Low Temperature Oxidation of CO over Cluster-Derived Platinum

Feb 22, 2006 - Lorna B. Ortiz-Soto, Oleg S. Alexeev, and Michael D. Amiridis*. Department of Chemical Engineering, UniVersity of South Carolina, Colum...
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Langmuir 2006, 22, 3112-3117

Low Temperature Oxidation of CO over Cluster-Derived Platinum-Gold Catalysts Lorna B. Ortiz-Soto, Oleg S. Alexeev, and Michael D. Amiridis* Department of Chemical Engineering, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed August 29, 2005. In Final Form: December 5, 2005 The structural and catalytic properties of SiO2- and TiO2-supported Pt-Au bimetallic catalysts prepared by coimpregnation were compared with those of samples of similar composition synthesized from a Pt2Au4(CtCBut)8 cluster precursor. The smallest metal particles were formed when the bimetallic cluster was used as a precursor and TiO2 as the support. FTIR data indicate that highly dispersed Au crystallites in these samples, presumably located in close proximity to Pt, are capable of linearly coordinating CO molecules with a characteristic vibration observed at 2111 cm-1. The cluster-derived Pt2Au4/TiO2 samples were the only ones exhibiting low-temperature CO oxidation activity, indicating that both the high dispersion of Au and the nature of the support are important factors affecting the catalytic activity for this system.

Introduction

Experimental Section

Initial reports by Haruta et al.,1,2 demonstrating that supported gold nanoparticles in the 3-5 nm size range are very active for the oxidation of CO even at temperatures below 0 °C, have led to extensive research efforts in this area. Subsequent reports have shown that not only the size of the gold nanoparticles but also the interaction between Au and the support, and hence the preparation method followed, are important factors affecting the catalytic performance of the resulting supported Au catalysts for various oxidation reactions.3 The ability of such catalysts to oxidize CO at room temperature can be potentially used for a number of applications, including CO detection devices, indoor air purification systems, and the purification of hydrogen streams from trace amounts of CO for fuel cell applications.4 The catalytic properties of bimetallic systems (i.e., activity and/or selectivity) can be substantially different from those observed with their monometallic counterparts.5 However, little has been reported so far in the literature regarding supported bimetallic catalysts containing gold. One such example is the supported bimetallic Pt-Au catalysts prepared from a Pt2Au4(CtCBut)8 cluster precursor that have been the focus of our recent work.6-8 Such cluster-derived Pt2Au4 catalysts supported on SiO2 were examined for several reactions including the selective catalytic reduction of NO by propylene, the oxidation and hydrogenation of propylene, and the 16O2/18O2 exchange. In all of these cases, it was shown that the presence of Au substantially affects the catalytic properties of Pt, despite the fact that Au itself is inactive for these reactions. In this paper, we report the results of our first attempts to characterize the catalytic behavior of Au in SiO2- and TiO2-supported Pt2Au4 catalysts using the oxidation of CO as a probe reaction, in which Au nanoparticles are expected to be more active than Pt.

