| In 2021,China proposed the strategic goal of"emission peak and carbon neutrality",so it is important to improve the utilization of fossil energy to produce clean energy and develop the low-carbon economy.The water gas shift(WGS)reaction is an important reaction for industrial large-scale production of H2,and hydrogen energy,as a kind of secondary clean energy with abundant reserves,is known as the most promising energy source in the 21st century.The CO2 hydroreduction to CO reaction,namely the reverse water gas shift(RWGS)reaction,is an important reaction for the efficient conversion of CO2 and developing the low-carbon economy.Therefore,it is of great importance to promote the progress of these reactions by developing efficient catalytic materials.This thesis focuses on the Cu-based catalysts supported by rare earth(Ⅲ)compounds,with emphasis on the application of rare earth oxycarbonates and rare earth oxides in WGS and RWGS reactions.The catalyst structure is analyzed through a combination of experimental and theoretical approaches to investigate the unique advantages of the rare earth oxycarbonates for dissociating H2O and dispersing surface metals.Then,the important role of the active interface of Cu catalysts supported by rare earth oxide(Ⅲ)in the low and medium-temperature WGS reactions and the specific catalytic mechanism that the catalyst can still efficiently catalyze the reaction in high temperature RWGS reactions despite with the sintering phenomenon are systematically investigated through a variety of in-situ and ex-situ characterization methods.This thesis expands the application of rare earth(Ⅲ)compounds in the field of catalysis and provides the important reference for the design of more efficient and stable catalysts for WGS and RWGS reactions.The specific research of this thesis is as follows:1.Construction of active copper oxide-rare earth oxycarbonates molecular exchange surface to accelerate the water gas shift reactionThe dissociation of H2O in the WGS reaction is generally considered as the rate-determining step,so the design and synthesis of catalysts for efficient dissociation of H2O to facilitate the WGS reaction is a key scientific issue for researchers.Rare earth oxide CeO2 is widely favored by researchers due to its unique redox properties,and the presence of Ce4+to Ce3+transition in the reducing atmosphere thus generating oxygen vacancies(Ov),which can provide active sites for the adsorption and dissociation of H2O molecules.However,it has a disadvantage that CO2 adsorption can easily cause the accumulation of carbonate species during the reaction process and cover the active site,which reduces the reactivity.Therefore,it is necessary to develop a class of catalyst materials that can not only activate and dissociate H2O,but also effectively desorb or recycle CO2.In this work,we explore and discovery a class of rare earth oxycarbonates RE2O2CP3(RE=Sm and La)with the layered structure,which exhibit the carbonate layer(CO32-)and the metal oxide layer(RE2O22+),where the CO32-layer is ordered arrangement between the RE2O22+ bilayer.Combining the density functional theory(DFT)calculations and experimental double argumentation,we observe that the presence of RE2O22+…OH to RE2O22+…CO32-during the cyclic switching of H2O and CO2 atmospheres on the RE2O2CO3 surface,which means that the RE2O2CO3 surface can dissociate H2O to produce*OH and then replace carbonate species produced by CO2 adsorption,so as to make the whole reaction cycle.Thus,we successfully prepare the Cu/RE2O2CO3 catalyst by loading non-noble metal Cu species onto the RE2O2CO3 surface.It exhibits excellent reaction performance in the WGS reaction,with a reaction rate of 1711μmolCO·gCu-1·s-1 at 300℃ of 5Cu/Sm2O2CO3 sample,which is 8-34 times higher than that of other Cu-based catalysts reported in the literature,while also providing excellent long-term stability.This work provides an effective reference for further applications of rare earth oxycarbonates in catalysis field.2.Efficient rare earth(Ⅲ)oxide supported Cu catalysts for WGS reactionRare earth(Ⅲ)oxides are rarely reported in the field of catalysis because of their non-valence.However,in recent years,we find that Ni species supported on the surface of rare-earth(Ⅲ)oxides have been gradually applied to several catalytic reactions such as CO2 methanation and methane oxidation coupling.This indicates that the application of rare-earth(Ⅲ)oxides in heterogeneous catalysis is gradually recognized,but their specific roles in catalytic reactions and the reaction mechanism are not yet clear.In this work,we selected rare-earth(Ⅲ)oxides Y2O3 and Gd2O3 as support materials and supported different contents of non-noble Cu species on their surfaces to catalyze the WGS reactions,respectively.Through the performance studies and systematic in-situ and ex-situ characterization means,we are surprised to find the high activity and stability of the Cu/RE2O3 samples for catalyzing WGS reaction,with reaction rates as high as 1000 and 1900 μmolCO·gcat-1·s-1 for 2Cu/Y2O3 and 2Cu/Gd2O3 at 300℃,respectively.In addition,it is confirmed that Cu+ at the interface is the active species for CO molecule adsorption,while H2O molecule also dissociate at the interface to generate*OH to react synergistically with CO to produce CO2 and H2,This work expands the application of Cu/RE2O3 catalysts in the field of heterogeneous catalysis.3.Construction of high active sites in partially sintered rare earth(Ⅲ)oxide supported Cu catalystsIn the context of China’s strategic goal of "peak carbon dioxide emissions and carbon neutrality",strengthening the resource utilization of CO2 is now a hot scientific issue studied by scientists.the reduction of CO2 to produce CO and then further conversion to more valuable chemicals by subsequent Fischer-Tropsch synthesis is considered as one of the reactions for efficient utilization of CO2.However,this reaction is heat-absorbing and favorable at high temperatures,so designing catalysts that remain stable under such harsh conditions of strong reduction atmosphere and high temperatures is now a research challenge.Here we have successfully applied the rare earth(Ⅲ)oxide supported Cu catalysts to the RWGS reaction and achieved excellent catalytic performance and stability.The reaction rate reached 136×10-5 molCo2·gcat-1·s-1 at 600℃,which is at least two times higher than that of other Cu-based catalysts reported in the literature,and is not inferior to the redox support CeO2 supported Cu catalysts.Furthermore,we observed an interesting phenomenon that even though the catalysts undergo sintering and growth after high temperature reduction treatment,this do not affect its activity,and achieving a strong interaction between the sintered Cu species and the rare earth oxides,which together promoted the effective RWGS reaction.And through the various characterization means,we confirmed that the rare earth(Ⅲ)oxides are the main active sites for the adsorption and dissociation of CO2,while the Cu species are responsible for the activation of H2.This work provides a new idea for designing and synthesizing more efficient RWGS reaction catalysts and lays a good foundation. |
PDF Full Text Request