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Novel nanostructred Pd–Zr oxides
Materials Science and Engineering A366 (2004) 248–253
Novel nanostructred Pd–Zr oxides Heng Zhang∗ , Jack Gromek, Gayanath Fernando, Harris L. Marcus Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA Received 5 April 2003; received in revised form 24 July 2003
Abstract Novel nanostructured Pd–Zr oxides have been synthesized using chemical processing. The composition and structure have been investigated using X-ray diffraction and electron microscopy. The phase stability, thermal behavior and catalytic property under methane atmosphere have been examined by using thermogravimetric analysis, in situ gas environment high temperature X-ray diffraction and gas mass spectroscopy. The study indicated that the alloying of Zr improves the thermal stability and modifies the hysteresis characteristic of the phase transition between Pd–Zr metal and oxide during the thermal cycling. © 2003 Elsevier B.V. All rights reserved. Keywords: Nanostructured Pd–Zr oxides; Thermal stability; Catalytic combustion of methane
1. Introduction Pd–PdO system has shown potential as a catalyst for methane combustion to minimize the formation of NOx [1–10]. It was demonstrated that Pd–PdO system results in the lean-premixed catalytic combustion of methane appearing at a few hundred degrees lower than the traditional combustion. Hence, lean-premixed catalytic combustion at lower operating temperature will dramatically reduce NOx emission. It was known both Pd and PdO have catalytic activity for methane combustion. PdO has higher activity at lower temperature, while Pd has higher activity at higher temperature [1]. However, there is phase decomposition for PdO to Pd in Pd–PdO system, which is dependent on oxygen partial pressure. PdO is stable at lower temperature and decomposes to Pd at high temperature [4]. Remaining the stable PdO structure at higher temperature has become one of the key factors for the successful application in catalytic combustion of methane. Alloying palladium oxide is likely a routine to modify its chemical and thermal behavior. A transition element could be one of the options for this purpose as transition metals often show catalytic behavior. With the addition of the tran∗ Corresponding author. Present address: Inframat Corporation, 156 River Road, Willington, CT 06279, USA. E-mail address: [email protected] (H. Zhang).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.07.004
sition metal element, the bond characteristic of the metal and oxide atoms will be adjusted in the pseudo-binary Pd–O system. Recent work in Pd–Mn oxides has shown the improvement in the thermal stability [11]. In this work, we report our investigation for a Pd–Zr pseudo-binary oxide system.
2. Experiments PdO and Pd1−x Zrx O (x = 0.05, 0.1 and 0.2) were prepared by using a chemical synthesis approach. The precursors used are PdCl4 (Acros, 99.9%), ZrCl4 (Acros 99.9%). The metal salts (0.1 mol/l) and sodium hydroxide (0.22 mol/l) were dissolved in water to form the desired metal hydroxide. For the Pd1−x Zrx O (x = 0.05, 0.1, 0.2), the used precursors of PdCl4 and ZrCl4 are in the mole ratio of the designed fraction. Then, the formed hydroxide is thoroughly aged to form nanometer oxides, and consequently filtered and washed using water to get chloride and sodium ions out. Powder and wafer samples of PdO and (Pd, Zr)O are prepared from the formed hydroxide in the aqueous solution. The powder samples are used for room temperature X-ray diffraction (XRD), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The wafer samples with the Pd–Zr oxides deposited on a nickel-based alloy substrate with an Al2 O3 support layer are used for in situ high temperature X-ray diffraction and catalytic activity study.
