Hydrogen Production from the Current-Enhanced Reforming and

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Hydrogen Production from the Current-Enhanced Reforming and Decomposition of Ethanol Lixia Yuan,† Tongqi Ye,† Feiyan Gong,† Qingxiang Guo,† Youshifumi Torimoto,‡ Mitsuo Yamamoto,§ and Quanxin Li*,† Department of Chemical Physics, Anhui Key Laboratory of Biomass Clean Energy, UniVersity of Science & Technology of China, Hefei, Anhui 230026, People’s Republic of China, Oxy Japan Corporation, 7# Floor, Miya Building, 4-3-4 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan, and College of Arts and Sciences, The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku 153-8902, Japan ReceiVed December 25, 2008. ReVised Manuscript ReceiVed March 18, 2009

Production of hydrogen from ethanol with high hydrogen yield and high energy efficiency was performed by using an electrochemical catalytic reforming (ECR) approach over the Ni-based catalyst. ECR was carried out in the fixed-bed continuous flow reactor, where an ac electronic current passed through the catalyst. It was found that the performance of the ethanol reforming not only depended on the reforming temperature, but also was remarkably enhanced by the current through the catalyst. In particular, the hydrogen yield and its selectivity significantly increased with an increase in the current in ECR. The promoting effects of the current on the decomposition of ethanol were also studied via the homogeneous experiments and low-pressure time-of-flight measurements. The alteration of the catalyst after the ECR processes was investigated via X-ray diffraction, transmission electron microscopy, and Brunauer-Emmett-Teller measurements. The mechanism of the electrochemical catalytic reforming of ethanol was discussed on the basis of the above investigation.

1. Introduction With the continuously increasing pressure on the pollution of the environment from the use of fossil fuels, searching for alternative energy sources has been made necessary. Fuel cell powered vehicles appear as good candidates to prevent automotive pollution. Hydrogen is a clean fuel with high energy and zero emission, and its production is a subject of current interest for fuel cell applications.1-4 Conventional methods for hydrogen production are based on gasoline and natural gas. However, the production of hydrogen from fossil fuels will increase the emission amounts of carbon dioxide and local pollution. Alternatively, production of hydrogen from lignocellulosic biomass, an environmentally friendly and rich feedstock, opens a new prospect for the utilization of the renewable biomass * To whom correspondence should be addressed. Telephone: 86-5513601118. Fax: 86-551-3606689. E-mail: [email protected]. † University of Science & Technology of China. ‡ Oxy Japan Corp. § The University of Tokyo. (1) Breen, J. P.; Burch, R.; Coleman, H. M. Metal-catalysed steam reforming of ethanol in the production of hydrogen for fuel cell applications. Appl. Catal., B 2002, 39 (1), 65–74. (2) Sun, J.; Qiu, X.; Wu, F.; Zhu, W.; Wang, W.; Hao, S. Hydrogen from steam reforming of ethanol in low and middle temperature range for fuel cell application. Int. J. Hydrogen Energy 2004, 29 (10), 1075–1081. (3) Frusteri, F.; Freni, S.; Chiodo, V.; Spadaro, L.; Di Blasi, O.; Bonura, G.; Cavallaro, S. Steam reforming of bio-ethanol on alkali-doped Ni/MgO catalysts: Hydrogen production for MC fuel cell. Appl. Catal., A 2004, 270 (1-2), 1–7. (4) Sun, J.; Qiu, X. P.; Wu, F.; Zhu, W. T. H-2 from steam reforming of ethanol at low temperature over Ni/Y2O3, Ni/La2O3 and Ni/Al2O3 catalysts for fuel-cell application. Int. J. Hydrogen Energy 2005, 30 (4), 437–445. (5) Comas, J.; Marino, F.; Laborde, M.; Amadeo, N. Bio-ethanol steam reforming on Ni/Al2O3 catalyst. Chem. Eng. J. 2004, 98 (1-2), 61–68. (6) Czernik, S.; French, R.; Feik, C.; Chornet, E. Hydrogen by catalytic steam reforming of liquid byproducts from biomass thermoconversion processes. Ind. Eng. Chem. Res. 2002, 41 (17), 4209–4215.

sources.5-11 Among the various oxygenated organic compounds which can be converted to hydrogen, ethanol is particularly important, since it is readily produced from renewable resources and has reasonably high hydrogen content, and it is suitable for feeding fuel cells.12-14 The intent of the ethanol steam reforming process is to make as much hydrogen and carbon dioxide as possible by the reforming reaction of ethanol in the presence of steam on a catalyst. Stoichiometrically, the overall steam reforming reaction of ethanol could be represented as follows: C2H5OH + 3H2O a 2CO2 + 6H2 (∆H298K,1bar ) +347.4 kJ/mol) (1) However, it has been revealed that the reaction pathways occurring in the ethanol steam reforming process are very (7) Marquevich, M.; Czernik, S.; Chornet, E.; Montane, D. Hydrogen from biomass: Steam reforming of model compounds of fast-pyrolysis oil. Energy Fuels 1999, 13 (6), 1160–1166. (8) Kechagiopoulos, P. N.; Voutetakis, S. S.; Lemonidou, A. A.; Vasalos, I. A. Hydrogen production via steam reforming of the aqueous phase of bio-oil in a fixed bed reactor. Energy Fuels 2006, 20 (5), 2155–2163. (9) Swami, S. M.; Abraham, M. A. Integrated catalytic process for conversion of biomass to hydrogen. Energy Fuels 2006, 20 (6), 2616–2622. (10) Bridgwater, A. V.; Czernik, S. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004, 18, 590–598. (11) Bridgwater, A. V.; Cottam, M. L. Opportunities for biomass pyrolysis liquids production and upgrading. Energy Fuels 1992, 6 (2), 113– 120. (12) Jamsak, W.; Assabumrungrat, S.; Douglas, P. L.; Laosiripojana, N.; Charojrochkul, S. Theoretical performance analysis of ethanol-fuelled solid oxide fuel cells with different electrolytes. Chem. Eng. J. 2006, 119 (1), 11–18. (13) Vaidya, P. D.; Rodrigues, A. E. Insight into steam reforming of ethanol to produce hydrogen for fuel cells. Chem. Eng. J. 2006, 117 (1), 39–49. (14) Morgenstern, D. A.; Fornango, J. P. Low-temperature reforming of ethanol over copper-plated raney nickel: A new route to sustainable hydrogen for transportation. Energy Fuels 2005, 19 (4), 1708–1716.

