Effects of Catalysts and Membranes on the Performance of Membrane Reactors in Steam Reforming of Ethanol at Moderate Temperature

Steam reforming of ethanol in the membrane reactor using the Pd77Ag23 membrane was evaluated in Ni/CeO2 and Co/CeO2 at atmospheric pressure. At 673 K, the H2 yield in the Pd77Ag23 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.


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][6][7][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,

Preparation of Catalysts
For the experiment, 15 wt% Ni/CeO 2 and 15 wt% Co/CeO 2 were prepared as the literature reported [31]. For Ni/CeO 2 , nickel acetate tetrahydrate was dissolved in deionized water and stirred at 343 K. After adding CeO 2 (mean particle size: 1 µm), the pH was adjusted to 9 by adding 0.25 M Na 2 CO 3 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/CeO 2 , the preparation procedure was the same to that for Ni/CeO 2 . Cobalt acetate tetrahydrate was used as the cobalt source.
Preparation of Ru-Co/CeO 2 and Pd-Co/CeO 2 was as follows. Ruthenium chloride or palladium chloride was dissolved in deionized water and stirred. Then, Co/CeO 2 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 N 2 flow for 5 h.

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 sputtered with Pd coating with the thickness of approximately 100 nm. The appearance of obtained membrane was shown in Figure 1.
Processes 2016, 4, 18 3 of 11 sputtered with Pd coating with the thickness of approximately 100 nm. The appearance of obtained membrane was shown in Figure 1.

Characterization
Hydrogen permeation tests. The amorphous alloy membranes (size: 10 mm × 10 mm) were placed in the separator sealed by Cu gaskets. The membrane was pre-heated in vacuum up to 673 K. Then, the membrane was cooled down to 623 K. After keeping the membrane at 673 K, H2 was introduced at the appropriate pressure (0.05, 0.10 and 0.15 MPa-G). The flow rate at the permeation side was measured by the soup-film flow meter.
Catalytic tests. The steam reforming of ethanol was carried out in a conventional reactor and membrane reactor. In the conventional reactor, the 10 g of catalysts was placed in the reactor and heated to 773 K under an N2 flow. Then, the catalyst was reduced under an H2 flow at 773 K for 1 h. After controlling the reaction temperature (623-773 K), a mixture of water and ethanol with the steam to carbon ratio (S/C) of two was fed into the reactor at atmospheric pressure. The products were analyzed with a GC-8A gas chromatograph (Shimadzu Corporation, Kyoto, Japan) and the outlet gas flow rate was measured by the soup-film flow meter.
In the membrane reactor, the catalysts and membrane were placed in the reactor. The reactor was heated to 773 K under an N2 flow, and catalyst was reduced for 1 h under an H2 flow. The Ar sweep gas was used at the permeation side in the reactor. A mixture of water and ethanol with the S/C of two was fed into the reactor after staying at the reaction temperature. The total pressure at both feed and permeate sides was maintained at atmospheric pressure. The gas composition in the retentate and permeate side was analyzed with the gas chromatograph and the outlet gas flow rate in both sides was measured by the soup flow meter. A Pd77Ag23 membrane (thickness: 20 μm, purchased from Tanaka Kikinzoku Kogyo K.K., Tokyo, Japan) was used as reference.
The conversion of ethanol to C1 products and H2 yield was calculated as follows: Additionally, we defined H2 removal ratio as follows to compare the membrane reactor performance:

Characterization
Hydrogen permeation tests. The amorphous alloy membranes (size: 10 mmˆ10 mm) were placed in the separator sealed by Cu gaskets. The membrane was pre-heated in vacuum up to 673 K. Then, the membrane was cooled down to 623 K. After keeping the membrane at 673 K, H 2 was introduced at the appropriate pressure (0.05, 0.10 and 0.15 MPa-G). The flow rate at the permeation side was measured by the soup-film flow meter.
Catalytic tests. The steam reforming of ethanol was carried out in a conventional reactor and membrane reactor. In the conventional reactor, the 10 g of catalysts was placed in the reactor and heated to 773 K under an N 2 flow. Then, the catalyst was reduced under an H 2 flow at 773 K for 1 h. After controlling the reaction temperature (623-773 K), a mixture of water and ethanol with the steam to carbon ratio (S/C) of two was fed into the reactor at atmospheric pressure. The products were analyzed with a GC-8A gas chromatograph (Shimadzu Corporation, Kyoto, Japan) and the outlet gas flow rate was measured by the soup-film flow meter.
In the membrane reactor, the catalysts and membrane were placed in the reactor. The reactor was heated to 773 K under an N 2 flow, and catalyst was reduced for 1 h under an H 2 flow. The Ar sweep gas was used at the permeation side in the reactor. A mixture of water and ethanol with the S/C of two was fed into the reactor after staying at the reaction temperature. The total pressure at both feed and permeate sides was maintained at atmospheric pressure. The gas composition in the retentate and permeate side was analyzed with the gas chromatograph and the outlet gas flow rate in both sides was measured by the soup flow meter. A Pd 77 Ag 23 membrane (thickness: 20 µm, purchased from Tanaka Kikinzoku Kogyo K.K., Tokyo, Japan) was used as reference.
The conversion of ethanol to C 1 products and H 2 yield was calculated as follows: Conversion o f ethanol to C 1 products " f low rate o f CO, CO 2 and CH 4 in products f eed rate o f ethanolˆ2 , Additionally, we defined H 2 removal ratio as follows to compare the membrane reactor performance: H 2 permeation f low rate rmol{mins Theoritical H 2 production rate rmol{minsˆ1 00.

Results and Discussion
3.1. Membrane Reactor Performance Using Pd 77 Ag 23 Membrane with Co/CeO 2 and Ni/CeO 2 Catalysts Figure 2 shows the comparison of reactor performance of the conventional reactor and membrane reactor with the Pd membrane over Co/CeO 2 and Ni/CeO 2 catalysts. In the conventional reactor, Ni/CeO 2 exhibited higher conversion of ethanol to C 1 products compared to Co/CeO 2 catalysts. However, the H 2 yield over Ni/CeO 2 did not exceed those over Co/CeO 2 at the reaction temperatures from 673 to 773 K. Regardless of catalysts, the membrane reactor exhibited higher conversion and H 2 yield than the conventional reactor, but the influence of Pd 77 Ag 23 membrane was different between the catalysts. The increase in conversion of ethanol to C 1 products over Ni/CeO 2 by the Pd 77 Ag 23 membrane was much higher than those over Co/CeO 2 catalysts, and it achieved more than 90% at 723 K, whereas the conversion was only 65% at 773 K over Co/CeO 2 . The H 2 yield over Co/CeO 2 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 H 2 yield over Ni/CeO 2 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 H 2 removal from the reaction zone through the Pd 77 Ag 23 membrane was higher over Ni/CeO 2 than Co/CeO 2 at the high reaction temperature. However, Co/CeO 2 was suitable for the membrane reactor at low reaction temperature. Figure 3 shows the selectivity of C 1 products. For Ni/CeO 2 , 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 H 2 removal by Pd 77 Ag 23 membrane greatly enhanced the CO 2 selectivity with decreasing of the methane selectivity. Furthermore, the membrane reactor showed lower methane selectivity of 25.9% at 773 K. For Co/CeO 2 , the main product in the conventional reactor was CO 2 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/CeO 2 . 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/CeO 2 . 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 H 2 yield over Ni/CeO 2 in the membrane reactor at 773 K was due to the shift of equilibrium by simultaneous H 2 removal through the Pd 77 Ag 23 membrane. However, at 673 K, it was easy for Ni/CeO 2 to produce methane and the methane steam reforming would be hardly promoted once methane produced even when H 2 was selectively removed from the reaction zone because the methane steam reforming is kinetically and thermodynamically unfavorable at low temperature. Therefore, the low H 2 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/CeO 2 , the Pd 77 Ag 23 membrane could remove produced H 2 from the reaction zone before H 2 was consumed by the reverse methane steam reforming. This means the selective H 2 removal through Pd 77 Ag 23 membrane kinetically inhibited the methane production, resulting in high H 2 yield at low reaction temperature. Indeed, Seelam et al. reported Co/Al 2 O 2 that showed high membrane performance at 673 K compared to Ni/ZrO 2 because of high methane selectivity in Ni/ZrO 2 [27], which is consistent with our experimental results.