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
The prepared samples were characterized using a D8adv X-ray diffractometer with Cu K␣ radiation. Particle size of the powder samples was observed by TEM analysis using a Philips EM-420 transmission electron microscopy with an accelerated voltage of 120 kV. The aqueous sample was diluted using methanol alcohol with 1:5 volume ratio. Then the particle in the solution was dropped to the copper disk with carbon support film. Semi-quantitative element analysis was carried out using an energy dispersive spectrometer (EDS) equipped in an Amray-1000A scanning electron microscopy with an accelerated voltage of 20 kV. Thermal behavior was analyzed using a TA Instruments Hi-Res TGA 2950 thermogravimetric analyzer. The TGA scans were performed with a rate of 20 ◦ C/min from room temperature up to 900 ◦ C, and cooling to suitable temperature under a flowing air gas with a rate of 40 ml/min. Five cycles were carried out in order to determine the decomposition and re-oxidation reversibility of the studied metal oxide systems. The multiple cycle TGA experiments are carried out continuously under the same air flow rate, heating rate of 20 ◦ C/min and cooling rate of 40 ◦ C/min. In situ high temperature X-ray diffraction and catalytic behavior tests were carried out using a recently established high temperature X-ray unit with gas control system and residual gas analyzer (RGA) (a gas mass spectroscopy) [12] in our laboratory. The chamber can be connected with different atmospheres including fuel mixture gas. The pressure control and gas delivery system allow experiments either under static or dynamic (gas flowing) conditions. Variant-temperature experiments were carried out under 1 atm of 1% CH4 + 10% O2 + 89% N2 from room temperature to 850 ◦ C at a heating rate of 100 ◦ C/min. The X-ray diffraction data collection starts after the selected temperature arriving 2 min. Each scan at the design temperature takes ∼0.5 h. Isothermal dynamic observations were carried out using a flow rate of 150 ml/min of the methane mixture gas.
3. Results and discussion Room temperature XRD patterns for the PdO and Pd1−x Zrx O (x = 0.05, 0.1, and 0.2) powder samples are shown in Fig. 1. The semi-quantitative EDS analysis results for the (Pd, Zr) oxides are shown in Table 1. Comparing to the XRD pattern for the PdO sample, no extra diffraction peaks are observed for all the three Pd1−x Zrx O samples. This indicates that a pseudo-binary (Pd, Zr)O oxide has
Table 1 Concentration of Pd1−x Zrx O from EDS analysis Sample
Pd
Zr
Pd0.95 Zr0.05 O Pd0.9 Zr0.1 O Pd0.8 Zr0.2 O
0.937 0.902 0.811
0.063 0.098 0.189
249
Fig. 1. XRD patterns for PdO (a) and Pd1−x Zrx O powder samples: (b) x = 0.05; (c) x = 0.10; (d) x = 0.20.
formed. On the other hand, all the peaks of Pd–Zr oxides shift to lower angles. This is attributed to the increase of lattice constant due to the replacement of Pd atoms by Zr atoms with larger atomic size. This demonstrates that Zr atoms substitute Pd in the PdO and a Pd–Zr oxide was formed. Through analysis of the broad diffraction peaks {1 0 1} at ∼33.6◦ , {1 1 0} at ∼41.9◦ , and {1 1 2} at ∼60.5◦ of 2θ using a three lines fitting program from the DiffracPlus package of Bruker Analytical X-ray systems, the grain sizes are calculated as 10–20 nm for the PdO and Pd1−x Zrx O oxides. Comparing the XRD patterns of Pd1−x Zrx O sample with that of PdO, no extra diffraction peaks were observed. Hence, this demonstrates that Pd1−x Zrx O has been formed through replacing partial Pd atoms by Zr atoms. The topography for the (Pd, Zr)O powder samples can be seen from the SEM images in Fig. 2. The element distribution for sample Pd0.8 Zr0.2 O has been examined by X-ray map element analysis and result indicates that the elements Pd and Zr are mixed homogeneously (see Fig. 2(d)). This result supports the X-ray diffraction results that a Pd–Zr oxide was formed. TEM results for the PdO and Pd0.8 Zr0.2 O powder samples are shown in Fig. 3. The nanometer particles aggregate to clusters. However, the 5–20 nm particles still can be identified from these images, supporting the peak broadening in XRD patterns. TGA analysis results for PdO and Pd–Zr oxides are shown in Fig. 4. All the TGA curves show continuous decrease in weight below 500 ◦ C. This phenomenon could be attributed to the evaporation of moisture and the burn lost of the residual ions. After a platform temperature region that appears between 500 and 800 ◦ C, a sharp weight drop that corresponds to the decomposition of PdO or (Pd, Zr)O is presented. The decomposition temperature (Td ) of PdO and (Pd, Zr)O was derived from the TGA curves by the tangent method. The obtained data are listed in Table 2. Compared to the PdO power, the decomposition temperatures of all the Pd–Zr
250
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
Fig. 2. SEM images for the Pd1−x Zrx O powders as-received: (a) x = 0.05; (b) x = 0.10; (c) x = 0.20. (d) X-ray map for element distribution of sample Pd0.8 Zr0.2 O (Pd: blue; Zr: red; O: green).
oxides are higher than that of PdO, and the decomposition temperature (Td ) of (Pd, Zr)O increases with the Zr concentration. For Pd0.8 Zr0.2 O, the Td increases from 802 to 845 ◦ C for PdO. On the other hand, the area of hysteresis increases
significantly with Zr concentration, and the transform reversibility for the decomposed PdO reverting to PdO in the presence of Zr is dramatically promoted. This characteristic offers a significant advantage for the practical application of
Fig. 3. TEM images for PdO (a) and Pd0.8 Zr0.2 O (b) powder samples.
