AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM2
Dynamic Imaging and Chemical Probing on the Atomic Scale: NO/H@sub 2@ and NO@sub 2@/H@sub 2@ on Pt

Monday, October 2, 2000, 8:40 am, Room 208

Session: Atomic-Scale Studies of Model Catalysts
Presenter: N. Kruse, Universit@aa e@ Libre de Bruxelles, Belgium
Authors: T. Visart de Bocarm@aa e@, Universit@aa e@ Libre de Bruxelles, Belgium
T. Baer, Universit@aa e@ Libre de Bruxelles, Belgium
C. Voss, Universit@aa e@ Libre de Bruxelles, Belgium
N. Kruse, Universit@aa e@ Libre de Bruxelles, Belgium
Correspondent: Click to Email
The reaction of NO and NO@sub 2@ with hydrogen has been studied on a Pt single crystal using field ion microscopy (FIM) and atom-probe field ion mass spectrometry (APFIMS) under conditions of applied temperatures (395-530 K), pressures (10@super -4@ - 10@super -2@ Pa) and field strength (8.7 V/nm). The 3D-Pt sample ("field emitter tip") has been found to undergo a transformation from a nearly hemispherical shape into a top- and edge-truncated pyramid. Kinetic instabilities, including rapid ignition and oscillations, were found for both reactions on this reshaped crystal. Video-FIM (time resolution: 20ms) revealed the rough areas lying between the (001) top plane and the (111) slopes of the pyramid to be catalytically most active. Catalytic ignition to water production was seen to occur in (012) areas with subsequent wave propagation either predominantly along the <100> (for NO/H@sub 2@) or along the <211> zone lines (for NO@sub 2@/H@sub 2@). Thus in either case a strongly anisotropic reaction behavior was observed. Under oscillatory conditions the catalytic cycles lasted for several 100ms only. The local chemical analysis of (012) and (133) planes (comprising several ten Pt atoms) demonstrated the formation of water product (desorption as H@sub 2@O@super +@ and H@sub 3@O@super +@) during the catalytic cycles. Major amounts of NO@sub x@@super +@ were registered due to surface diffusion of either NO or NO@sub 2@ into sites emptied by either reaction. Moreover, the mass spectrometric results strongly suggest NO@sub x@@super +@ to be the imaging species at otherwise rather low field strengths.