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CO oxidation of Pt thin films deposited on smooth and porous Au nanorods
Applied Catalysis A: General 367 (2009) 89–92
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
CO oxidation of Pt thin films deposited on smooth and porous Au nanorods Wei Sheng Tai, Sang-Hoon Yoo, Kwang-Dae Kim, Sungho Park *, Young Dok Kim * Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 March 2009 Received in revised form 16 July 2009 Accepted 24 July 2009 Available online 3 August 2009
This study compared the activity of electrochemically deposited Pt thin films on nanoporous and smooth Au rods with various Pt thicknesses for CO oxidation. With increasing Pt thickness from 1 to 4 monolayers (ML), the activity in CO oxidation increased. Nanoporous Au rods, on which 4 ML of Pt were deposited, showed 30 times higher CO oxidation activity than smooth ones with the same amount of Pt. A higher surface area of nanoporous Au resulted in a catalytic activity of the Pt films on porous Au superior to those of the respective smooth surfaces. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Pt CO oxidation Au Nanoporous surface
1. Introduction Structure and size dependence in catalytic activity of transition metal nanoparticles have recently been attracting attention. With decreasing diameter and thickness of supported transition metal nanoparticles below 5–10 nm, enhanced activity in heterogeneous catalysis can often be observed. For Au, which is inert as bulk, a sharp increase in activity towards various reactions including CO oxidation can be found as the particle size becomes smaller than 3–4 nm [1–6]. Similar size effects in catalytic activity were also reported for Rh and Ag [7,8]. Pd nanoparticles with a mean diameter of 6–8 nm were shown to be more active towards CO oxidation than larger particles [9]. In contrast to these results, Pt nanoparticles smaller than 3–4 nm were shown to be less active than larger particles, i.e. Pt particles larger than a critical size should be used for synthesizing an efficient Pt-based catalyst for heterogeneously catalyzed reactions [6]. The origin of the size effects in catalytic activity of transition metal nanostructures is still in debate: quantum-size effects, and change in the number of edge atoms, and metal-support interaction as a function of particle thickness have been suggested to be related to the size-selectivity in chemical properties [1–9]. Nanoporous metal surfaces have recently attracted much attention in heterogeneous catalysis [10–13]. Nanoporous Au is active for low temperature CO oxidation [10–12]. Pt thin films on nanoporous Au rods exhibit an electrocatalytic activity superior to that of the smooth counterpart [13]. The nanoporous surfaces were
* Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail addresses: [email protected] (S. Park), [email protected] (Y.D. Kim). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.036
shown to be catalytically active at room temperature, so that they can be used as electrocatalysts or catalysts for room-temperature reactions; however, it is not clear whether such a high surface porosity can survive at elevated temperatures. If so, the applications of the nanoporous surfaces in heterogeneous catalysis can be extended. This study compared the activity of electrochemically deposited Pt thin films on nanoporous and smooth Au rods for CO oxidation at 200 8C. High-surface area Pt films were created by electrochemical deposition of Pt on nanoporous Au rods [13]. With increasing amounts of Pt deposited, activity for CO oxidation was measured. Different activity patterns in CO oxidation were found when Pt films with dissimilar thicknesses were used as catalysts. Deposition of 4 monolayers (ML) of Pt on nanoporous Au rods resulted in the highest catalytic activity under our experimental conditions. When Pt was deposited on smooth Au rods, much lower activity for CO oxidation than that of the respective porous surface was found. Our results show that control of the Pt thickness on the atomic scale can be used for synthesizing a catalyst with a maximum activity for a given reaction. Moreover, nanoporous surface structures can show a superior catalytic activity to smooth surfaces not only at room temperature, but also at 200 8C. 2. Experimental 2.1. Preparation of Pt thin films on smooth and nanoporous Au nanorods In a typical experiment, as a first step, a thin layer of gold (1 mm) was thermally evaporated on one side of an anodized aluminum oxide (AAO) template (from Whatman International;
