(560io) Design, Synthesis and Evaluation of Ir@Pt Bimetallic Catalysts for High Temperature Decomposition of so3 to so2 in the Hys Process for Thermochemical Water Splitting

One of the most promising new technologies for green energy production is development of an economic process for the solar driven Hybrid Sulfur (HyS) process. The key to success for this process is the high temperature, catalytic decomposition of sulfuric acid in the high temperature section of this process. After thermal volatilization and cracking of H₂SO₄ to form steam and SO₃, catalytic decomposition of SO₃ to SO₂ and O₂ must occur to regenerate SO₂ for the formation of H₂ from the electrolyzer portion of the process. Supported Pt catalysts are the most active of Group VIII metals for decomposition of SO₃ to SO₂ + ½ O₂. However, the reaction is conducted at temperatures ≥ 800°C and conventional, supported Pt catalysts typically undergo sintering and loss of activity after only several hours at reaction conditions. Thus, the reaction and entire process becomes economically unattractive. Most of the external energy input (about 78%) comes in thermal form and is exchanged between the high temperature thermal source and the sulfuric acid section of the HyS process. Only 22% of the external input is in electric form to drive the low temperature electrolysis section. Thus, the efficiency of this thermochemical process is primarily affected by the performance of the high temperature catalytic decomposition section and having an efficient high temperature decomposition section is critical to reduce H₂ production costs. In addition, the decomposition section also affects the capital cost of the HyS chemical plant itself. Recent studies have demonstrated that the cost of the decomposition reactor represents about 53% of the overall HyS plant cost.

We have found that selective placement of Pt by electroless deposition, either as a partial or complete shell, on higher surface free energy (SFE) metal cores such as Ir (SFE = 3230 ergs/cm²) results in active and stable Pt catalysts for SO₃ decomposition. The thermodynamic stabilization of the lower SFE Pt component (SFE = 2691 ergs/cm²) on higher SFE Ir cores helps to constrain diffusion of Pt to form larger Pt aggregates. Evaluation data for long term decomposition of a gas stream containing up to 95% H₂SO₄ will be presented. Also discussed will be the enhanced stability and catalyst performance of non-oxidic supports such as BN, which maintains its surface area and pore structures at temperatures as high as 900°C.