Stabilization of Pt at the inner wall of hollow spherical SiO2 generated from Pt/hollow spherical SiC for sulfuric acid decomposition

Abstract

Catalysts for sulfuric acid (SA) decomposition, one of three reactions in Sulfur-Iodine (SI) cycle to produce hydrogen, should be active and stable up to 800–900 °C. Here, a SiC hollow sphere supported Pt catalyst (1 wt% Pt/hSiC) is prepared, and its catalytic activity and stability are monitored in SA decomposition at 850 °C. The initial SA conversion with the Pt/hSiC catalyst is ca. 80% at 850 °C and a GHSV of 76,000 mL/gcat/h. For comparison, a core-shell SiO2 supported Pt catalyst (1 wt% Pt/SiO2@mSiO2) is prepared and tested for the reaction. The core-shell SiO2 support has the structure of a dense core and a mesoporous shell. The initial SA conversion with the Pt/SiO2@mSiO2 catalyst is ca. 54% at 850 °C and a GHSV of 76,000 mL/gcat/h. The Pt/hSiC catalyst is transformed to the SiO2 hollow sphere supported Pt catalyst (Pt/hSiO2) within 6 h reaction. CO chemisorption and TEM analysis exhibit that Pt particles on the pristine and spent catalysts, pretreated at 850 °C, are encapsulated by SiC or SiO2 on the surfaces of SiC and SiO2 supports. When the encapsulated Pt particles are in contact with sulfuric acid vapor, the Pt particles are exposed to the reactants by the removal of SiO2 encapsulating Pt during the reaction. Pt particles at the outer wall of the pristine hSiC are partly lost via PtOx evaporation, while Pt particles at the inner wall of the hollow sphere supports are stabilized without the severe Pt loss and Pt sintering. In contrast, the Pt particles on SiO2@mSiO2 with the dense SiO2 core are severely lost via PtOx evaporation during the reaction resulting in severe Pt sintering. The high stability of Pt particles at the inner wall of the hollow support is attributed to the Pt encapsulation and Pt anchoring of the small Pt particles at the inner walls and the diffusion barrier role of the shell for the migration of Pt at the inner wall to the outer wall.