SYNTHESIS AND CHARACTERIZATION OF Cu-X/ O CATALYST BY

Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 24 (2) 169−178 (2018) CI&CEQ

169

SHILIN HUANG

JUAN LI CHANG-FENG YAN

ZHIDA WANG CHANGQING GUO

YAN SHI

Key Laboratory of Renewable Energy, Chinese Academy of

Sciences; Guangdong Key Laboratory of New and Renewable

Energy Research and Development, Guangzhou Institute

of Energy Conversion, Chinese Academy of Sciences, No.2,

Wushan, Tianhe District, Guangzhou, China

SCIENTIFIC PAPER

UDC 546.11:544.4:66

SYNTHESIS AND CHARACTERIZATION OF Cu-X/γ-Al2O3 CATALYST BY INTERMITTENT MICROWAVE IRRADIATION FOR HYDROGEN GENERATION FROM DIMETHYL ETHER STEAM REFORMING

Article Highlights • 2Cu-Fe/72γ-Al2O3 performs excellent for DME steam reforming • The intermittent microwave irradiation provides a rapid method for Cu-Fe/γ-Al2O3

synthesis • Adjusting micro-irradiation on and off time can effectively control the crystallite size of

CuO • Ferric oxide particles are active in water-gas shift reaction, which lower CO

concentration Abstract

A series of Cu-X/γ-Al2O3 (X = Fe, Co, Ni) catalysts were synthesized by a rapid intermittent microwave irradiation method for hydrogen generation from dimethyl ether steam reforming. Different parameters, such as the promoters of X (X = Fe, Co, Ni), microwave irradiation procedure and the ratio of metal to γ-Al2O3, were investigated. The results show that 2Cu-Fe/72γ-Al2O3 has the best performance, for which the agglomeration is prevented, CuO is well dispersed and the catalytic activity is improved. Promoter iron oxide in 2Cu-Fe/9γ-Al2O3 facilitates the water-gas shift reaction, which lead to an increase in the conversion of CO to CO2 and hydrogen yield. Particularly, the 2Cu-Fe/72γ-Al2O3 catalyst, with the best molar ratio of metal to γ-Al2O3, shows a dimethyl ether conversion of >99% and a hydrogen yield of >98% and produces the lowest CO content of 1.4%, indicating that the synergism between dimethyl ether hydrolysis and methanol reforming requires an appropriate balance between the metallic Cu-Fe and the acidγ-Al2O3. The intermittent microwave irradiation technique provides a simple but effective method of the Cu-Fe/γ-Al2O3 synthesis with a good catalyst perform-ance for the dimethyl ether steam reforming.

Keywords: hydrogen generation; dimethyl ether; intermittent microwave; catalytic steam reforming.

Polymer electrolyte membrane fuel cell (PEMFC) vehicles fueled by hydrogen are one of the most pro-mising candidates for the internal combustion engine automobiles due to their high efficiency and environ-mentally friendly properties [1]. Dimethyl ether (DME) Correspondence: C.-F. Yan, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangdong, 510640, China. E-mail: [email protected] Paper received: 2 March, 2016 Paper revised: 9 August, 2016 Paper accepted: 28 August, 2017

https://doi.org/10.2298/CICEQ160203029H

is an ideal liquid fuel as hydrogen carrier for its high H2 content (13.0 wt.% compared to 12.5 wt.% of methanol), easy storage and simple transportation [2]. Besides, DME is inert, non-carcinogenic, non-muta-genic, non-corrosive and virtually non-toxic. It can also be conveniently handled because its physical properties are very similar to liquid petroleum gas (LPG). However, DME now faces overcapacity in China with an annual production of 13 million metric tons per year. Over 90% of DME is used as a blend stock for LPG which is primarily used for combustion in cooking and irradiation [3]. Steam reforming of

