitates carbon dissolution during thermal treatment in a vacuum, leading to the formation of small and well‐protected α‐Fe‐based nanoparticles (Fe3C@C/CNFAC). In contrast, untreated CNF produce bigger nanoparticles, pure Fe3C phase, and less protected particles (Fe3C@C/CNF). The iron‐based hybrids obtained were evaluated for application in magnetic hyperthermia (MH) by using an AC magnetometer. The specific absorption rate (SAR) was calculated from hysteresis losses. The well‐protected α‐Fe‐based nanoparticles  show a higher SAR value than unprotected iron nanoparticles , thus confirming the possibility of SAR enhancement by an effective interaction between the carbon support and the iron nanoparticles. This was further demonstrated by the significant increase in SAR value of the material after purification with removal of the unprotected nanoparticles by using HCl. This work demonstrates a controlled design of new iron‐based confined nanostructures with high magnetic heating efficiency, which may be used for further devices in magnetic hyperthermia.A well‐protected reduced form of iron nanoparticles has been prepared at high temperatures by simply controlling the interaction of FeChemicals: PR‐24‐XT‐HHT‐AM (PR24) carbon nanofibers were purchased from Pyrograph Products by Applied Sciences. All chemicals were purchased from Sigma–Aldrich and were used without further purification.3O4NP)Synthesis of iron oxide nanoparticles .Milling of carbon nanofibers (CNF): 50 mg of PR24 were milled for 180 min at a frequency of 10 Hz in a mixer mill MM400.AC)Acid treatment of milled carbon nanofibers (CNF: 50 mg of milled carbon nanofibers (PR24) were treated with 25 mL of 6 M HNO3 for 2 h at reflux in air (100 °C). After that, it was cooled down to room temperature and the nanofibers were collected using a hydrophilic 0.45 μm PTFE membrane. They were washed with distilled water until pH 7 and dried on the filter.3C@C/CNF)Preparation of iron carbide particles encapsulated within graphitic carbon layers supported on modified carbon nanofibers  and carbon nanofibers (30 mg). It was suspended in hexane (30 mL) and sonicated for 1 min. The solvent was evaporated in a rotary evaporator at room temperature. The process was repeated 5 times. After that, the product was suspended one last time in hexane by 1 min sonication and filtered using a hydrophobic 0.2 μm PTFE membrane. The nanoparticles in excess were removed by washing the product with hexane on the filter. To evaluate the effect of the iron content, two samples with different Fe3O4NP:CNFAC ratio  were prepared. The obtained hybrid nanostructures were then heated at 1000 °C for 3 h inside quartz tubes sealed in vacuum, each tube containing 30 mg of sample. After that time, they were taken out of the oven and left to cool down to room temperature.3C@C/CNFAC in HCl (Fe3C@C/CNFACPurification of Fe_HCl): Approximately 10 mg of thermally treated hybrid with acid carbon nanofibers (Fe3C@C/CNFAC) were mixed in 20 mL of HCl (37 %) in a round flask and refluxed in air (100 °C) for 4 h. After the reaction, the mixture was cooled down to room temperature and the sample was collected by vacuum filtration using a hydrophilic 0.45 μm PTFE membrane. Finally, it was washed with distilled water until neutral pH and left to dry.Characterization of the hybrid materials: Thermogravimetric analysis were performed using a Mettler Toledo TGA/DSC 3+. The thermal program consisted of an isotherm at 30 °C for 10 min followed by a heating ramp of 5 °C min−1 until 1000 °C. A gas flow of 90 mL min−1 of air was employed as oxidant atmosphere. As reducing atmosphere, gas flow of 25 mL min−1 of nitrogen was applied. IR spectra were obtained with a universal attenuated total reflectance FTIR spectroscope (UATR Two PerkinElmer). The measurements were conducted in a spectral range from 400 to 4000 cm−1. A confocal Raman “Invia Reflex” was employed, with excitation lasers at 514 nm to observe carbon signals and at 785 nm for iron carbide signals. Samples were prepared by depositing a drop of a dispersion of the materials in hexane (∼1 mg mL−1) onto a microscope glass slide. TEM images were obtained with a field emission transmission electron microscope JEOL JEM‐F200 at an accelerating voltage of 200 kV. Samples were dispersed in hexane and drop casted onto holey carbon film on copper grids. The powder XRD patterns were obtained using a Bruker D8 Advance diffractometer equipped with a CuKα radiation source (λ=1.5418 Å) operating at 40 kV and 40 mA and a LYNXEYE XE‐T detector. Magnetic measurements on Fe3O4 nanoparticles and Fe3O4@CNF hybrid were carried out in a commercial Quantum Desing MPMS‐XL5 Superconducting Quantum Interference Device (SQUID) magnetometer. Fe3O4 NP and Fe3O4@CNF samples were carefully prepared using a plastic capsule with a negligible diamagnetic contribution. The electrochemical behavior of the hybrid nanostructures was measured using a potentiostat–galvanostat PGSTAT302 N Autolab. Each sample was suspended in hexane (0.5 mg in 0.5 mL) by 5‐min sonication, and 60 μL were drop cast on a clean glassy carbon electrode (5 mm diameter). The experiments were carried out in 100 mL of 0.1 M HClO4 electrolyte solution under nitrogen by using a reversible hydrogen electrode (RHE) as reference and a graphite rod as counter electrode. A scan rate of 50 mV s−1 was applied for cyclic voltammetry. AC magnetometry measurements were carried out by a Combi AC Hyster Series from Nanotech Solutions in the frequency range from 50 to 250 kHz and intensity range from 8 to 24 kA m−1. Liquid dispersions were prepared by sonication of 1 mg of material in 1 mL of 0.1 wt% SDS aqueous solution, then dilution of this dispersion to prepare 0.125 mg of material per mL of 0.1 wt% SDS solution. The specific absorption rate (SAR) values were calculated according to SAR=A⋅f, where A is the magnetic area and f is the AC magnetic field frequency. SAR values are mass normalized using the iron loading obtained from TGA measurements.The authors declare no conflict of interest.1As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.


However, heterogeneous catalysts efficient at low partial pressures of CO2 still need to be developed. Here, we explore Co/CeO2 as a catalyst for the methanation of diluted CO2 streams. This material displays an excellent performance at reaction temperatures as low as 175 °C and CO2 partial pressures as low as 0.4 mbar (the atmospheric CO2 concentration). To gain mechanistic understanding of this unusual activity, we employed in situ X‐ray photoelectron spectroscopy and operando infrared spectroscopy. The higher surface concentration and reactivity of formates and carbonyls—key reaction intermediates—explain the superior activity of Co/CeO2 as compared to a conventional Co/SiO2 catalyst. This work emphasizes the catalytic role of the cobalt‐ceria interface and will aid in developing more efficient CO2 hydrogenation catalysts.The direct catalytic conversion of atmospheric CO 2 is an efficient low‐temperature methanation catalyst at CO2 partial pressures down to atmospheric concentrations. Advanced spectroscopic investigations unveiled that the combination of a higher concentration and higher intrinsic activity of active intermediates (formates and carbonyls) explains the high activity, emphasizing the role of the metal‐ceria interface.Co/CeO Mitigation of the negative consequences from CO2 methanation. High activity and stability in a broad temperature range is achieved over costly noble metals .2 methanation . Cobalt is an alternative base metal catalyst for the low‐temperature methanation, although Co catalysts with high activity below 200 °C are yet to be identified . Furthermore, to date, numerous studies focused on the effect of increasing the reactant partial pressures on the performance of CO2 catalysts.2 remains largely underexplored, especially for heterogeneous systems .2, with no separate energy‐intensive separation and purification steps, which can offer environmental and economic benefits.3+) in wet impregnation mode. After calcination and reduction, the metal particle sizes for both catalysts were close to 7 nm, according to TEM and CO chemisorption . The even distribution of Co over both supports was confirmed by TEM‐EDX  and non‐reducible (SiO2) support was assessed  catalytic activity than previously reported Co and Ni catalysts .After in situ reduction in hydrogen at 500 °C, the catalytic performance of the cobalt nanoparticles deposited on a reducible CeO and non‐r Figure . Co/CeO22 activation between the studied catalysts, the reaction orders with respect to CO2 were determined at 175 °C . The methanation at low partial pressures of CO2 over Co/CeO2 proceeds with >95 % selectivity to CH4 . Under the same reaction conditions, the CH4 selectivity over Co/SiO2 was much lower. Interestingly, the reaction order in CO2 is higher for Co/SiO2 . The catalysts were reduced in H2 at 500 °C and subsequently exposed to a continuous flow of CO2+H2 (200 mL min−1) with either a high (25 mbar) or low (0.6 mbar) CO2 partial pressure at 175 °C.The activity of heterogeneous catalysts is governed by the transformation of surface intermediates.