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Author(s): C. Wu

Article title: Effect of process conditions on the steam reforming of ethanol with a nano-Ni/SiO2 catalyst

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Techset Composition Ltd, Salisbury TENT586731 Page#: 8 Printed: 5/7/2011

Environmental TechnologyVol. 00, No. 0, Month 2011, 1–8

Effect of process conditions on the steam reforming of ethanol with a nano-Ni/SiO2 catalyst

C. Wu∗ and P.T. Williams

Energy & Resources Research Institute, The University of Leeds, Leeds LS2 9JT, UK

(Received 10 March 2011; final version received 4 May 2011 )

In this paper, a nano-Ni/SiO2 catalyst was prepared by a sol-gel method and tested for hydrogen production from ethanol steamreforming using a two-stage fixed-bed reaction system. The reaction conditions, such as reaction temperature, water/ethanolratio and sample feeding rate, were investigated with the prepared nano-Ni/SiO2 catalyst. Brunauer–Emmett–Teller surfacearea and porosity, temperature-programmed oxidation, X-ray diffraction and focused ion beam (FIB)/scanning electronmicroscopy were used in this work to analysis the fresh and/or reacted catalysts. An extended catalyst stability test for ethanolsteam reforming with the Ni/SiO2 catalyst was carried out at a reaction temperature of 600 ◦C, when the water/ethanol ratiowas kept at 3.5 and sample feeding rate was 4.74 g h−1. The results showed that a stabilized gas and hydrogen productionwas obtained with a potential H2 production of about 40 wt.%. Increasing the reaction temperature during ethanol steamreforming with the Ni/SiO2 catalyst resulted in an increase of gas and hydrogen production. The gas yield was slightlyreduced when the water/ethanol ratio was increased from 2.0 to 3.5. However, the potential H2 production was increased.The investigation of the sample feeding rate showed that the gas production per hour was increased due to the higher samplefeeding rate, but the potential H2 production was reduced.

Keywords: nickel; ethanol; reforming; sol-gel; nano

IntroductionHydrogen production from ethanol steam reforming hascurrently attracted much attention [1–3] due to the inter-est in the utilization of bio-ethanol and the advantages ofhydrogen as a clean energy [4,5]. Catalysts play an impor-tant role during the ethanol steam reforming process, byincreasing hydrogen production and reducing the requiredreaction temperature. Among the catalysts researched, Ni-based catalysts have been widely investigated and acceptedas a promising candidate due to their comparatively lowcost and effective catalytic activity [2,3,6,7]. In addition,the Ni/Al2O3 catalyst is a typical catalyst used for ethanolsteam reforming [8–10]. However, Ni/Al2O3 catalysts suf-fer serious deactivation due to sintering of Ni particlesand coke deposition on the catalyst during the process ofethanol steam reforming. Therefore, various methods havebeen studied to improve the performance of Ni-based cat-alysts, including the addition of a second metal, such asPt [11], Rh [12] or Cu [13,14] into the Ni catalyst system,changes to the Ni catalyst support, such as MgO-CeO2 [15],CeO2-ZrO2 [16,17] and SiO2 [14], and modification of thepreparation method of the catalyst [18,19]. Uniform nano-Ni particles supported with highly porous materials havebeen reported to reduce the deactivation of catalysts dur-ing the steam reforming of hydrocarbons by increasing Nidispersion and pore volume [20,21].

∗Corresponding authors. Email: wcf1314@gmail.com (C. Wu); p.t.williams@leeds.ac.uk (P.T. Williams)

Recently, we produced a nano-Ni/SiO2 catalyst usinga sol-gel method and tested the catalyst for hydrogen pro-duction from ethanol steam reforming. The catalyst (withhigh Ni dispersion and uniform particle size) prepared bythe sol-gel method was reported to be more effective than aNi/SiO2 catalyst prepared by an impregnation method [22].In addition, process conditions are important for hydrogenproduction and catalyst performance during ethanol steamreforming [23,24]. In this paper, a nano-Ni/SiO2 catalystprepared by a sol-gel method was applied to investigatethe influence of reaction temperature, sample feeding rateand water/ethanol ratio for the production of hydrogenfrom the steam reforming of ethanol. In addition, the sta-bility of the catalyst in relation to hydrogen production andcoke deposition was also examined over an extended 7 hperiod.

