Introduction to Environmental Catalysis
CM3103
Environmental Technology
M A Morris
Rm. 109
Summary
- introduction to catalysis
activation energies, surface chemistry
- concept of supported catalysts
meso and microporous supports, small metal particles and sintering
- car exhaust technology
NOx reduction, HC and CO oxidation
- methane combustion
low temperature oxidation
- nitrogen oxide catalysis
for de-NOx and N2O abatement
INTRODUCTION TO CATALYSIS
Definition of catalysis
A catalyst was defined by J. J. Berzelius in 1836 as a compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction.
‘Many bodies...... have the property of exerting on other bodies an action which is very different from chemical affinity. By means of this action they produce decomposition in bodies and form new compounds into the composition of which they do not enter. This new power, hitherto unknown, is common both in organic and inorganic nature.....I shall call it catalytic power. I shall also call catalysis the decomposition of bodies by this force.’
J J Berzelius, Edinburgh New Philosophical Journal, XXI (1836) 223
This definition allows for the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly lost.
However, the catalyst affects only the rate of the reaction, it changes neither the thermodynamics of the reaction nor the equilibrium composition.
E.g. in ammonia synthesis:
N2 + 3H2 → 2NH3
Thermodynamically the reaction is moving from more stable reactants to less stable products – endothermic.
However, the reaction occurs the energy change will be the same.
The thermodynamic equilibrium is maintained.
ΔG° = -RT ln K
Where K = equilibrium constant
The time taken to reach equilibrium for a catalysed reaction is less than for the non-catalysed system.
Since rate is determined by the activation energy barrier the kinetics must be altered by changes in the activation energy.
The major component of the activation energy is dissociation of the N2 and H2.
N2 → 2N. H2 → 2H.
The iron surface used to catalyse the reaction provides a means to dissociate the molecules without having to input the energy needed to break the bond.
Generally speaking the process is not efficient and many attempts are needed to bring about a successful reaction. In terms of below not many molecules are transferred from the physisorbed state to the chemisorbed state.
Q1. What is the most important of the following in defining the effectiveness of the catalyst?
A: The number of collisions the gas molecules make with the catalyst surface
B: The temperature of the gas molecules
C: The interaction energy of chemisorbed gas atoms
We can sketch the potential energy of a nitrogen atom approaching an iron surface. The activation energy is defined by the energy needed to dissociate a nitrogen molecule
Repulsion is largely due to electron cloud overlap.
The nitrogen is described as being chemisorbed – strongly attached to the surface.
As a nitrogen molecule approaches the surface it feels a weak attraction due to van der Waals interactions (oscillating dipole interactions). This can be sketched on the PE diagram above.
This leads to a very weak adsorbed state called physisorption. However, the physisorbed and chemisorbed states cross over below the energy zero. This allows the nitrogen molecules to dissociate without being ‘activated’.
The importance of catalysis
Catalysis is of crucial importance for the chemical industry, the number of catalysts applied in industry is very large and catalysts come in many different forms.
Heterogeneous catalysis – where the reactants, products and the catalysts are in different phases. Most commonly gas and solid. Commonly, the catalyst is a porous high surface area material supporting a active material.
Homogeneous catalysis where the active material is dissolved in the liquid reaction mixture. The catalyst may sponsor activity or be directly involved and reformed as a reaction product.
Biological catalysis via enzymes which involve complex series of reaction steps.
Q2: Which of the following is an heterogeneous reaction?
A: Reaction of liquid ammonia with liquid acetone over a silver catalyst
B: Reaction of liquid acetylene with liquid ammonia chloride
C: Reaction of indium oxide with tin oxide using an iron oxide catalysts Reaction mechanisms
Catalysts accelerate reactions by becoming part of the transition state activated complex. In gas solid heterogeneous catalysis the weakly adsorbed physisorbed state is the transition state.
Q3. Catalysts works by effecting:
a) the rate of reaction
b) the equilibrium of the reaction
c) both the rate and the equilibrium
d) neither the rate or the equilibrium
Q4. In a catalytic reaction the catalyst changes the reaction so as to:
a) Alter the free energy of the products
b) Alter the free energy of the reactants
c) Alter the free energy of the products and reactants
d) Alter neither the free energy of the products and reactants
]Limitations of the catalytic effect
Thermodynamics frequently limits the concentration of a desired product. This is not altered by the catalyst.
However, catalysis can be used to effectively increase the equilibrium constant. The reaction conditions (temperature, pressure and reactant composition) can be optimized to maximize the equilibrium concentration of the desired product.
