Adsorption of NO2 on YSZ (111) and Oxygen-Enriched YSZ (111

Article pubs.acs.org/JPCC

Adsorption of NO2 on YSZ(111) and Oxygen-Enriched YSZ(111) Surfaces M. Breedon,*,†,∥ M. J. S. Spencer,§ and N. Miura‡ †

CSIRO, Materials Science and Engineering, Clayton, 3168 Victoria, Australia Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan § School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia ‡

ABSTRACT: The adsorption of NO2 onto yttrium-stabilized zirconia (YSZ) surfaces is rarely considered a necessary concern when designing devices or structures which rely on YSZ either as a high-temperature mechanical support or in its role as an oxide ion conducting electrolyte. However, in this work we show that the presence or absence of a subsurface oxygen ion vacancy can play a significant role in not only the type of adsorption which can occur but also the reaction mechanism of NO2 on the surface at different temperatures. In this work the binding energy, vibrational frequencies, density of states, magnetic moments, electron localization, and charge transfer, as determined using density functional theory calculations, are presented for all stable NO2 adsorption configurations on YSZ(111) and oxygen-enriched YSZ+O(111) surfaces. Additionally, ab initio molecular dynamics simulations performed at 298 and 773 K show how high temperatures influence the adsorption and desorption of NO2 onto YSZ(111) and YSZ+O(111) surfaces, with respect to their application as an oxide ion conducting electrolyte in high-temperature electrochemical gas sensors. It was found that, on an oxygen-enriched YSZ+O(111) surface, NO2 bonded with a surface oxygen atom, which could yield an adsorbed NO2, nitrate, or cis-peroxynitrite (OONO) molecule. Adsorbed NO2 was found to desorb from the YSZ(111) and YSZ+O(111) surfaces at 298 K. The adsorbed nitrate or cis-peroxynitrite species, however, are more stable, remaining on the surface at 298 K up to a simulation time of 3 ps. At 773 K, either NO3 desorbs from the surface or the OONO dissociates into O2 and NO, which can also desorb from the surface. Hence, the presence of subsurface O results in the oxidation of NO2, returning the surface to YSZ(111). A further oxide ion could then migrate to the vacancy, allowing reaction with further NO2 molecules. This work is of particular interest to those developing solid-state electrochemical NOx sensors and for understanding the adsorption and reaction of NO2 with YSZ surfaces.

1. INTRODUCTION Considering the relentless tightening of both automotive and industrial emission standards, the need for high-performance gas sensors, capable of selectively monitoring pollutant gases, is of particular interest to both the research community and industry alike. Among the different sensor technologies available for NOx sensing, high-temperature yttria-stablized zirconia (YSZ)-based electrochemical gas sensors are among the most popular, with well-established applications in several different fields. YSZ is a high-temperature oxide ion conductor which is capable of withstanding the harsh thermal, mechanical, and chemical environments found in automotive exhausts. These characteristics have popularized the use of YSZ in several fields, such as high-temperature fuel cells,1 thermal barrier coatings,2 robust oxide ion conducting electrolytes,3 and hightemperature solid-state electrochemical gas sensors.4 Hightemperature YSZ-based electrochemical gas sensors are the backbone of the current-generation lambda sensors,5 which are used in conjunction with an automotive electronic control-unit to optimize combustion, reducing hazardous exhaust gas emissions. These oxygen sensors, which operate at temperatures on the order of several hundreds of degrees, have been © XXXX American Chemical Society

used for decades, establishing themselves as a reliable technological solution.6 While still in their infancy, when compared with other YSZ-based sensors, amperometric NOx sensors utilizing an oxide ion conducting zirconia electrolyte have appeared on the market, based on the initial products offered by NGK Ltd.7 However, due to its chemical inertness and excellent thermal and mechanical stability, it is often assumed that YSZ is largely unaffected by gases other than O2. Here we will examine the adsorption of NO2 onto YSZ(111) and YSZ+O(111) surfaces to better understand the surface reactions which occur, and whether or not these surfaces could play a significant role in contributing to the observed sensing signal in solid-state electrochemical NOx sensors. This work builds upon previous ab initio studies of YSZ(111)/YSZ+O(111) and Ni/YSZ(111) surfaces by Shishkin and Ziegler8,9 who examined the adsorption of methane, oxygen, and hydrogen onto Received: October 10, 2012 Revised: May 14, 2013

A

dx.doi.org/10.1021/jp310016r | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Figure 1. (a) Relaxed YSZ(111) surface model; (b) relaxed YSZ+O(111) surface model; (c) NO2 molecule and calculated parameters; (d, e) initial adsorbate positions on YSZ surfaces (for clarity, only the uppermost layer of atoms are visible); blue spheres represent N, red spheres O, teal spheres Zr, green spheres Y.

