AVS 51st International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Paper SS1-TuM6
Iridium (210): Nanoscale Faceting and Structure-sensitivity of Surface Reactions

Tuesday, November 16, 2004, 10:00 am, Room 210B

Session: Catalytic Reactions: The Role of Surface Steps and Structure
Presenter: I. Ermanoski, Rutgers University
Authors: I. Ermanoski, Rutgers University
W. Chen, Rutgers University
T.E. Madey, Rutgers University
Correspondent: Click to Email
The stability of the atomically rough fcc Ir(210) surface depends on its cleanliness. The clean surface is planar and unreconstructed after heating in UHV up to 2100K. The oxygen-covered surface is, however, structurally unstable, and undergoes a dramatic faceting transformation: Nanometer-sized pyramids exposing {311} and (110) faces are formed upon deposition of more than 0.5 monolayers of oxygen and annealing to 600K, completely replacing the original planar surface. The oxygen-covered faceted surface reversibly reverts to planar at ~ 900K. LEED and STM experiments show that pyramid sizes increase with annealing temperature, and can reach ~25nm. This growth does not have an effect on the orientation of the facets. The complete thermal desorption of oxygen from iridium requires a temperature of ~1400K, and causes a complete destruction of the faceted structure. Nevertheless, by using catalytic CO oxidation (at ~550K) and reaction of H2 to form H2O (at ~400K), we have been able to routinely prepare a clean faceted surface in situ. Owing to the ability to prepare clean surfaces of different nano-scale structure (planar, faceted, varying facet sizes) from Ir(210), we have found it to be an excellent substrate for studying the kinetics of surface chemical reactions that depend on the surface structure. Our temperature programmed desorption (TPD) experiments show that thermal decomposition of acetylene, hydrogen recombinative desorption and ammonia decomposition all exhibit structure sensitivity, and proceed differently on the planar and faceted surfaces. Furthermore, differences are observed in reactions on clean faceted surfaces with different average facet sizes, showing that particle-size effects also play an important role in these reactions.