The Formation of Isocyanic Acid and Ammonia during the Reduction of No over Supported Platinum Group Metals

摘要

As is well-known, ammonia is a product of reactions occurring during the reduction of NO over three-way vehicle exhaust catalysts based on Pt, Pd and Rh and such emissions have increased as engine control systems are now set to operate somewhat richer. The usual assumption has been that NH_3 is formed by the reaction of NO with H_2. However work over the last decade has established the existence of an alternative route in which NO is first converted to gaseous isocyanic acid (HNCO) over the platinum group metal according to NO + CO + 1.5H_2 → HNCO + H_2O (1) The HNCO then undergoes hydrolysis to NH_3 HNCO + H_2O → NH_3 + CO_2 (2) in a separate step on oxide supports such as Al_2O_3, CeO_2 etc. Silica is the only support not active for hydrolysis and high yields of HNCO can be observed directly by on-line FTIR when using a Pt/SiO_2 catalyst.