A Research On Hydrogen Production By Ethanol Steam Reforming Over Nickel-based Catalysts

The demand for more clean and high efficient vehicles fuel and increasingly stringent environmental regulations has spurred intensive research toward catalytic hydrogen production for fuel cell uses. The key technology lie in the catalyst for hydrogen production of ethanol steam reforming. Comepared with noble catalysts, nickel-based catalysts has high price-performance ratio and have application prospect. The supporter and preparation method of nickel-based catalyst has great influence on its catalystic performance. At present, CeO2 and ZrO2 supported nickel-based catalysts have been more studied. However, these catalyst supports has the disadvantage of low specific surface. The catalyst with higher specific surface is easy to increase the dispersion of active components on the catalyst surface, which can improve the performance of catalysts. The catalyst with high specific surface can be easily prepared by using SiO2 as catalyst support, especially monodispersed Silica Sol, which has the characteristic of better chemical stability, particle uniformity and uniform structural channels, is a good catalyst support. There are more research on Ni/SiO2 methane catalytic reforming catalysts for syngas production, and at present Ni/SiO2 catalyst for ethanol steam reforming of hydrogen production has also less research, and meanwhile the preparation method of catalyst has influence on the catalytic properties. In order to investigate the effect of different SiO2 raw material type and preparation methods on the catalyst properties of ethanol steam reforming for hydrogen production, the 10wt% Ni/SiO2 catalysts were prepared by means of coprecipitation, immersion method and sol-gel process respectively. The performance of catalysts for hydrogen production of ethanol steam reforming were evaluated, and catalyst reaction paths and mechanism of ethanol steam reforming for hydrogen production were inferred according to the reaction products distribution. In addition, Ni-based catalysts with some composite components were also discussed.Ni/SiO2-CP catalyst, Ni/SiO2-IM catalyst and Ni/SiO2-SG catalyst were prepared using by coprecipitation method, impregnation method and sol-gel method respectively. Precipitation method Ni/SiO2-CP catalyst was prepared using monodispersed Silica Sol as SiO2 raw material, immersion method Ni/SiO2-IM catalyst using silica gel as SiO2 raw material for carrier, and sol-gel method Ni/SiO2-SG getting SiO2 from teos hydrolysis as the carrier material. The physicochemical characteristics and morphology and organization of catalysts were characterized by Specific surface area (BET) and X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy spectrum analysis (EDS). The performance of catalysts was evaluated in a fixed-bed reactor. The effects of reaction temperature in the low temperature range (300～550℃), ethanol to water molar ratio (mEtOH/mH2O=1:3.3～1:12) and liquid space velocity (LHSV=11.5 h-1-46.2 h-1) on catalytic performance were investigated. The coke degree of used catalysts was analysed by Thermogravimetric/differential thermal (TG/DTA) and SEM-EDS method. The main conclusions in this article are as follows:(1) Catalyst characterization. For Ni/SiO2-CP, Ni/SiO2-IM and Ni/SiO2-SG three kind of catalyst, the specific surface of three kinds of catalysts all reach more than 220m2/g. According to XRD patterns, Ni/SiO2-IM and Ni/SiO2-SG catalysts are respectively 11.4nm,9.0nm, the Ni/SiO2-SG catalyst of sol-gel method has nickel good dispersion and there are very weak NiO diffraction peaks can to be seen. The average diameter of NiO particles on Ni/SiO2-CP is less than 6.5nm. The sol-gel method Ni/SiO2-SG catalyst with the minimum NiO particle diameter indicates that sol-gel process has a active component good dispersion. According to the EDS test results, due to the reason of the carbon on test basement included, the nickel content of Ni/SiO2-CP catalyst is 9.51wt%, which is close to the theoretical 10wt%. The average nickel content of Ni/SiO2-IM from immersion method close to the theoretical value with the reason of uneven NiO scattered. the Ni/SiO2-SG surface nickel content of Sol-gel measurement was lower than the theoretical value.(2) Catalysts performance. The ethanol conversion all reached above 90% over three kind of Ni/SiO2 catalysts in the reaction temperature scope, in which Ni/SiO2-CP catalyst from coprecipitation method has the best hydrogen production performance with hydrogen hydrogen selectivities of is 60%～75% in the reaction range and with the hydrogen yield of 160 mmolH2/gcat.h under the condintion of 500℃, ethanol to water molar ratio of 1:9, and liquid volume space speed (LHSV) of 23.08 h-1. The optimum experimental process conditions for hydrogen production of ethanol steam reforming is:reaction temperature for (400-550)℃, ethanol to water mole ratio is 1:6～1:9, LHSV for (11.54～34.62) h-1. Under these conditions, high reaction rate, stable ethanol conversion rate and hydrogen produce rate, and higher hydrogen selectivity can be obtained. (3) Characterization of used catalysts. The SEM-EDS and TG/DTA analysis on used three kinds of catalysts show that the performance of Ni/SiO2-CP, Ni/SiO2-IM was better than Ni/SiO2-SG, but the coke deposit on the Ni/SiO2-CP, Ni/SiO2-IM catalyst surface are also higher. Under the research condition, Ni/SiO2-SG catalysts from sol-gel method has low activity, which indicates that the Ni/SiO2-SG catalyst needs to be improved.(4) Research on composite catalyst. In order to investigate the influence of the composite components and Cu addition on the nickel-based catalysts, the composite carrier (ZnO-Al2O3, ZnO-TiO2, ZnO-MgO, ZnO-CeO2 and ZnO-ZrO2) and compound active components (Ni-Cu) of nickel-based catalysts were tested about ethanol steam reforming for hydrogen production. Research results show that the nickel-based catalysts of ZnO-TiO2 compound carrier has the best performance with the hydrogen selectivivity of 82.05%, high ethanol conversion, and better stability. Bying comparing the Ni/ZnO-TiO2 catalyst with Ni-Cu/ZnO-TiO2 catalyst, the catalytic performance over Ni-Cu/ZnO-TiO2 has better results with ethanol conversion more than 90%, high hydrogen selectivity and better stability during continuous reaction period of 10 h.(5) Discussion of Reaction paths. Hydrogen production experiments from ethanol steam reforming were carried out over three kinds of catalysts of Ni/SiO2-CP, Ni/SiO2-IM and Ni/SiO2-SG. Gas products obtained have H2, CO, CO2, CH4 and C2H4, liquid products obtained have CH3CHO and CH3COCH3, and according to the reaction products that the reaction mechanism and the reaction paths could be speculated. The main reaction paths on Ni/SiO2 catalysts may be ethanol dehydrated into C2H4, ethanol dehydrogenation into acetaldehyde, ethanol being decomposed into CH4, CO, and H2, and the water gas shift reation (WGS). In addition, there are the disproportionated reaction of CO, methane cracking reaction and steam reforming reaction of CH4.The study on nickel-based catalysts for hydrogen production from ethanol steam reforming was carried out. The main conclusion was that coprecipitation method Ni/SiO2-CP catalyst has the peculiarities of simple preparation, easy to control, excellent catalyst performance and comprehensive superior performance, and that will be has a broad industrial application foreground.The further research contents including:catalyst preparation conditions and reaction conditions optimizing; Catalysts characterizing analysis, such as TEM, TPR and others; Catalyst evaluation of longer stability and life; carbon analysis and control of catalysts prepared.