Miguel A. Valenzuela

Preparation, characterization and photocatalytic activity of ZnO, Fe2O3 and ZnFe2O4

ZnO, Fe 2 O 3 and ZnFe 2 O 4 were synthesized by precipitation or coprecipitation at constant pH. After aging, washing and drying, the solids were calcined at 800 C. The samples were characterized by XRD, TGA, N 2 physisorption, UV-Vis and TPR. The photocatalytic degradation of phenol was studied in a batch reactor and TiO 2 (Degussa, P-25) was used as a reference catalyst. Pure ZnO and Fe 2 O 3 were obtained at 800 C, whereas ZnFe 2 O 4 and α-Fe 2 O 3 (segregated) were detected in the zinc ferrite sample. TPR experiments showed the reduction peaks in the range of 350-800 C only with Fe 2 O 3 and ZnFe 2 O 4 , attributed to Fe 3+ to Fe 2+ reduction and complete dehydroxylation. The photodegradation of phenol (samples calcined at 800 C) showed small but significant variations that decreased in the order: TiO 2 > ZnFe 2 O 4 > ZnO > Fe 2 O 3 . Subproducts such as hydroquinone, catechol and benzoquinone were mainly detected. In particular, attention was focused on our prepared materials because after 24 h they did not dissolve as happened with TiO 2 .

The influence of the preparation method on the surface structure of ZnAl2O4

Abstract The effect of preparation method (sol-gel, wet mixing or coprecipitation) on the surface structure of zinc aluminate is reported. Coprecipitated and sol-gel prepared ZnAl2O4 were found to be present spinel structure. If calcium was added during synthesis, it was found to be deposited on top of the spinel surface while, if tin was added, the surface of ZnAl2O4 was reconstructed. These aluminates were impregnated with platinum and tested in the isobutane dehydrogenation. The characterization techniques used were LEIS and X-ray diffraction.

Hydrogen interactions and catalytic properties of platinum-tin supported on zinc aluminate

Abstract Pt-Sn/ZnAl2O4 catalysts prepared by two-step impregnation (first tin), with a Sn/Pt atomic ratio ranging from zero to 6.72, were characterized by hydrogen chemisorption, temperature-programmed reduction and tested in isobutane dehydrogenation. Metal dispersion correlates linearly with reaction rate; both parameters reach a maximum when the Sn/Pt atomic ratio is about one. The activity of the sites capable of hydrogen chemisorption, as expressed in turnover frequency number, TOF, decrease as the tin concentration is increased. From theoretical ab-inito calculations, it is proposed that tin reduces the platinum-hydrogen charge transfer responsible for hydrogen dissociation through an orbital overlapping.

Comparison of the structural and acid–base properties of Ga- and Al-containing layered double hydroxides obtained by microwave irradiation and conventional ageing of synthesis gels

Structural and acid–base properties of Mg/Al and Mg/Ga layered double hydroxides (LDHs) obtained by microwave irradiation of the co-precipitated gels have been investigated and compared to those of samples conventionally aged by prolonged hydrothermal treatment of the gels. Similar crystallinities and chemical compositions were obtained whatever the synthesis method used. Besides, all samples, and remarkably the Ga-containing LDHs with a molar ratio Mg ∶ Ga = 4.5, exhibited pure lamellar phases. The acid–base properties of the mixed oxides obtained by calcination of the LDHs have been examined by microcalorimetric adsorption of CO2 and by FTIR spectroscopy upon CH3CN interaction. These techniques gave evidence that the number and strength of acid and basic sites were influenced by the nature and amount of the trivalent cation, as well as by the preparation method.

