Determination of stable surface orientation and the crystal morphology for considered WB5 is presented in detail in the Supporting Information. Below we discussed our results obtained for B-(010) and W-(101) surfaces.
To comprehensively study the adsorption of gases on the B-(010) and W-(101) surfaces we consider nonequivalent by symmetry adsorption sites, shown in Figure 1, and Fig. S2 in the SI. For B-(010) surface, in order to reveal the role of boron in the catalytic process, the adsorption sites (1, 4, 5, and 6 in Figure 1) over the B atoms or the B-B bonds were examined. We considered three additional sites (2, 3, and 7 in Figure 1) on top of the B-hexagons. The difference between these sites is the position of boron triangle. The site 3 is on top of the B-hexagon having the W atom and then the B-triangle underneath. Site 2 is above the B-hexagon having underneath the W atom only. Site 7 differs from others, placing the molecule above the B-hexagon with the B-triangle underneath (see Figure 1). The adsorption sites for W-(101) surface is described in the SI, Fig. S2.
It should be noted, that crystal structure of higher tungsten boride WB5-x is disordered with respect to location of B-triangles19,21. This leads to composition variation (0 < x < 2) of WB5‑x. One of limiting case is WB3 structure with alternation of B and W layers 7,11 (no B-triangles). Another limiting case is WB5, as detailed earlier11. The adsorption sites considered in our work include those typical for pure WB3 and WB5, and for the intermediate compositions, including the sites not typical for the limiting cases.
To further explore the adsorption of gases on the higher tungsten borides, we consider all possible configurations of adsorbates on the surfaces. For example, a СO2 molecule can adsorb in both linear 27 and triangular 28,29 configurations. At the same time, water H2O, with its strong dipole moment, can bind in four different configurations with respect to the surface 30–32. Here, the adsorption of 10 molecules is considered: СО, СO2, H2, N2, O2, NO, NO2, H2O, NH3, and SO2. The total number of considered configurations on B-(010) is 7 for CO, NO, and NH3; 18 for H2; 21 for O2 and N2; 28 for NO2, SO2 and CO2; and 62 for H2O molecules. Distributions of the adsorption energies for W-(101) and for B-(010) surfaces are shown in Figure 2 by orange and blue colors, respectively. The total numbers of considered configurations on this surface are: 10 for CO, NO, and NH3; 20 for H2; 15 for O2 and CO2; 28 for N2, NO2, and SO2; and 62 for H2O molecules.
For the adsorption on the B-(010) surface we have found that most favorable sites are at the B-B bonds (sites 4, 5, 6 in the Figure 1) and on top of the B atom (site 1 in the Figure 1). The adsorption energies of all molecules at all considered sites are given in Tables S2 and S3 in the SI. Possible adsorption configurations of each molecule are given on Figure S5 in the SI. Thus we find, in contrast to previous study 8, that the surface B atoms are actively involved in the adsorption. This also is corroborated by the relaxation of a molecule originally placed in the center of the B-hexagon yet shifting to the B atom or to the B-B bond, where they rest fixed. Furthermore, during the dissociation of the molecules, the products also bind to the B-B bonds.
NO. These molecules adsorb vertically with the N atom closer to the surface with the average adsorption energies of -1.66 and -1.62 eV for the B-(010) and W-(101) surfaces, respectively (Figure 2). The reference data for Cu20 clusters33 shows the higher adsorption energies of about -0.52 eV and on the PbAu it is about 0.72 eV34.
NO2 molecules can be adsorbed parallel to the surface or with its N directed to the surface. If it adsorbs in a parallel manner and bonds with the surface with two atoms, dissociation could occur, especially in the case of the B-(010) surface. However, none of the dissociated molecules were included in the distributions shown in Figure 2. Thus, the average energies of adsorption are -1.60 and -2.51 eV for the B-(010) and W-(101) surfaces, respectively. The adsorption energy on the B-(010) surface is similar to the one obtained for graphene where it is about 1.25 eV 35
NH3 molecule adsorbs mostly on the boron atoms with average adsorption energies of about -1.29 and -1.25 eV for the B-(010) and W-(101) surfaces, see Figure 2. The obtained numbers are similar to those found on the single-atom-embedded ternary B3C2P3 monolayers (-1.37 eV)36. The adsorption energies of NH3 on Pt(111) is -0.95 eV and on Co(0001) is -0.69 eV, which are higher 37 but still comparable to that on WB5-x.
