Jan., 1963
M E C H A N I S M FOR F O R M A T I O N O F X Y L Y L E N E S I N
GASP H A S E
73
THE CI-IEMISTRY OF XYLYLENES. XVII. THE MECHANISM FOR FORMATION OF XYLYLENES I N GAS PHASE BY L. A. ERR~EDE AND J. P. CASSIDY Contrzbution N o . d2S from the Central Research Laboratories of the Minnesota, Mining and Manufacturing Company,' St. Paul, M i n n . Received M a y IS,1968 Fast ffow co-pyrolysis of p-xylene and carbon tetrschloride a t low pressure was used to elucidate the mechanism for conversion of p-xylene to p-xylylene. It was demonstrated that p-methylbenayl radicals are converted catalytically t o p-xylylene as the hydrocarbon gas stream travels away from the furnace. Each p-xylylene molecule is formed from one p-methylbenzyl radical and not via collision and disproportionation of two radicals as heretofore believed. The catalyst for dehydrogenation of p-methylbenzyl radicals is the carbonized material deposited along the post-pyrolysis zone of the pyrolysis system. Coupling to give 1,2-di-p-tolylethane is a homogeneous second-order reaction that competes with dehydrogenation. The ratio of p-xylylene to 1,2-di-p-tolylethane produced via these two reactions increases with increase in catalyst surface and decreases with increase in pressure. The half-life of the p-methylbenzyl radicals in the hydrocarbon stream was about 0.01 sec. In contrast, trichloromethyl radicals disappeared completely from the chlorocarbon stream within 0.002 sec. after leaving the pyrolysis zone.
Introduction I n the preceding paper2 of this series, it was reported that fast flow co-pyrolysis of p-xylene and carbon tetrachloride a t low pressure yielded a pyrolyzate having pmethylbenzyl chloride and p,p-dichloro-p-methylstyrene as two major components. On the other haind, p-methylbenzyl chloride and p-xylylene dichloride were the two major products of interaction when p-xylene and carbon tetrachloride were pyrolyzed in separate ICOaxial tubes and the two streams were allowed to blend at a point one inch beyond the furnace. No products of interaction were produced in significant amount when the pyrolyzed stream of one was allowed to blend with an unpyrolyzed stream of the other, proving that the chlorohydrocarbons isolated were produced primarily via coupling of the radical fragments. These results suggested that p-xylylene might be produced in the hydrocarbon gas stream as the pyrolyzate traveled away from the furnace. It was reasoned that a systematic study of the product distribution as a function of blend point might help elucidate the mechanism whereby p-m.ethylbenzy1 radicals are converted to pxylylene. Szwarc has shown unequivocally that thermal rupture of the C-H bond of p-xylene to give a pmethylbenzyl radical and a hydrogen atom is a firstorder homogeneous gas phase r e a ~ t i o n . ~Subsequent reaction of the hydrogen atom with p-xylene gives another p-methylbenzyl radical and molecular hydrogen.
H2
Szwarc postulated that p-methylbenzyl radicals are converted to p-xylylene in one of two ways, namely, via disproportionation (3)and/or decomposition (4). Reaction 4 as written leads to a chain reaction. The work of Szwarc, however, had shown that the (1) This work was carried out in the laboratories of the M. W. Kellogg The data were acquired by the Minnesota Mining and Mfg. Co. with the purchase of the Chemical Manufacturing Division of the M. W. Keliogg Co. in March, 1957. ( 2 ) L. A. Errede and J. P. Cassidy, J. Phvs. Chem., 67, 69 (1963). (3) M. Szwarc, .Discussions Faradau Soc., 2, 48 (1947).
