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CeO2–ZrO2 catalysts
Catalysis Communications 3 (2002) 565–571 www.elsevier.com/locate/catcom
Hydrogen production by ethanol reforming over Rh=CeO2–ZrO2 catalysts C. Diagne a
a,1
, H. Idriss
b,* ,
A. Kiennemann
a
LMSPC (UMR 7515), Ecole de Chimie Polym eres et Mat eriaux, Universit e Louis Pasteur, 25 rue Becquerel, Strasbourg, 67087 Strasbourg Cedex 2, France b Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 7 August 2002; received in revised form 10 September 2002; accepted 13 September 2002
Abstract Reforming of ethanol in excess of water (1–8 molar ratio) has been investigated on Rh=CeO2 , Rh=ZrO2 and Rh=CeO2 –ZrO2 (Ce/Zr ¼ 4, 2 and 1). Catalysts characterization was conducted by X-ray diffraction, BET surface area measurements, CO2 adsorption, and temperature programmed reduction (TPR). At 400–500 °C all catalysts showed high activity and selectivity towards hydrogen production (between 5 and 5.7 mol of H2 per mol of ethanol inlet) despite the considerable textural differences of the oxides (fluorite, monoclinic and tetragonal). The large variations of Rh dispersion (as monitored by TPR) between all catalysts had a small effect on H2 production. Although it appears that the reaction is not sensitive to either the oxide or the metal structure Rh=CeO2 (the most basic catalyst investigated) was the least reactive. Ó 2002 Published by Elsevier Science B.V.
1. Introduction Hydrogen, as an alternative source of energy, is receiving increasing demand because of environmental reasons. Both methanol and ethanol are the most important candidates for an efficient production of hydrogen. Unlike methanol (mainly made from syn-gas conversion) ethanol can be produced either by hydrolysis of cellulosic materials followed by fermentation or by direct fermentation of sugar containing crops. Both processes result in a mixture of ethanol–water with *
1
Corresponding author. Fax: +64-9-373-7422. E-mail address: [email protected] (H. Idriss). On leave from the University of Dakar, Dakar, Senegal.
a ratio close to 1 ethanol for 10 water molecules. It is thus highly desirable to find efficient catalytic materials for the production of hydrogen in a high water to ethanol ratios. Cerium oxide has been shown active for direct CO oxidation and water gas shift reactions [1–3]. Addition of ZrO2 to CeO2 has resulted in enhancing the ceria activity for oxidation reactions [4,5]. This enhancement has been reported as due to an increasing oxygen mobility and the formation of a solid solution. Rh=CeO2 (alone or in presence of a second metal: Pd or Pt) has shown interesting activity for ethanol decomposition in presence of oxygen [6,7] and Rh=Al2 O3 [8,9] showed relatively good hydrogen production from ethanol reforming;
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although at relatively high temperatures (>700 °C). This activity is mainly due to the capacity of Rh to break the carbon–carbon bond necessary for an efficient total decomposition of ethanol. In this work we show that up to 5.7 mol of hydrogen can be produced per mol of ethanol at reasonable temperatures (350–450 °C) on Rh= CeO2 –ZrO2 in presence of excess of water.
2. Experimental CeO2 , ZrO2 and CeO2 –ZrO2 supports were prepared by precipitation with an ammonia solution (32 wt%) from 0.2 mol solutions of cerium and/or zirconium nitrates (Strem). After filtration, washing with de-ionized water and drying at 120 °C (8 h) the hydroxide precursors were calcined at 680 °C (8 h). Rh (2 wt%) was added by impregnation of an aqueous solution containing RhCl3 3H2 O under stirring at RT. After drying at 120 °C (8 h) and calcination at 680 °C (8 h) Rh-based catalysts were prepared. The BET surface area was conducted using a Coulter SA 3100 apparatus. X-ray diffraction (XRD) spectra were collected using a D5000 ) at a 2h Siemens with a Cu radiation (Ka ¼ 1:5406 A interval of 20–60° with a step size of 0.05° and a count time of 35 s per step. Catalytic reactions of ethanol reforming were conducted in a fixed-bed reactor with a molar ratio H2 O/ethanol ¼ 8, flow rate of 0.77 l h1 and an Ar flow rate of 3 l h1 . Prior to the catalytic reaction, all catalysts were reduced in a H2 flow (10% in Ar) at 320 °C (1 h). Reaction products were analyzed as a function of time at different temperatures. The
