ZnO catalyst for methanol steam reforming

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CeO2eZrO2-promoted CuO/ZnO catalyst for methanol steam reforming Lei Zhang a,b, Liwei Pan a,*, Changjun Ni a, Tianjun Sun a, Shengsheng Zhao a, Shudong Wang a,**, Anjie Wang b, Yongkang Hu c a

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China c Fushun Research Institute of Petroleum and Petrochemicals, Fushun 113001, China b

article info

abstract

Article history:

The Cu-based catalysts with different supports (CeO2, ZrO2 and CeO2eZrO2) for methanol

Received 27 September 2012

steam reforming (MSR) were prepared by a co-precipitation procedure, and the effect of

Received in revised form

different supports was investigated. The catalysts were characterized by means of N2

12 December 2012

adsorptionedesorption, X-ray diffraction, temperature-programmed reduction, oxygen

Accepted 9 January 2013

storage capacity and N2O titration. The results showed that the Cu dispersion, reducibility

Available online 28 February 2013

of catalysts and oxygen storage capacity evidently influenced the catalytic activity and CO selectivity. The introduction of ZrO2 into the catalyst improved the Cu dispersion and

Keywords:

catalyst reducibility, while the addition of CeO2 mainly increased oxygen storage capacity.

Methanol steam reforming

It was noticed that the CeO2eZrO2-containing catalyst showed the best performance with

Hydrogen

lower CO concentration, which was due to the high Cu dispersion and well oxygen storage

CO concentration

capacity. Further investigation illuminated that the formation of CO on CuO/ZnO/CeO2

CeO2eZrO2

eZrO2 catalyst mainly due to the reverse water gas shift. In addition, the CuO/ZnO/CeO2

CO formation

eZrO2 catalyst also had excellent reforming performance with no deactivation during 360 h run time and was used successfully in a mini reformer. The maximum hydrogen production rate in the mini reformer reached to 162.8 dm3/h, which can produce 160e270 W electric energy power by different kinds of fuel cells. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The proton exchange membrane fuel cells (PEMFC) have been investigated widely for electric power generation application in these days [1,2]. Pure hydrogen is the most suitable feed for PEMFC, but the supply and the storage of hydrogen raise safety problems. Liquid fuels, such as methanol and gasoline, are considered the most suitable at least in early stages of commercialization. For this reason, there is currently an ongoing development of new and more efficient fuel

reforming technologies. Methanol has more advantages compared with other liquid fuels because it offers a high hydrogenecarbon ratio, absence of carbonecarbon bonds, potentially high production capacity, and easy handling [3]. There are mainly four approaches in producing H2 from methanol, namely decomposition (DM), steam reforming (MSR), partial oxidation (POM) and autothermal reforming (ATM). Among them, the maximum hydrogen concentration (nearly 75%) and relatively small amount of carbon monoxide can be achieved by MSR, as shown in Eq. (1).

* Corresponding author. Tel.: þ86 411 84379332; fax: þ86 411 84662365. ** Corresponding author. Tel.: þ86 411 84379052; fax: þ86 411 84662365. E-mail addresses: [email protected] (L. Zhang), [email protected] (L. Pan), [email protected] (S. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.053

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Unfortunately, the MSR process produces CO as a byproduct in appreciable amounts. The precious metal electrocatalyst at the fuel cell anode is poisoned by CO already in concentrations higher than few ppms. There are two potential sources of CO in the reforming reaction network: the reverse wateregas shift (RWGS) and the direct methanol decomposition (DM), as shown in Eqs. (2) and (3) respectively, therefore the question how to decrease the CO concentration is an enormous problem in methanol reforming. CH3 OHðgÞ þ H2 OðgÞ/CO2 ðgÞ þ 3H2 ðgÞ DH298 ¼ 49:5 kJ=mol

(1)

H2 ðgÞ þ CO2 ðgÞ/COðgÞ þ H2 OðgÞ DH298 ¼ 41:2 kJ=mol

(2)

CH3 OHðgÞ/2H2 ðgÞ þ COðgÞ DH298 ¼ 92:0 kJ=mol

(3)

