Nickel/Molybdenum Bimetallic Alloy for Dry Reforming of Methane: A Coverage-Dependence Microkinetic Model Simulation Based on the First-Principles Calculation

Abstract

Ni-based catalysts have attracted much attention because of their high catalytic activity for dry reforming of methane (DRM) reaction and their low-cost characteristics. The deactivation caused by carbon deposition has become a major factor restricting its application. Mo doping is expected to improve the carbon deposition resistance of Ni-based catalysts. In this study, we used density functional theory (DFT) calculations and the microkinetic model to compare the catalytic activity of Ni and MoNi4 (as a Mo-doped Ni-based catalyst model) for the DRM reaction. By calculating the surface energies, Ni(111) and MoNi4(001) were determined as the calculation models. The DFT calculation results show that MoNi4 catalyst has a stronger adsorption capacity for intermediate species. Combining Bader charge analysis and charge density differences analysis, the stronger adsorption capacity comes from the stronger electron donating capacity brought about by the doping of Mo. According to our proposed reaction network, the energy barriers of 19 elementary reactions were calculated in detail. We found that there are similar CO2 dissociation modes (direct dissociation) and C–O bond formation modes (CH2/CH–O oxidation) on Ni(111) and MoNi4(001). Both CH4 dissociation and CO2 dissociation on MoNi4(001) have a lower energy barrier, while CO formation through CH2/CH–O oxidation has a higher energy barrier. The total free energy barrier of the DRM is lower than that of carbon deposition formation on MoNi4(001), whereas the opposite on Ni(111), means MoNi4(001) shows strongly carbon resistance properties. At 1073.15 K and 2 bar, the microkinetic simulation results showed that when the reaction reaches equilibrium, MoNi4(001) is deactivated due to excessive O*. Accordingly, the influence of O* coverage on the activation energy of CO 2 dissociation was considered in the MoNi 4 microkinetic model. In this way, the DRM reaction can be carried out steadily and continuously on MoNi4(001). And the simulation results show that MoNi4 catalyst has extremely high catalytic activity and selectivity and is basically not affected by carbon deposition and byproduct H2O. Under the same simulated conditions, MoNi4(001) showed better catalytic activity than Ni(111). In addition, degree of rate control (DRC) analysis showed that on MoNi4(001), the activation of CH4 is the rate-limiting step of the whole reaction. Through our study, one might be proposed that alloying Ni with a small amount of active metals such as Mo can increase the carbon deposition resistance by increasing the activation of CH4 and CO2 in an equal ratio.

Design Strategy, Synthesis, and Mechanism of Ni Catalysts for Methane Dry Reforming Reaction: Recent Advances and Future Perspectives

“…Right: Reaction rate of CO 2 with temperature. Reprinted with permission from ref . Copyright 2021, American Chemical Society.…”

“…Wang et al used DFT and the microkinetic model to compare MDR activities of Ni and MoNi 4 catalysts (Figure III) . The results showed that intermediates were more strongly adsorbed on the MoNi 4 catalyst than on the Ni catalyst, which was associated with the stronger electron-donating capacity in the former due to Mo doping.…”

Perspective on Sorption Enhanced Bifunctional Catalysts to Produce Hydrocarbons

“…Wan-Ying et al performed DFT calculations on DRM mechanisms of Ni and a Ni–Mo alloy. They showed that the activation of CH 4 and CO 2 is higher in the Ni–Mo compared to monometallic Ni . Turap et al tested a series of Co–Ni catalysts with different ratios .…”

The role of Mo species in Ni–Mo catalysts for dry reforming of methane

“…29 Moreover, Wang et al pointed out that the MoNi 4 (001) surface can improve the coke resistance by more efficient CH 4 and CO 2 activation. 30 However, there have been few detailed studies of electronic properties on the role of Mo (or Mo–O) in Ni–Mo catalysts for coke resistance, namely, how the Mo (or Mo–O) species achieve the activity and the carbon resistance.…”