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
Introduction V V V Supported monolayer vanadia catalysts are active for: CH 3 H + 2 CH 2 + H 2 CH 3 + 2 + 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
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 2 3 0.068 83.7 7 % V 2 5 /Nb 2 5 0.4 71.1 6 % V 2 5 /Ti 2 1.1 92.0 4 % V 2 5 /Zr 2 1.7 75.3 3 % V 2 5 /Ce 2 10.0 83.7 From: Wachs et al., in Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis Elsevier Science B.V., Amsterdam, 1997.
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 2 + 2 + 2 2* (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 2 + 2 + 2 2* 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
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
Vanadium Deposition on Ce 2 (111) XPS Ce 3d/ V 2p 3/2 intensity 1s 80 60 40 20 0 0 2 4 6 ML of vanadium deposited Layer-by-layer film growth V 2p 1/2 V 2p 3/2 Ce 2 (111) ML of V 5.5 1.0 0.5 0.25 0 V +3 V 2p 3/2 = 515.7 ev 540 530 520 510 Binding energy, ev 500 Atom-resolved NC-AFM image Namai et al. Catalysis Today 85 (2003) 79.
Metal xide ΔG f o kcal/mole of metal cation V 2 3-136.2 V 2 5-169.7 Ce 2-244.9 Ce 2 3-203.9 Ti 2-212.6 Ti 2 3-171.4 6 Ce 2 + 2 V V 2 3 + 3 Ce 2 3-26.3 ΔG o kcal/mole 10 Ce 2 + 2 V V 2 5 + 5 Ce 2 3 70.7 6 Ti 2 + 2 V V 2 3 + 3 Ti 2 3-25.1 10 Ti 2 + 2 V V 2 5 + 5 Ti 2 3 72.7 V 2 3 + 4 Ce 2 V 2 5 + 2 Ce 2 3 97.0 V 2 3 + 4 Ti 2 V 2 5 + 2 Ti 2 3 97.8
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 0 512.3 ev 10-7 torr 2 V +3 515.3 ev Just deposited V +4 515.7 ev V 2p 1/2 V 2p 3/2 525 520 515 510 Binding energy, ev V +5 516.8 ev V 2p 1/2 V 2p 3/2 d 10-3 torr 2 400 K c b 10-7 torr 2 575 K As deposited a Clean substrate 535 525 515 Binding energy, ev 505 540 530 520 510 Binding energy, ev 500
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 940 920 900 Binding energy, ev 880 860 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
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 540 530 520 510 Binding energy, ev 500
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
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 300 400 500 600 700 800 Temperature (K)
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 1 2 300 400 500 600 Temperature, K 700 800
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 2 300 400 500 600 700 800 Temperature, K
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 3/2 10-3 torr 2 400 K 10-7 torr 2 575 K CH 2 300 400 500 600 700 800 Temperature, K 535 525 515 Binding energy, ev 505
V x /Ti 2 (110) V x /Ce 2 (111) CH 2 460 K 585 K 2 Exposure CH 2 520 K 560 K 2 Exposure Torr, 30 sec 606 K 10-3 10-3 Torr, 3600 sec 10-6 10-3 Torr, 600 sec 10-7 10-3 Torr, 30 sec 10-6 Torr, 600 sec 10-8 reduced reduced 300 400 500 600 700 800 Temperature, K 300 400 500 600 700 800 Temperature, K
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 2 3 0.068 83.7 7 % V 2 5 /Nb 2 5 0.4 71.1 6 % V 2 5 /Ti 2 1.1 92.0 4 % V 2 5 /Zr 2 1.7 75.3 3 % V 2 5 /Ce 2 10.0 83.7
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 300 400 500 600 700 800 Temperature (K) Reduced by annealing at 750 K in vacuum
High Surface Area V 2 5 /Ce 2 catalyst (1s) Ce 2 Ce(3d) 529.6 ev (i) Intensity (a.u.) 6 wt.% Vx/Ce 2 (ii) Intensity (a.u.) 524.3 ev V(2p) 516.7 ev (a) (b) 523.5 ev 515.8 ev After CH 3 H TPD xidized sample 930 920 910 900 890 Binding Energy (ev) 880 870 540 535 530 525 520 Binding Energy (ev) 515 510
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.
