Typical supported metal catalysts consist of nonuniform nano-structures dispersed on supports. The nonuniformity has led to difficulties in the determination of structure-catalytic property relationships and understanding of the catalysts.  In contrast, supported catalysts with virtually molecular structures, incorporating supported metal complexes, offer the advantages of simplicity and, in prospect, uniformity of structure to facilitate fundamental understanding.  Our goals were to prepare and investigate well-defined and nearly uniform supported rhodium complex that is an active and stable catalyst and to determine relationships between catalyst structure (including the structure of the metal–support interface) and activity for a simple test reaction, hydrogenation of ethylene.  The supported rhodium complex was synthesized from Rh(C₂H₄)₂(acac) [acac is the bidentate ligand CH₃COCHCOCH₃] and highly dealuminated zeolite Y.  The supported sample was characterized by IR, EXAFS, and ¹³C MAS NMR spectroscopies, both in the as-synthesized form and in the working state. 

The EXAFS spectrum of the as-synthesized sample gives no evidence of a Rh–Rh contribution, consistent with the presence of the rhodium as site-isolated mononuclear species, consistent with the IR data.  The EXAFS data show that the Rh center was bonded to the support via 2 Rh–O bonds; the distance is 2.19 Å, consistent with a strong polar bond.  The EXAFS Rh–C contribution (coordination number N_Rh-C = 3.9, distance R_Rh-C = 2.09 Å) indicates two ethylene ligands bonded to the Rh atoms, in agreement with the IR spectra.  Thus, the supported rhodium complex is among the best-defined supported metal species.

When the initially formed sample was treated in flowing H₂ at 298 K, gas was evolved and analyzed with a mass spectrometer; the data indicated ethane formation even in the absence of ethylene in the feed.  This result indicates that ethylene ligands on the rhodium were hydrogenated on the site-isolated Rh centers bonded to the support; these ligands are candidate reaction intermediates.  The supported Rh complex was tested as a catalyst for ethylene hydrogenation at 298 K and 760 Torr in a flow reactor/EXAFS cell.  The EXAFS spectrum after the H₂ treatment indicates that the rhodium was slightly aggregated, forming clusters with a Rh–Rh bond distance of 2.66 Å and coordination number of 2.3.  The rhodium in these minute clusters was bonded to two oxygen atoms of the support (N_Rh–O = 2.0, R_Rh–O = 2.10 Å).  EXAFS spectra of the working catalyst in ethylene + H₂ show that the clusters had fragmented to again form mononuclear rhodium complexes; no Rh–Rh contribution was found, and the data indicated only bonding of the rhodium to the support and to hydrocarbon ligands (N_Rh–O = 2.1, R_Rh–O = 2.06 Å; N_Rh–C = 2.9, R_Rh–C = 2.25 Å).