−1 can be assigned to carbonyls adsorbed on metallic cobalt (*CO).−1 and higher are assigned to linear adsorption of CO (top‐CO).−1 are assigned to the bridged and hollow adsorption modes of CO (bridged‐CO and hollow‐CO).[−1 (C−O stretch I) and 1360–1390 cm−1 (C−O stretch II). The C−H stretch (2800–2890 cm−1) and O−C‐H bending (≈1320–1370 cm−1) formate vibrations were also identified.At both partial pressures Figure , the challow‐CO).21, 232 as manifested by several bands in the regions of 1200–1650 cm−1 and 950–1100 cm−1  over both catalysts. In situ formation of CO2 also led to the appearance of carbonate, bicarbonate, and formate bands .[To investigate the nature of the carbonyl species, CO‐FTIR experiments were performed at 50 °C on reduced Co/SiOociation.23, 282 by Oads was investigated by in situ NAP‐XPS experiments. To follow the oxidation state of cobalt and ceria upon exposure to CO, the percentage of Co0 and Ce3+ were determined from the Co 2p and Ce 3d core line spectra, respectively. After treatment in hydrogen (1 mbar) at 500 °C, the Co3O4 precursor was completely reduced . Additionally, the concentration of reduced Ce3+ species increased by ≈20 %, in line with the hydrogen temperature‐programmed reduction results . Following the procedure of the FTIR experiment, the cell was evacuated to a high vacuum (10−8 mbar) at 500 °C and then cooled down to 50 °C. Upon exposing the sample to CO, no significant oxidation of cobalt was observed. Although lab‐based NAP‐XPS does not provide the utmost surface sensitivity (inelastic mean free path of Co 2p photoelectrons is ≈15 Å), we infer that these results disfavor the hypothesis of the partial oxidation of the cobalt surface by Oads. The observed slight oxidation of Ce3+ (18.7 % to 16.8 % Ce3+) under the same conditions , however, indicates the dissociation of CO. In this process, Cads remains on the Co surface while Oads oxidizes the CeO2 support via oxygen spillover or reacts with *CO to form CO2 as observed by FTIR (Note S2).−1 (subsequently referred to as Cads‐CO) is likely caused by the carburization of the cobalt surface.The possibility of the partial surface oxidation of Co/CeO2 and 0.8 mbar H2 at 150 °C and 175 °C .[3+ continued during further cooling to 50 °C in the presence of H2 . Oxidation of cobalt (+6.2 % Co2+) observed by NAP‐XPS upon cooling to 150 °C in H2  leads to a gradual oxidation of the Co particles . At 150 °C we observed 87.0 % of Co0 which decreased to 83.7 % of Co0 at 175 °C . Under reaction conditions ceria undergoes slight oxidation .30, 312 Figure  can be lC Figure . Removinn Figure  due to t2 participate in the activation of CO2. Activation of CO2 at the Co‐CeO2 interface under reaction conditions involves time‐ and temperature‐dependent partial oxidation of both Ce3+ and Co0. With this better understanding of the active site speciation, we focus in on operando FTIR spectroscopy to monitor the reaction intermediates.Altogether, we conclude that both Co and CeO2 partial pressure of 0.6 mbar to investigate the influence of temperature on the steady‐state behavior of various surface species .2O3 catalyst these multibonded carbonyls were the main reaction intermediates under the applied reaction conditions.Operando FTIR spectroscopy experiments were performed between 125 and 185 °C with a CO2 Figure . The dec2 Figure . For top−1 and one at 1565 cm−1 , we found that peaks located at 2855, 1584, and 1369 cm−1 correspond to one formate species (formate‐I) and peaks located at 2838, 1565, and 1359 cm−1 to another formate species (formate‐II). Two types of formates were also reported previously for Ru/Al2O3,2,The ceria‐based catalyst contained two types of formates, one with a C−O stretch I vibration band at 1584 cm1 Figure . In the 2, the area of the carbonyl peaks on previously reported catalysts and Co/SiO2 .In striking contrast to Co/CeO2 Figure  increasea Figure . The tot2, the intensity of the formate bands on Co/SiO2 decreased over temperature  and formates (55 %) was not removed from Co/SiO2 after 15 minutes . In contrast, almost no carbonyls and formates remained on the surface after 15 minutes at 185 °C. Both carbonyl and formate species are significantly more dynamic on Co/CeO2 and decompose faster at all temperatures.Spectroscopic analysis of surface coverages during transient kinetic experiments is a powerful tool to reveal the mechanistic peculiarities of catalytic reactions.h Figure . For Co/2 display a fast initial decomposition rate followed by a period of slower decomposition .[−1) and C−H (2830–2910 cm−1) stretch vibrations on Co/SiO2 supports this explanation  FTIR experiment was performed. In SSITKA, a switch between chemically identical feeds containing 12CO2 and 13CO2 isotopologues is made. Thus, in SSITKA‐FTIR all spectra are acquired under steady‐state. SSITKA‐FTIR results at 175 °C  demonstrated that the isotope exchange of formates precedes that of carbonyls, in line with the observations based on the transient kinetic step‐responses. The step‐response experiments on Co/CeO2 demonstrate a faster decomposition of the carbonyls compared to Co/SiO2. At 155 °C, all the carbonyls were removed after 5 minutes on Co/CeO2, while for Co/SiO2 only 50 % of them were removed  . It should be noted that the fast removal of carbonyls from Co/CeO2 is not caused by their desorption as demonstrated by a control experiment in which the feed was rapidly changed to He instead of H2/He . The initial rate of carbonyl conversion was up to 5 times higher on the ceria‐supported catalyst .36, 42n Figure ,h. Othern Figure ,e. In th2 Figure . The fasd Figure ,e. Alreat Figure . As the 2 suggests that these species are active reaction intermediates. The step‐response and SSITKA‐FTIR experiments highlighted a difference in the transient behavior of the two different formates  are consistent with the effciient conversion of CO2 to *CO via *HCOO on Co/CeO2. In contrast to Co/SiO2, for which a shortage of reactive carbon species on the catalyst surface can be inferred from positive orders in CO2, the orders for Co/CeO2 are close to zero or even slightly negative . Carbon‐containing species formed upon adsorption and activation of CO2 on ceria  can act as precursors to the reactive formates and carbonyls. This point is supported by step‐response experiments at a higher partial pressure of CO2 . The increased partial pressure of CO2 leads to a significantly increased response time of the carbonyl and formate bands on Co/CeO2, whereas for Co/SiO2 the response was almost identical. Moreover, for Co/CeO2 a sustained methane production was observed by mass spectrometry for ≈50 s after the switch to H2, which was not observed for the Co/SiO2 catalyst, lacking the adsorbed carbon‐containing species necessary to sustain methane formation . In summary, CeO2 provided a sufficient pool of active formates that on SiO2 was much smaller. Moreover, significantly faster dynamics of carbonyls and formates on Co/CeO2 compared to Co/SiO2 explain the higher activity of the ceria‐supported catalyst.Comparing formate‐I on Co/CeO2 Figure , they ca2 displays a superior low‐temperature (<200 °C) CO2 methanation activity and selectivity at CO2 partial pressures down to the current concentration of CO2 in the atmosphere—0.4 mbar. The mechanistic origin of the unusual activity of Co/CeO2 was studied by detailed spectroscopic investigations and compared to a conventional Co/SiO2 catalyst. Reaction order studies and in situ NAP‐XPS, point at the importance of the CeO2 support and Co‐CeO2 interface in adsorbing and activating CO2. In addition, our NAP‐XPS data showcased the possibility of hydride formation on CeO2 with deposited cobalt nanoparticles. The combination of steady‐state, transient step‐response, and SSITKA‐FTIR experiments allowed for the identification of formates and carbonyls as the key reaction intermediates. Furthermore, an additional linear carbonyl species (Cads‐CO) was observed and, using 2D correlation analysis, linked to the facile CO dissociation on the catalyst surface. This Cads‐CO species can be used as a proxy for CO dissociation in future mechanistic studies. Our results point at a mechanism in which CO2 is activated on the support and at the metal‐support interface, leading to formates that further decompose into carbonyls. These carbonyls are subsequently hydrogenated to methane on the Co surface. We should note that we do not rule out a contribution of direct CO2 dissociation at Co‐CeO2 interface to form carbonyls.4 were significantly faster on Co/CeO2 than on Co/SiO2. Moreover, the formate pool was much larger for the CeO2‐based catalyst. The combination of a higher concentration of active intermediates and the higher intrinsic activity of formates and carbonyls explains the high activity of Co/CeO2 at low partial CO2 pressures and low temperatures. These results emphasize the role of the metal‐ceria interface in CO2 hydrogenation chemistry and provide a foundation for the development of better catalysts for the utilization of CO2.In this work, we demonstrated that Co/CeOThe authors declare no conflict of interest.1As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.