Many investigations on the MSR have been published which used Cu-based catalysts [4e9]. The high activity of Cu-based catalysts was attributed to high dispersion of Cu metal crystallites. Among Cu-based catalysts, the commercial CuO/ZnO/ Al2O3 catalyst has been used most widely in the MSR reaction in which ZnO is added to improve the dispersion and redox properties of the copper phases, and the Al2O3 is added to increase the surface area and prevent Cu sintering [10]. Only a small amount of Al2O3 is usually used in commercial CuO/ ZnO/Al2O3 catalyst, because high concentrations can have negative effect on the catalytic activity [11]. Due to the negative effect of Al2O3 support in methanol reforming catalysts, the trend in recent years has been the addition of another component to the CuO/ZnO/Al2O3 catalysts or simply use other supports rather than alumina. Compared with the conventional Al2O3-supported Cu catalysts, ZrO2- or CeO2containing catalysts have shown increased activity and reduced CO levels in the MSR reaction [12e15]. The promoting effect of ZrO2 has been attributed to improving the dispersion of Cu [16] and reducibility of catalyst [17], as well as preventing the sintering of Cu [8,16]. Agrell [17] used the reductioneoxidation cycle to confirm the enhanced reducibility of Zr-containing catalysts. Patel [18] proposed that ZrO2 increased the amount of Cuþ, improving the activity and stability of the catalysts. Likewise, the application of CeO2, as a carrier or a promoter, has been found to improve the activity of Cu-based catalysts and minimize the CO content in the reforming gas [19]. Moreover, the previous experiments have proved that CeO2 has a high oxygen storage capacity [20], which tends to have a beneficial effect on the catalytic performance [20]. The interaction between the CeO2 and ZrO2 can lead to the formation of a thermally stable solid solution, which has effect on increasing the mobility of oxygen in previous work of our team [21]. The present study focuses on the following: (1) Investigate the correlation between the catalyst structure and performance. (2) The function of ZrO2, CeO2 and CeO2eZrO2 support in CuO/ ZnO catalyst. (3) The reason for CO formation and the mechanism of MSR reaction by situ FT-IR. (4) Investigate the performance of CuZnCeZr catalyst in mini reformer.

2.

Experimental

2.1.

Catalyst preparation

The Cu-based catalysts were prepared by co-precipitation technique in aqueous solution. The solution of Cu(NO3)2$3H2O, Zn(NO3)2$6H2O, Ce(NO3)3$6H2O, and Zr(NO3)4$5H2O (0.1 M metal nitrate solution) was prepared in distilled water respectively and then mixed in a volume proportion corresponding to the final composition of the catalysts to be obtained. The resulting solution was stirred and heated to 60  C. Then the 0.5 M Na2CO3 aqueous solution was added to the nitrate solution under vigorous stirring until pH ¼ 8 was reached. The achieved precipitates were aged at 60  C for 2 h under stirring and then deposited at room temperature for 12 h without stirring. The excess solution was removed by filtering. The precipitates were thoroughly washed with deionized water in order to remove the sodium ions. After washing, the precipitates were dried at 110  C for 12 h. Finally, the precipitates were calcined at 400  C for 2 h. The obtained powders were pelletized, crushed, sieved to a particle of 0.28e0.45 mm, that was then used as catalysts for methanol steam reforming. The catalysts were named CuZn, CuZnCe, CuZnZr and CuZnCeZr. In CuZn catalyst the mass ratio of CuO/ ZnO was 45/20 (69 wt% CuO). In CuZnCe, CuZnZr and CuZnCeZr catalysts, the weight ratio of CuO/ZnO/support (CeO2, ZrO2 or CeO2eZrO2) was 45/20/35. And the molar ratio of CeO2/ZrO2 was 1 in the CuZnCeZr catalyst. The commercial CuO/ZnO/Al2O3 catalyst (CB-7) with a weight ratio of 60/25/15 was used as a reference.

2.2.

Characterization of catalysts

2.2.1.

N2 adsorptionedesorption

The specific surface areas of the calcined samples were calculated from N2 adsorptionedesorption isotherms acquired at liquid N2 temperature with a Quantachrome NOVA 2200E instrument. The powders were firstly outgassed at 300  C for 3 h prior to the measurement of the adsorption isotherm. The specific surface areas were calculated by method of Brunauer, Emmet, and Teller (BET isotherm).

2.2.2.

X-ray diffraction (XRD)

Power X-ray diffraction (XRD) patterns were collected on a Ru-200B with monochromatic Cu Ka radiation operating at 40 kV and 200 mA, with 2q from 20 to 70 and a scanning speed of 5 /min. Average particle sizes were calculated from the line-broadening of the XRD peaks by the Scherer equation [Eq. (4)]. d¼

Kl FW cos q

(4)

where K is a constant generally taken as unity; l is the wavelength of the incident radiation; FW is the full width at half maximum and q is the peak position.

2.2.3.

Temperature-programmed reduction (TPR)

TPR was performed on a Quantachrome ChemBET Pulsar instrument. 50.0 mg catalyst was heated in an Ar flow (30 ml/min) to 300  C with the heating rate of 10  C/min and kept there for

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 3 9 7 e4 4 0 6

30 min. TPR measurement was performed by heating the sample in a stream of 10% H2/Ar (30 ml/min) from room temperature to 500  C with 10  C/min heating rate. The hydrogen consumption was continuously monitored by means of a thermal-conductivity detector during the reduction.

2.2.4.

