Structure-activity correlation of Ti2CT2 MXenes for C–H activation

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.


Introduction
In the chemical industry, light olefins (C 1 to C 6 ) are considered as important building blocks for the synthesis of organic components [1]. Recently, such building blocks have * Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. registered a rapid growth on the global trade market due to the wide applications in further polymerization and functionalization [2]. Conventionally, the most common approaches for commercially producing industrial olefines are steam cracking of naphtha and fluid catalytic cracking of heavy oil [3,4]. Although the recovery rate of light olefins (especially for propylene) from a fluid catalytic cracking unit has increased by 29% in the past decades, the commercial methods for the synthesis of propylene are still not capable for closing the increasing 'propylene gap' [5,6]. Alternatively, a primary route for the synthesis of light olefins is via the direct C-H activation of natural abundant paraffin that can be easily obtained from shale gas deposits [7,8]. Generally, the C-H activations for saturated alkanes proceed with the presence of catalysts such as Pt and CrO x [9][10][11][12]. Nevertheless, the dehydrogenation is thermodynamically restricted to rather high temperatures (770 K for 20% conversion of propane) due to the chemical stability of the C-H bond [13]. Such high temperatures and its endothermic nature make the reaction energy-inefficient. Moreover, these catalysts still suffer from limitations including high costs, poor chemoselectivty and toxicity of active centers [14,15].
In the past decades, various strategies have been employed to improve the catalytic performance of catalysts towards dehydrogenation of alkanes. For example, the selectivity towards propylene can be effectively promoted by alloying another metal (e.g. Sn) into Pt catalysts in the propane dehydrogenation [16]. The underlying mechanism can be ascribed to a geometric effect, in which introduced metal would help generate dispersed Pt active sites [17]. Taking into account that the C-H cleavage would take place on every surface Pt atom, the catalytic performance of the Pt-metal alloy catalysts is still highly structure-sensitive [18]. On the other hand, metal-oxides catalyze the C-H bond activation via the socalled radical-like pathway, in which the M-O sites serve as the active sites [19]. However, due to the structural complexity, the mechanism of M-O sites for C-H activations are elusive [20]. For instance, the activity of the V-O and Cr-O sites are highly sensitive to the bonding nature between the metal ions and O sites [15,21]. Such diversity of active sites hinders the generation of uniformly dispersed M-O sites with high catalytic activity towards C-H activations [18]. Therefore, it has been long desired to obtain catalysts with uniformlydispersed active sites and high catalytic performance towards the dehydrogenation of light alkanes.
Two-dimensional materials have been considered as promising catalysts towards various heterogeneous catalysis due to unsaturated and uniformly-distributed active sites [22,23]. MXenes, with the general formula of M n+1 X n T z , have attracted tremendous interests in many different fields due to the tunable electronic structure and good thermal stability [24,25]. Moreover, the O termination groups of MXenes may serve as the active sites for various heterogeneous catalysis [26]. For instance, Jiang et al have reported that the Ti 3 C 2 O x exhibits high catalytic activity towards hydrogen evolution reaction, which is attributed to the highly-active O-sites on the basal plane [27]. Of importance, Diao et al have shown that the Ti 3 C 2 O 2 MXenes possess remarkable catalytic activity towards the ethylbenzene dehydrogenation [28]. Theoretical calculations reveal that the O termination groups are considered to account for the good catalytic performance. Nevertheless, three possible termination groups (O, OH and F) can be experimentally observed in the MXenes synthesized by HF etching [29]. Previous studies have shown that the co-existence of multiple termination groups are commonly observed via experimental characterization for different MXenes [30,31]. In addition, properties of MXenes such as electronic properties and catalytic activity are highly dependent on the surface chemistry [32]. For instance, the O termination always increases the work function while the OH termination would decrease it [33]. Furthermore, the configuration and defects of termination groups including O-vacancy not only tune the electronic structure but also enhance the catalytic activity [34]. Therefore, it is crucial to study how termination groups influence the property of MXenes as well as to explore the correlation between the termination configuration and the catalytic performance of the catalyst. Previous studies have shown that the catalytic performance of MXenes is closely related to the ratio of O terminations, in which more O terminations would lead to better catalytic activity [35]. Our preceding study has shown how the hydrogen affinity (E H )-the ability of an O active site to abstract a H atom-could be used to characterize the termination configurations of Ti 2 CT 2 MXenes and the activity towards C-H activations [36]. For the O/OH terminated Ti 2 C MXenes, the mean hydrogen affinity is linear to the overall ratio of O terminations. Moreover, the hydrogen affinity exhibits linear correlations with respect to the activation energies for C-H cleavage at both -CH 3 and -CH 2 -sites of propane. Note that the dehydrogenation at -CH 2 -site is energetically favorable, indicating high selectivity of the Ti 2 CO z (OH) 2−z MXenes towards propane dehydrogenation [36]. However, the influence of F terminations on the catalytic activity and selectivity is yet to be addressed, to obtain a complete understanding of the correlation between the termination configurations and the catalytic activity, including O, OH and F terminations.
The main purpose of the present study is to obtain a more complete understanding of the influence of the termination groups on the catalytic activity for the Ti 2 CT 2 MXene; in particular how the F terminations affect the hydrogen affinity of the MXenes, and whether or not the linear correlation between the hydrogen affinity and the catalytic activity is valid for generalized Ti 2 CT 2 MXenes. By performing first-principles calculations, we show that the mean hydrogen affinity of the Ti 2 CT 2 (T = O, OH and F) MXene is linear to the ratio of the O terminations to all terminations present on the surface. The influence of the F termination on the hydrogen affinity is weaker than that of OH termination, i.e. with the same O ratio, the O and F terminated Ti 2 C possesses lower hydrogen affinity compared to O and OH terminated ones. Furthermore, the catalytic activity of the Ti 2 CT 2 MXene towards C-H activations of propane is investigated. It is found that the validity of probing activity by hydrogen affinity can be extended to all three possible termination groups. Of importance, a universal linear correlation can be found between the activation energies of C-H bond at the -CH 2 -site and the hydrogen affinity, indicating that the catalytic activity can be solely characterized by the hydrogen affinity.

