Promoted Catalysis by Supported [Ru6N] Clusters in N2 and/or H2: Structural and Chemical Controls

Abstract

Catalysis (ammonia synthesis) on supported nitrido clusters [Ru~N] was investigated as a potential catalysis system of transition metal + main group element in relation to the framework structure (change) of [Rud] clusters in HZ or NZ as found in the accompanying paper. The [RU6N@-O)su)3] (Os,, oxygen atom at surface) clusters were prepared from the [RugN(C0)16]- cluster on MgO, K+-doped MgO, and Cs+-doped MgO, and the stability in reaction conditions of ammonia synthesis was probed by EXAFS (extended X-ray absorption fine structure). The reaction rates on these nitrido clusters were found to be faster than non-nitrido [Ru~] clusters prepared from [RU~(CO)~~]~-, degraded [Ru3@2-Osu)3] clusters or aggregated Ru clusters (NR-R,, = 6.2-6.6) prepared from [RugN(C0)16]-, or conventional Ru catalysts. Also, the H2-D2 exchange reactions (in the presence/absence of N2) proceeded faster on supported [Ru~N] clusters than the other catalysts. The Ru wt % dependence of ammonia synthesis activities on [RusN]/MgO suggested the importance of the Ruhexamer ensemble and cluster/support interface for the catalysis. Related to the coordination structures of H or NZ and structure changes of the [Rud] framework in H2 or N2 in the accompanying paper, the promoted reaction mechanism of ammonia synthesis on supported [Rua] clusters is discussed in terms of (1) nuclearity of Ru, (2) clustedsupport interface, (3) structural effect through expansionkontraction of the [RM] framework, and (4) electron donation by nitrido nitrogen, based on in-situ EXAFS, in-situ IR, H2-D2 exchange reactions, reaction orders, and WD isotope effects.