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Effects of Catalysts and Membranes on the Performance of Membrane Reactors in Steam Reforming of Ethanol at Moderate Temperature Manabu Miyamoto 1, *, Yuki Yoshikawa 2 , Yasunori Oumi 3 , Shin-ichi Yamaura 4 and Shigeyuki Uemiya 1, * 1 2 3 4

*

Department of Chemistry and Biomolecular Science, Gifu University, 1-1 Yanagido Gifu 501-1193, Japan Department of Materials Science and Technology, Gifu University, 1-1 Yanagido Gifu 501-1193, Japan; [email protected] Division of Instrument Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido Gifu 501-1193, Japan; [email protected] Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan; [email protected] Correspondence: [email protected] (M.M.); [email protected] (S.U.); Tel./Fax: +81-58-293-2588 (M.M.); +81-58-293-2583 (S.U.)

Academic Editors: Angelo Basile and Catherine Charcosset Received: 20 April 2016; Accepted: 26 May 2016; Published: 3 June 2016

Abstract: Steam reforming of ethanol in the membrane reactor using the Pd77 Ag23 membrane was evaluated in Ni/CeO2 and Co/CeO2 at atmospheric pressure. At 673 K, the H2 yield in the Pd77 Ag23 membrane reactor over Co/CeO2 was found to be higher than that over Ni/CeO2 , although the H2 yield over Ni/CeO2 exceeded that over Co/CeO2 at 773 K. This difference was owing to their reaction mechanism. At 773 K, the effect of H2 removal could be understood as the equilibrium shift. In contrast, the H2 removal kinetically inhibited the reverse methane steam reforming at low temperature. Thus, the low methane-forming reaction rate of Co/CeO2 was favorable at 673 K. The addition of a trace amount of Ru increased the H2 yield effectively in the membrane reactor, indicating that a reverse H2 spill over mechanism of Ru would enhance the kinetical effect of H2 separation. Finally, the effect of membrane performance on the reactor performance by using amorphous alloy membranes with different compositions was evaluated. The H2 yield was set in the order of H2 permeation flux regardless of the membrane composition. Keywords: amorphous alloy membranes; membrane reactor; steam reforming; ethanol

1. Introduction Hydrogen has been considered as one of the most promising clean energies because its combustion emits only water. Most of the hydrogen produced currently comes from catalytic steam reforming of natural gas [1]. Steam reforming of natural gas is a mature technology as a practical application, and it has been employed for hydrogen production from various hydrogen sources such as liquefied petroleum gas [2,3] iso-octane [4] and kerosene [5–8]. Considering the sustainable society, hydrogen production from fossil fuels is undesirable, and it would shift to renewable sources such as biomass-derived fuels. In particular, biomass-derived liquid fuels are preferable to direct hydrogen storage for on-site hydrogen production owing to their convenience for storage and transport. Among biomass-derived liquid fuels, bioethanol has been frequently studied for hydrogen production because it is easy to handle and distribute and it is readily available [9]. This process can be realized under far milder conditions than those of methane steam reforming. Therefore, the steam reforming of bioethanol is more attractive from practical and environmental view points. In the last decade, Processes 2016, 4, 18; doi:10.3390/pr4020018

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many researchers have made efforts to develop catalysts for steam reforming of ethanol aimed at hydrogen production [10–12]. Noble metal catalysts (Pd, Pt, Rh, Ru) exhibited high catalytic activity and stability [13–15]. Inexpensive catalysts involving Ni or Co have been also proposed as appropriate candidates in terms of activity and selectivity [16–19]. However, the process efficiency for hydrogen production must be further improved when hydrogen can be used as an energy source for fuel cells. Additionally, extremely purified hydrogen is required for the low-temperature fuel cells such as proton exchange membrane fuel cells because of irreversible catalyst poisoning even by a trace amount of CO [20,21]. Membrane reactors using hydrogen selective membranes such as Pd and Pd alloy membranes are expected to be one of the promising technologies to achieve high hydrogen production efficiency with high hydrogen purity, because extremely purified hydrogen can be obtained in one step with high hydrogen yield over the thermodynamic equilibrium, owing to simultaneous hydrogen separation from the reaction zone. Therefore, hydrogen production from several fuels using membrane reactors have been extensively studied so far [22–24]. Recently, steam reforming of ethanol has been also investigated by several research groups [25–30]. Basile and co-workers systematically investigated the performance of a membrane reactor using a Pd-Ag membrane packed with Co/Al2 O3 catalysts in steam reforming of ethanol [25,26], and achieved 100% ethanol conversion, 95.0% CO-free hydrogen recovery and up to 60% CO-free hydrogen yield at 673 K and 3.0 bar. They also investigated the effect of by-products such as acetic acid and glycerol as well [27]. Llorca and co-workers investigated the effect of reactor configurations over Co talc [28] and Co hydrotalcite [29] and demonstrated the higher performance of the catalytic membrane reactors using Pd-Ag membranes compared to the staged membrane reactor, where the catalyst has been placed in-series with the membrane. The enhancement of reactor performance by simultaneous hydrogen separation was also reported by Oyama’s group. They evaluated the effect of membrane performance on the reactor performance by comparison between Pd-Cu and SiO2 -Al2 O3 membranes and found that both permeance and selectivity had a favorable effect on steam reforming of ethanol in membrane reactors [30]. These studies clearly demonstrated the positive effect of membrane reactors in steam reforming of ethanol. However, there are only a few comparison studies on the effect of catalysts and membranes, and, in this study, we evaluated the Pd-Ag membrane reactor packed with Ni or Co based catalysts and then carried out the comparison study of the effect of membranes on the reactor performance using Pd-Ag and amorphous alloy membranes. 2. Experimental Section 2.1. Preparation of Catalysts For the experiment, 15 wt% Ni/CeO2 and 15 wt% Co/CeO2 were prepared as the literature reported [31]. For Ni/CeO2 , nickel acetate tetrahydrate was dissolved in deionized water and stirred at 343 K. After adding CeO2 (mean particle size: 1 µm), the pH was adjusted to 9 by adding 0.25 M Na2 CO3 aqueous solution. Then, the water was slowly vaporized at 373 K to obtain the precipitation, and the precipitation was calcined at 673 K for 5 h. For preparation of Co/CeO2 , the preparation procedure was the same to that for Ni/CeO2 . Cobalt acetate tetrahydrate was used as the cobalt source. Preparation of Ru-Co/CeO2 and Pd-Co/CeO2 was as follows. Ruthenium chloride or palladium chloride was dissolved in deionized water and stirred. Then, Co/CeO2 was added in the solution (M/Co = 0.003 w/w, M = Ru or Pd) and the solvent was slowly vaporized at 373 K. The obtained solid material was calcinced at 823 K under an N2 flow for 5 h. 2.2. Preparation of Amorphous Alloy Membranes Alloy ingots were prepared by arc melting a mixture of pure metals with the appropriate composition. After remelting the alloys several times to make better homogeneity, a ribbon sample was obtained by a single roller melt-spinning method. Both surfaces of the ribbon were polished and

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sputtered with Pd coating with the thickness of approximately 100 nm. The appearance of obtained sputtered with coating with the membrane was Pd shown in Figure 1. thickness of approximately 100 nm. The appearance of obtained membrane was shown in Figure 1.