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d = 13 mm, pore size = 200 nm) that served as a working electrode in a three-electrode electrochemical cell after making physical contact with a glassy carbon electrode. A Pt wire and an Ag/AgCl electrode were employed as a counter and a reference electrode, respectively. Au/Ag alloy nanorods were electrodeposited from a solution containing Au/Ag ions (mole ratio, Au+:Ag+ = 1:3, prepared from 50 mM of each aqueous solution, KAu(CN)2 and KAg(CN)2, with 0.25 M Na2CO3 as a supporting electrolyte, from Alfa Aesar) at the constant potential, 0.95 V vs. Ag/AgCl. After the alumina membrane was dissolved, the nanoporous Au nanorod array was formed from dealloying the Au/Ag alloys (selective dissolution of the less noble metal) with concentrated nitric acid [13]. The under potential deposition (UPD) layer of Cu was formed by holding the potential for 100 s at 0.1 V in 0.1 M H2SO4 as the supporting electrolyte (a supporting electrolyte solution was prepared from doubly-distilled H2SO4 from Sigma Aldrich using ultrapure water from a Millipore MillQ system) containing Cu2+ ions (5 mM CuCl2). An aqueous solution containing 5 mM H2PtCl6nH2O (n = 5.7), as the Pt metal ion source, was used in the Cu UPD layer replacement reactions. All the replacement reactions were carried out by immersing the Cu UPD layer coated Au nanorod arrays into a solution containing Pt metal ions for 10 min. 2.2. Catalytic activity measurements The activity of the various Pt/Au samples for CO oxidation was examined in a reaction chamber connected to a high-vacuum chamber equipped with a quadrupole mass-spectrometer [14]. Before the activity measurements for CO oxidation, each sample was annealed at 463 K for 2 h at a base pressure of <1.0 10 6 Torr. The chamber was filled with O2 and CO, and the change in the O2, CO and CO2 partial pressures in the reactor was monitored as a function of the reaction time at 463 K by leaking a small amount of gas from the reactor to the analysis chamber equipped with a quadruple mass-spectrometer. For all experiments, the initial partial pressure of CO was 20 mTorr, whereas that of O2 was 120 mTorr. The change in the partial pressures of gases with time was used to estimate the degree of CO–CO2 conversion. Pt/Au samples were placed on a Ta foil, and the sample temperature was measured by a thermocouple (C-type) spot-welded on the backside of the Ta foil. The Ta sample holder was resistively heated during the experiments.
Fig. 1. SEM images of smooth and nanoporous Au rods.
3. Results and discussion The structures and electrochemical properties of Pt layers of smooth and nanoporous Au rods have been published in more detail elsewhere [10]. Fig. 1 shows scanning electron microscopy (SEM) images of smooth and nanoporous Au rods, demonstrating a higher surface roughness of nanoporous Au rods. Fig. 2 shows cyclic voltammograms (CV) data of various Pt/nanoporous Au samples, indicating that the adsorption state of pure Au nearly completely disappeared after the deposition of 1 ML of Pt, i.e. most of the Au surface was covered by Pt after deposition of l ML of Pt at room temperature [10]. With increasing Pt thickness, Pt-related signals in the CV data increased in intensity, most likely due to a Stranski–Krastanov type growth of Pt on Au (3D-islands formation of Pt after completion of a layer of Pt), leading to the increase in the Pt surface area with increasing Pt thickness. Fig. 3 shows results of the CO oxidation experiments of bare smooth Au rods and those with different amounts of Pt deposited. Bare Au rods did not show any activity for CO oxidation under our conditions, and the activity in CO oxidation increased with increasing Pt thickness. When 1 or 2 ML of Pt were deposited, a
Fig. 2. Cyclic voltammograms (50 mV/s, in 0.1 M H2SO4) for nanoporous Au rod arrays before and after coating with 1–5 ML of Pt.
constant reaction rate for the CO oxidation could be found. Note that the first order differentiations of the curves in Fig. 3 reflect temporary reaction rates [14]. After 900 s, the CO–CO2 conversion of smooth Au rods with 4 ML of Pt was higher than that with 2 ML of Pt by a factor of 3, i.e. when the CO–CO2 conversion was normalized by the amount of Pt deposited, the sample with 4 ML of Pt showed the highest normalized activity in Fig. 3. The catalyticactivity-patterns of the surfaces with 2 and 4 ML of Pt were dissimilar; instead of a constant reaction rate, which could be found for the sample with 2 ML of Pt, an exponential increase in the CO/CO2 conversion was observed with increasing reaction time,
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Fig. 4. Results of the CO oxidation experiment, when Pt-bulk structure was used as catalyst.
