Catalysis in Frequency Space: Resolving Hidden Oscillating Minority Phases and Their Catalytic Properties

In situ catalysis studies typically correlate catalytic function and majority surface structures, but neglect that difficult-to-detect minority structures might govern catalysis. Here we use an oscillating CO:O2 gas composition to drive structural oscillations on a catalytically active Pd(100) surface and collect X-ray photoelectron spectroscopy data at high measurement frequency to demonstrate that the Fourier-transformed data selectively probe oscillations of minority surface structures and of the gas phase. Using the Fourier transform phase as well as work function shifts in the Fourier-transformed photoemission signal, we synchronize all signals and prove that most CO2 is produced above predominantly CO-covered areas.