Ni ratio

Applied Catalysis B: Environmental 106 (2011) 639–649

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

The ethanol steam reforming over Cu-Ni/SiO2 catalysts: Effect of Cu/Ni ratio Li-Chung Chen, Shawn D. Lin ∗ Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 26 August 2010 Received in revised form 19 May 2011 Accepted 14 June 2011 Available online 24 June 2011 Keywords: Ethanol Steam reforming Cu-Ni Impregnation Reaction pathway

a b s t r a c t The operating conditions of SRE (steam reforming of ethanol) reaction were evaluated by thermodynamics, in considering of the application requirements in hydrogen concentration and energy consumption. Under the select operating conditions, 5% CuNi/SiO2 catalysts with different Cu/Ni ratios prepared through incipient-wetness co-impregnation were tested for SRE. The catalysts were reduced with NaBH4 at room temperature, and again reduced by H2 at 623 K prior to temperature-programmed SRE testing to remove surface oxygen. The SRE reaction products indicate a reaction scheme involving ethanol dehydrogenation to acetaldehyde, wherein acetaldehyde steam reforming and acetaldehyde decomposition compete, and with subsequent CO conversion to CO2 via water gas shift reaction. The catalysts with Cu/Ni ≥ 1 showed higher ethanol conversion, higher acetaldehyde conversion, higher selectivity of acetaldehyde steam reforming, and lower coking at temperatures below 673 K than the Ni-rich catalysts. Analyses by XRD, XPS, and EXAFS indicate that the Cu-rich catalysts had formed an alloy structure with Ni-enriched surface. The catalyst with Cu/Ni = 1 showed the highest performance in ethanol conversion, acetaldehyde conversion, the selectivity of acetaldehyde steam reforming, and the stability against particle sintering. © 2011 Elsevier B.V. All rights reserved.

1. Introduction High efficiency production of hydrogen, with a low environmental impact is the Holy Grail of the growing fuel cell industry [1]. Bio-ethanol (ethanol produced from biomass) provides a renewable carbon cycle when it is used as the feedstock for hydrogen production [2]. The steam reforming of ethanol (SRE) reaction (C2 H5 OH + 3H2 O → 6H2 + 2CO2 ) has become an attractive alternative [3–7]. This multi-molecular reaction process involves several reaction pathways giving off hydrogen and numerous byproducts such as carbon oxides, methane, ethylene, acetaldehyde, coke, etc. The efficiency of hydrogen production from SRE depends on the control of the formation of these products. Therefore, a properly designed catalysts for SRE is important, to achieve high efficiency and selectivity for hydrogen production. Both noble metals (e.g., Pd, Pt, Ru, Rh) and non-noble metals (e.g., Co, Ni, Cu) have been studied for SRE [8–15]. Most studies carried out SRE at temperatures higher than 773 K. High temperature operations raise an obvious concern over the energy efficiency of the SRE, as well as the overall energy cycle. As a result, it is of great interest to lower the SRE reaction temperature. Copper catalysts can facilitate methanol steam reforming at temperatures below 573 K; however, Cu catalysts are not effective for SRE due to a lack of C–C bond breaking capability. Aside from noble metals, Ni is one of the best choices with such capability. For this reason, adding a

∗ Corresponding author. Tel.: +886 2 27376984; fax: +886 2 27376644. E-mail address: [email protected] (S.D. Lin). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.06.028

second metal such as Ni to Cu catalysts to assist in breaking the C–C bond of ethanol can be an effective strategy to lower the reaction temperatures. Although Cu-Ni bimetal catalysts for SRE have been previously reported [16–22], important catalyst design parameters such as the Cu/Ni ratio and the type of support are still not understood. Calles and co-workers [17] examined the SRE at 873 K with CuNi catalysts using a number of various supports; Cu-Ni/SBA-15 was found to be the most active; while Cu-Ni/Al2 O3 resulted in increased coking. These observations were attributed to the metal particle size and the acidity of Al2 O3 , respectively. Most of the previously reported Cu-Ni catalysts for SRE contained more Ni than Cu [16,17,21–24]. Adding Cu to Ni catalysts was found to improve the hydrogen selectivity and resistance to coke formation [17]. Marino et al. [18–20] examined the effect of adding Ni to Cu/K/␥-Al2 O3 on the SRE at 573 K; they suggested that Cu was the active agent and Ni promoted the rupturing of C–C bonds and acted as a hydrogenating/dehydrogenating site. Although optimal composition was proposed from each of these studies of bimetallic Cu-Ni catalysts, no systematic studies on the effect of Cu/Ni ratio on the Cu-Ni bimetal SRE catalysts have been found. In fact, comparison of already published results is difficult, due to the fact that steam/ethanol ratios and reaction temperatures varied considerably from one study to another. In this work, thermodynamic analysis was applied to identify the operation window for SRE whereupon we examined the effect of Cu/Ni ratio on SRE by using a relatively inert support, SiO2 . According to changes in the product distribution of SRE with reaction temperature, we examined the effect of Cu/Ni ratio and proposed the reaction pathways in consistent with experi-

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mental results. We characterized the morphology of the prepared CuNi/SiO2 by EXAFS, XPS, TPR, and XRD, and discussed the nanoscale morphology of the most active SRE catalyst.

