Differential Voltammetric Scanning Thermometry of Tenth Formal Formaldehyde Solution in Formal Perchloric Acid Bruce B. Graves Department of Chemistry, Eastern Michigan University, Ypsilanti, Mich. 48197
The concurrent use of DTA with cyclic voltammetry was further investigated using an improved electrode. Caloric standardization was approached from known Peltier AH of the thermistor enveloping hemispherical Pt/Pt-l3% Rh bimetal thermocouple electrode undergoing only electronic conduction producing a thermal peak via a thermistor bridge system. Cyclic voltammetry with Pt black in 0.1F HCHO in 1.OF HCIOl affords exothermic AHvalues for anodicand cathodic sweep oxidative processes of 24 i4 and 17 =t1Kcal/F, respectively, at scan rates of 50 mV/sec, the estimated error arising largely from uncertainties of planimeter area integrations. Methods are discussed for evaluation of overpotential heats from the same thermograms. Induction times associated with thermally evident nonelectrochemical electrode processes are shown to be sensitive indicators of electrode surface structures. The use of these induction times to interpret the state of Pt oxide layers or surface coverage by chloride is illustrated.
IN RECENT YEARS, there has been an increasing trend in electrochemistry toward measurement of in situ parameters other than the usual electrochemical ones. A few random examples are the use of A.C. capacitance ( I ) , specular reflectance (2-5), and ellipsometry (6) by several workers to study various properties of electrode films, and the use of photoemission measurements at organic electrodes was discussed recently by Bard (7). Another type of measurement recently demonstrated to be feasible is differential thermal analysis (8, 9) of a single microelectrode subject to concurrent cyclic voltammetric scanning. This latter technique represents a significant departure from other published work on thermal measurements in electrochemistry since only the electrode and its immediate solution environment are involved in the thermal picture rather than a large Dewar flask full of electrolyte. This makes work possible on transient electrode processes that was not possible before. The use of thermistor bridge DTA as has been done here is not unique as Joncich and Holmes have used it to measure CuS04 concentrations (IO) (1) M. W. Breiter, Electrochim. Acta, 8,925 (1963). (2) T. Takamura, K. Takamura, W. Nippe, and E. Yeager, J . Electrochem. SOC.,117,626(1970). (3) J. D. McIntyre, Paper 232, 135th National Meeting of the
Electrochem SOC.,New York, N.Y., May 4-9, 1969, Extended Abstracts, pp 578-9. (4) B. J. Holden and F. G. Ullman, ibid, Paper 52, Extended Abstracts, pp 61-4. ( 5 ) D. C . Walter, Can. J. Chem., 45, 807 (1967); ANAL.CHEM., 39, 896 (1967). (6) M. Novak, A. K. N. Reddy, and H. Wroblawa, J. Electrochem. Sac., 117,733 (1970). (7) A. J. Bard, “Modern Aspects of Electrochemistry,” Local ACS Meeting, University of Michigan, Ann Arbor, Mich., April 29, 1970. (8) S . L. Cooke, Jr., and B. B. Graves, Chem. Instrum., 1, 119 (1968). (9) B. B. Graves, Ph.D. Dissertation, University of Louisville, Ky., September 1966, University Microfilms Publication No. 68-8005, Ann Arbor, Mich. (1968). (10) H. F. Holmes and M. J. Joncich, ANAL.CHEM.,32, 1251 (1960).
