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γ-Al2O3 catalyst
Applied Catalysis B: Environmental 23 (1999) 159–167
Effect of the addition of Fe on catalytic activities of Pt/Fe/␥-Al2 O3 catalyst Y. Sakamoto ∗ , K. Higuchi, N. Takahashi, K. Yokota, H. Doi, M. Sugiura Toyota Central Research & Development Labs., Inc. Nagakute, Aichi, 480-1192, Japan Received 1 March 1999; received in revised form 17 June 1999; accepted 18 June 1999
Abstract The catalytic activities of stoichiometric and lean mixture simulated exhaust from automotive engines have been investigated on bimetallic Pt/Fe/Al2 O3 catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments and compared to that of Pt/Al2 O3 catalysts. The state of the Pt particles of the Pt/Fe/Al2 O3 catalysts was also investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The activity of the Pt/Fe/Al2 O3 catalyst was greater than that of the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment for the stoichiometric mixture. Also, the activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment was greater than that after the O2 and O2 –H2 –O2 treatments. Pt particles were found to react with Fe additives to form homogeneous Pt–Fe alloy particles on Al2 O3 under reducing conditions. Also, the Pt–Fe alloy particles on Al2 O3 were found to segregate into Pt and Fe2 O3 and to form a Fe2 O3 coverage layer on Pt particles so that Pt particles were prevented from sintering when heated at 800◦ C for a lean mixture. On the other hand, the activity of the Pt/Al2 O3 catalyst was greater than that of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for lean mixture. The layer of Fe2 O3 on Pt is responsible for the low activity. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Platinum; Iron; Sintering; Alloy; Alumina
1. Introduction Highly dispersed Pt particles supported on ␥-Al2 O3 are important for the present day automotive three-way catalysts (TWCs). The catalytic activity of aged catalysts becomes lower than a new one because chemical and thermal deactivations occur. Especially, thermal deactivation occurs mainly by Pt sintering. Initially, small Pt particles are highly dispersed on ␥-Al2 O3 , ∗ Corresponding author. Tel.: +81-561-63-5283; fax: +81-561-63-6150 E-mail address: [email protected] (Y. Sakamoto)
but the Pt particles become large when heated in air at high temperature for a long time. Many scientists have studied the process to prevent Pt particles from sintering. The sintering process of Pt particles was found to occur by several mechanisms such as the migration of crystallites and their coalescence [1], emission of single atoms by the small crystallites and their capture by a large one [2–4], and a combination of the two methods [5]. At high temperature under oxidizing conditions, sintering proceeds largely through vaporization of the platinum oxide [3]. Some bimetallic catalysts have a thermal stability larger than and a lifetime longer than the monometal-
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 7 4 - 0
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lic Pt catalyst. For example, bimetallic Pt/Rh catalysts have a very high thermal stability and are widely used for automotive catalysts. Pt particles are covered with Rh oxides, so those Pt particles are prevented from sintering at high temperature in an oxidizing atmosphere [6]. From the study of Pt–Fe alloys [7–11], the Pt–Fe alloy was found to be formed when a mixture of Pt and Fe oxide was heated at high temperature in a hydrogen atmosphere, and iron enrichment of the surface was found to occur at high temperature in an oxidizing atmosphere. As this phenomenon of the Pt–Fe alloy is similar to that for bimetallic Pt/Rh catalysts, bimetallic Pt–Fe catalysts are expected to have high thermal stability and catalytic activity. The purpose of this study clarified the fact that the addition of Fe to the Pt/Al2 O3 catalyst inhibits the growth of Pt particles heated in a lean mixture. The catalytic activities were investigated under stoichiometric and lean mixture simulated exhausts from an automotive engine on bimetallic Pt/Fe/Al2 O3 catalysts and compared to that of Pt/Al2 O3 catalysts. The relationship between the catalytic activity of the catalysts and the state of the Pt particles on Al2 O3 was clarified by the data obtained from X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM).
2. Experimental 2.1. Catalyst preparation Three catalysts, Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 , were prepared for use in this study. The Pt/Al2 O3 catalyst was prepared using the following method. ␥-Al2 O3 powder (Degussa, 180 m2 g−1 ) was brought into contact with an aqueous solution of Pt(NH2 )2 (NO2 )2 (Tanaka Precious Metal), followed by drying at 80◦ C for 6 h in air, calcining at 400◦ C for 1 h in flowing 50% H2 /N2 , and cooling to room temperature. The cooled powder was pressed in disks, crushed and sieved to a 0.3–0.7 mm size. The Pt loading amount of the Pt/Al2 O3 catalyst was 3.70 wt.%, based on induced coupled plasma (ICP) emission spectroscopy. The Pt/Fe/Al2 O3 catalyst was prepared using the following method. The ␥-alumina powder was
brought into contact with an aqueous solution of Fe(NO3 )3 ·9H2 O (Wako Pure Chemical Industries), followed by drying at 200◦ C for 2 h, and then calcining at 600◦ C for 3 h in air. After cooling to room temperature, the calcined powder was brought into contact with an aqueous solution of Pt(NH2 )2 (NO2 )2 , followed by drying at 200◦ C for 2 h in air, calcining at 400◦ C for 1 h in flowing 50% H2 /N2 , and cooling to room temperature. The obtained powder was pressed in disks, crushed and sieved to a 0.3–0.7 mm size. A 3.70 wt.% Pt loading of the Pt/Fe/Al2 O3 catalyst was chosen as it was comparable to that of the Pt/Al2 O3 catalyst described already. 1.10 wt.% Fe loading of the Pt/Fe/Al2 O3 catalyst was amounted. The Pt/5Fe/Al2 O3 catalyst was prepared in the same way as described above. The Pt and Fe loading amounts of the Pt/5Fe/Al2 O3 catalyst were 3.50 and 5.16 wt.%, respectively. The Fe loading amounts of 1.10 and 5.16 wt.% were chosen to investigate the effect of Fe addition to the Pt/Al2 O3 catalyst. All catalyst samples were evaluated as prepared and again evaluated following three kinds of thermal aging treatments. The thermal aging treatments were performed by placing the catalyst sample in the center of a tube in an electric furnace and downstream from a heat exchange zone. First, the catalyst samples were heated at 800◦ C for 5 h in flowing 7.5% O2 /N2 (O2 treatment). Second, the heated samples were heat-treated at 800◦ C for 5 h in flowing 6.0% H2 /N2 (O2 –H2 treatment). Finally, the heat-treated samples were again heated at 800◦ C for 5 h in flowing 7.5% O2 /N2 (O2 –H2 –O2 treatment).