2. Experimental Cu-Ni/SiO2 catalysts of 5 wt% metal loading were prepared by impregnating SiO2 support (Davison 952, 300 m2 /g, used after calcinations at 823 K) with a metal (Cu and Ni) nitrate (Aldrich, 98%) aqueous solution, to incipient wetness, and dried at 298 K under low vacuum. Thereafter, the catalyst was dispersed in H2 O, and stirred for 10 min. To the mixture, a 5% NaBH4 (diluted from Aldrich, 98.5%) solution was added drop-by-drop at room temperature until NaBH4 /metal = 5 (molar ratio). After additional stirring for 30 min, the solid was recovered by centrifugation and washed extensively with H2 O. The filtrate after NaBH4 reduction was examined by ICPMS and no loss of Cu or Ni was evident. Accordingly, the nominal loadings of Cu and Ni metals of the prepared CuNi/SiO2 catalyst with a Cu/Ni molar ratio of 1/0, 3/1, 2/1, 1/1, 1/2, 1/3, 0/1 are 5/0, 3.75/1.25, 3.33/1.67, 2.50/2.50, 1.67/3.33, 1.25/3.75, 0/5 wt%, respectively. XRD analysis was performed using the commercial instrument (Shimadzu XRD-6000, with Cu K␣ radiation). All spectra were taken at 298 K and a scan rate of 0.5◦ /min from 30◦ to 65◦ . TPR (Temperature-programmed reduction) analysis was performed at a heating rate of 5 K/min using 10% hydrogen in nitrogen and a TCD detector. TEM images were obtained using JOEL JEM-2100 instrument with an operating voltage of 80–200 kV. Elemental composition of the filtrate after NaBH4 reduction was analyzed by ICP-AES (Perkin-Elmer JY 2000-2) and the results were used to calculate the actual loading of the catalysts. XPS measurements were performed with a Kratos Axis Ultra DLD equipped with an Al K␣ source in a chamber with background pressure of 1 × 10−9 Torr. The measured spectra were deconvoluted after background subtraction by a least-squares fitting to Gaussian–Lorentzian functions using the software Peak Fit. EXAFS experiments were performed at the BL17C beamline of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan, with storage ring energy of 1.5 GeV and beam current between 150 and 200 mA. The Ni K-edge and the Cu K-edge absorbance were measured sequentially in transmission mode at room temperature. In situ analysis was carried out using in a stainless steel cell with Krypton film cap in two sides to prevent the beam path being exposed to air. EXAFS data analysis was carried out using the Artemis analysis programs with k2 -weighting of data in k-space ranging from 3 to 10.5 A˚ −1 and from 3.0 to 13.5 A˚ −1 , for the Ni K-edge and the Cu K-edge, respectively. A nonlinear leastsquares curve fitting was performed in r-space ranging from 1.0 to 3.0 A˚ for Cu and from 1.0 to 3.8 A˚ for Ni, respectively. Reference files were obtained from theoretical calculations using Feff 7.02 code for atom pairs such as Cu-O, Cu-Cu, Ni-O, Ni-Ni and Ni-Si. References of second-shell atom pairs were also made based on both theoretical and hypothetical bonding structures. Reference materials, Cu foil, Ni foil, CuO and NiO were checked to confirm the consistency of the calculated references. Metal dispersion was analyzed by N2 O adsorption and calculated by assuming an Oad /Ms ratio of 1/2, where Ms represents surface atom of either Cu or Ni. The catalysts were pretreated in the same way as the catalyst used for SRE reaction tests, cooled to 298 K under He, and then subjected to N2 O flow (50 mL/min) at 298 K for 30 min. Thereafter, the catalyst bed was purged with He and the Oad from N2 O adsorption was quantitatively analyzed by temperature-programmed reduction (TPR) using 10% H2 in N2 . Reaction tests were performed in a quartz-tube packed-bed reactor at atmospheric pressure. In each experiment, 0.1 g of the

catalyst was sandwiched between quartz wool, installed in a homemade system, and reduced with 20 vol% H2 /N2 (50 mL/min) at 623 K for 1 h. This in-line reduction was performed to reduce the surface oxide that might be formed due to air exposure. The ethanol aqueous solution with H2 O/ethanol = 6 (mol) was fed by a syringe pump and a He vapor flew into the reactor at a EtOH partial pressure of 5.67 kPa (42.5 Torr) and a WHSV (weight hourly space velocity, g EtOH/g catalyst/h) of 2 h−1 . Both He and H2 were of 99.995% purity and were flown through dryer and Oxytrap columns prior to entering the system. The WHSV was chosen based on preliminary tests to assure the absence of artifacts such as mass and heat transfer limitations. The reaction was carried out in a temperature-programmed mode at 1 K/min from 523 to 773 K to decrease the influence of coking on the reported catalytic performance. The reactor effluent was analyzed using a GC (Young Lin, YL6100 GC, with Pulsed Discharge Helium Ionization Detector) in line via a six-port valve (Valco). The mass balance of the effluents was typically within ±10% of the feed for most catalysts at temperatures below 673 K. The carbon balance from GC analysis was used as an index of coke formation.

3. Results 3.1. Thermodynamic analysis of SRE Both the effluent hydrogen concentration and the energy consumption per mole of hydrogen produced become important when the produced hydrogen is used for energy supplies. Thermodynamic analysis is able to provide guidelines for the selection of operational parameters, in order to meet the requirements of the application. Fig. 1(a) shows the effect of H2 O/EtOH (S/E) ratio and reaction temperature on the equilibrium ethanol conversion. With a stoichiometric feed, i.e., S/E = 3, the reaction reached nearly 100% EtOH conversion at temperatures higher than 850 K. Excess H2 O improved the equilibrium conversion and we were able to obtain an equilibrium conversion higher than 95% with S/E = 5 at 550 K. Results shown in Fig. 1(a) were evaluated for the SRE operating at atmospheric pressure. It can be expected from Le Chatelier’s principle that the equilibrium conversion will decrease as the reaction pressure increases. As a consequence, high pressure SRE is undesirable. Fig. 1(b) shows the calculated hydrogen concentration in the reactor effluent at equilibrium conversion conditions when no diluent (e.g., carrier gas) is used. Obviously a higher S/E ratio would result in a lower effluent hydrogen concentration because the excess H2 O acts as a diluent. In addition, the use of any diluents (e.g., carrier gas) also results in lower hydrogen concentration in the effluent. However, if excess water is completely removed from effluent, the effluent hydrogen concentration would increase with increasing S/E ratio. Fig. 1(b) shows both the situations when excess water in effluent is removed (dash lines) and when not removed (solid lines). It shows clearly the diluent effect of excess water on effluent hydrogen concentration. For applications such as hydrogen for fuel cells, the effluent hydrogen should be higher than 50%. The energy consumption, per mole of hydrogen produced, was calculated on the basis that the liquid feed was at room temperature and atmospheric pressure and that the effluent was at the reaction temperature. From the feed, the equilibrium conversion, and the resulted effluent compositions, the energy consumption per mole of hydrogen produced was calculated by: E = =

(HLiq.

