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Chemisorption: Active Sites

Chemisorption: Active Sites. A chemical reaction takes place: chemical bonds are formed Driving force: minimisation of Gibbs Free Energy (50 - 400 kJ/mol). Characterisation of surfaces: Specific surface area of phases Types of active sites Number of active sites Reactivity of active sites

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Chemisorption: Active Sites

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  1. Chemisorption: Active Sites A chemical reaction takes place: chemical bonds are formed Driving force: minimisation of Gibbs Free Energy (50 - 400 kJ/mol) Characterisation of surfaces: • Specific surface area of phases • Types of active sites • Number of active sites • Reactivity of active sites • Stability of active sites Chemisorption important for: • Adsorption process • Heterogeneous catalysis • Reduction of ores • Chemical Vapour Deposition • Coal gasification and combustion • Corrosion Catalysis and Catalysts - Chemisorption: Active Sites

  2. Metal Dispersion D ns = number of surface atoms nT = total number of atoms • Dispersion: • Chemisorption: titration of surface sites number of moles in monolayer nads Stoichiometry ?? ns p Catalysis and Catalysts - Chemisorption: Active Sites

  3. O O O O O C C O C C C C Various Modes of Adsorption a. b. c. d. e. a. linear or terminal (X = 1) b. bridged (X = 0.5) c. bridged (X = 0.67) d. valley or triple (X = 0.33) e. dissociative adsorption (X = 0.5) X = average number of adsorbed molecules per active site Catalysis and Catalysts - Chemisorption: Active Sites

  4. Crystallographic Planes Coordination number: 8 7 9 Catalysis and Catalysts - Chemisorption: Active Sites

  5. Adsorption Stoichiometry Catalysis and Catalysts - Chemisorption: Active Sites

  6. d VS ( nm) Pt Ni D Volume-Surface Mean Diameter dVS 15 10 5 0 0.0 0.5 1.0 D most fundamental parameter dVS most convenient for measuring directly (XRD, EM) Catalysis and Catalysts - Chemisorption: Active Sites

  7. Dispersion D versus Diameter d • Diameter d can be measured or calculated from several techniques • Electron Microscopy • X-ray Diffraction, X-ray Photoelectron Spectroscopy • Relation D and d (for spheres) ?? Catalysis and Catalysts - Chemisorption: Active Sites

  8. Number of Surface Atoms per Unit Area Dependent on particle size Catalysis and Catalysts - Chemisorption: Active Sites

  9. a. b. c. d. Shapes of Catalyst Particles on a Support poisoned part of surface Spherical Hemispherical Crystallite Complete wetting Catalysis and Catalysts - Chemisorption: Active Sites

  10. Chemisorption - Experimental Techniques • Gravimetry • Volumetry • Spectroscopy (Infrared, Raman) • Pulse techniques • Temperature Programmed Desorption (TPD) Gravimetry Catalysis and Catalysts - Chemisorption: Active Sites

  11. 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0  W (mg O/g Cu) 0 50 100 150 200 250 300 Time (min) Chemisorption of N2O on Cu Catalyst 2 Cu(s) + N2O Cu2O(s) + N2 Catalysis and Catalysts - Chemisorption: Active Sites

  12. 0.3 S = 12 m2/g Ni Total chemisorption D = 18 % Ni 0.2 Strong chemisorption nad (mmol/g) 0.1 Weak chemisorption 0 0 20 40 60 80 100 p (kPa) H2 Chemisorption on Ni/Al2O3 (323 K) after high-T evacuation Irreversible Reversible after subsequent low-T evacuation Catalysis and Catalysts - Chemisorption: Active Sites

  13. Results of Barometric H2 Chemisorption Catalysis and Catalysts - Chemisorption: Active Sites

  14. Pulse-Response Method Catalyst Detector CO Pulse Response Example: Ptsurface + CO Pt-CO Difference in total peak area nsurface Catalysis and Catalysts - Chemisorption: Active Sites

  15. 1.0 Detector signal 0.0 0 1 Time of analysis Pulse-Response Method CO chemisorption on reduced 5wt% Pt/Al2O3 TCD signals after CO pulses Catalysis and Catalysts - Chemisorption: Active Sites

  16. 0.08 0.06 SPt = 3 m2/g DPt = 24 % 2 S = 3 m /g (mmol/g) Pt 0.04 D = 24 % Pt ad n 0.02 0.00 0 0.5 1 1.5 Pulsed volume (ml) Pulse-Response Method CO chemisorption on reduced 5wt% Pt/Al2O3 Cumulative amount of chemisorbed CO Monolayer capacity: 0.06 mmol / g Pt Catalysis and Catalysts - Chemisorption: Active Sites

  17. Step-Response Method Example: 2 Cu(s) + N2O Cu2O(s) + N2 Mass Spectrometer Catalyst N2O N2 N2O t Step Response Catalysis and Catalysts - Chemisorption: Active Sites

  18. N N O 2 2 6 4 .) atm Concentration of gas-phase species (% 2 Dead time 0 t 0 Flow 0 300 600 900 Flow switch in switch out Time (s) Step-Response Method Quantification of metallic Cu surface area of reduced Cu-ZnO/Al2O3 catalyst by N2O chemisorption: 2 Cu(s) + N2O Cu2O(s) + N2 Catalysis and Catalysts - Chemisorption: Active Sites

  19. Temperature Programmed Desorption (TPD)Example: NH3 Desorption from H-ZSM-5 Weak acid sites Strong acid sites Catalysis and Catalysts - Chemisorption: Active Sites

  20. Silicalite 400 600 800 1000 Catalysis and Catalysts - Chemisorption: Active Sites T (K)

  21. Silicalite 400 600 800 1000 Catalysis and Catalysts - Chemisorption: Active Sites T (K)

  22. SpectroscopyExample: IR Absorption of Pyridine on Zeolite HY Catalysis and Catalysts - Chemisorption: Active Sites

  23. Chemisorption Techniques • Most techniques are rather “blind” but quantitative • Transient techniques also give kinetic information • pulse response, step response • TPD (NH3 also acid strength) • “In-situ” techniques also give qualitative information: • in-situ infrared spectroscopy • simultaneous measurement of heat effects Catalysis and Catalysts - Chemisorption: Active Sites

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