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Adsorption dynamics of CO on copper and gold clusters supported on silica – How special is nanogold?
Chemical Physics Letters 517 (2011) 59–61
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Adsorption dynamics of CO on copper and gold clusters supported on silica – How special is nanogold? J. Shan, M. Komarneni, U. Burghaus ⇑ Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND, USA
a r t i c l e
i n f o
Article history: Received 18 July 2011 In final form 30 September 2011 Available online 19 October 2011
a b s t r a c t A twist about an atomistic explanation for the unusual particle size-dependent reactivity enhancement of supported gold clusters still exists in the surface science and catalysis communities. A molecular beam scattering technique allowed the active sites of a catalyst to be mapped. In doing so, copper and gold clusters, both supported on silica, were studied. Indeed, Au clusters showed quite unusual behavior, however, that would be consistent with the influence of defects rather than quantum size effects. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The idea of discrete adsorption sites on surfaces dates back to Langmuir’s days. Related to that, the concept of active sites was introduced as early as 1925, [1] but it is still used in modern surface science, catalysis, and nanoscience [2–5]. A recent and impressive example of the ‘active site’ concept is the direct correlation of rim sites on supported MoSx clusters to their catalytic activity for the hydrodesulfurization of thiophene and electrochemical H2 formation [6,7]. Although the nanogold system in particular has been studied extensively [4,5,8,9], few studies have focused on the effect of supported nano-size clusters on gas-surface energy transfer processes (adsorption dynamics) [10–13]. The adsorption of gas-phase species, however, is always the first step in any heterogeneously catalyzed surface reaction. Therefore, the adsorption dynamics can influence the entire reaction mechanism. An efficient way to explore these effects is to measure the so-called initial adsorption probability. S0 quantifies the zero coverage (adsorbate surface density) reactivity of a catalyst toward the adsorption of a given gas-phase species. The zero coverage limit is particularly important for technical applications since industrial catalysis typically takes place at greater temperatures surmounting activation barriers and increasing reaction rates. S0 is determined by the gas-surface energy transfer processes and allows the number of active sites on a catalyst surface to be quantified. CO is traditionally used as a probe molecule. One of the most important prototype surface reactions is the conversion of CO to CO2. The first elementary step is the adsorption of CO by itself. Both systems studied show low temperature activity for the CO oxidation reaction [13,14]. However, for nanogold distinct cluster size effects are well-known [8]. The atomistic de-
tails of these cluster size effects are still debated as well as how unique nanogold actually is (see e.g. Refs. [10,15–20]). Indeed, we also see unusual adsorption dynamics on the gold system in contrast to nanocopper clusters that obey standard adsorption dynamics. Thus, nanogold is special also in this regard. Our results can naturally be explained by application of the active site concept in a sense that defect sites also dominate the reactivity of Au clusters in molecular beam scattering experiments. In this project, we utilized supersonic molecular beam scattering techniques to measure S0 for silica-supported Au and Cu clusters. The results obtained on the gold clusters were presented recently [21]. A direct comparison of these two systems, however, provides a clear and current example of the active sites concept on a catalyst surface. 2. Experimental procedures The measurements were conducted using a home-built, triplydifferentially pumped molecular beam scattering apparatus connected to an ultra-high vacuum scattering chamber [22]. Au and Cu were vapor deposited on the atomically clean amorphous silica support. Sample cleaning procedures are detailed in Ref. [23]. 3. Data presentation and discussion Two pieces of information are required to finally discuss the results for the silica-supported gold and copper clusters. First, how to interpret molecular beam scattering data such as S0 plots? Second, what does the morphology of the clusters look like? 3.1. Capture zone model
⇑ Corresponding author. Fax: +1 701 231 8831. E-mail address: [email protected] (U. Burghaus). URL: http://www.chem.ndsu.nodak.edu (U. Burghaus). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.09.076
The data shown here were collected at an adsorption temperature of negligible CO uptake on the silica supports [24,25].
