C-H activation of light alkanes is one of the most important reactions for a plethora of applications but requires catalysts to operate at feasible conditions. MXenes, a new group of two-dimensional materials, have shown great promise as heterogeneous catalysts for several applications. However, the catalytic activity of MXenes depends on the type and distribution of termination groups. Theoretically, it is desired to search for a relation between the catalytic activity and the termination configuration by employing a simple descriptor in order to avoid tedious activation energy calculations. Here, we show that MXenes are promising for splitting C-H bonds of light alkanes. Furthermore, we present how a quantitative descriptor - the hydrogen affinity - can be used to characterize the termination configuration of Ti2CTz(T = O, OH) MXenes, as well as the catalytic activity towards dehydrogenation reactions, using propane as model system. First-principles calculations reveal that the hydrogen affinity can be considered as an intrinsic property of O and OH terminated Ti2C MXenes, in which the mean hydrogen affinity for the terminated Ti2C MXenes is linearly correlated to the statistical average of their OH fraction. In addition, the C-H activation energies exhibit a strong scaling relationship to the hydrogen affinity. This quantity can therefore yield quick predictions of catalytic activity of terminated Ti2C MXenes towards C-H activations, and even predict their chemical selectivity toward scissoring different C-H bonds. We believe that the hydrogen affinity will accelerate the discovery of further applications of the broad family of MXenes in heterogeneous catalysis.

As a bourgeoning class of 2D materials, MXenes have recently attracted significant attention within heterogeneous catalysis for promoting reactions such as hydrogen evolution and C-H activation. However, the catalytic activity of MXenes is highly dependent on the structural configuration including termination groups and their distribution. Therefore, understanding the relation between the structure and the activity is desired for the rational design of MXenes as high-efficient catalysts. Here, we present that the correlation between the structure and activity of Ti2CT2 (T is a combination of O, OH and/or F) MXenes for C-H activation can be linked by a quantitative descriptor: the hydrogen affinity (E (H)). A linear correlation is observed between the mean hydrogen affinity and the overall ratio of O terminations (x (O)) in Ti2CT2 MXenes, in which hydrogen affinity increases as the x (O) decreases, regardless to the species of termination groups. In addition, the hydrogen affinity is more sensitive to the presence of OH termination than F terminations. Moreover, the linear correlation between the hydrogen affinity and the activity of Ti2CT2 MXenes for C-H activation of both -CH3 and -CH2- groups can be extended to be valid for all three possible termination groups. Such a correlation provides fast prediction of the activity of general Ti2CT2 MXenes, avoiding tedious activation energy calculations. We anticipate that the findings have the potential to accelerate the development of MXenes for heterogeneous catalysis applications.

On-surface synthesis is a powerful methodology for the fabrication of low-dimensional functional materials. The precursor molecules usually anchor on different metal surfaces via similar configurations. The activation energies are therefore solely determined by the chemical activity of the respective metal surfaces. Here, we studied the influence of the detailed adsorption configuration on the activation energy on different metal surfaces. We systematically studied the desulfonylation homocoupling for a molecular precursor on Au(111) and Ag(111) and found that the activation energy is lower on inert Au(111) than on Ag(111). Combining scanning tunneling microscopy observations, synchrotron radiation photoemission spectroscopy measurements, and density functional theory calculations, we elucidate that the phenomenon arises from different molecule-substrate interactions. The molecular precursors anchor on Au(111) via Au-S interactions, which lead to weakening of the phenyl-S bonds. On the other hand, the molecular precursors anchor on Ag(111) via Ag-O interactions, resulting in the lifting of the S atoms. As a consequence, the activation barrier of the desulfonylation reactions is higher on Ag(111), although silver is generally more chemically active than gold. Our study not only reports a new type of on-surface chemical reaction but also clarifies the influence of detailed adsorption configurations on specific on-surface chemical reactions.

We have computationally studied the formation mechanism of the biphenylene network via the intermolecular HF zipping, as well as identified key intermediates experimentally, on the Au(111) surface. We elucidate that the zipping process consists of a series of defluorinations, dehydrogenations, and C–C coupling reactions. The Au substrate not only serves as the active site for defluorination and dehydrogenation, but also forms C–Au bonds that stabilize the defluorinated and dehydrogenated phenylene radicals, leading to "standing" benzyne groups. Despite that the C–C coupling between the "standing" benzyne groups is identified as the rate-limiting step, the limiting barrier can be reduced by the adjacent chemisorbed benzyne groups. The theoretically proposed mechanism is further supported by scanning tunneling microscopy experiments, in which the key intermediate state containing chemisorbed benzyne groups can be observed. This study provides a comprehensive understanding towards the on-surface intermolecular HF zipping, anticipated to be instructive for its future applications.

The synthesis of ammonia (NH3) from nitrogen (N2) under ambientconditions is of great significance but hindered by the lack of highly efficient catalysts. Byperformingfirst-principles calculations, we have investigated the feasibility for employing atransition metal (TM) atom, supported on Ti3C2T2MXene with O/OH terminations, as asingle-atom catalyst (SAC) for electrochemical nitrogen reduction. The potential catalyticperformance of TM single atoms is evaluated by their adsorption behavior on the MXene,together with their ability to bind N2and to desorb NH3molecules. Of importance, the OHterminations on Ti3C2T2MXene can effectively enhance the N2adsorption and decrease theNH3adsorption for single atoms. Based on proposed criteria for promising SACs, ourcalculations further demonstrate that the Ni/Ti3C2O0.19(OH)1.81exhibits reasonablethermodynamics and kinetics toward electrochemical nitrogen reduction.