THE REACTION OF BROMINE AND OXYGEN WITH A TUNGSTEN SURFACE

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1 Philips'J. Res. 35, , 1980 R 1011 THE REACTON OF BROMNE AND OXYGEN WTH A TUNGSTEN SURFACE PART : MEASUREMENTS ON THE NFLUENCE OF HYDROGEN ON THE REACTON RATE by B. J. de MAAGT and G. C. J. ROUWELER Abstract The chemical reaction rate óf polycrystalline tungsten in mixtures of hydrogen-bromide, water and bromine or hydrogen was measured in a gas-flow system with argon as a carrier gas at atmospheric pressure. Hydrogen-bromide pressures were between 26.3 and 263 Pa, and water pressures between 0.2 and 2 Pa. The amounts of bromine or hydrogen did not exceed 20 Pa. Reaction temperatures were between 97~ and 1282K. The results in hydrogen-bromide, water and bromine are quantitatively the same as those in the tungsten-bromine-oxygen system for equivalent amounts of oxygen; in hydrogen-bromide, water and hydrogen a complicated reaction behaviour was found for which no explanation can be given. 1. ntroduetion n the recent past, several experimental studies have been published 1-8) on the chemical reaction of tungsten with halogen gas and/or halogen-oxygen mixtures. Recently, we have published results on the kinetics of the reaction of a polycrystalline tungsten wire with bromine-oxygen mixtures in a gas-flow system 7). n the present investigations these measurements are extended to the more complicated tungsten-bromine-oxygen-hydrogen system, thus incorporating species like HBr, H 2 0, Br, Br 2 and H 2 Measurements of this kind are performed in order to obtain a better insight into the mechanism of tail-end erosion 9) in halogen incandescent lamps containing hydrogen-bromide as the tungsten-transporting gas, and with oxygen-bearing species like water as common contaminants. The measurements of the reaction rate can be divided into two sections. n the region where the overall atomic ratio H:Br is less than one the results can be fully explained by the mechanism found in the tungsten-bromine-oxygen system 7). The reaction rate is quantitatively dependent on the same parameters as in the W-Br-O system and is independent of the amount of HBr. The data in the region where the H:Br ratio is larger than one shows a complicated reaction behaviour for which, in the field of our reaction conditions, no explanation can be given. Philips Journalof Research Vol. 35 No

2 B. J. de Maag! and G. C. J. Rouwe/er electrochemical cells soap-film flowmeter sieves argon cylinder Fig. 1. Schematic description of the measuring system. 2. Description of the experimental set-up The measurements of the reaction rate of tungsten with hydrogen-bromide, water and alternatively excess bromine or hydrogen, were carried out in the same experimental set-up as used for the experiments on the tungsten-bromineoxygen system 7). A schematic description is given in fig.:. Tungsten probes consisted oftwo straight polycrystalline wires (diameter 49.6!Lm, length 20 mm) of normal lamp quality. Results of a spectroscopie analysis are given elsewhere 7). The argon used as carrier gas (at atmospheric pressure) was obtained from a cylinder containing argon of lamp-production quality and was further purified by a Messer-Griesheim adsorption cell. The reactive gases were obtained by electrolysis: bromine from a melt of silver bromide (UeB, analytical quality), oxygen in a zirconia solid-electrolyte cell, and hydrogen from a water-h 2 S0 4 solution. The efficiency of generation was 99± J %, 100% and 100% for bromine, oxygen and hydrogen, respectively, as was found by titration. The part of the gas which had passed through the hydrogen generation cell was led - through an adsorption column containing molecular sieves. n this way the mole fraction of water in the gas could be kept at an acceptably low level of less than 0.5 x 10-6, as measured by means of a Keidel cell 10). Traces of oxygen were present in mole fractions up to a maximum of 0.2 x 10-6 in O 2 equivalents. Reducing components were present up to a maximum ofo.5 x 10-6 in H2 equivalents, as measured with a zirconia-oxygen sensor/generator 7). The gas flows containing bromine and hydrogen were combined and led into an electrical oven at 1020 K (fig. ). Here hydrogen-bromide formation took place with platinum gauze as a catalyst (efficiency 100%). This gas, 96 Philips Journalof Research Vol.35 No

