Terms Of Turnover Frequency Research Articles

Support mediated promotional effects of rare earth oxides (CeO2 and La2O3) on N2O decomposition and N2O reduction by CO or C3H6 over Pt/Al2O3 structured catalysts

The N2O decomposition and reduction by CO or C3H6 over rare earth oxides (REOs)-modified Pt/Al2O3 structured catalysts, i.e., coated on ceramic honeycomb monoliths, were comparatively investigated in a wide temperature interval of 100–600°C, either in the presence or in the absence of excess O2 and H2O. It was shown that the de-N2O efficiency can be remarkably enhanced via modification of Al2O3 support with rare earth oxides (REOs). In specific, complete conversion of N2O can be attained over REOs-modified catalysts at a relatively low temperature (ca. 480°C) even in the presence of excess O2, which in general depresses de-N2O efficiency, in opposite to the unmodified Pt/Al2O3 catalyst, over which, 20% N2O conversion is never exceeded for temperatures up to 600°C. In terms of turnover frequency (TOF), optimally modified (by REOs) Pt–Al2O3 catalyst exhibits one order of magnitude higher activity compared to that of the unpromoted Pt/Al2O3 sample. Under reducing conditions the N2O conversion is strongly enhanced by C3H6 and especially by CO, whereas marginal inhibition is induced by reducing agents under excess oxygen conditions. Water was found to induce a detrimental influence on N2O decomposition, with its effect however, to be partially reversible. An in situ diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) study, using CO as a probe molecule, was performed over both unmodified and REOs-modified Pt/Al2O3 catalysts to correlate their surface characteristics with their de-N2O efficacy. The results revealed that the superior catalytic performance of promoted samples could be mainly attributed to the increase of Pt dispersion as well as to the development of Pt sites with exceptional electron density, located on the metal–support interfacial area.

Effect of Y-stabilized ZrO2 as support on catalytic performance of Pt for n-butane oxidation

The effect of Y-stabilized ZrO2 as support on the Pt dispersion and catalytic activity for the total oxidation of n-butane was investigated. From XRD, TEM and CO chemisorption measurements, Pt particles were found to be well dispersed with a size of ca. 10nm over the surface of ZrO2 and Y-stabilized ZrO2, when Pt supported catalysts were calcined at 600°C. Although Pt particles supported on ZrO2 were significantly sintered by high temperature calcination above 800°C, no aggregation of Pt particles was observed for Pt/Y–ZrO2. When the catalyst was calcined at higher temperatures, Pt catalysts supported on Y-stabilized ZrO2 showed higher activity than Pt/ZrO2 for the total oxidation of n-butane. The intrinsic activity, expressed in terms of turnover frequency (TOF), increased with the decrease in Pt dispersion, indicating that n-butane oxidation over Pt catalysts supported on Y-stabilized ZrO2 is a structure sensitive reaction. Y2O3 additive plays a role in improving the thermal stability of Pt particles through the metal–support interaction, while it does not alter the catalytic performance of Pt as catalytic active sites.

Catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol by tert-butyl hydroperoxide over Pt/oxide catalysts

Heterogeneous oxidation of cyclohexane with tertiobutyl hydroperoxide was carried out on Pt/oxide (Al2O3, TiO2 and ZrO2) catalysts in the presence of different solvents (acetic acid and acetonitrile). The catalysts were prepared using Pt(NH3)2(NO2)2 as a precursor and characterized by chemical analysis using the ICP–AES method, XRD, TEM, FTIR and BET surface area determination. The oxidation reaction was carried out at 70°C under atmospheric pressure. The results showed the catalytic performance of Pt/Al2O3 as being very high in terms of turnover frequency.

