Conversion of NO into N2 over Î³-Mo2N - The Journal of Physical

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article 2

2

Conversion of NO Into N Over #-MoN Mohammednoor Altarawneh, Zainab Jaf, Hans Oskierski, Zhong-Tao Jiang, Jeff Gore, and Bogdan Z Dlugogorski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04107 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Conversion of NO into N2 over γ-Mo2N

Mohammednoor Altarawneha,∗, Zainab Jaf,a Hans Oskierskia, Zhong-Tao Jiang,a Jeff Goreb, Bogdan Z. Dlugogorskia

a

School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, WA 6150, Australia b

Dyno Nobel Asia Pacific Pty Ltd, Mt Thorley, NSW 2330, Australia

* Corresponding Author: * Phone: (+61) 8 9360-7507 E-mail: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

Abstract

Cubic molybdenum nitride (γ-Mo2N) exhibits Pt-like catalytic behaviour in many chemical applications, most notably as a potent catalyst for conversion of harmful NOx gases into N2. Guided by experimental profiles from adsorption of 15NO on γ-Mo214N, we map out plausible mechanisms for the formation of the three isotopologues of dinitrogen (14N2, 14

N15N) in addition to

15

N2 and

14

N15NO. By deploying cluster models for the γ-Mo2N(100) and γ-

Mo2N(111) surfaces, we demonstrate facile dissociative adsorption of NO on γ-Mo2N surfaces. Surfaces of γ-Mo2N clearly activate adsorbed

15

NO molecules, as evidenced by

high binding energies and the noticeable elongation of the N-O bonds.

15

NO molecule

dissociates through modest reaction barriers of 24.1 kcal/mol and 28.1 kcal/mol over γMo2N(100) and γ-Mo2N(111) clusters; respectively. Dissociative adsorption of a second 15

NO molecule produces the experimentally observed Mo2OxNy phase. Over the 100 surface,

subsequent uptake of 15NO continues to occur until the dissociated O and N atoms occupy all 4-fold hollow and top sites. We find that, the direct desorption of

15

N2 from the Mo2OxNy-

like phases phase requires a sizable energy barrier to precede. Considering a pre-oxygen surface covered cluster reduces this energy barrier only marginally. Desorption of

15

N2

molecules takes place upon combination of two adjacent N atoms from top sites via a lowenergy multi-step Langmuir-Hinshelwood mechanism. Dissociative adsorption of gaseous 15

NO molecules at surface Mo-N bonds weakens the Mo-N bonds and leads to formation of

14

N15N molecules (where

14

N denotes a nitrogen atom originated from surfaces of γ-Mo2N

crystals). Liberation of 14N2 molecules occurs via surface diffusion of two surface N atoms on the (111) N-terminated surface. Formation of 14N15NO proceeds via direct abstraction of a surface 14N atom by a gaseous 15NO adduct.


Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.

Introduction

Health and environmental concerns prompted by emission of nitrogen oxides (NOx) from thermal operations triggered the development of several abatement technologies.1

In

particular, the selective catalytic reduction2-6 and selective non-catalytic reduction (SNCR)7-12 have emerged as effective approaches to reduce NOx emission from thermal processes. The catalytic conversion of nitrogen monoxide (NO) into molecular nitrogen has led to a remarkable drop in emissions of NOx from mobile and stationary sources alike. However, the use of NH2-agents, such as ammonia and urea in SNCR processes, often results in the emission of secondary pollutants including N2O.

Emulating noble metals, cubic molybdenum nitride (γ-Mo2N) finds applications in heterogeneous catalysis spaning ammonia synthesis, hydrogenolysis and desulfurisation, in addition to abatement of NO emission.13 Despite three decades of applied and fundamental research, the exact mode of the catalytic reactivity of γ-Mo2N remains largely speculative. Contributing factors seem to stem from structural and electronic properties of γ-Mo2N. For example, relatively small pores in surfaces of γ-Mo2N prevent adsorption of larger hydrocarbon intermediates, leading to selective hydrogenation of smaller adsorbates.14 Dissociative adsorption over surfaces of γ-Mo2N originates from the strong interaction between the partially filled d orbitals of Mo and negatively charged atoms in the dissociating gaseous species.13,15 Furthermore, catalytic activity of transitional metal nitrides displays dependence on the number of vacant N sites that act as optimal adsorption centres. A recent theoretical study has demonstrated that, surfaces containing molybdenum nitrides could undergo a Mars–van Krevelen type mechanism in which N2 adsorbs at vacant surface



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

nitrogen sites activating them.16 Estimates of the equilibrium concentrations of vacant 3hollow nitrogen sites fall in the range of 1.6 × 1016 to 3.7 × 1016 cm–2.16

A definite experimental account of He et al.17 reported a 95 % selectivity of N2 formation from NO, at around 450 oC, over passivated and H2-treated γ-Mo2N. The XRD pattern presented by He at al. revealed the transformation of γ-Mo2N into a Mo2OxNy phase. Reduction of Mo2OxNy by H2 back to γ-Mo2N completed a single oxidation-reduction cycle. The reaction commenced at around 100 °C, with the selectivity to N2, N2O, and NO2 of about 20 %, 80 % and 0.3 %, respectively, indicating the existence of a low energy pathways, especially to N2O. In the experiments, the nitrogen atoms in γ-Mo2N entail

