MXenes as promising catalysts for water dissociation

Two-dimensional few-layered transition-metal nitrides and carbides, called MXenes, have attracted a great interest given their large surface areas and their unique physicochemical properties. Motivated by the known reactivity of surfaces of bulk transition metal carbides on the mechanism behind the water-gas shift (WGS) reaction, density functional theory (DFT) calculations were employed to investigate the bonding of water and its dissociation on a set of eighteen M2X MXene (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, while X = C or N) surfaces. Here it is shown that all the studied MXenes exothermically adsorb water, with adsorption energies ranging from -1.43 to -2.94 eV, and greatly facilitate its dissociation, with energy barriers below 0.44 eV. These results reinforce the role of MXenes in promoting water dissociation, effectively suggesting their potential as catalysts for industrially relevant processes such as the WGS reaction.


Introduction
Understanding the molecular mechanism of water dissociative chemisorption as catalysed by different types of surfaces is extremely important, not only owing to its noted implications in a plethora of industrial processes, but also in many areas of fundamental research. 1 For instance, water dissociation often plays a determining role in the water-gas shift (WGS) reaction, [2][3][4] and it has also been suggested to promote CO oxidation and O2 dissociation reactions catalysed by different surfaces. 5,6 Indeed, CeO2-supported singleatoms (Au, Cu, and Pt) [3][4][5][6] and other systems have shown high catalytic performance towards the WGS reaction just because of their demonstrated potential to dissociate water. Also, computational studies suggest that dissociation of the first O-H bond of water is the rate-determining step (RDS) of the WGS reaction on copper surfaces, 7,8 although detailed kinetic Monte Carlo simulations show that, despite the water dissociation is an important step, the RDS may change depending on the reaction conditions 9 and the surface morphology. 10 Water dissociation is also relevant in the generation of hydrogen by means of the hydrogen evolution reaction (HER), where some two-dimensional metal-free based catalysts appear to be promising because of their large specific surface areas and, concomitantly, the high number of active sites per material gram. 11,12 Apart from the widely used supported catalysts consisting of metallic nanoparticles supported on porous oxides or sulphides, 13,14 and inspired by the landmark article by Levy and Boudard, 15 transition metal carbides (TMCs) have been gaining interest momentum as active phases 16 as well as supports for a diversity of catalysed reactions. 17 In particular for the WGS catalysed by the TiC(001) surface and TiC nanoparticles, it has been shown that the redox mechanism may become competitive with the associative one. 18 The discovery of a new family of low-dimensional transition metal nitrides and carbides in 2011 19 provides a completely new scenario for such materials with unforeseen applications, 20 opening new possibilities for the use of carbides in catalysis. Inspired by the isolation of graphene, 21 these new twodimensional (2D) materials were termed MXenes. 19 MXenes can be obtained by selective etching of the A element from a precursor MAX phase under a top-down synthesis procedure. [22][23][24] This MAX phase is described as an hexagonal layered ternary transition metal nitride or carbide, where M corresponds to an early transition metal (e.g. Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), A stands for an element from a subset of groups XII-XVI, and X corresponds to either carbon (C) and/or nitrogen (N) elements. 25 The general formula of MXenes with 2n+1 atomic layers is Mn+1XnTx, where M and X are the same as in the MAX phases, and T stands for the surface termination, usually determined by the synthesis conditions (e.g. -OH, -O, -H, or -F groups when using the hydrofluoric acid synthesis procedure). 19 Such layered MXenes display high electrical and metallic conductivity, hydrophilicity, large surface areas, tuneable structures, and a high oxidation resistance. [26][27][28] All of these properties, however, can suffer modifications upon hybridization with other materials and underpin the huge application potential of MXenes in areas such as ecofriendly energy generation, 29 water splitting and hydrogen generation, [30][31][32] and other heterogeneous catalysed reactions, such as dehydrogenation reactions. 33 J o u r n a l P r e -p r o o f 4 In addition, MXenes have also been explored in applications including energy storage 34 -36 and gas sieving. [37][38][39] The synthesis of bare MXenes is far from being straightforward and often results in functionalized MXenes exhibiting a mixture of terminations. 19,20 Notably, recent advances on HF-free syntheses have led to MXenes terminated by H and OH only. 40 Furthermore, post-synthesis heating treatments 41 were shown to allow defunctionalising the MXene surface and, very recently, were explored to improve the electronic conductivity displayed by these materials 42 The increase in the MXenes conductivity upon surface defunctionalisation is, in principle, beneficial for catalysis. In fact, there is recent experimental evidence that clean Ti3C2 is able to adsorb and activate CO2, 43 confirming previous computational studies predicting that bare MXenes with M2X stoichiometry have a high capacity for CO2 abatement even at high temperature and low CO2 partial pressures. 44,45 Since CO2 is chemically inert, these results provide clear evidence that MXenes are very reactive for adsorbing and activating molecular species and pollutants. 46,47 The ability to produce MXenes exhibiting cleaned surfaces foresees a great potential in several chemical applications, from gas adsorption and separation, 48 to electrocatalysis, 49 heterogeneous catalysis 33 , and photocatalysis. 50 Motivated by earlier studies on the catalytic properties of bulk transition metal carbides, [16][17][18]51,52 here we investigate the possible application of MXenes as catalysts for the low temperature WGS reaction. To this end, we present a systematic study on the ability of a series of M2X MXenes to catalyse the water O-H bond scission, regarded as the energetically most demanding step. Using electronic density-functional theory calculations and suitable models, we report and analyse the energy profiles for water dissociation, including energy barriers, thus shedding light on the underlying molecular mechanisms. The viability of MXenes as heterogeneous catalysts for water dissociation is discussed by making use of a detailed comparison to data reported in the literature for the same reaction on pure metal and metal alloy surfaces.

