Reactions at well-defined surfaces

Oscillatory reactions on single crystal surfaces

Imaging of dynamic processes on surfaces by light

UNDERSTANDING of the mechanisms and kinetics of heterogeneous catalytic reactions has come largely from the study of gas–solid interactions on well defined single-crystal surfaces1,2. But real catalysts usually consist of nanometre-sized particles on which several different crystal planes are exposed. In general, it has been assumed that their properties can be regarded as a superposition of the contributions from each individual structural element. Here we show that this assumption may be invalid, even qualitatively, in certain cases. We have studied the oxidation of hydrogen on platinum surfaces at low pressure and room temperature. On a macroscopic Pt(lOO) single crystal the reaction reaches a steady state with a uniform distribution of adsorbates. But on the platinum tip of a field ion microscope, on which several different crystal planes are exposed, the reaction has a very different character. The tip contains a region of the (100) plane just 40 nm in diameter, on which the reaction rate displays sustained temporal oscillations. This effect is associated with continuously changing distributions of the adsorbed species in the form of propagating waves, which are generated by coupling of reactions occurring on adjacent crystal planes. This kind of interaction between different crystal planes may exert a profound influence on the kinetics of heterogeneous catalysis.

Coupling between adjacent crystal planes in heterogeneous catalysis by propagating reaction–diffusion waves

Nanosized 2D ceria islands randomly distributed over the Pt(111) surface have been prepared by oxidation of the nucleating Ce submonolayer and characterized using XPS and STM for coverages of 0.3 a...

CO Oxidation on a CeOx/Pt(111) Inverse Model Catalyst Surface: Catalytic Promotion and Tuning of Kinetic Phase Diagrams

The oxidation of carbon monoxide on platinum surfaces is investigated by field ion microscopy (FIM) and compared to earlier work by field electron microscopy (FEM). The reaction gas is used as the field ion imaging gas at pressures of < 10−4 mbar and at temperatures of the catalytic reaction. In FIM the surface is imaged by O2+-ions which are preferentially ionized at oxygen-covered surface areas. During temperature variations regions of bistability could be detected by FEM and FIM. Finally self-sustained isothermal oscillations of the catalytic CO-oxidation could be measured from the periodic fluctuations of the O2+-field ion current. Atomic resolution shows that the Pt-(331) planes are the pace makers of the oscillation.

V. Gorodetskii

Papers

Imaging the oscillating CO-oxidation on Pt-surfaces with field ion microscopy