Activity and selectivity of PtNi/TiO2 catalysts for hydrogenation of crotonaldehyde

The hydrogenation of crotonaldehyde over a series of titania supported PtNi catalysts was investigated. Pt/TiO₂ had the highest activity to hydrogenate the bond of crotonaldehyde. With the addition of Ni to Pt the activity for carbonyl group hydrogenation decreased, while that for the bond hydrogenation increased. Two different types of sites were established to be active for bond hydrogenation, located at (i) the Pt-TiO₂ interface and (ii) the PtNi bimetallic phase. For the Pt-TiO₂ interface the presence of accessible Ti ^x+ cations in close distance to Pt and for the PtNi phase a slightly positively charged Ni on the surface of these alloy particles were concluded to be responsible for the enhanced selectivity to crotylalcohol. The hydrogenation of the bond occurs mainly on the clean metal surfaces of Ni and Pt.     

Comportement électrochimique des variétés alpha et beta PbO2 et influence de l'antimoine sur leur reactivité électrochimique en milieu H2SO4, 8N

Résumé L'expérience a montré qu'il est possible d'obtenir par l'oxydation anodique des variétés- et-PbO₂ parfaitement pures au point de vue cristallographique, et que la réduction de-PbO₂ se déroule à un potentiel plus élevé et plus constant que celui observé sur-PbO₂. La réactivité électrochimique de-PbO₂ est plus importante que celle de-PbO₂. L'introduction de Sb dans les réseaux cristallins de ces variétés diminue fortement leur cristallinité et dans le cas de-PbO₂ on obtient toujours simultanément- et-PbO₂. Du point de vue réactivité électrochimique, l'accroissement dû à la présence de Sb est de l'ordre de 33%.

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The results demonstrate the possibility of preparing through anodic oxidation rigorously pure, from the crystallographic point view,- and-PbO₂ phases, and that the reduction of-PbO₂ takes place at a potential which is more positive and more constant than the one obtained with-PbO₂. In a battery, the electrochemical reactivity of-PbO₂ is more important. The introduction of Sb into the lattice of these forms of PbO₂ decreases their crystallinity, and for the case of-PbO₂ we obtained simultaneously- and-PbO₂. Their electrochemical reactivity can increase by about 33%.

     

Improvement of Catalytic Performance in Isobutane Oxidation to Methacrylic Acid of Keggin-Type Phosphomolybdates by Preparation via Lacunary Precursors: Nature of the Active Sites

A solvent-free method of preparation of a vanadium(V) phosphate is described and discussed. Reaction of V₂O₅ with H₃PO₄ in the absence of water at 150°C leads to the formation of a new catalytic material that is designated as anhydrous VOPO₄. The material readily hydrates to form VOPO₄2H₂O and has been characterised using powder X-ray diffraction, in situ Raman spectroscopy and ³¹P MAS NMR spectrometry. On activation in dry N₂ followed by reaction with butane/air another novel material is formed that has an intrinsic activity for maleic anhydride that is similar to catalysts derived from VOHPO₄0.5H₂O under comparable conditions. Activation of VOPO₄2H₂O under comparable conditions leads to the formation of _I-VOPO₄ which exhibits no partial oxidation activity. Reaction of anhydrous VOPO₄ with alcohols leads to the exclusive formation of VO(H₂PO₄)₂ in further contrast to VOPO₄2H₂O which under similar conditions leads to the synthesis of VOHPO₄0.5H₂O.     

