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Article

CO2 and H2O Coadsorption and Reaction on the Low-Index Surfaces of Tantalum Nitride: A First-Principles DFT-D3 Investigation

by
Nelson Y. Dzade
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
Catalysts 2020, 10(10), 1217; https://doi.org/10.3390/catal10101217
Submission received: 21 September 2020 / Revised: 13 October 2020 / Accepted: 16 October 2020 / Published: 20 October 2020
(This article belongs to the Section Computational Catalysis)
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Abstract

:
A comprehensive mechanistic insight into the photocatalytic reduction of CO2 by H2O is indispensable for the development of highly efficient and robust photocatalysts for artificial photosynthesis. This work presents first-principles mechanistic insights into the adsorption and activation of CO2 in the absence and presence of H2O on the (001), (010), and (110) surfaces of tantalum nitride (Ta3N5), a photocatalysts of significant technological interest. The stability of the different Ta3N surfaces is shown to dictate the strength of adsorption and the extent of activation of CO2 and H2O species, which bind strongest to the least stable Ta3N5(001) surface and weakest to the most stable Ta3N5(110) surface. The adsorption of the CO2 on the Ta3N5(001), (010), and (110) surfaces is demonstrated to be characterized by charge transfer from surface species to the CO2 molecule, resulting in its activation (i.e., forming negatively charged bent CO2−δ species, with elongated C–O bonds confirmed via vibrational frequency analyses). Compared to direct CO2 dissociation, H2O dissociates spontaneously on the Ta3N5 surfaces, providing the necessary hydrogen source for CO2 reduction reactions. The coadsorption reactions of CO2 and H2O are demonstrated to exhibit the strongest attractive interactions on the (010) surface, giving rise to proton transfer to the CO2 molecule, which causes its spontaneous dissociation to form CO and 2OH− species. These results demonstrate that Ta3N5, a narrow bandgap photocatalyst able to absorb visible light, can efficiently activate the CO2 molecule and photocatalytically reduce it with water to produce value-added fuels.

Graphical Abstract

1. Introduction

Photocatalytic reduction of carbon dioxide (CO2) with water (H2O) to produce value-added fuels such as such as CO, HCOOH, CH3OH, and CH4 is a promising route to reduce CO2 emissions and address the global energy crisis [1,2,3,4,5]. The activation and reduction of CO2 is, however, an energetically demanding process that involves multiple electron transfer reactions [6,7,8,9,10], hence highly efficient and robust photocatalysts are critical. Several photocatalysts, such as TiO2 [11,12,13], In2O3 [14,15], Ga2O3 [16,17], Al2O3 [18,19], ZnO [20,21], CeO2 [22,23], ZnGe2O4 [24], and BiVO4 [25,26], have been investigated for their performance in catalysing CO2 reduction. However, because of their large bandgaps and high charge-carrier recombination rates, most of these semiconductor materials have low CO2 conversion efficiencies. Therefore, there is continuous active research to find novel photocatalytic materials that are active under visible/solar light.
Recently, many active visible light absorbers with narrow bandgaps, in particular Ta-based materials, such as tantalum nitrides (Ta3N5) and tantalum oxynitrides (TaON), have attracted a lot of attention owing to their unique catalytic properties [27]. Various forms of nanostructured Ta3N5 including nanorod, nanoparticle, hollow sphere, and thin films have been considered as photocatalysts with reported enhanced photocatalytic activities [28,29,30,31,32]. Ta3N5 and TaON have narrow band gap energies of 2.1 and 2.4 eV, respectively, which make them suitable to absorb visible light to initiate photocatalysis [33]. Due to its narrow bandgap energy, Ta3N5 could generate a sufficient number of electrons and holes even under visible light, which could directly reduce CO2 into a radical anion (CO2−δ) and reduce H2O to protons (H+). Several studies have been conducted on the application of Ta3N5 for water splitting and photocatalytic degradation of organic pollutants [15,16,34,35]. The obtained results indicated that the T3N5 is a promising candidate as a visible light-driven photocatalysis. Recently, the photocatalytic reduction of CO2 to CO over Ta3N5 has been reported [36,37]. However, detailed mechanistic understanding of the interaction between CO2 and Ta3N5 photocatalyst is still limited.
The adsorption and activation of CO2 are the foremost and fundamental steps in the photocatalytic reduction of CO2 on the surface of a photocatalyst [38,39,40,41]. Compared to the linear gas-phase molecule, chemisorbed CO2 (mainly carbonate or CO2−δ anion) is characterized by a bent geometry with a decreased lowest unoccupied molecular orbital (LUMO), which favours charge transfer from the photoexcited semiconductors to the surface-adsorbed CO2 molecules [41]. Generally, exposed surfaces with a smaller work function provide greater activation for CO2 as they favour electron transfers [42]. In this work, a comparative first-principles density functional theory (DFT) study of the adsorption and activation of CO2 in the absence and presence of H2O on Ta3N5(001), (010), and (110) surfaces is presented. First, the structures and relative stabilities of the low-index (001), (010), and (110) surfaces were systematically characterized and the equilibrium crystal morphology of the Ta3N5 crystal was constructed based on calculated surface energies. Secondly, the fundamental adsorption and coadsorption geometries of CO2 and H2O, including the energetics and electronic properties are discussed. The stabilities of the coadsorbed CO2–H2O species on the various surfaces were also evaluated to determine the pathways for the surface reactions involving these species, and to characterize the stability of the different reduced forms of CO2, in particular the formate and bicarbonate species that were identified experimentally [37].

