Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

Isomers and electronic states of Ni2O2H2 and evaluation of the effect of charge on the electronic properties and reactivity of Ni2O2

Different isomers of Ni2O2H2 are investigated by multireference configuration interaction (MRCI) calculations, based on complete active space self-consistent field (CASSCF) calculations. The lowest-lying Ni2(OH)2 isomer has a rhombic shape with two OH(-) groups bridging two Ni(I) ions. Its ground term is a (1)Ag term. At a relative energy of 1.06 eV, there is a chain-like NiONi(OH2) isomer. A rhombic (NiH)2O2 isomer with Ni-H bonds has a considerably higher energy of 2.93 eV. Both Ni2(OH)2 and NiONi(OH2) feature a large number of low-lying electronic terms that in the case of Ni2(OH)2 form Heisenberg spin ladders due to the coupling of the electrons of two Ni(I) ions (3d(8)4s(1)) with S = 3/2. For the reaction Ni2O2 + H2 → Ni2(OH)2, the reaction energy is estimated to -2 eV. Finally, neutral and charged Ni2O2 and their hydrogenation products (Ni2O2H2(0/+)) are compared.

Cyclic and linear NiO2: a multireference configuration interaction study

Linear (ONiO) and triangular (Ni(O2))isomers of NiO2 are investigated by multiconfiguration self-consistent field (MCSCF) and multireference configuration interaction (MRCI) calculations. For ONiO, the ground electronic term is a 1Σg+ term. The lower-lying excited terms are 3Πg, 1Πg, and 5Πu at relative energies of 0.55, 0.95, and 1.20eV, respectively. For Ni(O2), the ground electronic term is a1A1 term with an energy of 1.53 eV with respect to the ONiO ground state. Lower-lying excited terms are 5B2, 5A1, and 3B2 at 0.58, 0.62, and 0.73 eV with respect to the 1A1 state, respectively.A transition structure between the ground states of both isomers has been located with an energy of 2.76 eV above the ONiO ground state. For the fragmentation Ni(O2) → Ni + O2 the (electronic) reaction energy is estimated to 1.15 eV. The wave function based results demonstrate the failure of previous density functional investigations and emphasize the importance of a multireference treatment.