S. HUANG et al.: SYNTHESIS AND CHARACTERIZATION OF Cu-X/γ-Al2O3… Chem. Ind. Chem. Eng. Q. 24 (2) 169−178 (2018)

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DME is an excellent process to produce hydrogen and an attractive route to provide hydrogen as fuel cells on a small or medium scale. An integrated sys-tem of DME steam reforming (Eq. (1)) for hydrogen generation usually involves two consecutive pro-cesses: the DME hydrolysis (Eq. (2)) and the MeOH steam reforming (Eq. (3)). Besides, the reverse water- -gas shift reaction (Eq. (4)) takes place. Generally, a solid acid catalyst such as γ-Al2O3, HZSM-5, zeolites, Ga2O3 or ZrO2 is needed to catalyze the hydrolysis of DME, whereas Cu-, Pt-, Ru-, Pd-, and Ni-based catalysts are used for MeOH steam reforming [4-9]. Consequently, suitable metallic and acid components are required to synthesize the bi-function catalyst for high DME conversion, high H2 selectivity and high stability.

DME steam reforming:

3 3(g) 2 (g) 2 2

O -1298

CH OCH 3H O 2CO 6H ,

122 kJ molKH

+ = +

Δ = (1)

DME hydrolysis:

3 3(g) 2 (g) 3 (g)

O -1298 K

CH OCH H O 2CH OH ,

24 kJ molH

+ =

Δ = (2)

MeOH steam reforming:

O -13 (g) 2 (g) 2 2 298 KCH OH H O CO 3H , 49 kJ molH+ = + = (3)

Reverse water-gas shift reaction:

O -12 2 2 (g) 298 KCO H H O CO, 41 kJ molH+ = + Δ = (4)

DME steam reforming bi-functional catalysts can be prepared by several methods including mechanical mixing [10,11], deposition-precipitation [12] and sol-gel [13]. The mechanical mixing method has been applied in CuZnAlX/HZSM-5 (X = Cr, Zr, Co or Ce) for hydrogen production from steam reforming of DME in our previous work [10,11]. The microwave technology has been used to prepare catalyst material, which reduced the preparation time and optimized the crys-tallization [14,15]. Li et al. used microwave irradiation method in the aged process that give great benefits to both activity and stability of Cu/ZnO/Al2O3 catalyst [16]. In intermittent microwave irradiation (IMI) tech-nique, the irradiation on and off, are alternatively per-formed at controllable temperatures. Compared with the conventional and continuous microwave heating technique, IMI is much easier to control the heating temperature for crystallization and provide relaxation time to protect the particle growth. Here we propose a rapid synthesis method based on the IMI technique for preparing the metal based catalyst Cu-X (X = Fe,

Co, Ni) supported on the most commonly used acid function γ-Al2O3 as the acid catalyst for DME steam reforming of hydrogen generation. To improve the catalytic performance, effects of irradiation procedure, ratio of metal to γ-Al2O3 and experimental condition were investigated in details.

EXPERIMENTAL

Catalyst preparation

2Cu-X/9γ-Al2O3 (X = Fe, Co, Ni) with a Cu: X: γ-Al2O3, mole ratio of 2:1:9, was prepared by the imp-regnation method: an aqueous precursor containing Cu(NO3)2, X(NO)n (X = Fe, Co, Ni) and γ-Al2O3 pow-der was made at room temperature and then was pro-cessed in the microwave (FT-2KW, Guangzhou Futao microwave equipment co. ltd at 2 kW). IMI was in the pulse form of 5 s-on and 5 s-off for 20 times (2Cu-Fe/9γ-Al2O3, 5 s). The catalyst was dried at 110 °C for 12 h and calcined at 400 °C for 5 h.

Effects of the microwave irradiation procedure were investigated by the different irradiation times with a total irradiation time of 100 s, irradiation and relaxation time remain the same ratio of 1:1. 2Cu-Fe/ /9γ-Al2O3 (0 s) meant that no microwave irradiation was applied. 2Cu-Fe/9γ-Al2O3 (20 s) was heated by intermittent microwave in the pulse form of 20 s-on and 20 s-off for 5 times. 2Cu-Fe/9γ-Al2O3 (50 s) was heated in the pulse form of 50 s-on and 50 s-off for 2 times.

