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The reaction of OH with H at elevated temperatures
Radiation Physics and Chemistry 64 (2002) 29–33
The reaction of OH with H at elevated temperatures T. Lundstro. ma,*, H. Christensenb, K. Sehestedc a
Department of Physics and Measurement Technology, Linko.ping University, SE-581 83 Linko.ping, Sweden b Studsvik Nuclear AB, SE-611 82 Nyko.ping, Sweden c Ris National Laboratory, DK-4000, Roskilde, Denmark Received 20 December 2000; accepted 15 January 2001
Abstract The temperature dependence of the rate constant for the reaction between OH radicals and H atoms has been determined in Ar-saturated solutions at pH 2. The reaction was studied in the temperature range 5–2331C. The rate constants at 201C and 2001C are 9.3 109 and 3.3 1010 dm3 mol1 s1, respectively. The activation energy was found to be 8.270.4 kJ mol1 (2.070.1 kcal mol1). This value is lower than that expected for a diffusion controlled reaction. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pulse radiolysis; Rate constant; Activation energy; H atom; OH radical
1. Introduction In order to make a reliable prediction of the concentrations of radiolytic species at various positions in a water-cooled reactor, it is necessary to have data on the rate constants of important chemical reactions at the operating temperature of the reactor. In recent years rate constants for several important reactions have been determined (Christensen and Sehested, 1980; Elliot et al., 1990; Buxton and Mackenzie, 1992; Elliot, 1994; Shiraishi et al., 1994; Christensen et al., 1994 and references therein). In the present investigation the reaction between OH radicals and H atoms has been studied in the temperature range 5–2331C OH þ H-H2 O:
ð1Þ
An attempt to determine the rate constant at 2501C failed due to a too high noise to signal ratio. The reaction has previously been studied by Buxton and Elliot (1993) in the temperature range 20–2001C. They found rate constants of 1.5 1010 and *Corresponding author. Tel.: +46-155-22-1407; fax: +46155-26-3150. E-mail address: [email protected] (T. Lundstro. m).
4.9 1010 dm3 mol1 s1 at 201C and 2001C, respectively, with a nonlinear dependance of rate constant on 1/T K1. Previously, a value for the rate constant at an ambient temperature of 7.0 109 dm3 mol1 s1 has been given (Buxton et al., 1988).
2. Experimental The experimental procedure has been described in detail previously (Christensen and Sehested, 1980; Sehested and Christensen, 1990). The experiments were performed in our high-temperature, high-pressure (HTP) cell of optical path length 2.5 cm. Some experiments in the temperature range 5–691C were carried out using a thermostated cell with an optical path length of 5.1 cm. The HRC linac at Ris delivered 10 MeV electrons in a 1-ms single pulse. The dose was 40–120 Gy per pulse as measured with the hexacyanoferrate(II) dosimeter at 201C using e420 ¼ 103 cm1 dm3 mol1 and G ¼ 5:9: The dose in the HTP cell was about half of that in the thermostated cell due to absorption of radiation in the thin steel window of the HTP cell. The optical absorption was measured using a Perkin Elmer double prism monochromator and a 1P28 photomultiplier. The
