Effects of alumina incorporation in coprecipitated NiO–Al 2O 3 catalysts

Abstract

The effect of Al 2O 3 levels on the properties of NiO in coprecipitated NiO–Al 2O 3 samples were investigated, using samples with up to 60.7 wt.% Al 2O 3 that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H 2. Only NiO was detected in samples with up to 4.1 wt.% Al 2O 3 for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al 2O 3-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al 2O 3 level. The NiO was nonstoichiometric (NiO 1+ x ), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al 2O 3 through a maximum at about 3 wt.% Al 2O 3. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al 2O 3 addition. At higher levels of Al 2O 3 (13.6 wt.% and above), surface areas increased with higher Al 2O 3 loadings, but NiO crystallite sizes remained approximately the same, independent of both Al 2O 3 content and calcination 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 Al 2O 3. Reducibility was more difficult than with low Al 2O 3 levels, and nonstoichiometry was low and independent of Al 2O 3 content. Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously.