Size-selected Metal Cluster Deposition on Oxide Surfaces: Impact Dynamics and Supported Cluster Chemistry

Transcription

1 Size-selected Metal Cluster Deposition on Oxide Surfaces: Impact Dynamics and Supported Cluster Chemistry Sungsik Lee, Masato Aizawa, Chaoyang Fan, Tianpin Wu, and Scott L. Anderson Support: AFOSR, DOE

2 CO oxidation on Aun on TiO2

3 Cluster Size Effects in Catalysis CO oxidation on bulk catalyst Bamwenda,..., Haruta. Catal. Lett. 44, 83, (1997) (as plotted by Goodman et al. Problems: TEM characterization 70 atoms Broad size distribution Other changes together with size

4 Cluster Size Effects in Catalysis CO oxidation on planar model catalyst Our size range Planar model support / Cluster grown by annealing Au/TiO2 Sized by STM - Considerable size distribution, especially in the small size range. Size proportional to metal dose Valden, Lai, Goodman, Science 281 (1998) 1647

5 Metal dose effect on TPD Ir 10 deposited at 1eV/atom Dose shift is in opposite direction to cluster size shift 0.05ML 0.1ML 0.2ML Temp.(K)

6 What we want to do: Independently vary cluster size, concentration, support structure and defects, morphology of deposit Characterize all of the above. Supported catalysts Size selected Metal clusters Embedded catalysts Metal oxide support Complete picture of electronic, geometry, and support effects in supported catalysts.

7 Utah Cluster Deposition Instrument

8 Instrument Design Pressure in main chamber during deposition 2 X 10-9 Torr Electron Multiplier Deposition Stage Focusing Lens Quadrupole Ion Guide 3 TPD/TPR EELS Chamber Load Lock/Anneal 9 stages differential pumping Mass range ~4000 d Lens Deflectors 0.1ML of M n (n=1-30) deposited within 30min Isolation Valve Nd YAG Laser 532nm/100Hz R R R Cross Sputter/Anneal QMS with Entrance and Exit Lens Quadrupole Ion Guide 2 Quadrupole Ion Guide 1 Laser Ablation Cluster Source Side View - Main Chamber XPS/ISS Auger Deposition Stage Similar source to Heiz STM

9 XPS : Ti Ti 4+ Ti 3+ O vacancy Clean TiO 2 With Au

10 O 2 interaction with TiO 2 vacancies annealed O 2 sputtered annealed O 2

11 O 2 at vacancies (Gyeong Hwang et al.) O 2 at vacancy Dissociation neighbor 2 nd neighbor O 2 at Ti centers

12 XPS : Au 5f ~0.3eV shift to higher B.E. Initial state chemical shift Final State Shift Au 4 Au 3 Au 2 Au Constant signal = constant sticking coefficient ~1.0 No shifts with cluster size not seen in any system in this size range

13 Morphology? Morphology: ISS 4 He + 1keV 45º incident angle Ion Beam Important: Besenbacher and co. see sintering at room temperature Small Clusters we should get ~20 atom clusters? Ion Beam Wahlstrom, E.; Lopez, N.; Schaub, R.; Thostrup, Large P.; Ronnau, A.; Africh, C.; Laegsgaard, E.; Norskov, J. K.; Clusters Besenbacher, F., Phys. Rev. Lett. 2003, 90, /1-01/4. Metal/Substrate Ratio Small Ion Beam Detection Direction Substrate signal Blocked/Shadowed in area around ad-atoms Metal/Substrate Ratio Large Only detect atoms in topmost layer SMSI or Embedded Clusters Metal/Substrate Ratio Near Zero Embedded SMSI

14 scattering from O ISS : cluster size scattering from Ti scattering from Au Au Au 2 Au 3 Au 4 clean TiO 2 Constant intensity: All Au in topmost layer for small clusters? Are clusters sintering?

15 ISS ratios: Cluster Size Effects Decrease in O/Ti ratio -Ir n preferentially attenuates scattering from O Anomalous behavior for atoms Metal / substrate ratio Ni / substrate Ir / substrate O / Ti for Ir n (O / Ti) as deposited / (O / Ti) cleantio C luster size (atom s) 12 14

16 ISS probing of CO binding sites as deposited sputtering 5 min sputtering 10 min sputtering 15 min Ir 10 at 1eV/atom ~70% of CO removed Ir 10 at 1eV/atom He + Ir ISS Intensity (arb.) He + Ir ISS Intensity (arb.) E/E E/E Aizawa, M.; Lee, S.; Anderson, S. L., Surf. Sci. 2003, 542,

17 How to study CO oxidation in UHV? CO sticks to small Au O 2 adsorbs on catalyst X CO 2 desorbs at lower temperature than CO doesn t poison catalyst All previous UHV studies have used atomic O

