Ultradispersed metal catalysts for energy applications involving hydrogen – UltraCat

Through this project, beyond scanning transmission electron microscopy (STEM), we have extensively used in situ and operando characterization techniques, which allow investigating catalyst structure and catalytic mechanisms in realistic operating conditions, i.e. in the presence of gases at controlled temperature: i) X-ray photoelectron spectroscopy (NAP-XPS) and X-ray absorption spectroscopy (XANES, EXAFS), both performed at synchrotrons (ESRF and SOLEIL, France; ALBA, Spain); ii) Diffuse reflectance infrared spectroscopy (DRIFTS). This has led us to unravel dynamical processes such as single metal atom aggregation, and to characterize catalyst active sites at the atomic scale.

The main results of UltraCat are categorized per application, itself related to a specific catalytic material:
• Elaboration and characterization of ultradispersed Mo/TiO2 as a stable catalyst for CO2 hydrogenation to methanol [1].
• Elaboration and characterization of Ir-N-C single-atom catalyst for selective hydrogenation of butadiene to 2-butene [2].
• Characterization of palladium nanoparticle size effect on hydrogen trapping capacity [3].
• Evidence of the aggregation of single platinum atoms under reaction conditions: CO oxidation [4] and hydrogen photoproduction [5,6].

Initial objectives regarding elaboration and characterization of ultradispersed catalysts for CO2 hydrogenation have been reached. The Mo/TiO2 system exhibits unexpected properties for methanol production, provided that the TiO2 type is suitably selected.
However, the methanol yield could possibly be increased through further work on the catalyst. In this regard, other types of supports, such as transition metal carbides, are currently investigated.
Besides, the Ir-N-C SAC has shown a high selectivity to 2-butene from butadiene hydrogenation, which is at variance from conventional metal catalysts leading to 1-butene. Future work will aim at substituting iridium for an abundant metal.

Six articles have been published in international peer-reviewed journals [1-6] and twelve communications have been given at conferences. The discovery of Mo/TiO2 catalyst for CO2 hydrogenation to methanol has led to several communicating actions to non-specialist audiences [7].

[1] Ultradispersed Mo/TiO2 catalysts for CO2 hydrogenation to methanol
T. Len, M. Bahri, O. Ersen, Y. Lefkir, L. Cardenas, I.J. Villar-Garcia, V. Pérez Dieste, J. Llorca, N. Perret, R. Checa, E. Puzenat, P. Afanasiev, F. Morfin, L. Piccolo
Green Chemistry 23, 7259-7268 (2021)
doi.org/10.1039/D1GC01761F

[2] Unveiling the Ir single atoms as selective active species for the partial hydrogenation of butadiene by operando XAS
W. Liu, F. Morfin, K. Provost, M. Bahri, W. Baaziz, O. Ersen, L. Piccolo, C. Zlotea
Nanoscale 14, 7641-7649 (2022)
doi.org/10.1039/D2NR00994C

[3] Size-dependent hydrogen trapping in palladium nanoparticles
W. Liu, Y. Magnin, D. Förster, J. Bourgon, T. Len, F. Morfin, L. Piccolo, H. Amara, C. Zlotea
Journal of Materials Chemistry A 9, 10354-10363 (2021)
doi.org/10.1039/D0TA12174F

[4] Dynamics of single Pt atoms on alumina during CO oxidation monitored by operando X-ray and infrared spectroscopies
C. Dessal, T. Len, F. Morfin, J.L. Rousset, M. Aouine, P. Afanasiev, L. Piccolo
ACS Catalysis 9, 5752-5759 (2019)
doi.org/10.1080/10.1021/acscatal.9b00903

[5] Influence of Pt nanoparticle size and reaction phase on the photocatalytic performances of ultradispersed Pt/TiO2 catalysts for hydrogen production
C. Dessal, L. Martinez, C. Maheu, T. Len, F. Morfin, J.L. Rousset, E. Puzenat, P. Afanasiev, L. Soler, J. Llorca, L. Piccolo
Journal of Catalysis 375, 155-163 (2019)
doi.org/10.1016/j.jcat.2019.05.033

[6] Operando X-ray absorption spectroscopy investigation of photocatalytic hydrogen evolution over ultradispersed Pt/TiO2 catalysts
L. Piccolo, P. Afanasiev, F. Morfin, T. Len, C. Dessal, J.L. Rousset, M. Aouine, F. Bourgain, A. Aguilar-Tapia, O. Proux, Y. Chen, L. Soler, J. Llorca
ACS Catalysis 10, 12696-12705 (2020)
doi.org/10.1021/acscatal.0c03464

[7] www.albasynchrotron.es/en/media/news/researchers-have-discovered-a-new-catalyst-for-the-conversion-of-carbon-dioxide-to-methanol

Heterogeneous catalysis, which is involved in about 80% of industrial processes, has a prominent role to play in the quest for clean energy technologies. A major challenge lies in the development of atom-efficient catalysts, i.e. with both maximum efficiency (activity, selectivity, stability) and minimum amount of rare and expensive materials (especially noble metals, which are used in many catalytic processes and in automotive converters). In recent years, due to the development of last-generation electron microscopies, a fast growing interest for so-called “single-atom catalysts”, i.e. catalysts constituted of metals atomically dispersed on a stabilizing support, is observed. Similarly to the “nano” wave which has drastically changed materials science and opened the way to a variety of applications, the additional downscaling may open a new era. As a matter of fact, subnanometric downsizing gives rise to a dramatic change in the electronic properties of metals, which in turn leads to promising catalytic performances. However, as synthesis and characterization of these materials are challenging, the works in this field are still restrained to a limited number of catalytic systems.
The UltraCat project aims at the design and investigation of new catalysts based on metals “ultradispersed” in the form of subnanometric particles down to isolated atoms, supported on mesoporous oxides for reactions with high interest for clean-energy-production processes involving hydrogen. The related challenges will be the use of non-noble metals, the tuning of metal loadings so as to increase the catalytic yields without significant decrease of the metal dispersion, and the softening of reactions conditions (lower operating temperatures and pressures).
The catalyst preparation methods envisaged in the project are both simple for implementation and original by making use of mixed-oxide and intermetallic alloy chemistry concepts through thermal post-treatments. The first considered reaction is the preferential oxidation of CO in H2 (CO-PROX), which allows ultimate purification of hydrogen for proton-exchange-membrane fuel cells. The second reaction is the hydrogenation of CO2 to methanol, which is a promising way of valorizing anthropogenic CO2 into a high-value platform molecule and energy carrier. Both reactions are known to benefit from a high intimacy between the metal and the support, making the ultradispersion approach relevant. The hydrogen sorption properties of the materials (adsorption, absorption, hydride formation), which are dispersion-dependent and strongly linked to catalytic ones (though often neglected), will be investigated in parallel to catalytic reactions. In order to better understand the dynamic gas-solid interaction phenomena involved in the reactions, and in turn guide the materials conception, the catalytic systems will be investigated by advanced in situ/operando techniques such as aberration-corrected environmental transmission electron microscopy, vibrational spectroscopies (infrared, Raman), and synchrotron X-ray spectroscopies (XAS, XPS). These methods will be complemented by materials and reaction modeling through computer-based simulations using the density functional theory (DFT) within rationalizing and predictive approaches.
The UltraCat consortium covers all the above aspects in an interdisciplinary manner (chemistry / physics, catalysis / hydrogen sorption, experiment / theory) while relying on preexisting informal collaborations and promising preliminary results, which should favor the success of this project.