2.2 Heterogeneous catalysis Flashcards
(22 cards)
What should the properties of a catalyst be?
-increase reaction rate -catalytic amounts- reused/regenerated -selectivity -non toxicity -low cost -incorporability into industry -stability/ lifetime
What are the two types of catalysis and describe
Homogeneous and heterogeneous Homo- cat and substrate are in same phase (eg. both aqeous) Hetero- cat and substrate are in different phases (eg. solid cat with liq/gas substrate)
3 typical forms of heterogeneous catalysis
1) porous material (eg zeolites) 2) surface (eg flat noble metals) 3) supported (eg graphene supporting nanoparticles)
advantages of homo
higher reaction rates High selectivity Optimal use of expensive catalysts
advantages of hetero
Facile isolation of product Catalyst easy to recycle or recover Can be non toxic Easier to control/handle
disadvantages of homo
Difficult to separate- leads to contamination cost of ligands- extra expense stability of catalyst can be poor under reaction conditions
disadvantages of hetero
Low reaction rates low selectivities link between catalyst structure and cat activity complex Sub optimal use of expensive metal
What are the steps to gas phase heterogeneous analysis?
1.ADSORPTION of reagents onto surface of catalyst 2.DIFFUSION of reagents along surface to an active site or other reactants 3.REACTION 4.DIFFUSION of product along the surface to a point where desorption is possible 5.DESORPTION of products and removal from surface
Extra steps in heterogeneous gas phase for porous materials
First step- reactants diffuse into pores last step products diffuse out of pores
How does the surface effect the catalytic activity
surface miller indicies and surface shape and defects all contribute to catalytic activity steps, ledges and kinks create better catalyst sites
What is the Haber Bosch basic equation? Why is it essential?
N2 + 3H2 ⇌ 3NH3 production of ammonia ammonia essential in fertiliser- feed the world and allows modern agricultural yields
Haber Bosch: Conditions and why
conditions= 650K and 200 bar strongly exothermic so eqm favours low T kinetic barrier v high pressure good solution but expensive
Catalyst system for Haber Bosch
Fe3O4 - active catalyst reduced in process Al2O3- structural promoter - increase SA CaO - structual promoter increase SA K2O chemical - ?maybe affects N binding energy? FACK
Mechanism of the Haber-Bosch
- ABSORBTION and LYSIS -N2(g) -> N2(ads) N2(ads)-> 2N(ads) (RDS) -H2(g) -> H2(ads) H2(ads)-> 2H(ads) 2.REACTION- COMBINATION -N+H-> NH NH+H->NH2 NH2+H->NH3 (all with (ads)) 3. DESORPTION NH3(ads)->NH3(g)
Synthesis of Methanol equation
CO2 +3H2 ⇌ 2CH3OH 2nd most produced after nh3
Synthesis conditions of Methanol
SYN gas ( H2 CO2 and CO mix) 600K 50-100bar Catalyst -CAZ Cu - main catalyst Al2O3-structural promoter ZnO -weakly catalytic-maybe structural Cu and ZnO nanoparticles on porous surface with 10% Al2O3 - high SA
Mechanism for synthesis of methanol
What is an active site
The region or specific area of the catalyst where the reactant molecule binds and a temporary bond is formed
What is Auto-exhaust catalysis?
Noble metals eg Pt used to prevent harmful emissions from automobiles
What does the Pt catalyse? (auto exhaust)?
Oxidation of CO -> CO2 Oxidation of hydrocarbons _> CO2 and H2O Reduction of NOx
Autoexhaust - conditions are v important for this catalyiss one in particular
Stoichiometric fuel/air ratio High fuel ratio- reduction reactions promoted High oxygen ratio -oxidation reactions promoted must find a compromise or “sweet spot”
Platinum surgace restructuring can occur
between faceted- (111) miller plane exposed and rounded (211) miller plane exposef rounded more undercoordinated and hence higher activity
