980 resultados para Territoire en transition
Resumo:
The objective of the article is to examine the way in which social work in Ireland evolved from practices of philanthropy in the late 19th century to a distinct professional strategy in the present. Results: The results of archival research show that philanthropy in Ireland was provided almost exclusively by religious organizations and was constructed within a discourse of sectarianism and rivalry between the two main denominations, Catholic and Protestant, up to the 1960s. It is only in the past 30 years that social work has become firmly established as a secular strategy. Conclusions: It is concluded that although social work is now clearly distinct from voluntary and religious-based social work practices, some of its present principles and practices remain continuous with its historical antecedents.
Resumo:
La necesidad de contar con un mapa de base científica que cubriera todo el territorio español llevó al Estado a crear en paralelo varias Comisiones con cometidos geodésicos, topográficos y cartográficos durante la década de 1850. La labor simultánea de estas Comisiones se prolongó hasta 1859, cuando se aprobó la Ley de Medición del Territorio, que las fusionó en un único organismo. Este artículo analiza aspectos técnicos de los trabajos que realizaron estas Comisiones a partir de la información contenida en algunos documentos que custodia el Archivo Topográfico del IGN. Las conclusiones que se extraen son que estas Comisiones acometieron operaciones geodésicas que resultaron cruciales en el establecimiento ulterior de la red de triangulación peninsular, realizaron mediciones topográficas que fueron reutilizadas veinte años después en el levantamiento del Mapa Topográfico Nacional, e idearon las características catastrales que fueron adoptadas durante todo el siglo posterior para el Catastro de España.
Resumo:
Spectroscopic measurements of NOAA AR 10871, obtained with the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument on 2006 April 12, reveal velocity oscillations in the He II 303.8 angstrom emission line formed at T approximate to 5; 10(4) K. The oscillations appear to arise in a bright active region loop arcade about 25 '' wide which crosses the EUNIS slit. The period of these transition region oscillations is 26 +/- 4 s, coupled with a velocity amplitude of +/- 10 km s(-1), detected over four complete cycles. Similar oscillations are observed in lines formed at temperatures up to T approximate to 4; 10(5) K, but we find no evidence for the coupling of these velocity oscillations with corresponding phenomena in the corona. We interpret the detected oscillations as originating from an almost purely adiabatic plasma, and infer that they are generated by the resonant transmission of MHD waves through the lower active region atmospheres. Through the use of seismological techniques, we establish that the observed velocity oscillations display wave properties most characteristic of fast body global sausage modes.
Resumo:
Transition metal catalyzed bond formation is a fundamental process in catalysis and is of general interest throughout chemistry. To date, however, the knowledge of association reactions is rather limited, relative to what is known about dissociative processes. For example, surprisingly little is known about how the bond-forming ability of a metal, in general, varies across the Periodic Table. In particular, the effect of reactant valency on such trends is poorly understood. Herein, the authors examine these key issues by using density functional theory calculations to study CO and CN formations over the 4d metals. The calculations reveal that the chemistries differ in a fundamental way. In the case of CO formation, the reaction enthalpies span a much greater range than those of CN formation. Moreover, CO formation is found to be kinetically sensitive to the metal; here the reaction barriers (E-a) are found to be influenced by the reaction enthalpy. CN formation, conversely, is found to be relatively kinetically insensitive to the metal, and there is no correlation found between the reaction barriers and the reaction enthalpy. Analysis has shown that at the final adsorbed state, the interaction between N and the surface is relatively greater than that of O. Furthermore, in comparison with O, relatively less bonding between the surface and N is observed to be lost during transition state formation. These greater interactions between N and the surface, which can be related to the larger valency of N, are found to be responsible for the relatively smaller enthalpy range and limited variation in E-a for CN formation. (C) 2007 American Institute of Physics.
Resumo:
Density functional theory calculations are used to study the stability of molecularly adsorbed CO and CN over transition metal surfaces. The minimum energy reaction pathways, corresponding reaction barriers (E-a), and reaction enthalpies (Delta H) for the dissociation of CO and CN to atomic products over the 4d transition metals from Zr to Pd have been determined. CO is found to be the more stable adsorbate on the right hand side of the period (from Tc onwards), whereas CN is the more stable surface species on the early metals (Zr, Nb and Mo). A single linear relationship is found to exist that correlates the barriers of both reactions with their respective reaction enthalpies. (c) 2006 Elsevier B.V. All rights reserved.