Catalyst Synthesis. The SiO2 (Syloid 74, Grace Davison) and TiO2 (AKZO Nobel) supports were calcined in air at 500 °C for 10 h prior to their use. After such treatment, the Brunauer-EmmettTeller (BET) surface areas of SiO2 and TiO2 were found to be 290 and 180 m2/g, respectively. Monometallic Pt/TiO2 and Pt/SiO2 samples were prepared by incipient wetness impregnation of the supports with an aqueous solution of H2PtCl6‚6H2O (Alfa Aesar). The amount of precursor was chosen to yield samples containing 1 wt % of Pt. A similar procedure was used for the preparation of bimetallic Pt-Au catalysts. However, in this case the aqueous solution contained both H2PtCl6‚6H2O and HAuCl4‚3H2O (Aldrich) precursors in appropriate concentrations to yield 1 wt % of Pt and an Au/Pt atomic ratio of 2. Following the impregnation step, all samples were dried in air at 110 °C for 24 h. Cluster-derived Pt2Au4 catalysts were prepared by slurrying the Pt2Au4(CtCBut)8 cluster precursor with the SiO2 or TiO2 supports in hexanes under N2 flow for 10 h at room temperature, followed by overnight evacuation at 25 °C to remove the solvent. Prior to further characterization, each sample was activated by treatment in H2 or in a 10% O2 in He mixture at temperatures ranging from 300 to 500 °C. Characterization Techniques. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) measurements were carried out with a Phillips CM 12 and Hitachi HD-2000 instrument, respectively. Average particle sizes were determined by measuring at least 200 particles. The statistical method used to determine the mean particle surface sizes was similar to that reported elsewhere.9 Infrared spectra were collected with a Nicolet Nexus 470 spectrometer equipped with a MCT-B detector that was cooled by liquid nitrogen. Powder samples were pressed into self-supported wafers and mounted in an IR cell that was connected to a gas distribution manifold. Spectra were recorded at a spectral resolution of 4 cm-1 accumulating 64 scans per spectrum. Catalytic Measurements. The catalytic oxidation of CO was performed in a quartz single-pass fixed-bed microreactor at atmospheric pressure, a space velocity of 120 000 mL/g‚h, and temperatures between 30 and 300 °C. The reaction feed contained 1% of CO balanced with dry air. The inlet and outlet of the reactor were analyzed with an on-line single beam NDIR analyzer (Ultramat 23, Siemens) capable of detecting CO with a limit of 1 ppm. Prior to the catalytic measurements, the samples were treated in flowing H2 or a 10% O2 in He mixture as the temperature was ramped at 5 °C/min and then held at the desired value for 2 h. No measurable conversion of CO was observed in the absence of a catalyst.

* Corresponding author. Tel.: 803-777-7294. Fax: 803-777-8265. Email: [email protected]. (1) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (2) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (3) Haruta, M. CATTECH 2002, 6, 102. (4) Choudhary, T. V.; Goodman, D. W. Catal. Today 2002, 77, 65. (5) Alexeev, O. S.; Gates, B. C. Ind. Eng. Chem. Res. 2003, 42, 1571. (6) Mihut, C.; Descorme, C.; Duprez, D.; Amiridis, M. D. J. Catal. 2002, 212, 125. (7) Mihut, C.; Chandler, B. D.; Amiridis, M. D. Catal. Commun. 2002, 3, 91. (8) Ortiz-Soto, L. B.; Monnier, J. R.; Amiridis, M. D. Catal. Lett., in press.

(9) Zaikovskii, V. I.; Ryndin, Yu. A.; Koval’chuk, V. I.; Plyasova, L. M.; Kuznetsov, B. N.; Ermakov, Yu. I. Kinet. Katal. 1981, 22, 443.

10.1021/la052358k CCC: $33.50 © 2006 American Chemical Society Published on Web 02/22/2006

Low Temperature Oxidation of CO

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Table 1. Average Metal Particle Size for Various Pt and Au Catalysts (Estimated from TEM Data) catalyst composition

pretreatment conditions

average particle size (nm)

Pt/SiO2

H2 at 300 °C O2 at 500 °C H2 at 300 °C O2 at 500 °C O2 at 500 °C H2 at 200 °C O2 at 500 °C H2 at 300 °C O2 at 500 °C H2 at 300 °C O2 at 500 °C H2 at 300 °C O2 at 300 °C H2 at 300 °C O2 at 300 °C