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
251
Fig. 4. TGA results for the PdO (a) and Pd1−x Zrx O powder samples using a heating rate of 20 ◦ C/min, under air gas flowing (40 ml/min): (b) x = 0.05; (c) x = 0.1; (d) x = 0.2.
the catalyst in case the catalyst oxides decomposing to metal due to overheating, can come back to the oxide status thoroughly once the temperature returning below the decomposition temperature point. This phenomenon could be related to the modified electronic configuration and kinetic barrier of the re-oxidation for the palladium-based oxide etc intrinsic factors due to the alloying of Zr. It will be further explored through thermodynamic analysis and micro-characterization in order to clarify the intrinsic factor. According to the above TGA data, a wafer sample Pd0.8 Zr0.2 O, which shows the largest increase in oxide decomposition temperature, is chosen for further in situ high temperature X-ray diffraction studies under the 1% CH4 Table 2 Thermal data for the PdO and Pd1−x Zrx O derived from the TGA results Sample
Td (◦ C)
PdO Pd0.95 Zr0.05 O Pd0.9 Zr0.1 O Pd0.8 Zr0.2 O
802 812 822 845
Fig. 5. In situ high temperature XRD patterns for Pd0.8 Zr0.2 O wafer sample under 1% CH4 + 10% O2 + 89% N2 gas mixture (under 1 atm).
252
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
Fig. 6. Methane concentration against isothermal heating time (at 700 ◦ C) under gas flowing rate of 150 ml/min (using a gas mixture of 1% CH4 + 10% O2 + 89%N2 , under 1 atm) for PdO and Pd0.8 Zr0.2 O wafers.
+ 10% O2 +89% N2 gas mixture. The experimental results are shown in Fig. 5, which indicates that the (Pd, Zr)O phase is stable until 820 ◦ C. The (Pd, Zr) metal phase can be seen at 820 ◦ C and becomes dominant at 850 ◦ C. Hence the Td for the Pd0.8 Zr0.2 O is ∼820–850 ◦ C. Fig. 5 also indicates that the decomposed (Pd, Zr)O phase can be re-oxided fully when the temperature decreases below Td , which is in good agreement with the TGA multiple cycle results in Fig. 4. Comparing to the Td derived from the TGA under 1 atm of air; current Td is somewhat lower than that under air atmosphere. This phenomenon is consistent with our recent phase diagram study for the Pd–PdO decomposition of Pd oxide as a function of oxygen partial pressure [13] and the study [11] on PdO in similar dynamic conditions. The dynamic study on PdO indicates that the PdO phase is stable until 750 ◦ C. When temperature increases to 780 ◦ C, the PdO phase is fully decomposed to Pd. Referring to the TGA results in Fig. 4(a), the Td for PdO under 1 atm of air, which is ∼0.21 atm of partial oxygen pressure, is ∼802 ◦ C. In the methane mixture gas experiments, the partial pressure of oxygen is only 0.1 atm, which explains the observed lower oxide transformation temperature. As shown in early work [12], a residual gas analyzer was used to monitor the gas pressures in the flowing gas stream during the catalytic runs. With the progress, the oxygen and methane concentration decreases, while the H2 O and CO2 are observed from none in initial. This is an apparent sign that the combustion appears at this temperature under the catalytic function of PdO and Pd–Zr oxide. As this isothermal dynamic catalytic combustion test was carried out in a chamber of high temperature X-ray unit with a volume over 4 l and a few of feet of connection tube to the RGA, in comparison to the large volume of the environment and the small catalyst (1 cm × 2 cm), the gas kind and gas concentration will take certain time to reach an equilibrium status. When using 100 ◦ C/min heating rate heat the wafer sample to 700 ◦ C, the catalytic combustion reaction will start. With time, the methane concentration in the chamber decreases
until the concentration out the measuring sensitivity of the RGA. The needed time, called ‘cutting time’, which corresponds to the time period that starts from the time at which set temperature arrived and ends at the time at which the methane concentration is down to zero. Hence, in this particular instrument set-up and test conditions, the methane cutting time is as an indirect parameter to evaluate the performance of catalytic combustion for methane at this study. The shorter the cutting time, the better the catalytic function. The experiment results are shown in Fig. 6. The PdO and (Pd, Zr)O results in Fig. 6 indicate that the presence of Zr does not significantly reduce the observed catalytic behavior of the PdO in promoting the low temperature oxidation of the methane. There were some indications of the oxidation reaction initiating more slowly, but further experiments will be required to quantify these observations.
