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Fundamental Calculations on the Surface Area Determination of Supported Gold Nanoparticles by Alkanethiol Adsorption Alexander Janz,*,† Angela K€ockritz,† Lide Yao,‡ and Andreas Martin† †
Department of Heterogeneous Catalytic Processes, Leibniz-Institut f€ ur Katalyse e.V. an der Universit€ at Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany and ‡Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Received October 30, 2009. Revised Manuscript Received December 21, 2009
Surface area determination is a crucial step for the characterization of the activity of noble metal catalysts. Not only the development of useful determination methods but foremost the understanding of surface properties and their conversion into mathematical expressions are essential to obtain reliable results. A selective method to gain access to the specific surface area of gold on oxidic supports is the chemisorption of alkanethiol from suspensions. Therefore, the concentration of a 1-dodecanthiol solution before and after immersion of supported gold catalysts was determined by gas chromatography. To convert the concentration information into a specific surface area, the surface coverage, the surface atom concentration, the interatomic Au-Au distance, and the particle morphology were considered. Further calculations afforded the determination of a mean particle diameter. A good agreement was found between gold particle sizes obtained from transmission electron microscopy and thiol adsorption. The given mathematical expressions are highly valuable for a broad range of chemisorption methods and noble metal catalysts.
Introduction Catalysis by gold has attracted increasing interest during the past decades. Today, it is recognized that gold has unique properties as a catalyst for many reactions but foremost for heterogeneously catalyzed oxidations.1,2 The outstanding activity exhibited by gold particles below a particle size of 5 nm is explained by the dramatic increase of the surface area and by the change in electronical properties due to quantum size effects.3 The dependence of the catalytic activity on the particle size of gold raises the question on the appropriate unit for the reaction rate. Regarding gold catalysis, often the specific rate rs is given in mol/s 3 gAu or mol/s 3 molAu, also denoted (but at least scientifically questionable) as turnover frequency (TOF). The more advanced quantity is the areal rate ra which is given in molar amount per time and surface area [mol/s 3 m2Au]. This quantity is appropriate, when the surface area is accessible but the number of active surface atoms and, as a result, the dispersion are not identified. Chemisorption methods are promising tools for the specific surface area determination. The adsorption of gaseous probe molecules for this purpose has been known since Brunauer, Emmett, and Teller developed a method (BET) to analyze the surface area by determining the amount of adsorbed N2 in 1938.4 However, the BET method does not allow the discrimination between a catalytic active species and the supporting material. *To whom correspondence should be addressed. Telephone: þ49(381)1281-283. Fax: þ49(381)1281-51283. E-mail: alexander.janz@ catalysis.de. (1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405–408. (2) Hashimi, A. S. K.; Hutchings, G. J. Angew. Chem. 2006, 118, 8064–8105. (3) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006; Vol. 6. (4) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309–319. (5) Iizuka, Y.; Fujiki, H.; Yamauchi, N.; Chijiiwa, T.; Arai, S.; Tsubota, S.; Haruta, M. Catal. Today 1997, 36, 115–123. (6) Berndt, H.; Pitsch, I.; Evert, S.; Struve, K.; Pohl, M. M.; Radnik, J.; Martin, A. Appl. Catal., A 2003, 244, 169–179.
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Chemisorption methods, employing CO,5 H2, and O26 as probe gases are in principle suitable for the selective characterization of surface properties, such as the degree of dispersion, that is, the percentage of surface atoms, and the specific surface area. Anderson et al. reviewed gas phase chemisorption methods with emphasis on Pt, Pd, Ph, Ni, and Cu.7 Though those methods are limited by the properties of the support and could not be generally used, for instance, on Au/TiO2 catalysts, CO is adsorbed both on gold and on titania. A recent study discusses the possibility of circumventing this problem.8 Au particle sizes determined by O2 chemisorption on Au/Al2O3 have shown good agreement with transmission electron microscopy results, but the method was not applicable to Au/CeO2 or Au/TiO2 due to the redox properties of those supports.6 Analogue limitations were found for H2. Alternatively, Clement reported on the adsorption of sulfur compounds in the liquid phase based on the principle of selfassembled monolayers (SAMs).9 Assuming that gold particles are spherical/hemispherical in shape and the adsorption on the surface is comparable to the one on flat single crystal model surfaces, specific surface areas were calculated.6,9 SAMs of alkanethiols on gold model surfaces were extensively studied.10 The binding angle of an n-alkanethiol on Au differs from 28° to 14° for {111} and {100} surfaces, respectively. Branched thiols are adsorbed with an angle of 15° more upright on a {111} surface than n-alkanethiols, so that they are packed closer with an intermolecular distance of 4.3 A˚, compared to 5.0 A˚ for n-alkanethiols.11 Consequently, the space requirement of alkanethiols depends on the orthogonal projection and the binding angle of the molecule onto the surface. The self-organization (7) Anderson, J. A.; Fernandez-Garcia, M.; Martinez-Arias, A. Supported Metals in Catalysis; Imperial College Press: London, 2005; Vol. 5, p 123. (8) Chiorino, A.; Manzoli, M.; Menegazzo, F.; Signoretto, M.; Vindigni, F.; Pinna, F.; Boccuzzi, F. J. Catal. 2009, 262, 169–176. (9) Clement, M.; Menard, H.; Rowntree, P. A. Langmuir 2008, 24, 8045–8049. (10) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (11) Chechik, V.; Schonherr, H.; Vancso, G. J.; Stirling, C. J. M. Langmuir 1998, 14, 3003–3010.
