CeO2 nanorods, nanocubes, and nanospheres were decorated with 1–4wt % Ni. The materials were structurally characterized using TEM and in situ XANES/EXAFS. The CO2 activation was analyzedby DFT and temperature-programmed techniques combined with MS-DRIFTS.Synthesized CeO2 morphologies expose {111} and {100} terminatingfacets, varying the strength of the CO2 interaction andredox properties, which influence the CO2 activation. Temperature-programmedCO2 DRIFTS analysis revealed that under hydrogen-lean conditionsmono- and bidentate carbonates are hydrogenated to formate intermediates,which decompose to H2O and CO. In excess hydrogen, methaneis the preferred reaction product. The CeO2 cubes favorthe formation of a polydentate carbonate species, which is an inertspectator during DRM at 500 °C. Polydentate covers a considerablefraction of ceria’s surface, resulting in less-abundant surfacesites for CO2 dissociation.Dry reforming of methane (DRM) is a promising way toconvert methaneand carbon dioxide into H Lately,many legislative and research efforts have been dedicated to the large-scaleutilization of CO2 for the production of fuels and chemicalsor enhanced biological and technological utilization.3 The main problem of CO2 utilization is thatthe molecule is very stable (ΔG298 K = −394 kJ/mol) and requires about 530 kJ/mol for the C=Obond to dissociate. Methane is an abundant hydrocarbon source thathas an even higher greenhouse effect than CO2. One of thepromising methods for large-scale methane and CO2 utilizationis the dry reforming of methane  with a H2/CO ratioof <1. The CO-rich syngas produced via DRM is suitable for thesynthesis of dimethyl ether or long-chain hydrocarbons via the Fischer–Tropschprocess.4In recent decades, economicgrowth has led to an increased demandfor energy, which is the driving force for human welfare.ane DRM, 1, which 4 and CO2 and a reactionthat is highly endothermic and thermodynamically limited, which requireshigh temperatures for substantial CH4 and CO2 conversion.2/CO ratio by lowering the H2 selectivity.5There are several drawbacksrelated to catalytic DRM: difficultactivation of both CHreaction 2–4). The6 which stronglydepends on the catalyst support and nickel ensemble size.8 Different threshold values for the nickel particle size are reported,above which carbon accumulation is initiated: about 5 nm for silica,8 10 nm on γ-Al2O39 and 20 nm for CeZrO2.7 The carbon accumulation on nickel is influenced by themetal–support interactions. Specifically in the case of ceria,a sufficient supply of oxygen species from the support has to be ensuredto gasify the carbon precursors accumulating on the nickel surface.7 The oxygen mobility in ceria and consequentlyits carbon gasification efficiency can be adjusted by doping11 or nanoparticle engineering to preferentially expose certain facets,namely, {100}, {110}, and {111}.13 These facetsare characterized by different energies of oxygen vacancy formation, resulting in different reactivity and mobilityof surface oxygen.Nickel is a promising metal for catalyzing the DRMbecause of itslow cost, abundance, and high activity. The main disadvantage of Niis its low carbon resistance,12 Over different CeO2 nanoshapes, Li et al.15 observeda beneficial effect of CeO2 {100} and {110} crystal planeson activity for CO oxidation, compared to CeO2 {111}. Asimilar activity dependency trend was reported by Trovarelli et al.16 for the soot oxidation reaction. Zhang et al.17 studied the DRM reaction over Ni supported onCeO2 nanorods and nanopolyhedra. They reported a highercatalytic activity and carbon accumulation resistance of nanorod-basedcatalysts, which they attributed to more abundant reactive oxygenspecies on CeO2 {100} and {110} crystal planes than on{111}.In recent years, a lot of research has focusedon understandingthe structure–activity relationships of nanoshaped ceria indifferent reactions.18 reported a decreasing abundanceof oxygen vacanciesas well as oxygen mobility in ceria in the following order: rods >octahedrons > cubes > particles. The catalytic activity forDRM overNi/CeO2 catalysts over the mentioned morphologies decreasedin the same order. In addition, both DFT and experiments suggest thatoxygen-assisted methane activation could be used to accelerate theC–H bond activation.19 Consequently,the ability to optimize ceria for accelerating CO2 activationand dissociation and initiating oxygen spillover could be exploitedto accelerate the DRM reaction and minimize carbon accumulation. Althoughceria is known as a basic support,20 itsbasic/acidic properties can vary depending on the exposed facet.21 Remarkably active catalysts for the low-temperaturemethane dry reforming reaction are of great academic and industrialinterest, which would increase the feasibility of decentralized naturalgas valorization. A detailed structure-dependent understanding ofCO2 activation pathways on such highly active catalyticsurfaces is important not only for avoiding carbon accumulation inDRM and improving stability but also for accelerating CO2 reduction to value-added chemicals such as CO, CH4, andmethanol.On the basis of UV/vis, Raman, and XPS analyses, Weiet al.2 rods, cubes, and spheres, which in the DRM reactionexhibited notablydifferent catalytic activity as well as the dynamics of carbon accumulation.Using temperature-programmed DRIFT-MS analyses complemented with DFT,we analyzed the CO2 activation pathways in the absenceand presence of hydrogen over the particularly promising 2Ni/CeO2 nanorod catalyst.In this work, 1–4 wt % nickel was depositedon CeO22.12–R)- and nanocube (CeO2–C)-shapedCeO2 was synthesized as reported by Zabilskiy et al.22 In a typical synthesis, 58.8 g of NaOH  was dissolved in 140 mL of ultrapure water. A separatesolution containing 4.9 g of Ce(NO3)3·6H2O  was dissolved in 84 mL of ultrapurewater. Both solutions were mixed for 0.5 h using a magnetic stirrerand transferred into Teflon-lined stainless steel autoclaves (∼35mL volume). The autoclaves were heated to 100 and 180 °C for24 h to stimulate the formation of ceria nanorods and cubes, respectively.The precipitate was separated from the supernatant by repetitive washingand centrifugation (7500 rpm for 90 s) using ultrapure water and finallyabsolute ethanol until complete sodium removal occurred. To lowerthe possibility of CeO2 nanorods breaking, the CeO2–R precursor was freeze-dried and the CeO2–C precursor was dried overnight in a laboratory dryer inair at 70 °C. The dried precursors were calcined in a staticair atmosphere at 500 °C for 4 h with a heating ramp of 5 °C/minusing a chamber furnace (Nabertherm P330).The nanorod(CeO2 spheres (CeO2–S) were synthesized using a modifiedprotocol reported by Yan et al.23 and cetyltrimethylammoniumbromide  as the structure-directingagent. In a typical synthesis, 0.91 g of CTAB and 2.18 g of Ce(NO3)3·6H2O  were dissolved in 10 mL of absolute EtOH and stirred for 1 hat room temperature in a 50 mL glass beaker covered with Parafilm.The solution was transferred into a laboratory drier and aged in twosteps: at 40 °C for 48 h with a controlled relative humidityof 25%. For the formation of ceria spheres, maintaining the mentionedrelative humidity is crucial. In the second step, the solution washeld at 100 °C for 48 h, independent of the relative humidity.After the appointed aging, CTAB was removed from the precipitate byextraction in absolute EtOH at 45 °C for 24 h. The collectedpowder was calcined in static air at 500 °C for 4 h with a heatingramp of 1 °C/min in a chamber furnace (Nabertherm P330).The CeO3)2·6H2O  was dissolvedin 40 mL of ultrapure water. After stirring for 15 min, 1 g of a powderedCeO2 support was added and stirred for an additional 15min. Afterward, a 2.5 wt % aqueous ammonia solution was added dropwiseuntil the pH reached 7.5. The beaker was covered with Parafilm andstirred for an additional 2 h. After the latter, the pH value of thesuspension was adjusted to 9 with a 25 wt % aqueous ammonia solutionand stirred for an additional 15 min. The final product was filtered,dried at 70 °C overnight, and calcined at 500 °C for 4 hwith a heating ramp of 5 °C/min.When depositing a nominal 2 wt % nickel content, 100 mg of Ni equaled 1.95 (2Ni–R), 1.80 (2Ni–C), and1.85 wt % (2Ni–S).The synthesized catalystsare denoted as 2.22.2.1The phase composition and crystal structure of the samples were analyzedby transmission electron microscopy  operatingat 200 kV and equipped with a slow-scan CCD camera . The powdered samples were dispersed in EtOH and sonicatedto prevent agglomeration and then transferred onto Cu-supported amorphouscarbon lacey grids. Raw image data was processed with Digital Micrographsoftware , and selected-area electron diffraction datawas simulated by electron microscopy software Java Version (JEMS 4.9).2.2.22 physisorption at −196°C . Before the analysis,the samples were degassed in a flow of N2  for 1 h at 90 °C, followed by 4 h at 300 °C. Thetotal pore volume and pore size distribution values were calculatedwith the BJH  method from thedesorption branch of the isotherms.The BET specific surface area, total pore volume, and pore sizedistribution were determined by N2.2.3The PANalyticalEmpyrean diffractometer using Bragg–Brentano geometry and CuKα1 radiation was used for the characterization of the crystallinephases. XRD patterns were recorded in the 2θ range from 10 to80°, with a measurement increment of 0.034° and a step timeof 100 s.2.2.42-TPR and CO2-TPD analyses were performedto (i) qualitatively and quantitatively assess the amount of removableoxygen as a function of ceria shape and (ii) qualitatively and quantitativelyassess the number of CO2 adsorption sites. Powdered samples(∼60 mg) were positioned inside a U-shaped quartz reactor ona flock of quartz wool (Micromeritics Autochem 2920 apparatus). Theanalytical procedure is shown in Figure S1A. The temperature range of the H2-TPR analysis (25–550°C) was selected to cover the range of temperatures, probed bythe catalytic tests (see below).The H2 bound to CeO2–R and 2Ni–R was analyzedby temperature-programmed in situ DRIFTS-MS (DiffusIRcell from PIKE Technologies) on reduced and oxidized samples. Evolvedgases were continuously analyzed by mass spectrometry. Three possiblescenarios were analyzed: (A) CO2 activation in the absenceof hydrogen, (B) CO2 activation with hydrogen species adsorbedon the catalyst, mimicked by formic acid (FA) as the probe molecule,and (C) CO2 activation in excess gaseous hydrogen. Seethe Supporting Information and Figure S1for more details.The dynamic structure of CO2.2.5in situ Ni K-edge XANES andEXAFS spectra were measured in transmission mode on the 2Ni–R,2Ni–C, 2Ni–S, and 4Ni–R catalysts, first at RTin air, then after reduction in a 5% H2/N2 stream(flow rate of 15 mL/min at 1 bar) for 60 min at 400 °C, and duringthe DRM reaction. For the latter, the catalyst was kept for 30 minat 400, 500, and 550 °C in a CH4/CO2 atmosphere.The We measured the XAS scans on different spots of the sample pelletto avoid potential measurement errors caused by radiation damage tothe catalyst. The exact energy calibration was established with anabsorption measurement on a 5-μm-thick Ni foil placed betweenthe second and third ionization