Acid-Base Properties of N-Doped Carbon Nanotubes: A Combined Temperature-Programmed Desorption, X-ray Photoelectron Spectroscopy, and 2-Propanol Reaction Investigation

摘要

Chemical and electronic properties of N-doped multiwalled carbon nanotubes (NCNTs) synthesized by NH3 treatment of preoxidized CNTs at 300, 500, and 700 degrees C have been investigated by a set of surface sensitive techniques. Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were applied to characterize the nature, thermal stability, and binding state of N and/or O containing surface functional groups. Acid base properties in aqueous phase were analyzed by potentiometric pH titration, while the catalytic reaction of 2-propanol probed the acid-base behavior of the materials in the gas phase. NH3 treatment at 300 degrees C leads to an acid-base bifunctional surface, predominantly decorated with imide species. Contrarily, pyrrolic N is the most abundant moiety present on the sample modified at 500 degrees C. Here, only small fractions of lactam groups and pyridinic species are present. The incorporation of N at 700 degrees C leads to a carbon nanotube (CNT) surface with a well-defined basicity due to pyridinic N, which serves as a Lewis basic site converting 2-propanol into acetone. Furthermore, the oxidative stability of NCNTs strongly depends on the nature of N-containing species. Regarding the oxidative stability, NCNTs obtained at 700 degrees C behave similar to the pristine CNTs, whereas the lower NH3 treatment temperatures are detrimental for this property. The combination of dedicated techniques reveals links between structural and functional properties of surface species that change dynamically with temperature.