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• 

  Hydrogen fuel cell has received particular attention as an alternative energy source to internal engines for transportation purpose^{[1, 2]}. To overcome the hydrogen storage, safety and handling problems, its on-board production from proper hydrogen sources can be used^{[3, 4]}. Methanol is a favorable option which can be converted to hydrogen by steam reforming^[5-7]:

  However, using the conventional Cu/ZnO/Al₂O₃ catalysts in this process CO was produced as by-product in appreciable amount. Since CO is a poison to the fuel cell electrodes, CO content in the reformate gas has to be removed before entering the fuel cell which requires additional reactors and complicates the overall process. Therefore, CO production is to be avoided as much as possible. Also, for mobile application the catalyst has to be stable for long periods of operation^{[1, 5]}. Copper has the highest activity and selectivity in the methanol steam reforming^{[5, 8]}, however, the nature and properties of support, promoter and catalyst composition have a significant effect on the catalyst performance^{[1, 9]}. Many researchers have attempted to develop a new catalyst with effective performance. In their studies various components such as La, Y^[5], Zr^{[1, 10]}, Ce^[1], Cr^[10], Pd^[11-13], Ni, Pt^[13] were investigated and supports other than alumina such as CeO₂^{[7, 14]}, ZrO₂^{[5, 6, 15]}, SiO₂^{[2, 13, 16]}, Cr₂O₃^{[2, 13]}, MnO₂, MgO, La₂O₃, NdO₃^[13] and Nb₂O₅were used. High activity and desired selectivity were reported for Cu-SiO₂ catalyst^{[17, 18]}. Two scheme of reaction on the Cu-SiO₂ catalyst was suggested in the literature. In the first mechanism no CO forms in the reaction^{[13, 18]}:

  The second scheme consists of the steam reforming of methanol and CO formation via reverse water-gas shift reaction^[17]:

  Due to the attractive properties of silica aerogel such as high surface area, large and open mesoporosity in catalytic applications^{[19, 20]}, in previous work^[21], we synthesized and characterized the copper-silica aerogel catalyst. In a typical condition its activity and selectivity was favorable in comparison with a commercial catalyst. However, in addition to catalyst properties, the reaction parameters are also important in the optimization of catalyst performance^{[3, 7]}. Using a commercial Cu/ZnO/Al₂O₃ catalyst, it is observed that even at 200 ℃, CO was formed and its concentration was increased by temperature. Also, at 260 ℃, in order to maintain 90% conversion the maximum feed flow rate may be restricted to 12 cm³/h, in which condition CO selectivity is about 6%^[22]. Inconsistency was observed in the steam to methanol ratio effect on the hydrogen production rate and CO selectivity^{[22, 23]}. It is reported that for Cu/ZnO/Al₂O₃ catalyst CO is formed under the condition that methanol conversion is higher than 70%^[23]. The effect of reaction temperature and weight hourly space velocity (WHSV) in methanol steam reforming using Mo₂C supported on-alumina and zirconia was investigated. The best result was obtained for Mo₂C/ZrO₂ in which at 400 ℃ and WHSV of 3 h^-1, 97% methanol conversion and 5.5 mmol/(h·g) hydrogen production rate could be achieved. However, CO production was very high and its selectivity varied from about 15% to 35% by increasing temperature from 300 to 500 ℃^[24]. The objective in the present study is to find the dependency of the activity and selectivity of Cu-SiO₂ aerogel catalyst to the operating conditions in the methanol steam reforming to gain enhanced understanding of the reaction mechanism using the Cu-SiO₂ aerogel catalyst, and to handle the process in the optimum conditions. Therefore, after investigation of catalyst stability, influence of reaction temperature, steam to methanol molar ratio, methanol feed rate or contact time and carrier gas flow rate on the reaction rate and CO selectivity were discussed.

  1   Experimental

  Copper-silica aerogel catalyst which was used in this study was synthesized using silicate sodium and copper nitrate trihydrates as silica and copper precursor, respectively. Cogelation and ambient pressure drying methods were used to incorporate the copper in the silica matrix and to dry the gels, respectively. Chemical surface modification of gels was performed using 20% HMDZ solution in hexane. Detailed synthesis procedure was reported in^[21].