2. Results and Discussion

2.1. Bulk and Surface Properties

Ta3N5 crystalizes in the orthorhombic structure, as shown in Figure 1a,b, with space group Cmcm (No. 63). The neutron diffraction-derived lattice parameters are a = 3.886 Å, b = 10.212 Å, c = 10.262 Å, and α = β = γ = 90° [43]. The structure of Ta3N5 is composed of octahedra of N atoms centred by Ta atoms. Since the N atoms are both three and four coordinated, the octahedra are irregular. The conventional unit cell consists of 32 atoms, where each Ta atom is bonded to six N atoms, while N atoms are bonded to three or four Ta atoms. The Ta–N distances in Ta3N5 range from 1.96 to 2.24 Å. From full relaxation (ions + cell shape + volume) until the required accuracy was reached, the lattice parameters of Ta3N5 were predicted at a = 3.921 Å, b = 10.317 Å, and c = 10.323 Å, in close agreement with experimental data [43] and earlier theoretical predictions [44,45,46,47]. The Ta–N distances for the N atoms coordinated to three Ta atoms were calculated at 2.000, 2.083, and 2.048 Å, whereas those for the N atoms coordinated to four Ta atoms are 2.117(×2) and 2.241(×2) Å. Figure 1c shows the partial density of states of Ta3N5, from which the band gap is estimated at 2.11 eV, which is in good agreement with ultraviolet–visible (UV–vis) spectroscopy measurements of Ta3N5 powders and thin films which estimate an optical gap of approximately 2.1 eV [48,49]. Previous theoretical studies based on GGA+U [45] and HSE06 [50,51] functionals predicted the bandgap in the range of 2.1–2.21 eV. It is evident from Figure 1c that the top of the valence band is mainly composed of N-2p orbitals, while the bottom of the conduction band is mainly composed of Ta-5d orbitals, indicating that transitions near the absorption edge occur between N-2p and Ta-5d orbitals, which is in agreement with earlier theoretical works [45,50,51].
Figure 2 shows the optimized structures of the most stable terminations of the Ta3N5(001), (010), and (110) surfaces, which have calculated surface energies of 2.33, 2.04, and 1.58, Jm−2, respectively. All surfaces are terminated by N atoms and exhibit step-like surface topologies. Using the calculated surfaces energies, the equilibrium Wulff shape of Ta3N5 nanocrystal was constructed, as shown in Figure 3. The Ta3N5 nanocrystal is found to exhibit an elongated shaped, with the (110) and (010) facets expressed in the rectangular crystal edges and the (001) facet enclosing the hexagonal edge. The predicted morphology is consistent with the elongated polyhedral crystal shape observed in experiments [30,52]. The differences in the structure, composition and stabilities of the (001), (010), and (110) surfaces are expected to dictate their reactivity towards CO2 and H2O molecules, which is investigated in detail and discussed in the following sections.

