In-situ ammonia generation over a three-way catalyst is an important part of the "passive ammonia SCR" technology being developed at GM Global R&D as a low-cost alternative to the current lean NO emission control technologies. The purpose of this laboratory reactor study is to clarify the reaction pathways and to identify/quantify the key controlling factors for the formation and destruction of ammonia over Pd-based three-way catalysts under realistic feedstream conditions simulating the exhaust from a lean gasoline engine operating at fuel-rich air-fuel ratios. In addition to the "full-feed" experiments, we also conducted some "simple-feed" experiments to better understand the NH3 formation process (via the NO + H2 reaction with and without CO) as well as the possible removal process of the NH3 formed (via the NH3 + 02 reaction or NH3 decomposition). Of particular interest in this study are the multiple roles that CO plays in both the formation and destruction of NH3. It is also found that the presence of Rh and/or Ce in a Pd-based three-way catalyst decreases the amount of NH3 formation significantly under fuel-rich (net-reducing) conditions of practical interest. (C) 2014 Elsevier B.V. All rights reserved.