The dimensions of modern energy regimes, on the one hand, and the necessary transport and trade of globally produced renewable energy, on the other, will require the use of chemical batteries in conjunction with the local production of renewable electricity. The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary, robust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO2 does not yet exist.Efforts to obtain raw materials from CO 2 is one promising option for chemical batteries. The intricate material science of Cu catalysts to control the selectivity of this reaction is discussed in detail in this Review.Sustainable energy systems cannot exist without the storage of volatile renewable electricity. Whereas electrical batteries can be used for small amounts of energy, chemical batteries are required for large amounts of energy. The hydrogenation of CO There are few chemical reactions that concern so many people outside of chemistry as does the future processing of CO2. In reference to these efforts, we use the abbreviation “CCU” (Carbon Capture and Use). The use of CO2 is first considered in the context of sustainable energy regimes. Here, the focus is directly on the storage and transport of sustainably produced hydrogen in the form of “synthetic fuels.” The notion should not be entertained here that hydrogen is a means of removing CO2 from the atmosphere. Rather, CO2 is a component of a circular economy for renewable energy. This concept will be expressed more thoroughly in the introduction to chemical batteries. As with electrical batteries, there is a chemical accumulator (or rechargeable battery) that can be repeatedly discharged and charged in a closed cycle. There is also a type of electrical battery that cannot be recharged. This device corresponds to the single‐use binding of CO2 to renewable hydrogen in synthetic fuels, which is referred to in the literature as “linear CCU” in contrast to the use of repeatedly  collected CO2 that leads to “circular CCU”.Current Research considers the hydrogenation of CO2 prices and taxes must be divided into three categories if a successful incentive is to be achieved. These categories are: no utilization (max price), linear use (discounted price), and circular use (no price). For each case, different definitions and rules apply.The concept of the battery is not only a quaint formulation, but also an attempt to draw the attention of energy regime regulators to the fact that CO2 as a raw material in the chemical industry. Here, many synthetic routes and concepts for novel production chains exist. They are of importance for today's chemical industry and will continue to be so in the future.A significant aspect—and one that is often treated in a cursory fashion—is the use of CO2 with hydrogen, whereby copper is used as a thermochemical or electrochemical catalyst. Beginning with a network of reactions, it will be shown that the term “copper metal” is too simple a term to describe the active component in the catalyst if the goal is to understand the different effects of nominally the same copper catalyst in the reaction network.A large part of this Review is devoted to understanding the reaction of COFigure 2Batteries are devices that store energy. The name comes from military speech and refers to the strengthening of an effect or action by organizing single units into groups. In electrochemical batteries,One disadvantage of both rechargeable and non‐rechargeable batteries is that the energy is stored in the device—specifically in the solid electrodes. The alternative redox‐flow batteryElectrochemical battery:Storage:Recovery:Equations (1) and (2) are simplified to illustrate the principle of storage using ions. Equation (1) implies that, in real batteries and accumulators, complex reactionsChemical battery:Primary conversion:Storage:Recovery:2 as the storage molecule. For this material, a closed cycle is certainly necessary because of its multiple other functions in the atmosphere (greenhouse gas). The industrial revolution and all the resulting developments have depended on the high energy density of fuels and the resulting large amount of CO2 released through the use of these fuels. Concepts for carbon cycles were first suggested by Asinger and Olah.Chemical batteries require a circular economy of storage molecules to enable a constant supply of energy; these molecules are a hallmark of a sustainable energy regime. Water, oxygen, and nitrogen molecules are present in such large quantities on Earth that no closed cycles are necessary. The purpose of the present Review is, however, to consider the suitability of CO2) reaction in an electrochemical reactor,At a first glance, the lengthy process chain makes it appear that chemical batteries should perhaps be limited to hydrogen. The concept of a hydrogen economyThere will be many application scenarios for a circular economy of energy carriers and will include centralized as well as decentralized solutions.32 has long been the subject of research.2 is an atrocity or even a “thermodynamic crime”.2”2 lies so far down the energy scale. Those opposed to the thought of a CO2 cycle suggest deposition and confinement of the material as the only scalable means of combatting the greenhouse effect.2 plays an important role in these cases. For this reason alone, it is advisable to thoroughly study the chemistry of this molecule and its applications as a chemical battery.The chemistry of CO2 is, at a value of −393 kJ mol−1, 318 kJ mol−1 more negative than the value of methane, which will be used here as a general, or standard, energy carrier. If CO2 is allowed to react with a typical base such as Ca(OH)2 to form the corresponding carbonate, 1207−393=814 kJ mol−1 are gained—a significant value for a supposedly low‐energy2. Although mineralization processes chemically bind the CO2, they may also have a large number of effects on the bedrock.To assess the thermodynamic arguments, several standard enthalpies of formation will be considered. In no way do such considerations substitute for a complete analysis of chemical energy storage processes. However, such analyses contain so many process‐specific values that the results lead to a few general conclusions. Detailed examinations of this kind can be found in the literature.17 The s2. It should be remembered here that the initial energy investment required for the synthesis of one mole of storage molecules (charging) is accompanied by a release of enthalpy during the reaction. The difference in these values is equal to the enthalpy which remains stored in the system. The energy source for the charging step is hydrogen, which must be obtained from sources and processes which release no CO2.Table Table 2 hydrogenation is still being debated, the atmospheric concentration of CO2 has climbed to 408 ppm and is growing at 2.17 ppm each year.2 as a raw material in the chemical industry, often viewed as a source of motivation in the literature, offers little in the way of greenhouse gas reduction when compared to the use of CO2 as an energy carrier. In Germany, 834 PJ equivalents of fossil resources were consumed by the chemical industry in 2018. This usage corresponded to 6.3 % of Germany's primary fossil energy requirement of 13 106 PJ. If the supply of raw materials for the chemical industry were shifted to CO2, a considerable increase in renewable electrical energy would be necessary to chemically reduce it. On average, three molecules of water would be needed to split one molecule of CO2, in addition to the energy required for the conversion processes themselves. The DECHEMA study “Roadmap Chemie 20502 to hydrocarbon feedstock, which corresponds approximately to Germany's entire electricity consumption.Although the value of CO2 if this results in simplified synthesis processes, or if waste and CO2 emissions can be reduced. This change is only sensible if the necessary energy (and hydrogen) are supplied solely2 emission on a per‐product basis. Chemistry can also establish new, less‐intensive CO2 pathways for the supply of essential compounds. This is, in fact, a classic area of research in chemistry[The products of the chemical industry then should be manufactured from COchemistry24 and h2 still contained in them is set free. If these practices were made to complete a cyclic process that used the burned waste as a source of CO2, the cycle would remain closed. If control over the chemical products is lost, for example through landfilling, the integrity of the closed cycle cannot be guaranteed and the reduction of CO2 emissions2.Many products of the chemical industry are burned at the end of their lifecycle, or after recycling, whereby the CO2 emissions in this area would be the application of hydrogen, which is used in large quantities in the petrochemical industry, from fossil‐free sources . This relationship is old2. It is for this reason that the chemically simple, but urgently needed products arising from the reduction of CO2 to “solar fuels” are the most valuable productsA central role for chemistry will be the storage and transport of renewable energy, which is initially produced as electricity, so that this energy can become a globally available traded commodity.2 following photochemical water splittingFinally, the direct replication of nature and the energy storage cycle , running facilities on sources of intermittent energyCentral questions surrounding the incorporation of CO2 is a reactive molecule. The carbon contained in these materials may not be able to make a transition to a higher oxidation state, but the molecule is still reactive in many ways. It can, for example, generate different forms of organic and inorganic carbonates.2 can form adsorbates with metal surfaces, which have already been investigated for some time.2 with hydrogen.The processes of photosynthesis, biomineralization, and also technical mineralization42. To provide an answer, an overview of the current literature will first be provided and discussed. Along the way, the meaning of the word “valuable” should be kept in mind. The view is widespread2. Inorganic carbonates,2 plays the role of a building block during synthesis, are also considered valuable. The view is also held that CO2 could be an important raw material2 building block. The molecular chemistry of CO2 has already been reviewed many times[2 reduction can be described in a similar way. The current Review uses the insight provided by these studies and makes an attempt at a critical analysis. This goal seems justified in light of the fact that the long history of research on the reduction of CO2 has led to a constriction of research foci and makes a satisfactory overview difficult. The formation of methanol will be considered here as an example of this situation.The question considered here will be which reactions lead to valuable products through the reduction of COany times45, 462. The size of today's oil and gas industry provides an impression of the required dimensions. One application of CO2 hydrogenation can be found in the production of synthetic fuels.