Coupled catalytic oscillators: CO oxidation over polycrystalline Pt

We have observed unusual oscillations in the rate of CO oxidation over a polycrystalline Pt wire loop. In contrast to simple oscillations between states of low and high reactivity, the unusual oscillations occurred between low, intermediate, and high values of the reaction rate. Experiments were performed at atmospheric pressure in a gas flow reactor over a range of gas temperatures and CO partial pressures. The reaction rate was measured using a thermocouple spotwelded to the loop. For a given CO partial pressure, oscillations occurred over a range of gas temperatures. Oscillations were simple at high gas temperatures and became complex as the temperature was lowered. When we monitored two thermocouples on opposite sides of the loop, we found that oscillations on the two sides occurred in phase, but differed in details of shape and amplitude. When we cut the loop and separated the halves, the complex oscillations at lower gas temperatures disappeared, and oscillations on the half-loops occurred with different frequencies. Results obtained by varying the separation showed that coupling between the half-loops occurred primarily through the gas phase. When the loop was spotwelded back together, oscillations became resynchronized, and complex waveforms reappeared at lower gas temperatures. To our knowledge, this is the first experimental demonstration that coupling chemical oscillators can increase the parameter space over which oscillatory instabilities occur.