0969-806X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 3 9 - X
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
30
signal was processed on a Lecroy 9400A digital storage oscilloscope (175 MHz) and an online PC. The experiments were carried out in Ar-saturated solutions at pH 2 (as measured at ambient temperature), using HClO4 (102 mol kg1 at all temperatures). The cell was pressurized with about 10 MPa of Ar. The rate constant was determined following the decay of the OH radical as measured at 240 nm. The extinction coefficient, e; of OH at this wavelength is 612 cm1 dm3 mol1 and it changes only little with temperature (Christensen
and Sehested, 1981). The rate constant of reaction 1 was determined by fitting the computed absorption curve to the experimental absorption curve using a mechanism with rate constants given previously (Lundstro. m et al., 2000). In this mechanism the absorptions of H2O2 and HO2 were taken into consideration using e-values of 50 and 1300 cm1 dm3 mol1, respectively (only small contributions to the total absorption). The rate constants for the various reactions and the g values of the various species were calculated at each temperature as described
Table 1 Rate constants at 201C and activation energiesa Reaction
Rate constant (dm3 mol1 s1)
Activation energy (J mol1)
OH+OH-H2O2 OH+E-OH OH+H-H2O OH+HO2-H2O+O2 OH+O 2 -O2+OH
5.38E+009 2.98E+010 6.84E+009 6.83E+009 9.52E+009
7.90E+03 1.47E+04 8.40E+03 1.42E+04 1.76E+04
OH+H2O2-HO2+H2O OH+H2-H+H2O OH+HO 2 -O2 +H2O OH+OH-H2O+O OH+O-HO 2
2.60E+007 3.22E+007 7.21E+009 1.15E+010 1.73E+010
1.42E+04 1.92E+04 1.42E+04 1.42E+04 1.42E+04
O+H2O-OH+OH E+E-H2+OH+OH E+H-H2+OH aH2O E+O 2 -HO2+OH aH2O E+HO2-HO 2
1.50E+008 5.80E+009 2.31E+010 1.25E+010 1.92E+010
na na 1.40E+04 1.42E+04 1.42E+04
E+H2O2-OH+OH E+O2-O 2 E+H+-H E +H2O-H+OH E+HO 2 -O +OH
1.15E+010 1.83E+010 2.22E+010 1.83E+001 3.36E+009
1.56E+04 1.36E+04 1.26E+04 1.42E+04 1.42E+04
H+H-H2 H+O 2 -HO2 H+HO2-H2O2 H+H2O2-H2O+OH H+O2-HO2
5.28E+009 1.92E+010 1.92E+010 5.10E+007 2.04E+010
1.46E+04 1.42E+04 1.42E+04 1.07E+04 1.03E+04
H+OH-E+H2O HO2+HO2-H2O2+O2 HO2+O 2 -O2+HO2 HO2-H++O 2 H++O 2 -HO2
1.98E+007 7.93E+005 9.40E+007 3.80E+005 4.81E+010
3.76E+04 2.06E+04 7.60E+03 naa 1.42E+04
H2O2+OH-HO 2 +H2O HO 2 +H2O-H2O2+OH H2O2-H2O+O O+O-O2 H2O-H++OH
4.75E+008 1.20E+007 3.36E-008 4.81E+009 1.47E001
1.88E+04 naa 6.30E+04 1.42E+04 naa
H++OH-H2O O 2 +O2 -HO2 +O2 H+ H +H2O-H2+OH O 2 a+H2O-HO2 +OH
1.38E+011 1.42E+009 9.61E+005 9.61E+005
1.42E+04 8.00E+04 1.42E+04 1.42E+04
a
na: Not applicable.
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
31
previously (Lundstro. m et al., 2000). The g values were also corrected for scavenging in the spur (Bjergbakke et al., 1984). The g values obtained in this way were checked for stoichiometry between reducing and oxidizing species, and if necessary adjusted. The rate contants and g values are shown in Tables 1 and 2, respectively. The computer program MAKSIMA-CHEMIST (Carver et al., 1979) was used in the modelling.
