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CeO2 nanocatalyst for low-temperature methane combustion
Author’s Accepted Manuscript Sintering Inhibition of Flame-Made Pd/CeO2 Nanocatalyst for Low-Temperature Methane Combustion Nafeng Wang, Shuiqing Li, Yichen Zong, Qiang Yao www.elsevier.com/locate/jaerosci
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S0021-8502(16)30033-7 http://dx.doi.org/10.1016/j.jaerosci.2016.11.017 AS5083
To appear in: Journal of Aerosol Science Received date: 29 January 2016 Revised date: 10 November 2016 Accepted date: 28 November 2016 Cite this article as: Nafeng Wang, Shuiqing Li, Yichen Zong and Qiang Yao, Sintering Inhibition of Flame-Made Pd/CeO2 Nanocatalyst for Low-Temperature Methane Combustion, Journal of Aerosol Science, http://dx.doi.org/10.1016/j.jaerosci.2016.11.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sintering Inhibition of Flame-Made Pd/CeO2 Nanocatalyst for Low-Temperature Methane Combustion
Nafeng Wang, Shuiqing Li*, Yichen Zong, Qiang Yao
Address: Key laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China
Fax: +86-10-62794068; Tel: +86-10-62773384. *
To whom correspondence should be addressed. Email: [email protected]
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Abstract Dispersed palladium on the high-surface-area ceria support is synthesized via one-step flame-assisted spray pyrolysis method, which applies a highly-quenched stagnation- point flame controlling catalyst sizes and structures. The X-ray photoelectron spectroscopy (XPS) spectra of Pd show a significantly high binding energy for flame-made Pd/CeO2 catalysts, possessing a value of 1.7 eV larger than the reference value in literature. It suggests that the partial electron transfer occurs from metal Pd to their supports during the synthesis process, which creates Pd electron-deficient (cationic Pdδ+) and Ce electron-rich (anion Ceδ-), respectively. The catalytic activities of CH4 oxidation are performed over the temperatures ranging from 200°C to 600°C. In comparison with inert support materials, the synergistic effect is found between palladium and support ceria that leads to the enhanced catalytic activity. During the heating and cooling cycles of CH4 oxidation, Pd/CeO2 catalysts exhibit an exceptional inhibition effect against the sintering of Pd cluster and its dispersion decrement, which is related to strong electronic interaction of metal-support interfaces induced by the aforementioned partial electron transfer.
Key words: Nano catalysts; Flame synthesis; Palladium; Ceria; Electron transfer
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Abstract Graphics
XPS spectra of Pd 3d in as-prepared Pd/CeO2 sample and electronic transfer from Pd to the support making electron-rich (anion Ceδ-) but making Pd electron-deficient (cationic Pdδ+)
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1. Introduction Catalytic natural-gas combustion is becoming an environmentally friendly alternative to conventional flame combustion for heat and power production due to its ultra-low pollutant emissions and high efficient conversion (Farrauto, 2012; Shimizu & Wang, 2011; Shimizu et al., 2010). Previous studies have demonstrated that catalyst supports play an important role in reaction and a number of support materials have been investigated, including Al2O3, TiO2, and CeO2, etc. (Ciuparu et al., 2002; Cargnello et al., 2012; Cargnello et al., 2012). Among these support martials, ceria has received particular attention as one of promising support candidates since 1980s, which is mainly due to the enhanced catalytic activities resulting from the reversible oxygen storage and release capacity (Hailstone et al., 2009). Moreover, ceria carries a high content of surface oxygen vacancies, which can serve as active sites and participate in a variety of chemical reactions for the enhanced catalytic activities (Schaub et al., 2001). It is expected that supported noble metal catalysts will combine both high catalytic activity and structurally thermal stability in cyclic operations. However, such heterogeneous catalysts unfortunately suffer from the performance degradation with time-on-stream due to the metal sintering under oxidative conditions. Interestingly, it has been reported that noble metal catalysts sinter more slowly and can maintain in small particle size when supported on ceria compared with other supports (Colussi et al., 2009 ). It has been proposed that oxygen vacancies formed at the surface of the CeO2 could serve as the anchoring sites of Pd, and bind the transition metal more strongly than normal oxide sites (Campbell & Peden, 2005). Furthermore, it is revealed that local stronger transition metal-ceria adhesion energy was 4
attributed to the unusual sintering resistance of transition metal catalysts ( Farmer & Campbell, 2010). However, the situation is more complex when both the metal and support ceria are ultrafine particles below 10 nm with strong interfacial interaction, and the mechanism of the sintering inhibition is still of a great interest that remains to be explored. Meanwhile, electron transfer phenomena, found between the species where the oxidation states are varying, is believed to occur in the supported metal catalysts that further enhances metal-support interaction. It has been reported that electron transfer occurred for both Pd0 and Pd2+ supported samples and show remarkable dependence on the support. Higher binding energy for Pd0 was found when Pd supported on carbon and the lower binding energy was also reported when supported on Al2O3 and SiO2 (Cárdenas-Lizana et al., 2013). For Pd2+, both higher and lower binding energies were reported as well (Priolkar et al., 2002; Hinokuma et al., 2014). However, the relationship between binding energies and sintering resistance performances of supported nanoparticles still needs deeply investigation. The panoramic morphology, as well as the interaction between metal and support, is the strong function of synthesis methods and conditions. Various methods have been reported for preparing supported catalysts. Among them, flame synthesis is highly promising method that can produce multicomponent and composite nanoparticle with relatively simple technique and high production rate (Tiwari et al., 2008; Aromaa et al., 2012; Bhanwala et al., 2009; Kumfer et al., 2010). For flame-made catalysts produced by the recent innovative flame-assisted spray pyrolysis (FASP) with highly-quenched premixed stagnation-point swirl flame (SSF) system, it is evidenced that these catalysts are endowed with particular structure and enhanced interfacial interaction by comparison with the conventional
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multi-stage solution-based methods, which could often be explained as the reasons for the uniqueness in their catalytic behaviors (Niu et al., 2014). Therefore, as a dry synthesis route based on the atomic-level assembly (via. controlling time scales of reaction, nucleation and coagulation of different metal oxides), the one-step SSF-based FASP is a promisingly scalable technology to design and fabricate the supported metal catalysts with tunable performances. The goal of this paper is to explore the high-efficient fuel-soluble catalysts for low-temperature methane combustion that can simultaneously achieve high catalytic activity and excellent thermal stability (i.e., sintering inhibition). Here, we particularly employ the one-step SSF-based FASP to synthesize nano-sized Pd/CeO2 catalysts, which are further characterized by Brunauer-Emmett-Teller (BET), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD). The catalytic activity of Pd/CeO2 is compared with those of pure CeO2 and inert support SiO2. More importantly, the particular concerns are given to the sintering inhibition of Pd supported on CeO2 for methane combustion undergoing three heating-cooling cycles, which is due to electronic interaction induced by partial electron transfer.
