Time-resolved surface reaction kinetics in the pressure gap

Abstract

We extend the use of our recently developed Near-Ambient Pressure Velocity Map Imaging (NAP-VMI) technique to study the kinetics and dynamics of catalytic reactions in the pressure gap. As an example, we show that NAP-VMI combined with molecular beam surface scattering allows the direct measurement of time- and velocity-resolved kinetics of the scattering and oxidation of CO on the Pd(110) surface with oxygen pressures at the surface up to 1 × 10⁻⁵ mbar, where different metastable surface structures form. Our results show that the c(2 × 4) oxide structure formed at low O₂ pressure is highly active for CO oxidation. The velocity distribution of the CO₂ products shows the presence of two reaction channels, which we attribute to reactions starting from two distinct but rapidly interconverting CO binding sites. The effective CO oxidation reaction activation energy is E_r = (1.0 ± 0.13) eV. The CO₂ production is suppressed at higher O₂ pressure due to the number of antiphase domain boundaries increasing, and the missing row sites are filled by O-atoms at O₂ pressures approaching 1 × 10⁻⁶ mbar. Filling of these sites by O-atoms reduces the CO surface lifetime, meaning the surface oxide is inactive for CO oxidation. We briefly outline further developments planned for the NAP-VMI and its application to other types of experiments.