12964
J. Phys. Chem. 1994, 98, 12964-12967
-
UV Absorption Study of the Thermal Decomposition Reaction H2S
H2
+ S(3P)
H. A. Olschewski, J. Troe,* and H. Gg. Wagner Institut fur Physikalische Chemie, Universitat Gottingen, Tammannstrasse 6, 0-37077 Gottingen, Germany Received: July 15, 1994; In Final Form: September 19, 1994@
The thermal decomposition of H2S was studied in shock waves by monitoring UV absorption signals in the range 210-330 nm. Following the decay of H2S at 215 nm, experiments with 200-5000 ppm of H S in Ar could be analyzed. First-order rate constants of W[Ar]= 4.0 x 1014 exp(-33000 WT)cm3 mol-' s-' were obtained between 1800 and 3300 K. As these results complement observations on the formation of S atoms (Woiki, D.; Roth, P. J . Phys. Chem., preceding paper in this issue), the thermal dissociation of H2S is proven to proceed by the following spin-forbidden elimination reaction: H2S M H2 S(3P) M with a rate constant k~ which is given by k~ = W2, i.e., kll[Ar] = 2.0 x 1014exp(-33000 WT)cm3 mol-' s-' between 1800 and 3300 K. The measured rate constants are analyzed in terms of unimolecular rate theory, and a threshold energy of EO= 295 kJ mol-' is derived which agrees with the endothermicity of the above reaction.
-
+
cs,
Introduction The thermal dissociation reactions of H2O and H2S exhibit an interesting property: the spin-allowed simple bond-fission processes are energetically less favorable than the spin-forbidden elimination reactions of the central atoms. For H20, one has'
H,O
-
+ O(3P)
H,
= +490.98 kJ mol-'
and
H,O
-
-
H
+ OH
AZP,,, = +499.1 kJ mol-'
H,
+ S(3P)
AZP,, = +297.61 kJ mol-' (1)
and
H,S
-
H
+ SH
Lw"298
= +381.6 k~ mol-'
(2)
One may be tempted to neglect the spin-forbidden in favor of the spin-allowed processes because of their electronically forbidden character. However, apart from energetic consequences, this aspect is of (nearly) no relevance for the low pressure limit of the thermal unimolecular reaction, whose rate is determined by collisional energy transfer and not by the electronic transition. For example, the thermal dissociation of N20 in the low pressure limit definitely proceeds via the spinforbidden channel2
N,O
-
N,
+ O(3P)
Therefore, only the question of the threshold energy of the H2S reaction has to be addressed: does the energy barrier of the spin-forbidden elimination process exceed the simple bondfission energy like in N20,, or is there a crossing of the singlet and triplet potential energy surfaces at energies close to or even below the energy of the separated products in their lowest energetic states such as in the reaction3 @
+
q3P)
In the preceding article4it is shown that the thermal dissociation of H2S occurs via the spin-forbidden elimination channel (1). This conclusion is derived by using atomic resonance absorption spectroscopy (ARAS) of S atoms which shows prompt formation of S atoms at such low initial H2S concentrations that secondary bimolecular reactions cannot be the source of S . At the same time the production of H atoms is slightly delayed and also much slower than the appearance of which rules out a major contribution from reaction 2. It appeared of interest to c o n f i i the reported rate of S formation by monitoring the rate of H2S disappearance. This is the issue of the present article. Detecting the W absorption of H2S behind shock waves, it was possible to lower the concentration to such an extent that secondary bimolecular reactions could be understood. In an earlier study monitoring H2S UV absorption: markedly higher concentrations were applied such that experiments at lower concentrations also appeared desirable. The measured rates of disappearance of H2S in the present work were found to be fully consistent with the observed appearance of S atoms from ref 4 which confirms the dissociation of H2S via channel 1. The rate coefficients of the unimolecular reaction in the low pressure limit are also analyzed in this work with the goal of identifying the position of the threshold energy between the reaction enthalpies of channels 1 and 2. S4p5
whereas, for H2S, one has
H,S
-cs +
+
Abstract published in Advance ACS Abstracts, November 1, 1994.
