TIME-RESOLVED FTIR STUDY OF THE CATALYTIC CO OXIDATION UNDER PERIODICVARIATION OF THE REACTANT CONCENTRATION

Oxidation of CO over a palladium/zirconia catalyst obtained from an am orphous Pd25Zr75 precursor was investigated by time resolved FTIR spec troscopy. Sine wave shaped modulation of the reactant concentration, i .e., variation of CO or O-2 partial pressure, was used to induce dynam ic variations of the IR signals of product (CO2) and unconverted react ant (GO), which were detected in a multi-pass absorption cell. The pha se shift, phi, between external perturbation and variation of the CO2 signal was examined in dependence on temperature (100 less than or equ al 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 m odulation frequency (1.39 x 10(-4) less than or equal to omega less th an or equal to 6.67 x 10(-2) Hz). Comparison of modulation and step do wn experiments, performed by rapidly decreasing the CO inlet concentra tion, allows discrimination of parameter regimes characterized by diff erent reaction orders. From the phase shift values, an Eley-Rideal mec hanism is excluded, and the rate limiting step of the Langmuir-Hinshel wood 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.