Modification of Au Nanoparticles Dispersed on Carbon Support Using Spontaneous Deposition of Pt toward Formic Acid Oxidation

This work presents formic acid oxidation on Pt deposits on Au nanoparticles dispersed on Vulcan XC-72R. The Pt deposits were produced using spontaneous deposition method contacting the Au nanoparticles with solutions containing Pt complex ions in various concentrations. The Pt deposits were characterized using CO stripping coulometry, X-ray photoelectron spectroscopy, and inductively coupled plasma atomic emission spectroscopy. When the Pt concentration is 10(-5)-10(-4) M, the Pt deposits are nanoislands of monatomic height. In the concentration range of 10(-4)-10(-3) M, the Pt deposits are most likely two-layer-thick nanofeatures. As Pt concentration increases further, the deposits become wider and thicker. Voltammetric behavior of Pt deposits reveals that on Pt deposits, dehydrogenation path is activated at the expense of poison-forming dehydration path. Furthermore, chronoamperometric measurement of the catalytic activity of Pt deposits supports that the two-layer-thick Pt deposits are most efficient in formic acid oxidation among the studied Pt deposits on Au nanoparticles. The enhancement factor of the particular Pt deposits is 2 in terms of turnover frequency, compared with a commercial Pt catalyst. Details are discussed in conjunction with Pt deposits on Au(111).

Efficient electrocatalytic oxygen reduction by the ‘blue’ copper oxidase, laccase, directly attached to chemically modified carbons

This discussion describes efforts to produce a stable, efficient electrocatalyst for four-electron O2 reduction through the direct attachment of fungal laccase, a 'blue' copper oxidase, to functionalised carbon electrode materials. Commercially available carbons, including fibrous and porous materials, offer important opportunities for achieving high conductivity over high surface areas that can be chemically functionalised. A promising approach for attaching laccase to a carbon surface is to use the diazonium coupling reaction to generate protrusive aromatic functionalities that can bind to hydrophobic residues close to the 'blue' Cu site: this site provides a fast, intramolecular electron relay into the buried trinuclear Cu active site that converts O2 rapidly and cleanly to H2O. This enhancement procedure makes possible the stable, direct electrocatalytic reduction of O2 at high potential with high efficiency in terms of turnover frequency per enzyme active site engaged with the electrode. The absence of electron-transfer mediators and simplicity of electrode system reveals the more inherent characteristics of the electrocatalytic mechanism that are masked in the waveform when a mediator is used. The study includes experiments to assess the effects of methanol and chloride ions on laccase electrocatalysis, complementing studies carried out by other groups, particularly those in which laccase is embedded in an electron-mediating gel.

Regio- and stereoselective terpene epoxidation using tungstate-exchanged takovites: a study of phase purity, takovite composition and stable catalytic activity

Nanocrystalline tungstate-exchanged layered double hydroxides of the takovite type, [Ni(x)Al(1-x)(OH)(2)](NO(3))(0.9-x)(WO(4))(0.05).mH(2)O, have been prepared with changing values of x. The morphology-texture parameters of the synthesized materials were characterized by using scanning electron microscopy and nitrogen adsorption techniques. X-Ray diffraction, diffuse reflectance spectroscopy, (27)Al MAS NMR and infrared spectroscopy were applied to study the structure and the phase purity of the tungstate exchanged materials. Identification of the tungsten state was attempted by infrared and Raman spectroscopy. The catalytic properties of the pure tungstate exchanged takovites were tested in the bromide-assisted epoxidation of terpenic olefins using H(2)O(2) as environmental benign oxidant. The bromide-assisted oxidation shows very interesting chemo-selectivity for various substrates with unique regio- and stereo-selectivity often opposite to that of many traditional methods. The activity of the tungstate catalyst was observed to highly depend on the Al content in the takovite support, most likely due to efficient charge shielding at the catalyst's surface. Tungstate exchanged on Al-rich takovite has a remarkable oxidation activity in terms of turnover frequency. Moreover, the takovite catalyst was found to be stable under reaction conditions and recyclable.

