Effect of nitric oxide on the formation of cobalt–aluminum oxide structure from layered double hydroxide and its further transformation during reductive activation

• 3% nitric oxide decelerates the Co–Al LDH decomposition, but causes better crystallized Co–Al oxide.
• Presence of nitric oxide in calcination medium causes also better crystallized metallic cobalt after reduction.
• The observed difference in the Co0 size does not affect the activity in FTS.
• The catalysts precalcined in inert gas with 3% of NO produce less methane and more olefins.

The presence of nitric oxide NO in the gas phase was shown to decrease the decomposition rate of the hydrotalcite-like Co–Al LDH phase in the catalysts prepared by co-precipitation or deposition on Al2O3 under the condition of urea hydrolysis. The decrease in LDH decomposition rate is related to the formation of a more crystallized phase of the spinel-like Co–Al oxide due to the ability of nitric oxide to transfer oxygen in consecutive reactions of the oxidation to NO2 and reduction to NO. The difference in the coherent scattering domain size of the catalyst samples precalcined in the presence of NO or in a pure inert gas is retained at all consecutive steps of the reductive heat treatment in hydrogen: after the formation of a cubic phase of the (Co, Al)O oxide and its reduction to metallic cobalt. The observed changes in the degree of crystallinity and dispersion of the active metal exert only a slight effect or even no effect on the activity of the catalysts in Fischer–Tropsch synthesis. Noteworthy are a lower selectivity for methane and a greater fraction of olefins in the products obtained on the catalysts precalcined in a flow of inert gas containing 3% NO.

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