C.A. Bernardo

Carbon deposition and methane steam reforming on silica-supported NiCu catalysts

Methane decomposition and steam reforming were studied over silica-supported copper-nickel alloy catalysts. Hydrogen chemisorption and X-ray photoelectron spectroscopy (XPS) measurements showed copper surface segregation after reduction at 773 K. The carbon formation/gasification equilibrium was the same for the alloy catalysts as for monometallic nickel catalysts except at the highest copper concentration (80 at.% Cu). The rate of carbon formation, for more than 10 at.% Cu, was more strongly decreased by alloying with Cu than the rate of steam reforming, which followed approximately the decrease in hydrogen capacity. On samples with less than 80 at.% Cu the carbon was deposited mainly as filaments with a metal particle at the end and with approximately the same diameter as the metal particle. At 80 at.% Cu thinner filaments of a new type were observed. A kinetic expression describing the rate of methane decomposition at low hydrogen pressure over the catalysts with 10 at.% Cu or lower indicates that a surface reaction step is rate controlling.

The reversibility of filamentous carbon growth and gasification

Controlled atmosphere electron microscopy observations of the nickel-catalyzed growth and gasification of carbon filaments have shown that these processes can be reversed. This supports the view that growth and gasification in either hydrogen or steam occur by similar mechanisms, where one of the steps involves the diffusion of carbon through the metal. It was observed that the small catalyst particles are the most active in filament formation and steam gasification, while the large particles are the most active for the hydrogenation reaction. This is explained in terms of different rate-controlling steps.

Reactivity of carbon deposited on nickel-copper alloy catalysts from the decomposition of methane

In order to evaluate the effect of the catalyst composition on the nature of the carbon deposits formed on it, and the reactivity of the latter towards hydrogen, a series of temperature-programmed reactions (TPR) was performed. Silica-supported Ni-Cu catalysts, prepared by dry impregnation and characterized in the same way as in a previous study were used. Carbon was deposited on them, from CH/sub 4/-H/sub 2/ mixtures in constant conditions (850 K, P/sub CH/sub 4// = 13 kPa, P/sub CH/sub 2// = 7 kPa, N/sub 2/ as balance), in a reaction system consisting of a C.I. Electronics MK2B microbalance and associated flow reactor. Gasification thermograms were obtained by heating the carbon deposit at constant heating rate under a flow of hydrogen, using a Mark III Stanton Redcroft Temperature Programmer. The derivative thermogravimetric curves that yield information on the reactivity of the deposits are presented in a figure for the different alloy compositions. Results are discussed. 13 references.

Coking and decoking during methanation and methane decomposition on Ni-Cu supported catalysts

The effect of the composition of silica supported Ni-Cu alloy catalysts on the process of coking and decoking during methane decomposition and during methanation was considered. The kinetics of methanation was studied and compared to those of carbon deposition and of strong adsorption of hydrogen. Initiation of the formation of filamentous carbon formation on mono-metallic surfaces may take place if the ratio of the partial pressures, pCO/pH2, is larger than 2 (T 673 K). Once the process starts, the chemical potential of the gas phase may be reduced to lower values without interruption of filament growth. Besides, it was concluded that the methanation reaction includes two steps: the dissociative adsorption of CO and the hydrogenation of the adsorbed species. It was possible to establish the mechanism through which Cu affects the activity of Ni. The effect of the composition of the alloy catalysts on the methane formation and on the simultaneous carbon deposition indicates that those reactions belong to group I and to group II, respectively, following Ponecs classification. It was possible to find the optimal Cu concentration that maximises methanation and minimises carbon deposition. The kinetics of methane decomposition was also considered and is well described by adapting a model developed by other authors for Fe catalysts. n n n nCokebildung und Entcoking wahrend der Methanbildung und des Methanzerfalls auf Ni-Cu-Tragerkatalysatoren n n n nDer Einflus der Zusammensetzung von Ni-Cu-Legierungen auf SiO2- Tragerkatalysator auf den Coking- und Entcokingprozes wahrend des Methanzerfalls und der Methanbildung wurde untersucht. Die Kinetik der Methanbildung wurde untersucht und verglichen mit der Kohlenstoffabscheidung und der Starke der Adsorption von Wasserstoff. Der Start der Bildung von fadenformigem Kohlenstoff auf einheitlichen Metalloberflachen kann stattfinden, wenn das Verhaltnis der Partialdrucke pCO/pH2 groser ist als 2 (T 673 K). Sobald der Prozes beginnt, kann das chemische Potential der Gasphase vermindert werden zu niedrigeren Werten, ohne das das Fadenwachstum abgebrochen wird. Weiterhin wurde geschlusfolgert, das die Methanbildungsreaktion zwei Schritte umfast: die dissoziative Adsorption von CO und die Hydrierung der adsorbierten Spezies. Es war moglich, den Mechanismus aufzustellen, durch den Cu die Aktivitat von Nickel beeinflust. Der Einflus der Zusammensetzung der Legierungskatalysatoren auf die Methanbildung und die gleichzeitige Kohlenstoffabscheidung zeigt an, das diese Reaktionen zu Gruppe I und zu Gruppe II gehoren gemas der Klassifizierung von Ponec. Es war moglich, die optimale Cu-Konzentration zu finden, die die maximale Methanbildung und minimale Kohlenstoffabscheidung bewirkt. Die Kinetik des Methanzerfalls wird auch betrachtet, diese wird gut beschrieben, in dem man ein Modell annimmt, das bereits von anderen Autoren fur Eisenkatalysatoren entwickelt wurde.

