Argo A M, Odzak J F, Lai F S, Gates B C
Department of Chemical Engineering and Materials Science, University of California, Davis 95616, USA.
Nature. 2002 Feb 7;415(6872):623-6. doi: 10.1038/415623a.
Homogeneous organometallic catalysts and many enzymes activate reactants through coordination to metal atoms; that is, the reactants are turned into ligands and their reactivity controlled through other ligands in the metal's coordination sphere. In the case of supported metal clusters, catalytic performance is influenced by the support and by adsorbed reactants, intermediates or products. The adsorbates are usually treated as ligands, whereas the influence of the supports is usually ascribed to electronic interactions, even though metal clusters supported on oxides and zeolites form chemical bonds to support oxygen atoms. Here we report direct observations of the structure of supported metal clusters consisting of four iridium atoms, and the identification of hydrocarbon ligands bound to them during propene hydrogenation. We find that propene and molecular hydrogen form propylidyne and hydride ligands, respectively, whereas simultaneous exposure of the reactants to the supported iridium cluster yields ligands that are reactive intermediates during the catalytic propane-formation reaction. These intermediates weaken the bonding within the tetrahedral iridium cluster and the interactions between the cluster and the support, while replacement of the MgO support with gamma-Al2O3 boosts the catalytic activity tenfold, by affecting the bonding between the reactant-derived ligands and the cluster and therefore also the abundance of individual ligands. This interplay between the support and the reactant-derived ligands, whereby each influences the interaction of the metal cluster with the other, shows that the catalytic properties of supported metal catalysts can be tuned by careful choice of their supports.
均相有机金属催化剂和许多酶通过与金属原子配位来活化反应物；也就是说，反应物转变为配体，其反应活性通过金属配位球中的其他配体来控制。对于负载型金属簇而言，催化性能受载体以及吸附的反应物、中间体或产物的影响。吸附物通常被视为配体，而载体的影响通常归因于电子相互作用，尽管负载在氧化物和沸石上的金属簇与载体氧原子形成化学键。在此，我们报告了由四个铱原子组成的负载型金属簇结构的直接观测结果，以及在丙烯加氢过程中与它们结合的烃类配体的鉴定。我们发现丙烯和分子氢分别形成次丙基和氢化物配体，而将反应物同时暴露于负载型铱簇会产生在催化丙烷形成反应中作为反应中间体的配体。这些中间体削弱了四面体铱簇内的键合以及簇与载体之间的相互作用，而用γ -Al₂O₃替代MgO载体可使催化活性提高十倍，这是通过影响反应物衍生配体与簇之间的键合，进而也影响单个配体的丰度来实现的。载体与反应物衍生配体之间的这种相互作用，即二者各自影响金属簇与对方的相互作用，表明通过仔细选择载体可以调节负载型金属催化剂的催化性能。
