There are a number of methods for preparing dihydrogen from metals and metal hydrides.
9.3.1 Laboratory Preparation of Dihydrogen
It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.
Zn + 2H+ → Zn2+ + H2
It can also be prepared by the reaction of zinc with aqueous alkali
Zn + 2NaOH → Na2ZnO2 + H2
9.3.2 Commercial Production of Dihydrogen
The commonly used processes are outlined below:
1.Electrolysis of acidified water using platinum electrodes gives hydrogen
2H2O (l) → 2H2 (g) + O2 (g)
2.High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
3.It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution. During electrolysis, the reactions that take place are:
At anode 2Cl- (aq) → Cl2(g) + 2e-
At cathode 2H2O (l) + 2e- → H2 (g) + 2OH- (aq)
The overall reaction is
2Na+ (aq) + 2Cl- (aq) + 2H2O(l)
Cl2(g) + H2(g) + 2Na+ (aq) + 2H2O- (aq)
4.Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.
CnH2n+2 + nH2O → nCO + (2n+1)H2
CH4 (g) + H2O (g) → CO (g) + 3H2 (g)
The mixture of CO and H2 is called water gas. As this 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. The process of producing 'syngas' from coal is called 'coal gasification'.
C (s) + H2O (g) → CO (g) + 3H2 (g)
The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.
CO (g) + H2O (g) → CO2 (g) + H2 (g)
This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.
Presently ~77% of the industrial dihydrogen is produced from petro-chemicals, 18% from coal, 4% from electrolysis of aqueous solutions and 1% from other sources.