Role of surface phosphorus complexes on the oxidation of porous carbons.

Abstract

Chemical activation of olive stone with phosphoric acid produces activated carbons with relatively high content of P surface groups that remain very stable on the carbon surface at relatively high temperatures. Changes in the surface chemistry of a phosphoric acid activated carbon after subjecting it to thermal treatments in oxidizing and inert conditions are studied by temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and adsorption/desorption of NH3. TPD and XPS results point out that P surface groups preferentially reacts with molecular oxygen, prior to carbon gasification, through the oxidation of C-P bond to form C-O-P ones, which are thermally stable at temperatures lower than 700 °C. At higher temperatures, these C-O-P type surface groups decompose to less oxygenated P groups on the carbon surface (of C-P type) generating CO (and CO2) in the gas phase. These C-P type surface groups seem to be very reactive and are (re)oxidized upon contact with air, even at room temperature, forming again C-O-P type groups. Thus, the presence of these oxygen-containing P surface groups with an interesting redox functionality of high chemical and thermal stability seems to be responsible of the high oxidation resistance and high oxygen content (once exposed to ambient air) of this type of porous carbons.