Catalytic Synthesis of Nitric Monoxide at AlN(0001) Surface: Ab Initio

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Catalytic Synthesis of Nitric Monoxide at AlN(0001) Surface: Ab Initio Analysis Pawel Strak, Konrad Sakowski, Pawel Kempisty, Izabella Grzegory, and Stanislaw Krukowski J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

Catalytic Synthesis of Nitric Monoxide at AlN(0001) Surface: Ab Initio Analysis Pawel Strak1, Konrad Sakowski1,2, Pawel Kempisty1, Izabella Grzegory1, Stanislaw Krukowski1* 1Institute

of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland

2Institute

of Applied Mathematics and Mechanics, University of Warsaw, Banacha 2, 02-097 Warsaw, Poland

ABSTRACT Molecular nitrogen and molecular oxygen adsorption at Al-terminated AlN(0001) surface was investigated using ab initio simulations. It was shown that both species undergo barrierless dissociation during attachment to the surface. The H3 adsorption site was identified as the most favorable for both O and N adatoms. The adsorption energy for O2 and N2 were found to be 14.8 eV and 6.0 eV, respectively. At clean surface separate N and O adatoms has their energy 4.37 eV lower than NO admolecule. At normal nitrogen pressures AlN(0001) surface is fully covered by nitrogen adatoms. It was shown for this coverage adsorption of oxygen leads to creation of NO admolecule which is necessary step for nitric monoxide synthesis. The energetics of NO molecules is related to electronic charge transfer: from the two possible configurations: T4 and H3 are donors and acceptors respectively. The resulting coverage is mixture of both configurations, controlled by electron charge balance. Thus the ab initio modelling provides indication that AlN(0001) is powerful catalyst for high pressure – high temperature synthesis of nitric monoxide (NO), indicating that AlN may be a candidate for applications in industrial mass synthesis of nitrogen based materials such as fertilizers or explosives.

* Corresponding author. Email: [email protected]

ACS Paragon Plus Environment

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INTRODUCTION. A sequence of the necessary steps for effective fabrication of fertilizers and explosives consists of two basic processes: synthesis of ammonia and combustion of ammonia to obtain nitric dioxide, targeting the ultimate goal of cost-effective fabrication of nitric acid, necessary substance for production of nitrites, know commercially as various kind of saltpeter or nitroglycerin, i.e. essentially dynamite, most widely used explosive. These two steps create formidable obstacle which was overcome at the beginning of twentieth century. In fact the second problem in the production chain was solved first by Wilhelm Ostwald who developed the ammonia combustion and patented it in 1902.1,2 Since then the ammonia oxidation is known as Ostwald process. The second problem was solved by Fritz Haber and his assistant Robert le Rossignol who invented platinum catalyzed synthesis of ammonia from molecular nitrogen and hydrogen in 1909.3 The patent was acquired by BASF where the industrial implementation was assigned to Carl Bosch. This was achieved in 1910, and the large scale production of nitric fertilizers and nitroglycerin was possible. Fritz Haber and Carl Bosch were awarded Noble Prizes in Chemistry in 1918 and 1931, respectively.4,5 Since then the ammonia synthesis is known as Haber-Bosch process. Generally, the invention of ammonia catalytic synthetic reaction and its conversion to nitric acid is one of the most beneficial achievements of the chemical sciences.1 The discovery paved the way to famine reduction on the global scale. In addition, the synthesis of nitrogen compounds is important for other industry branches, including fabrication of plastic materials or explosives.1 The success story of the ammonia synthesis has his own price. The synthesis is energy and hydrogen consuming. Even at small facilitation of the process, the scale of the gains in the production, and also reduction of the costs is enormous. That is why, after a century, the search of better, more effective catalysts is still very active. The investigations brought significant progress in terms of the pressure used for synthesis which dropped from original 100 MPa to 10-15 MPa. Simultaneously, the energy spending was reduced from 78 GJ to 27.2 GJ, fairly close to theoretical limit of 20.1 GJ.17 Platinum as ammonia synthesis catalyst was invented first. Later, the other, metal based catalysts were developed, including Gallivare magnetite iron,7 cobalt and ruthenium

8,9

or

osmium.10-12 Significant improvement brought use of combined ruthenium-molybdenum barium hydride (BaH2) – molybdenum (Mo) - carbon nanotubes (CNT) catalysts, developed recently.13-15


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Ruthenium nanoparticles combined with calcium amide or electride may facilitate synthesis considerably.17,18 The experiments of Ertl and Somorjai greatly enhanced understanding of surface reaction pathways and prediction of transition states controlling the reaction rates.19-25 These contribution were fundamental to general understanding of the surface reactions for which Gerhard Ertl was awarded Nobel Prize in Chemistry in 2007.26 A new approach was proposed by Norskov et al. based on ab intio calculations of surface reaction pathways.27-32 They proposed interpolation in the periodic table to create an alloy consisting of the elements very active and very inactive with nitrogen to create very active surface of the optimal performance. This assumption proved to be very fruitful in case of Co-Mo catalysts, more effective than ruthenium or osmium.30-33 Ab initio assessment of the activity of metal catalysts discerned distinctively different temporal behavior of metal wool catalysts: Pt, Pd, Ag, Cu and Ni increased their activity while Au, Fe, Mo, Ti, W and Al remained constant.34 The difference was attributed to initial nitridation of metal surface and creation of Me3N units at the surface.30 The invention of the complex of nitride acid synthesis reaction chain is one of the most beneficial achievements of the chemical sciences.6,35 The invention paved the way to famine reduction on the global scale. The synthesis of nitrogen compounds is important for other industry branches, including fabrication of plastic materials or explosives. Still the process is energy and hydrogen consuming. Even at small facilitation of the process, the scale of the gains in the production, and also reduction of the costs is enormous. Much greater saving may be achieved by removal of hydrogen from the nitric acid synthesis chain. In principle this is possible because hydrogen is not present in the final product, i.e. nitric dioxide. The direct synthesis of nitric monoxide may circumvent Haber-Bosch process altogether, Oswald process will be reduced to second stage, i.e. oxidation of nitric monoxide to dioxide. The basic problem is that oxygen and nitrogen have to be used simultaneously. This was not possible because some metal catalyst are oxidized easily and the removal of the attached oxygen is not possible as they form oxides. Other catalysts do not facilitate oxidation. Nevertheless the possibility exist as suggested by recently proposed application of AlN(0001) surface for ammonia catalyst.37 which is capable to dissociate molecular nitrogen during adsorption.38 Despite high bonding energy of nitrogen molecule, equal to 9.76 eV (941.636  0.60 kJ/mole),39 the energy gain in dissociative adsorption of N2 molecule at clean AlN(0001) surface is about 6 eV,38 much higher than for any


