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The reaction of carbon and atomic oxygen produced in a microwave discharge
Carbon
Pergamon Press Ltd.
1966, Vol. 4, pp. 467-472.
OF CARBON AND ATOMIC
THE REACTION
IN A MICROWAVE
PRODUCED
R. C. MELUCCIt Chemistry
Printed in Great Britain
Department,
OXYGEN
DISCHARGE*
and J. P. WIGHTMAN
Virginia Polytechnic (Rec&ed
Institute,
Blacksburg,
Virginia, U.S.A.
24 March 1966)
Abstract-The reaction of graphitic carbon with atomic oxygen was studied at room temperature as a function of oxygen pressure and distance of the carbon sample from the discharge. Significant concentrations of CO and CO, were produced on reaction. Supporting evidence indicated that a possible mode of formation of CO2 was the gas phase reaction between CO, the primary reaction product, and atomic oxygen. A gas chromatographic technique was developed whereby mixtures of 0,, CO and CO, at low pressures could be analyzed quantitatively. The concentration of atomic oxygen was established by product analysis and independently by isothermal calorimetry. Good agreement was obtained on comparison of results of the two techniques
(1) the identification of reaction products by gas chromatography;
1.INTRODUCTION THE REACTIONof atomic oxygen and carbon has been studied in several laboratories and divergent results have been reported.(‘-6) It is generally agreed that a reaction does occur at room temperature and that CO and CO2 are reaction points. MARSH et uZ.(~) however did not note any significant reaction at room temperature although a finite concentration of atomic oxygen was reported. Several factors including the type of discharge used to produce atomic oxygen appear significant and may account for some of the discrepancies in the reported BLACKWOOD and results. MCTAGGART(I) for example using a 30 MC radiofrequency discharge reported much higher concentrations of atomic oxygen at comparable oxygen pressures and distances downstream than VASTOLA et uZ.(~) The latter workers used a 2450 MC microwave discharge. The reaction of atomic oxygen generated in a microwave discharge with graphitic carbon at room temperature has been studied in this laboratory and the following areas investigated:
(2) the determination of the concentration atomic oxygen from product analysis;
(3) the determination of the concentration of atomic oxygen by isothermal calorimetry; and (4) the origin of carbon product. Quantitative results areas simultaneously previously.
dioxide as a reaction
encompassing all of these have not been reported
2. EXPERIMENTAL A high vacuum controlled gas flow apparatus was used. A typical mass flow rate was 1.92~ 1O-6 moles/min at an oxygen pressure of 0.07 torr corresponding to a linear flow rate of 10.8 cm/set. Oxygen was passed through a dry ice-acetone trap prior to dissociation in the microwave cavity. Oxygen pressures ranged from 0.03 to 0.11 torr and were measured on a calibrated Hastings thermopile gauge located near the discharge zone. Spectroscopically pure graphite rods (dia. 6 mm) obtained from the National Carbon Corpn. were suspended vertically above the discharge zone and could be positioned at varying distances from the discharge zone. The carbon rod was evacuated to
*This work is based on the M.S. thesis of one of us (R.C.M.) and was presented in part at the SoutheastSouthwest Regional Meeting of the American Chemical Society, Memphis, Tenn., Dec. 1965. +Present Address : Chemistry Department, Chester State College, West Chester, Penna.
of
West
467
468
R. C. MELUCCI
and J. P. WIGHTMAN
with the atomic oxygen. The reaction products were collected in a removable sample tube which was positioned approximately 100 cm from the discharge zone. A Beckman GC 2A gas chromatograph was used to identify and determine the concentration of the gaseous reaction products which was a mixture of CO and COa in addition to Oz. The reaction products were entrained in a He stream and were first passed through a Beckman 6 ft stainless steel ($ in.) silica gel column and after passage through the thermal conductivity detector, the gases passed through a Beckman 6 ft stainless steel ($ in.) molecular sieve 15A column and finally passed through the other side of the detector. The silica gel column provided a good separation of CO and 02 from COa however the separation of CO and 02 was poor. The CO and 0s were separated satisfactorily after passage through the molecular sieve column. The He flow rate was 60 cm3/min at 30 psig and the filament current in the detector was maintained at 350 mA. The concentration of atomic oxygen was calculated from the measured concentrations of the reaction products. Calorimetry was employed as an independent method of determining the concentration of atomic oxygen. The isothermal calorimetric technique used was an adaptation of the method described by ELIAS et a1j7) Atomic oxygen was passed over a spiral made from approximately 60 cm of No. 24 copper wire with an oxide coating produced by exposure of the copper spiral to a stream of atomic oxygen. A known electric current was passed through the wire and the resistance of the wire measured with a Wheatstone bridge circuit before and after initiation of the discharge. A 2450 MC Raytheon microwave generator (Model KV-104) was used as the power source for the discharge. The air-cooled cavity was obtained from the Ophthos Co. The power level was maintained at 85 r.f. watts. The discharge was initiated by a Tesfa coil. Oxygen (USP grade) was obtained from Airco and was purified by passage through a liquid air trap. 0s was analyzed on an 18 ft copper (i in.) molecular sieve 5A column at 70°C and a helium Aow rate of 60 cm3/min at 30 psig. The purified 02 had an average NZ content of 0.04 per cent. Carbon monoxide (CP grade) was obtained from Air Products and was purified by passage through a
