Study On The Coke Properties During The Hydrogen Production From Model Compounds Of Bio-oil By Steam Reforming

Hydrogen has many advantages due to its high combustion efficiency,pollution-free and light weight,which makes it an ideal energy medium and can replace fossil fuels such as petroleum and coal.Biomass is one of the most abundant renewable resources that can be used to produce hydrogen through thermochemical or biological processes.The bio-oil steam reforming technology is widely used to produce hydrogen energy because of its advantages such as lower transportation cost.In addition,bio-oil is a mixture of hundreds of components,and model compounds are usually used to study the catalytic activity of reforming catalysts.Noble metal-based catalysts limit their further application due to their high price.Transition metal-based catalysts are widely used because of their low price and good catalytic activity.However,the generation of carbon deposits is an important cause of catalyst deactivation.Therefore,research on the composition and properties of carbon deposits is very important for preparing catalysts with high catalytic efficiency.The research content and innovative results of this article are as follows:1.Steam reforming of acetic acid over the Ni/γ-Al₂O₃catalysts with varied nickel contents（from 2.5 to 40 wt%）was performed,aiming to investigate the effects of nickel loading on properties and catalytic behaviors of the catalysts.The results show that the interaction of nickel species with alumina varies with varied nickel loading.At lower nickel loadings,the nickel strongly interacting with alumina are the main species,while with the saturation of the reactive sites of alumina via loading more nickel,the proportion of the nickel species weakly interacting with alumina increased remarkably.Excess nickel on alumina can fill the pores,decreasing the reduction degree of nickel oxides,the specific area(from ca.277to 132 m²g^-1with the increase of nickel loading to 40 wt%),the pore volume,and the pore diameters.Nickel loading also significantly affects the catalytic activity,stability of the catalyst and the resistivity towards coking.With the increase of nickel loading to 10 wt%,the activity of the catalyst reached a plateau.The further increase of nickel loading did not further increase the activity,but increased the stability and the tolerance towards the deactivation induced by coking.In addition,with higher nickel loadings,the coke formed has more large aromatic ring system.2.Attapulgite clay（Atta）,a natural mineral,can be used as support of various catalysts.Nevertheless,the limited surface affects its applications.Therefore,the activation of Atta with KOH to enhance surface area was developed and supported nickel catalysts were evaluated in steam reforming of acetic acid or guaiacol.The results showed that the activation by KOH created abundant micropores in Atta via the solid phase reaction between KOH and SiO₂and Al₂O₃.The activation also enhanced reduction degree of nickel oxide,interaction between nickel and Atta,dispersion of nickel species and alkalinity of the catalyst,rendering the superior catalytic activity in steam reforming of either acetic acid or guaiacol.The in-situ Diffused Reflectance Infrared Fourier Transform Spectroscopy characterization of the steam reforming reaction indicated that the activation facilitated absorption of*OH and*CO₂species on catalyst for gasifying the coke precursors such as the ones containing C=O,enhancing resistivity of the catalyst towards coking.In addition,owing to the different molecular structures,steam reforming of acetic acid and guaiacol over the catalyst encountered the different steric hindrance,different tendencies towards coking and different properties of coke.Specially,the catalyst showed varied sensitivity towards coking in steam reforming reaction.3.The influence of barium addition to a Ni/Al₂O₃catalyst on the reaction intermediates formed,the activity,resistance of the catalyst to coking,and properties of the coke formed after acetic acid steam reforming were investigated.The results showed the drastic effects of barium addition on the physicochemical properties and performances of the catalyst.The solid-phase reaction between alumina and Ba O formed Ba Al₂O₄,which re-constructed the alumina structure,resulting in a decrease in the specific surface area and an increase in the resistance of metallic Ni to sintering.The addition of barium was also beneficial for enhancing the catalytic activity,resulting from the changed catalytic reaction network.The in-situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）study of the acetic acid steam reforming indicated that barium could effectively suppress the accumulation of the reaction intermediates of carbonyl,formate,and C=C functional groups on the catalyst surface,attributed to its relatively high ability to cause the gasification of these species.In addition,coking was considerably more significant over the Ba-Ni/Al₂O₃catalyst.Moreover,the Ba-Ni/Al₂O₃catalyst was more stable than the Ni/Al₂O₃catalyst,owing to the distinct forms of coke formed（carbon nanotube form over the Ba-Ni/Al₂O₃catalyst,and the amorphous form over the Ni/Al₂O₃catalyst）.4.Alkalinity is an important parameter affecting adsorption and evolution of reaction substrates in steam reforming reactions.Steam reforming of guaiacol,a model compounds in the volatiles in gasification of biomass,over Ni/SiO₂modified with Na,Mg or La was investigated.The results showed that the strong alkalinity induced by Na or Mg addition to Ni/SiO₂led to remarkably decreased catalytic activity.The high alkalinity also promoted formation and accumulation of the oxygen-containing species such as CO*,C-O-C aliphatic structure and carbonyl functionalities,which were precursors of the polymeric products with bigπ-conjugated structures and coke.La addition increased the alkalinity to a mild extent and promoted the nickel dispersion,enhancing the catalytic activity and the resistivity towards coking.The formation of the thermally stable amorphous coke in the Na or Mg modified catalyst wrapped nickel sites and led to rapid catalyst deactivation.Both amorphous form and in carbon nanotube form of coke defective structures were formed over La-Ni/SiO₂and Ni/SiO₂catalysts,resulting from the distinct reaction intermediates formed during the steam reforming of guaiacol.