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LOUIS SILVERMAN, ROBERT SALLACH, RACHEL SEITZ, and WANDA BRADSHAW’ Atomics International Division, North American Aviation, Inc., Canoga Park, Calif.
Catalytic Decomposition of Hydrogen Peroxide and Uranyl Peroxide These decomposition rates will be of prime interest in operating the newer homogeneous reactors fuel for homogeneous research reactors is enriched uranium present as uranyl sulfate in an ordinary (light) water solution. When these reactors are in operation hydrogen peroxide is produced by the radiolysis of the water a t a rate proportional to the energy adsorbed by the wateI. Hydrogen peroxide itself has little effect on the operation of a reactor, but it introduces serious problems by its reaction with uranyl ion to form relatively insoluble uranyl peroxide. Excessive precipitation of uranyl peroxide could so lower uranium concentration as to make the reactor inoperative (noncritical), and the accumulated precipitates might develop local hot spots and induce corrosion. Rapid decomposition rates of the two peroxides would eliminate these problems. Silverman, Watson, and McDuffie ( 2 ) investigated the decomposition rate of peroxide in dilute uranyl sulfate-hydrogen peroxide solutions. They tested a wide variety of catalysts and combination of catalysts but generally limited their investigations to the p H range of 1 to 3. , Many of the new homogeneous reactors will require more highly concentrated uranyl sulfate solutions. T H E
Experimental
To evaluate k, the specific reaction rate constant, the so-called “half life” method is used. Hydrogen peroxide is added to a solution containing uranyl sulfate, catalyst, and acid at a fixed temperature; at predetermined intervals, aliquots are withdrawn and titrated ( 2 ) . 1 Present address, Lockheed Aircraft Corp., Sunnyvale, Calif.
T o simulate reactor conditions, several experiments were performed using a pictorial, “steady-state” method to determine k. In one experiment (Figure 2) fixed amounts of hydrogen peroxide are repetitively pipetted into a solution containing selected amounts of uranyl sulfate, catalysts, and acid a t a fixed temperature; a t predetermined intervals, aliquots are withdrawn and titrated, as usual. By plotting hydrogen peroxide concentrations us. time, it is soon observed whether peroxide additions are too small or too large; by making proper adjustments in the volume of added peroxide, a “steady-state” copdition (such as would be attained in the reactor) is reached where peroxide addition rate is equal to peroxide decomposition rate. A t a n y time, addi-
Hydrogen peroxide concentration is calculated, and a plot of the logarithm of peroxide concentration us. time results in a straight line (Figure 1). The time interval in seconds for any selected value of hydrogen peroxide concentration to decrease by one half is the half life, to.& k is then calulated using Equation 1. k = 0.693/to.a
(1)
Silverman and others ( 2 ) discussed the application of decomposition rate data to reactor operation. They logically developed Equation 2 relating “steady-state” peroxide concentration and k to F, the rate of hydrogen peroxide formation, and thus indirectly to reactor power density:
F = k(H20z)
(2)
Figure 2. In this typical steady2state experiment, peroxide addition was stopped and k calculated from . the decay curve (right)
3.0
I
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l
’
l
~ & ~ O - o - ~ o - O ~ - * O
;STOPPED I ADDITION
I1
Addition rate, 0.55 gram of peroxide per minute; k from Equation 2, 0.22 min.-’; k from decay curve, 0.26 min.-l
4 Figure 1. “Half life” used in evaluating k in Equation 1 is the time interval in (seconds) for a given peroxide concentration to decrease by one half
F Figure 3. As expected, decomposition rate increased with temperature 300 grams of uranium per liter, 30 p.p.m. of iron
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2 80 290 3 00 RECIPROCAL TEMPERATURE x io3 VOL. 50, NO. 12
DECEMBER 1958
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16
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4
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A U=JOOgrn/l H2S04 = 0.54M
I .