Experimental detailsCatalyst preparationA 20 wt.% Ni/SiO2 catalyst was prepared by a sim-ple sol-gel method adapted from the literature [25].Ni(NO3)2·6H2O (Sigma-Aldrich), anhydrous citric acid(Alfa Aesar), deionized water, absolute ethanol (Sigma-Aldrich) and tetraethyl orthosilicate (TEOS) (Sigma-Aldrich) were used as raw materials. Ni(NO3)2·6H2O

ISSN 0959-3330 print/ISSN 1479-487X online© 2011 Taylor & FrancisDOI: 10.1080/09593330.2011.586731http://www.informaworld.com

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2 C. Wu and P.T. Williams

(0.01 mol) and 0.01 mol of citric acid were first dissolvedinto 200 ml of absolute ethanol and stirred at 60 ◦C for 3 h.Deionized water (25 ml) and 8.7 ml of TEOS were addedinto the solution. After drying at 80 ◦C in a water bath forabout 4 hours, the precursor was calcined at 450 ◦C in astatic air atmosphere for 3 h. A test of the catalyst thermalstability (using a thermogravimetric analyser at a heatingrate of 10 ◦C min−1 to 700 ◦C in an air atmosphere) hadshown that the non-calcined catalyst was stabilized after400 ◦C. The catalyst was crushed and sieved to granuleswith a size between 0.08 and 0.20 mm.

Characterization of catalystsThe Brunauer–Emmett–Teller (BET) surface area and theporosity of the prepared catalysts were determined using aNONA 2200e Surface Area and Pore Size Analyser. Sam-ples were initially degassed under vacuum (0.016 mm Hg)for 2 h at a temperature of 120 ◦C before surface analysis.The system operates by measuring the quantity of nitrogenadsorbed onto or desorbed from a solid sample at differentequilibrium vapour pressures. The BET analysis showedthat the BET surface area of the prepared Ni/SiO2 catalystwas about 386 m2 g−1, Barrett–Joyner–Halenda (BJH) poreQ1volume was about 0.108 cm3 g−1 and BJH pore diameterwas 3.4 nm.

The temperature-programmed oxidation (TPO) of thereacted catalysts was carried out using a Stanton-Redcroftthermogravimetric analyser (derivative thermogravimetric(DTG) and thermogravimetric analysis (TGA)) to deter-Q1mine the properties of the coked carbons deposited on thereacted catalysts. About 10 mg of the reacted catalyst washeated in an atmosphere of air at 15 ◦C min−1 to a finaltemperature of 800 ◦C, with a dwell time of 10 minutes.

X-ray diffraction (XRD) (Philips PW 1830) was car-ried out for the fresh and reacted catalysts. The sample wasloaded into the 20 mm aperture of an aluminium sampleholder. The data was collected by Hiltonbrooks’ HBX dataacquisition software. The phase identification was obtainedusing GBC Scientific Equipment Ltd TRACES softwareusing the ICDD PDF2 (International Centre for DiffractionData Powder Diffraction Files) database.

A high-resolution scanning electron microscope (SEM,LEO 1530) coupled to an energy-dispersive X-ray

spectroscope (EDXS) was used to characterize and exam-ine the characteristics of the carbon deposited on thecoked catalysts. A Nanolab Dualbeam focused ion beam(FIB)/SEM coupled to an EDXS was also used to anal-yse the cross section of the reacted catalyst. Transmissionelectron microscopy (TEM) (Philips CM200) was used todetermine the fresh and reacted catalysts. For the TEM anal-ysis, the samples were ground, dispersed with methanol, andthen deposited on a Cu grid covered with a perforated carbonmembrane.

Reaction system for the ethanol steam reformingTesting of the prepared Ni/SiO2 catalysts was carried outwith a two-stage reaction system consisting of a pre-heatedstage and a gasification stage. A schematic diagram of thereaction system is shown in our previous paper [22].

A mixture of absolute ethanol and deionized water wasintroduced into the pre-heated stage of the reaction system(190 ◦C) using a syringe pump. The pre-heated mixture ofgaseous feedstock then passed through the gasification stagewith N2 as carrier gas (80 ml min−1) and was reformed inthe presence of the prepared catalyst (0.5 g). The derivedgaseous products were condensed with an air-cooled con-denser and a dry ice-cooled condenser to collect the liquidproducts. The non-condensed gases were collected with aTedlar™ gas sample bag. The reproducibility of the reac-tion system was tested thoroughly to ensure the reliabilityof research results.

