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Probing Photocatalytic Nitrogen Reduction to Ammonia with Water on the Rutile TiO (110) Surface by First-Principles Calculations 2

Xiao-Ying Xie, Pin Xiao, Wei-Hai Fang, Ganglong Cui, and Walter Thiel ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01551 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Probing Photocatalytic Nitrogen Reduction to Ammonia with Water on the Rutile TiO2 (110) Surface by First-Principles Calculations Xiao-Ying Xie,† Pin Xiao,† Wei-Hai Fang,† Ganglong Cui,*† and Walter Thiel‡ †Key

Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of

Chemistry, Beijing Normal University, Beijing 100875, China and ‡ Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany Abstract Photocatalytic ammonia production from air and water at ambient conditions is ideally suited for artificial nitrogen fixation. It has been the subject of several recent experimental studies with titanium dioxide and titania-based semiconductors as catalysts. The TiO2-mediated photocatalytic NH3 production from H2O and N2 is a very complex process that is not yet well understood mechanistically, which hampers further advances. In the present work, we address the detailed mechanism of N2 reduction to NH3 driven by photolysis of water adsorbed on the rutile TiO2 (110) surface containing oxygen vacancies, by means of reliable density functional calculations (HSE06+D3//PBE+U+D3). We show that each major step of the reaction is driven by H2O photolysis and can proceed at ambient conditions. The initial N 2 adsorption, the activation of the inert NN bond, and the N-N cleavage are all efficiently promoted by TiO2 surface hydroxylation and photo-generated electrons, as well as their synergistic effects, while proton-coupled electron transfers play a decisive role in the N2 reduction to NH3. These mechanistic insights could probably guide further experimental studies of TiO2 photocatalytic nitrogen fixation and NH3 photosynthesis.

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Keywords: photocatalysis; nitrogen fixation; TiO2; DFT; oxygen vacancy; hydroxyl group Introduction Ammonia (NH3) is an indispensable chemical in modern society as an essential building block for manufacturing synthetic chemicals like fertilizers, drugs, dyes, explosives, and resins.[1-6] Because of its large hydrogen capacity and high energy density, NH3 could also be used as an alternative energy carrier to establish a low-carbon society.[7] Currently, half of the global ammonia production is derived from the industrial Haber–Bosch process operated at high temperature and pressure. [8-10] It is an energy- and resource-intensive process, which requires complex large-scale infrastructure and simultaneously produces a substantial amount of carbon dioxide.[11,12] The ever-increasing ammonia demand has stimulated significant research on artificial nitrogen fixation,[13-45] but the core issue is how to develop green and sustainable nitrogen-fixing strategies. Early experimental studies in the late 20th century targeted the photocatalytic reduction of molecular nitrogen to ammonia with titanium dioxide and titania-based catalysts,[46-55] which have been reviewed in two recent papers.[34,37] In the first attempt by Schrauzer and Guth in 1977, the photolysis of chemisorbed water on incompletely outgassed TiO2 powder was explored in the presence of molecular nitrogen. [46] The photocatalytic oxidation of water was found to produce O 2, with concomitant reduction of the adsorbed nitrogen to NH3 and a small quantity of N2H4.[46] Soon after this pioneering work, different spectroscopic techniques were used to detect the species produced by long-time irradiation of rutile TiO2 powders containing adsorbed water, with near UV photons (365 nm) in the presence of either oxygen or air. [47] Instead of NH3 and N2H4, hydrogen peroxide was initially observed on the surface of the TiO 2 powders during irradiation, and desorption spectra showed that nitric oxide was formed thereafter on the TiO 2 surface, presumably by reaction of nitrogen with the adsorbed H2O2.[47] Subsequently, several experimental studies were carried out to explore the photocatalytic nitrogen fixation on titania-based catalysts,[48-51] which