Cu surface area measurement by N2O titration

The copper surface area was measured on a Quantachrome ChemBET Pulsar instrument using the N2O titration method, as is typically done for copper based catalysts [22,23]. 50.0 mg catalyst was placed in a quartz tube reactor and held in place by quartz cotton. Prior to measurement, the samples were pre-reduced at 500  C with 10% H2 in Ar for 30 min, in order to reduce the copper species to Cu0. After the reduction, the gas flow was switched to Ar and the catalyst was cooled to room temperature. In the measurement, Ar flow was replaced by a mixture of 7.56%N2O in Ar at 90  C for 1 h, during which Cu0 on the surface was transformed to Cu2O (Eq. (5)). The nitrous oxide molecule was assumed to react selectively with the reduced surface copper atom with an O/Cu(s) stoichiometric ratio of 0.5, without oxidizing of the bulk copper. From the amount of chemisorbed oxygen (Cu2O(s)), Cu surface area was estimated by assuming the surface density of 1.46  1019 copper atoms per square meter. The dispersion is defined as the ratio of copper atoms on the surface of the catalysts to the total number of copper atoms in the catalyst. 2Cu þ N2 O/Cu2 O þ N2

2.2.5.

(5)

Oxygen storage capacity (OSC)

OSC measurements were performed in the same unit used for TPR runs. The sample (0.1 g) was previously dried at 300  C for 1 h under He flow, followed by reduction under 10% H2 in Ar at 800  C for 1 h. After the reduction, the gas flow was switch to He and the samples were cooled to room temperature. Then, the samples were exposed to a 3% O2 in He flow from room temperature to 550  C with 20  C/min heating rate and kept for 1 h.

2.2.6.

In-situ FT-IR spectroscopy

In-situ FT-IR spectra were collected on Nicolet 6700 FT-IR spectrometer with MCT/A detector. For adsorption experiment, pressed disk of 20 mg pure catalyst was contained in infrared cell with a ZnSe window which can work at high temperature. In-situ absorbance spectra were obtained by collecting 32 scans at 6 cm1 resolution. The catalyst was pretreated in Ar gas at 300  C for 30 min and then cooled to the adsorption temperature. The pure methanol was dosed via saturators in an Ar stream. Their concentrations in the combined stream were controlled via the adjustment of mass flow controllers. The overall concentration was approximately 5% in a total flow of 30 cm3/min. The measurements were conducted beginning at 30  C.

2.3.

Catalytic performance

Catalytic activity in methanol steam reforming (MSR) was evaluated in a fixed-bed quartz tube reactor (internal diameter: 8 mm) at 230e260  C in atmospheric pressure. 2.0 cm3 catalyst was packed between two quartz wool plugs to keep the catalyst bed in place. Prior to each run, the catalyst was

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reduced in-situ in a stream of 5%H2/N2 (flow rate 100 cm3/min) at 280  C for 2 h. When the temperature had been stabilized at the reaction temperature, the methanol/water mixture was fed to the evaporator by a liquid syringe pump. For all catalyst experiments of this study, the water/methanol molar ratio (W/M) was set at 1.2. The unreacted condensable species were collected in an ice bath condenser and trapped in a drying tube. The gas products were analyzed on-line by an Agilent7890D gas chromatograph (GC) equipped with a thermalconductivity detector (TCD). The conversion of methanol was calculated as
Xð%Þ ¼ FR  CCO þ CCO2  ð32 þ 18 wÞ=ðF  r  22:4  1000Þ  100% (6) The CO selectivity (SCO) is determined by
SCO ð%Þ ¼ CCO = CCO þ CCO2  100%

(7)

where FR is the flow rate of the reforming gas (cm3/min); F is the flow rate of the methanol and water solution (cm3/min); r is the density of methanol and water solution (g/cm3); w is the molar ratio of water to methanol and CCO as well as CCO2 represent the molar concentrations of CO and CO2 in the reforming gas.

3.

Results and discussion

3.1.

Surface area analysis and catalyst performance

The data of specific surface area, pore volume, copper surface area and copper dispersion are presented in Table 1. As expected, the specific surface area of prepared CuZn catalyst without any supports was the lowest one in all catalysts, only 42.0 m2/g. The specific surface area of CuZnCe catalyst was slightly ascension, which was 50.6 m2/g. The largest one was CuZnZr catalyst with 143.5 m2/g. The specific surface area of CuZnCeZr catalyst was 90.8 m2/g, which was almost the same as the commercial CuZnAl catalyst (95.6 m2/g). Moreover, the Cu surface area and Cu dispersion of prepared catalysts are also displayed in Fig. 1. The lowest Cu surface area was CuZn catalyst and the highest Cu surface area was CuZnZr, which was the same trend as the specific surface area of prepared catalysts. There was evident correlation between the specific surface area and the Cu dispersion of the catalysts. The larger specific surface area the catalyst had, the higher Cu dispersion it had. But the commercial CuZnAl catalyst was an exception with lower Cu dispersion compared with CuZnCeZr catalyst, even though it had the slightly higher specific surface area. It was because that the composition of commercial CuZnAl catalyst was not the same as our prepared catalysts. The larger specific surface area can be beneficial to active components dispersed and improve the utilization rate of catalysts. Furthermore, as expected, the lowest H2 production rate was obtained on the CuZn catalyst with the lowest Cu surface area, and the highest H2 production rate was obtained on the CuZnZr catalyst with the highest Cu surface area. The correlation between the Cu surface area and H2 production rate is shown in Fig. 1. A nearly linear correlation between the Cu

