XPS Characterization of reduced LaCoO3 perovskite

Three samples of LaCoO₃ were prepared by two different methods and calcined at 800 or 1000 °C. They had BET areas of 1, 12, and 16 m²/g and all of them showed pure perovskite X-ray diffraction patterns with identical unit cell dimensions. In a series of experiments the oxide, having larger surface area, was stepwise reduced in hydrogen at temperatures between 60 and 500 °C. The XPS spectra, taken at room temperature after evacuation at 400 °C at each reduction level, showed that the surface concentration of Co° was very low up to 300 °C but increased sharply between 300 and 350 °C (9–75%). This concentration further increased to 100% after 10 min reduction at 450 °C, but upon heating in hydrogen for an additional 10 min at 500 °C it decreased again to 75%. In another series of experiments the mixed oxide was reoxidized after each reduction. A fresh sample was reduced to 350 and 400 °C by contacting with hydrogen for 1 hr and evacuated at temperatures between 400 and 500 °C. Both high evacuation temperatures and reduction at 400 °C during 1 hr produced a sharp decrease in Co° surface concentration. These results are consistent with the catalytic behavior of this perovskite reported earlier by E. A. Lombardo et al. (4–7). A model is proposed to interpret the reduction of LaCoO₃.     

Conductivity of highly concentrated aqueous electrolyte solutions: NH4NO3 + H2O + LiNO3 system

Conductivity of ammonium nitrate + water (2 moles per NH₄NO₃) + lithium nitrate mixtures containing up to 0.225 stoichiometric mole fraction of lithium nitrate, at the temperatures ranging between 295—345 K, has been measured. At a given composition the variation of conductivity with temperature could be expressed by the equation.

Where A and B are empirical constants. Conductivity—composition isotherms are non-linear, indicating the change in the ordering tendency of the system.     

Interactions of acetone with MoO3

Interactions of acetone and acetone/oxygen mixtures with MoO₃ were studied at 215–300 °C by pulse method. It has been shown that acetone is deoxygenated to propylene or oxidized to acetic acid, CO, and CO₂. Moreover a part of acetone is reversibly adsorbed and several minutes are necessary for acetone to be removed with a flow of carrier gas. As the surface is endowed with deoxygenating and oxidizing sites the desorbing acetone leaves the reactor mainly in the form of CO. These results agree with theoretical predictions based on the bond-strength model of active sites, according to which the sites of the first type (conversion of acetone) are localized on (100) face of MoO₃ and those of the second type (adsorption) on (001) and (101) planes. Both types of sites consist of surface anion vacancies and differ in the strength of bonding of oxygen. On the (100) face the bond strength is sufficiently large to break CO bond in acetone. On the other faces the interactions between acetone and surface are much weaker. In this context the application of acetone as poison of other reactions (e.g., conversion of methanol) is discussed.     

Low energy ion scattering study of Ni-MoAl2O3 catalysts

The principles of low energy ion scattering (ISS) and its application to the study of real catalyst surfaces are discussed. As a special example, the effect of the calcination temperature of a Ni-Mo/ Al₂O₃ on the elemental distribution is studied. From the ISS “depth profiles” it can be concluded that at temperatures <770 K the molybdate phase on the Al₂O₃ support forms islands with three-dimensional extension while uncovered support surface remains even though the loading corresponds to the theoretical monolayer capacity. A spreading of the molybdate over the support surface occurs at 870 K to form a more or less closed monolayer. Simultaneously, Ni²⁺ ions are lost from the surface molybdate by penetration into subsurface layers of the Al₂O₃ matrix where they occupy octahedral and tetrahedral sites. The importance of the redistribution of promotor ions and the dispersion of the molybdate phase for the catalytic performance is emphasized.     

Hydrogenation of conjugated dienes over ZrO2 by H2 and cyclohexadiene

Hydrogenation of 1,3-butadiene and 2-methyl-1,3-butadiene and equilibration reaction of H₂D₂ were carried out over ZrO₂ using H₂ and Cyclohexadiene as hydrogen sources. Reaction rates were measured by changing the activation temperature of the catalyst. While the hydrogenation with H₂ and H₂D₂ equilibration reaction gave an optimum activity at 600 °C, another optimum was obtained for transfer hydrogenation at 800 °C. Hence it is concluded that the sites responsible for the transfer hydrogenation are not the same as those which catalyze hydrogenation with H₂ and H₂D₂ equilibration. Product distributions in n-butenes and methylated butenes were compared on ZrO₂, ThO₂, La₂O₃, and MgO and were also compared in direct hydrogenation with H₂ and transfer hydrogenation with Cyclohexadiene. Assuming an ionic intermediate, selectivity changes in the monoolefins produced over different catalysts and by different hydrogen sources were interpreted in terms of the variation of the anionic character of the intermediate and the shift of anionic to neutral or cationic intermediate, respectively.     

