Insights Into The Mechanism Of Formic Acid Dehydrogenation On Pd-Co@Pd Core-shell Catalysts:A Theoretical Study

Hydrogen energy has attracted wide attention due to its wide sources,high energy utilization rate,high efficiency and cleanness.It is regarded as the most potential clean energy in the 21st century.However,the controllable production,storage and transportation of hydrogen has always been a big challenge and bottleneck in the field of hydrogen energy utilization.Formic acid is considered to be one of the most potential chemical hydrogen storage materials because of its high H₂reserves?4.4wt%?,abundant sources and liquid state at room temperature.Formic acid can decompose in two ways:H₂ and CO₂ are generated by decarboxylation and CO is generated by decarbonylation.The latter needs to be strictly controlled because CO will poison the catalyst and lose its activity.Platinum based catalyst is the main active component of heterogeneous catalyst for hydrogen production from formic acid,in order to reduce the cost of production and use,and further improve the catalytic activity,the key to using formic acid as hydrogen storage material is to find a catalyst with high activity and selectivity under mild conditions.The results show that the modification of catalyst by adjusting the composition and morphology of catalyst can not only effectively improve the catalytic activity and stability of heterogeneous catalyst for the decomposition of formic acid,but also greatly reduce the occurrence of side reactions,thus prolonging the catalyst life.In this paper,Pd-Co@Pd is used as catalyst for dehydrogenation of formic acid.The decomposition mechanism of formic acid on the surface of catalyst is studied by using the periodic density functional theory?DFT?,and the effect of cobalt content in the core structure on the activity and selectivity of catalyst is discussed.Based on the first principle,we systematically studied the decomposition mechanism of formic acid on three Pd-based core-shell catalysts Pd_x Co_y@Pd?111??x:y=1:3,1:1 and 3:1?and pure Pd?111?surfaces.The possible dehydrogenation and dehydration pathways through the HCOO,COOH and HCO intermediates have been identified.It is found that the most favorable dehydrogenation pathways on Pd₁Co₃@Pd?111?and PdCo@Pd?111?are the COOH-mediated pathway,which are different from the HCOO-mediated pathway on Pd₃Co₁@Pd?111?and Pd?111?.The increase of Co content in the Pd-Co core inhibits HCOOH dehydrogenation,but promotes the H-H coupling to form H₂,and improves the anti-CO poisoning ability of the catalysts.Accordingly,three bimetallic Pd_xCo_y@Pd catalysts exhibit better overall catalytic activity and product selectivity toward H₂+CO₂ than pure Pd?111?,especially PdCo@Pd?111?has the best catalytic performance for HCOOH decomposition.The present calculations show that a suitable Co composition in Pd-Co core plays an important role in tuning the catalytic performance,which provides a basic theoretical guideline to design high performance bimetallic core-shell catalysts for other dehydrogenation reactions.