Applied Catalysis B, Environmental (v.55, #3)


In the present work, we have used Ph3SnOH as precursor for the synthesis of highly active Ti1−x Sn x O2 nanosized photocatalysts. This new preparation route is based on the reaction of TiCl4 with the organometallic compound in an organic medium to yield an amorphous precipitate of TiO2 which contains adsorbed tin species. Subsequent crystallisation of the precipitate under thermal or hydrothermal conditions determines the phase composition (rutile/anatase ratio) and the particle size of the final material. Characterisation data reveal that the incorporation of Sn promotes the anatase to rutile transformation but reduces the size of rutile crystallites. The activity of these samples has been tested for the photocatalytic oxidation of methylcyclohexane (MCH) vapors in an oxygen flow. This hydrocarbon can be considered representative of the volatile organic chemicals (VOC) present in urban atmospheres. The results obtained indicate that the Ti1−x Sn x O2 materials obtained under thermal conditions present higher specific photoactivity than the reference material TiO2 P25, especially when the reaction is performed in a stream of humid oxygen. On the other hand, the comparison with undoped TiO2 prepared in similar conditions shows that the incorporation of Sn significantly increases the photocatalytic oxidation rate. High crystallinity and an adequate anatase to rutile ratio seem to be beneficial for the removal of MCH. In contrast, pure rutile Ti1−x Sn x O2 nanoparticles prepared by autoclaving the amorphous precursor in HCl shows a quite limited photoactivity, despite its high surface area.
Keywords: Photocatalysis; TiO2; Sn; Mixed oxides; Nanoparticles; XRD; Raman; FTIR; Methylcyclohexane; VOC's;

Reaction mechanism of NO decomposition over alkali metal-doped cobalt oxide catalysts by Masaaki Haneda; Yoshiaki Kintaichi; Hideaki Hamada (169-175).
The kinetics of NO decomposition were investigated over alkali metal-doped Co3O4 catalysts. For all the alkali metal-doped Co3O4 catalysts tested, the presence of O2 caused a decrease in the N2 formation rate with reaction orders between −0.26 and −0.40. The reaction orders with respect to NO were between 1.21 and 1.47, which are higher than unity, suggesting that NO decomposition proceeds via a bimolecular reaction. The observation by in situ Fourier transform infrared (FT-IR) spectroscopy confirmed the presence of nitrite (NO2 ) species on the surface under NO decomposition conditions. Isotopic transient kinetic analysis performed using 14NO and 15NO revealed that a surface-adsorbed species, probably NO2 , serves as an intermediate during NO decomposition. We proposed a reaction mechanism in which the reaction is initiated by NO adsorption onto alkali metals to form NO2 species, which migrates to the interface between the alkali metals and Co3O4, the active sites, and then react with the adsorbed NO species to form N2.
Keywords: Direct decomposition; Nitrogen monoxide; Alkali metals; Co3O4; Isotopic transient kinetic analysis;

MgLa mixed oxides as highly active and selective heterogeneous catalysts for Wadsworth–Emmons reactions by M.L. Kantam; H. Kochkar; J.-M. Clacens; B. Veldurthy; A. Garcia-Ruiz; F. Figueras (177-183).
The Wadsworth–Emmons reaction usually run in a stoichiometric mode with a high ratio base/substrate can be achieved catalytically, minimizing the by-products. Diethyl(cyanomethyl)-phosphonate and triethylphosphonoacetate were reacted with different aldehydes on solid bases in batch conditions at 383 K. MgLa mixed oxide was found to be an efficient solid base for the selective synthesis of α-, β-unsaturated esters and nitriles with greater than 80% yields, and avoiding Knoevenagel condensation and aldolisation even with aliphatic aldehydes. The catalyst could be recycled once with high activity. The effect of substituents on phenyl ring proves that the reaction proceeds via a base catalysed mechanism with a low charge on intermediate species.
Keywords: Catalysis by solid bases; Wadsworth–Emmons reaction; Activated hydrotalcites; Fluorinated hydrotalcite; MgLa mixed oxides;

The temperature-programmed activity of a series of oxide-supported (TiO2, Al2O3 and SiO2) Cu catalysts formed from two different Cu precursors (Cu(NO3)2 and CuSO4) for the selective catalytic reduction of NO x using solutions of urea as a reductant have been determined. These activities are compared to those found using NH3 as a reducing agent over the same catalysts in the presence of H2O and it is found that catalysts that are active for the selective reduction of NO x with NH3 are inactive for its reduction using solutions of urea. Poisoning of the surface by H2Oads is not responsible for all of this decrease in activity and it is postulated that the urea is not hydrolysing to form NH3 over the catalysts but rather is oxidising to form N2 or forming passivated layers of polymeric melamine complexes on the surface. The catalysts were characterised by temperature-programmed reduction while temperature-programmed desorption and oxidation of NH3 and temperature programmed decomposition of urea are used to characterise the interaction of both reductants with the various catalysts.
Keywords: Copper; NO x ; SCR–NH3; Urea;

