(i) Hydrogen economy : The energy is transported and stored in the form of liquid or gaseous hydrogen is the principle due to which hydrogen is considered to be a possible source of clean energy. This proposal to use hydrogen as fuel is called hydrogen economy. (ii) Hydrogenation : The process of addition of hydrogen to unsaturatedhydrocarbons is known as hydrogenation. $\underset{\text{Ethene}}{\mathrm{H}_2\mathrm{C}=\mathrm{CH}_2}+\mathrm{H}_2$ $\xrightarrow[473\,\text{K}]{\text{Ni or Pt or Pd}}$ $\underset{\text{Ethane}}{\mathrm{H}_3\mathrm{C}-\mathrm{CH}_3}$ $\underset{\text{Ethyne}}{\mathrm{H}-\mathrm{C}\equiv \mathrm{C}-\mathrm{H}}+2\mathrm{H}_2$ $\xrightarrow[473\,\text{K}]{\text{Ni or Pt or Pd}}$ $\underset{\text{Ethane}}{\mathrm{H}_3\mathrm{C}-\mathrm{CH}_3}$ (iii) Syngas : The mixture of CO and H2 is used for the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas or syngas. Nowadays, syngas is produced from sewage, saw dust, scrap wood, newspapers, etc. (iv) Water-gas shift reaction : The production of dihydrogen can be increased by reacting CO of syngas mixtures with steam in the presence of iron chromate as catalyst. $\mathrm{CO}_{(g)}+\mathrm{H}_2\mathrm{O}_{(g)}$ $\xrightarrow[\text{Catalyst}]{673\,\text{K}}$ $\mathrm{CO}_{2(g)}+\mathrm{H}_{2(g)}$ This is called water-gas shift reaction. (v) Fuel cell : It is a commercial cell in which the chemical energy produced during the combustion of fuel is converted directly into electricity, e.g., $\mathrm{H}_2\ \text{--}\ \mathrm{O}_2$ fuel cell. A fuel cell is used as a source of electrical energy in the space vehicles.