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CO2 hydrogenation reaction over pristine Fe, Co, Ni, Cu and Al2O3 supported Ru: Comparison and determination of the activation energies

Mutschler, Robin, Moioli, Emanuele, Luo, Wen, Gallandat, Noris, Züttel, Andreas
Journal of catalysis 2018 v.366 pp. 139-149
activation energy, aluminum oxide, carbon dioxide, carbon monoxide, catalysts, catalytic activity, cobalt, copper, gases, glass, hydrocarbons, hydrogenation, iron, methanol, nanoparticles, nickel, reaction mechanisms, ruthenium, silica, spectrometers, temperature, titanium dioxide, zinc oxide
Fe, Co, Ni and Cu are the main non-noble industrially significant catalysts in the CO₂ and CO gas phase hydrogenation reaction towards hydrocarbons and alcohols. These catalysts are typically supported on metal oxides such as SiO₂, TiO₂, Al₂O₃ and ZnO, in order to maximize the activity towards the desired reaction. The role of the supporting material is to stabilize the catalytic nanoparticles and to prevent sintering at the elevated reaction temperatures and pressures. The supporting phase can improve the reaction activity or even have a crucial role in the reaction, as is the case, e.g. for the Methanol synthesis over Cu based catalysts supported on ZnO. Studying the metals without a supporting oxide phase is of great importance for the fundamental understanding of the catalytic activity of the metal phase. Therefore, we investigated the pristine transition metals Fe, Co, Ni and Cu (diluted with silica glass beads to avoid sintering) towards their activity in the CO₂ hydrogenation reaction and determined the activation energy. An Al₂O₃ supported Ruthenium catalyst with 0.5 mass percent of Ru loading was taken as reference system. It was found that Co, Ni and Ru/Al₂O₃ are mostly active in the Sabatier reaction, while Fe is active in the reverse water gas shift reaction. Cu as pristine metal shows no catalytic activity. C2+ hydrocarbons were formed on Co in low concentrations. For the calculation of the activation energy, the kinetically determined temperature range of the reaction is identified with a high resolution in time by means of a quantitative gas analysis method with an online mass spectrometer. The observation activation energy of the CO₂ hydrogenation reaction was determined to be 50 kJ/mol over Fe, 77 kJ/mol over Co, 74 kJ/mol over Ni and 73 kJ/mol over the Ru/Al₂O₃ catalyst. This indicates similar reaction pathways over Co, Ni and Ru/Al₂O₃ and a different reaction mechanism on Fe.