Examples of the use of XPS in Catalysis Research from the Vohs Group

Transcription

1 Examples of the use of XPS in Catalysis Research from the Vohs Group Supported monolayer vanadia catalysts Ceria/zirconia automotive emissions control catalysts Reaction of oxygenates on Zn Pd/Zn methanol steam reforming catalysts

2 Introduction V V V Supported monolayer vanadia catalysts are active for: CH 3 H + 2 CH 2 + H 2 CH H 2 CH 3 N + NH 3 N + 2 H 2 Reactivity depends on: - Identity of the support (Ti 2, Al 2 3, Zr 2, Mg, Ce 2, etc.) - Vanadia coverage Structure-activity relationships not well understood

3 V CH 3 H V H C H H H 2 Methanol oxidation activity for vanadia on various oxide supports Catalyst (monolayer vanadia coverage) TF for CH 3 H xidation to H 2 C (s -1 ) H 2 C + H 2 Apparent E a (kj/mol) 25 % V 2 5 /Al % V 2 5 /Nb % V 2 5 /Ti % V 2 5 /Zr % V 2 5 /Ce From: Wachs et al., in Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis Elsevier Science B.V., Amsterdam, 1997.

4 CH 3 H + 2 CH 2 + H 2 - Mechanism Hydride Transfer CH 3 H + V* + * V*-CH 3 + *-H V*-CH 3 CH 2 + V*-H V*-H + *-H V* + H * (1) (2 -rds) (3) (4) Proton/hydrogen Transfer CH 3 H + V* + * V*-CH 3 + *-H * + V*-CH 3 CH 2 + V* + *-H 2 *-H * + H * TF = k app P CH 3 H P 2 1/4 P H2 1/2 k app = (K 1 K 3 1/2 K 4 1/4 ) k rds E app ~ E rds + ΔH 1 + ½ΔH 3 + ¼ΔH 4

5 UHV sample manipulator Thermocouple wires Single crystal substrate Resistively heated vanadium source Model Systems: Sub-monolayer, monolayer, and multilayer vanadia films on single crystal metal oxide supports: Ti 2 (110), Ce 2 (111), Zr 2 (100), Mg(100) ~10-10 torr Film thickness monitor TPD AES XPS LEED HREELS Temperature Programmed Desorption Auger Electron Spectroscopy X-ray Photoelectron Spectroscopy Low Energy Electron Diffraction High Resolution Electron Energy Loss Spectroscopy

6 Vanadium Deposition on Ce 2 (111) XPS Ce 3d/ V 2p 3/2 intensity 1s ML of vanadium deposited Layer-by-layer film growth V 2p 1/2 V 2p 3/2 Ce 2 (111) ML of V V +3 V 2p 3/2 = ev Binding energy, ev 500 Atom-resolved NC-AFM image Namai et al. Catalysis Today 85 (2003) 79.

7 Metal xide ΔG f o kcal/mole of metal cation V V Ce Ce Ti Ti Ce V V Ce ΔG o kcal/mole 10 Ce V V Ce Ti V V Ti Ti V V Ti V Ce 2 V Ce V Ti 2 V Ti

8 0.5 ML V x - XPS 1s V x /Ce 2 (111) V x /Ti 2 (110) 1s 10-3 torr 2 V 2p 3/2 V 2p 3/2 V ev 10-7 torr 2 V ev Just deposited V ev V 2p 1/2 V 2p 3/ Binding energy, ev V ev V 2p 1/2 V 2p 3/2 d 10-3 torr K c b 10-7 torr K As deposited a Clean substrate Binding energy, ev Binding energy, ev 500

9 XPS 0.5 ML Vanadia/Ce 2 (111) Ce(3d) Ce(3d) 10-3 torr 2, 400K u' u o v' v o 10-5 torr 2, 400K u''' u'' u' u v''' v'' v' v 0.5 ML V as deposited Ce 2 (111) Peaks Initial State Final State Binding energy, ev Ce 3+ v o,u o 3d 10 4f 1 3d 9 4f 2 V n-1 Ce 3+ v',u' 3d 10 4f 1 3d 9 4f 0 V n Ce 4+ Ce 3+ Ce 4+ v,u 3d 10 4f 0 3d 9 4f 2 V n-2 Ce 4+ v'',u'' 3d 10 4f 0 3d 9 4f 1 V n-1 Ce 4+ v''',u''' 3d 10 4f 0 3d 9 4f 0 V n