Catalyst characterization

X-ray diffraction (XRD) analysis was performed in the PANalytical X’Pert diffractometer (X’Pert PRO MPD, PW3040/60) within the 2-θ ranged from 20 to 80° by a speed of 2° per min with Cu-Kα (λ = = 0.154060 nm) radiation (40 kV, 40 mA). The crys-tallite size of CuO and Fe2O3 was calculated by the Sherrer equation on the basis of the data of the line broadening at half the maximum intensity (full width at half-maximum, FWHM) and the Bragg angle (θ). The Brunauer-Emmett-Teller (BET) specific surface was determined from the adsorption and desorption iso-therms of nitrogen at -196 °C after outgassing pro-cedure under vacuum at 250 °C for 20 h, using a Quantachrome SI-MP-10/PoreMaster 33. The catalyst was characterized by the Scanning electron micro-scope (SEM) with a S-4800 instrument at 2.0 kW. Hydrogen temperature-programmed reduction (H2-TPR) was carried out to estimate the reduction per-formance of the catalyst by Quantachrome ASIQACIV200-2. H2-TPR was conducted in the fol-lowing method: 100 mg of a powder sample was


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heated at a rate of 10 °C/min up to 120 °C in helium atmosphere and kept for 1 h. The sample was cooled down to 40 °C in the He atmosphere, and then followed by a pure hydrogen flow for 1 h. Sub-sequently, H2-TPR was initiated by a heating rate of 10 °C/min up to 750 °C. The consumption of hyd-rogen was determined by a Thermal conductivity det-ector (TCD) and recorded by an online computer.

Catalytic reaction and analyses of the products

DME steam reforming was executed in a bench-scale fixed-bed reactor (a quartz tube with inner dia-meter of 20 mm) under ambient pressure. The fixed-bed reactor consists of a gas supplier, a catalyst reaction part, an analyzer, and a controller, as shown in Figure 1. The catalyst was placed in the quartz tube and was heated by an electric furnace equipped with a temperature controller. The catalyst (m = 2 g) was reduced in 5 vol.% H2 in N2, heating rate of 10 °C min-1 from room temperature up to 400 °C for 4 h, then the system was balance with N2 for 1 h at the same tem-perature. Standard reaction conditions for DME steam reforming were steam to DME molar ratio of 5 to 1 at a space velocity of 3600 ml/(gcat h). The system was run for 1 h then the product gas was cooled and dried before analyzing by the chromatograph (WUFENG GC522) equipped with a TCD detector and a Flame ionization detector (FID) detector. The steam reform-ing reaction performance over Cu-X/γ-Al2O3 catalysts was evaluated by conversion of the CO2 and selec-tivity CO, CH4. The representative values are given as follows:

DME,in DME,outDME

DME,in

100F F

XF

−= (5)

,out

DME,in DME,out

1002( )

XX

FS

F F=

− (6)

22

HH

DME,in

100

6

FY

F= (7)

where XDME is DME conversion, SX (X = CH4, CO) are CH4 selective and CO selectivity, FDME,,in and FDME,out are the inlet and outlet molar flow rates of DME, respectively. FH2 are the molar flow rate of H2 in the gas out reactant, FX,out (X = CH4, CO) are the molar flow rate of CH4 or CO in the gas out reactant.

