2436
THOMAS C. FRANKLIN AND DONALD C. MCCLELLAND
The constants A , B, C, and D represent heat and entropy changes for reactions 4 and 5. I n the computation, approximate values of the constants were first calculated by hand. These were fed into the computer and the iterative procedure was continued until the constants were changing only slightly with iteration number. At this point, the fit looked as good as could be expected and further iterations did not seem worthwhile. Thermodynamic values derived from the computation are AH01400 = Jr51.2 =t1.0 kcal./mole and A8'1400 = +22.5 f 1.0 e.u. for reaction 4, and AH'14oo = +22.5 f 2.0 kcal./mole and = -0.2 f 2.0 e.u. for reaction 5 . The uncertainty given for each thermodynamic value was derived from estimated uncertainties in the vapor pressure values. Using these thermodynamic values, the individual partial pressures of RuOs and Ru04 shown in columns 3 and 4 of Table I are calculated. The calculated sums of the individual pressures are given in column 5, and it can be seen that they are in good accord with the observed pressures in column 2 . Using the heat and entropy values computed for reaction 4 and an estimated ACp of -2.7 j= 1.0 cal./deg.mole (based on heat capacity values for gaseous metal oxides given by Kelley,1° the heat capacity for RuOa(g) was taken to be 20), we calculate AH0298 = $54.2 2.0 kcal./mole and AS0Z98 = $26.7 f 2.0 e.u. for reaction 4. AC, was assumed to be constant in the calculation. Combining A 8 O 2 9 8 , S"Z~S RuOz(s) = 12.5 f 2.0 e.u., calculated above, and s 0 2 9 8 0 2 = 49.01 f 0.01 e.u., given by Kelley and King,12 yields 8 ' 2 9 8 RuOa(g) = 63.7 f 4.0 e.u. This value appears reasonable when compared with 8°298values given by Kelley and King12 for other metal oxide gases: 55.80 f 0.10, TiO; 57.7 f 2.0, V02; and 70.1 f l.O,OsO4. By combining AHoZg8 and AS0298 values for reaction 4 with respective heat and entropy values given above
1701.
67
for reaction 1, we calculate AH'298 = -18.0 f 4.0 kcal./mole and A8'298 = -16.6 4.0 e.u. for the formation reaction
*
Ru(s)
+ 3 / ~=0 RuOdg) ~
(7) The AH0298 value for reaction 7 may be compared with AH 0298 = - 12.7 kcal./mole calculated by Schafer, Tebben, and Gerhardt5 from their data using a thirdlaw method. Based on statistical considerations, they took SoZ98 RuOs(g) t o be 68.3 f 2 e.u. Standard heats and entropies for reaction 5 are now calculated using the AH01400 and ASo1400 values computed for the reaction. ACp is estimated to be -2.7 f 1.0 cal./deg.-mole (based on the heat capacity for OsO&) given by Kelley,l0 the heat capacity for Ru04(g) was taken to be 24), and it is assumed to be constant with temperature. Thus, AH'298 = +25.5 f 3.0 kcal./ mole and ASoZg8= f4.0 j= 3.0 e.u. for reaction 5. Combining A8'298 with standard entropy values for RuOz(s) and O2 given above yields 8'298 RuOe(g) = 65.5 i 5.0 e.u. This value appears reasonable when compared with the S O 2 9 8 values given above for other metal oxide gases. By combining thermodynamic quantities for reactions 5 and 1, we obtain AH'z98 = -46.7 f 5.0 kcal./mole and A8'298 = -39.3 rt 5.0 e.u. for the formation reaction Ru(s)
+ 202
=
RuOd(g)
(8)
Schafer, Tebben, and Gerhardt calculated from their data by the third-law method that AH'298 = -43.2 kcal./mole for reaction 8. They used S O 2 9 8 RuOe(g) = 69.3 e.u., which they derived from statistical considerations. Acknowledgments.-The authors are indebted to Drs. Ulrich Merten and J. H. Norman for suggestions and helpful discussions, and to R. E. Inyard for performing part of the experimental work.
