Acetone hydrogenation catalysis is important in applications such as heat pumps and fuel cells or in fulfilling the sizable demand for the product of selective acetone hydrogenation, 2-propanol. Reported herein is the discovery of a superior acetone hydrogenation catalyst--superior in terms of activity at low temperature, selectivity at complete conversion, and total catalyst lifetime. The new catalyst system consists of Ir(0)(n) nanoclusters plus HCl easily and reproducibly formed from commercially available [(1,5-COD)IrCl](2) under H(2). The resultant, room temperature, high activity, and highly selective (2/n)Ir(0)(n) plus 2HCl catalyst system hydrogenates acetone at 22 degrees C and 40 psig of H(2) pressure to 95% 2-propanol and the rest diisopropyl ether at 100% conversion with 16400 total catalytic turnovers and with an initial turnover frequency of 1.9 s(-1) at 22 degrees C. When molecular sieves are added, the catalyst system becomes even more selective and long-lived, providing the complete and selective conversion of acetone to 100% 2-propanol with 188000 total turnovers. Also reported are initial kinetic, D-labeling and other mechanistic studies, a summary section detailing the four main findings, the "green chemistry" aspects, and the current main drawback (a limited catalytic lifetime due to nanocluster precipitation) of the present invention. A review of the extensive literature of acetone hydrogenation is also tabulated as part of the Supporting Information.