on Pristine and Al-Hydroxylated Î±-Alumina Surfaces - ACS Publications

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Understanding the Fate of Insensitive Munitions Compounds – Computational Study on Adsorption of Nitroguanidine (NQ) and 1,1-Diamino-2,2Dinitroethylene (FOX7) on Pristine and Al-Hydroxylated #-Alumina Surfaces Manoj K. Shukla, Jing Wang, and Jennifer M. Seiter J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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The Journal of Physical Chemistry

Understanding the Fate of Insensitive Munitions Compounds – Computational Study on Adsorption of Nitroguanidine (NQ) and 1,1-Diamino-2,2-dinitroethylene (FOX7) on Pristine and Al-Hydroxylated α-Alumina Surfaces Manoj K. Shuklaa*, Jing Wangb and Jennifer Seitera a

US Army Engineer Research and Development Center, Environmental Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180, USA b HX5, LLC, Vicksburg, MS, 39180, USA.

Abstract Plane-wave Density Functional Theory (DFT) level of investigation was performed on adsorption of two insensitive munition compounds, nitroguanidine (NQ) and 1,1-diamino-2,2dinitroethylene (FOX7), on the (0001) surfaces of (4X4) Al-terminated α-alumina (α-Al2O3) and its Al-hydroxylated form. Surface hydroxylation was obtained by using 16 water molecules. Full geometry optimizations were performed using the vdW-DF2 van der Waals functional. The interaction of electron with core was accounted using the Vanderbilt ultrasoft pseudopotentials. The charge-density difference maps were performed to characterize the nature of munitions adsorption on alumina surfaces. It was revealed that both of the munition compounds will be adsorbed in parallel orientations on alumina surfaces. It was found that deformation energy correction would be required for the reliable interaction energy prediction on pristine alumina surface. On the pristine surface adsorption will be characterized by the covalent type of interaction while on the Al-hydroxylated surface hydrogen bonding interaction will take place. Compared to the 2,4,6-trinitrotoluene (TNT) the binding of NQ and FOX7 on alumina surface will significantly be stronger.

*Corresponding author; Phone: 601-634-5431; Email: [email protected]

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Introduction Nitroguanidine (NQ) and 1,1-Diamino-2,2-dinitroethylene (FOX7) represent nitroamine class of energetic compounds which are insensitive in nature1-3. Armed forces around the world are developing insensitive class of energetic compounds to eliminate the unintentional detonation of munitions due the inherent problem associated with traditional munition compounds. US armed forces are developing insensitive high energy explosive formulations such as Insensitive Munitions Explosive (IMX) and Picatinny Arsenal Explosive (PAX) series which may contain either insensitive munition compounds or combination of both the traditional and insensitive munition compounds4. For example, NQ is an important components of some of the IMX formulations4 while FOX7 is currently being produced by Eureco Bofors AB in Sweden2. The production, storage and applications of these compounds will increase the likelihood of their presence in the surrounding environments. Thus, it is essential to understand the fate of these compounds when present in the environments. Alumina is an important metal oxide material in the earth’s surface with significant insulating property and finds application in various field5-8. Although, alumina has several phases, the αalumina is the most stable phase and also called as corundum and exists naturally9,10. Moreover, the Al-terminated (0001) surface of α-alumina is found to be more stable than the oxygenterminated surface11,12. The Al-terminated (0001) surface of α-alumina causes water to dissociate on its surface i.e. water on alumina surface undergoes hydroxylation13,14. Such hydroxylation is due the strong acidic nature of surface Al-atoms which are significantly under coordinated compared to the interior Al-atoms15. In our earlier investigations on adsorption of 2,4,6trinitrotoluene (TNT) on pure and hydroxylated surfaces of Al-terminated (0001) surfaces of αalumina, it was revealed that though on both surfaces preferred orientation of TNT would be in


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parallel to the alumina surface, but the nature of interaction will change from covalent type in the pure surface to hydrogen bonding type on the hydroxylated surface16,17. On the other hand, the adsorption

of

5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one

(NTO),

an

insensitive

high

performance explosive material, on Al-terminated (0001) surface has been predicted to be dissociative due to the deprotonation of NTO; a proton from the NTO moves to the nearby oxygen site of the alumina surface18.

