Giuseppe Marci

Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions : Correlation with electron-hole recombination kinetics

Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (<100 fs). The second-order rate constant (kr) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H2 and PCA production under deaerated conditions depended strongly on kr, but the relation between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different kr dependences of photoactivity on the reaction kinetics governed by e−–h+ recombination were attributed to the presence of O2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.

Oxidation of Aromatic Alcohols in Irradiated Aqueous Suspensions of Commercial and Home-prepared Rutile TiO2: A Selectivity Study

The photocatalytic oxidation of benzyl alcohol (BA) and 4-methoxybenzyl alcohol (MBA) has been performed in pure water by using commercial TiO(2) samples (Sigma-Aldrich, Merck, Degussa P25) and rutile TiO(2) prepared from TiCl(4) at low temperature. Particular attention has been devoted to the identification of the produced aromatic compounds along with the formed CO(2). Oxidation products such as the corresponding aromatic aldehyde and acid, as well as mono- and dihydroxylated aldehydes have been detected. The home-prepared rutile sample showed a marked selectivity towards the formation of the aromatic aldehyde (38 and 60 % for BA and MBA, respectively), resulting in a three- to sevenfold improvement relative to commercial samples, with the only byproduct being CO(2). This catalyst was found to be the most selective in the formation of aldehyde in water. By using the commercial or the calcined home-prepared samples, many hydroxylated aromatic compounds were detected besides the aldehyde and the acid. This finding points to a higher selectivity performance of the home-prepared rutile relative to the commercial TiO(2) samples. Some of the home-prepared samples were also dialysed to check the influence of the presence of Cl(-) species on catalyst reactivity and selectivity. We have attempted to explain the different reaction rate and selectivity observed for MBA and BA.

Photocatalytic activity of TiO2/SiO2 systems.

Silica-supported TiO(2) powders were synthesized by a wet method under mild conditions. The aim of the work was the preparation of TiO(2)/SiO(2) additives for photocatalytic cements. Three types of commercial SiO(2) were used as supports: Cabot, Axim and Fly Ash. Cabot silica was ultra-pure whereas the other two silica contained different percentages of various oxides. The TiO(2)/SiO(2) samples, denoted TiO(2)/Cabot, TiO(2)/Axim and TiO(2)/Fly Ash, were prepared by boiling suspensions obtained by addition of silica to a solution of TiCl(4) in water (volume ratio 1:10). The photocatalytic activity was evaluated in a gas-solid system both in batch and in continuous reactors using 2-propanol as probe molecule. SEM-EDX analysis revealed that titanium dioxide was quantitatively deposited on silica. TiO(2)/Axim and TiO(2)/Fly Ash were scarcely active whereas a good photoactivity was exhibited by the TiO(2)/Cabot sample both in the batch and in the continuous system. Consequently only the last sample was tested for both NO(x) abatement and for 4-nitrophenol photodegradation in a liquid-solid system.

Influence of tungsten oxide on structural and surface properties of sol–gel prepared TiO2 employed for 4-nitrophenol photodegradation

A set of tungsten oxide/TiO2 polycrystalline samples (W/Ti) prepared by the sol–gel method has been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, determination of specific surface area, scanning electron microscopy, Fourier-transform infrared spectroscopy and monitorisation of pyridine adsorption for surface acidity. Moreover the samples have been empolyed as catalysts for 4-nitrophenol photodegradation in aqueous suspensions used as a ‘probe’ reaction. Characterisation results indicate that the surface of the W/Ti particles is enriched with homogeneously dispersed tungsten as well as microcrystalline or amorphous species. Maximum photoactivity for 4-nitrophenol photodegradation was achieved for a sample containing 1.7 mol W per 100 mol Ti.

Transition metal doped TiO2: physical properties and photocatalytic behaviour

Two probe photocatalytic reactions, i.e. ethanoic acid and 4-nitrophenol photooxidation, were carried out in different experimental conditions by using suspensions of transition metal (Co, Cr, Cu, Fe, Mo, V and W) doped polycrystalline TiO2 powders in aqueous systems. A beneficial influence of the pres- ence of metal species was observed only with the samples containing copper and tungsten. In particular, the TiO2/Cu powders showed to be more photoactive of bare TiO2 for the ethanoic acid oxidation while the TiO2/W samples were more efficient for 4-nitrophenol degradation. A tentative interpretation is provided on the basis of the values of the points of zero charge of the powders and of the rate constants of recombi- nation of photogenerated electrons and holes, determined by femtosecond pump-probe diffuse reflectance spectroscopy (PP-DRS).

Sol-Gel Synthesis and Characterisation of TiO2-Anatase Powders Containing Nanometric Platinum Particles Employed as Catalysts for 4-Nitrophenol Photodegradation

A set of TiO2 and Pt-TiO2 polycrystalline samples were prepared by sol-gel method hydrolysing a modified alkoxide titanium precursors under acidic conditions. The Pt-TiO2 samples gave an homogeneous nanometric metal dispersion after drying heat treatment forming platinum particles in the range 2–4 nm. All the samples have been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction coupled with a Rietveld refinement procedure, X-ray photoelectron spectroscopy and determination of specific surface area. Moreover, the samples have been employed as catalysts for 4-nitrophenol photodegradation in aqueous suspension used as a “probe” reaction. Characterisation results indicate that the thermal and chemical treatments of the catalysts influenced the photocatalytic activity. In the Pt-TiO2 samples both Pt(0) and Pt(II) species are present in the catalyst particles and the most abundant phase is anatase for all of the samples. Doping with Pt beneficially influences the photo-oxidant properties of TiO2 while the presence of organic residual species on the surface, deriving from the preparation procedure of the catalyst particles interferes negatively in the kinetics of the photocatalytic process.

