J. Phys. Chem. C 2007, 111, 445-451
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Universal Phenomena of CO Adsorption on Gold Surfaces with Low-Coordinated Sites Wai-Leung Yim,† Tobias Nowitzki,‡ Mandus Necke,§ Hanno Schnars,§ Patricia Nickut,§ Ju1 rgen Biener,⊥ Monika M. Biener,⊥ Volkmar Zielasek,*,‡ Katharina Al-Shamery,§ Thorsten Klu1 ner,† and Marcus Ba1 umer*,‡ Institut fu¨r Reine und Angewandte Chemie, Theoretische Chemie, UniVersita¨t Oldenburg, D-26111 Oldenburg, Germany, Institut fu¨r Angewandte und Physikalische Chemie, UniVersita¨t Bremen, Postfach 330440, D-28334 Bremen, Germany, Institut fu¨r Reine und Angewandte Chemie, Physikalische Chemie, UniVersita¨t Oldenburg, Postfach 2503, D-26111 Oldenburg, Germany, and Nanoscale Synthesis and Characterization Laboratory, Lawrence LiVermore National Laboratory, 7000 East AVenue, LiVermore, California 94550 ReceiVed: October 6, 2006
Since Au turned out to be an active catalyst for CO oxidation at low temperatures, CO adsorption on various Au surfaces has been in the scope of numerous surface science studies. Interestingly, supported particles as well as stepped and rough single-crystal surfaces exhibit very similar adsorption behavior. To elucidate the origin of these similarities, we have performed temperature-programmed desorption and infrared absorption spectroscopy for a whole range of Au surfaces from nanoparticles grown on HOPG to Au(111) surfaces roughened by argon ion bombardment. In line with previous results, we have observed two desorption states at ∼130-145 and ∼170-185 K, respectively, and one infrared peak at around 2120 cm-1 in all cases. In addition to the experiments, we have carried out theoretical studies of CO adsorption on Au(332). The calculations show that CO desorption states above 100 K may be located at step-edges but not on terrace sites. Reducing the coordination of Au atoms further leads to successively higher binding energies with an unchanged anharmonic frequency. Therefore, we conclude that both desorption peaks belong to CO on low-coordinated Au atoms at steps and kinks. For the sputtered Au(111) surface, scanning tunneling microscopy reveals a rough pit-and-mound morphology with a large number of such sites. In annealing experiments we observe that the loss of these sites coincides with the loss of CO adsorption capacity, corroborating our conclusions.
Introduction Since Haruta et al. reported CO oxidation at temperatures as low as 200 K, catalyzed by nanosized Au particles on oxide supports,1 the mechanisms of the unexpected catalytic activity of gold, which is inert as a bulk material, have been the target of numerous surface science studies.2-10 Irrespective of the complexity of possible adsorption and reaction schemes on supported particles, which are being debated up to date, there is evidence that low-coordinated Au surface sites are a crucial prerequisite. At such sites, oxygen is more easily dissociated, as calculations indicate,11 and CO binds more strongly than at regular terrace sites, as theory and experiments have revealed.12-16 In particular, the CO binding energy was calculated to increase from 0.3 eV on densely packed Au(111) surfaces to values above 0.5 eV when adsorption on stepped surfaces or less densely packed Au surfaces is investigated.17,18 Previous calculations by Lopez et al. for the Au(211) surface and an Au10 cluster have found that the binding of CO to the surface is stronger as the coordination of the adsorption site is reduced,12 which Kim et al. confirmed experimentally.16 Experimental results for the CO binding energy, probed by temperature-programmed desorption (TPD) spectroscopy, have been reported for some selected Au surfaces with low* Address correspondence to these authors. E-mail: mbaeumer@ uni-bremen.de;
[email protected]. † Institut fu ¨ r Reine und Angewandte Chemie, Theoretische Chemie, Universita¨t Oldenburg. ‡ Institut fu ¨ r Angewandte und Physikalische Chemie, Universita¨t Bremen. § Institut fu ¨ r Reine und Angewandte Chemie, Physikalische Chemie, Universita¨t Oldenburg. ⊥ Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory.