2.2. CO2 Adsorption on (001), (010), and (110) Ta3N5 Surfaces

Considering that the initial step for CO2 reduction is its activation [53], the lowest-energy adsorption configurations of CO2 on the Low-Miller index (001), (010), and (110) Ta3N5 surfaces were first investigated in order characterize the strength of interaction and the extent of the C−O bond activation. Shown in Figure 4 are the lowest-energy adsorption geometries of CO2 on the different Ta3N5(001), (010), and (110) surfaces, with the calculated adsorption energy and structural parameters summarized in Table 1. The less stable CO2 adsorption geometries predicted on the different Ta3N5 surface are displayed in Supplementary Information Figures S1–S3. At the (001) surface, the CO2 molecule interacts through all three atoms, as shown in Figure 4a, with the C atom bound to N sites and the O atoms bound to Ta sites, releasing an adsorption energy of −2.73 eV. The interacting Ta−O and the N−C bonds were calculated at 2.176 and 1.361 Å, respectively. The strong adsorption induced structural transformation in the CO2 molecules with the C−O bonds elongated from 1.176 Å in the gas phase to 1.301 Å in the adsorbed state and the ∠ OaCOb angle reduced to 130.0°, indicating activation of the CO2 molecule. Compared to the (001), the CO2 molecule interacts with the (010) surface via the carbon and one oxygen atom, as shown in Figure 4b, releasing an adsorption energy of −1.89 eV. The C−O bonds are significantly elongated, in particular the surface-bound one (1.422 Å) compared to the unbound one (1.202 Å) and the ∠ OaCOb angle is reduced to 123.9°. Similar to the (001) surface, CO2 adsorption on the (110) surface involves all three atoms, as shown in Figure 4c, with the C atom binding at the N site and the O atoms bound to Ta sites, releasing an adsorption energy of −1.20 eV. Consistent with the weaker adsorption, longer Ta−O and the N−C bonds than on the (001) were calculated at 2.217 and 1.400 Å, respectively. The average C−O bond and ∠ OaCOb angle are calculated at 1.292 Å and 125.5°, respectively.
Partial density of states (PDOS) and differential charge density isosurface contours analyses provided further atomic-level insights into the mixing atomic orbitals and redistribution of electron density within the CO2-Ta3N5 systems. The adsorption of CO2 on Ta3N5 surface is characterized by a strong mixing of interacting surface (Ta-d and N-p) and CO2 (C-p and O-p) orbitals, as shown in Figure 4d–f. Consistent with chemisorption, electron density redistributions within the CO2-Ta3N5 systems is observed, which was analysed through differential charge density isosurface contours, obtained as:
Δ ρ = ρ C O 2 + s u r f a c e − ( ρ s u r f a c e + ρ C O 2 )
where ρ C O 2 + s u r f a c e , ρ s u r f a c e and ρ C O 2 are the electron density of the total CO2-Ta3N5 system, the bare Ta3N5 surface, and the isolated CO2 molecule as in the relaxed adsorbed configuration. The differential charge density iso-surface contours inserts in Figure 4d–f reveal a charge transfer from the bound surface ions into the π-antibonding orbital of the CO2 molecule via the newly formed Ta−O and the N−C bonds. The CO2 molecule gained a net charge of 0.40, 0.35 and 0.28 from the (001), (010), and (110) surfaces, resulting in the formation of negatively charged bent species (CO2δ−) with elongated C−O bonds, confirmed by vibrational frequencies analyses, as shown in Table 1.

