Surface Reactions Investigated at the Nanoscale by Field Emission Techniques: Nonlinear Dynamics of the Catalytic Hydrogenation of NO and NO2 Over Platinum Crystallites

Abstract

The catalytic hydrogenation of nitrogen oxide (NO and NO2) over platinum nanocrystals is investigated with nanoscale lateral resolution using video-field emission microscopy (field electron microscopy (FEM) and field ion microscopy (FIM)). The ongoing reaction is imaged at the extremity of sharp tip samples mimicking size and morphology of catalyst nanoparticles. The results show a strong sensitivity of the reaction to local surface structures. Fast transitions from nonreactive to reactive conditions, reminiscent of “surface explosions,” are observed. On (001)-oriented Pt samples, such transitions are mainly triggered along zone-lines involving {011}, {012} and kink sites around {001} facets. Explosions are spontaneously repeated without external modifications of the control parameters. FIM investigations demonstrate oscillatory-type behavior with irregular periods for the NO+H2 reaction. On the other hand, NO2 hydrogenation, investigated by FEM, shows highly regular surface explosions. Further data analyses indicate that this system bears the features of a noisy chemical oscillator with asymmetric peaks characteristic of relaxation-type oscillations. Surface reconstructions ranging from local surface atom displacements to the transformation of the hemispherical into a polyhedral crystal shape may be observed using FIM. Nonlinear behaviors occur on reconstructed samples as well as on nonreconstructed ones.