Fig. 3. (a) Catalytic activity of the Pt-thin films on smooth Au rods for CO oxidation was measured at 463 K with initial O2 and CO pressures of 120 and 20 mTorr, respectively. thickness of Pt film was varied from 1 to 4 ML. (b) A magnified view of the initial stage of the CO oxidation experiments for the samples with 2 and 4 ML of Pt.
when 4 ML of Pt were deposited on smooth Au rods, i.e., the reaction rate was accelerated with time for this sample. A close inspection of the initial stage of the CO oxidation experiments revealed that the sample with 4 ML of Pt resulted in a lower activity than that of the sample with 2 ML of Pt for the first 500 s of the CO oxidation experiment; however, the catalytic activity of the sample with 4 ML of Pt became much higher as the reaction time increased (Fig. 3b). For comparison, activity of a Pt-bulk structure (Pt-wire with a purity of 99.99%)) for CO oxidation was studied under the same experimental conditions as those of Pt/Au systems. Similar to the results of Au surfaces covered by 4 ML of Pt, an acceleration of the reaction rate with time was found on the Pt-bulk sample (Fig. 4). In contrast, a constant reaction rate could be observed under the same conditions, when Pt nanoparticles <5 nm in diameter supported by Ta-oxide were used as catalyst [15]. This result implies that the acceleration of the reaction rate with time is characteristic for Pt-bulk structures, i.e. the data in Fig. 3 correspond to the evolution of the bulk-characteristics of Pt in activity for CO oxidation with increasing Pt thickness. It is worth mentioning that results similar to ours were found for CO oxidation on Pd, Rh and Ru single crystal surfaces: the activity of these Pt-group metal surfaces for CO oxidation increased with increasing reaction time [9]. These results were attributed to the fact that oxygen coverage is increased as a function of reaction time, and the catalytic activity is enhanced when oxygen coverage is increased. Most likely, the lateral repulsion between chemisorbed oxygen atoms can decrease the binding energy of atomic oxygen on the surface, as previous studies have shown [9,16].
Consequently, CO can more easily react with atomic oxygen on metal surfaces to form CO2 with increasing reaction time and oxygen coverage [9,15]. A higher partial pressure of O2 with respect to that of CO resulted in an enhanced catalytic activity, in line with this scenario [9,15]. For comparison, nanoporous Au rods covered by Pt with different thicknesses were used as catalysts for CO oxidation (Fig. 5). With increasing Pt thickness, catalytic activity for CO oxidation increased, in agreement with the respective data of smooth Au rods. For thinner Pt films, the reaction rate was nearly constant during the reaction, similar to the results of smooth surfaces with the same Pt thickness. As the amount of Pt deposited exceeded 3 ML, bulk-like behavior of Pt in the CO oxidation experiment (increase in the reaction rate with time) could be found. When 5 ML of Pt were deposited on nanoporous Au rods, the CO oxidation rate was more slowly increased comparing to the sample with 4 ML of Pt. Nanoporous Au rods with 4 ML of Pt were shown to be the most active catalyst for CO oxidation among the samples studied here. In order to shed light on the structure of Pt on nanoporous Au rods as a function of amount of Pt deposited, we used X-ray photoelectron spectroscopy (XPS) (Fig. 6). The samples were characterized using XPS after CO oxidation experiments. The Pt(4f)/Au(4f) intensity ratio in XPS increased by a factor of 8, when the amount of Pt deposited was increased from 2 to 4 ML (Fig. 6a). This result implies that the number of active Pt sites for CO oxidation was drastically enhanced as the Pt thickness was
Fig. 5. (a) Catalytic activity of the Pt-thin films on nanoporous Au rods was measured at 463 K with initial O2 and CO pressures of 120 and 20 mTorr, respectively. The thickness of Pt film was varied from 1 to 5 ML.
