Improved CO Oxidation Activity in the Presence and Absence of

Paul A. Midgley,§ John Meurig Thomas,§ Richard D. Adams,‡ and Michael D. Amiridis*,†. Department of Chemical Engineering and Department of Chemi...
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Langmuir 2006, 22, 5160-5167

Improved CO Oxidation Activity in the Presence and Absence of Hydrogen over Cluster-Derived PtFe/SiO2 Catalysts Attilio Siani,† Burjor Captain,‡ Oleg S. Alexeev,† Eirini Stafyla,† Ana B. Hungria,§ Paul A. Midgley,§ John Meurig Thomas,§ Richard D. Adams,‡ and Michael D. Amiridis*,† Department of Chemical Engineering and Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208, and Department of Materials Science and Metallurgy, UniVersity of Cambridge, Cambridge, United Kingdom CB2 3QZ ReceiVed December 23, 2005. In Final Form: March 21, 2006 The catalytic performance of cluster-derived PtFe/SiO2 bimetallic catalysts for the oxidation of CO has been examined in the absence and presence of H2 (PROX) and compared to that of Pt/SiO2. PtFe2/SiO2 and Pt5Fe2/SiO2 samples were prepared from PtFe2(COD)(CO)8 and Pt5Fe2(COD)2(CO)12 organometallic cluster precursors, respectively. FTIR data indicate that both clusters can be deposited intact on the SiO2 support. The clusters remained weakly bonded to the SiO2 surface and could be extracted with CH2Cl2 without any significant changes in their structure. Subsequent heating in H2 led to complete decarbonylation of the supported clusters at approximately 350 °C and the formation of Pt-Fe nanoparticles with sizes in the 1-2 nm range, as indicated by HRTEM imaging. A few larger nanoparticles enriched in Pt were also observed, indicating that a small fraction of the deposited clusters were segregated to the individual components following the hydrogen treatment. A higher degree of metal dispersion and more homogeneous mixing of the two metals were observed during HRTEM/XEDS analysis with the cluster-derived samples, as compared to a PtFe/SiO2 catalyst prepared through a conventional impregnation route. Furthermore, the cluster-derived PtFe2/ SiO2 and Pt5Fe2/SiO2 samples were more active than Pt/SiO2 and the conventionally prepared PtFe/SiO2 sample for the oxidation of CO in air. However, substantial deactivation was also observed, indicating that the properties of the Pt-Fe bimetallic sites in the cluster-derived samples were altered by exposure to the reactants. The Pt5Fe2/SiO2 sample was also more active than Pt/SiO2 for PROX with a selectivity of approximately 92% at 50 °C. In this case, the deactivation with time on stream was substantially slower, indicating that the highly reducing environment under the PROX conditions helps maintain the properties of the active Pt-Fe bimetallic sites.

Introduction

* Corresponding author. † Department of Chemical Engineering, University of South Carolina. ‡ Department of Chemistry and Biochemistry, University of South Carolina. § University of Cambridge.

to improve their performance by the addition of promoters to the existing catalytic formulations.11 In this regard, supported bimetallic catalysts, incorporating more than one active component, may provide possibilities of synergism that lead to superior performance. Typically, the catalytic behavior of bimetallics is affected by the size and composition of the metal nanoparticles, interactions with the support, and interactions between the metal components.12 As a result, the catalytic properties are often better than the sum of the properties of the two individual components. For example, literature reports indicate that PtRu and PtSn bimetallic combinations supported on carbon were more active and selective in PROX than Pt/γ-Al2O3 at temperatures as low as 0-80 °C.13,14 Furthermore, the addition of iron or cerium oxides to Pt/γ-Al2O3 has been shown to enhance the activity and selectivity of Pt for this reaction.15-17 It was suggested that the promoting effect in the case of Pt-Fe2O3/γ-Al2O3 is related to the ability of the iron oxide species, located in close proximity to platinum, to provide adsorption sites for oxygen that can subsequently react with CO molecules adsorbed on adjacent Pt sites through a noncompetitive dual site mechanism.16,18 However, conventional preparative

(1) Santra, A. K.; Goodman, D. W. Electrochim. Acta 2002, 47, 3595. (2) Song, C. Catal. Today 2002, 77, 17. (3) Fierro, J. L. G.; Pena, M. A. Catal. Sci. Ser. 2005, 5, 229. (4) Adams, W. A.; Blair, J.; Bullock, K. R.; Gardner, C. L. J. Power Sources 2005, 145, 55. (5) Farrauto, R. J.; Flytzani-Stephanopulos, M. Fuel Processing and PEM Fuel Cells: AdVanced Catalysts, Adsorbents and Electrocatalysts; Elsevier: Amsterdam, 2005. (6) Choudhary, T. V.; Goodman, D. W. Catal. Today 2002, 77, 65. (7) Levec, J. In Opportunities in catalytic reaction engineering. Examples of heterogeneous catalysis in water remediation and preferential CO oxidation; Galan, M. A., Martin del Valle, E., Eds.; Wiley: Chichester, U.K., 2005; p 103. (8) Manasilp, A.; Gulari, E. Appl. Catal. B 2004, 37, 17. (9) Oh, S. H.; Sinkevitch, R. M. J. Catal. 1993, 142, 254.

