+
SYKTHETIC FORMALDEHYDE FRO11 CO HZ MECHAKISM O F T H E SEKSITIZED PHOTOCHEMICAL REACTIOK Contribution from the Laboratory of Physical Chemistry. Princeton University. BY ABRAHA4M LIh-COLS MARSHALL
This paper deals with a study of the kinetics of the photochemical reaction between hydrogen and carbon monoxide when sensitized by mercury vapor to light of wave length 2536.7 A from a water-cooled mercury arc. I n all the experiments made a circulatory system was used similar to that described in a previous paper' and the gases were saturated with mercury at 50°C.. the reaction
FIG.I
On account of the variations observed in previous work in the light emitted from a mercury arc completely immersed in water, the lamp in this investigation was only cooled for a distance of 2 cm. above the level of the mercury cathode. Measurements were made of the amount of energy absorbed while the reaction was proceeding in order to be certain that conditions did not change during a given experiment and to be able to calculate the rate for a given energy absorption. From the data so obtained it is possible to make a number of new calculations of the quantum yield t o supplement those alreacl? given in a previous *J. Phys C'hem., 30, 1078 (19261.
163 5
S I T T H E T I C FORMALDEHYDE
publication. The volume of the apparatus in all these experiments was 890 C . C . The experimental arrangement and technique were the same as already described. Table I gives the data for two experiments at slightly varying composition.
TABLE 1 Composition 491 mm. CO, 2 0 2 mm. H, Composition 433 mm. CO, 167 mm. H? time pressure AP time pressure AD 0
5 IO
15 20
25
30 41
60 r 1 -3
90 105
I20
150 I 80 250
3 I6 371 416
679 673 66 I 649 63 7 626 614 593 553 524
6
0
I2
>
I2
IO
I2
'5
I1
30 45 70
I2 21
40 29 28 26 24
496 469 445 402 372 316 279 273 276
43 30 56 37
85 115
130 I45 160 I7.i
60 I 592 583 574
9 9 9 30 29
544 515 490 46 5 42 0 399 381 362 345
2. i 45 31
I8 I9 17
I
LIGHTABSORPTIOX cell free from mercury 269 reaction proceeding I 60 ~
absorption
I09
LIGHTABSORPTIOX cell free from mercury 259 reaction proceeding I6I ~
absorption
98
The calibration of the thermopile gave 235 scale divisions for an energy density of 47.9 IO)-^ watt em.-?. In these experiments the thermopile was 3 0 em. from the arc. The average absorption in the two experiments was 1.68 ( 1 0 )quanta ~~ per minute. I n the first experiment 3.76 ( 1 0 ) ~molecules ~ of formaldehyde were produced per minute and in the second 2.82 (10)~~. These values correspond to a quantum yield of respectively 2.24 and 1.68. -1series of ten experiments was made starting with mixtures of varying compositions. The results are plotted graphically in Fig. I and are tabulated in Table 111. The numbers on the curves refer to the number of the experiment which is given in the first column of the table. The second and third columns give respectively the initial partial pressures of the hydrogen and carbon monoxide and the fourth column the initial rate of reaction in mm. per minute. The remaining columns give reaction velocity constants for various
1636
.iBRAH.IN LIKCOLN MARSHALL
kinetic equations, The experiments listed in this table took about a week to perform and it can be clearly seen from a reference to the rate in ( I ) and ( I O ) that the amount of energy absorbed had decreased to about half value in this period. The velocity constants for all four kinetic equations lie within this range and it is impossible to reach a decision from these data. The results for the final experiment performed are given in Table 11.
TABLE I1 Composition 510 mm. CO, 279 mm. HS Thermopile readings: calibration 185 scale divisions for 47.9(10)-~ watt 11 2j3 ” no mercury present cm.-2. 1) 188 ” reaction proceeding 71 absorbed 6j time pressure AP 0 788 8 ;80 18 7 ~
))
15 32 44 150 16.5
762 726 700
36 26 I 88
512
22
390
The absorption is a t the rate of 1.3 j IO)'^ quanta per minute. The maximum rate of reaction is 2.1 mm. per minute corresponding to 3.3 ( 1 0 ) l g molecules of formaldehyde formed or quantum yield of 2.44. At this point it might be well to point out that the reaction product obtained in the liquid air trap is largely formaldehyde or a polymer which readily yields this gas on heating. In experiment (3) the maximum yield of formaldehyde obtainable was 149 C.C.and I I 5 C.C. were actually obtained giving a yield of 7; percent, In experiment ( j ) the theoretical amount was 2 0 0 c.c. and 138 C . C . were obtained. Much weight however cannot be placed on the yield of formaldehyde but the results suffice to show that it is the major product of the reaction. In order to attempt to differentiate between the various possible kinetic equations it was necessary to obtain more reproducible results. It was decided to study only the initial rates of reaction and not t o attempt to calculate from point to point along a curve as it is still uncertain that the gases disappear in exactly the ratio of I : I and for this reason such a method would be indecisive. Two series of experiments were made which are listed in Tables IT’ and T’. Some of these results are plotted in Fig, 2 . It will be observed that for the first hour the reaction velocity is practically constant. On inspecting the reaction velocity constants in these tables it is soon seen that those depending on proportionality to (2/=) (pco)give a much steadier value than those depending on ( p H J (pco). I n Table IV the former vary between 2j-34 ( 1 0 ) ~and the latter over the range 13-34 IO)-^. In TableV the variation is 19-2 j (IO)-; and 8-29 IO)-^ respectively. It would thus seem
SYNTHETIC FORMALDEHYDE
1638
.iBR.IHAM L I N C O L S MARSHALL
;
-
I-
-
- 0 0
.
