An operando Raman study of molecular structure and reactivity of molybdenum(vi) oxide supported on anatase for the oxidative dehydrogenation of ethane

Abstract

Supported molybdenum oxide catalysts on TiO₂ (anatase) with surface densities in the range of 1.8–17.0 Mo per nm² were studied at temperatures of 410–480 °C for unraveling the configuration and molecular structure of the deposited (MoO_x)_n species and examining their behavior for the ethane oxidative dehydrogenation (ODH). In situ Raman and in situFTIR spectra under oxidizing conditions combined with ¹⁸O/¹⁶O isotope exchange studies provide the first sound evidence for mono-oxo configuration for the deposited (MoO_x)_n species on anatase. Isolated OMo(–O–)₃ tetra-coordinated species in C_3v-like symmetry prevail at all surface coverages with a low presence of associated (polymeric) species (probably penta-coordinated) evidenced at high coverages, below the approximate monolayer of 6 Mo per nm². A mechanistic scenario for ¹⁸O/¹⁶O isotope exchange and next-nearest-neighbor vibrational isotope effect is proposed at the molecular level to account for the pertinent spectral observations. Catalytic measurements for ethane ODH with simultaneous monitoring of operandoRaman spectra were performed. The selectivity to ethylene increases with increasing surface density up to the monolayer coverage, where primary steps of ethane activation follow selective reaction pathways leading to ∼100% C₂H₄ selectivity. The operandoRaman spectra and a quantitative exploitation of the relative normalized MoO band intensities for surface densities of 1.8–5.9 Mo per nm² and various residence times show that the terminal MoO sites are involved in non-selective reaction turnovers. Reaction routes follow primarily non-selective pathways at low coverage and selective pathways at high coverage. Trends in the initial rates of ethane consumption (apparent reactivity per Mo) as a function of Mo surface density are discussed on the basis of several factors.