coordinated sites, so far. On Au(332) Ruggiero and Hollins found two desorption states above 100 K.19 For Au(110)-(2 × 1), exhibiting a missing row surface reconstruction, only one TPD peak at ∼145 K was observed. Another at ∼180 K, however, was detected after the sample surface was roughened by argon ion bombardment.13 Also for Au nanoparticles on wellordered oxide supports20,21 and on highly orientated pyrolytic graphite (HOPG)22 two peaks have been identified in TPD spectra. Summing up experimental evidence, TPD indicates two CO species desorbing at about 120-145 and 170-210 K, respectively, almost independent of the system under study. To the best of our knowledge, atomic details of this kind of universal CO adsorption behavior have not been resolved, yet, neither theoretically nor experimentally. Reporting on a combined experimental and theoretical effort, we will demonstrate that two types of low-coordinated sites dominate the adsorption of CO at temperatures above 100 K for the whole range of Au substrates ranging from stepped single-crystal surfaces to small supported Au particles. TPD and infrared (IR) absorption spectroscopy data for CO binding energies and vibrational frequencies will be presented for CO adsorbed on Au(111) roughened by sputtering as well as on Au nanoparticles deposited on HOPG. By choosing HOPG as the substrate, we avoid any ambiguities that a strong particlesubstrate interaction present on some oxide substrates would impose on our analysis. Both types of substrates were studied in the same apparatus under the same conditions so that the universal behavior of CO adsorption on Au surfaces is unequivocally revealed. In addition, for the sputtered Au(111) surfaces, combined scanning tunneling microscopy (STM) and TPD studies enabled us to directly correlate surface morphology
10.1021/jp0665729 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2006
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and desorption states. Relating the experimental results to density functional theory (DFT) calculations for CO adsorption at Au(332) model surfaces, we find that CO presumably adsorbs on-top of Au step and kink sites. In contrast to previous calculations,12 we have varied the coordination of the adsorption site within a single underlying surface model to systematically extract the influence of coordination, and studied various adsorption sites and geometries. Furthermore, CO vibrational frequencies were determined for direct comparison with experiment. As reported previously and somehow puzzling at first sight, the IR data show a single absorption band while two TPD peaks are observed. Our calculations will solve this puzzle by showing that in contrast to the binding energy, the vibrational frequency of CO is hardly affected by the coordination of the adsorption site. Experimental Section The experiments were performed in an STM/TPD system and a separate Fourier-transforming (FT) IR system in ultrahigh vacuum (base pressure ∼10-10 mbar). The STM/TPD system comprises two compartments: One is equipped with a commercial STM/AFM instrument (Omicron), X-ray photoelectron spectroscopy (Leybold EA10 plus and VG dual anode X-ray tube), and a low-energy electron diffraction unit (Omicron SpectaLEED). The other compartment houses metal evaporators, a focusable ion gun (Specs), and a differentially pumped quadrupole mass spectrometer (Hiden) for TPD. The temperature ramp was driven by a digital temperature controller and power supply (Schlichting Physikalische Instrumente HS 130 and HS 720). In the STM/TPD system, samples could be transferred via a small, Omicron-type carrier plate between the two compartments. The HOPG sample was fastened to the plate by ceramic glue (T-E-Klebetechnik Ceramabond 503) and the Au(111) single crystal (Mateck) was wedged by molybdenum clips. NiCr/Ni thermocouples, glued or spot-welded to the samples, were used to control the temperature. The FTIR system comprises a Bruker IFS 66 v infrared spectrometer to take IR spectra in grazing reflection geometry, a LEED/Auger system, and a TPD facility. The Au(111) single crystal was cleaned by repeated cycles of sputtering with argon ions at 300 K, followed by annealing in vacuum until no contaminants were detected by XPS, and the herringbone reconstruction was visible in LEED. The surface was then roughened by sputtering with argon ions of 500 eV kinetic energy until about 4 monolayers (ML, 1 ML ) 1.4 × 1015 cm-2) were removed. The so produced surface morphologies were very similar to morphologies generated with higher ion energies (5 keV) and ion fluences (corresponding to the removal of roughly 600 ML) so that a “steady-state” (in terms of a saturation of the step density) is reached by removing 4 ML. Clean HOPG surfaces were obtained by cleaving the crystal ex situ by means of scotch tape, as described in the literature,23 and immediately transferred into the vacuum system. In vacuum, the samples were then heated to 1000 °C for 10 min. Care was taken to avoid any ion bombardment of the HOPG sample surfaces, because even low-energy ions emitted by ion pumps or ionization gauges can generate nucleation sites for the growth of metal clusters.24 Gold was evaporated from a graphite crucible with a commercial electron bombardment evaporator (Focus, EFM 3). The flux was calibrated by a quartz microbalance and a rate of 0.05 nm/min (about 1/5 ML min-1) was used for deposition. During deposition, the sample was biased with a retarding voltage to prevent Au ions emitted by the evaporator from hitting the sample surface. As will be shown later, such ions have a
Figure 1. (a) On-top adsorption sites on Au(332) surface. The periodic unit of p(1 × 2) Au(332) is highlighted by a rectangle on top of the surface. (b) Au(332) surface on which the coordination of step-edge atoms is reduced from 7 to 6. The periodic unit of p(1 × 3) Au(332) is highlighted by a rectangle on top of the surface.