2.3. CO2 Dissociation on (001), (010), and (110) Ta3N5 Surfaces

Shown in Figure 5 are the most stable coadsorption geometry of CO + O pair from CO2 dissociation (CO2→CO + O) reaction on the (001), (010), and (110) Ta3N5 surfaces. At the Ta3N5(001) surface, the oxygen atom binds at Ta site, whereas the carbon of the CO moiety bridges two N atoms, as shown in Figure 5a, releasing a coadsorption energy of −1.64 eV. At the Ta3N5(010) surface, the oxygen atom binds at bridge Ta sites, whereas the carbon of the CO moiety binds at the N site, as shown in Figure 5b, releasing a coadsorption energy of −2.99 eV. At the Ta3N5(110) surface, as shown in Figure 5c, the oxygen atom and the carbon of the CO moiety bind at Ta and N sites, respectively, releasing a coadsorption energy of −0.68 eV. The results show that the coadsorption of (CO + O) pairs is more favourable on the Ta3N5(010), followed by Ta3N5(001), and then the Ta3N5(110) surface. The geometrical parameters and calculated coadsorption (Ecoads), reaction (Erxn), and the activation energy barriers (Eact) of CO2 dissociation are shown in Table 2. The Ta3N5(010) surface exhibits the lowest CO2/Ta3N5(010)→(CO + O)/Ta3N5(010) reaction energy—exothermic by −1.09 eV. The CO2→CO + O reaction is, however, endothermic by 1.04 and 0.52 eV on the Ta3N5(001) and Ta3N5(110) surfaces, respectively. The activation energy barrier (Eact) for the dissociation of CO2 on the (001), (010), and (110) Ta3N5 surfaces are calculated at 1.34, 1.15 and 1.56 eV, respectively. The higher Eact and the endothermic Erxn for the dissociation of CO2 predicted on the (001) and (110) surfaces suggest that direct dissociation may be hindered at room temperature and without surface promoters such as H2O and H species. Direct CO2 dissociation may, however, occur at room temperature on the Ta3N5(010) surface owing to its calculated exothermic Erxn and the lower Eact.

2.4. CO2 and H2O Coadsorption and Reactions

Water is a suitable source of hydrogen for CO2 conversion via its hydrogenation to valued-added chemicals [54]. As such, investigations of H2O and CO2 coadsorption and the possible reactions between them are indisputable. Prior to investigating the coadsorption structures of CO2 and H2O, the most stable adsorption geometries of isolated H2O on the (001), (010), and (110) Ta3N5 surfaces were systematically characterized. The calculated energetics and structural parameters for molecular and dissociative water adsorption on the three Ta3N5 surfaces are presented in Table 3 and Figure 6. The most stable adsorption of water is predicted at the Ta sites via its O atoms. The adsorption energy of molecular water at the (001), (010), and (110) Ta3N5 surfaces is calculated to be −1.42, −1.07, and −1.08 eV, respectively, indicating that the interaction is strongest at the (001) surface and weakest on the (110) surface, similar to the trend predicted for CO2 adsorption. The most stable dissociative water adsorption configurations (OH + H pair) provided in the rightmost panel of Figure 6 show that when dissociated, the OH species preferentially bind to the Ta sites through the O atom, whereas the H atoms bind at the N sites. From Table 3, it is worth noting that the dissociative adsorption of water on three surfaces is thermodynamically more favourable than molecular adsorption as reflected is the larger coadsorption energies released by the OH + H pairs. The H2O→OH + O reaction energies on the (001), (010), and (110) surfaces can be calculated at −1.15, −0.28, and −1.03 eV, respectively, all of which are exothermic and thus indicate favourable dissociation of water. The interacting Ta−OH distances are calculated at 1.987, 2.297, and 1.978 Å, whereas the N−H distances were converged at 1.031, 1.056, and 1.022 Å on the (001), (010), and (110) surfaces, respectively. The transition states structures were determined (central panels, Figure 6) in order to estimate the activation energy barriers for the dissociation of water, which were predicted at 0.18, 0.34, and 0.27 eV at the (001), (010), and (110) surfaces, respectively. In a previous study the energy barriers for the dissociation of water was calculated at to be as low as 0.05 eV on the Ta3N5(110) surface [55]. Compared to direct CO2 dissociation, the predicted low activation energy barriers and the exothermic reaction energies suggest that spontaneous water dissociation will occur on the (001), (010), and (110) surfaces.
The lowest-energy coadsorption structures of CO2 + H2O on the (001), (010), and (110) surfaces of have been characterized, as shown in Figure 7. The coadsorption energy of CO2 + H2O on the different surfaces is calculated as follows:
E c o a d s = E ( C O 2 + H 2 O ) / s u r f a c e − ( E s u r f a c e + E C O 2 + E H 2 O )
where E ( C O 2 + H 2 O ) / s u r f a c e , E C O 2 , E H 2 O and E s u r f a c e are the total energy of the coadsorbed (CO2+H2O) surface system, the free CO2, the free H2O, and the bare Ta3N5 surfaces, respectively. The calculated CO2 + H2O coadsorption energies and the geometrical parameters are presented in Table 4. The coadsorption energy for the CO2 + H2O pair on the (001), (010), and (110) Ta3N5 surfaces were calculated at −3.94, −3.68, and −2.43 eV, respectively. Consistent with attractive interactions, the coadsorption energies are more exothermic than the sum of the separate CO2 and H2O adsorption energies.
Analysis of the differential charge density isosurface contours (Figure 8) shows accumulation of electron density within the C–N and O–Ta bonding regions, indicating chemisorption.
Hydrogen-bonded interactions between the CO2 and H2O species are observed at the Ta3N5(010) surface, evident by the close interaction of the electron density of the two species. This may favour proton transfer to the CO2 molecule, which has been investigated and found to results in spontaneous dissociation of the surface-bound C−O bond to form CO and 2OH− species (Figure 9a). Relative to the initial coadsorbed CO2 + H2O system, the reaction energy for the formation of the CO + 2OH− species on the Ta3N5(010) surface is calculated to be highly exothermic by 1.11 eV. When the proton is transferred to the unbound O of CO2, a stable carboxyl COOH species (Figure 9b) is formed with a reaction energy of −0.67 eV. No stable formate HCOO species is formed as the attached proton to the C atom detaches during energy minimisation to the surface N site, as shown in Figure 9c, with the reaction energy calculated to be −0.19 eV. These results suggest that the Ta3N5(010) surface favours the dissociation CO2 in the presence of H2O, and the resulting CO species could be further hydrogenated to form CH2O and CH3OH.
Compared to the Ta3N5(010) surface, the reaction energy for the proton transfer from H2O to CO2 to form COOH and HCOO species is found to be highly endothermic on the Ta3N5(001) and Ta3N5(110) surfaces, as shown in Figure 10. The less endothermic reaction energies predicted for the COOH species than for the HCOO species, however, suggest that further hydrogenation reactions to products will proceed via the carboxyl COOH route.

