Salvatore Coluccia

Metal sols as a useful tool for heterogeneous gold catalyst preparation: reinvestigation of a liquid phase oxidation

Differently stabilised metal sols have been used as precursors in the preparation of heterogeneous gold catalysts for liquid phase oxidation in water solution. The methodology of sols generation appears to be fundamental to obtaining nanoparticles; the support, instead, plays an important role in maintaining particle dimension and morphology. Three different materials (γ-Al2O3, SiO2 and activated carbon) have been used as the supporting agents for different gold sols that were obtained by reducing HAuCl4 with NaBH4 in the presence of polyvinylalcohol (PVA) or polyvinylpirrolidone (PVP) and with the tetrakis(hydroxymethyl)phosphonium chloride (THPC)/NaOH system. During the immobilisation step, the maintenance of the particle dimension observed in solution depends on both the support and the type of sol. The gold particle mean size of the colloidal suspension is more easily maintained on oxidic supports than on carbon, the latter apparently needing both steric and polar stabilisation of the gold particle. Comparison of Au/γ-Al2O3 and Au/C catalyst activity in the liquid phase oxidation of ethylene glycol to glycolate highlighted the peculiarity of gold on carbon catalysts; in fact, the normally observed trend of reactivity is partially reversed, medium sized gold particle being the most active.

Photocatalytic oxidation of toluene on irradiated TiO2: comparison of degradation performance in humidified air, in water and in water containing a zwitterionic surfactant

Photocatalytic degradation of toluene was carried out both in gas–solid and in liquid–solid regime by using polycrystalline samples of TiO2 Merck and TiO2 Degussa P25. For the gas–solid regime two types of continuous photoreactor were used, a fixed bed one of cylindrical shape and a Carberry type photoreactor, both irradiated by near-UV light. The inlet reacting mixture consisted of air containing toluene and water vapours. The influence of the gas flow rate and the presence of water vapour on the photocatalytic process was investigated. CO2 and benzaldehyde were the toluene degradation products detected in the gas phase by using TiO2 Merck. In the presence of water vapour this catalyst exhibited a stable activity, which greatly decreased in the absence of water vapour. On the contrary, TiO2 Degussa P25 produced CO2 and traces of benzaldehyde but it continuously deactivated even in the presence of water vapour. For the liquid–solid regime a batch photoreactor with immersed lamp was used. In order to increase the reaction rate, a zwitterionic surfactant, i.e. tetradecyldimethylamino-oxide, was added to the reacting mixture. A complete photo-oxidation of toluene was achieved after few hours of irradiation in the presence of both types of photocatalysts; longer irradiation times produced the photodegradation of surfactant. The main intermediates of toluene degradation were p-cresol and benzaldehyde while traces of pyrogallol and benzyl alcohol were also found. Benzoic acid, hydroquinone and trans, trans-muconic acid were detected only with TiO2 Merck. The reaction rate was higher in the presence of the surfactant suggesting that this compound acts as a sequestration agent. An FTIR study gave information on the role played by superficial hydroxyl groups both on the onset of activity and on the deactivation process. On the basis of photoreactivity results and of FTIR investigation the differences of activity and distribution and nature of toluene degradation products are critically discussed for the three reacting systems used.

The role of H2O in the photocatalytic oxidation of toluene in vapour phase on anatase TiO2 catalyst: A FTIR study

Photocatalytic oxidation of toluene has been carried out in a gas–solid regime by using polycrystalline anatase TiO2 in a fixed-bed continuous reactor. Air containing toluene and water vapours in various molar ratios was fed to the photoreactor irradiated by a medium pressure Hg lamp. Toluene was mainly photo-oxidised to benzaldehyde, and small amount of benzene, benzyl alcohol and traces of benzoic acid and phenol were also detected. In the presence of water, no decrease of photoreactivity was observed at steady-state conditions. By removing water vapour from the feed, the conversion of toluene to benzaldehyde was almost completely inhibited, and an irreversible deactivation of the catalyst occurred. FTIR investigations indicated that benzaldehyde is photoproduced on the TiO2 surface even in the absence of water vapour, but exposure of the catalyst to the UV light in a dry atmosphere results in an irreversible consumption of surface hydroxyl groups. As these species play a key role in the photoreactive process, this dehydroxylation should be the reason of the catalyst deactivation observed in the catalytic runs carried out in the absence of water vapour.

Structure–functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies

Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210–230 nm in diffuse reflectance (DR) UV–Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at λ250 nm in the DR UV–Vis and emit very weakly in the 500–600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. ⩽0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.

An infrared spectroscopic investigation of the surface properties of magnesium aluminate spinel

Abstract Magnesium aluminate (spinel) samples were prepared by decomposition of Mg, Al mixed oxalate and by thermal decomposition of a Mg(NO 3 ) 2 -impregnated transition alumina. In the latter preparation, which yields samples more suitable for ir work, transformation to the spinel phase occurs at ~400 °C, as monitored by the XRD spectrum, and by a sudden shift of the surface OH band to ~3740 cm −1 . Pyridine (Py), CO, and CO 2 were adsorbed in order to investigate the surface acidity and basicity. Py is most effective in revealing Lewis acidity, which is mostly ascribed to coordinatively unsaturated (cus) Mg IV ions, whereas CO and CO 2 σ-coordinate to only a few such sites. (The Roman numeral in Mg IV symbolizes tetrahedral coordination). CO 2 also adsorbs to give bicarbonates as well as monodentate and bidentate carbonates. Evidence is presented to show the importance of the octahedral coordination (Al IV ) and of the degree of surface hydration with respect to surface oxygen basicity. The tetrahedral coordination is shown to be important for the formation of surface bicarbonates, which probably have a bridged structure. The partly inverted cation distribution of some samples was monitored by spectral studies in the surface OH group region and by the adsorption behavior of Py, CO, and CO 2 .

