Periodic Reporting for period 1 - TMC4MPO (Transition metal carbides as efficient catalysts for methane partial oxidation)
Berichtszeitraum: 2021-03-01 bis 2023-02-28
Methane (CH4) is an extremely stable molecule, a major component of natural gas. There is therefore an imperative, at least in the short to medium term, for the capture and subsequent recycling of CH4. The most economically feasible route for the conversion of CH4 into more valuable chemicals is via syngas, a mixture of CO and H2, which can be achieved via the methane partial oxidation (MPO) reaction.
Nickel is the most common catalyst used for the MPO. It has the advantage of being inexpensive; however, it suffers from deactivation by carbon deposition, sintering, or volatilisation. Noble metal catalysts possess exceptional catalytic activity and stronger carbon deposition resistance; yet their scarcity and high prices make it necessary to optimise their use for industrial applications. Transition metal carbides (TMC), however, are resistant against carbon deposition and much cheaper than noble metals. Moreover, they are known to exhibit versatile catalytic properties, with the inherent benefit of their abundance and affordable cost. Apart from the activity of the TMCs per se, they are excellent supports for small metal particles. This line originated from the theoretical discovery of the ability of TiC to modify the electronic structure of supported Au particles, thereby drastically increasing their catalytic activity through strong metal-support interactions between Au and TiC.
Motivated by the challenges just noted, the present proposal envisions the computational prediction of TMC-supported metal catalysts for the MPO reaction. This high-level aim is achieved via systematic screening of a large set of such materials by means of state-of-the-art Density Functional Theory calculations and Kinetic Monte Carlo (KMC) simulations (Figure 1). In particular, the overall objectives of the project are:
1. Study the stability of a large set of materials consisting of metal clusters supported on TMCs.
2. Screen the set of TMCs based on an evaluation of their ability to activate CH4.
3. Explore the complete energy profiles for MPO of the selected TMC-based catalysts.
4. Investigate the performance of the most promising candidates by KMC simulations.
Nickel is the most common catalyst used for the MPO. It has the advantage of being inexpensive; however, it suffers from deactivation by carbon deposition, sintering, or volatilisation. Noble metal catalysts possess exceptional catalytic activity and stronger carbon deposition resistance; yet their scarcity and high prices make it necessary to optimise their use for industrial applications. Transition metal carbides (TMC), however, are resistant against carbon deposition and much cheaper than noble metals. Moreover, they are known to exhibit versatile catalytic properties, with the inherent benefit of their abundance and affordable cost. Apart from the activity of the TMCs per se, they are excellent supports for small metal particles. This line originated from the theoretical discovery of the ability of TiC to modify the electronic structure of supported Au particles, thereby drastically increasing their catalytic activity through strong metal-support interactions between Au and TiC.
Motivated by the challenges just noted, the present proposal envisions the computational prediction of TMC-supported metal catalysts for the MPO reaction. This high-level aim is achieved via systematic screening of a large set of such materials by means of state-of-the-art Density Functional Theory calculations and Kinetic Monte Carlo (KMC) simulations (Figure 1). In particular, the overall objectives of the project are:
1. Study the stability of a large set of materials consisting of metal clusters supported on TMCs.
2. Screen the set of TMCs based on an evaluation of their ability to activate CH4.
3. Explore the complete energy profiles for MPO of the selected TMC-based catalysts.
4. Investigate the performance of the most promising candidates by KMC simulations.
We started by investigating the structural and electronic properties of all 77 combinations between 7 transition metals supported on 11 TMC slabs by means of periodic DFT calculations. The stability of the supported clusters was quantified by determining the binding strength of the cluster and their resistance to metal aggregate formation and fragmentation. Several interesting stability trends were identified with the phase type (cubic vs hexagonal), termination (C vs metal) or position of the metal in the periodic table. The electronic properties of these materials were investigated by computing the atomic charges and electron density polarisation of the supported particles. These results allowed to rationalise the stability trends mentioned above. In addition, our results showed that surface C vacancies significantly increase the binding strength of these clusters, making them more stable.
We also developed a high-throughput framework to discover stable and active CH4 (and also CO2) conversion catalysts from the original set of 77 supported clusters (Figure 2). All clusters strongly bind the TMC support but, while those supported on cubic TMCs are very resistant against fragmentation and weak against aggregation, the opposite behaviour is observed for clusters supported on hexagonal TMCs. Weaker binding to O (thus preventing oxidation) can be achieved by combining Pd, Pt or Au clusters with Group 4 or 5 TMCs. We also observed that those supported clusters not involving Au, cubic MoC and cubic WC are in general very stable in the presence of adsorbates, with negligible displacement and deformation. Regarding their catalytic activity, many combinations can dissociate CO2 and CH4 with negligible energy barriers, with Pt clusters being in general the most reactive ones. By considering all stability and activity metrics, we identified 8 combinations as promising catalysts, all of them being new for experimental validation, thus expanding the chemical space for efficient conversion of CH4 and CO2. Among them, Ni@VC and Ni@NbC stand out as the only candidates that consist of earth abundant elements only.
