Anatase Titanium Dioxide with Exposed {001} Facets as a Support for Molecular Catalysts: Surface Characterization and Application in Photocatalysis

Abstract

A specific allotrope of titanium dioxide (anatase) was synthesized with a highly anisotropic morphology ({001}-anatase) dominated by the {001} facet (81%). its surface chemistry after dehydroxylation was studied by 1H NMR and FT-IR. Influence of surface fluorides on surface chemistry was also studied by 1H NMR, FT-IR and DFT. Full attribution of the IR and NMR spectra of anatase with dominant {001} facets could be provided based on experimental data and further confirmed by DFT. Our results showed that chemisorbed H2O are still present on anatase after dehydroxylation at 350°C, and that the type of surface hydroxyls present on the {001} facet is dependent on the presence of fluorides. They also provided general insight on the nature of surface species on both fluorinated and fluorine-free anatase. The use of vanadium oxychloride (VOCl3) allowed determining the accessibility of the various OH groups spectroscopically observed. A platinum complex, (CH3)2Pt(COD), is then grafted via surface organometallic chemistry (SOMC) on morphology-controlled Anatase TiO2 to generate single, isolated Pt atoms on TiO2 nano-platelets. The resulting material is characterized by FT-IR, High resolution scanning transmission electron microscopy (HRSTEM), NMR, and XAS, and then used to perform photocatalytic water splitting. The photocatalyst with SOMC-grafted Pt shows superior performance in photocatalytic hydrogen evolution and strongly suppresses backwards reaction of H2 and O2 forming H2O under dark conditions, compared to photocatalyst prepared by standard wet impregnation at the same Pt loading. However, single Pt atoms on this surface also rapidly coalesce into nanoparticles under photocatalytic conditions. It was also found that adsorbtion of carbon monoxide gas at room temperature also triggers the aggregation of Pt single atoms into nanoparticles. A detailed mechanism is investigated for the mobility of Pt in the formation of its carbonyls using density functional theory (DFT) calculations.