Field ion microscopy study of the structural changes in Rh crystals during the reaction of oxygen-hydrogen gas mixtures

The interaction of oxygen/hydrogen gas mixtures with Rh crystals was studied by means of video-FIM (field ion microscopy) at temperatures between 400 and 500 K and total pressures up to 10(-2) Pa. Rh samples were found to suffer considerable morphological changes during the reaction at 505 K and relative gas amounts of H-2/O-2= 0.1...9. Strong faceting of planes lying along the (110) zone lines was seen and interpreted as being oxygen-driven. The surface structures changed while removing hydrogen from the gas mixture. In situ video-FIM allowed us to monitor the time dependence of changes. Accordingly, planes lying along the (100) zone lines transformed first from O-ad/H-ad into O-ad-covered. The process started in {011} planes and finally reached the (001) Dole. A cross-like image feature thus appeared and divided the apex surface of the Rh crystal into four equivalent quadrants with {111} planes in their center. These quadrants reacted subsequently in an independent manner, that is, the transformation from an O-ad/H-ad mixed layer into an O-ad layer occurred with a delay period of up to several minutes while decreasing the hydrogen gas pressure. The structural changes observed were reversible upon increasing the hydrogen gas pressure. (C) 2000 Elsevier Science B.V. All rights reserved.