Small metal clusters supporting catalysis
To an inorganic chemist, a metal cluster may be a compound with two or more metal atoms. For the researchers working on the EU-funded UΝΙΜETCLUST project, a metal cluster can be dispersed and stabilised on a support, such as a porous metal oxide. When metal clusters are bonded to oxide or zeolite supports, they may combine the advantages of solid catalysts with the selectivity of soluble molecular catalysts. This was the driving force for the development of metal clusters that incorporate uniform active sites so small they could be considered molecular in character. UNIMETCLUST scientists successfully prepared extremely small clusters of rhodium and iridium bonded to ethylene and other ligands on the surface of magnesium oxide (MgO) and dealuminated zeolite HY, which acted as supports with contrasting chemical reactivities. More importantly, they closely followed the first steps of the metal cluster formation with infrared and extended X-ray absorption fine structure spectroscopy. Investigations of the interaction between the metal catalysts and reactive gases — ethylene, 1,3-butadiene, carbon monoxide, dihydrogen (H2) and molecular oxygen (O2) — allowed the transformation of one metal species into others, and the investigation of the influence of the catalytic structure in the rate and selectivity of relevant chemical reactions. Scientists used a wide range of techniques, including infrared spectroscopy and high-resolution microscopy, to determine the exact structure and nature of the active sites. They used the findings to synthesise active sites of catalysts with tuneable catalytic characteristics. The preciseness of the synthesis method developed within the UNIMETCLUST project for supported rhodium and iridium catalysts enabled drastic changes in catalytic activity (with 58-fold increases) and/or selectivity (from virtually zero to almost 100 %) through well-defined changes in the structure of the active sites. Metal catalysts have already made a respectable showing in the chemical industry and became the most important catalysts for processes such as refining petroleum and converting automobile exhaust. With the recent UNIMETCLUST results, new exciting possibilities should open up for catalysis.
Keywords
Metal clusters, catalysis, compounds, metal atoms, metal oxide, zeolite, solid catalysts, soluble molecular catalysts, rhodium, iridium, reactive gases, petroleum, automobile exhaust
