T H E REACTIONS OF HYDROGEN ACTIVATED BY EXCITED MERCURY ATOMS* BY HUGH STOTT TAYLOR AND ABRAHAM LINCOLON MARSHALL
Studies in contact catalysis have shown that reactions occur at contact surfaces and that the properties of the surface are fundamental to both the velocity an.d the nature of the reaction occurring. These studies have, however, failed to reveal, as yet, what is the mechanism of the activation of the reacting species, induced by association with the contact agent. It has, on several occasions, been pointed out by Bancroftl, that the solution to the problem of how activation occurs might be reached through a comparison of the results of catalytic and photo-chemical studies, Bancroft illustrated this point by reference to various decomposition processes which organic molecules undergo in contact with catalysts and also under the influence of ultraviolet light. There are, however, several reasons why such a comparison may involve problems of very great difficulty. A considerable body of experimental work in recent years2 has shown that catalytic decomposition of organic compounds at surfaces are complex processes, very sensitive in their progress to the catalyst preparation and the purity of the reacting species. The photochemical studies, also, show a wide variety of processes occurring simultaneously, involving not only decompositions but syntheses of new substances from the initial products of decomposition. It seems, therefore, worth while to attempt a comparison of catalytic and photo-effects in reactions which, from their simplicity and the smoothness with which they proceed catalytically, offered greater prospects of successful issue. Processes of hydrogenation seemed to present such an opportunity. Several catalytic hydrogenations have been shown to occur quantitavely, without complicating side reactions, and yith measurable velocities. Recent research has shown that at least two methods exist by which hydrogen can be brought in to the atomic condition by physical agencies. In one method, the passage of a powerful high tension discharge through molecular hydrogen at low partial pressures was shown by Wood3to yield hydrogen atoms. In the other method, the resonance radiation absorbed by mercury vapor from the light of a cooled mercury vapor arc can be transferred, as was shown by Cario and Franck4, to hydrogen present in the illuminated system with the production of atomic hydrogen. Such methods of production of atomic hydrogen seemed to offer a very interesting opportunity to learn more concerning the activation necessary in
* Contribution
from the Laboratory of Physical Chemistry, Princeton University. First Report, Committee on Contact Catalysis, National Research Council; See also, Ind. Eng. Chem., 16, 270 (1924). 2 See especially that of Adkins and his co-workers: J. Am. Chem. Soc., 44, 385, 2175 (1922); 45, 809 (1923) 46, 130 (1924); 47J 807 (192.5). 3Proc. Roy. Soc., 102A,I (1922). '2.Physik, 11, 162 (1922). 1
REACTIONS OF ACTIVATED HYDROGEN
1141
catalytic hydrogenation processes, from a study of the reactivity of such hydrogen atoms with a variety of materials known to be subject to catalytic hydrogenation, Reactivity with hydrogen atoms would indicate that adequate activation of hydrogen alone was a sufficient preliminary to reaction. Non-reactivity with the atoms would suggest that the catalytic process must involve activation of both molecular species. Marshall and Taylor have shown’ that hydrogen atoms by Wood’s method will produce hydrogen chloride when led into unilluminated chlorine. Further work by Marshall2 has shown that more hydrogen chloride is produced than corresponds with hydrogen atoms introduced and that therefore some chain mechanism, as in the photochemical combination of hydrogen and chlorine, is involved. Bonhoeffer3 has studied the reactivity of such hydrogen atoms with a number of solid substances such as sulphur, phosphorus and arsenic as well as with an unsaturated oil, showing that hydrogenation occurs in each instance. Oxides, sulphides, halides and oxygen salts were reduced by the atomic hydrogen. For kinetic studies with gases, the activation of hydrogen by the resonance radiation of mercury is more amenable to experimental manipulation. Cario and Franck showed that hydrogen atoms so produced would reduce copper oxide and tungstic oxide at room temperatures and would ‘clean up’ on the walls of the containing vessel. Dickinson has shown4that oxygen and hydrogen atoms so produced will react at 45OC. W. A. Noyes, Jr., has recently indicated5 that at the temperature of boiling mercury a nitrogen-hydrogen mixture in the presence of mercury vapor illuminated by the necessary radiation produces minute quantities of ammonia. In the experiments recorded in this paper we have studied the reaction of hydrogen activated by excited mercury atoms with ethylene, carbon monoxide, nitrous oxide and carbon dioxide. For comparison with the results of the work already recorded we have also made measurements with oxygen and nitrogen. Our experiments differ from those which have preceded in the working pressures over which the observations extend. Cario and Franck studied the rate of ‘clean up’ at hydrogen pressures of 0 . 2 mm. Dickinson showed that, with an initial pressure of 0.18mm., an hourly pressure decrease of 0.06 mm. was obtained. Our normal initial pressures have been in the neighbourhood of atmospheric pressure and the reaction concentrations have been suitably varied so that the reaction rates over wide pressure intervals and with varying ratios of reactants have been ascertained. Experimental Procedure The reaction vessel consisted of a horizontal transparent fused quartz tube, 175 ccs. in volume, connected to a manometer and also to a burette Yature, 112, 937 (1923). J. Phys. Chem., 28, 842 (1925). Z. physik. Chem., 113, 199 (1924). 4Pr0c. S a t . Acad. Sei., 10, 409 (1924). J. Am. Chem. SOC.,47, 1003 (1925).
