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Cataluminescence and catalytic reactions of ethanol oxidation over nanosized Ce1−xZrxO2 (0 ⩽ x ⩽ 1) catalysts
Catalysis Communications 7 (2006) 589–592 www.elsevier.com/locate/catcom
Cataluminescence and catalytic reactions of ethanol oxidation over nanosized Ce1 xZrxO2 (0 6 x 6 1) catalysts Qing Ye, Qi Gao, Xin-Rong Zhang, Bo-Qing Xu
*
Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, The Haidian District, Beijing 100084, China Received 28 July 2005; received in revised form 23 January 2006; accepted 24 January 2006 Available online 6 March 2006
Abstract Cataluminescence (CTL) and catalytic reactions of ethanol oxidation over nanosized (5–20 nm) Ce1 xZrxO2 (0 6 x 6 1) materials were studied at 170–300 °C. Characterization with XRD showed that the Ce1 xZrxO2 material was a CeO2-based solid solution in the cubic phase at x 6 0.15; it became a phase mixture of the solid solution and tetragonal ZrO2 at 0.15 < x < 1. The materials that were rich in the solid solution phase (i.e., x = 0.05–0.25) showed very high CTL activity at 220 °C and thus can be considered as efficient lowtemperature CTL sensors for ethanol. The CTL intensity of ethanol oxidation was closely related to the steady state activity of the Ce1 xZrxO2 catalysts for oxidative dehydrogenation of ethanol to form acetaldehyde. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Acetaldehyde; Cataluminescence; Ce1 xZrxO2 nanoparticles; Ethanol sensor; Phase structure; Oxidative ethanol dehydrogenation
1. Introduction Cataluminescence (CTL) is the chemiluminescence produced during catalytic oxidation reactions. The first observation of CTL was made by Breysse et al. [1] during the catalytic oxidation of CO on ThO2. Since the middle of 1990s, CTL has received much attention due to its potential applications in gas sensors for the detection of organic waste vapors [2–7]. CTL can be more suitable as a gas-sensor because the signal comes directly from CTL of a gas itself or its related surface intermediate during the catalytic reaction but not from change of the sensor properties, e.g., electric conductivity. A CTL-based gas-sensor may also have other advantages like fast response, high signal-to-noise (S/N) ratio, good reproducibility, and long-term stability. Until now, only a limited numbers of solid oxides, including c-Al2O3 and rare-earth (Dy, Eu, Lu) doped c-Al2O3 [2–4],
*
Corresponding author. Tel.: +86 10 6277 2592; fax: +86 10 6279 2122. E-mail address: [email protected] (B.-Q. Xu).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.01.023
TiO2, Y2O3 and ZrO2 [5–7], were studied as the catalysts for CTL of ethanol oxidation. These oxide catalysts seem inefficient since temperatures higher than 400 °C were required to achieve the catalysis for high CTL intensity [3– 6]. Such high temperatures would inevitably cause high incandescent radiation from the substrate and then lower the S/N ratio. Also, the documented studies were mainly devoted to the understanding of the sensor properties; little is known about the relationship between the CTL activity and conventional catalytic performance. Ce1 xZrxO2 samples of different compositions have appeared as oxygen storage material and excellent redox catalysts or catalyst supports in environmental catalysis research [8–10]. In the present study, nanosized Ce1 xZrxO2 powders are used as catalysts for low temperature CTL of ethanol oxidation. It is shown that the structure and composition of the Ce1 xZrxO2 samples influence greatly the CTL property of ethanol oxidation. We have found that the CTL property of the oxidation reaction is closely related to the oxidative dehydrogenation to form acetaldehyde from ethanol.
