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Título : Optimization of char for NOx removal
Autor : Phillips, Jonathan; Radovic, Ljubisa R.; Xia, Bo; Menéndez Díaz, José Ángel
Palabras clave : Carbón
Surface chemistry
NOx
Basic sites
Hydrogen
Spill over
Point of zero charge
microcalorimeter
Fecha de publicación : 28-jun-1998
Serie : DE-FG22-95PC95225--04
Resumen: Work performed for this study demonstrates that the temperature of treatment and the identity of the treatment gas both strongly impact the surface chemistry of activated carbon. Two commercial activated carbons were treated in either N2 or H2 at different temperatures up to 2600oC. Several techniques - including microcalorimetry, point of zero charge measurements, thermal desorption - were used to provide insight into important aspects of the chemical surface properties. The results show that activated carbons treated at high temperatures (ca. 950oC) in hydrogen will not react with oxygen and water at ambient temperatures; moreover, surfaces created in this fashion have stable properties in ambient conditions for many months. In contrast, the same carbons treated in an inert gas (e.g., N2) will react strongly with oxygen and water at ambient temperatures. In the presence of platinum (or any other noble metal), stable basic carbons, which will not adsorb oxygen in ambient laboratory conditions, can be created via a relatively low-temperature process. Treatment at higher temperatures (>1500oC) produced increasingly stable surfaces in either N2 or H2. A structural model is proposed. To wit: Treatment at high temperatures in any gas will lead to the desorption of oxygen surface functionalities in the form of CO and/or CO2. Absent any atom rearrangement, the desorption of these species will leave highly unsaturated carbon atoms (“dangling carbons”) on the surface, which can easily adsorb O2 and H2O. In an inert gas these “dangling carbons” will remain, but hydrogen treatments will remove these species and leave the surface with less energetic sites, which can only adsorb O2 at elevated temperatures. Specifically, hydrogen reacts with any highly unsaturated carbons in the surface to form methane. At temperatures greater than 1500oC (e.g., 1800oC, 2600oC), structural annealing takes place and the consequent growth in the size of graphene layers eliminates the highly energetic dangling carbon sites. The basicity of the surface originates from two types of Lewis base sites: the localized electron pairs at the edges of the graphene layers and the delocalized p electrons on the basal planes. A hydrogen spillover mechanism was proposed here to explain the lowtemperature process for the stable basic carbon. The role played by platinum (or any noble metal) is to produce atomic hydrogen, which spills over onto the carbon surface. This atomic hydrogen hydrogasifies the most reactive, unsaturated carbon atoms at far lower temperatures than molecular hydrogen, thus leading to surface stabilization at relatively low temperatures.
Descripción : En el informe se recoge un resumen de parte del trabajo realizado por el Dr. J. Ángel Menéndez, durante su estancia en Penn State University (PSU), Pennsylvania, EE.UU. Esta estancia posdoctoral (años 1995 y 1996 fue financiada por el CSIC. Los datos y resultados que se muestran en este informe han sido recogidos en al menos 4 publicaciones de gran repercusión (alguna de las cuales ha recibido mas de 100 citaciones).
URI : http://hdl.handle.net/10261/3757
Aparece en las colecciones: (INCAR) Informes y documentos de trabajo
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