Influence of alloying on the catalytic performance of Ni–Al catalyst prepared from hydrotalcite-like compounds for methane decomposition

Cobalt-, iron-, and copper-substituted nickel-aluminum hydrotalcite-like compounds (Ni_2.7Co_0.3Al, Ni_2.7Fe_0.3Al, Ni_2.7Cu_0.3Al HTlcs) have been synthesized and used as precursors to prepare Ni–Co, Ni–Fe, and Ni–Cu alloy catalysts for methane decomposition. The catalysts before and after reaction were characterized with various techniques including XRD, H₂-TPR, HAADF-STEM-EDX, SEM, TEM, and Raman. The characterization results indicate that upon calcination HTlcs are transformed into a mixed oxide solid solution, where cobalt, copper, and iron ions are incorporated into the nickel oxide, and the reduction treatment leads to composition-uniform alloy particles. In methane decomposition at 600 °C, alloying Ni with Co, Fe, and especially Cu is found to enhance the catalytic life and carbon yield. The order of activity is Ni_2.7Cu_0.3Al >> Ni_2.7Fe_0.3Al > Ni_2.7Co_0.3Al > Ni₃Al in terms of carbon yield, highlighting that Ni–Cu alloying is the most effective. Besides, Ni–Cu alloying remarkably changes the carbon morphology, giving carbon nanofibers as the main product. TEM and STEM measurements suggest that Ni–Cu alloy particles are readily aggregated into big particles (>60 nm) under the reaction conditions, which may be responsible for the significant effect of Ni–Cu alloying.