Intrinsic kinetics of CO₂ methanation on low-loaded Ni/Al₂O₃ catalyst: Mechanism, model discrimination and parameter estimation

Highlights

• CO₂ methanation kinetics on low-loaded Ni/Al₂O₃ catalyst is described based on DRIFTS and kinetic data.

• LHHW rate equations are deduced including the role of water, carbonyls and formates.

• Formates decomposition as RDS rate equation represents most accurately the kinetic performance.

• The participation of both carbonyls and formates in the CO₂ hydrogenation is confirmed.

Abstract

The mechanism and kinetic of CO₂ methanation reaction of 9.5 % Ni/Al₂O₃ catalyst is analysed under a wide range of operating conditions. Once the catalyst activity is stabilized, the influence of temperature, total pressure and space velocity is studied for kinetic characterisation. A data set comprising of 153 experimental runs has been used to develop a kinetic model capable to accurately predict the reaction rate. Ni/Al₂O₃ catalyst shows an apparent activation energy of 80.1 kJ mol⁻¹ in CO₂ hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H₂O inhibition, with a water adsorption constant of 3.1 atm⁻¹ and apparent orders of 0.24 and 0.27 for H₂ and CO₂, respectively. Based on DRIFTS results, we propose for the first time the H-assisted CO formation route, which is compared with the more conventionally reported carbonyl route, and describe the corresponding reaction rate LHHW equation, resulting in notable improvement for mean deviation (D) of 7.0 % in our model related to that based on the carbonyl route (D = 20.1 %) usually suggested for catalysts with higher Ni loads around 20 %. The H-assisted CO formation route considers the formate species decomposition into carbonyls via H-assisted CO formation mechanism and further carbonyls hydrogenation into CHO as the rate determining step. Thus, the LHHW mechanism, in which carbonyls as well as formate species participate in CO₂ methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al₂O ₃ catalyst with high accuracy under relevant process conditions (315−430 °C, 1−6 bar, H₂ to CO₂ molar ratios between 1–16 and, different reagents and products partial pressures).