dry ice-acetone trap. No impurities were detected by gas chromatographic analysis. Carbon dioxide (bone dry grade) was obtained from the Matheson Co. and was purified by passage through a dry iceacetone trap. Gas chromatographic analysis showed no detectable impurities. Argon (USP grade) obtained from Airco was used without further purification. 3. RESULTS 3.1 Atomic Olga-carbon reaction The atomic oxygen-carbon reaction was studied at oxygen pressures of 0.03, 0.07 and 0.11 torr. The distance of the carbon rod from the discharge varied from 1 to 15 cm. The temperature in the reaction zone was measured with a thermometer suspended in place of the carbon rod. The highest temperature observed was 18” above room temperature which occurred just outside the discharge zone. The increase in temperature is due primarily to radiation from the discharge since the residence time of gaseous species in the discharge is too short to significantly effect the neutral gas temperature. The background contribution to the product analysis of the atomic oxygen-carbon reaction was assessed in the following way: the carbon sample was removed and oxygen passed through the discharge zone thus allowing the atomic oxygen produced to react with any extraneous carbon sources present in the system. The background contribution was negligible since neither CO or CO2 was detected in any of the background runs. Preliminary experiments indicated that constant gaseous concentrations were insured if each reaction was allowed to proceed for at least 5 min before collecting the product gases for analysis. Average concentrations of the reaction products at several oxygen pressures and with the carbon rod at a distance of 6 cm from the discharge are shown in TabIe 1. TABLEI
PRODUCTANALYSISOFATOMICOXYGEN--CARBON REACTION
Product gases (mole %) CO CO,
Oxygen pressure (torr)
0,
0.11
94.3
2.9
1.2
0.07
91.9
5.9
2.2
0.03
89.0
6.9
4.1
THE
REACTION
OF CARBON
0 .03 A .07
AND ATOMIC
469
OXYGEN
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0 .03
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A .07
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FIG. 1. Percent atomic oxygen (f*) determined from product analysis as a function of distance from the discharge at several oxygen pressures.
FIG. 2. Percent atomic oxygen (fL> determined from caiorimetry as a function of distance from the discharge at several oxygen pressures.
In every case, the concentration of CO was greater than the concentration of COz. Concentrations of atomic oxygen were calculated from the concentrations of the reaction products assuming that the major reactive species of an oxygen discharge was atomic oxygen, that CO2 is formed by the reaction between CO and atomic oxygen, and that back diffusion was negligible. The percent calculated from product atomic oxygen (f;) analysis is given by equation (1)
discharge. Concentrations of atomic oxygen of approximately 20 percent were realized under the present experimental conditions just outside the discharge zone.
f= r
(cwww 2(02)-t(C0)+2(c0~)
x100
(1)
where the bracketed quantities are concentrations of product gases expressed in moles. The percent atomic oxygen calculated from product analysis as a function of distance of the carbon sample from the discharge and at several oxygen pressures is shown in Fig. 1. At a given oxygen pressure, the concentration of atomic oxygen decreased exponentially with increasing distance from the discharge. Further, at short distances, the concentration of atomic oxygen decreased with increasing oxygen pressure at a given distance from the
3.2 Isothermal calorimetry The concentration of atomic oxygen was determined independently by isothermal calorimetry assuming that the major reactive species of an oxygen discharge was atomic oxygen. The percent atomic oxygen (fc) was calculated following the method outlined by ELIAS.")The percent atomic oxygen calculated from calorimetric data as a function of the distance of the copper coil from the discharge and at several oxygen pressures is shown in Fig. 2. The scatter here is signi~cant~y less than in Fig. 1, even at low pressures. At a given oxygen pressure, the concentration of atomic oxygen decreased exponentially with increasing distance from the discharge. Further, the concentration of atomic oxygen decreased with increasing oxygen pressures at a given distance from the discharge. These results are consistent with a higher rate of recombination of atomic oxygen with decreasing mean free path at increasing pressures. At an oxygen
R. C. MELUCCI
470
and J. P. WIGH’IMAN
pressure of 0.07 torr, the calculated value for the mean free path of atomic oxygen is 4.5 mm. Satisfactory agreement is seen between the results of the two techniques if Fig. 1 and 2 are compared at a constant oxygen pressure. 3.3 Gas phase reactions The origin of CO2 observed as a reaction product in the atomic oxygen-carbon reaction was investigated by studying the reaction between atomic oxygen and carbon monoxide introduced at a distance of 3 cm from the discharge zone. The oxygen pressure was 0.07 torr and the flow of CO was increased until the total pressure was 0.11 torr. The product analysis is shown in Table 2. The average deviation of oxygen/carbon ratios calculated from the results reported in Table 2 was 0.62.
