Insights into the Molecular Structure of MoO_x Catalysts via Static and Transient Raman Experimentation

A comprehensive framework is developed for the study of the molecular configuration of (MoO_x)_n species supported on pure anatase-TiO₂ as well as on mixed CeO₂-TiO₂. The framework first employs the equilibrium deposition filtration method to molecularly control the nature of the active molybdenum sites on the surface of the supports while ensuring loadings below monolayer coverage. Next, we deploy in situ Raman spectroscopic characterization in combination with the isotope ¹⁸O₂/¹⁶O₂ exchange technique for the molecular-level identification of (MoO_x)_n surface configurations. Results show that the distribution of (MoO_x)_n species depends strongly on the pH of the precursor solution and that on both the TiO₂ and CeO₂-TiO₂ supports, the dominant configuration pertains to a mono-oxo arrangement. Distinctive spectral behaviors of a multicomponent band in the vicinity of the ∼900 cm^-1 band for supported (MoO_x)_n on CeO₂-TiO₂ are assigned to two separate vibrational modes that involve different anchoring Mo-O-Support bonds. The framework also extends to the coupling of pulse experimentation with operando Raman spectroscopy (transient operando spectroscopy) to distinguish the reactivity among oxygen sites. From the rationalization of combined results, we show that upon H₂ exposure, the initial removal of surface oxygen predominantly happens at the terminal (Mo═O) site, which is then followed by the breaking of some Mo-O-Support bonds. This mechanism allows for oxygen swapping between different Mo-O bonds during reoxidation.