A lean NO x trap (LNT) catalyst has been widely used for removing NO x exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NO x removal efficiency during lean-rich cycle operation, a Cu/CeO2 (CC) catalyst was added to a Pt-BaO/CeO2 (PBC) catalyst. In comparison to PBC- or CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NO x storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt-BaO-Cu/CeO2 (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NO x removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NO x storage on PBC. Under rich conditions, H2 was generated in situ on the CC by a water-gas shift reaction then accelerated the reduction of NO x, which had been stored on PBC, with a higher selectivity to N2. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.