Showing papers in "Journal of Molecular Catalysis A-chemical in 2003"

A review of the problem of distinguishing true homogeneous catalysis from soluble or other metal-particle heterogeneous catalysis under reducing conditions

Abstract: This review considers cases in which a discrete transition-metal complex is used as a precatalyst for reductive catalysis; it focuses on the problem of determining if the true catalyst is a metal-complex homogeneous catalyst or if it is a soluble or other metal-particle heterogeneous catalyst. The various experiments that have been used to distinguish homogeneous and heterogeneous catalysis are outlined and critiqued. A more general method for making this distinction is then discussed. Next, the circumstances that make heterogeneous catalysis probable, and the telltale signs that a heterogeneous catalyst has formed, are outlined. Finally, catalytic systems requiring further study to determine if they are homogeneous or heterogeneous are listed. The major findings of this review are: (i) the in situ reduction of transition-metal complexes to form soluble-metal-particle heterogeneous catalysts is common; (ii) the formation of such a catalyst is easy to miss because colloidal solutions often appear homogeneous to the naked eye; (iii) a variety of experiments have been used to distinguish homogeneous catalysis from heterogeneous catalysis, but there is no single definitive experiment for making this distinction; (iv) experiments that provide kinetic information are key to the correct identification of the true catalyst; and (v) a more general approach for distinguishing homogeneous catalysis from heterogeneous catalysis has been developed. Additionally, (vi) the conditions under which a heterogeneous catalyst is likely to form include: (a) when easily reduced transition-metal complexes are used as precatalysts; (b) when forcing reaction conditions are employed; (c) when nanocluster stabilizers are present; and (d) when monocyclic arene hydrogenation is observed. Finally, (vii) the telltale signs of heterogeneous catalysis include the formation of dark reaction solutions, metallic precipitates, and the observation of induction periods and sigmoidal kinetics.

Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes

Abstract: Electrochemical reduction of carbon dioxide was studied with various series of copper single crystal electrodes in 0.1 M KHCO3 aqueous solution at constant current density 5 mA cm−2; the electrodes employed are Cu(S)-[n (1 0 0)×(1 1 1) ], Cu(S)-[n (1 0 0)×(1 1 0) ], Cu(S)-[n (1 1 1)×(1 0 0) ], Cu(S)-[n (1 1 1)×(1 1 1) ] and Cu(S)-[n (1 1 0)×(1 0 0) ]. The electrodes based on (1 0 0) terrace surface give ethylene as the major product. The ethylene formation is further promoted by the introduction of (1 1 1) or (1 1 0) steps to the (1 0 0) basal plane. The highest C2H4 to CH4 formation ratio amounts to 10 in terms of the current efficiency for the (7 1 1) (=4(1 0 0) − (1 1 1)) surface as compared with the value 0.2 for the (1 1 1) electrode. The n(1 1 1)−(1 1 1) surfaces favor the formations of acetic acid, acetaldehyde and ethyl alcohol with the increase of the (1 1 1) step atom density. CH4 formation at the n(1 1 1)−(1 1 1) electrodes decreases with the increase of the (1 1 1) step atom density. The n(1 1 1)−(1 0 0) surfaces give higher gaseous products; the major product is CH4 with lower fraction of C2+ compounds.

A review of soluble transition-metal nanoclusters as arene hydrogenation catalysts

Abstract: A critical review of the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds is presented. The review begins with a brief introduction to arene hydrogenation and to nanocluster science. The introductory material is followed by a detailed discussion of the approximately 20 papers in the literature that deal with the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds. Metal particle catalysts on solid supports are not reviewed herein, and are considered only as far as they serve to compare and contrast with soluble transition-metal nanoclusters. The major findings of this review are: (i) soluble nanocluster catalysts are implicated as the true catalysts in many putatively “homogeneous” arene hydrogenations; (ii) with few exceptions, nanocluster catalysts used for arene hydrogenation are poorly characterized; (iii) soluble nanocluster catalysts for arene hydrogenation have modest activity and lifetime; (iv) Rh and Ru are used almost exclusively as the active metals; (v) two catalyst systems, one developed by Roucoux and co-workers and the other by our own research group, stand out from the rest in terms of activity and lifetime; (vi) selective arene hydrogenation, especially for the synthesis of the all-cis diastereomer of substituted cyclohexanes, has received considerable attention and is a promising area for future study and, perhaps, fine chemical applications (selectivities >90% for the all-cis diastereomer have been achieved by several groups).