Conversion of carbon dioxide to cyclic carbonates using diimine Ru(II) complexes as catalysts

AbstractCationic diimine Ru(II) complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from CO2 and liquid epoxides (propylene oxide, epichlorohydrine, 1,2‐epoxybutane and styrene oxide) which served as both reactant and solvent. The reaction rates not only depended on the type of ligand, but also on reaction conditions such as temperature, pressure, base, the epoxide substrates and the use of an additional solvent. Reaction rates in terms of turnover frequencies up to 4050 molproduct molcat.−1 h−1 at 99% selectivity were achieved by optimizing the diimine ligand as well as the reaction temperature and CO2 pressure. Consistent with CV measurements, the electron donating group on the p‐position of the aryl ring accelerated the reaction rate. Copyright © 2008 John Wiley & Sons, Ltd.

Catalytic performance of rhodium supported on ceria–zirconia mixed oxides for reduction of NO by propene

The catalytic activity of Rh/CeO 2–ZrO 2 for NO reduction by C 3H 6 under stoichiometric conditions was found to depend strongly on the Ce/Zr composition. Rh/CeO 2–ZrO 2 with a Ce/Zr molar ratio of 50/50 (Rh/CZ-50/50) had the greatest activity; Ce/Zr = 74/26 (Rh/CZ-74/26), the least. The Rh/CZ-50/50 also showed specifically high intrinsic activity expressed in terms of turnover frequency (TOF). Different characterization techniques revealed that the Rh and CeO 2–ZrO 2 support interaction varied according to Ce/Zr composition, and high TOF on Rh/CZ-50/50 was accounted for by the strong Rh and CeO 2–ZrO 2 support interaction. Although the Rh/CZ-74/26 catalyst was the least active, it was able to catalyze NO reduction with C 3H 6 when O 2 was eliminated from the reaction gas. In situ FT-IR spectroscopy suggested that formate species formed and stabilized on the Rh/CZ-74/26 catalyst poison the catalytically active sites for NO reduction with C 3H 6.

Solvent-free selective oxidation of alcohols by molecular oxygen over gold nanoparticles supported on β-MnO2 nanorods

Abstract The performances of a series of MnO 2 -supported gold nanocatalysts with varying gold loading were studied in the solvent-free selective oxidation of alcohols by molecular oxygen. The gold was deposited onto the β-MnO 2 support, featuring one dimensional rod-like morphology, by means of homogeneous deposition–precipitation. The catalytic activity of the catalyst for the solvent-free aerobic oxidation of benzyl alcohol was found to increase with increasing gold loading, with the highest specific activity in terms of turnover frequencies (TOF) being achieved over the catalyst with 5 wt.% gold loading. Both XRD and TEM results showed that the majority of Au particles of various catalysts have diameters in the range of 2–6 nm. Temperature-programmed reduction (TPR) results indicated that the reducibility of MnO 2 was significantly enhanced with the surface-deposited gold nanoparticles. X-ray photoelectron spectroscopy (XPS) revealed the coexistence of both oxidized and metallic gold species on the surface of MnO 2 , the composition of which depends critically on the gold loading. The superior specific activity of the catalyst with 5% gold loading was attributed to the presence of higher amounts of positive gold species and more surface oxygen vacancy sites resulting from stronger metal–support interactions as compared to other catalysts.

Mn(III)(salen)-catalyzed synthesis of cyclic organic carbonates from propylene and styrene oxide in “supercritical” CO 2

Various homogeneous and immobilized manganese-salen complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from two liquid epoxides (propylene oxide and styrene oxide) and CO 2, which served as reactant and solvent. Reaction rates in terms of turnover frequencies up to 255 mol product mol Mn −1 h −1 at 98% selectivity were achieved by optimizing the salen ligand as well as the reaction temperature and CO 2 pressure. The reaction rates did not only strongly depend on the kind of salen ligand, the phase behaviour, and the reaction conditions but significant differences were also observed between the epoxide reactants. In addition, two different and simple ways for the immobilization of Mn-salen complexes were examined, among which one led to a highly active, stable and reusable heterogeneized catalyst. In this heterogeneous catalyst the salen ligand was covalently bound to the silica surface and showed low deactivation and almost no leaching of Mn during repetitive use. A coordinatively bound Mn-salen complex resulted in a high loading of the Mn-salen complex on a specially modified silica surface, but was not stable enough. Furthermore, phase behaviour studies and ATR-IR spectroscopic investigations on the product formation are reported that gave further insight into this catalytic reaction taking place at elevated pressure.