14

N isotope,

whereas the isotopologue form of gaseous NO is that of 15NO. Pulsing 15NO over H2-treated γ-Mo214N between 300 – 450 oC resulted in the formation of 15N2, 14N15N, 14N2 and 14N15NO molecules, with trace emissions of NO2 but only below 350 °C. Emission of 14N15N and 14N2 molecules signified desorption of surface 14N atoms. Trends in the yields of 15N2 and 14N15N followed each other up to 400 °C, but then

15

N2 exceeded

14

N15N.

This implies the

appearance of three pathways. Along the first corridor, the sharp decline in the yield of surface-originated

14

N2 revealed the relatively weakly bounded

14

N atoms departing the γ-

Mo2N structure merely under thermal influence without involvement of 15NO. The other two pathways led to

14

N15N and

15

N2, both corridors characterised by higher activation energies

than that for the formation of 14N2.

The mechanisms proposed by He et al.17 comprise (i) the attack of the O in NO on a surface Mo atom and (ii) a bridge-like adsorption of NO on adjacent surface atoms of Mo and N:


Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N O N

O

Mo

N

NOg

+

Mo

N

Mo

N

Whereas experimental results of He et al. have unequivocally established γ-Mo2N as a potent catalytic material for purification of automobile exhaust gases, they have also left open several intriguing questions:

(i) What is the mechanism operating in the dissociative

adsorption of 15NO over neat γ-Mo214N surfaces and the subsequent release of 15N2, 14N15N, 14

N2 and 14N15NO molecules and formation of the Mo2OxNy phase? (ii) Would adsorption of

two

15

NO molecules at neighbouring sites facilitate NO decomposition into N2? (iii) How

does γ-Mo2N activate adsorbed NO moieties? (iv) What is the kinetic and thermodynamic feasibility of creation of a surface nitrogen vacancy? (v) How could the NO adsorption process continue, once all hollow sites become occupied by nitrogen and oxygen atoms? To address these questions and resolve the mechanism of catalytic conversion of NO to N2, this contribution studies the adsorption of NO on a neat γ-Mo2N surface by means of density functional theory (DFT) calculations.

2.

Computational methodology

2.1

Structural optimisation

The DMol3 code18 executed all unrestricted spin polarised optimisations and total energy computations. The calculation methodology relied on a generalised gradient approximation (GGA-PW91),19 along with the Grimme dispersion correction,20 and a double-polarised 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

numeric basis set of DNP. The energies and total forces converged with the precision of 2×10-6 eV and 5×10-4 eV/Å, respectively, with a real space cutoff distance of 4.4 Å. We located the transition states using the complete linear synchronous and quadratic synchronous transit approaches (LST/QST). Charge analysis on some structures followed the Hirshfeld formalism.21 During optimisation, the third and fourth atomic layers were frozen; while allowing the rest of the clusters and the adsorbents to fully relax.

2.2

The γ-Mo2N structures

Cleavage of the γ-Mo2N unit cell along the three low Miller indices affords only three distinct surface configurations denoted as (100), (110) and (111). Terminations along the (100) and (110) directions produce mixed Mo/N layers whereas the (111) surface yields slabs truncated with either N (111_N) or Mo (111_N) atoms.

In this regard, the ab initio atomistic

thermodynamics approach identify the most stable γ-Mo2N configurations. In supporting information (SI), we derive stability curves for the four possible terminations of Mo2N along the low miller indices. Our recent study on stability of copper chloride surfaces illustrates in details the underlying equations.22 The SI document enlists the most important governing equations and the optimisation methodology in case of Mo2N. Figure S1 in the SI document depicts optimised geometries for the four considered terminations; namely 100, 110, 111_N and 111_Mo. Figure S2 plots calculated surface energies between 300 – 1000 K at 1.0 atm. Between 10-3 – 103 atm and 300 – 1000 K (bounded T-P conditions encountered in deploying Mo2N as catalyst for NO capture and conversion), the 111_N holds profound stability whereas the nitrogen-deficient 111_Mo (i.e., N/Me ratio of 0.44) is the least stable configuration. Calculated surface energies for the 100, 110, 111_Mo and 111_N at 1 atm and 600 K amount 0.14, 0.15, 0.72 and -0.49 eV/ Å2; respectively. The two mixed stoichiometric


Page 6 of 34

Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mo/N terminations 100 and 110 (i.e., N/Me ratio of 0.50) display matching surface energies within 0.01 eV/ Å2; a marginal difference which most likely falls within the accuracy limit of the adapted methodology. The noticeable stability of the nitrogen-rich 111_N (i.e., N/O ratio of 0.57) termination (i.e., lower surface energy) is in line with a recent experimental findings23 pointing out to the superior catalytic activity of MoN2 surfaces (i.e., N/Me ratio of 2) towards CO uptake and hydro desulfarisation reactions.

