Toward Computational Screening of Bimetallic Alloys for Methane Activation: A Case Study of MgPt Alloy

Abstract

CH4 is the main component of natural gas; there is a need for heterogeneous catalysts that can directly convert it into useful substances. On active metal surfaces, e.g., Pt surfaces, CH4 is sequentially dehydrogenated to CH or C. It is very difficult to obtain useful C2 products from them. We here present a catalytic informatics strategy with DFT calculations and databases to discover bimetallic alloy catalysts for selective methane coupling, which cannot be achieved with monometal catalysts. Considering two properties required for a methane conversion catalyst, i.e., reactivity and selectivity, alloy surfaces that allow the initial C–H bond cleavage reaction of methane to proceed and that stabilize CH2 and CH3 species more than CH and C species will be suitable catalysts for direct methane conversion. An exhaustive screening of alloys satisfying such conditions is carried out using density functional theory calculations. As a result, MgPt is predicted to be one of the most useful catalysts; on its surface, the activity of Pt is moderately suppressed due to Mg, and CH3 and CH2 species get more stable than CH and C species. The calculations predict that the C–C coupling reaction with the lowest activation barrier on the MgPt surface occurs for the pair of CH3 and CH2, producing the C2H5 adsorbed species; it becomes ethane if hydrogenated and ethylene if dehydrogenated. In addition, the optimal Mg/Pt ratio for the reaction is computationally explored, and it is found that the Mg/Pt ratio of 1:1 is the best. Eventually, experimental verification is carried out by actually synthesizing an alloy satisfying this ratio; the nonoxidative coupling reaction of methane molecules is tested in the presence of the MgPt catalyst, and the formation of C2 hydrocarbons as primary products is confirmed.

Oxidative Addition of Methane and Reductive Elimination of Ethane and Hydrogen on Surfaces: From Pure Metals to Single Atom Alloys

“…The catalyst was hardly deactivated in an 8-h test, indicating excellent stability and prevention of coke formation. We theoretically predicted that Pt–Mg bimetallic catalysts would be active for the NOCM reaction, working with experimentalists to validate our prediction by using a temperature-programmed reaction under a pure CH 4 gas stream . The amount of carbon deposition on the Pt–Mg catalyst was found to be much lower than that on the Pt catalyst.…”

“…This trend is in good agreement with the trend reported in preceding theoretical calculations. 34,72 If the first activation barrier can be overcome, it is expected that the subsequent activation barriers can also be overcome. Thus, these potential energy diagrams suggest that it is very difficult to stop the overoxidation of methane on the Pt surface.…”

“…We theoretically predicted that Pt−Mg bimetallic catalysts would be active for the NOCM reaction, working with experimentalists to validate our prediction by using a temperatureprogrammed reaction under a pure CH 4 gas stream. 34 The amount of carbon deposition on the Pt−Mg catalyst was found to be much lower than that on the Pt catalyst. These previous studies suggest that the NOCM reaction can be improved by adding another metal to Pt.…”

“…Of course, this is because the activation energy of each dehydrogenation step is lower than that of those coupling reactions. 34 If one performs first-principles calculations of the Pt surface, one can obtain fairly accurate energy profiles of the reactions, but we do not think that one can understand all of what is going on just by doing calculations ab initio.…”

“…But that is not the only problem. Coke formation is inevitable in reactions at such high temperatures and can lead to practical limitations, such as catalyst deactivation, process impairments, and loss of carbon yield. ,, To improve the NOCM reaction, great efforts have been made to find catalyst systems that work efficiently to activate the C–H bond of methane and promote C–C coupling, as well as to inhibit coking. ,− …”