The effect of Nb2O5 and ZrO2 additions on the behaviour of Li/MgO and Li/Na/MgO catalysts for the oxidative coupling of methane

Incorporation of Nb2O5 or ZrO2 into both Li/MgO and Li/Na/MgO systems produced ternary and quaternary catalysts, respectively, capable of attaining optimal C2 yields and selectivities at lower temperatures relative to the unpromoted materials. The degree of enhancement effected by these metal oxide additives was compared to that produced by Li/MgO and Li/Na/MgO catalysts promoted with SnO2 or Co3O4. At reaction temperatures < 700°C, the Li/Co/MgO ternary system showed marked differences in behaviour compared to the other ternary catalysts tested. This was particularly evident in the variation in C2 selectivity with time on stream during ageing studies of (i) untreated materials, (ii) materials pretreated in CO2, and (iii) materials dosed periodically with CHCI3

The oxidative coupling of methane and the oxidative dehydrogenation of ethane over a niobium promoted lithium doped magnesium oxide catalyst

The promoting effect of niobium in a Li/MgO catalyst for the oxidative coupling of methane (OCM) and for the oxidative dehydrogenation of ethane (ODHE) has been studied in some detail. It has been found that a Li/Nb/MgO catalyst with 16 wt % niobium showed the highest activity for the C2 production in the OCM reaction; the activity at 600 °C was ten times that of the Li/MgO catalyst at the same temperature. The Li/Nb/MgO catalyst was also slightly more active for the ODHE reaction than was the Li/MgO catalyst. However, the Li/Nb/MgO catalyst produced considerably more carbon dioxide in the both reactions. Structural investigation of the catalyst showed that the addition of niobium to the Li/MgO catalyst increased the surface area and gave an increase in the lithium content of the calcined catalysts. Two niobium phases, LiNbO3 and Li3NbO4, were formed; it is shown that the first of these probably causes the increased activity. Ageing experiments showed that the activity of the catalyst was lost if the catalyst was used above 720 °C, the melting point of the lithium carbonate phase. The catalyst showed a decrease of surface area after ageing and a sharp decrease of the amount of the two niobium phases. The addition of carbon dioxide to the feed could not prevent the deactivation of the Li/Nb/MgO catalyst

The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on ¿-Al2O3, TiO2 and ZrO2 are active catalysts for the CO2 reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/¿-A12O3 < Pt/TiO2 < Pt/ZrO2. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO2 and Pt/ZrO2, deactivation started immediately after the start of the reaction, while the Pt/¿-A12O3 catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity

The effect of addition of a third component on the behaviour of the lithium doped magnesium catalysts for the oxidative dehydrogenation of ethane

The oxidative dehydrogenation of ethane was studied with the use of promoted Li/MgO catalysts at temperatures of 600¿650°C. The addition of known promoters, cobalt and tin, gave a slight Increase In activity but a strong decrease in selectivity to ethylene under the conditions used. The addition of sodium improved the selectivity to ethylene and suppressed the formation of carbon monoxide. Using a feed of 12 vol% ethane and 6 vol% oxygen, the U/Na/MgO catalyst with 3.2wt% sodium showed a selectivity of 86 % to ethylene at 38 % conversion of ethane; the Li/MgO catalyst showed a selectivity of 80 % at similar conversions Thermal Investigations of the Li/Na/MgO catalyst showed that an eutectic melt of LINaCO3 is formed at 490°C; the existence of this molten phase is probably the cause of the Increased selectivity

Сущность и виды операций перестрахования

В статье проанализированы сущность и понятие перестрахования, рассмотрены участники перестраховочных операций и их функции. Подробно описана страховая терминология, которая используется в перестраховании. Выделены преимущества перестраховочных отношений при развитии страхового рынка, их роль при осуществлении страховых операций. Рассмотрены особенности типов перестраховочных договоров и существующих форм перестрахования, их положительные и отрицательные стороны для страховщиков и перестраховщиков, сфера использования.У статті проаналізовані сутність та поняття перестрахування, розглянуті учасники перестрахувальних операцій та їх функції. Докладно описана страхова термінологія, що використовується в перестрахуванні. Виділені переваги перестрахувальних відносин при розвитку страхового ринку, їх роль при здійсненні страхових операцій. Розглянуті особливості типів перестрахувальних договорів та існуючих форм перестрахування, їх позитивні і негативні сторони для страховиків і перестрахувальників, сфера використання.Essence and conception of reassurance, members of reassurance operations had been analyzed and examined in the article. Insurance terminology, which used in reassurance, had been described in details. The author of the article had picked out the advantages of reassurance operations and their role in it under the development of the insurance market. Properties of types of reassurance contracts and relevant forms of reassurance, their positive and negative sides for insurers and reinsures, the field of their using had been described in this article

The Exotic World of Carolus Clusius 1526-1609.Catalogue of an exhibition on the quatercentenary of Clusius' death, 4 April 2009. With an introductory essay by Florike Egmond

bookUB publicatie

Io. Grolieri & Amicorum. A Stroll through Leiden's Book History

UB publicatie

The role of potassium as a promoter in iron catalysts for ammonia synthesis

Five ammonia synthesis catalysts, mainly differing in potassium content, were prepared from a commercial doubly promoted iron catalyst. The activities of these catalysts were measured at 350–450 °C and 5–200 atm. The experimental reaction rates were fitted to the modified Temkin rate equation. Increasing the potassium content from 0.1 to 3.8 wt% results in increasing the order in H2 from 0.7 to 1.5. The change from singly to doubly promoted behavior is gradual. The nature of catalysts with a relatively high K-content is changed, preventing the formation of NH (or NH2) groups so that N-atoms become the main species on the surface, which explains the higher order in H2

Oxidative coupling of methane over doped Li/MgO catalysts

A series of zirconia doped Li/MgO catalysts with a fixed amount of zirconia and varying concentrations of lithium was used for the oxidative coupling of methane. It was found that an increase in lithium concentration resulted in a decrease in initial activity, while the selectivity was not affected. The life-time of Zr doped Li/MgO catalysts with a fixed concentration of ZrO2 is a function of the lithium concentration. Previous results have shown that Li2Mg3ZrO6 is active and selective but it is now shown to be instable under reaction conditions