Neubauer, LR
1992
Direct MAS/MES Evidence for Electronic Metal-Support Interaction in Dilute Co-57 and Fe-57 Carbon and Alumina-Supported Catalysts
Bartholomew, C.H.; Neubauer, L.R. and Smith, P.A.
Tenth International Catalysis Congress, Budapest, Hungary, January 1992. Funded by US Department of Energy/Basic Energy Services and Brigham Young University.
Mössbauer absorption spectroscopy (MAS) and Mössbauer emissions spectroscopy (MES) studies of 1-3% 57Fe and 1% Co-57 on carbon and alumina supports were conducted as a function of a reduction temperature. Catalysts were prepared by nonaqueous evaporative deposition to maximize the reduction of cobalt and iron to the metal. Metal surface areas of the catalysts were determined by H2 adsorption, while extents of reduction to the metal were determined by both Mössbauer spectroscopy and by titration of reduced catalysts with oxygen at 673 K. MAS/MES data for 1 and 3% Fe-57/C and 1% Co-57/C catalysts reduced at 773 K indicate the presence of only one phase-superparamgnetic (SP) clusters of metal having diameters of about 1-2 nm. Room temperature isomer shifts for these carbon supported metal clusters of 0.10-0.14 mm/s indicate a decrease in electron density of the metal nuclei relative to the bulk metals. MES data for Co-57/Al2O3 suggest the existence of three phases: Cosp metal, Co(II) oxide, and Co(III) oxide, while MAS generally shows only the Fesp metal clusters and Fe(III) oxides to be present in 1-2% Fe-57/Al2O3, except for some ferromagnetic Fe metal in 2% 57Fe/Al2O3 reduced at 873 K. Isomer shifts for the metal clusters in the Al2O3-supported Co-57 and Fe-57 catalysts are -0.05 to -0.15 mm/s indicating an increase in the electron density at metal nuclei. The presence of small metals cluster of 1-5 nm in these catalysts is confirmed by H2 absorption. Moreover, Debye temperatures measured by Mössbauer are significantly lower than for bulk iron consistent with the lattice dynamics expected for small metal clusters having a large fraction of surface atoms. The very significant isomer shifts observed for SP metal phases by Mössbauer are consistent with electronic modification of small metal clusters in supported Co or Fe. That the isomer shift is positive for metal/C catalysts and negative for metal/Al2O3 catalysts indicates this effect must be due to metal-support interactions.
Neubauer, LR
1992
Direct MAS/MES Evidence for Electronic Metal-Support Interaction in Dilute Co-57 and Fe-57 Carbon and Alumina-Supported Catalysts
Bartholomew, C.H.; Neubauer, L.R. and Smith, P.A.
Tenth International Catalysis Congress, Budapest, Hungary, January 1992. Funded by US Department of Energy/Basic Energy Services and Brigham Young University.
Mössbauer absorption spectroscopy (MAS) and Mössbauer emissions spectroscopy (MES) studies of 1-3% 57Fe and 1% Co-57 on carbon and alumina supports were conducted as a function of a reduction temperature. Catalysts were prepared by nonaqueous evaporative deposition to maximize the reduction of cobalt and iron to the metal. Metal surface areas of the catalysts were determined by H2 adsorption, while extents of reduction to the metal were determined by both Mössbauer spectroscopy and by titration of reduced catalysts with oxygen at 673 K. MAS/MES data for 1 and 3% Fe-57/C and 1% Co-57/C catalysts reduced at 773 K indicate the presence of only one phase-superparamgnetic (SP) clusters of metal having diameters of about 1-2 nm. Room temperature isomer shifts for these carbon supported metal clusters of 0.10-0.14 mm/s indicate a decrease in electron density of the metal nuclei relative to the bulk metals. MES data for Co-57/Al2O3 suggest the existence of three phases: Cosp metal, Co(II) oxide, and Co(III) oxide, while MAS generally shows only the Fesp metal clusters and Fe(III) oxides to be present in 1-2% Fe-57/Al2O3, except for some ferromagnetic Fe metal in 2% 57Fe/Al2O3 reduced at 873 K. Isomer shifts for the metal clusters in the Al2O3-supported Co-57 and Fe-57 catalysts are -0.05 to -0.15 mm/s indicating an increase in the electron density at metal nuclei. The presence of small metals cluster of 1-5 nm in these catalysts is confirmed by H2 absorption. Moreover, Debye temperatures measured by Mössbauer are significantly lower than for bulk iron consistent with the lattice dynamics expected for small metal clusters having a large fraction of surface atoms. The very significant isomer shifts observed for SP metal phases by Mössbauer are consistent with electronic modification of small metal clusters in supported Co or Fe. That the isomer shift is positive for metal/C catalysts and negative for metal/Al2O3 catalysts indicates this effect must be due to metal-support interactions.