In addition to its profound thermodynamic stability, the facile release of

14

N2 and

14

N15NO

molecules at temperature as low as 100 oC provide another experimental support for the likely presence of 111_N facets in which surface N atoms may combine to yield 14N2. As the γ-Mo2N(111) surface contains exposed N atoms in positions that correspond to frequently unoccupied 3-fold hollow sites, it seems likely for the N-truncated γ-Mo2N(111) cluster to liberate surface N2 (i.e.,

14

N2).

However, it must be noted that the ab initio atomistic

thermodynamic approach does not account for plausible kinetic factors and the results are exclusively limited to the surveyed structures.

In view of its very high surface energy, interaction of NO with the 111_Mo termination, has not been considered herein. Similarly, γ-Mo2N(110) and γ-Mo2N(100) configurations incur very similar surface energies and both contain mixed Mo/N atomic termination. Consequently, interaction behaviour of NO with γ-Mo2N(100) reflects that of γ-Mo2N(110). Hence, calculations involving the Mo2N(110) were not considered herein.

We have adapted cluster models for the γ-Mo2N(100) and γ-Mo2N(111) surfaces with four layers of atoms in the [100] and [111] directions, respectively based on a 2×2 supercell. Figure 1 portrays geometries of the two clusters. The adapted cluster models account for all



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 34

potential adsorption sites that appear on surfaces of γ-Mo2N(100). Cleavage along the [100] direction leaves partly mixed uncoordinated Mo and N exposed at the surface. The lengths of Mo-Mo and N-N surface bonds reasonably match the corresponding segments obtained for an analogous (100) periodic boundary system,24 i.e., 3.09 Å and 2.12 Å compared to 2.97 Å and 2.11 Å, respectively.

In addition to its profound thermodynamic stability, the facile release of

14

N2 and

14

N15NO

molecules at temperature as low as 100 oC prompted us to consider N-terminated γMo2N(111) cluster in which surface N atoms may combine to yield

14

N2.

As the γ-

Mo2N(111) surface contains exposed N atoms in positions that correspond to frequently unoccupied 3-fold hollow sites, it seems likely for the N-truncated γ-Mo2N(111) cluster to liberate surface N2 (i.e., 14N2).

3.

Results and discussion

As mentioned previously, in the experiments, the γ-Mo2N surfaces comprised

14

N atoms

whereas molecules of gaseous NO included 15N, prompting us to adopt the same isotopes in computations.17 The presentation of results is organised as follows: Section 3.1 accounts for the strong molecular adsorption of NO over the γ-Mo2N(100) and γ-Mo2N(111) clusters. Section 3.2 discusses the subsequent uptake of

15

NO molecules by the γ-Mo2N(100) and γ-

Mo2N(111) clusters and the eventual release of N2 (i.e.,

15

N2) via a low-energy multi-step

Langmuir-Hinshelwood mechanism. In Section 3.3, we examine three single-step channels for emission of N2 and O2 from the Mo2OxNy phase, but find all three corridors energetically


Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


unfeasible. Section 3.4 develops another more-direct Langmuir-Hinshelwood mechanism that requires intermediate activation energy, between those associated with pathways of Sections 3.2 and 3.3. Sections 3.5 and 3.6 investigate low-energy reactions for the formation of

14

N15N and

14

N2 over γ-Mo2N(100) and γ-Mo2N(111) clusters, respectively.

Finally,

Section 3.7 demonstrates a bimolecular reaction leading to the formation of 14N15NO. Figure 2 summarises the low-energy pathways for the appearance of 15N2, 14N15N, 14N2 and 14N15NO molecules.

3.1

Molecular adsorption of NO

Molecular adsorption of NO radicals on the γ-Mo2N surface constitutes the first step in their conversion to nitrogen molecules. For that reason, we report the different adsorption modes of NO on γ-Mo2N(100) and γ-Mo2N(111) clusters. The γ-Mo2N(100) cluster embodies three bridge sites (B1, B2 and B3), two top sites TN and TMo) and the 4-fold hollow site (H), see Figure 1. The γ-Mo2N(111) cluster has five distinct adsorption sites, namely top TMo site, bridge site between two surface Mo atoms (B), a 3-fold hollow fcc site (H1, with an underneath Mo atom in the fourth sublayer (see the structure in the top right corner in Figure 1 for definition of the sublayers), and two 3-fold hollow sites. One of these two hollow sites borders an underneath N atom in the third sublayer (H2) and the other adjoins an underneath Mo atom in the fourth sublayer (H3).

Figure 3 presents optimised geometries of six molecular adsorption structures on the γMo2N(100) cluster, while Table 1 enlists their calculated adsorption energies (Eadd), elongation of the N-O bonds and charge transfer from the γ-Mo2N cluster to the adsorbed NO



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

radicals. In M1 configuration, the NO molecule adsorbs vertically at the H site with the N atom in NO pointing downward. In the M2 structure, a slightly titled NO molecule is chemisorbed at a B3 site. The M3 structure characterises a tilted NO molecule above TMo site in which the O atom points downward. Table 1 assembles the other three structures (M4M6).