Studies on nonisothermal solid state galvanic cells — effect of gradients on EMF

An expression for the EMF of a nonisothermal galvanic cell, with gradients in both temperature and chemical potential across a solid electrolyte, is derived based on the phenomenological equations of irreversible thermodynamics. The EMF of the nonisothermal cell can be written as a sum of the contributions from the chemical potential gradient and the EMF of a thermocell operating in the same temperature gradient but at unit activity of the neutral form of the migrating species. The validity of the derived equation is confirmed experimentally by imposing nonlinear gradients of temperature and chemical potential across galvanic cells constructed using fully stabilized zirconia as the electrolyte. The nature of the gradient has no effect on the EMF.Nomenclature J _i flux of speciesi - X _i generalized forces - L _ij Onsagar coefficient - ₁ electrochemical potential of ions - ₂ electrochemical potential of electrons - T absolute temperature - U ₁ ^* total energy of transfer of the ion - partial molar enthalpy of the ion - Q ₁ ^* heat of transport of the ion - Z ₁ charge on the ion - F Faraday constant - electrostatic potential - ₂ chemical potential of the electron - ₁ chemical potential of the ion - partial entropy of the ion - E _SE EMF developed across the solid electrolyte - E _Pt EMF developed across the platinum lead - ( ₂)_Pt chemical potential of electrons in platinum - partial entropy of electrons in platinum - (Q ₂ ^* )_Pt heat of transport of electrons in platinum - E _cell EMF developed across the whole cell - chemical potential of oxygen - chemical potential of oxygen in its standard state - R universal gas constant - partial pressure of oxygen - relative chemical potential of oxygen - _M relative chemical potential of metal M - a _M activity of metal M     

Diffusion effects in the hydrogenation of 3-methyl crotonaldehyde over zeolite-supported Ru

The activity and selectivity in gas-phase hydrogenation of the unsaturated aldehyde 3-methyl crotonaldehyde (UAL) over zeolite-supported Ru was found to depend markedly on the partial pressures of the two reactants in the gas phase, with the rate increasing as the ratio decreased. Anomalously low apparent activation energies were found compared to the same reaction over Ru/SiO₂, and secondary reactions to produce the totally hydrogenated product occurred much more readily in the zeolite-supported Ru. These observations led to the suggestion that these reactions were under diffusion control.     

Temperature programmed oxidation of Cu/ZnO catalysts with N2O: an attempt for characterization of copper catalysts

For characterization of the surface structure of metallic copper formed on the support, temperature programmed oxidation (TPO) with N₂O was carried out over various Cu/ZnO catalysts. Four peaks of the N₂ formation (, , and ) were observed at 223, 400, 545 and 600 K in the TPO runs. The average copper crystallite size estimated from the sum of the amount of - and -peaks agreed fairly with those determined by X-ray diffraction and transmission electron microscopy. It was concluded that - and -peaks resulted from the oxidation of metallic copper atoms on the steps, corners and/ or defect sites, and on the flat sites of the surface of copper crystallites, respectively, while - and -peaks resulted from the bulk oxidation of copper.     

Isotopic exchange between H2 and D2 by the Rideal-Eley mechanism

The rate of the reaction, H₂+D₂ 2HD, is very rapid at –160 °C on the catalyst Cp₂Th-(CH₃)₂/dehydroxylated Al₂O₃, activated in H₂+D₂ and believed to contain supported Cp₂ThH⁺(Cp = ⁵-pentamethylcyclopentadienyl). Even at –195 °C, the turnover frequency is greater than 0.1 sec^–1 at atm. Strong evidence is adduced that the exchange proceeds by a Rideal-Eley process, i.e. by a four-center transition state involving Th-H+D-D.     

Mass transfer at the electrodes of the ‘falling-film cell’

In the falling-film cell the electrolyte flows as a thin film in the channel between an inclined plane plate and a sheet of expanded metal which work as electrodes. The present work gives the mass transfer coefficients at both electrodes; the experimental variables are the electrolyte flow-rate, the angle of inclination of the channel and the interelectrode distance. The results allow three different flow regimes to be characterized. At low flow rates, there exists a particular regime where capillary effects are present; in this regime the mass transfer coefficient decreases with increasing flow rate, which is interesting from the point of view of possible industrial electrolytic applications.Nomenclature b width of the inclined channel - D diffusion coefficient - d interelectrode distance - e _m mean film thickness - Grashof number, based ond - Grashof number, based onL - ¯k overall mass transfer coefficient, defined by Equation 9 - L electrode length - Q _v volumetric flow rate - volumetric flow rate per unitQ _vl width of channel - Reynolds number - Schmidt number - Sherwood number, based ond - Sherwood number, based onL - mean velocity of the liquid film - inclination angle of the channel with respect to the horizontal - kinematic viscosity of them electrolyte     