10 Thermal Stability of V 2 5 /Ce 2 (111) 0.5 ML V x /Ce 2 (111) 1s V 2p 3/2 750 K 650 K 550 K 450 K 300 K Reduction is evident at T 650 K Binding energy, ev 500

11 Summary of Film Growth Characterization Layer-by-Layer vanadia film growth on Ce 2 (111), Ti 2 (110), and Zr 2 (110). The oxidation state of the vanadium cations in monolayer films can be varied from +3 to +5 Unable to grow multilayer films of V 2 5 on Ce 2 (111) and Ti 2 (110) in UHV

12 Temperature Programmed Desorption (TPD) E a and A for for desorption and surface reaction Surface-adsorbate bonding Structure of adsorbed intermediates Reaction pathways and mechanisms Procedure Dose reactant at low temperature Heat sample with a linear temperature ramp Monitor desorbing species with mass spec Temperature (K)

13 CH 3 H TPD 0.5 ML V 2 5 /Ce 2 (111) 1st run after oxidation 2 H 2 C E a,1 ~ 140 kj/mole E a,2 ~ 155 kj/mole 2nd run CH 3 H 2 E a for C-H bond cleavage from adsorbed methoxides is much greater than E app for high surface area catalysts. H 2 C CH 3 H Temperature, K

14 Kinetic Isotope Effect CH 3 H/CD 3 D TPD 0.5 ML V 2 5 /Ce 2 (111) H 2 C C-H bond cleavage is rate limiting step on both V +5 and V +3 D 2 C V H C H H H 0.5 ML V 2 3 /Ce 2 (111) H 2 C D 2 C H 2 C + H Temperature, K

15 0.5 ML V x /Ce 2 (111) - CH 3 H TPD CH 3 H TPD XPS V 2 5 /Ce 2 (111) 1s Desorption Rate V 2 3 /Ce 2 (111) CH 2 V 2p 1/2 V 2p 3/ torr K 10-7 torr K CH Temperature, K Binding energy, ev 505

16 V x /Ti 2 (110) V x /Ce 2 (111) CH K 585 K 2 Exposure CH K 560 K 2 Exposure Torr, 30 sec 606 K Torr, 3600 sec Torr, 600 sec Torr, 30 sec 10-6 Torr, 600 sec 10-8 reduced reduced Temperature, K Temperature, K

17 Summary of Activation energies V*-CH 3 CH 2 + V*-H Model Catalysts V 2 3 V 2 5 Ti 2 (110) ~160 kj/mole 120 kj/mole Zr 2 (110) 158 kj/mole 124 kj/mole Ce 2 (111) 155 kj/mole 140 kj/mole High surface area catalysts Catalysts Catalyst (monolayer vanadia coverage) TF for CH 3 H xidation to H 2 C (s -1 ) Apparent E a (kj/mol) 25 % V 2 5 /Al % V 2 5 /Nb % V 2 5 /Ti % V 2 5 /Zr % V 2 5 /Ce

18 V 2 5 /Ce 2 - Powder Samples - CH 3 H TPD CH 2 520K 550K Fresh Sample fully oxidized Slightly reduced 610K Highly reduced Reoxidation treatment 0.1 Torr 2, 300 K 0.1 Torr 2, 525 K 0.1 Torr 2, 750 K 1.0 Torr 2, 750 K Temperature (K) Reduced by annealing at 750 K in vacuum

19 High Surface Area V 2 5 /Ce 2 catalyst (1s) Ce 2 Ce(3d) ev (i) Intensity (a.u.) 6 wt.% Vx/Ce 2 (ii) Intensity (a.u.) ev V(2p) ev (a) (b) ev ev After CH 3 H TPD xidized sample Binding Energy (ev) Binding Energy (ev)

20 Single Crystal Model Systems Summary Reactivity trends are consistent with those of high surface area analogues Monolayer vanadia films are active for the oxidation of methanol to formaldehyde Multilayer films are inactive Activation energy for the dehydrogenation of methoxide to produce formaldehyde is a function of the oxidation state of the vanadium cations Methanol can be used as a chemical probe of the vanadium oxidation state.

