A catalyst changes the activation energy, E_a, of a reaction by providing an alternate pathway for the reaction. The rate and rate constant k of a reaction are related to E_a in the following ways:

A catalyst in the same phase (usually liquid or gas solution) as the reactants and products is called homogeneous catalyst.

A catalyst that is in a separate phase from the reactants is said to be a heterogeneous, or contact, catalyst. Contact catalysts are materials with the capability of adsorbing molecules of gases or liquids onto their surfaces. An example of heterogeneous catalysis is the use of finely divided platinum to catalyze the reaction of carbon monoxide with oxygen to form carbon dioxide. This reaction is used in catalytic converters mounted in automobiles to eliminate carbon monoxide from the exhaust gases.

Promoters are not catalysts by themselves but increase the effectiveness of a catalyst. For example, alumina Al₂O₃, is added to finely divided iron to increase the ability of the iron to catalyze the formation of ammonia from a mixture of nitrogen and hydrogen. A poison reduces the effectiveness of a catalyst. For example, lead compounds poison the ability of platinum as a catalyst. Thus, leaded gasoline shall not be used for automobils equiped with catalytic converters.

Because heterogeneous catalysts often are used in high temperatures reactions, they are usually high melting (refractory) materials, or else they can be supported by refractory materials such as alumina.

Today, catalysts design is a challenge for chemists and engineers for effective productions, pollution prevention, and waste treatments.

Inorganic Chemistry by Swaddle (page 117) gives an excellent example to illustrate the chemisorption of hydrogen by a nickel catalyst. The bond energy of H₂ is 435 kJ/mol. Thus, in a hydrogenation reaction, energy has to be made available for the reactions:

In the activation of O₂ by a metal M, the O=O bond is weakend or broken via these steps:

With sophiscated experimental techniques, we can study the chemisorbed species in details. For example, the chemisorbed ethylene is believed to be an ethylidyne radical

     H  H  H
      \ | /
        C
        |
        C
       /|\
    PtPtPtPtPt
Metal Metal Metal

The chemisorbed ethylidune radical.

A poison reduces the effectiveness of a catalyst. Tetraethyl lead has allways been additive to the gasoline. For environmental protection, catalytic converters have been installed in automobiles to oxidize carbon monoxide and hydrocarbons. However, lead compounds poison the ability of platinum as a catalyst. Thus, leaded gasoline should not be used for automobiles equipped with catalytic converters.

There are many types of catalyst in the market place, for example MIRATECH oxidation catalyst can also reduce carbon monoxide and hydrocarbon emissions. The most common catalytic converter uses Pt metal.

Recently, there is a concern over the reduction of sulfur in gasoline and other engine fuels for the purpose of reducing sulfur oxides emission. Technically, sulphur compounds are not catalyst poisons (i.e. they do not cause an irreversible reduction in catalyst efficiency). However, they will occupy part of the precious metal surface, thereby reducing the active conversion of exhaust emissions until the sulphur gets de-sorbed from the precious metal sites again (short-term effect).

Tranisition metals are used in hydrogenation reactions mentioned earlier. These reaction are represented by

Other processes catalyzed by transition metals are oxidation-reduction reactions:

For most transition metals except gold, the chemisorption strength follows a general sequence for gaseous reagents:

• Fe, Ru, and Os chemisorb most gases firmly, and barely chemisorb N₂.
• Chemisorb strengths for Co and Ni are weaker than those of the Fe group. Their absorptions for CO₂ and H₂ are very weak.
• Rh, Pd, Ir, and Pt barely chemisorb H₂, but not CO₂.
• Cu, Ag, barely chemisorb CO and ethylene.

For hydrogenation reactions, the catalyst must chemisorb H₂. Metals Co, Rh, Ir, Ni, Pd, and Pt are suitable. Availability and costs are additional factors for the consideration. Nickel is actually a good choice, all considered.

These guidelines are very crude, and each case must be carefully studied. Fortunately, many catalysts are commercially available. The research and development of catalyst are left for many companies.