Can write:
-ΔG°/RT = ln K
For an exothermic reaction ΔG° is negative. Thus as T increases K decrease. Thus the highest equilibrium constant is achieved at lowest temperatures. However, the rate may be vanishingly small. The catalyst may increase the rate at this temperature to workable. Thus, we have ‘traded’ catalyst rate for equilibrium concentration.
Catalyst mass. Catalyst activity – the ability to promote a reaction – is determined by the specific activity (chemical activity) and the amount of material present (the number of active sites). Thus, a catalyst with extremely high reaction rate will certainly exhibit large mass. The activity of a catalyst is defined by its surface area – the number of sites available for reaction with the gas phase. The easiest way to double the activity is to use twice as much catalyst. For large quantities of material thermal and mass transfer become important and these can be come rate limiting. In simple terms can’t feed solid into a pipe without expecting it to block!
Heterogeneous Catalysis
We shall exclusively deal with heterogeneous catalysis where gas molecules react at a surface to produce a product.
The surface plays three distinct roles in the reaction.
1. It lowers the energy barrier to dissociation of the reactants.
2. It acts to concentrate molecules (since the gas may be of low concentration and the surface saturated with material)
3. It can activate molecules. On collision the translational energy (kinetic) can be transferred to internal energies – electronic, vibrational and rotational. This is important in reactions of methane. These internal energies weaken bonds in the molecule.
Environmental Catalysis
Environmental catalysis has been recently defined as the development of catalysts to either decompose environmentally unacceptable compounds or provide alternative catalytic syntheses of important compounds without the formation of environmentally unacceptable by-products.
Catalysts
It is not enough to simply make an active material – it is also necessary to make it in a form that is thermally and mechanically robust enough to survive in a reactor for the time required. Catalyst lifetimes can be many years. Catalyst forms are determined by careful design in terms of the reaction. It involves very complex engineering. The catalyst is normally prepared to exhibit as large a surface area as is possible. This maximises the number of sites. Note that all sites on the surface might not be active – some merely act as adsorbent sites not the centres of reactivity. The reactive sites are known as ACTIVE SITES.
The physical shape and size of the catalyst is defined by practicality. Powders are not used because they can block pipes etc. The rest is engineering to maximise contact between the gas and the catalyst whilst reducing the pressure needed to move the gas through the REACTOR BED. Very careful consideration of the catalyst process is required.
The Catalyst Process.
1. Reactants are transferred from the gas flow towards the gas surface
Conditions (I.e. pressure, stirring, flow rates) are changed which allow gases to be mixed and to be move towards the catalyst surface. Imagine water going down a sink – not all of the gas contacts the walls.
2. Gas close to the surface enters the phsyisorbed state
3. Physisorbed molecules become chemisorbed
Not all of them do – often 1 in 10,000 attempts are successful
Implies many collisions with a surface can be required
The turnover number defines a number of collisions needed to produce one product molecule
4. Chemisorbed atoms diffuse across the surface to the active site
5. If right molecules are present and given success a product molecule is formed
6. The product molecule leaves active site and diffuses to a site where it may desorb (enter the gas phase) before it possible dissociates or reacts further
‘Contact time’ may be important in defining success
7. The molecule leaves the physisorbed state enters the gas phase close to the surface
8. Molecule is transferred into normal gas flow
Q5. What factors are critical in defining the collision rate (no of molecules per unit surface area per unit time)?
A) The gas pressure alone
B) The gas pressure and the surface area
C) The surface area
D) Neither are important
Q6. The turnover number is a measure of (choose a number of these):-
A) The surface area
B) The gas pressure
C) The number of active sites
D) The number of desorbing sites
Q7. The flow of gas (i.e. m3 s-1 or moles s-1) can determine the success of a catalytic process. We would expect the reaction effectiveness to increase as
A) The flow rate decreases
B) The flow rate increases
C) Neither
D) Depends on how effectiveness is defined flow will not effect the turnover number but will determine the collision rate to a large extent
Q8. A reaction is run at a certain temperature so that the energy defined by RT is equivalent to 100 kJ mol-1. The activation energy is measured at 200 kJ mol-1. If a catalyst is used in same conditions so that the activation energy is lowered to 100 kJ mol-1, the rate of reaction (assuming all terms are constant):-
A) increases by a factor of about 2.7
B) decreases by a factor of about 2.7
C) decreases by a factor of about 2.0
D) increases by a factor of about 2.0