J . Phys. Chem. 1984, 88, 4104-41 11
4104
measurements above 1000 K are too low. Below 500 K there is evidence for a water complex taking part in the reaction mechanism. Acknowledgment. We acknowledge the helpful discussions of the Argonne Theoretical Group, Thom Dunning, A1 Wagner, and Larry Harding. We had helpful discussions concerning the chemical systems with Myran Sauer and Ron Cooper. We are grateful to A. R. Ravishankara for sharing his data for OH CO with us prior to publication, to Bill Felder for his suggestion, both chemical and experimental, and to Bob Gorse who built the original resonance lamp. None of these experiments could have
+
been done without the able assistance of the Argonne Linac Group, Don Ficht and George Cox. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, U S . Department of Energy under Contract NO. W-3 1- 109-ENG-38. Registry No. OH, 3352-57-6; CO, 630-08-0; CH4, 74-82-8; H20, 7732-18-5; deuterium, 7782-39-0.
Supplementary Material Available: An Appendix containing rate constants as a function of temperature for the reactants OH CO, OD + CO, and OH CH4 (6 pages). Ordering information is available on any current masthead page.
+
+
An X-ray Photoelectron and Auger Electron Spectroscopic Study of the Adsorption of Molecular Iodine on Uranium Metal and Uranium Dioxide J. G. Dillard,+H. Moers, H. Klewe-Nebenius, G . Kirch, G . Pfennig, and H. J. Ache* Institut fur Radiochemie, Kernforschungszentrum Karlsruhe GmhH, D - 7500 Karlsruhe, Federal Republic of Germany (Received: September 28, 1983)
The adsorption of molecular iodine on uranium metal and on uranium dioxide has been investigated at 25 "C. Clean surfaces were prepared in an ultrahigh vacuum apparatus and were characterized by X-ray photoelectron (XPS) and X-ray and electron-induced Auger electron spectroscopies (AES). Adsorption of I2 was studied for exposures up to 100 langmuirs (1 langmuir = 10" torr s) on uranium metal and to 75 langmuirs on uranium dioxide. Above about 2-langmuir I, exposure on uranium, spectroscopic evidence is obtained to indicate the beginning of U13 formation. Saturation coverage for I2adsorption on uranium dioxide occurs at approximately 10-15 langmuirs. Analysis of the XPS and AES results as well as studies of spectra as a function of temperature lead to the conclusions that a dissociative chemisorption/reaction process occurs on uranium metal while nondissodative adsorption occurs on uranium dioxide. Variations in the iodine Auger kinetic energy and in the Auger parameter are interpreted in light of extra-atomic relaxation processes.
Introduction The investigation of diatomic halogen adsorption on metals has occupied the attention of several In previous studies it was of interest to discover periodic trends exhibited by halogens in adsorption kinetics and adatom substrate interaction^,'^^^^ to probe the variety of complex structures that are and to evaluate the application of surface techniques in elucidating the nature of adsorbatesubstrate bonds.68 Adsorption of iodine on uranium and on uranium dioxide is of fundamental and practical interest. Surface chemical changes that accompany molecular iodine adsorption on uranium and uranium dioxide are unexplored, the comparative uptake of I2 on the two substrates is unknown, the kinetics of the adsorption process have not been investigated, and the behavior of the surface adsorbate layer as a function of temperature has not been reported. From a practical point of view knowledge regarding the fate of iodine produced in nuclear reactors and adsorbed on aerosols is of particular interest with regard to health hazards:-*' and studies of the reactions of iodine and iodine-containing molecules are important for an understanding of corrosion reactions involving reactor components."*'* In the present study the adsorption and subsequent reaction processes of molecular iodine on uranium and uranium dioxide have been investigated. The principal goals of this work have been to characterize the substrate surface and the adsorbate via X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) following the adsorption of I2 on clean uranium and on clean, stoichiometric uranium dioxide surfaces, to obtain information regarding the occurrence of dissociative or nondissociative processes, to describe the chemical nature of the adsorbate-sub+Permanent address: Blacksburg, VA 2406 1.
Department of Chemistry, Virginia Tech,
0022-3654/84/2088-4104$01SO/O
strate interaction, and to consider the application of various kinetic models for describing molecular iodine adsorption. Experimental Section The experimental procedures and the details for characterizing the substrates and adsorbate-substrate surfaces have been described in earlier p~b1ications.l~Details are given here where significant differences in the experimental procedures and/or methods were adopted compared to the previous work.I3 Poly(1) P. A. Dowben and R. G. Jones, Surf. Sci., 88, 348 (1979), and references therein. (2) S. A. Cochran and H. H. Farrell, Surf. Sci., 95, 359 (1980). (3) P. A. Dowben and R. G. Jones, Surf.Sci., 84, 449 (1979). (4) R. G. Jones and D. L. Perry, Surf. Sci., 88, 331 (1979). ( 5 ) K. J. Rawlings, G. G. Price, and B. J. Hopkins, Surf. Sci., 95, 245 (1980). (6) G. K. Wertheim, S. B. DiCenzo, and D. N. E Buchanan, Phys. Rev. B, 25, 3020 (1982). (7) S. B. DiCenzo, G. K. Wertheim, and D. N. E. Buchanan, Phys. Rev. B, 24, 6143 (1981). (8) S. B. DiCenzo, G. K. Wertheim, and D. N. E. Buchanan, Surf. Scr., 121, 411 (1982). (9) D. K. Craig, Healrh Phys., 12, 1047 (1966). (10) "Control of Iodine in the Nuclear Industry", Technical Reports Series No. 148, International Atomic Energy Agency, Vienna, 1973. (11) D. 0. Campell, A. P. Malinauskas, and W. R. Stratton, Nucl. Technol., 53, 111 (1981). (12) S. H. Shann and D. R. Olander, J . Nucl. Mater., 113,234 (1983). (13) (a) J. G. Dillard, H. Moers, H Klewe-Nebenius, G . Kirch, G. Pfennig, and H. J. Ache, KfK-3575 (1984); (b) J. G. Dillard, H. Moers, H. Klewe-Nebenius, G. Kirch, G. Pfennig, and H. J Ache, submitted for publication; (c) J. G. Dillard, H. Moers, H. Klewe-Nebenius, G. Kirch, G. Pfennig, and H. J. Ache, Conference abstract, SCADEC-AMSEL Conference, De Eemhof, The Netherlands, May 3-6, 1983, to be published in Spectrochim. Acta, Part B.
0 1984 American Chemical Society
I, Adsorption on U and UO,
The Journal of Physical Chemistry, VoI. 88, No. 18, 1984 4105
crystalline uranium foil was cleaned via repeated Ar' ion etching, heating, and annealing cycles until the surface atomic percent oxygen and/or carbon was
