Supported cobalt catalysts - preparation, characterization and reaction studies

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Doctoral thesis (article-based)
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Date
2009
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Language
en
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Verkkokirja (440 KB, 59 s.)
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TKK dissertations, 162
Abstract
The aim of this work was to understand on the effect of thermal treatments, precursor and support on the interaction between the support and cobalt species, and further how the interaction affects the reducibility and dispersion of the catalyst. Silica and alumina supported cobalt catalysts were prepared, characterised and tested for catalytic activity. The catalysts were prepared by gas phase deposition techniques from cobalt acetylacetonate and cobalt carbonyl and by incipient wetness impregnation from cobalt nitrate. One of the goals was to investigate whether atomic layer deposition (ALD) using cobalt acetylacetonate precursors can produce well dispersed reducible cobalt catalysts. The cobalt acetylacetonates, Co(acac)2 and Co(acac)3, were found suitable for ALD. Silica supported catalysts were prepared by chemisorption of Co(acac)3, while Co(acac)2 was used on alumina. The main mode of interaction on silica was the ligand exchange reaction with OH groups. On alumina both the ligand exchange reaction and dissociative adsorption occurred. Steric hindrance limited the amount of precursor on the support. The acac ligands were removed through calcination at 450 °C. The cobalt loading was increased by repeating the precursor addition and air calcination steps up to five times; samples with about 2 to 8 Co atoms per nm² were achieved on both silica and alumina. Calcination of the cobalt acetylacetonate modified samples led to the formation of silicate- or aluminate-type species, which decreased the reducibility of the catalysts. The reducibility was enhanced when the calcination step after the last precursor reaction step was omitted ('uncalcined' catalysts). High reduction temperatures were still needed: the maximum metal surface area was obtained after reduction at 500-600 °C. The cobalt dispersion on the uncalcined ALD catalysts was, in general, higher than on the calcined catalysts. Furthermore, the dispersion was higher on the alumina supported catalysts than on corresponding silica supported ones. The interaction between cobalt and silica on the nitrate based catalysts was found to be weak, which led to high reducibility but modest dispersion. The main cobalt species on the catalysts was Co3O4. The use of reduction temperatures above 400 °C induced sintering or migration of silica, which decreased the cobalt surface area significantly. Dicobalt octacarbonyl, Co2(CO)8, was adsorbed on silica by vapour-phase adsorption in a fluidised bed reactor under CO. The carbonyl interacted through hydrogen bonding and rearranged to Co4(CO)12 on the support. The amount of precursor that adsorbed on the support was limited by steric hindrance. Decarbonylation was achieved by heat treatment, and higher cobalt loadings were obtained by repeating the deposition and decarbonylation steps. Chemisorption of hydrogen on cobalt was found to be activated and highly reversible. The effect was stronger on alumina than on silica supported samples. The ALD and nitrate based catalysts were tested for gas phase hydrogenation of toluene, and the activity was found to correlate with the available surface area of metallic cobalt.
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Keywords
cobalt catalysts, cobalt acetylacetonate, reducibility, dispersion, hydrogenation, ALD
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Parts
  • [Publication 1]: M. K. Niemelä, L. Backman, A. O. I. Krause, and T. Vaara. 1997. The activity of the Co/SiO2 catalyst in relation to pretreatment. Applied Catalysis A: General, volume 156, number 2, pages 319-334.
  • [Publication 2]: Aimo Rautiainen, Marina Lindblad, Leif B. Backman, and Riikka L. Puurunen. 2002. Preparation of silica-supported cobalt catalysts through chemisorption of cobalt(II) and cobalt(III) acetylacetonate. Physical Chemistry Chemical Physics, volume 4, number 11, pages 2466-2472.
  • [Publication 3]: L. B. Backman, A. Rautiainen, M. Lindblad, O. Jylhä, and A. O. I. Krause. 2001. Characterisation of Co/SiO2 catalysts prepared from Co(acac)3 by gas phase deposition. Applied Catalysis A: General, volume 208, numbers 1-2, pages 223-234.
  • [Publication 4]: L. B. Backman, A. Rautiainen, A. O. I. Krause, and M. Lindblad. 1998. A novel Co/SiO2 catalyst for hydrogenation. Catalysis Today, volume 43, numbers 1-2, pages 11-19.
  • [Publication 5]: Sari Suvanto, Tapani A. Pakkanen, and Leif Backman. 1999. Controlled deposition of Co2(CO)8 on silica in a fluidized bed reactor: IR, chemisorption and decomposition studies. Applied Catalysis A: General, volume 177, number 1, pages 25-36.
  • [Publication 6]: L. B. Backman, A. Rautiainen, M. Lindblad, and A. O. I. Krause. 2000. Effect of support and calcination on the properties of cobalt catalysts prepared by gas phase deposition. Applied Catalysis A: General, volume 191, numbers 1-2, pages 55-68.
  • [Publication 7]: L. B. Backman, A. Rautiainen, M. Lindblad, and A. O. I. Krause. 2009. The interaction of cobalt species with alumina on Co/Al2O3 catalysts prepared by atomic layer deposition. Applied Catalysis A: General, volume 360, number 2, pages 183-191.
  • [Errata file]: Errata of publication 6
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