  Catalytic tests were performed in a fixed bed Pyrex tubular reactor (i.d=4 mm) which was immersed in a temperature-controlled furnace. Liquid feed mixture (methanol: water) was introduced with a given flow rate to the reactor entrance on the inert Pyrex bed using a syringe pump (702 SM Titrino, Metrohm), and was carried by argon to the reactor, which evaporated before reaching the catalyst bed. The reactor effluent gas was dried by passing through two condensers in the line and then was analyzed using an on-stream gas chromatography (Agilent Technologies 7890AGC) equipped with TCD detector and HP-Plot/Q (30 m, 0.53 mm, 40 μm) capillary column. The flow rates of water and methanol entrance were known and the product gas flow rate and CO₂, CO and H₂ concentration in the product gas were measured. The methanol conversion was determined from the material balance. Prior to MSR reaction the catalyst was reduced by H₂ stream at 300 ℃ for 2 h.

  2   Results and discussion

  2.1   Catalyst stability

  In our previous work^[21], the performance of catalyst in terms of activity and selectivity was compared with commercial CuO/ZnO/Al₂O₃ which showed better results. Another important requirement in development of a new catalyst is its stability. To investigate the synthesized copper-silica aerogel stability under normal reaction condition the methanol conversion and hydrogen production rate as a function of time on stream over a period of 60 h was obtained and is shown in Figure 1.

  Figure 1.  Catalytic activity of copper silica aerogel in methanol steam reforming.

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  As can be seen, no deactivation of the catalyst was observed during this time. Even this sample was used several days at different conditions for various catalytic tests experiments but decreased activity in catalysts was not observed. Comparison of various catalysts stability in the methanol steam reforming which reported in literature is shown in Table 1. These results showed relatively good stability of prepared copper silica aerogel catalyst in methanol steam reforming process.

  Table 1.  Stability comparison of various methanol steam reforming catalysts

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  Catalyst	Reaction temp. t/℃	Time on stream t/h	Decrease in MeOH conversion w/%
  CuO/Cr2O3/F2O3/Graphite[2]	220	100	about 30%-40% of its initial activity
  CuO/ZrO2/CeO2[2]
  CuO/ZnO/Al2O3/Graphite[2]
  Cu/La2O3/ZrO2 [5]	300	10	about 18%-24% of its initial activity
  Cu/Y2O3/ZrO2 30/20/50[5]	300	10	stable at about 70%
  Cu/Y2O3/ZrO2[5]	300	10	about 11%-32% of its initial activity
  Cu/CeO2/ZrO2[5]	300	10	about 30% of its initial activity
  Cu/Al2O3/ZrO2[5]	300	10	about 21%-32% of its initial activity
  CuO/CeO2[14]	300	40	from 98% to 90% in the first 10 h, then stable at 90%
  Cu/ZnO/Al2O3[15]	400	8	from 82% to 52%
  Cu/ZrO2[15]	400	8	from 64% to 22%
  Cu/ZnO/25%ZrO2[15]	400	24	from 93% to 70%
  40-Cu/SiO2[17]	300	6	from 95% to 84%
  Mo2C/ZrO2[24]	400	10	stable at about 90%
  Cu/ZnO/Al2O3[25]	250	8	from 39% to 33%
  50%Cu/ZnO[25]	250	8	stable at 22%-23%
  40%Cu/ZnO/ZrO2[25]	250	8	stable at 31%-32%
  50%Cu/ZnO[25]	400	7	from 89% to 69%
  40%Cu/ZnO/ZrO2[25]	400	28	from 100% to 80%
  Cu/ZnO/ZrO2/Al2O3[26]	260	5	from 98% to 92%
  ZnO/Cu/SiO2[27]	300	6	stable at 87%-88%
  CuO/ZnO/ZrO2/Al2O3[28]	250	110	about 18%-31% of its initial activity
  CuO/ZnO/Al2O3[29]	250	320	from 60% to 52%
  CuO/ZnO/Al2O3[30]	250	24	stable at about 92%-94%
  This work	300	60	stable at about 88%-92%

  Table 1.  Stability comparison of various methanol steam reforming catalysts

  2.2   Effect of feed flow rate

  Methanol conversion and CO selectivity at different temperatures vs feed flow rate (H₂O/CH₃OH=2) is shown in Figure 2. Water: methanol molar ratio in these experiments was 2. Obviously, by increasing the feed flow and decreasing the contact time, the methanol conversion or hydrogen production per mole of methanol was reduced; however, the hydrogen production rate was increased.