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metal catalysts.40 The N adatoms are located at H3 sites at AlN(0001) surface. Based on these findings, the patent for use of AlN as catalysts for ammonia synthesis was submitted to the patent office.41 The energy gain remain almost unchanged up to the critical N = 0.25 ML coverage. For higher coverages nitrogen is adsorbed molecularly with small energy gain of about 1 eV, not suitable for ammonia synthesis. The semiconductor surfaces are generally very active. They catch any species from the vapor, most frequently oxygen or water. In consequence of oxidation they become inactivated. Aluminum nitride has the largest bonding energy of all nitrides, it is exceptionally hard, capable to withstand most harsh conditions, far superior to any refractory metals. Surface oxidation of AlN(0001) surface was investigated recently.42 It was shown that oxygen dissociates at AlN(0001) surface, the O adatoms are located in H3 sites. The adsorption energy is 6.24 eV/molecule. In addition, the oxidized AlN(0001) surface could be reactivated by nitridation in ammonia flow at 1000C. Sapphire crystals are nitrided in ammonia flow, in temperature close to 1000C.43 In the result, the AlN layer is obtained, indicating that Al-O bonds and oxygen can be removed. The poisoning of the catalyst by oxygen can be avoided and the catalysts could be regenerated. The present paper is devoted to investigations of the possible reaction between adsorbed oxygen and nitrogen, and the creation of the nitric monoxide (NO) molecules. The energy of the adsorbed NO molecules will be obtained and the reaction path investigated and the energy barriers determined. In addition the equilibrium pressures will be determined using recently formulated determination procedure of the equilibrium pressures at semiconductor surfaces.44 This information allows to determine the possibility and the optimal thermodynamic conditions for effective synthesis of nitric monoxide.

METHOD A freely-accessible ab initio density functional theory (DFT) code SIESTA was used in the calculations reported below.45,46 The solution procedure used in Generalized Gradient Approximation (GGA) calculations employs norm conserving pseudopotentials, in the determination of the system wavefunction expressed as a finite combinations of local basis molecular orbitals. The norm-conserving Troullier-Martins pseudopotentials, in the Kleinmann-


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Bylander factorized form47,48 for Al, N and O atoms were generated using ATOM program provided by the authors of SIESTA code. The atomic basis sets were: Al - 3s: DZ (double zeta), 3p: TZ, 3d DZ; N - 2s: DZ, 2p: TZ, 3d: SZ and H (real plus termination atoms) 1s: TZ, 2s DZ, 2p: SZ. The exchange-correlation functional was adopted in revised modification of Perdew, Burke and Ernzerhof (PBE)49,50 functional for solids and surfaces (PBEsol).51,52 A convergence criterion for the termination of the self-consistent field (SCF) loop was that the maximum difference between the output and the input of each element of the density matrix was to be equal or smaller than 10-4. Relaxation of the atomic position was terminated when the force acting on any atom becomes smaller than 0.005 eV/Å. The ab initio lattice parameters of bulk AlN were: aAlN = 3.116 Å and cAlN = 4.97 Å, reasonably not far from the experimental values: aAlN = 3.111 Å and cAlN = 4.981 Å.53 The surface is represented by (4 x 4) AlN slab of the thickness of 8 Al-N double atomic layers (DALs). The periodic boundary conditions are enforced in the plane parallel to the surface. The thickness of the slab is sufficient to avoid the significant overlap of wavefunctions of the atoms on both surfaces of the slab: the real and termination one. The broken bonds at the opposite side of the slab were saturated by fractional (Z = 0.75) charge hydrogen atoms.54,55 This construction is sufficient to remove most drastic effect related to the arbitrary termination of the solid body. DFT ab intio simulations of the three molecules: nitrogen (N2), oxygen (O2) and nitric oxide (NO) serve as the test of compatibility of the DFT parameterization for molecular species. This is important as the typical choice for simulation of solids and semiconductors in particular, work poorly for small, covalently bound molecules. The compromise solution is naturally inferior from the optimized for molecules only, and naturally much worse than advanced ab intio calculations, such as W1 or CCSD(T).56-58 The test values for the molecules, obtained in our calculations are dissociation energy ∆𝐸𝑑𝑖𝑠𝑠 and the bond length d. The N2 dissociation energy obtained in the presently used parameterization is ∆𝐸𝐷𝐹𝑇 𝑑𝑖𝑠𝑠 (𝑁2) = 9.798𝑒𝑉 which is in a good agreement with the 𝐷𝐹𝑇 59 experimental value ∆𝐸𝑒𝑥𝑝 𝑑𝑖𝑠𝑠(𝑁2) = 9.756𝑒𝑉. The DFT obtained bond length 𝑑𝑁 ― 𝑁 = 1.099Å is 56 in excellent agreement with the experimental value 𝑑𝑒𝑥𝑝 𝑁 ― 𝑁 = 1.098Å. The additional insight may

be obtained from the diagram of bonding states in N2 molecule, presented in Fig. 1.


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Figure 1. Ab intio obtained energy states of the nitrogen molecule (N2). The left panel presents the molecule quantum states Crystal Orbital Hamiltonian Population (COHP) of the two N atoms, the other panels present projected density of states (PDOS) of Ns and Np orbitals. Fermi level is at zero energy.

The data presented in Figure 1 indicate that the nitrogen N2s bonding and antibonding states have the lowest energy and accordingly, they are occupied. In addition the bonding N2p states (both 2 and 2 bonds) are occupied that contributes to high dissociation energy of the molecule. Differently to nitrogen, ab initio simulations of O2 molecule encountered serious obstacles due to the electronic configuration of the molecule.60 Oxygen atoms couple in the molecule by bonding and antibonding orbitals with antibonding 2πp* orbitals filled only in half. The molecule ―

remains in trifold degenerated triplet ground state 3Σ𝑔 , found experimentally to be the most stable.61 The two other states were found to have higher energy: twofold degenerate1∆𝑔 state and 1

+

the ∑𝑔 singlet state of their energies 0.975 eV and 1.624 eV higher.61 Hartree –Fock (HF) or post Hartree-Fock methods, such as several variants of Configuration Interaction (CI) gives the


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𝑒𝑥𝑝 61 62 interatomic distance 𝑑𝐶𝐶 𝑂 ― 𝑂 = 1.207Å, close to experimental value 𝑑𝑂 ― 𝑂 = 1.208Å. Our test

calculations give 𝑑𝐶𝐶 𝑂 ― 𝑂 = 1.353Å, which is rather high as compared to the experiment. HF and CI methods underestimate O2 binding energy by several electronvolts.62 This is caused partially by incorrect representation of separate atoms by CI approximation. Our DFT calculations also overestimate the exact oxygen binding energy, experimentally determined to ∆𝐸𝑒𝑥𝑝 𝑑𝑖𝑠𝑠(𝑂) = 5.116𝑒𝑉 58

𝐷𝐹𝑇 59 or ∆𝐸𝑒𝑥𝑝 𝑑𝑖𝑠𝑠(𝑂2) = 5.123𝑒𝑉 by about 1eV giving ∆𝐸𝑑𝑖𝑠𝑠 (𝑂2) = 5.994𝑒𝑉. An additional data,

required for analysis of the oxygen bonding at AlN(0001) surface are given in Figure 2.

Figure 2. Ab intio obtained energy states of the oxygen molecule (O2). The left panel presents the molecule states Crystal Orbital Hamiltonian population (COHP) of the two oxygen atoms, the other panels present projected density of states (PDOS) of O2s and O2p orbitals. Fermi level is at zero energy. The data in Figure 2 indicate that the 2s oxygen bonding and antibonding states have the lowest energy and accordingly they all are occupied. Bonding O2p states (both 2 and 2 states) and are also fully occupied. In addition the antibonding 2πp* states are occupied in half which explains much lower bonding energy of the oxygen molecule.


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Finally, the nitric oxide molecule was analyzed. Nitric oxide plays important role in the physiology of living organism, therefore it was investigated intensively. A number of quantum mechanical calculation showed that the high precision CCSD(T) method gives interatomic distance = 1.169Å 𝑑𝐶𝐶𝑆𝐷(𝑇) 𝑁―𝑂

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63,64 Our in good agreement with the experimental value 𝑑𝑒𝑥𝑝 𝑁 ― 𝑂 = 1.151Å.

DFT calculations give 𝑑𝐷𝐹𝑇 𝑁 ― 𝑂 = 1.191Å which is also in reasonably good agreement with the 65 experiment. The NO dissociation energy experimental value is ∆𝐸𝑒𝑥𝑝 𝑑𝑖𝑠𝑠(𝑁𝑂) = 6.515𝑒𝑉. The DFT

results show considerable dependence of the NO dissociation energy on the basis set.58 Our result is ∆𝐸𝑒𝑥𝑝 𝑑𝑖𝑠𝑠(𝑁𝑂) = 6.654𝑒𝑉 in good agreement with the experimental value. An important result is the bonding structure of NO molecule presented in Figure 3.

Figure 3. Ab intio obtained energy states of the nitric oxide molecule (NO). The left panel presents the molecule states Crystal Orbital Hamiltonian population (COHP) of the nitrogen and oxygen atoms, the other panels present projected density of states (PDOS) of s and p orbitals of both atoms. The calculations shown here were made in spin-polarized version.


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The data in Figure 3 presents more complex diagram, basically similar to oxygen. As above the 2s oxygen and nitrogen bonding and antibonding states have the lowest energy and accordingly they all are occupied. The energy difference between these states is much higher than for oxygen. Bonding 2p states (both 2 and 2 states) and are also fully occupied The antibonding 2πp* states are fractionally occupied in one fourth which gives the bonding energy of NO higher than the oxygen but much lower than nitrogen molecule. Fermi level is at zero energy.

The reaction path was determined using Born-Oppenheimer approximation employing nudged elastic band (NEB) method.66-68 In the present formulation NEB module of Atomistic Simulation Environment69 was linked to SIESTA package paving the way to determination of the energy and conformation of the species along the optimized pathways. Doping simulation was conducted by adding a charge density with SIESTA native routine that determines the charge of the system, and adds a compensating uniform background charge that makes the system neutral.

RESULTS Properties of clean AlN(0001) surface.

Since the properties of clean AlN(0001) surface were already determined, they are only mentioned here.44 According to the ab initio simulations, the clean AlN(0001) surface undergoes no reconstruction. The Fermi level at AlN(0001) surface is pinned by a highly dispersive Al broken bond surface state of the energy located below conduction band minimum (CBM). The surface electric neutrality energy level is located about 0.4 eV below CBM.70 The catalytic activity for NO synthesis reaction could be hampered by oxidation of the AlN(0001) surface. Therefore the effective sustainable synthesis is possible in nitrogen rich conditions when the surface is protected from oxidation by dense coverage of the nitrogen adatoms. Therefore the data for high N coverage are pertinent.

AlN(0001) surface under N coverage


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Molecular nitrogen (N2) is necessary in direct synthesis of nitric monoxide. In the reaction catalysis AlN surface has to be covered by nitrogen adatoms, admolecules or derived adducts. The nitrogen coverage is defined as a ratio of nitrogen atoms attached to the surface to the number of Al top surface atoms, i.e.  N 

NN NN  , where NN and NAl are the numbers of nitrogen adatoms N Al N o

and Al atoms. The latter is assumed to be equal to number of sites at the surface No. The adsorption of nitrogen, already investigated using ab initio calculations, was reported in Refs. 37 and 38. The results presented there will be shortly summarized only. Nitrogen molecule adsorbs dissociatively at clean AlN(0001) surface with the net energy gain of ∆𝐸𝐷𝐹𝑇 𝑎𝑑𝑠 (𝑁2) = 6.0 𝑒𝑉. The process is essentially barrierless, the energy barrier is not larger than 0.1 eV, i.e. is of no importance to catalytic synthesis which should be conveyed at high temperatures. The created single N atoms are located in H3 sites, saturating three Al surface broken bonds. The N2s states of N adatoms preserve their molecular character, with their energy located deep in valence band (VB). The N2p states overlap with the neighboring Al atoms, creating states in the bandgap. The higher energy is obtained for N2pz state and the lower for N2px and N2py states. These states accept electrons from broken bond states of Al surface atoms without coverage. At the critical coverage 𝑁 1

= 4𝑀𝐿, the donating electron Al broken states are fully emptied, i.e. no further electron donation is possible. The Fermi level is depinned from Al broken bond state and moves down, still above the upper Npz-Al state in the bandgap. The transition predicted by application of extended electron counting rule71-73 was confirmed by ab initio calculations.37,38 Since the number of electrons available is greater than 6, the Fermi level remains always above the uppermost Npz-Al state. For 1

the coverage above the critical one, i.e. 𝑁 = 4𝑀𝐿, the donation of the electrons, is not possible. It 1

was shown that for coverage 𝑁 ≥ 3𝑀𝐿, the nitrogen is adsorbed molecularly, with the energy gain of about 1 eV/molecule. The molecular regime is not presented in full extent as it is not suitable for application as a catalyst for ammonia synthesis reaction. The equilibrium pressure of molecular nitrogen was also determined, using full expression derived from equality of chemical potential of nitrogen at the surface and in the vapor.44 These data 1

are limited by the configurational entropy factor to 𝑁 < 3𝑀𝐿 for which all sites are covered. The equilibrium pressure of nitrogen at AlN(0001) surface is very low, at the temperature 𝑇 = 1000𝐾