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Table 1 e The physical characteristics and performances of the prepared catalysts for methanol steam reforming. Catalysts

CuZn

CeZr

CuZnCe

CuZnZr

CuZnCeZr

CuZnAl

a

42.0 0.12 20.7 1.3 0.37 309

99.4 0.10 e e e e

50.6 0.12 11.7 2.3 0.98 392

143.5 0.21 14.3 13.2 5.68 553

90.8 0.12 15.6 9.6 4.13 510

95.6 0.20 e 7.3 2.35 340

2

SBET (m /g) Pore volume (cm3/g) b Crystallite size of CuO (nm) c Cu surface area (m2/g) c Cu dispersion (%) d H2 production rate (cm3/kgcat/s) a

a b c d

BET surface area and pore volume determined by N2 adsorptionedesorption. Determined by Scherer equation at the Cu (111) peaks from XRD. Determined by N2O titration at 333 K. Results at T ¼ 240  C, W/M ¼ 1.2, GHSV ¼ 1200 h1, no carrier gas.

surface area and H2 production rate was obtained in CeO2 and/ or ZrO2 support catalyst. Thus, the Cu surface area may be one of the important factors for structureeactivity correlations of prepared support catalysts. But there was a significant jump in activity with addition of CeO2 and/or ZrO2 support to the CuZn catalyst that was not due solely to an increase in Cu surface area. It was due to the fact that the CeO2 and/or ZrO2 support can also beneficial to the catalyst activity. The commercial CuZnAl catalyst was also an exception (not shown in Fig. 1), although it had higher Cu surface area than that of CuZnCe catalyst, the H2 production rate was lower. This may be due to the fact that Al2O3 as support can weaken the MSR reaction [11].

3.2.

Crystalline structure

The X-ray diffractograms of all prepared catalysts are shown in Fig. 2. The characteristic peaks of CuO were evident in the spectra obtained from the fresh catalyst samples except CeZr catalyst. But the intension and width of CuO peaks were very different. In the CuZn catalyst, the diffraction peaks of CuO and ZnO were sharp and the CuO diffraction peaks separated from the ZnO diffraction peaks apparently, indicating that the size of CuO and ZnO particle was the largest among all the catalysts prepared. The dispersions of CuO and ZnO on the surface were lower, which was consistent with the N2O titration. However, using the CeO2, ZrO2 and CeZr as supports

in the CuO/ZnO catalysts, the diffraction patterns of CuO and ZnO were weakened and widened, indicating that the crystallite size of CuO and ZnO decreased and the dispersion of CuO and ZnO was improved in the catalysts. These transformations were all beneficial to the catalytic activity. Meanwhile, the diffraction peaks of CuO and ZnO evidently overlapped, and the part of ZnO diffraction peaks disappeared. It indicated that the interaction between CuO and ZnO was strengthened. Moreover, as shown in the X-ray diffractograms of CuZnCe catalyst, the characteristic peaks of CeO2 was at 2q ¼ 28.4 . However, in the CeZr and CuZnCeZr catalysts, the characteristic peaks shifted to 2q ¼ 28.9 and the separate phases of CeO2 or ZrO2 did not appear, which implied that the CexZr1xO2 solid solution was formed. Hori [24] studied the effect of ZrO2 addition to CeO2 properties by XRD and found that the addition of ZrO2 (25%) to CeO2 did not cause the appearance of a separate ZrO2 phase, whereas the observed shift in Bragg angles was attributed to the formation of a solid solution of CexZr1xO2. Additionally, Pantu [25] verified that for Ce0.5Zr0.5O2, the addition of ZrO2 shifted CeO2 Bragg angles to higher values, and also interpreted their results by the formation of a solid solution. The formation of CexZr1xO2 solid solution can promote the oxygen storage capacity of CeO2 carrier which will be discussed in Section 3.4. The particle size information presented in Table 1 was calculated using the (111) peak position and the full width at

600

500

Intensity

3

H2 production rate (cm /kg/s)

CuO ZnO CexZr1-xO2 CeO2

400

a b c

300

d e

200 0

3

6 9 2 Cu surface area (m /g)

12

Fig. 1 e H2 production rate as a function of specific Cu surface area.

15

20

30

40

2 (o)

50

60

Fig. 2 e XRD patterns of prepared catalysts. a: CuZn, b: CeZr, c: CuZnCe, d: CuZnZr, e: CuZnCeZr.

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half maximum of CuO from XRD data. The CuO particle sizes of the catalysts became smaller and the dispersion of CuO was more uniform when adding different support materials, which were beneficial to the catalyst activity.

catalysts [31,32]. Evidently, the catalysts in this study were consistent with these trends.

3.3.

The values obtained during OSC experiments are listed in Table 2. As expected, CuZn, CuZnZr and commercial CuZnAl catalysts did not present any oxygen storage capacity. Several studies indicated that zirconia addition to ceria improves the oxygen storage capacity and therefore improves the redox properties [33]. Thus, the total OSC of CuZnCeZr catalyst was higher than that of the CuZnCe catalyst. The high OSC was beneficial to suppress the CO level by promoting the WGS reaction in our previous research [21].

