Thermocatalytic decomposition (TCD) of methane offers a promising approach for CO_x-free production of H₂. Techno-economic analysis suggests that TCD can become competitive with state-of-the-art steam methane reforming, if high-value, crystalline carbon co-products are generated to reduce the net cost of H₂ production. Different transition metal-supported catalysts, especially Ni-based, have been investigated for TCD reaction. However, carefully controlled studies evaluating the effect of systematic changes in catalyst properties on catalytic performance is lacking. As a result, direct correlation between catalyst properties, such as Ni⁽⁰⁾ particle size, for example, and its effect on TCD activity, stability, and carbon selectivity have not been reported, to the best of our knowledge. Here we report on a series of Al₂O₃-based and MgAl₂O₄-based catalysts, Ni/Al₂O₃ aerogel, Ni/γ-Al₂O₃, Ni-Al₂O₃ spinel and Ni/MgAl₂O₄for the TCD reaction. Surface and structural properties of the fresh reduced and spent catalysts were investigated by performing nitrogen physisorption, H₂-chemisorption, XRD, H₂-temperature programmed reduction (H₂-TPR) and transmission electron microscopy (TEM). Crystalline properties and morphologies of carbon by-products were identified by temperature programmed oxidation (TPO), Raman spectroscopy, and TEM. We report how initial methane turnover increases as Ni⁽⁰⁾ particle size increases, without influence of catalyst support. Furthermore, we show how catalytic deactivation rates decrease with an increase in Ni⁽⁰⁾ particle size, which is in agreement with literature reports. Moreover, large Ni particles (>20 nm) were found to be highly selective to the formation of carbon nanotubes, whereas small Ni particles (<10 nm) favored formation of graphitic layers of carbon. Resulting carbon product morphology also influences catalyst stability. The formation of graphitic carbon layers block access to Ni active sites, thus rendering the catalyst inactive more quickly than when CNTs were produced. Additionally, the catalyst deactivation observed with time-on-stream is due to the fragmentation of Ni particles into smaller Ni particles followed by their encapsulation with graphitic carbon layers. Taken together, these results show how under systematic investigation the Ni⁽⁰⁾ particle size dictates both i) methane turnover, and ii) carbon product selectivity, which also influences catalytic stability.

Robert Weber

Pacific Northwest National Laboratory

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