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Fig. 7. Catalytic activity for CO oxidation of smooth Au rods covered by 4 ML of Pt and that of the nanoporous counterpart are compared.
nanoporous surface was shown to be higher than that of the smooth surface by a factor of 30. Primarily, the higher surface area of nanoporous Au rods should be responsible for the enhanced catalytic activity of Pt layers on nanoporous Au rods. It is worth mentioning that the dissimilarity in optical properties (color) of smooth and nanoporous Au rods was preserved after the CO oxidation experiments. This result also implies that the nanoporous surface structure of Au rods survived after the CO oxidation reactions at elevated temperatures. 4. Conclusion Fig. 6. XPS spectra of nanoporous Au rods with 1,2 and 4 ML of Pt deposited. XPS data were taken after CO oxidation experiments. (a) Changes in Pt (4f)/Au((4f) peak intensity ratio as a function of amount of Pt deposited are summarized. (b) Each Pt 4f2/7 spectrum was normalized with respect to its maximum intensity. In the parentheses, the full-width of half maximum (FWHM) values of each peak is given.
increased from 2 to 4 monoalyers (ML). It can be suggested that, during heat treatment, a significant Pt/Au-mixing took place when Pt thickness was low. Consequently, the sample surface with 2 ML of Pt deposited was only partially covered by Pt. Additionally deposited Pt could undergo less Pt/Au-mixing, and as a consequence, the number of active Pt-sites could be largely enhanced as the amount of Pt deposited was increased from 2 to 4 ML. When 1 ML of Pt was deposited on nanoporous Au, the Pt 4f7/2 level was centered at 71 eV (Fig. 6b). When 2 ML of Pt were deposited, the Pt 4f7/2 level was shifted to the higher binding energy by 0.2 eV with respect to the sample with 1 ML of Pt (Fig. 6b). Additional deposition of Pt did not result in any change in the binding energy of the Pt 4f7/2 level; however, the peak became narrower when the amount of Pt deposited was increased from 2 to 4 ML (Fig. 6b). The dissimilar electronic properties of Pt for the sample with 1 ML of Pt can be attributed to electronic modification of Pt by Au. Narrowing of the Pt 4f7/2 level by increasing the amount of Pt from 2 to 4 ML implies that the electronic nature of Pt became less heterogeneous with increasing amount of Pt, i.e. with increasing amount of Pt deposited, terrace-size of Pt domains may have been increased. As the metal domain size increases, the relative amount of edge atoms decreases, and as a consequence, the electronic properties of metal can become more homogeneous [17]. Fig. 7 compares initial stages of the CO oxidation experiments of smooth Au rods and nanoporous Au rods covered with 4 ML of deposited Pt, showing a much higher catalytic activity for the nanoporous surfaces. For the first 270 s, the catalytic activity of the
In order to synthesize a highly efficient Pt-based catalyst for CO oxidation, we have deposited Pt on porous Au rods by using electrochemical deposition. Catalytic activity for CO oxidation was measured as a function of thickness of Pt at 463 K. Deposition of 4 ML of Pt resulted in the highest activity for CO oxidation. Porous surfaces could show a higher catalytic activity than their nonporous counterparts. Acknowledgements This study was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Government (MOST) (R01-2007-000-10280-0). References [1] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301–309. [2] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175–192. [3] M.S. Chen, D.W. Goodman, Science 306 (2004) 252–255. [4] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647–1650. [5] R. Meyer, C. Lemire, Sh.K. Shaikhutdinov, H.