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Q. Ye et al. / Catalysis Communications 7 (2006) 589–592
2. Experimental 2.1. Catalyst preparation and characterization The Ce1 xZrxO2 samples were prepared by citric acid solgel method, in which stoichiometric amounts of Ce(NO3)3 Æ 6H2O and ZrO(NO3)2 Æ 2H2O were dissolved in deionized water by addition of aqueous citric acid. The mole ratio of citric acid to the total of Ce and Zr was 1.0. The solution was vigorously stirred and dehydrated at 100 °C. The obtained solid was dried overnight and then calcined at 800 °C for 5 h. The composition of Ce1 xZrxO2 sample was analyzed by X-ray fluorescence analysis on an XRF-1700 spectrometer. The composition of Ce1 xZrxO2 sample was found in good agreement with the theoretical values as shown in Table 1. BET surface areas of the samples were measured at 77 K using a Micromeritics ASAP 2010C instrument. XRD patterns were collected on a Bruker D8 diffractometer in the range of 2h = 25°–65° with a scan step of 0.02° using Cu Ka radiation at k = 0.15418 nm. 2.2. Measurements of CTL and catalytic reaction A schematic diagram of the CTL detection system is shown in Fig. 1. Ce1 xZrxO2 catalyst was coated on a heating rod and placed in a quartz tube (i.d. 12 mm). The temperature of the sample layer was adjusted by controlling the voltage exerted to the heating tube. The gas feed was a mixture of air and ethanol vapor, which was allowed to flow through the quartz tube at different catalyst temperatures to generate CTL. The CTL intensity was measured with a BPCL Ultra Weak Chemiluminescence Analyzer, a product of Biophysics Institute of Chinese Academy of Sciences (Beijing). Conventional catalytic tests of ethanol oxidation were studied at ambient pressure in a tubular quartz reactor using 100 mg Ce1 xZrxO2 sample as the catalyst. The reactant ethanol was introduced into the reactor by air (320 ml/ min) bubbling through a saturator containing anhydrous liquid ethanol at room temperature. Reaction products were analyzed by an on-line gas chromatograph (SP GC1490), equipped with a thermal conductivity detector (TCD) and a GDX-403 column. In all of the reaction tests,
Fig. 1. Schematic cataluminescence.
experimental
setup
for
the
detection
of
the material balance by carbon was always very good (100 ± 5%). 3. Results and discussion 3.1. Crystal structure, specific surface area and particle size of Ce1 xZrxO2 samples Fig. 2 shows the XRD patterns of the Ce1 xZrxO2 powders. The patterns indicate that the samples with x 6 0.15 are in the cubic phase of ‘‘pure’’ CeO2 (PDF34-0394). The main diffractions for CeO2 at 2h 28.6° and 47.5° were shifted to 2h 28.9° and 47.9° for Ce0.85Zr0.15O2, and lattice constant (a) of the cubic crystallites reduced from 0.5427 nm for CeO2 to 0.5391 nm for Ce0.85Zr0.15O2 ˚ ) by smal(Table 1) due to the replacement of Ce4+ (1.09 A ˚ ) ions [11]. These observations indicate that ler Zr4+ (0.86 A Zr4+ ions are incorporated into the CeO2 lattice and the Ce1 xZrxO2 samples are cubic ZrO2–CeO2 solid solutions at x 6 0.15 [12]. For the samples with x > 0.15, we detected asymmetric diffraction peaks at ca. 2h 29° and 2h 49°. The deconvolution of these peaks reveals that they are combinations of the diffractions from cubic CeO2/ZrO2–CeO2 and tetragonal ZrO2 phases, as shown by the inset of Fig. 2 at around 2h 29° for the sample of x = 0.25 (Ce0.75Zr0.25O2, trace 4). The positions of the cubic component were shifted from 2h 29.0° and 48.0° for Ce0.75Zr0.25O2 to 2h 29.8° and 49.8° for Ce0.2Zr0.8O2 and the lattice constant ‘‘a’’ for the cubic crystallites reduced from 0.5388 nm to 0.5176 nm, also due to the replacement of Ce4+ by the smaller Zr4+
Table 1 Composition, specific surface area and average crystallite size of Ce1 xZrxO2 samples Samples
xa
Surface area (m2/g)
Crystal sizeb (nm)
Lattice constant a (nm)
ZrO2 Ce0.2Zr0.8O2 Ce0.5Zr0.5O2 Ce0.75Zr0.25O2 Ce0.85Zr0.15O2 Ce0.95Zr0.05O2 CeO2
1.00 0.80 0.51 0.25 0.16 0.06 0.00
2.63 3.00 17.06 32.12 34.31 23.05 8.86
19 9b 7b 10b 9b 14b 28b
– 0.5176 0.5289 0.5388 0.5391 0.5418 0.5427
a b
x in Ce1 xZrxO2 were measured by XRF. Average crystal sizes were obtained according to the Scherrer equation by measuring the (1 1 1) diffraction of the cubic crystallites.
Q. Ye et al. / Catalysis Communications 7 (2006) 589–592
24
28
29
30
31
7 6 5 4
x=0 x = 0.05 x = 0.15 x = 0.25 x = 0.5 x = 0.8 x=1
20 CTL Intensity / a.u.
Intensity / a.u.
monoclinic tetragonal cubic 27
591
16 12 8 4
3 2
0
1 480
25
30
35
40
45
50
55
60
520
65
560 600 Wavelength /nm
640
680
2 theta / Degrees Fig. 3. Cataluminescence spectra of ethanol oxidation over Ce1 xZrxO2 at 220 °C.