4. Conclusions Novel Pd–Zr oxides have been successfully synthesized using a chemical processing method. The Pd–Zr oxide has nanostructure with gain size of 5–20 nm. The partial replacement of Pd atoms by Zr atoms improves the thermal stability of the Pd based oxide. The decomposition temperature of Pd based oxide increases with the Zr concentration within the concentration range studied. The transform reversibility of Pd–PdO as measured with TGA has been promoted in the Pd–Zr oxide system. The alloying of PdO by Zr also affects the catalytic behavior of methane combustion but still behaves as a catalyst. Further studies are required to verify this behavior.
Acknowledgements This material is based upon work supported by Connecticut Innovations Inc. under the grant entitled, “Development
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
of in situ materials analysis techniques for materials optimization of methane combustion catalysts”. References [1] P. Salomonsson, S. Johansson, B. Kasemo, Catal. Lett. 33 (1995) 1. [2] J.H. Lee, D.L. Trimm, Fuel Process. Technol. 42 (1995) 339. [3] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control Commercial Technology, Van Nostrand Reinhold, New York, 1995. [4] M. Lyubovsky, L. Pfefferle, Catal. Today 47 (1999) 29. [5] R.B. Anderson, K.C. Stein, J.J. Feenan, L.E.J. Hofer, Ind. Eng. Chem. 53 (1961) 809. [6] M. Valden, J. Aaltonen, E. Kusisto, M. Pessa, C.J. Barnes, Surf. Sci. 307–309 (1994) 193.
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[7] S.T. Kolaczowski, Trans. Inst. Chem. Eng. 73 (1995) 168. [8] R.A. Dalla Betta, Catal. Today 35 (1997) 129. [9] R.J. Farrauto, J.K. Lampert, M.C. Hobson, E.M. Waterman, Appl. Catal. B: Environ. 6 (1995) 263. [10] R.J. Farrauto, M.C. Hobson, T. Kennelly, E.M. Waterman, Appl. Catal. A: Gen. 81 (1992) 227. [11] H. Zhang, J. Gromek, G. Fernando, R.S. Boorse, H.L. Marcus, Physica B, in press. [12] H. Zhang, J. Gromek, M. Augustine, G.W. Fernando, Marwan Rasamny, R. Samuel Boorse, Harris L. Marcus, in: F.D.S. Marquis, N. Thadhani, E.V. Barrera (Eds.), Powder Materials, TMS, The Minerals, Metals & Materials Society, Indianapolis, Indiana, 2001, p. 59. [13] H. Zhang, J. Gromek, G. Fernando, R.S. Boorse, H.L. Marcus, J. Phase Equilib. 23 (2002) 246.
Novel nanostructred Pd–Zr oxides Heng Zhang∗ , Jack Gromek, Gayanath Fernando, Harris L. Marcus Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA Received 5 April 2003; received in revised form 24 July 2003
Abstract Novel nanostructured Pd–Zr oxides have been synthesized using chemical processing. The composition and structure have been investigated using X-ray diffraction and electron microscopy. The phase stability, thermal behavior and catalytic property under methane atmosphere have been examined by using thermogravimetric analysis, in situ gas environment high temperature X-ray diffraction and gas mass spectroscopy. The study indicated that the alloying of Zr improves the thermal stability and modifies the hysteresis characteristic of the phase transition between Pd–Zr metal and oxide during the thermal cycling. © 2003 Elsevier B.V. All rights reserved. Keywords: Nanostructured Pd–Zr oxides; Thermal stability; Catalytic combustion of methane
1. Introduction Pd–PdO system has shown potential as a catalyst for methane combustion to minimize the formation of NOx [1–10]. It was demonstrated that Pd–PdO system results in the lean-premixed catalytic combustion of methane appearing at a few hundred degrees lower than the traditional combustion. Hence, lean-premixed catalytic combustion at lower operating temperature will dramatically reduce NOx emission. It was known both Pd and PdO have catalytic activity for methane combustion. PdO has higher activity at lower temperature, while Pd has higher activity at higher temperature [1]. However, there is phase decomposition for PdO to Pd in Pd–PdO system, which is dependent on oxygen partial pressure. PdO is stable at lower temperature and decomposes to Pd at high temperature [4]. Remaining the stable PdO structure at higher temperature has become one of the key factors for the successful application in catalytic combustion of methane. Alloying palladium oxide is likely a routine to modify its chemical and thermal behavior. A transition element could be one of the options for this purpose as transition metals often show catalytic behavior. With the addition of the tran∗ Corresponding author. Present address: Inframat Corporation, 156 River Road, Willington, CT 06279, USA. E-mail address: [email protected] (H. Zhang).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.07.004
sition metal element, the bond characteristic of the metal and oxide atoms will be adjusted in the pseudo-binary Pd–O system. Recent work in Pd–Mn oxides has shown the improvement in the thermal stability [11]. In this work, we report our investigation for a Pd–Zr pseudo-binary oxide system.
