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The preferential oxidation of CO (CO-PrOx) to CO2 is an effective catalytic process for purifying the H2 utilized in proton-exchange membrane fuel cells for power generation. Our current work reports on the synthesis, characterization and CO-PrOx performance evaluation of unsubstituted and magnesium-substituted iron- and cobalt-based oxide catalysts (i.e., Fe3O4, Co3O4, MgFe2O4 and MgCo2O4). More specifically, the ability of Mg to stabilize the MgFe2O4 and MgCo2O4 structures, as well as suppress CH4 formation during CO-PrOx was of great importance in this study. The cobalt-based oxide catalysts achieved higher CO2 yields than the iron-based oxide catalysts below 225 °C. The highest CO2 yield (100%) was achieved over Co3O4 between 150 and 175 °C, however, undesired CH4 formation was only observed over this catalyst due to the formation of bulk fcc and hcp Co0 between 200 and 250 °C. The presence of Mg in MgCo2O4 suppressed CH4 formation, with the catalyst only reducing to a CoO-type phase (possibly containing Mg). The iron-based oxide catalysts did not undergo bulk reduction and did not produce CH4 under reaction conditions. In conclusion, our study has demonstrated the beneficial effect of Mg in stabilizing the active iron- and cobalt-based oxide structures, and in suppressing CH4 formation during CO-PrOx.

Transition metal alloys are receiving considerable attention in heterogeneous catalysis as they hold promise to combine advantageous properties of the constituting metals and, therefore, provide attractive avenues for targeted catalyst design. The present study concerns the effect of Co and Ni substituents in the ferrite (Fe 3 O 4 ) structure used as a catalyst precursor for medium-temperature Fischer−Tropsch (MTFT) synthesis, in anticipation of enhanced oxygenate selectivities. The ferrites were synthesized by co-precipitation and characterized in detail before and after exposure to MTFT conditions, employing both conventional ex situ and state of the art in situ techniques. The complex product spectrum from the MTFT was analyzed by combining off-line one-dimensional and on-line two-dimensional gas chromatography. The latter was used specifically to investigate the formation of minority species, such as oxygenates, which are often disregarded in literature. In situ XRD and magnetometry showed no notable change in the reduction behavior of the ferrites with a cobalt substituent, but substituting with Ni decreased the reduction temperature drastically from 315 to 250 °C, most likely due to the increased hydrogen dissociation activity of Ni. The activity, CO conversion, in MTFT increased in the order Fe ≪ CoFe < NiFe < CoNiFe. Incorporation of Co and Ni in the catalysts makes them less prone to deposition of inactive carbon. The addition of Ni specifically, also results in a significant shift in selectivity toward a shorter average chain length, lower olefinicity and higher water−gas shift activity. Interestingly, these shifts are paralleled by a 76% or 170% increase in C 2+ oxygenates selectivity or yield, respectively. The increase in hydrogenation activity of substituted (i.e., Co and/or Ni) Fe-based catalysts, plays a critical role in the Fischer−Tropsch synthesis activity and selectivity to the different product classes (i.e., paraffins, olefins, and oxygenates) and the findings reported here provide valuable insights of key importance for further development and optimization of FT catalysts.

Mesoporous 2D NiO-Nb2O5-Al2O3 nanorods (and, for the first time, template free ordered mesoporous alumina (OMA)) were prepared via glycol-thermal synthesis for the direct transformation of octane to octenes via CO2...