The interaction of surface sulfur with carbon on Fe(110)

The interaction of carbon and sulfur was studied on an iron single crystal with a (110) surface orientation. Sulfur was deposited from the decomposition of H2S at 150 °C and first formed a P(2×2) followed by a C(3×1) LEED pattern. The fully developed patterns correspond to 1/4 and 1/3 sulfur atom per surface iron atom. The ease of graphite formation from the dehydrogenation of acetylene as well as the decomposition of surface graphite are inhibited by the presence of a three-dimensional carbide, as compared to the initially clean surface. Once formed, surface graphite facilitates carbide dissolution. The formation and decomposition of surface graphite is inhibited by the presence of surface sulfur. Carbon surface segregation measurements at constant sulfur coverage were evaluated using the Langmuir–McClean equation. As the concentration of surface sulfur increases up to its saturation value of 1/3 monolayer, there is a corresponding decrease in the enthalpy of segregation of carbon. An order transition from a C(18×3) to a C(3×1) LEED pattern, concomitant with a discontinuous change in the concentration of carbon with temperature, is observed near 3/4 surface sulfur saturation, precluding analysis within an ideal solution framework.