Abstract
The effect of the addition of phosphoric acid at different stages of the preparation on the properties of VMoTiO 2 catalysts has been studied by ESR. The evolution of the V 4+ and Mo 5+ signals after different reduction treatments indicates that, depending on the procedure followed, either only the surface or the whole catalyst can be affected. By impregnating with H 3PO 4 after calcination, a surface layer of chemisorbed phosphates is formed, protecting the sample bulk against reduction. If the impregnation takes place before calcination of the precursor material, the dispersion of the vanadium and molybdenum ions is observed and is ascribed to the formation of mixed phosphate phases.
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