Periodic Reporting for period 1 - MEXCAT (Metal EXsolved CATalysts for the CO2 valorisation to methanol: design, synthesis, and characterisation of next-generation catalysts, unravelling their structure-activity relationship)
Reporting period: 2022-11-01 to 2024-10-31
With more than 60% of greenhouse gas emissions produced by the energy sector, increased efforts are required to meet the 2050 climate goals established by the European Union (EU) to become the world’s first climate-neutral continent, as energy demand is not anticipated to decline. Moreover, while the transition to renewable energy will extensively contribute to lowering such emissions, CO2 sources from certain power and process industries will likely persist in the long term. Recent initiatives developed by the European Commission are specifically focussing on CO2 chemical valorisation, aiming to convert CO2 into various useful products, including chemicals and fuels. Specifically, conversion by hydrogenation of excess CO2 would potentially decrease emissions related to chemical feedstock synthesis by ∼90% compared to current industrial solutions. In this context, converting CO2 to methanol (MeOH), a widely applicable component for fuels, plastics, paints, and textile chemicals, is gaining significant interest for industrial implementation, owing to its significant economic potential. However, the major challenge related to the process is the low performance, mainly caused by limited catalyst selectivity and deactivation caused by active metal nanoparticles (NP) sintering.
MEXCAT aimed at developing new stable and selective catalysts for the efficient catalytic CO2 hydrogenation to MeOH, employing the novel method of NP “exsolution”, where catalytic nanoparticles are generated in situ from an active metal-substituted oxide. Subjecting this tailor-designed material to a thermal reduction causes metal ion diffusion to the oxide surface, subsequently nucleating or “exsolving” into anchored, and hence highly stable, nanoparticles. Although several literature examples reported the successful use of exsolved materials for catalytic applications, the exsolution pathway had barely been explored for the catalytic CO2 hydrogenation to MeOH. Building upon the proven compositions of state-of-the-art catalysts for this hydrogenation reaction, the project aimed at designing and synthesising novel exsolved materials as next-generation catalysts for the CO2 hydrogenation to MeOH, to reach the ambitious goal of a 10% MeOH yield. To then address the knowledge gap in the understanding of the exsolved materials superior catalytic properties, the project also aimed to quantitatively analyse the chemical, structural, and microstructural features governing their catalytic mechanisms using a set of in situ/operando cutting-edge characterisation techniques. The ultimate goal was to create a combined method for identifying the principles that would determine an enhancement of the catalytic activity of the developed materials. This set of principles would then enable the rational design of an optimised catalyst, informed by this assessment.
MEXCAT aimed at developing new stable and selective catalysts for the efficient catalytic CO2 hydrogenation to MeOH, employing the novel method of NP “exsolution”, where catalytic nanoparticles are generated in situ from an active metal-substituted oxide. Subjecting this tailor-designed material to a thermal reduction causes metal ion diffusion to the oxide surface, subsequently nucleating or “exsolving” into anchored, and hence highly stable, nanoparticles. Although several literature examples reported the successful use of exsolved materials for catalytic applications, the exsolution pathway had barely been explored for the catalytic CO2 hydrogenation to MeOH. Building upon the proven compositions of state-of-the-art catalysts for this hydrogenation reaction, the project aimed at designing and synthesising novel exsolved materials as next-generation catalysts for the CO2 hydrogenation to MeOH, to reach the ambitious goal of a 10% MeOH yield. To then address the knowledge gap in the understanding of the exsolved materials superior catalytic properties, the project also aimed to quantitatively analyse the chemical, structural, and microstructural features governing their catalytic mechanisms using a set of in situ/operando cutting-edge characterisation techniques. The ultimate goal was to create a combined method for identifying the principles that would determine an enhancement of the catalytic activity of the developed materials. This set of principles would then enable the rational design of an optimised catalyst, informed by this assessment.