as a linear combination of plane waves and truncated to include those with energies less than 520 eV. 2.2. Surface Models. YSZ surfaces were modeled using the supercell approach, where periodic boundary conditions are applied to a central cell so that it is repeated periodically throughout three-dimensional space.16 The (111) surface was cleaved from the bulk cubic crystal structure of ZrO2 to form a [1 × 1] supercell with optimized lattice constants of a = b = c = 5.150 Å, which compares well to the experimental value of a = b = c = 5.135 Å (Inorganic Crystal Structure Database entry #89429). Yttrium doping was introduced into the surface model by substituting two Zr atoms with two Y atoms in the uppermost and second surface layers, as previously described by Shishkin and Ziegler,8,9 resulting in an Y-doping concentration of 9 mol %; and for surfaces containing an oxygen vacancy, a single oxygen atom was removed from the second surface layer (subsurface) to create a defect which occurs when a Zr4+ ion is replaced with an Y3+ ion. A vacuum spacer of 10 Å was introduced in the z-direction for all surfaces to prevent interactions between periodic images in this direction. Modeling of the adsorption process was achieved by placing an NO2 molecule ∼3 Å above a prerelaxed (35 or 36 atom) YSZ(111) or YSZ+O(111) surface model. Several initial orientations and adsorption sites were investigated, namely, atop O, Y, and Zr sites, as can be seen in Figure 1. The top surface layers, as well as the adsorbate atoms, were allowed to relax in the x-, y-, and z-directions, while keeping the bottom two surface layers fixed. The atomic positions were optimized until the total energy was converged to 10−4 eV, and the Hellmann−Feynman forces were less than 0.01 eV/Å. K-space sampling was performed using the Monkhorst and Pack scheme17 with a mesh of 3 × 3 × 1, as it has been previously shown to be more than sufficient to accurately represent the YSZ(111) surface.8 The isolated NO2 molecule was modeled in a 10 Å × 10 Å × 10 Å supercell using a Monkhorst and Pack mesh of 9 × 9 × 9 with Gaussian

YSZ(111) and oxygen-rich YSZ+O(111) surfaces, primarily for fuel cell applications. It is well-known that the oxygen ion transport mechanism in YSZ is a discrete hoping process3 in which oxygen ion vacancies (which occur in the YSZ lattice due the substitution of Zr4+ ions with Y3+) can be filled by oxide ions migrating through the crystal lattice of YSZ. These O2− ions originate from the electrochemical reaction of O2 at an electrode, or the dissociation products from other adsorbed species, including O2. These migrating oxide ions effectively disrupt the stoichiometry of the region around the oxygen vacancy site. From an oxide ion conducting perspective, the filling of an oxygen vacancy blocks the defect site from further oxide ion adsorption until the initial ion can migrate away from or deeper into the YSZ surface. Therefore, it is also pertinent to consider the possibility that a YSZ(111) surface can become an oxygenenriched YSZ surface, YSZ+O(111). In this study only the migration of O2‑ ions from a Pt/YSZ/air reference electrode to the YSZ surface will be considered, rather than the dissociation of O2 at the sensing electrode. In this work, the adsorption of NO2 onto a YSZ(111) surface containing a naturally occurring oxygen vacancy, and an oxygen-enriched YSZ(111)+O surface have been studied using density functional theory (DFT) calculations and ab initio MD simulations, with respect to general interest in its surface science, as well as potential insights into relevant gas sensing related phenomena.

2. METHOD 2.1. Computational Methodology. All calculations were performed using the Vienna Ab-initio Simulation Package (VASP),10,11 which performs fully self-consistent DFT calculations to solve Kohn−Sham equations.12 The exchange correlation functional of Perdew−Burke−Ernzerhof13 (PBE) was employed for all simulations, and the projector augmented wave scheme was used to account for electron−ion interactions.14,15 The electronic wave functions were expanded B



Article

Figure 2. Calculated minimum energy structures of NO2/YSZ(111).

YSZ(111). The relaxed NO2/YSZ(111) surface models and the calculated properties of these structures are presented in Figure 2. It should be noted that the distance indicators in Figure 2, as well as in all subsequent figures, are not necessarily to scale. The five unique NO2/YSZ(111) minimum energy structures (a−e) are presented in descending order of stability with binding energies of −0.20 to −0.05 eV. These low binding energies, in conjunction with the large adsorbate−surface distances, indicate that NO2 physisorbs onto the YSZ(111) surface. It is clear from Figure 2 that the interaction between the adsorbate and the surface for the two most stable structures occurs preferentially via the O atom of the NO2 molecule and the Y atom of the YSZ(111) surface. The adsorption process does not result in any significant surface reconstruction or relaxation, most likely due to the NO2 adsorbing directly above the Y atom at a distance between 1.75 and 2.58 Å, and the weak calculated binding energies. The adsorbate bond lengths and angle in structures a and b were little changed after adsorption, with the N−O bond length extending or contracting by at most 0.02 Å, and the bond angle changing by only a few degrees, compared with the free NO2 molecule. The NO2 molecule in the remaining weakly bound structures (c−e) did not undergo any appreciable lengthening or bond angle deviations when compared with the free NO2 molecule. The distance between the adsorbate oxygen atom and the closest surface yttrium atom ranged from 2.66 to 4.69 Å, which is considerably longer than the Y−O bond length found in bulk Y2O3 (2.26−2.35 Å). The binding energies of structures a−e, as well as their corresponding adsorption geometries, are all indicative of a physisorbed surface species. Due to the weak adsorption, we considered whether it would be favorable for NO2 to dissociate on the YSZ(111) surface, filling the vacancy site with an oxygen

smearing. Spin polarization was included for all the structures as well as the clean YSZ(111) and YSZ+O(111) surfaces. The binding energy (BE) was calculated as follows: BE = [Eads + surf − (Eads+Esurf )]

