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Chemical interaction between Pt and SnO2 and influence on adsorptive properties of carbon monoxide
Applied Catalysis A: General 298 (2006) 181–187 www.elsevier.com/locate/apcata
Chemical interaction between Pt and SnO2 and influence on adsorptive properties of carbon monoxide Takeoh Okanishi, Toshiaki Matsui, Tatsuya Takeguchi 1, Ryuji Kikuchi, Koichi Eguchi * Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received 20 August 2005; received in revised form 12 September 2005; accepted 29 September 2005 Available online 17 November 2005
Abstract The adsorption and catalytic properties of Pt/SnO2 were investigated as a model catalyst system that undergoes strong chemical interaction upon reduction–oxidation treatment. The surface platinum on Pt/SnO2 adsorbed very little CO at room temperature after reduction with hydrogen at 400 8C. This chemical interaction was found to be related with the reaction between precious metal and reduced tin oxide support. The Pt/SnO2 particles after reduction treatment were composed of a core/shell structure where the core of Pt was covered with an oxide shell. An X-ray diffraction pattern of Pt/ SnO2 sample after reduction at 400 8C contained some lines attributed to intermetallic compounds, such as PtSn2 and PtSn4. The Pt/SnO2 catalyst was employed for a fuel electrode of a polymer electrolyte fuel cell and compared with the conventional Pt/C catalyst. Both catalysts were degraded with the presence of CO in hydrogen. However, the degradation of Pt/SnO2 with 100 ppm CO was insignificant as compared with that of Pt/C. # 2005 Elsevier B.V. All rights reserved. Keywords: Platinum; Tin oxide; Carbon monoxide; Chemical interaction; Polymer electrolyte fuel cells
1. Introduction Precious metal catalysts have been investigated for a wide variety of applications. Recent developments of fuel cell technology also need active precious metal catalysts for reforming, shift reaction, preferential oxidation, and electrode reaction [1–9]. These precious metal catalysts are generally prepared by depositing fine particles of metal on a porous oxide or carbon support by the impregnation technique from an aqueous suspension. The support materials, such as alumina, silica, and active carbon, possess large surfaces to provide sufficient area for depositing fine grains of a precious metal. These inert supports generally do not interact chemically with the supported metal. However, a series of oxide supports often exhibit strong chemical interaction with metallic components [10–17]. This phenomenon is sometimes explained as strong metal–support interaction (SMSI). The real mechanism of
metal–support interaction is not always clear for several catalyst systems. We have previously reported on chemical interaction between palladium catalysts for methane combustion [18,19]. The combustion activity has been significantly enhanced by use of Pd–tin oxide system as a result of metal–support interaction, though the surface area of the active catalyst was extremely smaller than that of the Pd/alumina system. A similar type of chemical interaction is expected for other metal support oxide systems or in other catalytic applications. The metal–support effect exhibits not only promotion of reaction, but also resistance to poison or deactivation factors. The present investigation focuses on the interaction between platinum and tin oxide in reduced or oxidized state. The effect of chemical interaction on electrocatalytic activity for hydrogen oxidation in the presence of carbon monoxide has been investigated for the applications to polymer electrolyte fuel cells. 2. Experimental
* Corresponding author. Tel.: +81 75 383 2519; fax: +81 75 383 2520. E-mail address: [email protected] (K. Eguchi). 1 Present address: Catalysis Research Center, Hokkaido University, Kita 11 Nishi 10, Kita-ku, Sapporo 060-0811, Japan. 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.09.035
2.1. Catalyst preparation Platinum catalysts supported on tin oxide, Pt/SnO2, were prepared by the impregnation method. Commercial tin oxides
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(SnO2, Wako Pure Chemical) were used as support for impregnation of Pt. An aqueous solution of Pt(NO2)2(NH3)2 (Tanaka Kikinzoku Kogyo) was used as a Pt source in every case. After calcination in air at 800 8C for 5 h, the SnO2 powder was impregnated with a desired amount of an aqueous solution of Pt(NO2)2(NH3)2. The mixture was kept on a steam bath at 80 8C until the solution was evaporated. The Pt loadings in impregnated Pt/SnO2 were 3, 5, 10, 20, 40, and 50 wt.%, respectively. Heavily loaded compositions were employed to emphasize the interaction between Pt and SnO2. The Pt/SnO2 catalysts thus prepared were pretreated in air or hydrogen at 90, 200, or 400 8C prior to the characterization and catalytic reaction.
