Density Functional Theory Study Of CO And H₂Adsorption And Dissociation On Nickel Surfaces

The methanation reaction, namely, methane formation through the reaction of carbon monoxide(include CO2) and hydrogen over transition metal catalysts, commences through CO activation overtransition metal surfaces. However, the mechanism of the interaction of Ni-based catalysts and gas underexperimental conditions has not been able to reach a consensus. And therefore, it is utmost necessaryusing the atomic-scale simulation methods to understand this dynamic process to optimize catalysts andreaction conditions.In this paper, the adsorption and dissociation of CO and H2on the transition metal Ni were simulatedusing density functional theory.First of all, the adsorption properties including adsorption energies, sites, structures and net charges ofcarbon monoxide and atomic C and O on the (111),(100), and (110) surfaces of nickel was investigatedusing spin-polarized density functional theory calculations (DFT) within the slab model approximation atfour coverage levels, namely,0.11,0.25,0.5, and1monolayer (ML). The adsorption ability on three Nisurfaces followed the order: C> O> CO, which attributed charge donation from Ni. The most favorableactive sites for adsorbates were the threefold hollow sites on (111), fourfold hollow sites on (100), and thelong, short-bridge sites on (110). In all the adsorption sites. CO had been activated at different degrees. At0.25and0.5ML, the adsorption ability of CO and O was in the sequence (100)>(111)>(110), and(100)>(110)>(111) for C.(110) was the most favorable for the chemisorption of carbon, oxygen atom,and CO molecule at saturation coverage. At0.25and0.5ML coverage, the Ni(100) crystal face was morefavorable for the dissociation of CO than (110) and (111), and this moment CO dissociation with respectto CO adsorption on Ni (100) was an exothermic process. At1ML coverage, the CO dissociation wasdifficult because of the stronger repulsive interaction and weaker activation of CO, and (110) was themost preferred.Then, the Nin(n=1-13) clusters had been constructed. The absorption of H2at different angles on Nin(n=1-13) clusters was investigated using density functional theory. By analyzing adsorption energies, thenet charge and the optimized structures, it indicated that the adsorption of H2molecules on Nin(n=4-13)clusters in parallel at top sites was still non-dissociate when at bridge and hollow sites was dissociative,while the direction of charge transfer was contrary. Horizontal H2molecules on Ni1, Ni2and Ni3at anysites were dissociative and combine the resulting of hydrogen atom getting electrons. The result of theabsorption of vertical H2molecules on Ni clusters showed that the interaction of them was weak physicaladsorption. Dissociative chemisorption was more likely to occur by comparing the energies ofdissociation and non-dissociation adsorption. So H2on Ni clusters was mainly in the form of atomic statesin the process of synthesis gas methanation. The adsorption sites of H2in parallel determine the type ofadsorption, and it was a critical for morphology of absorbate after activation. It indicated the transition ofH2on Nin(n=4-13) clusters from molecule adsorption states to atomic adsorption states needed toovercome the energy of barrier roughly3.5~3.953kcal/mol range. The change was an exothermic processand thermodynamically favorable because energy of reaction was negative.Finally, the adsorbate adsorption on cluster models and periodic slab models was compared. It indicatedthe cluster models for evaluation of adsorption had a slightly higher. The adsorption ability on Nin(n=4-13) clusters followed the order: C> O> H> CO; The average binding energies were in the sequenceNinCO> NinC> NinO> NinH and Nin, and after adsorption, the stability of clusters generally increased.