(113h) Perovskite Sorbents for Removal of Sulfur, Arsenic and Phosphorus

Solid sorbents are being developed to reduce sulfur, arsenic, and phosphorus concentrations in synthesis gas and water-gas shift mixtures from parts per million by volume to at most 1?2 parts per billion by volume in the presence of steam. To prevent poisoning of nickel catalysts used in solid-oxide fuel cells, concentrations of sulfur likely must be reduced to below 60 ppbv; arsenic to below 5 ppbv and phosphorus to below 1?2 ppbv. The platinum catalysts of proton exchange membrane (PEM) fuel cells are even more susceptible to poisoning, unlikely to withstand greater than 4 ppbv of sulfur and 200 ppbv of carbon monoxide. The PEM fuel cells require hydrogen of the highest purity?likely requiring hydrogen purification membranes, which in turn require protection from sulfur, arsenic, and phosphorus. Commercial palladium-silver membranes are poisoned by H₂S concentrations as low as 4 ppbv?forming a bulk Pd₄S. The sorbents under development bind sulfur more tightly relative to conventional zinc- or iron-based sorbents, thus greatly reducing the vapor pressures of H₂S and COS. In addition, these sorbents are designed to form some of the most thermally stable phosphates and arsenates known?greatly reducing the vapor pressures of elemental arsenic and vapor-phase species such as HPO₂ and HPO₃. These polishing sorbents are designed to operate at 300°C, which complements other bulk sorption, water-gas shift, and palladium membrane technologies. Thermogravimetric analysis shows that preferred sorbents capture over 7 wt% sulfur at 300°C and that sorbents can be regenerated by temperature-swing desorption at 500°C under H₂, through 20 cycles. Sorbents can also be reprocessed by heating in air at 1200°C to remove sulfur. Breakthrough tests under simulated sulfur-contaminated water-gas shift conditions including steam indicate total sulfur levels below the current limits of detection (sub-70 ppbv). Calculations indicate that strong binding of sulfur will reduce the equilibrium partial pressure of H₂S even in the presence of high-pressure steam to immeasurably low concentrations. Control breakthrough experiments using commercial zinc-based sorbents under identical conditions show total sulfur concentrations well above 150 ppbv?indicating that the Eltron Research & Development sorbents are superior to zinc-based sorbents when steam is present.