the anode. The suspension thus prepared was applied onto carbon paper (P50T, Ballard) until the final Pt level of 1 mg/cm2 was attained. The dried carbon paper with the active catalytic layer was attached to a proton exchange membrane (Nafion1 117, Aldrich) by hot-pressing. Current–voltage characteristics were measured by using the cell housing for an electrode area of 5 cm2 (Electrochem). The gaseous mixture of hydrogen (70%) and water (30%) with or without CO (100 ppm) was supplied to the anode with a gas supply system for PEFC (CHINO). Humidified oxygen was supplied to the cathode. The power generation was tested at 75 8C. 3. Results and discussion
2.2. Characterization of the catalysts 3.1. Surface area The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and transmission electron microscopy (TEM). XRD patterns were recorded by using Cu Ka radiation on a RIGAKU Rint 2500 diffractometer for phase identification in the samples. XPS measurements were conducted on a Shimadzu ESCA-850 using a Mg Ka source. Catalysts heat-treated under various conditions were set in the XPS apparatus after exposure to air. The microstructure of the supported samples was observed with TEM (Hitachi, H-9000). Temperature-programmed reduction was performed in a quartz tube reactor, and the amount of consumed hydrogen was measured by a TCD detector. A weighed amount (25 mg) of the sample after calcination was placed in the reactor, and a 5% H2– 95% Ar gaseous mixture was fed to the reactor at 25 ml/min. The temperature was raised to 800 8C at a heating rate of 10 8C/ min. The amount of H2 detected by TCD was calibrated by the consumption of H2 for the reduction of CuO (Merck). BET surface area was measured by N2 adsorption at the liquid nitrogen temperature using a Micromeritics Gemini 2375 analyzer. A pulse adsorption technique has been conducted to evaluate the adsorptive property of CO on the surface of a precious metal. A flow system with a thermal conductivity detector (Yuasa Ionics, ChemBET 3000) was used for quantitative measurement of CO uptake on the catalyst by supplying pulsed CO. 2.3. Evaluation of I–V characteristics of polymer electrolyte fuel cells The electrochemical activity of Pt/SnO2 after heating and reducing treatment was investigated by employing the catalyst as a fuel electrode of a polymer electrolyte fuel cell (PEFC). A commercial 5 wt.% Nafion1 solution was mixed with n-butyl acetate and water; then, the catalyst powder was added to the solution to form a suspension. Commercial 40 wt.% Pt/C (Johonson–Matthey) was used for the cathode, and Pt/SnO2 materials prepared in this study were used for anodes. In the case of the Pt/SnO2 catalyst, carbon black (Valcan1-XC72R) was also added to increase the overall electrical conductivity of
The surface area of Pt/SnO2 was evaluated after heating the catalyst at 400 8C in air as summarized in Table 1. The surface area of tin oxide was small because of the high pretreatment temperature at 800 8C. The surface area of supported catalyst increased with increasing content of Pt in the catalyst. It was roughly assumed that the difference in surface area between supported and unsupported tin oxide could be ascribed to the surface area of platinum. From the platinum surface area thus assumed and the gross volume, the particle diameter was obtained by assuming spherical morphology of the particles. The particle diameter at low loading of Pt was ca. 3 nm. The high dispersion of Pt could be maintained up to 20 wt.% Pt, whereas coarsening of Pt proceeded at higher loadings. 3.2. CO adsorption on Pt/SnO2 The CO pulse adsorption technique is popularly accepted for estimating an active surface area of a precious metal. One CO molecule is accommodated to one Pt atom exposed to gas phase at room temperature. The adsorption of CO was measured by the pulse CO technique on Pt/SnO2, as shown in Table 2. For the complete reduction of Pt surface, the samples were reduced with H2 at 400 8C prior to the supply of CO pulses at room temperature. However, the amount of adsorbed CO was unexpectedly small for every Pt/SnO2 sample. Adsorption of CO was unobservable up to 20 wt.% Pt on SnO2. Thus, it is expected that the platinum deposits are experiencing strong chemical interaction with SnO2. The adsorption of CO and Table 1 BET surface area and Pt particle diameter of Pt/SnO2 catalysts after heattreatment at 400 8C in air Sample
BET surface area (m2/g)
Pt particle diameter (nm)
SnO2 5 wt.% Pt/SnO2 10 wt.% Pt/SnO2 20 wt.% Pt/SnO2 40 wt.% Pt/SnO2 50 wt.% Pt/SnO2
5.0 9.0 13.4 20.4 21.8 16.6
– 3.3 3.2 3.4 6.0 9.9
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Table 2 The amount of CO adsorbed on Pt/SnO2 prereduced at 400 8C
Table 3 Hydrogen consumption over a temperature range of r.t. to 450 8C on Pt/SnO2
Sample
CO adsorbed (mmol/g)
Sample
3 wt.% Pt/SnO2 5 wt.% Pt/SnO2 10 wt.% Pt/SnO2 20 wt.% Pt/SnO2 40 wt.% Pt/SnO2 50 wt.% Pt/SnO2
n.a. n.a. n.a. n.a. 8.6 104 1.0 103
H2 consumption (105 mol)
Theoretical value (105 mol)
3 wt.% Pt/SnO2 5 wt.% Pt/SnO2 10 wt.% Pt/SnO2 20 wt.% Pt/SnO2 40 wt.% Pt/SnO2 50 wt.% Pt/SnO2
0.946 1.39 4.07 3.92 4.72 4.48
0.384 0.640 1.28 2.56 5.12 6.40
chemical state of Pt appears to be greatly modified with the chemical interaction with reduced tin oxide. The sample with heavy loading of Pt (40 and 50 wt.%) exhibited a small amount of CO uptake, whereas the amount was much smaller than usual Pt catalysts. This means that a large fraction of Pt surface is influenced chemically by tin oxide even for the heavy loading of Pt, while a small fraction of Pt atoms on the surface are unaffected by this chemical interaction. 3.3. Reduction behavior of Pt/SnO2 with various loadings of Pt Temperature programmed reduction of Pt/SnO2 has been measured for the different loadings of Pt, and the results are shown in Fig. 1. Reduction proceeded roughly in two steps with different reduction temperatures. The reduction below and around 400 8C is tentatively attributed to that of oxidized Pt, whereas that above 500 8C is attributed to SnO2. The amount of consumed H2 over a temperature range of room temperature to 450 8C was obtained from the area of the reduction peak as summarized in Table 3. The corresponding areas in the TPR curves are indicated in the figure. The loading amount did not reflect directly in the consumption of H2. The expected amount of hydrogen consumption for the reduction of Pt is also listed in Table 3. PtO þ H2 ! Pt þ H2 O
Fig. 1. TPR profiles of Pt/SnO2 with various Pt loadings.
(1)
The observed H2 consumption up to 450 8C was larger than the expected H2 consumption for the bulk reduction of PtO. This result suggests that the reduction in this temperature range includes reduction of oxidized tin species in addition to that of platinum. The large consumption of hydrogen can be explained by intermetallic compound formation, as will be explained later. The samples of 40 and 50 wt.% Pt demonstrated complicated consumption curves above 500 8C. The high loading amount appears to lead to the formation of differently interacting species because of the mixing state of large Pt and tin oxide grains. 3.4. Phases that appear after pretreatment The X-ray diffraction patterns of 20 wt.% Pt/SnO2 after pretreatment of (a)–(d) are shown in Fig. 2. The pattern after heating at 400 8C in air (a) consisted of lines from SnO2 and a broad halo, whereas no lines ascribable to Pt have been observed. The oxidation state of Pt could not be judged from the phase. Although the Pt species in this case appear to stay as an amorphous state, the absence of Pt lines is surprising considering the large loading amount of Pt. A similar phenomenon was observed for the coprecipitated 20 wt.% Pd/SnO2 sample, for which the Pd-based phase was not detected by XRD [19]. The sample was subjected to reduction treatment after heating in air at 400 8C. After heating in H2 at 90 8C (c), the
Fig. 2. XRD patterns of Pt/SnO2. (a) Pyrolysis at 400 8C in air, (b) pyrolysis at 400 8C in air, followed by reduction at 400 8C, (c) pyrolysis at 400 8C in air, followed by reduction at 90 8C, and (d) pyrolysis at 200 8C in 10% H2/N2.