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complex and a dozen potential products may form, depending on the catalysts and reforming conditions used.15-17 Therefore, it is important to reduce the production of undesirable intermediate compounds, e.g., CH4 and C2H4. The presence of C2H4 especially hinders the overall H2 production reaction by inducing the pathways toward carbon production and thus causing “coking” of the catalysts.18,19 Among different technologies proposed, attention was focused on the development of highly active, stable, and selective catalysts for ethanol steam reforming reactions. It has been reported that the transition-metal-supported catalysts, especially Ni, Cu, Co, Pt, Pd, Rh, and Ru, etc., are good candidates for the steam reforming of ethanol.20-25 At the moment, lower energy cost in the reforming process, less byproduct, lower metal sintering, and minimal coke formation seem to represent the main goals. Much of our attention in our previous work has been paid to the production of bio-oil, hydrogen, and liquid biofuels from biomass.26-28 Production of hydrogen from electrochemical catalytic reforming (ECR) of bio-oil and acetic acid has been investigated in our previous work, in which the hydrogen yield and carbon conversion were both significantly enhanced by the current.29,30 The ECR proposal rooted in the study of the anionic (15) Barroso, M. N.; Gomez, M. F.; Arrua, L. A.; Abello, M. C. Reactivity of aluminum spinels in the ethanol steam reforming reaction. Catal. Lett. 2006, 109 (1-2), 13–19. (16) Homs, N.; Llorca, J.; de la Piscina, P. R. Low-temperature steamreforming of ethanol over ZnO-supported Ni and Cu catalysts. The effect of nickel and copper addition to ZnO-supported cobalt-based catalysts. Catal. Today 2006, 116 (3), 361–366. (17) Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Current status of hydrogen production techniques by steam reforming of ethanol: A review. Energy Fuels 2005, 19 (5), 2098–2106. (18) Cavallaro, S. Ethanol steam reforming on Rh/Al2O3 catalysts. Energy Fuels 2000, 14, 1195–1199. (19) Fatsikostas, A. N.; Kondarides, D. I.; Verykios, X. E. Production of hydrogen for fuel cells by reformation of biomass-derived ethanol. Catal. Today 2002, 75 (1-4), 145–155. (20) Marin˜o, F.; Boveri, M.; Baronetti, G.; Laborde, M. Hydrogen production from steam reforming of bioethanol using Cu/Ni/K/Al2O3 catalysts. Effect of Ni. Int. J. Hydrogen Energy 2001, 26, 665–668. (21) Llorca, J.; Homs, N.; Sales, J.; Ramı´rez de la Piscina, P. Efficient production of hydrogen over supported cobalt catalysts from ethanol steam reforming. J. Catal. 2002, 209 (2), 306–317. (22) Velu, S.; Satoh, N.; Gopinath, C. S.; Suzuki, K. Oxidative reforming of bio-ethanol over CuNiZnAl mixed oxide catalysts for hydrogen production. Catal. Lett. 2002, 82 (1-2), 145–152. (23) Cavallaro, S.; Chiodo, V.; Vita, A.; Freni, S. Hydrogen production by auto-thermal reforming of ethanol on Rh/Al2O3 catalyst. J. Power Sources 2003, 123 (1), 10–16. (24) Freni, S.; Cavallaro, S.; Mondello, N.; Spadaro, L.; Frusteri, F. Production of hydrogen for MC fuel cell by steam reforming of ethanol over MgO supported Ni and Co catalysts. Catal. Commun. 2003, 4 (6), 259–268. (25) Barroso, M. N.; Gomez, M. F.; Arrua, L. A.; Abello, M. C. Hydrogen production by ethanol reforming over NiZnAl catalysts. Appl. Catal., A 2006, 304 (1), 116–123. (26) Wang, Z.; Pan, Y.; Dong, T.; Zhu, X.; Kan, T.; Yuan, L.; Torimoto, Y.; Sadakata, M.; Li, Q. Production of hydrogen from catalytic steam reforming of bio-oil using C12A7-O- based catalysts. Appl. Catal., A 2007, 320, 24–34. (27) Wang, X. Z.; Dong, T.; Yuan, L. X.; Kan, T.; Zhu, X. F.; Torimoto, Y.; Sadakata, M.; Li, Q. X. Characteristics of bio-oil-syngas and its utilization in Fischer-Tropsch synthesis. Energy Fuels 2007, 21, 2421– 2432. (28) Dong, T.; Wang, Z.; Yuan, L.; Torimoto, Y.; Sadakata, M.; Li, Q. Hydrogen production by steam reforming of ethanol on potassium-doped 12CaO · 7Al2O3 catalyst. Catal. Lett. 2007, 119, 29–39. (29) Chen, Y. Q.; Yuan, L. X.; Ye, T. Q.; Qiu, S. Q.; Zhu, X. F.; Torimoto, Y.; Yamamoto, M.; Li, Q. X. Effects of current upon hydrogen production from electrochemical catalytic reforming of acetic acid. Int. J. Hydrogen Energy 2009, 34, 1760–1770. (30) Yuan, L. X.; Chen, Y. Q.; Song, C. F.; Ye, T. Q.; Guo, Q. X.; Zhu, Q. S.; Torimotob, Y.; Li, Q. X. Electrochemical catalytic reforming of oxygenated-organic compounds: A highly efficient method for production of hydrogen from bio-oil. Chem. Commun. 2008, 5215–5217.