The present research is devoted to understand how two insensitive munition compounds NQ and FOX7 of nitroamine class are adsorbed on the metal oxide such as α-alumina surface; the effect of hydroxylation has also been investigated by the plane-wave theory using the van der Waals functional and ultrasoft pseudopotentials. It was found that due to the presence of water on Alterminated (0001) surface of α-alumina, the nature of adsorption is changed from the covalent on pristine surface to hydrogen bonding on the hydroxylated surface. Since, TNT is a legacy explosive compound which has been extensively used for both military and civilian applications, while, NQ and FOX7 are emerging compounds for use in insensitive munitions. Therefore, the similarity and differences with respect to TNT absorption are also discussed. Computational Details The three dimensional periodic boundary conditions under the generalized-gradient approximations (GGA) using the high accuracy van der Waals density functional (vdW-DF2) was applied in the present investigation19-22. The ultrasoft pseudopotentials available for the PBE functional were used to describe the electron-core interaction. The wave function cutoff of 25 Ry and the kinetic energy cutoff for charge density and potential as 250 Ry were used. The Alterminated (0001) surface of (4X4) α-alumina that was used in our earlier investigations6-18, was 3 ACS Paragon Plus Environment


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used in the present investigation also. We would like to mention that the computed lattice parameters of the hexagonal (4X4) unit cell of α-alumina using the vdW-DF2 van der Waals functional were found to be a = 19.0884 Å and c = 13.0258 Å and are used in the present investigation also.18 These parameters were found to be in an excellent agreement with the corresponding experimental parameters (a = 19.0408 Å and c = 12.9933 Å; here parameter “a” was obtained by multiplying 4 times the corresponding experimental value of (1X1) α-Al2O3.)23 Reason for the consideration of such a large size cell was to avoid interaction of adsorbate with its periodic image in the calculation; distance between the adsorbate with its periodic image in all studied complexes was found to be more than 13 Å. Moreover, enough vacuum space (over 20 Å) along the c-direction was maintained for surface relaxation and also to avoid any interaction of adsorbate with the bottom layer of the adsorbent in the periodic image. The binding energy or adsorption energy (∆Ead) energy of adsorbate on the alumina surfaces were computed using following equation: ∆Ead = - (Ecomplex – Eadsorbate – Eadsorbent) Where, Ecomplex represents the total energy of the complex and Eadsorbate and Eadsorbet represent the total energy of the adsorbate (NQ or FOX7) and adsorbent (pristine or Al-hydroxylated alumina), respectively, within the respective complex geometry. The deformation energies were computed as the difference between the total energies of the separately optimized monomer to that within the complex geometry. Full geometry optimizations of investigated system were performed and the plane-wave Density Functional Theory (DFT) based quantum espresso program package was used in the current investigation24. Due to the large size of the system, geometry optimizations were performed only at the gamma point.


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Results and Discussion The considered (4X4) super cell of α-Al2O3 has 18 layers stacked in the c-direction with AlOAl repetitive layers as discussed in our earlier publications16-18. We have shown that at the vdWDF2 level in the bulk α-alumina the Al and oxygen containing sequential layers are separated by 0.83 Å while Al-containing successive layers are separated by 0.51 Å. However, the surface relaxed geometry obtained by adding 20 Å of vacuum in the c-direction has shown significant inward relaxation limited to few layers from the top layer. Consequently, the distance between the top layer (Al-containing plane) and the layer below it (O-containing plane) is reduced to 24 Å. The Al-hydroxylated (4X4) α-alumina was obtained by using 16 water molecules16. It should be noted that in this structure all surface Al atoms were hydroxylated while only one third of the oxygen atoms from the top most oxygen layer (second layer from top) were protonated. In the surface relaxed pristine alumina, each of the surface Al atoms is tri-coordinated with oxygen atoms from the second layer with corresponding Al-O distance of about 1.696 Å. Therefore, it is not unexpected that Al-hydroxylation leads to asymmetric changes in the Al-O surface bond distances. In this case each of the hydroxylated Al atom is coordinated with two oxygen atom and another protonated oxygen from the second layer of pristine alumina. The Al-OH distance was computed to be 1.861 Å, while the other two Al-O distances were predicted to be 1.752 and 1.764 Å, respectively. Detailed information about the structure of Al-hydroxylated α-alumina can be found in our earlier publication16.



