AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA10
Dissociative Chemisorption of Chloromethanes on Ir(110), Ir(111) and Oxygen Modified Ir(111)

Tuesday, October 3, 2000, 5:00 pm, Room 208

Session: Mechanisms and Control of Surface Reactions
Presenter: R.J. Meyer, University of Texas at Austin
Authors: R.J. Meyer, University of Texas at Austin
C.T. Reeves, University of Texas at Austin
D.J. Safarik, University of Texas at Austin
D.T. Allen, University of Texas at Austin
C.B. Mullins, University of Texas at Austin
Correspondent: Click to Email
Chlorinated hydrocarbons are a primary component of the waste streams from the manufacture of many commodity chemicals. Noble metal catalysts, including iridium on alumina, have been shown to be active for both oxidative and reductive catalytic treatment methods. However, the causes of deactivation and mechanisms by which these reactions proceed are not precisely known. Dissociative chemisorption of CH@sub 3@Cl, CH@sub 2@Cl@sub 2@, CHCl@sub 3@, and CCl@sub 4@ on Ir(110) and Ir(111) has been examined at surface temperatures from 77-1200 K using molecular beam methods. From our ongoing studies two particularly exciting results have emerged. (1) We have identified the reactive site for methyl chloride on Ir(110). An examination of CH@sub 3@Cl on hydrogen precovered Ir(110) indicates that the reaction pathway for dissociative chemisorption can be shut off by filling the high temperature desorption state. This result implies that methyl chloride chemisorbs in the hollow site of the rows of the (110) surface. (2) The production of phosgene was observed when a CCl@sub 4@ beam was impinged on an oxygen modified Ir (111) surface. We propose that phosgene forms via a Langmuir-Hinshelwood mechanism from the reaction of absorbed oxygen with CCl@sub 2@ surface species formed during the decomposition of carbon tetrachloride. Competing reactions involving the oxidation of completely dissociated carbon tetrachloride become significant as the surface temperature increases. COCl@sub 2@ formation was maximized at an oxygen surface coverage of approximately 0.3 ML. At higher oxygen coverages, the production of phosgene decreases due to a diminished reactive adsorption of carbon tetrachloride but the efficiency of COCl@sub 2@ formation per dissociatively chemisorbed CCl@sub 4@ increases dramatically.