(10) Ito, S.; Fujimori, T.; Nagashima, K.; Yuzaki, K.; Kunimori, K. Catal. Today 2000, 57, 247. (11) Suh, D. J.; Kwak, C.; Kim, J.-H.; Kwon, S. M.; Park, T.-J. J. Power Sources 2005, 142, 70. (12) Alexeev, O. S.; Gates, B. C. Ind. Eng. Chem. Res. 2003, 42, 1571. (13) Snytnikov, P. V.; Sobyanin, V. A.; Belyaev, V. D.; Shlyapin, D. A. Khimiya Interesakh UstoichiVogo RazVitiya 2003, 11, 297. (14) Schubert, M. M.; Kahlich, M. J.; Feldmeyer, G.; Huttner, M.; Hackenberg, S.; Gasteiger, H. A.; Behm, R. J. Phys. Chem. Chem. Phys. 2001, 3, 1123. (15) Korotkikh, O.; Farrauto, R. Catal. Today 2000, 62, 249. (16) Kotobuki, M.; Watanabe, A.; Uchida, H.; Yamashita, H.; Watanabe, M. J. Catal. 2005, 236, 262. (17) Son, I. H.; Lane, A. M. Catal. Lett. 2001, 76, 151.

The CO oxidation in the absence and presence of H2 has attracted significant attention recently because of its potential application in indoor/cabin air cleanup and in the purification of hydrogen streams used in proton exchange membrane (PEM) fuel cells.1,2 Hydrogen is currently produced by catalytic steam reforming, partial oxidation, and auto-thermal reforming of hydrocarbons.3 However, CO is formed as a byproduct in all of these processes and must be subsequently removed prior to the introduction of hydrogen to PEM fuel cells, due to the high sensitivity of the Pt-based PEM electrocatalysts to poisoning by CO.4-6 The preferential oxidation (PROX) of CO in the presence of hydrogen is currently used commercially for this application due to its efficiency and relative simplicity. Supported metal catalysts incorporating Pt and other noble metals have been extensively investigated for PROX and have exhibited substantial activity for this reaction.7-10 Further attempts have been made

10.1021/la053476a CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006

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techniques used for the above-mentioned samples apparently lead to the formation of bimetallics with a relatively nonuniform size and composition. Therefore, it becomes difficult to correlate the structure of these samples to their catalytic performance. In this paper, we report the synthesis of PtFe/SiO2 bimetallic samples from discrete organometallic Pt5Fe2(COD)2(CO)12 and PtFe2(COD)(CO)8 (COD ) 1,5-cyclooctadiene) cluster precursors with preformed Pt-Fe bonds. We use these bimetallic clusters as precursors in an attempt to gain better control of the stoichiometry and morphology of the resulting Pt-Fe active sites. This in turn will allow us to gain a mechanistic insight of the CO oxidation process over Pt-Fe systems. A detailed characterization of the various steps involved in the synthesis of these catalysts by FTIR is reported in this paper, along with HRTEM measurements for the resulting catalysts and preliminary catalytic performance data for the oxidation of CO in the presence and absence of H2. Experimental Methods Reagents and Materials. The synthesis of the organometallic clusters was performed under dry N2 using standard Schlenk techniques. The toluene solvent was dried by standard procedures over sodium benzophenone and was freshly distilled prior to use. Na2Fe(CO)4‚3/2C4H8O2 was purchased from Aldrich and was stored and handled in a drybox. Pt(COD)Cl2 and Pt5Fe2(COD)2(CO)12 were prepared according to published procedures.19,20 Silica gel (60-200 µm, 70-230 mesh) used for chromatographic separations was purchased from Silicycle. The SiO2 support (Engelhard) with a BET surface area of 100 m2/g was calcined in air at 500 °C overnight prior to use. Dry methylene chloride (Fisher Scientific) was used as purchased. N2, H2, He, and the CO/N2 mixture (all UHP grade, Airstar) were additionally purified prior to use by passage through oxygen/moisture traps (Agilent) capable of removing traces of O2 and water to 15 and 25 ppb, respectively. In addition, the CO/N2 mixture was heated to 350 °C in a trap filled with quartz particles (60-80 mesh) to eliminate any carbonyls that may have been formed in the storage cylinder. Synthesis of Organometallic Cluster Precursors. The reaction of Na2Fe(CO)4‚3/2C4H8O2 with Pt(COD)Cl2 was performed in a 100 mL three-neck round-bottom flask. The Pt(COD)Cl2 (300 mg, 0.80 mmol) complex was dissolved in 20 mL of toluene and the solution was stirred, while the temperature was maintained at approximately 0 °C using an ice-water bath. When Na2Fe(CO)4‚3/2C4H8O2 (290 mg, 0.84 mmol) was quickly added to the solution, the color immediately turned to dark red. The reaction mixture was further stirred for approximately 6 h, while the temperature was slowly raised to 25 °C. The reaction solution was then filtered through a silica gel plug eluting with methylene chloride solvent until no more colored solution was obtained. The filtrate was evaporated to dryness using a rotary evaporator and the residue was separated on a silica gel column to yield in the order of elution: a green band of Fe3(CO)12 (8 mg) and an orange band of Pt3Fe3(CO)15 (7 mg, 1%) eluted by pure hexane,20 a red band of PtFe2(COD)(CO)8 (80 mg, 16%) eluted by a hexane/CH2Cl2 (10:3) mixture,21 and a brown band of Pt5Fe2(COD)2(CO)12 (31 mg, 2%) eluted by pure CH2Cl2.20 All compounds were identified by FTIR spectroscopy in the νCO region. Sample Preparation. Supported catalysts were prepared by slurrying of the Pt5Fe2(COD)2(CO)12 or PtFe2(COD)(CO)8 precursors in CH2Cl2 with the SiO2 support in powder form for 24 h under nitrogen flow in the absence of light. The solvent was allowed to evaporate slowly during this period of time to ensure complete uptake (18) Liu, X.; Korotkikh, O.; Farrauto, R. Appl. Catal. A: General 2002, 226, 293. (19) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem. Soc. 1976, 98, 6521. (20) Adams, R. D.; Arafa, I.; Chen, G.; Lii, J. C.; Wang, J. G. Organometallics 1990, 9, 2350. (21) Farrugia, L. J.; Howard, J. A. K.; Mitrachachon, P.; Stone, F. G. A.; Woodward P. J. Chem. Soc., Dalton Trans. 1981, 1134.