10
ions. Therefore, Ce1 xZrxO2 samples with x > 0.15 are mixtures of cubic ZrO2–CeO2 solid solution and tetragonal ZrO2. The reference ZrO2 sample prepared in this work is identified as mixed monoclinic (JCPDS 37–1484) and tetragonal ZrO2 phases. Table 1 gives the specific surface area, average crystallite size (XRD sizes) and lattice constant for the cubic crystallites of the Ce1 xZrxO2 samples. Clearly, the sample specific surface area was affected significantly by the sample composition: the specific surface area increased from 9 to 34 m2/g when x is increased from 0 to 0.15, whereas further increase in x caused continued decrease in specific surface area to ca. 3 m2/g of the ‘‘pure’’ ZrO2. The average sizes of the cubic crystallites were obtained according to the Scherrer equation by measuring the (1 1 1) diffraction of the cubic crystallites. ‘‘Pure’’ CeO2 and ZrO2 crystallites are considerably larger than those of the binary oxides Ce1 xZrxO2, indicating that the incorporating Zr4+ ions into CeO2 inhibits the growth of cubic crystallites. 3.2. CTL and catalytic study of ethanol oxidation Shown in Fig. 3 are the CTL spectra in the range of 450– 700 nm of catalytic ethanol oxidation at 220 °C over Ce1 xZrxO2. Any contribution from blank radiation of the samples, though it was very weak at 220 °C (Fig. 4), was deducted in the intensity measurements. Although the catalyst composition, namely x in Ce1 xZrxO2, has a significant effect on the CTL intensity, there was always only a single maximum at around 620 nm for all the samples. The very similar spectra over all the Ce1 xZrxO2 samples would suggest that the light emission is associated with a common chemiluminescent species in the reaction system. The Ce0.85Zr0.15O2 catalyst, which exhibited the highest CTL activity among the Ce1 xZrxO2 samples (Fig. 3), was
Relative CTL Intensity / a.u.
24 8
20 16
6
12 4 8 2
4
0
0 180
Yield of Acetaldehyde / %
Fig. 2. XRD patterns of Ce1 xZrxO2: (1) x = 0, (2) x = 0.05, (3) x = 0.15, (4) x = 0.25, (5) x = 0.5, (6) x = 0.8, (7) x = 1. The inset shows the fitted and experimentally measured diffraction peak at around 2h = 29° 2h of trace 4 (x = 0.25).
200
220
240
260
280
300
Temperature / ˚C Fig. 4. Influence of reaction temperature on the cataluminescence intensity at 620 nm (dark symbols) and the yield of acetaldehyde (open symbols) over Ce0.85Zr0.15O2 (d and s) and CeO2 (j and h ) catalysts. D ata represented by ‘‘+’’ show the luminescent signals of blank radiation.
used to study the reaction temperature effect on the CTL intensity at 620 nm. Fig. 4 shows the results in comparison with those over the reference CeO2 catalyst. While the Ce0.85Zr0.15O2 catalyst was hardly active at temperatures lower than 185 °C, its CTL actvity increased rapidly with increasing the reaction temperature in the range of 190– 220 °C. But, the detected CTL intensity turned to diminish quickly at still higher temperatures, showing a favorable CTL window at 210–225 °C with the maximum appearing at 220 °C. The optimum CTL temperature (220 °C) of catalytic ethanol oxidation over the present Ce1 xZrxO2 catalysts is much lower than those reported in the earlier publications using c-Al2O3(450 °C) [2,3] or TiO2 (420 °C) [7] for the catalysts. A high reaction temperature would always result in high incandescent radiation which could significantly disturb the CTL detection. As shown by the data
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Q. Ye et al. / Catalysis Communications 7 (2006) 589–592
connected with a dotted line in Fig. 4, the blank radiation became significantly high when temperature increase to 300 °C. Thus, the present Ce1 xZrxO2 samples, especially those with x in the range of 0.05–0.25 (Fig. 3), could be considered as efficient lower-temperature CTL sensors for ethanol. In order to understand the nature of cataluminescent species, catalytic ethanol oxidation over the Ce1 xZrxO2 catalysts was also investigated on a conventional catalytic reactor under conditions similar to those for the cataluminescence study. As in the above CTL study, conversion of ethanol was observed at above 185 °C over all of the Ce1 xZrxO2 catalysts and the products appeared in the reactor outlet were acetaldehyde and water at temperatures up to 220 °C, indicating a pure oxidative dehydrogenation catalysis in the catalytic system. Formation of CO2 became detectable at still higher temperatures and the yield of CO2 continuously increased while that of acetaldehyde decreased with increasing the reaction temperature up to 240 °C, at which the formation of acetaldehyde became undetectable and catalytic combustion of ethanol to form CO2 and water turned to be the only reaction for ethanol. Fig. 4 also shows the reaction temperature effect on the yield of acetaldehyde during the catalytic reaction over Ce0.85Zr0.15O2 and CeO2 catalysts. It seems that the variation in reaction temperature produced very similar effects on the CTL intensity and yield of acetaldehyde. Using the CTL (at 620 nm) and catalytic reaction data at 220 °C, we attempted in Fig. 5 to gain a general relationship between the yield of acetaldehyde and CTL intensity over all of the Ce1 xZrxO2 samples. The linear relationship in Fig. 5 clearly suggests that the CTL performance of ethanol oxidation would be directly correlated with the formation of acetaldehyde during the reaction. Since the disappearance of acetaldehyde in the reaction product and the combustion of ethanol to form CO2 over the cata-
Yield of Acetaldehyde / %
8
3 6
4 2
4
1 2 6 7 0 0
5
10
15
20
25
Relative CTL Intensity / a.u. Fig. 5. Correlation between the cataluminescence intensity and acetaldehyde yield of ethanol oxidation over Ce1 xZxO2 catalysts at 220 °C. (1) x = 0; (2) x = 0.05; (3) x = 0.15; (4) x = 0.25; (6) x = 0.8; (7) x = 1.
lysts at above 240 °C resulted in extinction of the CTL activity (Fig. 4), it could be rationalized that the chemiluminesent species is closely associated with the formation of acetaldehyde at the surface of the Ce1 xZrxO2 catalysts. The reaction responsible to the CTL process might be expressed as: C2H5OH + 1/2O2 = CH3CHO* + H2O = CH3CHO + H2O + ht. By showing that the chemiluminesence spectrum of formaldehyde resembles the CTL spectrum of ethylene oxidation by ozone, Nakagawa et al., proposed that formaldehyde produced in the oxidation of ethanol-derived ethylene was the chemiluminesent species of ethanol oxidation over c-Al2O3 [2,3]. However, these authors made no detection of formaldehyde in their CTL study. In the work of Zhang et al., ethylene and formaldehyde were both considered as chemiluminesent species of ethanol oxidation over SrCO3 catalyst [6]. However, ethylene and acetaldehyde were considered as the chemiluminesent species when TiO2 was used as the catalyst in their CTL study of ethanol oxidation [5,7]. Obviously, information about the nature of chemiluminesent species in the CTL of ethanol oxidation over oxide catalysts were not consistent in the open literature. Further work on experimental identification of the chemiluminesent species or intermediate of the CTL process is still required in the future. In conclusion, nanosized Ce1 xZrxO2 materials of x = 0.05–0.25 are efficient catalysts for low temperature (220 °C) CTL of ethanol oxidation. The CTL intensity of ethanol oxidation is closely related to the catalyst activity for oxidative dehydrogenation of ethanol to form acetaldehyde. Acknowledgments The authors acknowledge the financial support of this work from the National Natural Science Foundation of China (Grants: 20125310 and 20590362) and the National Basic Research Program of China (Grant G2003CB615804). References [1] M. Breysse, B. Claudel, L. Faure, M. Guenin, R.J.J. Williams, J. Catal. 45 (1976) 137. [2] M. Nakagawa, Sens. Actuat. B 29 (1995) 94. [3] M. Nakagawa, T. Okabayashi, T. Fujimoto, et al., Sens. Actuat. B. 51 (1998) 159. [4] M. Nakagawa, I. Yamamoto, N. Yamashita, Anal. Sci. 14 (1998) 209. [5] Z.Y. Zhang, C. Zhang, X.R. Zhang, Analyst 127 (2002) 792. [6] J.J. Shi, J.J. Li, Y.F. Zhu, F. Wei, X.R. Zhang, Anal. Chim. Acta. 466 (2002) 69. [7] Y.F. Zhu, J.J. Shi, Z.Y. Zhang, C. Zhang, X.R. Zhang, Anal. Chem. 74 (2002) 120. [8] L.F. Liotta, A. Macaluso, A. Longo, et al., Appl. Catal. A. 240 (2003) 259. [9] R. Rajasree, J.H.B.J. Hoebink, J.C. Schouten, J. Catal. 223 (2004) 36. [10] R. Di Monte, J. Kaspar, Catal. Today 100 (2005) 27. [11] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [12] E. Tani, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. 66 (1983) 506.