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provided further evidence that NH3 molecules can be produced from molecular nitrogen under a variety of conditions. In addition, the rutile phase and iron doping were found to enhance the efficiency of the N2 reduction.[46,48-50] On the contrary, Edwards and coworkers failed to fix nitrogen as ammonia, [52] when they performed photosynthesis of NH3 from N2 and H2O in the presence of iron-doped TiO2 or other metal oxides under conditions similar to those reported in the previous studies, [46,50] which gave rise to a fierce debate in the mid-1990s.[53,54] In the following two decades, the field remained rather dormant. Recently, the photocatalytic N2 reduction under ambient conditions has again attracted much interest, and there are numerous experimental studies with titanium dioxide and other semiconductor materials as catalysts.[20,22,24-30,34-36,40,42-45] Among these recent studies, Hirakawa and co-workers reported a much high solar-to-chemical energy conversion efficiency of 0.02% for the TiO2 photocatalytic system.[26] In their experiment, ammonia is successfully produced at atmospheric pressure and room temperature upon UV irradiation of pure water and nitrogen on several commercially available TiO2 samples with a large number of surface oxygen vacancies. The Ti3+ species at oxygen vacancies were found to behave as active sites for N2 adsorption and reduction by ESR (electron spin resonance) and DRIFT (diffuse reflectance infrared Fourier transform) spectroscopies. After H2O photolysis, the N-H, TiO-H, and N-N stretching vibrations on the TiO2 surface were identified through the time-dependent changes in the DRIFT spectra. In addition, the N2 gas was confirmed to be the source of NH3 by isotope-labeling experiments.[26] On the basis of these experimental findings, a possible mechanism was proposed for photocatalytic nitrogen-to-ammonia conversion on TiO2 surfaces with oxygen vacancies, which involves three basic steps. Each step is initially driven by photolysis of H2O to release O2. Then, the adsorbed molecular nitrogen (*N2) is reduced stepwise to adsorbed diazene (*N2H2), hydrazine (*N2H4), and ammonia (*NH3) in the first, second, and third steps, respectively. The mechanism proposed by Hirakawa and co-workers is not consistent with recent computational work on photocatalytic N2 reduction to NH3 with H2O on the rutile TiO2 (110) surface.[56] Their computational


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model was based on DFT (density functional theory) calculated voltages (relative to the reversible hydrogen electrode) for possible redox reactions, and applied to analyze the expected dinitrogen coverage at the TiO2 surface and the overpotentials for electrochemical N2 reduction and oxidation. Their calculations indicate that the TiO2 (110) surface is unlikely to support nitrogen reduction, because of the low stability of the adsorbed N2Hx and NHx (x=1 or 2) species. Moreover, the thermodynamic limiting potential for nitrogen oxidation on the TiO2 (110) surface was determined to be very low, and the oxidation pathway was suggested to be relevant on the TiO2 (110) surface. A complex balance of oxidative and reductive processes was proposed to be responsible for nitrogen fixation on the TiO2 (110) surface.[56] Photocatalytic NH3 production from H2O and N2 with noble-metal-free TiO2 as catalyst is an ideal artificial photosynthetic pathway for solar-to-chemical energy conversion. With water and air as raw materials, a bright future for green and sustainable NH3 photosynthesis has been envisioned in previous experimental studies.[26,46,48-51,54] However, due to the great complexity of the TiO2-mediated processes, there is a lack of detailed molecular-scale mechanistic information, which hampers the further development of photocatalytic nitrogen fixation. It is generally accepted that a combined experimental and theoretical approach can help resolving mechanistic issues and thus promote faster progress in NH3 photosynthesis.[34,37] Actually, numerous DFT calculations have addressed catalytic N2 fixation, but most of them were focused on the thermochemical or electrochemical conversion of nitrogen to ammonia in model systems. It is well established experimentally that the photolysis of adsorbed water on TiO 2 precedes the subsequent N2 reduction to NH3.[26,46-52] The photocatalytic water splitting on TiO2 has been studied intensely both experimentally and theoretically,[57,58] and oxygen evolution is known to be the bottleneck of the overall reaction: 2H2O2H2+O2. Very recently, Hu and co-workers performed a systematic computational investigation on the mechanism of the oxygen evolution reaction at the water/TiO 2 (110)


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interface, and the low concentration of photo-generated holes in the TiO2 surface was identified as the key obstacle to oxygen evolution.[58] In the present work, we explore the TiO2-mediated photocatalytic NH3 production from N2 and H2O by focusing on how the photo-generated electrons and protons make it possible to reduce N2 to NH3. Computational Methods All spin-polarized DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP5.4) interfaced with the MedeA2.20 suites.[59,60] The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was chosen to describe the electronic exchange-correlation interaction.[61] The projector augmented-wave (PAW) method was employed to represent the core-valence electron interaction.[62,63] All DFT calculations were augmented by including long-range dispersion interactions (DFT+D3) with Becke-Johnson damping.[64,65] The ion-electron interaction was described by the PAW pseudopotential. A cutoff energy of 460 eV was adopted for the plane-wave basis sets. The rutile TiO2 (110) surface was modeled as periodic slab with three O-Ti-O trilayers. The accuracy of such slab models has been demonstrated in recent calculations.[66-69] The vacuum layer between slabs was set to 15 Å and the adsorption was modeled on one side of the slabs. A 4 × 2 surface cell and 1 × 1 × 1 k-point mesh were used in all calculations. During structural optimizations, all atoms except those in the bottom layer of the TiO2 slab were allowed to relax until all energy gradients were less than 0.05 eV Å −1.[58,68-71] The GGA-based PBE functional cannot provide an accurate description of trapping electrons on the surface and sub-surfaces of material,[72,73] due to the self-interaction error.[74] In order to overcome this shortcoming, we either applied a simple U correction to the 3d orbital of the Ti atom in the DFT calculations (DFT+U method)[75] or employed the more accurate HSE06 hybrid functional, [76,77] in analogy to previous studies.[78] Because HSE06 calculations are very time-consuming, we used the corrected PBE+U+D3 functional to optimize stationary structures and the HSE06+D3 functional to refine their energies in single-


