Electrochemical study of modified non-functional bis-silane layers on Al alloy 2024-T3

In the last few years great efforts have been made in order to find and to develop environmentally friendly substitutes for Cr6+ pre-treatments applied on aluminium alloys used in the aircraft industry. Among the potential substitutes, silane layers have attracted considerable interest from researchers and from the industry. The present work investigates the anti-corrosion behaviour of (bis-1, 2-(triethoxysilyl) ethane (BTSE)) silane layers modified with Ce ions and/or silica nanoparticles applied on Al alloy 2024-T3 substrates. The corrosion behaviour was investigated in 0.1 M NaCl solution via d.c. polarization and electrochemical impedance spectroscopy (EIS). Contact angle measurements and XPS were used to assess information on the chemistry of the silane pre-treated surfaces. The results have shown that the introduction of additives improves the corrosion protection properties of the silane layer. (c) 2008 Elsevier Ltd. All rights reserved.

Ultra-thin films of alternating semi-interpenetrating layers of a conducting polymer with thermosetting phenolic resins for sensor application

In this work a new method for crosslinking ultra-thin films with potential applications in sensor systems is proposed. The films were produced by layer-by-layer (LbL) assembly using a conducting polymer, poly(o-ethoxyaniline) (POEA), alternated with a thermosetting resin, novolac-type phenolformaldehyde (PF), crosslinked by a simple thermal treatment. The PF resin served as both alternating and crosslinking agents. The films were characterized by Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopy, thermogravimetry (TG), desorption, doping/dedoping cycling and electrical measurements. The results showed that film architecture and crosslinking degree can be controlled by the conditions used for film deposition (number of bilayers, polymer concentration, pH, and deposition time), and crosslinking time. Moreover, this approach offers several advantages such as fast curing time and low cost, indicating that these films can be used to produce sensors with improved stability.

Evaluation of Characterization and Performance Modeling of Cementitiously Stabilized Layers in the Mechanistic-Empirical Pavement Design Guide

Cementitious stabilization of aggregates and soils is an effective technique to increase the stiffness of base and subbase layers. Furthermore, cementitious bases can improve the fatigue behavior of asphalt surface layers and subgrade rutting over the short and long term. However, it can lead to additional distresses such as shrinkage and fatigue in the stabilized layers. Extensive research has tested these materials experimentally and characterized them; however, very little of this research attempts to correlate the mechanical properties of the stabilized layers with their performance. The Mechanistic Empirical Pavement Design Guide (MEPDG) provides a promising theoretical framework for the modeling of pavements containing cementitiously stabilized materials (CSMs). However, significant improvements are needed to bring the modeling of semirigid pavements in MEPDG to the same level as that of flexible and rigid pavements. Furthermore, the MEPDG does not model CSMs in a manner similar to those for hot-mix asphalt or portland cement concrete materials. As a result, performance gains from stabilized layers are difficult to assess using the MEPDG. The current characterization of CSMs was evaluated and issues with CSM modeling and characterization in the MEPDG were discussed. Addressing these issues will help designers quantify the benefits of stabilization for pavement service life.

Catalyst Writing Syndicate Meetings Agenda 2007

Meetings take place 1-2 pm Wednesdays. The April/May convenor is Nicki Sochacka.

Fixed bed reactors with catalyst poisoning - first order kinetics.

The long performance of an isothermal fixed bed reactor undergoing catalyst poisoning is theoretically analyzed using the dispersion model. First order reaction with dth order deactivation is assumed and the model equations are solved by matched asymptotic expansions for large Peclet number. Simple closed-form solutions, uniformly valid in time, are obtained.

Substrate-inhibited kinetics with catalyst deactivation in an isothermal CSTR

Analytical expressions are developed for the time-dependent reactant concentration and catalyst activity in an isothermal CSTR with Langmuir-Hinshelwood kinetics of deactivation and reaction. Several parallel and series posioning mechanisms are considered for a reactor which, without poisoning, would operate at a unique steady state. The use of matched asymptotic expansions and abandonment of the usual initial-steady-state assumption give results, valid from startup to final loss of activity, whose accuracy can be improved systematically.

Substrate-inhibited kinetics with catalyst deactivation in an isothermal CSTR. II. Multiple pseudosteady states and reactor failure

Analytical expressions are derived for the time and magnitude of failure of an isothermal CSTR with substrate-inhibited kinetics, caused by slow catalyst deactivation under three types of parallel and series mechanisms. Reactors operating at high space velocity are found to be most susceptible to early failure and poisoning by product is more dangerous than by reactant. The magnitude of the jump across steady states depends solely on the Langmuir-Hinshelwood kinetic parameters and a detailed analysis of reactor behavior during the jump itself is given.

