Surface Complexes in Catalysis

Transcription

1 Surface Complexes in Catalysis David Karhánek Ústav organické technologie, VŠCHT Praha Institut für Materialphysik, Universität Wien XXXVII Symposium on Catalysis, Prague, October 7-8, 2005.

2 Research Methodologies: Catalytic Hydrogenation Competitive kinetic measurements relative adsorptivities (K A /K B ) a reactivities (r A /r B ) of the substrates Physical-Chemical methods FT-IR, MAS-NMR, TPD, LEED, SEM, TEM, STM, XRD, properties of the catalyst surface Pt(111), Au(100) Molecular modeling estimation of geometry, IR vibrations, adsorption enthalpy binding energies, transition state structure a) molecular dynamics (stat. thermodynamics) b) quantum mechanics (Schrödinger eqn.) CH 2 =CH Pt(111)

3 Catalytic Hydrogenations: Adsorption Mechanism: Hoiruti - Polanyi (1934) reactants catalyst surface Surface complex: changes of adsorptive geometry: distortion of substituents from the C=C bond plane elongation of C=C bond changes of adsorbent geometry: relaxation and reconstruction at the active site changes in hybridization, IR, NMR parameters el. density shifted from π-bond towards the metal surface 4 5 reaction mixture catalyst surface

4 Catalytic Hydrogenations: Adsorption Metal surface

5 Application of the Quantum Chemistry Schrödinger equation H i$ Ψ ( x) = EΨ( x) Molecular Structures (cluster): DFT Methods Periodical Structures (slab): Gaussian 03W Vienna Ab-initio Simulation Package (VASP)

6 Model Compounds ethylene prop-2-en-1-ol (allylalcohol) 2-methylbut-3-en-2-ol C 38 H 34 P 2 Pt hex-1-en-3-ol hept-1-en-4-ol [C 2 H 4 Pt 36 ] C 2 H 10 P 2 Pt c(3x3)-pt(111) ethylene-bis(phosphin)platinum

7 Platinum Crystal and Surface FCC cell Lattice constant a = Å (calc.) a = Å (exp.) Description of the periodical structure: FCC Pt bulk cartesian Surface energy: 100 plane 110 plane 111 plane σ = ev/a 2 σ = ev/a 2 σ = ev/a 2

8 Geometries of the Adsorbed Complexes Ethene Prop-2-en-1-ol 2-Methylbut-3-en -2-ol Hex-1-en-3-ol Hept-1-en-4-ol d(c=c)* [Å] di-σ adsorbed state: * d(c=c)** [Å] di-σ coordinated state: ** Dd* [Å] d* [Å] Dd** [Å] 0,16 0,155 0,15 0,145 0,14 0,135 0,13 0,125 Absolute Absolutní elongation prodloužení of vazeb the C=C bonds 0,098 0,096 0,094 0,092 0,09 d** [Å] Ethene Prop-2-en-1-ol 2-Methylbut-3-en -2-ol Hex-1-en-3-ol Hept-1-en-4-ol

9 Energetics of the Surface Complex TS 1 Energy Profile of Hydrogenation E A1 TS 2 TS 3 H ads E A2 E A3 CH 2 =CHR + 2* H H H R * * H 2 CH 3 -CH 2 R (110) Freely exposed (111) crystallographic planes (100) H ads = E complex - (E metal + E olefin ) Organometallic compound Surface complex

10 Adsorption vs. Dissociation Energies E ads [kj/mol] E diss [kj/mol] K rel [-] Ethene Prop-2-en-1-ol Methylbut-3-en-2-ol Hex-1-en-3-ol Hept-1-en-4-ol ,0 20,0 E [kj/mol] 200,0 150,0 100,0 15,0 10,0 K [-] H 2 / Pt O 50,0 5,0 E ads E diss K rel 0,0 E(ads) [kj/mol] E(diss) [kj/mol] K(rel) [-] adsorption enthalpy of the di-σ adsorbate at c(3x3) surface of platinum Pt(111) cell dissociation energy of the di-σ bond in (olefin)pt(ph 3 ) 2 complex relative adsorption coefficient of the adsorptive with respect to standard on 5%-Pt/C catalyst 0,0

11 Impact of the Level of Theory on the Calculation Accuracy Coordinated olefin Ethene Prop-2-en-1-ol 2-Methylbut-3-en-2-ol Hex-1-en-3-ol Hept-1-en-4-ol Adsorption / dissociation energies E diss [kj/mol] E diss [kj/mol] E ads [kj/mol] B3LYP / 6-31G(d) MP2 / 6-31G(d) GGA-PAW / Γ-point , , E ads [kj/mol] GGA-PAW / k(2x2x1) Stretch-vibrational wavenumber of the coordinated C=C bond Coordinated olefin Ethene Prop-2-en-1-ol 2-Methylbut-3-en-2-ol Hex-1-en-3-ol Hept-1-en-4-ol ν(c=c) [cm -1 ] B3LYP / 6-31G(d) ν(c=c) [cm -1 ] GGA-PAW / Γ-point

12 Correlation of the Applied Methods Adsorption vs. dissociation energies Coordinated olefin E diss [kj/mol] E ads [kj/mol] MP2 / 6-31G(d) GGA-PAW / k(2x2x1) Ethene Prop-2-en-1-ol Methylbut-3-en-2-ol Hex-1-en-3-ol Hept-1-en-4-ol Dis o ciační vs. ads o rpční energie Dissociation vs. adsorption energy H 2 / Pt O E ads E dis Dissociation Disociační vs. adsorption adsorpční en. en. Linear Lineární regression regrese R 2 = 0,6209 Solution: Correlation coefficient after neglection of allylalcohol: R 2 =

13 Electronic density profile

14 Conclusions Geometries of adsorbed structures of unsaturated compounds on Pt(111) surface were described. Enthalpy changes of the chemisorption reactions were evaluated. Group of organometallics, suitable for good approximative description of surface adsorbates was found. Raising the level of theory more reliable geometries of adsorbates and adsorption enthalpy values. Applied methods may enable to predict the chemisorption of a mixture of substances and estimate the selectivity of the catalytic reaction. Calculation of entire reaction heats, activation energies and IR vibrational spectra rate constants real reaction kinetics. Comparison with experimental data formulation of LFER equation (Linear Free Energy Relationship).

15 Acknowledgements My Supervisors and Colleagues: Ing. Petr Kačer, PhD. Ing. Marek Kuzma Prof. Ing. Libor Červený, DrSc. O. Univ. Prof. Dipl.-Ing. Dr. Jürgen Hafner V.-Ass. Dipl.-Ing. Dr. Florian Mittendorfer