detectors. The absolute energy reproducibilityof the measured spectra was ±0.02 eV.24 in combinationwith FEFF6 program code25 for the ab initio calculation of photoelectron scattering paths.More details on XAS analyses are shown in the Supporting Information.The quantitativeanalysis of XANES and EXAFS spectra was performedwith the Demeter program package2.2.628 The periodicityof the crystals was accounted for using the plane-wave formalism ofthe density functional theory with the projector augmented wave method.30 The energy cutoff of 500 eV was tested and found to suffice forwell-converged results (<1 meV). The geometric optimizations wereperformed at the GGA+U level with the Perdew–Burke–Ernzerhof(PBE) functional31 in the single effectparameter (U–J) approximation,as proposed by Dudarev et al.32 The on-siteCoulomb interaction of localized electrons was applied to Ce 4f orbitalsonly with U–J = 4.5 eV, asdone for CeO2 by Penschke and Paier.33 The 4.5 eV value for ceria in PBE+U had also been calculatedself-consistently by Fabris et al.34 andused as such.35 The Grimme D3 correctionfor van der Waals interactions was employed.36Electronic structurecalculations were performed using VASP 5.4.1.k mesh was used. For the p(2 × 2)slabs, a 2 × 2 × 1 mesh was found to suffice for well-convergedresults. For molecules in vacuum, a box with dimensions of 20 ×21 × 22 Å3 was used with gamma-point meshing.For the bulk calculations, a 8 × 8 × 8 Monkhorst–Pack z axis were usedto describe the slabs.38 Spin polarization was used whenrequired. Activation barriers for the diffusion of oxygen vacancieswere calculated with the climbing-image nudged elastic band methodwith seven images.40During the GGA+U relaxations, the threshold was set at 0.02 eV/Å.All geometries were confirmed to represent local minima by performinga vibrational analysis, where no imaginary frequencies were found.Standard dipole corrections on the Eads = Eadsorbed – Eslab – Egaseous, where Eadsorbed denotes the energyof the slab with the adsorbed adsorbate, Eslab is the energy of the empty relaxed slab, and Egaseous is the energy of the relaxed adsorbate in vacuum. Theexperimentally determined entropy of gasesous CO2 is 213.8J/(mol K).41 During the adsorption, CO2 loses approximately one-third of its entopy, yielding 143J/(mol K).42 The temperature of desorptionis estimated from ΔG(T)ads = 0, where G = H – TS = Eads – TS + PV.The adsorption energieswere calculated as 2(111) and CeO2(110) structure,12-layer slabs (four O–Ce–O trilayers) with the bottom6 layers fixed in their bulk positions were constructed. For CeO2(100), an eight-layer slab was used with the bottom four layersfixed. However, because CeO2(100) is a polar structure,43 the stable geometry was compensated for by movinghalf of the top oxygen atoms to the bottom of the slab, effectivelymaking a nine-layer slab.44 For preciseelectronic energy calculations on previously relaxed structures, thetetrahedron method with Blöchl corrections was used for smearing.45For the CeO2.2.7Figure S3), a powderedsample (20 mg) was activated in 5% H2/N2 (15mL/min) at 400 °C for 1 h. After pretreatment, the activity wasanalyzed in 50 °C increments between 400 and 550 °C in anequimolar flow of CH4 and CO2 (30 mL/min each)in a tubular quartz reactor (5 mm i.d.) at atmospheric pressure. Forcatalytic activity and stability tests, catalysts were activated for1 h at 500 °C and the catalytic activity was investigated in20 °C decrements or isothermally at 500 °C, respectively.The reaction products were analyzed by gas chromatography . To ensure thatthe intrinsic activity is reported, operation in the kinetic regimewas experimentally confirmed . The amount of carbon accumulated during the DRM reaction was quantifiedusing a CHNS elemental analyzer .In preliminarycatalytic tests , pure CH4, or a CO flow (WHSV= 120 L/gcat·h), for 1 h. The reactivity of carbonwas analyzed using a TGA-TPO technique by heating the samples to 800 °C in air (25 mL/min) at a rateof 10 °C/min.Toevaluate the reactivity of the carbon which accumulated during DRM,methane cracking, and the Boudouard reaction, the 2Ni–R, 2Ni–C,and 2Ni–S samples were analyzed. The catalysts were first activatedin 5% H33.1Figure S3. The following increasing trend in catalytic activity (basedon CH4 and CO2 conversions) at identical nickelcontent was observed: Ni–S < Ni–C < Ni–R.By increasing the amount of nickel from 1 to 4 wt %, the conversionof CH4 and CO2 increased for all three CeO2 morphologies, as did the H2/CO ratio. The carbonaccumulation rate increases with increasing nickel content . As a result, further tests werefocused on catalysts containing 2 wt % Ni.Preliminary catalyticresults of the DRM reaction are presented in 4 and CO2 rates of 15 and 30 mmol/gcat·min were achieved at 500 °C, which were higherthan over 2Ni–C (13 and 23 mmol/gcat·min) andabout 5 times higher than over 2Ni–S (3 and 7 mmol/gcat·min) . The CH4 and CO2 rates over the 2Ni–Rcatalyst in the temperature range between 420 and 500 °C aremore than an order of magnitude higher compared to values in the literature(Table S1 in the Supporting Information). It is evident that we are working with an exceptionally activecatalyst.Over the 2Ni–Rcatalyst, CHcat·min) 1A,B. The2/CO ratio , and the Boudouard reaction (pure CO feed) were compared .Carbon can exhibit markedly different reactivity, which differentiatesit from spectator and reaction intermediates. The latter are able to react with surfacehydroxyl or (lattice) oxygen species, thus forming CO.48 Carbon from CO  oxidizesat a lower temperature and is more reactive than carbon produced frommethane cracking (red trace). Carbon accumulated during DRM  is the most stable,regardless of the CeO2 morphology.The TGA-TPO experiment was used to probe the reactivity ofcarbonoriginating from CO, CH3.33.3.1S6) revealed CeO2 cubes ranging from 10 to 50 nm in size .Visualization of the2Ni–C catalyst 2A and S6 in size 2F.The Che cubes 2B,C. TheS7) identifiedceria rods measuring about 6–10 nm in diameter and 100–200nm in length  and a higher alkali concentration (10 vs 2 M). The extendedhydrothermal digestion time apparently retains the overall nanorodshape, but the surface energy minimization favors surface restructuringand termination from {110} to {111}.The prevailing facet in CeO2 spheresmeasuring up to 200 nm were visualizedin the 2Ni–S catalyst (S8). The individualCeO2 crystallites measuring about 5 nm are closely packed,forming larger spheres ,whereas CeO2–R and 2Ni–R as well as CeO2–C and 2Ni–C exhibit type II and III isotherms,characteristic of mesoporous materials.The morphological properties are summarized in 2–S materials shows a dominant contributionof micropores . For CeO2–R andCeO2–C materials, the bimodal pore size distributionlikely occurs as a result of interparticle porosity among randomlystacked rods and cubes as well as their aggregates. After the depositionof 2 wt % Ni, the BET specific surface area, pore size distribution,and volume change only marginally.The pore size distributionof CeOFigure S10A. No differences in crystal structurebetween different CeO2 morphologies were observed, suggestingthat all consist of the same fcc crystal structure. After a 2 wt %deposition of nickel, no change in the characteristic CeO2 [111] diffraction position is visible , suggesting that there is no (or that there is belowdetection limit) NiCeOx solid solutionformation. Furthermore, no diffraction lines of NiO or metallic nickelwere observed .The XRD results before andafter nickel deposition are shown in 3.3.32-TPR was used to analyzethe reactivity of oxygen in the materials, which has important consequencesfor carbon oxidation during the DRM reaction. The reduction of bareCeO2 is initiated at about 250 °C, regardless of itsmorphology, and increases progressively until ∼520 °C. After the sample was heldat 550 °C for 30 min, no further H2 consumption wasnoticed, revealing that quasi-steady-state reduction was achieved.The H2 consumption, normalized per mass of the sample,is very similar for the CeO2–S and CeO2–R morphologies, whereas reduction is less intense for CeO2–C. This reflects the fraction of reduced Ce3+ , compared to 9% for CeO2–C  followed by rods and spheres (5.8 and 4.0μmol/m2). Because CeO2 cubes have thelowest surface area (Table S2), absolutelyspeaking they are the least susceptible to reduction, despite beingterminated by (100) facets, which exhibit the lowest oxygen vacancyformation energy.14Hr CeO2–C 1. One sh2 supports,the reduction starts at ∼60 °C for 2Ni–R and 2Ni–Sand at ∼90 °C for 2Ni–C and continues as a clusterof overlapping peaks for up to ∼500 °C, which containsnickel reduction as well as surface and partial bulk ceria reduction.51 The role of nickel during H2-TPR is to facilitate H2 dissociation at lower temperatures compared to those forceria, thus strongly improving the low-temperature oxygen removalfrom CeO2. The highest fraction of Ce3+ normalizedper mass was achieved on 2Ni–R, followed closely by 2Ni–S,while the lowest was measured for 2Ni–C  is evident among differentceria morphologies: CeO2–C > CeO2–R> CeO2–S  show that all three CeO2 morphologies (with or without Ni) possess weak (30–150 °C),intermediate (150–400 °C), and strong (above 500 °C)adsorption sites. At 500 °C, a considerable fraction of CO2 remains bound as polydentate carbonates (see below).The CO2 increases only marginally, whereas the fraction of CO2 which remains adsorbed at 500 °C increases. Apparently,nickel notably increases the CO2 binding strength. Thisis likely caused by electron displacement from the nickel clustersto adjacent Ce3+ sites (causing increased electron density),which is due to lowering the work function of nickel (5.01 eV) comparedto that of partially reduced ceria nanorods (5.34 eV).52After the deposition of nickel, the total amount of adsorbed CO2-TPD, carbonatescan dissociate to CO andOL (lattice oxygen), leading to catalyst reoxidation andrelease of gaseous CO  shows that these morphologies are able to (partly)reoxidize via the decomposition of adsorbed carbonates. This drawsa parallel with a higher reducibility of CeO2–Rand CeO2–S, compared to CeO2–C.During COseous CO 5. The id3.3.5S13 and scenario A; Supporting Information). Prior to theexperiments, the samples were reduced in situ andflushed with argon at 500 °C for 30 min.The reconstruction and desorption of surfacecarbonates in the absence of hydrogen on the 2Ni–R, 2Ni–C,and 2Ni–S catalysts were analyzed by temperature-programmedDRIFTS-MS , bidentate (B), polydentate (P), and hydrogen carbonate (HC).53 Between 25 and 200 °C, 2Ni–S and2Ni–R are populated with bidentate carbonate (1550–1570cm–1). The stretching of monodentate carbonate andhydrogen carbonate 54 occurs in a similar region. Onthe contrary, on the 2Ni–C catalyst, bands at 1490 and 1385cm–1 are more pronounced compared to 2Ni–Sand especially 2Ni–R, identifying a considerable populationof polydentate carbonates on ceria nanocubes.In the region between 1200 and 1700 cm–) stretchingintensities of the B and M species decrease,whereas the intensity of bands between 1490 and 1385 cm–1 increases. This reveals a reconstruction of M and B into polydentatecarbonate (P) species on all three ceria morphologies.As