Effect of Promoters on Co/Al2O3 Catalysts for Partial Oxidation of Methane: Structure–Activity Correlations

The development of cost-effective non-noble metal catalysts for the partial oxidation of methane (POM) remains a key strategy for producing hydrogen-rich syngas while mitigating greenhouse gas emissions. In this study, cobalt-supported alumina (Co/Al2O3) catalysts were prepared using 5 wt.% of Co and calcined at 600, 700, and 800 °C. Subsequently, Co/Al2O3 catalysts were promoted with 10 wt.% Mg, Si, Ti, and Zr at the optimized calcination temperature. The catalysts were systematically characterized by FT-IR, XRD, N2 physisorption, H2-TPR, and XPS analyses. Catalytic activity tests for POM of CH4 were conducted at 600 °C (CH4/O2 = 2 and GHSV = 14,400 mL g−1 h−1). Catalysts calcined at 700 °C (5Co/Al_700) exhibited the highest activity among unpromoted samples, with CH4 conversion of 43.9% and H2 yield of 41.8%. The superior performance was attributed to its high surface area and the abundance of reducible Co3+ species, generating a greater number of Co0 active sites. XPS results confirmed the structural stability of γ-Al2O3 and preserved Co–Al interactions across calcination temperatures, while promoters mainly modulated Co dispersion and redox accessibility. Among the promoted catalysts, the activity order followed: 5Co/10ZrAl > 5Co/10MgAl> unpromoted-5Co/Al_700 > 5Co/10SiAl > 5Co/10TiAl. Si and Ti promoted catalysts acquired less concentration of active sites and less activity as well. The concentration of reducible species as well as initial activity towards POM are comparable over Zr and Mg-promoted catalysts. However, earlier one has a higher edge of reducibility and sustained constant activity over time in a stream study. The Zr-promoted catalyst exhibited superior reducibility and remarkable stability, achieving 47.3% CH4 conversion and 44.4% H2 yield sustained over 300 min time-on-stream. TEM analysis of spent 5Co/10ZrAl indicated that Zr promotion suppressed graphitic carbon formation.