Density Functional Theory Study Of Co Methanation On Ni/zrO₂ Catalysts

Natural gas（NG） is a type of clean fuel which is the crucial impetus of the Chinese economy. The development of Coal-to-SNG technology is an important strategy to realize the utilization of coal efficiently and friendly, to regulate the structure of consumption scientifically. The methanation process and its corresponding catalyst design are the key technologies of Coal-to-SNG. The exploration of methanation catalyst has been undergone a long-time investigation, among which, the Ni-based catalyst has been widely used in industry production due to its good catalytic activity and high selectivity. Generally, Al2O3, SiO₂, TiO₂ and MgO are used as supports in the Ni-based catalyst. It has been reported that ZrO₂ has three different phases（cubic（c）, monoclinic（m） and tetragonal（t））, and Ni supported on different phase of ZrO₂ carrier pose a significant impact on the catalytic performance of CO methanation. Since it is hard to obtain pure phase of ZrO₂ support experimentally, the influence of support phase on Ni/ZrO₂ catalyst and its performance become more difficult to reveal essentially.In this paper, density functional theory（DFT） is applied to explore the growth of Ni_n clusters and their interaction with cubic（c）, monoclinic（m）, and tetragonal（t） ZrO₂ surfaces. The phase effect of ZrO₂ to the adsorption and nucleation of Ni_n clusters have been analyzed systematically. The Ni/Zr O₂ with single phase has been successfully synthesized by elaborately selecting and controlling preparation parameters. Based on the results experimentally and theoretically, the mechanism of CO methanation on the Ni₄/t-ZrO₂ catalysts and the behavior of surface oxygen vacancy and hydroxyl group have been calculated and analyzed using DFT method. The main conclusions are presented as follows:（1） According to interatomic potential and quantum chemical methods, the most stable surfaces of c-ZrO₂, m-ZrO₂ and t-ZrO₂ are（111）,（-111） and（101）, respectively. The geometries and energetics of Ni_n cluster adsorbed on c-ZrO₂（111）, m-ZrO₂（-111）, and t-ZrO₂（101） surfaces were calculated under the DFT framework; The energy of adsorption of individual Ni atom is higher than that of adsorption of any Ni_n（n = 2-4） cluster onto the zirconium dioxide surface. Stabilities of adsorption of the Ni atom and Ni_n（n = 2-4） clusters on the zirconium dioxide surface follow the trend: m-ZrO₂（-111） > t-ZrO₂（101） > c-ZrO₂（111）. The results of Egrow indicated that the m-ZrO₂ has better dispersion for Ni_n cluster than t-ZrO₂ and c-ZrO₂.（2） The combined approach of DFT and hirshfeld charges are applied to discriminate the possible mechanisms for CO hydrogenation on Ni₄/t-ZrO₂ catalysts. Methane formation from CO hydrogenation was suggested based on 12 elementary reaction steps. CO undergoes stepwise hydrogenation to form CHO, and the resultant CHO hydrogenates to CH₂ O. CH₂ O further undergoes stepwise hydrogenation to form CH₃ O, then dissociated into CH₃. CH₄ formation can easily happen according to CH₃ hydrogenation.（3） Furthermore, the highest barrier for the conversion of CO to CH₄ on Ni₄/t-ZrO₂（101） catalyst is 258.0 kJ?mol^-1, whereas that for CH₃ OH formation is 187.7 kJ?mol^-1, indicating that the formation of CH₃ OH becomes more favorable. As for Ni₄/VO-t-ZrO₂（101） catalyst, the highest barrier for the conversion of CO to CH₄ is 147.6 kJ?mol^-1, whereas that for CH₃ OH formation is 196.4 kJ?mol^-1, indicating that the formation of CH₄ becomes more favorable both thermodynamically and kinetically, and the selectivity of CH₄ formation is highly improved. Moreover, the highest barrier for the conversion of CO to CH₄ on Ni₄/H-t-ZrO₂（101） catalyst is 233.6 kJ?mol^-1, whereas that for CH₃ OH formation is 254.0 kJ?mol^-1, indicating that the formation of CH₃ OH and CH₄ becomes competitive.（4） Overall, the productivity and selectivity of methane on Ni/t-ZrO₂ surface is controlled by CH₃ formation; as a result, we can predict that to achieve high productivity and selectivity for methane, O vacancy sites promote to break the O-C bond of CH₃ O and/or decrease CH₃ OH formation.