Temperature-programmed reduction

Intensity

In order to probe the relationship between the catalyst activity and the catalyst reducibility, temperature-programmed reduction (TPR) is performed on the prepared catalysts. The TPR profiles of different catalysts are shown in Fig. 3. The profile of the CuZn catalyst was the same as that of the pure CuO. There was only one reduction peak at around 280  C, which was considerably lower than the reduction temperature reported for pure CuO shown at around 340  C [26e28]. Except for the CuZn catalyst, the TPR profiles of the other three prepared catalysts showed two overlapped reduction peaks, which included one lower reduction temperature peak (the shoulder peak) and one higher reduction temperature peak (the main peak). The two reduction peaks were due to the reduction of different Cu species. The temperature of the shoulder peak which shifted from 180 to 260  C corresponded to the reduction process of the highly dispersed copper oxide species and the temperature of the main peak at around 270  C was ascribed to the reduction process of the body phase of copper species or the reduction of the crystalline Cu(Ⅱ)O [29,30]; compared to the supported catalysts, the CuZn catalyst only appeared one main reduction peak. This may be the reason why the CuZn catalyst had the lowest methanol conversion. Combined with the Cu dispersion, there was the lowest dispersion of copper oxide in CuZn catalyst. Moreover, the two most active catalysts, CuZnZr and CuZnCeZr, had the lowest temperature reduction peaks of all the catalysts. Therefore, the ease of reducibility is likely to be one of important factors in determining the activity of the catalyst. This was one of the reasons that the CuZnZr and CuZnCeZr catalysts showed high methanol conversion compared to CuZnCe catalyst. The study on impregnated and precipitated CuOeZnO based catalysts done by Breen and Song indicated that catalysts which reduced at lower temperatures were more active methanol steam reforming

CuZn CuZnZr CuZnCe CuZnCeZr 100

200

300

Fig. 3 e H2-TPR profiles of the prepared catalysts.

400

3.4.

3.5.

Oxygen storage capacity (OSC)

Catalyst performance

Catalytic activity was evaluated in terms of methanol conversion (mol%) and level of CO in the reforming gas.

3.5.1.

Methanol conversion

Methanol conversion as a function of temperature at a constant liquid feed flow rate for each catalyst tested is shown in Fig. 4. As shown in Fig. 4, when the temperature was at 230  Ce260  C, the water/methanol molar ratio (W/M) was constant 1.2 and the methanol gas hourly space velocity (GHSV) was 1200 h1, the thermodynamic equilibrium conversions of methanol all reached 100%. Moreover, for all the catalysts, the activity increased monotonically with the temperature because of the endothermic property of MSR. However, when the temperature was above 300  C, the Cu-based catalysts were prone to deactivation [12]. Decreasing the reaction temperature and increasing the activity at low temperature is one of the methods to prevent deactivation. The CuZn catalyst had the lowest catalytic activity. The methanol conversion was only 39% at 230  C and 85% at 260  C. The activity of the commercial catalyst using Al2O3 as support increased a little. Adding the CeO2 support material, the catalyst activity was further improved. The methanol conversion increased 5% compared to the commercial catalyst at each reaction temperature. It implied that the CeO2 was a more suitable support material compared with Al2O3 for MSR reaction. Patel and Pant [27,28] found that the MSR reaction was promoted as a part of Al2O3 was replaced with CeO2 in the CuO-based catalysts, which was consistent with our conclusion. In contrast, Chen [11] made an opposite conclusion that the weakening effect by CeO2 on the MSR reaction may have been caused by the increase of Al2O3 and the decrease of ZnO on the catalyst. However, the difference of conclusions may be due to the fact that the composition of prepared catalysts was not the same. There have no Al2O3 material in our prepared catalysts. Moreover, it was evident from Fig. 4 that ZrO2-containing catalysts were a much better promoter for catalytic activity compared with CeO2 and Al2O3. The ZrO2-containing catalysts can evidently improve the Cu dispersion and reducibility of the catalyst as discussed above. The CuZnZr and CuZnCeZr catalysts had the best catalytic activity for the MSR reaction. The methanol conversion was above 90% when the temperature

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Table 2 e Oxygen storage capacity (OSC) of the prepared catalysts. Catalysts O2 uptake (mmol/gcat)

CuZn

CeZr

CuZnCe

CuZnZr

CuZnCeZr

CuZnAl

0

812

332

0

486

0

was higher than 230  C. With the reaction temperature increase, the methanol conversion reached 100% at 260  C.

