samples closely paralleled the relative rate constants listed in Table 11. Extension of Kinetic Model to Platinum Monolithic Catalysts. Since i t was desirable t o extend t h e mathematica! model of pelleted platinum catalytic converters t o platinum monolithic converters, i t was necessary t o establish some kinetic parameters f o r platinum monolithic catalysts. The kinetic parameters for fresh Engelhard PTX catalyst were estimated by comparing limited kinetic data for pieces crushed to 14-25 mesh size with the corresponding kinetic data for t,he fresh pelleted platinumalumina catalyst previously described. These comparisons indicated that the activity of the crushed platinum monolithic catalyst was about 3.5 t’imes that of the base case pelleted platinum catalyst. The adsorption effects were similar for both catalysts. Therefore, the kinetic model gave a reasonable fit of these data when the frequency factors @Or, and korr?)for CO and C3& were 3.5 times those used for the pelleted catalyst (Table I) while the adsorption constants and activation energies were kept the same. This scaling factor of 3.5 mas confirmed by independent Hz chemisorption measurements of platinum area on the two catalysts. Some preliminary kinetic d a h and chemisorption measurements on whole PTX monolithic catalysts also confirmed that a scaling factor of 3.5 was reasonable to map over the oxidation kinetics from the pellet’ed platinum-alumina catalyst to a platinum monolithic catalyst (Engelhard PTX). Conclusions
Kinetic data for the oxidation of CO and C3H6 on a pelleted platinum-alumina catalyst were determined betlveen 400 and iOO”F, and the result’s were utilized to formulate a system of reaction rat’e equations and to evaluate the key kinetic parameters. The kinetic model accounts for the enhaticemelit’ effect of O2 and the inhibition effect’s of CO, C3H6,and
NO on the reaction rates. Both the experimental results from the synthetic gas mixtures and the predictions of the kinetic model agree favorably with the observed oxidation rates of CO and hydrocarbons in real engine exhausts. The kinetic model has also been successfully applied to aged pelleted platinum-alumina and fresh platinum monolithic catalysts. literature Cited Belousov, V. hl., Gershingorina, A. V., Kinet. Katal., 12, 614
(1971). Harned, J. L., Paper No. 720320, SAE National Automotive Engineering Meeting, Detroit, Mich., May 1972. Hougen, 0. A., Watson, K. 11. “Chemical Process Principles,” Part 111, Wiley, New York, Y., 1947. Jacob, S. AI., Voltz, S. E., unpublished results, 1969. Jagel, K. I., Dwyer, F. G., Paper No. 710290, SAE Automotive Engineering Congress, Detroit, Mich., Jan 1971. KUO,J. C. W., Morgan, C. It., Lassen, H. G., Paper No. 710289, SAE Automotive Engineering Congress, Detroit, Mich., Jan 1971. Kuo, J. C. W.,Prater, C. D., Osterhout, 11. P., Snyder, P. W., Wei, J., Paper No. 2/14, 14th International Automobile Technical Congress of FISITA, London, June 1972. Langmuir, I., Trans. Faraday Soc., 17, 621 (1922). Patterson, U’.R., Kemball, C., J . Catal., 2, 463 (1963). Schwartz, A., Holbrook, L. L., Wise, H., ibid.,21, 199 (1971). Shishu, R. C., Ph.D. Dissertation, Universit,y of Det,roit, 1972. Sklyarov, A. V., Tret’yakov, I. I., Shab, B. R., Roginski, S. Z., Dokl. Ph,y*. Chem., 189, 829 (1969). Smith, J. hl., “Chemical Engineering Kinetics,” 2nd ed, McGrawHill, Xew York, S . Y., 1970. Solov’eva, L. S., Russ. J . Phys. Chem., 34, ,586 (1960). Su, E. C., Shishu, R. C., private communication, 1972. Taylor, K. C., unpublished results, 1971. Walas, S. 1L, “Reaction Kinetics for Chemical Engineers,” 1IcGraw-Hill, Xew York, Pi. Y., 1939. Wu,E., unpublished results, 1971.
s.