2.2 2.0 2.5 3.0 >100 22 100 10 >100 17 >100 6.4 2.0 1.7 1.2

Pt/TiO2 Au/SiO2 Au/TiO2 Pt-Au/SiO2 Pt-Au/TiO2 Pt2Au4/SiO2 Pt2Au4/TiO2

Results and Discussion Metal Dispersion. The average particle sizes for the various SiO2- and TiO2-supported monometallic (Pt or Au) and bimetallic (PtAu) catalysts pretreated in H2 or O2 at different temperatures are summarized in Table 1. Histograms showing the distribution of metal particle sizes for various samples after different treatments are compared in Figures 1 and 2. When the Pt/SiO2 sample prepared from H2PtCl6‚6H2O was treated with H2 at 300 °C, approximately 99% of the observed platinum particles were in the size range of 1-3 nm and only 1% of the particles were as large as 4.5 nm (Figure 1A). The surface-averaged size of the Pt particles in this sample was found to be 2.2 nm (Table 1). Similarly, when the freshly prepared Pt/SiO2 was exposed to oxidation treatment at 500 °C, all particles observed in the TEM images were in the size range of 1-3 nm (data not shown for brevity), and the surface-averaged size was found to be approximately 2 nm (Table 1). However, in this case, the observed particles represent not metallic platinum but platinum oxide species, presumably formed on the SiO2 surface after such treatment. The very similar sizes of the metal particles observed for Pt/SiO2 following reduction and oxidation treatments suggest a similar thermal stability of metallic and oxidized platinum species on the SiO2 surface. The analysis of the TEM images of the Pt/TiO2 sample prepared from H2PtCl6‚6H2O and reduced in H2 at 300 °C indicates the absence of metal particles larger than 4 nm (Figure 2A). However, in this case, the fraction of particles with sizes ranging between 3 and 4 nm is evidently larger than for Pt/SiO2, yielding a surfaceaveraged size of approximately 2.5 nm (Table 1). When freshly prepared Pt/TiO2 was treated in O2 at 500 °C, the particle size distribution remained relatively narrow with all particles ranging between 1 and 4 nm (data not shown for brevity). In this case, the calculated surface-averaged size of the platinum oxide particles was found to be 3.0 nm (Table 1). The comparison of the TEM data obtained for the Pt/TiO2 and Pt/SiO2 samples indicates that the mobility of the Pt and PtO species on the TiO2 surface during thermal treatments is somewhat higher than that on SiO2. However, it is evident that the metal-support interactions on both supports were strong enough to prevent the agglomeration of platinum into particles larger than 2-3 nm on the average. In contrast, thermal treatment in either H2 or O2 of SiO2- and TiO2-supported Au monometallic catalysts prepared from HAuCl4‚3H2O led to the formation of large Au particles. The data of Table 1, for example, indicate that Au particles with an average diameter of 22 nm were formed on the TiO2 surface

Figure 1. Metal particle size distributions estimated from TEM measurements for (A) Pt/SiO2, (B) Pt-Au/SiO2, and (C) Pt2Au4/ SiO2 after treatment in H2 at 300 °C.

following reduction in H2 even at temperatures as low as 200 °C, whereas oxidation at 500 °C led to the formation of Au particles with diameters larger than 100 nm on both SiO2 and

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Figure 2. Metal particle size distributions estimated from TEM measurements for (A) Pt/TiO2, (B) Pt-Au/TiO2, and (C) Pt2Au4/ TiO2 after treatment in H2 at 300 °C.