MXene models
A total of eighteen MXenes with M2X stoichiometry (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W; and X = C or N) were selected to investigate the adsorption and dissociation of water, see Fig. 1. It is worth pointing out that the basal (0001) surface exhibited by the MXenes is equivalent to the (111) surface of bulk transition metal carbides (TMCs) with rocksalt structure. For these TMCs, the (111) surfaces have a considerable higher surface energy than the most stable (001) one. 53 Therefore, MXenes are predicted to be quite reactive and, on the other hand, allow one to investigate surfaces which could hardly exist otherwise in those bulk TMCs. To represent a situation with a small water coverage, each surface was modelled by a periodic p(3×3) supercell.
Since a three-dimensional periodic symmetry is used for convenience, the dimension of the supercell in the direction perpendicular to the surface plane of the MXene was set to 16 Å. This ensures the presence of at least 10 Å of vacuum between periodic copies of the system, even after the adsorption of the H2O molecule.
J o u r n a l P r e -p r o o f Table 1 compiles the calculated lattice parameter (a) of each studied MXene. MXenes composed of d 2 metals (Ti, Zr, and Hf) show the largest lattice parameters, followed by d 3 metals (V, Nb, and Ta), and d 4 ones (Cr, Mo, and W), regardless of whether they are N-or C-based. The calculated lattice parameters of Ti2C (3.06 Å) and Ti2N (2.98 Å) are in very good agreement with the respective values, 3.04 Å and 2.98 Å, found in the literature. 54 Finally, it is worth pointing out that these MXenes have been reported to be mechanically and dynamically stable. 55

