Structure of vanadia-titania catalysts doped with alkali metals according to solid-state NMR, ESR spectroscopies and DFT

Abstract

In this work, V/M/TiO₂ catalysts (M = Li, Na, K, Rb and Cs) were investigated with Solid-state nuclear magnetic resonance (SSNMR), electron paramagnetic resonance (ESR) and first-principles calculations. The total vanadium content corresponds to half monolayer (4 V atoms·nm⁻² of TiO₂ support surface), with V/M atomic ratio being 4/0.6. Static ⁵¹V and MAS ¹H, ⁷Li, ⁵¹V, ²³Na, ¹³³Cs NMR spectra were acquired. Surface moieties of vanadium oxide on (001) anatase surface were modeled using density functional theory (DFT); their ⁵¹V NMR parameters were calculated with the Gauge-Including Projected Augmented Wave (GIPAW) method and compared to experimental data obtained with ⁵¹V SSNMR spectroscopy. It was found that mainly strongly bound vanadium sites (V₃) are formed on anatase samples, located on TiO₂ terminal oxygen atoms. Weakly bound vanadium sites (V₁, V₂) are formed on bridged anatase oxygen atoms. Supported alkali metals (Li, Na, K, Rb and Cs) on TiO₂ were found to interact with protons located on the terminal and bridged TiO₂ oxygen atoms. The relative ratio of weakly bound vanadium sites (V₁, V₂) increased on alkali-containing samples. Calculations performed showed lithium cation to prefer V-O-Ti or V-O-V bonds over VO, unsystematically deshielding vanadium nuclei. The presence of V³⁺ is likely to cause a loss of intensity in ⁵¹V NMR spectra.