Carbon Nanofibers-Supported Ni Catalyst for Hydrogen Production from Bio-Oil through Low-Temperature Reforming

XU Yong JIANG Pei-Wen LI Quan-Xin

(Anhui Key Laboratory of Biomass Clean Energy,Department of Chemical Physics,University of Science&Technology of China,Hefei 230026,P.R.China)

1 lntroduction

Hydrogen is a clean energy without generating any environmental pollutants,which is identified to be an ideal energy carrier with high efficiency.There is a great interest in use of hydrogen as fuel,especially in high-efficiency systems such as fuel cells,and its application in the transportation sector.1Hydrogen is also one of the most important chemicals and has been widely used for ammonia production,oil refineries,and methanol production,which may play the indispensable role in the future global economy.2-4Currently,main processes for commercial hydrogen production are catalytic steam reforming of natural gas and oil-derived naphtha,partial oxidation of heavy oils,gasification of coal as well as electrolysis of water.5-7

In recent years,the increase in crude oil price and the limited petroleum resources have led to an increased interest in the alternative routes for the production of bio-fuels and chemicals using biomass as feedstocks.8,9As the only renewable carbon resource on the earth,biomass,as well as liquids derived from fast pyrolysis of various plant components of lignocellulosic biomass(bio-oil),has been proved to produce hydrogenviabiomass gasification,biomass catalytic pyrolysis or bio-oil steam reforming.10-12Compared with solid biomass as the raw material,liquid bio-oil,which can be readily stored and transported,is more suitable for the production of bio-fuels or chemicals on a large scale.Production of hydrogen from bio-oil reforming is probably one of the most promising options because it can achieve higherhydrogenyieldandhighercontentof hydrogen.13-20

Bio-oil is a dark organic liquid derived from fast pyrolysis of biomass,which contains numerous and complex organic compounds including acids,alcohols,aldehydes,ketones,substituted phenolics,and other oxygenates.21As mentioned above,bio-oil can be further converted into hydrogenviacatalytic steam reforming.Generally,bio-oil steam reforming can be simplified as the steam reforming of the oxygenated organic compounds(CnHmOk)by the following reactions:22,23

Thus,the overall steam reforming reaction can be given by:

During the bio-oil steam reforming,the decomposition of the organic compounds in the bio-oil as well as the Boudouard reaction may simultaneously occur.

High hydrogen yield and low operation temperature are essential for production of hydrogen using bio-oil steam reforming.Even though high temperature and highnS/nC(the molar ratio of steam to carbon)are favor for bio-oil steam reforming,high temperature not only increases the energy consumption and cost,also easily causes coke deposition on the catalyst.24,25The remaining challenges for production of hydrogen from bio-oil include realizing low-temperature reforming and suppressing catalyst deactivation.

With regards to the reforming catalysts,noble metal catalysts(e.g.,Pd,Ru,and Rh-based catalyst)26-29and non-noble metal catalysts(e.g.,Ni-,Co-,Fe-based catalysts)2,11,30,31were explored for bio-oil steam reforming.Generally,noble metal catalysts are more efficient than non-noble metal catalysts in terms of H2yield and bio-oil conversion at lower reaction temperatures.In view of the economy,bio-oil steam reforming using non-noble metal catalysts should be more economical and attractive.Among the non-noble metal catalysts,the Ni-based catalysts have been widely used for studying the reforming reactions of bio-oil as well as bio-oil model compounds.1,2,10,11,22

Carbon materials have been widely used as catalyst supporter in heterogeneous catalysis.As a unique form of carbon,the carbon nanofibers(CNFs)have been demonstrated to be promising alternative support materials owing to their intrinsic properties such as higher surface area,unique electronic properties,and thermal stability.32-35The objective of this paper is to prepare a high active carbon nanofibers-supported Ni catalyst for producing hydrogen from bio-oilviasteam reforming,and investigate the effects of temperature andnS/nCon the reforming.

