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

Nitrogen-containing functional groups were generated on the surface of partially oxidized multi-walled carbon nanotubes (CNTs) via post-treatment in ammonia. The treatment temperature was varied in order to tune the amount and type of nitrogen- and oxygen-containing functional groups, which were studied using high-resolution X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The surface defects on CNTs due to the incorporation of nitrogen were investigated by Raman spectroscopy. Deconvoluted XP N1s spectra were used for the quantification of different nitrogen-containing functional groups, and TPD studies were performed in inert and ammonia atmosphere to investigate the surface reactions occurring on the oxidized CNT surfaces quantitatively. Nitrile, lactam, imide and amine-type functional groups were formed in the presence of ammonia below 300 degrees C. When the OCNTs were treated in the medium temperature range between 300 degrees C to 500 degrees C, mainly pyridine-type nitrogen groups were generated, whereas pyridinic, pyrrolic and quaternary-type nitrogen groups were the dominating species present on the CNT surface when treated above 500 degrees C. It was found that about 38% of the oxygen functional groups react with ammonia below 500 degrees C.

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The role of the interface between a metal and oxide (CeO x −Cu and ZnO−Cu) is critical to the production of methanol through the hydrogenation of CO 2 (CO 2 + 3H 2 → CH 3 OH + H 2 O). The deposition of nanoparticles of CeO x or ZnO on Cu(111), θ oxi < 0.3 monolayer, produces highly active catalysts for methanol synthesis. The catalytic activity of these systems increases in the sequence: Cu(111) < ZnO/Cu(111) < CeO x /Cu(111). The apparent activation energy for the CO 2 → CH 3 OH conversion decreases from 25 kcal/mol on Cu(111) to 16 kcal/mol on ZnO/Cu(111) and 13 kcal/mol on CeO x /Cu(111). The surface chemistry of the highly active CeO x −Cu(111) interface was investigated using ambient pressure X-ray photoemission spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (AP-IRRAS). Both techniques point to the formation of formates (HCOO −) and carboxylates (CO 2 δ−) during the reaction. Our results show an active state of the catalyst rich in Ce 3+ sites which stabilize a CO 2 δ− species that is an essential intermediate for the production of methanol. The inverse oxide/metal configuration favors strong metal−oxide interactions and makes possible reaction channels not seen in conventional metal/oxide catalysts.

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