Reducing Air Pollution

Introduction

Earlier in this unit you met the main air pollutants from burning fuels: carbon monoxide, soot, unburnt hydrocarbons, sulfur dioxide and nitrogen oxides. Knowing where pollution comes from is only half the story. Chemistry also provides the solutions, from devices bolted onto car exhausts to giant towers at power stations that wash sulfur dioxide out of chimney gases. This page explains the main methods and weighs up their strengths and limits.

Core explanation

There are three broad strategies: clean up the waste gases, use cleaner fuels, or burn less fuel altogether.

1. Catalytic converters. Almost every modern petrol car has a catalytic converter in its exhaust system. Inside is a ceramic honeycomb coated with a very thin layer of precious metals such as platinum, palladium and rhodium . The honeycomb gives a huge surface area, so gases meet the catalyst quickly. As hot exhaust gases pass through, three harmful pollutants are converted into less harmful gases:

- Carbon monoxide is oxidised to carbon dioxide: 2CO + O₂ → 2CO₂ - Nitrogen monoxide is reduced to nitrogen, often by reacting with carbon monoxide: 2NO + 2CO → N₂ + 2CO₂ - Unburnt hydrocarbons are oxidised to carbon dioxide and water.

The metals are catalysts, so they are not used up, although they can be "poisoned" by some impurities. Converters work best when hot, so they are less effective on very short journeys. Note that they still release CO₂, a greenhouse gas.

2. Cleaner fuels.

- Low-sulfur fuels. Oil refineries now remove most sulfur from petrol and diesel, so little SO₂ forms when they burn. - Switching fuels. Natural gas produces far less soot and sulfur dioxide than coal, and hydrogen produces only water when it burns. - Electric vehicles produce no exhaust at the roadside; their overall emissions depend on how the electricity is generated. - Renewable electricity from wind, solar and hydroelectric power avoids combustion entirely.

3. Scrubbers at power stations. Many coal-burning power stations use flue gas desulfurisation . Waste gases are passed through a spray or slurry of a base, such as powdered calcium carbonate (limestone) or calcium oxide. Sulfur dioxide, an acidic oxide, reacts and is removed:

calcium carbonate + sulfur dioxide → calcium sulfite + carbon dioxide CaCO₃ + SO₂ → CaSO₃ + CO₂

The calcium sulfite is then oxidised to calcium sulfate, which is gypsum, and can be sold for making plasterboard. Other devices, such as electrostatic precipitators and filters, remove particulates from chimney gases, and diesel vehicles use particulate filters to trap soot.

4. Using less fuel. Better insulation, efficient engines, public transport, cycling and walking all reduce the amount of fuel burned, which cuts every pollutant at once.

Method Pollutants reduced --- --- Catalytic converter CO, NOₓ, hydrocarbons Low-sulfur fuel SO₂ Flue gas desulfurisation SO₂ Particulate filters Soot and particulates Renewables and less fuel use All, including CO₂

Step-by-step reasoning

To choose a method for a given pollutant:

1. Identify the pollutant and its source. 2. Ask whether it can be prevented (change the fuel, remove sulfur). 3. If not, ask whether it can be converted or captured (catalyst, scrubber, filter). 4. Check what new product forms, such as CO₂ or gypsum. 5. Consider cost and practicality.

Visual explanation

Picture a cut-away catalytic converter: exhaust enters from the engine on the left carrying CO, NO and hydrocarbons, flows through the narrow channels of a honeycomb coated in silvery metal, and leaves on the right as CO₂, N₂ and H₂O.

Real-world analogy

A catalytic converter is like a busy matchmaker at a party. It brings the right pairs of molecules together on its surface so they react, then sends them on their way and is ready for the next pair — never pairing up itself.

Real-world example

London introduced an Ultra Low Emission Zone in which older, more polluting vehicles must pay a daily charge. Combined with cleaner buses and taxis, this has led to measurable falls in nitrogen dioxide levels along busy roads, showing how rules and technology work together.

Why?

Why is a honeycomb structure used inside a catalytic converter instead of a solid block of metal? Reactions happen on the catalyst surface. A honeycomb coated thinly with metal gives an enormous surface area using only a small, affordable mass of expensive platinum group metals, while letting exhaust gases flow through freely.

Common misconception

"Catalytic converters make car exhaust completely harmless." They greatly reduce CO, NOₓ and hydrocarbons, but they do not remove carbon dioxide — in fact they produce it — so cars with converters still contribute to climate change.

Worked example

Question: Balance the equation for the reaction in a catalytic converter: NO + CO → N₂ + CO₂.

Reasoning: Two nitrogen atoms are needed on the left, so use 2NO. This gives two oxygen atoms from NO; 2CO gives two more oxygen and two carbon atoms, making 2CO₂ on the right.

Answer: 2NO + 2CO → N₂ + 2CO₂

Quick check

1. Name one metal used as a catalyst in catalytic converters. Answer: Platinum (or palladium or rhodium).

Exam focus

Be specific: say which pollutant each method removes and what it becomes. Examiners reward equations such as 2CO + O₂ → 2CO₂ and a clear link between flue gas desulfurisation and reducing acid rain. Remember that catalysts are not used up.

Advanced insight

In the reaction between NO and CO, one gas is oxidised and the other reduced at the same time. Nitrogen monoxide loses oxygen (reduction) while carbon monoxide gains oxygen (oxidation). Catalytic converters on petrol engines need the fuel-to-air ratio to be controlled precisely by sensors, so that enough oxygen is present for oxidation but not so much that NO cannot be reduced.

Summary

Air pollution can be cut by cleaning waste gases, using cleaner fuels or burning less. Catalytic converters use platinum, palladium and rhodium to turn CO, NOₓ and hydrocarbons into CO₂, N₂ and H₂O. Low-sulfur fuels and flue gas desulfurisation with limestone remove SO₂. Filters trap particulates, while renewables and efficiency reduce every pollutant, including CO₂.

Practice questions

1. Write a balanced equation for the oxidation of carbon monoxide in a catalytic converter. Answer: 2CO + O₂ → 2CO₂ 2. Explain how flue gas desulfurisation reduces acid rain. Answer: Sulfur dioxide is reacted with a base such as calcium carbonate and removed before the gases leave the chimney, so less SO₂ forms sulfuric acid in clouds. 3. Why do catalytic converters work less well on very short car journeys? Answer: They need to be hot to work effectively, and on a short journey they do not warm up fully. 4. Give one reason why electric cars may still cause some air pollution. Answer: The electricity may be generated by burning fossil fuels in power stations; tyre and brake wear also produce particulates.