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 H2/O2=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 Oad/Had into Oad-covered. The process started in {011} planes and finally reached the (001) pole. 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 Oad/Had mixed layer into an Oad 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.