DFT Study of 17O NMR Spectroscopy Applied to Zirconia Surfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A DFT Study of 17O NMR Spectroscopy Applied to Zirconia Surfaces and Nanoparticles Farahnaz Maleki, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06162 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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The Journal of Physical Chemistry

A DFT Study of 17O NMR Spectroscopy Applied to Zirconia Surfaces and Nanoparticles Farahnaz Maleki and Gianfranco Pacchioni* Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy

Abstract Solid state

17O

NMR is emerging as a new tool to characterize the nature of active sites on the

surface of oxide materials. In particular, the identification and quantification of low-coordinated sites can provide useful information to assess the chemical properties and the chemical reactivity of oxide nanostructures. In this modeling study we have considered regular and stepped surfaces of tetragonal ZrO2 as well as zirconia nanoparticles, either stoichiometric or oxygen-deficient. To this end, we have performed DFT calculations and determined the

17O

chemical shift and the

quadrupolar coupling constants of the various surface sites. The results show that the 17O chemical shift can clearly distinguish the O2C, O3C and O4C sites on the surface, steps, or bulk regions of zirconia. Since oxide surfaces exposed to atmospheric ambient conditions reacts with water and carbon-dioxide, we have also considered the adsorption properties of the t-ZrO2 (101) surface towards these molecular species, and the corresponding 17O and 13C chemical shifts.

*Corresponding author: [email protected]

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1. Introduction Solid-state nuclear magnetic resonance (NMR) is one of the most powerful analytical techniques available today for the structural characterization and the detailed investigations of a wide range of solid materials. The existence of several NMR active nuclei allows for deep studies to be carried out.

17O

NMR is an emerging area that adds to other experimental techniques for the

characterization of oxide materials and of their surfaces.1,2

17O

is a low-abundance isotope

(0.037%), and its absolute receptivity is also low. Nevertheless,

17O

(spin 5/2) is a nucleus with a

moderate quadrupole moment and a large chemical shift range of more than 1000 ppm, making it a sensitive structural probe.3 This characterization method is well established for several classes of materials, as it gives access to precise structural information.4,5,6 For instance,

17O

NMR

spectroscopy has been used as structural characterization technique for regular oxides and nanooxides,7,8,9 zeolites,10,11 metal organic frameworks (MOFs).12,13 Also,

17O

NMR is used to

study catalytic processes,14,15 Brønsted acid sites,16 and acid/basic properties.17 Recently, density functional theory (DFT) calculations of

17O

NMR properties, in combination with experimental

measurements, have shown that quite reliable results can be obtained from this computational technique, thus providing a solid basis for spectral assignments.18,19 ZrO2 is a material with broad technological applications such as fuel cell electrolytes,20 active catalysts,21 or catalysts supports, as a chromatographic support,22 and solid electrolyte.23 However, the properties of zirconia depend on the different structural polymorphs, and also on nanostructuring.24, 25, 26 Zirconia has at least five stable polymorphs.27,28 Because of brittleness of the structure when cooling down from the tetragonal phase, the most stable monoclinic polymorph has few practical applications.29 The most stable facet of tetragonal zirconia, (101), is the same as the most stable cubic zirconia (111) surface.30 Samples made up of nanocrystallized ZrO2 are probably complex mixtures of monoclinic, tetragonal and amorphous regions, with the possible presence of some cubic and orthorhombic components.31 Thus, obtaining a thorough understanding of metal oxide nanoparticles and nanostructures by applying a characterization methodology that can provide detailed info on the structural nature of these particles is challenging but also quite relevant. With the developments in high external magnetic field strength and fast magic-angle spinning (MAS) techniques,32 17O solid-state NMR spectroscopy became an interesting method for obtaining detailed information on the structure of oxide surfaces, subsurfaces and nanostructures. Theoretical and experimental studies have been reported on various structures of zirconia by

17O

NMR.9,33,34,35,36 However, an extension to zirconia nanostructures is lacking. In this study, the oxygen anions at different positions and with different coordination in ZrO2 surfaces and nanostructures will be characterized according to their 17O NMR chemical shifts 2 ACS Paragon Plus Environment

