Modeling Negative Temperature Coefficient region in methane oxidation

Standard kinetic models are essential tools for predicting and interpreting the evolution of oxidation processes and obtain useful information for designing and dimensioning practical combustion facilities. Quite often a large part of the development work consists in the determination of the most suited chemical kinetics scheme to use in numerical simulations. This step is even more critical in the case of innovative technologies. In fact, in this case, models are required to work in extrapolative conditions, i.e. in range of parameters outside the ones for which they have been optimized. This is the case of prediction methane autoignition at atmospheric pressure, in diluted conditions, corresponding to MILD combustion conditions, where no experimental data are available. The aim of the present work is to compare the efficacies in predicting the existence of Negative Temperature Coefficient (NTC) behavior of ignition time of methane at atmospheric pressure of several kinetic models available in the literature. Such phenomenology is extensively described in the literature for high molecular weight paraffin but few experimental evidences are reported about its occurrence in methane oxidation. Methane autoignition time in dependence of temperature, reaction pathways with rate of production, sensitivity and flow diagram analysis have been exploited in order to highlight the kinetic controlling steps of methane autoignition at different temperature ranges. It has been shown that the prevalence of either the oxidation or the recombination results in a speeding or a slowing down of the reactive process. In this reactive network, a key role is covered by the active oxidation pathway. At the same time, in dependence of working temperature, the branching routes of H₂/O₂ reaction mechanism supply a great part of radicals needed for ignition. Thus, numerical results presented in the paper clearly show that the Negative Temperature Coefficient region in the Arrhenius plot of methane ignition delay marks the shift from one principal reaction route to the others.     

Synthesis and characterization of nanosized lithium manganate and its derivatives

Spinel lithium manganese oxide, LiMn₂O₄ and its derivatives are prepared by the sol–gel method. The lattice constant of the pure material is calculated as 8.23 Å. Different transition metal cations of chromium, iron, cobalt, nickel, copper and zinc (0.05 and 0.15 M) are doped in place of manganese in the LiMn₂O₄. X-ray powder diffraction data show that the spinel framework preserved its integrity upon doping. Formation of a single phase and the purity of the samples are confirmed by X-ray powder diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The crystallite size of the samples is calculated by use of the Scherrer formula and is found to be within a range of 43–66 nm. The electrical conductivity of the samples is determined over a temperature range of 200–300 K by means of four-point probe method. An increasing trend of conductivity with increase in temperature is noted for all the samples. The parent compound LiMn₂O₄ has a conductivity value of 3.47 × 10⁻⁴ ohm⁻¹ cm⁻¹ at room temperature. This value increases on doping with the above-mentioned transition metal cations.     

Simulations of the Oxidation and Degradation of Platinum Electrocatalysts

Improved understanding of the fundamental processes leading to degradation of platinum nanoparticle electrocatalysts is essential to the continued advancement of their catalytic activity and stability. To this end, the oxidation of platinum nanoparticles is simulated using a ReaxFF reactive force field within a grand‐canonical Monte Carlo scheme. 2–4 nm cuboctahedral particles serve as model systems, for which electrochemical potential‐dependent phase diagrams are constructed from the thermodynamically most stable oxide structures, including solvation and thermochemical contributions. Calculations in this study suggest that surface oxide structures should become thermodynamically stable at voltages around 0.80–0.85 V versus standard hydrogen electrode, which corresponds to typical fuel cell operating conditions. The potential presence of a surface oxide during catalysis is usually not accounted for in theoretical studies of Pt electrocatalysts. Beyond 1.1 V, fragmentation of the catalyst particles into [Pt₆O₈]^4? clusters is observed. Density functional theory calculations confirm that [Pt₆O₈]^4? is indeed stable and hydrophilic. These results suggest that the formation of [Pt₆O₈]^4? may play an important role in platinum catalyst degradation as well as the electromotoric transport of Pt^2+/4+ ions in fuel cells.     

The influence of loading rates on hardening effects in elasto/viscoplastic strain-hardening materials

In this paper the influence of increasing loading rates on hardening effects is analyzed for rate-dependent elastoplastic materials. The effects of different loading rates on hardening rules are discussed with regard to the constitutive behavior of strain-hardening materials in elasto/viscoplasticity. A suitable procedure for the numerical simulation of rate-sensitive material behavior is illustrated. A comparative analysis is presented on constitutive relations in strain-hardening plasticity without rate effects and with rate effects in order to show the different role played by hardening rules in the rate-sensitivity analysis of elasto/viscoplastic strain-hardening materials. By reporting suitable numerical simulations for the adopted constitutive relations it is shown that when the rate of application of the loading is increased the influence of hardening has a different effect in the mechanical behavior of structures. Computational results and applications are finally illustrated in order to show numerically the different role played by hardening on the plastic strains when the loading rates are incremented for elasto/viscoplastic strain-hardening materials and structures.     