(1)

where Eads+surf is the total energy of the relaxed adsorbate− surface system (NO2 on YSZ(111) or YSZ+O(111) surfaces), Eads is the total energy of the free adsorbate molecule, and Esurf is the total energy of the relaxed YSZ(111) or YSZ+O(111) surface. Therefore, a negative binding energy is indicative of energetically favorable adsorption, whereas a positive binding energy is indicative of an unstable system. To classify the type of stationary points determined, vibrational frequency calculations were performed by permitting directional freedom to the adsorbate in the x-, y-, and z-directions, while fixing all of the surface atoms and diagonalizing a finite difference construction of the Hessian matrix with displacements of 0.015 Å, as implemented in VASP. Only minimum energy structures (i.e., those with only real vibrational frequencies) are reported here. The perpendicular height of the adsorbate molecule was calculated between the closest surface and adsorbate atoms, which in some instances were measured between atoms which have been incorporated into the adsorbate molecule during the adsorption process. Ab initio molecular dynamics calculations were also performed at 298 and 773 K, with a time step of 1 fs for at least 3 ps. A Verlet algorithm was used to integrate Newton’s equations of motion,18 and the temperature was controlled with a Nose thermostat.19

3. RESULTS AND DISCUSSION 3.1. Adsorption of NO2 onto YSZ(111) Surfaces. 3.1.1. Binding Energy and Adsorbate Geometry NO2/ C



Article

transfer being greater for the more strongly bound structures. It is interesting to note that, despite adsorbing to the Y atom (for structures a and b), there is negligible change in the partial Bader charges of these dopant atoms, which indicates that, irrespective of where NO2 adsorbs on the YSZ(111) surface, the charge that is donated to the surface is spread out over several atoms. 3.1.4. Electron Localization Functions of NO2/YSZ(111). The electron localization function (ELF) plot of the most stable NO2/YSZ(111) structure is presented in Figure 3(a).

atom and yielding an adsorbed NO molecule, according to the following reaction: NO2(g) + YSZ(111) → NO(ads) + YSZ + O(111)

(2)

Our calculations, however, reveal that the energy difference for this reaction is +0.07 eV, using the respective energies for each optimized component in reaction 2; which for the most stable adsorbed NO structure was with an N atom adsorbed over a surface Y atom. Further details about the interaction of NO with this surface will be presented in our future work. We also considered formation of NO/YSZ+O(111) via the dissociation of NO2 that is already adsorbed onto the YSZ(111) surface; this energy difference is also positive: +0.27 eV. The positive calculated value of these reactions indicates that at 0 K this reaction is unfavorable; however, any barrier to dissociation may be overcome if sufficient energy was supplied, which may be possible at elevated temperatures. We investigate this via ab initio molecular dynamics simulations in section 3.3 and find that NO2 dissociation is unfavorable for this system at elevated temperatures within our simulation time frame and parameters. 3.1.2. Vibrational Frequencies of NO2/YSZ(111). The calculated vibrational frequencies of NO2 adsorbed onto YSZ(111) are presented in Figure 2. The theoretically calculated vibrational frequency values for the NO2 molecule (1347, 1684, and 728 cm−1 for the symmetric stretch and the asymmetric stretch and bend, respectively) are in good agreement with experimental values (1318, 1618, and 750 cm−1).21 The vibrational frequency shifts within this paper shall be defined as being either hypsochromically (blue) shifted or bathochromically (red) shifted. The N−O symmetric stretch frequencies of the NO2/YSZ(111) structures were all calculated to bathochromically shift by 13−74 cm−1 when compared with the isolated NO2 molecule. It was found that the shift of the symmetric stretch of the more weakly adsorbed structures c and e was negligible, consistent with the weak adsorption. The asymmetric stretch values were also found to bathochromically shift, albeit by a smaller amount: 15−31 cm−1 for all NO2/ YSZ(111) structures, with the bending mode also calculated to be bathochromically shifted for all structures by 7−42 cm−1. These small changes are another clear indication that NO2 is physisorbed on this surface. Furthermore, it would be very difficult to distinguish the adsorption geometries from their IR spectra. 3.1.3. Bader Charge Analysis of NO2/YSZ(111). The calculated charge transfer (Δq) values between the NO2 and the YSZ(111) surface after adsorption are presented in Table 1.

Figure 3. ELF contour plots taken along the shortest adsorbate− surface bonds of (a) NO2/YSZ(111) and (a and c) NO2/YSZ +O(111). The dashed lines indicate the plane of the ELF slices (blue spheres represent Zn, green O, and red N). Insets of ELF plots are the adsorbate/immediate surface atom configurations provided for visual reference only.

This plot was obtained by slicing through the atoms with the shortest adsorbate−substrate distance (i.e., O on the NO2 molecule and Y on the surface). The direction of the slice is identified by the dashed black line in the corresponding topdown surface image. In these plots, regions of high electron localization (indicated in red) can be interpreted as bonding and nonbonding electron pairs.22 The red regions between the adsorbate atoms of the NO2 molecule highlight the covalent bonding between N and O. It is apparent in the ELF plot of Figure 3(a) that there is little interaction between NO2 and the surface atoms, consistent with the weak binding of these structures. 3.1.5. Magnetic Moments of NO2/YSZ(111). Adsorption of NO2 on the surface was found to induce minor surface magnetism. For all structures, the total magnetic moment was calculated to be ∼0.77 μB. Prior to adsorption, the spin on the NO2 molecule was calculated to be distributed with 35.4%

Table 1. Calculated Charge Transfer (Δq) of the NO2/ YSZ(111) Minimum Energy Structures NO2/YSZ(111) NO2 Δq

structure a

structure b

structure c

structure d

structure e

0.10

0.09

0.04

0.04

0.02

These Δq values are calculated by subtracting the relevant YSZ(111) and NO2 Bader charges from the corresponding values in the NO2/YSZ(111) structure. As can be seen, all NO2/YSZ(111) structures were calculated to have small positive Δq values, indicating a small donation of charge to the surface from the NO2 molecule. The amount of the charge transfer decreases with the binding energy, with the charge D