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J. Shan et al. / Chemical Physics Letters 517 (2011) 59–61
However, even in that case, the lifetime of the probe molecules on the surface is sufficiently long enough to allow for diffusion of the CO molecules from the support to the metal deposits [24]. In other words, the support provides most of the landing sites for the gasphase species that dock on the high-energy binding sites, the clusters, as airplanes in hangars. This idea is referred to as ‘capture zone models’ (CZM) and was introduced early on [26]. Numerous applications of the CZM to various systems can be found in the literature [24,27,28]. The CZM is simply a precursor model (similar to the historic Kisliuk model [29]) that takes the effect of the supported clusters into account. Thus, the CZM provides information primarily about the precursor lifetime. 3.2. Cluster growth mode Irrespective of the differences in the fine details, the cluster growth mode for gold [30–32] and copper [33–40] on silica is very similar. In the nucleation phase (at small total metal coverages), initially small 2D clusters form. Therefore, increasing the amount of deposited metal, v, initially results in an increase in the cluster density, while keeping the cluster size fairly constant. When all nucleation sites are saturated, the clusters start to grow and start to form larger and taller 3D structures. Finally, when v is increased further, thick films are built up. This cluster growth mode was also evident in our study [41] applying Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy, and scanning electron microscopy [21]. The labels ‘nucleation phase,’ ‘growth phase,’ etc., added to Figure 1, are mostly based on our AES data, [41] compared to what is known from the literature. (See Supplemental document) At low Cu exposures, a typical cluster size amounts to 0.1–0.3 nm, whereas at large Cu coverages a continuous copper grain structure (approximately 20 nm) was reported [33,35]. Similarly, in the nucleation phase sub 10 nm Au clusters form [21]. Generally, adsorbates (e.g., CO) can adsorb along the rim and edges and on the terrace sites of the supported clusters. However, in contrast to copper and other systems, for gold large clusters are unreactive. In simple terms, CO does not adsorb on Au terrace sites. For example, the total coverage of CO on small Au clusters is larger than on large clusters [19] since large clusters are dominated by terrace sites rather than rim sites. The opposite holds true for Cu clusters. The initial adsorption probability, S0, is basically the ratio of the exposed gas-phase species to the adsorbed species. Therefore, measuring S0 precisely with molecular beam scattering techniques determines basically the number of adsorption sites on the catalyst surface. 3.3. Copper clusters Figure 1 depicts the initial adsorption probability, S0, as a function of cluster deposition time, v, for copper (Figure 1A) and gold (Figure 1B) clusters supported on silica. The S0(v) curves obtained for Au and Cu look very different. Whereas Au clusters show a maximum in S0(v) curves at v = 10 s, S0(v) is a smooth step-like function for supported Cu clusters. The shape of S0(v) seen for copper is predicted by the CZM. The gold data, however, are very unusual. The initial increase in S0 as a function of Cu coverage (Figure 1A) is predicted by the CZM. Accordingly, S0 initially increases since the combined capture zone formed around all Cu deposits increases within the nucleation regime. If the cluster’s coverage and their dispersion become large enough, the individual capture zones finally overlap. At that point, a further increase (or change) in catalyst activity (i.e., in S0) with increasing v cannot be expected, assuming that Cu clusters remain catalytically active, irrespective of their size. The initial increase in S0 is a kinetics effect since the
Figure 1. Initial adsorption probability, S0, for CO on two different model catalysts, as a function of metal deposition time. (A) copper clusters on silica, (B) gold clusters on silica. Several independent measurements were averaged. The growth mode was determined by Auger electron spectroscopy in comparison with the literature. (Surface temperature Ts 90 K, CO impact energy Ei = 0.39 eV.)
size of an individual capture zone is given by the ratio of the surface residence time and the diffusion time (both depend on the adsorption temperature). The absolute value of S0, however, is determined by the dynamics of the energy transfer processes. In fact, S0 for large Cu coverages coincides with the values obtained for copper single crystals [42]. 3.4. Gold clusters Explaining the shape of the S0(v) curve for gold, however, requires invoking the unique cluster-size-dependent reactivity of nanogold. During the nucleation stage (v < 10 s) of the Au clusters, the cluster density increases rapidly with cluster deposition time v. Therefore, S0 (see Figure 1B) increases initially with v, since the number of catalytically active sites increases, similarly to the Cu system described above. This effect is in contrast to site blocking effects [43], well-known from studies about catalyst poisoning. In this case, S0 drops due to the blocking of catalytically active sites.
