Industrial syntheses of Hydrogen
|Several methods are employed in the preparation of hydrogen on the large scale. |
- When steam is passed over carbon, in the form of coke1 or coal, the so-called " water-gas " is formed. Two strongly endothermic reactions are involved:
[C]+2(H2O)=2(H2) + (CO2)-21.700 Cal.;
[C] + (H2O)=(H2)+(CO) – 31.700 Cal.
As indicated by the equations, heat is absorbed and increase in volume accompanies both reactions, so that they are promoted by rise in temperature and reduction of pressure. Lang 2 found that the higher the temperature the greater is the proportion of carbon monoxide at 600° C. the product consists almost wholly of hydrogen and carbon dioxide, but at 1000° C. the composition of the equilibrium mixture expressed in atmospheres is
CO2=0.012, CO =0.296, H2=0.303, H2O =0.031.
Bay gives a summary of methods described in certain French patents for removing the oxides of carbon from water-gas. In one process these gases are eliminated by treatment with calcium carbide:
In another method the poisonous carbon monoxide is oxidized by ferric oxide to innocuous carbon dioxide, the product containing 85 per cent, of hydrogen, and having a lower density and higher calorific value than ordinary water-gas -
Carbon monoxide can also be removed by passing the water-gas three times over powdered calcium oxide or hydroxide, or a mixture of these substances, at 400° C. The carbon monoxide is replaced by hydrogen, methane, and ethylene, the reaction proceeding in three stages -
By Claude's method hydrogen containing not more than 1.5 per cent, of carbon monoxide, and suitable for the production of synthetic ammonia, can be prepared from water-gas. Part of the carbon monoxide is removed by preliminary cooling, and the residual gas is allowed to expand whilst doing external work.
- Calcium hydride, CaH2, known as "hydrolite," is decomposed by water, with evolution of hydrogen:
Jaubert considers this method well adapted for preparing hydrogen to fill balloons in the field. It was employed with very satisfactory results by Dr George C. Simpson in Captain Scott's last Antarctic expedition (1910-1912). In a modified form of this process, the hydride is mixed with sodium hydrogen carbonate, boric acid, or soda-lime, and heated at 80° C. Namias has pointed out that the use for balloons of hydrogen evolved from sulphuric acid and cast iron involves damage to the envelopes, the arsine and phosphine present becoming oxidized to arsenic acid and phosphoric acid respectively, and these acids exert a corrosive action on the material of the gas-bag. Jorissen has given a useful summary of the methods employed in the preparation of gas for balloons.
- Jaubert described a process involving the use of "hydrogenite," a mixture of silicon, calcium hydroxide, and sodium hydroxide. At red heat it reacts in accordance with the equation
This method is also employed for balloon-gas.
- At red heat calcium carbide reacts with water-vapour, liberating hydrogen and carbon dioxide -
The carbon dioxide is absorbed by lime. The yield is excellent; the hydrogen produced is very pure, and is well adapted for heating and lighting, and for filling balloons.
- Lavoisier's work on the composition of water showed that steam reacts with iron at 150° C., liberating hydrogen and forming ferroso-ferric oxide, Fe3O4:
Deville investigated the reaction, and proved it to be reversible. The proportion of hydrogen in the equilibrium mixture is greatest at 800° C., a higher temperature shifting the equilibrium to the left, and causing reduction of the iron oxide. Deville believed the composition of the solid phase to correspond with the formula Fe4O5, but later work by Preuner leaves it still undecided between Fe,FeO; FeO,Fe3O4; and Fe,Fe3O4.
The interaction of steam and iron takes place in three stages -
- Dissociation of steam - H2O ⇔ 2H+O.
- Combination of the nascent oxygen with the iron to form ferrous oxide -
Fe+O ⇔ FeO.
After one hour at 350° C. this reaction becomes perceptible.
- The further oxidation to ferroso-ferric oxide - 3FeO+O=Fe3O4.
- A mixture of aluminium with small proportions of mercuric oxide and sodium hydroxide reacts with water instantaneously, and yields 1500 times its volume of hydrogen.
Laboratory preparation of hydrogen
|The choice of a method for the preparation of hydrogen in the laboratory is decided by the degree of purity required in the gas, but by suitable means the product obtained by the interaction of a metal and an acid can be sufficiently purified for ordinary use. |
- One of the best methods for the preparation of pure hydrogen is the electrolysis of water containing sulphuric acid or potassium hydroxide to augment the conductivity. The acid or alkali plays an important part in the process, as indicated in the schemes
(a) H2SO4=2H+SO4''; H2O+SO4''=H2SO4+O.