Study of ternary-component bismuth molybdate catalysts by 18O2 tracer in the oxidation of propylene to acrolein

Participation of lattice oxide ions of ternary-component bismuth molybdate catalysts MBiMo O (M = Ni, Co, Mg, Mn, Ca, Sr, Ba, and Pb) was investigated using the ¹⁸O₂ tracer in the selective oxidation of propylene to acrolein. The participation of the lattice oxide ions in the oxidation is prominent on every catalyst but the extent of the participation varies significantly depending on the structure of the catalyst. Only lattice oxide ions in the bismuth molybdate phase are incorporated into the oxidized products on the catalysts (M = Ni, Co, Mg, and Mn) where M have smaller ionic radius than Bi³⁺; catalyst particles are composed of a shell of bismuth molybdates and a core of MMoO₄. On the other hand, whole oxide ions in the active particles are involved in the oxidation on catalysts having a scheelite-type structure (M = Ca, Sr, Ba, and Pb) where M has a comparable ionic radius to Bi³⁺.     

Correlation between electrode surface coverage and the rate of inhibition of Pb(II) reduction process at the DME in HClO4 solutions

This study was conducted to ascertain that the main factor governing the inhibition of the Pb(II) reduction process in 1 N HClO₄ solutions was the extent of electrode surface coverage by coumarin molecules. The electrode surface coverage was calculated from the results of electrocapillary and differential capacitance measurements. These results indicated the applicability of the Flory—Huggins equation in case of adsorption of coumarin in 1 N HClO₄ solution at the dropping mercury electrode. The rate constants for the Pb(II) reduction process in the same solutions were calculated from results of electrode impedance measurements. The direct relation between surface coverage and the decreasing rate constants is in agreement with the postulate of their interdependence.     

Preparation of glass-ceramics Pb5Ge3-xSixO11 (0 ≤ x ≤ 1.75)

Glass-ceramics with a high transparency were prepared by the crystallization of 5PbO·(3-X)GeO₂·XSiO₂ glass. From the results of temperature dependence of the permittivity, the Curie temperatures of the products were estimated to be 175°C, 115°C and 65°C for the glass-ceramics 5PbO·3GeO₂, 5PbO·2.5·GeO₂·O.5SiO₂ and 5PbO·2GeO₂·SiO₂, respectively.     

Studies on MnO2—III. The kinetics and the mechanism for the catalytic decomposition of H2O2 over different crystalline modifications of MnO2

The catalytic decomposition of aqueous H₂O₂ solution over MnO₂ of various crystalline modifications has been studied in the temperature and the pH ranges of 20–35°C and 3–14 respectively. The active site in the pH range of 3–8 is considered to be Mn⁴⁺ and in the range 8–14 is Mn³⁺ ion. Catalytic activity attains maximum at pH of about 11.5–12.0 and then decreases with further increase in pH. The experimental rate data agrees satisfactorily with the proposed mechanism based on the above considerations. Catalytic activity has been correlated with some physico-chemical characteristics of the samples such as surface oxygen, surface OH groups, lattice parameters and in somewhat complex manner with electrochemical or discharge capacity of MnO₂. Although the discharge capacity increases with increase in catalytic activity for some samples (mostly γ-MnO₂) a number of samples belonging to α, β and δ-variety show sharp deviation from the above behaviour. This is explained from the state of occurrence of MnOOH in the host lattice.     

The electrochemical oxidation of lead in various H2OH2SO4 mixtures—II. Ring-disc electrode study

The rotating ring-disc electrode technique was used to study the generation of soluble Pb(II) species during the anodic oxidation of the lead electrode in various H₂OSO₄ media. The concentration range extended from 0.06 to 10.3 M H₂SO₄. For every concentration it was possible to detect a small cathodic ring current variation which was attributable to the reduction of Pb(II) to Pb. Quantitative measurements of the collection efficiency showed that the small cycle life of the lead-acid battery in 5.6 M H₂SO₄ cannot be ascribed to the dissolution step.     

Carbon dioxide transport through enzymatically active synthetic membranes

The facilitated transport of CO₂ through thin liquid membranes, Millipore filter membranes, and cross-linked protein membranes has been investigated using a tracer ¹⁴CO₂ technique. Both the uncatalyzed reactions and the enzymatic reactions catalyzed by carbonic anhydrase in homogeneous solution and immobilized in several membrane configurations were studied. The steady-state transport data were reduced in terms of an analytical model for simultaneous reaction and diffusion which permitted the direct determination of the diffusional and enzyme kinetic parameters. The experimental method illustrates a powerful technique for measuring rapid reaction kinetics. In addition, a general multi-layer membrane model is developed which is capable of treating membrane kinetic heterogeneities. The multi-layer model yields a useful definition for the reaction boundary layer and provides a comparison among different membrane kinetic configurations for membrane design purposes.     

