Oxidation of H_2S by coadsorbed oxygen on the a-Cr_2O_3(0001) surface

摘要

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 dean a-Cr_2O_3 (0001) was dissociative, populating the surface with chromyl groups (Cr=O) in the former case (corresponding to roughly 10 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 SQ_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.