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point calculations. This combined HSE06+D3//PBE+U+D3 strategy has previously been demonstrated to be reliable.[79,80] Transition states were optimized with the dimer method.[81,82] The forward finite-difference method was utilized to compute the curvature along the dimer direction. The step size for numerical differentiation was 0.01 Å and the dimer was rotated merely when the predicted rotation angle was greater than 0.01 rad. The trial optimization step size was set to 0.01 Å and the trust optimization radius was chosen to be 0.1 Å . Adsorption energies (𝐸𝑎𝑑𝑠 ) were computed using the formula 𝐸𝑎𝑑𝑠 = −(𝐸𝑠𝑙𝑎𝑏+𝑁2 − 𝐸𝑠𝑙𝑎𝑏 − 𝐸𝑁2 ) , where 𝐸𝑠𝑙𝑎𝑏+𝑁2 , 𝐸𝑠𝑙𝑎𝑏 , and 𝐸𝑁2 were potential energies of the TiO2 model with N2 adsorbed, the pure TiO2 slab model, and the N2 molecule, respectively. Frontier molecular orbitals were plotted on the basis of single-point energy calculations with the CP2K software.[83-85] The PBE functional as well as DZVP-MOLOPT-SR-GTH basis sets and Goedecker-Teter-Hutter (GTH) pseudopotentials were used.[61, 86-90] Results and Discussion Adsorption and initial activation of N2 by photolysis of the first H2O molecule The adsorption of N2 is the prerequisite for its reduction to ammonia, and hence we started our study with structural optimizations of periodic slab (Per-slab) models with N2 adsorbed at the surface oxygen vacancy (Ov) and penta-coordinated Ti atom (Ti5c). In addition, two different orientations for N2 adsorption on TiO2, referred to as end-on and side-on, are characterized in the present work, since they have been used for the subsequent N2 reduction in the previous studies.[29,34] The PBE+U+D3 optimized structures are shown in Figure 1, Figure S1 or Figure S2 and the HSE06+D3//PBE+U+D3 calculated adsorption energies are listed in Table S1. The calculated adsorption energies indicate that the end-on structure is more stable than the side-on structure for N2 adsorption at different sites. For example, the end-on structure has its adsorption energies of 0.31 and 0.40 eV at Ov and Ti5c respectively, while the adsorption energy of the side-on structure becomes 0.05 and 0.10 eV for N2 adsorption at Ov and Ti5c respectively.


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Figure 1. Optimized structures of the pristine-surface model (a), the pristine-surface model with N2 adsorbed at Ti5c (b), Per-slab model (c), Per-slab model with N2 adsorbed at Ov (d), Hyd-per-slab model (e), and Hyd-per-slab model with N2 adsorbed at Ov (f). Also shown are the electron spin density (isovalue: 0.005 e/Å 3) and key bond lengths in Å . Gray, red, blue and white balls represent Ti, O, N and H atoms, respectively. These color conventions are used throughout the paper. In the end-on structure, the NN bond is nearly perpendicular to the TiO2 (110) surface. The proximal N atom, labeled as *N, is located on the top of the Ti5c atom of the pristine surface or in the middle between


