Simulation of NO Oxidation Catalysis over Oxygen Covered Transition Metal Surfaces

Lean burn (excess O₂) automobile engines are more energy efficient than their stoichiometric or rich (O₂ starved) burn counterparts, but technologies do not exist to effectively remediate harmful NO_x (x = 1,2) compounds from lean exhaust. Current removal strategies rely in part on the catalytic oxidation of NO to NO₂<br><br> NO + 1/2O₂ ? NO₂.<br><br> Pt is the most active metal, but there is a strong drive to use less expensive materials. Understanding how Pt functions is a key step in catalyst design.<br><br> Prior experiments and theory indicate the catalysis is promoted at high O coverage (Ì_åü_O = N_O_Pt ), but too much O is inhibitive: Pt is prone to oxidative deactivation. The rate is promoted by high O₂ pressures and inhibited by product NO₂ . The latter is true even after correcting for approach to equilibrium, suggesting NO₂ hinders the reaction kinetics. <br><br>In this work, we attempt to understand these phenomena with molecular simulation. We use density functional theory, first principles thermodynamics, and mean field microkinetic modeling to elucidate the catalysis under actual reaction conditions. We find the reaction occurs at 0.25Ì¢‰âÂ'0.50 monolayer O. At these Ì_åü_O, the kinetics of O₂ dissociation (O₂ + 2* ? 2O*) are strongly inhibited due to repulsive interactions on the surface, but the OÌ¢‰âÂ'NO bond formation (NO* + O* Ì¢'Á' NO₂ + 2*) kinetics are facile. In contrast to prior reports, we show O₂ dissociation is rate limiting, and OÌ¢‰âÂ'NO bond formation is equilibrated. The rate is strongly dependent on p_O2 , and the O coverage is governed by p_NO2/p_NO, leading to the observed rate inhibition by NO₂ . These observations are in excellent agreement with experiment. <br><br>We apply our models to other transition metals and transition metal alloys to facilitate new catalyst design. Analysis indicates such materials should exhibit nearly identical behavior to Pt, offering no improvements in rate or propensity to oxidize. Screening the catalytic properties of Au nanoparticles and the O buffering properties of Co₃O₄/metal oxide supports is recommended for future work.