Experimental Technique H2S was pyrolyzed in reflected shock waves under high dilution in the carrier gas Ar. Our general technique has been described b e f ~ r e ~ ,such ~ , ' that only a few specific details are given in the following. Mixtures of H2S (199.9%) and Ar (99.999%) were prepared outside the shock tube in large glass mixing vessels. Great precautions were taken to avoid contamination with traces of 0 2 and HzO. Mixtures with HZSlAr concentrations in the range 200-5000 ppm were introduced into the shock tube and heated to temperatures in the range 1800-3300 K in the reflected wave. The progress of reaction was monitored by means of timeresolved W absorption spectroscopy in the spectral range 210330 nm. The light source for the absorption measurements was a xenon high pressure arc lamp (Osram XBO 150). The light
0022-365419412098-12964$04.5010 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 49, 1994 12965
Thermal Decomposition of H2S of this lamp passed the shock tube and was dispersed by a quartz double monochromator and recorded with a photomultiplier and an oscilloscope. As H2S most sensitively was detected near 215 nm, stray light had to be carefully eliminated. Absorption signals recorded over the range 210-230 nm could predominantly be attributed to HzS: absorption steps at the arrival of the incident and reflected shock wave and absorption decay behind the reflected wave showed the expected behavior, see below. However, the signals did not decay to zero; furthermore, the residual absorptions irreproducibly varied in height when the shock tube was not evacuated long enough. This problem was attributed to solid deposits on the wall of the tube (probably sulfur) which, on the one hand, could be removed from the wall by the incident shock, vaporized, and dissociated to strongly absorbing S2 molecules; on the other hand, long pumping times between shots resulted in a reduction the residual absorptions to a small, reproducible level which apparently arose from secondary reactions of H2S decomposition and not from solid deposits from previous shocks. Because of these problems, the tube was always evacuated for a long period (2-30 h) and the extent of the “cleaning” procedure was recognizable by the height of the residual absorption relative to the initial H2S absorption signals. Because of the superposition of the H2S absorption and other more long-lived absorptions, the latter were recorded separately at A > 240 nm where H2S absorption decreases strongly with increasing whvelength. S2 absorption and emission signals in this range have been recorded p r e v i o ~ s l y ; ~however, * ~ * ~ conclusions8 on reaction 2 from S2 signals, particularly in experiments with very high H2S concentrations, on the basis of our present work appear impossible. In order to subtract the superimposed S2 absorptions from our H2S absorption signals at 215 nm, we further investigated absorption signals between 240 and 330 nm. We found two components contributing to the absorption in this range, probably mostly S2 and some SH (one may also think of HS2 and H2S2). At low H2S concentrations, S2 formation showed marked induction times, see below, in contrast to the failure to observe induction times in the high concentration experiments of ref 8. We did not attempt to separate S2 and SH signals in detail. Instead, we simultaneously recorded absorption signals at 215 nm and at one longer wavelength in the range 240-330 nm. Because of the similarity of the (presumably) S2 signals at all wavelengths (apart from superimposed, presumably SH, contributions at some wavelengths), the S2 absorption component at 215 nm could be subtracted after calibrating the height of the S2 signal at 215 nm through the residual absorption level. The detailed account for the small S2 (or other) absorptions superimposed on the H2S signals at 215 nm considerably improved the accuracy of our data. The detection of H2S at 248.2 nm in ref 6 , because of the much weaker absorption of HzS at this wavelength, required the use of at least 10 times larger H2S concentrations. It also did not allow us to separate the two contributions (H2S and S2) to the absorption signal; residual absorptions appeared here as well. Results
In the following, only absorption-time profiles are shown where the residual absorption level had reached a reproducible minimum level such that perturbations from sulfur deposits were eliminated. Figure 1 gives three examples in which absorptions at 215 and 300 nm were simultaneously recorded. Attributing the absorption at 300 nm to S2, one clearly notices an induction time of S2 formation. The appearance of Sz induction times is also manifested by small absorption minima of the superimposed
/---i 1
I
1
1
1
I
I
1
1
1
1
1
I
1
1
1
1
1
L
1
J
I
I 1
1
1
I
Figure 1. Absorption-time profiles in the dissociation of HzS. Upper traces: at 300 nm. Lower traces: at 215 nm. Time marks, 100 ps; [H2S]/[Ar] = lo00 ppm if not stated otherwise; absorption base lines are traced; conditions behind reflected shock, from top to bottom: (a) [H*S]/[Ar] = 500 ppm, T = 2240 K, [Ar] = 8.5 x mol ~ m - (b) ~; T = 2150 K, [Ar] = 9.5 x loM5 mol ~ m - (c) ~ ; T = 1927 K, [Ar] = 10.4 x mol ~ m - ~ .
