High Catalytic Performance of Ruthenium‐Doped Mesoporous Nickel–Aluminum Oxides for Selective CO Methanation

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. Preferential oxidation of CO (PROX) has been proposed as a “deep-cleaning” process: CO is oxidized to CO2 with air supplied downstream from the WGS reactor; it has succeeded in meeting the requirements of the PEFC. 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. Moreover, the CH4 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 CO2 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 CO2 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 CO2. 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. Nickeland ruthenium-based catalysts have been reported to be the most effective ones for selective CO methanation by Takenaka et al. They reported that Ni/ ZrO2 and Ru/TiO2 showed the highest catalytic activities among a series of catalysts studied for this reaction, reducing CO levels from 0.5 vol% to 20 ppm, accompanied by low CO2 conversion in the presence of 25 vol% CO2, but in a narrow reaction temperature range. Such catalytic performance has been related to the size and shape of the metal nanoparticles and the interactions between the metals and the oxide supports. Recently, we reported superior selective CO methanation with H2 in reforming gas on Ni-Al mixed oxides modified by 1 wt% Ru (surface area 130 mg ) synthesized by a solution-spray plasma technique. The best catalyst can decrease CO levels from 1 vol% to 13 ppm at about 210 8C with a reaction selectivity of 80%. We concluded that ruthenium plays an important role, not only enhancing the formation of CO methanation active sites of nanosized nickel particles formed on the surface of NiAl2O4 by reduction with spill-over hydrogen, but also improving the selectivity by the suppression of CO2 dissociation over nickel metal sites. Herein, we demonstrate for the first time that mesoporous Ni-Al oxides with high surface areas synthesized by the sol– gel method, doped with a small amount of ruthenium through a conventional impregnation process, successfully avoid the formation of NiAl2O4 and show excellent catalytic performance for selective CO methanation. A series of mesoporous Ni-Al oxides, denoted as MA-xNi (x is defined as the nickel mole percent relative to Ni plus Al; 100 Ni/(Ni+Al)), was prepared by a sol–gel method using evaporation-induced self-assembly, following the method proposed by Morris et al. Wide-angle X-ray diffraction (XRD) patterns of the powder indicate the amorphous nature of MA-xNi (x= 0, 10, and 20) calcined at 400 8C.When x= 33, broad peaks assigned to NiO are observed, which become sharper at higher nickel fractions (Supporting Information, Figure S1). The presence of uniform, hexagonally ordered mesopores for the samples up to 20% Ni was confirmed by TEM analysis (Supporting Information, Figure S2). The [*] Dr. A. Chen, Prof. T.Miyao, Prof. K. Higashiyama, Prof. H. Yam ...