Feed,298→T

T + HReaction × Xconversion )

nH2 ,Effluent 298 × Xconversion + HEffluent,298→T ) (HReaction

nH2 ,Effluent

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Fig. 2. Thermodynamic evaluation of the energy consumption per mole of H2 produced by SRE, where the feed was assumed liquid at 298 K, 1 atm and the effluent was at the reaction temperature, 1 atm.

Fig. 1. Thermodynamic evaluation of (a) the equilibrium ethanol conversion and (b) the effluent hydrogen concentration as a function of SRE reaction temperature and H2 O/EtOH ratio. The reaction was assumed at atmospheric pressure and without carrier gas.

This includes the heat of vaporization of liquid feed and vaporphase reaction enthalpy. Fig. 2 shows that the energy consumption per mole of hydrogen produced would increase with an increase in either the reaction temperature or the S/E ratio. Figs. 1 and 2 indicate that the use of high S/E feed in SRE is undesirable not only for the low hydrogen concentration but also for the high energy consumption. The catalyst for SRE had better targeting operations at lower temperatures and lower S/E ratios. In this study, SRE tests were carried out at atmospheric pressure and S/E = 6; the theoretical EtOH conversion could exceed 99% at 550 K with an effluent hydrogen concentration of 54.5%. A small amount of carrier gas was used in this study to maintain flow stability, which reduces the maximum effluent hydrogen concentration to 37.7%. According to Figs. 1 and 2, the energy consumption per mol of H2 produced is higher using a water/ethanol mol ratio of 6 at 550 K than using a S/E ratio of 3 at 850 K, where ethanol conversion is 100% and H2 concentration is considerably higher. However, high operating temperatures may produce undesired by-products such as coke. 3.2. Catalyst characterization The 5% Cux Niy /SiO2 catalysts were prepared by co-impregnation and then reduced by aqueous NaBH4 solution. The as-prepared

Fig. 3. XANES of the air-exposed CuNi/SiO2 catalysts in (a) Cu K-edge and (b) Ni K-edge. All spectra were recorded at 298 K.

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Fig. 4. EXAFS analysis of the air-exposed CuNi/SiO2 catalysts in r-space of (a) Cu K-edge and (b) Ni K-edge. Lines represent experimental data and symbols represent fittings using models presented in Table 1. All spectra were recorded at 298 K.

catalysts were exposed to air and XRD analysis found no obvious diffraction peak related to Cu or Ni. This indicates that all of the prepared catalysts were in very small crystalline phase or amorphous phase. XANES of the air-exposed samples are shown in Fig. 3. Fig. 3(a) shows that all catalysts, except the Cu1 Ni1 /SiO2 catalyst, had characteristic pre-edge absorption of Cu metal. However, the white line region indicated that all catalysts contained oxidized Cu. The most likely morphology was that the NaBH4 -reduced nanoparticles were exhibiting an oxidic skin due to the exposure to air. From the intensity of the white line, the extent of Cu oxidation increased from Cu-only to Cu/Ni = 1/1, and then decreased with a further increase in Ni content. The Ni-edge XANES in Fig. 3(b) show a similar trend in the changes to the extent of Ni oxidation with an increase in Ni content. These results suggest that Cu1 Ni1 /SiO2 had the highest affinity for oxygen. EXAFS analyses of the air-exposed samples are shown in Fig. 4 and Table 1. All catalysts contained Cu–O bond at distance of approximately 1.9 A˚ and metallic Cu-Cu coordination of approx˚ except the Ni1 Cu1 /SiO2 catalyst, which contained imately 2.56 A, ˚ attributable to CuO. As to the Ni-edge, Cu-O and Cu-Cu at 2.99 A, ˚ The all catalysts contained Ni-O near 2.05 A˚ and Ni-Ni near 3.10 A. Ni-Ni coordination near 3.10 A˚ was reported in Ni(OH)2 [25,26] and Ni silicate [27]. The Ni-rich catalysts, i.e., Cu1 Ni2 , Cu1 Ni3 , and

Ni-only, contained an additional metallic Ni-Ni coordination (at ˚ The Cu-rich catalysts (with Cu/Ni > 1) conapproximately 2.56 A). tained oxidized Ni, oxidized Cu and metallic Cu; this suggests that Ni may be distributed more toward the surface according to the model of the presence of oxidic skin over bimetal nanoparticles. The Cu1 Ni1 /SiO2 was the only catalyst containing mainly oxidized Cu and oxidized Ni; this might be due to relatively small particles with high oxygen affinity. Cu2 Ni1 /SiO2 was analyzed additionally after H2 reduction at 623 K with an environment-controlled sample holder. Fitting results indicated that Cu could be completely reduced but Ni was only partially reduced after H2 reduction at 623 K. The particle size estimated from the metallic Cu-Cu coordination number was approximately 3 nm [28], which was consistent with the particle size observed in TEM (Fig. 5). However, it is not possible to determine from EXAFS if Cu and Ni formed an alloy due to the difficulty in differentiating between Cu backscatter and Ni backscatter. TPR (temperature-programmed reduction) analysis of the airexposed catalysts is shown in Fig. 6. The hydrogen consumption was typically less than 10% of the expected consumption from the CuO and NiO loadings. This indicated that the catalysts were only partly oxidized by air-exposure, which was consistent with the proposed model of the presence of oxidic skin over bimetal nanoparticles. The Ni-only catalyst showed two obvious H2 consumption features at around 673 K and 923 K respectively. The former was consistent with the reduction of NiO on SiO2 support [29], while the latter indicated the presence of a Ni-support interaction [30,31]. The presence of a Ni-SiO2 interaction was also observed in the XPS analysis of the 623K-reduced catalysts, again after airexposure, as shown in Fig. 7. The Cu 2p peaks fit well with the combination of Cu(0) and Cu(II) peaks. However, the Ni 2p peaks contained a main peak attributable to NiSiO3 [31,32] and a small fraction of Ni(II). The significant contribution of NiSiO3 suggests that Ni may distribute more toward the perimeter of the particles in contact with the SiO2 . Table 2 compares the XPS-resolved surface Cu/Ni ratio, the bulk Cu/Ni ratio from ICP analysis and metal dispersion analysis based on N2 O adsorption. The surface Cu/Ni ratio from XPS is different from that of the bulk. The CuNi/SiO2 catalysts with Cu/Ni ≥ 1 showed a lower surface Cu/Ni ratio than the bulk Cu/Ni ratio. This indicated that presence of Ni-enrich surface, in consistent with the observation from EXAFS analysis. Fig. 8 shows the powder XRD patterns of the 623 K-reduced CuNi/SiO2 catalysts before and after SRE reaction; all samples were exposed to air during measurement. The results show that the diffraction peaks include metallic Cu and metallic Ni but no oxides. This suggests that the metal particles had been reduced and that the oxide skin might have been too thin to be detected in XRD. When the Cu/Ni ratio was decreased, the Cu(1 1 1) peak position shifted to higher 2. This indicated the formation of alloy [33,34]. When the Ni/Cu ratio exceeded 1, a separate metallic Ni(1 1 1) peak could be seen, in consistent with the observation of additional metallic Ni-Ni coordination in EXAFS results. This indicated phase separation and the catalysts contained both Ni (or Ni-rich) and Cu (or Cu-rich) particles. The Cu1 Ni1 /SiO2 catalyst showed a very broad Cu(1 1 1) diffraction peak and no CuO or Ni diffraction peaks, indicating the presence of very small particles. After SRE reaction, the catalyst with Cu/Ni = 1 showed no obvious change in peak broadening; whereas, the other catalysts showed an increase in the size of crystals. This indicated that the alloy particles of Cu1 Ni1 /SiO2 were stable. The crystal sizes calculated from both Cu(1 1 1) and Ni(1 1 1) diffraction are listed in Table 3. The calculated crystal sizes show a trend in consistent with the metal dispersion calculated from N2 O adsorption (Table 2). The Cu1 Ni1 /SiO2 catalyst had the smallest crystal size of approximately 3.5 nm and the highest metal dispersion.

