Elucidating the role of CO2 in the soft oxidative dehydrogenation of propane over ceria-based catalysts

A mixed oxide support containing Ce, Zr, and Al was synthesized using a physical grinding method and applied in the oxidative dehydrogenation of propane using CO₂ as the oxidant. The activity of the support was compared with that of fully formulated catalysts containing palladium. The Pd/CeZrAlO_x material exhibited long-term stability and selectivity to propene (during continuous operation for 140 h), which is not normally associated with dehydrogenation catalysts. From temperature-programmed desorption of NH₃ and CO₂ it was found that the catalyst possessed both acidic and basic sites. In addition, temperature-programmed reduction showed that palladium promoted both the reduction and reoxidation of the support. When the role of CO₂was investigated in the absence of gas-phase oxidant, using a temporal analysis of products (TAP) reactor, it was found that CO₂ dissociates over the reduced catalyst, leading to formation of CO and selective oxygen species. It is proposed that CO₂ has the dual role of regenerating selective oxygen species and shifting the equilibrium for alkane dehydrogenation by consuming H₂through the reverse water-gas-shift reaction. These two mechanistic functions have previously been considered to be mutually exclusive.