N2 molecule adsorbs mostly on top of the tungsten atom. The high adsorption energies about -0.14 and -0.24 eV for the B-(010) and W-(101) surfaces, Figure 2, indicate a weak interaction between the molecule and the surface, and as a result, dissociation of this molecule does not occur. The obtained data are similar to those for Si-doped graphene (-0.15 eV)38. However, the obtained energies are higher than those for other transition metals, such as Mo-doped Fe2P (-0.96 eV) 39.
H2 molecules also primarily interact with tungsten atoms, with average adsorption energies of about -0.57 and -0.54 eV for the B-(010) and W-(101) surfaces, Figure 2. This number for W-(101) surface is very similar to the one for the Ti2AC (A = Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) 40 where it is equal to -0.40 eV. Dissociation of the molecule can only occur at sites on the B-B bond. The energy of the dissociated molecule is represented in the distribution on Figure 2 as eV per atom of dissociated molecule.
SO2 molecules can be adsorbed in a parallel manner, with the S atom closer to the surface. Its interaction with the W-(101) surface is stronger than with the B-(010) surface, as shown by the difference in the average adsorption energies +1.19 and -1.39 eV, respectively, Figure 2. Although the dissociation of the molecule during adsorption in a parallel manner could occur while interacting with the tungsten atoms on the W-(101) surface, these cases are not considered in the distributions shown in Figure 2. The adsorption energy of SO2 molecule is also found to be positive (about +0.29 eV) for TiO2 surface 41, but there are some catalysts, for example metal organic frameworks, where SO2 could be adsorbed and the adsorption energy is about -0.59 eV 42.
H2O molecule has four different adsorption configurations relative to the surface. Tables S2 and S3 in the Supporting Information contain all of them. The molecule primarily interacts with boron atoms, which is the reason of good adsorption on the B-(010) surface with an average adsorption energy of about -0.88 eV. Conversely, it mostly does not adsorb onto the W-(101) surface, with an average adsorption energy of about -0.63 eV. These values of adsorption energies correlate well with the values for Cu (-0.33 eV) 43 and for Ru (0001) surface where the adsorption energy is -0.29 eV 31.
CO gas molecules adsorb vertically with the C atom positioned closer to the surface, primarily on the B-B bonds. The average adsorption energies for the B-(010) and W-(101) surfaces are -1.55 and -1.83 eV respectively, see Figure 2. This values are much lower than that found for Si-doped graphene (-0.19 eV38), indicating that WB5-x is a better potential catalyst for reactions starting from CO gas.
O2 molecule dissociate without any energy barrier in most cases. The energies of the dissociated oxygen atoms are shown in the distribution in Figure 2, measured in eV per atom. The average adsorption energies are equal to -1.56 and -4.60 eV for the B-(010) and W-(101) surfaces, respectively. The adsorption energy of O2 on the W-(101) surface as low as for silicene (−5.38 eV) 44, where it is also dissociates.
CO2 molecule was analyzed in its linear form, as it exists in the atmosphere. The average energies of its adsorption are approximately -0.37 and -0.7 eV for the B-(010) and W-(101). Figure 2. The average value obtained for the B-(010) surface is similar to that obtained for the C6N6 monolayer embedded with Fe single-atom (-0.46 eV) 45. However, it is incomparable with the value obtained for the Si-doped graphene (-0.18 eV)38. It is worth noting, that the distributions shown in Figure 2 do not take into account dissociation of the molecule.
