An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane

The structural and catalytic properties of supported MoO3/Al2O3 catalysts with Mo surface densities, nsns, in the range 1.1–12.5 Mo/nm2 were studied for the oxidative dehydrogenation of ethane by in situ Raman spectroscopy with simultaneous catalytic measurements at temperatures of 400–550 °C. Isolated mono-molybdates (MoO4) and polymolybdates are formed at various proportions (depending on the loading) on the catalyst surface under dehydrated conditions; bulk Al2(MoO4)3 crystals are formed at nsns exceeding the monolayer. Under reactive environment (C2H6/O2), the Raman features attributed to MoO and MoOMo modes appear to be perturbed, indicating that surface MoOx species are reduced to lower valence states. The combined information on catalyst structure under reaction conditions and catalytic data shows a concurrence between the trends for activity per Mo (turnover frequency [TOF]) and for the number of MoOAl bonds per Mo as a function of nsns. The catalytic activity is related to the anchoring MoOsupport bonds. Structure–activity/selectivity relations are derived and discussed.