CHEMICAL ACTION I N T H E GLOW DISCHARGE
VI. The Oxidation of Carbon Monoxide BY A. KEITH BREWER ASD PAUL D. KUECK*
The oxidation of carbon monoxide in the glow discharge is of particular interest from two angles. First, it shows the physical state in which the molecules must be placed to become chemically active. Second, it enables the rezction to be studied in steps from the simple non-propagating oxidation, through various lengths of reaction chains, until ignition point is reached. The cathodic combustion of carbon monoxide has been studied by Finch and Hodge.' The investigators conclude the combustion to be determined by ions of both constituent molecules, and that metal atoms sputtered from the cathode are of vital importance in aiding the reaction by overcoming the electrostatic repulsion between the ions. Recently Finch and Thompson2 failed to find bands due to ionized carbon monoxide in the cathode zone of an electric discharge in a ZCO/OZmixture. From this they conclude, in contradiction to their previous contentions, that the ionization of carbon monoxide is unnecessary in this reaction. At the same time they state that the failure to find bands due to ionized oxygen is no indication of the absence of these ions. Lind and B a r d ~ e l lstudying ,~ this reaction in the presence of radon, found that carbon monoxide, when sufficient oxygen was present for combustion, was completely oxidized to carbon dioxide. The maximum ratio of molecules formed to ions reacting was obtained with the combining mixture; from this it was concluded that CO+ and 0 2 + ions are equally effective. Average values of M(c~+o,,/N(co+o~, = 6, or Mco,/N(co+o,, = 4, were obtained. Hunt and S c h ~ m binvestigating ,~ the carbon monoxide oxygen equilibrium found in general that the rate of oxidation was increased by excess carbon monoxide and retarded by excess oxygen. The ignition of carbon monoxide oxygen mixtures in the condensed electric discharge has been studied by several investigators5 with the apparent agreement t h x propagation takes place when the ion density in the discharge reaches a definite value depending on the conditions involved. * Fertilizer and Fixed Nitrogen Investigations, Bureau of Chemistry and Soils, Washington, D. C. Finch and Hodge: Proc. Roy. SOC.,124, 303 (1929). * Finch and Thompson: Proc. Roy. Soc., 129, 314; Finch and Patrick: 656 (1930). 3 Lind and Bardwell: J. Am. Chem. SOC., 47, 2675 (1925). 4 Hunt and Schumb: J. Am. Chem. Soc., 52, 31j 2 (1930). 6 Finch and Cowen: Proc. Roy. Soc., 116, 5 2 9 (192;). 5 Brewer: Proc. Nat. Acad. Sei., 13, 689 (1927). 7 Brewer and Deming: J. Am. Chem. Soc., 52, 422j ( 1 9 3 0 ) .
1282
A. KEITH BREWER AND PAUL D. KUECK
Apparatus The apparatus and general method of procedure employed were described* in earlier article appearing in this series.8~9 The carbon monoxide used was prepared by the action of formic acid on phosphoric acid. The other gases were purified as described in previous communications. Results The oxidation of carbon monoxide follows much more closely the oxidation of hydrogens than it does the oxidation of nitrogen.1° This is due to the fact that at pressures higher than about one centimeter propogation or chain reactions occur with both hydrogen and carbon monoxide. An electric discharge in a zCO/02 mixture a t liquid air temperatures gives carbon dioxide as the reaction product, while in the presence of excess carbon monoxide or of neutral gases, carbon suboxide is deposited on the walls of the tube along with carbon dioxide. Carbon monoxide alone dissociates into carbon dioxide and carbon suboxide; the rate is from a quarter to a half that for the oxidation. Reaction i n the Negative Glow. A typical curve for the rate of carbon dioxide synthesis from a 2CO/O2 mixture in the negative glow is illustrated in Fig. I , curve I . Curves of this type have been taken with discharge tube No. I, a t pressures as high as 30 mm, without any further increase in rate than that occurring a t 15 mm., and with no tendency towards ignition. Indeed it has never been possible to produce ignition of a 2C0/02 or 2H2/02 mixture in the negative glow; propagation appears to start only in the positive column. At pressures below about 9 mm., the rate accelerates in a manner very different from that observed for the previous reactions of this series. It will be noted that the voltage as given by curve I V increases simultaneously with the increase in rate. The rise in voltage a t these pressures appears to be due to a change in the normal cathode fall of potential occasioned by the deposition of carbon dioxide and possibly the suboxide on the cathode. Since the rate of ion production is very nearly proportional to the normal cathode fall, the observed increase in synthesis is to be expected. The effect of current on the rate of synthesis was investigated over the range from I O ma to 50 ma. The rate was found proportional to the current in tubes I and I1 over the entire current and pressure range for the negative glow. The results obtained by varying the ratio of carbon monoxide to oxygen are illustrated in Fig. 2 . Curve 2 was taken a t 3.5 mm. in tube No. 11, equipped with aluminum electrodes, while curve I was taken at I O mm. with iron electrodes in tube No. I . 8Brewer and Westhaver: J. Phys. Chem., 34, 153 (1930). 9 Brewer and Westhaver: J. Phys. Chem., 34, 2,343 (1930). '0 Westhaver and Brewer: J. Phys. Chem., 34, 554 (1930).
1283
CHEMICAL ACTION IN THE GLOW DISCHARGE
I
0
10
I
20
I
30
I
40
% Carbon
I
SO
FIG.2
I
60
Monoxide
I
I
70
I 80
I
90
I
1W
1284
A . KEITH BREWER AND PAUL D. KUECK
The difference in rate for curves I and 2 is due primarily to a difference in the voltage characteristics of the two tubes rather than to the difference in pressure. The cathode drop in potential was about twice as great for tube I1 as for tube I. The effects of excess carbon monoxide and oxygen are shown further in Fig. 3; the results with helium and argon are also given. These data were taken under the same conditions as was curve z of Fig. 2. It will be noted from Figs. 2 and 3 that excess carbon monoxide has an accelerating effect on the rate in all mixtures in which it constitutes less than
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