Photoelectrochemical oxidation of methylene blue is investigated, with particular focus on the difference in kinetics and thermodynamics of decoloration and mineralization employing photoinduced absorption spectroscopy. Hematite and titania photoanodes are used for the comparison of both reactions, which is determined to be associated with the depth of the valence band . Methylene blue is mineralized by the titania photoanode, however it is only oxidized to small fragments by hematite. Such difference is related to the valence band potential that provides the thermodynamic driving force for photogenerated holes in both materials. In addition, the kinetic competition of water oxidation is found to occur on titania by controlling the pH of the electrolyte. In the pH 14 electrolyte, mineralization of methylene blue is suppressed due to the faster and dominant kinetics of water oxidation, in contrast to the complete mineralization in the near neutral electrolyte where water oxidation kinetics are modest. These results clearly address the importance considering both thermodynamic and kinetic challenges of methylene blue oxidation, which has been thought to be an easy molecule to oxidize, as the model reaction in the application of photo(electro)catalysis using metal oxides. 2 by deep valence band potential, whereas the oxidation stops at catechol by hematite modest valence band potential.Photoinduced absorption spectroscopy is employed to distinguish decoloration and mineralization of methylene blue using titania and hematite photoanodes. The valence band potential is found to be directly related to oxidizability of the organic molecule. Titania can oxidize the methylene blue to CO Typical oxidations include water oxidation to form oxygen or hydrogen peroxide, organic oxidation with high selectivity or complete alcohol oxidation to CO2 for alcohol‐hydrogen reforming, bio‐mass and plastic reforming to produce hydrogen and mineralization of organic pollutants or toxins for water purification. The reaction pathway therefore has to be precisely controlled according to each specific reaction. For example, the PEC alcohol oxidation requires the termination of oxidation process at aldehyde formation, thus avoiding further oxidation to acid or CO2 by the photogenerated holes. On the other hand, the decontamination of water specifically needs to oxidize all organic molecules to CO2 regardless the intermediate formed during the oxidation. These two types of reactions illustrate the importance of considering the reaction pathway for the desired oxidation product via PEC oxidation.Photoelectrochemical (PEC) oxidation reaction using semiconducting photoelectrodes is central for the application of solar energy conversion and storage.1 The 2 or carbonate) by the photogenerated holes. The oxidation process involves a first‐step oxidation to break down the organic molecule to small organic fragments. This first‐step reaction has been often applied in the study of water purifications using photocatalysis or photoelectrocatalysis as the indication of reaction process characterized by UV–vis spectroscopy or chromatography. On the other hand, mineralization requires further oxidation of these small molecules to inorganic carbon, where the detection of total organic carbon (TOC) or chemical oxygen demand (COD) is more meaningful. However, the small molecules only indicate that the structure of toxins or pollutants has been partially decomposed. The potential hazard still remains to the environment, which requires the further oxidation to CO2. Such process, however, has not been significantly considered to date.Amongst the above‐mentioned applications, the mineralization of organic substances has been used in the application of environmental photocatalysis. It is often coupled with hydrogen production technique to achieve both water purification and hydrogen production within one system. The mineralization requires complete oxidation of organic molecules to inorganic carbon  with the absorption edge ≈500–700 nm. In addition, most of these materials have shown their excellent water oxidation efficiency, giving rise to the potential of organic oxidation with higher thermodynamic driving force than water oxidation (1.23 VRHE). The oxidation of organic molecules, for example methylene blue (MB) by bismuth vanadate (BiVO4) or hematite (α‐Fe2O3), has been reported with 100% decoloration efficiency. However, the secondary oxidation of the decolorated small molecules has not been frequently reported or reported with modest ability of mineralization . Similar results have also been observed using carbon nitride (g‐C3N4), bismuth oxobromide (BiOBr) or bismuth oxide (Bi2O3), further demonstrating the challenge in complete oxidation of organic molecules.One of the challenges in mineralization is often related to the n‐type semiconducting materials where oxidations take place. Semiconductors with the property of visible light absorption have been widely used for both photocatalysts and photoelectrodes. These materials often consist of the valence band potential between 1.5‐2.5 Vficiency.9 Howe2) is a typical n‐type semiconductor that has been reported with 97–100% TOC removal of persistent pollutants . The oxidation potential of the photogenerated holes is ≈3.2 VRHE due to the deeper valence band in TiO2, significantly larger than the visible light absorbing materials ≈1.5‐2.5 VRHE. Such strong oxidation power suggests a potential advantage in the mineralization of organic molecules. Pervious water oxidation studies indicated that the deep valence band potential is able to greatly enhance the water oxidation kinetics by the higher thermodynamic driving force between the valence band potential of TiO2 and oxygen evolution redox potential (1.23 VRHE). In addition, several studies have reported that TiO2 has lower selectivity in PEC oxidation of organic synthesis than α‐Fe2O3, including alcohol oxidation to aldehyde and methyl phenyl sulfide (MPS) oxidation to methyl phenyl sulfoxide (MPSO), suggesting that secondary or further oxidation is likely to occur on TiO2, opposite to the results employing α‐Fe2O3. Mechanistic investigations have been carried out to uncover the underlying phenomenon between decolaration and mineralization. Active oxygen or hydroxyl radicals as the intermediate of water oxidation have been proposed to oxidize organic molecules. However, the oxidation by radicals suggest that the oxidation of organic molecules should be independent of photocatalysts because the same type of radicals must have identical redox potential to the targeted organic molecule, thus having the invariant oxidation power for organic oxidation. The difference in TOC removal, on the other hand, strongly suggests that there is a clear difference in the oxidation power, likely dependent on different semiconductors. Therefore, the inherent mechanisms, especially including the identification of the active species for organic oxidation, the correlation between the thermodynamic driving force and ability of mineralization, and the kinetic competition between water oxidation and organic oxidation, have not been greatly discussed, which is the focus of this study reported herein.In contrast to the visible light absorbing materials, semiconductors with UV light absorption have illustrated remarkable abilities for organic mineralization. Titania .14 The2, in comparison with α‐Fe2O3. Methylene blue was used as the indication of decoloration and mineralization. Although MB has been used commonly in photocatalysis of dye degradation mimicking water purification, and considered as an easy dye to decompose, the mineralization shows significant difficulties with certain conditions that must be considered, giving it a perfect model system to demonstrate the challenges in organic mineralization. Operando photoinduced absorption (PIA) spectroscopy was employed to investigate the charge carrier dynamics in TiO2 and α‐Fe2O3 photoanodes for MB oxidation in decolaration and mineralization. Correlation between the reaction kinetics and their valence band potential was established to further illustrate the limitation of mineralization in the semiconductors with shallow valence band potential, and to provide the modification strategies for both materials to achieve 100% mineralization capability.This study is primarily focused upon revealing the underlying mechanism of PEC organic oxidation using TiO22 and α‐Fe2O3. Despite broad interests in the MB photocatalytic oxidation, the concentration of MB in the electrolytes has not been considered to affect the reaction pathway. One of the significant challenges for mineralization is the low concentration of organic substrate where water oxidation becomes kinetically competitive. Therefore, the low concentration of MB employed in this study is to distinguish whether or not mineralization of MB is able to take place. Such phenomenon is thought to be more meaningful for real application in complete removal of organic compounds in water purification. In this study, low concentration (5 ppm) of MB was used to investigate the oxidation pathway with kinetic competition with water oxidation.This study primarily focuses upon the selectivity of MB oxidation between decoloration and mineralization on TiO2 and α‐Fe2O3 photoanodes were fabricated on fluorine‐doped tin oxide (FTO) using a modified metal‐organic deposition method from the previous report of BiVO4. X‐ray diffraction results indicate that both TiO2 and α‐Fe2O3 are polycrystalline shown in Figure 2 and α‐Fe2O3, respectively.The TiOFigure 2 and α‐Fe2O3 photoanodes measured under 365 nm illumination. It is clear that both photoanodes show an enhanced photocurrent in the presence of a trace amount of MB (5 ppm), suggesting that MB is more in favor of being oxidized on α‐Fe2O3. We note that the photocurrent of MB oxidation measured in the α‐Fe2O3 photoanode is marginally higher than that of water oxidation. In order to confirm this, oxygen and hydrogen evolution during bulk PEC MB oxidation was carried out and the faradaic efficiency (FE) was calculated shown in Figure α‐Fe2O3 and TiO2 photoanodes show 100% FE for H2 evolution and 0% for O2 evolution, strongly suggesting that MB oxidation is kinetically competitive even at low concentration against water oxidation on the surface of these two materials. However, it is noted that the MB oxidation observed in the PEC measurements is indicative of a decoloration process, where the chemical structure of MB is decomposed during the PEC oxidation. Such oxidation is thermodynamically more favorable than water oxidation at 1.23 VRHE, consistent with general organic oxidation with greater thermodynamic driving force using metal oxide photoanodes. However, it is yet unclear whether this decomposition results in the MB structure to be oxidized to small organic molecules or inorganic carbon, such as CO2.2 and α‐Fe2O3, the oxidizability of MB on these two photoanodes still remains unclear. Figure 2 and α‐Fe2O3 measured under continuous 365 nm illumination. It is clear that both photoanodes can decolorate the MB electrolyte at low 5 ppm MB concentration measured by the UV–vis absorption as a function of time  is much slower than that of decoloration (1.5 h), suggesting that the MB was firstly oxidized to small molecules (denoted as MB*) and secondly further oxidized to inorganic carbon. The O2 and H2 generated during bulk PEC MB and MB* oxidation were quantified as the faradaic efficiency to indicate whether photogenerated holes fully participated in the MB and MB* oxidation, without water oxidation as the side reaction. No oxygen was detected during the bulk PEC oxidation of MB and MB* in neutral pH  for 24 h, then using TiO2  to continue oxidizing the remaining electrolyte. It is clear, as shown in Figure 2 can effectively remove all organic carbon from the MB intermediate originally generated by α‐Fe2O3. Such method of MB* synthesis was also used to prepare the electrolyte of MB* for the PEC oxidation of MB* using TiO2 as shown below.Although TiO2. Figure 2. It is clear that TiO2 can decolorate the MB electrolyte completely with 100% FE shown in Figure 2 in different pH also shows that mineralization of MB* was unable to be proceeded in pH 14, where oxygen evolution dominated the reaction with 100% FE opposite to the 0% FE in neutral pH for MB* oxidation. Such difference suggests that there is an underlying reaction competition between water oxidation and MB mineralization on TiO2 that determines the reaction pathway and oxidation