Carbon monoxide-selective oxidation catalysts

A method of removing carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NX) from an exhaust stream of an internal combustion engine that is powering an automotive vehicle, the engine being configured and controlled to primarily operate fuel-lean, but also to periodically operate fuel-rich to generate ammonia (NH3) in the exhaust stream, the method comprising:

• controlling the engine during fuel-rich operation so that the exhaust stream from the engine contains nitrogen oxides (NX), carbon monoxide (CO), and hydrogen (H2), but little or no oxygen (O2);
• passing the exhaust stream from the engine in contact with particles of a platinum group metal (PGM) catalyst to convert an amount of NX, CO, and/or H2 in the exhaust stream to ammonia (NH3);
• and thereafter
• adding oxygen to the exhaust stream and passing the exhaust stream in contact with particles of a CO-selective oxidation catalyst to oxidize CO in the exhaust stream to carbon dioxide (CO2) without oxidizing the NH3, the CO-selective oxidation catalyst comprising particles of mixed oxides of cerium (Ce), zirconium (Zr), and copper (Cu), the mixed oxides of cerium, zirconium, and copper consisting of CeaZrbCucOx, which is formulated such that a has a value in the range of 0.50 to 0.70, b has a value in the range of 0.10 to 0.20, and c has a value in the range of 0.15 to 0.35, with the sum of these molar ratios of Ce, Zr, and Cu being equal to 1, and the value of x is in the range of one to three.