Water gas is produced when superheated steam is passed over red hot coke or coal at 1270 K in presence of nickel as catalyst.

$$\begin{aligned} & \mathrm{C}(\mathrm{s}) \\ & \text { Coke } \end{aligned}+\underset{\text { Steam }}{\mathrm{H}_2 \mathrm{O}(\mathrm{g})}+121.3 \mathrm{~kJ} \xrightarrow[\text { Nickel }]{1270 \mathrm{~K}} \underbrace{\mathrm{CO}(\mathrm{g})+\mathrm{H}_2(\mathrm{~g})}_{\text {Water gas }}$$

It is inconvinient to obtain pure $\mathrm{H}_2$ from water gas as CO is difficult to remove. Hence, to increase the production of $\mathrm{H}_2$ from water gas, CO is oxidised to $\mathrm{CO}_2$ by mixing it with more steam and passing the mixture over $\mathrm{FeCrO}_4$ catalyst at 673 K .

$\underbrace{\mathrm{CO}(g)+\mathrm{H}_2(g)}_{\text {Water gas }}+\mathrm{H}_{\text {Steam }}^{\mathrm{H}_2 \mathrm{O}}(\mathrm{g}) \xrightarrow[\mathrm{FeCrO}_4]{673 \mathrm{~K}} \mathrm{CO}_2(g)+2 \mathrm{H}_2(g)$

This is called water-gas shiff reaction. Carbon dioxide is removed by scrubbing with mixture of sodium arsenite solution or by passing the mixture through water under 30 atm pressure when $\mathrm{CO}_2$ dissolves leaving behind $\mathrm{H}_2$ which is collected.