Co 3 O 4 as an NH 3-oxidation catalyst may transform reversibly to CoO under reaction conditions, even in the presence of excess oxygen. The use of alumina may then result in the formation of cobalt aluminate rendering the catalyst inactive. The formation of cobalt aluminate can be avoided by using ZnAl 2 O 4 as a support.

The performance of pellets of unsupported and silica-supported Co 3 O 4 in the ammonia oxidation was investigated as a function of the particle size to investigate the utilization of the catalytically active phase in these materials. The obtained activity in terms of ammonia conversion over the silica-supported Co 3 O 4 is higher compared to the conversion over the unsupported Co 3 O 4 , despite a lower cobalt oxide loading and more severe diffusional limitations. The effectiveness factor for the silicasupported catalyst is slightly lower than the effectiveness factor for the unsupported catalyst in the form of pellets of similar size. However, the effective utilization of cobalt within the catalyst is higher for the silica-supported catalyst, mainly due to the higher dispersion of the catalytically active phase.