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1

that reaches 𝑝𝑁2 = 10 ―19𝑏𝑎𝑟 for 𝑁 = 4𝑀𝐿. In order to reach the molecular adsorption regime, 1

corresponding to 𝑁 > 3𝑀𝐿 the pressure should be increased to about 𝑝𝑁2 = 1𝑏𝑎𝑟. That effectively corresponds to complete saturation of Al broken bonds by nitrogen. The large span of the pressures opens the possibility of use of relatively low pressure of nitrogen which could assure high coverage of nitrogen of the surface sites. Still this opens the possibility of dissociative adsorption of oxygen at the remaining sites and possible creation of NO admolecules. In addition, the excess of nitrogen is necessary to protect AlN surface from reaction with the oxygen atoms and conversion of the AlN(0001) surface into oxide. The obtained pressures, not exceeding 𝑝𝑁2 = 1𝑏𝑎𝑟 opens the technical possibility of preserving that regime for very long time. Thus, chemical stability of AlN(0001) catalyst may be assured so the catalyst may serve for a very long time.

AlN(0001) surface under O coverage

Oxygen adsorption at AlN(0001) surface was investigated by Ye at al.74 They have found that oxygen is adsorbed at H3 site at the surface. The adsorption energy was found to change from 6.6 eV for 0.25 ML to 2.6 for 1 ML coverage. The change was attributed the strong interaction between O adatoms though the presented diagrams indicate the change of the sites and the bonding with the surface for 0.5 ML coverage or higher. Full discussion of the oxygen adsorption at AlN(0001) surface is beyond scope of the present paper. Since the subject is limited to interaction of oxygen with N-covered AlN(0001) surface, only adsorption of oxygen at bare AlN(0001) is considered as a guidance. Fractional coverage by oxygen adatoms will not be investigated. In accordance to Ref. 74, the adsorption position of atomic oxygen is H3 site. The scheme is presented in Figure 4.


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Figure 4. Slab scheme representing the stable H3 site of oxygen atom at AlN(0001) surface: (a) – side view, (b) – top view. The blue, yellow and red balls represent Al, N and O atoms, respectively. The adsorption energy for O single atom is relatively large, ∆𝐸𝐷𝐹𝑇 𝑎𝑑𝑠 (𝑂) = 10.4 𝑒𝑉. Since the oxygen dissociation energy was ∆𝐸𝐷𝐹𝑇 𝑑𝑖𝑠𝑠 (𝑂2) = 5.994𝑒𝑉, the adsorption energy of molecular 𝐷𝐹𝑇 oxygen is ∆𝐸𝐷𝐹𝑇 𝑎𝑑𝑠 (𝑂2) = 14.8 𝑒𝑉, far exceeding the value of molecular nitrogen ∆𝐸𝑎𝑑𝑠 (𝑁2)

= 6.0 𝑒𝑉. That confirms well known propensity of aluminum oxide surface to create oxygen layer, extremely difficult to remove. It is of considerable interest to determine the electronic properties of oxygen adsorbed at AlN(0001) surface. The band structure of the 8 atomic layer 2 3 × 2 3 slab system with single O 1

adatom is presented in Figure 5. As the slab has 12 adsorption sites, the oxygen coverage is 𝜃𝑂 = 12 𝑀𝐿.


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Figure 5. Band structure of the 8 atomic layer 2 3 × 2 3 slab system with single oxygen adatom, 1

representing AlN(0001) surface under 𝜃𝑂 = 12𝑀𝐿 coverage. The panel from the left represent bands in momentum and real space, and the O adatom density of states. Fermi level is at zero energy.

As it is shown the oxygen O2s bonding states are below the valence band and always occupied. The O2p bonding states are located about 0.2 eV above valence band maximum (VBM) and are also occupied. Denoting the O coverage by α and the fraction of bare Al sites by β , the condition of normalization is: 3𝛼 + 𝛽 = 1

(1)

because single O adatom saturates three Al broken bonds. The electron counts assumes that all saturated states are occupied, the bare are empty. Each Al surface atom contributes ¾ electron and the O adatom 6 electrons, the extended electron counting rule (EECR)71-73 takes the form:

(3 × 34 + 6)𝛼 + 34𝛽 = 8𝛼 + 2𝛽


(2)

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Solving these equation we obtain 𝛼 = 16 and 𝛽 = 16. Therefore the critical coverage is 𝜃𝑂 = 16𝑀𝐿. Naturally, the possible jump could be observed only in the case when the Al broken states are separated from the conduction band. In fact it is not and the Fermi level is located in the band with partial occupation of these states. Thus the jump will not have large influence on the adsorption energy of oxygen at relatively high coverage, close to 1 3 𝑀𝐿𝑠 .

AlN(0001) surface under mixed O and N coverage

As it was shown above, basic configurations of the adsorbed nitrogen and oxygen at low coverage on AlN(0001) surface are single atoms, both of them located at H3 site. The simultaneous presence of both species at the surface may lead to the possible creation of NO admolecule, i.e. the configuration of the oxygen and nitrogen atoms covalently bound and attached at the surface. This was verified by ab initio calculations where the O and N adatoms are located separately and then moved closer to create the NO molecule using the same rate for each atom. Initial and final configurations are presented in Figure 6. All calculations reported in this Section were made in spin nonpolarized version.


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Figure 6. Atomic configurations presenting oxygen and nitrogen atoms: (a) separated; (b) in NO admolecule. The blue, yellow and red balls represent aluminum, nitrogen and oxygen atoms, respectively. The energetic stability of the configuration of these atoms was calculated using NEB procedure. The energy change in function of the N-O interatomic distance is presented in Figure 7.

Figure 7. Energy E of the pair of oxygen and nitrogen attached at AlN(0001) surface, in function of the interatomic N-O distance d. As it is demonstrated in Figure 7, the minimal energy configuration is attained for the separated atoms. The motion towards the bound configuration initially increases its energy which is related to the shift from the optimal energy position at H3 sites. The drastic increase of the energy is related to reconfiguration of the bonding which leads to the energy increase by 5.13 eV. The bonding into


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NO molecule reduced the energy excess to ∆𝐸𝐷𝐹𝑇 𝑑𝑖𝑠 (𝑁𝑂) = 4.37 𝑒𝑉, i.e. 0.76 eV lower. Therefore NO molecule corresponds to local energy minimum. The considerable energy difference proves that for low coverage, the separated atom configuration is preferred energetically. As it was shown before, AlN(0001) attains considerable atomic nitrogen coverage, even at relatively low pressure of nitrogen in the vapor. The route to NO effective synthesis leads via adsorption of O2 molecule at such, nitrogen-covered surface. The process is presented schematically in Figures 8 and 9. In Figure 8, the three NO configurations are presented: initial – for oxygen molecule located far away from the surface, intermediate when NO molecule and O adatom are located in H3 sites, and final in which one molecule is located in H3 and the second in T4 site. As shown in Figure 8 the H3 and T4 sites are located in the center of triangle created by topmost Al atoms. They are located above topmost layer of Al atoms, i.e. approximately at the center of regular tetrahedron, so they are denoted as tetrahedral sites. The H3 sites is located above empty channel in wurtzite structure, i.e. it has 3 nearest neighbors. The T4 site is located above N atom in the topmost N layer, i.e. it has 4 nearest neighbors. In Figure 8(c), the oxygen atoms is shifted from T4 site, thus it is in slightly skewed position.


