Spectroscopic investigation of the influence of Ni particle size in CO ₂ methanation

CO₂ methanation was investigated over Ni/Al₂O₃ catalysts with Ni nanoparticle sizes ranging from 4.6 to 8.4 nm. Particle sizes were determined using electron microscopy and CO chemisorption measurements. The turnover frequency (TOF) increased from 0.005 s^-1 to 0.21 s^-1 with increasing Ni particle size. To understand the origin of this trend, <i>in situ</i> FTIR studies were performed. The results revealed that smaller Ni particles predominantly followed a direct formate pathway, while larger particles exhibited a combined CO and formate-mediated mechanism. Although surface basicity varied among the catalysts, methanation activity was not correlated with this basicity. However, the ability of the catalysts to undergo reduction and oxidation was suggested to cooperatively enhance methanation activity.