Theodore E. Madey. Department of Physics and Astronomy, and Laboratory for Surface Modification

Transcription

1 The Science of Catalysis at the Nanometer Scale Theodore E. Madey Department of Physics and Astronomy, and Laboratory for Surface Modification Rutgers, The State University of New Jersey Piscataway, NJ

2 Motivation Catalysis huge economic and societal impact chemical industry petroleum refining energy conversion environmental cleanup pharmaceuticals

3 Why nanoscience of catalysis? Selectivity (choosing specific reaction pathway) much less understood than activity in catalysis The goal: 100% selectivity in catalytic processes G.A. Somorjai and Y.G. Borodko, Catal. Lett. 76 (2001) 1

4 1. Introduction - background, materials - particle size effects 2. Fabrication of Model Nanocatalysts - e-beam - deposition of nanoclusters - theory 3. Self-assembly of nanocatalysts - clusters on oxides - nanoscale faceting of bimetallics - metals on sulfides 4. Outlook real catalysts model catalysts ~ 100 m 2 /g ~ 1 cm 2

5 Chemical Processes and Catalytic Materials (Somorjai, 1981)

6 Structure-Sensitive Reactions: rates depend on particle size, surface crystallography N 2 + 3H 2 2NH 3 over Fe C 2 H 6 + H 2 2CH 4 1 Ir / Al 2 O TON (s -1 ) 0.01 Rh / TiO 2 Rh / SiO 2 1E-3 Ni / Al 2 O 3 1E Diameter (nm) (M.Boudart, 1975) (Che and Bennett, 1989) Structure-insensitive reactions: little dependence of rate on particle size, surface structure C 2 H 4 + H 2 C 2 H 6 (hydrogenation of ethylene) 2CO + O 2 2CO 2 (oxidation of CO) 3H 2 + CO CH 4 + H 2 O (catalytic methanation)

7 Selectivity - the ability to turn reaction on particular pathways, to form specific products Example: catalytic conversion of ethanol vapor at ~ 600 K (C 2 H 5 OH) Catalyst Products ( ) Cu metal Acetaldehyde C + H 2 Al 2 O 3 ZSM-5 Zeolite H O CH 3 H H Ethylene( ) C C H H + H O 2 Synthetic gasoline, H 2 O

8 Possible origins for size effects - Geometric effects - Electronic effects Small clusters do not have bulk electronic structure - Valence band sharpens, DOS at E F decreased Limit for quantum size effects N ~ E F / 2kT ~ 100 atoms (d ~ 1.5 nm)

9 Use of e-beam lithography to fabricate nanoparticle arrays as model catalysts Pt / SiO 2 G. Rupprechter, A.S.Eppler, A.Avoyan, G.A.Somorjai, Studies in Surf. Sci. and Catalysis / Elsevier, 2000, p.215

10 Monte Carlo simulations of kinetics on nanometer catalyst particles Typical shape of supported metal particle 2D lattice mimicking supported catalyst particle Zhdanov and Kasemo, Surf. Sci. Rep. 39 (2000) 25 Model reaction: Reaction steps: 2A + B 2 2AB A gas A ads (B 2 ) gas 2B ads A ads + B ads (AB) gas Pd/MgO (C.Henry)

11 Snapshot of central and peripheral facets during MC run

12 Reaction Rate for 2A + B 2 fi 2AB as a function of impingement rate p where p = F A F A + F B 2 Zhdanov and Kasemo, Phys. Rev. Lett. 81 (1998) 2487 Facet communication (via diffusion) leads to new, unique kinetics Adsorbate-induced catalyst reshaping can dramatically change kinetics

13 Soft-landing of mass-selected clusters Pd + n MgO at 90K Cyclization of acetylene over nanoclusters 3C 2 H 2 -> C 6 H 6 S. Abbet et al. / Thin Solid Films 400 (2001) One Pd atom is enough!

14 When gold is not noble: nanoscale Au catalysts CO oxidation (2CO + O 2 fi CO 2 ) at 300 K! CO oxidation on Au 8 supported on (a) defect-rich and (b) defect poor MgO(100) Theory: CO and O 2 react on Au 8 to form CO 2 A.Sanchez et al., J.Phys. Chem. A 103 (1999) 9373

15 Nucleation and Growth of Au clusters on TiO 2 (110) STM image of Au clusters Scanning Tunneling Spectroscopy (STS) of individual clusters C.C.Chusuei et al. / Topics in Catalysis 14 (2001) 71

16 Correlation of catalytic activity and electronic properties with Au cluster size C.C.Chusuei et al. / Topics in Catalysis 14 (2001) 71

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18 Pd / SiO 2 catalyst after oxidation and reduction A. K. Datye / Topics in Catalysis, 13 (2000)

19 Rh / TiO 2 catalyst after reduction at 473K (Low-T reduction) A. K. Datye / Topics in Catalysis, 13 (2000)

20 Rh / TiO 2 catalyst after reduction at 473K (Low-T reduction) A. K. Datye / Topics in Catalysis, 13 (2000)

21 Pt / SiO 2 Model catalyst after reduction at 923K (High -T reduction) A. K. Datye / Topics in Catalysis, 13 (2000)

22 Self-ordered arrays of S atom nanoclusters on strained Cu / Ru(0001) as deposited anneal to 700 K nm nm The hope: a nanotemplate for cluster deposition with extremely uniform size distribution J. Hrbek, J. de la Figuera, K. Pohl, T. Jirsak, J. A. Rodriguez, A. K. Schmid, N. C. Bartelt, and R. Q. Hwang, J. Phys. Chem. B103 (1999)

23 Challenges - How many catalyst atoms are needed to make, break chemical bonds? - Achieve high selectivity make uniform nanoparticle catalysts three dimensional arrays active sites role of promoters, site-blockers - Catalyst characterization under reaction conditions - Instrumentation: high spatial and temporal resolution - Electronic structure theory