Hydrogen Production Via Cyclohexane Dehydrogenation Over Non-precious Metal Ni,Cu And Their Bimetal Catalyst

As a kind of liquid organic hydride,cyclohexane holds high hydrogen content about 7.1 wt.%,easy storage and transportation,recyclable,CO-free and no greenhouse-gas CO2 emission during the whole process.Moreover,the hydrogenation process from the dehydrogenation product benzene is mature.Thus,cyclohexane is an efficient hydrogen and transport medium on a large scale.The development of catalysts with high performance for cyclohexane dehydrogenation is vital to realize the reversible catalytic cyclohexane hydrogenation-dehydrogenation reactions.In recent years,Pt-based catalysts have been studied for cycloalkane dehydrogenation reaction.However,as the reverse reaction of hydrogenation,dehydrogenation could be suitable for a non-noble metallic catalyst with medium hydrogenation activity,rather than Pt-based catalysts which have good hydrogenation performance.Therefore,the non-precious metal catalysts could be more benefit to dehydrogenation.In addition,it is important to decrease the cost of hydrogen storage for the development of non-precious metal catalysts.Thus,this paper focused on non-precious metal catalysts for cyclohexane dehydrogenation,the synergistic effect between Ni and Cu due to their better catalytic performance,and the structure-activity relationships of Ni-Cu and dehydrogenation reaction.The main research contents of this article and the innovation are mainly manifested in the following aspects:(1)The catalytic performances of non-precious monometallic and bimetallic catalysts for cyclohexane dehydrogenation were investigated.The monometallic Ni catalyst exhibits excellent catalytic activity but low selectivity.Compared to single metallic Ni catalyst,the dehydrogenation selectivity of bimetallic Ni-based catalysts are drastically improved.And their catalytic performances are in the order of Ni0.8Cu0.2>Ni0.8Mn0.2>Ni0.8Co0.2>Ni0.8Zn0.2 ? Ni0.8Fe0.2.(2)The NiCu/SiO2 catalyst parepared by sol-gel has been optimized and studied with characterizations.The result reveals that the synergistic effect existed obviously between Ni and Cu.Ni stables Cu,while Cu improves the reduction properties of Ni.And the Ni-Cu nanoparticles have been formed with a uniform composition and narrow size distribution(2-6 nm),and exhibited high dispersion.This results in the reduction of the non-selectivity sites made up with monometallc Ni.The Ni0.85Cu0.15/SiO2 catalyst presents the highest yield of benzene with 94.9%cyclohexane conversion and 99.5%selectivity at 350 ?,and is closed to that of Pt-based catalysts.(3)Due to the high selectivity(close to 100%)of cyclohexane dehydrogenation and the improvement on dehydrogenation selectivity for Ni-based catalyst,the effects of Cu metal particle size on catalytic activities of Cu/SBA-15 with different Cu content were investigated.Overall,the smaller Cu nanoparticles exhibit higher hydrogen evolution rate or lower active energy barrier.But,the smaller CuO nanoparticles have formed on the catalyst with lower Cu content during the calcination,and they would be more prone to sinter after reduction.An appropriate Cu content could lead to form amounts of stable and small Cu nanoparticles after high-temperature treatment with the space limitation by ordered channels of SBA-15.(4)The catalytic reaction mechanism and inactivation reasons for Ni-Cu/SBA-15 catalyst were studied.The catalysts prepared with the same metal loading by impregnation,shows very close pore properties,and keeps the pore structure of SBA-15 well after calcination.The addition of Cu into Ni changes the crystal structure of metal Ni,and forms Ni-Cu alloy.This could not only suppress the non-selectivity sites which breaks the C-C bond and lead to produce methane,but also prevent the coke formation.The in-situ DRIFT studies have shown the vinyl species,indicating the existence of alkenes in the reactive intermediates.Additionally,the strong absorption of benzene on the metal(especially on Ni)could result in that benzene was dehydrogenated further to carbon.(5)The interations for Ni and Cu,and the internal relations between NiCu and cyclohexane dehydrogenation were investigated by Dmol3 through density functional theory(DFT)computations.The first C-H bond cleavage of cyclohexane and the formation of C6H11· adsorption on metal surface at same time has the maximum reaction energy barrier,and is the rate determining step.And,the synergistic effect between Ni and Cu was verified.At Ni-Cu bimetals,Ni gave electrons to Cu,and enlarged Cu 3d span at the whole energy interval.Thus,the Cu addition to Ni,not only decreased the adsorption strength of cyclohexane,but also reduced minimum distance of the H atom of cyclohexane and metal surface.These leaded to that the C-H bond was broken more easily.Moreover,cyclohexane is adsorped more stable above Cu atoms.This revealed that cyclohexane trended to adsorp on the sites of Cu or Ni-Cu rather than that of Ni,as a result of the suppression to hydro-cracking and carbon deposition catalized by only Ni.