J. Shan et al. / Chemical Physics Letters 517 (2011) 59–61
Here, S0 initially increases since active sites are formed while Au clusters are deposited. However, increasing v after the nucleation regime also results in an increase in the cluster size (growth regime). The particle density remains essentially constant [31,32]. Hence, the relative fraction of low-coordinated gold sites decreases. (This can also be shown theoretically by employing the so-called Wulf construction of clusters [44].) Therefore, for the supported Au clusters, S0 decreases within the cluster growth regime, since Au terrace sites are unreactive, but low-coordinated sites are reactive. (Or, in simple terms, large 2D Au clusters are less reactive than small ones.) Interestingly, the maximum in S0(v), corresponds to an Au cluster size of approximately 2 nm as determined by statistical analysis of SEM images [25]. This size has been identified numerous times as the catalytically most active phase of gold clusters [45]. The variations in S0 are, however, rather small. Obviously, not only the adsorption dynamics of CO governs the catalytic activity of nanogold in the CO oxidation reaction.
4. Conclusions The variation in S0(v) for copper and gold clusters nicely reflects the variation in the density of catalytically active sites. Indeed, nanogold is also special when it comes to the adsorption dynamics of a standard probe molecule such as CO. Two limiting cases were considered: a system that clearly shows cluster size effects and one with weak cluster size dependent adsorption dynamics. The interpretation proposed here would not directly require inclusion of electronic effects or quantum size effects, although these certainly exist.
Acknowledgements Financial support by an NSF-CAREER award (CHE-0743932) is acknowledged. In addition, a supplemental equipment grant (for acquiring the 2nd hand XPS system) from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy is acknowledged (Project DE-FG02-08ER15987).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2011.09.076.
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References [1] H.S. Taylor, Proc. R. Soc. London, Ser. A 108 (1925) 105. [2] T. Zambelli, J. Wintterlin, J. Trost, G. Ertl, Science 273 (1996) 1688. [3] J.K. Nørskov, T. Bligaard, B. Hvolbæk, F. Abild-Pedersen, I. Chorkendorff, C.H. Christensen, Chem. Soc. Rev. 37 (2008) 2163. [4] U. Heinz, U. Landman, in: U. Heiz, U. Landman (Eds.), Nanocatalysis (NanoScience and Technology), Springer, 2007 (ISBN 978-3-540-74551-8). [5] B. Zhou, S. Hermans, G.A. Somorjai (Eds.), Nanotechnology in Catalysis, Springer series: nanostructure science and technology, Springer, 2004 (ISBN 0-306-48323-8). [6] J. Kibsgaard, J.V. Lauritsen, E. Laegsgaard, B.S. Clausen, H. Topsoe, F. Besenbacher, J. Am. Chem. Soc. 128 (2006) 13950. [7] T.F. Jaramillo, K.P. Jorgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Charkendorff, Science 317 (2007) 100. [8] M. Haruta, Catal. Today 36 (1997) 153. [9] T.V. Choudhary, D.W. Goodman, Top. Catal 21 (2002) 25. [10] T.S. Kim, J.D. Stiehl, C.T. Reeves, R.J. Meyer, C.B. Mullins, J. Am. Chem. Soc. 125 (2003) 2018. [11] J.D. Stiehl, T.S. Kim, S.M. McClure, C.B. Mullins, J. Am. Chem. Soc. 126 (2004) 1606. [12] J.D. Stiehl, T.S. Kim, C.T. Reeves, R.J. Meyer, C.B. Mullins, J. Phys. Chem. B 108 (2004) 7917. [13] S. Lee, C. Fan, T. Wu, S.L. Anderson, J. Am. Chem. Soc. 126 (2004) 