temperatures. It is also evident that Au sinters faster in O2 than in H2 under our experimental conditions. Furthermore, the possible presence of residual Cl- ions in the samples can promote the aggregation of Au, as it was reported earlier in the literature.10 When the Pt-Au/SiO2 sample prepared by coimpregnation of aqueous solutions of H2PtCl6‚6H2O and HAuCl4‚3H2O was exposed to H2 at 300 °C, the metal particle size distribution was substantially broader than that observed for the monometallic Pt/SiO2 sample after similar treatment (Figure 1B). In this case, only 80% of the observed particles were in the 1-4 nm size range and approximately 20% were in the 5-12 nm range. An average value of approximately 10 nm was obtained for the surface-averaged size of metal particles (Table 1). Furthermore, when the freshly prepared Pt-Au/SiO2 sample was treated in O2 at 500 °C, the observed particle size distribution was even broader and the surface-averaged size of the particles observed was larger than 100 nm (Table 1). The comparison of the TEM data obtained for the Pt-Au/SiO2 sample after various treatments further indicates that the H2 treatment resulted in less sintering of metals on the SiO2 surface as compared to the O2 treatment, which took place also at a higher temperature. When TiO2 was used as the support for the preparation of the coimpregnated bimetallic PtAu/TiO2 samples, the surface-averaged sizes of the particles were found to be 17 nm and larger than 100 nm after the treatments in H2 at 300 °C and O2 at 500 °C, respectively. These values are similar to those observed for the SiO2-supported samples of similar composition (Figure 2B and Table 1). Unfortunately, the TEM data for bimetallic Pt-Au samples do not allow us to differentiate between Pt and Au due to the similar contrast and very small difference in the d spacing between (111) and (200) planes of Pt, Au, and bimetallic Pt-Au species in the optical diffraction patterns,11 which in any case, was below the line to line resolution of the TEM instrumentation used in this experiments. Elemental mapping by EDX of various particles in these samples was also ineffective due to the close proximity of the characteristic lines for gold and platinum in the X-ray spectrum. However, a simple comparison of the TEM data from Table 1 for the Au/TiO2 and Pt-Au/TiO2 samples after similar treatments indicates that the coimpregnated bimetallic sample has nearly the same average size of metal particles as the Au/ TiO2 sample. Taking into account the TEM data indicating that the surface-average size of Pt species for Pt/TiO2 and Pt/SiO2 does not exceed 3 nm after either treatment (Table 1), we can conclude that the formation of the large metal particles observed in the TEM images for the SiO2- and TiO2-supported Pt-Au samples is primarily due to the agglomeration of Au. Furthermore, it is reasonable to assume that Au is largely segregated from Pt in the coimpregnated samples. At the same time, we cannot exclude the possibility that some of the large particles formed in these samples may incorporate both metals and have in fact some bimetallic character. Finally, it is evident from our TEM data that the agglomeration of metals observed for the coimpregnated samples on both supports was substantially higher in an O2 than in a H2 environment. When the Pt2Au4(CtCBut)8 bimetallic cluster was used as a precursor and SiO2 as the support, the particle size distribution remained relatively wide following the 300 °C reduction treatment (Figure 1C). However, in this case, a larger fraction of the metal particles was located within the 1-3 nm size range, yielding a surface-averaged size for the metal particles in this sample of 6.4 nm, which is somewhat smaller than that observed for the catalyst

TiO2. These results clearly indicate that, regardless of the nature of the support, the Au-support interactions are weak and the Au particles have a high mobility on both supports at elevated

(10) Oh, H.-S.; Yang, J. H.; Costello, C. K.; Wang, Y. M.; Bare, S. R.; Kung, H. H.; Kung, M. C. J. Catal. 2002, 210, 375. (11) Sachdev, A.; Schwank, J. J. Catal. 1989, 120, 353.

Low Temperature Oxidation of CO

Figure 3. Infrared spectra of CO adsorbed at room temperature on (a) Pt2Au4/TiO2, (b) Pt-Au/TiO2, (c) Pt/TiO2, and (d) Au/TiO2 reduced in H2 at 300 °C. Spectra were recorded immediately after the removal of CO from the gas phase.

