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Changing the composition and structure of a catalyst to obtain a positive impact on its performance is challenging [...]

The specific roles played by both support and noble metals in light absorption, charge separation, and the formation of ·OH and O2− (ROS) are analyzed for light-triggered oxidation of phenol (Ph) over pristine and over noble metal (Ag, Au, Pt) -loaded TiO2. Experiments show that the supported noble metals act as a light visible absorber, assist the separation of photo-charges and reduction of O2 to O2−. The O2− oxidizes mildly Ph to oxygenated products (hydroquinone, benzoquinone, and 1,2-dihydroxibenzene). In a parallel process, ·OH radicals, yielded by TiO2, mineralize Ph to CO2 by fast reaction sequences. Radical quenching and photo electrochemical measurements (surface photovoltage) confirm independently that the production of ·OH and O2− scale with oxidative conversion of Ph. The selectivity to CO2 and mild oxidation products is the result of the interplay between catalyst activity for ·OH and for O2− production.

Volatile organic compound (VOC) abatement has become imperative nowadays due to their harmful effect on human health and on the environment. Catalytic oxidation has appeared as an innovative and promising approach, as the pollutants can be totally oxidized at moderate operating temperatures under 500 °C. The most active single oxides in the total oxidation of hydrocarbons have been shown to be manganese and cobalt oxides. The main factors affecting the catalytic performances of several metal-oxide catalysts, including CoOx and MnOx, in relation to the total oxidation of hydrocarbons have been reviewed. The influence of these factors is directly related to the Mars–van Krevelen mechanism, which is known to be applied in the case of the oxidation of VOCs in general and hydrocarbons in particular, using transitional metal oxides as catalysts. The catalytic behaviors of the studied oxides could be closely related to their redox properties, their nonstoichiometric, defective structure, and their lattice oxygen mobility. The control of the structural and textural properties of the studied metal oxides, such as specific surface area and specific morphology, plays an important role in catalytic applications. A fundamental challenge in the development of efficient and low-cost catalysts is to choose the criteria for selecting them. Therefore, this research could be useful for tailoring advanced and high-performance catalysts for the total oxidation of VOCs.

Studies on the nitrate reduction reaction have shown that the bimetallic catalysts may be more efficient compared to the monometallic ones. To reduce nitrate, it is necessary to activate the precious metal by addition of a promoting second metal. Some studies have pointed out the relevance of the structure and geometry of the metal particles in the final mechanism of nitrate reduction. Well-defined nanoparticles could be used to catalyze structure-sensitive reactions with practical importance (pollution control). Our investigation aimed to make deeper insight into NO3- catalytic reduction and photocatalytic reduction mechanisms. Two types of Pt-Cu supported on TiO2 materials were employed in order to study the nitrate removal from aqueous phase and the ability of obtaining and using the in situ generated solar H2 as reducing agent. The prepared catalysts were characterized and tested in photocatalytic/catalytic reduction of nitrate in aqueous solution in water splitting reaction. A correlation between the catalytic/photocatalytic performances of the supported Pt-Cu catalysts and their associated physicochemical and adsorption properties has been made. The catalysts activity for the nitrate reduction can be directly related to the interactions between platinum and copper. Deposition with Pt nanoparticles was found to contribute to enhance the photocatalytic activity toward H2 production from aqueous solution. Our future research aims at optimizing the photocatalytic system having as future possible applications for hydrogen production using sunlight and water as the hydrogen source.

In this paper, the CO oxidation on Pd/TiO 2 and Pd/SnO 2 /TiO 2 catalysts was studied using a non-conventional tool: AC electrical measurements in operando conditions. The structural and textural properties of the obtained powders have been characterized using BET, XRD, SEM, and CO chemisorption. Their redox properties were tested using the TPR technique. The surface dynamics was studied by electrical conductivity measurements under similar conditions with those encountered in the practical use in catalysis and correlated with their performances in CO oxidation.

