The thermal stability and the reducibility of oxygen-containing functional groups on the surface of nitric acid-treated multiwalled carbon nanotubes (CNTs) have been studied using temperature-programmed desorption and reduction (TPD and TPR) and high-resolution X-ray photoelectron spectroscopy (XPS). The thermal treatments up to 720 °C were carried out in the XPS setup, either under ultrahigh vacuum (UHV) or in diluted hydrogen. Deconvoluted XP spectra were used for the quantitative determination of the amount of the different functional groups on the CNT surfaces as a function of the pretreatment. The number of the oxygen atoms per unit surface area was obtained from the oxygen to carbon (O/C) ratio derived from the corresponding peak areas in the XP spectra. The results obtained by XPS agree quantitatively with the observations by TPD and TPR. The acid treatment not only introduced carboxyl, carbonyl, and phenol groups on the surface but also generated ether-type oxygen groups between the graphitic layers as indicated by the oxygen balance. Generally, the presence of hydrogen decreased the thermal stability of the oxygen-containing functional groups. Both XPS and TPR provided evidence for the reduction of carboxylic groups to phenolic groups at 300 °C in hydrogen. Heating in hydrogen was found to be more effective in removing the oxygen-containing functional groups compared to heating in UHV but did not allow either to remove all oxygen species even at 720 °C.

Nitrogen-containing carbon nanotubes (NCNTs) were prepared via pyrolysis of acetonitrile over cobalt catalysts at different temperatures to control the nitrogen content. The changes in the chemical and structural properties of undoped CNTs and NCNTs were investigated using high-resolution X-ray photoelectron and Raman spectroscopy. The NCNTs prepared at 550°C had a higher amount of pyridinic groups and edge plane exposure than the ones prepared at 750°C. The thermal stability and transformation of these nitrogen functional groups was studied using deconvoluted XP N 1s spectra. The NCNTs show a considerably higher activity in the oxygen reduction reaction in acidic electrolyte compared with undoped CNTs as demonstrated by cyclic voltammetry, rotating disk electrode measurements, and the redox-competition mode of scanning electrochemical microscopy (RC-SECM). Particularly, the NCNT sample prepared at 550°C exhibited the highest activity, which was about 1 order of magnitude lower than that of a commercial Pt/C sample containing 20 wt % Pt. The oxygen reduction reaction (ORR) performance of this sample showed hardly any signs of deterioration after 3 days, as determined by voltammetric stability tests in H 2 SO 4 .

Where oxide and metals meet: The activation of an efficient associative mechanistic pathway for the water-gas shift reaction by an oxide-metal interface leads to an increase in the catalytic activity of nanoparticles of ceria deposited on Cu(111) or Au(111) by more than an order of magnitude (see graph). In situ experiments demonstrated that a carboxy species formed at the metal-oxide interface is the critical intermediate in the reaction