significant influence on the growth of Au clusters. Gold was deposited at either 100 or 300 K onto HOPG and could be removed by heating the sample to 1000 °C. Cleanness was checked by XPS. We have determined the sticking coefficient of Au on graphite as only 0.06-0.18 even at 100 K, in good agreement with the work by Anton and Schneidereit.25 In the following, the total amount of gold hitting the surface is given as the nominal film thickness (in nm) determined by the microbalance for a sticking coefficient of ∼1. Computational Methods Density functional theory (DFT) calculations were performed within generalized gradient approximations (GGA), using the Vienna Ab Initio Simulation Package (VASP).26-29 PBE exchange and correlation functionals30,31 were employed. We adopted the PAW pseudopotentials for Au, C, and O, which were supplied in VASP.32,33 The plane wave cutoff was set to 400 eV, and an augmentation charge cutoff of 645 eV was applied. Full geometry optimizations were carried out by the conjugate gradient minimization scheme in which the convergence thresholds for geometry optimizations and electronic structure calculations were set to 10-4 eV. Calculations were performed for a Au(332) surface model generated by Cerius 2 (accelrys) software. Two substrate surface modifications were chosen for the calculations: a well-ordered Au(332) surface as depicted in Figure 1a, exhibiting 4-atomic-layer thick Au(111) facets separated by regular steps, and a p(1 × 3) Au(332) surface with kink sites, generated by removing each third atom from the steps (see Figure 1b). The next neighboring images along the z-direction are separated by 13 Å, which is large enough to avoid spurious interactions and allows for a faithful neglect of a dipole correction. As to be shown later, CO-CO coupling interaction must be considered only if two on-top CO are adsorbed on next-neighbor Au sites. Therefore, we have considered adsorption of a single CO molecule on one side of the slab within a p(1 × 2) unit cell of the regular Au(332) surface to avoid significant CO-CO interaction. The p(1 × 2)
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Au(332) model was put in an orthogonal supercell of 0.59 × 1.38 × 2.28 nm3. A 6 × 3 × 1 k-mesh generated by the Monkhorst-Pack method34 was used. The p(1 × 3) Au(332) surface model was located in a 0.88 × 1.38 × 2.28 nm3 orthogonal supercell using a 4 × 3 × 1 k-mesh. The transition structure of CO migration (atop CO f bridge CO) on p(1 × 2) Au(332) was obtained by using the Nudged Elastic Band (NEB) method within VASP.35 The adsorption energy is defined as:
Ead ) E(CO/Au(332)) - E(free CO) - E(Au(332)) Vibrational frequency calculations were performed by fitting the potential energy curve as a function of CO distance with a Morse potential. The Morse potential is expressed as:
V(r) ) hcDe(1 - exp(-ar))2 where h is the Planck constant and c is the velocity of light. De and a are obtained by curve fitting. The harmonic frequency, νh, is expressed as:
νh )
1 2πc
x
2Dea2 µ
where µ is the reduced mass of CO. The anharmonic frequency, νah, can be obtained by the following expression:
νah ) νh(1 - 2χe) where
νhχe )
a2h 2µ
Since the coupling between the CO stretching mode and substrate phonons is small, we fixed all atoms of the metal substrate. Furthermore, the coupling between C-O stretching and bending modes was neglected. To compare absolute values of vibrational frequencies, we introduce a factor of 1.01056 to scale the calculated anharmonic frequency of free CO to the experimental value of 2143 cm-1. In the present study, also infrared absorption intensities were investigated which are proportional to the square of the dynamic dipole moment. The intensity can be related to the change of work function of the corresponding vibrational motions,36 which was estimated by the corresponding change of the Fermi energy, dEfermi/drCO. Results and Discussion Ar+-Sputtered Au(111). Starting with a well-ordered flat Au(111) surface we have generated low-coordinated Au atoms by argon ion bombardment at room temperature (RT). The sputter process induces a rough pit-and-mound structure with a lateral size of ∼10-15 nm, exhibiting a lot of low-coordinated atoms at step-edges and kinks, as the STM reveals (see Figure 2a). The tentatively hexagonal shape of the pits and mounds still reflects the (111) orientation of the terraces. The surface was then saturated with CO by exposure to 10 Langmuir (L, 1 L ) 10-6 Torr s) at 110 K. A subsequently taken TPD spectrum is shown in the inset of Figure 2a. A broad intense peak at ∼145 K and a second peak at ∼185 K are visible, indicating two desorption states. In contrast, TPD spectra of freshly prepared and annealed Au(111) surfaces do not show any CO desorption
Figure 2. STM images of the Au(111) surface obtained (a) after Ar+sputtering at room temperature and after heating to (b) 400 and (c) 500 K. The insets show the corresponding TPD spectra. Note that the pressure scale is always the same. The decrease in TPD intensity goes along with the loss of surface roughness and the formation of terraces during annealing.