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Figure 8. Atomic configurations during adsorption of molecular oxygen at 1/3 ML N covered AlN(0001) surface: (a) – initial configuration with molecule located far from the surface, (b) – intermediate configuration with NO molecule and O adatom located in H3 positions, (c) – final configuration with NO molecules located in H3 and T4 positions. The blue, yellow and red balls represent aluminum, nitrogen and oxygen atoms, respectively.

The energy change during adsorption of oxygen molecule at 1/3 ML N covered AlN(0001) surface is presented in Figure 9. The oxygen molecule is attracted by the surface at all distances i.e. energy barrier is absent. During adsorption, the O2 molecule is decomposed into two separate O atoms that are attached to the Al top layer atoms and to neighboring N adatoms, i.e. the NO


17


Page 18 of 44

admolecules are created. The intermediate configuration consists of the oxygen atom attached to N atom, creating NO admolecule, located in H3 site. The second oxygen atom moves to neighboring H3 site where it is attached to Al top atoms. Also the neighboring N atom is dragged from the H3 position into T4 position, closer to the oxygen adatom. The final configuration is attained by further motion of the latter oxygen atom towards the nitrogen adatom creating the second NO molecule located in the T4 position.

Figure 9. Energy E, of the oxygen molecule approaching AlN(0001) surface fully covered by N adatoms, in function of the molecule-surface distance h. The distance is measured with respect to minimum energy configuration presented in Figure 10(c). The labels (a), (b) and (c) mark the configurations presented in Figure 8. The initial configuration of the oxygen, at the distance 3.32Å from the surface, represents the O2 molecule in the vapor phase, having its energy 8.20 eV above the final configuration energy. Hence spontaneous creation of NO admolecule occurs during adsorption of molecular oxygen at Ncovered AlN(0001) surface. Synthesis of nitric oxide admolecules is therefore potentially effective at AlN(0001) surface provided that the thermodynamic conditions prevent oxidation of the surface. The intermediate position is 0.82 eV higher than the final configuration energy. The energy barrier


18

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equal to 68 meV is negligible as it is easily overcome by the kinetic energy acquired in the initial stage of adsorption process. The huge energy gain in the process is compounded by the negligent energy barrier indicating that the creation of NO admolecules is efficient process, potentially suitable for manufacturing of the nitric oxide.

Figure 10. Atomic configurations of NO molecules obtained from adsorption of oxygen molecule O2 at the 2 3 × 2 3 slab representing 1/3 ML N covered AlN(0001) surface: top row – top view, bottom row – side view. The oxygen molecule decomposes creating to NO admolecules located in: (a) both in H3 position , (b) both in T4 positions, (c) in H3 and T4 positions. The remaining two N adatoms are located in H2 positions. The blue, yellow, red and cyan balls represent aluminum, nitrogen and oxygen atoms, and hydrogen pseudoatoms respectively.


19


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The role of electronic degrees of freedom can be elucidated using the data on electronic states of NO molecules attached to the surface. The three configurations of the pairs of NO molecules resulting from decomposition of O2 molecule and bonding with the N adatoms were selected: (a) both in H3 sites (2xH3 configuration), (b) both in T4 sites (2xT4 configuration), and (c) one in H3 and one in T4 sites (H3&T4 configuration). These slabs are presented in Figure 10. The N-O interatomic distances in the NO molecules in 2xH3 configuration are: 𝑑𝐻3 𝑁 ― 𝑂 = 1.434Å and 𝑑𝐻3 𝑁 ― 𝑂 = 1.423Å. These distances are not identical as these two NO molecules are not symmetry-related. These data differ considerably from our DFT value for separate NO molecule 𝑑𝐷𝐹𝑇 𝑁 ― 𝑂 = 1.191Å, indicating that the interaction with the surrounding strongly affects the molecule, still preserving it as bound entity. The data for 2xT4 configuration are similar: 𝑑𝑇4 𝑁 ― 𝑂 = 1.434Å and 𝑑𝑇4 𝑁 ― 𝑂 = 1.457Å. Again they are different from separate molecule value confirming strong interaction with the neighboring atoms. Finally, in the H3&T4 configuration these distances are: 𝑇4 𝑑𝐻3 𝑁 ― 𝑂 = 1.482Å and 𝑑𝑁 ― 𝑂 = 1.427Å. As in the previous cases, these distances are similar and

relatively distant from the separate molecule value. It is important to note that the mixed configuration is different from the uniform ones: the distance for H3 site is the highest, and the distance for T4 is the smallest, suggesting strong influence, possibly by charge transfer. These configurations have the following total energies: (a) 2xH3 - 𝐸2𝑥𝐻3 𝑡𝑜𝑡 = ―35960.874 𝑒𝑉, (b) 𝐻3&𝑇4 2xT4 - 𝐸2𝑥𝑇4 = ―35962.825 𝑒𝑉. The more interesting 𝑡𝑜𝑡 = ―35961.290 𝑒𝑉 and (c) H3&T4 - 𝐸𝑡𝑜𝑡

are their relative energies, the lowest energy has the mixed H3&T4 configuration at value reference level at 𝐸𝐻3&𝑇4 = 0, 2xT4 configuration has its energy higher 𝑎𝑡 𝐸2𝑥𝑇4 𝑟𝑒𝑓 𝑟𝑒𝑓 = 1.535 eV , and finally 2xH3 has its energy the highest at 𝐸2𝑥𝐻3 𝑟𝑒𝑓 = 1.950 eV. Thus the stability of the investigated configuration is not related solely to the bonding as the mixed configuration should be between two uniform ones. In fact the mixed configuration is the most stable which indicates again on the charge transfer and the compensation.


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Figure 11. Band structure of the 8 atomic layer 2 3 × 2 3 slab system with two NO admolecules located in H3 positions, presented in Fig 10 (a). The panels from the left represent bands in momentum and real space, and the density of states of N separate adatom (third panel), and N and O adatoms in NO admolecule (fourth and fifth panel). Fermi level is at zero energy.

