REMPI as a tool for studies of OH radicals in catalytic reactions

The catalytic reaction H-2 + 1/2O(2) --> H2O on poly crystalline Pt foils has been studied by detection of desorbing OH radicals using the Resonance Enhanced Multiphoton Ionization, REMPI, technique. The measurements were performed at catalyst temperatures of 1000-1400 K and a total pressure below 10(-4) mbar. The studies of OH desorption by REMPI were achieved using a two-photon excitation D-2 Sigma(-)-X-2 Pi (1-0), followed by one-photon ionization. The ions were detected in a time-of-flight mass spectrometer, TOE-MS, in order to avoid interference from non-resonantly ionized molecules. By applying TOF-MS, a simultaneous nonresonant ionization and detection of H-2, O-2 and H2O was achieved. Recorded REMPI spectra were compared with spectra simulated using known molecular constants. The kinetics of the reaction derived from the measurements were compared with what was obtained in earlier LIF detection of OH, performed at higher total reactant pressure using the A-X transition. REMPI TOE-MS is shown to be a complement to LIF for reaction studies below 5 x 10(-4) mbar total pressure, where LIE is too insensitive for quantification. The reaction kinetics was found to be in agreement with a theoretic model and previous LIF studies.