  Figure 2.  Effect of the feed flow rate on the methanol conversion, H₂ production and CO selectivity at different temperatures (H₂O/CH₃OH (molar ratio)=2)

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  For example, hydrogen production rate was increased from 69 to 200 mmol/(h·g) when WHSV was increased from 1.6 to 5.3 h^-1 at 350 ℃, while methanol conversion was decreased from complete conversion to 88%. Also, by increasing the feed flow rate the CO formation was decreased. CO production was observed at nearly complete conversions. These results may indicate that on the copper-silica aerogel catalyst CO does not form in the reaction mechanism or as a by-product from the reactants, but is produced as a consecutive product from the main reaction products through the reverse water-gas shift reaction. The higher the conversion, the higher the main product (CO₂ and H₂) concentration and thus the possibility of CO production is increased. Even at high temperatures which have lower conversions the formation of CO was not observed and as the methanol exists in the reaction space the CO formation is not desirable. As a result, the copper-silica aerogel catalyst in the methanol steam reforming CO does not participate in the reaction mechanism and the direct conversion of reactant to CO is unlikely. This result was in good agreement with the observations and proposed mechanism for Cu-SiO₂ catalyst in^{[13, 17, 18, 31]}.This is an advantage for the catalyst in MSR process which has no tendency to produce CO from reactants and also its selectivity to the consecutive CO production reaction is very low.

  2.3   Effect of water: methanol ratio

  Hydrogen production rate as a function of methanol inlet at different molar ratios of water to methanol at 300 ℃ was plotted in Figure 3. As seen, when this ratio is increased the hydrogen production per mole of methanol inlet was increased slightly. These results show that the presence of steam in the reaction environment has a favorable effect on the catalyst activity. This becomes even more important when we know that, in this case fixed rate of methanol does not mean the same total feed rate; however, for a constant methanol inlet, by increasing the water to methanol ratio the total feed flow was increased, and by increasing the water: methanol ratio at a constant methanol feed, although the total liquid feed flow is increased or retention time was decreased, the hydrogen production or methanol conversion was increased. In other words, at a constant methanol feed, with higher ratios of water to methanol despite the retention time of methanol in the reactor being lower, the methanol conversion is higher. For instance, at about the same methanol feed rate, 63.5 and 62.8 mmol/(h·g) for steam to methanol ratio of 3.0 and 2.0, respectively, WHSV is 5.4 and 4.3 h^-1 and hydrogen production rate is 151.3 and 134.3 mmol/(h·g), respectively. This favorable effect of steam concentration was reported in^{[23, 32]}, too. This favorable effect is not limited to catalyst activity, as the higher ratios of water to methanol lead to desired selectivity. So by increasing this ratio the minimum temperature in which the formation of CO was observed was increased. In other words, by increasing this ratio CO production at the same reaction temperature was reduced. This result is in agreement with the obtained result in the section 2.2 which indicates CO was produced as a secondary product through reverse water gas shift reaction. Indeed, by increasing the amount of water in the reaction environment by increasing its molar ratio in the feed, the progress of the water gas shift reaction toward CO production side will be inhibited or decreased.

  Figure 3.  Hydrogen production rate using different molar ratio (R) of steam to methanol

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  The lowest reaction temperature at which CO was formed and the corresponding CO selectivity for different water: methanol molar ratio was given in Table 2. For molar ratios of 1.2 and 2 the formation of CO was observed at 325 ℃, however, for ratio of 2 CO was not formed at higher feed flow rates. This indicates that CO selectivity is lower for a ratio of 2 rather than 1.2. For water: methanol molar ratio of 3.0 and 5.0 the lowest temperature of CO formation was 350 and 375 ℃, respectively. Thus, the CO selectivity decreases with water: methanol molar ratio.

