On the Simple Complexity of Carbon Monoxide on Oxide Surfaces: Facet‐Specific Donation and Backdonation Effects Revealed on TiO₂ Anatase Nanoparticles

Abstract

Atomic-scale relationships between the structure of TiO2 surfaces and the physicochemical properties of surface sites, functional for titania-based applications, can be obtained from IR spectroscopy by using carbon monoxide (CO) as a molecular probe. In the literature, it is reported that strongly unsaturated cationic Ti sites (Lewis acid), which are important for reactivity, should cause a large upshift of the CO stretching frequency. By using IR spectroscopy of CO on TiO2 nanomaterials and theoretical analyses, here this model is challenged. It is shown that the stretching frequency of adsorbed CO results from a facet-dependent and synergic CO-surface donation (upshift) - surface-CO backdonation (downshift) mechanism. These results imply that the interaction of adsorbed molecules with the Ti centers is tuned by the surface oxygen atoms of the first coordination sphere, which play an active role as indirect electron density donors (Lewis base).

Unravelling the High-Pressure Behaviour of Dye-Zeolite L Hybrid Materials

“…Pseudopotentials and basis set expansion were the same adopted in [56]. Such a computational approach has demonstrated to provide a satisfactory description of diverse systems involving adsorbate-surfaces [108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] or host-guest interactions [123][124][125][126][127][128][129][130][131][132][133][134] including also processes at zeolite interfaces [32,59] and high pressure conditions [97,[135][136][137][138][139][140].…”

Few-Layer MoS2 Nanodomains Decorating TiO2 Nanoparticles: A Case Study for the Photodegradation of Carbamazepine

“…First of all, the main features are due to the typical adsorptions of CO on the different Ti sites of the TiO 2 surface. Specifically, the intense main peak centered at 2178 cm −1 can be explained with the building up of parallel CO oscillators interacting with five-fold coordinated Ti 5c 4+ sites located on flat (101) surfaces [48], while the one at 2155 cm −1 is explained with hydrogen interactions of CO with residual OH groups, even present after the thermal treatment [25,27,48]. It is noteworthy that, even if not reported in Figure 4, for the sake of simplicity, the previously discussed 2178-cm −1 band shifts to higher frequencies, when CO pressure is decreased, due to the changes of the lateral interactions between adjacent CO molecules on the surface of TiO 2 .…”

“…Finally, the weak band at 2208 cm −1 is assigned to CO adsorbed on Ti Lewis acidic defective sites, such as steps, corners and edges, with higher coordinative unsaturations [48]. …”

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

“…[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.…”

“…Hence,w e explored different anchoring geometries for HCOOH with first-principles molecular dynamics (FPMD), [11,12] as thermal effects play akey role at the molecule-material interfaces. [8,13] Thestarting point was astructure proposed on the basis of IRRAS spectra, [9] depicting the dissociation of HCOOH as am onodentate HCOO À bonded to as urface Ti (Ti s )a nd ap roton transferred to an oxygen atom of the surface (O s ), not interacting with the formate (see the Supporting Information for details on the setup). This system proved to be thermally unstable:a fter af ew ps of FPMD at 300 K, the HCOO À (Ti) and H + (O s )a dducts relaxed to an undissociated monodentate HCOOH bonded to aT i s .M oreover,t he H + (O s )s tretching mode shows aw ell defined frequency (3621 cm À1 ), whereas no n(OH) signal was detected by IRRAS.…”