Lookup NU author(s): Emeritus Professor Mark Thomas
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The interpretation of temperature-programmed desorption (TPD) data relating to the decomposition of oxygen surface complexes on carbonaceous materials is complex. Oxygen K-edge XANES spectroscopy was used to characterize a series of acenaphthylene-derived chars oxidized by either partial combustion or low-temperature oxygen chemisorption prior to their study by TPD. Evidence for the presence of mobile hydrogen species was provided by the observation that H2 and H2O were evolved at high temperatures during TPD. A constant CO:CO2 ratio for the desorption products evolved during the TPD of the partially combusted chars at desorption temperatures below approximately 1000 K was attributed to surface reactions of mobile oxygen surface species controlling the product ratio. This indicates that desorption product ratios may be an unreliable guide to the functional groups undergoing decomposition, especially for chars with high hydrogen contents. Above approximately 1000 K, the CO:CO2 ratio was observed to increase sharply. This was attributed to scavenging of C(O) by mobile hydrogen species, although the possibility that different functional groups might have different mobilities was also considered. The TPD profiles of the surface oxygen complexes formed during oxygen chemisorption at 473 K contained additional peaks not present in the profiles of the samples combusted at 773 K. These additional peaks were attributed to the formation of a wider range of acid-like surface complexes during low-temperature oxygen chemisorption than during combustion. This suggests that care must be taken when applying low-temperature chemisorption data to combustion scenarios.
Author(s): Turner JA, Thomas KM
Publication type: Article
Publication status: Published
Print publication date: 14/09/1999
ISSN (print): 0743-7463
ISSN (electronic): 1520-5827
Publisher: American Chemical Society
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