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Table 1 Results of EXAFS model fitting of the air-exposed CuNi/SiO2 catalysts. Catalyst

Shell

˚ R (A)

Coordination number

Cu

Cu-O Cu-Cu

1.934 ± 0.014 2.564 ± 0.009

1.9 ± 0.5 4.7 ± 0.6

Cu3 Ni1

Cu-O Cu-Cu Ni-O Ni-Ni

1.951 2.569 2.053 3.102

± ± ± ±

0.018 0.014 0.020 0.037

2.5 3.7 3.0 5.9

± ± ± ±

0.7 0.7 0.9 4.3

Cu-O Cu-Cu Ni-O Ni-Ni

1.968 2.586 2.057 3.103

± ± ± ±

0.023 0.021 0.017 0.030

2.6 2.3 2.2 5.9

± ± ± ±

Cu-O Cu-Cu Ni-O Ni-Ni

1.946 2.987 2.068 3.105

± ± ± ±

0.005 0.009 0.007 0.010

3.6 3.0 5.5 5.1

Cu-O Cu-Cu Ni-O Ni-Ni Ni-Ni

1.914 2.541 2.071 2.567 3.151

± ± ± ± ±

0.027 0.013 0.054 0.055 0.056

Cu-O Cu-Cu Ni-O Ni-Ni Ni-Ni

1.937 2.546 2.050 2.558 3.139

± ± ± ± ±

Cu-Cu Ni-O Ni-Ni Ni-Ni

2.536 2.032 2.520 3.093

± ± ± ±

Cu2 Ni1

Cu1 Ni1

Cu1 Ni2

Cu1 Ni3

Cu2 Ni1 in situ

 2 × 103

Eo

r-Factor

5.7 ± 2.9 8.8 ± 1.0

−10.0 ± 1.5

0.003

7.4 9.9 4.0 20.3

± ± ± ±

3.1 1.7 3.5 9.6

−8.8 ± 2.3

0.008

−6.1 ± 2.7

0.036

0.7 0.9 0.8 3.4

6.6 10.1 4.4 19.6

± ± ± ±

3.0 3.2 2.9 7.6

−5.7 ± 3.4

0.019

−6.3 ± 2.2

0.025

± ± ± ±

0.2 0.7 0.6 1.0

5.8 13.1 7.2 12.4

± ± ± ±

0.6 2.0 1.3 2.1

−10.3 ± 0.7

0.002

−7.4 ± 0.9

0.003

1.0 6.4 2.7 3.7 0.8

± ± ± ± ±

0.7 1.3 1.3 6.5 1.3

2.3 10.2 5.5 15.9 3.3

± ± ± ± ±

5.3 1.6 4.7 18.6 9.9

−12.9 ± 2.3

0.005

−2.8 ± 5.8

0.008

0.027 0.011 0.091 0.084 0.098

1.5 6.3 2.1 3.1 0.3

± ± ± ± ±

0.9 0.9 1.8 6.3 1.2

7.8 10.7 5.9 12.7 −1.3

± ± ± ± ±

6.8 1.2 8.8 21.0 22.6

−11.8 ± 1.9

0.003

−3.9 ± 9.7

0.012

0.008 0.042 0.025 0.056

8.0 3.5 3.2 1.5

± ± ± ±

0.9 1.9 1.9 2.5

9.8 10.3 7.8 10.4

± ± ± ±

0.9 7.6 4.5 15.0

−12.7 ± 1.3 −7.3 ± 4.1

0.002 0.010

3.3. SRE analysis of CuNi/SiO2 The SRE reaction conversion changed with respect to the Cu/Ni ratio of CuNi/SiO2 catalysts during temperature-programmed SRE, as shown in Fig. 9(a). The Cu-only catalyst had nearly 100% ethanol conversion at approximately 663 K; whereas, the Ni-only catalyst needed 733 K to approach complete conversion. Obviously, Cu was more active than Ni at lower temperatures. Besides H2 , the Cu-only catalyst released mainly acetaldehyde and a small amount of CO2 at 663 K when the ethanol conversion was close to 100%. In comparison, the Ni-only catalyst gave acetaldehyde at temperatures below 593 K but the product gradually shifted to CO, CH4 and CO2 at higher temperatures. The C-balance between the feed and the effluent was

nearly 100% fulfilled at lower temperatures, but not at higher reaction temperatures. The missing carbon from the C-balance analysis was used to evaluate the extent of coking. The Cu-only catalyst showed no coking up to 663 K, but the Ni-only catalyst started to show significant coking at 663 K. From Fig. 9(a), the CuNi/SiO2 with Cu/Ni ≥ 1 had a higher ethanol conversion than the Cu-only catalyst at temperatures below 633 K. This suggests a synergistic effect between Cu and Ni, and the difference in metal dispersion (Table 2) cannot account for the different activity. The Cu1 Ni1 /SiO2 catalyst showed the highest activity. Alloy formation may contribute to its high activity. More Ni, i.e., at Cu/Ni < 1, resulted in lower metal dispersion, phase separation, and lower activity at lower temperatures.