[2 is not only its economical nature, but also pertains to the defossilization of the energy regime. It is noted here that this viewpoint is not shared by all[2 as the energy carrier has also been criticized2 and that the origin of the CO2 (from the air or biomass (green), or from fossil sources (black)) plays a role.[2‐based processes. A fundamental examination of the hydrogenation of CO2 has been reported by Bardow et al.A carbon cycleic fuels.21, 49The selection of a reaction for a chemical battery is currently in no way straightforward. High‐level discussions surrounding the structures of sustainable energy regimes currently include critical considerations of the use of chemical batteries. Unfortunately, it must be stated that none of the reactions discussed above have been tested in the form of a chemical battery for the storage of hydrogen with charging, transformation, and discharging on the scale required for a global technology. In Table 2, the central quantity for battery applications—the storage capacity—is given with a negative weighting to take into account the loss of hydrogen in the form of water molecules during the conversion of CO2. Consideration of these weighted capacities is quite sobering for all the CO2‐based processes. The comparatively favorable values for ammonia , methane (17 %), higher alcohols (16 %), and alkanes (14 %). The analysis underscores the view that the hydrogenation of CO2 is specifically geared toward the preparation of synthetic fuels.Figure Web of Science is used  is approximately 54 TWh/annum, or about 10 % of Germany's total consumption of electricity. In this project, only currently existing technologies can be employed to at a minimum partially redirect emissions into chemical applications in the next 10 years. Nevertheless, significant challenges appear during these operations in connection with the realities of fluctuating primary renewable energy sources. These challenges lead, with respect to opportune economical solutions, to fundamental questions about the dynamic operation of methanol synthesis or the configuration of minimal gas purification methods which do not harm the subsequent catalytic processes and also leave the products free of unwanted trace substances. The argument that other processes may also be involved in the reduction of emissions in heavy industry is taken into account in the C2C project by diversification of the product portfolio to include urea, higher alcohols, and reactive intermediates from CO2 as well as in using unavoidable CO2 sources such as lime production and waste incineration.This argument can be elucidated with an example. In the project C2C,72 begins with the transfer of negative charge to the initially linear CO2 molecule. As a result, the molecule becomes bentThe activation2 with hydrogen results in the accessibility of a large number of molecules which are both valuable for chemistry and suitable for renewable energy storage. The energy is delivered via hydrogen. A common complaint in the discussion on the production of resources from CO2 is that the processes are too “energy hungry.” All reduction reactions produce water, with the exception of the formal addition of hydrogen to CO2 to form formic acid , no water is produced. The “storage effect” is smaller in this case—but the energetic losses are also lower. For this reason, the formic acid molecule is also a valuable storage medium[It is apparent that the conversion of COge medium55 and o2 initially involves a series of acid–base reactions. Carboxylate (a) can either dimerize[c) or it can react with a Brønstedt or Lewis base (BOx) to produce carbonate (b). If water is added, carbonic acid is produced, which, if it is not esterified, will decompose into CO2 and water. The process can proceed spontaneously or with metal catalysts which poorly bind atomic oxygen.6). If hydrogen is present, H‐CO (d) is easily formed and can, via n, react to produce methoxy compounds (g) and methanol. The reaction need not proceed along the “formate,” or “reverse water‐gas shift” (RWGS) route . This fact is important, because it means a mechanistic pathway to methanol is available, which is consistent with the stepwise hydrogenation of CO and yet contains the carbon atom from the CO2 molecule. Isotope marker experiments designed to explain the reaction pathway of methanol synthesis are, therefore, ambiguous. A precondition for this reaction pathway is that the generated carbonate decomposes back to CO2 to close the catalytic cycle. This is the case for the catalyst system Cu/ZnO under the reaction conditions, as was shown through in situ investigations on the formation of this catalyst.2) from the intermediate (n). This process does not take place under normal reaction conditions because the additional energy of the subsequent reaction to generate methoxy compounds (g) is significantly higher than the energy maximum[From Figure  dimerize56 (a rea) to formate (f). From this point, the process splits and results in the hydrogenation of either the carbon (j) or oxygen (k); the latter can lead to the production of formic acid (3). The intermediates j and m are identical, which makes experimental differentiation problematic. It has been observed that both formate (f), when used as a precursor for the synthesis of the Cu/ZnO catalyst, and formate co‐adsorbed with oxygen on copper decompose at 473 K.The reaction pathway60, 621) is formed from intermediate g, which, in the presence of acid sites, reacts further to generate dimethyl ether. Selectivity for methane (5) has not been observed under normal conditions with a copper catalyst. Under the conditions for electroreduction,g or via intermediates n and h. In contrast, the reactions between intermediates h and methanol to generate higher alcoholse. To observe these reaction products on copper, on the other hand, co‐doping with a transition metal (oxide) or electroreduction is needed. The innumerable resulting reactions, when starting from CO or methanol,2 is highly efficient; it would require, however, the design of a dedicated combustion process to avoid its blending with other molecules, which diminishes the synthetic advantage. The combustion of pure methanol can be very clean and minimize local emissions.Under the conditions of gas‐phase catalysis, methanol  and to “charge” the battery with hydrogen from renewable sources (“green hydrogen”). Concern with respect to this concept has been expressed2 were chosen as the source of carbon instead of synthesis gas.Today, methanol is produced industrially from synthesis gas and hydrogen. Since the proposals of AsingerHowever, the situation is more complex if the reaction is made to proceed in a way that achieves a maximum space‐time yield. In this case, the catalyst operates near an equilibrium line which is defined through a network of reactions Eqs. –8.(6)C2 conversion, there is always also a partial pressure of CO and water in the catalyst bed. If pure CO is used as the initial molecule, very harsh conditions are required (BASF process) to produce methanol.2. In this process, the multifunctionality of the ZnO componentsThus, under the conditions of high CO2/H2 input gas is used. In technical processes, the synthesis gas can be chosen such that only small amounts of water are produced to protect against possible decomposition of the delicate nanostructures of technical catalysts. It should be noted that a catalyst decomposition through reduction and resulting in brassIf the fact is considered that the conversion into methanol is restricted under practical, relevant conditions,2 to methanol has been determined as a function of temperature. Figure 2 with a relevant space‐time yield.The performance of technical Cu/ZnO catalysts during the reduction of CO2 system can exhibit noticeably increased productivities of up to 1.2 g g−1 h if the space velocity is increased by about a factor of 10 compared to normal values.Figure 2, along with water, are present along most of the length of the catalyst bed. It is, therefore, not strictly necessary[2 to methanol because the performance and stability of the Cu/ZnO system approach the reference values under synthesis gas conditions. It seems possible to influence how completely the equilibrium is reached through modification of the composition of the Cu/ZnO system. Systems with large amounts of ZnOThe relatively marginal sensitivity of the reaction to the nature of the catalytic systemnecessary58, 712, and hydrogen, it would be desirable to increase the catalyst activity so that methanol synthesis can take place at temperatures below 473 K, where methanol is in a liquid state (slurry).2 and CO.As a consequence of the position of the chemical equilibria between CO, CO92 due, among other things, to its ability to catalyze a gas‐phase reaction with a high selectivity for the production of methanol. If solid acids are present, the reaction can also produce dimethyl ether.[2 (CO). It is certainly possible to modify copper with co‐catalysts to “toggle” it into catalyzing the formation of C−C bonds.2.[The element copper has achieved a distinguished position in the catalytic reduction of COyl ether.83, 912 with copper can lead to an entire series of hydrocarbons is, therefore, even more striking.