Published on Web 01/20/2010
DOI: 10.1021/la9041277
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of the surfactants is kinetically determined.12 Within a few minutes, 80-90% of the monolayers built up. The complete surface coverage, however, takes several hours.13 Furthermore, the self-organization depends on the polarity of the solvent: less polar solvents lead to a fast but less organized self-assembly. It also depends on the temperature: as expected, increased temperature results in a faster self-assembly with a reduced number of defects.10 Surfaces of small nanoparticles, exemplarily discussed on thiol capped colloids, contain low valence sites to a great extent in contrast to flat model surfaces. Additionally, the curvature causes more space for the chain moieties of alkanethiols. Therefore, this work deals with the question whether the additional space causes a higher surface coverage. However, the additional space for the surfactants is regarded in a model SAM.14 When alkanethiols are exposed to oxidic supports, no adsorption is observed except for CuO and AgO.15 Hence, it can be estimated that alkanethiols adsorb selectively only on gold nanoparticles and not on the support. Alternatively, these metal oxide support surfaces could be blocked before adsorption of thiols by carboxylic acids, phosphoric acids, or hydroxamic acids.15 The aim of the present work is to develop a simple, reliable, and general method for the determination of the specific surface area of supported gold catalysts. By using 1-dodecanethiol as the probe molecule in the liquid phase and developing the mathematical approach, the experimental results of the adsorption will be evaluated on the basis of a standard method, that is, electron microscopy.
Experimental Section and Characterization Materials. HAuCl4 3 xH2O was synthesized according to a 16
known procedure. Al2O3 (Puralox HP 14/150, Sasol), TiO2 (P25, Evonik), CeO2 (Acros), and 1-dodecanthiol (Aldrich, g98%) were received from the given manufacturers. All other reagents were acquired from standard sources and used as received. Synthesis. The syntheses of the gold catalysts followed standard procedures for incipient wetness impregnation (IMP),17 deposition precipitation, both NaOH and urea versions (DPN and DPU),6 and direct anionic exchange (DAE).18 Characterization. The chemical compositions of the gold catalysts were measured using an Optima-3000-XL inductively coupled plasma optical emission spectrometer (Perkin-Elmer, ICP-OES). Transmission electron microscope (TEM) analyses were performed using different devices: (A) Au/Al2O3-DPN samples were dispersed in ethanol (absolute for analysis); this suspension was ultrasonicated to minimize clustering of samples. A drop of the suspension was then deposited on a circular copper grid covered by a thin holey carbon film. High resolution TEM (HRTEM) observations were performed using a Titan 80-300 (FEI) instrument with an image Cs corrector operated at 300 kV. The spatial resolution in TEM mode can obtain 0.8 A˚. (B) Sample Au/CeO2-IMP was analyzed by using a Titan 80-300 (FEI) environmental TEM (ETEM) with the correction for objective (12) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145–1148. (13) Han, Y.; Uosaki, K. Electrochim. Acta 2008, 53, 6196–6201. (14) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem.;Eur. J. 1996, 2, 359–363. (15) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813–824. (16) Gross, S. Colloidal Dispersion of Gold Nanoparticles. In Materials Syntheses; Schubert, U., H€using, N., Laine, R. M., Eds.; Springer: Vienna, 2008; pp 157-158. (17) Delannoy, L.; El Hassan, N.; Musi, A.; Le To, N. N.; Krafft, J.-M.; Louis, C. J. Phys. Chem. B 2006, 110, 22471–22478. (18) Ivanova, S.; Petit, C.; Pitchon, V. Gold Bull. 2006, 39, 3–8.
