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Bifunctional catalysts for hydrogen production from dimethyl ether
Natural Gas Conversion VIII F.B. Noronha, M. Schmal, E.F. Sousa-Aguiar (Editors) © 2007 Published by Elsevier B.V.
445
Bifunctional Catalysts for Hydrogen Production from Dimethyl Ether Galina Volkova, Sukhe Badmaev, Vladimir Belyaev, Lyudmila Plyasova, Anna Budneva, Evgeny Paukshtis, Vladimir Zaikovsky, Vladimir Sobyanin Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva 5, Novosibirsk, 630090, Russia
Abstract The production of hydrogen directly from dimethyl ether (DME) was performed on bifunctional Cu-Ce/Al2O3 catalysts. The catalysts were characterized by XRD, HRTEM, EDX and IR spectroscopy of low-temperature CO adsorption. The high hydrogen productivity up to 600 mmol g-1h-1 may be explained by assuming that (1) DME dehydration occurs on acid sites of Ȗ-Al2O3 and (2) methanol steam reforming takes place on mixed oxide phase CuO-CeO2, solid solution of copper ions in cerium dioxide with ratio Cu/Ce from 12/86 to 33/67 at.%. 1. Introduction Dimethyl Ether is expected to become an important energy resource as a raw material for hydrogen production for fuel cells. DME as well as methanol can be easily and selectively converted to hydrogen-rich gas at relatively low temperature (250-350oC) compared to other fuels such as natural gas, gasoline and LPG [1-3]. DME is relatively inert, non-corrosive, non-carcinogenic. DME is more favorable for steam reforming (SR) than methanol from the economical viewpoint because it can be produced from syn-gas more effectively [4-5]. Overall DME SR is expressed by the equations: CH3OCH3 + H2O = 2CH3OH CH3OH + H2O = 3H2 + CO2 CO2 + H2 = CO + H2O
446
G. Volkova et al.
The first step of DME SR is hydration of DME into methanol the second step is methanol steam reforming into hydrogen-rich gas. Besides, during DME SR a reverse water gas shift reaction may occur to produce carbon monoxide. The mechanical mixtures of two catalysts: solid-acid for DME hydration and copper-containing oxide for methanol steam reforming, are usually used for DME SR [6-9]. The main disadvantage of such catalysts is their layering that leads to decrease of activity. The design of a bifunctional catalyst for DME SR seems to be a promising way to overcome this problem. Cu/Al2O3, Cu-Zn/Al2O3, Cu-Pd/Al2O3, Cu-Ru/Al2O3, Cu-Pt/Al2O3, Cu-Rh/Al2O3, Cu-Au/Al2O3 and Cu/Ga8Al2O15 systems were studied as bifunctional catalysts for DME SR reaction [10-13] but demonstrated low hydrogen productivity. Researchers from the Boreskov Institute of Catalysis have developed the bifunctional Cu-Ce/Al2O3 catalysts for DME SR providing ten-fold increase of H2 productivity [14]. Here we present the performance and physical characterization of Cu-Ce/Al2O3 catalysts. 2. Experimental The Cu-Ce/Al2O3 catalysts were synthesized by treating Ȗ-alumina in solutions of copper and cerium salts taken at the given ratio. Samples were dried at 100oC and calcined at 450oC for 3 hours. Catalysts were evaluated in fixed bed flow reactor with gas analysis on line. Reaction conditions: H2O/DME/N2=20/60/20, GHSV=10000 h-1, pressure 1 atm, temperature 250-370oC. Prior to reaction, catalysts were reduced at 300oC in stream of H2-N2 for 1 hour. HRTEM images were obtained on a JEM-2010 electron microscope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm at an accelerating voltage of 200 kV. The high-resolution images of periodic structures were analyzed by the Fourier method. Local energy-dispersive X-ray analysis (EDXA) was carried out on an EDAX spectrometer (EDAX Co.) fitted with a Si (Li) detector with a resolution of 130 eV. FT-IR spectroscopy was used to determine the acidity of the samples by monitoring the low temperature CO adsorption. The samples were degassed in the IR cell at 400oC and then cooled to -173 oC and treated with low doses of CO from 0.1 to 10 torr. The spectra were recorded with 4 cm-1 resolution using Shimadzu FT-IR-8300 spectrometer. The number of CO adsorption sites was calculated by fitting the data to equation: N=A/UAo, where A (cm-1) is the integrated intensity of band, U (g cm-2) is the mass of material normalized on pellet surface, Ao is the molar integrated absorption coefficient equal to 0.8 cm ȝmol-1 for band 2185-2200 cm-1. Procedure for measurement of Ao is described in [15]. XRD studies were carried out on a diffractometer URD-63 with CuKĮ radiation using graphite monochromator with reflected beam. The lattice parameters and dispersion of CeO2 were determined from line (111), ǻĮ ± 0.003 Å.
