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U3O8, MnO, Mп3O4, Mn2O3, MnO2, Fe2O3 and the oxides of nickel and cobalt(?). While a catalytic decomposition of barium peroxide takes place also with several of these oxides, a purely catalytic evolution of oxygen without a chemical reaction of any sort1 was proved with CuO, MgO, CaO,2 CdO, La2O3(?), and CeO2. Of all the oxides examined the only ones to have no effect were SnO, SnO2, and ZrO2. In some cases enormous reaction velocities were observed in the temperature region of about 200-300°, particularly with Cu2O, V2O5, Sb2O3, Cr2O3, MoO3, and MnO. The reaction spread almost explosively through the mass and the rate of temperature rise increased often forty-fold."

Specific Activation

The effect of a catalyst does not depend exclusively upon the increased concentration due to adsorption. All the people agree that the effect of the catalyst is specific. Armstrong and Hilditch3 confirm the experiments of Sabatier that copper and nickel do not behave alike towards alcohol at the same temperature. Though both tend to convert alcohol into acetaldehyde and hydrogen, nickel is much more likely to decompose the aldehyde into carbon monoxide and methane. "In the case of copper, not only is the ratio of aldehyde to hydrogen close to that calculated but the unchanged alcohol may be recovered almost quantitatively, the yield of aldehyde being about 90-85 percent of that to be expected from the amount of alcohol used. There is a striking absence of the secondary products observed when aldehyde together with an excess of hydrogen is passed over the metal at the same temperature."

Of course, this is not what Armstrong and Hilditch are trying to say. The unchanged alcohol can also be recovered quantitatively from the experiments with nickel and also the question whether the yield of aldehyde is high or low has

1 It is possible that solid solutions may be formed.

2 Cf. Wöhler and Liebig: Pogg. Ann., 24, 172 (1832).

3 Proc. Roy. Soc., 97A, 259 (1920).

nothing to do with the recovery of the unchanged alcohol. What the experiments actually showed was that the ratio of aldehyde to hydrogen was about 36% of the theoretical with nickel at 250° and about 97% with copper at 300°; and that with nickel the gas contained 60% H2, 20% CO, and 15-17% methane. The specific effect occurs also during hydrogenation, though the difference is not so marked. "Aldehyde may be converted into alcohol by passing the vapor, together with hydrogen, over either copper or nickel (Sabatier); but in presence of the latter metal, probably owing to the special affinity of nickel for the carbonyl group, the aldehyde is prone to undergo decomposition into carbon monoxide and methane." With copper at 200° less than four percent of carbon monoxide is formed; but the amount increases markedly at 300°, the decomposition of the aldehyde being greater with copper at 300° than with nickel at 120°-150°. On the other hand, with hydrogenation of alcohol by copper at 300°, "there is a striking absence of the secondary products observed when aldehyde together with an excess of hydrogen is passed over the metal at the same temperature."

Armstrong and Hilditch consider that both alcohol and water are adsorbed selectively by copper, thus decreasing the adsorption of the aldehyde and protecting it from decomposition. The presence of water in the hydrogenation of aldehyde is not an advantage. Although it decreases the decomposition of the aldehyde by copper at 300°, it cuts down the rate of hydrogenation very much.

Armstrong and Hilditch do not agree absolutely with Sabatier as to the cause of dehydrogenation. "Sabatier has attributed the dehydrogenation of alcohol as well as of hydrocarbons by finely divided metals to the supposed aptitude of such metals to form hydrides, the attraction of the metal for hydrogen being regarded as the impelling force. But it is obvious that the relative affinities of all the agents concerned in such interactions must come into play. In the case of aldehyde, we are inclined to regard the affinity of the carbon compound rather than that of the hydrogen to the metal as of

prime importance, indeed, as the determining factor; the marked influence of the metal in causing secondary decomposition is in itself a clear indication that the attraction exercised upon the aldehyde at the metallic surface is considerable."

