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+single bondCO and Ti4+single bondCO 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+single bondCO and Zr4+single bondCO 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+single bondNO (1932 cm−1, negligible amount), Ni2+single bondNO (1878 cm−1), Ni2+(NO)2 (1897 and 1851 cm−1), and Ni+single bondNO (1855 cm−1) complexes. Only Ni2+single bondNO (1840 cm−1) and Ni+single bondNO (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+single bondNO 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.