Surface chemistry of oxides: Basics and applications€¦ · Winterschool on the Theory of...

Winterschool on the Theory of Hetero-Interfaces and Surfaces

Surface chemistry of oxides: Basics and applications

Joachim Paier

Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin,

Germany

Department of Materials Science and Engineering, Yonsei University 11/11/16

Motivation. (Why an entire lecture on oxides?) Relevant properties.

• atomic structure• ((vibrational (phonon) properties → IRRAS, HREELS))• electronic/magnetic properties

How to tackle oxides computationally (ab initio, first principles)?

Polarity and stability of surfaces.

Defects.

Applications:

1. Adsorption of water on iron oxides.2. The support effect: vanadium oxide deposited on cerium dioxide.3. Surface terminations of V2O3: A longstanding problem revisited.

Overview

2

• Transition metal and rare-earth oxides

Problems in vibrational/electron spectroscopy

3See, e.g., Y. Wang, Christof Wöll, Chemical reactions on metal oxide surfaces investigated by vibrational spectroscopy, Surf. Sci. 603 (2009) 1589-1599.

• In oxides, electron-based spectroscopic methods hampered due to experimentaldifficulties such as charging problems caused by the poor electric conductivities.

• HREEL spectra of single crystal oxide surfaces governed by the intense opticalFuchs-Kliewer phonons.

• Fuchs-Kliewer phonons are optical branches (ions oscillate out of phase ⇒dipole changes huge ⇒ huge coupling to external el. fields (IR) ⇒ molecularadsorbates’ intensities tiny).

• IR intensities (mostly via reflectance mode) very small due to missing imagedipole as on metal surfaces, which amplifies the intensity normal to surface(surface selection rule).

• IR: small reflectivity at grazing incidence (featuring max. sensitivity)

Literature

4

(1) V. E. Henrich and P. A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, 1994.

(2) J. Jupille and Geoff Thornton (eds.), Defects at Oxide Surfaces, Springer, 2015.

(3) A. Groß, Theoretical Surface Science: A Microscopic Perspective, Springer, 2003.

(4) L. Pauling, The Nature of the Chemical Bond, Cornell University Press, 1993.

Books - Monographs

Review Articles

(1) Charles T. Campbell, Joachim Sauer (Eds.), Surface Chemistry of Oxides, Chemical Reviews, vol. 113 (2013).

a) Site Requirements for the Adsorption and Reaction of Oxygenates on Metal Oxide Surfaces, John M. Vohs.

b) Well-Ordered Transition Metal Oxide Layers in Model Catalysis – A Series of Case Studies, Helmut Kuhlenbeck, Shamil Shaikhutdinov, and Hans-Joachim Freund.

c) Oxygen Defects and Surface Chemistry of Ceria: Quantum Chemical Studies Compared to Experiment, Joachim Paier, Christopher Penschke, and Joachim Sauer.

⇒ Technical applications• Chemical industry, heterogeneous catalysis

− aluminosilicates (microporous), i.e. zeolites, 3d metaloxides, rare-earth oxides

• Energy conversion, storage− photocatalysis (titanium oxide)

• Electronics, magnetic recording, information storage− iron oxides

• Corrosion sciences (incl. geochemistry (soil formation))

Motivation

5

Surface Chemistry of Oxides, - Charles T. Campbell, Joachim Sauer (Eds.), Chemical Reviews, vol. 113 (2013).

⇒ collection of reviews on the cutting edge oxide-related science.

- H.-J. Freund, H. Kuhlenbeck, and V. Staemmler, Oxide Surfaces, Rep. Prog. Phys. 59, 283-347 (1996).

MotivationImportant phenomena at interfaces between

a) two different oxidesb) an oxide and a metal

Examples:1. Semiconductor physics:

• heterojunctions (p-n-junctions, doping)MOS-FET (metal-oxide-semiconductor field effect transistor)

Modulation of the electronic structure (due to band bending) at theinterface (Schottky barrier).