within this temperature regime at all. The adsorption energies can be determined via the Redhead formula:
Edes ) RTdes[ln(νTdes/β) - 3.46] Here, Tdes is the temperature at peak maximum, β is the heating rate (1 deg/s in our experiments), and ν is the pre-exponential factor. Assuming a pre-exponential factor of 1015, as used as an estimate by Ruggiero et al.,19 we calculate the adsorption energies as -0.46 (-44 kJ/mol) and -0.56 eV (-54 kJ/mol) for the low-temperature TPD peak (LTP) and high-temperature peak (HTP), respectively. The energy difference of 0.1 eV (∼10 kJ/mol) between both states does not change significantly when ν is varied in the range 1013 to 1015. The density of low-coordinated sites was varied by annealing the sputtered surface up to various temperatures. Panels b and
448 J. Phys. Chem. C, Vol. 111, No. 1, 2007 c of Figure 2 show STM images of the morphologies obtained after heating to 400 and 500 K, respectively. After heating to 400 K the surface still exhibits pits with diameters in the range of 10 nm, but extended terraces start to form and the density of atomic steps has decreased. After annealing up to 500 K, finally, the small pits have vanished and vacancy islands of about 50 nm diameter remain on the terraces. The insets in panels b and c of Figure 2 show the CO-TPD spectra obtained from the imaged structures. For comparison, the scale of the CO partial pressure is the same in all spectra. While the capacity for CO adsorption clearly decreases when the surface is annealed and the number of low-coordinated sites is reduced, maxima at the position of the LTP and the HTP can be identified in all spectra, even in Figure 2c, although there they appear as very weak and broad desorption features. Consequently, on the one hand both desorption peaks must be due to under-coordinated adsorption sites, because no desorption is detected from the well-ordered Au(111) surface before ion bombardment (data not shown). On the other hand, the predominant adsorption sites must be the same for the whole range of surface morphologies depicted in Figure 2. Au on HOPG. Complementing the spectrum of Au surfaces, CO adsorption was also studied on Au nanoparticles which are relevant for supported catalysts. Au was deposited on HOPG under different conditions to vary the morphology. First, the deposition temperature was varied between RT and 110 K. Previous STM results indicated the growth of two different kinds of particles: large aggregates and small, defect-rich clusters, the density of which increases as the deposition temperature is lowered.22 Second, substrate bombardment by Au ions during deposition was admitted by switching off the retarding voltage between gold evaporator and sample. As described in the Experimental Section, ion bombardment of HOPG is expected to generate nucleation sites for metal clusters, favoring growth of a high density of small Au clusters. Figure 3 shows CO TPD spectra for increasing amounts of gold deposited on HOPG at RT (panels a and b) and at 110 K (panel c). For the depositions represented in Figure 3b Au ions were admitted to the HOPG, for the depositions represented in Figure 3a,c they were not. The samples were always exposed to 10 L of CO at 100 K prior to TPD to induce saturation coverage. As for the Au single-crystal surfaces, two CO desorption peaks can be identified in all TPD spectra:22 the LTP at ∼130 K and the HTP at ∼170 K. The relative peak heights depend on the amount of Au and the preparation conditions, as Figure 3 shows. For small Au coverages deposited at RT (Figure 3a) the HTP is more prominent, but as the amount of gold is increased and the clusters grow in size or form aggregates, the HTP intensity saturates and the LTP becomes dominant. Obviously, both TPD peaks represent two different lowcoordinated Au adsorption sites, the relative density of which varies with the particle size. For Au deposited at 110 K, we expect preferred growth of small clusters. In line with the above-mentioned interpretation, the TPD spectra shown in Figure 3c are dominated by the HTP, and the ratio of HTP and LTP intensity appears almost independent of Au coverage in the range 0.1-1 nm. Furthermore, as expected due to a higher capacity for CO adsorption on small clusters, the total TPD intensity is higher by a factor of 2-3 when compared to CO-TPD from the same amount of gold deposited at RT (Figure 3a). Au ion bombardment during Au deposition is considered to induce nucleation sites, leading to the generation of a high density of small clusters even at RT. A comparison of panels a and b of Figure 3 reveals that, again, the HTP is more intense when the density of small clusters is presumed to be high.