CO is the only significant gaseous product of the CO discharge. Appreciable amounts of 02, CO and COa were observed as the gaseous products of the COa discharge. A 76 percent 0 a and 24 percent CO mixture was passed through the discharge at a total pressure of 0.11 torr. The variation in oxygen content of the reactant mixture was estimated at 76&5 percent. The product analysis is shown in Table 4. The average deviation of o~~n~c~bon ratios calculated from the results in Table 4 was 0.79. CO2 is observed as a reaction product. TABLE 4. PRODUCTANALYSISOFCARBON MONOXIDE-OXYGENDISCH!.ItGE
0, TAELE 2. PRODUCTANALYSISOFATOMICOXYGEN-URBON MONOXIDEREACTION
GP
Product gases (mole %) co
CO%
56.8
32.1
11.1
59.4
31.7
9.9
62.4
27.6
10.0
63.7
26.5
9.8
71.0
21.3
7.7
CO2 was noted as a reaction product thus suggesting that a possible mode of formation of CO2 involved the reaction between CO (g) and atomic oxygen. Additional experiments were done in order to establish the validity of the suggested mode of formation of COz. Both CO and COa were passed separately through the discharge at a pressure of 0.11 torr. The product analysis for the two gases is shown in Table 3.
63.5 70.8 69.4
21.7 20.5 23.3
14.8 8.7 7.4
76.0
16.5
7.5
co
CO*
0,
5. PRODUCT
Product gases (mole %) CO CO*
Trace
99.0
Trace
27.5
46.7
26.6
Definite carbon deposit in the discharge zone No carbon deposit observed
No carbon deposit in the discharge zone
The formation of CO2 could occur by an alternate path namely by the back diffusion of CO into an oxygen discharge. To investigate this possibility, CO2 was introduced at a distance of 3 cm from an Ar discharge. The Ar pressure was 0.07 torr and the flow of CO2 was increased until the total pressure was 0.11 torr. These conditions correspond to those used in the atomic oxygencarbon monoxide reaction. CO2 was chosen as an indicator of back diffusion since it was observed to decompose extensiveiy in the discharge as shown in Table 3. The product analysis for the Ar discharge with inbled CO is shown in Table 5. The color of the Ar discharge was purplish; however, when CO2 was introduced downstream, the color changed to bluish-white. TABLE
Gas
Product gases (mole %) CO CGr
ANALYSIS OF AR INBLEDCOP
DISCHARGE
Product gases (mole %-argonand oxygen-free co CO* 70.1
29.9
70.5
29.5
basis)
WITH
THE REACTION
OF CARBON AND ATOMIC OXYGEN
Since CO is observed as a reaction produet, these resuits indicate that under the present experimental flow conditions back diffision does occur. The observed color change of the discharge is consistent with this conclusion. 4. DISCUSSION 4.7 Atomic o~g~~ur~~ reaction The results presented for the atomic oxygencarbon reaction are in general agreement with previous studiesf2*4*s) using microwave discharges in that an extensive reaction occurs between atomic oxygen and carbon at room temperature. There are certain discrepancies which indicate that a degree of caution is needed in making quantitative comparisons. VASTOLAel”a1.@1 reported an extensive reaction at oxygen pressures from 0.03-0.06 torr and carbon sample distances of l-30 cm. In the absence of carbon approximately 10 percent of the oxygen was consumed presumably due to reaction with extraneous carbon sources present in the system. The present results indicate that no detectable reaction occurred when the carbon sample was absent. The concentration of atomic oxygen at pressures and distances ~orrespond~g to the results of Vastola et al. was significantly less. Since a correlation was obtained in the present work between the amount of atomic oxygen present by two independent techniques, the discrepancy could be attributed to difIerences in the efficiency of coupling of the microwave energy in the discharge zone. The efficiency of coupling is determined partly by the type of cavity and indeed two different types of cavities were used in these two investigations. The satisfactory agreement between the concentrations of atomic oxygen as determined from product analysis and from isothermal calorimetry suggests that product analysis is a valid approach to the calculation of the concentration of atomic oxygen. A significant amount of recombination prior to reaction of atomic oxygen with either the carbon sample or CO is not consistent with the present results. The greater scatter in the product analysis data may possibly be attributed to back diffusion discussed below. The calorimetric technique is subject to error in the measurement of atomic oxygen if electronically excited molecular oxygen is presentf7) The concentration of excited 02 was assumed to be
