Oxygen : Catalysis

The action of manganese dioxide, copper oxide, and ferric oxide in promoting the decomposition of potassium chlorate by heat, and the similar effect of cobalt oxide on bleaching powder, have been described. These substances appear to act by contact, hence their effect was called contact action by Mitscherlich; the usual name, due to Berzelius, is catalytic action or catalysis.

A catalyst is a substance which alters the speed of a chemical reaction without itself undergoing permanent chemical change; in most cases it accelerates the reaction, but in some cases it retards it, when it is called a negative catalyst. It is essential that a true catalyst shall undergo no permanent chemical change; it must be left after the reaction of the same chemical composition as at the beginning, but not necessarily in the same physical state. Very small quantities of a catalyst therefore serve to bring about the decomposition, or other chemical change, of large quantities of materials.

The first reasonable theory of catalytic action was due to Clement and Desormes (1806), who assumed that the reacting substances and the catalyst form an unstable intermediate compound, which then breaks up, reproducing the catalyst in its original chemical composition, and liberating the products of reaction. This series of alternating, or cyclic reactions, so called because the catalyst goes through a series of complete cycles of changes and returns to its original state after each, is regarded by this theory as the cause of catalytic action. Thus. in the present case, J. Mercer (1842) assumed that manganese dioxide in presence of a powerful oxidising agent, such as potassium chlorate, tends to pass into a higher stage of oxidation, say Mn₂O₇. At the high temperature, however, this higher oxide can hold its oxygen only transiently; it breaks up, giving gaseous oxygen, and forming manganese dioxide again:

KClO₃ + 2MnO₂ = KCl + Mn₂O₇ = KCl + 2MnO₂ + 3O.

Fowler and Grant (1890) showed that only oxides which can form unstable higher oxides, again decomposed by heat, can act catalyti-cally in the decomposition of potassium chlorate. Thus, MnO₂-> MnO₃ or Mn₂O₇; Cr₂O₃->CrO₃; Fe₂O₃->FeO₃; all these higher oxides are known in the form of salts: K₂O,MnO₃ (manganate); K₂O,Mn₂O₇ (permanganate); K₂O,CrO₃ (chromate); K₂O,FeO₃ (ferrate) Copper oxide probably forms an imperfectly known higher oxide (? CuO₂) Oxides which do not form higher oxides, such as zinc oxide or magnesium oxide, act only very feebly (to the same extent as powdered glass), whilst acidic oxides, such as alumina, Al₂O₃, vanadium pentoxide, V₂O₅, or tungsten trioxide, WO₃, give both chlorine and oxyden:

K₂O, Cl₂O₅ + WO₃ = K₂O, WO₃ + Cl₂O₅ = K₂O, WO₃ + Cl₂ + 5O.

Expt. 10. Fuse some potassium chlorate in two hard glass tubes. To one add a very small quantity of manganese dioxide, to the other a very small quantity of chromium sesquioxide, Cr₂O₃. Observe that (i) oxygen is evolved; (ii) the fused salt becomes permanently pink KMnO₄) and yellow (K₂CrO₄), respectively. KMnO₄ cannot exist alone at the temperature of the fused chlorate, hence it must be continuously decomposed and reproduced by a series of cyclic actions. A little ferric oxide, Fe₂O₃, produces a violent effervescence, and on cooling the mass is slightly pink, from the formation of ferrate, K₂FeO₄.

McLeod (1889) observed that pieces of manganese dioxide put into fused chlorate break up into a very fine powder. The physical state of the manganese dioxide changes, which suggests that it has entered into reaction and been reproduced. Traces of chlorine are always evolved in the preparation of oxygen from chlorate, and McLeod suggested that chlorine and potassium permanganate are intermediate products in the decomposition:

(1) 2KClO₃ + 2MnO₂ = 2KMnO₄ + Cl₂ + O₂.
(2) 2KMnO₄ = K₂MnO₄ + MnO₂ + O₂.
(3) K₂MnO₄ + Cl₂ = 2KCl + MnO₂ + O₂.

If chlorine escapes, however, the residue should contain manganate; this is never found, so that probably the chlorine is produced by a secondary reaction of KCl with MnO₂, which is known to take place. Reactions (2) and (3) are also known to take place, but it is doubtful if (1) occurs. This reaction, however, is the basis of McLeod's scheme. No perchlorate is formed.

It may be difficult to see how manganese dioxide can exert any action on solid chlorate, since the catalytic effect occurs below the fusion point of the latter. But some local fusion probably occurs on account of the heat evolved in the reaction (flashes of light are always seen), and in any case L. H. Parker (1914-18) has shown that chemical action may occur between solids. The reaction:

BaCO₃ + Na₂SO₄ = BaSO₄ + Na₂CO₃,

and the reverse reaction, take place to a limited extent when the dry powdered mixture is heated short of fusion, or simply triturated in a dry mortar. Reaction also occurs in the dry powder when it is strongly compressed, as was shown by Spring.

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