Characterization of fractal surface roughness and its influence on diffusion and reaction

To investigate the effect of surface roughness on diffusion and reaction in porous 'amorphous' catalysts, a realistic model of roughness is necessary. By combining small-angle and wide-angle X-ray scattering, the suggestion f rom adsorption experiments that the surface is often fractal can be confirm ed, since these techniques allow to measure the surface and bulk properties over a wider range of scales, so that the complete scaling regime, if pres ent, can be revealed. The fractal dimension can be determined, as well as t he inner and outer cutoff of the fractal scaling regime. No significant fra ctal surface roughness is found for zeolites, but is shown for such materia ls as a mesoporous gamma -Al2O3, catalytic reforming and hydrotreating cata lysts. Results from the simulation of aggregation phenomena similar to thos e occurring during the sol-gel synthesis used in catalyst preparation lead to additional insights. The fractal surface roughness of a catalyst or cata lyst support on molecular scales has a significant influence on the convers ions and selectivities of chemical reactions, especially when they are limi ted by Knudsen diffusion. This effect is felt up to the scale of industrial reactors, which implies that changing the fractal surface roughness can im prove the performance of an industrial process. (C) 2001 Elsevier Science B .V. All rights reserved.