2. Experimental method 2.1 Catalysts preparation Pd/CeO2 nanoparticles were synthesized by using a one-step flame-assisted spray pyrolysis system, which mainly includes two parts, an ultrafine spray setup and a SSF burner. A detailed description of the synthesis system can be found in our previous literature on synthesis of titania-based Pd catalysts (Niu et al., 2014). The precursor mixture was prepared by dissolving palladium acetate (Pd(OAc)2, Aladdin, 99%) and Cerium(III) 2-ethylhexanoate (Strem, 99%) in the solvent of xylene. The total mass fraction of metal Pd was maintained at 10%. The solution was injected into the spray setup through a syringe pump 6
at 20 mL·h-1 and dispersed into fine droplets by a gas-assist nozzle fed by 2 L·min-1 of nitrogen. The droplet-laden gas was then mixed with another premixed flow, which is composed of methane/oxygen/nitrogen at 2 L·min-1, 6.6 L·min-1 and 21.9 L·min-1, respectively, throughout all experiments. The setup of flame synthesis is schematically illustrated in Fig.1. A stable flame for long-time operation was achieved by using the swirl-stabilized stagnation flow burner. The temperature profile measured using a K-type thermocouple shows that it is about 600 K at the nozzle exit, and the maximum temperature is found as 1598 K at a distance of 13.2 mm from the nozzle centerline. And then, it significantly decreases due to the quenching effect (Zong et al., 2015). The particles were collected from the water cooling substrate which is fixed at 18 mm downward the nozzle exit.
Fig. 1. Schematic of small Pd clusters supported on CeO2 during flame synthesis process
2.2 Catalysts characterization The specific surface area (SSA) of the catalysts was evaluated from N2 adsorption isotherms with the BET method, using a Micromeritics ASAP 2000 apparatus. The phase composition of the samples was determined by XRD, using a D8 ADVANCE instrument from Bruker. Diffraction patterns were obtained for 2θ between 200 and 800, using a step of 0.05 and a counting time of 1 s·step-1. HRTEM 7
investigations were performed on JEM-2100F (JEOL Ltd.), where nanoparticles were deposited onto a carbon foil supported on a copper grid. Phase identification was carried out using the reference database (JCPDS-files). The catalyst surface composition was revealed by XPS on an EscaLab 250Xi instrument. 2.3 Catalytic performance evaluation The catalytic activity of the flame-made catalysts was measured in a lab-scale fixed-bed quartz reactor (i.d. 12 mm) at atmospheric ambience. The reactive gas mixture, consisting of 2% CH4 and 8% O2, balanced by N2, was led over a thin layer of the catalyst (15 mg) at a flow rate of 130 mL·min-1. A K-type thermocouple was inserted into the quartz tube to record the catalyst temperature. The programmable temperature was recorded using three separated thermocouples outside the quartz reactor. The oven temperature was programmed at a ramp of 4 °C · min-1 from 200°C to 600 °C. The inlet and outlet gas compositions were analyzed by the on-line infrared ray gas analyzer. The catalytic activity was evaluated in terms of CH4 conversion defined as (cin−cout)/cin×100%, where cin and cout is methane concentration corresponding to the inlet and outlet, respectively. The catalytic performance in this study was indicated by the maximum methane conversion under reaction conditions. 3 Results and discussion 3.1 Morphology characterization Well-dispersed Pd clusters (diameter estimated as 2~3 nm) supported on the surface of the CeO2 are obtained in this SSF-based FASP, which is similar to our previous report on TiO2-supported Pd (Niu et al., 2014). At the same time, the overlapping of the lattices is possible to exist in our catalysts as well, due to the atomic scale assembly of Pd and Ce in flame synthesis. Dark-field STEM and EDS images show the spatial distributions of Ce, Ti and Pd in as-prepared catalyst, as displayed in Fig.2. 8
The specific surface areas of as-prepared CeO2 and Pd/CeO2 nanoparticles are 243 m2·g-1 and 333 m2·g-1, respectively, which were derived from the BET measurements. It is seen that the loading of CeO2 with Pd leads to an apparent increase of the specific surface area because of the small sizes of Pd clusters. The use of stagnation-point flame with very high temperature decrement gradient (at a magnitude of 104 K·cm-1) can result in a substantial increase in surface area of CeO2, which is much larger than those synthesized by wet-chemistry method (Tang et al., 2004 ) or even other free-jet flame synthesis (Pisduangdaw et al., 2015 ).
Fig. 2. Dark-field STEM-EDS elemental mapping for Ce and Pd of Pd/CeO2 shown in 2(a)
The XRD analyses are used to investigate the nature of the Pd/PdO particles and the possible modifications of the CeO2-based support. Fig.3 shows XRD patterns of as-prepared CeO2 and Pd/CeO2. Only the cubic Pd (111) plane on fresh Pd/CeO2 is identified by its XRD reflection at 2θ=40.115° (Thevenin et al., 2002). The characteristic peaks of bare CeO2 and Pd/CeO2 are assigned to (111), (200), (220), (311) planes, respectively. CeO2 nanoparticles preferentially exposes the most stable plane (111), which is indicated by the most intensive peak. A new f.c.c. structure appears in the XRD pattern of Pd/CeO2, which is inferred as palladium carbide PdCx as a result of carbon dissolution in palladium (Bychkov et al., 2009).
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Fig. 3.
XRD patterns of as-prepared CeO2 and Pd/CeO2
Ce 3d, Pd 3d, and O 1s XPS core-level spectra for CeO2 and Pd/CeO2 are shown in Fig. 4. Apparently, cerium is known to exhibit a very complicated 3d spectrum that lanthanides usually have, due to hybridization with ligand orbitals and fractional occupancy of the valence 4f orbitals. This fact can also be found from other report (Thevenin et al., 2003). The two sets of spin–orbital multiplets, corresponding to the 3d3/2 and 3d5/2 contributions, are labeled as u and v, respectively(Colussi et al., 2009). The peaks labeled v and v'' are assigned to a mixing of Ce 3d9 4f2 O 2p4 and Ce 3d9 4f1 O 2p5, and peak v''' is to 3d9 4f0 O 2p6 of Ce4+ final state. On the other hand, lines v0 and v' are assigned to Ce 3d9 4f2 O 2p5 and Ce 3d9 4f1 O 2p6 of Ce3+( Ji et al., 2008 ). Usually, the Ce3+/(Ce4+ + Ce3+ ) atomic ratio in various catalysts indicates the concentration of surface oxygen vacancies. A favorable method, which is proposed by Hilaire et al.( Hilaire et al., 2001) for describing the relative area of the u0 (v0) and u' (v') peaks to the area of Ce 3d region , is applied to roughly estimate Ce3+ content, as shown in equation (1).
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Ce3+ (%)
S (u0 ) s(u ') s(v0 ) s(v ') 100% [s(u) s(v)]
(1)
The deconvolution results show that there are up to 37% and 38% Ce3+ detected for CeO2 and Pd/CeO2, respectively. It shows that flame-made CeO2 has a higher Ce3+ ratio compared with the 25% of CeO2 prepared by deposition–precipitation method (Huang et al., 2009). Thus it demonstrates the significant reduction of the CeO2 and mixed valent state of Ce derived from flame synthesis. In general, the Pd 3d core level of the XPS shows that the binding energy of 335.0 eV is assigned to metallic Pd, and is shifted to a higher binding energy value of 336.8 eV, which corresponds to PdO (Iwasa et al., 1995 ). It is found that Pd species in Pd/CeO2 is present in the metallic oxidation and its spectrum is dominated by a binding energy peak of 338.5eV, as compared with the reference binding energy peak of PdO at 336.8eV. It suggests that the binding energy peak of PdO in Pd/CeO2 sample is shifted positively by a value of 1.7 eV. Therefore, it implies that a partial electron transfer occurs from Pd species to Ce species in Pd/CeO2 system, making Ce electron-rich (anion Ceδ-) and making Pd electron-deficient (cationic Pdδ+). This will further result in a strong electronic interaction between Pd species and support, as schematically presented in the inset of Fig. 4. As discussed above, the XPS spectra of Pd indicate that Pd/CeO2 contains primarily PdO. By comparison to Pd/TiO2 synthesized under the same condition (Niu et al., 2014), Pd/CeO2 exhibits a higher concentration of cationic Pd2+. It indicates the ceria can provide an additional channel for oxygen on the Pd and oxygen migration may occur through the strong interface between CeO2 and supported Pd under the synthesis condition. In the O 1s XPS spectra, it shows three spectral features, i.e., the peaks at around 531.8 eV, 532.3 11
eV and 533 eV, respectively, which are ascribed to Ce-O and Pd-O, and to -OH groups caused by the chemisorbed H2O or and CO2 molecule to the surface (Zhang & Xu, 2013), although these peaks values are higher than those indicated in the reference (Divins et al., 2014). Then, in comparison to bare CeO2, positive peak shift of 0.2eV is observed for Pd/CeO2. In summary, from XPS characterization, electron transfer is observed in Pd/CeO2 supported catalysts for the first time, which is believed to be a direct result of the well-dispersed doping effect at atomic scale in high-temperature flame aerosol synthesis. This kind of electronic effect resulting from the electron transfer may fundamentally alter the physical and chemical properties of the as-prepared catalysts and play a key role in reaction process. It is expected to bring out some distinguished behaviors in catalytic reactions subsequently.