%Conversion 100 Three-Way Automotive Emissions Control Catalysts Conversion 75 50 25 HC C N x 0 13.5 14.0 14.5 15.0 15.5 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 2 3 0.97 1.0 1.03 Stoichiometry Computer Control 2 sensor
Dense Alumina Monolith Porous Alumina Washcoat Impregnated with: Ce 2, Zr 2, La 2 3 Rh, Pd, Pt
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
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
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)
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
Model Catalysts Polycrystalline Ce 2 Al 2 3 (0001) Epitaxial Ce 2 Film Zr 2 (100)
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) 200 300 400 500 600 700 800 900 Temperature (K)
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 1 1 2 3 2 3 4 4 5 5 6 6 7 7 300 400 500 600 700 800 900 1000 Temperature (K) 300 400 500 600 700 800 900 1000 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
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 1 1 2 2 1 3 2 3 3 4 4 4 300 400 500 600 700 800 900 1000 Temperature (K) 300 400 500 600 700 800 900 1000 Temperature (K) 300 400 500 600 700 800 900 1000 Temperature (K) High oxygen vacancy concentration in 2 annealed epitaxial ceria film Heating to 900 K causes significant reduction of the ceria film?
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 1000 900 800 700 600 500 400 300 200 100 0 Binding Energy (ev)
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 2 550 K 750 K 825 K 900 K 1000 K Ce 2 Ce 2 3 450 K 600 K 750 K 825 K 900 K 940 930 920 910 900 890 880 870 940 930 920 910 900 890 880 870 Binding Energy (ev) Binding Energy (ev)
Ceria/Zirconia Results from Pulsed Neutron Diffraction Studies Mamontov, Egami et al., J. Phys. Chem. B 104 (2000) 11110. - 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
Enhanced Reducibility of Ce 2 /Zr 2 4Ce Ce + 2 2 2 3 2 Bulk Thermodynamics: T red ~ 1300 K TPD & XPS Results: T red ~ 750 to 900 K Ce 2 750 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
Steam Reforming of Methanol CH 3 H + H 2 C 2 + 3 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
CH 3 H xidation on Zn(0001) TPD Pathways and Intermediates A - Zn(0001) H 2 Zn 2+ 2- Zn(0001) Intensity (Arb. Units) C C 2 H 2 C CH 3 H 300 400 500 600 Temperature (K) 700
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) 510-530 K CH 3 (ad) CH 2 (g) + H(ad) 560-580 K HC(ad) C 2 (g) + H(ad) HC(ad) C (g) + H(ad) 2 H(ad) H 2 (g) + (l)
CH 3 H xidation on Zn(0001) TPD Pathways and Intermediates H C 660 K CH 3 520 K 495 K H CH 3 400 K Dosed with CH 3 H at 160 K 294 290 286 282 Binding Energy, ev
Methanol Steam Reforming Methanol Dehydrogenation Pd/Zn % Conversion % Selectivity Reduction Temperature (K) 10.1 kpa CH 3 H 10.1 kpa H 2 493 K C 2 Selectivity = C + C2
Steam Reforming of Methanol Pd/Zn
Intensity (Arb. Units) 1.5 ML 0.75 ML 0.35 ML 345 3 ML 2 ML 1 ML 0.5 ML Pd(3d 3/2 ) 342 XPS Pd(3d5/2) 339 336 333 Binding Energy (ev) 330 Pd Film Growth on Zn(0001) Zn(2p)/Pd(3d) Peak Area Ratio Zn(2p)/Pd(3d) Peak Area Ratio 3.5 3 2.5 2 1.5 1 0.5 0 0 0.5 1 1.5 2 2.5 3 Pd Coverage (ML) 1.4 1.2 1 0.8 0.6 0.4 0.2 0 300 400 500 600 700 800 900 Annealing Temperature (K) Zn(0001) 3 ML Pd/Zn(0001) 300 K 3 ML Pd/Zn(0001) 850 K LEED
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
Zn(0001), Pd/Zn(0001) CH 3 H TPD A - Zn(0001) B - 0.35 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 300 400 500 600 Temperature (K) 700 300 400 500 600 Temperature (K) 700
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 300 400 500 600 700 Temperature (K) 300 400 500 600 700 Temperature (K)
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 345 342 339 336 333 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
Mechanism of the reaction of CH 3 H on Pd 250-300 K CH 3 (ad) CH x (ad) + H(ad) CH x (ad) C(ad) + H(ad) H(ad) + H(ad) H 2 (g) 450-500 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) 400-550 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.