RESULTS

Effect of promoter on steam reforming of DME

Figure 2a shows the XRD patterns of 2Cu-X/9γ- -Al2O3 (X = Fe, Co, Ni) and γ-Al2O3. The peaks of γ-Al2O3 and CuO can be seen for all the 2Cu-X/9γ- -Al2O3 (X = Fe, Co, Ni) catalysts. The strong peaks assigned to CuO appears at 2θ 35.6 and 38.5°. The X-ray phase analysis results for the 2Cu-Fe/9γ-Al2O3 (Figure 2a, e) shows the composition of the sample includes the phase CuO, Fe2O3 and γ-Al2O3. A broad-ened low-intensity peak in the range of 2θ 33° indicate the presence of a hematite phase Fe2O3 (2θ = 33.18°). The low-intensity of reflection from crystallized ferric oxide phase allows us to assume that in 2Cu-Fe/9γ- -Al2O3, ferric compounds are found in an X-ray amor-phous or highly dispersed state [17]. The peaks of Co3O4 crystals can be found in 2Cu-Co/9γ-Al2O3 (Fiure 2a, d)). There is no peak of NiO, which may be existed in amorphous or microcrystal state.

Figure 2b shows the results of the H2-TPR of the catalysts. Cu species are the main center of the acti-vity for methanol steam reforming. It was reported that usually there are two CuO species in copper based catalysts, the highly dispersed CuO reacts at lower reduction temperature (peak α) and the bulk CuO species reacts at higher temperature (peak β) [18]. This 2Cu-X/9γ-Al2O3 (X = Fe, Co, Ni) series show that introducing a second metal into the copper

Figure 1. Schematic diagram of DME catalytic reaction apparatus. (MFC: mass flow controller, GC: gas chromatograph).


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Figure 2. a) XRD patterns and b) H2-TPR profiles of the Cu- based catalysts (a: γ-Al2O3, b: 2Cu/9γ-Al2O3, c: 2Cu-Ni/9γ-Al2O3,

d: 2Cu-Co/9γ-Al2O3, e: 2Cu-Fe/9γ-Al2O3); c) XRD patterns and d) H2-TPR profiles of 2Cu-Fe/9γ-Al2O3 catalysts by different microwave irradiation time. a : 2Cu-Fe/9γ-Al2O3 (0 s), b : 2Cu-Fe/9γ-Al2O3 (5 s), c : 2Cu-Fe/9γ-Al2O3 (20 s), d : 2Cu-Fe/9γ-Al2O3 (50 s); e) XRD

patterns and f) H2-TPR profiles of the catalysts derived from the Cu-Fe/γ-Al2O3 by different loading of 2Cu-Fe andγ-Al2O3 (a: γ-Al2O3, b: 2Cu-Fe/72γ-Al2O3, c: 2Cu-Fe/36γ-Al2O3, d: 2Cu-Fe/18γ-Al2O3, e: 2Cu-Fe/9γ-Al2O3. g) SEM images of 2Cu-Fe/9γ-Al2O3 (5 s) and

h) 2Cu-Fe/9γ-Al2O3 (20 s).


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based catalyst change the copper reduction property. The change of the reduction temperature for bicom-ponent catalysts can be explained by the several pos-sible factors: the dispersity of the copper oxide in the presence of the second metal, formation of mixed oxides (leading to changing the metal-oxygen bonds). In the presence of the promoter Fe, Co and Ni, the reduction of dispersed CuO species all shifts to higher temperature (Table 1).

2Cu-X/9γ-Al2O3 (X = Fe, Co, Ni) was tested in steam reforming of DME at the condition of H2O to DME of 5:1, temperature of 400 °C and space velocity of 3600 ml g-1 h-1 in Figure 3. All the catalysts show the DME conversions of >99%, while the hydrogen yield differ from each other. 2Cu/9γ-Al2O3 achieved hydrogen yield of 93%, 2Cu-Ni/9γ-Al2O3 and 2Cu- -Co/9γ-Al2O3 obtained 69 and 86%. The results obtained are in agreement with the H2-TPR profiles, according to which for 2Cu-Ni/9γ-Al2O3, 2Cu-Co/9γ- -Al2O3 and 2Cu/9γ-Al2O3, we observe an increase in the CuO reduction temperature. Usually the higher reduction temperature presents lower reducibility and catalytic activity [11], whereas the 2Cu-Fe/9γ-Al2O3 has the highest activity of H2 yield 97% whose reduction temperature is slightly higher than 2Cu/9γ- -Al2O3. It may be due to the presence of ferric oxides within the composition of the catalyst, which are active catalysts for the water-gas shift reaction. As a result, it leads to an increase in the conversion of CO to CO2 and an increase in the hydrogen yield. The effect of ferric additives on the water-gas shift react-ion has also been noted for Fe-Cu/ZrO2 [17], Ni- -Fe/La2O2CO3 [19] and Fe/Ca-Al2O3 [20]. The addition of ferric promotes leading to the better catalytic per-formance and the lowest CO selectivity of 1.4% in the case of 2Cu-Fe/9γ-Al2O3. Therefore, the following study will focus on ferric as the promoter.

