Catalysis (from the Gr. κατά, down, and λύειν, to loosen), in chemistry, the name given to chemical actions brought about by a substance, termed the “catalyst,” which is recovered unchanged after the action. The term was introduced by Berzelius, who first studied such reactions. It is convenient to divide catalytic actions into two groups:—(1) when the catalyst first combines with one of the reaction components to form a compound which immediately reacts with the other components, the catalyst being simultaneously liberated, and free to react with more of the undecomposed first component; and (2), when the catalyst apparently reacts by mere contact. The theory of catalysis is treated under Chemical Action; in this article mention will be made of some of the more interesting examples.
A familiar instance of a catalytic action is witnessed when a mixture of potassium chlorate and manganese dioxide is heated to 350°, oxygen being steadily liberated, and the manganese dioxide being unchanged at the end of the reaction. The action may be explained as follows:—part of the chlorate reacts with the manganese dioxide to form potassium permanganate, chlorine and oxygen, the chlorine subsequently reacting with the permanganate to produce manganese dioxide, potassium chloride and oxygen, thus
2KClO3 + 2MnO2 = 2KMnO4 + Cl2 + O2 = 2KCl + 2MnO2 + 3O2.
This explanation is supported by the facts that traces of chlorine are present in the gas, and the pink permanganate can be recognized when little dioxide is used. Other oxides bring about the same decomposition at temperatures below that at which the chlorate yields oxygen when heated alone; but since such substances as kaolin, platinum black and some other finely powdered compounds exercise the same effect, it follows that the explanation given above is not quite general. Another example is Deacon’s process for the manufacture of chlorine by passing hydrochloric acid gas mixed with air over heated bricks which had been previously impregnated with a copper sulphate solution. The nitrous gases employed in the ordinary chamber process of manufacturing sulphuric acid also act catalytically. Mention may be made of the part played by water vapour in conditioning many chemical reactions. Thus sodium will not react with dry chlorine or dry oxygen; carbon, sulphur and phosphorus will not burn in perfectly dry oxygen, neither does nitric oxide give red fumes of the peroxide. In organic chemistry many catalytic actions are met with. In the class of reaction known as “condensations,” it may be found that the course of the reaction is largely dependent upon the nature of some substance which acts catalytically. One of the most important is the Friedel and Craft’s reaction, in which an aromatic compound combines with an alkyl haloid in the presence of aluminium, zinc or ferric chloride. It seems in this, as in other cases, that additional compounds are first formed which subsequently react with the re-formation of the catalyst. The formation of benzoin from benzaldehyde in the presence of potassium cyanide is another example; this action has been investigated by G. Bredig and Stern (Zeit. Elektrochem., 1904, 10, p. 582).
The second class of catalytic actions, viz. those occasioned by the presence of a metal or some other substance which undergoes no change, is of especial interest, and has received much attention. The accelerating influence of a clean platinum plate on the rate of combination of hydrogen and oxygen was studied by Faraday. He found that with the pure gases the velocity of reaction increased until the mixture exploded. The presence of minute quantities of carbon monoxide, carbon disulphide, sulphuretted hydrogen and hydrochloric acid inhibited the action; in the case of the first two gases, there is no alteration of the platinum surface, since the plate brings about combination when removed to an atmosphere of pure hydrogen and oxygen; with the last two gases, however, the surface is altered, since the plate will not occasion the combination when placed in the pure gases. M. Bodenstein (Zeit. phys. Chem., 1904, 46, p. 725) showed that combination occurs with measurable velocity at ordinary temperatures in the presence of compact platinum. More energetic combination is observed if the metal be finely divided, as, for instance, by immersing asbestos fibres in a solution of platinum chloride and strongly heating. The “spongy” platinum so formed brings about the combination of ammonia and oxygen to form water and nitric acid, of nitric oxide and hydrogen to form ammonia (see German Patent, 1905, 157,287), and of sulphur dioxide and oxygen to form sulphur trioxide. The last reaction, which receives commercial application in the contact process of sulphuric acid manufacture, was studied by M. Bodenstein and W. Pohl (Zeit. Elektrochem., 1905, 11, p. 373), who found that the equilibrium followed the law of mass-action (see also F. W. Küster, Zeit. anorg. Chem., 1904, 42, p. 453, R. Lucas, Zeit. Elektrochem., 1905, 11, p. 457). Other metals, such as nickel, iron, &c., can also react as catalysts. The use of finely divided nickel (obtained by reducing the oxide in a current of pure hydrogen at a temperature of 350°) has been carefully studied by P. Sabatier and J.B. Senderens; a summary of their results is given in the Ann. Chim. Phys., 1905 (viii.) 4, pp. 319-488. Of special interest is the condensation of acetylene. If this gas mixed with hydrogen be passed over the reduced nickel in the cold, the temperature may rise to as high as 150°, the acetylene disappearing and becoming replaced by a substance like petroleum. If the nickel be maintained at 200°, and the gases circulated for twenty-eight hours, a product, condensible to a yellow liquid having a beautiful fluorescence and boiling at 45°, is obtained. This substance closely resembles ordinary Pennsylvanian petroleum. If acetylene be passed alone over nickel heated to 200°-300°, a mixture, boiling at 60°-70° and having a green colour by diffused and a red by transmitted light, was obtained. This substance closely resembles Caucasian petroleum. The decomposition of carbon monoxide according to the reaction 2CO ⇄ C + CO2 is purely catalytic in the presence of nickel and cobalt, and also in the presence of iron, so long as the amount of carbon dioxide present does not exceed a certain amount (R. Schenck and W. Heller, Ber., 1905, 38, pp. 2132, 2139). It is of interest that finely divided aluminium and magnesium decompose methane, ethane, and ethylene into carbon and hydrogen in the same way as nickel. Charcoal at 350° also reacts catalytically; for example, Senderens found that ethyl alcohol was decomposed by animal charcoal into methane, ethylene, hydrogen, carbon monoxide and a little carbon dioxide, and propyl alcohol gave propylene, ethane, carbon monoxide and hydrogen, while G. Lemoine obtained from ethyl alcohol and wood charcoal a mixture of acetaldehyde and hydrogen.