3. Results and discussion At temperatures below 701C the rate constant for reaction (1) was determined using both the thermostated cell and the HTP cell. The agreement between the results obtained using these two cells was good. In Fig. 1 the experimental data at 34.51C, obtained in the thermostated cell and the corresponding computer fit are shown. In Fig. 2 similar results obtained at 2001C, in the HTP cell, are shown. A number of experiments were carried out at 20–221C. In Fig 3 the mean value of these results is plotted in an Arrhenius plot at 211C, together with results from experiments between 51C and 2331C. In Table 3 all the results are compiled, specifying the type of cell used in each experiment. From the straight line in Fig. 3 an activation energy of 8.2 kJ mol1 (2.0 kcal mol1) has been calculated. There is an indication of a slight curvature in the plot. This curvature is in qualitative agreement with the results of Buxton and Elliot (1993) and may be caused by the combination of reaction rate and diffusion rate constants as discussed by Buxton and Elliot (1993). Thus, it may be difficult to make a reliable extrapolation of the results above 2331C. Our result obtained at 2501C by a direct plot without using a computer fit gave a rather high value, but with a high degree of uncertainty. Thus, from this experiment the downward bending of the curve at higher temperatures can neither be confirmed nor disproved. A sensitivity analysis of the importance of the competing reactions 2–5 has been carried out. The rate constants at 1251C are given in parenthesis. H þ OH-H2 O2 ð1:266E þ 10Þ;
ð2Þ
H þ H-H2 ð2:567E þ 10Þ;
ð3Þ
Fig. 1. Computer fit to experimental results, obtained at 34.51C in the thermostated cell. FFF: fitted curve; +: experimental points.
Fig. 2. Computer fit to experimental results obtained at 2001C in the HTP cell. FFF: fitted curve; +: experimental points.
Table 2 G-values (molecules/100 eV) and their change with temperature
+
H , OH H H2 OH H2O2 H2O
251C
dG=dT
401C
901C
0.1 3.55 0.4 2.83 0.76 4.45
3.27E003 4.62E003 5.77E004 6.54E003 3.85E004
0.15 3.61 0.41 2.93 0.75 4.58
0.31 3.85 0.44 3.25 0.74 5.04
Fig. 3. Arrhenius plot of the rate constant kðOH þ HÞ: x: Thermostated cell; *: Mean value of experiments at 20–221C; +: HTP cell.
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
32
Table 3 Rate constants for the reaction of OH with H atoms Temperature (1C)
Cella
kðOH þ HÞ ( 1010 dm3 mol1 s1)
5 5 5 13 13 14 14 20 21 22 22 34.5 34.5 50 50 55 55 69 69 80 100 100 125 125 150 150 175 175 200 200 207 233
T T T T T T T T T H H T T T T H H T T H H H H H H H H H H H H H
0.7 0.7 0.7 0.7 0.7 1.0 1.0 0.7 1.0 0.8 0.9 1.1 1.1 1.2 1.4 1.4 1.5 1.9 2.0 1.8 2.2 1.9 2.0 2.0 2.0 2.5 2.6 3.0 3.4 3.6 3.0 4.0
Fig. 4. (a) Sensitivity analysis. The effect on the fit at 1251C. kðOH þ OHÞ was increased or decreased by a factor of 2. (b) Sensitivity analysis. The effect on the fit at 1251C. kðH þ HÞ was increased or decreased by a factor of 2.
a
T: Thermostated cell; H: High-temperature high-pressure cell.
OH þ H2 O2 -H2 O þ HO2 ð1:208E þ 08Þ;
ð4Þ
H þ H2 O2 -H2 O þ OHð1:577E þ 08Þ:
ð5Þ
The change in the fit of the calculated absorption to the experimental absorption is quite large for reaction (2) and moderate for (3). For reactions (4) and (5) no effect of changing the rate constant on the plot could be seen. The change in fit by variation of reaction 2 and 3 are shown in Fig. 4. An increase in k2 by a factor of 2 resulted in a two times lower best fit for reaction 1. Experimentally it is not possible to study reaction 1 without interference of reaction 2 and 3. However, these latter reactions were studied without interference of reaction 1(Christensen and Sehested, 1981; Sehested and Christensen, 1990). Thus the uncertainty in the values of the rate constants k2 and k3 should be moderate. If a straight line were drawn through the experimental points of Buxton and Elliot (1993), an activation energy
of 7.7 kJ mol1 could be obtained, in fair agreement with our value. Our rate constants at 201C and 2001C, as obtained from Fig. 3, are 9.3 109 and 3.3 1010 dm3 mol1 s1, respectively. These values are lower than the corresponding values of Buxton and Elliot (1993) (1.5 1010 and 4.9 1010 dm3 mol1 s1, respectively). Our value at 211C ((9.371.5) 109) is in fair agreement with the value 7.0 109 dm3 mol1 s1 given by Buxton et al. (1988). Initially, we determined the rate constants from a direct plot of absorption against time, without corrections for absorption of H2O2 and HO2 and without including the effect of competing reactions. The rate constants determined in this way were higher and the resulting activation energy was lower than our values given above.