signals at 215 nm which at higher temperatures were observed before the residual absorption was reached (near 200 p s after reflected shock in Figure lb). However, the position of these minima was found at longer times than deduced from the S2 signals at 300 nm. Apparently, the 300 nm signal at higher temperatures contained another absorption component which appeared and disappeared during the reaction and which could have been due to SH. This component apparently was not present at 215 nm such that a minor uncertainty remained about the subtraction of the S2 contribution at 215 nm at higher temperatures. Figure 2 demonstrates the temperature dependence of the absorption signals at 215 nm. The small absorption minima before reaching the residual absorption again are recognized in Figure 2a (after 10 p s ) and b (after 20 p s ) whereas they are absent at the lower temperatures of Figure 2c-e. After subtracting the small Sz contributions from the 215 nm signals, clean first-order concentration-time profiles were obtained for temperatures above about 2200 K. At lower temperatures, some accelerations of the reaction with time were observed, see Figure 2e; however, the initial rate could easily be obtained by extrapolation in this case. Within the range of our conditions 200-5000 ppm HzS in Ar, the derived first-order rate constants k did not depend on the H2S concentration. Only in a few shots did we check for the dependence of k on [Ar],which always indicated that k was proportional to [Ar]. The investigations of ref 4 led to a value of the rate constant of the reaction S H2S products of k =
+
-
Olschewski et al.
12966 J. Phys. Chem., Vol. 98, No. 49, 1994
,
10’0 I
I
I
I
a /
‘$2 9
I
bl
?
- 1 L
9 N
lo7
t
0
I
1
0.3
1
1
1
1
I
0.4
0.5
0.6
1000 K / T Figure 3. Rate coefficients of HzS disappearance, experiments with [HzS]I[Ar] = 200 ppm (A),500 ppm (v), loo0 ppm (O), and 2000 ppm (0)from this work; dashed line, results from ref 4; full line, results from ref 6 to be reinterpreted see text.
reactions H2S
+ S -HS, + H
(44
-SH+SH
(4b)
+ H,
(4c)
-
S,
Monitoring H profiles in refs 4 and 5 , the ratio k4a/(k4a -I- k4b k k ) was established as 0.52(+0.05/-0.17). Our observation of marked induction times of the S2 signals, under conditions where the reactions 4 rapidly established quasi-stationary S concentrations, suggests only minor contributions from channel 4c. Reaction steps following the reactions 4, such as reactions of SH and HS2, at present are not known ~ompletely.~ For this reason, we do not further inspect influences of these processes on the measured k values. These reactions undoubtedly play a role such as manifested by the acceleration of the reaction at lower temperatures shown in Figure 2. However, the agreement between k/2 from this work and kl from ref 4 indicates that perturbations of this kind do not influence the presence interpretation of our data. Figure 3 includes results from ref 6, which were derived from experiments at higher H2S concentrations, in which the superimposed absorptions of H2S and S2 or SH at 248 nm apparently could not be separated satisfactorily. By monitoring H2S at 215 nm, where H2S absorbs much stronger, in the present work this problem could be overcome.
