Details

Autor(en) / Beteiligte

• Deutsch, S.E.
• Xiao, F.-S.
• Gates, B.C.

Titel

Near Absence of Support Effects in Toluene Hydrogenation Catalyzed by MgO-Supported Iridium Clusters

Ist Teil von

• Journal of catalysis, 1997-08, Vol.170 (1), p.161-167

Ort / Verlag

Amsterdam: Elsevier Inc

Erscheinungsjahr

1997

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Iridium cluster catalysts were prepared by reaction of [Ir4(CO)12] with MgO that had been calcined at 300, 500, or 700°C to vary to degree of hydroxylation, followed by removal of the CO ligands from the iridium by treatment in He. The catalysts were tested for toluene hydrogenation at 60, 80, and 100°C and 1 atm. Extended X-ray absorption fine structure (EXAFS) spectra of each catalyst after 120 h of continuous operation in the absence of deactivation showed that they had changed little, if any, nearly retaining the Ir4tetrahedra of the precursor [Ir4(CO)12]. The catalyst with the most highly hydroxylated support (calcined at 300°C) evidently underwent slight aggregation, giving iridium clusters with nuclearities of about 6±2. Thus the data allow a determination of the influence of the degree of hydroxylation of the support on the catalytic activity in the near absence of metal cluster size effects. The turnover frequency at 60°C decreased from 7.5×10−4to 6.2×10−4s−1as the MgO calcination temperature increased from 300 to 700°C. The change is negligible in view of the experimental errors.These results, combined with similar values of the turnover frequency for toluene hydrogenation catalyzed by γ-Al2O3-supported clusters that were also nearly Ir4, indicate that the effects of changes in nonreducible metal oxide supports are very small. The Ir4clusters are regarded as quasi-molecular in character, with the metal oxide supports being multidentate ligands; the ligand effects are inferred to be similar for the γ-Al2O3and MgO with various densities of surface O and OH groups.