results reported here are those taken after 90 min of reactions (a steady state condition was usually obtained after ca. 30 min). Products were analyzed using three GC apparatus. One equipped with a FI detector for CH4 and ethanol (small traces of acetone and acetaldehyde were observed only on Rh=CeO2 and neglected). A micro GC equipped with a TC detector was used for H2 , CH4 and CO separation (molecular sieve 5 A with Ar as the carrier gas, at 110 °C) and a second GC also equipped with TC detector for CO2 separation (Poraplot, with He as the carrier gas, at 85 °C). Temperature programmed reduction were conducted using a 50 mg of catalyst (in a fixed-bed quartz reactor) under 10% H2 in Ar (2 l h1 ) with a ramping rate of 15 °C min1 from 25 to 950 °C. CO2 adsorption was conducted at 0 °C in a quartz micro-balance connected to a high vacuum system (<105 Torr). Catalysts were heated (500 °C) in vacuum over night prior to CO2 adsorption. Calculation of the equilibrium constant, K was computed from the reciprocal slope of the plot 1=hf ð1=PCO2 Þ (for each catalyst); where h is the saturation coverage following Langmuir isotherm: h¼
KPCO2 1 1 ) ¼ þ 1: h KPCO2 1 þ KPCO2
ð1Þ
3. Results and discussion 3.1. BET, XRD, TPR and CO2 adsorption Table 1 shows the BET surface areas of the different catalysts used in this work. The surface
Table 1 BET surface area of the CeO2 –ZrO2 and 2 wt% Rh=CeO2 –ZrO2 catalysts CeO2 –ZrO2; Ce=Zra ¼ Catalyst ðm2 g1 Þ
CeO2 22.7
ZrO2 55.7
4 49.5
2 50.8
1 36.6
2 wt% Rh=CeO2
2 wt% Rh=ZrO2
17.6
46.7
a
2 wt% Rh=CeO2 –ZrO2 , Ce=Zr ¼ Catalyst ðm2 g1 Þ a b
4 42.8 (23.4)b
Atomic ratio. Expected based on a linear contribution from both oxides.
2 43.6 (27.3)b
1 28.5 (32.2)b
C. Diagne et al. / Catalysis Communications 3 (2002) 565–571
Fig. 1. X-ray diffraction Rh=CeO2 –ZrO2 catalysts.
of
Rh=CeO2 ,
Rh=ZrO2
and
area of ZrO2 is more than twice higher than of CeO2 . Interestingly the addition of small amounts of ZrO2 to CeO2 (about 20%) increases considerably the surface area of the catalyst and then decreases it again. This may indicate a change of material structure (see below). Fig. 1 shows XRD of the series of oxides investigated in this work. CeO2 shows the expected diffraction of the fluorite structure while the monoclinic + tetragonal diffraction lines are shown
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for ZrO2 . The absence of ZrO2 diffraction lines at low loading of ZrO2 into CeO2 (Ce/Zr ¼ 4 and 2) indicates the presence of a solid solution (this has been further corroborated by Raman analyses of the same samples [10]). The shift of the diffraction lines to higher degrees has been recently reported [11] and is attributed to shrinking of the lattice due ) by Zr ions to the replacement of Ce ions (1.09 A ) [12]. At a higher loading of ZrO2 (Ce/ (0.86 A Zr ¼ 1) an additional tetragonal phase is seen as reported by other workers [11]. Thus, XRD shows three different structures: pure cubic (CeO2 ), a mixture of cubic and tetragonal (Ce/Zr ¼ 1) and a mixture of monoclinic and tetragonal (ZrO2 ). Rh is not seen due to its low percentage. Comparing Fig. 1 to Table 1 indicates the following. The solid solutions (for Ce/Zr ¼ 4 and 2) have higher surface areas than expected from a linear combination of the surfaces of the two oxides separately. On the contrary, the catalyst with Ce/Zr ¼ 1 has a surface area very close to the combined areas of the two different oxides (0:5 17:6 þ 0:5 46:7 ¼ 32:2 m2 g1 to be compared with 28.5 m2 g1 ). Fig. 2 shows TPR of the different Rh catalysts. In the absence of Rh [13] no peaks below 200 °C are observed. Reduction of Rh starts at about 70 °C and ends below 200 °C. Variations (peak
Fig. 2. Temperature programmed reduction of Rh=CeO2 , Rh=ZrO2 and Rh=CeO2 –ZrO2 catalysts. Inset: computed H2 consumption (arbitrary units) of the Rh region (<200 °C) per m2 as a function of percentage of Ce.
