PHOTOSENSITIZATION BY OPTICALLY EXCITED MERCURY ATOMS, T H E HYDROGEN OXYGEN REACTION BY ABRAHAM LINCOLN MARSHALL
Kinetic studies of gas reactions have played a very important role in the development of theories concerning the mechanism of chemical reactions. There are, however, very few such reactions occurring in the gas phase free from disturbances due to the walls of the containing vessel. Bodenstein and his eo-workers have made many comprehensive studies regarding the behavior of thermal gas reactions and have accumulated a large amount of valuable data. I n all of this work remarkably few truly homogeneous reactions have been found, by far the greater number taking place at some solid surface. Hinshelwood and Hughes1have shown that the data on the thermal decomposition of nitrous oxide, hydrogen iodide and chlorine monoxide can be completely interpreted in terms of simple thermal activation by collision, with every collision between two molecules of joint energy exceeding an amount Q leading to reaction. The thermal decomposition of nitrogen pentoxide studied by Daniels and Johnston2 has been shown t o be a unimolecular homogeneous gas reaction3. The studies on heterogeneous gas reactions have thrown a great deal of light4 on the influence of the physical condition and chemical nature of the surface on the course of the reaction but have yielded practically no information on the mechanism by which the reaction proceeds a t the surface. Photochemical studies of gas reactions have given us considerable additional information on reaction mechanism in the gas phase. The number of such reactions available for exact quantitative study has in the past been limited, however, by the small absorption coefficient of many of the reactants in the wave length range available. Other complications have been secondary reactions induced in the primary products due primarily to the lack of intense sources of monochromatic radiation. The Einstein Law of Photochemical Equivalence between quanta absorbed and molecules reacting has been of the greatest value in correlating the results of experiment. I n a number of cases approximate equivalence has been found5 and in quite a few others wide divergence. But in all these cases it has served as a starting point in studying the mechanism of these reactions. The hydrogen chlorine combination is one of the cases of wide divergence; in this case a million molecules react per quantum absorbed. It has, however,
.
J. Chem. SOC.125, 1841 (1924); See however H. A. Taylor; J. Phys. Chem. 28, 984 . . (1924). . . J. Am. Chem. SOC.43; 53 ( I ~ Z I ) . ~ 8 White and Tolman; J. Am, Chem. SOC. 47,1240 (1925);Hirst: J. Chem. SOC.127, 657
(1424). ~
~
I
I
Rideal and Taylor: Catalysis in Theory and Practice; Taylor: “Treatise on Physical Chemistry”, Chap XV. 6 “Treatise on Physical Chemistry”, p. 1212. 4
PHOTOSENSITIZATION BY MERCURY ATOMS
35
been possible to evolve a quite satisfactory mechanism for this reaction. Recently Bodenstein and Lutkemeyerl have studied the photochemical reaction between hydrogen and bromine and confirmed the mechanism2 previously postulated to account for the thermal reactions. They have also obtained valuable data on the rate of combination of bromine atoms: all this from an interpretation based on the Photochemical Equivalence Law. Weigert4 has shown that a number of gas reactions can be sensitized to light in a spectral region easily available. By the addition of chlorine to the reaction mixture, he has studied the decomposition of ozone and the combination of hydrogen and oxygen in visible light. One must remember that in all photochemical gas reactions conditions are considerably simplified over those obtaining in purely thermal reactions. The reactions are homogeneous and the method of activation is known. If the primary action of the light can be interpreted it is usually possible to obtain a fairly complete idea of the mechanism of the reaction. A large number of workers, since the time of Davy, have investigated the thermal reaction between hydrogen and oxygen. This work has clearly shown that the controlling influence for this reaction is the nature of the surface exposed, the reaction being hetereogeneous. Bodensteins has investigated the reaction over a glazed porcelain surface in the temperature range 48zo-68g0C and at ordinary temperatures with a platinum catalyst. Bone and Wheelera have made a very complete study with a large variety of catalytic material and have demonstrated that the reaction depends primarily on the condensation of one or the other of the reactants on the heated surface. Langmuir’ has shown that with platinum as a catalyst in the temperature range 300’600°K the reaction velocity is proportional to the oxygen pressure and inversely to that of the hydrogen. He interprets this as meaning that adsorbed oxygen atoms on the platinum surface are very reactive towards hydrogen, every collision between a hydrogen molecule and an oxygen atom resulting in combination. Under certain conditions, he states, adsorbed hydrogen atoms are relatively inactive towards oxygen molecules. Direct photochemical investigation of this reaction is very difficult since oxygen does not commence to absorb appreciably above 195pp and hydrogen is diactinic down to 103pp. Any photochemical reaction observed must be due t o activation of the oxygen present. Andreew8 has observed that under the influence of ultraviolet light the rate of reaction is independent of the concentration of the reacting gases. Coehn and Groteghave studied this reaction Z. physik. Chem. 114, 208 (1924). Christiansen: Dansk. Vid. Math. Phys. Med. 1, 14 (1919); Polanyi; Z. Elektrochem. 26, 50 (1920); Herzfeld: 25, 301 (1919). Bodenstein and Lind: Z. physik. Chem. 57, 118 (1906). Ann. Physik, (4) 24, 5 5 , 243 (1907). Z. physik. Chem. 29, 66j (1899). Phil. Trans. 206A, I (1906). ’Trans. Faraday Soc. 17, 621 (1921). J. Russ. Phys. Chem. Ges. 43, 1342 (1911). “Nernst Festschrift”, p. 136.