Synthesis and characterization of nanocrystallite MgAl2O4 spinels as catalysts support

Because of the high melting point, good chemical stability and mechanical strength, spinel-type solid, MgAl2O4, is widely applied as ceramic materials. Recently, there has been growing interest in the utilization of magnesium-aluminate spinel as catalysts or catalysts support in the fields of environmental catalysis, petroleum processing and fine chemicals production [1–3]. For example, MgAl2O4 is an important base catalyst which efficiently catalyzes the coupling reactions of short chain linear alcohols [4] and mercaptan oxidation [5]. Due to its strong basicity and the high capacity of SO2 picking-up, magnesium-aluminate is also used as a co-catalyst, mixed with FCC catalysts for controlling SOx emission in the fluid catalytic cracking units [6–8]. It is reported that when Pt-Sn metals are supported on MgAl2O4, instead of alumina, in the case of propane dehydrogenation, the Pt-Sn/MgAl2O4 catalysts show a high resistance-sintering ability [9]. Catalytic tests also confirm that this kind of catalysts have satisfactory activity and selectivity in propane dehydrogenation reaction in steam as reaction media [10]. Magnesium-aluminate spinel is, therefore, of considerable interest in catalysis from a technical point of view. Differing from ceramic materials, magnesiumaluminate spinel used as catalysts support or catalysts by itself, show a relatively large surface area, small crystalline size and special active sites, which can be controlled by its preparation method. The ceramic magnesium-alumina spinels can be prepared by wet mixing of magnesium oxide and aluminum hydrogel, coprecipitation and sol-gel techniques with the calcination temperatures between 1200 and 2000 ◦C, resulting in a bulk-sintered body with very small surface area [11–14]. In the catalysis field, however, magnesiumaluminate spinel is generally prepared at a moderate temperature range from 600 to 900 ◦C to obtain a suitable pore distribution and large surface area. In such case, coprecipitation and sol-gel methods are often used [3, 6, 8, 10]. In the present work, we focus on the preparations of magnesium-aluminate spinels which will be used as Pt-Sn catalysts support for propane dehydrogenation, by using traditional coprecipitation techniques with low cost precursors. Samples A, B and C were prepared by using Mg(NO3)2·5H2O and Al(NO3)3·9H2O as precursors and concentrated ammonia solution as precipitating agent. Two solutions were respectively prepared by dissolving a given amount of magnesium and aluminum nitrates which was dependent on the designed composition of the samples. At the same rate, these two solutions were added into a 2000 ml flask containing 500 ml of deionized water. During the addition, the formed slurry was stirred and the pH value was respectively controlled at about 7, 10 or 12 by adding ammonia solution. Afterwards, the slurry was continuously stirred for an hour and then aged at room temperature overnight. The aged slurry was filtered and washed with deionized water. The filtered cake was dried at 120 ◦C for 16 h and then calcined at 800 ◦C for 8 h in air in a furnace. Samples D and E were also prepared by coprecipitation of magnesium nitrate and aluminum nitrate solutions but with (NH4)2CO3 as precipitating agent. The preparation procedures were the same as shown above. Samples F, G and I which respectively correspond to a nominal atomic ratio of Al to Mg of 1.8, 2.0 and 2.3, were prepared by wet mixing method. Alumina hydrogel (boehmite) and magnesium oxide were mixed in 1 liter of deionized water with continuous stirring to obtain a suspended solution. The filtered materials were dried at 120 ◦C for 16 h and then calcined at 800 ◦C for 8 h. The real chemical compositions of the samples were determined by using atomic absorption spectroscopy (AAS). Measurements were carried out on atomic absorption apparatus (Perkin Elmer Company model 5000). The specific surface area, pore volume and pore size distribution of the various samples were obtained from nitrogen adsorption–desorption isotherms measured on a model Digisorb 2600 analyzer.

Comparison between sol-gel, coprecipitation and wet mixing synthesis of ZnAl2O4

ZnAl2O4 was prepared by hydrolyzing a mixture of aluminum alkoxide with zinc nitrate dissolved in hexylene glycol and calcining at 800°C. The results are compared with those obtained by wet mixing and coprecipitation. The sol-gel method produces solids whose surface areas and pore volumes are 100% larger and with a more homogeneous pore size distribution.