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Fig. 4.
XPS spectra of Ce 3d, Pd 3d, and O 1s in CeO2 and Pd/CeO2 samples
3.2 Catalytic performances 3.2.1 Catalytic activity over methane The catalytic performances of pure CeO2, Pd/CeO2 and Pd/SiO2 (synthesized with precursor mixtures of Pd(OAc)2 and hexamethyl disiloxane ) are compared and shown in Fig. 5. The pure CeO2 exhibits catalytic activities even in the absence of noble metal Pd, which is in accordance with previous 13
report on CeO2 activity made by wet co-precipitating method (Mayernick & Janik, 2011). The methane conversion initiates at 425°C and increases to 11% when the temperature is raised to 600°C. Oxygen vacancies accompanied by the formation of Ce3+ cation as intrinsic defects are considered as the active sites on the surfaces of metal oxides (Schaub et al., 2001), which is responsible for the catalytic activities of pure CeO2. When CeO2 is used as a support, it is considered as a catalytically active catalyst component. It is expected that the introduction of a small amount of noble metal Pd to CeO2 can yield substantially higher CH4 oxidation activity than those expected simply by summing the contribution from the CeO2 support and the pure Pd. Indeed, as shown in Fig. 5, the observed high activity is identical to the expectation when Pd is deposited on CeO2 support. The effect of Pd/CeO2 on the activity is compared to inert support SiO2 with the same Pd loading. It is generally known that SiO2 does not contribute to the catalytic activity (An et al., 2013). Catalytic performance of Pd/CeO2 is notably higher than that of Pd/SiO2. Therefore, it may be deduced that the overlap of Pd-CeO2 interaction, and the activities of support CeO2 and Pd, leads to a cooperative control of surface reactivity and catalytic activity, and thus plays an intrinsic role for the enhanced catalytic performances.
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Fig. 5. Conversion of methane as a function of temperature over CeO2, Pd/CeO2 and Pd/SiO2
Based on the discussion mentioned above, here, a scheme describing the possible processes was proposed in Fig. 6. Firstly, CH4 could react with the oxygen atoms in the topmost surface layer of CeO2. Secondly, reaction also occurs on the surface of PdO, because previous studies indicated that for methane combustion, oxygen from PdO is more efficiently than oxygen from the gas phase (Müller et al., 1997). Thirdly, the Pd/CeO2 can provide high mobility of oxygen at the metal-oxide interface, which can participate in the reaction (Descorme et al., 2002).
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Fig. 6.
Schematic representation of possible reaction path involved for CH4 oxidation over Pd/CeO2
And also, the catalytic activity of flame synthesized Pd/CeO2 nanocatalyst has been compared against those used in some recent work on low-temperature methane combustion. It has been confirmed that total oxidation of methane can be achieved with Pd/CeO2 prepared by wet methods (Miller et al., 2015; Guo et al., 2016). For flame-made catalysts, it should be noted that carbon deposition as discussed in the XRD analysis may lead to catalyst poisoning, which is one of the reasons for the lower activities. Though flame-made Pd/CeO2 exhibiting relatively lower catalytic properties, flame synthesis offers a much more convenient route to generate nanoparticles in large quantities. What’s more, some special properties have been noted for flame-made particles due to the occurrence of atomic fabrication in the high-temperature flame. 3.2.2 Thermal stability in heating-cooling cycles In order to further investigate the thermal stability of Pd/CeO2 catalysts, the catalytic performances towards CH4 oxidation during three heating-cooling cycles were evaluated under the same conditions as described earlier. Each cycle consisted of heating from 200°C to 600°C and cooling down to 200 °C, both at a rate of 4°C·min-1. Fig. 7 displays the typical activity profiles as a function of temperature during the cycles. The important reaction characteristics, temperature of 10%, 20%, 30% conversion (as expressed by T10%, T20%, T30% respectively) and the maximum conversion under the test temperature range are listed in Table 1. Two kinds of different hysteresis behaviors are clearly shown here. For the first heat-cooling cycle, the hysteresis effect with a higher conversion upon cooling is observed in the temperature range between
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400°C and 500°C. However, during the next two cycles, the conversion is always lower upon cooling and the hysteresis area tends to decrease between the heating and cooling ramps. A comparison of the first and the further two cycles reveals a significant change in the behavior of methane oxidation. The peculiar hysteresis behavior in the first cycle might be attributed to the restructuring of Pd under the reacting condition (Datye et al., 2000). In addition, the hysteresis effect of Pd/CeO2 becomes much smaller, compared with the apparently pinched hysteresis loop of catalytic activity of Pd/TiO2 in literature (Niu et al., 2014). The difference can be attributed to the distinct interface effect for Pd supported on CeO2 and TiO2.
Fig. 7.
Comparison of hysteresis behaviors of methane conversion over three heating and cooling cycles ( Filled and unfilled symbols refer to heating and cooling ramp, respectively).
Table 1 Summary of methane oxidation characteristics 1st cycle
2nd cycle
3rdcycle
Parameters Heating
Cooling
Heating
Cooling
Heating
Cooling
T10
330°C
360°C
390°C
420°C
415°C
430°C
T20
395°C
400°C
420°C
450°C
465°C
475°C
T30
430°C
420°C
450°C
480°C
500°C
510°C
Max conv
64.59%
63.79%
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57.27%
The data on dispersion of Pd in Pd/CeO2 is present in Table 2 and compared with the previous work conducted under the similar experimental conditions (Niu et al., 2014). The indicative I(Pd3d)/I(Ce3d) demonstrates that good dispersion of Pd in Pd/ CeO2 can almost be pertained even after three heating-cooling cycle. Comparatively, the severe dispersion decrement of Pd in Pd/TiO2 was previously reported, in which the dispersion dropped by 55% and 57% after the second and the third cycle, respectively. The good thermal stability of ceria-based catalysts offers the potential of the reutilization under the actually cyclic operations. Table 2 Comparison of Pd dispersion between Pd/CeO2 and Pd/TiO2 after each heating-cooling cycle I(Pd3d)/I(Ce3d)
Sample
1st
2nd
3rd
Pd/CeO2
0.110
0.111
0.108
Pd/TiO2
0.134
0.060
0.057
To investigate this behavior in more detail, samples were exposed to TEM analysis and ex situ XPS measurement after each heating-cooling cycle, as shown in Fig. 8 and Fig. 9. From the TEM images, spherical particles are observed with a slight larger particle size (~8.6 nm, ~9.1nm and ~9.5nm, respectively) compared with that of the as-prepared particles. XPS spectra show that the higher binding energy of PdO is still found, which indicates the existence of electronic interaction during the cycles. In contrast, the electron transfer after each cycle was not found in the surface characterization of Pd/TiO2. The electronic interaction is proposed to explain the substantial difference in resistances to sintering between Pd supported on CeO2 and TiO2. On one hand, Pdδ+ effectively creates a repulsive interaction 18
between neighboring Pdδ+ nanoparticle. Such repulsion force acts as an activation barrier to prevent two Pdδ+ nanoparticles from diffusing together and agglomerating. On the other hand, strong electrostatic attraction between Pdδ+ species and Ceδ- species is also conducive to stabilizing the Pd species against sintering. It is inferred that the sintering process will be largely inhibited, where the sintering rate is much slower and the sintering extend is significantly reduced. In other words, this kind of electronic interaction offers an effective energy barrier to prevent the migration of individual Pd atoms or Pd clusters, which helps to maintaining the well dispersion of Pd when supported on the CeO2 even under the cyclic reaction conditions.