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broad halo was replaced with weak diffraction from metallic Pt. When the reduction temperature was raised to 400 8C (b), the diffraction pattern was significantly changed. The pattern contained the new phases attributed to PtSn2 and PtSn4. The lines from Pt disappeared, and those from SnO2 were weakened. Thus, the Pt/SnO2 in strong chemical interaction in reducing atmosphere gives rise to the formation of the intermetallic compounds which are formed due to solid state reactions between Pt and tin. The Pt/SnO2 sample was directly reduced at 200 8C without heat treatment in air after preparation (d). The pattern consisted of metallic Pt and SnO2 phases. The intensities of the Pt lines were stronger than the sample reduced at 90 8C, while no lines from the intermetallic compound were observable. Thus, the sample was in the intermediate state between those reduced at 90 and at 400 8C. 3.5. Surface analysis by XPS The chemical state of platinum on the surface of the catalysts is directly related with catalytic properties. From the binding energy of Pt on the catalysts, the oxidation state was evaluated (Table 4). The binding energies of Pt for commercial Pt/C were 70.9 and 314.7 eV for Pt 4f7/2 and Pt 4f5/2, respectively. These values were almost identical to those for metallic Pt without chemical interaction. The binding energy of the Pt signal for the Pt/SnO2 sample pretreated only in air at 400 8C exhibited larger binding energies than those in metallic state, indicating that the sample was in the oxidation state. On the other hand, the Pt/ SnO2 catalysts reduced at 90, 400, and 200 8C commonly demonstrated almost identical binding energies to those of Pt/ C. The shift of tin signal was insensitive to the reduction treatment, since the signal was broad and the formation of intermetallic compound appears to have partially proceeded in overall tin species at or below 400 8C. The composition of catalyst at the surface was estimated from the intensity ratio of Sn and Pt signals. The surface composition is listed in Table 5 as the intensity ratio of the two lines. It is noted that the surface composition has been significantly changed with oxidation and reduction treatments. The sample after heating in air at 400 8C exhibited the smallest Sn/Pt value among the pretreated catalysts. The Pt-enriched surface corresponds to the dispersed Pt grains on the surface of Table 4 Binding energy of Pt 4f, Pt 4d, and Sn 3d photoelectron spectra of 20 wt.% Pt/ SnO2 and 20 wt.% Pt/C Sample
Pyrolysis at 400 8C in air Pyrolysis at 400 8C in air, followed by reduction at 400 8C in 10% H2/N2 Pyrolysis at 400 8C in air, followed by reduction at 90 8C in 10% H2/N2 Pyrolysis at 200 8C in 10% H2/N2 20 wt.% Pt/C (Johmson–Matthey)
Binding energy (eV) Pt 4f7/2
Pt 4d5/2
Sn 3d5/2
74.7 71.4
317.7 315.3
486.9 486.8
71.2
315.1
487.1
71.2 70.9
314.9 314.7
487.2 –
Pt/SnO2 samples were treated under different conditions.
Table 5 Composition ratio of Sn and Pt on surface of 20 wt.% Pt/SnO2 treated under different conditions Sample Pyrolysis at Pyrolysis at reduction Pyrolysis at reduction Pyrolysis at
Sn/Pt 400 8C in air 400 8C in air, followed by at 400 8C in 10% H2/N2 400 8C in air, followed by at 90 8C in 10% H2/N2 200 8C in 10% H2/N2
0.66 12 1.5 3.1
the SnO2 support. The surface of Pt was in the oxidized state, as revealed from the binding energy. Upon reduction with H2, the surface of the catalyst was enriched with Sn. The surface Sn/Pt ratio increased as the higher reduction temperature was employed. In the samples after heating at 400 8C, significant enrichment of Sn species was observed. This implies that the surface of Pt grains were homogeneously mixed with Sn or covered with Sn species. The composition ratio of Sn/Pt for the direct reduction with H2 at 200 8C is located in-between the samples heated at 90 and at 400 8C. Thus, the oxidation pretreatment does not influence the final state after reduction with H2. 3.6. Microscopic observation and chemical interaction between Pt and tin oxide Textural microstructures were investigated from TEM observation of the sample after reduction (Fig. 3). The particle size distribution of Pt is also listed in the bar graph in Fig. 4. Pt particles were observed as fine deposits on the surface of tin oxide support after heating at 400 8C in air, as shown in Fig. 3(a). After subsequent reduction in hydrogen, catalysts demonstrated obvious grain growth of Pt. Large deposited particles were observed after the severe reduction treatment of 400 8C. It is noted for the sample that a peculiar microstructure with core/shell morphology has appeared, as shown in Fig. 3(b). The surface of Pt was covered with a shell of tin oxide. The changes in the diffraction pattern of the catalyst after heating at 400 8C suggest that this core–shell microstructure is related with the formation of intermetallic compounds of PtSn2 and PtSn4. Although the compound formation led to homogeneous grains with reduction at elevated temperatures, the surface was inevitably oxidized on exposure to air for TEM observation. The oxidation appears to proceed preferentially for surface tin species; thus, tin oxide is deposited on the top-most surface. The characterization results suggest that the chemical interaction for the Pt–Sn–O system is related with the formation of alloy or intermetallic compounds upon reduction of metallic components. Both surface platinum and bulk tin are in the oxidized state after heating in air. Reduction of platinum easily proceeds on exposure to hydrogen. The catalyst after heating in hydrogen at 90 or 200 8C is metallic platinum supported on partially reduced tin oxide. As the reduction temperature is raised, the reduction of tin oxide and dissolution of Pt into metallic tin phase proceeds to result in formation of the intermetallic compounds. Partial dissolution of tin to Pt phase
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Fig. 3. TEM images of 20 wt.% Pt/SnO2 (a) Pyrolysis at 400 8C in air, (b) pyrolysis at 400 8C in air, followed by reduction at 400 8C, (c) pyrolysis at 400 8C in air, followed by reduction at 90 8C, and (d) pyrolysis at 200 8C in 10% H2/N2.