pyrolysis of xylenes does not involve a chain mechanism. Furthermore, the results of kinetic studies3?4appeared to favor reaction 3. Consequently, it was concluded tentatively that under the usual conditions of fast flow pyrolysis, T-xylylene was formed primarily via reaction 3. This conclusion was supported by the observation that fast flow pyrolysis of 1,2-bis-ptolylethane gave, among other products, p-xylylene and p-xylene in about equal amount^.^^^ Results and Discussion If it is assumed that p-methylbeneyl radicals are converted to p-xylylene in the gas phase via disproportionation as indicated in reaction 3, then it follows that the concentration of reactive species (i.e,, pmethylbenzyl radicals plus p-xylylene) should decrease as the gas stream travels away from the furnace. Hence, the yield of chlorocarbons obtained by quenching this hydrocarbon stream with a gas stream of chlorocarbon radicals should decrease as the blend point is moved farther into the post-pyrolysis zone. Accordingly, fast flowing gas streams of p-xylene and carbon tetrachloride were pyrolyzed in separate concentric tubes, as shown schematically in Fig. 1. The two streams were made to blend at a k e d point having the temperature indicated by the temperature profile shown directly below the diagram of the pyrolysis system. At a point 3 ft. beyond the furnace, the resulting gas stream was collected in hexane kept Much of the chlorine generated in this rea t -78'. action was condensed in the second cold trap cooled by liquid nitrogen. The reaction products were separated as described in detail in our preceding publication.2 A series of these coaxial pyrolyses was carried out a t approximately the same temperature and residence time. I n successive experiments the two gas streams were mixed a t -4.5, -2.5, 0.5, 1.5, 3.5, 4.5,
Co.
(4) M. Levy, M. Szwaro, and J. Throssell, J . Chem. Phga., 22, 1904
(1954). ( 5 ) J. R. Schaefgen, J . Polymer Sci., 15, 230 (1955). (6) L. A. Errede and J. P. Cassidy, J. Am. C k m . SOC.,82, 3653 (1960).
74
L. A. ERREDE AND J. P. CASSIDY
Vol. 67
TABLE I PRODUCTS ISOLATED via COAXIALPYROLYSIS OF P-XYLENEAND CARBON TETRACHLORIDE Pyrolysis temp., "C. Residence time, 10-3 sec. Blend point," in. Temp. of blend point Moles p-xylene used Moles C c 4 used
1030 6 -4.5 1030 12.1 13.9
Reaction conditions 1000 1010 6 6 0.5 1.5 990 950 11.1 8.3 12.0 5.6
1020 6 - 2.5 1020 9.2 7.3
1000 6 3.5 700 18.5 25.4
Products isolated (moles) 0 0 0.04 0.30 0.20 .34 .26 .13 .05 .22 .I6 .21 .32 .37 .24 -1.10 0.86 0.88
0 0.57 .44 .37 .39
__
1.77
0.15 .74 0 0.38 .39 ~
1.66
1040 5 4.5 600 30.6 35.6
1030 6 7.5 400 7.18 9.00
1030 8 10.5 300 5.75 7.27
0.45 1.65 0 1.51 1.18
0.25 .38 0 0.22 .17
0.33 .23 0 0.32 .13
4.79
1.02
-
__
-
1.01 15 12 8 11 9 16 14 18 r 13 7 10 9 15 13 16 a Point of mixing measured in inches away from pyrolysis zone (see Fig. 1). Mixture of 1,2-di-p-tolylethane and methylated diphenylmethane. Residue isolated as decomposition products formed during separation by vacuum distillation. d Moles of p-xylyl equivalents isolated X 100 + moles of p-xylene pyrolyzed. Based on p-xylene pyrolysis data that related conversion of p-xylene to p-methylbenzyl radicals aa a function of temperature and residence time' (see eq. 5, 6, and 7).
PYROLYSISSYSTEM
C = Atexp(-Bt) (5) where t is the residence time in seconds, and A and B are temperature dependent variables given by
4.3 CM
A
=
2.6 X 1016exp(-83/RT)
B = 8.5 X 1014exp(-8O/RT)
. l000'
L)
80O0
' 2 a!
600'
400' Q
er:
ZOO'
0'
- Ib
I
-17.
-8
I
I
6
I2 16
BLEND POINT IN
IUCHES.
-4
0
4
I
I
20
24
Fig. 1.-Diagram of pyrolysis system showing the corresponding temperatures in the pyrolysis zone C and the post-pyrolysis zones B and A with internal diameters of 2.5 and 4.3 cm., respectively.
