Chemisorption of nitric oxide on copper–alumina catalysts

Abstract

Nitric oxide chemisorption on an oxidized and an H₂-reduced surface copper spinel containing 10 wt.% CuO has been studied by IR transmission and thermal desorption spectroscopy (TDS). A Cu²⁺—NO complex and nitrito–nitrato complexes were identified, at 308 K on the oxidized surface. The initial evolution of N₂O during dynamic adsorption experiments is related to the existence of cus Cu⁺ sites on the surface. The Cu²⁺—NO complexes dissociate with evolution of NO between 350 and 450 K, while nitrito complexes decompose with evolution of NO and simultaneous formation of nitrato complexes between 450 and 600 K. The latter groups finally decompose between 600 and 750 K with evolution of NO and O₂.

NO is chemisorbed on metallic Cu^o on the reduced materials at 308 K. On increasing exposure and at higher temperatures, Cu²⁺—CO complexes are additionally formed, this being suggestive of surface reoxidation. Initially, the reduced surface is active for NO dissociation as evidenced by N₂ and N₂O evolution. Oxygen ad-atoms are retained on the surface and lead to surface and bulk reoxidation. N₂O evolution is correlated with the presence of Cu^– sites. The surface and bulk metal particles are continuously oxidized during adsorption–desorption cycles and the TDS profiles for NO and N₂O progressively approach those observed for the original oxidized sample. This suggests a reconstitution of the orginal surface spinel phase even under comparative mild reoxidation conditions. Evidence for the formation of a copper nitride, CuN, phase is also reported, which can be observed as long as metallic Cu^o is still present in the material. It is believed that these results bear relevance for the redox processes possibly occurring during catalytic reduction of nitric oxide.