21 %Conversion 100 Three-Way Automotive Emissions Control Catalysts Conversion HC C N x Air/Fuel Reactions C + 2 C 2 Hydrocarbons + 2 C 2 + H 2 N x N 2 Catalysts Pt, Pd, Rh - Ce 2, Zr 2, La Stoichiometry Computer Control 2 sensor

22 Dense Alumina Monolith Porous Alumina Washcoat Impregnated with: Ce 2, Zr 2, La 2 3 Rh, Pd, Pt

23 Issues in Automotive Catalysis Tighter emissions standards require better catalysts Cold start emissions Close-coupled converter Thermal stability Sulfur tolerance Durability N x reduction for lean burn engines

24 Role of Ceria and Zirconia in Automotive Emissions Control Catalysts Ce 2 Enhance activity for water gas shift reaction Stabilization of metal dispersion Stabilization of γ-al 2 3 support Enhance metal reactivity Provide oxygen storage capacity Zr 2 Stabilization of Ce2 dispersion Enhance Ce 2 oxygen storage capacity

25 Ceria Zirconia Interfaces Fundamental question: How do interactions at the ceria-zirconia interface influence the redox and catalytic properties of ceria? Experimental Approach: Model systems consisting of metal oxide single crystals and thin films supported on metal oxide single crystals Kinetic measurements Surface-sensitive spectroscopic probes Microscopy Model Systems Studied Ce 2 (100) Ce 2 (111) Ce 2 /Zr 2 (100) Ce 2 /Zr 2 (110) Ce 2 /Zr 2 (111) Ce 2 /Al 2 3 (0001)

26 Model Catalysts xide Single Crystals Ce 2 Al 2 3 Zr 2 Ceria Thin-Films xide-supported Rh particles Film Growth sample Film Growth effusion source Quartz crystal film thickness monitor Film thickness monitor

27 Model Catalysts Polycrystalline Ce 2 Al 2 3 (0001) Epitaxial Ce 2 Film Zr 2 (100)

28 Ce 2 (111) - CH 3 H TPD (a) Ce 2 (111) CH 3 H (x10) Vacuum Annealed (nearly stoichiometric) H 2 C (x10) (b) CH 3 H H 2 C Ar + Sputtered (reduced) Temperature (K)

29 Ce 2 /α-al 2 3 (0001) CH 3 H TPD Ce 2 /α-al 2 3 (0001) CH 3 H Ce 2 /α-al 2 3 (0001) H 2 C Temperature (K) Temperature (K) Results similar to those obtained for Ce 2 (111) Dissociative adsorption of CH 3 H on surface oxygen vacancies Concentration of oxygen vacancies increases with each TPD run Annealing temperature does not affect properties

30 Ce 2 /YSZ(100) CH 3 H TPD Ce 2 /YSZ(100) CH 3 H Ce 2 /YSZ(100) H 2 C Ce 2 /YSZ(100) C Temperature (K) Temperature (K) Temperature (K) High oxygen vacancy concentration in 2 annealed epitaxial ceria film Heating to 900 K causes significant reduction of the ceria film?

31 X-ray Photoelectron Spectroscopy - XPS X-ray photon hν 2P 2s Photoelectron Electron Binding Energy = hν E KE Ψ 1s Ce 3d XPS spectrum of Ce 2 (111) Ce (MNN) Ce (MNV) C Ce 4p Ce 4d Surface composition Morphology and thickness of thin films xidation states Adsorbate structure Binding Energy (ev)

32 XPS: Thermal Stability of Ceria Films Ce 2 / α-al 2 3 (0001) Ce 2 / Zr 2 (100) Ce 2 Zr 2 (100) Ce 2 α-al 2 3 (0001) XPS: Ce 3d XPS: Ce 3d Ce 2 Ce K 750 K 825 K 900 K 1000 K Ce 2 Ce K 600 K 750 K 825 K 900 K Binding Energy (ev) Binding Energy (ev)

33 Ceria/Zirconia Results from Pulsed Neutron Diffraction Studies Mamontov, Egami et al., J. Phys. Chem. B 104 (2000) xygen vacancies - xygen interstitials After annealing at 900 C After annealing at 900 C Ceria sample is nearly stoichiometric Ceria-zirconia sample is oxygen deficient Ce 2 oxygen vacancy-interstitial defect concentration decreases upon heating to 700ºC

34 Enhanced Reducibility of Ce 2 /Zr 2 4Ce Ce Bulk Thermodynamics: T red ~ 1300 K TPD & XPS Results: T red ~ 750 to 900 K Ce to 900 K Ce 2 3 Zr 2 Zr 2 Lattice mismatch at interface maintains small ceria grain size a = 5.4 Å a = 5.1 Å Ceria grain size ~75 Å Bonding at interface prevents Ce 2 phase transformation Zr 2, Ce 2 Ce 2 3 cubic fluorite trigonal Ce 2 3 /Zr 2, cubic fluorite? Interfacial strain alters vacancy and interstitial concentrations in Ce 2