447
Bifunctional catalysts for hydrogen production from DME
3. Results and Discussion Table 1. Performance of bifunctional Cu-Ce/Al2O3 catalysts in DME SR at 350oC, pressure 1 atm, H2O/DME/N2=60/20/20; GHSV= 10000 h-1
Catalyst
DME
Outlet gas composition, %
H2
conv. %
H2
CO2
CO
CH4
CH3OH
mmol/g h
4Cu-4Ce/Al2O3
71
50
17.3
0.3
0.08
0.64
473
8Cu-4Ce/Al2O3
80
55
18.5
0.7
0.13
0.35
523
12Cu-4Ce/Al2O3
81
57
18.5
0.7
0.25
0.6
520
8Cu-2Ce/Al2O3
98
59,5
19.3
0.7
1.6
0.07
610
Equilibrium
100
60
15.7
6.7
0
0.00013
The high hydrogen productivity up to 600 mmol g-1h-1 and low CO concentration in hydrogen-rich gas were observed for all tested catalysts (Table 1). The H2 productivity is more than one order of magnitude higher than the early reported (Table 2). The content of copper and cerium and their ratio slightly affected the catalyst activity, it reduced by 30% at the lowest concentration of CuO+CeO2. Performance of an alumina–free catalyst 10Cu/CeO2 was very poor; the DME conversion was within 1%. Table 2. Comparison of bifunctional catalysts in DME SR
Catalyst
T,oC
GHSV h-1
Inlet gas DME/H2O/N2
DME conv.,%
mmol/g h
H2
References
Cu-Ce/Al2O3
350
10000
20/60/20
71-98
470-610
Present work
Cu-Zn/Al2O3
350
180
25/75/0
97
11.3
[10]
Cu-Zn/Al2O3
350
1400
20/60/20
58
48.2
[11]
Ga8Al2O15
350
20000
1/3/96
80
43.0
[12-13]
Equilibrium conversion of DME and concentration of H2 over 8Cu-2Ce/Al2O3 catalyst are reached at temperature 350oC (Figure 1). The CO concentration is lower than 1% and remains below the equilibrium value at 250-370oC. FT-IR spectra of Ȗ-Al2O3 and Cu-Ce/Al2O3 catalysts in the hydroxyl region (Figure 2a) showed that the bands at 3730 and 3770 cm-1 are decreased (curve 2) and disappeared (curve 3) under loading of Cu-Ce oxides. However, no change in bands intensity of the most acidic hydroxyl groups at 3675 and 3690 cm-1 was detected.
448 100
80
80
Concentration, vol.%
100
60
60 H2
40
40
20
20
DME conversion, %
G. Volkova et al.
CO2
0 250
300
0
CO
350
Figure 1. DME conversion and H2, CO2, CO concentrations vs. temperature on 8Cu-2Ce/Al2O3 catalyst at H2O/DME = 3, GHSV= 10000 h-1. Points – experimental data, dash lines – calculated equilibrium data.
