Oxidation activity and 18O-isotope exchange behavior of nickel oxide-stabilized cubic zirconia

A series of NiO–ZrO2 samples with 2 to 40 mol% of NiO were prepared using a sol–gel synthesis technique and calcined at 873 K. XRD characterization of the samples revealed the stabilization of a cubic zirconia- (fluorite) phase containing nickel oxide up to 20 mol%. Bulk NiO characteristics were observed above 20 mol% loading of NiO. The linear decrease in lattice parameter up to 20 mol% of NiO indicates the probable incorporation of Ni2+ into the lattice position of Zr4+ ions. The NiO–ZrO2 sample with 20% NiO retained its cubic phase even after prolonged heating at 1273 K, indicating 20 mol% as an optimum content of NiO for a thermally stable cubic zirconia phase. The BET surface areas of these samples were in the range of 40 to 70 m2 g−1. XPS spectra along with XRD data indicated that at low (5 mol%) concentrations of NiO, Ni2+ enters ZrO2 lattice substitutionally creating oxygen vacancies. TPR of NiO–ZrO2 showed the reduction of nickel oxide at 533 and 633 K, indicating nickel in two different environments, at a substitutional position and at a surface/interstitial position. 18O-isotope exchange studies of these samples showed a partial heterogeneous exchange and the Tonset was found to be lowest for the sample containing 20 mol% NiO. The activity for CH4 and CO oxidation was investigated by 18O-isotope exchange as well as catalytic studies in complete oxidation of CH4 and CO. The activity for CH4 oxidation was highest for the NiO–ZrO2 sample with 20 mol% NiO. CH4 and CO oxidation with 18O isotope over NiO–ZrO2 catalysts showed the formation of CO2 with 16O (amu 44), suggesting that the bulk oxygen is acting as an active species for methane oxidation. The structure of NiO–ZrO2 samples with varying NiO content and its correlation with catalytic activity mechanism is discussed.