and electrode enthalpies for Cu and Ag systems as well as for water decompositions (11). While it is useful to know the properties of electrode films in understanding electrode mechanisms, and while photoemission techniques can provide useful information about electron transition in specific electrode processes, we also note that DTA shows promise not only in affording enthalpy data for steady state and transient electrode processes, but also in detection of electrode processes that are not electrochemical and are therefore undetected by electrochemical means. The earlier referenced work on DTA with cyclic voltammetry obtains enthalpy information by correlating DTA thermal areas with coulomb areas of the cyclic voltammogram. A really satisfying means to standardize the thermal areas in terms of calories was lacking in this work since the standardization depended on the enthalpy of the hydrogen evolution reaction. This being a single electrode reaction, the electron transferred between electrode and solution represented transfer of an unknown amount of entropy. The need was thus established for a completely separate means of standardization, one which did not involve such a single electrode reaction. A new electrode has now been developed which permits such standardization by use of bimetal Peltier effects in a bimetal thermocouple hemisphere, the outer surface of which is also the electrode in solution. This approach has now been supplied to the analysis of electrode processes in acidic formaldehyde solutions which analysis comprises the subject of this report. THEORETICAL
As with any calorimetric method, we are concerned in this work, with the amount of heat produced for each unit of reaction. The unit of reaction can be evaluated from the electrical charge passed, a quantity derived from peak areas of time/current curves typical of cyclic voltammetry, while the heat produced can be obtained from peak areas of differential thermograms recorded concurrently. We must start, then with the equation given by Speil and Kerr et al. (12, I.?), relating the thermal area to the specific reaction enthalpy, AHs,
The conditions under which this equation holds are those of a typical DTA process in which the reactant mass, m’, while surrounding a thermocouple detector is subjected, along with a dummy “sample” surrounding its reference thermocouple detector, to a programmed linear temperature rise in a surrounding furnace. A thermal area of interest will generally (11) M. J. Joncich and H. F. Holmes, “Proceedings of the First Australian Conference on Electrochemistry,” J. A. Friend and F. Gutman, Ed., Pergamon Press, New York, N.Y., 1965, p 138. (12) S. Speil, L. H. Berkelhamer, J. A. Pask, and B. Davis, US. Bur. Mines Tech. Papers, 664 (1945). (13) P. F. Kerr and J. L. Kulp, Amer. Mineral., 33, 387 (1948). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
993
course, can be evaluated from the area of the current time curve, thus,
Z and so,
=
1
Idt
LH [Idt = C
(5)
ATdt
d
=
Q
nF This equation, now represents only the ideal situation of a perfectly reversible reaction involving no overpotentials at the electrode. In fact, this situation is never obtained, although it may be approached in rare instances. The total heat Q t actually realized experimentally at the electrode will consist of two components, one due to electrochemical change, Q, and the other due to overpotential, Q’, a quantity to which van Rysselberghe refers as “uncompensated heat” due to irreversibility thus, Qt
=
Q
+ Q’
(7)
Now van Ryssellberghe has shown (14, 15) that the uncompensated heat consists of several components, namely, dQ’ - CONSTANT TEMPERATURE BATH
-
MECHANICAL HEAT PUMP
CIRCULATING HzO BATH
nF[rla
+ Ival + Rlld5
(8)
where qa and o b represent the positive anodic and negative cathodic overpotentials respectively, R the internal resistance, and dQ’ and d t represent infinitesimal increments of uncompensated heat and electrochemical advancement, respectively. We are interested, now, in the Q and Q’ terms in the vicinity of only the one microelectrode and so the internal resistance term and one of the overpotential terms will vanish leaving, for the small system under consideration,
__
CONSTANCY 2 O.O0WoC -
2S.0°C.,
=
dQ‘
=
nFq,d[
(9)
or
Figure 1. Thermoelectrochemical system
dQ’ = nFlv,/dZ: be the time integral of the differential temperature os. time plot between the time limits c and d. Terms g and k represent experimental constants of heat capacity and thermal conductivity of the sample. The reaction is electrochemical, now, so the mass, m’, can be related to the charge passed, thus,
z = -nFm’
depending on the polarity of the voltammetric scan. From Equation 3, however, nF[ = 2,so
Z nF
(3)
and thus we can modify Equation 1 to the form
-AHZ ~
nF
C l ATdt
[-
(4)
where C = gk and AH is the molar reaction enthalpy with the usual negative sign for an exothermic process. 2, of (14) P. van Rysselberghe, “Electrochemical Affinity,” Hermann,
Paris, 1955. (15) P. van Rysselberghe, “Thermodynamics of Irreversible Pro-
cesses,” Blaisdell, New York, and Hermann, Paris, 1963. 994
(11)
- Q cathodic
= (q@
(12)
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
1 1
+Q’ cathodic
= qa
Idt
- Q’cathodic
= lqb/
(13)
Zdt
(14)
and substituting these with Equation 6 into Equation 7 we have,
d
=
= qaZ
or
where n and M have their usual significance. Rearranging, we obtain an expression for van Rysselberghe’s “degree of advancement” (14, IS), m’ M
+ Q anodic
whereupon, using Expressions 5 and 8 we find
M
[ = - = -
(10)
4.184
+
( 3 11 Id nF
Zdt
=
C
ATdt
(1 5 )
and
which is the operational equation used in this study that relates the areas under the thermogram to the areas under the voltammogram. In principle, the overpotential term should
EXPERIMENTAL
vary with current according to the Tafel equation, 7 = a
+ blogZ
(17)
However, it is not always feasible to determine the constants, a and b, for a transient voltammetric process, and graphical analysis may often be required. For a more detailed discussion of this derivation, the reader is referred to the original publication (9). One other theoretical aspect that must be considered is the assignment of caloric values to the areas under the thermograms. It is, of course, well known that differential thermograms do not afford caloric values directly but rather areas that must be assigned caloric units by standardization with a process of known enthalpy. The fist attempts at such calibration used the enthalpy of the hydrogen evolution reaction (HER) for this purpose with appropriate corrections for hydrogen overpotentials (even though small). It is probably not valid to ignore thermal effects due to the electron transfer required on the balancing of this or any half reaction even though these are also small. Recalling that electron flow across a bimetal junction also causes thermal effects, the same should be true of metal/solution junction even though the electrons may originate from, or be supplied to, an ion. To standardize the thermal areas independently of these metal/ solution junction electron transfers would thus be highly desirable, and one approach would be to use the known thermal (Peltier) effects of electron transfer across a bimetal junction, one of the metals of the couple being also the electrode under study. In this approach, care must be taken that the bimetal couple has high thermal conductivity relative to the surrounding media of electrolyte and incidental materials of construction in order that the spatial distribution of the bimetal Peltier enthalpy is not significantly different from the electrochemical reaction enthalpy. With this in mind, we know the enthalpy for passage of one Faraday of positive charge from metal B to metal A in a thermocouple junction is given by the Peltier coefficient (16). IIBA = AH = TAS
(18)
We also know from the same source that,
EA
=
--AB
or l L , =
-nendo
(19)
and that the Peltier coefficients are related to the slope of the potential/temperature plot for that thermocouple by the relation.