  Table 2.  Lowest temperature of the CO formation and corresponding CO selectivity at different steam to methanol molar ratios

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  Steam to methanol molar ration	Lowest temperature of CO formation t/℃	Liquid feed flow q/(mL·h-1)	CO selectivity s/%
  1.2	325	1.2	8.8
  		2.4	6.9
  		4.8	5.2
  		6.0	4.5
  2	325	1.2	8.2
  		1.8	4.4
  		3.0	0
  		4.8	0
  3	350	3.0	3.4
  		6.0	2.8
  		9.0	2.0
  5	375	8.4	2.5

  Table 2.  Lowest temperature of the CO formation and corresponding CO selectivity at different steam to methanol molar ratios

  2.4   Effect of reaction temperature

  Effect of reaction temperature on the reaction rate is shown in Figures 4 and 5. Results of Figures 4 and 5 are obtained at water to methanol ratios of 5 and 2, respectively. As seen, temperature had a significant effect on the methanol conversion. For instance, for molar ratio of 5.0 the methanol conversion increases from 49% to 100% as the temperature was increased from 210 to 275 ℃ at feed rate of 1.8 mL/h, and increases from 21% to 84% as the temperature was increased from 210 to 300 ℃ at feed rate of 8.4 mL/h. At molar ratio of 2.0 and feed rate of 3 mL/h the methanol conversion increases from 30% to 80% as the temperature was increased from 240 to 300 ℃. In the case of selectivity, for all performed experiments no CO formation was observed at temperatures lower than 300 ℃ and the lowest temperatures of CO formation and CO selectivity for various conditions are given in Table 2. Generally, it is observed that CO formation increases with reaction temperature which is compatible with the CO formation through reverse water gas shift reaction. Indeed, reverse water gas shift reaction is an endothermic reaction with CO₂ and H₂ as reactants. In addition to the direct influence of rising temperature, the methanol conversion and also CO₂ and H₂ concentration in the reactor were increased as the temperature rose. Therefore, as the temperature is increased the conditions for this reaction became favorable. The influence of temperature on the CO selectivity is also shown in Figure 4.

  Figure 4.  Effect of reaction temperature on the methanol conversion using H₂O/CH₃OH molar ratio of 5.0 at different liquid feed rates

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  Figure 5.  Effect of reaction temperature on the methanol conversion and CO selectivity using H₂O/CH₃OH molar ratio of 2.0 at different liquid feed rates

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  It is found that to maintain the methanol conversion of at least 95%, and to produce the highest hydrogen amount without CO formation, feed rates of 3.0 and 8.4 mL/min and temperatures of 325 and 350 ℃ can be used for steam to methanol ratio of 2.0 and 5.0, respectively. In these conditions the hydrogen production rates were 114 and 186 mmol/(h·g).

  2.5   Effect of carrier gas flow

  The effect of carrier gas flow rate variation on the methanol conversion at a constant feed flow rate and various conditions is shown in Figure 6.

  Figure 6.  Effect of carrier gas flow rate on the methanol conversion at different conditions

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  In all conditions, increasing the carrier flow rate decreases the methanol conversion and this is due to the reduction of reactant residence time. However, this effect is not significant, so when the carrier flow rate increases 3, 4 times, the methanol conversion only reduces about 5%-10%. Although increasing the carrier flow decreases the hydrogen production, on the other hand, it slightly reduces CO selectivity. The effect of carrier flow rate on the CO selectivity is given in Figure 7. Here, CO₂ and H₂ concentration is reduced in the product and according to CO formation through reverse water gas shift reaction the possibility of reaction between CO₂ and H₂ and CO formation is lessened.

  Figure 7.  Effect of carrier gas flow rate on CO selectivity at different conditions (R: H₂O/CH₃OH molar ratio, F: liquid feed flow: (mL/h), t: reaction temperature (℃))

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  3   Conclusions

  The stability of the prepared copper-silica aerogel catalyst in the normal reaction condition of methanol steam reforming was established. Reaction parameters showed a significant effect on the hydrogen production rate and CO selectivity. The most effective parameter on the catalyst performance was reaction temperature. On the basis of feed flow and steam to methanol molar ratio the complete conversion of methanol can be achieved at about 300-350 ℃ without any CO formation. Increasing the steam to methanol molar ratio had a favorable effect on the reaction, and CO formation was decreased when this ratio was increased. To maintain the methanol conversion value higher than 95% and to achieve the highest rate of hydrogen production with no CO formation, the best result was obtained at 350 ℃ with liquid feed flow of 8.4 mL/min and steam to methanol ration of 5.0. In this condition the WHSV was 7.85 h^-1 and hydrogen production rate was 186 mmol/(h·g). Lower CO selectivity was obtained at higher contact time, higher liquid feed flow rate or carrier gas flow, and lower concentration of H₂ and CO₂ in the reactor effluent as well as lower reaction temperature. The most likely path to CO formation is reverse water gas shift reaction. This scheme is a consecutive reaction and takes place about complete conversions in which no methanol presents within the reactor. The Cu-SiO₂ aerogel catalyst had very low selectivity to CO formation.