Catalytic activity of LaMnO3 and LaCoO3 perovskites towards VOCs combustion

Abstract: The catalytic combustion of some volatile organic compounds (VOCs) has been investigated over LaMnO3 and LaCoO3 perovskite-type oxides. Redox titration has shown that cobalt is present in LaCoO3 exclusively in the 3+ oxidation state whereas LaMnO3 contains considerable amount of Mn4+ (35%) in addition to Mn3+. VOCs reactivity towards total oxidation follows the trend: acetone > isopropanol > benzene. The oxidation of isopropanol occurs through the formation of acetone in the homogeneous reaction. The increase of the oxygen partial pressure is beneficial for total oxidation of acetone. The adsorptive properties of the catalysts towards the VOCs and H2 have been examined by means of temperature programmed desorption. The LaMnO3 surface is the most reactive to the adsorption of VOCs and H2. The role of the adsorbed oxygen has been studied by examining the variations of the electrical conductivity of the catalysts during the processes of oxygen adsorption–desorption.

The catalytic methanol synthesis over nanoparticle metal oxide catalysts

Abstract: Several nanoparticle metal oxides were prepared and studied for the catalytic production of methanol from hydrogen and carbon dioxide. These catalysts include: ZnO, CuO, NiO, and a binary system CuO/ZnO. The catalysts were prepared through sol–gel synthesis and were found, via TEM and BET, to have high surface areas and small crystallite sizes. With this in mind, the catalytic production of methanol was studied at various temperatures in a flow reactor. The percent conversion and turnover numbers were calculated for each sample, and it was found that the nanoparticle ZnO, CuO/ZnO and NiO were much more active catalysts than the commercially available materials. The nanocrystalline CuO sample was found to rapidly reduce to Cu, where it lost all activity. The results suggest that the catalytic process is efficient for several nanoparticle metal oxide formulations, however, copper metal is not active, but small copper particles in a CuO/ZnO matrix is a very active combination.

Liquid phase hydrogenation of furfural to furfuryl alcohol over the Fe-promoted Ni-B amorphous alloy catalysts

Abstract: The ultrafine Fe-doped Ni-B amorphous catalyst (Ni-Fe-B) was prepared by reducing mixed FeCl3 and NiCl2 with KBH4 in aqueous solution. At suitable Fe-content (χFe), the Ni-Fe-B amorphous catalyst exhibited much higher activity than the corresponding undoped Ni-B in the liquid phase hydrogenation of furfural (FFR) to furfuryl alcohol (FFA). With the increase of χFe, the activity first increased and then decreased while the selectivity to FFA changed in a contrast way. The Fe-B amorphous alloy itself was inactive for the FFR hydrogenation. The optimum χFe was determined as 0.51, at which the FFA yield reached 100% after reaction for 4 h. Based on various characterizations, the promoting effect of the Fe-dopant was discussed by considering (1) the increase in the surface area, more highly unsaturated Ni active sites, and the more homogeneous distribution of the Ni active sites owing to the presence of Fe2O3 as a dopant; (2) the affinity of the Fe3+ for the oxygen in the carbonyl group which strengthened the adsorption of the CO bond by the catalyst and thus facilitated its hydrogenation; (3) the electron donation of the metallic Fe to the metallic Ni, making Fe electron-deficient while Ni electron-enriched which activated the CO bond towards the hydrogenation.

Preparation of supported gold catalysts by gas-phase grafting of gold acethylacetonate for low-temperature oxidation of CO and of H2

Abstract: In order to clarify the support effect on CO oxidation over Au catalysts without the influence of the size effect of Au, Au was deposited on a variety of supports with high dispersion by gas-phase grafting (GG) of an organo-gold complex. Comparison of Au/SiO2 with Au/Al2O3 and Au/TiO2, which were prepared by both GG and liquid-phase methods and were active for CO oxidation at 273 K, showed that there were no appreciable differences in their catalytic activities as far as Au is deposited as nanoparticles with strong interaction. However, Au deposited onto the acidic supports, such as silica–alumina and activated carbon (AC), exhibited a much lower CO oxidation catalytic activity than those of the active Au catalysts, such as Au/TiO2.

