Temperature-programmed desorption of oxygen surface complexes on acenaphthylene-derived chars: Comparison with oxygen K-edge XANES spectroscopy

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 se ries of acenaphthylene-derived chars oxidized by either partial combustion or low-temperature oxygen chemisorption prior to their study by TPD. Eviden ce for the presence of mobile hydrogen species was provided by the observat ion that H-2 and H2O were evolved at high temperatures during TPD. A consta nt CO:CO2 ratio for the desorption products evolved during the TPD of the p artially combusted chars at desorption temperatures below similar to 1000 K was attributed to surface reactions of mobile oxygen surface species contr olling 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 similar to 1000 K, the CO:CO2 ratio was observed to increase sharply. This was attributed to s cavenging of C(O) by mobile hydrogen species, although the possibility that different functional groups might have different mobilities was also consi dered. The TPD profiles of the surface oxygen complexes formed during oxyge n chemisorption at 473 K contained additional peaks not present in the prof iles of the samples combusted at 773 K. These additional peaks were attribu ted 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 t o combustion scenarios.