J. Phys. Chem. 1985,89, 3392-3394
3392
Adsorption of Carbonyl Sulfide on Nickel and Tungsten Fflms Jalal M. Saleh* and Faik A. K. Nasser Department of Chemistry, College of Science, University of Baghdad, Baghdad, Jadiriya, Republic of Iraq (Received: September 7, 1984; In Final Form: February 21, 1985)
The interaction of carbonyl sulfide with evaporated nickel and tungsten films has been investigated in the temperature range 195-450 K using gas pressures ranging from 1 to 13 N m-,. Rapid but mainly associative chemisorption of COS occurred on both metals at 195 K. Further adsorption of COS on W at temperatures 293-450 K was extremely slow and accompanied by more CO desorption than COS adsorbed. Sulfidation of Ni film by COS occurred at temperatures 3 293 K with the liberation of carbon monoxide. The rate of adsorption increased with temperature but was independent of COS pressure. The activation energy (E,) increased with extent ( X ) of sulfidation to a limiting value of 97 kJ mol-I. A linear relationship was obtained from the plot of E, against 1/X, suggesting the applicability of Cabrera-Mott theory to the sulfidation of Ni film by COS.
Introduction There have been no previous investigations on the interaction of COS with metal films compared with the iiumerous investigations on the adsorption of COzi and CS22-5despite a number of existing similarities among these three compounds. We have selected COS in order to find if there were obvious differences between the behavior of this gas with the metal films and those of CO, and CS,, which have been studied before, which might be attributed to the heterogeniety of the bonds in the COS molecule. The system COS metal film is of interest because of the different reactions which may occur. These may involve molecular and dissociative adsorption in addition to the sulfidation of the metal through the rupture of the carbon-sulfur bonds and the subsequent evolution of carbon monoxide.
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Experimental Section The apparatus and general technique employed in this work have been described,e6 as have the preparation and sintering of the evaporated Nickel films were prepared from Johnson Matthey spectroscopically standardized nickel wire (0.5 mm diameter) which was first reduced at 1000 O C under low pressure (1 .O kN m-2) of pure hydrogen, then degassed at a slightly higher temperature for no less than 6 h. Tungsten films were formed from "pure grade" wire (0.1 mm diameter) obtained from British Tungsten Manufacturing Co. The film of each metal was formed at a pressure of < 10" N m-, with the reaction vessel maintanined at 78 K;it was subsequently annealed under vacuum at 80 O C for 30 min. After the film was annealed, the general procedure for determining the surface area with krypton and for subsequent adsorption and incorporation was as given b e f ~ r e . ~ - ~ Carbonyl sulfide, more than 99% pure, was obtained from British Oxygen Co. Ltd., and it was subjected to successive melting and freezing in vacuo before it was stored. New samples of the gas were prepared frequently; its vapor pressure at 78 K as measured by a Mcleod gauge was N m-,. Mixtures of COS + C O were analyzed by condensing the former gas in a cold glass finger immersed in liquid nitrogen. Calibration experiments showed that condensation occurred within 1 min, provided the total gas pressure was not greater than 13.3 N m-2. The composition of the gas phase at each temperature was also analyzed (1) Quinn, C. N.; Roberts, M. W. Trans. Faraday SOC.1962, 58, 569. (2) Saleh, J. M. Trans. Faraday SOC.1969, 65, 259. (3) Saleh, J. M. Trans. Faraday SOC.1970, 66, 242. (4) Saleh, J. M. Trans. Faraday SOC.1971, 67, 1830. (5) Saleh, J. M.; Hikmat, N. A. Iraqi J . Sci. 1978, 19, 47. (6) Saleh, J. M.; Kemball, C.; Roberts,M. W. Trans. Faraday Soc. 1961, 57. 1771.
0022-3654/85/2089-3392$01.50/0
TABLE I: Adsorption of COS on Ni and W Filmo at 195 K film vm/mm3 v,/mm3 X Ni 15.0 21.7 1.44 W 38.4 38.9 1.01 with a 4 2 0 0 mass spectrometer partial pressure gauge which operated on the quadrupole principle and covered the mass range 1-200 amu. The mass spectrometer was combined with a pressure converter which could reduce the total gas pressure to yeron 1 cm2 of surface and thereafter multiplying the resulting data by the value of a. Another method for calculating the value of y was to find the value of A in units of cm s-' from)the.,value of E, and the rate of increasing thickness of the sulfide layer (dxldt), in cm s-l, at a given temperature T by using eq 2. By these procedures, it wasgossible to obtain values of and lo4 for y on Ni and W, respectively. The lower value may thus of A in the case of W ( lozo)as compared with Ni ( be attributed to the lower number of special sites per unit area ( N - ) from which tungsten ions move into the sulfide layer. This is likely to arise from the low mobility of the metal atoms in the high melting tungsten metal. The constancy of A values for each metal suggests that the number of special sites present at the interface must be statistically constant over the whole period of the reaction, Le., the sites must be self-generating as the sulfidation proceeded. This suggests that the growth of the sulfide layer should start at localities of the surface where spiral dislocations emerge from the metal. By such a procedure, the growth can spread throughout the volume of the metal while the number of the isolated sites at the metal/sulfide interface remains substantially constant during the progress of the reaction. Registry No. W,7440-33-7; Ni,7440-02-0; COS, 463-58-1. (20) Roberts, M. W.; Tompkins, F. C. Proc. R. SOC.London, Ser. A 1959, 251, 369.
