Physicochemical characterization of carbon-coated alumina

A commercial alumina powder was coated with carbon via deposition from liquefied petroleum gas (LPG) at 800°C. The chemical and thermal properties of the resulting material were evaluated via (a) the selective neutralization of surface groups; (b) electrophoresis; (c) mass titration; (d) thermogravimetric analysis; (e) carbon gasification reactivity in air; and (f) temperature-programmed desorption (TPD) and oxidation (TPO). The air-exposed material was found to contain predominantly basic and weakly acidic surface sites, in the latter case mainly through the contribution of surface hydroxyl groups on uncoated Al₂O₃. These groups desorb as H₂O upon heating and lead to an overestimate of carbon deposition from weight loss data. Temperature-programmed oxidation estimates of carbon deposition are more consistent with those of other techniques after correction for heteroatom (O, H) content of the deposit. The deposit was thus found to comprise ca. 0.30% wt. of a material having an empirical formula close to CH_0.4O_0.115 and covering <30% of the Al₂O₃ surface. Preferential coating of the inner pore walls by deposited material (which rendered the internal surface of Al₂O₃ basic) is inferred from a combination of electrophoresis and mass titration data. In addition, the appearance of a low- (420°C) and a high-temperature (580°C) CO₂ peak upon TPO is ascribed, in the absence of metallic impurities, to carbon deposits on Lewis acid sites distant from or close to electron-withdrawing (e.g., chlorine) ions, respectively.