2.2. Catalytic activity measurement The thermally-aged catalyst samples were evaluated for light-off activity using a previously developed laboratory reactor [12]. Catalytic activity data were obtained using a conventional fixed-bed flow reactor at atmospheric pressure. A quartz tube with an inner diameter of 18 mm was chosen as the reactor tube. A 0.1 g catalyst sample mixed with 0.9 g ␥-Al2 O3 crushed and sieved to 0.3–0.7 mm size (∼1 cm3 ) was placed on a quartz filter in the middle part of the reactor. The upper part of the catalyst bed was packed with 7 cm3 of inactive 3 mm o.d. SiC spheres for preheating the feedstream gas. The gas leaving the reactor was
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led to a condenser to remove any water vapor. The remaining components were analyzed continuously by non-dispersive infrared (CO and CO2 ), flame ionization (hydrocarbon, HC), magnetic susceptibility (O2 ), and chemiluminesence (NOx ) equipped with a gas analyzer (Horiba MEXA-8120). The feedstream compositions and reaction conditions used in this study were as follows: 1. Simulated stoichiometric mixture (stoichiometric feedstream). It consisted of 10.0% CO2 , 0.70% CO, 0.23% H2 , 1600 ppm C3 H6 , 1200 ppm NO, 0.64% O2 , 3% H2 0, and the balance N2 . This mixture simulated an air to fuel (A/F) ratio of 14.6. 2. Simulated lean mixture (oxidizing feedstream). It consisted of 11.9% CO2 , 0.12% CO, 0.04% H2 , 1600 ppm C3 H6 , 1200 ppm NO, 4.3% O2 , 3% H2 O, and a balance of N2 . This mixture simulated an A/F ratio of 18.0. To investigate the TWC behavior of the samples for each light-off test, catalysts were exposed to the simulated exhaust at 3.3 l min−1 while the temperature was decreased from 600 to 100◦ C at 5◦ C min−1 at about a 200,000 h−1 space velocity. Conversion data were measured at temperatures of 500–100◦ C. Catalytic activity of the catalysts was expressed as the temperature at 50% HC (C3 H6 ) conversion because the temperature at 50% HC conversion is related to the Pt mean diameter [13]. 2.3. XRD XRD measurements were carried out with a Rigaku RU-3L X-ray diffractometer using Co K␣ radiation to determine the Pt particle sizes on the fresh catalyst sample and the samples after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. To calculate the particle size from the diffractogram, the Scherrer equation was used. 2.4. XPS The XPS measurements were done under 1 × 10−7 Torr O2 pressure at room temperature by the conventional method which involved exposure of the sample to air for a few minutes when placing it on the sample holder. XPS measurements were conducted using a V.G. Scientific Escalab MK III with Mg K␣ X-rays for
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Fig. 1. Temperature at 50% HC conversion under stoichiometric mixture after the O2 and O2 –H2 –O2 treatments.
the measurements of the oxidized and reduced samples after the O2 ,O2 –H2 and O2 –H2 –O2 treatments, as described already. The Pt4d5 and Fe2p3 binding energies were calibrated using Si2p (103.4 eV). The concentration of Pt and Fe on the surface of the samples was calculated from the ratio of peak areas of the XPS data of the samples to that of pure Pt, Fe, FeO and Fe2 O3 . 2.5. HRTEM The HRTEM was carried out using a JOEL JEM-2000 electron microscope operated at 200 kV. The sample powders after the O2 and O2 –H2 –O2 treatments, as described above, were supported on carbon-coated copper grids by simply bringing the powders into contact with the grid.
3. Result and discussion 3.1. Catalytic activity after the O2 –H2 –O2 treatment Fig. 1 shows the temperature at 50% HC conversion of the Pt/Al2 O3 and Pt/Fe/Al2 O3 catalysts in the stoichiometric mixture after the O2 and O2 –H2 –O2 treatments. The activity of both catalysts decreased due to the O2 –H2 –O2 treatment. The activity of Pt/Fe/Al2 O3 is greater than that of the Pt/Al2 O3 catalyst. Moreover, the decreasing ratio of activity of Pt/Fe/Al2 O3 is smaller than that of the Pt/Al2 O3 catalyst. These re-
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Fig. 2. Temperature at 50% HC conversion under lean mixture after the O2 and O2 –H2 –O2 treatments.
Fig. 3. Temperature at 50% HC conversion under stoichiometric and lean mixture after the O2 , O2 –H2 and O2 –H2 –O2 treatments on Pt/Fe/A12 O3 catalysts.
sults indicate that the addition of Fe to the Pt/Al2 O3 catalyst plays a significant role in the reactivity of Pt on the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for the stoichiometric mixture. Fig. 2 shows the temperature at 50% HC conversion of the Pt/Al2 O3 and Pt/Fe/Al2 O3 catalysts in the lean mixture after the O2 and O2 –H2 –O2 treatments. The activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment was slightly smaller than that of the Pt/Al2 O3 catalyst. These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst plays no significant role in the reactivity of Pt on the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for a lean mixture.
3.3. Characterization of catalysts
3.2. Catalytic activity of Pt/Fe/Al2 O3 after the O2 –H2 treatment As shown in Fig. 3, the temperature at 50% HC conversion of the Pt/Fe/Al2 O3 catalysts after the O2 –H2 treatment was higher than those after the O2 and O2 –H2 –O2 treatments. However, the activity of Pt/Fe/Al2 O3 after the O2 treatment was nearly equal to that after the O2 –H2 –O2 treatment. These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst plays a more significant role in the reactivity of Pt on the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment for the stoichiometric mixture than those after the O2 and O2 –H2 –O2 treatments.