Ni based mixed oxide materials for CH4 oxidation under redox cycle conditions

Abstract: The preparation, characterization and redox properties of Ni–Al–O and Ni–Mg–Al–O mixed oxides for CH 4 chemical looping combustion (CLC) is addressed in this study. Ni–Al–O samples having different Ni/Al ratios (0.5–2.25), prepared by coprecipitation, consist after calcination at 1000 °C of cubic NiO and NiAl 2 O 4 spinel. A similar phase composition is obtained for Ni–Mg–Al–O, with Mg partitioned in the two phases. The presence of NiAl 2 O 4 prevents the crystal size growth of NiO with respect to pure NiO; further limit of the sintering of the cubic oxide was observed in presence of Mg. Reduction of the samples by H 2 occurs in two steps, associated with reduction of Ni 2+ in NiO and NiAl 2 O 4 . Mg stabilizes Ni 2+ in both the cubic oxide and the spinel phase and improves regenerability upon repeated redox cycles. Temperature programmed reduction with CH 4 (CH 4 -TPR) experiments showed poor selectivity to CO 2 and H 2 O, being CO and H 2 the most abundant products. Also, formation of coke is observed over the samples. The same behavior is observed in CH 4 /O 2 pulse experiments; however, in the case of the Mg-containing system, coke formation can be avoided by co-feeding H 2 O along with CH 4 .

Alumina-supported Mn(II), Co(II), Ni(II) and Cu(II) bis(2-hydroxyanil)acetylacetone complexes as catalysts for the oxidation of cyclohexene with tert-butylhydroperoxide

Abstract: New Mn(II), Co(II), Ni(II) and Cu(II) complexes of a tetradentate Schiff base ligand [bis(2-hydroxyanil)acetylacetone],“2-[{1-methyl-3-[(2-hydroxyphenyl)imino]butylidene}amino]phenol”, H 2 haacac, have been prepared and characterized by elemental analyses, IR and conductometry. The results suggest that the Schiff base is a bivalent anion with tetradentate ONNO donors derived from the phenolic oxygen and azomethine nitrogen. The formulae was found to be [M(haacac)] for the 1:1 non-electrolytic complexes. Alumina-supported [M(haacac)] complexes catalyze the oxidation of cyclohexene with tert -butylhydroperoxide (TBHP). The major products of the reaction were 2-cyclohexene-1-ol (OH), 2-cyclohexene-1-one (CO) and 2-cyclohexene-1-( tert -butylperoxy) (OO t Bu). The influence of temperature, solvent and time for the oxidation reaction has been studied. The selectivity of 2-cyclohexene-1-( tert -butylperoxy) varied with reaction temperature. Mn(haacac)-alumina shows significantly higher catalytic activity than other alumina-supported complexes.

On the acid-dealumination of USY zeolite: a solid state NMR investigation

Abstract: The dealumination of USY (ultrastable Y) zeolites by nitric acid and oxalic acid treatment was systematically investigated by multinuclear solid-state NMR and MQ MAS NMR experiments. The results show that both acids are very effective in removing non-framework Al as well as framework Al but that aluminum is extracted from the lattice at a higher rate by oxalic acid even at low concentrations. The presence of different species (e.g. silanol nest, Al–OH, four-coordinated framework Al, six-coordinated framework Al, six-coordinated non-framework Al and five-coordinated non-framework Al) was detected, and their changes were followed during the dealumination. The investigation gives evidence that the breakdown of the parent USY zeolite mainly depends on the degree of dealumination and that non-framework Al exerts a great effect on the acidity of the USY zeolite. Leaching-induced increase in the Bronsted acidity of the USY zeolite was also observed by 1 H MAS NMR spectroscopy. The different distribution of Al species in these samples accounted for the different catalytic performance of n-dodecane cracking. © 2002 Published by Elsevier Science B.V.