Decomposition of nonionic surfactant on a nitrogen-doped photocatalyst under visible-light irradiation by Sylwia Mozia; Maria Tomaszewska; Beata Kosowska; Barbara Grzmil; Antoni W. Morawski; Kazimierz Kałucki (195-200).
A visible-light-active N-containing TiO2 photocatalysts were prepared from crude amorphous titanium dioxide by heating amorphous TiO2 in gaseous NH3 atmosphere. The calcination temperatures ranged from 200 to 1000 °C, respectively. UV–vis/DR spectra indicated that the N-doped catalysts prepared at temperatures <400 °C absorbed only UV light (E g  = 3.3 eV), whereas samples prepared at temperatures ≥400 °C absorbed both, UV (E g  = 3.10–3.31 eV) and vis (E g  = 2.54–2.66 eV) light. The chemical structure of the modified photocatalysts was investigated using FT-IR/DRS spectroscopy. All the spectra exhibited bands indicating nitrogen presence in the catalysts structure. The photocatalytic activity of the investigated catalysts was determined on a basis of a decomposition rate of nonionic surfactant (polyoxyethylenenonylphenol ether, Rokafenol N9). The most photoactive catalysts were those calcinated at 300, 500 and 600 °C. For the catalysts heated at temperatures of 500 and 600 °C Rokafenol N9 removal was equal to 61 and 60%, whereas TOC removal amounted to 40 and 35%, respectively. In case of the catalyst calcinated at 300 °C surfactant was degraded by 54% and TOC was removed by 35%. The phase composition of the most active photocatalysts was as follows: (a) catalyst calcinated at 300 °C—49.1% of amorphous TiO2, 47.4% of anatase and 3.5% of rutile; (b) catalyst calcinated at 500 °C—7.1% of amorphous TiO2, 89.4% of anatase and 3.5% of rutile; (c) catalyst calcinated at 600 °C—94.2% of anatase and 5.8% of rutile.
Keywords: Visible-light-active photocatalyst; Nitrogen doping; Ammonia; Titanium dioxide; Nonionic surfactant; FT-IR study;

Photocatalytic degradation of iron–cyanocomplexes by TiO2 based catalysts by Rafael van Grieken; José Aguado; María-José López-Muñoz; Javier Marugán (201-211).
The removal of iron–cyanocomplexes in industrial effluents is a difficult process, due to the resistance of these compounds to conventional treatments for cyanide wastewater detoxification. The mechanism of both the homogeneous photolysis of these compounds and their heterogeneous photocatalytic oxidation with Degussa P25 TiO2 and silica-supported TiO2 photocatalysts has been investigated. The activities of the tested catalysts for complexed cyanide degradation were found to be different from those observed for free cyanide photo-oxidation. The best activity was found for the photocatalyst synthesized by supporting 20 wt.% of TiO2 on SBA-15 silica as compared with the commercial catalyst Degussa P25 and the other supported catalysts tested. On the basis of detected intermediate species, a mechanism for iron–cyanocomplexes photodegradation is suggested. The influence of the textural properties of the support and titania loading on the process is discussed. The results point out that the high activity observed, when SBA-15 is used as support of TiO2, seems to be related to the microporosity of the material acting as molecular sieve, which avoids the deactivation of the semiconductor. The porous structure of the SBA-15 material limits the access of the iron–cyanocomplexes to the TiO2 particles whereas the free cyanides homogeneously released can reach the semiconductor surface, being subsequently oxidized to cyanate.
Keywords: Photocatalysis; Supported-TiO2; SBA-15; Cyanide; Ferricyanide; Ferrocyanide; Hexacyanoferrate;

The interaction of DeNO x components comprising NO, NO2, propane, O2 and H2 with a selected Ag/Al2O3 catalyst was studied by in situ FTIR spectroscopy at a fixed temperature where a promoting effect of H2 admixture on the catalytic NO x reduction has been reported to occur. The significant enhancement of nitrato and acetate ad-species from NO and propane, respectively, could be confirmed. New findings are the relative inertness of these species for further reaction progress. Instead, nitrite, nitrito, and nitro species are reactive, and the H2 co-fed favours the formation of those species. Nevertheless, the way H2 interferes with the kind of species formed includes the promotion of oxidative reaction steps evidenced by different effect of H2 on the interaction of the catalyst with NO/O2 and NO2, respectively. During NO2/H2 adsorption adsorbed NO x species and Ag+ ions at the surface are reduced. This prevents abundant formation of stable nitrato species and favour the formation of largely unstable but reactive nitro and nitrito species. Otherwise, NO2 is able to oxidize pre-reduced Ag/Al2O3. Furthermore, indications were found for minor propene formation from propane in the presence of hydrogen.
Keywords: NO; NO2; Propane; Ag/Al2O3; In situ FTIR spectroscopy;

TiO2 photocatalysts promoted by alkali metals by Beata Zielińska; Antoni Waldemar Morawski (221-226).
The new photocatalysts have been obtained by calcination titanium dioxide of anatase type (Tytanpol A11, “Police” Chemical Factory, Poland) with hydroxides (Li, Na, K, Ba) and carbonate (Sr). The obtained materials have been characterised by several analytical methods, like XRD, FT-IR/DRS, DR–UV–vis. The photocatalytic oxidation of phenol as model contamination has been investigated over obtained alkali metals–TiO2. From all investigated photocatalysts only materials based on titanates in perovskite forms (BaTiO3, SrTiO3) have higher activity than pure titanium dioxide A11.
Keywords: Photocatalytic oxidation; Modified titanium dioxide; Phenol;