Characterization of the Li1−x H x AlO2 system; 0.00 ⩽x ⩽ 0.90

The mechanism of cation replacement in the Li_1–x H _x AlO₂; 0.00 x 0.90 system was investigated with XRD. Examination of the peak position and intensity associated with the 018 and 110 Bragg reflections in a series of partially replaced samples showed that the cation replacement process proceeded by a two phase mechanism. Catalytic characterization of LiAlO₂ with the 2-propanol probe reaction revealed the formation of the condensation products methyl-cyclopentene, 4-methyl-2-pentanone, and 4-methyl-2-pentanol. These products were seen in addition to propylene and acetone. Catalytic characterization of Li_1–x H _x AlO₂;x = 0.90 with 2-propanol showed a significant decrease in condensation activity and no change in the propylene/acetone ratio relative to LiAlO₂. This suggests that the decrease in the amount of lithium eliminated the basic sites necessary for the condensation reactions.     

Mathematical modelling of electrode growth

It is known that during electrodeposition or dissolution electrode shape change depends on the local current density (Faraday's law in differential form). Assuming that concentration gradients in the bulk of the solution may be neglected, the current distribution in an electrochemical system can be modelled by a Laplace equation (describing charge transport) with nonlinear boundary conditions caused by activation and concentration overpotentials on the electrodes. To solve this numerical problem, an Euler scheme is used for the integration of Faraday's law with respect to time and the field equation is discretized using the boundary element method (BEM). In this way, and by means of a specially developed electrode growth algorithm, it is possible to simulate electrodeposition or electrode dissolution. In particular, attention is paid to electrode variation in the vicinity of singularities. It is pointed out that the angle of incidence between an electrode and an adjacent insulator becomes right (/2). This is confirmed by several experiments.List of symbols x _i coordinates of a point i belonging to a boundary (m) - t time (s) - h thickness variation at a point belonging to an electrode (m) - M molecular weight (kgmol^–1) - m specific weight (kgm^–3) - z charge of an ion (C) - F Faraday's constant (C mol^–1) - R _a2 impedance of the linearized activation overvoltage on cathode (S2 cm^–2) - efficiency of the reaction - electric conductivity (^–1 m^–1) - U electric potential (V) - rate of mechanical displacement of a point (m s^–1) - V applied potential on an electrode (V) - W Wagner number defined as the ratio of the mean impedance of the reaction and the mean ohmic resistance of the cell given by L/ with L a characteristic length of the cell. - overvoltage (V) - ₁ overvoltage on anode (V) - ₂ overvoltage on cathode (V)     

Spin Labels as the Instrument for Analysis of Topochemistry of Polymer-Immobilized Ziegler Catalytic Systems

A spin labeling technique was used to investigate the topochemical characteristics of polymer carriers and immobilized metal complexes. Functionalized polyethylenes (PE) such as PE–grafted-polyallylamine (1), PE–grafted-polydiallyl-amine (2), and PE–grafted-poly-4-vinylpyridine (3), obtained by grafting polymerization of the corresponding monomers, were used as polymer carriers. Metal-containing polymers were obtained by immobilization to 2 with either TiCl₄ (2/Ti) or Al(C₂H₅)₂ Cl (2/Al). The stable nitroxyl radical 2,2,6,6-tetramethyl-4-(2-oxy-4,6-dichlorotriazine)piperidine-1-oxyl was used for spin labeling 2 to give , while 2,2,5,5,tetramethyl-3-(N-acetoamidiiodine)-pyrrollidine-1-oxyl was used for spin labeling 3 to give . Radical 2,2,6,6-tetramethyl-4-hydroxy-pyperidine-1-oxyl was bonded to 2/Ti and 2/Al to give and , respectively. The accessibility of the functionalized polymers 1–3 to interact with the spin labels and the immobilized metal complexes was examined. Estimation of the effective distances between the spin labels and the dynamic behavior of nitroxyl radicals in the functionalized polymer matrixes and metal-containing polymers revealed several important features of spin-labeled systems. Metallation of a functional covering makes the polymer more accessible for spin labeling and has a considerable effect on the dynamic characteristics of polymer matrix. A change of the rotational activation energy of the spin labels from 15 kcal/mol in to 44 kcal/mol for in the temperature range 100–150°C was observed.     

