Effects of Ligands, Cluster Size, and Charge State in Gas-Phase Catalysis: A Happy Marriage of Experimental and Computational Studies

Since the seminal publication of Kappes and Staley in 1981 on “Gas-Phase Oxidation by Transition-Metal Cations” [ 1 ], various aspects of this topical problem have been addressed in numerous reviews [ 2 – 25 ]. The enormous interest is due to the fact that gas-phase studies on ‘isolated’ reactants provide an ideal arena for probing experimentally the energetics and kinetics of a chemical reaction in an unperturbed environment at a strictly molecular level without being obscured by difficult-to-control or poorly understood solvation, aggregation, counterions and other effects, thus providing an opportunity to explore the concept of single-site catalysts directly [ 26 – 33 ]. Further, in these experiments reactive intermediates can be characterized in detail, mechanisms uncovered, and questions addressed on how factors such as cluster size and dimensionality, stoichiometry, oxidation state, degree of coordinative saturation, aggregation, or charge state affect the outcome of a chemical process. Active or single-sites in heterogeneous catalysis are usually rather ill-defined and often characterized by dangling bonds, kinks, steps, defects, or nano-sized particles; probing them experimentally is all but trivial [ 31 , 32 , 34 ] and their identification constitutes one of the intellectual cornerstones in contemporary catalysis. As ‘naked’ gas-phase species are, in general, much more reactive than their condensed-phase counterparts, these studies will, in principle, of course never account for the precise kinetic and mechanistic details which prevail at a surface or in the condensed phase. Yet, complemented by appropriate computational studies, gas-phase experiments have proved meaningful, on the ground that they permit a systematic approach to address the above mentioned questions and provide a conceptual framework. The DEGUSSA process, that is the platinum-mediated coupling of CH 4 and NH 3 to generate HCN [ 35 ], may serve as a good example. Mass-spectrometry based experiments [ 36 , 37 ] suggested (i) the key role of CH 2 NH as a crucial transient, and (ii) the advantage of using a bimetallic rather than a pure platinum-based catalysts for the C–N coupling step in competition with undesired soot formation; the existence of CH 2 NH was later confirmed by in situ photoionization studies [ 38 ] and currently used catalysts contain silver–platinum alloys. Obviously, each and every information and insight that help to optimize or improve the often trial-and-error based strategies on catalyst developments [ 39 ] are highly welcome.