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Addition of potassium enhanced the activity of preferential CO oxidation on Pt/Al 2 O 3. The additive effect of potassium weakened the interaction between CO and Pt, and it also changed the CO adsorption site. FT-IR observation under the PROX condition suggests the presence of adsorbed species derived from O 2 and H 2 such as OH species, which can be related to the promoting effect of H 2 presence in the PROX reaction.

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As fuel-cell research and development has become a flourishing area in recent years, fuel processing, including hydrogen generation, purification, and storage is drawing a great deal of attention. At present, most hydrogen is synthesized through the steam reforming of hydrocarbon fuels, and the water-gas shift of CO (WGS) inevitably coproduces 0.5-1 vol % of CO. However, the polymer electrolyte fuel cell (PEFC) is poisoned easily if the CO concentration is higher than 10 ppm. [1][2][3][4] Preferential oxidation of CO (PROX) has been proposed as a "deep-cleaning" process: CO is oxidized to CO 2 with air supplied downstream from the WGS reactor; it has succeeded in meeting the requirements of the PEFC.[5-10] However, this process requires an external air supplier, a cooling system, and a mixer for reformate gas and air, which makes it necessary to explore other more costeffective approaches. The process of CO methanation, that is, direct hydrogenation of CO to methane and water by consumption of three moles of hydrogen, has been investigated as a less costly, space-saving substitute for PROX that requires no additional reactants. [11][12][13][14][15][16][17][18] Moreover, the CH 4 produced by this reaction can be reused by recirculating the anode off-gas into the reformer as a combustion fuel for reforming. However, to date, there is still a major challenge to remove 1 vol % CO down to lower than 10 ppm under standard operating conditions. Furthermore, maintaining the selectivity of CO methanation is another challenge owing to the presence of about 20 vol % of CO 2 in reformate hydrogen fuels, which will also generate methane by consuming four moles of hydrogen at relatively high temperatures, and which is often accompanied by another side reaction, the reverse water-gas-shift (RWGS) reaction by converting CO 2 into CO. The exothermic character of both methanation reactions also causes problems with the exact control of the reaction temperature, which can result in a further increase in conversion of CO 2 . For the sake of maintaining selectivity, the reaction temperature should be controlled to be as low as possible; specifically, lower than 250 8C. Moreover, the equilibrium temperature for the WGS reactor in the case of 1 vol % residual CO is around 230 8C. Considering practical applications, the most suitable temperature range for CO methanation is 200-250 8C. Furthermore, the long-term stability of the catalyst is another important factor. Consequently, for the effective removal of CO by means of catalytic methanation, the following three requirements should be met: 1) high performance, including activity and selectivity; 2) a wide working temperature window, including the range of 200-250 8C; and 3) good stability.Nickel-and ruthenium-based catalysts have been reported to be the most effective ones for selective CO methanation by Takenaka et al. [19] They reported that Ni/ ZrO 2 and Ru/TiO 2 showed the highest catalytic activities among a series of catalysts studied for this reaction, reducing CO levels from 0...