Figure 3. Performance of DME steam reforming over

2Cu-X/9γ-Al2O3 (X = Fe, Co, Ni) catalysts: mcat = 2 g, Tempera-ture = 400 °C, H2O/DME = 5/1, space velocity = 3600 ml g-1 h-1.

Effect of microwave irradiation procedure on steam reforming of DME

Effects of the IMI procedure on 2Cu-Fe/9γ-Al2O3 were studied using different irradiation procedure with a total irradiation time of 100 s as described in the catalysts preparation part. A typical XRD pattern of CuO and Fe2O3 crystal samples is shown in Figure 2c, the major peaks located at 2θ values of 30-70° correspond to the characteristic diffraction of the monoclinic phase of CuO and Fe2O3. No other peak is observed for the impurities such as Cu2O, indicating the obtained products have a high purity. The micro-wave irradiation time affects little on the BET and pore size distribution, but the average crystallite sizes of Cu and Fe2O3 increase from 41.8 and 20.9 nm for 2Cu-Fe/9γ-Al2O3 (5 s) to 52.4 and 31.2 nm for 2Cu- -Fe/9γ-Al2O3 (50 s) (Table 1).

For the 2Cu-Fe/9γ-Al2O3 synthesized using 5 s intermittent irradiation, the average crystallite size of CuO and Fe2O3 were 41.8 and 20.9 nm, respectively, significantly smaller than those synthesized in longer

Table 1. Specific surface area, pore structure and reduction peak temperature and ratio of dispersed CuO to bulk CuO of different catalysts prepared in this work. Chemical composition referred as mole ratio

Catalyst BET (m2 g-1) V / cm3 g-1 d / nm Temperature, °C Dispersed

CuO/Bulk CuO Crystallite size, nm

Dispersed CuO Bulk CuO CuO Fe2O3

γ-Al2O3 256.7 0.50 7.86 - - - - -

2Cu-Fe/9γ-Al2O3 195.7 0.41 8.02 282 319 1.79 41.8 20.9

2Cu-Co/9γ-Al2O3 182.9 0.38 8.40 302 326 1.04 36.4 -

2Cu-Ni/9γ-Al2O3 181.9 0.39 8.47 286 322 0.79 51.1 -

2Cu/9γ-Al2O3 197.7 0.42 8.49 273 323 0.98 62.7 -

2Cu-Fe/9γ-Al2O3 (0 s) 195.6 0.40 8.21 257 319 0.73 42.8 31.5

2Cu-Fe/9γ-Al2O3 (20 s) 193.7 0.38 8.00 259 306 1.04 49 27.8

2Cu-Fe/9γ-Al2O3 (50 s) 192.1 0.39 8.20 258 305 1.04 52.4 31.2

2Cu-Fe/18γ-Al2O3 206.0 0.42 8.00 283 319 2.30 - -

2Cu-Fe/36γ-Al2O3 233.2 0.46 7.96 287 320 2.61 - -

2Cu-Fe/72γ-Al2O3 237.6 0.48 7.95 317 343 4.03 - -


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intermittent irradiation. The microwave irradiation of short on and off time results in lower reaction tempe-rature of the synthesis reaction system, which limits the nucleation and the grain growth reaction of the CuO and Fe2O3 particles. Shorter irradiation time is required for the dispersion of CuO and Fe2O3 on the γ-Al2O3 supports [21-23]. The results indicate that regulating micro irradiation on and off-time can effect-ively control the crystallite size. From Table 1, 2Cu- -Fe/9γ-Al2O3 (5 s) has the highest dispersed CuO/ /bulk CuO ratio of 1.79. Further increasing the irradi-ation time, the dispersed CuO concentration dec-reases, indicating that the shorter the irradiation time, the higher the ratio of CuO/bulk CuO.