3 The reaction of bromine and oxygen with a tungsten surface eventually containing excess bromine or hydrogen, was mixed with the gas flow containing oxygen. The latter gas reacted to form water in the first part of the reaction tube. Reaction with tungsten took place in the central part of the reaction vessel, which was surrounded by an electrical furnace having a temperature constancy at the reaction zone of about 0.5 K (see ref. 7). The rate of gas flow was chosen large enough to ensure that the physical transport of reagents and reaction products towards and away from the probes was only of minor influence on the measured reaction rate 7) (typical value 10 cm S-1 at the reaction temperature). The rate of tungsten removal jw (mol W cm- 2 s" 1) was determined by monitoring the voltage change across the probes at a constant direct current of 10 ma. The inaccuracy in the values of jw was at most about 30 %. This situation arises at high hydrogen-bromide pressures and at an overall atomic ratio of H:Br of approximately one, where the reaction rate is a sensitive function ofthis ratio. The reason for this effect is the uncertainty in the efficiency of bromine generation (see above). The experimental variables were the partial pressures of hydrogen-bromide and water, the reaction temperature and the excess amount of bromine and hydrogen, which is defined by LlH2-Br2 = ".EP H2 - ".EPBr2. The symbol ".EP. is the equivalent sum pressure of element 1 of all compounds containing. 3. Results The main reactive component during our investigations was hydrogenbromide. Hydrogen, or, alternatively, atomic and molecular bromine were present as well, depending on the ratio of the reducing element hydrogen and the oxidizing elements bromine and oxygen. Water is formed under all conditions whenever oxygen is present. The results may be presented in different ways each with their own merits. We chose the value of LlH2-Br2 as the variable. This has the main advantage of making a direct comparison possible with the results of Zubler's experiments 8). From the numerous experimental data we have selected a few characteristic results. for graphical presentation. n figure 2 the tungsten reaction rate jw is plotted as a function of LlH2-Br2 at a fixed hydrogen-bromide pressure and a constant temperature. The parameter is the water pressure, which ranges from 0.2 to 2 Pa. A solid line is drawn through the measuring points. At the lefthand part ofthe figure the dashed lines represent the values ofjw in the W-Br-O system (i.e. without HBr) for equivalent quantities of oxygen. The dashed lines have been derived from fig. 6b in ref. 7. From the kinetic point of view, (P ar 2 +!PBr) is the effective parameter instead of LlH2-Br2 (see discussion). For the reason of comparison, therefore, the Philips Journalof Research Vol. 35 No

4 B. J. de Maagt and G. C. J. Rouwe/er jw (mol cm- 2 s- 1 ) t P HBr =79Pa T=1169K /7.'~- /j r ~ ' : Li H2-8r2 (Pa) Fig. 2. Tungsten reaction rate jw as a function of.ó. H -Br2 at a hydrogen-bromide pressure of79 Paand a reaction temperature of 1169 K. Water pfessures: 0.2 (L), 0.6 (0) and 2 Pa (.). Dashed curves represent measurements in the W-Br-O system (ref. 7) for equivalent amounts of oxygen. dashed curves have been shifted with respect to the abcissa over a distance PH 2 0, which is indicated by an arrow. Figures 3 and 4 show the measured dependence on HBr pressure and temperature. The dashed lines have the same meaning as in fig. 2. A selection of reaction rates for LlH2-Br2 > 0 is given in table. 4. Discussion On thermodynamical grounds it can be derived that all oxygen is bonded as water. When (LlH2-Br,) - PH 2 0 is negative (roughly the left-hand part of figs 2, 3 and 4) Br and Br 2 are also present. When (P Br 2 + tpbr) is taken as variable, which quantity is nearly equal to (Ll H2 -Br,) - PH20, it is apparent from figs 2 and 4 that the reaction rate here is identical to the rate found in the W-Br-O system for equivalent amounts of oxygen 7). 98 Philips Journalof Research Vol. 35 No

