A. Monzón

Catalytic decomposition of methane over Ni-Al2O3 coprecipitated catalysts: Reaction and regeneration studies

Abstract The catalytic decomposition of methane over nickel catalysts is a potential alternative route to steam reforming or partial oxidation for the production of hydrogen from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of a Ni(30%)/Al2O3 catalyst during the catalytic decomposition of methane. The influence of the operating and reduction temperatures and feed composition on the methane conversion, hydrogen production and coking rate has been studied. The effects of the regeneration cycles with oxygen on activity and carbon formation are also investigated. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. An increase in the reaction temperature increases both the deactivation rate and the growth rate of filaments. However, at high reduction temperatures, there is a decrease in the number of filaments formed due to sintering of the Ni particles. A kinetic model has been developed for the prediction of H2 production and of carbon, taking into account both stages of carbon formation, nucleation and filament growth.

Improved explicit equations for estimation of the friction factor in rough and smooth pipes

Abstract The most common correlations for calculating the friction factor in rough and smooth pipes are reviewed in this paper. From these correlations, a series of more general equations has been developed making possible a very accurate estimation of the friction factor without carrying out iterative calculus. The calculation of the parameters of the new equations has been done through non-linear multivariable regression. The better predictions are achieved with those equations obtained from two or three internal iterations of the Colebrook–White equation. Of these, the best results are obtained with the following equation: 1 f =−2.0 log e/D 3.7065 − 5.0272 Re log e/D 3.827 − 4.567 Re log e/D 7.7918 0.9924 + 5.3326 208.815+Re 0.9345 .

Dehydrogenation of isopropylic alcohol on a Cu/SiO2 catalyst: a study of the activity evolution and reactivation of the catalyst

The activation-deactivation behaviour of a Cu/SiO2 catalyst during isopropyl alcohol (IPA) dehydrogenation to acetone has been studied. Simultaneous reduction and sintering processes are responsible for the activation-deactivation behaviour observed. The reducing H2 atmosphere impulses the transformation of CuO into Cu0 and at the same time the sintering of the Cu0 phase formed. The reoxidation treatment in air at 573–673 K is able to regenerate a finely dispersed CuO phase from the sintered Cu0 phase. As a result, the active phase is redispersed and the dehydrogenation activity is recovered. The results indicate the existence of two CuO phases on the catalyst, with different properties. The CuO phase with a low reduction temperature, a small particle size and a poor crystallinity, is the one responsible for the appearance of the finely dispersed Cu0, which is the active phase for this reaction.

Acetylene hydrogenation over Ni-Si-Al mixed oxides prepared by sol-gel technique

Abstract The effect of the support (here silica and silica-alumina) and of the composition of Ni-based hydrogenation catalysts, elaborated by sol–gel method, on their physico-chemical characteristics and on their activity and selectivity (towards ethylene) for acetylene hydrogenation has been studied with the help of temperature programmed reduction (TPR) and oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and reaction tests (acetylene to ethylene hydrogenation). These techniques have shown the existence of two different types of sites. Firstly, the hydrogenolytic (naked) metallic centres, corresponding to nickel without (or little) interaction with the support, are responsible for a large part of the coke and are active for side-reactions. Secondly, the hydrogenating sites correspond to nickel in interaction with the support and are active for the main reaction. Three types of coke have been detected: filaments, amorphous coke, and partially hydrogenated carbons strongly adsorbed on the surface. The latter are facilitated by the acidity of the silica-alumina. Because the metal–support interactions related to a better dispersion of reduced nickel, silica-alumina-based catalysts produce a much smaller amount of coke than silica-based catalysts.

Acetylene hydrogenation on Ni–Al–Cr oxide catalysts: the role of added Zn

Abstract Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis–UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni≈4 (molar ratio).

Use of hydrotalcites as catalytic precursors of multimetallic mixed oxides. Application in the hydrogenation of acetylene

Abstract The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO·ZnO·Al2O3 catalysts (modified with Fe3+ or Cr3+) during acetylene hydrogenation has been studied. Coke formation is decreased significantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ratio for activity, selectivity and coke formation performance has been found. The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, if the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned optimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidines. A kinetic model of coke growth is proposed assuming the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively.

