The preferential oxidation of CO (CO-PROX) has been identified as one route of further reducing the trace amounts of CO (approx. 0.5 – 1 vol%) in the H2-rich reformate gas after the high- and low-temperature water-gas shift reactions. CO-PROX makes use of air to preferentially oxidise CO to CO2, reducing the CO content to below 10 ppm while minimising the loss of H2 to H2O. In this study, a Co3O4/γ-Al2O3 model catalyst was investigated as a cheaper alternative to the widely used noble metal-based ones. The CO oxidation reaction in the absence of hydrogen has been reported to be crystallite size-dependent when using Co3O4 as the catalyst. However, studies looking at the effect of crystallite size during the CO-PROX reaction are very few. Metal-support interactions also play a significant role on the catalyst’s performance. Strong metal-support interactions (SMSI) in Co3O4/Al2O3 catalysts give rise to irreducible cobalt aluminate-like species. Under CO oxidation and CO-PROX reaction conditions, such strong interactions in a similar catalyst can have a negative effect on the performance of Co3O4 but can keep its chemical phase intact i.e., help prevent the reduction of the Co3O4 phase. The catalysts used to investigate these two effects (i.e., crystallite size and metal-support interactions) were synthesised using the reverse micelle technique from which nanoparticles with a narrow size distribution were obtained. Certain properties of the microemulsions prepared were altered to obtain five catalysts with varying Co3O4 crystallite sizes averaging between 3.0 and 15.0 nm. Four other catalysts with different metal-support interactions were also synthesised by altering the method for contacting the support with the cobalt precursor. The crystallite size of Co3O4 in these four catalysts was kept in the 3.0 – 5.0 nm size range. Catalytic tests for the first series of catalysts showed that the mass-specific CO oxidation activity increased with a decrease in the starting crystallite size of Co3O4. However, the surface area-specific CO2 formation rate increased with an increase in the crystallite size up to 8.5 nm. Above 8.5 nm, the crystallites possess relatively few surface active sites due to their low massspecific surface areas. In situ characterisation in the UCT-developed magnetometer and PXRD capillary cell instruments at temperatures between 50 and 350 °C revealed that at elevated temperatures the catalysts were partially reduced to metallic Co with CoO being the other cobalt phase present. The formation of metallic cobalt resulted in the formation of methane and in the decrease of the CO2 selectivity. Larger crystallites were reduced to a greater extent compared to smaller crystallites possibly due to the existence of weaker metal-support interactions in the catalysts with the large crystallites. Upon decreasing the reaction temperature to below 350 °C, both in situ techniques also revealed that all the catalysts were partially re-oxidised to CoO with no Co3O4 observed. The complete re-oxidation of the surface metallic Co species terminated the formation of methane and restored the exclusive conversion of CO to CO2. It is possible that at the end of each experiment the catalysts possess crystallites with a double-shell structure i.e., crystallites with a CoO core, a metallic Co inner-shell and a CoO outer-shell. For the second series of catalysts, those that had SMSI were much harder to reduce to metallic Co as expected, with the methane yields observed barely reaching 20% at 350 °C (as opposed to reaching 100% as observed with the first series of catalysts). When decreasing the reaction temperature to below 350 °C, the CO oxidation activity over these catalysts was restored but was unexpectedly higher than the activity initially observed along the heating profile. The reason for this enhanced activity may be that the nanoparticles had segregated from the support along the heating profile as they were being partially reduced. The segregation weakened the metal-support interactions availing more active surface area. The catalysts with weaker metalsupport interactions displayed higher mass-specific CO oxidation activities but were much easier to reduce to metallic Co and as a result, formed much more methane along the heating profile (i.e., 50 - 350 °C). These catalysts were later partially re-oxidised at decreased reaction temperatures (i.e., below 350 °C) and were also thought to have crystallites with a double-shell structure at the end of the tests.

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