DFT Study on Reaction Mechanism of Nitric Oxide to Ammonia and

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DFT Study on Reaction Mechanism of Nitric Oxide to Ammonia and Water on a Hydroxylated Rutile TiO(110) Surface 2

Xiao-Ying Xie, Qian Wang, Wei-Hai Fang, and Ganglong Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04811 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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The Journal of Physical Chemistry

DFT Study on Reaction Mechanism of Nitric Oxide to Ammonia and Water on a Hydroxylated Rutile TiO2(110) Surface Xiao-Ying Xie, Qian Wang, Wei-Hai Fang, and Ganglong Cui∗ Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China E-mail: [email protected]

Abstract Nitric oxide (NO) is an important air pollutant. Its chemical conversion to ammonia (NH3 ) and water (H2 O) molecules recently attracts a lot of experimental attention. In this work, we have employed periodic density functional theory method combined with a slab model to study this catalytic reaction of NO adsorbed on a hydroxylated rutile TiO2 (110) surface. We have obtained two favorable NO adsorption structures: in the first one, the terminal N atom is bonded with a Ti5c surface atom (Nad O); in the second one, both N and O atoms are bonded with two nearby Ti5c surface atoms (Nad Oad ). Interestingly, Nad Oad becomes more stable than Nad O with the increasing of coverage of hydroxyl groups, i.e. more than three hydroxyl groups in our slab model, which demonstrates that hydroxyls can seriously influence surface electronic structures and thus surface catalysis. Mechanistically, we have found that the N-O bond should be first weakened prior to its dissociation. In the Nad O adsorption structure, this weakening is achieved through a hydrogen atom transfer to the N atom of the NO molecule; in

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the Nad Oad adsorption structure, this N-O bond is already activated upon adsorbed on the surface. After the N-O bond is broken, a series of hydrogen atom transfers to either N or O atom take place in order, which eventually produce the final products. Our present computational results provide important mechanistic insights into NO removal from TiO2 surfaces.

Introduction NOx is a generic term of nitrogen oxides in atmospheric chemistry, which are most relevant to air pollution, such as nitric oxide (NO) and nitrogen dioxide (NO2 ). These gases contribute to the formation of smog and acid rain as well as tropospheric ozone. NOx are usually produced from reactions among nitrogen and oxygen in combustion of fuels such as hydrocarbons in air at high temperatures, especially in car engines. In large cities of high motor vehicle traffic, nitrogen oxides emitted can be a significant source of air pollution. 1 Therefore, developing efficient catalytic processes converting these NOx pollutants to nitrogen and water molecules represents one of critical challenges in modern catalysis. 2,3 Previous works have shown that NOx can be reduced through catalytic reactions over oxide catalysts 4–7 for example Al2 O3 , 8 V2 O5 , 9 WO3 , 10 and V2 O5 /WO3 . 11–18 TiO2 has already emerged as a promising oxide material with numerous applications in diverse and important areas such as solar cells, 19–21 photocatalysts, 22,23 gas sensors, 24 etc. In recent years, heterogeneous catalytic reactions of various environmental pollutants (e.g. NOx and volatile organic compounds) on the surfaces of TiO2 , one of abundant atmospheric aerosols, have attracted rapidly growing attention because of their significant importances in atmospheric chemistry. 25–27 In the past years, mechanistic aspects of NO reduction to N2 O on TiO2 surfaces have been revealed. 28–31 For example, Sorescu et al. have studied chemisorption properties of NO on TiO2 (110) surface by temperatureprogrammed desorption measurements. 32,33 They have found that NO reacts with each 2 ACS Paragon Plus Environment

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other to form N2 O, which eventually leaves an oxygen atom on TiO2 (110) surface. Rusu et al. explored photodecomposition of chemisorbed NO on TiO2 (110) surface and TiO2 powders to generate N2 O, which desorbs from the surface when heated. 34 In contrast, experimental studies of NO with surface hydroxyl groups are less reported although their interaction is fundamentally important. Surface hydroxyls have been shown to be able to trap NO on TiO2 at room temperature, demonstrating that their interaction is strong and could benefit dissociation reactions of NO on these surfaces. 35,36 Recently, Kim et al. have investigated reduction reaction of NO on a hydroxylated TiO2 (110) surface using temperature programmed desorption. 37 They found that NO reaction with hydroxyl groups (HOb ) on the hydroxylated TiO2 (110) surface leads to NH3 , which desorbs at ca. 400 K. Production amount of NH3 depends nonlinearly on the dose of NO. Preadsorbed water molecules are found not to react with NO, therefore having negligible effects on NH3 yield. Additionally, NH3 is not observed in the absence of surface hydroxyls on a stoichiometric TiO2 (110) surface although both NO and H2 O are coadsorbed. On the basis of these observations, they proposed a possible reaction mechanism for the formation of NH3 . NO first dissociates to N and O adatoms that subsequently react with HOb and lead to NH3 and H2 O; then, the N adatoms readily abstract hydrogen from HOb until NH3 is formed. On the computational side, to our best knowledge, there is no computational work that has focused on this catalytic reaction up to date. Thus, in this work we have employed firstprinciples density functional theory to systematically study catalytic reaction mechanism of NO on the hydroxylated TiO2 (110) surface, which eventually leads to NH3 and H2 O.