A comparative study of gas phase esterification on solid acid catalysts

For the first time, a comprehensive comparison of the intrinsic activities of solid acid catalysts in terms of turnover frequency (TOF) is reported for the gas-phase esterification of acetic acid with methanol. The catalysts studied included a zeolite (Hβ), two modified zirconias (sulfated zirconia, SZ; and tungstated zirconia, WZ), and an acidic resin-silica composite (Nafion/silica, SAC-13). Activities on a per weight basis decreased in the following order: Hβ ∼ SAC-13 ≫ SZ > WZ at 130 °C. However, on a rate-per-site basis (TOF), all catalysts showed comparable activities. The TOF results suggest that the acid sites of these catalysts have similar capacity for effectively catalyzing esterification. All catalysts deactivated to a quasi-steady-state rate with TOS. Regeneration experiments suggested that catalyst deactivation was due mainly to site blockage by carbonaceous deposits. Selective poisoning experiments showed that the reaction predominately took place on Bronsted acid sites.

A highly active Au/Al2O3 catalyst for cyclohexane oxidation using molecular oxygen

The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol has been investigated over Au/Al2O3 catalysts using molecular oxygen in a solvent-free system. The catalysts were prepared by a modified direct anionic exchange method and characterized by AAS, N2 adsorption and TEM. The results showed that the catalytic performance of Au/Al2O3 is very high in terms of turnover frequency. Moreover, the nano-size effect of gold is also reported in the reaction.

Effect of iridium dispersion on the catalytic activity of Ir/SiO 2 for the selective reduction of NO with CO in the presence of O 2 and SO 2

The effect of iridium dispersion on the activity of Ir/SiO 2 for the selective reduction of NO with CO in the presence of O 2 and SO 2 has been investigated. The intrinsic activity, expressed in terms of turnover frequency (TOF), increases with decreasing iridium dispersion, indicating that NO reduction with CO over Ir/SiO 2 is a structure-sensitive reaction. Characterization of Ir/SiO 2 using TPO, XRD and FT-IR spectroscopy following CO adsorption shows that larger iridium crystallites are more difficult to be oxidized and much easier to be reduced under reaction conditions, resulting in the formation of stable iridium metal sites on which NO reduction occurs.

Behavior of homogeneous and immobilized zinc-based catalysts in cycloaddition of CO 2 to propylene oxide

Homogeneous and heterogeneous zinc-based catalysts were tested in the reaction of propylene oxide and CO 2 to propylene carbonate without the use of additional solvents. For the preparation of the heterogeneous catalysts, the homogeneous zinc pyridine bromide and acetate complexes were modified for chemical anchoring on a silica surface. All intermediate stages of the series, starting with the zinc pyridine-based complex and ending with the anchored complex, were catalytically tested and characterized. Rates in terms of turnover frequencies (TOFs) > 1100 h −1 were obtained with the homogeneous zinc pyridine bromide-based catalysts, and TOFs of 330 h −1 were obtained with the corresponding heterogeneous catalyst. Using acetate instead of bromide as a ligand significantly decreased the performance. The differences in activity could be traced to structural differences unraveled by X-ray absorption spectroscopy (XAS). For the heterogeneous catalysts, special attention was paid to the deactivation behavior and the reusability. The catalyst structures were followed in situ by XAS using a specially designed batch reactor cell. This provided important information on the fate of zinc and bromide under reaction conditions and also gave new insight into the reaction mechanism.

