Tuning Transition Metal Carbides Activity by Surface Metal Alloying: Case Study on CO2 Capture and Activation

the possible of metal of these by a textbook periodic slab models with largesupercells and state-of-the-art ABSTRACT CO 2 is one of the main actors in the greenhouse effect and its removal from the atmosphere is becoming an urgent need. Thus, CO 2 capture and storage (CCS) and CO 2 capture and usage (CCU) technologies are intensively investigated as technologies to decrease the concentration of atmospheric CO 2 . Both CCS and CCU require appropriate materials to adsorb/release and adsorb/activate CO 2 , respectively. Recently, it has been theoretically and experimentally shown that transition metal carbides (TMC) are able to capture, store, and activate CO 2 . To further improve the adsorption capacity of these materials, a deep understanding of the atomic level processes involved is essential. In the present work, we theoretically investigate the possible effects of surface metal doping of these TMCs by taking TiC as a textbook case and Cr, Hf, Mo, Nb, Ta, V, W, and Zr as dopants. Using periodic slab models with large supercells and state-of-the-art density functional theory based calculations we show that CO 2 adsorption is enhanced by doping with metals down a group but worsened along the d series.

. Bader charges on all bare surfaces. Table S2. Binding modes, and adsorption energies with and without van der Waals corrections (vdW), as obtained PBE-D3, as well as geometry parameters, including CO2 angle, C-C bond distances, d(C-C), and CO2 molecule C-O bond distances, d(C-O). Table S3. Bader charges for all the studied surfaces with adsorbed CO2.     ABSTRACT CO2 is one of the main actors in the greenhouse effect and its removal from the atmosphere is becoming an urgent need. Thus, CO2 capture and storage (CCS) and CO2 capture and usage (CCU) technologies are intensively investigated as technologies to decrease the concentration of atmospheric CO2. Both CCS and CCU require appropriate materials to adsorb/release and adsorb/activate CO2, respectively. Recently, it has been theoretically and experimentally shown that transition metal carbides (TMC) are able to capture, store, and activate CO2. To further improve the adsorption capacity of these materials, a deep understanding of the atomic level processes involved is essential. In the present work, we theoretically investigate the possible effects of surface metal doping of these TMCs by taking TiC as a textbook case and Cr, Hf, Mo, Nb, Ta, V, W, and Zr as dopants. Using periodic slab models with large supercells and state-of-the-art density functional theory based calculations we show that CO2 adsorption is enhanced by doping with metals down a group but worsened along the d series.
-2 -Adsorption sites, dispersion and coverage appear to play a minor, secondary constant effect.
The dopant-induced adsorption enhancement is highly biased by the charge rearrangement at the surface. In all cases, CO2 activation is found but doping can shift the desorption temperature by up to 135 K. * Corresponding authors: francesc.vines@ub.edu, michael.nolan@tyndall.ie