C
.
.
r,r N
k
e x
.
' N N
. e. -.
cc -
b
-
X
c , c . * . - o ~ .* .* o. o. h.
-
e
A
A
A
0
0
i
N
w,*wic
Pi
N
N
N
- -
m
A
A
A
A
A
A
h
.I N Cl v v v v v v v v v v N
N
I639
SPNTHETI C FORMALDEHYDE
that the velocit>-depended in some way on the one-half power of the hydrogen and the first power of the carbon monoxide concentration. This would seem to mean that the velocity depended on the concentration of atomic hydrogen. The mechanism which has previously been postulated for this reaction leads to a kinetic equation of this type, Hg' HZ 2H Hg ( I ) kl H CO = HCO ( 2 ) k? HCO HZ = HCHO H (3) k3 H + H = H ? (4) k4
+ + +
+ d dt (H)
+
7
+
Hz
=
ki 1 (HZ+ co)
+
+ k~ (HCO)iH?)
(CO) k, (H)' k? (H) (CO) - d d t (HCO) = k3 (HCO) (Hz) I n the steady state ( j ) = (6) and ( 7 ) = (8). From this it iq that the rate of reaction (3) is governed by the equation - d d t (H) = k, (H)
+ d 'dt (HCO)
d dt (HCOH) =
=
(K
1I
Ii(d/H2) iCo)l
=
H?
H:!+CO 1 H Z+ C O
(5) (6) (7)
(8) easy to show
) ( C O ) (9) (10)
I t Ti-as not possible to determine the dependence of the reaction velocity on light intensity with the experimental arrangement employed but by varying the total pressure a test could be made on the variation with pressure. Experiments (8),( 2 5 ) and (26) were made to test this point. K i t h the slow velocities observed in these caseq the results are somewhat uncertain but seem to indicate that the rate for a given composition is not inversely proportional to the square root of the total pressure as deduced in ( I O ) but is given by the equation ti dt (HCHO)= k ( v ' E ) ( p c o ) If this is the correct equation for the rate the velocity with a given gas mixture should depend on p 2. It therefore becomes important t o investigate this reaction a t high pressures. Boehm and Bonhoeffer' have studied the reactions between atomic hydrogen produced by Kood's method and carbon monoxide or dioxide. Their total pressure was Z. phgsik. Chem., 119, 385 (1926).
3 ?3
Izo
0
20
40 FIG.2.
60
1
about 0.4 mm. and they obtain a yield of about a milligram of formaldehyde in I O minutes. in the two cases. The apparent discrepancy between their results and those of Taylor and Marshall' who observed no reaction with carbon dioxide and a relatively rapid reaction with carbon monoxide at 50-60 rm. pressure is easily understood. Since the latter reaction depends on p3 one would expect that at a fraction of a millimetre the rate would be negligible. The reaction b e h e e n carbon dioxide and hydrogen has been reinvestigated with the present set up and it has been found that reaction proceeds at a relatively slow rate. Pressure changes of the order of 2-4 nini. per hour have been observed with a total volume of 19.30 C . C .
Summary A study has been made of the dependence of the rate of formation of formaldehyde on the concentrations of hydrogen and carbon monoxide in the photochemical reaction sensitized by mercury. X theoretical equation has been deduced of the forin 2. I.
3 . The results obtained indicate that the-velocity is determined by the equation d d t (HC'HO) = K(V'&~) ( p c o ) . 4. Such a kinetic equation points clearly to the fact that atomic hydrogen is one of the constituents taking part in the reaction.
I n conclusion we would like to acknowledge our indebtedness t o the Hanovia Chemical and lllanufacturing Company and the General Electric Company for much of the quartz apparatus which has made this and previous investigations possible. 1
J. Phys. Chem., 29, 1140 (19251.
