1703
Kinetics of the Tungsten-Oxygen-Hydrogen Bromide Reaction
Kinetics of the Tungsten-Oxygen-Hydrogen Bromide Reaction E. G. Zubler Lamp Phenomena Research Laboratory, GeneralElectric Company, Neb Park, Clevelsnd, Oh& 44 112 (Received December 30, 1974) Publication costs assisted by the Lamp Business Division, General Nectric Company
The reaction rate of W a t 500-1000° in He or N2 containing 0.003-0.014Torr of 0 2 and 0.13-1.2Torr of HBr was measured by a microbalance-flow technique. As the HBr was increased at constant 0 2 , the reaction rate increased and then decreased beyond a maximum. At the rate maxima which were linear with 0 2 , the PHBr21P02 ratio was constant. From 500 to 70O0,the rate-controlling step had an apparent activation energy of 13.6kcal/mol. At 750°,a transition occurred to a diffusion-controlled region. At 1000°,there was some indication of a return to a regime with a significant activation energy. The addition of 0.1-0.7Torr of H2 decreased the rate maxima above 700O. The results are explained qualitatively by a two-layer adsorption model, which has been used for tungsten oxidation a t higher temperatures.
Introduction A recent investigation' of the kinetics of the tungstenoxygen-bromine reaction has indicated complex adsorption effects of the bromine which were not revealed in earlier work.2 A microbalance-flow technique was used to determine the reaction rate of tungsten foil a t 700-900' in Ar, He, and N2 at 1 atm pressure and containing to Torr of oxygen and 0.01-3 Torr of bromine. The reaction rate was linear with oxygen but a complex function of the bromine and the carrier gas. Except a t the lowest bromine pressures, adsorbed bromine inhibited the reaction and the rate was higher in N2 than Ar or He where the rate was equal. One of the interpretive problems in the previous was the inability to distinguish between the effects of bromine atoms and molecules on the reaction kinetics. Within the range of experimental conditions, the gaseous bromine dissociation was 0.2-0.98which means that both atomic and molecular species were present but indistinguishable. This ambiguity has been reduced in the current work by the use of HBr which is dissociated only 0.005 a t 1000°. Some information is available on the adsorption of HBr on W. McCarrolP has observed both dissociative and nondissociative chemisorption of HBr with the latter dominant below 165O0K as indicated by the preferred desorption of HBr over Br atoms. In the current work, the reaction rate of tungsten at 500-1O0O0 in a flow of He or N2 a t 1 atm pressure and containing 0.003-0.014 Torr of 0 2 and 0.13-1.2 Torr of HBr has been measured. The results are compared with the previous work involving bromine. Experimental Section Method. The experimental technique was essentially the same as described in the previous work.2 In gas flows of 260-480 ml/min, the desired partial pressures of 0 2 and HBr were obtained by the addition of measured flows of Ar + 0.1% 0 2 and Ar + 0.46% HBr to the carrier gas, 99.999% He or Nz. The residual 0 2 level in the carrier gas was 0.20.4 ppm. The 0 2 level was set and measured continuously by a solid electrolyte sensor4 while the HBr level was set by flow rates and measured by a wet chemical technique based on a bromide ion electrode. The tungsten foil with a minimum purity of 99.95% was
obtained from Rembar Co. During the investigation, seven different samples (0.0025cm X 1.00 cm X 1.90 cm) with initial and final weights of 70-75 and 35-40 mg, respectively, were used. After a reaction series, microscopic examination showed a uniformly roughened surface with no indication of any preferential attack of the surface or edges. In a typical run, the gas flow of He 0 2 HBr was adjusted and then the reaction chamber was brought to temperature. After temperature stabilization, the weight loss was recorded for 30-90 min depending on the reaction rate. From the slope of the weight loss and the geometric dimensions of the foil, the reaction rate in mol/(cm2 sec) was calculated.