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emission materials in our previous work,31-34 in which we developed an approach to selectively store and emit the anions of O-, OH-, H2-, and F2- via the Ca-Al-O materials (C12A7-X-, X ) O, OH, H, and F, etc.). When a current was applied on these materials, the electrons provided by the negative power source were supplied onto the material surface and entered into the materials’ body. The electrons provide a substitute for the lost anions due to the emission and keep the charge neutrality in the material. To obtain a stable anionic beam, the consumed anions (X-) can be regenerated via an implantation method by the reactions of molecules with thermal electrons (e.g., O2(surface) + 2e-(surface) f 2O-(surface)33,34). In fact, it is well-known that when an electrified metal or metal oxide is heated, electrons can boil off its surface, leading to thermal emission of electrons from the surface,35 and electrons on a metal or a metal oxide surface play an important role in the reduction process (e.g., O2 + 4e- f 2O2-), occurring on the cathode of a solid oxide fuel cell (SOFC).36-39 In this work, we investigated hydrogen production via ECR of ethanol. In the ECR approach, an electrified Ni-Cr wire, which passed through a given ac electronic current, entwined around a quartz column, was installed in the catalyst bed, and was used for heating the catalyst and synchronously providing the thermal electrons onto the catalyst during the reforming. The catalyst was uniformly embedded around the Ni-Cr wire. To obtain a certain reforming temperature, the catalyst bed was heated by a supplementary outside furnace. Production of hydrogen from ethanol via ECR was performed over the Nibased catalyst. The main aim in this work is to promote reforming of ethanol and the decomposition of ethanol via the current applied to the catalyst, which is useful for increasing the hydrogen selectivity, hydrogen yield, and energy efficiency. The effects of the current on the reforming of ethanol and its decomposition and the microcosmic properties of the catalyst were studied. The mechanism of the electrochemical catalytic reforming for ethanol was also discussed on the basis of these investigations. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. Nickel catalyst supported on Al2O3 was prepared by the impregnation method (31) Huang, F.; Li, J.; Wang, L.; Dong, T.; Tu, J.; Torimoto, Y.; Sadakata, M.; Li, Q. X. Features and mechanism of H- anion emission from 12CaO · 7Al2O3 surface. J. Phys. Chem. B 2005, 109, 12032. (32) J. Li, F. H.; Wang, L.; Yu, S. Q.; Torimoto, Y.; Sadakata, M.; Li, Q. X. High density hydroxyl anions in a microporous crystal: [Ca24Al28O64]4+ · 4(OH-). Chem. Mater. 2005, 17, 2771. (33) Li, Q. X.; Hayashi, K.; Nishioka, M.; Kashiwagi, H.; Hirano, M.; Torimoto, Y.; Hosono, H.; Sadakatad, M. Absolute emission current density of O- from 12CaO/7Al2O3 crystal. Appl. Phys. Lett. 2002, 80, 4259. (34) Song, C. F.; Sun, J. Q.; Qiu, S. B.; Yuan, L. X.; Tu, J.; Torimoto, Y.; Sadakata, M.; Li, Q. X. Atomic fluorine anion storage-emission material [Ca24Al28O64]4+ · 4F- and semiconductor etching by atomic fluorine anions. Chem. Mater. 2008, 20, 3473–3479. (35) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994; p 36. (36) Bebelis, S.; Kotsionpoulos, N. Electrochemical characterization of perovskite-based. J. Appl. Electrochem. 2007, 37, 15–20. (37) Jamsak, W.; Assabumrungrat, S.; Laosiripojana, N.; Suwanwarangkul, R.; Charojrochkul, S.; Croiset, E. Performance of ethanol-fuelled solid oxide fuel cells: Proton and oxygen ion conductors. Chem. Eng. J. 2007, 133, 187–194. (38) M. S.; Mitrovsk, C. L.; Elliot, C.; Nuzzo, R. G. Microfluidic devices for energy conversion: Planar integration and performance of a passive, fully immersed H2-O2 fuel cell. Langmuir 2004, 20, 6974–6976. (39) Zhenhua, Wang; Sunb, K.; Zhang, N.; Qiao, J.; Xua, P. Preparation of YSZ thin films for intermediate temperature solid oxide fuel cells by dip-coating method. J. Membr. Sci. 2008, 320, 500–504.

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Figure 1. (Left) Schematic setup of the fixed-bed flow reaction system for ethanol. (Right) (a) ECR mode: the catalyst was uniformly embedded around the Ni-Cr wire, which passed through an ac electronic current for heating the catalyst and synchronously providing the electrons onto the catalyst. (b) CSR mode: the current was shut off, and the reactor was homogeneously heated by an outside furnace.

using nickel nitrate as the metal precursor. The prepared process details are described elsewhere.26 The nickel loading in the resulting sample was about 18 wt % NiO measured by inductively coupled plasma and atomic emission spectroscopy (ICP/AES), and the BET (Brunauer-Emmett-Teller) surface area was about 115.2 m2/g. Before reforming was run, the NiO/Al2O3 catalysts were reduced at 650 °C for 4 h in the flowing H2 in situ. The condition is sufficient to ensure NiO reduction (as confirmed by X-ray diffraction, XRD) but not so high as to cause Ni sintering. XRD measurements were employed to investigate the diffraction structure changes of the catalysts. X-ray diffraction patterns of the catalysts were recorded on an X’pert Pro Philips diffractrometer, using Cu KR radiation. Transmission electron microscopy (TEM) images directly yield the size and shape of the Ni particles. TEM measurement was done on a Japan H-800 electron microscope, which has a primary electron energy of 200 keV and a point resolution of 0.45 nm in TEM mode. The powder sample was dispersed in ethanol and kept in an ultrasonic bath for 3 h, and then the sample was deposited onto a carbon-covered Cu supporting grid and dried at 25 °C for TEM analysis. The BET surface area and pore volume were determined by the N2 physisorption at -196 °C using a Coulter SA 3100 analyzer. 2.2. Reaction System and Operating Procedure. As shown in Figure 1, the ethanol steam reforming experiments were carried out in the continuous-flow systems, using a quartz fixed-bed reactor under atmospheric pressure. The ethanol was fed into the reactors using a multisyringe pump (TS2-60, Baoding Longer Precision Pump), the steam from a steam generator was simultaneously fed into the reactors, the steam amount was controlled by the mass flow controller, and the effluent gases from the reactors were measured by flow display. Temperature and its distribution were measured by the thermocouples inserted into the catalyst beds. We performed the reforming experiments with the following two modes, i.e., the common steam reforming (CSR) model and the electrochemical catalytic reforming (ECR) mode. For the ECR mode, an annular Ni-Cr wire, which passed through a given ac electronic current, entwined around a quartz column for heating the catalyst and synchronously providing the electrons onto the catalyst, was installed in the center of the reactor. The catalyst was uniformly embedded around the Ni-Cr wire. To obtain a certain reforming temperature, the catalyst bed was heated by a supplementary outside furnace. For the CSR mode, the ac current was shut off and the catalyst bed was homogeneously heated by an outside furnace.

Table 1. Typical Temperature Distributions in the Catalyst Bed for the CSR and ECR Modes at 1 atm positiona (mm, mm)

T(CSR)b (°C) 30 mL/min

300 mL/min

T(ECR)c (°C) 30 mL/min

300 mL/min

(0, 0) (25, 0) (-25, 0) (0, 1.5) (0, -1.5)

400 406 397 403 399

399 405 394 402 397

401 405 393 391 406

403 408 396 395 412

Tav std devd

401.0 3.5

399.4 4.3

399.2 6.9

402.8 7.4

a The coordinates in the CSR bed and in the ECR catalyst bed are shown in Figure 1a,b. The coordinate (0, 0) stands for the center of the bed. b Iinside ) 0 A, and ftotal ) 30 and 300 mL/min. c Iinside ) 3.2 A, and ftotal ) 30 and 300 mL/min. d Std dev refers to standard deviation.