Catalyst Development for Improving the Membrane Reactor Performance
Comparing Co/CeO 2 and Ni/CeO 2 , Co/CeO 2 was preferable to Ni/CeO 2 for steam reforming of ethanol at low temperature, and the H 2 removal through the Pd 77 Ag 23 membrane was very effective at achieving high H 2 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/CeO 2 and Pd-Co/CeO 2 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 C 1 products. However, the H 2 yield in the conventional reactor was not changed in Ru-Co/CeO 2 and significantly decreased in Pd-Co/CeO 2 compared to Co/CeO 2 (here, it should be noted that the H 2 yield and conversion to C 1 product on Co/CeO 2 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 H 2 yield can be explained by the thermodynamic equilibrium. The C 1 selectivity at the thermodynamic equilibrium is CO 2 :CO:CH 4 = 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 H 2 yield because produced H 2 was consumed by reverse methane steam reforming from CO 2 and CO. In the membrane reactor, the H 2 yield in both Ru-Co/CeO 2 and Pd-Co/CeO 2 was increased by the simultaneous H 2 separation although the conversion was not changed. In addition, the methane selectivity was decreased by the H 2 removal, and it was much lower than that at the thermodynamic equilibrium as shown in Table 1. This indicates that the H 2 removal by the Pd 77 Ag 23 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 H 2 dissociation/association ability and H 2 spillover effect. Otsuka et al. reported that Pt accelerated the formation rates of H 2 and CO in the partial oxidation of methane by the reverse spillover of H 2 [32]. Lei et al. investigated the effect of Rh in the high temperature water-gas shift reaction, and they found that Rh greatly enhances H 2 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 H 2 molecules from the catalyst through the reverse spillover mechanism. Comparing Ru-Co/CeO 2 and Pd-Co/CeO 2 , Ru-Co/CeO 2 exhibited high H 2 yield and low methane selectivity. From the investigation in C 1 selectivity with the conversions of ethanol as shown in Figure 4, the methane selectivity in Pd-Co/CeO 2 was increased at lower conversions compared to Ru-Co/CeO 2 . This clearly indicates that the promotion effect of methane-forming reaction was higher in Pd than Ru, probably caused by high H 2 storage capacity of Pd. Thus, the Pd membrane could effectively remove H 2 through the reverse spillover on Ru before they reacted with CO 2 or CO to methane, resulting in higher H 2 yield in Ru-Co/CeO 2 . On the other hand, a certain part of hydrogen would react with CO 2 and CO due to relatively high hydrogen concentration on Pd before desorption of H 2 molecules by the reverse spillover mechanism, although the Pd 77 Ag 23 membrane removed H 2 from the reaction zone.   Figure 5 shows the H2 permeability of amorphous alloy membranes. The H2 permeability of Ni-Nb-Zr ternary alloy membrane was increased with increasing Zr content as the literature reported [34,35]. The H2 permeability of (Ni0.6Nb0.4)70Zr30 was approximately 8.8 × 10 −9 mol·m −1 ·s −1 ·Pa −0.5 , which is consistent with the value reported by the researchers [36]. Paglieri et al. reported that the addition of Ta lowered the H2 permeability, although this slightly improves the thermal stability [37]. We have found that the increase in Ta content slightly decreased the H2 permeability of Ni-Nb-Ta-Zr quaternary alloy membranes [38]. Indeed, Ni-Ta-Zr ternary alloy membranes showed the lower H2 permeability in our study as well. In contrast, the addition of a small amount of Zr and Ta increased the H2 permeability in Nb-Ni-Co alloy membranes because the introduction of larger atoms expanded the amorphous structure, resulting in an increase in H2 diffusivity [39]. Thus, the Ni40Nb20Ta5Zr30Co5 alloy membrane exhibited the highest H2 permeability that was comparable to the Pd77Ag23 membrane. The steam reforming of ethanol over Ru-Co/CeO2 was evaluated in the membrane reactors with amorphous alloy membranes. Table 2 summarizes the membrane reactor performance. The H2 yield was clearly related to H2 removal ratio. Indeed, the amorphous membranes with the lowest H2 removal ratio of approximately 10%, such as (Ni0.67Ta0.33)80Zr20 and Pd78Cu6Si16, exhibited the same membrane reactor performance, and a similar H2 yield was obtained on the (Ni0.6Nb0.4)70Zr30 and Ni40Nb20Ta5Zr30Co5 with the highest H2 removal ratio of approximately 50%. However, the H2 removal ratio was not the same order of the H2 permeability. For example, the H2 yield and H2 removal ratio in the (Ni0.6Nb0.4)70Zr30 membrane was higher than that in the (Ni0.5Nb0.5)80Zr20 and was almost comparable to Ni40Nb20Ta5Zr30Co5, although the H2 permeability was on the order of Ni40Nb20Ta5Zr30Co5 > (Ni0.5Nb0.5)80Zr20 ≅ (Ni0.6Nb0.4)70Zr30. This could be owing to their membrane thickness that is in inverse proportion to the H2 permeation flux when the membrane is thick enough.    Figure 5 shows the H 2 permeability of amorphous alloy membranes. The H 2 permeability of Ni-Nb-Zr ternary alloy membrane was increased with increasing Zr content as the literature reported [34,35].

