Abstract
<jats:p> Thermocatalytic methane pyrolysis (TMP), a process in which methane is decomposed into solid carbon and hydrogen, is of high interest for low-carbon H <jats:sub>2</jats:sub> production and solid carbon valorization. Understanding the mechanism of formation of solid carbon on metal-based catalysts is essential for the rational design of improved TMP catalysts and for controlling carbon properties. Previous studies have shown that carbon growth depends strongly on temperature, with two distinct carbon growth regimes, below 500 °C and between 550 and 600 °C. However, the effect of the TMP temperature regime on carbon properties remain underexplored. In this work, we investigate the type of solid carbon formed on a Ni-Cu/γ-Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> catalyst material at different temperatures using operando Raman spectroscopy coupled with mass spectrometry (MS), and corroborate the results with X-ray diffractometry (XRD), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The Raman spectra evolved during reaction time and with temperature, indicating the formation of carbon structures with different degrees of graphitization and crystallite size, as revealed by the ratio of defective (D) over graphitic (G) Raman band areas. The obtained D/G ratios at the different temperature regimes revealed low defect densities and a low H <jats:sub>2</jats:sub> yield at a reaction temperature of 400 °C, with higher defect densities and a higher H <jats:sub>2</jats:sub> yields at 500 or 600 °C. Extensive measurements at 400 °C revealed pronounced heterogeneity in the type of solid carbon formed. In some cases, D/G ratios below 1 were obtained, consistent with the formation of more ordered carbon with a high degree of graphitization, likely enabled by the slower growth rate at 400 °C. When the temperature was ramped from one regime to the other (400 to 600 °C), the resulting solid carbon could be tuned in size and D/G ratio relative to isothermal reaction conditions. Together, these findings provide insights into the temperature dependence of carbon growth during methane pyrolysis over metal-based catalysts and suggest that non-isothermal operation of TMP can be used to tune carbon deposition. </jats:p>