COMMUNICATIONS TO THE EDITOR
4048
atoms and COS molecules. These complexes can stabilize on the walls, e.g., by dimerizing into an oxalate. Once formed, the deposit can then photochemically decompose, especially easily with excited Hg atoms. Gas phase CO seems to be produced from such decomposition. This is also supported by measurements of gas phase CO yield as a function of C02 pressure. After an initial rise in the CO yield a nearly constant rate was observed. Carbon dioxide is linear in its ground state (‘2,+). Both the lowest singlet excited states, ‘B2 and lA2, are bent and probably have their vibrational zero levels near 7 e.v.’ Similarly, the lowest triplet excited states, 3Bzand 3,42,are also bent and probably have their vibrational zero levels near 5 to 5.5 e . ~ . 7 - ~I n the light of these spectroscopic estimates, the Hg 6 ‘PI atoms are capable of producing the 1A2 state of C02. Thus one would be able to place an upper limit of 6.7 e.v. on the lowest vibrational energy level. An excited Conmolecule, on the other hand, is hardly expected to produce our observed solid phase products since it would lose its energy in collisions with other molecules and walls. Our postulated collision complex survives more readily near the walls as was shown by the data. When the resonance lamp was used directly, without the filter, the ratio of the Hg 6 1P1 atoms to the Hg 6 3P1 atoms in the reaction cell was about 0.11. Carbon monoxide was found as the gas phase product. An orange-yellow deposit on the cell walls decomposed into oxygen and carbon dioxide when heated. The amount of COS was considerably less than the comparable runs with filtered radiation. With a methanol filter, which allowed only the production of Hg 6 3P1atoms, no COn decomposition would be detected under conditions of similar intensity and reaction time. We conclude that a collision complex is formed only with singlet excited mercury atoms. The triplet excited mercury atoms seem to decompose a previously formed solid product into CO and HgO. From the spectroscopically assumed excited state configurations of COn and our observed product-presumably an oxalate-the collision complex probably has one of the configurations
o
o
/ 2. *y Hg
c
.Hg. or O
’0
\. .y
c
(7) R. S. Mullken, Can. J . Chem., 36, 10 (1958). (8) A. D. Walsh, J . Chem. Soc., 2266 (1953). (9) R.N.Dixon, Proc. Roy. SOC.(London), A275,431 (1963). (10) To whom inquiries concerning this communication should be directed at the Department of Chemistry, University of Alberta, Edmonton, Alberta.
The Journal of Physical Chemistry
Dimerization of either of these complexes on the walls can now readily produce an oxalate. DEPARTMENT OF CHEMISTRY OF HOUSTON UNIVERSITY TEXAS77004 HOUSTON,
C. M. WOLFF PERTEL~O RICHARD
RECEIVED AUQUST12, 1963
Effects of Oxygen Absorbed in the Skin of a Platinum Electrode on the Determination of Carbon Monoxide Adsorption Sir: Investigationsl-’o have shown that hydrogen and oxygen absorbed in the “skin” of a platinum electrode can significantly affect its electrochemical behavior. We9 named this type of absorption “dermasorption.” We d e m o n ~ t r a t e dthat ~ ~ ~anodic ~~ and cathodic pretreatment to remove surface impurities can result in effectually changing the electrode material (by alloying metal atoms in the skin with H or 0). Dermasorbed 0 or H also can lead to erroneous determinations of the identity and surface coverage of chemisorbed species by anodic stripping because some of the dermasorbed atoms may migrate to the surface.6 As an example of the effects of dermasorbed 0, we shall discuss the differences in the degree of coverage of a platinum electrode with CO found by Brummer and Ford” and our~elves.~We shall also attempt to correct some of their misinterpretations of our The central difference between these two studies is the determined value of Qc0, the charge required to remove adsorbed CO; Brummer and Ford” found 365 f 36 pcoulombs/cm.*, wes found 453 =k 20. These values depend critically on the shape and on the assumptions made about the significance of the various regions of the anodic charging curve. Since the voltage-time relation observed by them (Figure 1 in ref. 11) and by us (Figure 1 in ref. 5) differ greatly, it is not (1) S. Schuldiner, J. Electrochem. Soc., 107,452 (1960). (2) C. H.Presbrey, Jr., and S. Schuldiner, ibid., 108, 985 (1961). (3) S. Schuldiner and R. M.Roe, (bid., 110, 332 (1963). (4) S. Schuldiner and T. B. Warner, J . Phys. Chem., 68, 1223 (1964). (5) T. B. Warner and S. Schuldiner, J . Electrochem. SOC.,111, 992 (1964). (6) 5. Schuldiner and T. B. Warner, Anal. Chem., 36, 2510 (1964). (7) V. I. Lukyanycheva and V. 5. Bagotskii, DokE. Akad. Nauk SSSR, 155, 160 (1964). (8) 9. Schuldiner and T. B. Warner, J. Electrochem. SOC.,112, 212 (1965). (9) T.B. Warner and S. Schuldiner, ibid., 112, 853 (1965). (10) S. Schddiner and T.B. Warner, Electrochim. Acta, in press. (11) S. B. Brummer and J. I. Ford, J . Phys. Chem., 69, 1355 (1965).