of similar composition prepared by coimpregnation (Table 1). In contrast, when an oxidation treatment at 300 °C was used, no particles larger than 3 nm were formed, and the surface-averaged size of the metal particles was found to be on the order of 2 nm (Table 1), consistent with our previous reports.6,7 These data do show that the close proximity between Pt and Au, which is originally present in the Pt2Au4(CtCBut)8 bimetallic cluster precursor, prevents sintering for both metals on the SiO2 surface. In this case, the treatment in O2 at 300 °C appears to be more favorable than the corresponding treatment in H2. Once again, we were not able to differentiate between Au and Pt crystallites in this sample or to provide solid evidence for the formation of Pt-Au bimetallic species due to the reasons mentioned above. However, we can infer that the formation of particles with sizes larger than 4 nm during the H2 treatment at 300 °C can be due to the aggregation of Au. This process may include the aggregation of entire Pt2Au4 fragments enriched with Au into core-shell structures with Pt being located primarily inside the particles due to the lower surface tension of Au. Alternatively, the partial disintegration of the Pt-Au cluster core during the ligand removal step leading to the formation and sintering of the monometallic Au species can also not be excluded. Substantial differences were observed with the TiO2-supported cluster-derived sample. The particle size distribution characterizing the Pt2Au4/TiO2 sample treated in H2 at 300 °C was very narrow with all particles observed within the 1-3 nm size range (Figure 2C). The surface-averaged particle size in this case following either reduction or oxidation at 300 °C remained below 2 nm, which is smaller than what was observed even for the monometallic Pt/TiO2 sample (Table 1). These data clearly show that in the case of Pt2Au4/TiO2 not only the original close proximity between Pt and Au, but also the nature of the support assisted in preventing sintering and maintaining both metals in a highly dispersed state under our experimental conditions. Infrared Spectra of Adsorbed CO. Spectra collected following the adsorption of CO at room temperature on reduced TiO2-supported Pt, Au, Pt-Au, and Pt2Au4 samples are shown in Figure 3. No characteristic νCO vibrations were observed in the spectra of the Au/TiO2 sample (Figure 3, spectrum d), which had an average Au particle size of approximately 22 nm (Table 1). This result is consistent with previous literature reports

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demonstrating that the adsorption of CO on Au depends on the particle size of the Au crystallites and is observed only on Au nanoparticles with sizes below 5 nm.12-14 When the Pt/TiO2 sample was exposed to CO under similar conditions, a strong band was observed at 2077 cm-1, which can be assigned to terminal CO species adsorbed on fully reduced Pt sites (Figure 3, spectrum c).15 A small shoulder, observed in the spectrum at approximately 2111 cm-1, indicates the presence of small amounts of Ptn+ cations on the catalyst surface due to the possible incomplete reduction of Pt.16 The CO adsorption on these sites is relatively strong and the band at 2111 cm-1 remained visible in the infrared spectrum even at elevated temperatures. Similar results were also obtained with the Pt-Au/TiO2 sample prepared by coimpregnation. The formation of terminally bonded CO species on fully reduced Pt and Ptn+ sites was indicated by the presence of characteristic bands at 2065 and 2111 cm-1, respectively (Figure 3, spectrum b). Since the spectra shown on Figure 3 have been collected from samples of different density, we cannot provide an accurate estimate whether the chemisorptive properties of Pt were altered or not after the addition of Au. However, a comparison of the spectrum characterizing the PtAu/TiO2 sample with the one obtained for Pt/TiO2 indicates that in this case the terminal νCO band was shifted to lower wavenumbers by 12 cm-1. Such a shift could be related to a variety of different reasons. The dispersion of Pt for example, can play a role as it was observed earlier for Pt/SiO2 and Pt/ Al2O3 samples.17 Indeed, the TEM data show that after reduction at 300 °C the Pt-Au/TiO2 sample had a larger average metal particle size when compared to Pt/TiO2 (Table 1). However, in this case, a blue shift in the νCO would be expected due to the increased coordination number of the surface Pt atoms in large particles, leading to the reduction of back-donation of electrons into the antibonding molecular orbitals of CO,18 which contradicts our observations. Another possible explanation could be related to a geometric effect, caused by a dilution of Pt surface sites by Au. If such dilution was to take place, the decreased dipoledipole coupling between adsorbed CO molecules would result in a red shift in the νCO, consistent with our observations. We also cannot ignore the possibility that PtAu bimetallic species may be formed to a certain extent in the coimpregnated PtAu/TiO2 sample and the electronic interactions between Pt and Au could drive the νCO band to lower frequencies, thus complimenting the geometric effect.18,19 However, our catalytic data discussed below, together with infrared data reported in the literature for coimpregnated Pt-Au/SiO2 samples,19 suggest that the effect of Au on the properties of Pt in coimpregnated samples are mainly geometric in nature. When similar FTIR experiments were repeated with the Pt2Au4/TiO2 sample, the terminal νCO band adsorbed on Pt was red-shifted even further to 2057 cm-1 (Figure 3, spectrum a). The observed pattern clearly indicates a stronger impact of Au on the CO chemisorption on Pt as compared to the coimpregnated samples of similar composition. Similar to the Pt-Au/TiO2 sample, this shift could be attributed at least in part to a decreasing dipole-dipole coupling between CO molecules adsorbed on Pt, (12) Haruta, M.; Date, M. Appl. Catal. A: General 2001, 222, 427. (13) Haruta, M. Catal. Today 1997, 36, 153. (14) Iizuka, Y.; Fujiki, H.; Yamauchi, N.; Chijiiwa, T.; Arai, S.; Tsubota, S.; Haruta, M. Catal. Today 1997, 36, 115. (15) Jin, T.; Zhou, Y.; Mains, G. J.; White, J. M. J. Phys. Chem. 1987, 91, 5931. (16) Daniel, D. W. J. Phys. Chem. 1988, 92, 3891. (17) Solomennikov, A. A.; Lokhov, Yu. A.; Davidov, A. A.; Ryndin, Yu. A. Kinet. Katal. 1979, 20, 714. (18) Balakrishnan, K.; Schwank, J. J. Catal. 1992, 138, 491. (19) Balakrishnan, K.; Sachdev, A.; Schwank, J. J. Catal. 1990, 121, 441.

