- Email: [email protected]
Study of novel nanostructured Pd–Mn oxides
ARTICLE IN PRESS
Physica B 344 (2004) 278–283
Study of novel nanostructured Pd–Mn oxides Heng Zhanga,*, Jack Gromeka, Matthew Augustinea, Gayanath Fernandoa, R. Samuel Boorseb, Harris L. Marcusa a
Institute of Materials Science, University of Connecticut Storrs, 156 River Road, Willington, CT 06269-3136, USA b Precision Combustion Inc., North Haven, CT 06473, USA Received 24 July 2002; received in revised form 2 September 2002; accepted 15 September 2002
Abstract Novel Pd–Mn oxides have been synthesized using chemical processing. The study indicated that the Pd–Mn oxide powders have nanometer structure. The alloying of Mn improves the thermal stability and modifies re-oxidation characteristic of the palladium oxide during the thermal cycling. r 2003 Elsevier B.V. All rights reserved. PACS: 81.20.Ka; 61.46+w; 65.80.tn; 68.18.Jk Keywords: Pd–Mn oxide; Nanocrystalline materials; Thermal stability; Decomposition; Catalysis
1. Introduction Reduction of the NOx emission of industrial combustion has become a crucial task for environment protection. It is known that the emission of NOx is dependent on the temperature, and the lower the combustion temperature, the less the NOx emission will be. The palladium–palladium oxide system has shown the potential as a catalyst for methane combustion minimizing the formation of NOx through the lean-premixed catalytic combustion [1–10]. The lean-premixed catalytic combustion operates at much lower temperature, hence, dramatically reduces NOx emission. PdO–Pd system leads to combustion of the methane appearing at a few hundred C lower than the traditional combustion. Both Pd and PdO *Corresponding author. Tel.: +1-8604873838; fax: +18604295911. E-mail address: [email protected] (H. Zhang).
have catalytic activity for methane combustion. PdO has higher activity at lower temperature, while Pd has higher activity at higher temperature [1]. There is a phase decomposition temperature for Pd–PdO system, which is dependent on oxygen partial pressure. PdO is stable at lower temperature, and decomposes to Pd at high temperature [4]. Alloying PdO keeps the same crystal structure as PdO, but could modify its chemical and thermal behavior of PdO. Transition elements are likely to be one of the options for this purpose as they often show some catalytic behavior. In this work, we report our study for a Pd–Mn alloy oxide system.
2. Experiments PdO and Pd1 xMnxO (x=0.05, 0.1 and 0.2) were prepared by using a chemical synthesis
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.08.131
ARTICLE IN PRESS H. Zhang et al. / Physica B 344 (2004) 278–283
in an Amray-1000A scanning electron microscopy with an accelerated voltage of 20 kV. Thermal behavior was analyzed by using a TA Instruments Hi-Res TGA 2950 Thermo-gravimetric Analyzer (TGA). 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 PdO–Pd decomposition and reoxidation reversibility. In-situ high-temperature XRD and catalytic behavior tests were carried out using a hightemperature X-ray unit with gas control system and gas mass spectroscopy (Fig. 1). 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. Variable-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
approach. The precursors used are PdCl4 (Acros, 99.9%), MnCl2 (Acros 99.9%). The metal salts and sodium hydroxide were dissolved in water to form the desired metal hydroxide. Then, the formed hydroxide is thoroughly aged to form nanometer oxides. The powder samples are used for room temperature X-ray diffraction (XRD) and thermalgramic analysis. The coupon samples with the Pd/Mn oxides deposited on an Al2O3 support layer are used for the in-situ hightemperature XRD and catalytic activity study. The prepared samples were characterized using a D-5005 XRD with Cu Ka radiation. Particle size of the powder samples was observed by TEM analysis using a Philips EM-420 transmission electron microscopy (TEM) 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. Semiquantitative element analysis was carried out using an energy dispersive spectrometer (EDS) equipped
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Fig. 1. Schematic for the in-situ high-temperature X-ray instrument with vacuum and gas system: (1) gas tank, (2) regulator, (3) needle valve, (4) flow meter, (5) insulation valve, (6) relief valve, (7) pressure meter, (8) XRD/HTU, (9) heat exchanger, (10) thermal couple gauge, (11) connective valve, (12) RGA analyzer, (13) sub-/over atmosphere selective valve, (14) ball valve, (15) dynamic control valve, (16) exhaust connective valve, (17) molecular trap, (18) exhaust trap, (19) rotary pump, (20) pressure transducer, (21) throttle control valve, (22) PID pressure controller, and (23) connective valve.