Fig. 5. (a) TEM image and (b) the particle size distribution of the air-exposed Cu1 Ni1 /SiO2 catalyst.

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Table 2 XPS analysis, the bulk composition from ICP and the metal dispersion by N2 O adsorption of CuNi/SiO2 catalysts. Catalyst

Binding energy (eV) 0

+1

Cu + Cu Cu Cu3 Ni1 Cu2 Ni1 Cu1 Ni1 Cu1 Ni2 Cu1 Ni3 Ni a b

CuO

932.9 (71.0) 932.6 (86.3) 932.9 (87.3) 933.1 (81.6) 933.0 (93.2) 933.0 (100) –

Db (%)

Cu/Ni a

934.9 (29.0) 934.5 (13.7) 935.0 (12.7) 935.2 (18.4) 934.7 (6.8) – –

Ni 2p

XPS

ICP

– 856.6 856.6 856.8 856.5 856.6 856.5

– 2.2 1.4 0.8 0.5 0.4 –

– 3.0 2.0 1.0 0.5 0.33 –

15.6 16.0 15.3 20.2 7.8 7.4 1.9

Numbers in parentheses represent the relative contribution obtained from peak deconvolution. Dispersion, analyzed by the N2 O adsorption-decomposition method.

Table 3 XRD analysis of the 623K-reduced CuNi/SiO2 catalystsa before and after SRE tests. Catalyst

Before reaction

After reaction

Cu(1 1 1) 2 Cu Cu3 Ni1 Cu2 Ni1 Cu1 Ni1 Cu1 Ni2 Cu1 Ni3 Ni a

43.30 43.47 43.50 43.55 43.47 43.36 –

Ni(1 1 1) d (nm) 16.1 6.0 4.4 3.5 6.9 5.7 –

2 – – – – 44.28 44.33 –

Cu(1 1 1) d (nm) – – – – 6.6 6.2 –

2 43.30 43.44 43.45 43.42 43.59 43.28 –

Ni(1 1 1) d (nm) 16.1 11.9 7.1 3.5 8.8 7.4 –

2

d (nm)

– – – – 44.20 44.64 44.53

– – – – 9.7 7.2 8.1

All samples were air-exposed prior to measurement. The particle sizes were calculated based on the Scherrer equation and the broadening of select peaks.

The product selectivity of SRE over Cu1 Ni1 /SiO2 and that over Cu1 Ni3 /SiO2 are shown in Fig. 9(b); all other catalysts gave similar products under the test conditions of this study. Acetaldehyde was the main product at lower temperatures, evolving to CO and CH4 , then CO2 formed at higher temperatures. The reaction pathway of SRE has been discussed extensively [5–7,11,33–38]. It is generally agreed that ethanol would first undertake either a dehydrogenation path leading to acetaldehyde or a dehydration path leading to ethylene. The formation of ethylene or acetone was not detected in this study, although they were often reported in SRE [34,39,40]. The dehydration pathway did not occurred over the CuNi/SiO2 catalysts in this study. From Fig. 9(b) and the previously reported SRE mechanism [6,36–38], the SRE reaction pathway over our CuNi/SiO2 catalysts could be proposed as shown in Scheme 1.

Acetaldehyde from the ethanol dehydrogenation (ED) was the only initial product. This decreased with an increase in temperature, while at the same time CO and CH4 were formed although not at 1:1 ratio as expected from the acetaldehyde decomposition (AD, CH3 CHO → CO + CH4 ). Therefore, the acetaldehyde steam reforming (ASR, CH3 CHO + H2 O → 2CO + 3H2 ) was proposed to compete with AD. The ASR resulted in a higher H2 yield and more CO than CH4 , which was consistent with observations during the experiment. Thereafter, the WGS (water gas shift, CO + H2 O → CO2 + H2 ) reaction was considered as the main reaction pathway for the formation of CO2 from CO. Based on Scheme 1, the fractional conversion of acetaldehyde (per mole of converted ethanol), the selectivity of ASR over AD, and the fractional conversion of CO to CO2 were calculated quantita-

Fig. 6. TPR profiles of the air-exposed CuNi/SiO2 catalysts, with 10% H2 /N2 at 5 K/min heating rate.

Fig. 7. XPS of Cu 2p and Ni 2p core level of the 623 K-reduced CuNi/SiO2 catalysts. The samples were air-exposed prior to measurements.

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Scheme 1. Proposed reaction pathways of the SRE reaction over CuNi/SiO2 catalysts.

Fig. 8. XRD patterns of the 623 K-reduced Cux Niy /SiO2 catalysts before (solid lines) and after SRE reaction test (dash lines). All samples were air-exposed prior to measurements.

