Cryo kinetics of N₂ adsorption onto bimetallic rhodium-iron clusters in isolation

We report the N₂ cryo adsorption kinetics of selected gas phase mixed rhodium-iron clusters [Rh_iFe_j]⁺ in the range of i = 3-8 and j = 3-8 in 26 K He buffer gas by the use of a cryo tandem RF-hexapole trap-Fourier transform ion cyclotron resonance mass spectrometer. From kinetic data and fits, we extract relative rate constants for each N₂ adsorption step and possible desorption steps. We find significant trends in adsorption behavior, which reveal adsorption limits, intermittent adsorption limits, and equilibrium reactions. For those steps, which are in equilibrium, we determine the Gibbs free energies. We conclude on likely ligand shell reorganization and some weakly bound N₂ ligands for clusters where multiple N₂ adsorbates are in equilibrium. The relative rate constants are transferred to absolute rate constants, which are slightly smaller than the collision rate constants calculated by the average dipole orientation (Langevin) theory. The calculated sticking probabilities increase, in general, with the size of the clusters and decrease with the level of N₂ adsorption, in particular, when reaching an adsorption/desorption equilibrium. We receive further evidence on cluster size dependent properties, such as cluster geometries and metal atom distributions within the clusters through the accompanying spectroscopic and computational study on the equiatomic i = j clusters [Klein et al., J. Chem. Phys. 156, 014302 (2022)].