Impact of Potassium Addition on the Performance of Ni/MgAl2O4 Catalysts in Steam Reforming of Bio-Oil Model Compounds

Abstract

Ni/MgAl₂O₄ catalysts with and without K promotion were tested in steam reforming of phenol (SRP), ethanol (SRE), and butanol (SRB), to evaluate the effect of K on catalytic activity and methane formation. The catalysts were prepared by a wet impregnation method and were characterized using nitrogen adsorption, in situ XRD, H₂-TPR, TEM, XPS, and XANES techniques. Catalytic evaluations were performed at temperatures ranging from 250 to 650 °C. DFT calculations were employed to study the hydrogenation of CH_x species on Ni modified by K. The addition of K to the Ni catalysts weakened the NiO-support interaction, causing NiO agglomeration and an increase in Ni particle size. The effect of K on CH₄ formation was strongly influenced by the structure of the reformed molecule, leading to the formation of different CH_x species during the reaction. The introduction of K into the Ni catalyst suppressed formation of CH₄ by hydrogenation of CH, with this effect diminishing for CH₂ and being absent for CH₃ species. DFT calculations of the interaction between CH_x species absorbed in an Ni₄ cluster (CH_x-Ni₄) and K, particularly KOH, indicated that species such as HOKH_xC–Ni₄ were stabilized, with decreased energies of −291.5, −242.4, and −27.7 kJ/mol for CH, CH₂, and CH₃, respectively. The increased heat of adsorption for CH and CH₂ species reduced their hydrogenation activity toward methane.