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

Abstract

Chemical and electronic properties of N-doped multiwalled carbon nanotubes (NCNTs) synthesized by NH3 treatment of preoxidized CNTs at 300, 500, and 700 °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 °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 °C. Here, only small fractions of lactam groups and pyridinic species are present. The incorporation of N at 700 °C leads to a carbon nanotube (CNT) surface with a well-defined basicity due to ...