Acknowledgements We would like to thank the Swedish Centre for Nuclear Technology and the Svensk Ka¨rnbra¨nslehantering AB for financial support. H. Corfitzen and
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
T. Johansen are gratefully acknowledged for technical assistance.
References Buxton, G.V., Elliot, A.J., 1993. Temperature dependence of the rate constant for the reaction H+OH in liquid water up to 2001C. J. Chem. Soc. Faraday Trans. 89 (3), 485–488. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical rewiev of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O) in aqueous solution. J. Phys. Chem. Ref data 17 (2), 513–886. Buxton, G.V., Mackenzie, S.R., 1992. Non-linear Arrhenius behaviour of the rate constant of some activationcontrolled reactions of the hydrated electron in the temperature range 20–2001C. J. Chem. Soc., Faraday Trans. 88, 2833–2836. Bjergbakke, E., Sehested, K., Rasmussen, O.L., Christensen, H., 1984. Input files for computer simulation of water radiolysis. Ris National Laboratory, Ris -M-2430. Carver, M.B., Hanley, D.V., Chaplin, K.R., 1979. MAKSIMACHEMIST. A program for mass action kinetics simulation by automatic chemical equation manipulation and integration using stiff techniques. Atomic Energy of Canada, AECL-6413.
33
Christensen, H., Sehested, K., 1980. Pulse radiolysis at high temperatures and high pressures. Radiat. Phys. Chem. 16, 183–186. Christensen, H., Sehested, K., 1981. Pulse radiolysis at high temperatures and high pressures. Radiat. Phys. Chem. 18, 723–731. Christensen, H., Sehested, K., Lo. gager, T., 1994. Temperature dependence of the rate constant for reactions of hydrated electrons with H, OH and H2O2. Radiat. Phys. Chem. 43 (6), 527–531. Elliot, A.J., 1994. Rate constants and G-values for the simulation of light water over the range 0–3001C. Atomic Energy of Canada, AECL-11073. Elliot, A.J., McCracken, D.R., Buxton, G.V., Wood, N.D., 1990. Estimation of the rate constants for near-diffusion controlled reactions in water at high temperature. J. Chem. Soc., Faraday trans. 86, 1539–1547. Lundstro. m, T., Christensen, H., Sehested, K., 2002. The reaction of hydrogen atoms with hydrogen peroxide as a function of temperature. in Radiat. Phys. Chem. 61, 109–113. Sehested, K., Christensen, H., 1990. The rate constant of the bimolecular reaction of hydrogen atoms at elevated temperatures. Radiat. Phys. Chem. 36 (3), 499–500. Shiraishi, H., Sunaryo, G.R., Ishigure, K., 1994. Temperature dependence of equilibrium and rate constants of reactions inducing conversion between hydrated electron and atomic hydrogen. J. Phys. Chem. 98, 5164–5173.