selectivity, requiring detailed kinetic analyses and will be discussed below.We now turn to consider the pH impact upon the TOC removal on TiO2 and α‐Fe2O3. Photoinduced absorption (PIA) spectroscopy and transient photocurrent (TPC) were employed to determine the reaction kinetics of TiO2 and α‐Fe2O3 MB oxidation. Scheme 2 and α‐Fe2O3 photoanodes, respectively, following excitation on/off by 365 nm LED at each 5 s, which have been determined to be the photogenerated holes. Previous transient and photoinduced absorption studies have indicated that the spectral feature of photogenerated holes was independent of the excitation intensities, suggesting that the photogenerated holes primarily accumulated in the valence band where the quasi Fermi level was approaching to the valence band edge. Therefore, the thermodynamic driving force for these holes to oxidize water or MB/MB* molecules is invariant of the excitation intensity employed in this PIA study herein. The PIA data were calculated to the surface hole density of both TiO2 and α‐Fe2O3 using extinction coefficient of 2200 M−1 cm−1 and 672 M−1 cm−1, respectively , k is the reaction rate constant, α is the reaction order, and [h+] is the density of surface holes (nm−2) accumulated at the photoanode surface.We now consider the oxidation selectivity on TiOed holes.19 PreFigure 2 photoanodes. On the log‐log scale, all PEC oxidations  show a linear relationship with photocurrent. It is clear that TiO2 shows a second‐order water oxidation kinetics in pH 6 electrolyte, in good agreement with a previous PIA study by Kafizas et al. Both MB and MB* oxidations also show the second order with higher reaction rate constant than water oxidation  was calculated from the steady‐state TPC data, and the surface hole density (h+ nm−2) was calculated from the steady‐state PIA amplitude at 5s.Figure us study.22 The2 and α‐Fe2O3 is higher than the TOF for water oxidation. However, the MB* oxidation on TiO2 is significant faster than water oxidation, confirming the further oxidation of MB* to CO2. In contrast, α‐Fe2O3 can oxidize MB only to MB* according to the TOC measurement shown in Figure α‐Fe2O3 shows an identical third order to water oxidation shown in Figure 2 evolution FE in Figure α‐Fe2O3 photoanodes after the MB molecules are oxidized to small organic catechol molecules (as MB*), clearly demonstrating the reaction and selectivity challenges for mineralization of MB using photocatalyst.It is apparent that the TOF for MB oxidation on TiO2, there is still a kinetic challenge using TiO2 for MB oxidation to CO2. Figure 2 for water oxidation, MB oxidation and MB* oxidation. The water oxidation by TiO2 shows a third order reaction, consistent with the previous study. MB oxidation also exhibits a second order, in agreement with the MB oxidation measured in pH 6 shown in Figure 2 as illustrated in Scheme Despite the mineralization of MB using TiOus study.15 MB 33.12 and α‐Fe2O3 photoanodes for water and alcohol oxidations. We first of all consider the photogenerated active species responsible for MB decoloration and mineralization.Previous studies have assigned the PIA absorption to photogenerated holes at 460 and 650 nm for TiOidations.4a We 2O molecules, (3) singlet oxygen, or (4) by active oxygen by the reduction of oxygen molecules generated from water oxidation similar to a Fenton pathway. In our study reported herein, both TiO2 and α‐Fe2O3 photoanodes have been reported being able to oxidize water molecules to hydroxyl radicals by the photogenerated holes, showing as the first‐order oxidation pathway. In the PEC measurement in Figure RHE for hydroxyl radicals) regardless the difference of the valence band potential for photogenerated holes in TiO2 and α‐Fe2O3, which is in contrast to the variation of MB mineralization results using these two photoanodes. Therefore, the photogenerated holes are responsible for direct MB oxidation shown in PEC and PIA studies herein. However, the oxidizability of the photogenerated holes in TiO2 and α‐Fe2O3 varies with their respective valence band potential , that alters the oxidation pathway as we discuss in the following section.Studies in literature have suggested that the main process of MB oxidation has been assigned to 1) direct oxidation by photogenerated holes, (2) by hydroxyl radicals from the first oxidation of H pathway.23 In  direct o3.22 and α‐Fe2O3 are able to oxidize MB to a small organic molecule even at very low concentration of MB. This oxidation is attributed to breaking the chromophore monitored by UV–vis absorption and characterized by HPLC‐MS shown in supporting information Figures RHE.We now consider the reaction thermodynamics and kinetics that control the reaction pathway. Figure ures RHE.262, has however attracted little attention in terms of the reaction mechanism. The observation, that the reaction kinetics change following the oxidation of MB* in TiO2, suggests that mineralization of MB on the surface of TiO2 takes place whilst the same reaction is unable to occur on the surface of α‐Fe2O3. Such observation is also supported by the TOC monitored during bulk PEC oxidation shown in Figure Scheme 2 (3.2 VRHE) and α‐Fe2O3 (2.5 VRHE). The photogenerated holes in TiO2 are 700 mV deeper than that in α‐Fe2O3, providing significantly stronger oxidation power and thermodynamic driving force. The oxidation of MB* using TiO2 therefore remains higher reaction kinetics than water oxidation as shown in Figures 2 possibly enables the photogenerated holes to overcome the thermodynamic barrier as in α‐Fe2O3 to further oxidize MB*. In α‐Fe2O3, the water oxidation are kinetically challenging compared to TiO2. The MB* oxidation is kinetically more favorable for TiO2 in pH 6 due to the slower water oxidation. However, the slower kinetics of water oxidation on α‐Fe2O3, even in pH 14, should give rise to the oxidation of catechol. In contrast, the 100% O2 FE shown in Figure α‐Fe2O3 is likely to associate with the lack of thermodynamic driving force by the shallow valence band potential. We are also aware of the challenge in direct comparing the activation energies between MB and MB* oxidations due to the inactivity of MB* oxidation on α‐Fe2O3. However, we are able to qualitatively determine that TiO2 is more catalytic than α‐Fe2O3 according to the faradaic efficiencies and kinetic analyses, implying smaller activation energy required on TiO2. This observation is also consistent with our previous investigation of PEC alcohol oxidation using α‐Fe2O3 photoanodes, where aldehyde molecules oxidized from alcohol on α‐Fe2O3 is unable to be further oxidized due to significantly higher activation barrier (400 meV). It is therefore clear that the oxidation of MB* also requires higher thermodynamic driving force, either by overcoming the activation barrier or by deeper valence band potential such as TiO2 that is intrinsically more in favor for mineralization.The further oxidation, often associated with the continuous oxidation of the small molecules from MB and complete oxidation to CO to TiO2.15 The2 photoanodes is kinetically faster than that on α‐Fe2O3 due to the deeper valence band potential at 3.2 VRHE. Such valence band potential not only provides higher thermodynamic driving force for water oxidation, but also efficiently suppresses the surface recombination, that severely limits the water oxidation efficiency on BiVO4 and α‐Fe2O3 with valence band potential ≈2.5 VRHE. However, in our study herein, it is striking to us that the amplitude of valence band potential completely alters the reaction pathway of organic oxidations, strongly suggesting that the thermodynamic challenge is the key consideration for the functionality of photocatalysts, especially for the oxidation of organic molecules.Previous kinetic studies have indicated that water oxidation on TiO2.5 VRHE.27 How2 is able to fully oxidize MB molecules to inorganic carbon in pH 6 electrolyte. Rate law analyses based upon the PIA results also show that the MB oxidation is kinetically faster than water oxidation despite the low concentration of MB employed in this study. The reaction kinetics shown in Figure Apart from the semiconductor's valence band potential that provides the thermodynamic driving force for the photogenerated holes, the reaction kinetics are considered for the reaction pathway of PEC MB oxidation. Our PEC results have indicated that TiO2 photoanodes between pH 6 and 14 suggests that there is a kinetic difference for the photogenerated holes to oxidize MB molecules. It is particularly important to note that the photogenerated holes are unable to oxidize the MB molecule to CO2 in the pH 14 electrolyte. The third‐order reaction of water oxidation on TiO2 in pH 14 shown in Figure 2 photoanodes, regardless the sufficient thermodynamic driving force from the valence band potential in TiO2.The comparison of the MB mineralization using TiO2 photoanodes suggests that the kinetics of water oxidation differs from electrochemical environment. Water oxidation on metal oxides often requires proton‐coupled electron transfer (PCET) to facilitate charge transfer and energetically regulate the surface, for example redox levelling. This PCET process is promoted at higher pH due to the more favorable reaction to extract protons and to reach the rate‐determining step of water oxidation, which has been observed and characterized on the surface of α‐Fe2O3 and TiO2. Our observation of water oxidation kinetics on TiO2 photoanodes is also in good agreement with these previous studies in terms of turnover frequency and reaction order, indicating that such conclusion of water oxidation is independent of the method of photoanode fabrication and the semiconductor properties .Our observation of the kinetic competition of MB* oxidation versus water oxidation on TiOevelling.22 Thi2 is significantly affected by the kinetics of water oxidation. It has been observed above, that the first oxidation of MB to MB* is a kinetically faster reaction than water oxidation, which has been also observed with decoloration independent of pH, also consistent with the literature employing MB as the indicator for photocatalysis.  However, our PIA and rate law analyses clearly indicate that there is a kinetic challenge of MB* oxidation in which the reaction pathway is determined by the kinetic competition of water oxidation. The oxidation of MB* is not a kinetically fast reaction to competing with water oxidation, based upon the difference in TOC measured in pH 6 and 14. The point of zero charge of TiO2 is c.a. 6–7. Previous studies have suggested that the 2nd order water oxidation on TiO2 in the pH neutral electrolyte is associated with the 2nd hole reacting with the surface hydroxyl radicals. In our work, since the TiO2 surface is dominated by the water molecules in pH 6, the dissociation and deprotonation of water molecule becomes a kinetically challenging reaction, thus giving rise to the oxidation of catechol proceeding as the mineralization. On the other hand, in pH 14, the surface is fully covered by OH−, the water oxidation is therefore proceeded with three neighboring holes, even in the presence of catechol (MB*), shown in Figure  the MB* is therefore to be oxidized by the photogenerated holes. However, at pH 14, the water oxidation shifts to a faster reaction pathway, resulting in the MB* oxidation being no longer kinetically competitive. Photogenerated holes therefore are more in favor of oxidizing water molecules rather than MB* despite more favorable thermodynamic driving force. Our observation of this kinetic control by the reaction kinetics is also in agreement with the previous study of reaction competition of ethanol and acetaldehyde oxidation on hematite photoanodes. The reaction pathway is controlled by the kinetics of the two competing reactions, as the result of different activation energies. In our study herein, it is then clear that the MB* oxidation is more catalytic than the second‐order water oxidation in pH 6, however less than the third‐order water oxidation in pH 14, also indicating that the importance of the kinetic consideration for effective and efficient organic oxidations.On the other hand, the oxidation of MB* to COtalysis. 