Ab initio thermodynamics examination of sulfur species present on Rh, Ni, and binary Rh-Ni surfaces under steam reforming reaction conditions

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.