545035 Enhanced Methane Reforming over Hierarchical Structured Supported Ni Based Catalyst

Monday, June 3, 2019: 5:30 PM
Texas Ballroom D (Grand Hyatt San Antonio)
Anup Tathod1, Naseem S. Hayek1, David Simakov2 and Oz M. Gazit1, (1)Chemical Engineering, Israel Institute of Technology - Technion, Haifa, Israel, (2)Chemical Engineering, University of Waterloo, Waterloo, ON, Canada

The reforming of methane, using CO2 as an oxidizer, to make syngas (H2 and CO) is an intriguing route to produce value added chemicals. For this reaction to become viable the catalyst needs to be based on nonprecious transition metals that show high activity and good resistance to deactivation induced by metal sintering and coke formation. Supported nickel catalysts were identified as a cost effective and active catalyst for MDR. However, the high concentrations of carbon species in MDR and the absence of water makes carbon deposition on the nickel catalyst the thermodynamically favored path. This stimulates the use of even higher reaction temperatures, which was shown thermodynamically to limit carbon formation. Unlike most rare earth metals, for which the TTam (Tammann temperature) is above the desired reaction temperature (600 oC - 800 oC), Ni has a lower TTam (590 oC), making it highly prone to sintering. Supporting nano-sized Ni catalysts on a strongly interacting support (SIS) will produce a stable catalyst but, may also block important active sites. In contrast, the same nano-sized Ni particles supported on a weekly interacting support (WIS) will be less stable but, will also be more active. This highlights the challenge of balancing the degree of interaction between the metal catalyst and the underlying support material, which is pivotal for obtaining optimal catalytic performance.

To mitigate this challenge, we used a ″surface phase oxide″ in the form of MgAl hydrotalcite nanosheets (<2 nm thick) as an interlayer SIS to mediate the interaction between nickel nanoparticles and an underlying WIS (i.e. ZrO2 or SiO2). We show that this hierarchical configuration, in which the Ni is supported only on the hydrotalcite SIS nanosheets, the obtained reaction performance, for an unoptimized reaction conditions, enhanced the conversion by up to 2-fold, as compared to Ni supported on ZrO2, and by up to 15-fold as compared to Ni on bulk Mg-Al hydrotalcite. Moreover, selectivity of the hierarchical catalyst was similar to that of the Ni on ZrO2, despite the fact that the Ni phase had virtually no physical interaction with the underlying ZrO2. We found that the interaction of the Ni nanoparticles with the hydrotalcite surface phase oxide maintained a stable 2 nm Ni catalyst for a total of 250 h on stream. H2-TPR analysis showed that the presence of the SIS surface phase oxide increased the Ni reduction temperature above that of the underlying ZrO2 but kept it below that of Ni supported on bulk hydrotalcite. To gain better understanding into the mechanism of enhanced catalytic performance we used a combination of PXRD, SAXS, Cryo-TEM, H2-TPR, STEM-EDS, and XPS in conjunction with the results for the catalytic performance and kinetic analysis. The combined results show that the amplified catalytic performance is strongly correlated with the modification of the metal-support interaction (MSI) by the MgAl hydrotalcite nanosheets interlayer. The changes in MSI interactions is postulated to arise either due to the ability of the Ni sites to electronically communicate with the underlying ZrO2 or due to the effect of the new interface between the hydrotalcite nanosheets and the underlying ZrO2. Molecular level studies into these effects are being conducted using atomic friction force microscopy, in situ-spectroscopy analysis and temperature programmed adsorption measurements.


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