Langmuir 2003, 19, 717-721
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Carbon Monoxide on γ-Alumina Single Crystal Surfaces with Gold Nanoparticles Carsten Winkler,* Alexander J. Carew, Sam Haq, and Rasmita Raval Surface Science Research Centre, The University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K. Received June 11, 2002. In Final Form: July 18, 2002 The adsorption and oxidation reaction of CO on γ-Al2O3 single-crystal thin films with Au nanoparticles were studied as a function of the particle size using reflection-adsorption infrared spectroscopy, temperatureprogrammed desorption (TPD), and the molecular beam technique in a temperature range of 93 K < Ts < 213 K. It was found that CO undergoes a precursor adsorption on the Au particles ending up in an on-top position. The activation energy of this process is calculated from the molecular beam data to (Ed - Ea) ) 3.53 kJ/mol. The amount of CO adsorbed increases with increasing particle size. Two types of adsorption sites could be deduced from the TPD data. The oxidation of CO, however, was never observed in the present experiments.
1. Introduction The chemical reactivity of carbon monoxide is generally of interest and has been studied in great depth over recent years. Besides applied chemistry, where, e.g., the catalytic oxidation of CO in the methanol synthesis1 or environmental issues such as the removal of the poisonous carbon monoxide2 play an important role, the simplicity of the diatomic CO molecule is fully utilized in fundamental research, and so carbon monoxide is considered as a model system here. In surface science, the adsorption of CO under different experimental conditions and on various substrates is extensively studied. With respect to substrates, gold had attracted very little attention in surface chemistry until some time ago, because of its inert chemical character and its low dispersion properties on common support materials.3 Recently, however, it was found that gold could exhibit remarkably high activity in many reactions, when it is highly dispersed on metal oxide supports.4 Since unique physical and chemical properties specific to their size are well known for gas-phase clusters,5 the question came up, how far the reactivity on, and with that the catalytic activity of, nanoparticles could be size-sensitive. First results for the change of the catalytic activity by varying the cluster size are reported by Valden and coworkers6 and Heiz et al.7 Valden and co-workers studied the CO oxidation reaction on Au/TiO2(110) and observed a remarkable huge turnover frequency for gold clusters at a specific particle size of about 2.7 nm in diameter at room temperature. The CO oxidation reaction was also investigated by Heiz et al. for the case of monodispersed platinum clusters on thin MgO(110) films. The particle sizes here were up to N ) 20 atoms per cluster, and it was seen that the amount of CO2 formed increases by about a factor of 6 for N > 15 atoms. Our present paper was inspired by these observations. The adsorption and chemical reactivity of carbon monoxide (1) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (2) Ponec, V.; Bond, C. G. Studies in Surface Science and Catalysis; Elsevier: New York, 1994; Vol. 95. (3) Park, E. D.; Lee, J. S. J. Catal. 1999, 186, 1. (4) Haruta, M. Catal. Today 1997, 36, 153. (5) An overview can be found in: Z. Phys. 1989, D20. (6) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (7) Heiz, U.; Sanchez, A.; Abbet, S.; Schneider, W.-D. J. Am. Chem. Soc. 1999, 121, 3214.
