2.2 Heterogeneous catalysis

Question 1

What should the properties of a catalyst be?

Answer 1

-increase reaction rate -catalytic amounts- reused/regenerated -selectivity -non toxicity -low cost -incorporability into industry -stability/ lifetime

Question 2

What are the two types of catalysis and describe

Answer 2

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)

Question 3

3 typical forms of heterogeneous catalysis

Answer 3

1) porous material (eg zeolites) 2) surface (eg flat noble metals) 3) supported (eg graphene supporting nanoparticles)

Question 4

advantages of homo

Answer 4

higher reaction rates High selectivity Optimal use of expensive catalysts

Question 5

advantages of hetero

Answer 5

Facile isolation of product Catalyst easy to recycle or recover Can be non toxic Easier to control/handle

Question 6

disadvantages of homo

Answer 6

Difficult to separate- leads to contamination cost of ligands- extra expense stability of catalyst can be poor under reaction conditions

Question 7

disadvantages of hetero

Answer 7

Low reaction rates low selectivities link between catalyst structure and cat activity complex Sub optimal use of expensive metal

Question 8

What are the steps to gas phase heterogeneous analysis?

Answer 8

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

Question 9

Extra steps in heterogeneous gas phase for porous materials

Answer 9

First step- reactants diffuse into pores last step products diffuse out of pores

Question 10

How does the surface effect the catalytic activity

Answer 10

surface miller indicies and surface shape and defects all contribute to catalytic activity steps, ledges and kinks create better catalyst sites

Question 11

What is the Haber Bosch basic equation? Why is it essential? 

Answer 11

N2 + 3H2 ⇌ 3NH3 production of ammonia ammonia essential in fertiliser- feed the world and allows modern agricultural yields

Question 12

Haber Bosch: Conditions and why

Answer 12

conditions= 650K and 200 bar strongly exothermic so eqm favours low T kinetic barrier v high pressure good solution but expensive

Question 13

Catalyst system for Haber Bosch

Answer 13

Fe3O4 - active catalyst reduced in process Al2O3- structural promoter - increase SA CaO - structual promoter increase SA K2O chemical - ?maybe affects N binding energy? FACK

Question 14

Mechanism of the Haber-Bosch

Answer 14

1. 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)

Question 15

Synthesis of Methanol equation

Answer 15

CO2 +3H2 ⇌ 2CH3OH 2nd most produced after nh3

Question 16

Synthesis conditions of Methanol

Answer 16

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

Question 17

Mechanism for synthesis of methanol

Answer 17

Question 18

What is an active site

Answer 18

The region or specific area of the catalyst where the reactant molecule binds and a temporary bond is formed

Question 19

What is Auto-exhaust catalysis?

Answer 19

Noble metals eg Pt used to prevent harmful emissions from automobiles

Question 20

What does the Pt catalyse? (auto exhaust)?

Answer 20

Oxidation of CO -> CO2 Oxidation of hydrocarbons _> CO2 and H2O Reduction of NOx

Question 21

Autoexhaust - conditions are v important for this catalyiss one in particular

Answer 21

Stoichiometric fuel/air ratio High fuel ratio- reduction reactions promoted High oxygen ratio -oxidation reactions promoted must find a compromise or “sweet spot”

Question 22

Platinum surgace restructuring can occur

Answer 22

between faceted- (111) miller plane exposed and rounded (211) miller plane exposef rounded more undercoordinated and hence higher activity