35 Steam Reforming of Methanol CH 3 H + H 2 C H 2 Cu/Zn is used commercially Has high activity and selectivity but - Cu sinters at T>575 K - Pyrophoric once reduced Pd/Zn has been proposed as an alternative catalyst Has high activity and selectivity Structure-activity relationships are poorly understood

36 CH 3 H xidation on Zn(0001) TPD Pathways and Intermediates A - Zn(0001) H 2 Zn Zn(0001) Intensity (Arb. Units) C C 2 H 2 C CH 3 H Temperature (K) 700

37 CH 3 H xidation on Zn(0001) TPD Pathways and Intermediates >150 K CH 3 H(g) CH 3 (ad) + H(ad) > 400 K CH 3 (ad) + (l) HC(ad) + 2 H(ad) K CH 3 (ad) CH 2 (g) + H(ad) K HC(ad) C 2 (g) + H(ad) HC(ad) C (g) + H(ad) 2 H(ad) H 2 (g) + (l)

38 CH 3 H xidation on Zn(0001) TPD Pathways and Intermediates H C 660 K CH K 495 K H CH K Dosed with CH 3 H at 160 K Binding Energy, ev

39 Methanol Steam Reforming Methanol Dehydrogenation Pd/Zn % Conversion % Selectivity Reduction Temperature (K) 10.1 kpa CH 3 H 10.1 kpa H K C 2 Selectivity = C + C2

40 Steam Reforming of Methanol Pd/Zn

41 Intensity (Arb. Units) 1.5 ML 0.75 ML 0.35 ML ML 2 ML 1 ML 0.5 ML Pd(3d 3/2 ) 342 XPS Pd(3d5/2) Binding Energy (ev) 330 Pd Film Growth on Zn(0001) Zn(2p)/Pd(3d) Peak Area Ratio Zn(2p)/Pd(3d) Peak Area Ratio Pd Coverage (ML) Annealing Temperature (K) Zn(0001) 3 ML Pd/Zn(0001) 300 K 3 ML Pd/Zn(0001) 850 K LEED

42 Growth of Pd Films on Zn(0001) - Summary Pd XPS, LEED, HREELS indicate a 2D island growth mode Zn Critical island size ~0.5 ML Heat

43 Zn(0001), Pd/Zn(0001) CH 3 H TPD A - Zn(0001) B ML Pd/Zn(0001) H 2 H 2 Intensity (Arb. Units) C C 2 H 2 C Intensity (Arb. Units) C C 2 H 2 C CH 3 H CH 3 H Temperature (K) Temperature (K) 700

44 0.75 ML Pd/Zn(0001) CH 3 H TPD A - C B - C 2 Annealed 850 K Intensity (Arb. Units) Annealed 850 K Annealed 825 K Annealed 800 K Intensity (Arb. Units) Annealed 825 K Annealed 800 K 800 K 800 K 750 K as deposited as deposited Temperature (K) Temperature (K)

45 Pd/Zn high surface area catalyst Pd/Zn(0001) model catalyst Pd(3d 3/2 ) Pd(3d 5/2 ) PdZn PdZn 900 K Intensity (Arb. Units) 800 K 700 K 500 K 300 K Binding Energy (ev) 330 XPS spectra of 10 wt.% Pd/Zn reduced in H 2 at various temperatures XPS spectra of 10 wt.% Pd/Zn(0001) as a function of annealing temperature in UHV

46 Mechanism of the reaction of CH 3 H on Pd K CH 3 (ad) CH x (ad) + H(ad) CH x (ad) C(ad) + H(ad) H(ad) + H(ad) H 2 (g) K C(ad) C(g) Proposed Mechanism for the production of C 2 of CH 3 H on Pd/Zn(0001) 300 K CH 3 H + Pd C-Pd + 2 H 2 (g) K C-Pd + Zn C 2 (g) + PdZn In Pd/Zn methanol steam reforming catalysts Zn may act as a redox site that provides oxygen for reaction with C adsorbed on the metal to produce C 2. These Zn sites could be located either at the Pd/Zn interface or on the surface of the PdZn alloy.