Vibrational Properties of CO Adsorbed on Au Single Atom Catalysts on TiO2(101), ZrO2(101), CeO2(111), and LaFeO3(001) Surfaces: A DFT Study

Abstract

The nature and local environment of Au single atoms supported and stabilized on four different oxides is studied by means of DFT + U calculations using CO as probe molecule and its stretching frequency, ωe, as a fingerprint of the site where the Au atom is bound. Four oxides are considered, anatase TiO2, tetragonal ZrO2, cubic CeO2, and a perovskite LaFeO3. In this latter case a recently reported experimental study has detected a stretching mode for CO adsorbed on Au1/LaFeO3 of 2215 cm−1, with a large blue shift, ∆ω(CO) = 72 cm−1 with respect to free CO. In order to identify the Au adsorption site that can give rise to this large blue-shift we have considered five cases: (a) Au replacing a lattice cation, (Au)subM; (b) Au replacing a lattice O anion, (Au)subO; (c) Au adsorbed on the surface, (Au)ads; (d) Au bound to an extra O atom on the surface, (AuO)ads, or (e) Au bound to two extra O atoms on the surface, (AuO2)ads. It turns out that the correct reproduction of ∆ω for CO adsorbed on positively charged gold, Auδ+, is challenging for DFT. Therefore, we have performed a comparative study of Auδ+-CO molecular compounds for which ωe(CO) is known experimentally using various kinds of DFT functionals and accurate CCSD and CCSD(T) quantum chemistry methods. Also based on this comparison we propose a tentative assignment for the observed frequency of CO adsorbed on Au1/LaFeO3 single atom catalyst. Graphic Abstract

A Few Questions about Single-Atom Catalysts: When Modeling Helps

“…The resulting spectra can be used to verify the predictions of DFT calculations. When this is the case, this is strong proof that the structure of the SAC used in the calculations coincides with or is very close to that present on the real samples. − …”

Atomic‐level insights into single Pt atom catalysts on CeO2 support

“…A plane wave basis set with a cutoff energy of 450 eV was applied for all calculations. [ 16 ] The geometrical structures were optimized and converged with the ionic forces threshold of less than 0.01 eV/Å.…”

Modeling Single‐Atom Catalysis

“…A comparison of standard and hybrid DFT functionals with high‐level quantum chemistry methods has shown the difficulty of advanced DFT approaches to reproduce quantitatively the large blue shifts of the C─O vibrational frequency observed in the experiments, a problem that emerges when the CO molecule is bound to Au δ + atoms in a positive oxidation state. [ 51 ]…”

“…[44,45] Recently we have shown that the combination of Fourier transform infrared spectroscopy and thermal desorption spectroscopy of adsorbed probe molecules with DFT calculations can be extremely powerful to identify the bonding environment of a metal atom. [46][47][48][49][50][51] CO is widely used to probe the nature of a surface site thanks to its high sensitivity and low reactivity. The combined measurement of CO vibrational properties and adsorption energy, and the comparison of these quantities with DFT calculations provides a powerful way to unambiguously identify the nature of the active sites on a solid surface.…”