The first part of the fellowship focussed on the development of materials with simpler compositions, to evaluate whether it would be possible to exsolve from structures different than perovskite, with which the MSCA Fellow had in-depth previous experience. Cu-based single-phase fluorite materials were hence developed, studied and tested. Three distinct wet-synthesis methods were assessed and compared based on the materials obtained. A thorough multiscale characterisation study allowed to identify the most successful synthesis method in terms of: phase purity, microstructure, specific surface area, and composition. The best exsolution conditions were then identified mainly by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning-TEM energy dispersive spectroscopy (STEM-EDX) after targeting specific temperature and dwell-time ranges. To verify and understand the influence of the properties of the different materials on the catalytic activity during the CO2 hydrogenation to MeOH reaction, the most promising materials were tested in a high-pressure test rig in isothermal conditions for ~20 h, to evaluate their stability during operation, and over a range of temperatures relevant to industrial application (180-300 °C) to assess the materials activity and selectivity performances. This allowed to identify the behaviour of the different developed catalysts during the reaction. To better understand the reaction mechanism, operando FTIR tests were then performed. These tests allowed to elucidate the reaction pathway of the different catalysts while identifying the reaction intermediates developing while flowing the reagent mixture (3H2:CO2) in a heated FTIR cell, at 7 bar and in a temperature range as studied in the test rig. The best-performing material was monitored in real time with a high-resolution electron microscope in operando conditions. Using this combined approach, an experimental and analysis protocol for the characterisation of the catalytic activity mechanism of exsolved materials was proposed.
The second part of the fellowship focussed on the design of an optimised material building upon proven compositions of state-of-the-art catalysts for the investigated reaction. The aim was to investigate the possibility of obtaining pure-phase spinel structures from which co-exsolve Cu and Zn/ZnO and to evaluate and compare their overall performance in high-pressure hydrogenation tests. By tuning the composition and the exsolution temperature, it was possible to tune the catalysts’ selectivity and activity. These materials were then studied mechanistically, specifically by performing operando FTIR studies at pressure (7 bar). Due to the novelty of these systems, the exsolution mechanism was also investigated in situ, by performing electron spectroscopy experiments (EELS) while heating the materials in ultra-high vacuum using a heating TEM holder, and simultaneously acquiring high-resolution images and videos of the NP formation process.
The second part of the fellowship focussed on the design of an optimised material building upon proven compositions of state-of-the-art catalysts for the investigated reaction. The aim was to investigate the possibility of obtaining pure-phase spinel structures from which co-exsolve Cu and Zn/ZnO and to evaluate and compare their overall performance in high-pressure hydrogenation tests. By tuning the composition and the exsolution temperature, it was possible to tune the catalysts’ selectivity and activity. These materials were then studied mechanistically, specifically by performing operando FTIR studies at pressure (7 bar). Due to the novelty of these systems, the exsolution mechanism was also investigated in situ, by performing electron spectroscopy experiments (EELS) while heating the materials in ultra-high vacuum using a heating TEM holder, and simultaneously acquiring high-resolution images and videos of the NP formation process.
The work carried out within MEXCAT allowed to advance in the development of novel, more stable and active catalysts for CO2 hydrogenation to MeOH. By controlling the materials properties through material design for optimum efficiency, two optimised new catalysts were proposed. The use of advanced operando characterisation techniques allowed to significantly advance our knowledge in exsolved catalysts activity mechanism. All the novel samples successfully showed enhanced stability, with no sintering/coalescence of the exsolved nanoparticles observed after long-term catalytic testing, hence contributing to scientific advances both in the field of CO2 valorisation for carbon-neutral fuel production and in the field of exsolved materials. Overall, this work highlights the potential of new exsolved catalysts for the efficient CO2 valorisation to methanol, opening the possibility of overcoming some of the challenges hindering the implementation of such catalytic processes at the industrial level, potentially advancing the technology of MeOH synthesis catalysts through achieving competitive activity and stability. A lower MeOH yield than the initial goal was achieved; however, when calculating the thermodynamic equilibrium values at our operating conditions, which used considerably lower pressures than industrial conditions (23 vs. 50-100 bar), it was found that all the developed materials consistently achieved yields close to or equal to the thermodynamic equilibrium. By developing new stable materials for methanol synthesis from CO2, these results are expected to significantly impact the economy, society and technology in the long term. Having explored the potential of exsolved materials for this CO2 valorisation reaction, by tailoring the properties of these materials even further, we anticipate obtaining novel catalysts with even higher efficiency, reduced costs, and increased yields, eventually resulting in profits for industries as well as the societal impact of the reduced presence of atmospheric carbon dioxide, in line with EU priorities.