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Fig. 8. TEM images after the each cyclic process ( a, b, and c refer to cycle 1, 2, and 3 respectively)
Fig. 9.
XPS spectra of Ce 3d, (b) Pd 3d of Pd/CeO2 after each heating-cooling cycle PdO
Conclusion A one-step dry synthesis of nanostructured Pd/CeO2 particles is demonstrated by using a premixed stagnation swirl flame with an ultra-fine spray feeding system. Compared with the inert support material, the as-synthesized catalysts on ceria exhibit a much higher catalytic activity. Pd species dispersed on oxide supports is highly positively charged, leading to the strong covalent metal-support bonding interactions between the Pd clusters and the support. In terms of the sintering behavior of the catalysts, 20
the remarkably improved Pd sintering inhibition on CeO2 is found. The activation barrier among the neighboring Pdδ+ nanoparticles, as well as the strong electrostatic attraction between Pd clusters and support, together suppress the diffusion and sintering of the Pd over ceria surface. Acknowledgements The authors acknowledge the National Natural Science Fund of China (Grant No. 51676109 and Grant No. 51390491) and the National Key Basic Research and Development Program (Grant No. 2013CB228506) for financial support. We would like to thank Prof. Yujun Wang from Tsinghua University for his assistance with CH4 catalytic oxidation measurement. Special thanks are given to Prof. Jun Huang from the University of Sydney and Prof. Xueqing Gong from East China University of Science and Technology for helpful discussion when accomplishing this paper. References An, K., Alayoglu, S., Musselwhite, N., Plamthottam, S., Melaet, G., & Lindeman, A. E., et al. (2013). Enhanced CO oxidation rates at the interface of mesoporous oxides and Pt nanoparticles. Journal of the American Chemical Society, 135, 16889–16696. Aromaa, M., Arffman, A., Suhonen, H., Haapanen, J., Keskinen, J., & Honkanen, M., et al. (2012). Atmospheric synthesis of super hydrophobic TiO2 nanoparticle deposits in a single step using liquid flame spray. Journal of Aerosol Science ,52, 57–68. Bhanwala, A. K., Kumara, A., Mishrab, D. P., & Kumara, J. (2009). Flame aerosol synthesis and characterization of pure and carbon coated titania nano powder. Aerosol Science, 40, 720–730. Bychkov, V. Y., Tyulenin, Y. P., Slinko, M. M., Shashkin, D. P., & Korchak, V. N. (2009). The study of the oscillatory behavior during methane oxidation over Pd catalysts. Journal of Catalysis, 267, 181−187. Campbell, C. T., & Peden, C. H. F. (2005). Oxygen vacancies and catalysis on ceria surfaces. Science, 309, 713−714. Cárdenas-Lizana, F., Hao, Y., Crespo-Quesada, M., Yuranov, I., Wang, X., & Keane, M. A., et al. (2013). Selective gas phase hydrogenation of p- chloronitrobenzene over Pd catalysts: role of the support. ACS Catalysis, 3, 1386−1396. Cargnello, M., Doan-Nguyen, V. V. T., Gordon, T. R., Diaz, R. E., Stach, E. A., & Gorte, R. J., et al. (2012). Control of metal nanocrystal size reveals metal-support interface role for ceria catalysts. Science, 341, 771–773. Cargnello, M., Jaén, J. J. D., Garrido, J. C. H., Bakhmutsky, K., Montini, T., & Gámez, J. J., et al. (2012). Exceptional activity for methane combustion over modular Pd@CeO2 subunits on functionalized Al2O3. Science, 337, 713– 717. Ciuparu, D., Lyubovsky, M. R., Altman, E., Pfefferle, L. D., & Datye, A. (2002). Catalytic combustion of methane over palladium-based catalysts. Catalysis Reviews: Science and Engineering, 44 , 593−649. Colussi, S., Gayen, A., Camellone, F. M., Boaro, M., Llorca, J., & Fabris, S., et al. (2009). Nanofaceted PdO sites in Pd-Ce surface superstructures: enhanced activity in catalytic combustion of methane. Angewandte Chemie
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International Edition, 48, 8481−8484. Datye, A. K, Bravo, J., Nelson, T. R., Atanasova, P., Lyubovsky, M., & Pfefferle, L. (2000). Catalyst microstructure and methane oxidation reactivity during the Pd↔PdO transformation on alumina supports. Applied Catalysis AGeneral, 198, 179–196. Descorme, C., Taha, R., Mouaddib-Moral, N., & Duprez, D. (2002). Oxygen storage capacity measurements of three-way catalysts under transient conditions, Applied Catalysis A: General, 223, 287–299. Divins, N. J., Angurell, I., Escudero, C., Pérez-Dieste, V., & Llorca, J. (2014). Influence of the support on surface rearrangements of bimetallic nanoparticles in real catalysts. Science, 346, 620−623. Farmer, J. A., & Campbell, C. T. (2010). Ceria maintains smaller metal catalyst particles by strong metal-support bonding. Science, 329, 933−936. Farrauto, R. J. (2012). Low-temperature oxidation of methane. Science, 337, 659–660. Guo, T., Du, J., Wu, J., Li, J.(2016). Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation. Applied Catalysis A: General, 524, 237–242. Hailstone, R. K., DiFrancesco, A. G., Leong, J. G., Allston, T. D., & Reed, K. J. (2009). A study of lattice expansion in CeO2 nanoparticles by transmission electron microscopy. Journal of Physical Chemistry C, 113, 15155−15159. Hilaire, S., Wang, X., Luo, T., Gorte, R. J., & Wagner, J. (2001). A comparative study of water-gas-shift reaction over ceria supported metallic catalysts. Applied Catalysis A- General, 215, 271−278. Hinokuma, S., Fujii, H., Katsuhara, Y., Ikeue, K., & Machida, M. (2014). Effect of thermal ageing on the structure and catalytic activity of Pd/CeO2 prepared using arc-plasma process. Catalysis Science & Technology, 4, 2990–2996. Huang, X., Sun, H., Wang, L., Liu, Y., Fan, K., & Cao, Y. (2009). Morphology effects of nanoscale ceria on the activity of Au/CeO2 catalysts for low-temperature CO oxidation. Applied Catalysis B- Environmental, 90, 224−232. Iwasa, N., Masuda, S., Ogawa, N., & Takezawa, N. (1995). Steam reforming of methanol over Pd/ZnO: effect of the formation of PdZn alloys upon the reaction. Applied Catalysis A- General, 125, 145–157. Kumfer, B. M., Shinoda, K., Jeyadevan, B., & Kennedy, I. M. (2010). Gas-phase flame synthesis and properties of magnetic iron oxide nanoparticles with reduced oxidation state. Journal of Aerosol Science ,41, 257–265. Mayernick, A. D., & Janik, M. J. (2011). Methane oxidation on Pd-Ceria: A DFT study of the mechanism over PdxCe1-xO2, Pd, and PdO. Journal of Catalysis, 278, 16−25. Miller, J. M., Malatpure M. (2015). Pd catalysts for total oxidation of methane: support effects. Applied Catalysis AGeneral, 495, 54–62. Müller, C. A., Maciejewski, M., Koeppel, R. A., & Baiker, A. (1997). Combustion of methane over palladium/zirconia derived from a glassy Pd–Zr alloy: effect of Pd particle size on catalytic behavior. Journal of Catalysis ,166, 36– 43. Niu, F., Li, S., Zong, Y., & Yao, Q. (2014). Catalytic behavior of flame-made Pd/TiO2 nanoparticles in methane oxidation at low temperatures. Journal of Physical Chemistry C, 118 (33), 19165−19171. Pisduangdaw, S., Mekasuwandumrong, O., Fujita, S., Arai, M., Yoshida, H., & Panpranot, J. (2015). One step synthesis of Pt–Co/TiO2 catalysts by flame spray pyrolysis for the hydrogenation of 3-nitrostyrene. Catalysis Communications, 61, 11−15. Ji, P., Zhang, J., Chen, F., & Anpo, M. (2008). Ordered mesoporous CeO2 synthesized by nanocasting from cubic Ia3d mesoporous MCM-48 silica: formation, characterization and photocatalytic activity. Journal of Physical Chemistry C, 112, 17809−17813. Priolkar, K. R., Bera, P., Sarode, P. R., Hegde, M. S., Emura, S., & Kumashiro, R., et al. (2002). Formation of
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Highlights
Dramatically higher binding energy of palladium is found to be ~1.7eV.
