Catalysis by supported metal clusters and model systems

Transcription

1 Catalysis by supported metal clusters and model systems Gianfranco Pacchioni Dipartimento di Scienza dei Materiali Università Milano-Bicocca Part I Catalysis by supported metal clusters and model systems Part II - CO on MgO: lessons from 25 years of interplay between theory and experiment (the role of surface science ) Part III Oxide surfaces: role of morphology, defects, sample preparation Part IV The unusual properties supported gold: size, charging, support effects Part V New phenomena: metal clusters on ultra-thin oxide films

2 Nobel prizes in catalisys and surface chemistry P. Sabatier (hydrogenation of organic compounds with dispersed metals) F. Haber (ammonia synthesis on iron catalysts) I. Langmuir (contributions to surface chemistry) K. Ziegler e G. Natta (ethylene polimerization)

3 1909 Fritz Haber develops the industrial process of ammonia synthesis on Fe catalysts 2005 The process is completely elucidated at a first-principles level of theory

4 THE ROLE OF SURFACE SCIENCE Nobel Prize in Chemistry 2007 Gerhard Ertl has succeeded in providing a detailed description of how chemical reactions take place on surfaces and has in this way laid the foundation of modern surface chemistry. Fritz-Haber-Institut der Max-Planck- Gesellschaft Berlin

5 ENVIRONMENT: AUTOMOBILE CATALYTIC CONVERTERS Car exhaust catalyst: monolithic backbone covered internally with alumina+ceria+zirconia (microporous) Support for metal particles (Rh, Pt) of nanometer size Diffusion from gas phase N 2 NO Product desorption Dissociative CO Surface adsorption CO diffusion 2 Molecular adsorption Noble metal reaction Adsorbed product Three-way catalysis: CO converted to CO 2, C n H 2n+2 to CO 2, NO x to N 2 Support

6 How is the exhaust converted? TWC - three way catalyst CO oxidation: CO + ½ O 2 CO 2 HC oxidation: HC + O 2 CO 2 + H 2 O NO reduction: NO + ½ CO ½ N 2 + CO 2 HC + 2 NO CO 2 + N 2 + H 2 O Catalyst: Pt (Pd) / Rh / Al 2 O 3 / Ce x Zr 1-x O 2

7 Oxygen-poor Oxygen-rich

8 Aging effects Noble metal support Ceramic substrate Monolita in cordierite Sintering and incapsulation of the noble metal Sintering of the support (loss of surface area) 1273 K Durability - High cost

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10 J. Libuda, ChemPhysChem, 2004

11 METAL OXIDE SUPPORTED METAL CATALYSTS

12 METAL OXIDE SUPPORTED METAL CATALYSTS

13 Supported metal nanoparticles: preparation

14 NANOLITOGRAPHY

15 ELECTROCHEMICAL NANOSTRUCTURING Tip-induced cluster formation Metal is deposited onto the tip The metal-loaded tip approaches the surface for a short time depositing small amounts of material Left: array of 400 Cu clusters on Au(111); the Cu clusters are about 0.6 nm in height Kolb et al. Science 275 (1997) 1097

16 SELF-ASSEMBLY OF METAL CLUSTERS FROM ATOMS DEPOSITION Knudsen cell Metal clusters on oxide thin films in UHV deposition OXIDE FILM nucleation METAL SUBSTRATE re-evaporation diffusion UHV chamber Metal clusters formed on oxide thin films from vapor deposition ultra-thin oxide films grown on a metal substrate in UHV allow use of electron spectroscopies, STM, etc.

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18 WELL-DEFINED SUPPORTED METAL PARTICLES Rh clusters on Al 2 O 3 Metal deposited from vapor on Al 2 O 3 ultra-thin films grown on a metal in UHV: allows use of Scanning Tunneling Microscopy (STM). STM images of: (a) clean Al 2 O 3 thin film surface (b) deposition of Rh at 90 K (c) deposition of Rh at 300 K (d) deposition of Rh at 300 K on hydroxylated surface Nucleation occurs at steps and defects; the particles have a nearly uniform size (few nm). Bäumer, Freund, Progr. Surf. Sci. 61 (1999) 127.

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20 FABRICATION OF NANO-CATALYSTS ATOM BY ATOM Nd:Yag Laser (Infinity ) 532nm / 100Hz Analysis Chamber Epitaxial MgO thin films on Mo(100) grown by vaporizing Mg in O 2 atmosphere Gas-phase metal clusters generated by laser vaporization Clusters are ionized, massselected and deposited at low kinetic energy (<0.2 ev/atom) (soft landing) Low cluster concentration (<0.1 mono-layer) and low substrate temperature (90 K) to prevent cluster diffusion and aggregation Quadrupole II Sample Transport for STM Quadrupole I Quadrupole Deflector Cluster Source Mass Spectrom. (TDS) FTIR MgO film UPS/XPS AES Heiz et al. Rev. Sci. Instrum. 68 (1997) 1986 Heiz et al. JACS 120 (37) (1998) 9668 Heiz et al. JACS 121 (13) (1999) 3214

