Catalyst

CHM 1046
General Chemistry II
Dr. Michael Blaber

Chemical Kinetics

Catalysis

A catalyst is a substance that changes the speed of a chemical reaction (i.e. increasing the reaction rate) without undergoing a permanent chemical change itself during the process

A chemical reaction may be energetically favorable (i.e. may be exothermic), but if the activation barrier is high (i.e. the activation energy is high) the reaction rate may be extremely slow

A lot of research is performed to identify new catalysts for chemical reactions of commercial interest (time is money, after all)
Also, a lot of research is performed to develop methods to inhibit the action of certain catalysts, so that some reactions will not occur (e.g. certain biochemical reactions that rely upon the action of catalytic molecules)

It turns out that while may reactions and physical processes required for living systems are thermodynamically favored (i.e. spontaneous) their rate is so slow that a living system will die just waiting for such processes to occur. Thus, catalysts are essential to living systems - they are necessary in order to get required reactions to occur on a timescale that is useful for living systems. Proteins form the basis of almost all catalysts and are known as enzymes. For example, viruses also rely on certain enzymes for function. If such enzymes can be selectively inhibited, then a virus will not be able to reproduce or will not infect a host cell (i.e. you and I).
Homogenous Catalysis

A homogenous catalyst is present in the same phase as the reactants
The Arrhenius equation states that the rate constant, k, of a reaction is directly proportional to the frequency factor (A), inversely proportional to the activation energy E_a, and directly proportional to the temperature:

note: the negative exponent means that k is large when the value of E_a/RT is small (and vice versa). E_a/RT will be small when E_a is small or T is large (R is a constant). Thus, k is large when T is large or E_a is small. As a biological system utilizing chemical reactions you can prove the relationship of k to T. Next time you are in the snow (or in a walk-in freezer) see how rapidly you can open and close your fist. Try this again in a warm environment. You will find that it is much slower in the cold.

If a catalyst is to increase the reaction rate, k, it would appear to be able to do so by one of two ways:

Increase the frequency factor, A (i.e. in some way increase the rate of successful molecular collisions)
Decrease the activation energy, E_a

Generally speaking, a catalysts typically increases reaction rates by lowering the activation energy, E_a. This is related to the energy required to stretch and break a bond, thus, catalysts must facilitate this process in some way.

Also, a catalyst often lowers the overall activation energy for a reaction by providing a completely different reaction mechanism for the reaction. In other words, a different set of underlying elementary reaction steps.

Heterogeneous Catalysis

A heterogeneous catalyst exists in a different phase from the reactant molecules - often as a solid in contact with either a gaseous or liquid reactants
Many important industrial reactions are catalyzed by the surfaces of special solid materials
Heterogeneous catalysts are often composed of metals or metal oxides
The greater the surface area of a heterogeneous catalyst, the more reactions can take place. Thus, in manufacturing heterogeneous catalysts techniques are used to maximize the surface area (e.g. using highly porous structures)

Adsorption: the binding of molecules to a surface

Absorption: refers to the movement of molecules into the interior of a material

The initial step in heterogeneous catalysis is the adsorption of reactants onto the surface of a catalyst
The surface of metal catalysts are highly reactive in comparison to interior atoms

Interior atoms have fully satisfied bonding interactions with neighbor atoms
Atoms on the surface lack a complete set of bonding partners, and thus can bind and react with other molecules in the environment

The reaction of ethylene and hydrogen gas to form ethane, and the catalysis of this reaction by platinum metal

The reaction of ethylene gas (C₂H₄) and hydrogen gas to form ethane gas (C₂H₆) is exothermic, but occurs very slowly (due to a large activation energy)

C₂H₄(g) + H₂(g) à C₂H₆(g) (kinetically slow)

In the presence of a metal surface (e.g. in the presence of platinum powder) the reaction proceeds rapidly even at room temperature

Binding and interaction with the metal atoms promotes dissociation of the hydrogen atoms (lowering the activation energy associated with this process
Furthermore, binding of both reactants can increase the probability of collision (increasing the frequency factor A). In other words, the diffusion of the molecules on the surface is diffusion in 2-dimensions, rather than 3. This "contains" the reacting molecules in a smaller area and increases the likelyhood of collision and therefore, reaction

Some commercially important metal catalysts:

Platinum can catalyze the oxidation of CO to CO₂

CO(g) + 1/2O₂(g) à CO₂(g) reaction catalyzed in presence of Pt(s)

Rhodium can catalyze the decomposition of NO

2NO(g) à N₂(g) + O₂(g) reaction catalyzed in presence of Rh(s)

CO(g) and NO(g) are environmentally toxic gasses produced by the incomplete combustion of hydrocarbons in a nitrogen-containing atmosphere. Rhodium and platinum catalysts in an automobile exhaust can perform chemistry, at the necessary kinetic rate, to render these as less harmful gasses

The interaction of the metal catalyzes the reaction. There are several things to consider:

The reaction rate is increased, by lowering of the overall activation energy, but the energy difference between the reactants and products has not been altered
The reaction requires specific bonds to be broken (H₂) and this requires the input of energy. The formation of specific bonds (C-H) is exothermic and releases energy.

The difference between the two processes is the overall reaction enthalpy (which a catalyst does not change)
If the catalyst makes it easier to break a specific bond, then to maintain the overall reaction enthalpy, the formation of the appropriate new bonds must not result in the release of as much energy (as in the absence of the catalyst). In other words, if it takes less energy to break a bond, then conversely, not as much energy is released when forming a new bond.

Enzymes

Many chemical reactions associated with life are energetically favorable, but kinetically slow (i.e. exothermic, but high activation energy and therefore a slow reaction rate)

Biological catalysts are needed

Proteins have evolved to serve as specific and effective catalysts for many biological reactions (reactions to break down food to produce energy and building products, and reactions to use this energy and building materials to build a new you; reactions to replicate your genetic material; reactions that let you contemplate the words you are now reading). These protein catalysts are called enzymes.

Proteins are polymers of amino acids connected by a peptide bond (NH-CO)
Each amino acid is about 10 atoms or so in size, consisting of combinations of N, C, O and S atoms. Each amino acid is about 110 amu in mass. The average protein is a linear polymer of about 150 amino acids (some smaller, some much larger)
The protein polymer typically folding up into a unique 3-D structure. For enzymes, the "active" site (where the catalysis occurs) is a very small region of the overall protein structure

The "Lock and key" model for enzyme based catalysis describes the protein active site as being complementary (i.e. the "lock") to the reactant (also called a substrate) that it will catalyze (i.e. the "key")

The reactive groups in the enzyme important for catalysis are located in or near the substrate binding site. The region of catalysis is called the active site

The number of catalyzed reaction events, called the turnover number, is on the order of 1,000 to 10 million per second. These high rates indicate that enzymes have reduced the energy barrier to a very low activation energy
The proteins in your body will help catalyze the oxidation of hydrocarbons (fats) or carbohydrates (sugars), and both a form of reduced carbon, to produce CO₂(g) and H₂O(g) and this will occur in an aqueous environment

C₆H₁₂O₆(aq) + 6O₂(aq) à 6CO₂ + 6H₂O(aq) (release of much energy)