Considering the adsorption energies, it can be expected that the CO and NO oxidation reactions are possible on B-(010) surface according to both the Eley-Rideal and Langmuir–Hinshelwood mechanisms. The adsorption of the reaction products (NO2 and CO2) is significantly weaker than adsorption of the reagents (NO and CO), which suggests the catalytic effectiveness of B-(010) surface of WB5. Furthermore, based on the strong adsorption of NO and CO, as well as the weak adsorption of N2 and CO2, we suggest that WB5 could be used as active material for an automotive catalytic converter. In addition to its efficient production 19, the advantage of WB5‑x over currently used catalysts is its resistance to poisoning by sulfur gases, particularly SO2 46, whose adsorption energy is calculated to be positive on this surface, see Figure 2.
It should be pointed out, there is a high probability of dissociation of CO2 during the adsorption on both surfaces. Thus, we can further suggest a potential application of WB5-x as effective catalyst or co-catalyst for the conversion of carbon dioxide into fuels and other vital products 6,23.
Moreover, opposite to the B-(010) surface, in most cases H2O does not bond to the W-(101) surface. But if the H2O molecule attaches the surface, then the distance between the surface and the molecule is about 2.25 Å, hence, the interaction is weak causing the low adsorption energy. Therefore, we can assume about the potential application of WB5 as a functional part of filters for water purification from heavy metals and halogenated organics including pesticides 47 as water will not significantly interact with catalyst made of WB5.
According to obtained information about stability and adsorption properties of WB5 surfaces, we examined the CO reduction reaction on B-(010) and W-(101). The most energetically favorable positions of CO and O2 molecules on the surfaces were determined above.
To gain better insight into low adsorption energies of CO and O2 molecules we calculate the Bader charges 48 of considered structures, Figure 3. It is known that there is a charge transfer from the W to B atoms in the tungsten boride due to electronegativity differences. According to the Bader charge analysis, each W atom of the B-(010) surface loses approximately -1.12 e, while on the W-(101) surface each W atom loses -1.05 e. It is found that an excess of electrons at each boron atom of the B-(010) surface equal to +0.17 e, while larger excess of +0.21 e was found for W-(101) surface. Thus, the differences in charge redistribution on various surfaces are small. The direction and the magnitude of the charge transfer can be related to smaller work function of tungsten compared to boron 49. Similar effect was observed for other bicomponent systems with different work functions of constituent elements 50,51.
The low adsorption energy of CO on the W-(101) originates from the CO preference attaching to positively charged sites, rather than negatively charged, due to more pronounced electron donation nature of CO 52,53. Positively charged W sites are referred for adsorption of CO over the negatively charged B. In the case of B-(010) surface the CO donation is 0.13 e, which is much smaller than for W-(101) where it is about 0.54 eV.
For the adsorption of O2 molecule we calculate Bader charge redistributions for oxygen after dissociation on both surfaces (B-(010) and W-(101)). Our calculations show that surface donates to oxygen 1.52 e in the case of B-(010) and 1.21 e in the case of W-(101). It is well known that O2 molecule acts as an electron acceptor and interacts strongly with those sites capable of donating electrons easily to the antibonding orbitals of the O2 54. In our case the sites with B atoms can donate electrons. Electron density difference Δρ (the density of the combined system minus the sum of the densities of the isolated slab and adsorbate at the same positions as in the combined system) for oxygen molecule is shown in Figure 3a,b. It is clearly seen that on B-(010) there is a greater redistribution of the charge on the surface and the bond between the surface and oxygen is stronger, as the boron atoms are charged negatively. For CO on W-(101), Figure 3d, the redistribution occurs more intensively on the bond between CO and the surface than on B-(010) surface (Figure 3c). This indicates a stronger binding of CO with W-(101) surface.
Obtained information shows that W-(101) surface is better suited for CO oxidation reaction because there are sites (positively charged, with W atoms) with low CO adsorption energy, and for adsorption and dissociation of oxygen (negatively charged sites with B atoms).
