Commercial Production of Dihydrogen

Industrial Routes

Several large-scale processes supply dihydrogen commercially.

1. Electrolysis of acidified water. Using platinum electrodes, water is split directly:

2. High-purity electrolysis. Electrolysing warm aqueous barium hydroxide between nickel electrodes gives dihydrogen of very high purity (>99.95%).

3. By-product of the chlor-alkali process. When brine (aqueous NaCl) is electrolysed to manufacture NaOH and $Cl_2$, hydrogen appears at the cathode as a by-product:

• Anode: $2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$
• Cathode: $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$
• Overall: $2Na^+(aq) + 2Cl^-(aq) + 2H_2O(l) \rightarrow Cl_2(g) + H_2(g) + 2Na^+(aq) + 2OH^-(aq)$

4. Steam reforming of hydrocarbons or coke. Passing steam over hydrocarbons or coke at high temperature, with a catalyst, produces a mixture of CO and $H_2$:

For methane specifically:

This CO + $H_2$ mixture is called water gas, or more generally synthesis gas ("syngas"), since it feeds into methanol and hydrocarbon synthesis; syngas can also be made from waste materials such as sewage, sawdust, scrap wood and newspaper. When made from coal, the process is called coal gasification:

Boosting hydrogen yield — the water-gas shift reaction. Reacting the CO from syngas further with steam, using an iron chromate catalyst, converts more CO into extra hydrogen: …