Haber process

The Haber process, also called the Haber–Bosch process, is the main way we make ammonia. Ammonia helps plants grow because it is used to make fertilizers.

The process mixes nitrogen from the air with hydrogen and uses a special helper called a catalyst, made from iron, to create ammonia through a chemical reaction.

Fritz Haber, 1918

This reaction needs a lot of pressure and high temperature to work well. It was invented by two German chemists, Fritz Haber and Carl Bosch, in the early 1900s. They made it better than older ways of making ammonia, like the Birkeland–Eyde and Frank–Caro processes.

The Haber process can work with another process called steam reforming to make ammonia using water, natural gas, and air. Both Haber and Bosch won the Nobel Prize in Chemistry for their work.

History

Main article: History of the Haber process

Carl Bosch, 1927

In the 1800s, people needed more nitrates and ammonia to help plants grow and make things. They got these from special rocks and bird droppings. But by the early 1900s, people worried these supplies would run out. Scientists searched for new ways to make ammonia from the air.

A scientist named Haber, with help from Robert Le Rossignol, created a special machine that could turn air into ammonia. They showed it worked in 1909. A big company in Germany bought the idea and made it bigger for factories to use. By 1913, they were making ammonia on a large scale. This was very important during World War I because it helped Germany make things they needed.

Process

Making ammonia uses a lot of energy. It uses about 1–2% of all the energy in the world, and it creates some pollution. To make ammonia, we need hydrogen gas and nitrogen from the air. Hydrogen is usually made from natural gas, but it can also come from coal, water, or other sources.

A historical (1921) high-pressure steel reactor for the production of ammonia via the Haber process is displayed at the Karlsruhe Institute of Technology, Germany.

Special materials called catalysts help turn hydrogen and nitrogen into ammonia. Scientists have found new catalysts that work better or need lower temperatures. The main way to make hydrogen right now is by using natural gas, but there are other ways being studied too.

Hydrogen production

The main source of hydrogen is natural gas. We heat and treat the gas to get hydrogen out of it. There are also ways to make hydrogen without using fuels that create pollution, like using water and electricity.

Illustrating inputs and outputs of steam reforming of natural gas, a process to produce hydrogen

Ammonia production

Hydrogen and nitrogen are mixed together and passed over a catalyst to make ammonia. This process needs high pressure and temperature to work well. The gases are cooled after each step so ammonia can be separated out as a liquid. The leftover gases go back into the process to make more ammonia.

K(T) for N
₂ + 3 H
₂ ⇌ 2 NH
₃
Temperature θ (°C)	Equilibrium constant K
300	434 × 10⁻⁵
400	16.4 × 10⁻⁵
450	4.51 × 10⁻⁵
500	1.45 × 10⁻⁵
550	0.538 × 10⁻⁵
600	0.225 × 10⁻⁵

Catalysts

First reactor at the Oppau plant in 1913

The Haber–Bosch process uses special materials called catalysts to help turn nitrogen from the air into ammonia. These catalysts are solid pieces that work with gases.

The most common catalyst is made from tiny pieces of iron. This iron is prepared very carefully to make it work well. It starts as a special kind of iron ore and is treated to create small, porous pieces that help the reaction happen faster. Other materials are added to help the iron stay effective.

Catalysts other than iron

Scientists have tried many different metals to use besides iron. Some, like ruthenium, work well but need special treatments to be useful. Ruthenium can work at lower pressures and temperatures.

Catalyst poisons

Certain impurities in the gases used can hurt the catalysts. Some of these impurities, like sulfur and chlorine, can stop the catalyst from working. Others, like water or carbon monoxide, can slow it down. Even harmless gases can build up and make the reaction slower.

Typical catalyst composition	Iron (%)	Potassium (%)	Aluminium (%)	Calcium (%)	Oxygen (%)
Volume composition	40.5	00.35	02.0	1.7	53.2
Surface composition before reduction	08.6	36.1	10.7	4.7	40.0
Surface composition after reduction	11.0	27.0	17.0	4.0	41.0

Industrial production

Synthesis parameters

The Haber process makes ammonia from nitrogen and hydrogen gases. This happens through a special reaction that needs high pressure and temperature to work well. The basic reaction is: nitrogen plus three hydrogen molecules can change into two ammonia molecules.

To make this reaction work faster, we use a special material called a catalyst, usually made from iron. The process needs high temperatures around 450 to 550 °C and a lot of pressure, between 250 to 350 bar, to get the best results. The gases are mixed in a ratio of one part nitrogen to three parts hydrogen.

Large-scale implementation

Modern factories can make more than 3000 tons of ammonia every day using one production line. Before the reaction, the gases need to be very clean. Impurities can stop the catalyst from working well.

To get the hydrogen needed, methane gas is mixed with water vapor. This makes carbon monoxide and hydrogen. Then, more methane is mixed with oxygen to make even more hydrogen. Finally, carbon monoxide is changed into carbon dioxide, which is removed.

The gases are then compressed to the right pressure and sent into a reactor where the ammonia is made. The ammonia is separated from the other gases and cleaned before it’s ready to use.

Change of the equilibrium constant K as a function of temperature
Temperature θ (°C)	Equilibrium constant K
300	434 × 10⁻⁵
400	16.4 × 10⁻⁵
450	4.51 × 10⁻⁵
500	1.45 × 10⁻⁵
550	0.538 × 10⁻⁵
600	0.225 × 10⁻⁵

Mechanism

Elementary steps

The Haber process makes ammonia from nitrogen and hydrogen gas. It uses a special material called a catalyst to help the reaction happen. The process has several steps:

The gases move to the catalyst surface.
The gases stick to the catalyst.
The gases react on the catalyst.
The ammonia leaves the catalyst.

The slowest step is breaking apart the nitrogen molecule, which needs a lot of energy. This step decides how fast the whole process goes.

Energy diagram

The energy needed for the reaction can be shown in a diagram. Without a catalyst, breaking apart nitrogen would need too much energy. The catalyst helps by holding onto the nitrogen atoms, making the reaction possible at normal temperatures. Even with the catalyst, breaking apart nitrogen is still the slowest step because it needs a lot of energy to start.

Economic and environmental aspects

Further information: Ammonia production § Sustainable production

When it was first made, the Haber process was compared to another method called the cyanamide process. The cyanamide process used more electricity and more work than the Haber process.

Severnside fertilizer plant northwest of Bristol, UK

By 2018, the Haber process made 230 million tonnes of ammonia per year. This ammonia is mostly used as a fertilizer in three forms: as ammonia itself, as ammonium nitrate, and as urea. The process uses some of the world's natural gas, which is part of our energy use. Together with better plant breeding and plant protection chemicals, these fertilizers have helped farms grow more food on the same land.

However, the process uses a lot of energy, which can harm the environment. It can cause problems like extra nitrates getting into water. Almost half of the nitrogen in our bodies today comes from the Haber process. It has helped the world’s population grow from 1.6 billion in 1900 to 7.7 billion by 2018. New technology could make ammonia without needing to create hydrogen first.

Using synthetic fertilizers means farmers sometimes do not use crop methods that naturally add nitrogen to the soil, like planting certain plants together.