FTIR study of the influence of sulfate species on the adsorption of NO, CO and NH3 on CuO/Al2O3 catalysts

CuO/Al2O3 catalysts have been sulfated by oxidation of various amounts of SO2. The surface properties of the samples were studied using probe molecules. The IR spectra of CO species adsorbed on the non-sulfated sample show an strong band at 2125 cm–1 with a shoulder at 2160 cm–1, which shift to 2165 and 2180 cm–1, respectively, with a concomitant decrease in intensity as the amount of sulfate increases. Adsorbed NO species on the non-sulfated catalyst give rise to two weak bands at 1754 and 1873 cm–1. On sulfated samples, the first band disappears whereas that at 1873 cm–1 shifts towards higher wavenumber and reaches 1917 cm–1 for large amounts of sulfate. These results show that the copper electronic state is affected by sulfation. The positive charge on the copper sites increases with the amount of sulfate. NH3 adsorption is also influenced by sulfation. Sulfate species are adsorbed on basic sites and high sulfation prevents NH3 dissociative adsorption while it favours coordinated and protonated species on acid–base pair sites.