1 1 1
1
I
1
h
1
I
I
Figure 2. Absorption-time profiles in the dissociation of HzS (only traces for 215 nm are shown). For details, see caption to Figure 1. Time marks, 100 ps if not stated otherwise; conditions from top to bottom: (a) time marks, 10 ps; T = 3145 K, [Ar] = 4.8 x mol mol ~ m - (b) ~ ; time marks, 10 ps; T = 2715 K, [Ar]= 6.2 x ~ m - (c) ~ ;T = 2360 K, [Ar]= 7.8 x mol ~ m - (d) ~ ; T = 2140 K, [Ar]= 9.3 x mol ~ m - (e) ~ ; time marks, 200 ps; T = 1795 K, mol ~ m - ~ . [Ar]= 11.9 x
5.7 x 1014 exp(-7600 W7‘)cm3 mol-’ s-l. This value is so high that, even with our lowest concentrations of 200 ppm, the apparent fist-order rate constant k after very short initial periods approached twice the rate constant of the dissociation process, i.e., k = 2kl. Only with the still lower concentrations of 5-50 ppm of ref 4 could the transition of k between kl and 2kl be resolved. Therefore, in Figure 3 we present an arrhenius plot of W2[Ar]= kl/[Ar]. The agreement of this plot with the results from ref 4, derived from S atom formation, is very good, see Figure 3. This proves that indeed reaction channel 1 dominates the thermal dissociation of H2S while channel 2 is at least a factor of 20 s l ~ w e r . The ~ ~ ~results of Figure 3, over the temperature range 1800-3300 K and at [Ar] in the range (5mol cm-l, are represented by 10) x
Discussion In the following we briefly analyze the expression for kl from the present work and from ref 4. Assuming that the reaction involves a spin-forbidden intersystem crossing from the Morsetype potential energy surface of the singlet ground state to an either repulsive or attractive excited triplet state, the low pressure rate constant of the unimolecular reactionlo
k/2[Ar]= k,/[Ar] =
2.0 x 1014exp(-33000 K/T)cm3 mol-’
s-l
(3)
with an estimated uncertainty of the activation energy of A(El R) = f500 K. The elimination channel 1 is followed4 by the
can be simplified (see ref 10 for the meaning and calculation of all factors in (5)). To a first approximation, the “correction
J. Phys. Chem., Vol. 98, No. 49, 1994 12967
Thermal Decomposition of H2S
TABLE 1: Experimental Conditions and Apparent First-Order Rate Constants k T/K mol cm-3 [HzS]/[Ar] ppm W[Ar]cm3 mol-' 1835 11.4 200 1.1 x 107 2015 1790 1950 2140 2180 2240 2700 2930 3170 1795 1920 2150 2160 2270 2360 2455 2480 2640 2715 2770 2800 2930 3075 3145
3345 2085 2375 2480 3170
10.2 11.4 10.2 9.5 8.8 8.5 6.6 5.3 4.9 11.9 9.9 9.0 9.3 8.6 7.8 7.8 7.4 6.7 6.2 5.4 6.2 5.7
5.1 4.8 4.7 9.4 8.2 3.5 4.9
200 500 500 500 500 500 500 500
1.2x 6.1x 2.5x 6.0x 9.7 1.9x 1.6 x 4.0x
500
1.4x 1Olo 1.1 x 107
lo00
1000 1000 lo00
1000 1000 1000 1000 1000 lo00 1000 1000 1000 1000 lo00 lo00 2000 2000 5000 5000
107
lo6 107 107 107
lo8 109 109
2.4 8.7x 1.0 x 1.9x 3.7x 6.0x 5.8x
107 107 107
1.5 x
109 109
2.1 3.0 x 2.4 4.9x 8.5x 8.8x 2.0x
s-l
108
108
lo8 lo8 109 109 109
Acknowledgment. Financial support of our work by the Deutsche Forschungsgemeinschaft (SFB 357 "Molekulare Mechanismen unimolekularer Prozesse") and discussions with C. T. Bowman, D. Woiki, P. Roth, and H. Matsui are gratefully acknowledged.