Yim et al.
Figure 3. TPD spectra of 10 L CO adsorbed at 115 K on increasing amounts of gold deposited on HOPG. Au was deposited (a) at room temperature, (b) at room temperature without retarding voltage (so gold ions from the evaporator can hit the sample), and (c) at 110 K (cooled by liquid N2). In all spectra two desorption states are observable at about ∼130 and ∼170 K. Depending on the preparation conditions and the amount of gold deposited the intensity ratio of the two peaks differs.
As an estimate for the adsorption energies on Au/HOPG, the Redhead formula yields -0.4 (-39 kJ/mol) and -0.53 eV (-51 kJ/mol) for both TPD peaks, respectively, indicating slightly weaker bonds between CO and low-coordinated gold sites than in the case of sputtered Au(111). The energy difference between both desorption states, however, is about the same for sputtered Au(111) and Au nanoparticles on HOPG (0.1-0.13 eV). Theory. To elucidate which types of low-coordinated Au sites are involved in CO adsorption we have carried out DFT calculations for Au(332) surface models. For Au(332), Ruggiero and Hollins have reported two CO-TPD peaks at temperatures comparable to our results for sputtered Au(111) and Au nanoparticles on HOPG. Furthermore, they observed only one CO-induced IR absorption band shifting to lower wavenumbers with increasing CO coverage.19,37 In our own experiments we also find only one IR absorption band at ∼2120 cm-1 for sputtered Au(111) (see also next section) and Au/HOPG,22 adding to the evidence for a universal scheme of CO adsorption on the entire spectrum of Au surfaces. As a consequence, Au(332) is an appropriate model system for the experiments described above. Figure 1a shows five possible adsorption sites for CO on the regular Au(332) surface. Calculations were performed for structure 1, representing CO on-top adsorption at the step edge, and structures 2-5, representing CO on-top adsorption on terrace sites. The energetics of CO adsorption, together with vibrational properties of chemisorbed CO, are summarized in Table 1. The chemisorption energy of CO at the step edge (1) is found to be -0.63 eV, which represents a significantly stronger bond than the chemisorption energy of CO on the Au(111) terrace sites (2-5) ranging from -0.13 to -0.18 eV. In good agreement with another theoretical study12,17 for Au(111) and Au(211) and with our experimental results on annealed Au(111), we find that desorption temperatures above 120 K are too high for CO adsorption on terrace sites. This implies that the interaction of CO with Au surfaces is sensitive
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TABLE 1: Chemisorption Energies and Vibrational Properties for CO on p(1 × 2) Au(332) Surface structure
Ead (eV)
1 2 3 4 5 flat bridge CO
-0.63 -0.18 -0.16 -0.18 -0.13 repulsive -0.51
a
dEfermi/drCO IR rel intensity νah shift νah (cm-1)a (cm-1) (eV/Å) (arb. unit) 2072 2071 2071 2070 2068 2115 1950
-0.5593 -0.5238 -0.4856 -0.4708 -0.4045 -0.1646 -0.5746
-71 -72 -72 -73 -75 -28 -193
0.95 0.83 0.71 0.67 0.50 0.08 1.00
Reference value ) 2143 cm-1.