Methane coupling at low temperatures on Ru(0001) and Ru(11¯20) catalysts

The conversion of methane to higher hydrocarbons on single crystal Ru catalysts has been investigated using combined elevated-pressure kinetic measurements/surface science studies. The reaction consists of activation of methane on Ru(0001) and Ru(11¯20) surfaces to produce carbonaceous intermediates at temperatures between 350 and 700 K and rehydrogenation of these species to ethane and propane at 370 K. It is found that under the reaction conditions employed, the maximum yield in ethane/propane production occurs at 500 K on both surfaces. Influence of the hydrogenation temperature on the production of ethane and propane is also examined. On Ru(0001), the yields of ethane and propane maximize at = 400 K, whereas no maximum yield was observed on Ru(11 0) in the 300–500 K temperature range. Under optimum reaction conditions, hydrocarbon products consist of 16% ethane and 2% propane. High-resolution electron energy-loss spectroscopy (HREELS) has been used to identify various forms of hydrocarbonaceous intermediates following methane decomposition. An effort is made to relate the hydrocarbon intermediates identified by HREELS to the gas phase products observed in the elevated pressure experiments.     

Poly(β-malic acid) : a new polymeric drug-carrier

Summary Poly(-malic acid) is a new synthetic functional polyester of the poly(-hydroxy-acid)-type whose properties are investigated in regard to possible uses as bioresorbable polyvalent drug-carrier. Degradation of polymer chains in 0.15 N phosphate buffer at pH=7.5 is monitored by aqueous GPC on SEPHADEX gels and by enzymatic titration of ultimate degradation products. It is shown that the rate of degradation obeys first order kinetics at the begining and that poly(-malic acid) degrades to malic acid at last.Presented at the 26th IUPAC Microsymposium on Macromolecules: Polymers in Medicine and Biology, Prague, July 9–12, 1984     

The linear master dependence for polymer melts in the non-Newtonian range

Summary The multivariable power function · _o = B( _o)^b ₁· q^b ₂· G^b ₃ (symbols are explained in the text) is proposed as the general linear master dependence for the behaviour of linear and branched polymer melts in the non-Newtonian range. It can be applied for generation of any flow curve for a given polymer sample provided that constants B, b₁, b₂ and b₃ are known. It has been found, using the experimental data on polycarbonate, that B is approximately equal to 3×10^–5 being the universal constant, and b₁=0.77 for linear polymers. Lower values of b₁ indicate the existence of branched macromolecules, and the relationship of b₁ vs G can be used for determination of branching degrees from the rheological data.     

Correlation between CO frequency and Pt coordination number. A DRIFT study on supported Pt catalysts

The infrared stretching frequencies of linear-bonded carbon monoxide on face, edge and corner atoms have been identified with four platinum catalysts. The metal particles were supported on different absorbentia and each having a different average particle size, ranging from about 10 to 200 Å. The values of the various absorption bands correlate linearly with the number of nearest neighbours of the different surface sites (n in C _n ). Effects of an interaction between Pt clusters and support on the linear-CO stretching frequency have not been observed. This empirical correlation between and C_n, and the advantages ofin situ Diffuse Reflectance Fourier transform Infrared Spectroscopy in combination with derivative spectrometry opens the possibility to monitor the dispersity of supported platinum in a simple and relatively quick way.     