RECEIVED for review June 11, 1973 ACCEPTEDAugust 24, 1973 Presented at the Division of Industrial and Engineering Chemistry, 165th rational Meeting of the American Chemical Society, Dallas, Tex., April 1973.
Vapor Pressure of Tungsten Oxytetrabromide Alfred Pebler Tl-estinghouse Research Laboratories, Pittsburgh, Pa. 15235
The vapor pressure of WOBrl was measured tensimetrically with a sensitive quartz Bourdon gauge. The second-law enthalpies of sublimation and vaporization were determined to be 28.6 i 0.8 kcal/mol at 250” and 13.0 i. 1.5 kcal/mol at 325”, respectively. Standard enthalpy and entropy data for WOBrr(g) were derived or estimated which gave a standard free enthalpy of formation of -98.4 i 2.0 kcol/mol. The results were compared with those of other vapor pressure determinations and with thermochemical data of other tungsten oxytetrahalides.
Thermochemical data of tungsten halides and oxyhalides are important in understanding the reactions in incandescent tungsten-halogen l a m p , specifically those which contain bromine in the inert-gas lamp atmosphere. I n the presence of residual oxygen (as 0 2 , H90, or COJ, osyhalides can form in addition to halides. One of the specles of interest in tungstenbromine lanips is WOBr4. The heat of formation of solid
WOBr4 \vas reported by Shchukarev and Kokovin (1964). I n the present work, the vapor pressure of W013r4has been measured tensimetrically. The results enabled us to arrive a t a value for the heat of formation of WOBr*(g). The data have been used for thermodynamic equilibrium calculations involving the system W-O-13r (Yannopoulos and Pebler, 1971). Since the present author carried out this work, the vapor Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No.
4, 1973 301
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Ior
1
-35
-40
1 1 1W
1
I 150
200 t.
250
oc
Figure 1 . Bourdon g a u g e readings a s a function of sample temperature pressure of WOBr4 has also been measured tensimetrically by Kokovin (1967) and by Oppermann and Stover (1971) and mass spectrometrically by Gupta (1971) using the Knudsen effusion technique. Experimental Section
Preparation of Sample. Tungsten oxytetrabromide was prepared by allowing a n excess amount of bromine (10-20%) t o react with a stoichiometric mixture of tungsten and tungsten dioxide in sealed-off Pyrex ampoules a t 400'. Tungsten dioxide was made by partial reduction of tungsten trioxide WOS with a n H2-H20 mixture a t 800-900" (Brauer, 1965). The W-WOZ mixture was filled into a heavywall reaction tube and vacuum degassed at 400'. T h e required amount of bromine, which had been dried over P205, was vacuum distilled into the heavy-wall tubing which then was sealed off. I n order t o reduce the likelihood of a n explosion, the ampoule was inserted into a steel cylinder together with some water so that the bromine pressure was partially compensated by outside water vapor pressure while the cylinder was heated to 400' and kept there overnight. The W-WOZ mixture reacted nearly quantitatively to form black needles of W o k 4 with a high luster. The reaction product was shaken to the bottom of the tube and the excess bromine was condensed in the upper ampoule section and sealed off from the rest. The woB1-4 was purified by subliming i t in a temperature gradient 250"/150' to the other end of the tube and sealing off the residue. The remaining ampoule was opened in a n argon-filled drybox and 1t70Br4transferred into break-seal tubes, which were sealed off under vacuum and stored for future use. The compound was identified by X-ray analysis, the result of which agreed with the data given in the literature (Hartung, 1964; Kokovin and Toropova, 1965). Llaterials used were high-purity grade tungsten powder from the Restinghouse Lamp Division, purified tungsten trioxide from Fisher, and bromine containing 99.7% Brz, 0.15% CL, and