on Al2O3 (a widly used support in catalysis) supported gold nanoparticles were studied as a function of the substrate temperature Ts and the mean particle size N0 using the molecular beam (MB) technique, reflectionabsorption infrared spectroscopy (RAIRS), and temperature programmed desorption (TPD). The aim was to check, if the chemical reactivity of CO also depends significantly on the particle size in the case of Au clusters on alumina. 2. Experimental Methods A detailed description of the experimental setup is already given elsewhere.8 Briefly, all experiments were carried out under ultrahigh vacuum (UHV) conditions in a stainless steel vessel. In there, a NiAl(110) single crystal was mounted on a x-y-z-Θ manipulator. The sample temperature could be varied in a range of 90 K < Ts < 1200 K, and it was determined with a thermocouple. Furthermore, the experiment was equipped with a low-energy electron diffraction (LEED) unit which was used to control the alumina formation. The gas and vapor fluxes during the sample preparation were measured with a quadrupole mass spectrometer (QMS). For the RAIRS experiments, a MATTSON Galaxy 6020 FTIR spectrometer was used. The Al2O3 was prepared as a thin film close to 5 Å thickness by oxidizing the clean NiAl(110) single crystal in UHV. This technique is described in detail by Freund and co-workers,9-11 and it was shown recently by Ceballos et al.12 that the oxide yield with this method is γ-Al2O3. The gold particles were then grown on the alumina by exposing the sample at Ts ) 300 K to gold vapor. The mean cluster size N0 on the alumina substrate was calculated from the amount of Au dosed. This method is based on previous results where scaling parameters were deduced for N0 as a function of the metal vapor exposure from scanning tunneling microscopy (STM) studies.13
3. Results First, some experiments were carried out in order to check if CO and O2 show any sticking on the clean alumina (8) Raval, R.; Harrison, M. A.; King, D. A.; Caine, G. J. Vac. Sci. Technol., A 1991, 9, 345. (9) Jaeger, R. M.; Kuhlenbeck, H.; Freund, H.-J.; Wuttig, M.; Hoffmann, W.; Franchy, R.; Ibach, H. Surf. Sci. 1991, 259, 235. (10) Libuda, J.; Winkelmann, F.; Ba¨umer, M.; Freund, H.-J.; Bertrams, Th.; Neddermeyer, H.; Mu¨ller, K. Surf. Sci. 1994, 318, 61. (11) Bertrams, Th.; Brodde, A.; Neddermeyer, H. J. Vac. Sci. Technol., B 1994, 12, 2122. (12) Ceballos, G.; Song, Z.; Pascual, J. I.; Rust, H. P.; Conrad, H.; Ba¨umer, M.; Freund, H. J. Chem. Phys. Lett. 2002, 359, 41. (13) Winkler, C.; Carew, A. J.; Ledieu, J.; McGrath, R.; Raval, R. Surf. Rev. Lett. 2001, 8, 693.
10.1021/la026054z CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002
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Figure 1. Measured sticking probabilities (points) of CO on alumina-supported gold clusters with N0 = 700 atoms. The curve profiles indicate precursor adsorption. The amount of CO adsorbed generally increases for lower surface temperatures Ts and with increasing cluster size. The lines are results of fits to the data that were performed on the basis of eq 1.
substrate. In aggreement with the results of Hsaio et al.,14 a significant sticking was never observed, neither in the MB nor in the RAIRS data, and so any adsorption of CO on the sample can clearly be attributed to the presence of the Au nanoparticles. 3.1. Molecular Beam Data. Within the MB experiments, CO was beamed at temperatures between 93 K e Ts e 148 K to samples with mean cluster sizes of N0 = 10 (onset of cluster formation), N0 = 350, and N0 = 700 atoms, respectively. The evolution of the partial pressures with increasing dosage was monitored with the QMS for the dosed molecules at the masses m ) 28 amu (CO) and m ) 32 amu (O2), and the possible reaction product CO2 at m ) 44 amu. These data were then analyzed with respect to the sticking probability S and the coverage of the sample according to a method developed by King and Wells15 on the basis of the Kisliuk formalism.16,17 A typical example of the measured sticking probabilities as function of the CO coverages is given in Figure 1. The mean cluster size in this figure is N0 = 700 atoms. To summarize the MB results: it was seen that the sticking coefficient decreases generally with decreasing particle size. This observation confirms the very first experiments done on the clean alumina, where no CO adsorption was observed, and it shows that the CO adsorbes on the Au particles. Furthermore, the sticking becomes less likely at higher temperatures. This behavior indicates a precursor sticking mechanism for the adsorption of CO, where the CO molecule first adsorbes in a weakly bond physisorbed state on the nanoparticles, before it overcomes an activation energy E and either desorbes again or chemisorbes into a final state. A precursor sticking process can also be deduced from the profiles of the sticking probalility curves in Figure 1, since S is accurately independent of the surface coverage in the case of low coverages.15 The corresponding activation energies can be obtained from initial sticking probability at zero coverage S0 with
S0 )
R νd Ed - Ea 1 + exp νa RTs
(
(
))
(1)
where νd and νa are pre-exponentials for desorption and (14) Hsiao, G. S.; Erlay, W.; Ibach, H. Surf. Sci. Lett. 1998, 405, L465. (15) King, D. A.; Wells, M. G. Proc. R. Soc. London, Ser. A 1974, 339, 245. (16) Kisliuk, P. J. J. Phys. Chem. Solids 1957, 3, 95. (17) Kisliuk, P. J. J. Phys. Chem. Solids 1958, 5, 78.
