Time resolved FTIR study of the catalytic CO oxidation under periodic variation of the reactant concentration

Oxidation of CO over a palladium/zirconia catalyst obtained from an amorphous Pd25Zr75 precursor was investigated by time resolved FTIR spectroscopy. Sine wave shaped modulation of the reactant concentration, i.e., variation of CO or O-2 partial pressure, was used to induce dynamic variations of the IR signals of product (CO2) and unconverted reactant (GO), which were detected in a multi-pass absorption cell. The phase shift, phi, between external perturbation and variation of the CO2 signal was examined in dependence on temperature (100 less than or equal to T less than or equal to 350 degrees C), partial pressure (1 x 10(-4) less than or equal to p(CO)/p(O2) less than or equal to 2), and modulation frequency (1.39 x 10(-4) less than or equal to omega less than or equal to 6.67 x 10(-2) Hz). Comparison of modulation and step down experiments, performed by rapidly decreasing the CO inlet concentration, allows discrimination of parameter regimes characterized by different reaction orders. From the phase shift values, an Eley-Rideal mechanism is excluded, and the rate limiting step of the Langmuir-Hinshelwood mechanism for the CO oxidation may be identified. Adsorption and possible surface movement of CO to the actual reaction site determines the rate of the CO oxidation on the palladium/zirconia catalyst used in our study.