471
negligible in the present work. Definitive evidence as to the concentration of excited 02 is lacking in the pressure range studied in the present work. HERFCON and SCHII;‘F@)have shown that the concentrations of excited 02 and atomic oxygen from an oxygen discharge were about equal (10 percent) over the range 0.1 ta 2 torr. No pressure dependence of the ratio of these two species was indicated. 4.2 Gas phase reactions The observed decomposition of CO in the dis-. charge noted in Table 3 agrees with the results of ~~TA~~~RT~~) who reported that at CO pressures of 1-3 torr, deposits formed on the walls in the discharge region. CO, CO2 and 02 were reported present in the effluent gas mixture. Only trace quantities of CO2 and 02 were observed in the present work but a definite deposit was noted indicating decomposition of CO. The tenfold lower pressure and the lower power level used in the present work probably accounts for the observed difference in the product gases. The fact that no deposit was observed in the COa discharge {Table 3) whereas a definite deposit was formed in the case of CO is significant. The energy required to break one C-O bond in CO2 is 191.7 kcal/mole whereas for CO the corresponding value is 256.6 kcal/mole. Since CO was observed to decompose, it is reasonable to assume that CO1 also underwent decomposition. The CO2 discharge however is oxygen-rich relative to the CO discharge and an oxygen-rich discharge gives rise to gaseous products rather than a carbon deposit. This conclusion is further supported by the results of the product analysis on passing a mixture of CO and 02 through the discharge (Table 4). Again, no deposit was observed to form in the presence of excess oxygen Since a measurable amount of CO2 is formed in the atomic oxygen-carbon reaction, the question arises as to the possible mode of formation of COa. A possible mode of formation of CO2 is a primary reaction between electronically excited molecular oxygen with carbon, Since no definitive evidence exists as to the concentration of the excited molecuiar species from oxygen discharge at pressures comparable to the range used in this work, this reaction remains a plausible one. A second possible mode of formation of CO2 is a
472
R. C. MELUCCI
and J. P. WIGHTMAN
secondary reaction between atomic oxygen and the primary reaction product, CO. Supporting evidence for this mode is found on examination of the results shown in Table 2. An appreciable amount of CO2 was produced by the reaction of CO and atomic oxygen in the absence of the carbon sample. The surface of carbon may play a part in the formation of CO2 but it should not be regarded as essential to its formation. The results of this study suggest that the primary reaction is between atomic oxygen and carbon and CO formed in this reaction reacts with excess atomic oxygen to form COz. There are two complicating factors in the above analysis of the formation of COz. Back diffusion of CO, the primary reaction product, into the oxygen discharge could produce CO2 as is indicated by the results in Table 4. The results of the back diffusion studies unfortunately do not lead to an estimate of the extent of back diffusion. GESSER and HUSSAIN(‘~~noted in a recent paper that photochemical reactions may occur outside of the discharge zone of different types of discharges, the discharge itself serving as a photolytic source. Such a photochemical effect is not exclusively operative in the present system since the color of the Ar dis-
charge was observed to change when CO was introduced downstream. Photochemical reactions downstream would not alter the color of the Ar discharge. Acknowledgments-One of us (R.C.M.) was a NASA Trainee Fellow for the duration of this work. Partial financial support for equipment by a Grant-in-Aid of Research from the Society of the Sigma Xi is gratefulfy acknowledged. REFERENCES 1. BLACKWOODJ. D. and MCTAGGART F. K., Aust. J. Ckem. 12,114 (1959). 2. VA~TOLAF. J., WALKER P. L. JR. and WIGHTE~~AN J. P., Carbon 1, 11 (1963). 3. MARSH H., O’HAIR E., REED R. and WYNNE-JONES W. F. K., Nature 198, 1195 (1963). 4. OTTERBEIN M. and BONNETAINL., Compt. ReBd 258, 2563 (1964). 5. OTTE~EIN M. and BONNETAINL., Compt. Rend. 259,
791 (1964). 6. ROSNERD. E. and ALLENDORFH. D., Carbon 3,153 (1965). 7. ELIAS L., OGRYZLOE. A. and SCHIFF H. I., Can. r. Chem. 37, 1680 (1959). 8. HERRON J. T. and SCHIFF H. I., Can. J. Ckem. 36, 1159 (1958). 9. MCTAGGART F. K., Awt. J. Cttm. 17, 1182 (1964). 10. GENDERH. and HUSSAINS., Nature 201,290 (1964).