(b) KOH=K•+OH'; K+H2O=KOH+H; 2OH'=H2O+O.
When sulphuric acid is employed, the SO4'' ions liberated at the anode interact with the water to regenerate the sulphuric acid and liberate oxygen, while hydrogen is evolved at the cathode; with potassium hydroxide, the potassium ions at the cathode act on the water with formation of potassium hydroxide and evolution of hydrogen, the hydroxyl ions uniting at the anode with production of water and oxygen. The regeneration of the acid and alkali respectively enables the electrolysis to proceed without further addition of either. Platinum electrodes should be employed in the electrolytic cell. Morley has shown that hydrogen evolved from alkali solutions is always contaminated by organic substances. The exclusion of air is attained by making all the joints of the apparatus air-tight, avoiding the use of long pieces of rubber tubing, and allowing the electrolysis to proceed for some time before collecting the gas. When diffusion of oxygen from the anode to the cathode is prevented, the evolved hydrogen contains only from 0.001 to 0.0005 per cent, of nitrogen, von der Pfordten recommends the removal of oxygen by passing the gas through a solution of chromous chloride in hydrochloric acid, another method being to lead it through a heated tube containing palladized asbestos.
- Several experimenters advocate preparing pure hydrogen by the interaction of aluminium-amalgam or magnesium-amalgam and water, and consider the method very convenient -
- Mauricheau-Beaupre has shown that treatment of aluminium filings with a small proportion of mercuric chloride and potassium cyanide yields a powder stable in dry air. He called this substance "hydrogenite," and found that one kilogram of it treated with water yields 1300 litres of pure hydrogen, measured at 15° C. and 760 mm.
- The ordinary laboratory method for the preparation of hydrogen consists in the decomposition of dilute sulphuric acid or hydrochloric acid by zinc -
Zn+H2SO4=H2+ZnSO4; Zn+2HCl =H2+ZnCl2.
Since the crude materials employed often contain arsenic, phosphorus, sulphur, and carbon, such impurities as arsine, phosphine, hydrogen sulphide, and hydrocarbons are frequently present in the hydrogen evolved. Morley found that zinc occludes carbon dioxide, which contaminates the hydrogen. If the sulphuric acid employed is too concentrated, the hydrogen contains hydrogen sulphide and sulphur dioxide, formed by partial reduction of the acid by the nascent gas -
To remove these impurities, the hydrogen can be washed with caustic alkali and with an oxidizing-solution such as acidified potassium permanganate. To remove arsenic, Reckleben and Lockemann recommend passing the gas through a saturated solution of potassium permanganate or a 5-10 per cent, solution of silver nitrate, or over cupric oxide; for use on the manufacturing scale they advise employing a solution of bleaching-powder.
It is noteworthy that pure zinc decomposes dilute acids very slowly, but that addition of a few drops of a solution of cupric sulphate or platinum chloride greatly augments the velocity of the reaction. A similar effect is produced by amalgamating the metal. In both instances the acceleration is due to electrolytic action, the copper and platinum being deposited on the zinc.
For any metal the power of decomposing water and acids is determined by two factors - the potential of the metal must be more negative, and its solution-pressure must be higher, than the corresponding constants of the hydrogen evolved from the water or dilute acid.
Victor Meyer and von Recklinghausen have pointed out that contact with hydrogen materially increases the tendency of potassium permanganate to evolve oxygen, so that in washing with this reagent there is risk of introducing oxygen as an impurity. They explain the slow absorption of hydrogen by potassium permanganate by assuming oxidation to hydrogen peroxide, which is then decomposed by the permanganate with evolution of oxygen.
An improved type of generator for producing hydrogen from zinc and an acid has been described by Edwards. It is said to be superior to the ordinary Kipp apparatus in rapidity of furnishing a supply of hydrogen free from air.
- Solution of zinc, aluminium, and tin in concentrated caustic alkalis evolves hydrogen, with formation of the zincate, aluminate, and stannate of the alkali-metal -
Sn+4KOH = 2H2+SnO2,2K2O.
- Hydrogen is evolved from aqueous solutions of strong reducing- agents, such as chromous salts, potassium cobaltocyanide, chloro- molybdenum chloride (Mo3Cl6), and from all reducing-agents with a reduction-potential higher than that of hydrogen. The velocity of the gaseous evolution is considerably accelerated by addition of finely divided platinum or palladium, especially from chromous chloride.
- Bruno has prepared hydrogen by agitating iron filings with water saturated with carbon dioxide, the operation lasting 20-40 hours -