Effect of the Gd substitution by Bi in the Bi xGd 1− xVO 4 catalysts on the catalytic behaviour of methanol oxidation

Abstract

A series of Bi x Gd 1− x VO 4 mixed oxide has been prepared by coprecipitation and characterised by XPS, IR spectroscopy and by the reaction of methanol oxidation used as a catalytic surface probe. Within the tetragonal phase domain (0 ≤ x ≤ 0.64), zircon-type and scheelite-type structures are observed for x less than 0.3 or higher than 0.3, respectively. The zircon-type phase shows higher selectivities in methyl formate and carbon dioxide than the scheelite-type phase which mainly forms formaldehyde. The selectivities in methyl formate and carbon dioxide increase with the bismuth content in the zircon-type phase domain. Whatever these structures, the gadolinium substitution by the larger size bismuth cation induces an effect similar to that of the external pressure, leading to a shortening of the Bi–O and Gd–O distances which tends to increase the electronic density on these cations. However, a Bi → VO 4 charge transfer which occurs in the Bi–O–V group partly compensates the preceding effect.