-J. Freund, Gold Bull. 37 (2004) 72–124. [6] M. Haruta, Gold Bull. 37 (2004) 27–36. [7] L.A. de Oliveira, A. Wolf, F. Schu¨th, Catal. Lett. 73 (2001) 157–160. [8] M.E. Grass, Y. Zhang, D.R. Butcher, J.Y. Park, Y. Li, H. Bluhm, K.M. Bratlie, T. Zhang, G.A. Somorjai, Angew. Chem. Int. Ed. 47 (2008) 8893–8896. [9] M.S. Chen, Y. Cal, Z. Yan, K.K. Gath, S. Alexandra, D.W. Goodman, Surf. Sci. 601 (2007) 5326–5331. [10] V. Zielasek, B. Jurgens, C. Schulz, J. Biener, M.M. Biener, A.V. Hamza, M. Baumer, Angew. Chem. Int. Ed. 45 (2006) 8241–8244. [11] C. Xu, J. Su, X. Xu, P. Liu, H. Zhao, F. Tian, Y. Ding, J. Am. Chem. Soc. 129 (2007) 42–43. [12] C. Xu, X. Xu, J. Su, Y. Ding, J. Catal. 252 (2007) 243–248. [13] S.H. Yoo, S. Park, Adv. Mater. 19 (2007) 1612–1615. [14] H.O. Seo, S.H. Jeong, H.J. Lee, H.-G. Jee, J.-H. Boo, D.C. Lim, Y.D. Kim, Appl. Catal. A. Gen. 347 (2008) 112–116. [15] W.S. Tai, Y.D.Kim, unpublished results. [16] Y.D. Kim, S. Wendt, S. Schwegmann, H. Over, G. Ertl, Surf. Sci. 418 (1998) 267–272. [17] M. Mavrikakis, P. Stoltze, J. Nørskov, Catal. Lett. 64 (2001) 101–106.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
CO oxidation of Pt thin films deposited on smooth and porous Au nanorods Wei Sheng Tai, Sang-Hoon Yoo, Kwang-Dae Kim, Sungho Park *, Young Dok Kim * Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 March 2009 Received in revised form 16 July 2009 Accepted 24 July 2009 Available online 3 August 2009
This study compared the activity of electrochemically deposited Pt thin films on nanoporous and smooth Au rods with various Pt thicknesses for CO oxidation. With increasing Pt thickness from 1 to 4 monolayers (ML), the activity in CO oxidation increased. Nanoporous Au rods, on which 4 ML of Pt were deposited, showed 30 times higher CO oxidation activity than smooth ones with the same amount of Pt. A higher surface area of nanoporous Au resulted in a catalytic activity of the Pt films on porous Au superior to those of the respective smooth surfaces. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Pt CO oxidation Au Nanoporous surface
1. Introduction Structure and size dependence in catalytic activity of transition metal nanoparticles have recently been attracting attention. With decreasing diameter and thickness of supported transition metal nanoparticles below 5–10 nm, enhanced activity in heterogeneous catalysis can often be observed. For Au, which is inert as bulk, a sharp increase in activity towards various reactions including CO oxidation can be found as the particle size becomes smaller than 3–4 nm [1–6]. Similar size effects in catalytic activity were also reported for Rh and Ag [7,8]. Pd nanoparticles with a mean diameter of 6–8 nm were shown to be more active towards CO oxidation than larger particles [9]. In contrast to these results, Pt nanoparticles smaller than 3–4 nm were shown to be less active than larger particles, i.e. Pt particles larger than a critical size should be used for synthesizing an efficient Pt-based catalyst for heterogeneously catalyzed reactions [6]. The origin of the size effects in catalytic activity of transition metal nanostructures is still in debate: quantum-size effects, and change in the number of edge atoms, and metal-support interaction as a function of particle thickness have been suggested to be related to the size-selectivity in chemical properties [1–9]. Nanoporous metal surfaces have recently attracted much attention in heterogeneous catalysis [10–13]. Nanoporous Au is active for low temperature CO oxidation [10–12]. Pt thin films on nanoporous Au rods exhibit an electrocatalytic activity superior to that of the smooth counterpart [13]. The nanoporous surfaces were
* Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail addresses: [email protected] (S. Park), [email protected] (Y.D. Kim). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.036