5682. [14] T. Sueyoshi, T. Sasaki, Y. Iwasawa, Surf. Sci. 343 (1995) 1. [15] M.S. Chen, D.W. Goodman, Science 306 (2004) 252. [16] C. Lemire, R. Meyer, S. Shaikhutdinov, H.J. Freund, Angew. Chem. Int. Ed. 43 (2004) 118. [17] C.T. Campbell, Science 306 (2004) 234. [18] A. Vittadini, A. Selloni, J. Chem. Phys. 117 (2002) 353. [19] W.L. Yim et al., J. Phys. Chem. C 111 (2007) 445. [20] G. Pacchioni, A.M. Ferrari, P.S. Bagus, Surf. Sci. 350 (1996) 159. [21] E. Kadossov, S. Cabrini, U. Burghaus, J. Mol. Catal. A Chem. 321 (2010) 101. [22] J. Wang, U. Burghaus, J. Chem. Phys. 122 (2005) 044705. [23] S. Funk, T. Nurkic, U. Burghaus, Appl. Surf. Sci. 253 (2007) 4860. [24] J. Wang, U. Burghaus, J. Chem. Phys. 123 (2005) 184716. [25] E. Kadossov, U. Burghaus, Chem. Phys. Lett. 483 (2009) 250. [26] F. Rumpf, H. Poppa, M. Boudard, Langmuir 4 (1988) 722. [27] M. Bowker, P. Stone, R. Bennett, N. Perkins, Surf. Sci. 497 (2002) 155. [28] C.R. Henry, C. Chapon, C. Duriez, J. Chem. Phys. 95 (1995) 700. [29] P. Kisliuk, J. Phys. Chem. Solids 3 (1957) 95. [30] K. Luo, D.Y. Kim, D.W. Goodman, J. Mol. Catal. A 167 (2001) 191. [31] W.T. Wallace, B.K. Min, D.W. Goodman, J. Mol. Catal. A 228 (2005) 3. [32] B.K. Min, W.T. Wallace, A.K. Santra, D.W. Goodman, J. Phys. Chem. B 108 (2004) 16339. [33] X. Xu, S.M. Vesecky, D.W. Goodman, Science 258 (1992) 788. [34] X. Xu, J.W. He, D.W. Goodman, Surf. Sci. 284 (1993) 103. [35] X. Xu, D.W. Goodman, J. Phys. Chem. 97 (1993) 683. [36] X. Xu, D.W. Goodman, Appl. Phys. Lett. 61 (1992) 1799. [37] D.E. Piercea, R.P. Burnsa, K.A. Gabrielb, Thin Solid Films 206 (1991) 340. [38] P. Dumas, R.G. Tobina, P.L. Richardsa, Surf. Sci. 171 (1986) 579. [39] J. Cechal, J. Polcak, M. Kolibal, P. Babor, T. Sikola, Surf. Sci. 256 (2010) 3636. [40] J.B. Zhou, T. Gustaffsson, E. Garfunkel, Surf. Sci. 372 (1997) 21. [41] M. Komarneni, J. Shan, U. Burghaus, J. Phys. Chem. C 115 (2011) 16590. [42] S. Kneitz, J. Gemeinhardt, H. Koschel, G. Held, H.P. Steinrück, Surf. Sci. 433 (1999) 27. [43] U. Burghaus, J. Ding, W.H. Weinberg, Surf. Sci. 384 (1997) L869. [44] S. Schimpf et al., Catal. Today 72 (2002) 63. [45] Y. Iizuka, H. Fujiki, N. Yamauchi, T. Chijiiwa, S. Arai, S. Tsubota, M. Haruta, Catal. Today 36 (1997) 115.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Adsorption dynamics of CO on copper and gold clusters supported on silica – How special is nanogold? J. Shan, M. Komarneni, U. Burghaus ⇑ Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND, USA
a r t i c l e
i n f o
Article history: Received 18 July 2011 In final form 30 September 2011 Available online 19 October 2011
a b s t r a c t A twist about an atomistic explanation for the unusual particle size-dependent reactivity enhancement of supported gold clusters still exists in the surface science and catalysis communities. A molecular beam scattering technique allowed the active sites of a catalyst to be mapped. In doing so, copper and gold clusters, both supported on silica, were studied. Indeed, Au clusters showed quite unusual behavior, however, that would be consistent with the influence of defects rather than quantum size effects. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The idea of discrete adsorption sites on surfaces dates back to Langmuir’s days. Related to that, the concept of active sites was introduced as early as 1925, [1] but it is still used in modern surface science, catalysis, and nanoscience [2–5]. A recent and impressive example of the ‘active site’ concept is the direct correlation of rim sites on supported MoSx clusters to their catalytic activity for the hydrodesulfurization of thiophene and electrochemical H2 formation [6,7]. Although the nanogold system in particular has been studied extensively [4,5,8,9], few studies have focused on the effect of supported nano-size clusters on gas-surface energy transfer processes (adsorption dynamics) [10–13]. The adsorption of gas-phase species, however, is always the first step in any heterogeneously catalyzed surface reaction. Therefore, the adsorption dynamics can influence the entire reaction mechanism. An efficient way to explore these effects is to measure the so-called initial adsorption probability. S0 quantifies the zero coverage (adsorbate surface density) reactivity of a catalyst toward the adsorption of a given gas-phase species. The zero coverage limit is particularly important for technical applications since industrial catalysis typically takes place at greater temperatures surmounting activation barriers and increasing reaction rates. S0 is determined by the gas-surface energy transfer processes and allows the number of active sites on a catalyst surface to be quantified. CO is traditionally used as a probe molecule. One of the most important prototype surface reactions is the conversion of CO to CO2. The first elementary step is the adsorption of CO by itself. Both systems studied show low temperature activity for the CO oxidation reaction [13,14]. However, for nanogold distinct cluster size effects are well-known [8]. The atomistic de-
tails of these cluster size effects are still debated as well as how unique nanogold actually is (see e.g. Refs. [10,15–20]). Indeed, we also see unusual adsorption dynamics on the gold system in contrast to nanocopper clusters that obey standard adsorption dynamics. Thus, nanogold is special also in this regard. Our results can naturally be explained by application of the active site concept in a sense that defect sites also dominate the reactivity of Au clusters in molecular beam scattering experiments. In this project, we utilized supersonic molecular beam scattering techniques to measure S0 for silica-supported Au and Cu clusters. The results obtained on the gold clusters were presented recently [21]. A direct comparison of these two systems, however, provides a clear and current example of the active sites concept on a catalyst surface. 2. Experimental procedures The measurements were conducted using a home-built, triplydifferentially pumped molecular beam scattering apparatus connected to an ultra-high vacuum scattering chamber [22]. Au and Cu were vapor deposited on the atomically clean amorphous silica support. Sample cleaning procedures are detailed in Ref. [23]. 3. Data presentation and discussion Two pieces of information are required to finally discuss the results for the silica-supported gold and copper clusters. First, how to interpret molecular beam scattering data such as S0 plots? Second, what does the morphology of the clusters look like? 3.1. Capture zone model
⇑ Corresponding author. Fax: +1 701 231 8831. E-mail address: [email protected] (U. Burghaus). URL: http://www.chem.ndsu.nodak.edu (U. Burghaus). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.09.076
The data shown here were collected at an adsorption temperature of negligible CO uptake on the silica supports [24,25].