All quantities here are readily accessible except the enthalpy which is sufficient for its determination. The only fly in the ointment is the ever present resistance component of the enthalpy arising whenever a standardizing current is passed through the bimetal couple. This can, however, be eliminated by current reversal during standardization since the resistive enthalpy is always exothermic while the Peltier enthalpy changes according to Equation 19. Thus if HI =
ne,, + 12Rand H z =
- H2
=
nendo
+ 12R
(21)
then HI
nexo
+ Z2R -
nendo
- Z2R = 2IIexo
(22)
(16) G. N. Lewis and M. Randall, “Thermodynamics,” 2nd ed., McGraw-Hill Book Co., New York, N.Y., 1961, p 465.
Apparatus. A block diagram of the electrical and electrochemical system in use is given in Figure 1. The zero equilibrium potential electrochemical cell is a three-electrode cell where the counter electrode, represented by the square plate, has an area about 100 times that of the indicator electrode which is represented by the outer Pt surface of the complex center electrode. The single bead represents the reference electrode, and all three of these electrodes are appropriately connected to a solid state potentiostat constructed in a fashion similar to circuits 10 and 15b given by Schwarz and S h a h (17), circuits first proposed by DeFord (18) for a general purpose potentiostat. The latter is driven by a solid state triangle wave generator constructed along the lines of a circuit suggested by Schroeder and Modderman (19) that consists essentially of a bistable square wave oscillator and integrator. The f 1 5 volt power sources for these modules are Model CA20-1 Deltron (20) solid state units Poaffording output regulation to *0.1 mV or + O . l % . tentiostat, function generator, and power supplies are all housed in a 6 in. X 6 in. X 17 in. chassis mounted on a 6-in. X 19-in. standard relay rack panel. The output of the electrochemical chassis then appears at the X and YI, inputs of the two pen XYIYP Hewlett-Packard (21) Model 136AM plotter. The thermal signal appears on the YS channel of the same plotter at the same time as the X axis is driven by the triangular voltage signal applied to the cell (also in effect a time base). This thermal signal is obtained by use of a Model 120 Princeton Applied Research (22) lock-in amplifier and A.C. Wheatstone bridge combination in which two Veco No. 41A2 thermistors (23), a “sample” and reference thermistor, serve as two of the bridge resistances. In Figure 1, these two thermistors are represented by the single beads each with two electrical leads, the “sample” bead being at the mid point of the complex central electrode. The other half of the temperature bridge with its coarse and fine adjustment potentiometers is mounted immediately adjacent to the cell to minimize lead capacitance and noise pick-up. Variations in the temperature of the “sample” thermistor are recorded relative to the reference thermistor, the latter being immersed in constant temperature surroundings on the right in Figure 1. These variations now, may arise either from electrochemical processes that occur at the central indicator electrode or from thermal pulses generated in the bimetal hemisphere electrode for the purpose of the Peltier standardizations. These thermal pulses are caused by passing current pulses through the bimetal hemisphere either from the function generator as shown or simply by manual switching. Standardization currents can be measured easily with a standard resistor and laboratory potentiometer of the usual types, or recorded on the Y1 channel of the plotter. The obviously important temperature control is provided by the Tamson-Neslab ( 2 4 ) Model TEV-40 bath and TE9 circulator with a converted refrigerator condensing unit (17) W. M. Schwarz and I. Shain, ANAL.CHEM., 35,1770(1963). (18) D. D. DeFord, Anal. Chem. Div., 133rd Natl. Meeting, ACS, San Francisco, Calif., April 1958. (19) R. R. Schroeder and J. Modderman, Department of Chemistry, Wayne State University, Detroit, Mich. 48202, private
communication, June 1969. (20) Deltron, Inc., Wissahickon Ave., N. Wales, Pa. 19454. Model CA20-1, (21) Hewlett-Packard, Inc., Midwest Sales, 24315 Northwestern Highway, Southfield, Mich. 48075. (22) Princeton Applied Research Corp., P.O. Box 565, Princeton, N. J. 08540, Model 120,1000Hz. (23) Victory Engineering Corp., 136-50 Springfield Ave., Springfield, N.Y. 07081, No. 41A2-10K. (24) Neslab Instruments, Inc., 871 Islington St., Portsmouth, N.H.. 03801. ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
995
b 1
40
0
R-13%R h lead wire
*
\
kadr
,
Approrimate acale
1-1
i m m =
Figure 2. Bimetal thermistor electrode
AT
'1
bridge
01
A v
I.
