Study of the influence of the In2O3 loading on γ-alumina for the development of de-NOx catalysts

Alumina-supported indium oxide (In2O3) catalysts with In loadings between 2 and 22 wt% were prepared by impregnation and characterized for their main properties. Surface properties, such as the surface area; surface In content, measured by X-ray photoelectron spectroscopy (XPS); and acidity and basicity, measured both by adsorption microcalorimetry using ammonia and sulfur dioxide as probe molecules and by infrared spectroscopy of pyridine adsorption, were determined. Bulk properties, namely the crystallographic structure as determined by powder X-ray diffraction (XRD) analysis and the redox character of the In2O3-dispersed phase as determined by redox cycles performed both in a flow apparatus and in a thermobalance coupled with a differential scanning microcalorimeter (TG-DSC), were studied. The results obtained were interpreted in terms of In2O3 surface dispersion or aggregation. The catalysts tested in the reduction of NOx by ethene in an oxygen-rich atmosphere showed an interesting ability to selectively reduce NOx to N2, independently of the In loading. The weakness of the oxidative properties of the In2O3 phases limited the ethene combustion and made the catalysts able to reduce NOx even at high temperature (up to 550 °C). The rates of nitrogen formation depended strongly on the In2O3 aggregation state; the In centers were very active even at low amounts.