DFT Study On Water Gas-shift Reaction Catalyzed By Ruthenium Carbonyl Complexes And Cu-based Clusters

The water-gas-shift reaction?CO+H₂O?CO₂+H₂?,is an important industrial reaction,it mainly used for fuel cells and industrial productions.The important research topic of the fuel cell is the source of hydrogen.At present,the main source of hydrogen is steam reforming of hydrogen rich gas,but which contains a lot of impurity gas,such as CO and CO₂gas.The water gas-shift reaction?WGSR?can remove the poisonous CO,and produce extra H₂ fuel.So we calculate the WGSR catalyzed by ruthenium carbonyl complexes and Cu-based clusters in this work,and explore the reverse water-gas shift reaction?RWGSR?on the Cu₁₂TM clusters.All of the optimized structures of WGSR are computed by using spin-polarized density functional theory?DFT?in the Gaussian 09 programs level.The contents are listed as follows:1.Investigated the WGSR in the gas and solvent phase for two ruthenium carbonyl species Ru₃?CO?₁₂ and Ru?CO?₅.The Ru₃?CO?₁₂ and Ru?CO?₅ catalyze WGSR where structure is D_3h symmetry.According to the Torrent,Barrows,Rozanska and Zhang successively put forward four reaction mechanisms catalyzed by Fe?CO?₅ for the WGSR,we use them to calculate WGSR catalyzed by Ru₃?CO?₁₂ and Ru?CO?₅.The results find that the mechanism D plays an important role in WGSR.In mechanism A,because high barrier of the OH^-desorption makes mechanism A unlikely to proceed.However,mechanism B avoided the important intermediate Fe?CO?₄H^-observed experimentally.Both mechanism C and D overcomes above-mentioned condition,but the last proposal-mechanism D is more favorable.So we think that the mechanism D is the best pathway.For the first time,we calculate turnover frequency?TOF?values to compare accurately the efficiency of catalysts by energetic span model?ESM?.We calculate the four mechanisms,and find that the TOF value of Ru₃?CO?₁₂ in mechanism B show(3.11×10^-12s^-1,for Ru₃?CO?₁₂;3.66×10^-16s^-1,for Ru?CO?₅)higher catalytic performance using ESM,which thus indicate the mechanism B is the main pathway for WGSR.The Ru₃?CO?₁₂ is more effective than Ru?CO?₅ as catalyst for WGSR.We also find that the catalytic efficiency of Ru₃?CO?₁₂ is much more favored in solvent phase,and verify the experiment results.2.We learn the cluster of Cu₁₂TM by systematized discussion,including nine transition metal species referring rows 3d⁵d with groups VIII?Co,Rh,Ir,Ni,Pd,Pt?and IB?Cu,Ag,Au?to discuss catalytic activity,as well as compare three reaction mechanisms.In the redox mechanism,Firstly,a coadsorption of the CO and H₂O molecules on the Cu₁₂TM surfaces has been described as the intermediate 1.Then the H₂O molecule dissociate to OH and H.Secondly,the CO adsorbed on the metal surface is oxidized by atomic O obtained from H₂O by two successive H abstraction steps,finally,two atom H combine to H₂.In the carboxyl mechanism,the first two steps,are similar to redox mechanism,then the oxidation of CO by OH and the formation of H and COOH intermediates,and the releasing of CO₂ and H₂ finally.In the formate mechanism,the steps 1-3,are consistent with redox mechanism.The next step is different,namely,the CO interacts with single H to form HCO and with H to form HCOO group,and finally,the HCOO decomposes into CO₂ and H_2.Our results show that the carboxyl mechanism regards as prior pathway and plays an important role.We think the carboxyl mechanism is most suitable pathway in the WGSR,because the redox and formate mechanisms need the higher energy barrier in the WGSR.We compare the efficiency of catalysts by ESM.The Cu₁₂TM?TM=Co,Rh,Ir,Ag?cluster show better catalytic activity of the left and lower-right metals in the d block.Exceptionally,the Cu₁₂Ni cluster shows the best catalytic activity because the highest TOF value,so we suggest that the Cu₁₂Ni is a promising catalyst toward the WGSR.3.Carried out the Cu₁₂TM?TM=Co,Rh,Ir,Ni,Pd,Pt,Cu,Ag,Au?catalysts for the reverse water-gas shift reaction?RWGSR?in this work,three possible reaction mechanisms are discussed in detail,including the CO₂ dissociation,carboxyl,and formate mechanisms.They are different from the three mechanisms of WGSR.Our results indicate that RWGSR prefers to follow the CO₂ dissociation mechanism on Cu₁₂TM clusters,which includes the CO and H₂O on the Cu₁₂TM cluster,a coadsorption has been found,then CO₂ cleavage to form CO and O,the two successive oxidation of O by atomic H obtained from H₂ abstraction steps,and the formation of CO and H₂O.In addition,we calculate the efficiency of catalysts by ESM.The results find that the TOF value of Cu₁₂Co is the highest value,which thus is more efficient catalyst to RWGSR.Moreover,we explain the CO₂ activation by the position of the d-band center of metal.The position of d-band center??_d?can be described by the energy of the d-band center relative to the Femi level.The position of d-band center??_d?moving toward Femi level affects the activity of the catalyst and leads to a stronger chemisorption.The relevance is applicable descriptor to explain catalytic reactivity of catalysis and adsorption properties.