2. Catalysis:• At metal-oxide interfaces strong-metal-support interactions, i.e. SMSI,

occur (part of it EMSI = electronic metal-support interaction).• Encapsulation or migration of oxides onto the metal nanoparticles

(spill-over effect)• Modulating catalytic activity at oxide-oxide interfaces.

6

Crystal structures of binary metal oxides

7

rocksalt (NaCl) corundum (Al2O3) rutile (r-TiO2)

wurtzite (w-ZnO) fluorite (CeO2)

Figures adapted after V. E.Henrich and P. A. Cox, TheSurface Science of MetalOxides, Cambridge UniversityPress, 1994.

Crystallographic unitcells are shown exceptfor corrundum.

• Metal oxides occurr in many different crystallographic structures.

Fe3O4 (magnetite) – high-temperature bulk phase

Fe3+A = tet Fe2+,3+

B = oct O2-

• O atoms: close packed fcc lattice

• Fe2+ at interstitial octahedral [B] sites

• Fe3+ at interstitial oct. and tetrahedral [A] sites

• ⇒ inverse spinel

• a0 = 8.396 Å

• UC contains 8 formula units

• (Fe83+)A[Fe3+Fe2+]8,BO32

• A-site and B-site magnetic moments are antiferromagnetically coupled

𝑡𝑡2𝑔𝑔↓3 𝑒𝑒𝑔𝑔↓

2𝑡𝑡2𝑔𝑔↑3 𝑡𝑡2𝑔𝑔↓ 𝑒𝑒𝑔𝑔↑

2Fe2+

Fe3+ (S = 5/2)(S = 2)

Magnetism in transition metal oxidesor: On the importance of electron correlation effects

[1] M. Imada, A. Fujimori, Y. Tokura, Metal-insulator transitions, Rev. Mod. Phys. 70, 1039-1263 (1998). 7

Electrical dc conductivity of Fe3O4; Miles et al., Rev. Mod. Phys. 1957, 29, 279.

Charge ordering in Fe3O4 – Verwey phase transition

Very “rich” physics of Fe3O4

• Conductivity drastically drops (ca. 2 orders of magnitude) at 122 K (ambient pressure).

• Comes together with a change in the crystal structure.

• Verwey: First attempt to explain the metal-insulator phase transition.

Verwey, E.J.W. "Electronic conduction of magnetite (Fe3O4) and its transition point at low temperatures". Nature 1939, 144, 327.

Magnetism in transition metal oxidesor: On the importance of electron correlation effects

9

10

• Two unit cell vectors determine the periodicdirections.

• One „large“ vector orthogonal to the surfacedetermines the so-called vacuum gap.

• Along the surface normal (i.e. surfacevector): sufficient number of atomic layers.

Alternative: Cluster models

• Very useful, when effects are ratherlocal, e.g. core-level excitations.

• More sophisticated wavefunction-based methods like, e.g., coupled-cluster techniques doable.

Modelling of oxide surfaces: periodic slab models

Figure adapted from: H.-J. Freund, H. Kuhlenbeck, and V. Staemmler, Oxide Surfaces, Rep. Prog. Phys. 59, 283-347 (1996).

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Modelling of oxide surfaces: periodic slab models

Common criterion to ensure appropriate number of atomic layers in the slab is theconvergence of the surface energy with respect to the number of layers.

Reproduced from: A. Kiejna, T. Pabisiak, S. W. Gao, The energetics andstructure of rutile TiO2(110), J. Phys. Condens. Matter 18, 4207 (2006).

GGA-PW91

GGA-PBE

GGA-PBE

GGA-PW91

unrelaxed slab.

relaxed slab.

Metal oxide surfaces require a large number of atomic layers to capture(vertical) relaxation effects.

Not every surface resulting from cleavage of a crystal is stable.

Reason: The overall electroneutral crystal consists of charged ions.Example: MgO consists of Mg2+ and O2- ions.

MgO(111): polar (i.e. charged)surface, implies huge dipolemoment (alternating positivelyand negatively charged layers).