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the two Ti atoms for N2 adsorption at the surface Ov site. The *NN bond length is in the range of 1.113 1.116 Å in the optimized end-on and side-on structures, which is nearly the same as that in the isolated N2 molecule. This shows that N2 adsorption at Ov or Ti5c has little influence on the inert NN bond. The Per-slab model contains one oxygen vacancy on the TiO2 (110) surface and two unpaired or excess electrons. The PBE+U+D3 computed electron spin densities are shown in Figure 1(c) and (d) for the optimized structures. Evidently, the distribution of the two unpaired electrons remains nearly unchanged upon N2 adsorption at the Ov site, indicating that the N2 molecule is adsorbed physically. As shown in Figure 1 (b), the *N-Ti distance is 2.422 Å in the optimized end-on structure for N2 adsorption at the Ti5c atom of the pristine surface, which is much shorter than the corresponding value of 2.804 Å for N2 adsorption at the Ov site. Actually, the interaction of the adsorbed *N2 with Ti5c is partially of chemisorbed character. It should be pointed out that the surface hydroxylation of TiO2 leads to a significantly stronger chemisorption of N2 at the Ov site, as will be discussed below. Experimentally, Ti-O-H species were inferred to be generated on the rutile TiO2 (110) surface.[26] This is in line with the PBE+U+D3 optimized structures for the hydroxylated Per-slab (Hyd-per-slab) model without and with N2 adsorption at the Ov site [see Figure 1(e) and (f)]. Importantly, the TiO2 surface hydroxylation plays a pivotal role for N2 adsorption and activation according to the HSE06+D3//PBE+U+D3 results. The computed adsorption energy for the end-on structure is increased from 0.31 eV in Per-slab to 0.61 eV in Hyd-per-slab. The N2 adsorption is thus significantly strengthened by the TiO2 surface hydroxylation, and *N-Ti bonds are partially formed between the adsorbed *N atom and the two nearest Ti atoms [2.438 and 2.418 Å , see Figure 1(f)]. The NN bond in the end-on structure is elongated from 1.114 Å in the Per-slab model to 1.228 Å in the Hyd-per-slab model, indicating considerable activation of the inert NN bond upon N2 adsorption at the Ov site of the hydroxylated surface.


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Figure 2. Charge density differences (a) for N2 adsorption in the Hyd-per-slab model (yellow: charge accumulation; blue: charge depletion), schematic orbital interactions (b) based on the relevant frontier molecular orbitals (c). Further evidence for the NN activation comes from the charge density differences and the relevant molecular orbitals shown in Figure 2 and from the distribution of the electron spin densities in Figure1(e) and (f). Two photo-generated electrons are distributed in the vicinity of the Ov site, and there are two excess electrons in the sub-surface. Under these conditions, the tetravalent Ti4+ species at the Ov site are reduced to the trivalent Ti3+ species by the photo-generated electrons. The exposed Ti3+ species were suggested to serve as active sites for N2 adsorption and reduction in several previous studies:[26,91-93] the photo-generated electrons around the Ov site strengthen the N2 adsorption on the TiO2 surface, and they are partially transferred to the NN π* orbital during N2 adsorption, which results in the initial N2 activation. Hence, the photo-generated electrons distributed in the Ov site and the photo-generated protons trapped by the bridging surface oxygen atoms play an essential role for the adsorption of N 2 and its initial activation, which is the main reason why the H2 evolution is inhibited upon H2O photolysis in the presence of N2 adsorbed on the Ov site of the TiO2 surface. It is noteworthy that the surface hydroxylation and its synergistic interaction with the photo-generated electrons were not considered in the previous computational model,[56] which may explain why that study did not support the experimental observation of


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photocatalytic NH3 production or the proposed mechanism.[26] Finally, it should be pointed out that the TiO2 surface hydroxylation has little influence on the side-on structure and that the NN bond in the side-on structure is less activated by the photo-generated electrons (see Figure S1 of Supporting Information). The end-on structure will thus serve as the initial state for the subsequent N2 reduction, in view of its larger adsorption energy and significant NN activation. Upon irradiation of the TiO2 semiconductor, positive holes (h+) are generated in the valence band, which can oxidize the H2O molecule to generate protons via: H2O + 2 h+  ½ O2 + 2 H+. Previous studies[94,95] have shown that the produced protons can transfer very easily among the surface oxygen atoms, due to their interaction with H2O molecules adsorbed on the penta-coordinated Ti (Ti5c) atoms of the TiO2 surface. After the H2O photolysis with release of O2, the two photo-generated electrons are distributed in the defect state and become localized mainly in the region of the Ov site (see Figure 1). The two photogenerated protons (H+) are preferentially trapped by the bridging oxygen atoms adjacent to the Ov site, producing two Ti-O-H groups on the TiO2 surface. The surface hydroxylation is driven by photolysis of the first H2O molecule and can be formulated as: Per-slab + H2O + 2 hv  Hyd-per-slab + 1/2 O2. This reaction releases some excess energy, which is available for the subsequent N2 reduction. On the basis of the HSE06+D3//PBE+U+D3 calculated enthalpy change (2.9 eV) for the stoichiometric reaction, Per-slab + H2O  Hyd-per-slab + 1/2 O2, we estimate the excess energy to be larger than 3.0 eV (depending on the chosen photo-excitation energy). It should be emphasized that the surface hydroxylation, together with the formation of photo-generated electrons at the Ov site, is the main reason why H2 generation is inhibited upon photolysis of H2O in the presence of N2 adsorbed on the TiO2 surface. Before ending this section, we pay a little attention to the H2O adsorption at the Ov site, since it is generally accepted that H2O molecules interact with the oxygen vacancy of the TiO2 surface more preferentially than N2 molecules. On the basis of the PBE+U+D3 optimized structures for the H2O