The gases collected in the sample bag were analysedoff-line by packed column gas chromatography (GC). C1–C4 hydrocarbons were analysed using a Varian 3380 gaschromatograph with a flame ionization detector, with an80–100 mesh Hysep column and nitrogen carrier gas. Per-manent gases (H2, CO, O2, N2 and CO2) were analysed bya second Varian 3380 gas chromatograph with two separatecolumns. Hydrogen, oxygen, carbon monoxide and nitro-gen were analysed on a 60–80 mesh molecular sieve columnwith argon carrier gas, whilst carbon dioxide was analysedon a Hysep 80–100 mesh column with argon carrier gas.

Table 1. Mass balance and hydrogen production for the 7 h testing in relation to ethanol steam reforming.

Reaction time (h)1 2 3 4 5 6 7

Gas production (g h−1) 2.26 2.12 2.20 1.92 2.13 2.14 2.11H2 production (g h−1) 0.21 0.20 0.22 0.19 0.22 0.21 0.21Potential H2 production (wt.%) 40.9 38.1 43.0 36.6 41.4 40.7 39.4Average gas yield (wt.%) 44.9Total liquid yield (wt.%) 45.4

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Environmental Technology 3

Results and discussionSeven-hour testing of Ni/SiO2 catalyst for the ethanolsteam reformingThe Ni/SiO2 catalyst prepared by the sol-gel method wastested for ethanol steam reforming for 7 h while the sam-ple (water and ethanol, molar ratio 3.5:1) feeding rate wasmaintained at 4.74 g h−1 and the reforming temperature waskept at 600 ◦C. The gas sample was collected and analysed

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Figure 1. Gas compositions from ethanol steam reforming withthe Ni/SiO2 catalyst.

at hourly intervals. Mass balance data and hydrogen pro-duction data are presented in Table 1. As shown in Table 1,the gas production was around 2.10 g h−1, the hydrogenproduction was around 0.20 g h−1 and the potential H2 pro-duction was around 40 wt.%, respectively, during the 7 hethanol steam reforming process. In this paper, the potentialhydrogen production was calculated from the H2 produc-tion divided by the maximum hydrogen production fromthe steam reforming of the injected ethanol (0.26 g maxi-mum H2/g ethanol). Therefore, it is shown from Table 1that the gas and hydrogen production were quite stable overthe 7 h ethanol steam reforming period. From Table 1, theaverage gas yield related to the mass of injected samplewas about 44.9 wt.% and the liquid production (mass ofcondensed products divided by the mass of injected sam-ple) was about 45.4 wt.%. Song et al. [26] investigatedethanol steam reforming over supported cobalt catalysts;a 38.1 wt.% H2 yield (equates to the potential H2 produc-tion in this paper) was obtained at a reforming temperatureof 550 ◦C, H2O/ethanol ratio of 10 and weight hourly spacevelocity of 0.54 (g ethanol per g of catalyst per hour). Incomparison, the weight hourly space velocity in this workis about 4.0 (g ethanol per g of catalyst per hour).

The gas concentrations from the 7 h testing of the cata-lyst in relation to ethanol steam reforming are presented inFigure 1. From Figure 1, the H2, CO, CO2 and CH4 concen-trations were stabilized at around 65, 11, 18 and 4 Vol.%,respectively. Therefore, a stable gas concentration could

Figure 2. SEM results of the reacted Ni/SiO2 catalyst after 7 h testing in relation to ethanol steam reforming.

Figure 3. Cross-section images of the reacted Ni/SiO2 catalyst after 7 h testing in relation to ethanol steam reforming.

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4 C. Wu and P.T. Williams

also be concluded, indicating the stable performance of theprepared Ni/SiO2 catalyst.

