NOVEL METHOD FOR STUDYING THE KINETICS OF SURFACE-REACTIONS AT SUBMONOLAYER EXPOSURES - DECOMPOSITION OF ACETIC-ACID ON PALLADIUM FOIL

The experimental set-up for relaxation spectroscopy with a newly constructed time-of-fright mass spectrometer (ToF-MS) is described. The response to single gas pulses is measured with the ToF-MS, which is equipped with a special storing ion source and an electrostatic ion reflector. The gas pulses, with rise and fall times of less than 2 ms, are generated by a solenoid valve in a differentially pumped system. Conventional ToF instruments with ionization by electron pulses detect only a small fraction of the incoming molecules. Therefore our instrument with the storing (integrating) ion source is more sensitive, typically by a factor of 1000. Pulses of deuterated acetic acid (AcOD) with a length of 20 ms are used to produce exposures of 1/1000 monolayer (ML) per pulse on a polycrystalline Pd foil. On clean Pd the formation of CO2 is observed. An activation energy of 0.78 eV and a pre-exponential factor of 3 x 10(8) s-1 are derived from the transients in the temperature range between 550 and 650 K. After a total dose of 1 ML of AcOD the CO2 signal ceases and the AcOD signal emerges, yielding an activation energy of 0.44 eV and a pre-exponential factor of 9 x 10(8) s-1. It is shown that even exposure to small doses of AcOD decreases the efficiency of CO2 formation. After a dose of 2 ML the CO2 formation is completely suppressed. To determine the energy and the pre-exponential factor for the investigated surface reaction, an exposure dose of less than 1/100 ML is sufficient.