Combustion is a complex chemical system which involves thousands of chemical reactions and generates hundreds of molecular species and radicals during the process. In this work, a neural network-based molecular dynamics (MD) simulation is carried out to simulate the benchmark combustion of methane. During MD simulation, detailed reaction processes leading to the creation of specific molecular species including various intermediate radicals and the products are intimately revealed and characterized. Overall, a total of 798 different chemical reactions were recorded and some new chemical reaction pathways were discovered. We believe that the present work heralds the dawn of a new era in which neural network-based reactive MD simulation can be practically applied to simulating important complex reaction systems at ab initio level, which provides atomic-level understanding of chemical reaction processes as well as discovery of new reaction pathways at an unprecedented level of detail beyond what laboratory experiments could accomplish.
We perform ab initio DFT+U calculations and experimental studies of the partial oxidation of methane to syngas on iron oxide oxygen carriers to elucidate the role of oxygen vacancies in oxygen carrier reactivity. In particular, we explore the effect of oxygen vacancy concentration on sequential processes of methane dehydrogenation, and oxidation with lattice oxygen. We find that when CH adsorbs onto Fe atop sites without neighboring oxygen vacancies, it dehydrogenates with CH radicals remaining on the same site and evolves into COvia the complete oxidation pathway. In the presence of oxygen vacancies, on the other hand, the formed methyl (CH) prefers to migrate onto the vacancy site while the H from CH dehydrogenation remains on the original Fe atop site, and evolves into CO via the partial oxidation pathway. The oxygen vacancies created in the oxidation process can be healed by lattice oxygen diffusion from the subsurface to the surface vacancy sites, and it is found that the outward diffusion of lattice oxygen atoms is more favorable than the horizontal diffusion on the same layer. Based on the proposed mechanism and energy profile, we identify the rate-limiting steps of the partial oxidation and complete oxidation pathways. Also, we find that increasing the oxygen vacancy concentration not only lowers the barriers of CH dehydrogenation but also the cleavage energy of Fe-C bonds. However, the barrier of the rate-limiting step cannot further decrease when the oxygen vacancy concentration reaches 2.5%. The fundamental insight into the oxygen vacancy effect on CH oxidation with iron oxide oxygen carriers can help guide the design and development of more efficient oxygen carriers and CLPO processes.
The
cyclic redox reactivity of metal oxides plays an important
role in many energy fields such as fuel cells, photocatalysis, and
chemical looping. In chemical looping systems, oxygen carriers are
required to have high reactivity, recyclability, and high oxygen carrying
capacity. We utilize catalytic lanthanum dopants to dramatically change
the reactivity with carbonaceous fuels while maintaining or even improving
the recyclability of iron-based oxygen carriers. A low concentration
of La dopant is applied to maintain the high oxygen carrying capacity.
These results are substantiated by ab initio DFT+U and thermochemistry
analysis and will have a significant impact on future chemical looping
oxygen carrier design.
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