Mechanistic investigations of chemical reactions on 2D MXenes and metal surfaces from first-principles

Abstract: Chemical reactions on surfaces play a central role both for our daily life and industrial purposes, including the storage and release of energy, as well as the formation of new materials. To achieve high efficiency, catalysis lies in the heart of chemical reactions as it plays a critical role in accelerating the chemical transformation to target products. However, environmental issues arise as the applications of catalytic technologies and current synthetic approaches such as pollution from undesirable byproducts and massive emission of carbon dioxides due to the usage of fossil fuels. This calls for developing improved strategies for fabricating new materials with highly efficient catalytic properties. In recent years, on-surface chemical reactions have also been used to synthesize new low-dimensional materials with atomic precision, by coupling molecules into nanostructures. It is crucial to not only obtain high activity for chemical reactions, but also achieve distinct selectivity towards desired products. For this purpose, understanding mechanisms of target chemical reactions and origins of catalysts’ activity are of great significance to facilitate chemical processes.In this thesis, three types of chemical reactions are investigated within the framework of density functional theory (DFT), in which chemical reactions relevant for both heterogeneous catalysis and electrochemical synthesis are considered on two-dimensional transition metal carbides (2D MXenes), and chemical reactions for synthesizing organic nanostructures are studied on metal surfaces. Focusing on one of the most fundamental chemical reaction, C(sp3)-H activation, we demonstrate that MXenes can serve as highly efficient heterogeneous catalysts and exhibit high activity. The thermally triggered C-H activations are shown to follow the “radical-like” mechanism on MXenes, in which O terminations serve as active sites. By adopting the hydrogen affinity (EH) as a descriptor, both the geometry configuration and the catalytic activity of MXenes can be quantitatively characterized.In the context of on-surface synthesis, we theoretically propose reaction mechanisms of two types of chemical reactions on surface. A new strategy for constructing C-C bonds via the desulfonylation reaction was achieved experimentally for the first time by collaborators. With DFT calculations, an observed discrepancy between Ag(111) and Au(111) is ascribed to interactions between surfaces and molecules. Secondly, the formation mechanism of the 2D biphenylene network (BPN), a recently realized carbon allotrope formed by intermolecular HF zipping on Au(111), has been computationally investigated.With the tool of DFT calculations, a single Ni atom catalyst supported by Ti3C2T2 MXenes for electrochemical nitrogen reduction has been theoretically proposed. Such single atom catalyst (SAC) is computationally screened from three aspects including stability, activity, and selectivity. Our theoretical results show that not only the catalytic performance of the Ni SAC predicted by screening criteria can be verified, but also a H rich environment can be beneficial for the electrochemical nitrogen reduction on such SACs.In summary, first-principles calculations have been performed to evaluate the catalytic performance of 2D MXenes towards C-H activation, unravel formation mechanisms of organic materials synthesized via on-surface reactions, and design effective catalysts towards the synthesis of ammonia. It is anticipated that this thesis can pave the way for the rational design of high-efficient catalysts for various reactions and shed lights on developing synthetic strategies of unprecedented organic materials.