Figure 3. Hemisphere electrode bimetal Peltier calibration thermogram Current pulses shown produce endotherms or exotherms depending on current direction. Current-time areas are related to thermal areas for calibration
serving as the final heat-sink. Short term temperature stability of the order of +0.0005 "C with this system is no problem; however, long term stability required daily attention or maintenance of the surrounding room air a t constant temperature. The crucial bimetal hemisphere electrode thermistor probe is shown schematically in Figure 2. The electrode proper is the outer 10-square millimeter Pt surface area of the bimetal hemisphere, the inner layer being Pt-13 Rh. Together this metal pair comprises a thermocouple having well known properties with a hemispherical configuration nearly identical with that of the electrode solution interface. It is important to note that the electrical lead wires for the hemisphere must be placed on opposite sides to avoid localized configurational distortion of bimetal Peltier standardization heats due to the 996
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
electric current flow patterns that would occur were both lead wires on the same side. The hemisphere is joined by a metal-to-glass seal on the inside to the Veco 41A2 thermistor bead of nominal 10,000 ohms room temperature resistance. Details on the techniques of construction of this electrode will be published elsewhere in the near future. The cell compartments are made of Pyrex No. 774 glass and electrode or inlet seals are made either with commercially available all-Teflon (Du Pont) packing glands or with glass standard taper joints and Teflon sleeves. Reagent additions and withdrawals are made through glass/Teflon sleeve joints or through commercially available glass/Teflon threaded needle valves, items not shown in Figure 1. With this arrangement, various gas atmospheres can be maintained in any of the three cell compartments, the reference electrode being in the third compartment with Luggin capillary, (also not shown in the figure for simplicity). Reagents. All reagents were of reagent grade quality except for the formaldehyde which was prepared by decomposition of paraformaldehyde to circumvent the alcohol preservative in reagent grade formaldehyde solutions. All solutions were made using distilled water passed through a Barnstead deionizing column. Procedure. Voltammetric scans were all taken in the conventional manner a t a scan rate a t 50 mV/sec. During each scan, the thermogram of the electrode processes was simultaneously recorded with the second plotter channel. For a given process, areas were measured under the appropriate pair of peaks giving coulombs of charge and thermal units from the two simultaneous plots. The base lines chosen for the two types of recordings were necessarily based on different criteria. The thermal areas were taken as simply the area under a given peak but above the ambient base line that would correspond to absence of any thermal process. Since this system was essentially isothermal compared to conventional temperature programmed DTA, there was no problem with base-line drift except that due to an interfering peak of a nearby thermal process. In such cases, the peaks were separated by visual extrapolation to the base line, a process which can lead to considerable uncertainty in complex plots where adjacent peaks overlap. Areas under electrochemical plots were delineated by similar methods except that the chosen base line corresponded to the zero current axis unless the charging current was measurable in a region of the cyclic voltammogram devoid of electrochemical processes. In that case, that charging current was taken as a constant and as the base line. After a Peltier calibration, the thermal units can be assigned a value in calories, thus allowing determination of enthalpy for that process. For studies of non-electrochemical induction times involving open circuit switching or alteration of scan reversing potential, the cell was simply manually switched to open circuit at the desired potential or the scan reversing limit control was appropriately altered on the function generator panel. The data for thermal calibration are given in Figure 3 which shows both endotherms and exotherms produced by the DTA system upon passing known current pulses through the bimetal thermocouple interface. In other words, these represent Peltier heat absorbed or evolved as a result of strictly electron flow across the boundary between the two thermocouple metals and do not refer to any electrochemical process involving species in solution. For this particular system as an example, these heats were found to be,
HI= 8.34 X Hz= -5.28 x
volt sec coulomb-'
(23)
volt sec coulomb-l
(24)
and by applying Equations 19,21, and 22, we find
/IT( = 681 pVsec coulomb-'
Now, from our knowledge of the Pt/Pt-13% Rh thermocouple, we know the temperature dependence of potential is 6.175 microvolts per degree. From Equation 20, then, we have 6.175 X lo-' V deg-1
=
n
-
FT
=
n 6,487 X 298
3 -
AT bridge . 2 -
(26)
vonsdo3 -
I 11
whence
n
=
~ 1 7 7 . JF-I 7
=
h42.43 cal F-'
exolhcnnlc
=
441 pcal coulomb-1
-
sgnol
(27)
This result with Equation 25 affords a standardization of the thermal area in terms of the conversion factor, y,
y =
441 Fcal coulomb-' 681 pVsec coulomb-1
=
0.647 pcal per pVsec
(28)
a result valid, of course, only for the specific solution studied.