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with the help of DFT calculations. Recently, we have reported the structure, stability, reactivity, electronic and magnetic properties of a series of zirconia nanoparticles of size ranging between 1.5 and 2 nm, (ZrO2±x)n with n = 13 to n = 85.37,38,39,40 In this paper, the regular tetragonal ZrO2 (101) surface will be compared with stoichiometric and non-stoichiometric (oxygen deficient) ZrO2 nanostructures. Low-coordinated O sites at step edges of zirconia surfaces will also be studied. Then, the interaction of the regular t-ZrO2 (101) surface with H2O and CO2 molecules will be discussed in terms of 17O NMR chemical shifts. 2. Computational details All the calculations were performed by means of the Vienna Ab-initio Simulation Package (VASP 5.4.4)41,42,43 where plane waves were used as basis set with a kinetic energy cutoff of 400 eV. To describe the effect of the core electrons, the projector-augmented wave (PAW) method44,45 was used where H (1s), C (2s, 2p), O (2s, 2p) and Zr (4s, 5s, 4p, 4d) were considered as valence electrons and therefore treated explicitly. The PBE functional was employed to compute the exchange-correlation energy.46 For the purpose of partially correcting the self-interaction error, the PBE+U47,48 approach was adopted. Here, we set the Hubbard parameter to U-J = 4 eV for d-states of Zr.37 The D3 dispersion energy was included by means of the Becke-Johnson damping.49, 50 The lattice parameters of bulk tetragonal zirconia were fully optimized with a kinetic energy cut-off of 600 eV and a 8 × 8 × 8 Monkhorst−Pack k-point grid; the resulting parameters were fixed in all slab calculations. For the (101) surface, a 3 × 2 supercell with 5 layers of Zr and 10 layers of O (5 ZrO2 trilayers) was used (Zr60O120). For the stepped surfaces, on the basis of the previous work,51 we adopted 1 × 1 supercells of t-ZrO2 (134), (145) and (4 3) surfaces with 3 layers of Zr and 6 layers of O (3 ZrO2 trilayers); the corresponding formulas are Zr24O48, Zr30O60 and Zr24O48, respectively. The t-ZrO2 (101) surfaces only contain 7-fold coordinated Zr atoms (Zr7c) and O3c atoms. The edge of the t-ZrO2 (134) and (145) surfaces contain Zr6c and O3c atoms. The edge of tZrO2 (4 3) is made up of even lower coordinated Zr5c and O2c atoms. For all cases, a vacuum region (>10 Å) is present to remove the slab−slab interactions. For nanostructured zirconia, the stoichiometric nanoparticles Zr16O32 and Zr40O80, and the oxygen deficient Zr19O32 and Zr44O80 nanoparticles were considered.37 Clusters and nanoparticles have properties that can scale linearly with size (scalable regime) or that differ case by case in a non predictable way (non-scalable regime).52 The particles considered in this study, of average diameter of about 1 nm, belong to the second category, as reported in ref. 37. The size of the nanoparticles considered is about one order of magnitude smaller than those experimentally studied so far. On the



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other hand, it is precisely in the non-scalable regime that interesting and uncommon properties of nanoscale materials appear. For all models, we performed structural relaxations of all atoms with convergence criteria of 10−5 eV and 10−2 eV/Å for the electronic and ionic loops, respectively. The sampling of the reciprocal space is set to the Γ-point for t-ZrO2 (101) surfaces and zirconia nanoparticles: this choice still ensures reasonable accuracy, given the large size of the adopted supercells. For t-ZrO2 (134) and (4 3) stepped surfaces and t-ZrO2 (145) surface, we used 3 × 2 × 1 and 2 × 1 × 1 k-point meshes, respectively. The calculated chemical shielding value, , describes the level of magnetic shielding at a nuclear centre in a molecule or a solid with respect to the bare nucleus, and is related to an experimentally observable chemical shift,  = (ref - )/(1 - ref), where ref is the shielding value associated with a nucleus for a specific reference sample. The isotropic chemical shift (δiso) can then be computed as δiso = ref – cal, where cal is the chemical shielding obtained in VASP and ref is the reference for chemical shift. For 13C NMR, on the basis of experimental reports,53 we consider tetramethylsilane (TMS) as the reference with 178 ppm chemical shielding calculated by VASP. For

17O

NMR, on the basis of previous reports,7,8,9 considering that the bulk oxygen anions have

more regular arrangements than the oxygen anions at the surface, this constant value can be used to determine the reference chemical shift (ref): ref = δiso (bulk (Exp)) – m × calc (bulk(Calc))

(1)

