The influence of a cycled gas feed on reactions of the automotive catalytic converter has been investigated under low pressure conditions ( p < 10 −3 mbar) with a Pt(100) surface as catalyst. Catalytic NO reduction with CO or H 2 as reducing agent was studied in binary mixtures. The reactions were forced around the unstable high rate branch of the rate vs. T hysteresis by applying a square-wave modulation of p CO ( p H2 ). For the NO + CO reaction the yield vs frequency dependence exhibits a pronounced maximum. The pressure modulation causes a periodic structural transformation 1 × 1 ↔ hex. The frequency dependence of the yield is largely related to the average concentration of defects which are created during the phase transition.