Kinetics and mechanism of the heterogeneous catalyzed oxidative degradation of indigo carmine

Abstract: The kinetics of the oxidative degradation of the indigo carmine (IC) dye (disodium salt of 3,3-dioxobi-indolin-2,2-ylidine-5,5-disulfonate) with hydrogen peroxide catalyzed with the supported metal complexes have been investigated. The complexes used are [Cu(amm)4]2+, [Co(amm)6]2+, [Ni(amm)6]2+, [Cu(en)2]2+, and [Cu(ma)4]2+ (amm=ammonia, en=ethylenediamine, and ma=methylamine). Silica, alumina, silica-alumina (25% Al2O3), and cation-exchange resins (Dowex-50W, 2 and 8% DVB) are used as supports. The reaction is first order with respect to [IC] while the order with respect to [H2O2] was dependent on the initial concentration and the type of the catalyst used. At lower [H2O2]0 the order was first, which then decreases with increasing [H2O2]0, finally reaching zero. This aspect is consistent with the formation of a colored peroxo-complex on the catalysts surface. The reactivity of catalysts is dependent on the redox potential of the metal ions, the amount of complex loaded per gram of dry catalyst, the type of ligand, and the support. Moreover, the reaction rate was strongly dependent on the pH of the medium, the cationic and anionic surfactants, and the irradiation with UV-light. The reaction is enthalpy controlled as confirmed from the isokinetic relationship. A reaction mechanism was proposed with the formation of free radicals as reactive intermediates.

Pd/SiO2 catalysts: synthesis of Pd nanoparticles with the controlled size in mesoporous silicas

Abstract: Synthesis of Pd nanoparticles with controlled size (d(Pd) = 1-3.6 nm) was carried out within the pores of the mesoporous HMS and SBA-15 silicas. Pd was ion-exchanged on non-calcined silicas, prepared by solvent extraction of the templates. A high concentration of silanol groups on the mesopore surface allowed attaining Pd loading up to 4.4. wt.%. The Pd/HMS and Pd/SBA-15 were characterised by chemical analysis, XRD, N2 adsorption-desorption and transmission electron microscopy (TEM) methods. The materials possess a high SSA and narrow pore size distribution. Introduction of Pd nanoparticles in HMS resulted in a progressive loss of the regularity in the mesoporous structure. On the contrary, all Pd/SBA-15 composites retained the original well-ordered 2D hexagonal structure of SBA-15. The thick walls of the SBA-15 framework are accounted for the higher stability observed. The TEM investigations confirmed that the Pd nanocrystals were located within the SBA-15 mesoporous framework channels. The particle size did not exceed the mesopore diameter (2-6 nm) at Pd loading of 0.1-4.4wt.%. Pd clusters were found to be resistant against sintering during air-calcination (550 degreesC, 4h). The catalyst 2.1%Pd/SBA-15 used in methane combustion at 520 degreesC demonstrated stable activity during 6h on stream.

Selective synthesis of isobutanol by means of the Guerbet reaction Part 2. Reaction of methanol/ethanol and methanol/ethanol/n-propanol mixtures over copper based/MeONa catalytic systems

Abstract: The synthesis of isobutanol via the Guerbet condensation between methanol and ethanol was studied by using sodium methoxide (MeONa) as soluble basic component and copper-based catalysts as heterogeneous dehydrogenating/hydrogenating metal species. The effect of the nature of the catalyst and the relative amount of its individual components with respect to the reacting alcohols as well as of temperature on productivity and selectivity of the process was investigated. The collected data indicated that the copper chromite/MeONa was more active than Cu-Raney/MeONa system. The reaction was shown to proceed with the formation only of n-propanol and isobutanol. Ethanol conversion up to 61% with selectivity to isobutanol up to 98.4% was obtained. The same catalytic systems were also employed in the reaction of the methanol/ethanol/n-propanol ternary mixture. Again copper chromite/MeONa resulted more active than the Cu-Raney/MeONa system. Finally, experiments were carried out on methanol/n-propanol mixtures in the presence of the copper chromite/MeONa catalytic system by recycling both the recovered solid copper component and the liquid reaction mixture for evidencing eventual copper leaching by MeONa. On the basis of the obtained results it was concluded that in the Guerbet reaction copper chromite works as heterogeneous catalyst.