The data given in Table 1 characterise the evident activation of adsorbed NO

molecules. In the six structures associated with the 100 surface, the noticeable elongation of the N-O bonds (8.3 – 18.2%) and the pronounced charge transfer from the surface (see Table 1) concur with the relatively significant adsorption energies (-13.4 – -38.0 kcal/mol).

Figure 4 portrays molecular adsorption over the five different sites in the γ-Mo2N(111) cluster. Adsorption energies for the

15

NO molecule over the three 3-fold hollow sites (M9-

M11) incur very comparable adsorption energies in the narrow range of -56.0 to -58.0 kcal/mol. Elongation of the N-O bond and charge transfer in these 3-fold hollow structures are lower than the corresponding values of the M1 structure, in which the adsorption occurs over the 4-fold hollow site of the γ-Mo2N(100) termination. Energies listed in Table 1 show stronger molecular adsorption over the N -terminated surface of γ-Mo2N(111) than that on the mixed N/Mo-truncated γ-Mo2N(111) cluster. Nonetheless, based on calculated adsorption energies in Table 1, the NO radical bounds strongly with both 100 and 111 clusters.

Adsorption of a CO molecule on a γ-Mo2N(100) surface at the H site and on the H1-H3 sites in a γ-Mo2N(111) surface were found to be exothermic by 34.3 kcal/mol.24,25 The stronger interaction of the NO radical with the γ-Mo2N surfaces in reference to the corresponding adsorption of a CO molecule stems from the nature of NO as a free radical. The density of states (DOS), illustrated in Figure 5, further explains the interaction between molecularly adsorbed NO and the γ-Mo2N(100). Adsorbed NO in structure M1 reduces the DOS intensity


Page 10 of 34

Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of d orbitals in surface Mo atoms (Figure 5b) in reference to the clean cluster (Figure 5a) near the Fermi level. Likewise, the sharp DOS peaks between 0.5 and 1.0 Ha of the gaseous NO radical (Figure 5c) translate into a broad peak located between 1 to 2 Ha (Figure 5d). Having established a rather strong molecular NO adsorption on the γ-Mo2N(100) surface, a question now arises of whether binding of the NO radical is strong enough to facilitate rupture of the N-O bonds and then the formation of N2 from two adsorbed N atoms; i.e., with the reaction proceeding via the Langmuir-Hinshelwood mechanism.

3.2

Dissociative adsorption of NO and low-energy pathway to 15N2

Figure 6 portrays a mechanism of the subsequent uptake of NO by the γ-Mo2N(100) cluster. Starting with the molecular adsorption structure of M1, dissociation of the NO radical into adsorbed O and N atoms at two neighbouring H sites overcomes a barrier of 24.1 kcal/mol via transition structure TS1. The facile nature of the dissociative adsorption of NO supports the experimentally reported Pt-like catalytic properties of γ-Mo2N toward mitigating emission of NOx. The D1 configuration resides 77.9 kcal/mol below the M1 structure. Addition of a second NO radical to the H site in D1 is exothermic by 32.0 kcal/mol and results in the formation of the D2 structure (The reaction energies presented here are always with respect to separated reactants). The adsorbed NO adduct in D2 dissociates into O and N atoms passing a modest barrier of 25.4 kcal/mol that accompanies the appearance of TS2. This process produces the D3 structure in which the adsorbed O and N atoms occupy all H sites. Structure D3 reflects the early Mo2OxNy phase that arises at the initial stage of NO pulsing over γMo2N.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 34

According to the experimental results of He et al.,17 the uptake of NO continues after the formation of the Mo2OxNy phase and the occupation of all apparently optimal H adsorption sites. The D4 structure forms upon addition of NO molecule at the B2 site (surface Mo-N bond) in the D3 configuration. In the subsequent step, the adsorbed NO molecule dissociates into two O and N atoms at two surface Mo atoms. This process occurs through a reaction barrier of 26.2 kcal/mol characterised by TS3. A fourth NO molecule adsorbs dissociatively on the surface according to the reaction sequence NO + D5 → D6 → TS6 → D7. In D7, adsorbed N and O atoms occupy neighbouring H and TMo sites.

15

N atoms at TMo sites adsorb

weakly and readily depart the D7 structure as 15N2 molecules. In TS5, two 15N atoms at two neighbouring TMo sites combine to form an adsorbed

15

N2 molecule.

This process

necessitates a barrier of 28.3 kcal/mol. In the final step, the weakly adsorbed N2 molecule (i.e., in form of 15N2) desorbs from D8 in a modestly endothermic process of 28.3 kcal/mol. The desorption of the N2 molecule from D8 makes the two neighbouring TMo sites accessible for further uptake of NO molecules and subsequent desorption of 15N2, until the process stops due to the profound stability of the Mo2OxNy phase, necessitating its reduction to γ-Mo2N by hydrogen. The comparable activation barriers of TS1-TS4 indicates subsequent uptake of 15

NO proceeds largely unaffected by the degree of N/O coverage.