A CORRELATION BETWEEN THE CATALYTIC ACTIVITY OF PLATINUM AND ITS ABILITY TO ADSORB HYDROGEN' BY THOMAS C. FRAXKLIN AKD DONALD H. RICCLELLAND Chemistry Department, Baylor University, Waco, Texas Received M a y 31, 1963 A study was made of the catalytic activity of platinized platinum used for the decomposition of hydrogen peroxide and the hydrogenation of quinone. The surface area of the platinum was measured by its ability to adsorb hydrogen. It was concluded that the hydrogenation of quinone occurred on the whole surface, whereas the decomposition of the hydrogen peroxide only occurred on certain sites.
Recently, a series of studies has been made of different processes occurring on platinum electrodes using the ability of the electrode to adsorb hydrogen as a measure of the surface area of the electrodes2 It has been observed repeatedly that there are two surface forms of h y d r ~ g e n . ~ -As ~ part of these studies, (1) The hydrogen peroxide portion of this paper was presented a t the Southwest Regional Meeting of the Amencan Chemical Society a t Tulsa,
Oklahoma, in 1957. (2) (a) T. C. Franklin and R. D. Sothern, J . P h w . Chen., 58, 951 (1954): (b) M. W. Breiter and S. Gilman, J . Electrochem. Soc., 109, 622 (1962). (3) A. Eucken and B. Weblus. 2. Elektrochem., 55, 114 (1951).
an examination was made of other surface processes to see which of the two sites was involved. Therefore, a comparison was made between the catalytic activity of a platinum electrode and its ability to adsorb hydrogen in the two forms. A study was made of two types of (4) M. Breiter, C. A. Knorr, and V. Volkl, ibid., 59, 681 (1955). (5) M. Breiter, H. Kammermaier, and C . A. Knorr, ibid., 60, 37 (1956). (6) M. Breiter. H. Kammermaier, and C. Knorr, ibid., 6 0 , 119 (1956). (7) T. C. Franklin and S. L. Cooke, Jr., J . Electrochem. SOC.,107, 555
A.
(1960).
(8) C . H. Presbrey, Jr., and S. Sohuldiner, U. S. Naval Research Laboratory Report 5472, Washington, D. C., 1962.
CATALYTIC ACTIVITY OF PLATINUM
Nov., 1963
catalytic processes, the catalytic decomposition of hydrogen peroxide and the catalytic hydrogenation of pbenzoquinone. Both processes havle been investigated previously by a number of workers. The decomposition of hydrogen peroxideg-l* has been found to be first order with xespect to hydrogen peroxide composition and first order with respect to the amount of platinum. The catalytic hydrogenation of p-benzoquinone also has been investigated1g-21 and has been found to be aero order with respect to the quinone.
Experimental The electrodes were 20-gage platinum wires sealed in glass. The electrodes were chemically cleaned with aqua regia and then electrolytically cleaned b y alternately generating hydrogen and oxygen on the electrodeu in 2 N sulfuric acid. After rinsing in distilled water, the electrodes were plated in a 3% chloroplatinic acid solution containing various amounts of lead acetate. The electrodes were plated a t 2-25 ma. for 60-600 sec. to obtain a wide range of catalytic a,ctivity. After plating, the electrodes were placed in 2 N sulfuric acid and again alternately hydrogen and oxygen were electrolytically generated on the electrode until the time for the potential to pass from the oxygen evolution potential to the hydrogen evolution potential became constant. The cell for the hydrogen peroxide experiments was a test tube connected with a stopcock to a large nonpolarizable mercury-mercury(1) sulfate reference electrode. In the reaction cell were the platinized platinum electrode to be tested, a small rotating bright platinum electrode, and a hydrogen bubbler. The stopcock was open only during the time it took to make an electrical measurement. Hydrogen was bubbled through the solution until the potential of' the platinized platinum electrode became constant. The bubbling was stopped and the system was allowed to come to equilibrium for several minutes. The potential of the electrode was determined with a student potentiometer and the amount of hydrogen on the electrode was measured by the previously described coulometric technique' in which the hydrogen was oxidized polarographically and the large