Magnesium as a Methanation Suppressor for Iron- and Cobalt-Based Oxide Catalysts during the Preferential Oxidation of Carbon Monoxide

Abstract

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.

Co3O4/TiO2catalysts studiedin situduring the preferential oxidation of carbon monoxide: the effect of different TiO2polymorphs

“…However, the continued decrease in the outlet flow of O 2 suggests the simultaneous occurrence of H 2 oxidation (eqn (2)), which is a well-known competing reaction in CO-PrOx. 23,24,27–33 This is also confirmed by the S O 2 →CO 2 , which is below 100% from 75 °C for all three catalysts and continues to decrease as the reaction temperature is increased (Fig. 8(b)).…”

“…Upon forming Co 0 , CO is converted to CH 4 , which is an undesired CO conversion pathway because valuable H 2 is being consumed (eqn (3)). [27][28][29][30][31][32][33] However, methanation only takes place over the fcc and hcp Co 0 formed on P25 (max. X CO→CH 4 : 69.7% at 450 °C) and rutile (max.…”

“…Moreover, the ultimate formation of Co 0 changes the conversion pathway of CO from oxidation to hydrogenation, which produces undesired methane (CH 4 ). [27][28][29][30][31][32][33] Our previous study reported the effect of different support materials (viz., CeO 2 , ZrO 2 , SiC, SiO 2 , and Al 2 O 3 ) on the performance and phase stability of Co 3 O 4 nanoparticles during CO-PrOx. 32 More specifically, we proposed that weak NPSI (e.g., in Co 3 O 4 /ZrO 2 ) allow for high CO 2 yields to be achieved via the MvK mechanism, which relies on the high surface reducibility (and reoxidation) of the nanoparticles.…”

“…Moreover, the ultimate formation of Co 0 changes the conversion pathway of CO from oxidation to hydrogenation, which produces undesired methane (CH 4 ). 27–33…”

Cobalt–Magnesium Oxide Catalysts for Deep Oxidation of Hydrocarbons

“…Other studies note the high thermal stability of CoO-MgO solid solutions and their low tendency to coking, which determines their high potential in the dry reforming of methane [12]. Recently, the number of studies on MgCo 2 O 4 spinel has increased in connection with the search for active components and supports for catalysts [24][25][26][27], as well as nanostructured compositions for electrocatalysis [28], thermochemical energy storage cycles [29], and electrodes for Li-ion batteries and supercapacitors [30,31]. However, studies examining the genesis of the Co-Mg oxide system during its heat treatment with the transition of the spinel into the solid solution are few and contradictory [12,31,32].…”