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Energies 2019, 12, 469 2 of 15 100 ppm for PtRu-anodes [4,5]. Hence, the CO content in the operating gas has to be further reduced below these threshold values, preventing a degradation of the fuel cell catalyst. CO preferential oxidation (CO-PROX) has so far been a reliable technique as a fine purification step of the feed gas downstream the WGS unit [2,6–10]. In order to achieve the required low CO concentration permanently, a two-step system is usually needed [4]. In addition, this process requires an additional, closely controlled, low rate O2 supply to keep the unwanted oxidation of hydrogen as low as possible [2,10]. Selective CO methanation is considered an attractive alternative to preferential CO oxidation for the removal of CO from H2 rich gases for PEMFC applications [11,12]. Here, no additional reactant is needed such as air for CO-PROX, but the risk of undesired CO2 methanation, which also consumes additional H2, is likely to occur. In the course of selective CO methanation of a reformate gas, three main reactions can take place, CO methanation, Equation (1), CO2 methanation, Equation (2), and the reverse-water-gas-shift (RWGS) reaction, Equation (3). CO + 3H2 ↔ CH4 + H2O CO2 + 4H2 ↔ CH4 + 2H2O CO2 + H2 ↔ CO + H2O ∆H0298 = −206 kJ·mol−1 (1) ∆H0298 = −165 kJ·mol−1 (2) ∆H0298 = 41 kJ·mol−1 (3) Both CO2 methanation and reverse water gas shift (RWGS) are unwanted and have to be suppressed, as valuable H2 is consumed and CO is even produced, respectively. The development of a catalyst with high activity and selectivity for CO methanation at low temperatures, optimally at temperatures of the off-gas of the WGS unit of around 200 ◦C, is therefore important. Ruthenium catalysts show these characteristics for selective CO methanation [13–18]. Based on own screening experiments with a variety of Co and Ru catalysts with regard to activity and selectivity for selective CO methanation [19], a 2 wt% Ru/γ-Al2O3 catalyst was selected as most suitable for this application. This Ru catalyst was prepared and applied for the investigation of the CO methanation reaction under varying reaction conditions, starting from gas mixtures containing only CO and H2 as well as CO2 and H2, respectively, to gas mixtures including H2O and both CO and CO2, representing a more realistic reformate gas comparable to practical applications [20]. In addition to 0.5–1 vol% CO, gas streams leaving the RWGS unit typically contain 10–25 vol% CO2 and 5–20% H2O (rest H2) [12,21,22]. Kinetic studies were conducted to investigate the influence of temperature and concentration of each gas component on the reaction rate. Based on the respective results, a kinetic model was developed to predict the conversions of CO and CO2. Finally, an adiabatic fixed reactor was simulated to determine the amount of a methanation catalyst needed for a typical household fuel cell system. 2. Experimental 2.1. Catalyst Preparation and Characterization The 2 wt% Ru/γ-Al2O3 catalyst was prepared by incipient wetness impregnation (Table 1). Al2O3 spheres with a diameter of 2.5 mm (Sasol Germany GmbH) were impregnated with the respective amount of Ru(NO)(NO3)3 solution in dilute nitric acid. The catalyst was dried under vacuum in a rotary evaporator (60 ◦C, 8 h). After reduction with H2 (10% in N2, 350 ◦C, 4 h), the catalyst was utilized in the methanation of CO or CO2. The catalyst was characterized by energy dispersive X-ray spectroscopy (EDX), indicating that a core shell catalyst with a thickness of the Ru containing shell of 0.35 mm was obtained. The shell represents 62% of the particle volume, and the Ru content in this shell is therefore about 3 wt% compared to the average content of 2%. H2 pulse chemisorption experiments were performed after reduction in H2 (350 ◦C, 30 min). The Ru particle size of 6 nm was calculatedPDF Image | Selective Methanation of CO over a Ru-y-AI2O3 Catalyst in CO2 H2
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