2. Experiments PdO and Pd1−x Zrx O (x = 0.05, 0.1 and 0.2) were prepared by using a chemical synthesis approach. The precursors used are PdCl4 (Acros, 99.9%), ZrCl4 (Acros 99.9%). The metal salts (0.1 mol/l) and sodium hydroxide (0.22 mol/l) were dissolved in water to form the desired metal hydroxide. For the Pd1−x Zrx O (x = 0.05, 0.1, 0.2), the used precursors of PdCl4 and ZrCl4 are in the mole ratio of the designed fraction. Then, the formed hydroxide is thoroughly aged to form nanometer oxides, and consequently filtered and washed using water to get chloride and sodium ions out. Powder and wafer samples of PdO and (Pd, Zr)O are prepared from the formed hydroxide in the aqueous solution. The powder samples are used for room temperature X-ray diffraction (XRD), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The wafer samples with the Pd–Zr oxides deposited on a nickel-based alloy substrate with an Al2 O3 support layer are used for in situ high temperature X-ray diffraction and catalytic activity study.
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
The prepared samples were characterized using a D8adv X-ray diffractometer with Cu K␣ radiation. Particle size of the powder samples was observed by TEM analysis using a Philips EM-420 transmission electron microscopy with an accelerated voltage of 120 kV. The aqueous sample was diluted using methanol alcohol with 1:5 volume ratio. Then the particle in the solution was dropped to the copper disk with carbon support film. Semi-quantitative element analysis was carried out using an energy dispersive spectrometer (EDS) equipped in an Amray-1000A scanning electron microscopy with an accelerated voltage of 20 kV. Thermal behavior was analyzed using a TA Instruments Hi-Res TGA 2950 thermogravimetric analyzer. The TGA scans were performed with a rate of 20 ◦ C/min from room temperature up to 900 ◦ C, and cooling to suitable temperature under a flowing air gas with a rate of 40 ml/min. Five cycles were carried out in order to determine the decomposition and re-oxidation reversibility of the studied metal oxide systems. The multiple cycle TGA experiments are carried out continuously under the same air flow rate, heating rate of 20 ◦ C/min and cooling rate of 40 ◦ C/min. In situ high temperature X-ray diffraction and catalytic behavior tests were carried out using a recently established high temperature X-ray unit with gas control system and residual gas analyzer (RGA) (a gas mass spectroscopy) [12] in our laboratory. The chamber can be connected with different atmospheres including fuel mixture gas. The pressure control and gas delivery system allow experiments either under static or dynamic (gas flowing) conditions. Variant-temperature experiments were carried out under 1 atm of 1% CH4 + 10% O2 + 89% N2 from room temperature to 850 ◦ C at a heating rate of 100 ◦ C/min. The X-ray diffraction data collection starts after the selected temperature arriving 2 min. Each scan at the design temperature takes ∼0.5 h. Isothermal dynamic observations were carried out using a flow rate of 150 ml/min of the methane mixture gas.