Effect of Fluorination on the Surface Properties of Titania P25 Powder: An FTIR Study

A study was carried out on the consequences of the -OH(surf)/F(-) exchange occurring at the surface of TiO(2) P25 when suspended in HF/F(-) solutions. The maximum extent of fluorination was reached at pH 3.0, resulting in the fixation on the surface of ca. 2.5 F(-)/nm(2). The surface features of fluorinated samples under two selected conditions were investigated by IR spectroscopy, in comparison with pristine TiO(2). The collected data suggested that bridged -OH(surf), likely located on regular facets, was more resistant to exchange with F(-). Combined high resolution transmission electron microscopy (HRTEM), inductively coupled plasma mass spectrometry (ICP-MS) and IR measurements indicated that the fluorination performed in the adopted condition did not induce any etching of TiO(2) particles, and the -OH(surf)/F(-) exchange appeared reversible by treatment in concentrated basic solutions. Furthermore, fluorination resulted in an increase of the Lewis acid strength of surface Ti(4+) sites, which, as a consequence, retained adsorbed water molecules even after outgassing at 423 K. Such an effect involved the overwhelming majority of cations exposed on regular facets.

Surface Characteristics of Nanocrystalline Apatites: Effect of Mg Surface Enrichment on Morphology, Surface Hydration Species, and Cationic Environments

The incorporation of foreign ions, such as Mg2+, exhibiting a biological activity for bone regeneration is presently considered as a promising route for increasing the bioactivity of bone-engineering scaffolds. In this work, the morphology, structure, and surface hydration of biomimetic nanocrystalline apatites were investigated before and after surface exchange with such Mg2+ ions, by combining chemical alterations (ion exchange, H2O-D2O exchanges) and physical examinations (Fourier transform infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM)). HRTEM data suggested that the Mg2+/Ca2+ exchange process did not affect the morphology and surface topology of the apatite nanocrystals significantly, while a new phase, likely a hydrated calcium and/or magnesium phosphate, was formed in small amount for high Mg concentrations. Near-infrared (NIR) and medium-infrared (MIR) spectroscopies indicated that the samples enriched with Mg2+ were found to retain more water at their surface than the Mg-free sample, both at the level of H2O coordinated to cations and adsorbed in the form of multilayers. Additionally, the H-bonding network in defective subsurface layers was also noticeably modified, indicating that the Mg2+/Ca2+ exchange involved was not limited to the surface. This work is intended to widen the present knowledge on Mg-enriched calcium phosphate-based bioactive materials intended for bone repair applications.

Characterisation of microporous and mesoporous materials by the adsorption of molecular probes: FTIR and UV–Vis studies

This contribution describes with a few, properly selected, examples the spectroscopic characterisation of acid, base and redox centres in microporous and mesoporous molecular sieves. FTIR studies of the adsorption of molecular probes that lead to weak H-bonding (N2 and CO), medium–strong H-bonding (H2O) and proton-transfer reactions (NH3) with the Bronsted acid hydroxyl groups of zeolites and aluminophosphates (HSAPO-34 and CoAPO-18) are illustrated. The acid Lewis sites (countercations) and the acid–base couples in NaX and NaY zeolites are monitored by CO and CO2 adsorption respectively. Finally, FTIR and UV–Vis studies of the H2O and NH3 adsorption that probe the transition metal cation centres in mesoporous Ti–MCM41 and microporous CoAPO-18 catalysts are described.

The hydroxylated surface of MgO powders and the formation of surface sites

The structure of the hydroxylated surface of MgO powders has been studied by infrared spectroscopy and microgravimetry. It is shown that: 1. 1) OH− groups on corner and edge sites absorb at higher frequency than those on extended {100} faces; 2. 2) the former hydroxyls are more stable than the latter; 3. 3) some surface reconstruction may occur upon hydroxylation-dehydroxylation cycles; 4. 4) neutral anion vacancies might be formed in the last stages of surface dehydroxylation.

Elucidating the local environment of Ti(IV) active sites in Ti-MCM-48 : a comparison between silylated and calcined catalysts

Abstract A mesoporous Ti-MCM-48 material was silylated using hexamethyldisilazane (HDMS) and a remarkable increase in the catalytic performances in the cyclohexene epoxidation with tert-butylhydroperoxide obtained. We report on a combined FTIR, diffuse reflectance UV–Vis–NIR, photoluminescence and 29Si MAS NMR study, augmented by TGA and XRD analysis, of the local environment of Ti sites in both calcined and silylated Ti-MCM-48 catalysts. Spectroscopic studies were also performed on the as-synthesised material and showed that both silanols and titanols are formed during the decomposition of the cetyltrimethylammonium hydroxide surfactant used to template the mesoporous structure. Unequivocal evidences that a further silica condensation of the MCM-48 framework structure occurs both during thermal treatments in vacuum and after calcination at 540°C are also reported. Upon silylation, a more hydrophobic surface is obtained where trimethylsilyl groups (Si(CH3)3) are anchored on to the silica surface by reaction of the silylating agent, HDMS, with silanols, and in addition, (SiO)3TiOH sites transformed into (SiO)3TiOSi(CH3)3. It is proposed that the more hydrophobic character of the silylated material prevents the deactivation of the catalyst by decreasing the amount of glycols which may be formed by reaction of the epoxide with water adsorbed on the surface of the Ti-MCM-48, and that poison the titanium catalytic centres.