29 How2, has demonstrated the great difficulty due to either lack of thermodynamic driving force (by α‐Fe2O3) or sluggish oxidation kinetics (by TiO2 in pH 14). Previous study of methanol oxidation has suggested that H‐C activation in the CH3OH molecule is essential for the formation of ‐C = O bond as the rate determining step. However, in our study reported herein, the alcohol carbon center in the catechol are fully occupied with C‐C and C = C bond. Therefore, the oxidation of catechol molecule would directly involve with the breakage of C‐C or C = C bond first in order for the ‐OH group to be oxidized, which apparently requires higher energy and faster reaction to overcome both thermodynamic and kinetic barriers when competing with water oxidation. This conclusion is also extended to a more general concern of organic oxidation, especially to the breakage of C‐C bond in organic molecules, using semiconductors for photoinduced oxidations. Our previous PIA study of ethanol oxidation on α‐Fe2O3 photoanodes has shown that the acetaldehyde is the final product from ethanol with a second‐order reaction mechanism, which is consistent with the methanol oxidation to formaldehyde, indicating that the oxidation by the photogenerated holes in α‐Fe2O3 does not involve with C‐C bond breaking. These previous studies further support our conclusion of this study that the oxidation of C‐C bond must consider both the thermodynamic and kinetic advantages to provide sufficient driving force, and to accelerate reaction to minimize the side reaction pathway and the resulted by‐product formation.Our kinetic study herein demonstrates that although the photogenerated holes in metal oxide semiconductors are highly oxidizing due to the deep valence band potential, the oxidation of organic molecules is still of great challenge due to unexpected kinetic competition. The MB employed in this study as the model reaction has been thought to be relatively easy to be oxidized and thus frequently used as the evidence for photocatalytic dye degradation. This easy compound is even instable over prolonged storage in the air as the UV–vis absorption changes shown in Figure ing step.4a How2 and α‐Fe2O3 for MB oxidation strongly suggest the consideration of thermodynamic and kinetic competition for the mineralization reaction, rather than simply decolorating the MB electrolyte. The limitation for the deep oxidation of organic molecules has been reported with incomplete TOC removal for many organic substrates including organic dyes and anti‐biotics, using the photocatalysts with modest valence band potential ≈1.6‐2.5 VRHE. Such oxidation power, however, is insufficient to oxidize the small organic intermediate molecules to CO2. On the other hand, Li et al reported a nanocone structure of TiO2 to oxidize persistent pollutants of 4‐cholorophenol with 100% TOC removal. In addition, they also showed that WO3 is able to mineralize bisphenol A (BPA) in a PEC configuration. Both valence band potentials of TiO2 and WO3 are > 3 VRHE, in perfect agreement with our proposed thermodynamic driving force required for organic oxidation.Our optical spectroscopic study of TiOanic dyes30 and2 clearly demonstrate that water oxidation is a dominant reaction pathway in pH 14 simply due to faster reaction kinetics than MB mineralization. On the other hand, the kinetics of organic oxidation on photocatalysts with shallow valence band potential  can be accelerated by modifying the photoanode surface with co‐catalysts. Stolarczyk et al. demonstrated the full oxidation of methanol to carbonate using Pt as the co‐catalyst on g‐C3N4. Since g‐C3N4 is thermodynamically unfavorable for complete oxidation of alcohol, the primary function of Pt is to change the reaction pathway of methanol oxidation with 100% efficiency. Semiconducting photocatalysts with visible light absorption have great absorption range, giving rise to the maximum efficiency of organic oxidation, the valence band potential is however often ≈1.5‐2.5 VRHE, that is difficult to fully oxidize organic molecules. The function of co‐catalysts, therefore, is promising for the broad spectrum of light absorption and meanwhile the mineralization of organics with maximum efficiency.In addition to the thermodynamic requirement, the kinetic competition is also often overlooked in organic oxidations. Water oxidation in the presence of organic oxidation is considered as a side reaction that consumes photogenerated holes thus limiting the efficiency. However, our results of MB oxidation using TiOn g‐C3N4.5 Sinc2 is not as simple as it decolorates, clearly showing the underlying limitation and challenges in the inherent properties of photocatalysts in their thermodynamics and kinetics. This model concluded from the spectroscopic results can also be extended to other PEC or photocatalytic organic oxidation systems such as methanol and biomass reforming for hydrogen production,  environmental catalysis of pollutants or persistent compounds from chemical industry, all of which require to completely break down the organic molecules to CO2.This study addressed the challenges for alternative oxidation using metal oxide photoanodes. We used methylene blue as the model molecule that has been considered a simple reaction. However, the PIA studies reveal that the complete oxidation to COduction, 32 env42 and α‐Fe2O3 photoanodes employing photoinduced absorption spectroscopy. The decoloration of MB to small molecules is proceeded with direct oxidation by the photogenerated holes in both TiO2 and α‐Fe2O3. The further mineralization of these small molecules to CO2, on the other hand, exhibits different reaction pathways depending on oxidation conditions applied in the system. The mineralization on α‐Fe2O3 is unable to occur due to the modest valence band potential (2.5 VRHE) that is insufficient to provide thermodynamic driving force. TiO2 can mineralize MB in neutral pH electrolyte whilst water oxidation occurs after decoloaration in pH 14 electrolyte due to faster kinetics of photogenerated holes for water oxidation. Our results indicate that the oxidation of organic molecules is not a simple reaction using photocatalysis and photoelectrochemistry due to the thermodynamic and kinetic challenges. Proposed solutions to these challenges include employing deep valence band semiconductors to have greater driving force between the oxidation potential of organics and the photogenerated holes, or modifying the modest valence band semiconductor with efficient cocatalysts to completely change the reaction pathway and to have absolute kinetic advantage of the oxidation on the cocatalyst. Careful consideration should be given to the relative kinetics and the thermodynamic driving force by the semiconductor and photocatalyst.In this paper, we have focused on the reaction pathway of MB oxidation on TiO52SO4, >99.0%) were purchased from Sinopharm Chemical Reagent. Iron(III) acetylacetonate (>98%) was purchased from Shaoyuan. Titanium butoxide (≥99%) was purchased from Macklin. Fluorine‐doped tin oxide  substrates were purchased from Kaivo Optoelectronic Technology.Acetic acid (>99.5%) was purchased from Titan. Sodium hydroxide , acetylacetone (99%), and sodium sulfate  in acetic acid (1.5 mL) and acetylacetone (10 mL). A piece of FTO (3 cm × 2.5 cm) was cleaned by sonication with glass cleaner (Shanghai Johnson) in deionized water . The FTO was then sonicated in 95% ethanol before calcined at 450 °C  for 30 min to remove all organic residues.The method of fabrication of undoped TiOrication.27 A s2 films were fabricated by depositing 100 µL of the precursor solution on the FTO by a spincoater . Spincoating parameters were set at 1000 rpm for 30 s for each layer of deposition. For each layer, a small piece of the FTO substrate was covered with a piece of tape to leave blank FTO (0.5 cm ×3 cm) for electrical connection. After each layer was deposited, the film was calcined at 580 °C for 10 min and cooled at room temperature. This spincoating/calcination procedure was repeated 12 times. After depositing the 12th layer, the coated film was calcined at 580 °C for 3 h.TiO−1) in acetic acid (1.5 mL) and acetylacetone (10 mL). The precursor solution was magnetically stirred at room temperature (25 °C) for 30 min. The size and preparation process of FTO are the same as above TiO2 photoanodes.A sol‐gel precursor was obtained by dissolving iron(III) acetylacetonate . Spincoating parameters were set at 1000 rpm for 30 s for each layer of deposition. For each layer, a small piece of the FTO substrate was covered with a piece of tape to leave blank FTO (0.5 cm × 3 cm) for electrical connection. After each layer was deposited, the film was calcined at 580 °C for 10 min and cooled at room temperature. This spincoating/calcination procedure was repeated 9 times. After depositing the 9th layer, the coated film was calcined at 580 °C for 3 h.The  for 3 h.4bα radiation (λ = 1.5406 Å). UV–vis spectra were recorded using UV–vis spectrophotometer . The SEM images of the TiO2 and α‐Fe2O3 samples were recorded by a Hitachi field‐emission scanning electron microscope (S‐4800) with acceleration voltage of 5 kV. Total organic carbon (TOC) of the solution was measured via a total organic carbon analyzer . The product of PEC oxidation of MB was separated and determined using the high‐performance liquid chromatography‐mass spectroscopy . The column (ZORBAX Eclipse Plus C‐18 RRHD) temperature was 30 °C. The carrier was a methanol‐water mixture . The injection volume was 2 µL with the flow rate of 0.4 mL min−1.The XRD data were recorded by an X‐ray diffractometer  using Cu K2 or α‐Fe2O3 as the working electrode, a platinum mesh (1.5 cm × 1.5 cm) as counter electrode and a Ag/AgCl as reference electrode. The electrolyte (50 mL) consisted of 5 ppm MB and 0.1 m Na2SO4 for PEC measurements using TiO2 and 1 m NaOH for α‐Fe2O3. Back‐side illumination was used in the PEC and PIA measurements, where the electrode was fixed on the wall of the customized quartz electrolytic cell. The TiO2 or α‐Fe2O3 was in contact with the electrolyte with a fixed area of 0.785 cm2. In PEC MB degradation of TOC measurements, the illuminated photoelectrode area was enlarged to 6.25 cm2 for both TiO2 and α‐Fe2O3 photoanodes with the front‐side illumination.The PEC oxidation of water and MB measurements were carried out in a custom‐made transparent quartz reactor. Autolab potentiostat (PGSTAT204) was used to record the electrochemical responses. One 365 nm LED light  was used as the light source with the intensity indicated where needed. The three‐electrode configuration was constructed using the TiO2 and H2 evolution during PEC MB and MB* oxidation using TiO2 and α‐Fe2O3 photoanodes, and the calculation of corresponding faradaic efficiency were reported previously. In brief, the PEC oxidation in the three‐electrode configuration was carried out using a gas‐tight gas product circulation and collection system . The gas product was generated in the system and sent automatically to a gas chromatography . The system was initially vacuumed to 1 kPa for the O2 and H2 continuously generated to accumulate in the system. Due to the low pressure in the system, the temperature of the reactor was estimated to be 278 K. The sampling volume of the gas mixture for GC quantification was 0.6 mL compared to 600 mL of the total volume. The sampled H2 and O2 were sent to a RubyBond 5A column by argon carrier at 373 K, and detected by a thermal conductivity detector at 423 K. The calibration of H2 and O2 was carried out using 99.999% H2 from an alkaline electrolyzer  and 99.999% O2 (Air Liquide).The quantification of Oeviously.33 In n (mol) is the moles of H2 or O2 generated, I (A) is the photocurrent during the measurement, t (s) is the reaction time, z is the number of electrons in the O2 (z = 4) and H2 (z = 2) evolution reactions, and F is the faradaic constant (96 485 C mol−1).The faradaic efficiency was therefore calculated using the following equation: The above‐mentioned quartz PEC cell was used for PIA measurement. The applied potential was controlled and transient photocurrent data recorded using a CHI potentiostat (1140C). The long pulse 365 nm LED was used to excite the TiO2 and α‐Fe2O3 photoanode. The 5 s on/off pulse was controlled by a frequency generator . A 150 W tungsten filament lamp with a monochromator  used as the probe light source. The monochromatic probe light was filtered through long‐pass filters (Hengyang Optics) and a band‐pass filter (Rayan Optics) at the specified probe wavelength and then recorded by a homemade silicon photodiode (Hamamatsu S5973). Optical data was sent to a data acquisition device  for processing on the Labview platform.The operando spectroelectrochemical photoinduced absorption spectroscopy was modified from the previous report.21a ThThe authors declare no conflict of interest.Supporting InformationClick here for additional data file.