3. Results and discussion Room temperature XRD patterns for the PdO and Pd1−x Zrx O (x = 0.05, 0.1, and 0.2) powder samples are shown in Fig. 1. The semi-quantitative EDS analysis results for the (Pd, Zr) oxides are shown in Table 1. Comparing to the XRD pattern for the PdO sample, no extra diffraction peaks are observed for all the three Pd1−x Zrx O samples. This indicates that a pseudo-binary (Pd, Zr)O oxide has
Table 1 Concentration of Pd1−x Zrx O from EDS analysis Sample
Pd
Zr
Pd0.95 Zr0.05 O Pd0.9 Zr0.1 O Pd0.8 Zr0.2 O
0.937 0.902 0.811
0.063 0.098 0.189
249
Fig. 1. XRD patterns for PdO (a) and Pd1−x Zrx O powder samples: (b) x = 0.05; (c) x = 0.10; (d) x = 0.20.
formed. On the other hand, all the peaks of Pd–Zr oxides shift to lower angles. This is attributed to the increase of lattice constant due to the replacement of Pd atoms by Zr atoms with larger atomic size. This demonstrates that Zr atoms substitute Pd in the PdO and a Pd–Zr oxide was formed. Through analysis of the broad diffraction peaks {1 0 1} at ∼33.6◦ , {1 1 0} at ∼41.9◦ , and {1 1 2} at ∼60.5◦ of 2θ using a three lines fitting program from the DiffracPlus package of Bruker Analytical X-ray systems, the grain sizes are calculated as 10–20 nm for the PdO and Pd1−x Zrx O oxides. Comparing the XRD patterns of Pd1−x Zrx O sample with that of PdO, no extra diffraction peaks were observed. Hence, this demonstrates that Pd1−x Zrx O has been formed through replacing partial Pd atoms by Zr atoms. The topography for the (Pd, Zr)O powder samples can be seen from the SEM images in Fig. 2. The element distribution for sample Pd0.8 Zr0.2 O has been examined by X-ray map element analysis and result indicates that the elements Pd and Zr are mixed homogeneously (see Fig. 2(d)). This result supports the X-ray diffraction results that a Pd–Zr oxide was formed. TEM results for the PdO and Pd0.8 Zr0.2 O powder samples are shown in Fig. 3. The nanometer particles aggregate to clusters. However, the 5–20 nm particles still can be identified from these images, supporting the peak broadening in XRD patterns. TGA analysis results for PdO and Pd–Zr oxides are shown in Fig. 4. All the TGA curves show continuous decrease in weight below 500 ◦ C. This phenomenon could be attributed to the evaporation of moisture and the burn lost of the residual ions. After a platform temperature region that appears between 500 and 800 ◦ C, a sharp weight drop that corresponds to the decomposition of PdO or (Pd, Zr)O is presented. The decomposition temperature (Td ) of PdO and (Pd, Zr)O was derived from the TGA curves by the tangent method. The obtained data are listed in Table 2. Compared to the PdO power, the decomposition temperatures of all the Pd–Zr
250
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
Fig. 2. SEM images for the Pd1−x Zrx O powders as-received: (a) x = 0.05; (b) x = 0.10; (c) x = 0.20. (d) X-ray map for element distribution of sample Pd0.8 Zr0.2 O (Pd: blue; Zr: red; O: green).
oxides are higher than that of PdO, and the decomposition temperature (Td ) of (Pd, Zr)O increases with the Zr concentration. For Pd0.8 Zr0.2 O, the Td increases from 802 to 845 ◦ C for PdO. On the other hand, the area of hysteresis increases
significantly with Zr concentration, and the transform reversibility for the decomposed PdO reverting to PdO in the presence of Zr is dramatically promoted. This characteristic offers a significant advantage for the practical application of
Fig. 3. TEM images for PdO (a) and Pd0.8 Zr0.2 O (b) powder samples.
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
251
Fig. 4. TGA results for the PdO (a) and Pd1−x Zrx O powder samples using a heating rate of 20 ◦ C/min, under air gas flowing (40 ml/min): (b) x = 0.05; (c) x = 0.1; (d) x = 0.2.
the catalyst in case the catalyst oxides decomposing to metal due to overheating, can come back to the oxide status thoroughly once the temperature returning below the decomposition temperature point. This phenomenon could be related to the modified electronic configuration and kinetic barrier of the re-oxidation for the palladium-based oxide etc intrinsic factors due to the alloying of Zr. It will be further explored through thermodynamic analysis and micro-characterization in order to clarify the intrinsic factor. According to the above TGA data, a wafer sample Pd0.8 Zr0.2 O, which shows the largest increase in oxide decomposition temperature, is chosen for further in situ high temperature X-ray diffraction studies under the 1% CH4 Table 2 Thermal data for the PdO and Pd1−x Zrx O derived from the TGA results Sample
Td (◦ C)
PdO Pd0.95 Zr0.05 O Pd0.9 Zr0.1 O Pd0.8 Zr0.2 O
802 812 822 845
Fig. 5. In situ high temperature XRD patterns for Pd0.8 Zr0.2 O wafer sample under 1% CH4 + 10% O2 + 89% N2 gas mixture (under 1 atm).