Article

located on the N atom and 32.3% on each of the O atoms, which is in good agreement with previous theoretical calculations.23 For structures a and b, approximately 88% of the spin remains on the NO2 molecule after adsorption, indicating that very little spin has been redistributed to the surface atoms. For structures a and b, this small surface magnetism is restricted to the p orbitals of the oxygen atoms in the two uppermost surface layers, with negligible contributions from the Zr and Y atoms. For structures c−e, the majority of the spin (at least 95%) remains on the NO2 molecule, consistent with the extremely weak interaction with the surface. 3.1.6. DOS of NO2/YSZ(111). While some DFT functionals are known to underestimate band gaps, the trends in band gap changes as well as information regarding the enhancement or addition of extra electronic states remain a useful method for understanding changes relating to conductivity and bonding. The partial density of states (PDOS) of both the clean YSZ(111) and YSZ+O(111) surfaces, presented in Figures 4

Figure 5. Orbital resolved PDOS of the clean YSZ+O(111) surface, and structures a and c of NO2/YSZ+O(111).

+O(111). The relaxed NO2/YSZ+O(111) structures and their calculated properties are presented in Figure 6. Six unique minimum energy structures (Figure 6) were found and are presented in order of decreasing stability. The NO2/YSZ+O(111) systems were calculated to have different binding energies, with the five most stable structures being chemisorbed on the surface (with BE values from −2.42 eV to -0.75 eV) and the least stable being physisorbed (with a BE of −0.15 eV). The adsorbate−surface interactions occurred between one or more adsorbate atoms (either O, N) and one or more surface atoms (O, Zr, Y). The most stable structures (a, b) were those where the N atom of the NO2 molecule bonded to a surface O atom and a Y or Zr surface atom. For structures c−e, which all have similar binding energies (−1.02 to −0.75 eV), adsorption occurs such that one or both of the adsorbate O atoms interacts with surface O and Y atoms. For structures c and e, the surface O atom relaxes outward on the surface, forming a bond with the adsorbate O atom that is 1.45 Å (a distance only 0.022 Å longer than the calculated bond distance of O2 (1.23 Å)). This interaction results in a small increase in the adsorbate N−O bond length of 0.08 Å. Structure d is similar to structure c; however, in structure d only the surface oxygen atom previously adjacent to the uppermost yttrium surface atom interacts with the NO2 molecule. It is noteworthy that despite the different adsorption configurations

Figure 4. Orbital resolved PDOS of the clean YSZ(111) surface, and the most stable NO2/YSZ(111) structure.

and 5, respectively, are similar to those described by Shiskin and Ziegler,8 showing a calculated band gap of ∼3 eV. The highest occupied level in each PDOS plot has been aligned to zero. The orbital resolved partial density of states (PDOS) of the most stable NO2/YSZ(111) structure is presented in Figure 4. The DOS are little changed after adsorption of NO2, and there are no new states within the band gap region, consistent with the weak binding of NO2 on this surface. There is, however, an increase in the intensity of states around 0 eV as well as at approximately −1.6, and −4.4 eV below the Fermi level (EF). Some of these states are due to the NO2 orbital contributions. As they do not all align with the states of the free molecule,24 however, the changes are consistent with some mixing of the NO2 and surface states, and the small calculated transfer of charge we calculated to occur from NO2 to the surface. 3.2. Adsorption of NO2 onto YSZ+O(111) Surfaces. 3.2.1. Binding Energy and Adsorbate Geometry NO2/YSZ E



Article

Figure 6. Calculated minimum energy structures of NO2/YSZ+O(111).

From the chemisorbed NO2/YSZ+O(111) structures, it is apparent that the surface may be trying to re-equilibrate itself to a YSZ(111) surface by forming an adsorbed NO3 molecule (which could desorb from the surface, as will be discussed in section 3.2). A similar observation with CO adsorbing onto

the Y−O bond lengths in structures c−e are all approximately 2.55 Å, which is longer than the bulk Y2O3 bond length, which varies between 2.26 and 2.35 Å,20 indicating a weaker bond than that found in Y2O3 (due to the interaction of the O atom with the N atom). F



Article

Table 2. Calculated Charge Transfer (Δq) of the NO2/YSZ+O(111) Minimum Energy Structures NO2/YSZ+O(111) NO2 Δq