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caused by a dilution of the Pt ensembles by Au. However, such an interpretation was previously excluded for Pt2Au4/SiO2 samples prepared from a similar cluster precursor by determination of the CO singleton frequency through the use of 12CO/13CO mixtures.6 As a result, electronic interactions between Pt and Au are most likely responsible for the observed differences in these clusterderived samples. Such electronic interactions between Pt and Au atoms located in close proximity to each other would likely cause a shift of electron density from Au to Pt. Such a shift would in turn increase the back-donation of electrons from the filled d orbitals of Pt to the CO 2π* antibonding molecular orbitals, resulting in a weakening of the CdO bond. The observed shift of the terminal νCO band to lower wavenumbers is consistent with bonding of CO on more electron-rich Pt. The results obtained suggest that such an electronic effect is more pronounced in the case of the cluster-derived Pt2Au4/TiO2, indicating closer PtAu interactions in this sample. Furthermore, in this case, the terminal νCO band exhibits an asymmetric tailing toward lower wavenumbers, suggesting the presence of a distribution of Pt sites with lower coordination numbers (i.e., reduced CO-CO dipole effect) and possibly a closer interaction with Au (i.e., increased electronic effect). Finally, the band at 2111 cm-1 appeared much stronger in the spectrum of the Pt2Au4/TiO2 sample (Figure 3, spectrum a). Furthermore, the properties of this band were not consistent with the behavior of the Ptn+-CO species observed in small amounts in the spectra of the PtAu/TiO2 and Pt/TiO2 samples (Figure 3, spectra b and c). More specifically, the intensity of this band was relatively strong only in the presence of gas-phase CO, whereas it was removed from the spectrum during flushing with He even at room temperature. Consequently, the band at 2111 cm-1 in the spectrum of Pt2Au4/TiO2 can be assigned to CO weakly adsorbed on new surface sites that are not present on the PtAu/TiO2 and Pt/TiO2 samples. These sites are different in nature from the partially reduced Ptn+ cations observed with Pt/TiO2 and Pt-Au/TiO2, even though the νCO vibrations coincide at the same frequency. On the basis of our previous work with Pt2Au4/SiO2,6 as well as other literature reports for the adsorption of CO on Au nanoparticles,20 Au atoms,21 and small Au clusters,22 we can assign the band at 2111 cm-1 in the case of the Pt2Au4/TiO2 sample to CO adsorbed on highly dispersed Au ensembles, which could constitute a fraction of the surface of bimetallic PtAu nanoparticles formed on TiO2. Such an assignment is also in agreement with literature reports indicating that heats of CO adsorption on Au are low, which is consistent with weak bonding of CO to the Au surface.23 Catalytic Oxidation of CO. Light-off curves showing the temperature dependence of CO oxidation over various TiO2and SiO2-supported catalysts, that were pretreated in either O2 or H2 as described above, are shown in Figures 4 and 5, respectively. Complete CO conversion was observed with the reduced Pt/TiO2 and Pt/SiO2 catalysts at approximately 180 and 200 °C, respectively, indicating that the nature of the support plays a role even in the case of these relatively large (i.e., 2-3 nm) Pt particles. Oxidation at 500 °C prior to the reaction shifts the light-off curves to higher temperatures with complete CO conversion being observed at 210 and 220 °C for the TiO2- and SiO2-supported samples, respectively. Consistent with previous (20) Boccuzzi, F.; Chiorino, A.; Monzoli, M. Surf. Sci. 2002, 502-503, 513. (21) Liang, B.; Andrews, L. J. Phys. Chem. A 2000, 104, 9156. (22) Jiang, L.; Xu, Q. J. Phys. Chem. A 2005, 109, 1026. (23) Derrouiche, S.; Gravejat, P.; Bianchi, D. J. Am. Chem. Soc. 2004, 126, 13010.

