mixed oxide layers on TiO 2 (110) using a

Transcription

1 Preparation of ordered Mo+Ti and V+Ti mixed oxide layers on TiO 2 (110) using a W+Ti oxide diffusion blocking layer E. Primorac, O. Karslioglu, l M. Naschitzki, H. Kuhlenbeck, H.-J. Freund. D. Löffler, J. Uhlrich helped at BESSY Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, Berlin, Germany

2 Supported catalyst with part of the active component in the support. Mixed oxide with phase separation. Chemical activity of the mixed phase? What phases are to be expected? How are the atoms of the active component embedded into the substrate lattice? How does the matrix modify the chemical properties of the embedded atoms? Oxidation states? t Equilibrium between surface and bulk component? Influence of gases (oxygen!)? How to prepare? p

3 I. E. Wachs, Catalysis Today 100 (2005) 79 94

4 Preparation strategies Deposition of metal onto support followed by annealing. Direct preparation of a mixed oxide. Interested in the properties of the mixed phase direct preparation. Better control of composition. Co-deposition of two metals in an oxygen atmosphere. Concentration of the mixed-in metal: not too high. Systems: TiO 2 mixed with Mo and V.

5 Rutile unit cell Rutile(110) a a c Lattice parameter for different rutile-type oxides oxide a c CrO MoO RuO MoO 2 and dvo 2 both exhibit rutile structure: t good SnO mixing with TiO 2. Other oxidation states have a TiO tendency for phase separation. VO WO

6 Pt(110) Au(111) Au(111) TiO 2 preparation investigated on several Au and Pt surfaces. Thin layers: strange structures; t thicker layers: dewetting, faceting, one-dimensional i disorder. d Probable bl problem: lattice mismatch Pt(211) Pt(211) Pt(211)

7 Preparation strategy t -2- To improve lattice match: use TiO 2 (110) substrate High-quality TiO 2 (110) layers Stable layers of Mo in TiO 2 (110). Problem: vanadium diffuses into the bulk. Prepare a diffusion blocking layer: (Ti+W)O X Good quality of TiO 2 (110) on the blocking layer TiO 2 (110) substrate TiO 2 (110) I WO X I TiO(110) 100 ev 100 ev

8 Short summary - the systems are: Mo X Ti Y O Z layer V X Ti Y O Z layer W X Ti Y O Z blocking layer TiO 2 (110) substrate t TiO 2 (110) substrate t

9 Mo X Ti Y O X Mo3d, Mo 4+ Mo 0.1 Ti 0.9 O X Mo 5+ (?) Oxygen treatment produces hν=330 ev Mo 6+ Mo 6+ at the surface. In the bulk: Mo annealed in O2 Part of the Mo 6+ and dmo 5+ possibly also in the bulk. annealed in vacuum 80 After oxidation in the high- 0 pressure cell 80 0 as prepared Binding Energy (ev) Binding Energy [ev]

10 Vacuum annealed TiO (110) 2 Vacuum annealed Mo+Ti mixed oxide film on TiO Vacuum annealed Mo+Ti mixed oxide d=2.8 1 Å 40nm 60nm Mo/(Mo+Ti)= 42% 10nm d=6.2 2 Å 1.5nm [001] [-110] 1.5nm 2.8 Å 6.2 Å [001] [-110] Protrusions on the surface are probably due to MoO 3.

11 50 ev 50 ev Mo/(Mo+Ti) = 2% Mo/(Mo+Ti ) = 38% LEED pattern resembles TiO 2 (110) pattern.

12 Short summary - Mo X Ti Y O X Mo 6+ TiO 2 + Mo 4+, Mo 5+, Mo 6+ TiO 2 + Mo 4+, Mo 5+, Mo 6+ TiO 2 (110) substrate Annealing in vacuum TiO 2 (110) substrate

13 Short summary - Mo X Ti Y O X Mo 6+ TiO 2 + Mo 4+, Mo 5+, Mo 6+ TiO 2 + Mo 4+, Mo 5+, Mo 6+ TiO 2 (110) substrate Annealing in TiO 2 (110) substrate 3e-7 mbar O 2

14 Short summary - Mo X Ti Y O X Mo 6+ Mo 6+ TiO 2 + Mo 4+, Mo 5+, Mo 6+ TiO 2 (110) TiO 2 (110) substrate Annealing in high-pressure cell TiO 2 (110) substrate

