114 zyxwvutsrqponml E. Bertel et al.: Tailoring Surface Electronic ProDerties to Promote Chemical Reactivity zyxw Tailoring Surface Electronic Properties to Promote Chemical Reactivity E. Bertel and P. Sand1 Max-Planck-Institut fur Plasmaphysik, Abt. Oberflachenphysik, EURATOM-Association, P.O. Box 1533, zyxw D-85740 Garching, Germany K.D. Rendulic and M. Beutl Institut fur Festkorperphysik, Technische Universitat Graz, Petersgasse zyxwv 16, A-8010 Graz, Austria zyxw Key Words: Catalysis / Epitaxy / Spectroscopy, Photoelectron / Surfaces One of the foremost objectives in the search for improved catalysts is the promotion of molecule dissociation, which in many catalytic reactions is the rate limiting step. It is shown that modification of electronic surface states by surface dopants allows to control the dissociation barrier. The relation of this new type of promotion mechanism to the effect of surfactants in metal epitaxy opens a new perspective in the search for both, promoters and surfactants. Many chemical reactions, which would proceed very slowly or not at all in the gas phase, take place readily if the reacting molecules are brought into contact with a suitably selected catalyst. This observation forms the basis of heterogeneous catalysis. In most catalytic reaction chains, dissociative adsorption of molecules is the rate limiting step. Considerable efforts have therefore been invested to promote dissociation by modifying catalysts in a suitable way. The results reported here shed new light on the mecha- nisms of catalytic promotion. Dissociative adsorption can be initiated in different ways. A molecule arriving from the gas phase may be reactive enough to form right away a chemical bond with the substrate. Usually, low lying, originally unoccupied molec- ular orbitals are partially populated in this process. If these orbitals are antibonding with respect to intramolecular bonds, the molecule is prone to dissociate. Thermal activa- tion or the interaction with a coadsorbate may then suffice to take the molecule from the chemisorbed precursor state over the energy barrier into the dissociatively chemisorbed state [ 1,2]. Such a case is sketched in Fig. 1 A. The molecule is adsorbed close to an alkali atom. The charge transfer A B from the alkali atom to the substrate causes a local electric field, which in turn enhances charge transfer from the substrate into an antibonding orbital of the molecule. Con- sequently, the barrier for dissociation is lowered. This mechanism of alkali promoted dissociation works only at close distance, because the electric field of the promoter atom is effectively screened by the substrate electrons [3]. It is straightforward to extend the model to the inhibition of catalytic reactions by electronegative adsorbates, which have the opposite effect on closeby adsorbed molecules. Dissociation of catalytically important molecules, such as H, or N,, however, has been observed to be hindered by alkali metals in some cases and to be promoted by oxygen. This is obviously due to a different mechanism, which we explore below. In H2 and N, the high energy of the lowest unoccupied orbitals prevents a facile population. Here the barrier is met before a chemisorption bond is established (Fig. 1 B). The approaching molecule is attracted to the surface by the long-range van-der-Waals force. Upon close approach the molecular charge density starts to overlap with the charge density of the substrate. The Pauli exclusion principle re- C Fig. 1 Different scenarios on a molecule’s way to dissociative adsorption: (A) A coadsorbed alkali metal sets up an electric field which helps to populate an antibonding orbital (shown in red) of the nearby chemisorbed molecule. (B) A molecule impinging on the catalyst surface experiences the Pauli repulsion as soon as the charge density of the molecule (green) and the substrate start to overlap. The outermost tail of the substrate charge density contains a considerable surface state contribution (shown in yellow, as opposed to the bulk state contribution, which is shown in blue). (C) Depopulation of the surface state reduces the repulsion and allows a closer approach to the surface, where the molecule interacts with the entire manifold of bulk states to form the (dissociative) chemisorption bond Ber: zyxwvutsrqpon Bunsenges. Pliys. Chem. 100, 114-118 (1996) No. 2 zyxwvutsr 0 VCH VeragsgeseNSchaft mbH, D-694Sl Weinheim, 1996 OOOS-9021/96/0202-0114 $ zyx 10.00+ ,2510