CO molar concentration and selectivity

The route of CO formation was investigated in our experiments. Initially, the mechanism of the methanol decomposition (DM) followed by water gas shift reaction (WGS) [34] has been assumed. According to the mechanism, the CO concentration in the product gas should be higher than the equilibrium concentration. However, this result became unacceptable since many results have shown that the CO level in the reforming gas was dramatically lower than the predicted value from MSR equilibrium [35]. Fig. 5 showed the CO molar concentration as a function of reaction temperature. The CO concentration in the reforming gas was far below the value than obtained from MSR equilibrium, indicating that the reaction pathway of methanol steam reforming did not go through DM and WGS. The CO molar concentration increased along with the temperature. This was because that CO as the by-product was mainly produced by the endothermic RWGS reaction which will be discussed in detail in Section 3.8. Although the temperature increase can promote the catalytic activity, it also significantly increased the CO concentration in the reforming gas. Increasing the catalytic activity at lower temperature can be favorable to depress the CO level. Moreover, the commercial CuZnAl catalyst had the highest CO concentration in the reforming gas. With the temperature increase, the CO concentration presented linear increased and was close to the equilibrium concentration, which was from 0.7% at 230  C to 2.3% at 260  C. On the contrary, CuZnCe had the lowest CO concentration in reforming gas, which was from 0.17% at 230  C to 0.5% at 260  C. It was much lower than that over the commercial CuZnAl catalyst. Compared with the

3.0

Methanol conversion (%)

100 90 80 70 60 50 40 30

CuZn CuZnZr CuZnAl

20 10

CuZnCe CuZnCeZr Equil

0 230

235

240

245 T ( oC)

250

255

260

Fig. 4 e The profiles of the catalyst activity as a function of the reaction temperature. W/M [ 1.2, GHSV [ 1200 hL1, no carrier gas.

CO concentration (mol%)

3.5.2.

CuZn catalyst, the adding of CeO2 as support promoted the activity of catalyst, and the CO concentration slightly decreased. The phenomena were mainly due to the fact that CeO2 had the function of oxygen storage capacity which was confirmed by Fernandez-Garcia [36]. The CeO2 as support mainly has the effect of decreasing the CO concentration in the reforming gas. However, the CuZnCe catalyst still had lower methanol conversion compared with Zr-containing catalysts. The CuZnZr and CuZnCeZr catalysts had the highest activity as mentioned above. However, the increased methanol conversion produced more hydrogen and carbon dioxide, which promoted the RWGS reaction. It was the reason that the CO concentration also significantly increased as the methanol conversion increased. Compared with the CuZnZr catalyst, the CuZnCeZr catalyst had almost the same methanol conversion, but much lower CO concentration. The CO concentration produced over CuZnCeZr catalyst was 3300 ppm lower than that over CuZnZr catalyst at 260  C, which was mainly due to the high oxygen storage capacity and lattice oxygen activities ability of CexZr1xO2 solid solution. Since the CO level was greatly dependent on the methanol conversion, it was useful to compare the CO selectivity of different catalysts at a specific conversion. Table 3 showed the CO selectivity and the reaction temperature at around 85e91% methanol conversion. The CuZnCeZr catalyst exhibited the lowest CO selectivity and reaction temperature. Moreover, the reaction temperature of the CuZnAl commercial catalyst was much higher than that of the CuZnCeZr catalyst and the CO selectivity was about eight times as high as for the CuZnCeZr catalyst. According to Agrell [37], lowering the maximum reaction temperature in the reactor can further decrease the CO level, which was consistent with the experiments. The CeZr support not only can promote the Cu-based catalyst activity,

CuZn CuZnZr CuZnAl

2.5

CuZnCe CuZnCeZr Equil

2.0 1.5 1.0 0.5 0.0 230

235

240

245 T ( oC)

250

255

260

Fig. 5 e The profiles of the CO molar concentration as a function of the reaction temperature. W/M [ 1.2, GHSV [ 1200 hL1, no carrier gas.

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Table 3 e Comparison of CO selectivity at around 85e90% conversion. Catalysts

Conversion (%)

Selectivity (%)

Temperature ( C)

CuZn CuZnCe CuZnZr CuZnCeZr CuZnAl

84 87 90 91 90

2.3 2.0 1.9 1.4 10.5

260 260 230 230 270

but can also suppress the CO formation. Thus, it is the suitable support for the MSR.

3.6.

Mechanism of MSR reaction

3.7.

Stability of catalyst

The stability of the CuZnCeZr catalyst was evaluated by a test run of 360 h under the following reaction conditions: 230e260  C, W/M ¼ 1.2, and GHSV ¼ 600 h1. As shown in Fig. 8, the reaction started at 230  C and the methanol conversion was nearly 97%. However, the catalytic activity evidently decreased and the reaction temperature gradually increased during the first 50 h of testing (Fig. 8A). The methanol conversion decreased from 98% to 85%, and the reaction temperature increased from 230  C to 240  C. After 50 h, the catalytic activity was stabilized at methanol conversion of 85% and the CO concentration was stable at 0.3% (Fig. 8B). For the first 50 h of testing, the catalytic activity decreased gradually because of the instability of the Cu particle after reduction [42,43]. In order to evaluate the stability of CuZnCeZr catalyst at different reaction temperature, the temperature increased to 260  C (Fig. 8C), then decreased to 250  C (Fig. 8D), at last returned to 240  C (Fig. 8E). When the stability test at 260  C was done during 100 h (Fig. 8C), the methanol conversion maintained almost at 100% and the CO concentration