109
lo9 10'0
5.1 107
3.8x lo8 6.7x lo8 1.4x 1Olo
factors" FE, F d , and Frotare all set equal to unity. Putting Z = and pC= T', from eq 5 one would expect an apparent activation energy for ko of
E, = E, - 20.1 k~ mol-'
known details of the potential such as the geometry of the crossing of the singlet and triplet potentials. Calculating FE at 2300 K yields 1.lo,', F d = 1.59 for the H2S singlet surface,12 and Frot% 1.8 as for CO2,'O pc decreases to pc % 0.022 at 2300 K (corresponding to - ( A 9 = 45 cm-') which agrees with the pCvalue of N20 dissociation at 2300 K.Io Using the same pc value for the reaction channel 2, Le., for the simple bond dissociation 2, at 2300 K, one would have predicted a rate constant which is 10 times smaller than the actually observed value of k1.l0 In a two-channel reaction, however, the upper channel (2) in the low pressure limit would be highly ~nderpopu1ated.l~Therefore, we conclude that H2S decomposition nearly exclusively proceeds via channel 1. The Eo value of 293 kJ mol-' suggests a potential where the triplet surface intersects with the singlet ground state surface at energies close to (or below) the energy of the separated dissociation products of the triplet surface. With respect to the singlet-triplet transition, the situation appears quite comparable to the dissociation of C S Z ,although, ~ for CS2, dissociation of a C-S bond is observed while elimination of the central S atom occurs in H2S. Quantum chemical calculations are desirable to confirm the present conclusions.
(6)
between 1800 and 3200 K such that EO becomes close to 293.3(&4) kJ mol-', Le., equal to the reaction enthalpy of reaction 1 at 0 K'," of 294.7 kJ mol-'. Calculation of the preexponential factor in eq 5 with F f l d F r o t = 1 and comparison with the experiments leads to pC= 0.07 at 2300 K and -(A,??) = 160 cm-'. This value is already of the usual magnitude', for reactions of this type in the weak collision limit of collisional energy transfer; it decreases to some extent when the product F E F ~ more F ~ realistically ~ ~ is increased above the value of unity assumed so far. Its accurate value depends on not so well
References and Notes (1) Atkinson, R.;Baulch, D. L.; Cox, R.A.; Hampson, R.F.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Datu 1992,21, 450. (2) Olschewski, H. A.; Troe, J.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1966,70,450. (3) Olschewski, H. A.; Troe, J.; Wagner, H. Gg. Z. Phys. Chem. NF
1965,45, 329. (4)Woiki, D.; Roth, P. J. Phys. Chem., preceding paper in this issue. (5) Roth, P.; Loehr, R.;Bamer, U. Combust. Flame 1982,45,273. (6) Bowman, C. T.; Dodge, L. G. 16rh Symp. (In?.) on Comb.; The Combustion Institute: Pittsburgh, 1976;p 971. (7) Muller-Markgraf,W.; Troe, J. J. Phys. Chem. 1988,92,4899,4914. (8) Higashihara, T.; Saito, K.; Yamamura, H. Bull. Chem. SOC.Jpn.
1976,49,965. (9) Yoshimura, M.; Koshi, M.; Matsui, H. Chem. Phys. Lett. 1992, 189,199. (10) Troe, J. J. Chem. Phys. 1977,66,4745,4758; J. Phys. Chem. 1979, 83,114. (11) JANAF Thermochemical Tables (NSRDS-NBS37, Washington DC, 1971). (12) Troe, J. Chem. Phys., in press. (13) Just, Th.; Troe, J. J. Phys. Chem. 1980,84,3068.
