Sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride photocatalyst: insights from first principles

We present here results of our first principles studies of the sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride (g-C3N4). Using the Ab initio thermodynamics approach combined with some kinetic analysis, we reveal the favorable S-doping configurations By analyzing the valence charge densities of the doped and un-doped systems, we find that sulfur partially donates its px- and py- electrons to the system with some back donation to the S pz-states. To obtain accurate description of the excited electronic states, we calculate the electronic structure of the systems using the GW method. The band gap width calculated for g-C3N4 is found to be equal to 2.7 eV that is in agreement with experiment. We find the S doping to cause a significant narrowing the gap. Furthermore, the electronic states just above the gap become occupied upon doping that makes the material a conductor. Analysis of the projected local densities of states provides insight into the mechanism underlying such dramatic changes in the electronic structure of g-C3N4 upon the S doping. Based on our results, we propose a possible explanation for the S doping effect on the photo-catalytic properties of g-C3N4 observed in the experiments

First principles studies of the size and shape effects on reactivity of the Se modified Ru nanoparticles

We present here the results of our density-functional-theory-based calculations of the electronic and geometric structures and energetics of Se and O adsorption on Ru 93- and 105-atom nanoparticles. These studies have been inspired by the fact that Se/Ru nanoparticles are considered promising electrocatalysts for the oxygen reduction reaction (ORR) on the direct methanol fuel cell cathodes and the oxygen binding energy is a descriptor for the catalyst activity towards this reaction. We find the character of chemical bonding of Se on a flat nanoparticle facet to be ionic, similar to that obtained earlier for the Se/Ru(0001) surface, while in the case of a low coordinated Ru configuration there is an indication of some covalent contribution to the bonding leading to an increase in Se binding energy. Se and O co-adsorbed on the flat facet, both accept electronic charge from Ru, whereas the adsorption on low-coordinated sites causes more complicated valence charge redistribution. The Se modification of the Ru particles leads to weakening of the oxygen bonding to the particle. However, overall, O binding energies are found to be higher for the particles than for Se/Ru(0001). High reactivity of the Se/Ru nanoparticles found in this work is not favorable for ORR. We thus expect that larger particles with well-developed flat facets are more efficient ORR catalysts than small nanoparticles with a large fraction of under-coordinated adsorption sites

