The unusual properties supported gold: size, charging, support effects

Transcription

1 The unusual properties supported gold: size, charging, support effects Gianfranco Pacchioni Dipartimento di Scienza dei Materiali Università Milano-Bicocca, Milan (Italy) 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 The strange story of gold chemistry...

3 CATALYTIC METALS

4 OXIDATION OF CO AND OF H2

5 REACTIONS ARE STRUCTURE SENSITIVE OVER Au CATALYSTS

6 Population (%) Activity / molco 2 s -1 site Cluster diameter (nm) Effect of Particle Size and Morphology on Reactivity of Au/TiO 2, Au/Fe 2 O 3 CO + ½ O 2 CO 2

7 NUMBER OF PAPERS ON Au CATALYSTS

8 CATALYST PREPARATION: COPRECITATION

9 Au/Fe 2 O 3 PREPARED BY COPRECITATION

10 SUBNANOMETER GOLD IN CATALYSIS 1% weight 99% Science, 321, 1331 (2008) High resolution TEM (with aberration correction): Au atoms & Au nanoclusters ( nm) are present on active Au/Fe 2 O 3 catalyst Activity connected to the presence of 0.5 nm 2-layer Au clusters (<10 atoms) Active Au catalysts <1% of the total gold present on the surface. Not detectable with normal spectroscopies!

11 Au clusters (Haruta s catalyst): role of support we have been lead to believe that for the genesis of high activity of Au catalysts, the contact structure between Au particles and support is more important than the mean diameter of Au particles [Haruta et al. J. Mol. Catal. A 199 (2003) 73] Charge state of supported clusters? Positive, negative or neutral?

12 IR spectra of CO on supported metal clusters CO adsorbed on Au nanoparticles supported on titania CO IR spectra as a function of sample thermal treatment Boccuzzi, Chiorino, Manzoli, Surf. Sci (2000) 942 Metallic Au 0 particles Oxidized Au δ+ particles Reduced Au δ particles

13 Reactivity of gas-phase anionic gold clusters Stolcic et al. JACS 125, 2848 (2003); Wallace and Whetten JACS 124, 7499 (2002); Hagen et al. PCCP 4, 1707 (2002); Socaciu et al. JACS 125, (2003).

14 Is charging of gold clusters important in nanocatalysis? Can we design procedures to prepare supported gold cluster anions? GENERATE DEFECT RICH MgO THIN FILMS (1) Prepare a perfect, defect free MgO film, then bombard the film by electrons or Ar + ions (2) Prepare a slightly under-stoichiometric film controlling the Mg and O 2 fluxes

15 Ni clusters grown on regular and defect sites of MgO Vibrational properties of CO adsorbed on free and MgO supported Ni clusters 3-hollow CO Ni 8 (CO) [Ni 8 (CO) 4 ] 1690 MgO(F 5c )/Ni 8 (CO) MgO(F 5c+ )/Ni 8 (CO) 4 ω e (CO), cm-1 From Ni 8 (CO) 4 to [Ni 8 (CO) 4 ] : red shift of 59 cm -1 same shift observed for Ni 8 (CO) 4 adsorbed on neutral F center net charge transfer from the surface to the Ni cluster shift of 17 cm -1 computed for Ni 8 (CO) 4 adsorbed on charged F + center Del Vitto, Giordano, Pacchioni, Rösch, Surf. Sci. 575, 103 (2005)

16 Au CLUSTERS ON DEFECTIVE AND NON-DEFECTIVE MgO FILMS deposition Knudsen cell UHV re-evaporation nucleation diffusion OXIDE THIN FILM METAL SUBSTRATE Grow MgO thin films on Ag(100); generate F centers by electron bombardment, then deposit Au clusters; measure vibrational properties of CO on Au clusters A: STM of 7L MgO (clean surface) B: STM of Au on 7L MgO Au clusters form on defects created on lowcoordinated sites (steps, edges) Beinin, Lin, Gao, Nilius, Freund, J. Phys. Chem. C 111, (2007)