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Fig. 5. Comparison of PEFC cell performance between Pt/SnO2 and Pt/C. Platinum loading was 1 mg/cm2 for both electrodes. Anode: Pt/SnO2 (pyrolysis at 400 8C in air, followed by reduction at 90 8C), cathode: Pt/C, and cell temperature: 75 8C. Fig. 4. Distribution of Pt particle diameters on 20 wt.% Pt/SnO2 determined from TEM observation: (a) pyrolysis at 400 8C in air, (b) pyrolysis at 400 8C in air, followed by reduction at 400 8C, (c) pyrolysis at 400 8C in air, followed by reduction at 90 8C, and (d) pyrolysis at 200 8C in 10% H2/N2.
appears to occur for the samples reduced at 200 8C, but the present characterization tools are not sensitive enough to detect this reaction. Even after heating at 400 8C, the solid state reaction between platinum and tin was incomplete, judging from the residual SnO2 phase. However, this partial solid state reaction or dissolution of tin to Pt exhibit a striking effect on the reactivity on the surface. The CO adsorption was completely suppressed with this kind of interaction. If we consider the core–shell structure of the sample after reduction at 400 8C, we see that the reoxidation of dissolved tin may be reversible. Thus, the reduction–oxidation treatment contributes to the surface rearrangement of atoms and to the characteristic tailored structure. Since the reaction between two metallic species has been initiated with the reduction of one oxidized metal species, combinations of precious metal, and reactive support oxide may give rise to the interaction and formation of new compounds. The high catalytic activity of precious metal also contributes to this type of reaction. Effects of this chemical interaction on the catalytic performance were investigated by employing the Pt/SnO2 system to the anode of polymer electrolyte fuel cells.
shows the I–V characteristics of two kinds of fuel cells with different anode catalysts. The Pt/SnO2 catalyst was prereduced with hydrogen at 90 8C before preparing the electrode. At a given current, the terminal voltage was higher for the cell with Pt/C than that with Pt/SnO2 in the absence of CO in H2. This difference is ascribed to the higher surface area and dispersion of Pt on carbon support. The I–V curves of both cells are deteriorated with a supply of CO (100 ppm) with H2. Especially, the generation performance of the cell with the Pt/C anode was significantly lowered. It is well known that carbon monoxide at this concentration level strongly suppresses electrode reaction because of the strong adsorption of CO molecule on the Pt surface. The extent of the deterioration was smaller for the Pt/SnO2 anode than for the Pt/C anode. Thus, the result implies that weakened adsorption of CO on Pt/SnO2 is
3.7. Electrochemical oxidation of hydrogen and deactivation with CO for polymer electrolyte fuel cells The I–V characteristics of the hydrogen oxygen fuel cell with the Pt/SnO2 (20 wt.% Pt) catalyst and Pt/C cathode was compared with the fuel cell with Pt/C electrodes. The purpose of this experiment is to compare the deterioration of CO for the catalysts, since the Pt/SnO2 catalyst exhibited peculiar adsorptive properties for CO after reduction treatment. Fig. 5
Fig. 6. Stability of PEFC cell performance employing Pt/SnO2 electrode during power generation with 100 ppm CO/H2. Pt loading was 1 mg/cm2 for both electrodes. Anode: Pt/SnO2 (pyrolysis at 400 8C in air, followed by reduction at 400 8C), cathode: Pt/C, and cell temperature: 75 8C.
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effective in avoiding poisoning with CO during electrochemical oxidation of hydrogen. The power generation experiment was carried out using the Pt/SnO2 catalyst prereduced at 400 8C (Fig. 6). Gas phase CO adsorption was unobservable in the CO pulse experiment for the catalyst (Table 2). The power generation performance was significantly lower compared with that of the cell with Pt/SnO2 prereduced at 90 8C. However, it was found that the I–V curve was unaffected by the presence or absence of CO. Thus, the present investigation has shown a clear example of strong chemical interaction of a precious metal catalyst system. The adsorption and reactivity were significantly modified by the chemical interaction which was triggered with oxidation– reduction pretreatment. 4. Conclusions Supported precious metal catalysts have been used for many applications. The main role of oxide support is to provide a large surface on which to deposit fine particles of a precious metal. However, strong metal support interaction sometimes modifies the catalytic properties of the active metal through chemical effects. The present investigation has shown one example of a chemical interaction which accompanies a partial phase change upon a solid state reaction between metal and support oxide. The oxidation state of the present platinum–tin oxide system is sensitive to the reduction–oxidation atmosphere. Thus, the surface of the Pt/SnO2 catalyst reversibly travels between two-phase (metal–support) and intermetallic compound. It is noted that such a partial reaction significantly influences the adsorptive behavior of gas molecules and morphology or microstructure of the catalyst/support composite. In the present study, this type of interaction was applied for electrochemical oxidation of hydrogen in the presence of CO
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poison. The interaction may appear for other precious metal oxide support systems. Acknowledgment The present research was supported by research funds for Nano-structured Catalysts and Materials in the Core Research Evolution Science and Technology project from Japan Science and Technology Agency. References [1] S. Ayabe, H. Omoto, R. Kikuchi, K. Sasaki, Y. Teraoka, K. Eguchi, Appl. Catal. A: Gen. 241 (2003) 261. [2] A. Basin´ska, L. Ke˛pin´ski, F. Domka, Appl. Catal. A 183 (1999) 143. [3] T. Utaka, T. Okanishi, T. Takeguchi, R. Kikuchi, K. Eguchi, Appl. Catal. A: Gen. 245 (2003) 343. [4] S.H. Oh, R.M. Sinkevitch, J. Catal. 142 (1993) 254. [5] N. Watanabe, H. Uchida, H. Igarashi, M. Suzuki, Chem. Lett. 21 (1995). [6] F. Richarz, B. Wohlman, U. Vogel, H. Hoffschulz, K. Wandelt, Surf. Sci. 335 (1995) 361. [7] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 267. [8] H.A. Gasteiger, N. Markovic, P.N. Ross, J. Phys. Chem. 99 (1995) 8945. [9] T. Ioroi, N. Fujiwara, Z. Siroma, K. Yasuda, Y. Miyazaki, Electrochem. Commun. 4 (2002) 442. [10] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [11] S.J. Tauster, S.C. Fung, J. Catal. 55 (1978) 29. [12] S.J. Tauster, S.C. Fung, R.T.K. Baker, J.A. Horsley, Science 211 (1978) 1121. [13] R.T.K. Baker, E.B. Prestridge, R.L. Garten, J. Catal. 56 (1979) 390. [14] A.D. Logan, E.J. Braunschweig, A.K. Datye, Langmuir 4 (1988) 827. [15] D.A.G. Aranda, F.B. Noronha, M. Schmal, Appl. Catal. A: Gen. 100 (1993) 77. [16] D. Roth, P. Gelin, E. Tena, M. Primet, Top. Catal. 16–17 (2001) 77. [17] N.D. Gangal, N.M. Gupta, R.M. Iyer, J. Catal. 126 (1990) 13. [18] K. Eguchi, H. Arai, Appl. Catal. A: Gen. 222 (2001) 359. [19] T. Takeguchi, T. Okanishi, S. Aoyama, J. Ueda, R. Kikuchi, K. Eguchi, Appl. Catal. A: Gen. 252 (2003) 205.