[The observationstriking.95 This It can be concluded from these considerations that the branching of the reaction network in Figure 2 and to the intermediates in Figure The different chemical states of the copper metal can be understood by remembering that electrochemical reduction takes place at room temperature and diffusion of oxygen atoms is slow. Rapid and deep diffusion is, however, very much a possibility2 with molecular catalysts at interfaces.2 compounds can be traced to the surface oxide,1 compounds.Comprehensive studies on the selectivity of Cu metal in differing preparations as well as Cu oxides and alloys are described in an exhaustive review on the electroreduction of CO2O.This hypothesis differs from an assumption in gas‐phase catalysis that adsorbed atomic oxygen is necessary as an oxidant for CO in the synthesis gas to enable formate intermediates. Pulse kinetic measurements have clearly shownx1− can “creep” onto and around a Cu cluster that has been, for example, contaminated with and roughened by dissolved oxygen.The question of the chemical nature of the active copper has long been the subject of investigation.105 It i2 can be further clarified if another element,2‐hybridized carbon was used. With respect to the latter, a heterolytic H2 dissociation seems to occur.It is questionable whether the functionality of copper in the electroreduction of CO102 . In contrast, the embedding of copper in ZnO creates lattice defects in the copper, thereby resulting in a significantly higher lattice constant of 0.3625 nm.A richly featured nanostructure and a strong dependence of the reactivity on copper particles have been identified as important characteristics105c for2O crystallites by topotactic reduction. The structural motif of CuO, on the other hand, requires a non‐topotactic conversion where the roughening is a consequence of the formation of polycrystalline particlesThe astonishing diversity of the answers to the seemingly simple question about the nature of the catalyst surely means that there are different forms of methanol copper. The reaction environment may be partially responsible for the variety of active states. In particular, the presence of oxygen in the many manifestations of activated catalysts plays an important role along with the non‐translational structure of the copper. These two factors are linked, because oxygen causes stress and strain in copper which results in a roughening of the copper surface. Additionally, an effect of the initial state of the oxide on the formation of the roughened surface is also relevant. The initial state  is dictated by the activity of oxygen during formation of the preceding compound. For example, smooth metallic particles in all orientations can be formed from Cu2, CeO2), perimeter states[2 on an Au/MgO model system, has been published by the Freund group.The following overall picture of methanol copper during gas‐phase catalysis arises: in all cases, metallic copper forms the matrix phase—all analytical methods show this to be the quantitatively predominant phase. The simplest case is metallic copper with a roughened surface. The roughness is likely caused by internal forces of stress and strain;er states135 will2 activation. Oxygen atoms are required for the partially reduced and porous2 reduction.If ZnO is present, it can be chemically reduced through very dry conditions or a high chemical potential of CO. The result is the local generation of a brass crystal structure.107 Unde2 as probe molecules. For such an experiment, nanostructured Cu was synthesized by using ZnO doped with Al or Mg (3 % by weight in each case) using the conventional co‐precipitation method. As a comparison, Cu nanopowder was also produced through precipitation and activation.The existence of methanol copper can be verified, beyond the controversial discussion summarized here, by microcalorimetric experiments with CO and CO2 on the Cu catalyst with inhibited ZnO reduction is shown in Figure 3, pure nanocrystalline MgO was measured as a comparison. The behavior of CO2 and CO are largely the same with regards to the dynamic exposure of the adsorption sites. In the case of CO2, however, the absence of the reductive interaction between CO2 and ZnO results in the more expected behavior of gradually weakening adsorption.It is evident from Figure 112 and its products during a catalytic reaction. The quantitative determination of the reactive surface of the Cu/ZnO system provides an example. Unspecific adsorption of nitrogen generates the geometrical surface. Using other probe molecules with specific chemisorption results in significantly smaller “active surfaces” which can, as expected, be differentiated through selection of the probe molecule. Table Molecular catalysts with copper show a diverse chemistry of interaction with the element oxygen depending on the different geometries and oxidation states.It is unlikely that pure copper is the only active catalyst, because both samples in Table 2 represents a case of redox chemistry. In the formal simple reaction [Eq. The ability of copper to stabilize multiple chemical states of oxygen is significant because the reduction of COtion Eq. , two elef to k) is shown in Equation f were identical, it would be difficult to understand why an oxygen atom would remain bound, especially when the large number of active hydrogen atoms present under the reaction conditions is considered. If, on the other hand, a step exists on the metal surface or an impurity atom (a ZnO site for example) is next to a Cu adsorption site, the likelihood of reaction (10) in Figure f are no longer identical.A further reaction  and are converted with atomic hydrogen. This situation occurs during electroreduction. Such hydrides cannot, however, be produced though synthesis from the elements. It is not known whether hydrides are formed during the reduction of CuxO under a high hydrogen pressure. In this context it is interesting to note that the polymeric substance “CuH” exhibits the same red‐brown coloration as Cu2O and even contains embedded residual oxygen. Thus, the idea that electron‐poor copper is important for the reduction of CO2 benefits from a new aspect resulting from the so far largely overlooked possibility of CuH formation  as the active form of the copper MeOH catalyst.An analogous consideration holds for the form of the reacting hydrogen. It can be transferred as an H atom , as a hydride , or as a proton (on an oxo group) plus an electron. In each case, the participation of the catalyst is necessary, as it, at the very least, exchanges charge with the adsorbates. These processes must also be investigated in the context of a microkinetic description to clarify, for example, whether some of the processes are simultaneously active. Illustrating the fine differences in the reactivity of metal–hydrogen species, studiesThe reactions in Figure 2. For example, this happens in electroreduction, with its low thermal excitation for restructuring at the electrode. In situ XAS measurements on the deposition of Cu on a gold electrode showed[+ and some Cu0 arise from the disproportionation of Cu+ into Cu2+ and Cu0 instead of only metallic copper, which makes up the main phase. If this mixture is used under electrochemical reduction conditions for CO2 in an alkaline electrolyte and a strong negative potential, the resulting material may not only be copper, but rather a metal modified with oxygen (OH).[3O+.2.Under mild reaction conditions, different surface structures co‐exist and will, therefore, generate a number of reaction products from COde showed116 that2‐H2 system.Under harsh conditions, on the other hand, only states such as “alloy,” “surface oxide,” or “perimeter oxide” will exist and, therefore, only a small number of products will be generated. An example of this is the high‐pressure synthesis of methanol. If, however, stable co‐catalysts such as transition‐metal atoms and their low valency oxo compounds are present,66e high122 leads to a wide array of products. As fuels, some of the products will play a strategic role in future energy regimes along with a circular economy and the storage of fluctuating renewable electricity. The products make possible a concept such as the chemical battery, with which nearly an unlimited supply of renewable energy can be stored and transported. Only thanks to the chemical battery concept will a global trade of renewable energy be possible and replace the trade of fossil energy carriers. Furthermore, the chemical battery enables the use of renewable energy in the mobility sector, where, most notably, high‐performance applications with electric batteries are difficult to implement. The similarities of the physicochemical material characteristics of chemical batteries and fossil fuels allow the further use of contemporary converters (motors and turbines). The chemical battery “hydrogen” is only able to fill this role in a limited way and in addition requires new infrastructure for transport to the user.The reduction of CO2 will change parts of the resource infrastructure of the chemical industry. In some cases, the use of CO2 as a building block for chemical synthesis will lead to new methods for the production of complex chemical products. Such processes will contribute to a reduction of CO2 emission from the chemical industry by simplifying synthetic routes and thus saving fossil raw materials and process energy. All of these applications are based on the chemistry of CO2 in different reaction environments, which, at a fundamental level, is well‐understood.Other products resulting from the hydrogenation of CO2 is to use it as an energy carrier in the carbon cycle economy. Its use as a scavenger for fossil emissions would be way too energy‐intensive and costly and could thus not contribute to the mass‐reduction problem of reducing emissions (Gt per annum). The prime objective must be to stop the emission of CO2 from fossil sources, and this can only happen if we replace fossil by renewable energy carriers on a global scale. This in turn requires the concept of chemical batteries as a basis and with it the application of