Cataluminescence and catalytic reactions of ethanol oxidation over nanosized Ce1 xZrxO2 (0 6 x 6 1) catalysts Qing Ye, Qi Gao, Xin-Rong Zhang, Bo-Qing Xu
*
Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, The Haidian District, Beijing 100084, China Received 28 July 2005; received in revised form 23 January 2006; accepted 24 January 2006 Available online 6 March 2006
Abstract Cataluminescence (CTL) and catalytic reactions of ethanol oxidation over nanosized (5–20 nm) Ce1 xZrxO2 (0 6 x 6 1) materials were studied at 170–300 °C. Characterization with XRD showed that the Ce1 xZrxO2 material was a CeO2-based solid solution in the cubic phase at x 6 0.15; it became a phase mixture of the solid solution and tetragonal ZrO2 at 0.15 < x < 1. The materials that were rich in the solid solution phase (i.e., x = 0.05–0.25) showed very high CTL activity at 220 °C and thus can be considered as efficient lowtemperature CTL sensors for ethanol. The CTL intensity of ethanol oxidation was closely related to the steady state activity of the Ce1 xZrxO2 catalysts for oxidative dehydrogenation of ethanol to form acetaldehyde. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Acetaldehyde; Cataluminescence; Ce1 xZrxO2 nanoparticles; Ethanol sensor; Phase structure; Oxidative ethanol dehydrogenation
1. Introduction Cataluminescence (CTL) is the chemiluminescence produced during catalytic oxidation reactions. The first observation of CTL was made by Breysse et al. [1] during the catalytic oxidation of CO on ThO2. Since the middle of 1990s, CTL has received much attention due to its potential applications in gas sensors for the detection of organic waste vapors [2–7]. CTL can be more suitable as a gas-sensor because the signal comes directly from CTL of a gas itself or its related surface intermediate during the catalytic reaction but not from change of the sensor properties, e.g., electric conductivity. A CTL-based gas-sensor may also have other advantages like fast response, high signal-to-noise (S/N) ratio, good reproducibility, and long-term stability. Until now, only a limited numbers of solid oxides, including c-Al2O3 and rare-earth (Dy, Eu, Lu) doped c-Al2O3 [2–4],
*
Corresponding author. Tel.: +86 10 6277 2592; fax: +86 10 6279 2122. E-mail address: [email protected] (B.-Q. Xu).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.01.023
TiO2, Y2O3 and ZrO2 [5–7], were studied as the catalysts for CTL of ethanol oxidation. These oxide catalysts seem inefficient since temperatures higher than 400 °C were required to achieve the catalysis for high CTL intensity [3– 6]. Such high temperatures would inevitably cause high incandescent radiation from the substrate and then lower the S/N ratio. Also, the documented studies were mainly devoted to the understanding of the sensor properties; little is known about the relationship between the CTL activity and conventional catalytic performance. Ce1 xZrxO2 samples of different compositions have appeared as oxygen storage material and excellent redox catalysts or catalyst supports in environmental catalysis research [8–10]. In the present study, nanosized Ce1 xZrxO2 powders are used as catalysts for low temperature CTL of ethanol oxidation. It is shown that the structure and composition of the Ce1 xZrxO2 samples influence greatly the CTL property of ethanol oxidation. We have found that the CTL property of the oxidation reaction is closely related to the oxidative dehydrogenation to form acetaldehyde from ethanol.
