The surface structure of the active bismuth molybdate catalyst

Abstract

Bi 2Mo 3O 12(α), Bi 2Mo 2O 9(β), Bi 2MoO 6(γ), and mixtures of α and γ were investigated as to their activity for the oxidative dehydrogenation of but- 1-ene to butadiene. Pure γ was found to be almost inactive, α was only moderately active, but β and all mixtures of α and γ showed considerable and approximately equal activity. XPS measurements showed the surface of α, β, and γ to have similar Bi Mo ratios to the bulk. However, for mixtures of α and γ with average ratios 2 3 < Bi Mo < 2 1 , the Bi Mo ratio at the surface was found to be approximately equal to 1, decreasing steeply at the Mo-rich side and increasing even more steeply at the Bi-rich edge. The catalytic activity therefore seems to be connected with a surface layer with a Bi Mo ratio near 1. Adsorption studies on inactive and active catalysts near the Bi Mo = 2 edge show that B-type adsorption (weak olefin adsorption) is already found on inactive catalyst (stoichiometric γ). A-Type adsorption (strong butadiene adsorption) only becomes observable when a considerable amount of excess MoO 3 is introduced; the amounts adsorbed increase almost linearly with the increase in activity of the catalyst. In infrared and Raman spectroscopic studies, the theoretical positions for the bands related to Mo-O vibrations were determined, using atomic coordinates from the literature and stretching force constants obtained by the application of the Cotton-Wing relation and finally computing the position via a modified Schachtschneider program. The information thus acquired for compounds with known structure was used to analyze the spectra of Bi-molybdates with unknown structure. The results indicate that the active nonstoichiometric catalysts are characterized by the presence of a surface layer on γ. The new bands formed in catalysts containing small amounts of excess MoO 3 were found to be similar to those of β. It is concluded that the surface layer has a structure closely related to that of β. A model of the surface of an active catalyst is given.