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

Abstract

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.

Complete cleavage of the N≡N triple bond by Ta2N+via degenerate ligand exchange at ambient temperature: A perfect catalytic cycle

“…For example, the Haber-Bosch (H-B) process operates at elevated temperature (300°C to 500°C) and relatively high pressure (200 to 350 atm), and including the energy-intensive production of H 2 from CH 4 , it has been estimated to consume 1 to 2% of the world's annual fossil energy output and generates about 670 million metric tons of the greenhouse gas CO 2 (4). Despite extensive experimental and theoretical studies (3,(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) that have been carried out in the century following the invention of the H-B process, only little has changed in industrial nitrogen activation chemistry.…”

“…Clearly, knowledge of the elementary steps that prevail at the molecular level is of crucial importance. In this context, metal clusters that bring about scission of the N≡N bond have served as prototypical systems to probe the active sites in heterogeneous catalysis (9,20,21). Indeed, recent combined experimental/ theoretical gas-phase studies provided a conceptual understanding of dinitrogen activation and conversion (22)(23)(24)(25).…”

Ta2+-mediated ammonia synthesis from N2and H2at ambient temperature

“…While some progress has been made on the daunting road to artificial nitrogen activation, the number of well-defined complexes that bind N 2 and ultimately, lead to a complete cleavage of the N≡N triple bond is rather limited so far. For instance, complexes with single (14-32) and multiple (15,(33)(34)(35)(36)(37)(38)(39) transition metal centers, small metal clusters (40)(41)(42)(43)(44)(45)(46)(47)(48), and also, main group compounds (49-52) have been found to be able to split dinitrogen. The activation of N-N bonds by oriented external electric fields (53,54) followed by insertion of, for example, a nitrogen atom in the C-C bonds of alkanes has been reported as well (55,56).…”

Low‐Energy Electronic Excitations of N‐Substituted Heteroacene Molecules: Matrix Isolation Spectroscopy in Concert with Quantum‐Chemical Calculations

“…The matrix isolation technique with noble gases is a powerful tool for studying the electronic structure and molecular properties to minimise extrinsic effects, such as solvation or aggregation . It allows for the application of standard spectroscopic methods, for example, IR, UV/Vis or Raman scattering measurements, that are not readily applicable in the gas phase.…”

“…The matrix isolationt echnique with noble gases is ap owerful tool for studying the electronic structure and molecular properties to minimise extrinsic effects, such as solvation or aggregation. [41][42][43][44] It allows for the application of standard spectroscopic methods, for example, IR, UV/Vis or Raman scattering measurements, that are not readily applicable in the gas phase. The most useful features of absorbance measurements on substances trapped in solid noble-gas matrices include narrowb and widths and the avoidance of hot bands.…”