60
J. Shan et al. / Chemical Physics Letters 517 (2011) 59–61
However, even in that case, the lifetime of the probe molecules on the surface is sufficiently long enough to allow for diffusion of the CO molecules from the support to the metal deposits [24]. In other words, the support provides most of the landing sites for the gasphase species that dock on the high-energy binding sites, the clusters, as airplanes in hangars. This idea is referred to as ‘capture zone models’ (CZM) and was introduced early on [26]. Numerous applications of the CZM to various systems can be found in the literature [24,27,28]. The CZM is simply a precursor model (similar to the historic Kisliuk model [29]) that takes the effect of the supported clusters into account. Thus, the CZM provides information primarily about the precursor lifetime. 3.2. Cluster growth mode Irrespective of the differences in the fine details, the cluster growth mode for gold [30–32] and copper [33–40] on silica is very similar. In the nucleation phase (at small total metal coverages), initially small 2D clusters form. Therefore, increasing the amount of deposited metal, v, initially results in an increase in the cluster density, while keeping the cluster size fairly constant. When all nucleation sites are saturated, the clusters start to grow and start to form larger and taller 3D structures. Finally, when v is increased further, thick films are built up. This cluster growth mode was also evident in our study [41] applying Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy, and scanning electron microscopy [21]. The labels ‘nucleation phase,’ ‘growth phase,’ etc., added to Figure 1, are mostly based on our AES data, [41] compared to what is known from the literature. (See Supplemental document) At low Cu exposures, a typical cluster size amounts to 0.1–0.3 nm, whereas at large Cu coverages a continuous copper grain structure (approximately 20 nm) was reported [33,35]. Similarly, in the nucleation phase sub 10 nm Au clusters form [21]. Generally, adsorbates (e.g., CO) can adsorb along the rim and edges and on the terrace sites of the supported clusters. However, in contrast to copper and other systems, for gold large clusters are unreactive. In simple terms, CO does not adsorb on Au terrace sites. For example, the total coverage of CO on small Au clusters is larger than on large clusters [19] since large clusters are dominated by terrace sites rather than rim sites. The opposite holds true for Cu clusters. The initial adsorption probability, S0, is basically the ratio of the exposed gas-phase species to the adsorbed species. Therefore, measuring S0 precisely with molecular beam scattering techniques determines basically the number of adsorption sites on the catalyst surface. 3.3. Copper clusters Figure 1 depicts the initial adsorption probability, S0, as a function of cluster deposition time, v, for copper (Figure 1A) and gold (Figure 1B) clusters supported on silica. The S0(v) curves obtained for Au and Cu look very different. Whereas Au clusters show a maximum in S0(v) curves at v = 10 s, S0(v) is a smooth step-like function for supported Cu clusters. The shape of S0(v) seen for copper is predicted by the CZM. The gold data, however, are very unusual. The initial increase in S0 as a function of Cu coverage (Figure 1A) is predicted by the CZM. Accordingly, S0 initially increases since the combined capture zone formed around all Cu deposits increases within the nucleation regime. If the cluster’s coverage and their dispersion become large enough, the individual capture zones finally overlap. At that point, a further increase (or change) in catalyst activity (i.e., in S0) with increasing v cannot be expected, assuming that Cu clusters remain catalytically active, irrespective of their size. The initial increase in S0 is a kinetics effect since the
Figure 1. Initial adsorption probability, S0, for CO on two different model catalysts, as a function of metal deposition time. (A) copper clusters on silica, (B) gold clusters on silica. Several independent measurements were averaged. The growth mode was determined by Auger electron spectroscopy in comparison with the literature. (Surface temperature Ts 90 K, CO impact energy Ei = 0.39 eV.)
size of an individual capture zone is given by the ratio of the surface residence time and the diffusion time (both depend on the adsorption temperature). The absolute value of S0, however, is determined by the dynamics of the energy transfer processes. In fact, S0 for large Cu coverages coincides with the values obtained for copper single crystals [42]. 3.4. Gold clusters Explaining the shape of the S0(v) curve for gold, however, requires invoking the unique cluster-size-dependent reactivity of nanogold. During the nucleation stage (v < 10 s) of the Au clusters, the cluster density increases rapidly with cluster deposition time v. Therefore, S0 (see Figure 1B) increases initially with v, since the number of catalytically active sites increases, similarly to the Cu system described above. This effect is in contrast to site blocking effects [43], well-known from studies about catalyst poisoning. In this case, S0 drops due to the blocking of catalytically active sites.
