(363h) On the Role of Reverse Water Gas Shift Reaction in Catalytic CO2 Hydrogenation over Ru- and Ni-Based Catalysts
CO2 hydrogenation using renewable hydrogen is a way for utilization of carbon dioxide in the optic of Carbon Capture and Recycling technologies . CO2 hydrogenation is considered as promising for the production of bulk chemicals and fuels, i.e. methane, higher hydrocarbons, CO, methanol and formic acid , even though one of the technology bottleneck resides in the wide availability of renewable hydrogen at reduced prices. In recent years, a great interest was centered on the development of processes and catalytic materials devoted to CO2 activation, but at the present, commercial formulations for CO2 hydrogenation catalysts are still lacking. Scientific development is still needed for reaction mechanisms, kinetics, reaction intermediates and interplay in-between reverse water gas shift (rWGS) and hydrogenation reactions either of CO or of CO2. Moreover, the role of promoters is still not clear and we recently focused on the addition of Lanthanum to the formulation of Ni/Al2O3 based catalysts.
Different catalytic systems based on nickel [3,4,5] and ruthenium [4,6] both in commercial and home-made formulations were used. Catalysts characterization was performed through XRD, UV-vis, IR, FE-SEM and IR of adsorbed CO. Catalytic experiments were carried out in a fixed-bed tubular silica glass flow reactor, operating isothermally and at atmospheric pressure, loaded with 700 mg of silica glass particles (60-70 mesh sieved) as an inert material mixed with variable amounts of catalyst. Different gaseous mixtures of CO2 and H2 (with excess H2) diluted with nitrogen were fed, with a total gas flow of 75 mlNTP/min. Temperature was varied step by step in-between 523 K and 773 K and back down to 523 K.
3. Results and discussion
For home-made Ni/Al2O3 (alumina with 5% SiO2, Siralox 5/170 from Sasol) catalysts  the activity towards rWGS and methanation is dependent from the Ni loading as well as from the redox state of Ni species. In particular, rWGS activity is higher as much as the Nickel loading is increased. From the data obtained it seems that the reduction of moderate loading NiO/Al2O3 catalyst is completely selective to methane with no CO coproduction and fast methanation occurs at the expense of CO intermediate on the corners of nanoparticles interacting with alumina, likely with a âvia oxygenateâ mechanism.
Considering Ru and Ni commercial formulations, i.e. 3%Ru/Al2O3 and 20%Ni/Al2O3 from ACTA spa, the best results are obtained on Ru-based material with 96% yield to methane and any CO coproduction at 573 K and a GHSV of 15000 h-1. For commercial nickel based catalyst, the best performance is obtained at 673 K achieving 80% methane yield coupled with some CO coproduction. At high temperatures, i.e. 723-773 K, both catalytic systems approach the thermodynamic equilibrium .
In both cases, reaction kinetic orders and apparent activation energies have been determined in the conditions where the hypothesis of differential reactor is applicable. In particular, for Ru/Al2O3 catalyst the measured kinetic orders are 0 for CO2 and slightly positive for H2 (0.39)  with an apparent activation energy ranging in-between 60-75 kJ/mol . In the case of 20% Ni/Al2O3 catalyst, the obtained reaction orders are 0.17 and 0.32 for CO2 and hydrogen respectively, with an apparent activation energy of 80 kJ/mol.
Lanthanum addition to Ni/Al2O3 promotes CO2 hydrogenation. In particular, an enhancement of CO2 methanation at low temperature is found and the best methane yield arises at 573 K at 55000 h-1 for Ni14La-Al2O3 (71% methane yield, 14 wt.% La2O3 in catalyst formulation) followed by Ni4La-Al2O3 and Ni37La-Al2O3 (53% and 52%, respectively) and Ni-Al2O3 (39%). The obtained results suggest  that lanthanum provides more sites to absorb CO2 and that it could activate Ni in CO2 hydrogenation. Moreover, La addition, hinders the production of CO by reverse water gas shift reaction at low temperature. Preliminary data suggests that on silica containing Ni/La-Si-Al2O3 (NixLSA, with x La2O3 wt. %), the same promoting effect is achieved upon increasing lanthanum loading but with a noticeable difference on the optimum lanthanum content. This might suggests that the interaction of both nickel and lanthanum with the support have a crucial role in CO2 methanation and metal support interaction is very important for this reaction. The unpromoted Ni0LSA have similar CO2 conversion and methane yield as the commercial Ni/Al2O3 catalyst reported in the literature.
The effect of SiO2 and/or La2O3 addition on Ni/Al2O3 catalyst in the methanation of CO2 has been investigated and the results were compared with Ru/Al2O3. It appears clear that the tailoring of the support properties might give rise to Ni/Al2O3 modified catalysts that might be competitive with more expensive Ru/Al2O3 ones.
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