CHARACTERIZATION OF NICKEL SPECIES AT NI/GAMMA-AL2O3 AND NI/FAUJASITE CATALYSTS BY CARBON-MONOXIDE ADSORPTION

The catalysts Ni/gamma-Al2O3, Ni/HY(deal) and Ni/NaX are studied by carbon monoxide adsorption and desorption at 30-degrees-C and a broad range of pressures. Novel bands of carbon monoxide stretching vibrations and assignments to carbon monoxide complexes are considered for (i) Ni/gamma-Al2O3 at 2082 cm-1 related to Ni(delta+) sites at metallic Ni clusters-NiAl2O4 boundaries, (ii) Ni/HY(deal) at 2100 + 2140 cm-1 (adsorption) and 2118 cm-1 (desorption), referred to bicarbonyls and monocarbonyls, respectively, on Ni(delta+) sites stabilized by nonskeletal Al species, and (iii) Ni/NaX at 1812-2068 cm-1 formed after the decomposition of Ni(CO)4 and attributed to polynuclear nickel carbonyls. The reducibility of the Ni species characterized by the maxima of TPR peaks in starting materials increases in the sequence: cationic Ni2+ in Ni(II)NaY and Ni(II)NaX < NiO clusters on Al2O3 < NiO in Ni(II)HY(deal). The degrees of reduction of the individual catalysts are reflected by their methanation activities.