Partial electron transfer occurs from metal Pd to the support CeO2.
An exceptional inhibition effect against sintering and resulted dispersion decrement of Pd supported on CeO2 is evidenced.
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PII: DOI: Reference:
S0021-8502(16)30033-7 http://dx.doi.org/10.1016/j.jaerosci.2016.11.017 AS5083
To appear in: Journal of Aerosol Science Received date: 29 January 2016 Revised date: 10 November 2016 Accepted date: 28 November 2016 Cite this article as: Nafeng Wang, Shuiqing Li, Yichen Zong and Qiang Yao, Sintering Inhibition of Flame-Made Pd/CeO2 Nanocatalyst for Low-Temperature Methane Combustion, Journal of Aerosol Science, http://dx.doi.org/10.1016/j.jaerosci.2016.11.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sintering Inhibition of Flame-Made Pd/CeO2 Nanocatalyst for Low-Temperature Methane Combustion
Nafeng Wang, Shuiqing Li*, Yichen Zong, Qiang Yao
Address: Key laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, China
Fax: +86-10-62794068; Tel: +86-10-62773384. *
To whom correspondence should be addressed. Email: [email protected]
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Abstract Dispersed palladium on the high-surface-area ceria support is synthesized via one-step flame-assisted spray pyrolysis method, which applies a highly-quenched stagnation- point flame controlling catalyst sizes and structures. The X-ray photoelectron spectroscopy (XPS) spectra of Pd show a significantly high binding energy for flame-made Pd/CeO2 catalysts, possessing a value of 1.7 eV larger than the reference value in literature. It suggests that the partial electron transfer occurs from metal Pd to their supports during the synthesis process, which creates Pd electron-deficient (cationic Pdδ+) and Ce electron-rich (anion Ceδ-), respectively. The catalytic activities of CH4 oxidation are performed over the temperatures ranging from 200°C to 600°C. In comparison with inert support materials, the synergistic effect is found between palladium and support ceria that leads to the enhanced catalytic activity. During the heating and cooling cycles of CH4 oxidation, Pd/CeO2 catalysts exhibit an exceptional inhibition effect against the sintering of Pd cluster and its dispersion decrement, which is related to strong electronic interaction of metal-support interfaces induced by the aforementioned partial electron transfer.
Key words: Nano catalysts; Flame synthesis; Palladium; Ceria; Electron transfer
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Abstract Graphics
XPS spectra of Pd 3d in as-prepared Pd/CeO2 sample and electronic transfer from Pd to the support making electron-rich (anion Ceδ-) but making Pd electron-deficient (cationic Pdδ+)
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1. Introduction Catalytic natural-gas combustion is becoming an environmentally friendly alternative to conventional flame combustion for heat and power production due to its ultra-low pollutant emissions and high efficient conversion (Farrauto, 2012; Shimizu & Wang, 2011; Shimizu et al., 2010). Previous studies have demonstrated that catalyst supports play an important role in reaction and a number of support materials have been investigated, including Al2O3, TiO2, and CeO2, etc. (Ciuparu et al., 2002; Cargnello et al., 2012; Cargnello et al., 2012). Among these support martials, ceria has received particular attention as one of promising support candidates since 1980s, which is mainly due to the enhanced catalytic activities resulting from the reversible oxygen storage and release capacity (Hailstone et al., 2009). Moreover, ceria carries a high content of surface oxygen vacancies, which can serve as active sites and participate in a variety of chemical reactions for the enhanced catalytic activities (Schaub et al., 2001). It is expected that supported noble metal catalysts will combine both high catalytic activity and structurally thermal stability in cyclic operations. However, such heterogeneous catalysts unfortunately suffer from the performance degradation with time-on-stream due to the metal sintering under oxidative conditions. Interestingly, it has been reported that noble metal catalysts sinter more slowly and can maintain in small particle size when supported on ceria compared with other supports (Colussi et al., 2009 ). It has been proposed that oxygen vacancies formed at the surface of the CeO2 could serve as the anchoring sites of Pd, and bind the transition metal more strongly than normal oxide sites (Campbell & Peden, 2005). Furthermore, it is revealed that local stronger transition metal-ceria adhesion energy was 4
attributed to the unusual sintering resistance of transition metal catalysts ( Farmer & Campbell, 2010). However, the situation is more complex when both the metal and support ceria are ultrafine particles below 10 nm with strong interfacial interaction, and the mechanism of the sintering inhibition is still of a great interest that remains to be explored. Meanwhile, electron transfer phenomena, found between the species where the oxidation states are varying, is believed to occur in the supported metal catalysts that further enhances metal-support interaction. It has been reported that electron transfer occurred for both Pd0 and Pd2+ supported samples and show remarkable dependence on the support. Higher binding energy for Pd0 was found when Pd supported on carbon and the lower binding energy was also reported when supported on Al2O3 and SiO2 (Cárdenas-Lizana et al., 2013). For Pd2+, both higher and lower binding energies were reported as well (Priolkar et al., 2002; Hinokuma et al., 2014). However, the relationship between binding energies and sintering resistance performances of supported nanoparticles still needs deeply investigation. The panoramic morphology, as well as the interaction between metal and support, is the strong function of synthesis methods and conditions. Various methods have been reported for preparing supported catalysts. Among them, flame synthesis is highly promising method that can produce multicomponent and composite nanoparticle with relatively simple technique and high production rate (Tiwari et al., 2008; Aromaa et al., 2012; Bhanwala et al., 2009; Kumfer et al., 2010). For flame-made catalysts produced by the recent innovative flame-assisted spray pyrolysis (FASP) with highly-quenched premixed stagnation-point swirl flame (SSF) system, it is evidenced that these catalysts are endowed with particular structure and enhanced interfacial interaction by comparison with the conventional
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multi-stage solution-based methods, which could often be explained as the reasons for the uniqueness in their catalytic behaviors (Niu et al., 2014). Therefore, as a dry synthesis route based on the atomic-level assembly (via. controlling time scales of reaction, nucleation and coagulation of different metal oxides), the one-step SSF-based FASP is a promisingly scalable technology to design and fabricate the supported metal catalysts with tunable performances. The goal of this paper is to explore the high-efficient fuel-soluble catalysts for low-temperature methane combustion that can simultaneously achieve high catalytic activity and excellent thermal stability (i.e., sintering inhibition). Here, we particularly employ the one-step SSF-based FASP to synthesize nano-sized Pd/CeO2 catalysts, which are further characterized by Brunauer-Emmett-Teller (BET), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD). The catalytic activity of Pd/CeO2 is compared with those of pure CeO2 and inert support SiO2. More importantly, the particular concerns are given to the sintering inhibition of Pd supported on CeO2 for methane combustion undergoing three heating-cooling cycles, which is due to electronic interaction induced by partial electron transfer.