Apart from that, we have also studied in detail the adsorptive properties of clean TMC and metal clusters supported on TMCs. We showed that most TMC are able to activate stable CO2 molecules by charge transfer and that hexagonal carbides not only interact strongly with supported metal clusters than cubic carbides but also with most reaction intermediates. Unlike metals, the interaction between TMC and adsorbates is highly influenced by the electrostatic interactions. Most importantly, the use of TMCs as supports for small metal particles constitute a plethora of opportunities for catalyst design, as these systems cover a broad range of formation energies for the different adsorbates and are completely oblivious to the limitations imposed by the LS relations, allowing to tune the adsorption strength of key species.
Finally, we have also studied in detail the catalytic activity of Ni@VC for the MPO and other important industrial processes involving similar elementary steps. Ni@VC is chosen because its good stability and reactivity metrics, and the fact that it is formed by earth-abundant elements only. We computed the potential energy diagrams for Ni@VC and clean VC, as it can also be active for the reaction, and we are currently running the KMC simulations for the reaction.
The outcomes of this project have been shared in several national and international scientific conferences and meetings, in a UCL WordPress Blog (https://blogs.ucl.ac.uk/tmc4mpo/) and with the publication of a journal article (DOI: 10.1039/D1TA08468B). Another publication will come very soon as it is currently under review (see preprint at DOI: 10.26434/chemrxiv-2023-f13jf) and finally two more publications will come later this year.
We also developed a high-throughput framework to discover stable and active CH4 (and also CO2) conversion catalysts from the original set of 77 supported clusters (Figure 2). All clusters strongly bind the TMC support but, while those supported on cubic TMCs are very resistant against fragmentation and weak against aggregation, the opposite behaviour is observed for clusters supported on hexagonal TMCs. Weaker binding to O (thus preventing oxidation) can be achieved by combining Pd, Pt or Au clusters with Group 4 or 5 TMCs. We also observed that those supported clusters not involving Au, cubic MoC and cubic WC are in general very stable in the presence of adsorbates, with negligible displacement and deformation. Regarding their catalytic activity, many combinations can dissociate CO2 and CH4 with negligible energy barriers, with Pt clusters being in general the most reactive ones. By considering all stability and activity metrics, we identified 8 combinations as promising catalysts, all of them being new for experimental validation, thus expanding the chemical space for efficient conversion of CH4 and CO2. Among them, Ni@VC and Ni@NbC stand out as the only candidates that consist of earth abundant elements only.
Apart from that, we have also studied in detail the adsorptive properties of clean TMC and metal clusters supported on TMCs. We showed that most TMC are able to activate stable CO2 molecules by charge transfer and that hexagonal carbides not only interact strongly with supported metal clusters than cubic carbides but also with most reaction intermediates. Unlike metals, the interaction between TMC and adsorbates is highly influenced by the electrostatic interactions. Most importantly, the use of TMCs as supports for small metal particles constitute a plethora of opportunities for catalyst design, as these systems cover a broad range of formation energies for the different adsorbates and are completely oblivious to the limitations imposed by the LS relations, allowing to tune the adsorption strength of key species.
Finally, we have also studied in detail the catalytic activity of Ni@VC for the MPO and other important industrial processes involving similar elementary steps. Ni@VC is chosen because its good stability and reactivity metrics, and the fact that it is formed by earth-abundant elements only. We computed the potential energy diagrams for Ni@VC and clean VC, as it can also be active for the reaction, and we are currently running the KMC simulations for the reaction.
The outcomes of this project have been shared in several national and international scientific conferences and meetings, in a UCL WordPress Blog (https://blogs.ucl.ac.uk/tmc4mpo/) and with the publication of a journal article (DOI: 10.1039/D1TA08468B). Another publication will come very soon as it is currently under review (see preprint at DOI: 10.26434/chemrxiv-2023-f13jf) and finally two more publications will come later this year.
Overall, our results suggest that the deposition of small metal particles on TMCs can lead to stable catalysts with unique catalytic properties. Our calculations can provide guidance to experimentalists, who could synthesise and test the most promising materials. In addition to predicting the activity of these TMC-based catalysts, this project also provides important fundamental insights into the stability of TMC-supported metal particles, as well as their performance as catalysts in the MPO and other important industrial reactions within the context of CH4 and CO2 conversion. Moreover, our study identifies several key features determining the stability and catalytic activity of small metal clusters in general. Thus, the outcomes of the proposed project will have a broader impact within a wide range of practical catalysis applications.
The knowledge gained from this project will aid in the rational design and fabrication of more active and selective carbide catalysts for the conversion of CH4 and CO2 to valuable fuels and chemicals, making the process more efficient. By using CH4 and CO2 that are captured from large point sources, including power generation or industrial facilities that use fossil fuels or biomass, the production of syngas and its derived chemicals can be done in a more carbon neutral way, and the adverse environmental and health effects of its synthesis will be mitigated.
The knowledge gained from this project will aid in the rational design and fabrication of more active and selective carbide catalysts for the conversion of CH4 and CO2 to valuable fuels and chemicals, making the process more efficient. By using CH4 and CO2 that are captured from large point sources, including power generation or industrial facilities that use fossil fuels or biomass, the production of syngas and its derived chemicals can be done in a more carbon neutral way, and the adverse environmental and health effects of its synthesis will be mitigated.