Dimitris Vattis

Decomposition of N2O on Fe2O3/Al2O3 catalysts. Relationships between physicochemical and catalytic properties

The influence of the iron(III) content and the calcination temperature on the dispersion of supported Fe3+ species on alumina, on the semiconducting properties and on the catalytic activity of Fe2O3/Al2O3 catalysts has been studied, with the decomposition of N2O into N2 into N2 and O2 being used as a probe reaction. Above 250 °C iron(III) is located on the surface of the carrier as α-Fe2O3 and ‘strongly associated iron(III)’(denoted by Fe3+–S). The ratio of the amounts of these species remains constant from X= 0.172 to X= 0.615 mmol Fe3+ per g of alumina and then changes favouring Fe3+–S. The transformation of α-Fe2O3 into Fe3+–S is not accompanied by any detectable change in the Fe3+ dispersion but causes an increase in the activation energy of conduction. A rise in the calcination temperature and iron(III) content brings about an increase in the size of supported α-Fe2O3 crystals shown by a decrease in the dispersion of Fe+. It has been demonstrated that α-Fe2O3 and Fe3+–S exhibit similar activity. This increases linearly with the dispersion of the Fe3+ ions. Moreover, no relationship was found to exist between the semiconducting properties of the catalysts estimated by the activation energy of conduction and the catalytic activity. X-ray photoelectron spectroscopy was used to estimate the dispersion of the supported iron(III), conductivity experiments were performed to determine the activation energy of conduction and catalytic tests were carried out to determine catalytic activity.

Catalytic deactivation of Co-Mo hydrodesulphurization catalysts supported on γ-Al2O3 doped with Li+ ions

Abstract A series of hydrodesulphurization (HDS) catalysts containing 12% MoO3 and 2.8% Co3O4 were prepared using γ-Al2O3 modified with various amounts of Li+. The effect of the Li+ cations on the catalytic activity with respect to the HDS of thiophene was studied. A marked decrease in the catalytic activity with increasing Li+ content was found. An explanation based on the Co-Mo bilayer model proposed recently is suggested. This work provides more information about the well-known synergistic effect. Diffuse reflectance spectroscopy and X-ray spectroscopy were used to characterize the specimens.

Diffusion coefficients from stopped-flow gas chromatography

SummaryMutual diffusion coefficients of two gases A and B can be determined in an empty gas chromatographic column by letting component B enter at an intermediate position of the column and continuously flow through a part only of it, as a carrier gas. The other component A is injected in a small amount instantaneously at the closed end of the column with the detector placed at the other end. By repeatedly stopping and then restoring after a short time the flow of B, narrow extra peaks are produced on the chromatographic elution curve, owing to diffusion of A into B. An equation is derived giving the area under the curve of each stop-peak as a function of time of the corresponding stop. Plotting the experimental data according to this equation permits the determination of the diffusion coefficient of A into B. Some results obtained by this method show negligible variations with changes in the experimental parameters.

Hydrodesulfurization of thiophene over Na-doped CoMo/γ-Al2O3 catalysts prepared by inverse impregnation

The effects of Na+ ions on the physicochemical properties and catalytic activity of CoMo/γ-Al2O3 hydrodesulfurization catalysts prepared by inverse impregnation has been studied. The catalytic activity was found to be almost independent of the priority of impregnation, it is correlated with the distribution of promoter species in the oxidic precursors as well as with the selectivity of the catalysts. The Na doping of the support brings about a progressive decrease in the catalytic activity due to the partial scavenging of MoVI by Na+ to form Na2MoO4.AbstractБыли исследовани эффекты ионов Na+ на физико-химические свойства и каталитическую активность катализаторов гидродесульфурирования. CoMo/γ-Al2O3, приготовленных инверсным импрегнированием. Каталитическая активность почти не зависит от очередности импрегнирования. Она коррелируется с распределением промоторных частиц в окисляющих предшественниках, а также с селективностью катализаторов. Дотирование носителя натрием приводит к прогрессивному уменьшению каталитической активности, вследствие частичного захвата MoVI ионами Na+, образуя Na2MoO4.

Temperature programmed reduction study of alumina supported Fe3+ catalysts

The influence of the calcination temperature and Fe3+ content on the solubility of iron(III) in the alumina lattice has been investigated, using temperature programmed reduction (T.P.R.). It was found that the main quantity of Fe3+ species is dissolved in the region 200–320 °C. The solubility of the Fe3+ was almost constant in the range of 320–665 °C. This is found to be the case irrespective of the Fe3+ content at 665 °C. The surface Fe3+ species generally differs from the bulk α-Fe2O3.AbstractБыло исследовано влияние температуры обжига и содержания Fe3+ на растворимость железа(III) в решетке окиси алюминия с помощью температурно-программированного восстановления. Было найдено, что в большинстве частицы Fe3+ разбавлены в области 200–320 °C. Растворимость Fe3+ держалась почти постоянной в интервале температур 320–665 °C. Оказалось, что при 665 °C это не зависит от содержания Fe3+. Поверхностные частицы Fe3+ обычно отличаются от блочных α-Fe2O3.

