B. van der Linden

Inhibiting and deactivating effects of water on the selective catalytic reduction of Nitric Oxide with ammonia over MnOx/Al2O3

The effect of water on the selective catalytic reduction (SCR) of nitric oxide with ammonia over alumina supported with 2–15 wt.-% manganese oxide was investigated in the temperature range 385–600 K, with the emphasis on the low side of this temperature window. Studies on the effect of 1–5 vol.-% water vapour on the SCR reaction rate and selectivity were combined with TPD experiments to reveal the influence of water on the adsorption of the single SCR reactants. It turned out that the activity decrease due to water addition can be divided into a reversible inhibition and an irreversible deactivation. Inhibition is caused by molecular adsorption of water. TPD studies showed that water can adsorb competitively with both ammonia and nitric oxide. Additional kinetic experiments revealed that adsorbed ammonia is present in excess on the catalyst surface, even in the presence of water. Reduced nitric oxide adsorption is responsible for the observed reversible decrease in the reaction rate; the fractional reaction order changes from 0.79 in the absence of water to 1.07 in its presence. Deactivation is probably due to the dissociative adsorption of water, resulting in the formation of additional surface hydroxyls. As the amount of surface hydroxyls formed is limited to a saturation level, the deactivating effect on the catalyst is limited too. The additional hydroxyls condense and desorb in the temperature range 525–775 K, resulting in a lower degree of deactivation at higher temperature. A high temperature treatment at 775 K results in a complete regeneration. The amount of surface hydroxyls formed per unit surface area decreases at increasing MnOx-loading. The selectivity to the production of nitrogen is enhanced significantly by the presence of gas phase water.

Equilibrium adsorption of linear and branched C6 alkaneson silicalite-1 studied by the tapered element oscillating microbalance

The equilibrium adsorption of linear and branched C6 alkanes n-hexane, 2-methylpentane, 3-methylpentane and 2,3-dimethylbutane on silicalite-1 has been investigated using a novel technique—the tapered element oscillating microbalance (TEOM). For n-hexane, a small “kink ” in the isotherm is observed at about 4 molecule (unit cell of silicalite-1)−1. The measured isotherms of both 2-methylpentane and 3-methylpentane at 303 K for the first time show a second-step adsorption at loadings over 4 molecule (unit cell)−1. A two-step adsorption behavior is confirmed for single branched C6 alkanes. This observation is in good agreement with the picture of two distinct adsorption locations for single branched alkanes in silicalite-1 indicated by other techniques. The maximum loading for 2,3-dimethylbutane is about 4 molecule (unit cell)−1 under the conditions investigated and the molecules reside completely in the intersections. A dual-site Langmuir expression appropriately describes the equilibrium data for n-hexane, 2-methylpentane and 3-methylpentane, while the isotherms of 2,3-dimethylbutane can be described by the Langmuir model. The derived thermodynamic properties such as adsorption enthalpy and entropy agree with those available, determined by other techniques. The observed two-step adsorption behavior for single branched C6 alkanes on silicalite-1 is attributed to the large difference in the adsorption entropy between the molecular locations in the channel intersections and in the zigzag channels.

MultiTRACK and operando Raman-GC study of oxidative dehydrogenation of propane over alumina-supported vanadium oxide catalysts

Combined operando Raman-GC and MultiTRACK studies provide new insights into the interaction of propane with a V/alumina catalyst. The Raman-GC analysis showed that the catalyst is essentially in the oxidized state during oxidative dehydrogenation reaction conditions, while stable intermediates and/or carbonaceous deposits are not observed on the catalyst surface. In the absence of oxygen, the catalyst is reduced by propane, and two types of carbonaceous deposits can be observed: one with a more aliphatic character at low temperatures, and one with a graphitic character at higher temperatures, of which the particle size increases as a function of increasing temperature. In agreement with the Raman studies, evidence for carbonaceous deposits was also provided by the MultiTRACK experiments. From CO2 response profiles of the oxidation of these deposits, it was concluded that increasing temperature of operation and increasing propane/oxygen ratio enhance the amount and stability of the surface carbonaceous species formed. Based on the MultiTRACK studies also the participation of two types of oxygen species in the reaction of propane was evident: a highly reactive super-surface oxygen mainly yielding CO2, and a catalytic oxygen, associated with the vanadia phase. In MultiTRACK conditions, the extent of participation of vanadia-associated oxygen increases with reaction temperature and/or the propane/oxygen ratio, enhancing the selectivity of the reaction to propene.