400
2110
3400
3500
3600
3700
3
2
2
1
1 2000
3800
2050
2188
3
2150
3770
Absorbance
Absorbance
3730
3690
3675
Temperature, ɨɋ
2100
2150
2200
2250
-1
-1
Wavenumber, cm
Wavenum ber, cm
a
b
Figure 2. FT-IR spectra of (1) Al2O3, (2) 4Cu-4Ce/Al2O3 and (3)12Cu-4Ce/Al2O3 catalysts in: (a) hydroxyl region and (b) adsorbed CO.
FT-IR spectra of adsorbed CO (Figure 2b) revealed bands characteristic of both Lewis (2185-2200 cm-1) and Brønsted (2110-2175 cm-1) forms of adsorption. Table 3 collects the acidity data calculated from bands 2185-2200 cm-1 for Lewis acid sites. It can be seen that amount of acid sites on the surface of Ȗ-Al2O3 significantly reduced after loading of copper-cerium oxides. Nevertheless, the Lewis acid sites typical for Ȗ-alumina were registered on the surface of all bifunctional Cu-Ce/Al2O3 catalysts. Table 3. Acidity of the catalysts from FTIR spectra of CO adsorption
Catalyst
ȞCO, cm-1
Lewis acid sites, ȝmol g-1
Ȗ-Al2O3
2184-2200
600
4Cu-4Ce/Al2O3
2190
110
8Cu-2Ce/Al2O3
2190
70
12Cu-4Ce/Al2O3
2190
50
449
Bifunctional catalysts for hydrogen production from DME Table 4. Characterization of Cu-Ce/Al2O3 catalysts by XRD and EDX analysis
Catalyst
CeO2 4Cu-4Ce/Al2O3 8Cu-2Ce/Al2O3 12Cu-4Ce/Al2O3
Phase
a (111)
D (111)
Cu/Ce
composition
CeO2, Å
CeO2, Å
at. %
CeO2 CeO2 +CuO+Al2O3 CeO2 +CuO+Al2O3
5.417 5.381 5.378 5.402
80 40 35 42
33/67 no data 12/88
CeO2 +CuO+Al2O3
Results of XRD and EDX analysis for Cu-Ce/Al2O3 catalysts are presented in Table 4 and on Figure 3a. Three crystalline phases: CuO, CeO2 and Al2O3 were identified in bifunctional Cu-Ce/Al2O3 catalysts. Content of CuO phase decreased with decreasing of copper concentration in the samples; in 4Cu-4Ce/Al2O3 catalyst only traces of CuO were observed. It can be seen from Table 4 that the unit cell parameter of CeO2 decreased from 5.417 Å to 5.4025.378 Å. It indicates the formation of the solid solution of copper oxide in cerium dioxide. The exact atomic ratio of metals Cu/Ce in CuO-CeO2 solid solutions was determined from the EDX spectra (Figure 3a). It was shown that the atomic ratio of Cu/Ce ranged from 12/88 to 33/67 at. % (Table 4). The dispersion of CeO2 in the range of 35-42 Å has been registered by XRD for all bifunctional catalysts (Table 4). TEM observations presented in Figure 3b show the formation of 50-100 nm aggregates of mixed CuO-CeO2 oxide. It means that CuO-CeO2 solid solution is composed of elementary particles (35-42 Å) agglomerated in large particles of 50-100 nm in size. These particles of mixed CuO-CeO2 oxide are located within the large pores of Ȗ-alumina.
