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Applied Catalysis B, Environmental (v.57, #4)
Catalytic hydrogenation of nitrates in water over a bimetallic catalyst by Jacinto Sá; Hannelore Vinek (pp. 247-256).
Bimetallic (Pd/Cu) catalysts supported on alumina, which promote the liquid-phase hydrogenation of nitrates, were synthesized using catalytic reduction and consecutive impregnation and were characterized by TEM with EDX, XRD, TPR, BET, and AAS. The catalysts prepared by catalytic reduction have very small particle sizes that are unchanged even after reduction. The two metals are located near each other, as revealed in the EDX profile. Due to the close contact between Pd and Cu, Cu is stabilized in a low oxidation state. The active sites responsible for activity and selectivity in the reduction of nitrates are discussed. These catalysts show high activity for nitrate degradation and good selectivities to nitrogen.
Keywords: Pd-Cu/Al; 2; O; 3; Catalytic reduction synthesis procedure; TEM–EDX; XRD; TPR
Mechanistic approach of the combined (iron–TiO2) photocatalytic system for the degradation of pollutants in aqueous solution: an attempt of rationalisation by Hana Měšt’ánková; Gilles Mailhot; JaromÃr Jirkovský; Josef Krýsa; Michèle Bolte (pp. 257-265).
Kinetics of oxidative photodegradation of Monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in different photocatalytic systems (iron, TiO2 and combined system iron+TiO2) were investigated and compared. The influence of iron addition on TiO2 photocatalyst and of TiO2 on the photocatalytic cycle Fe(III)/Fe(II) were carefully studied. A very positive effect of iron addition was observed. This phenomenon was more and more pronounced when TiO2 concentration was lower. In a suspension of TiO2 (24mgL−1) with addition of Fe(III) (3×10−4molL−1) the measured rate constant was similar to that obtained in a suspension of TiO2 with a concentration more than 20 times higher (500mgL−1). The mechanistic approach carried out in this study allows us to identify the main reactions governing the combined system and a photochemical cycle was proposed. The optimisation of the photocatalytic systems was obtained when each photocatalyst plays a specific role: Fe(III) as a mainOH radicals source and TiO2 as an oxidizing agent of Fe(II).
Keywords: Iron; TiO; 2; Monuron; Photodegradation
Redox behavior of palladium at start-up in the Perovskite-type LaFePdO x automotive catalysts showing a self-regenerative function by Mari Uenishi; Masashi Taniguchi; Hirohisa Tanaka; Mareo Kimura; Yasuo Nishihata; Junichiro Mizuki; Tetsuhiko Kobayashi (pp. 267-273).
In order to elucidate the superior start-up activity of LaFePdO x catalysts in practical automotive emission control, the redox property of Pd species in a Perovskite-type LaFe0.95Pd0.05O3 catalyst was studied at temperatures ranging from 100 to 400°C using X-ray spectroscopic techniques. In a reductive atmosphere, and even at temperatures as low as 100°C, Pd0 species is partially segregated out onto the catalyst surface from the B-site of the Perovskite-type matrix of LaFe0.95Pd0.05O3. Passing through successive oxidizing atmospheres, the segregated Pd0 species is re-oxidized into Pd2+ at 200–300°C. The formation of a solid solution between the re-oxidized Pd species and the Perovskite-type matrix begins to be seen at around 400°C and accelerates at higher temperatures. Thus a quasi-reversible redox reaction between the surface Pd0 and the cationic Pd in the LaFe0.95Pd0.05O3 matrix takes place. The start-up activity of LaFePd xO x catalysts can be attributed to Pd0 that segregates under the reductive atmosphere which is a natural part of the redox fluctuation in automotive exhaust gases at 100–200°C.
Keywords: Perovskite; Three-way catalyst; Self-regenerative function; Crystal structure; Automotive emission; The intelligent catalyst
Photocatalytic decomposition of methyl tert-butyl ether in aqueous slurry of titanium dioxide by Yujing Zang; Ramin Farnood (pp. 275-282).
The photocatalytic degradation of methyl tert-butyl ether (MTBE) was investigated in the aqueous slurry of titanium dioxide (TiO2) particles irradiated with xenon lamp in a batch reactor. The reaction was found to follow a pseudo-first-order kinetics and the reaction rate increased by raising the TiO2 loading and the UV light intensity. The reaction rate constant was proportional to the square root of the UV intensity. An upper plateau level was observed for the reaction rate constant, corresponding to a steady photoproduction of hydroxyl radicals at high catalyst concentrations. The experimental data was analyzed using a modified Turchi–Ollis model to account for changes in the catalyst loading. It was found that this model predicted the observed trend in the pseudo-first-order rate constant if the reaction pathway involves the interaction between adsorbed hydroxyl radicals and solvated molecules.
