Paper SS1-ThA9
The Adsorption and Desorption of H₂O, H₂S and SO₂ on Amorphous Carbon Films

Thursday, October 18, 2007, 4:40 pm, Room 608

There is a renewed interest in the development of efficient catalyst-sorbents for the capture and conversion of sulfur containing compounds that poison the catalysts and separation membranes, often used in modern processes for hydrogen generation. It is well known that sulfur-containing molecules interact with activated carbon surfaces by both physisorption and chemisorption, where surface chemistry and pore structure appear to contribute to the strength of the interaction. However, because it is difficult to control and characterize their surface properties, the fundamental nature of the adsorption onto activated carbons is still not well understood. In this work, we obtain precise control of surface properties by modeling activated carbons with amorphous carbon (a-C) films deposited under ultra high vacuum conditions. Carbon films with controlled and defined microstructure (sp²/sp³ ratio), surface chemistry (dangling bonds, oxidation state), and morphology (roughness, porosity) were deposited by DC magnetron sputtering. Temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) were used to study H₂O, H₂S and SO₂ surface chemistry on these films. For comparison, experiments were also carried out on a highly oriented pyrolytic graphite (HOPG) surface. H₂O, H₂S and SO₂ physisorb on HOPG and on a-C films by a cluster growth mechanism. At lower coverages, the molecules adsorbed in the first two-dimensional layer and on the edges of the three dimensional (3D) islands have a weaker interaction (lower binding energy) than molecules within 3D islands. Water desorbs from a-C with one peak, while H₂S and SO₂ desorb with two features, corresponding desorption from the monolayer and the multilayer. The three adsorbates interact with the a-C surface with different strengths: at low coverage, energies of desorption, E_des, from a-C films were 11.3, 8.2 and 10.2 kcal/mol for H₂O, H₂S and SO₂, respectively. XPS analysis after TPD experiments on HOPG and a-C films revealed no reactions between the carbon surfaces and the adsorbates. On the other hand, H₂S adsorption on oxidized films, a-CO_x, revealed the presence of residual elemental sulfur, thereby indicating that the oxygen of the films had reacted with the adsorbate.