Fractal dimension of zirconia nanopowders and their activity in the CO oxidation

We present a study of the CO oxidation on zirconia nanopowders consisted of nanoparticles with a size in a range of 3-7 nm. The fractal characteristics of zirconia nanopowders were studied both by nitrogen adsorption and small-angle X-ray scattering. The surface fractal dimension of zirconia is approximately constant and equals to 2 indicating that zirconia nanoparticles have a smooth surface in all solids. The mass fractal dimension is in the range between 2.1 and 2.5 showing a difference in nanoparticle aggregation depending on their size. Catalytic tests indicate that the activation energy of the CO oxidation on zirconia nanopowders (E-a) is constant. We associate this observation with constancy of the surface fractal dimension. The pre-exponential factor of the rate constant (k(o)) varies significantly decreasing with increasing the mass fractal dimension of solids. This finding is in a good agreement with theoretical approach developed in this communication. (c) 2011 Elsevier B.V. All rights reserved.