The reaction of OH with H at elevated temperatures T. Lundstro. ma,*, H. Christensenb, K. Sehestedc a
Department of Physics and Measurement Technology, Linko.ping University, SE-581 83 Linko.ping, Sweden b Studsvik Nuclear AB, SE-611 82 Nyko.ping, Sweden c Ris National Laboratory, DK-4000, Roskilde, Denmark Received 20 December 2000; accepted 15 January 2001
Abstract The temperature dependence of the rate constant for the reaction between OH radicals and H atoms has been determined in Ar-saturated solutions at pH 2. The reaction was studied in the temperature range 5–2331C. The rate constants at 201C and 2001C are 9.3 109 and 3.3 1010 dm3 mol1 s1, respectively. The activation energy was found to be 8.270.4 kJ mol1 (2.070.1 kcal mol1). This value is lower than that expected for a diffusion controlled reaction. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pulse radiolysis; Rate constant; Activation energy; H atom; OH radical
1. Introduction In order to make a reliable prediction of the concentrations of radiolytic species at various positions in a water-cooled reactor, it is necessary to have data on the rate constants of important chemical reactions at the operating temperature of the reactor. In recent years rate constants for several important reactions have been determined (Christensen and Sehested, 1980; Elliot et al., 1990; Buxton and Mackenzie, 1992; Elliot, 1994; Shiraishi et al., 1994; Christensen et al., 1994 and references therein). In the present investigation the reaction between OH radicals and H atoms has been studied in the temperature range 5–2331C OH þ H-H2 O:
ð1Þ
An attempt to determine the rate constant at 2501C failed due to a too high noise to signal ratio. The reaction has previously been studied by Buxton and Elliot (1993) in the temperature range 20–2001C. They found rate constants of 1.5 1010 and *Corresponding author. Tel.: +46-155-22-1407; fax: +46155-26-3150. E-mail address: [email protected] (T. Lundstro. m).
4.9 1010 dm3 mol1 s1 at 201C and 2001C, respectively, with a nonlinear dependance of rate constant on 1/T K1. Previously, a value for the rate constant at an ambient temperature of 7.0 109 dm3 mol1 s1 has been given (Buxton et al., 1988).
2. Experimental The experimental procedure has been described in detail previously (Christensen and Sehested, 1980; Sehested and Christensen, 1990). The experiments were performed in our high-temperature, high-pressure (HTP) cell of optical path length 2.5 cm. Some experiments in the temperature range 5–691C were carried out using a thermostated cell with an optical path length of 5.1 cm. The HRC linac at Ris delivered 10 MeV electrons in a 1-ms single pulse. The dose was 40–120 Gy per pulse as measured with the hexacyanoferrate(II) dosimeter at 201C using e420 ¼ 103 cm1 dm3 mol1 and G ¼ 5:9: The dose in the HTP cell was about half of that in the thermostated cell due to absorption of radiation in the thin steel window of the HTP cell. The optical absorption was measured using a Perkin Elmer double prism monochromator and a 1P28 photomultiplier. The
0969-806X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 3 9 - X
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
30
signal was processed on a Lecroy 9400A digital storage oscilloscope (175 MHz) and an online PC. The experiments were carried out in Ar-saturated solutions at pH 2 (as measured at ambient temperature), using HClO4 (102 mol kg1 at all temperatures). The cell was pressurized with about 10 MPa of Ar. The rate constant was determined following the decay of the OH radical as measured at 240 nm. The extinction coefficient, e; of OH at this wavelength is 612 cm1 dm3 mol1 and it changes only little with temperature (Christensen