Surface energy of polar surfaces diverges! ⇒ unstable!

MgO(001): non-polar, local dipole moments in surface annihilate each other (between layers).

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Charge distribution, i.e., polarity affects the surface stability

Figures adapted fromWikipedia.

H.-J. Freund, H. Kuhlenbeck, and V. Staemmler, Oxide Surfaces, Rep. Prog. Phys. 59, 283-347 (1996).

Example: Hydroxilated MgO(001) vs MgO(111)–stabilities

Source: Noguera and co., Phys. Chem. Chem. Phys. 11, 2228 (2009).

Wulff G., Zeitschriftfür Krystallographieund Mineralogie, vol.34 (1901) 449–530.

Adapted from: C. Noguera, Polar Oxide Surfaces, J. Phys.: Condens. Matter 12 (2000) R367.

14

Charge distribution, i.e., polarity affects the surface stability→ Classification after Tasker

Defects in ionic solids

Point defects (0-dim. defects)

a) vacanciesb) interstitial defectsc) substitutional defects (“antisite defects”)

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• Affect mechanical properties of the material.• Affect transport properties as well as reactivity.• They offer pathways of “high mobility” for atoms and ions.

Oxygen point defects in a surface

Example: CeO2 (111)Uncharged surface O-vacancies (= electroneutral point defect)

[111]

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- O relax

⊗[111]

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Oxygen point defects in a surface

Example: CeO2 (111)Uncharged surface O-vacancies (= electroneutral point defect)

Electron localization + lattice relaxation = small polaron

O vacancies in the surface layer:[2Ce4+, 4O2-] → [2Ce3+, 3O2-,VO] + 1/2 O2

[1] JP, C. Penschke, and J. Sauer, Oxygen Defects and Surface: Chemistry ofCeria: Quantum Chemical Studies Compared to Experiment, Chem. Rev., 2013, 113, 3949-3985.

∆E(1/2O2), [1] (eV) 2.76 2.49 2.44

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• Defects involved in surface chemical reactions.• Reducibility of a surface determines its catalytic activity.

Mars van Krevelen mechanism

i) Ag → Aadsii) Aads + Osurf → AOadsiii) AOads → AOg + OViv) OV + O2(g) → 2 Osurf

Langmuir-Hinshelwood mechanism“adsorption – diffusion/reaction – desorption”

Eley-Rideal mechanism“adsorption – reaction with gas-phase – desorption”

Reaction mechanisms inHeterogeneous catalysis.

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Application 1:Water adsorption on structurally distinct iron oxide surfaces:Fe2O3, hematite (corundum) versus Fe3O4, magnetite (spinel)

- Initial steps of hydration on the Fe3O4(111) surface.- Molecular beam/surface calorimetry and IRAS.- Density functional theory.

Collaborations, Experimental Partners:

Prof. Joachim Sauer (Humboldt-Universität zu Berlin)

Petr Dementyev, Shamil Shaikhutdinov, Swetlana Schauermann, Prof. Hans-Joachim Freund (Fritz-Haber-Institut)

• P. Dementyev, …, S. Schauermann, X. Li, JP, J. Sauer, H.-J. Freund, Angew. Chem. Int. Ed.2015, 54, 13942-13946.

• X. Li, JP, J. Phys. Chem. C, 2016, 120, 1056-1065.• R. Ovcharenko, E. Voloshina, J. Sauer, Phys. Chem. Chem. Phys. 2016, 18, 25560-25568. 20

[1] Campbell, Schauermann,Freund and coworkers,Rev. Sci. Instrum. 2011,82, 024102.

[2] Dementyev,…, Schauermann,Li, Paier, Sauer, Freund,Angew. Chem. Int. Ed.,2015, 54, 13942-13946.

Advantages• Direct probe for molecule-

surface interactions.• No assumptions required.• Any surface processes can be

studied.