Figure 4. Typical differential voltammetric scanning thermogram Solution surrounding electrode-O.1F HCHO in 1.OF HClOa with helium atmosphere. Scan rate 50 mV/sec
RESULTS AND DISCUSSION
Enthalpies of Transient Processes. The dashed line plot of Figure 4 is the cyclic voltammogram typical of formaldehyde in perchloric acid using the platinized platinum hemisphere electrode described above. Shown also is the solid line plot representing the simultaneous differential thermogram for the scan. One will note that nearly all processes are exothermic, specifically,
Process I: Anodic scan formaldehyde oxidation process. Process 11: Anodic scan mixed oxidation/Pt oxide formation. Oxygen Evolution. Process I11 : Cathodic scan formaldehyde oxidation. Hydrogen Evolution: Thermal process not evident. Earlier work (9) has shown the HER to be endothermic, but this is not evident in Figure 4, since a higher detection sensitivity than used here is required. From appropriate areas for Processes I and 111, and using the Peltier standardization process outlined above, the following enthalpies have been calculated: Process I : 24 h 4 Kcal per F of charge passed. Process 111: 17 i 1 Kcal per F of charge passed. As an initial demonstration of the utility of these measurements, one can calculate the enthalpies for electrochemical oxidation of CH20(,,) to various reasonable products such as HzC03, HCOO-, COz.6HZO, COz,,,,, or COz,,, and if electron flow enthalpies are neglected, conclude that a significant portion of electrode enthalpy arises from sources not involving chemical transformation. Of the products mentioned, the largest exothermic enthalpy is about zero for COz. 6HzO(,)which leads to the conclusion that a reasonably high degree of irreversibility characterizes this oxidation. The excess heat evolved above the chemical enthalpy change may be regarded a consequence of overpotentials and other factors as follows. a. b. c. d.
Electron flow Peltier enthalpy. Reactant adsorption enthalpy. Product desorption enthalpy. Error due to assumption that S O B + ( , , , std state m = 1 is zero. e. Activation overpotential. f. Kinetic or transfer overpotential. g. Accompanying nonelectrochemical transformation leading to mixed processes (NEAT).
b
AOPH = UmPJ I i d t
Ill!
(
b
C
0.0
(-1
Y
' I
d.oc+, [ A&lied I
0
.
time,t
0.5 .
"
1.5
1.0
p t e n t i o l , Volts ~
VI '
S.H.E. ~
10 seconds 20
I
"
.
~
30
Figure 5. Hypothetical diagrammatic representation of reversible and overpotential heat sources
With respect to item (f), we should note that a process having no kinetic overpotential would be expected to occur with infinite rate and be complete at the initial decomposition potential thus presenting a very high but narrow peak. It is obviously impossible to separate all these component enthalpies for this formaldehyde system without additional information from complementary experiments. For the present, however, we might suppose item (a) to be quite small if we presume it is of similar magnitude to bimetal thermocouple Peltier enthalpies. Our measurements show these to be nearly two orders of magnitude smaller than the enthalpies of chemical transformations usually encountered. Items (b) and (c), the adsorption/desorption heats, probably do not enter the picture in this instance since we are looking at the overall transformation to carbon dioxide. Alternatively, electrode adsorbed species before and after a peak would likely have similar heats of adsorption/desorption and tend to cancel each other compared to the larger heat sources. Item (d) would also account for only an additional exothermic 1.61 Kcal/F if COz,,,, is the product and if we take the value reported by Lee and Tai (25) for S'A+(,,)= -5.4 eu rather than zero. (25) F. H. Lee and Y . K. Tai, J . Chiiz. C/zem. SOC.(Peipiizg), 8, 60 (1941). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
997
'
-
U, awlied, Volts vs SHE.