the samplesare heated to 400 and 500 °C, the , identifying the significance of Ce3+ and Ov (oxygen vacancy) sites for CO2 adsorption andactivation and more numerous adsorption modes of CO2 onreduced ceria surfaces.On oxidizedbare CeO2-TPD in argon  and bidentate carbonate intensities decrease.On the other hand, the polydentate carbonate  intensity increases  is characterized bythe absence of the monodentate carbonate (1600 cm–1) species and a less pronounced polydentate carbonate gain due tolower initial carbonate coverage .The CO2 activation on the reduced2Ni–R catalyst, two possibilities were analyzed: scenario B,where formic acid was used as a probe molecule because it containstwo H atoms per one molecule of CO2 (S16B). TheFA vapors were injected over the sample which was held in a flow ofargon, followed by sample heating. In scenario C, the temperature-programmedCO2 activation was performed while the 2Ni–R samplewas held in a flow of H2 (S16C).For hydrogen-assisted CO–1) coexists with the COO–carbonate stretchings, identifying the presence of bidentate and monodentateformate species .Scenario B: Initially at RT, the characteristic stretching C–H compared to the asymmetric C–H stretch (2970–2920cm–1). Simultaneously, the C–O stretchingof the H2COO* increases. This reveals that two formatespecies are transformed to the dioxymethylene H2COO* groupas a result of the C–H bond cleavage and the transfer of Hto the carbon atom with the simultaneous release of CO gas between100 and 200 °C . At 300°C, all formate and H2COO* are decomposed and C–Hbands disappear. Because formates are less thermally stable than carbonates,they decompose at lower temperatures and the population of inert polydentatecarbonates stabilizes at about 300 °C in scenario B, comparedto at 500 °C in scenario A.Changes in the νC–H stretching andC–O stretchingabove 200 °C are caused by restructuring of the BF, MF, and H2 activation in a flowof H2 , the intensity of C–H bands starts to increaseat about 100 °C, reaches a maximum at 150 °C, and completelydecays at 250 °C  in the presenceof hydrogen increase CO formation and correlate well with the temperature-programmedexperiments described above (scenarios A–C).PulseCO3.3.62 at 500 °C  to populate its surface with polydentate carbonates,H2 pulses were injected at the same temperature, and DRIFTS-MSwas used to analyze the catalyst surface and evolved gases. Duringthe H2 pulses, no decrease in the polydentate signal wasobserved  and the trace signal for CO wereobserved during H2 pulsing  and (b) DFTcalculations reported in Contraryto hydrogen, oxygen pulses at 500 °C caused a noticeable dropin the polydentate band intensity 6D. Catal3.3.72+ on all three CeO2 morphologies (S18–S25). After activation in a 5% H2/N2 flow, between 60 and 70% of the Ni2+ is reduced to metallic Ni (Ni0). This fraction of Ni0 is retained during the DRM reaction at 400 °C.At RT in air, nickel is present exclusivelyas Ni0 increased to 85, 85, and 90% in 2Ni–C, 2Ni–S,and 2Ni–R, respectively. At 550 °C, the fraction of Ni0 exceeds 90% in all catalysts. A very similar trend in thetemperature-dependent evolution of the nickel oxidation state wasalso observed for the 4Ni–R catalyst .During the DRM reaction at 500 °C, the fractionof NiIn situ Ni K-edge EXAFS analysiswas used to monitorthe changes in the local structure around nickel under the above-mentionedconditions. In all studied catalysts in air at RT, Ni2+ is coordinated with oxygen atoms in the first coordination shelland with Ni and O in more distant coordination shells at distancescharacteristic of NiO . In the 2Ni–R sample, the coordination numbers are significantlylower than in the bulk NiO (Tables S3–S5), indicating that nickel is in the form of small clusters, measuringbelow 1 nm in diameter.63 The estimatedsize of NiO clusters does not change when the nickel content is increasedfrom 2 to 4 wt %: their structural parameters are the same (Table S5). In the 2Ni–C catalyst, thesize of NiO nanoparticles is estimated to be about 2 nm.2+ cations are connectedto Ce atoms (forming Ni–O–Ce bridges), indicating thatnickel is in the form of two-dimensional thin islands, coating theCeO2 surface.Inthe case of 2Ni–S, the NiO nanoparticle size, estimatedfrom the average coordination number, is smaller compared to thosein the 2Ni–R sample. All NiEXAFS is a bulk analysis method andis not as localized as TEMvisualization. The variation in the NiO particle size, estimated byTEM and EXAFS, likely originated from the polydisperse size distributionof NiO in the catalysts. The NiO crystallites measuring about 6–10nm  coexist with the subnanometer NiO clusters,which are the majority phase (detected by EXAFS).2+ state, as demonstrated by the XANES analysis,and the local structure remains the same as in the NiO crystal . During the DRM reaction at 400 °C,there are no significant structural changes in the Ni metal or NiOclusters (Tables S6–S9).During catalystactivation at 400 °C, the nearest-neighborNi–Ni distances are the same as in Ni metal fcc (2.48 and 3.52Å). More distant Ni neighbors expected in the Ni fcc metal structureare below the limit of detection, indicating that the average sizeof Ni metal clusters is below 1 nm. A portion of Ni cations remainin the Ni2 morphologies. In the case of 2Ni–R, the Ni–Ni coordinationnumber of the metallic species becomes higher than that in 2Ni–C.The lowest Ni–Ni coordination remains in 2Ni–S. Thisreveals that under the DRM conditions at 500 and 550 °C the restructuringof the nickel phase is most prominent on ceria nanorods. Mao and Campbell64 report that an oxophilic metal such as nickelis repelled by surface oxygen vacancy sites of ceria and that nickelprefers to be associated with surface oxygen atoms. The most extensivereduction of the 2Ni–R catalyst ), the surface with 0.25 ML of subsurfacevacancies (CeO2(111)(1/4ss)), and the surfacewith 1.00 ML of subsurface vacancies and the stacking fault (CeO2(111)(1ss+stacking)).We investigate the stoichiometricsurface (CeO2(111)(1/4s)). Overcoming a smallbarrier of 0.27 eV, the subsurface oxygen migrates to the surface,forming CeO2(111)(1/4ss), which is 0.28 eV morestable. However, the vacancies in the subsurface are immobile (Ea = 2.81 eV for diffusion), and the less stablesurface vacancies can diffuse at high temperature (Ea = 1.94 eV for diffusion). This explains why vacanciesdo not agglomerate.69When oxygen is removedfrom the surface, a surface vacancy is firstcreated (CeO2(111)(1ss) restructuresto CeO2(111)(1ss+stacking) upon overcoming abarrier of 1.05 eV, lowering its energy for 0.38 eV. This is in agreementwith the findings from Lustemberg et al.70 All structures are shown in Figure S29.A fully reducedsurface CeO3.3.8.22(110), oxygen vacancies can also form on the surfaceand in the subsurface.71 A surface oxygenvacancy is thermodynamically 0.85 eV more stable than a subsurfaceone. Subsurface vacancies are also kinetically unstable, requiringonly 0.04 eV to migrate to the surface. Diffusion of the oxygen vacancieson the surface can happen along (Ea =1.76 eV) or across (Ea = 0.92 eV) theCe–Ce rows. Thus, we will limit our calculations to the stoichiometricsurface and surface vacancies. All structures are shown in Figure S29.On stoichiometricCeO3.3.8.32(100) surface, half of the surfaceoxygen atoms must be displaced to the bottom of the slab .43 For theB0 surface, we investigate the structure with one surface oxygen vacancy(B1s) and with one subsurface oxygen vacancy (B1ss). For the C0 surface, two surface oxygen vacancies (C2) are considered.Becauseof the polarity of the CeOfrom ref ). AddingEa = 0.23 eV). Hence, bothstructures can exist under oxygen-lean experimental conditions. Inthe subsurface, there are two unequivalent sites for oxygen vacancies:below the surface oxygen atoms (structure B1ss) and belowthe Ce–Ce bridge position, with the latter being much lessstable (+1.33 eV). Consequently, diffusion in the subsurface is notlikely (Ea = 1.37 eV), whereas it proceedsreadily on the surface (Ea = 0.63 eV). Figure S30 shows all five mentioned surface structuresof ceria.On B0, a surface oxygen vacany is slightly (0.14 eV) more stablethan a subsurface one. The barrier for its migration in the subsurfaceis also low , which are assignedto the monodentate (M), bidentate (B), and polydentate (P) species.This is in excellent agreement with the DFT data for the (111) surfaceat all levels of oxidation (CeO2(111), CeO2(111))(1/4ss), and CeO2(111))(1ss+stacking)).CO2 is planarly positioned above the surface,the geometric and electronic structures are not perturbed noticeablyand a very weak interaction of −0.15 eV (irrespective of thesurface) results, which is merely physisorption.When CO2 can bind in a monodentate fashion in the tiltedorientation atop the Ce atom, forming an angle of 54° with thesurface at a O–Ce distance of 2.96 Å. The interactionenergy is −0.24 eV on all surfaces. There is negligible chargetransfer from the catalyst to CO2, no geometric distortionof CO2, and no perturbation in the density of states .CO2 can also adsorbin a bidentate fashion through Ce–Oand Osurf–C with physisorption , CeO2(111)(1/4ss), and CeO2(111)(1ss+stacking)),thus effectively forming a carbonate species with one free (dangling)oxygen. In CO2, the O–C–O angle is distortedfrom 180 to 131° and the O–C bonds are elongated to 1.29and 1.22 Å. The Ce–O bond length is 2.38 Å, and theOsurf–C bond length is 1.43 Å. There is somecharge transfer as the CeO2(111) surface donates 0.15e– to CO2, the CeO2(111)(1/4ss) surface donates 0.21e–, and the CeO2(111)(1ss+stacking) surface donates0.27e–.CO2 with CeO2(111) occurs when it adsorbsas a polydentate/tridentate species, CeO2(111)(1/4ss), and CeO2(111)(1ss+stacking), respectively, forming one Ce–Oand two Osurf–C interactions. The resulting carbonatespecies has the original C–O bonds elongated to 1.27 Åand the O–C–O angle is further decreased to 129°.The Ce–O bond lengths are 2.54 Å and the Osurf–C bond length is 1.38 Å. The charge transfer is 0.17 e– from CeO2(111) and 0.27 e– from CeO2(111)(1ss+stacking). As shown in Figure S31, the electronicdensity of the surface is not noticeably perturbed by the adsorbedCO2 molecule. The three adsorption modes are shown in The strongest interactionof CO2 is released, setting ΔG(T)ads = 0. The monodentate species would desorbfrom CeO2(111) at 99 °C, the bidenatate species ataround 316 °C, and the polydentate/tridentate/carbonate speciesat 781 °C. On the reduced surfaces, these values are comparable.Note that the values are strongly dependent on the accuracy of DFTdata and can easily vary for ±100 K because of the uncertaintyof the DFT data. Nevertheless, they agree qualitatively with Figure S12, which shows three distinct peaksduring CO2-TPD.We estimate the approximate temperatures, wheredifferently boundCO3.3.8.52(110),three proper adsorption modes are possible, which is again in agreementwith the CO2-TPD . However, CO2 can bindas a monodentate species in a vertical position at a O–Ce distanceof 2.84 Å (−0.22 eV) without a geometry distortion. Whenbound as a bidentate species (−0.46 eV), two O–Ce bondsare formed (2.9 Å). The strongest interaction (−1.53 eV)occurs when CO2 forms a tridentate (carbonate) specieswith