Computational Approach To The Problems Of Electro- And Photo-catalysis

The main objective of this work is to gain basis for rational design of catalysts used in fuel cells for conversion of chemical energy stored in hydrogen molecules into electric energy, as well as photo-catalysts used for hydrogen production from water under solar irradiation. This objective is achieved by applying the first principles computational approach to reveal relationship among compositions of materials under consideration, their electronic structure and catalytic activity. A major part of the work is focused on electro-catalysts for hydrogen fuel cells. Platinum (Pt) is widely used in the electrodes of fuel cells due to its good catalytic properties. However, Pt is an expensive and scarce element, its catalytic activity is not optimal and also it suffers from CO poisoning at anode. Therefore the search for new catalytic materials is needed for large scale implementation of fuel cells. The main direction of search of more efficient electro-catalysts is based in the design in which an active element monoatomic layer (AE) is deposited on a metal substrate (MS) made of a cost-effective material. Two goals are achieved by doing this: on the one hand, the cost of the catalytic system is reduced by reducing the amount of the AE in the system and on the other hand the catalytic properties of the AE can be tuned through its interactions with the MS. In the first part of this work the Pd-based alloys and layered structures have been studied as promising electro-catalysts for the ORR on the fuel cell cathodes, more precisely Pd-Co alloys and Pd/M/Pd (M=Co,Fe). There exists a robust model linking the activity of a surface toward ORR to computable thermodynamic properties of the system and further to the binding energies iv of the ORR intermediates on the catalyst surface. A more challenging task is to find how to tune these binding energies through modification of the surface electronic structure that can be achieved by varying the surface composition and/or morphology. To resolve this challenge, the electronic structure, binding energies of intermediates and the ORR free energies have been calculated within the density functional theory (DFT) approximation. The results presented in this work show that in contrast to the widely accepted notion, the strain exerted by a substrate on AE hardly affects the surface activity toward ORR, while the hybridization of the electronic states of the AE-and MS-electronic states is the key factor controlling the catalytic properties of these systems. Next it is shown that the catalytic activity of the promising anode electrocatalysts, such as Pt/M, M=Au, Ru and Pd, is also determined by the AE-MS hybridization with a minor effect of the strain. Furthermore, we have shown that, if AE is weakly bound to the substrate (as it is for Pt/Au), surface reconstruction occurs. This leads to the breaking of the relation between the electronic structure of the clean surface and the reactivity of the sytem. Other kind of promising ORR catalysts is designed in the form of Ru nanoparticles modified by chalcogens. In this work, I present the results obtained for small Ru clusters and flat Ru facets modified with chalcogens (S, Se and Te). The O and OH binding energies are chosen as descriptors of the ORR. The results on the two systems are compared, concluding that large clusters with relative large flat facets have higher catalytic activity due to the absence of low coordinated and thus high reactive Ru atoms. Regarding the problem of the hydrogen production via photo-catalytic splitting of water, one of the challenges is tuning the band gap of the photo-anodes to optimal levels. Graphitic carbon nitride (g-C3N4) is a promising material to be used as a photo-anode, however, a v reduction of the band gap width by rational doping of the material would improve the efficiency significantly. This issue is addressed in the last chapter of this work. Two problems are considered: a) the stability of the doped system and b) the band gap width. To address the first problem the ab-initio thermodynamics approach has been used, finding that the substitution of C and N with the doping agent (B, C, N, O, Si and P) is thermodynamically preferred over the interstitial addition of dopant to the g-C3N4 structure. However, due to high kinetic energy barriers for the detachment of C and N atoms, involved in the substitution doping, the interstitial addition found to be kinetically more favorable. Since the density functional theory fails to reproduce the band gap of semiconductors correctly, the GW approximation was used to study the band gap of the system. The results indicate that the g-C3N4 system maintain its semiconductor character if doped with B, O and P under certain conditions, while reducing the band gap

Análisis de estrategias óptimas de pago de dividendos cuando el excedente sigue un proceso Browniano Drift

En este trabajo se estudia el caso concreto cuando el excedente de una compañía de seguros se comporta de acuerdo a un movimiento Browniano Drift, y a partir de allí se deduce el comportamiento del excedente modificado para una estrategia de barrera y para una estrategia de pago continuo con el objetivo de determinar los parámetros óptimos que hacen que cada una de ellas maximize la expectativa del valor presente de los dividendos descontados. Además, se comparan los resultados obtenidos con una estrategia de pago continuo con aquellos que se obtienen asumiendo una estrategia de barrera. Los experimentos numéricos mostraron que la estrategia de barrera produce más dividendos que la estrategia de pago continuo bajo diferentes escenarios de parámetros (incluyendo diferentes niveles de volatilidad), razón por la cual se concluye que independientemente del perfil de aversión al riesgo del inversionista, si éste decide invertir, entonces siempre preferirá que sus dividendos sean pagados acordes con una estrategia de barrera.In this paper, we study the case when the surplus of an insurance company behaves according to a Brownian Drift motion, subsequently we derive the modified surplus dynamics assuming both a barrier strategy and a continuous payment strategy in order to determine the optimal parameters which make that each of these strategies maximize the mathematical expectation of the present value of the discounted dividends. In addition, we compare the results obtained with a continuous payment strategy with those obtained with a barrier strategy. The numerical experiments suggest that the barrier strategy is more profitable than the continuous payment one under different constellations of parameters (including different levels of volatility), and therefore it may be concluded that independently of the risk aversion profile of the investor, if he decides to invest, then he will always prefer his dividends to be paid according with a barrier strategy

Factors controlling the energetics of the oxygen reduction reaction on the Pd-Co electro-catalysts: Insight from first principles

