Sulfur-compounds are ubiquitous in fossil fuels processing. In addition to their potential for adverse environmental impact, sulfur-compounds poison catalysts and foul process equipment. There is, therefore, significant interest in development of efficient catalyst-sorbents for their capture and conversion. Activated carbons are well known for their ability to adsorb sulfur-compounds; both morphology and surface chemistry contribute to their performance. However, primarily because control and characterization of activated carbon's surface properties are difficult, the interactions between sulfur-compounds and carbon surfaces are not understood at a fundamental level that enables rational design of new materials.

In this work we used ultra-high vacuum (UHV) techniques to carefully prepare and thoroughly characterize amorphous carbon (a-C) thin films as models of activated carbon sorbent-catalysts. Films with controlled and defined microstructure (sp2/sp3 bonding ratio), surface chemistry (dangling bonds, oxidation state), and morphology (roughness, porosity) were prepared 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 the films. For comparison, we performed similar experiments on a highly oriented pyrolytic graphite (HOPG) surface.

Water desorbs from both the a-C and HOPG surfaces with zero-order kinetics and desorption energy, E_des, that increases with coverage, indicative of initial condensation via a cluster growth mechanism. On the a-C surface, SO₂ and H₂S both display TPD features consistent with first-order desorption from both mono- and multilayers. At sub-monolayer coverages, SO₂ (E_des ~ 10 kcal/mol) interacts more strongly with a-C than H2S (E_des ~ 8 kcal/mol ). XPS analysis after TPD experiments on HOPG and a-C films provided no evidence of reaction between the carbon surfaces and the adsorbates. On the other hand, tightly bound residual sulfur was found on the surfaces of oxidized films, a-COx, after H₂S TPD, suggesting that chemical reaction between H₂S and the surface had occurred. This result illustrates the potential for chemical modification of the surface for altering adsorption/reaction pathways.

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