富山大学研究推進機構水素同位体科学研究センター研究報告 38：19－24，2018.

ノート

Fe3C を構築した鉄のメタン分解活性田口 明*，米山優紀

富山大学

研究推進機構〒930-8555

水素同位体科学研究センター
富山市五福 3190

Catalytic methane decomposition over iron grains with and without Fe3C
prepared via an ex situ process

Akira Taguchi*, Yuki Yoneyama

Hydrogen Isotope Research Center
Organization for Promotion of Research, University of Toyama
Gofuku 3190, Toyama 930-8555(Received April 8, 2019; accepted October 31, 2019)

Abstract
Fe3C supported on iron grains (Fe3C/Fe) was prepared by the solid-solid reaction of iron grains(Fe-pw) and carbon that had been sputter deposited on the Fe-pw surface. Fe3C/Fe showedslightly higher methane decomposition activity as compared to the bare Fe-pw at 750 ºC.

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田口 明・米山 優紀

Research note
Over the last several decades, hydrogen has received increasing attention as a potential
energy carrier, using methane (natural gas) or water as hydrogen sources [1-3]. Although water
is a plentiful hydrogen source, its standard enthalpy of formation (ΔfHº: -285.8 kJ/mol) is lower
than that for methane (-74.4 kJ/mol) [4], indicating that the conversion of water to H2 requires
a greater energy input. While methane is currently obtained as a fossil fuel, recent progress inresearch regarding biomass methane will likely increase its importance as a hydrogen source.
Even though the ΔfHº value for methane is relatively small, the poor reactivity of methane
requires the application of high temperatures. As a result, the most cost effective means ofhydrogen production is currently the steam reforming of methane [5,6].
Another option that has become of interest is catalytic methane decomposition to generatehydrogen and carbon nanomaterials. This process does not produce carbon monoxide and sodoes not require the separation of hydrogen and carbon monoxide. Many studies of this process
have been performed, and have considered the use of nickel, iron and cobalt as active metals
for catalytic methane decomposition, as well as the use of activated carbon [1,7]. In work withFe2O3 supported on Al2O3 (with Fe2O3 loadings above 38.5 wt%), the iron species in thedeactivated catalyst following methane decomposition at 800 °C was found to be Fe3C, and so
this has been proposed as the active component [8]. However, Fe3C alone does not exhibit
methane decomposition activity [9]. Thus, the conditions used to prepare Fe3C, such as theprecursor materials or the thermal treatment (including pre-activation), may affect its catalytic
activity. In the present study, the ex situ preparation of Fe3C on an iron grain surface wasattempted, based on the barrel-sputter deposition of carbon on the iron grain followed bythermal annealing. The methane decomposition activity of the resulting iron-supported Fe3C
was subsequently investigated.
Iron powder with grain diameters in the range of 3–5 μm (referred to as Fe-pw herein) was20

Fe3C を構築した鉄のメタン分解活性

purchasedfromtheKOJUNDO

ChemicalLaboratory Co., Ltd., and stored under N2. Carbon
was deposited on the Fe-pw using a barrel
sputtering apparatus [10,11], employing a Fe-pw
mass of approximately 3.0 g, a radio frequency
power of 200 W, an Ar pressure of 2.0 Pa and a
sputtering duration of 4.5 h. The resulting material
is referred to herein as C/Fe. The X-ray diffraction(XRD) pattern of Fe-pw was shown in Figure 1(a).

