Effects of alumina incorporation in coprecipitated NiO-Al2O3 catalysts

The effect of Al2O3 levels on the properties of NiO in coprecipitated NiO-A l2O3 samples were investigated, using samples with up to 60.7 wt.% Al2O3 th at had been calcined in the range 300-700 degrees C. Characterization techn iques included BET surface area of fresh and reduced catalysts, X-ray diffr action analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H-2. Only NiO was detect ed in samples with up to 4.1 wt.% Al2O3 for all sample calcination temperat ures. Surface areas were similar for all fresh samples but decreased rapidl y after calcination at high temperatures. The surface area loss was less fo r the higher Al2O3-containing samples. Nickel oxide crystallite sizes incre ased at higher calcination temperatures, but remained approximately the sam e for each Al2O3 level. The NiO was nonstoichiometric (NiO1+x), with x decreasing at higher calcina tion temperatures and increasing with small amounts of added Al2O3 through a maximum at about 3 wt.% Al2O3. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al2O3 addition. At higher levels of Al2O3 (13.6 wt.% and above), surfa ce areas increased with higher Al2O3 loadings, but NiO crystallite sizes re mained approximately the same, independent of both Al2O3 content and calcin ation temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al2O3. Reducibility was more difficult than with low Al2O3 levels, and nonstoichiometry was low and independent of Al2O3 content. Reducibilities of all samples calcined at 300 degrees C correlated well wit h the final BET surface area of the reduced samples, indicating that more d ispersed NiO crystallites are more difficult to reduce, a conclusion that s upports a model for reduction proposed previously. (C) 2000 Elsevier Scienc e B.V. All rights reserved.