Catalysis

 

Introduction

The main purpose of a catalyst is to ensure a reaction is completed more quickly than it otherwise would. For a reaction to occur, the particles involved need a certain amount of activation energy, which is the threshold which, if crossed, can ensure the reaction can occur. With catalysts, this threshold is reduced, which thus means the reaction can occur at a faster rate, though it will still end at the same time as before.  

I've attempted to show this on a very crude, simple and not correctly annotated enthalpy profile (the arrows would usually go on the curves, not beside them, however it would have made the profile even more confusing). Catalysts are depicted as red - the activation energy is lower, at least the diagram makes that clear.

Catalysts speed up a reaction without being used up. Essentially, they'll merely increase the rate, but at the end of the reaction, they are unchanged and can be reused continuously, provided they're not deformed due to external factors. 

This entire process is known as catalysis - the act of the rate of reaction increasing due to catalysts.

What a catalyst you turned out to be

There are two main types of catalyst - homogeneous and heterogeneous. 

• Homogeneous catalysts are of the same phase as the reactants - for example, if two gases react with each other, and the catalyst is also a gas, this reaction would occur as one homogenous mixture. Therefore, the reactants and catalysts in phase with each other.
• Heterogeneous catalysts are of a different phase as the reactants. An obvious example would be a metal catalyst reacting with two liquids or gases - they're in different states, so are obviously out of phase with each other. However, the same principle would apply in a reaction between water and oil - oil doesn't dissolve in water, instead it forms a separate layer, so the two are therefore out of phase with each other.
• Source - https://www.chemguide.co.uk/physical/catalysis/introduction.html
  

Homogeneous catalysts have a main advantage, which is that they are in full contact with the reactants, whereas heterogeneous catalysts can only interact with the surface layer atoms. For this reason, reactions with homogeneous catalysts are often quicker than those with heterogeneous catalysts. 

That's not to say that heterogeneous catalysts are irrelevant. The Haber process, for instance, uses an iron catalyst which helps produce ammonia far more effectively than otherwise. The production of ethene from ethane and water is also helped through using a nickel catalyst. It is also far easier to extract a heterogeneous catalyst since they're out of phase, whereas you may need to distill a solution to extract a homogeneous catalyst. 

Quite often, hetereogeneous catalysts will require catalyst supports which improve their effectiveness through maximising the catalyst's surface area and stabilising the catalyst so it can work under intense conditions. Aluminium oxide, for example, acts as a support in the Haber process.

The active sites of catalysts

Perhaps the greatest place where catalysis occurs is inside you. Countless enzymes break down lengthy chains of molecules into smaller monomers which can be digested. They end with the suffix -ase, and include amylase (breaking down starch into simple sugars) and catalase (which breaks down H₂O₂ into water and oxygen). Each enzyme breaks down a certain molecule, and can do this at a very fast rate - catalase breaks down millions of H₂O₂ molecules a second. However, if an enzyme becomes denatured due to a difference in pH or temperature, it can no longer break down the molecule and becomes redundant - this process is irreversible.

There are seven categories of enzymes, all given a unique EC number which classifies what its role is. EC 4.1.1.9, for example, is malonyl-CoA decarboxylase, which helps in the metabolism of fatty acids. It's a lyase, which catalyses reactions in ways other than through oxidation or hydrolysis, that's what the lead 4 means. Rubisco is similar; its role is to catalyse the fixation of carbon so that it can be converted into glucose in photosynthesis. It's the most abundant enzyme on the planet, even if it's not the fastest catalyst (even a paper says it's "not so bad", such is its lethargic reputation). 

There's an entire list of the EC numbers here.

Economic benefits

Catalysts are very vital economically, contributing up to 40% of the world's global GDP - in fact, I'm also writing an article about that. The Haber process is no easy process, and to react nitrogen with hydrogen, you will need lots of energy to enable them to form ammonia. That, combined with the need to produce a crucial fertiliser in a large enough yield that is used in countless locations, means there has to be a balancing act in both time used and money spent. Luckily, the iron catalyst means this reaction is both faster, more efficient, and less costly. And since over 157 billion tonnes of ammonia is produced annually, that can be crucial (source - https://www.sciencedirect.com/science/article/abs/pii/S0195925514000717 - info in introduction).

There are also catalytic converters, which break down any toxic gases that may form in a car engine with a ceramic support. For example, the oxidation of carbon monoxide (which can cause blood poisoning) causes carbon dioxide (a safer gas, even if still harmful), as in this equation:

2CO + O₂ --> 2CO₂ 

Similarly, hydrocarbons are oxidised to form carbon dioxide and water. Both of these reactions can reduce the levels of harmful pollutants released by a car engine into the atmosphere. In fact, they're so effective all cars must have a catalytic converter unless it's a classic car, whatever that means.

Catalysts can also indirectly impact agriculture, such as through the reaction for anaerobic respiration in yeast: 

C₆H₁₂O₆ --> 2C₂H₅OH + 2CO₂

This is fermentation, and it helps produce alcohols. You can produce alcohol in industry using ethene and steam, along with a nickel catalyst, but this tried and tested method is far more efficient. It's partly due to the yeast acting as biological catalysts, which also helps produce carbon dioxide used to make bread rise. It's not hard to imagine the economic impact of this reaction. And I could carry on discussing how catalysts are involved in medicine, as well as this article on catalytic antibodies which create new catalysts, but I'd be here for a while. I might write some more on catalysts, including the aforementioned article. 

Concluding remark

These are just a few ways in which catalysts have shaped our lives. They're around us and inside us, making our lives cheaper and less harmful - personally, I'm more interested in the chemistry aspect.