252
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
Fig. 6. Methane concentration against isothermal heating time (at 700 ◦ C) under gas flowing rate of 150 ml/min (using a gas mixture of 1% CH4 + 10% O2 + 89%N2 , under 1 atm) for PdO and Pd0.8 Zr0.2 O wafers.
+ 10% O2 +89% N2 gas mixture. The experimental results are shown in Fig. 5, which indicates that the (Pd, Zr)O phase is stable until 820 ◦ C. The (Pd, Zr) metal phase can be seen at 820 ◦ C and becomes dominant at 850 ◦ C. Hence the Td for the Pd0.8 Zr0.2 O is ∼820–850 ◦ C. Fig. 5 also indicates that the decomposed (Pd, Zr)O phase can be re-oxided fully when the temperature decreases below Td , which is in good agreement with the TGA multiple cycle results in Fig. 4. Comparing to the Td derived from the TGA under 1 atm of air; current Td is somewhat lower than that under air atmosphere. This phenomenon is consistent with our recent phase diagram study for the Pd–PdO decomposition of Pd oxide as a function of oxygen partial pressure [13] and the study [11] on PdO in similar dynamic conditions. The dynamic study on PdO indicates that the PdO phase is stable until 750 ◦ C. When temperature increases to 780 ◦ C, the PdO phase is fully decomposed to Pd. Referring to the TGA results in Fig. 4(a), the Td for PdO under 1 atm of air, which is ∼0.21 atm of partial oxygen pressure, is ∼802 ◦ C. In the methane mixture gas experiments, the partial pressure of oxygen is only 0.1 atm, which explains the observed lower oxide transformation temperature. As shown in early work [12], a residual gas analyzer was used to monitor the gas pressures in the flowing gas stream during the catalytic runs. With the progress, the oxygen and methane concentration decreases, while the H2 O and CO2 are observed from none in initial. This is an apparent sign that the combustion appears at this temperature under the catalytic function of PdO and Pd–Zr oxide. As this isothermal dynamic catalytic combustion test was carried out in a chamber of high temperature X-ray unit with a volume over 4 l and a few of feet of connection tube to the RGA, in comparison to the large volume of the environment and the small catalyst (1 cm × 2 cm), the gas kind and gas concentration will take certain time to reach an equilibrium status. When using 100 ◦ C/min heating rate heat the wafer sample to 700 ◦ C, the catalytic combustion reaction will start. With time, the methane concentration in the chamber decreases
until the concentration out the measuring sensitivity of the RGA. The needed time, called ‘cutting time’, which corresponds to the time period that starts from the time at which set temperature arrived and ends at the time at which the methane concentration is down to zero. Hence, in this particular instrument set-up and test conditions, the methane cutting time is as an indirect parameter to evaluate the performance of catalytic combustion for methane at this study. The shorter the cutting time, the better the catalytic function. The experiment results are shown in Fig. 6. The PdO and (Pd, Zr)O results in Fig. 6 indicate that the presence of Zr does not significantly reduce the observed catalytic behavior of the PdO in promoting the low temperature oxidation of the methane. There were some indications of the oxidation reaction initiating more slowly, but further experiments will be required to quantify these observations.
4. Conclusions Novel Pd–Zr oxides have been successfully synthesized using a chemical processing method. The Pd–Zr oxide has nanostructure with gain size of 5–20 nm. The partial replacement of Pd atoms by Zr atoms improves the thermal stability of the Pd based oxide. The decomposition temperature of Pd based oxide increases with the Zr concentration within the concentration range studied. The transform reversibility of Pd–PdO as measured with TGA has been promoted in the Pd–Zr oxide system. The alloying of PdO by Zr also affects the catalytic behavior of methane combustion but still behaves as a catalyst. Further studies are required to verify this behavior.
Acknowledgements This material is based upon work supported by Connecticut Innovations Inc. under the grant entitled, “Development
H. Zhang et al. / Materials Science and Engineering A366 (2004) 248–253
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