Modelling the economic and environmental performance of engineering products: a materials selection case study

PurposeLife cycle assessment (LCA) studies allow understanding all relevant processes and environmental impacts involved in the life cycle of products. However, in order to fully assess their sustainability, these studies should be complemented by economic (LCC) and societal analyses. In this context, the present work aims at assessing all costs (internal and external) and the environmental performance associated to the full life cycle of specific engineering products. These products are lighting columns for roadway illumination made with three different materials: a glass fibre reinforced polymer composite, steel and aluminium.MethodsThe LCA/LCC integrated methodology used was based in a “cradle-to-grave” assessment which considers the raw materials production, manufacture, on-site installation, use and maintenance, dismantlement and end-of-life (EoL) of the lighting columns. The fossil fuels environmental impact category was selected as the key environmental impact indicator to perform the integrated environmental and cost analysis.ResultsThe potential total costs obtained for the full life cycle of the lighting columns demonstrated that the one made in steel performs globally worse than those made in composite or aluminium. Although the three systems present very similar internal costs, the steel column has higher external costs in the use phase that contribute for its higher total cost. This column has very high costs associated to safety features, since it constitutes a significant risk to the life of individuals. The raw material and column production stages are the main contributors for the total internal life cycle costs. The EoL treatment is a revenue source in all systems because it generates energy (in the case of the composite incineration) or materials (in the case of metal recycling). The composite and aluminium lighting columns present similar “cradle-to-grave” life cycle total cost. However, until the dismantlement phase, the aluminium column presents the highest environmental impact, whereas in the EoL treatment phase this scenario is reversed. The “cradle-to-grave” life cycle potential total cost and the environmental impact (fossil fuels) indicator of the steel lighting column are higher than those of the other columns.ConclusionsEven though the uncertainties in the LCC are larger if external costs are included, their consideration when modelling the economic performance of engineering products increases the probability of developing a more sustainable solution from a societal perspective.

Gasification of Carbon Deposited on Metallic Catalysts

The catalytic gasification of carbon and coal is a subject of great industrial importance, with applications in the production of fuel gases, regeneration of coked catalysts and decoking of heat transfer surfaces and reactor walls. On a more theoretical basis, it is also relevant in the investigation of the mechanisms of important catalytic processes, like the methanation, steam-reforming and Fischer-Tropsch reactions. Thus, it is not surprising that a recent computer search produced 1567 references, for the period 1982–1984 only.

FILAMENTOUS CARBON FORMATION ON METALS AND ALLOYS

Carbonaceous materials are often deposited on catalysts, reactor surfaces and heat exchanger tubes in the course of hydrocarbon processing operations. Such deposits, commonly referred to as “coke”, contain a variety of carbons of different structures and origins which may be classified as pyrolytic or catalytic carbons, as well as condensed high molecular weight aromatic compounds, or tars [1].