Fig. 9. (a) The ethanol conversion of CuNi/SiO2 catalysts and (b) the distribution of C-containing products on Cu1 Ni1 /SiO2 during temperature-programmed SRE tests. The tests were carried out at H2 O/EtOH = 6, WHSV = 2 g EtOH/g cat h, and 1 K/min.

tively from the observed SRE product distribution of each catalyst. The calculation is illustrated in the Appendix A. These calculated results at reaction temperatures of 628 and of 733 K and the coking analysis from both C-balance and the TGA of spent catalysts are shown in Fig. 10. At 628 K, Fig. 10(a) shows that the Cu1 Ni1 /SiO2 catalyst had the highest fractional conversion of acetaldehyde during SRE. A higher acetaldehyde fractional conversion suggests superior ability in breaking C–C bonds. The fractional conversion of acetaldehyde increased with an increase in temperature, and the effect of the Cu/Ni ratio of CuNi/SiO2 catalysts was less significant at 733 K. Fig. 10(b) shows that the selectivity of ASR was higher over the CuNi/SiO2 catalysts with Cu/Ni ≥ 1 at 628 K; but the selectivity became almost independent of Cu/Ni ratio at 733 K. The Cu-only catalyst produced only H2 , acetaldehyde and trace amount of CO2 . The calculated fractional conversion of acetaldehyde was very low but the calculated selectivity of ASR was very high because no CH4 had been produced. The selectivity of ASR of the CuNi/SiO2 was at most 0.3. In order to produce H2 more efficiently, the need to enhance ASR and to suppress AD was obvious. Fig. 10(c) shows that the fractional conversion of CO over CuNi/SiO2 increased with a decrease in Cu/Ni ratio. Both Cu and Ni catalysts [41] catalyzed WGS effectively; the former were effective for alcohol dehydrogenation but ineffective for C–C bond breaking. The fractional conversion of CO over Ni-rich catalysts was higher. This suggests that ethanol dehydrogenation, C–C breaking, and WGS activities should be balanced. Fig. 10(d) compares catalysts in terms of coke formation, measured as mole percentages of carbon missing from reactor effluent based on C-balance. The CuNi/SiO2 with Cu/Ni > 1 had low coking. TGA analysis of the catalysts after SRE indicated a similar trend, in which the Cu-rich catalysts were less likely to produce coke than the Ni-rich catalysts. The higher extent of coking in TGA than that could be expected from carbon balance was due to the fact that spent catalysts contained coke accumulated from the entire SRE test. Scheme 1 describes the SRE in the absence of coking. Coke formation was the reason that the calculated fractional conversion of acetaldehyde was less than 1 at 733 K in Fig. 10(a). Scheme 1 could be used to discuss possible coking mechanisms. Three coking reactions related to the observed products in this study could occur: (i) CH4 → C + 2H2 , (ii) 2CO → C + CO2 , and (iii) COx + H2 → C + H2 O. These three coking reactions would result in an increase, no change or a decrease in H2 yield, respectively. Consequently, the possible coking mechanism can be analyzed by comparing the calculated hydrogen yield of the observed products based on coke-free situation (Scheme 1) to the experimentally observed hydrogen yield. The calculation could be found in the Appendix A. Fig. 11 shows that the Ni-rich catalysts yielded more hydrogen at 628 K than the calculated yield and, therefore, their coking could be attributed to CH4 (or the corresponding surface intermediate) dehydrogenation. On the other hand, Cu-rich catalysts had a low coking tendency at 628 K and the calculated hydrogen yields were close to the experimentally observed values. At 733 K, all CuNi/SiO2 catalysts showed

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Fig. 10. Calculated (a) fractional conversion of acetaldehyde, (b) selectivity of ASR over AD, (c) fractional conversion of CO to CO2 , per mole of reacted EtOH, and (d) the coke deposition over CuNi/SiO2 catalysts during temperature programmed SRE tests.

a lower hydrogen yield than the calculated values and therefore the coke formed via the reduction of COx . Bartholomew [42,43] showed that the rate of CO dissociation (to form ␣-C) is less than the gasification rate of ␣-C at low temperatures and that the hydrogenation

rate of ␤-C exceeds that of its formation. This implies that CO reduction to coke would not occur at lower temperatures and that CH4 dehydrogenation to coke would not occur at higher temperatures. This may explain why we found that coke formation was due to

Table 4 Comparison of SRE performance over supported Cu-Ni bimetal catalysts. Catalyst CuNi/SiO2 CuNi/SiO2 CuNi/SBA-15 CuNi/SBA-15 CuNi/SiO2 CuNi/SiO2 CuNi/Al2 O3 CuNi/Al2 O3 CuNi/CeZrOx CuNi/CeZrOx CuNi/Ce20-SBA CuNi/La20-SBA CuNi/Mg10-S CuNi/Ca10-S a b

Cu/Ni (wt%) 2.5/2.5 2.5/2.5 2/7 1.1/10.9 1.5/13.1 1.5/13.1 2.17/4 6.6/6 5/30 5/30 2/7 2/7 2/7.5 1.8/6.6

Reaction temperature (K)

S/E (mole ratio)

Activitya (moleEtOH/gmetal h)

H2 yield

Ref.

623 733 873 673 673 773 573 573 873 673 873 873 873 873

6 6 3.7 3.7 3.7 3.7 2.5 2.5 8 8 3.7 3.7 3.7 3.7

0.83 0.87 1.26 0.94 0.77 0.77 0.28 0.15 0.08 0.07 1.26 1.26 1.26 1.26

1.2 1.8 3.2b 1.5b 1.1b 1.3b 0.7 1.3 5.2 2.4 2.9 3.8 4.0 3.6

This study This study [16] [16] [17] [17] [18] [19] [23] [23] [44] [44] [45] [45]

Calculated based on the data reported in the paper according to Activity [mole EtOH/gmetal h] = Calculated according to SH2 (%) =

FH

2

[3(FEtOH,in −FEtOH,out )+(Fwater,in −Fwater,out )]

× 100%.

(WHSV[g EtOH/g catalyst h]×XEtOH )/(Metal loading) . MWEtOH

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Fig. 11. Comparison of experimentally observed hydrogen yield and the calculated hydrogen yield of CuNi/SiO2 catalysts under a coke-free scenario.

