R. XELSONSMITH,JAMES SWINEHART AND DAVID LESNINI
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It should be possible to add a third and fourth The low -AH1 for (-CHZOCHZCH~NH~)Z is' molecule of this amine to the copper ion, but con- somewhat surprising. Even if the oxygens are stants for these reactions could not be calculated. very weakly coordinated, a - AH1 of about 12 kcal. Probably the amine concentrations used in this for the two nitrogens would have been predicted. study were too low to give appreciable concentra- It is possible that the 8-membered chain between tions of the tri- and tetraamine complexes. the amine groups is flexible enough to allow rather The tetradentate amine (-CHZNHCHZCHZNHZ)Zfree movement over a considerable area. This shows normal Cu-N bond strengths. For its disul- would tend both to weaken the Cu-N bonds and to fur analog (-CHaSCHzCH2NH2) 2- AH1 exceeds that hinder the approach of a second ligand molecule. for two Cu-N bonds by 3 or 4 kcal. On this basis, The behavior of H2NCHzCHzSCH2CHzOCHzCH~the strength of a Cu-S bond is of the order of 2 kcal. NHZ is in line with this hypothesis. The C u S The mixed compound HzNCHZCHZSCH&HZOCHZ-bond, even though comparatively weak, is sufficient CHZNHZhas AH1greater by only 1 or 2 kcal. than to restrain free movement of the amine molecule that for two Cu-N bonds. This figure is in good with respect to the copper ion, and the strength of agreement with the estimate of - A H just given for the Cu-N bonds is preserved. a single Cu-S bond, and suggests that, as was the Acknowledgment.-The authors are indebted to case with O(CHnCHaNH2)z and HOCHzCHzSCH2CHZNHZ,oxygen is not bound strongly enough to Miss Helen A. Carbone for assistance with the give a measurable - A H . calculations involved in this work.
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THE OXIDATION OF CARBON BY NITRIC OXIDE BY R. NELSONSMITH,JAMES SWINEHART AND DAVIDLESNINI Contribution from the Chemistry Department, Pomona College, Claremnt, California Received Auguat 19, 1968
Below 200° NO reacts with an ashless sugar charcoal to give N2and carbon-oxygen surface complexes, the reaction ceasing when the surface is saturated with oom lexes. I n the temperature range 450-600Othe reaction is b t order proceeds continuously and yields N2, Con, CO an$ carbon-oxygen surface complexes. The activation energy varies from 15.0-18.8 kcal./moie, depending on the nature of the surface. It is postulated that a NO molecule reacts with another adsorbed a t the site of a surface oxygen complex to give Ns and C02, and an oxygen complex on an adjacent carbon atom. The new oxygen complex usually serves as a site for further adsorption of NO and continuing reaction, but its location may be unstable and CO may be the product instead. H2-treatment of the surface decreases the rate, and Optreatment increases the rate of reaction.
A previous study' of the adsorption of nitric oxide on carbon showed that adsorption isotherms a t temperatures above about - 150" were without meaning because of the very rapid reaction of NO with the carbon to form Nz gas and carbon-oxygen surface complexes. This paper reports a further study of this complicated reaction of NO with carbon, a t elevated temperatures where the additional reaction products are CO and COZ. Experimental The reaction system provided for the continuous circulation of NO through a heated bed of carbon, and in design it was much the same as that used in earlier work.2 The major portion of the reaction proceeds with no pressure change and the reaction products vary with conditions 80 that rate measurements cannot be made manometrically. Instead, samples were removed at regular intervals for analysis by gas chromatographic methods.' The decrease in ressure which would have occurred after the removal of ea& sample was prevented by adding an equal volume of mercury to a com ensation bulb. The design of the compensation bulb angcirculation system was such as to eliminate dead space. . A small dead volume of about 2 ml. (in a system volume of about 800 ml.) existed where a manometer was connected to the system. This manometer was used to ascertain when a proper volume of mercury had been added in cornpensstion for Sam le removal, and.to follow any small changes in pressure w&ch occurred during the course of the reaction. (1) R. N. Smith, D. Lesnini and J. Mooi, Tare JOURNAL, 60, 1063 (1956). (2) R. N. Smith and J. Mooi, ibid., 69, 814 (1955). (3) R. N . Smith, J. Swinehart and D. Leanini, A n d . Chem., 80, 1217 (1958).