SEM and FIB/SEM analysis were used to character-ize the reacted Ni/SiO2 catalyst after the 7 h investiga-tion of ethanol steam reforming. The SEM results areshown in Figure 2 and the FIB/SEM results are shown inFigure 3. Filamentous carbons were observed from both theSEM and FIB/SEM analysis (Figures 2 and 3). Figure 2shows cracking of the catalyst particles. The cracking ofthe catalyst after the ethanol steam reforming was alsoobserved in the FIB/SEM analysis (Figure 3), where crack-ing lines were obvious. The filamentous carbons were notonly deposited on the top of the surface of the catalyst(Figures 2 and 3), but also were found in the gaps ofcracked particles (Figure 3). The cracking of the Ni-Mg-Al catalyst after the pyrolysis gasification of polypropylenehas been reported by Wu and Williams [27]. It is sug-gested that some gaseous intermediates might permeateinto the catalyst during the reforming and generate aninitial force to crack the catalyst for the Ni/SiO2 cata-lyst used in this work. However, the initially generated

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Figure 4. TGA and DTG analysis results of the reacted Ni/SiO2after 7 h testing in relation to ethanol steam reforming.

layered carbons (metal carbides, non-reacted hydrocar-bons or reactive carbons), regarded as essential for theproduction of the filamentous carbons [21,27,28], weredifficult to find by SEM analysis. In addition, the lackof layered carbons is further shown by the TPO experi-ments. As shown in Figure 4, the oxidation peak (around500 ◦C) assigned to the layered carbons [29] in the TPO-DTG analysis results was difficult to observe. In addition,TEM analysis (Figure 5) show that filamentous carbonswere formed on the reacted Ni/SiO2 catalyst after theethanol steam reforming; catalyst sintering was obtainedfrom Figure 5, where Ni particles were found to be slightlyincreased.

As was indicated from the TPO-TGA and TPO-DTGanalysis results (Figure 4), the first weight loss peak (around100 ◦C) was assigned to water evaporation and the sec-ond weight loss peak (around 600 ◦C) was assigned tothe oxidation of filamentous carbons [29,30]. In addition,from Figure 4, about 4.5 wt.% of coke deposition wasobtained for the reacted Ni/SiO2 catalyst. The amountof the coke deposition was calculated by the mass differ-ence between the sample weight after water evaporationand the mass of residue divided by the sample weightafter water evaporation. Vizcaíno et al. [31] investigatedethanol steam reforming using a Ni/Al2O3 catalyst. Theyreported that about 50 wt.% of coke deposition (mass ofcoke divided by the used catalyst) was obtained for a14-hour reaction at the reaction temperature of 600 ◦C.Carrero et al. [32] studied a Cu-Ni/SBA-15 catalyst forethanol steam reforming: high coke deposition (around50 wt.%) was reported for a 3 h reaction time at a tem-perature of 600 ◦C. In addition, higher coke deposition hasalso been reported in other work [14,33] for ethanol steamreforming.

XRD analysis was carried out on the fresh and reactedNi/SiO2 catalyst after 7 h ethanol steam reforming. Theresults are shown in Figure 6. From Figure 6, only NiOcrystals could be found in the fresh Ni/SiO2 catalyst. Inaddition, an amorphous peak at around 23 ◦C was obtained Q2

Figure 5. TEM analysis of the fresh Ni/SiO2 and reacted Ni/SiO2 catalyst after 7 h steam reforming.

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Environmental Technology 5

and assigned to SiO2 present in the catalyst. Similar XRDpatterns for the Ni/SiO2 catalyst have been reported in otherwork [34,35]. After ethanol steam reforming, Ni crystalswere identified in the reacted Ni/SiO2 catalyst (Figure 6). Itis suggested that the non-pre-reduced catalyst was reducedby the product gas reducing agents, such as H2 and CO, dur-ing the steam reforming of ethanol. In addition, the massincreasing peak at around 400 ◦C in the TPO-DTG analy-sis result (Figure 4) is suggested to be the oxidation of Niparticles during the TPO experiment; this is consistent withthe observation of Ni crystals in the reacted catalyst by theXRD analysis (Figure 6).

Influence of reforming temperaturesReforming temperatures of 500, 550, 600 and 650 ◦C wereinvestigated in this work for hydrogen production from thesteam reforming of ethanol with the prepared Ni/SiO2 cata-lyst, when the sample feeding rate and water/ethanol molarratio were kept at 4.74 g h−1 and 3.5, respectively. The gassample was collected when the process was stabilized (afterabout 30 minutes) for the experiments investigating processconditions.

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Figure 6. XRD results for the fresh (a) and reacted (b) Ni/SiO2after 7 h testing in relation to ethanol steam reforming.