CH4 dehydrogenation at 628 K and it was due to the reduction of COx at 733 K. 4. Discussion Most previously reported SRE listed ethylene and acetone among the numerous products. Ethylene was considered a possible precursor to coke and should be avoided as much as possible, not to mentioned that its presence would decrease the hydrogen yield. Bimetallic CuNi/Al2 O3 produced ethylene during SRE [6,10,12,13,17,21–23,36,38]. The absence of ethylene during the SRE over the CuNi/SiO2 in this study could be attributed to the nonacidic support. The SiO2 support used in this study was relatively neutral compared to the acidic Al2 O3 used in previous studies. Furthermore, the NaBH4 pretreatment used in this study may have eliminated the surface acid sites if there had been any. Table 4 compares the SRE activity of our Cu1 Ni1 /SiO2 catalyst with previously reported CuNi catalysts [16–19,23,44,45]. Despite differences in the steam ratio and in the Cu/Ni ratio, the Cu1 Ni1 /SiO2 was among the more active catalysts, particularly at temperatures below 673 K. Carrero et al. [16] proposed the SRE performance as a combined effect of metal particle size, metal loading and Cu/Ni ratio. Vizcaíno et al. [17] examined the effect of the support, but attributed the SRE activity mainly to the metal particle size. The Cu1 Ni1 /SiO2 in this study was the most active among the catalysts we examined, and also had the smallest metal particle size. We observed a synergistic effect in the SRE activity over catalysts with Cu/Ni of 3, 2, and 1 at temperatures below 633 K (Fig. 9(a)). The addition of Ni to Cu resulted in enhanced activity as well as in increase in C–C bond breaking, leading to CO and CH4 when compared with the Cu-only catalyst. Marino et al. [18,19] reported similar observations when adding Ni to Cu/K/Al2 O3 catalyst. In comparison, almost all other previous reports [16,17,21–24] used Ni-rich Cu-Ni catalysts. The different optimal Cu/Ni ratios in different studies could be due to variations in the morphology of the metal particles resulting from differences in the preparation of the catalysts. For example, most previous studies calcined the freshly prepared catalysts at high temperatures [6,8,9,11–22,32–40,46]. However, a calcination would change the TPR of CuNi/SiO2 [29] and CuNi/ZnO [35]. This suggests that the morphology and consequently the catalytic performance of bimetallic Cu-Ni catalysts depend on both the Cu/Ni ratio and the preparation procedures.

647

The catalysts with Cu/Ni of 3, 2, and 1 formed alloy according to XRD shown in Fig. 7. Alloy formation in bimetallic CuNi catalysts was previously mentioned based on TPR after the catalyst was reduced [29,35]. We demonstrate in this study that alloy nanoparticles of these Cu-rich catalysts had Ni-enriched surface, where the surface Cu/Ni ratio of the most active Cu1 Ni1 /SiO2 was around 0.8 from XPS. Both the bulk Cu/Ni ratio and the surface Cu/Ni ratio are higher than the optimal Cu/Ni ratio reported previously based on SRE activity analysis [22]. This could be either due to the initiative of adding Cu to Ni catalysts adopted in these earlier studies or due to the formation of extra phase such as spinel [18–20]. It should be noted that at low Cu/Ni ratio a separate Ni phase might occur (Fig. 8). Figs. 9 and 10 indicate that at temperatures below 673 K the Ni-only and the Ni-rich bimetallic catalysts had lower activity and more coke formation than the Cu-rich catalysts. The Ni-rich catalysts showed phase separation although the Cu-Ni phase diagram indicates homogeneous solid solution at all compositions. Wang and Gao [35] reported that Ni4Cu8/ZnO required a higher calcination temperature to induce phase separation than Ni8Cu4/ZnO, in agreement with the higher tendency of phase separation in Nirich catalysts observed in this study. Previous studies indicate that phase separation of Cu-Ni binary system occurred depending on the preparation method [47] and on the substrate [48]. As the Cu content increased to Cu/Ni ≥ 1, no phase separation was found and the coke formation was largely suppressed. The copper atoms might preferentially eliminate large ensembles of Ni metal atoms required for carbon deposition [49]. The absence of ethylene by-product and the low coking tendency of the CuNi/SiO2 with Cu/Ni ≥ 1 suggest energetic C–C bond breaking. However, more surface Ni should promote C–C bond breaking activity. Vizcaíno et al. [17] even proposed that Ni was the primary phase responsible for the hydrogen production for SRE at a higher temperature of 873 K. This may be one of the reasons why Cu1 Ni1 /SiO2 is more active than Cu2 Ni1 /SiO2 or Cu3 Ni1 /SiO2 catalyst. Results of this study indicate that at temperatures below 673 K the Cu1 Ni1 /SiO2 catalyst had the greatest activity, the highest fractional conversion of acetaldehyde (i.e., the highest C–C bond breaking activity), a relatively high selectivity of acetaldehyde steam reforming, and a relatively low coking tendency, from among all the CuNi/SiO2 catalysts examined in this study. This highest performance of Cu1 Ni1 /SiO2 catalyst is attributed to the high metal dispersion and the alloy formation whose surface Cu/Ni ratio might result in good balance of ethanol dehydrogenation activity and C–C bond breaking activity, and low coking tendency. The reasons why Cu1 Ni1 /SiO2 had the smallest particle size and why phase separation occurred when Ni/Cu > 1 are not clear at this moment. From the literature, the particle size seems to depend more on the preparation method than on the Cu/Ni ratio. For example, Cu1 Ni1 nanoparticles prepared by a sol–gel method had a smaller size of than that of Cu1 Ni3 or Cu3 Ni1 . [50] On the other hand, the CuNi nanoparticles prepared by Zhang et al. [51] had similar sizes over a wide range of Cu/Ni ratio. An interesting finding with this best Cu1 Ni1 /SiO2 catalyst is its high O2 affinity as indicated from XPS (Table 2) and XANES (Fig. 3) of air-exposed samples. How this O2 affinity may relate to the SRE performance awaits further study. However, the hydrogen yield of this best Cu1 Ni1 /SiO2 catalyst at temperatures below 673 K was still low. This would need to be improved by increasing ASR and WGS activity in Scheme 1. An ideal Cu-Ni catalyst should convert ethanol dehydrogenation and acetaldehyde steam reforming selectively, in addition to high WGS activity. The SRE reaction pathway shown in Scheme 1 is not new in comparison to those reported in the literature [5–7,11,33–38]. The absence of ethylene dehydration makes Scheme 1 a simple pathway that can be ideal for SRE. The first three reactions, namely, ED, AD, and ASR are thermodynamically favorable at high temper-

