Oxidation of H2S by coadsorbed oxygen on the α-Cr2O3(0001) surface

The interactions of H2S and oxygen have been explored on the α-Cr2O3(0001) surface using temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and sticking coefficient measurements. H2S adsorbs with near unity sticking on the clean α-Cr2O3(0001) surface at 125 K up to a coverage of ~ 1.6 ML (where 1 ML is defined as the surface areal density of Cr3+ sites). Reversible adsorption/desorption of H2S was evidenced in TPD by three desorption states evolving between 150 and 315 K. Although no S-containing decomposition products were observed in TPD, AES detected S on the surface after TPD indicating that some degree of irreversible decomposition occurred. The level of H2S decomposition on the clean surface was estimated to be between 0.2 and 0.5 ML using water TPD as an indicator of S site blocking. In contrast, preadsorbed O2 at three temperatures (125, 400 and 800 K) exerted drastic changes in the surface chemistry of H2S. At 400 and 800 K, O2 adsorption on clean α-Cr2O3(0001) was dissociative, populating the surface with chromyl groups (Cr = O) in the former case (corresponding to roughly 1 O per Cr3+ surface site) and resulting in a nearly complete O-termination sheet (~ 3 O per Cr3+) in the latter case. Little or no H2S chemistry was observed on the O-terminated surface based on TPD and AES. However, the availability of some Cr-coordination sites on the chromyl-terminated surface facilitated H2S adsorption and oxidation during TPD to SO2 (445–470 K) and H2O (320 K). Isotopic-labeling studies suggest that the oxygen atom in the water product originated from the dosed oxygen whereas that in the SO2 product came from the lattice. Similar results were obtained from H2S dosed on the surface pretreated with O2 at 125 K, where O2 adsorption was predominately molecular, except that S2 was also detected in TPD at 525 K and the amount of SO2 produced at 445 K decreased. These results suggest that atomically adsorbed oxygen effectively oxidized H2S to SOx surface species, but that molecularly adsorbed O2 was the key to the partial oxidation of H2S to elemental sulfur.