TABLE 2: Comparisons of CO Adsorption Properties Calculated for Different Au Surfaces structure
coverage (%)
Ead (eV)
νah (cm-1)
p(1 × 2) Au(332) p(1 × 2) Au(110)-(1 × 1)a,b p(1 × 1) Au(110)-(1 × 2)a,c p(1 × 1) Au(23h0)a
50 50 50 39.25
-0.63 -0.69 -0.52 -0.70
2072 2079 2079 2077
a
Reference 18. b Unreconstructed surface. c Reconstructed surface.
Figure 4. Reaction energy profile of atop CO to bridge CO transformation on 0.5 ML p(1 × 2) Au(332) surface. For bridge CO a shallow energy minimum is calculated, indicating that this could be stable on the surface.
to the local coordination environment of the surface atoms. This is also corroborated by Table 2 demonstrating that not densely packed surfaces exposing low-coordinated sites with comparable coordination numbers exhibit all similar adsorption properties. Apart from CO adsorbed on-top at step sites, we have also considered CO bridge adsorption sites at the step edge. The energy profile of CO on the Au step edge obtained by NEB calculations is shown in Figure 4, revealing that the bridge CO adsorption site is 0.12 eV higher in energy than the on-top CO adsorption site. The energy profile along the migration pathway from “on-top” to “bridge” shows that the bridge CO is trapped by a shallow energy well of 0.07 eV. According to our calculation, CO bonded at bridge sites would lead to an intense IR signal between 1900 and 2000 cm-1 that significantly deviates from the experimental result (2120 cm-1). Consequently, although the energy difference between CO on-top and bridge species corresponds to the observed separation of the TPD peaks, significant CO bridge adsorption at step sites can be ruled out. For Au deposits on oxide supports it was speculated that one of the two observed TPD peaks represents CO lying flat on the surface.20 Due to surface selection rules this species would not be visible in IR spectra, leading to the single-band spectrum. Therefore, we studied CO adsorption in the groove of the mon-
TABLE 3: Chemisorption Energies and Vibrational Properties for CO on Perfect and Defective p(1 × 3) Au(332) Surfaces site 1 adsorption
Au coord no.
Ead (eV)
νah (cm-1)
p(1 × 3) p(1 × 3) p(1 × 3)
5 6 7
-0.88 -0.75 -0.65
2072 2071 2071
atomic step to simulate the flat-lying CO. In fact, the calculated IR intensity is less than 10% of that of adsorption structure 1 (see Table 1). However, the interaction between a flat-lying CO molecule and the surface turns out to be purely repulsive, excluding the presence of such a species on the surface. Au atoms at perfect step edge sites are low-coordinated, with 7 next-neighboring Au atoms. To evaluate the role of coordination of Au sites for CO adsorption, binding energies and CO vibrational frequency were calculated for the p(1 × 3) Au(332) substrate model in which kink sites were introduced by removing step atoms. The results are shown in Table 3. Starting with perfect steps, we find the adsorption energy of one on-top CO within a p(1 × 3) unit cell as -0.65 eV, very close to the value calculated for the p(1 × 2) unit cell. The scaled anharmonic frequency is 2071 cm-1, which is also similar to that on the p(1 × 2) cell. When one next-neighboring Au step atom is removed from the adsorbate site, i.e., the coordination number is decreased to 6, the CO adsorption energy increases to -0.75 eV, while the calculation reveals that the scaled anharmonic frequency of CO remains unchanged. Further removal of an Au atom on the p(1 × 3) Au(332) surface will result in a 5-fold coordinated Au site for CO adsorption. The adsorption energy on this 5-fold coordinated Au is still larger at -0.88 eV, while there is only a very minor red-shift of 1 cm-1 for the CO stretching vibration compared to the 6-fold coordinated Au site. We estimate that the binding energy of CO on 5-fold coordinated Au is so large that the desorption temperature should be higher than 200 K, which was not observed in the TPD results. The invariance of the vibrational frequency in connection with a binding energy difference of 0.1 eV between 6-fold coordinated kink sites and 7-fold coordinated step sites is in very good agreement with the experimental observations. With respect to the absolute value of the vibrational frequency, our calculations yield 2071 cm-1, which is smaller than the experimental value by ∼50 cm-1. A similar tendency of the calculations to underestimate the frequency was also observed for Au(110) by Loffreda and Sautet.18 This observation can be traced back to the underestimation of CO 2π* orbital energy by DFT. With respect to the absolute binding energies, the calculations yield values that are significantly higher than our experimental estimates (-0.65 and -0.75 eV compared to -0.39/-0.44 and -0.51/-0.54, respectively). It should