Effect of crystallite size on the performance and phase transformation of Co 3 O 4 /Al 2 O 3 catalysts during CO-PrOx – an in situ study

The preferential oxidation of carbon monoxide has been identified as an effective route to remove trace amounts of CO (approx. 0.5–1.0 vol%) in the H 2 -rich reformate gas stream after the low-temperature water–gas shift. Instead of noble metal-based catalysts, Co 3 O 4 -based catalysts were investigated in this study as cheaper and more readily available alternatives. This study aimed at investigating the effect of crystallite size on the mass- and surface area-specific CO oxidation activity as well as on the reduction behaviour of Co 3 O 4 . Model Co 3 O 4 catalysts with average crystallite sizes between 3 and 15 nm were synthesised using the reverse micelle technique. Results from the catalytic tests revealed that decreasing the size of the Co 3 O 4 crystallites increased the mass-specific CO oxidation activity in the 50–200 °C temperature range. On the other hand, the surface area-specific CO oxidation activity displayed a volcano-type behaviour where crystallites with an average size of 8.5 nm were the most active within the same temperature range. In situ characterisation in the magnetometer revealed that the Co 3 O 4 crystallites are partially reduced to metallic Co above 225 °C with crystallites larger than 7.5 nm showing higher degrees of reduction under the H 2 -rich environment of CO-PrOx. In situ PXRD experiments further showed the presence of CoO concurrently with metallic fcc Co in all the catalysts during the CO-PrOx runs. In all experiments, the formation of fcc Co coincided with the formation of CH 4 . Upon decreasing the reaction temperature below 250 °C under the reaction gas, both in situ techniques revealed that the fcc Co previously formed is partially re-oxidised to CoO.