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.     

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.     

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.     

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.     

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.     

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.     

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.     

A tracer study of the metathesis of simple olefins over molybdena-alumina catalysts

Metathesis of perdeuterio- and perhydroethene, propene, 1-butene, and their mixtures with cis-2-butene was studied over a molybdena-alumina catalyst using recirculation and pulse systems. No metathesis was observed over a fresh unreduced catalyst, but after pretreatment with cis-2-butene the reactivity of olefins in metathesis reactions increased in the sequence of ethene ? 1-butene < propene < cis-2-butene. The activity was higher for the metathesis of propene or 1-butene after the catalyst was pretreated with the corresponding olefin instead of cis-2-butene, but no metathesis was induced by ethene. Metathesis of cis-2-butene was suppressed by the addition of equal amounts of olefins in the sequence: ethene ? 1-butene < propene. Studies of mixtures revealed that the rate of cis- to trans-2-butene isomerization by metathesis ～- the rate of degenerate metathesis of 1-butene or the cross metathesis of 1-butene with 2-butene.     

Different behavior between Pt and Pd catalysts in the hydrogenolysis of cyclohexanediones and hydroxycyclohexanones

Hydrogenolysis reactions of cyclohexanediones, hydroxycyclohexanones, and some related alicyclic ketones were studied over Pt, Pd, Ir, and Rh catalysts at atmospheric hydrogen pressure in t-butyl alcohol as a solvent. Pt and Pd had high catalytic activities for the hydrogenolysis of carbon-oxygen bonds. However, Ir and Rh scarcely had any activity unless 1,3-cyclohexanedione and 3-hydroxycyclohexanone were involved. The mechanisms of the hydrogenolysis differed with Pt and Pd. In the hydrogenation of 4-methoxycyclohexanone, Pt afforded cyclohexyl methyl ether as the hydrogenolysis product; while Pd afforded cyclohexanone, which was then hydrogenated to cyclohexanol. Thus Pt cleaved the carbon-oxygen double bond, and Pd cleaved the carbon-oxygen single bond. Deuterolysis of cyclohexanone and 4-methoxycyclohexanone on Pt gave mainly d₂ species of cyclohexane and cyclohexyl methyl ether as the hydrogenolysis products. This indicated that the carbon-oxygen double bonds were directly cleaved to yield methylene groups on Pt. Almost of all 3-hydroxycyclohexanone was hydrogenolyzed to cyclohexanone on Pd; whereas cyclohexanone as well as cyclohexanol was not hydrogenolyzed at all. In the case of Pd, the carbon-oxygen single bond was cleaved when it was activated by formation of π-oxoallyl adsorbed species on the catalyst at the carbon-oxygen double bond.     

Study of Pb(II) in various H2O-H2SO4 mixtures by differential pulse polarography: solubility of lead sulphate,diffusion coefficient of Pb(II) and half-wave potential of Pb(Hg)/Pb(II)

A study of some properties of Pb(II) in solutions of sulphuric acid was carried out by means of differential pulse polarography. The range of concentrations of acid extended from 0.6 to 70 weight % of H₂SO₄ (0.06-11.5 M). Determination of the solubility of lead sulphate was found to be in good agreement with NBS's results in the range of H₂SO₄ concentrations they studied. With increasing concentration in H₂SO₄, the diffusion coefficient was found to decrease more than expected from the Stokes-Einstein equation, while the standard potential of Pb/Pb(II) related to she was found to increase. This led to a more important self-discharge of the negative electrode of the lead-acid battery.     

The liquid-phase dehydration of tert.-butyl alcohol on B-P-O catalysts

The dehydration of tert.-butyl alcohol in the liquid phase has been investigated on BPO catalysts with phosphorus/boron molar ratios from 0.8 to 1.4 and pretreatment temperatures up to 400 °C. A maximum in the initial reaction rate occurs at a P/B of 1.25 and a pretreatment temperature of 100 °C. The reaction is catalyzed by Brønsted acid sites and not Lewis acid centers and appears to proceed through a carbonium ion mechanism.     

Preparation and surface characterization of nonsupported ruthenium catalysts

A study of the preparation of ruthenium powder catalysts, starting from ruthenium compounds from various sources, has been made. In many cases it is found that the extent of the chemisorbed hydrogen monolayer is lower than expected on the grounds of the BET surface area of the powder. From TGA, XPS, and EM observations it is concluded that this is caused by the presence of tightly bound chlorine and oxygen on the surface. These poisoning substances can be removed by reduction in hydrogen at room temperature, but above 700 K the metal surface is repoisoned by chlorine and oxygen originating from the bulk of the metal and diffusing to the surface. Methods are indicated to circumvent this difficulty and to prepare very pure samples.     

Heat and mass transfer in monolithic honeycomb catalysts—I.

The body of information presented in this paper is directed to engineers and scientists concerned with control of automobile emissions and exhaust gases from some industrial processes. The differential equations describing heat and mass transfer in a monolithic honeycomb catalyst are developed. Following transport mechanism is considered: convective heat and mass transfer in the holes of the structure, longitudinal thermal conductivity of the honeycomb support and gas-to-solid heat and mass transfer. The magnitudes of governing parameters for monolithic modules in use are discussed. Two methods for the numerical solution of a system of coupled, nonlinear ordinary differential equations with split boundary conditions are proposed. The first method-shooting procedure can be used only for problems with low values of Peclet number. For high values of Peclet number finite-difference approach along with the Newton-Raphson algorithm is suggested. It is shown that two stable steady states exist in certain regions of operation of a particular monolithic structure. The all-metal monolithic supports are more prone to multiplicity of steady states than the ceramic ones. For ceramic supports the two-phase piston-flow model is sufficiently accurate.     

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.     

Experimental study of traveling waves in nonadiabatic fixed bed reactors for the oxidation of carbon monoxide

An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occuring on a Pt/alumina catalyst has been used. Unlike the adiabatic conditions the velocity of a traveling temperature wave in a nonadiabatical arrangement depends on its axial position. In certain regions of inlet concentration multiple temperature fronts have been observed. For low inlet CO concentration a downstream temperature wave results and the lower (kinetic) steady state is dominant. For high inlet CO concentration an upstream propagating front results and the upper steady state is dominant. For a downstream moving wave oscillations of wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions periodic behavior of temperature profiles in the reactor has been observed.     

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.