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NQ

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FOX7

Scheme 1. Optimized structures and atom labels used for NQ and FOX7. Adsorption on Al-terminated (0001) surface of α-alumina The labels for the NQ and FOX7 discussed in this study are displayed in Scheme 1. Two orientations (vertical and parallel) with respect to the (0001) plane of alumina were considered for NQ in the adsorption investigation. In the vertical orientation (Figure 1a), the approximate plane of adsorbate (NQ) is perpendicular to the surface with oxygen from the nitro group interacts directly with the surface Al atom at the atomic distance of 1.913 Å. The interaction pulls the coordinated Al atom by about 0.25 Å towards the NQ from the approximate plane containing the rest of Al atoms. On the other hand, two complexes of NQ in the parallel orientation with respect to the (0001) plane of alumina were investigated. In the first complex (I), an oxygen atom from the nitro group and a nitrogen atom from one of the amino groups interact directly with two separate Al atoms from the surface (Figure 1b). The Al…O and Al…N coordination distances (shown as bonds a and e, respectively in Figure 1b) were computed to be about 1.951 Å and 2.117 Å, respectively. Due to the direct interaction of the NQ to the surface Al atoms, the coordinated Al atoms are pulled by about 0.11 Å and 0.23 Å, respectively from the rest of the approximate plane. 6 ACS Paragon Plus Environment

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Moreover, the formation of three hydrogen bonds between surface oxygen atoms (from the second layer) and hydrogen atoms from adsorbate with distances ranging from 1.813 to 2.653 Å are also revealed in this complex (Figure 1b). In the second complex (II), the oxygen from the nitro group makes direct interaction with a surface Al atom with coordination distance of 1.882 Å as depicted by bond “a” in the Figure 1c. Further, both of amino groups interact with oxygen atoms from the second layer through four hydrogen bonds with distance ranging from 2.054 to 2.638 Å (Figure 1c). Moreover, as a consequent to the adsorption, the interacting surface Al atom is pulled by about 0.34 Å towards the adsorbate from the plane containing rest of Al atoms as shown in the Figure 1c. However, it is interesting to note that the relative orientation of the adsorbate has significant effect on the surface geometry. For example, in the second complex, the Al atom as identified in the Figure 1c is pushed down by about 0.76 Å below the Al surface leading to localized structural distortion. Such structural distortion is caused by proximity of amino hydrogen with this surface Al atom and importantly the presence of four hydrogen bonds due to the amino hydrogens and surface oxygen atoms hinders the lateral movement of the adsorbate on the surface. Moreover, similar to TNT adsorption17 an adsorption of NQ also does not appear to have noticeable effect on the inter-plane distance of alumina. Computed bond lengths of NQ in the isolated and within complex geometries are shown in the Table 1. Consequent to the adsorption significant geometrical change has been found in NQ and such geometrical changes are dependent upon the orientation of adsorbate in the complex. For example, in the complex with perpendicular orientation of NQ (Figure 1a) the N4-O1 bond length is increased by about 0.067 Å while N4-N3 and C-N2 bond lengths are decreased by about 0.063 and 0.038 Å respectively compared to the isolated NQ. Similarly, in the first complex of NQ in parallel orientation (Figure 1b), the N4-O1 7 ACS Paragon Plus Environment


