bae 820 physical principles of environmental systems

Biological and Agricultural Engineering

BAE 820 Physical Principles of Environmental Systems

Catalysis of environmental reactions

Dr. Zifei Liu


Catalysis and catalysts

2

• Catalysis is the increase in the rate of a chemical reaction due to the

participation of an additional substance called a catalyst. With a catalyst,

reactions occur faster and with less energy.

• Catalyst has been defined as a substance that changes the rate of reaction

but that is not itself consumed in the process. In fact, catalysts

participate in a reaction, but are eventually regenerated in the system

such that there is no net concentration change.

• Catalyzed reactions are prevalent in the natural environment and

pollution prevention processes. Most industrial reactions are run with

catalysts. Over 90% of chemical and petroleum products are made via

catalytic processes.


Reactivity and selectivity

3

• Sometimes the selectivity to form a desired product is more

important than reactivity; a catalyst can be used provide a lower

barrier for the desired reaction, leaving the undesired reaction

rate unchanged.

• The search for this active and selective catalyst in biology or in

chemical synthesis is perhaps the central goal in most biological

and physical sciences research.


Homogeneous and heterogeneous catalysis

4

• Homogeneous catalysis:

– The catalysts are in the same phase as the reactants.

• Heterogeneous catalysis:

– The catalysts are in a separate phase. They act at the

boundary of the phase of reactants (usually a gas or liquid

solution) to promote reactions (e.g. solid particles in water).

– Most surface reactions belong to the heterogeneous catalysis.


Examples of homogeneous and heterogeneous catalysis in environmental engineering

5

Reaction Application Catalysis type

Enzyme-catalyzed biodegradationAquatic, soil chemistry,

waste treatmentHomogeneous

Ozone destruction in gas phase Atmospheric chemistry Homogeneous

Hydroxylation of N2O over zeolite

catalystsRemoval of N2O Heterogeneous

Oxidation of organics in water on

TiO2

Removal of organic

pollutantsHeterogeneous

NOx formation in combustion Atmospheric chemistryHomogeneous and

heterogeneous

Acid and base hydrolysis of

pesticides and esters

Aquatic, soil and

sediment chemistry

Homogeneous and

heterogeneous


General rate expressions for catalyzed reactions

6

• Consider reactant A and catalyst X. The first step is the formation of

a complex Z. The complex Z further reacts with another reactant C

to give desired products and regenerate the catalyst X.

A(reactant)+X(catalyst) Z(complex)+B

Z(complex)+C products + X(catalyst)

• The overall equilibrium constant for the formation of a complex Z is

Keq=𝑘𝑓

𝑘𝑏

= 𝐶𝑍𝐶𝐵

𝐶𝑋𝐶𝐴

=𝐶𝑍𝐶𝐵

(𝐶𝑋0

−𝐶

𝑧)(𝐶

𝐴0−𝐶

𝑧)

• The rate of product formation is proportional to the concentration of

the complex Z.

-r = k2CZCC

k2


When the reactant is in large excess

7

• When CA0>>CX0, CA≈CA0,

Keq ≈ 𝐶𝑍𝐶𝐵

𝐶𝑋0

−𝐶

𝑧𝐶𝐴0

Cz ≈ 𝐾𝑒𝑞𝐶𝑋0𝐶𝐴0

𝐾𝑒𝑞𝐶𝐴0

+𝐶𝐵

If 𝐾𝑒𝑞𝐶𝐴0 ≫𝐶𝐵,


𝐾𝑒𝑞𝐶𝐴0

= 𝐶𝑋0

-r = k2CZCC ≈ k2CX0CC

In this case, the rate of product formation is linear to the

concentration of the catalyst and independent of CA0.


When the catalyst is in large excess

8

• When CA0<<CX0, CX≈CX0,

Keq ≈ 𝐶𝑍𝐶𝐵

𝐶𝐴0

−𝐶

𝑧𝐶𝑋0


𝐾𝑒𝑞𝐶𝑋0+𝐶

𝐵

If 𝐾𝑒𝑞𝐶𝑋0 ≫ 𝐶𝐵,


𝐾𝑒𝑞𝐶𝑋0

= 𝐶𝐴0

-r = k2CZCC ≈ k2CA0CC

In this case, the rate of product formation is linear to CA0.


Catalysis and activation energy

9

• Catalyzed reactions have a lower activation energy (rate-limiting free energy of

activation) than the corresponding uncatalyzed reaction, resulting in a higher

reaction rate at the same temperature and for the same reactant concentrations.