Comparison of Amorphous Alloy Membranes and Pd 77 Ag 23 Membrane in the Membrane Reactor
The H 2 permeability of (Ni 0.6 Nb 0.4 ) 70 Zr 30 was approximately 8.8ˆ10´9 mol¨m´1¨s´1¨Pa´0 .5 , which is consistent with the value reported by the researchers [36]. Paglieri et al. reported that the addition of Ta lowered the H 2 permeability, although this slightly improves the thermal stability [37]. We have found that the increase in Ta content slightly decreased the H 2 permeability of Ni-Nb-Ta-Zr quaternary alloy membranes [38]. Indeed, Ni-Ta-Zr ternary alloy membranes showed the lower H 2 permeability in our study as well. In contrast, the addition of a small amount of Zr and Ta increased the H 2 permeability in Nb-Ni-Co alloy membranes because the introduction of larger atoms expanded the amorphous structure, resulting in an increase in H 2 diffusivity [39]. Thus, the Ni 40 Nb 20 Ta 5 Zr 30 Co 5 alloy membrane exhibited the highest H 2 permeability that was comparable to the Pd 77 Ag 23 membrane.  Figure 5 shows the H2 permeability of amorphous alloy membranes. The H2 permeability of Ni-Nb-Zr ternary alloy membrane was increased with increasing Zr content as the literature reported [34,35]. The H2 permeability of (Ni0.6Nb0.4)70Zr30 was approximately 8.8 × 10 −9 mol·m −1 ·s −1 ·Pa −0.5 , which is consistent with the value reported by the researchers [36]. Paglieri et al. reported that the addition of Ta lowered the H2 permeability, although this slightly improves the thermal stability [37]. We have found that the increase in Ta content slightly decreased the H2 permeability of Ni-Nb-Ta-Zr quaternary alloy membranes [38]. Indeed, Ni-Ta-Zr ternary alloy membranes showed the lower H2 permeability in our study as well. In contrast, the addition of a small amount of Zr and Ta increased the H2 permeability in Nb-Ni-Co alloy membranes because the introduction of larger atoms expanded the amorphous structure, resulting in an increase in H2 diffusivity [39]. Thus, the Ni40Nb20Ta5Zr30Co5 alloy membrane exhibited the highest H2 permeability that was comparable to the Pd77Ag23 membrane. The steam reforming of ethanol over Ru-Co/CeO2 was evaluated in the membrane reactors with amorphous alloy membranes. Table 2 summarizes the membrane reactor performance. The H2 yield was clearly related to H2 removal ratio. Indeed, the amorphous membranes with the lowest H2 removal ratio of approximately 10%, such as (Ni0.67Ta0.33)80Zr20 and Pd78Cu6Si16, exhibited the same membrane reactor performance, and a similar H2 yield was obtained on the (Ni0.6Nb0.4)70Zr30 and Ni40Nb20Ta5Zr30Co5 with the highest H2 removal ratio of approximately 50%. However, the H2 removal ratio was not the same order of the H2 permeability. For example, the H2 yield and H2 removal ratio in the (Ni0.6Nb0.4)70Zr30 membrane was higher than that in the (Ni0.5Nb0.5)80Zr20 and was almost comparable to Ni40Nb20Ta5Zr30Co5, although the H2 permeability was on the order of Ni40Nb20Ta5Zr30Co5 > (Ni0.5Nb0.5)80Zr20 ≅ (Ni0.6Nb0.4)70Zr30. This could be owing to their membrane thickness that is in inverse proportion to the H2 permeation flux when the membrane is thick enough.   The steam reforming of ethanol over Ru-Co/CeO 2 was evaluated in the membrane reactors with amorphous alloy membranes.  30 . This could be owing to their membrane thickness that is in inverse proportion to the H 2 permeation flux when the membrane is thick enough. Finally, we carried out the steam reforming of ethanol in the Pd 77 Ag 23 membrane reactor with different sweep Ar flow rate to understand the effect of H 2 removal by the membrane on H 2 yield. Figure 6 shows the H 2 yield and C 1 selectivity as a function of H 2 removal ratio. The solid lines were interpolated from the experimental results on the Pd 77 Ag 23 membrane reactor by varying the sweep Ar flow rate to obtain different H 2 removal ratio. Interestingly, the H 2 yield and C 1 selectivity in the amorphous membranes fitted the curve well. This clearly shows that the membrane reactor performance was determined by the H 2 removal ratio. Considering the reaction mechanism mentioned in Section 3.1, at the low reaction temperature, the catalytic performance such as C 1 selectivity was kinetically controlled by the H 2 permeation rate through the membrane. Thus, it is not an unexpected result that the membrane reactor performance using different amorphous membranes fitted those with the Pd 77 Ag 23 membrane. In other words, we can roughly predict the membrane reactor performance based on their actual H 2 permeation flux regardless of the metal composition of the alloy membranes. Indeed, the H 2 removal ratio is clearly related to the H 2 permeation flux of the membrane as shown in Figure 7.  Finally, we carried out the steam reforming of ethanol in the Pd77Ag23 membrane reactor with different sweep Ar flow rate to understand the effect of H2 removal by the membrane on H2 yield. Figure 6 shows the H2 yield and C1 selectivity as a function of H2 removal ratio. The solid lines were interpolated from the experimental results on the Pd77Ag23 membrane reactor by varying the sweep Ar flow rate to obtain different H2 removal ratio. Interestingly, the H2 yield and C1 selectivity in the amorphous membranes fitted the curve well. This clearly shows that the membrane reactor performance was determined by the H2 removal ratio. Considering the reaction mechanism mentioned in Section 3.1, at the low reaction temperature, the catalytic performance such as C1 selectivity was kinetically controlled by the H2 permeation rate through the membrane. Thus, it is not an unexpected result that the membrane reactor performance using different amorphous membranes fitted those with the Pd77Ag23 membrane. In other words, we can roughly predict the membrane reactor performance based on their actual H2 permeation flux regardless of the metal composition of the alloy membranes. Indeed, the H2 removal ratio is clearly related to the H2 permeation flux of the membrane as shown in Figure 7.