Temperature distributions in the catalyst bed under different conditions were first measured by the thermocouples inserted into different positions in the bed before the reforming test was run. We have carefully detected the temperature distribution in the catalyst bed for both the ECR mode and the CSR mode. As shown in Table 1, the flow speed of the mixture gas and the heating modes (e.g., ECR and CSR) affected the temperature distribution in the catalyst bed. When the temperature in the center of the ECR catalyst bed was near 400 °C, the maximal temperature gradients in the transverse orientation and in the lognitudinal orientation were about 12 and 15 °C, respectively, for the low flow speed condition (total flow 30 mL/min). With increasing flow speed, both maximal temperature gradients in the transverse and lognitudinal orientations increased, reaching about 12 and 17 °C, respectively, for a total flow speed of 300 mL/min. On the other hand, the temperature gradients for the CSR mode are smaller than those in the ECR mode. The relationship between the averaged temperature in the catalyst bed (Taverage) and the center temperature (Tcenter) was derived from the temperature distribution measured under different experimental conditions. Thus, the averaged temperature needed can be adjusted by the center temperature through the power applied to the Ni-Cr wire and/or the outside oven. Moreover, the temperature in the center of the catalyst bed, generally, is almost close to the average value (the deviations between the center temperature and the average value in the bed were less than 5 °C) in our investigated range (400-600 °C). Generally, the averaged temperature in the catalyst bed was approximately used as the reaction temperature in the ECR or CSR reforming experiments.

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2.3. Analysis of the Products. The products of the reforming reactions were analyzed by two online gas chromatographs using TDX-01and Porapak Q columns and a thermal conductivity detector. Ultra-high-purity argon (99.999%) was used as the carrier gas. The reforming performance of ethanol was studied by measuring the ethanol conversion, hydrogen yield, and product selectivity under different reforming conditions. The conversion of ethanol is denoted as XEtOH, the hydrogen yield as YH2, the carboncontaining product selectivity (CO, CO2, CH4, etc.) as SC, and the hydrogen selectivity as SH2. According to a previous study on the reforming of ethanol,40 these are calculated according to the following equations:

XEtOH )

moles of EtOHin - moles of EtOHout × 100 (2) moles of EtOHin YH2 )

SC ) SH2 )

moles of H2 moles of EtOHin

(3)

moles of C × 100 (4) χ(moles of EtOHin - moles of EtOHout)

moles of H2 × 100 3(moles of EtOHin - moles of EtOHout) + (moles of waterin - moles of waterout) (5)

where C stands for the carbon-containing products and χ is the stoichiometry factor. All of the data presented are given as average values. Generally, the experiments were repeated three times. The difference for each repetition, in general, ranged from 0% to 10%. The intermediates desorbed from the catalyst surface were mass analyzed by a time-of-flight (TOF) mass spectrometer. The experimental setup of the TOF system has been described in detail elsewhere.33,34 Here, a quartz-tube reactor was installed in the center of the sample chamber of the TOF system. For the ECR mode, an electrified Ni-Cr wire was embedded in the catalyst, used for heating the catalyst and providing the electrons onto the catalyst. The reactants or carrier gas (argon) was fed onto the quartz-tube reactor by a nozzle with a total pressure of about 10-1 Pa in the reactor tube. A small amount of the products (including intermediates formed in the reforming reactions) passed through a pinhole of about 100 µm (located in the center of the reactor), which allows a simultaneous analysis via TOF mass spectrometry (Figure 9).

3. Results and Discussion 3.1. Effect of the Current on the Reforming of Ethanol. The performance of the ethanol reforming (e.g., the ethanol conversion, hydrogen yield, and product selectivity) using the conventional steam reforming method, generally, is mainly controlled by the reforming temperature and the S/C ratio (the ratio of steam to carbon fed) for a given catalyst. In this work, it was observed that the behavior of the ethanol reforming was very sensitive to the current (I) through the catalyst, which is described as electrochemical catalytic reforming. To investigate the features of the electrochemical catalytic reforming for ethanol, the reforming was performed under different currents; meanwhile other experimental conditions (i.e., T (temperature), S/C (the ratio of stream to carbon fed), and LHSV (liquid hourly space velocity)) were maintained at given constant values. Figure 2a presents the influence of the current on the ethanol conversion for the reforming of ethanol over Ni/Al2O3, which was measured as a function of the current through the catalyst at different fixed temperatures. The ethanol conversion has no (40) Goula, M. A.; Kontou, S. K.; Tsiakaras, P. E. Hydrogen production by ethanol steam reforming over a commercial Pd/γ-Al2O3 catalyst. Appl. Catal., B 2004, 49 (2), 135–144.

Figure 2. Influence of the current on (a) the ethanol conversion and (b) the hydrogen yield for the ethanol reforming over Ni/Al2O3. Reforming conditions: T ) 400, 450, 500, 550, and 600 °C, respectively, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm.

obvious change with increasing current. However, the hydrogen yield was significantly enhanced by the current through the catalyst (Figure 2b). With an increase of the current from 0 to 3.2 A, the yield of hydrogen increased from 1.66 to 3.45 mol at 400 °C. The hydrogen yield reached 5.57 mol at I ) 3.2 A and 600 °C. Figure 3 shows the effect of the current on the selectivity of products from ethanol reforming over Ni/Al2O3. The main products are H2, CO2, CO, and CH4. It was found that the selectivity of H2 and CO increased with increasing current, accompanied by a decrease of the CO2 and CH4 selectivity. For example, the selectivity of hydrogen at 500 °C increased from 74.8% to 87.2% with an increase of the current from 0 to 3.2 A, while the selectivity of methane decreased from 34.4% to 21.9%. The selectivity of carbon monoxide increased from 8.5% to 20.5% accompanied by a slight decrease of CO2 from 58.3% to 57.8% under the above conditions. High selectivity of hydrogen (95.5%) was obtained at 600 °C and 3.2 A. The product selectivity in the reforming processes of ethanol was mainly determined by the steam reforming reactions of ethanol and its hydrocarbon fragments, the dissociation of ethanol, and the water-shift reaction. The decrease of the CH4 content in the products at high current may reflect the thermal electrons from the Ni-Cr wire promoted the steam reforming and dissociation of hydrocarbons, which will be described more in detail in section 3.5. The stability of the catalyst during the ECR of ethanol was tested by measuring the ethanol conversion, the yield of hydrogen, and the product compositions as a function of the time on stream. As shown in Figure 4a, both the ethanol conversion and the hydrogen yield almost remained constant during our investigated ranges (for 53 h). On the other hand, no obvious changes of the products’ compositions were observed (Figure 4b). Generally, the catalyst deactivation is mainly caused



Figure 3. Effect of the current on the selectivity of the products for the ethanol reforming over Ni/Al2O3: (a) H2 selectivity, (b) CH4 selectivity, (c) CO selectivity, (d) CO2 selectivity. Reforming conditions: water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm.