reference electrode was used as a nonpolarizable electrode. The area under the hydrogen maxima was used as a measure of the amount of hydrogen adsorbed on the electrode. With some of the large electrodes the currents were so large that the voltage no longeir increased linearly with time and the two hydrogen maxima became hard to separate. Therefore, the normal polarographic circuit was modified with a very simple onetransistor current amplifier. The oxidation procedure was repeated until reproducible results were obtained. In the hydrogen peroxide experiments, the solution then was flushed with oxygen until the hydrogen was removed, leaving the solut,ion saturated with oxygen. The concentration of the hydrogen peroxide was measured voltammetrically during the decomporsition using a rotating microplatinum electrode.22,23 The residual current was determined in the oxygen-saturated solution, then a known amount of hydrogen peroxide was added to the solution. Polarograms were obtained every 3 to 4 hr. during the course of the decomposition. In the hydrogeioation iyxperiments, p-benzoquinone of a certain concentration was introduced into the reaction cell and hydrogen was bubbled a t at constant and reproducible rate (150160 bubbles/min.). Prior to the introduction of the p-benzo(9) G. Bredig, 2. E'lektrochem., 12, 581 (1900). (10) J. G. Telotov, J . Russ. Phus. Chem. Soc., 39, 1146 (1906). (11) X. Barnmann, 2. Elektrochem., 15, 673 (1909). (12) D. .4. MaoInnis, J. A m . Chem. Soc., 36, 878 (1914). (13) G. Bianohi, G. Caprioglio, G. Davolio, F. Ahfama,and T. Mussini, Chem. Ind. (Milan), 48, 146 (1961). (14) J. Giner, 2. Elektrochem., 64, 491 (1960). (15) R. Gerishoher and H. Gerishoher, 2. physik. Cham., 6, 178 (1956). (16) D. Winklemen, 2. Elekkrochem., 60, 731 (1956). (17) J. Weiss, Trar,s. Faraday Soc., 31, 1547 (1935). (18) J. Lopia and J. M. Guillen, Anales real soc. espan. fis. quim. (Madrid), 83B,5 (1957). (19) E. E'. Rosenblatt, J. Am. Chem. SOC.,62, 1092 (1940). (20) M. M. Popovs and D. 1 '. Sokolskii, Tr. I n s t . K h i m . N a u k Akad. N a u k , Kozakh., S S R , 2 , 70, 84 (1958). (21) M. M. Popova and D. V. Sokolskii, Zh. Fiz. R h i m . , 33, 2573 (1959). (22) D. Winkelmann, Z. Elektrochem., 60, 731 (1956). (23) 4.Hamamoto and T. Ansco. Oyo Den& Kenkyusho Iho, 9, 206 (1957).
2
g.50
pi
%
2437
-
L.40 0
E30-
e
6.200
6 0
-
(I
s
I 0
Fig. 1.-The
I 30
I
10
20 TIME
I
so
40
[ HOIJRS).
log of the hydrogen peroxide concentration us. time for the decomposition of hydrogen peroxide.
301 (D
0 0
Q 0
1
0
I
I
20
10
1
1
30
40
I
50
1
60
I 70
BO
Hydrogen on electrode, mcoulombs.
Fig. 2.-Rate constants for the heterogeneous decomposition of hydrogen peroxide on platinized platinum as a function of the amount of hydrogen. 30
1
pi c
0
I
I
30
40
Hydrogen adsorbed in the second form on platinized platinum, mcoulombs,
Fig. 3.-Rate constants for the heterogeneous decomposition of hydrogen peroxide on platinized platinum as a function of the amount of hydrogen adsorbed in the second form. quinone, the amount of hydrogen on the electrode was determined coulometrically. The cell consisted of a 1.5-nil. vessel containing the platinized platinum electrode and a hydrogen bubbler. It was connected through stopcocks and capillary tubing to a large nonpolariaable mercury-mercury(1) sulfate electrode and a Beckman reference calomel electrode. After the introduction of the p-benzoquinone, the electrode was introduced into the solution and the potential difference between the platinum and calomel electrodes was recorded as a function of time. The electrode remained a t the potential for the p-benzoquinone until almost all of the quinone was gone, then returned to the potential of the hydrogen electrode. The length of time from the insertion of the electrode until the return of the potential to 0 volts was used as the time for the reaction to proceed to a specific fraction of completion. After rinsing the cell and electrode with 2 N sulfuric acid, the procedure was repeated using varying amounts of p-benzoquinone The p-benzoquinone was purified by recrystallization from petroleum ether (b.p. 80-110") and sublimation. The purity, as
.
