Theoretical Study On Metal Alloy Nacatalysts For The Development And Utilization Of Hydrogen Energy Source

It is well known that hydrogen energy source is an important research hotspot in view of its high calorific value,wide source,low ignition point and good cleanliness.However,the widely utilization of hydrogen energy is limited by its storage,transportation and safety.For the development of hydrogen energy,hydrogen production by decomposition of formic acid under mild conditions is regarded as an origin of hydrogen energy,which reduces its difficulty in storage and transportation and improves its safety.For the utilization of hydrogen energy,proton exchange membrane fuel cells(PEMFCs)is an excellent energy conversion device due to its high efficiency,simple operation conditions and good cleanliness.However,the main reason of limiting the development and untilization of hydrogen energy is that the performance of catalysts could not meet the application requirements.It has been widely confirmed that the catalytic performance of metal alloy nanocatalysts composed of two or three different metals is significantly higher than that of the corresponding single metal nanocatalysts.The possible reason of the increase of catalytic activity is the synergistic catalytic effect of two or three different metals.Therefore,metal alloy nanocatalysts have an important application prospect in the development and utilization of hydrogen energy source.In this thesis,the hydrogen production by formic acid decomposition and the oxygen reduction reaction in fuel cell system are regared as the application background..The reaction mechanism and catalyst design on metal alloy nanocatalysts applied for these two reactions are studied using density functional theory(DFT)calculations,which provide theoretical guidance for the development of new catalysts.The main innovation points are as follows:(1)Design of Pd-based alloy catalysts for hydrogen production via efficient formic acid decompositionPd-based alloy nanocatalysts are of potential application background for hydrogen production via efficient formic acid decomposition,whereas improving their activity and selectivity is still an ongoing challenge.We have performed a comprehensive study,on the basis of available experimental data,of the relationship between surface structure of Pd-based alloy catalysts and their catalytic activities for formic acid decomposition by using DFT and Sabatier analysis.Importantly,the average Bader charge of Pd atoms on the surface and the average bond length of the surface atoms are identified as two quantitative descriptors to analyze the effect of charge redistribution and surface tension on reactivity of the Pd-based alloy catalysts.Based on the two descriptors,we propose a strategy to rationally engineer the surface structure of Pd-based alloy catalysts by introducing suitable dopants and by devising optimal atomic arrangements.The strategy allows us to identify the potential candidates-Pd-Au and Pd-Ag alloy with specific atomic arrangement.We find that these alloy catalysts are superior to the state-of-the-art systems tested in previous experiments.Our strategy may be generalized for designing heterogeneous alloy catalysts beyond formic acid decomposition.(2)Study on reaction mechanism of hydrogen generation from formic acid decomposition on Pd-Cu nanoalloys and catalyst designIn view of the potential application background of Pd-Cu nanoclusters during hydrogen generation from formic acid decomposition,we investigate HCOOH decomposition mechanism on Pd,Cu and three Pd-Cu nanoclusters by DFT calculations and solvation model.After the analysis of reaction mechanism on these nanoclusters,it is found that the dehydration process is preferable on Pd55,and the dehydrogenation process occurs on Cu55.Alloying promotes the increase of selectivity of H2 generation for monometallic Pd system and decreases the activation energy of rate-limiting step for monometallic Cu system.Among the five clusters,Pd43Cu12 is of the highest activity for H2 production from HCOOH decomposition.In addition,the effect of pre-adsorbed H20 molecule during the whole reaction is discussed.Among all elementary reactions,the COOH*dehydrogenation is deeply impacted except Pd54Cu1 in the present of pre-adsorbed H20 molecule,which is explained by the elongation of O-H band and the less charge transfer from H to O atom.Our results would shed new light on the design of Pd-Cu nanoalloys for hydrogen generation from HCOOH decomposition.(3)A full understanding of oxygen reduction reaction mechanism on Au(111)surfaceIn fuel cell system,the development of oxygen reduction reaction catalyst is of great significance.However,the mechanism of oxygen reduction reaction is different on different catalyst surfaces.In this work,a full understanding of oxygen reduction reaction(ORR)mechanism on Au(111)surface is investigated by DFT calculations,including the reaction mechanisms of O2 dissociation,OOH dissociation,and H2O2 dissociation.Among these ORR mechanisms on Au(111),the activation energy of O2*hydrogenation reaction is much lower than that of 02*dissociation,indicating that O2*hydrogenation reaction is more appropriate at the first step than O2*dissociation.In the following,H2O2 can be formed with the lower activation energy compared with the OOH dissociation reaction,and finally H2O2 could be generated as a detectable product due to the high activation energy of H2O2 dissociation reaction.Furthermore,the potential dependent free energy study suggests that the H2O2 formation is thermodynamically favorable up to 0.4 V on Au(111),reducing the overpotential for 2e-ORR process.The elementary step of first H20 formation becomes non-spontaneous at 0.4 V,indicating the difficulty of 4e-reduction pathway.Our DFT calculations show that H2O2 can be generated on Au(111)and the first electron transfer is the rate determining step.Our results show that 4e-reduction pathway is difficult on gold surface and provide a theoretical basis for the further design of surface alloy nano-catalysts for oxygen reduction reaction.(4)The stability and oxygen reduction reaction performance of Pt-Ni nanoclustersPt-Ni cluster nanoalloy catalysts stand out among many fuel cell catalysts because of their high stability and high activity.And understanding the stability and adsorption properties of O atom can be considered as the first step to understand the mechanism of ORR on Pt-Ni nanoclusters.In this work,the equilibrium structures,stability,adsorption properties and deformation energies of medium-sized Pt-Ni nanoclusters are studied by global optimization method and DFT calculations.It is found that Pt-Ni nanoclusters are of more stable structure and larger oxygen adsorption energy than Pt nanoclusters,and the enhanced stability and oxygen adsorption capacity of Pt-Ni nanoclusters originate from both the strain and electronic factors.Based on the strain effect,the local pressures are on average much better equilibrated on Pt-Ni nanoclusters,bringing about the more stable structure.The elongation of the metal-metal bond distances results in the increase of adsorption energy of the O atom on Pt-Ni nanoclusters.Considering the electronic effect,the more charge interaction between the LDOS(d orbital)of metal atoms adjacent to the adsorbed O atom and the DOS(p orbital)of the O atom gives rise to the increase of oxygen adsorption capacity on Pt-Ni nanoclusters.Simultaneously,the charge density difference analysis shows that the Ni atoms doping is conducive for O atom adsorption.Our results show that both strain and electronic factors are of important effects on the stability and adsorption properties of nanoalloys.