Structural Investigation of the Carbon Deposits on Ni/Al₂O₃ Catalyst Modified by CaO-MgO for the Biogas Dry Reforming Reaction

Ni/Al₂O₃ and Ni/CaO-MgO-Al₂O₃ catalysts were investigated for the biogas dry reforming reaction using CH₄/CO₂ mixtures with minimal dilution. Stability tests were conducted between 600 and 800 ^oC and TGA/DTG, Raman, STEM-HAADF, HR-TEM, XPS techniques were used to characterize the spent samples. Graphitized carbon allotrope structures, carbon nanotubes (CNTs) and amorphous carbon were formed on all samples. Metallic Ni⁰ was recorded for all (XPS), whereas a strong peak corresponding to Ni₂O₃/NiAl₂O₄ was observed for the Ni/Al₂O₃ sample (650–750°C). Stability tests confirmed that the Ni/CaO-MgO-Al₂O₃ catalyst deactivates at a more gradual rate and is more active and selective in comparison to the Ni/Al₂O₃ for all temperatures. The Ni/CaO-MgO-Al₂O₃ exhibits good durability in terms of conversion and selectivity, whereas the Ni/Al₂O₃ gradually loses its activity in CH₄ and CO₂ conversion, with a concomitant decrease of the H₂ and CO yield. It can be concluded that doping Al₂O₃ with CaO-MgO enhances catalytic performance by: (a) maintaining the Ni⁰ phase during the reaction, due to higher dispersion and stronger active phase-support interactions, (b) leading to a less graphitic and more defective type of deposited carbon, and (c) facilitating the deposited carbon gasification due to the enhanced CO₂ adsorption on its increased surface basic sites.