Effect of the Gd substitution by Bi in the BixGd1-xVO4 catalysts on the catalytic behaviour of methanol oxidation

A series of Bi-x Gd1-x VO4 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 less than or equal to x less than or equal to 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 --> VO4 charge transfer which occurs in the Bi-O-V group partly compensates the preceding effect. (C) 2000 Elsevier Science B.V. All rights reserved.