Accessibility of Reactants and Neighborhood of Mo Species during Methane Aromatization Uncovered by Operando NAP-XPS and MAS NMR

One-step nonoxidative methane dehydroaromatization is a facile process to generate aromatics and CO-free hydrogen. Despite their high activity and aromatics selectivity, Mo/HZSM-5 catalysts suffer from a continuous deactivation, hampering their application, yet the cause is intensively debated. Employing a combination of characterizations including, but not restricted to, high-resolution electron microscopy, operando NAP-XPS, and MAS NMR spectroscopy, we endeavored in this contribution to get deeper insight into the nature of active sites and origin of catalyst deactivation. Our results indicated (i) an irreversible reaction-induced MoOx particle sintering, (ii) reversible buildup/removal of coke species, (iii) no quantitative correlation between the deactivation rate and the presence/loss of Brønsted acid sites, and (iv) that coke accumulation occurs almost exclusively on Mo instead of Brønsted acid sites. Deactivation is explained by partial blocking of Mo species by coke, which diminishes the accessibility of methane to active sites and successive narrowing and/or blocking of pores hindering the diffusion of larger reaction products (e.g., naphthalene) to the outer surface. Active sites for aromatics formation are referred to as highly dispersed Mo species (mononuclear and tiny subnanometer oxy- and/or oxycarbidic Mo clusters) located inside the micropores on/or close to Brønsted sites.