6784 DOI: 10.1021/la9041277
lens spherical aberration. Finely ground powder of the sample was applied directly to a standard TEM lacey carbon Cu grid. Au/CeO2 was studied by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) as a complementary technique. (C) All other gold catalysts were investigated by using a CM20 TEM (Philips, LaB6) using a voltage of 200 kV. In order to ensure the reliability of statistic analysis, more than 100 Au particles were measured for each sample during TEM experiments. General Procedure of Thiol Adsorption. A 2.5 mM 1-dodecanethiol solution in acetonitrile (MeCN) was prepared. Each of the solid samples, 200 mg both of supported Au catalysts and of pure supports as blanks, was suspended in 2 mL of the alkanethiol solution. After stirring for 20 h, the self-assembly of thiol molecules onto the gold surface was completed. The suspension was filtered over kieselguhr. Consecutively, the filter cake was washed with 7 mL of MeCN to remove thiol molecules from SAM domains and from kieselguhr, which were only physisorbed. A total of 0.5 mL (10 mM) of a 1-undecanol solution as internal standard was added to the filtrate. The resulting solution was filled up to 10 mL with MeCN and was analyzed by GC-MS (Shimadzu GCMS 2010). Quantitative Analysis. The GC-MS instrument was calibrated with seven solutions of various 1-dodecanthiol concentrations between 10 and 2000 μmol/L, each containing 500 μmol/L of the internal standard 1-undecanthiol. The obtained calibration curve shows an excellent linearity (R2 = 0.9996) and a y-intercept near zero (b = -0.02). Samples of the filtrates obtained from both the catalysts and the blanks were analyzed by GC-MS to determine the concentration of 1-dodecanethiol. The amount retained by gold particles was calculated from the difference of the thiol amounts of the fresh solution and the solution with the catalyst considering the amount potentially adsorbed on the support (blank sample).
Results and Discussion From the thiol adsorption method, the amount of adsorbate is obtained as quantitative information. To calculate the specific surface area and the particle diameter from that information, first the polyhedral structures of gold clusters are compared to the simplified supposition of a spherical/hemispherical particle morphology with regard to their adsorption properties. This is followed by the discussion of the particle size dependence of the surface coverage. After that, the surface atom concentration, taken as a constant, is questioned by a geometrical analysis of the different surface atom species and the statistical weighting of respective species. The contraction of the interatomic distance with decreasing particle size is also taken into account. Lastly, the resulting specific surface areas and particle diameters are verified by TEM results. Calculation Basis. Reactions catalyzed by supported noble metals are characterized in most cases by the rate of reaction which is given as the amount of reactant per time and mass of the active metal. That habit is more of a practical approach. In fact, the reaction solely takes place on the surface of the active metal, but the mass includes surface and bulk atoms. Therefore, the catalytic activity should be normalized either to the number of surface atoms, to the degree of dispersion, or to the related value of the specific surface area as. The specific surface area is usually obtained from the specific 6 amount of probe molecules nA s (eq 1). Therefore, two values need to be known: the number of adsorbates NA per surface atom which is the surface coverage θ, and the number of surface atoms NS per area unit, which is the surface atom concentration σ (Figure 1). Both, θ and σ, were assumed to be constant values in several publications:6,9 In the case of gold as the active metal, σ is Langmuir 2010, 26(9), 6783–6789
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Figure 1. Specific surface area as as a function of the specific adsorption (bold line) and as a function of the particle diameter (dashed line). Spherical morphology and adsorption on a {111} gold model surface were assumed.
Figure 3. Influence of assuming nonspherical gold particle morphologies, icosahedron (Ih), truncated octahedron (TO), and cuboctahedron (Co), on the particle diameter calculation.
Figure 4. Surface coverage on gold nanoclusters (bold line) versus the assumption of adsorption on a flat model surface (dashed line) according to Hostetler.24
Figure 2. TEM images of Au/Al2O3-DPN.
11.5 nm-2;19 in the case of n-alkanethiols as probe molecules, θ is determined by SAM investigation on a flat gold model surface and equals 0.33.20 as ¼
NL A n θσ s
ð1Þ
For the reason of comparison with other size determining techniques, as for instance TEM, the conversion of the specific surface area into a particle diameter is often performed. The combination of the surface area A, volume V, mass m, and density F of a sphere results in eq 2 (Figure 1). Even though gold nanoparticles are in a first approximation described as hemispheres, there is no effect on the calculation of the specific surface area, because both surface area and weight are halved. as ¼
A 6 -1 ¼ d m F
ð2Þ
The above-mentioned calculations suffer from some impreciseness, especially for small gold nanoparticles (
