Molecular structure and activity of molybdena catalysts supported on zirconia for ethane oxidative dehydrogenation studied by operando Raman spectroscopy

Molybdena/zirconia catalysts with surface densities, nsns, in the range of 1.7–10.5 Mo/nm2 were studied by in situ Raman spectroscopy and simultaneous measurement of their catalytic performance for the oxidative dehydrogenation of ethane at temperatures of 420–540 °C. Isolated monomolybdates [O(MoO)4] and polymeric MoOx units with distorted octahedral (CNMo = 5) configuration coexisted on the support at various proportions depending on loading; bulk Zr(MoO4)2 crystals were formed at surface densities exceeding the monolayer (>7 Mo/nm2). The surface composition and the structural properties of the dispersed species were found to respond to changes in the catalyst gas environment; by exploiting the Raman band intensities, it was concluded that reduction of MoO and MoOMo sites under steady state-reaction or reducing conditions was facilitated with increasing loading and that the MoO sites were perturbed and reduced to a greater extent compared with MoOMo. The selective reactivity to ethylene was found to increase up to the monolayer coverage, with high ethylene selectivity persistent at high conversions. The combination of structural and catalytic data revealed a concurrence between the trends for the apparent activity per Mo atom and for the number of MoOZr bonds per Mo versus nsns. The anchoring MoOsupport bonds were shown to be of relevance for the catalytic activity.