+S
+O
liO
0.5 T
(t) 0.0 (->
11
-- 0
,
O
,
,
time
,
,
,
,
,
,
seconds
,
,
,
20
,
,
(
30
Figure 6. Typical differential voltammetric scanning thermogram with interrupted electrolytic scan Solution-0.1F HCHO in 1.OF 50 mVisec
under He atm. Scan rate
Items (e) and (f), the enthalpies due to overpotentials, are best discussed relative to Figure 5 which represents a generalized voltammetric peak for a process that might have a theoretical reversible potential or decomposition potential at point (c). If the reaction occurred as a very high but narrow peak at this point, we would expect practically no heat from overpotential. If a similar type of peak were realized at point (a), we would expect the heat equivalent of the overpotential, U,,,, times the coulombs obtained from the peak area at (a) to be added to the heat of chemical transformations. If, in addition to this, the peak is spread out over a finite potential interval as shown (also a time interval in voltammetry), we would expect an additional quantity of heat from overpotential due to the volt-coulomb product for each increment of potential between (a) and (b),the coulomb aspect of the product being the area of the time increment, ( A t ) , times the average current for the increment. “TOPH” in the figure is this quantity referring to “transfer overpotential heat” and similarly “AOPH” refers to the “activation overpotential heat.” Using these concepts, we have calculated item (f) for Process I11 to be approximately -7.1 Kcal/F. Still assuming the overall oxidation to result in COP,,,, according to the reaction, HCHO(,,,
+ HzO
+
COP,,,,
+ 4H+ + 4e-,
(29)
we should obtain an endothermic process for A H of +3.07 KcallF, when, in fact, we obtain AH = -17 Kcal/F which amounts to 20.1 Kcal/F due to items (a) through (g). As explained below, item (g) corresponds to about -3.4 Kcal/F which together with -7.1 Kcal/F for transfer overpotential enthalpy (TOPH) and -9.6 Kcal/F for activation overpotential enthalpy (AOPH) compares reasonably with this -20.1 KcallF. This assumes an activation overpotential, UAO,of 0.42 volt. A similar analysis applied to Process I, still taking COP,,,, as oxidation product, requires the same f3.07 Kcal/F enthalpy, but now a measured exotherm of -24 Kcal/F is obtained which requires accounting for -27.1 Kcal/F of heat. Transfer overpotential calculation for this case affords - 10.2 Kcal/F, there is no known NEAT, so we must again assume a degree of activation overpotential enthalpy, in this case -16.9 KcallF. This corresponds to an activation overpotential of 0.73 volt. By using these calculated overpotentials together with the decomposition potentials obtained from 998
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
the Z/U plots, one can determine the theoretical reversible potential, U d , and these are shown in the last column of Table I which summarizes the results of these calculations. The disparity of 0.12 V between these reversible potentials probably does not represent a real difference, in our opinion, but rather indicates the necessity for further refinement of this type of analysis, which for one thing assumes arbitrarily that no sources of heat have been overlooked, that several sources mentioned are very small, and that capacitative charging is a relatively linear function of potential. This analysis of processes I and I11 has been pointed out during review to be an oversimplification which may well be the case since we have avoided drawing conclusions not warranted by our own data. It may well be worth pointing out, however, that both processes I and I11 may contain not only the direct reaction of CHzO to COS, but also the probably lesser oxidation and formation (respectively) of adsorbed carbonaceous species formed between about 0.5 and zero volts. These factors may be partly responsible for the differences in the two values of theoretical Un in Table I, columnJ. Similarly, it might be mentioned that the direct CH20 oxidation to COSis inhibited by [O]ad at potentials more positive than 0.8 volt which should remind us that the overvoltage items (e) and (f) refer to a Pt surface covered in part by [o]s.d.
