DFT-based characterization of the multiple adsorption modes of nitrogen oxides on Pt(111)

Pt is the most common catalyst for NO oxidation to NO2, a key reaction in NOx remediation chemistry. In this work, density functional theory calculations and plane-wave supercell models are used to calculate the energies, charge distributions, and vibrational spectra of the stable and metastable states of adsorbed NO, NO2, and NO3 on Pt( 111), the most likely active metal face for this catalytic oxidation. NO, NO2, and NO3 are all strong electron acceptors and bind to the Pt( 111) surface via charge donation from the surface. NO and NO2, in particular, exhibit a variety of adsorption geometries, the most favorable at low coverage being those that maximize surface-adsorbate charge transfer through binding to multiple surface Pt. At low coverage, the order of binding energies is NO > NO3 > NO2, and the oxidation of adsorbed NO to NO2 is endothermic by 0.78 eV. Higher surface coverages favor migration of NO and NO2 to lower-coordination surface sites due to competition for metal d charge density. These changes in surface binding configurations, along with the general decrease in surface-adsorbate bond energies associated with higher surface coverages, both tend to energetically promote NO conversion to NO2 and are important in describing this catalytic chemistry.