a surface oxygen atom. The O–C–O angle approachesthe ideal carbonate angle, and the O–C bonds are elongated(1.28 Å). There is a considerable charge transfer of (−0.21e0) from the surface to the CO2. Theentire carbonate species is negatively charged (−1.39e0). On the surface with a surface vacancy (CeO2(110)(1s)), the adsorption modes are analogousbut the interaction is slightly weaker. The monodentate species bindswith 0.22 eV, the bidentate species with 0.42 eV, and the carbonatespecies with 1.16 eV.On the stoichiometric CeO CO2-TPD 5 and 6A.3.3.8.62(100) structures,the situation is more complex. First, we treat B0 and B1ss structures concomitantly because they differ only in one subsurfaceoxygen and feature the same adsorption modes. When CO2 isplanarly above the surface Ce atom, the purely physisorption interactionis weak (−0.07 eV). When CO2 binds as a monodentatespecies, it sits in the bridge position of the removed surface oxygenatom, forming an angle of 37.4° with the surface at a O–Cedistance of 2.76 Å. The geometry of the molecule is not distorted,and there is no charge transfer. The adsorption interaction is −0.49eV.On different CeOss, respectively). Here, upon the formation of the Ce–O andOsurf–C bonds, CO2 is adsorbed as a carbonatespecies with one free (dangling) oxygen. The O–C–O angleis reduced to 131 °C, and the O–C bonds are elongatedto 1.31 Å (toward C(−Osurf)) and 1.21 Å(toward the dangling oxygen). CO2 interacts most stronglywith CeO2(100) when it binds as a tridentate species withone Ce–O and two Osurf–C interactions. Incorporatinginto the lattice horziontally, the adsorption energies are −2.45and −1.66 eV on B0 and B1ss, respectively. A propercarbonate species with three equally long C–O bonds (1.30 Å)is formed. There is a considerable charge transfer from the surfaceto the CO2 molecule (−0.27e0), and the entire carbonate molecule is negatively charged(−1.33e0). Experimentally, twodistinct adsorption modes can be observed: the monodentate and thecarbonate species. However, under oxygen-lean conditions there willbe surface oxygen vacancies present, which introduces additional possibilities,as shown below.Somewhat surprisingly, the bidentate mode of adsorptionis weaker , as a monodentate species (−0.60eV), as a bidentate species (−0.91 eV), or as a carbonate species(−3.28 eV). The two additional adsorption modes of CO2 include a bent geometry: the carbon atom can occupy the hollow (−0.57eV) or the bridge surface site (−0.87 eV), with the CO2 molecule positioned horizontally.On the B12. Whenlocated horizontally above the surface Ce atom, the CO2 interaction is negligible (−0.02 eV). It can also adsorbas a monodentate species atop the Ce (−0.21 eV) without noticeablegeometry distortion, electronic perturbation, or charge transfer.On C2, a bridge position for the monodentate species is also possible(−0.51 eV), while on C0 the bridge position is not particularlystable (−0.16 eV). CO2 can be positioned betweenthe adjacent Ce adatoms . On C0, there are two distinct, equally strong adsorptionmodes. CO2 can bind as a bidentate (Ce–O and Osurf–C) or a carbonate (Ce–O and two Osurf–C) species, with both exhibiting an adsorption energy of−1.09 eV. On C2, the carbonate species is bound similarly strongly(−0.91 eV). However, because of two missing surface oxygenatoms, there is a particularly strong (−1.96 eV) adsorptionmode with CO2 incorporating into the lattice on the surface.All adsorption modes are shown in C0 and C2 exhibitsimilar adsorption modes of CO3.3.8.72 on nickel has alreadybeen thoroughly researched by Wang et al.72 Using the GGA level of DFT, adsorption energies on Ni(111), Ni(110),and Ni(100) were calculated. The authors found that on Ni(111), whichis the most stable and common nickel surface, CO2 can adsorbin four different ways with similar interactions (0.31–0.46eV). Our results confirmed these and found that the adsorption energyof the lowest-lying intermediate is 0.53 eV, which is in reasonableagreement with the data from Wang et al. In the most favorable configuration(4-5fcc), CO2 assumes a bent geometry with a O–C–Oangle of 132°. The carbon atom is positioned between the fccand bridge sites, and the oxygen atoms interact with two neighboringNi atoms (d(Ni–O) = 2.11 Å). The otheradsorption modes are similar in energy (0.3–0.5 eV). See Figure S32 and Table S10 for geometries and energies.The adsorption of CO2 and the formationof different surface motifs, the decomposition of CO2 intoCO has been studied elsewhere. Because of a strong repulsion betweenthe surface oxygen atoms and the newly formed O*, this is feasibleon oxygen-deficient surfaces. Kildgaard et al. studied this reactionon CeOx2–(111) and found that carbonatecannot decompose to CO because of a prohibitively high activationbarrier, whereas the less stable bidentate CO2– could decompose with a barrier of 0.57 eV.73 Similarly, CeO2(110) is conducive for this reaction onlywhen oxygen vacancies are represent. Kumari et al. have shown thatCeO2(110) with a single oxygen vacancy cannot decomposeCO2 to CO because of a barrier of 2.6 eV. Hence, a secondarymetal (Ni) is required.74Although we studied the adsorption of CO4in situ EXAFS analysis, was found to be verysimilar on all ceria shapes (∼1 to 2 nm), as was the fractionof metallic nickel which reaches 90–95% at 550 °C duringthe DRM reaction.Chemically identical catalysts,all containing 2 wt % nickel onceria rods, cubes, and spheres exposing predominantly (111) and (100)facets, provide very different activity, stability, and carbon accumulationresistance in the low-temperature DRM reaction. We established thefollowing decreasing trend in catalytic activity: 2Ni–R >2Ni–C> 2Ni–S. The initial size of the nickel clusters, estimatedvia 3+ quantifiedduring H2-TPR analysis. The facile reduction of the 2Ni–Rcatalyst terminated predominantly with the (111) facet enables efficientcarbon gasification during DRM with mobile oxygen species originatingfrom CO2.51A strong inverse correlation exists betweenthe carbon accumulatedduring the DRM reaction and the fraction of CeThe lowest catalytic activityand carbon deposition on 2Ni–Sare due to the partial inaccessibility of Ni active sites to gaseousreactants, caused by the close packing of ceria crystallites.The catalyst deactivation strongly depending on the surface morphologyof nanoshaped ceria is most extensive over 2Ni–C terminatedwith the (100) facet. During methane cracking in the absence of oxidant,the highly active 2Ni–R sample was most extensively coked.However, the 2Ni–C accumulated by about an order of magnitudemore carbon during DRM compared to two other tested catalysts.The following phenomena likely contribute to extensive coking anddeactivation over 2Ni–C: (i) the redox chemistry of ceria cubesis least prominent, leading to lagging oxygen supply to the nickel,resulting in slow carbon removal, and (ii) the surface of cubes duringthe DRM reaction is most densely populated with stable polydentatecarbonate spectator species, which additionally hinders oxygen mobilityand the participation of oxygen species in the catalytic cycle.2(100) (2.45eV) than on CeO2(111) (0.7 eV) or CeO2(110)(1.53 eV).Our DFT calculations reveal a much stronger adsorption interactionof the carbonate species on stoichiometric CeO2-TPD analysesshow that oxygen-deficient{100} facets provide stronger binding sites compared to {111}.Moreover, both DFT and CO2 moleculesbind per nm2 of CeO2  and 1.2–1.3 bind per nm2 in CeO2 spheres, which is due to the close stacking of individualceria crystals and the poor accessibility of its surface.According to experimental data shown in 2 has been computedto be roughly 0.5 eV, whereas on various CeOx2– surfaces this interaction can exceed 1.0 eV forpolydentate species.Theinteraction between Ni(111) and CO2 predominantlyadsorbs and undergoesactivation on CeOx2– and not nickel.This is further confirmed by the similarity of DRIFT spectra of pureceria supports and Ni–CeO2 catalysts as well asthe difference spectra at 500 °C .Consequently, CO2 sample under identical conditions as forNi/CeO2–R for the carbonate population . We can see a much lower (negligible) carbonate signalon Ni/SiO2. This indicates a fundamentally different chemistryof both materials (Ni/SiO2 and Ni/CeO2) forCO2 activation and adsorption.75For the sake of completeness, we also analyzeda 2 wt % Ni/SiOThe function of nickel is methane activation and the supplyofhydrogen species via spillover to facilitate the reaction of differentsurface carbonates (excluding polydentate).The following argumentsregarding the reactivity of different carbonates,populating fully oxidized or partially reduced (111) ceria nanorodsurfaces, can be put forward.2 activation on a partially reduced ceriasurface in the absence or presence of hydrogen is proposed . Upon heating in the absence of hydrogen , the M and B carbonates rearrange into the P carbonateor desorb as CO (minor pathway) or CO2 (major pathway).CO2 and partially rearrange and hydrogenate into H2COO* intermediates . The H2COO* group (position 4) can then further decompose to CO and H2O by weakening of the C–O bond in the carbonate species,arriving at position 7. (ii) In excess hydrogen, ,the formate readily converts to H2COO* and further to methoxygroups . The methoxy can further hydrogenate to CH4 withbyproducts of CO and H2O. This is supported by a DRIFTS-MSexperiment, where a simultaneous decrease in bands characteristicof C–O and C–H stretching takes place together withthe identification of methane gas. In parallel, MF and BF can alsodesorb as CO2 and H2O or convert to P carbonates. To further substantiate the conversion ofmethoxy to methane, Wang et al.76 observedthat the hydrogenation of methanol to methane occurs easily over theRu/CeO2 catalyst at 300 °C. Also, Kawi et al.77 observed that formate and methoxy species areimportant intermediates during CO2 hydrogenation to methaneover Ni/CeO2 catalysts.In the presence of H species, bidentate and monodentate formateare formed . The hydrogenation of Bcarbonate to BF is shown above the blue-green arrow. With increasingtemperature, two possibilities exist: (i) Under hydrogen-lean conditions(green arrows), the formates partially decompose to CO and CO2 pulses over the 2Ni–R catalystat 500 °C (2 activation at 500 °C(S16).No decrease in the polydentateband intensity was detected duringthe injection of Ht 500 °C 6D. This t 500 °C1. The pr52 binding strength of the CeO2 nanorod surfacefor the stabilization of reactive M and B carbonates as well as highreducibility for facile CO2 dissociation. Catalytic stabilityis influenced by the terminating ceria facets: the jagged (100) surfacepresent in ceria nanocubes helps maintain the structural integrityof nickel by preventing its sintering but causes severe coking dueto poor redox activity.Nickel crystallites measuringabout 1 nm in size were dispersedon ceria nanorods which predominantly expose {111} crystal facets.This produced an exceptionally active and stable DRM catalyst withlow carbon accumulation, compared to ceria nanocube and nanospheremorphologies. This can be attributed to the synergetic actions ofhighly dispersed nickel for methane activation and the intermediateCO2 adsorbs on (sub)stoichiometricceria surfaces asmonodentate, bidentate, and hydrogen carbonate species at room temperature.The polydentate carbonate population is favored over the {100} facetof ceria, which prevails in the ceria nanocube surface. On the contrary,monodentate and bidentate carbonates prevail on the {111} facet. Themonodentate or bidentate species and the polydentate species on fullyoxidized CeO2(111) are bound more weakly than on CeO2(100). The oxygen-deficient  ceria surfacesprovide the strongest interaction with CO2, especiallyCeO2(100).CO2, revealing P to be a spectator rather than an activespecies during the DRM reaction at 500 °C. The presence of hydrogenincreases the reactivity of the surface carbonates and partially preventstheir restructuring to inert polydentate, supporting the hydrogen-assistedCO2 activation pathway on Ni/CeOx2–. In a surplus of hydrogen, the dioxymethylene converts tomethane and water via methoxy, whereas under H2-lean conditions,dioxymethylene preferentially decomposes to CO and water.The most thermally stable polydentate(P) carbonate which coversa large fraction of the partially reduced ceria surface reacts sluggishlywith H


