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Mechanisms in heterogeneous catalysis. Langmuir-Hinshelwood. Rideal-Eley. Adsorption. Migration. Desorption. Surface reaction ...
Typology: Exercises
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Homogeneous x Heterogeneous Catalysts Homogeneous Heterogeneous Active All atoms Surface atoms sites Concentration Low High (variable) Diffusion disguises No Important Reaction conditions 50 - 200 oC 200 - 1000 oC Application Limited Large Characterization Structure, composition Well defined No clear defintion Modification Easy Difficult Temperature stability Low High Separation Difficult Easy packed beds Recycling Feasible Feasible
Steps in a catalytic reaction
Fluid phase Products Reactants Desorption Surface reaction Adsorption Dissociation Migration Elementary steps of catalytic reaction
Adsorption x Chemisorption Energy of biatomic molecule A 2 Adsorption (physical phenomenon) Chemisorption (chemical phenomenon) Distance from catalyst surface Emc, Emp – activation energy of migration in adsorbed and chemisorbed state ED – energy of dissociation of molecule A 2 Ed – energy of desorption of A 2 Ea – energy of activation of transition from adsorbrd to chemisorbed state 2 A 2[] 2 A * A 2^ 2[]^2 A *
Adsorption x Chemisorption Adsorption Chemisorption Principal van der Waals forces covalent or ionic bonds no electron transfer! electron transfer Adsorbent all solids specific sites Adsorbat gases T < Tc reactive components Temperature low higher Enthalpy 10 - 40 kJ/mol 80 - 600 kJ/mol Rate high depends on T Activation energy low high Occupancy multilayer monolayer Reversibility YES YES but … Use BET method surface concentration pore size distribution of active sites
Irving Langmuir 1920 - adsorption isotherm
Langmuir adsorption isotherm Brunauer-Emmett-Teller (BET) adsorption isotherm
Asociative chemisorption Langmuir adsorption (chemisorption) isotherm
Multicomponent associative chemisorption on ideal catalytic surface A(g) + * A*
B(g) + *
Occupancy of A A A B B B B B 1 K .p K .p K .p Θ A A B B A A A 1 K .p K .p K .p Θ Occupancy of B KA = ka / kd KB = kb / kd Irreversible chemisorption catalyst poisoning