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• 

  Figure 1  Catalytic activity of copper silica aerogel in methanol steam reforming.

  (t=300 ℃, H₂O/CH₃OH (molar ratio)=2, Ar flow=30 mL/min, MeOH Feed=23.5 mmol·h^-1·g_cat^-1)

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  Figure 2  Effect of the feed flow rate on the methanol conversion, H₂ production and CO selectivity at different temperatures (H₂O/CH₃OH (molar ratio)=2)

  ■: 240 ℃; ▲: 300 ℃◇: 325 ℃; ●: 350 ℃

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  Figure 3  Hydrogen production rate using different molar ratio (R) of steam to methanol

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  Figure 4  Effect of reaction temperature on the methanol conversion using H₂O/CH₃OH molar ratio of 5.0 at different liquid feed rates

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  Figure 5  Effect of reaction temperature on the methanol conversion and CO selectivity using H₂O/CH₃OH molar ratio of 2.0 at different liquid feed rates

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  Figure 6  Effect of carrier gas flow rate on the methanol conversion at different conditions

  (R: H₂O/CH₃OH molar ratio, F: liquid feed flow: (mL/h), t: reaction temperature (℃))

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  Figure 7  Effect of carrier gas flow rate on CO selectivity at different conditions (R: H₂O/CH₃OH molar ratio, F: liquid feed flow: (mL/h), t: reaction temperature (℃))

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  Table 1.  Stability comparison of various methanol steam reforming catalysts

  Catalyst	Reaction temp. t/℃	Time on stream t/h	Decrease in MeOH conversion w/%
  CuO/Cr2O3/F2O3/Graphite[2]	220	100	about 30%-40% of its initial activity
  CuO/ZrO2/CeO2[2]
  CuO/ZnO/Al2O3/Graphite[2]
  Cu/La2O3/ZrO2 [5]	300	10	about 18%-24% of its initial activity
  Cu/Y2O3/ZrO2 30/20/50[5]	300	10	stable at about 70%
  Cu/Y2O3/ZrO2[5]	300	10	about 11%-32% of its initial activity
  Cu/CeO2/ZrO2[5]	300	10	about 30% of its initial activity
  Cu/Al2O3/ZrO2[5]	300	10	about 21%-32% of its initial activity
  CuO/CeO2[14]	300	40	from 98% to 90% in the first 10 h, then stable at 90%
  Cu/ZnO/Al2O3[15]	400	8	from 82% to 52%
  Cu/ZrO2[15]	400	8	from 64% to 22%
  Cu/ZnO/25%ZrO2[15]	400	24	from 93% to 70%
  40-Cu/SiO2[17]	300	6	from 95% to 84%
  Mo2C/ZrO2[24]	400	10	stable at about 90%
  Cu/ZnO/Al2O3[25]	250	8	from 39% to 33%
  50%Cu/ZnO[25]	250	8	stable at 22%-23%
  40%Cu/ZnO/ZrO2[25]	250	8	stable at 31%-32%
  50%Cu/ZnO[25]	400	7	from 89% to 69%
  40%Cu/ZnO/ZrO2[25]	400	28	from 100% to 80%
  Cu/ZnO/ZrO2/Al2O3[26]	260	5	from 98% to 92%
  ZnO/Cu/SiO2[27]	300	6	stable at 87%-88%
  CuO/ZnO/ZrO2/Al2O3[28]	250	110	about 18%-31% of its initial activity
  CuO/ZnO/Al2O3[29]	250	320	from 60% to 52%
  CuO/ZnO/Al2O3[30]	250	24	stable at about 92%-94%
  This work	300	60	stable at about 88%-92%

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  Table 2.  Lowest temperature of the CO formation and corresponding CO selectivity at different steam to methanol molar ratios

  Steam to methanol molar ration	Lowest temperature of CO formation t/℃	Liquid feed flow q/(mL·h-1)	CO selectivity s/%
  1.2	325	1.2	8.8
  		2.4	6.9
  		4.8	5.2
  		6.0	4.5
  2	325	1.2	8.2
  		1.8	4.4
  		3.0	0
  		4.8	0
  3	350	3.0	3.4
  		6.0	2.8
  		9.0	2.0
  5	375	8.4	2.5

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