J. Shan et al. / Chemical Physics Letters 517 (2011) 59–61
Here, S0 initially increases since active sites are formed while Au clusters are deposited. However, increasing v after the nucleation regime also results in an increase in the cluster size (growth regime). The particle density remains essentially constant [31,32]. Hence, the relative fraction of low-coordinated gold sites decreases. (This can also be shown theoretically by employing the so-called Wulf construction of clusters [44].) Therefore, for the supported Au clusters, S0 decreases within the cluster growth regime, since Au terrace sites are unreactive, but low-coordinated sites are reactive. (Or, in simple terms, large 2D Au clusters are less reactive than small ones.) Interestingly, the maximum in S0(v), corresponds to an Au cluster size of approximately 2 nm as determined by statistical analysis of SEM images [25]. This size has been identified numerous times as the catalytically most active phase of gold clusters [45]. The variations in S0 are, however, rather small. Obviously, not only the adsorption dynamics of CO governs the catalytic activity of nanogold in the CO oxidation reaction.
4. Conclusions The variation in S0(v) for copper and gold clusters nicely reflects the variation in the density of catalytically active sites. Indeed, nanogold is also special when it comes to the adsorption dynamics of a standard probe molecule such as CO. Two limiting cases were considered: a system that clearly shows cluster size effects and one with weak cluster size dependent adsorption dynamics. The interpretation proposed here would not directly require inclusion of electronic effects or quantum size effects, although these certainly exist.
Acknowledgements Financial support by an NSF-CAREER award (CHE-0743932) is acknowledged. In addition, a supplemental equipment grant (for acquiring the 2nd hand XPS system) from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy is acknowledged (Project DE-FG02-08ER15987).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2011.09.076.
61
References [1] H.S. Taylor, Proc. R. Soc. London, Ser. A 108 (1925) 105. [2] T. Zambelli, J. Wintterlin, J. Trost, G. Ertl, Science 273 (1996) 1688. [3] J.K. Nørskov, T. Bligaard, B. Hvolbæk, F. Abild-Pedersen, I. Chorkendorff, C.H. Christensen, Chem. Soc. Rev. 37 (2008) 2163. [4] U. Heinz, U. Landman, in: U. Heiz, U. Landman (Eds.), Nanocatalysis (NanoScience and Technology), Springer, 2007 (ISBN 978-3-540-74551-8). [5] B. Zhou, S. Hermans, G.A. Somorjai (Eds.), Nanotechnology in Catalysis, Springer series: nanostructure science and technology, Springer, 2004 (ISBN 0-306-48323-8). [6] J. Kibsgaard, J.V. Lauritsen, E. Laegsgaard, B.S. Clausen, H. Topsoe, F. Besenbacher, J. Am. Chem. Soc. 128 (2006) 