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Rational Design of Ceria-Supported Non-Noble Metal Nanoalloys as Catalysts for the Selective Direct Conversion of Methane to Methanol

Periodic Reporting for period 1 - 4lessCH4 (Rational Design of Ceria-Supported Non-Noble Metal Nanoalloys as Catalysts for the Selective Direct Conversion of Methane to Methanol)

Reporting period: 2019-09-01 to 2021-08-31

Catalysis has contributed strongly to human well-being as a result of the concomitant great advances made in recent years in industrial, agricultural, energy and health technologies. However, rapid global industrialization, coupled with population growth and a concomitant increase in energy consumption, have caused a major threat to the environment due to a massive increase in the production and ventilation of poisonous and dangerous pollutants.

Therefore, it is necessary to rethink chemical production and refinery methodologies, with the consequent need for new catalytic approaches. Future chemical production requires radically new types of catalysts and catalytic technologies. In this sense, the advanced design of new catalysts is one of the most fundamental objectives, but it is extremely challenging due to the complexity of the powder-based catalysts of industrial relevance. In general, fundamental studies are performed using experimental or theoretical model catalysts of increasing complexity to disentangle the effects of the multiple variables that influence catalytic performance. The activity of heterogeneous catalysts depends on an optimized interaction of their microstructure and chemical interface with the reaction environment. Achieving the optimal microstructure for any particular catalytic process, therefore, requires a substantial effort in terms of synthesis and characterization, and is very challenging. Furthermore, fundamental knowledge of the structure ↔ reactivity relationships that can be obtained prior to catalyst synthesis from computational studies is essential for the rational design of efficient catalysts.

In this sense, the 4lessch4 project addresses the conversion of methane (CH4), which is a powerful greenhouse gas, to methanol (CH3OH) that can be used as a liquid fuel as well as a solvent for the production of inks, adhesives, and colorants. The direct oxidation of methane into methanol at low temperatures has long been a holy grail of catalysis. The conversion of methane to methanol is difficult because the reaction typically progresses all the way to yield CO and/or CO2.

We show theoretical results that indicate that a catalyst consisting of bimetallic NiCu nanoparticles supported on CeO2 can convert methane to methanol with higher selectivity than other catalysts such as Ni/CeO2. This is due to the fact that two metals are combined where Ni favors, while Cu disfavors, the breaking of the C-H bond. By selecting the correct composition of the NiCu nanoparticles, we are able to convert methane to methanol in a more efficient way.
The execution of the 4lessch4 project suffered some delays due to the global pandemic situation caused by COVID. Although the project is essentially theoretical, it has a strong experimental contribution that was delayed due to the closure of the laboratories in the US during 2020 and the partial opening in 2021.
In view of this situation, it was decided to expand the project and study other relevant reactions such as the steam reforming of methane (SMR, CH4 + H2O → CO + 3 H2), the steam reforming of ethanol (SRE, C2H5OH+ 3 H2O → CO2 + 6 H2), and the reverse water gas shift (RWGS, CO2 + H2 → CO + H2O) for which our network of experimental collaborators had obtained results before the pandemic.

Objective 1: Structure: Computational models
The Fellow develop atomistic models of differently oriented CeO2 nanofacets, Ce1-xZrxO2, CeGaO2 Pt-, Pd, Fe-, and bimetallic Ni-Cu-CeO2 catalysts. Once having identified the most stable structure, their electronic and magnetic properties were investigated. The electronic structure was interpreted in terms of the calculated electronic densities of states, spin densities, Bader charges, etc.

Objective 2: Reactivity: Chemical properties and reaction mechanisms
The adsorption and activation of several molecules on the catalysts described in objective 1 were analyzed: CH4, CH3OH, H2O, CO, CO2, and CH3CH2OH. In particular, the Fellow studied the chemical properties of ethoxy species resulting from the SRE reaction over Ga-doped CeO2 as well as over differently oriented CeO2 nanofacets. The SMR reaction was studied on a low-loaded Ni 3-D catalyst supported on CeO2, finding that strong interaction between Ni and the ceria support, as well as the low metal loading, are crucial for the reaction to proceed in a coke-free and efficient way. Our calculations of the elementary steps of the RWGS reaction on model Ni/La-doped-ceria catalysts aid toward the microscopic understanding of the nature of the active sites.

The activation of methane was also studied as the necessary step in the conversion of methane into methanol over the following model catalysts: Pt-, Pd-, Fe-, and bimetallic NiCu-CeO2 catalysts. In the cases of catalysts with a single metal, it is observed that methane is easily activated and fully decomposes whereas, in the NiCu catalyst, the activation and extent of decomposition depend on the concentration of each metal.

Objective 3: Sinergy between theory and experiment
Most of the objectives proposed have been motivated by recent cutting-edge experimental insight. In particular, this project has been carried out in close collaboration with Dr. J. A. Rodriguez’s group at BNL in the US, Dr. C. T. Campbell group at the University of Washington in the US, Dr. C. Woell group at KIT in Germany, Dr. F. Prado group at the UNS in Argentina, Dr. A. Bonivardi at the UNL in Argentina as well as Dr. C. Alvarez-Galvan and Dr. A. Martínez-Arias at the ICP in Spain.

The high impact of the joint publications generated in this project is a good indicator of the quality of the numerous obtained results and the excellent synergy between experiment and theory.

Objective 4: Dissemination and public engagement
The Fellow has participated as co-author in 6 scientific events and 5 outreach events to the general public. The oral participation in the ACS Fall 2021 meeting and the oral invitation to the RIVA Online 2021 Iberian Vacuum Conference (Reunion Ibérica de Vacío, RIVA) stand out.

Regarding the public outreach, the fellow has participated in European Researchers' Night 2021 and the Satellite Meeting to the SAFIS (Argentine Society of Physiology) 2021 Annual Meeting, under the topic “Access to international scholarships”. He has also participated in Science week 2019, 2020, and 2021 as well as in the “ICP Open Doors Days”.
It is expected that 4 more papers will be published in high impact journals and that the results will be presented in the following congresses: PSI-K 2022 conference at Lausanne (Switzerland); International Conference on Theoretical Aspects of Catalysis (ICTAC-2022), Lyon (France); ACS Spring 2022, San Diego, US; ZCAM-ASEVA, Zaragoza (Spain).
The Fellow has been advising a full-time PhD student at the ICP-CSIC and a Postdoctoral student at the UNS-CONICET. In both cases, the results have been published where the Fellow has been the corresponding author.
It is also important to mention that thanks to this project and his capacity, the fellow is in a position to request promotion to an independent researcher at the National Council for Scientific and Technical Research (Argentina).
The Fellow led computer time projects, and has been awarded an Extension of Individual Projects of the MSCA-IF, in order to reinforce the excellence of the Spanish National Research Council (CSIC).
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