Nanoclusters of Iridium Oxide and of Rhodium Oxide Supported on MgO

Abstract

MgO-supported Ir 4 and Rh 6 , prepared from [Ir 4 (CO) 12 ] and [Rh 6 (CO) 16 ], respectively, on MgO, were partially oxidized at 323 K, with partial destruction of the metal frames, and reformed by treatment in H 2 . Oxidation at higher temperatures resulted in total oxidation of the clusters with breakup of the metal frames, and treatment in H 2 led to near reconstruction of the metal clusters, but with some migration and aggregation. The results imply that the oxidized nanoclusters were site isolated on the support surface; this is the first report of oxide clusters of noble metals. The metal oxide clusters are catalytically active for CO oxidation.Nanoclusters or nanoparticles dispersed on porous supports are the common form of solid catalyst, offering the advantages of high surface area per unit volume of the catalyst and a large fraction of the catalytically active material at surfaces, where it is accessible to reactants. Typical catalysts for oxidation reactions are metal oxides and noble metals, and the latter, in operation, typically consist of nanoparticles of metal covered in part with surface oxide layers. Gas-phase oxide nanoclusters 1 have been investigated as structurally well-defined models of oxidation catalysts, but these have been restricted to oxides of nonnoble metals. Supported nanoclusters of noble metals have been thoroughly investigated, 2-4 but there are only few reports of supported nanoclusters of metal oxides, and these are oxides of non-noble metals (Fe, 5 Mo, 6 and W 7 ) supported in zeolites. We now report oxide nanoclusters of noble metals dispersed on oxide supports and their catalytic performance for oxidation of CO.The supported samples were prepared with standard airexclusion techniques by formation of nanoclusters of metal on a high-area MgO support followed by oxidation to convert the metal nanoclusters into metal oxide nanoclusters. The supported nanoclusters Ir 4 /MgO and Rh 6 /MgO were prepared from the precursors [Ir 4 (CO) 12 ] (Strem, 98%) and [Rh 6 -(CO) 16 ] (Strem, 98%), respectively. Rh 6 /MgO was synthesized by adsorption of [Rh 6 (CO) 16 ] on MgO calcined at 673 K (MgO 673 ), followed by decarbonylation in He at 573 K; Ir 4 /MgO was synthesized by adsorption of [Ir 4 (CO) 12 ] on MgO 973 followed by decarbonylation in He at 593 K. The methods are described elsewhere. 2,8-10 The decarbonylated samples were pretreated in H 2 at 573 K prior to oxidation and reduction treatments. Reagent-grade n-hexane (Aldrich), used as a solvent to bring the metal carbonyl precursors in contact with the support, was dried over sodium benzophenone ketyl. MgO powder (97%, EM Science) was used as received. The BET surface area of MgO 673 was approximately 60 m 2 g -1 , 11 and that of MgO 973 was approximately 18 m 2 g -1 . 2 The treatment gases used in the sample preparations were He (Matheson, 99.999%, purified by passage through traps to remove traces of O 2 and moisture) and H 2 (Matheson, 99.999%, or generated by electrolysis of

Supported iridium catalysts prepared by atomic layer deposition: effect of reduction and calcination on activity in toluene hydrogenation

“…After deposition of the metal compound, the catalyst is commonly activated through removal of the ligands in treatments with hydrogen (reduction) or oxygen (calcination). The effect of the activation procedure on iridium catalyst has earlier been studied by Lai et al [4] and Cunha et al [5]. Lai et al [4] showed that, at 250°C, oxygen causes oxidation and fragmentation of Ir 4 clusters prepared from Ir 4 (CO) 12 .…”

“…The effect of the activation procedure on iridium catalyst has earlier been studied by Lai et al [4] and Cunha et al [5]. Lai et al [4] showed that, at 250°C, oxygen causes oxidation and fragmentation of Ir 4 clusters prepared from Ir 4 (CO) 12 . After exposure of the oxidized and fragmented Ir 4 clusters to hydrogen at 300°C, the original Ir 4 clusters are reconstructed with a minor degree of aggregation [4].…”

“…Lai et al [4] showed that, at 250°C, oxygen causes oxidation and fragmentation of Ir 4 clusters prepared from Ir 4 (CO) 12 . After exposure of the oxidized and fragmented Ir 4 clusters to hydrogen at 300°C, the original Ir 4 clusters are reconstructed with a minor degree of aggregation [4]. Cunha et al [5] showed that, iridium on alumina exhibits larger particle sizes after reduction at 400°C than after calcination followed by reduction at 400 or 500°C.…”

“…The activation of iridium catalysts, prepared by impregnation, has been studied earlier [4,5], but since the mechanism of iridium catalyst preparation is different when done by ALD and impregnation, the behavior in activation may well differ too. The activation of Cr(acac) 3 has been studied earlier [8][9][10][11][12], but as the noble nature affects the preparation process [3], the behavior of iridium in activation can be expected to be different from that of chromium.…”

CO oxidation on MgAl2O4 supported Irn: activation of lattice oxygen in the subnanometer regime and emergence of nuclearity-activity volcano

“…76 The most energetically favorable was the near tetrahedral configuration with three iridium atoms bound to the surface. 77,78 ESI Fig. S21a† was chosen to be our representative Ir 4 cluster.…”