TPD and XPS studies of O2, CO2, and H2O adsorption on clean polycrystalline graphite

Abstract

Temperature programmed desorption (TPD) and X-Ray photoelectron spectroscopy (XPS) studies on clean polycrystalline graphite under Ultra High Vacuum conditions are described. The same three strongly bound oxygenated species are formed after O 2 , CO 2 and H 2 0 adsorption. They decompose to give CO at 973, 1093 and 1253 K. Small amounts of CO 2 are also produced after adsorption of these gases, with desorption temperatures at 463, 573, 693, 793 and 793 K. Attempts are made to ascribe these TPD features more precisely. After H 2 0 adsorption, some H2 is evolved at ca. 1300 K. Hydrocarbons (C l -C 6 ) are also produced, but is smaller amounts. A general mechanism is proposed for the gasification reactions of graphite with O 2 , CO 2 and H 2 0. Physical wetting of the clean graphite surface leads to a H 2 0 molecule re\'ersibly bound to the carbon surface. According to XPS data, a hydrate type of bond is proposed. Considerations on the non-catalytic as well as on the catalytic steam gasification of graphite are made. It is suggested that in both cases the reaction is not only controlled by the desorption of the products, i. e. the decomposition of the surface intermediates, but also by the sticking probability of the H 2 0 on the graphite edges.

Combined XPS and TPD study of oxygen-functionalized carbon nanofibers grown on sintered metal fibers

“…Water can originate either from adsorbed water or from a chemically bound form of aliphatic alcohol (in accordance with the model of Clark). We are aware that this assignment of water is in contrast to Marchon et al [21] who indicated a binding energy of approximately 533 eV for water ''wetting'' graphite, with successively lower binding energies for higher heating temperatures (532.5 eV after flashing to 593 K).…”

“…Considering CO 2 evolution, this was observed from ozone-treated sample, D3, mainly in the lower temperature range (<800 K), with a maximum at about 600 K. A broader but less intense CO 2 evolution from the H 2 O 2 -treated sample, C1, is observed at low temperature, but is also observed at higher temperature (up to 1000 K). In the literature, the evolution of CO 2 at low temperature is usually assigned to the carboxylic functional group, while at temperatures higher than 700 K CO 2 is attributed to the decomposition of anhydride and lactone groups [21][22][23]. CO desorption is dominated by the CNF burn-off at higher temperatures.…”

“…We tentatively assign the peak at BE 530.3 eV of sample D4 to oxygen atoms bonded to the residual metal present on the sample, which should be insensitive to the heating. Water adsorbed on polycrystalline graphite was found to have a binding energy of around 533 eV [21]. Therefore, we should include adsorbed water in our region C, vide infra.…”

Importance of the oxygen bond strength for catalytic activity in soot oxidation

“…This value is relatively high compared to reported activation energies for O 2 dissociation on defected graphite [72], but in the low end of the reported activation energies for CO and CO 2 desorption from semiquinone and lactone species in graphite surfaces [72][73][74]. It is therefore difficult on this basis to unambiguously assign a rate limiting step for the non-catalytic oxidation.…”

“…The reaction leaves a reduced surface site in the metal oxide, which is reoccupied according to R1. It is known from TPD studies [73,74] that the semi-quinone species can desorb as CO (R3), but the activation energy for desorption of CO 2 from carbon bound to two oxygen atoms is significantly lower [73,74], and, when possible, a second transfer of oxygen (R4) should be a more favourable pathway. In the catalytic oxidation the fraction of carbon released as CO is low compared to the non-catalytic oxidation (supplementary information, figure S13), but as secondary catalytic oxidation of released CO is also a possibility, it is difficult to identify the immediate product of the carbon oxidation.…”