Ortiz-Soto et al.

Figure 4. Light off curves characterizing the oxidation of CO by air as a function of temperature over Pt/TiO2, Pt-Au/TiO2, and Pt2Au4/TiO2 treated in (A) O2 and (B) H2 at the temperatures shown in Table 1.

literature reports,24 our data also show that reduced samples were more active than oxidized ones, suggesting that metallic Pt surfaces are preferable for this reaction. A completely different pattern was observed when the clusterderived Pt2Au4/TiO2 was used. Complete conversion of CO was observed in this case at approximately 80 °C when the sample was pretreated in O2 at 300 °C and at 110 °C when the sample was pretreated in H2 at 300 °C (Figure 4, panels A and B). Since the data collected for the Pt/TiO2 sample clearly show that Pt is inactive at such low temperatures, we can confidently attribute the observed catalytic activity of the Pt2Au4/TiO2 sample to the contributions of Au. Such a conclusion is consistent with the FTIR data presented above (Figure 3), which show that Au in the Pt2Au4/TiO2 sample is capable of chemisorbing CO. Both, the O2 and H2 treatments can obviously lead to the formation of active Au sites, although the former appears to be preferable, since smaller particles are formed in this case (Table 1). On the basis of these, as well as our previous results,6 we can speculate that the metal particles formed in the cluster-derived Pt2Au4/ (24) Alexeev, O. S.; Chin, S. Y.; Engelhard, M. H.; Ortiz-Soto, L.; Amiridis, M. D. J. Phys. Chem. B 2005, 109, 23430.

Low Temperature Oxidation of CO

Figure 5. Light off curves characterizing the oxidation of CO by air as a function of temperature over Pt/SiO2, Pt-Au/SiO2, and Pt2Au4/SiO2 treated in (A) O2 and (B) H2 at the temperatures shown in Table 1.