Although these processes are also characterized by a structural reorganisation of the initially chelated diamide spectator ligand, computational quantum chemical studies indicate that intramolecular electron transfer is abetted by the frontier molecular orbitals (HOMO/LUMO) of the {Mg2Na2} ensemble, which arise exclusively from the 3s valence atomic orbitals of the constituent sodium and magnesium atoms.Ionic compounds containing sodium cations are notable for their stability and resistance to redox reactivity unless highly reducing electrical potentials are applied. Here we report that treatment of a low oxidation state {Mg 2Na2} assembly with basic molecules that are less prone to reduction than the sodium cation itself induces the complete extrusion of the elemental alkali metal.Treatment of a low oxidation state {Mg While this latter behaviour necessarily represents a process of intermetallic electron transfer, the resistance of the Na+ ion toward well‐defined molecular redox processes continues to prevail.As the archetypal alkali metal, the chemistry of sodium is dominated by ionic compounds, NaX , comprising Na2+ ions also invariably result from oxidation of the alkaline earth elements of group 2 , Jones and co‐workers’ isolation of Mg−Mg bonded β‐diketiminate (BDI) and guanidinate derivatives MgNa]2 N(DiPeP)}2; DiPeP=2,6‐(3‐pentyl)‐phenyl), which exploits the extreme kinetic protection provided by the BDI* anion.3 was sufficiently stable to allow an initial assessment of its highly reducing reactivity and behaviour as a magnesium‐centered nucleophile. It was reported to decompose, however, either to unidentified species in the presence of ethereal bases or in benzene solution at moderately elevated temperatures 2 }2; Dipp=2,6‐i‐Pr2C6H3)].6 was prepared by reduction of the magnesium(II) precursor (5)  in comparison to those observed in MgI derivatives such as 1 and 2 [ca. 2.81 Å], quantum theory of atoms in molecules (QTAIM) analysis of 6 revealed a bond critical point  consistent with the maintenance of a covalent Mg−Mg interaction. Two further bond paths were located between the Na cations and the Mg−Mg BCP itself. While weak (ρ=0.0034), these were estimated by perturbation energy analysis to represent an overall σ‐donation strength between the Mg−Mg bond and each Na+ cation of ΔE(2)≈25 kcal mol−1. While its reactivity with CO highlighted that 6 is a potent two‐electron reductant in which the Na and Mg centers operate in harness, this initial computational assay hinted that the {Na2Mg2} unit is better considered as a contiguous ensemble rather than as a series of independent s‐block centers. In the current study, we provide an experimental and theoretical examination of this supposition, which demonstrates that perturbation of the electronic structure of 6 by its treatment with a range of non‐reducible neutral or anionic nucleophiles induces a counterintuitive, but facile and molecularly‐defined, NaI→Na0 redox process.The apparent significance of the Na‐arene interactions to the structural integrity of compound 6 did not tolerate even trace quantities of basic co‐solvents. Addition of a drop of THF, for example, induced the initial formation of a red/purple coloration and a subsequent bleaching of the solution over the course of several seconds, a process which occurred with the simultaneous deposition of a metallic mirror on the surface of the reaction vessel , which was assessed to contain two molecules of coordinated ether by relative integration (4H) of the THF methylene 1H NMR resonances at δ 2.33 and 1.26 ppm. Consistent with the operation of a completely selective NaI→Na0 redox process, quantitative analysis by ICP‐OES demonstrated that the metallic mirror consisted solely (and of the entirety) of the sodium content of 6 employed in the reaction. Furthermore, the fate of the constituent magnesium of 6 was revealed by crystallization of compound 7, which was isolated as colorless single crystals from the initial filtrate in high yield (83 %).It was noted during the early stages of our study that bright yellow arene solutions of l Figure . Perform6 occurs through the oxidation of its {MgI−MgI} unit and ring opening of the 7‐membered disilazide chelate structures, which now adopt alternative {Mg‐μ‐κ1‐N,μ‐κ1‐N′‐Mg′} bridging modes between the magnesium centers to provide a 14‐atom macrocycle. The 3‐coordinate Mg1 and Mg2 are each further ligated by a single molecule of THF, while the relevant Mg−N  and Mg−O  bond lengths are consistent with an assignment of the MgII oxidation state to both atoms. Although it cannot be ascertained with any level of confidence whether the disilazide ligands of 7 originate from a single parent molecule of 6, the atomic specificity of this process convincingly advocates the reaction stoichiometry shown in Equation The resultant X‐ray analysis Figure  revealed6 was treated with two molar equivalents of the charge neutral, but more sterically encumbered, N‐ and C‐donor bases, quinuclidine, 1,3‐di‐isopropyl‐4,5‐dimethyl‐2‐ylidene (i‐Pr2NHC(Me)2) and 1,3‐bis‐2‐ylidene (IPr) in d6‐benzene. Although no discernible changes were observed by 1H NMR spectroscopy, heating of these reactions to 40 °C for four weeks resulted in the similar formation of a grey powder. This process occurred alongside the deposition of subsequently insoluble colorless single crystals of compound 8 , provided similarly slow deposition at 40 °C of metallic sodium and single crystals of compound 8  by 1H and 13C NMR spectroscopy. Compound 10 was isolated by removal of volatiles, washing with hexane and crystallization from hot benzene, which provided colorless single crystals suitable for X‐ray diffraction analysis .Like that of 6 were judged to be EPR silent, albeit such samples were consistently observed to contain variable quantities of unidentifiable paramagnetic impurities. A further reaction performed by addition of THF to a frozen solution of 6 within the cavity of a CW EPR spectrometer and then warmed to the melting point of the solvent also provided no definitive evidence for the production of observable radical intermediates. Development of the sodium mirror, however, was accompanied by the appearance of a broad signal (giso=2.0068), which persisted after completion of the reaction. While sodium atoms trapped in noble gas  or hydrocarbon matrices have been observed to provide well resolved spectra displaying the expected 23Na (I=3/2) couplings,g≈2 have been ascribed to the formation of the particulate metal after atomic diffusion.6 to a similar phenomenon.As a further assessment of this behavior, benzene solutions of compound 6 in both solution and the solid state provides a notable feature for further investigation of its electronic structure. Accordingly, a UV/Vis spectrum (250–500 nm) was recorded of an 8.5×10−8 M solution in benzene /BS2//BP86/BS1 level of theory (see the ESI for full details). Multiple scenarios were assessed, including the thermodynamics associated with the removal of one or both of the Na atoms, either as single Na0 atoms or NaI cations (lone cation or solvated {Na(THF)4}+). The loss of NaI from 6  was, despite still being endergonic, thermodynamically favoured over the removal of an Na0 atom . To interrogate how the formation of 7 may be initiated, therefore, coordination of an initial single molecule of THF to 6 at both MgI (A) and NaI (B) was computed   to form B , rather than coordination at MgI via TS(6‐A)   with the {Mg−Mg} unit is augmented, resulting in a contraction of the Mg⋅⋅⋅Na′ distances to 3.737 and 3.776 Å and a shortening of the Mg−Mg bond to 3.270 Å. QTAIM and NBO analyses were performed on B to probe the effects on this now perturbed tetrametallic core for comparison with the previously described data for 6. The results of this study are outlined in Figure A significant kinetic and thermodynamic preference was identified towards THF coordination at the NaB has a larger density at the bond critical point (BCP) with respect to 6, where ρ(r)=0.0216 (0.0194 in 6), and more negative Laplacian (∇2ρ(r)=−0.0155 for B, versus −0.0136 in 6) and energy density values (where H(r)=−0.00412 in B vs. −0.00362 in 6). These data are consistent with an augmentation of the Mg1−Mg2 covalent bond in B as a result of THF coordination to the NaTHF cation. Furthermore, the Mg1−Mg2 bond path now displays a mild curvature towards the Na′ center, reinforcing the inference that formation of the NaTHF unit has disrupted the previously characterized equivalence of the interactions between the Mg−Mg bond and Na cations in 6.B revealed a small, yet appreciable, Na′−Mg1 BCP that is primarily ionic in nature, where ρ(r)=0.005 and ∇2ρ(r)=+0.0074. While the density at this BCP has, thus, increased from that characterized in 6 (ρ(r)=0.0034),THF unit and the Mg−Mg bond could now be characterized. This is also supported by second‐order perturbation energy analysis of the Fock matrix in NBO basis, where two sets of σMgMg→nNa* interactions are captured =31.6 kcal mol−1 .B is now better considered as comprising a cooperative {Mg2Na′} trimetallic subunit, wherein the shortening of the interatomic distances may facilitate a viable pathway for initiation of the redox chemistry observed upon addition of THF or other non‐reducible bases .QTAIM analysis reveals that the Mg1−Mg2 bond in d Figure b, with s6 is delicately poised. It is also notable that the formation of 8 is effectively the reverse of the process involved in synthesis of 6 [Figure 5 is disrupted during MgI to MgII oxidation. Any further theoretical analysis of the formation of 7 is, thus, necessarily speculative and complicated by the observed macrocyclization of the disilazido magnesium chelate structure of 6 and the coordination of a molecule of THF to both magnesium centers. We have not yet observed any evidence, either thermally or under the stimulus of other reagents, for the direct conversion of compound 5 to compound 8. The calculated free energies of both species, however, not only indicate that macrocycle formation is