3. Summary and Conclusions

This work presents comprehensive first-principles density functional theory analyses of the adsorption and activation of CO2 in the absence and presence of H2O on the (001), (010), and (110) surfaces of Ta3N5, a photocatalyst able to absorb visible light to initiate photocatalysis. The strength of adsorption and extent of CO2 activation is found to be influenced by the stability of the different Ta3N surfaces, where it adsorbs most strongly onto the least stable Ta3N5(001) surface and most weakly onto the most stable Ta3N5(110) surface. Direct dissociation of CO2 is suggested to occur on the Ta3N5(010) surface owing to the calculated exothermic reaction energy and lower activation energy barrier. In contrast, direct CO2 dissociation would be hindered on the Ta3N5(001) and Ta3N5(110) surfaces without surface promoters such as H2O and H species. Spontaneous water dissociation is predicted occur on the (001), (010), and (110) surfaces, providing the necessary hydrogen source for CO2 reduction reactions. The strongest attractive interaction between coadsorbed CO2 and H2O is predicted on the Ta3N5(010) surface, which gave rise to proton transfer to the CO2 molecule, causing its spontaneous dissociation to form CO and 2OH− species with an exothermic reaction energy of −1.11 eV. The formation of COOH* and HCOO* intermediates is found to be highly endothermic on the Ta3N5(001) and Ta3N5(110) surfaces, although the COOH* species are less endothermic, indicating that further hydrogenation reactions will proceed via the carboxyl COOH* route. A further hydrogenation of the OH end of the COOH* intermediate may lead to CO + H2O formation, where the formed CO species could be further hydrogenated towards methane of methanol formation. This is consistent with the findings of Lu et al. [37], who, based on their detected intermediates, suggested that the possible reaction pathway for CO2 reduction over the Ta3N5 catalysts is CO2→COOH*→CO→CH*x→CH4. The present results demonstrate that Ta3N5 can efficiently activate the CO2 molecule and photocatalytically reduce it with water to produce value-added fuels. Future investigations of the Eley–Rideal type of mechanism will be important to draw a direct comparison with the Langmuir–Hinshelwood mechanism unravelled in the present study. Further investigations of the effects of transition metal doping on the electronic structure and CO2 conversion reactions over Ta3N5 catalyst under visible light will also be important.

