IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA10
The Structures and Phase Transformations of CO and NO on Rh(111) in the Torr Pressure Range Studied by Scanning Tunneling Microscopy

Thursday, November 1, 2001, 5:00 pm, Room 121

Session: Catalysis on Model Systems
Presenter: K.S. Hwang, University of California, Berkeley
Authors: K.S. Hwang, University of California, Berkeley
K.B. Rider, University of California, Berkeley
M. Salmeron, Lawrence Berkeley National Laboratory
G.A. Somorjai, Lawrence Berkeley National Laboratory
Correspondent: Click to Email
Using scanning tunneling microscopy (STM) in a high-pressure reactor cell, we have studied, for the first time, the molecular structure and reaction of CO and NO on Rh(111) in the Torr pressure range. This is a model system for the automobile catalytic converter, where CO is oxidized to CO@sub 2@ and NO is reduced to N@sub 2@. Numerous reaction studies have been done in various temperature and pressure regimes,@footnote 1@ but they generally do not yield direct information about molecular surface structure. Traditionally, molecular surface structure studies have been done at low temperature and pressure. These structures are kinetically frozen however, and may be different from high-temperature, high-pressure structures that are in equilibrium with the gas phase. At high coverage, CO forms a (2x2)-3CO structure on Rh(111) with one top-site and two hollow-site molecules in the unit cell.@footnote 2@ This structure forms at 300 K from low pressure to at least atmospheric pressure. NO at 300 K forms an analogous structure below 0.03 Torr. Above 0.03 Torr we have discovered a new structure with a (3x3) unit cell. By directly observing the phase transformation between the two structures, we have found the heat of adsorption of the new structure to be 0.9 eV and an energy barrier between the two structures of 0.7 eV. When CO and NO are coadsorbed on Rh(111) at low partial pressures of NO, NO appears to mix randomly among the CO molecules. As the partial pressure of NO increases, the NO segregates into islands. The (3x3)-NO structure nucleates on these islands, though the presence of CO on the surface inhibits the phase transition until the NO partial pressure is three to five times that of CO. At temperatures above 300 K, we have seen evidence of the reaction between CO and NO occurring. @FootnoteText@ @footnote 1@ V. P. Zhdanov and B. Kasemo, Surf. Sci. Rep. 29 (1997) 31. @footnote 2@ M. A. Van Hove, R. J. Koestner, and G. A. Somorjai, Phys. Rev. Lett. 50 (1983) 903.