+ +
Results and Discussion Under all conditions, the tungsten weight loss was linear with time and the reaction rate was independent of gas flow in the range 260-480 mllmin. The precision of the rate measurement was about f10% after the tungsten foil had been conditioned by reaction with 0 2 and HBr a t 1000°. After a reaction series which corresponded to a tungsten weight loss of about 35 mg, the condensate on the wall downstream of the furnace was characterized by three distinct zones. The major deposit was located at the edge of the furnace area and consisted of a green band which was identified as WOs by X-ray diffraction. Further downstream, there was a yellow brown band which was identified as WOzBrz by X-ray diffraction. Between these two major deposits, there was a narrow black band but the quantity of material was insufficient for identification. The red crystals of W02Br2 observed further downstream in the previous work1p2with bromine were not observed here. The wo3 resulted from the reaction product, W02Br2, W 0 3 + WOBr4, either by disproportionation, 2W02Br2 or decomposition to WO2 followed by oxidation. The decomposition reaction has been reported by Korovin5 and Gupta.6 As shown in Figure 1, the reaction rate increased as the 0 2 level was increased. The series of curves in Figure 1 resulted from the decrease in 0 2 level due to dilution as the HBr (in Ar) was added to the flow mixture. For a given 0 2 level range, the reaction rate increased and then decreased beyond a maximum as the HBr level was increased. This HBr dependency indicates competitive chemisorption of
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The Journal olPhysical Chemistry, Vol. 79, No. 16, 1975
1704
E. G. Zubler 44
L
40
-
36
-
32
-
P
28 -
-
24
Q
20: 16 12 8 -
4 0,
x
IO3 Torr
0
I
I
I
2
4
6
Flgure 1. Rate dependence on O2 and HBr pressures at 800"; HBr (Torr): X, 0.13; @, 0.21;0,0.31;0,0.42;A, 0.53;H, 0.66;A,
I
.e
I
1
1.0
1.2
0.78:V,0.89:8 , 0.99;V, 1.10:9,1.20. Flgure 3. Rate dependence on HBr pressure for various temperatures ("C)and 02 pressures (Torr): A, 800°, 0.0122;@, 700°,
0.9
0.0122;0 ,800°, 0.0084;0,700°, 0.0084;X, 800°, 0.0019,
0.8
linear dependency on 0 2 in the immediate vicinity of the rate maxima, the rate data (of the type shown in Figure 1) can be corrected to a constant 0 2 pressure and displayed as a function of HBr pressure as shown in Figure 3. As shown in Figure 3, the curves are slightly broader a t lower temperatures but the P H B r 2 / P 0 2 ratio a t the rate maximum is nearly constant. Over the range 600-900" the P H B r 2 / P 0 2 ratio at the rate maximum increased very slightly with an increase in temperature. This P H B r 2 / P 0 2 constancy and the observed reaction products are consistent with the overall reaction
0.7
0.E L
c" 2 I
0.5 0,4
N
a 0.2
W(s) ioz(g) + 2HBr(g)
0.2 0.1
2
4
6
8
IO
12
14
Po? x IO3 Torr Figure 2. HBr dependency on
the 0 2
O2at the rate maxima.
