SURFACE-STRUCTURE AND REACTIVITY OF CRO3/SIO2 CATALYSTS

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% CrO3/SiO2 also contain crystalline α-Cr2O3 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 HCOOCH3, CO, and C02 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 HCOOCH3. 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 HCOOCH3 are produced on two different catalytic sites: HCHO is formed on the Cr site, whereas HCOOCH3 is produced via hemiacetal intermediates, which are formed by interaction between HCHO adsorbed on the Cr site and CH3O adsorbed on the silica site. The CO and CO2 combustion products are produced on both the Cr and silica site. © 1992.