Preparation and characterization of Al2O3 supported TiO2 catalysts employed for 4-nitrophenol photodegradation in aqueous medium

Abstract In this paper the preparation and some bulk and surface characterizations of set of Al2O3 supported TiO2 photocatalysts are reported. The powders were home prepared by a wet impregnation method using titanium (IV) isopropylate. The impregnation was followed by thermal treatments and this procedure was repeated several times in order to obtain samples with different amounts of TiO2. The solids were characterized by X-ray diffraction, specific surface area determination, scanning electron microscopy coupled with an electron microprobe used in an energy-dispersive mode and diffuse reflectance spectroscopy. Moreover, all of them were tested as photocatalysts by using different kinds of batch photoreactors for 4-nitrophenol photodegradation in aqueous medium chosen as a ‘probe’ reaction. The characterization results indicate that a layer of anatase TiO2 formed on the surface of Al2O particles, its extent depending on the number of impregnations. The photoactivity of the powders increased by increasing the number of impregnations up to seven times. For samples obtained by impregnating seven times, the photoreactivity was higher, but not much different from that observed by using samples impregnated five times. The photoactivities of the samples obtained with five and seven impregnations were found to be not much lower than that of a pure home prepared TiO2 sample, but quite lower than that of a Degussa P25 TiO2 sample. Finally, it is worth noting that the alumina supported TiO2 catalysts are easily separated from water by decantation and no significant deactivation due to disaggregation of the particles and/or erosion of their surface was shown to occur in some of the used experimental conditions, i.e. when photoreactors coupled with an electric impeller were used.

Keggin heteropolyacid H3PW12O40 supported on different oxides for catalytic and catalytic photo-assisted propene hydration

Catalytic and catalytic photo-assisted hydration of propene to form 2-propanol in gas-solid regime at atmospheric pressure and 85 °C were carried out by using a heteropolyacid (POM) supported on different oxides. Binary materials were prepared by impregnation of H3PW12O40 on different commercial and home prepared supports (TiO2, SiO2, WO3, ZrO2, ZnO, Al2O3). Some of the composites were active both for catalytic and catalytic photo-assisted reactions. The Keggin type POM was completely and partially degraded, when supported on ZnO and Al2O3, respectively, and these binary solids always resulted as inactive for both catalytic and catalytic photo-assisted reactions. The supported Keggin POM species played a key role both for the catalytic and the photo-assisted catalytic reactions, due to their strong acidity and ability to form strong oxidant species under UV irradiation, respectively. The contemporary presence of heat and UV light improved the activity of almost all POM supported materials. All materials were characterized by X-ray diffraction (XRD), scanning electron microscopy observations (SEM), diffuse reflectance spectroscopy (DRS), determination of the conduction and valence band energy by photovoltage measurements, Fourier transform infrared spectroscopy (FTIR), NH3-TPD experiments and time resolved microwave conductivity (TRMC).

Co3O4 particles grown over nanocrystalline CeO2: influence of precipitation agents and calcination temperature on the catalytic activity for methane oxidation

Crystalline cobalt oxides were prepared by a precipitation method using three different precipitation agents, (NH4)2CO3, Na2CO3 and CO(NH2)2. Cobalt oxide nanoparticles corresponding to a Co3O4 loading of 30 wt% were also deposited over high-surface area nanocrystalline ceria by the same precipitation agents. The effect of calcination temperature, 350 or 650 °C, on the morphological and structural properties was evaluated. Characterization by BET, XRD, SEM, TEM, Raman spectroscopy, H2-TPR, XPS and NH3-TPD was performed and the catalytic properties were explored in the methane oxidation reaction. The nature of the precipitation agent strongly influenced the textural properties of Co3O4 and the Co3O4–CeO2 interface. The best control of the particle size was achieved by using CO(NH2)2 that produced small and regular crystallites of Co3O4 homogeneously deposited over the CeO2 surface. Such a Co3O4–CeO2 system precipitated by urea showed enhanced low-temperature reducibility and high surface Co3+ concentration, which were identified as the key factors for promoting methane oxidation at low temperature. Moreover, the synergic effect of cobalt oxide and nanocrystalline ceria produced stable full conversion of methane in the entire range of investigated temperature, up to 700–800 °C, at which Co3O4 deactivation usually occurs.

Influence of some aromatic and aliphatic compounds on the rate of photodegradation of phenol in aqueous suspensions of TiO2

Abstract The influence of 3-nitrophenol, 4-nitrophenol, pentachlorophenol, ethanol, 2-propanol, ethanoic acid and ethanedioic acid on the photodegradation rate of phenol in an aqueous suspension of TiO2 (anatase) irradiated in the near-UV region was investigated. Binary mixtures of organic compound and phenol in molar ratios of 1:1 and 10:1 were used for the photoreactivity experiments performed in a batch reactor. The mineralization rate of all the mixtures was also monitored by measuring the total organic carbon (TOC) concentration. The reactivity results indicate that the photodegradation rate of phenol is mainly determined by the total amount of aromatic substrates present in the reacting medium independently of their nature, while it is scarcely affected by the presence of aliphatic compounds. The TOC results indicate that the mineralization rate of aromatic compound-phenol mixtures is sensitive to the mixture composition, and the simultaneous presence of phenol and aliphatic compounds negatively affects the mineralization rate. A tentative explanation of the above behaviour is proposed by taking into account (i) the similar manner of photoadsorption of phenol molecules and of the other aromatic compounds on the active surface site(s), (ii) the different sites of photoadsorption for aromatic and aliphatic substrates and (iii) the possible formation of stable intermediate compounds from the degradation products of phenol and aliphatic compounds.