Au Coatings on Oxide Carriers for Low Temperature Oxidation of CO

1. Mean dia. = 2.26 nm StdDev = 0.23 nm Sample = 943 Mean dia. = 1.999 nm StdDev = 0.4161 nm Sample = 112 Figure 2. Gold LIII edge spectra of supported Au catalysts and reference compounds. The Au chemical state was determined using x-ray absorption spectroscopy. As prepared samples from deposition-precipitation contained Au in a +3 oxidation state, but subsequently reduced to predominantly metallic state following pretreatment in He at 623K. The as-prepared samples from LAD contained Au in a metallic state. High purity targets and oxide carrier are placed in processing chamber Processing chamber is evacuated and backfilled to desired pressure LAD is initiated, powders are uniformly coated Deposition is completed samples are ready for testing 10nm 10nm (c) (d) Au Coatings on Oxide Carriers for Low Temperature Oxidation of CO J.T. Callaa, C.E. Allmondb, M.C. Raphuluc, R.J. Davisa, and J.M. Fitz-Geraldb Departments of a Chemical and b Materials Science Engineering, University of Virginia, 116 Engineer’s Way, Charlottesville, Virginia 22904 C Department of Chemistry, University of Witwatersrand, Johannesburg, South Africa Introduction Engineered particulates affect a wide range of advanced existing and emerging products used in aircraft, cutting tools, lithium-ion rechargeable batteries, superconductors, pharmaceutical drug formulations that employ micron to submicron sized particulate precursors. Bulk gold is considered the least reactive metal in heterogeneous catalysis due to repulsion between the orbitals of the adsorbate and gold’s filled d states, but Haruta, et al. groundbreaking work presented gold’s unique ability for CO oxidation at ambient temperaure [1,3]. Gold’s catalytic potential is not realized until it is present as supported nanoparticles [4,5]. Supported gold nanoparticles are recognized as active catalysts for a variety of reactions, such as oxidation [2], epoxidation of propene [6,7], hydrogenation [8], water – gas shift [9,10], and NO reduction [11]. Despite intense effort, the underlying principles and the degree of their contribution to the activity of gold catalysts are currently debated. Although not conclusive, several factors have been proposed to explain the high activity of gold nanoparticles. These factors can be generalized into three classifications; metal particle size, metal oxidation state, and synergy between the metal particle and the support. Typically, supported Au nanoparticles are prepared with techniques using molecular precursors in wet suspensions. However, it is difficult to control the factors that are suspected to impact activity. Laser assisted deposition (LAD) provides an alternative, dry method to prepare supported Au nanoparticles while controlling the parameters that impact activity. By attaching atomic cluster to 1-3 nm particles in discrete or continuous forms on the surface of the core particles, i.e. nano - functionalization of the particulate surface, materials and products with significantly enhanced properties can be obtained. XANES and Catalytic Results Table 1. Effect of Preparation Method and Support Composition on the Rate of CO Oxidation over Supported Au Catalysts. The catalytic activity of the samples for CO oxidation was evaluated in a single-pass, fixed bed reactor. Between 5 and 50 mg of sample was diluted with up to 400 mg SiC and loaded into a quartz reactor operating at atmospheric pressure and at temperature 373 K. Gas flows of 40 to 150 mL min-1 composed of 2.0 mole % CO and O2 and balance He were used. Experimental The preparation technique using a molecular precursor is a deposition – precipitation methodology. In this process, the molecular precursor is ‘precipitated’ in the presence of the metal-oxide support. The support acts as a nucleating agent upon which the precursor is ‘deposited.’ To deposit Au onto Al2O3 and TiO2 an aqueous solution of HAuCl4 adjusted to pH 7 using NaOH is contacted with an aqueous slurry of the support at 343 K for 2 h. The samples are then rinsed with H2O and suction filtered, followed by drying in air at 313 K for 24 h. Prior to any kinetic studies the samples are pretreated in flowing He at 623 K for 4 h. Laser-assisted deposition (LAD) offers an innovative and unique method for gold deposition. It presents a novel method of controlling factors that affect the catalytic activity: size and oxidation state. By adjusting fundamental variables, cluster or particle size can be tailored to narrow size distributions. In these experiments Au and Pd targets (99.99% at.) were ablated in inert environments at pressures ranging from 0.1 mTorr – 10 mTorr at a fluences of 1 to 3 J/cm2. The oxide carriers were mechanically agitated to allow uniform particle coating. The samples were characterized with Inductively coupled plasma mass spectroscopy (ICP-MS) metal loading measurements, X-ray Absorption Near Edge Structure (XANES), and high resolution transmission electron microscopy (HRTEM) utilizing a JEOL 4000 EX TEM. HRTEM and FE-AEM Dried at 308 – 318 K For ~ 24 h Suction filtered then resuspended in ~ 100 mL DI H2O At 343 K: repeated 4 times 80 mL 8.4 mM HAuCl4 At 343 K Adjusted to pH 7 5 g g-Al2O3 or TiO2 suspended in 120 mL DI H2O At 343 K Figure 3.Rate of CO oxidation at 373K during activation. No catalytic activity for CO oxidation was observed over the Au/Al2O3 sample prepared by LAD. At hour 1, H2 was added to the reactor feed, which resulted in an observable steady-state rate of selective oxidation of CO in H2. The flow of H2 was stopped at hour 6 and the rate decreased to a new steady-state value. For the Au/TiO2 sample prepared by LAD, a low level of catalytic activity was observed without the addition of H2 at hour 2. The CO oxidation rate also increased substantially after addition of H2 at hour 2. The CO oxidation rate also increased after the H2 flow was stopped at hour 10. It appears that H2 was involved in an activation process for the samples prepared by LAD. We suspect that small amounts of water produced by oxidation of H2 assist in the removal of surface contaminants deposited on the Au particles during LAD. Mean dia. = 2.081 nm StdDev = 0.563 nm Sample = 936 5 nm (a) (b) (b) (a) (b) (a) Figure 4. Micrographs of Pd nanoparticles deposited onto oxide supports under identical deposition conditions for Au: (a) particle size distribution of Pd on amorphous carbon support 250 pulses 3 mTorr of Ar; (b) HRTEM image of Pd on amorphous carbon support 250 pulses 3 mTorr of Ar; (c) HRTEM image of Pd deposited onto TiO2 (250 pulses, 3 mTorr); (d) HRTEM image of Pd onto Al2O3 (250 pulses, 3 mTorr Ar). Figure 5. Size distributions of Pd particles: (a) 100 mTorr Ar; Pd on TiO2, 250 pulses, 3 mTorr Ar; (b) Pd on Al2O3 250 pulses, 3 mTorr Ar. Again these size distributions are of nanoparticles deposited under similar deposition parameters as the Au on oxide carriers. Figure 1.Laser assisted deposition systemfor coating particulate and irregularly shaped materials. • Conclusions • Laser assisted deposition is a viable technique to prepare highly dispersed, zero valent, supported noble metal nanoparticles. • Coatings of Au, Pd, Pt, and Ce on Al2O3 and TiO2 have been performed. • Narrow (1-3 nm) size distributions with no measurable evidence of agglomeration have been produced. • Supported Au catalysts prepared by laser assisted deposition are active for CO oxidation. Future Work • Completion of HRTEM for Au, Pd, Pt, and Ce. • Comparison of reactivity of nanoparticle to atomic coatings. • In-situ plasma cleaning of support materials during deposition is being investigated. References [1] Thompson, D.T., Appl. Catal. A: Gen. 2003, 243, 201. [2] Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal.1989, 115, 301. [3] Hammer, B.; Norskov, J.K. Nature1995, 376, 238. [4] Lopez, N.; Janssens, T.V.W.; Clausen, B.S.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Norskov, J.K. J. Catal.2004, 223, 232. [5] Haruta, M. Catal. Today1997, 36, 153. [6] Zwijnenburg, A.; Saleh, M.; Makkee, M.; Moulijn, J.A. Catal. Today2002, 72, 59. [7] Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566 [8] Hutchings, G.J.; Catal. Today2002, 72, 11. [9] Fu, Q.; Saltsburg, H.; Flytzani – Stephanopoulos, M. Science2003, 301, 935. [10] Andreeva, D.; Tabakova, T.; Idakiev, V.; Christov, P.; Giovanoli, R. Appl. Catal. A: Gen.1998, 169, 9. [11] Kung, M.C.; Lee, J.H.; Chu-Kung, H.; Kung, H.H. 11th. International Congress on Catalysis – Studies in Surface Science and Catalysis1996, 101, 701. Acknowledgements The authors wish to acknowledge the National Science Foundation and the National Synchrotron Light Source for funding and beam-time. The authors also wish to acknowledge Prof. Jim Howe and Dr. Vladimir Oleshko for useful discussions and microscopy.