Computational details
The overall study relies on first-principles calculations carried out in the framework of density functional theory (DFT) using the Vienna Ab initio Simulation Package (VASP). 56 The calculations were carried out within the generalized gradient approximation (GGA) to the many-body exchange-correlation potential, namely using the functional introduced by Perdew, Burke, and Ernzerhof (PBE), 57 with the contribution of dispersion terms approximated through the D3 method as proposed by Grimme. 58 The valence electron density was expanded in a plane wave basis set with an energy cutoff of 415 eV. The effect of the atomic inner cores on the valence electron density was taken into account by means of the projector augmented wave (PAW) method. 59 The convergence criterion for the self-consistent field energies was set to 10 -6 eV and structures were considered relaxed when the forces acting on all atoms were lower than 0.01 eV/Å. To carry out the necessary numerical integrations in the reciprocal space, the Brillouin zone was sampled using a Monkhorst-Pack 551 grid of special k-points. 60 Systematic convergence tests showed that, with this computational setup, calculations are converged within an accuracy of about 1 meV/atom. Spin polarisation was considered in all cases. This did not affect results on the M2N systems but introduced some noticeable changes on a few M2C MXenes, as discussed in more detail below.
For each species S, namely, H, OH, or H2O, the adsorption energy (Eads) is defined as where E(S/MXene), EMXene, and ES correspond to the total energy of the MXene containing the adsorbed species, the bare pristine MXene, and the isolated species, respectively. Note that the energies of the isolated H or OH species, ES, were calculated as EH = 1 /2 2 and = 2 − 1 2 2 , respectively. The local minimum character of the ground state configurations was further characterized by calculating the normal modes of vibration of the adsorbed species and ensuring that all the eigenvalues of the dynamic matrix were positive, and, by that, that they correspond to real frequencies. The vibrational frequencies were obtained by diagonalization of the corresponding block of the Hessian matrix with elements computed as finite differences of 0.015 Å of analytical gradients. The calculated frequencies were also used to calculate the zero-point energy (ZPE) contribution to the total energy of both adsorbed and gas phase species. While the ZPE contributions J o u r n a l P r e -p r o o f 6 to the adsorption energies were all lower than 0.1 eV, they reduced some dissociation energy barriers by almost 0.2 eV.
The saddle point configuration for the minimum-energy pathway for H2O dissociation on each MXene was located by using the dimer method. 61 The transition state (TS) structure characterized by the frequency analysis ensured a single imaginary frequency corresponding, precisely, to one O-H bond scission. This was verified by following bidirectionally the eigenvector of the dynamic matrix corresponding to the imaginary frequency, which yielded two minima adjacent to the saddle point consisting of a molecular and a dissociated adsorbed H2O molecule. The dissociation energy barrier is defined as the amount of energy required to dissociate the adsorbed H2O molecule into OH + H fragments, and is calculated, for each MXene, as the difference between the energy of the saddle-point configuration and that of the ground state adsorbed configuration of H2O, both including the ZPE term.

Results and discussion
The dissociation of water on the explored MXene (0001) surfaces involves several steps, as depicted in Fig. 2. These are the adsorption of the water molecule onto the MXene surface, the dissociation of the water molecule into OH and H moieties, plus the eventual migration of OH and H fragments away from each other, i.e., their diffusion on the surface towards their respective most stable adsorbed sites. In this study, we mainly focus on the first two steps. To simplify the discussion of our results, the results for H, OH, and H2O adsorption on MXene surfaces are separated from those corresponding to the dissociation step.

Adsorption of H, OH, and H2O on MXene surfaces
The study of H adsorption constitutes the simplest to analyse, as the most favourable adsorption site of the H atom is over the centre of an HM site, see adsorbs on top of a metal atom, which interacts with the O atom of the molecule. On this MXene, the O atom of OH is located about 1.9 Å away from the nearest W atom, in a configuration with C1 symmetry. Indeed the adsorption of OH on the W2N surface involves a distortion of the neighbouring lattice, characterized especially by the nearest W atom moving more than 1 Å away from the MXene plane. In general, the OH adsorption energy ranges from -1.5 to almost -3.0 eV, showing that the process is energetically favourable. Spin polarisation affects the adsorption of OH on Ti2C and Cr2C surfaces, causing it to become 0.16 eV weaker and stronger, respectively, Table 3. The absolute value of the adsorption energy is larger on d 2 MXenes (Ti, Zr, and Hf) and becomes smaller for d 3  It is interesting to compare the present results for the MXenes with those in a very recent computational work concerning the H2O adsorption on the (111) surfaces of TiC, VC, ZrC, and NbC. 62 In that work, it was predicted that water adsorption would occur preferably on an HC site. We tested this possibility, along with the adsorption on an HM site, but both were found unstable for all MXenes and, after geometry optimization, the molecule relaxed to either the T-or B-adsorbed structures. This shows that the adsorption configuration can change not only by switching the X atom but also by changing the number of atomic layers and/or the stoichiometry, which can affect concomitantly the electronic structure. Nevertheless, weak bonding with adsorption energies just below 1 eV are a common feature between both types of systems.