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intensity and peak positions) exist between catalysts with different Zr loading. The total amount of H2 consumed, attributed to reduction of Rh, is indicated in the inset. Assuming that the consumption is directly related to the dispersion of Rh it is clear that Rh particles in Rh=ZrO2 are by far the least dispersed. It also appears that catalysts with both CeO2 and ZrO2 behave similarly with respect to reduction of Rh ions (dispersion). The two peaks, clearly observed in the case of Rh=CeO2 , indicate that Rh ions existed in two distinct states. These may not necessarily be two distinct electronic states. Similar results were observed by other workers. The authors [14,15] attributed the low-temperature peak to uniformly distributed Rh2 O3 particles on the support, while the second one is attributed to a bulk-like crystalline Rh2 O3 on the surface: large particles. The peak at ca. 850 °C (not observed in the case of ZrO2 alone) is clearly due to bulk reduction of Ce ions (Ce4þ to Cexþ ; x < 4). Reversible and irreversible CO2 adsorption at 0 °C on all catalysts was conducted. A representative data is shown in Fig. 3 for Rh=CeO2 . Both the surface-uptake (molecules/m2 ) and coverage (h) are given. The estimated errors are in the order of 10% of each value. From a plot of 1=hf ð1=PCO2 Þ the binding constant KCO2 was computed for all catalysts. This is shown in the inset of Fig. 4 for CeO2 , while the figure presents KCO2 (computed from the reversible data) as a function of percentage of Ce for Rh=CeO2 –ZrO2 .
The higher the electronic charge around the oxygen (electron withdrawing) the higher the basicity, the higher is the reaction with incoming CO2 molecules according to Eq. (2) (see [16] and [17] for more details). CO2 ðgÞ þ M–O MCO3
ð2Þ
CeO2 is the most basic oxide and the basicity decreases with increasing the amount of ZrO2 . No particular effect was seen for the solid solution. 3.2. Catalytic reactions Catalytic reactions have been performed on a Rh=CeO2 –ZrO2 series with varying Ce/Zr ratios. The results are shown in Tables 2 and 3 and Fig. 5. Below 200 °C negligible reactions were observed
Fig. 4. Binding constant, KCO2 (kPa1 ) as a function of percentage of Ce for Rh=CeO2 –ZrO2 catalysts. Inset: plot of 1=hf ð1=PCO2 ) for CeO2 .
Fig. 3. CO2 uptake at 0 °C for Rh/CeO2 . (}) Total adsorption under the indicated (x-axis) PCO2 ; (s) irreversible adsorption (105 Torr); (M) reversible adsorption (computed from the difference between total adsorption and irreversible adsorption); () reversible CO2 coverage, h (data of (M) normalized to 1).
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Table 2 Composition of reaction products at 450 °C and at 100% conversion for the Rh=CeO2 –ZrO2 catalysts series Composition (%)
Rh=CeO2
Rh=ZrO2
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 4)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 2)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1)
H2 CH4 CO CO2
69.1 8.2 3.5 19.2
71.7 6.0 2.1 20.2
70.3 7.3 1.6 20.8
69.2 8.5 1.6 20.7
70.3 7.3 1.5 20.9
Table 3 CO2 =CO molar ratio as a function of temperature for ethanol/water (1/8) reaction over Rh=CeO2 –ZrO2 catalysts series CO2 =CO 300 350 400 450 500
°C °C °C °C °C
Rh=CeO2
Rh=ZrO2
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 4)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 2)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1)
0.36 3.14 5.1 5.5 5.6
0.87 4.3 12.7 9.4 5.9
1.09 6.9 27.7 13.1 7.0
0.49 3.9 11.1 13.1 7.9
0.84 28 32.5 14.1 7.5
In italics: best temperature and best catalyst.
Fig. 5. Hydrogen yield as a function of reaction temperature for Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1) per mole of ethanol inlet. Inset: moles of hydrogen per mole of ethanol inlet, as a function of percentage of Ce for the series of catalysts at 450 °C.