Single Crystal Adsorption Calorimetry for H2O/Fe3O4(111)• Molecular Beam Experiments[1,2]

100 ± 2 kJ⋅mol-1

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PBE+U Adsorption enthalpies [kJ/mol]: Calorimetry

1/n Fe3O4 (111) + H2O → 1/n [Fe3O4 (111), n H2O]; n = 1,2,3.

Single metal (Fe-tet1) termination:

Double metal (Fe-oct2, Fe-tet1) termination:

• X. Li, JP, J. Phys. Chem. C, 2016, 120, 1056-1065.

−66/H2O −75/H2O −59/H2O

−136/H2O −109/H2O −84/H2O

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[1] Dementyev, …, Schauermann, Li, Paier, Sauer, Freund, Angew. Chemie Int. Ed. 2015, 54, 13942-13946.

IR Absorption Spectroscopy[1]

16O → 18O exchange ?∆ = 25 cm-1

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16O → 18O exchange ?

ν1 ν2

∆ = 25 cm-1

24



16O → 18O exchange

∆ = 25 cm-1

ν1 ν2ν1

ν2

(D2O)2/Fe-oct, Fe-tet

(D2O)2/Fe-tet

2740 (-17) 2707 (-17)2710 (-18) 2692 (-17)

ν1ν2

25




∆ = 25 cm-1

ν1 ν2ν1

ν2

(D2O)2/Fe-oct, Fe-tet

(D2O)2/Fe-tet

2740 (-17) 2707 (-17)2710 (-18) 2692 (-17)

ν1ν2

∆Hads/H2O: -109 kJ⋅mol-1 -75 kJ⋅mol-1

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[2] C. M. Huggins, G. C. Pimentel, J. Phys. Chem.1956, 60, 1615-1619.



∆ = 25 cm-1

ν1 ν2ν1

ν2

∆Hads/H2O: -109 kJ⋅mol-1 -75 kJ⋅mol-1

H-bonded OH vibrations are not observed due to broadening [2].

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Summary - Outcome

rather than

• In its initial stage, water adsorption on Fe3O4(111) follows

Application 1:

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This structure featuresa) adsorption enthalpies,b) IR wavenumbers (taking DFT errors into account), as well as,c) 18O isotopic shifts

that are fully reconcilable with observation.

Application 2: Surface structure of V2O3(0001)

FHI: Felix E. Feiten; H. Kuhlenbeck, H.-J. Freund;

HU (Physics): J. Seifert, H. Winter;

HU (Chemistry): JP, J. Sauer;

a) If we want to understand the chemistry on V2O3surfaces, we need to have a clear picture of their atomic structures.

b) Ambiguous interpretations of experimental results and DFT calculations published in the literature.

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Surface structural models

V2O3 hexagonal bulk unit cell:

3 different terminations in (0001)2 metal terminations: …O3-V2-O3-V2

…O3-V2-O3-V “single” or “half-metal”

1 oxygen termination:…O3-V2-O3

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…O3-V2-O3-V=O “vanadyl”

…O3-V-O3V3O3 “O3”

V2O3 hexagonal bulk unit cell:

3 different terminations in (0001)2 metal terminations: …O3-V2-O3-V2

…O3-V2-O3-V “single” or “half-metal”

1 oxygen termination:…O3-V2-O3

Experiment / DFT:


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V: grayO: red

…O3-V2-O3-V=O …O3-V-O3V3O3Ordered superstructure with2/3 and 1/3 of full V=O coverage.


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Surface Stabilities

[1] Kresse et al., Surf. Sci. 555, (2004), 118.

• Range of “stability plot” determined by the heats of formation of VxOy!

3x(VO2)-trilayer

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Medium energy ion-scattering experiments (100 keV H+)[Window et al., Surf. Sci. 2012, 606, 1716.]

34

Surface phases as a function of conditions

Experiment by

Felix E. Feiten,H. Kuhlenbeck,H.-J. Freund, FHI.

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Characterization techniques

(empty state) atomically resolved STM (FHI):

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V=O O3


37


V=O O3

Infra-red absorption!