be noted, however, that the energies have been determined under different adsorption conditions: Experimental values correspond to CO saturation of the sample, while the calculation refers to the adsorption energy of a single molecule per unit cell. In our calculations the CO-CO coupling interaction was simulated by increasing the one-dimensional CO density on the step edge. The results are shown in Table 4. A repulsive CO-CO coupling becomes non-negligible only when two on-top CO are adsorbed on the next-neighboring Au atoms. If repulsion is included, a binding energy of -0.49 eV is calculated for CO at a step-edge atom, which is quite comparable at least to the energy for the LTP observed for the Au(111) surface. Experimental CO dosage series will show in detail the influence of repulsive interactions on the TPD spectra. Variation of CO Dosage. To obtain information about COCO interaction and the development of the two adsorption states,
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Figure 5. Series of TPD (a) and IR (b) spectra obtained from the argon sputtered Au(111) surface for increasing the amount of CO below 120 K. IR spectra were also taken at that temperature. The dotted lines indicate the peak positions. The TPD shows peak shifts for unsaturated peaks due to repulsive interaction. In the IR spectra a continuous increase of the intensity is observed until full coverage of the surface is reached, indicating that both desorption states are active with respect to IR absorption. No other peaks are detected in IR, excluding CO adsorbed on a bridge site.
TABLE 4: Chemisorption Energies and Vibrational Properties for CO on p(1 × n) Au(332) Surface, n ) 1-3 CO site 1 adsorption
Ead (eV)
νah (cm-1)
νah shift (cm-1)
p(1 × 1) p(1 × 2) p(1 × 3)
-0.49 -0.63 -0.65
2069 2072 2071
-74 -71 -72
CO dosage series were measured for the sputtered Au(111) surface and Au nanoparticles on HOPG. The results for sputtered Au(111) are plotted in Figure 5a (TPD) and Figure 5b (FTIR). In TPD only the HTP at ∼185 K, representing desorption from kink sites, is observable for the initial dosage of 0.1 L CO. The HTP seems to saturate for an exposure of 0.15 L and does not significantly change in position at higher CO dosage. A second peak becomes visible at ∼170 K for 0.2 L CO and shifts to lower temperatures as the amount of CO is increased. Saturation is reached only after dosing 1.0-2.0 L CO to the surface. Then, the second peak can be clearly identified as the LTP at a position of 145 K, which does not change significantly for higher CO doses. Since the LTP represents desorption from step-like sites, its dramatic shift to lower maximum desorption temperatures
as the CO density is increased clearly indicates repulsive COCO interaction along the steps. As one would expect, desorption from step-like sites is only observed after the kink sites are saturated, which, as the experiment indicates, is the case after a dosage of only 0.15-0.20 L of CO. In accordance, also the evolution of the IR spectra with increasing CO dosage, shown in Figure 5b, changes at 0.2 and 1.0-2.0 L. For the IR experiments, the sample was exposed to CO always at 120 K and the spectra were taken at the same temperature. All spectra exhibit a single absorption band, the intensity of which continuously increases from 0.1 to 1.0 L. In addition, a shift of the frequency is observed. Up to 0.2 L of CO, as long as only one type of adsorption site is populated, the IR signal is located at 2126 cm-1. Beyond 0.2 L, when also step-like sites are being populated, the signal shifts to lower wavenumbers as the amount of CO is increased until the surface is saturated at ∼1.5 L. At saturation the IR frequency is 2115 cm-1. Our results are in very good agreement with an IR spectroscopic study for CO on Au(332). Ruggiero and Hollins also observed a single IR peak, shifting from ∼2125 to ∼2110 cm-1 and increasing in intensity with coverage.37 The continuous increase of intensity indicates that both adsorbed CO species are active with respect to IR absorption, meaning that the molecular axis must be oriented upright with respect to the Au(111) surface, in accordance with our calculations. Since the anharmonic frequency of adsorbed CO is essentially the same for both, step and kink sites, the red-shift of 15 cm-1 is probably not due to the subsequent growth of two peaks with different frequencies not resolved in the spectrum. Rather, it is likely that the observed red-shift is a particular feature to CO adsorption on gold surfaces, as already proposed in the literature.37 Before turning to the particles, it should be noted that in spite of the large similarities between sputtered Au(111) and Au/ HOPG deviations are expected to some extent. On the one