2438
THOMAS C. FRANKLIN AND DONALD C. MCCLELLAND
Vol. 67
manganate. It had a maximum conductivity of 3 X 10-5 mho/ cni. The commercial tank hydrogen was purified by passing it over hot copper wire, hot platinized asbestos, sodium hydroxide pellets, and finally 1-16 mesh silica gel. All glassware was cleaned first with kerosene, then chromic acid, and, after rinsing thoroughly with triply distilled water, was soaked in the water for several hours. All connections were made with Tygon tubing. The temperature was maintained a t 25 i 0.2O.
T I M E IMIN).
Fig. 4.-The potential of the electrode (vs. saturated calomel electrode) 21s. time for varying amounts of added quinone.
Time, min.
Fig. 5.-The effect of added quinone on the time necessary for the potential of the electrode to return to 0.00 v.
Rate constant (g./min.) X 10%
Fig. 6.-The effect of the ability of the electrode t o adsorb hydrogen on the hydrogenation rate constants. The different symbols indicate different platinum base electrodes. determined by an amperometric titration with sodium thiosulfate, was 99+ %. The solutions were prepared fresh for each run since the quinone underwent a slow polymerization in the 2 N sulfuric acid. All other chemicals were the best grade available commercially. The water was triply distilled, once over basic potassium per-
Data and Discussion of Results Hydrogen Peroxide Decomposition.-Figure 1 shows a plot of the log of the concentration of the hydrogen peroxide vs. time for a typical run. This plot shows the reaction to lie first order with respect to hydrogen peroxide. Figures 2 and 3 are plots of the rate constants for the decomposition of hydrogen peroxide vs. the activities of the electrode as measured by its ability to adsorb hydrogen in the various forms. It is seen that there is no relationship between the rate constant and the total ability of the electrode to adsorb hydrogen or its ability to adsorb hydrogen in the first form (the one oxidized a t the lowest potentials). However, it is seen that the decomposition is first order with respect to the ability of the electrode to adsorb hydrogen in the second form. Hydrogenation of p-Benzoquinone.-The potential vs. time curves for a particular electrode using various amounts of benzoquinone are shown in Fig. 4. In Fig. 5 it is seen that a plot of time necessary to return to zero potential (vs. saturated calomel electrode) against the weight of p-benzoquinone added to the cell is linear, indicating a reaction that is zero order with respect to quinone. The zero potential was chosen because the curve has its maximum slope in this region. This leaves a constant amount of unreacted material in each case. The only effect this has, since it is a zero-order reaction, is to cause the curve to not have a zero intercept. Figure 6 is a plot of the zero-order rate constant (as determined from the slopes of plots similar to that in Fig. 5 ) against the total amount of hydrogen adsorbed on the electrode. It is seen that the reaction is first order with respect to the total amount of hydrogen in the cases where the electrode area was varied by varying the plating conditions. However, the rate was also influenced by the area of the base electrode, as is seen from the fact that one obtains four different straight lines with four different base electrodes. The four lines are parallel. If one plots the intercepts against the geometric areas of the wires used, one observes a straight line. This indicates that there is a small secondary reaction or reactions removing the quinone, but that the primary surface hydrogenation is first order with respect to the total area occupied by the hydrogen. Conclusions The two reactions studied here have been studied previously by several workers and have some similarities in mechanism. Tretter2* has put forth an electrontransfer mechanism for the catalytic reduction of the quinone on the bare platinum electrode. The hydrogenation involves an anodic and cathodic reaction with the bare platinum acting as an electron acceptor and donor. Similarly, the catalytic deconiposition of hy(24) K. J. Vetter. 2. Elektrochem.. 66, 797 (1952).
Nov., 1963
VISCOELASTIC PROPERTIES OF A SIMPLE ORGANIC GLASS
drogen peroxide has been indicated to be an electrochemical reaction taking place on an oxide of platinum, an anodic reaction and a cathodic reaction; however, in this case the platinum oxide acts as an electron acceptor and donor. 14.16.17.18.25 The data in these experiments are explainable in terms of this mechanism. The quinone is a strong enough oxidizing agent to reduce both forms of hydrogen. Therefore, the total surface takes part in .the catalytic reduction of the quinone. The hydrogen peroxide data indicate that the two forms of adsorbed hydrogen are significantly different. (26) I. A. Bagotskaya and I. E. Yablokova, Dokl. Akad. Nauk SSSR,95, 1219 (1984).
