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 NOx (x = 1,2) compounds from lean exhaust. Current removal strategies rely in part on the catalytic oxidation of NO to NO2
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.
Prior experiments and theory indicate the catalysis is promoted at high O coverage (ÌåüO = NOPt ), 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.
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 ÌåüO, 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 pO2 , and the O coverage is governed by pNO2/pNO, leading to the observed rate inhibition by NO2 . These observations are in excellent agreement with experiment.
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.