An activated sugar cliarcoal of extreme1 low ash content (less than 0.005%) waa prepared from AA suqar furnished by the California and Hawaiian Sugar Re finin Cor oration, San Francisco, California. It is designate! as #u-SO and the method of its re aration and activation is described elsewhere.' The Bkgnitrogen surface area is 1060 m.* per gram. An 0.85- sample was used. For many of the runs the sample was &$-treated" prior to the addition of the nitric oxide. This hydrogen treatment consisted of three a proximately 15-minute treatments with HIgas a t 1000". %he Hn waa pumped out between each treatment and after the last treatment the sample was cooled slowly with continuous pumping. Nitric oxide was prepared by the method of Marqueyrol and Florentin' followin a detailed procedure described elsewhere.' The analyticaf method used showed this gas to be pure NO. Initial pressures of 200 mm. or less were used in each run. Carbon monoxide was obtained by dropping powdered, outgassed sodium formate onto Outgassed concentrated sulfuric acid in an evacuated system. The CO so produced was passed through a Dry Ice trap before storing in a Pyrex reservov. The analytical method used showed this gas to be pure CO.
confectioners
Results The rate of disappearance of NO is most rapid on an 02-treated surface (one which has been treated with Oaand outgassed for several hours a t the same temperature as the run), and least rapid on a HZtreated surface. Regardless of the nature of the carbon surface, the rate of disappearance of NO is first order with respect to the partial pressure of NO. (4) M. Marqueyrol and D. Florentin, Bull. 8oc. chim. Francs, [41 11,804 (1912).
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OXIDATION OF CARBON BY NITRICOXIDE
April, 1959
Figure 1 shows typical firstlorder plots for a variety of temperatures on a surface that had already been exposed to and had reacted with NO. This surface is designated as an "oxide surface'' to distinguish it from a HAreated surface which has no carbon-oxygen complexes or an 02-treated surface which has a very high concentration of carbon-oxygen complexes. The initial rate on an 02-treated or an oxide surface is always somewhat greater than the first-order rate by which the reaction continues after this short initial period. Table I gives the rate constants for this reaction for a variety of temperatures at a variety of surface conditions. The activation energy for this reaction over the temperature range 450-600" varies from 15 kcal./mole on an 02-treated surface to 18.3 kcal./mole on a Hztreated surface. It should be emphasized that the rate constants for both the 02-treated and the Hztreated surfaces, as well as the activation energies calculated from them, must be considered as qualitative only since the nature of the surface is continually changing during the course of these runs. It is also of interest to note that a t 600" the rate is substantially the same regardless of the form of surface pre-treatment. TABLE I RATECONSTANTS T y p e of surface
Ortreated
Temp. (°C1)
450 500 550 ti00
Oxidized (Run after that on 02-treated surface)
450 500 550 600
HI-treated
Oxide (Run after that on Hz-treated surface)
450 500 550 600 450 500 550 600
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(sec.
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.. 19.2 38.5 38.9 5.1 13.0 22.4 3ti.7 5 1 7.2 20.2 35.6 .
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The products of the reaction of NO with carbon vary with temperature and the previous treatment of the carbon surface. From below Dry Ice temperatures to approximately 200" the sole products are Nz gas and carbon-oxygen surface complexes (which can be removed from the surface only a t higher temperatures in the form of carbon dioxide and carbon monoxide). With increasing temperature above 200", more COz gas appears as one of the major products and carbon-oxygen surface complexes become less important as a product. The approximate percentage of NO which is used in making oxygen complexes varies with temperature as follows: 200" and lower, 100%; 250", 75%; 450", 35%; 500", 32%; 550", 16%; 600", 7%. If the carbon surface has been Hz-treated prior to the addition of NO, no gaseous products other than Nz arLdCOZ appear up to a temperature of 500". At 550" and higher, CO also appears as a product; at 600" about one-tenth of the oxygen from the de-
545
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200
300 400 500 600 Minutes. Fig. 1.-A typical semi-log plot of Po/P us. time showing, a t various tem eratures, the first-order nature of the reaction on an ox& surface. Po = initial pressure of NO and P = partial pressure of NO a t time t. 0,450'; 0 , 500°, 0,550°, 8, 600".
