New EXAFS Measurements by XEOL and TEY on Porous Silicon

Results of an EXAFS investigation on porous Silicon carried out by X-ray Excited Optical Luminescence (XEOL) and Total Electron Yield (TEY) techniques, at the Si K absorption edge, are reported. For the first time XEOL spectra of porous silicon have been recorded in a wide energy range (1800–2500 eV) and EXAFS signals have been singled out from them. Simultaneous TEY and XEOL measurements yield to different results: in particular TEY-EXAFS is sensitive up to the third coordination shell of Si, while XEOL-EXAFS reveals only the contributions of the first two coordination shells; moreover they show a different dependence on changes of the etching parameters. This evidences the sensitivity of XEOL technique to the local structure of the quantum confined luminescent sites. The dependence of the light emission properties on the main preparation parameters and their influence on the short-range structure of red and yellow porous silicon samples are also investigated.     

Redispersion of Pd on acidic supports and loss of methane combustion activity during the selective reduction of NO by CH4

The CH₄ combustion, occurring under selective NO reduction conditions, has been investigated over several Pd catalysts. In the presence of NO and support acidity, the combustion activity of Pd is drastically inhibited. Highly dispersed Pd/acidic zeolites compare favorably with other SCR catalysts. At 500°C, a 0.3% Pd/H-ZSM-5 sample exhibited a better performance than a typical Co-ZSM-5 catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamic “operando” observation of 1 wt% Pd-based TWCs: Simultaneous XAS/DRIFTS/mass spectrometry analysis of the effects of Ce0.5Zr0.5O2 loading on structure, reactivity and performance

The behavior of 1 wt% Pd-TWCs (three-way catalysts), containing up to 33 wt% Ce_0.5Zr_0.5O₂ is followed under reducing (CO) and oxidizing (NO) cycling conditions. The dynamic behavior of these systems is analyzed using a synchronous, time-resolved energy dispersive X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) set-up with subsecond time resolution. Two main physico-chemical phenomena corresponding to noble metal morphological (size/shape) changes and the redox behavior of the noble metal–promoter interface are shown to control the TWC response to NO/CO cycling conditions. Metal-only aspects strongly influence N–O dissociation and N–N coupling steps while the metal–promoter interface has a global influence on both N₂ and CO₂ formation via oxygen handling (storage/release) properties. The relative importance of these two phenomena is studied as a function of the Ce_0.5Zr_0.5O₂ promoter content of the catalysts.     

Changes in metal–sulfur bond energy in promoted and unpromoted molybdenum catalysts

Insight into the changes in the metal–sulfur bond energy in unpromoted and promoted (Co, Ni, or Fe) MoS₂ catalysts is obtained by temperature‐programmed reduction (TPR) studies of in situ sulfided catalysts. The experiments reveal a significant decrease in the metal–sulfur bond energy for the more active Co–Mo–S and Ni–Mo–S structures as compared to MoS₂. A similar decrease is not observed for the less active Fe–Mo–S structure. The sulfur coordination is directly measured by in situ EXAFS and the results support that one of the roles of the promoter atoms is to facilitate vacancy formation. The findings are in accord with recent density functional theory calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic Activity of Palladium Supported on Mesoporous Zirconium Oxide in Low-Temperature Methanol Decomposition

Methanol decomposition to hydrogen and carbon monoxide can be effectively catalyzed at 160–220°C over palladium supported on mesoporous zirconium oxide by the deposition–precipitation method. The electronic state and the particle size of palladium in the catalyst are very similar to those of palladium supported on non-porous zirconium oxide, but the palladium surface area of the former sample is smaller than that of the latter. However, the activity of the mesoporous catalyst is significantly higher than that of the non-porous catalyst, implying a promotional effect of the mesoporous structure.     