Selection of a catalyst is important in industrial productions. For example, using rhodium or platinum as catalysts have shown to give very different distribution of products when methane or ethane were used.

Clusters are the limiting sizes of metal particles, each of which are made up only a few atoms. There is no need to rigorously define the number of atoms in a particulate to be called clusters, but a general view is that when the number of atoms at the surface of the particle is more than the nuber of atoms in the interior, the particle is a cluster. Thus, a cluster can have as few as 3 atoms, and as large as a few tens of atoms.

By the way, the term cluster have been used in other areas of study. For example, in organometallic chemistry, compounds with a few metals bonded together by metal-metal bonds are also called metal clusters. Many carbonyl compounds belong to this category. For example,

All catalytic activities occur at the surface, because the surface atoms have tendencies for chemisorption of gas molecules. Thus, clusters will naturally be excellent potential catalysts. Thus, the study of heterogeneous catalysts may involve the study of metal cluster ion chemistry, and encapsulated silver clusters as oxidation catalysts. Clusters can be made from vapour deposition. The title of this link sounds very interesting: Metal Atom Vapor Chemistry: A Field Awaits Its Breakthrough.

A p-type metal oxide have excess positive charges in the solid, and they can adsorb oxygen to form anions such as O^-, O^2-, O₂^-, and O₂^2- on their surfaces. Nickel oxide is such an oxide. It turns out that the adsorbed O^- species is the most active,

A sulfide, such as MoS₂, can loose sulfur atoms to become a n-type solid, Mo_1+xS₂ or gain a sulfur atom to become a p-type solid Mo_1-xS₂, depending on the vapour pressure of S₂ gas surrounding the solid. By doping MoS₂ with oxide can also make it a p-type solid for a catalyst.

One of the useful applications of MoS₂ as a catalyst is to reduce sulfur in gasoline. For example, the cyclic thiophene C₄H₄S can be converted to a hydrocarbon by using a p-type MoS₂,

Acidic oxides

Zeolites, which are alumniosilicates, function as acidic catalysts. They also catalyze isomerization, cracking, alkylation and other organic reactions.

Basic oxides

Mixtures of basic oxides have been used as catalysts in the oxidative coupling of methane. In some cases, special reactors and catalysts are designed for this type of application. TAP Reactor is one such an application. In this case, zeolites were used.

The simulation below shows that when the yellow beam strikes the semiconductor TiO₂, an electrons are excited from the valance band into the conduction band. This band gap is 3.2 V. The excited electron then promote the production of H₂. The holes take electrons from OH^- groups converting them to active OH radicals. The radicals break up forming O₂ or react with CHCl₃ converting it into the harmless CO₂, H⁺, and Cl^-. This simulation of photocatalysts is prepared by a Japanese group, and it illustrates the concept rather well. In reality, the process is rather complicated.

In the photodecomposition of water, the excited electrons react with hydrogen ions (protons)

As another example, fluoroboric acid is used in electroplating and metal finishing. To treat wastewater from these industries requires the removal of fluoroboric acid. Existing methods of adsorption, coagulation, precipitation methods do not work. Thus, photocatalytic decomposition of fluoroboric acid has been studied, and it showed that TiO₂ being rather effective when it doped with Cr and Fe oxides. The above link showed that doping of Cr or Fe drastically enhanced the activity. Moreover, 0.5 wt% Cr/TiO2 and 1.0 wt% Fe/TiO2 showed maximum activities of 61 % and 41 %, respectively.

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Solution
The energy to excite an electron up 3.2 V is 3.2 eV.

Discussion
The wavelength of these photons are

Solution
The energy is

Discussion
When we discuss Gibbs energy, we have learned that the enthalpy of formation for H₂O is - 285.83 kJ. This means that we need a minimum of 286 kJ to decompose water. Thus, 1 einstein of photons has more energy to decompose a mole of water than the minimum. However, an overpotential is required to decompose water. The titanium oxide is mixed with platinum metal and ruthenium oxide to facilitate the formation of bubbles in these process. (See Inorganic Chemistry by Swaddle(page 125).