Theoretical And Experimental Studies On Structural Regulation Of Metal-based Catalysts And The Catalytic Mechanism For Water-gas Shift Reaction

The water-gas shift reaction(WGS:CO+H₂O（?）CO₂+H₂;ΔH^?=-41.2 k J mol^-1)is mainly used to produce high-purity H₂ and eliminate residual CO in the fuel cell feedstock gas stream to prevent Pt electrode poisoning,which have shown important application values.Due to the exothermic nature of the reaction,a two-shift process WGS opration is normally employed:a high-temperature shift（HT,300-450°C）process is used to ensure fast reactions rate;whilst a low-temperature shift（LT,200-250°C）is applied to guarantee a high conversion.Metal catalysts play an important role in this reaction,for example,Ni-based catalysts have attracted much attention due to low cost and excellent HT-WGS reactivity;and Au-based catalysts have been demonstrated with good LT-WGS reactivity.Although much research progress has been made,the following scientific issues remain unresolved:firstly,the influence of fine structure of catalysts on the activity,selectivity and stability as well as the structure-property correlation are still unclear;secondly,the catalytic mechanism and pathway for the water-gas shift reaction are still controversial.These limit the structural design of efficient catalysts to some extent.Based on the above key issues,in this dissertation,studies on the active site structure regulation and catalytic mechanism of Ni,Au catalysts toward WGS reaction were carried out based on density functional theory calculations,microkinetic simulations and experimental verification methods,from the perspective of adjusting doping elements and carrier crystal type.The main research contents and conclusions of this work are as follows:（1）Through doping non-metallic P element into Ni-based catalysts,their catalytic performances toward the WGS reaction were systematacially studied.The WGS reaction mechanism and the process of coke formation on Ni_xP_ysurfaces(Ni₃P（001）,Ni₁₂P₅（001）and Ni P₂（100）)were investigated via DFT approach.The adsorption energies of reaction species（H₂O,CO,H₂,OH,etc.）,electron density difference,bader charge and reaction paths were systematically calculated.The results reveal that the introduction of P element separates the continuous Ni sites to be more dispersed in geometry,accompanied with charge transfer from Ni to P(Ni^δ+)and enhanced the energy barriers of coke formation and CO methanation.But an excessive content of P is disadvantageous for the dissociation of H₂O;Ni₁₂P₅（001）is thus determined as the best surface among the calculated ones which not only facilitates the dissociation of H₂O but also inhibits carbon deposition.For OH*and H*,P-top sites act as the active sites,which favors the dissociation of OH*and the oxidation of CO to CO₂.The mechanism calculated and micro-kinetic simulation for WGS reaction on the Ni₁₂P₅（001）surface confirm that the redox path is the most dominant,with the dissociation of water as the rate determining step.The micro-kinetic simulation further verifes that the introduction of an appropriate proportion of P improves the catalytic activity of Ni-based catalysts at medium-high temperature.（2）Based on Ni-based catalysts,the second metal was introduced with equal proportion,and the synertistic effect as well as reaction mechanism over bimetallic catalysts were further explored.The mechanism of WGS reaction and the main side-reaction（coke formation）on the Ni M surfaces（Ni Fe（111）,Ni Co（111）,Ni Cu（111）and Ni Zn（111））were studied by DFT method.The results show that the incorporation of the second metal element（Fe,Co,Cu and Zn）leads to a high-dispersion of active Ni;and meanwhile M sites can also act as the active sites,which increases the energy barriers of coke formation reaction.From the electronic structure analysis,the transfer of electrons from the second metal M to Ni results in the partial negative charge(Ni^δ-)of the Ni site.The adsorption energies of the reactants H₂O and CO on the Ni M（111）surfaces are both reduced compared with those on the pure Ni（111）surface,and the binary linear regression calculation shows that the electronic effects（ligand effect,adsorption site charge）play a more important role in determining the adsorption strength than geometric effects（strain at adsorption site）on the Ni Fe（111）and Ni Co（111）surfaces,the positive charge weakens the adsorption of the substrate;while for Ni Cu（111）and Ni Zn（111）surfaces,the compressive strain weakens the substrates adsorption,which has a decisive effect on the adsorption energy.The WGS mechanism was further calculated over Ni Co（111）and Ni Cu（111）with less coke formation and better H₂O dissociation activity.On the surface of Ni Co（111）,the redox path is the most favorable and the CO association with O is the rate-determining step.Whilst on Ni Cu（111）,the dominant pathway is the carboxyl path and the association of CO with OH to COOH serves as the rate-determining step.In addition,experimental studies verify that the Ni Co and Ni Cu samples exhibit the optimum performance,selectivity,resistance against carbon deposition and stability,which accords well with the theoretical prediction.（3）By introducing reducible supports with different crystal structures,the strong metal-support interaction and interfacial active sites were regulated,and their effects on the catalytic performance for WGS were investigated.A combination study including DFT calculation,microkinetic simulation and experimental verification for the LT-WGS reaction mechanism over Au₈ cluster supported on Ti O₂ with three crystalline phases（Au/ana-O_v,Au/rut-O_v and Au/bro-O_v,O_v stands for oxygen vacancy）has been systematically performed.The results show that in terms of geometric structure,the reducible oxide support Ti O₂ can be modified on the surface of Au nanoparticles to inhibit the agglomeration of metals and maintain the stability of the catalyst.From the electronic structure analysis,electron transfer occurs at the Au-Ti O₂ interface site,and H₂O is adsorbed and dissociated on the carrier O_v site;the interface Au sites are partially negatively charged(Au^δ-),which act as active sites to enhance the adsorption of CO,thereby improving the catalytic performance.Au/ana-O_vgives the lowest energy barrier at each step of WGS reaction,indicating a superior catalytic activity;and a redox pathway is confirmed.The results of microkinetic simulation confirm the favorable operation condition(p _H2O/p _CO≤4)for this reaction.Furthermore,experimental investigations verify that the Au/ana-O_v sample exhibits the optimum catalytic activity,in agreement with the calculation results.This dissertation focused on tunning of Ni,Au metal catalyst geometry and electronic structure,so as to study structure-property correlation and and reveal WGS reaction mechanism.This work provides theoretical information for improving the activity,selectivity and stability of metal-based catalysts,and offers useful guidance for the design and preparation of high-performance heterogeneous catalysts for WGS reaction.