Carbon-based monolithic supports for palladium catalysts:: The role of the porosity in the gas-phase total combustion of m-xylene

Three different carbon-based monoliths have been studied in their performance as Pd catalyst supports in the total gas-phase combustion of m-xylene at low temperatures. The first monolithic support (HPM) was a classical square channel cordierite modified with alpha-Al2O3, blocking the macroporosity of the cordierite and rounding the channel cross-section, on which a carbon layer was applied by carbonization of a polyfurfuryl alcohol coating obtained by dipcoating. The other two monolithic supports were composite carbon/ceramic monoliths (MeadWestvaco Corporation, USA), microporous (WA) and a mesoporous (WB) sample. All the catalysts have a comparable total Pd loading and very similar Pd particle size (around 5-6 nm). In sample Pd/WA the Pd is situated only in the macropores, while in the case of Pd/WB the Pd is distributed throughout the mesoporous texture. In the case of Pd/HPM, Pd particles are clearly situated at the external surface of the carbon layer. The catalytic activities of the samples were very different, decreasing in the order: Pd/WB > Pd/WA > Pd/HPM. These results show that the carbon external surface area, the macropores and mainly mesopores, play an important role in this kind of gas-phase reactions, improving the contact between the I'd particles and the m-xylene molecules. The catalytic activity of the Pd supported on carbon-based monoliths correlates with the surface area developed in macro- and mesopores of the monolithic support. (C) 2007 Elsevier B.V. All rights reserved.