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and C-N3 bond lengths are increased by about 0.116 and 0.051 Å, respectively and N4-N3 and C-N2 bonds are decreased by about 0.091 and 0.033 Å, respectively. In the second complex (Figure 1c), the N4-O1 and C-N1 bond lengths are increased by 0.072 and 0.087 Å, respectively while N4-N3 and C-N2 bond lengths are decreased by 0.046 and 0.057 Å, respectively as compared to that predicted in the isolated NQ. Thus, in all complexes, the N4-O1 bond length shows significant increase and N4-N3 bond length shows significant decrease compared to the isolated species. The computed binding energies are shown in the Table 2. Obviously, the computed binding energy of parallel complexes is approximately two times larger than that predicted for the vertically oriented complex. Significant increase in binding of parallel oriented NQ is due to the presence of several hydrogen bonds between amino hydrogen atoms and the oxygen sites from the alumina surface and the presence of additional coordination of amino nitrogen with surface Al atom in the first complex (Figure 1b). However, surprisingly the second complex of NQ shows larger binding energy than the first complex (Table 2). Due to the presence of Al…O and Al…N coordination, the first complex is expected to have larger binding energy than the second complex which has only Al…O coordination; although both complexes have several additional hydrogen bonds between amino hydrogens of NQ and oxygen atoms of alumina surface (Figure 1(b,c), Table 2). The presence of Al…N coordination should be the driving factor for the larger binding energy of the first complex than the second one. As discussed earlier that second complex showed inward displacement of an Al atom due to the complex formation. Therefore, we also computed the energy of deformation for adsorbate and the surface due to the complex formation and they are presented in the Table 2. It is evident from the data shown in the Table 2 that both the surface and adsorbate undergo deformation due to the complex formation. But, surface deformation 8 ACS Paragon Plus Environment

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energy in the second complex is almost two times larger than that was revealed in the first complex. Thus, the total deformation energies amounts to 23.9 and 41.9 kcal/mol for the first and second complex, respectively. The relatively larger deformation energy in the surface of the second complex originates from the inward displacement of surface Al-atom and resultant localized deformation due to the adsorption as discussed earlier. The deformation corrected interaction energies were predicted to be 45.6 and 36.6 kcal/mol for the first and second complex, respectively (Table 2). Thus, the first parallel complex of NQ on alumina surface is about 9 kcal/mol more stable than the second complex.

a. Vertical NQ adsorption on α-Al2O3 surface

b. Parallel NQ adsorption I on α-Al2O3 surface (1st complex)

c. Parallel NQ adsorption II on α-Al2O3 surface (2nd complex)

d. Parallel FOX7 adsorption on α-Al2O3 surface

Figure 1. Adsorptions of NQ and FOX7 on α-Al2O3 surface showing only the first layer of alumina. For complete structure, see Supporting Information. The labels of NQ and FOX 7 are the same as those in Scheme 1. 9 ACS Paragon Plus Environment


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Table 1. Bond lengths (in Å) of NQ and FOX7 in the isolated form (Scheme 1) and their various complex geometries. NQ C-N1 C-N2 C-N3 N3-N4 N4-O1 N4-O2

1.360 1.385 1.342 1.422 1.286 1.248 FOX7

C1-C2 C1-N1 C1-N2 C2-N3 C2-N4 N3-O1 N3-O2 N4-O3 N4-O4

1.431 1.359 1.358 1.453 1.451 1.289 1.251 1.251 1.290

α-Al2O3- α-Al2O3α-Al2O3NQ(┴) NQ(=I) NQ(=II) 1.361 1.346 1.447 1.347 1.352 1.328 1.367 1.393 1.353 1.359 1.331 1.376 1.353 1.402 1.358 1.232 1.236 1.227 α-Al2O3HydroxylatedFOX7 α-Al2O3-FOX7 1.445 1.434 1.332 1.360 1.432 1.356 1.431 1.427 1.446 1.464 1.353 1.295 1.231 1.262 1.300 1.261 1.261 1.273

Hydroxylated- αAl2O3-NQ(=) 1.362 1.371 1.356 1.409 1.294 1.247

Hydroxylatedα-Al2O3-NQ(┴) 1.348 1.375 1.356 1.377 1.281 1.276

Table 2. Computed binding energy (kcal/mol) and deformation energy (surface/adsorbate; kcal/mol) of NQ and FOX7 on pristine and Al-hydroxylated alumina surface in vertical (┴) and parallel (=) orientations. The corresponding deformation corrected binding energies are shown in the parentheses.