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adsorption at the Ov and Ti5c sites of the TiO2 surface with and without the hydroxylation, the H2O adsorption energies were calculated at the HSE06+D3 level and the calculated results are listed in Table S2. The first observation is that the surface hydroxylation has little influence on the H2O adsorption energies at the Ov and Ti5c sites. The calculated adsorption energies are 1.17 and 0.98 eV for H2O molecules at the Ov and Ti5c sites of the hydroxylated TiO2 surface, respectively, which indicates that H2O molecules have comparable probabilities to be adsorbed at the Ov site and the Ti5c atom. However, it can be expected that the H2O adsorption at the Ti5c atom is still preferred because of much more Ti5c atoms than the Ov site on the TiO2 surface. It is clear that both oxygen vacancy (Ov) and hydroxyl group synergistically increase the adsorption of N2 at the Ov site (see Tables S1 and S2). Thus, a N2 molecule will obviously prefer to occupy the Ov site. Differently, for H2O, both Ti5c and Ov sites have similar adsorption energies. In our TiO2 model, there are 8 Ti5c sites and 1 Ov site, so the probability of a H2O molecule occupying the Ov site is ca. 10%, which could become much smaller because of more Ti5c sites in realistic catalysts. Therefore, if there exist comparable numbers of N2 and H2O molecules, the N2 molecule has much larger probability occupying the Ov site than the H2O molecule. Of course, this probability will be reduced to certain extent considering little different absorption energies of N2 and H2O. Nonetheless, there should be non-negligible N2 molecules adsorbed on the Ov sites. Importantly, the N2 adsorption on the Ov site could be enhanced from specific experimental conditions. For example, recent experiments by Hirakawa et al. [26] are carried out in a glass vessel with 200 mL solution under N2 bubbling of 0.3-1.0 L/min at 313 K for several tens of hours (even up to 100 hours). As a result, there are about 11 mol water molecules in the vessel and there are 1.2 mol N 2 molecules added in the vessel per hour for 0.5 L/min N2 bubbling (12 mol for 10 hours). Moreover, the N2 amount in solution should be raised with the time and amount of the N2 bubbling albeit some N2 molecules will escape from solution, because there is an obvious increase of ammonia produced [see Fig. 1 in recent experiments[26]]. Considering these reasons, there should be comparable probability for N 2 adsorbed on the


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Ov site in recent experiments.[26] The Ov and Ti5c sites are re-exposed after the photolysis of H2O adsorbed on the TiO2 surface to release O2. The subsequent surface hydroxylation takes place easily, as discussed before, which provide a good chance for N2 adsorbed at the Ov site. Upon UV irradiation of a commercially available TiO2 in pure water with N2 flow, the rate of NH3 formation was experimentally observed to be enhanced with an increase in the rate of N2 bubbling. [26] Meanwhile, the N2 reduction was determined to occur at oxygen vacancies by ESR and DRIFT spectra.[26] After the H2O photolysis to release O2, the N-H, TiO-H, and N-N stretching vibrations on the TiO2 surface were identified through time-dependent changes in DRIFT spectra. In addition, the N2 molecules were confirmed to be the source of NH3 by isotope-labeling experiments and the chemisorbed nitrogen is reduced to NH3 according to the stoichiometry of reaction, 2N2+6H2O+nh 4NH3+3O2.[26,46] All these experimental findings provide strong evidence that there are some N2 molecules adsorbed at the Ov sites of the TiO2 surface after the photolysis of adsorbed H2O to release O2.

Figure 3. Relative energies (in eV) of the stationary structures on the pathway from Hyd-per-slab with N2 adsorbed to the *N-NH2 species, together with some selected internuclear distances (in Å ). Two Ti atoms at the oxygen vacancy are highlighted.


