Catalytic Reaction Mechanism And High Efficient Catalysts For Oxidative Dehydrogenation Of Light Alkane

Ethylene and propylene are two of the most important building blocks in the chemical industry.With the continuous development of modern industry,the demand of these building blocks has increased obviously for the past few years.Oxidative dehydrogenation(ODH)of light alkanes to alkenes,which is a low energy consumption,low cost,green and environmentally friendly process has reveived considerable attention.However,it still turns out to be challenging to explore effective catalysts for ODH reaction with high alkane conversion,high alkene selevtivity and high stability at telatively low temperature.Meanwhile,there are still many controversies about the ODH reaction mechanism and reaction pathway with different types of catalysts,which makes the fundamental understanding-guided structural designs of efficient ODH catalysts unlikely.This thesis targets at reaction mechanisms and efficient catalysts of ODH reactions using CeO2 and CeO2-supported(NbOx/CeO2 and Ni/CeO2)catalysts with different morphologies and h-BN catalysts.Roles of oxygen vacancies of CeO2 in CO,methane and propane combustion reactions were investigated,and the metal-support interaction,structure-property relations and reaction mechanism of NbOx/CeO2,Ni/CeO2 and h-BN catalysts in oxidative dehydrogenation of propane were comprehensively studied.Meanwhile,a new strategy for synthesizing phase-pure M1 MoVTeNbOx catalyst with excellent catalytic performance for ODHE reaction was established.The main results are listed as the following:1.CeO2 nanorods exposing different different concentration of oxygen vacancies were synthesized and used as catalysts for CO,methane and propane oxidation reactions.Roles of oxygen vacancies and active oxygen species in different combustion reactions were observed.Porous nanorod ceria(PN-CeO2)catalyst with the highest concentration of oxygen vacancies is most active in catalyzing CO oxidation reaction at low temperatures but least active in catalyzing CH4 and C3H8 combustions at high temperatures.It was found that adsorbed oxygen species at oxygen vacancy sites are stable at low temperatures and the active oxygen species in low-temperature CO oxidation reaction,whereas they become unstable at high temperatures and lattice oxygen is the active oxygen species for high-temperature CH4 and C3H8 combustion reactions.Too-high concentrations of oxygen vacancies in CeO2 can suppress reactivity of surface lattice oxygen species and improve the stabiliry of carbonate and format surface intermediates,which inhibites the carbonate-to-CO2 surface reaction and subsequently decreases the catalytic performance.2.A series of NbOx/CeO2-rod catalysts were synthesized and employed as catalysts for oxidative dehydrogenation of propane(ODHP)reaction.Synchrotron radiation photoemission spectroscopy and reso-nant photoemission spectroscopy results demonstrate that the Nb5+ precursor preferentially interact with surface oxygen vacancie on CeO2 rods.Surface redox reactions occul between Nb5+ and Ce3+(surface oxygen vacancy)to produce Nb4+ and Ce4+,resulting in decreased surface oxygen vacancy concentrations on CeO2 rods.CeO2 rods are active in catalyzing propane combustion at low reaction temperatures(below 400?)only via MvK mechanism.The ODHP reaction catalyzed by CeO2 rods and NbOx/CeO2 rod catalysts at high reaction temperature(above 450?)involves the radical mechanism with the unamniguously identified formation of gas-phase propyl radicals by the activation of propane on the catalyst surface.3.CeO2 nanocrystals exposing different morphologies and crystal planes were used as supports to synthesize a series of Ni/CeO2 catalysts for the propane combustion and ODHP reactions.The Ni-CeO2 interaction greatly promotes the reducibility of CeO2,but CeO2 morphology-dependent Ni-CeO2 interaction was observed to form different speciation of Ni species and oxygen species.Ni-CeO2 interaction is stronger in Ni/c-CeO2 catalysts than in other Ni/CeO2 catalysts.Different morphology-dependences of Ni/CeO2 catalysts in propane combustion and ODHP reactions were observed.The Ni/r-CeO2-500 catalyst with the largest strongly-activated oxygen species is most catalytic active in the propane combustion reaction while the Ni/c-CeO2 catalyst with the largest amount of weakly-activated oxygen species exhibits the best catalytic performance in the ODHP reaction.Thus,the CeO2 morphology engineering strategy is effective in finely tuning the metal-CeO2 interaction and the reactivity of oxygen species to meet the requirements of different types of catalytic oxidation reactions.4.The ODHP reaction catalyzed by both h-BN and suppored boron oxide catalysts were successfully demonstrated to involve the gas-phase radical mechanisms and pathways with unambiguously identified gas-phase methyl radicals(CH3·)by using the SVUV-PIMS.By coupling the results from kinetic and SVUV-PIMS studies with DFT calculations,detailed reaction pathways are proposed for the various products from ODHP over boron-based catalysts.Propene is mainly formed from surface reaction via the cleavage of C-H bonds of propane.Both surface-mediated and gas-phase reactions pathways can contribute to the C1 and C2 products.Our findings provide new insights towards understanding the ODHP reaction mechanisms and pathways over boron-based catalysts and are of significance for developing highly selective catalysts for alkane ODH.5.A new synthesis strategy was developed for phase-pure M1 MoVTeNbOx catalysts with high specific surface are and pore volume.The prepared catalysts exhibit excellent catalytic performance for the ODHE reaction.Under the high ethane contact time reaction conditions,73.7%ethane convertion,85%ethylene and 1.92 kgc2H4/kgcat/h C2H4 producti'vity were obtained;meanwhile,this M1 catalyst showed good stability under different reaction conditions.The excellent catalytic performance was found not only due to the high specific surface area but also to the preferential exposures of catalytically active sites.