4. Computational Details

The density functional theory (DFT) calculations were performed within the VASP package [55,56,57,58]. The projected augmented wave (PAW) method [59] was employed to describe the interactions between the valence electrons and the ionic core. Geometry optimisations were carried out using the Perdew−Burke−Enzerhof (PBE) generalized gradient approximation (GGA) functional [60], while the Hubbard U correction (PBE+U) was employed for accurate determination of the electronic structures [61,62,63,64]. From an analysis of how the electronic band gap increases with increasing strength of the on-site Coulomb repulsion, it was found that an effective U of 6.5 eV gives an accurate description of the structural parameters and the electronic properties of Ta3N5 [65]. Dispersion forces were accounted for through the Grimme DFT-D3 scheme [66]. The kinetic energy cut off was set to 600 eV, which ensured convergence of the total energy of the Ta3N5 to within 10−6 eV and the residual Hellman−Feynman forces to within 10−3 eV Å−1. A 7 × 3 × 3 mesh of Monkhorst−Pack [67] k-points was used to sample the Brillouin zone of Ta3N5. For accurate determination of the electronic structure of Ta3N5, a higher mesh of 9 × 5 × 5 was used.
The (001), (010), and (110) surfaces, which are commonly observed in Ta3N5 nanoparticles, were created from the optimized bulk material using the METADISE code [68,69], which ensures the creation of surfaces with zero dipole moment perpendicular to the surface plane. In each simulation cell (slab thickness of at least 15 Å), a vacuum region of 20 Å was added in the z direction to avoid interactions between periodic slabs. The relative stabilities of the (001), (010), and (110) Ta3N5 surfaces were determined according to their relaxed surface energy ( γ r ), calculated as:
γ r = E s l a b r e l a x e d − n E b u l k 2 A
where E s l a b r e l a x e d is the energy of the relaxed slab, n E b u l k is the energy of an equal number (n) of the bulk Ta3N5 atoms, and A is the area of the slab surface. The adsorption energy (Eads) of CO2 and H2O species is determined as follows:
Eads (M) = EM+surface – (Esurface + EM)
where EM+surface is the total energy of the relaxed M+Ta3N5 systems (M = CO2 and H2O), Esurface the total energy of the naked Ta3N5 surfaces alone, and EM the total energy of the isolated adsorbates (CO2 and H2O). Because of the adsorption of reactant molecules at only one side of the surface, Makov–Payne dipole correction was applied perpendicular to each surface [70]. In order to determine the preferred adsorption sites and lowest energy adsorption modes of CO2 and H2O molecules on the Ta3N5 surfaces, different initial orientations of the molecules were optimized without any symmetry constraints. Charge transfer between the surfaces and adsorbates is quantified via Bader charge analysis [71]. Transition states (TS) along reaction pathways were determined using the climbing-image nudged elastic band (CI-NEB) method [72], wherein six images were generated between the states of reactants (IS) and products (FS) in each elementary process. Located TS were characterized by only one imaginary frequency, corresponding to the reaction coordinate. The reaction activation energy barrier (EA) is determined by EA = TS – IS, whereas the reaction energy (ER) is determined by ER = FS – IS.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/10/1217/s1, Figure S1: Contains the relaxed structures of all possible adsorption CO2 geometries on Ta3N5 (001), (010), and (110) surfaces.

Funding

This research was funded by the UK’s Engineering and Physical Sciences Research Council (EPSRC), grant number EP/S001395/1.

Acknowledgments

The simulations were performed using the computational facilities of the Advanced Research Computing @ Cardiff (ARCCA) Division, Cardiff University. This work also made use of the facilities of ARCHER (http://www.archer.ac.uk), the UK’s national supercomputing service, via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202). Information on the data that underpins the results presented here, including how to access them, can be found in the Cardiff University data catalogue at http://doi.org/10.17035/d.2020.0119220581

