(72d) Low Temperature Catalytic Hydrolysis of Carbonyl Sulfide (COS) during Hydrogen Production for Fuel Cell Applications

After hydrocarbon fuel is reformed (NG, LPG) or gasified (Liquid fuels), sulfur appears primarily as H₂S (10 ppmv for regulated diesel or gas – 10,000 ppmv from an oxygen blown coal gasifier) while presence of Carbonyl sulfide (COS) depends on operating conditions of the desulfurizer & an equilibrium between H₂S and CO/CO₂. Post reformer desulfurization is an important unit operation in fuel processing of these hydrocarbons during H₂ production for PEMFCs operated at low temperature (<100°C). Efficient H₂S removal can be achieved via highly dispersed ZnO supported on porous silica. COS is a relatively neutral molecule compared to highly acidic H₂S and ZnO based adsorbents suffer from almost no capacity for COS at ambient conditions. Another concern is that presence of COS is known to cause compliance-testing failures while using LPG as a raw material for H₂ production. Hydrolysis of COS to H₂S followed by H₂S adsorption is an attractive means to control the sulfur content of reformed/gasified fuel. Most studies are focused on hydrolysis at temperatures in excess of 200°C and COS concentration in excess of 5000 ppmv. The current work is, however, focused on COS removal via catalytic hydrolysis with 200 ppmv COS fed to the reactor operating at ambient (20°C) to mild temperatures (150 °C). These conditions were found to be close to industrially relevant conditions (~150 ppm & 30-60°C). As a result, there is need to develop novel materials to overcome the issue of initial catalyst activation and dissociative chemisorption responsible for active site blockage at ambient conditions. A series of catalysts consisting of Al₂O₃, TiO₂ and SiO₂ impregnated with promoter ions such as transition metals (Cu, Mn, Zn, Fe), alkaline metals (Na, K, Li), alkali earth (Mg, Ca, Ba), rare earth metals (Y, Gd, Nd, La) and Layered Double Hydroxides were tested in terms of their specific activity for COS hydrolysis at ambient conditions. GC-PFPD was used to estimate the H₂S and COS concentration simultaneously at the reactor outlet. Effect of COS concentration (200-1000 ppmv) and H₂O concentration in feed on hydrolysis rate are also discussed. Finally, in order to understand the deactivation mechanism of these catalysts the performance is monitored in presence of 10 ppmv H₂S in feed stream.