The Os+-catalytic reduction of N2O by H2 in gas phase has been theoretically investigated with B3LYP method.The reaction mechanisms on the sextet and quartet surfaces were found to be similar.The calculated sextet potential energy profiles show that the two reactions involved in the catalytic cycle,Os+ + N2O → OsO+ + N2 and OsO+ + H2 → Os+ + H2O,have barriers of 28.3 and 123.3 kJ/mol,respectively.In contrast,the reactions on the quartet surfaces are energetically much more favorable.These results rationalize the experimentally observed low catalytic reactivity of sextet(ground-state) Os+.Further,the crossing between the sextet and quartet surfaces are also suggested and qualitatively discussed. The Os + -catalytic reduction of N2O by H2 in gas phase has been theoretically investigated with B3LYP method. The reaction mechanisms on the sextet and quartet surfaces were found to be similar. The calculated sextet potential energy profiles show that the two reactions involved in the catalytic cycles, Os + + N2O → OsO + + N2 and OsO + + H2 → Os + + H2O, have barriers of 28.3 and 123.3 kJ / mol, respectively.In contrast, the reactions on the quartet surfaces are energetically much more favorable. These results rationalize the experimentally observed low catalytic reactivity of sextet (ground-state) Os + .Further, the crossing between the sextet and quartet surfaces are also suggested and qualitatively discussed.