V=O always observed!

[UHV grown V2O3(0001)]


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Thermochemistry of bulk VxOy phases [eV/f.u.]

Reaction 1: V2O3 → 2 VO + 1/2 O2Reaction 2: V2O3 + 1/2 O2 → V2O4Reaction 3: V2O4 → 1/2 V2O3 + 1/2 V2O5

a) NIST-JANAFb) W. Brückner et al., Vanadium

oxide, Akademie Berlin, 1983.

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PBE surface stability as a function of ∆µO

40

HSE surface stability as a function of ∆µO

41

Conclusions

DFT results have to be interpreted carefully!See, e.g., JP, Catal. Lett. (2016), 146:861.

However, as the thermochemistry of VxOy bulk phases improves upon utilization of the HSE functional, weinfer improved surface stabilities.

Support by experimental observation of dewetting as V2O5 at higher T and p(O2).

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Application 3: Supported Transition Metal Oxide Catalysts

TEM of ceria nanocrystallites; bySchneider et al., Beilstein J. of Nanotechnology 2, 776 (2011).

- Important catalyst for selective oxidation reactions:

• chemical (alkane/alkene),• petroleum,• environmental industries.

o-xylene → V2O5/TiO2 → phthalic anhydride2D surface vanadium oxidephase: monolayer

- methanol → formaldehyde:

• simple probe reaction for C-H activation• oxidative dehydrogenation (H-transfer, 2 e-’s)• Mars-van Krevelen mechanism

- surface (lattice ) oxygen atoms involved!

- selectivity is an issue.43

Application 3: Supported Transition Metal Oxide Catalysts

TEM of ceria nanocrystallites; bySchneider et al., Beilstein J. of Nanotechnology 2, 776 (2011).

- Important catalyst for selective oxidation reactions:

• chemical (alkane/alkene),• petroleum,• environmental industries.

o-xylene → V2O5/TiO2 → phthalic anhydride2D surface vanadium oxidephase: monolayer

- methanol → formaldehyde:

• simple probe reaction for C-H activation• oxidative dehydrogenation (H-transfer, 2 e-’s)• Mars-van Krevelen mechanism

- surface (lattice ) oxygen atoms involved!

- selectivity is an issue. CH3OH + 3/4 O2 CO2 + 1/2 H2O; ∆Hro = -309 kJ/mol

CH3OH + 1/2 O2 HCOH + H2O; ∆Hro = -153 kJ/mol44

Support Effect: CH3OH Oxidation at VOx/MnOm

• data from Wachs & Deo1994;

• Wachs et al. 1997; Wachs et al. 2005.

• assumed all sites active (94, 97).

• “high-03”: discriminates active and inactive sites.

• “high”: near monolayer concentration, but no V2O5NCs (no X-ray detection).

[*] Courtesy of J. Sauer, Erkner 2011.

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Complex Surface Structure of Supported TMOs

3 nm

• Supported oxide catalysts nottranslationally ordered.

• Size, structure, distribution of active sites not known.

• Assignment of spectroscopic signatures not save;e.g. structure-IR relationships.

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[1] Baron, … Freund,… Sauer, Angew. Chem. Int. Ed. 48, 8006 (2009).

3 nm

direct relationship between nuclearity of vanadiaclusters and the V=O frequency, reproduced by DFT

IR: V=O stretching mode blue shift uponaggregation (25 cm-1)

STM: VOx monomers,dimers, trimers;

similar apparent height →„flat“ structures

Assignment of IR-active V=O Stretching Mode

47

[1] Baron et al., Angew. Chem. Int. Ed. 48, 8006 (2009).[2] Ganduglia-Pirovano et al., J. Am. Chem. Soc. 132,

2345(2010).

