Electronic and Geometric Effects on Chemical Reactivity of 3d-Transition-Metal-Doped Silver Cluster Cations toward Oxygen Molecules

Abstract

We report electronic and geometric structures of 3d-transition-metal-doped silver cluster cations, Ag N−1 M + (M = Sc−Ni), studied by chemical reaction with oxygen molecules. The evaluated reaction rate coefficients for small sizes, N, are 2− 6 orders of magnitude higher than those of undoped Ag N + , whereas those for large N are comparable with those of Ag N + .The low reactivity at large sizes is attributed to a geometric effect, that is, encapsulation of the dopant atom, which provides an active site located on the surface of the cluster in small sizes. In addition, a reactivity minimum is observed for Ag N−1 M + with M = Sc, Ti, V, Fe, Co, and Ni at a specific size, where the cluster possesses 18 valence electrons including 3d electrons. With the aid of density functional theory calculations, the reactivity minimum is suggested to be due to an electronic effect, that is, formation of a closed electronic shell by the 18 valence electrons, implying delocalized 3d electrons. Ag 13 Cr + and Ag 12 Mn + , possessing 18 valence electrons as well, are noted to be exceptions, where d electrons are supposed to be localized on the dopant atom because of the half-filled nature of Cr and Mn 3d orbital.

Reaction Kinetics of Nitric Oxide on Size-Selected Silver Cluster Cations

“…Finally, the overall reaction rate coefficient was evaluated for each size of Ag n + to show size dependence of the reactivity; the extinction rate constants of the reactant clusters were divided by the number density of NO in the ion trap under the assumption that the elementary reaction is first order for NO. Reaction rate coefficients thus obtained are plotted in Figure , along with reaction rate coefficients against O 2 that were previously measured . Note that the reactivities of Ag 3 + and Ag 5 + are so low that no product ion was observed in the present measurement, which is also the case for Ag 5 + , Ag 7 + , and Ag 9 + reacting with O 2 .…”

“…Reaction rate coefficients thus obtained are plotted in Figure 5, along with reaction rate coefficients against O 2 that were previously measured. 67 Note that the reactivities of Ag 3 + and Ag 5 + are so low that no product ion was observed in the present measurement, which is also the case for Ag 5 + , Ag 7 + , and Ag 9 + reacting with O 2 . In general, the reactivity toward NO was found to be about 3 orders of magnitude higher, except for n = 3 and 5, than that toward O 2 .…”

“…The experimental setup used for the reaction kinetics measurement has been described in detail elsewhere. 67 Briefly, Ag n + (n = 3−18) was generated by a magnetron-sputter cluster-ion source, where the sputtering of a silver plate (Toshima Manufacturing Co., Ltd.; 99.99%) was followed by aggregation of sputtered atoms and ions through collisions with a buffer helium gas (99.99995%) cooled by liquid nitrogen. After thermalization by collisions with a helium gas at liquid nitrogen temperature, the cluster cations were mass selected by a quadrupole mass filter (MAX-4000, Extrel CMS, LLC).…”

Exploring s–d, s–f, and d–f Electron Interactions in AgnCe+ and AgnSm+ by Chemical Reaction toward O2

“…Time-dependent measurements were performed to evaluate the depletion rate constant of the reactants, Ag n Ce + and Ag n Sm + , which was divided by the oxygen concentration to obtain a reaction rate coefficient in the same way as reported for 3d-transition-metal-doped clusters . The reaction rate coefficients thus obtained for Ag n Ce + and Ag n Sm + are shown in Figure a,b, respectively, along with those of undoped Ag n + .…”

“…In this context, we have reported systematic measurements of reactivity of the doped clusters against O 2 ; oxygen etching is frequently employed to examine the stability of the clusters. − When multiple-collision reaction experiments are performed, not only the reactivity but also successive elementary reaction processes can be elucidated. − We have found that the doped anionic clusters, Ag 14 Sc – , Ag 13 Ti – , and Ag 12 V – , as well as cationic clusters, Ag 16 Sc + , Ag 15 Ti + , Ag 14 V + , Ag 11 Fe + , Ag 10 Co + , and Ag 9 Ni + , exhibit a reactivity minimum in the size-dependent measurement of reaction rate coefficients along with reaction kinetics; − the low reactivity suggests that these clusters are stable due to the 18-electron shell closure including delocalized 3d electrons of the dopant atom. In contrast, Ag 9 Fe – , Ag 8 Co – , and Ag 7 Ni – , possessing an exohedral dopant geometry, were highly reactive even with 18 valence electrons, which suggests that an endohedral geometry plays a key role in 3d delocalization …”