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Hydrogenation of adsorbed N2 by proton-coupled electron transfer The initial hydrogenation leads to further activation of the adsorbed *N-N species. It is an important elementary process for N2 reduction in most cases.[13,40] In principle, two pathways are possible for the initial hydrogenation, i.e. *N-N  *N-NH and *N-N  *NH-N. The relatively large steric hindrance[96] at the *N atom and the transfer of the photo-generated electron toward the distal N atom (see above) let us expect that the distal N atom is in a favorable position to accept a proton from the Ti-O-H species. This is confirmed to be the minimum-energy pathway for the initial N2 reduction by the HSE06+D3//PBE+U+D3 results. As shown in Figure 3, the initial hydrogenation reaction, *N-N  *N-NH, is endothermic by 0.08 eV at this level, and the energy of the optimized transition state is 0.39 eV higher than that of the initial state and 1.00 eV higher than that of the Hyd-per-slab with N2 adsorbed. The corresponding barrier can be overcome easily due to the excess energy available from photolysis and surface hydroxylation (>3.0 eV, see above). As shown in Figure 1, the two unpaired electrons are mainly distributed in the sub-surface, while the two photo-generated electrons at the Ov site are fairly delocalized into the adsorbed N-N region. Once the proton of the Ti-O-H species is transferred to the distal N atom, one electron is transferred to the *N-NH species. This is confirmed by the calculated charge transfer from TiO2 to *N-NH in the process of *N-N  *N-NH. Therefore, the initial hydrogenation reaction of the *N2 molecule is driven by protoncoupled electron transfer (PCET). The resulting adsorbed *N-NH species is formed as a stable intermediate, with *N-Ti1 and *N-Ti2 lengths of 2.189 and 2.139 Å , respectively (see Figure 3). Further reduction takes place via a second proton transfer from the surface Ti-O-H species to either the *N or N atom of the *N-NH species, forming either *NH-NH or *N-NH2 at the Ov site. As discussed above, the proton transfer to the *N atom has larger steric hindrance, and the barrier for *NH-NH generation is thus much higher than that for formation of the *N-NH2 species, although both the adsorbed *NH-NH and *N-NH2 species have nearly equal energies (see Figure S5 of the Supporting Information).


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Like the initial hydrogenation, the reduction of *N-NH to *N-NH2 is a PCET process. The relevant potential energy profile from HSE06+D3// PBE+U+D3 is shown in Figure 3. The second PCET process has a rather small barrier of 0.51 eV, indicating that it can occur more easily than the initial hydrogenation from *N-N to *N-NH. It leads to further activation of the NN bond, which is reflected in the NN bond length of 1.280 Å in the adsorbed *N-NH2 species, indicating N=N double-bond character. NN cleavage assisted by photolysis of a second H2O molecule As discussed above, the two protons produced by photolysis of a first H2O molecule are trapped by two bridging oxygen atoms to generate the Ti-O-H species in the vicinity of the Ov site. After the initial reduction of *N-N to *N-NH2, the bridging oxygen atoms are again exposed and can serve as active sites to capture protons from photolysis of a second H2O molecule. Thereby, the Ti-O-H species are regenerated in

Figure 4. Relative energies (in eV) of the stationary structures on the pathways for N-N cleavage of the adsorbed *N-NH2 and for the subsequent hydrogenation to produce adsorbed *NH2 + *NH2 species, together with some selected internuclear distances (in Å ). Optimized structures for *N-NH2, *N + *NH2, *NH


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+ *NH2, and *NH2 + *NH2 species can be found in Figure S4 of the Supporting Information, along with the calculated electron spin densities. the vicinity of the Ov site, accompanied with redistribution of the two photo-generated electrons. According to the HSE06+D3//PBE+U+D3 results, the *N-NH2 decomposition into adsorbed *N and *NH2 species is endothermic by 2.31 eV when the surface is not hydroxylated. By contrast, the N-N cleavage becomes exothermic by 0.08 eV at the same level of theory, when the Ti-O-H species are regenerated close to the Ov site. This dramatic change in the energy required for N-N cleavage originates mainly from the redistribution of the two photo-generated electrons due to surface hydroxylation: one is distributed in the Ov site after Ti-O-H regeneration (strengthening the interaction of the adsorbed *N-NH2 with the Ti1 and Ti2 atoms), while the other one is transferred into the N-N * orbital (further weakening the N-N bond of the *NNH2 species, elongation from 1.280 to 1.364 Å ). It should be emphasized that the adsorbed *N and *NH2 species, which are produced by the N-N cleavage of the adsorbed *N-NH2 species, are again significantly stabilized by redistribution of the photo-generated electrons. With respect to the adsorbed *N-NH2 species, the barrier height of the N-N cleavage is predicted to be 1.28 eV at the HSE06+D3//PBE+U+D3 level (see Figure 4). Other conceivable pathways starting from the *N-NH2 species are described in Figures S9 – S11 of the Supporting Information; they are not competitive with the N-N cleavage discussed above.


