Pollutants and Catalytic Converters Revision Notes | A Level AQA Chemistry

1. Where the Problem Starts: Inside the Engine

Peak temperatures can exceed 2000 C inside the engine cylinder during combustion, although this is not the sustained bulk temperature of the whole exhaust system.

Air is approximately 78% nitrogen. Under normal conditions, N2 is almost entirely unreactive. At the extreme temperatures inside a combustion chamber, however, nitrogen and oxygen from the air react directly to form nitrogen oxides — collectively called NOx. This is a reaction that would not occur at room temperature; the thermal energy of the engine forces it.

At the same time, the fuel itself does not always combust completely. When oxygen supply is restricted — for example during a cold start or under heavy load — carbon and hydrogen in the fuel are only partially oxidised. The result is a cocktail of pollutants leaving the exhaust pipe. Understanding each one is the first step to understanding how to remove them.

2. The Four Key Pollutants from Internal Combustion Engines

AQA requires you to know four categories of pollutant produced by internal combustion engines: NOx, carbon monoxide (CO), carbon particulates, and unburned hydrocarbons. Each has a distinct origin and a distinct set of consequences.

Carbon monoxide forms when there is insufficient oxygen for complete combustion. Complete combustion of a hydrocarbon produces CO2 and H2O; incomplete combustion produces CO instead. CO is particularly dangerous because it binds to haemoglobin in red blood cells with a much greater affinity than oxygen does, reducing the blood's capacity to carry oxygen to tissues.

Carbon particulates — tiny solid particles of soot — form when combustion is severely oxygen-limited, particularly in diesel engines. These fine particles are small enough to penetrate deep into lung tissue and are classified as a respiratory hazard. Unburned hydrocarbons are fuel molecules that pass through the engine without reacting at all; they contribute to photochemical smog and some are carcinogenic.

3. How a Catalytic Converter Works

Now that you can see what leaves the exhaust, the question becomes: how do we intercept those pollutants before they reach the atmosphere? The answer fitted to virtually every modern petrol vehicle is the catalytic converter — and the chemistry inside it is a beautiful example of heterogeneous catalysis.

Picture the device as a ceramic honeycomb structure coated with a thin layer of platinum and rhodium metals. The honeycomb design maximises surface area — the greater the surface area, the more pollutant molecules can adsorb onto the catalyst at any one time. Exhaust gases flow through the channels, and the reactions happen on the metal surface rather than in the gas phase. Modern converters can also contain palladium, but AQA requires platinum and rhodium as the key named catalysts.

The catalytic converter carries out three types of reaction simultaneously. First, it oxidises CO to CO2. Second, it oxidises unburned hydrocarbons to CO2 and H2O. Third — and this is the most chemically interesting — it reduces NOx back to N2, using CO as the reducing agent. These reactions occur as exhaust gases pass through; the air-fuel ratio is carefully managed to allow both oxidation and reduction to proceed effectively. This is why the platinum-rhodium combination is used: platinum is particularly effective for oxidation reactions, while rhodium is particularly effective for the reduction of NOx.

3a. The Key Reactions Inside the Converter

Worked example: Writing and Balancing the Catalytic Converter Reactions

Write balanced equations for the three main reactions occurring in a catalytic converter, including state symbols.

1. $2\text{CO}(g) + \text{O}_2(g) \rightarrow 2\text{CO}_2(g)$ — CO is oxidised to CO2. Check: left side has 2 C, 2 O (from CO) + 2 O (from O2) = 4 O total; right side has 2 C and 4 O. Balanced.
2. $\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g)$ — Unburned hydrocarbons are fully oxidised to CO2 and H2O. The exact equation depends on the hydrocarbon; AQA may use a general or specific example. Check: 3 C each side; 8 H each side (as 4 H2O); O: 10 left, 10 right. Balanced.
3. $2\text{CO}(g) + 2\text{NO}(g) \rightarrow 2\text{CO}_2(g) + \text{N}_2(g)$ — This is the key reaction for removing NOx. CO is oxidised (C goes from +2 to +4 in CO2) and NO is reduced (N goes from +2 to 0 in N2). Check: 2 C, 2 N, 4 O each side. Balanced.
4. All species are gaseous (g) under operating conditions inside the converter. — State symbols are required for full marks in AQA equations. The converter operates at high temperature, so water is produced as steam (g), not liquid (l).

Answer: Oxidation of CO: $2\text{CO}(g) + \text{O}_2(g) \rightarrow 2\text{CO}_2(g)$. Reduction of NO: $2\text{CO}(g) + 2\text{NO}(g) \rightarrow 2\text{CO}_2(g) + \text{N}_2(g)$. Oxidation of hydrocarbons: $\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g)$.

4. Why the Converter Cannot Remove Everything

The catalytic converter is highly effective at removing CO, NOx, and unburned hydrocarbons from exhaust gases. There is, however, one pollutant it cannot address: carbon particulates (soot). Solid particles cannot undergo the surface reactions that gaseous molecules can, and they pass straight through the converter. Diesel particulate filters (DPFs) are a separate technology used to trap these particles — but that is beyond the scope of this specification point.

There is also a cold-start problem. Catalytic converters only operate efficiently above approximately 300 °C. During the first few minutes after a cold engine starts, the converter has not yet reached operating temperature, and pollutants pass through unreacted. This is why a disproportionate fraction of a vehicle's total emissions occur in the first minute of a journey.

Additionally, catalytic converters are poisoned by lead — this is one key reason why leaded petrol was phased out, alongside the direct health hazards of lead emissions to humans. Lead deposits on the platinum-rhodium surface and permanently deactivates the catalyst. The introduction of unleaded petrol was therefore a prerequisite for catalytic converters to function at all.

5. Exam Technique and Common Mistakes

AQA questions on this topic tend to fall into two categories: recall of pollutant sources and consequences, and equation writing for the catalytic reactions. Both are very accessible once you know the patterns — but there are several traps that cost students marks every year.

6. Evaluating Catalytic Converters: A Balanced Assessment

AQA 'evaluate' questions require you to weigh evidence on both sides before reaching a conclusion. For catalytic converters, there is a genuine tension: they are highly effective at removing three of the four major gaseous pollutants, but they do not solve the problem of carbon particulates, they produce CO2 (a greenhouse gas) as a product, and they are only effective once warmed up.

From a mark-scheme perspective, a strong evaluative answer will acknowledge the effectiveness of the converter for CO, NOx, and hydrocarbons, then explicitly identify the limitations — particulates, cold-start inefficiency, and lead poisoning — before reaching a reasoned conclusion about overall effectiveness. A conclusion that says 'they are good' without qualification will not access the top marks.

7. Bringing It Together: Spec-Point Summary

Both specification requirements for this topic are now fully covered. You can identify the four pollutants from internal combustion engines — NOx (from N2 and O2 reacting at high temperature), CO (from incomplete combustion), carbon particulates (from severely incomplete combustion), and unburned hydrocarbons (from fuel passing through unreacted) — and you can explain the consequences of each.

You can also describe how a catalytic converter removes the gaseous pollutants through heterogeneous catalysis on a platinum-rhodium surface, write balanced equations for the oxidation of CO and hydrocarbons and the reduction of NOx, and critically evaluate the converter's limitations. These two spec points connect directly to broader themes of alkane combustion and industrial catalysis elsewhere in the AQA specification.