There are many and still increasing number of possible CO oxidation mechanisms, some are complicated and less probable, hard to identify and explore without detailed computational simulations 46,55. Generally, however, there are two traditional mechanisms to take place on the surfaces without oxygen atoms.
The first one is the Eley–Rideal (ER) mechanism 56,57. The CO oxidation could occur through ER mechanism if the oxygen has a strong adsorption on the surface. This way the association of the CO gas molecule to the pre-adsorbed O2 molecule happens.
On the contrary the Langmuir–Hinshelwood (LH) mechanism 56,58 can proceed when both CO and O2 have a strong adsorption on the surface and can move across the surface with relatively small energy barrier. In this case the CO molecule moves to the adsorbed O2 molecule to make the OCOO complex. This complex then dissociates into the CO2 gas molecule and adsorbed O atom that could react with another CO molecule.
Therefore, the mechanism of CO oxidation is primarily defined by the adsorption possibilities of CO and O2. As mentioned above, both CO and O2 can be easily adsorbed on either of B-(010) or W-(101) surfaces. Consequently, the CO oxidation reaction can take place through both ER and LH mechanisms. The O2 has a stronger adsorption on both surfaces, hence the ER mechanism is more likely.
The O-O bond in the O2 is known as very strong, making it challenging to successfully apply in the oxidative chemistry. Different catalysts were considered to overcome the high barrier of O2 dissociation, for instance, the AuxPdx clusters 54. In our case, O2 dissociates on the both B-(010) and W-(101) surfaces without any energy barrier, see Fig. S5 in SI. Consequently, WB5-x could be proposed as the catalyst for environmentally and industrially important reactions such as ethylene epoxidation, hydrocarbon oxidation, and so on 59–61.
Thus, in case of CO oxidation over the WB5-x surfaces, the mechanisms become less complicated and the only difference between ER and LH mechanisms is the state of CO (gas or pre-adsorbed). We carefully study CO oxidation mechanisms for each surface, see Figure 4.
For B-(010) surface the rate-limiting step is the bonding of the CO and O*; see transition state (TS) in the Figure 4a. The CO oxidation takes place through the ER and LH mechanisms with the reaction barriers of 1.29 and 2.29 eV, respectively.
For the W-(101) surface there are different rate-limiting steps for the different reaction mechanisms, Figure 4b. If the CO oxidation goes through the ER mechanism (red color in Fig. 4b), the rate-limiting step is the bonding between the CO and O* with the reaction barrier of 1.35 eV. If the CO oxidation goes through the LH mechanism (black color in Figure 4b), the rate-limiting step is the CO2 release, and the energy barrier of this step is 1.83 eV.
Obtained information about reaction barriers of CO oxidation on considered surfaces indicates that this reaction will take place on the WB5 exterior. The major mechanisms of reactions in this case could be both the ER and LH mechanisms. Surprisingly, the W-(101) surface has the lowest energy barrier for CO oxidation of 1.35 eV. Nevertheless, the B-(010) surface without any metal atoms on the surface has the lowest energy barrier 1.29 eV for the CO oxidation. Such a small difference in the energy barriers may indicate the active sites without any metal atoms and the boron participation in the catalytic process.
It should be noted that the energy barrier for the CO oxidation reaction may seem high in comparison to that of Au nanoparticles, which is approximately 0.4 eV 62. However, when considering the overall structure of the noble metal, the range of values is significantly different. Both bulk Au and Pd have energy barriers of approximately 0.93 63,64 and 1.02 eV 65, respectively. It should be noted that the O2 molecule is adsorbed onto these surfaces in its initial state, and there is an additional energy barrier to dissociate it. The Pt surface exhibits a lower barrier of 0.47-0.63 eV, but it is easily poisoned by oxygen, resulting in an increased reaction barrier of 1.14 eV. 66.
Based on this data, it can be concluded that investigating transition metal borides as potential catalysts is a promising direction. Possible modifications to this material include doping with other transition metals, considering nanoparticles, forming single atom alloys, and catalysis on the surfaces.