The phenol steam reforming reaction over MgO-based supported Rh catalysts

Abstract

The phenol steam reforming reaction toward H2 formation has been investigated in the 575-730°C range over MgO, CeO2, and ZrO 2 single metal oxides and various mixed metal oxides such as Mg-Ce-O, Mg-Zr-O, and Mg-Ce-Zr-O, all prepared by the sol-gel method. The same reaction has also been studied over Rh supported on these metal oxides. The effects of synthesis method of the Mg-Ce-Zr-O support (mechanical mixing versus sol-gel) and of Rh deposition (wet impregnation versus sol-gel) on the activity and H2 selectivity of the reaction over the Rh/Mg-Ce-Zr-O catalyst have also been investigated. It was found that 0.5 wt% Rh/MgO and 0.1 wt% Rh/Mg-Ce-Zr-O catalysts, the supports of which were prepared by the sol-gel method, exhibit better performance when compared to a commercial Ni-based catalyst used for tar steam reforming. In particular, Rh/MgO presented the highest H2 product concentrations and selectivities in the 575-730°C range and no more than 20% drop in activity after 24 h of continuous reaction (0.5% C6H5OH/40% H2O/He). It was found that steam reforming of phenol is favored over very small Rh particles in the case of 0.1 wt% Rh/Mg-Ce-Zr-O catalyst. In particular, the specific integral reaction rate of H2 production (mmol-H 2/(m2 Rh s)) significantly increased by decreasing the Rh particle size from 2.7 to 1.2 nm. The effect of the partial pressure of water and phenol in the feed stream on catalyst activity was found to strongly depend on support chemical composition. More than one kind of carbonaceous species forming during reaction has been identified with varying compositions and reactivities toward oxygen and steam. XPS studies suggested that the MgxZr1-xO2 phase present in the Mg-Ce-Zr-O support is reduced by H2 at 300°C in the presence of very small Rh particles leading to the creation of oxygen vacant sites and diffusion of Zr toward the bulk of the crystal. As a result of this, the surface acidity and basicity of the Rh/Mg-Ce-Zr-O catalyst are expected to alter, thus influencing the hydrogen activity and selectivity of the reaction. © 2004 Elsevier Inc. All rights reserved.