Structural and morphological investigation of ceria-promoted Al(2)O(3) under severe reducing/oxidizing conditions

A series of CeO(2)/Al(2)O(3) samples with different ceria loadings in the range 0-25 wt % (0, 2, 5, 7.5, 15, and 25%) were prepared by incipient wetness and studied using several complementary techniques such as Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman, high-resolution transmission electron microscopy (HRTEM), and extended X-ray absorption fine structure (EXAFS). The aim of the investigation was to understand the behavior of ceria when deposited on alumina and treated under oxidizing and reducing conditions at high temperature (T >/= 1273 K). It is shown that ceria can partially stabilize alumina toward the formation of low-surface-area phases up to 1373 K under oxidizing conditions, while enhanced stabilization is observed under reducing conditions, being effective up to 1473 K. A detailed quantitative temperature-programmed reduction (TPR) analysis made at different loadings and calcination temperatures allowed us to identify three characteristic regions where the reduction of small and large ceria crystallites occurs with the formation of CeAlO(3) crystallites at high temperature. These are likely responsible for surface-area stabilization. For dispersed ceria samples, reduction takes place almost exclusively at low temperature (<700 K), while a shift to higher temperatures is observed upon increasing the ceria particle size. A fraction of Ce, in samples at low loadings, is stable in the lower oxidation state, even if subjected to strongly oxidizing conditions.     

On the optimization of a conservation law

Consider the Cauchy problem for a conservation law and assume that an integral functional on its solution is defined. In this note we obtain an Euler-Lagrange equation for the stationary points of this functional. An application to the optimal management of traffic flows is considered.Received: 29 April 2002, Accepted: 24 October 2002, Published online: 4 September 2003Mathematics Subject Classification (2000): 35L65, 49K20, 90B20The authors thank Stefano Bianchini for useful discussions.     

Characterization of Pd-Cu alloy nanoparticles on gamma-Al2O3-supported catalysts

Cu-Pd/Al2O3 bimetallic catalysts have been characterized by XRD, TEM, and EDX techniques. The surface structure has been investigated by FT-IR spectroscopy of low-temperature adsorbed CO in the reduced and in the oxidized state. Evidence has been provided of the formation of Cu-Pd alloy nanoparticles, both of the alpha-phase (disordered fcc) and of the beta-phase (ordered CsCl-type). IR spectra suggest that Cu likely decorates the edges while Pd mostly stays at the main faces. Part of copper disperses as Cu+ on the support even after reduction. The presence of copper seems to modify strongly the sate of oxidized Pd centers in oxidized high-Pd content materials. The redox chemistry of the system, where Pd is reduced more easily than Cu, appears to be very complex.     

Dielectric properties of selected laterally fluoro-substituted 4,4′′-dialkyl,dialkoxy and alkyl-alkoxy [1:1′;4′:1′′]terphenyls

Measurements of the complex permittivity, ε*?=?ε′ – iε″, within the frequency range 200 Hz to 10 MHz for 15 laterally fluoro-substituted terphenyls have been conducted. In most cases the substances exhibited the nematic phase over a broad temperature range. All substances were characterised by negative dielectric anisotropy, and are potentially useful for vertical alignment mode systems. The static permittivity tensor components have been analysed in relation to the dipole structure of the molecules. Dielectric relaxation processes observed in the liquid crystalline (LC) and solid rotator (R) phases (obtained by slow cooling of the samples) are characterised by calculation of the relaxation times and activation barriers. The rotation motions around the short axes are typical for LC phases, whereas rotations about the long axes, accompanied in some cases by internal motions, are present in the R phase.     

Synthesis of novel 1,4‐dihydropyrido[3′,2′:5,6]thiopyrano[4,3‐c]‐pyrazoles and 5H‐pyrido[3′,2′:5,6]thiopyrano[4,3‐d]pyrimidines as potential antiproliferative agents

The synthesis of new planar derivatives characterized by the presence of a pyridothiopyranopyrazole or pyridothiopyranopyrimidine nucleus, carrying a substituted aryl group, is reported. The novel 1,4‐dihydropyrido[3′,2′:5,6]thiopyrano[4,3‐c]pyrazole derivatives were obtained by condensation of 2,3‐dihydro‐3‐hydroxymethylenethiopyrano[2,3‐b]pyridin‐4(4H)‐ones with appropriate hydrazines. The preparation of 2‐substituted pyrido[3′,2′:5,6]thiopyrano[4,3‐d]pyrimidines was accomplished from the intermediate 2,3‐dihy‐dro‐3‐dimethylaminomethylenethiopyrano[2,3‐b]pyridin‐4(4H)‐ones by reaction with the appropriate binucleophile amidines. The antiproliferative activity of some new products was tested by an in vitro assay on human tumour cell lines (HL‐60 and HeLa), but none of them showed any significant effects in the tests performed. Accordingly, linear flow dichroism measurements indicated their inability to form a molecular complex with DNA.     

