Oxygen assisted reconstructions of rhodium and platinum nanocrystals and their effects on local catalytic activity of hydrogenation reactions

Abstract

• Morphological changes of single nanoparticles occur during exposure to reactive gases. • The reconstructions are imaged operando by field emission techniques. • On rhodium, reconstructions have only a limited influence on the structure sensitivity of the O 2 –H 2 reaction at T > 500 K. • On platinum, the NO 2 –H 2 reaction occurs first on {0 1 1} facets, whereas (0 0 1) plane becomes active as the reconstruction proceeds. The reconstruction of rhodium and platinum crystals of some tens of nanometres diameter was investigated during the ongoing hydrogenation of oxygen atoms resulting from the dissociation of O 2 and NO 2 species. Field ion and field emission electron microscopies (FIM and FEM) were used to characterise the apex of tip samples before, during and after the catalytic reactions. On rhodium samples, the exposure of less than 10 Langmuir of O 2 is sufficient to induce significant morphological changes. At higher exposures, the presence of subsurface oxygen causes surface reconstructions illustrated with atomic resolution by FIM at 50 K. The same pattern is also visible at 505 K in the presence of H 2 and O 2 during water production. Upon the decrease of H 2 pressure, surface oxidation shows a strong sensitivity to the local surface initiated along the 〈0 0 1〉 zone lines. On platinum, the kinetic instabilities of the NO 2 –H 2 reaction are followed by FEM at 390 K starting from a hemispherical tip sample. The instabilities are expressed as surface explosions occurring randomly in time, but synchronised over {0 1 1} facets. These instabilities expand along the 〈0 0 1〉 lines over the (0 0 1) pole and exhibit self-sustained kinetic oscillations. The analysis of the tips by FIM after the reaction shows dark regions over the {1 1 3} facets, suggesting the extension of those to the detriment of vicinal ones. A well-controlled field evaporation procedure reveals that these regions appear dark due to the presence of surface oxygen. Structural reconstructions are observed but do not lead to the drastic morphological changes suggested by the FIM and FEM patterns. Nanoparticle dynamics must be accounted in models describing the non-linear features of catalytic reactions and more generally included in the description of catalytic properties of nanosized particles.