Figure 2. Initial sticking probability of CO on Au/Al2O3 as function of the substrate temperature. The data for S0 (points) are taken from the analysis to the measured data as shown in Figure 1. The lines show the extrapolation of S0 for Ts f 0 K.
Figure 3. The gradient of a semilog plot according to eq 3 delivers the activation energy (Ed - Ea) ) 3.53 kJ/mol for the chemisorption of CO on Al2O3 with Au particles. The difference in the intercept for smaller and bigger clusters is discussed in the text.
adsorption from the physisorbed precursor state, Ed and Ea are the respective activation energies, and R is the gas constant. The factor R can be estimated from eq 1 as the low-temperature limit for S0
lim S0 ) R Tsf0
(2)
and so (Ed - Ea) can be calculated from rearranging eq 1 to
ln
(
)
Ed - Ea 1 R -1 ∝S0 R Ts
(3)
With R = 0.34, taken from the low-temperature limit for S0 that is plotted in Figure 2, the semilog plot according to eq 3 yields (Ed - Ea) ) 3.53 kJ/mol from the gradients of the fitted lines; see Figure 3. Since the data points of the different samples follow the same gradient, one can assume that the final adsorption state is the same in all cases. However, it is interesting to note that the different intercepts indicate a bigger absolute sticking probability for bigger particles. This can be understood from the adsorption kinetics. In the weakly bond precursor physisorbed state, the CO molecule diffuses on the surface, and the mean diffusion path xj that a molecule has to travel before it stabilizes into the final chemisorbed state is apparently bigger than the diameter of the smallest clusters in the present experiments. In that case, the annihilation of excess energy of the incoming CO molecule leading to chemisorbed state is less likely, and so de-
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sorption becomes dominant. The mean diffusion distance can be estimated, if the size and shape of the cluster are known. Previous STM experiments13 have almost shown earlier that clusters grown by vapor deposition on the alumina can be described in a good approximation with a simple geometrical approach. This approach deals with a hemispherical particle with an face-centered cubic (fcc) structure similiar to that of the bulk,18 and so the radius r of a particle consisting of N0 atoms is given by
r ) (2N0)1/3 r0
(4)
with r0 for the radius that is taken by an Au atom in the cluster. With eq 4, the reduced diffusion distance becomes (xj/r0) = (r/r0) ) 3 atoms for the present case with N0 = 10 atoms. From that and using the results of the studies by Gomer19 on the surface mobility
()
Ed - Em ) RTs ln
xj r0
(5)
(Em ) migration energy), one gets a lower limit of (Ed Em) ) 820 J/mol from the data at Ts ) 90 K. This is just about 25% of the value for (Ed - Ea) calculated from eq 3, which indicates a relatively high mobility of the physisorbed molecule on the cluster. With that, it also confirms the bigger S0 for the CO adsorption on bigger clusters as discussed above, since it is more likely for the traveling molecule to find an unoccupied adsorption site on bigger particles. This and the results of Figure 3 lead finally to the conclusion that the adsorption of CO on bigger Au particles is very similiar to that on an extended Au surface. Within the MB experiments, O2- and CO/O2-mixed beams were also dosed to the samples; however, the adsorption of O2 or even the CO oxidation reaction was never observed for the present temperature range and cluster sizes. This supplies the results from a fixed-bed reactor of Park and Lee3 which showed that aluminasupported pure gold nanoparticles act as a poor catalyst under high-pressure conditions. 3.2. RAIRS Data. In addition to the MB measurements, RAIRS experiments were carried out on the adsorption of CO on alumina-supported gold nanoparticles. Samples with particle sizes between the onset of cluster formation up to N0 = 600 atoms were prepared and with that, RAIR spectra were taken for different doses of CO at Ts ) 90 K, e.g., Figure 4 for N0 = 350 atoms. Figure 5 shows data for the same sample with maximum uptake of CO (saturation dosing) at Ts ) 93 K, followed by sample heating to 213 K. In both RAIR spectra, the change of the reflectivity of the sample as function of the wavenumber is plotted. The change is normalized to the corresponding initial reflectivity and given in percent. The spectra in Figures 4 and 5 are dominated by a single vibrational band around 2100 cm-1. This was generally observed, as well as an increase of the peak intensity for higher CO doses and for lower Ts. A complete desorption of CO was seen for Ts > 210 K. This temperature dependence clearly confirms the adsorption mechanism, which is discussed above on the basis of the MD data. The peak position shifts to higher wavenumbers with increasing particle size. With increasing CO coverage, the peak position remains constant around 2098 cm-1 for smaller Au-cluster (N0 < 100) atoms. In the case of a bigger cluster, (18) Kittel, C. Introduction to Solid State Physics, 6th ed.; Wiley: New York, 1986. (19) Gomer, R. Discuss. Faraday Soc. 1959, 28, 23.