1032

1445

1143

1048

1375 1310

1545

1345

1454

1765 1740

303K

303K

3500 3500

2359 2320

Intensity

523K

1765 1753 1740

1062 1032 1004

1600

2183

673K 623K 573K 523K 473K 423K 373K

2074 2052

Intensity

2359 2320

1623

The CeZr support was suitable to MSR reaction which is discussed above. In this section, the mechanism of MSR reaction on the CuZnCeZr catalyst was explored by in-situ FT-IR. The in-situ FT-IR spectrum of MSR reaction is shown in Fig. 6. The bands at 2800e3000 cm1 and 1000e1100 cm1 were assigned to the adsorption of methoxy groups on different species, such as Cu and CeO2eZrO2. The bands at around 2359 cm1 and 2320 cm1 can be attributed to the two branches of the rotation vibration spectrum of gaseous carbon dioxide. The feature adsorption peak of gaseous carbon monoxide was at 2183 cm1. The surface copper carbonyls were observed with a feature at 2074 cm1 and 2052 cm1. The bands at 1765 cm1, 1740 cm1, 1600 cm1 and 1345 cm1 were all the feature adsorption peaks of formate ions. As shown in Fig. 6, the adsorption peak of methoxy and formate can be observed at room temperature. It is implied that methanol dehydrogenates to form the methoxy and then the portion of methoxy converts to methyl formate at room temperature (303 K). With the temperature increase, the successive hydrolysis of formates occurs, giving rise to evolution of CO2 at relatively low temperature (473 K), while at higher temperature CO is evolved. If the mechanism of the methanol decomposition (DM) followed by water gas shift reaction (WGS) [34] was assumed, CO should be firstly formed by DM. But in our experiment, the CO2 was formed previously. The results showed that the steam reforming of methanol and

RWGS reaction was involved in the pathway of MSR system. The formation of CO was mainly due to the RWGS reaction. This is consistent with the research of Jiang [38,39]. In order to further investigate the function of the CeZr support, we also performed methanol adsorption experiment on the CeZr support. The in-situ FT-IR spectrum of methanol adsorption is shown in Fig. 7. It can be seen that the methanol also can dehydrogenate and transform to methyl formate on the CeZr support at room temperature. It is implied that the methanol can be activated not only on the Cu species, but also on the CeZr solution solid. The first and rate-determining step of MSR reaction is the dehydrogenation of surface methanol [40,41]. The function of methanol dehydrogenation can accelerate the MSR reaction rate and then promote the catalytic activity. The adsorption peak of gaseous CO2 was obviously seen at 523 K. It is implied that the methanol decomposed on the CeZr support which was also useful to the MSR reaction. Moreover, as shown in Figs. 6 and 7, the intermediate production was formate ions in the MSR reaction.

3000

2500 2000 1500 Wavenumbers ( cm-1)

1000

Fig. 6 e FT-IR spectra of the MSR on CuZnCeZr catalyst.

3000

2500 2000 1500 Wavenumbers ( cm-1)

1000

Fig. 7 e FT-IR spectra of the adsorbed species formed upon methanol adsorption on CeZr carrier.

4404

2.5 2.0

H concentration

A

B

C

D

E

CO concentration

1.5 1.0 0.5

3.0

90

2.7

80

2.4

0.0 0

50

100 150 200 250 300 350 Time on stream (h)

CuZnCeZr catalyst performance in the mini reformer

Used

1.5

40

1.2

30

0.9

20

0.6

10

0.3 0.0

2.0

2 (o)

50

60

Fig. 9 e XRD patterns of fresh and used catalysts.

70

235

240

245 250 T ( oC)

255

260

230 oC 250 oC

240 oC 260 oC

1.5

1.0

0.5

0.0 0.001

Fresh 40

1.8

50

chamber. The design of the mini reformer was reported in a previous paper from our group [44]. Fig. 10 shows methanol conversion and CO concentration as a function of temperature and methanol gas hourly space velocity (GHSV) for CuZnCeZr catalyst in the mini reformer. The methanol conversion increased with the reforming temperature, but it was slightly lower than that in the case of a fixed-bed quartz tube reactor. This phenomenon was due to the non-uniform temperature distribution in the mini reformer. Moreover, the methanol conversion decreased with increasing GHSV. Increasing the GHSV decreased the contact time (W/F ), which resulted in a lower conversion. The lowest methanol conversion was 77.5% at 230  C and 1200 h1. The highest methanol conversion reached 100% at 260  C and 300 h1. Furthermore, as shown in Fig. 10, it is evident that the CO concentration in the reforming gas declined gradually with increasing GHSV, especially from 300 h1 to 600 h1. As to the reason of CO formation, there were two mechanisms according to the literature [31,45]: RWGS and DM. In the first mechanism, CO concentration in the reformer gas decreased

CO concentration (mol%)

CuO ZnO Cu CexZr1-xO2

30

2.1

600 h-1 1200 h-1

Fig. 10 e The profiles of the catalysts activity and CO molar concentration as a function of the reforming temperature and GHSV. W/M [ 1.2, no carrier gas.