Weiss and Downs' showed that as yet it is only with molybdenum oxide or vanadium oxide that one can stop the oxidation of benzene at all conveniently at the maleic anhydride stage. This is in part a question of temperature regulation, but probably not entirely. "The total amount of oxygen needed to burn one molecule of benzene completely is fifteen atoms. It is conceivable that the intermediate products previously mentioned are formed and burned immediately to the more highly oxidized form even in the ordinary combustion of benzene but that they cannot be isolated as such because of the extreme velocity of the combustion reaction. The function of a catalyst, which by its use allows the separation of the intermediate products of combustion is, of course, problematical; but it would appear that by its presence the range of temperature between the formation of one intermediate product and its combustion to a higher degree of oxidation is broadened, and that, by a close temperature regulation within this range, throughout which this intermediate is stable, it may be isolated. Another contact substance which either shows no reaction or produces complete combustion at a higher temperature does not provide this sufficiently broad temperature range and the only products of reaction are those of complete combustion. An example of the former contact substance is vanadium oxide, and examples of the latter are cerium oxide, platinum, and numerous others. Even with these it is possible that sufficiently close regulation will result in the isolation of intermediate products and it is further possible and even probable that in other cases suitable control would make possible the isolation of other products, such as phenol. . . . The heat developed in the formation of maleic acid from benzene is nearly sixty percent of that developed by a benzene flame."

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Burdick' finds that the catalytic oxidation of nitric oxide to nitrogen peroxide is extremely specific. "The only materials possessing catalytic properties of any pronounced value were the special charcoals and the activated coke. The two samples which were the most reactive were the cocoanut and peachkernel charcoals. A sample of coke subjected to steaming at 600° to 800° was found to possess a pronounced but much lower catalytic activity. It is to be expected that the highly absorptive silica gels would act in the same manner as the charcoals; but this material was not available for testing at the time these experiments were performed. The order of catalytic activity that these materials possess is quite remarkable; thus, the specific reaction velocity of cocoanut charcoal was found to be about 11000, or more than five hundred times that observed for ordinary porous materials. The time required therefore to produce a given degree of oxidation in a given gas mixture is less in the same proportion. For example, to secure the conversion to nitrogen peroxide of ninety percent of the nitric oxide in a gas containing three percent of nitric oxide and three percent of oxygen requires five hundred seconds with inert material and only one second in the presence of the catalyzer.

"Water vapor has a very considerable effect on the catalytic properties of the charcoals. To retain activity the catalyzer must be maintained at a temperature above the condensation point of the aqueous nitric acid which would be in equilibrium with the gas phase in contact with it. Condensation on the catalyzer or soaking in nitric acid had no permanent harmful effect, as the material resumes its activity when dried. . . . With dry gases the catalytic activity is only slightly decreased by an increase of temperature. When, however, moisture is introduced into the gases, the reaction rate drops greatly, in the case of the cocoanut charcoal at 25°, one percent of water vapor in the gas causing a decrease in the rate of reaction constant from 11000 to 1100. With moist gases the effect of

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temperature is the opposite of that found with dry gases, the rate of reaction increasing as the temperature difference above the condensation point of the aqueous nitric acid becomes greater.

"A slight amount of water vapor present in the gases inhibits the activity of the catalysis to a considerable degree; but, except in the immediate neighborhood of the condensation point where the catalysts are very sensitive to differences in water vapor concentrations, concentrations of water vapor in excess of one percent seem to exert no further depressing effect. Even with a water vapor content as high as fifteen percent, as would be encountered, for instance, in the gases from ammonia oxidation, the catalytic activity of the charcoals is well maintained, provided the temperature of the reaction space is sufficiently elevated. The decrease in the rate of reaction with elevation of temperature, observed in the case of the dry gases, is probably occasioned by the decreased absorptive ability of the charcoal at the higher temperatures. The increase in the rate of reaction with temperature rise in the presence of water vapor is likewise probably to be explained by the lowering of the surface concentration of absorbed water in the charcoal caused by the temperature increase, thus enabling a relatively greater proportion of the active mass of the catalyzer to take up its duty.

"With respect to the life of the charcoal catalysts, their properties do not seem to be impaired at all with time, tests over many weeks' duration showing no decrease in activity. The charcoals were also tested for possible slow combustion; but in no case was there any evidence of carbon dioxide formation."

The best catalytic agent for forming phosgene, COCl2, is charcoal; but Atkinson, Heycock and Pope' report that certain charcoals are much better than others. "On the introduction of carbonyl chloride as a weapon of chemical warfare, it becomes necessary to ascertain which method of preparation was most adaptable as a works process for manufacture, and

1 Jour. Chem. Soc., 117, 1410 (1920).

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