Inspection of reactions presented in Figure 6 leads to two important mechanistic points: (i) the potent H sites in the neat γ-Mo2N structure are occupied and deactivated during at the initial stages of NO uptake, (ii) desorption of N2 molecules takes place when two N atoms at TMo sites combine. Reduction with hydrogen may liberate adsorbed O and N atoms in forms of water (or hydroxyl) and ammonia (or amine). Mechanisms of the interaction between the Mo2OxNy phase and hydrogen requires further investigations.


Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7a shows energies for the analogous first

15

NO dissociative adsorption over the γ-

Mo2N(111) cluster. Dissociation of 15NO via TS13 proceeds via a barrier of 28.1 kcal/mol; a value that slightly overshoots the corresponding barrier of the analogous transition structure TS1 (24.1 kcal/mol) over the γ-Mo2N(100). It can be inferred that both N/Mo mixed- and purely N-terminated γ-Mo2N facets dissociates NO molecules via similar activation barriers.

3.3

Fate of the Mo2OxNy phase and high-energy pathways to 15N2 and 14N15N

Experimental measurements17 have demonstrated that, the γ-Mo2N structure arises from the Mo2OxNy phase by reducing Mo2OxNy with hydrogen. Herein, we investigate the possibility of direct emission of N2 and O2 molecules from the Mo2OxNy phase (structure D3) as an alternative mechanism for regenerating the γ-Mo2N structure.

Figure 8 presents two

pathways for elimination of 15N2 molecules. The first corridor proceeds via TS6 and signifies desorption and combination of two adsorbed N atoms directly forming 15N2 molecule. TS6 incurs a very high activation barrier of 125.2 kcal/mol. The analogous process over the γMo2N(111) occurs through a very matching barrier of 124.8 kcal/mol (TS12 in Figure 7b). Clearly, formation of 15N2 from adsorbed 15N atoms via this pathway is not a feasible process. Intuitively, energy requirement for the removal of N atoms from the surface could be reduced if other elector negatively charged atoms were incorporated in the surface layers at greater loads.

In this regard, MoOx may represent potential candidates.

To the best of our

knowledge, literature presents no account for the interaction of NO with any MoOx-based materials. To investigate the effect of higher surface oxygen concentrations on the activation energies of dissociative adsorption of 15NO and subsequent elimination of 15N2 molecules, we consider a pre-oxygen covered γ-Mo2N(100) cluster in which all surface nitrogen atoms are



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

replaced by oxygen atoms. Figure S3 provides reaction and activation energies for these two steps over a pre-oxygen covered γ-Mo2N(100) cluster. Reaction barrier for the 15N2 removal step amounts to 109.4 kcal/mol. This indicates that doping the surface layer with high oxygen coverage slightly reduces the tremendous barrier required for the departure of

15

N2

from Mo2OxNy-like phase. It will be insightful to map out all possible reaction pathways of NO with pure MoOx surfaces.

In the alternate process, a

14

N15N molecule departs the D3 structure in a two-step process.

The first reaction characterises surface diffusion of one of the adsorbed N atom toward a surface-bound

14

N atom through TS9. The barrier of TS9 attains a value of 43.6 kcal/mol,

significantly lower than that of TS6. Desorption of the adsorbed N2 molecule from D15 is modestly endothermic by 32.1 kcal/mol.

The high barriers associated with the two desorption processes in Figure 7 indicate the highly unlikely emission of N2 molecules (in form of

15

N2 or

14

N15N) directly from the Mo2OxNy

phase. Analogously, we find that, desorption of an oxygen molecule from the Mo2OxNy phase incurs high endothermic penalty of 97.9 kcal/mol. Careful scanning of the potential energy surface reveals that, this process does not encounter an intrinsic reaction barrier. This finding is in line with the stability of the Mo2OxNy phase, elucidated experimentally17 through the XRD measurements, and no detection of O2 in laboratory measurements.

This

necessitates the reduction of stable Mo2OxNy phase to Mo2N with H2, complicating a practical implementation of this NO remediation process.

As illustrated in Section 3.1, the γ-Mo2N cluster activates adsorbed NO molecules. Here, we investigate the formation of

15

N2 molecules from adsorbed neighbouring molecules of NO.


Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9 draws a reaction energy profile for such a process. A 15N2 molecule forms via TS8 that involves a combination of two N atoms from adjacent adsorbed NO molecules leaving two adsorbed O atoms. A sizable reaction barrier of 45.6 kcal/mol signifies TS8. As this barrier is lower than that of the previous section (i.e., 125.2 kcal/mol), but higher than that of Figure 6, it prompts us to conclude that, the Langmuir-Hinshelwood mechanism depicted in Figure 8 might contribute modestly to the transformation of NO into 15N2.