1142
HUGH STOTT TAYLOR AND ABRAHAM LINCOLN MARSHALL
system and oil pump for evacuation, A 7cm. Hanovia quartz mercury vapor arc was mounted horizontally and in contact with the reaction tube throughout its length. The whole system was kept in a water bath through which water a t approximately I ~ O C .was rapidly passing. In this manner the mercury arc system was kept cold, a necessary precaution if the radiation, X = 2536.7 b, is to be obtained from the arc. I n a hot mercury arc all the radiation of this frequency is absorbed by the mercury vapor in the arc and none is transmitted. We have specially verified this point by illuminating our reaction system with the light from the hot arc transmitted through a quartz window in our water bath. In such circumstances none of the effects recorded below were obtained. The reaction vessel always contained as much as ICC. of liquid mercury. Compressed ethylene, oxygen, nitrous oxide and carbon dioxide were used as the source of these gases in the experiments, without any special purification. Nitrogen and hydrogen from cylinders were freed from oxygen. Carbon monoxide was prepared from formic acid and concentrated sulphuric acid. All the reactions studied proceed with a decrease in the number of molecules so that the rate of the reaction could be followed manometrically at constant volume. In all of the experiments the arc burned at approximately 2 5 volts and consumed some 2 0 0 watts. The deviations from these magnitudes never exceeded 20 percent. Experimental Results Hydrogen and Ethylene. The data for the hydrogenation of ethylene at 15. 5°C. are shown in Table I. The mercury arc burned at 2 8 volts and consumed 2 1 0 watts with variations amounting to ==I I O percent.
TABLE I
+
C2H4 Hz = CzHs, a t 15.j'C. Initial Hydrogen Pressure 34.5 om. Pressure Decrease Time in Hours 0.25
0.90
1.35 2.38 4.08 5.92
7.28 9.18
I . 05 cm. 2.8 4.25 6.85 10.8 15.5 18.7 23.0
Initial Ethylene Pressure 32.3 cm. Pressure Decrease Time in Hours 10.05
24.95
11.07 12.66 13.05 13.47 13.73 14.0 14.25
30.5 31.3 32.0 32.25 32.3 32.3
27.25
Since the decrease in pressure corresponds exactly to the initial ethylene pressure it is evident that the reaction proceeds smoothly according to the usual equation. It will be noted that over the first thirteen and one quarter hours the change in rate with time is relatively small. Initially the rate of
I143
REACTIONS OF ACTIYATED HYDROGEN
pressure decrease amounts to 3 cm. per hour. From the tenth to the thirteenth hours the rate averages 2 . 1 5 cm. decrease per hour. Between 1 3 . 2 5 and 13.75 hours the velocity falls suddenly to zero. A further experiment in which ethylene was present in excess (27.8 cm. G H 4 ;2 1 . 5 cm. Hz) showed the same initial rate as the above. Measurements were discontinued after 1.5 hours. We have already pointed out that the above reaction was quite absent when the mercury arc was operated hot, outside the water bath and illuminating the reaction system through a quartz window in the water-bath. This result can be further emphasized by our experimental observation that a hydrogen-ethylene mixture containing mercury vapor and maintained a t IOOOC. showed no change in pressure when illuminated by the hot mercury arc for I j hours. Hydrogen and Oxygen: In Table I1 are recorded experimental data for the reaction between hydrogen and oxygen. In this experiment, results were first obtained with an initial total pressure of 36 cm. After a steady value for the rate had been determined the pressure was reduced by evacuation and the rate at the end of the reaction was observed. This latter rate is recorded in the right half of the following table.