and Sehested, 1981). The rate constant of reaction 1 was determined by fitting the computed absorption curve to the experimental absorption curve using a mechanism with rate constants given previously (Lundstro. m et al., 2000). In this mechanism the absorptions of H2O2 and HO2 were taken into consideration using e-values of 50 and 1300 cm1 dm3 mol1, respectively (only small contributions to the total absorption). The rate constants for the various reactions and the g values of the various species were calculated at each temperature as described
Table 1 Rate constants at 201C and activation energiesa Reaction
Rate constant (dm3 mol1 s1)
Activation energy (J mol1)
OH+OH-H2O2 OH+E-OH OH+H-H2O OH+HO2-H2O+O2 OH+O 2 -O2+OH
5.38E+009 2.98E+010 6.84E+009 6.83E+009 9.52E+009
7.90E+03 1.47E+04 8.40E+03 1.42E+04 1.76E+04
OH+H2O2-HO2+H2O OH+H2-H+H2O OH+HO 2 -O2 +H2O OH+OH-H2O+O OH+O-HO 2
2.60E+007 3.22E+007 7.21E+009 1.15E+010 1.73E+010
1.42E+04 1.92E+04 1.42E+04 1.42E+04 1.42E+04
O+H2O-OH+OH E+E-H2+OH+OH E+H-H2+OH aH2O E+O 2 -HO2+OH aH2O E+HO2-HO 2
1.50E+008 5.80E+009 2.31E+010 1.25E+010 1.92E+010
na na 1.40E+04 1.42E+04 1.42E+04
E+H2O2-OH+OH E+O2-O 2 E+H+-H E +H2O-H+OH E+HO 2 -O +OH
1.15E+010 1.83E+010 2.22E+010 1.83E+001 3.36E+009
1.56E+04 1.36E+04 1.26E+04 1.42E+04 1.42E+04
H+H-H2 H+O 2 -HO2 H+HO2-H2O2 H+H2O2-H2O+OH H+O2-HO2
5.28E+009 1.92E+010 1.92E+010 5.10E+007 2.04E+010
1.46E+04 1.42E+04 1.42E+04 1.07E+04 1.03E+04
H+OH-E+H2O HO2+HO2-H2O2+O2 HO2+O 2 -O2+HO2 HO2-H++O 2 H++O 2 -HO2
1.98E+007 7.93E+005 9.40E+007 3.80E+005 4.81E+010
3.76E+04 2.06E+04 7.60E+03 naa 1.42E+04
H2O2+OH-HO 2 +H2O HO 2 +H2O-H2O2+OH H2O2-H2O+O O+O-O2 H2O-H++OH
4.75E+008 1.20E+007 3.36E-008 4.81E+009 1.47E001
1.88E+04 naa 6.30E+04 1.42E+04 naa
H++OH-H2O O 2 +O2 -HO2 +O2 H+ H +H2O-H2+OH O 2 a+H2O-HO2 +OH
1.38E+011 1.42E+009 9.61E+005 9.61E+005
1.42E+04 8.00E+04 1.42E+04 1.42E+04
a
na: Not applicable.
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
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previously (Lundstro. m et al., 2000). The g values were also corrected for scavenging in the spur (Bjergbakke et al., 1984). The g values obtained in this way were checked for stoichiometry between reducing and oxidizing species, and if necessary adjusted. The rate contants and g values are shown in Tables 1 and 2, respectively. The computer program MAKSIMA-CHEMIST (Carver et al., 1979) was used in the modelling.
3. Results and discussion At temperatures below 701C the rate constant for reaction (1) was determined using both the thermostated cell and the HTP cell. The agreement between the results obtained using these two cells was good. In Fig. 1 the experimental data at 34.51C, obtained in the thermostated cell and the corresponding computer fit are shown. In Fig. 2 similar results obtained at 2001C, in the HTP cell, are shown. A number of experiments were carried out at 20–221C. In Fig 3 the mean value of these results is plotted in an Arrhenius plot at 211C, together with results from experiments between 51C and 2331C. In Table 3 all the results are compiled, specifying the type of cell used in each experiment. From the straight line in Fig. 3 an activation energy of 8.2 kJ mol1 (2.0 kcal mol1) has been calculated. There is an indication of a slight curvature in the plot. This curvature is in qualitative agreement with the results of Buxton and Elliot (1993) and may be caused by the combination of reaction rate and diffusion rate constants as discussed by Buxton and Elliot (1993). Thus, it may be difficult to make a reliable extrapolation of the results above 2331C. Our result obtained at 2501C by a direct plot without using a computer fit gave a rather high value, but with a high degree of uncertainty. Thus, from this experiment the downward bending of the curve at higher temperatures can neither be confirmed nor disproved. A sensitivity analysis of the importance of the competing reactions 2–5 has been carried out. The rate constants at 1251C are given in parenthesis. H þ OH-H2 O2 ð1:266E þ 10Þ;
ð2Þ
H þ H-H2 ð2:567E þ 10Þ;
ð3Þ
Fig. 1. Computer fit to experimental results, obtained at 34.51C in the thermostated cell. FFF: fitted curve; +: experimental points.