well‐understood catalysis technologies.The objective of the hydrogenation of CO2 chemistry has devoted much attention to a small number of molecules such as methanol and olefins. A wide range of other possible sophisticated reactions has received much less focus in research. This choice can be justified by the challenge of both the urgency and the scale of the processes based on CO2 as a raw material. In this field, as well as with the production and purification of CO2 , there are a multitude of scientific problems still to be solved. At the same time, though, many synthetic and catalytic studies in CO2 chemistry do not identify any pathways to scalable processes for the next two decades. One suggestion to overcome this is that, in the future, authors should either realistically evaluate their prospects for contributing to climate protection and reflect on these considerations during their experiments, or the generation of knowledge should be expressed as the motivation of the work.Scientific research into CO2, which is, depending on the conditions and catalyst, branched in different ways. The multiple chemical states of the central element copper have only now begun to be recognized with respect to the molecular geometry and the presence of modifying atoms such as oxygen and structural dynamics. Some controversies in the literature have arisen from the interpretation of experimental results that have been reported without adequate critical reflection on the rich and dynamic structural details of the copper. The methods were not surface‐sensitive enough, did not contain enough evidence to support the conclusions, or did not sufficiently consider gaps in chemical potential connected with the dynamic reaction of an active catalyst. These insights have been possible thanks to many challenging operando experiments. Theory has also been responsible for supplying critical motivation for the atomic‐level understanding of reaction pathways and the structural peculiarities that lie at their core. Nevertheless, today, a comprehensive description of the fundamental aspects of a reacting system consisting of a catalyst, carrier, and the reaction conditions is still not available.This applies also to the literature on methanol synthesis and higher alcohols reviewed here. Comprehensive and highly detailed studies with seemingly contradictory conclusions describe a reaction network for the hydrogenation of CO2 to methanol proceeds by the formate/RWGS route  to produce a mixture of methanol and higher alcohols. An opportune use of the resulting products could be fuels[2 hydrogenation chemistry may then approach the well‐known FT chemistry.The assumption cannot be substantiated here be fuels53a to a2 reduction also used here for discussion. The intended reduction of complexity excludes the detection of significant aspects of the reactivity under high‐performance conditions that depend on the chemical constitution and dynamics of the metal surface of the working catalyst. This shortcoming results in the well‐known “science gaps” in catalysis research. To overcome this challenge, the application of artificial intelligence and its diverse methods may result in decisive contributions leading to structure–function correlations that can bridge the gaps.If, as is the case here, the fundamentals of a complex reaction sequence in a network of processes are unclear, an effective recourse is the employment of model systems . Complexity can be reduced in this way to gather detailed information149 on e2 reduction at interfaces using modern methods of digital catalysis research would quickly fail for this reason. Thus, one point of urgency in the current Review is to declare that a standardization of methods and descriptions in future theoretical and experimental studies on the hydrogenation of CO2 should be introduced as a precedent for the execution and documentation of future research on catalytic processes. The result would be a “handbook” defining standards and general research guidelines for synthesis, testing, and functional characterization as well as documentation in the framework of a certified metrology. In this way, a minimal standard would be established for future work without restricting the creative developments of tomorrow's research. All research carried out according to these norms would offer a fount of data2 for our future justifies this undertaking without hesitation.A major impediment in this respect is the incomplete and poorly structured presentation of comprehensive and high‐quality results. Attempts at collecting all the descriptions of COThe authors declare no conflict of interest.Robert Schlögl (born in Munich in 1954) has been director at the Fritz Haber Institute of the Max Planck Society in Berlin since 1994 and founding director of the MPI for Chemical Energy Conversion in Mülheim a.d. Ruhr since 2011. In 2020, he was elected Vice President of the National Academy Leopoldina. He is Honorary Professor at the TU Berlin, Humboldt University of Berlin, University of Duisburg‐Essen, and Ruhr University of Bochum as well as member of acatech and BBAW. His research focuses on heterogeneous catalysts, with the aim to combine scientific with technical applicability, as well as nanochemically optimized materials for energy storage.


Implementing heating and cooling set-point temperature modulations in buildings can promote energy savings and boost energy flexibility. However, time and time-of-day requirements in current indoor climate regulations are either overly simplified or ignored completely. A better understanding of how human thermal responses vary throughout the day is useful to effectively design and operate energy-flexible buildings. To date, only a handful of studies have looked at diurnal changes in thermal perception and mostly near steady-state neutrality without controlling for light exposure. This is the first experimental investigation aimed at understanding how the time of the day influences physiological and subjective human sensory responses to a localized dynamic thermal stimulus under constant light rich in long wavelengths (red). Results indicated that humans responded physiologically differently depending on the time of the day with a higher rate of change in the skin temperature in the evening compared with the afternoon. Furthermore, the increase of thermal sensation during the warming skin temperature transients was found to be greater in the evening. No differences were observed under steady-state thermal conditions. This evidence suggests that accounting for the time of the day is important when dynamically operating buildings, such as during demand-response programs. Heating and cooling set-point temperature modulations can also be useful for enhancing building occupants’ comfort8, well-being9, and health12. However, thermal comfort has been mostly studied under steady-state conditions and near neutrality13; the effect of the time-of-day on human thermal perception has been even less often addressed14. As a consequence, time and time-of-day requirements in current indoor climate regulations are either over simplistic or not taken into account at all16. The supposition that thermal comfort does not depend on the time-of-day is primarily based on experimental studies conducted in the 1970s and 1980s22. More recent evidence suggests that diurnal variations in human thermal perception exist24. These observations highlight the importance of re-evaluating and re-quantifying the impact played by the time-of-day factor on human thermal comfort responses, especially under the dynamic conditions typically encountered during demand-management electricity programs, such as demand-response (DR) events. If diurnal variations in human thermal perception were confirmed, the way we currently operate buildings could be significantly modified.The implementation of time-varying indoor set-point temperatures can contribute to reducing buildings’ energy consumption and promoting their energy flexibility25. The extent of this change varies among individuals but it is usually around 1 °C. The nadir in core body temperature occurs during sleep time at about 04:00, see Fig. 29. The nadir of the core body temperature is preceded by the peak in distal skin temperature by an average of 4 h30. These diurnal rhythms in core body and skin temperatures are due to the circadian variation of the balance between body heat generation and dissipation. Concerning heat production, the basal metabolic rate (i.e. the fasted and resting metabolic rate) is highest during the late afternoon and lowest during the late night31. Regarding heat loss, core body temperature is defended at a lower level during the night due to the modification of the thresholds regulating the autonomic thermoregulatory responses of vasodilation and sweating33. These diurnal variations are coordinated by the circadian system which makes it possible for humans to synchronize their physiological functions and behaviours to the 24-h solar day by using signals from the environment, such as light exposure, activity, body position, environmental temperature, and sleep. The circadian system consists of a central circadian clock located in the hypothalamus and peripheral clocks present in every cell of the body34. The period of the central clock is slightly longer than 24 h and, hence, this diurnal rhythm is referred to as circadian, i.e. “circa diem” or “about a day”.The lowering of the core body temperature at night is a well-known physiological phenomenon documented since 184214, we identified only 21 experimental and observational thermal comfort studies including the “time-of-day” variable49. We could observe a preference of building occupants for higher temperatures in the late afternoon (17:00 to 18:00) at a time when the core body temperature is about to reach its peak42. However, the results from at least four other studies22 were found to contradict these findings. The review outlined some possible reasons for the lack of concordance between studies, including a potential major bias associated with the intensity and spectrum of the experimental lighting conditions. Daylight