Transfer hydrogenation of a variety of ketones catalyzed by rhodium complexes in aqueous solution and their application to asymmetric reduction using chiral Schiff base ligands

Abstract: The rhodium complex [Cp ∗ Rh(bpy)Cl]Cl shows efficient catalytic activity in the transfer hydrogenation of a wide variety of ketones in aqueous formic acid solution under mild reaction conditions. In the asymmetric reduction using chiral Schiff base ligands, the asymmetric induction was observed in reduction of dialkyl ketones as well as aryl ketones, among which ortho-chloroacetophenone gave the highest ee of 84%. © 2002 Elsevier Science B.V. All rights reserved.

Kinetics of the photocatalytic water-splitting reaction on TiO2 and Pt/TiO2 studied by time-resolved infrared absorption spectroscopy

Abstract: The present account describes how the water-splitting reaction on TiO2 (P-25) and Pt/TiO2 catalysts is traced by time-resolved infrared (IR) absorption spectroscopy. The ac-coupled amplification of the IR signal allows detection of transient absorbance-change as small as 10−6 with a time-resolution of 50 ns. The TiO2 or Pt/TiO2 catalyst irradiated by a 355 nm UV-pulse at time = 0 presents a transient IR absorption which monotonically increases in intensity with decreasing wavenumber from 3000 to 1000 cm−1. Photogenerated electrons trapped in shallow mid-gap states are proposed to cause the absorption. The decay kinetics of the electrons traced by monitoring the IR absorbance at 2000 cm−1 is sensitive to vapor atmospheres. The electrons recombine with the complementary holes along a multi-exponential rate law in the catalysts placed in a vacuum. Dioxygen arriving from the gas-phase captures the electrons at delay time of 10–100 μs following the UV irradiation. In the presence of water vapor, holes in the TiO2 catalyst are captured by an adsorbed reactant (probably hydroxyl species) within 2 μs and the recombination is obstructed thereafter. The excess electrons cannot transfer to another adsorbate to be reduced (probably proton) and hence reduce the catalyst itself. On the Pt/TiO2 catalyst exposed to water, the electron transfer takes place at 10 μs or later, following the hole transfer completed within 2 μs. The hole transfer on Pt/TiO2 is insensitive to the pressure of water vapor whereas the rate of the electron transfer is enhanced with the increasing pressure at 1–10 Torr. Transient response of an adsorbate vibration is also observed. The OH stretching band of an adsorbed hydroxyl species at 3677 cm−1 thermally shifted to the low-wavenumber side, when the TiO2 catalyst is irradiated by the UV-pulse.

Structure and photocatalytic characteristics of TiO2 film photocatalyst coated on stainless steel webnet

Abstract: High photocatalytic activities of TiO 2 films (50–360 nm) were coated on stainless steel webnet by the sol–gel method. Stainless steel webnet is a good substrate due to its large surface area, good ventilation for gases passing and good utilization of UV light. The TiO 2 films exhibited a uniform anatase structure with particle size about 10 nm. The photocatalytic activity of the TiO 2 film coated on stainless steel webnet can be related to its crystallization and thickness. With increasing of thickness of the TiO 2 film, the diffusion of the substrate was restricted from the surface and a more perfect crystallization of TiO 2 film was formed. Thicker and better crystallization of TiO 2 film resulted in its higher photocatalytic activity, which was evaluated by the photocatalytic oxidation of gaseous formaldehyde in a cylindrical flowing photoreactor. When the TiO 2 film thickness exceeds a certain value (about 250 nm), its photocatalytic activity remains unchanged. This indicated that only a certain thickness of the TiO 2 film was effective on the photocatalytic reaction.