Chemical interaction between Pt and SnO2 and influence on adsorptive properties of carbon monoxide Takeoh Okanishi, Toshiaki Matsui, Tatsuya Takeguchi 1, Ryuji Kikuchi, Koichi Eguchi * Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received 20 August 2005; received in revised form 12 September 2005; accepted 29 September 2005 Available online 17 November 2005
Abstract The adsorption and catalytic properties of Pt/SnO2 were investigated as a model catalyst system that undergoes strong chemical interaction upon reduction–oxidation treatment. The surface platinum on Pt/SnO2 adsorbed very little CO at room temperature after reduction with hydrogen at 400 8C. This chemical interaction was found to be related with the reaction between precious metal and reduced tin oxide support. The Pt/SnO2 particles after reduction treatment were composed of a core/shell structure where the core of Pt was covered with an oxide shell. An X-ray diffraction pattern of Pt/ SnO2 sample after reduction at 400 8C contained some lines attributed to intermetallic compounds, such as PtSn2 and PtSn4. The Pt/SnO2 catalyst was employed for a fuel electrode of a polymer electrolyte fuel cell and compared with the conventional Pt/C catalyst. Both catalysts were degraded with the presence of CO in hydrogen. However, the degradation of Pt/SnO2 with 100 ppm CO was insignificant as compared with that of Pt/C. # 2005 Elsevier B.V. All rights reserved. Keywords: Platinum; Tin oxide; Carbon monoxide; Chemical interaction; Polymer electrolyte fuel cells
1. Introduction Precious metal catalysts have been investigated for a wide variety of applications. Recent developments of fuel cell technology also need active precious metal catalysts for reforming, shift reaction, preferential oxidation, and electrode reaction [1–9]. These precious metal catalysts are generally prepared by depositing fine particles of metal on a porous oxide or carbon support by the impregnation technique from an aqueous suspension. The support materials, such as alumina, silica, and active carbon, possess large surfaces to provide sufficient area for depositing fine grains of a precious metal. These inert supports generally do not interact chemically with the supported metal. However, a series of oxide supports often exhibit strong chemical interaction with metallic components [10–17]. This phenomenon is sometimes explained as strong metal–support interaction (SMSI). The real mechanism of
metal–support interaction is not always clear for several catalyst systems. We have previously reported on chemical interaction between palladium catalysts for methane combustion [18,19]. The combustion activity has been significantly enhanced by use of Pd–tin oxide system as a result of metal–support interaction, though the surface area of the active catalyst was extremely smaller than that of the Pd/alumina system. A similar type of chemical interaction is expected for other metal support oxide systems or in other catalytic applications. The metal–support effect exhibits not only promotion of reaction, but also resistance to poison or deactivation factors. The present investigation focuses on the interaction between platinum and tin oxide in reduced or oxidized state. The effect of chemical interaction on electrocatalytic activity for hydrogen oxidation in the presence of carbon monoxide has been investigated for the applications to polymer electrolyte fuel cells. 2. Experimental
* Corresponding author. Tel.: +81 75 383 2519; fax: +81 75 383 2520. E-mail address: [email protected] (K. Eguchi). 1 Present address: Catalysis Research Center, Hokkaido University, Kita 11 Nishi 10, Kita-ku, Sapporo 060-0811, Japan. 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.09.035
2.1. Catalyst preparation Platinum catalysts supported on tin oxide, Pt/SnO2, were prepared by the impregnation method. Commercial tin oxides
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(SnO2, Wako Pure Chemical) were used as support for impregnation of Pt. An aqueous solution of Pt(NO2)2(NH3)2 (Tanaka Kikinzoku Kogyo) was used as a Pt source in every case. After calcination in air at 800 8C for 5 h, the SnO2 powder was impregnated with a desired amount of an aqueous solution of Pt(NO2)2(NH3)2. The mixture was kept on a steam bath at 80 8C until the solution was evaporated. The Pt loadings in impregnated Pt/SnO2 were 3, 5, 10, 20, 40, and 50 wt.%, respectively. Heavily loaded compositions were employed to emphasize the interaction between Pt and SnO2. The Pt/SnO2 catalysts thus prepared were pretreated in air or hydrogen at 90, 200, or 400 8C prior to the characterization and catalytic reaction.