0 2 and HBr and is similar to that reported for the W + + Clp reaction. Rosner and Allendorf7,s found that at
constant 0 2 pressure, the apparent reaction order with respect to Cl2 passed from large positive values to large negative values beyond the rate maximum. For the same system, McKinleyg observed a rapid decrease from first- to zero-order dependence above some total 0 2 Cl2 pressure. In Figure 1, the rate maxima show a linear dependence on the 0 2 level. A similar linear dependence on 0 2 was observed in the previous work with Brp in the region where adsorbed bromine inhibited the reaction. A plot of the HBr vs. 0 2 levels at the rate maxima is given in Figure 2 and shows a linear relation between PHB~' and Po2.Assuming a
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The Journal of Physical Chemistry, Vol. 79,No. 16, 1975
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WOzBrz(g) + Hz(g)
The temperature dependence of the rate maxima is shown in Figure 4. A reactivity parameter e, the ratio of the tungsten removal rate to the 0 2 impingement rate, has been employed to indicate the linear dependence on 0 2 over the temperature range 500-1000°. The Arrhenius plot indicates that two and possibly three different kinetic regimes are involved. From 500 to 700°, the rate-controlling step has an apparent activation energy of 13.6 kcal/mol. At about 750°, a transition occurs and from 800 to 950°, the rate-controlling step has an activation energy near zero which suggests a diffusion-controlled process, i.e., the diffusion of reactant molecules through a product layer at the reaction interface. Duplicate runs a t gas flows of 280 and 460 ml/min indicated that gas flow did not affect the reaction rate in this regime. Transitions to diffusion-controlled regions have been reported (although at higher temperatures) for similar systems, W + 02,10J1 Re 0 2 , l 2 and W + 0 2 C ~ ZA t. ~ 1000°, which was the upper limit for the experimental technique, the limited data (actually each point represents three runs) indicate the possibility of another transition to a regime with a significant activation energy. Such a transition might be associated with the volatility of
+
+
1TC5
Kinetics of the Tungsten-Oxygen-Hydrogen Bromide Reaction 16
v
%
14
12
N
'\
I6 a
W + W
I
~
a a
6
10
I
\
W
g
E,
= 13.6 Kcol/MOLE
P x
8 6
n
FW
>-
' L 4
t
?
+ u
2
a W a
0
1
2
\ 3
4
5
6
7
pH2
Figure 5. Rate maxima dependence on H2 for 0.0032 Torr of 0 2 and 0.40 Torr of HBr at various temperatures. 1
I6 1
l
75 80
1
1
1
1
1
1
1
1
1
I
8 5 9.0 9 5 10.0 105 110 115 120 125 I30 IO?T ( OK )
Figure 4. Temperature dependence of reactivity parameter e (reaction rate maximum/02 impingement rate) for various 0 2 pressures (Torr):X , 0.0031;0 ,0.0046;A,0.0096;0,0.0139.
the tungsten oxides above about 1000' and suggests that the diffusion-limited region, 800-950', might be the result of an adsorbed product layer. In the previous work with Big (0.01-1 Torr), the reaction rate was affected by the carrier gas. At Brz pressures above about 0.02 Torr where the reaction was inhibited by adsorbed bromine, the reaction rate was higher in Nz than in He (or Ar). The higher rates in Nz were explained by adsorbed nitrogen which could be displaced by oxygen13J4 but not by bromine. The proposal was consistent with the binding energy of nitrogen on tungsten15J6 which is comparable to that of bromine3 but less than that of 0xygen.l' In the current work with HBr, duplicate runs in He and N2 over the range of experimental conditions indicated that both the reaction rate maxima and the corresponding HBr/02 ratios were the same within experimental error. The limited and less precise reaction rate data beyond the rate maxima, where adsorbed HBr inhibited the reaction, indicated no significant Nz effect. Consequently, in spite of the large excess of N2, competitive chemisorption between the NZ and HBr was not significant. Either different surface sites were involved or Nz was unable to compete for the same sites. The HBr-N2 system may be similar to 02-Nz where chemisorbed oxygen on tungsten prevents chemisorption