composed NO appears as CO. If the outgassed carbon surface possesses carbon-oxygen complexes as a result of previous reaction with NO, the addition of NO will cause production of some CO a t temperatures as low as 450". 'At 450", 8% of the oxygen from the decomposed NO may be combined as CO, while a t 600" there may be as much as 20% as CO. CO is formed even more readily if, prior to the addition of NO, the carbon surface has been treated with oxygen (and also outgassed). The ratio of CO to C02is much greater in the initial stages of a reaction carried out over an oxidized surface than over a Hz-treated surface (see Fig. 2). It is not possible to make precise statements about the fraction of the NO which reacts to form CO because NO also reacts with CO a t these temperatures in the presence of carbon surfaces to produce Nz and C02. Thus during the course of a given run, the partial pressure of GO builds up rather rapidly during the early stages of the reaction, levels off during the middle portion, and then decreases during the later stages (see Fig. 3). An equimolar mixture of CO and NO passed over carbon a t 550" reacts in such a manner that about one-half of the NO reacts with the carbon surface and one-half with the CO. Higher temperatures and excess CO favor the reaction with CO. Discussion Certain conclusions about the mechanism of the reaction of NO with carbon may be quickly drawn from the experimental observations. 1. The first step in the reaction must be the formation of carbon-oxygen surface complexes, for if these are not already present on the surface the
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4. CO does not arise from the thermal decomposition of carbon-oxygen surface complexes, for a well-outgassed oxidized surface which has resided in a closed system for several hours at a given temperature without giving any gaseous products will 0.6 subsequently react with NO to give CO. 5. CO, once produced, may react with NO to produce NZand COz as evidenced by the fact that (a) CO will not react at the temperatures of this study with carbon or with carbon possessing carbon-oxygen surface complexes, (b) CO is consumed when mixed with NO and passed over carbon a t these temperatures, especially readily when CO is in excess, (c) the CO content of the reaction mixture increases during the initial part of the reL I I action, and then decreases during the latter part of 0.2 0.4 0.6 0.8 the reaction when the CO becomes large relative to Fraction of reaction time. the NO. Fig. 2.-The ratio of the partial pressures of CO and COZ 6. The carbon-oxygen complexes made by oxyas a function of reaction time on a variety of surfaces a t gen seem to be somewhat greater in quantity and 550’: 8, 02-treated surface; 0 , run following a run on 0 2 treated surface; e, oxide surface; 0,H2-treated surface. somewhat different in nature from those produced There is essentially no difference between an oxide and a H2- by NO as evidenced by the fact that (a) the reactreated surface. tion goes fastest on an 02-treated surface, (b) with an 02-treated surface there is an excess of oxygen in the products over that available in the initial NO used, (c) the ratio of CO to C 0 2in the gas products from the 02-treated surface is greater, and that (d) in a succession of reactions, the first being over an 02-treated surface, the reaction rate and the CO-to-COz ratio approach those observed with a non-oxygen-treated surface in about three runs. These six reaction steps appear to account for all of the observed experimental data I
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0.2 0.4 0.6 0.8 Fraction of reaction time. Fig. 3.-The partial pressure of CO, in mm., as a function of reaction time on a variety of surfaces a t 550”: 8, 0 2 treated surface; 0 , run following a run on an 02-treated surface; e, oxide surface; 0,Hz-treated surface. There is essentially no difference between an oxide and a H2treated surface. Initial NO pressure in each case was approximately 150 mm.
initial reaction product is only N 2 a t temperatures below 200”, or a t higher temperatures Nz along with insufficient CO2 to account for all the NO decomposed. 2. Carbon-oxygen surface complexes are important intermediates in the reaction, as evidenced by the fact that (a) the rate on a Hz-treated surface is slowest, (b) the initial important process on a H2-treated surface is the production of Nz without COZ, (c) there is an excess of oxygen in the products (over that contained in the NO which decomposed) if the reaction is carried out on an oxygentreated surface. 3. The complexes from which CO is produced must be more stable or less reactive than those from which COZ is produced as evidenced by the fact that (a) CO is produced only a t temperatures above 500” on Hz-treated surfaces, whereas COZ production on similar surfaces commences at about 200”, (b) less CO is always produced than CO2, (c) in the thermal decomposition of carbon-oxygen surface complexes COZ is the predominant product a t lower temperatures and CO at higher temperatures.