on all catalysts. At 300 °C all catalysts but Rh=CeO2 ( 60%) showed complete conversion. Between 400 and 500 °C the catalysts showed similar behavior with respect to H2 production. Table 2 presents the product composition (mol%) for all the series at 450 °C. The main difference is in the CO2 and CO distribution. The presence of ZrO2 alone or in addition to CeO2 decreases the amount of CO. The CO2 =CO is sensitive not only
to the reaction temperature but also to the relative amount of ZrO2 (or CeO2 ). The effect is not linear and it appears that the best catalyst (higher CO2 =CO) contains equal amounts of Ce and Zr (Ce/Zr ¼ 1). Rh=CeO2 is a good catalyst for water gas shift reaction (WGSR) [22,23]. The addition of ZrO2 has shown enhancement of its activity for oxidation reactions [4,5]. Table 3 shows that ZrO2 alone is more efficient for CO2 formation than
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CeO2 . The reason may not be due to direct oxidation of CO to CO2 but to a high water WGS activity. We have not tested the series for WGSR and have not found any systematic work addressing the catalytic activity of Rh=CeO2 –ZrO2 series with respect to WGSR. It is worth reminding the elementary steps of ethanol decomposition in order to gain some insights into the reasons behind the products distribution. In the decomposition pathway of ethoxy species Rh has a unique effect. It abstract H from the CH3 -group making a stable oxametallacycle intermediate ((a)CH2 –CH2 –O(a)) [18,19] and the following elementary steps may describe the complete process: CH3 CH2 OH ! CH3 CH2 OðaÞ þ HðaÞ
ð3Þ
CH3 CH2 OðaÞ ! ðaÞCH2 –CH2 OðaÞ þ HðaÞ
ð4Þ
ðaÞCH2 –CH2 OðaÞ ! CH4 ðgÞ þ COðgÞ
ð5Þ
CH4 þ H2 OðaÞ ! 3H2 ðgÞ þ COðgÞ
ð6Þ
2CO þ 2H2 O ! 2CO2 þ 2H2
ð7Þ
2HðaÞ ! H2 ðgÞ
ð8Þ
(a): adsorbed. The sum of Eqs. (3)–(8) gives Eq. (9) (see below); more information on Eq. (6) can be found in [18] and [19], on ethoxy decomposition on metals in general in [20], and on reforming of CH4 in [21]. Fig. 5 shows the product yield from ethanol reforming as a function of temperature for Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1), as an example of the catalytic series. Products yield is calculated per mole of inlet ethanol. A complete reforming of CH3 CH2 OH would yield 6H2 and 2CO2 . CH3 CH2 OH þ 3H2 O ! 6H2 þ 2CO2
ð9Þ
The presence of methane and CO indicates that reforming of ethanol occurs via other reaction intermediates (methane since acetaldehyde formation was negligible). The selectivity to hydrogen initially increases with temperature due to decreasing amounts of CH4 at CO. Reforming of methane (Eq. (6)) and WGSR (Eq. (7)) are most likely the reasons. Still at higher temperatures
>400 °C, CO2 =CO shows a decreasing trend. This may indicate that WGSR is not effective any more: WGSR is exothermic and is thus disfavored by increasing the temperature. The continuing increase of H2 is thus most likely occurring by more reforming of CH4 rather than by WGSR. Reforming of CH4 by CO2 may also occur. However, it is 10 kcal mol1 higher than that by H2 O (59 compared to 49 kcal mol1 ). The excess of water used in this work in addition to the fact that CO2 is actually formed from reforming of CO makes this route unlikely. From Table 2, Fig. 5 and the above equations of the elementary reactions it appears that the rate limiting steps for the formation of H2 from ethanol is making CH4 from ethanol (most likely via oxametallacycle intermediate (Eq. (5))). In summary, results of this work indicate the following three points. (1) The best catalysts (in the series Rh=CeO2 –ZrO2 ) are not necessarily the most basic ones but those that can bind CO2 mildly. (2) Ethanol reforming on Rh=CeO2 –ZrO2 does not appear to be sensitive to Rh dispersion. (3) The CO2 =CO is sensitive to the Ce/Zr in the Rh=CeO2 –ZrO2 series investigated.
Acknowledgements The authors thank Peter Buchanan for conducting the CO2 adsorption experiments, Dr. Jean Seakins for Raman analyses and Suzanne Libs for her help in gas-phase analyses.
References [1] G. Kim, Ind. Eng. Chem. Procd. Res. Div. 21 (1982) 267. [2] G.B. Hoflund, S.D. Gardner, D.R. Schryer, B.T. Upchurch, E.J. Kielen, React. Kinet. Catal. Lett. 58 (1996) 19. [3] T.X.T. Sayle, S.C. Parker, C.R.A. Catlow, Surf. Sci. 316 (1994) 329. [4] C. Leitenburg, A. Trovarelli, J. Llorca, F. Cavani, G. Bini, Appl. Catal. A-Gen. 139 (1996) 161. [5] K. Tomishige, Y. Furusawa, Y. Ikeda, M. Asadulah, K. Fujimoto, Catal. Lett. 151 (1995) 151. [6] A. Yee, S. Morrison, H. Idriss, Catal. Today 63 (2000) 30. [7] P.-Y. Sheng, A. Yee, G.A. Bowmaker, H. Idriss, J. Catal. 208 (2002) 393. [8] S. Freni, J. Power Sources 94 1 (2001) 14. [9] S. Cavallaro, Energy Fuels 14 (2000) 1195.