Figure 1. Image Image of of amorphous amorphous alloy alloy membrane prepared by by single single roller roller melting-spinning melting-spinning method. method. Figure 1. membrane prepared

2.3. Characterization 2.3. Characterization Hydrogen permeation alloy membranes membranes (size: (size: 10 10 mm mm ˆ × 10 mm) were were Hydrogen permeation tests. tests. The The amorphous amorphous alloy 10 mm) placed in the separator sealed by Cu gaskets. The membrane was pre-heated in vacuum up to 673 K. placed in the separator sealed by Cu gaskets. The membrane was pre-heated in vacuum up to 673 K. Then, the the membrane membrane was was cooled cooled down down to to 623 K. After After keeping keeping the membrane at at 673 673 K, K, H H2 was was Then, 623 K. the membrane 2 introduced at the appropriate pressure (0.05, 0.10 and 0.15 MPa-G). The flow rate at the permeation introduced at the appropriate pressure (0.05, 0.10 and 0.15 MPa-G). The flow rate at the permeation side was was measured measuredby bythe the soup-film soup-filmflow flowmeter. meter. side Catalytic tests. tests. The The steam steam reforming reforming of of ethanol ethanol was was carried carried out out in and Catalytic in aa conventional conventional reactor reactor and membrane reactor. In the conventional reactor, the 10 g of catalysts was placed in the reactor and membrane reactor. In the conventional reactor, the 10 g of catalysts was placed in the reactor and heated to to 773 773 K K under under an an N N2 flow. Then, the catalyst was reduced under an H2 flow at 773 K for 1 h. heated 2 flow. Then, the catalyst was reduced under an H2 flow at 773 K for 1 h. After controlling the reaction temperature (623–773 a mixture water and ethanol withthe thesteam steam After controlling the reaction temperature (623–773 K),K), a mixture of of water and ethanol with to carbon ratio (S/C) of two was fed into the reactor at atmospheric pressure. The products were to carbon ratio (S/C) of two was fed into the reactor at atmospheric pressure. The products were analyzed with with aa GC-8A GC-8A gas gas chromatograph chromatograph(Shimadzu (ShimadzuCorporation, Corporation,Kyoto, Kyoto,Japan) Japan)and andthe theoutlet outletgas gas analyzed flow rate was measured by the soup-film flow meter. flow rate was measured by the soup-film flow meter. In the the membrane membranereactor, reactor,the thecatalysts catalysts and membrane were placed in the reactor. reactor In and membrane were placed in the reactor. The The reactor was was heated K under N2 flow, and catalyst was reduced 1 h an under an H2The flow. Ar heated to 773toK773 under an N2an flow, and catalyst was reduced for 1 hfor under H2 flow. ArThe sweep sweep gas was used at the permeation side in the reactor. A mixture of water and ethanol with the gas was used at the permeation side in the reactor. A mixture of water and ethanol with the S/C of S/C of two was fed into the reactor after staying at the reaction temperature. The total pressure at two was fed into the reactor after staying at the reaction temperature. The total pressure at both feed bothpermeate feed andsides permeate sides was maintained at atmospheric The gas in composition inand the and was maintained at atmospheric pressure. Thepressure. gas composition the retentate retentate and permeate side was analyzed with the gas chromatograph and the outlet gas flow rate permeate side was analyzed with the gas chromatograph and the outlet gas flow rate in both sides was in both sides was measured by the soup flow meter. A Pd77Ag23 membrane (thickness: 20 μm, measured by the soup flow meter. A Pd 77 Ag23 membrane (thickness: 20 µm, purchased from Tanaka purchased from Tanaka Kikinzoku Kogyo K.K., Tokyo, Japan) was used as reference. Kikinzoku Kogyo K.K., Tokyo, Japan) was used as reference. The conversion conversion of of ethanol ethanol to to C C1 products and H H2 yield as follows: The products and yield was was calculated calculated as follows: 1

2

𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑜𝑜𝑜𝑜 𝐶𝐶𝐶𝐶,𝐶𝐶𝑂𝑂2 𝑎𝑎𝑎𝑎𝑎𝑎 𝐶𝐶𝐶𝐶4 𝑖𝑖𝑖𝑖 𝑝𝑝𝑝𝑝𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑜𝑜𝑜𝑜 𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝑡𝑡𝑡𝑡 𝐶𝐶1 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = rate o f CO, CO2 and CH4 in products , f low 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑜𝑜𝑜𝑜 𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 × 2 Conversion o f ethanol to C1 products “ , f eed rate o f ethanol ˆ 2 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑜𝑜𝑜𝑜 𝐻𝐻2 𝑖𝑖𝑖𝑖 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝐻𝐻2 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 = . f low 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 rate o𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 f H2 𝑜𝑜𝑜𝑜 in 𝑒𝑒𝑒𝑒ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 products× 6 H2 yield “ . rateaso ffollows ethanol to ˆ 6compare the membrane reactor Additionally, we defined H2 removalf eed ratio

Additionally, we defined H2 removal ratio as follows to compare the membrane performance: reactor performance: 𝐻𝐻2 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 [𝑚𝑚𝑚𝑚𝑚𝑚/𝑚𝑚𝑚𝑚𝑚𝑚] 𝐻𝐻2 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 [%] = × 100. 𝑇𝑇ℎ𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝐻𝐻2 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 [𝑚𝑚𝑚𝑚𝑚𝑚/𝑚𝑚𝑚𝑚𝑚𝑚] H2 permeation f low rate rmol{mins H2 removal ratio r%s “ ˆ 100. Theoritical H2 production rate rmol{mins

