M. Macias

Bulk and surface characterization of powder iron-doped titania photocatalysts

Some additional contributions to structural and surface features of powder iron-doped titania photocatalysts have been investigated by applying a variety of complementary techniques: temperature programmed desorption-mass spectrometry analysis, non-isothermal thermogravimetric analysis, differential thermoanalysis, infrared spectroscopy, scanning electron microscopy and X-ray diffraction analysis. The effects of iron concentration and pretreatment temperature, over the polymorphic transformation anatase-rutile, were analysed. The results obtained were used in an attempt to correlate structural and surface aspects of catalysts, with their reported activity versus inactivity to photoreduce dinitrogen to ammonia.

Formation of zirconium titanate powder from a sol-gel prepared reactive precursor

A hydroxoperoxo compound of Zr and Ti (HXPZT) was prepared as a reactive powder by a sol-gel method. X-ray diffraction and SEM studies showed it to be formed from small (ca. 10–15 μm) amorphous particles. The effects of progressive thermal treatment were determined by differential thermal analysis, thermogravimetric analysis, temperature programmed desorption, X-ray diffraction, energy-dispersive spectrometry and SEM. The features observed have been attributed to the removal of physically and chemically bonded water and of some nitrogen-containing compound. Crystallization of zirconium titanate occurs at low temperature (650 °C), giving rise to spherical (ca. 2 μm dia.) regular particles.

Simultaneous use of isothermal, nonisothermal, and constant rate thermal analysis (CRTA) for discerning the kinetics of the thermal dissociation of smithsonite

The mechanism of the thermal decomposition of smithsonite has been determined from a comparison of the results obtained from isothermal, linear heating rate (TG), and Constant Rate Thermal Analysis (CRTA) experiments. Two important precautions have been taken in this work. Firstly, the chemical composition of the sample has been checked in order to be sure that pure anhydrous zinc carbonate has been used. Secondly, the experimental conditions have been selected in such a way to avoid the influence of heat and mass transfer phenomena on the forward reaction. It has been shown that the mechanism for the thermal decomposition of smithsonite depends upon temperature. Thus, at temperatures lower than 650 K, approximately, an A0.5 kinetic model describes the reaction, whereas, at temperatures roughly higher than 690 K the above reaction obeys a F1 kinetic law. An interpretation of this behavior is given.

On the influence of chemical processing in the crystallization behaviour of zirconium titanate materials

The aim of this study is to relate the crystallization phenomena of Zr TiO 4 powders starting from different forming gels as precursors. The presence of hydrogen peroxide in the chemical processing of amorphous reactive precursors plays a key role in controlling the crystallization temperature. Thus, the temperature can be as low as 640°C

ZrO2SiO2 mixed oxides: surface aspects, photophysical properties and photoreactivity for 4-nitrophenol oxidation in aqueous phase

Mixed oxides of ZrO2 and SiO2 were synthesized by a sol-gel technique by using zirconyl chloride and commercial silica gel as starting materials. The paper reports some surface features of the powders and the photoreactivity results obtained in the oxidation of 4-nitrophenol in aqueous suspensions of powdered photocatalyst. The optical characterization of the suspension was also performed by measuring the photon flows reflected and absorbed by the reacting system. The photoactivity exhibited by the ZrO2SiO2 samples is very scarce especially if compared with that shown by a TiO2 sample tested at equal experimental conditions. The low photoreactivity can be justified on the basis of two main reasons: (i) the unfavourable optical features of ZrO2SiO2 suspensions in comparison with those of TiO2 ones; and (ii) the lower degree of hydroxylation of ZrO2SiO2 samples with respect to that of TiO2 and, consequently, the less significant oxygen adsorption.

Combined use of XPS, IR and EDAX techniques for the characterization of ZrO2-SiO2 powders prepared by a sol-gel process

Abstract Powder precursor gels with composition x ZrO 2 -(100 × x )SiO 2 , with selected values of x = 8, 14 and 75 mol%, were prepared by a sol-gel chemical processing route, using commercial silica gel and zirconyl chloride. A combination of X-ray photoelectron (XPS) and infrared (IR) spectroscopies has been applied for studying the chemical bondings in silicon, zirconium and oxygen as a function of the nominal zirconia compositions. Furthermore, ZrO 2 /SiO 2 ratios based on semiquantitative XPS and energy-dispersive X-ray analyses (EDAX) showed changes of the surface chemical composition in the as-prepared samples as compared with the thermal-treated powders. These changes have been ascribed to some peculiarities in the processing chemical route when it is applied to this kind of composites.