Fig. 1 The XRD patterns of the (a) Fe-pw, (b)The original Fe-pw showed the signals at 2θ values

C/Fe and (c) Fe3C/Fe samples. The triangleof 44.6° and 65.0º that can be assigned to the 111

marks the peaks that can be assigned to α-Fe.

and 220 planes of α-Fe, respectively (Figure 1(a)). The carbon coating to form the C/Fe did notlead to any significant differences in the diffraction pattern (Figure 1(b)). Also, neither sample
showed the presence of impurities such as iron oxides.
The solid-solid reaction of the carbon and iron on the Fe-pw surface to form iron carbide
was achieved by annealing the C/Fe under an Ar flow (50 cm3/min) at 500 ºC for 30 min. TheXRD pattern of the resulting material is also provided in Figure 1(c). This pattern exhibits
signals assignable to Fe3C in the 2θ region of 35°–60º, based on indexing to the JCPDS 340001 standard. These results suggest the successful formation of Fe3C on the Fe-pw, and thissample is referred to as Fe3C/Fe hereafter. Quantitative analyses of the carbon concentrationsin the Fe3C/Fe and C/Fe samples by combustion were unsuccessful because the Fe-pw could
not be fully oxidized to iron oxides even at 1000 ºC, but rather produced a mixture of metallic
Fe and Fe2O3. It was also difficult to determine whether Fe3C was present on the iron grainsurface as a complete uniform layer or as inhomogeneous patches. Further experiments arerequired to obtain a better understanding of this material.

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Catalytic methane decomposition was performed over both the Fe3C/Fe and Fe-pw using aconventional fixed-bed flow reactor. The gas-phase product was analyzed via an on-line gas
chromatographs (Shimadzu; GC-8A) equipped with an activated carbon column and a thermal
conductivity detector (TCD) or a Porapak Q column and flame ionization detector (FID). In
this study, the catalyst (approximately 0.5 g) was pre-treated by heating to 750 °C at a rate of
10 ºC/min under a H2(50 %)/N2-balance gas (50 cm3/min) followed by a hold at that temperaturefor 0.5 h. Subsequently, the gas flow was switched to a CH4(90 %)/N2-balance gas (50 cm3/min)at the same temperature. The effluent gases were analyzed using the gas chromatograph, so asto evaluate the decomposition activity. The extent of methane conversion was calculated fromthe equation:
CH4 conversion =ሺሾCH4 ሿin - ሾCH4 ሿout ሻሾCH4 ሿin× 100

where [CH4]in and [CH4]out are the concentrations ofCH4 in the inflow and the effluent gases,respectively. The gas phase product was found to beprimarily H2 (as determined by TCD), with small
amounts of ethane and ethene (as determined byFID).
The methane conversions obtained over the Fepw and Fe3C/Fe catalysts as functions of time areplotted in Figure 2(A). It is evident that the methane
conversion initially increased slightly over time inboth cases. However, during the final stage of the22reaction, the gas flow was found to decrease

Fig. 2 (A) Methane conversion and (B) H2 andsignificantly because the reactor was blocked byhydrocarbon production rates as functions ofcarbon deposition. As a result, the methane

Fe-pw (empty symbols).

time, over the Fe3C/Fe (filled symbols) andFe3C を構築した鉄のメタン分解活性

conversion rose almost to 100 %, at which point the reaction was stopped. A greater degree ofcarbon deposition was observed when using the Fe3C/Fe, suggesting higher methane
decomposition activity over this material. This superior activity is confirmed by the H2
production rate data in Figure 2(B). The H2 production rate over the Fe3C/Fe is seen to begreater than that over the Fe-pw, indicating that the presence of Fe3C on the Fe effectively
promoted this reaction. In addition, both catalysts were found to produce ethane and ethene,
although at rates three orders of magnitude smaller than the H2 production rate (Figure 2(B)).
Propane and propene were also found in the effluent, but at very low generation rates (below
0.01 μmol/min·g-cat).
In conclusion, Fe3C synthesized via a solid-solid reaction on an iron grain surface showed
improved methane decomposition activity, giving a higher H2 production rate than that obtained
from the bare Fe-pw. The results of this work suggest that decreasing the Fe3C/Fe grain size
might enhance the methane decomposition activity of this material. This study also
demonstrates that new catalysts can be fabricated by reforming catalytic surfaces through solidsolid reactions.

Acknowledgements
This work was partly supported by the research found from the Nippon Sheet Glass (NSG)
Foundation for Materials Science and Engineering.
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