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Interactions of SO2 and H2S with Modified Amorphous Carbon Films

W. Michalak, E. Broitman, J.B. Miller, and A.J. Gellman. Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

In addition to their potential for adverse environmental impact, sulfur-compounds commonly encountered in processing of fossil fuels can poison catalysts and foul process equipment. Therefore, there is significant interest in development of efficient processes 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, 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 use 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 modified surface chemistries were prepared by post-deposition oxidation of a sputtered carbon film (a-COx) and by sputtering in the presence of N2 (for a-CNx) or methane (for a-CHx). Temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) were used to study H2S and SO2 surface chemistry on the films. For comparison, we performed similar experiments on a highly oriented pyrolytic graphite (HOPG) surface.

SO2 and H2S readily adsorb on the unmodified a-C surface at low temperature in UHV. In TPD experiments, both desorb with first-order kinetics and desorption energies (Edes) that are higher than their respective heats of sublimation. The chemically modified a-COx surface adsorbs less H2S and SO2 than a-C, but desorption kinetics and energetics are unaffected, suggesting a simple site blocking mechanism. In contrast, modification as a-CNx and a-CHx significantly alters the nature of the interactions between the adsorbed molecules and the carbon surface. SO2 continues to adsorb in significant quantities on these surfaces, but desorbs from both with zero order kinetics, indicative of adsorption as islands or droplets on the surfaces. H2S does not adsorb onto either a-CNx or a-CHx in appreciable quantities, mirroring the behavior of the inactive HOPG surface.

These results illustrate the potential for chemical modification of carbon surfaces for altering adsorption/reaction pathways.