Kinetic Study And Mechanistic Interpretation Of CO Hydrogenation On Supported Ru Catalyst

CO and H₂,the main components in syngas,can be used to produce liquid fuels,solid wax,light olefins,and other oxygenated compouds under different conditions on surfaces of various metals,such as Fe,Co,Ni,Ru,and Rh.When the reaction occurs at high temperature and low CO pressure,known as CO methanation,methane is the main product.Meanwhile,Fishcer-Tropsch synthesis?FTS?refers to the reaction between CO and H₂ at low temperature and high CO pressure with the production of hydrocarbons with high carbon numbers.In this dissertation,CO chemisorption and CO hydrogenation surface reaction were studied on supported Ru catalysts,with the combination of kinetic and in-situ infrared studies.C-O activation and C-C bond formation were comprehensively investigated and the effects of CO*coverage on CO chemisorption and CO hydrogenation surface reaction,together with the factors determing products distribution,were highlighted.First,triethanolamine?TEA?was used to prepare supported Ru catalyst samples using the incipient wetness impregnation method,and the effects of TEA amount,support properties,and treatment conditions on Ru cluster size distribution were studied.The results indicate that the addition of TEA can fix and uniformly distribute surface Ru atoms during impregnation,dry treatment and high temperature treatment processes,leading to the formation of supported Ru catalyst samples with uniform size distribution.More concretely,air treatment temperature and time can significantly alter Ru cluster size,while the H₂ treatment has less effect.In this study,catalyst samples with diffident mean cluster size were prepared by changing the air treatment temperature and time.Next,CO chemisorption on Ru nanocluster during CO hydrogenation reaction was characterized using in-situ infrared technique.The results indicate that CO chemisorption at low CO pressures can be characterized using ideal Langmuir chemisorption model.With increasing CO pressure,Ru surfaces are covered with more chemisorbed CO*and gradually reaching monolayer adsorption.The amount of CO*increases only slightly when CO pressure continuously increases after the monolayer adsorption.Meanwhile,the obvious blue shifts of C-O infrared frequencies suggest the presence of strong lateral interactions between chemisorbed species,and CO chemisorption,in this case,is no longer Langmuir chemisorption.These interactions can alter the relative stability of CO*and other surface species,which can consequencely change the reactivity of CO hydrogenation surface reaction.Then,C-O activation was studied using kinetic experiments.The experimental results indicate that the effects of reactants,including H₂ and H₂O,on CO hydrogenation turnover rates are consistent with H-assisted CO activation mechanism with*HCOH*formation or*HCOH*dissociation as the rate-limiting step.At low CO pressures,the Langmuir-Hinshelwood?L-H?based rate equation following H-assisted CO activation mechanism can accurately describe the experimental rate data.The lateral interactions between surface adsorbed species become dominating with increasing CO pressure,leading to the formation of dense CO*adlayer and,further,the deviation of CO chemisorption and CO hydrogenation turnover rates from the ideal models.The effect of this dense CO*adlayer on CO hydrogenation surface reaction was analyzed using transition state theory,combined with the conception of activation area.The dense CO*adlayer effect on CO*-CO*was stronger than that on the transition state because of the higher surface area of CO*-CO*and,thus,the negative activation area of CO hydrogenation surface reaction on Ru surfaces covered with CO*.Consequently,these effects favor the formation of transition state and,thus,the products,leading to the higher observed rates than the predicted values using ideal model,which is consistent with the observed less inhibition effect of CO pressure on CO hydrogenation turnover rates at high CO*coverages.At last,the effects of CO conversion,reactant concentrations,and reaction temperature on C-C bond formation and products distribution were also studied using kinetic experiments.The results suggest that higher CO conversion and CO pressure,and lower H₂ pressure and reaction temperature favor C-C bond formation?the production of hydrocarbons with high carbon numbers?.On the contrary,CH₄ is preferable at lower CO conversion and CO pressure,and higher H₂ pressure and reaction temperature.Both CH-CH coupling and CO insertion mechanisms can describe the chain termination and chain growth processes of C₁ intermediate,while for C₂₊intermediate,CO insertion mechanism is more reasonable.