INTRODUCTION
Every year increasing evidence of the significant impact of global warming on Earth are being reported. 1,2 The environmental prediction models show a non-optimistic future if no urgent measures are taken to face this issue. 3,4 Atmospheric CO2 is one of the greenhouse gases with highest impact, and its environmental effects are particularly problematic. Apart from wellknown ones such as ocean acidification, 5 there are predictions that while the anthropogenic emissions CO2 rise into the atmosphere, the CO2 concentration and the concomitant global warming will rise exponentially through carbon-cycle feedbacks. 4 Despite worldwide efforts in controlling and reducing CO2 emissions, as exemplified in active environmental protocols such as Copenhagen, Kyoto, or Paris, 6-8 recently the International Energy Agency (IEA), in its global energy and CO2 status reports, announced an increase of 1.4% in energy-related CO2 emissions. 9 Among many strategies oriented at reducing CO2 emissions or at decreasing its atmospheric concentration, an appealing one is the removal of atmospheric CO2 via scrubber materials. 10,11 Spurred on by the urgent requirements of the Paris protocol, CO2 surface chemistry 12 is experiencing a renewed interest focusing on characterizing scrubber materials and to optimize their properties in CO2 capture and storage (CCS). 13,14 Along this line, a desirable advance is not only in CO2 capture, but its chemical activation and eventually re-use as a carbon feedstock to synthesize other valuable chemicals through the commonly known CO2 capture and usage (CCU) strategies. 15 Given the high CO2 chemical stability, only a few privileged materials are able to selectively adsorb CO2 strong enough for CCS or CCU technologies. Moderate to high CO2 adsorption energies usually indicate an activation of the molecule, 12 which often occurs through a charge transfer from the surface of a material to the CO2 6a1 anti-bonding molecular orbital, thus weakening the C-O bonds and leading to a bent geometry. 10 From this point on the CO 2 δmolecule is much more reactive than the neutral one and the starting point for any industrial process to reuse CO2. based computational studies, mostly focused on early TMCs, proved that these materials display significant potential for CO2 activation. 18,19 In particular, TiC, WC, and especially -Mo2C excel among the family of TMCs, and so, have been studied in deeper detail. These TMCs have proven their CCS potential but also their CCU capabilities, as highlighted by their CO2 based catalytic activity, 17 including CO2 hydrogenation towards methanol. 20 Additional systematic DFT based studies report a strong CO2 adsorption on most stable (001) surface of rocksalt crystal group IV (TiC, ZrC, WC), group V (NbC, TaC), and group VI (δ-MoC) TMCs with a concomitant activation of the adsorbed molecule, 10 indicating that these materials may be adequate for CCU technologies.
In the search for tailor-made materials for CCS and/or CCU, different ways of tuning the surface activity towards CO2 can be envisaged, including the use of surface doping agents.
This is a common practice when considering metal and metal oxide properties, thus locally tuning the chemical activity or stabilizing certain facets as in F-doped TiO2 nanoparticles. 21,22 It has also been shown that the presence of surface dopants promotes the oxygen vacancy formation on CeO2 nanoparticles. 23,24 Also, CO adsorption on ceria is easily enhanced by adding Zr or Ti on the bulk, improving CO oxidation and releasing CO2 to the atmosphere. 25 In the case of metals, metal alloying is a standard, but not yet fully understood or controlled, approach to modulate the surface activity, although there have been clear improvements in the understanding of nanoalloys, 26 thus opening the door to future control.
In the present work we explore transition metal doping or surface metal alloying on TMCs surfaces in the context of CO2 capture, storage, and activation. Here, TiC has been chosen to understand how the CO2 adsorption energy is affected by the presence of a surface or subsurface doping agent. The dopants scrutinized are Zr, Hf, V, Nb, Mo, Cr, Ta, and W; a -5 -subset of transition metals that are known to form metal carbides. This work therefore aims to assess the significant possibility of bimetallic/doped TMCs for use in CCU and the implications for design of materials with enhanced CO2 adsorption.

SURFACE MODELS AND COMPUTATIONAL DETAILS
Calculations reported in the present work have been carried out within the density functional theory (DFT), 27

Dopant effects on the TiC surface
The substitution of Ti atom of the TiC (001) Table S1 of the Supporting Information.

Binding modes of CO2 on doped TiC (001)
A previous thorough study explored several binding modes of CO2 on a set of TMC (001) surfaces and considered situations in which the molecular axis lies perpendicular or parallel to the carbide surface. 10 Approaching the CO2 molecule to the surface in a perpendicular interactions with the dopant site. In pristine TiC, TopC is found to be slightly more stable, and hence used for the oncoming detailed exploration of the doped systems. 10

CO2 adsorption strength
The CO2 adsorption energy, , on the pristine or doped TiC(001) surface, is obtained as in where is the energy of the surface with adsorbed CO2, the energy of the clean pristine or doped TiC surface, and the energy of the gas phase molecule, respectively.
Within this definition, the more negative the , the stronger the adsorption. From all the set of investigated dopants, the case with Hf located in the first atomic layer exhibits the largest adsorption energy (-0.96 eV for TopC). This is interpreted in term of the larger positive charge on Hf compared to Ti (see Table S1), which induces a stronger stabilization through a coulombic Hf-O interaction as clearly seen in Figure 2. This implies that the adsorption energies involving early TMs as dopants are larger than those involving late TMs (Table 2), with one of the weakest situations being W (-0.22 eV). However, the case of Ta represents an exception to this rule since even with a quite a large charge of +1.91 e, it displays a slightly smaller (-0.57 eV) than pristine TiC. In any case, it is clear that the dopant charge is a determining factor although not the only one.
Concerning doping with W, the TopC adjacent situation implies a slightly higher adsorption energy (-0.  Table S1), we consider in the following that the doping effect is completely local and focus on the situations where CO2 is close to the doping atom as in Figure 1.
Finally, the same two doping agents (Hf and W) have been used to evaluate possible effects due to dopant saturation/coverage on the surface. To this end, an extra CO2 with its correspondent dopant have been added to the (2√2×2√2)R45º unit cell, effectively increasing the doping concentration and also the CO2 coverage (see Figure S2). The results, as expected, reveal a reduction of the mean adsorption energies to -0.77 eV for the case of Hf and -0.13 eV for that of W. Interestingly, the local nature of the dopant effect prevails, a strengthening of the interaction is observed for Hf, and a weakening for W. The reduction with respect to the low coverage case is mostly due to steric repulsion between adsorbates.

