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CATALYSIS AND CATALYTIC REACTION MECHANISM PART 1. BY PUAN AZDUWIN KHASRI 26 NOVEMBER 2012. CATALYST-DEFINITION. A catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged.

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catalysis and catalytic reaction mechanism part 1

CATALYSIS AND CATALYTIC REACTION MECHANISMPART 1

BY PUAN AZDUWIN KHASRI

26 NOVEMBER 2012

catalyst definition
CATALYST-DEFINITION
  • A catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged.
  • A catalyst usually changes a reaction rate by promoting a different molecular path ("mechanism") for the reaction.
  • Catalyst affect yield and selectivity
  • Changes only the rate of reaction; it does not affect the equlibrium.
catalyst example
CATALYST-EXAMPLE

Example- H2 and O2 to form water; with Platinum as catalyst

slide4

CATALYST-TYPE

HOMOGENEOUS

HETEROGENEOUS

  • In solution with at least one of the reactants
  • Example;

0xo process for manufacturing normal isobutylaldehyde

  • more than one phase – typical solid catalyst in liquid/gaseous reactants
  • Example;

The dehydrogenation of cyclohexane

catalyst properties
CATALYST-PROPERTIES
  • Porous-Catalyst with large area resulting from pores
  • Molecular sieve - Small pores
  • Monolithic- can be either porous or nonporous
  • Supported-consists of minute particles of an active material dispersed over a less active substance.
    • Unsupported- mainly promoters that increase activity
steps in catalytic reaction
Steps in Catalytic Reaction

External diffusion

External diffusion

Internal diffusion

Internal diffusion

Adsorption

Desorption

Surface reaction

slide7

Steps in Catalytic Reaction

  • Mass transfer (difussion) of the reactant from the bulk to the external transport.
  • Internal diffusion
  • Adsorption
  • Reaction on the surface
  • Desorption
  • Diffusion of the reactant
  • Mass transfer of the product

* Steps 3, 4 and 5 are MOST important

slide10

STEP 3:Adsorption Isotherms

A atom,molecule,or atomic combination

One unit of species A will be adsorbed on the site S

S

Active site

slide11

STEP 3:Adsorption Isotherms

SITE BALANCE:

Ct =total concentration of sites

Cv= Vacant site

slide12

STEP 3:Adsorption Isotherms

MODEL 1

MOLECULAR

(NONDISSOCIATED ADSORPTION)

CO is adsorbed as molecules CO

MODEL 2

DISSOCIATIVE ADSORPTION

CO is adsorbed as C and O

LANGMUIR ISOTHERM

slide13

STEP 4:SURFACE REACTION

SINGLE SITE

DUAL SITE

ELEY-RIDEAL

slide14

STEP 5:DESORPTION

The rate of desorption of C

the rate limiting step
The Rate-Limiting Step
  • When heterogeneous reactions are carried out at steady state. The rates of each of the three reaction steps in series (adsorption, surface reaction and desorption) are equal to one another:
  • One particular step in the series is usually found to be rate-limiting or rate-controlling.
synthesizing rate law mechanism and rate limiting step
Synthesizing Rate law, Mechanism and Rate Limiting step

EXAMPLE: Cumene decomposition

CUMENE (C)

BENZENE (B) + PROPYLENE (P)

slide18

EXERCISE 1

Synthesize the rate law for each mechanism;

ADSORPTION

SURFACE REACTION

DESORPTION

algorithm for determining rxn mechanism and rate limiting step
ALGORITHM FOR DETERMINING RXN MECHANISM AND RATE LIMITING STEP

Select a mechanism

Assume a rate limiting step:

Choose the surface rxn first

Find the expression for concentration of adsorbed species.

Write a site balance

Derive the rate law

Compare with data

slide20

Is the Adsorption of Cumene Rate-Limiting?

Rate of adsorption;

Assume adsorption is rate-limiting, thus;

kA is small and ks and kD are large

The ratio very small ~approximately zero;

slide21

Surface reaction

Desorption

Rearrange;

slide23

To eliminate Cv ;

Cumenerxn rate law if

adsorption are limiting step

slide24

Initially, no products are present; consequently,

PP = PB = 0.

If the cumene decomposition is adsorption rate limited, the initial rate will be linear with the initial partial pressure of cumene

is the surface reaction rate limiting
Is the Surface Reaction Rate-Limiting?

Rate of surface reaction;

Assume surface reaction is rate-limiting, thus;

kS is small and kA and kD are large

Cumenerxn rate law if

Surface reaction are limiting step

The initial rate;

is the desorption of benzene rate limiting
Is the Desorption of Benzene Rate-Limiting?

Rate of desorption of benzene;

Assume desorption is rate-limiting, thus;

kD is small and kA and kS are large

Cumenerxn rate law if

desorption are limiting step

The initial rate;

summary of the cumene decomposition
SUMMARY OF THE CUMENE DECOMPOSITION

Surface limited reaction mechanism is consistent with experimental data