36
ABRAHAM LINCOLN MARSHALL
in the temperature range I ~ O ~ - Z ~They O ~ Cobtain . a velocity constant of the first order from their kihetic studies. Also they report a trace of hydrogen peroxide in the initial stages of the reaction. The temperature coefficient is given as 1.04. They put forward a reaction mechanism of the type
+
Hz 0 2 = HzOz HzOz Hz = ~ H z 0 Weigertl has observed that the reaction can be sensitized by chlorine and Norrish and Ridea12 have investigated this reaction in detail and suggested a possible mechanism. The reaction is completed by the formation of hydrogen chIoride but they have shown that the rate of reaction is independent of the hydrogen concentration and can be represented by the equation
+
d[HzOl = K [ClzJ [Oz]
at Griffith and Shutt3 have studied the photochemical reaction between hydrogen and ozone-oxygen mixture in visible light. They find that the presence of hydrogen causes the reaction 302
203
to take place many times faster. In a mixture 6 x 0 3 , 40% Hz, 54% Oz it goes five times as fast as in the absence of hydrogen. The secondary reactions may be either 0 Hz-+HzO or O3 (activated) Hz --t HzO O2
+
+
+
depending on the mechanism postulated for the primary action of the light. They suggest some sort of a chain mechanism involving energy-rich water molecules to explain the effect of the hydrogen. Taylor and Marshall4 have observed that a large number of hydrogenation reactions will proceed in the gas phase when the mixture contains mercury vapor and is illuminated by the light of a cold mercury arc. This paper will give an account of a more complete investigation of the reaction between hydrogen and oxygen. The method of sensitization by mercury has already been described. In our previous experiments a pressure decrease of thirty millimeters per hour was the maximum rate observed. The apparatus now to be described was designed to enable one to utilize completely all the radiation, from the quartz mercury arc, that is absorbable by mercury vapor. A compartment for a light filter has been provided and it is possible to control the temperature of the reaction system independently from that of the arc. A diagram of the cell is given in Fig. I . The inner compartment contains the quartz mercury arc C which is cooled by a flow of tap water from A to B; electrical connections are made as indicated. The compartment D is evacuated and enables one to control the temperature of E 'Ann. Physik, (4) 24, 55, 243 (1907). 2 J. Chem. SOC. 127, 787 (1925). a J. Chem. SOC.123, 2752 (1923). J. Phys. Chem. 29, 1140 (1925).
PHOTOSENSITIZATION BY MERCURY ATOMS
37
and F independently from C. The compartment E is 6 mm. in thickness and in some of the experiments contained chlorine a t two atmospheres pressure together with about 60 mm. pressure of bromine. The compartment F is 55 mm. inside diameter and 71 mm. outside giving a thickness of 6 mm; it is IOO mm. long. The capillary G connects to gas holders and a manometer. The whole apparatus was placed in a thermostat to give accurate temperature control. The arc is 90 mm. long between the tungsten spiral and the mercury surface and is centered in the tube by quartz lugs fused to it. Wood1 gives the following table for the relative intensity of the lines in the cold mercury arc.