Dissociation of H2O on MXene surfaces
In the quest of using the dimer method to search for transition states, two different types of saddlepoint structures were located, depending on the molecular water adsorption site, see Fig. 3. Please note that in Fig. 3, the top row panels are representative of water dissociation reactions on MXene surfaces where the adsorbate is preferentially located at the B site, whereas the bottom row panels are representative of MXene surfaces preferentially adsorbing water at the T sites. The dissociative mechanism of water initially adsorbed on a B site, found in Ti2N, Zr2C, and Hf2C MXenes, involves a rotation of the H2O molecule by about 15 o around the axis perpendicular to the molecular plane, followed by a displacement of the whole molecule towards the nearby HM site. The resemblance between the ground state and the transition state is reflected in dissociation energy barriers ( diss ) lower than 0.1 eV (see Table 5), practically unaffected by the ZPE term.
On the other hand, the dissociation mechanism for surfaces where H2O is initially adsorbed at the T site involves one of the H atoms of water being pulled away from the O atom and being drawn closer to the surface, towards a nearby HM site, which, indeed, is the corresponding preferred adsorption site for atomic H. The dihedral angle between the molecular and surface planes is around 40 o . For the MXenes where water adsorbs at the T site, the dissociation energy barriers are larger than the ones corresponding to the B sites (with the exception of Hf2N) and up to 0.44 eV (see Table 5). Here the ZPE term on diss decreases the barriers by 0.17-0.19 eV.
Hence, the main difference between the two dissociative mechanisms is the orientation of H2O with respect to the (0001) MXene surface. As clearly seen in Fig. 3, when the molecule is adsorbed at a B site, the H2O molecular plane is perpendicular to the surface, whereas for the adsorption on a T site both planes are almost parallel. In general, dissociation of molecules siting at the T site requires larger energy barriers. For this reason, in the cases where H2O is preferentially adsorbed at a T site, the molecule could migrate to (or close to) a B site and then dissociate almost spontaneously. To test this hypothesis, we calculated the dissociation energies of water at the B configuration on the MXene surfaces where the most stable adsorption occurs on a T site. In all cases, the calculated energy barriers were of at least 0.2 eV, which is in clear contrast with the values below 0.1 eV found for the corresponding MXenes where H2O is preferentially adsorbed at B sites. Therefore, the hypothesis of migration from B to T was ultimately discarded since the migration energy from T to B, added to the dissociation energy on site B, exceeds the dissociation energy on site T by at least 0.1 eV, and by that implying that migration to B site followed by dissociation is a less likely process.  Table 5 show that the d 2 MXenes exhibit the smallest energy barriers followed by the d 3 and d 4 MXenes. In general, the dissociation of water is energetically slightly easier in carbide MXenes than in their nitride counterparts. Regardless of the mechanism, the dissociation energy J o u r n a l P r e -p r o o f barriers range from 0.05 to 0.44 eV, and so, are minimal, highlighting the great H2O breaking power of MXenes.
After H2O dissociation, the products of the reaction are not necessarily located at their most stable sites. Indeed, the absolute values of the reaction in Table 5 are larger when the dissociation products end up farther from each other. Nevertheless, in all cases water dissociation is thermodynamically favoured. The energy difference between Eads(OH) + Eads(H) and Eads(H2O) + Ereaction, labelled as ∆E∞, see Fig. 2, stands for the amount of energy that the system can still gain after the dissociation of H2O into OH and H when moving the two product species farther apart from each other into their own ground state adsorption sites. Defined like this, a value of ∆E∞ close to zero represents an OH+H configuration that is already stable immediately after dissociation. Contrarily, values of ∆E∞ much smaller than zero can be visually confirmed to be associated with configurations after the dissociation of H2O, in which the OH and H moieties are still too close to each other and not on their most favourable adsorbed sites. Fig. 4a presents a plot of activation energies (Ediss) versus the full reaction energy (Ereaction +∆E∞); the rather good linear correlation implies that a Brønsted-Evans-Polanyi (BEP) relationship holds, [63][64][65] which may be useful to explore trends in other MXenes. In addition, Fig. 4b shows a second plot corresponding to activation energies (Ediss) versus the d-band centre (εd), 66 These findings also reinforce the common assumption that TMCs are catalysts similar, or even better, than Ptgroup metals. 15 Note also that the dissociation reaction was predicted to be endothermic on three of the five Pt surfaces studied, 70 while here it is clearly exothermic in all the investigated MXenes. The molecular dissociation mechanism is also similar to that reported for a Pt79 nanoparticle, for a Pt8 cluster in gas phase, this Pt8 deposited on a CeO2(111) surface, and also for a Ce40O80 nanoparticle. 4 For these systems, the water J o u r n a l P r e -p r o o f adsorption energies range between -0.5 and -1.0 eV, a similar order of magnitude as the one found for the MXene surfaces, see Table 4. However, the corresponding energy barriers are in the 0.5-0.7 eV range, 4 thus significantly larger than the barriers predicted here for the scrutinized MXene surfaces, see Table 5. Finally, note that water dissociation on a TiC(001) extended surface implies an energy barrier of 0.37 eV, 18 quite low, but still larger than the barrier reported for the MXene surfaces.
Boumaza et al. reported that the ternary catalyst based on the Cu0.5Zn0.5Al2O4 spinel oxide shows high activity for the WGS reaction at low temperature. 71 A recent computational study compared the activities displayed by different ternary catalysts models and showed that Cu surfaces doped with Zn and Al atoms were quite active in the dissociation of the H2O molecule into OH and H species. 72 The energy barriers, It is pertinent to point out that, in practice, the WGS reaction requires temperatures of a few hundred Celsius. On the other hand, it has been shown that the Ti3C2 MXene is stable up to 800 o C in Ar atmosphere 73 and that high temperatures contribute to maintain some regions of the MXene surfaces clean enough. 41 Hence, the MXenes are expected to be stable (and very active) at the temperatures required for the WGS reaction. It is worth mentioning here that, if the MXene surface is fully terminated, the catalyst loses its outstanding reactivity and molecules adsorb less exothermically. Nevertheless, preliminary calculations with oxygenterminated MXenes show that the dissociation reactions are practically unaffected provided the O coverage is below 66 %. Let us now consider the subsequent steps of the WGS reaction taking the Ti2C catalyst surface as an example. It is well known that OH-terminated MXenes are quite stable, and the terminating OH groups are almost not reactive. The results from the preliminary DFT calculations also revealed an activation energy for OH dissociation into O+H of 0.53 eV (expectedly, this is higher than the first O-H bond scission requiring 0.09 eV only) and a reaction energy of -1.87 eV. The adsorption of CO is highly exothermic (-3.05 eV) and