Confirmation of sulfur tolerance of bimetallic Pd–Pt supported on highly acidic USY zeolite by EXAFS

The sulfur tolerance (i.e., degree of sulfidation) of Pd and Pt in sulfided bimetallic Pd–Pt catalysts (Pd : Pt mole ratio of 4 : 1) supported on USY (ultrastable Y) zeolites (SiO₂/Al₂O₃ = 10.7, 48, and 310) was investigated using an extended X‐ray absorption fine structure (EXAFS) method. The sulfidation of the catalysts was done in a 1000 ppm H₂S–2% H₂/N₂ stream at 573 K for 0.5 h. In the Fourier transforms of Pd K‐edge and Pt L_III‐edge EXAFS spectra, both of the peaks due to metallic Pd and to metallic Pt for the Pd–Pt/USY (SiO₂/Al₂O₃ = 10.7) catalyst remained most after sulfidation. Further, the results of the Fourier transforms confirmed that the sulfur tolerance of both Pd and Pt decreased with increasing SiO₂/Al₂O₃ ratio, suggesting that Pd and Pt become sulfur‐tolerant when Pd–Pt bimetallic particles are supported on highly acidic USY zeolite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Estimation of equilibrium surface precipitation constants for trivalent metal sorption onto hydrous ferric oxide and calcite

Equilibrium constants for modeling surface precipitation of trivalent metal cations (M³⁺) onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energies of formation, () of the surface precipitates. The surface precipitation reactions were derived from Farley et. al. [K.J. Farley, D.A. Dzombak, F.M.M. Morel, J. Colloid Interface Sci. 106 (1985) 226] surface precipitation model, which are based on surface complexation model coupled with solid solution representation for surface precipitation on the solid surface. The values were correlated through the following linear free energy relations and where ‘ss’ stands for the end-member solid component of surface precipitate, is in kJ/mol, r_M³⁺ is the Shannon-Prewitt radius of M³⁺ in a given coordination state (nm), and is the non-solvation contribution to the Gibbs free energy of formation of the aqueous M³⁺ ion. Results indicate that the above surface precipitation correlations are useful tools where experimental data are not available.     

EXTRACTION OF HEAVY METAL IONS WITH 3-[(DIOCTYLAMINO)METHYL]ALIZARIN

3-[(Dioctylamino)methyl]alizarin (C₈AL), which is endowed with high solubility in nonpolar organic solvents through introduction of dioctylaminomethyl group into the alizarin nucleus was studied for extracting heavy metal ions such as Cu(II), Zn(II), Cd(II), Co(II), Mn(II), and Ni(II), from aqueous solution.Cu(II) was most readily extracted into chlorobenzene at low pH and thus was separated from other metal ions. The metal ions stabilized in alkaline solution in the presence of water-soluble chelating agents were found to be extracted by this, alizarin-type extraction agent into chlorobenzene with the assistance by lipophilic quaternary ammonium salt, Capriquat? (methyltrioctylammonium chloride, Q^?C1^?).The proton dissociation process of CRAL was studied, and the mechanism of these metal extractions was discussed.The separation of Cu(II) and Ni(II) from the mixture with other divalent metal ions was also studied.     

Ternary CdxZn1−xSe deposited on Ag (1 1 1) by ECALE: Electrochemical and EXAFS characterization

This paper reports the characterization of ternary II-VI semiconductor nanocrystals, deposited by the electrochemical atomic layer epitaxy (ECALE) technique.In particular, morphological and structural properties of the ternary compounds of formula Cd_xZn_1−xSe deposited on Ag (1 1 1) have been characterized as a function of composition. The number of the attainable x values is limited by the necessity of using well-defined ZnSe/CdSe deposition sequences. However, the quantitative analysis carried out on the basis of both electrochemical and extended X-ray absorption fine structure (EXAFS) experiments indicates that the ECALE method is a successful way of controlling the composition of Cd_xZn_1−xSe. In addition, the electrochemical measurements show that the amount of deposition is minimum in correspondence to the compound with x = 0.5, thus corroborating the hypothesis of a higher degree of disorder suggested both by morphological and structural investigation. The morphology was studied by atomic force microscopy (AFM). The structure of the films is estimated by EXAFS which is a powerful technique for the analysis of the local structure around chosen atoms.     

A Comparative Study of Palladium and Copper Catalysts in Methanol Synthesis

Catalytic activity of copper supported on cerium oxide (Cu/CeO₂) in methanol synthesis from carbon monoxide and hydrogen at 473 K is similar to that of ceria-supported palladium (Pd/CeO₂). Both catalysts contain 0.3 mmol g^-1 of the active metals and the activities on a mole basis of the active metals are significantly higher than that of a commercial copper catalyst. Analyses bt EXAFS suggest that the particle size of copper in Cu/CeO₂ is similar to that of palladium in Pd/CeO₂. The activity of copper supported on silica is very low even at 523 K although the particle size of copper is close to that in Cu/CeO₂. Hence, cerium oxide promotes the activity of copper as can be seen in Pd/CeO₂.