Surface knowledge from ultra-high vacuum to technically-relevant - - PowerPoint PPT Presentation

Surface knowledge from ultra-high vacuum to technically-relevant conditions

O + Cu

70µm Slides from June 2000

O + Ru

O + Ru O + Cu

70µm Matthias Scheffler Fritz-Haber-Institut der MPG, Berlin

in collaboration with: Xiao-Gang Wang, Catherine Stampfl, Karsten Reuter, Artur Böttcher, Robert Schlögl

Surface knowledge from ultra-high vacuum to technically-relevant conditions

part one: Oxidation Catalysis, mainly Ru and RuO2 under high O2 and CO pressure

• -removed--

Commercial Process of Styrene Production

Dehydrogenation of Ethylbenzene to Styrene C6H5CH2CH3 C6H5CH=CH2 + H2 (∆H0298 = 124.9 kJ/mol)

• reaction temperature: 580-640°C
• pressure:

atmospheric

• component of feed gas:

H2O / EB = 6 / 9

• selectivity to styrene: 95%

Parameters of the commercial process:

0,5 1 1,5 2 2,5 3 3,5

Catalytic Activity

Styrene Production Rate (%) Fe3O4 Fe2O3 K FexOy Fe2O3 K FexOy 5 4 3 2 1 3 2 1 0 40 80 120 160 Time (min)

Stoichiometry and Structure

• f the Surface depend on the Environment

(atomic chemical potentials) Esurface = Etotal - NO µO - NFe µFe

O2 gas surface Fe2 O3 substrate

2mFe + 3mO = Ebulk (Fe2O3)

Fe2O3 (0001) Surface Terminations

O3FeFe-R FeO3Fe-R FeFeO3-R O2FeFe-R O1FeFe-R Oxygen chemical potential (eV) Surface energy (meV/Å2)

• 3.0 -2.0
• 1.0

0.0

X.G. Wang et al., PRL 81 (1998)

Fe2O3 (0001) O-terminated Surface

O Fe

X.G. Wang et al., PRL 81 (1998)

The Role of Carbon

• The surface is fully covered

with carbon (and oxygen)

• Ethylbenzene does not

interact with iron oxide

• Thus, the catalytic activity

is not related to Fe-oxide

So, what is the active catalyst?

100 300 500 Temperature (K) Desorption of Ethylbenzene 3.83 L 3.22 L 2.58 L 1.29 L 0.64 L 0.32 L 0.16 L

• C. Kuhrs and R. Schlögl,

to be published

Carbon Nanotubes for EB Conversion

• R. Schlögl et al., to be published

1 7 3 4 5 1 6 8 8 5 1 2 1 1 9 1 3 6 1 5 3 1 2 3 4 5 6 7 8 9 1

S T yie ld S e le ctivity to S T

% T im e , m in

Selectivity to Styrene: 100% Styrene yield: 50% 1.0 0.8 0.6 0.4 0.2 0.0 17 51 85 119 153 Time (min)

Reaction temperature 450°C Concentration of ethylbenzene in the stream 1·10-5 mol/ml Flow of the stream 10 ml/min

Conclusions

• The technique of calculating free energies and

predicting the lowest-energy structures in equilibrium with multiple species in the environment is applicable to a wide variety of gas-phase and solution-phase chemistry.

• The ideal models are re-structured and the

composition is changed into the suitable real structure (new material). This happens with a "fast" kinetics only at "high" temperature and pressure.

• Complexity is essential for understanding the

function of surfaces.

Some references (updated: Jan. 2002)

• N. Keller et al., Angew. Chem., in print (2002): The

Catalytic Use of Onion-Like Carbon Materials for the Styrene Synthesis by Oxidative Dehydrogenation of Ethylbenzene.

• X.-G. Wang, W. Weiss, Sh.K. Shaikhutdinov, M. Ritter,
• M. Petersen, F. Wagner, R. Schlögl, and M. Scheffler,
• Phys. Rev. Lett. 81, 1038 (1998): The hematite (Alpha-

Fe2O3)(0001) surface: Evidence for domains of distinct chemistry.

• R. Schloegl, CATTECH 5 (3), 146 (2001): Theory in

heterogeneous catalysis. An experimentalist's view.

• S. Strano, Jane Rempel, John Halverson, Chris Burket,

Jonathan Mathews and Henry C. Foley, in print (2002): Structural modeling of nanoporous carbon: A review of approaches to simulating an aperiodic and non- equilibrium solid.

• K. Reuter and M. Scheffler, Phys. Rev. B 65, 035406

(2002). Composition, structure, and stability of RuO2(110) as a function of oxygen pressure (11 pages).

• S. Wilke, M.H. Cohen, and M. Scheffler, Phys. Rev. Lett.

77, 1560-1564 (1996): Local reactivity of solid surfaces,

• Michael S. Kane, Lien C. Kao, Ravindra K. Mariwala,

David F. Hilscher, and Henry C. Foley, Ind. Eng. Chem.

• Res. 35, 3319 (1996): Effect of porosity of carbogenic

molecualar sieve catalysts on ethylbenzene oxidative dehydrogenation.

• V. Petkov, R.G. DiFrancesco, S.J.L. Billinge, M. Acharya

and H.C. Foley, Philosophical Magazine B 79, 1519 (1999): Local structure of nanoporous carbons.

• Madhaw Acharya, Michael S. Strano, Jonathan P.

Mathews, Simon J.L. Billinge, Valeri Petkov, Shekhar Subramoney, and Henry C. Foley, Philosophical Magazine B 79,1499 (1999): Simulation of nanoporous carbons: a chemically constrained structure.