Topics in Catalysis Vol. 14, No. 1-4, 2001 35 Imaging and chemical probing on the atomic scale: reconstruction and dynamics of the systems O 2 /Rh and NO 2 –H 2 /Pt T. Visart de Bocarmé and N. Kruse ∗ Université Libre de Bruxelles, Chimie Physique des Matériaux, Campus Plaine CP243, B-1050 Bruxelles, Belgium E-mail: nkruse@ulb.ac.be Dedicated to Professor Gabor Somorjai on the occasion of his 65th birthday This paper presents two case studies of adsorbate-induced surface reconstruction, on the one hand, and dynamical reaction imaging along with local chemical probing, on the other hand. The first one deals with the oxygen-induced reshaping of 3D Rh crystals. Field ion microscopy (FIM) was applied to image in real-space the change from a nearly hemispherical shape in the absence of oxygen toward a polyhedral one in the presence of oxygen. Shape transformation occurs at temperatures of 380–550 K and is associated with the appearance of facets with {111} and {001} orientation. The only high-index planes present in the polyhedral form are of {137} symmetry. (1 × 2) and (1 × 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 NO 2 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. One-dimensional wavefronts were subsequently observed to move along the 〈211〉 zone lines. These studies were performed by video-FIM and could be correlated with a local chemical analysis by time-of-flight mass spectrometry of ionised species. The mass spectrum provided information on water product (H 2 O + and H 3 O + ) and NO intermediate formation. Strong fluctuations in the NO + 2 current indicated the occurrence of NO 2 surface diffusion. These species are most likely responsible for the field ion image formation. KEY WORDS: atom-probe field ion microscopy; hydrogen; oxygen; nitrogen dioxide; rhodium; platinum 1. Introduction The structure of metal surfaces is closely related to their chemical reactivity. Thus it might be suspected, in an unso- phisticated approach, that with a given arrangement of sur- face atoms a chemical reaction would run similar to check- ers moving on a checkerboard. The situation is much more complicated, however. Rather than being fixed in certain po- sitions, surface atoms may become mobile in the presence of adsorbed species. The displacement of surface atoms or metal–adsorbate complexes can lead to reconstruction forms which differ significantly from the bulk-truncated forms. Metal surfaces undergoing reconstruction have been coined “flexible surfaces” by Gabor Somorjai, one of the great pi- oneers in the surface science approach to heterogeneous ca- talysis. An excellent review of metal reconstruction under either clean or adsorbate-covered conditions was given by Somorjai and Van Hove [1]. The invention of scanning tunneling and atomic force mi- croscopy (STM and AFM) has considerably contributed to developing a proper understanding of the atomic structure of surfaces. Most of the work was so far executed with 2D ex- tended single crystal surfaces. A prominent example of ma- jor reconstruction is the Pt(110) crystal face which forms mi- crofacets of different orientation in the presence of oxygen at atmospheric pressure and at 425 K [2,3]. It is remarkable that the oxygen-induced structural features can be replaced ∗ To whom correspondence should be addressed. by missing-row reconstructions in the presence of hydrogen. Much attention was also devoted to the oxygen/Rh(110) sys- tem. Murray et al. [4] have demonstrated the occurrence of (1 × 2) missing row reconstructions for this system. Inter- estingly, not every second row of atoms along the [110] di- rection is missing on that surface plane but, depending on the oxygen coverage, only every third, fourth or fifth row is absent. In heterogeneous catalysis with metals we are usually concerned with small 3D particles on a support material. Again, local probe methods can be used to make visible changes in the particle size or dispersion due to the presence of adsorbates or the occurrence of a catalytic reaction [5–7]. Details of the particle morphology and the atomic-scale structure of individual planes are not easily accessible, how- ever. With this background it seems that other local probe methods such as field ion microscopy (FIM) gain signifi- cance. It has been demonstrated during the past years that FIM can provide atom-scale structural information [8,9] of metal particles given the form of tips. Indeed, a tip can be considered an excellent model of a single catalyst grain in the absence of any insulating support material. One of the most attractive features of the FIM method is that it can be quite easily combined with a time-of-flight mass spectrome- ter to construct a local chemical probe [10–12]. With such a device available we can perform experiments which touch on one of the ultimate goals in the fundamental research of het- erogeneous catalysis. Gabor Somorjai writes [3]: “We need 1022-5528/00/1200-0035$18.00/0  2000 Plenum Publishing Corporation