On the distribution of oxygen-containing surface groups in carbons and their influence on the preparation of carbon-supported molydenum catalysts

Abstract

Activated carbon was modified by thermal treatment in order to obtain carbon samples with a very different surface chemistry and similar textural properties. The distribution of oxygen-containing surface groups in these carbons was assessed. The difference between the point of zero charge and the isoelectric point (pH PZC−pH IEP) showed the same trend as the difference between the oxygen content on the external surface, determined by XPS, and that in the bulk, obtained by chemical analysis (%O surface−%O bulk). Thus, pH PZC−pH IEP can be used as a parameter for indicating the radial distribution of oxygen-containing groups in carbons. Carbon samples were loaded with molybdenum, using the procedure of incipient-wetness impregnation. Hydrophilicity conferred by the oxygen functionalities to the carbon substrates promotes the formation of small, well dispersed crystals of the metal precursor on the support. Results seem to indicate that oxygen-containing surface groups work as anchorage centres for molybdenum complexes. The activated catalyst prepared with the most acidic carbon showed a higher amount of molybdenum than in the case of the other carbon supports. This may be due to the presence of metal sulphides that are bound to the oxygen groups of the support.