• STM images [1] show mono- and oligomers of VOx/CeO2(111) (lower left)

• Monomers display higher ODH activity (α peak) according to TPD [2] (right)

Low-temperature ODH activity of VOx monomers

3 nm

48

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linear regime saturation regime(Henry regime)

Langmuir adsorption isotherm

T1 < T2 < T3

number of molecules in the monolayer

Saturation pressure, ps

ps

𝜃𝜃 =𝐾𝐾𝑎𝑎𝑎𝑎𝑎𝑎 � 𝑝𝑝

1 + 𝐾𝐾𝑎𝑎𝑎𝑎𝑎𝑎 � 𝑝𝑝; 𝜃𝜃 =

𝑁𝑁𝑁𝑁𝑚𝑚

(𝜃𝜃 = 𝐾𝐾𝑎𝑎𝑎𝑎𝑎𝑎 � 𝑝𝑝)Kads … equilibrium constant for

adsorption;p … pressure

50

Langmuir adsorption isotherm

A surface chemical reaction requiresi) adsorption from the gas-phase (pre-equilibrium) andii) „turn-over“ to product.

Figure: Courtesy of Prof. Joachim Sauer.

… Turn-Over-Frequency

Computational Details

• PBE+U(4.5 eV ) for Ce 4f states;• HSE hybrid by Heyd, Scuseria, Ernzerhof [1]

• B3LYP hybrid• ZPVE correction included (enthalpies!)• Grimme dispersion for vdW forces [2]

• NEB and improved dimer [3] method for transition structures

[1] S. Grimme, J. Comput. Chem. 27, 1272 (2006); T. Kerber, M. Sierka, J. Sauer, J. Comput. Chem. 29, 2088 (2008).

[2] J. Heyd, G. E. Scuseria, M. Ernzerhof J. Chem. Phys. 118, 8207 (2003).[3] G. Henkelman, B. P. Uberuaga, H. Jonsson, J. Chem. Phys. 113, 9901

(2000).

8 Å

9L

• projector augmentedplane waves (PAW);

p(4x4) CeO2(111)

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Vorführender

Präsentationsnotizen

ZPVE affects barriers more strongly; adsorption energies essentially unaffected.

V(d1)O2 Mono-, Di-, and Trimer [1]

[1] C. Penschke, J. Paier, and J. Sauer, J. Phys. Chem. C 117, 5274 (2013).

1 x Ce3+ 2 x Ce3+ 3 x Ce3+

Monomer: VO2 Dimer: (VO2)2 Trimer: (VO2)3

52

Partial Oxygen Spillover upon V(d1)O2 Deposition [*]

[*] J. Paier, T. Kropp, C. Penschke, and J. Sauer, Faraday Discuss. 162, 233 (2013).

• Tetrahedral coordination of V5+(!) causes partial O-spillover.

• VO2 at the CeO2(111) surface leads to formation of a “pseudo-vacancy”.

Perspective side view: Top view:

pseudo-vacancy

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What’s the most stable VOx at CeO2(111)?

1. C. Popa, M. V. Ganduglia-Pirovano, and J. Sauer, J. Phys. Chem. C 115, 7399 (2011).

2. C. Penschke, J. Paier, and J. Sauer, J. Phys. Chem. C 117, 5274 (2013).

3. J. Paier, T. Kropp, C. Penschke, and J. Sauer, Faraday Discuss. 162, 233 (2013).

VO2

VO

VO4

VO3V

UHV

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Which one agglomerates? V(d3)O or V(d1)O2?

Energies in kJ/mol (in parentheses: with dispersion). 100 kJ/mol≈ 1.04 eV[*] C. Penschke, J. Paier, and J. Sauer, J. Phys. Chem. C 117, 5274 (2013).