ARTICLE IN PRESS H. Zhang et al. / Physica B 344 (2004) 278–283
280
of 100 C/min. Each scan at temperature takes B0.5 h. Isothermal dynamic observations were carried out using a flow rate of 150 ml/min of the methane mixture gas.
Intensity (arb. units)
a
b c d
30
40 50 60 Two Theta (degrees)
70
Fig. 2. XRD patterns for PdO (a) and (Pd1 xMnx)O powder samples (b) x= 0.05; (c) x=0.10; (d) x=0.20.
Table 1 Concentration of Pd1 xMnxO from EDS analysis Sample
Pd
Mn
Pd0.95Mn0.05O Pd0.9Mn0.1O Pd0.8Mn0.2O
0.927 0.904 0.805
0.073 0.096 0.195
3. Results and discussion Room temperature XRD patterns for the PdO and Pd1 xMnxO (x=0.05, 0.1, and 0.2) powder samples are shown in Fig. 2. The semi-quantitative EDS analysis results for the Pd–Mn alloy oxides are shown in Table 1. By comparing the PdO sample with the XRD pattern, no extra diffraction peaks were observed for all the three Pd1 xMnxO samples. On the other hand, all the peaks of Pd– Mn alloy oxides shifted to higher angles. This is attributed to the decrease of lattice constant due to the replacement of Pd atoms by the smaller atomic size Mn atoms. This indicates that Mn substitutes for Pd in PdO and a solid solution of Pd–Mn oxide was formed. Analysis of the broad diffraction peaks results in the grain sizes in the range of 10– 20 nm for the PdO and Pd–Mn alloy oxides. TEM results for the PdO and Pd0.8Mn0.2O powder samples are shown in Fig. 3. The nanometer particles aggregate to form clusters. However, the 5–20 nm particle still can be identified from these images, supporting the line broadening XRD analysis. Thermal analysis (TGA) results for PdO and Pd–Mn oxides are shown in Fig. 4. The decomposition temperature of PdO and (Pd, Mn)O, Td was derived from the TGA curves by the tangent method as shown in Fig. 4. The obtained data are listed in Table 2. The decomposition temperatures of all the Pd–Mn alloy oxides are higher than that
Fig. 3. TEM images for PdO (a) and Pd0.8Mn0.2O (b) powder samples.
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Fig. 4. TGA results for the PdO (a) and Pd1 xMnxO (b) x=0.05; (c) x=0.1; (d) x = 0.2 powder samples using a heating rate of 20 C/min under air gas flowing (40 ml/min).
Table 2 Thermal data for the PdO and Pd1 xMnxO derived from the TGA results Sample
Td ( C)
PdO Pd0.95Mn0.05O Pd0.9Mn0.1O Pd0.8Mn0.2O
802 813 817 833
of PdO, and the decomposition temperature of (Pd, Mn)O, Td, increases with the Mn concentration. For Pd0.8Mn0.2O, the Td increases to 833 C from 802 C for PdO. On the other hand, the reoxidation percentage for the decomposed PdO in the presence of Mn is dramatically increased. At present, we have not identified the causes of these changes. From the above TGA data, we chose Pd0.8Mn0.2O, which shows the largest increase in oxide decomposition temperature, for further in-
situ high-temperature XRD studies under the 1% CH4+10% O2+89% N2 gas mixture. The experimental results are shown in Fig. 5. For Pd0.8Mn0.2O coupon, the (Pd, Mn)O phase is stable until 820 C. The (Pd, Mn) phase can be seen at 820 C and becomes dominant at 850 C. Hence the Td for the (Pd0.8Mn0.2)O is B820–850 C. 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 PdO–Pd [11] decomposition of Pd oxide as a function of oxygen partial pressure. Based on the set up of Fig. 1, a residual gas analyzer (RGA) was used to monitor the gas pressures in the flowing gas stream during the catalytic runs. The catalytic activity of methane combustion can be approximately estimated from the cutting time of methane. The shorter the cutting time, the more active the catalyst is. Comparison of the PdO and Pd0.8Mn0.2O is given
ARTICLE IN PRESS 282
H. Zhang et al. / Physica B 344 (2004) 278–283
in Fig. 6, it is found that the presence of Mn did not significantly reduce the observed catalytic behavior of the PdO in promoting the lowtemperature oxidation of the methane. This is what is expected for the alloying design for the palladium-based oxide catalyst. However, more critical measurement is needed to confirm the catalytic activity. There were some indications of the oxidation reaction initiating more slowly, but further experiments will be conducted to quantify these observations. This work is in process and result will be reported elsewhere.