Fig. 4 shows the XRD data in the 37–43◦ region of diffraction angle 2θ of the Pt/Al2 O3 , Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts after the (a) O2 , (b) O2 –H2 and (c) O2 –H2 –O2 treatments. As shown in Fig. 5(a) and (c), the Pt (111) peak for the Pt particles in all three catalysts after the O2 and O2 –H2 –O2 treatments was at 2θ = 39.8◦ . On the other hand, as shown in Fig. 5(b), the Pt (111) peak for the Pt particles in the Pt/Al2 O3 catalyst after the O2 –H2 treatment was at 2θ = 39.8◦ . However, that in the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts shifted to an angle higher than 39.8◦ after the O2 –H2 treatment. From these results, Pt is found to react with Fe to form a Pt–Fe alloy during the O2 –H2 treatment and the length of the shift increases with increasing amount of Fe. This result is in agreement with the formation of Pt–Fe alloy particles on (Pt + Fe)–SiO2 catalysts in flowing hydrogen, as Kovalchuk and Kuznetosov have already reported [11]. No diffraction line due to Fe and Fe2 O3 was directly observed in this study. It seems that the iron oxides exit as small particles or amorphous materials. Fig. 5 shows the lattice constant of Pt calculated from the XRD spectra data for the Pt/Al2 O3 , Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts. The decrease in the lattice constant of Pt with Fe content means the dissolution of Fe into the Pt lattice, since the radius of the Fe atom (1.24 Å) is smaller than that of the Pt atom (1.39 Å) [14]. On the other hand, no change in the lattice constant of Pt in the Pt/Al2 O3 ,
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Fig. 4. The XRD spectra of Pt(111) in the 37–43◦ region of diffraction angle, 2θ, of the Pt/A12 O3 , Pt/Fe/A12 O3 and Pt/5Fe/A12 O3 catalysts after the (a) O2 , (b) O2 –H2 and (c) O2 –H2 –O2 treatments.
Fig. 5. The lattice constant of Pt(111) calculated from the XRD spectrum data; Pt/A12 O3 , Pt/Fe/A12 O3 and Pt/5Fe/A12 O3 catalysts after the O2 , O2 –H2 and O2 –H2 –O2 treatments.
Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts with the change in Fe content must be responsible for no reaction of Pt with Fe during the O2 and O2 –H2 –O2 treatments (Fig. 5(a) and (c)). Fig. 6 shows the mean diameter of Pt particles in the Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 catalysts after the O2 and O2 –H2 –O2 treatments. The mean diameter of the Pt particles in the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment was greater than that after the O2 treatment. The mean diameter of the Pt particles in the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 cata-
Fig. 6. The mean diameter of Pt particles in the Pt/A12 O3 , Pt/Fe/A12 O3 , and Pt/5Fe/A12 O3 catalysts after O2 and O2 –H2 –O2 treatments.
lysts after the O2 –H2 –O2 treatment was nearly equal to that after the O2 treatment. These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst inhibits the growth of the Pt particles in the catalyst heated at 800◦ C in flowing 7.5% O2 /N2 . Fig. 7 shows the XPS spectra in the region of the Pt4d5 emission band of the Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. The peak position (at 315 eV
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Fig. 7. XPS spectra in the region of the Pt 4d5 emission band of the catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. (a) Pt/A12 O3 , (b) Pt/Fe/A12 O3 , (c) Pt/5Fe/A12 O3 .
[15,16]) and area of the XPS spectra in the region of the Pt4d5 emission band of the Pt/Al2 O3 catalyst after the O2 –H2 and O2 –H2 –O2 treatments were about the same as that after the O2 treatment, as shown in Fig. 7(a). These results indicate that the concentration of Pt on the surface of the Pt/Al2 O3 catalyst after the O2 –H2 and O2 –H2 –O2 treatments is about the same as that after the O2 treatment. The peak position (at 315 eV) of the XPS spectra in the region of the Pt4d5 emission band of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 and O2 –H2 –O2 treatments was about the same as that after the O2 treatment. However, the peak area at 315 eV of Pt on the Pt/Fe/Al2 O3 catalyst after O2 –H2 and O2 –H2 –O2 treatments was smaller than that after the O2 treatment, as shown in Fig. 7(b). These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst affects the concentration of Pt on the surface of the catalyst after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. As shown in Fig. 7(c), the peak position (at 315 eV) of Pt on the Pt/5Fe/Al2 O3 catalyst after the O2 treatment was about the same as that after O2 –H2 treatment; however, the peak area after O2 –H2 treatment was smaller than that after the O2 treatment. In particular, the peak area after the O2 –H2 –O2 treatment is almost zero. This result indicates that Pt particles on the Pt/5Fe/Al2 O3 catalyst are completely covered with the Fe2 O3 layer, as will be mentioned later. Fig. 8 shows the XPS spectra in the region of the Fe2p3 emission band of the Pt/Al2 O3 , Pt/Fe/Al2 O3 ,
and Pt/5Fe/Al2 O3 catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. As shown in Fig. 8(a), the peak positions at 712, 709 and 708 eV, respectively, in the region of the Fe2p3 emission band are due to Fe2 O3 , FeO, and Fe. Therefore, the state of Fe on the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts after the O2 and O2 –H2 –O2 treatments must be due to Fe2 O3 . The peak of FeO on the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts after the O2 –H2 treatment increased more than that after the O2 and O2 –H2 –O2 treatments. Such a peak shift must be due to the reduction of Fe2 O3 . As shown in Fig. 8(b), the position and shape of the XPS spectra in the region of the Fe2p3 emission band of the Pt/5Fe/Al2 O3 catalyst are similar to those of the Pt/Fe/Al2 O3 catalyst, as shown in Fig. 8(a). However, the intensity of the XPS spectra in the region of the Fe2p3 emission band for the Pt/5Fe/Al2 O3 catalyst is higher than that of the Pt/Fe/Al2 O3 catalyst because of the large amount of Fe2 O3 on the Pt/5Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment of Pt particles on the catalyst which must have been covered with Fe2 O3 layers, so that Pt was not observed by the XPS measurement, as mentioned in Fig. 7(c). Fig. 9(a), (b) and (c) show the concentrations of Pt and Fe on the surface of the Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 catalysts, respectively, after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. These concentrations were estimated from the ratio of peak area of the XPS data of the samples shown in Figs. 7 and 8 to that of pure Pt, FeO and Fe2 O3 .
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Fig. 8. XPS spectra in the region of the Fe2p3 emission band of the catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. (a) Pt/Fe/A12 O3 , (b) Pt/5Fe/A12 O3 .