Figure 4. Stability curves measured under a typical electrochemical catalytic reforming of ethanol over the Ni/Al2O3 catalyst: (a) hydrogen yield and ethanol conversion, (b) product distribution. Reforming conditions: I ) 3.2 A, T ) 450 °C, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm.

by the deposition of carbon (coke formation) on the catalyst.41-44 The present results indicated that no serious deposition of carbon (41) Amphlett, J. C.; Leclerc, S.; Mann, R. F.; Peppley, B. A.; Roberge, P. R. Fuel cell hydrogen production by catalytic ethanol steam reforming. Proceedings of the 33rd Intersociety Energy ConVersion Engineering Conference, Colorado Springs, CO, August 1998; pp 98-269. (42) Fatsikostas, A. N.; Kondarides, D. I.; Verykios, X. E. Production of hydrogen for fuel cells by reformation of biomass-derived ethanol. Catal. Today 2002, 75, 145–155.

occurred in the ethanol reforming via ECR. The amount of the carbon deposition estimated by the TGA (thermogravimetry analysis) measurements was about 0.7% after the ECR of ethanol for 53 h of reforming. To further quantitatively study the reforming product distribution, we evaluated the carbon balance, oxygen balance, and hydrogen balance in the ECR and CSR processes. The carbon balance is defined as the ratio of all carbon-containing product moles to the consumed moles of the ethanol, accounting for stoichiometry. The oxygen balance and the hydrogen balance are calculated by using a definition similar to that of the carbon balance. Table 2 summarizes the results of the overall mass balance for the ethanol reforming in three ECR and CSR conditions. The overall mass balances of hydrogen and oxygen fluctuated around 100%. On the basis of the estimation of the carbon balance and the oxygen balance, most of the carbon or oxygen in the ethanol was converted into CO2 and CO. The influence of the Ni-Cr wire on the ethanol reforming was also investigated. Figure 5 presents the effects of Ni-Cr wire on the ethanol conversion and hydrogen yield from the following tests, including (1) the empty bed test (i.e., neither Ni-Cr wire nor catalyst), (2) only the Ni-Cr wire (INi-Cr ) 0 A) installed, (3) only the electrified Ni-Cr wire (INi-Cr ) 3.2 A) installed in the reactor, (4) the CSR mode (INi-Cr ) 0 A) over the catalyst, and (5) the ECR mode (INi-Cr ) 3.2 A) over the catalyst. The experimental results show that both the conversion of ethanol and hydrogen yield over the single Ni-Cr wire are much lower than those from the reforming via the catalyst, indicating that the contribution from the Ni-Cr wire is minor. 3.2. Effect of the Current on the Decomposition of Ethanol. The influence of the current on the decomposition of ethanol was tested via homogeneous experiments with and (43) Fatsikostas, A. N.; Verykios, X. E. Reaction network of steam reforming of ethanol over Ni-based catalysts. J. Catal. 2004, 225, 439– 452. (44) Llorca, J.; Homs, N.; Sales, J.; Fierro, J. G.; Piscina, P. R. Effect of sodium addition on the performance of Co-ZnO-based catalysts for hydrogen production from bioethanol. J. Catal. 2004, 222, 470–480.

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Table 2. Mass Balance of C, H, and O in the Ethanol Reforming under Three ECR and CSR Conditionsa ECR temp (°C) current (A) carbon concn (%) in CO2 product carbon concn (%) in CO product carbon concn (%) in CH4 product coke concn (%) carbon balance (% feed) hydrogen concn (%) in H2 product hydrogen concn (%) in CH4 product hydrogen balance (% feed) oxygen concn (%) in CO2 product oxygen concn (%) in CO product oxygen balance (% feed) a

400 3.2 59.3 7.60 32.3 0.7 99.9 72.2 24.9 97.1 92.9 4.90 97.8

500 3.2 57.8 20.5 21.9 0.4 100.6 87.2 15.4 102.6 85.2 10.8 96.0

CSR 600 3.2 54.5 44.3 2.40 0.2 101.4 95.5 2.60 98.1 72.5 25.6 98.1

400 0 59.8 0.60 40.9 0.9 102.2 48.9 48.0 96.9 95.7 0.60 95.3

500 0 58.3 8.50 34.4 0.6 101.8 74.8 23.8 98.6 91.5 7.1 98.6

600 0 57.1 33.7 7.60 0.5 98.9 90.9 4.50 95.4 80.9 18.6 99.5

T ) 400, 500, and 600 °C, respectively, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm.

Figure 5. Ethanol conversion (I) and hydrogen yield (II) measured as a function of temperature for (9) the empty bed test (i.e., neither Ni-Cr wire nor catalyst), (∆) only the Ni-Cr wire (I ) 0 A) installed in the reactor, (2) only the electrified Ni-Cr wire (I ) 3.2 A) installed in the reactor, (0) the CSR mode (I ) 0 A) over the catalyst, and (•) the ECR mode (I ) 3.2 A) over the catalyst. Other reforming conditions: water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm.

without the current through the quartz bed (only quartz powder and Ni-Cr wire were installed in the reactor). As shown in Figure 6a, the ethanol conversion for the decomposition of ethanol depends both on the temperature and on the current. Without the current supplied (i.e., I ) 0 A) over the quartz bed, for example, the carbon conversion was very low (about 10%) at 500 °C. However, the carbon conversion increased to 27.3% with an increase of the current from 0 to 3.2 A. It was also observed that the hydrogen yield was enhanced by the current over the quartz bed (Figure 6b). Figure 7 presents the effect of the current on the selectivity of the products from the decomposition of ethanol. The main products from the decomposition of ethanol observed are H2, CH4, CO, CH3CHO, C2H4, and H2O. It was found that the selectivity of H2, CO, and CH4 slightly increased with increasing current, accompanied by a decrease of the selectivity of C2H4, CH3CHO, and H2O. The above results indicated that the current promoted the decomposition of ethanol. To further study the dissociation and reforming processes, the species desorbed from the catalyst surface were detected

Figure 6. Effect of the current on (a) the ethanol conversion and (b) the hydrogen yield from the decomposition of ethanol. Other conditions: ethanol/Ar (mole ratio) ) 5.3, LHSV ) 3.0 h-1, and P ) 1 atm.