Pergamon Press Ltd.
1966, Vol. 4, pp. 467-472.
OF CARBON AND ATOMIC
THE REACTION
IN A MICROWAVE
PRODUCED
R. C. MELUCCIt Chemistry
Printed in Great Britain
Department,
OXYGEN
DISCHARGE*
and J. P. WIGHTMAN
Virginia Polytechnic (Rec&ed
Institute,
Blacksburg,
Virginia, U.S.A.
24 March 1966)
Abstract-The reaction of graphitic carbon with atomic oxygen was studied at room temperature as a function of oxygen pressure and distance of the carbon sample from the discharge. Significant concentrations of CO and CO, were produced on reaction. Supporting evidence indicated that a possible mode of formation of CO2 was the gas phase reaction between CO, the primary reaction product, and atomic oxygen. A gas chromatographic technique was developed whereby mixtures of 0,, CO and CO, at low pressures could be analyzed quantitatively. The concentration of atomic oxygen was established by product analysis and independently by isothermal calorimetry. Good agreement was obtained on comparison of results of the two techniques
(1) the identification of reaction products by gas chromatography;
1.INTRODUCTION THE REACTIONof atomic oxygen and carbon has been studied in several laboratories and divergent results have been reported.(‘-6) It is generally agreed that a reaction does occur at room temperature and that CO and CO2 are reaction points. MARSH et uZ.(~) however did not note any significant reaction at room temperature although a finite concentration of atomic oxygen was reported. Several factors including the type of discharge used to produce atomic oxygen appear significant and may account for some of the discrepancies in the reported BLACKWOOD and results. MCTAGGART(I) for example using a 30 MC radiofrequency discharge reported much higher concentrations of atomic oxygen at comparable oxygen pressures and distances downstream than VASTOLA et uZ.(~) The latter workers used a 2450 MC microwave discharge. The reaction of atomic oxygen generated in a microwave discharge with graphitic carbon at room temperature has been studied in this laboratory and the following areas investigated:
(2) the determination of the concentration atomic oxygen from product analysis;
(3) the determination of the concentration of atomic oxygen by isothermal calorimetry; and (4) the origin of carbon product. Quantitative results areas simultaneously previously.
dioxide as a reaction
encompassing all of these have not been reported
2. EXPERIMENTAL A high vacuum controlled gas flow apparatus was used. A typical mass flow rate was 1.92~ 1O-6 moles/min at an oxygen pressure of 0.07 torr corresponding to a linear flow rate of 10.8 cm/set. Oxygen was passed through a dry ice-acetone trap prior to dissociation in the microwave cavity. Oxygen pressures ranged from 0.03 to 0.11 torr and were measured on a calibrated Hastings thermopile gauge located near the discharge zone. Spectroscopically pure graphite rods (dia. 6 mm) obtained from the National Carbon Corpn. were suspended vertically above the discharge zone and could be positioned at varying distances from the discharge zone. The carbon rod was evacuated to
*This work is based on the M.S. thesis of one of us (R.C.M.) and was presented in part at the SoutheastSouthwest Regional Meeting of the American Chemical Society, Memphis, Tenn., Dec. 1965. +Present Address : Chemistry Department, Chester State College, West Chester, Penna.
of
West
467
468
R. C. MELUCCI
and J. P. WIGHTMAN
with the atomic oxygen. The reaction products were collected in a removable sample tube which was positioned approximately 100 cm from the discharge zone. A Beckman GC 2A gas chromatograph was used to identify and determine the concentration of the gaseous reaction products which was a mixture of CO and COa in addition to Oz. The reaction products were entrained in a He stream and were first passed through a Beckman 6 ft stainless steel ($ in.) silica gel column and after passage through the thermal conductivity detector, the gases passed through a Beckman 6 ft stainless steel ($ in.) molecular sieve 15A column and finally passed through the other side of the detector. The silica gel column provided a good separation of CO and 02 from COa however the separation of CO and 02 was poor. The CO and 0s were separated satisfactorily after passage through the molecular sieve column. The He flow rate was 60 cm3/min at 30 psig and the filament current in the detector was maintained at 350 mA. The concentration of atomic oxygen was calculated from the measured concentrations of the reaction products. Calorimetry was employed as an independent method of determining the concentration of atomic oxygen. The isothermal calorimetric technique used was an adaptation of the method described by ELIAS et a1j7) Atomic oxygen was passed over a spiral made from approximately 60 cm of No. 24 copper wire with an oxide coating produced by exposure of the copper spiral to a stream of atomic oxygen. A known electric current was passed through the wire and the resistance of the wire measured with a Wheatstone bridge circuit before and after initiation of the discharge. A 2450 MC Raytheon microwave generator (Model KV-104) was used as the power source for the discharge. The air-cooled cavity was obtained from the Ophthos Co. The power level was maintained at 85 r.f. watts. The discharge was initiated by a Tesfa coil. Oxygen (USP grade) was obtained from Airco and was purified by passage through a liquid air trap. 0s was analyzed on an 18 ft copper (i in.) molecular sieve 5A column at 70°C and a helium Aow rate of 60 cm3/min at 30 psig. The purified 02 had an average NZ content of 0.04 per cent. Carbon monoxide (CP grade) was obtained from Air Products and was purified by passage through a