Methods
All density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package together with the atomic simulation environment [37,38]. The projected augmented wave potentials were employed for describing electron-ion interactions [39]. The exchange-correlation interactions were treated by van der Waals density functional (vdWDF) with the version of rev-vdWDF2 proposed by Hamada [40]. The energy cutoff for the plane wave was set as 400 eV. A 20 Å vacuum layer was adopted to prevent the periodic image interactions. A combined method of climbimage nudged elastic band (CI-NEB) and dimer method was employed for the search of transition states [41][42][43]. Firstly, 20 images were generated between the initial and final state. Secondly, the central image was used as the input for the further dimer method in order to obtain the transition state. The structure of local minima and saddle point were optimized until the average atomic force was lower than 0.02 eV Å −1 . As illustrated in scheme 1, three types of p(4 × 4) supercells of Ti 2 CT 2 were employed as catalysts: (a) O/F terminated, (b) O/OH terminated, and (c) a mixture of all three terminations. The calculation of the hydrogen affinity and the dehydrogenations were performed on the top side of the Ti 2 CT 2 MXenes (upper panel of scheme 1), while the bottom surface remained unchanged in order to be consistent with our preceding investigation [35]. The Brillouin zone was modelled by gammacenter Monkhorst-Pack scheme, in which the Γ point and 4 × 4 × 1 grid were adopted for geometry optimization and electronic structure calculations, respectively [44].

Hydrogen affinity for the O and F terminated Ti 2 C MXenes
As suggested by our preceding study, the hydrogen affinity (E H ) can be considered as a function of the termination configuration for O/OH terminated Ti 2 C. Herein, the validity of the correlation between the E H and termination configurations is extended by including F terminations. The hydrogen affinity is defined as the ability of O termination on the top surface for abstracting one H atom from the molecule: are referred to the potential energy of the catalyst with an extra H atom, the original catalyst, a water molecule and an oxygen molecule, respectively [36,45]. Such descriptor is expected to be related to the termination configuration of the Ti 2 C MXenes that can be characterized the by ratio of O terminations to all terminations present on the surface:

The hydrogen affinity for MXenes with mixture of terminations
The co-existence of all possible termination groups is commonly observed in experiments [46]. Therefore, our subsequent analysis is focused on the hydrogen affinity for Ti 2 C MXenes with O, OH and F terminations. In addition, the termination groups are randomly distributed on the top and bottom surfaces of Ti 2 C MXenes, and their structure exhibits high degree of freedom including the ratio of termination groups as well as their distribution on both top and bottom side [47].
Thus, it is difficult to obtain every possible structure for Ti 2 CT 2 MXenes. To begin with, the relation between the hydrogen affinity and the termination configurations is investigated by fixing the ratio of one termination group (F or OH) to 50%. As seen, the mean E H is linear to the ratio of the O termination in the Ti 2 CT 2 (figure 2). The hydrogen affinity is more sensitive to the ratio of OH terminations, in which the range of the mean E H for Ti 2 CT 2 with the F terminations fixed to 50% is significantly larger than that of Ti 2 CT 2 with 50% of OH terminations. Further calculations have shown that the change of E H is solely determined by variation of ratio of O termination when x F < 50%. The scaling relations for Ti 2 CT 2 MXenes with x F < 50% are parallel to each other, in which the slopes of the linear regressions are almost the same (figure S1). Such results agree to our discussion above where effect of the F termination on the hydrogen affinity is less significant. Furthermore, it has been reported that the hydrogen affinity can be employed for probing the catalytic activity, in which the low E H leads to high activity [45]. Therefore, the Ti 2 C MXenes with F termination may possess high catalytic activity towards C-H activation.