Langmuir, Vol. 22, No. 11, 2006 5161 of the precursor by the support. When the support became nearly dry, it was further evacuated at room temperature for 12 h to remove any remaining traces of the solvent. Finally, the prepared samples were stored in the absence of light to prevent possible decomposition of the supported species. The amount of each precursor was chosen to yield samples containing 1 wt % Pt after all ligands were removed. The corresponding Fe content was 0.57 and 0.11 wt % for PtFe2(COD)(CO)8/SiO2 and Pt5Fe2(COD)2(CO)12/SiO2, respectively, as determined based on the stoichiometry of the clusters. A reference 1 wt % Pt/SiO2 sample was prepared by conventional incipient wetness impregnation of SiO2 with an aqueous solution of H2PtCl6‚ 6H2O. A bimetallic sample denoted as PtFe/SiO2 was also prepared by co-impregnation. An aqueous solution precursor containing a mixture of H2PtCl6‚6H2O and Fe(NO3)3‚9H2O, in amounts calculated to yield the same final composition as the Pt5Fe2(COD)2(CO)12/ SiO2 sample (i.e., 1 wt % Pt and 0.11 wt % Fe), was used in this case. FTIR Spectroscopy. FTIR experiments were performed with both liquid and solid samples. A Thermo Nicolet Nexus 470 spectrometer was used to record spectra with a resolution of 2 cm-1 averaging 64 scans per spectrum. Liquid samples were scanned in a transmission cell equipped with NaCl windows and having a path length of 0.5 mm. Solid samples were pressed into self-supported wafers with a diameter of 12 mm and a density of approximately 30 mg/cm2 and were mounted in the IR cell. The cell construction allowed treating samples at different temperatures, while various gases were flowing through the cell. Extraction of Surface Species. Freshly prepared PtFe2(COD)(CO)8/SiO2 and Pt5Fe2(COD)2(CO)12/SiO2 samples were soaked with CH2Cl2 in a Schlenk flask under dry nitrogen flow. The suspension was stirred for 1 h, and the liquid was isolated and transferred by syringe into the IR cell. Spectra were then recorded as stated above. HRTEM. Z-contrast images and X-ray energy dispersive spectra (XEDS) were recorded on a 200kV FEI Tecnai F20 STEM/TEM electron microscope equipped with an EDAX r-TEM ultrathin window (UTW) X-ray detector. The probe size for XEDS analysis was 1 nm, allowing the composition of isolated individual particles to be quantified. Specimens were prepared by depositing the particles of the samples onto a copper grid supporting a perforated carbon film. Deposition was achieved by dipping the grid directly into the powder of the samples to avoid contact with any solvent. Catalytic Measurements. Catalytic activity measurements for the preferential oxidation of CO under excess H2 (PROX) were performed in a quartz single-pass fixed-bed microreactor at atmospheric pressure. The temperature inside the reactor was monitored by a thermocouple extended into the catalyst bed. Samples in a powder form (0.077 g) were diluted 90 times by weight with quartz particles (60-80 mesh) to maintain the catalyst bed isothermal. The total volumetric flow rate of the reactant mixture was held at 154 mL/min (1 atm, 25 °C) yielding a corresponding gas hourly space velocity (GHSV) of 120 000 mL/g‚h. Reacting gases were mixed in a gas distribution system, while the flow of each gas was controlled by a mass flow controller (model 201, Porter) to create an accurate and reproducible feed containing 0.5% CO, 0.5% O2, 45% H2, and balance N2. Before mixing, the CO/N2 mixture was heated to 350 °C in a quartz trap to eliminate any carbonyls that may have been formed in the storage cylinder. The feed and the reaction products were analyzed with on-line single beam NDIR CO (Ultramat 23, Siemens) and O2 (model 201, AMI) analyzers capable of detecting CO and O2 in the 0-250 and 0-1000 ppm ranges, respectively. The outputs from both analyzers and the temperature controller were linked to a user interface with Labview software. The reaction selectivity toward the formation of CO2 was calculated as the amount of O2 consumed in the CO oxidation reaction (calculated from the CO balance) over the total amount of O2 consumed. The catalytic oxidation of CO in the absence of H2 was carried out in equipment similar to that described above, at atmospheric pressure, a GHSV of 120 000 mL/g‚h, and temperatures between 30 and 300 °C. The reaction feed contained 1% of CO balanced with air. Both the reaction feed and the products were analyzed with an on-line single beam NDIR (Ultramat 23, Siemens) analyzer capable