C. Diagne et al. / Catalysis Communications 3 (2002) 565–571 [10] J. Seakins, H. Idriss, work in progress. [11] S. Pengpanich, V. Meeyoo, T. Rirksomboon, K. Bunyakiat, Appl. Catal. A-Gen. 234 (2002) 221. [12] Handbook of Chemistry and Physics, 76th ed., CRC Press, Baton Rouge, FL, 1995. [13] M. Angel, A. Kiennemann, work in progress. [14] P. Fornasiero, R. Di Monte, G.R. Rao, J. Kaspar, S. Meriani, A. Trovarelli, M. Graziani, J. Catal. 151 (1995) 168. [15] J.C. Vis, H.F.J. vanÕt Blik, T. Huizinga, J. van Grondelle, R.J. Prins, J. Catal. 95 (1985) 95. [16] H. Idriss, E.G. Seebauer, Catal. Lett. 66 (2000) 139.
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[17] H. Idriss, M.A. Barteau, Adv. Catal. 45 (2000) 261. [18] G.S. Jones, M. Mavrikakis, M.A. Barteau, J.M. Vohs, J. Am. Chem. Soc. 120 (1998) 3196. [19] M. Mavrikakis, D.J. Doren, M.A. Barteau, J. Phys. Chem. 102 (1998) 394. [20] Y. Cong, R.I. Masel, Surf. Sci. 396 (1998) 1. [21] A.N. Fatsikostas, D.I. Kondarides, X.E. Verykios, Catal. Today 75 (2002) 145, and references therein. [22] A. Trovarelli, Catal. Rev. Sci. Eng. 38 (1996) 439. [23] T. Shido, A. Yamaguchi, K. Asakura, Y. Iwasawa, J. Mol. Catal. A-Chem. 163 (2000) 67.
Hydrogen production by ethanol reforming over Rh=CeO2–ZrO2 catalysts C. Diagne a
a,1
, H. Idriss
b,* ,
A. Kiennemann
a
LMSPC (UMR 7515), Ecole de Chimie Polym eres et Mat eriaux, Universit e Louis Pasteur, 25 rue Becquerel, Strasbourg, 67087 Strasbourg Cedex 2, France b Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 7 August 2002; received in revised form 10 September 2002; accepted 13 September 2002
Abstract Reforming of ethanol in excess of water (1–8 molar ratio) has been investigated on Rh=CeO2 , Rh=ZrO2 and Rh=CeO2 –ZrO2 (Ce/Zr ¼ 4, 2 and 1). Catalysts characterization was conducted by X-ray diffraction, BET surface area measurements, CO2 adsorption, and temperature programmed reduction (TPR). At 400–500 °C all catalysts showed high activity and selectivity towards hydrogen production (between 5 and 5.7 mol of H2 per mol of ethanol inlet) despite the considerable textural differences of the oxides (fluorite, monoclinic and tetragonal). The large variations of Rh dispersion (as monitored by TPR) between all catalysts had a small effect on H2 production. Although it appears that the reaction is not sensitive to either the oxide or the metal structure Rh=CeO2 (the most basic catalyst investigated) was the least reactive. Ó 2002 Published by Elsevier Science B.V.
1. Introduction Hydrogen, as an alternative source of energy, is receiving increasing demand because of environmental reasons. Both methanol and ethanol are the most important candidates for an efficient production of hydrogen. Unlike methanol (mainly made from syn-gas conversion) ethanol can be produced either by hydrolysis of cellulosic materials followed by fermentation or by direct fermentation of sugar containing crops. Both processes result in a mixture of ethanol–water with *
1
Corresponding author. Fax: +64-9-373-7422. E-mail address: [email protected] (H. Idriss). On leave from the University of Dakar, Dakar, Senegal.
a ratio close to 1 ethanol for 10 water molecules. It is thus highly desirable to find efficient catalytic materials for the production of hydrogen in a high water to ethanol ratios. Cerium oxide has been shown active for direct CO oxidation and water gas shift reactions [1–3]. Addition of ZrO2 to CeO2 has resulted in enhancing the ceria activity for oxidation reactions [4,5]. This enhancement has been reported as due to an increasing oxygen mobility and the formation of a solid solution. Rh=CeO2 (alone or in presence of a second metal: Pd or Pt) has shown interesting activity for ethanol decomposition in presence of oxygen [6,7] and Rh=Al2 O3 [8,9] showed relatively good hydrogen production from ethanol reforming;
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although at relatively high temperatures (>700 °C). This activity is mainly due to the capacity of Rh to break the carbon–carbon bond necessary for an efficient total decomposition of ethanol. In this work we show that up to 5.7 mol of hydrogen can be produced per mol of ethanol at reasonable temperatures (350–450 °C) on Rh= CeO2 –ZrO2 in presence of excess of water.
