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

Abstract

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

Mechanism of ammonia decomposition on clean and oxygen-covered Cu (1 1 1) surface: A DFT study

“…Especially, periodic DFT calculations with the slab model have been a powerful approach to study the adsorption and dissociation of small molecules on the metal surfaces. Much attention has been paid to ascertaining the adsorption and dehydrogenation mechanism of NH 3 on numerous metal surfaces, such as Fe [11,12], Co [12], Ni [13][14][15][16], Ir [5,[17][18][19][20][21][22], Cu [23,24], Pd [25][26][27], Pt [28][29][30][31][32], Au [33]. Most of these works were based on density functional theory (DFT) http calculations.…”

NH3–NO Coadsorption System on Pt(111). II. Intermolecular Interaction

“…In Table 1 we summarize the most significant structural parameters and other relevant data obtained from DFT calculations for the triangular unit, in comparison with the 2 × 2 mixed layer and the constituent molecules, whose data are reported from the preceding article. 4 The main structural parameters for NO in the triangular unit (namely, the N−O distance d N−O and the height h N of N with respect to the average height of the surface Pt atoms) are in between the values for the 2 × 2 α-NO and mixed NH 3 −NO layers. At variance with the N−O distance, which slightly elongates upon coadsorption, the characteristic distances of NH 3 do not change.…”

“…The details concerning the STM experiments can be found in the preceding article. 4 Theoretical. DFT calculations were performed with the plane-wave-based suite QUANTUM ESPRESSO, 11 as detailed in the preceding article.…”

“…DFT calculations were performed with the plane-wave-based suite QUANTUM ESPRESSO, 11 as detailed in the preceding article. 4 To get more insight concerning the intermolecular bonding, for some selected structures we have also computed the maximally localized Wannier functions (MLWFs). 12 Essentially they consist of a change of the basis set, passing from Bloch functions built on plane waves, not localized in real space, to a superposition of localized Wannier functions.…”

“…Rather, we define the cohesion energy E coh of the triangular unit, in analogy to eq 2 of the preceding article, 4 as the difference between the adsorption energy of the unit itself and those of ammonia and nitric oxide separately adsorbed in the same adsorption sites a Columns refer to NO and NH 3 individually adsorbed in 2 × 2 unit cells (constituents), to the same molecules coadsorbed in the triangular NH 3 − 3NO isolated unit in a 4 × 4 cell, and to the same molecules coadsorbed in the 2 × 2 mixed layer. Symbols denote: h height of an atom with respect to the average height of surface atoms; d distance between two atoms; α angle between three atoms; Δn difference of Loẅdin charges with respect to the gas-phase molecules frozen in the adsorbed geometry; Since there are three NH 3 −NO bonds both in the triangular unit and in the 2 × 2 unit cell of the mixed layer, the cohesion energy for the two cases is directly comparable and, divided by three, gives the strength of the NH 3 −NO bond in the two structures.…”