2 Experimental

2.1 Materials

Bio-oil from sawdust was produced in a circulating fluidized bed with a capacity of 120 kg·h-1of oil in our lab(Anhui Key Laboratory of Biomass Clean Energy,Hefei,China),whose main characteristics are listed as follows:C/H/O,54.5/6.7/38.7(w,%),pH=2.1,moisture:21%(w),density:1300 kg·m-3,Lower heat value:18.2 MJ·kg-1;CNFs were purchased from Shenzhen Nanotech Port Co.,Ltd.China;γ-Al2O3(analytically pure),ethanol(analytically pure),HNO3(analytically pure),H2SO4(analytically pure),and Ni(NO3)2·6H2O(analytically pure)were obtained from Sinopharm Chemical Reagent Co.,Ltd.,Shanghai,China.

2.2 Catalyst preparation

CNFs-supported Ni catalysts were prepared by impregnation.First of all,CNFs were pre-oxidized in a mixed acid of HNO3and H2SO4(V/V=3:1)at 120°C for 4 h,and then washed with deionized water till the pH value of the filtrate was about 7,and then the filtered solid was overnight dried at 100°C.Secondly,the purified CNFs and Ni(NO3)2·6H2O were dispersed in ethanol respectively,and then dropped the prepared nickel nitrate solution into CNF solution under a moderate magnetic stirring for 4 h.After that,the mixture solution was aged for 1 h,and followed by an overnight drying at 100°C.Finally,the dried precursor was subjected to a calcination at 350 °C for 4 h.Besides,the 20%Ni/γ-Al2O3catalyst used as a comparison was also prepared by impregnating the nickel nitrate aqueous solution intoγ-Al2O3,and the ultrasonic was used for a well dispersion of Ni onγ-Al2O3.The pH value was kept at 7 during the impregnation,after 2 h ultrasonic impregnation,the precursor was overnight dried at 120°C,and then calcinated at500°C for 4 h.

2.3 Catalyst characterization

The metallic element contents in the prepared catalysts were measured by inductively coupled plasma and atomic emission spectroscopy(ICP/AES,Atom Scan Advantage of Thermo Jarrell Ash Corporation,USA).The Brunauer-Emmett-Teller(BET)surface area and pore volume were evaluated from the N2adsorption-desorption isotherms obtained at 77 K over the whole range of relative pressures by using a COULTER SA 3100 analyzer.The X-ray diffraction(XRD)was measured on an X'pert Pro Philips diffractrometer with a CuKαradiation.The microstructure of the catalyst and dispersion of Ni was evaluated by conventionalscanning electron microscope(SEM,XL-30 ESEM).

2.4 Reaction system and analysis of products

The bio-oil steam reforming was carried out in a fixed bed under atmosphere,and the schematic diagram of continuous fixed bed reaction system has been reported elsewhere.2,10,11Briefly,bio-oil was fed into the quartz reactor using a micro-injection pump(TS2-60,Baoding Longer Precision Pump).The inner diameter of the quartz reactor was 40 mm,the catalyst with a mean size of 40-60 mesh was mixed with sand and installed in the center of the reactor.The steam was produced from a stream generator,it was simultaneously fed into the reactor to carry the bio-oil into catalyst bed,as well as to adjust the molar ratio of steam to carbon(nS/nC),and the steam and gas flow was controlled and measured by mass flow controllers.Before the bio-oil was fed into the reactor,the catalysts used for bio-oil steam reforming were pre-reduced in H2(99.9%)for 3 h,the gas hourly space velocity(GHSV)was 10000 h-1during the process of reduction.The online analyses of gaseous products were carried out using two gas chromatographs(GC).The hydrocarbons(CH4etc.)were detected by GC1(Model:SP6890,column:PorapakQ-S)with a flame ionization detector(FID),H2,CO,and CO2were detected by GC2(Model:SP6890,column:TDX-01)with a thermal conductivity detector(TCD).The performance of bio-oil steam reforming was evaluated by the carbon conversion of bio-oil(Eq.(6))and the yield of hydrogen(Eq.(7)).All the tests were repeated for three times.