We report here results of our density functional theory based computational studies of the electronic structure of the Pd-Co alloy electrocatalysts and energetics of the oxygen reduction reaction (ORR) on their surfaces. The calculations have been performed for the (111) surfaces of pure Pd, Pd0.75Co0.25 and Pd0.5Co0.5 alloys, as well as of the surface segregated Pd/Pd0.75Co0.25 alloy. We find the hybridization of dPd and dCo electronic states to be the main factor controlling the electrocatalytic properties of Pd/Pd0.75Co0.25. Namely the dPd - dCo hybridization causes low energy shift of the surface Pd d-band with respect to that for Pd(111). This shift weakens chemical bonds between the ORR intermediates and the Pd/Pd0.75Co0.25 surface, which is favorable for the reaction. Non-segregated Pd0.75Co0.25 and Pd0.5Co0.5 surfaces are found to be too reactive for ORR due to bonding of the intermediates to the surface Co atoms. Analysis of the ORR free energy diagrams, built for the Pd and Pd/Pd0.75Co0.25, shows that the co-adsorption of the ORR intermediates and water changes the ORR energetics significantly and makes ORR more favorable. We find the onset ORR potential estimated for the configurations with the O - OH and OH - OH co-adsorption to be in very good agreement with experiment. The relevance of this finding to the real reaction environment is discussed

Determinantes de la demanda por educación superior en Colombia.

Este artículo estudia los factores que en Colombia determinan la decisión de las personas de ingresar a la educación superior. Se utiliza información del Ministerio de Educación Nacional, del Instituto Colombiano de Crédito Educativo y Estudios Técnicos en el Exterior (Icetex) y de la Encuesta Continua de Hogares (ECH) para estimar un modelo de datos de panel al nivel de cada institución de educación superior (IES), para las 13 principales ´áreas metropolitanas, en el período 2000-2006. Los principales resultados del estudio sugieren que la oferta educativa tiene una gran influencia sobre las tasas de matrícula en las IES, particularmente en el sector publico. A su vez, la dinámica del empleo, especialmente el de las personas con educación secundaria, desempeña un papel significativo en las decisiones de los individuos para ingresar a los estudios superiores y en la escogencia del nivel educativo

El modelo “Customer Relationship Management” como herramienta para gestionar la relación con los clientes en las grandes empresas de servicios no financieros de la ciudad de Medellín

105 páginasTo determine the viability and efficiency in the implementation of strategies for the Customer Relationship Management in the large non-financial companies in the city of Medellín, the first step was taking a look at North America –United States and Canada–, to have a reference point in a dynamic market where CRM is the fundamental basis of relationship strategies. Some cases of successful implementation of this methodology in the companies were analyzed, as well as theories held by highly regarded authors. It was also necessary to determine the advantages and disadvantages that come with the implementation of a CRM strategy; therefore, secondary information was analyzed to explain why it is good to implement relationship strategies and the obstacles or problems that brings its execution. Using in-depth interviews, the existing CRM theory and the reality of its implementation in large non-financial services companies of Medellin was corroborated and contrasted.Para determinar qué tan viable y eficiente resulta la implementación de estrategias de Administración de la Relación con el Cliente en las grandes empresas de servicios no financieros de la ciudad de Medellín, se comenzó realizando un recorrido por Norteamérica –Estados Unidos y Canadá–, para tener el referente de un mercado dinámico donde el CRM es base fundamental de las estrategias de relacionamiento; igualmente se analizaron casos de éxito de la implementación de esta metodología en dichas empresas y teoría de autores norteamericanos de gran reconocimiento y trayectoria. Era indispensable también determinar las ventajas y desventajas que trae consigo la implementación de una estrategia CRM, para lo cual se accedió a información secundaria que expuso y sustentó por qué es bueno implementar estrategias de relacionamiento y los obstáculos o problemas que trae su ejecución. Con el trabajo de campo –entrevistas a profundidad– se pudo corroborar y contrastar la teoría existente sobre CRM y la realidad de su aplicación en las grandes empresas de servicios no financieros de Medellín.PregradoIngeniero(a) Administrativo(a