Multicomponent reversible reactions inside a porous catalyst pellet

This work deals with a solution method to handle multicomponents reversible reactions occurring inside a porous catalyst pellet. The complexity of this problem arises from the fact that the effective diffusivities and Biot number, which characterizes the external mass transfer, are different for each chemical species. In mathematical terms, this means that each chemical species has its own subspace and, therefore, when the technique of finite integral transform is applied to solve this multicomponent problem, each chemical species is associated with its own integral transform kernel. The analytical solutions obtained for this problem are compact and simple for any further manipulation. Application of this result to the catalytic reforming of C7 hydrocarbon system is shown in this paper.

The transient response of a CSTR containing porous catalyst pellets. Asymptotic solutions

Transient response of a CSTR containing porous catalyst pellets is analyzed theoretically using a matched asymptotic expansion technique. This singular perturbation technique leads directly to the conditions under which the minima of reservoir concentration occur. The existence of the minima may be used to estimate some inherent parameters of the catalyst pellet.

Hysteresis and phase transitions in a single partially internally wetted catalyst pellet: Thermogravimetric studies

The hydrogenation of cyclohexene over palladium supported in a microporous gamma-alumina pellet is studied thermogravimetrically with a view to measuring the extent of partial internal wetting associated with the different steady state branches. As many as three steady state branches having significantly different degrees of internal wetting and reaction rates, with transitions between them, are confirmed from observations of catalyst weight change. It is seen that with reduction in catalyst activity the middle branch, obtained by condensation from a vapor filled pellet, is much more prominent without showing an evaporative transition for the range of hydrogen partial pressures used here. The catalyst activity is therefore an important parameter affecting the structure of the steady state branches. Hysteresis effects are found to occur, and the thermogravimetric results also confirm the importance of history in determining the catalyst state. The measured degree of wetting is in accordance with that estimated from a mathematical model incorporating capillary condensation effects in addition to reaction-diffusion phenomena. The same model also satisfactorily interprets the reaction rate variations and transitions seen in the present work.

Effects of acidic treatments on the pore and surface properties of Ni catalyst supported on activated carbon

Activated carbon as catalyst support was treated with HCl, HNO3, and HF and the effects of acid treatments on the properties of the activated carbon support were studied by N-2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). Ni catalysts supported on untreated and treated activated carbons were prepared, characterized and tested for the reforming reaction of methane with carbon dioxide. It is found that acid treatment significantly changed the surface chemical properties and pore structure of the activated carbon. The surface area and pore volume of the carbon supports are generally enhanced upon acid treatment due to the removal of impurities present in the carbon. The adsorption capacity of Ni2+ on the carbon supports is also increased, and the increase can be closely correlated with the surface acidity. The impregnation of nickel salts decreases the surface area and pore volume of carbon supports both in micropores and mesopores. Acid treatment results in a more homogeneous distribution of the nickel salt in carbon. When the impregnated carbons are heated in inert atmosphere, there exists a redox reaction between nickel oxide and the carbon. Catalytic activity tests for methane reforming with carbon dioxide show that the activity of nickel catalysts based on the acid-treated carbon supports is closely related with the surface characteristics of catalysts. (C) 1998 Elsevier Science Ltd. All rights reserved.

Catalytic conversion to N2O to N2 over potassium catalyst supported on activated carbon

Catalytic conversion of N2O to N-2 With potassium catalysts supported on activated carbon (K/AC) was investigated. Potassium proves to be much more active and stable than either copper or cobalt because potassium possesses strong abilities both for N2O chemisorption and oxygen transfer. Potassium redispersion is found to play a critical role in influencing the catalyst stability. A detailed study of the reaction mechanism was conducted based upon three different catalyst loadings. It was found that during temperature-programmed reaction (TPR), the negative oxygen balance at low temperatures (< 50 degrees C) is due to the oxidation of the external surface of potassium oxide particles, while the bulk oxidation accounts for the oxygen accumulation at higher temperatures (below ca. 270 degrees C). N2O is beneficial for the removal of carbon-oxygen complexes because of the formation of CO2 instead of CO and because of its role in making the chemisorption of produced CO2 on potassium oxide particles less stable. A conceptual three-zone model was proposed to clarify the reaction mechanism over K/AC catalysts. CO2 chemisorption at 250 degrees C proves to be an effective measurement of potassium dispersion. (C) 1999 Academic Press.

The role of carbon surface chemistry in N2O conversion to N2 over Ni catalyst supported on activated carbon

The effect of acidic treatments on N2O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N-2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N2O-carbon reaction catalyzed by Ni catalyst. HNO3 treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N2O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

A dual catalyst bed system for the elimination of hydrogen from CO2 feed gas in urea synthesis

A dual catalyst bed system (Au/Fe2O3 + Pt-Pd/Al2O3) for eliminating hydrogen from the CO2 feed gas in urea synthesis is found to be far superior to commercially available and patented catalysts in catalytic activity. At relatively low temperatures, hydrogen is eliminated and coexistent CO is also oxidized completely to useful CO2. This can avoid effectively the accidental explosion of hydrogen-oxygen-ammonia mixed gases, thus ensuring the safety of urea synthesis.