4. Conclusions Novel Pd–Mn oxides have been successfully synthesized using a chemical processing method. The Pd–Mn oxide has nano-structure with gain size of 5–20 nm. The partial replacement of Pd atoms by Mn atoms improves the thermal stability of the Pd based oxide. The decomposition temperature of Pd oxide increases with the Mn concentration within the concentration range studied. The re-oxidation behavior of palladiumbased oxides as measured with TGA has been modified in the Pd–Mn oxide system. The alloying of PdO by Mn has not apparently changed the catalytic behavior of methane combustion but with better thermal stability.
Acknowledgements This material is based upon work supported by Connecticut Innovations Inc. under the grant entitled, ‘‘Development of In Situ Materials Analysis Techniques for Materials Optimization of Methane Combustion Catalysts’’.
References Fig. 5. In situ high-temperature XRD patterns for Pd0.8Mn0.2O coupon sample under 1% CH4+10% O2+89% N2 gas mixture (under one atmosphere).
[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.
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).
ARTICLE IN PRESS H. Zhang et al. / Physica B 344 (2004) 278–283 [3] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control Commercial: Technology, Van Nostrsnd 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.
283
[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, J. Phase Equilibria 23 (2002) 246.
Physica B 344 (2004) 278–283
Study of novel nanostructured Pd–Mn oxides Heng Zhanga,*, Jack Gromeka, Matthew Augustinea, Gayanath Fernandoa, R. Samuel Boorseb, Harris L. Marcusa a
Institute of Materials Science, University of Connecticut Storrs, 156 River Road, Willington, CT 06269-3136, USA b Precision Combustion Inc., North Haven, CT 06473, USA Received 24 July 2002; received in revised form 2 September 2002; accepted 15 September 2002
Abstract Novel Pd–Mn oxides have been synthesized using chemical processing. The study indicated that the Pd–Mn oxide powders have nanometer structure. The alloying of Mn improves the thermal stability and modifies re-oxidation characteristic of the palladium oxide during the thermal cycling. r 2003 Elsevier B.V. All rights reserved. PACS: 81.20.Ka; 61.46+w; 65.80.tn; 68.18.Jk Keywords: Pd–Mn oxide; Nanocrystalline materials; Thermal stability; Decomposition; Catalysis
1. Introduction Reduction of the NOx emission of industrial combustion has become a crucial task for environment protection. It is known that the emission of NOx is dependent on the temperature, and the lower the combustion temperature, the less the NOx emission will be. The palladium–palladium oxide system has shown the potential as a catalyst for methane combustion minimizing the formation of NOx through the lean-premixed catalytic combustion [1–10]. The lean-premixed catalytic combustion operates at much lower temperature, hence, dramatically reduces NOx emission. PdO–Pd system leads to combustion of the methane appearing at a few hundred C lower than the traditional combustion. Both Pd and PdO *Corresponding author. Tel.: +1-8604873838; fax: +18604295911. E-mail address: [email protected] (H. Zhang).
have catalytic activity for methane combustion. PdO has higher activity at lower temperature, while Pd has higher activity at higher temperature [1]. There is a phase decomposition temperature for Pd–PdO system, which is dependent on oxygen partial pressure. PdO is stable at lower temperature, and decomposes to Pd at high temperature [4]. Alloying PdO keeps the same crystal structure as PdO, but could modify its chemical and thermal behavior of PdO. Transition elements are likely to be one of the options for this purpose as they often show some catalytic behavior. In this work, we report our study for a Pd–Mn alloy oxide system.