21 IMPORTANCE OF CLUSTER SOFT-LANDING Ion Signal (Arb. Units) Rh Rh Rh 2 Rh Rh 6 Rh 7 Rh 8 Rh 9 Rh 10 Rh 11 Rh 12 Rh Mass (amu) Cluster ions accelerated and mass-selected in a massspectrometer Soft-landing: electric field reduces kinetic energy of accelarated particle down to 0.2 ev/atom CLUSTER SURFACE IMPACT Effect of high kinetic energy of a Cu cluster of 2000 atoms impinging on a tilted Cu(001) surface at 10 kev of energy (molecular dyamics simulation) Use of thin layer of weakly adsorbed raregas atoms also possible to achieve softlanding of deposited clusters

22 Supported metal nanoparticles: characterization and properties

23 PARTICLES SIZE, SHAPE, MORPHOLOGY AND DISPERSION Pd particles on MgO Transmission Electron Microscopy of Pd particles grown on MgO single crystal (insulating) generated by metal deposition on the oxide surface in ultra-high vacuum (UHV) Various sizes of nanocrystals can be identified; average size 150x150 nm 2 Goyenex, Henry, Urban, Phil. Mag. A 69 (1994) 1073

24 ATOMIC RESOLUTION OF CLUSTER STRUCTURE: SCANNING TUNNELING MICROSCOPY (STM) (a) Atomically resolved image of 27 Pd atoms arranged in two layers on MoS 2 substrate (b) Atomically resolved Pd nanocrystallite grown on a thin alumina film

25 METAL-METAL DISTANCES IN NANOCLUSTERS Lattice constants (from HRTEM) and interatomic distances of Pt particles grown on Al 2 O 3 /NiAl(110) as function of their size (horizontal bars represent the difference of the widths and the lengths of the clusters, vertical bars represent error bars)

26 INSULATOR TO METAL TRANSITION: HOW MANY ATOMS ARE REQUIRED? Spectroscopic measurements of gap in inorganic transition metal carbonyl compounds show that 70 atoms are sufficient to close the gap Property strongly metal-dependent Quantum-size effect

27 LOCAL MEASURE OF PARTICLE GAP Scanning tunneling spectroscopy (STS) of single Au clusters deposited on TiO 2 (110); recording of current-voltage curves (I-V). Large particles do not exhibit a plateu near I = V = 0, smaller clusters do show the behaviour expected for a system with a gap

28 CHEMISTRY ON SIZE-SELECTED CLUSTERS 0.28 % ML Pd 30 Acetylene trimerization to form benzene on MgO supported Pd clusters C 6 H 6 + Ion Signal [Arb. Units] Pd 20 Pd 13 Pd 8 Pd 7 Pd 6 Pd 5 Pd 3 3 C 2 H 2 C 6 H 6 up to Pd 6 benzene produced at T 300 K from Pd 7 to Pd 30 additional peak observed at 430 K Pd 2 Pd 1 MgO temperature programmed reaction (TPR) for clusters from Pd 1 to Pd 30 ; peak in TPR corresponds to benzene formation different mechanisms for smalland medium-siize clusters Abbet, Sanchez, Heiz, Schneider, Ferrari, Pacchioni, Rösch, J. Am. Chem. Soc. 122, 3453 (2000) one Pd atom is enough to catalyze the reaction!

29 ROLE OF SUBSTRATE IN CATALYTIC REACTIONS Pd atoms deposited on MgO films, then exposed to acetylene; benzene forms and desorbs at 300 K. Reaction: 3 C 2 H 2 C 6 H 6 Pd acts as catalyst (MgO inactive) Gas-phase Pd atom is inactive because there is not enough electron density Pd is active when charged Pd δ (DFT B3LYP calculations) Pd ATOMS ADSORBED ON F CENTERS OF MgO Reaction path: only on F centers activation energy of 1 ev, compatible with measured desorption barrier (T des 300 K, E act = 1 ev) 0 CH # (C 2H 2)(C2H 2) 0.87 CH (C 4H 4)(C 2H 2) # (C4H 4)(C2H 2) 0.86 F + 5c All other MgO sites (terraces, lowcoordinated ions, etc.) not active because Pd is neutral Energies in ev Abbet, Sanchez,Heiz, Schneider, Ferrari,Pacchioni, Rösch, J. Am. Chem. Soc (2000) Ferrari, Giordano, Pacchioni, Abbet, Heiz, J. Phys. Chem. B, 106, 3173 (2002) Charging is essential to turn inactive Pd atoms into active catalysts

30 DEFECTS AND SURFACE IRREGULARITIES role of surface defects: they act as nucleation centers where cluster grow begins they can alter the properties of nano-size metal clusters deposited on the surface Pd Pd defect MgO MgO regular MgO surface inactive defective MgO surface promotes catalytic reactions

31 STABLE (AND VERY ACTIVE!) NANOCATALYSTS Pt 8-10 clusters on alumina - Mass selected, soft-landing deposition - Thermally stable up to 500 C times more active than conventional catalysts in alkane dehydrogenation Vajda et al. Nature Materials, 2009

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