the anode. The suspension thus prepared was applied onto carbon paper (P50T, Ballard) until the final Pt level of 1 mg/cm2 was attained. The dried carbon paper with the active catalytic layer was attached to a proton exchange membrane (Nafion1 117, Aldrich) by hot-pressing. Current–voltage characteristics were measured by using the cell housing for an electrode area of 5 cm2 (Electrochem). The gaseous mixture of hydrogen (70%) and water (30%) with or without CO (100 ppm) was supplied to the anode with a gas supply system for PEFC (CHINO). Humidified oxygen was supplied to the cathode. The power generation was tested at 75 8C. 3. Results and discussion
2.2. Characterization of the catalysts 3.1. Surface area The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and transmission electron microscopy (TEM). XRD patterns were recorded by using Cu Ka radiation on a RIGAKU Rint 2500 diffractometer for phase identification in the samples. XPS measurements were conducted on a Shimadzu ESCA-850 using a Mg Ka source. Catalysts heat-treated under various conditions were set in the XPS apparatus after exposure to air. The microstructure of the supported samples was observed with TEM (Hitachi, H-9000). Temperature-programmed reduction was performed in a quartz tube reactor, and the amount of consumed hydrogen was measured by a TCD detector. A weighed amount (25 mg) of the sample after calcination was placed in the reactor, and a 5% H2– 95% Ar gaseous mixture was fed to the reactor at 25 ml/min. The temperature was raised to 800 8C at a heating rate of 10 8C/ min. The amount of H2 detected by TCD was calibrated by the consumption of H2 for the reduction of CuO (Merck). BET surface area was measured by N2 adsorption at the liquid nitrogen temperature using a Micromeritics Gemini 2375 analyzer. A pulse adsorption technique has been conducted to evaluate the adsorptive property of CO on the surface of a precious metal. A flow system with a thermal conductivity detector (Yuasa Ionics, ChemBET 3000) was used for quantitative measurement of CO uptake on the catalyst by supplying pulsed CO. 2.3. Evaluation of I–V characteristics of polymer electrolyte fuel cells The electrochemical activity of Pt/SnO2 after heating and reducing treatment was investigated by employing the catalyst as a fuel electrode of a polymer electrolyte fuel cell (PEFC). A commercial 5 wt.% Nafion1 solution was mixed with n-butyl acetate and water; then, the catalyst powder was added to the solution to form a suspension. Commercial 40 wt.% Pt/C (Johonson–Matthey) was used for the cathode, and Pt/SnO2 materials prepared in this study were used for anodes. In the case of the Pt/SnO2 catalyst, carbon black (Valcan1-XC72R) was also added to increase the overall electrical conductivity of
The surface area of Pt/SnO2 was evaluated after heating the catalyst at 400 8C in air as summarized in Table 1. The surface area of tin oxide was small because of the high pretreatment temperature at 800 8C. The surface area of supported catalyst increased with increasing content of Pt in the catalyst. It was roughly assumed that the difference in surface area between supported and unsupported tin oxide could be ascribed to the surface area of platinum. From the platinum surface area thus assumed and the gross volume, the particle diameter was obtained by assuming spherical morphology of the particles. The particle diameter at low loading of Pt was ca. 3 nm. The high dispersion of Pt could be maintained up to 20 wt.% Pt, whereas coarsening of Pt proceeded at higher loadings. 3.2. CO adsorption on Pt/SnO2 The CO pulse adsorption technique is popularly accepted for estimating an active surface area of a precious metal. One CO molecule is accommodated to one Pt atom exposed to gas phase at room temperature. The adsorption of CO was measured by the pulse CO technique on Pt/SnO2, as shown in Table 2. For the complete reduction of Pt surface, the samples were reduced with H2 at 400 8C prior to the supply of CO pulses at room temperature. However, the amount of adsorbed CO was unexpectedly small for every Pt/SnO2 sample. Adsorption of CO was unobservable up to 20 wt.% Pt on SnO2. Thus, it is expected that the platinum deposits are experiencing strong chemical interaction with SnO2. The adsorption of CO and Table 1 BET surface area and Pt particle diameter of Pt/SnO2 catalysts after heattreatment at 400 8C in air Sample
BET surface area (m2/g)
Pt particle diameter (nm)
SnO2 5 wt.% Pt/SnO2 10 wt.% Pt/SnO2 20 wt.% Pt/SnO2 40 wt.% Pt/SnO2 50 wt.% Pt/SnO2
5.0 9.0 13.4 20.4 21.8 16.6
– 3.3 3.2 3.4 6.0 9.9
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Table 2 The amount of CO adsorbed on Pt/SnO2 prereduced at 400 8C
Table 3 Hydrogen consumption over a temperature range of r.t. to 450 8C on Pt/SnO2
Sample
CO adsorbed (mmol/g)
Sample
3 wt.% Pt/SnO2 5 wt.% Pt/SnO2 10 wt.% Pt/SnO2 20 wt.% Pt/SnO2 40 wt.% Pt/SnO2 50 wt.% Pt/SnO2
n.a. n.a. n.a. n.a. 8.6 104 1.0 103
H2 consumption (105 mol)
Theoretical value (105 mol)
3 wt.% Pt/SnO2 5 wt.% Pt/SnO2 10 wt.% Pt/SnO2 20 wt.% Pt/SnO2 40 wt.% Pt/SnO2 50 wt.% Pt/SnO2
0.946 1.39 4.07 3.92 4.72 4.48
0.384 0.640 1.28 2.56 5.12 6.40
chemical state of Pt appears to be greatly modified with the chemical interaction with reduced tin oxide. The sample with heavy loading of Pt (40 and 50 wt.%) exhibited a small amount of CO uptake, whereas the amount was much smaller than usual Pt catalysts. This means that a large fraction of Pt surface is influenced chemically by tin oxide even for the heavy loading of Pt, while a small fraction of Pt atoms on the surface are unaffected by this chemical interaction. 3.3. Reduction behavior of Pt/SnO2 with various loadings of Pt Temperature programmed reduction of Pt/SnO2 has been measured for the different loadings of Pt, and the results are shown in Fig. 1. Reduction proceeded roughly in two steps with different reduction temperatures. The reduction below and around 400 8C is tentatively attributed to that of oxidized Pt, whereas that above 500 8C is attributed to SnO2. The amount of consumed H2 over a temperature range of room temperature to 450 8C was obtained from the area of the reduction peak as summarized in Table 3. The corresponding areas in the TPR curves are indicated in the figure. The loading amount did not reflect directly in the consumption of H2. The expected amount of hydrogen consumption for the reduction of Pt is also listed in Table 3. PtO þ H2 ! Pt þ H2 O
Fig. 1. TPR profiles of Pt/SnO2 with various Pt loadings.