3 Results and discussion

3.1 Characterizations of catalysts

From the ICP-AES analysis,as shown in Table 1,the real contents of Ni loading on CNFs were 11.3%,22.1%,and 37.2%(w)respectively,referred as the 11%Ni/CNFs,22%Ni/CNFs,and 37%Ni/CNFs catalysts.For the typical 22%Ni/CNFs catalyst,the BET surface area and pore volume were 75.6 m2·g-1and 0.33 cm3·g-1,respectively.With increasing the Ni loading from 11.3%to 37.2%,the BET surface area of the CNFs-supported Ni catalysts slightly decreased from 93.4 to 60.3 m2·g-1,accompanied by a decrease in the pore volume from 0.35 to 0.29 cm3·g-1.This observation suggested that the part of the pores in CNFs was occupied by the Ni particles when Ni was added to CNFs,especially at a higher Ni loading amount.Because the most of the Ni particles were well dispersed on the carbon nanofiber surface,no significant decline in the BET surface area and pore volume occurred in our investigated range.

The XRD analysis was carried out to investigate the diffraction structures of the different Ni-based catalysts and supporter.Fig.1 displays the typical XRD spectra from the purified CNFs,the fresh 20%Ni/γ-Al2O3catalyst,the fresh 22%Ni/CNFs catalyst,the reduced 20%Ni/γ-Al2O3catalyst by 20%(volume fraction)H2at 500°C for 3 h,and the reduced 22%Ni/CNFs catalyst by 20%H2at 350°C for 3 h.For the purified CNFs,no obvious diffraction peaks were observed,indicating that the purified carbon nanofibers belong to an amorphous structure.For the Ni/γ-Al2O3and Ni/CNFs catalysts before the reduction treatment,three peaks at 2θ=37.5°,43.3°,62.9°were assigned to the NiO phases of NiO(222),NiO(400),and NiO(440),respectively.After the Ni-based catalysts were reduced by 20%H2,new peaks corresponding to the diffractions of the Ni(111),Ni(200),and Ni(220)at 2θ=44.4°,51.9°,76.4°were identified,indicating that NiO in the Ni/CNFs catalysts was reduced into metallic Ni.Moreover,the SEM measurement was employed for the morphologies of catalysts,as shown in Fig.2.For the sample of 11%Ni/CNFs,some scattered Ni particles were covered on the surface of carbon nanofibers(Fig.2(a)).When the Ni loading was increased over 22%,the most of the Ni particles were also uniformly dispersed on the surface of CNFs along with a small amount of agglomeration of the Ni particles(Fig.2(b,c)).However,it was observed that agglomeration of the Ni particles in 20%Ni/γ-Al2O3catalyst was muchmoreserious as comparedwiththeNi/CNFs catalysts.

Table 1 BET surface area and pore volume of catalysts

3.2 Catalytic activities of different Ni-based catalysts for bio-oil reforming

In this work,we performed the comparative tests on the production of hydrogen from the bio-oil using different Ni-based catalysts including:the purified CNFs;20%Ni/γ-Al2O3catalyst;11%Ni/CNFs catalyst;22%Ni/CNFs catalyst;37%Ni/CNFs catalyst.As shown in Fig.3,the hydrogen yield from the bio-oil reforming with different catalysts decreased in the following order:37%Ni/CNFs≈22%Ni/CNFs＞11%Ni/CNFs＞20%Ni/γ-Al2O3.The purified CNFs without Ni loading were nearly inactive for the bio-oil steam reforming in our investigated ranges.When the Ni was added onto CNFs,the performance of bio-oil steam reforming was significantly improved,and the carbon conversion and the H2yield reached about 94.7%and 92.1%respectively with the 22%Ni/CNFs catalyst at 550°C.Further increasing Ni content from 22%to 37%,there was no obvious increase in carbon conversion and H2yield,indicating that the optimized Ni loading content was around 22%.As a contrast,the conventional reforming catalyst of 20% Ni/γ-Al2O3catalyst was also tested under the same operation conditions.It was found that both bio-oil conversion and the H2yield using the 20%Ni/γ-Al2O3catalyst were significantly lower than the level over the 22%Ni/CNFs catalyst,especially at lower reforming temperature(350-450°C).Higher efficiency of production hydrogen with Ni/CNFs catalyst was attributed to highly dispersed Ni active sites when the carbon nanofibers were used as a supporter(see Fig.2).