7.5, and 10.5 in. beyond the end point of the pyrolysis zone. The numbers of moles of various components isolated in each run are shown in Table I. The per cent yield of total products of interaction is independent of the distance the hydrocarbon gas stream travels ,before it is quenched with the chlorocarbon stream. I n fact, the yield of interaction products in each run is about equal to the corresponding per cent conversion of p-xylene to p-meth ylbenxyl anticipated on the basis of an earlier study. The latter demonstrated that the fractional conversion of pxylene to p-methylbenzyl radicals (C) is given by
(6)
(7)
These results prove that eq. 3 does not represent the mechanism for conversion of p-methylbenzyl radicals to p-xylylene, since this equation requires that the amount of interaction products decrease as the quench point is moved farther away from the pyrolysis zone. It also was shown7 that the ultimate distribution of products from fast flow pyrolysis of p-xylene does not change appreciably as conversion increases a t the same reaction pressure. If it is assumed that this also is true in co-pyrolysis,the data given in Table I can be used to calculate the number of moles of each component that would be isolated when 10 moles of p-xylene are pyrolyzed to give 1 mole of p-methylbenzyl radicals for interaction with the chlorocarbon radicals. Accordingly, the data in Table I were recalculated to determine the yield of individual reaction products a t 10% conversion of p-xylene, and the results are summarized in Fig. 2 . This diagram shows a continuous change in product distribution as a function of blend point. No pxylylene derivative was isolated until the blend point of the two pyrolysis streams was outside the pyrolysis zone; thereafter, the amount of p-xylylene dichloride isolated increased linearly with distance away from the pyrolysis zone. These results demonstrate clearly that p-xylylene is formed as the pyrolyzate streams away from the furnace. Apparently any p-xylylene that might form in the pyrolysis zone is destroyed before it can couple with chlorine or chlorocarbon radicals to give stable compounds. For example, p-xylylene produced in the pyrolysis zone could rearrange to cyclooctatctraene, which in turn is known to undergo (7) L. A. Errede and F. DeMaria, J. Phys. Cham., 66, 2664 (1962). (8) E. J. Prosen, W. H. Johnson, and F. D. Rossini, J. Am. Chem. SUC. 69, 2068 (1947).
MECHAXISM FOR FORMATION OF XYLYLENES IN GASPHASE
Jan., 1963
thermal rearrangement to styrene.8 Indeed, styrene was identified as one of the products of fast flow pyrolysis of p-xylene, and cyclooctatetraene probably was another product of this reaction6 as indicated by mass spectrometric analysis. Moreover, other experiments showed that the proportion of styrene (and cyclooctatetraene) produced from p-xylene increases with increase in severity of the pyrolysis condition^.^ Another possibility is that p-xylylene generated in the pyrolysis zone is decomposed to non-condensable gases such as ethylene and a ~ e t y l e n e . ~ The amount of P,P-dichloro-p-methylstyrene isolated in these experiments remained constant so long as the blend point was located within the pyrolysis zone. It fell off rapidly when the blend point was extended beyond the furnace. Undoubtedly, P,p-dichloro-p-methylstyrene is formed via coupling of pmethylbenzyl radicals with trichloromethyl radicals to afford p-(p1/3,p-trichloroethyl)-toluene,which splits out hydrogen chloride a t the high temperatures that exist in the zone of coupling, as indicated by reaction 8.
+
-
CC13
CH3OCH=CCI2
+
+ Clz CC13. + C1. or ((212) + CCL + ((21.)
2cc13.
CCl3CC13 +CClz=CC12
.CI(C12)
CH23(>Cr-r2 -
+
--
C I C H 2-~ C H 2 C I
-+TO
COLDTRAP
OL 5
5 0.4 a
E
“0 0.3
5- 0.2 8
0.1
-2 0 2 4 6 8 10 12 Blend point in inches away from end of pyrolysis zone.
-4
Fig. 2.-Product distribution as a function of the point where the pyrolyzed p-xylene stream was allowed to blend with the pyrolyzed carbon tetrachloride stream. POST- PYROLYSIS
$ 5 O s d 7 O 0 ‘
1.0
-Y :.r c
I
O O‘O
-
ZONE 3To COLD TRAP 300’
5EC.--------1 I
(9) (10)
(12)
Clz
It is conceivable that p-CH3CaH4CH2CC13was produced in small amount despite the fact that it was not isolated, since an appreciable fraction of the condensate of each experiment was obtained as the ill-defined, non-distillable residue having an average empirica,l formula of (CeH6CIo7)ca. This residue was shown by infrared analysis to be a mixture of alkylated aromatics. It was found that residues of this type are formed when a synthetic mixture of the compounds isolated in these experiments is separated by vacuum distillation. Thie amount of this chlorine containing residue decreased as the proportion of p-methylbenzyl halide derivatives decreased in the mixture. Figure 2 shows that the amount of p-methylbenzyl 0.7.
