The Research Of Modified Co₃O₄-CeO₂ Catalysts For CO Preferential Oxidation

The content of CO in H₂ feeding gas of PEMFC must be below 10¹00ppm,since the anodes of fuel cells can easily be poisoned by CO. But H₂-rich gasesproduced by steam reforming and water gas shift reaction typically contain about 1vol.% CO. Therefore, it is essential to remove the trace CO from H₂-rich gases.Among the promising methods for removing CO from H₂-rich gases, CO preferentialoxidation (CO-PROX) is the simplest and the most effective one. So far, researcherspay much attention on noble metal catalysts and CuO catalysts for this purpose. Thiswork is to develop a new economic catalyst with high catalytic performance for COpreferential oxidation. Based on this opinion, a series of FeO_x, CuO, ZrO₂ and MnO_xmodified Co₃O₄-CeO₂ catalysts were prepared by co-precipitation method. Thecatalytic performance of these catalysts was tested for CO preferential oxidation. Thedeactivation mechanism of 8Co-1Ce-1Mn catalyst in the CO-PROX reaction was alsoinvestigated. The catalysts were characterized by XRD, H₂-TPR, TPO, TEM, XPS,FT-IR and TPD techniques.The effect of the additive of FeO_x on the catalytic performance of Co₃O₄-CeO₂catalysts for CO preferential oxidation was examined. After adding a small mount ofFeO_x, the activity for CO preferential oxidation was highly improved, which resultedin a larger"temperature window"for 100% CO conversion. CO could be completelyremoved by 8Co-1Ce-1Fe catalyst at 160¹80oC when the feeding gases containedCO₂ and H₂O. But this catalyst exhibited less stability for CO preferential oxidation. Itwas found that adding FeO_x into Co₃O₄-CeO₂ led to smaller crystal sizes of Co₃O₄ andCeO₂ and could enhance the interaction between Co₃O₄ and CeO₂, which contributedto higher catalytic performance for CO preferential oxidation.CuO has significant influence on the catalytic performance of Co₃O₄-CeO₂catalysts. Although CuO could improve the activity of Co₃O₄-CeO₂ for CO oxidation,CH4 could also be detected at about 150oC, which decreased the selectivity of O₂ toCO oxidation. CH₄ formation could be avoided by changing the ratio of Co: Ce: Cu.However, it was found that Co₃O₄ had little influence on the activity and CuO was themain active site for CO preferential oxidation. This might be caused by the weakinteraction between Co₃O₄ and CeO₂ and the change of the property of Con+ reductionafter adding CuO, which resulted in the decrease of the activity of Co₃O₄. ZrO₂ could improve the activity of Co₃O₄-CeO₂ catalysts for CO preferentialoxidation when it had high Co/Ce ratios. 100% CO conversion could be obtained overthis catalyst with the reaction gas of 20 vol.% CO₂, 10 vol.%H₂O, 1 vol.%CO, 1vol.%O₂, 50 vol.%H₂ and N₂ balance under the space velocity of 40,000 ml·h^-1·gcat^-1 at the temperature of 160¹80oC. A little higher calcined temperature led to higheractivity for CO preferential oxidation, since higher calcined temperature wasbeneficial for the formation of Ce_xZr_1-xO₂ solution which enhanced the oxygenstorage capacity.8Co-1Ce-1Mn catalyst showed the best catalytic performance for CO preferentialoxidation among the tested catalysts of this work. Complete oxidation of CO wasachieved over this catalyst with the reaction gas of 20 vol.% CO₂, 10 vol.% H₂O, 1vol.% CO, 1 vol.% O₂, 50 vol.% H₂ and N2 balance under high space velocity of80,000 ml·h^-1·gcat^-1 at the temperature range of 160¹80 oC. Adding MnO_x intoCo₃O₄-CeO₂ led to more uniform mixing of Co₃O₄ and CeO₂ particles and led tofinely dispersed and high valence state cobalt oxides species, which contributed tohigh catalytic activity of Co-Ce-Mn mixed oxides catalysts.8Co-1Ce-1Mn catalyst exhibited less stability in the CO-PROX reaction, while itcould be reactivated after oxidized at 300oC. CO₂ in the feeding gases has largeinfluence on the stability of 8Co-1Ce-1Mn catalyst. It could occupy the active surfacesites of the catalyst by adsorption as carbonate, which led to the deactivation of8Co-1Ce-1Mn catalyst. During the deactivation process, the ratio of Co³⁺/Co²⁺decreased and the number of Co3+ active sites reduced accordingly, which wasprobably responsible for the deactivation of 8Co-1Ce-1Mn catalyst.