Surface Structure Design And Performance Study For CO₂ Reduction Catalysts

The fixation,conversion and utilization of CO2 has become a global concern.The conversion of CO2 into chemicals through catalytic hydrogenation not only fixes CO2 but also gives it high value,making it one of the most effective ways to convert and utilize CO2.Among them,the reverse water gas shift reaction(RWGS)reaction can convert CO2 to important platform CO molecule,and the generated CO can not only produce alkanes and olefins through Fischer-Tropsch(F-T)synthesis,but also further prepare methanol through further hydrogenation.Meanwhile,the RWGS reaction is also a key step in CO2 hydrogenation to produce other products.Therefore,the development of advanced strategies to construct effective catalysts for catalyzing the RWGS reaction and the study of the related catalytic reaction mechanisms are essential for the catalytic conversion and utilization of CO2.In this thesis,the design of the surface structure over the efficient RWGS reaction catalyst is taken as the guide,and based on the activation mechanism of CO2,the efficient catalytic active sites are constructed through two pathways,H2-assisted CO2 activation and the direct dissociation of CO2,respectively,and then the strategy of constructing efficient catalyst is further summarized.Firstly,the active metal cluster-oxygen vacancy synergistic sites are constructed by utilizing the active metal-support interaction and the redox properties of reducible oxides(CeO2,MoO3).In the presence of H2,CO2 is effectively activated at the active metal-oxygen vacancy interface.Meanwhile,with the assistance of a series of in situ characterization techniques and theoretical simulations,the catalytic mechanism of active metal cluster-oxygen vacancy synergistic site is investigated,and then the active metal cluster-oxygen vacancy synergistic strategy is proposed.In addition,we exploit the stress in MoO3/Mo2N heterostructures and the reversible switch between different Mo oxidation states to construct a catalytic surface structure with high density and efficient vacancy structure on the bulk Mo2N surface without loaded active metal.The obtained catalyst achieves the direct dissociation of CO2 to gaseous CO.The specific systems explored in this thesis are as follows.1.The construction of copper cluster-oxygen vacancy synergistic sites on the surface of CeO2 to catalyze the high-temperature RWGS reactionFor H2-assistanted CO2 activation,a catalyst needs to possess the ability to adsorb CO2 and dissociate H2,so that active intermediates can be formed with the adsorbed CO2 and dissociated H species.We utilized the interaction between Cu and CeO2 to construct abundant Cu clusters and oxygen vacancies on the CeO2 surface to achieve an efficient catalytic performance for the high-temperature RWGS reaction.The catalyst exhibited an excellent kinetic reaction rate(146.6 × 10-5 molCO2/gcat/s)at 600℃ and demonstrated an excellent stability over 240 h of long-term reaction.With the help of serial characterization,we found that partial sintering of the CeO2 support occurred under the harsh reaction conditions,but the copper species remained mainly dispersed on the surface of CeO2 in the form of clusters.The high stability of the copper clusters was attributed to the strong electronic interaction between copper and ceria that was difficult to be broken.Besides,copper clusters are able to facilitate the in situ formation of abundant surface oxygen vacancies on CeO2 during the pretreatment and the RWGS reaction processes.Meanwhile,CO2 could consume oxygen vacancies and H2 regenerated the consumed oxygen vacancies.A large number of stable copper clusters combined with regenerative oxygen vacancies to form the copper cluster-oxygen vacancy synergistic sites.CO2 was more easily activated at the interface between copper cluster and oxygen vacancy than single oxygen vacancy,and further formed reactive intermediates and catalyzed the reaction efficiently.In this work,a catalytic surface with abundant and stable copper clusters and oxygen vacancies was constructed by the metal-support interaction.And the obtained catalyst achieved a combination of high activity and high stability in the high-temperature RWGS reaction.Furthermore,we revealed the catalytic mechanism of the metal cluster-oxygen vacancy synergistic interface.2.The construction of Pt cluster-oxygen vacancy synergistic sites on the surface of Mo2N to catalyze the low-temperature RWGS reactionFor conventional reducible oxides,such as CeO2 and TiO2,the formation energy of surface oxygen vacancies is relative large,and without the influence of transition metals,the formation of oxygen vacancies is very limited.Mo2N is a synthetic interstitial compound that has been used in a variety of important catalytic reactions due to its inherent noble metal-like catalytic properties.We used the stress between the MoO3 surface passivation layer and the Mo2N bulk phase structure to construct abundant surface oxygen vacancies in the absence of supported active metal.Under reductive atmosphere,the MoO3 surface can spontaneously form MoOx(2<x<3)structure,resulting in forming rich oxygen vacancies.Then we loaded Pt species on the MoO3/Mo2N surface to construct a Pt cluster-MoOx catalytic interface.By controlling the experimental conditions,we confirmed that when the Pt cluster or MoOx structure was removed,the catalysts were much less active than the catalyst with Pt cluster-MoOx catalytic interfaces.These results demonstrated that the catalytic interface between Pt clusters and oxygen vacancies was the important active sites in the RWGS reaction.Meanwhile,the Pt-MoOx/Mo2N catalyst constructed in this work showed an excellent low-temperature RWGS reaction activity with a kinetic reaction rate of 17.4 × 10-5 molCO2/gcat/s at 300℃,which exceeded almost all the RWGS reaction catalysts reported in the literatures.In this work a MoOx surface with rich vacancy structures was constructed by utilizing the stress of heterogeneous structures,and the Pt cluster was stabilized by the interaction between Pt species and MoOx,thus constructing Pt cluster-oxygen vacancy synergistic sites.This work achieved the construction of the catalyst with an excellent performance in the low-temperature RWGS reaction and further refined the metal cluster-oxygen vacancy synergistic strategy.3.The construction of the catalytic surface with high-efficiency and high-density vacancy sites to catalyze the high-temperature RWGS reactionIn above two works,we focused on the H2-assisted CO2 activation approach to induce the RWGS reaction,but for this CO2 activation pathway,the catalyst often requires a stable active metal-support interface.The agglomeration of the active metal or the sintering of the support lead to the deactivation of the catalytic inteerface.Therefore,it is more valuable to develop effective catalysts without supported active metals.In addition to modulating the geometric and electronic structure of supported active metals,oxygen vacancies are able to adsorb reactant molecules themselves.However,the conventional oxygen vacancy is difficult to directly dissociate CO2 to form gaseous CO,which directly limits the oxygen vacancy as an independent active site to complete the whole catalytic cycle of the RWGS reaction.By further investigating the redox behavior of the MoO3 passivated layer on the Mo2N surface,we found that in reductive or oxidative atmospheres,there is a reversible shift between different Mo oxidized states.In reductive atmosphere,the reduction of surface Mo-O species was accompanied by the generation of abundant oxygen vacancies,which could directly cut the stable C=O bond in CO2 to form gaseous CO.Subsequently,the O atoms left behind after the dissociation of CO2 could be removed by H2 to form H2O,while regenerating the oxygen vacancies.The active redox properties of the MoOx(x<3)surface accelerated the transfer of oxygen atoms from the input CO2 to the output H2O,thus promoting the reaction equilibrium This catalyst exhibited high activity in the high temperature RWGS reaction and showed no apparent deactivation in several rounds of long-term stability tests(900 h),demonstrating a high cycling ability and good application potential.This work develop the advanced defect engineering on MoO3/Mo2N heterostructural surface to construct a rich vacancy structure for efficiently catalyzing CO2 reduction to CO.This work provided a reference for the construction of efficient solid catalysts without supported active metals.