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3. Results and Discussion 3.1. Membrane Reactor Performance Using Pd77 Ag23 Membrane with Co/CeO2 and Ni/CeO2 Catalysts Figure 2 shows the comparison of reactor performance of the conventional reactor and membrane reactor with the Pd membrane over Co/CeO2 and Ni/CeO2 catalysts. In the conventional reactor, Ni/CeO2 exhibited higher conversion of ethanol to C1 products compared to Co/CeO2 catalysts. However, the H2 yield over Ni/CeO2 did not exceed those over Co/CeO2 at the reaction temperatures from 673 to 773 K. Regardless of catalysts, the membrane reactor exhibited higher conversion and H2 yield than the conventional reactor, but the influence of Pd77 Ag23 membrane was different between the catalysts. The increase in conversion of ethanol to C1 products over Ni/CeO2 by the Pd77 Ag23 membrane was much higher than those over Co/CeO2 catalysts, and it achieved more than 90% at 723 K, whereas the conversion was only 65% at 773 K over Co/CeO2 . The H2 yield over Co/CeO2 increased by almost 13% regardless of temperature, e.g., 44.4% and 57.8% at 773 K for the conventional reactor and membrane reactor, respectively. In contrast, it was found that the increase in H2 yield over Ni/CeO2 was significantly improved with an increase in the reaction temperature (from 27.4% at 673 K to 64.5% at 773 K). These results indicate that the influence of H2 removal from the reaction zone through the Pd77 Ag23 membrane was higher over Ni/CeO2 than Co/CeO2 at the high reaction temperature. However, Co/CeO2 was suitable for the membrane reactor at low reaction temperature. Figure 3 shows the selectivity of C1 products. For Ni/CeO2 , the main product at 673 K was methane (58.1%) and a slight decrease in the methane selectivity was observed (51.1%) in the membrane reactor at this reaction temperature. The methane selectivity was slightly decreased to 41.4% at 773 K in the conventional reactor, and the simultaneous H2 removal by Pd77 Ag23 membrane greatly enhanced the CO2 selectivity with decreasing of the methane selectivity. Furthermore, the membrane reactor showed lower methane selectivity of 25.9% at 773 K. For Co/CeO2 , the main product in the conventional reactor was CO2 regardless of the reaction temperature and the methane selectivity was very low (28.8% and 18.9% at 673 K and 773 K, respectively) compared to Ni/CeO2 . In the membrane reactor, the methane selectivity was decreased, and it was noting that the decrease in methane selectivity was much higher at 673 K than 773 K in Co/CeO2 . Torres et al. reported the difference in reaction path in the steam reforming of ethanol [16], and their reaction scheme is summarized in Scheme 1. In the literature, at high reaction temperature, the steam reforming of ethanol was dominant over both Ni and Co catalysts. However, the reaction path was largely different at the moderated reaction temperature between Ni and Co catalysts. The initial reaction was ethanol dehydrogenation to acetaldehyde over both catalysts. The product selectivity approached the thermodynamic equilibrium in Ni catalysts because of the methane-forming reaction such as ethanol cracking, acetaldehyde decarbonilation and the reverse methane steam reforming. On the other hand, the Co catalysts did not promote such methane-forming reactions and the steam reforming of acetaldehyde was preferably occurred. Considering their reaction scheme, the highly increased H2 yield over Ni/CeO2 in the membrane reactor at 773 K was due to the shift of equilibrium by simultaneous H2 removal through the Pd77 Ag23 membrane. However, at 673 K, it was easy for Ni/CeO2 to produce methane and the methane steam reforming would be hardly promoted once methane produced even when H2 was selectively removed from the reaction zone because the methane steam reforming is kinetically and thermodynamically unfavorable at low temperature. Therefore, the low H2 yield was owing to the preferential production of methane in both conventional and membrane reactors at low temperature. In contrast, because of the low reaction rate of methane-forming reactions in Co/CeO2 , the Pd77 Ag23 membrane could remove produced H2 from the reaction zone before H2 was consumed by the reverse methane steam reforming. This means the selective H2 removal through Pd77 Ag23 membrane kinetically inhibited the methane production, resulting in high H2 yield at low reaction temperature. Indeed, Seelam et al. reported Co/Al2 O2 that showed high membrane performance at 673 K compared to Ni/ZrO2 because of high methane selectivity in Ni/ZrO2 [27], which is consistent with our experimental results.

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Processes 2016, 4, 18 Processes 2016, 4, 18 Processes 2016, 4, 18 100 100 100 80 80 80 60 60 60 40 40 40 20 20 20 0 0650 0 650 650

H2 yield yield [%][%] HH [%] 2 2yield

Conversion to products C1 products Conversion [%][%] Conversion totoCC [%] 1 1products

100 100 100 80 80 80 60 60 60 40 40 40 20 20 20 0 0650 0 650 650

5 of 11 5 of 11 5 of 11 Co/CeO2 CR CR Co/CeO2 Co/CeO2CR CR Co/CeO2 CR Ni/CeO Ni/CeO2 2 CR Co/CeO Co/CeO2 CR 2 CR Ni/CeO2 CR CR Ni/CeO2 Ni/CeO2 CR CR Ni/CeO2

Co/CeO2MR MR CoCeO2 Co/CeO2MR MR CoCeO2 MR Ni/CeO Ni/CeO2 2 MR Co/CeO CoCeO2 MR 2 MR Ni/CeO2 MR MR Ni/CeO2 Ni/CeO2 MR MR Ni/CeO2

Co/CeO2 CV CR Co/CeO2MR MR CoCeO2 Co/CeO2 Co/CeO2CV CR Co/CeO2MR MR CoCeO2 Co/CeO2 CV Ni/CeO CR Ni/CeO Ni/CeO2 Ni/CeO2 2 CV 2 MR Co/CeO Co/CeO CoCeO2 Co/CeO2 2 CR 2 MR Ni/CeO2 CV CR Ni/CeO2 MR Ni/CeO2 Ni/CeO2 Ni/CeO2 CV CR Ni/CeO2 MR Ni/CeO2 Ni/CeO2 700 750 800 700 750 800 700 temperature 750 [K] 800 700 temperature 750 [K] 800 Reaction Reaction 700 750 800 700 750 800 Reaction temperature [K] Reaction temperature [K] Reaction temperature [K] Reaction temperature [K] Figure 2. Conversion of ethanol to C1 products and H2 yield over Co/CeO2 and Ni/CeO2 catalysts in Figure 2. Conversion of ethanol to C1 products and H2 yield over Co/CeO2 and Ni/CeO2 catalysts in Figurereforming 2. Conversion of ethanol C1 productsreactor, and H2 MR: yieldmembrane over Co/CeO 2 and W/F: Ni/CeO in steam of ethanol. CR: to conventional reactor, 1.02 catalysts × 10 4 g-cat 4 g-cat 2. Conversion of ethanol to C1 productsreactor, and H2 MR: yield membrane over Co/CeO 2 and Ni/CeO steamFigure reforming of ethanol. CR: conventional reactor, W/F:2 catalysts 1.0 ˆ 10in