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Table 1. Characteristic FTIR Bands in the νCO Region of Pt-Fe Clusters Supported on Silica and in Various Solvents cluster/sample

solvent

νCO bands (cm-1)

ref

Pt5Fe2(COD)2(CO)12/SiO2 Pt5Fe2(COD)2(CO)12 Pt5Fe2(COD)2(CO)12 Pt5Fe2(COD)2(CO)12/SiO2 PtFe2(COD)(CO)8/SiO2 PtFe2(COD)(CO)8 PtFe2(COD)(CO)8 PtFe2(COD)(CO)8/SiO2

none C6H14 CH2Cl2 treated with CH2Cl2 none C6H12 CH2Cl2 treated with CH2Cl2

2065(s), 2026(s), 2003(s), 1951(m), 1890(w) 2068(m), 2029(m), 2009(s), 2003(sh), 1962(w) 2064(s), 2024(s), 2006(sh), 1956(m), 2063(s), 2023(s), 2005(sh), 1955(m), (extracted species) 2062(s), 2024(sh), 2014(s), 1974(sh), 1889(w) 2065(m), 2013(s), 1995(w), 1981(m), 1969(m), 1945(w) 2062(s), 2010(s), 1975(s) 2062(s), 2010(s), 1975(s), (extracted species)

present work 20 present work present work present work 21 present work present work

of detecting CO in the 0-500 ppm and 0-5% ranges and CO2 in the 0-5% range. Prior to any catalytic measurements, all samples were treated with H2, while the temperature was ramped at 5 °C/min to 350 °C and held at this temperature for 2 h. Following the reduction treatment, the reactor was purged with N2 and cooled to room temperature. The reaction mixture was introduced at that point and data were collected at different times, while the temperature was raised every 2 h in 10-20 °C increments. In the absence of a catalyst, there was no measurable conversion of CO for either of the reactions examined.

Results and Discussion Deposition of Pt-Fe Clusters on the SiO2 Surface. The interaction of metal carbonyl clusters with oxide supports typically produces a variety of new surface species as indicated in the literature for monometallic and bimetallic metal carbonyls.12,22 Some carbonyl clusters can be adsorbed intact on metal oxide supports,12,22 whereas in other cases, the cluster core readily disintegrates upon impregnation.23 These results indicate that the nature of the cluster, as well as the support surface chemistry and strength of the metal-support interactions are important factors for the successful preparation of catalysts from metal carbonyl precursors. When the Pt5Fe2(COD)2(CO)12 cluster was brought in contact with the silica surface and the solvent was slowly removed by evaporation at room temperature, a brown-yellow powder with characteristic FTIR bands in the νCO region at 2065(s), 2026(s), 2003(sh), 1951(m), and 1890(w) cm-1 was obtained (Table 1 and Figure 1, spectrum 1). Some of these bands are slightly shifted from the positions observed for the same cluster in various solvents (Table 1). Moreover, a new low intensity band was observed at 1890 cm-1 in the spectrum of Pt5Fe2(COD)2(CO)12/ SiO2 but was not present in the spectra of this cluster in any of the solvents used. Differences between spectra of solid and liquid samples can be explained in part by the presence of vibrations that are allowed by the different symmetries of the molecule in different environments.24 However, the very close general agreement between the bands observed for the Pt5Fe2(COD)2(CO)12 cluster supported on silica and dissolved in CH2Cl2 or C6H14 (Table 1) suggests that the majority of the cluster species were deposited intact on the SiO2 surface and the cluster-support interactions were weak and did not alter substantially the structure of the original cluster precursor. To verify this hypothesis, the solid Pt5Fe2(COD)2(CO)12/SiO2 sample was brought in contact with CH2Cl2 in order to extract the surface species back into solution. Following such a procedure, the color of the support became once again white, whereas the color of the solution became dark brown. The infrared spectrum of the remaining solid did not include any characteristic bands of the carbonyl cluster, indicating that the cluster was completely extracted into the solution. Furthermore, the spectrum of the extracted species was (22) Alexeev, O.; Gates, B. C. Top. Catal. 2000, 10, 273. (23) Guczi, L.; Beck, A. Polyhedron 1988, 7, 2387. (24) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London, 1975.