α-Al2O3 Alhydroxylated α-Al2O3

Binding Energy NQ ┴ ═ I 41.1 69.5 (24.7) (45.6) 22.2 22.2 (15.1) (17.0)

FOX7 II 78.5 (36.6)

83.4 (54.1) 25.1 (17.5)

Deformation Energy NQ FOX7 ┴ ═ I II 12.6/3.9 16.4/7.5 31.4/10.5 20.1/9.3 5.2/1.9


4.5/0.7

-

2.5/5.1

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Three different parallel adsorption orientations of FOX7 on the α-Al2O3 (0001) surface investigated by Sorescu et al.28 revealed the maximum coordination involving two surface Al atoms of alumina. Moreover, as expected, they have found that parallel orientation of FOX7 will form stronger complex than the vertical orientation with alumina surface. Therefore, in the present study, we only selected parallel orientation of FOX7 which was not investigated earlier (Figure 1d). The present adsorbed complex of FOX7 in parallel orientation on the alumina surface depicted a different adsorption which showed the coordination of N atom of an amino group and an oxygen atom from each of the nitro groups with the surface Al atoms (depicted with bonds f, a and e, respectively in Figure 1d). Moreover, both hydrogen atoms of the second amino group form hydrogen bonds with nearby oxygen atoms from the second layer of alumina (with distances of 2.543, 1.897 and 2.682, Å, depicted as bonds b, c, and d, respectively in Figure 1d). Thus, in addition to hydrogen bonds, FOX7 is found to be tri-coordinated with surface Al atoms. It is clear from the Figure 1d that the Al…O coordination distances are in the range of 1.966 – 2.032 Å, the Al…N coordination distance is predicted to be about 2.122 Å while one of the O..H hydrogen bonds is relatively stronger (1.897 Å) than the other two hydrogen bonds. Due to the FOX7 adsorption the coordinated Al atoms are pulled towards the adsorbates which are predicted to be in the range of 0.18 – 0.21 Å. Moreover, such pulling is larger for the O…Al coordination. Other sites of alumina including the inter layer spacing did not show significant change due to FOX7 adsorption. Since, both of the nitro and amino groups of FOX7 are involved in interaction with alumina surface consequent to adsorption, significant change in the geometry of adsorbate was revealed. The two interacting NO bond lengths (N4-O3 and N3-O1) were found to be increased by about 0.049 and 0.064 Å, respectively. And the interacting NC bond lengths (C1-N1 and C2-N3) were decreased by about 0.027 and 0.022 Å, respectively compared to the



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corresponding values in the isolated FOX7 (Table 1). The most significant change was found in the C1-N2 bond whose length was predicted to be increased by about 0.074 Å after the complexation (Table 1). The deformation energy for FOX7 due to the adsorption was predicted to be 9.3 kcal/mol (Table 2). Moreover, due to the coordination of three Al surface atoms and the presence of hydrogen bonds, the deformation energy for alumina surface was also predicted to be about 20 kcal/mol due to the FOX7 adsorption. Therefore, due to the presence of relatively larger number of coordination for adsorption of FOX7, it is not surprising that the computed binding energy of 83.4 kcal/mol (deformation corrected binding energy is 54.1 kcal/mol) is found to be larger than that predicted for NQ on the alumina surface. We would like to mention that the binding energy of TNT on α-alumina surface was predicted to be 44.7 kcal/mol at the same level of theory16. Thus, both the NQ and FOX7 would be adsorbed on alumina surface much stronger than TNT. Adsorption on Al-hydroxylated (0001) surface of alumina Both the vertical and parallel orientations of NQ and parallel orientation of FOX7 with respect to the (0001) surface of Al-hydroxylated alumina were considered. The optimized geometries of NQ adsorbed on Al-hydroxylated alumina are shown in the Figure 2a and 2b for vertical and parallel adsorptions, respectively while that of FOX7 is shown in the Figure 2c. The computed binding energies are shown in the Table 2. In the complex of NQ on Al-hydroxylated alumina surface in vertical orientation, one hydrogen bond (depicted as ‘b’ in Figure 2a) is observed between one oxygen atom from the nitro group with the protonated oxygen site from the second layer of alumina with a distance of 1.834 Å. This oxygen atom also forms a relatively weaker hydrogen bond with one hydroxyl group on the surface (labeled as c in Figure 2a) with distance of 2.494 Å. The other oxygen atom from the same nitro group interacts with one hydroxyl groups 12 ACS Paragon Plus Environment