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atures and they can be considered as nearly irreversible since the equilibrium conversions are all close to 1 at temperatures ≥573 K [52]. The equilibrium conversion of WGS is near 1 at low temperatures but it gradually decreases when temperature is higher than 573 K. The presence of excess water can enhance WGS equilibrium conversion. Although WGS becomes reversible at high temperatures, its extent of reaction from our quantitative calculations should represent the net results of both directions of reversible reaction. Based on the product distribution shown in Fig. 9(b), we proposed in Scheme 1 that CO is the primary product from reactions of acetaldehyde and CO2 is formed subsequently. Thermodynamic analysis [52] indicates that the combination of AD + WGS is still favorable at high temperatures. Furthermore, the ratio of CO/CH4 which is larger than 1 at low temperatures (Fig. 9(b)) indicates that ASR and AD compete with each other. CO is still considered as the primary product of ASR instead of CO2 , because the former is a bimolecular reaction (CH3 CHO + H2 O → 2CO + 3H2 ) which is a more probable reaction pathway than the latter four-molecular reaction (CH3 CHO + 3H2 O → 2CO2 + 5H2 ). From the previously reported SRE reaction pathways [1,5,7,36,37], methane steam reforming and methanation (from CO and H2 ) may be involved during SRE. The methanation from the reaction between CO and H2 is thermodynamically favorable at temperatures ≤600 K [52]. However, it is excluded from Scheme 1 because its occurrence should result in more CH4 than CO + CO2 , which is inconsistent with the experimental results of this study. The methane reforming (CH4 + H2 O → CO + 3H2 ) is favored at high temperatures but its calculated equilibrium conversion is only 14% at 673 K when in the presence of 30% hydrogen (presumed to occur from the hydrogen generation pathways in Scheme 1). If CO comes from methane steam reforming following a dominating AD pathway, the CO/CH4 ratio would not exceed 1.14/0.86 at temperatures ≤673 K. This is inconsistent with the CO/CH4 ratio near 3 at 560 K over Cu1 Ni1 /SiO2 shown in Fig. 9(b). Consequently, the methane reforming pathway is not considered in Scheme 1. Accordingly, we consider that Scheme 1 and the derived quantitative calculations should represent the reaction pathways of SRE in this study.

The SRE over the CuNi/SiO2 catalysts could be explained, qualitatively and quantitatively, by a reaction scheme of ethanol dehydrogenation to acetaldehyde, wherein acetaldehyde decomposition (to CH4 and CO) and acetaldehyde steam reforming (to CO and H2 ) compete with each other, and a subsequent CO conversion to CO2 via water gas shift reaction. Accordingly, the extent of reaction of each reaction pathway could be calculated and the factional conversion of acetaldehyde, the selectivity of acetaldehyde steam reforming over decomposition, and the fractional conversion of CO were analyzed. The Cu-rich catalysts had a higher selectivity of ASR and resistance of coking. On the other hand, the Ni-rich catalysts had higher CO conversion. The coke formation could be analyzed based on the proposed reaction scheme. At lower reaction temperatures, no significant coke formation was found over the catalysts with Cu/Ni ≥ 1 but the Ni-rich catalysts produced coke via CH4 dehydrogenation. At higher temperatures, all catalysts generated coke via the reduction of COx . The Cu1 Ni1 /SiO2 catalyst had the best catalytic performance, with the highest fractional conversion of acetaldehyde, the highest selectivity of steam reforming of acetaldehyde and lower degree of carbon deposition. Acknowledgments The financial support of the National Science Council of Taiwan (NSC 98-2120-M-011-001) is acknowledged. We are also grateful to Precious Instrument Center of National Taiwan University of Science and Technology (NTUST) for the support in TEM analysis. Appendix A. Quantitative reaction performance evaluation based on Scheme 1 The four reaction paths in Scheme 1 are: C2 H5 OH → CH3 CHO + H2

(ED, ethanol dehydrogenation) (i)

CH3 CHO + H2 O → 3H2 + 2CO (ASR, acetaldehyde steam reforming)

(ii)

5. Conclusions The thermodynamic analysis of SRE operating conditions indicated that a higher H2 O/EtOH ratio would increase the equilibrium conversion of EtOH, decrease the effluent hydrogen concentration, and increase the energy consumption per mole of hydrogen formed. The desired conditions for efficient production of H2 from SRE should target the operation at a lower temperature and with a lower H2 O/EtOH ratio. The CuNi/SiO2 catalysts prepared by co-impregnation and by NaBH4 reduction contained reduced CuNi alloy nanoparticles. A separate Ni phase would occur when the Cu/Ni ratio dropped below 1, especially after SRE tests. The CuNi alloy nanoparticles had a Nienriched surface of the CuNi/SiO2 with Cu/Ni ≥ 1. The Cu1 Ni1 /SiO2 catalyst had the smallest metal particle size of around 3 nm. The presence of a Ni-SiO2 strong interaction was observed both in TPR and in XPS, and NiSiO3 might be formed which could not be reduced by H2 at 623 K. The steam reforming of ethanol reaction over the CuNi/SiO2 catalysts produced H2 , acetaldehyde, CH4 , CO and CO2 , but no ethylene or acetone. The Cu-only catalyst actively catalyzed ethanol dehydrogenation to acetaldehyde; while the Ni-only catalyst was less active at temperatures below 673 K. Adding Ni to Cu catalysts could improve C–C bond rupture to produce CH4 , CO and CO2 . A synergistic enhancement in ethanol conversion at temperatures below 673 K was observed over the catalysts with Cu/Ni ≥ 1. The Cu1 Ni1 /SiO2 catalyst had the best ethanol conversion activity.

CH3 CHO → CO + CH4 (AD, acetaldehyde decomposition)

(iii)

CO + H2 O → CO2 + H2

(iv)

(WGS, water gas shift)

Per mole of reacted EtOH, the number of moles of each product could be expressed in terms of the extent of reaction of each reaction path as the following: n-CH3 CHO = 1 −  ASR −  AD n-CO = 2 ASR +  AD −  WGS n-CH4 =  AD n-CO2 =  WGS Thus, the extent of reaction of ASR can be obtained as  ASR = (n-CO − n-CH4 + n-CO2 )/2. From the experimentally measured products per mole of ethanol reacted, the extent of reaction of every reaction path can be calculated. The calculated extent of reaction can reveal the selectivity of each pathway when ethanol is converted. Subsequently, the reaction performance can be compared by the following index. Fractional conversion of Acetaldehyde, fAA =  ASR +  AD

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