Fig. 2. Computer fit to experimental results obtained at 2001C in the HTP cell. FFF: fitted curve; +: experimental points.
Table 2 G-values (molecules/100 eV) and their change with temperature
+
H , OH H H2 OH H2O2 H2O
251C
dG=dT
401C
901C
0.1 3.55 0.4 2.83 0.76 4.45
3.27E003 4.62E003 5.77E004 6.54E003 3.85E004
0.15 3.61 0.41 2.93 0.75 4.58
0.31 3.85 0.44 3.25 0.74 5.04
Fig. 3. Arrhenius plot of the rate constant kðOH þ HÞ: x: Thermostated cell; *: Mean value of experiments at 20–221C; +: HTP cell.
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
32
Table 3 Rate constants for the reaction of OH with H atoms Temperature (1C)
Cella
kðOH þ HÞ ( 1010 dm3 mol1 s1)
5 5 5 13 13 14 14 20 21 22 22 34.5 34.5 50 50 55 55 69 69 80 100 100 125 125 150 150 175 175 200 200 207 233
T T T T T T T T T H H T T T T H H T T H H H H H H H H H H H H H
0.7 0.7 0.7 0.7 0.7 1.0 1.0 0.7 1.0 0.8 0.9 1.1 1.1 1.2 1.4 1.4 1.5 1.9 2.0 1.8 2.2 1.9 2.0 2.0 2.0 2.5 2.6 3.0 3.4 3.6 3.0 4.0
Fig. 4. (a) Sensitivity analysis. The effect on the fit at 1251C. kðOH þ OHÞ was increased or decreased by a factor of 2. (b) Sensitivity analysis. The effect on the fit at 1251C. kðH þ HÞ was increased or decreased by a factor of 2.
a
T: Thermostated cell; H: High-temperature high-pressure cell.
OH þ H2 O2 -H2 O þ HO2 ð1:208E þ 08Þ;
ð4Þ
H þ H2 O2 -H2 O þ OHð1:577E þ 08Þ:
ð5Þ
The change in the fit of the calculated absorption to the experimental absorption is quite large for reaction (2) and moderate for (3). For reactions (4) and (5) no effect of changing the rate constant on the plot could be seen. The change in fit by variation of reaction 2 and 3 are shown in Fig. 4. An increase in k2 by a factor of 2 resulted in a two times lower best fit for reaction 1. Experimentally it is not possible to study reaction 1 without interference of reaction 2 and 3. However, these latter reactions were studied without interference of reaction 1(Christensen and Sehested, 1981; Sehested and Christensen, 1990). Thus the uncertainty in the values of the rate constants k2 and k3 should be moderate. If a straight line were drawn through the experimental points of Buxton and Elliot (1993), an activation energy
of 7.7 kJ mol1 could be obtained, in fair agreement with our value. Our rate constants at 201C and 2001C, as obtained from Fig. 3, are 9.3 109 and 3.3 1010 dm3 mol1 s1, respectively. These values are lower than the corresponding values of Buxton and Elliot (1993) (1.5 1010 and 4.9 1010 dm3 mol1 s1, respectively). Our value at 211C ((9.371.5) 109) is in fair agreement with the value 7.0 109 dm3 mol1 s1 given by Buxton et al. (1988). Initially, we determined the rate constants from a direct plot of absorption against time, without corrections for absorption of H2O2 and HO2 and without including the effect of competing reactions. The rate constants determined in this way were higher and the resulting activation energy was lower than our values given above.
Acknowledgements We would like to thank the Swedish Centre for Nuclear Technology and the Svensk Ka¨rnbra¨nslehantering AB for financial support. H. Corfitzen and
T. Lundstro.m et al. / Radiation Physics and Chemistry 64 (2002) 29–33
T. Johansen are gratefully acknowledged for technical assistance.
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