Where δiso (bulk (Exp)) is the experimental chemical shift of bulk ZrO2 (378 ppm),9 calc (bulk(Calc)) refers to the chemical shielding of oxygen atoms obtained in VASP that have constant value in the inner layers (-414 ppm for t-ZrO2 (101) surface, see Table S1) and m is a gradient assumed to be equal to −1.7,8,9 Thus, ref can be determined as 36 ppm, and this reference was used for all the calculations of 17O NMR chemical shifts in this study. For the electric field gradient (EFG) calculations to obtain the quadrupolar parameters, quadrupole coupling constant (CQ) and asymmetry parameter (η), we used the experimental quadrupole moment (Q) of −0.02558 barns.54 The center of gravity, δCG, which takes into account both the chemical shift and quadrupolar induced shift, is calculated for a magnetic field of 9.4 T

9

according to Lippmaa method.55 In cubic oxides the high symmetry of the oxygen sites ensures that both shielding anisotropy and quadruple coupling vanish for

17O

except in vicinity of point or

extended defects.56 As the difference between the chemical shift and the center of gravity is very 4 ACS Paragon Plus Environment

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small in bulk and surfaces of zirconia, in the paper we only report the chemical shift, while all the details of quadrupolar coupling can be found in the Supporting Information. The adsorption energies were calculated according to the following equation: EADS = E(x/ZrO2) − E(x)(g)− E(ZrO2)

(2)

where x is H2O or CO2. 3. Results and discussion 3.1. Regular t-ZrO2 (101) surface On the t-ZrO2 (101) surface there is only one kind of oxygen, the 3-coordinated (O3c) oxygen anion; in the sub-surface and in the bulk the oxygen anions are 4-coordinated (O4c). Figures S1 and S2 show the optimized structure and density of states (DOS) of the t-ZrO2 (101) surface, respectively. The results of 17O NMR chemical shift (δiso) of the t-ZrO2 (101) surface are summarized in Fig. 1 and Table 1 (see also Table S1 for a detailed list of NMR and EFG parameters). The calculated chemical shifts (δiso) of the surface O3c sites is 454 ppm (small oscillations between 451−456 ppm are observed, due to slight in homogeneities in the positions of the surface O3c atoms). This is significantly different from the chemical shift of O4c sites in the bulk, Fig. 1 and Table 1. Here values in the range 370−385 ppm are found, with an average of 376 ppm. So, the change in coordination from O3c to O4c results in a sizable shift of nearly 80 ppm. We also notice that going from the inner “bulk like” layer to sub-surface layers a change of 8 ppm is found, Fig. 1, showing that O atoms near the surface experience a slightly different electric field than bulk atoms. These results are consistent with recent experimental measurements and previous DFT calculations done on the very same system, see Table 1.9 This provides a validation of the computational set-up used, and allows us to extend the analysis to other ZrO2 structures. Table 1.17O NMR average chemical shift (δisoav, ppm) of surface and bulk oxygen atoms of the t-ZrO2 (101) slab.

Layers

previous work9

this work δisoav layers δisoav Bulk

1 (O3C)

454

2 (O4C)

383

3 (O4C)

371

4 (O4C)

377

5 (O4C)

375

6 (O4C)

375

δisoav calc

δiso Exp

-

447

440

376

375 378



7 (O4C)

377

8 (O4C)

371

9 (O4C)

383

10 (O3C)

454

-

447

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440

Fig 1. 17O NMR average chemical shift (δisoav, ppm) of oxygen atoms in various layers of a t-ZrO2 (101) slab.

3.2. Stepped t-ZrO2 surfaces 3.2.1. t-ZrO2 (134) surface In this section we consider the

17O

NMR properties of oxygen atoms at the stepped t-ZrO2 (134)

surface. This surface is shown in Fig. 2, together with the values of the 17O chemical shifts, see also Table 2 (further details in Table S2). As for the regular t-ZrO2 (101) surface, also the stepped surface presents only O3c ions, but these atoms feel a different electrostatic potential depending on the position, due to the presence of a step, resulting in different chemical shifts, Fig. 2. On the other hand, all the other O atoms in the inner layers (“bulk”) are O4c. As can be seen in Fig. 2, the average of chemical shifts (δisoav) for surface O3c and bulk O4c sites are 440 and 364 ppm, respectively. These two values are quite close to those computed for the regular t-ZrO2 (101) surface, Fig. 1, and are also similar to those reported experimentally.9 However, in the stepped t-ZrO2 (134) surface one can distinguish two different O3c anions, either on the flat terrace region, with 425 and 428 ppm chemical shift, or at the step sites, where the value is 454 ppm, Fig. 2. Thus, the 17O chemical shift is sensitive not only to the coordination of the oxygen atom but to its environment. Here a