Computational Details All density functional theory (DFT) calculations were performed using Vienna ab initio simulation package (VASP5.4) interfaced with MedeA5.3 modeling suites. 38–42 The electronic exchange-correlation interaction was described by Perdew, Burke, and Ernzerhof (PBE) 3 ACS Paragon Plus Environment


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functional within generalized gradient approximation (GGA). 43 The projector augmentedwave (PAW) method was used to represent the core–valence electron interaction. 44 All calculations were also conducted involving long-range dispersion interactions [DFT-D3] of Grimme in which Becke-Jonson (BJ) damping was invoked. 45,46 The ion-electron interaction was described by PAW pseudo-potential. 42,44 A cutoff energy of 400 eV was used for the plane-wave basis set. The rutile TiO2 (110) surface was modeled as a periodic slab with three O–Ti–O trilayers of oxide, which is demonstrated accurate enough in recent calculations. 47,48 The minimal i.e. five hydrogen atoms were added to the bridged oxygens to model chemical transformation of NO to NH3 and H2 O on the hydroxylated rutile TiO2 (110) surface. The vacuum between slabs was set to 15 Å. A 4x2 surface cell and 1x1x1 k-point mesh were used in calculations. The adsorption was modeled on one side of the slab, and during structural optimizations all of the atoms except those in the bottom −1

of the TiO2 slab were allowed to relax until all atom forces reached < 0.05 eV Å . Transition states were optimized using the improved dimer method implemented in VASP5.4 package. 49,50 The forward finite difference formula was used to compute the curvature along the dimer direction. The step size for numerical differentiation was set to 0.01 Å and the dimer was rotated only if predicted rotation angle is greater than 0.01 radian. The trial optimization step size was 0.01 Å and the trust optimization radius was 0.1 Å. In our work, absorption energies are estimated according to

Eads = −(Eslab/N O − Eslab − EN O ) in which Eslab/N O , Eslab , and EN O are potential energies of NO-adsorbed TiO2 model, pure TiO2 slab model, and NO molecule, respectively.


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Figure 1: Three adsorbed structures of NO molecule on the hydroxylated rutile TiO2 (110) surface. In Nad O and NOad , the N and O terminals of the NO molecule are respectively bonded with the Ti5c atom; while, in Nad Oad , the N and O terminals are concurrently bonded with the two nearby Ti5c atoms. Also shown are some selected bond lengths in Å. N, O, Ti, and H atoms are in blue, red, gray, and white. See discussion in text.

Figure 2: Frontier molecular orbitals of different adsorbed structures of NO molecule on the hydroxylated rutile TiO2 (110) surface. See discussion in text.

Table 1: Computed Adsorption Energies for Different Adsorption Structures (Energy in eV). See Text for Discussion. structure Nad O NOad energies 1.43 0.69