Effectiveness of high-resolution SAR for water resource management in low-income semi-arid countries

This article presents an efficient framework and a sustainable pilot project on the effective use of spaceborne synthetic aperture radar (SAR) in low-income countries and semi-arid climatic contexts. The technical efficiency was pursued by integrating SAR models and hydrological assessment methods; the socio-economical sustainability was guaranteed by the joint work of scientists, technicians, and volunteers. The pilot project was developed in the Yatenga region, a Sahelian area in northern Burkina Faso. In particular, an original development of SAR interferometry algorithms was tailored to the peculiar climate, the soil characteristics, and the land cover of the semi-arid regions. A digital elevation model (DEM) was derived, and an original approach based on the use of SAR amplitude images is proposed for its validation. The achieved resolution (9 m) is significantly better than that of the previously available DEMs in the study area (30 m). Based on the DEM, the soil sedimentation rate of small reservoirs was estimated together with the average soil loss in the contributing catchments due to the erosion process. A multi-temporal filter was implemented on the SAR images for monitoring of water intake volume in small reservoirs, and its seasonal evolution. The developed tools provide an innovative contribution for the improvement of water resource management in the study area. This approach is repeatable and scalable to suit situations with similar economic and climatic conditions.     

The Juxtaposed approximate PageRank method for robust PageRank approximation in a peer-to-peer web search network

We present Juxtaposed approximate PageRank (JXP), a distributed algorithm for computing PageRank-style authority scores of Web pages on a peer-to-peer (P2P) network. Unlike previous algorithms, JXP allows peers to have overlapping content and requires no a priori knowledge of other peers’ content. Our algorithm combines locally computed authority scores with information obtained from other peers by means of random meetings among the peers in the network. This computation is based on a Markov-chain state-lumping technique, and iteratively approximates global authority scores. The algorithm scales with the number of peers in the network and we show that the JXP scores converge to the true PageRank scores that one would obtain with a centralized algorithm. Finally, we show how to deal with misbehaving peers by extending JXP with a reputation model. Partially supported by the EU within the 6th Framework Programme under contract 001907 “Dynamically Evolving, Large Scale Information Systems” (DELIS).     

Transforming paper documents into XML format with WISDOM++

The transformation of scanned paper documents to a form suitable for an Internet browser is a complex process that requires solutions to several problems. The application of an OCR to some parts of the document image is only one of the problems. In fact, the generation of documents in HTML format is easier when the layout structure of a page has been extracted by means of a document analysis process. The adoption of an XML format is even better, since it can facilitate the retrieval of documents in the Web. Nevertheless, an effective transformation of paper documents into this format requires further processing steps, namely document image classification and understanding. WISDOM++ is a document processing system that operates in five steps: document analysis, document classification, document understanding, text recognition with an OCR, and transformation into HTML/XML format. The innovative aspects described in the paper are: the preprocessing algorithm, the adaptive page segmentation, the acquisition of block classification rules using techniques from machine learning, the layout analysis based on general layout principles, and a method that uses document layout information for conversion to HTML/XML formats. A benchmarking of the system components implementing these innovative aspects is reported. Received June 15, 2000 / Revised November 7, 2000     

Heterogeneous Photooxidation of Phenol by Catalytic Membranes

In this work the heterogenization in polymeric membranes of decatungstate, a photocatalyst for oxidation reactions, was reported. Solid state characterization techniques confirmed that the catalyst structure was preserved within the polymeric membranes. The catalytic membranes were successfully applied in the aerobic photo-oxidation of phenol, one of the main organic pollutants in wastewater, providing stable and recyclable photocatalytic systems. The dependence of the phenol degradation rate by the catalyst loading and transmembrane pressure was shown. By comparison with homogeneous reaction, the catalyst heterogenised in membrane appears to be more efficient concerning the rate of phenol photodegradation and mineralization.     

Fluorescent polymer nanofibers based on polycaprolactone and dansyl derivatives for development of latent fingerprints

This work provides a convenient low-cost strategy for fast fabrication of dansyl-based fluorescent nanofibers for development of latent fingerprints aiming forensic applications. Polycaprolactone (PCL) and dansyl cadaverine (DnsCad) or dansylglycine (DnsGly) nanofibers are prepared by electrospinning technique and characterized by SEM, FTIR, thermal analysis, and fluorescence spectroscopy. The electrospun PCL/DnsCad and PCL/DnsGly nanofibers are fluorescent with greenish blue and yellowish green emission, respectively. Such nanofibers are useful for enhancement of the visual contrast of latent fingerprints on metallic surfaces when exposed to UV light. These findings are particularly important for development of latent fingerprints on cartridge cases.