appreciably exergonic , but also highlight the significant stabilization provided to the structure of 8 by the computed dispersion correction, the absence of which maintains the equilibrium significantly toward the chelated structure of 5 (Table S14). In further mitigation of this hypothesis, and although the calculations do not take into account possible crystal packing effects, the optimized structure of 8 identifies close Mg⋅⋅⋅HCβ contacts (Mg⋅⋅⋅HCβ=2.38–2.39 Å) that are expedited by the conformational flexibility of the macrocyclic structure. On this basis, therefore, it appears likely that related macrocyclization processes contribute significantly to the overall exothermicity of the transformation of 6 to compounds 7, 8 and 10. Furthermore, we have been able to assess the thermodynamic viability of the formation of both 7 and 8 and note the exergonic stability of both species to be −11.2 and −18.1 kcal mol−1, respectively .Together, these observations indicate that the thermodynamic stability of compound 2Na2} assembly with basic molecules that are less prone to reduction than the sodium cation itself induces the complete extrusion of the elemental alkali metal. Although this process also results in a significant reorganisation of the initially chelating diamide supporting ligand, quantum chemical calculations indicate that the origin of this process may be traced to a perturbation of the electronic structure of the tetra‐metallic ensemble. We are continuing to further characterize, generalize and utilize this unusual s‐block element derived redox reactivity.In summary, we have observed that treatment of a low oxidation state {Mg7, 8 and 10,Full experimental and instrumental details, NMR, UV/Vis and EPR spectral data, details of the X‐ray analysis of compounds The authors declare no conflict of interest.1As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.Supporting InformationClick here for additional data file.Supporting InformationClick here for additional data file.Supporting InformationClick here for additional data file.Supporting InformationClick here for additional data file.


This variability can affect ocean‐climate feedbacks and has been shown to influence marine ecosystems. The seasonal variability of the ocean CO2 system has already changed, with enhanced seasonal variations in the surface ocean pCO2 over recent decades and further amplification projected by models over the 21st century. Mesocosm studies and CO2 vent sites indicate that diurnal variability of the CO2 system, the amplitude of which in extreme events can exceed that of mean seasonal variability, is also likely to be altered by climate change. Here, we modified a global ocean biogeochemical model to resolve physically and biologically driven diurnal variability of the ocean CO2 system. Forcing the model with 3‐h atmospheric outputs derived from an Earth system model, we explore how surface ocean diurnal variability responds to historical changes and project how it changes under two contrasting 21st‐century emission scenarios. Compared to preindustrial values, the global mean diurnal amplitude of pCO2 increases by 4.8 μatm (+226%) in the high‐emission scenario but only 1.2 μatm (+55%) in the high‐mitigation scenario. The probability of extreme diurnal amplitudes of pCO2 and [H+] is also affected, with 30‐ to 60‐fold increases relative to the preindustrial under high 21st‐century emissions. The main driver of heightened pCO2 diurnal variability is the enhanced sensitivity of pCO2 to changes in temperature as the ocean absorbs atmospheric CO2. Our projections suggest that organisms in the future ocean will be exposed to enhanced diurnal variability in pCO2 and [H+], with likely increases in the associated metabolic cost that such variability imposes.Our understanding of how increasing atmospheric CO 2 system responds to historical and future projections of climate change. In a high 21st‐century emission scenario, the global mean diurnal amplitude of pCO2 increases threefold compared to preindustrial values, with a dramatic increase in the probability of extreme diurnal events. The main driver of heightened pCO2 diurnal variability in the surface open ocean is the enhanced sensitivity of pCO2 to changes in temperature as the ocean absorbs anthropogenic carbon.Using a global ocean biogeochemical model, we project how diurnal variability of the surface ocean CO Extreme diurnal cycles are generally thermally driven in the open ocean but biologically driven in coastal environments  How are mean and extreme diurnal cycles of the ocean CO2 system likely to respond to rising atmospheric CO2 and associated climate change? (ii) What are the physical and biogeochemical drivers of these changes and how do they manifest regionally? (iii) How does accounting for increased diurnal variability of key ocean CO2 system variables influence the probability that ecosystems are exposed to conditions that may impact organism fitness?Here, we perform multi‐centennial simulations with a global ocean biogeochemical model that includes ocean physics, biogeochemistry, and sea ice, and has been adapted to resolve diurnal cycles of marine net primary production (NPP). The model is forced with 3‐h atmospheric fields derived from past simulations of a fully coupled Earth system model, which, in addition to ocean physics, biogeochemistry, and sea ice, also contains an atmospheric general circulation model and land surface/terrestrial biosphere model. This combination permits the global ocean biogeochemical model to simulate the diurnal variability of the surface ocean CO22 system are influenced by ocean acidification and climate change, we use the NEMO version 4.0 ocean modelling platform  protocols  and nitrate (only possible during the day).To evaluate how diurnal cycles of the COnninkhof . Alongsi2.1z‐coordinate vertical levels with thicknesses increasing from 1 m in the surface ocean to a maximum of 204 m in the abyssal ocean. The model was forced with 3‐h atmospheric outputs  derived from concentration‐driven simulations of the IPSL‐CM6‐LR Earth system model . The model configuration is discretized into 75 2 concentration was an annually prescribed global value, as in IPSL‐CM6‐LR, in accordance with CMIP6 guidelines , and salinity were used to compute surface ocean CO2 system variables offline with the mocsy software package  observations of the CO2 system exist  in line with approaches that have been applied to studies of marine heat waves , total alkalinity (AT), temperature (T), salinity (S), phosphate, and silicate were computed for the surface ocean using 3‐h model output for the last 20 years of both the historical simulation (1995–2014) and SSP5‐8.5 (2081–2100). For each grid cell in the two 20‐year periods, we distinguished contributions to the mean diurnal cycle of pCO2 with a Taylor‐series deconvolution  are estimated numerically from the mean diurnal cycle of absolute values of CT, AT, T, S, phosphate, and silicate using mocsy  a nonthermal component that is the sum of the three other deconvolution terms. Thus, the diurnal variability of surface ocean pCO2 was partitioned into thermally and nonthermally driven contributions at the grid‐cell scale for both the recent past and the projected end‐of‐century conditions under the high‐emission scenario SSP5‐8.5. The performance of the deconvolution was assessed by comparing the sum of its four terms to the mean diurnal cycle of pCO2 anomalies actually simulated by NEMO‐PISCES‐QUOTA and found to differ by typically <1%.In order to assess the thermal and nonthermal drivers of diurnal 33.13.1.13−) concentrations. The model broadly captures spatial patterns in DIC and alkalinity in the surface ocean. Although it overestimates both, spatial patterns of mean surface ocean pCO2 are globally well produced, albeit with slight negative biases in the subtropical southern hemisphere and Arctic, and slight positive biases in the subtropical northern hemisphere and certain parts of the Southern Ocean. Simulated global mean, depth integrated, NPP also compares favorably to observations, with high productivity in tropical regions, low productivity in subtropical gyres, and reasonable values in the major high‐nutrient, low‐chlorophyll regions.The annual mean state of physical and biogeochemical surface ocean fields simulated by NEMO‐PISCES‐QUOTA is similar to observations Figures  and S2. 3.1.2pCO2 and pH time series for which the CO2 system and its associated diurnal and seasonal variability have been characterized previously  would act to produce negative pCO2 anomalies during the day and positive pCO2 anomalies at night. The simulated diurnal cycle of Ωarag is also driven by temperature variability with maximum Ωarag values at 15:00 and minimum values at 06:00 coincident with maximum and minimum SST anomalies. However, observations show greater diversity in the timing of diurnal Ωarag cycles across stations . Thus, nonthermal processes may play a greater and more variable role in observed diurnal cycles of Ωarag.Long‐term observations of diurnal physical and biogeochemical variability in the surface ocean are sparse, being typically associated with specific moored observational platforms. In order to evaluate model performance at capturing diurnal variability, we use multiple in situ autonomous m Figure . Both obpCO2, pH, and Ωarag, it underestimates corresponding diurnal amplitudes , where despite the model being in phase with observed diurnal SST variability, it is in antiphase with observed diurnal cycles of pCO2, pH, and Ωarag, and it has a much larger negative bias with respect to simulated diurnal amplitudes . Under the high‐emission scenario SSP5‐8.5, global annual mean surface ocean pCO2 is projected to increase to 1090 μatm by 2100. In contrast under the high mitigation scenario SSP1‐2.6, it reaches a maximum of 462 μatm in 2063 and decreases thereafter to 439 μatm by 2100. Annual mean surface ocean pH decreases from 8.17 in 1850 to 8.06 at the end of the historical period, a reduction that is