TABLE I1 2Hz
+ O2 =
Initial Hydrogen Pressure, 2 0 . 1 5 em. Time in Hours Pressure Decrease
2H20
at 15.5'C. Initial Oxygen Pressure, 16.0 em. Time in Houre Pressure Decream
0.32
0.05
0.66
1.15
0.55
1.02
0.4 0.65
1.66
0.85
1.65
0.9
2.92
1.25
2.0
1.1
3.55
1.85
4.0
1.4
4.93
2.55
5.0
1.4
6.07
3.05
Within the experimental error the velocity of reaction was, in this case, constant over the whole pressure range covered by the experiment. It is to be noted, however, that the rate is considerably less than was found in the case of the hydrogen-ethylene reaction. The reaction system contained liquid water, so that, throughout the experiment, water vapor was present at the saturation concentration.
Hydrogen and Carbon Monoxide: The data in Table I11 present the experimental results obtained when hydrogen and carbon monoxide in the presence of mercury vapor were exposed to the resonance radiation of mercury from the arc. In the first case the gases were present in approximately equal concentrations. In the second case hydrogen was present in pronounced excess.
1144
HUGH STOTT TAYLOR AND ABRAHAM LINCOLN MARSHALL
TABLE 111
Hz
+ CO = HCHO etc. at 15.5’C.
Initial Hydrogen Pressure 3 3 . 2 cm. Initial Carbon Monoxide Preswre 32.3 cm. Pressure Drcrrase Time in Hours
0.25 0.55
0 . 8 5 cm.
0.77
2.3 3.3
1.16 1.89 2.37 3.35 3.96 4.69 5.60 5.96 7.0
1.70
.”,
Initial Hydrogen Pressure 44.35 Initial Carbon Monoxide Pressure 2 j.25 Time in Hours Pressure Decrease
0.23 0.42 0.73 1.0
0 . 6 0 cm. 2.0
4.35 7.45
5 .OS
1.58
10.35
6.15 7.9 8.75 9.65
2.08 2.66 3.0 3.96 4.98 6.0 6.75
13.05 15.5 16.85 19.45 21.45 23 .os 24. I 5
IO.
85
11.35 12.45
These reactions have not been followed to their end. Test showed that the reaction was complex, formaldehyde, solid polymerised products and methane being among the identified reaction products. A further study of this reaction is deferred until a method has been devised of protecting the products of reaction against further action of the ultra-violet light and of hydrogen atoms. The experiments performed suffice to settle the immediate problem of interest, namely, that unactivated carbon monoxide will react readily with hydrogen atoms. Formaldehyde is undoubtedly the primary reaction product and this synthesis represents a new method of achieving its production at atmospheric pressure and room temperature. That the carbon monoxide was not activated by the light employed is suggested by the experimental observation that mixtures of carbon monoxide and oxygen when illuminated gave no reaction, in marked contrast to the case of hydrogen and oxygen already considered. The data for ethylene, oxygen, and carbon monoxide are presented graphically in Figure I . Hydrogen and Carbon Dioxide: Two experiments were carried out, one with 32.7 cm. COz and 2 1 . 1 cm. Hz as dry gases, the other with 35.8 cm. COz and 28.1 cm. Hz in presence of a small amount of liquid water, In neither case was any pressure change detectable over a period of several hours exposure to the resonance radiation, although the manometric system permitted a change of 0.5 mm. to be very definitely recorded. Hydrogen and Nitrogen: Mixtures of nitrogen with excess hydrogen in presence of mercury vapor showed no measurable pressure change when illuminated with resonance radiation, the working temperature being I 7°C. Any pressure change occurring was less than 0.5 mm. of mercury in a system at atmospheric pressure containing 2 0 percent nitrogen. No special attention was paid in these experiments to secure thoroughly dry gases. In some of the experiments, the reaction vessel was screened from the lamp by a quartz mantle carrying ethyl alcohol, with a view to cutting out the radiation from
1145
REACTIONS O F ACTIVATED HYDROGEN
the mercury arc capable of decomposing any ammonia that might be formed. Tests with Nessler’s reagent upon completion of a run failed to reveal any ammonia formed. Hydrogen and Kitrous Oxide: A mixture of hydrogen and nitrous oxide reacted more rapidly when illuminated with resonance radiation than did similar mixtures of hydrogen and oxygen. That water vapor is one of the reaction products is evident from the initial slow rate of pressure decrease during the first hour as shown in Table IV and in Fig, 2 , during which time a water film was forming. The reaction is complicated by the fact that
*
FIG.I (1) Hydrogen-Ethylene; (11) Hydrogen-Oxygen; (111) and (IV) Hydrogen-Carbon
Monoxide.
