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. 2022 Oct 3;27(19):6538.
doi: 10.3390/molecules27196538.

Effect of Formic Acid on the Outdiffusion of Ti Interstitials at TiO2 Surfaces: A DFT+U Investigation

Daniel Forrer  1   2 , Andrea Vittadini  1   2
Affiliations

Effect of Formic Acid on the Outdiffusion of Ti Interstitials at TiO2 Surfaces: A DFT+U Investigation

Daniel Forrer et al. Molecules. .

Abstract

Ti interstitials play a key role in the surface chemistry of TiO2. However, because of their elusive behavior, proof of their participation in catalytic processes is difficult to obtain. Here, we used DFT+U calculations to investigate the interaction between formic acid (FA) and excess Ti atoms on the rutile-TiO2(110) and anatase-TiO2(101) surfaces. The excess Ti atoms favor FA dissociation, while decreasing the relative stability of the bidentate bridging coordination over the monodentate one. FA species interact significantly with the Ti interstitials, favoring their outdiffusion. Eventually, Ti atoms can emerge at the surface forming chelate species, which are more stable than monodentate FA species in the case of rutile, and are even energetically favored in the case of anatase. The presence of Ti adatoms that can directly participate to surface processes should then be considered when formic acid and possibly carboxylate-bearing species are adsorbed onto TiO2 particles.

Keywords: density functional theory; formic acid; interstitial defects; titanium dioxide.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Side views for M and BB species at the rutile (110) surface (a,b) and at the anatase (101) surface (c,d). Blue: Ti; red: O; brown: C; white: H.
Figure 2
Figure 2
(a) Relative energy for an excess Ti atom at the TiO2(110) surface as a function of the occupied layer. Layer = 0 stands for adatom sites. Solid lines indicate A-type sites, dashed lines indicate B-type sites (see text). (b): lateral view of the slab, where A-type interstitial sites are shown as large grey spheres. U values in eV.
Figure 3
Figure 3
Adsorption energy at the TiO2(110) surface for formic acid in the monodentate form (M, solid line) and in the bridging bidentate form (BB, dotted line) as a function of the site for an excess TI atom (see text). BC indicates a formate chelating species. U values in eV.
Figure 4
Figure 4
Side sketches of Ti adatoms coordinated by a bidentate chelating formate anion at (a) the rutile surface and (b) at the anatase surface. Atom colors are the same as in Figure 1. The Ti adatom is shown as a green sphere. Distances in Å refer to calculations with U = 3.5 eV.
Figure 5
Figure 5
(a) Relative energy for an excess Ti atom at the a-TiO2(101) surface as a function of the occupied site, Tn. (b) Lateral view of the slab showing the positions of the examined interstitial sites as grey spheres. Colors are the same as in Figure 4. U values in eV.
Figure 6
Figure 6
Adsorption energy at the a-TiO2(101) surface for formic acid in the monodentate form (M, solid line) and in the bidentate form (BB, dotted line) as a function of the site for the excess Ti atom. BC indicates a bidentate chelating species where the excess Ti is an adatom.
See this image and copyright information in PMC

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