Kinetics of steam reforming of methane on Ru/Al2O3 catalyst promoted with Mn oxides

New types of solar heated reformers have been developed in recent years. In these reactors, called volumetric reformers, concentrated solar radiation illuminates directly the catalyst through a transparent window. These solar reformers can operate at higher temperatures (1000-1100 degrees C) since the surface temperature limit of a regular metal tubular reactor used in the industry is eliminated and a much higher heating rate is feasible. The main restriction, so far, of the solar volumetric reformers to operate at these temperatures is caused by the thermal stability of the current available catalysts. A new catalyst, thermally stable at these temperatures, based on Ru supported on a-alumina and promoted with Mn oxides, has been investigated. The experimental results show that the activity of this catalyst in steam reforming of methane was practically unchanged after operation at 1100 degrees C for 100 h. The structure of the catalyst was studied by XRD. The fresh sample contained alpha-Al2O3 and MnO2. Under the reaction conditions at high temperatures MnO2 is reduced to form Mn3O4 and spinel MnAl2O4. SEM micrographs show large islands of Mn oxides over alpha-Al2O3 particles. The kinetics of the steam reforming of methane on Ru/(alpha-Al2O3 + MnOx) catalysts was studied in a flow reactor operated in differential mode at the temperature range of 500-900 degrees C and total pressure of 1-7 atm. Conversion of methane was 2-8%. The results show that the reaction order with respect to methane is < 1 at 450-500 degrees C and close to 1 at 700-900 degrees C. The reaction order with respect to steam is negative at all temperature range. The kinetic equation was derived from the mechanism of the steam reforming reaction as developed in this paper. This mechanism involves the following main stages: (a) dissociative adsorption of methane on Ru surface; (b) adsorption of steam on the catalyst support in a molecular form; (c) dissociative adsorption of steam on the Ru surface; (d) oxidation of the surface carbon. (c) 2004 Elsevier B.V. All rights reserved.