2. Experimental method 2.1 Catalysts preparation Pd/CeO2 nanoparticles were synthesized by using a one-step flame-assisted spray pyrolysis system, which mainly includes two parts, an ultrafine spray setup and a SSF burner. A detailed description of the synthesis system can be found in our previous literature on synthesis of titania-based Pd catalysts (Niu et al., 2014). The precursor mixture was prepared by dissolving palladium acetate (Pd(OAc)2, Aladdin, 99%) and Cerium(III) 2-ethylhexanoate (Strem, 99%) in the solvent of xylene. The total mass fraction of metal Pd was maintained at 10%. The solution was injected into the spray setup through a syringe pump 6
at 20 mL·h-1 and dispersed into fine droplets by a gas-assist nozzle fed by 2 L·min-1 of nitrogen. The droplet-laden gas was then mixed with another premixed flow, which is composed of methane/oxygen/nitrogen at 2 L·min-1, 6.6 L·min-1 and 21.9 L·min-1, respectively, throughout all experiments. The setup of flame synthesis is schematically illustrated in Fig.1. A stable flame for long-time operation was achieved by using the swirl-stabilized stagnation flow burner. The temperature profile measured using a K-type thermocouple shows that it is about 600 K at the nozzle exit, and the maximum temperature is found as 1598 K at a distance of 13.2 mm from the nozzle centerline. And then, it significantly decreases due to the quenching effect (Zong et al., 2015). The particles were collected from the water cooling substrate which is fixed at 18 mm downward the nozzle exit.
Fig. 1. Schematic of small Pd clusters supported on CeO2 during flame synthesis process
2.2 Catalysts characterization The specific surface area (SSA) of the catalysts was evaluated from N2 adsorption isotherms with the BET method, using a Micromeritics ASAP 2000 apparatus. The phase composition of the samples was determined by XRD, using a D8 ADVANCE instrument from Bruker. Diffraction patterns were obtained for 2θ between 200 and 800, using a step of 0.05 and a counting time of 1 s·step-1. HRTEM 7
investigations were performed on JEM-2100F (JEOL Ltd.), where nanoparticles were deposited onto a carbon foil supported on a copper grid. Phase identification was carried out using the reference database (JCPDS-files). The catalyst surface composition was revealed by XPS on an EscaLab 250Xi instrument. 2.3 Catalytic performance evaluation The catalytic activity of the flame-made catalysts was measured in a lab-scale fixed-bed quartz reactor (i.d. 12 mm) at atmospheric ambience. The reactive gas mixture, consisting of 2% CH4 and 8% O2, balanced by N2, was led over a thin layer of the catalyst (15 mg) at a flow rate of 130 mL·min-1. A K-type thermocouple was inserted into the quartz tube to record the catalyst temperature. The programmable temperature was recorded using three separated thermocouples outside the quartz reactor. The oven temperature was programmed at a ramp of 4 °C · min-1 from 200°C to 600 °C. The inlet and outlet gas compositions were analyzed by the on-line infrared ray gas analyzer. The catalytic activity was evaluated in terms of CH4 conversion defined as (cin−cout)/cin×100%, where cin and cout is methane concentration corresponding to the inlet and outlet, respectively. The catalytic performance in this study was indicated by the maximum methane conversion under reaction conditions. 3 Results and discussion 3.1 Morphology characterization Well-dispersed Pd clusters (diameter estimated as 2~3 nm) supported on the surface of the CeO2 are obtained in this SSF-based FASP, which is similar to our previous report on TiO2-supported Pd (Niu et al., 2014). At the same time, the overlapping of the lattices is possible to exist in our catalysts as well, due to the atomic scale assembly of Pd and Ce in flame synthesis. Dark-field STEM and EDS images show the spatial distributions of Ce, Ti and Pd in as-prepared catalyst, as displayed in Fig.2. 8
The specific surface areas of as-prepared CeO2 and Pd/CeO2 nanoparticles are 243 m2·g-1 and 333 m2·g-1, respectively, which were derived from the BET measurements. It is seen that the loading of CeO2 with Pd leads to an apparent increase of the specific surface area because of the small sizes of Pd clusters. The use of stagnation-point flame with very high temperature decrement gradient (at a magnitude of 104 K·cm-1) can result in a substantial increase in surface area of CeO2, which is much larger than those synthesized by wet-chemistry method (Tang et al., 2004 ) or even other free-jet flame synthesis (Pisduangdaw et al., 2015 ).
Fig. 2. Dark-field STEM-EDS elemental mapping for Ce and Pd of Pd/CeO2 shown in 2(a)
The XRD analyses are used to investigate the nature of the Pd/PdO particles and the possible modifications of the CeO2-based support. Fig.3 shows XRD patterns of as-prepared CeO2 and Pd/CeO2. Only the cubic Pd (111) plane on fresh Pd/CeO2 is identified by its XRD reflection at 2θ=40.115° (Thevenin et al., 2002). The characteristic peaks of bare CeO2 and Pd/CeO2 are assigned to (111), (200), (220), (311) planes, respectively. CeO2 nanoparticles preferentially exposes the most stable plane (111), which is indicated by the most intensive peak. A new f.c.c. structure appears in the XRD pattern of Pd/CeO2, which is inferred as palladium carbide PdCx as a result of carbon dissolution in palladium (Bychkov et al., 2009).
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Fig. 3.
XRD patterns of as-prepared CeO2 and Pd/CeO2
Ce 3d, Pd 3d, and O 1s XPS core-level spectra for CeO2 and Pd/CeO2 are shown in Fig. 4. Apparently, cerium is known to exhibit a very complicated 3d spectrum that lanthanides usually have, due to hybridization with ligand orbitals and fractional occupancy of the valence 4f orbitals. This fact can also be found from other report (Thevenin et al., 2003). The two sets of spin–orbital multiplets, corresponding to the 3d3/2 and 3d5/2 contributions, are labeled as u and v, respectively(Colussi et al., 2009). The peaks labeled v and v'' are assigned to a mixing of Ce 3d9 4f2 O 2p4 and Ce 3d9 4f1 O 2p5, and peak v''' is to 3d9 4f0 O 2p6 of Ce4+ final state. On the other hand, lines v0 and v' are assigned to Ce 3d9 4f2 O 2p5 and Ce 3d9 4f1 O 2p6 of Ce3+( Ji et al., 2008 ). Usually, the Ce3+/(Ce4+ + Ce3+ ) atomic ratio in various catalysts indicates the concentration of surface oxygen vacancies. A favorable method, which is proposed by Hilaire et al.( Hilaire et al., 2001) for describing the relative area of the u0 (v0) and u' (v') peaks to the area of Ce 3d region , is applied to roughly estimate Ce3+ content, as shown in equation (1).