dry ice-acetone trap. No impurities were detected by gas chromatographic analysis. Carbon dioxide (bone dry grade) was obtained from the Matheson Co. and was purified by passage through a dry iceacetone trap. Gas chromatographic analysis showed no detectable impurities. Argon (USP grade) obtained from Airco was used without further purification. 3. RESULTS 3.1 Atomic Olga-carbon reaction The atomic oxygen-carbon reaction was studied at oxygen pressures of 0.03, 0.07 and 0.11 torr. The distance of the carbon rod from the discharge varied from 1 to 15 cm. The temperature in the reaction zone was measured with a thermometer suspended in place of the carbon rod. The highest temperature observed was 18” above room temperature which occurred just outside the discharge zone. The increase in temperature is due primarily to radiation from the discharge since the residence time of gaseous species in the discharge is too short to significantly effect the neutral gas temperature. The background contribution to the product analysis of the atomic oxygen-carbon reaction was assessed in the following way: the carbon sample was removed and oxygen passed through the discharge zone thus allowing the atomic oxygen produced to react with any extraneous carbon sources present in the system. The background contribution was negligible since neither CO or CO2 was detected in any of the background runs. Preliminary experiments indicated that constant gaseous concentrations were insured if each reaction was allowed to proceed for at least 5 min before collecting the product gases for analysis. Average concentrations of the reaction products at several oxygen pressures and with the carbon rod at a distance of 6 cm from the discharge are shown in TabIe 1. TABLEI
PRODUCTANALYSISOFATOMICOXYGEN--CARBON REACTION
Product gases (mole %) CO CO,
Oxygen pressure (torr)
0,
0.11
94.3
2.9
1.2
0.07
91.9
5.9
2.2
0.03
89.0
6.9
4.1
THE
REACTION
OF CARBON
0 .03 A .07
AND ATOMIC
469
OXYGEN
tofr
0 .03
tar-r
t
A .07
t Oi-f-
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11
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6 DISTANCE
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km)
FIG. 1. Percent atomic oxygen (f*) determined from product analysis as a function of distance from the discharge at several oxygen pressures.
FIG. 2. Percent atomic oxygen (fL> determined from caiorimetry as a function of distance from the discharge at several oxygen pressures.
In every case, the concentration of CO was greater than the concentration of COz. Concentrations of atomic oxygen were calculated from the concentrations of the reaction products assuming that the major reactive species of an oxygen discharge was atomic oxygen, that CO2 is formed by the reaction between CO and atomic oxygen, and that back diffusion was negligible. The percent calculated from product atomic oxygen (f;) analysis is given by equation (1)
discharge. Concentrations of atomic oxygen of approximately 20 percent were realized under the present experimental conditions just outside the discharge zone.
f= r
(cwww 2(02)-t(C0)+2(c0~)
x100
(1)
where the bracketed quantities are concentrations of product gases expressed in moles. The percent atomic oxygen calculated from product analysis as a function of distance of the carbon sample from the discharge and at several oxygen pressures is shown in Fig. 1. At a given oxygen pressure, the concentration of atomic oxygen decreased exponentially with increasing distance from the discharge. Further, at short distances, the concentration of atomic oxygen decreased with increasing oxygen pressure at a given distance from the
3.2 Isothermal calorimetry The concentration of atomic oxygen was determined independently by isothermal calorimetry assuming that the major reactive species of an oxygen discharge was atomic oxygen. The percent atomic oxygen (fc) was calculated following the method outlined by ELIAS.")The percent atomic oxygen calculated from calorimetric data as a function of the distance of the copper coil from the discharge and at several oxygen pressures is shown in Fig. 2. The scatter here is signi~cant~y less than in Fig. 1, even at low pressures. At a given oxygen pressure, the concentration of atomic oxygen decreased exponentially with increasing distance from the discharge. Further, the concentration of atomic oxygen decreased with increasing oxygen pressures at a given distance from the discharge. These results are consistent with a higher rate of recombination of atomic oxygen with decreasing mean free path at increasing pressures. At an oxygen
R. C. MELUCCI
470
and J. P. WIGH’IMAN
pressure of 0.07 torr, the calculated value for the mean free path of atomic oxygen is 4.5 mm. Satisfactory agreement is seen between the results of the two techniques if Fig. 1 and 2 are compared at a constant oxygen pressure. 3.3 Gas phase reactions The origin of CO2 observed as a reaction product in the atomic oxygen-carbon reaction was investigated by studying the reaction between atomic oxygen and carbon monoxide introduced at a distance of 3 cm from the discharge zone. The oxygen pressure was 0.07 torr and the flow of CO was increased until the total pressure was 0.11 torr. The product analysis is shown in Table 2. The average deviation of oxygen/carbon ratios calculated from the results reported in Table 2 was 0.62.