POST-PYROLYSISZONE
HCl (8)
The rapid decrease in formation of P,/3-dichloro-pmethylstyrene accompanies a corresponding increase in p-methylbenzyl chloride formation, which occurs via coupling of niethylbenzyl radicals with chlorine (rleaction 11). Apparently all of the trichloromethyl radicals in the chlorocarbon stream are consumed via eq. 9 and 10 before the gas stream travels 2 in. beyond the pyrolysis zone. Thereafter, the only reactive species in the chlorocarbon stream is chlorine. Consequently, p-methylbenzyl chloride and p-xylylene dichloride are the only interaction products isolated (eq. 11 and 12) when the two streams are made to blend ab a distance greater than 2 in. beyond the ,3 zone, pyrolysi(”
+
PYROLYSIS ZONE
75
-4 -2 0 2 4 6 8 10 12 Blend point in inchee away from end of pyrolysis zone.
Fig. 3.-Reletive concentration of reactive species as a function of distance away from the end of the pyrolysis zone.
chloride isolated decreases steadily and the amount isolated as p-xylylene dichloride increases as the blend point is moved beyond the furnace. The proportion of p-methylbenzyl radical isolated as 1,Bdi-p-tolylethane and methylated diphenylmethanes does not increase appreciably until the blend point is well past the pyrolysis zone. The diarylmethanes were consistently a major portion of this fraction when the blend point was less than 2 in. beyond the pyrolysis zone. The proportion of ditolylethane isolated, however, increased as the blend point was moved beyond 2 in. This is consistent with earlier results which proved that dibenzyls are converted in good yield at 1000° to diarylmethanes via fast flow pyrolysis.6 Most of the p-methylbenzyl units isolated as diarylmethanes must have formed within the pyrolysis zone. The increase in proportion of 1,2-ditolylethane as the distance to the blend point increases can be attributed to the greater time allowed for coupling of p-methylbenzyl radicals before quenching with the chlorocarbon pyrolyzate. The relative amount of p-methylbenzyl radicals still present in the hydrocarbon stream a t the blend point in question is given by the sum total yield of interaction products isolated as p-methylbenzyl chloride, p,Bdichloro-p-methylstyrene, and units of non-volatile residue. The yield of p-xylylene dichloride represents the corresponding relative concentration of p-xylylene. I n Fig. 3 the sum total moles of p-methylbenzyl radical equivalents and p-xylylene equivalents are plotted as a function of blend point. The p-methyl-
L. A. ERREDE AND J. P. CASSIDY
76
benzyl units isolated as l12-di-p-tolylethane and methylated diphenylmethane have not been included in the data shown in Fig. 3, since these products result from side reactions that consume p-methylbenzyl radicals via coupling and subsequent rearrangement6 in the pyrolysis zone. Figure 3 illustrates the relative stability (with respect to reactivity) of p-methylbenzyl and trichloromethyl radicals. At the fast flow pyrolysis conditions described for the present experiments, p-methylbenzyl radicals have a half-life of about 0.01 sec. in the hydrocarbon stream, whereas the trichloromethyl radicals disappear completely from the chlorocarbon stream within 0.002 sec. Figure 3 also shows that the disappearance of pmethylbenzyl radicals in the hydrocarbon pyrolyzate is accompanied by a corresponding equivalent increase in p-xylylene, implying that each p-methylbenzyl radical is converted to one p-xylylene molecule. Equation 4, however, does not represent the true picture, since this equation as written is part of a chain sequence that includes eq. 2 and 4. As mentioned earlier, it was shown by Szwarca14that the pyrolysis of p-xylylene does not involve a chain mechanism. The data shown in Fig. 3 for conversion of p-methylbenzy1radicals