is the primary synchronizer of the central circadian clock to the 24-h solar day and its effect on the circadian rhythm depends on the time of day at which light exposure occurs50. In the late evening and early night, bright light, and in particular short-wavelength (blue) radiation, suppresses the release of nocturnal melatonin and slows down the increase in distal skin temperature and the associated decline in core body temperature51, which induces phase delays of the circadian rhythm52. The maximum melatonin-suppressing effect is achieved at the shortest wavelengths 53. In the late night and early morning, bright light exposure anticipates the increase in core body temperature and the decline in distal skin temperature, which results in phase advances54. Despite the well-documented role of light in synchronizing the human circadian rhythmicity, only three out of the 15 reviewed experimental studies reported the characteristics of the lighting conditions and only in terms of intensity (lux)43. None of the six reviewed observational studies conducted any light measurements. Furthermore, the majority of the studies were conducted in thermoneutral steady-state conditions. The effect of the time-of-day on thermal comfort may be more significant under dynamic conditions and at the boundaries of the thermoneutral zone while being insignificant near the centre of thermoneutrality. Thus, the circadian effect on human thermal perception may be masked by the fact that previous studies were mostly conducted in steady-state thermoneutral conditions.Despite variations in core body and distal skin temperatures and autonomic thermoregulatory responses across the day being well-identified, only a few studies have examined diurnal changes in thermal perception and/or behavioural thermoregulatory responses. In a previous review57. We set the following a priori null hypotheses:The time of the day does not affect human dynamic thermal perception.Human dynamic thermal perception does not differ between females and males.The primary research aim of this study is to understand whether the time of the day (afternoon and evening) influences physiological (skin temperature) and subjective  human thermal responses to a fan-induced dynamic localized thermal stimulus. As secondary objectives, this study also aims to investigate sex differences and the effect of thermal alliesthesia on human dynamic thermal perception. The experiment is performed under constant light condition rich in long wavelengths (red) that is not capable of suppressing the decline of core body temperature in the evening“Mauro Felli” in Perugia, Italy. The mean daily outdoor temperature was 22.8 °C during the experiments in June and 20.5 °C in September.The study was conducted during June and September 2021 (northern hemisphere summer) in the NEXT.ROOM, a test room developed for human comfort studies at the CIRIAF Institute, the interuniversity research centre on Pollution and Environment A fan was used to expose the participants to four sequences of partial-body cooling over 130 min under a neutral whole-body thermal condition as predicted by Fanger’s PMV model. The active cooling of the skin induced by the fan was followed by a passive re-warming towards the neutral condition. The four cycles were selected to induce thermal alliesthesia by alternating displeasure and pleasure thermal states. After the first two cycles, the body was allowed to return to a steady state for 30 min. The fan was mounted 125 cm away from the back of the neck of the participant when seated straight. The anemometer was 35 cm in front of the fan and 90 cm behind the participant’s neck. Seat height was adjusted to ensure that each participant’s neck was directly in the path of the airflow. The experimental procedure and setup are shown in Fig. 57. The most often used vertical illuminance at the eye was equal to 500 lx which can be considered a reasonable level for office tasks. The horizontal and vertical spectral irradiances are reported respectively in Supplementary Table The same experimental procedure was repeated at two different times of the day . Eight participants  were exposed to the neutral condition starting at 12:00 pm, and six other participants  at 19:00. The light condition was identical at the different times of the day. We used a polychromatic light with an illuminance of about 200 lx, a Dominant Wavelength (DWl) of about 580 nm and a Correlated Colour Temperature (CCT) of about 3400 K (red colour) as measured at the level of the eye of the subject in the main direction of gaze. We selected a light condition with long wavelengths to limit the effect of short-wavelength radiation on the suppression of the decline of the core body temperature in the eveningOne participant at a time took part in the experiment and only at one time of the day . On the day of the experiment, the participant arrived at least 2 h before the beginning of the study and was asked to remain in a normal office for metabolic adaptation purposes before being transferred to the test room. The office was monitored in terms of air temperature but the environmental conditions were not controlled. For the tests starting at 12:00 pm, participants were advised not to have any food from 10:00. For those starting at 19:00, they were advised not to have any food from 17:00. An energy bar  was provided at the beginning of the test. Bottled water was also made available. For the first 70 min of the exposure, the thermal conditions were kept constant to allow the occupant to reach steady-state thermal conditions before the start of the fan-induced active cooling and passive warming transients. During the first 30 min in the test room, each participant filled out a questionnaire about personal information and was briefed verbally about the study’s requirements and allowed to ask any questions but without detailing the thermal conditions that he/she was going to experience, thus following a single-blind procedure. This time was also used to put the sensors on the participant’s skin. The first thermal perception questionnaire was completed 40 min after entering the test room and this was considered the actual start of the test. The principal investigator of the study (i.e. the first author) was continuously present in the test room during all experimental sessions to operate the fan and make sure the participants were answering the questionnaires at the right times..58.The experiment took place in the NEXT.ROOM, a test room developed for human comfort studies at the CIRIAF Institute in Perugia, Italy. The test room is located within the laboratories in the basement of the main building of the Institute and presents an internal volume of 4 × 4 × 2.7 m. It includes a unique window, aligned to the building openings to provide a view of the outdoors from inside the room. During the experiments, the window was closed by a 0.05 m thick opaque panel of polystyrene to avoid daylight access to the space and limit heat fluxes through the envelope. The lighting scenario as described in the previous section was realized by controlling the six luminaires at the ceiling of the NEXT.ROOM. The internal thermal conditions were achieved thanks to the control of the radiant surfaces  and a heat-recovery HVAC system. The conditioned supply air was distributed from four inlet nozzles located on one side of the room . On the opposite side of the test room’s ceiling, two air extractors took away the exhaust air. Detailed technical descriptions of the NEXT.ROOM  are provided in Vittori et al15.Fourteen adults  participated in the experiment. The participants were recruited by word of mouth, mostly within the University of Perugia. Only participants between 20 and 40 years old were included in the study to avoid any confounding effect of the age variable. They were asked to wear similar clothing outfits, consisting of short trousers or a skirt, a shirt with short sleeves, ankle-length socks and shoes with total clothing insulation estimated to be about 0.6 clo (including the insulation of the office chair), based on the tabulated clo values given in ANSI/ASHRAE Standard 55Avoid heavy exercise,Avoid alcoholic or stimulating drinks,Avoid eating large meals,Maintain a regular sleep schedule (do not stay overnight the night before).During the experiments, they were allowed to perform office tasks  but were not permitted to move around the room. Their metabolic rate was estimated to be constant and equal to approximately 1 met. For at least 24 h before the experiment, they were requested to:All the participants were paid for their participation. The anthropometric characteristics of the participants are reported in Table 59, and an average air temperature was calculated over the three values. The mean radiant temperature was computed from the globe temperature using the function psychrometrics.t_mrt from the pythermalcomfort python package60. Fanger’s Air temperature Skin temperature was measured in 16 areas of the body: neck  is important for activating purposive thermoregulatory behaviours which have been described as “an attempt to avoid what humans call thermal discomfort or displeasure and to obtain thermal pleasure”64. The Thermal Sensation Vote (TSV) was asked on the classical ASHRAE 7-point thermal sensation scale ranging from “Cold” (− 3) to “Hot” (+ 3)15. The Thermal Pleasure Vote (TPV) was also asked on a 7-point scale: “Very pleasant” (+ 3), “Pleasant” (+ 2), “Slightly pleasant” (+ 1), “Indifferent” (0), “Slightly unpleasant” (− 1), “Unpleasant” (− 2), and “Very unpleasant” (− 3). The questions were either in English or Italian depending on the language preferred by the participants and were presented through an Internet browser. Only discrete votes were allowed. The two questions translated into Italian are reported in the Supplementary Note.The participants filled in a questionnaire describing their whole-body thermal sensation and thermal pleasure at 10 min intervals, except during the partial-body cooling and warming periods where the questions were asked every minute. The exact timing of the questionnaires is shown in Fig. The research