(573e) Methane Steam Reforming: Using External Electric Fields to Enhance the Catalytic Performance of Ni-Based Catalysts

According
to the Annual Energy Outlook for 2015, natural gas production in the US is
projected to continue rising through 2040. [1] To make the most of
this abundant natural resource and at the same time reduce emissions of harmful
greenhouse gases it is imperative that we fully understand the catalytic
reactions which are used in methane processing – particularly in the case of
methane steam reforming (MSR). MSR produces around 98% of the world’s hydrogen
gas supply. Additionally, MSR is one
of main reactions that occur on the Ni-based anode for direct methane solid oxide
fuel cells (SOFCs). [2] MSR is our reaction of interest because
the conversion of methane to syngas greatly affects the charge-transfer
chemistry and consequently influences the SOFCs' performance. There are two
significant issues facing MSR: (i) Coke formation; (ii) High temperatures of
above 900 K. [3] To address these issues, we are
interested in the effect of an electric field on this process. A large electric
field can rearrange the potential energy states of molecular orbitals, alter
adsorbate-surface interactions, and directly change the overall catalytic
activity of Ni-based catalysts. [4-8]

Based on the DFT
results, an electric field can greatly influence the stabilities of
intermediates, the activation energies and the reaction energies of the
MSR-related elementary reactions. Both the established microkinetic model and
the corresponding experimental evidence of the field-dependent MSR-over-Ni have
shown that the presence of a positive electric field can decrease the formation
of coke and reduce the MSR reaction temperature significantly while maintaining
the overall methane conversion (Figure. 1).[9-14] Therefore, the
importance of considering the electric field effects on the catalytic reactions
in a fuel cell system is self-evident. This information can also be used to
design new electrochemical systems and to enhance the catalytic performance of
Ni-based steam reforming operations.

Figure 1. (a) Experimental results showing the
field effects on the methane conversion at initial H₂O/CH₄
flow rates of 2 (solid line) and 1 (dotted line) in the presence (red) and the absence
(blue) of an electric field. (b) Kinetic modeling results for methane
conversion at steady state at the experimental conditions with a positive
electric field (red) and no field (blue) and initial ratios of H₂O/CH₄
of 2 (solid line) and 1 (dotted line).

For
our future work, we will focus on the field effects on the catalytic activities
of Ni/YSZ catalysts since the MSR processes in SOFCs
typically use Ni/YSZ as an anode material. Our preliminary results show that
the oxygen vacancy formation energy of Ni/YSZ is largely influenced by the presence
of an electric field [15] Moreover,
the various oxidation states of the Ni atoms within the cluster will be altered
by the presence of an electric field and the oxygen vacancy concentration, which
can significantly alter the first C-H bond cleavage of methane (the rate-limiting
step in the MSR reaction). [16] Such
strong field effects increase strongly suggest  that the MSR mechanism at the interface
of the Ni/YSZ catalysts should have significant field effects as compared to those on a
pure Ni surface. This
ongoing work is important as it marks the first time that such complex and
realistic electrocatalytic modeling has been undertaken.

References

[1]
D. Energy, Annual energy outlook with projections 2015, Energy Dept, 2015.

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[9]
F. Che, S. Ha, J.-S. McEwen, Appl. Catal. B, doi:10.1016/j.apcatb.2016.04.026(2016).

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[12]
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[13]
F. Che, J. T. Gray, S. Ha, J.-S. McEwen, In preparation, (2016).

[14]
F. Che, J.T. Gray, S. Ha, J.-S. McEwen, In preparation, (2016).

[15]
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[16]
F. Che, J.T. Gray, S. Ha, J.-S. McEwen, In preparation, (2016).