of nitrogen13J4J8 and oxygen can displace chemisorbed nitrogen on tungsten.13J4 A comparison of the rate maxima with HBr and Br2l provides some information on the relative efficacies of the two reactants. At 700-900°, and 0.0087 Torr of 02, the rate maxima occurred at about 0.02 Torr of Brz (mostly atomic under these conditions) compared with 0.7 Torr of HBr while the absolute rates were higher in Brz by a factor of nearly 2. The higher rates and the substantially greater inhibitive effect of the bromine are consistent with the dissociative and nondissociative adsorption of Br:! and HBr, respectively, that have been r e p ~ r t e d . ~
The addition of 0.10-0.70 Torr of Hz resulted in a decrease in the rate maxima at temperatures above 700'. The decrease in the reaction rate was a function of both the partial pressure of H2 and the temperature as shown in Figure 5. In the range 750-950°, the decrease in the rate maxima was a linear function of the added Hz and the slope - A R ~ ~ ~ / A P was H ~ proportional to Po21/2a t a given temperature. The dependency of the rate decrease by H2 on Po:iz suggests that the mechanism may involve the removal of adsorbed oxygen through the formation of HOH. This mechanism is consistent with the fact that the Hz effect was significant only above 700' which is the minimum temperature required for the Hz reduction of oxides on the surface of tungsten. In view of the relatively low binding energy of H2 (-45 kcal/mol) compared with HBr (88 kcal/moP) it is unlikely that the H2 affected the concentration of adsorbed HBr by competing for available sites. A two-layer adsorption model that was developed for the W 0 2 reaction17 by Schissel and Trulson has been applied to the W 0 2 Clz r e a ~ t i o n It . ~ was concluded that the first layer consisted of atomic oxygen1g while the second layer consisted of both oxygen and chlorine with the oxygen more extensively adsorbed. Recent studiesz0Vz1involving Ne scattering measurements during the oxidation of W and Mo support the two-layer adsorption model. In the W 0 2 HBr reaction here, it is reasonable, also, to interpret the results in terms of a two-layer adsorption where the first layer is predominantly oxygen and the second layer is oxygen and hydrogen bromide. However, the data provide no confirming evidence for the two-layer model and can be explained satisfactorily by assuming a model based on competitive adsorption within a single adsorbed layer. A t the lowest HBr levels for a given 0 2 level in Figure 1, the rate increases with increasing HBr or 0 2 which indicates that adsorption sites are available. As the HBr level is increased, the adsorption of oxygen and hydrogen bromide is completed and the reaction becomes zero order with respect to HBr. As the HBr is increased further, the HBr displaces some of the oxygen in the second layer and the reaction rate decreases. A t higher 02 levels, the transitions to zero order occur at higher HBr levels which indicates that additional sites are available for the oxygen
+
+ +
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The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
1706
Charles J. Marzzacco
even in the presence of excess HBr. This suggests that either different sites are involved for the oxygen and hydrogen bromide or that the first layer contains some hydrogen bromide which can be displaced by additional oxygen. The condensate downstream of the reaction zone indicated that W02BrAg) was the principal reaction product. The reaction mechanism, however, cannot be deduced from , ~ abthe available data. In the W 0 2 Cln ~ y s t e m the sence of gaseous intermediates, e.g., WOCl(g) and WO2Cl(g) suggested that the WO2C12 may have resulted from the direct combination of W02 and Cl2. A similar situation involving WOa and HBr may be involved here.
+
+
Acknowledgments. Drs. S.K. Gupta and D.A. Lynch contributed to many helpful discussions and reviewed the manuscript along with Dr. J.S. Saby.
References and Notes (1) E. G. Zubler, J. Phys. Chem., 76, 320 (1972). (2) E. G. Zubler, J. fhys. Chem., 74, 2479 (1970).
