J. Phys. Chem. C 2008, 112, 16723–16724
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Reply to “Comment on ‘Formation and Thermal Stability of Au2O3 on Gold Nanoparticles: Size and Support Effects’” Luis K. Ono and Beatriz Roldan Cuenya* Department of Physics, UniVersity of Central Florida, Orlando, Florida 32816 ReceiVed: May 27, 2008; ReVised Manuscript ReceiVed: July 24, 2008 In his comment on our recent publication,1 J. M. Gottfried criticizes the temperature programmed desorption (TPD) data analysis used and the activation energy obtained for the desorption of oxygen from large Au nanoparticles (NPs). On the basis of our results, he calculates a frequency factor with a magnitude that he considers unrealistic. His main point of criticism concerns the nonlinear behavior of the Arrhenius curve observed near the temperature of maximum desorption Tmax in our original Figure 9c for any chosen reaction order.1 We emphasize that this nonlinear behavior appears only for the large NPs (∼5 nm high), while for the small NPs (∼1.5 nm high) our data in Figure 9d clearly follow a linear behavior at the peak maximum. The latter fact is ignored in Gottfried’s comment. After receipt of the comment, we recalculated the data in Figure 9, and a numerical error was found in the procedure used to derive the Arrhenius graph of the desorption rate for large NPs (∼5 nm) presented in Figure 9c. Therefore, we retract Figure 9c from ref 1. Figure 1a shows the O2 TPD spectra of these ∼5 nm high particles, and Figure 1b shows the correct Arrhenius graph. In the corrected data shown in Figure 1b, the curve obtained for the large NPs is not linear for any of the chosen reaction orders n, which prevents us from using this graph to extract an activation energy (Edes) and pre-exponential factor (ν) for this complex process.2-4 Consequently, we also retract the following sentence from ref 1: “As can be seen in Figure 9c, for the large NPs, the best fit to a straight line was obtained for n ) 1, with a desorption energy of 1.0 ( 0.1 eV”. On the other hand, we consider our data analysis and results obtained for the small (∼1.5 nm) NPs correct and a good approximation to describe the complex system studied here. We emphasize that complex reaction kinetics are expected in our system, which consists of supported nanometer-sized catalysts, due to at least well-known size-effects and supportmetal interactions. In addition, the presence of two heterogeneous surfaces, the SiO2 support and Au NPs, makes the analysis challenging, and very few quantitative studies are available in the literature on this kind of systems.5,6 Therefore, the question arises whether it is reasonable to directly compare the kinetic parameters obtained from our TPD experiments to those previously reported on gold single-crystal surfaces with different surface terminations.5,7-11 For example, a wide range of activation energies (Edes ) 1.0-1.5 eV) and pre-exponential factors (ν ) 8 × 109 s-1 to 6 × 1013 s-1) have been reported in the literature for different surface terminations of Au single crystals,2,3,7-9,11 and our faceted Au NPs include several of these orientations.12 A second question relates to which is the most adequate TPD analysis method for these type of systems. * To whom correspondence should be addressed. E-mail: roldan@ physics.ucf.edu.
Figure 1. (a) O2 TPD spectra obtained on ∼5 nm high Au NPs (sample #1) deposited on SiO2. The data were obtained after sample exposure to an in situ O2 plasma treatment at room temperature (2.3 × 10-5 mbar, 15 min). A heating ramp of 5 K/s was used. (b) Plots of ln(-dθ/ dt) - n ln θ versus 1/T are shown for several choices of desorption order, n.
Gottfried mentions in his comment that the heating rate variation (HRV) method is the most suitable method to investigate complex systems. This is not always the case. From the standard heat rate analysis, wherein only the peak maximum is followed versus heating rate, no information can be extracted on the possible dependence of the desorption energy and preexponential factor on the coverage. A comparison of the applicability of different analysis methods (complete analysis,13 leading edge,14 Chan-Aris-Weinberg,15 Redhead,16 and HRV17) to extract kinetic parameters can be found in refs 18 and 19. On the basis of the modeling of simulated TPD spectra, de Jong and Niemantsverdriet19 demonstrated that only the complete analysis13 and the leading edge method14 can properly reproduce their simulated TPD data over the entire coverage range. For our complex system, where two desorption features are observed in the TPD spectra in close proximity (molecular oxygen desorption from SiO2 and from Au clusters), the leading edge analysis method is not the best choice, since there will be an ill-defined background contribution from the SiO2 peak to the leading-edge portion of the Au desorption peak. Even if the background is subtracted and the SiO2-related peak is removed, there is an additional point of uncertainty, since possible interactions between oxygen adsorbed on SiO2 and the Au NPs will be neglected. In order to obtain further information on the kinetic parameters of our SiO2-supported size-selected gold NPs, new experiments are presently being conducted on this system in our group. The new data together with a comparative analysis
10.1021/jp804679w CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
16724 J. Phys. Chem. C, Vol. 112, No. 42, 2008 using different TPD evaluation procedures (Redhead, HRV, and complete analysis) will be presented in a separate paper.20 References and Notes (1) Ono, L. K.; Roldan Cuenya, B. J. Phys. Chem. C 2008, 112, 4676. (2) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2002, 511, 65. (3) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 525, 184. (4) Gottfried, J. M.; Elghobashi, N.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 523, 89. (5) Bondzie, V. A.; Parker, S. C.; Campbell, C. T. J. Vac. Sci. Technol. A 1999, 17, 1717. (6) A private communication from the authors of ref 5 indicated that a numerical mistake was made in their calculation of the activation energies from O2-desorption based on the Redhead method. The correct Edes values in ref 5 are 1.4 and 1.7 eV for six- and two-layer-thick Au islands, respectively. For further details see Erratum to ref 5 by C. T. Campbell, J. Vac. Sci. Technol. A 2008, in press.
Comments (7) Sault, A. G.; Madix, R. J.; Campbell, C. T. Surf. Sci. 1986, 169, 347. (8) Saliba, N.; Parker, D. H.; Koel, B. E. Surf. Sci. 1998, 410, 270. (9) Deng, X. Y.; Min, B. K.; Guloy, A.; Friend, C. M. J. Am. Chem. Soc. 2005, 127, 9267. (10) Choi, K. H.; Coh, B. Y.; Lee, H. I. Catal. Today 1998, 44, 205. (11) Kim, J.; Samano, E.; Koel, B. E. Surf. Sci. 2006, 600, 4622. (12) Ono, L. K.; Sudfeld, D.; Roldan Cuenya, B. Surf. Sci. 2006, 600, 5041. (13) King, D. A. Surf. Sci. 1975, 47, 384. (14) Habenschaden, E.; Kuppers, J. Surf. Sci. 1984, 138, L147. (15) Chan, C. M.; Aris, R.; Weinberg, W. H. Appl. Surf. Sci. 1978, 1, 360. (16) Redhead, P. A. Vacuum 1962, 12, 203. (17) Falconer, J. L.; Madix, R. J. Surf. Sci. 1975, 48, 393. (18) Miller, J. B.; Siddiqui, H. R.; Gates, S. M.; Russell, J. N.; Yates, J. T.; Tully, J. C.; Cardillo, M. J. J. Chem. Phys. 1987, 87, 6725. (19) de Jong, L. J.; Niemantsverdriet, J. W. Surf. Sci. 1990, 233, 355. (20) Ono, L. K.; Roldan Cuenya, B. J. Phys. Chem. C 2008, in press.
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