Reduction of NO by H2 on PdO-MoO3/γ-Al2O3 of low molybdena loading

The catalytic activity and selectivity of three PdO-MoO₃/-Al₂O₃ catalysts containing about 2% Pd and 2% Mo were studied for the reduction of NO by h₂ in the presence of varying amounts of oxygen at temperatures from 50 to 550 °C. The results are compared with those for PdO/-Al₂O₃, PdO-MoO₃/-Al₂O₃ containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. In the absence of oxygen, the conversion of NO to N₂ and N₂O is higher on the three catalysts than it is on PdO/-Al₂O₃ at 500 and 550 °C. In the presence of oxygen, the yields of N₂ and N₂O are generally lower on two of the PdO-MoO₃/-Al₂O₃ catalysts than on PdO/-Al₂O₃.     

Initiation of living polymerization of isobutyl vinyl ether by the “H2O”/n-Bu4NTiCl5 system

Summary Living carbocationic polymerization (LCPzn) of isobutyl vinyl ether (IBuVE) is readily initiated by adventitious moisture (H₂O) in the presence ofn-Bu₄NTiCl₅ in CH₂Cl₂ solvent at-20 °C to yield high molecular weight polymers (up to 100,000 g/mole) with narrow molecular weight distributions . The number of macromolecules remains constant and reflects the number of H₂O molecules in the charge. Thede facto initiator is most likely HCl formedin situ by the partial hydrolysis ofn-Bu₄NTiCl₅. Surprisingly the overall polymerization rates obtained in the presence of purposely added initiator, MeCH(Oi-Bu)Cl, were lower than in the absence of it,i. e., with initiator free system. Experiments carried out in the presence of the proton trap, 2,6-di-tert-butylpyridine (DtBP), and alcohols were analyzed to explain the findings. The observed rates may be explained by reactions between H₂O and the various ingredients in the system prior to initiation.     

Comparison between the impedance spectra of Li/SOCl2 batteries obtained using the time and the frequency domain measurement techniques

A comparison between the impedance spectra of Li/SOCl₂ batteries obtained in the time and frequency domains is reported. It is demonstrated that by averaging over several responses the accuracy in the time domain is greatly improved. On the other hand, it was found that the time domain technique caused nonlinearity in the system response even at very small amplitudes of excitation (for example corresponding to a potential drop of 30 mV). The method is useful for routine characterization of the quality of galvanic cells in industrial production. The accuracy compared with market-available impedance spectrometers operating in the frequency domain is satisfactory (±10%), the price being much lower.Nomenclature g _i parameters of model frequency response - g _id parametersg _i obtained by deconvolution - g _ir parametersg _i obtained by frequency domain method - H frequency response - H _m model frequency response - H _r reference frequency response - i(t) excitation current - I() Fourier transform of thei(t) - Im imaginary part - j imaginary unit - M Number of independent measurements - N number of samples - Re real part - R resistance - R ₀ ohmic resistance - T sampling time - u(t) response voltage - U() Fourier transform ofu(t) - dispersion factor - angular frequency - () phase spectrum - _a relative amplitude error - _gi relative error ofg _i - average relative amplitude error - _p relative phase error - average relative phase error - ₀ mean relaxation time     

Heterogeneous asymmetric reactions. 23. Enantioselective hydrogenation of ethyl pyruvate over cinchonine- and α-isocinchonine-modified platinum catalysts

The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al₂O₃ catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm^–3, 1 bar H₂, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm^–3, 1 bar H₂, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations.     

The IR spectroscopy of methane and hydrogen adsorbed on α-chromia

The vibrational perturbations induced in the IR spectra of methane and of dihydrogen by the non-dissociative adsorption on the (01 2) and (11 0) faces of -chromia are illustrated and compared. It is concluded that both molecules are adsorbed on Cr³⁺ sites and that the involved forces are mainly of the electrostatic type. The resulting situation is compared with that found on the more basic MgO. The problems associated with the C–H and H–H bond activations on transition metal oxides are also discussed.