CO2 Assisted Ethane Oxidative Dehydrogenation over MoO3 and V2O5 Catalysts Supported on Reducible CeO2-TiO2

Abstract

<p>Supported MO<sub>x</sub> (M= Mo, V) on mixed CeO<sub>2</sub>-TiO<sub>2</sub> were investigated for the oxidative dehydrogenation of ethane (ODHE) using CO<sub>2</sub> as a mild oxidant. Raman spectroscopic characterization of the synthesized catalysts under dehydrated conditions suggest that surface MoO<sub>x</sub> species prefer to anchor on the crystalline domains of TiO<sub>2</sub>. Upon increasing the amount of CeO<sub>2</sub> in the mixed oxide support, an intense and broad band at ~930cm<sup>-1</sup> underscored that the prevalent species tend to be polymeric (MoO<sub>x</sub>)<sub>n</sub> domains. On the other hand, in the case of VO<sub>x</sub> catalysts, a gradual shift in the symmetric stretching of the vanadyl (V=O) Raman band with increasing CeO<sub>2</sub> content was observed that points at the gradual anchoring of the surface vanadia species on both TiO<sub>2</sub> and CeO<sub>2</sub> thus highlighting the possible existence of the amorphous VO<sub>x</sub> to be located at the interface of the two mixed oxides. The catalytic behavior of Mo and V were distinct. As the ceria content in the support increased, MoO<sub>x</sub> catalysts promoted the ODHE via Mars van Krevelen mechanism while VO<sub>x</sub> catalysts appeared to favor ethane direct dehydrogenation. Investigation of structure-function relationships via in-situ Raman spectroscopic efforts revealed that adding ceria not only changed the redox properties of the support but also improved those of the deposited metal oxide. We also show that upon incorporation of ceria into the support, CO<sub>2</sub> directly participates in the reoxidation of the dispersed MoO<sub>x</sub> species during catalysis. This effect was distinct from the reaction of CO<sub>2</sub> in the reverse water gas shift reaction. Operando Raman spectra revealed that the presence of CO<sub>2</sub> enhances the stability of the bridging Mo–O–Mo bond of polymeric molybdena domains, which is proposed to affect the relative contribution of oxidative versus non-oxidative pathways in ethane dehydrogenation.</p>