to pxylylene as a function o€ distance along the post-pyrolysis zone were studied from the standpoint of elapsed time for the pyrolyzate to reach the respective quench points. It was noted that the rate of conversion of pmethylbenzyl radicals to p-xylylene in the post-pyrolysis zone of a tube having a 1-in. diameter was virtually constant despite large changes in temperature and concentration of p-methylbenzyl radical. On the other hand, the dehydrogenation rate decreased sharply when the pyrolyzate entered the expanded post-pyrolysis tube of 4.3-cm. diameter (Table 11) beginning at a point 8 in. away from the pyrolysis zone, as shown in Fig. 1. These results suggest that the dehydrogenation is heterogeneous and zero order. TABLE I1 RELATIVE EFFICIENCY FOR DEHYDROGENATION OF METHYLBENZYL RADICALS TO p-XYLYLENE IN P-XYLENE GASSTREAMAT 4 MM. PRESSURE Dehydrogenation zone"
Surface/volume ratio of dehydrogenation zone
Relative zero order rate oonstant
A
0.18/1 25 0.59/1 91 c 0.91/1 200 " Refers to pyrolysis tube system shown in Fig. 1: (A) portion of the post-pyrolysis zone having a 4.3-em. diameter; (B) portion of the post-pyrolysis zone having a 2.5-cm. diameter; (C) post-pyrolysis zone B fitted with 5 tubes (4in. long, 8 mm. o.d., and 6 mm. i.d.) inserted in the form of a bundle around the inner pyrolysis tube between the pyrolysis zone and the quench point.
B
To test this hypothesis, the surface area in the path of the pyrolyzed hydrocarbon stream was increased by packing the reaction tube between the pyrolysis zone and the quench point with small quartz tubes. Pyrolysis of p-xylene again was carried out under conditions that produce 10% conversion to p-methylbenzyl radicals in the pyrolysis zone. The pyrolyzate, after flowing through the quartz tube packing, was quenched with pyrolyzed carbon tetrachloride at a point 6 in. beyond the pyrolysis zone. The resulting gas stream
Vol. 67
was collected in hexane kept a t -78" and the products were separated in the usual way, as described in detail in a previous publication.2 Thirty-one per cent of the p-methylbenzyl radicals were isolated as p-xylylene dichloride, 27% as p-methylbenzyl chloride, 24% as methylated diphenylmethanes, 5% as 1,2-di-p-tolylethane, and 14% as the ill-defined residue2**believed to be produced from chlorohydrocarbon products during vacuum distillation of the reaction mixture. Thus, a t a point 6 in. beyond the pyrolysis zone, the ratio of p-xylylene to p-methylbenzyl radicals in this experiment was 31/27 (or 1.15 to l), whereas Fig. 2 shows that without the packing in the post-pyrolysis zone, the corresponding ratio a t the same quench point was only 19/27 (or 0.70/1). The relative zero-order rate constants for dehydrogenation of p-methylbenzyl radicals to p-xylylene were calculated as a function of the surface t o volume ratio along the post-pyrolysis zones A and B of the apparatus shown in Fig. 2 and for the zone B when modified by addition of the quartz packing. The data, summarized in Table 11, show that the dehydrogenation efficiency increases with increase in surface to volume ratio of the post-pyrolysis zone. These results virtually prove that the conversion of p-methylbenzyl radicals to p-xylylene is a heterogeneous reaction. This conclusion can be tested further by noting the product distribution as a function of reaction pressure. It was demonstratedg that the p-xylylene produced in this dehydrogenation can be collected in a cold solvent and then polymerized at -78' to give insoluble poly-(pxylylene) almost exclusively. It also was demonstrated6 that coupling of two p-methylbenzyl radicals gives 1,Z-di-p-tolylethane that can rearrange under the existing pyrolysis conditions to give o-methylated diarylmethanes which in turn give anthracenes.
