Vladimiro Dal Santo

Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO2 Nanoparticles

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO(2) nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO(2) nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.

Bimetallic Au–Pt/TiO2 photocatalysts active under UV-A and simulated sunlight for H2 production from ethanol

A new class of bimetallic Au–Pt/TiO2 photocatalysts were developed and employed in H2 photoassisted production using ethanol as a sacrificial reagent both under UV-A and simulated sunlight irradiation. Remarkably, preliminary experiments show promising hydrogen evolution under visible light using Au0.5–Pt0.5/TiO2 with methanol. The presence of bimetallic Au–Pt nanoparticles and the TiO2 visible light absorption, induced by the presence of oxygen vacancies and/or Ti3+, are the two parameters accounting for the difference in activity.

H2 Production by Renewables Photoreforming on Pt–Au/TiO2 Catalysts Activated by Reduction

Bimetallic Pt-Au nanoparticles supported on reduced anatase nanocrystals represent a new class of promising photocatalysts with high activity in hydrogen production by photoreforming of aqueous solution of renewable feedstock, such as ethanol and glycerol. The catalysts are easily obtained by metal impregnation of commercial TiO₂, followed by a reductive treatment. Remarkably, deeper catalyst pre-reduction results in enhanced photoactivity. When ethanol is used as sacrificial agent, under both UV-A or simulated sunlight irradiation, H₂ is the most abundant product in the gas stream whereas, in the case of glycerol, significant amounts of CO₂ have also been detected, indicating a more efficient oxidation of the organic sacrificial agent. The presence of bimetallic Pt-Au nanoparticles and of Ti³⁺ sites/O²⁻ vacancies in the bulk structure of titania are two key parameters to maximize light absorption and feedstock activation, finally resulting in good photocatalytic performances.

A comparison between silica-immobilized ruthenium(II) single sites and silica-supported ruthenium nanoparticles in the catalytic hydrogenation of model hetero- and polyaromatics contained in raw oil materials

A comparative study of the hydrogenation of various heterocycles, model compounds in raw oil materials, by either Ru(II) complex immobilized on mesoporous silica or Ru(0) nanoparticles deposited on the same support has been performed. The single-site catalyst contains the molecular precursor [Ru(NCMe)3(sulphos)](OSO2CF3) tethered to partially dehydroxylated high-surface-area silica through hydrogen bonds between silanol groups of the support and SO3− groups from both the sulphos ligand [−O3S(C6H4)CH2C(CH2PPh2)3] and the triflate counter anion. Highly dispersed ruthenium nanoparticles were prepared by calcination/reduction of silica-supported Ru3(CO)12. The heterocycles (benzo[b]thiophene, quinoline, indole, acridine) are hydrogenated to cyclic thioethers or amines. The Ru(II) single-site catalyst is active for both benzo[b]thiophene and the N-heterocycles, while the Ru(0) catalyst does not hydrogenate the S-heterocycle, yet is efficient for the reduction of the N-heterocycles and simple aromatic hydrocarbons. The surface silanols promote the hydrogenation of indole via NH⋯ O(H)Si hydrogen bonds and can interact with the π-electron density of all substrates.

Design and use of nanostructured single-site heterogeneous catalysts for the selective transformation of fine chemicals.

Nanostructured single-site heterogeneous catalysts possess the advantages of classical solid catalysts, in terms of easy recovery and recycling, together with a defined tailored chemical and steric environment around the catalytically active metal site. The use of inorganic oxide supports with selected shape and porosity at a nanometric level may have a relevant impact on the regio- and stereochemistry of the catalytic reaction. Analogously, by choosing the optimal preparation techniques to obtain spatially isolated and well-characterised active sites, it is possible to achieve performances that are comparable to (or, in the most favourable cases, better than) those obtained with homogeneous systems. Such catalysts are therefore particularly suitable for the transformation of highly-functionalised fine chemicals and some relevant examples where high chemo-, regio- and stereoselectivity are crucial will be described.

Hierarchical Hematite Nanoplatelets for Photoelectrochemical Water Splitting

A new nanostructured α-Fe2O3 photoelectrode synthesized through plasma-enhanced chemical vapor deposition (PE-CVD) is presented. The α-Fe2O3 films consist of nanoplatelets with (001) crystallographic planes strongly oriented perpendicular to the conductive glass surface. This hematite morphology was never obtained before and is strictly linked to the method being used for its production. Structural, electronic, and photocurrent measurements are employed to disclose the nanoscale features of the photoanodes and their relationships with the generated photocurrent. α-Fe2O3 films have a hierarchical morphology consisting of nanobranches (width ∼10 nm, length ∼50 nm) that self-organize in plume-like nanoplatelets (350-700 nm in length). The amount of precursor used in the PE-CVD process mainly affects the nanoplatelets dimension, the platelets density, the roughness, and the photoelectrochemical (PEC) activity. The highest photocurrent (j = 1.39 mA/cm(2) at 1.55 VRHE) is shown by the photoanodes with the best balance between the platelets density and roughness. The so obtained hematite hierarchical morphology assures good photocurrent performance and appears to be an ideal platform for the construction of customized multilayer architecture for PEC water splitting.

