985
Sept., 1955
NOTES HETEROGENEOUS DECOMPOSITION OF NITROUS OXIDE AND THE THETA RULE BY RICHARD J. MIKOVSKY AND ROBERT F. WATERS Research Depaitmenl, Standard Oil Company (Indiana), Whiting, Indiana Received April 1.6, 1966
The heterogeneous decomposition of nitrous oxide has been the subject of investigations resulting in a number of proposals concerning the mechanism.1s2 Weyl suggests that the reaction proceeds by means of a transfer of electrons from the catalyst. If this extraction were the rate-determining step, other reactions, possessing the same ratecontrolling step, should exhibit kinetics similar to the nitrous oxide decomposition over the same catalysts. The present work was initiated to study the nitrous oxide decomposition over platinum-onalumina catalysts which had been used for more complex hydrocarbon conversions. The experiments were carried out in a flow system in which the temperature and flow rate necessary to yield an arbitrary 30% decomposition were measured.
Experimental Twenty-four platinum-on-alumina catalysts were studied. Fifteen contained a third component, such as Moos, Ruz03 ZrOz, MgO, TiOz, CrzOa or VzO6. Runs with alumina alone showed that less than 10% of the measured decomposition took place on the bare alumina surface. Nitrous oxide (Mathieson) was cooled to Dry Ice temperatures to remove water vapor and, after reheating to room temperature, was metered by a glass capillary flow meter. I t was then passed upward through a reactor containing 5 to 10 g. of catalyst. Unreacted NzO was frozen out in a constant-level liquid-oxygen trap, and the non-condensable reaction products, after reheating to room temperature, were metered by a second flow meter. The first flow meter was calibrated for NzO flow and the second for the stoichiometric 2 : l mixture of nitrogen and oxygen. The relative readings of the flow meters indicated when 30% decomposition occurred. In each experiment the NaO flow rate was adjusted to a set of predetermined values between 1.2 and 4.4 liters STP/hr., and the catalyst temperature was varied by means of a manometric thermoregulator until 30% of the NzO decomposed. The required temperatures lay between 430 and 530”. With most of the catalysts, successive runs were made until the activation energy was reproduced within 5%.
Results A plot of the logarithm of the NzO flow rate (log IC) versus the reciprocal absolute temperature for 30% reaction gives the frequency factor (ko) and the activation energy (AE,) in accordance with the Arrhenius kinetic equation. I n the absence of surface-area data for the platinum crystallites, the rate constants are reduced to unit weight of cata(1) C. N. Hinshelwood and C. R. Prichard, J . Chem. Soc., 127. 327 (1925); G. M. Schwab, et al., 2.physik. Chem., 8 9 , 265 (1930); B21, 65 (1933); B26, 418 (1934); E. W. R. Steacie and J. McCubbin, J . Chem. Phus., 2 , 585 (1934); C. Wagner, ibid., 18, 69 (1950); M. Boudart, ibid., 18, 571 (1950); K. Hauffe, el al., 2. physik. Chem.,201, 223 (1952). (2) W. A. Weyl, “A New Approach to Surface Chemistry and to
Heterogeneous Catalysis,” The Pennsylvania State College Bulletin No. 57, 1951.
lyst. The resultant dimensional units of the rate constants are liters hour-l gram-’. I n Fig. 1, the logarithms of the frequency factors are plothed against the energies of activation. The points define a straight line in accordance with the Theta Rule.3 The best line, determined by the method of least squares, may be represented by log,, ko = 0.468
+ 0.272AEa
Some of the catalysts became more active in successive runs with a corresponding drop in activation energy. The initial run for each catalyst is represented by a solid point. All of the data follow the same Theta Rule relationship. 30
25
t
2c
a?
P
e’
I3
I(
I
20
30
40
SO
60
70
80
90
100
-+ rule for decomposition of NzO over Pt-A1208 AEo(kcal /mol*)
Fig. 1.-Theta
catalysts.
Discussion Johnston4 points out the anomalous behavior of
NzO and notes that the activation energy of the
thermal decomposition ranges from 48 to 65 kcal./ mole. The fact that some of our observed activation energies for the heterogeneous reaction lie outside this range focusses attention on the nature of the N-0 bond. Pauling6 reports that the N-0 and N-N bonds in NzO have properties approaching those of a double and triple bond, respectively. Briner and Karbassie unexpectedly found that the threshold quantum of energy necessary for the photolytic decomposition of NzO corresponds to 142 kcal./mole. Thus, the observed high activation energies for the heterogeneous reaction seem reasonable. (3) G. M. Schwab, Proc. Intern. Congr. Pure and Appl. Chem., 11th Congr., London, 1947, Vol. I, 621 (1950); Advances in Catalysis, 2, 251 (1950). E. Cremer, for reasons discussed in Advances in Catalyads, 7 , in press (1955), prefers t o use the term “Compensation Effect (CE)” instead of Theta Rule. (4) H.9. Johnston, J . Chem. Phys., 19, 663 (1951). (5) L. Pauling, “The Nature of the Chemical Bond,” Cornell University Press, Ithaca, N. Y., 1945, p. 124. (6) E. Briner and H. Karbassi, Helv. Chem. Acta, 28, 1204 (1945).
