Study On The Stability Of Ni/Biochar Catalysts For Catalytic Steam Reforming Of Toluene

Biomass gasification is a technology for converting solid biomass to gaseous products.However,the existence of tar in gas production is one of the main challenges in the industrial operation of biomass gasification technology.The real tar is a complex mixture,mainly including single to multiple ring aromatic compounds,along with oxygenates.Toluene represents the stable aromatic structure and apparent in tar formed with high-temperature processes.It is considered as a model compound for biomass gasification tar in this paper.In recent years,biochar supported nickel(Ni/BC)catalysts have been widely used in the study of tar steam reforming due to their advantages of low price,easy preparation,good catalytic activity and direct combustion or gasification after deactivation.However,compared with the carbon deposition and nickel particle sintering of traditional nickel-based catalysts,the gasification consumption of the biochar supports in the steam is a major problem for catalysts supported by biochar.Among them,the factors affecting the reactivity of biochar gasification mainly include the alkali and alkaline earth metals(AAEMs)and char structure inherent in the biochar supports.The presence of AAEMs(particularly K)may enhance the steam reforming activity of catalysts;on the other hand,it may accelerate the gasification reaction between steam and the biochar support.In addition,the boron can improve the stability of carbon material and the coking resistance of Ni-based catalysts.Therefore,this paper studies the effects of the inherent K on the catalytic performance of Ni/BC catalysts in the steam reforming of toluene and the influences of B on the stability of boron-doping Ni/BC(Ni-B/BC)catalysts in the steam reforming of toluene,and established a method to improve the stability of Ni/BC catalyst in the tar steam reforming reaction.In this work,firstly,three Ni/BC catalysts with different amount of K were prepared from raw,water-washed and acid-washed biomass,and used in toluene steam reforming in the temperature range 450–600°C to elucidate the effects of inherent K on the catalytic activity and stability.Secondly,Ni/BC-RAW catalysts and Ni-B/BC-RAW catalysts with different B doping amount were prepared by the incipient wetness impregnation method with raw biomass.The effect of B doping on the catalytic performance of Ni/BC-RAW catalysts was investigated by comparing the toluene steam reforming activity of catalysts and stability.Finally,Ni/BC-AW catalysts and Ni-B/BC-AW catalysts with different B doping amount were prepared by the incipient wetness impregnation method with acid-washed biomass.The effect of B doping on the catalytic performance of Ni/BC-AW catalysts was investigated by comparing carbon resistance ability and the toluene steam reforming activity of catalysts.The main results and conclusions are as follows:(1)Detailed characterization indicated that K enhanced water adsorption due to its hydroscopicity and lowered the condensation and graphitization degrees of biochar,but the alteration to the electronic state of Ni was not observed.These effects led K to have different effects on the catalyst at different temperatures.(2)At relatively low temperature of 450 and 500°C,toluene conversion rate was significantly increased in the presence of K,due to the strengthened water adsorption.When increasing the reaction temperature to 550 and 600°C,100%initial conversion of toluene was observed for Ni/BC catalysts with K.However,they experienced fast deactivation,since the more disordered biochar support was consumed more rapidly by the high amount of adsorbed water.By contrast,Ni/BC without K performed more stably at 550 and 600°C.(3)An appropriate amount of B doping can improve the condensation and graphitization degrees of biochar in Ni/BC-RAW catalyst,and effectively inhibit the catalytic activity of K in the biochar,which is conducive to improving the stability of Ni/BC-RAW catalyst.Ni-B/BC-RAW pyrolyzed at 600 ℃ with a Ni loading of 5%and B doping of 3.5%(Ni-3.5B/BC-RAW)catalyst displayed lower gasification reaction and higher graphitization degrees than other catalysts.Therefore,Ni-3.5B/BC-RAW catalyst showed superior catalytic performance and stability than other catalysts in steam reforming of toluene.(4)B doping lowered the condensation and graphitization degrees of biochar in Ni/BC-AW catalyst,but it can improve its carbon resistance ability and anti-sintering properties,which is conducive to improving the stability of Ni/BC-AW catalyst.Ni-B/BC-AW pyrolyzed at 600 ℃ with a Ni loading of 5%and B doping of 2%(Ni-2B/BC-AW)catalyst has a lower Ni particle size and lower carbon content than other catalysts after used.Therefore,Ni-2B/BC-AW catalyst showed superior catalytic performance and stability than other catalysts in steam reforming of toluene.