The mass balance and hydrogen production from theethanol steam reforming at different reforming temperaturesare shown in Table 2. As shown in Table 2, gas productionwas increased from 1.41 to 2.68 g h−1, hydrogen productionwas increased from 0.12 to 0.26 g h−1 and the potential H2production was increased from 23.8 to 50.4 wt.%. In addi-tion, the gas yield related to the mass of injected sample wasalso increased, when the reforming temperature increasedfrom 500 to 650 ◦C.

The gas concentrations obtained at various reformingtemperatures are shown in Figure 7(a). The results showthat CO, H2, CO2 and CH4 are the main gas products duringethanol steam reforming at different reforming tempera-tures. Similar results were also presented by Li et al. [36].It is shown from Figure 7(a) that the H2 concentration wasincreased when the reforming temperature was increasedfrom 500 to 600 ◦C; however, with the further increase intemperature to 650 ◦C, the H2 concentration reduced. FromFigure 7(a), the CO concentration was increased from 2.1 to16.8 Vol.%, the CO2 concentration was reduced from 23.0to 15.8 wt.%, the CH4 concentration reduced from 11.8 to1.7 Vol.% and concentrations of C2–C4 hydrocarbon gaseswere at a negligible level when the reforming tempera-ture was increased from 500 to 650 ◦C. Wang et al. [37]researched ethanol steam reforming over Ni and Ni-Cu cat-alysts and reported that H2 concentration was increased,CO2 and CH4 were reduced and CO concentration wasincreased with the increase of reforming temperature from400 to 650 ◦C with both Ni and Ni-Cu catalysts, while thewater/ethanol ratio was kept at 3.0. Investigation of reform-ing temperature has also been carried out by Deng et al. [38]for hydrogen production over a NiO/ZnO/ZrO2 catalyst;the selectivity towards CO was found to be increased and theselectivity to CO2 and CH4 was reported to be reduced withincreasing reforming temperature from 300 to 650 ◦C; how-ever, the H2 selectivity was stabilized after 550 ◦C. It seemsthat gas compositions were largely affected by the watergas shift reaction (exothermic), which favours the produc-tion of CO at a higher reaction temperature and reduces theH2 and CO2 concentration. It is also suggested that methanesteam reforming and methane dry reforming were promotedat a higher reaction temperature during the ethanol steamreforming process [1,36]. Therefore, CO and H2 concentra-tions were increased and the CH4 concentration was reducedat higher reforming temperature. In this work, the increasing

Table 2. Mass balance and hydrogen production for ethanol steam reforming at various reaction conditions.

Temperature (◦C) 500 550 600 650 600 600 600 650 650Feeding rate (g h−1) 4.74 4.74 4.74 4.74 1.90 6.64 9.49 4.74 4.74H2O/ethanol ratio 3.5 3.5 3.5 3.5 3.5 3.5 3.5 2.0 3.0

Gas production (g h−1) 1.41 1.76 2.20 2.68 1.19 2.63 3.18 3.01 2.75H2 production (g h−1) 0.12 0.17 0.22 0.26 0.12 0.25 0.30 0.27 0.26Gas yield (wt.%) 29.8 37.1 46.4 56.4 25.0 55.6 67.0 63.5 57.9Potential H2 production (wt.%) 23.8 32.2 43.0 50.4 58.1 34.7 28.5 39.4 46.5

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6 C. Wu and P.T. Williams

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Figure 7. Gas compositions from ethanol steam reforming at different reaction conditions: (a) reaction temperature; (b) sample feedingrate; (c) H2O/ethanol ratio.

H2 concentration might be due to the promotion of ethanolsteam reforming and steam reforming intermediates, suchas methane steam reforming, when the reforming tempera-ture was increased from 500 to 600 ◦C. However, when thereaction temperature further increased to 650 ◦C, the watergas shift reaction might be more dominant for the hydrogenpartial pressure in the experiment, and hence results in lowerH2 concentration compared to 600 ◦C. Maximum concen-tration of H2 during hydrocarbon gasification at differentreaction temperatures have also been presented in otherwork [3,39].

