NH3-NO Coadsorption System on Pt(111). I. Structure of the Mixed Layer

In the selective catalytic reduction (SCR) process, nitrogen oxides are selectively transformed to N2 by reductants such as ammonia. The specificity of this reaction on platinum-based catalysts was tentatively attributed to the formation of NH3-NO coadsorption complexes, as indicated by several surface science techniques. Here we combine scanning tunneling microscopy (STM) and density functional theory (DFT) calculations to characterize the NH3-NO complex at the atomic scale on the (111) surface of platinum, investigating the intermolecular interactions that tune the selectivity. In this first article, we analyze the structures that arise upon coadsorption of NH3 and NO in terms of adsorption sites, geometry, energetics, and charge rearrangement. An ordered 2 × 2 adlayer forms, where the two molecules are arranged in a configuration that maximizes mutual interactions. In this structure, NH3 adsorbs on-top and NO on fcc-hollow sites, leading to a cohesional stabilization of the extended layer, calculated to be 0.29 eV/unit cell. The calculated vibrational energies of the coadsorption structure agree with the experimental values found in the literature.