DFT Studies Of Acetic Acid Steam Reforming Over Ni-based Catalysts

Renewable biomass could be efficiently converted into bio-oil with higher energy density via fast pyrolysis,and the liquid yield is up to 80%.Due to the high water content,high oxygen content and moderate acidity of bio-oil,its direct utilization is limited.Thus,it is necessary to make full use of bio-oil.The methods of utilizing bio-oil mainly include bio-oil steam reforming and bio-oil upgrading by hydro-treating.Bio-oil steam reforming is a sustainable and promising process for hydrogen production,which can take full advantage of water in bio-oil aqueous fraction and provide hydrogen for hydrogenation process of oil phase.The low cost and readily available Ni-based catalysts are widely used in bio-oil steam reforming studies.However,catalyst deactivation caused by carbon deposition is the main problem in this process.As the composition of bio-oil is extremely complex,it is difficult to have a comprehensive understanding of the nature of the steam reforming process and the carbon deposition via experimental studies.While selecting the model compound is an effective way to reveal the mechanism of complicated bio-oil steam reforming,particularly for the mechanism of carbon deposition formation.In this work,acetic acid is selected as the model compound of bio-oil aqueous fraction for two aspects.One is that organic acids account for the major part of bio-oil aqueous fraction,and acetic acid is the main component of acid compounds.The other is that CH₃COOH contains various functional groups,including C-O,C-C,C-H,O-H and C=O bonds.Generally speaking,acetic acid steam reforming can be basically divided into acetic acid decomposition and water gas shift reaction?WGS?.In this paper,density functional theory method is used to investigate all of the possible decomposition pathways of acetic acid and the formation mechanism of important intermediates acetone and acetaldehyde on Ni?111?.Moreover,the mechanism of water dissociation,the effect of water on the carbon elimination and WGS reaction are further explored.In addition,due to the participation of water in acetic acid steam reforming,the effects of water derived oxygen species?O*/OH*?and the solvent effect of water on some key dehydrogenation steps are studied in detail.Thus,H2O*,CH₃COOH*,trans-COOH* and CH₃CO*,CH3C*,CH₂C* are selected to represent O-H and C-H bond scission reactions.Analogously,the solvent effect of acetic acid on O-H bond scission is also investigated.By studying the above studies,this work illustrates the mechanism of acetic acid steam reforming,the rate determining step?RDS?and the root cause of carbon formation.The main results and conclusions are as follows:1.The most favorable reaction pathway of acetic acid steam reforming on Ni?111?is suggested as CH₃COOH* ? CH₃COO* ? CH₃CO* ? CH₂CO* ? CH₂* + CO* ? CH* ? CHOH* ? CHO* ? CO*,followed by WGS reaction to produce CO2 and H2.CH* species is identified as an important carbon precursor produced by deep dissociation of acetic acid,and the enriched OH* derived from water dissociation is the active species to eliminate CH*.The RDS of CH₃COOH decomposition is the dehydrogenation of CH₃CO* into CH₂CO* with an activation energy of 1.33 eV.While the RDS of WGS reaction is CO* + OH* ? cis-COOH*,which needs to overcome an activation barrier of 1.85 eV,higher than that of acetic acid decomposition.Therefore,WGS determines the reaction rate of overall acetic acid steam reforming process.2.The formation mechanism of the important intermediate acetone is identified as the indirect formation path: CH₃CO* + CH* ? CH₃COCH* ? CH₃COCH₂* ? CH₃COCH3*,instead of the direct formation path: CH₃CO* + CH3* ? CH₃COCH3*;Another important intermediate acetaldehyde can be easily formed via CH₃CO* hydrogenation step?0.49 eV?.CH₃CHO desorbs easily due to the weak adsorption on Ni catalyst,while its readily decomposed property is the reason why acetaldehyde is difficult to observe in experimental studies.3.Water derived O* and OH* species can be involved in dehydrogenation steps to form OH* and H2O*.The results show that O* and OH* species can both facilitate the O-H bond scission,and the promotional effect of O* is superior to OH*.However,C-H bond scission reactions are inhibited with only one exception that the enriched OH* species lowers the reaction barrier of the rate determining step?CH₃CO* dehydrogenation step?in acetic acid decomposition.4.Water acts as a “solvent”,and its solvent effect has significant influence on some key dehydrogenation steps during acetic acid steam reforming.The results show that the pre-adsorbed water molecule is not truly involved in the reactions,but interacts with oxygen-containing species through hydrogen bond.On one hand,the adsorption stability of oxygen-containing species is remarkably improved.On the other hand,the solvent effect of water notably weakens O-H bonds,yet exhibits negligible effect on the C-H bond breakage.Furthermore,the solvent effect of acetic acid favors the O-H bond scission as well.