Nad Oad 1.77



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Results and Discussion Three different adsorption structures of NO molecule on the hydroxylated rutile TiO2 (110) surface have been optimized and referred to as Nad O, NOad , and Nad Oad in Fig. 1. In Nad O, the terminal N atom is bonded with the exposed Ti5c surface atom. The NO molecule is almost perpendicular to the rutile TiO2 (110) surface. In this conformation, there exist some hydrogen bonds between the N atom of the NO molecule and the nearby H atoms on the bridged oxygen atoms of the rutile TiO2 (110) surface. The N-Ti bond length is computed to be 1.839 Å; the corresponding N-O bond length is 1.222 Å, which is elongated in comparison with that of gas phase NO molecule (exp. 1.15 Å). These structural changes can be understood very well considering electronic structure changes of NO molecule. For isolated NO molecule, it has odd electrons. Its highest occupied molecular orbital (HOMO) is a typical π ∗ antibonding MO, which is occupied by an unpaired single electron (Fig. S1). When the NO molecule is bound with the hydroxylated TiO2 system, the unpaired electron is transferred into the HOMO orbital of the NO molecule. Then, two paired electrons occupy the π ∗ antibonding HOMO orbital. Thus, the N-O bond order decreases; accordingly, its bond length increases. However, this HOMO orbital has a π bonding feature between the N and Ti5c surface atoms; therefore, the N-Ti5c bond order increases (see the left panel of Fig. 2). So, we can see that the N atom is bonded with the Ti5c surface atom. In NOad , the terminal O atom of the NO molecule is bonded with the Ti5c surface atom. The terminal N atom of the NO molecule leans toward the nearby H atoms due to stronger hydrogen-bonding interaction. Similar to the above situation, the N-O bond is also activated to certain extent and its bond length increases to 1.231 Å, which is about 0.081 Å longer than that of NO molecule in vacuo. The O-Ti5c bond length is also a little longer than that of the N-Ti5c bond length in Nad O; but, the O atom of the NO molecule is still bonded with the Ti5c surface atom. The right panel of Fig. 2 shows HOMO in the NOad adsorbed 6 ACS Paragon Plus Environment

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structure. It is clear that there is antibonding character between the N and O atoms of the NO molecule; while, it gives a visible bonding character between the O and Ti5c surface atoms. This situation is similar to that in the Nad O adsorbed structure discussed above. In addition to the Nad O and NOad adsorption structures in which only a terminal atom is bonded with the Ti5c surface atom, we have optimized the adsorption structure in which both N and O atoms of the NO molecule are concurrently bonded with two nearby Ti5c surface atoms (referred to as Nad Oad in Fig. 1). In this structure, the NO molecule is overall parallel the hydroxylated rutile TiO2 (110) surface, unlike the Nad O and NOad adsorption structures in which the NO molecule overall stands up on the hydroxylated TiO2 surface. In Nad Oad , the N-Ti and O-Ti distances are predicted to be 1.834 and 1.931 Å, respectively, which clearly demonstrate that the O and N atoms of the NO molecule are bonded with the Ti5c surface atoms. This adsorption structure is also further stabilized by the nearby hydrogen bonds. Electronic structure analysis shows that the antibonding π ∗ molecular orbital of the NO molecule overlaps with the d orbitals of the Ti5c surface atoms, thereby increasing the bond order between the N or O atom with the Ti5c surface atom. In our hydroxylated rutile TiO2 (110) slab model in which five H atoms are added to the bridged O atoms, the Nad Oad adsorption structure has the largest adsorption energy of 1.77 eV, which is 0.34 and 1.08 eV higher than those of the Nad O and NOad ones (Table 1). In contrast, the NOad adsorption structure has the smallest adsorption energy, 0.69 eV. Finally, it should be noted that both Nad O and NOad adsorption structures have been found in previous computational studies. 35,36 Here we have shown that the Nad Oad adsorption structure becomes stable with the increasing of the coverage of hydroxyl groups on the rutile TiO2 (110) surface. Further calculations show that this Nad Oad adsorption structure is unstable with low coverage of hydroxyl groups, i.e. 1-2 hydrogen atoms in our model, until 3 hydrogen atoms added to the bridged O atoms (see Fig. S2).



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Reaction Pathways As discussed above, the Nad O adsorption structure is always more stable than the NOad one in our slab model, which is consistent with previous theoretical studies. 35,36 On the other hand, the Nad Oad adsorption structure becomes the most stable one when more than three hydroxyl groups adsorbed on the rutile TiO2 (110) surface. Therefore, both Nad O and Nad Oad adsorption structures are chosen as starting structures to explore catalytic reaction pathways of NO on the hydroxylated rutile TiO2 (110) surface that lead to NH3 and H2 O. According to our present calculations, the total catalytic reaction can be divided into two stages. In the first stage, the NO molecule adsorbed on the hydroxylated TiO2 (110) surface is activated, which produces the HN + O and HNOH intermediates, from which the final NH3 and H2 O products are generated. Therefore, we first discuss reaction pathways to form the HN + O and HNOH intermediates and then those to form the final NH3 and H2 O products.