17 Properties of CO/Au n /MgO (regular & defective surface) CO dosed on Au clusters formed on defectfree and defect-rich MgO films (F centers) (a) IR of defective MgO surface (b) IR of defect-free MgO (c) IR of highly defective surface Defect free MgO: ω e = 2120 cm -1 : neutral Au n clusters on terrace sites Defect rich MgO: ω e 2070 cm -1 : Au n clusters on F centers CO frequency red-shift: 50 cm -1 Sterrer, Yulikov, Fischbach, Heyde, Rust, GP, Risse, Freund, Angew. Chem., 45, 2630 (2006)

18 Experimental IR spectra of free Au n -CO clusters Free Electron Laser for Infrared experiments (FELIX) gas-phase clusters Very accurate measurements of CO frequency as a function of cluster nuclearity and net charge Sterrer CO/Au n /MgO Sterrer CO/Au n /F MgO Au 8 CO Fielicke, Mejer & al. J. Phys. Chem. B, 109, (2005) Charge transfer ( 0.6 electrons) occurs from F centers to Au clusters

19 FABRICATION OF NANO- CLUSTERS Epitaxial MgO thin films on Mo(100) (Mg vaporizing in O 2 background) Quadrupole II Nd:Yag Laser (Infinity ) 532nm / 100Hz Quadrupole I Analysis Chamber Mass Spectrom. (TDS) Gas-phase metal clusters generated by laser vaporization Quadrupole Deflector FTIR MgO film Clusters cations are massselected and soft-landed (low kinetic energy <0.2 ev/atom) Sample Transport for STM Cluster Source UPS/XPS AES Low cluster concentration (<0.1 mono-layer) and low substrate temperature (90 K) to prevent aggregation 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

20 CO OXIDATION ON Au CLUSTERS ON MgO CO + ½ O 2 CO 2 Reaction studied on Au clusters on MgO thin films Au 1 to Au 7 inactive; Au 8 promotes CO oxidation! Cluster-size effect. defect-rich MgO defect-poor MgO Different reactivity observed for differently prepared MgO substrate (defect rich and defect poor) Sanchez et al. J. Phys. Chem. A 103 (1999) 9573 Reactivity is sensitive to both cluster size and adsorption site! Identification of oxide sites where nucleation and growth occur essential

21 Is vibrational frequency of adsorbed CO a probe of the charge state of atoms or clusters? THE SIMPLEST CASE: Au ADATOM ON MgO(100) Au O O O Mg Mg Mg DFT plane wave calculations preferred adsorption site O anions binding energy 1 ev spin largely on Au no evidence of charge transfer Del Vitto, GP, Delbeq, Sautet, J. Phys. Chem. B 109, 8040 (2005)

22 ELECTRON PARAMAGNETIC RESONANCE OF Au 1 /MgO Experiments (MgO films, UHV, 30 K): Au atoms deposited on MgO terraces; Theory (DFT Relativistic Spin- Orbit calculations, ADF-ZORA) 495 G Spin density on Au = 0.8 a iso (G) Experiment Theory Au Au 1 /MgO hfcc reduced by Pauli repulsion with substrate (expansion of Au 6s electron) No charge transfer despite large change in hyperfine coupling constants Yulikov, Sterrer, Heyde, Rust, Risse, Freund, GP, Scagnelli, PRL 96, (2006) Au atom on MgO(100) is neutral!