Rate of enhancement with catalysts

10

• Consider the decomposition of hydrogen peroxide in the aqueous

phase. Under normal condition, the activation energy is 76 kJ/mol.

Presence of bromide can reduce it to 57 kJ/mol. The enhancement

in rate at 298K can be estimated from

𝑘1

𝑘2

= exp(−

𝐸1

𝑅𝑇)

exp(−𝐸2

𝑅𝑇)

=exp(−

57

𝑅𝑇)

exp(−76

𝑅𝑇)

= 2140

Reaction Catalyst

Ea

uncatalyzed

Kcal/mol

Ea

catalyzed

Kcal/mol

Rate of

enhancement

calculated at 500K

H2+I22HI Pt 44 14 1013

2N2O2N2+O2 Au 58 29 1013


Catalysis and reaction temperature

11

• From a practical standpoint, one of the key roles of a catalyst is

to lower the temperature required for a reaction.

• E.g. a reaction has an activation barrier of 53 kcal/mol in the

absence of a catalyst, and 38 kcal/mol in the absence of a

catalyst. With an activation barrier of 53 kcal/mol you need to

run the reaction at about 800K, while with an activation barrier

of 38 kcal/mol you can run the reaction at about 600K.

Tminute = Ea15 K mol/kcal

In which, Tminute is the temperature needed for a reaction to get a

half-life of about one minute.


Acids or bases catalysis

12

• The most prevalent reactions in the environment are catalyzed by

acids or bases.

• If a catalyzed reaction is carried out at a high enough [H+] such

that [OH-] is negligible, the rate of reaction will be directly

proportional to [H+] and CA.

-r= kH[H+]CA

• Similarly, the rate of a base-catalyzed reaction is given by

-r= kOH [OH-]CA

• The overall rate of an acid-base catalyzed reaction is therefore

-r = (k0+kH[H+]+ kOH [OH-])CA


Enzymes

13

• Enzymes are a very important class of homogeneous catalysts. Action

of enzymes is an important aspect of environmental bioengineering.

• Enzymes are complicated proteins. They are usually classified

according to what they do rather than how they are constructed.– Oxidoreductasea are enzymes that promote oxidation-reduction reactions

– Transferases are enzymes that promote transfer of functional groups from

a donor molecule to a acceptor molecule.

– Hydrolases are enzymes that promote hydrolysis reactions.

• Enzymes work principally by four routes:– Bind to the reactants in a way that key bonds in the reactants are stretched.

That makes bonds easier to break.

– Lower the energy of the transition state.

– Stabilize key intermediates.

– Push the reactants together for a bond-forming reaction.


Heterogeneous catalysis (surface reactions)

14

• Many reactions in environment occur at the surfaces of either

solids or liquids, and they are significantly influenced by the

nature and property of the surface.

• Particulate-mediated catalysis plays a large role in many

atmospheric photochemical reactions.

• Removal of VOCs from automobile exhaust involves the use of

sophisticated catalysts.

• Many waste treatment processes rely on reactions as surfaces.


General mechanisms of surface catalysis

15

• A surface reaction involves a series of successive steps. The first

step is bulk diffusion of reactant, which is generally fast. Then

the process of adsorption, reaction, and desorption is usually

considered a single rate-limiting step.

• A heterogeneous surface reaction mechanism involves

postulating a surface-adsorbed molecule which further becomes

aa activated complex that then breaks down to give the products.

A(reactant)+X(surface) Z(complex)

Z(complex) products + X

A

PZ

P

Adsorption

Reaction

Desorption


Reaction rate of surface catalysis

16

• The rate of conversion of the adsorbed complex to products is

-r= kS0θA = kS0

𝐾𝐿𝐶𝐴

1+𝐾𝐿𝐶

𝐴

In which, θA is surface coverage of the reactant A, which is

estimated using the Langmuir isotherm; KL is Langmuir adsorption

constant; S0 is the total binding sites available on the surface.

• At high reactant concentration, KLCA>>1, -rkS0 and is

independent of the concentration of A.

• At low reactant concentration, KLCA<<1, -rkS0KLCA and the

rate is first order in A.


Two competing species

17

• Consider the Langmuir isotherm for two competing species A and B.

• If only A reacts, and B is nonreactive, it can act as an inhibitor since it

reduces the surface coverage of A.

