J. Phys. Chem. B 2005, 109, 14581-14587
14581
Hydrogen Chemisorption on Al2O3-Supported Gold Catalysts Eveline Bus, Jeffrey T. Miller,† and Jeroen A. van Bokhoven* Institute for Chemical and Bioengineering, Swiss Federal Institute of Technology, ETH, CH-8093 Zurich, Switzerland ReceiVed: April 1, 2005; In Final Form: June 1, 2005
Hydrogen is dissociatively adsorbed on the gold particles in Au/Al2O3 catalysts, as demonstrated by a combination of in-situ X-ray absorption spectroscopy, chemisorption, and H/D exchange experiments. This chemisorption of hydrogen induces changes in the Au L3 and L2 X-ray absorption near-edge structures. The gold atoms on corner and edge positions dissociate the hydrogen, which does not spill over to the face sites. Therefore, the average number of adsorbed hydrogen atoms per surface gold atom increases with decreasing particle size. With temperature, the hydrogen uptake by supported gold increases or remains constant, whereas it decreases for platinum. Furthermore, in H/D exchange experiments, the activity of Au/Al2O3 increases strongly with temperature. Thus, the dissociation and adsorption of hydrogen on gold is activated.
Introduction Bulk gold is the least active metal in the adsorption of atoms and molecules,1 but its properties change remarkably when it is highly dispersed and supported on a metal oxide.2 These supported gold nanoparticles catalyze the oxidation of CO, the epoxidation of propylene, and the low-temperature WGS reaction.2,3 For the most part, oxidation reactions have been studied.2-4 Gold, however, is also active in reactions involving hydrogen, such as hydrogenation of alkenes, alkadienes, alkynes, and ketones, as well as in hydrogenolysis and in dehydrogenation reactions, as reviewed by Bond and Thompson.3 The selective hydrogenation of hydrocarbons with more than one unsaturated bond is of particular interest. Acetylene,5 1,3butadiene, crotonaldehyde, acrolein,6-8 and other R,β-unsaturated aldehydes9,10 are selectively hydrogenated by supported gold nanoparticles. Despite hydrogen being an important reactant in these reactions, there are only few studies dealing with the interaction of gold and hydrogen. On Au(110)(1 × 2), no chemisorption of hydrogen molecules was observed. Hydrogen atoms adsorb on this Au surface and desorb again at 226 K.11 Stobin´ski et al.12 showed that hydrogen molecules chemisorb dissociatively on thin, unsintered gold films and suggested that hydrogen reacts with gold atoms on corners and edges. The hydrogen coverage was less than 0.015 at a pressure of 0.3 Pa and a temperature of 78 K. Okada et al.13 found that hydrogen molecules adsorb dissociatively on thin Au films grown on an Ir(111) surface. They attributed this to localized sites such as defects and relaxed structures. These results show that, under certain conditions, hydrogen chemisorbs on the surface of gold. However, these studies were performed under conditions and with structures that differ from those in supported gold catalysis. Chemisorption of hydrogen by supported gold has been occasionally mentioned, and some or no chemisorbed hydrogen has been reported.3 Lin and Vannice14 did not find measurable amounts of chemisorbed hydrogen on Au/SiO2 and Au/TiO2 at 298 K. At 473 K, one Au/TiO2 sample adsorbed a small amount of hydrogen of about 1 atom-% of the amount of gold. These * Author to whom correspondence should be addressed. Phone: +411-632-5542; fax: +41-1-632-1162; e-mail:
[email protected]. † BP Research Center, E-1F, Naperville, IL 60563.
authors suggested that, for this catalyst, the adsorption process is activated and that the hydrogen is dissociated. In their samples, the gold particle size was around 30 nm, which is too big to show catalytic activity.2 Jia et al.5 measured a H/Au of 14% for 3.8-nm Au particles on Al2O3, and this catalyst was active in the acetylene hydrogenation. Because the activity of gold catalysts in hydrogenation reactions suggests the dissociative adsorption of hydrogen, our goal is to gain insight into the chemisorption of hydrogen on supported gold nanoparticles under catalytically relevant conditions by studying chemisorption and the Au-H interaction. To determine whether the hydrogen is chemisorbed on the gold particles, the Au-H bond must be observed. This can be achieved with X-ray absorption spectroscopy (XAS). It is well established that the chemisorption of hydrogen induces significant changes in the Pt L2 and L3 X-ray absorption near-edge structures (XANES) of supported Pt particles.15-31 Here, we report a very similar change in the Au L2 and L3 XANES. Quantitative measurements of the adsorption of hydrogen have been performed in the temperature range 298-523 K, and the hydrogen uptake is compared with that of Pt/SiO2. To determine whether dissociation of hydrogen takes place, H/D exchange experiments were performed. Experimental Section Sample Preparation and Characterization. Gold particles supported on γ-Al2O3 were prepared by adsorbing HAuCl4 on a γ-Al2O3 support (Condea or Catalpal, pore volume 0.5 cm3/ g, specific surface area 230 m2/g) by incipient wetness impregnation with an aqueous solution or by depositionprecipitation. In the deposition-precipitation method, 50 mL of an aqueous solution containing 0.8 g HAuCl4 was added to a slurry of 20 g Al2O3 in 300 mL water. The pH was adjusted to 4 with HNO3 and subsequently the solution was stirred at room temperature. The resulting yellow powder was recovered by filtering and washing with water. In both methods, chlorine was removed by stirring the AuClx/Al2O3 powder in 10 mL water per gram powder at 343 K and maintaining a pH of 8 using NaOH. The resulting white powder was dried at 363 K and stored in the dark. The initial gold content in the sample
10.1021/jp051660z CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
14582 J. Phys. Chem. B, Vol. 109, No. 30, 2005
Bus et al.
