A Comprehensive Treatise on Inorganic and Theoretical Chemistry

154 INORGANIC AND THEORETICAL CHEMISTRY
A catalyst may exert a very specific action so that the catalyst directs the
reaction in one direction in preference to another. This is exemplified by
P. Sabatier's work on the catalysis of organic compounds. Thus, the vapour of
formic acid is decomposed into hydrogen and carbon dioxide when passed over
zinc oxide, and into water and carbon monoxide when passed over titanic oxide :
HCOOH JTt02 -^ H2O-I-CO
Ethyl alcohol in the presence of nickel decomposes into acetaldehyde and hydrogen ;
and in the presence of alumina, it forms ethylene and water :
O W OYf$ Ni "* CH..COH + H,
O2U6OIi^03 ^ C2H4H-H2O
Again, a mixture of carbon monoxide and hydrogen, at 300°, furnishes chiefly
formaldehyde in the presence of copper at 300° ; chiefly methyl alcohol in the
presence of a mixture of zinc and chromium oxides at 300° to 358° ; and chiefly
methane in the presence of finely-divided nickel at 150° to 200° :
(Cu
CO + wHj J ' Cr2O3
(Ni
H.COH
CH4-HH2O
Again, the presence of "water vapour favours the oxidation of carbon monoxide ;
the union of hydrogen and oxygen ; the union of the hydrogen and chlorine ; and
the union of hydrogen chloride and ammonia. In fact, if these gases be intensely
dried, the reactions may not occur under ^conditions where the moist gases readily
combine. The catalyst is here supposed to act by a cycle or chain of reactions,
say : CO-J-H2O=CO2+H2 ; followed by : 2H2 + 02=2H20. At high temperatures,
the reaction : 2CO+O2 = 2CO2, may proceed directly. On the other
hand, a catalyst may retard the progress of a reaction, and it is then called a
negative catalyst. Thus, K. Than (1864) found that the presence of water vapour
retards the decomposition of ammonia ; W. A. Shenstone (1887), that dry ozone
at 0° decomposes 30 times as rapidly as the moist gas at 26-4° ; and H. G. van de
Stadt (1893), that moisture retards the oxidation of phosphorus.
The presence of certain impurities may reduce the chemical activity of the
catalyst resulting in what is metaphorically called a poisoning of the catalyst.
E. Turner (1823), M. Faraday (1834), and W. C. Henry (1836), for example, noticed
that finely-divided platinum becomes less active in stimulating the reaction between
hydrogen and oxygen if certain foreign gases are present—the presence of carbon
monoxide, and ethylene act as inhibitors of the reaction. Again, the oxidation of
sulphur dioxide in the presence of a catalyst was not successful commercially until
it was found that the reacting gases must first be freed from arsenical compounds
which poison the catalyst ; sulphur compounds also act as poisons in the synthesis
of ammonia, and in hydrogenation processes. The poison is thought to act by
being preferentially adsorbed on the surface of the catalyst.
In some cases, the activity of a catalyst is enhanced by admixture with another
catalyst so that the activity of the mixture is greater than the sum of the activities
of the individual constituents. This is the so-called promotor action. For example,
dehydrating agents act as promotors in the catalytic hydrogenation of carbon
monoxide or dioxide. Finely-divided manganese oxide favours the oxidation of
carbon monoxide at temperatures as low as —30°, but the catalyst is poisoned by
alkali, and promoted by cupric oxide. The catalyst called hopcalite is manganese
and cupric oxides in the proportions 3:2. The poisoning by alkali does not occur
so readily with the promoted catalyst. In some cases the promotor acts by increasing
the available catalytic surface ; or by reducing the tendency of the catalyst
to sinter by heat. The promotor may also favour the decomposition of the inter-