ESR spectra of metal oxide catalysts during propylene oxidation

Abstract

The oxidation of propylene was investigated on silica-supported bismuth molybdate catalysts (MoO 3, Bi 2O 3 · 3MoO 3 + 0.25Bi 2O 3, 3Bi 2O 3 · MoO 3, and Bi 2O 3) by simultaneous measurement of the ESR spectra of the catalysts during the oxidation reaction at 325 and 390 °C and the product distribution. As the oxygen/propylene ratio was increased in the range of 0.25 to 3, the Mo 5+ signal intensity decreased and the total conversion increased. However, the acrolein selectivity remained almost constant. The Mo 5+ signal intensity was found to be inversely proportional to the total propylene conversion. The experimental data are developed by assuming that the molybdenum ions detectable by ESR are primarily at the surface. These molybdenum ions in the Mo 6+ valence state may be the surface sites necessary for the initial catalytic conversion step. Another type of site is postulated to account for selective oxidation to acrolein. ESR was also employed to investigate the rates of oxidation and reduction of the catalysts by measurements of the Mo 5+ signal and of the relative electrical conductivity of the samples. Very fast oxidation and reduction reactions with time constants of seconds are associated with oxidation and reduction involving the surface. Slow reduction reactions with time constants of half an hour are associated with reduction of the bulk with bulk diffusion as the probable rate-limiting process.