Activated CO2 structure
In all cases where the surface-CO2 interaction leads to a C-C type of bond, similar structural changes appear for the adsorbed CO2 molecule. have an impact in the subsequent CO2 usage since one of the C-O bonds appears to be more -11 -activated with a preference for further reactions as the CO2 dissociation upon hydrogenation. 10

Charge transfer
Additional information to better understand the observed trends in CO2 adsorption energy triggered by the presence of the dopant is gained by analyzing the net charges on relevant atoms as obtained from the Bader's atoms-in-molecules analysis of the total electron density. 36 To this end two possibilities are investigated involving the relationship between Eads and the initial Bader charges of the doped surface (Table S1), or with the net charge transfer from the doped-surface to CO2 (Table S3). The first relationship, summarized in Figure 3, strongly supports that the more oxidized the dopant, then the stronger is the adsorption of CO2. This is in agreement with the previously discussed charge modifications on the surface electronic structure due to the presence of the dopant.
The chemistry behind this trend is quite simple. A more oxidized cation leads to higher negative charge in the neighboring C atoms (Table S1) directly interacting with the adsorbate; this is a feature that favors electron transfer to the CO2. On the other hand, comparing the local charges with and without the adsorption, one observes that the charge transferred to the CO2 strongly depends on the binding mode rather than on the dopant (see Table S3). Upon CO2 adsorption, the surface C involved in the bonding gets less negatively charged, and CO2 is highly activated exhibiting a net charge in the -0.76 to -0.98 e range. The emerging picture of the adsorption process can be thought as a Lewis acid-base reaction, 38 a feature also reported for the interaction of CO2 with some oxides. 39 The surface acts as a base transferring charge to the CO2 acting as an acid.
To further confirm the role played by the charge transfer into CO2 adsorption on the doped TMCs surfaces, the same analysis has been repeated for other binding modes with a smaller adsorption energy which all display a smaller degree of charge transfer. Thus, a general trend is present where stronger adsorption energies involve concomitant larger charge transfers. Lastly, we note that the negatively charged adsorbate would attract other positively charged species such as H + with implications in the CO2 conversion, especially in its electroreduction.

Work function analysis
The bonding mechanism between doped TiC and CO2 which we have discussed above and which involves a charge transfer may have observable consequences. In particular, it is likely to affect the surface work function (), a descriptor for basic-acid characterization of the surfaces. 38 Table 3. These results show that low/high work functions tend to favor/disfavor CO2 adsorption, as expected. However, one must also caution that the relationship between workfunction and CO2 adsorption is not strong enough so as to conclude a definitive clear direct link between them, which is consistent with previous work. 44,45 Hence, the dopant effects appear to be too local so as to strongly modify the overall surface work function.

Adsorption/desorption rates
Pressure and temperature are important factors to control the CO2 adsorption capacity of such metal doped TMCs. The rates of the adsorption and desorption, evaluated through transition state theory, determine whether the CO2 will be quantitatively stored on the surface or will be desorbed as fast as it will contact on the surface. The adsorption and desorption rate estimates, rads and rdes, respectively, have been obtained following the set up described by Kunkel et al., 10  Eads with values reported in Table 4. Notice that the main effect of the ZPE correction is to reduce the adsorption strength, but it does by up to, at most, 0.03 eV, and therefore can be considered negligible. showing that a small increase on the adsorption energies implies a slight increase of the limit temperature between adsorption and desorption regions. Indeed, for Hf-doped TiC, the adsorption energy is 0.34 eV stronger than for undoped TiC(001), implying an equilibrium temperature between adsorption and desorption of 370 K; this is 135 K higher than the corresponding value for stoichiometric TiC estimated as 235 K. 10 Consequently, Hf-doped TiC would lead to CO2 capture at higher temperature conditions. For other doping metals, such as W, the temperature decreases, but, in such situations, the previous analysis indicates - 14 -that CO2 will preferentially occupy dopant-free surface regions and, as a result, no change in the equilibrium temperature is to be expected.

SUMMARY AND CONCLUSIONS
The present DFT based study reports the effects caused by a dopant in the TiC (001)

NOTES
The authors declare no competing financial interest.