The effect of fractal surface roughness on diffusion and reaction in porous catalysts - from fundamentals to practical applications

'Amorphous' porous catalysts and catalyst supports have a rough internal su rface. Small-angle and wide-angle X-ray scattering results confirm the orig inal but inconclusive suggestion from adsorption experiments that this surf ace is often fractal on molecular scales. Not only does this imply that the accessible surface area depends on the size of the molecules moving throug h the pores, but there also exists an accessibility distribution along the surface. Transport and reaction processes may therefore depend on surface r oughness, in a way that can be quantified using fractal geometry. The effect of fractal surface roughness on Knudsen diffusion is discussed. An analytical expression for the Knudsen diffusivity is derived and the res idence time distribution of the molecules is obtained from Monte-Carlo simu lations. The equations for diffusion and reaction in fractal pores and in p orous catalysts with a fractal internal surface are given. Practical exampl es - vinyl acetate production and the catalytic reforming of naphtha - illu strate the significant influence of fractal roughness on product yields and distributions of industrial processes, from the microscale up to the obser vable reactor scale. This indicates how conversions and selectivities may b e increased by modifying the catalyst surface morphology. (C)1999 Elsevier Science B.V. All rights reserved.