Detection of Non-Electrochemical Electrode Processes. Taking the same conditions as above, Figure 6 shows what happens when the voltammetric scan is interrupted by placing the electrode on open circuit at the point indicated. In this particular case, one notes that an exotherm we shall label NEAT, occurs after a time interval, t,, following the interruption despite the fact that electrochemical processes requiring external circuit electron flow do not take place. One concludes therefore that this exotherm arises from a local electrochemical or catalytic surface reaction, and for the purposes of this discussion, we shall simply refer to this reaction as “chemical” or “catalytic” in order to distinguish it in a word from electrochemical processes requiring external circuit electron flow. One logical choice for this net reaction might be, 3Pt(OH)z
+ CHzO
COz
+ 3Pt + 4 H ~ 0
(30)
where the platinum oxide is available for reaction as a result of anodic scan preconditioning. Other processes could be proposed, of course, depending on the nature of the platinum oxide and the degree of adsorption, hydration, or hydrolysis of reactants and products. It is not intended to embark on a discussion of these factors here, but rather to emphasize by this example that it appears one component of the mixed Process I11 has been separated, at least thermally. Buck and Griffith (26) have suggested I11 is a mixture of Pt oxide reduction and formaldehyde oxidation, a view consistent with occurrence of Reaction 30 along with additional electrochemical formaldehyde oxidation. In this study, the NEAT was an exotherm about one-fifth (0.201) the size of the Process I11 exotherm corresponding to -3.4 Kcal/FIII. We feel that the NEAT can be related to the Process I11 charge passed in that these reactions, both chemical and electrochemical, all depend on the quantities of reactants adsorbed, therefore on the surface area, and therefore on the amount and history of the platinum black on the electrode. This consideration also includes the amount of (26) R. P. Buck and L.R.Griffith, J. Nectrochem. SOC.,109, 1005 (1962).
Table I. Summary of Voltammetric/ThermometricData on HCHO in 1 F HClO4 B A
Process I 111
Measured apparent AH (1) -24 =!= 4 -17 =!= 1
C AH
D
J
Total HCHO + C o n exotherm (Tables) required (2) (B-C) +3.07 +3.07
-27.1 -20.1
G
E Ud, V
F
v
Measured
SHE(3)
(4)
NEAT (AH)
+O. 52 +0.32
$0.735 +0.42
0 -3.4
GS.
UAO,
H TOPH calculated (5 ) -10.2
I AOPH (6)
Theor. U D (Reversible) US. SHE, V
-16.9 -9.6
-7.1
(7)
-0.22 -0.10
NOTES (1) All enthalpies are in Kcal/F. (2) Electrons are ignored. (3) Ud = initial decomposition potential from I / U plots. (4) UAO= activation overpotential, calculated from heat not accounted for otherwise. i
CIi(Ui
-
UdW
( 5 ) TOPH = Transfer overpotential enthalpy,
4.185 J/cal (6) AOPH = Activation overpotential enthalpy. (7) Uo(the0r.) = Theoretical reversible decomposition obtained by subtracting UAOfrom
platinum oxide formed as a result of the anodic scan. Results, again, are summarized inTable I. Induction Time Dependence on Interruption Potential. Holding the scan reversal potential relatively constant at about f1.9 volts (all potentials here are referred to the SHE), the potential corresponding with definite onset of oxygen evolution, the potential of interruption was varied from this extreme to about +0.8 V, the potential of onset of Process I11 in a normal scan. The results of this study are presented in Figure 7. It will be noted that the interruption potential has little apparent effect on ti until a potential of about f l . 1 V is reached. Reference to Figure 4 shows this to be the beginning of the cathodic prewave that just precedes Process I11 and ti decreases dramatically to zero as the interruption potential is advanced through the region of this cathodic prewave. It is felt that these results offer important clues to at least some of the qualitative nature of this electrode surface. If the basic source of heat for the NEAT is indeed Reaction 30, one could question why there should be an induction time at all for it if reactant HCHO can be in direct contact with the platinum oxide layer by adsorption. Since ti exists, however, it is suggested that there is another substance, perhaps a layer of adsorbed oxygen interposed between the two reactant layers of oxide and HCHO, or perhaps [ O ] a d adsorbed in juxtaposition to platinum oxide both residing on the metallic platinum substrate, and that the surface [ o ] a d inhibits Reaction 30. Presumably, then, the cathodic prewave to Process I11 would represent the electrochemical reduction of [O],d and with the attendant progressive removal of this inhibitor at various interruption potentials, ti also becomes shorter. Since the NEAT occurs suddenly when all the surface [ o ] a d is consumed, it would not be necessary for the surface [O],d to be a completely intact layer (if it is a layer covering the platinum oxide). Rather the action of the surface [o],d in protecting the (underlying) platinum oxide might be likened to cathodic protection of iron by active metals like Mg or Zn which need not completely cover the iron to prevent corrosion. This in turn would require the substrate platinum oxide to be a reasonable electronic conductor. That such is the case has been pointed out by Hoare (27). Further, (27) J. P. Hoare, J . Electroanal. Chem., 12,26@4 (1966).
ud.