Thrombocytopenia, an early common complication in heatstroke (HS), has been widely considered as a mortality predictor of HS. The mechanism underlying thrombocytopenia in HS remains unknown. It is not known whether NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is activated in HS platelet, which, in turn, induces platelet activation and thrombocytopenia. This study tried to clarify the activation of the NOD-like receptor signaling pathway under HS conditions and investigate its roles in mediating HS-induced thrombocytopenia.Rat HS models were established in a certain ambient temperature and humidity. Platelets, isolated from blood, were counted and CD62P, an index of platelet activation, was measured by flow cytometry in all rats. The colocalization of NLRP3 inflammasome in platelet was detected by confocal fluorescence microscopy. Mitochondrial-derived reactive oxygen species (ROS) was detected using the molecular probes. Plasma HMGB1 and IL-1β levels were measured by ELISA.Platelet activation, showed by upregulated CD62P, and thrombocytopenia were observed in HS rats. HS activated the NLRP3 inflammasome, which was induced by elevated levels of ROS, while the upregulated CD62P and thrombocytopenia triggered by NLRP3 inflammasome were attributed to the high mobility group box protein 1 (HMGB1) inplasma. Moreover, inhibition of the NOD-like receptor signaling pathway in rats with HS suppressed platelet activation and the decline of platelet count. Similar results were obtained when the receptor toll-like receptor 4 (TLR4)/advanced glycation end product (RAGE) was blocked.The NOD-like receptor signaling pathway induces platelet activation and thrombocytopenia in HS rats. These findings suggested that the NLRP3 inflammasome might be the potential target for HS treatment. Heatstroke (HS) is a life-threatening condition characterized by a core body temperature over 40.5 °C . which wDuring the past few decades, evidences suggest that multi-organ damage correlates with the development of systemic inflammatory response syndrome (SIRS) that is stimulated by cytokines and other immune modulators in HS . PlateleNOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome has been extensively investigated in the processes of dysbalanced inflammation and coagulation cascade . NLRP3 iHigh mobility group box protein 1 (HMGB1), a typical damage-related molecular pattern molecule, is often transferred from the nucleus to the cytoplasm and can be extracellularly released under many stimuli . We haveIn this study, using a rat model of HS, we tried to investigate whether HMGB1-induced activation of NLRP3 inflammasome occurs via its receptors including toll-like receptor 4 (TLR4) and the receptor for advanced glycation end product (RAGE) signaling pathway, and contributes to platelet activation and thrombocytopenia.Because estrogen has certain effects on HS pathogenesis and organ injury , only adRats housed for 6 h at ambient temperature (25 °C ± 0.5 °C) with a humidity of 35% ± 5%, 126 rats were randomly divided into 21 groups, with 6 rats per group. Animals have free access to standard food and water. To induce HS, rats were placed in a prewarmed incubator maintained at 39.5 °C ± 0.2 °C with a relative humidity of 60.0% ± 5.0%. The rectal temperature (Tr) was monitored at 10-min intervals using a thermocouple , which was inserted 5.5 cm into the rectum of each rat. The time point at which the Tr reached 43 °C was considered a reference point of HS onset . The rat50 µL of whole blood was collected into a sodium citrate anticoagulation tube to measure the platelet count using a blood cell analyzer .3, 138 mM NaCl, 5.5 mM glucose, 2.9 mM KCl, 2 mM MgCl2, 0.42 mM NaH2PO4, 10 mM HEPES, pH 7.4). The mixture was centrifuged at 37 °C at 500 g for 8 min in the presence of PGE1  and apyrase , washed twice with CGS buffer , and resuspended in modified Tyrode buffer.Platelets were isolated as described previously . BrieflyOne volume of PRP prepared above was added into 45 volumes of 3% glutaraldehyde and to stand for 4 h. The samples were centrifuged at 800 g for 10 min and the supernatant was discarded. The pellet was washed with 0.1 M phosphate buffer for twice, postfixed with 1% osmium tetroxide at 4 °C for 1 h, and dehydrated in a graded ethanol series and embedded in Epon. Epon-embedded platelets were cut into sections (70–80 nm) with an Ultracut UCT ultramicrotome, transferred to copper grids, and stained with uranyl acetate and lead citrate. The sections were then observed with transmission electron microscope (TEM) .6 to 107) were resuspended in one mL of modified Tyrode buffer, and treated with 0.2% triton before to measure intracellular cytokines. A minimum of 10,000 events per gate was acquired using a flow cytometer (BD LSR Fortessa). P-selectin (CD62P) was determined using the anti-CD62P monoclonal antibody , and TLR2 (toll-like receptor 2), TLR4, and RAGE were assessed with antibodies targeted to TLR2 , TLR4 , and RAGE . NLRP3 expression was evaluated by an NLRP3 antibody , and activation of caspase-1 was assessed by the fluorescent probe green fluorescent-labeled inhibitor of caspase-1 , which irreversibly binds to activated caspase-1. IL-1β was evaluated by the anti–IL-1β antibody . Mitochondrial-derived reactive oxygen species (ROS) were detected using the molecular probes Mito-SOX , DHE , and DCF-DA . All indices were doubly labeled with a phycoerythrin- or fluorescein isothiocyanate–conjugated CD61 monoclonal antibody  to label platelets. The results were analyzed using FlowJo software 7.6.Freshly isolated platelets , and blocked with 10% fetal calf serum (FCS) in PBS. Then, platelets were incubated with goat anti-rat NLRP3 antibody  or rabbit anti-rat ASC antibody  for 30 min, washed twice with PBS, and labeled with corresponding Dlight 488 and Dlight 594 conjugated secondary antibodies  for 30 min respectively. Preparations were analyzed under a laser scanning confocal microscope , and FV1000 operations software was used for recording.Immunofluorescence for NLRP3 and apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC) was performed according to previous research . PlateleAccording to the manufacturer’s instructions, plasma HMGB1 levels were measured by a commercially available HMGB1 enzyme-linked immunosorbent assay kit , and IL-1β levels were measured by a commercially available IL-1β enzyme-linked immunosorbent assay kit .t-test or one-way analysis of variance (ANOVA). IBM SPSS Statistics 26 software was used for data. P < 0.05 was considered statistically significant.All data were presented as the mean ± SD unless stated otherwise. All data followed a normal distribution except for one group, which was analyzed with Kruskall-Wallis. For the data followed a normal distribution, statistical significance was determined with the least significant difference P < 0.01). Therefore, for the remaining experiments, we observed heat-stressed animals only at the time point of 9 h after HS onset.The platelet count began to decline at 3 h after HS onset , whereasThe association of NLRP3 and ASC as well as caspase-1 represents activation of the inflammasome. The colocalization of NLRP3 (green) and ASC (red) in platelets was observed under a confocal microscopy . In addiTo investigate whether HS-activated NLRP3 inflammasome mediates platelet activation and thrombocytopenia, we injected rats i.p. with MCC950, a specific inhibitor of the NLRP3 inflammasome, before the rats subjected to heat stress. The results showed that inhibited NLRP3 inflammasome significantly down-regulated the levels of cleaved caspase-1 , IL-1β sPrevious studies found that early increased extracellular HMGB1 levels in HS were negatively correlated with its prognosis . HMGB1 aEP inhibiting or neutralizing antibody against HMGB1 significantly reduced the expression of NLRP3  and cleaIn the early stages of sterile inflammation, HMGB1 activates the NLRP3 inflammasome through TLR4 or RAGE receptors . As show2O2 in platelets in HS rats. We found that mitochondrial O2•- (2O2 (ROS mainly includes the superoxide anion (O2–), hydrogen peroxide (H2O2), and the hydroxyl radical (HO–) . Many stial O2•- , cytoplaial O2•- , and H2O2•-  HS activatied the NOD-like receptor signaling pathway, which induced thrombocytopenia and platelet activation in HS rats. (2) High levels of extracellular HMGB1, through TLR4 and RAGE pathway, induced NLRP3 inflammasome activation via ROS possibly in HS rats.MPV and PDW are indices of average platelet volume and variation in platelet size respectively, both of which are markers of platelet activation and thrombocytopenia . In thisHMGB1, a class of non-histone chromosomal binding protein presenting in eukaryotic cells, has been extensively studied as an important molecule activating NLRP3 inflammasome. Our preROS, taking an important role in the homeostasis of aging and death of platelets , are invPyroptosis is a caspase-1 dependent programmed cell death . In the In conclusion, this study preliminarily demonstrates that elevated extracelluar HMGB1 induced high levels of ROS via both TLR4 and RAGE receptors of platelets, which in turn participates NLRP3 inflammasome activation leading to thrombocytopenia in HS rats. The recognition of this study may aid to develop novel therapeutic agents in future.10.7717/peerj.13799/supp-1Figure S1Click here for additional data file.10.7717/peerj.13799/supp-2Figure S2Click here for additional data file.10.7717/peerj.13799/supp-3Figure S3Click here for additional data file.10.7717/peerj.13799/supp-4Figure S4Click here for additional data file.10.7717/peerj.13799/supp-5Figure S5Click here for additional data file.10.7717/peerj.13799/supp-6Supplemental Information 6(A). Changes of platelet aggregation in HS rats (n=6). *p ¡ 0.005 vs sham. (B). Changes of heat exposure time in HS rats (n=6). *p ¡ 0.001 vs sham.Click here for additional data file.10.7717/peerj.13799/supp-7Supplemental Information 7Click here for additional data file.10.7717/peerj.13799/supp-8Supplemental Information 8Click here for additional data file.