2Cu-Fe/9γ-Al2O3 (5 s) and 2Cu-Fe/9γ-Al2O3 (0 s) obtain over 99% DME conversion in Figure 4. The advantages of optimized IMI technique compared with the conventionally heated one can be seen in the following: 2Cu-Fe/9γ-Al2O3 (5 s) achieves a hydrogen yield of 97%, but the conventionally heated 2Cu- -Fe/9γ-Al2O3 (0 s) achieves 86%. It is noted that the CO concentration of 2Cu-Fe/9γ-Al2O3 (5 s) is 2.3% while of 2Cu-Fe/9γ-Al2O3 (0 s) is 7.2%. Optimized microwave irradiation technique can decrease the CO selectivity of the 2Cu-Fe/9γ-Al2O3 catalyst. Further increasing the irradiation time leads to a lower DME conversion and hydrogen yield, and decreases the CO selectivity. Usually the micro structure such as

BET, pore size distribution and high disperse CuO reduction property affect the catalyst activity. Since the microwave irradiation procedure has little influ-ence on the BET and pore size distribution of the cat-alysts (Table 1), it is the reducibility of CuO that affects the activity of 2Cu-Fe/9γ-Al2O3. 2Cu-Fe/9γ- -Al2O3 (5 s) achieve the >99% DME conversion, 97.5% hydrogen yield and 2.5% CO selectivity. The elevated catalytic property might be due to the high-est ratio of dispersed CuO/bulk CuO of 1.79.

Influence of reaction condition over 2Cu-Fe/9γ-Al2O3

The effect of temperature, steam to DME ratio and space velocity were investigated for the 2Cu- -Fe/9γ-Al2O3 (Figure 5). The runs were conducted in a fixed bed reactor in the 300-400 °C range, as shown in Fig. 5a, where the DME conversion increases with the increase of the reaction temperature. γ-Al2O3 typically shows an excellent DME hydrolysis activity at the temperature above 300 °C due to its moderate acid amount of weak acid strength [24]. The steam reforming reaction is a strong endothermic reaction. Increasing the reaction temperature will facilitate the reaction.

Figure 5b shows the effect of steam to DME ratio on SRD over 2Cu-Fe/9γ-Al2O3. The DME con-version and hydrogen yield increases sharply from 51 and 45% to 95 and 92% as the stoichiometric ratio of steam to DME was 1 to 3. DME conversion and hyd-

Figure 4. Performance of DME steam reforming over 2Cu-Fe/9γ-Al2O3 by different microwave irradiation time: mcat

= 2 g, H2O/DME = 5/1, space velocity = 3600 ml g-1 h-1.


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rogen yield flatten out as continued increasing the steam to DME ratio. The CO concentration clearly decreases when the steam to DME ratio was raised to 5. The hydrogen yield never reaches 100% for it is consumed by the reverse water-gas shift reaction [25].

Figure 5c shows the influence of space velocity on the catalytic activity. The DME conversion, hyd-rogen yield and CO decrease with the increase of space velocity. An over 99% DME conversion is seen in 3600 ml g-1 h-1 but only 46% in 18000 ml g-1 h-1. The increase of the space velocity leads to an insufficient residence time for the catalytic reaction, which dec-reases the DME conversion and hydrogen yield.

Effect of weight ratio of γ-Al2O3 to 2Cu-Fe on Steam reforming of DME

γ-Al2O3 and copper is used as the active com-ponent of the catalyst for DME hydrolysis and the steam reforming of methanol, respectively. The sel-ection of appropriate ratio of γ-Al2O3 to 2Cu-Fe will significantly improve the performance of the catalyst in the DME reforming reaction.