30r
7353
L
1200
20
4
\
Figure 7. Dependence of induction time, ti, on scan interruption potential, Ui
Solution4.1F HCHO in 1.W ticlo4 under He atm. Scan rate 50 mV/sec
if this is indeed a case of "anodic protection," it may only be necessary that [ o l a d exist in the surface of platinum oxide (rather than over it) in which case the requirement for electronic conduction is met by substrate metallic platinum. On the other hand, if the mixed surface film is some monolayers thick as some workers suggest (28), then electronic conduction of platinum oxide must again be invoked. A further obvious requirement of the fact that ti is not infinite is that there must be a slow chemical removal of the surface [old when the electrode is on open circuit, perhaps a reaction like HCHO
+ 20-S
-+
Hz0
+ COz + 2 s
(31)
where S represents the substrate for the [o]& It has been suggested in review that mention should be made of the probable mixed potential nature of Reaction 29 and that the [o],d layer may correspond to at least a monolayer coverage. This is a view I tend to favor although our data here do not allow definite conclusions to be drawn about (28) A. J. Appleby, J. Electrochem. SOC.,117,641 (1970). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
999
I -$odic
.i2
-el i o.
B
‘
:I
’ I 12
i cathodic
Figure 8. Dependence o f t,, NEAT size and U,,, for process I11 on scan reversal potentials Solution-0.1F HCHO in 1.OF HC104 with [Cl-] atm. Scan rate 50 mVisec
5
under He
detailed mechanisms. Oxley et al. (29) for example quite acceptably have proposed that the reactions
+ 2H+ + 2e- Pt + HzO HCOOH C o n + 2H+ + 2e-
PtO
4
-P
(32) (33)
comprise the mixed process with HCOOH in 5N H2S04. A similar sequence could apply of course for formaldehyde. At the same time, these workers propose the reaction PtO
+ HCOOH
+
COz
+ H20 + Pt
(34)
as the slow reaction preceding the fast reaction we have referred to here as “NEAT.” Reaction 34 is thus proposed instead of one like Reaction 31. The suggested sequence of events is, then, that only when the potential decay due to the slow Reaction 34 has reached a point where Reaction 32 can occur, a type of rapidly accelerating chain reaction involving both Reactions 32 and 33 takes place a t a time r , the delay time. In other similar and more recent studies of the delay time for these inhibited reactions, Burke and Moynihan (30) have found that the fast reaction is triggered at that point where the slow reaction has removed all but about 40 % of the original oxide as determined by cathodic stripping. This acceleration is attributed to a likely increase in catalytic activity of the electrode surface with decreasing oxide coverage. I assume a slow reaction like 34 is also implied here in view of their reference to “oxide” and to “catalytic activity,” presumably of Pt. Vertes and Horfinyi (31) incidentally, are not so specific on this point and refer to both “oxide” and adsorbed oxygen. It appears that some problems arise in using Reaction 34 for the slow process. It seems probable that Oxley’s chain reaction amounts to an autocatalytic reaction involving catalyst Pt in which case the question arises as to why the plentiful Pt produced in slow Reaction 34 does not initiate the autocatalytic process much earlier. The Pt produced in Reaction 34 would not have been subjected to the previous anodic polarization and would probably not be oxide-covered al(29) J. E. Oxley, G. K. Johnson, and B. T. Buzalski, Electrochim. Acta, 9,897 (1964). (30) L. D. Burke and A. Moynihan, ibid., 16,167 (1971). (31) G. Vertes and G. Horhyi, ibid., p 1823. 1000
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
though it may be buried under other adsorbed entities up to the trigger potential in which case Reaction 34 really should not give bare Pt as a product. It also does not seem reasonable for an autocatalytic reaction to rapidly accelerate when only 40% of the original oxide remains if there is already considerable Pt catalyst present as would be dictated by Reaction 34. It appears much more plausible for the fast autocatalytic reaction (also producing our NEAT) to be triggered by the first appearance of catalytic Pt as we suggest by Reaction 31 where substrate ( S ) is not Pt, but the oxide. An alternative possibility is that a mechanism suggested by James (32) is operating in which slow Reaction 34 product Pt may be formed between the oxide layer and the bulk electrode surface where it presumably would be protected from the bulk solution. The Pt would thus be unavailable as catalyst until imperfections arise in the outer oxide layer due to its disappearance via Reaction 34. It is of interest for example to note that if the slow and fast reactions involve [Olad and atoms Pt(OH)2 (or PtO), respectively, in the ratio of 1.5 [o]%d per Pt(OH)2, then one would expect the first production of catalytic Pt to occur at what appears to be 40% total apparent oxide coverage as determined by the cathodic stripping methods used by these workers. Various electrode pretreatments may of course affect this ratio, and if the mechanism of James (32)applies, this would mean a coverage by the oxide 2.5 monolayers thick. No attempt has yet been made in this survey study to measure dependence of t