Sn-exchanged hydrotalcites as catalysts for clean and selective Baeyer–Villiger oxidation of ketones using hydrogen peroxide

Abstract: A Sn-doped hydrotalcite (Sn/HT) catalyst prepared by ion-exchange technique is found to be an active and selective catalyst for the liquid phase Baeyer–Villiger (BV) oxidation of cyclic ketones in acetonitrile using hydrogen peroxide (H 2 O 2 ) as oxidant. Different reaction parameters such as effect of solvent, Sn content, reaction temperature and catalyst-to-substrate ratio are studied. The activity of the catalyst for the selective BV oxidation of ketones is attributed to the presence of active Sn sites in the interstitial position of hydrotalcite support. Sn sites activate the carbonyl group of ketones followed by a nucleophilic attack by the active peroxide species (peroxycarboximidic acid by reaction of acetonitrile and H 2 O 2 ) to form a Criegee adduct that rearranges to give the lactone.

Nb(V) compounds as epoxides carboxylation catalysts: the role of the solvent

Abstract: The synthesis of organic cyclic carbonates from epoxides and CO2 is described. The role of the solvent is discussed, together with that of Nb(V) catalysts (Nb2O5 and NbCl5). The reaction is very selective and affords the conversion of epoxides into the relevant carbonates in high yield. © 2003 Elsevier Science B.V. All rights reserved.

Characterization of chromium species in catalysts for dehydrogenation and polymerization

Abstract: Chromium catalysts, prepared with different chromium contents, supports and precursor compounds, were characterized by O2 and CO chemisorption, in order to quantify the Cr3+ and Cr2+ sites, active species to dehydrogenation and ethylene polymerization reactions. Both probe molecules, O2 and CO, showed to be selective to the chromium active species, Cr3+ and Cr2+, respectively, and a valid method to quantify these sites. Distinct chromium species, Cr6+, amorphous Cr3+, crystalline Cr3+ and Cr2+, were characterized by XRD, DRS, TPR, FT-IR and DRIFTS in the calcined and reduced catalysts. The distribution of chromium species, which depends on the content, precursor compound and support, was related to the catalytic activity in the dehydrogenation and polymerization reactions. The catalytic activity to cyclohexane dehydrogenation depends on the dispersion of amorphous Cr3+ species. The activity in the ethylene polymerization is related to the amount of CrA2+ and CrB2+ species. Both sites showed similar activities.

Electrochemical behaviour of pseudomorphic overlayers: Pd on Au(1 1 1)

Abstract: The electrochemical properties of palladium adlayers on Au(1 1 1) are compared with those of massive Pd(1 1 1). Electro-deposited Pd forms a pseudomorphic overlayer on the unreconstructed Au(1 1 1) surface following the stacking sequence of the substrate. The altered electrochemical behaviour of the overlayer can be ascribed to changes in the electronic structure due to lateral strain. A gradual change to bulk properties is observed with increasing Pd coverage. Besides hydrogen adsorption, sulphate adsorption and surface oxidation, formic acid oxidation was chosen as a test reaction. The influence of lateral strain on the reaction kinetics of formic acid oxidation is discussed.

Production of hydrogen by steam reforming of methanol on CeO2 promoted Cu/Al2O3 catalysts

Abstract: Catalytic production of hydrogen by steam reforming of methanol reaction has been developed on a series of co-precipitated Cu/Al2O3 catalysts promoted with CeO2 at atmospheric pressure in a microreactor. Effects of CeO2 content, reaction temperature, methanol space velocity and H2O/CH3OH molar ratio on the catalytic activity have been investigated. CeO2 promoted Cu/Al2O3 catalysts exhibited higher activity and stability as compared to the unpromoted ones. The catalyst containing 20 wt.% of CeO2 was the most active one with a methanol conversion of 95.5 mol%, H2 selectivity of 99.9 mol% and the outlet CO concentration of 0.14 mol% at 250 °C. After 200 h of reaction, methanol conversion was still over 90.0% with the catalyst containing 20 wt.% of CeO2, while Cu/Al2O3 catalyst deactivated rapidly after 100 h of reaction. Results of X-ray diffraction (XRD) and the surface element distribution of catalysts showed that CeO2 not only greatly improved the surface copper dispersion and prevented copper crystallites from conglomeration or sintering, but also made copper crystallites relatively smaller. The improvement in activity and stability of the promoted catalysts was attributed to higher copper dispersion and smaller copper crystallites, and the synergetic effect of ceria.

