Abstract
Adsorption of CO on Ni/TiO 2 sample results in the formation of Ni 2+ CO and Ti 4+ CO species which are observed in the 2210–2185 cm −1 region and are characterized by similar stabilities. This makes unambiguous assignment of the carbonyl bands problematic. An analogous situation is found with Ni/ZrO 2 sample (carbonyls in the 2190–2155 cm −1 region), where distinguishing between Ni 2+ CO and Zr 4+ CO species is even more difficult. Some Ni + sites were easily monitored on both samples by bands in the 2135–2110 cm −1 region. Adsorption of NO on Ni/TiO 2 results in the formation of Ni 3+ NO (1932 cm −1, negligible amount), Ni 2+ NO (1878 cm −1), Ni 2+(NO) 2 (1897 and 1851 cm −1), and Ni + NO (1855 cm −1) complexes. Only Ni 2+ NO (1840 cm −1) and Ni + NO (1815 cm −1) species are detected after NO adsorption on the Ni/ZrO 2 sample. In this case, however, formation of surface nitrates as a result of NO disproportionation (at high NO equilibrium pressures) strongly affects the Ni 2+ NO nitrosyls: their stretching frequencies are blue-shifted due to the enhanced acidity of the Ni 2+ ions. It has been concluded that coadsorption of CO and NO permits simultaneous and selective registration of (i) Ni 2+ and Ti 4+ (Zr 4+), and (ii) Ni 2+ and Ni + surface cations. This is based on the fact that CO is adsorbed more strongly on Ti 4+, Zr 4+, and Ni + ions while NO forms a stronger bond with Ni 2+ ions.
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