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Figure 5. Schematic diagram of the available energies from TiO2 surface hydroxylation driven by H2O photolysis. After the N-N cleavage of the *N-NH2 species, the formed NH2 group is bound to the Ti5c atom that is closest to the Ov site. In this species, referred to as *NH2-Ti5c, more electron density is transferred into the vicinity of the *N atom adsorbed at the Ov site, and hence the *N atom becomes a proton acceptor. Consequently, two protons are successively transferred from the Ti-O-H species to the *N atom, forming an *NH2 group adsorbed at the Ov site (*NH2-Ov), with concomitant redistribution of the unpaired electrons. As shown in Figure 4, the two proton transfers are exothermic by 0.52 and 0.72 eV, respectively, and the energies of the corresponding transition states (1.21 and 0.76 eV above the *N-NH2 species) are slightly lower than that of the transition state for N-N cleavage (1.28 eV). According to the HSE06+D3//PBE+U+D3 results, the overall reaction from *N- NH2 to *NH2 + *NH2 is exothermic by 1.32 eV with a barrier of 1.28 eV for the rate-determining step of N-N cleavage.

Figure 6. Relative energies (in eV) of the stationary structures on the pathways for reduction of adsorbed *NH2 to *NH3, together with some selected internuclear distances (in Å ). The optimized structures for the


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four minima are shown in Figure S5 of the Supporting Information, along with the calculated electron spin densities. As shown in Figure 5(a), the energy (𝐸 𝐴 ) available from TiO2 surface re-hydroxylation and H2O photolysis can be calculated from 𝐸 𝐴 = 2ℎ𝑣 − ∆𝐻, where ℎ𝑣 is the electronic excitation energy and ∆𝐻 is the enthalpy change of the stoichiometric reaction, Per-slab+*N-NH2 + H2O  Hyd-per-slab+*N-NH2 + 1/2O2, where Per-slab+*N-NH2 and Hyd-per-slab+*N-NH2 denote the periodic species with N-NH2 adsorbed on the oxygen vacancy of the rutile TiO2 surface and the hydroxylated surface, respectively. At the HSE06+D3//PBE+U+D3 level, ∆𝐻 is 1.1 eV for the stoichiometric reaction, and the available energy is thus estimated to be at least 4.9 eV (using a reference band gap of 3.0 eV for rutile TiO 2). This is sufficient to overcome the barriers on the pathways shown in Figure 4. Therefore, the N-N cleavage of the *N-NH2 species and the subsequent proton transfer processes will proceed easily. Since the photo-generated electrons are fully used for formation of the N-Ti bonds and the reduction of the N-N bond, more excess energy is available after TiO2 surface re-hydroxylation, as compared with the initial hydroxylation, where the photo-generated electrons are mainly distributed in the defect state. Reduction of *NH2 to *NH3 by photolysis of a third H2O molecule The NN bond cleavage and the further reduction to two *NH2 groups make full use of the photogenerated protons and electrons from photolysis of the second H 2O molecule. Thereafter, the surface bridging oxygen atoms are re-exposed and can again accept protons from photolysis of a third H 2O molecule, leading to re-hydroxylation of the TiO2 surface. Subsequently, there are a few pathways to generate adsorbed NH3 molecules (*NH3) by hydrogenation of the *NH2 groups. The most feasible pathway at the HSE06+D3//PBE+U+D3 level is schematically shown in Figure 6, while other conceivable pathways are presented in Figure S12 of the Supporting Information. The *NH2-Ti5c group is rather far away from the Ti-O-H moiety, and hence the *NH2-Ov group acts as a relay for hydrogenation of the *NH2-Ti5c group,