Synthesis,coordination to Rh(I), and hydroformylation catalysis of new beta-aminophosphines bearing a dangling nitrogen group: an unusual inversion of a Rh-coordinated P center

Variants of the beta-aminophosphine L(1) [Ph(2)PCH(2)CH(Ph)NHPh] containing additional nitrogen donor functions have been prepared. These functions are branched off the C atom adjacent to the P atom, or the P atom itself. Ligand [Ph(2)PCH(o-C(6)H(4)NMe(2))CH(Ph)NHPh] has been obtained as a mixture of two diastereomers L(3A) and L(3B) by lithiation of L(2) [Ph(2)PCH(2)(o-C(6)H(4)NMe(2))] with n-BuLi followed by PhCH=NPh addition and hydrolysis. The diastereomers have been separated by fractional crystallization from ethanol. Ligand Et(2)NCH(2)P(Ph)CH(2)CH(Ph)NHPh has been obtained as a mixture of two diastereomers L(5A) and L(5B)(starting with P-Ph reductive cleavage of L(1) by lithium and subsequent hydrolysis to give PhP(H)CH(2)CH(Ph)NHPh (mixture of two diastereomers L(4A) and L(4B)). The latter reacts with diethylamine and formaldehyde to afford the L(5) diastereomeric mixture. Complexes RhCl(CO)(L) (L = L(3A), 1(A); L(3B), 1(B); L(5A/B), 2(A/B)) were obtained by reaction of [RhCl(CO)(2)](2) and the appropriate ligand or ligand mixture. Complexes 1(A), 1(B), and 2(A) have been isolated in pure form and characterized by classical techniques and by single-crystal X-ray diffraction. All structures exhibit a bidentate kappa-P,kappa-N(NHPh) mode similar to the complex containing L(1). While complexes 1(A) or 1(B) are stable in CDCl(3) solution, complex 2(A) slowly converts to its diastereomer 2(B). This unexpected epimerization appears to take place by inversion at the Rh-coordinated P center, an apparently unprecedented phenomenon. A mechanism based on a reversible P-C bond oxidative addition is proposed. The influence of the pendant nitrogen function of the diaminophosphines L(3A) and L(5A/B) on the rhodium catalytic activity in styrene hydroformylation has been examined and compared to that of the aminophosphines L(1) or L(2). The observed trends are related to the basicity of the dangling amine function and to its proximity to the metal center.     

Switchable Polymerization Triggered by Fast and Quantitative Insertion of Carbon Monoxide into Cobalt–Oxygen Bonds

A strategy that uses carbon monoxide (CO) as a molecular trigger to switch the polymerization mechanism of a cobalt Salen complex [salen=(R,R)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine] from ring‐opening copolymerization (ROCOP) of epoxides/anhydrides to organometallic mediated controlled radical polymerization (OMRP) of acrylates is described. The key phenomenon is a rapid and quantitative insertion of CO into the Co?O bond, allowing for in situ transformation of the ROCOP active species (Salen)Co^III‐OR into the OMRP photoinitiator (Salen)Co^III‐CO₂R. The proposed mechanism, which involves CO coordination to (Salen)Co^III‐OR and subsequent intramolecular rearrangement via migratory insertion has been rationalized by DFT calculations. Regulated by both CO and visible light, on‐demand sequence control can be achieved for the one‐pot synthesis of polyester‐b‐polyacrylate diblock copolymers (?<1.15).     

Synthesis of the novel conjugated ω,ω′-diaryl/heteroaryl hexatriene system with the central double bond in a heteroaromatic ring: photochemical transformations of 2,3-divinylfuran derivatives

New β,β′-aryl/heteroaryl 2,3-divinylfuran derivatives (9a-d) in which a hexatriene system is a part of heteroaromatic ring have been synthesized and their photochemical properties were investigated. The primary process observed was the isomerization to trans,trans-isomers 9a-d followed by photochemical rearrangement of the furan ring giving the phototransposition products (I-IV). Stilbenes (20, 21) and phenanthrenes (22, 25, and 26), formed as secondary products from the competitive intermolecular cycloadditions, were also observed.     

New Phenomena in Organometallic‐Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers

The impact of reversible bond formation between a growing radical chain and a metal complex (organometallic‐mediated radical polymerization (OMRP) equilibrium) to generate an organometallic intermediate/dormant species is analyzed with emphasis on the interplay between this and other one‐electron processes involving the metal complex, which include halogen transfer in atom transfer radical polymerization (ATRP), hydrogen‐atom transfer in catalytic chain transfer (CCT), and catalytic radical termination (CRT). The challenges facing the controlled polymerization of “less active monomers” (LAMs) are outlined and, after reviewing the recent achievements of OMRP in this area, the perspectives of this technique are analyzed.