structure a

structure b

structure c

structure d

structure e

structure f

0.06

0.06

0.15

0.13

0.14

0.05

oxygen-enriched (YSZ+O) has been made previously.25 In this study it was found that CO adsorbed and removed an O atom from the YSZ+O surface to form CO2, which then desorbed, returning the surface to YSZ. The existence of NO3 radical species has been well documented previously, showing it can exist in asymmetrical and symmetrical configurations; the symmetric N-centered molecule is known as the nitrate radical (NO3), whereas the less stable (by 13 kcal/mol)26 asymmetric isomer is called the peroxynitrite radical (OONO).27 It is clear from Figure 6 that structures a and b are consistent with formation of a surface bound nitrate species, whereas in structures c−e the adsorbate can be described as adopting a cis-peroxynitrite configuration. The cis-isomer has been described as a cyclic conformer, with weak bonding envisaged between the two proximal oxygen atoms.28 These adsorbate oxygen atoms in structures c and e have interacted with the Y dopant atom in the uppermost surface layer, forming bonds ∼2.55 Å in length. In structure d the distal oxygen atom of the peroxynitrite species is coordinated to both the surface Zr and Y atoms, which is a unique configuration among the data set. It is noteworthy that no stable trans-peroxynitrite species were observed on the YSZ +O(111) surfaces, despite previous ab initio Hartree−Fock calculations which predicted that both the free cis- and transOONO have approximately the same stability.26 We suggest the cis- may be favorable, as it allows a greater coordination between the adsorbate and surface atoms. Hence, the results presented in this study suggest OONO stereospecificity for this particular surface. 3.2.2. Vibrational Frequencies of NO2/YSZ+O(111). It was observed that the adsorption geometry had a significant effect on all vibrational frequency characteristics of the NO2/YSZ +O(111) structures. It should be noted that the surface oxygen atom which formed a surface bound NO3 molecule was not included in the vibrational frequency calculation. The symmetric stretches of the NO2/YSZ+O(111) structures were calculated to bathochromically shift by 40−466 cm−1, when compared with the free NO2 adsorbate molecule. As the two most stable structures (a and b) share a common adsorption configuration, where one of the adsorbate O atoms is not bonded to the surface, the symmetric stretch was bathochromically shifted by 130−150 cm−1. Similarly, as structures c−e are adsorbed in a similar orientation to each other, where one or both of the adsorbate O atoms bond to the surface, this results in significant bathochromic shifts of the symmetric stretch of up 466 cm−1. As would be expected, for the weakly bound structure f, the stretch was bathochromically shifted by only 40 cm−1. The asymmetric stretch shifts of the NO2/YSZ+O(111) structures were again all shifted bathochromically, with shifts ranging from 23 to 129 cm−1, with respect to the free molecule. Finally, the bending mode of the NO 2 /YSZ+O(111) structures was calculated to either hypsochromically or bathochromically shift, depending upon the adsorption geometry. Structures a and b, with their unrestrained O atom projected away from the surface, were the only two adsorption configurations which resulted in a hypsochromic bending shift of 106−129 cm−1. For the

remaining structures, c−f, the bend was calculated to bathochromically shift by 32−95 cm−1. Thus, the distinction between the adsorbed nitrate species and the NO2 and cisperoxynitrite species via spectroscopic methods should be easily achieved, as the bending modes shift in opposite directions on the YSZ+O(111) surface. Additionally, the magnitude of the symmetric stretch could be used to distinguish the three adsorption geometries. 3.2.3. Bader Charge Analysis of NO2/YSZ+O(111). The calculated charge transfer (Δq) values of the NO2/YSZ +O(111) structures after adsorption are presented in Table 2. The Δq values were also calculated to be positive and similar to the values of the NO2/YSZ(111) structures, indicating that there is a transfer of charge from the adsorbate to the surface. It is clear that the structures with similar adsorption geometry also have similar Δq values, such as structures a and b, which have an adsorbed nitrogen-centered nitrate radical molecule and corresponding Δq of 0.06e, and structures c−e, which all feature an adsorbed cis-peroxynitrite molecule and equivalent adsorption configurations with Δq values of ∼0.14e. It is interesting to note that the surface oxygen atom, which is incorporated to form the adsorbed NO3 molecule on the YSZ +O surface, loses charge, some ∼10 times greater in magnitude than the other upper layer oxygen atoms. This larger transfer may be synonymous with back-bonding from the surface O to the adsorbate. Together with a redistribution of charge within the adsorbate itself, this facilitates the formation of a covalent bond required to form an adsorbed NO3 molecule. We suggest such a redistribution of charge results in the small overall Δq values calculated for structures a and b. Hence, while the overall charge transfer is similar on both surfaces, the stronger binding on the YSZ+O surface is believed to be due to charge donation back to the adsorbed molecule. 3.2.4. Electron Localization Functions of NO2/YSZ+O(111). Electron localization function (ELF) plots of the NO2/YSZ +O(111) minimum energy structures a and c are presented in Figure 3(b, c). Due to the similarity between some of the adsorbed species, only the ELF plots of the most stable nitrate and cis-peroxynitrite molecules are presented. The physisorbed structure f displays similar characteristics to the NO2/YSZ(111) systems and, therefore, is also not shown. The high degree of electron localization between the N and surface O atoms (structure a) and between the O atom in NO2 and a surface O atom (structure c) is indicative of the formation of an NO3 molecule, be it either a nitrate species (structure a) or a cisperoxynitrite molecule (structure c). As can be seen in Figure 3(b, c) the bond between the NO2 and the closest oxygen surface atom is covalent in nature due to the high degree of electron localization and sharing of electron density, also confirming the formation of either an adsorbed nitrate molecule in structure a or a cis-peroxynitrite molecule in structure c. 3.2.5. Magnetic Moments of NO2/YSZ+O(111). Adsorption of NO2 on the YSZ+O(111) surface was also found to induce magnetism. The total magnetic moment for structures a−f was ∼0.82 μB; however, the distribution of the spin can be used to distinguish structures a−e from structure f. The adsorbed NO2 molecules in structures a−e were determined to transfer >99% G



Article

Figure 7. Ab initio molecular dynamics trajectory at 773 K of the adsorbate N atom, and the associated snapshots of the cis-peroxynitrite molecule in NO2/YSZ+O(111) structure c.

