Study On Cu-ZnO-ZrO₂ Interface Interaction And Its Catalytic CO₂ Hydrogenation For Selective Methanol Synthesis

Energy crisis and global warming pose serious challenges to human sustainable development.Chemical conversion of CO₂ not only reduces CO₂ emissions,but also produces carbon-containing compounds,which are used as precursors for the production of chemicals and fuels,and are of great practical significance for solving increasingly serious environmental and energy problems.Methanol is an extremely important basic organic chemical raw material and large-tonnage chemical product.It can be used to produce chemicals such as olefins and aromatic hydrocarbons,as well as gasoline and diesel oil,and can also be directly used as fuel or fuel additive.Therefore,the methanol synthesis technology by CO₂ hydrogenation can provide a new way for the strategic adjustment of the energy structure.Cu-based catalysts are the most widely used in methanol synthesis.Among them,the Cu-ZnO-ZrO₂ catalysts is particularly concerned due to its excellent economy and high catalytic performance（CO₂ conversion and methanol selectivity）.The traditional view is that Cu species are the active sites of such catalysts.In the Cu/ZnO or Cu/ZrO₂ binary catalysts,the parameters such as particle size,dispersion and surface area of Cu are closely related to their catalytic activities.However,a large number of studies have proved that a large number of proven structure-activity relationships in Cu-based binary catalytic systems are not applicable to the Cu-ZnO-ZrO₂ ternary system,indicating that the catalyst has a completely different action mechanism.However,there is no deep understanding of the role of different species in the Cu-ZnO-ZrO₂ system for CO₂ activation,conversion pathways and methanol selectivity.In this paper,a series of binary（Cu-ZnO,Cu-ZrO₂）and ternary（Cu-ZnO-ZrO₂）catalysts were prepared by co-precipitation method and colloidal crystal template method and the typical ones were selected as model catalysts.The effects of the physical properties of Cu（particle size,dispersion and surface area）,the interaction modes of Cu-ZnO,Cu-ZrO₂,ZnO-ZrO₂ binary interfaces and the physicochemical characteristics of Cu-ZnO-ZrO₂ on catalyst activity and CO₂ conversion pathway were discussed through physicochemical characterization,in situ diffuse reflectance（DRIFTS）experiments and catalytic activity tests under different conditions.The active centers of CO₂ adsorption and H₂ dissociation were deeply analyzed,and the generation and evolution of reaction intermediates were discussed.The reaction mechanism of CO₂ hydrogenation over Cu-ZnO,Cu-ZrO₂ and Cu-ZnO-ZrO₂ under different reaction conditions was described.The relationship between material structure and CO₂ conversion pathway was revealed.A series of Cu-ZrO₂ catalysts with different specific surface area(S_BET),copper surface area(S_Cu),oxygen vacancy concentration were obtained by co-precipitation at different calcination temperatures.It is found that the interaction between Cu and ZrO₂ is the key factor controlling the activity of the catalyst.The interaction between Cu and ZrO₂is depended on the degree of Cu entering ZrO₂.It has been found that a copper-zirconia catalyst with strong interaction contributes to the H overflows from the copper surface to the surface of ZrO₂,promoting the conversion of CO₂ adsorbed on the zirconia surface to formate,methoxy,and finally methanol.Formate hydrogenation is the rate-control step of the reaction.The binary（Cu-ZnO,Cu-ZrO₂,ZnO-ZrO₂）and ternary（Cu-ZnO-ZrO₂）model catalysts were synthesized.The CO₂ adsorption and conversion pathways of the model catalysts were compared by in-situ DRIFTS technique and correlated them to the activity of the catalyst.It was found that the CO₂ hydrogenation on Cu-ZnO-ZrO₂ catalysts followed the formate path.During the conversion of CO₂ to methanol,the ZnO-ZrO₂ interface is the active site for CO₂ adsorption,activation and further conversion,while the metal Cu species is responsible for the H₂ dissociation,the synergistic effect of them can contribute to high methanol yield.It’s reveal that tuning the interaction between ZnO and ZrO₂ can be considered as another important factor for designing high performance catalysts for methanol generation from CO₂.It’s found that methoxy,which derived from the hydrogenation of formate,is a precursor to the synthesis of methanol by DRIFTS experiments.Combined isotope（D₂O）experimental results confirmed that the desorbed water vapor reacted with the methoxy to form methanol,while the adsorbed water vapor promoted the decomposition of the carboxylate into CO.The macroporous catalysts exhibited much higher methanol selectivity than the conventional ones due to the macroporous structure contributes to the desorption and diffusion of water vapor.In situ DRIFTS experiments also show that Cu-ZnO and Cu-ZnO-ZrO₂ have different CO₂ hydrogenation pathways.On the Cu-ZnO surface,the intermediate carboxylate is rapidly decomposed into the adsorbed CO and hydrogenated to methanol.On the Cu-ZnO-ZrO₂ surface,CO₂ is first converted to intermediate species such as formate and methoxy and then the methanol was formed by hydrolysis of the methoxy.