TABLEI Relative Intensities of Lines in Cold Mercury Arc x Intensity x Intensity 2536 2652
400
2753
2
2805
I
2894 2967 3023 3125
8
4 I2 IO
30
3131 3341
50 8
3650 3654 3663 4046 4077 43 58
64 30 30
200
546 1
400
IO0
30
The chlorine-bromine light filter absorbs all the radiation in the range 2900-5000 A" so that apart from the yellow and green lines the bnly light of any intensity reaching the vessel is 2536 A" which is the wave length employed. The volume of the reaction chamber was about 1 2 0 C.C. and the capillary connecting to the manometer Fro. I which was external to the thermostat has a volume of 2 C.C. Experiments were carried out in the temperature range 25-70°C. The hydrogen and oxygen were prepared electrolytically, passed over heated palladized asbestos and dried over phosphorous pentoxide. In some cases the mixtures used were prepared directly by electrolysis by the use of auxiliary electrodes. This ensured complete mixing before introducing into the reaction chamber. This factor was of importance since the reactions proceeded so rapidly. Some seventy-five experiments in all have been performed and the results of these will be given in a series of tables and curves. A number of experiments a t 50°C with gas mixtures ranging in composition from 406 mm. Hz and 148 mm. 0 2 to 572 mm. HZand 156 mm. O2gave reactions taking place a t the same rate, the pressure decrease being 24.7 mm. Proc. Roy. Soc. 106A,679 (1924).
ABRAHAM LINCOLN MARSHALL
38
per minute, The pressure-time curves showed an initial break a t a point corresponding to saturation with water vapor and the curve was then linear till the reaction was complete, the rate thus being independent of the total pressure to a partial pressure of hydrogen or oxygen of the order of a few millimeters. I n all these cases there was an excess of hydrogen over the amount required to combine with all the oxygen present. Fig. 2 (a) illustrates the type of curve obtained. The curves were quite different in character when oxygen was present in excess. The rate of combination decreased rapidly as the reaction proceeded. Typical results are given in Fig. 2 (b) and (c).
FIG.2 The Compositions of the Reacting Mixtures are (a) 520 mm HZ 131 mm 0 2 (b) 294 mm H Z 481 mm 0 2 (c) 2 0 8 m m H 2 229 mm 0 2
A large number of experiments were made with mixtures containing approximately two volumes of hydrogen to one of oxygen in the temperature range 25°-700C. Table I1 summarizes these results. The rate of pressure change was that determined after saturation had been reached. TABLE I1 Temperature
Rate of Pressure Change mm./min.
SO0
38.3
60' 70'
45 48
Table I11 gives similar results where the experiments had been sandwiched in order to obtain some idea of the reproducibility of the results. The numbers in brackets give the order in which the experiments were performed.
PHOTOSENSITIZATION BY MERCURY ATOMS
39
TABLE I11 Temperature
40°
60"
2 9
36.8 one 18.6 two 2.72
39. o four
three
24.0 five 2 5 . 7 six
I n the first five of these experiments a chlorine-bromine light filter was used. For experiment (6) this was removed and the results show that it exerted no appreciable influence. The results in these two tables indicate that the temperature coefficient obtained in this way is a very involved quantity. There seem to be a t least two factors entering; firstly the increased vapor pressure of mercury at the higher temperatures giving an increased light absorption and secondly the increased yield per quantum absorbed. Table IV gives the vapor pressures of mercury in the temperature range under consideration.
TABLE IV Vapor Pressures. of Mercury Temperature 25
40
Vapor Pressure
.oo17 mm. 11 *0057
50
.OI2
)'
60 70
.025
)'
.06
11
From the measurements given by Woodl on the extinction coefficient of mercury vapor for X2536.7 A" it appears that at temperatures slightly above 25' there should be complete absorption. These values however were obtained for pure mercury vapor. Later results by the same author2indicate that the presence of other gases may greatly modify the width of the absorption band and very probably the magnitude of the extinction coefficient as well. In pure helium a t 14 cm. pressure and room temperature the resonance radiation is enormously enhanced and the width of the line is broadened. Wood estimated a thirty fold increase in intensity, From this work it is evident that it will be necessary to obtain exact experimental evidence on the quantum yield a t the various temperatures before any definite evidence can be given on the temperature coefficient of this reaction. The fact that the values a t 60" and 70" agree so closely seems to indicate that the true temperature coefficient will be found to be very close to unity. Another complication enters at the higher temperatures from the fact that the reaction mixture cannot be completely homogeneous. The energy absorption will be very high at the inner Phil. Mag. 23, 689 (1912). Wood: Proc. Roy. Soc. 106, 674 (1924).