From the XRD patterns in Figure 2e, 2Cu-Fe/9γ- -Al2O3 (e) clearly shows the characteristic diffraction of the monoclinic phase CuO and Fe2O3. As the ratio of γ-Al2O3 increases, the diffraction from CuO and Fe2O3 becomes so weak, only γ-Al2O3 phase could be detected in 2Cu-Fe/36γ-Al2O3 (c) and 2Cu-Fe/72γ- -Al2O3 (d), indicating that they exist as small crystals

or in amorphous phase [12,26]. The results are also confirmed by the H2-TPR. With the increase of γ-Al2O3 loading, the CuO reduction peak is weakening and shifted to higher temperature. The CuO reduction peak of 2Cu-Fe/72γ-Al2O3 is broader than that of 2Cu- -Fe/36γ-Al2O3, indicating a higher dispersion of CuO in 2Cu-Fe/72γ-Al2O3. Meanwhile, as the Cu-Fe con-centration decreases, CuO reduction peak shifts to higher temperature, which reveals that it enhances the interaction of CuO and γ-Al2O3. As discussed pre-viously in part 3.2, the ratio of dispersed CuO and bulk CuO strongly affects the catalytic performance. 2Cu-Fe/72γ-Al2O3 has the highest ratio of 4.03 pre-senting the best activity and selectivity (Table 1).

The catalytic performance of different weight ratio of γ-Al2O3 to Cu-Fe starts with 2Cu-Fe/9γ-Al2O3 (Figure 6). High γ-Al2O3 content leads to high DME conversion and hydrogen yield at a temperature below 400 °C. When the reaction temperature reaches 400 °C, all the 2Cu-Fe/Yγ-Al2O3 (Y = 9, 18, 36 or 72) catalysts achieve >99% DME conversion and >96% hydrogen yield. Moreover, the 2Cu-Fe/72γ--Al2O3 catalyst presented the least CO selectivity of 1.4%.

The CH4 concentration in the outlet doesn’t change in the temperature range of 300-400 °C (Fig-ure 6d), where usually methanation occurs in alu-mina-rich composite catalysts at above 350 °C [27].

Figure 5. a) Effect of temperature (H2O/DME = 5/1, space velocity = 3600 ml g-1 h-1), b) steam to DME ratio (temperature = 400 °C,

space velocity = 3600 ml g-1 h-1) and c) space velocity (temperature = 400 °C, H2O/DME = 5/1) in DME steam reforming over 2Cu-Fe/9γ-Al2O3.


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The best performance of 2Cu-Fe/72γ-Al2O3

comes from: 1) its higher γ-Al2O3 content, which is the effective component for DME hydrolysis; 2) the highest ratio of dispersed CuO to Bulk CuO. γ-Al2O3

rich samples possess a large amount of acid sites, which result in high DME conversion; 3) the well dis-persed CuO and Fe2O3 on the γ-Al2O3 phase. Hence, the weight balance between γ-Al2O3 and Cu-Fe should be regulated very carefully.

Figure 6e shows dimethyl ether conversion as a function of reaction time over the 2Cu-Fe/72γ-Al2O3 catalyst. 2Cu-Fe/72γ-Al2O3 had good stability over 50 h and dimethyl ether conversion stayed around 99%. The 2Cu-Fe/72γ-Al2O3 catalyst had been stable at 400 °C, while usually ordinary Cu-supported catalysts were gradually deactivated [27].

CONCLUSIONS

A series of 2Cu-X/γ-Al2O3 (X = Fe, Co, Ni) cat-alysts were synthesized by a rapid intermittent micro-wave irradiation (IMI) technique for hydrogen gener-ation in dimethyl ether (DME) steam reforming. The promoter of ferric oxides within the composition of 2Cu-Fe/γ-Al2O3 are active catalysts for the water-gas shift reaction which leads to an increase in the con-version of CO to CO2 and an increase in the hydrogen yield.