In order to investigate the CuZnCeZr catalyst performance, the 45.0 mL CuZnCeZr catalyst was placed in the reforming

Intensity

300 h-1 900 h-1

60

230

maintained at 0.6e0.7%. No obvious deactivation phenomenon was observed. When the reaction temperature decreased to 250  C (Fig. 8D), the methanol conversion stayed at 90% and CO concentration was 0.35e0.47%. At last, the reaction temperature returned to 240  C, the methanol conversion was stable at 84% and CO concentration was 0.3%. It was almost the same as the initial test (Fig. 8B). During the 360 h stability test, the CuZnCeZr catalyst did not show obvious deactivation. The H2 concentration was nearly 75% and the CO concentration was less than 0.8% in the reforming gas. The XRD technique is also used to characterize CuZnCeZr catalyst after 360 h stability test, the XRD patterns of fresh and used for 360 h catalysts are shown in Fig. 9. It can be seen that the diffraction lines of the CuO phase almost completely disappeared and were replaced by the diffraction lines of Cu phase after 360 h stability test. The Cu particle size of 360 h used catalyst calculated by the Scherrer equation was 22.7 nm.

20

70

0

Fig. 8 e Stability test of CuZnCeZr catalyst. W/M [ 1.2, GHSV [ 600 hL1, no carrier gas. A: 230e240  C, B: 240  C, C: 260  C, D: 250  C, E: 240  C.

3.8.

100

Methanol conversion (%)

Methanol conversion/ H2 concentration (mol%)

Methanol conversion

CO concentration (mol%)

3.0

100 90 80 70 60 50 40 30 20 10 0

CO concentration (mol%)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 3 9 7 e4 4 0 6

0.002

0.003 0.004 0.005 W/F( g·h/cm3)

0.006

0.007

Fig. 11 e The profiles of the CO molar concentration as a function of the reforming temperature and W/F. W/ M [ 1.2, no carrier gas.

Hydrogen production rate ( dm3·h-1)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 3 9 7 e4 4 0 6

Moreover, we found that the RWGS reaction was mainly involved in the formation of CO on CuZnCeZr catalyst. A life test was carried out to check the stability of CuZnCeZr catalyst. During the 360 h continuous operation, no noticeable deactivation was observed. The H2 concentration remained constant and the CO concentration was lower than 0.8%. At last, the CuZnCeZr catalyst also had excellent performance in the mini reformer. The maximum hydrogen production rate was 162.8 dm3/h, which can produce 160e270 W electric energy power by different kinds of fuel cells.

250 300 h-1 900 h-1

200

600 h-1 1200 h-1

150 100 50 0 230

235

240

245 T ( oC)

250

255

260

Fig. 12 e The profiles of the hydrogen production rate as a function of the reforming temperature and GHSV. W/ M [ 1.2, no carrier gas.

with the decreasing contact time, whereas in the later one CO concentration was independent of the contact time. In order to clarify the route responsible for CO formation, the effect of contact time (W/F ) on the CO concentration was investigated by varying the GHSV. Fig. 11 presents a plot of CO concentration versus the contact time (W/F ). The CO concentration declined gradually with the decrease of contact time. It indicated that the reaction sequence was such that CO2 and H2 were the primary products of the reaction and the CO was formed by the RWGS reaction. As shown in Fig. 10, the lowest CO concentration was 0.24% at 230  C and 1200 h1. The highest CO concentration was 1.7% at 260  C and 300 h1. The hydrogen production rate as a function of reforming temperature and GHSV for CuZnCeZr catalyst in the mini reformer is shown in Fig. 12. The maximum hydrogen production rate was 162.8 dm3/h and the electric energy power was 160e270 W in our experimental conditions.

4.

4405

Conclusions

Compared with the CuZn and CuZnAl catalysts, adding the CeO2, ZrO2 and CeO2eZrO2 as support to CuO/ZnO catalysts resulted in promoting the catalytic activity. The introduction of ZrO2 into the catalyst significantly promoted the catalytic activity and the addition of CeO2 mainly suppressed the CO concentration in the reforming gas. The CeO2eZrO2 solid solution had the function of surface methanol dehydrogenation and high oxygen storage capacity, which can promote the catalytic activity and suppress CO formation. For CuZnCeZr catalyst, the methanol conversion reached 95% at 240  C, which is 30% higher than that of the commercial catalyst. The CO concentration was only 0.46%, which is one third of the CO concentration in the case of the commercial catalyst. Combined with characterizations by BET, XRD, TPR, OSC and N2O titration, indicated that the Cu dispersion, the reducibility of catalysts and the oxygen storage capacity evidently influenced the catalytic activity and CO selectivity.

Acknowledgments The work was supported by the National Natural Science Foundation of China (21076206), the Natural Basic Research Program of China (973 Program, 2010CB732302), and the National High Technology Research and Development Program (863 Program, 2011AA050706).

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