Catalysis over Porous Anodic Alumina Catalysts

Abstract

Porous alumina films with a high pore surface density, of the order of 1010/cm2, were prepared on Al metal by the galvanostatic anodic oxidation of Al metal in a thermostated, nonstirred bath of H2SO4 solution, 15% w/v, at various bath temperatures and current densities that resulted in different oxide film thicknesses, in order to be used as catalysts. Structural features such as pore base diameter, porosity, and total and specific real surface were determined by means of kinetic data obtained from the Al2O3 film growth. The catalytic behaviour of the porous anodic Al2O3 films was investigated by using the decomposition of HCOOH as a test reaction in the temperature range 270-390°C. The catalytic decomposition of HCOOH was found to be a dehydration reaction of zero order with respect to the partial pressure of HCOOH at an operation pressure of 1 atm. After some initial catalyst deactivation, the kinetic parameters of the catalytic dehydration, namely activation energy, frequency factor, total activity (referred to a constant geometric film surface area), and specific activity (expressed either per g of oxide mass or per m2 of real surface) at 350°C were found to be strongly affected by film thickness and bath temperature. Anodic Al2O3 films modified by hydrothermal treatment gave consistently higher values of the kinetic parameters, while the span of their variation with film thickness was significantly reduced in comparison to the untreated films. Also, the initial deactivation was prevented to a significant extent. This catalytic behaviour was satisfactorily explained on the basis of the microcrystalline nature of the oxide, the presence of electrolyte anions incorporated inside the oxide bulk during film growth, and the heterogeneity of the oxide surface present along the walls of conical pores.