Nickel Catalysts Supported Over TiO2, SiO2 and ZrO2 for the Steam Reforming of Glycerol

Ni‐based catalysts supported on TiO2, ZrO2 and SiO2 (in the form of mesoporous Santa Barbara Amorphous 15 (SBA‐15) and amorphous dense nanoparticles), were employed in the steam reforming of glycerol. Each sample was prepared by liquid phase synthesis of the support followed by impregnation with the active phase and calcination at 800 °C or by direct synthesis through flame pyrolysis. Many techniques have been used to assess the physical chemical properties of both the fresh and spent catalysts, such as atomic absorption, N2 adsorption/desorption, XRD, SEM, TEM, temperature‐programmed reduction (TPR), X‐ray photoelectron spectroscopy (XPS), Micro‐Raman and FTIR spectroscopy. The samples showed different textural, structural and morphological properties, as well as different reducibility and thermal resistance depending on the preparation method and support. Some of these properties were tightly bound to catalyst performance, in terms of H2 productivity and stability towards coking and sintering. A key parameter was the metal–support interaction, which strongly depended on the preparation procedure. In particular, the stronger the interaction, the more stable the metallic Ni clusters, which in turn lead to a higher catalytic activity and stability. Surface acidity was also taken into account, in which the nature of the acid sites was differentiated (silanols, titanols or Lewis acid sites). The characterisation of the spent catalysts also allowed us to interpret the deactivation process. The formation of multi‐walled nanotubes was observed for every sample, though it was only in some cases that this led to severe deactivation.

Probing Long‐Lived Plasmonic‐Generated Charges in TiO2/Au by High‐Resolution X‐ray Absorption Spectroscopy

Exploiting plasmonic Au nanoparticles to sensitize TiO2 to visible light is a widely employed route to produce efficient photocatalysts. However, a description of the atomic and electronic structure of the semiconductor sites in which charges are injected is still not available. Such a description is of great importance in understanding the underlying physical mechanisms and to improve the design of catalysts with enhanced photoactivity. We investigated changes in the local electronic structure of Ti in pure and N-doped nanostructured TiO2 loaded with Au nanoparticles during continuous selective excitation of the Au localized surface plasmon resonance with X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Spectral variations strongly support the presence of long-lived charges localized on Ti states at the semiconductor surface, giving rise to new laser-induced low-coordinated Ti sites.

Rational design of single-site heterogeneous catalysts: towards high chemo-, regio- and stereoselectivity

The main methods for the design and preparation of single-site heterogeneous catalysts on inorganic oxide supports are described and reviewed. Catalytically active metal sites can be either introduced into the framework of porous materials via direct synthesis or added to a pre-existing support by post-synthesis techniques. Particular attention is paid to selected examples where the geometry, the nature and the chemical surroundings of the active single site is a key factor to obtain catalytic systems with enhanced chemo-, regio- and stereoselectivity. The ever-increasing capabilities of ‘nanoarchitecture’ at molecular level enable chemists to build ideal catalysts for the sustainable transformation of bulky and high added-value molecules.

Influence of TiO2 electronic structure and strong metal–support interaction on plasmonic Au photocatalytic oxidations

Aiming at understanding how plasmonic reactions depend on important parameters such as metal loading and strong metal–support interaction (SMSI), we report the plasmonic photodegradation of formic acid (FA) under green LED irradiation employing three TiO2 supports (stoichiometric TiO2, N-doped TiO2, black TiO2) modified with Au nanoparticles (NPs) 3–6 nm in size. The rate of FA photooxidation follows different trends depending on Au loading for stoichiometric and doped Au/TiO2 materials. In the first case, the only contribution of hot electron transfer produces a volcano-shaped curve of photoreaction rates with increasing Au loading. When TiO2 contains intra-bandgap states the photoactivity increases linearly with the amount of Au NPs due to the concomitant enhancement produced by hot electron transfer and plasmon resonant energy transfer (PRET). The role of PRET is supported by finite element method simulations, which show that the increase in both Au NP inter-distance and SMSI enhances the probability of charge carrier generation at the Au/TiO2 interface.