The interaction of H2O, CO2, H2 and CO with the alkali-carbonate/carbon system : a thermogravimetric study

Abstract The mass changes of an alkali-carbonate/carbon sample were studied in various gas mixtures during temperature programmed gravimetric analysis (TPGA), isothermal adsorption and desorption and temperature programmed reduction and desorption experiments at (sub)gasification temperatures. Both CO 2 and CO show a strong interaction with the alkali/carbon system, resulting in reversible mass changes, which are ascribed to changes in the catalytically active alkali species present on the carbon surface. The extent of reversible mass change is strongly dependent on temperature, gas phase composition and pretreatment of the catalyst/carbon sample. In the presence of H 2 O or CO 2 addition or removal of H 2 shows no significant effect on the sample mass, whereas in the absence of an oxidizing agent H 2 acts as a strong reducing agent. As is known, H 2 O is capable of oxidizing or gasifying the catalyst/carbon sample, but no H 2 O chemisorption is observed. The alkali-catalysed oxygen exchange reactions in H 2 O-, CO 2 -, H 2 - and CO-containing gas mixtures e.g. the water gas shift reaction, can be described by a three step model in which empty ( ∗ ), oxidized (O- ∗ ) and chemisorbed CO 2 (CO 2 - ∗ ) intermediates are involved. The H 2 O/H 2 oxygen exchange proceeds through (O- ∗ ) and ( ∗ ) intermediates, whereas the CO 2 /CO oxygen exchange proceeds through the CO 2 - ∗ intermediate. The inhibiting role of CO 2 on all oxygen exchange rates can be explained by the presence of CO 2 - ∗ sites. The model proposed provides a basis for the kinetic modelling of the steam gasification process, taking into account changes in catalytic activity in various gas mixtures.

Structure-activity relation and ethane formation in the hydrogenolysis of methyl acetate on silica supported copper catalysts.

Abstract The activity and selectivity of silica-supported copper catalysts in the hydrogenolysis of methyl acetate were studied in relation to the structure of the active phase. The activity was found independent of the copper loading, when the weight of copper in the reactor was kept constant. Copper catalysts were prepared by homogeneous precipitation. In the range of reduction temperatures between 543 K and 743 K, the activity was constant, in spite of considerably different copper metal surface areas. The absence of a correlation between activity and copper surface area is assigned to the formation of highly dispersed copper not active in methyl acetate hydrogenolysis. In contrast to the equal activity, catalysts of low copper loading produced more ethane than the higher loaded catalysts, based on the weight of copper. A remarkable parallel between the amount of ammonia chemisorbed on the reduced catalyst and the ethane selectivity suggests that a dehydration reaction to ethene on acid sites located at the silica support, followed by hydrogenation on the copper metal phase, is responsible for the formation of ethane.

Occurrence of dammar-13(17)-enes in sediments: Indications for a yet unrecognized microbial constituent?

A pair of isomeric dammarenes has been identified in numerous Pleistocene to Jurassic sediment samples, most of them from continental margins. Their structures have been determined to be (20R)- and (20S)-dammar-13(17)-ene based on comparison of mass spectra and Chromatographic data with those of synthetic standards. Their saturated counterparts have also been found in a few samples. Based on rigorous nuclear magnetic resonance (NMR) studies with one synthetic standard and molecular mechanics calculations, it is suggested that geologically occurring dammaranes possess 13β,17α(H) stereochemistry. Compounds with a dammarane skeleton are well known natural products of several families of land plants, but the widespread occurrence of dammarenes in marine sediments suggests an alternative origin, supported by their carbon isotopic signature. Dammarenes can be envisaged as direct proton-induced cyclisation products from squalene and may represent a contribution from microorganisms. Although we are in favour of this hypothesis, an origin from geologically induced rearrangements of other compounds cannot, at present, be excluded.

Forced Concentration Oscillations of CO and O2 in CO oxidation over Cu/Al2O3

Abstract The kinetics of CO oxidation over an alumina supported Cu catalyst are examined using successive oxidation and reduction cycles. Experiments were done at a temperature of 493 K with isotopically labelled gases in a tubular reactor. Surface species were monitored during transients in an FTIR flow cell. For the reoxidation of the catalyst a three-step mechanistic model is proposed. The kinetic constants are determined by mathematical modelling. The role of carbonates is found to be minor in the production of CO 2 in contrast to carbonyls which are shown to be the intermediates. Anet dissociation of the CO bond was observed during reduction caused by adsorption of CO 2 on a partly reduced catalyst under formation of carboxylates and its subsequent decomposition to CO, thereby leaving oxygen on the catalyst. A complete mechanistic scheme is presented which allows us to describe qualitatively and in part quantitatively the experimental results. This study shows that the use of forced oscillations is a powerful tool with strong elucidating abilities in mechanistic investigations in heterogeneous catalysis.

High temperature gasification of coal under severely product inhibited conditions: the potential of catalysis

Abstract The CO2-gasification reactivity of three widely differing coals was examined under severely product inhibited conditions. To enhance the reactivity K2CO3 was added as a catalyst. Two types of experiments were performed in a thermobalance: temperature programmed experiments to characterize the gasification behaviour of the different samples relative to each other; and isothermal experiments to determine the overall reactivity of the samples. Relatively low temperature data were extrapolated to predict high temperature reactivity. Extrapolation was carried out according to the Arrhenius equation. The validity of this simple model was experimentally verified for gasification in pure CO2 by using an entrained flow reactor system. The results showed that extrapolation of low temperature reactivity data over a 200–250 K range, according to the Arrhenius equation and using a realistic value for the apparent activation energy of the reaction examined, gives useful information on high temperature reactivity. Under product inhibited conditions complete gasification of coal on a time scale of seconds was found to be possible from 1350 K for K2CO3/lignite. Much higher temperatures were needed for the other coal samples studied, whether catalysed or uncatalysed. The consequences of the results in relation to a new type of iron oxide reduction process are briefly discussed.