Fig. 9. The concentrations of Pt and Fe on the surface of (a) Pt/A12 O3 , (b) Pt/Fe/A12 O3 , and (c) Pt/5Fe/A12 O3 catalysts, after the O2 , O2 –H2 , and O2 –H2 –O2 treatments estimated by XPS data.
The concentration of Pt on the surface of the Pt/Al2 O3 catalyst is not altered by the O2 , O2 –H2 , and O2 –H2 –O2 treatments, while that of the Pt/Fe/Al2 O3 catalyst decreased in the order O2 > O2 –H2 > O2 –H2 –O2 treatments. Nevertheless, the catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is greater than that of the Pt/Al2 O3 catalyst in the stoichiometric mixture, as shown in Fig. 1. Also, the activity of the Pt/Fe/Al2 O3 catalyst is great in the order O2 –H2 > O2 ; O2 –H2 –O2 treatments, as shown in Figs. 1 and 3. The high activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2
treatment must be responsible for the Pt–Fe alloy particles on the catalyst. The catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is lower than that of the Pt/Al2 O3 catalyst in the lean mixture, as shown in Fig. 2. Also, the activity of the Pt/Fe/Al2 O3 catalyst in the lean mixture is low in the order O2 –H2 –O2 < O2 < O2 –H2 treatments, as shown in Figs. 2 and 4. The low activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment must be responsible for the Fe2 O3 layer on the Pt particles in the catalyst.
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Fig. 10. Dark field TEM observation of Pt/Fe/A12 O3 after the O2 treatments.
Fig. 12. Dark field TEM observation of Pt/Fe/A12 O3 after the O2 –H2 –O2 treatments. High contrast metal core (A): Pt particle. Lower contrast shell (B): Fe oxides.
and O2 –H2 –O2 treatments. As shown in Fig. 10, Pt particles on the Pt/Fe/Al2 O3 catalyst after the O2 treatment are about 10–30 nm crystallites. This particle size of Pt corresponds to the average size of Pt (∼20 nm) on the Pt/Fe/Al2 O3 catalyst after the O2 treatment from the XRD data, as shown in Fig. 6. As shown in Fig. 11, some particles are observed on the surface of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment. These particles seem to be Pt–Fe alloy particles of 20–35 nm in size, based on the XRD data as shown in Fig. 5(b). As shown in Fig. 12, some particles are observed on the surface of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment. These particles seem to be covered Pt particles, based on the Fe2 O3 from XPS data, as shown in Figs. 7, 8 and 9. 3.4. A model for the Pt/Fe/Al2 O3 catalyst
Fig. 11. Dark field TEM observation of Pt/Fe/A12 O3 after the O2 –H2 treatments.
Figs. 10, 11 and 12 show dark field TEM observations of the Pt/Fe/Al2 O3 catalyst after the O2 , O2 –H2 ,
Fig. 13 shows a simple model for the Pt/Fe/Al2 O3 catalyst, which can describe all the experimental results obtained with the measurements mentioned above. The Pt particles on Al2 O3 grow and the Fe additives reacts with O2 to form Fe2 O3 during the O2 treatment. Pt reacts with the Fe2 O3 particles on Al2 O3
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Fig. 13. A model for the Pt/Fe/A12 O3 catalyst after the O2 , O2 –H2 , and O2 –H2 –O2 treatments.
to form homogeneous Pt–Fe alloys covered with a thin Fe2 O3 layer during the O2 –H2 treatment. The Fe in the Pt–Fe alloys segregates from them to form a Fe2 O3 layer on Pt particles during the O2 –H2 –O2 treatment so that Pt particles covered with an Fe2 O3 layer are prevented from growing even if they are exposed to the lean mixture. The Fe2 O3 layer is reduced in order to remove it from the surface of the Pt particles under stoichiometric conditions. Therefore, under the stoichiometric conditions, the catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is higher than that of the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment under the stoichiometric conditions. On the other hand, the Fe2 O3 layer remains unchanged on the surface of the Pt particles of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for the lean condition. Therefore, the catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is lower than that of the Pt/Al2 O3 catalyst after the same treatments under lean conditions. 4. Summary We found that Pt/Fe/Al2 O3 catalysts have characteristics different from conventional Pt/Al2 O3 catalysts, based on the same points as described below. 1. The activity of Pt/Fe/Al2 O3 catalyst was greater than that of the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment at the stoichiometric point, while that of the Pt/Fe/Al2 O3 catalyst was lower
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than that of the Pt/Al2 O3 catalyst after the same treatment under oxidizing conditions. 2. The activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment was greater than that after the O2 and O2 –H2 –O2 treatments, while that of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment was lower than that after the O2 and O2 –H2 treatments. 3. Pt particles were found to react with Fe additives to form homogeneous Pt–Fe alloy particles on Al2 O3 under reducing conditions. 4. Pt–Fe alloy particles on Al2 O3 were found to segregate into Pt and Fe2 O3 and form a cover of the Fe2 O3 layer on the Pt particles so that the Pt particles were prevented from sintering when heated at 800◦ C under oxidizing conditions.
Acknowledgements We would like to thank Noritomo Suzuki, Toyota Central Research & Development Labs. Inc., for taking the HRTEM images of the samples with the JOEL JEM-2000 electron microscope. References [1] E. Reckenstein, B. Pulvermacker, J. Catal. 29 (1973) 224. [2] B.K. Chakraverty, J. Phys. Chem. Solids 28 (1967) 2401. [3] P. Wynblatt, N.A. Gjoostein, Progr. Solid State Chem. 9 (1975) 21. [4] P.C. Flynn, S.E. Wanke, J. Catal. 34 (1974) 390. [5] E. Ruckenstein, D.B. Dadyburjor, J. Catal. 33 (1977) 233. [6] H. Muraki, H. Sobukawa, M. Kimura, A. Isogai, SAE paper 900610, 1990. [7] A. Ati, M. Abon, P. Beccat, J.C. Bertolini, B. Tardy, Surf. Sci. 302 (1973) 121. [8] G.W.R. Leibbrandt, F.H.P.M. Habraken, J. Catal. 143 (1993) 102. [9] S.G.O.N.M. Rodriguez, R.T.K. Baker, J. Catal. 136 (1992) 584. [10] F. Delbecq, P. Sautet, J. Catal. 152 (1996) 164. [11] V.I. Kovalchuk, B.N. Kuznetsov, J. Mol. Catal. A 102 (1995) 103. [12] H. Muraki, K. Yokota, Y. Fujitani, Appl. Catal. 48 (1989) 93. [13] S. Matsnaga, K. Yokota, S. Hyodo, T. Suzuki, H. Sobukawa, SAE paper 982706, 1997. [14] N.V. Agreev, V.S. Sekhtman, Proc. Acad. Sci. USSR 143 (1962) 300. [15] U. Bardi, B.C. Beard, P.N. Ross, J. Catal. 124 (1990) 22. [16] Z. Zsoldos, L. Guczi, J. Phys. Chem. 96 (1992) 9393.
