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.