hand, particle surfaces may not be considered solely as a sequence of (111)-oriented terraces and (100)-type steps and kinks but are composed of faces of various orientations, obviously exhibiting sites of step-like and kink-like coordination. On the other hand, particle size effects on charging, strain, and electronic structure may play a role for very small clusters.12,38 The results of CO dosage series on 0.2 nm Au deposited on HOPG at RT and at 110 K are shown in Figure 6, panels a and b, respectively. The Au particles prepared at 110 K were heated once to 210 K prior to TPD to make sure that the morphology does not change during the CO dosage series. Accordingly, the total number of low-coordinated sites probably changes but is still higher than that for the particles generated at RT, as indicated by the significantly higher overall TPD intensity. For both samples, only the HTP is observable at the lowest CO dosage of 0.1 L. Its intensity increases with the CO coverage until saturation is observed at ∼0.3 L for Au/HOPG prepared at 110 K while saturation is observed already at 0.15 L for the RT deposit, in line with a higher concentration of the corresponding low-coordinated sites. Similar to sputtered Au(111), the second desorption state at step-like sites is significantly populated after the HTP is saturated. The CO-CO repulsioninduced shift of the LTP to lower temperatures apparently saturates at 0.5 L of CO for Au deposited at RT. Since our experiments were limited to initial conditions at temperatures above 110 K, however, the final desorption temperature at saturation may be even somewhat lower. For Au deposited at 110 K, saturation is obtained after exposing the surface to 1 L of CO. A closer inspection of the position of the HTP in Figures 3 and 6 reveals that the temperatures of maximum desorption at
CO Adsorption on Gold Surfaces
J. Phys. Chem. C, Vol. 111, No. 1, 2007 451 be ruled out. The total CO binding energies for both lowcoordinated sites are in good agreement with the calculated energies if CO-CO repulsion is taken into account. Therefore, we attribute the two adsorption states to two kinds of lowcoordinated Au atoms with the low temperature state belonging to CO at a step or a very similar site and the high-temperature peak stemming from CO at a site with kink-like coordination. Acknowledgment. Financial support by Deutsche Forschungsgemeinschaft (DFG) and Fonds der Chemischen Industrie (FCI) is gratefully acknowledged. Calculations were performed with use of the Opteron cluster allocated in the chemistry department of the University of Oldenburg. W.-L. Yim and J. Biener acknowledge support by the Hanse Wissenschaftskolleg and W.-L. Yim acknowledges support by the Alexander von Humboldt Foundation. References and Notes
Figure 6. TPD series for increasing CO dosage at 115 K for 0.2 nm Au deposited on HOPG (a) at room temperature and (b) at 110 K and heating to 225 K. The dotted lines indicate the peak positions. Due to repulsive interaction of the adsorbed molecules, the desorption peaks shift to lower temperatures as long as they are not saturated. For the room temperature deposit the high-temperature peak saturates already after exposure of 0.15 L, which indicates a smaller number density of the corresponding adsorption site as for the particles deposited at liquid N2 temperature.
saturation coverage can differ by about 10 K when the three preparation conditions of Au nanoparticles are compared. These variations of the binding energy may be due to particle size effects mentioned above. Conclusion A universal scheme of CO adsorption on supported Au nanoparticles and stepped or rough single-crystal surfaces is identified by comparing our experimental results obtained via TPD and FTIR for various Au deposits on HOPG and argonion sputtered Au(111) and discussed in the view of previous reports for selected gold surfaces. Two different adsorption states with binding energies in the range of -0.4 and -0.5 eV, respectively, are observed while only a single CO absorption peak is detected in FTIR at around 2120 cm-1. The DFT calculations presented for Au(332) surface models provide a conclusive solution to this puzzle by demonstrating that for CO adsorption at low-coordinated Au sites, the binding energy significantly depends on the order of coordination while the anharmonic frequency of the CO stretching vibration does not. Large-scale consistency between experimental results and theory is achieved by assuming that CO binds on-top of 7-fold coordinated step-edge sites and 6-fold coordinated kink sites. Significant CO adsorption at terrace sites, at bridge sites on step-edges, or as flat-lying species, as previously suggested, can
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