Adsorption equilibria and rates of reactions of adsorbed compounds on reduced and oxidized Bi-Mo catalysts

The adsorption equilibria of the butenes, butadiene and H2O were determined on fully oxidized or partially reduced Bi---Mo catalysts as a function of the degree of reduction. The rate of reduction of the catalysts by butene and by butadiene and the rate of oxidation of reduced catalysts by O2 were also studied in their dependence on the degree of reduction. The adsorption isotherms were found to follow simple Langmuir laws, either of the single or of the dual site type. This behavior was reflected in a linear or quadratic decrease or increase of the maximal volume adsorbable as a function of the degree of reduction. The adsorption of the hydrocarbons decreased with increasing reduction; that of water and oxygen proved to be zero on oxidized catalysts but to increase with increasing reduction. Two types of adsorptive centers could be distinguished in this way, viz., 1. (1) a single site that adsorbs butadiene in a slow but strong adsorption and that also acts as the adsorption site for H2O and in the reduced state (A center). 2. (2) a combination of three sites, two of which adsorb 1-butene, cis-2-butene but also butadiene in a fast but weak adsorption, the third one adsorbing trans-2 in a single site adsorption (B center). The conversion of butene to butadiene was found to be a bifunctional process that involves the simultaneous cooperation of A and B centers. Combustion on the other hand starts from single A centers. Reoxidation also proceeds from the vacant A centers provided the temperature does not exceed 400 °C. Above this limit a new process sets in, the rate of which is not determined by a diffusional process but instead by a surface reaction. The nature of the latter process is still obscure. The heat of adsorption of H2O on the reduced A centers was found to be so low as to provide an explanation for the absence of an inhibition by water in the conversion to butadiene.