Reactivity of NiO for 2,4-D degradation with ozone: XPS studies.

2,4-Dichlorophenoxyacetic acid (2,4-D) is usually used as a refractory model compound that requires a prolonged reaction time for mineralization. In this study, we found that nickel oxide (NiO) significantly improved 2,4-D degradation and mineralization in reaction with ozone. Other metal oxides, such as titania, silica and alumina, were also tested in this reaction, so that, the mineralization degree was almost the same for all of them (ca. 25%), whereas NiO showed more than 60% in 1h. These outstanding results led us to study in more depth the role of NiO as catalyst in the degradation of 2,4-D. For instance, the optimum NiO loading amount was 0.3 g L(-1). The catalytic ozonation showed a high stability after three reaction cycles. With the aim of identifying the surface species responsible for the high activity of NiO, besides knowing the byproducts during the degradation of 2,4-D, XPS and HPLC were mainly used as analytical tools. According to the results, the mineralization of 2,4-D was directly influenced by the adsorbed chlorate organic compounds and oxalate group onto NiO. Therefore, NiO plays a true role as a catalyst forming surface compounds which are subsequently decomposed causing an increase in the mineralization efficiency. In addition, it was possible to identify several degradation byproducts (2,4-diclorophenol, glycolic, fumaric, maleic and oxalic acids) that were included in a rational reaction pathway. It was proposed that 2,4-D elimination in presence of NiO as catalyst is a combination of processes such as: conventional ozonation, indirect mechanism (OH) and surface complex formation.

Selective hydrogenation of carvone and o-xylene on Pd–Cu catalysts prepared by surface redox reaction

A series of Pd and Pd–Cu bimetallic catalysts were prepared by surface redox reaction and coimpregnation methods. A comparison between the preparation methods in the activity and selectivity for o-xylene and carvone hydrogenation was done. FTIR studies showed that the Cu is deposited mainly in the low coordination Pd sites when the catalyst was prepared by redox, whereas a random distribution of Cu on the Pd surface was obtained for the catalyst prepared by coimpregnation. The activity and selectivity of bimetallic catalysts, were found to depending on the method of preparation used. For o-xylene hydrogenation in redox catalyst, the selectivity to trans-1,2-dimethylcyclohexane was lower than that obtained in the Pd/SiO2 reference catalyst, whereas in the coimpregnated catalyst, the trans isomer selectivity was higher. In the redox preparation, the selective hydrogenation of carvone favors the formation of carvotanacetone, whereas the coimpregnated catalyst favors the selectivity to carvomenthone.

Effect of microwave irradiation time on the synthesis of zirconia-pillared clays

In this work, we present the synthesis, at different zirconia/clay ratios, of zirconia-pillared montmorillonite in presence of microwave irradiation. The effect of irradiation time (5, 10, 20 and 30 min) on the structure and properties of the solids was also studied. The samples were characterized by X-ray diffraction, nitrogen adsorption (BET surface area) and thermo-programmed desorption of ammonia. It is shown that the irradiation time is critical between 5 and 10 min. For times longer than 10 min, no effect on the pillared clay features is observed.

Propane dehydrogenation activity of Pt and Pt–Sn catalysts supported on magnesium aluminate: influence of steam and hydrogen

Propane dehydrogenation was carried out in hydrogen and steam as reaction media on Pt/MgAl2O4 and Pt–Sn/ MgAl2O4 catalysts. A wide range of Pt and Pt–Sn concentrations was explored. Monometallic Pt catalysts were completely poisoned by steam. Concerning bimetallic Pt–Sn catalysts, tin played an important role related to the activation of platinum particles when the reaction was carried out in steam. On the other hand, tin inhibited cracking reactions leading to an increase of catalysts stability. Activation energy in hydrogen was the same for monometallic and bimetallic catalysts: 22 kcal/mol; while for the reaction in steam, values ranging from 10 to 15 kcal/mol were obtained.