5 The reaction of bromine and oxygen with a tungsten surface jw T=1169K (mol cm- 2 1) PH20=0.6Pa t o Fig. 3. Tungsten reaction rate as a function of,ó.h 2 -Br2 at constant temperature (1169 K) and constant water pressure (0.6 Pa). Hydrogen-bromide pressures: 26.3 (.6), 79 (0) and 263 Pa (.). Under these circumstances, then, HBr can be considered as a non-reactive gas. From the measurements in the W-Br-O system it is known that above some critical ratio Po) (PBr2 + 1PBr) one can expect oxidation of tungsten and thus a diffusion-limited reaction rate. Therefore, the rate of tungsten removal at ~H2-Br2 near zero is presumably not determined by chemical reaction kinetics. A comparison with Zubler's data 8) can be made only for ~H2-Br2 ~ 0, since his experiments were carried out in this region. At ~H2-Br2 = 0 Zubler's values for the reaction rate, at temperatures comparable with ours, are a factor of 5 to 20 smaller than ours. n our opinion, therefore, also in his experiments diffusion-limited processes, possibly even in the gas-phase, are also rate determining. These differences in rate values may be ascribed to differences in probe geometry. At (~H2-Br2) - P H 20 > 0 the reagent gas has reducing features, H2 being present. From the right-hand parts in figs 2, 3 and 4 it can be derived that the reaction rate decreases with increasing values of ~H2-Br2 (figs 2 to 4), and increases with partial water pressure (fig. 2) and also slightly with partial HBr Philips Journalof Research Vol.3S No

6 B. J. de Maagt and G. C. J. Rouwe/er t PH o=0.6pa "2 Á v ~ o Fig.4. Tungsten reaction rate as a function of ~H2-Br2 at constant hydrogen-bromide pressure (79 Pa) and fixed water pressure (0.6 Pa). Reaction temperatures: 974 (.6.), 1066 (0), 1169 (0) and 1282 K (.). pressure (fig. 3). Furthermore the reaction rate varies with temperature in the way shown in fig. 4. These trends can also be recognized in the survey in table, where a selection is given of measurements at LlH2-Br2 > O. A comparison with Zubler's results reveals that under comparable conditions at.llh2-br2 > 0 his values for the reaction rate are roughly a factor of three smaller than ours. There is only qualitative agreement with our experiments, i.e. the fall-off of the reaction rate with increasing values of LlH2-Br2' 5. Relation to halogen incandescent.lamps t is not simple to draw conclusions from our experiments that can be of general value for the practice of lamp manufacturing. This is related to the complex and transient processes which may take place in lamps during lifetime. First of all, the chemical conditions at the tail-ends in the lamps depend not only on filling doses and impurity level, but also on lamp geometry, burning position and burning time. When e.g. hydrogen and bromine are introduced in the lamp in the form of organic compounds the oxidation of carbon by metal oxides from the filament creates a reducing environment in the lamp ). This is followed by bromination ofthe resulting metals and the liberation of'hydrogen. 100 Philips Journalof Research Vol.35 No

7 The reaction of bromine and oxygen with a tungsten surface TABLE Reaction ratesjw(x mol W cm- 2 s" ') in excess hydrogen. PHDr (Pa) PH 2 (Pa) T(K) "0" O 20 "0" "0" PH,O (Pa) < ,4 1, , < < , Oxidizing conditions may prevail when tungsten oxides, formed during seal-off, are not thoroughly reduced or when the quartz wall is not freed from water during the lamp processing. Gas-phase separation processes arising from normal and thermal diffusion tend to transport hydrogen to the wall region 12). Separation effects may be even greater in a vertical burning position when convection and radial separation cooperate 13). Finally, hydrogen may gradually escape from the lamp by diffusion through the quartz wall if the latter is sufficiently hot 14). So the value of~ph2 and certainly that of ~H2-Dr2 are not known with sufficient precision. t seems rather difficult to include all the effects mentioned above in a single theoretical -model of a halogen incandescent lamp. On the other hand, no Philips Journalof Research Vol.35 No