shown to be catalytically active at room temperature, so that they can be used as electrocatalysts or catalysts for room-temperature reactions; however, it is not clear whether such a high surface porosity can survive at elevated temperatures. If so, the applications of the nanoporous surfaces in heterogeneous catalysis can be extended. This study compared the activity of electrochemically deposited Pt thin films on nanoporous and smooth Au rods for CO oxidation at 200 8C. High-surface area Pt films were created by electrochemical deposition of Pt on nanoporous Au rods [13]. With increasing amounts of Pt deposited, activity for CO oxidation was measured. Different activity patterns in CO oxidation were found when Pt films with dissimilar thicknesses were used as catalysts. Deposition of 4 monolayers (ML) of Pt on nanoporous Au rods resulted in the highest catalytic activity under our experimental conditions. When Pt was deposited on smooth Au rods, much lower activity for CO oxidation than that of the respective porous surface was found. Our results show that control of the Pt thickness on the atomic scale can be used for synthesizing a catalyst with a maximum activity for a given reaction. Moreover, nanoporous surface structures can show a superior catalytic activity to smooth surfaces not only at room temperature, but also at 200 8C. 2. Experimental 2.1. Preparation of Pt thin films on smooth and nanoporous Au nanorods In a typical experiment, as a first step, a thin layer of gold (1 mm) was thermally evaporated on one side of an anodized aluminum oxide (AAO) template (from Whatman International;
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d = 13 mm, pore size = 200 nm) that served as a working electrode in a three-electrode electrochemical cell after making physical contact with a glassy carbon electrode. A Pt wire and an Ag/AgCl electrode were employed as a counter and a reference electrode, respectively. Au/Ag alloy nanorods were electrodeposited from a solution containing Au/Ag ions (mole ratio, Au+:Ag+ = 1:3, prepared from 50 mM of each aqueous solution, KAu(CN)2 and KAg(CN)2, with 0.25 M Na2CO3 as a supporting electrolyte, from Alfa Aesar) at the constant potential, 0.95 V vs. Ag/AgCl. After the alumina membrane was dissolved, the nanoporous Au nanorod array was formed from dealloying the Au/Ag alloys (selective dissolution of the less noble metal) with concentrated nitric acid [13]. The under potential deposition (UPD) layer of Cu was formed by holding the potential for 100 s at 0.1 V in 0.1 M H2SO4 as the supporting electrolyte (a supporting electrolyte solution was prepared from doubly-distilled H2SO4 from Sigma Aldrich using ultrapure water from a Millipore MillQ system) containing Cu2+ ions (5 mM CuCl2). An aqueous solution containing 5 mM H2PtCl6nH2O (n = 5.7), as the Pt metal ion source, was used in the Cu UPD layer replacement reactions. All the replacement reactions were carried out by immersing the Cu UPD layer coated Au nanorod arrays into a solution containing Pt metal ions for 10 min. 2.2. Catalytic activity measurements The activity of the various Pt/Au samples for CO oxidation was examined in a reaction chamber connected to a high-vacuum chamber equipped with a quadrupole mass-spectrometer [14]. Before the activity measurements for CO oxidation, each sample was annealed at 463 K for 2 h at a base pressure of <1.0 10 6 Torr. The chamber was filled with O2 and CO, and the change in the O2, CO and CO2 partial pressures in the reactor was monitored as a function of the reaction time at 463 K by leaking a small amount of gas from the reactor to the analysis chamber equipped with a quadruple mass-spectrometer. For all experiments, the initial partial pressure of CO was 20 mTorr, whereas that of O2 was 120 mTorr. The change in the partial pressures of gases with time was used to estimate the degree of CO–CO2 conversion. Pt/Au samples were placed on a Ta foil, and the sample temperature was measured by a thermocouple (C-type) spot-welded on the backside of the Ta foil. The Ta sample holder was resistively heated during the experiments.
Fig. 1. SEM images of smooth and nanoporous Au rods.