TABLE 1: Particle Sizes as Determined with STEM and EXAFS particle size (nm) -Aa
Pt/SiO2 Pt/SiO2-Bb Au/Al2O3-Ac Au/Al2O3-Bc Au/Al2O3-Cc Au/Al2O3-De Au/Al2O3-Ee,f
metal loading (wt %)
STEM
EXAFS
1.0 1.0 0.58 0.92 1.21 0.43 0.43
2 20 1.2 3 10 1.4 2
1.2 5 n.d.d n.d. 1.3 1.1 n.d.
a Prereduced at 423 K. b Prereduced at 873 K. c Prepared by deposition-precipitation. d n.d. ) not determined. e Prepared by incipient wetness impregnation. f Au/Al2O3-E is Au/Al2O3-D reduced at 673 K.
prepared by impregnation was 2 wt %, and after the treatment with base, it was 0.43 wt %. The gold content varied from 0.4 to 1.2 wt %. It was determined by atomic absorption spectrometry (AAS) on a Varian SpectrAA instrument after dissolving the samples in concentrated HF and HNO3 or by inductively coupled plasma spectrometry after dissolving in HF. The chlorine content in all the samples was negligible, as confirmed with X-ray fluorescence. The samples were reduced at 473 K, with the exception of Au/Al2O3-E, which was reduced at 673 K (Table 1). The reduction and metal particle formation were done prior to analysis. Pt/SiO2 was prepared by adding an aqueous solution of tetraammine platinum nitrate (PTA) to a slurry of SiO2 (Davison 644, pore volume 1 cm3/g, specific surface area 290 m2/g) in an NH4OH solution with a pH of 10. After adsorption of the PTA, the powder was filtered, washed, and dried at 373 K. The 1 wt % Pt/SiO2 samples were prereduced at 423 K (Pt/SiO2-A) or 873 K (Pt/SiO2-B). The metal particle size was determined by extended X-ray absorption fine structure analysis (EXAFS, see XAS section) and by scanning transmission electron microscopy (STEM). For STEM, the material was dispersed in ethanol and was deposited onto a perforated carbon foil supported on a copper grid. The investigations were performed with a Tecnai F30 microscope operating with a field emission cathode at 300 kV. STEM images, obtained with a high-angle annular dark-field detector, reveal the metal particles with a bright contrast (Z contrast). The images were taken after using the samples in the hydrogen chemisorption experiments. X-ray Absorption Spectroscopy (XAS). The sample was pressed into a self-supporting wafer and was placed in a stainless-steel in-situ cell.18 The cell was connected to a gasflow system, temperature controller, and vacuum pump. The XAS experiments were performed at beamline X1 of HASYLAB (Hamburg, Germany) and at the SNBL (BM01B) and DUBBLE (BM26A) stations at the ESRF (Grenoble, France). At beamline X1, the double-crystal monochromator was equipped with Si(111) crystals that were detuned to 70% to remove higher harmonics. At the SNBL, the incident X-rays were monochromated by a Si(111) channel-cut monochromator, and harmonics were rejected by a chromium-coated mirror. At the DUBBLE station, a double-crystal monochromator was used, followed by two vertically focusing Pt- and Si-coated mirrors to reject the harmonics. All the spectra were recorded in transmission mode using ion chambers for detection. Prior to measurement, the samples were reduced at 473 K in a 20 mL/min H2 flow and were cooled in this flow to 298 K. At this temperature, XAS data for the L3 and L2 edges were collected in static hydrogen atmosphere. Subsequently, the cell was evacuated at 473 K for 2 h to remove the adsorbed hydrogen. L3 and L2 XAS spectra were collected in dynamic vacuum (
