Enhanced Fisher-Tropsch Chemistry Inside Carbon Nanotubes Containing Iron Nitride Nanoparticles

Title: FeN nanoparticles confined in carbon nanotubes for CO hydrogenation

Authors: Zhiqiang Yang, Shujing Guo, Xiulian Pan, Junhu Wang, and Xinhe Ba

Journal: Energy and Environmental Science

Affiliation: * State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Above: Cartoon depicting Iron Nitride (FeN) particles deposited inside multi-walled carbon nanotubes (MWCNTs)

Fisher-Tropsch chemistry is the process of mixing carbon monoxide (CO) and dihydrogen (H2) in the presence of a catalyst in order to make hydrocarbons like methane, ethane, etc., potentially decreasing our dependence on oil.  Although most syngas (a mixture of CO and H2) is made from coal gassification and steam reforming of methane, which are fossil fuels, greener approaches to syngas production, such as using biomass as the source and garbage gassification are being investigated as possible alternatives.

Most Fisher Tropsch chemistry is performed at high temperature and pressure, resulting in intensive energy inputs and difficulty in controlling the product mixtures.  These researchers have studied how the activity, product ratio, and stability of Iron (Fe) and Iron Oxide (FenOm ) nanoparticles is affected when they are placed inside carbon nanotubes.  In this article, they describe similar studies with related Iron Nitride (FeN) nanoparticles.

The synthesis of the FeN nanoparticles begins with the deposition of Fe2O3 nanoparticles inside the MWCNTs, which is accomplished by heating a solution of NaOH, FeCl3, and MWCNTs for a set amount of time.  This process resulted in a high percentage of the nanoparticles situated inside the nanoparticles.  Using H2, they were able to chemically reduce the nanoparticles resulting in Fe nanoparticles in the MWCNTs (below).

Above: Transmission electron microscope images (TEM) of Fe2O3 and Fe nanoparticles inside a MWCNT.  The resolution is high enough to see the nanoparticle lattice spacing.  This image is from another paper by this group.

The authors were also able to prepare MWCNTs containing Fe2O3 nanoparticles on the outside.  Nitrogen can be introduced into the Fe2O3 nanoparticles simply be heating under an atmosphere of ammonia.  This process resulted in the first ever synthesis of FeN in which the atoms were packed in a face-centered cubic arrangement, which only occurred with FeN particles contained within the MWCNTs.  The size of the FeN nanoparticles  could be controlled by varying the temperature at which the Fe2O3 was “nitrided” (their word).  At 350oC, the nanoparticles inside the MWCNTs (denoted FeN-in by the authors) were 4-6 nanometers (nm) in diameter, whereas increasing the temperature to 500oC resulted in nanoparticles 6-12 nm in diameters.  Interestingly, (or maybe as expected) nanoparticles on the outside of the MWCNTs grew much larger at the same temperatures.

Using TEM, the authors were able to obtain impressive images MWCNT-FeN mixtures (below).

Above: TEM images of four different samples of FeN-MWCNT and particle size distribution inlaid.  The number on the right is the “nitridation” temperature.

For assessing the catalytic activity of the nanoparticles and the effect of the MWCNTs, the catalyst was exposed to a 1:1 mixture of CO:H2 at 300oC.  A few important trends were observed during these studies:

1) An increase the particle size (or increase in the temperature of “nitridation”) increases the consumption of CO and the turnover frequency (speed of catalysis).  This is attributed to the lower stability of Fe nanoparticles, which form carbides or are oxidized under these conditions.

2) FeN-in consumes about 1.5 times more active than FeN-out and about 8 times as much as Fe-in (iron nanoparticles in the MWCNTs).  The authors believe this is due to the tendency for exposed FeN (and its non-cubic packing) to lose nitrogen and become less active.

3) Fe-in produces a larger percentage of hydrocarbons from CO and H2 (75%) than FeN-in (60%) and FeN-out (62%).  The rest of the CO is oxidized to CO2.

4) Fe-in produces a higher percentage of hydrocarbon containing 5 or more carbons (22%) than FeN-out (19%) and  Fe-in (18%).

The first ever synthesis of cubic FeN resulted in some interesting Fisher-Tropsch chemistry.  The authors do not explicitly state what future directions they plan to take this work, but the list of known of nanoparticles is sizable and constantly growing, so they do not have to worry about running out of ideas. The eventual goal would be to develop a system that gave product specificity and could function at (relatively) low temperatures.

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