Preparation, Properties, and ESCA Characterization of Vanadium Surface Compounds on Silicagel. II

Abstract

Abstract Binding energy (BE) behavior and signal intensity as function of V conc. and ligands L were critically examined in V n /SiO 2 species (where n = +5, +4, and +3) and securing of optimized products and ESCA samples is described. The results moreover demonstrate conditions and approaches whereby ESCA can provide reliable information on highly diluted surface compound systems. From aquous media 1.0–1.2 wt. % V V and 0.4–1.2 wt. % V III proved to be preferred ranges whereas in non aqueous (metalorganic) preparates the surprisingly low concentration of 0.4–0.15 wt. % V III gave usable spectra and even 0.4 wt. % V V could be measured. ESCA data with the peroxo complex (SiO) 3 V(O 2 ) (I) confirmed these trends. Superiorly subdivided surface compounds (reflected in the quality of spectra) are obtainable from V[N(SiMe 3 ) 2 ] 3 impregnations where 0.4–0.5 wt. % V III (or V V ) are maximum/optimum conc. limits. Evidence for formulating V n /SiO 2 as three legged surface compounds is summarized and diagnostic/predictive uses of the additivity principle for surface electron states illustrated. In the (SiO) 3 VL series, where L = none, O, and (O 2 ) positive ΔBE shifts for SiOV (0.7 eV); VO (0.6 eV); and V(O 2 ) (0.4 eV) were estimated. Tentative BE contributions of some donor ligands (and Cl) are also suggested. In (I) ESCA indicated a formal oxidation state of approx. +4. Certain limits (precautions) with ESCA are noted and extensions of the additivity relationship discussed.