protocol was approved by the Ethics Committee at the University of Tours and Poitiers in France (Protocol No. CER-TP 2021-06-02) and adhered to the principles of the Declaration of Helsinki. Each participant was provided with written instructions and an information sheet and gave written informed consent to participate in the study.Mann–Whitney U test. The common language effect size (CLES) is used as a measure of effect size and indicates the probability that a random value or score from one group will be greater than a random value or score from the other65. A Mixed-effects Linear Model (MLM) with possible two-way interactions is employed to model the thermal sensation vote as a function of the skin temperature, the rate of change of skin temperature, sex, and the time of day that are included as fixed effects. The participants are treated as random factors (random intercept model) due to the longitudinal nature of the collected time-series data. The maximum likelihood is the chosen estimation method for the parameters in the MLM model. A “top-down” modelling strategy is used, starting with the maximum model followed by a stepwise backward elimination procedure with only significant covariates kept in the model at the end of the procedure. The open-source Python packages pingouin and statsmodels are used for the statistical analyses. Differences at p ≤ 0.05 are considered statistically significant.Statistical differences between the means of two independent samples (from two groups) at each time point are calculated with the non-parametric p < 0.05 , for a nondirectional (two-tailed) Mann–Whitney U test, the required sample size corresponded to fourteen participants66.The sample size of fourteen participants for the testing of the first null hypothesis was chosen based on a prospective power analysis calculation. None of the papers identified in the literature review reported effect size so the rationale for the selection of the effect size was to have a study capable of detecting a shift effect of 1 vote on the Likert scale. With the power levels set to 0.80 , the alpha levels set at The mean measured environmental conditions during the four experimental exposures are reported in Table 8. This phenomenon appears to occur at a central level, rather than being related to the dynamic activity of sensory neurons as the dynamic thermal overshoot phenomena  with possible two-way interactions is employed to model the thermal sensation vote as a function of the skin temperature, the rate of change of skin temperature, sex, and time-of-day that are included as fixed effects. Given that “alliesthesia” to refer to “the property of a given stimulus to arouse pleasure or displeasure according to the internal state of the subject”78. Any thermal stimulus that minimizes the “thermoregulatory load error” emanating from the body core is perceived as pleasant , while any stimulus that exacerbates the discrepancy is perceived as unpleasant 78. However, extant empirical evidence now shows that positive/negative alliesthesia can also be induced in the thermoneutral zone without any deviation in body core temperature82. This phenomenon is known as spatial thermal alliesthesia and occurs when the mean skin temperature is displaced from its “neutral threshold” and one or more body parts are heated or cooled to reduce/increase the whole-body discomfort. Here we show evidence that positive alliesthesial thermal states can also arouse during environmental transients when localized thermal discomfort is suddenly removed. This phenomenon can be referred to as temporal thermal alliesthesia. On the left of Fig. Cabanac originally coined the term .79.Finally, we employed MLM with possible two-way interactions to test whether i.e., between 14:00 and 14:50 (afternoon) and 21:00 and 21:50 (evening) under sequences of partial-body active cooling and passive warming induced by a fan. We found that participants felt warmer during the evening warming transients and these results could not be explained in terms of differences in mean skin temperature which was the same in the afternoon and the evening. This was further confirmed by the fact that the relationship between mean skin temperature and the thermal sensation vote was not found to depend on the time of the day, with the same skin temperatures eliciting warmer sensory responses in the evening. However, we observed that the cutaneous vascular responses to the fan exposure were more pronounced in the evening. By analysing the times during which the experiments were conducted against the normalized mean course of the core body temperature as measured over several experiments35 and illustrated in Fig. transition” periods83: a “heat gain” phase in the afternoon (as the core body temperature climbs) and a “heat loss” phase in the evening (as the core body temperature starts to drop). Our findings suggest that humans are more sensitive to warming discomfort during the “heat loss” phase. This could be needed to favour the decrease of body core temperature during the cooling down phase. Similarly, the influence of light on thermophysiology has been mainly observed in the “transition” periods of the body core temperature54. Therefore, it seems particularly important to focus future thermal comfort studies on these periods. Furthermore, our results suggest that perceptual differences are only evident under dynamic conditions, while no differences are observed under steady-state conditions. This indicates that future investigations are to be aimed at the edges of the thermoneutral zone, as thermal sensory differences may be insignificant near the centre of neutrality.This study aimed to understand whether the time of the day affects human dynamic thermal perception. We have analysed thermal sensation votes collected after 120 min from the beginning of the afternoon and evening tests, 88, including our previous study89. The lower metabolic rate of females means they have lower metabolic heat-production capability during cold exposures and, thus, a poorer ability to thermoregulate compared to males which makes them more vulnerable to cool conditions. Hence, females must be more responsive to cooling transients through vasoconstriction to activate behavioural thermoregulatory responses earlier, which explains the observed higher cooling rates of the skin temperature at the neck.In our study, we could also observe that females have a greater local vascular response at the neck than males. This corroborates previous evidenceIn this study, the thermal adaptation time was equal to 70 min which is more than double the 30 min considered the norm in thermal comfort experiments. Despite this long adaptation time, skin temperatures up to 70 min from the start of the experiment were influenced by the previous thermal exposure in the office where the participants were required to stay for two hours to ensure that they were all having the same initial metabolic rate. As the main limitation of this study, we did not control the thermal conditions in the offices because we did not anticipate they could influence the results of the experiment. Because of this limitation and to eliminate any effect of the initial thermal state we only analysed time-of-day and sex effects on the physiological and subjective data collected 120 min from the start of the tests.We did not measure core body temperature which would have allowed us to have more insights into the circadian phase of the participants.Finally, the between-subject design and the small sample size of this study imply that there is a risk of making a Type II error and, thus, missing real effects . This may have led to a failure in detecting differences in skin temperature between the afternoon and evening periods. A further limitation of the study related to the small sample size is that we could only focus on studying interindividual differences due to sex. Other than sex, individual differences can also be due to factors such as age, body build, fitness level, and geographic/ethnic acclimatization. The fact that the small sample is quite homogenous in terms of age and body build means that we could not study the impact of these factors and, thus, limits the generalisability of our results to the entire population.This study aimed to understand whether the time of the day (afternoon and evening) influences physiological (skin temperature) and subjective  human thermal responses to a fan-induced dynamic localized thermal stimulus. It also investigated sex differences in human dynamic thermal perception. It was found that participants respond physiologically differently depending on the time of the day with a higher rate of change in the skin temperature observed in the evening compared with the afternoon. It was also observed that participants felt warmer in the evening during the warming skin temperature transients. This evidence suggests that humans are more sensitive to warming in the evening when the core body temperature starts to decline. This could be needed to favour the decrease of body core temperature. No differences were observed under steady-state thermal conditions. Furthermore, we found that time-of-day differences in thermal sensation could not be fully explained in terms of skin temperature, thus suggesting that equal skin temperatures elicit different sensory responses at different moments of the day. These results point to the importance of accounting for the time of the day when studying thermal comfort under dynamic conditions. Furthermore, given that the effect sizes observed were large, these results can contribute to developing novel building energy management algorithms for demand response. In particular, the shape of the set point temperature modulations could be optimized for thermal comfort and energy needs depending on the time of the day. The important practical implications of these findings highlight the need to conduct replication studies to confirm and extend these results and, therefore, facilitate applications in the built environment.Supplementary Table 1.Supplementary Table 2.Supplementary Table 3.Supplementary Table 4.Supplementary Information.