2. Experiments PdO and Pd1 xMnxO (x=0.05, 0.1 and 0.2) were prepared by using a chemical synthesis
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.08.131
ARTICLE IN PRESS H. Zhang et al. / Physica B 344 (2004) 278–283
in an Amray-1000A scanning electron microscopy with an accelerated voltage of 20 kV. Thermal behavior was analyzed by using a TA Instruments Hi-Res TGA 2950 Thermo-gravimetric Analyzer (TGA). 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 PdO–Pd decomposition and reoxidation reversibility. In-situ high-temperature XRD and catalytic behavior tests were carried out using a hightemperature X-ray unit with gas control system and gas mass spectroscopy (Fig. 1). 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. Variable-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
approach. The precursors used are PdCl4 (Acros, 99.9%), MnCl2 (Acros 99.9%). The metal salts and sodium hydroxide were dissolved in water to form the desired metal hydroxide. Then, the formed hydroxide is thoroughly aged to form nanometer oxides. The powder samples are used for room temperature X-ray diffraction (XRD) and thermalgramic analysis. The coupon samples with the Pd/Mn oxides deposited on an Al2O3 support layer are used for the in-situ hightemperature XRD and catalytic activity study. The prepared samples were characterized using a D-5005 XRD with Cu Ka radiation. Particle size of the powder samples was observed by TEM analysis using a Philips EM-420 transmission electron microscopy (TEM) 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. Semiquantitative element analysis was carried out using an energy dispersive spectrometer (EDS) equipped
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3 2
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15 1 23 22
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18 19
Fig. 1. Schematic for the in-situ high-temperature X-ray instrument with vacuum and gas system: (1) gas tank, (2) regulator, (3) needle valve, (4) flow meter, (5) insulation valve, (6) relief valve, (7) pressure meter, (8) XRD/HTU, (9) heat exchanger, (10) thermal couple gauge, (11) connective valve, (12) RGA analyzer, (13) sub-/over atmosphere selective valve, (14) ball valve, (15) dynamic control valve, (16) exhaust connective valve, (17) molecular trap, (18) exhaust trap, (19) rotary pump, (20) pressure transducer, (21) throttle control valve, (22) PID pressure controller, and (23) connective valve.
ARTICLE IN PRESS H. Zhang et al. / Physica B 344 (2004) 278–283
280
of 100 C/min. Each scan at temperature takes B0.5 h. Isothermal dynamic observations were carried out using a flow rate of 150 ml/min of the methane mixture gas.
Intensity (arb. units)
a
b c d
30
40 50 60 Two Theta (degrees)
70
Fig. 2. XRD patterns for PdO (a) and (Pd1 xMnx)O powder samples (b) x= 0.05; (c) x=0.10; (d) x=0.20.
Table 1 Concentration of Pd1 xMnxO from EDS analysis Sample
Pd
Mn
Pd0.95Mn0.05O Pd0.9Mn0.1O Pd0.8Mn0.2O
0.927 0.904 0.805
0.073 0.096 0.195
3. Results and discussion Room temperature XRD patterns for the PdO and Pd1 xMnxO (x=0.05, 0.1, and 0.2) powder samples are shown in Fig. 2. The semi-quantitative EDS analysis results for the Pd–Mn alloy oxides are shown in Table 1. By comparing the PdO sample with the XRD pattern, no extra diffraction peaks were observed for all the three Pd1 xMnxO samples. On the other hand, all the peaks of Pd– Mn alloy oxides shifted to higher angles. This is attributed to the decrease of lattice constant due to the replacement of Pd atoms by the smaller atomic size Mn atoms. This indicates that Mn substitutes for Pd in PdO and a solid solution of Pd–Mn oxide was formed. Analysis of the broad diffraction peaks results in the grain sizes in the range of 10– 20 nm for the PdO and Pd–Mn alloy oxides. TEM results for the PdO and Pd0.8Mn0.2O powder samples are shown in Fig. 3. The nanometer particles aggregate to form clusters. However, the 5–20 nm particle still can be identified from these images, supporting the line broadening XRD analysis. Thermal analysis (TGA) results for PdO and Pd–Mn oxides are shown in Fig. 4. The decomposition temperature of PdO and (Pd, Mn)O, Td was derived from the TGA curves by the tangent method as shown in Fig. 4. The obtained data are listed in Table 2. The decomposition temperatures of all the Pd–Mn alloy oxides are higher than that
Fig. 3. TEM images for PdO (a) and Pd0.8Mn0.2O (b) powder samples.