by TOF mass spectrometry under the low-pressure (10-1 Pa) condition (Figure 9). Without the current applied to the catalyst (Figure 8a), no TOF signal was observed. When the current passed through the catalyst in argon, as shown in Figure 8b, only one peak near the mass numbers of zero was observed, corresponding to the thermal electrons desorbed from the electrified Ni-Cr wire and the catalyst surface. When the current passed through the Ni-Cr wire without catalyst in argon, the thermal electron peak was slightly enhanced, as shown in Figure 8c. When water/argon was fed onto the electrified catalyst, three new peaks appeared, corresponding to H- (m/z ) 1), OH- (m/z ) 17), and H2O- (m/z ) 18) (Figure 8d). These anionic fragments would form by the dissociation of water with the thermal electrons on the catalyst surface (e.g., e-(s) + H2O(s) f OH-(s) + H(s), where s represents the surface). As ethanol and argon were fed onto the electrified catalyst, a series of new peaks appeared, corresponding to H- (1), H2- (2), CHx- (x ) 0-4, m/z ) 12-16), C2HxO- (x ) 0-4), OH- (17), and CO(28) (Figure 8e). The anionic hydrocarbon fragments would form via the dissociation of ethanol which were caused by the thermal electrons on the catalyst surface (i.e., e-(s) + C2H5OH(s) f CHx-(s) +...). As the mixture of H2O/C2H5OH/Ar was injected



Figure 7. (a) Selectivity of the products at 600 °C and (b) selectivity of the products at 700 °C from the decomposition of ethanol. Other conditions: ethanol/Ar (mole ratio) ) 5.3, LHSV ) 3.0 h-1, and P ) 1 atm.

Figure 10. (a) Effect of the current passed through the Ni-Cr wire on the current emitted from the Ni-Cr wire surface at different temperatures. (b) Effect of the distance from the Ni-Cr wire surface to the Faraday plate on the number density of electrons (other conditions: T ) 600 °C, I ) 82.0 µA). (c) Effect of the current passed through the Ni-Cr wire on the number density of electrons at a position 1 mm away from the Ni-Cr wire surface.

Figure 8. Typical TOF spectra: (a) without the current applied to the catalyst, I ) 0 A, T ) 580 °C, P(Ar) ) 2.5 × 10-1 Pa, (b) current passed through the Ni/Al2O3 catalyst in argon (I ) 3.2 A, T ) 575 °C, P(Ar) ) 2.7 × 10-1 Pa), (c) current passed through the wire without the catalyst in argon (I ) 3.1 A, T ) 580°C, P(Ar) ) 2.8 × 10-1 Pa), (d) H2O/Ar mixture fed onto the electrified catalyst (I ) 3.2 A, T ) 575 °C, P(H2O) ) 1.6 × 10-1 Pa, P(Ar) ) 1 × 10-1 Pa), (e) C2H5OH /Ar mixture fed onto the electrified catalyst (I ) 3.1 A, T ) 580 °C, P(C2H5OH) ) 5 × 10-2 Pa, P(Ar) ) 2.3 × 10-1 Pa), (f) mixture of C2H5OH /H2O/Ar fed onto the electrified catalyst (I ) 3.0 A, T ) 580 °C, P(H2O) ) 1.4 × 10-1 Pa, P(Ar) ) 1.0 × 10-1 Pa). P(C2H5OH) ) 5 × 10-2 Pa.

Figure 9. Schematic setup of the TOF system for detecting the decomposition fragments when the current passed through the catalyst.

onto the electrified catalyst, the peak of CO2- was obviously observed (Figure 8f), indicating that the reforming reaction of

Figure 11. (a) Schematic setup for detecting the current emitted from the Ni-Cr wire surface. (b) Distance x from the Ni-Cr wire surface toward the Faraday plate.

ethanol (i.e., C2H5OH(s) + H2O(s) f CO2(s) + H2(s)) occurred. The TOF results confirmed that the dissociation of molecules via the thermal electrons occurred on the catalyst surface. To further investigate the current flowing through the catalytic bed (Ibed), the current emitted from the Ni-Cr wire surface was collected by a columned Au-coated Faraday plate and detected by a Keithley model 6485 amperometer (please see Figure 11a). The experimental conditions were fAr ) 370 mL/min, fH2O ) 0.01 mol/min, and P ) 1 atm. The background including a trace amount of the leak current and the thermal electron emission from the wire was first measured under the Ni-Cr wire without applying a current (INi-Cr ) 0). As is shown in Figure 10a, the background current was about 3.0 µA at 620 °C; when the current was applied to the wire, the space current flowing through the catalytic bed (i.e, Ibed) was significantly increased.

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Yuan et al. Table 3. Macroscopic Structure Parameters of Each Catalyst under Various Conditions sample no.a S 1 2 3 4 5 6

Figure 12. XRD spectra for (a) the fresh sample (NiO/Al2O3), (b) the reduced catalyst in H2 before the reforming test was run (Ni/Al2O3), (c) the used Ni/Al2O3 catalyst after CSR at 400 °C for 8 h, (d) the used Ni/Al2O3 catalyst after ECR at 400 °C and 3.2 A for 8 h, (e) the used Ni/Al2O3 catalyst after ECR at 450 °C and 3.2 A for 53 h, and (f) NiO/Al2O3 treated via a current passing through the catalyst under argon at 600 °C and 3.2 A for 8 h.

For example, with an increase of the current INi-Cr from 0 to 3.2 A, the Ibed values increased from 3.0 to 82.0 µA at 620 °C. According to the TOF results (please see Figure 8), the charge carrier through the bed should be the thermal electrons. Moreover, we calculated the number density of thermal electrons flowing from the wire on the basis of the Fick law and the theory of molecular thermodynamic movement and thermal diffusion:

( )

dne dx I I ) j e ) n eV ) se 2πxle

Fick law: je ) -D

( )

1 k3 2 D ) Vjλ ) 2 3 3πd p πme Vj )

8kT πme

(6) (7)

1/2

T 3/2

(8) (9)

where k is the Boltzmann constant, me is the electron mass, d is the radius of the gas molecule, D is the diffusion coefficient, l is the length of the Faraday plate, and x refers to the distance from the central axis to the Faraday plate. On the basis of the above formulas, first, we calculated the influence of the distance from the Ni-Cr wire surface to the Au-coated Faraday plate on the number density of thermal electrons flowing from the wire, as shown in Figure 10b; the number density of thermal electrons flowing from the wire significantly decreased with the distance (x) from the Ni-Cr wire surface to the Au-coated Faraday plate in logarithmic form. Second, we calculated the different number densities of thermal electrons corresponding to the different currents emitted from the Ni-Cr wire at different conditions (Figure 10c). With the same trend as Figure 10a, the number density of thermal electrons increased with increasing current of the Ni-Cr wire; e.g., with the current increasing from 0.9 to 3.2A, the number density of electrons flowing through the bed increased from 0.048 × 109 to 1.30 × 109 cm-3 at 620 °C at a position 1 mm away from the Ni-Cr wire surface. The diffusion velocitiy of thermal electrons at 620 °C was estimated to be 5.87 × 105 m/s. 3.3. Influence of the Current on the Catalyst Properties. The alterations of the catalyst after CSR and ECR of ethanol were investigated by XRD measurements. Figure 12 shows typical XRD spectra from six different catalyst samples, i.e., (a) the fresh sample (NiO/Al2O3), (b) the reduced catalyst in H2 before the reforming test was run (Ni/Al2O3), (c) the used Ni/Al2O3