Figure 1. FTIR spectra obtained during the decarbonylation of Pt5Fe2(COD)2(CO)12/SiO2 in H2 at various temperatures: (1) 25 °C, (2) 50 °C, (3) 100 °C, (4) 150 °C, (5) 200 °C, (6) 250 °C, (7) 300 °C, (8) 350 °C.

found to be almost identical to that of Pt5Fe2(COD)2(CO)12 dissolved in CH2Cl2 (Table 1), confirming that the precursor can be extracted intact from the silica surface. Similar results were also obtained for the PtFe2(COD)(CO)8 cluster. More specifically, when this cluster was deposited on the SiO2 surface by slow evaporation of the solvent at room temperature, the bright red powder obtained was characterized by FTIR bands in the νCO region at 2062(s), 2024(sh), 2014(s), 1974(sh), and 1889(w) cm-1 (Table 1 and Figure 2, spectrum 1). With only minor differences, this spectrum resembles that of the PtFe2(COD)(CO)8 cluster dissolved in various solvents (Table 1), suggesting that the majority of this cluster remained intact on the silica surface as well. Following treatment with CH2Cl2, the spectrum of the species extracted from the SiO2 surface was indistinguishable from the spectrum of the original PtFe2(COD)(CO)8 cluster dissolved in the same solvent (Table 1), further indicating that the cluster integrity was preserved during the interaction with SiO2. Similar to the previous case, the infrared spectrum of the remaining solid did not include any characteristic bands of the carbonyl cluster, illustrating that this cluster was also completely extracted back into the solution. In summary, the infrared data described above provide strong evidence that both the PtFe2(COD)(CO)8 and Pt5Fe2(COD)2(CO)12 clusters remained intact on the silica surface, and no reaction took place between the carbonyl or COD ligands of both clusters and the functional groups of the support. These results indicate that both clusters were simply physisorbed on

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Figure 2. FTIR spectra obtained during the decarbonylation of PtFe2(COD)(CO)8/SiO2 in H2 at various temperatures: (1) 25 °C, (2) 50 °C, (3) 100 °C, (4) 150 °C, (5) 200 °C, (6) 250 °C, (7) 300 °C, (8) 350 °C.

the silica surface and the observed infrared bands in the νCO region represent their unique fingerprints on the silica support. Decarbonylation of Pt5Fe2(COD)2(CO)12/SiO2 in H2. FTIR spectra collected during the decarbonylation of Pt5Fe2(COD)2(CO)12/SiO2 in H2 at different temperatures are shown in Figure 1. The flow of H2 over this sample at room-temperature did not affect the intensity of the different bands in the νCO region. However, when the temperature was increased to 50 °C and held constant for 60 min, the intensities of the bands at 2065, 2026, 2003, and 1951 cm-1 decreased significantly (Figure 1, spectrum 2). Analysis of the area under the different bands indicates that approximately 15% of CO ligands were removed at this temperature. The most dramatic change in the spectra was observed when the temperature was further increased to 100 °C. After steady-state conditions were reached at this temperature (i.e., after 60 min), the bands at 2065, 2026, 2003, and 1951 cm-1 completely disappeared from the spectrum, whereas the band at 1890 cm-1 declined in intensity, and a new band became evident at 2048 cm-1 (Figure 2, spectrum 3). The cluster was decarbonylated by approximately 70% at this temperature. The band at 1890 cm-1 gradually decreased in intensity and shifted to 1885 cm-1 as the temperature was further increased to 300 °C (Figure 1, spectra 4-7). In contrast, the intensity of the band at 2048 cm-1 first decreased, when temperature was raised to 150 °C, then slightly increased in the 200-250 °C temperature range, and finally, decreased once again in the 250-350 °C range. A red shift of approximately 15 cm-1 was also observed in band position between 250 and 300 °C (Figure 1, spectrum 7). When the temperature reached 350 °C, carbonyl bands were no longer present in the spectrum, indicating the complete decarbonylation of the cluster at that point (Figure 1, spectrum 8). Decarbonylation of PtFe2(COD)(CO)8/SiO2 in H2. FTIR spectra collected at various temperatures during the decarbonylation of PtFe2(COD)(CO)8/SiO2 in H2 are shown in Figure 2. Similar to the previous cluster, the flow of H2 over this sample at room temperature does not affect the intensity of the different bands in the νCO region. However, the bands at 2062, 2014, and