The electrochemical oxidation of lead in various H2OH2SO4 mixtures—I. Linear sweep voltammetry

The linear sweep voltammetry technique was used in order to detect a change in the lead oxidation mechanism around 5 M H₂SO₄. This change could explain the strong decrease in cycle life of the lead-acid battery when increasing H₂SO₄ concentration from 4.7 to 5.6 M. In the concentration range studied (3.6–10 M) no change was detected. The same behaviour was observed when cycling the electrode, when increasing sweep rate and when rotating the electrode.     

Forced cycling of the catalytic oxidation of CO over a V2O5 catalyst—II Temperature cycling

An experimental study was made of the effects of temperature cycling on carbon monoxide oxidation over a vanadium pentoxide catalyst. Square-wave temperature cycles at periods above 5 hr provided reaction rates which, when time-averaged, were closely approximated by quasi-steady state reaction rates. As the period decreases, reaction rates fall off indicating that time-average rates under transient conditions are lower than those acheived at quasi-steady state. At a period of one hour, the time-average cycling rate falls below the rate at steady state at the mean temperature of the cycle used.     

Electrochemical kinetic study of the Fe3+Fe2+ system in oxalate medium

The overall processes were determined for the reduction of Fe³⁺ to Fe²⁺, in an oxalate medium and at oxalate concentrations of less than 0.5M. The coordination numbers of the oxidized and reduced species were calculated, together with the number of intervening protons that depend on the oxalate concentration and the pH.In addition, the species that reduces at the electrode is determined, together with the intensity equations for each process.     

Studies on MnO2—II. Relationship between physicochemical properties and electrochemical activity of some synthetic MnO2 of different crystallographic forms

The electrochemical activity of a number of synthetic MnO₂ samples of various crystalline modifications has been determined by discharging a thin pellet containing 0.150 g MnO₂ and 0.015 g acetylene black in both NH₄ClZnCl₂ (pH 4.1) and 9M KOH medium. An attempt has been made to find correlations between depolarizing capacity (time to reach 0.6 V) and various physico-chemical properties; such as, density, surface area, lattice parameters, surface OH groups, surface oxygen, magnetic susceptibility etc. Whereas depolarizing capacity for γ-MnO₂ increases with increase in both true and packing (powder) density, capacity tends to decrease with increase in surface area. For other modifications of MnO₂ there exist direct relationships between depolarizing capacity and the various properties of the samples mentioned above. The relationships between lattice parameters and depolarizing capacity indicate that samples with an extended ab plane in general possess higher activity. It has been observed that surface OH groups and surface oxygen when normalised to unit weight are not only important properties in active MnO₂ but are also interrelated.     

Reforming of C6 hydrocarbons on a PtAl2O3 catalyst

Isomerization, dehydrocyclization and hydrocracking of C₆ hydrocarbons on a commercial reforming PtAl₂O₃ catalyst were studied in a tubular reactor. The temperature was varied from 420 to 500°C, the pressure from 1.6 to 16 bar, the molar H₂/hydrocarbon inlet ratios from 1.5 to 20. Reaction rate equations of the Hougen—Watson type, corresponding to a bifunctional mechanism, were found to describe the experimental data. Parameter estimates for the reforming reactions were obtained by application of a generalized least square criterion to the calculated and observed production rates of the gas phase components. Electrobalance experiments showed that both the reforming reaction rates and the coking rate decrease exponentially with coke content. Multiplying the reaction rates with one common exponential deactivation function allowed the prediction of the observed conversions by one single set of kinetic parameters. The coking rate equation was derived by fitting the final coke content profiles obtained in the tubular reactor. The main contribution to coke formation was attributed to Me-cyclopentadienes.     

In situ laser raman spectroscopy of the sulfiding of Moγ-Al2O3 catalysts

Raman spectra of sulfided $\text{Mo}\text{γ-}\text{Al}{}_2\text{O}{}_3$ catalysts were obtained using in situ techniques for two sulfiding methods. For samples sulfided by 10% $\text{H}{}_2\text{S}\text{H}{}_2$ at 400 °C, MoS₂ structures were observed. A stepwise sulfiding using 10% $\text{H}{}_2\text{S}\text{H}{}_2$, with spectra recorded at 150, 250, and 350 °C, resulted in observation of molybdenum oxysulfide, reduced molybdate, and surface “MoS₂” phases. Reexposure of these samples to air led to radical modification of the oxysulfide structures as well as transformation of some sulfide phases. A model incorporating terminal and bridging MoS bonding and anion vacancies is proposed. This model is based on the conversion of isolated and aggregated molybdate and MoO₃ species to oxysulfide and reduced molybdenum phases. Conversion of reduced molybdenum phases to sulfides is observed to be slow.