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Ce3+ (%)
S (u0 ) s(u ') s(v0 ) s(v ') 100% [s(u) s(v)]
(1)
The deconvolution results show that there are up to 37% and 38% Ce3+ detected for CeO2 and Pd/CeO2, respectively. It shows that flame-made CeO2 has a higher Ce3+ ratio compared with the 25% of CeO2 prepared by deposition–precipitation method (Huang et al., 2009). Thus it demonstrates the significant reduction of the CeO2 and mixed valent state of Ce derived from flame synthesis. In general, the Pd 3d core level of the XPS shows that the binding energy of 335.0 eV is assigned to metallic Pd, and is shifted to a higher binding energy value of 336.8 eV, which corresponds to PdO (Iwasa et al., 1995 ). It is found that Pd species in Pd/CeO2 is present in the metallic oxidation and its spectrum is dominated by a binding energy peak of 338.5eV, as compared with the reference binding energy peak of PdO at 336.8eV. It suggests that the binding energy peak of PdO in Pd/CeO2 sample is shifted positively by a value of 1.7 eV. Therefore, it implies that a partial electron transfer occurs from Pd species to Ce species in Pd/CeO2 system, making Ce electron-rich (anion Ceδ-) and making Pd electron-deficient (cationic Pdδ+). This will further result in a strong electronic interaction between Pd species and support, as schematically presented in the inset of Fig. 4. As discussed above, the XPS spectra of Pd indicate that Pd/CeO2 contains primarily PdO. By comparison to Pd/TiO2 synthesized under the same condition (Niu et al., 2014), Pd/CeO2 exhibits a higher concentration of cationic Pd2+. It indicates the ceria can provide an additional channel for oxygen on the Pd and oxygen migration may occur through the strong interface between CeO2 and supported Pd under the synthesis condition. In the O 1s XPS spectra, it shows three spectral features, i.e., the peaks at around 531.8 eV, 532.3 11
eV and 533 eV, respectively, which are ascribed to Ce-O and Pd-O, and to -OH groups caused by the chemisorbed H2O or and CO2 molecule to the surface (Zhang & Xu, 2013), although these peaks values are higher than those indicated in the reference (Divins et al., 2014). Then, in comparison to bare CeO2, positive peak shift of 0.2eV is observed for Pd/CeO2. In summary, from XPS characterization, electron transfer is observed in Pd/CeO2 supported catalysts for the first time, which is believed to be a direct result of the well-dispersed doping effect at atomic scale in high-temperature flame aerosol synthesis. This kind of electronic effect resulting from the electron transfer may fundamentally alter the physical and chemical properties of the as-prepared catalysts and play a key role in reaction process. It is expected to bring out some distinguished behaviors in catalytic reactions subsequently.
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Fig. 4.
XPS spectra of Ce 3d, Pd 3d, and O 1s in CeO2 and Pd/CeO2 samples
3.2 Catalytic performances 3.2.1 Catalytic activity over methane The catalytic performances of pure CeO2, Pd/CeO2 and Pd/SiO2 (synthesized with precursor mixtures of Pd(OAc)2 and hexamethyl disiloxane ) are compared and shown in Fig. 5. The pure CeO2 exhibits catalytic activities even in the absence of noble metal Pd, which is in accordance with previous 13
report on CeO2 activity made by wet co-precipitating method (Mayernick & Janik, 2011). The methane conversion initiates at 425°C and increases to 11% when the temperature is raised to 600°C. Oxygen vacancies accompanied by the formation of Ce3+ cation as intrinsic defects are considered as the active sites on the surfaces of metal oxides (Schaub et al., 2001), which is responsible for the catalytic activities of pure CeO2. When CeO2 is used as a support, it is considered as a catalytically active catalyst component. It is expected that the introduction of a small amount of noble metal Pd to CeO2 can yield substantially higher CH4 oxidation activity than those expected simply by summing the contribution from the CeO2 support and the pure Pd. Indeed, as shown in Fig. 5, the observed high activity is identical to the expectation when Pd is deposited on CeO2 support. The effect of Pd/CeO2 on the activity is compared to inert support SiO2 with the same Pd loading. It is generally known that SiO2 does not contribute to the catalytic activity (An et al., 2013). Catalytic performance of Pd/CeO2 is notably higher than that of Pd/SiO2. Therefore, it may be deduced that the overlap of Pd-CeO2 interaction, and the activities of support CeO2 and Pd, leads to a cooperative control of surface reactivity and catalytic activity, and thus plays an intrinsic role for the enhanced catalytic performances.
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Fig. 5. Conversion of methane as a function of temperature over CeO2, Pd/CeO2 and Pd/SiO2
Based on the discussion mentioned above, here, a scheme describing the possible processes was proposed in Fig. 6. Firstly, CH4 could react with the oxygen atoms in the topmost surface layer of CeO2. Secondly, reaction also occurs on the surface of PdO, because previous studies indicated that for methane combustion, oxygen from PdO is more efficiently than oxygen from the gas phase (Müller et al., 1997). Thirdly, the Pd/CeO2 can provide high mobility of oxygen at the metal-oxide interface, which can participate in the reaction (Descorme et al., 2002).
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Fig. 6.
Schematic representation of possible reaction path involved for CH4 oxidation over Pd/CeO2
And also, the catalytic activity of flame synthesized Pd/CeO2 nanocatalyst has been compared against those used in some recent work on low-temperature methane combustion. It has been confirmed that total oxidation of methane can be achieved with Pd/CeO2 prepared by wet methods (Miller et al., 2015; Guo et al., 2016). For flame-made catalysts, it should be noted that carbon deposition as discussed in the XRD analysis may lead to catalyst poisoning, which is one of the reasons for the lower activities. Though flame-made Pd/CeO2 exhibiting relatively lower catalytic properties, flame synthesis offers a much more convenient route to generate nanoparticles in large quantities. What’s more, some special properties have been noted for flame-made particles due to the occurrence of atomic fabrication in the high-temperature flame. 3.2.2 Thermal stability in heating-cooling cycles In order to further investigate the thermal stability of Pd/CeO2 catalysts, the catalytic performances towards CH4 oxidation during three heating-cooling cycles were evaluated under the same conditions as described earlier. Each cycle consisted of heating from 200°C to 600°C and cooling down to 200 °C, both at a rate of 4°C·min-1. Fig. 7 displays the typical activity profiles as a function of temperature during the cycles. The important reaction characteristics, temperature of 10%, 20%, 30% conversion (as expressed by T10%, T20%, T30% respectively) and the maximum conversion under the test temperature range are listed in Table 1. Two kinds of different hysteresis behaviors are clearly shown here. For the first heat-cooling cycle, the hysteresis effect with a higher conversion upon cooling is observed in the temperature range between
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400°C and 500°C. However, during the next two cycles, the conversion is always lower upon cooling and the hysteresis area tends to decrease between the heating and cooling ramps. A comparison of the first and the further two cycles reveals a significant change in the behavior of methane oxidation. The peculiar hysteresis behavior in the first cycle might be attributed to the restructuring of Pd under the reacting condition (Datye et al., 2000). In addition, the hysteresis effect of Pd/CeO2 becomes much smaller, compared with the apparently pinched hysteresis loop of catalytic activity of Pd/TiO2 in literature (Niu et al., 2014). The difference can be attributed to the distinct interface effect for Pd supported on CeO2 and TiO2.