Efficient preparation of stable colloidal suspensions of platinum nanoparticles was achieved by reduction of Pt precursor (H 2 PtCl 6 ·6H 2 O) in the presence of new thermosensitive polymers with triblock copolymer structure consisting of a poly(ethylene glycol) (PEG) central block and poly(N-isopropylacrylamide-co-N-t-butylacrylamide) statistical copolymer as side chains. The predominant shape and size of platinum nanoparticles was controlled by changing the N-t-butylacrylamide (TB)/N-isopropylacrylamide (NIPAM) mole ratio in the capping polymer. The triblock copolymer side chain composition was varied by increasing gradually the TB molar percentage from 0, 3 and 6 mol%. The molar ratio between the capping polymer, calculated based on the monomer unit, and platinum cation was in all cases 10:1. The morphology, shape and mean size of colloidal particles was determined from transmission electron microscopy (TEM) micrographs. The best morphological control was obtained using the thermosensitive polymer with 3 mol% TB within the side blocks, and the optimal reduction temperature for this polymer was 34 °C (below the critical temperature). The surface area of exposed platinum nanoparticles dispersed on TiO 2 was estimated by CO pulse chemisorption. The photocatalytic hydrogen generation over Pt/TiO 2 materials under solar irradiation was evaluated. The hydrogen production rate was found to be sensitive to the particle size.

AC electrical conductivity of CeO2 catalyst samples containing 5, 10 and 20 wt% SnO2 prepared by co-precipitation and of SnO2 and CeO2 pure oxides (prepared under the same conditions) was measured in operando conditions. The measurements were carried out at a frequency of 1592 Hz in the temperature range from room temperature up to 400 °C, using successively programmed heating–cooling cycles under various atmospheres. The catalyst samples were tested in the catalytic oxidation of CO in presence and in absence of oxygen in the same temperature range (30–400 °C). An improved deep oxidation of carbon monoxide has been found with these co-precipitated mixed oxides compared to pure components, leading to a better catalytic activity at lower temperature. The band gap energies were estimated based on the UV–Vis spectra and correlated with XRD results and AC electrical conductivity data. These results show the influence of the tin oxide on the electrical properties and oxygen mobility of the catalytic systems studied and on the surface behaviour of ceria.

The surface behavior of a series of SnO2 (5, 10 and 20 wt.%) - CeO2 catalyst samples prepared by impregnation method and also pure SnO2, CeO2, was studied based on the response of the ac electrical conductivity. The electrical measurements were carried out in operando conditions in order to obtain information about the surface interaction with reactants during CO/hydrocarbon oxidation over these catalysts. The apparent activation energies of conduction in presence of CO:O2 and C3H6:air mixtures were presented. The catalyst samples were tested in catalytic oxidation of CO and C3H6 at temperature range of room temperature up to 400 C. The catalytic activity in both oxidation reactions and selectivity towards CO2 in propylene oxidation at 400 C exhibited by the SnO2-CeO2 systems were significantly higher to those exhibited by the pure ceria catalyst.

Newly prepared SnO2-CeO2 catalyst samples used as depollution catalysts were characterized applying low-temperature nitrogen adsorption (BET), X-ray diffraction (XRD), thermo-gravimetry (TG-dTG) and temperature-programmed reduction (TPR) methods. Pure SnO2 has higher surface area (17 m2/g) than the pure CeO2 (8 m2/g). Addition of organic tin oxide precursor to ceria in amount of 5 wt% slightly decreases the surface area of CeO2 (SSn5-Ce= 7 m2/g). The increase of tin oxide content to 10 and 20 wt% increases the surface area of the catalyst (SSn10-Ce= 9 m2/g; SSn20-Ce= 10 m2/g). A similar effect was observed for pores of 1.7-300nm size. Catalyst sample Sn5-Ce exhibited the lowest pore volume, which increases with increasing the amount of tin oxide. Tin dioxide in Sn-Ce samples with lower loadings of SnO2 (.10 wt%) were well dispersed showing amorphous structure. High loading (20 wt%) of tin dioxide in Sn-Ce showed XRD lines of formation of cassiterate crystalline structure of SnO2 without evidence of solid solution formation. In case of SnO2 the TPR profile exhibits a major peak at about 545 C with much greater intensity than in case of CeO2, revealing that SnO2 was more easily reducible than CeO2. Sn-Ce samples exhibit reducibility at lower temperatures (between 545-635 C) compared to the single tin dioxide (750 C).