Metal oxide-supported cluster catalysts derived from organometallic precursors: Spectroscopic characterization in reactive environments

Metals are among the most important catalysts. Catalysis of CO oxidation by metals is important for emissions control and potentially for fuel cell applications. Bimetallic catalysts have found useful applications by improving the selectivity or lifetime over what is attainable with monometallic catalysts. This thesis reports an evaluation of temperature-programmed desorption for characterizing dispersed bimetallic catalysts; characterization of structurally simple site-isolated iridium oxide and rhodium oxide species made from supported clusters; and investigation of site-isolated iridium clusters and rhodium clusters, Ir₄ and Rh₆, as well as the oxide species formed from them as CO oxidation catalysts. The highly dispersed supported bimetallic catalysts were prepared from organometallic precursors incorporating the oxophilic tungsten bonded to the noble platinum. Site-isolated iridium oxide and rhodium oxide species were made by treatment in O₂ of nearly uniform supported metal clusters, synthesized from metal carbonyl precursors, Ir₄, Ir₆, and Rh₆, supported on MgO and on Al₂O₃.; X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to show that ensembles of site-isolated oxide complexes were made from Ir₄ and Rh₆ clusters supported on MgO; the metal clusters could be reformed from the ensembles. The rhodium samples were found to be active for CO oxidation catalysis, exhibiting light-off behavior at temperatures between 398 and 473 K. The order of activity of the rhodium species is Rh(0) > Rh(I) >> Rh(III), corresponding to zerovalent rhodium clusters, rhodium centers associated with gem-dicarbonyl and oxide species similar to those in the ensembles of oxide complexes, respectively. Light-off behavior was also observed for the iridium samples at temperatures between 473 and 548 K. The oxide species were found to be inactive for CO oxidation catalysis, but zerovalent iridium is active; Ir(I) centers cannot be ruled out as catalytically active species.