Imaging and chemical probing on the atomic scale: reconstruction and dynamics of the systems O-2/Rh and NO2-H-2/Pt

This paper presents two case studies of adsorbate-induced surface reconstru ction. on the one hand, and dynamical reaction imaging along with local che mical probing, on the other hand. The first one deals with the oxygen-induc ed reshaping of 3D Rh crystals. Field ion microscopy (FIM) was applied to i mage in real-space the change from a nearly hemispherical shape in the abse nce of oxygen toward a polyhedral one in the presence of oxygen. Shape tran sformation occurs at temperatures of 380-550 K and is associated with the a ppearance of facets with {111} and {001} orientation. The only high-index p lanes present in the polyhedral form are of {137} symmetry, (1 x 2) and (1 x 3) missing-row reconstructions appear in the {113} and {011} planes. The polyhedral form has also been imaged under in situ conditions of the oxygen -hydrogen reaction on Rh at 505 K. The second case study deals with kinetic non-linearities occurring in the NO2 reaction with hydrogen on the surface of a 3D Pt crystal reconstructed to a top- and edge-truncated pyramid. The reaction was found to ignite in the {012} corner planes of the crystal. On e-dimensional wavefronts were subsequently observed to move along the {211} zone lines. These studies were performed by video-FIM and could be correla ted with a local chemical analysis by time-of-flight mass spectrometry of i onised species. The mass spectrum provided information on water product (H2 O+ and H3O+) and NO intermediate formation. Strong fluctuations in the NO2 current indicated the occurrence of NO2 surface diffusion. These species a re most likely responsible for the field ion image formation.