Catalytic combustion of hydrocarbons over perovskites

The advantages and disadvantages of the catalysts so far employed or proposed for the low temperature catalytic combustion of hydrocarbons, in both static and mobile energy production devices, are discussed. Furthermore, a La0.9Ce0.1CoO3\ub1\u3b4 perovskite has been prepared by a recently proposed new flame-hydrolysis (FH) method. This proved a high surface area, thermally highly resistant catalyst. The partial substitution of Ce for La in such a cobaltite led to a relatively low suprafacial activity, but to a high bulk oxygen mobility, leading to high intrafacial activity for the catalytic flameless combustion of methane. The best operating conditions have been also found, for supporting the so prepared active phase, by dip-coating of a cordieritic honeycomb support, after deposition of an alumina primer. A very active and durable catalyst was so obtained, useful for practical application in the environmentally friendly low temperature combustion of methane

Effect of Ru loading and of Ru precursor in Ru/C catalysts for ammonia synthesis

One of the key points in the study of Ru/C catalysts for ammonia synthesis is the structure sensitivity of the reaction, which usually determines the proper metal loading, so governing the cost of the final catalyst. In the present work, the effect of Ru loading on catalytic activity and thermal stability (i.e. catalyst resistance against support methanation and metal sintering) was investigated. The optimal Ru loading was found to be around 3.5 wt.%, a value considerably lower with respect to that commonly found in literature. Furthermore, top activity was obtained with 10-15% Ru dispersion, higher dispersion values leading to less performing catalysts. Finally, the effect of some different water-soluble Ru precursors on catalyst behaviour was also investigated

LIMIT THEOREMS FOR HOROCYCLE FLOWS

International audienceThe main results of this paper are limit theorems for horocycle flows on compact surfaces of constant negative curvature. One of the main objects of the paper is a special family of horocycle-invariant finitely additive Hölder measures on rectifiable arcs. An asymptotic formula for ergodic integrals for horocycle flows is obtained in terms of the finitely-additive measures, and limit theorems follow as a corollary of the asymptotic formula. The objects and results of this paper are similar to those in [15], [16], [4] and [5] for translation flows on flat surfaces. The arguments are based on the classification of invariant distributions for horocycle flows established in [12]

Characterisation of Ru/C catalysts for ammonia synthesis by oxygen chemisorption

A standard chemisorption procedure has been set up for the determination of Ru dispersion in Ru/C catalysts. Pulse chemisorption of oxygen was carried out at 0 \ub0C, after having proved that no corrosive chemisorption phenomena are present. An average chemisorption stoichiometry was experimentally determined through measurements on Ru black. The procedure was applied to the investigation of promoted and unpromoted Ru/C catalysts for ammonia synthesis, supported on two different carbon supports. The main factor influencing Ru dispersion showed to be Ru loading, while the addition of even large amounts of Ba\u2013Cs\u2013K promoters has practically no influence. It is also briefly discussed how such results can help in elucidating several aspects of the behaviour of Ru/C as catalyst for ammonia synthesis

Promoters state and catalyst activation during ammonia synthesis over Ru/C

Carbon-supported, promoted Ru-based catalysts for ammonia synthesis proved to be interesting substitutes for the traditional Fe-based ones. A debate recently arose on the active state of promoters, mainly Cs and Ba, and on the effect of the latter on Ru active sites. In the present work a set of Ba-, Cs- and K-promoted samples has been characterised by various techniques. Higher H2 and O2 uptakes have been observed during reduction and chemisorption, respectively, on Cs- and K-promoted samples supported on graphitised carbon. No evidence of this has been observed with samples supported on active carbon. This is in line with the hypothesis of alkaline promoters partial reduction under the ammonia synthesis conditions, favoured by the formation of graphite intercalation compounds. Furthermore, some suggestions are here introduced on the beneficial role of Ba, especially in increasing the support resistance to methanation. Finally, the efficacy of catalyst activation was found to depend on the nature of Ru precursor. Indeed, a prolonged activation at relatively high temperature is usually needed with chloride precursors, to remove the counterion, a poison for the catalyst, whereas less dramatic conditions are required for different precursors, such as nitrosylnitrate