18 ISS : 16 O, 18 O exchange 1. Au 3 on Ti 16 O 2 3. CO L 18 O 2 ~20% 18 O bound at O vacancies ~9% of surface O is 18 O Some O bound at Au cluster sites attenuation too low to be atop

19 CO dose effect on ISS: Au 1 /TiO 2 Au 1 /TiO 2 CO ~5 L CO binds at vacancies why nothing here? CO on Au

20 CO dose effect on ISS: Au 2 /TiO 2 Au 2 /TiO 2 CO CO on Au

21 CO dose effect on ISS: Au 3 /TiO 2 First sign of chemical differences CO at vacancies Au 3 /TiO 2 CO Why No CO on Au?

22 Chemical Probing - TPD TPD: Only probes what desorbs (CO on Ir 2 /TiO 2 ) heating rate : 3K/sec dose temp. : 300K ~0.1 L C 16 O plus 0.5 L C 18 O 5L( L(30) 5L(28) 0.5L( L(30) 0.5L(28) 0.1L 28 ~0.1 L C 16 O exposure Red = C 16 O Blue = C 18 O Black = all CO Temp.(K)

23 Bad News Ir 2 deposited at 1eV/atom after CO TPD scans Ir 4f XPS O 1keV He + ISS Ti Intensity (arb.) Intensity (arb.) Ir Binding energy (ev) E/E

24 Chemical Probing II Single Crystal Small Clusters TPD: Only probe what desorbs Destructive 1 shot expt Pulsed dosing Only probe what desorbs Less destructive if low T?

25 CO oxidation reaction I : size 600L pre-dose of 18 O L CO pulses Monitor C 16 O 18 O Au 4 Au 3 Au 2 Au TiO 2

26 CO oxidation size effect Au 3 Au 4 TiO 2 Au Au 2

27 CO oxidation size effect Substantial activity for much smaller clusters than in bulk or model catalyst studies Au 7 dose ~10 times lower than others

28 CO oxidation reaction : Au 3 decay half width ~ 8 L CO Decay: O depletion? Clusters changing?

29 CO pulse reaction II : atom annealing Still much smaller than Au 3 700K 600K 500K 400K Growing larger clusters? Unblocking defects? Au on TiO 2

30 CO pulse reaction III : pulse shape ~300msec CO pulse Possible origins of slow CO 2 evolution slow CO Au 2 thermal desorption rate diffusion of active O to reactive site mass lifetime of CO on small Au n Au 3 actual CO oxidation kinetics 100 Au 2 50 Au time(msec.) clean TiO 2 TiO2 Au1 Au2 Au3 Au4 CO pulse

31 What happens with Au n+ are deposited Expts of Wahlström et al. Should have Au n, n ~ 20 for all samples CO oxidation & binding strongly dependent on deposited size. WHY? Catalysts are stable on an few hour timescale for many cycles of O 2 / CO Samples do not reach an equilibrium state.

32 Needed 13 CO + 18 O 2 to allow measurement of both C 16 O 2 and C 16 O 18 O Only seeing the tip of the iceberg? Low temperature deposition and analysis to reduce/change sintering Probe steady state species on surface not just species desorbing

33 New Endstation Continuous T control IRAS capability Retain load/lock Greatly simplified manipulations tip o the hat - Wayne Goodman

34 Utah Cluster Deposition Instrument

35 Ti XPS Ti 4+ Mg Kα ev clean TiO2 with 0.1ML Ir 2, 1eV/atom Ti 3+ Ti 0 Intensity (arb.) Freshly Annealed ~5% missing O defects 1 minute 1 kev He + ~18% missing O defects Binding energy (ev)

36 ISS cluster size effect O Ir 15 Ti Ir n deposited at 1eV/atom Ir Note much lower intensity compared to gold Intensity (arb.) Ir 10 Ir 5 Ir 2 Ir E/E fig. 4

37 Ir ISS v.s. Sputter Time Slow sputtering for large n --Multilayer clusters --Stronger Ir-Ir binding

38 Effects of TPD on the sample 5 L dose of C 18 O at room temperature Solid = 1st TPD run Open = 2nd TPD run CO 18+ Intensity (arb.) 0.1ML Ir n at 1eV/atom Ir 15 Ir 10 Ir 5 Ir 2 clean TiO 2 518K Ir Temp. (K) clean TiO 2

39 Cluster Size Effects in Catalysis Example: Gold cluster catalyst, from Haruta, nm gold clusters on TiO 2 (background) Variation in product yield with cluster size. Propene/O 2 /H 2 mixture over Au/TiO 2 catalyst (from Haruta 1997)