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. The orthorhombic crystal structure of Ta3N5 in terms of TaN6 octahedra viewed in the (a) c–b and (b) b–a planes. (c) The electronic density of state of Ta3N5 showing the total and projection on the Ta d states and N p states. (Colour scheme: Ta = pale olive and N = blue.)
Figure 1. The orthorhombic crystal structure of Ta3N5 in terms of TaN6 octahedra viewed in the (a) c–b and (b) b–a planes. (c) The electronic density of state of Ta3N5 showing the total and projection on the Ta d states and N p states. (Colour scheme: Ta = pale olive and N = blue.)
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Figure 2. Optimized structures of the (a) {001}−(2 × 1), (b) {010}−(2 × 1), and (c) {110}−(1 × 1) surfaces of Ta3N5, in top (top) and side (bottom) views. (Atomic colour scheme: Ta = pale olive and N = blue.)
Figure 2. Optimized structures of the (a) {001}−(2 × 1), (b) {010}−(2 × 1), and (c) {110}−(1 × 1) surfaces of Ta3N5, in top (top) and side (bottom) views. (Atomic colour scheme: Ta = pale olive and N = blue.)
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Figure 3. Equilibrium morphology of Ta3N5 nanocrystal derived from Wulff construction.
Figure 3. Equilibrium morphology of Ta3N5 nanocrystal derived from Wulff construction.
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Figure 4. Lowest-energy CO2 adsorption structures on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces. The DOS projected on the surface Ta d states and N p states interacting with the C and O p states of CO2 (d–f) and the corresponding isosurface contours of the differential charge density are shown as inserts in (d–f), where green contours denote electron density increase and the red contours denote electron density decrease by 0.02 e/Å3, respectively. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
Figure 4. Lowest-energy CO2 adsorption structures on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces. The DOS projected on the surface Ta d states and N p states interacting with the C and O p states of CO2 (d–f) and the corresponding isosurface contours of the differential charge density are shown as inserts in (d–f), where green contours denote electron density increase and the red contours denote electron density decrease by 0.02 e/Å3, respectively. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
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Figure 5. Lowest-energy coadsorption structures of CO + O pairs on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces.
Figure 5. Lowest-energy coadsorption structures of CO + O pairs on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces.
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Figure 6. Optimized structures for the initial (left panels), transition (middle panels), and final (right panels) states of the most favourable path for the H2O→OH + H reaction on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces. (Atomic colour scheme: Ta = pale olive, N = blue, O = red, and H = white.)
Figure 6. Optimized structures for the initial (left panels), transition (middle panels), and final (right panels) states of the most favourable path for the H2O→OH + H reaction on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces. (Atomic colour scheme: Ta = pale olive, N = blue, O = red, and H = white.)
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Figure 7. Lowest energy of coadsorption structures of CO2 + H2O on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, O = red, and H = white.)
Figure 7. Lowest energy of coadsorption structures of CO2 + H2O on (a) (001), (b) (010), and (c) (110) Ta3N5 surfaces. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, O = red, and H = white.)
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Figure 8. Differential charge density isosurface contours of the CO2 + H2O coadsorption on Ta3N5 (a) (001), (b) (010), and (c) (110) surfaces. The green contours denote electron density increase whereas the red contours denote electron density decrease by 0.02 e/Å3, respectively. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
Figure 8. Differential charge density isosurface contours of the CO2 + H2O coadsorption on Ta3N5 (a) (001), (b) (010), and (c) (110) surfaces. The green contours denote electron density increase whereas the red contours denote electron density decrease by 0.02 e/Å3, respectively. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
