Synthesis and Characterization of High-Surface-Area Silica–Titania Monoliths

Hybrid SiO₂–TiO₂ mesoporous monoliths were synthesized via sol–gel method. The water for hydrolysis of titanium precursor has been obtained in situ via esterification reaction between acid and alcohol without use of additional agents. The titania content in the samples has been varied from 2 to 40 M% molar content with respect to silica. The nitrogen adsorption isotherms showed that monoliths containing 2 M% of Ti retain a large surface area and micropores with size peaking in the 1 to 2 nm interval. For larger Ti content, a bimodal average micropores distribution is observed. The surface area decreases for titania content larger than 10 M%. The Raman spectra taken with excitation lines in the 244–785 nm interval and the UV–vis measurements demonstrate that in the 2 M% sample Ti is atomically dispersed and fully incorporated in the SiO₂ framework and that the local structure is similar to that of titanium silicalite. This conclusion is further confirmed by XAS measurements. Also, the reactivity toward hydrogen peroxide is similar. For larger Ti concentrations, the presence of segregated anatase particles grown in restricted space is observed, as demonstrated by XRD, TEM, Raman, and UV–vis measurements. The surface properties of the silica matrix and of the embedded anatase nanoparticles have been fully characterized by FTIR spectra of CO probe adsorbed at 100 K. Unlike the samples with 5–20 M% Ti, the 40 M% Ti system is fully amorphous, and only small traces of crystalline TiO₂ are present

Rutile Surface Properties Beyond the Single Crystal Approach: New Insights from the Experimental Investigation of Different Polycrystalline Samples and Periodic DFT Calculations

The relationships between particle morphology and surface properties for different kinds of rutile polycrystalline samples have been thoroughly investigated, combining FTIR spectroscopy of CO adsorbed at 60 K through the carbon end on surface Ti⁴⁺ centers, electron microscopy, and periodic DFT calculations. We provide a complete assignment of the FTIR spectra of CO adsorbed on micro- and nanorutile crystals with well-defined morphology, on commercially sourced nanorutile, and on the P25 rutile component. On the basis of these results, the spectrum of CO on native P25 is revisited. Of special interest is the fact that the (110) rutile main surface undergoes a thermally induced relaxation process, leading to the shielding of exposed Ti⁴⁺ sites and consequently to the reduction of the polarizing power. This process can be reversed by inducing an outward relaxation of the shielded Ti atoms by treating the sample in water at 573 K. A red-shifted band ascribed to CO adsorbed through the oxygen end on the low polarizing sites of relaxed surfaces provides the signature of surface relaxation. CO species interacting through the oxygen end have already been studied for CO on zeolites exchanged with low polarizing alkaline cations but not yet properly discussed for CO on metal oxides

Defect Sites in H₂‑Reduced TiO₂ Convert Ethylene to High Density Polyethylene without Activator

We show the unprecedented potential of commercially available TiO₂ materials reduced in H₂ (H₂-reduced TiO₂) in the conversion of ethylene to high density polyethylene (HDPE) under mild conditions (room temperature, low pressure, absence of any activator), with the consequent formation of HDPE/TiO₂ composites, which have been characterized by electron microscopy. Combination of UV–vis and IR spectroscopies allows one to demonstrate that ethylene polymerization occurs on Ti^4–<i>n</i> defect sites, which behave as shallow-trap defects located in the band gap and, differently from the active sites in the widely used Ziegler–Natta catalysts, do not contain any alkyl (Ti–R) or hydride (Ti–H) ligands. These results represent a step forward the understanding of ethylene polymerization mechanism and open valuable perspectives for commercial TiO₂ materials as catalysts for polyethylene production under mild conditions

Particles Morphology and Surface Properties As Investigated by HRTEM, FTIR, and Periodic DFT Calculations: From Pyrogenic TiO₂ (P25) to Nanoanatase

By combining electron microscopy, FTIR spectroscopy of different CO isotopic mixtures, and DFT calculations, a complete assignment of the IR spectrum of CO adsorbed on P25 (a mixture of 85% anatase and 15% rutile) at various dehydration states, on its pure rutile component, and on nanoanatase was obtained. It is shown that the measurements at 60 K provide IR spectra of unprecedented quality and that the spectroscopic method is extremely powerful not only to study the surface Lewis and Brønsted acidity but also to investigate the particles morphology. Indeed, as CO adsorbed on different faces is characterized by different stretching frequencies, the IR spectrum contains information on the exposed faces and hence on particles morphology. This information, combined with the study of the IR spectrum of CO adsorbed on defects, of dipole–dipole interactions between parallel oscillators adsorbed on extended faces and with HRTEM results allowed us to fully explore the relations between particles morphology and surface properties. By comparing the spectra of CO obtained on P25, on the pure rutile fraction of P25, and on nanoanatase, the high crystalline character of P25 is inferred, which is likely the key of its outstanding photocatalytic activity. It is also found that the IR spectrum of CO on P25 is the sum of the rutile and anatase contributions and that no additional surface features ascribable to anatase–rutile junctions are noticeable, thus contributing to the strong debate present in the literature

MoS₂ Nanoparticles Decorating Titanate-Nanotube Surfaces: Combined Microscopy, Spectroscopy, and Catalytic Studies

MoS₂/TNTs composites have been obtained by impregnation of titanate nanotubes (TNTs) with a centrifuged solution of nanosized MoS₂ particles in isopropyl alcohol (IPA). The characterization has been performed by combining UV–vis–NIR, Raman, AFM, and HRTEM analyses, before and after impregnation. HRTEM images show that the contact between single-layer MoS₂ nanoparticles and the support is efficient, so justifying the decoration concept. The volatility of IPA solvent allows the preparation of composites at low temperature and free of carbonaceous impurities. MoS₂ nanoparticles have strong excitonic transitions, which are only slightly shifted with respect to the bulk because of quantum size effects. Concentrations of MoS₂, less than 0.1 wt %, are enough to induce strong absorption in the visible. Photodegradation of methylene blue (MB) has been performed on TNTs and MoS₂/TNTs to verify the effect of the presence of MoS₂. The first layer of adsorbed MB is consumed first, followed by clustered MB in the second and more external layers. The presence of low concentrated MoS₂ nanoparticles does not substantially alter the photocatalytic properties of TNTs. This result is due to poor overlapping between the high frequency of MoS₂ C, D excitonic transitions and the TNTs band gap transition