Site dependent reactivity of Pt single atoms on anatase TiO2(101) in an aqueous environment

Distribution of Pt single atom coordination environments on anatase TiO2 supports controls reactivity

Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO2 supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO2 are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.

Hydrogen Bonds and H3O+ Formation at the Water Interface with Formic Acid Covered Anatase TiO2

Carboxylic acid-modified TiO₂ surfaces in aqueous environment are of widespread interest, yet atomic-scale understanding of their structure is limited. We here investigate formic acid (FA) on anatase TiO₂ (101) (A-101) in contact with water using density functional theory (DFT) and ab initio molecular dynamics (AIMD). Isolated FA molecules adsorbed in a deprotonated bridging bidentate (BD) form on A-101 are found to remain stable at the interface with water, with the acid proton transferred to a surface oxygen to form a surface bridging hydroxyl (O_brH). With increasing FA coverage, adsorbed monolayers of only BD and successively of alternating monodentate (MD) and BD species give rise to a higher concentration of surface O_brH's. Simulations of these adsorbed monolayers in water environment show that some protons are released from the surface O_brH's to water resulting in a negatively charged surface with nearby solvated H₃O⁺ ions. These results provide insight into the complex acid-base equilibrium between an oxide surface, adsorbates and water and can also help obtain a better understanding of the wetting properties of chemically modified TiO₂ surfaces.

Ag5-induced stabilization of multiple surface polarons on perfect and reduced TiO2 rutile (110)

The recent advent of cutting-edge experimental techniques allows for a precise synthesis of subnanometer metal clusters composed of just a few atoms, opening new possibilities for subnanometer science. In this work, via first-principles modeling, we show how the decoration of perfect and reduced TiO₂ surfaces with Ag₅ atomic clusters enables the stabilization of multiple surface polarons. Moreover, we predict that Ag₅ clusters are capable of promoting defect-induced polarons transfer from the subsurface to the surface sites of reduced TiO₂ samples. For both planar and pyramidal Ag₅ clusters, and considering four different positions of bridging oxygen vacancies, we model up to 14 polaronic structures, leading to 134 polaronic states. About 71% of these configurations encompass coexisting surface polarons. The most stable states are associated with large inter-polaron distances (>7.5 Å on average), not only due to the repulsive interaction between trapped Ti³⁺ 3d¹ electrons, but also due to the interference between their corresponding electronic polarization clouds [P. López-Caballero et al., J. Mater. Chem. A 8, 6842-6853 (2020)]. As a result, the most stable ferromagnetic and anti-ferromagnetic arrangements are energetically quasi-degenerate. However, as the average inter-polarons distance decreases, most (≥70%) of the polaronic configurations become ferromagnetic. The optical excitation of the midgap polaronic states with photon energy at the end of the visible region causes the enlargement of the polaronic wave function over the surface layer. The ability of Ag₅ atomic clusters to stabilize multiple surface polarons and extend the optical response of TiO₂ surfaces toward the visible region bears importance in improving their (photo-)catalytic properties and illustrates the potential of this new generation of subnanometer-sized materials.