Apostichopus japonicus (Selenka) is among the oldest echinoderm fauna and has unique regenerative potential, but the catalytic mechanism of iron oxidation in A. japonicus ferritin (AjFER) remains elusive. We previously identified several potential metal-binding sites at the ferroxidase center, the three- and four-fold channels in AjFER. Herein, we prepared AjFER, AjFER-E25A/E60A/E105A, AjFER-D129A/E132A, and AjFER-E168A mutants, investigated their structures, and functionally characterized these ferritins with respect to Fe2+ uptake using X-ray techniques together with biochemical analytical methods. A crystallographic model of the AjFER-D129A/E132A mutant, which was solved to a resolution of 1.98 Å, suggested that the substitutions had a significant influence on the quaternary structure of the three-fold channel compared to that of AjFER. The structures of these ferritins in solution were determined based on the molecular envelopes of AjFER and its variants by small-angle X-ray scattering, and the structures were almost consistent with the characteristics of well-folded and globular-shaped proteins. Comparative biochemical analyses indicated that site-directed mutagenesis of metal-binding sites in AjFER presented relatively low rates of iron oxidation and thermostability, as well as weak iron-binding affinity, suggesting that these potential metal-binding sites play critical roles in the catalytic activity of ferritin. These findings provide profound insight into the structure–function relationships related to marine invertebrate ferritins.Ferritin is widely acknowledged as a conservative iron storage protein found in almost all living kingdoms.  The content of secondary structure elements, including α-helixes, β-sheets, β-turns, and random coils, was analyzed using Yang’s method by Jasco-Corp.The protein concentrations were adjusted to 0.1 mg/mL with binding buffer . The CD spectra of AjFER and its variants were examined using a Jasco J-1500 CD spectrometer  with a 0.1 cm path length at room temperature. The far-UV region was scanned from 190 to 260 nm in triplicate with a bandwidth of 1 nm. The CD spectral data are presented as the molar ellipticity (deg∙cmg for 10 min at room temperature and then loaded into a quartz cuvette prior to measurement. The hydrodynamic diameter (DH) distribution of the prepared samples was calculated using Omni SIZE 2.0 software. Three measurements were performed with the instrument optimizing the number of runs for each measurement.The DLS data were collected on a Zetasizer Nano Zs instrument  using disposable polystyrene microcuvettes (VWR). The protein samples (1 mg/mL) in 25 mM Tris–HCl, pH 8.0, containing 150 mM NaCl were centrifuged at 12,000× The purified recombinant AjFER and its variants were placed on carbon-coated copper grids. After removing the excess solution by blotting with filter paper, the samples were negatively stained with 1% uranyl acetate for 1 min. The TEM images were captured at 80 kV using a Model H-7650 transmission electron microscope .v/v) glycerol, the crystals were immediately flash-cooled and stored in liquid nitrogen. X-ray data were collected on a BL19U1 beamline with a Pilatus3 6M detector at SSRF  to evaluate the shape and fold of the proteins, providing information regarding the oligomeric state of the biomolecule [To study the conformation of ferritins in solution, SAXS experiments were performed at the BL19U2 beamline of the National Center for Protein Science Shanghai (NCPSS) at the Shanghai Synchrotron Radiation Facility (SSRF). The SAXS data were collected as 20 × 1 s exposures, and the scattering profiles for the 20 passes were compared at 10 °C using 60 μL sample in 25 mM Tris-HCl, pH 8.0, containing 150 mM NaCl. A total of 1500 successive frames were recorded using a Pilatus 1 M detector  with an exposure time of 1.0 s for each frame. The wavelength of the incoming monochromatic X-ray radiation was 1.24 Å with a  package . The scain ATSAS . The datmolecule . The molmolecule . Ten lowmolecule  and the molecule . The docmolecule . The str2+ ion (7.2 mM) samples were prepared by dissolving FeSO4·7H2O in 0.1% (v/v) HCl under anaerobic conditions. The protein samples were diluted to a final concentration of 0.5 μM in 20 mM HEPES, pH 7.0, containing 150 mM NaCl. The ferritins and Fe2+ ions were added to a 1 mL quartz cuvette (Hellma) at final concentrations of 0.357 and 205.7 μM, respectively. The Fe/protein molar ratio was 576. All experiments were repeated in triplicate.The iron oxidation assay for AjFER and its variants was conducted by measuring an increase in ultraviolet (UV) absorbance at 315 nm with a UV–visible (UV–vis) spectrophotometer  [The metal ion content of the protein samples was then quantified with three replicates using a Thermo X Series II ICP–MS instrument . The standard curves for the Fe atoms were obtained using multielement standard solutions. The atoms per ferritin cage were calculated as previously described . Then, t2+ ions toward AjFER and its variants, MST experiments were performed using a Monolith NT.115  and standard capillaries. Each protein sample was labeled using a Protein Labeling Kit RED-NHS 2nd Generation  according to the manufacturer’s instructions. Briefly, 90 µL of protein solution (10 µM) in 20 mM HEPES, pH 7.0, was mixed with 10 µL of 300 µM RED-NHS fluorophore in labeling buffer and incubated for 30 min in the dark at room temperature. Then, the labeled protein was centrifuged at 12,000× g at 4 °C for 10 min to remove the protein aggregates. Five microliters of the labeled protein (a concentration of approx. 200 nM) and increasing concentrations of non-labeled ferrous solution (from 3.05 nM to 10 µM) were loaded into NT.115 standard treated capillaries . Subsequently, the capillaries were inserted into the chip tray of the MST instrument, followed by thermophoresis analysis and the appraisal of the binding affinity (Kd) values. The MST measurements were performed in triplicate at 25 °C, with medium MST power and the red channel using 20% excitation power. Normalization of the fluorescence signal and curve fitting were carried out using MO Affinity Analysis v2.3 software .MST, a powerful technique for characterizing biomolecular interactions, depends on the thermophoresis principle of detecting optical fluorescence properties to quantify the binding affinity of different molecules . To detei) were assessed based on the changes in the 350/330 nm fluorescence emission ratio values. Thereafter, AjFER and its variants were heated at 90 °C for 10 min, and their structural integrity was analyzed using TEM and DLS methods, as described above.The thermostability of the AjFER, MF, M3, and M4 proteins was determined using a label-free thermal shift assay with the Tycho NT.6  via the intrinsic tryptophan and tyrosine fluorescence. The 10 µL solutions of protein samples (approx. 5 mg/mL) were prepared and loaded into Tycho NT.6 capillaries . The thermal profiles of the proteins were recorded during a quick thermal ramp from 35 to 95 °C, with a heating rate of 30 °C/min. The inflection unfolding temperatures  and OriginPro 9.0  software. Significant differences were determined by one-way ANOVA. All data were collected in triplicate and are presented as the mean ± standard deviation (SD). A α-helix, β-sheet, β-turn, and random coil contents were calculated, as shown in α-helix content decreased/increased non-significantly after mutation (p > 0.05). However, the MF protein displayed a decrease of 4.1% for the β-turn content and an increase of 3.1% for the β-sheet content and 2.8% for the random coil content, compared to AjFER. As a next step, the quaternary structure of the proteins was analyzed in solution at pH 8.0 by DLS analysis . Subsequently, TEM was used to determine the inherent shell-like structural integrity of the proteins, as depicted in Based on the known structure of AjFER , the MF,analysis B. The re2+ ions from the outside environment into the cage; however, the limited crystal data were insufficient to elucidate the underlying ion transport mechanism. On this basis, we expressed and purified the M3 protein, successfully obtained the crystal of the M3 protein, and solved the three-dimensional structure at a resolution of 1.98 Å. The diffraction dataset of the M3 protein was integrated into the monoclinic space Group C121, with unit cell parameters of a = 192.83 Å, b = 130.95 Å, c = 122.62 Å, α = 90.00°, γ = 90.00°, and β = 119.40° and with twelve monomers in the asymmetric unit. The crystallographic parameters for the collected diffraction data and structure refinement are summarized in Based on the previously reported crystal structure of AjFER , the res2+ movement, they also prevent the guiding of carboxylate side chains that seemingly dominates Fe2+ ion traffic  , the mac traffic B. Only o traffic C,D. Furtresidues D. Conventionally, it is a widespread belief that electrostatic gradients are of crucial importance for the overall functioning of proteins . The mol2+ ions were bound to the three-fold channel of the M3 protein nanocage by metal coordination bonds, in contrast to the coordination by three Mg2+ ions in AjFER   . Structu 0.10 Å) C. Notable center D–F. As ie center . Fe2 wasq < 0.18 Å−1, which did not decay to near zero at higher q-values and maintained a slight elevation, as expected for hollow spheres (p(r) can provide the radius of gyration (gR), which is based on the full scattering curve and gives the maxD value of a protein as the distance where p(r) approaches zero  of the AjFER, MF, M3, and M4 proteins were 120, 121, 125, and 126 Å, respectively (maxD/2 [p(r) calculation by GNOM (ranging from 52.61 to 53.54 Å)  . When fi53.54 Å) D–G.2+ ions, as shown in 2+ ions. Notably, the oxidized activity of the M3 and M4 proteins decreased by more than 50%, evidencing their crucial importance in promoting the oxidation of Fe2+ into Fe3+ ions. The iron content of AjFER and its variants was determined by means of ICP–MS, revealing 67.3 ± 1 atoms per cage for AjFER, 3.1 ± 0.46 atoms per cage for MF protein, 27.2 ± 0.8 atoms per cage for M3 protein, and 3.7 ± 0.4 atoms per cage for M4 protein (2+ (p < 0.05). The order of iron content among these proteins from high to low was AjFER (2947 ± 54) > M3 (2747 ± 50) > M4 (2064 ± 56) > MF (1474 ± 57) atoms per cage, which was consistent with the color order of the ferritin solutions, which presented from yellow to colorless (AjFER > M3 > M4 > MF)  . CD spec+ uptake A. The CDd 222 nm A, similad 222 nm A. Nevert protein .2+ oxidation, we examined the binding affinity of Fe2+ ions toward the AjFER, MF, M3, and M4 proteins using MST  and right-shifted the binding curve by approx. two-fold to an apparent Kd = 230.6 ± 9.1 nM for the M3 protein and Kd = 199.1 ± 8.6 nM for the M4 protein. In contrast, Kd = 616.9 ± 14 nM for the MF protein.As a biophysical assay, MST can sensitively measure molecular interactions between target proteins and metal ions in solution . Evidentsing MST B. Apart i, for these protein nanocages using a label-free Tycho measurement, and then confirmed their structural integrity at 90 °C using TEM and DLS analyses , M3 (190.1 ± 7.9 nm), and M4 (122.4 ± 5.4 nm) proteins were greater than that of the AjFER protein (13.5 ± 1.0 nm) in solution  [To identify the structure of AjFER before and after mutation, several supporting experimental methods were performed, including CD spectrum, DLS, and TEM analyses. The CD spectra revealed that the AjFER, MF, M3, and M4 proteins all had two very similar negative absorption peaks in the far-UV spectrum A, which n (BfMF) . Accordin (BfMF) ,35. Howen (BfMF) .Fo–Fc maps and the coordination environment of cadmium, three Cd2+ ions were observed in the three-fold channel of the M3 protein, and each Cd2+ ion was coordinated with the residues His116, Asp120, and Cys128, as well as three water molecules. A metal ion binding to the Cys128 residue (forming the Cd–S–Cys charge transfer complex) at the three-fold channel of ferritin revealed that metal ion coordination to sulfur can complement ionic binding during Fe2+ transit through ion channels [α-helical bundles per subunit in the M3 protein  were similar and consistent with those from the crystal structures  distribution by GNOM, were in agreement with the Guinier analysis, varying between 52.61 and 53.81 Å  distribution curves and the Kratky plots indicated that all of the proteins were well folded and had a globular shape (2 of 52.7 using FoXS)  . Howeverng FoXS) . The resS. monotuberculatus), AjFER also plays a dominant role in iron detoxification and the immune response [2+ into Fe3+ via UV absorption at 315 nm, aiming to elucidate the relationship between iron oxidation activity and the protein–metal binding sites  than for the other variants, and the order of the binding affinity was AjFER > M4 > M3 > MF  were prepared and their structural and biochemical features were further characterized. The crystal structure of the M3 protein was solved at a resolution of 1.98 Å and the results suggested that D129A/E132A substitutions significantly affected the quaternary structure of the three-fold channel in contrast with that of AjFER. The structural parameters in solution obtained from the SAXS data were mostly in accordance with those stemming from the crystal structure of AjFER. Compared to AjFER, the site-directed mutagenesis of the MF, M3, and M4 proteins seriously affected the thermostability and catalytic activities of these proteins for Fe