Figure 3. EDX spectra (a) and TEM micrograph (b) of 4Cu-4Ce/Al2O3 catalyst
450
G. Volkova et al.
It is known that the activity of copper containing mixed oxides for methanol synthesis and for water gas shift reaction is determined by the formation of solid solution of copper ions in zinc-alumina mixed oxides [16-18]. These catalysts are also very active for methanol steam reforming. Our aim was to provide the formation of the CuO-CeO2 solid solution on the surface of alumina and at the same time to prevent the blocking of alumina acid sites. We have succeeded for the first time in design of bifunctional catalysts for DME SR with two types of active sites on the surface of alumina: solid solution of copper-cerium oxides and acid sites. 4. Conclusion It was shown that bifunctional Cu-Ce/Al2O3 catalysts (CuO+CeO2 10-20 wt. %, Cu/Ce wt. ratio from 1/1 to 4/1) are novel and effective catalysts for hydrogen production from DME. Two types of active sites were registered on the surface of these catalysts: acid sites for DME dehydration into methanol and CuO-CeO2 solid solutions for steam reforming of methanol to hydrogen rich gas. The formation of solid solutions CuO-CeO2 is a key factor responsible for high performance of Cu-Ce/Al2O3 catalysts. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
V.A. Sobyanin, S. Cavallaro, S. Freni, Energy Fuels 14 (2000) 1139. T.H. Fleisch, R.A. Sills, M.D. Briscoe, J. Natural Gas Chem., 11 (2002) 1. International DME Association Website: www.aboutdme.org T. Shikada, Y. Ohno, T. Ogawa, M. Ono, M. Mizuguchi, K. Tomura, K. Fujimoto, Stud. Surf. Sci. Catal. 119 (1998) 515. T. Ogawa, N. Inoue, T. Shikada, O. Inokoshi, Y.Ohno, Stud. Surf. Sci. Catal. 147 (2004) 379. V.V. Galvita, G.L. Semin, V.D. Belyaev, T.M. Yurieva, V.A. Sobyanin, Appl. Catal. A: Gen. 216 (2001) 85. Y. Tanaka, R. Kikuchi, T. Takeguchi, K. Eguchi, Appl. Catal. B: Env. 57 (2004) 211. T. Nishiguchi, K. Oka, T. Matsumoto, H. Kanai, K. Utani, S. Imamura, Appl. Catal. A: Gen. 301 (2006) 66. T.A. Semelsberger, K.C. Ott, R.L. Borup, H.L. Greene, Appl. Catal. B: Env. 65 (2006) 291. K. Takeishi, H. Suzuki, Appl. Catal. A: Gen. 260 (2004) 111. T.A. Semelsberger, K.C. Ott, R.L. Borup, H.L. Greene, Appl. Catal. A: Gen. 309 (2006) 210. T. Mathew, Y. Yamada, A. Ueda, H. Shioyama, T. Kobayashi, Catal. Lett. 100 (2005) 247. T. Mathew, Y.Yamada, A.Ueda, H. Shioyama, T. Kobayashi, Appl. Catal. A: Gen.286 (2005) 11. S.D. Badmaev, G.G. Volkova, V.D. Belyaev, L.M. Plyasova, V.A. Sobyanin, Rassian Patent 2 286 210 (2006). E.A. Paukshis, IR spectroscopy for heterogeneous acid-base catalysis. Nauka, Novosibirsk, 1992 (in Russian). J.B. Hansen, in G.Ertl, H. Knozinger, J.Weitkamp (Eds.) Handbook of Heterogeneous Catalysis, Vol. 5, Wiley-VCH, Weinheim, 1997, p.1856. K. Klier, Adv. Catal. 31 (1982) 243. T.M. Yurieva, Catal. Today 51 (1999) 457.
