Surface‐Mediated Synthesis of Dimeric Rhodium Catalysts on MgO: Tracking Changes in the Nuclearity and Ligand Environment of the Catalytically Active Sites by X‐ray Absorption and Infrared Spectroscopies

Abstract

The preparation of dinuclear rhodium clusters and their use as catalysts is challenging because these clusters are unstable, evolving readily into species with higher nuclearities. We now present a novel synthetic route to generate rhodium dimers on the surface of MgO by a stoichiometrically simple surface-mediated reaction involving [Rh(C(2)H(4))(2)] species and H(2). X-ray absorption and IR spectra were used to characterize the changes in the nuclearity of the essentially molecular surface species as they formed, including the ligands on the rhodium and the metal-support interactions. The support plays a key role in stabilizing the dinuclear rhodium species, allowing the incorporation of small ligands (ethyl, hydride, and/or CO) and enabling a characterization of the catalytic performance of the supported species for the hydrogenation of ethylene as a function of the metal nuclearity and ligand environment. A change in the nuclearity from one to two Rh atoms leads to a 58-fold increase in the catalytic activity for ethylene hydrogenation, a reaction involving unsaturated, but stable, dimeric rhodium species.

New trends in tailoring active sites in zeolite-based catalysts

“…In some cases, however, this procedure may yield unpredictable results as the stoichiometry and structure of the formed cluster often depends on the nature of the metal, the metal ligands, and the support host. For example, Ir(C2H4)2 complexes are converted into Ir4 clusters in H2 at 80 °C on HY zeolite,267 while the use of Rh(C2H4)2 leads to a mixture of small Rh2-4 clusters on HY268 but neatly generates Rh2 clusters on MgO 269. Interestingly, it was possible to stabilize either the mononuclear Ir(C2H4)2 and Rh(C2H4)2 complexes or the metal Ir4 and Rh2 clusters in the supercages of HY by simply tuning the H2/C2H4 ratio in the reactive gas mixture (see further discussion in Section 3.2) 267,268,270.…”

Tracking Rh Atoms in Zeolite HY: First Steps of Metal Cluster Formation and Influence of Metal Nuclearity on Catalysis of Ethylene Hydrogenation and Ethylene Dimerization

“…There are still only a small number of catalysts in this class. [12][13][14][15][16] We now report supported metal catalysts consisting initially of isolated Rh centers on a support, zeolite HY, and its transformation in the presence of H 2 to form rhodium pair sites and then larger rhodium clusters. The data reported here indicate how the transformation of the rhodium species leads to changes in the catalytic properties of the rhodium, and they thus lay a foundation for how to tailor supported rhodium catalysts.…”

Single‐Site vs. Cluster Catalysis in High Temperature Oxidations

“…Previous work by Gates et al. illustrates the effect that a single metal–metal bond can exert on (hydrogenation) catalysis [23, 30–32] . A single Rh−Rh bond in MgO‐supported Rh dimers, for example, is responsible for ca.…”

“…A single Rh−Rh bond in MgO‐supported Rh dimers, for example, is responsible for ca. 60 times activity boost in ethylene hydrogenation when compared to single Rh atoms, due to the improvement in the rate of H 2 dissociation [32] . Interestingly, the effect is much less pronounced when the metal sits on an electron‐withdrawing support such as HY zeolite, because then the resultant electron‐deficient single Rh atoms activate H 2 more effectively, making the role of the metal‐metal bond secondary [31] .…”