Chemical state

About: Chemical state is a research topic. Over the lifetime, 2378 publications have been published within this topic receiving 78183 citations.

Chapter Thirteen – Surface Chemical Characterization of Carbons from Adsorption Studies

Abstract: The adsorption behavior of carbons is affected to a considerable extent by the chemical state of their surfaces, which is also of great practical importance in many other applications of carbon materials such as for catalysts and catalyst supports, and carbon-polymer composites. In the surface of carbon materials, the regular network of covalent C-C bonds is broken, and reactive sites result as a consequence. Usually “free valences,” also called “dangling bonds,” are saturated with foreign elements, in first line hydrogen and oxygen. In the case of carbon structures derived from the graphite lattice, the surface is inhomogeneous and is constituted to variable fractions of basal faces, i.e., honeycomb-like graphene layers, and of the edges of the graphene layers. While the basal faces are quite inert, the edge sites are reactive and can chemisorb other elements such as hydrogen, oxygen, nitrogen species, and halogens. In contrast, the surface of diamond is much more homogeneous and has a comparatively simpler chemical behavior.

XPS, AFM and nanoindentation studies of Ti1−xAlxN films synthesized by reactive unbalanced magnetron sputtering

Abstract: A combined investigation of chemical analysis and bonding states in Ti1−xAlxN films sputter-deposited on Si(1 0 0) in an Ar–N2 gas mixture by X-ray photoelectron spectroscopy and of surface morphology and mechanical properties by atomic force microscopy (AFM) and nanoindentation measurements is reported. It was found that a linear increase in the Al concentration of the films was observed with increasing Al target current up to 7 A, while the reverse trend was seen for the Ti concentration. The Al and Ti in the films presented in a form of stoichiometric TiN and AlN at different atomic concentration of Al. Several types of chemical states, such as Al2O3, TiNxOy, Ti2O3 and TiO2 have been identified. However, no unbound Al and Ti atomic species were detected in films. By applying the height–height correction functions to the measured AFM images, a steady growth roughness exponent α=0.94±0.03 was determined for all the Ti1−xAlxN films. The value of α is consistent with growth model predictions incorporating surface diffusion. It was also found that the improved mechanical properties of Ti1−xAlxN films with the addition of Al into TiN matrix were attributed to their densified microstructure with development of fine grain size and reduced surface roughness. The effect of aluminium in stabilizing the Ti–Al–N structure was also elucidated and explained on the basis of the adatom mobility and the surface diffusion of atoms.

Surface Heterogeneity of Zirconia-Supported V2O5 Catalysts. The Link between Structure and Catalytic Properties in Oxidative Dehydrogenation of Propane†

Abstract: Undoped and alkali-doped zirconia-supported vanadia catalysts for the oxidative dehydrogenation of propane were prepared by impregnation and characterized by various techniques. The chemical state of surface and bulk vanadium was investigated as a function of the calcination temperature, V2O5 loading, and the nature and content of alkali-metal additives. It is found that on the surface vanadium is present in the form of isolated vanadyl species or oligomeric vanadates, or as nanocrystalline V2O5 and that V5+ and V4+ ions coexist in octahedral and tetrahedral coordination, while within the bulk of zirconia matrix, V4+ ions are stabilized in a VxZr1-xO2 solid solution. Presence of the alkali-metal additives and water changes the dispersion of surface vanadium species favoring, in both cases, formation of mononuclear vanadyl surface complexes. Surface heterogeneity plays a vital role for the catalytic performance of V2O5/ZrO2 catalysts in oxidative dehydrogenation of propane. Catalytic properties are related...

Au/CeO2 Catalysts: Structure and CO Oxidation Activity

Abstract: In this comprehensive review, the main aspects of using Au/CeO2 catalysts in oxidation reactions are considered. The influence of the preparation methods and synthetic parameters, as well as the characteristics of the ceria support (presence of doping cations, oxygen vacancies concentration, surface area, redox properties, etc.) in the dispersion and chemical state of gold are revised. The proposed review provides a detailed analysis of the literature data concerning the state of the art and the applications of gold–ceria systems in oxidation reactions.