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

Abstract

Carboxylic-acid adsorption on anatase TiO 2 is ar elevant process in many technological applications.Y et, despite several decades of investigations,t he acid-proton localization-either on the molecule or on the surface-is still an open issue.Bymodeling the adsorption of formic acid on top of anatase(101) surfaces,wehighlight the formation of ashort strong hydrogen bond. In the 0Klimit, the acid-proton behavior is ruled by quantum delocalization effects in asingle potential well, while at ambient conditions,t he proton undergoes ar apid classical shuttling in as hallowt wo-well freeenergy profile.T his picture,s upported by agreement with available experiments,s hows that the anatase surface acts like ap rotecting group for the carboxylic acid functionality.S uch an ew conceptual insight might help rationalizec hemical processes involving carboxylica cids on oxide surfaces.Atomistic insight of adsorbed ÀCOOH groups on TiO 2 is of key relevance in photocatalysis and environmental remediation processes. [1] Moreover,t he interaction of carboxylic groups with TiO 2 governs the anchoring of solar-cell sensitizers. [2] Also,TiO 2 catalyzes the solvent-free direct amidation of RÀCOOH groups with amines, [3] and amino-acid oligomerization in prebiotic conditions. [4] Atmospheric carboxylic acids show an impressive adsorption selectivity on rutile TiO 2 (110) with respect to alcohols,which are present in much higher concentrations than the carboxylic acids. [5] This behavior, proposed to affect self-cleaning and photocatalytic properties of TiO 2 ,was rationalized by the atomistic details of the dissociative adsorption of formate in abidentate mode. [5,6] Conversely,f or anatase,w hich is the preferred TiO 2 form in many applications,t he adsorption of small carboxylic acids still shows puzzling aspects. [7] This holds true also for the most stable (101) surface-the principal termination of anatase nanoparticles [7,8] -even for stoichiometric samples with alow density of defects.One of the main issues stems indeed from IRRAS and STM experiments dealing with the adsorption of HCOOH [9] and H 3 C À COOH, [10] respectively,o nn on-defective terminations of anatase(101) single crystals.Inboth cases, the features related to the HCOO/H 3 CÀCOO moieties pointed towards dissociative adsorption. However,n either IRRAS nor STM studies gave clear indications of the fate of the acid proton:n on(OH) signal was detected by IRRAS, and no surface OH groups were found by STM. So,asimple but fundamental question arises:where is the missing proton?Here we report that the acid proton is shared between the ÀCOO À group and asurface oxygen, and forms ashort strong hydrogen bond (SSHB). Thep roton behavior is ruled by quantum delocalization at low temperatures and by thermally activated shuttling at room temperature.T his picture, obtained from modeling,y et in line with experiments, suggests that the catalytic oxide surface acts as ap rotecting group of the Brønsted-acid functionality.

High photovoltaic performance (23.75) of triazatruxene-based dye-sensitized solar cells containing different π bridges: computational investigation

“…Generally, the bidentate chelation mode, in which two oxygen atoms are coupled to the same Ti atoms, is considered the most stable mode compared to the other modes. [40][41][42] Therefore, in this work, we used the bidentate bridged mode to achieve optimal coupling in the dye@(TiO 2 ) 9 system (see Fig. S3, ESI †).…”

Controlled synthesis of Bi- and tri-nuclear Cu-oxo nanoclusters on metal–organic frameworks and the structure–reactivity correlations

“…Major features have been observed at around 1380 cm -1 and 1570 cm -1 that can be assigned to the symmetric stretch of O-C-O, ν s (COO), and the asymmetric stretch of O-C-O, ν a (COO), respectively. 45 Notably, in 2CuO, we have also observed peaks at 2920 cm -1 and 2850 cm -1 which can be assigned to C-H stretch, ν(C-H), and a combination band consisting of asymmetric O-C-O stretch and in-plane C-H bend, ν comb = ν a (COO) + 𝛿(C-H), as discussed by Hayden et al ν a (COO) + 𝛿(C-H) cannot be observed individually as they belong to the irreducible representation of C 2v , which can only be attributed to the bidentate adsorption configuration of FA in the form of formate (*HCOO) on the surface. [46][47][48] In 1CuO, from the absence of ν(C-H), the adsorption configuration can be assigned to carboxyl (*COOH) on the surface.…”

Adsorption-Controlled Wettability and Self-Cleaning of TiO2

“…On the other hand, since the experimental study of the wetting behaviors on surfaces with controllable molecular adsorption state is still challenging, molecular simulation is a promising tool for quantitatively investigating such systems. For example, the adsorption state of water on the TiO 2 surface is also a long-standing topic, which has been studied by numerous theoretical simulations. − Selloni et al employed the first-principles calculations and molecular dynamics (MD) simulations to demonstrate that water molecules on various TiO 2 surfaces can adopt both dissociative adsorption (generated −OH groups) and molecular adsorption (in the H 2 O form) depending on the coverage. − Furthermore, surface defects, environmental conditions, surface treatment, and UV radiation can strongly increase the proportion of dissociated adsorbed water on TiO 2 surfaces. , The adsorption of carboxylic acids has also been confirmed by some theoretical studies. − According to these pioneer works, the hydrophilicity of TiO 2 is strongly affected by the species and adsorption state of surface molecules, and the adsorption of carboxylic acids could promote the self-cleaning effect of titanium dioxide. However, the quantitative relationship between the surface adsorption state and the wettability is still lacking, which might be the main reason for the debating of different wetting mechanisms of TiO 2 .…”