Effect of the addition of Fe on catalytic activities of Pt/Fe/␥-Al2 O3 catalyst Y. Sakamoto ∗ , K. Higuchi, N. Takahashi, K. Yokota, H. Doi, M. Sugiura Toyota Central Research & Development Labs., Inc. Nagakute, Aichi, 480-1192, Japan Received 1 March 1999; received in revised form 17 June 1999; accepted 18 June 1999
Abstract The catalytic activities of stoichiometric and lean mixture simulated exhaust from automotive engines have been investigated on bimetallic Pt/Fe/Al2 O3 catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments and compared to that of Pt/Al2 O3 catalysts. The state of the Pt particles of the Pt/Fe/Al2 O3 catalysts was also investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The activity of the Pt/Fe/Al2 O3 catalyst was greater than that of the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment for the stoichiometric mixture. Also, the activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment was greater than that after the O2 and O2 –H2 –O2 treatments. Pt particles were found to react with Fe additives to form homogeneous Pt–Fe alloy particles on Al2 O3 under reducing conditions. Also, the Pt–Fe alloy particles on Al2 O3 were found to segregate into Pt and Fe2 O3 and to form a Fe2 O3 coverage layer on Pt particles so that Pt particles were prevented from sintering when heated at 800◦ C for a lean mixture. On the other hand, the activity of the Pt/Al2 O3 catalyst was greater than that of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for lean mixture. The layer of Fe2 O3 on Pt is responsible for the low activity. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Platinum; Iron; Sintering; Alloy; Alumina
1. Introduction Highly dispersed Pt particles supported on ␥-Al2 O3 are important for the present day automotive three-way catalysts (TWCs). The catalytic activity of aged catalysts becomes lower than a new one because chemical and thermal deactivations occur. Especially, thermal deactivation occurs mainly by Pt sintering. Initially, small Pt particles are highly dispersed on ␥-Al2 O3 , ∗ Corresponding author. Tel.: +81-561-63-5283; fax: +81-561-63-6150 E-mail address: [email protected] (Y. Sakamoto)
but the Pt particles become large when heated in air at high temperature for a long time. Many scientists have studied the process to prevent Pt particles from sintering. The sintering process of Pt particles was found to occur by several mechanisms such as the migration of crystallites and their coalescence [1], emission of single atoms by the small crystallites and their capture by a large one [2–4], and a combination of the two methods [5]. At high temperature under oxidizing conditions, sintering proceeds largely through vaporization of the platinum oxide [3]. Some bimetallic catalysts have a thermal stability larger than and a lifetime longer than the monometal-
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 7 4 - 0
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lic Pt catalyst. For example, bimetallic Pt/Rh catalysts have a very high thermal stability and are widely used for automotive catalysts. Pt particles are covered with Rh oxides, so those Pt particles are prevented from sintering at high temperature in an oxidizing atmosphere [6]. From the study of Pt–Fe alloys [7–11], the Pt–Fe alloy was found to be formed when a mixture of Pt and Fe oxide was heated at high temperature in a hydrogen atmosphere, and iron enrichment of the surface was found to occur at high temperature in an oxidizing atmosphere. As this phenomenon of the Pt–Fe alloy is similar to that for bimetallic Pt/Rh catalysts, bimetallic Pt–Fe catalysts are expected to have high thermal stability and catalytic activity. The purpose of this study clarified the fact that the addition of Fe to the Pt/Al2 O3 catalyst inhibits the growth of Pt particles heated in a lean mixture. The catalytic activities were investigated under stoichiometric and lean mixture simulated exhausts from an automotive engine on bimetallic Pt/Fe/Al2 O3 catalysts and compared to that of Pt/Al2 O3 catalysts. The relationship between the catalytic activity of the catalysts and the state of the Pt particles on Al2 O3 was clarified by the data obtained from X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM).
2. Experimental 2.1. Catalyst preparation Three catalysts, Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 , were prepared for use in this study. The Pt/Al2 O3 catalyst was prepared using the following method. ␥-Al2 O3 powder (Degussa, 180 m2 g−1 ) was brought into contact with an aqueous solution of Pt(NH2 )2 (NO2 )2 (Tanaka Precious Metal), followed by drying at 80◦ C for 6 h in air, calcining at 400◦ C for 1 h in flowing 50% H2 /N2 , and cooling to room temperature. The cooled powder was pressed in disks, crushed and sieved to a 0.3–0.7 mm size. The Pt loading amount of the Pt/Al2 O3 catalyst was 3.70 wt.%, based on induced coupled plasma (ICP) emission spectroscopy. The Pt/Fe/Al2 O3 catalyst was prepared using the following method. The ␥-alumina powder was
brought into contact with an aqueous solution of Fe(NO3 )3 ·9H2 O (Wako Pure Chemical Industries), followed by drying at 200◦ C for 2 h, and then calcining at 600◦ C for 3 h in air. After cooling to room temperature, the calcined powder was brought into contact with an aqueous solution of Pt(NH2 )2 (NO2 )2 , followed by drying at 200◦ C for 2 h in air, calcining at 400◦ C for 1 h in flowing 50% H2 /N2 , and cooling to room temperature. The obtained powder was pressed in disks, crushed and sieved to a 0.3–0.7 mm size. A 3.70 wt.% Pt loading of the Pt/Fe/Al2 O3 catalyst was chosen as it was comparable to that of the Pt/Al2 O3 catalyst described already. 1.10 wt.% Fe loading of the Pt/Fe/Al2 O3 catalyst was amounted. The Pt/5Fe/Al2 O3 catalyst was prepared in the same way as described above. The Pt and Fe loading amounts of the Pt/5Fe/Al2 O3 catalyst were 3.50 and 5.16 wt.%, respectively. The Fe loading amounts of 1.10 and 5.16 wt.% were chosen to investigate the effect of Fe addition to the Pt/Al2 O3 catalyst. All catalyst samples were evaluated as prepared and again evaluated following three kinds of thermal aging treatments. The thermal aging treatments were performed by placing the catalyst sample in the center of a tube in an electric furnace and downstream from a heat exchange zone. First, the catalyst samples were heated at 800◦ C for 5 h in flowing 7.5% O2 /N2 (O2 treatment). Second, the heated samples were heat-treated at 800◦ C for 5 h in flowing 6.0% H2 /N2 (O2 –H2 treatment). Finally, the heat-treated samples were again heated at 800◦ C for 5 h in flowing 7.5% O2 /N2 (O2 –H2 –O2 treatment).
