Construction And Performances Of New Nickel Foam Supported Ruthenium Catalysts For CO Selective Methanation In Hydrogen-rich Gas

CO selective methanation processes has an attractive prospect in fuel cell applications for deep-removing low concentration level CO in hydrogen-rich feed gases, due to its kinds of merits such as simple, non-toxic side effects of the product and without additional reactants.In this work, theoretical analysis of CO selective methanation reaction system in hydrogen-rich mixture gas was done. The results show that both CO methanation reaction and CO₂ methanation reaction can be occurred in thermodynamics at 100-320oC, and CO methanation is preferentially carried out. High temperatures are in favor of reversed water gas shift reactions. In additional, due to the differences in molecular structure, electronic states and the activation process of adsorption of CO and CO₂, metal Ru has been screening to be the the best CO selective adsorption properties on low-temperature conditions.A design for constructing in-situ a load-layer on NF surface by using evaporation-induced self-assembly was proposed. Therefore, new type of Ru/Ni-Al-oxide/NF and RuNi-Al₂O₃/NF composite monolithic catalysts were designed. In this new catalyst, Ni-Al-oxide oxide composite as transition layers are used to disperse active species, while RuNi-Al₂O₃ catalytic layers are anchored in-situ on the surface of NF to obtain high catalytic activities. Meanwhile, metal skeleton NF as a carrier matrix provide high thermal conductivity and the microreactor channels to eliminate possible local hot spots of reaction system.Ru/Ni-Al-oxide powder catalysts were used to the CO selective methanation, which were prepared by evaporation-induced self-assembly method. The catalyst materials were studied by using XRD, SEM, TEM, N₂ adsorption-desorption, H₂-TPR and other characterization techniques, and associated the catalyst material structure and its catalytic properties. The results show that Ni-Al-oxide composite with high surface area and mesoporous structure were successfully prepared by an evaporation-induced self-assembly method. Then, Ru/Ni-Al-oxide catalyst was prepared by impregnation method using Ni-Al-oxide as support, which shows a very good catalytic activity and obtained low CO outlet concentration below 10 ppm under a wide reaction temperature window at 190-240oC. Addition of Ti during preparation process of Ni-Al-oxide can effectively improve the activity and selectivity with a wider reaction temperature window at 220-280oC, but the lowest reaction temperature for CO concentration below 10 ppm has increased. This could be attributed to that Ti weaken the interaction between the active ingredient Ru and the carrier Ni-Al-oxide, restrained adsorption dissociation of CO₂ on the nickel species, lead to suppress CO₂ methanation reaction.Evaporation induced self-assembly methods were used to anchor Ni-Al-oxide to NF surface to fabricate Ni-Al-oxide/NF composite material, which has a high specific surface area and pore structure of load layer. A new Ru/Ni-Al-oxide/NF composite monolithic catalyst, which was prepared by an impregnation method using Ni-Al-oxide/NF composite as support, exhibits excellent catalytic activity for CO methanation. Ru/50Ni-Al-oxide/NF composite monolithic catalysts exhibits the best methanation performance, which decreased CO outlet concentration to less than 10 ppm in a wide temperature range（180-280 oC）, and maintain a high CO selectivity（SCO>50%）, CH4 outlet concentration of less than 2%. Under 120 h continuous methanation testing, it exhibits excellent catalytic stability and remains high catalytic activity and selectivity. NiO of Ru/Ni-Al-oxide/NF catalysts is partial reduced to metal Ni, and the synergy effect of Ni and Ru is beneficial to obtain high CO methanation activity. The larger Ni particles and smaller Ru particles are favor of high selectivity.A novel RuNi-Al₂O₃/NF monolithic catalyst was fabricated by employing a simple one step evaporation-induced self-assembly method. The results show that RuNi-Al₂O₃/NF catalyst has better low-temperature activity with increases of Ni contents, but promote CO₂ competitive methanation at the same time, so CO selectivity decreases. Within the scope of research and exploration, RuNi（20）-Al₂O₃/NF composite monolithic catalysts with 20% Ni content exhibits the best methanation performance, which decreased CO outlet concentration to less than 10 ppm in a wide temperature range（190-240oC）, and maintain a high CO selectivity（SCO>50%）, CH4 outlet concentration of less than 2%. As increases of Ru content, the activity and selectivity of RuNi（20）-Al₂O₃/NF catalyst were significantly improved. A high selectivity was obtained for all the RuNi（20）-Al₂O₃/NF catalyst when Ru content is more 3%, but the catalytic activity has a little improvement. 3% RuNi（20）-Al₂O₃/NF catalyst with a suitable content of Ru, exhibits the best methanation performance, which decreased CO outlet concentration to less than 10 ppm in a wide temperature range（185-265oC） and maintain a high CO selectivity（SCO>50%）, CH4 outlet concentration of less than 2%. For the RuNi-Al₂O₃/NF monolithic catalyst, most of the Ru particles localized on the Ni surface, and CO would be preferentially adsorbed Ru surface active component to form CH4, while dissociation H on Ni surface can overflow to the surface of Ru, and this synergy effect of Ni and Ru obtain high catalytic activity. Compared with the Ni-Al₂O₃/NF catalyst, decomposition of formic acid species as CO₂ conversion intermediate species was slow, resulting in CO₂ methantion decrease, so as to obtain a high selectivity. In addition, NF matrix with good thermal conductivity can lead away the heat resulting from methanation, effectively eliminate hot spots problems in the reaction system and prevent catalyst deactivation, while improving the selectivity.