Contains reports on four research projects.National Science Foundation (Grant DMR81-19292)National Science Foundation (Grant CHE82-06422)Research CorporationCamille and Henry Dreyfus FoundationMonsant

Beckerle, John D.

Ceyer, Sylvia T.

Gladstone, David J.

Hines, Melissa A.

Lee, Myung B.

McGonigal, Marianne

Simonson, Robert J.

Tang, Sau Lan

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Chemical Reaction Dynamicsat Surfaces4. Chemical Reaction Dynamics at SurfacesAcademic and Research StaffProf. S.T. CeyerGraduate StudentsJ.D. Beckerle, D. Gladstone, M.B. Lee, M. McGonigal, R.J. Simonson, S.L. TangUndergraduate StudentsM. Hines4.1 Dynamics of Activated Dissociative AdsorptionNational Science Foundation (Grant DMR81-19292)Sylvia T. Ceyer, John D. Beckerle, Melissa Hines, Myung B. Lee, Sau Lan TangOur efforts this year have centered around the construction, assembly and testing of the molecularbeam-EELS apparatus which will enable us to study the dynamics of molecule-surface interactions.In particular, we are interested in understanding why dissociative chemisorption does not occur ontransition metal surfaces where the adsorption is known to be exoergic. In many of these cases it islikely that dissociative adsorption is activated and thus, is not observed when the adsorption isattempted by allowing ambient molecules, whose energies are characterized by a 300 KMaxwell-Boltzmann distribution to impinge on the surface. The existence of a barrier along thelowest energy pathway is the condition for activated dissociative chemisorption. On many metalsurfaces, however, dissociative adsorption is observed when the surface has been modified with lowcoordination number sites or promoter atoms. Whether these modifications lower the barrier fordissociative adsorption, increase the atom-surface binding energy or affect both quantities isunknown. In short, the features of the potential energy surfaces which are modified to promotedissociation are simply not known experimentally.Because activation energy barriers are extrema on the potential surface they are experimentallyaccessible. It is proposed to verify the existence of and to measure the magnitude of these dominantfeatures in an experimental arrangement in which a beam of molecules that are to be adsorbed on asurface and high resolution electron energy loss spectroscopy are coupled. With this arrangement,the effect of the energy of the incoming molecule on its. probability of dissociatively chemisorbing canbe determined. If there exists a barrier to dissociation, there will be a sharp onset in intensity of thedissociated species at some incident beam energy. This beam energy then constitutes theappearance threshold or magnitude of the energy barrier to dissociation. The orientation of thebarrier relative to the surface can be determined by variation of the incident beam angle. The effectRLE P.R. No. 126... ni~R ./ e ction Dynamicsat Surfacesof lateral interactions on the height of the barrier can be investigated by variation of the surfacecoverage.Carbon monoxide dissociation on Ni(1 11) is the rate limiting step in the methanation reaction, CO +3H2--CH4 + H20. This reaction occurs with measurable rates only at high surface temperatures(>500 K) and at high pressures (>1 atm). This observation suggests that CO dissociative adsorption isan activated process since the pressure must be high enough for the CO molecules to gain energythrough multiple inelastic collisions with the hot surface prior to the collision which leads todissociation. We have chosen to investigate this system on an apparatus which we have recentlyfinished constructing. Preliminary results indicate that the dissociative adsorption of CO is indeedactivated and that the barrier is at least 25 kcal. If we confirm this result, then the rate of themethanation reaction could be greatly increased by heating the reactants prior to their encounter withthe nickel catalyst. We also have measured the decrease in the barrier height when a small quantity(<10% of a monolayer) of potassium is doped on the surface. It appears that this small amount ofpotassium decreases the barrier by two orders of magnitude. We are presently confirming thesepreliminary results.4.2 Dynamics of Dissociative AdsorptionNational Science Foundation (Grant CHE82-06422)Research CorporationCamille and Henry Dreyfus FoundationSylvia T. Ceyer, John D. Beckerle, Melissa Hines, Myung B. Lee, Sau Lan TangInvestigations into the dynamics of the dissociative chemisorption process on transition metalsurfaces are planned. The collision energy of the incident adsorbate is a convenient probe of thedominant features of the molecule-surface potential energy surface which governs the adsorptiondynamics. High resolution electron energy loss spectroscopy and photon detection are sensitivedetectors of the result of the dissociative adsorption event. The effect of translational energy and,where appropriate, internal energy on the extent of dissociative chemisorption will be studied formolecule-metal systems that are believed to dissociate via a precursor state, a direct, an activatedand an electronic mechanism or are believed not to dissociate. The emphasis of these studies is onestablishing a correlation between the nature of the potential energy surface for the molecule-surfaceinteraction and the dynamics of dissociative adsorption.4.3 Chemical Reaction Dynamics at Semiconductor SurfacesSylvia T. Ceyer, Marianne McGonigal, David GladstoneAn investigation into the reaction dynamics of atomic oxygen and atomic halogens withRLE P.R. No. 126Chemical Reaction Dynamicsat Surfacessemiconduct'or surfaces will be carried out with molecular beam-surface scattering techniques.These techniques enable the measurement of the mass distribution, the residence time distribution,the angular distribution, and the energy distribution of the product molecules. The goals of thisresearch are to develop a kinetic model for the surface reaction, to determine the chemical identitiesof the reaction intermediates, to understand the mechanism of the desorption step and how it relatesto the potential energy surface in the exit channel region and to investigate whether simple gas phaseunimolecular decay theories may be applied to describe the angular and energy distributions of thedesorbing molecule after chemical activation. The plans for this apparatus have been submitted tovarious machine shops.4.4 Spectroscopic Studies of Small Molecule Adsorption on RareEarth Single Crystal Metal and Metal Oxide SurfacesMonsantoSylvia T. Ceyer, Robert J. SimonsonAn investigation aimed at a spectroscopic study of the adsorption of small molecules on singlecrystal rare earth metal and metal oxide surfaces has begun. Such techniques as X-ray andultraviolet photoelectron spectroscopy, Auger and high resolution electron energy loss spectroscopy,low energy electron diffraction and thermal desorption are being employed to determine thedissociative nature of the adsorption, the electronic structure and the binding site of the molecule, thekinetics of the desorption process and the binding energy of the molecule on the surface. Thesestudies represent the first investigations into adsorption on single crystal rare earth metals eventhough these metals, their oxides and their intermetallics exhibit catalytic activity comparable totransition metal catalysts.At the present time, we have temporarily side-tracked from these goals to investigate someinteresting inner shell spectroscopy of the rare earths. We have observed and identified inner core to4f transitions in an electron energy loss spectrum of a Gd single crystal. These resonances havebeen previously observed in X-ray absorption studies of polycrystalline rare earth metals. Since boththese resonances and the Auger transitions can be observed in a single spectrum, we are attemptingto identify the Auger transitions that can be attributed to direct recombination from the bound 4fexcitations versus those that arise from continuum excitations.PublicationsCeyer, S.T., W. Guthrie, T.H. Lin, and G.A. Somorjai, "D 20 Product Angular and Translational EnergyDistributions from the Oxidation of Deuteriumn on Pt(1 11)," J. Chem. Phys. 78, 6982 (1983).Ceyer, S.T. and J.T. Yates, J. Elect. Chem. 150, 17 (1983).RLE P.R. No. 126RLE P.R. No. 126 22

Chemical Reaction Dynamics at Surfaces

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Chemical Reaction Dynamics at Surfaces

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