NO 99K HN + O Fig. 3 shows ground-state energy profile for the reaction pathway from the Nad O adsorbed structure to the HN + O intermediate. In the first step, the H8 atom is transferred to the N atom of the NO molecule forming a stable HNO intermediate in which the N-O bond is still intact (N5-O5: 1.293 Å) and the H atom is attached with the N5 atom (N5-H8: 1.041 Å). In this structure, the O atom of the NO molecule is oriented toward the neighboring H atoms to form hydrogen bonds. This reaction step is thermodynamically exothermic, which is computed to be -0.16 eV. In addition, it only needs to overcome a barrier of 0.61 eV. In the second step, the N-O bond is broken. At the HN + O structure, the N5-O5 bond length is calculated to be 3.077 Å, which is much longer than 1.293 Å at the HNO structure and 1.906 Å at the TS-a2 transition state. The second step is also an exothermal reaction 8 ACS Paragon Plus Environment

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Figure 3: Energy profile (in eV) of one reaction pathway related to the conversion of NO molecule to the HN + O intermediate starting from Nad O adsorption structure. Also shown are optimized stationary points with some selected key geometric parameters in Å. See text for discussion.



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(-0.97 eV; Fig. 3). In addition, the second one has a smaller barrier of 0.22 eV comparied with that of the first step.

Figure 4: Energy profile (in eV) of a reaction pathway related to the conversion of NO molecule to the HN + O intermediate starting from Nad Oad adsorption structure. Also shown are optimized stationary points with some selected key geometric parameters in Å. See text for discussion. In addition to starting from the Nad O adsorbed structure, the HN + O intermediate can also be formed from the Nad Oad adsorbed structure. This is a concerted reaction step in which the H8 atom transfer to the N5 atom of the NO molecule occurs concurrently with the N5-O6 bond fission. As shown in Fig. 4, the N5-O6 bond length is 1.359 in Nad Oad , which gradually increases to 1.921 Å at the TS-b1 transition state and is eventually broken to 3.077 Å in the HN + O intermediate. Moreover, one can also find that the H8 atom has been already transferred to the N5 atom at the TS-b1 transition state, which demonstrates that this bond-fission reaction is induced by the H8 atom transfer. Energetically, this reaction process is predicted to be an exothermal reaction (-0.79 eV) and associated with a small barrier of 0.38 eV. 10 ACS Paragon Plus Environment

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Figure 5: Energy profile (in eV) of reaction pathway related to the conversion of NO molecule to the HNOH intermediate starting from Nad O adsorption structure. Also shown are optimized stationary points with some selected key geometric parameters in Å. See text for discussion.



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Figure 6: Energy profile (in eV) of water-assisted reaction pathway related to the conversion of NO molecule to the HNOH intermediate starting from Nad O adsorption structure. Also shown are optimized stationary points with some selected key geometric parameters in Å. See text for discussion.


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NO 99K HNOH Next, we discuss two energetically feasible reaction pathways to form the HNOH intermediate, which is accessible only from the Nad O adsorbed structure (in Nad Oad , it is unstable). In the first pathway, the HNO intermediate is first formed via overcoming a barrier of 0.61 eV at the TS-a1 transition state (see Fig. 5). From the HNO intermediate, the H2 atom is further transferred to the O5 atom producing the HNOH intermediate. This reaction step is a little exothermic, -0.24 eV and has a small barrier of 0.05 eV; thereby, the second H atom transfer is very easy once the HNO intermediate is formed. There exists another pathway to generate the HNOH intermediate from the Nad O adsorbed structure. This needs assistance of a water molecule as bridge to shuttle a hydrogen atom in the beginning. In this adsorbed structure, the O atom of the NO molecule is a little far away from the H atom in comparison with the N atom, so the direct H atom transfer is not feasible. Instead, a water molecule must be placed in a proper position to assist the H atom transfer between the O atom of the NO molecule and the H atom of the OH group (see Fig. S3). This step is a little endothermic, 0.25 eV and has a barrier of 0.28 eV. In the following, the H8 atom is directly transferred to the N5 atom to generate the HNOH intermediate. This process is very much exothermic, which is computed to be -0.65 eV. In addition, its barrier can be readily overcome (0.33 eV; see Fig. 6).

HN + O 99K NH3 + H2 O Starting from the HN + O intermediate, there are three energetically feasible reaction pathways available to generate the final products of NH3 and H2 O. In the first pathway, the H9 atom is first transferred to the dissociated O atom producing a HN + OH intermediate in which the HN and OH species are located on the two nearby Ti5c atoms (see top panel in Fig. 7). This step is thermodynamically allowed with a release of 0.54 eV energy and merely needs to surmount a barrier of 0.18 eV. In the subsequent step, the H2 atom 13 ACS Paragon Plus Environment


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Figure 7: Energy profile (in eV) of reaction pathway for the conversion of HN + O intermediate to final NH3 + H2 O products. Also shown are optimized stationary points with some selected key geometric parameters in Å. See text for discussion.


