Carbon Dioxide conversion to hydrocarbons with reactivated iron-carbon nanotube catalysts

The use of carbon dioxide to make valuable products can reduce greenhouse gas
emissions, whilst allo Conversion of carbon dioxide into hydrocarbon products
can be achieved via the two step process of the water gas shift reaction,
followed by Fischer-Tropsch catalysis [1]. For the latter, carbon
nanotube-supported iron catalysts have been used in the past [2, 3].
Typically this involves purifying the nanotubes, and subsequently adding an
iron catalyst in the form of nanoparticles (NPs) tethered to the tube
surface. This is an inefficient process as carbon nanotubes are commonly
synthesised using an iron catalyst, which is then removed during several
purification steps. We have by-passed all these steps by re-utilising the
iron used to synthesize the CNTs as a catalyst for carbon dioxide conversion.
Multi-walled carbon nanotubes (MWCNTs) have been produced using an
aerosol-based method using ferrocene as an iron source. The resulting CNTs
have a large number of iron nanoparticles formed in-situ on their walls.
After the synthesis process, the iron NPs are coated with a graphitic layer,
which is removed by a mild oxidation process, without damaging the tubes. The
nanoparticles are subsequently reactivated in an inline reaction to act as
catalysts for CO2 reduction [4]. Conversions of the reactivated iron were
significantly higher than achieved by impregnated iron nanoparticles, due to
the close interaction of the iron nanoparticles formed in-situ with the
carbon nanotube, allowing hydrogen absorbed onto the MWCNT surface to
interact with the absorbed carbon species.
Carbon nanotube powders are not well suited for larger scale catalysis as
they have a high pressure drop. Using a similar aerosol based method we have
grown arrays of MWCNT with iron NPs grown in situ directly onto a structured
cordierite monolith, creating a carbon nanotube composite with low pressure
drop [5]. This method produces significantly thicker MWCNT composites than
has previously been achieved, whilst using a much simpler one step process,
with no preparation of the monolith [6]. We can control the thickness of the
MWCNT coating by changing the synthesis conditions. Activating the iron
nanoparticles in-situ creates an iron based MWCNT supported on cordierite in
two simple steps, rather than the many steps which would otherwise be
required to create a similar support. Here we report on these structured
carbon nanotube supports, and their effectiveness as catalysts for CO2
conversion.
References:
[1] Dorner RW, Hardy DR, Williams FW, Willauer HD. K and Mn doped iron-based
CO2 hydrogenation catalysts: Detection of KAlH4 as part of the catalyst's
active phase. Applied Catalysis A: General. 2010;373(1-2):112-21.
[2] Torres Galvis HM, Bitter JH, Khare CB, Ruitenbeek M, Dugulan AI, de Jong
KP. Supported Iron Nanoparticles as Catalysts for Sustainable Production of
Lower Olefins. Science. 2012;335(6070):835-8.
[3] van Steen E, Prinsloo FF. Comparison of preparation methods for carbon
nanotubes supported iron Fischer-Tropsch catalysts. Catal Today.
2002;71(3-4):327-34.
[4] O'Byrne JP, Owen RE, Minett DR, Pascu SI, Plucinski P, Mattia D, et al.
High CO2 and CO conversion to hydrocarbons using bridged Fe nanoparticles on
carbon nanotubes. Catalysis Science and Technology. 2013;under review.
[5] Minett DR, O’Byrne JP, Jones MD, Ting VP, Mays TJ, Mattia D. One-step
production of monolith-supported long carbon nanotube arrays. Carbon.
2013;51(0):327-34.
[6] García-Bordejé E, Kvande I, Chen D, Rønning M. Synthesis of composite
materials of carbon nanofibres and ceramic monoliths with uniform and
tuneable nanofibre layer thickness. Carbon. 2007;45(9):1828-38.