Resumo:
The dissociative adsorption of N-2 has been studied at both monatomic steps and flat regions on the surfaces of the 4d transition metals from Zr to Pd. Using density functional theory (DFT) calculations, we have determined and analyzed the trends in both straight reactivity and structure sensitivity across the periodic table. With regards to reactivity, we find that the trend in activation energy (Ea) is determined mainly by a charge transfer from the surface metal atoms to the N atoms during transition state formation, namely, the degree of ionicity of the N-surface bond at the transition state. Indeed, we find that the strength of the metal-N bond at the transition state (and therefore the trend in Ea) can be predicted by the difference in Mulliken electronegativity between the metal and N. Structure sensitivity is analyzed in terms of geometric and electronic effects. We find that the lowering of Ea due to steps is more pronounced on the right-hand side of the periodic table. It is found that for the early transition metals the geometric and electronic effects work in opposition when going from terrace to step active site. In the case of the late 4d metals, however, these effects work in combination, producing a more marked reduction in Ea.
Resumo:
The density functional theory (DFT) based hard-soft acid-base (HSAB) reactivity indices, including the electrophilicity index, have been successfully applied to many areas of molecular chemistry. In this work we test the applicability of such an approach to fundamental surface chemistry. We have considered, as prototypical surface reactions, both the hydrogenation of atomic nitrogen and the dissociative adsorption of the NH molecular radical. By use of a DFT methodology, the minimum energy reaction pathways, and corresponding reaction barriers, of the above reactions over Zr(001), Nb(110), Mo(110), Tc(001), Ru(001), Rh(111), and Pd(111) have been determined. By consideration of the chemical potential and chemical hardness of the surface metal atoms, and the principle of electronegativity equalization, it is found that the charge transferred to the NH radical during the process of dissociative adsorption correlates very well with that determined by Mulliken population analysis. Furthermore, it is found that the stability of the NH/surface transition state complex relates directly to this charge transfer and that the trend in transition state stability predicted by a HSAB; treatment correlates very strongly with that determined by DFT calculations. With regards to N hydrogenation, we find that during the course of the reaction, H loses cohesion to the surface, as it must migrate from a 3-fold hollow site to either a bridge or top site, to react with N. Partial density of states (PDOS) and Mulliken population analysis reveal that this loss of bonding is accompanied by charge transfer from H to the surface metal atoms. Moreover, by simple modeling, we show that the reaction barriers are directly proportional to this mandatory charge transfer. Indeed, it is found that the reaction barriers correlate very well with the electrophilicity index of the metal atoms.
Resumo:
A catalyst preparation by design is one of the ultimate goals in chemistry. The first step towards this goal is to understand the origin of reaction barriers. In this study, we have investigated several catalytic reactions on some transition metal surfaces, using density functional theory. All the reaction barriers have been determined. By detailed analyses we obtain some insight into the reaction barrier. Each barrier is related to (i) the potential energy surface of reactants on the surface, (ii) the total chemisorption energy of reactants, and (iii) the metal d orbital occupancy and the reactant valency. (C) 2001 American Institute of Physics.
Resumo:
Dissociative adsorption is one of the most important reactions in catalysis. In this communication we propose a model aiming to generalize the important factors that affect dissociation reactions. Specifically, for a dissociation reaction, say AB -->A + B, the model connects the dissociation barrier with the association barrier, the chemisorption energies of A and B at the final state and the bonding energy of AB in the gas phase. To apply this model, we have calculated CO dissociation on Ru(0001), Rh(111), Pd(111) (4d transition metals), Os(0001), Ir(111), and Pt(111) (5d transition metals) using density function theory (DFT). All the barriers are determined. We find that the DFT results can be rationalized within the model. The model can also be used to explain many experimental observations. (C) 2001 American Institute of Physics.
Resumo:
A new route to the isolation of the enantiopure tris- chelate complex (Delta/Lambda)- fac-[Ru( L-1)(3)] 21 (where L-1 is 2,2'-bipyridine-5-carboxylic acid) is demonstrated, where the transition metal centre retains the memory of the chirality present in a simple tripodal tether used to control the metal centred geometry.
Resumo:
Purpose – This paper aims to examine the growing incidence of judicialisation of politics in Nigeria’s democratisation experience against the backdrop of questionable judicial accountability. Design/methodology/approach – The article draws on legal and political theory as well as comparative law perspectives. Findings – The judiciary faces a daunting task in deepening democracy and (re) instituting the rule of law. The formidable challenges derive in part from structural problems within the judiciary, deficient accountability credentials and the complexities of a troubled transition. Practical implications – Effective judicial mediation of political transition requires a transformed and accountable judiciary. Originality/value – The article calls attention to the need for judicial accountability as a cardinal and integral part of political transitions. Keywords Democracy, Politics, Law, Nigeria, Africa Paper type Viewpoint