COMMUNICATIONS TO THE EDITOR
surprising that determined values of QCO also differ. Temperature and electrolyte differences would not likely cause the discrepancy. The anodic cleaning pulse of 20-100 ma./cm.2 applied for about 1 sec. by Brummer and Ford prior to CO adsorption may account both for their lower value of QCO and for the different appearance of the charging curves. Dermasorbed 0 from the cleaning pulse (a) could affect CO oxidation kinetics and the shape of the anodic charging curve, and (b) 0 atoms could migrate to the surface and react with adsorbed CO reducing the charge necessary to strip
co.
Brummer and Ford’s anodic charging curve shape is such that quantitative separation of QCO from Qo is virtually impossible. Brummer and Ford indicate that our QCO region corresponds to their AB region (Figure 1 in ref. 11). Comparing our Figure l5 with theirs” shows that this is not accurate; we extrapolate two linear regions to find an intersection point and no point on their curve can be compared with ours due to the important differences in curve shapes. Also, they suggest that we found that the oxidation of the adsorbed CO and of the electrode occur completely separately. In fact, we5 postulated that a small amount of 0 formed in the CO oxidation region oxidized adsorbed CO. The 0 concentration remained small because its reaction with adsorbed CO mas much faster than the oxidation of mater to 0 atoms. Brunimer and JIakridesl? show an anodic charging curve for CO oxidation ,similar in shape to ours.5 Specific conditions were not given by Brummer and Nakrides, but the same anodic pretreatment sequence used by Bruniiiier and Ford” is unlikely; otherwise, the transients would have looked the same. The presence of derniasorbed 0 in Brummer and Ford’s electrode is suggested also by the values of QO in Figure 6 of ref. 11. Oxygen determined following anodic pulses where less than a nionolayer of CO was removed led thein to suspect concomitant electrode oxidation and thus they subtracted these observed QO values when determining Qco from Q&. However, they may have detected dermasorbed 0 which migrated to the surface; this would invalidate their “correction.” This is supported by the close correspondence of our5 Qo values in the presence and absence of CO in solution (Figure 3d in ref. 5 ) which showed that virtually as much 0 can be deposited after stripping CO as can be deposited on a clean electrode. Brummer and Ford’l agree with us5 that physically adsorbed CO can exist on top of chemisorbed CO, but they feel that me used invalid evidence whereas their finding was based on solid facts. They state that physically adsorbed CO would be easier to oxidize
4049
than chemisorbed CO. How these authors arrive at this unsupported statement is unknown. They may suspect that physically adsorbed CO oxidizes more easily than chemisorbed CO, but before generalizing, substantiating evidence is required. There is no reason, per se, why chemisorbed CO should not be oxidized more readily than physically adsorbed CO. Brummer and Ford’’ conclude “. . . Warner and Schuldiner were incorrect in their assignment of charge to the various processes which occur during anodic transients with CO.” They further qtate that they “ . .can see clearly the quantitative limit on the aswniptionS involved in deriving QCO, whereas in the previous work this was not possible.” We feel that such conclusions are premature. Any decision a i to the type of CO bonding with the surface niuqt await a clear esperimental answer to the question of how much CO is, in fact, adsorbed. We suggest that derniasorbed 0 formed during electrode “activation” may well account for many of the phenomena which are dependent on the time following such “activation” and for discrepant rewlt. obtained using different “activation” procedures. It appear. that there is no easy way of avoiding expenditure of the time and effort required to make aiid maintain the clean electrocheniical systems required for fundamental research. (12) S. B. Brummer and A. C. Makrides, J . Phys. Chem.. 68, 1448 (1964).
U.S. NAVALRESEARCH LABORATORYTHEODORE B. WARNER D. C. 20390 SIGMUND SCHCLDINER WASHINGTON, RECEIVED JULY19, 1965
The Correction for Electrode Oxidation in the Estimation of Adsorbed CO on Smooth Platinum by Anodic Stripping
Siy: ,4nodic galvanostatic stripping of CO adsorbed on Pt occurs a t potentials where the electrode should normally be oxidized.’-4 Consequently, some correction niust be made for the oxidation which occurs when stripping CO. Three methods of doing this have been a p ~ l i e d . ~ Warner -~ and Schuldiners suggest that (1) S. B. Brummer and A. C. Makrides, J . P h y s . C h e m . , 68, 1448
(1964). (2) S. B. Brummer and J. I. Ford, ibid., 69, 1355 (1965). (3) T.B. Warner and S. Schuldiner, J . Electrochem. SOC.,111, 992 (1964). (4) S. Gilman, J . Phys. Chem., 66,2657 (1962). (5) T.B.Warner and S. Schuldiner, ibid., 69,4048 (1965).
Voliime 69, S z t m b e r 11
.?-onember 1966