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which yields an *NH3 molecule adsorbed at the Ti5c atom (labeled as *NH3-Ti5c). Overall, the reduction from *NH2 to *NH3 is exothermic by 0.63 eV with a rate-limiting barrier of 0.99 eV for the first proton transfer. The desorption energies are calculated to be 1.44 and 1.19 eV for *NH 3-Ti5c and *NH3-Ov, respectively. Referring to Figure 5(b), the energy available for the entire reduction and the subsequent *NH3 desorption is estimated to be larger than 5.2 eV, based on the formula 𝐸 𝐴 = 2ℎ𝑣 − ∆𝐻, where ∆𝐻 = 0.8 eV is obtained from HSE06+D3//PBE+U+D3 calculations. Therefore, the *NH2 reduction and desorption should occur easily. Conclusions In the present work, we have employed reliable PBE+U+D3 and HSE06+D3 exchange-correlation functionals, together with periodic slab models, to explore the detailed mechanism for the TiO2-catalyzed N2 reduction to NH3 driven by H2O photolysis. A new mechanism is proposed for this reaction (see Figure 7), which is different from those proposed for the thermochemical or electrochemical conversion of nitrogen to ammonia in previous studies.[34,56] Computationally, we show that the N2 adsorption, the N-N cleavage, and the N2 reduction to NH3 are efficiently promoted by surface defects, photo-generated protons and electrons, and their synergistic effects. The overall photocatalytic reaction from N 2 adsorption to NH3 production is exothermic by ~2.0 eV. It is feasible because of three H2O photolysis events that provide the energy input to overcome the barriers (0.4-1.3 eV) to each elementary step of the N2 reduction of N2 to NH3. Therefore, the TiO2-catalyzed N2 fixation driven by H2O photolysis is a downhill process, although the dissociation energy of the NN triple bond is extremely high (~230.0 kcal/mol or ~10.0 eV).


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Figure 7. Detailed mechanism for the reduction of N2 to NH3 driven by photolysis of water adsorbed on the TiO2 surface, together with the computed values (HSE06+D3//PBE+U+D3) for the N2 adsorption energy (Eads/eV), the barrier heights (E/eV), the enthalpy changes for three basic processes (I, II, and III), the desorption energies (Edes/eV) of the adsorbed NH3, and the energies available for each process (EA/eV).

Because the photo-generated electrons are distributed in the vicinity of the surface oxygen vacancies (the surface defect states), the TiO2 surface hydroxylation occurs preferentially at the bridging surface oxygen atoms that are adjacent to oxygen vacancies. Importantly, the surface hydroxylation not only strengthens the N2 adsorption, but also plays a decisive role in the N2 reduction, the NN cleavage, and the final NH3 production through proton-coupled electron transfers. In addition, excess energy is released by the surface hydroxylation and the subsequent PCET reactions, which is the main reason why H2 generation is inhibited upon photolysis of H2O in the presence of N2 adsorbed on the TiO2 surface. These unique roles of surface hydroxylation are uncovered here for the first time by the present first-principles calculations. Finally, it should be pointed out that the photocatalytic NH3 production from N2 and H2O on the TiO2 surface is a complicated heterogeneous process, which may be influenced by several other factors (for example, solvent effects on N2 adsorption, the role of surface defects on proton and electron transfer


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processes, photo-triggered redox reactions, and so on). Here, as a first step, we investigate a simplified model system for the TiO2-catalyzed N2 reduction to NH3 driven by H2O photolysis, by means of firstprinciples calculations. We believe that the new mechanistic insights gained in the present work will be helpful for guiding further experimental studies of photocatalytic nitrogen fixation and green NH3 photosynthesis with noble-metal-free TiO2 and titania-based catalysts.


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Additional figures with optimized structures and calculated electron spin densities. Corresponding Author Email: [email protected] (G.C.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by grants from the NSFC (Grant Numbers.: 21522302, 21590801, 21688102, 21520102005, and 21421003). G.C. is also grateful for financial support from the "Fundamental Research Funds for Central Universities". This work is dedicated to the memory of Prof. Walter Thiel. REFERENCES (1) Smil, V. Detonator of the Population Explosion. Nature 1999, 400, 415. (2) Schlö gl, R. Catalytic Synthesis of Ammonia - A "Never-Ending Story"? Angew. Chem. Int. Ed. 2003, 42, 2004–2008. (3) Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. M. Nitrogen Cycle Electrocatalysis. Chem. Rev. 2009, 109, 2209–2244. (4) Service, R. F. New Recipe Produces Ammonia from Air, Water, and Sunlight. Science 2014, 345, 610. (5) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a Century of Ammonia Synthesis Changed the World. Nat. Geosci. 2008, 1, 636–639.


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SYNOPSIS


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Probing Photocatalytic Nitrogen Reduction to ... - ACS Publications

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