Moreover, γ- and δ-FeOOH were prepared by precipitation methods through slow and quick oxidation, respectively. On this basis, their crystal structure, morphology, and surface area were measured. Then, all the synthesized materials were applied to activate peroxymonosulfate (PMS) to generate sulfate radicals (SO4−˙) for acid orange 7(AO7) degradation. Compared with α-FeOOH, β-FeOOH, and γ-FeOOH, δ-FeOOH showed more efficient decolorization of AO7 in the catalytic system because of its abundant surface area and crystalline structure. The effects of several parameters in the δ-FeOOH/PMS/AO7 system were investigated. The results show that the initial pH, which is related to the features of surface hydroxyl groups, is the decisive factor, and excellent catalytic activity is maintained in the pH range 5–8. The increase of catalyst dosage and appropriate increase of PMS concentration contributed to promote the degradation effect. However, self-quenching was observed in a high PMS concentration system. Moreover, δ-FeOOH was stable after six consecutive cycles, and the leaching of iron ions was negligible. According to the quenching test and electron spin resonance analysis, both SO4−˙ and ˙OH were the dominant radicals for AO7 degradation.In this study, different crystalline structures of FeOOH have been prepared. α-FeOOH was synthesized through a hydrothermal method, whereas β-FeOOH was synthesized  In this study, different crystalline structures of FeOOH have been prepared. The common radicals used in wastewater treatment are hydroxyl radical (˙OH) and sulfate radical , hydroxyl iron oxide (FeOOH) has been reported to be used in different heterogeneous catalysis systems to remove dyes and other refractory contaminants from aqueous solutions. Silva 10–12 Unlike persulfate, which requires other auxiliary methods , peroxymonosulfate (PMS) is more easily activated in a heterogeneous system in a neutral medium, especially by iron-based catalysts such as α-Fe2O3,13,14 Fe3O4,14,15 MnFe2O4,16 and Fe(0).17 However, no study has been reported on the activation of PMS with FeOOH to produce a sulfate radical. On the other hand, based on the existing literature, there are differences in the efficiencies of degradation when FeOOH with different structures is used.5,8 However, the influence of their different crystal structures on the degradation process has been rarely analyzed in detail.The sulfate radical mainly obtained by activating peroxymonosulfate and persulfate has the great advantage of its stabile oxidation reduction potential (2.01 eV at pH 7 and 1.96 eV at pH 4).In the present study, FeOOH nanoparticles with different crystal structures  were synthesized and characterized. Then, the obtained solids were used as PMS activators for the first time to degrade acid orange 7 (AO7), a carcinogenic azo dye. After the activation effect was estimated, the catalytic mechanism was proposed according to the results.2.2.1.3)3·9H2O), ferrous sulfate (FeSO4·7H2O), polyvinylpyrrolidone (PVP K30), ethylenediaminetetraacetic acid disodium salt (EDTA), urea, hydrochloric acid (HCl), sodium hydroxide (NaOH), hydrogen peroxide solution (30%), and ammonia solution (30%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai China). Acid orange 7 (AO7) anhydrous ethanol, tertiary butyl alcohol (TBA), and oxone  were purchased from Aladdin Industrial Co. Ltd. . All the solutions were prepared with deionized water.Ferric nitrate  or Fe(ii) systems. Due to the need for careful control to prevent other oxidation products for Fe(ii) systems, the Fe(iii) system is recommended. In the Fe(iii) system, α-FeOOH can be formed over a wide pH range, either in acidic or alkaline media.18 Because of the rapid formation of precipitates in the alkaline media, it is generally used. In the alkaline media, synthesis involves holding freshly prepared ferrihydrite, which is the precursor of α-FeOOH, at pH > 12 for several days.18 Recently, to decrease the aging time, a hydrothermal method was adopted.19,20α-FeOOH can be synthesized from either Fe was added to a 25 mL solution containing 1.7 g Fe(NO3)3·9H2O. Then, a 9 mL solution of NaOH (5 M) was added to the mixture under vigorous stirring. After 3 h, stirring was stopped, and the stable suspension was transferred into a 100 mL Teflon-lined stainless-steel autoclave that was maintained at 120 °C for 12 h.Briefly, 0.2 g of PVP  chloride solution. In the synthesis process, chloride ion occupies the 0.5 × 0.5 nm2 interstices in the tunnels of the structure and appears to direct this structure and stabilize it. β-FeOOH cannot be prepared at pH > 5 because OH− ion is far more competitive than chloride ion for structural sites.18β-FeOOH was prepared by the hydrolysis of a Fe was added to a 100 mL solution containing 3.97 g of FeSO4·7H2O. Then, the pH of the solution was adjusted to 6.5–7.5 by adding NaOH dropwise under vigorous stirring. Moreover, the bubbling started with an air rate of 2 L min−1 for 12 h. The solid precipitate was obtained by centrifugation and washing 3 times with deionized water. The final product was obtained after drying at 60 °C for 6 h.The γ-FeOOH catalyst was synthesized by an easy precipitation method. Herein, 0.05 g of EDTA 2 at a high pH. Very rapid oxidation is essential for the formation of δ-FeOOH because if the oxidation rate is lowered, γ-FeOOH or Fe3O4 may form.18δ-FeOOH is a ferrimagnetic mineral that is usually produced by the H4·7H2O was dissolved in 100 mL of deionized water. Then, a 20 mL solution of NaOH (5 M) was immediately added to the metal ion solution under vigorous stirring. After this, 5 mL of 30% H2O2 was injected into it to provide the necessary rapid oxidation for the formation of crystalline structures.24–26 After 1 min, the precipitate was centrifuged and washed 3 times with deionized water. The final product was obtained after drying at 60 °C for 6 h.The δ-FeOOH catalyst was synthesized by a modified precipitation method. Typically, 3.97 g of FeSO2.3.θ range 10–80°. X-ray photoelectron spectroscopy (XPS) experiments were used to identify the valency of elements . The Brunauer–Emmett–Teller (BET) method was used to measure the specific surface area and the pore structure of the catalysts . Moreover, the isoelectric point (pHpzc) was measured by zetasizer . Thermogravimetric analysis (TGA) was carried out using a TGA/DSC1 STAR thermogravimetric analyzer from 50 to 400 °C at a heating rate of 3 °C min−1 in a N2 flow.Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were employed to investigate the morphology and microstructure of the catalysts . The X-ray powder diffraction (XRD) pattern was employed to determine the crystallinity of the catalysts  at 40 kV and 30 mA over the 22.4.−1) and oxone solution (0.1 M) were added to the mixture. Moreover, the pH of the reaction mixture was adjusted using 0.1 M HCl and 0.1 M NaOH. After all the abovementioned steps, the total volume of each suspension was adjusted to 100 mL with deionized water.Catalytic experiments were conducted in common 250 mL conical flasks at 25 °C with 120 rpm. The catalyst suspensions were prepared by dispersing the catalysts in flasks with deionized water under ultrasonication . Then, some AO7 solution  .The AO7 concentration was analyzed using a spectrophotometer  at 484 nm immediately after the suspensions were filtered through 0.22 μm hydrophilic polyethersulfone membranes . The concentration of iron ion was analyzed by inductively coupled plasma atomic emission spectroscopy . Moreover, the generated radical products were analyzed 3.3.1.The crystal structures of the four FeOOH catalysts were analyzed by wide-angle XRD patterns, as shown in The morphologies and microstructures were observed by TEM and HRTEM. As can be seen in 2 physisorption isotherm and pore diameter distribution of various FeOOH solids. Both the BET surface areas and average pore diameters are listed in 27 On the other hand, β-FeOOH is rich in micropores, which are induced by the slow process of hydrolysis.28 It is interesting to note the highly centralized distribution of mesoporous structures of δ-FeOOH. This can be further confirmed by the type IV isotherm of its adsorption–desorption curve.29,30 The abundant mesopores may be related to the microbubbles produced by H2O2 decomposition during the preparation. The pHpzc values of the four materials  were 6.82, 7.31, 6.38, and 5.84, which were roughly consistent with the values reported in literature.31 All the results may be associated with different crystal types.3.2.The adsorption and degradation of AO7 on various FeOOH solids are displayed in Moreover, similar trends could be observed for degrees of discoloration when different forms of FeOOH were combined with PMS. It can be seen from et al. have found that the higher PMS activation capacity of the prepared porous α-Fe2O3 as compared to that of the commercial α-Fe2O3 may be attributed to the larger surface area of the former.32 Saputra et al. found that α-MnO2 exhibited higher adsorption due to its larger surface area, which promoted the reaction between the sulfate radical and phenol.16 Wang et al. reported that meso-CuFe2O4 with a high surface area displayed a higher catalytic activity than commercial CuFe2O4.33 Among the four FeOOH solids, δ-FeOOH has more surface area than the other three, which provides more active sites in the adsorption and heterogeneous catalytic reaction.Ji 3+ in crystals is hexagonal close packed to make [FeO3(OH)3] an octahedral structure with anions around.34 β-FeOOH belongs to the tetragonal system with a (2 × 2) tunnel structure.18 γ- and δ-FeOOH are layered crystal structures constituted by octahedral [FeO6], belonging to the orthorhombic system and hexagonal system, respectively.35 Because of the abovementioned different structures, the bound water has different locations in the crystal; this leads to hydration of different strengths. Therefore, TGA analysis was performed to demonstrate the location of the structural water in different FeOOH solids. As shown in 36,37 Therefore, the more amount of surface water may induce a higher catalytic activity. δ-FeOOH showed the highest amount of adsorbed water loss from TGA, which was coincident with the result of its highest catalytic efficiency. Due to its excellent specific surface area and crystalline structure, δ-FeOOH owns most active sites among the four solids, leading to the best catalytic activity. However, α-FeOOH with a lower surface area presented a higher catalytic efficiency than γ-FeOOH although their hydrations were similar. This could be attributed to the relatively weak surface FeO–H bonds of α-FeOOH that seemed to favor the interaction of surface hydroxyl groups with HSO5−.31 The relatively weak surface FeO–H bonds of the hydroxylated α-FeOOH lead to a high affinity of its electrophilic H; this makes the surface OH-PMS combination easy. Therefore, the surface hydroxyl groups of α-FeOOH exhibited higher catalytic activity than γ-FeOOH in promoting PMS decomposition.Moreover, the effect of the crystalline structures of the catalysts cannot be ignored. The atomic configurations of all four crystal structures of FeOOH polymorphs are given in Fig. S1.3.3.In the following experiments, δ-FeOOH was mainly used as a heterogeneous catalyst to investigate the influence of various factors, including catalyst dosage, oxidant dosage, and pH, on the catalytic process.3.3.1.