Anion treatment (F− or SO42−) of AlPO4-Al2O3 (25 wt.-% Al2O3) catalysts: IV. Catalytic performance in the alkylation of phenol with methanol

Abstract Vapour-phase catalytic alkylation of phenol with methanol was carried out in a microcatalytic pulse reactor on AlPO 4 -Al 2 O 3 catalysts, containing various amounts (1–3 wt.-% ) of fluoride or sulfate anions. The methylation reaction gave a mixture of O- and C-alkylated products (C-alkylation taking place preferentially at the ortho -position). The influences of the methanol/phenol molar ratios (0.5–8), the reaction temperature ( 523–673 K ) and both anion type and anion loading upon the conversion of phenol and the selectivities of the products were investigated. The increase in surface acidity by anion loading (especially in the case of fluoride ion) increases both the O-alkylation selectivity (mainly to anisole) and dealkylation processes; so that a lower phenol conversion (smaller pseudokinetic constant) and higher anisole selectivity is found for anion-modified AlPO 4 -Al 2 O 3 catalysts related to unmodified ones. From the distribution of methylated products and their OPE curves, an outline of catalyzed alkylation was presented.

The effect of dosage on the photocatalytic degradation of organic pollutants

Heterogeneous photocatalytic degradation of many organic pollutants, such as phenol and phenol derivatives, may be optimised if the catalyst surface saturation and the appearance and accumulation of non-photocatalytically degradable intermediates is avoided. It has been shown that under certain concentration threshold the highest degradation efficiencies are achieved. Over these concentrations, degradation rates become constant owing to the limited catalyst surface. By the dosage of the contaminant, currently in an aqueous solution, the process may be optimised, thus avoiding the formation of inert intermediates which may be more toxic than the parental compound. The effect of dosage on the photocatalytic degradation of phenol and phenol derivatives, such as salicylic acid and 4-aminophenol has been studied. Comparatively notably higher efficiencies have been obtained compared to those of the high initial single dose experiments (non-dosage), for which high initial concentrations of the organics resulted in the catalysts poisoning. Degussa P-25 and its combination with 13% (w/w) activated carbon, namely AC−TiO2, have been used as catalysts. Almost complete degradations are achieved at low dosage rates (1–2 pmm/min). At higher dosage rates, different processes such as catalyst poisoning predominate, resulting in lower degradation efficiencies.

Structure, texture, acidity and catalytic performance of AlPO4-caesium oxide catalysts in 2-methyl-3-butyn-2-ol conversion

A series of aluminium orthophosphate-caesium oxide systems with various caesium oxide loadings (5-30 wt%) were prepared by impregnation of AlPO 4 with a methanolic solution of caesium acetate and characterized by TG/DTA, XRD, DRIFT, Raman, SEM-EDX, XPS, 27 Al and 31 P MAS NMR and nitrogen adsorption. Aluminium orthophosphate remained amorphous with caesium oxide incorporation when calcined at 873 K for 3 h. After thermal treatment at 1423 K it crystallized in the tridymite form except for materials containing 30 wt% caesium oxide. DRIFT spectroscopy showed that the P-OH stretching vibration at 3670 cm –1 decreased in intensity with caesium oxide loading. Materials at 20-30 wt% caesium oxide did not exhibit any hydroxyl bands. Besides, Al and P atoms remained in tetrahedral coordination as in unmodified aluminium orthophosphate. Moreover, the incorporation of caesium oxide leads, simultaneously, to a progressive decrease in surface area and pore volume (larger at 30 wt% caesium oxide) as well as to an increase in the most frequently occurring pore radius. On the other hand, caesium oxide reduced both the number and strength of acid sites as the caesium content increased. Consequently, 2-methylbut-3-yn-2-ol (MBOH) underwent almost exclusively dehydration to 3-methyl-3-buten-1-yne (acid activity) on pure AlPO 4 , whereas its modification with increasing amounts of caesium oxide developed AlPO 4 -based materials with increased basic properties and hence high selectivities (99 mol%) to the base-catalysed cleavage of MBOH yielding acetone and acetylene.

Critical examination of the DTA techniques used to study the kinetics of solid-state transformations

The use of the different equations proposed to study the kinetics of the glass-to-crystal transition from DTA curves has been extended to include the evaluation of the kinetic parameters of polymorphic transitionS. The transformation kinetics of CaCO3 (aragonite to calcite) and PbO (litharge to massicot) are the examples studied. The kinetic results of both transformations, obtained by the analysis of DTA curves, differ widely from those revealed by the analysis of the isothermal kinetic data. This discrepancy is probably due to erroneous simplifications used in deriving the equations applied in non-isothermal conditions.