15 TiO 2 Methanol adsorption (XPS) MeOH + MeO MeO OH MeOH H 2 O Probably methoxy formation MeOH ads. at 100 K Analyzer at C -70 C -35 C 0 C C 1s O1s Contamination??? Binding Energy [ev] Binding Energy [ev]

16 QMS Signal (a.u.) CH3O CHO Pure TiO2 vacuum annealed 6.5% Mo after e irradiation Methanol adsorption (TDS) Damping of the ~270K peak with increased Mo content. Large part of the signal due to TiO 2 (110). Electron irradiated surface gives a formaldehyde peak. irradiation Electron irradiated surface gives a CH3O CHO 12% Mo, without e irradiation Henderson et al., Surf. Sci., 1998, 412/413, pp CH3O CHO 24% Mo, oxidized Temperature (K) 16

17 Thermal stability of the V+Ti mixed oxide layer V X Ti Y O Z TiO 2 (110) WO X TiO 2 (110) 800 K 900 K 13 % V 850 K 11 % V sputtering Inten nsity 900 K AES Ti 6 % V V Intens sity AES 850 K V Kinetic energy [ev] Kinetic energy [ev]

18 Thermal stability of the V+Ti mixed oxide layer V X Ti Y O Z TiO 2 (110) WO X TiO 2 (110) 800 K 13 % V 850 K 11 % V Inte ensity 900 K 6 % V V AES Ti Kinetic energy [ev]

19 V X Ti Y O X on the blocking layer V 0.25 Ti 0.75 O X (110) V2p 80 hν=630 ev 514.3eV ev After annealing 0 in 3x10-7 mbar O 2 at 800 K (15 min) 517 ev V X Ti Y O Z on blocking layer V 5+ : ev V 4+ : ev V 3+ : ev V 0 : ev Silversmit et al, J. Electr. Spectrosc. Relat. Phenom. 135 (2004) ev 80 After annealing in vacuum at 800 K (15 min) 515 ev Binding energy [ev] Oxygen produces V 5+ at the surface. 0 1x2 LEED pattern 100 ev V 3+ and V 4+ below.

20 Does the blocking layer block defect diffusion? Valence band, (5 Å W + 28 Å Ti) oxide on TiO 2 (110) hν=180 ev θ=0 Annealing in vacuum, 800 K, 10 min Yes, somewhat W interlayer (5 Å) + 28 Å TiO 2 (110) layer reduced surface Binding energy [ev]

21 Thermal stability of the W+Ti oxide layer WO X TiO 2 (110) TiO 2 (110) WO X TiO 2 (110) annealing in 1x10-6 mbar O annealing in 1x10-6 mbar O K 800 K K K ty Intensi K charging? AES W Ti Kinetic energy [ev] O Intensit ty K No W AES Kinetic energy [ev]

22 Tungsten oxide on TiO 2 (110) WO X ITiO(110) 2 reciprocal space real space 100 ev C(4x1) structure, 6.8 Å lattice constant LEED simulation with LEEDpat 2.1, K. Hermann and M. A. van Hove

23 Thermal stability of the W+Ti oxide layer -2- W4f (5 Å W + 28 Å Ti) oxide on TiO 2 (110) hν=950 ev W 6+ ~355eV 35.5 W 5+ ~ 34.5 ev W 4+ ~ 32.7 ev W 0 ~ 31.2 ev (Bigey et al, J. Phys IV, France 8 (1998) PR annealing in 3x10-7 mbar O 2 at 800 K Annealing at 800 K in oxygen leads to the pile up of W 6+ at the surface [ est.: some percent t] Mainly W 4+ in the bulk. 0 Try another material as blocking layer 80 annealing in vacuum at 800 K Binding energy [ev]

24 Summary Preparation of TiO 2 (110) layers on Au and Pt substrates was not successful. TiO 2 (110) and mixed oxide layers [ V, Mo in TiO 2 (110) ] can be prepared and stabilized on TiO 2 (110). Tungsten oxide diffusion blocking layer can hinder V and defect diffusion. Blocking layer not fully stable. Tendency for Mo, V, and W mixed with TiO 2 (110): oxygen treatment pulls out higher oxidation states [ phase separation ]. More stable when annealed in vacuum. Current effort Check whether lead [ larger ionic diameter ] can be used for the blocking layer. Methanol adsorption. Future