(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 [1]. CO₂ hydrogenation is considered as promising for the production of bulk chemicals and fuels, i.e. methane, higher hydrocarbons, CO, methanol and formic acid [2], 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.

2. Experimental

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 ml_NTP/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 [3] 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 “via oxygenate” 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 [4].

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) [4] with an apparent activation energy ranging in-between 60-75 kJ/mol [5]. 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 [5] 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[4].

4. Conclusions.

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.

References

1. G. Centi, S. Perathoner, Green Carbon Dioxide, Advances in CO₂ Utilization, Wiley 2014
2. W. Wang, J. Gong, Front. Chem. Sci. Eng. 5 (2011) 2-10
3. G. Garbarino, P. Riani, L. Magistri, G. Busca, Int. J. Hydrogen En. 39 (2014) 11557-11565
4. G. Garbarino, D. Bellotti, P. Riani, L. Magistri, G. Busca, Int. J. Hydrogen En. 40 (2015) 9171-9182
5. G. Garbarino, C. Wang, T. Cavattoni, E. Finocchio, P. Riani, M. Flytzani-Stephanopoulos, G. Busca, Appl. Catal. B: Environ. 248 (2019) 286-297
6. G. Garbarino, D. Bellotti, E. Finocchio, L. Magistri, G. Busca, Catalysis Today, 277 (2016) 21-28