steam reforming ethanol. CR:Pd conventional reactor, MR: reactor, W/F: 1.0 × 10 4 g-cat 77 Ag23 , Sweep Ar flow rate:membrane 500 mL/min. min/C-mol. S/C = of 2. Membrane: steam reforming ethanol. CR:Pd conventional reactor, MR: membrane reactor, W/F: 1.0 × 10 g-cat min/C-mol. S/CS/C = =2.of Ag , Sweep 500 mL/min. 77Ag 77 2323 , Sweep Ar Ar flowflow rate:rate: 500 mL/min. min/C-mol. 2.Membrane: Membrane: Pd 4

100 100 100 80 80 80 60 60 60 40 40 40 20 20 20 0 0 0

100 100 100 80 80 80 60 CH460 60 CH440 CO CH4 CO 40 CO2 CO 40 CO220 CO220 20 0 0 0

C selectivity [%][%] CC [%] 1 selectivity 1 1selectivity

C selectivity [%][%] CC [%] 1 selectivity 1 1selectivity

min/C-mol. S/C = 2. Membrane: Pd77 Ag23 , Sweep Ar flow rate: 500 mL/min.

CR 673 CRK CR 673 K 673 K

MR 673 MRK MR 673 K 673 K

CR 773 CRK CR 773 K 773 K

MR 773K MR MR 773K 773K

CH4 CH4 CH4 CH CO CO CH4 CH44 CO CO CO2 CO2 CO CO CO2 CO22 CO2 CR 673 CRK CR 673 K 673 K

MR 673 MRK MR 673 K 673 K

CR 773 CRK CR 773 K 773 K

MR 773K MR MR 773K 773K

Figure 3. Selectivity of C1 products over Co/CeO2 (left) and Ni/CeO2 (right) catalysts in steam

Figure 3. of Selectivity of C 1 products over Co/CeO 2 (left) and Ni/CeO 2 (right) catalysts in steam 4 g-cat min/C-mol. reforming ethanol. conventional reactor, MR: membrane reactor, W/F: × 10 Figure 3. Selectivity of CR: C products over (left) and and Ni/CeO (right) catalysts in steam Figure 3. Selectivity of1 C 1 products overCo/CeO Co/CeO Ni/CeO 2 (right) catalysts in steam 22 (left) 21.0 4 g-cat min/C-mol. reforming of ethanol. CR: conventional reactor, MR: membrane reactor, W/F: 1.0 × 10 4 77 Ag23 , Sweep Ar flow rate: 500 mL/min. S/C = 2. Pd 4 g-cat min/C-mol. reforming of ethanol. CR: conventional reactor,MR: MR:membrane membranereactor, reactor, W/F: W/F: 1.0 1010 reforming ofMembrane: ethanol. CR: conventional reactor, 1.0׈ g-cat min/C-mol. S/C = 2. Membrane: Pd77 Ag23 , Sweep Ar flow rate: 500 mL/min. 77 Ag23,, Sweep 500 mL/min. 2. Membrane: S/C =S/C 2. =Membrane: PdPd Arflow flowrate: rate: 500 mL/min. 77 Ag 23 Sweep Ar

High reaction temperature High High reaction reaction temperature temperature

(steam reforming of ethanol) (steam reforming reforming of of ethanol) ethanol) (water (steam gas shift) (water gas shift) (water gas shift)

Low reaction temperature Low Low reaction reaction temperature temperature (dehydrogenation) (dehydrogenation) (dehydrogenation)

Ni/CeO2 Ni/CeO Ni/CeO22

Co/CeO2 Co/CeO Co/CeO22

(decarbonilation) (decarbonilation) (decarbonilation) (reverse methane steam reforming) (reverse methane steam (reverse steam reforming) reforming) (water gasmethane shift) (water (water gas gas shift) shift) (Cracking) (Cracking) (Cracking) (steam reforming of acetaldehyde) (steam reforming of (steam reforming of acetaldehyde) acetaldehyde) (water gas shift) (water gas shift) (water gas shift)

Scheme 1. Difference in reaction path over Ni and Co catalysts in steam reforming of ethanol [16]. Scheme 1. Difference in reaction path over Ni and Co catalysts in steam reforming of ethanol [16]. Scheme 1. Difference in reaction pathover overNi Ni and and Co reforming of ethanol [16]. [16]. Scheme 1. Difference in reaction path Co catalysts catalystsininsteam steam reforming of ethanol

