TEMPERATURE-PROGRAMMED REDUCTION AND OXIDATION OF NICKEL SUPPORTED ONWO3-AL2O3 COMPOSITE OXIDES

A series of Ni/WO3-Al2O3 catalysts have been prepared by systematicall y varying the weight loading of WO3-Al2O3 and the pH during adsorption /impregnation. The catalysts developed an increased interaction betwee n the impregnated nickel and the supported metal oxide; the charge on the mixed oxide resulted in the impregnated nickel cation to be prefer entially adsorbed onto tungsten sites of the composite supports. The a mount of nickel associated with tungsten oxide was found to be a funct ion of both the tungsten oxide weight loading and the impregnation pH. An increase in the negative charge due to hydrolysis of the tungsten oxide phase in the aqueous impregnant is proposed to result in a tungs ten oxide associated nickel that is twice as large for the catalysts p repared at pH 6 than that at pH 4. The Ni/support interaction formed d uring adsorption/impregnation and drying were studied by temperature-p rogrammed reduction (TPR) and temperature-programmed oxidation (TPO). The tungsten oxide and associated nickel were found to form a nickel t ungstate compound which reacted with H-2 at ca. 1050 K but could be re generated by TPO. An Ni/Al2O4 type surface compound was also formed th at was reduced at ca. 11 50 K.