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Figure 9. Optimized structures of the reaction products of proton transfer from H2O to CO2 on Ta3N5(010) surface. (a) Proton transfer to surface-bound O atom, (b) proton transfer to unbound O atom and (c) proton transfer to the C atom. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
Figure 9. Optimized structures of the reaction products of proton transfer from H2O to CO2 on Ta3N5(010) surface. (a) Proton transfer to surface-bound O atom, (b) proton transfer to unbound O atom and (c) proton transfer to the C atom. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
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Figure 10. Optimized structures of the reaction products of proton transfer from H2O to CO2 to COOH and HCOO species on (a,b) Ta3N5(001) and (c,d) Ta3N5(110) surfaces. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
Figure 10. Optimized structures of the reaction products of proton transfer from H2O to CO2 to COOH and HCOO species on (a,b) Ta3N5(001) and (c,d) Ta3N5(110) surfaces. (Atomic colour scheme: Ta = pale olive, N = blue, C = green, and O = red.)
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Table
Table 1. Adsorption energies, geometrical parameters, charges, and vibrational frequencies of molecular CO2 on Ta3N5(001), (010), and (110) surfaces.
Table 1. Adsorption energies, geometrical parameters, charges, and vibrational frequencies of molecular CO2 on Ta3N5(001), (010), and (110) surfaces.
ParameterFree CO2Ta3N5(001)Ta3N5(010)Ta3N5(110)
Eads (eV)-−2.73−1.89−1.20
Δq(CO2) (|e|)0.00.400.340.31
d(C−Oa) (Å)1.1761.3021.4221.293
d(C−Ob) (Å)1.1761.3011.2021.290
∠ OaCOb (o)180.0130.0123.9125.5
d(C−N) (Å)-1.3611.3961.400
d(Oa−Ta) (Å)-2.1762.0122.217
d(Ob−Ta) (Å)-2.180-2.230
υas (cm−1)2373146518041718
υs (cm−1)13231265911881
υb (cm−1)631819737691
Table
Table 2. Coadsorption (Ecoads), reaction (Erxn), activation (Eact) energies and geometrical parameters of (CO + O) pairs on Ta3N5(001), (010), and (110) surfaces.
Table 2. Coadsorption (Ecoads), reaction (Erxn), activation (Eact) energies and geometrical parameters of (CO + O) pairs on Ta3N5(001), (010), and (110) surfaces.
ParameterTa3N5(001)Ta3N5(010)Ta3N5(110)
Ecoads (eV)−1.69−2.99−0.68
Erxn (eV)1.04−1.090.52
Eact (eV)1.341.151.56
d(C−N) (Å)1.467/1.3881.2371.242
d(Oa−Ta) (Å)1.7882.126/1.9791.788
d(C−Ob) (Å)1.2271.1731.170
Table
Table 3. Coadsorption (Ecoads), reaction (Erxn), activation (Eact) energies and geometrical parameters of (OH + H) pairs on Ta3N5(001), (010), and (110) surfaces.
Table 3. Coadsorption (Ecoads), reaction (Erxn), activation (Eact) energies and geometrical parameters of (OH + H) pairs on Ta3N5(001), (010), and (110) surfaces.
ParameterTa3N5(001)Ta3N5(010)Ta3N5(110)
StateMolecularDissociativeMolecularDissociativeMolecularDissociative
Eads (eV)−1.42−2.16−1.07−1.35−1.08−2.12
Erxn (eV)-−0.74-−0.28-−1.04
Eact (eV)0.18-0.34-0.27-
d(O−Ta) (Å)2.3071.9872.3952.2972.2951.978
d(H−N) (Å)-1.031-1.056-1.022
d(O−H) (Å)0.977/0.9770.9690.980/0.9790.9760.986/1.0060.970
Table
Table 4. Coadsorption energies and structural parameters for CO2 + H2O on Ta3N5(001), (010), and (110) surfaces.
Table 4. Coadsorption energies and structural parameters for CO2 + H2O on Ta3N5(001), (010), and (110) surfaces.
Parameter Ta3N5(001)Ta3N5(010)Ta3N5(110)
Ecoads (eV)−3.94−3.68−2.34
Δq(CO2+H2O) (|e|)0.400.350.31
Δq(H2O) (|e|)0.030.030.02
d(C−Oa) (Å)1.3051.4601.292
d(C−Ob) (Å)1.3071.2021.292
∠ OaCOb (o)125.6122.1125.5
d(C−N) (Å)1.3541.3781.399
d(Oa−Ta) (Å)2.1872.0452.231
d(Ob−Ta) (Å)2.188-2.244
d(Ow−H) (Å)0.9811.0220.978
d(Ow−Ta) (Å)2.2992.2262.315
d( O–H) (Å)3.6191.6063.454
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Dzade, N.Y. CO2 and H2O Coadsorption and Reaction on the Low-Index Surfaces of Tantalum Nitride: A First-Principles DFT-D3 Investigation. Catalysts 2020, 10, 1217. https://doi.org/10.3390/catal10101217

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Dzade NY. CO2 and H2O Coadsorption and Reaction on the Low-Index Surfaces of Tantalum Nitride: A First-Principles DFT-D3 Investigation. Catalysts. 2020; 10(10):1217. https://doi.org/10.3390/catal10101217

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Dzade, Nelson Y. 2020. "CO2 and H2O Coadsorption and Reaction on the Low-Index Surfaces of Tantalum Nitride: A First-Principles DFT-D3 Investigation" Catalysts 10, no. 10: 1217. https://doi.org/10.3390/catal10101217

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