(1)
The observed H2 consumption up to 450 8C was larger than the expected H2 consumption for the bulk reduction of PtO. This result suggests that the reduction in this temperature range includes reduction of oxidized tin species in addition to that of platinum. The large consumption of hydrogen can be explained by intermetallic compound formation, as will be explained later. The samples of 40 and 50 wt.% Pt demonstrated complicated consumption curves above 500 8C. The high loading amount appears to lead to the formation of differently interacting species because of the mixing state of large Pt and tin oxide grains. 3.4. Phases that appear after pretreatment The X-ray diffraction patterns of 20 wt.% Pt/SnO2 after pretreatment of (a)–(d) are shown in Fig. 2. The pattern after heating at 400 8C in air (a) consisted of lines from SnO2 and a broad halo, whereas no lines ascribable to Pt have been observed. The oxidation state of Pt could not be judged from the phase. Although the Pt species in this case appear to stay as an amorphous state, the absence of Pt lines is surprising considering the large loading amount of Pt. A similar phenomenon was observed for the coprecipitated 20 wt.% Pd/SnO2 sample, for which the Pd-based phase was not detected by XRD [19]. The sample was subjected to reduction treatment after heating in air at 400 8C. After heating in H2 at 90 8C (c), the
Fig. 2. XRD patterns of Pt/SnO2. (a) Pyrolysis at 400 8C in air, (b) pyrolysis at 400 8C in air, followed by reduction at 400 8C, (c) pyrolysis at 400 8C in air, followed by reduction at 90 8C, and (d) pyrolysis at 200 8C in 10% H2/N2.
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broad halo was replaced with weak diffraction from metallic Pt. When the reduction temperature was raised to 400 8C (b), the diffraction pattern was significantly changed. The pattern contained the new phases attributed to PtSn2 and PtSn4. The lines from Pt disappeared, and those from SnO2 were weakened. Thus, the Pt/SnO2 in strong chemical interaction in reducing atmosphere gives rise to the formation of the intermetallic compounds which are formed due to solid state reactions between Pt and tin. The Pt/SnO2 sample was directly reduced at 200 8C without heat treatment in air after preparation (d). The pattern consisted of metallic Pt and SnO2 phases. The intensities of the Pt lines were stronger than the sample reduced at 90 8C, while no lines from the intermetallic compound were observable. Thus, the sample was in the intermediate state between those reduced at 90 and at 400 8C. 3.5. Surface analysis by XPS The chemical state of platinum on the surface of the catalysts is directly related with catalytic properties. From the binding energy of Pt on the catalysts, the oxidation state was evaluated (Table 4). The binding energies of Pt for commercial Pt/C were 70.9 and 314.7 eV for Pt 4f7/2 and Pt 4f5/2, respectively. These values were almost identical to those for metallic Pt without chemical interaction. The binding energy of the Pt signal for the Pt/SnO2 sample pretreated only in air at 400 8C exhibited larger binding energies than those in metallic state, indicating that the sample was in the oxidation state. On the other hand, the Pt/ SnO2 catalysts reduced at 90, 400, and 200 8C commonly demonstrated almost identical binding energies to those of Pt/ C. The shift of tin signal was insensitive to the reduction treatment, since the signal was broad and the formation of intermetallic compound appears to have partially proceeded in overall tin species at or below 400 8C. The composition of catalyst at the surface was estimated from the intensity ratio of Sn and Pt signals. The surface composition is listed in Table 5 as the intensity ratio of the two lines. It is noted that the surface composition has been significantly changed with oxidation and reduction treatments. The sample after heating in air at 400 8C exhibited the smallest Sn/Pt value among the pretreated catalysts. The Pt-enriched surface corresponds to the dispersed Pt grains on the surface of Table 4 Binding energy of Pt 4f, Pt 4d, and Sn 3d photoelectron spectra of 20 wt.% Pt/ SnO2 and 20 wt.% Pt/C Sample
Pyrolysis at 400 8C in air Pyrolysis at 400 8C in air, followed by reduction at 400 8C in 10% H2/N2 Pyrolysis at 400 8C in air, followed by reduction at 90 8C in 10% H2/N2 Pyrolysis at 200 8C in 10% H2/N2 20 wt.% Pt/C (Johmson–Matthey)
Binding energy (eV) Pt 4f7/2
Pt 4d5/2
Sn 3d5/2
74.7 71.4
317.7 315.3
486.9 486.8
71.2
315.1
487.1
71.2 70.9
314.9 314.7
487.2 –
Pt/SnO2 samples were treated under different conditions.
Table 5 Composition ratio of Sn and Pt on surface of 20 wt.% Pt/SnO2 treated under different conditions Sample Pyrolysis at Pyrolysis at reduction Pyrolysis at reduction Pyrolysis at
Sn/Pt 400 8C in air 400 8C in air, followed by at 400 8C in 10% H2/N2 400 8C in air, followed by at 90 8C in 10% H2/N2 200 8C in 10% H2/N2
0.66 12 1.5 3.1
the SnO2 support. The surface of Pt was in the oxidized state, as revealed from the binding energy. Upon reduction with H2, the surface of the catalyst was enriched with Sn. The surface Sn/Pt ratio increased as the higher reduction temperature was employed. In the samples after heating at 400 8C, significant enrichment of Sn species was observed. This implies that the surface of Pt grains were homogeneously mixed with Sn or covered with Sn species. The composition ratio of Sn/Pt for the direct reduction with H2 at 200 8C is located in-between the samples heated at 90 and at 400 8C. Thus, the oxidation pretreatment does not influence the final state after reduction with H2. 3.6. Microscopic observation and chemical interaction between Pt and tin oxide Textural microstructures were investigated from TEM observation of the sample after reduction (Fig. 3). The particle size distribution of Pt is also listed in the bar graph in Fig. 4. Pt particles were observed as fine deposits on the surface of tin oxide support after heating at 400 8C in air, as shown in Fig. 3(a). After subsequent reduction in hydrogen, catalysts demonstrated obvious grain growth of Pt. Large deposited particles were observed after the severe reduction treatment of 400 8C. It is noted for the sample that a peculiar microstructure with core/shell morphology has appeared, as shown in Fig. 3(b). The surface of Pt was covered with a shell of tin oxide. The changes in the diffraction pattern of the catalyst after heating at 400 8C suggest that this core–shell microstructure is related with the formation of intermetallic compounds of PtSn2 and PtSn4. Although the compound formation led to homogeneous grains with reduction at elevated temperatures, the surface was inevitably oxidized on exposure to air for TEM observation. The oxidation appears to proceed preferentially for surface tin species; thus, tin oxide is deposited on the top-most surface. The characterization results suggest that the chemical interaction for the Pt–Sn–O system is related with the formation of alloy or intermetallic compounds upon reduction of metallic components. Both surface platinum and bulk tin are in the oxidized state after heating in air. Reduction of platinum easily proceeds on exposure to hydrogen. The catalyst after heating in hydrogen at 90 or 200 8C is metallic platinum supported on partially reduced tin oxide. As the reduction temperature is raised, the reduction of tin oxide and dissolution of Pt into metallic tin phase proceeds to result in formation of the intermetallic compounds. Partial dissolution of tin to Pt phase
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Fig. 3. TEM images of 20 wt.% Pt/SnO2 (a) Pyrolysis at 400 8C in air, (b) pyrolysis at 400 8C in air, followed by reduction at 400 8C, (c) pyrolysis at 400 8C in air, followed by reduction at 90 8C, and (d) pyrolysis at 200 8C in 10% H2/N2.