3. Results and discussion The structures and electrochemical properties of Pt layers of smooth and nanoporous Au rods have been published in more detail elsewhere [10]. Fig. 1 shows scanning electron microscopy (SEM) images of smooth and nanoporous Au rods, demonstrating a higher surface roughness of nanoporous Au rods. Fig. 2 shows cyclic voltammograms (CV) data of various Pt/nanoporous Au samples, indicating that the adsorption state of pure Au nearly completely disappeared after the deposition of 1 ML of Pt, i.e. most of the Au surface was covered by Pt after deposition of l ML of Pt at room temperature [10]. With increasing Pt thickness, Pt-related signals in the CV data increased in intensity, most likely due to a Stranski–Krastanov type growth of Pt on Au (3D-islands formation of Pt after completion of a layer of Pt), leading to the increase in the Pt surface area with increasing Pt thickness. Fig. 3 shows results of the CO oxidation experiments of bare smooth Au rods and those with different amounts of Pt deposited. Bare Au rods did not show any activity for CO oxidation under our conditions, and the activity in CO oxidation increased with increasing Pt thickness. When 1 or 2 ML of Pt were deposited, a
Fig. 2. Cyclic voltammograms (50 mV/s, in 0.1 M H2SO4) for nanoporous Au rod arrays before and after coating with 1–5 ML of Pt.
constant reaction rate for the CO oxidation could be found. Note that the first order differentiations of the curves in Fig. 3 reflect temporary reaction rates [14]. After 900 s, the CO–CO2 conversion of smooth Au rods with 4 ML of Pt was higher than that with 2 ML of Pt by a factor of 3, i.e. when the CO–CO2 conversion was normalized by the amount of Pt deposited, the sample with 4 ML of Pt showed the highest normalized activity in Fig. 3. The catalyticactivity-patterns of the surfaces with 2 and 4 ML of Pt were dissimilar; instead of a constant reaction rate, which could be found for the sample with 2 ML of Pt, an exponential increase in the CO/CO2 conversion was observed with increasing reaction time,
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Fig. 4. Results of the CO oxidation experiment, when Pt-bulk structure was used as catalyst.
Fig. 3. (a) Catalytic activity of the Pt-thin films on smooth Au rods for CO oxidation was measured at 463 K with initial O2 and CO pressures of 120 and 20 mTorr, respectively. thickness of Pt film was varied from 1 to 4 ML. (b) A magnified view of the initial stage of the CO oxidation experiments for the samples with 2 and 4 ML of Pt.
when 4 ML of Pt were deposited on smooth Au rods, i.e., the reaction rate was accelerated with time for this sample. A close inspection of the initial stage of the CO oxidation experiments revealed that the sample with 4 ML of Pt resulted in a lower activity than that of the sample with 2 ML of Pt for the first 500 s of the CO oxidation experiment; however, the catalytic activity of the sample with 4 ML of Pt became much higher as the reaction time increased (Fig. 3b). For comparison, activity of a Pt-bulk structure (Pt-wire with a purity of 99.99%)) for CO oxidation was studied under the same experimental conditions as those of Pt/Au systems. Similar to the results of Au surfaces covered by 4 ML of Pt, an acceleration of the reaction rate with time was found on the Pt-bulk sample (Fig. 4). In contrast, a constant reaction rate could be observed under the same conditions, when Pt nanoparticles <5 nm in diameter supported by Ta-oxide were used as catalyst [15]. This result implies that the acceleration of the reaction rate with time is characteristic for Pt-bulk structures, i.e. the data in Fig. 3 correspond to the evolution of the bulk-characteristics of Pt in activity for CO oxidation with increasing Pt thickness. It is worth mentioning that results similar to ours were found for CO oxidation on Pd, Rh and Ru single crystal surfaces: the activity of these Pt-group metal surfaces for CO oxidation increased with increasing reaction time [9]. These results were attributed to the fact that oxygen coverage is increased as a function of reaction time, and the catalytic activity is enhanced when oxygen coverage is increased. Most likely, the lateral repulsion between chemisorbed oxygen atoms can decrease the binding energy of atomic oxygen on the surface, as previous studies have shown [9,16].