445
Bifunctional Catalysts for Hydrogen Production from Dimethyl Ether Galina Volkova, Sukhe Badmaev, Vladimir Belyaev, Lyudmila Plyasova, Anna Budneva, Evgeny Paukshtis, Vladimir Zaikovsky, Vladimir Sobyanin Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva 5, Novosibirsk, 630090, Russia
Abstract The production of hydrogen directly from dimethyl ether (DME) was performed on bifunctional Cu-Ce/Al2O3 catalysts. The catalysts were characterized by XRD, HRTEM, EDX and IR spectroscopy of low-temperature CO adsorption. The high hydrogen productivity up to 600 mmol g-1h-1 may be explained by assuming that (1) DME dehydration occurs on acid sites of Ȗ-Al2O3 and (2) methanol steam reforming takes place on mixed oxide phase CuO-CeO2, solid solution of copper ions in cerium dioxide with ratio Cu/Ce from 12/86 to 33/67 at.%. 1. Introduction Dimethyl Ether is expected to become an important energy resource as a raw material for hydrogen production for fuel cells. DME as well as methanol can be easily and selectively converted to hydrogen-rich gas at relatively low temperature (250-350oC) compared to other fuels such as natural gas, gasoline and LPG [1-3]. DME is relatively inert, non-corrosive, non-carcinogenic. DME is more favorable for steam reforming (SR) than methanol from the economical viewpoint because it can be produced from syn-gas more effectively [4-5]. Overall DME SR is expressed by the equations: CH3OCH3 + H2O = 2CH3OH CH3OH + H2O = 3H2 + CO2 CO2 + H2 = CO + H2O
446
G. Volkova et al.
The first step of DME SR is hydration of DME into methanol the second step is methanol steam reforming into hydrogen-rich gas. Besides, during DME SR a reverse water gas shift reaction may occur to produce carbon monoxide. The mechanical mixtures of two catalysts: solid-acid for DME hydration and copper-containing oxide for methanol steam reforming, are usually used for DME SR [6-9]. The main disadvantage of such catalysts is their layering that leads to decrease of activity. The design of a bifunctional catalyst for DME SR seems to be a promising way to overcome this problem. Cu/Al2O3, Cu-Zn/Al2O3, Cu-Pd/Al2O3, Cu-Ru/Al2O3, Cu-Pt/Al2O3, Cu-Rh/Al2O3, Cu-Au/Al2O3 and Cu/Ga8Al2O15 systems were studied as bifunctional catalysts for DME SR reaction [10-13] but demonstrated low hydrogen productivity. Researchers from the Boreskov Institute of Catalysis have developed the bifunctional Cu-Ce/Al2O3 catalysts for DME SR providing ten-fold increase of H2 productivity [14]. Here we present the performance and physical characterization of Cu-Ce/Al2O3 catalysts. 2. Experimental The Cu-Ce/Al2O3 catalysts were synthesized by treating Ȗ-alumina in solutions of copper and cerium salts taken at the given ratio. Samples were dried at 100oC and calcined at 450oC for 3 hours. Catalysts were evaluated in fixed bed flow reactor with gas analysis on line. Reaction conditions: H2O/DME/N2=20/60/20, GHSV=10000 h-1, pressure 1 atm, temperature 250-370oC. Prior to reaction, catalysts were reduced at 300oC in stream of H2-N2 for 1 hour. HRTEM images were obtained on a JEM-2010 electron microscope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm at an accelerating voltage of 200 kV. The high-resolution images of periodic structures were analyzed by the Fourier method. Local energy-dispersive X-ray analysis (EDXA) was carried out on an EDAX spectrometer (EDAX Co.) fitted with a Si (Li) detector with a resolution of 130 eV. FT-IR spectroscopy was used to determine the acidity of the samples by monitoring the low temperature CO adsorption. The samples were degassed in the IR cell at 400oC and then cooled to -173 oC and treated with low doses of CO from 0.1 to 10 torr. The spectra were recorded with 4 cm-1 resolution using Shimadzu FT-IR-8300 spectrometer. The number of CO adsorption sites was calculated by fitting the data to equation: N=A/UAo, where A (cm-1) is the integrated intensity of band, U (g cm-2) is the mass of material normalized on pellet surface, Ao is the molar integrated absorption coefficient equal to 0.8 cm ȝmol-1 for band 2185-2200 cm-1. Procedure for measurement of Ao is described in [15]. XRD studies were carried out on a diffractometer URD-63 with CuKĮ radiation using graphite monochromator with reflected beam. The lattice parameters and dispersion of CeO2 were determined from line (111), ǻĮ ± 0.003 Å.