The correlation between catalytic activity and hydrogen affinity
The hydrogen affinity is an intrinsic property for not only characterizing the termination configuration, but also probing the catalytic activity towards the C-H activations of propane [36]. We begin by studying the thermodynamics of the C-H activations on Ti 2 CT 2 MXenes. Based on the Brønsted-Evans-Polanyi relation, the energy of transition states (E a ) is proportional to the reaction energies (ΔE) of the chemical reactions [48][49][50]. Herein, catalytic activity of 24 O/OH terminated Ti 2 C and 16 O/F terminated Ti 2 C is investigated by calculating the reaction energies. As shown in figure 3, the thermodynamics of the C-H activations on Ti 2 CT 2 MXenes can be lumped into a simple linear correlation, in which the reaction energy increases as the hydrogen affinity increases for both -CH 3 and -CH 2 -sites. Note that the hydrogen affinity exhibits different scaling relations with the reaction energy at the -CH 3 site ( figure 3(a)). Such discrepancy may be related to the steric hindrance of final states of C-H cleavage. The Ti 2 C MXenes with low O termination exhibit larger strain, in which the formation of the C-O bond would elongate the Ti-O bonds, resulting in higher potential energy (see figure S2). Such distortion of the final states would decrease the accuracy of the linear regression, leading to different scaling relations with respect to different termination configurations. As seen in figure 3(b), however, the O/OH terminated Ti 2 C and O/F terminated Ti 2 C can be grouped into one linear regression for the -CH 2 -site (black line). Such result indicates that the catalytic activity of the terminated Ti 2 C MXenes can be accurately predicted by the hydrogen affinity. In addition, six Ti 2 C MXenes with all three possible termination groups are selected to evaluate the accuracy of the prediction (squares in figure 3, data in tables S1-S3). The mean absolute errors (MAE) for the -CH 3 and -CH 2 -sites are 0.19 and 0.12, respectively. Such small MAE indicates that the hydrogen affinity exhibits high accuracy in the prediction the reaction energy for both -CH 3 and -CH 2 -sites. Furthermore, DFT calculations have shown that the hydrogen affinity can be utilized as a quantitative descriptor for probing the activity of Ti 2 CT 2 MXenes towards the propane dehydrogenation regardless to the termination configuration. The circles, triangles and squares represent the Ti 2 CO z (OH) 2−z , Ti 2 CO z (F) 2−z , and Ti 2 CT 2 (T = O, OH and F, 0 z 2), respectively. The color bar from blue to green corresponds to the ratio of O terminations in Ti 2 CT 2 from fully OH and/or F termination (0%) to Ti 2 CO 2 . The scaling relations between the activation energy and the hydrogen affinity at (a) -CH 3 and (b) -CH 2 -in the propane on the Ti 2 CO z (OH) 2−z (circles) and Ti 2 CO z F 2−z (triangles). The color bar from blue to green corresponds to the ratio of O groups in Ti 2 CT 2 from fully OH and F termination (0%) to Ti 2 CO 2 .
Note that the C-H activation exhibits similar reaction pathways on Ti 2 CT 2 MXenes independent of the termination stoichiometry, in which the reaction initiates at the physisorption of the propane and finalizes at the co-adsorption of the radical and H atom (exemplified in figure S3). Figure 4 summarizes the activation energies for C-H cleavage on 24 O/OH terminated Ti 2 C and 8 O/F terminated Ti 2 C MXenes with respect to E H . As seen, there is no significant difference between the linear regression for the O/OH terminated Ti 2 C and O/F terminated Ti 2 C MXenes, indicating that the termination configuration would not affect the scaling relation between the catalytic activity and the hydrogen affinity. Moreover, the validity of such scaling correlation can be extended to both -CH 3 and -CH 2 -site, suggesting that the hydrogen affinity is an intrinsic property of Ti 2 CT 2 MXenes. To this end, it is reasonable to extend the application of the hydrogen affinity to the general Ti 2 CT 2 MXenes (T = O, OH and F).

Conclusions
In conclusion, the termination configuration and the catalytic activity of the Ti 2 CT 2 (T = O, OH and F) MXenes are investigated by DFT calculations. The termination configuration of the Ti 2 CT 2 MXenes can be characterized by a quantitative descriptor, hydrogen affinity (E H ), which is defined as the ability for an O active site to abstract a H atom from the propane.
Our calculations show that the mean hydrogen affinity is linear to the overall O ratio of the Ti 2 C MXenes, in which high O ratio leads to E H . By considering either O/OH or O/F terminated Ti 2 C MXenes, the E H is more sensitive to the OH termination groups, while the influence of the F termination groups on the E H is less significant. Such result can be further demonstrated by the E H of Ti 2 CT 2 (T = O, OH and F). For the Ti 2 C MXenes with low F terminations (x F < 50%), the scaling relations between the hydrogen affinity and the O ratio are parallel, indicating that the effect of F termination can be considered as a constant. Of importance, the linear correlation between the hydrogen affinity and the catalytic activity can be extended to the general Ti 2 CT 2 MXenes, in which the low activation energy and/or reaction energy of C-H activations at both -CH 3 and -CH 2 -site of the propane can be found on the surface with low hydrogen affinity.