2. Experimental CeO2 , ZrO2 and CeO2 –ZrO2 supports were prepared by precipitation with an ammonia solution (32 wt%) from 0.2 mol solutions of cerium and/or zirconium nitrates (Strem). After filtration, washing with de-ionized water and drying at 120 °C (8 h) the hydroxide precursors were calcined at 680 °C (8 h). Rh (2 wt%) was added by impregnation of an aqueous solution containing RhCl3 3H2 O under stirring at RT. After drying at 120 °C (8 h) and calcination at 680 °C (8 h) Rh-based catalysts were prepared. The BET surface area was conducted using a Coulter SA 3100 apparatus. X-ray diffraction (XRD) spectra were collected using a D5000 ) at a 2h Siemens with a Cu radiation (Ka ¼ 1:5406 A interval of 20–60° with a step size of 0.05° and a count time of 35 s per step. Catalytic reactions of ethanol reforming were conducted in a fixed-bed reactor with a molar ratio H2 O/ethanol ¼ 8, flow rate of 0.77 l h1 and an Ar flow rate of 3 l h1 . Prior to the catalytic reaction, all catalysts were reduced in a H2 flow (10% in Ar) at 320 °C (1 h). Reaction products were analyzed as a function of time at different temperatures. The
results reported here are those taken after 90 min of reactions (a steady state condition was usually obtained after ca. 30 min). Products were analyzed using three GC apparatus. One equipped with a FI detector for CH4 and ethanol (small traces of acetone and acetaldehyde were observed only on Rh=CeO2 and neglected). A micro GC equipped with a TC detector was used for H2 , CH4 and CO separation (molecular sieve 5 A with Ar as the carrier gas, at 110 °C) and a second GC also equipped with TC detector for CO2 separation (Poraplot, with He as the carrier gas, at 85 °C). Temperature programmed reduction were conducted using a 50 mg of catalyst (in a fixed-bed quartz reactor) under 10% H2 in Ar (2 l h1 ) with a ramping rate of 15 °C min1 from 25 to 950 °C. CO2 adsorption was conducted at 0 °C in a quartz micro-balance connected to a high vacuum system (<105 Torr). Catalysts were heated (500 °C) in vacuum over night prior to CO2 adsorption. Calculation of the equilibrium constant, K was computed from the reciprocal slope of the plot 1=hf ð1=PCO2 Þ (for each catalyst); where h is the saturation coverage following Langmuir isotherm: h¼
KPCO2 1 1 ) ¼ þ 1: h KPCO2 1 þ KPCO2
ð1Þ
3. Results and discussion 3.1. BET, XRD, TPR and CO2 adsorption Table 1 shows the BET surface areas of the different catalysts used in this work. The surface
Table 1 BET surface area of the CeO2 –ZrO2 and 2 wt% Rh=CeO2 –ZrO2 catalysts CeO2 –ZrO2; Ce=Zra ¼ Catalyst ðm2 g1 Þ
CeO2 22.7
ZrO2 55.7
4 49.5
2 50.8
1 36.6
2 wt% Rh=CeO2
2 wt% Rh=ZrO2
17.6
46.7
a
2 wt% Rh=CeO2 –ZrO2 , Ce=Zr ¼ Catalyst ðm2 g1 Þ a b
4 42.8 (23.4)b
Atomic ratio. Expected based on a linear contribution from both oxides.
2 43.6 (27.3)b
1 28.5 (32.2)b
C. Diagne et al. / Catalysis Communications 3 (2002) 565–571
Fig. 1. X-ray diffraction Rh=CeO2 –ZrO2 catalysts.
of
Rh=CeO2 ,
Rh=ZrO2
and
area of ZrO2 is more than twice higher than of CeO2 . Interestingly the addition of small amounts of ZrO2 to CeO2 (about 20%) increases considerably the surface area of the catalyst and then decreases it again. This may indicate a change of material structure (see below). Fig. 1 shows XRD of the series of oxides investigated in this work. CeO2 shows the expected diffraction of the fluorite structure while the monoclinic + tetragonal diffraction lines are shown
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for ZrO2 . The absence of ZrO2 diffraction lines at low loading of ZrO2 into CeO2 (Ce/Zr ¼ 4 and 2) indicates the presence of a solid solution (this has been further corroborated by Raman analyses of the same samples [10]). The shift of the diffraction lines to higher degrees has been recently reported [11] and is attributed to shrinking of the lattice due ) by Zr ions to the replacement of Ce ions (1.09 A ) [12]. At a higher loading of ZrO2 (Ce/ (0.86 A Zr ¼ 1) an additional tetragonal phase is seen as reported by other workers [11]. Thus, XRD shows three different structures: pure cubic (CeO2 ), a mixture of cubic and tetragonal (Ce/Zr ¼ 1) and a mixture of monoclinic and tetragonal (ZrO2 ). Rh is not seen due to its low percentage. Comparing Fig. 1 to Table 1 indicates the following. The solid solutions (for Ce/Zr ¼ 4 and 2) have higher surface areas than expected from a linear combination of the surfaces of the two oxides separately. On the contrary, the catalyst with Ce/Zr ¼ 1 has a surface area very close to the combined areas of the two different oxides (0:5 17:6 þ 0:5 46:7 ¼ 32:2 m2 g1 to be compared with 28.5 m2 g1 ). Fig. 2 shows TPR of the different Rh catalysts. In the absence of Rh [13] no peaks below 200 °C are observed. Reduction of Rh starts at about 70 °C and ends below 200 °C. Variations (peak
Fig. 2. Temperature programmed reduction of Rh=CeO2 , Rh=ZrO2 and Rh=CeO2 –ZrO2 catalysts. Inset: computed H2 consumption (arbitrary units) of the Rh region (<200 °C) per m2 as a function of percentage of Ce.