Catalytic deamination on solid surfaces. Part 1.—Kinetic studies by stopped-flow gas chromatography

Stopped-flow gas chromatography was used to study the detailed kinetics of formation of unsaturated hydrocarbons by deamination on aluminium oxide and on porous glass of aminocyclohexane, 2-aminopropane, 1-aminobutane, 2-aminobutane, 1-amino-2-methylpropane and 2-amino-2-methylpropane. The results are consistent with a proposed mechanism in which the reacting amine is rapidly adsorbed on two or three kinds of active sites of aluminium oxide or porous glass, respectively, leading to unsaturated hydrocarbons through the formation of an intermediate adsorbed product on each site. Based on this mechanism, theoretical equations are derived which adequately describe the experimental reaction rate as a function of time. Analysis of the data by means of these equations permits the calculation of the various rate constants for the surface processes. True activation energies and frequency factors for the deamination of aminocyclohexane on aluminium oxide have also been determined.

Conversions in catalytic deamination calculated by stopped-flow gas chromatography

Abstract By calculating absolute values for the intercepts of the rate equation that describes the deamination of aminocyclohexane on aluminium oxide, the components of a weighted mean rate constant can be found. From these the conversion of the reactant amine to product(s) can be computed, and this is in good agreement with the conversion determined directly by separate experiments. This is an indication of the internal consistency of the theoretical rate equation, derived on the basis of a particular reaction model with many kinds of active sites. The conversions are much less than 100%, showing that a considerable fraction of the reactant is adsorbed on sites which transform it to irreversibly adsorbed products. The conversion to gaseous cyclohexene increases with increasing flow-rate of the carrier gas. Finally, it was found that the effective duration of the stops is very different from the real duration.

Catalytic Deamination on Solid Surfaces

The Kinetics of the deamination of aminocyclohexane on silica gel have been investigated. The results are consistent with a mechanistic scheme in which the aminocyclohexane is rapidly adsorbed on three kinds of active sites of Si02, leading to various saturated and unsaturated hydrocarbons through the formation of an intermediate adsorbed product on each site. True activation energies and frequency factors have been determined. A comparison of the results with those obtained for the deamination of the same amine on y-AI203 and porous glass has been attempted.

Adsorption of thiophene and model hydrocarbons on MoO3/γ-Al2O3 catalysts studied by gas–solid chromatography

A number of MoO3/γ-Al2O3 catalysts of varying Mo content have been prepared and the adsorption of some probe molecules (thiophene, benzene, cyclohexane, cyclohexene, pentane, hexane, heptane and octane) has been studied using the gas–solid chromatographic technique. Isotherms for the adsorption of thiophene at 250 °C as well as enthalpies of adsorption of the probe molecules in the range 140–300 °C have been determined. Thiophene is adsorbed on three distinct kinds of site. On the first kind it is reversibly adsorbed. There was no preference for the development of this type of site on a particular Mo supported species. On the second type of site, located on octahedral MoVI ions of the polymeric Mo, reversible adsorption takes place, but the molecules adsorbed on these sites were desorbed very slowly. It was concluded that the third type of site, on which thiophene is irreversibly adsorbed, is generated exclusively on the bulk MoO3 supported on the carrier. Only reversible adsorption of instantaneous desorption type of the probe hydrocarbons was observed on the catalysts prepared. On the basis of linear relations between the enthalpy of adsorption and polarizability of the non-polar hydrocarbons used it was inferred that the MoO3/γ-Al2O3 catalysts exert non-specific (London or electrostatic) forces on these non-polar molecules. In the case of benzene, specific interactions resulting in the formation of a surface π-complex are involved in addition to the non-specific ones. These specific interactions are promoted by the supported MoVI.

Catalytic deamination on solid surfaces part 2.-Deamination of dicyclohexylamine on aluminium oxide

Deamination of dicyclohexylamine gives the same final products as those obtained from the respective primary amine on a similar surface. A theoretical rate equation derived permits the calculation of various rate constants from the experimental data. It is concluded that one of the intermediates formed from the two amines must be identical.AbstractДеаминирование дициклогексиламина дает те же самые конечные продукты, которые были получены из подобного первичного амина на подобной поверхности. Полученное теоретическое уравнение скорости позволяет рассчитать из экспериментальных данных различные константы скорости. Было заключено, что одии из промежуточных продуктов, поплчамых из двух аминов, должен быть идентичным.