Sorption Enhanced Methane Steam Reforming Using Ni/CaO And In-Situ Conversion Of CO₂ By DFT Study

Sorption enhanced steam methane reforming（SESMR）using CaO-based materials provides a promising method to produce high purity hydrogen by in-situ CO2 capture.Ni-doped CaO（Ni/CaO）can effectively improve the hydrogen production in the SESMR process.However,Studies on the most possible SESMR reaction path using Ni/CaO are lacking.Integrated CO2 capture and in situ utilization technology refers to the use of CH4/H2 to convert captured CO2 into value-added industrial raw materials,which reduces the cost of CO2 compression and transportation in traditional CCUS,enables CaO regeneration at lower temperatures,and reduces energy consumption while effectively reducing the sintering of CaObased materials.At present,there are no reports on the mechanism of in-situ conversion of captured CO2 by CaO-based materials in presence of CH4/H2.To solve the above problems,the possible reaction paths of SESMR reaction using Ni/CaO and the strengthening mechanism of Ni on SESMR reaction were investigated by DFT study.In addition,the reaction paths of insitu conversion of captured CO2 in the presence of CH4/H2 during calcination stage of calcium looping process were studied by DFT study.The strengthening mechanisms of CH4/H2 on the CaO-based materials regeneration were revealed on DFT study.The possible reaction paths in SESMR reactionon Ni-CaO surface were studied at atomic level.The adsorption energies and bond structures of reactants,intermediates and products were calculated.The most possible reaction path on Ni-CaO was determined by analyzing the energy barriers on different reaction paths in SESMR reaction.H2O dissociates into hydroxyl and atomic H spontaneously.The OH-assisted dissociation assists in the breaking of first C-H bond in CH4*.Process CH3*→CH2*→CH*is accomplished by direct dissociation in two steps.Next,CHO*is spontaneously formed from CH*and O*.CHO*is oxidized to generate HCOO*.Finally,the CO2*is formed by the HCOO*dehydrogenation path.Afterwards,H2*molecule generates from two atomic H*.The rate-limiting step of SESMR reaction is CH*dehydrogenation（3.215 eV）on CaO surface.The presence of Ni transforms the rate-limiting step into CH3*dehydrogenation and the energy barrier decreases to 2.030 eV.Besides,Ni reduces the adsorption energy of CH4*（-0.106 eV）and H2O*（-1.32 eV）.Therefore,Ni alters the reaction path and promotes the reaction of SESMR on Ni-CaO surface.The most probable reaction path of direct reaction of CH4 with CaCO3 was determined.The influence of C and CH intermediates on the decomposition characteristics of Ni-CaCO3 were analyzed by combining charge distributions and reaction energy barriers.In the calcination stage of calcium cycle,the most possible reaction path for in-situ CO2 conversion in the presence of CH4 is the decomposition path of CaCO3 influenced by CH intermediates.The rate limiting step is the dehydrogenation process of CH4（4.860 eV）.CH4 reduces the energy barrier of CaCO3→CaO reaction from 3.47 eV to 2.733 eV.Electron cloud perturbations around C and O atoms confirm that the presence of CH4 weakens the interaction between C and O atoms in the carbonates,making CaCO3 easier to decompose.The reaction mechanism for in-situ conversion of captured CO2 by CaO in presence of H2 during calcium looping process was investigated.The CaCO3 direct decomposition was compared to clarify the effect of H2 on CaCO3 decomposition.The electron differential densities（EDD）and partial density of states（PDOS）for HCO3*and CO3*models were also compared.The results show the detailed reaction pathway for in-situ conversion of the capture CO2 by CaO in presence of H2 in the calcination stage of CaCO3.H2 is adsorbed on the CaCO3 surface to form OH*,CO2*and HCO3*.CO2*further is decomposed into CO and O*.HCO3*is decomposed into CO2*and OH*.Then,two OH*are attracted to each other to form H2O and O*.The overlap of C-O orbitals and the disturbance of electron clouds around O atom all confirm that promoting mechanism of CaCO3 decomposition by the presence of H₂.