The new concept 3-way catalysts for a lean-burn engine have been developed, and their NOx purification mechanisms have been studied. The catalysts consist of precious metals, aluminum oxide and some other metal compounds such as NOX storage compounds. NOx is oxidized over the precious metals and stored as nitrate ion combined with NOx storage compounds under oxidizing conditions. The stored NOx is reduced to N-2 under stoichiometric and reducing conditions. The NOx storage capacity is deteriorated by sulfur. The improved catalysts showed sufficient NOx conversion durability in the Japanese 10-15 mode test.