40
ABRAHAM LINCOLN MARSHALL
surface of the reaction vessel and decrease rapidly as the light progresses thru the chamber. Hence the rate of reaction will be different in various parts of the vessel and this will cause concentration differences to be set up. At the end of experiment (28) the apparatus was dismantled and the reaction chamber was found to be heavily coated with yellow mercuric oxide. This will undoubtedly account for some of the variability in rate between successive experiments. It was later observed that the mercuric oxide was formed to the greatest extent in experiments using excess of oxygen and more particularly towards the end of these runs. This point will be discussed in more detail later, Another factor of importance in causing variability is the gradual deterioration of the quartz mercury arc. In later experiments with a new arc rates twice as great as those mentioned so far were obtained. Fig. 3 illustrates some of the results obtained in later experiments. In these cases the reaction mixture (2Hz:102)was prepared by direct electrolysis of caustic soda solution and dried over phosphorous pentoxide. The reaction proceeded at a constant rate until the very end, the final pressure in every case being the vapor pressure of water at the temperature of the experiment. Table V gives a more complete list of the experiments. The experiments are numbered in the order of their occurrence. Experiments 1-9 and 10-3 2 were carried out on consecutive days. After experiments 2 1 , 23, 2 5 , 2 7 , 28 and 30 pure hydrogen was admitted to the reaction chamber and illuminated for about half an hour in order to remove any mercuric oxide from the vessel walls. After experiment 3 I , the vessel was thoroly cleaned. From these experiments it is evident that a considerable portion of the variability is due to the coating of mercuric oxide on the walls, which apparently cuts down the transmission of the active radiation. Figs. 4 and 5 illustrate experiments with mixtures containing approximately equal amounts of hydrogen and oxygen. Tangents have been drawn to these curves at a number of points to determine the rate at these points. Table VI presents the values obtained. The first column gives the Hz pressure in mm., the second the oxygen, the third, fourth and fifth the ratios H2/Oz, Hz/(Hz 0 2 ) and Hz/ (Hz 0 2 HzO) the sixth the rate of reaction in
+
+ +
PHOTOSENSITIZATION BY MERCURY ATOMS
41
1 3 1, FIG.
4
Composition of Gas 40' =359 mm Hz, 364 mm O 2 70"=336 mm Hz, 341 mm O2
FIG.5
30
42
ABRAHAM LINCOLN MARSHALL
Temperature 70"
mm. pressure change per minute and the remaining columns this rate divided by the above ratios. The numbers in brackets refer to the points on the curves in Figs. 4 and 5. The last three columns give the constants for the kinetic equations
The degree of constancy is about the same in each case so that the data from this type of experiment with excess of oxygen will not permit us to differentiate between these three equations. The method used to determine the hydrogen pressure-calculation from point to point on the curve assuming no side reactions-makes all the errors cumulative. It is proposed to repeat these experiments with a modified apparatus, employing a circulatory system which will enable samples to be taken from time to time during the course of the reaction. This will enable one to obtain a much more accurate idea regarding
PHOTOSENSITIZATION BY MERCURY ATOMS
0 h
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43
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44
ABRAHAM LINCOLN MARSHALL
the value of K. The degree of constancy obtained is all one can expect under present conditions, The results obtained at the higher temperature with the zHz 1 0mix~ tures seem to rule out equation (3). In these cases the water vapor pressure was as high as 2 5 0 mm. but in no case did the reaction rate show any decrease until the very end. Some of the earlier experiments at 50°C using excess hydrogen over that necessary for a two t o one mixture showed practically no increase in rate towards the end of the reaction as predicted by equation (I). So it can be said that the results at present available favor the kinetic equation ( 2 ) .