CO is the only significant gaseous product of the CO discharge. Appreciable amounts of 02, CO and COa were observed as the gaseous products of the COa discharge. A 76 percent 0 a and 24 percent CO mixture was passed through the discharge at a total pressure of 0.11 torr. The variation in oxygen content of the reactant mixture was estimated at 76&5 percent. The product analysis is shown in Table 4. The average deviation of o~~n~c~bon ratios calculated from the results in Table 4 was 0.79. CO2 is observed as a reaction product. TABLE 4. PRODUCTANALYSISOFCARBON MONOXIDE-OXYGENDISCH!.ItGE
0, TAELE 2. PRODUCTANALYSISOFATOMICOXYGEN-URBON MONOXIDEREACTION
GP
Product gases (mole %) co
CO%
56.8
32.1
11.1
59.4
31.7
9.9
62.4
27.6
10.0
63.7
26.5
9.8
71.0
21.3
7.7
CO2 was noted as a reaction product thus suggesting that a possible mode of formation of CO2 involved the reaction between CO (g) and atomic oxygen. Additional experiments were done in order to establish the validity of the suggested mode of formation of COz. Both CO and COa were passed separately through the discharge at a pressure of 0.11 torr. The product analysis for the two gases is shown in Table 3.
63.5 70.8 69.4
21.7 20.5 23.3
14.8 8.7 7.4
76.0
16.5
7.5
co
CO*
0,
5. PRODUCT
Product gases (mole %) CO CO*
Trace
99.0
Trace
27.5
46.7
26.6
Definite carbon deposit in the discharge zone No carbon deposit observed
No carbon deposit in the discharge zone
The formation of CO2 could occur by an alternate path namely by the back diffusion of CO into an oxygen discharge. To investigate this possibility, CO2 was introduced at a distance of 3 cm from an Ar discharge. The Ar pressure was 0.07 torr and the flow of CO2 was increased until the total pressure was 0.11 torr. These conditions correspond to those used in the atomic oxygencarbon monoxide reaction. CO2 was chosen as an indicator of back diffusion since it was observed to decompose extensiveiy in the discharge as shown in Table 3. The product analysis for the Ar discharge with inbled CO is shown in Table 5. The color of the Ar discharge was purplish; however, when CO2 was introduced downstream, the color changed to bluish-white. TABLE
Gas
Product gases (mole %) CO CGr
ANALYSIS OF AR INBLEDCOP
DISCHARGE
Product gases (mole %-argonand oxygen-free co CO* 70.1
29.9
70.5
29.5
basis)
WITH
THE REACTION
OF CARBON AND ATOMIC OXYGEN
Since CO is observed as a reaction produet, these resuits indicate that under the present experimental flow conditions back diffision does occur. The observed color change of the discharge is consistent with this conclusion. 4. DISCUSSION 4.7 Atomic o~g~~ur~~ reaction The results presented for the atomic oxygencarbon reaction are in general agreement with previous studiesf2*4*s) using microwave discharges in that an extensive reaction occurs between atomic oxygen and carbon at room temperature. There are certain discrepancies which indicate that a degree of caution is needed in making quantitative comparisons. VASTOLAel”a1.@1 reported an extensive reaction at oxygen pressures from 0.03-0.06 torr and carbon sample distances of l-30 cm. In the absence of carbon approximately 10 percent of the oxygen was consumed presumably due to reaction with extraneous carbon sources present in the system. The present results indicate that no detectable reaction occurred when the carbon sample was absent. The concentration of atomic oxygen at pressures and distances ~orrespond~g to the results of Vastola et al. was significantly less. Since a correlation was obtained in the present work between the amount of atomic oxygen present by two independent techniques, the discrepancy could be attributed to difIerences in the efficiency of coupling of the microwave energy in the discharge zone. The efficiency of coupling is determined partly by the type of cavity and indeed two different types of cavities were used in these two investigations. The satisfactory agreement between the concentrations of atomic oxygen as determined from product analysis and from isothermal calorimetry suggests that product analysis is a valid approach to the calculation of the concentration of atomic oxygen. A significant amount of recombination prior to reaction of atomic oxygen with either the carbon sample or CO is not consistent with the present results. The greater scatter in the product analysis data may possibly be attributed to back diffusion discussed below. The calorimetric technique is subject to error in the measurement of atomic oxygen if electronically excited molecular oxygen is presentf7) The concentration of excited 02 was assumed to be