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moves to the nearby N5 atom; as a consequence, the H2 N + OH intermediate is obtained. This process is much more efficient than the previous one due to its smaller barrier of 0.06 eV. Followed is the H7 atom transferred to the N5 atom again. Essentially, there are two means to fulfill this purpose. In the first way, a water molecule is used as bridge to shuttle the hydrogen atom from the O7 to the N5 atom (see Fig. S4a). This hydrogen transfer only needs to overcome a barrier of 0.06 eV but with an energy release of 0.75 eV. In the second way, the H7 atom is first transferred to the bridged O8 atom with water assistance. This step is also facile because of a barrier of 0.27 eV (Fig. S4b). Finally, this transferred H atom moves to the N5 atom producing the NH3 + OH intermediate (only 0.04 eV barrier; Fig. S4c). Once the NH3 molecule is formed, the H3 atom migrates from the O3 to O6 atom producing the H2 O molecule. Although this step is associated with a relatively larger barrier of 0.44 eV, it remains fast because this barrier can be overcome easily due to a lot of accumulated internal energy in the preceding reaction steps. The second and third reaction pathways are overall similar to the first one except having different orders of hydrogen atom transfers to the dissociated N and O atoms adsorbed on the hydroxylated rutile TiO2 (110) surface (Figs. S5-S6).

HNOH 99K NH3 + H2 O In this route, the N-O bond of the adsorbed HNOH intermediate will be broken first; then, the dissociated OH moiety is bonded with the nearby Ti5c atom forming the HN + OH intermediate. This reaction step is rather exothermic, more than 0.9 eV but has a relatively large barrier of 0.79 eV. At the HN + OH intermediate, it is clear that the central N-O bond is cleaved, as demonstrated by the distance between them, 2.779 Å. From the HN + OH intermediate, there are two reaction pathways to form the final NH3 and H2 O molecules. 15 ACS Paragon Plus Environment


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Figure 8: Energy profile (in eV) of reaction pathway for the conversion of HNOH to HN + H2 O intermediate. See supporting information for the remaining ones. Also shown are optimized stationary points with some selected key geometric parameters in Å. See text for discussion.


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The first one has been discussed above in the first pathway of HN + O 99K NH3 + H2 O. In the second one, the H7 atom first transfers to the O4 atom of the OH group generating a water molecule. This process has a barrier of 0.17 eV and releases 0.13 eV energy (see Fig. 8). However, the resultant configuration of hydrogen atoms at the HN + H2 O intermediate is not so good for direct hydrogen atom transfers to the NH group to form the NH3 molecule. From this intermediate, there are two ways to produce the final products. In the first way, water molecules are used as bridges to shuttle hydrogen atoms between the hydroxyl groups and the HN and H2 N species. As shown in Fig. S7, these hydrogen atom transfer reactions are much exothermic and are merely associated with small barriers (less than 0.2 eV). In the second way, water molecules also play as bridges; however, they are used to shuttle hydrogen atoms between the bridged oxygens, i.e. regenerating a proper configuration of hydrogen atoms. It can be found that these reactions merely need to overcome small barriers again (less than 0.3 eV, Fig. S8). When the proper configuration of hydrogen atoms is produced, two facile hydrogen atom transfer reactions happen in a sequential means, thereby generating the final NH3 molecule (see Fig. S9). Taken together, both chemical reaction pathways are feasible in the view of energy.

Suggested Mechanism Fig. 9 summarizes two main efficient reaction pathways suggested based on our present computational results. The first one starts from the Nad O adsorbed structure, from which the nearby hydrogen atom attached on the bridged oxygen atom of the TiO2 (110) surface is first transferred to the N atom of the NO molecule producing HNO species. In such way, the N-O bond is significantly weakened, so the N-O bond can be easily broken to generate HN and O fragments. Then, two hydrogens from the nearby bridged oxygen atoms are transferred to the HN group, eventually producing the NH3 molecule. Finally, 17 ACS Paragon Plus Environment


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Figure 9: Suggested three efficient reaction pathways to generate the final NH3 and H2 O molecules from NO adsorbed on the hydroxylated rutile TiO2 (110) surface. Also shown are computed reaction barriers in eV. See text for discussion.


