545435 A Stability Analysis of Electroless Deposition-Derived Ni-Pt Catalysts for the Dry Reforming of Methane

Tuesday, June 4, 2019: 11:51 AM
Texas Ballroom D (Grand Hyatt San Antonio)
Benjamin T Egelske, Jayson Keels, John R. Monnier and John R. Regalbuto, Chemical Engineering, University of South Carolina, Columbia, SC

Dry reforming of methane (DRM) by CO₂ to form CO and H₂ is an opportunity to blend sustainable initiatives with inplace hydrocarbon infrastructure. To overcome sluggish reactivity methane reforming must take place at temperatures exceeding 700 °C which leads to rapid deactivation of current Ni-based catalyst technology.

Past studies have shown improved stability and performance when Group VIII metals are paired with Ni, but these results often suffer from two deficiencies. First, samples have been prepared using traditional dry impregnation which randomizes metal-metal contact making it difficult to determine correlations between catalyst composition and performance [1]. Secondly, evaluation data is reported over a wide range of temperatures often below 700 °C resulting in a lack of industrial relevance [1].

In this study Electroless Deposition (ED), where one metal is placed directly and only on a second metal, is used to prepare a series of Ni-Pt catalysts. Evaluations were performed at both low temperature (<= 600 °C) and high temperature (700 °C) conditions in a continuous flow reactor. Low temperature data agrees with the promotional effects previously reported in literature but at high temperature conditions performance is reversed, i.e. all bimetallic samples exhibit lower conversion and higher deactivation rates compared to monometallic Ni.

Figure 1: Activities of Ni-Pt catalysts at 550 °C (A) and 700°C (B). All experiments performed at identical conditions so instantaneous CH₄ conversion is proportional to reaction rate.

X-ray Diffraction data suggests high temperature deactivation results from Pt-assisted methane decomposition due to separation of the Ni-Pt alloy above 630 °C. Pre and post reaction STEM imaging showed a slight growth in monometallic Ni particles but large Pt clusters and the presence of whisker coke for bimetallic samples. TPO analysis on spent samples shows a direct relationship between Pt loading and surface carbon. These results suggest that coke formation is the primary cause for deactivation with only small effects from sintering of Ni.

An analysis of low temperature kinetics indicates the presence of a compensation effect or the offset of apparent activation energies by pre-exponential factors up to an isokinetic temperature at which point the order of catalytic performance is reversed [2]. For the Ni-Pt series experimental data suggests a link between the calculated isokinetic temperature of 624 °C and the historically reported phase separation point at 630 °C [3]. The findings in this study highlight the benefits of evaluating catalysts of known compositions and the importance of including practical reaction conditions in scientific studies.

Figure 2: Ni-Pt compensation plot under stable operation (T <= 600 °C).

[1] A. N. Şener, M. E. Günay, A. Leba, and R. Yıldırım, "Statistical review of dry reforming of methane literature using decision tree and artificial neural network analysis," Catalysis Today, vol. 299, pp. 289-302, 2018.

[2] G. C. Bond, M. A. Keane, H. Kral, and J. A. Lercher, "Compensation Phenomena in Heterogeneous Catalysis: General Principles and a Possible Explanation," Catalysis Reviews, vol. 42, no. 3, pp. 323-383, 2000.

[3] H. Okamoto, "Ni-Pt (Nickel-Platinum)," Journal of Phase Equilibria and Diffusion, vol. 31, no. 3, pp. 322-322, 2010.

 


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