447
Bifunctional catalysts for hydrogen production from DME
3. Results and Discussion Table 1. Performance of bifunctional Cu-Ce/Al2O3 catalysts in DME SR at 350oC, pressure 1 atm, H2O/DME/N2=60/20/20; GHSV= 10000 h-1
Catalyst
DME
Outlet gas composition, %
H2
conv. %
H2
CO2
CO
CH4
CH3OH
mmol/g h
4Cu-4Ce/Al2O3
71
50
17.3
0.3
0.08
0.64
473
8Cu-4Ce/Al2O3
80
55
18.5
0.7
0.13
0.35
523
12Cu-4Ce/Al2O3
81
57
18.5
0.7
0.25
0.6
520
8Cu-2Ce/Al2O3
98
59,5
19.3
0.7
1.6
0.07
610
Equilibrium
100
60
15.7
6.7
0
0.00013
The high hydrogen productivity up to 600 mmol g-1h-1 and low CO concentration in hydrogen-rich gas were observed for all tested catalysts (Table 1). The H2 productivity is more than one order of magnitude higher than the early reported (Table 2). The content of copper and cerium and their ratio slightly affected the catalyst activity, it reduced by 30% at the lowest concentration of CuO+CeO2. Performance of an alumina–free catalyst 10Cu/CeO2 was very poor; the DME conversion was within 1%. Table 2. Comparison of bifunctional catalysts in DME SR
Catalyst
T,oC
GHSV h-1
Inlet gas DME/H2O/N2
DME conv.,%
mmol/g h
H2
References
Cu-Ce/Al2O3
350
10000
20/60/20
71-98
470-610
Present work
Cu-Zn/Al2O3
350
180
25/75/0
97
11.3
[10]
Cu-Zn/Al2O3
350
1400
20/60/20
58
48.2
[11]
Ga8Al2O15
350
20000
1/3/96
80
43.0
[12-13]
Equilibrium conversion of DME and concentration of H2 over 8Cu-2Ce/Al2O3 catalyst are reached at temperature 350oC (Figure 1). The CO concentration is lower than 1% and remains below the equilibrium value at 250-370oC. FT-IR spectra of Ȗ-Al2O3 and Cu-Ce/Al2O3 catalysts in the hydroxyl region (Figure 2a) showed that the bands at 3730 and 3770 cm-1 are decreased (curve 2) and disappeared (curve 3) under loading of Cu-Ce oxides. However, no change in bands intensity of the most acidic hydroxyl groups at 3675 and 3690 cm-1 was detected.
448 100
80
80
Concentration, vol.%
100
60
60 H2
40
40
20
20
DME conversion, %
G. Volkova et al.
CO2
0 250
300
0
CO
350
Figure 1. DME conversion and H2, CO2, CO concentrations vs. temperature on 8Cu-2Ce/Al2O3 catalyst at H2O/DME = 3, GHSV= 10000 h-1. Points – experimental data, dash lines – calculated equilibrium data.
400
2110
3400
3500
3600
3700
3
2
2
1
1 2000
3800
2050
2188
3
2150
3770
Absorbance
Absorbance
3730
3690
3675
Temperature, ɨɋ
2100
2150
2200
2250
-1
-1
Wavenumber, cm
Wavenum ber, cm
a
b
Figure 2. FT-IR spectra of (1) Al2O3, (2) 4Cu-4Ce/Al2O3 and (3)12Cu-4Ce/Al2O3 catalysts in: (a) hydroxyl region and (b) adsorbed CO.