Effect of sulfation on the photoactivity of TiO2 sol–gel derived catalysts

Abstract: Titania and sulfated-titania were prepared by the sol–gel method using titanium alkoxide as titania precursor. BET specific surface areas on samples calcined at 600 °C were 57 and 62 m2/g for titania and sulfated-titania, respectively. FTIR spectra of sulfated samples show the 1360–1370 cm−1 absorption band assigned to SO vibrations. Moreover, by means of FTIR-pyridine absorption the strength of the sulfate bonded on the TiO2 surface was studied. Pyridine absorption bands assigned to Lewis and Bronsted acidic sites were identified on sulfated TiO2. A diminution of the band gap (Eg) by effect of sulfation was observed by UV-Vis spectroscopy. X-ray diffraction patterns show that sulfate ions stabilize the anatase phase. The photoactivity of the samples determined in the 2,4-dinitroaniline decomposition was found to be higher in sulfated samples. A model is proposed, in which the sulfate anchored to the titania surface acts as an electron trap during photo activation leading to an enhancement of activity.

AuPd bimetallic nanoparticles on TiO2: XRD, TEM, in situ EXAFS studies and catalytic activity in CO oxidation

Abstract: Monometallic Au, Pd and bimetallic AuPd nanoparticles were prepared from Au(III), Pd(II) precursor ions in aqueous media with reduction by a mixture of Na-citrate and tannin producing stable metal sols of narrow size distribution. The nanoparticles of 4–7 nm in diameter as indicated by transmission electron microscopy were adsorbed on TiO2. In situ X-ray absorption spectroscopy (XAS) at Au LIII and Pd K-edges and X-ray diffraction (XRD) techniques evidenced the presence of bimetallic particles in the supported AuPd/TiO2 sample. The high amount of organic residues of the samples was removed by calcination at 400 °C proved by temperature programmed oxidation (TPO). After reduction in H2 the TEM, XRD and the CO chemisorption showed some enlargement in the metal particle sizes. The catalytic activity of the bimetallic AuPd/TiO2 in the CO oxidation revealed a slight synergistic effect compared to the activity of monometallic analogous referred to the estimated surface area of Au and Pd in the bimetallic sample.

Partial hydrogenation of 1,3-cyclooctadiene using dendrimer-encapsulated Pd–Rh bimetallic nanoparticles

Abstract: This article reports the first effort for the preparation of Pd–Rh bimetallic nanoparticles in the presence of poly(amidoamine) dendrimers with surface hydroxyl groups (fourth generation, PAMAM-OH). The resulting dendrimer-encapsulated Pd–Rh bimetallic nanoparticles show a promising catalytic activity in partial hydrogenation of 1,3-cyclooctadiene.

Photocatalytic oxidation of cyanide: kinetic and mechanistic studies

Abstract: The kinetics and mechanism of the photocatalytic oxidation of cyanide in the presence titanium dioxide catalyst were investigated in this study. By displacing the surface hydroxyl groups on the surface of titanium dioxide with fluoride ions, it was deduced that cyanide is oxidised via a pure heterogeneous pathway, i.e. oxidised by the holes trapped at the surface hydroxyl groups TiO . The quantum efficiency of the photocatalytic oxidation was found to be low (ca. 0.1–0.2%) and this was mainly due to (1) the low adsorption of cyanide ions onto the titanium dioxide surface, (2) the absence of homogenous reaction between cyanide ions and diffused hydroxyl radicals, and (3) the high electron hole recombination rate within the titanium dioxide photocatalyst. A kinetic model was developed to describe the mechanism involved in the photocatalytic oxidation of cyanide.

