1 Special Properties of Au Nanocatalysts Maryam Ebrahimi Chem 750/7530 March 30 th, 2006

2 Outline Introduction Goodmans Research Laboratory Gold Nanoparticles Research Proposal References

3 Introduction Metal oxide interface, metal coatings or dispersed metals on oxide supports play an important role in many technological areas. One of the areas where deposited metal particles are technically employed to a large extent is heterogeneous catalysis. There is still a lack of fundamental knowledge about the essential properties of thin metal films and small metal particles on oxide supports. So, an increasing number of model studies like model catalysis have been introduced. One approach comes from ultrahigh vacuum (UHV) surface science aiming at an understanding of the elementary steps involved on a microscopic level. Particle-size effects and the role of metal-support interactions

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5 Gold Nanoparticles Au has long been known as being catalytically far less active than other transition metals. Because of its inertness, Au was formerly considered as an ineffective catalyst. This assumption was based on studies where Au was present as relatively large particles (diameter > 10 nm) or in bulk form such as single crystal. Haruta et al. have shown exceptionally high CO oxidation activity on supported nano-Au catalysts even at sub-ambient temperatures (200 K).

6 Gold Nanoparticles Supported Nano-Au catalysts exhibit: an extraordinary high activity for low-temperature catalytic combustion Partial oxidation of hydrocarbon Hydrogenation of unsaturated hydrocarbons Reduction of nitrogen oxides Propylene epoxidation Methanol synthesis Environmental catalysis

7 The structure of Catalytically Active Gold on Titania Cluster size and morphology, particle thickness and shape Support effects: Nature of the support material, Surface defects, Metal-Support charge transfer, Au- support interface. Metal oxidation state Au-oxide contact area

8 The Most Active Size: nm Science, 281 (1998) 1647

9 The Most Active Size: nm Catalysis Letters,99 (2005) 1 Catalysis Today,111 (2006) 22-33

10 Gold monolayers & bilayers that completely wet the oxide support, eliminate direct support effects. Science, 306 (2004) 252

11 Particle thickness and shape (CO Adsorb strongly on the Au bilayer structure) On the basis of kinetic studies and scanning tunneling microscopy (STM): Au consists of bilayer islands that have distinctive electronic and chemical properties compared to bulk Au. Two well-ordered Au films (monolayer and bilayer) completely wet an ultrathin titania surface.

12 Science, 306 (2004) 252 Catalysis Today,111 (2006) 22-33

13 Strong metal support interaction (SMSI) A key feature of Au grown on TiO x /Mo(112) is the strength of the interaction between the overlayer Au and the support comprised of strong bonding between Au and reduced Ti atoms of the TiO x support, yielding electron-rich Au. Recent theoretical studies: importance of reduced Ti defect sites at the boundary between Au clusters and a TiO x interface in determining the Au cluster shape and electronic properties via transfer of charge from the support to Au.

14 Surface Defects The introduction of defects into a crystal can dramatically change its electronic properties Defects can affect the chemistry of bare metal-oxide surfaces Au particles bind more strongly to a defective surface than to a defect deficient surface. There is significant charge transfer from the support to the Au particles. Au particles dont bind to a perfect TiO 2 surface. Defect sites on the oxide support play an important role in the wetting of Au particles yielding electron-rich Au. But the support itself need not be directly involved in the CO oxidation reaction sequence.

15 Essential Features of the Interaction of Au with TiO 2 (1) wetting of the support by the cluster (2) strong bonding between the Au atoms at the interface with surface defects (reduced Ti sites) (3) electron-rich Au (4) annealing at temperatures in excess of 750 K, sufficient to create and mobilize surface and bulk defects, is crucial in preparing an active catalyst (5) oxidation leads to deactivation via sintering of Au Goodman: Au particle size is related to activity, bilayer Au structure and the strong interaction between Au and defect sites on the TiO 2 surface and critical for CO oxidation activity.

16 Research Proposal Electronic properties of deposited metal clusters and thin films: how does the electronic structure develop with increasing size/thickness? Metal-oxide interface: what is the nature and strength of the bonding? Adsorption and adhesion energies. Diffusion of metal atoms on oxide supports. Nucleation and growth: what are the activation energies for the elementary steps involved? What is the prevailing nucleation mechanism? Under which conditions are ordered/disordered particles formed? Is the growth process influenced by an ambient of certain gases? Interaction with gases: in which way does the interaction strength/adsorption energy change with size? Is the particle shape altered by gas adsorption? Catalytic activity: how does the activity/selectivity change with dispersion. Are metal-support interactions of relevance?

17 The purpose of this program is to explore and manipulate the size, morphology and chemical environment of gold-containing nanoparticles with the goal of optimizing their reactivity with respect to elementary reactions that are of widespread interest in heterogeneous catalysis. The materials focus is on nanoscale molecular catalysts incorporating the early transition metals (like: Ti, V, Cr, Mn, Fe) or late transition metals (like: Rh, Pd, Pt) which may have promising catalytic properties and may offer significant advantages over more commonly used noble metals. Research Proposal

18 The main steps of the research program involve: (1) the development of new methodologies for the preparation of well-defined nanoparticles (2) reactivity studies as a function of size, morphology and chemical environment chemical environment : modification of the surface (a) adding electropositive or electronegative elements (b) Deposing transition metals, rare earth elements (Ce in the metalic or oxidized form) (c) depositing Au nanoparticles on the functionalized substrate (3) the development and application of new theoretical methods for understanding and predicting the structure and reactivity of metal-containing nanoparticles. Current methods being explored for nanoparticle preparation include templating on strained metal surfaces, deposition of size-selected clusters and impregnation into nanoporous materials (collaboration with Prof. Uzi Landman at Georgia Tech., Prof. Jense Norskov in Denmark, and Dr. Pacchioni in Italy) (4) Methods of characterization: STM, STS, UPS, XPS,FT-IR, HREELS, STM-IETS, TPD Research Proposal

19 References 1. D.W. Goodman et al., Science 281 (1998) D.W. Goodman et al., Science 306 (2004) D.W. Goodman et al., Catalysis Today 111 (2006) D.W. Goodman et al., Catalysis Letters 99 (2005) D.W. Goodman et al., J. Phys. Chem. B 108 (2004) D.W. Goodman et al., Surface Science 600 (2006) L7-L11 7. D.W. Goodman et al., Applied Catalysis A 291 (2005) D.W. Goodman et al., Science 310 (2005) M. Baumer & H-J Freund, Progress in Surface Science 61 (1999) G.A. Somorjai et al., Topics in Catalysis 24 (2003) 61-72