TABLE IV NzO Initial Hydrogen Pressure 9.7 cm. Initial Nitrous Oxide Pressure 8.85 em. Time in Hours Pressure Decrease
+ Hz at I~OC. Initial Xitrous Oxide Pressure 42.5 em. Time in Hour*
Pressure Increase
0.5
0 . 3 5 cm.
0.20
0.45 cm.
0.86
1.1
0.50
1.1
I .o
1.4
1.62
2.8
0.75 1.75
3.0
2.0
3.4 4.25 5.5 6.95 7.3
2.58 3.5 4.84
5.84
1.4
1146
HUGH STOTT TAYLOR AND ABRAHAM LINCOLN MARSHALL
nitrous oxide alone in presence of mercury vapor is decomposed by the light of the cooled mercury arc, an increase in pressure resulting. We are planning a special study of the photochemical decomposition of this gas.
6 k Aq
2;/
1
readily achieved, a conclusion of importanceinrespect to the production of methane and of ammonia from these respective gases by contact catalytic methods. On the quantitative side, the experimental results disclose a state of affairs for which there is little precedent
/T/ME /N HOURS
Phys. Rev., (2) 23, 466 (1924). See for example, Photochemistry, Chapter 18, p. 1222-1230, Treatise on Physical Chemistry, Van Nostrand, 1924. Cario and Frank, Z. Physik, 17, 209 (1923) discuss the possibility, even with their low reaction yields, of a chain mechanism in the reduction of metallic ox;des.
REACTIONS O F ACTIVATED HYDROGEN
1147
existed in the present case we should have the entirely parallel case of hydrogen atoms functioning as a photochemically produced catalyst for the combination of hydrogen and a reducible gas. The decision on such matters must await a more quantitative study of the reactions. To that end we are employing a more refined technique and a more suitable arrangement of reaction system and exciting arc. With the new experimental arrangement it will be possible greatly to extend the range of reactions studied, the temperatures at which they proceed and the sensitising energies at our disposal. We wish to acknowledge our indebtedness to Professor Karl T. Compton for advice and assistance which led to the right choice of experimental conditions.
Summary Ethylene, carbon monoxide, oxygen and nitrous oxide have been shown to react at room temperatures with hydrogen atoms produced by collisions of molecular hydrogen with mercury atoms excited by illumination with the resonance line, X = 2 53 7.6 A. of the mercury arc. Carbon dioxide and nitrogen do not react with hydrogen atoms so far as can be observed under the given experimental conditions. The velocities of reaction measured are some 100-40000times more rapid than in previous studies of the same type. The present experiments were conducted at pressures averaging 0.5 atm. as opposed to earlier work with pressures of a few mm. or less. It is suggested that these high yields may resdt from a chain mechanism of the type obtaining in the photochemical hydrogen-chlorine combination. The reactions studied form additional examples of photosensitisation, with mercury atoms as the sensitiser. One of the reactions, that between hydrogen and carbon monoxide, yields, inter alia, formaldehyde, by a new process of photosynthesis at room temperatures. The bearing of these photochemical studies on the problem of mechanism in contact catalysis has been indicated. Princeton, New Jersey.
