Supported molecular gold complexes and clusters: Synthesis, characterization, and catalysis

Molecular catalysts, ranging from mononuclear metal complexes to metal nanoclusters supported on oxides and zeolites, have surface catalytic groups that are close analogues of molecular complexes in solution with uniqueness of structure, that in prospect, could promote high activity and selectivity with robustness in operation, sometimes the properties of these molecular catalysts differ from those of conventional catalysts. Supported single-site metallocenes for alkene polymerization, platinum nanoclusters supported in zeolite LTL for naphtha dehydrocyclization, and highly dispersed supported gold nanoclusters for CO oxidation and propylene epoxidation are technological examples of these supported molecular catalysts.; Gold complexes and clusters on the surface of γ-Al₂O ₃, TiO₂, and MgO were prepared by adsorption of Au III(CH₃)₂(C₅H₇O₂ ) on the support surface followed by treatment in He or H₂ at various temperatures ranging from 273 to 573 K and 1 atm. The structures and chemical composition of these catalysts were determined spectroscopically in reactive atmospheres and during catalysis, primarily by infrared (IR), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) spectroscopies, temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO). The catalytic activity of the supported gold species was tested for CO oxidation and ethylene hydrogenation.; A structural model of a MgO-supported complex AuIII(CH ₃)₂{lcub}OMg{rcub}₂ (where the {lcub}OMg{rcub} ligands are part of the MgO support) has been established based on distances and coordination numbers determined by EXAFS spectroscopy. This compound is the precursor of a mononuclear gold complex that catalyzes ethene hydrogenation. For comparison, a family of supported gold clusters was also prepared. Furthermore, the surface chemistry for the formation of a related sample, {lcub}AlO)₂-Au III(CH₃)₂, was established on the basis of IR, EXAFS, XANES spectroscopies during the reaction of AuIII(CH ₃)₂(C₅H₇O₂) with both surface OH groups and coordinatively unsaturated aluminum sites of the support surface.; Data obtained during steady-state CO oxidation indicate the presence of gold clusters with an average diameter of about 30 Å, regardless of the initial size of the supported clusters. The XANES results demonstrate the simultaneous presence of both zero-valent and cationic gold in these catalysts during CO oxidation.