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3.2. Catalyst Development for Improving the Membrane Reactor Performance Comparing Co/CeO2 and Ni/CeO2 , Co/CeO2 was preferable to Ni/CeO2 for steam reforming of ethanol at low temperature, and the H2 removal through the Pd77 Ag23 membrane was very effective at achieving high H2 yield because of the kinetic inhibition of methane-forming reactions. To improve the membrane reactor performance, we developed the catalysts with addition of a trace amount (M/Co = 0.003 (w/w)) of precious metals such as Ru and Pd. Table 1 shows the performance of Ru-Co/CeO2 and Pd-Co/CeO2 in steam reforming of ethanol in the conventional and membrane reactors. It was interestingly found that the addition of a very small amount of Ru or Pd greatly enhanced the conversion of ethanol to C1 products. However, the H2 yield in the conventional reactor was not changed in Ru-Co/CeO2 and significantly decreased in Pd-Co/CeO2 compared to Co/CeO2 (here, it should be noted that the H2 yield and conversion to C1 product on Co/CeO2 was higher than those in Figure 2 even at the same reaction condition. This was owing to the refinement of flow system on the reactor in Figure S1). The low H2 yield can be explained by the thermodynamic equilibrium. The C1 selectivity at the thermodynamic equilibrium is CO2 :CO:CH4 = 30.32:0.15:69.53. Indeed, the methane selectivity approached the thermodynamic value with the increased conversion by the addition of Ru and Pd, resulting in low H2 yield because produced H2 was consumed by reverse methane steam reforming from CO2 and CO. In the membrane reactor, the H2 yield in both Ru-Co/CeO2 and Pd-Co/CeO2 was increased by the simultaneous H2 separation although the conversion was not changed. In addition, the methane selectivity was decreased by the H2 removal, and it was much lower than that at the thermodynamic equilibrium as shown in Table 1. This indicates that the H2 removal by the Pd77 Ag23 membrane kinetically inhibits the methane-forming reaction but does not shift the equilibrium as mentioned above. The platinum group is well-known to show high H2 dissociation/association ability and H2 spillover effect. Otsuka et al. reported that Pt accelerated the formation rates of H2 and CO in the partial oxidation of methane by the reverse spillover of H2 [32]. Lei et al. investigated the effect of Rh in the high temperature water-gas shift reaction, and they found that Rh greatly enhances H2 release during reoxidation by water, presumably by recombining hydrogen atoms transferred from oxide to metal by reverse spillover [33]. Therefore, the Ru and Pd might accelerate the association of hydrogen atom and desorption of H2 molecules from the catalyst through the reverse spillover mechanism. Comparing Ru-Co/CeO2 and Pd-Co/CeO2 , Ru-Co/CeO2 exhibited high H2 yield and low methane selectivity. From the investigation in C1 selectivity with the conversions of ethanol as shown in Figure 4, the methane selectivity in Pd-Co/CeO2 was increased at lower conversions compared to Ru-Co/CeO2 . This clearly indicates that the promotion effect of methane-forming reaction was higher in Pd than Ru, probably caused by high H2 storage capacity of Pd. Thus, the Pd membrane could effectively remove H2 through the reverse spillover on Ru before they reacted with CO2 or CO to methane, resulting in higher H2 yield in Ru-Co/CeO2 . On the other hand, a certain part of hydrogen would react with CO2 and CO due to relatively high hydrogen concentration on Pd before desorption of H2 molecules by the reverse spillover mechanism, although the Pd77 Ag23 membrane removed H2 from the reaction zone. Table 1. Performance of conventional and membrane reactors over Ru-Co/CeO2 and Pd-Co/CeO2 in steam reforming of ethanol. Catalysts Co/CeO2 Ru-Co/CeO2 Pd-Co/CeO2

Conversion to C1 (%) CR CR MR CR MR

63.4 86.8 87.9 92.7 94.2

C1 Selectivity (%)

H2 Yield (%) 47.6 42.3 70.3 30.8 52.9

CO2

CO

CH4

70.3 59.0 81.8 51.6 65.3

6.0 3.3 4.0 1.9 2.4

23.6 37.7 14.2 46.5 32.3

Reaction temperature: 623 K, W/F: 1.0 ˆ 104 g-cat min/C-mol. S/C = 2. Membrane: Pd77 Ag23 , Sweep Ar flow rate: 500 mL/min. CR: conventional reactor, MR: membrane reactor.

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80 100 60 80 40 60 20 40 0 20 0

100

20 40 60 80 Conversion to C1 products [%]

Product selectivity [C-mol%] [C-mol%] Product selectivity

Product selectivity [C-mol%] [C-mol%] Product selectivity

100 Processes 2016, 4, 18

CH3CHO 80 (CH3)2CO 100 CH4 60 CH3CHO CO2 80 (CH3)2CO CO 40 CH4 60 CO2 20 CO 40 0 100 20 0

0

7 of 11 CH3CHO CH3CHO (CH3)2CO (CH3)2CO

CH4 CH4 CH3CHO CH3CHO CO2 CO2 (CH3)2CO (CH CO CO 3)2CO CH4 CH4 CO2 CO2 CO CO 20 40 60 80 Conversion to C1 products [%]

100

0

Figure 4. Product distribution over Ru-Co/CeO2 (left) and Pd-Co/CeO2 (right) in steam reforming of

0 20distribution 40 60over Ru-Co/CeO 80 100 0 20 40 60 80 100 Figure 4. Product 2 (left) and Pd-Co/CeO2 (right) in steam reforming of ethanol in Conversion conventional Reaction temperature: 623 Conversion K, S/C = 2. to C1 products [%] to Creactor. 1 products [%] ethanol in conventional reactor. Reaction temperature: 623 K, S/C = 2.

Figure 4. Product distribution over Ru-Co/CeO 2 (left) and Pd-Co/CeO2 (right) in steam reforming of 3.3. Comparison of Amorphous Alloy Membranes and Pd77Ag23 Membrane in the Membrane Reactor

ethanol of in conventional Reaction temperature: K, S/C = 2. 3.3. Comparison Amorphous reactor. Alloy Membranes and Pd77623 Ag23 Membrane in the Membrane Reactor

Figure 5 shows the H2 permeability of amorphous alloy membranes. The H2 permeability of Ni-

H2 Permeability H2 Permeability -1 Pa-0.5 -8 s m-1 [×10-8 mol mol m-1 s] -1 Pa-0.5] [×10