Fig. 7.
Comparison of hysteresis behaviors of methane conversion over three heating and cooling cycles ( Filled and unfilled symbols refer to heating and cooling ramp, respectively).
Table 1 Summary of methane oxidation characteristics 1st cycle
2nd cycle
3rdcycle
Parameters Heating
Cooling
Heating
Cooling
Heating
Cooling
T10
330°C
360°C
390°C
420°C
415°C
430°C
T20
395°C
400°C
420°C
450°C
465°C
475°C
T30
430°C
420°C
450°C
480°C
500°C
510°C
Max conv
64.59%
63.79%
17
57.27%
The data on dispersion of Pd in Pd/CeO2 is present in Table 2 and compared with the previous work conducted under the similar experimental conditions (Niu et al., 2014). The indicative I(Pd3d)/I(Ce3d) demonstrates that good dispersion of Pd in Pd/ CeO2 can almost be pertained even after three heating-cooling cycle. Comparatively, the severe dispersion decrement of Pd in Pd/TiO2 was previously reported, in which the dispersion dropped by 55% and 57% after the second and the third cycle, respectively. The good thermal stability of ceria-based catalysts offers the potential of the reutilization under the actually cyclic operations. Table 2 Comparison of Pd dispersion between Pd/CeO2 and Pd/TiO2 after each heating-cooling cycle I(Pd3d)/I(Ce3d)
Sample
1st
2nd
3rd
Pd/CeO2
0.110
0.111
0.108
Pd/TiO2
0.134
0.060
0.057
To investigate this behavior in more detail, samples were exposed to TEM analysis and ex situ XPS measurement after each heating-cooling cycle, as shown in Fig. 8 and Fig. 9. From the TEM images, spherical particles are observed with a slight larger particle size (~8.6 nm, ~9.1nm and ~9.5nm, respectively) compared with that of the as-prepared particles. XPS spectra show that the higher binding energy of PdO is still found, which indicates the existence of electronic interaction during the cycles. In contrast, the electron transfer after each cycle was not found in the surface characterization of Pd/TiO2. The electronic interaction is proposed to explain the substantial difference in resistances to sintering between Pd supported on CeO2 and TiO2. On one hand, Pdδ+ effectively creates a repulsive interaction 18
between neighboring Pdδ+ nanoparticle. Such repulsion force acts as an activation barrier to prevent two Pdδ+ nanoparticles from diffusing together and agglomerating. On the other hand, strong electrostatic attraction between Pdδ+ species and Ceδ- species is also conducive to stabilizing the Pd species against sintering. It is inferred that the sintering process will be largely inhibited, where the sintering rate is much slower and the sintering extend is significantly reduced. In other words, this kind of electronic interaction offers an effective energy barrier to prevent the migration of individual Pd atoms or Pd clusters, which helps to maintaining the well dispersion of Pd when supported on the CeO2 even under the cyclic reaction conditions.
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Fig. 8. TEM images after the each cyclic process ( a, b, and c refer to cycle 1, 2, and 3 respectively)
Fig. 9.
XPS spectra of Ce 3d, (b) Pd 3d of Pd/CeO2 after each heating-cooling cycle PdO
Conclusion A one-step dry synthesis of nanostructured Pd/CeO2 particles is demonstrated by using a premixed stagnation swirl flame with an ultra-fine spray feeding system. Compared with the inert support material, the as-synthesized catalysts on ceria exhibit a much higher catalytic activity. Pd species dispersed on oxide supports is highly positively charged, leading to the strong covalent metal-support bonding interactions between the Pd clusters and the support. In terms of the sintering behavior of the catalysts, 20
the remarkably improved Pd sintering inhibition on CeO2 is found. The activation barrier among the neighboring Pdδ+ nanoparticles, as well as the strong electrostatic attraction between Pd clusters and support, together suppress the diffusion and sintering of the Pd over ceria surface. Acknowledgements The authors acknowledge the National Natural Science Fund of China (Grant No. 51676109 and Grant No. 51390491) and the National Key Basic Research and Development Program (Grant No. 2013CB228506) for financial support. We would like to thank Prof. Yujun Wang from Tsinghua University for his assistance with CH4 catalytic oxidation measurement. Special thanks are given to Prof. Jun Huang from the University of Sydney and Prof. Xueqing Gong from East China University of Science and Technology for helpful discussion when accomplishing this paper. References An, K., Alayoglu, S., Musselwhite, N., Plamthottam, S., Melaet, G., & Lindeman, A. E., et al. (2013). Enhanced CO oxidation rates at the interface of mesoporous oxides and Pt nanoparticles. Journal of the American Chemical Society, 135, 16889–16696. Aromaa, M., Arffman, A., Suhonen, H., Haapanen, J., Keskinen, J., & Honkanen, M., et al. (2012). Atmospheric synthesis of super hydrophobic TiO2 nanoparticle deposits in a single step using liquid flame spray. Journal of Aerosol Science ,52, 57–68. Bhanwala, A. K., Kumara, A., Mishrab, D. P., & Kumara, J. (2009). Flame aerosol synthesis and characterization of pure and carbon coated titania nano powder. Aerosol Science, 40, 720–730. Bychkov, V. Y., Tyulenin, Y. P., Slinko, M. M., Shashkin, D. P., & Korchak, V. N. (2009). The study of the oscillatory behavior during methane oxidation over Pd catalysts. Journal of Catalysis, 267, 181−187. Campbell, C. T., & Peden, C. H. F. (2005). Oxygen vacancies and catalysis on ceria surfaces. Science, 309, 713−714. Cárdenas-Lizana, F., Hao, Y., Crespo-Quesada, M., Yuranov, I., Wang, X., & Keane, M. A., et al. (2013). Selective gas phase hydrogenation of p- chloronitrobenzene over Pd catalysts: role of the support. ACS Catalysis, 3, 1386−1396. Cargnello, M., Doan-Nguyen, V. V. T., Gordon, T. R., Diaz, R. E., Stach, E. A., & Gorte, R. J., et al. (2012). Control of metal nanocrystal size reveals metal-support interface role for ceria catalysts. Science, 341, 771–773. Cargnello, M., Jaén, J. J. D., Garrido, J. C. H., Bakhmutsky, K., Montini, T., & Gámez, J. J., et al. (2012). Exceptional activity for methane combustion over modular Pd@CeO2 subunits on functionalized Al2O3. Science, 337, 713– 717. Ciuparu, D., Lyubovsky, M. R., Altman, E., Pfefferle, L. D., & Datye, A. (2002). Catalytic combustion of methane over palladium-based catalysts. Catalysis Reviews: Science and Engineering, 44 , 593−649. Colussi, S., Gayen, A., Camellone, F. M., Boaro, M., Llorca, J., & Fabris, S., et al. (2009). Nanofaceted PdO sites in Pd-Ce surface superstructures: enhanced activity in catalytic combustion of methane. Angewandte Chemie
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Highlights
Dramatically higher binding energy of palladium is found to be ~1.7eV.
Partial electron transfer occurs from metal Pd to the support CeO2.
An exceptional inhibition effect against sintering and resulted dispersion decrement of Pd supported on CeO2 is evidenced.
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