THE CA%TXLTTICOXIDaiTIOX OF CXRBOX LPOKOXIDE BY J. C. a.FRAZER
Introduction The catalytic oxidation of carbon monoxide is of interest largely because of the unusual and interesting properties of carbon monoxide. 3 l y interest in this subject began with the development some years ago of a catalyst which would bring about the complete oxidation of this gas at o°C and below a t an extremely rapid rate. The first’ catalyst which we developed consisted of manganese dioxide and silver oxide. Shortly after this we found it possible to prepare a catalyst equally good from manganese dioxide and copper oxide.’ In both mixtures the manganese dioxide was readily recognized as the more active of the two constituents. Finally we fount it possible to prepare an equally good catalyst from manganese dioxide alone* by using a method for its preparation which eliminated the possibility of contamination with adsorbed materials, particularly alkalies which manganese dioxide is so prone to adsorb from solution and from which it cannot be freed by ordinary methods of washing. Subsequently it was found possible to prepare finely divided cobaltic oxide3 in a state of purity and of equal catalytic activity for the oxidation of carbon monoxide. Finally, Bennett4 working in this laboratory, has succeeded in preparing so-called “nickelic oxide” finely divided and free from adsorbed materials and again the metallic oxide so purified has been found to be an extremely active catalyst for the reaction under considerat,ion. In all three of these cases the essential characteristics of the catalyst are the existence of the metallic oxide in finely divided condition and free from adsorbed materials. I n all three cases the use of other oxides and so-called “promoters,” originally thought by many necessary to obtain catalysts of high catalytic activity, has lost its significance, freedom from “poisoning” impurities being much more important. Having carried the experimental work this far it became a t once a matter of interest to see what degree of success would attend the preparation of catalysts from other metallic oxides in a pure and finely divided condition. For this purpose a method was devised for electrolysing the impurities from the finely divided oxides suspended in water.4 This was the only method which could be relied on t o free many of these oxides from adsorbed materials. Sone of the seventeen oxides as prepared was found to be in the same class as catalysts for this reaction as the three already mentioned, although some of them as for example ferric oxide show some activity below 100°C. Roger, Piggot, Bahlke, and Jennings: J. Am. Chem. SOC.,43, 1973 (1921). Whitesell and Frazer: J. Am. Chem. SOC., 45, 2841 (1923). Williams: Dissertation, Johns Hopkins University (1928). Dissertation, Johns Hopkins University (1930).
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J. C. W. FRAZER
I t appears on studying the properties of these highlyactiveoxides that they are characterized by one common property which distinguishes them from the other oxides and to which we are inclined to ascribe their high catalytic activity. This property is their indefinite composition. They all behave as mixtures of more than one oxide which mutually dissolve to form solid solutions. This appears not to be true of the other oxides tested under the conditions of their use in these experiments. This property enables these oxides to function as catalysts over a wide range of oxygen pressure allowing them to give up or take on oxygen with great readiness, the oxygen as given up
FIG.I
amounting essentially t o nascent oxygen is activated and readily reacts with the carbon monoxide which is likewise probably activated by adsorption on the catalyst. This peculiar property was first' studied in the case of finely divided manganese dioxide. E n g l i ~ h in , ~ an unpublished investigation, showed in this laboratory that finely divided manganese dioxide readily dissociated and the oxygen pressure in equilibrium with the oxide depended on the composition of the oxide. English's apparatus shown in Fig. I, consisted of a closed system. The part of this system containing the sample of oxide under investigation was kept in an electric furnace the temperature of which was regulated. A t the beginning of the experiment the system was filled with pure nitrogen which was passed backward and forward at regular and frequent intervals over the oxide in the furnace. The mechanism b y means of which this was accom6
Dissertation, Johns Hopkins University (1922).
C.4TALYTIC OXIDATIOS OF C A R B O S MONOXIDE
407
plished is shown in the illustration referred to. Provision was made for keeping the temperature of the gas in contact with the oxide the same as that of the oxide and also for removing the water vapor given off from the oxide. I n order to follow the course of the reaction it is only necessary to know the capacity of the gas space, the original composition and weight of the oxide used. At intervals a sample of the gas was removed for analysis and the partial pressure of oxygen therein determined. After the oxygen determination the nitrogen was returned to the system or this could be done after restoring the amount of oxygen it contained when it was removed from the
FIG.2 Osygen Pressure over
20
Gm. Fremy Oxide
system. I n this way the results given in the following series of curves, Fig. 2 , were obtained. It will be seen that as oxygen is removed from the system and the oxygen content of the oxide diminished the dissociation pressure of the oxide diminishes. With each removal of oxygen the dissociation-pressure curve drops and finally approaches the lowest curve which is that of *4skenasy6 for the dissociation of pyrolusite after it has been strongly heated. I n 1924, Weld’ working in this laboratory made a much more careful study of the dissociation of manganese dioxide using a static method. The apparatus used is shown in Fig. 3. The oxide was kept for long periods of time a t constant temperature, especially a t the low temperatures. This was accomplished simply by using various pure liquids with appropriate boiling points b
7
Askenasy and Nonowski: 2. Elektrochemie, 16, 107 (1910). Dissertation, Johns Hopkins University (1924).
