Formation of carbonyl and nitrosyl complexes on titania- and zirconia-supported nickel: FTIR spectroscopy study

Adsorption of CO on Ni/TiO2 sample results in the formation of Ni2+-CO and Ti4+-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/ZrO2 sample (carbonyls in the 2190-2155 cm-1 region), where distinguishing between Ni2+-CO and Zr4+-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/TiO2 results in the formation of Ni3+-NO (1932 cm-1, negligible amount), Ni2+-NO (1878 cm-1), Ni2+(NO)2 (1897 and 1851 cm-1), and Ni+-NO (1855 cm-1)complexes. Only Ni2+-NO (1840 cm-1)andNi+-NO (1815 cm-1)species are detected after NO adsorption on the Ni/ZrO2 sample. In this case, however, formation of surface nitrates as a result of NO disproportionation (at high NO equilibrium pressures)strongly affects the Ni2+-NO nitrosyls: their stretching frequencies are blue-shifted due to the enhanced acidity of the Ni2+ ions. It has been concluded that coadsorption of CO and NO permits simultaneous and selective registration of (i)Ni2+ and Ti4+ (Zr4+), and (ii)Ni2+ and Ni+ surface cations. This is based on the fact that CO is adsorbed more strongly on Ti4+, Zr4+, and Ni+ ions while NO forms a stronger bond with Ni2+ ions.