471
negligible in the present work. Definitive evidence as to the concentration of excited 02 is lacking in the pressure range studied in the present work. HERFCON and SCHII;‘F@)have shown that the concentrations of excited 02 and atomic oxygen from an oxygen discharge were about equal (10 percent) over the range 0.1 ta 2 torr. No pressure dependence of the ratio of these two species was indicated. 4.2 Gas phase reactions The observed decomposition of CO in the dis-. charge noted in Table 3 agrees with the results of ~~TA~~~RT~~) who reported that at CO pressures of 1-3 torr, deposits formed on the walls in the discharge region. CO, CO2 and 02 were reported present in the effluent gas mixture. Only trace quantities of CO2 and 02 were observed in the present work but a definite deposit was noted indicating decomposition of CO. The tenfold lower pressure and the lower power level used in the present work probably accounts for the observed difference in the product gases. The fact that no deposit was observed in the COa discharge {Table 3) whereas a definite deposit was formed in the case of CO is significant. The energy required to break one C-O bond in CO2 is 191.7 kcal/mole whereas for CO the corresponding value is 256.6 kcal/mole. Since CO was observed to decompose, it is reasonable to assume that CO1 also underwent decomposition. The CO2 discharge however is oxygen-rich relative to the CO discharge and an oxygen-rich discharge gives rise to gaseous products rather than a carbon deposit. This conclusion is further supported by the results of the product analysis on passing a mixture of CO and 02 through the discharge (Table 4). Again, no deposit was observed to form in the presence of excess oxygen Since a measurable amount of CO2 is formed in the atomic oxygen-carbon reaction, the question arises as to the possible mode of formation of COa. A possible mode of formation of CO2 is a primary reaction between electronically excited molecular oxygen with carbon, Since no definitive evidence exists as to the concentration of the excited molecuiar species from oxygen discharge at pressures comparable to the range used in this work, this reaction remains a plausible one. A second possible mode of formation of CO2 is a
472
R. C. MELUCCI
and J. P. WIGHTMAN
secondary reaction between atomic oxygen and the primary reaction product, CO. Supporting evidence for this mode is found on examination of the results shown in Table 2. An appreciable amount of CO2 was produced by the reaction of CO and atomic oxygen in the absence of the carbon sample. The surface of carbon may play a part in the formation of CO2 but it should not be regarded as essential to its formation. The results of this study suggest that the primary reaction is between atomic oxygen and carbon and CO formed in this reaction reacts with excess atomic oxygen to form COz. There are two complicating factors in the above analysis of the formation of COz. Back diffusion of CO, the primary reaction product, into the oxygen discharge could produce CO2 as is indicated by the results in Table 4. The results of the back diffusion studies unfortunately do not lead to an estimate of the extent of back diffusion. GESSER and HUSSAIN(‘~~noted in a recent paper that photochemical reactions may occur outside of the discharge zone of different types of discharges, the discharge itself serving as a photolytic source. Such a photochemical effect is not exclusively operative in the present system since the color of the Ar dis-
charge was observed to change when CO was introduced downstream. Photochemical reactions downstream would not alter the color of the Ar discharge. Acknowledgments-One of us (R.C.M.) was a NASA Trainee Fellow for the duration of this work. Partial financial support for equipment by a Grant-in-Aid of Research from the Society of the Sigma Xi is gratefulfy acknowledged. REFERENCES 1. BLACKWOODJ. D. and MCTAGGART F. K., Aust. J. Ckem. 12,114 (1959). 2. VA~TOLAF. J., WALKER P. L. JR. and WIGHTE~~AN J. P., Carbon 1, 11 (1963). 3. MARSH H., O’HAIR E., REED R. and WYNNE-JONES W. F. K., Nature 198, 1195 (1963). 4. OTTERBEIN M. and BONNETAINL., Compt. ReBd 258, 2563 (1964). 5. OTTE~EIN M. and BONNETAINL., Compt. Rend. 259,
791 (1964). 6. ROSNERD. E. and ALLENDORFH. D., Carbon 3,153 (1965). 7. ELIAS L., OGRYZLOE. A. and SCHIFF H. I., Can. r. Chem. 37, 1680 (1959). 8. HERRON J. T. and SCHIFF H. I., Can. J. Ckem. 36, 1159 (1958). 9. MCTAGGART F. K., Awt. J. Cttm. 17, 1182 (1964). 10. GENDERH. and HUSSAINS., Nature 201,290 (1964).