NOTES
986
The slope of the line in Fig. 1 is inversely proportional to what has been termed a “preparation temp e r a t ~ r e . ” ~It corresponds to the reciprocal of the absolute temperature at which the specific reaction rate constants of all the catalysts are equal. Presumably the distributions of active centers on the catalysts are all equivalent at this temperature. Hence, the slope of the line may be common to other NzO studies and to other reaction studies involving a presumably similar electron transfer which might be rate determining. The data reported in the literature, however, give little positive support to this idea. The authors appreciate the assistance of G. S. Marlow with the experimental work.
THE LIMITING CURRENT ON A ROTATING DISC ELECTRODE I N SILVER NITRATEPOTASSIUM NITRATE SOLUTIONS. THE DIFFUSION COEFFICIENT OF SILVER ION BY MARTINB. KRAICHMAN AND ERNEST A. HOGGE
Vol. 59
to a variable speed motor. The anode consisted of silver foil whose area was about 20 times that of the cathode and was placed 5 cm. from it. A silver wire potential probe was positioned near the disc. The vessel containing the electrodes and probe held 600 ml. of electrolyte and was approximately 9 cm. in diameter. Nitrogen was led into the vessel so that a Dositive Dressure was exerted above the e1ect)rolyteat all times. Kinematic viscositv measurements of the 0.2 N KNOa solution a t room temperature were made using an Ostwaldi Fenske viscometer. Procedure.-Current and voltage measuring apparatus, consisting of a precision d.c. milliammeter and vacuum tube voltmeter, was connected to the electrolysis cell. Known amounts of AgN03 solution were added to the supporting electrolyte, KNOI by means of calibrated pipets. The voltage across the diffusion boundary layer a t the cathode was maintained at 0.2 volt for all readings by compensating for the ohmic drop in the solution. Limiting current va!ues were then read for each concentration of AgNOa using various cathode speeds. All measurements were made in a temperature-controlled room.
. 6
Results and Discussion The theory of the ternary system2 gave the following expression for the limiting current in c.g.s. units
U.S.Naval Ordnance Laboratory, Silver Spring, Maryland Received March 18, 1966
Equations for the steady-state rate of diffusion a t a rotating disc electrode have been solved by Levich’ for a binary electrolyte system. A previous study by the authors2 extended these equations to the ternary system KI-K13 with K I as an indifferent electrolyte. Using this system with platinum electrodes, experimental data confirmed the relationships derived and yielded a value of the diffusion coefficient of the single ionic species 13-. LIMITINGCURRENTS IN MILLIAMPERES AT 0.2 v. (r.p.8.) ‘12
2.00
3.99
1.22 1.87 2.30 3.15 3.63 4.28 4.73 5.31 6.22 6.66
0.0189 .0282 .0347 .0467 .0533 .0627 .0693 .0776 .0898 .0960
0.0377 .0565 .0690 .0932 .lo8 .127 .140 .156 ,180 ,192
FOR
TABLE I VARIOUS CONCENTRATIONS OF AgN03 A N D VARIOUS CATHODE SPEEDS
Conon. of AgNOa in normality X 106 5.96 7.92 9.87
0.0567 .0841 .lo3 ,140 .161
.188 .208 .232 .268 .288
Further experimental work by the authors has confirmed the relationships of the ternary system for the case of deposition using silver electrodes and the electrolyte system AgNO3--KNO3with the addition of small amounts of gelatin. The data also yielded a value for the diffusion coefficient of Ag+. Experimental Preparation of Materials.-Boiled distilled water was used in the preparation of all solutions. Reagent grade chemicals were used in the preparat,ion of stock solutions of 0.2 iV KNO3 and 0.00602 N AgIV03. Oxygen was removed by bubbling with tank nitrogen. Using U.S.P. grade gelatin (Knox), O . O l ~ owas added to the electrolyte and the system used for a single run. Apparatus.-The silver rotating disc cathode, approximately 2 cm. in diameter, WAS mounted on a shaft connected B. Levioh, Acta Physieochim. U.R.S.S., 17, 257 (1942). (2) E. A. Hogee and h‘l. B. Kraichrnan, J. Am. Chem. S a c . , 76, 1431 (1954). (1)
where n3is the valence of the reacting ion species; D3is the diffusion coefficient of this species; e is the unit electronic charge; 1230 is the concentration of the reacting ion species expressed in normality; s is the rate of rotation of the rotating disc electrode in revolutions per second; A is the area of the disc electrode; N is Avogadro’s number; v is the kinematic viscosity of the electrolyte. For fitting a regression line by the method of least
0.0756 .113 .140 .189 .214 ,251 ,278 .309 .358 .382
0.943 .141 ,172 .234 ,268 .312 .345 .385 ,452 .482
33.2
54.7
74.7
0.320 .477 1.585 .793 ,905 1.08 1.20 1.35 1.57 1.67
0,530 .790 .974 1.32 1.52 1.80 1.98 2.20 2.55 2.72
0.731 1.09 1.35 1.82 2.08 2.43 2.68 2.97 3.37 3.66
squares, the product cBsl/’ may be considered as the independent variable. The regression coefficient is then given by from which we can solve for the diffusion coefficient Table I shows the experimental data for various disc speeds and h g N 0 3 concentrations. Plots of these data show a linear relationship between the limiting current and the 4 s for different concentrations of AgNO,. Extrapolations of these lines back to zero speed pass through the origin within experimental error. Fitting a regression line to the data in Table I yielded a calculated value 21 = 0.07495 f 0.00045
b
a