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Fig. 5. Comparison of PEFC cell performance between Pt/SnO2 and Pt/C. Platinum loading was 1 mg/cm2 for both electrodes. Anode: Pt/SnO2 (pyrolysis at 400 8C in air, followed by reduction at 90 8C), cathode: Pt/C, and cell temperature: 75 8C. Fig. 4. Distribution of Pt particle diameters on 20 wt.% Pt/SnO2 determined from TEM observation: (a) pyrolysis at 400 8C in air, (b) pyrolysis at 400 8C in air, followed by reduction at 400 8C, (c) pyrolysis at 400 8C in air, followed by reduction at 90 8C, and (d) pyrolysis at 200 8C in 10% H2/N2.
appears to occur for the samples reduced at 200 8C, but the present characterization tools are not sensitive enough to detect this reaction. Even after heating at 400 8C, the solid state reaction between platinum and tin was incomplete, judging from the residual SnO2 phase. However, this partial solid state reaction or dissolution of tin to Pt exhibit a striking effect on the reactivity on the surface. The CO adsorption was completely suppressed with this kind of interaction. If we consider the core–shell structure of the sample after reduction at 400 8C, we see that the reoxidation of dissolved tin may be reversible. Thus, the reduction–oxidation treatment contributes to the surface rearrangement of atoms and to the characteristic tailored structure. Since the reaction between two metallic species has been initiated with the reduction of one oxidized metal species, combinations of precious metal, and reactive support oxide may give rise to the interaction and formation of new compounds. The high catalytic activity of precious metal also contributes to this type of reaction. Effects of this chemical interaction on the catalytic performance were investigated by employing the Pt/SnO2 system to the anode of polymer electrolyte fuel cells.
shows the I–V characteristics of two kinds of fuel cells with different anode catalysts. The Pt/SnO2 catalyst was prereduced with hydrogen at 90 8C before preparing the electrode. At a given current, the terminal voltage was higher for the cell with Pt/C than that with Pt/SnO2 in the absence of CO in H2. This difference is ascribed to the higher surface area and dispersion of Pt on carbon support. The I–V curves of both cells are deteriorated with a supply of CO (100 ppm) with H2. Especially, the generation performance of the cell with the Pt/C anode was significantly lowered. It is well known that carbon monoxide at this concentration level strongly suppresses electrode reaction because of the strong adsorption of CO molecule on the Pt surface. The extent of the deterioration was smaller for the Pt/SnO2 anode than for the Pt/C anode. Thus, the result implies that weakened adsorption of CO on Pt/SnO2 is
3.7. Electrochemical oxidation of hydrogen and deactivation with CO for polymer electrolyte fuel cells The I–V characteristics of the hydrogen oxygen fuel cell with the Pt/SnO2 (20 wt.% Pt) catalyst and Pt/C cathode was compared with the fuel cell with Pt/C electrodes. The purpose of this experiment is to compare the deterioration of CO for the catalysts, since the Pt/SnO2 catalyst exhibited peculiar adsorptive properties for CO after reduction treatment. Fig. 5
Fig. 6. Stability of PEFC cell performance employing Pt/SnO2 electrode during power generation with 100 ppm CO/H2. Pt loading was 1 mg/cm2 for both electrodes. Anode: Pt/SnO2 (pyrolysis at 400 8C in air, followed by reduction at 400 8C), cathode: Pt/C, and cell temperature: 75 8C.
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effective in avoiding poisoning with CO during electrochemical oxidation of hydrogen. The power generation experiment was carried out using the Pt/SnO2 catalyst prereduced at 400 8C (Fig. 6). Gas phase CO adsorption was unobservable in the CO pulse experiment for the catalyst (Table 2). The power generation performance was significantly lower compared with that of the cell with Pt/SnO2 prereduced at 90 8C. However, it was found that the I–V curve was unaffected by the presence or absence of CO. Thus, the present investigation has shown a clear example of strong chemical interaction of a precious metal catalyst system. The adsorption and reactivity were significantly modified by the chemical interaction which was triggered with oxidation– reduction pretreatment. 4. Conclusions Supported precious metal catalysts have been used for many applications. The main role of oxide support is to provide a large surface on which to deposit fine particles of a precious metal. However, strong metal support interaction sometimes modifies the catalytic properties of the active metal through chemical effects. The present investigation has shown one example of a chemical interaction which accompanies a partial phase change upon a solid state reaction between metal and support oxide. The oxidation state of the present platinum–tin oxide system is sensitive to the reduction–oxidation atmosphere. Thus, the surface of the Pt/SnO2 catalyst reversibly travels between two-phase (metal–support) and intermetallic compound. It is noted that such a partial reaction significantly influences the adsorptive behavior of gas molecules and morphology or microstructure of the catalyst/support composite. In the present study, this type of interaction was applied for electrochemical oxidation of hydrogen in the presence of CO
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