ELSEVIER Surface Science 360 (1996) 255 260 surface science The coadsorption of hydrogen and potassium on Ir(111) M. Schick, J. Lauterbach, W.H. Weinberg * Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106, USA Received 28 December 1995; accepted for publication 4 March 1996 Abstract The adsorption of gas-phase atomic hydrogen on potassium-precovered Ir(111) surfaces was investigated. Even very low coverages of potassium adatoms strongly inhibit the dissociative adsorption of molecular hydrogen. However,using gas-phase atomic hydrogen allows us to overcome the activation barrier for dissociative hydrogen adsorption. In addition, abstraction of hydrogen adatoms by impinging atomic hydrogen occurs. The probabilities and cross-sections for both reactions and the maximum number of hydrogen adsorption sites are derived and compared to data obtained on other surfaces. Furthermore, a kinetic isotope effectin the desorption of hydrogen and deuterium was observed. Implications of these results with respect to the potassium hydrogen interaction are discussed. Keywords: Adsorption kinetics; Alkali metals; Atom-solid reactions; Deuterium; Hydrogen; Iridium; Solid-gas interfaces; Thermal desorption spectroscopy 1. Introduction In recent years, the coadsorption of potassium with various gases has been studied on a wide variety of transition metal surfaces. Our work was motivated by the promoter properties of potassium in many catalytic reactions, in particular in Fischer~Tropsch synthesis and ammonia synthesis [1,2]. Many investigations have focused on the CO/K and N2/K coadsorption systems on trans- ition metals I-3-5]. However, only in relatively few cases has the influence of potassium on the dissoci- ative adsorption kinetics of hydrogen, the other reactant in Fischer-Tropsch and ammonia synthe- sis, been characterized [6-13]. In general, the influence of potassium adatoms * Corresponding author. Fax: + 1 805 8934731; e-mail: chari@engineering.ucsb.edu. on the dissociative adsorption of hydrogen is char- acterized by a large decrease in the adsorption probability. This behavior can be explained by an increased electron density in the metal due to electron donation from the electropositive potas- sium adatoms. This causes a large work-function decrease, enhancing the Pauli repulsion between the lo bonding level of the impinging H 2 molecules and the metal surface [7]. In addition, hydrogen generally desorbs at higher temperatures in the presence of potassium as compared to the clean surface, i.e. adsorbed hydrogen is stabilized by the presence of potassium on the surface. In this work, the potassium-precovered Ir(111) surface was exposed to gas-phase atomic hydrogen and deuterium in order to overcome the potas- sium induced activation barrier for dissociative hydrogen adsorption [ 11 ]. This approach permits not only study of the hydrogen-potassium inter- 0039-6028/96/$15.00 Copyright © 1996 ElsevierScience B.V. All rights reserved PII S0039-6028 ( 96 ) 00666-8