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

The interaction of oxygen/hydrogen gas mixtures with Rh crystals was studie d 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 suf fer considerable morphological changes during the reaction at 505 K and rel ative gas amounts of H-2/O-2= 0.1...9. Strong faceting of planes lying alon g the (110) zone lines was seen and interpreted as being oxygen-driven. The surface structures changed while removing hydrogen from the gas mixture. I n situ video-FIM allowed us to monitor the time dependence of changes. Acco rdingly, planes lying along the (100) zone lines transformed first from O-a d/H-ad into O-ad-covered. The process started in {011} planes and finally r eached the (001) Dole. A cross-like image feature thus appeared and divided the apex surface of the Rh crystal into four equivalent quadrants with {11 1} planes in their center. These quadrants reacted subsequently in an indep endent manner, that is, the transformation from an O-ad/H-ad mixed layer in to an O-ad layer occurred with a delay period of up to several minutes whil e decreasing the hydrogen gas pressure. The structural changes observed wer e reversible upon increasing the hydrogen gas pressure. (C) 2000 Elsevier S cience B.V. All rights reserved.