Investigation of the Structure and Active Sites of TiO₂ Nanorod Supported VO_x Catalysts by High-Field and Fast-Spinning ⁵¹V MAS NMR

Abstract

Supported VO x /TiO2-rod catalysts were studied by 51V MAS NMR at high field using a sample spinning rate of 55 kHz. The superior spectral resolution allows for the observation of at least five vanadate species. The assignment of these vanadate species was carried out by quantum chemical calculations of 51V NMR chemical shifts of model V surface structures. Methanol oxidative dehydrogenation (ODH) was used to establish a correlation between catalytic activity and the various surface V sites. It is found that monomeric V species are predominant at low vanadium loadings with two 51V NMR peaks observed at about −502 and −529 ppm. V dimers with two bridged oxygens result in a peak at about −555 ppm. Vanadate dimers and polyvanadates connected by one bridged oxygen atom between two adjacent V atoms resonate at about −630 ppm. A positive correlation is found between the V dimers, giving rise to the −555 ppm peak, and the ODH rate, and an even better correlation is obtained by including V monomer contributions. This result suggests that surface V dimers related to the −555 ppm peak and monomers are the primary active sites for the methanol ODH reaction. Furthermore, a portion of the V species is found to be invisible to NMR and the level of such invisibility increases with decreasing V loading levels, suggesting the existence of paramagnetic V species at the surface. These paramagnetic V species are also found to be much less active in methanol ODH.

Mechanism by which Tungsten Oxide Promotes the Activity of Supported V2O5/TiO2 Catalysts for NOX Abatement: Structural Effects Revealed by 51V MAS NMR Spectroscopy

“…1 monolayer coverage) catalysts reveals apparent, distinct structural differences. The supported 5 % V 2 O 5 /TiO 2 catalyst primarily contains features below the 51 V NMR −544 ppm peak (96 %), similar to those we previously reported . A relatively narrow feature at −612 ppm accounts for 13 % of the total vanadia species in the supported 5 % V 2 O 5 /TiO 2 catalyst that is the approximate position of V 2 O 5 ‐like nanoparticles, but may also be related to large, 2D oligomeric surface vanadia structures at these higher loadings.…”

“…Previous technological limitations in the magnetic field, sample spinning rate, and spectral interpretation have impaired a detailed analysis of the surface vanadia sites present on oxide supports . When sufficiently high fields and fast spinning rates are applied, it has now become possible to distinguish between unique surface vanadia sites on titania for enhanced species identification and quantification …”

Polymeric vanadyl species determine the low-temperature activity of V-based catalysts for the SCR of NO x with NH 3

“…The 51 V NMR spectra (fig. S5) show that, with increasing sulfur content, the intensity of the central-band peak at lower chemical shifts (i.e., −655 ppm) was significantly increased, suggesting that the polymerization of vanadyl species was enhanced with sulfate introduction ( 30 ). The transformation of monomeric vanadyl to polymeric vanadyl species caused by sulfate introduction may be due to the surface sites of TiO 2 being partially occupied by sulfate, which enables the vanadyl species to be close to each other ( 16 ).…”

Supported Vanadium Oxide Clusters in Partial Oxidation Processes: Catalytic Consequences of Size and Electronic Structure

“…However, in this case a relative low selectivity to acetaldehyde was also reported and mainly acetic acid was observed as the main reaction product at 200 °C . Owing to a lack of control of the exact vanadia cluster size, the changes in the reactivity of VO x clusters with different morphologies is still subject of controversy and the exact determination of the active site is a topic of intense research . This determination demands a clear molecular structure for the VO x model representing the active site.…”