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on the surface of the hydroxylated alumina through a hydrogen bonding (depicted as bond a in Figure 2a, with distance of 2.133 Å). On the other hand, in the complex with parallel orientation of NQ (Figure 2b), the adsorbate forms four hydrogen bonds. Two hydrogen bonds (a and b in Figure 2b) are formed between the two hydrogens from the one amine group of NQ and the two oxygens of the hydroxylated alumina surface with distances by 2.185 Å and 2.060 Å, respectively. A third hydrogen bond ‘d’ is observed between one oxygen of nitro group from NQ and one hydrogen from the hydroxylated alumina surface with distance of 2.116 Å. The fourth hydrogen bond ‘c’ is revealed between the nitrogen N3 from NQ and the hydrogen from one hydroxyl group attached to Al atoms in hydroxylated alumina with distance of 2.089 Å. Although the parallel orientation compound has one more hydrogen bond than that of the vertical orientation compound, one hydrogen bond in the latter compound reveal a stronger binding with a relatively shorter distance of 1.834 Å. Therefore, it is not surprising that computed binding energies of NQ on Al-hydroxylated surface in both orientations were predicted to be about 22.2 kcal/mol. On the other hand deformation corrected binding energies for vertical and parallel orientations were predicted to be 15.1 and 17.0 kcal/mol respectively. As expected, the deformation energies in the Al-hydroxylated complexes are significantly smaller than those obtained in the pristine alumina. Thus, taking into account of deformation energy, the parallel orientation of NQ would lead to the formation of slightly stronger complex than the vertical orientation on the Al-hydroxylated alumina surface. Moreover, it is also evident that the stronger hydrogen bond (bond distance 1.834 Å) formed between nitro group oxygen and hydroxyl group obtained by the protonation of oxygen of second layer of pristine alumina during hydroxylation is a main contributing factor for binding in vertical orientation.



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a. Vertical NQ adsorption on hydroxylated αAl2O3 surface

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b. Parallel NQ adsorption on hydroxylated α-Al2O3 surface

c. Parallel FOX7 adsorption on hydroxylated α-Al2O3 surface Figure 2. Adsorptions of NQ and FOX7 on Al-hydroxylated α-Al2O3 surface showing only the first layer of alumina. For complete structure, see Supporting Information. The optimized geometry of FOX7 on Al-hydroxylated alumina in parallel orientation show the existence of three hydrogen bonds formed between nitro and amino groups of adsorbate and hydroxyl groups attached to the surface Al atoms. The length of such hydrogen bonds are predicted to be in the range of 1.886 – 2.275 Å. The computed binding energy is predicted to be about 25.1 kcal/mol (deformation corrected binding energy 17.5 kcal/mol) which is over three times smaller than that predicted for pristine alumina surface. Thus, hydroxylation provides screening effect and NQ and FOX7 are weakly adsorbed on the hydroxylated alumina surface. However, compared to TNT adsorption on Al-hydroxylated alumina surface which was predicted


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to be about 8.3 kcal/mol,16 the FOX7 and NQ adsorption would be about three time stronger on such surface. Significant change among the geometrical parameters of neither the adsorbates nor the adsorbent (except some rotation of the hydroxyl groups involved in hydrogen bonding interaction with adsorbates) after adsorption on Al-hydroxylated surface compared to the respective isolated geometries were not revealed. Surface hydroxylation of oxide surfaces have also been found to influence catalytic activities,29,30 proton transport and wetting in graphene. 31,32

For example, Ganesh et al.29 by performing DFT level of theoretical calculation using the