The electronic structure of the 2xH3 configuration is presented in Fig. 11. As it is shown the Al broken bond states are absent. They are replaced by the bonding states created between topmost Al atoms and the N adatom and NO admolecule. As shown in Ref 37, the N adatom N2s states are located deep below valence band maximum so they are not shown in Fig. 11. The do not create covalent bonds with the neighboring Al surface atoms states. The bonding states created due to overlap between Al3p and N2p states are all located in the bandgap. These states have different energies: the states created by N2px and N2py states are degenerate and have lower energy than the state related to N2pz orbital. The balance of the charge in the N adatom configuration could be determined using extended electron counting rule (EECR).71-73 In total the 8 atomic states are present which could


21


be potentially occupied. The number of electrons is 5 + 3 ×

Page 22 of 44

3

1 3 4 = 7 4, i.e. 4 electron is

missing. Out of these two electrons are located on N2s atomic states, and the others are located on N2p derived states. As shown in Fig. 11 nitrogen PDOS panel, the N2px and N2py states are fully occupied while N2pz states are occupied in part, pinning the Fermi level at the surface. The other important part of the analysis is related to the states of the NO admolecule, presented in Fig 12. As the molecule remains intact, it has 16 common states, derived from s and p states of the oxygen and nitrogen atoms. The total number of electrons is: 5 + 6 + 3 ×

3

1 4 = 13 4. It is expected

that N2s-O2s bonding and antibonding states remain intact, very low in energy i.e. relatively unaffected by interaction with the neighboring states due to energy difference, and accordingly 1 they absorb 4 electrons. From the remaining 9 4 electrons, 6 are allocated in the N2p - O2p 1 bonding states and the 3 4 electron are allocated in the N-O antibonding states. As it is shown in Fig 12 (a), the bonding states are degenerate with valence band (VB) and therefore they are characterized by high dispersion.

Figure 12. The electronic properties of NO admolecule located in H3 position (Fig. 10(a)). The two left panels (a) and (b) represent partial and total density of states (PDOS and TDOS), the right two panels (c) and (d) represent crystal orbital Hamilton population (COHP).73,74 The positive and ACS Paragon Plus Environment

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negative sign of COHP overlap corresponds to antibonding and bonding overlap, respectively. The red, green and blue lines in (a) represent TDOS, and PDOS of O and N in N-O admolecule, respectively. The red, orange and cyan lines in (b) represent TDOS, and PDOS of Al surface atoms adjacent to O and N adatoms, respectively. The magenta, navy-blue and brown lines in (c) represent COHP of Np and Op orbitals, parallel (magenta) and perpendicular (navy-blue and brown) to N-O molecule axis, respectively. The yellow and dark-green lines in (d) represent COHP of Al - O and Al - N atoms respectively. Horizontal black line denotes Fermi level: 𝐸2𝑥𝐻3 = ―4.473 𝑒𝑉. 𝐹 Note that the N2p – O2p states, perpendicular to the admolecule axis, are antibonding as presented in Figure 12 (c), having the considerable overlap. They have higher energy and accordingly, they located in the bandgap. They are also characterized by considerable bonding overlap with the neighboring Al surface atoms that binds NO molecule to the surface (Figure 12 (d)) . Nevertheless it has to be noted that despite that the antibonding N-O states preserve molecular character i.e. they have relatively small dispersion in the energy scale. As it turns out both states have the energy, close to the Fermi level, thus the upper states in N adatom and the antibonding states of NO admolecule are partially filled, pinning the Fermi level at the surface at 𝐸2𝑥𝐻3 𝐹 = ―4.473 𝑒𝑉. The antibonding state, parallel to the molecule axis, has much higher energy, is located outside the energy scale, not depicted in Figure 12. As shown in Ref. 37, the N adatom N2s states are located deep below valence band maximum. Thus they are not depicted in Figure 13. The other aspects of the electronic structure of the 2xT4 configuration, presented in Figure 13, is different from 2xH3 configuration. As it is shown in Figure 10(b) a part of the Al broken bond states is not saturated, their energies are located at the bottom of conduction band as shown in Figure 13. Other broken bond states are saturated to create the bonding states between topmost Al atoms and either the N adatoms or NO admolecules. The states, created due to overlap between Al3p and N2p states, are located in the lower part of the bandgap: the states created by N2px and N2py states are split by the interaction with the Al surface atoms. Similarly to 2xH3 case, the state related to N2pz orbital has the highest energy. As it is shown in Fig 13 these three states are in the lower part of the bandgap, below Fermi level thus all of them are fully occupied. They absorb 6 electrons, with the missing

3

4 electron shifted from

NO molecule.


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Figure 13. Band structure of the 8 atomic layer 2 3 × 2 3 slab system with two NO admolecules located in T4 positions, presented in Fig 10 (b). The panels from the left represent bands in momentum and real space, and the PDOS of: the N separate adatom, and the N and O atoms in NO admolecule. Fermi level is at zero energy.

The analysis of the occupation of the quantum states in 2xT4 configuration of NO 1 admolecule has to take into account the mentioned charge transfer which leaves 12 2 electrons to distribute among NO states. The N2s-O2s bonding and antibonding states absorb 4 electrons. Also 6 more electrons occupy N2p-O2p bonding states, overlapping the valence band as presented in Fig 14 (a). Actually one of the bonding states is slightly higher in energy, extending to the bandgap. The antibonding states are totally different from 2xH3 case, the N2p-O2p antibonding state, parallel to the molecule axis, remains in the bandgap (Figure 14 (c)). It has considerable antibonding overlap with the adjacent Al surface atom. These all states are occupied, absorbing 12 electrons. The N2p-O2p antibonding states, perpendicular to the molecule axis, are strongly affected by the overlap with the Al atoms that increases their energy. They are degenerate with the


24

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conduction band and remain unoccupied. The remaining

1

2 electron is shifted to the Al broken

bond state, located close to conduction band bottom. Accordingly, the Fermi level is raised up to the energy level of this state, remains pinned at very high level at 𝐸2𝑥𝑇4 = ―2.758 𝑒𝑉. This causes 𝐹 considerable increase of the energy of the system as it contains 2 NO admolecules, because in total 1 electron is promoted to high energy state.

Figure 14. The electronic properties of NO admolecule located in T4 position (Fig. 10(b)). The two left panels, (a) and (b), present partial and total density of states (PDOS and TDOS), the right two panels, (c) and (d) present crystal orbital Hamilton population (COHP).75,76 The positive and negative sign of COHP overlap corresponds to antibonding and bonding overlap, respectively. The red, green and blue lines in (a) represent TDOS, O adatom and N adatom PDOS, respectively. The red, orange and cyan lines in (b) represent TDOS, Al surface atoms adjacent to O and N adatoms, respectively. The magenta, navy-blue and brown lines in (c) represent COHP overlap of Np and


25


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Op orbitals, parallel (magenta) and perpendicular (navy-blue and brown) to N-O molecule axis, respectively. The yellow and dark-green lines in (d) represent COHP of Al-O and Al-N atoms respectively. Horizontal black line denotes Fermi level at: 𝐸2𝑥𝑇4 = ―2.758 𝑒𝑉. 𝐹

The third, mixed H3&T4 configuration is the most complex as it contains both types of NO configurations, as shown in Fig 10(c). The basic electronic properties of this case are presented in Figure 15.

