The Case of Formic Acid on Anatase TiO2 (101): Where is the Acid Proton?

Gloria Tabacchi; Marco Fabbiani; Lorenzo Mino; Gianmario Martra; Ettore Fois

doi:10.1002/anie.201906709

The Case of Formic Acid on Anatase TiO₂ (101): Where is the Acid Proton?

Angew Chem Int Ed Engl. 2019 Sep 2;58(36):12431-12434. doi: 10.1002/anie.201906709. Epub 2019 Aug 7.

Authors

Gloria Tabacchi¹, Marco Fabbiani², Lorenzo Mino², Gianmario Martra², Ettore Fois¹

Affiliations

¹ Department of Science and High Technology, University of Insubria and INSTM, via Valleggio 9, I-22100, Como, Italy.
² Department of Chemistry and Nanostructured Interfaces and Surfaces NIS interdepartmental centre, University of Torino, via P. Giuria 7, I-10125, Torino, Italy.

PMID: 31310450
DOI: 10.1002/anie.201906709

Abstract

Carboxylic-acid adsorption on anatase TiO₂ is a relevant process in many technological applications. Yet, despite several decades of investigations, the acid-proton localization-either on the molecule or on the surface-is still an open issue. By modeling the adsorption of formic acid on top of anatase(101) surfaces, we highlight the formation of a short strong hydrogen bond. In the 0 K limit, the acid-proton behavior is ruled by quantum delocalization effects in a single potential well, while at ambient conditions, the proton undergoes a rapid classical shuttling in a shallow two-well free-energy profile. This picture, supported by agreement with available experiments, shows that the anatase surface acts like a protecting group for the carboxylic acid functionality. Such a new conceptual insight might help rationalize chemical processes involving carboxylic acids on oxide surfaces.

Keywords: density functional calculations; molecular dynamics; surface chemistry; vibrational spectroscopy; zero-point energy.