Adsorptive properties of magnetite surfaces as studied by temperature-programmed desorption: Studies of O 2, NO, CO 2, and CO adsorption

Abstract

The adsorptive properties of magnetite surfaces of differing oxidation state were studied with respect to various probe molecules (O 2, NO, CO 2, and CO) using temperature-programmed desorption (TPD). Oxygen existed on these surfaces as molecular species ( Q O 2,≅ 61 and 78 kJ mol −1 , where Q is the heat of adsorption), atomic species, and lattice-like species. The number of these various oxygen species decreased as the surface was vacuum-annealed at higher temperatures. Correspondingly, the number and coordinative unsaturation of surface cations increased. Nitric oxide adsorbed as both weakly bound nitrosyl and strongly bound dissociated species. Nitrosyl species ( Q NO 2 ≅ 60–130 kJ mol −1) desorbed as NO and simultaneously reacted to form a small amount (ca. 1%) of desorbed N 2O and surface oxygen. Dissociated species reacted to form N 2 (with an activation energy of ca. 200 kJ mol −1) and surface lattice oxygen. The surface coverages of both species increased with increasing extent of surface reduction, and total NO coverages correlated with the number of exposed cation sites. Carbon dioxide adsorbed as both monodentate ( Q CO 2 ≅50–60 kJ mol −1) and bidentate ( Q O 2 = 60–120 kJ mol −1) carbonate species. Bidentate carbonate formation, which required cation-anion pair sites, inhibited NO adsorption on cation sites and was blocked by NO preadsorption on these sites. The CO 2 forming these carbonate species was capable of exchanging one oxygen with the surface upon desorption. Total CO 2 coverages correlated with the number of reactive surface oxygen species. Carbon monoxide weakly adsorbed ( Q O 2≌ 46 kJ mol −1 ) in a carbonyl fashion on cation sites of low coordination.