590
Q. Ye et al. / Catalysis Communications 7 (2006) 589–592
2. Experimental 2.1. Catalyst preparation and characterization The Ce1 xZrxO2 samples were prepared by citric acid solgel method, in which stoichiometric amounts of Ce(NO3)3 Æ 6H2O and ZrO(NO3)2 Æ 2H2O were dissolved in deionized water by addition of aqueous citric acid. The mole ratio of citric acid to the total of Ce and Zr was 1.0. The solution was vigorously stirred and dehydrated at 100 °C. The obtained solid was dried overnight and then calcined at 800 °C for 5 h. The composition of Ce1 xZrxO2 sample was analyzed by X-ray fluorescence analysis on an XRF-1700 spectrometer. The composition of Ce1 xZrxO2 sample was found in good agreement with the theoretical values as shown in Table 1. BET surface areas of the samples were measured at 77 K using a Micromeritics ASAP 2010C instrument. XRD patterns were collected on a Bruker D8 diffractometer in the range of 2h = 25°–65° with a scan step of 0.02° using Cu Ka radiation at k = 0.15418 nm. 2.2. Measurements of CTL and catalytic reaction A schematic diagram of the CTL detection system is shown in Fig. 1. Ce1 xZrxO2 catalyst was coated on a heating rod and placed in a quartz tube (i.d. 12 mm). The temperature of the sample layer was adjusted by controlling the voltage exerted to the heating tube. The gas feed was a mixture of air and ethanol vapor, which was allowed to flow through the quartz tube at different catalyst temperatures to generate CTL. The CTL intensity was measured with a BPCL Ultra Weak Chemiluminescence Analyzer, a product of Biophysics Institute of Chinese Academy of Sciences (Beijing). Conventional catalytic tests of ethanol oxidation were studied at ambient pressure in a tubular quartz reactor using 100 mg Ce1 xZrxO2 sample as the catalyst. The reactant ethanol was introduced into the reactor by air (320 ml/ min) bubbling through a saturator containing anhydrous liquid ethanol at room temperature. Reaction products were analyzed by an on-line gas chromatograph (SP GC1490), equipped with a thermal conductivity detector (TCD) and a GDX-403 column. In all of the reaction tests,
Fig. 1. Schematic cataluminescence.
experimental
setup
for
the
detection
of
the material balance by carbon was always very good (100 ± 5%). 3. Results and discussion 3.1. Crystal structure, specific surface area and particle size of Ce1 xZrxO2 samples Fig. 2 shows the XRD patterns of the Ce1 xZrxO2 powders. The patterns indicate that the samples with x 6 0.15 are in the cubic phase of ‘‘pure’’ CeO2 (PDF34-0394). The main diffractions for CeO2 at 2h 28.6° and 47.5° were shifted to 2h 28.9° and 47.9° for Ce0.85Zr0.15O2, and lattice constant (a) of the cubic crystallites reduced from 0.5427 nm for CeO2 to 0.5391 nm for Ce0.85Zr0.15O2 ˚ ) by smal(Table 1) due to the replacement of Ce4+ (1.09 A ˚ ) ions [11]. These observations indicate that ler Zr4+ (0.86 A Zr4+ ions are incorporated into the CeO2 lattice and the Ce1 xZrxO2 samples are cubic ZrO2–CeO2 solid solutions at x 6 0.15 [12]. For the samples with x > 0.15, we detected asymmetric diffraction peaks at ca. 2h 29° and 2h 49°. The deconvolution of these peaks reveals that they are combinations of the diffractions from cubic CeO2/ZrO2–CeO2 and tetragonal ZrO2 phases, as shown by the inset of Fig. 2 at around 2h 29° for the sample of x = 0.25 (Ce0.75Zr0.25O2, trace 4). The positions of the cubic component were shifted from 2h 29.0° and 48.0° for Ce0.75Zr0.25O2 to 2h 29.8° and 49.8° for Ce0.2Zr0.8O2 and the lattice constant ‘‘a’’ for the cubic crystallites reduced from 0.5388 nm to 0.5176 nm, also due to the replacement of Ce4+ by the smaller Zr4+
Table 1 Composition, specific surface area and average crystallite size of Ce1 xZrxO2 samples Samples
xa
Surface area (m2/g)
Crystal sizeb (nm)
Lattice constant a (nm)
ZrO2 Ce0.2Zr0.8O2 Ce0.5Zr0.5O2 Ce0.75Zr0.25O2 Ce0.85Zr0.15O2 Ce0.95Zr0.05O2 CeO2
1.00 0.80 0.51 0.25 0.16 0.06 0.00
2.63 3.00 17.06 32.12 34.31 23.05 8.86
19 9b 7b 10b 9b 14b 28b
– 0.5176 0.5289 0.5388 0.5391 0.5418 0.5427
a b
x in Ce1 xZrxO2 were measured by XRF. Average crystal sizes were obtained according to the Scherrer equation by measuring the (1 1 1) diffraction of the cubic crystallites.
Q. Ye et al. / Catalysis Communications 7 (2006) 589–592
24
28
29
30
31
7 6 5 4
x=0 x = 0.05 x = 0.15 x = 0.25 x = 0.5 x = 0.8 x=1
20 CTL Intensity / a.u.
Intensity / a.u.
monoclinic tetragonal cubic 27
591
16 12 8 4
3 2
0
1 480
25
30
35
40
45
50
55
60
520
65
560 600 Wavelength /nm
640
680
2 theta / Degrees Fig. 3. Cataluminescence spectra of ethanol oxidation over Ce1 xZrxO2 at 220 °C.