Keywords: Photocatalyst; AOP; Ultraviolet light; MTBE; Titanium dioxide; Reaction kinetics
Carbon supported Pt–Co alloys as methanol-resistant oxygen-reduction electrocatalysts for direct methanol fuel cells by José Ricardo Cezar Salgado; Ermete Antolini; Ernesto Rafael Gonzalez (pp. 283-290).
The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co (Pt/C and Pt–Co/C) alloy electrocatalysts was investigated in sulphuric acid (both in the absence and in the presence of methanol) and in direct methanol fuel cells (DMFCs). In pure sulphuric acid Pt–Co/C alloys showed improved specific activity towards the oxygen reduction compared to pure platinum. In the methanol containing electrolyte a higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. The onset potential for methanol oxidation at Pt–Co/C was shifted to more positive potentials. Accordingly, Pt–Co/C electrocatalyts showed an improved performance as cathode materials in DMFCs.
Keywords: Pt–Co/C alloy; Oxygen reduction electrocatalyst; Direct methanol fuel cells
Immobilized Fe(III)-HY: an efficient and stable photo-Fenton catalyst by M. Noorjahan; V. Durga Kumari; M. Subrahmanyam; Lipsa Panda (pp. 291-298).
This article presents preparation, characterization and evaluation of an efficient heterogeneous Fe(III)-HY catalyst for photo-assisted Fenton reaction. Fe(III) ions are immobilized on HY zeolite using different loadings by impregnation, calcination and the activity of the catalyst is evaluated by the degradation of phenol. To initiate a photo-Fenton reaction, suspended Fe(III)-HY in solution is irradiated using UV light to form Fe(II)-HY necessary for the reaction to go. The effect of Fe loadings, H2O2 concentration, pH and quenching on photo-Fenton reaction are studied. The results obtained clearly show that 0.25wt.% Fe(III)-HY is efficient in the degradation of phenol at pH=6. Further the efficiency of Fe(III)-HY is compared with that of a homogeneous photo-Fenton reaction and the increased rate of reaction on Fe(III)-HY highlights the synergistic role of zeolite. Heterogeneous Fe(III)-HY in photo-Fenton reaction allows a wide range of pH for reaction against the narrow pH range in homogeneous system. The system is further subjected to evaluate its stability in solid state. Firstly the reaction solution containing Fe(III)-HY catalyst on irradiation is analyzed for Fe ions with atomic absorption spectroscopy (AAS) and also by calorimetry using 1,10-phenanthroline ( o-phen) to find out any Fe leaching from the catalyst and the results show insignificant leaching of Fe (<0.3ppm) at maximum loading of Fe under experimental conditions. Secondly, the irradiated Fe(III)-HY is complexed with o-phen and it is subjected to Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance spectroscopy (DRS) and electron spectroscopy for chemical analysis (ESCA) studies to detect and confirm the oxidation state of Fe in solid state. Critical analysis of these studies clearly show that Fe(III)-HY on irradiation changed to Fe(II)-HY and it is intact with the surface during the course of the reaction. The DRS spectra further evidences complexation of Fe(II) with o-phen. The stability of the catalyst is established by recycling studies.
Keywords: Heterogeneous photo-Fenton; Homogenous photo-Fenton; Fe(III)-HY; Phenol degradation
Influence of the preparation method on the structure–activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation by Theodora Ataloglou; John Vakros; Kyriakos Bourikas; Christina Fountzoula; Christos Kordulis; Alexis Lycourghiotis (pp. 299-312).
In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.Three series of catalysts of varying Co content (up to 21wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–pvi≫pvi≫edf, nta–pvi≫edf≈pvi and edf>nta–pvi>pvi.The catalysts prepared were characterized using various techniques (BET, UV–vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on Co3O4-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned Co3O4 crystallites: the ratio ‘amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step’ closely related with the ratio ‘amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step’ as well as the composition of the precipitated phase.The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)62+·2NO3−. Upon calcination it is transformed into loosely bounded Co3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts.The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800°C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content.The application of the nta–pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(II)–nta]− and [Co(II)–2nta]4− complexes, in which the Co(H2O)62+ complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta–pvi catalysts a precipitated phase containing the [Co(II)–nta]−·NH4+(or H+) and [Co(II)–2nta]4−·4NH4+ (or 4H+) complex salts predominates. Upon calcination these are transformed into Co3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity.
Keywords: Cobalt oxide catalysts; Preparation method; Equilibrium adsorption; Chelating agent; VOCs combustion; Benzene oxidation; DRS; XPS; TPR