3.5

14

Formation of 14N15N molecule in low energy pathways

N15N molecules arise from combination of nitrogen atoms from adsorbed

15

NO molecules

with surface N atoms. This could occur via many possible reactions, two of which we study in detail. Figure 10a demonstrates that, liberation of a surface 14N atom via the attack of NO molecule in the M6 structures requires a reaction barrier of 42.3 kcal/mol, associated with the transition state of TS7. In a subsequent step, the weakly attached

14

N15N molecule departs

the D11 configuration overcoming modest endothermicity of 34.0 kcal/mol. In an alternative process (Figure 10b), dissociative adsorption of NO molecule on a surface N-O bond in the Mo2OxNy phase releases a

14

N15N molecule through a barrier that lies at the level of the

entrance channel. This process may indeed be responsible for emission of temperatures.

3.6

Formation of 14N2 molecule


14

N15N at low


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To simulate the process of desorption of

14

Page 16 of 34

N2 molecules, we consider a nitrogen-terminated

111 surface as shown in Figure 1. We selected this surface because the nitrogen atoms weakly bound in this surface in reference to incompletely coordinated N atoms in the 100terminated slab.

In Figure 11,

14

N2 molecule is formed through diffusion followed by

combination of two surface N atoms in the γ-Mo2N(111) cluster. Reaction barrier for this process amounts to only 14.5 kcal/mol (TS10). This barrier allows the desorption of

14

N2

molecules to continue until consumption of all surface nitrogen atoms. Desorption of the formed

14

N2 molecule is endothermic by of 32.5 kcal/mol and appears to be the rate

determining step in Figure 11. This explains the sharp decrease in emission of temperature.17

On the γ-Mo2N(111) surface, liberation of surface

accessible for 15NO uptake, which in turn sustains its conversion into

14

14

N2 with

N atom makes Mo

15

N2. While we have

only focused on energy terms, entropic factors associated with the species loss in Figure 11 will most likely result in enhancing the process for 14N2 removal.

3.7

Direct abstraction of a surface N atom leading to formation of 14N15NO molecules

Apart from the three isotopologues of dinitrogen, He at al.17 detected a large amount of 14

N15NO, as reported in their Figure 7. The intensities of the 14N2O indicated minor emission

of this species and NO2 only evolved in a trace amount.17 Selectivity toward formation of N2O sharply decreased with temperature; i.e., mirroring the emission profile of N2 (see Figure 5 in He at al.).

A plausible pathway initiates by adsorption of a 15NO molecule on a Mo surface atom in the γ-Mo2N(111) surface followed by combination of the adsorbed


15

NO adduct with a surface

Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

N atom. Figure 12 demonstrates a corresponding reaction pathway. Based on the M8

structure, the transition state TS11 signifies a 14N-like diffusion reaction toward the adsorbed 15

NO. TS11 is associated with a reaction barrier of 33.1 kcal/mol (in reference to D8). The

formation of

14

N15NO via this reaction is considerably exothermic by 24.4 kcal/mol. The

lower energy pathway depicted in Figure 12 clearly illustrates a potentially accessible corridor for the formation of

14

N15NO at temperature as low as 100 oC, in accord with the

product profiles (see Figure 5 in He at al.).17 Emission of

14

N15NO significantly decreases

with the increase in temperature.17 This appears consistent with the pathway proposed in Figure 12. Removal of

14

N atoms under thermal influence (Figure 11) consumes surface

nitrogen atoms that are available for abstraction by 15NO, leading to lower emission profiles for 14N15NO as the temperature is ramped up.

4.

Conclusions and future directions

In this contribution, we explained the conversion of NO into N2 over the γ-Mo2N surface by deploying a cluster model. Strong NO molecular adsorption facilitates its dissociation into adsorbed O and N atoms. Over the (100) surface, emission of N2 (as 15N2) commences from neighbouring top Mo sites.

Subsequent uptake of NO by the (100) surface is rather

exothermic and precedes via facile reaction barriers. Dissociated oxygen and nitrogen atoms become incorporated into the lattice of γ-Mo2N forming the Mo2OxNy phase. The stability of the Mo2OxNy phase manifests itself by the very high-energy requirement for desorption of a nitrogen and oxygen molecules. Formation of

15

N2 via dissociation of two adjacent NO

molecules is marginal, with a reported activation energy at 45.6 kcal/mol (i.e., LangmuirHinshelwood mechanism). Formation of 14N15N commences with dissociative adsorption of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

NO molecules on Mo—N surface bonds rather than through the attack of 15NO on a top 14N site.

The

14

N2 isotopologue evolves via direct bimolecular combination of two surface

nitrogen atoms in N-terminated configurations; i.e., γ-Mo2N(111)-N. Finally, we identified a low energy pathway that permits the release of N2O at low temperature, in agreement with experimental observations.

In this contribution, we have limited our mechanistic analysis to mixed Mo/N- and Nterminated clusters. Desorption of surface nitrogen atoms in form of 14N2 is expected to yield pure Mo-terminated phases. Along the same line of enquiry, γ-Mo2N may follow a Mars–van Krevelen mechanism by releasing and re-capturing nitrogen molecules.

The plausible

occurrence of such process warrants further investigations in the pursuit of elucidating potentials pathways and species that prevent γ-Mo2N-based catalysts from being de-activated. The significant capacity of γ-Mo2N material to capture oxygen may also play a role in its performance in NOx reduction.