10
ions. Therefore, Ce1 xZrxO2 samples with x > 0.15 are mixtures of cubic ZrO2–CeO2 solid solution and tetragonal ZrO2. The reference ZrO2 sample prepared in this work is identified as mixed monoclinic (JCPDS 37–1484) and tetragonal ZrO2 phases. Table 1 gives the specific surface area, average crystallite size (XRD sizes) and lattice constant for the cubic crystallites of the Ce1 xZrxO2 samples. Clearly, the sample specific surface area was affected significantly by the sample composition: the specific surface area increased from 9 to 34 m2/g when x is increased from 0 to 0.15, whereas further increase in x caused continued decrease in specific surface area to ca. 3 m2/g of the ‘‘pure’’ ZrO2. The average sizes of the cubic crystallites were obtained according to the Scherrer equation by measuring the (1 1 1) diffraction of the cubic crystallites. ‘‘Pure’’ CeO2 and ZrO2 crystallites are considerably larger than those of the binary oxides Ce1 xZrxO2, indicating that the incorporating Zr4+ ions into CeO2 inhibits the growth of cubic crystallites. 3.2. CTL and catalytic study of ethanol oxidation Shown in Fig. 3 are the CTL spectra in the range of 450– 700 nm of catalytic ethanol oxidation at 220 °C over Ce1 xZrxO2. Any contribution from blank radiation of the samples, though it was very weak at 220 °C (Fig. 4), was deducted in the intensity measurements. Although the catalyst composition, namely x in Ce1 xZrxO2, has a significant effect on the CTL intensity, there was always only a single maximum at around 620 nm for all the samples. The very similar spectra over all the Ce1 xZrxO2 samples would suggest that the light emission is associated with a common chemiluminescent species in the reaction system. The Ce0.85Zr0.15O2 catalyst, which exhibited the highest CTL activity among the Ce1 xZrxO2 samples (Fig. 3), was
Relative CTL Intensity / a.u.
24 8
20 16
6
12 4 8 2
4
0
0 180
Yield of Acetaldehyde / %
Fig. 2. XRD patterns of Ce1 xZrxO2: (1) x = 0, (2) x = 0.05, (3) x = 0.15, (4) x = 0.25, (5) x = 0.5, (6) x = 0.8, (7) x = 1. The inset shows the fitted and experimentally measured diffraction peak at around 2h = 29° 2h of trace 4 (x = 0.25).
200
220
240
260
280
300
Temperature / ˚C Fig. 4. Influence of reaction temperature on the cataluminescence intensity at 620 nm (dark symbols) and the yield of acetaldehyde (open symbols) over Ce0.85Zr0.15O2 (d and s) and CeO2 (j and h ) catalysts. D ata represented by ‘‘+’’ show the luminescent signals of blank radiation.
used to study the reaction temperature effect on the CTL intensity at 620 nm. Fig. 4 shows the results in comparison with those over the reference CeO2 catalyst. While the Ce0.85Zr0.15O2 catalyst was hardly active at temperatures lower than 185 °C, its CTL actvity increased rapidly with increasing the reaction temperature in the range of 190– 220 °C. But, the detected CTL intensity turned to diminish quickly at still higher temperatures, showing a favorable CTL window at 210–225 °C with the maximum appearing at 220 °C. The optimum CTL temperature (220 °C) of catalytic ethanol oxidation over the present Ce1 xZrxO2 catalysts is much lower than those reported in the earlier publications using c-Al2O3(450 °C) [2,3] or TiO2 (420 °C) [7] for the catalysts. A high reaction temperature would always result in high incandescent radiation which could significantly disturb the CTL detection. As shown by the data
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Q. Ye et al. / Catalysis Communications 7 (2006) 589–592
connected with a dotted line in Fig. 4, the blank radiation became significantly high when temperature increase to 300 °C. Thus, the present Ce1 xZrxO2 samples, especially those with x in the range of 0.05–0.25 (Fig. 3), could be considered as efficient lower-temperature CTL sensors for ethanol. In order to understand the nature of cataluminescent species, catalytic ethanol oxidation over the Ce1 xZrxO2 catalysts was also investigated on a conventional catalytic reactor under conditions similar to those for the cataluminescence study. As in the above CTL study, conversion of ethanol was observed at above 185 °C over all of the Ce1 xZrxO2 catalysts and the products appeared in the reactor outlet were acetaldehyde and water at temperatures up to 220 °C, indicating a pure oxidative dehydrogenation catalysis in the catalytic system. Formation of CO2 became detectable at still higher temperatures and the yield of CO2 continuously increased while that of acetaldehyde decreased with increasing the reaction temperature up to 240 °C, at which the formation of acetaldehyde became undetectable and catalytic combustion of ethanol to form CO2 and water turned to be the only reaction for ethanol. Fig. 4 also shows the reaction temperature effect on the yield of acetaldehyde during the catalytic reaction over Ce0.85Zr0.15O2 and CeO2 catalysts. It seems that the variation in reaction temperature produced very similar effects on the CTL intensity and yield of acetaldehyde. Using the CTL (at 620 nm) and catalytic reaction data at 220 °C, we attempted in Fig. 5 to gain a general relationship between the yield of acetaldehyde and CTL intensity over all of the Ce1 xZrxO2 samples. The linear relationship in Fig. 5 clearly suggests that the CTL performance of ethanol oxidation would be directly correlated with the formation of acetaldehyde during the reaction. Since the disappearance of acetaldehyde in the reaction product and the combustion of ethanol to form CO2 over the cata-
Yield of Acetaldehyde / %
8
3 6
4 2
4
1 2 6 7 0 0
5
10
15
20
25
Relative CTL Intensity / a.u. Fig. 5. Correlation between the cataluminescence intensity and acetaldehyde yield of ethanol oxidation over Ce1 xZxO2 catalysts at 220 °C. (1) x = 0; (2) x = 0.05; (3) x = 0.15; (4) x = 0.25; (6) x = 0.8; (7) x = 1.
lysts at above 240 °C resulted in extinction of the CTL activity (Fig. 4), it could be rationalized that the chemiluminesent species is closely associated with the formation of acetaldehyde at the surface of the Ce1 xZrxO2 catalysts. The reaction responsible to the CTL process might be expressed as: C2H5OH + 1/2O2 = CH3CHO* + H2O = CH3CHO + H2O + ht. By showing that the chemiluminesence spectrum of formaldehyde resembles the CTL spectrum of ethylene oxidation by ozone, Nakagawa et al., proposed that formaldehyde produced in the oxidation of ethanol-derived ethylene was the chemiluminesent species of ethanol oxidation over c-Al2O3 [2,3]. However, these authors made no detection of formaldehyde in their CTL study. In the work of Zhang et al., ethylene and formaldehyde were both considered as chemiluminesent species of ethanol oxidation over SrCO3 catalyst [6]. However, ethylene and acetaldehyde were considered as the chemiluminesent species when TiO2 was used as the catalyst in their CTL study of ethanol oxidation [5,7]. Obviously, information about the nature of chemiluminesent species in the CTL of ethanol oxidation over oxide catalysts were not consistent in the open literature. Further work on experimental identification of the chemiluminesent species or intermediate of the CTL process is still required in the future. In conclusion, nanosized Ce1 xZrxO2 materials of x = 0.05–0.25 are efficient catalysts for low temperature (220 °C) CTL of ethanol oxidation. The CTL intensity of ethanol oxidation is closely related to the catalyst activity for oxidative dehydrogenation of ethanol to form acetaldehyde. Acknowledgments The authors acknowledge the financial support of this work from the National Natural Science Foundation of China (Grants: 20125310 and 20590362) and the National Basic Research Program of China (Grant G2003CB615804). References [1] M. Breysse, B. Claudel, L. Faure, M. Guenin, R.J.J. Williams, J. Catal. 45 (1976) 137. [2] M. Nakagawa, Sens. Actuat. B 29 (1995) 94. [3] M. Nakagawa, T. Okabayashi, T. Fujimoto, et al., Sens. Actuat. B. 51 (1998) 159. [4] M. Nakagawa, I. Yamamoto, N. Yamashita, Anal. Sci. 14 (1998) 209. [5] Z.Y. Zhang, C. Zhang, X.R. Zhang, Analyst 127 (2002) 792. [6] J.J. Shi, J.J. Li, Y.F. Zhu, F. Wei, X.R. Zhang, Anal. Chim. Acta. 466 (2002) 69. [7] Y.F. Zhu, J.J. Shi, Z.Y. Zhang, C. Zhang, X.R. Zhang, Anal. Chem. 74 (2002) 120. [8] L.F. Liotta, A. Macaluso, A. Longo, et al., Appl. Catal. A. 240 (2003) 259. [9] R. Rajasree, J.H.B.J. Hoebink, J.C. Schouten, J. Catal. 223 (2004) 36. [10] R. Di Monte, J. Kaspar, Catal. Today 100 (2005) 27. [11] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [12] E. Tani, M. Yoshimura, S. Somiya, J. Am. Ceram. Soc. 66 (1983) 506.