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CH3OH Oxidation on VOx/SiO2

Döbler, Pritzsche, Sauer, J. Am. Chem. Soc. 127, 2005, 10861

SiV

O

OO

OCH3

SiO

H

1st Step chemisorption of substrateaddition on to V−O(−Si) bond (heterolytic)

+ CH3OH V

O

OO

O- 40 kJ/mol

2nd Step redox stepH abstraction by metal-oxo bond (homolytic)

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Oxidation Step: CH3OH to CH2O

VV

CH2

OO

H

VIV

•CH2

OO

H

O

VIII

CH2

O

H

d0 d1- biradicaloid d2 - triplet

Observed (kJ/mol) Calcd. Barrier (kJ/mol)Heat of adsorption (503K) 50 B3LYP 154Apparent barrier (503 K) 82±10 CCSD(T)-corr -17 Intrinsic barrier (0K) 137±10 Total Calcd 137

Requires broken symmetry solution for DFTRequires hybrid functionals

Döbler, Pritzsche, Sauer, J. Am. Chem. Soc. 127, 2005, 10861

57

58

H2 Bond Dissociation (Szabo-Ostlund)

Apparently better answer-but not for the right reason

Wrong dissociationbehaviour of RHF

Reactivity Parameters (Mars van Krevelen Mechanism)

HO−V-d2

H2O

OV-d0

[ ]V-d2

CH3O−

CH2O

½ O2

HOV-d1/CH2O−

CH3OH + ½ O2 CH2O + H2O

OV-d0 [ ]V-d2 + ½ O2

O-defect formation

OV-d0 + ½ H2 HOV-d1

Hydrogenation

+H+

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Apparent Barriers vs O-Defect Formation Energies

from impedance spectroscopy

Beck, …, Freund, Schlögl, Sauer, Schomäcker, J. Catal. 296, 2012, 120.

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Data from Schomäcker et al., Schlögl et al., CRC 546

Same variation for ethanol compared to methanol, but- change of the relative activity of TiO2 and CeO2 supports- different TiO2 supports show large variation

Ethanol Oxidation on Supported Vanadia

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(VO2)n/CeO2(111) – Stability and Reactivity

PBE+U (1/2 O2) Penschke, Paier, Sauer

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Synergy on Defect Formation in VO2/CeO2(111)

Popa, Ganduglia-Pirovano, Sauer,J. Phys. Chem. C 115, 2011, 7399 1xCe3+

VO/CeO2(111)

– ½ O2

+ 0.79

3xCe3+

V5+ V5+relax

VO2/CeO2(111)

CeO2(111)

– ½ O2

+ 1.87

CeO2-x(111)

2xCe3+

vacrelax

PBE+U (1/2 O2; eV)

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VOx/CeO2(111): TPD of Methanol

Tdes/K Ea/kJ/molα ~ 370 96β ~ 475 125γ ~ 600 150

Powder catalysts (Vohs)V2O5/TiO2 525 148V2O5/CeO2 525 148V2O5/SiO2 675 192

CH2O formation on VOx/CeO2 atmuch lower T than on VOx/SiO2

Ganduglia-Pirovano,...Sauer,...Freund, J. Am. Chem. Soc., 132, 2010, 2345

64

Si4+OH-/[CH3O-V5+]Ce4+OH-/[CH3O-V5+]Ce4+OH-/[CH3O-Ce4+]-169 -165 - 40

kJ/mol; ZPVE includedPseudo O-vacancy

VOxSiO2VOxCeO2(111)

CeO2 vs SiO2 Support: Chemisorption

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Si4+OH-/[CH3O-V5+]Ce4+OH-/[CH3O-V5+]Ce4+OH-/[CH3O-Ce4+]

V4+OH−Ce3+OH−

-169 -165 - 40

131

kJ/mol; ZPVE included

137

Pseudo O-vacancy

VOxSiO2VOxCeO2(111)

CeO2 vs SiO2 Support: Cooperation in H transfer step

66

Si4+OH-/[CH3O-V5+]Ce4+OH-/[CH3O-V5+]Ce4+OH-/[CH3O-Ce4+]

V4+OH−Ce3+OH−V5+OH−/Ce3+

-169 -165 - 40

100 131

kJ/mol; ZPVE included

137

Pseudo O-vacancy

VOxSiO2VOxCeO2(111)

CeO2 vs SiO2 Support: Cooperation in H transfer step

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CeO2-x

IntegrationPolanyi-Wigner eq.QM energiesand pre-exp

Kropp, Paier, Sauer,J. Am. Chem. Soc.

