Modulation Of Ni Based Catalyst With Theoretical Studies For Hydrocarbon Steam Reforming

Hydrogen is an environmental friendly energy resource with a variety of feedstocks and applications,which will be a crucial part of the energy structure in the future.Hydrogen is industrially produced mainly via steam reforming of alkanes and catalyst is pivotal to the process.Hence,improving the performance of catalyst is essential for steam reforming.This dissertation focuses on modulation of Ni based catalyst for hydrocarbon steam reforming and studies on reaction mechanism,with the aim to obtain Ni-based steam reforming cata-lyst with both high activity and stability.Planar rhombohedral（Pl）and pyramidal Ni₄ clusters supported on CeO₂（111）and CeO₂（110）catalytic systems were designed,and the influences of cluster structure and CeO₂crystal facets on metal-support interaction（MSI）,as well as elementary reaction of methane steam reforming（SMR）were studied by density functional theory（DFT）.Compared with Ni（111）,Pl-Ni₄/CeO₂（111）exhibited strengthened MSI,with all Ni atoms existing as oxi-dized state,which enhanced the activity of SMR,especially for C-O bond formation.The energy barriers of C-O bond formation on Pl-Ni₄/CeO₂（111）reduced to 0.46 e V from 1.24e V on Ni（111）.The use of CeO₂（110）as support induced stronger MSI and the higher ac-tivity of SMR than that CeO₂（111）,but the activity of carbon deposition increased.Furthermore,a series of Ni-NiO/CeO₂ model catalysts with various ratios of NiO/（Ni+NiO）were established.Binding energy of the Ni cluster and the energy barrier of the rate-limiting step were compared with Pl-Ni₄/CeO₂（111）.The results showed that the model catalysts with the low ratio of NiO/（Ni+NiO）had the significantly high stability and comparable activity of SMR.Among them,Ni₄O₁/CeO₂ possessed both the best stability and the highest activity.The binding energy of Ni₄O₁/CeO₂was 1 e V lower than that of Pl-Ni₄/CeO₂（111）.On Pl-Ni₄/CeO₂ and Ni-NiO/CeO₂ model catalysts,the dissociation of H₂O,H₂ and oxygen vacancies formation were studied.Oxygen vacancies can promote the adsorption and dissociation of H₂O.Adsorbed H₂O is more easily dissociated on the interface of Ni-NiO/CeO₂.The active oxygen generated at the interface can oxidize the clusters,and the clusters also can be reduced by the interface oxygen vacancies as accepting the oxygen at-oms of the clusters through oxygen spill over.Based on this,the mechanism that H₂O,ox-ygen vacancies and the reforming reaction atmosphere synergistically stabilize the compo-sition of the Ni-NiO clusters is proposed.The adsorption,successive dehydrogenations and C-C cleavage of n-butane on Ni（111）were studied by DFT,with a focus on the comparison of two possible pathways of deep dehydrogenations and C-C cleavage and analysis of reaction mechanism and kinetics.The results indicate that Ni is favorable to the deep dehydrogenation.The energy barrier of the first dehydrogenation step is higher than those of deep dehydrogenations,but lower than those of C-C cleavage.The C-C cleavage serves as the rate-determining step.The 1-butyne pathway(deep dehydrogenation of n-butane to form 1-butyne and then（C-C）_α,βcleavage)was found to be more favorable than that of 2-butyne(deep dehydrogenation of n-butane to form 2-butyne and then（C-C）_β,β’)in thermodynamics and kinetics.To enhance C-C bond cleavage as the rate limiting step of n-butane and higher alkanes,a series of doped bimetal M-Ni（111）catalytic surfaces（M=W,Cr,Mo,Co）were designed and the C-C bond breaking on these surfaces were studied.It was found that the W-Ni（111）to be the highest activity for C-C bond breaking.Furthermore,W-Ni-CeO₂ catalysts pre-pared by sol-gel method and the effects of the W contents and calcination temperature on the steam reforming of n-dodecane were investigated.The results showed that the catalyst with 10%W calcined at 600°C has the best activity and stability in n-dodecane steam re-forming due to strong MSI interaction and high oxygen vacancy.