of their spin to the surface, almost exclusively to surface O and Zr atom p-orbitals located close to the adsorbate. It is interesting to note that there is insignificant spin on the surface O atom that forms part of the adsorbed NO3 species, keeping the net spin of the NO3 species at ∼0 μB. For structure f, the total spin on the adsorbed NO2 molecule is −0.7 μB (compared to 0.74 μB for the free NO2 molecule), which indicates that only 0.7% of the spin is transferred to the surface, consistent with the weak binding of this structure, but in contrast to adsorption on the YSZ(111) surface, the electrons are in the spin down state. 3.2.6. DOS of NO2/YSZ+O(111). The orbital resolved PDOS of the most stable NO2/YSZ+O(111) structures featuring a nitrate (structure a) and a cis-peroxynitrite (structure c) adsorbed species are presented in Figure 5. For both structures, changes to the PDOS are more significant than on the YSZ(111) surface, as expected due to the stronger interaction with the surface. Formation of either species on the surface results in an increase in states around the Fermi energy (EF); however, no new states are introduced in the midgap region, indicating that the electronic component of conductivity will remain unchanged. Changes deeper within the valence band, however, can be used to distinguish the two adsorption geometries. Specifically, formation of the adsorbed nitrate

species results in an increase in the states located 2−3 eV below EF, which is primarily due to mixing of the 2p orbitals on the NO2 atoms, and the surface p and (to a lesser extent) d orbitals. Formation of the adsorbed cis-peroxynitrite structure, in contrast, results in an increase in states primarily around −4 eV, which is also attributed to mixing of N 2p and O 2p orbitals with surface p (followed by d) orbitals. Hence, these changes are consistent with the formation of a bond between the NO2 molecule and the surface O atom. 3.3. Ab Initio Molecular Dynamics and Reactivity of Adsorbed NO2 Species. In order to examine the stability of adsorbed NO2 and possible removal of oxidized surface NO2 species on the YSZ(111) and YSZ+O(111) surfaces, ab initio molecular dynamics simulations were performed at 278 and 773 K. The operational temperature of an YSZ-based electrochemical gas sensor must be in excess of the minimum oxide ion conduction threshold of approximately 350 °C; otherwise, it will not be possible to measure a stable potential or current. Considering that the typical operational temperature for engine exhaust gas sensing is in excess of 450 °C, we suggest it is possible that the nitrate and cis-peroxynitrite molecules may split into their constituent NO and O2 components as per eq 3: H



Article

Figure 8. Visual summary of NO2/NO3 stability on the different YSZ surfaces investigated in this study; arrows indicate surface adsorption (down) or desorption (up).

For structure c at 773 K, a different surface reaction was observed. The key stages in the MD simulation are illustrated in Figure 7. After ∼1.5 ps of simulation time, the bond between the adsorbate and surface O atoms breaks (stage 2), followed by breaking of the N−O bond after 2.5 ps to yield a surface bound O atom and desorbed NO molecule (stage 3). This is followed by desorption of an O2 molecule after 3.5 ps (stage 4). While both species do not move far away, they diffuse across the surface, later readsorbing as a surface bound O2 and NO species (stages 5−8). For structures d and e, the MD simulations showed similar surface reactions. A summary of the reaction mechanism of NO2 with the YSZ(111) and YSZ +O(111) surfaces at the two temperatures simulated is presented in Figure 8; the black arrows indicate desorption or readsorption of species at 298 or 773 K. If YSZ is utilized as a mechanical support, in which oxide ions do not migrate through the surface, then the results presented here show that when NO2 adsorbs onto YSZ(111) in one of the five different unique adsorption configurations, then it will readily desorb at either 298 or 773 K, with a small donation of charge to the YSZ surface from the NO2 atoms. However, as YSZ is a poor electrical conductor, negligible measurable electrical changes would be observed experimentally. Thus, YSZ(111) is a relatively inactive surface toward NO2, from a materials science perspective. When considering the role that YSZ could play as a component in a high-temperature electrochemical gas sensor, the filling of an oxygen ion vacancy, either from the dissociation of atmospheric oxygen or from the migration of O2− through the YSZ, will significantly alter the surface reactivity. For the two surfaces examined here, the oxygen-enriched YSZ+O(111) surface exhibits a greater number of unique adsorption configurations for the NO2 molecule, with mostly chemisorbed configurations identified on the surface. At room temperature the adsorbed NO2 species is stable on the YSZ+O(111) surface in the majority of the orientations we found. Considering that O2− ion conduction occurs at higher temperatures, the surface reactivity is ultimately limited to the number of filled vacancies which will be rapidly “consumed” by NO2, effectively blocking the site for further adsorption. At higher temperatures, we show that the adsorbed species is

NO2(g) + YSZ + O(111) → NO3(ads) + YSZ(111) heat

⎯⎯⎯→ NO(g/ads) + O2(g/ads) + YSZ(111)

(3)