2.2. Catalytic activity measurement The thermally-aged catalyst samples were evaluated for light-off activity using a previously developed laboratory reactor [12]. Catalytic activity data were obtained using a conventional fixed-bed flow reactor at atmospheric pressure. A quartz tube with an inner diameter of 18 mm was chosen as the reactor tube. A 0.1 g catalyst sample mixed with 0.9 g ␥-Al2 O3 crushed and sieved to 0.3–0.7 mm size (∼1 cm3 ) was placed on a quartz filter in the middle part of the reactor. The upper part of the catalyst bed was packed with 7 cm3 of inactive 3 mm o.d. SiC spheres for preheating the feedstream gas. The gas leaving the reactor was
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led to a condenser to remove any water vapor. The remaining components were analyzed continuously by non-dispersive infrared (CO and CO2 ), flame ionization (hydrocarbon, HC), magnetic susceptibility (O2 ), and chemiluminesence (NOx ) equipped with a gas analyzer (Horiba MEXA-8120). The feedstream compositions and reaction conditions used in this study were as follows: 1. Simulated stoichiometric mixture (stoichiometric feedstream). It consisted of 10.0% CO2 , 0.70% CO, 0.23% H2 , 1600 ppm C3 H6 , 1200 ppm NO, 0.64% O2 , 3% H2 0, and the balance N2 . This mixture simulated an air to fuel (A/F) ratio of 14.6. 2. Simulated lean mixture (oxidizing feedstream). It consisted of 11.9% CO2 , 0.12% CO, 0.04% H2 , 1600 ppm C3 H6 , 1200 ppm NO, 4.3% O2 , 3% H2 O, and a balance of N2 . This mixture simulated an A/F ratio of 18.0. To investigate the TWC behavior of the samples for each light-off test, catalysts were exposed to the simulated exhaust at 3.3 l min−1 while the temperature was decreased from 600 to 100◦ C at 5◦ C min−1 at about a 200,000 h−1 space velocity. Conversion data were measured at temperatures of 500–100◦ C. Catalytic activity of the catalysts was expressed as the temperature at 50% HC (C3 H6 ) conversion because the temperature at 50% HC conversion is related to the Pt mean diameter [13]. 2.3. XRD XRD measurements were carried out with a Rigaku RU-3L X-ray diffractometer using Co K␣ radiation to determine the Pt particle sizes on the fresh catalyst sample and the samples after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. To calculate the particle size from the diffractogram, the Scherrer equation was used. 2.4. XPS The XPS measurements were done under 1 × 10−7 Torr O2 pressure at room temperature by the conventional method which involved exposure of the sample to air for a few minutes when placing it on the sample holder. XPS measurements were conducted using a V.G. Scientific Escalab MK III with Mg K␣ X-rays for
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Fig. 1. Temperature at 50% HC conversion under stoichiometric mixture after the O2 and O2 –H2 –O2 treatments.
the measurements of the oxidized and reduced samples after the O2 ,O2 –H2 and O2 –H2 –O2 treatments, as described already. The Pt4d5 and Fe2p3 binding energies were calibrated using Si2p (103.4 eV). The concentration of Pt and Fe on the surface of the samples was calculated from the ratio of peak areas of the XPS data of the samples to that of pure Pt, Fe, FeO and Fe2 O3 . 2.5. HRTEM The HRTEM was carried out using a JOEL JEM-2000 electron microscope operated at 200 kV. The sample powders after the O2 and O2 –H2 –O2 treatments, as described above, were supported on carbon-coated copper grids by simply bringing the powders into contact with the grid.
3. Result and discussion 3.1. Catalytic activity after the O2 –H2 –O2 treatment Fig. 1 shows the temperature at 50% HC conversion of the Pt/Al2 O3 and Pt/Fe/Al2 O3 catalysts in the stoichiometric mixture after the O2 and O2 –H2 –O2 treatments. The activity of both catalysts decreased due to the O2 –H2 –O2 treatment. The activity of Pt/Fe/Al2 O3 is greater than that of the Pt/Al2 O3 catalyst. Moreover, the decreasing ratio of activity of Pt/Fe/Al2 O3 is smaller than that of the Pt/Al2 O3 catalyst. These re-
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Fig. 2. Temperature at 50% HC conversion under lean mixture after the O2 and O2 –H2 –O2 treatments.
Fig. 3. Temperature at 50% HC conversion under stoichiometric and lean mixture after the O2 , O2 –H2 and O2 –H2 –O2 treatments on Pt/Fe/A12 O3 catalysts.
sults indicate that the addition of Fe to the Pt/Al2 O3 catalyst plays a significant role in the reactivity of Pt on the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for the stoichiometric mixture. Fig. 2 shows the temperature at 50% HC conversion of the Pt/Al2 O3 and Pt/Fe/Al2 O3 catalysts in the lean mixture after the O2 and O2 –H2 –O2 treatments. The activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment was slightly smaller than that of the Pt/Al2 O3 catalyst. These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst plays no significant role in the reactivity of Pt on the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for a lean mixture.