BET

(m2/g) PV (cm3/g) PS (nm) dXRD (nm) dTEM (nm)

115.2 114.9 113.2 109.2 105.2 114.8

0.28 0.33 0.33 0.35 0.34 0.35

9.69 11.5 11.7 12.9 13.1 12.1

20.1 22.7 28.0 37.9 23.0

24.5 28.2 34.3 45.9 29.6

a Samples: no. 1, freshly prepared NiO/Al O ; no. 2, reduced catalyst in 2 3 H2 before the reforming test was run (Ni/Al2O3); no. 3, used Ni/Al2O3 catalyst after CSR at 400 °C for 8 h; no. 4, used Ni/Al2O3 catalyst after ECR at 400 °C and 3.2 A for 8 h; no. 5, used Ni/Al2O3 catalyst after ECR at 450 °C and 3.2 A for 53 h; no. 6, NiO/Al2O3 treated via a current passing through the catalyst under argon at 600 °C and 3.2 A for 8 h.

catalyst after CSR for 8 h, (d) the used Ni/Al2O3 catalyst after ECR for 8 h, (e) the used Ni/Al2O3 catalyst after ECR for 53 h, and (f) NiO/Al2O3 treated via a current passing through the catalyst under argon. For the fresh NiO/Al2O3 (Figure 12a), the diffraction structure was assigned to two components: the NiO phase at 2θ ) 37.5°, 43.3° and 62.9° and the Al2O3 phase at 2θ ) 25.4° and 67.3°. For samples b-e, almost all of the NiO phase was converted into the metallic Ni (i.e., Ni(111), Ni(200), and Ni(220) peaks at 44.7°, 52.1°, and 76.6°). Moreover, NiO reduction into Ni was also observed when a current passed through the NiO/Al2O3 catalyst under argon, which would be attributed to the reduction of the ionic state Ni2+ with the thermal electrons, i.e., Ni2+ + 2e- f Ni0. From the XRD diffraction patterns, the average crystalline sizes of Ni particles were estimated by the Scherrer equation. The results are summarized in Table 3. It can be seen that the Ni particle size slightly increased with increasing reforming time and/or current applied. Parts a-e of Figure 13 show typical TEM images from the different catalyst samples mentioned above. The Ni particles for the reduced catalyst in H2 and the used one after CSR for 8 h showed quite narrow size distributions of 18-30 and 20-34 nm (Figure 13a,b), respectively. However, the Ni particles in the used catalyst after ECR for 8 h exhibited a wide size distribution from 28 to 42 nm, whereas those after ECR for 53 h had a much wider distribution of 36-56 nm. We measured the size of more than 100 particles of Ni for each catalyst and plotted the particle size distributions in various conditions, from which the average size of the Ni particles can be estimated (Figure 13f).The results are summarized in Table 3. The Ni particle size slightly increased with increasing reforming time and/or current applied, which agreed with the XRD results. The BET surface areas and pore volume were also measured for the six samples mentioned above (Table 3). In comparison with the fresh catalyst, the BET surface areas and pore volume from the used ones after CSR or ECR slightly decreased, accompanied by an increase of the pore diameter and the size of the particles. After a current passed through the catalyst under argon, a small decrease of the BET surface area together with an increase of the particle size was also observed. Generally, a decrease of the BET surface area or a increase of the particle size will lead to a decrease of the catalyst activity. However, it was observed that the current applied in ECR obviously promoted the production of hydrogen from the ethanol. The above results indicate that the alterations of the catalyst properties induced by the current would have a minor influence on the production of hydrogen from the electrochemical catalytic reforming of ethanol. 3.4. Energy Efficiency and Hydrogen Cost. The energy efficiency and hydrogen cost for production of hydrogen from ethanol were estimated in both the CSR and ECR processes.



Figure 13. Typical TEM images of the catalyst under different conditions including (a) the reduced catalyst in H2 before the reforming test was run (Ni/Al2O3), (b) the used Ni/Al2O3 catalyst after CSR at 400 °C for 8 h, (c) the used Ni/Al2O3 catalyst after ECR at 400 °C and 3.2 A for 8 h, (d) the used Ni/Al2O3 catalyst after ECR at 450 °C and 3.2 A for 53 h, and (e) NiO/Al2O3 treated via a current passing through the catalyst under argon at 600 °C and 3.2 A for 8 h. (f) Statistical histogram of the Ni particle size under the different conditions of (a)-(e) mentioned above.

According to refs 45 and 46, we defined the energy efficiency and hydrogen cost by the following formula: energy efficiency: η (%) )

∑E ∑E

output

× 100

(10)

W wH2

(11)

input

hydrogen cost: C((kW h)/kg of H2) )

where, ∑Einput and W stand for all energy consumed per hour, ∑Eoutput stands for all the energy covered in the products produced per hour, and wH2 is the hydrogen mass production weight per hour. We have measured the total lowest energy consumed for both CSR and ECR under medium and higher hydrogen yields. For ECR, the consumed energy included the energy for the inner and outside heaters and that for producing the steam from the steam generator. For CSR, the consumed (45) Sekine, Y.; Urasaki, K.; Asai, S.; Matsukata, M.; Kikuchi, E.; Kado, S. A novel method for hydrogen production from liquid ethanol/water at room temperature. Chem. Commun. 2005, 1, 78–79. (46) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Dissociation of vibrationally excited methane on Ni catalyst, Part 1. Application to methane steam reforming. Catal. Today 2004, 89, 57–65.