Graphitised carbon as support for Ru/C ammonia synthesis catalyst

In the present work, we compared the catalytic activity and mainly the stability under the usual ammonia synthesis conditions, of some carbon supports, differing as for their nature, purity and temperature of pretreatment. The effect of catalyst composition (metal and promoters loading) on stability was also investigated. XRD and N-2 adsorption/desorption analysis helped in elucidating the effect of carbon treatment. It was found that only after the support has been heated at least at 1900 degrees C the stability of the tri-promoted catalyst becomes fully satisfactory and virtually independent of Ru loading. For practical purposes such high temperature treatment must be compatible with the preservation of a sufficiently high surface area

Study of the deactivation of a commercial catalyst for ethylbenzene dehydrogenation to styrene

The catalytic performance and characteristics of an industrial catalyst, fresh and spent, i.e. downloaded from an industrial reactor after a normal lifetime cycle, were compared. Some different causes of catalyst deactivation, such as loss or redistribution of promoters, active phase modification, Fe 3+ reduction, coke deposition on catalyst surface and physical modifications, were evidenced by means of several techniques. All the mentioned causes of deactivation showed strictly interconnected and concurred to the decrease of conversion. However, the key factors leading to irreversible deactivation showed potassium migration towards the inner part of the extrudate particle, its agglomeration into concentrated spots and its volatilisation from the surface, together with the progressive Fe3+ reduction to Fe 2+. These modifications led to the enhancement of coking activity and to the loss of mechanical properties, so making deactivation irreversible

Solvent nature effect in preparation of perovskites by flame pyrolysis: 2 : Alcohols and alcohols plus propionic acid mixtures

The effect of either pure alcohols or alcohols + propionic acid mixtures as solvents for the preparation by flame pyrolysis of a standard LaCoO3 catalyst, to be employed for the catalytic flameless combustion of methane, has been investigated. All the catalysts proved very active for the mentioned reaction. Low-MW pure alcohols showed however less suitable than alcohols-propionic acid mixtures, leading to lower perovskite phase purity, less particle size homogeneity and lower specific surface area. The high volatility of the solvent seems to be the major cause, together with the improper behaviour of nitrates (forced by solubility reasons) as perovskite metals precursors. However, the addition of propionic acid to the alcohols allowed to use the acetates as precursors and hence to obtain high perovskitic phase purity, high SSA and Uniform particle size. Moreover, the increase of combustion enthalpy of the solvent, through the addition of higher-MW alcohols, leading to progressively higher flame temperature, strongly improved the thermal resistance of the catalyst, without lowering catalytic performance

Solvent nature effect in preparation of perovskites by flame pyrolysis: 1: carboxylic acids

The effect of a series of carboxylic acids (C(2)-C(8)), as solvents for the preparation by flame spray pyrolysis of LaCoO(3) catalyst for the flameless combustion of methane, has been investigated. Acetic acid showed to be unsatisfactory from several points of view: low phase purity of the catalyst, higher amount of unburnt carbonaceous residua, lower catalytic activity and low thermal stability. By increasing the carbon chain length of the solvent, the consequent increase of flame temperature led to an increase of crystal phase purity and of particle size and to a decrease of specific surface area of the catalyst. Catalytic activity showed only marginally affected by the last parameter, phase purity seeming more important. Thermal resistance showed directly related to flame temperature, i.e. to the combustion enthalpy of the solvent, but a relatively high amount of residual organic matter can negatively affect this property

Square-tiled cyclic covers

A cyclic cover of the complex projective line branched at four appropriate points has a natural structure of a square-tiled surface. We describe the combinatorics of such a square-tiled surface, the geometry of the corresponding Teichm\"uller curve, and compute the Lyapunov exponents of the determinant bundle over the Teichm\"uller curve with respect to the geodesic flow. This paper includes a new example (announced by G. Forni and C. Matheus in \cite{Forni:Matheus}) of a Teichm\"uller curve of a square-tiled cyclic cover in a stratum of Abelian differentials in genus four with a maximally degenerate Kontsevich--Zorich spectrum (the only known example found previously by Forni in genus three also corresponds to a square-tiled cyclic cover \cite{ForniSurvey}). We present several new examples of Teichm\"uller curves in strata of holomorphic and meromorphic quadratic differentials with maximally degenerate Kontsevich--Zorich spectrum. Presumably, these examples cover all possible Teichm\"uller curves with maximally degenerate spectrum. We prove that this is indeed the case within the class of square-tiled cyclic covers.Comment: 34 pages, 6 figures. Final version incorporating referees comments. In particular, a gap in the previous version was corrected. This file uses the journal's class file (jmd.cls), so that it is very similar to published versio