136, 14616 (2014).

Kropp, Paier,J. Phys. Chem. C

118, 23690 (2014).

TDS: Experiment vs Simulation

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Oxidative Dehydrogenation on Supported Transition Metal Oxides

• Initial step: Chemisorption of methanol as methoxy on surface– TM−O−S interphase bond or S−O−S support

• Intrinsic barrier: CH bond dissociation• H abstraction by

– TM=O bonds (VOx/SiO2)– TM−O−S or S−O−S bonds (VOx/CeO2)

• CeO2 is a very special supportIt easily accomodates electrons in f-states.In the VOx/CeO2 system this leads to stabilization of V+5 also underreducing conditions.

• The VOx/CeO2 system is more reactive than each component alone.

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Computing time: HLRN, The North-German Supercomputing Alliance JUROPA, Supercomputer Center − Forschungszentrum Jülich

Acknowledgements

Funding: • DFG – CRC1109• Fonds der Chemischen Industrie (FCI),

Habilitandenstipendium• COST action CM1104

Coworkers:

Thomas Kropp, Dipl.-Chem.

Christopher Penschke, Dipl.-Chem.

XiaoKe Li,M. Sc.

Prof. Joachim SauerProf. Hans-Joachim Freund

Appendix :The Problem of Localized Electrons:Electron Self-Interactions in KS-DFT (LDA,GGA)

HSE, PBE0, … (hybrid) & DFT+UPBE (GGA)

Spin-Density of a Surface O-Defect in CeO2(111)[1]

[1] J. Paier, C. Penschke, J. Sauer, Chem. Rev. 113, 3949 (2013).

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CeO2-x

IntegrationPolanyi-Wigner eq.QM energiesand pre-exp

Kropp, Paier, Sauer,J. Am. Chem. Soc.

136, 14616 (2014).

Kropp, Paier,J. Phys. Chem. C

118, 23690 (2014).

TDS: Experiment vs Simulation

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Desorption-Temperatures: PBE+U vs HSELDA/GGA underestimates barriers

PBE+U

HSE

73

Delocalized vs. Localized Electronic States

TS4 (2 x Ce3+)

74JP, Catal. Lett. (2016), 146:861.


TS4 (2 x Ce3+)

75JP, Catal. Lett. (2016), 146:861.


PBE+U(4.5;Ce f) 171 116HSE06 176 148B3LYP 198 144

Intrinsic Barriers (+D) (kJ/mol)

76JP, Catal. Lett. (2016), 146:861.


PBE+U(4.5;Ce f) 171 116HSE06 176 148B3LYP 198 144

Intrinsic Barriers (+D) (kJ/mol)

SiO2191-17 ∆CCSD(T)

174 Sauer & co., J. Am. Chem. Soc. 127, 2005, 10861

77JP, Catal. Lett. (2016), 146:861.

LYP and the “uniform electron gas limit” (UEG)

S. Kurth, J. P. Perdew, and P. Blaha, Int. J. Quant. Chem. 75, 889 (1999).

= B3LYP for the UEG

high density low density

78JP, Catal. Lett. (2016), 146:861.

Atomization energies for solids

JP, M. Marsman, G. Kresse, J. Chem. Phys. 127, 024103 (2007)

79JP, Catal. Lett. (2016), 146:861.

Atomization energies for solids

JP, M. Marsman, G. Kresse, J. Chem. Phys. 127, 024103 (2007)

B3PW91

80JP, Catal. Lett. (2016), 146:861.

Summary

• Accuracy of barriers obtained using DFT+U (PBE+U[4.5; Ce f]) is very reasonable.

• No qualitative differences compared with more computationally expensive barriers from hybrid functionals.

• However, hybrid functional results compare de facto quantitatively with observed TDS.

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Summary




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Summary




Thank you for your kind attention!

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