The height of the N atom (in the adsorbate molecule) above the surface was used as an indicator as to the behavior of the molecule at both simulated temperatures. The height was calculated with respect to the initial position of the N atom (shown as zero in Figure 7). For the structures we identified as being physisorbed (structures a−e on YSZ(111) and structure f on YSZ +O(111)), the MD simulations all showed that NO2 desorbed from the surface within 0.5 ps (3 ps for structure f) at 298 K. This finding confirms the weak binding of NO2 on the YSZ(111) surface. Increasing the temperature to 773 K, as expected, decreased the time required for NO2 to desorb from the YSZ(111) surface. On the YSZ+O(111) surface, significantly different behavior was observed for the more strongly adsorbed species. At 298 K, adsorbed NO2 is stable on the surface and does not desorb, during the simulation time of ≥3.0 ps. The adsorbed NO2 (structure a) was calculated to remain adsorbed on the atop Zr site, with the furthest deviation from its original position being 0.25 Å (which is within the expected vibrations of the atoms involved). Similarly, the adsorbed NO2 in structure c was also found to remain adsorbed on the surface during the simulation, remaining over its initial atop O site. The displacement of the N atom along the z-axis did not exceed 0.3 Å, during the 3 ps simulation time. These results indicate that the adsorbed nitrate and the cis-peroxynitrite species are relatively stable on the surface at this temperature. Increasing the temperature to 773 K showed that the adsorbed NO3 species in structure a was stable on the surface for approximately 4 ps, after which it diffused across the surface, from a surface Zr atop site to a surface O atop site, where it then desorbed as an intact nitrate species after reaching the uppermost surface Y atom; structure b also desorbed in a similar manner. I



Article

NO2 onto YSZ(111), the majority of the spin remains on the adsorbate atoms, consistent with the weak bonding. For all structures, NO2 was found to donate charge to the surface. The calculated density of states revealed that, for both the YSZ(111) and YSZ+O(111) surfaces, adsorption of NO2 did not induce any midgap states. The absence of such states suggests that the electrical component of conductivity should remain unchanged after adsorption. Ab initio molecular dynamics simulations of the NO2/ YSZ(111) minimum energy structures at 298 and 773 K showed that NO2 readily desorbs from the surface. This is in good agreement with the binding energy values, which indicated NO2 is only physisorbed on the surface. With the exception of the physisorbed structure we found for NO2/YSZ +O(111), NO2 was stable on the YSZ+O(111) surface at 298 K within our simulation time of at least 3 ps. At 773 K the adsorbed NO2 and nitrate molecule were both found to desorb intact from the surface, while the cis-peroxynitrite molecule dissociated into NO and O2, which either both remained close to the surface or desorbed. These results indicate that the oxide ion conducting YSZ surface can participate in surface bound chemical reactions with NO2. The presence of the extra surface O atom facilitates oxidation of surface adsorbed NO2, which can result in desorption of either NO3 or O2 and NO. Future work will focus on the adsorption of NO onto YSZ(111) and YSZ+O(111) to better understand NOx adsorption on these surfaces.

readily desorbed from the surface, with the mechanism differing depending on its adsorbed configuration, as depicted in Figure 8. For an adsorbed nitrate molecule, it desorbs intact, returning the surface to YSZ(111), permitting another oxide ion to migrate into the vacancy, restoring the surface so that the oxidation of another NO2 molecule can occur. This is similar to the oxidation of other molecules (such as H2, CH4) that has been demonstrated previously on this surface.8 For the adsorbed cis-peroxynitrite molecule, it can follow either of two pathways: one where the adsorbed ONOO dissociates into NO and OO, which, while not being strongly adsorbed, remain in association with the surface; or the other, where the ONOO dissociates into NO and O2 molecules which desorb from the surface. Either process results in returning the YSZ +O(111) surface YSZ(111) and again allowing another O atom to migrate to the vacancy site, for further oxidation of adsorbed NO2. Thus, at 773 K, NO2 adsorption on the YSZ+O(111) surface can result in either adsorbed NO and O2; or desorbed NO, O2, NO2, or NO3 (nitrate) species. These results indicate that the absence of an oxygen vacancy in YSZ(111) results in a surface which can participate in surface reactions that may disrupt electrochemical reactions occurring at the triple phase boundary (sensing electrode/YSZ/gas phase) in potentiometric sensors; such as the conversion of NO2 into NO3, O2, and NO. We note that the YSZ used in high-temperature electrochemical gas sensors is typically employed as a low surface area sheet/ tube, so the comparatively voluminous sensing electrode structure is likely to dominate the “gas-phase reaction” of adsorbate molecules. However, our finding should be considered in the development of high-temperature electrochemical NO2 sensors, as we have shown that the oxygen enriched YSZ is highly active toward NO2 species and could alter the response of the sensor. Additionally, further work into understanding the behavior of the complementary NO molecule is required to fully understand the complex equilibrium reactions which can occur at the triple phase boundary (sensing electrode/YSZ/gas phase), especially at elevated temperatures.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. ∥ M.B.: Work conducted during previous position at Kyushu University.

■

ACKNOWLEDGMENTS This research was partially supported by a Grant-in-Aid for Scientific Research (B) (No. 22350095) and JSPS fellows (No. 22-0353). Computations were mainly carried out using the computer facilities at the Research Institution for Information Technology, Kyushu University. The National Computational Infrastructure (NCI) National Facility is also acknowledged for providing computational facilities under the Merit Allocation Grant Scheme.

