PNNL

Abstract

The TPD-FTIR technique was used to investigate the relative thermal stabilities and decomposition reactions of different oxalate complexes adsorbed on the dry goethite surface. The measurements showed that important differences in coordination have a considerable impact on the thermal stability of the surface complexes. Three important stages of desorption were identified from both TPD (Figure 1) and FTIR data in the 300-900 K range. Stage I (300-440 K) corresponds to the desorption of weakly-bound oxalate molecules with decomposition pathways characteristic of oxalic acid. Stage II (440-520 K) corresponds to the dehydration of key surface OH2 groups responsible in stabilizing hydrogen-bonded surface complexes. These species can either decompose via typical oxalic acid decomposition pathways or convert to metal-bonded surface complexes. Finally, Stage III (520-660 K) corresponds to the thermal decomposition of all metal-bonded oxalate complexes, proceeding through a two-electron reduction pathway that converts oxalate to CO2. Experiments in the absence of oxalate were used to assess the contributions from dehydration, dehydroxylation and decarbonation reactions from the goethite bulk.