DFT study into the reaction mechanism of CO methanation over pure MoS₂

Abstract

Mo-based catalysts are commonly used in the direct methanation of CO; however, no integrated mechanism has been proposed due to limits in characterizing the nano-sized active structures of MoS 2 . Thus, we report our investigation into the mechanism of CO methanation over pure MoS 2 through density functional theory simulations, considering that only MoS 2 edge sites exhibit catalytic activity. Simulations revealed the presence of (010) and (110) surfaces on the MoS 2 edges.Both surfaces are reconstructed by the redistribution of surface sulfur atoms upon exposure to H 2 /H 2 S, and after sulfur coverage redistribution, S vacancies are generated for CO hydrogenation.The reaction mechanisms on both surfaces are discussed, with the S-edge being better suited to CO methanation than Mo-edge on the (010) surface. The rate-controlling step differs between surfaces, and corresponds to the initial activation reaction, which was achieved more easily on the (110) surface. K E Y W O R D S density functional theory calculation, first principles methods, heterogeneous catalysis, Mo-based catalyst, sulfur-resistant methanation | I N TR ODU C TI ONThe extensive use of natural gas to reduce the pollution caused by coal combustion has resulted in an increased global demand for this fuel. To meet such a demand, a novel coal gasification and syngas methanation technology has been adopted in China and other coal-rich countries to produce substitute natural gas, [1] thereby rendering the mechanistic characterization of methanation an important research area. [2] In this context, the sulfur-resistant nature of Mo-based catalysts [3,4] allows their use in the direct methanation of syngas, bypassing the desulfurization and water-gas shift reactions, [5,6] but requiring further experimental and theoretical investigations with regards to their corresponding reaction mechanisms.The activities and selectivities of Mo-based catalysts have been extensively studied to date, [7][8][9][10] with MoS 2 being currently recognized as the active phase of Mo-based catalysts. [11][12][13] However, despite various of experimental techniques (ie, Transition electron microscopy (TEM), [14,15] Xray photoelectron spectroscopy (XPS), [16] laser Raman spectroscopy, [17] extended X-ray absorption fine structure (EXAFS), [18] and IR spectroscopy [19] ) involved in characterizing the crystal structure and active phase roughly, it still has difficulty in determining active sites of nano-crystals presenting active MoS 2 moieties, which has led to a lack of integrated and detailed mechanistic investigations into the sulfur-resistant methanation reaction from a microscopic viewpoint. To circumvent the difficulties of experimental characterization, density functional theory (DFT) simulations [20] can be used to probe the reaction mechanism by determining the active site(s) and possible intermediates involved in the process, and subsequently searching for a suitable reaction path to calculate the reaction energy barrier and elucidate the rate-controlling step.Similar ...

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“…Schematic diagram showing reactants, intermediates, and products in three different pathways (a) and comparison of the energy profiles of optimal reaction paths for edge site and S-vacancy site on MoS 2 catalysts (b) . Reproduced with permission from ref . Copyright 2010 John Wiley and Sons.…”

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