4. CONCLUSIONS Several unique adsorption configurations of NO2 adsorbed on the YSZ(111) and YSZ+O(111) surfaces were identified. Five different NO2/YSZ(111) minimum energy structures were determined, all showing weak binding energies, indicating physisorption of NO2 on the surface. Six unique adsorption configurations were found for NO2 adsorbed on the oxygen rich YSZ+O(111) surface, with several chemisorbed structures. When exposed to NO2, the YSZ+O(111) surface donates the excess oxygen atom in the crystal lattice to form an adsorbed NO3 species, either as an adsorbed nitrate (NO3) or a cisperoxynitrite (ONOO) species. This oxidation of NO2 rebalances the stoichiometry of the surface to yield the YSZ(111) surface with a naturally occurring subsurface oxygen defect. The differences in the calculated vibrational frequency values could be used to distinguish different adsorption geometries of NO2 on the surface and, in particular, whether it forms a surface nitrate species, a cis-peroxynitrite species, or an adsorbed NO2 molecule. When NO2 adsorbs onto YSZ+O(111), it will induce a magnetic moment caused by spin redistribution within the uppermost surface layer atoms. Conversely, after adsorption of

■

REFERENCES

(1) Gorski, A.; Yurkiv, V.; Starukhin, D.; Volpp, H.-R. H2O Chemisorption and H2 Oxidation on Yttria-stabilized Zirconia: Density Functional Theory and Temperature-programmed Desorption Studies. Proc. 2010 Eur. Solid Oxide Fuel Cell Forum 2011, 196, 7188− 7194. (2) Chen, L. B. Yttria-stabilized Zirconia Thermal Barrier Coatings A Review. Surf. Rev. Lett. 2006, 13, 535−544. (3) Pramananda Perumal, T.; Sridhar, V.; Murthy, K. P. N.; Easwarakumar, K. S.; Ramasamy, S. Molecular Dynamics Simulations of Oxygen Ion Diffusion in Yttria-stabilized Zirconia. Physica A 2002, 309, 35−44. (4) Miura, N.; Plashnitsa, V. V.; Ueda, T., Wama, R.; Utiyama, M. Solid-State Electrochemical Gas Sensing. In Solid State Gas Sensing; Springer: 2009. (5) Riegel, J.; Neumann, H.; Wiedenmann, H. M. Exhaust Gas Sensors for Automotive Emission Control. Solid State Ionics 2002, 152−153, 783−800. J



Article

(6) Visser, J. H.; Soltis, R. E. Automotive Exhaust Gas Sensing Systems. IEEE Trans. Instrum. Meas. 2001, 50, 1543−1550. (7) Kato, N.; Kokune, N.; Lemire, B.; Walde, T. Long Term Stable NOx Sensor with Integrated In-Connector Control Electronics. SAE Tech. Pap. 1999, 1999-01-0202−. (8) Shishkin, M.; Ziegler, T. The Oxidation of H2 and CH4 on an Oxygen-Enriched Yttria-Stabilized Zirconia Surface: A Theoretical Study Based on Density Functional Theory. J. Phys. Chem. C 2008, 112, 19662−19669. (9) Shishkin, M.; Ziegler, T. Oxidation of H2, CH4, and CO Molecules at the Interface between Nickel and Yttria-Stabilized Zirconia: A Theoretical Study Based on DFT. J. Phys. Chem. C 2009, 113, 21667−21678. (10) Kresse, G.; Furthmuller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−60. (11) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (12) Kohn, W.; Sham, L. Self-Consistent Equations Including Exchange and Correlation Effects. J. Phys. Rev. 1965, 140, 1133−1138. (13) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (14) Blochl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, 17953−17979. (15) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (16) Sholl, D.; Steckel, J. A. Density Functional Theory: A Practical Introduction; Wiley: 2009. (17) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (18) Kresse, G.; Marsman, M.; Furthmüller, J. VASP the Guide; Universität Wien: Wien, Austria, 2012. (19) Nose, S. Constant Temperature Molecular Dynamics Methods. Prog. Theor. Phys. Suppl. 1991, 103, 1−46. (20) Paton, M. G.; Maslen, E. N. A Refinement of the Crystal Structure of Yttria. Acta Crystallogr. 1965, 19, 307−310. (21) Fundamental Vibrational Frequencies of Small Molecules. In Handbook of Chemistry and Physics, 91st ed.; Haynes, W. M., Ed.; CRC Press: 2010. (22) Terriberry, T. B.; Cox, D. F.; Bowman, D. A. A Tool for the Interactive 3D Visualization of Electronic Structure in Molecules and Solids. Comp. Chem. 2002, 26, 313−319. (23) Breedon, M.; Spencer, M. J. S. Yarovsky, Adsorption of NO2 on Oxygen Deficient ZnO(2110) for Gas Sensing Applications: A DFT Study. I. J. Phys. Chem. C 2010, 114, 16603−16610. (24) Eid, K. M.; Ammar, H. Y. A Density Functional Study of NO2 Adsorption on Perfect and Defective MgO (100) and Li/MgO (100) Surfaces. Appl. Surf. Sci. 2012, 258, 7689−7698. (25) Yurkiv, V.; Gorski, A.; Bessler, W. G.; Volpp, H.-R. Density Functional Theory Study of Heterogeneous CO Oxidation Over an Oxygen-enriched Yttria-stabilized. Chem. Phys. Lett. 2012, 543, 213− 217. (26) Morris, V. R.; Bhatia, S. C.; Hall, J. H. Ab Initio Self-consistent Field Study of the Vibrational Spectra for Nitrate Radical Geometric Isomers. J. Phys. Chem. 1990, 94, 7414−7418. (27) Wayne, R. P.; Barnes, I.; Biggs, P.; Burrows, J. P.; Canosa-Mas, C. E.; Hjorth, J.; Le Bras, G.; Moortgat, G. K.; Perner, D.; Poulet.; et al. The Nitrate Radical: Physics, Chemistry, and the Atmosphere. Atmos. Environ., Part A 1991, 25, 1−203. (28) Symons, M. C. R. Cis- and Trans-Conformations for Peroxynitrite Anions. J. Inorg. Biochem. 2000, 78, 299−301.

K


Adsorption of NO2 on YSZ (111) and Oxygen-Enriched YSZ (111

Recommend Documents