The 1,2-di-p-tolylethane1 diarylmethanes, and anthracenes are collected in the cold solvent along with p-xylylene, but these components remain soluble after polymerization of p-xylylene is completed. Hence, the secondary products of p-methylbenzyl radicals that form via bimolecular coupling and those that form via heterogeneous dehydrogenation can be separated easily by filtration. The latter are isolated as poly-(pxylylene) by filtration and the former are isolated as a mixture by evaporating the mother liquor to constant weight. The weight ratio of soluble products ( p ) to
ki
2CHs e C -H P *
C H S- ~ C H P C H ~ ~ C ~H JC
surface)
H
I
~
1-
HP (9) L. A. Errede, R. S. Gregorian, and J. M. Hoyt, J . Am. Chem. Soc., 81, 5218 (1960).
~
C
H
Jan., 1963
MECHANISM FOR FORMATION O F X Y L Y L E N E S IN
77
GAS P H A 4 S E
insoluble products ( W ) is then a measure of the relative rates for bimolecular coupling (RJ and dehydrogeiaation (22,) which are concurrent reactions as indicated schematically by ICl and kz. It follows that
Bimolecular coupling, a second-order reaction, should increase as the square of the reaction pressure. Dehydrogenation, here suggested to be zero order, should be essentially independent of reaction pressure. Hence, (u/ W )' h should increase linearly with increase in pressure ( P ) as indicated by
( u / W ) ~=/ ~K P
+C
(14)
where IC and C are constants. Accordingly, p-xylene was subjected to fast flow pyrolysis conditions that afford 3-4Oj, conversion to p-meth ylbenzyl radicals, and the weight ratio of products produced via bimolecular reaction (u) to those produced via dehydrogenation ( W ) was determined as described in the Experimental section. Pyrolyses were carried out at pressures ranging from 1.5 to 45 mm. and the results are summarized in Fig. 4. It is seen that (u/ W )'Iz increases linearly with pressure as required by eq. 13 and 14. Extrapolation of the line to zero pressure gives an intercept for this particular pyrolysis system a t 0.51. The value is a function of the geometry of the systern as shown in Table 11. Probably these results can be improved in favor of dehydrogenation by increasing the surface to volume ratio in the postpyrolysis zone. It was surmised immediately that the carbonized material deposited as a film along the walls of the postpyrolysis zone might be the active catalyst for dehydrogenation, since the p-methylbenzyl radicals generated in the carbon-free pyrolysis zone were not converted to p-xylylene until the pyrolyzate entered the post-pyrolysis zone of carbonized film. Szwarc had shown that p-methylbenzyl radicals generated from p-methylbenzyl bromide via relatively mild pyrolysis conditions ( 140”) then was separated by distillation a t 3.5 mm. to give three main fractions: (1) p methylbenzyl chloride, 59 g., b.p. 63-67’; (2) 1,2-di-p-tolyIethane, 21 g., b.p. 125-135”, m.p. 77-78” after one recrystallization from methanol; (3) residue (2 g.), b.p. > 135’. The infrared spectrum of this residue indicated a mixture of 1,2-dip-tolylethane, diarylmethanes, anthracenes, and a small amount of another component that could be cyclo-tri-p-xylylene. Thus,
Vol. 67
9.3 moles of pyrolysis feed stock was converted to about 0.2 mole of p-methylbenzyl units that were isolated as l,2-p-tolylethane (2% yield) and as polymerization products of p-xylylene (trace amounts only). I n order to test the effect of freshly deposited carbon char, the apparatus waa used for pyrolysis of 10 moles of p-xylene a t 1050’ and 0.01 sec. residence time. This “conditioning run” caused a uniform layer of carbon char to be deposited over the entire quartz surfaces of the post-pyrolysis zone. The pyrolysis apparatus then was used to repeat the original pyrolysis experiment. An equal amount of pyrolysis feed stock solution (8.6 moles of p-xylene and 0.86 mole of p-methylbenzyl chloride) was pyrolyzed as described before, to generate the equivalent amount of p-methylbenzyl radicals (about 0.2 mole) in the pyrolysis zone. The reaction products were isolated as described previously and the 0.2 mole of p-methylbenzyl radicals, generated in the pyrolysis zone, were isolated as poly-(p-xylylene) (0.11 mole), cyclo-tri-(p-xylylene) (0.03 mole), and 1,2-bisp-tolylethane (0.06 mole of p-methylbenzyl equivalents).