FT-IR spectra of adsorbed CO (Figure 2b) revealed bands characteristic of both Lewis (2185-2200 cm-1) and Brønsted (2110-2175 cm-1) forms of adsorption. Table 3 collects the acidity data calculated from bands 2185-2200 cm-1 for Lewis acid sites. It can be seen that amount of acid sites on the surface of Ȗ-Al2O3 significantly reduced after loading of copper-cerium oxides. Nevertheless, the Lewis acid sites typical for Ȗ-alumina were registered on the surface of all bifunctional Cu-Ce/Al2O3 catalysts. Table 3. Acidity of the catalysts from FTIR spectra of CO adsorption
Catalyst
ȞCO, cm-1
Lewis acid sites, ȝmol g-1
Ȗ-Al2O3
2184-2200
600
4Cu-4Ce/Al2O3
2190
110
8Cu-2Ce/Al2O3
2190
70
12Cu-4Ce/Al2O3
2190
50
449
Bifunctional catalysts for hydrogen production from DME Table 4. Characterization of Cu-Ce/Al2O3 catalysts by XRD and EDX analysis
Catalyst
CeO2 4Cu-4Ce/Al2O3 8Cu-2Ce/Al2O3 12Cu-4Ce/Al2O3
Phase
a (111)
D (111)
Cu/Ce
composition
CeO2, Å
CeO2, Å
at. %
CeO2 CeO2 +CuO+Al2O3 CeO2 +CuO+Al2O3
5.417 5.381 5.378 5.402
80 40 35 42
33/67 no data 12/88
CeO2 +CuO+Al2O3
Results of XRD and EDX analysis for Cu-Ce/Al2O3 catalysts are presented in Table 4 and on Figure 3a. Three crystalline phases: CuO, CeO2 and Al2O3 were identified in bifunctional Cu-Ce/Al2O3 catalysts. Content of CuO phase decreased with decreasing of copper concentration in the samples; in 4Cu-4Ce/Al2O3 catalyst only traces of CuO were observed. It can be seen from Table 4 that the unit cell parameter of CeO2 decreased from 5.417 Å to 5.4025.378 Å. It indicates the formation of the solid solution of copper oxide in cerium dioxide. The exact atomic ratio of metals Cu/Ce in CuO-CeO2 solid solutions was determined from the EDX spectra (Figure 3a). It was shown that the atomic ratio of Cu/Ce ranged from 12/88 to 33/67 at. % (Table 4). The dispersion of CeO2 in the range of 35-42 Å has been registered by XRD for all bifunctional catalysts (Table 4). TEM observations presented in Figure 3b show the formation of 50-100 nm aggregates of mixed CuO-CeO2 oxide. It means that CuO-CeO2 solid solution is composed of elementary particles (35-42 Å) agglomerated in large particles of 50-100 nm in size. These particles of mixed CuO-CeO2 oxide are located within the large pores of Ȗ-alumina.
Figure 3. EDX spectra (a) and TEM micrograph (b) of 4Cu-4Ce/Al2O3 catalyst
450
G. Volkova et al.
It is known that the activity of copper containing mixed oxides for methanol synthesis and for water gas shift reaction is determined by the formation of solid solution of copper ions in zinc-alumina mixed oxides [16-18]. These catalysts are also very active for methanol steam reforming. Our aim was to provide the formation of the CuO-CeO2 solid solution on the surface of alumina and at the same time to prevent the blocking of alumina acid sites. We have succeeded for the first time in design of bifunctional catalysts for DME SR with two types of active sites on the surface of alumina: solid solution of copper-cerium oxides and acid sites. 4. Conclusion It was shown that bifunctional Cu-Ce/Al2O3 catalysts (CuO+CeO2 10-20 wt. %, Cu/Ce wt. ratio from 1/1 to 4/1) are novel and effective catalysts for hydrogen production from DME. Two types of active sites were registered on the surface of these catalysts: acid sites for DME dehydration into methanol and CuO-CeO2 solid solutions for steam reforming of methanol to hydrogen rich gas. The formation of solid solutions CuO-CeO2 is a key factor responsible for high performance of Cu-Ce/Al2O3 catalysts. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
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