Infra-red study of adsorption and oxidation of ammonia on silica-supported platinum and silica

Abstract

An infra-red study of the adsorption of ammonia on silica-supported platinum in the region 3600-1900 cm^–1 over a temperature range 25–130°C yields four spectral features: a strong broad absorption at 2960 cm^–1 due to the perturbed ν(OH ···) in a surface SiOH ··· NH₃ complex, a shoulder at 3280 cm^–1 and two fairly sharp medium bands at 3425 and 3345 cm^–1. Only the 3345 and 3280 cm^–1 bands remain at 130° and are assigned to the asymmetric and symmetric stretching modes of NH₃ chemisorbed at Pt sites. The 3425 cm^–1 band is attributed to a stretching mode of NH₃ adsorbed at surface hydroxyl sites.

The spectra of NH₃+ O₂ mixtures over silica-supported platinum have been recorded at 25°C intervals over the temperature range 25–200°C. Up to 150°C the same pattern of behaviour is observed as for NH₃ adsorbed on silica-supported platinum. Above this temperature a doublet at 2235 and 2215 cm^–1 appears due to gaseous N₂O. On cooling, a new strong band is found at 3480 cm^–1 which is due to the H₂O formed by oxidation and subsequently adsorbed on the catalyst. The same experiments performed over silica in the temperature range 25–320°C show that silica itself is also able to catalyze ammonia oxidation at temperatures above 220°C. The presence of bands at 3350 and 3280 cm^–1 renders suspect the use of such a doublet as a criterion for the presence of Lewis acid-sites.