A STUDY OF THE MECHANIS-M OF CARBON MONOXIDE ADSORPTION ON PLATINUM BY A NEW ELECTROCHEMICAL PROCEDURE’ BY S. GILMAN Research Laboratory, General Electric Go., Schenectady, N . Y Received M a y 16, 1962 The adsorption of GO has been studied by means of a new electrochemical procedure utilizing a complex potential-time wave form. By means of this method, information on the type of bonding is obtained as a function of total surface coverage. Also, a minimum rate constant is estimated for the rapid chemisorption of CO from solution. Information is presented to reveal unusual reproducibility of the Pt surface before adsorption. Hydrogen adsorption is shown to be measurable with high precision using linear sweep speeds of up to 800 v./sec.
Introduction There is increasing interest in the mechanism of electrochemical oxidation of carbonaceous fuels in connection with fuel cell technology. A knowledge of the mechanism of fuel adsorption is of fundamental importance to the elucidation of the over-all electrode reaction mechanism. The electrochemical techniques previously described2 for measuring CO adsorption have now been further developed and make possible a more detailed study of the system. The bonding of CO t o platinum a t the gas-solid interface has been studied by Eischens and Pliskin3using infrared techniques. These investigators uncovered evidence for both a linear (one-site) and bridged (two-site) adsorption structure on cabosil and alumina-supported platinum. The linear structure was found to account for approximately 15% of the total coverage on the former and 50% of the total coverage on the latter support. G r ~ b e r ,using ~ conventional gas-phase volumetric techniques, studied the total adsorption of CO and, alternately, hydrogen on alumina-supported platinum of varying dispersion. Assuming a combination of linear and bridged structures, he derived the contribution of each a t saturation coverage as a function of platinum dispersion. Xone of the previous approaches to the problem could easily lead to information on the type of bonding as a function of total coverage. (1) This work was made possible by the support of the Advanced Research Projects Agency (Order No. 247-61) through the United States Army Engineer Research and Development Laboratories under Contract Number DA-44-009-ENG-4853. (2) 8 , Gilman. J . Phys. C h e n . , 66, 2657 (1962). (3) R. D. Eischens and W. Pliskin, “Advances in Catalysis,” Vol. X, Aoademic Press, Inc., New York, N.Y . , 1958, p. 18. (41 H. L. Gruber, J . P h w . Chsnz., 66, 48 (1962).
I n this work, the rapid electrochemical techniques employed permit us to determine the contribution of the bridged and linear structures to the coverage as it is increased from zero to saturation. It also allows us to place a minimum limit on the rate of the rapid chemisorption of CO from solution. Experimental The electrochemical cel and reagents have escribed previously.2 The electrolyte was 1 N HCIO,; the working electrode had a geometric area of 0.08 cm.2. All potentials are referred to a hydrogen electrode in 1 N HC101. A diagram of the circuit used appears in Fig. 1. The signal generator consisted of two Exact Model 250 signal generators whose ramp output m a added by means of a network. Separation between ramps was achieved by means of a Tektronix Type 161 delay pulse generator. Potential “staircases” were generated using relays and batteries for signals of longer than 15-sec. duration and with Tektronix Type 161 delay units for step signals of shorter duration. The potentiostat used was a Wenking “Fast Rise” potentiostat. The current-time (potential) signals were measured with a Tektronix Type 536 oscilloscope using type D and type T plug-ins. The potential function employed in this work is diagrammed in Fig. 2. The significance of steps A-E is covered below. I. For Adsorption of 100% CO by Linear Diffusion.-(A) Pretreatment step (15-sec.) to remove adsorbed oxidizable impurities, and to produce a layer of adsorbed oxygen which serves to block adsorption. GO is kept bubbling through the solution. Some molecular oxygen is evolved at a steady-state rate of approximately 1ma./cm.z. (B) Potential step during which the oxygen layer formed in (A) is maintained, molecular oxygen from (A) is swept away, and the concentration of the adsorbate is brought to its bulk value. CO is kept bubbling through the solution for 0.5 min. The flow of gas is then stopped and the solution allowed to become quiescent for 1.5 additional min. to allow for linear diffusion. S o measurable current (less than 1 pa,/cm.z) flows after the first second of pretreatment during this step. (C) Reduction and adsorption step. The adsorbed oxygen layer is reduced within a few milliseconds, exposing a reproducibly
