Apparent Absorption Spectra of Silica Supported Vanadium–Titanium Oxide Catalysts: Experimental Study and Modeling

Abstract

The structures of different titania, vanadia, and vanadia–titania clusters located on the surface of mesoporous silica SBA-15 are optimized using density functional theory (DFT). The apparent absorption spectra arising from these clusters are calculated with the aid of the ORCA package. The silica support is shown to contribute to the absorption spectra at wavelengths much shorter than those observed for vanadia and titania clusters located on the SBA-15 surface. The comparison of calculated and experimental apparent absorption spectra of supported vanadia and titania catalysts reveals that titania species generally show a higher nuclearity compared to vanadia species at similar low loadings. SBA-15 based catalysts loaded with both vanadia and titania are supposed to contain two types of species: species in which the V ions are anchored to the titania ones and those in which V and Ti ions alternate and are mainly coupled to the support through M–O–Si (M = V, Ti) bridges. The latter provide the major contribution to the apparent absorption spectra at not very high Ti loadings

Acid sites on silica-supported molybdenum oxides probed by ammonia adsorption: Experiment and theory

“…Density functional theory (DFT) calculations were performed to simulate Lewis and Brønsted acidity of the silica-supported molybdenum oxides by applying cluster models adapted from previous studies [42][43][44][45][46]. The Becke three-parameter hybrid functional with Lee-Yang-Parr correlation functional (B3LYP) together with Ahlrichs' triple-zeta split-valence basis set augmented by polarization functions (def2TZVP or def2TZVPP, respectively) were chosen for the calculations [47,48].…”

“…As shown in Fig. 9 A, supported Mo oxide species are represented by a tetrahedral di-oxo (Si-O-)2Mo(=O)2 unit anchored on top of a silsesquioxane cluster (Si8O12H8), which has been used to represents the SBA-15 support in previous studies [42][43][44]46]. To simulate the Lewis acidity, an ammonia molecule was located in the vicinity of the molybdenum atom in the molybdenum dioxo species as an initial guess for geometry optimization.…”

Operando Multiwavelength and Time-Resolved Raman Spectroscopy: Structural Dynamics of a Supported Vanadia Catalyst at Work

“…1,9,13 CT transitions in the range of 300−400 nm are related to tetrahedrally coordinated long-chain or large 2D VO x oligomers. 1,13 According to recent theoretical considerations of Klokishner et al 9 and Maganas et al, 10 absorption bands of monomeric VO x structures tend to be located in the range 200−220 nm, whereas dimeric, trimeric, and tetrameric VO x structures are found at around 250 nm.…”

“…Therefore, these transitions can be associated with reduced vanadia species, exhibiting the oxidation state +4 or +3. 9,20 Operando Multiwavelength Raman Spectroscopy. To gain insight into the structure of the dehydrated catalyst, Raman spectra were recorded at 128 °C prior to reaction using three different excitation sources (Figure 3).…”

“…Depending on the wavelength used, particular vanadia surface structures such as long-chain or large 2D oligomers, short-chain and monomeric structures, and crystallites (V 2 O 5 ) were probed. In fact, Klokishner et al 9 and Maganas et al 10 have theoretically shown that the positions of the absorption maxima of different structural groups follow a trend, according to which small VO x units exhibit blueshifted absorption maxima in comparison to larger VO x structures. This idea is supported by the investigation of UV−vis absorption spectra of various crystalline vanadates, which show a similar trend.…”