Figure 15. Band structure of the 8 atomic layer 2 3 × 2 3 slab system with two NO admolecules located in H3 and T4 positions, respectively, as presented in Fig 10 (c). The panels from the left represent bands in momentum and real space, and the density of states of N separate adatom (third panel) , and N and O atoms in NO admolecule in: H3 position (fourth panel) and T4 position (fifth panel), respectively. Fermi level is at zero energy.


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As it is shown in Figure 15, the electronic states present considerably complex picture. The broken bond state of Al top surface atoms is not present, in accordance with the diagram presented in Figure 10(c). The states related to N adatoms are present in the bandgap. As in the case of 2xT4 configuration, the states have three different energies. Similarly to 2xH3 case, the highest energy state, related to N2pz orbital is fractionally occupied, pinning the Fermi level. This is different from 2xT4 configuration in which this state is fully occupied. The same similarity 2xH3 case is observed for N-O states which are fractionally occupied. Thus the Fermi level is pinned by the admolecule/adatom states relatively low at 𝐸𝐻3&𝑇4 = ―3.893 𝑒𝑉, similarly to 2xH3, differently 𝐹 from 2xT4 case.

Figure 16. The electronic properties of NO admolecule located in T4 position (Fig. 10(c)). The two left panels represent partial and total density of states (PDOS and TDOS), the right two panels


27


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represent crystal orbital Hamilton population (COHP).75,76 The positive and negative sign of COHP overlap corresponds to antibonding and bonding overlap, respectively. The red, green and blue lines in leftmost panel represent TDOS, O adatom and N adatom PDOS, respectively. The red, orange and cyan lines in left-center panel represent TDOS, Al surface atoms adjacent to O and N adatoms, respectively. The magenta, navy-blue and brown lines in right-center panels represent COHP of Np and Op orbitals, parallel (one) and perpendicular (two) to N-O molecule axis, respectively. The yellow and dark-green lines in rightmost panel represent COHP of Al-O and Al-N atoms respectively. Horizontal black line denotes Fermi level 𝐸𝐻3&𝑇4 = ―3.893 𝑒𝑉. 𝐹

The electronic states are direct combination of the states obtained in the 2xH3 and 2xT4 configurations. The quantum states of the NO molecule, located in T4 position are presented in Figure 16. Similarly to Figure 14, the N2p-O2p one bonding state is totally in valence band and the second is shifted up to the bandgap (Figure 16 (a) and (c)). At approximately the same energy level, the antibonding state of the orbitals parallel to the molecule axis is observed (Figure 16 (c)). The state have considerable bonding to neighboring Al surface atoms (Figure 16 (d). .


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Figure 17. The electronic properties of NO admolecule located in H3 position (Fig. 10(c)). The two left panels represent partial and total density of states (PDOS and TDOS), the right two panels represent crystal orbital Hamilton population (COHP).75,76 The positive and negative sign of COHP overlap corresponds to antibonding and bonding overlap, respectively. The red, green and blue lines in leftmost panel represent TDOS, O adatom and N adatom PDOS, respectively. The red, orange and cyan lines in left-center panel represent TDOS, Al surface atoms adjacent to O and N adatoms, respectively. The magenta, navy-blue and brown lines in right-center panels represent COHP of Np and Op orbitals, parallel (one) and perpendicular (two) to N-O molecule axis, respectively. The yellow and dark-green lines in rightmost panel represent COHP of Al-O and Al-N atoms respectively. Horizontal black line denotes Fermi level 𝐸𝐻3&𝑇4 = ―3.893 𝑒𝑉. 𝐹


29


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The properties of the states of NO molecule located in H3 position are presented in Figure 17. Again similarity to the 2xH3 case is almost complete. The bonding N2p-O2p states are in the valence band. The energy of the antibonding N2p-O2p state, parallel to the molecule axis is high in the conduction band while the energies of the other two states, perpendicular to the molecule locate them close to the midgap in full similarity to the diagram in Figure 12. The occupation of the states is different. The N2s bonding state of each N adatom remain unchanged accepting 2 electrons. The N2px and N2py states of each adatom N absorb 4 electrons 1 3 laving 1 4 electron for N2pz , i.e. 4 electron is missing. In total for 2 N adatoms that amounts 1 1 to 2 2 electron available for 2 states, and 1 2 electron missing. Accordingly the Fermi level is pinned by N2pz derived state. The bonding and antibonding N2s-O2s states of each NO molecule absorb 4 electrons. And N2p-O2p bonding state of each NO molecule accept 6 electrons. The antibonding state of N2pO2p orbitals, parallel to the axis of NO molecule in T4 position absorbs 2 electrons. In total we 1 1 1 have 1 4 +3 4 = 4 2 electrons which should be allocated into 4 states. That creates an excess of

1

2electrons and both these states should be fully occupied. In fact N2pz derived state of two

1 adatom is at approximately the same energy level, missing 1 2 electrons. Thus it may accommodate

1

2 electrons from the pair of NO molecules, with still 1 electron missing Thus

Fermi level is pinned by these three states, located at relatively low energy at 𝐸𝐻3&𝑇4 = ―3.893 𝑒𝑉. 𝐹 The position of Fermi level assures the N2pz derived and N2p antibonding states, perpendicular to NO admolecule located in T4 configuration partially occupied, in full agreement with the above analysis. Note that the energy of NO molecule in T4 configuration is increased by the promotion of 1 electron to higher energy, it confirms that the T4 configuration correspond to optimal energy structure, and the total energy should be maximized in the mixed N and NO coverage. The argument may be sketched as follows: the 2xH3 configuration has higher energy of the bonding states., thus it should have higher energy than 2xT4 configuration. But the latter has lower number of the states and some charge are shifted to antibonding states, that rises the Fermi energy and accordingly increases its total energy. The mixed H3xT4 configuration allow to transfer charge


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from antibonding states of T4 configuration to bonding state of H3 configuration. Thus the Fermi level is low and the total energy is the lowest. Generally, this argument may be applied to the various coverage of the surface by N adatoms, NO admolecules in H3 and T4 positions. Denote by α the coverage of the surface by N adatoms, and by β and , the coverage by NO admolecules in H3 and T4 positions, respectively. Assumptions that the surface is completely covered leads to the following normalization condition: (3)

3𝛼 + 3𝛽 + 3𝛾 = 1 From the above analysis it follows that

i/ N adatom creates 8 states in total (2 N2s atomic and 6 N2p-Al bonding states) and contributes 7 1

4 electrons (5 from N adatom and 3 ×

3

4 electron from Al broken bond states), i.e.

