CATALYTIC ACTIVITY BY C. 0. H E N K E AND 0. W. BROWN
Catalytic Activity and Overpotential Rideal,’ from theoretical deductions, arrived a t the conclusion that metals with low overpotentials were catalytically active while metals having high overpotentials were catalytically inactive. He states that metals having an overpotential “equal to or exceeding o .455 volt, should, if used as catalysts in hydrogenation processes, show no activity since the energy necessary for desorption exceeds that necessary for the activation of hydrogen in the gaseous state in the absence of a catalytic material”. Rideal cites the fact that the well known catalysts like nickel, copper, platinum, etc., have low overpotentials. Tin, lead, mercury and zinc however have high potentials and he states that these according to Sabatier, are inactive in hydrogenation processes. In a previous paper2 we have given the results of several experiments with tin as catalyst. It was found to be an excellent catalyst for the reduction of nitrobenzene t o aniline. Material yields up to 99% aniline were secured. Yet, according to Rideal’s calculation, tin should have no catalytic activity or a t the most very low activity. Thus Rideal lists tin as having an overvoltage of 0 . 4 3 to 0.53 volt. Caspari found the overvoltage t o be 0.53 volt. Lead likewise has a high overpotential and should, according to this idea, be inactive catalytically. Yet we have shownS that lead is an excellent catalyst for hydrogenation processes. Not only does lead produce aniline but under some conditions 5 5 % yields of azobenzene were secured. With a suitable catalyst yields of over 96% aniline are obtained. Thus lead does not act as Rideal would predict from its overpotential which he lists as 0 . 4 2 to 0.78 volt. The overvoltage as determined by Caspari is 0.64 volt, According to Rideal’s idea both tin and lead should be inactive catalytically or a t the most their catalytic activity should be very low. However we have shown that these metals are not only catalytically active but in fact are excellent catalysts. This then would indicate that there is no relation between overpotential or overvoltage and catalytic activity. Specific Action of Catalysts The catalytic production of good yields of azobenzene has only been secured with lead, bismuth and thallium4 as catalysts. The production of azobenzene seems t o be specific, a t least to quite a large extent. Small amounts of azobenzene were secured with gold, antimony, chromium and manganese. The amount of azobenzene produced by these catalysts was J. Am. Chem. SOC.,42,94 (1920) *Brown and Henke: J. Phys. Chem. 27,739 (1923) a Henke and Brown: J. Phys. Cherni. 26, 324 (1922) Henke and Brown: J. Phys. Chem. 26,324,636 (1922)
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extremely small. Nickel, copper and silver produce no azobenzene. We have used about 33 different copper catalysts, with a wide range of activity, and have carried out nearly 600 experiments with copper as catalyst, yet, we have never secured any solid red azobenzene. Likewise nickel and silver do not give it. Sabatier’ states that with copper as catalyst and an insufficient excess of hydrogen the product obtained is colored an orange red by a certain amount of azobenzene or even of untransformed nitrobenzene. A colored product is easily obtained with any catalyst, the color possibly being due to azobenzene but we have never been able to obtain any solid red azobenzene with copper, nickel or silver as catalysts. Lead, bismuth and thallium on the other hand give azobenzene in quantity. Lead catalysts have been prepared giving as high as 55% yields of azobenzene. Also other lead catalysts (prepared differently) have given over 96% yields of aniline. Bismuth and thallium catalysts have given over 90% yields of azobenzene. With thallium we have never secured as high as IO% yields of aniline. Thus these three catalysts, lead, bismuth, and thallium, give azobenzene and aniline but do not give cyclo compounds. Copper and silver give amines but do not give azo compounds or cyclo compound$. Sabatier2 states that copper does not attack the aromatic nucleus. Nickel and cobalt give amines and cyclo compounds but do not give azo compounds. This indicates that in the reduction of nitrobenzene these catalysts are quite specific. Their activity and even the. product formed may be varied but only within certain limits. Thus lead may be prepared to give aniline or to give in addition to aniline large amounts