Consequently, CO can more easily react with atomic oxygen on metal surfaces to form CO2 with increasing reaction time and oxygen coverage [9,15]. A higher partial pressure of O2 with respect to that of CO resulted in an enhanced catalytic activity, in line with this scenario [9,15]. For comparison, nanoporous Au rods covered by Pt with different thicknesses were used as catalysts for CO oxidation (Fig. 5). With increasing Pt thickness, catalytic activity for CO oxidation increased, in agreement with the respective data of smooth Au rods. For thinner Pt films, the reaction rate was nearly constant during the reaction, similar to the results of smooth surfaces with the same Pt thickness. As the amount of Pt deposited exceeded 3 ML, bulk-like behavior of Pt in the CO oxidation experiment (increase in the reaction rate with time) could be found. When 5 ML of Pt were deposited on nanoporous Au rods, the CO oxidation rate was more slowly increased comparing to the sample with 4 ML of Pt. Nanoporous Au rods with 4 ML of Pt were shown to be the most active catalyst for CO oxidation among the samples studied here. In order to shed light on the structure of Pt on nanoporous Au rods as a function of amount of Pt deposited, we used X-ray photoelectron spectroscopy (XPS) (Fig. 6). The samples were characterized using XPS after CO oxidation experiments. The Pt(4f)/Au(4f) intensity ratio in XPS increased by a factor of 8, when the amount of Pt deposited was increased from 2 to 4 ML (Fig. 6a). This result implies that the number of active Pt sites for CO oxidation was drastically enhanced as the Pt thickness was
Fig. 5. (a) Catalytic activity of the Pt-thin films on nanoporous Au rods was measured at 463 K with initial O2 and CO pressures of 120 and 20 mTorr, respectively. The thickness of Pt film was varied from 1 to 5 ML.
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Fig. 7. Catalytic activity for CO oxidation of smooth Au rods covered by 4 ML of Pt and that of the nanoporous counterpart are compared.
nanoporous surface was shown to be higher than that of the smooth surface by a factor of 30. Primarily, the higher surface area of nanoporous Au rods should be responsible for the enhanced catalytic activity of Pt layers on nanoporous Au rods. It is worth mentioning that the dissimilarity in optical properties (color) of smooth and nanoporous Au rods was preserved after the CO oxidation experiments. This result also implies that the nanoporous surface structure of Au rods survived after the CO oxidation reactions at elevated temperatures. 4. Conclusion Fig. 6. XPS spectra of nanoporous Au rods with 1,2 and 4 ML of Pt deposited. XPS data were taken after CO oxidation experiments. (a) Changes in Pt (4f)/Au((4f) peak intensity ratio as a function of amount of Pt deposited are summarized. (b) Each Pt 4f2/7 spectrum was normalized with respect to its maximum intensity. In the parentheses, the full-width of half maximum (FWHM) values of each peak is given.
increased from 2 to 4 monoalyers (ML). It can be suggested that, during heat treatment, a significant Pt/Au-mixing took place when Pt thickness was low. Consequently, the sample surface with 2 ML of Pt deposited was only partially covered by Pt. Additionally deposited Pt could undergo less Pt/Au-mixing, and as a consequence, the number of active Pt-sites could be largely enhanced as the amount of Pt deposited was increased from 2 to 4 ML. When 1 ML of Pt was deposited on nanoporous Au, the Pt 4f7/2 level was centered at 71 eV (Fig. 6b). When 2 ML of Pt were deposited, the Pt 4f7/2 level was shifted to the higher binding energy by 0.2 eV with respect to the sample with 1 ML of Pt (Fig. 6b). Additional deposition of Pt did not result in any change in the binding energy of the Pt 4f7/2 level; however, the peak became narrower when the amount of Pt deposited was increased from 2 to 4 ML (Fig. 6b). The dissimilar electronic properties of Pt for the sample with 1 ML of Pt can be attributed to electronic modification of Pt by Au. Narrowing of the Pt 4f7/2 level by increasing the amount of Pt from 2 to 4 ML implies that the electronic nature of Pt became less heterogeneous with increasing amount of Pt, i.e. with increasing amount of Pt deposited, terrace-size of Pt domains may have been increased. As the metal domain size increases, the relative amount of edge atoms decreases, and as a consequence, the electronic properties of metal can become more homogeneous [17]. Fig. 7 compares initial stages of the CO oxidation experiments of smooth Au rods and nanoporous Au rods covered with 4 ML of deposited Pt, showing a much higher catalytic activity for the nanoporous surfaces. For the first 270 s, the catalytic activity of the
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