3 4

electron deficiency, ii/ NO admolecule in H3 position creates 14 states in total (4 N2s-O2s bonding and antibonding states, 6 N2p-O2p bonding states and 4 N2p-O2p antibonding state - perpendicular) and contributes 1 3 13 4 electrons (5 from N atom, 6 from O atom and 3 × 4 electron from Al broken bond states), i.e.

3

4 electron deficiency,

ii/ NO admolecule in T4 position creates 12 states in total (4 N2s-O2s bonding and antibonding states, 6 N2p-O2p bonding states and 2 N2p-O2p antibonding states - parallel) and contributes 13 1

4 electrons (5 from N atom, 6 from O atom and 3 ×

3

4 electron from Al broken bond states),

1 i.e. 1 4 electron surplus, Based on this, the condition for filling all these states is: 1

1

1

74𝛼 + 134𝛽 + 134𝛾 = 8𝛼 + 14𝛽 + 12𝛾 5

(4)

1

Solving these equation we arrive at 𝛼 + 𝛽 = 24 and  = 8. Naturally the latter are summing as they both contribute the same deficiency of electrons. The dependence divides the state of the surface into three regions


31


5

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1

i/ 𝛼 + 𝛽 > 24 and  < 8 - the Fermi level is pinned by N and NO states in the bandgap. 5

1

ii/ at 𝛼 + 𝛽 = 24 and  = 8 - the Fermi level is free, above all N and NO states in the bandgap and below conduction band minimum or Al broken bond state 5

1

ii/ 𝛼 + 𝛽 < 24 and  > 8 - the Fermi level is pinned by conduction band or Al broken bond state. The verification of the dependence may be made using the above considered cases: 2xH3 1

1

1

configuration has 𝛼 = 𝛽 = 12 and  = 0 i.e. case (i), 2xT4 configuration has 𝛼 = 12, 𝛽 = 0 and  = 6, 1

i.e. case (iii), and H3&T4 configuratio has 𝛼 = 𝛽 = 12 and  = 0 i.e. case (i). As it is known from the simulation results, all are in full agreement with these predictions.

CONCLUSIONS AlN(0001) surface in contact with pure nitrogen and oxygen and mixed nitrogen/oxygen vapors was investigated. At clean AlN(0001) surface molecular nitrogen decomposes and is adsorbed at H3 positions without energy barrier with the adsorption energy ∆𝐸𝐷𝐹𝑇 𝑎𝑑𝑠 (𝑁2) = 6.0 𝑒𝑉. The adsorption at H3 site leads to deficit of

3

4 electron which is transferred from Al broken bond 1

state, constituting its considerable part. At critical 4𝑀𝐿 nitrogen coverage, the donating electron Al broken states are fully emptied, and no electron transfer is possible. Nevertheless, nitrogen adsorption is energetically favorable so at normal pressure of pure nitrogen vapor, AlN(0001) surface is covered by nitrogen adatoms. In contact with clean AlN(0001) surface molecular oxygen decomposes, and adsorbs atomically, without energy barrier, with the adsorption energy equal to ∆𝐸𝐷𝐹𝑇 𝑎𝑑𝑠 (𝑂2) = 14.8 𝑒𝑉, i.e. far exceeding that of molecular nitrogen. Thus the surface is readily covered by atomic nitrogen at normal pressures. Naturally low pressure of mixed oxygen/nitrogen vapor leads to essentially identical results. Atomic oxygen and nitrogen in H3 sites are preferable energetically at clean AlN(0001) surface.


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The NO admolecule has its energy ∆𝐸𝐷𝐹𝑇 𝑑𝑖𝑠 (𝑁𝑂) = 4.37 𝑒𝑉 higher than the separate O and N adatoms. Critical factor determining the behavior of oxygen at the AlN(0001) surface is the presence of relatively high nitrogen pressure. At normal pressure AlN(0001) surface is fully covered by atomic nitrogen that shifts the preferential reaction path of adsorption of molecular oxygen. Molecular oxygen in contact with the N-covered AlN(0001) surface decomposes and adsorbs without energy barrier. The resulting coverage of the oxygen adsorption is mixed NO coverage, at T4 and H3 positions. The energy of adsorption of energy ∆𝐸𝐷𝐹𝑇 𝑎𝑑𝑠 (𝑁2 ― 2𝑁𝑂) = 8.20 𝑒𝑉, with negligible energy barrier of 68 meV. Thus creation of NO molecules is highly effective for oxygen adsorption at AlN(0001) surface in N-rich conditions. NO admolecules may be located in H3 or T4 positions. The new bonding states are located in the valence band. The antibonding states are located in the bandgap. These two states are created for H3 position, sufficient to accommodate all electrons. The single state is created for T4 position, insufficient for the electrons donated, that leads to considerable increase of the energy. The mixed H3 and T4 coverage is optimal as H3 states accommodate surplus electrons from T4 configuration. The conditions for optimal coverage was derived using EECR. Synthesis of NO admolecules, attached to AlN(0001) surface is the first necessary stap to effective synthesis of nitric oxide for many industrial application, including fertilizes, explosives, etc. The necessary condition to effectively detach these admolecules requires nitrogen pressure and high temperature, which are beneficial for stability of AlN(0001) surface. Thus effective catalyst for NO industrial synthesis may be developed using aluminum nitride.

PATENT INFORMATION Based on the results in the present paper, the patent for use of AlN as catalysts for NO synthesis was submitted to the patent office.77

ACKNOWLEDGMENT.


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The research was partially supported by Polish National Science Centre grants number DEC2015/19/B/ST5/02136 and 2017/27/B/ST3/01899. This research was carried out with the support of the Interdisciplinary Centre for Mathematical and Computational Modelling at the University of Warsaw (ICM UW) under grant no G15-9.


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TOC


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Figure 7. Energy E of the pair of oxygen and nitrogen attached at AlN(0001) surface, in function of the interatomic N-O distance d 279x215mm (150 x 150 DPI)



Figure 9. Energy E, of the oxygen molecule approaching AlN(0001) surface fully covered by N adatoms, in function of the molecule-surface distance h. The distance is measured with respect to minimum energy configuration presented in Figure 10(c). The labels (a), (b) and (c) mark the con-figurations presented in Figure 8. 279x215mm (150 x 150 DPI)


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Catalytic Synthesis of Nitric Monoxide at AlN(0001) Surface: Ab Initio

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