energy included the energy for the outside heaters and that for producing the steam from the steam generator. The amount of hydrogen formed from the reforming of ethanol per hour was derived from the dry gaseous volume collected and the content of hydrogen in the mixtures. Table 4 shows the results of the energy efficiency and cost estimated from the selected technologies to produce hydrogen. The energy efficiency estimated from ECR ranged from 80% to 86%; the corresponding cost ranged from 49 to 45 (kW h)/kg of H2. The energy efficiency estimated from CSR ranged from 58% to 72%; the corresponding cost ranged from 67 to 54 (kW h)/kg of H2. As can be seen, the energy efficiency in the ECR reforming process was higher than that in the CSR process. This may be attributed to the electrified Ni-Cr wire installed in the reactor, playing a dual role both in heating the catalyst and in providing the thermal electrons onto the catalyst at the same time. 3.5. Explanation for the Effects of the Current on the Reforming and Decomposition of Ethanol. This study shows that the performance of the reforming of ethanol was very prominently enhanced by the current through the catalyst. In particular, the hydrogen yield and its selectivity significantly increased with increasing current in the electrochemical catalytic reforming of ethanol (ECR mode), as shown in Figures 2 and 3. It was also observed that the current applied promoted the decomposition of ethanol (Figures 6 and 7). It was noticed that the ECR mode (i.e., inner heating) and CSR mode (i.e., outside heating) have different temperature distributions in the catalyst bed (Table 1). The maximal temperature gradients in ECR were higher than those in CSR. Even though the temperature in the center of the catalyst bed is almost close to the average value in the catalyst bed in our investigated range (400-600 °C), the local temperature on the surface of the electrified Ni-Cr wire or near the wire, generally, was obviously higher than the averaged temperature. Accordingly, the activity of the catalyst reforming near the electrified Ni-Cr wire should be significantly higher than that at other positions in the bed, partly leading to the enhancement of the apparent overlap reforming effect. On the other hand, the enhancement of the decomposition and reforming in the ECR process may be partly caused by the thermal electrons. The presence of the thermal electrons both on the electrified Ni-Cr wire and on the electrified catalyst surface was observed by the anionic TOF measurements (Figure 8b). It is well-known that when an electrified metal or a metal oxide is heated, electrons can boil off its surface, leading to thermal emission of electrons from the surface (i.e., thermal electron emission).35 It has been reported that thermal electrons on a metal or a metal oxide surface play an important role in the reduction process (e.g., O2 + 4e- f 2O2-).37,39 The promoting effect of the current on the EtOH decomposition and its reforming and the catalyst reduction is supported by the following observations. First, the dissociation of water into OHand the dissociation of ethanol into the anionic hydrocarbon fragments (CHx-) were observed from the electrified catalyst surface (Figure 8d,e). The anionic fragments of OH- would form by the dissociation of water with the thermal electrons on the catalyst surface, i.e. e-(s) + H2O(s) f OH-(s) + H(s)

(12)

Here s represents the surface. The anionic hydrocarbon fragments of CHx- would originate from the dissociation of ethanol through the following process: e-(s) + C2H5OH(s) f CxHy-(s) + ...

(13)

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Yuan et al.

Table 4. Energy Efficiency (ηenergy) and Hydrogen Cost (C) Estimated from the Selected Technologies To Produce Hydrogen from Ethanol reaction type

power (W)

timereaction (h)

Econsumed (kJ)

YH2 (mol of H2/h)

ηenergy (%)

C ((kW h)/kg of H2)

ECR-1a ECR-2b CSR-1c CSR-2d

82.9 126.2 99.2 138.8

2.0 2.0 2.0 2.0

0.60 0.91 0.71 1.00

0.84 1.38 0.73 1.27

80 86 58 72

49 45 67 54

a Electrochemical catalytic reforming of ethanol under the reforming conditions T ) 460 °C, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm. b Electrochemical catalytic reforming of ethanol under the reforming conditions T ) 566 °C, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm. c Common steam reforming of ethanol under the reforming conditions T ) 512 °C, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm. d Common steam reforming of ethanol under the reforming conditions T ) 610 °C, water/ethanol ) 2/1 (volume ratio), LHSV ) 3.8 h-1, and P ) 1 atm.

The small molecular dissociation of O2, H2, F2, H2O, and C6H6 via the thermal electrons has also been observed on the Ca-Al-O oxide surface.31-34 On the basis of the homogeneous experiments over the quartz bed, the selectivity toward H2, CO, and CH4 increased with increasing current, accompanied by a decrease of the selectivity of C2H4, CH3CHO, and H2O (Figure 7). The synchronous increase of the selectivity of H2, CO, and CH4 may imply that the current promoted the decomposition of ethanol via the thermal electrons, i.e. -

C2H5OH + e f CH4 + CO + H2

(14)

The decrease of CH3CHO with increasing current in the homogeneous decomposition of ethanol may be attributed to the ethanol dehydrogenation to acetaldehyde (CH3CHO), followed by the decomposition of acetaldehyde into CH4 and CO via the following route: C2H5OH + e- f CH3CHO + H2 -

CH3CHO + e f CH4 + CO

(15) (16)

The decrease of C2H4 with increasing current would be caused by ethanol dehydration and would further decompose into small hydrocarbon fragments: C2H5OH + e- f C2H4 + H2O -

C2H4 + e f CxHy + ...

(17) (18)

Accordingly, the thermal electrons may play a role in promoting the ethanol decomposition and reforming, leading to an increase of the hydrogen yield and selectivity in the ECR process. Partial or complete reduction from the oxidation state to the metallic phase in the NiO/Al2O3 catalyst was observed when a current passed through the catalyst under argon or helium ambience (Figure 12f). This would be attributed to the reaction of Ni2+ with the thermal electrons (i.e., Ni2+ + 2e- fNi0).

Moreover, it was found that the energy efficiency in the ECR reforming process was higher than that in the CSR process (Table 4), which may be explained by multiple effects including the local temperature effect, the effect of thermal electrons, and the effect of less heat loss in the inner heating mode. Further work is still required to clearly understand the reforming mechanism in the ECR process. 4. Conclusions This work presents an efficient method for hydrogen production via the electrochemical catalytic reforming of ethanol over the Ni/Al2O3 catalyst, giving good reforming performance with high conversion of ethanol, selectivity of hydrogen, and yield of hydrogen. The selectivity of hydrogen and yield of hydrogen were about 72.2% and 3.45 mol of H2/mol of EtOH at 400 °C and 3.2 A, reaching about 95.5% and 5.57 mol of H2/mol of EtOH at 600 °C and 3.2 A, respectively. The energy efficiency in the present ECR process was significantly higher than that in the common steam reforming process. It was observed that the current applied to the catalyst significantly promoted the ethanol decomposition and reforming and the reduction of NiO in the catalyst. The local temperature and thermal electrons from the Ni-Cr wire may play a promoting role in increasing the ethanol conversion and the hydrogen yield in the ECR process. Acknowledgment. This work was supported by the National Basic Research Program of China (Grant 2007CB210206), the National High Tech Research and Development Program (Grant 2006AA05Z118), the General Program of the National Natural Science Foundation of China (Grant 50772107), and the Green Agriculture Scientific Research Demonstration Program (Grant 2007-15). EF801131A

Hydrogen Production from the Current-Enhanced Reforming and

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