12.7 Catalysis

• Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction.
• Catalysts change the mechanism by which the process occurs.

Fig. 12.48

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Types of  Catalysts

1. Homogeneous catalysts
2. Heterogeneous catalysts
3. Enzymes

Homogeneous Catalysts

The reactants and catalyst are in the same phase.

Many times, reactants and catalyst are dissolved in the same solvent, as seen below

Fig. 12.49

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Heterogeneous Catalysts

• The catalyst is in a different phase than the reactants.
• Often, gases are passed over a solid catalyst.
• The adsorption of the reactants is often the rate-determining step.

Fig. 12.50

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• Enzymes are biological catalysts.
• They have a region where the reactants attach. That region
  is called the active site. The reactants are referred to as substrates.

Fig. 12.51

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What are catalysts?

Catalysts have no effect on the equilibrium constant and thus on the equilibrium composition.

Catalysts are substances that speed up a reaction but which are not consumed by it and do not appear in the net reaction equation. Also — and this is very important — catalysts affect the forward and reverse rates equally; this means that catalysts have no effect on the equilibrium constant and thus on the composition of the equilibrium state.

Thus a catalyst (in this case, sulfuric acid) can be used to speed up a reversible reaction such as ester formation or its reverse, ester hydrolysis:

Fig. 12.52

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The catalyst has no effect on the equilibrium constant or the direction of the reaction. The direction can be controlled by adding or removing water (Le Châtelier principle).

Catalysts provide alternative reaction pathways

Catalysts function by allowing the reaction to take place through an alternative mechanism that requires a smaller activation energy. This change is brought about by a specific interaction between the catalyst and the reaction components.

You will recall that the rate constant of a reaction is an exponential function of the activation energy, so even a modent reduction of E_a can yield an impressive increase in the rate.

Catalysts are conventionally divided into two categories: homogeneous and heterogeneous. Enzymes, natural biological catalysts, are often included in the former group, but because they share some properties of both but exhibit some very special properties of their own, we will treat them here as a third category.

Some common examples of catalysis

How to burn a sugar cube

When heated by itself, a sugar cube (sucrose) melts at 185°C but does not burn. But if the cube is rubbed in cigarette ashes, it burns before melting owing to the catalytic action of trace metal compounds in the ashes.

Platinum as an oxidation catalyst

The surface of metallic platinum is an efficient catalyst for the oxidation of many fuel vapors. This property is exploited in flameless camping stoves (left).

The image at the right shows a glowing platinum wire heated by the slow combustion of ammonia on its surface. [link].

..But if you dip a heated Pt wire into liquid ammonia, you get a miniature explosion: see video.

 Fig. 12.51, 52

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Decomposition of hydrogen peroxide

Hydrogen peroxide is thermodynamically unstable according to the reaction

2 H₂O₂ → 2 H₂O + O₂     ΔG = –210 kJ mol^–1    

Potassium iodide efficiently catalyzes H₂O₂ deposition. This short video shows what happens when some colored soap, H₂O₂, and KI are combined. But “don’t try this at [your] home”!

In the absence of contaminants this reaction is very slow, but a variety of substances, ranging from iodine, metal oxides, trace amount of metals, greatly accelerate the reaction, in some cases almost explosively owing to the rapid release of heat. The most effective catalyst of all is the enzyme catalase, present in blood and intracellular fluids; adding a drop of blood to a solution of 30% hydrogen peroxide induces a vigorous reaction.

↑ Rapid liberation of O₂ can result in a spectacular bubblebath if some soap is added. [link]

This same reaction has been used to power a racing car!

[link]

Relative rates of H₂O₂ decomposition →

Each kind of catalyst facilitates a different pathway with its own activation energy. Because the rate is an exponential function of E_a (Arrhenius equation), even relatively small differences in E_a‘s can have dramatic effects on reaction rates.

Note especially the values for catalase; the chemist is still a rank amateur compared to what Nature can accomplish through natural selection!

Table 12.7

How catalytic activity is expressed

Changes in the rate constant or of the activation energy are obvious ways of measuring the efficacy of a catalyst. But two other terms have come into use that have special relevance in industrial applications.

Turnover number

The turnover number (TON) is an average number of cycles a catalyst can undergo before its performance deteriorates (see below). Reported TONs for common industrial catalysts span a very wide range from perhaps 10 to well over 10⁵, which approaches the limits of diffusion transport.

Turnover Frequency

This term, which was originally applied to enzyme-catalyzed reactions, has come into more general use. It is simply the number of times the overall catalyzed reaction takes place per catalyst (or per active site on an enzyme or heterogeneous catalyst) per unit time:is defined as

The number of active sites S on a heterogeneous catalyst is often difficult to estimate, so it is often replaced by the total area of the exposed catalyst, which is usually experimentally measurable.

TOFs for heterogeneous reactions generally fall between 10^–2 to 10² s^–1.

2  Homogeneous catalysis

As the name implies, homogeneous catalysts are present in the same phase (gas or liquid solution) as the reactants. Homogeneous catalysts generally enter directly into the chemical reaction (by forming a new compound or complex with a reactant), but are released in their initial form after the reaction is complete, so that they do not appear in the net reaction equation.

Iodine-catalyzed cis-trans isomerization

Unless you are taking an organic chemistry course in which your instructor indicates otherwise, don’t try to memorize these mechanisms. They are presented here for the purpose of convincing you that catalysis is not black magic, and to familiarize you with some of the features of catalyzed mechanisms. It should be sufficient for you to merely convince yourself that the individual steps make chemical sense.

You will recall that cis-trans isomerism is possible when atoms connected to each of two doubly-bonded carbons can be on the same (cis) or opposite (trans) sides of the bond. This reflects the fact that rotation about a double bond is not possible.

Conversion of an alkene between its cis– and trans forms can only occur if the double bond is temporarily broken, thus freeing the two ends to rotate. Processes that cleave covalent bonds have high activation energies, so cis-trans isomerization reactions tend to be slow even at high temperatures. Iodine is one of several catalysts that greatly accelerate this process, so the isomerization of butene serves as a good introductory example of homogeneous catalysis.

The mechanism of the iodine-catalyzed reaction is believed to involve the attack of iodine atoms (formed by the dissociation equilibrium  on one of the carbons in Step :

 Fig. 12.53

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During its brief existence, the free-redical activated complex can undergo rotation about the C—C bond, so that when it decomposes by releasing the iodine (), a portion of the reconstituted butene will be in its trans form. Finally, the iodine atom recombine into diiodine. Since processes  and  cancel out, iodine does not appear in the net reaction equation — a requirement for a true catalyst.

 Fig. 12.54

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3  Heterogeneous catalysts

As its name implies, a heterogeneous catalyst exists as a separate phase (almost always a solid) from the one (most commonly a gas) in which the reaction takes place. The catalytic affect arises from disruption (often leading to dissociation) of the reactant molecules brought about by their interaction with the surface of the catalyst.

← Model of a catalyst consisting of clusters of 8-10 platinum atoms (blue) deposited on an aluminum oxide surface. This catalyst efficiently removes hydrogen atoms from propane, converting it into the industrially-important propylene. [source]

Fig. 12.55

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