458756 Methane Steam Reforming: Using External Electric Fields to Enhance the Catalytic Performance of Ni-Based Catalysts

Wednesday, November 16, 2016: 4:27 PM
Franciscan C (Hilton San Francisco Union Square)
Fanglin Che1, Jake T Gray1, Su Ha2 and Jean-Sabin McEwen1, (1)The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, (2)Chemical Engineering Department, Washington State University, Pullman, WA

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.

Picture3

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.

[2] K. Oshima, T. Shinagawa, M. Haraguchi, Y. Sekine, Int. J. Hydrogen Energy, 38 (2013) 3003-3011.

[3] E. Nikolla, A. Holewinski, J. Schwank, S. Linic, J. Am. Chem. Soc., 128 (2006) 11354-11355.

[4] E.M. Stüve, Chem. Phys. Lett., 519-520 (2012) 1-17.

[5] H.J. Kreuzer, Surf. Sci. Anal., 36 (2004) 372-379.

[6] G. Pacchioni, J.R. Lomas, F. Illas, J. Mol. Catal. A: Chem., 119 (1997) 263-273.

[7] K.-Y. Yeh, M.J. Janik, CHAPTER 3 Density Functional Theory Methods for Electrocatalysis,  Computational Catalysis, The Royal Society of Chemistry (2014), pp. 116-156.

[8] C.D. Taylor, S.A. Wasileski, J.-S. Filhol, M. Neurock, Phys. Rev. B, 73 (2006) 165402.

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

[10] F. Che, J. Gray, S. Ha, J.-S. McEwen, J. Catal., 332 (2015) 187-200.

[11] F. Che, R. Zhang, A.J. Hensley, S. Ha, J.-S. McEwen, Phys. Chem. Chem. Phys., 16 (2014) 2399-2410.

[12] F. Che, A. Hensley, S. Ha, J.-S. McEwen, Catal. Technol., (2014) 4020-4035.

[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] F. Che, S. Ha, J.-S. McEwen, J. Phys. Chem. C, under review (2016).

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

 


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