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281
Fig. 4. TGA results for the PdO (a) and Pd1 xMnxO (b) x=0.05; (c) x=0.1; (d) x = 0.2 powder samples using a heating rate of 20 C/min under air gas flowing (40 ml/min).
Table 2 Thermal data for the PdO and Pd1 xMnxO derived from the TGA results Sample
Td ( C)
PdO Pd0.95Mn0.05O Pd0.9Mn0.1O Pd0.8Mn0.2O
802 813 817 833
of PdO, and the decomposition temperature of (Pd, Mn)O, Td, increases with the Mn concentration. For Pd0.8Mn0.2O, the Td increases to 833 C from 802 C for PdO. On the other hand, the reoxidation percentage for the decomposed PdO in the presence of Mn is dramatically increased. At present, we have not identified the causes of these changes. From the above TGA data, we chose Pd0.8Mn0.2O, which shows the largest increase in oxide decomposition temperature, for further in-
situ high-temperature XRD studies under the 1% CH4+10% O2+89% N2 gas mixture. The experimental results are shown in Fig. 5. For Pd0.8Mn0.2O coupon, the (Pd, Mn)O phase is stable until 820 C. The (Pd, Mn) phase can be seen at 820 C and becomes dominant at 850 C. Hence the Td for the (Pd0.8Mn0.2)O is B820–850 C. 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 PdO–Pd [11] decomposition of Pd oxide as a function of oxygen partial pressure. Based on the set up of Fig. 1, a residual gas analyzer (RGA) was used to monitor the gas pressures in the flowing gas stream during the catalytic runs. The catalytic activity of methane combustion can be approximately estimated from the cutting time of methane. The shorter the cutting time, the more active the catalyst is. Comparison of the PdO and Pd0.8Mn0.2O is given
ARTICLE IN PRESS 282
H. Zhang et al. / Physica B 344 (2004) 278–283
in Fig. 6, it is found that the presence of Mn did not significantly reduce the observed catalytic behavior of the PdO in promoting the lowtemperature oxidation of the methane. This is what is expected for the alloying design for the palladium-based oxide catalyst. However, more critical measurement is needed to confirm the catalytic activity. There were some indications of the oxidation reaction initiating more slowly, but further experiments will be conducted to quantify these observations. This work is in process and result will be reported elsewhere.
4. Conclusions Novel Pd–Mn oxides have been successfully synthesized using a chemical processing method. The Pd–Mn oxide has nano-structure with gain size of 5–20 nm. The partial replacement of Pd atoms by Mn atoms improves the thermal stability of the Pd based oxide. The decomposition temperature of Pd oxide increases with the Mn concentration within the concentration range studied. The re-oxidation behavior of palladiumbased oxides as measured with TGA has been modified in the Pd–Mn oxide system. The alloying of PdO by Mn has not apparently changed the catalytic behavior of methane combustion but with better thermal stability.
Acknowledgements This material is based upon work supported by Connecticut Innovations Inc. under the grant entitled, ‘‘Development of In Situ Materials Analysis Techniques for Materials Optimization of Methane Combustion Catalysts’’.
References Fig. 5. In situ high-temperature XRD patterns for Pd0.8Mn0.2O coupon sample under 1% CH4+10% O2+89% N2 gas mixture (under one atmosphere).
[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.
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).
ARTICLE IN PRESS H. Zhang et al. / Physica B 344 (2004) 278–283 [3] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control Commercial: Technology, Van Nostrsnd 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.
283
[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, J. Phase Equilibria 23 (2002) 246.