Supporting Information

Schematic of Mo2N surfaces (Figure S1), T-P surface energies for Mo2N surfaces (Figure S2); and energy profile for reaction of

15

NO with oxygen covered γ-Mo2N(100) cluster

(Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

Conflict of Interest: The authors declare no conflict of interest.

Acknowledgement


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This work has been supported by the Australian Research Council (ARC) and Dyno Nobel Asia Pacific, via a Linkage Project. The National Computational Infrastructure (NCI) and Pawsey Supercomputing Centre in Perth, Australia have provided grants of computational resources.

References

(1) Oluwoye, I.; Altarawneh, M.; Gore, J.; Bockhorn, H.; Dlugogorski, B. Z. Oxidation of Polyethylene Under Corrosive NOx Atmosphere. J. Phys. Chem. C. 2016, 120, 3766-3775. (2) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. Rev. 1995, 95, 209-225. (3) Amiridis, M. D.; Zhang, T.; Farrauto, R. J. Selective Catalytic Reduction of Nitric Oxide by Hydrocarbons. Appl. Catal. B 1996, 10, 203-227. (4) Niksa, S.; Fujiwara, N. A Predictive Mechanism for Mercury Oxidation on Selective Catalytic Reduction Catalysts under Coal-Derived Flue Gas. J. Air. Water. Waste. Manag. Ass. 2005, 55, 1866-1875. (5) Baleta, J.; Vujanović, M.; Pachler, K.; Duić, N. Numerical Modeling of Urea Water Based Selective Catalytic Reduction for Mitigation of NOx from Transport Sector. J. Clean. Prod. 2015, 88, 280-288. (6) Liang, X.; Zhong, Z.; Jin, B.; Chen, X.; Li, W.; Wei, H.; Guo, H. Experimental Study of the Influence of Sodium Salts as Additive to NOxOUT Process. Kor. J. Chem. Eng. 2010, 27, 1483-1491. (7) Muzio, L. J.; Quartucy, G. C.; Cichanowicz, J. E. Overview and Atatus of Post-combustion NOx Control: SNCR, SCR and Hybrid Technologies. Int. J. Environ. Pollut. 2002, 17, 4-30. (8) Baltasar, J.; Carvalho, M. G.; Coelho, P.; Costa, M. Flue Gas Recirculation in a Gas-Fired Laboratory Furnace: Measurements and Modelling. Fuel 1997, 76, 919-929. (9) Lee, J. B.; Kim, S. D. Kinetics of NOx Reduction by Urea Solution in a Pilot Scale Reactor. J. Chem. Eng. Jpn 1996, 29, 620-626. (10) Wang, Q.; Wang, Q.; Cai, J. Experimental study of Urea on SNCR Removal of NOX. J. Chem. Pharama. Res. 2014, 6, 2541-2546. (11) Lee, S.; Park, K.; Park, J.-w.; Kim, B.-H. Characteristics of Reducing NO Using Urea And Alkaline Additives. Combust. Flame. 2005, 141, 200-203. (12) Caton, J. A.; Narney, J. K.; Cariappa, H. C.; Laster, W. R. The Selective nonCatalytic Reduction of Nitric Oxide Using Ammonia at up to 15% oxygen. Can. J. Chem. Eng. 1995, 73, 345-350. (13) Nagai, M. Transition-metal Nitrides for Hydrotreating Satalyst—Synthesis, Surface Properties, and Reactivities. Appl Catal A: General 2007, 322, 178-190.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(14) Ranhotra, G. S.; Bell, A. T.; Reimer, J. A. Catalysis over Molybdenum Carbides and Nitrides: II. Studies of CO Hydrogenation and C2H6 Hydrogenolysis. J. Catal. 1987, 108, 40-49. (15) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides; 1st ed.; Blackie Academic and Professional London, UK, 1996; Vol. DOI: 10.1007/978-94-0091565-7 (16) Zeinalipour-Yazdi, C. D.; Hargreaves, J. S. J.; Catlow, C. R. A. Nitrogen Activation in a Mars–van Krevelen Mechanism for Ammonia Synthesis on Co3Mo3N. J. Phys. Chem. C. 2015, 119, 28368-28376. (17) He, H.; Dai, H. X.; Ngan, K. Y.; Au, C. T. Molybdenum Nitride for the Direct Decomposition of NO. Catal. Lett 2001, 71, 147-153. (18) B. Delley. From Molecules to Solids with the DMol3 Approach. J Chem Phys 2000, 113, 7756-7764. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys Rev Lett 1996, 77, 3865-3868. (20) Grimme, S. "Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction". J. Comput. Chem. 2006, 27, . (21) Hirshfeld, F. L. Bonded-Atom Fragments for Describing Molecular Charge Densities. Theoret. Chem. Acta 1997, 44, 129-138. (22) Altarawneh, M.; Jiang, Z.-T.; Dlugogorski, B. Z. The Structures and Thermodynamic Stability of Copper(II) Chloride Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 24209-24215. (23) Wang, S.; Ge, H.; Sun, S.; Zhang, J.; Liu, F.; Wen, X.; Yu, X.; Wang, L.; Zhang, Y.; Xu, H.; et. al. A New Molybdenum Nitride Catalyst with Rhombohedral MoS2 Structure for Hydrogenation Applications. J. Am. Chem. Soc. 2015, 137, 4815-4822. (24) Frapper, G.; Pélissier, M.; Hafner, J. CO Adsorption on Molybdenum Nitride's γ-Mo2N(100) Surface: Formation of NCO Species? A Density Functional Study. J. Phys. Chem. B. 2000, 104, 11972-11976. (25) Zaman, S. F. A DFT study of CO Adsorption and Dissociation Over γMo2N(111) Plane. Bulgarian. Chem. Comm. 2015, 47, 125-132.