Sintering and redispersion of Pt/γ-Al2O3 catalysts: a kinetic model

Abstract The sintering and redispersion kinetics of a Pt/Al2O3 naphtha reforming catalyst have been studied. The effect of the operating conditions, temperature, and oxygen and HCl concentration on the sintering and redispersion rates have been investigated. It was found that the rate of sintering depends on temperature and oxygen partial pressure. The rate of redispersion depends both on oxygen and HCl concentrations. A new kinetic model to study the sintering and redispersion phenomena has been developed. The model considers the evolution of the metallic dispersion as a reversible process and includes the effect of the operating conditions on the dispersion variation rate.

Deactivation by coking and poisoning of spinel-type Ni catalysts

Abstract The effect of the addition of structure-modifying agents such as ZnO on the processes of coke formation and sulphur poisoning of NiO/A1 2 O 3 catalyst during ethyne hydrogenation has been studied. With or without the presence of modifiers, the addition of H 2 S to the feed gives rise to a strong increment in the rate of catalyst coking. However, pretreating the catalyst with H 2 S has a different effect when ZnO is added to the catalyst. In a NiO/A1 2 O 3 catalyst, sulphur preferentially deactivates metallic sites which are very active for coke formation. However, when ZnO is present in the catalyst structure, the addition of sulphur gives rise to a lower hydrogenation yield, and a greater rate of coke formation with respect to the fresh catalyst. Pretreating the catalyst with H 2 S also changes the coke morphology.

Texturising and structurising mechanisms of carbon nanofilaments during growth

The question of how the texture and structure of carbon nanofilaments (CNTs) are determined during growth is addressed via their preparation using the vapour phase method over Ni–Cu–Mg–Al catalysts. The CNTs formed and the related catalyst particles were investigated by high resolution transmission electron microscopy, electron diffraction, and X-ray energy dispersive spectroscopy. The nanofilament features were found to directly relate to the catalyst particle size and morphologies, which in turn depend on both the Ni/Cu ratio in the Ni–Cu alloy that forms the catalyst particles and the route by which they were prepared. The extent and orientation of graphenes within the carbon nanofilaments were found to be controlled by the extent of the related catalyst crystal faces and the angle value between the latter. It is proposed that energetics of graphenes, basically involving the ratio of the edge over the core carbon atoms, the energetic cost of heterocycles (pentagon), and that of the stress induced by the strain at graphene bending sites, determine whether the carbon nanofilaments would actually grow as nanotubes (i.e., hollow) with the dual “herringbone–bamboo” texture, or as nanofibres (i.e., not hollow) with either the “herringbone” texture or the platelet texture. Only the latter allowed the genuine graphite (3D periodicity) structure to develop, while the other nanofilament types could merely adopt the turbostratic structure. Meanwhile, it was demonstrated that the herringbone nanotubes and nanofibres here prepared are of “cup-stack” rather than “single helix” type.

Relationship between the kinetic parameters of different catalyst deactivation models

Abstract This paper presents a mathematical relationship between the parameters of Levenspiel’s Deactivation Kinetic Model (LDKM) and those of the Deactivation Models with Residual Activity (DMRA) and their evolution over time. This correlation provides an explanation for the erroneous variation obtained in the kinetic parameters (deactivation order and deactivation function) over time when LDKM is used to fit deactivation data having a certain level of residual activity, which frequently leads to systematic errors in estimating intrinsic parameters, such as activation energies. The variations of the LDKM parameters cannot in fact be related to a physical phenomenon, but are only a consequence of a mathematical artifact. The methodology developed in this work provides a valuable tool for the comparison and discrimination between different models used in kinetic studies. The equations here presented are applied to analyze the deactivation by fouling of Pt/Al2O3 reforming catalysts during methyl cyclohexane dehydrogenation.