Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: IV. Surface characterization and redox behaviour of iron-containing bismuth molybdovanadate catalysts by X-ray photoelectron spectroscopy

Abstract

X-ray photoelectron spectroscopy has been applied to the study of the surface composition and the redox behaviour of multicomponent oxide catalysts having the scheelite structure and general formula Bi 1−x 3 □ x 3−y Me y(V 1−xMo x−yFe y)O 4 (Me = Fe or Bi, □ = cation vacancies), before and after exposure to hydrogen, propene, and a propene + oxygen mixture. The results point to a different reactivity toward the various reactants. By interaction with hydrogen, an extensive reduction is obtained with considerable change of the catalyst surface composition. The reduction behaviour depends on the sample composition, the iron-containing catalysts being reduced at low temperature (140 °C). As regards propene treatment, a preferential reactivity of bismuth with propene has been found, regardless of the sample composition. Bismuth is reduced at low temperature (175 °C) with ensuing migration to the surface. Under conditions leading to the catalytic oxidation of propene, a high surface stability is exhibited by these compounds. The catalysts remain oxidized and the surface composition reflects that of the bulk. A correlation with the catalytic properties has also been made on the basis of the different mobility of the oxygen ions among the samples examined.