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1974 cm-1 decreased in intensity after H2 treatment at 50 °C for 60 min (Figure 2, spectrum 2). When the temperature was further increased to 100 °C and held constant for 60 min, the bands at 2062 and 1974 cm-1 completely disappeared from the spectrum (Figure 2, spectrum 3). A new broad band observed in the spectrum at 2020 cm-1 probably includes contributions from both the 2024 and 2014 cm-1 bands, the intensities of which also decreased at this temperature. The changes in area of the different bands indicate that approximately 60% of the carbonyl ligands were removed from the cluster following H2 treatment at 100 °C. As the temperature was increased further, the intensities of the bands at 2020 and 1889 cm-1 continued to decrease until both bands finally disappeared at 350 °C (Figure 2, spectra 3-8), indicating that complete decarbonylation of the cluster was achieved at that point. Formation of Pt-Fe Bimetallic Nanoparticles. Z-contrast HRTEM images collected for the Pt5Fe2(COD)2(CO)12/SiO2 and PtFe/SiO2 samples after the treatment with H2 at 350 °C are shown in Figure 3, panels A and B. The majority of observed metal particles were found to have sizes ranging between 1 and 2 nm for the cluster-derived Pt5Fe2/SiO2 sample. Larger particles were also observed in the HRTEM images, indicating that some metal aggregation also took place under these experimental conditions. However, the degree of sintering was apparently small. XEDS analysis performed on at least 35 randomly selected particles with sizes in the range of 1-2 nm indicates that all of these particles were bimetallic in nature. The Pt/Fe atomic ratio was approximately 2.5, identical to the Pt/Fe ratio in the original Pt5Fe2(COD)2(CO)12 cluster precursor. Therefore, these results suggest that small nanoparticles were formed by condensation of several Pt5Fe2 units that takes place during ligand removal and likely proceeds without disintegration of the cluster core. The larger particles observed in the images of Pt5Fe2/SiO2 appear to be also bimetallic in nature, as the XEDS analysis indicates the presence of both metals. However, in this case, the Pt/Fe atomic ratio was found to be on the order of 10-60, suggesting that these larger particles are enriched with Pt and were likely formed following the disintegration of some Pt5Fe2 units and the subsequent aggregation of the Pt component. Similar HRTEM data collected for the PtFe/SiO2 sample, prepared from individual Pt and Fe precursors, indicate that larger metal particles were formed after reduction at 350 °C (Figure 3B). Furthermore, the XEDS analysis indicated the presence of Pt-Fe bimetallic species in this sample with an average composition of 85% Pt and 15% Fe on a atomic basis. A substantial fraction of monometallic Pt and Fe particles was also detected by XEDS (Figure 3D), which indicates that a large fraction of both metals remained segregated from each other. In summary, the HRTEM data demonstrate that the use of bimetallic clusters as precursors for the preparation of these PtFe catalysts offers a higher degree of metal dispersion and more homogeneous mixing of the two metals, as compared to the conventional coimpregnation technique. CO Oxidation in Air. The CO conversions observed after 2 h on stream at different reaction temperatures over Pt/SiO2 and the two cluster-derived Pt-Fe catalysts are shown in Figure 4. In agreement with previous literature reports,25,26 under our experimental conditions the Pt/SiO2 catalyst pretreated in H2 at 350 °C does not exhibit any measurable activity at temperatures up to approximately 130 °C. Complete conversion of CO to CO2 (25) Gracia, F. J.; Bollmann, L.; Wolf, E. E.; Miller, J. T.; Kropf, A. J. J. Catal. 2003, 220, 382. (26) Alexeev, O. S.; Chin, S. Y.; Engelhard, M. H.; Ortiz-Soto, L.; Amiridis, M. D. J. Phys Chem. B 2005, 109, 23430.

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Figure 3. HRTEM images of (A) cluster-derived Pt5Fe2/SiO2 decarbonylated in H2 at 350 °C and (B) co-impregnated PtFe/SiO2 pretreated in H2 at 350 °C; Corresponding Pt/Fe atomic ratios measured by XEDS in randomly selected particles for (C) Pt5Fe2/SiO2 and (D) PtFe/SiO2.

was observed over this sample at approximately 200 °C (Figure 4). The light-off curve for the PtFe2/SiO2 sample is shifted to lower temperatures, and complete conversion of CO was observed in this case at approximately 160 °C. Furthermore, measurable conversion of CO was observed over this catalyst at temperatures as low as 50 °C. The Pt5Fe2/SiO2 sample exhibited even higher catalytic activity, which also surpassed that of the conventionally prepared PtFe/SiO2 sample of the same composition. A CO conversion of approximately 30% was observed over the Pt5Fe2/SiO2 sample in the 40-100 °C temperature range and complete conversion of CO was achieved at 140 °C (Figure 4). The unusual behavior observed in the low temperature regime (i.e., the conversion of CO does not appear to change with temperature) is due to the slow partial deactivation of the sample under these conditions, as discussed below. As a result, the conversions obtained after 2 h on stream and shown in Figure 4 do not necessarily represent steady-state behavior. At this point, we can only speculate on the reasons of the significant improvement of the catalytic behavior of Pt in the presence of Fe for this reaction. Our infrared data indicate that both the Pt5Fe2(COD)2(CO)12 and PtFe2(COD)(CO)8 clusters were adsorbed intact on the SiO2 surface. It is likely that the cluster core also remained mostly intact following the decarbonylation step, yielding bimetallic Pt-Fe sites in which the Pt atoms remained bonded to Fe. The subsequent nucleation of

such Pt-Fe sites at elevated temperatures would lead to the formation of Pt-Fe nanoparticles with intimate contact between the two metal components. The HRTEM data obtained for the Pt5Fe2/SiO2 sample largely support this suggestion. Close proximity between the two metals can alter the electronic state of Pt, affect the CO chemisorption on Pt, and therefore, change the reaction rate for the oxidation of CO. Similar relationships between the chemisorption of reactants and the activity for different catalytic reactions on bimetallic particles have been reported in the literature.18,27,28 It has also been reported that iron oxide species can be reduced to metallic Fe only at temperatures above 400 °C.29,30 However, TPR data reported for PtFe/γ-Al2O3 indicate that Pt promotes the reduction of supported iron oxide species by H2 at temperatures as low as 300 °C.30 Therefore, any Pt-Fe bimetallic sites formed under our experimental conditions may have incorporated Fen+ ions along with completely reduced metallic Fe, both of which could have participated in the oxidation of CO. In this case, it is possible that the catalytic oxidation of CO could proceed through (27) Sinfelt, J. H. Bimetallic Catalysts: DiscoVeries, Concepts, and Applications; Wiley: New York, 1983. (28) Ponec, V.; Bond, G. C. Catalysis by Metals and Alloys; Elsevier: Amsterdam, 1995. (29) Lobree, L. J.; Hwang, I.; Reimer, J. A.; Bell, A. T. J. Catal. 1999, 186, 242. (30) Sirijaruphan, A.; Goodwin, J. G.; Rice, R. W. J. Catal. 2004, 224, 304.