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intensity and peak positions) exist between catalysts with different Zr loading. The total amount of H2 consumed, attributed to reduction of Rh, is indicated in the inset. Assuming that the consumption is directly related to the dispersion of Rh it is clear that Rh particles in Rh=ZrO2 are by far the least dispersed. It also appears that catalysts with both CeO2 and ZrO2 behave similarly with respect to reduction of Rh ions (dispersion). The two peaks, clearly observed in the case of Rh=CeO2 , indicate that Rh ions existed in two distinct states. These may not necessarily be two distinct electronic states. Similar results were observed by other workers. The authors [14,15] attributed the low-temperature peak to uniformly distributed Rh2 O3 particles on the support, while the second one is attributed to a bulk-like crystalline Rh2 O3 on the surface: large particles. The peak at ca. 850 °C (not observed in the case of ZrO2 alone) is clearly due to bulk reduction of Ce ions (Ce4þ to Cexþ ; x < 4). Reversible and irreversible CO2 adsorption at 0 °C on all catalysts was conducted. A representative data is shown in Fig. 3 for Rh=CeO2 . Both the surface-uptake (molecules/m2 ) and coverage (h) are given. The estimated errors are in the order of 10% of each value. From a plot of 1=hf ð1=PCO2 Þ the binding constant KCO2 was computed for all catalysts. This is shown in the inset of Fig. 4 for CeO2 , while the figure presents KCO2 (computed from the reversible data) as a function of percentage of Ce for Rh=CeO2 –ZrO2 .
The higher the electronic charge around the oxygen (electron withdrawing) the higher the basicity, the higher is the reaction with incoming CO2 molecules according to Eq. (2) (see [16] and [17] for more details). CO2 ðgÞ þ M–O MCO3
ð2Þ
CeO2 is the most basic oxide and the basicity decreases with increasing the amount of ZrO2 . No particular effect was seen for the solid solution. 3.2. Catalytic reactions Catalytic reactions have been performed on a Rh=CeO2 –ZrO2 series with varying Ce/Zr ratios. The results are shown in Tables 2 and 3 and Fig. 5. Below 200 °C negligible reactions were observed
Fig. 4. Binding constant, KCO2 (kPa1 ) as a function of percentage of Ce for Rh=CeO2 –ZrO2 catalysts. Inset: plot of 1=hf ð1=PCO2 ) for CeO2 .
Fig. 3. CO2 uptake at 0 °C for Rh/CeO2 . (}) Total adsorption under the indicated (x-axis) PCO2 ; (s) irreversible adsorption (105 Torr); (M) reversible adsorption (computed from the difference between total adsorption and irreversible adsorption); () reversible CO2 coverage, h (data of (M) normalized to 1).
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Table 2 Composition of reaction products at 450 °C and at 100% conversion for the Rh=CeO2 –ZrO2 catalysts series Composition (%)
Rh=CeO2
Rh=ZrO2
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 4)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 2)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1)
H2 CH4 CO CO2
69.1 8.2 3.5 19.2
71.7 6.0 2.1 20.2
70.3 7.3 1.6 20.8
69.2 8.5 1.6 20.7
70.3 7.3 1.5 20.9
Table 3 CO2 =CO molar ratio as a function of temperature for ethanol/water (1/8) reaction over Rh=CeO2 –ZrO2 catalysts series CO2 =CO 300 350 400 450 500
°C °C °C °C °C
Rh=CeO2
Rh=ZrO2
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 4)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 2)
Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1)
0.36 3.14 5.1 5.5 5.6
0.87 4.3 12.7 9.4 5.9
1.09 6.9 27.7 13.1 7.0
0.49 3.9 11.1 13.1 7.9
0.84 28 32.5 14.1 7.5
In italics: best temperature and best catalyst.