TiO2 catalyst are likely bimetallic in nature and the close proximity of Pt and Au helps sustain Au in a highly dispersed state that is required to efficiently catalyze the CO oxidation reaction at temperatures below 100 °C. In contrast, the light-off curves characterizing the Pt-Au/ TiO2 sample prepared by coimpregnation were very similar to those of Pt/TiO2 (Figure 4, panels A and B), indicating that the presence of Au in this sample has no substantial effect on Pt activity, whereas Au remains inactive. This is not surprising, since the TEM data in this case indicate the formation of large crystallites (Table 1) and possible segregation of Pt and Au. Even though the FTIR data showed a red shift for the terminal νCO species, suggesting that some part of Au may be involved in electronic interactions with Pt, the fraction of such Au atoms is probably small. The large Au particles, which constitute the majority of Au, are not capable of adsorbing CO, so in the end, only the Pt sites remain active in this sample. Some significant differences were observed when the CO oxidation reaction was performed over SiO2-supported samples, as shown in Figure 5, panels A and B. Regardless of the conditions used to pretreat these samples, the temperature of complete CO

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conversion was increased (or the activity was decreased) in the following order: Pt/SiO2 < PtAu/SiO2 < Pt2Au4/SiO2. Previous reports have suggested that oxygen species donated by the support play a significant role in the Au-catalyzed low-temperature CO oxidation process.25-29 Silica has been frequently mentioned as the support with the least ability to provide such species. However, due to the difficulty of stabilizing Au nanoparticles on SiO2 because of the weak Au-silica interactions, the separation of the size effect from that of oxygen availability has not been easy. This task has been accomplished in the present report by the use of the cluster-derived catalysts. In particular, our characterization results presented here, as well as in our previous reports,6,7 clearly demonstrate that finely dispersed Au ensembles capable of chemisorbing CO are present in the Pt2Au4/SiO2 sample. Nevertheless, as shown in Figure 5, panels A and B, these Au sites are completely inactive for the low temperature oxidation of CO. Therefore, our results indicate that the presence of Au in a highly dispersed state is a necessary but not sufficient condition for low-temperature CO oxidation activity. Finally, the catalytic behavior of the cluster-derived Pt2Au4/ SiO2 catalyst further suggests that the presence of Au in this sample inhibits the CO oxidation ability of Pt. This behavior is consistent with our understanding of the properties and morphology of this system, as described in our previous work.6 In particular, kinetic and characterization experiments have shown that true bimetallic PtAu particles are formed in this case and that the Pt ensembles are relatively small. Pt in this configuration is believed to have a decreased ability to activate molecular oxygen, as indicated by kinetic results obtained for a number of reactions (i.e., oxidation of propylene, 16O2/18O2 homoexchange reaction, and the selective reduction of NO by propylene).6 An insufficient concentration of active oxygen species on the Pt surface can also reduce the CO oxidation rate, consistent with our current observations. In summary, the use of bimetallic Pt2Au4(CtCBut)8 clusters as precursors for the preparation of supported PtAu catalysts is a convenient way to form and maintain Au in a highly dispersed state on the surfaces of metal oxides. However, the requirement of high Au dispersion is not sufficient for the CO oxidation reaction to proceed at substantial rates in the low-temperature regime (i.e., below 100 °C). Since only the TiO2-supported clusterderived catalyst exhibited such low temperature activity for the oxidation of CO, these results appear to support previously published reports regarding the role of the support as an additional source of oxygen. Acknowledgment. This work was supported by the US Department of Energy (Grant DE-FG02-96ER14666). TEM experiments at the Oak Ridge National Laboratory (ORNL) High Temperature Materials Laboratory (HTML) were sponsored by the Assistant Secretary for Energy-Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, U.S. Department of Energy under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. The HRTEM image in the Table of Contents entry was obtained by Drs. Steve Pennycook and Albina Borisevich (ORNL). LA052358K (25) Boccuzzi, F.; Chiorino, A. Catal. Lett. 1994, 29, 225. (26) Boccuzzi, F.; Chiorino, A.; Tsubota, S.; Haruta, M. J. Phys. Chem. 1996, 100, 3625. (27) Lopez, N.; Janssens, T. V. W.; Clausen, B. S.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Norskov, J. K. J. Catal. 2004, 223, 232. (28) Liu, Z.-P.; Gong, X.-Q.; Kohanoff, J.; Sanchez, C.; Hu, P. Phys. ReV. Lett. 2003, 91, 266102. (29) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113.