The sternocleidomastoid (SCM) flap has been used for a long time in protective coverage of major vessels, reconstruction of intraoral pharyngeal, closure of pharyngo-cutaneous fistulas, and augmentation of soft tissue defects in the oral and maxillofacial region. However, this flap is not yet commonly used due to doubtful blood supply to the flap. This flap offers favorable esthetic results, combined flap, rich vascularization, and the possibility of shifting the two heads of the muscle. Thus, this flap has been used broadly in the maxillofacial region to reconstruct the defects of the post-parotidectomy, mandible, pharynx, and floor of the mouth defects. Previous studies discussed the use of a SCM flap following parotidectomy. However, few studies explained the use of SCMs in facial reconstruction. This study aims to review published articles discussing the use of SCMs for facial reconstruction. Head and neck defects are considered a challenge among surgeons due to the need for reconstruction of the defect according to the complexity of organs involved in the head and region. In addition to concerns related to the capacity to restore function and esthetics -2. The sMethodsThis article involved a review of published studies discussing the use of SCM flaps in facial reconstruction. Databases such as PubMed and Google Scholar were used to gather the most relevant studies. A search set was applied to combine a range of keywords: SCM flaps, facial reconstruction, and complication. By using this process, all the papers discussing the use of SCM flaps in facial reconstruction were obtained. In the inclusion criteria, we included all the relevant studies discussing SCM flaps and their complications. The studies that had poor methodological quality and insufficient data were excluded. The initial screening revealed 181 papers. After applying our inclusion criteria, the most relevant articles were selected and used in our current review. This study was conducted by reviewing 32 studies related to the use of SCM flaps in facial reconstruction.Facial reconstructionFacial reconstruction has been a debatable topic for surgeons and the public for a long time. Facial reconstruction's main goal is to restore function, while also ensuring a better esthetic outcome. Currently, facial reconstruction has progressed due to detailed anatomic knowledge, which permits tissue transmission from the local or distant region. The chief concern in esthetic facial reconstruction is the analysis of detailed defects. This also involves all reconstructive surgery that requires restoration of function. In defect analysis, a list of the problems that include missing tissues and functional impairment needs to be considered. When all the requirements have been acknowledged, a treatment plan can be prepared. Reconstruction of function must be the cornerstone of facial reconstruction and the starting point for every treatment plan for every surgeon, then the reconstruction of esthetics can be considered . For manAnatomy of the sternocleidomastoid muscleThe SCM is a long strap-shaped muscle that originates from the medial third of the clavicle and the superficial part of the manubrium of the sternum. This muscle passes posteriorly and superiorly in the neck region and is inserted into the lateral third of the superior nuchal line and the mastoid process . It starSternocleidomastoid flap The SCM flap was first demonstrated by Jianu in 1882 as the treatment of choice for facial paralysis, nowadays, it can be used for various purposes , 17-18. Treatment planning and requirements of reconstructionThe SCM flap is currently used for head and neck reconstruction, closure of tracheal or pharyngoesophageal fistulas, reconstruction of the oral cavity including the floor of the mouth and tongue, reconstruction of the lower jaw, correction of facial paralysis, large vascular neck protector, and coverage of defects after parotidectomy , 8, 26. Surgical approachFlap thickness varies between cases corresponding to the size of the gap and type of surgery , and the flap length is determined by measuring the defect length before cutting the SCM. Care should be acquired to avoid any injury to the accessory spinal nerve during the flap's cutting and dissection. The flap should be sutured to cover the parotid bed from the zygomatic arch above to the level below the mandible. Then the suction drainage should be placed deep on the flap before closing the wound . The desComplicationsThe most common postoperative complications that affects an individual's quality of life can be divided into both cosmetic and functional complications. The most common cosmetic defect associated with SCM is in the donor area due to its association with vast tissue loss . A deforThe standard treatment for benign parotid masses has been superficial parotidectomy, which allows for complete tumor removal while reducing the possibility of recurrence, however, these treatments come with consequences . The parThe SCM flap has been linked to some complications. As previously discussed with the various approaches, the skin paddle in the conventional superiorly based SCM flap typically undergoes partial or total necrosis because of its far location from the occipital artery, thus limiting its clinical utilization . InfectiThe SCM flap has proven its convenient and reliable properties that offer favorable esthetic results and rich vascularization. In addition to its various uses in the maxillofacial region to reconstruct various defects such as post-parotidectomy and floor of the mouth defects. However, a few considerations must be carefully understood by the practicing surgeon. This flap can form a deep deformity in the superior part of the neck at the donor site, particularly in young patients. In addition, care should be taken by the practicing surgeon to avoid any injury to the accessory spinal nerve during the flap's dissection. Current literature suggests maintaining at least two arteries for the SCM flap while retaining the thyroid artery intact to improve the flap's survival rate. The skin should also be sutured to the muscle to protect the skin's small perforators from being lost. Moreover, surgeons should be aware of the common complications associated with this flap as well as the appropriate management of these complications which include facial nerve paralysis, Frey’s syndrome, first bite syndrome, and loss of auditory sensation.


Machine learning as a service (MLaaS) plays an essential role in the current ecosystem. Enterprises do not need to train models by themselves separately. Instead, they can use well-trained models provided by MLaaS to support business activities. However, such an ecosystem could be threatened by model extraction attacks—an attacker steals the functionality of a trained model provided by MLaaS and builds a substitute model locally. In this paper, we proposed a model extraction method with low query costs and high accuracy. In particular, we use pre-trained models and task-relevant data to decrease the size of query data. We use instance selection to reduce query samples. In addition, we divided query data into two categories, namely low-confidence data and high-confidence data, to reduce the budget and improve accuracy. We then conducted attacks on two models provided by Microsoft Azure as our experiments. The results show that our scheme achieves high accuracy at low cost, with the substitution models achieving 96.10% and 95.24% substitution while querying only 7.32% and 5.30% of their training data on the two models, respectively. This new attack approach creates additional security challenges for models deployed on cloud platforms. It raises the need for novel mitigation strategies to secure the models. In future work, generative adversarial networks and model inversion attacks can be used to generate more diverse data to be applied to the attacks. It is expected to reach $12.1 billion by 2026, growing at a compound growth rate of 39.86% over the period 2021–2026. This is a very high growth rate, yet machine learning as a service platforms are at serious risk of data and model leakage.Machine Learning as a Service  helps cuPrevious work ,3,4 has We focus on model extraction attacks in the black-box case, which is a more practical setup where the attacker has a general understanding of the task and data used by the target model, but no knowledge of the architecture of the target model, how it is trained, and the specific training data. The attacker can only obtain the model output by querying a large number of samples from the target model to build a substitute model training dataset to train the substitute model. Theoretically, more queries lead to better extraction attacks. However, a large number of queries will not only increase the query cost paid by the attacker to the platform, which is more expensive, but also easy to be restricted by the platform. Some studies show that the data distribution of some attacks that reach a certain number of queries is not consistent with the data distribution of normal queries. How to select more representative samples to query the target model and reduce the number of queries while being able to guarantee the availability of substitute models is a current research hotspot.Model extraction attacks are also the process of building and training substitute models locally to obtain the functionality of the target model ,8. We waA general preprocessing step for model extraction attacks is proposed. We introduce the idea of instance selection into the attack process, which is particularly useful when an attacker can find a lot of data but wants to query only a few, and can make the most of this large data set while greatly reducing the amount of data to be queried.We propose the pre-trained model and the target model task-relevant data as a better choice for the substitute model architecture with training data, which reduce the queries and improve the performance of the substitute model. We also experimentally find it helpful to use semi-supervised learning of consistent regularization to train substitute models more efficiently, which can be learned from unlabeled data without querying the target model.We found the crucial role of model classification confidence in the attack. In the attack, we reduced the query by having the substitute model labeled before querying the target model, and we reduced the query by querying only the low-confidence data of the substitute model; after getting the output of the target model, we improved the efficiency and performance by training the substitute model using only the high confidence data of the target model.We have studied these problems faced by model extraction attacks and proposed several ideas to reduce the cost of attacks and improve model performance in a more practical setting. Finally, we have proposed a complete model extraction framework. We make the following contributions:We attacked two image classification models on Azure, a machine learning as a service provider, and achieved good experimental results with low cost and high accuracy, demonstrating the excellence of our solution.Attack. The concept of model extraction was first introduced by Tramer et al.  do  11:        ▹ Train substitute model12:        ▹ supervised loss13:        ▹ unsupervised loss14:        update 15:end for    16:    ▹ accumulate ensemble predictions17:    ▹ bias correction18:until 19:return We combine the previous methods and optimize the process to obtain a model extraction attack framework. As described in the relevant subsection, the framework fully considers the characteristics of the model extraction attack. It fully uses the training data while reducing the number of queries to the target model to reduce the query budget. The two parts of the training data used different training methods to train substitute models simultaneously in one go. Evaluation metric. We define the purpose of the model extraction attack as obtaining the target model decision capability as much as possible, i.e., the substitute model should be as close as possible to or even exceed the performance of the target model on the test dataset, and this is carried out because we stand for a practical point of view and want the extracted model to have more satisfactory usability. Thus, we use such an evaluation metric—the ratio of the substitute model to the target model in test accuracy, as shown in the following equation, where f is the target model. This is different from the way some papers evaluate extraction attacks, Ref. [ks, Ref.  is more ks, Ref.  is more Target models and their data selection. We use Microsoft’s Azure Custom Vision image recognition service, allowing users to build, deploy, and improve their own image identifier models. Image identifiers apply labels to images based on detected visual features, with each label representing a classification or object. We randomly selected 5000 image data from the training datasets of Intel-image and Fashion-MNIST, respectively, ensured a balanced number of data in each category, and trained two separate models using the service as targets for the attack. The models were trained on the platform for a few minutes. We needed to gain knowledge of the model architecture, weights, and hyperparameters. We could not change the models after training; only queries and the models would return output vectors. The trained model also gives the accuracy, which is 94.7% and 91% on the two datasets, respectively.Substitute models and their data selection. The substitute model is based on Squee-zenet [ee-zenet , a lightAs we discussed in the threat model regarding the attacker’s capabilities, an attacker can collect a dataset of waiting queries for the target model oriented to that task based on the knowledge of the task the model is oriented to. We consider the following two types of data relevant to the target model’s task for the training dataset of the substitute model.The first type of data comes from the validation set of Intel-image and part of the test dataset of Fashion-MNIST with 7301 data and 10,000 data, respectively. We strictly partitioned the datasets to ensure that the partial data from the validation set of Intel-image and the test dataset of Fashion-MNIST do not intersect with the training dataset of the two target models and the test dataset to be used later, respectively.The second type of data comes from the image data relevant to the target model task we searched using Google. Some keywords relevant to the task are searched on Google. These include the specific type of data used by the target model and some other keywords relevant to the task. After the search, the images displayed by Google are obtained. Then, these data are downloaded using a bulk download tool. For an attacker, searching for specific data with Google is a better option than finding data from some large datasets because the desired data can be easily found, and the data quality is not low. We experimentally query the target model with a small portion of the Google search data to see how much data can be classified with high confidence by the target model. Using our metric for measuring model confidence, the percentage of high confidence classifications by the two target models for the two datasets obtained from the search is shown in In our experiments, we considered two substitute model training datasets using a combination of the above two types of task-relevant data for each of the two target models.The first substitute model training dataset consists of two types of task-relevant data. The ratio of the number of the two types is related to the percentage of the target model’s high confidence classification of the dataset obtained from the search in The second substitute model training dataset consists of only the second type of data described above, i.e., using only task-relevant image data searched with Google. For Intel-image, the training dataset of the substitute model is 9149 data searched with Google. For Fashion-MNIST, the training data set of the substitute model is 30,626 data searched with Google. For Intel-image and Fashion-MNIST, the second substitute model training dataset is identified with The reason for considering the above two substitute model training datasets is that we want to explore the effect of data with different degrees of relevance to the target model task on the attack effectiveness. The effect of the attack is confirmed using the evaluation metrics described above. The first substitute model training dataset is more relevant to the target model task compared to the second substitute model training dataset.To minimize queries and to be consistent with our proposed scheme, the substitute models are trained using five substitute epochs. Using the updated dataset, the substitute model is trained for 100 epochs in each epoch.(1) Effect of data with different degrees of correlationAs we described, for Intel-image and Fashion-MNIST, the substitute models use two training datasets, respectively. (2) The important role of high confidence dataFirst, our method of evaluating model confidence is valid. In the experiments, the softmax function is applied to the output of the substitute model to calculate the probability, following the practice of existing work. When a component of the output after the action is greater than or equal to 0.9, the model is considered confidential in classifying that data. To demonstrate the validity of this measure, one can look at the training data of the untrained substitute model based on the pre-trained model SqueezeNet for classification confidence. Both figures in Second, the model trained with high confidence data only performs better on the test dataset than the model trained with all confidence data, as shown in (3) Effect of the ratio of labeled and unlabeled dataWe experimentally demonstrate the effectiveness of introducing consistency regularization into the model extraction attack, which fully uses the substitute model’s high-confidence data while reducing the query to the target model. The substitute model effectively learns from this unlabeled data. The substitute model trained by adding this part of unlabeled data obtained the highest test accuracy compared to the substitute model trained with high-confidence data of the target model only. The effect of the ratio of labeled and unlabeled data on the test accuracy of the substitute model was also experimentally explored, as shown in The reason for obtaining such results may be that the substitute model is trained with both labeled and unlabeled data and should learn knowledge mainly from supervised learning. More unlabeled data will interfere with the substitute model’s effective learning of labeled data, so more labeled data than unlabeled data are involved in the training. On the other hand, it is known from the experimental results that adding a small amount of unlabeled data are superior to completely using only labeled data, which indicates that the substitute model does learn knowledge from unlabeled data and the training of the substitute model needs the participation of unlabeled data.(4) Extent of reduction of queriesOur solution is dedicated to reducing queries and costs in several aspects. First, the found data are selected using the idea of instance selection. Two datasets, Intel-image and Fashion-MNIST, are reduced from 16,450 and 40,626 data to 3786 and 10,759 data, respectively. We also examined the case where, instead of using instance selection to pick, 3786 and 10,759 samples were randomly selected. The final impact of the two cases on the attack experiments is shown in Second, the substitute model’s approach of querying only low-confidence data to reduce the budget greatly reduces the number of queries to the target model. To achieve the purpose of budget reduction, instead of querying all the training data of the substitute model at one time, we use five substitution epochs in batches to train the substitute model. Such an approach is beneficial to reduce the number of queries to the target model. As shown in Finally, we reduce the required queries from 16,450 and 40,626 to 1204 and 2150 on the Intel-image and Fashion-MNIST datasets, requiring only 7.32% and 5.30% of the original data to be queried, which significantly improves the efficiency and reduces the expense.(5) Degree of substitution of substitute modelsTwo target models trained on Azure using Intel-image and Fashion-MNIST, respectively, were attacked using our scheme. The degree of substitution of the obtained substitute models is shown in (6) Evade an advanced defense methodA well-known defense method against model extraction attacks, PRADA, was proposed by Juuti et al. . They obFor a query sequence The Shapiro–Wilk Test  is then used to determine whether it is a potential attack sequence.We applied PRADA to detect our attack and obtained histograms of the distribution of query sequences This paper proposed a novel model extraction framework to reduce the cost of attacks and improve substitute models’ performance in a more practical setting. In particular, we use task-relevant data and pre-trained models of the target model as a better choice for substitute model training data and architecture. We use an instance selection method to select representative data in the dataset for the subsequent attack, reducing the use of data. In the attack, the substitute model queries only its low-confidence data, reducing queries; it also trains the substitute model using only the high-confidence data of the target model, significantly improving the model’s performance. For substitute models’ high-confidence data, which are unlabeled, we use consistency regularization to exploit these valuable data, which further enhances the attack. In addition, we use multiple substitution epochs for the attack. Such an approach fits well with the characteristics of model extraction attacks and can effectively reduce the query budget. We incorporate the above methods in a general model extraction framework and experimentally validate the framework’s effectiveness. The results show that the substitute models achieved 96.10% and 95.24% accuracy while querying only 7.32% and 5.30% of their training data on two target models, respectively. In future work, some techniques, such as generative adversarial networks and model inversion attacks, can be applied to the framework to generate more diverse query data to improve the attacks’ performance.