Oxygen Vacancies Promoted Hydrogen Spillover Over Ru-based Catalysts

Many chemical reactions,although intrinsically feasible,often require catalyst as the assistance to make it efficient,especially for some hydrogenation reactions just like the ammonia synthesis reaction.These hydrogenation reactions often need the hydrogen to be broken at the catalytic site into atoms before further reaction.The hydrogen-hydrogen bond in H2 is stable and is difficult to be cleaved under normal reaction conditions,but it is pretty easy to dissociate at the transition metal surface.In recent years,studies found that,for the supported transition metal hydrogenation catalyst,the hydrogen in the metal after activation will migrate to the second active center for the next reaction.This process is called hydrogen spillover.Hydrogen spillover not only can accelerate the reaction but also change the reaction selectivity.It often occurs in systems which use reducible oxides such as titanium oxide as the support.The traditional studies on hydrogen spillover both in theory and experiment are based on the complete surface as a precondition.However,it is well known that the surface of such oxides such as titanium oxide is extremely prone to emerge defects,which can absorb reactants and alter the reaction pathway.Therefore,it is necessary to investigate the effect of defects on hydrogen spillover.In this thesis,we chose the oxygen vacancies(OVs),a common surface defect,as the starting point,and focused on the role of OVs on hydrogen spillover.The confirmation of OVs function in this thesis can deepen the understanding of hydrogen spillover process,but also provide new ideas for exploring the catalytic mechanism of hydrogenation reaction.The main contents of this thesis are summarized as follows:1.First of all,we used Ru/TiO2 as the model to study the effect of OVs on the hydrogen spillover by theoretical and experiment methods.Theoretical calculations showed that on oxygen-deficient titanium surfaces,the transfer of hydrogen proceeds more easily and tends to form Ti-H intermediates.By the hydrogen adsorption experiment we found that the existence of oxygen vacancies greatly promoted the hydrogen spillover process and transferred more hydrogen from the ruthenium to the support titanium dioxide and the hydrogen formed Ti-H intermediate on the oxygen-deficient titanium dioxide support when it was transferred.Subsequently,the ammonia synthesis experiment of Ru/TiO2 showed that the existence of oxygen vacancies could promote the hydrogen spillover and make the surface of titanium dioxide contain more hydrogen,and this part of hydrogen could migrate to metal ruthenium for the combination of dissociated nitrogen,which promotes the ammonia reaction.These results reveal the mechanism of the interaction between oxygen vacancies and hydrogen spillover,and provide a new idea for further study of hydrogen spillover process.2.Based on the above work,then we extended the oxide support to CeO2 and explored the promoting effect of oxygen vacancies on the reaction of ammonia synthesis mainly from the viewpoint of hydrogen spillover.Through a series of characterization tests,we found that the promoting effect of oxygen vacancies on ammonia synthesis was not the electron-donating effect of traditional cognition but rather that oxygen vacancies promoted the hydrogen spillover process which make the surface of cerium oxide contain more hydrogen.Part of the hydrogen could be migrated to the metal ruthenium and combined with activated nitrogen atoms to form ammonia.This study confirms the conclusion that oxygen vacancies promote hydrogen spillover and provides a new strategy for the design of highly efficient ammonia synthesis catalysts and also provides new ideas for the study of the mechanism of other hydrogenation catalysts.