Construction Of Ruthenium-nickel Based Graphene-mixed Metal Oxide Composite Catalyst For CO Selective Methanation In Hydrogen-rich Gas

Proton exchange membrane fuel cells（PEMFCs）are a new type of fuel cell that uses platinum as the electrode and high-quality hydrogen as the fuel.PEMFCs is the motive power source of hydrogen fuel cell vehicles（FCEV）,which also is a kind of efficient and environment friendly motive power source system.However,hydrogen fuel contains trace of carbon monoxide（CO）,which will poison the Pt electrode during the energy supply process.Therefore,the hydrogen fuel needs to be further purified to deeply remove trace of CO.CO selective methanation has become an effective method for deep removal of trace of CO in hydrogen-rich gas due to its convenient,green,few side effects,and no need to add reactants,etc.Currently the key to this approach is to develop a CO selective methanation catalyst with high reaction activity,selectivity and stability.The growth of nickel-aluminum hydrotalcite（Ni Al Layered double hydroxides,LDHs）on the surface of graphene oxide（GO）by in-situ growth method was studied.A series of ruthenium-nickel/graphene-mixed metal oxide catalyst（Ru-Ni/r GO-MMO catalyst）with hydrotalcite（LDHs）as precursors were prepared.Also explored a way for fabricating 3D GO hydrogels and aerogels via the self-assembly cross-linked of 2D GO sheets with layered double hydroxides.A series of ruthenium-nickel/graphene aerogels-mixed metal oxide catalyst（Ru-Ni/GA-MMO）was prepared.The selective methanation of CO in hydrogen-rich gas was used as a probe reaction to investigate their catalytic performance.The specific surface area structure,structure of catalyst,etc.,were characterized by XRD,BET,TG-DSC,Raman,FT-IR,N₂adsorption-desorption,SEM,TEM,H2-TPR,CO-TPD,XPS and other characterization methods.The following conclusions were obtained:（1）Ru-Ni/r GO-MMO catalyst was successfully synthesized by coprecipitation method.The effects of the molar ratio of hydrotalcite,calcination of support and Ru loading on the structure and catalytic performance of the catalysts were investigated.The experimental results show that:firstly,increasing the Ni to Al ratio of LDHs can increase the number of Ni active sites,but too high Ni to Al molar ratio will lead to the decrease of crystallinity of hydrotalcite,and the migration and aggregation of Ni active components and the growth of particles are easy to occur;Secondly,because Ni（OH）₂is easier to be reduced to Ni component than Ni O,the catalyst supported on GO/LDHs composite has more Ni active sites than the catalyst supported on r GO-MMO;Thirdly,the experimental results show that there is a synergistic effect between the Supported Ru component and the Ni component in the support:Ru adsorbs and dissociates H₂to produce atomic hydrogen,and the partially activated hydrogen will migrate to the nearby Ni O,thus promoting the reduction of the active component of the catalyst.Among a series of Ru-Ni/r GO-MMO catalysts prepared,the catalyst prepared with 4 mole ratio of hydrotalcite and 1.2 wt.%Ru directly supported has the best catalytic performance for CO selective methanation,which can remove CO from hydrogen rich gas（<10 ppm）in a wide reaction temperature range（190-290℃）and maintain more than 50%selectivity for CO methanation.（2）graphene composite metal oxide（GA-MMO）aerogel catalyst carrier materials were successfully synthesized by layer by layer self-assembly method.The effects of the delamination of hydrotalcite and the mass ratio of hydrotalcite and graphene oxide on the structure of aerogel catalyst carrier were systematically investigated.The experimental results show that when the mass ratio of peeled layered double hydroxides and graphene oxide is within 1-4,a GA-MMO aerogel catalyst carrier with high specific surface area can be obtained after roasting under nitrogen protection.（3）using GA-MMO composite aerogel as catalyst carrier,the Ru-Ni/GA-MMO catalyst was prepared by impregnation reduction process.The influence of GA-MMO aerogel carrier on the structure and catalytic performance of the catalyst was investigated.Meanwhile,the mass ratio of hydrotalcite to graphene oxide was systematically investigated.The effects of Ni/Al molar ratio of hydrotalcite precursor and Ru loading on the catalytic performance of the catalysts were investigated.The experimental results show that the three-dimensional structure support with good thermal stability and high specific surface area is formed by the cross-linking of layered hydrotalcite and graphene oxide,which can significantly improve the thermal stability and specific surface area of the catalyst,expose more Ni O sites,and form more Ru-Ni O structure,At the same time,due to the excellent electron conduction ability of graphene,it is conducive to the electron transfer from Ru⁰to Ni⁰,and the electron cloud density on the surface of Ni⁰increases,which is conducive to the dissociation of CO on Ni and enhances the adsorption capacity of the catalyst for Carbon monoxide.In addition,the introduction of graphene can effectively reduce the intermediate and strong basic sites on the surface of the catalyst,thus reducing the adsorption and dissociation of CO₂,and improving the selectivity of CO methanation.Among a series of Ru-Ni/GA-MMO catalysts prepared,the best activity was obtained when the mass ratio of hydrotalcite to graphene oxide was 3,the molar ratio of nickel to aluminum was 4,and the Ru loading was 0.8 wt.%.The catalyst could remove CO from hydrogen rich gas at 220-280℃and maintain more than 50%selectivity for CO methanation,Moreover,the catalyst can work continuously at 240℃for 120 h and still maintain excellent activity and selectivity.