Steam Reforming Of Toluene Over Ni-Fe Bimetallic Supported Ordered Mesoporous Alumina Catalysts

The biomass gasification process is usually accompanied by the generation of tar,and the existence of tar severely affects the efficiency of biomass gasification and the downstream utilization of gas products.Therefore,it is necessary to remove tar to meet the application demands.Among the various tar removal technologies,steam reforming can transform tar into CO and H_2,thereby improving the overall gasification efficiency.Suitable catalysts are key to the steam reforming process.Conventional Ni-based catalysts suffere from the problems of coke deposition and metal sintering,which jeopardize the activity and stability of catalysts rigorously.To solve these problems,this study proposed the preparation of ordered mesoporous alumina（OMA）supported Ni-Fe bimetallic catalysts（Ni-Fe/OMA）,wherein Ni is the main active sites,Fe doping can enhance the adsorption and activation of steam,and the OMA supports can suppress the sintering of Ni particles owing to their channel restriction effects.With toluene as the model compound of tar,we investigated the effects of Fe doping amount on the activity of toluene reforming.Combined with a series of structural characterization and Density Functional Theory（DFT）calculation results,the mechanism of high activity and high stability of Ni-Fe/OMA catalysts during toluene steam reforming was elucidated.The problems of coking and sintering in the steam reforming of toluene have seriously affected the catalytic activity and stability of traditional Ni based catalysts.This study aims to solve the carbon-resistant problem of Ni based catalyst,improve the life of the catalyst,and make it maintain a high toluene conversion for a long time.（1）Preparation and catalytic activity evaluation for Ni-Fe/OMA catalysts.One-pot evaporation induced self-assembly（One-pot EISA）method was adopted for the preparation of OMA series catalysts.Small-angle XRD,TEM and BET characterization proved that the as-prepared catalysts had the regular ordered mesoporous channels.Fe addition led to the formation of Ni-Fe alloy,which further reduced the metal particle size and improved the metal dispersion.By changing the molar ratio of Fe/Ni,the type of Ni-Fe alloy also changed correspondingly.With the increase of Fe/Ni molar ratio from 0 to 2,the catalytic activity of OMA series catalysts increased at first and then decreased.When the Fe/Ni molar ratio was 1,Fe₁Ni/OMA catalyst had the highest toluene conversion activity.The H₂O-TPD and Toluene-TPD experiments were used to verify the adsorption and activation effects of catalysts on the reactants.The experimental results show that the Ni-Fe bimetallic catalyst has stronger adsorption and activation effects on water than the monometallic Ni catalyst,while the difference of adsorption effect on toluene was insignificant between the two catalysts.The theoretical calculation results are consistent with the experimental results.（2）Stability tests of various catalysts.With a Ni/γ-Al₂O₃ catalyst prepared by the incipient-wetness impregnation method as the comparison,lifetime tests of 50 h were carried out for the Ni/OMA and Fe₁Ni/OMA catalysts.The stability of the catalysts followed the order of Fe₁Ni/OMA>Ni/OMA>Ni/γ-Al₂O₃.The results of TG and TPO-MS showed that the Ni/γ-Al₂O₃ catalyst had the highest content of coke with the highest graphitization degree However,the lowest amount of coke with the lowest graphitization degree appeared on the Fe₁Ni/OMA catalyst.TEM characterization of the spent Ni/γ-Al₂O₃ and Ni/OMA catalysts showed the presence of filamentous coke with high graphitization degree,and coke deposition covered some parts of the active sites.However,the presence of filamentous coke on the Fe₁Ni/OMA catalyst was not observed,which indicates that the coke deposition mainly existed in the form of amorphous coke.（3）Influence of Fe doping on the mechanism of toluene steam reforming reaction.The evolution of functional groups during toluene cracking and steam reforming was observed by（DRIFTS）experiments.In the cracking experiment,toluene did not react below 300 ^oC.When the temperature is greater than or equal to 300 ^oC,toluene was cracked into alkene species.In the steam reforming experiment,toluene can react with water over the Fe₁Ni/OMA catalyst at300 ^oC,whereas at the same temperature,insignificant reaction took place on the Ni/OMA catalyst.Furthermore,carboxyl group was found to be the important oxygen-containing intermediate species,indicating that toluene steam reforming is composed of two major steps,including toluene cracking and water-gas shift reaction.Combined with the theoretical calculation results in our previous study,the reaction path of toluene steam reforming over Ni-Fe/OMA catalysts was proposed.In summary,the addition of Fe can improve the dispersion of the catalyst and enhance the adsorption and activation of steam.