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Figure 4. CO conversions observed during the oxidation of CO in air over ([) Pt5Fe2/SiO2, (2) PtFe2/SiO2, (1) PtFe/SiO2, and (b) Pt/SiO2 pretreated in H2 at 350 °C for 2 h. (Conditions: 1% CO, balance air, GHSV of 120 000 mL/g‚h.)

the interaction of CO adsorbed on Pt with oxygen species adsorbed on the adjacent Fe0 or Fen+ sites, as it has been previously suggested for Au/Fe2O3 and PtFe/γ-Al2O3 catalysts.16,18,30,31 Close proximity between Pt and Fe is a necessary condition for such a scheme, and this was apparently achieved more efficiently with the bimetallic PtFe2/SiO2 and Pt5Fe2/SiO2 samples used in our experiments. The differences observed between the PtFe2/SiO2 and Pt5Fe2/SiO2 samples may be related to differences in the stability of the metal cluster cores derived from the two cluster precursors on the silica surface, following the decarbonylation step. It is noteworthy, that the initial activity for the oxidation of CO at each temperature examined was substantially higher for both the PtFe2/SiO2 and Pt5Fe2/SiO2 samples than that observed after 2 h on stream and shown in Figure 4. An example of the changes observed in CO conversion over the 2 h period for the Pt5Fe2/ SiO2 sample at 30 °C is shown in Figure 5. This behavior could be related, at least in part, to the partial disintegration of PtFe bimetallic sites under reaction conditions. Alternatively, it could be also related to the reoxidation of initially reduced Fe sites by air, taking place under reaction conditions, similar to what has been reported elsewhere.30 Selective Oxidation of CO in H2-Rich Environment (PROX). The conversions of CO and O2 observed at different temperatures during the selective oxidation of CO in the presence of excess hydrogen over the Pt/SiO2 and Pt5Fe2/SiO2 samples are shown in Figure 6. The results obtained for the Pt/SiO2 catalyst are similar to those reported previously for Pt/γ-Al2O3 for the same reaction.26 More specifically, under our experimental conditions, the Pt/SiO2 sample is not active at temperatures below 120 °C. As the temperature was further increased, the O2 and CO conversions were also increased in parallel, reaching maxima of 100% and 94%, respectively, simultaneously at 236 °C, and yielding a CO2 selectivity of approximately 50% at that point (Figure 7). Further increase of the reaction temperature leads to (31) Liu, H.; Kozlov, A. I.; Kozlova, A. P.; Shido, T.; Iwasawa, Y. Phys. Chem. Chem. Phys. 1999, 1, 2851.

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Figure 5. CO conversion as a function of time on stream during CO oxidation in air at 30 °C over Pt5Fe2/SiO2 decarbonylated in H2 at 350 °C. (Conditions: 1% CO, balance air, GHSV of 120 000 mL/g‚h.)

Figure 6. CO (2, b) and O2 (4, O) conversions observed at different temperatures during the PROX reaction over (2, 4) Pt5Fe2/SiO2 and (b, O) Pt/SiO2 pretreated in H2 at 350 °C. (Conditions: 0.5% CO, 0.5% O2, 45% H2, and balance N2, GHSV of 120 000 mL/g‚h.)

a decrease in the CO conversion (Figure 6) most probably due to the higher rate of the competing H2 oxidation reaction and/or a greater effect of the reverse water-gas-shift reaction at elevated temperatures.18,32 A similar behavior has been also reported for various supported metals in the literature.18,26,33 In contrast to Pt/SiO2, the Pt5Fe2/SiO2 sample exhibits nearly 40% CO conversion at temperatures as low as 50 °C (Figure 6). (32) Ouyang, X.; Bednarova, L.; Besser, R. S.; Ho, P. AIChE J. 2005, 51, 1758. (33) Chin, S. Y.; Alexeev, O. S.; Amiridis, M. D. Appl. Catal. A: General 2005, 286, 157.

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Siani et al.

Figure 7. CO2 selectivity as a function of temperature during PROX over (2) Pt5Fe2/SiO2 and (b) Pt/SiO2 pretreated in H2 at 350 °C. (Conditions: 0.5% CO, 0.5% O2, 45% H2, and balance N2, GHSV of 120 000 mL/g‚h.)

Figure 8. CO conversion as a function of time on stream during PROX at 46 °C over Pt5Fe2/SiO2 decarbonylated in H2 at 350 °C. (Conditions: 0.5% CO, 0.5% O2, 45% H2, and balance N2, GHSV of 120 000 mL/g‚h.)