Fig.2 Typical SEM images of(a)11%Ni/CNFs,(b)22%Ni/CNFs,(c)37%Ni/CNFs,and(d)20%Ni/γ-Al2O3catalysts

Fig.3 Performance of bio-oil steam reforming measured over different catalysts

3.3 lnfluence of reaction conditions on production of hydrogen from bio-oil

Reforming temperature plays a key role in the production of hydrogen from the bio-oil.Fig.4 shows the effect of the reforming temperature on the hydrogen yield,the carbon conversion,and gas compositions of the reforming products with the selected 22%Ni/CNFs catalyst.At 400°C,the yield of hydrogen and the conversion of bio-oil were about 45.1%and 40.7%respectively,indicating that the 22%Ni/CNFs catalyst has a good low temperature catalytic activity.With increasing temperature to 550°C,both the bio-oil conversion and hydrogen yield remarkably increased to 94.7%and 92.1%respectively.On the other hand,the reforming temperature also affected the compositions of gaseous products.As shown in Fig.4b,with increasing the temperature from 350 to 550°C,the content of H2increased from 41.2%to 69.1%,while the concentrations of CO and CH4show negative impact to the increasing temperature.Specifically,the CH4content decreased from 8.5%to 0.9%,and the CO content reduced from 22.5%to 1.5%.The steam reforming of bio-oil mainly involves in the reforming reactions of the oxygenated organic compounds(CnHmOk)along with the bio-oil cracking and the water-gas shift reaction.36The reforming and cracking reactions of bio-oil are endothermic processes in thermodynamics,and increasing temperature enhances the conversion of bio-oil as well as the hydrogen yield.The decrease in the methane content at higher temperature reflects that the cracking fragments of the bio-oil will be further reformed with the following reactions.

Fig.5(a-c)shows the yield of hydrogen,the carbon conversion,and the distribution of the products as a function of thenS/nCratio.Both the bio-oil conversion and the hydrogen yield gradually increased with increasing thenS/nCratio from 2 to 7.The hydrogen content also increased along with a decrease in the methane and CO contents at a highernS/nCratio.Increasing thenS/nCratio promoted the bio-oil conversion and the hydrogen yield,which can be attributed to that the reforming reaction(CnHmOk+(n-k)H2O→nCO+(n+m/2-k)H2)and the watergas shift reaction(CO+H2O→CO2+H2)are shifted to hydrogen formation at a highernS/nCratio.

3.4 Stability of carbon nanofibers supported-Ni catalysts

The stability of the carbon nanofibers supported-Ni catalyst during the bio-oil steam reforming was tested by measuring the carbon conversion and the yield of hydrogen as a function of the time on stream.The bio-oil steam reforming was tested over the 22%Ni/CNFs catalyst.As shown in Fig.6,the hydrogen yield initially increased for about 15 min as the bio-oil was fed into the reactor,and reached a maximum after almost 60 min.In the following 600 min,no obvious decrease in carbon conversion and hydrogen yield were observed,which indicated that no obvious inactivation of the 22%Ni/CNFs catalyst occurred in the process of bio-oil steam reforming.However,the carbon conversion and the H2yield decreased to 39.2%and 36.0%respectively after 1200 min.The deactivation was caused by coke deposition on the catalyst.

Fig.4 Effects of temperature on the performance of bio-oil reforming over the 22%Ni/CNFs catalyst

Fig.5 Effects of nS/nCon the performance of bio-oil reforming over the 22%Ni/CNFs catalyst

Fig.6 Effects of the time on stream on(a)the carbon conversion and(b)the hydrogen yield for bio-oil steam reforming over the 22%Ni/CNFs catalyst

4 Conclusions

In conclusion,we presented a low-temperature bio-oil reforming process using well dispersed and high activity Ni/CNFs catalysts.The prepared Ni/CNFs catalyst shows a good hydrogen yield as well as a higher bio-oil conversion even at lower temperature(350-450°C).The performance of bio-oil reforming over the Ni/CNFs catalyst was much higher than that from the Ni/γ-Al2O3catalyst.Results also show that the reaction temperature,the molar ratio of steam to carbon,and the content of Ni significantly affect the performance of bio-oil steam reforming in terms of hydrogen yield and products distribution.High hydrogen yield and lower operation temperature may potentially provide a promising candidate for the production of hydrogen using renewable bio-oil feedstocks.

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