Suitable ligands for homogeneous ruthenium-catalyzed hydrogenolysis of esters

Abstract: Effective hydrogenolysis of dimethyl oxalate to ethylene glycol has been obtained using a catalyst prepared in situ from Ru(acac)3 with the facially coordinating tridentate phosphine ligand CH3C(CH2PPh2)3. This catalyst enabled full and selective conversion in 16 h at [S]/[Ru]=500 at 80–100 bar hydrogen pressure at 120 °C. This catalyst is far more active than any known homogeneous catalyst able to hydrogenate dimethyl oxalate to ethylene glycol. Several mono-, di- and tridentate P- and N-ligands have been selected and were evaluated, several of which showed (almost) no reactivity. In some cases, for instance when using the meridional coordinating ligand PhP(C2H4PPh2)2, selectivity can be directed toward the semi-hydrogenolysis product methyl glycolate.

Oxidation of alkylaromatics with hydrogen peroxide catalysed by manganese(III) porphyrins in the presence of ammonium acetate

Abstract: The oxidation of toluene (1), ethylbenzene (2) and cumene (3) with hydrogen peroxide, in the presence of several manganese(III) porphyrins with electron-withdrawing substituents, was studied using ammonium acetate as a co-catalyst. All products were characterised and their formation was justified by studying the oxidation of primary precursors, under the same conditions. The formation of the nitrate compounds was shown to be dependent on the presence of ammonium acetate. The oxidation of cumene and ethylbenzene afford products resulting from dehydrogenation reactions.

Hydrogenation of olefins over hydrido chlorocarbonyl tris-(triphenylphosphine) ruthenium(II) complex immobilized on functionalized MCM-41 and SBA-15

Abstract: Hydrido chlorocarbonyl tris-(triphenylphosphine) ruthenium(II) complex [RuHCl(CO)(PPh3)3] has been immobilized inside the pores of amine functionalized MCM-41 and SBA-15 materials. These grafted complexes were characterized by XRD, FTIR, N2 adsorption measurements, XPS and tested as catalyst for the hydrogenation of olefins. Immobilization and pore size has marked effects on the catalytic activity in the hydrogenation of olefins and the complex immobilized on functionalized SBA-15 was found to be catalytically more active than the one immobilized on MCM-41.

Ethylene and propylene polymerization behavior of a series of bis(phenoxy–imine)titanium complexes

Abstract: This contribution reports ethylene and propylene polymerization behavior of a series of Ti complexes bearing a pair of phenoxy–imine chelate ligands. The bis(phenoxy–imine)Ti complexes in conjunction with methylalumoxane (MAO) can be active catalysts for the polymerization of ethylene. Unexpectedly, this C2 symmetric catalyst produces syndiotactic polypropylene. 13 C NMR spectroscopy has revealed that the syndiotacticity arises from a chain-end control mechanism. Substitutions on the phenoxy–imine ligands have substantial effects on both ethylene and propylene polymerization behavior of the complexes. In particular, the steric bulk of the substituent ortho to the phenoxy–oxygen is fundamental to obtaining high activity and high molecular weight for ethylene polymerization and high syndioselectivity for the chain-end controlled propylene polymerization. The highest ethylene polymerization activity, 3240 kg/mol-cat h, exhibited by a complex having a t-butyl group ortho to the phenoxy–oxygen, represents one of the highest reported to date for Ti-based non-metallocene catalysts. Additionally, the polypropylene produced exhibits a Tm, 140 °C, and syndioselectivity, rrrr 83.7% (achieved by a complex bearing a trimethylsilyl group ortho to the phenoxy–oxygen) that are among the highest for polypropylenes produced via a chain-end control mechanism. Hence, the bis(phenoxy–imine)Ti complexes are rare examples of non-metallocene catalysts that are useful for the polymerization of not only ethylene but also propylene.

Gold catalyzed oxidation of aldehydes in liquid phase

Abstract: Gold on carbon oxidizes aldehydes to carboxylic acids in water solution under mild conditions without loss of activity on recycling, as does not occur for platinum on carbon. Also scarcely soluble aldehydes can be oxidized but the reaction rate is slower. Experiments carried out in an organic solvent such as CCl4 show a speed up of the reaction, in this solvent being recyclable both Au/C and Pt/C. Another explored possibility was represented by carrying out the reaction in the absence of solvent. However, for solid aldehydes like p- and o-OH-PhCHO the water solvent represent the unique alternative.