3.3. Characterization of catalysts
3.2. Catalytic activity of Pt/Fe/Al2 O3 after the O2 –H2 treatment As shown in Fig. 3, the temperature at 50% HC conversion of the Pt/Fe/Al2 O3 catalysts after the O2 –H2 treatment was higher than those after the O2 and O2 –H2 –O2 treatments. However, the activity of Pt/Fe/Al2 O3 after the O2 treatment was nearly equal to that after the O2 –H2 –O2 treatment. These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst plays a more significant role in the reactivity of Pt on the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment for the stoichiometric mixture than those after the O2 and O2 –H2 –O2 treatments.
Fig. 4 shows the XRD data in the 37–43◦ region of diffraction angle 2θ of the Pt/Al2 O3 , Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts after the (a) O2 , (b) O2 –H2 and (c) O2 –H2 –O2 treatments. As shown in Fig. 5(a) and (c), the Pt (111) peak for the Pt particles in all three catalysts after the O2 and O2 –H2 –O2 treatments was at 2θ = 39.8◦ . On the other hand, as shown in Fig. 5(b), the Pt (111) peak for the Pt particles in the Pt/Al2 O3 catalyst after the O2 –H2 treatment was at 2θ = 39.8◦ . However, that in the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts shifted to an angle higher than 39.8◦ after the O2 –H2 treatment. From these results, Pt is found to react with Fe to form a Pt–Fe alloy during the O2 –H2 treatment and the length of the shift increases with increasing amount of Fe. This result is in agreement with the formation of Pt–Fe alloy particles on (Pt + Fe)–SiO2 catalysts in flowing hydrogen, as Kovalchuk and Kuznetosov have already reported [11]. No diffraction line due to Fe and Fe2 O3 was directly observed in this study. It seems that the iron oxides exit as small particles or amorphous materials. Fig. 5 shows the lattice constant of Pt calculated from the XRD spectra data for the Pt/Al2 O3 , Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts. The decrease in the lattice constant of Pt with Fe content means the dissolution of Fe into the Pt lattice, since the radius of the Fe atom (1.24 Å) is smaller than that of the Pt atom (1.39 Å) [14]. On the other hand, no change in the lattice constant of Pt in the Pt/Al2 O3 ,
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Fig. 4. The XRD spectra of Pt(111) in the 37–43◦ region of diffraction angle, 2θ, of the Pt/A12 O3 , Pt/Fe/A12 O3 and Pt/5Fe/A12 O3 catalysts after the (a) O2 , (b) O2 –H2 and (c) O2 –H2 –O2 treatments.
Fig. 5. The lattice constant of Pt(111) calculated from the XRD spectrum data; Pt/A12 O3 , Pt/Fe/A12 O3 and Pt/5Fe/A12 O3 catalysts after the O2 , O2 –H2 and O2 –H2 –O2 treatments.
Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts with the change in Fe content must be responsible for no reaction of Pt with Fe during the O2 and O2 –H2 –O2 treatments (Fig. 5(a) and (c)). Fig. 6 shows the mean diameter of Pt particles in the Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 catalysts after the O2 and O2 –H2 –O2 treatments. The mean diameter of the Pt particles in the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment was greater than that after the O2 treatment. The mean diameter of the Pt particles in the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 cata-
Fig. 6. The mean diameter of Pt particles in the Pt/A12 O3 , Pt/Fe/A12 O3 , and Pt/5Fe/A12 O3 catalysts after O2 and O2 –H2 –O2 treatments.
lysts after the O2 –H2 –O2 treatment was nearly equal to that after the O2 treatment. These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst inhibits the growth of the Pt particles in the catalyst heated at 800◦ C in flowing 7.5% O2 /N2 . Fig. 7 shows the XPS spectra in the region of the Pt4d5 emission band of the Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. The peak position (at 315 eV
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Fig. 7. XPS spectra in the region of the Pt 4d5 emission band of the catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. (a) Pt/A12 O3 , (b) Pt/Fe/A12 O3 , (c) Pt/5Fe/A12 O3 .
[15,16]) and area of the XPS spectra in the region of the Pt4d5 emission band of the Pt/Al2 O3 catalyst after the O2 –H2 and O2 –H2 –O2 treatments were about the same as that after the O2 treatment, as shown in Fig. 7(a). These results indicate that the concentration of Pt on the surface of the Pt/Al2 O3 catalyst after the O2 –H2 and O2 –H2 –O2 treatments is about the same as that after the O2 treatment. The peak position (at 315 eV) of the XPS spectra in the region of the Pt4d5 emission band of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 and O2 –H2 –O2 treatments was about the same as that after the O2 treatment. However, the peak area at 315 eV of Pt on the Pt/Fe/Al2 O3 catalyst after O2 –H2 and O2 –H2 –O2 treatments was smaller than that after the O2 treatment, as shown in Fig. 7(b). These results indicate that the addition of Fe to the Pt/Al2 O3 catalyst affects the concentration of Pt on the surface of the catalyst after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. As shown in Fig. 7(c), the peak position (at 315 eV) of Pt on the Pt/5Fe/Al2 O3 catalyst after the O2 treatment was about the same as that after O2 –H2 treatment; however, the peak area after O2 –H2 treatment was smaller than that after the O2 treatment. In particular, the peak area after the O2 –H2 –O2 treatment is almost zero. This result indicates that Pt particles on the Pt/5Fe/Al2 O3 catalyst are completely covered with the Fe2 O3 layer, as will be mentioned later. Fig. 8 shows the XPS spectra in the region of the Fe2p3 emission band of the Pt/Al2 O3 , Pt/Fe/Al2 O3 ,
and Pt/5Fe/Al2 O3 catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. As shown in Fig. 8(a), the peak positions at 712, 709 and 708 eV, respectively, in the region of the Fe2p3 emission band are due to Fe2 O3 , FeO, and Fe. Therefore, the state of Fe on the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts after the O2 and O2 –H2 –O2 treatments must be due to Fe2 O3 . The peak of FeO on the Pt/Fe/Al2 O3 and Pt/5Fe/Al2 O3 catalysts after the O2 –H2 treatment increased more than that after the O2 and O2 –H2 –O2 treatments. Such a peak shift must be due to the reduction of Fe2 O3 . As shown in Fig. 8(b), the position and shape of the XPS spectra in the region of the Fe2p3 emission band of the Pt/5Fe/Al2 O3 catalyst are similar to those of the Pt/Fe/Al2 O3 catalyst, as shown in Fig. 8(a). However, the intensity of the XPS spectra in the region of the Fe2p3 emission band for the Pt/5Fe/Al2 O3 catalyst is higher than that of the Pt/Fe/Al2 O3 catalyst because of the large amount of Fe2 O3 on the Pt/5Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment of Pt particles on the catalyst which must have been covered with Fe2 O3 layers, so that Pt was not observed by the XPS measurement, as mentioned in Fig. 7(c). Fig. 9(a), (b) and (c) show the concentrations of Pt and Fe on the surface of the Pt/Al2 O3 , Pt/Fe/Al2 O3 , and Pt/5Fe/Al2 O3 catalysts, respectively, after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. These concentrations were estimated from the ratio of peak area of the XPS data of the samples shown in Figs. 7 and 8 to that of pure Pt, FeO and Fe2 O3 .