−1 to 0.3 g L−1, the degradation rate increased from 0.055 min−1 to 0.088 min−1, and the decolorization efficiency was promoted from 76.8% to 91.4% in 30 min. The notable improvement might be attributed to more active sites provided by more catalysts such that more radicals could be produced in a short time.38 However, the decolorization efficiency only increased to 92.8%, and the degradation rate increased to 0.092 min−1 when the catalyst dosage was increased to 0.5 g L−1. This might be related to the insufficient concentration of PMS in the reaction systems with high catalyst dosages.The effect of catalyst dosage on AO7 degradation in the δ-FeOOH/PMS system is presented in 3.3.2.−1. However, when the mole ratio was increased from 30 : 1 to 50 : 1, the rate decreased from 0.099 to 0.093 min−1. This phenomenon is consistent with many other heterogeneous catalytic reactions for the activation of PMS.39 The increase in PMS concentration within a certain range is conducive for producing more free radicals to attack pollutants. However, too many unreacted PMS in the solution will quench the produced free radicals as shown in the following reaction:40Oxidant dosage can be represented by the mole ratio of oxidant (PMS) and substrate (AO7). The influence of the mole ratio of PMS/AO7 on degradation process is illustrated in Moreover, the limited active sites of the catalyst hinder the increase in the degradation rate. On the other hand, the degradation efficiency of AO7 was 91.7% for the molar ratio of 20 : 1 and 93.6% for 30 : 1; thus, the optimal molar ratio was 20 : 1.3.3.3.−1 and 91.7%, respectively. When pH was reduced to 3, the degradation rate decreased to 0.054 min−1, and the decolorization efficiency was 79.3%. When the solution alkalinity was increased, the degradation rate started to decrease. The value was 0.080 min−1 at pH 7 (decolorization efficiency was 88.1%), whereas it underwent a sharp decrease to 0.028 min−1 at pH 9 (decolorization efficiency was 60%). These results may be related to the charge state of catalyst surface and the species of PMS in the aqueous solution.23,41 The pHpzc of δ-FeOOH is 5.84. Most of the surface hydroxyl groups are at a neutral state when pH is close to pHpzc. When pH is far below or above the pHpzc, the surface will be charged as follows:31The effect of initial pH on the degradation process has been demonstrated in 42 On the other hand, since the catalyst surface is heavily negatively charged at pH 9, HSO5− and organic matter hardly interact with the catalysts. Moreover, at pH 9, HSO5− will further transform into SO52−; thus, the electrostatic repulsion between the anion and catalysts becomes stronger.23 In addition, when solution pH exceeds 9, ˙OH would scavenge SO4−˙ and become the dominant active species,31 which possesses reduced oxidative capacity. These findings can explain the sharp decrease in the decolorization efficiency at pH 9.Thus, when the pH is 3, the catalyst surface is highly protonated, which is unfavorable for the non-polar ends of the accessing organic matter. When pH was 5 and 7, both acid centers and alkaline centers existed on the surface of catalysts, which induced the organic matter to easily access the interface.3.4.43,44 Therefore, successive experiments were conducted to explore the reusability of δ-FeOOH under the same conditions. After each trial, the used catalysts were obtained followed by washing with ethanol and deionized water, separation by centrifugation, and then drying at 60 °C. The catalysts were repeatedly used six times. 3+ in solution at each catalyst cycle. After recycling for six times, the δ-FeOOH/PMS system could still maintain a high catalytic efficiency. The decolorization efficiency of AO7 only decreased from 91.7% to 84.2%. Furthermore, the leached concentration of Fe3+ in the solution after each reaction cycle was determined using ICP-MS. As can be seen from −1. Both these results revealed the high stability of the catalysts, and the degradation reaction occurred at the interface of δ-FeOOH.45 In addition, the same successive experiments were carried out on the other three types of crystals, and the decolorization efficiency and the leached concentration of Fe3+ in each experiment are shown in the Tables S1 and S2,Reusability is an important factor that evaluates the performance of the catalyst in practical applications.via XRD analysis. As can be seen from The stability of the crystal structure of catalysts was determined via XPS analysis. As can be seen from 3/2 peak was present at 711.0 eV for the fresh catalyst, whereas it was at 711.2 eV for the catalyst after six cycles. The appearance of 0.88% Fe(ii) in the catalysts after six cycles indicated the occurrence of the reduction process during the reaction. The results indicated that δ-FeOOH was suitable to be used as a PMS activator.The elemental changes on the surface of catalysts before and after the reaction cycle could be confirmed 3.5.4−˙, ˙OH, and SO5−˙.46 Among them, SO5−˙ cannot decolorize AO7 owing to its low oxidative potential.47 To explore which radical (SO4−˙ or ˙OH) played the major role in the degradation process of AO7, both ethanol and tert-butanol (TBA) were added to the solution as radical quenching agents. Ethanol can react at a high rate with both SO4−˙ and ˙OH . However, TBA can only react rapidly with ˙OH .48 The effect of different quenchers on the degradation of AO7 in the δ-FeOOH/PMS process is shown in 4−˙ and ˙OH are generated in δ-FeOOH/PMS to attack AO7.According to previous reports, PMS can produce multiple radicals such as SO4−˙ and ˙OH were generated in the δ-FeOOH/PMS system, ESR tests were conducted to detect SO4−˙ and ˙OH during the catalytic process. DMPO was used as the spin-trapping agent, which formed complexes with SO4−˙ and ˙OH. Then, SO4−˙ and ˙OH could be detected by measuring the signals of DMPO-SO4 adducts and DMPO-OH adducts, respectively. As shown in a(N) 1.49 mT, a(H) 1.49 mT, obtained by simulation) are completely consistent with those of DMPO-OH.49 Moreover, the special hyperfine coupling constants of DMPO-SO4 (a(N) 1.38 mT, a(H) 1.02 mT, a(H) 0.14 mT, a(H) 0.08 mT) were obtained by simulation from the spectra.50 All the results further confirmed that both SO4˙− and ˙OH were generated in the δ-FeOOH/PMS system.To further strongly prove that both SO3.6.51 PMS can combine with the metal oxide through the surface hydroxyl groups and then undergo a redox reaction with the surface metal of oxide to produce the sulfate radical. Moreover, the oxidation state on the surface metal will consistently change with the surface hydroxyl groups.52–54 Thus, the in situ spectroscopic analysis could detect the intermediates related to PMS decomposition on the surface of metal oxide.It has been reported that the hydroxyl groups on the surface of the metal oxide play an important role in the heterogeneous oxidation reaction.in situ characterization of δ-FeOOH surface during catalytic decomposition of PMS was conducted via ATR-FTIR. As shown in −1 and 1249 cm−1,55 which originated from S–O of either HSO5− or SO42−. However, the intensity of 1249 cm−1 had a significant decline when PMS was mixed with δ-FeOOH. Thus, the band at 1249 cm−1 is related to HSO5−. The intensity decline implied the decomposition of PMS on the oxide surface. Moreover, a red-shift by 19 cm−1 of S–O crest at 1249 cm−1 occurred with the addition of δ-FeOOH as compared to the case of the pure PMS solution. This demonstrates that –OH in HSO5− attracts more electron density from the near S–O bond making it weaker;53 this may indicate that the surface metal captures electron from –OH, and an increase in the electron attraction from neighboring S–O leads to the generation of a sulfate radical.The et al., the stretching vibration of surface hydroxyl is around 3100 cm−1.56 In −1, which indicates the presence of surface –OH groups on δ-FeOOH. In the presence of HSO5−, this band was red-shifted by 6 cm−1. It is a symbol for the replacement or complexation of the surface –OH groups by HSO5−;53 this reveals the formation of a complex between HSO5− and metal oxide, and HSO5− loses an electron to the surface Fe(iii) to generate SO5−˙.According to Zhang 5−˙ is generated, whereas  PBM data was replaced with SVG by xgml2pxml:<glyph-data id="z.tbd" format="PBM" resolution="300" x-size="17" y-size="11" xml:space="preserve">0000000000000000000000000000000000000000000000000000111111111111111000000000000000000011111111111111100000000000000000001111111111111110000000000000000000000000000000000000000000000000000</glyph-data><svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="23.636364pt" height="16.000000pt" viewBox="0 0 23.636364 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>Created by potrace 1.16, written by Peter Selinger 2001-2019</metadata><g transform="translate scale" fill="currentColor" stroke="none"><path d="M80 600 l0 -40 600 0 600 0 0 40 0 40 -600 0 -600 0 0 -40z M80 440 l0 -40 600 0 600 0 0 40 0 40 -600 0 -600 0 0 -40z M80 280 l0 -40 600 0 600 0 0 40 0 40 -600 0 -600 0 0 -40z"/></g></svg>Fe(iii) is reduced to Fe(ii), and the generated Fe(ii) can form a complex with HSO5−Fe(ii) can be oxidized to Fe(iii); this releases SO4−˙ into the solution 57 Finally, SO4−˙ can react with OH− to produce ˙OH 53Based on the analysis of the obtained results, the possible activation mechanism of PMS by δ-FeOOH was proposed. PMS initially conjuncts with surface –OH of FeOOH 4−˙ and ˙OH, can then attack the chromophore of AO7. All the processes are radical reactions. The proposed activation mechanism of the δ-FeOOH/PMS process can be described as shown in The produced radicals, including SO4.2 adsorption/desorption, TEM, HRTEM, and other simple methods. Then, they were used as PMS activators for the degradation of AO7. According to the tests, δ-FeOOH was confirmed as a potential catalyst due to its much higher efficiency for PMS activation than others. The catalyst showed stability in element valences and catalytic activity during successive repeated reactions. The degradation reaction was a radical reaction, and both SO4−˙ and ˙OH were suggested as radical species in the PMS/δ-FeOOH system. The catalytic mechanism was proposed with the aid of the in situ ATR-FTIR analysis and XPS spectra. These results reveal that δ-FeOOH is an effective, environmentally friendly, and low cost catalyst for the efficient generation of sulfate radicals and hydroxyl radicals from PMS to degrade organic pollutants. They own a great potential in the advanced oxidative treatment of industrial wastewater and other contaminated water.In this study, FeOOH with four different crystalline phases  was synthesized and characterized by XRD, NThere are no conflicts to declare.RA-008-C7RA12615H-s001