Figure4. Rate con- .E stant increases lin- early up to 90 to 120 p.p.m. of iron, ci I but rate of increase N0 is proportionally x less above this s i concentration z E
Iff-B U=IOOgrn/l
'5
HZSO4: 0.36M
w0 LL LL W
0 0
2
2
Figure 5. hcrea! ing amounts of SUIfuric acid adversely affect catalytic decomposition rates
CATALYST CONCENTRATION (p.p.m. (ppm. Fe)
tion of peroxide may be stopped, and k determined in the same solution as in the first (half-life) method described. ResuIts
'
Effect of Temperature. Most experiments were run a t 60", 80°, and 90" C., and as expected, decomposition rate increases with temperature. Straight lines (Figure 3) are obtained when the logarithm of the rate constant is plotted against the reciprocal of the absolute temperature, T . Using the Arrhenius equation, the activation energy was calculated to be 28 to 29 kcal. per mole, which compares with Silverman's (2) values of 24 to 25. Effect of Catalyst Concentration. The rate constant increases linearly with increase of catalyst concentration u p to 90 to 120 p.p.m. of iron, but the rate increase is proportionally less above this catalyst concentration. Typical curves are shown in Figure 4. To compare various solutions k - ko is graphed; ko is the small, but varying rate constant in a solution free of catalyst. An empirical equation fits data for solutions of varying acidities and uranium concentrations. bc-ko=
a([Fe]
- 4.5
I-
X 10-e[U]1'2[Fe]S) ( 3 )
The second term appears independent of acidity, but a much closer fit is obtained when the square root of uranium concentration is included. For practical reasons iron concentration i s expressed in parts per million although uranium concentration is in moles per liter. a , the catalytic coefficient, depends on acidity and uranium concentrations. Effect of Acidity. Increasing amounts of sulfuric acid adverselv affect the catalytic decomposition rates (Figure 5 ) , more in lower uranyl sulfate concentrations (100 grams of uranium per liter) than in higher concentrations (300 grams
per liter). a is the initial slope of rate us. catalyst concentration curves and is inversely proportional to acidity. Effect of Uranium Concentration. With 0.72M sulfuric acid ( B , Figure 6): decomposition rate increases with increasing uranyl sulfate concentration. However, if acidity is 0.54M ( A ) , decomposition rate is maximum when uranyl sulfate concentration is about 325 grams of uranium per liter. Decomposition rate then decreases rather rapidly. Behavior at other acidities was not
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ACIDITY
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0 8 - A = 0 5 4 M H SO4 - B=0.72M H 2 f -
Y
0
0
0.2 0.4 0.6 0.8 1.0 HZSO, -MOL A R CO NC E N T R A TlON
1.2
studied completely for several reasons: The effect at acidities somewhat greater than 0.72M sulfuric acid could reasonably be expected to be similar to B, Figure 6, and solutions of high uranium concentrations and acidities lower than 0.5M have such a low tolerance for peroxide that investigations are more difficult. These latter solutions are therefore very unsuitable for use as reactor fuels.
~
~
~
~
An empirical expression relates several variables to peroxide decomposition rate.
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-d [ H 2 0 2 ]- A ( e - T)([Fe] dt
:
T E M'PER A T uR E 8ooc
-
o'2ww
/
- 4.5 X 10-8 [L]1'2[Fe]3)[H202] (4) [HzSOd] Once a value of A has been determined for a particular uranium concentration, then decomposition rates a t other acidities, temperatures, and iron concentrations (within the range covered by this study) can be calculated. At low iron concentrations and fixed temperature Equation 4 reduces to Equation 4a. -d
[HZOZ] = A'[Fe] [ H ~ O Z ] (4a) dt W2S041
which compares with Equation 5,
This is the established equation for decomposition of hydrogen peroxide with iron(II1) ion (7). Thus? there is an indication that the decomposition of hydrogen peroxide, rather than uranyl peroxide, is the predominant reaction. literature Cited
2.52 0
0.2
0
100
200
300
400
URANIUM CONCENTRATlON-gm.Li/I Figure 6. Acidity determines effect of uranium concentration on catalytic coefficient
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INDUSTRIAL A N D ENOINEERING CHEMISTRY
(1) Frankenburg, W. G., others, eds., Advances i n Catalvszs 4. 56 (1952). (2) Silverman, M. D.; Watson,' G. M., McDuffie, H. F., I m . EKG. CHEM.48,
1238 (1956). RECEIVED for review September 27, 1957 ACCEPTED August 1, 1958
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