Effects of Surface Coordination on the Temperature-Programmed Desorption of Oxalate from Goethite

The temperature-programmed desorption (TPD) of electrostatically, hydrogen-, and metal-bonded oxalate complexes at the goethite surface was investigated with concerted Fourier transform infrared (FTIR) measurements (TPD-FTIR) to 660 K and mass spectrometer analyses of the evolved gases to 900 K. These reactions took place with the concomitant dehydroxylation reaction of goethite to hematite and decarbonation of bulk-occluded carbonate. The measurements revealed three important stages of desorption. Stage I (300−440 K) corresponds to the desorption of electrostatically and/or unbound oxalate molecules in the goethite powder with a thermal decomposition reaction pathway characteristic of oxalic acid. Stage II (440−520 K) corresponds to the desorption of hydrogen-bonded surface complexes due to the loss of surface hydroxyls at 3660 and 3550 cm -1, leading to a partial desorption via oxalic acid thermal decomposition pathways and to a partial conversion to metal-bonded surface complexes. Finally, Stage III (520−660 K) corresponds to the thermal decomposition of the metal-bonded oxalate complex, proceeding through a two-electron reduction pathway.