2439
The decomposition of the hydrogen peroxide occurred on an oxidized electrode. The fact that the rate of decomposition is related only to the second form of hydrogen indicates that the difference in energy for bonding of hydrogen to the metal is basically a property of the metal surface and the same difference extends to the bonding of oxygen on the surface. Acknowledgment.-We wish to acknowledge the fellowship given by Phillips Petroleum Company in support of this work. In addition, the research reported in this paper has been sponsored in part by the Geophysics Research Directorate of the Air Force Cambridge Research Laboratories, Office of Aerospace Research, under contract AF 19(604)-8414.
'VISCOELASTIC PROPERTIES OF A SIMPLE ORGANIC GLASS BY A. V. TOBOLSKY AXD R. B. TAYLOR Department of Chemistry, Princeton University, Princeton, New Jersev Received M a y 31, 1963 The shear compliance of a three-dimensional glass-forming substance, Galex, composed largely of dehydroabietic acid, has been studied as a function of time and temperature by a ball indentation method, and the results are expressed in terms of a master curve of shear compliance against time a t 15'. This curve has been analyzed to separate the various components of the compliance and it is found that a glassy compliance, a single retardation time, and a steady flow shear viscosity are sufficient to describe the viscoelastic behavior of the compliance over the entire time scale. This is in striking contrast to the broad distribution of retardation timerr exhibited by polymeric materials. The glass transition temperature has been determined by volume temperature measurements and found to be 7'. 'The variation of the shear viscosity and the viscoelastic shift factors as a function of temperature are also discussed.
Introduction Although many investigations have been reported on the viscoelastic behavior of high polymer systems, very few have dealt with low molecular weight glass-forming materials. Benbowl has studied such systems by dynamic methods using 2-hydroxypentamethyl flavan as material and the rheological behavior of glucose2has also been investigated. The material chosen for the present study was dehydroabietic acid (Galex). I n molecular structure it may be represented as a roughly spherical molecule, so that the intermolecular interactions must be described as three-dimertsional, similar in kind to those that exist in a molecular crystal structure. However, the regularity of a crystal structure is absent in this glassforming material. Plastic flow by dislocations, so characteristic of crystals, is not pertinent here. I n this study we were particularly interested in the viscoelastic properties of Galex in the region of transition between glass and liquid. Linear amorphous polymers such as polystyrene also undergo a glass-to-liquid transition. The interactions between segments of neighboring polymer molecules are of the same type as found in Galex. I n the region of transition between glass and rubber, however, intramolecular segmental motion is of great significance. This type of molecular motion has been treated by Rouse and Bueche3* as being analogous to a one dimensional array of masses coupled by springs, the entire array being (1) J. J. Renbow, Pror. Phga. SOC.(London), B67, 120 (1954). ( 2 ) G. S. Park8, 1,. E. Barton, M. E. Spanht, and J. W. Richardson, Phusics, 5, 143 (1934). (3) f. E. Rouse, .I. Chem. Phys., 21, 1272 (1953). (4) F. Beuche zbzd. 22. 603 (1954).
immersed in a viscous fluid. This type of one dimensional motion gives rise to a very broad distribution of relaxation or retardation times. Tobolskys has calculated that the distribution of relaxation times in a three dimensional array should be very much narrower than that for a linear array and considered Galex as a substance by means of which this prediction might be verified. Experimental Galex is the trade name of EL dehydrogenated rosin supplied by the National Rosin Oil Products Co. It is a stable nonoxidizing material. The stability is obtained by dehydrogenation of abietic acid, the principal constituent of rosin, t o dehydroabietic acid, which contains the benzenoid nucleus.
Abietic acid (rosin)
Dehydroabietic acid (Galex)
The Galex, contained in shallow glass vessels, was heated in an oven at 100" until completely liquid and allowed to cool slowly to room temperature in order to minimize local stresses due to uneven contraction. This method produced short cylinders having parallel ends. The method chosen t o measure the viscoelastic behavior was a ball indentation method. By applying a force t o a sphere of known diameter and measuring the indentation 8s a function of time, the shear compliance J ( t ) could be calrulated. The incten( 5 ) A.
V. Tobolsky, ibid., 87, r575
(1962).