-r= kS0θA = kS0

𝐾𝐿,𝐴𝐶𝐴

1+𝐾𝐿,𝐴𝐶𝐴+𝐾

𝐿,𝐵𝐶𝐵

If KL,BCB>>1+ KL,ACA,

-r ≈ kS0

𝐾𝐿,𝐴𝐶𝐴

𝐾𝐿,𝐵𝐶𝐵

The rate is first order in A and inversely proportional to concentration of B.

• If Both A and B react,

-r= kS0θAθB = kS0

𝐾𝐿,𝐴𝐾𝐿,𝐵𝐶𝐴𝐶𝐵

(1+𝐾𝐿,𝐴𝐶𝐴+𝐾

𝐿,𝐵𝐶𝐵)2

The equation indicates the competition for surface sites between A and B. If CA

is held constant, the rate will go through a maximum as CB is varied.


Heterogeneous catalysts

18

• Most heterogeneous catalysts are solids that act on substrates in a

liquid or gaseous reaction mixture. The total surface area of solid

has an important effect on the reaction rate. The smaller the

catalyst particle size, the larger the surface area for a given mass

of particles.

• Heterogeneous catalysts are very important in industry.

Heterogeneous catalysts are generally powders or pellets that one

can add to a reacting mixture to speed up the reaction. They are

used extensively in chemical processing because heterogeneous

catalysts are easier to separate from the products of a reaction

mixture than a homogeneous catalyst.


Surface reaction rate

19

• The rate of a heterogeneously catalyzed reaction is usually proportional

to the surface area of the catalysts. The reaction rate scales as the surface

area of the catalyst, not the volume of the reactor. So it is common to

express the rate in units of mol per unit time per surface area.

r = 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎

𝑣𝑜𝑙𝑢𝑚𝑒r′

In which r′ is the surface reaction rate in units of mol per unit time per

surface area. r is a “pseudohomogeneous rate” in units of mol per unit time

per volume.

𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎

𝑣𝑜𝑙𝑢𝑚𝑒= S g ρ(1 − ε)

In which Sg is the surface area per unit weight of catalyst; ρ is density of the

catalysts; ε is porosity, which is given by ε = void volume/total volume


Surface area of catalysts

20

• The highest possible area is desired to attain the highest rate with

minimum total reactor volume. This is usually achieved by

formulating the catalyst as powder, which is then pressed into

porous pellets that are packed into the packed bed reactor.

• A number of special materials (e.g. activated carbon) have been

developed to squeeze as much surface area as possible in a

certain volume. It is possible to squeeze the surface area of 5000

m2 (about the area of a football field) into 100 cm3 or less of

material!


Supports of catalysts

21

• Heterogeneous catalysts are typically "supported," which means

that the catalyst is dispersed on a second material that enhances

the effectiveness or minimizes their cost.

• Supports prevent or reduce agglomeration of the small catalyst

particles, exposing more surface area, thus catalysts have a higher

specific activity (per gram) on a support.

• Support can be merely a surface on which the catalyst is spread to

increase the surface area.

• Supports can be porous materials with a high surface area, most

commonly alumina, zeolites or various kinds of activated carbon.

Specialized supports include silicon dioxide, titanium dioxide,

calcium carbonate, and barium sulfate.


Catalytic reactors

22

• Packed bed reactor:

– This is typically a tank or tube filled with catalyst pellets with

reactants entering at one end and products leaving at the other

end.

• Slurry reactor and fluidized bed reactor

– The fluid and the catalyst are stirred instead of having the

catalyst fixed in a bed. The stirring must be fast enough to mix

the fluid and particles.

– It is called a slurry reactor if the fluid is a liquid.

– It is called a fluidized bed reactor if the fluid is a gas and it

flows such that the particles are lifted and gas and particles

swirl around the reactor.


Limitations of catalysts

23

• Catalysts do not work over a wide range of conditions. At very high

temperature, catalysts may slow down the reaction by promoting termination

reactions.

• The effect of a catalyst may vary due to the presence of other substances known

as inhibitors (if reversible) or poisons (if irreversible) which reduce the

catalytic activity. The opposite of a catalyst, a substance that reduces the rate of

a reaction, is an inhibitor.

• Although catalysts are not consumed by the reaction itself, they may be

inhibited, deactivated, or destroyed by secondary processes. In heterogeneous

catalysis, typical secondary processes include coking where the catalyst

becomes covered by polymeric side products. Additionally, heterogeneous

catalysts can dissolve into the solution in a solid–liquid system or sublimate in

a solid–gas system.

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