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.