8 B. J. de Maagt and G. C. J. Rouwe/er diagnostic means are available for determining local gas densities in a working lamp. One has, therefore, to rely on phenomenological observations. An oxygen level around 10 Pa is not unusual in halogen incandescent lamps 15). f tail-end erosion is limited by the rate of the chemical reaction, then in the temperature range of K and in stoichiometrie conditions, a rate value of the order of JO-8 mol W cm- 2 S-1 is to be expected (extrapolation from the values given in table ). This means that a tungsten support of 200 (.Lm diameter at these temperatures would be consumed within about 30 hours of burning, if all the oxygen at the tail-end is available in the form of water. Since lifetime experiments show that the tail-ends have substantially longer lifetimes, it seems that the rate of chemical reactions at the tungsten surface is not the rate-limiting step in tail-end erosion. This is very plausible in halogen incandescent lamps because of the high gas-pressures and low gas-flow rates, factors which favour diffusion-limited processes. For this reason, prevention of tail-end erosion should be directed towards diminishing gas-phase transport processes, for instance by applying diffusion and convection barriers or by lowering temperature gradients in the critical regions of the lamp. Acknowledgements We are indebted to Messrs P. 't Lam, W. v. Leeuwen, J. Lunter, and J. de Nijs for carrying out parts of the experimental work and to G. Dittmer, S. Garbe and W. van den Hoek, for the critical reading of the manuscript. Philips Lighting Division Eindhoven, October 1980 REFERENCES 1) D. E. Rosner and H. D. Allendorf, Ablation rates in mixtures of reactive gases, AAA J. 5, 1489, ) J. D. McKinley, Mass spectrometric investigation of the surface reaction of tungsten with chlorine-oxygen mixtures, Proc. 6th nt. Symp. React. Sol., Schenectady N.Y. (USA), Aug , ) D. L. Fehrs and R. E. Stickney, Contact-potential measurements of the adsorption of 12, Br2' and Cl 2 on bare and cesiated W (100), Surf. Sci. 17, 298, ) E. G. Zubler, The kinetics of the tungsten-oxygen-bromine reaction, J. Phys. Chem. 74,2479, ) E. G. Zubler, Adsorption effects in tungsten-oxygen-bromine reaction, J. Phys. Chem. 76,320, ) V. W. Goddard and C. Pett, Kinetic investigation of the reaction between tungsten and bromine, J. Chem. Soc. Dalton Trans. 9,767, ) G. Rouweier and B. J. de Maagt, The reaction ofbromine and oxygen with a tungsten surface, Part : Measurement of the rate of reaction in a gas-flow experiment, Philips J. Res. 33, 1, ) E. G. Zubler, Kinetics of the tungsten-oxygen-hydrogen-bromide reaction, J. Phys. Chem. 79, 1703, Philills Journalof Research Vol.35 No

9 The reaction of bromine and oxygen with a tungsten surface 9) See e.g. J. R. de Bie and J. C. M. A. Ponsioen, Life and luminous flux of halogen incandescent lamps related to filament temperature, pressure and CHzBrz content, Lighting Res. Techn. 9, 141, ) F. A. Keidel, Determination of water by direct ampèrornetric measurement, Anal. Chem. 31, 2043, ) W. J. van den Hoek and E. G. Berns, Emission lines in the spectra of halogen incandescent lamps, Lighting Res. Techn. 7, 143, ) W. J. van den Hoek and G. Rouweier, On thermodynamic calculations of chemical transport in halogen incandescent lamps, Philips Res. Repts 31, 23, ) E. Fischer, Axial segregation of additives in mercury-metal-halide arcs, J. Appl. Phys. 47,2954, ) R. W. Lee and D. L. Fry, A comparative study of the diffusion of hydrogen in glass, Phys. Chem. Glasses, 7, 19, ) P. Eckerlin and S. Garbe, to be published. Philips Journalof Research Vol.35 No

THE REACTION OF BROMINE AND OXYGEN WITHA TUNGSTEN SURFACE PART I: MEASUREMENT OF THE RATE OF REACTION IN A GAS-FLOW EXPERIMENT

Philips J. Res. Vol., 33, 1-19, 1978 R977 THE REACTION OF BROMINE AND OXYGEN WITHA TUNGSTEN SURFACE PART I: MEASUREMENT OF THE RATE OF REACTION IN A GAS-FLOW EXPERIMENT by G. ROUWELER and B. J. de MAAGT