Page 20 of 34

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1: Properties of molecular structures on the γ-Mo2N(100) cluster. Refer to Figures 1 (M1-M6) and 2 (M7-M11) for graphical illustration of adsorption sites.

Structure

Description

Adsorption energy,

Elongation of

Charge transfer

kcal/mol

NO bond (%)

from the cluster to adsorbed NO (e)

M1

Vertical configuration at H site

-38.0

16.0

-0.38

Slightly tilted adsorption at B3 site

-31.6

13.2

-0.27

Tilted adsorption at TMo site

-13.4

8.3

-0.30

M4

Vertical configuration at TMo site,

-26.0

10.7

-0.35

γ-Mo2N(100)

with 15N pointing downwards

M5

Tilted adsorption at B1 site

-24.0

9.5

-0.34

M6

Near horizontal configuration at H

-33.8

18.2

-0.32

γ-Mo2N(100)

site.

M7

Near horizontal configuration at B

-30.1

5.1

-0.21

γ-Mo2N(111)

site.

M8

Near vertical configuration at TMo

-44.1

4.4

-0.25

γ-Mo2N(111)

site.

γ-Mo2N(100) M2 γ-Mo2N(100) M3 γ-Mo2N(100)

γ-Mo2N(100)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

M9

Page 22 of 34

Vertical adsorption at H3 site

-57.8

10.4

-0.29


-56.7

11.2

-0.27


-56.1

10.8

-0.31

γ-Mo2N(111) M10 γ-Mo2N(111) M11 γ-Mo2N(111)

Figure 1: Construction of the γ-Mo2N(100) and γ-Mo2N(111) clusters and the possible adsorption sites.

Mo and

14

N atoms are denoted by cyan- and blue-coloured spheres,

respectively. Atoms in the first two layers appear in a bigger size. In top views, inner layers are shown by lines for better representation of the first layer.


Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: Summary of lower energy pathways leading to formation of 15N2 and 14N15N over a γ-Mo2N(100) cluster (a) and 14N2 and 14N15NO over γ-Mo2N(111).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3: Structures of molecular adsorptions on the γ-Mo2N(100) cluster. Circles denote the adsorbed NO molecules. Mo, O,

14

N and

15

N atoms are denoted by cyan-, red-, blue and

yellow-coloured spheres, respectively. Only the first two layers are shown in side views. In top views, lines represent second-fourth layers. This colour code and representation of layers apply to all figures depicting the γ-Mo2N(100) cluster.


Page 24 of 34

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Structures of molecular adsorptions on the γ-Mo2N(111) cluster. Only the first atomic layer is shown in side views.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5: Density of states (DOS in arbitrary units) for (a) the d-orbitals in the four neighbouring surface Mo atoms in the clean γ-Mo2N(100) cluster, (b) the d-orbitals in the four surface Mo atoms enclosing the NO molecule in structure M1, (c) gaseous NO and (d) adsorbed NO in the M1 structure. Fermi level is at 0.0 eV. Only two layers were considered in the DOS calculations.


Page 26 of 34

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: Pathways for the subsequent uptake of 15NO by the γ-Mo2N(100) cluster. Circles denote the adsorbed species. Values are in kcal/mol in reference to each separated reactants. Yellow spheres portray the adsorbed N atoms (i.e.,

15

N). Oxygen atoms are red- coloured,

respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b) Figure 7: Dissociative adsorption of 15NO over γ-Mo2N(111) surface (a) and formation of 15 N2 (b).


Page 28 of 34

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8: Desorption of 15N2 molecules from the Mo2OxNy phase. Values are in kcal/mol.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9: Formation of

15

N2 from adjacent adsorbed

15

NO molecules.

kcal/mol.


Page 30 of 34

Values are in

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b) Figure 10: Formation of 14N15N molecule through attack of 15NO on a surface N atom (a) and dissociative adsorption of 15NO on the surface N-O bond in the Mo2OxNy phase (b). Values are in kcal/mol.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11: Formation of 14N2 molecules via bimolecular combination of two surface N atoms in the γ-Mo2N(111) surface.

Figure 12: Formation of 14N15NO molecules via abstraction of a surface 14N atom abstraction by gaseous 15NO molecule. Only the first atomic layer is shown.


Page 32 of 34

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

166x139mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 34 of 34

Conversion of NO into N2 over Î³-Mo2N - The Journal of Physical

Recommend Documents