Surface structure and reactivity of CrO 3/SiO 2 catalysts

Abstract

The molecular structure of the two-dimensional chromium oxide overlayer on the silica support at different chromium oxide contents has been investigated by in situ Raman spectroscopy. Under dehydrated conditions, the surface chromium oxide species on silica consists of a highly distorted, tetrahedral monochromate species regardless of chromium oxide content. The catalysts above 2% CrO 3/SiO 2 also contain crystalline α-Cr 2O 3 particles in addition to the surface monochromate species. The methanol oxidation studies reveal that the catalytic activity per Cr atom, the turnover number (TON), decreases as the chromium oxide content increases. The somewhat higher TON of the initial rate as compared to the steady state rate for the methanol oxidation reaction reflects the higher activity of the fully oxidized chromium oxide species relative to the partially reduced chromium oxide species for the methanol oxidation reaction. The major reaction product is HCHO, while HCOOCH 3, CO, and C0 2 are next in abundance. The selectivity for HCHO increases as the chromium oxide content increases and the precalcination temperature is increased. The opposite trend is observed for the selectivity of HCOOCH 3. These selectivity changes are due to dehydroxylization of the silica surface by precalcining at elevated temperatures and increasing the surface chromium oxide species. The current results suggest that HCHO and HCOOCH 3 are produced on two different catalytic sites: HCHO is formed on the Cr site, whereas HCOOCH 3 is produced via hemiacetal intermediates, which are formed by interaction between HCHO adsorbed on the Cr site and CH 3O adsorbed on the silica site. The CO and CO 2 combustion products are produced on both the Cr and silica site.