Conclusions
The adsorption and dissociation of water on eighteen MXenes (0001) surfaces with M2X (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W; and X = C or N) stoichiometry have been investigated by using first-principles DFT based calculations, including the approximate contribution of dispersion. The calculated results show that the water molecular adsorption is an exothermic process on all the analysed MXenes. Depending on the considered surface, the water molecule adsorbs either on a bridge or a top position. Ti2N and Zr2C emerge as the most reactive substrates of all nitrides or carbides, respectively. No trend was observed between H2O adsorption energies along periods or groups, or with the distance between the adsorbed molecule and the surface. In all cases, H2O dissociation on MXenes is exothermic with absolute reaction energies higher than 1 eV on most MXenes, up to 3.35 eV for Hf2N. Different transition state configurations were found depending on the preferential bridge or top H2O adsorption site. In all MXenes, the reaction activation energy barriers were found to be relatively small, i.e., below 0.44 eV. Barriers tend to be slightly larger for nitride MXenes than for the corresponding carbide counterparts, and to increase along the periodic table periods. The MXenes which adsorb water into a bridge configuration (Zr2C, Hf2C, and Ti2N) display very small dissociation energy barriers, below 0.1 eV. Finally, the reaction energy and the d-band centre constitute good descriptors that may be used to efficiently screen the capability of other MXenes for water dissociation. Overall, the present results reinforce the view of MXenes as active catalysts for water dissociation.

Conflicts of interest
There are no conflicts to declare      Step i) corresponds to molecular adsorption 2 , ii) to dissociation into coadsorbed OH+H, Ediss, and iii) to separation of the reaction products to their most stable adsorbed sites, ∆E∞. The value of + is the sum of the adsorption energies of OH and H on the MXene.
J o u r n a l P r e -p r o o f 20 Fig. 3 Top and side views of the molecular, transition and dissociated states for the water dissociation reaction on bridge sites of the Zr2C surface (first row panels) and on top sites of the Mo2N surface (second row panels).
Colour code: oxygen is red; hydrogen is white; carbon is grey; nitrogen is navy blue; zirconium is teal; and molybdenum is green.