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Abstract

The rates of chemical reactions may exhibit temporal oscillations, even when the reaction parameters are kept constant. These oscillations might be associated with spatial pattern formation owing to local variations of concentrations or temperature, or both, which propagate as chemical waves^1,2. An example is the catalytic oxidation of carbon monoxide on the surfaces of platinum single crystals. The mechanism of this reaction is well known, and under isothermal, low-pressure conditions, many types of temporal self-organization, ranging from harmonic oscillations to chaotic behaviour, have been observed^3–6. Spatial patterns have also been demonstrated by scanning beams of either electrons³ or ultraviolet photons⁶ across the surface, and by monitoring the electrons reflected or photoemitted, respectively. However, both scanning techniques are hampered by limited temporal as well as lateral resolution. Here we present observations of an unexpectedly rich variety of self-sustained spatio-temporal patterns by continuous imaging (on a timescale of ≳1 ms) of the surfaces using a newly developed, photoemission electron microscope⁷ with a spatial resolution of a few micrometres. Pattern evolution can be followed on a much finer timescale than is available from other imaging techniques. We observe a new type of structure with strong similarities to those found in the Belousov–Zhabotinsky oscillatory reaction.