(411c) Investigating the Redox Behavior of MoOx Catalysts Supported on CeO2-TiO2 Via in-Situ Raman and FTIR Spectrokinetics

Catalytic oxidative dehydrogenation (ODH) of alkanes is an alternative to the direct thermal dehydrogenation route towards the production of olefins. In light of the recent shale gas revolution, abundant ethane resources serve as a strong incentive for academic and industrial communities to advance and evolve research strategies for the production of ethylene. Utilizing carbon dioxide as an oxidant has been recognized in recent years as a promising alternative approach that offers an attractive solution to severe total oxidation of ethylene resulting in higher selectivity to desired products. However, the redox behavior of metal oxides in the presence of CO₂ as soft oxidant remains still elusive.

In this study, a series of CeO₂-TiO₂ supports have been synthesized via sol-gel synthesis and MoOx sites with different surface densities have been deposited via wet impregnation. In-situ Raman spectra revealed that the molecular structure of MoOx species depends significantly on the amount of CeO₂ in the mixed support. Specifically, unique features were identified in the 900-950cm^-1 spectral envelope that develop upon increasing the CeO₂ content, but for constant Mo surface density. These features belonging to the Mo-O stretching region, appear to be more stable than Mo=O when catalysts are exposed to H₂, C₂H₆ or C₂H₆/CO₂ mixture. Interestingly, under real reaction conditions (Figure 1), operando Raman measurement reveal that the presence and amount of CO₂ in the feed does not affect the reduction rate of Mo=O but significantly prolongs the existence of Mo-O bonds. By comparing the spectral changes under reduction-reoxidation (with CO₂) cycles, we observed an enhanced redox behavior of MoOx sites upon increasing CeO₂ content in the mixed support. This improvement may be also associated with the larger contribution of oxidative as opposed to direct dehydrogenation pathways at initial reaction conditions, as revealed by kinetic measurements in CO₂-assisted ODH of ethane.