Adsorption and reaction of adsorbed species on Bi2MoO6 catalyst Influence on sintering and of temperature of reduction

Abstract

The adsorption of butene (B-sites), the strong adsorption of butadiene (A-sites) and the sorption of O2 were studied on (partly) reduced and sintered samples of Bi2MoO6, in connection with studies on the rate of reduction of the catalyst by butene and the rate of reoxidation of reduced samples. The A-site concentration (A) was found to be independent of the surface area of the catalyst and to decrease linearly with the degree of reduction. The B-site concentration (B) falls to half its original value if samples are preheated to 600 °C. B-sites remain present if the catalyst is reduced below 400 °C but disappear if the reduction is performed above 400 °C or when a reduced sample is heated above 400 °C under vacuum. O2-sorption is fast on samples reduced below 400 °C but considerably slower on samples preheated above 400 °C. For the latter samples a new weak, molecular type of O2 adsorption was observed. The rate of reduction is dependent on (A) · (B)2. A mechanism for the catalytic oxidation of butene to butadiene was proposed in which butene is first adsorbed on a B-site by σ-allyl adsorption, the allyl then moving over to an A-site and donating its second H-atom to another B-site. Reoxidation of a reduced sample starts at the A-site but at higher temperatures of reduction an internal rearrangement in the catalyst leads to a transfer of O2− from MoO2 layers to Bi2O2 layers, the A-sites being on the edges of the Bi2O2 layers.