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Fig. 8. XPS spectra in the region of the Fe2p3 emission band of the catalysts after the O2 , O2 –H2 , and O2 –H2 –O2 treatments. (a) Pt/Fe/A12 O3 , (b) Pt/5Fe/A12 O3 .
Fig. 9. The concentrations of Pt and Fe on the surface of (a) Pt/A12 O3 , (b) Pt/Fe/A12 O3 , and (c) Pt/5Fe/A12 O3 catalysts, after the O2 , O2 –H2 , and O2 –H2 –O2 treatments estimated by XPS data.
The concentration of Pt on the surface of the Pt/Al2 O3 catalyst is not altered by the O2 , O2 –H2 , and O2 –H2 –O2 treatments, while that of the Pt/Fe/Al2 O3 catalyst decreased in the order O2 > O2 –H2 > O2 –H2 –O2 treatments. Nevertheless, the catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is greater than that of the Pt/Al2 O3 catalyst in the stoichiometric mixture, as shown in Fig. 1. Also, the activity of the Pt/Fe/Al2 O3 catalyst is great in the order O2 –H2 > O2 ; O2 –H2 –O2 treatments, as shown in Figs. 1 and 3. The high activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2
treatment must be responsible for the Pt–Fe alloy particles on the catalyst. The catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is lower than that of the Pt/Al2 O3 catalyst in the lean mixture, as shown in Fig. 2. Also, the activity of the Pt/Fe/Al2 O3 catalyst in the lean mixture is low in the order O2 –H2 –O2 < O2 < O2 –H2 treatments, as shown in Figs. 2 and 4. The low activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment must be responsible for the Fe2 O3 layer on the Pt particles in the catalyst.
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Fig. 10. Dark field TEM observation of Pt/Fe/A12 O3 after the O2 treatments.
Fig. 12. Dark field TEM observation of Pt/Fe/A12 O3 after the O2 –H2 –O2 treatments. High contrast metal core (A): Pt particle. Lower contrast shell (B): Fe oxides.
and O2 –H2 –O2 treatments. As shown in Fig. 10, Pt particles on the Pt/Fe/Al2 O3 catalyst after the O2 treatment are about 10–30 nm crystallites. This particle size of Pt corresponds to the average size of Pt (∼20 nm) on the Pt/Fe/Al2 O3 catalyst after the O2 treatment from the XRD data, as shown in Fig. 6. As shown in Fig. 11, some particles are observed on the surface of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment. These particles seem to be Pt–Fe alloy particles of 20–35 nm in size, based on the XRD data as shown in Fig. 5(b). As shown in Fig. 12, some particles are observed on the surface of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment. These particles seem to be covered Pt particles, based on the Fe2 O3 from XPS data, as shown in Figs. 7, 8 and 9. 3.4. A model for the Pt/Fe/Al2 O3 catalyst
Fig. 11. Dark field TEM observation of Pt/Fe/A12 O3 after the O2 –H2 treatments.
Figs. 10, 11 and 12 show dark field TEM observations of the Pt/Fe/Al2 O3 catalyst after the O2 , O2 –H2 ,
Fig. 13 shows a simple model for the Pt/Fe/Al2 O3 catalyst, which can describe all the experimental results obtained with the measurements mentioned above. The Pt particles on Al2 O3 grow and the Fe additives reacts with O2 to form Fe2 O3 during the O2 treatment. Pt reacts with the Fe2 O3 particles on Al2 O3
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Fig. 13. A model for the Pt/Fe/A12 O3 catalyst after the O2 , O2 –H2 , and O2 –H2 –O2 treatments.
to form homogeneous Pt–Fe alloys covered with a thin Fe2 O3 layer during the O2 –H2 treatment. The Fe in the Pt–Fe alloys segregates from them to form a Fe2 O3 layer on Pt particles during the O2 –H2 –O2 treatment so that Pt particles covered with an Fe2 O3 layer are prevented from growing even if they are exposed to the lean mixture. The Fe2 O3 layer is reduced in order to remove it from the surface of the Pt particles under stoichiometric conditions. Therefore, under the stoichiometric conditions, the catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is higher than that of the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment under the stoichiometric conditions. On the other hand, the Fe2 O3 layer remains unchanged on the surface of the Pt particles of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment for the lean condition. Therefore, the catalytic activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment is lower than that of the Pt/Al2 O3 catalyst after the same treatments under lean conditions. 4. Summary We found that Pt/Fe/Al2 O3 catalysts have characteristics different from conventional Pt/Al2 O3 catalysts, based on the same points as described below. 1. The activity of Pt/Fe/Al2 O3 catalyst was greater than that of the Pt/Al2 O3 catalyst after the O2 –H2 –O2 treatment at the stoichiometric point, while that of the Pt/Fe/Al2 O3 catalyst was lower
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than that of the Pt/Al2 O3 catalyst after the same treatment under oxidizing conditions. 2. The activity of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 treatment was greater than that after the O2 and O2 –H2 –O2 treatments, while that of the Pt/Fe/Al2 O3 catalyst after the O2 –H2 –O2 treatment was lower than that after the O2 and O2 –H2 treatments. 3. Pt particles were found to react with Fe additives to form homogeneous Pt–Fe alloy particles on Al2 O3 under reducing conditions. 4. Pt–Fe alloy particles on Al2 O3 were found to segregate into Pt and Fe2 O3 and form a cover of the Fe2 O3 layer on the Pt particles so that the Pt particles were prevented from sintering when heated at 800◦ C under oxidizing conditions.
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