of azobenzene, but it does not give cyclo compounds. Nickel may be made to give cyclo compounds or to give amines, with but a small amount of cyclo compounds, but it does not give azo compounds. Copper and silver, on the other hand, give only amines and neither azo compounds or cyclo compounds. Thus catalysts, in the reduction of nitro compounds, seem to be quite specific. On the other hand Adkins and Krause8 in a study on the decomposition of ethyl acetate by alumina, titania and thoria come to the following conclusion : “Experimental confirmation of the statement that alumina, titania and thoria catalyze specific decompositions of ethyl acetate, has not been obtained. The results obtained by us indicate that in determining the order of efficiency of these catalysts for these reactions, the method of preparation of the catalyst is of equal if not greater importance than the particular metallic element present in the catalyst.” Again Adkins4 in a later paper states: “Alumina has been preferentially activated for decarboxylation and for dehydration by modifying its method of preparation.” However it would be interesting to know the highest percentage yield obtainable in each of the three reactions by each of the three catalysts, using each catalyst under the best conditions for that catalyst for each reaction. One catalyst would probably work best a t one temperature and another a t a different temperature. 2 J
Compt,. rend., 133,321 (190:) “Catalysis in Organic Chem~stry,”p. 181 (1922) J. Am. Chem. SOC.,44, 385 (1922) J. Am. Chem. SOC.,44,2186(1922)
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Also one catalyst would give the highest percentage yield of reaction I (as numbered by Adkins and Krause) under one set of conditions, while an entirely different set of conditions might be required to give the highest percentage yield of reaction 2 and still another set for reaction 3. It might thus be found that one catalyst would give the highest yield of one reaction while another catalyst would give the highest yield of another reaction. Catalytic Activity and Atomic Weight In connection with the production of azobenzene it was pointed out in a previous paper1 that the three catalysts which produce azobenzene, thallium, lead and bismuth, fall together when the elements are arranged according to their atomic weights. Thus thallium has an atomic weight of 204, lead 2 0 7 and bismuth 208. No other element has an atomic weight between 204 and 208. No other catalyst has been found to give azobenzene in appreciable amounts. However these three catalysts under suitable conditions give azobenzene in large quantities. Thus there seems t o be a relation between atomic weight and the property of producing azobenzene. Catalytic Activity and Oxidation I n discussing the action of iron and antimony in a previous paper2, it was pointed out that apparently the metal partially reduced the nitrobenzene and was itself converted to the oxide. Thus a t the lower temperatures these catalysts would quickly lose their activity. Their activity however was quickly restored by heating to a higher temperature in hydrogen. This would indicate that a t the lower temperature the nitrobenzene oxidizes the metal to the oxide, and hence the catalyst loses its activity. Then a t this lower temperature the oxide is not reduced or only slowly reduced by the hydrogen. When the temperature is raised the oxide is reduced and hence the catalyst regains its activity. While a t the higher temperatures the catalyst does not, lose its activity or a t least much more slowly, presumably because at this temperature the oxide is reduced as fast as it is formed.
Summary The fact that tin and lead have high overvoltages and are excellent catalysts while the previously known catalysts, as nickel and platinum, have relatively low overvoltages indicates that there is no relation between catalytic activity and overvoltage. 2. In the reduction of nitrobenzene catalysts are specific. Their activity and even the product obtained may be varied but only within certain limits. 3. There seems to be a relation between atomic weight and the property of acting as a catalyst in the formation of azobenzene. 4. The behavior of iron and antimony catalysts indicatesoxidationof the metal by the nitrobenzene and subsequent reduction of the oxide by hydrogen. I.
Laboratory of Physical Chemistry Indiana Uni!iersil?j Bloomington Henke and Brown: d. Phys. Chem., 26,636 (1922) Brown and Henke: J. Phys. Chem. 26,278 (1922)