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a hydrogen transfer takes place to generate the H2 O molecule. In this pathway, the ratelimiting reaction step corresponds to the first hydrogen transfer from the bridged oxygen atom to the N atom, i.e. from Nad O to HNO. Its barrier is calculated to be 0.61 eV. In addition, the final step producing the H2 O molecule is also associated with a considerable barrier, 0.44 eV, which is the rate-limiting step for the H2 O formation. The second pathway is initiated with the Nad Oad adsorbed structure. In this structure, the N-O bond is already activated. Its bond length is 1.359 Å, which is longer than 1.222 Å in the Nad O adsorbed structure. In the first step, a hydrogen is transferred to the N atom of the adsorbed NO molecule. In this process, the N-O bond is also correspondingly cleaved, generating the HN + O species. Then, two hydrogens sequentially move to the HN species to produce the NH3 molecule. Finally, the H2 O molecule is also formed through additional two hydrogen transfer reactions. In this pathway, the rate-limiting step for the NH3 formation is related to the first hydrogen transfer, which is computed to be 0.38 eV. Similarly, the rate-limiting step for the H2 O formation is the second hydrogen transfer, which is estimated to be 0.44 eV. On the other hand, the generated H2 N + O species can be first converted into the H2 N + OH species if the third hydrogen is transferred to the O atom, not the N atom. This step needs a small barrier of 0.19 eV. From the H2 N + OH species, the reaction pathway is the same as that in the first pathway. Experimentally, it is speculated that the NO molecule first dissociates into two single N and O atoms on the hydroxylated rutile TiO2 (110) surface; then, from these two adatoms, a series of hydrogen atom transfer reactions happen producing the final NH3 and H2 O products. Our computational results improve their mechanistic proposal and provide many mechanistic details at atomistic scale. First, the direct N-O bond dissociation processes from both adsorption structures i.e. Nad O and Nad Oad are unfavorable. In the Nad O structure, the N-O bond remains very strong, as demonstrated by its bond length. In this adsorption structure, the N-O bond length is computed to be 1.222 Å, which is close to that of isolated NO molecule in vacuo (exp. ca. 1.15 Å). In other words, this N-O bond 19 ACS Paragon Plus Environment


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is not weakened at all upon adsorbed on the hydroxylated rutile TiO2 (110) surface. It is commonly known that directly breaking the N-O bond of the NO molecule is very hard. Therefore, the N-O bond should be first weakened to certain extent before its rupture. This weakening is realized by a hydrogen atom transfer to the N atom of the NO molecule generating the HNO species. In this structure, the N-O bond length is increased to 1.293 Å, which is 0.071 Å longer than that in the Nad O adsorption structure. Unlike the Nad O adsorption structure, the N-O bond in the Nad Oad adsorption structure is already weakened when the NO is adsorbed in the surface. Its bond length is elongated to 1.359 Å, much longer than 1.222 Å in the Nad O adsorption structure. Thus, the N-O bond fission is energetically feasible starting from the Nad Oad adsorption structure. However, this N-O dissociation process should be accompanied with a hydrogen atom transfer to the N atom of the NO molecule.

Conclusion In summary, we have employed periodic density functional theory method to study the catalytic reaction mechanism of NO adsorbed on a hydroxylated rutile TiO2 (110) surface that lead to NH3 and H2 O molecules. We have obtained two stable Nad O and Nad Oad adsorption structures and have found that from either Nad O or Nad Oad adsorption structure, the N-O bond of the NO molecule must be activated first (rate-limiting reaction steps). In the Nad O adsorption structure, this weakening is achieved through a hydrogen atom transfer to the N atom of the NO molecule; in the Nad Oad adsorption structure, this N-O bond is already activated upon adsorbed on the surface. After the N-O bond is broken, a series of hydrogen atom transfers to either N or O atom take place in a proper order, which eventually produce the final products. The present work contributes valuable mechanistic information on NO removal from metal oxides’ surfaces.


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Supporting Information Schematic molecular orbitals of nitric oxide molecule, adsorption structures and energies with different hydroxyls on TiO2 (110) surface, and additional reaction pathways.

Acknowledgments This work has been supported by the National Key Research and Development Program of China (2016YFC0202600) and the National Natural Science Foundation of China (21522302, 21520102005, and 21421003).

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TOC Graphic


DFT Study on Reaction Mechanism of Nitric Oxide to Ammonia and

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