Systematic investigation of supported transition metal oxide based formulations for the catalytic oxidative elimination of (chloro)-aromatics -: Part I:: Identification of the optimal main active phases and supports

This paper reports, for the first time, a systematic investigation (screening) of the catalytic activity of 40 different formulations of transition metal oxides-based supported catalysts in the course of the total oxidation of benzene as a model molecule for dioxin. The catalysts consisted in 10 different transition metal oxides (CrOx, MnOx, VOx, SnOx, WOx, NbOx, TaOx, MoOx, ZrOx, and BiOx) supported on 4 different supports (2 kinds of TiO2, Al2O3 and SiO2). A theoretical coverage of 0.75 monolayer of active phase was chosen to minimize the formation of crystallites. XPS and XRD characterizations demonstrated the better spreading as monolayer of the active phases at the surface of titania than of Al2O3 and SiO2. The latter induces a poor dispersion of almost all the active phases as crystallites. The variation of spreading of the active phases on the different supports is governed by the difference in surface free energy and is fully explained by the "solid-solid wetting" concept. For almost all active phases, the conversion of benzene progressively improves when the support is changed from SiO2 to Al2O3 and, finally, titanias. The performances of the active phases exhibiting this behavior, are clearly dictated by their presence as well-dispersed monolayers at the surface of the support. The screening revealed CrOx, VOx and MnOx as the most active phases. The first two exhibit their best activity when spread as monolayer at the surface of TiO2 based supports (classical behavior). At the opposite, MnOx works best when present in the form of Mn3O4 crystallites on the SiO2 support. (c) 2006 Elsevier B.V. All rights reserved.