Influence of water/ethanol ratiosIn this section, water/ethanol ratios (2.0, 3.0 and 3.5) wereinvestigated with the prepared Ni/SiO2 catalyst in rela-tion to hydrogen production from the steam reforming ofethanol. Throughout the experiments, the reforming tem-perature and the sample feeding rate were controlled at650 ◦C and 4.74 g h−1, respectively. The results in termsof mass balance and hydrogen production are shown inTable 2. With increasing water/ethanol ratio in the sam-ple feed, the gas production was reduced from 3.01 to2.68 g h−1 and the H2 production rate was only changedslightly (Table 2). However, as shown in Table 2, the poten-tial H2 production was increased from 39.4 to 50.4 wt.%with increasing water/ethanol ratio from 2.0 to 3.5. Theincreasing of hydrogen selectivity (related to the poten-tial hydrogen production in this paper) has also beenreported to be increased with the increase of water/ethanolratio during ethanol steam reforming by Goula et al. [23]using a commercial Pd/γ -Al2O3 catalyst. Hydrogen selec-tivity was also reported to be increased with increasing

water/ethanol ratio from 3.0 to 6.0; however, with the fur-ther increase of water/ethanol ratio, hydrogen selectivitywas stabilized [40]. The promotion of hydrogen productionat higher water/ethanol ratio has also been observed in otherreports [38,41].

In addition, the gas concentrations from the ethanolsteam reforming at different water/ethanol ratios are pre-sented in Figure 7(b). The results show that H2 and CO2 con-centrations were increased and CO and CH4 concentrationswere decreased when the water/ethanol ratio was increasedfrom 2.0 to 3.5. It seems that at higher water/ethanol ratios,the water gas shift reaction was promoted, and resultedin higher concentrations of H2 and CO2 and lower con-centrations of CO. In addition, methane steam reformingand methane dry reforming also seemed to be improved athigher water content during the ethanol steam reformingprocess, and resulted in lower concentration of CH4 andhigher production of hydrogen.

Influence of sample feeding ratesIn this paper, sample feeding rates of 1.90, 4.74, 6.64 and9.49 g h−1 were investigated using the prepared Ni/SiO2catalyst in relation to their influence on hydrogen produc-tion from the steam reforming of ethanol. During the studyof various sample feeding rates, the reforming temperatureand water/ethanol molar ratio were kept at 600 ◦C and 3.5,respectively.

The mass balance and the hydrogen production resultsare presented in Table 2, which shows that the gas andhydrogen production appeared to be increased with theincrease of the sample feeding rate. For example, gas pro-duction was increased from 1.19 to 3.18 g h−1, and the

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Environmental Technology 7

hydrogen production increased from 0.12 to 0.30 g h−1,when the sample feeding rate increased from 1.90 to9.49 g h−1. However, the potential H2 production wasreduced from 58.1 to 28.5 wt.% with increasing samplefeeding rate. The reduction of the potential H2 productionmight be due to the reduced contact time between reactantsand the catalytic sites during the process of ethanol steamreforming due to the increased sample feeding rate.

The gas concentrations from ethanol steam reformingat various sample feeding rates are shown in Figure 7(c).The results show that H2 concentration was slightly reducedfrom 67.6 to 63.6 Vol.% and CH4 concentration increasedfrom 2.5 to 6.8 Vol.% with the increase of the sample feed-ing rate from 1.90 to 9.49 g h−1. However, the concentrationof CO, CO2 and C2–C4 hydrocarbons were stabilized ataround 11, 18 and 0.6 Vol.%, respectively.

ConclusionsThe main objective of this work was to investigate the pro-cess conditions during ethanol steam reforming with a nano-Ni/SiO2 catalyst. Stabilized gas production (2.10 g h−1) andpotential H2 production (40 wt.%) were observed for 7 hethanol steam reforming with the Ni/SiO2 catalyst, andfilamentous carbons were observed on the surface of thecatalyst and inside the gaps of the cracked catalyst by usingSEM and FIB/SEM analysis. With the increase of reactiontemperature from 500 to 650 ◦C, gas production and hydro-gen production were found to be increased. Potential H2production was reduced from 58.1 to 28.5 wt.% when thesample feeding rate was increased from 1.19 to 9.49 g h−1.In addition, the increase of water/ethanol ratio resulted inan increase in the potential H2 production.

AcknowledgementThis work was supported by the UK Engineering and PhysicalSciences Research Council under EPSRC Grant EP/D053110/1.

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