Fig. 5. Hydrogen yield as a function of reaction temperature for Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1) per mole of ethanol inlet. Inset: moles of hydrogen per mole of ethanol inlet, as a function of percentage of Ce for the series of catalysts at 450 °C.
on all catalysts. At 300 °C all catalysts but Rh=CeO2 ( 60%) showed complete conversion. Between 400 and 500 °C the catalysts showed similar behavior with respect to H2 production. Table 2 presents the product composition (mol%) for all the series at 450 °C. The main difference is in the CO2 and CO distribution. The presence of ZrO2 alone or in addition to CeO2 decreases the amount of CO. The CO2 =CO is sensitive not only
to the reaction temperature but also to the relative amount of ZrO2 (or CeO2 ). The effect is not linear and it appears that the best catalyst (higher CO2 =CO) contains equal amounts of Ce and Zr (Ce/Zr ¼ 1). Rh=CeO2 is a good catalyst for water gas shift reaction (WGSR) [22,23]. The addition of ZrO2 has shown enhancement of its activity for oxidation reactions [4,5]. Table 3 shows that ZrO2 alone is more efficient for CO2 formation than
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CeO2 . The reason may not be due to direct oxidation of CO to CO2 but to a high water WGS activity. We have not tested the series for WGSR and have not found any systematic work addressing the catalytic activity of Rh=CeO2 –ZrO2 series with respect to WGSR. It is worth reminding the elementary steps of ethanol decomposition in order to gain some insights into the reasons behind the products distribution. In the decomposition pathway of ethoxy species Rh has a unique effect. It abstract H from the CH3 -group making a stable oxametallacycle intermediate ((a)CH2 –CH2 –O(a)) [18,19] and the following elementary steps may describe the complete process: CH3 CH2 OH ! CH3 CH2 OðaÞ þ HðaÞ
ð3Þ
CH3 CH2 OðaÞ ! ðaÞCH2 –CH2 OðaÞ þ HðaÞ
ð4Þ
ðaÞCH2 –CH2 OðaÞ ! CH4 ðgÞ þ COðgÞ
ð5Þ
CH4 þ H2 OðaÞ ! 3H2 ðgÞ þ COðgÞ
ð6Þ
2CO þ 2H2 O ! 2CO2 þ 2H2
ð7Þ
2HðaÞ ! H2 ðgÞ
ð8Þ
(a): adsorbed. The sum of Eqs. (3)–(8) gives Eq. (9) (see below); more information on Eq. (6) can be found in [18] and [19], on ethoxy decomposition on metals in general in [20], and on reforming of CH4 in [21]. Fig. 5 shows the product yield from ethanol reforming as a function of temperature for Rh=CeO2 –ZrO2 (Ce/Zr ¼ 1), as an example of the catalytic series. Products yield is calculated per mole of inlet ethanol. A complete reforming of CH3 CH2 OH would yield 6H2 and 2CO2 . CH3 CH2 OH þ 3H2 O ! 6H2 þ 2CO2
ð9Þ
The presence of methane and CO indicates that reforming of ethanol occurs via other reaction intermediates (methane since acetaldehyde formation was negligible). The selectivity to hydrogen initially increases with temperature due to decreasing amounts of CH4 at CO. Reforming of methane (Eq. (6)) and WGSR (Eq. (7)) are most likely the reasons. Still at higher temperatures
>400 °C, CO2 =CO shows a decreasing trend. This may indicate that WGSR is not effective any more: WGSR is exothermic and is thus disfavored by increasing the temperature. The continuing increase of H2 is thus most likely occurring by more reforming of CH4 rather than by WGSR. Reforming of CH4 by CO2 may also occur. However, it is 10 kcal mol1 higher than that by H2 O (59 compared to 49 kcal mol1 ). The excess of water used in this work in addition to the fact that CO2 is actually formed from reforming of CO makes this route unlikely. From Table 2, Fig. 5 and the above equations of the elementary reactions it appears that the rate limiting steps for the formation of H2 from ethanol is making CH4 from ethanol (most likely via oxametallacycle intermediate (Eq. (5))). In summary, results of this work indicate the following three points. (1) The best catalysts (in the series Rh=CeO2 –ZrO2 ) are not necessarily the most basic ones but those that can bind CO2 mildly. (2) Ethanol reforming on Rh=CeO2 –ZrO2 does not appear to be sensitive to Rh dispersion. (3) The CO2 =CO is sensitive to the Ce/Zr in the Rh=CeO2 –ZrO2 series investigated.
Acknowledgements The authors thank Peter Buchanan for conducting the CO2 adsorption experiments, Dr. Jean Seakins for Raman analyses and Suzanne Libs for her help in gas-phase analyses.
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