Identifying the functions of nickel in the attenuation of H 2S emissions from three-way automotive catalysts

Abstract

The design of new automotive catalyst formulations requires development of laboratory tests which mimic the vehicle exhaust-gas fluctuations encountered by a three-way converter. Under specific vehicle operating conditions, an unmodified three-way catalyst (comprising precious metals on CeO 2-Al 2O 3) can emit a ‘spike’ of hydrogen sulphide. A laboratory test to emulate these spike conditions requires a gas-feed that contains SO 2, a sudden change in gas composition from fuel-lean (oxidising) to fuel-rich (reducing), and an operating temperature > ca.500°C. The emission of H 2S can be prevented by adding a nickel-containing precursor to the catalyst. Using a simplified gas mixture (H 2S/N 2) it can be shown that surface NiO is capable of acting as an ‘H 2S getter’ and that it can be reactivated by exposure to an oxidising atmosphere. However, tests in which spike conditions have been emulated do not show a simple correlation between attenuation and sulphur retention by NiO. The results are considered in the light of a recent computational study which indicated that NiO should form NiSO 4 under lean conditions, and then release oxygen (or consume hydrogen) when the gas-stream becomes reducing. We conclude that nickel-containing automotive catalysts can attenuate H 2S in the following ways: 1. by preventing its formation; 2. by trapping it once it appears in the gas-phase.