23 Vibrational frequency of CO adsorbed on supported metal atoms: proof of charge transfer? An instructive case: CO/Au 1 /MgO Experiments (30 K): MgO/Ag(100) thin films Au atoms formed at terraces (from STM and EPR spectra) CO adsorption at 30 K IR spectrum shows a band due to CO interacting with Au at 2120 cm -1 Shift with respect to free CO (2143 cm -1 ): 20 cm -1 Risse, Sterrer, Freund et al. Shift in line with literature values for Au n on oxides Compute vibrations of CO/Au 1 /MgO: a simple problem for theory

24 Vibrational frequency of CO/Au 1 /MgO Periodic calculations (VASP, PW91) Free CO ω e : 2129 cm -1 MgO/Au 1 /CO: ω e (C-O) = 1848 cm -1 Vibrational shift 281 cm -1, 15 times the experimental one!!!! Typical of CO on three-hollow sites on metals!!! MgO/Au 1 /CO: bent complex; D e (CO)= 0.96 ev Such a low frequency never been observed for CO/Au/oxide complexes Something is wrong with the calculation DFT overestimates back-donation? Need of hybrid functionals? Failure of pseudo-potentials for Au? Fully relativistic calculations including spin-orbit coupling required? Supercell problem? Need to go to embedded clusters? Other methods and models have to be considered

25 Properties of MgO/AuCO (surface) MgO/Au-CO method Supercell PW91 PP α D e (ev) r(c-o) ω e ω e Cluster PW91 PP Cluster B3LYP PP Cluster CCSD(T) PP Huge shifts in CO ω e confirmed with all methods Nearly one electron transferred to CO Predicted frequency 1850 cm -1, measured frequency 2120 cm -1 : too large discrepancy to be due to approximations in the calculations Conclusion: what is computed is not what is measured C O Au Giordano, Carrasco, Di Valentin, Illas, GP, J. Chem. Phys., 124, (2006) What is the nature of the IR band at 2120 cm -1?

26 What about Au dimers? Matrix isolated Au 2 CO: ω e = 2132 cm -1 ( ω to free CO -11 cm -1 ) Matrix isolated AuCO: ω e = 2039 cm -1 ( ω to free CO -104 cm -1 ) L. Jiang and Q. Xu, J. Phys. Chem. A, 109, 1026 (2005) DFT(PW91) Au 2 CO: ω e = 2092 cm -1 ( ω to free CO -37 cm -1 ) DFT(PW91) AuCO: ω e = 1979 cm -1 ( ω to free CO -150 cm -1 ) Au 2 CO on MgO assumes the same structure as in gasphase. Computed frequencies (DFT): DFT ω e =2065 cm -1 ( ω to free Au 2 CO -27 cm -1 ) Exp. ω e =2120 cm -1 ( ω to free Au 2 CO -12 cm -1 ) Conclusion: band at 2120 cm -1 due to Au 2 CO (or larger clusters) not to AuCO

27 The low frequency band exists! IR spectrum of MgO/Au 1 CO: second band at ω e = 1852 cm -1 ω to free CO -290 cm -1 Theoretical predictions MgO/Au 1 CO ω e = 1868±20 cm -1 ω to free CO -289±10 cm -1 Origin of such a large shift? Au on MgO is neutral!!! CO acts as Lewis acid and promotes charge transfer from O 2- to Au and from Au to CO. Charge transfer occurs ONLY AFTER CO adsorption! CO vibrational frequency measures a FINAL STATE effect, not an INITIAL STATE interaction Sterrer, Yulikov, Risse, Freund, Carrasco, Illas, Di Valentin, Giordano, GP, Angew. Chem. Int. Ed., 45, 2633 (2006) Band at 2122 cm -1 supported Au n CO Final assignments Band at 1852 cm -1 supported AuCO

28 Au CLUSTERS ON OXIDE SURFACES ROLE OF SUBSTRATE AND OF CHARGING Au nanoparticles: active catalyst but reactivity depends on oxide support. Probelm of charging: positive? Negative? Probably depends on operating temperature and reaction conditions MgO surface: charging occurs through defects (oxygen vacancies, electron traps, etc.) where atoms or clusters are stabilized (stronger bonding) Charging modifies physical properties and chemical reactivity Charging can be proven experimentally by using CO as a probe molecule BUT GREAT CARE IS NEEDED (and theory )

29 END OF LECTURE 4