Conclusions
We evaluated the Pd membrane performance over Ni/CeO2 and Co/CeO2 in steam reforming of ethanol. In the conventional fixed bed reactor, Ni/CeO2 showed low H2 yield compared to Co/CeO2 although the conversion to C1 products was much higher at the temperatures. In the membrane reactor, the simultaneous H2 separation improved both conversion of ethanol to C1 products and H2 yield. For Ni/CeO2, the H2 yield exceeded that in Co/CeO2 at 773 K. However, at 673 K, although H2 yield over Ni/CeO2 was slightly increased by H2 removal, it was lower than that over Co/CeO2. The difference of the H2 removal effect in those catalysts could be due to their reaction mechanism. At high reaction temperature, the higher the reaction rate of steam reforming of ethanol in Ni/CeO2 compared to Co/CeO2, the higher the increase rate of H2 yield that was achieved due to the higher equilibrium shift effect by H2 removal. However, at low reaction temperature, the methane-forming reaction in Ni/CeO2 inhibits the H2 permeation, resulting in a low H2 yield. From these results, the H2 separation membrane can improve the H2 yield thermodynamically at high reaction temperature, but the simultaneous H2 separation kinetically inhibited methane formation by H2 removal at a low reaction temperature.
The amorphous alloy membranes with different compositions were employed in the steam reforming of ethanol, and the membrane reactor performance was compared with the Pd77Ag23 membrane. Regardless of membrane composition, the membrane reactor performance could be set in the order of H2 permeation flux.