(3) B. McCarroll in “Structure and Chernlstry of Solid Surfaces”, G. A. Sornorjal, Ed., Wiley, New York, N.Y., 1969. (4) S. S. Lawerence, H. S. Spacil, and D. Schroeder, Autornatica (Sept 1967). ( 5 ) G. A. Korovin, Russ. J. lnorg. Chem. (Engl. Edit), 12, 7 (1967). (6) S. K. Gupta, J. Phys. Chem., 75, 112 (1971). (7) D. E. Rosner and H. D. Allendorf, AlAA J., 5, 1469 (1967). (8) D. E. Rosner and H. D. Allendorf, “Proceedings of the Third International Symposium on High Temperature Technology“, Butterworths, London 1969, p 707. (9) J. D. McKinley, “Proceedlngs of the Sixth lnternatlonal Symposium on Reactivity of Sollds”, Wiley-lnterscience, New York. N.Y., 1969, pp 345-351. (10) R. W. Bartlett, Trans. AIM€, 230, 1097 (1964). (11) P. N. Walsh, J. M. Quets, and R. A. Graff, J. Ch8m. fhys., 46, 1144 (1967). (12) E A. Gulbransen and F. A. Brassart, J. Less Common Metals, 14, 217 (1968). (13) J. T. Yates and T. E. Madey, J. Chem. fhys., 45, 1623 (1966). (14) H. F. Winters and D. E. Horne, Surface Sci., 24, 587 (1971). (15) G. Ehrlich. J. Phys. Chem., 60, 1386 (1956). (16) G. Ehrlich, Roc. lnt. Congr. Catal., 3rd, 113 (1965). (17) P. 0. Schissel and 0. C. Trulson, J. Chem. fhys., 43, 737 (1965). (16) R. E. Schlier, J. Appl. fhys., 29, 1162 (1958). (19) B. McCarroll, J. Chem. Phys., 46, 863 (1967). (20) H. G. Lintz, A., Pentenero, and P. LeGoff, J. Chim. fhys., 67, 487 (1970). (21) D. F. Ollis, H. G. Lintz, A. Pentenero. and A. Cassuto, Surface Sci., 26, 21 (1971).
Effects of Metal Complexation on the Photophysical Properties of Pyrazinel Charles J. Marzzacco Department of Physical Science, Rhode Island College, Providence, Rhode lsland 02908 (ReceivedJanuary 3 1, 1974; Revised Manuscript Received April 14, 1975)
The 77°K absorption, phosphorescence, and phosphorescence excitation spectra of pyrazine in ethanol with various amounts of Li+, Na+, K+, and Zn2+ salts are presented. It is shown that in each case the pyrazine exists as three distinct ground state species. These are interpreted as pyrazine uncomplexed with metal (species I), pyrazine complexed with one metal ion (species 11),and pyrazine complexed with two metal ions (species 111). The relative phosphorescence quantum yields and the lifetimes of the various species are presented and the results are compared with previous work on pyrazine in a mixed hydroxylic solvent of ethanol and water.
Introduction
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The effects of hydrogen bonding complexation on the n P* transitions of azines and ketones have been investigated by many workers.2-6 The changes in transition energies resulting from changes in the nature of the solvent have been of particular interest. Recently we reported on the 77°K electronic spectra of pyrazine (1,4-diazine) in a mixed hydroxylic solvent of ethanol and water.‘ The absorption, phosphorescence, and phosphorescence excitation spectra of pyrazine in this solvent clearly indicate that pyrazine exists as three distinct hydrogen bonded species a t 77°K. These have been interpreted as pyrazine hydrogen bonded to ethanol a t both nitrogens (species I), pyrazine hydrogen bonded to ethanol a t one nitrogen and water at the other (species 11), and pyrazine hydrogen bonded to water at both nitrogens (species 111).The phosphorescence lifetimes of each of the three species have been reported and their relative quantum yields have been discussed qualitatively. The results of similar studies on benzopheThe Journal of Physical Chemistry, Vol. 79, No. 16, 1975
nones and substituted benzophenones have also been reported.8 In this paper we report on the phosphorescence of pyrazine in ethanol with various amounts of metal salts such as LiCl, NaBr, KI, and ZnCls. It will be shown that in each of these solvent mixtures the pyrazine exists as three distinct ground state species. These will be interpreted as pyrazine uncomplexed with metal (species 1): pyrazine complexed with a metal ion at one nitrogen (species 11),and pyrazine complexed with two metal ions, one at each nitrogen (species 111). We will also report on the lifetimes and the relative quantum yields of each of the various species and compare these results with those of the pyrazine ethanolwater system.
Experimental Section Pyrazine (Aldrich Chemical Co.) was purified by repeated zone melting. All metal salts were reagent grade (Mallinckrodt Chemical Co.) and were used without further pu-
