Preparation,characterization And Catalytic Performance Of Nano Ruthenium And Its Alloy Catalysts Supported On Porous G-C₃N₄-based Sheet Materials

As one of the globally recognized sustainable energy sources,hydrogen is considered to be a potential alternative energy for traditional fossil fuels,due to its cleaness,non-pollution and extremely high energy value.Nevertheless,it is urgent to solve the problems of inconvenient transportation and difficult storage.Among various hydrogen storage materials,ammonia borane(NH3BH3,AB)has attracted much attention because of its superor advantages,such as high hydrogen content(19.6 wt%),high water solubility and low molecular weight.Compared with the pyrolysis procedure,AB hydrolysis possesses the advantages of lower energy consumption and more convenient operation.However,the hydrolysis reaction cannot take place kinetically at room temperature.Impressively,the supported heterogeneous catalysts can be easily recovered and highly utilize the active components.Therefore,developing stable and efficient supported catalysts for AB hydrolysis has become one of the research hotspots in the field of hydrogen energy.Graphite carbon nitride(g-C3N4)has excellent thermal and chemical stability,good visible light absorption properties,endowing g-C3N4-based sheet materials broad application prospects in many fields,including heterogeneous catalysis.Moverover,compared with other noble metals used for catalyzing AB hydrolysis,Ruthenium(Ru)has the advantages of low cost and high catalytic activity.Therefore,it is of great significance to develop efficient supported Ru-based catalysts for AB hydrolysis.In this thesis,porous g-C3N4 is prepared via calcinating melamine by using NH4Cl as the dynamic gas templates.Then,the obtianed g-C3N4 is assembled with reduced graphene oxide(rGO)thorugh different strategys to form sheet composite materials.Subsequently,g-C3N4 and the composites are used as supports to prepare the supported Ru and alloy catalysts with an adsorption-in situ chemical reduction method by using NaBH4 as the reducing agents.Many characterization techniques are presented for elucidating the structure-performance relationships of the as-preared catalysts in AB hydrolysis.Also,the corresponding kinetic studies are carried out.The main research contents are as follows:1.A series of g-C3N4 materials with different porous structure are prepared by using different amounts of NH4Cl.Also,the corresponding supported Ru catalysts are fabricated.The results show that the catalysts supported on g-C3N4,which are prepared with a mass ratio of 1:3 for melamine to NH4Cl,display the highest catalytic activity for AB hydrolysis.This is because the optimal supports has the highest specific surface area(SBET=45.4 m2g-1)and the largest total pore volume(VP=0.254 cm3 g-1)among the as-prepared g-C3N4 materials,endowing Ru nanoparticles(NPs)high dispersion.Catalyzed by the optimal Ru/g-C3N4 catalysts,the hydrolytic reaction is suitable to the 1.17-and 0.06-orders,depending on the concentrations of the catalysts and AB,respectively.The corresponding maximum turnover frequency(TOF)(in alkaline solution,303 K)and apparent activation energy(Ea)are 498.1 min-1 and 35.6 kJ mol-1,respectively.In addition,the Ru/g-C3N4 catalysts maintain stable catalytic activity even after four runs,suggesting excellent cycling stability.2.A series of Ru1-xPdx/g-C3N4 catalysts are prepared with the above-mentioned optimal g-C3N4 as the supports.Benefitting from the synergistic effect of the RuPd alloy NPs,the obtained RuPd bimetallic catalysts display higher catalytic activity for AB hydrolysis than the corresponding monometallic ones,and the Ru0.85Pd0.15/g-C3N4 catalysts are the best.Catalyzed by the optimal Ru0.85Pd0.15/g-C3N4,the hydrolysis reaction is suitable to the 0.99-and 0.16-orders,depending on the concentrations of the catalysts and AB,respectively.The corresponding TOF(in alkaline solution,303 K)and Ea values are 948.2 min-1 and 24.2 kJ mol-1,respectively.Specifically,the Ru0.85Pd0.15/g-C3N4 catalysts demonstrate good durability,and its catalytic activity remains stable even after five runs in the cycling experiments.3.A series of RuNi alloy catalysts supported on the optimal g-C3N4 are prepared by controlling different molar ratios of Ru to Ni.The results show that Ru0.5Ni0.5/g-C3N4 delivers the highest catalytic activity.Catalyzed by Ru0.5Ni0.5/g-C3N4,the reaction orders for AB hydrolysis to the concentrations of the catalysts and AB are 1.01 and 0.16,respectively,and the corresponding TOF(in alkaline solution,303 K)and Ea values are 840.3 min-1 and 14.1 kJ mol-1,respectively.4.The above optimal g-C3N4 and rGO are assembled into sheet composites with the hydrothermal reduction,thermal reduction and chemical reduction processes,respectively.Also,the corresponding supported Ru catalysts are prepared.The results show that the catalysts supported on the composites with the rGO doping amount of 5.0 wt%and prepared with the hydrothermal reduction method(denoted as Ru/g-C3N4-rGO)demonstrate the highest activity for AB hydrolysis.This is because the support materials have the largest specific surface area(SBET=87.9 m2 g-1)and the highest total pore volume(Vp=0.436 cm3 g-1)among the obtained sheet composites,endowing Ru NPs high dispersion(particle size range:1.4-3.2 nm,average particle size 1.9 nm).Catalyzed by the optimal catalysts,the reaction orders for AB hydrolysis to the concentrations of the catalysts and AB are 0.95 and 0.12,respectively.The corresponding TOF(in alkaline solution,303 K)and Ea values are 887.2 min-1 and 20.9 kJ mol-1,respectively.Interestingly,even after five cycles,the reaction rate is still stable in the presence of Ru/g-C3N4-rGO.5.A series of Ru1-xNix/g-C3N4-rGO catalysts are prepared by using the above-mentioned optimal g-C3N4-rGO as the supports.The results show that Ru0.8Ni0.2/g-C3N4-rGO exhibits the highest catalytic activity,and the reaction orders for AB hydrolysis to the concentrations of the catalysts and AB are 0.91 and 0.26,respectively.The corresponding TOF(in alkaline solution,303 K)and Ea values are 905.0 min-1 and 24.5 kJ mol-1,respectively.After five cycles,the catalysts still maintain stable catalytic activity,indicating satisfactory cycling stability.In summary,the porous g-C3N4 and its composite sheet materials with rGO can effectively support the highly dispersed Ru NPs and its urtrafine alloys with Pd and Ni.Compared with most of the Ru-based catalysts previously reported,the as-prepared catalysts display competitive activity,due to the suitable geometry and alloy effects.Specifically,the Ru0.8Ni0.2/g-C3N4-rGO catalysts are expected to apply in the future commercial hydrolytic dehydrogenation of solid chemical hydrogen storage materials.