The stable form of adsorbed sulfur species and their coverage were investigated on Rh, Ni, and Rh-Ni binary metal surfaces using density functional theory calculations and the ab initio thermodynamics framework. S adsorption, SO x (x = 1-4) adsorption, and metal sulfide formation were examined on Rh(111) and Ni(111) pure metals. Both Rh and Ni metals showed a preference for S surface adsorption rather than SO x adsorption under steam reforming conditions. The transition temperature from a clean surface (< 1/ 9 ML) to S adsorption was identified on Rh(111), Ni(111), Rh 1Ni 2(111), and Rh 2Ni 1(111) metals at various P(H 2)/P(H 2S) ratios. Bimetallic Rh-Ni metals transition to a clean surface at lower temperatures than does the pure Rh metal. Whereas Rh is covered with 1/ 3 ML of sulfur under the reforming conditions of 4-100 ppm S and 800 °C, Rh 1Ni 2 is covered with 1/ 9 ML of sulfur at the lower end of this range (4-33 ppm S). The possibility of sulfate formation on Rh catalysts was examined by considering higher oxygen pressures, a Rh(221) stepped surface, and the interface between a Rh 4 cluster and CeO 2(111) surface. SO x surface species are stable only at high oxygen pressure or low temperatures outside those relevant to the steam reforming of hydrocarbons.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Surfaces and Interfaces