+ [C’ + C”] +[(C’O) + (C”O)] + N2 + [(C‘O) + C”‘] _r [(C‘O.*-ON)+ C”’] + [(C‘O. . *ON) + C”’] + (C“‘0) + C’Oz + Nz NO + [(C’O) + C””] [(C/O.. .ON) + C””] NO + [(C/O.. .ON) + C””] -+ C””0 + C’02 + N2 2 N 0 + 2C””O +2C””02 + N2
2N0 NO NO
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Step 1 occurs readily a t temperatures from below -80” and up, and below 200” it is the only step C”] which leads to the formation of N2. [C’ represents two adjacent surface carbon atoms, and (C”O)] represents the oxygen complex [(C’O) on C’ and C”. The oxygen complexes may then serve as sites for physical adsorption of NO, and thus steps 2 and 4 are reversible. It is believed that it is the oxygen atom of the NO molecule which is held to fhe surface. The only difference between steps 2 and 4 is in the nature of the adjacent carbon atoms, C”’ and C””. It is not known just what the real difference between C”’ and C”” is, but it is postulated here that C”” will yield CO gas, whereas C”’ will form just another surface complex, (C”’O), with the same properties as (C’O) and (C”0). If no CO gas were formed, steps 1, 2 and 3 would be sufficient to explain the reaction at temperatures above 200”. Since considerably less CO is formed than CO2 it is apparent that C”” atoms are not the most common, and perhaps they are located a t corners, edges, or cracks. Step 6 will not take place homogeneously at the temperature studied for it
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OXIDATIONOF CARBON BY NITRIC OXIDE
April, 1959
547
has an activation energy of 49.6 kcal./m~le.~It is catalyzed by carbon surfaces and hence step 6 is really a summary step for a reaction which needs further study. Steps 4 and 5 represent a minor fraction of the over-all reaction, and step 6 involves only part of the CO produced in steps 4 and 5. Steps 3 and 5 seem reasonable if NO is adsorbed by the oxygen end of the molecule and if the carbon atoms C’ and C’” or C’ and C”” are adjacent. I n this way two nitrogen atoms may conveniently unite, oxygen atoms from two different NO molecules are available to a single carbon atom for COz, and the necessary intermediate carbon-oxygen surface complex is regenerated for further reaction by steps 2 and 4. This mode of adsorption of NO and the decomposition of the resulting complex is different from that which Shahs suggested to explain the low-temperature formation of a trace of NO2. This mechanism also satisfactorily accounts for the first-order nature of the reaction. On a surface which has not been H2-treated step 1is unimportant because the complexes (C’O) and (C”0) have already been made in previous reactions, and step 6 is also unimportant in the over-all picture. Omitting steps 1 and 6 from serious consideration, then, leads to the rate expression
ing oxygen complexes a t the given temperature have obtained their oxygen. I n the temperature range of this study, the total amount of nitrogen produced by step 1 is small compared to that produced by steps 2 to 5 of the continuing reaction. Thus, even on a H2-treated surface, step 1 is not important in the total picture for the continuing reaction of NO. An additional term should be added to equation 3 to include the removal of NO by step 6 but, as pointed out earlier, this step plays a minor role (in the later stages of the reaction) in the over-all reaction, and the nature and order of this reaction are not sufficiently well-known to include such a term. When the surface has been treated with oxygen (and outgassed) prior to the addition of NO, the surface has more than the usual concentration of (C’O) and (C”0) and the rate is greater. In addition there are apparently more sites involving C”” atoms because the rate of production of CO is greater. It is of some interest to compare NO with N20. with respect to oxidation of carbon. Though both processes require carbon-oxygen surface complexes as intermediates, the NzO oxidation mechanism is far simpler.’ This is due primarily to the fact that in NzO both nitrogen atoms are already joined together and the reaction is merely a two-step trans-dP = klVP Kle”P (1) fer of oxygen atoms to carbon to form C02. The dt first step is where P is the partial pressure of NO, and 8‘ and N20 C +(CO) Nz 8” are the fractions of the surface holding NO by the [(C’O) C”’] and [(C’O) C””] complexes, and the second is respectively. Applications of the simple Langmuir N2O (CO)+COz Nz concept for the fraction of the surface covered leads Physical adsorption of NzO does not seem to be into volved as it is with NO. The oxidation of CO by NzO over carbon2 involves the physical adsorption of CO at some “acIf C I3 and K3P are both substantially greater than tive carbon” atom, not a carbon-oxygen complex, unity, then equation 2 reduces to the simple first- and it may be that the oxidation of CO by NO requires a similar active site. Further study will be order expression needed to determine this. _ -dP = krP+KP=KP Acknowledgment.-The authors gratefully ac(3) dt knowledge the support of the Office of Naval Step 1 is interesting in that it goes quite rapidly, Research for this work which was done under coneven at very low temperatures, but at each tem- tract N8onr54700. Production in whole or in part perature below 200” it comes to a standstill when is permitted for any purpose of the United States the surface carbon atoms which are capable of form- Government.
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(5) C . P. Fenimore, J . A m . Chem. Soc., 69,3143 (1947). (6) M.S. Shah, J . Chem. Soc., 2676 (1929).
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(7) R. N. Smith, D. Lesnini and J. Mooi, THISJOURNAL, 61, 81 (1957).