Figure 5 shows the H2 permeability of amorphous alloy TheReactor H2 permeability Nb-Zr ternary alloy membrane was increased with increasing Zr membranes. content as the literature reported 3.3. Comparison of Amorphous Alloy Membranes and Pd77 Ag23 Membrane in the Membrane −1·s −1·Pa −0.5, which [34,35]. Theternary H2 permeability of (Ni0.6Nb 0.4)70was Zr30 was approximately 8.8 × 10−9 mol·mZr of Ni-Nb-Zr alloy membrane increased with increasing content as the Figure 5 shows the H2 permeability of amorphous alloy membranes. The H2 permeability of Niis consistent with the value reported by the researchers [36]. Paglieri et al. reported that the addition literature [34,35]. The of (Ni Nb0.4 )70 was approximately 2 permeability 30literature Nb-Zr reported ternary alloy membrane was H increased with increasing Zr0.6content asZr the reported of the 2 permeability, although this slightly improves thereported thermal stability [37]. We have ´9lowered ´1 ¨ H ´ 1 ¨ Pa´0.5 , which −9 −0.5, which [36]. 8.8 ˆ[34,35]. 10Ta mol¨ m s is consistent with the value by researchers The H2 permeability of (Ni0.6Nb 0.4)70Zr30 was approximately 8.8 × 10 mol·m−1the ·s −1·Pa found that the increase in Ta content slightly decreased the H2 permeability of Ni-Nb-Ta-Zr Paglieri et al. reported the addition of researchers Ta lowered[36]. the Paglieri H2 permeability, although this slightly is consistent with thethat value reported by the et al. reported that the addition quaternary alloy membranes [38]. Indeed, Ni-Ta-Zr ternary alloy membranes showed the lower H2 of Ta lowered the Hstability 2 permeability, although this slightly improves the in thermal stability [37]. Wedecreased have improves the thermal [37]. We have found that the increase Ta content slightly permeability in our study as well. In contrast, the addition of a small amount of Zr and Ta increased 2 permeability of Ni-Nb-Ta-Zr that the increase in Ta content slightly decreased the H the Hfound permeability of Ni-Nb-Ta-Zr quaternary alloy membranes [38]. Indeed, Ni-Ta-Zr ternary 2 H2 permeability in Nb-Ni-Co alloy membranes because the introduction of larger atoms alloy the quaternary alloy membranes [38]. Indeed, Ni-Ta-Zr ternary alloy membranes showedthe the addition lower H2 of a membranes showed the lowerstructure, H study asin well. In contrast, 2 permeability [39]. Thus, the expanded the amorphous resulting in inour an increase H2 diffusivity permeability in our study as well. In contrast, the addition of a small amount of Zr and Ta increased smallNi amount and Tamembrane increasedexhibited the H2 permeability Nb-Ni-Cothat alloy 40Nb 20Taof 5ZrZr 30Co 5 alloy the highest H2 in permeability wasmembranes comparable tobecause the the H2 permeability in Nb-Ni-Co alloy membranes because the introduction of larger atoms the introduction of larger atoms expanded the amorphous structure, resulting in an increase in H2 Pd77Ag23 membrane. expanded the amorphous structure, resulting in an increase in H2 diffusivity [39]. Thus, the diffusivity [39]. Thus, the Nimembrane Zr30 Co5 alloy membrane exhibited the highest H2 permeability 40 Nb20 Ta5 exhibited Ni40Nb 20Ta5Zr30Co5 alloy1.2 the highest H2 permeability that was comparable to the Pd77Ag23 Pd77Ag23 that was comparable to the Pd Ag membrane. 77 23 Pd77Ag23 membrane. (Ni0.5Nb0.5)80Zr20 (Ni0.5Nb0.5)80Zr20 (Ni0.6Nb0.4)80Zr20 (Ni0.6Nb0.4)80Zr20 (Ni0.6Ag Nb0.4)70Zr30 (Ni0.6Nb0.4)70Zr30 Pd Pd77Ag23 77 23 (Ni0.67Ta0.33)80Zr2 (Ni0.67Nb Ta0.33 )80Zr20 (Ni (Ni0.5Nb0.5)80Zr20 0.5 0.5)80Zr20 (Ni0.67Ta0.33)70Zr3 (Ni Ta 0.67 0.33 20 )80)70 ZrZr (Ni0.6 Nb0.4 (Ni0.6Nb0.4)80Zr20 20 Ni40Nb20Ta5Zr30C Ni40Nb 20Ta 5Zr 20Co5 (Ni Nb ) (Ni0.6Nb0.4)70Zr30 0.6 0.4 70Zr30 Pd78Cu6Si16 Pd78Cu (Ni0.67Ta0.33)80Zr2 (Ni Ta6Si16) Zr

1

1.2 0.8 1 0.6 0.8 0.4

0.67

0.6 0.2 0.40 0.2

50

100

150

200

0.33 80

20

(Ni0.67Ta0.33)70Zr3 (Ni0.67Ta0.33)70Zr20 Ni40Nb20Ta5Zr30C Ni40Nb20Ta5Zr20Co5 Pd78Cu6Si16 Pd78Cu6Si16

Ph0.5-Pl0.5 [Pa0.5]

0 50of amorphous 100 alloy membranes 150 200 Figure 5. H2 permeability at 623 K. Ph = 0.15, 0.20 and 0.25 MPa. Ph0.5-Pl0.5 [Pa0.5]

The steam reforming of ethanol over Ru-Co/CeO2 was evaluated in the membrane reactors with Figure 5. H2 permeability of amorphous alloy membranes at 623 K. Ph = 0.15, 0.20 and 0.25 MPa. amorphous membranes. Table 2 summarizes the membrane reactor performance. The MPa. H2 yield Figure 5. Halloy of amorphous alloy membranes at 623 K. Ph = 0.15, 0.20 and 0.25 2 permeability was clearly related to H2 removal ratio. Indeed, the amorphous membranes with the lowest H2 The steam reforming of ethanol over Ru-Co/CeO2 was evaluated in the membrane reactors with removal ratio of approximately 10%, such as (Ni0.67Ta0.33)80Zr20 and Pd78Cu6Si16, exhibited the same amorphous membranes. Table 2over summarizes the membrane reactor performance. The H2 yield The steam alloy reforming of ethanol Ru-Co/CeO was evaluated in the membrane reactors membrane reactor performance, and a similar H2 yield2was obtained on the (Ni0.6Nb 0.4)70Zr30 and clearly related to H2 removalTable ratio.2 Indeed, the amorphous membranes with the lowest H 2 with was amorphous alloy membranes. summarizes the membrane reactor performance. H2 H2 Ni40Nb 20Ta5Zr30Co5 with the highest H2 removal ratio of approximately 50%. However, the The 0.67Ta 0.33) 80Zr20 and Pd78Cu6Si16, exhibited the same removal ratio of approximately 10%, such as (Ni yieldremoval was clearly H2same removal the amorphous membranes withand the H lowest ratio related was nottothe orderratio. of theIndeed, H2 permeability. For example, the H2 yield 2 membrane reactor performance, and a similar H2 yield was obtained on the (Ni0.6Nb 0.4)70Zr30 and H2 removal approximately such aswas (Nihigher )80that Zr20inand exhibited 30 membrane than the Pd (Ni78 0.5Cu Nb 0.5 )8016 Zr, 20 and was the removalratio ratio of in the (Ni0.6Nb 0.4)70Zr10%, 0.67 Ta0.33 6 Si Ni40Nb 20Ta5Zr30Co5 with the highest H2 removal ratio of approximately 50%. However, the H2 comparable Ni40Nb 20Ta5Zr 30Co although H2was permeability on(Ni the0.6order samealmost membrane reactor to performance, and a5,similar H2 the yield obtained was on the Nb0.4 )of Zr30 removal ratio was not the same order of the H2 permeability. For example, the H2 yield and H70 2 40 Nb 20 Ta 5 Zr 30 Co 5 > (Ni 0.5 Nb 0.5 ) 80 Zr 20 ≅ (Ni 0.6 Nb 0.4 ) 70 Zr 30 . This could be owing to their membrane Ni and Ni40 Nb20 Ta5 Zr30 Co5 with the highest H2 removal ratio of approximately 50%. However, the removal ratio in the (Ni0.6Nb 0.4)70Zr30 membrane was higher than that in the (Ni0.5Nb 0.5)80Zr20 and was thickness thatwas is in not inverse proportion to of thethe H2 permeation flux when membrane isH thick enough. H H2 removal ratio order H2 permeability. Forthe example, 2 yield 2 almost comparable to the Ni40same Nb 20Ta 5Zr30Co5, although the H2 permeability was the on the order and of removal ratio in5Zr the (Ni Nb Zr in theto (Ni 3020membrane 0.5 Nb 0.5 )80 Zr20 and 20Ta 30Co 5 >0.6 (Ni 0.50.4 Nb)70 0.5) 80Zr ≅ (Ni0.6Nb 0.4was )70Zrhigher 30. This than couldthat be owing their membrane Ni40Nb was almost comparable to Niproportion Zrthe , although flux the H was on the order of thickness that is in inverse H25 permeation when the membrane is thick enough. 40 Nb20 Ta5to 30 Co 2 permeability

