Twenty-five atomic cations, M+, that lie within the thermodynamic window for O-atom transport catalysis of the oxidation of hydrogen by nitrous oxide, have been checked for catalytic activity at room temperature with kinetic measurements using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Only 4 of these 25 atomic cations were seen to be catalytic: Fe+, Os+, Ir+, and Pt+. Two of these, It(+) and Pt+, are efficient catalysts, while Fe+ and Os+ are not. Eighteen atomic cations (Cr+, Mn+, Co+, Ni+, Cu+, Ge+, Se+, Mo+, Ru+, Rh+, Sn+, Te+, Re+, Pb+, Bi+, Eu+, Tm+, and Yb+) react too slowly at room temperature either in their oxidation with N2O to form MO+ or in the reduction of MO+ by H-2. Many of these reactions are known to be spin forbidden and a few actually may lie outside the thermodynamic window. Three alkaline-earth metal monoxide cations, CaO+, SrO+, and BaO+, were observed to favor MOH+ formation in their reactions with H-2. A potential-energy landscape is computed for the oxidation of H-2 with N2O catalyzed by Fe+(D-6) that vividly illustrates the operation of an ionic catalyst and qualitatively accounts for the relative inefficiency of this catalyst.