Reconstructive adsorption of H Rh(110)—a multiphase study

• Published in: Vacuum 1990, Vol.41 (1), p.297-300
• Main Authors: Nichtl-Pecher, W, Oed, W, Landskron, H, Heinz, K, Müller, K
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemistry; Exact sciences and technology; General and physical chemistry; Metals. Metallurgy; Solid-gas interface; Surface physical chemistry
• ISSN: 0042-207X; 1879-2715
• DOI: 10.1016/0042-207X(90)90340-5

The adsorption of hydrogen on a clean rhodium(110) surface develops—with increasing coverage—a number of commensurate phases (1 × 3-H, 1 × 2-H, 1 × 3-2H, 1 × 2-3H, 1 × 1-2H). Low Energy Electron Diffraction (LEED) was used to investigate both the phase diagram and the position of adsorbate and substrate atoms. Hydrogen adsorption not only weakens the multilayer relaxation but also causes a slight reconstruction of the rhodium substrate which is reduced with increasing coverage. At low coverages hydrogen atoms occupy threefold co-ordinates sites with a Rh-H bond length of 1.84 Å ± 0.1 Å and a hydrogen radius of 0.54 Å ± 0.1 Å. At higher coverages repulsive interactions between adjacent hydrogen atoms make adatoms move up to allow for maximum horizontal distances. Coverage dependent transition temperatures and angular beam profile analyses allow the determination of the phase diagram. The critical temperatures for the primitive phases 1 × 3-H and 1 × 2-H are T C = 132 ± 5 K and T C = 150 ± 5 K, respectively. The 1 × 3-H phase transforms to lattice gas disorder through a continuous phase transition, whereas in the case of the 1 × 2-H phase, a reversible transition to an incommensurate phase is observed. For the non-primitive phases T C is near or above the desorption boundary and thermal desorption is a competing process to the order-disorder transition. Order-order transitions are of first order as observed for the transitions of the primitive phases 1 × 3-H to 1 × 2-H as well as for the non-primitive phases 1 × 3-2H to 1 × 2-3H.