Figure 4. Typical RAIR spectra for up to 1.0 langmuir of CO dosed on alumina-supported Au nanoparticles (N0 = 350 atoms) at Ts ) 90 K. The spectra are dominated by a single vibrational band around 2100 cm-1, which indicates an adsorption of CO on top of the gold atoms.
Figure 5. RAIR spectra obtained for saturation dosing of CO onto Au/Al2O3 (again N0 = 350 atoms) at Ts ) 93 K, followed by heating to Ts ) 213 K. Again, the spectra are dominated by a single vibrational band around 2100 cm-1.
however, the peak position shifts about 3 cm-1 to smaller wavenumbers for higher CO doses (Figure 6). On the basis of the data from Ruggiero and Hollins20 for CO on an Au(332) single-crystal surface and those of Boccuzzi et al.21 on FTIR studies of CO on titaniasupported Au nanoparticles, the band around 2098 cm-1 can be associated with the stretch mode of the CO molecule adsorbed straight on top of an Au0 atom. The shift of the peak position to lower wavenumbers with increasing CO coverage on bigger clusters can be understood as the dominance of the chemical shift, which results from chemical changes induced in the adsorbed molecule by neighboring molecules, over the vibrational shift due to direct electromagnetic interaction.22 The same effect was almost seen for the CO adsorption on thin Au films.23 The (20) Ruggiero, C.; Hollins, P. Surf. Sci. 1997, 377-379, 538. (21) Boccuzzi, F.; Chiorino, A.; Manzoli, M. Surf. Sci. 2000, 454456, 942. (22) Bond, G. C.; Thompson, D. T. Catal. Rev. Sci. Eng. 1999, 41, 319.
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Figure 6. Observed peak position in the RAIR spectra as function of the particle size and the CO coverage in comparison to the data for Au(332) taken from ref 20. The lines are added to guide the eye. The curvatures show a very similiar situation only for bigger Au particles and an extended Au single-crystal surface, but not for the case of small cluster.
Winkler et al.
shoulder) and at Ts = 185 K, respectively. Ruggiero and Hollins assigned these two peaks to different adsorption sites on the surface with different adsorption enthalpies and therefore with different activation energies due to a nonideal (332) surface. They are not sure about the shoulder, and for that they do not exclude contamination, especially hydrogen. Different adsorption sites can generally also be found in the case of nanoparticles, i.e., different facettes of the particles26 and/or the surface sites of the clusters compared to the edge sites of the gold to the alumina substrate. Since the present TPD spectrum was taken for the onset of efficient cluster growth, i.e., for very small particles, a significant formation of facettes is not very likely, but the differences in surface and edge sites have to be taken seriously into account. For small particles, the number of edge sites (with lower coordination number and therefore higher adsorption enthalpie with respect to the surface sites) is comparable to the number of surface sites. The ratio of the available edge sites Ne to the surface sites Ns can be estimated from the geometrical approach for the cluster shape that is discussed more detailed in section 3.3.1. The ratio is then given by
Ns/Ne ) 0.46N01/3
Figure 7. The TPD spectrum of CO on Al2O3-supported Au particles for N0 = 10 atoms indicates the existence of two different adsorption sites. These can be assigned to the surface and the edge sites of the Au particles.