The maximum in the CO conversion was reached in this case at approximately 140 °C and the conversion remained in the 90-96% range at temperatures between 140 and 170 °C. Further increase of the reaction temperature leads to a decrease in the CO conversion similar to that observed for the Pt/SiO2 sample (Figure 6). These results demonstrate that the presence of Fe in close proximity to Pt in the cluster-derived sample, not only improves the catalytic activity of Pt for this reaction, but also expands the temperature range where the maximum conversion of CO can be maintained. It is interesting to note that the CO conversion was roughly twice that of O2 at temperatures up to 60 °C, suggesting that nearly all of the oxygen was consumed in the oxidation of CO, yielding a selectivity in excess of 90% (Figure 7). This result is significant because no improvement in the PROX selectivity was previously reported for PtFe/γ-Al2O3 samples.18,30 When the temperature was increased to 100 °C, the conversion curves for CO and O2 nearly overlapped and the selectivity decreased to approximately 50%, indicating that the CO oxidation and the competing H2 oxidation reactions proceed at similar rates under these conditions. Similar to the oxidation of CO in air, the enhancement of the catalytic properties of Pt in the presence of Fe for the PROX reaction could be attributed to the operation of a noncompetitive dual site adsorption pathway leading to an overall activity increase, as suggested previously by Farrauto et al. for commercial Pt-Fe catalysts.18 This suggestion was further confirmed by isotopic transient kinetic analysis experiments on model Pt-Fe/γ-Al2O3 samples.16,30 However, in our case, the presence of Pt-Fe interactions also improved the low temperature selectivity of the Pt5Fe2/SiO2 sample during PROX. A slow partial deactivation of the Pt5Fe2/SiO2 catalyst was also observed under PROX conditions, but the magnitude of the effect was smaller in this case (Figure 8). Deactivation in other related systems has been previously attributed to the deposition of carbon on the platinum surface, leading to a decrease in the concentration of active sites.30,34 It is therefore possible that the

partial deactivation observed with the Pt5Fe2/SiO2 catalyst is not related to any structural changes. Indeed, measurements conducted with the used Pt5Fe2/SiO2 sample reproduced the results obtained with the fresh catalyst, suggesting that the exposure of this sample to the highly reducing PROX conditions does not change the structure of the active bimetallic Pt-Fe sites.

(34) Sirijaruphan, A.; Goodwin, J. G.; Rice, R. W. J. Catal. 2004, 221, 288.

Conclusions Pt5Fe2/SiO2 and PtFe2/SiO2 samples prepared from bimetallic Pt5Fe2(COD)2(CO)12 and PtFe2(COD)(CO)8 cluster precursors decarbonylated in H2 at 350 °C were found to be highly active for the oxidation of CO in the presence and absence of H2. Infrared data indicate that both cluster precursors are deposited intact on the SiO2 surface. The interactions between these clusters and the support appeared to be relatively weak and do not involve any reactions between the carbonyl or COD ligands of the clusters with functional groups of the support. HRTEM and XEDS data obtained for the cluster-derived Pt5Fe2/SiO2 sample indicate that the majority of Pt-Fe nanoparticles formed have sizes in the range of 1-2 nm. The formation of such nanoparticles is likely the result of the nucleation of Pt5Fe2 units, since the atomic Pt/Fe ratio in them is similar to the 2.5 ratio in the original Pt5Fe2(COD)2(CO)12 cluster precursor. A few larger Pt-Fe nanoparticles enriched in Pt were also observed, indicating that a small fraction of Pt5Fe2 units was segregated to the individual components following the hydrogen treatment. A higher degree of metal dispersion and more homogeneous mixing of the two metals were observed with the cluster-derived sample as compared to the conventionally prepared one. Kinetic data for the oxidation of CO in air show that both the Pt5Fe2/SiO2 and PtFe2/SiO2 samples were more active for this reaction than Pt/SiO2. The effect was more pronounced when the Pt5Fe2(COD)2(CO)12 cluster was used as the precursor. This sample showed a significant conversion of CO even at room temperature, although it deactivates with time on stream, indicating that the Pt-Fe bimetallic sites that presumably yield the enhanced reaction rate are sensitive to the reaction conditions. The Pt5Fe2/SiO2 sample also exhibited superior activity to that

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of Pt/SiO2 for the selective oxidation of CO under H2-rich conditions. In addition to the high activity for this reaction, the Pt2Fe5/SiO2 sample exhibited a selectivity of approximately 92% at temperatures up to 60 °C, indicating that the competitive reaction of H2 oxidation was substantially suppressed in this case. The enhancement of the catalytic activity in both reactions and the high selectivity in PROX could be related to the formation of PtFe bimetallic sites on the SiO2 surface, in which Pt atoms are directly bonded to Fen+ ions or reduced Fe0 atoms. The presence of iron clearly enhances the activity of platinum for CO

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oxidation, whereas it also appears to be particularly crucial for the high PROX selectivity at low temperatures. Structural characterization of the Pt5Fe2/SiO2 and PtFe2/SiO2 samples by EXAFS and other techniques is under way and will be reported separately. Acknowledgment. This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences (DE-FG02-00ER14980). LA053476A