Kinetics of N2O decomposition on the surface of γ-Al2O3 doped with sodium ions

Abstract

The kinetics of N₂O decomposition on a series of specimens prepared by doping γ-Al₂O₃ with various amounts of Na⁺ ions has been studied at various temperatures using a flow-bed reactor working under atmospheric pressure. This doping promotes the adsorption of oxygen anions produced from surface decomposition, presumably via the formation of surface species [Na⁺⋯O^–⋯Na⁺], bringing about a transformation of the rate equation from R=k into R=kb_N2OP_N2O/b^½_O2P^½_O2(where b_N2O and b_O2 are adsorption coefficients and P_N2O and P_O2 are partial pressures). Moreover, a decrease in catalytic activity, expressed either as fractional conversion or rate of reaction, was observed on increasing the surface coverage C, of γ-Al₂O₃ with Na⁺ ions determined by X-ray photoelectron spectroscopy. Specifically, the dependence of the catalytic activity on the surface coverage of γ-Al₂O₃ is described by the relationship ln (1/R)= 15.4 +(281/K)C(where K is a proportionality constant) and it was concluded that the deactivation observed is due to the promotion of the O₂ adsorption caused by the Na⁺ ions. Finally, the linear dependence of the surface coverage of γ-Al₂O₃ on the sodium content strongly suggests that the dispersion of the sodium supported species is constant irrespective of the surface concentration of sodium.