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 from adsorption experiments that the surface is often fractal can be confirmed, since these techniques allow to measure the surface and bulk properties over a wider range of scales, so that the complete scaling regime, if present, can be revealed. The fractal dimension can be determined, as well as the inner and outer cutoff of the fractal scaling regime. No significant fractal surface roughness is found for zeolites, but is shown for such materials as a mesoporous γ-Al2O3, catalytic reforming and hydrotreating catalysts. Results from the simulation of aggregation phenomena similar to those occurring during the sol–gel synthesis used in catalyst preparation lead to additional insights. The fractal surface roughness of a catalyst or catalyst support on molecular scales has a significant influence on the conversions and selectivities of chemical reactions, especially when they are limited by Knudsen diffusion. This effect is felt up to the scale of industrial reactors, which implies that changing the fractal surface roughness can improve the performance of an industrial process.