The preparation of 2Cu-Fe/γ-Al2O3 catalysts by IMI technique changes the average crystallite size of Cu and Fe. The optimized IMI procedure is in the pulse form of 5 s-on and 5 s-off for 20 times. The 2Cu-Fe/72γ-Al2O3 (5 s) catalyst provides >99% DME

Figure 6. Performance of DME steam reforming over 2Cu-Fe/Yγ-Al2O3 (Y = 9, 18, 36, 72). mcat

= 2 g, Temperature = 400 °C, H2O/DME = 5/1, Space Velocity = 3600 ml•g-1•h-1, (e) Time course of 2Cu-Fe/72γ-Al2O3 catalytic activity (H2O/DME = 5/1, Space Velocity = 3600

ml•g-1•h-1)


177

conversion, 97% hydrogen yield and 1.4% CO per-cent by volume. It may be due to the higher ratio of dispersed CuO to bulk CuO leads to better activity and selectivity. The optimal condition IMI technique gives great benefits to hydrogen yield and CO and CH4 selectivity.

Acknowledgments

The authors are grateful for the financial support of CAS Renewable Energy Key Lab., Natural Science Foundation of China (51576201), Natural Science Foundation of Guangdong province (2015A030312007, 2015A030313716), Guangdong Science and Technology Project (2013B091300001, 2014A020216030) Guangzhou Science and Techno-logy Project (2013J4500027).

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178

SHILIN HUANG

JUAN LI CHANG-FENG YAN

ZHIDA WANG CHANGQING GUO

YAN SHI

Key Laboratory of Renewable Energy, Chinese Academy of Sciences;

Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of

Energy Conversion, Chinese Academy of Sciences, No.2, Wushan, Tianhe

District, Guangzhou, China

NAUČNI RAD

SINTEZA I KARAKTERIZACIJA KATALIZATORA Cu-X/γ-Al2O3 INTERMITENTNIM MIKROTALASNIM ZRAČENJEM ZA PROIZVODNJU VODONIKA REFORMOVANJEM DIMETIL ETRA VODENOM PAROM

Serija katalizatora Cu-X/γ-Al2O3 (X = Fe, Co, Ni) je sintetizovana metodom brzog inter-mitentnog mikrotalasnog zračenja za generisanje vodonika reformovanjem dimetil-etra vodenom parom. Ispitani su različiti parametri, kao što su: promoteri X (X = Fe, Co, Ni), postupak mikrotalasnog zračenja i odnos metal/Cu-X/γ-Al2O3. Rezultati pokazuju da 2Cu--Fe/72γ-Al2O3 ima najbolje performanse, jer sprečava aglomeraciju, CuO je dobro disper-govan, a katalitička aktivnost je poboljšana. Promotor oksida gvožđa u 2Cu-Fe/9γ-Al2O3 olakšava reakciju vodene pare, što dovodi do povećanja konverzije CO u CO2 i prinos vodonika. Naročito, katalizator 2Cu-Fe/72γ-Al2O3, sa najboljim molskim odnosom metala i γ-Al2O3, omogućava konverziju dimetil etra veću od 99%, prinos vodonika veći od 98% i najniži sadržaj CO od 1,4%, što ukazuje da sinergizam između hidrolize dimetil-etra i refor-movanja metanozahteva odgovarajuću ravnotežu između metalnog dela Cu-Fe i kiseleγ-Al2O3. Tehnika intermitentnog mikrotalasnog zračenja obezbeđuje jednostavnu, ali efi-kasnu metodu sinteze Cu-X/γ-Al2O3 sa dobrim performansama katalizatora za reformiranje dimetil etra vodenom parom.

Ključne reči: generisanje vodonika; dimetil etar; intermitentno mikrotalasno zra-čenje; katalitička reformiranje vodenom paroma.

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SYNTHESIS AND CHARACTERIZATION OF Cu-X/ O CATALYST BY