PBE functional have shown that hydroxyls enhances the catalytic activity of gold clusters on rutile (TiO2) surface towards the oxidation of CO. In another investigation hydroxyls were found to increase CO oxidation activity of Au/ZrO2 catalysts. Thus, it is clear that surface hydroxylation significantly influence the bonding affinity of oxide surfaces. Charge density difference maps and the nature of adsorption The charge density difference maps were obtained by subtracting the charge densities of adsorbate and adsorbent from the charge density of the adsorbed complex and were generated using the VESTA program25. The charge density difference maps of NQ and FOX7 adsorbed on pristine and Al-hydroxylated α-Al2O3 surface in the parallel orientation are shown in the Figure 3. In these maps, the isosurface with yellow region shows the region of electron gain while that with green region shows the region of electron loss. For both of the NQ and FOX7 adsorbed on pristine alumina surface as shown in Figures 3a and 3b respectively, buildup of significant amount of electron density at the center of coordination region involving nitro group oxygen of adsorbates and the coordinated Al atom of surface are revealed. In the case of FOX7, such buildup of charge density in the coordination region involving amino nitrogen and the interacting Al atom of the surface is also found. Buildup of charge density at the center of bonding region is 15 ACS Paragon Plus Environment


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an indicative of covalent type of interaction26,27. However, we would also like to point out that in these complexes, buildup of small amount of positive charge on amino hydrogen and negative charge on oxygen which are involved in interaction are also revealed and such charges are in the orientation of the bonding direction. These type of charge build up are characteristics of hydrogen bond16. Thus, based upon the charge density different map, it is clear that there would be some hydrogen bond interaction also driving the adsorption of NQ and FOX7 on pristine alumina surface and the major role will be played by the covalent type of interaction in the complex formation. In the case of adsorption of NQ and FOX on Al-hydroxylated surface as shown in Figures 3c and 3d respectively, the charge density difference maps show buildup of small amount of positive charge on hydrogen sites while small amount of negative charge on complementary oxygen sites; where these sites (oxygen and hydrogen) are involved in interaction, showing that only hydrogen bond type of interactions would be involved in these complexes. Moreover, the existence of only hydrogen bonding type of interactions for the adsorption of NQ and FOX7 on Al-hydroxylated alumina surface is also consistent with the computed adsorption energies which were predicted to be few times smaller than predicted on pristine alumina surfaces.

(a)

(b)


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(c)

(d)

Figure 3. Isosurface depicting change in electron density after adsorption of (a) NQ and (b) FOX7, both in parallel orientation, on Al-terminated (0001) surface of (4X4) α-Al2O3 and (c) NQ and (d) FOX7 both in parallel orientation on Al-hydroxylated alumina surface using the vdW-DF2 functional. Yellow region shows isosurface for gain and green region shows the isosurface for the loss of charge density. Isosurface corresponds to 0.005 e/Å3 for isolated and 0.002 e/Å3 for Al-hydroxylated alumina surfaces.

Conclusions We investigated adsorption of NQ and FOX7 on Al-terminated (0001) surface of (4X4) α-Al2O3 and Al-hydroxylated (4X4) α-Al2O3 surface using vdW-DF2 van der Waals functional. The nature of interaction was characterized from the charge-density difference map. Our calculation predicted that both compounds will be strongly adsorbed on the pristine alumina surface and binding will be characterized and dominated by covalent type of interaction involving oxygen and amino nitrogen sites of adsorbates and complementary surface Al sites. On the Alhydroxylated alumina surface adsorption will be characterized by the hydrogen bonding interactions and subsequently the adsorption energy is significantly reduced compared to that on pristine surface. On both types of alumina surfaces adsorbates will be adsorbed in parallel orientation. Compared to the adsorption of TNT the NQ and FOX7 would form much stronger complex on alumina surface. Moreover, due the binding the surface Al atoms involved in direct coordination will be pulled towards the adsorbate in pristine alumina. Further, due to adsorption on pristine alumina surface significant geometrical changes were observed for adsorbates, but 17 ACS Paragon Plus Environment


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similar around of geometrical change was not revealed when adsorbed on Al-hydroxylated alumina surface.

Supporting Information: Figures S1 and S2 and completed list of refs. 20, 31 and 32 are given in the supporting information.

ACKNOWLEDGMENT The use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. The tests described and the resulting data presented herein, unless otherwise noted, were obtained from research conducted under the Environmental Quality Technology Program of the United States Army Corps of Engineers and the Environmental Security Technology Certification Program of the Department of Defense by the USAERDC. Permission was granted by the Chief of Engineers to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.


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