constant peak position in the case of small gold particles, i.e., around the onset of cluster formation indicates a singleton CO frequency. Here the density of the adsorbed Au and with that the number of adsorption sites for the CO is too low to lead to an effective interaction of the CO molecules. A singleton vibrational band is also discussed by Jakob and Persson for CO adsorbed on Ru(001).24 Finally, the shift of the peak position to higher wavenumbers with increasing cluster size shows again that bigger Au clusters behave, with respect to the CO adsorption, similiar to an extented gold surface. The data for an Au(332) single crystal taken from ref 20 are also shown in Figure 6. 3.3. TPD Data. Finally, a TPD spectrum was taken in the range of 90 K e Ts e 200 K with a heating rate of 1 K/min after saturation beaming of CO at Ts ) 90 K to a sample with N0 = 10 atoms (onset of effective cluster formation). Again, the CO partial pressure was monitored with the QMS. The TPD spectrum in Figure 7 is quite broad and shows at least two resolved maxima at Ts = 105 K and Ts = 143 K. There could be another peak at Ts = 122 K, but it is uncertain because of the noise level. A very similiar structure was almost observed by Ruggiero and Hollins25 for the CO desorption from a Au(332) single crystal surface, but peaking at higher temperatures, namely, at Ts = 140 K (with a significant low temperature (23) Stephan, J. J.; Ponec, V. J. Catal. 1976, 42, 1. (24) Jakob, P.; Persson, B.N. J. J. Chem. Phys. 1998, 109, 8641. (25) Ruggiero, C.; Hollins, P. J. Chem. Soc., Faraday Trans. 1996, 92, 4829.
(6)
The assumption that the two peaks at Ts = 105 K and Ts = 143 K in Figure 7 can be assigned to the desorption from surface and edge sites of the cluster requires that the number of surface and edge sites becomes comparable: Ns = Ne, since also the peak intensities are comparable. With this, eq 6 yields N0 = 10 atoms, which is in excellent aggreement with the mean cluster size calculated from the scaling laws for the vapor deposition technique.13 The activation energy Ea′ for the desorption of CO from the Au nanoparticles can be obtained from an Arrhenius plot constructed from the leading edge of the curve in Figure 7 on the basis of formalism discussed by Falconer and Madix27
ln
Ea′
∝(dN dt ) RT
(7)
s
From that approach, the activation energy for the desorption state peaking at Ts = 105 K (which is assumed to be desorption from the surface sites of the Au particles) is calculated to Ea′ ) 8.3 kJ/mol. This energy is about a factor of 5 smaller than the corresponding one calculated by Ruggiero and Hollins for the case of Au(332). 4. Conclusion The adsorption and oxidation reaction of CO on Al2O3supported Au nanoparticles were studied as function of the particle size and the substrate temperature using MB, RAIRS, and TPD techniques. The MB data show a precursor sticking of CO on the Au particles for Ts < 210 K. The activation energies of this process were calculated to (Ed - Ea) ) 3.53 kJ/mol and (Ed - Em) ) 0.82 kJ/mol. The RAIRS data indicate an adsorption of the CO molecule on top of a gold atom with an Au0 oxidation state. The MB and the RAIRS data show clearly that the CO adsorption on bigger Au particles (N0 > 500 atoms) is very similiar to the process on an extented single crystal. For smaller (26) Frank, M.; Ba¨umer, M. Phys. Chem. 2000, 2, 4265. (27) Falconer, J. L.; Madix, R. J. J. Catal. 1977, 48, 262.
CO on γ-Al2O3
clusters, however, significant differences have been observed in these experiments. The TPD data for small nanoparticles show two different adsorption sites which can be associated to the surface and the edge sites of the Au cluster. From these data, the activation energy for the desorption of an CO molecule is calculated to: Ea′ ) 8.3
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kJ/mol. With respect to the CO oxidation, however, there was no dependence on the Au particle size found in the present experiments, since neither the adorption of O2 nor the CO oxidation were ever observed. LA026054Z
