Adsorption/Desorption of H 2 and O 2 at a p-Cr 2 O 3 Surface: Electrical Conductivity Studies

Abstract

The behaviour of H₂ and O₂ flows over a porous p-Cr₂O₃ pellet have been studied by means of an in situ electrical conductivity technique. Changes in the electrical conductivity associated with such gas flows were attributed to changes in the concentration of positively charged holes in the material studied. In the presence of an H₂ flow, the electrical conductivity of the p-Cr₂O₃ surface first increased and then decreased sharply. The initial increase was attributed to the chemisorption of H₂ leading to surface hydroxylation. The formation of O–H bonds during this process resulted in a decrease in the electron density at the surface and thereby increased the contribution of positively charged holes towards the total conductivity. In contrast, the subsequent sharp decrease in conductivity was explained by the elimination of chemisorbed oxygen which had originally been adsorbed dissociatively at the surface. During this process, elimination of chemisorbed oxygen atoms led to the presence of electrons in the chromia lattice which generated a sharp decrease in the hole concentration via an electron–hole recombination process. In an O₂ atmosphere, the conductivity of the deoxygenated chromia increased exponentially to a maximum value and then decreased until it reached a value similar to that for stoichiometric chromia. The increase in conductivity observed in this case was assigned to the adsorption of oxygen on the chromia surface while the subsequent decrease was attributed to hydrogen desorption.

The kinetics of the four processes observed, i.e. hydrogen adsorption, oxygen desorption, oxygen adsorption and hydrogen desorption, have been analyzed.