Oxidation of H 2S by coadsorbed oxygen on the α-Cr 2O 3(0001) surface

Abstract

The interactions of H 2S and oxygen have been explored on the α-Cr 2O 3(0001) surface using temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and sticking coefficient measurements. H 2S adsorbs with near unity sticking on the clean α-Cr 2O 3(0001) surface at 125 K up to a coverage of ~ 1.6 ML (where 1 ML is defined as the surface areal density of Cr 3+ sites). Reversible adsorption/desorption of H 2S 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 H 2S 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 O 2 at three temperatures (125, 400 and 800 K) exerted drastic changes in the surface chemistry of H 2S. At 400 and 800 K, O 2 adsorption on clean α-Cr 2O 3(0001) was dissociative, populating the surface with chromyl groups (Cr = O) in the former case (corresponding to roughly 1 O per Cr 3+ surface site) and resulting in a nearly complete O-termination sheet (~ 3 O per Cr 3+) in the latter case. Little or no H 2S 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 H 2S adsorption and oxidation during TPD to SO 2 (445–470 K) and H 2O (320 K). Isotopic-labeling studies suggest that the oxygen atom in the water product originated from the dosed oxygen whereas that in the SO 2 product came from the lattice. Similar results were obtained from H 2S dosed on the surface pretreated with O 2 at 125 K, where O 2 adsorption was predominately molecular, except that S 2 was also detected in TPD at 525 K and the amount of SO 2 produced at 445 K decreased. These results suggest that atomically adsorbed oxygen effectively oxidized H 2S to SO x surface species, but that molecularly adsorbed O 2 was the key to the partial oxidation of H 2S to elemental sulfur.