Decomposition of N2O on high surface area CoO-MgO catalysts investigated by electron spin resonance spectroscopy

Abstract

Oxygen radicals, formed during the decomposition of N₂O on samples containing Co²⁺ ions dispersed on high-surface-area (h.s.a.) MgO, have been studied by e.s.r. spectroscopy. Specimens with different amounts of cobalt (0.2, 1 and 5 atoms of Co/100 Mg atoms) were activated under vacuum at 873 or 1173 K, before exposure to N₂O. On addition of N₂O at 273 K and subsequent rapid evacuation at the same temperature, (O^–)_s radicals were the only oxygen species detected. By contrast, in the presence of N₂O, depending upon temperature and time, the following species were detected on the surface: (O^–)_s, (O^–₂)_s, (O^–₃)_s and Co³⁺… O^–₂. The e.s.r. signal of the Co³⁺‥ O^–₂ adduct clearly showed hyperfine interaction with a cobalt nucleus. No similar evidence was found for the other radicals, which consequently are thought to be adsorbed on sites of the matrix. The surface processes by which oxygen radicals, first formed on surface cobalt centres, then migrate to sites of the matrix, are discussed. It is suggested that the migration occurs through reaction of oxygen radicals with lattice O^2– ions yielding intermediate species such as O^2–₂. Diamagnetic forms of oxygen, O^2–₂ and O^2–, can be inferred by the fact that the oxygen radicals are only a small fraction of the total oxygen adsorbed during the N₂O decomposition.

The catalytic decomposition of N₂O on h.s.a. CoO-MgO samples has been studied in the temperature range 525–625 K. The turnover numbers [N₂O molecules decomposed s^–1(Co ion)^–1] at a given temperature are found to decrease with cobalt content, thus confirming the existence of the same “dilution effect” as observed previously for low surface area (l.s.a.) CoO-MgO specimens. This behaviour is briefly discussed in the light of a knowledge of the forms of oxygen produced on the surface during the reaction.