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J.
C. W. FRAZER
to fix and maintain the desired temperatures. The results obtained are shown for manganese dioxide of a certain known composition in the following series of curve, Figs. 4 and 5. The two remaining oxides, “nickelic” oxide and cobaltic oxide have been more recently studied by Le Blanc and Sachse.’ I n both cases there is ample evidence that both of these oxides show a similar variable oxygen content and give up or take up oxygen continuously according to conditions.
It is assumed that this property is responsible for the very great catalytic activity of these oxides and distinguishes them in their catalytic behavior from other metallic oxides investigated and gives a mechanism by means of which oxygen is activated at such a surface. Owing to the fine porosity of these oxide catalysts they are also characterized by their ability to condense vapors within their pores. For example, unless water vapor is removed from the gases in contact with them these Z. Elektrochemie, 32, 58, 204 (1926); 2. physik. Chem., 142, 151 (1929)
CATALYTIC O X I D A T I O S O F C A R B O S MOKOXIDE
409
catalysts lose their catalytic properties through capillary condensation of water within the porous oxide. In testing the catalytic properties of these oxides they and the gases passing over them are carefully dried. The effect of traces of water vapor on many reactions is well known. This effect on the oxidation of carbon monoxide has been noted and carefully studied by Dixon and other^.^ The investigations of greatest interest to us here are those which show the effect of small traces of water vapor on the explosibility of a mixture of carbon monoxide and oxygen and also on the rate of flame propagation in such an explosive mixture.
ZO
EO
50
40
0
300
300
400
FIG.4
I t was thought that a trace of water on these catalysts might exert a similar effect on their activity. I n 1925 Blitch'O in this laboratory undertook to investigate this point. A sample of active manganese dioxide was heated for one week in an all glass apparatus in a current of oxygen dried over phosphorus pentoxide. At the end of this time the sample showed its former activity. It was then dried similarly for another week a t 240'. At the end of this time it still retained its activity. It was bhen dried continuously for another week at 300' and a t the same time the apparatusfrequentlyevacuated. At the end of the experiment it had apparently lost none of its original activity. More recently Bone" has investigated the effect of extreme drying on the catalytic oxidation of carbon monoxide using such substances as copper and 9Dixon: Brit. Assocn. Adv. Sci. Repts., 503 (1880); Phil. Trans., 175, 617 (1884); Baker:
J. Chem. Soc., 47,349 (188j);Phil. Trans., 179,A571 (1888);Boneand Weston: Proc. Roy. SOC., llOA, 615; Bone, Frazer, and Newitt: 634 (1926); Fenning: Phil. Trans., 225A, 31 (1926). 10 Dissertation, Johns Hopkins University ( 192j). l 1 Proc. Roy. Soc., 112A,474 (1926).
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J. C.
W.
FRAZER
nickel oxides and metallic gold and silver. I n all the cases the first effect of drying was an increase of catalytic activity due to the removal of the water condensed by capillarity as discussed above. I n the experiments with gold (at 2 5 0 ' ) and silver (at 365') the drying was carried to great extremes and the remarkable fact was observed that the catalytic action practically ceased. The introduction of small amounts of water was sufficient to restore the high catalytic activity. The velocity constant in the case of gold was 0.0155 (moist) and 0.00045 (dry). I n the case of the experiment with silver the constant varied from 0.1208 to 0.0039. This apparent discrepancy in the effects of extreme drying on the catalytic activity of extremely active catalysts such as manganese dioxide on the one hand and poor catalysts such as metallic
FIG.5
silver and gold on the other may be reconciled if we assume that in the one 0 % COnand case oxygen is activated and the reaction catalysed is 2CO where the effects of extreme drying are negligible while in the other case which is almost completely inhibited by extreme drying the reaction catalysed is the water-gas reaction CO Hz0 COS HZ
+
+
+
Hz
+ 02
*Hz0
The catalytic oxidation of carbon monoxide is of technical interest in connection with the risks which industrial workers in several fields run from exposure to dangerous concentrations of this gas. X great step forward in this direction was taken during the war by the production of catalysts of the Hopcalite type for use in gas masks. When properly used these masks give complete protection against carbon monoxide. Many suggestions have been made to eliminate carbon monoxide from the exhaust gases of internal com-
CATALYTIC O X I D A T I O S O F CARBON M O S O X I D E
‘41 I
bustion engines. At the present time, however, there is no device of this kind available, but we have demonstrated that it is possible to produce a catalyst which will completely remove carbon monoxide from the exhaust gases of automobiles and some of these catalysts have been tested in a tempoi-ary device through which all of the exhaust gases were passed. The longest of these was a regular road test covering a distance of 1500 miles. Whether the possibilities of such catalysts can be fully utilized and applied in a commercial way remains shortly to be seen as work on the mechanical applications are now well under way. Department of Chemzstry, The John Hopkzns L1naverszty,
Baltamore, Maryland.
