Heterogeneous catalysis: the fundamentals Kinetics

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1 Heterogeneous catalysis: the fundamentals Kinetics Prof dr J W (Hans) Niemantsverdriet Schuit Institute of Catalysis

2 Catalysis is a cycle A B separation P catalyst P bonding catalyst A B reaction catalyst

3 Energy profile of a catalytic reaction non-catalytic reaction E non-catalytic energy barrier energy barrier reaction adsorption reaction coordinate desorption energy barrier of the catalytic route much lower!

4 How to translate the catalytic cycle into kinetics? relate surface concentration of adsorbed species to its gas phase pressure Langmuir Adsorption Isotherm express rate of reaction in terms of surface concentrations translate this to an expression in terms of gas phase partial pressures

5 Surface = collection of adsorption sites Sites are either occupied A or empty A Molecules and atoms adsorb on these sites A Fraction occupied: θ A All sites are equivalent A Adsorbates do not interact Catalyst surface with adsorbed species

6 Adsorption = equilibrium reaction A A A + * gas; partial pressure p A K A free sites; concentration: N s θ * A ads adsorbed; concentration: N s θ A with θ * + θ A = 1 A Equilibrium: K A = N s θ A p A N s θ * A or: θ A = K A p A θ * Langmuir adsorption isotherm: catalyst surface with N s sites fraction occupied by A ads is θ A θ A = K A p A 1+ K A p A

7 Langmuir Adsorption Isotherm θ A NB! Useful form: θ A = K A p A θ * θ B = K B p B θ * θ C = K C p C θ *..... θ A +θ B +θ C + +θ * = 1 Surface concentration as function of partial pressure

Langmuir - Hinshelwood Kinetics 1915 Langmuir: Adsorption Isotherm 1927 Hinshelwood: Kinetics of Catalytic Reactions Cyril Norman Hinshelwood 1897-1967 Nobel

8 Langmuir - Hinshelwood Kinetics 1915 Langmuir: Adsorption Isotherm 1927 Hinshelwood: Kinetics of Catalytic Reactions Cyril Norman Hinshelwood Nobel Prize 1956 Irving Langmuir Nobel Prize 1932 Consistent with Sabatier s Principle Coverage dependence: Volcano plot Temperature dependence: Volcano plot

9 Reaction Mechanism: A + * A ads equilibrium; K A B + * B ads equilibrium; K B A ads + B ads AB ads + * r.d.s; k AB ads AB + * fast Coverages: A = K A p A * B = K B p B * * = 1 1+K A p A +K B p B Reaction rate: r = N s k K AK B p A p B (1 + K A p A + K B p B ) 2

10 Rate of the catalytic reaction Pressure dependence: Let s do a simulation

11 , normalized rate Rate of a Catalytic Reaction: Pressure Dependence 1,2 1.0 rate 1, ,8 rate E ads (A) = E ads (B) = 125 kj/mol s(b) = s(a) ; E act = 50 kj/mol T = 600 K; p B is fixed 0.6 0,6 B A A 0.4 0, ,2 B 0.0 * 0,0 reaction order reaction order -1,0-0,8-0,6-0,4-0,2 0,0 0,2 0,4 0,6 0, positive in p A negative in p B negative in p A positive in p B p A /p B

12 Rate of the catalytic reaction Temperature dependence: Let s do a simulation

13 , normalized rate Rate of a Catalytic Reaction: Temperature Dependence E ads (A) E ads (B) s(b) p B E act = 135 kj/mol = 125 kj/mol = 10 s(a) p A = 50 kj/mol 1,0 0,8 A rate * 0,6 0,4 0,2 0,0 reaction order negative in p A positive in p B B reaction order positive in p A and p B T (K) right coverage at right temperature is the key!!!!!

14 Optima in rate at the optimal coverages partial pressures temperature: r r p A /p B Temperature and.. also an optimum in rate for the optimum metal adsorbate bond strength:

15 catalytic activity The Sabatier Effect optimum interaction catalyst - adsorbate: not too strong not too weak metal - adsorbate bond strength optimum coverage at right surface

16 The Sabatier Effect: Methanation CO + 3 H 2 = CH 4 + H 2 O optimum interaction catalyst - adsorbate: not too strong not too weak optimum coverage at right surface

17 The Sabatier Effect: interaction too strong Strong interaction catalyst - adsorbate: increases the activation energy for the surface reaction!!

18 Energy profile methanation on metals Ag Au Cu Pd, Pt Ru Fe W Ni, Rh Co Prediction: Ni, Rh, Co, perhaps Ru are good methanation catalysts G Jones, T Bligaard, F Abild-Pedersen, JK Norskov, J.Phys: Condens. Matter 20 (2008)

19 Trends in chemisorption Catalysis by Metals: Trends in Reactivity strong, dissociative adsorption stable oxides, carbides, nitrides Cr Mn Fe Co Ni Cu Mo Tc Ru Rh Pd Ag W Re Os Ir Pt Au Weak, molecular adsorption stable against oxide, carbide, nitride formation

20 Download free pdf handout and mp4 movie from: and read more in Concepts of Modern Catalysis and Kinetics, 2nd, Revised and Enlarged Edition Ib Chorkendorff and Hans Niemantsverdriet ISBN: Hardcover, 477 pages; August

Heterogeneous catalysis: the fundamentals

www.catalysiscourse.com Heterogeneous catalysis: the fundamentals Introduction Prof dr J W (Hans) Niemantsverdriet Schuit Institute of Catalysis What is Catalysis? a phenomenon in which a small quantity