NiAl2O4-derived active sites over Ni/Al2O3 catalysts for dry reforming of methane: Temperature-dependent interplay between support morphology and thermodynamic driving forces

The formation of NiAl2O4 during calcination and the subsequent generation of Ni–oxygen vacancy (Ni–Ov) interfaces during H2 reduction play a crucial role in suppressing coke formation in the dry reforming of methane (DRM). We discovered that these catalyst characteristics are significantly influenced by calcination temperatures and macroporosity. In this study, we synthesized macroporous and non-macroporous Ni/Al2O3 catalysts, which were then calcined at both mild and high temperatures to examine their structural evolution and catalytic performance in DRM. At a calcination temperature of 600 °C, the macroporous Al2O3 enhanced the formation of NiAl2O4 due to geometric effects and a higher AlO4/AlO6 ratio. This provided a structurally favorable environment for Ni–Al interaction, even under thermodynamically limited conditions. Conversely, at 800 °C, the energy barrier for NiAl2O4 formation was removed, resulting in similar spinel NiAl2O4 formation irrespective of support morphology. Complementary calculations using a pretrained atomistic model supported our experimental findings, indicating that Ni incorporation is energetically more favorable in Al2O3 structures with higher octahedral vacancy proportions, as seen with the macroporous support. Catalytic evaluation in DRM at 800 °C showed that the catalysts calcined at 800 °C exhibited the highest carbon balances (98–99%) and negligible coke deposition, whereas at 600 °C the macroporous catalyst showed a higher spinel fraction than the non-macroporous catalyst (0.24 vs 0.18 from H2-TPR) together with improved coke resistance and more stable performance. Catalysts rich in NiAl2O4 and Ni–Ov interfaces promoted efficient CO2 activation and coke gasification, leading to improved reaction stability and coke resistance.