Accessibility and Dispersion of Vanadyl Sites of Vanadium Silicate-1 Nanoparticles Deposited in SBA-15

Vanadium silicalite-1 (VS-1) nanoparticles dispersed onto the mesoporous walls of SBA-15 silica have characteristics different from those of the full-grown VS-1. In particular, the vanadium-binding site differs extensively. Here, we monitor the local environment of VO²⁺ with different electron paramagnetic resonance (EPR) techniques to obtain information about the heterogeneity of the vanadyl sites, the effect of a calcination/reduction cycle, and the subsequent adsorption of ¹³CO₂ and NH₃ to the vanadyl sites for both full-grown VS-1 and VS1-SBA15. Furthermore, two-cycle CO₂ and NH₃ adsorption experiments were performed. While the latter showed no chemisorption of CO₂ at 373 K for both materials, clear evidence was found for ligation of CO₂ to vanadyl in full-grown VS-1 by the detection of a strong ¹³C hyperfine value. This strong binding was not detected for the vanadyl centers in VS1-SBA15. Chemisorption of NH₃ was detected for all samples both by two-cycle adsorption experiments and by EPR.