+
Experiments by H. A. Stuart’ on the quenching of resonance fluorescence in mercury vapor by various gases indicate that hydrogen, oxygen and water vapor all have a very strong effect in quenching the fluorescence. In the light of this knowledge, it seems that these gases are all able to take energy from excited mercury atoms by collisions of the “second kind,” and in amounts proportional to their concentration. Cario and Franck2 have shown that hydrogen atoms are formed by collisions between molecular hydrogen and excited mercury molecules. Apparently the energy absorbed by the oxygen is degraded as heat. The probability of the collision of a hydrogen molecule with an excited mercury atom is proportional to the hydrogen pressure divided by the sum of the hydrogen and oxygen pressures and it is this probability which dctermines the rate of the reaction. It is possible to put forward several suggestions concerning possible meclanisms but the thermal data are entirely lacking to test these thermodynamically. The first step is of course the dissociation of hydrogen then
+ + + + +
Hz Hg (excited) = zH H 0 2 = HOz HO2 =HO+O HOz Hz = HzOz H 0 H2 = Hz0 OH Hz = Hz0 H HzOz = HzO 0
+ + +
+ Hg
(1)
(2)
(31 (4)
(5) (6)
(7) Such a series of reactions would give a “chain mechanism.” If (4) is one of the stepsin the reaction it should be possible by use of a rapid circulatory system toremove the hydrogen peroxide from the reaction chamber and obtain sufficknt quantities for a qualitative test. There is a distinct possibility also of ozoie taking part in the reaction. If hydroxyl is present in one stage it might be possible to show its presence by a study of the reactions in a mixture of ethylene, hydrogen and oxygen. There is one other possible factor ‘Z. Physik 33, 262 (1925). *Z. Physik 11, 162 (1922).
PHOTOSENSITIZATION BY MERCCRY ATOMS
45
that has not been discussed so far. There is at present no experimental evidence on the intensity of the 1849 A" line in the water cooled arc altho Wood' has evidence of its presence. This light also excites resonance radiation, of greater energy, in mercury and the quantum ( I 50 Kg. cals.) may be sufficient to dissociate oxygen. Further work will be necessary to settle this point.
It has been mentioned that at times considerable amounts of mercuric oxide are formed. The rate of formation is greatest a t 70°C, where there is a relatively high concentration of mercury vapor. A high mol fraction of oxygen greatly favors its formation. Under these conditions it is possible for the reaction Hg 0 = HgO
+
to proceed to a considerable extent and this may be the mechanism of its formation. A very rough calculation shows that a considerable portion of the energy of X 2 53 6 emitted by the arc must be absorbed in the reaction system to obtain equivalence between quanta absorbed and molecules reacting. The lamp was burning on 20 volts and I O amps. The intensity of the visible radiation is very much less than with the arc burning at 6 0 volts and 3 amps. Let us assume that 10% of the energy, some 20 watts is radiated and that ~ 5 of 7~ this is in the 2536 A" line. This amounts to 5 watts with a quantum of 7.8 (1o)-I2 ergs or 3.8 ( 1 0 )quanta. ~~ The maximum rate observed was 8 cm. pressure change per minute in a volume of 120 C.C. which corresponds to 2/3 X 8/760 X
120/22400
X 6
(10)~= ~ 2 . 2 (10)'~
molecules of water formed per minute. This calculation shows that it is necessary for large absorption of the 2536 A" line to obtain equivalence. Unless the width of the absorption band
of mercury is very greatly increased by the presence of the reacting gases such equivalence is impossible. In a previous communication2 it has been pointed out that the reaction between hydrogen and ethylene does not take place when the mixture is illuminated by a hot mercury arc. This is attributed to self-reversal of the resonance line in the arc. Such reversal, however, cannot be detected with an ordinary quartz spectrograph. This seems to indicate that in this case at least only a very small fraction of the energy in the line 2536 A" is useful in exciting the mercury atoms. At present it seems necessary to postulate a "chain mechanism," such as that given, to account for the rate of reaction observed.
Summary Photochemical studies of gas reactions should yield valuable information on the mechanism of chemical reactions. Proc. Roy. SOC. 106,687 (1924). This Journal 29, I 140 (1925).
46
ABRAHAM LINCOLN XARSHALL
A quartz reaction system has been developed t o study reactions sensitized by optically excited mercury atoms. A more complete study of the reaction between hydrogen and oxygen has been made. The kinetic equation has been shown to be of the form
It seems probable from energy considerations that we are dealing with a “chain reaction” and several possible mechanisms are advanced. Research Laboratory GenLral Electric Company Schenectady N e w York
