Structure, mécanisme de formation et stabilité de la couche d'adsorption du soufre sur le nickel
TL;DR: In this paper, a study has been carried out on the (111, (100) and (110) face using LEED combined with a radioactive tracer technique S 35.
About: This article is published in Surface Science.The article was published on 1970-03-01. It has received 224 citations till now. The article focuses on the topics: Chemisorption & Adsorption.
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TL;DR: The literature treating mechanisms of catalyst deactivation is reviewed in this paper, which can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor solid and/or solid solid reactions, and (vi) attrition/crushing.
Abstract: The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts. © 2001 Elsevier Science B.V. All rights reserved.
2,526 citations
TL;DR: In this article, a review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing).
Abstract: Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism.
1,173 citations
TL;DR: Based on density functional theory calculations, kinetic measurements, microkinetic and Monte Carlo simulations, thermogravimetric analysis (TGA) experiments, extended X-ray absorption spectroscopy (EXAFS) measurements, and experimental results from the literature, this paper presented a detailed and comprehensive mechanistic picture of the steam reforming process on a Ni catalyst.
1,002 citations
TL;DR: In this article, the authors provide a coherent description of the catalysis of the steam reforming reactions, focusing on the role of the catalyst and problems related to carbon formation, and provide a summary of the empirical evidence of the reforming reactions.
Abstract: Steam reforming reactions will play a key role in new applications of synthesis gas and in a future hydrogen economy. The aim of this review is to provide a coherent description of the catalysis of the reforming reactions. The review is not comprehensive. The first section deals with the applications of synthesis gas and hydrogen and the various synthesis gas technologies. The optimum choice of technology depends on the requirements of the gas composition and the scale of operation. Two examples are included for illustration: synthesis gas for gas-to-liquid plants and hydrogen for fuel cells. The steam reforming process is described in the second section with emphasis on the role of the catalyst and problems related to carbon formation. The third section is a summary of the empirical evidence of the catalysis of the reforming reactions. The methods of characterization are discussed, and data representing sintering, activity trends, and promotion are summarized. The fourth section is a description of the mechanism based on a combination of empirical knowledge with recent data from studies of well-defined surfaces, in-situ high-resolution electron microscopy, and calculations based on density functional theory. The central concept is the role of surface defects as the source of reactivity and the nucleation centers for whisker carbon formation.
711 citations
01 Jan 1984
TL;DR: The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane as discussed by the authors, which is used also for the well-knowm process for improvement of the octane number of gasoline.
Abstract: The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane
$$C_n H_m + nH_2 O \to nCO + \left( {n + \frac{m}{2}} \right)H_2 \left( { - \Delta H_{298}^0 < 0} \right) $$
(1)
$$ CO + H_2 O \rightleftarrows CO_2 + H_2 \left( { - \Delta H_{298}^0 = 41.2kJ\,mol^{ - 1} } \right) $$
(2)
$$ CO + 3H_2 \rightleftarrows CH_4 + H_2 O\left( { - \Delta H_{298}^0 = 206.2kJ\,mol^{ - 1} } \right) $$
(3)
The expression reforming is misleading since it is used also for the well-knowm process for improvement of the octane number of gasoline [1]. In the gas industry, reforming has generally been used for “the changing by heat treatment of a hydrocarbon with high heating value into a gaseous mixture of lower heating value” [2].
655 citations
References
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TL;DR: A set of empirical atomic radii has been set up, such that the sum of the radii of two atoms forming a bond in a crystal or molecule gives an approximate value of the internuclear distance.
Abstract: A set of empirical atomic radii has been set up, such that the sum of the radii of two atoms forming a bond in a crystal or molecule gives an approximate value of the internuclear distance. These radii give fair agreement with experiment in over 1200 cases of bonds in all types of crystals and molecules, with an average deviation of about 0.12 A. The radii are similar to a set suggested by W. L. Bragg in 1920, but refined by consideration of many more crystals. They hold for covalent, metallic, and ionic binding equally well. These radii agree remarkably well with calculated radii of maximum radial charge density in the outermost shells of the atoms, as taken from previously unpublished calculations of D. Liberman, J. T. Waber, and D. T. Cromer, of the Los Alamos Laboratory, by relativistic self‐consistent field calculation, using the exchange correction suggested in 1951 by the present author. There is discussion of the probable reason for this agreement, and discussion of the relation of these radii to the ionic radii of Pauling, Zachariasen, and others, and the tetrahedral and metallic radii of Pauling.
1,864 citations
TL;DR: In this article, the authors discuss the applicability of the seventeen two-dimensional space groups and the eighty diperiodic groups in three dimensions to the description of a surface structure relative to a triperiodic substrate.
Abstract: The recent increase of activity in the field of slow‐electron diffraction has resulted in a need for agreement on conventions of terminology in the description of a diperiodic surface structure relative to a triperiodic substrate. Vector description of the surface structure net in terms of the substrate surface net is suggested. Fractional Miller indices result when the surface structure mesh is larger than the substrate surface mesh. The surface structure may be either that of a deposit or that of the selvedge of the substrate.With the relaxation of one of the diffraction conditions in the diperiodic structure the reciprocal lattice becomes a set of parallel rods intersecting the Ewald sphere continuously with change of wavelength of the incident radiation or change of orientation of the crystal.The paper discusses the applicability of the seventeen two‐dimensional space groups and the eighty diperiodic groups in three dimensions to these diperiodic structures. A comparison is made between the different ...
329 citations
TL;DR: In this paper, the authors used low-energy electron diffraction to define the structure and the growth mechanism of the adsorbed layers of two-dimensional compounds, based on the mean distance between atoms.
177 citations
TL;DR: The complex low energy electron diffraction pattern shown in Fig. 1 was observed during an investigation of the reactions of Ni(111) with various carbon or sulphur-containing gases, including ethylene, carbon monoxide, benzene and hydrogen sulphide as discussed by the authors.
Abstract: THE complex low energy electron diffraction pattern shown in Fig. 1, previously described tentatively as Ni(111) (16√(3)×16√(3)) R 30°—C (ref. 1), has been observed during an investigation of the reactions of Ni(111) with various carbon or sulphur-containing gases, including ethylene, carbon monoxide1, benzene and hydrogen sulphide. The pattern is produced at a variety of surface temperatures, depending on the reactant, from 20° to 350° C and in exposure times from 10 to 1,000 Langmuirs (L=10−6 torr s). The patterns are not identical in every case but are consistent geometrically and in the grouping of the diffraction beams into two distinct levels of intensity. The strong features predominate at 70 V and above; the weaker spots are more clearly seen at lower voltages (Fig. 2). The differences between the patterns obtained with different reactants are in the relative intensities of the diffraction spots within each category. Both Fig. 1 and Fig. 2 are the products of reaction with ethylene. The action of H2S produced a series of patterns, one of which was the complex pattern, but diffuse and complicated by the presence of traces of the succeeding patterns. The same strong characteristic features were undoubtedly present, however.
76 citations
TL;DR: The isotherms of reversible chemisorption of sulphur on the (111, (100) and (110) faces of silver have been measured at different temperatures in hydrogen-hydrogen sulphide mixtures as discussed by the authors.
67 citations