Catalysis is playing a pivotal role in the efficient implementation of chemical reactions and in the establishment of a modern chemical industry. However, most industrial heterogeneous catalysts work in extreme conditions of temperature and pressure far removed from thermodynamic equilibrium.[1] Consequently, achieving high activity and selectivity remains still highly challenging for many catalytic conversion processes with undesirable side-reactions, which in addition to selectivity issues lead often to deactivation on-stream that requires (costly) process downtime for catalyst regeneration.

The primary motivation for coupling thermal and photo-excitation in a one-pot operation – and giving rise to the photothermal catalysis field – is driven by the imperative of reducing those drawbacks, and more globally of improving the chemical process sustainability, ie. with a reduction in environmental and energetic footprint.[2]

Indeed, solar energy is considered as one of the ideal (abundant) sources of renewable energy for the future to be linked with chemical processing, with the potential to meet the projected energy demand, as well as the global needs of the society. Its harnessing and integration into catalytic processing will undoubtedly play a major role in the development of more sustainable chemical processes, and in the establishment of a low carbon energy economy. The so-called solar-driven catalysis – and more precisely the photothermal catalytic strategy – is consequently an emerging area being pivotal to industrial renewal, that arouses a lot of hope for realizing the difficult transition from fossil-based resources towards an environmentally benign, CO2-neutral 'renewable-based chemicals' industry. This strategy aims at accelerating reaction rates and globally enabling higher performances – under fixed conditions, and/or at orientating the reaction selectivities enabling similar performances to be achieving under milder conditions, such as eg. at a lower temperature.

The gas phase decomposition of formic acid into hydrogen was taken as model reaction. It is also a reaction of high interest, as formic acid is a promising hydrogen carrier for hydrogen storage and can be used as internal hydrogen source (hydrogen donor) for performing catalytic transfer hydrogenation reactions. This hydrogen donor-mediated approach instead of using external pressurized hydrogen is a step forward in the design of sustainable hydrogenation processes enabling the production of high value-added chemicals. Formic acid, a glucose derivative issued from the lignocellulosic biomass hydrolytic conversion chain, can decompose into hydrogen and carbon dioxide via dehydrogenation (1) or generate carbon monoxide and water via dehydration (2).

HCOOH → CO2 + H2  (1)
HCOOH → CO + H2O (2)

Ru/TiO2 catalysts have been recently reported as very promising systems capable of meeting the challenge of catalyzing both the dehydrogenation of formic acid to hydrogen and the hydrogenation of valuable substrates under similar reaction conditions.[3] Therefore, this communication aims at reporting on the behavior of TiO2 and g-C3N4 supported Ru photothermal catalysts in the formic acid decomposition.

In addition, the use of semiconductor materials as catalyst support – namely medium surface area g-C3N4 and TiO2 – allows the catalyst preparation to be achieved via a photo-assisted method that avoids the use of external reducing agents or harsh temperature conditions, and that further allows a fine control of the Ru nanoparticle size distribution till the sub-nanometric level.

Combining UV-A light and heat as excitation source increased considerably the hydrogen production rate of the catalysts studied. The main findings are that Ru/g-C3N4 catalysts with highly dispersed Ru nanoparticles appear for the first time to be remarkable photo-thermal-catalysts, that clearly outperform the Ru/TiO2 reference catalyst in terms of hydrogen production as well as of selectivity to H2 and CO2, with the ability to produce larger amounts of CO-free hydrogen flows. The influence of the light irradiance on the catalytic behaviour and performances will be detailed, and the results will be discussed in terms of H2 and CO production rates, as well as of apparent activation energy of the reaction. This open the door to the implementation of hydrogenation reactions in a more sustainable way at softer/milder conditions.

The IdEx Program of the University of Strasbourg is thanked for funding the PhD
fellowship of Javier Ivanez.

[1] Imbihl, R.; Behm, R. J.; Schlögl, R. Phys. Chem. Chem. Phys. 2007, 9 (27), 3459. https://doi.org/10.1039/b706675a.
[2] Tang, S.; Sun, J.; Hong, H.; Liu, Q. Front. Energy 2017, 11 (4), 437–451. https://doi.org/10.1007/s11708-017-0509-z.
[3] Ruppert, A. M.; Grams, J.; Jȩdrzejczyk, M.; Matras-Michalska, J.; Keller, N.; Ostojska, K.; Sautet, P. ChemSusChem 2015, 8 (9), 1538–1547. https://doi.org/10.1002/cssc.201403332.