Effect of surface structure of supported palladium catalysts on the activity for direct decomposition of nitrogen monoxide

The active sites for direct decomposition of NO on supported palladium catalysts were investigated by a comparative study with single-crystal palladium surfaces. In the study of powder catalysts, the activity of Pd/Al 2O 3 was found to be significantly affected by the type of palladium precursor. Pd/Al 2O 3 catalysts prepared from palladium nitrate showed the highest activity, expressed in terms of turnover frequency (TOF), for NO decomposition to N 2. In studies using single-crystal model catalysts reported previously [I. Nakamura, T. Fujitani, and H. Hamada, Surf. Sci. 514 (2002) 409], the adsorption, dissociation, and desorption behavior of NO was closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. NO adsorption by FT-IR spectroscopy confirmed the presence of the stepped palladium sites on Pd/Al 2O 3 catalysts, and the specific activity (TOF step) for NO decomposition to N 2, calculated on the basis of number of step sites estimated from the amount of chemisorbed NO, was not changed for all the Pd/Al 2O 3 catalysts. These results suggest that direct decomposition of NO mainly proceeds on the step sites of palladium. This is in agreement with the results obtained for single-crystal model catalysts. The present study shows that the activity of powder catalyst can be well interpreted by its surface structure, indicating the usefulness of surface science techniques.

Activity of Co Ion Sites in ZSM-5, Ferrierite, and Mordenite in Selective Catalytic Reduction of NO with Methane

Selective catalytic reduction of NO with methane (CH4-SCR) in an excess of oxygen was investigated over CoH–, CoNa–, CoNaK–, CoBa–, and CoxOy–Co-zeolites of ZSM-5, ferrierite, and mordenite structures. The exchanged Co ions have been shown to exhibit activity in CH4-SCR of NO, while protons or oxide-like Co species in Co–zeolites contribute to the CH4-SCR activity mostly by enhancement of the oxidation of NO to NO2. The sequence of Co–zeolites activity in CH4-SCR of NO (per zeolite weight) was Co–ZSM-5≅Co–ferrierite⪢Co–mordenite for the highest Co loadings at Si/Al of 12, 8.4, and 9.2, but for the average Co ion site in the zeolite (in terms of turnover frequency, TOF, values) it was Co–ZSM-5>Co–ferrierite⪢Co–mordenite. From a comparison of the dependence of TOF values for NO conversion to N2 per Co ion and of the relative concentrations of the exchanged α-, β-, and γ-type Co ions (determined from quantitative analysis of the characteristic Co(II) Vis spectra) on the total Co concentration, the most active Co sites were estimated. The α-type Co ions, bound to framework oxygens of the wall of the main channel of mordenite and ferrierite, are the most active among the Co ions in these zeolites. On the other hand, in ZSM-5 the β-type Co ions, coordinated to the deformed six-membered ring at the intersection of straight and sinusoidal channels possess the highest activity. The main factors contributing to the individual Co ions' activity are suggested to be cation location in the inner zeolite volume, coordination to the framework, and distances between the Co ions.

Infrared studies of the effects of promoters on CO-induced structural changes in Rh

The catalytic activity of Rh/CeO2–ZrO2 for NO reduction by C3H6 under stoichiometric conditions was found to depend strongly on the Ce/Zr composition. Rh/CeO2–ZrO2 with a Ce/Zr molar ratio of 50/50 (Rh/CZ-50/50) had the greatest activity; Ce/Zr = 74/26 (Rh/CZ-74/26), the least. The Rh/CZ-50/50 also showed specifically high intrinsic activity expressed in terms of turnover frequency (TOF). Different characterization techniques revealed that the Rh and CeO2–ZrO2 support interaction varied according to Ce/Zr composition, and high TOF on Rh/CZ-50/50 was accounted for by the strong Rh and CeO2–ZrO2 support interaction. Although the Rh/CZ-74/26 catalyst was the least active, it was able to catalyze NO reduction with C3H6 when O2 was eliminated from the reaction gas. In situ FT-IR spectroscopy suggested that formate species formed and stabilized on the Rh/CZ-74/26 catalyst poison the catalytically active sites for NO reduction with C3H6.