consistent with observed pH decline of 0.018 per decade between 1991 and 2011  to 57 μatm (1995–2014), with most of this 21% increase occurring in the late 20th and early 21st century  by the end of the historical simulation, 0.005 (−8%) by the end of SSP1‐2.6, and 0.016 (−27%) by the end of SSP5‐8.5  project a greater relative increase in annual mean [H+] than in the seasonal amplitude of [H+], and thus, the seasonal amplitude of pH is attenuated  while also showing high interannual and multi‐decadal variability . That reduction under high emissions is much like the −9 ± 8% attenuation seen for the CMIP5 multi‐model mean under RCP8.5  to 2.7 μatm (1995–2015) (+26%) over the historical simulation. By the end of the 21st century, the global average diurnal amplitude of pCO2 is projected to increase to 3.3 μatm under SSP1‐2.6 and 6.9 μatm under SSP5‐8.5. Relative to 1850–1869 values, this represents an increase in the diurnal amplitude of +55% and +226% under SSP1‐2.6 and SSP5‐8.5, respectively. The global mean diurnal amplitude of Ωarag exhibits much smaller relative changes, declining by 0.0004 (−4%) over the historical period. By the final 20 years of SSP1‐2.6, the decrease in the Ωarag diurnal amplitude is 0.0006 (−5%), while under SSP5‐8.5, it reaches 0.0025 (−22%). Trends in the diurnal amplitude of pH are limited, particularly in the historical and SSP1‐2.6 simulations. There is, however, a slight decline in the diurnal amplitude of pH in SSP5‐8.5 (−4%).The global mean diurnal amplitudes of +] to the diurnal mean [H+]. Relative to 1850–1869 historical values, the diurnal amplitude of [H+] is amplified 23% by 1995–2014, 45% by 2081–2100 under SSP1‐2.6, and 170% by 2081–2100 under SSP5‐8.5. These diurnal increases in [H+] are slightly lower than those of pCO2, consistent with the near‐linear relationship between pCO2 and [H+] for long‐term trends  show high spatial variability  enhanced diurnal amplitudes of pCO2, pH, and Ωarag in coastal regions and the eastern equatorial upwelling systems of the Pacific and Atlantic basins. These features are mainly attributable to patterns in the simulated diurnal amplitude of SST, the principal driver of diurnal variability in the surface ocean CO2 system in the model.The simulated diurnal amplitudes of ocean COpCO2 is projected to amplify globally under both SSP1‐2.6 and SSP5‐8.5, each with similar amplification everywhere except in the Arctic where it is substantially larger  under unperturbed preindustrial conditions were characterized using 245 years of 3‐h model output. Simulated 99th percentile diurnal cycles, which by definition are 1 in 100‐day events, correspond to global mean diurnal amplitudes for SST, pCO2 diurnal amplitudes, with a global mean Pr of 2.2 indicating approximately a doubling in the likelihood of 99th percentile preindustrial amplitudes. In the final 20 years of SSP1‐2.6, the Pr increases to 4.3, while under SSP5‐8.5, it increases to 22 (2081–2100 mean). Thus, 1 in 100‐day pCO2 diurnal amplitudes in the preindustrial become 4.3 times more likely (approximately 1 in 23‐day events) under SSP1‐2.6 and 22 times more likely (approximately 1 in 5‐day events) under SSP5‐8.5.By the last 20 years of the historical simulation, there is an increase in the probability of extreme +] is 1.8, but in the last 20 years of this century, it rises to 3.3 under SSP1‐2.6 and to 15 under SSP5‐8.5  in the last 20 years of the historical, and by 2081–2100 that rises to 1.2 under SSP1‐2.6 and 1.7 under SSP5‐8.5. These global changes are dominated by high values in the Arctic Ocean where the Pr can exceed 30, particularly in areas of sea ice cover with minimal diurnal SST variability under preindustrial conditions. The reductions in sea ice in these regions during the historical and SSP simulations enhance diurnal variability and result in large increases in the Pr. Such findings are consistent with studies projecting dramatic amplification of SST seasonal cycles in the Arctic Ocean as a consequence of reduced sea ice cover . These reductions in the probability of Ωarag extreme diurnal amplitudes are broadly consistent across the global ocean with only a few isolated regions such as in the Arctic exhibiting limited increases in Pr values during historical and SSP simulations.The Pr of 99th percentile pCO2, SST, [H+], Ωarag, and pH global mean diurnal amplitudes of 19 μatm, 1.4°C, 0.4 nmol kg−1, 0.11 and 0.02 units, respectively, in the preindustrial control , historical (34.4 ± 1.8), SSP1‐2.6 (34.5 ± 1.7), and SSP5‐8.5 (34.5 ± 1.6) simulations. Relative to SST in the preindustrial (17.6 ± 9.9°C), the distribution of SST exhibits increases in mean values in the historical (18.4 ± 10.1°C), SSP1‐2.6 (19.3 ± 10.1°C), and SSP5‐8.5 (21.9 ± 10.4°C) simulations; however, the standard deviations of the distributions remain similar. In contrast, the distributions of CO2 system variables respond in terms of both the mean and standard deviation. For pCO2, [H+], and pH, end‐of‐century distributions under SSP5‐8.5  are distinct from those in the final 20 years of the preindustrial , historical , and SSP1‐2.6  simulations. There is effectively no overlap in conditions. Furthermore, while the standard deviation of surface temperature and salinity is broadly constant across simulations, there is more than a twofold increase in the standard deviation of pCO2 and [H+] under SSP5‐8.5.Probability density functions derived from binned monthly model output highlight how absolute physical and carbonate chemistry conditions change in the surface ocean over historical and future simulations Figure . The dis2 system is simulated to change dramatically, the contribution of this variability to extremes in absolute values is generally minimal. Simulated and observed surface ocean 99th percentile pCO2 values are well captured by monthly model outputs , although increases can be 10–15 μatm in certain coastal regions. In contrast, the use of 3‐h, instead of daily, model output generally has no impact on 99th percentile pCO2 values, with the exception of isolated parts of the Arctic Ocean and Southern Ocean.Although the diurnal variability of the surface ocean COs Figure . The use44.1pCO2 anomalies, relative to daily mean values, are thermally driven and in some regions attenuated slightly by a coincident nonthermal contribution , but that effect is secondary. The nonthermal contribution is consistently negative as it is primarily driven by NPP in the model. At the time of diurnal pCO2 maxima in the early afternoon, SST anomalies are at their maxima and photosynthetic active radiation is high. This is therefore consistently a time of the day when simulated NPP is positive and acts to reduce the concentration of DIC, and consequently pCO2. The enhanced negative contribution in equatorial waters is due to this being a region of high phytoplankton productivity , maximum diurnal n Figure . As prevy Figure .pCO2 maxima increase by up to 4 μatm in the low and mid‐latitude oceans. These increases are driven by an enhanced thermal contribution to pCO2 maxima and despite a coincident but lesser enhancement in the attenuating nonthermal contribution  increases as ocean carbon content increases. Although the nonthermal sensitivities ∂pCO2/∂CT and ∂pCO2/∂AT increase more  and a reduced fraction in the form of CO32−. Similar dominance of the thermal component of pCO2 variability combined with increases in ∂pCO2/∂T has previously been shown to explain projected amplification of the pCO2 seasonal cycle by CMIP5 models in the same subtropical and equatorial regions  as a result of the reduced dominance of the thermal contribution relative to the nonthermal contribution.The simulated magnitude of anomalies relative to the daily mean are smaller for diurnal 4.2pCO2 simulated by the model are well represented when using only monthly model output. Using finer temporal resolution, daily means and indeed 3‐h model output have limited effect on the magnitude of extreme absolute pCO2  and 0.07 nmol kg−1 (+170%), respectively, by 2081–2100 in the high‐emission scenario SSP5‐8.5, with most of that amplification occurring in the second half of the 21st century. Over the same period, the global mean diurnal amplitude of pH and Ωarag decreases by 0.0001 (−4%) and 0.0025 (−22%), respectively. Projected changes are smaller under the high‐mitigation scenario SSP1‐2.6, with the increase in the diurnal amplitudes of pCO2 and [H+] reaching 1.2 μatm (+55%) and 0.018 nmol kg−1 (+45%), respectively, and that of Ωarag declining by 0.0006 (−5%) while for pH, there is no change. The principal driver of amplified pCO2 diurnal variability is the enhanced sensitivity of ocean pCO2 to diurnal variations in temperature as the anthropogenic carbon content of the ocean increases. In our simulations, diurnal variability has very little impact on absolute extremes of the surface ocean CO2 system over most of the global ocean. However, the occurrence of extreme diurnal amplitudes of the CO2 system substantially changes, with 30‐ to 60‐fold increases in the frequency of preindustrial extreme diurnal amplitudes of pCO2 and [H+] under high 21st‐century emissions. Our projections suggest that the metabolic cost that diurnal variability of the CO2 system imposes on organisms will increase this century, with potential consequences for marine ecosystem health.Here, we present results from simulations of a global ocean biogeochemical model that resolves both physically and biologically driven diurnal cycles of the ocean COLester Kwiatkowski, James C. Orr, and Olivier Aumont conceived the study. Olivier Torres adapted the model code and performed the simulations and analysis. Lester Kwiatkowski led the writing of the manuscript with all co‐authors contributing.The authors declare that they have no conflict of interest.This article reflects only the authors' views; the funding agencies and their executive agencies are not responsible for any use that may be made of the information that the article contains.Appendix S1.Click here for additional data file.