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Ni40 Nb20 Ta5 Zr30 Co5 > (Ni0.5 Nb0.5 )80 Zr20 – (Ni0.6 Nb0.4 )70 Zr30 . This could be owing to their membrane thickness that is in inverse proportion to the H2 permeation flux when the membrane is thick enough. Table 2. Comparison of the membrane reactor performance using amorphous alloy membranes and Pd77 Ag23 membrane in the steam reforming of ethanol. Processes 2016, 4, 18Membrane

Membrane

Thickness (µm)

H2 removal (%)

H2 Yield (%)

CO2

8 of(%) 11 C1 Selectivity CO

Table 2. Comparison of the membrane reactor performance using amorphous alloy membranes and

CH4

(Ni0.5 Nb0.5 )80 Zr20 41.6 40.0 58.1 72.9 3.7 23.4 77 Ag23 membrane in the steam reforming of ethanol. (Ni0.6 Nb0.4 )80 ZrPd 40.6 37.0 57.7 71.8 3.4 24.7 20 (Ni0.6 Nb0.4 )70 Zr30 27.7 Membrane 49.2 65.6 75.8 2.8(%) 21.4 C1 Selectivity H2 removal (%) 47.9 H2 Yield (%) 66.3 (Ni0.67 Ta0.33 )80 ZrMembrane 35.7 10.0 5.1 29.0 20 Thickness (μm) CO2 CO CH4 (Ni0.67 Ta0.33 )70 Zr30 38.7 36.6 55.7 69.3 5.2 25.5 (NiCo 0.5Nb 0.5) 80Zr20 41.6 72.9 3.7 3.8 23.4 Ni40 Nb20 Ta5 Zr 30.4 50.9 40.0 67.2 58.1 79.0 17.2 30 5 0.6Nb 0.4) 80Zr2028.1 40.6 37.0 71.8 3.4 5.4 24.7 Pd78 Cu6 Si(Ni 9.6 46.5 57.7 65.8 28.8 16 27.7 75.8 2.8 4.0 21.4 Pd77 Ag23(Ni0.6Nb 0.4)70Zr3020.0 58.0 49.2 70.3 65.6 81.8 14.2 (NiRu-Co/CeO 0.67Ta 0.33) 80Zr20 35.7 10.0 47.9 66.3 5.1 29.0 4 Catalyst: 2 , reaction temperature: 623 K, 1.0 ˆ 10 g-cat min/C-mol. S/C = 2, sweep Ar flow rate: (Ni0.67Ta0.33)70Zr30 38.7 36.6 55.7 69.3 5.2 25.5 500 mL/min. Ni40Nb 20Ta5Zr30Co5 30.4 50.9 67.2 79.0 3.8 17.2 Pd78Cu6Si16 28.1 9.6 46.5 65.8 5.4 28.8 Finally, wePdcarried out the steam of ethanol in 70.3 the Pd77 Ag 77Ag23 20.0 reforming58.0 81.8 4.0 14.2reactor with 23 membrane Catalyst: Ru-Co/CeO , reaction temperature: 623effect K, 1.0 ×of 10 4H g-cat min/C-mol.by S/Cthe = 2, sweep Ar flow on H2 yield. different sweep Ar flow rate 2to understand the membrane 2 removal rate: 500 mL/min. Figure 6 shows the H2 yield and C1 selectivity as a function of H2 removal ratio. The solid lines Finally, we carried out the steam reforming of ethanol Ag23 membrane reactorby with were interpolated from the experimental results on the Pdin AgPd reactor varying the 77the 23 77membrane different sweep Ar flow rate to understand the effect of H2 removal by the membrane on H2 yield. sweep Ar flow rate to obtain different H2 removal ratio. Interestingly, the H2 yield and C1 selectivity Figure 6 shows the H2 yield and C1 selectivity as a function of H2 removal ratio. The solid lines were in the amorphous membranes fitted the curve well. clearly shows the membrane reactor Ag23 membrane reactor that by varying the sweep interpolated from the experimental results on the Pd77This performance wasrate determined by the H H22removal removal ratio. Considering reaction mechanism Ar flow to obtain different ratio. Interestingly, the H2the yield and C1 selectivity in thementioned fitted the curve well. clearly performance shows that the such membrane in Section amorphous 3.1, at the membranes low reaction temperature, theThis catalytic as C1reactor selectivity was was determined by the H2 removal ratio. Considering the reaction mechanism kineticallyperformance controlled by the H2 permeation rate through the membrane. Thus, it is not an unexpected mentioned in Section 3.1, at the low reaction temperature, the catalytic performance such as C1 result that selectivity the membrane reactorcontrolled performance using different amorphous membranes fitted those with was kinetically by the H 2 permeation rate through the membrane. Thus, it is not the Pd77 Ag membrane. Inthat other words, we can roughly predict membrane reactor performance an23unexpected result the membrane reactor performance using the different amorphous membranes fittedactual those with Pd77Ag23 membrane. In other of words, we cancomposition roughly predictofthe based on their H2 the permeation flux regardless the metal themembrane alloy membranes. reactor performance based on their actual H2 permeation flux regardless of the metal composition of Indeed, the H2 removal ratio is clearly related to the H2 permeation flux of the membrane as shown the alloy membranes. Indeed, the H2 removal ratio is clearly related to the H2 permeation flux of the in Figure 7. membrane as shown in Figure 7. 100

H2 yield [%]