13950. [7] T.F. Jaramillo, K.P. Jorgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Charkendorff, Science 317 (2007) 100. [8] M. Haruta, Catal. Today 36 (1997) 153. [9] T.V. Choudhary, D.W. Goodman, Top. Catal 21 (2002) 25. [10] T.S. Kim, J.D. Stiehl, C.T. Reeves, R.J. Meyer, C.B. Mullins, J. Am. Chem. Soc. 125 (2003) 2018. [11] J.D. Stiehl, T.S. Kim, S.M. McClure, C.B. Mullins, J. Am. Chem. Soc. 126 (2004) 1606. [12] J.D. Stiehl, T.S. Kim, C.T. Reeves, R.J. Meyer, C.B. Mullins, J. Phys. Chem. B 108 (2004) 7917. [13] S. Lee, C. Fan, T. Wu, S.L. Anderson, J. Am. Chem. Soc. 126 (2004) 5682. [14] T. Sueyoshi, T. Sasaki, Y. Iwasawa, Surf. Sci. 343 (1995) 1. [15] M.S. Chen, D.W. Goodman, Science 306 (2004) 252. [16] C. Lemire, R. Meyer, S. Shaikhutdinov, H.J. Freund, Angew. Chem. Int. Ed. 43 (2004) 118. [17] C.T. Campbell, Science 306 (2004) 234. [18] A. Vittadini, A. Selloni, J. Chem. Phys. 117 (2002) 353. [19] W.L. Yim et al., J. Phys. Chem. C 111 (2007) 445. [20] G. Pacchioni, A.M. Ferrari, P.S. Bagus, Surf. Sci. 350 (1996) 159. [21] E. Kadossov, S. Cabrini, U. Burghaus, J. Mol. Catal. A Chem. 321 (2010) 101. [22] J. Wang, U. Burghaus, J. Chem. Phys. 122 (2005) 044705. [23] S. Funk, T. Nurkic, U. Burghaus, Appl. Surf. Sci. 253 (2007) 4860. [24] J. Wang, U. Burghaus, J. Chem. Phys. 123 (2005) 184716. [25] E. Kadossov, U. Burghaus, Chem. Phys. Lett. 483 (2009) 250. [26] F. Rumpf, H. Poppa, M. Boudard, Langmuir 4 (1988) 722. [27] M. Bowker, P. Stone, R. Bennett, N. Perkins, Surf. Sci. 497 (2002) 155. [28] C.R. Henry, C. Chapon, C. Duriez, J. Chem. Phys. 95 (1995) 700. [29] P. Kisliuk, J. Phys. Chem. Solids 3 (1957) 95. [30] K. Luo, D.Y. Kim, D.W. Goodman, J. Mol. Catal. A 167 (2001) 191. [31] W.T. Wallace, B.K. Min, D.W. Goodman, J. Mol. Catal. A 228 (2005) 3. [32] B.K. Min, W.T. Wallace, A.K. Santra, D.W. Goodman, J. Phys. Chem. B 108 (2004) 16339. [33] X. Xu, S.M. Vesecky, D.W. Goodman, Science 258 (1992) 788. [34] X. Xu, J.W. He, D.W. Goodman, Surf. Sci. 284 (1993) 103. [35] X. Xu, D.W. Goodman, J. Phys. Chem. 97 (1993) 683. [36] X. Xu, D.W. Goodman, Appl. Phys. Lett. 61 (1992) 1799. [37] D.E. Piercea, R.P. Burnsa, K.A. Gabrielb, Thin Solid Films 206 (1991) 340. [38] P. Dumas, R.G. Tobina, P.L. Richardsa, Surf. Sci. 171 (1986) 579. [39] J. Cechal, J. Polcak, M. Kolibal, P. Babor, T. Sikola, Surf. Sci. 256 (2010) 3636. [40] J.B. Zhou, T. Gustaffsson, E. Garfunkel, Surf. Sci. 372 (1997) 21. [41] M. Komarneni, J. Shan, U. Burghaus, J. Phys. Chem. C 115 (2011) 16590. [42] S. Kneitz, J. Gemeinhardt, H. Koschel, G. Held, H.P. Steinrück, Surf. Sci. 433 (1999) 27. [43] U. Burghaus, J. Ding, W.H. Weinberg, Surf. Sci. 384 (1997) L869. [44] S. Schimpf et al., Catal. Today 72 (2002) 63. [45] Y. Iizuka, H. Fujiki, N. Yamauchi, T. Chijiiwa, S. Arai, S. Tsubota, M. Haruta, Catal. Today 36 (1997) 115.