Characterization of cobalt-impregnated VPO catalysts

Cobalt-impregnated VPO catalysts containing 1-6% of the metal by weight wer e prepared. The catalytic tests showed that cobalt impregnation significant ly increased the overall activity while slightly decreasing the maleic anhy dride selectivity To investigate the origin of the cobalt effect, the solid s were characterized using XRD, Raman spectroscopy, FT-IR, TPR and XPS. No structural effects were detected through XRD. TPR showed that the non-equil ibrated catalysts contain cobalt oxides which react with the VPO solids to yield a hard to reduce system. Non-equilibrated umpromoted and promoted (lo w cobalt loadings) catalysts contain small proportions of V(V) phases. Afte r several 100 h on stream, the only phase detected in all cases was V(IV) v anadyl pyrophosphate. The surface oxidation state of vanadium was V(IV). Th e maleic anhydride yield correlates with the increased exposure of cobalt o n the catalysts surface. (C)2000 Elsevier Science B.V. All rights reserved.