Pd77Ag23

Total

80

Pd77Ag23 Pd 77Ag23 Pd77Ag23 (Ni 0.5Nb0.5)80Zr20 (Ni0.6Nb0.4)80Zr2 (Ni 0.6Nb0.4)80Zr20 (Ni0.6Nb0.4)70Zr3 (Ni 0.6Nb0.4)70Zr30 (Ni0.67Ta0.33)80Z (Ni 0.67Ta0.33)80Zr20 (Ni0.67Ta0.33)70Z (Ni 0.67Ta0.33)70Zr20 NiNi40Nb20Ta5Zr30 40Nb20Ta5Zr20Co5 Pd Pd78Cu6Si16 78Cu6Si16

60 40 20 0

Permeate 0

20 40 H2 removal ratio [%]

60

C1 selectivity [%]

100 CO2

80 60 40

CH4

20 0

CO 0

20 40 H2 removal ratio [%]

60

Figure 6. H2 yield and C1 selectivity in the membrane reactors using Pd77 Ag23 and amorphous alloy

Figure 6. H C1 selectivity in the membrane reactors using Pd × Ag and amorphous alloy 2 yield and membranes in steam reforming of ethanol at 623 K. Catalyst: Ru-Co/CeO2 , 1.077 10 4 23 g-cat min/C-mol. membranes in steam reforming of ethanol at 623 K. Catalyst: Ru-Co/CeO , 1.0 ˆ 104 g-cat min/C-mol. S/C = 2. 2 S/C = 2.

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70

Pd Pd77Ag23 77Ag23 (Ni (Ni0.5Nb0.5)80Zr20 0.5Nb0.5)80Zr20 (Ni0.6Nb0.4)80Zr20 (Ni 0.6Nb0.4)80Zr20 (Ni0.6Nb0.4)70Zr30 (Ni 0.6Nb0.4)70Zr30 (Ni0.67Ta0.33)80Zr20 (Ni 0.67Ta0.33)80Zr20 (Ni0.67Ta0.33)70Zr30 (Ni 0.67Ta0.33)70Zr20 Ni40Nb20Ta5Zr30Co5 Ni 40Nb20Ta5Zr20Co5 Pd78Cu6Si16 Pd 78Cu6Si16

H2 removal ratio [%]

60 50 40 30 20 10 0

0

1 2 3 4 H2 permeation flux [cm3 cm-2 min-1]

5

Figure ofof H2Hpermeation flux in the amorphous alloy membranes and Figure 7.7.HH22 removal removalratio ratioasasa afunction function 2 permeation flux in the amorphous alloy membranes Pd Ag membrane at 623 K and 0.05 MPa-G in the feed side.side. 77 23 and Pd77 Ag23 membrane at 623 K and 0.05 MPa-G in the feed

4. Conclusions 4. Conclusions We insteam steam reforming reforming of We evaluated evaluatedthe thePd Pdmembrane membraneperformance performanceover overNi/CeO Ni/CeO22and andCo/CeO Co/CeO22 in ethanol. showedlow lowHH22 yield yieldcompared comparedto toCo/CeO Co/CeO22 ethanol. In In the the conventional conventionalfixed fixedbed bedreactor, reactor,Ni/CeO Ni/CeO2 2showed although the conversion to C products was much higher at the temperatures. In the membrane C11 temperatures. reactor, separationimproved improvedboth bothconversion conversionofofethanol ethanoltotoCC11products productsand andH H22 reactor,the thesimultaneous simultaneousHH22separation yield. For For Ni/CeO Ni/CeO2,2the , theHH exceeded that Co/CeO at 773 K. However, at 673 K, although 2 yield exceeded that inin Co/CeO 2 at2 773 K. However, at 673 K, although H2 2 yield H Ni/CeO slightly increased by H2 removal, it was lower Co/CeO 2 yield 2 was 2. 2 was slightly increased by H 2 removal, it was lower thanthan thatthat overover Co/CeO 2. The yield overover Ni/CeO The difference of H the H2 removal effect in those catalysts could be due to their reaction mechanism. difference of the 2 removal effect in those catalysts could be due to their reaction mechanism. At At high reaction temperature, the higher the reaction rate of steam reforming of ethanol in Ni/CeO high reaction temperature, the higher the reaction rate of steam reforming of ethanol in Ni/CeO22 compared thehigher higherthe theincrease increase rate of H22 yield yield that that was was achieved due to the higher compared to to Co/CeO Co/CeO22, ,the equilibrium However, at at low lowreaction reaction temperature, temperature, the the methane-forming methane-forming removal. However, equilibrium shift shifteffect effectby byH H22 removal. reaction in Ni/CeO inhibits the H permeation, resulting in a low H yield. From these results, reaction in Ni/CeO22inhibits the H2 2permeation, resulting in a low H2 yield. From these results, thethe H2 2 H membrane improve thermodynamically at high reaction temperature, 2 separation 2 yield yield thermodynamically at high reaction temperature, but separation membrane cancan improve thethe H2H but simultaneous kinetically inhibited methaneformation formationby byH H22removal removal at at a low the the simultaneous H2 H separation kinetically inhibited methane low 2 separation reaction temperature. reaction temperature. The amorphous alloy membranes with in the the steam steam The amorphous alloy membranes with different different compositions compositions were were employed employed in reforming of ethanol, and the membrane reactor performance was compared with the Pd 7777Ag reforming of ethanol, and the membrane reactor performance was compared with the Pd Ag2323 membrane. Regardless of membrane composition, the membrane reactor performance could be setset in membrane. Regardless of membrane composition, the membrane reactor performance could be the order of H permeation flux. in the order of2H 2 permeation flux. Acknow ledgements:ThisThis was performed under the inter-university cooperative research Acknowledgments: workwork was performed under the inter-university cooperative research program(Proposal No. 10G0045) ofNo. the10G0045) Advanced Center of Metallic Institute forInstitute materials program(Proposal of Research the Advanced Research CenterGlasses, of Metallic Glasses, for Research, materials Tohoku University Research, Tohoku University Author AuthorContributions: Contributions:M.M. M.M.and andS.U. S.U.conceived conceivedand anddesigned designedthe theexperiments; experiments;Y.Y. Y.Y.performed performedthe theexperiments experiments and analyzed the data; Y.O. contributed analysis tools; S.Y. provided the amorphous membranes; M.M. wrote and analyzed the data; Y.O. contributed analysis tools; S.Y. provided the amorphous membranes; M.M. wrote the paper. the paper.. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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4. 5. 6. 7. 8. 9.

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18. 19. 20. 21. 22. 23.

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