Insights into the Molecular Structure of MoO<sub><i>x</i></sub> Catalysts via Static and Transient Raman Experimentation
Thu D. Nguyen, Alfred Worrad, Dhanush Thirulogachandar, Fuat E. Celik, Stavros Caratzoulas, George Tsilomelekis
Abstract
A comprehensive framework is developed for the study of the molecular configuration of (MoO x ) n species supported on pure anatase-TiO 2 as well as on mixed CeO 2 –TiO 2 . 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 18 O 2 / 16 O 2 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 TiO 2 and CeO 2 –TiO 2 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 CeO 2 –TiO 2 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 H 2 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.