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difference of 30 ppm is simply due to the different electrostatic potential felt by the ion. More homogeneous are the values of the chemical shift in the sub-surface layers of the slab, Fig. 2, with average chemical shifts of about 364 ppm. However, even inside the second layer the O atoms exhibit chemical shifts ranging from 354 to 375 ppm (Table S2), showing that even in the subsurface layer the O ions feel the presence of the step. Table 2.17O NMR average chemical shift (δisoav, ppm) of surface and bulk oxygen atoms of the stepped t-ZrO2 (134) and (145) slabs. δisoav t-ZrO2 (134)

δisoav t-ZrO2 (145)

Layers

Layers

bulk

layers

bulk

1 (O3C)

440

-

437

-

2 (O4C)

363

3 (O4C)

365

4 (O4C)

365

5 (O4C)

363

6 (O3C)

440

361 364

364 364

363

361 -

437

-

Fig 2. 17O NMR average chemical shift (δisoav, ppm) of oxygen atoms in various layers of a stepped t-ZrO2 (134) slab.

3.2.2. t-ZrO2 (145) surface We also considered a second stepped surface, the t-ZrO2 (145) surface, Fig. 3 and Table 2 (see also Table S3). The δisoav values of O ions in the various layers are not significantly different from what found for the (134) stepped surface, Table 2. Also here there are two types of O3c anions, and their 17O

chemical shift, in the range of 424−431 ppm and 449-453 ppm, respectively, is practically the



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same found for the other stepped surface considered. The averaged δisoav of the bulk part, 363 ppm, is also virtually the same.

Fig 3. 17O NMR average chemical shift (δisoav, ppm) of oxygen atoms in various layers of a stepped t-ZrO2 (145) slab.

3.2.3. t-ZrO2 (4 3) surface The third stepped surface considered is the t-ZrO2 (4 3) surface, Fig. 4. On this surface one can identify two types of surface oxygen anions based on the different coordination, O2c and O3c. The O2c anions are located at the step sites, while all the oxygen anions in the terrace sites are O3c. The 17O

NMR results, Fig. 4, Table 3, and Table S4, show that the low-coordination of some of the O

atoms of the surface results in significant changes in the chemical shifts. In fact, the 17O chemical shift for some of the O2c step sites is the highest among the cases considered so far: 543 ppm. On the other hand, we also found some O2c sites which exhibit a much smaller chemical shift, 451 ppm, Fig. 4. This is connected to the very large rearrangements that occur once the surface is cut along the (4 3) directions, with some O atoms that move from the second layer to the top one. The δiso for O3c sites on the surface, terrace part, is in the range 431−438 ppm, with an average value of 435 ppm, similar to previous cases. The averaged δiso for the entire surface, containing both O2c and O3c sites, is 460 ppm, but we have seen that there is a large spread of values, depending on the position


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and coordination of the O atom. The δiso for O4c sites in the sub-surface regions is in the range 322363 ppm, with average at 341 ppm. Table 3.17O NMR average chemical shift (δisoav, ppm) of surface and bulk oxygen atoms of the stepped t-ZrO2 (4

3) slab. Layers

δisoav layers

1 (O2C)

497

1 (O3C)

435

2 (O4C)

335

3 (O4C)

343

4 (O4C)

343

5 (O4C)

335

6 (O3C)

435

6 (O2C)

497

460

δisoav bulk -

341

460

-

Fig 4. 17O NMR average chemical shift (δisoav, ppm) of oxygen atoms in various layers of a stepped t-ZrO2 (4 3) slab.

3.3. ZrO2 nanoparticles 3.3.1 Stoichiometric nanoparticles In this section we consider zirconia stoichiometric nanoparticles. Details on how these have been obtained and on their electronic structure can be found in refs. 37-40. We start from the small Zr16O32 unit, Fig. 5 and Table 4 (see also Table S5). In the surface structure of Zr16O32 one can recognize two different types of oxygen anions, O2c and O3c. The δiso for O2c surface sites has an 9 ACS Paragon Plus Environment


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average value of 545 ppm; however, this is not really indicative of the two-fold coordinated nature of the site, as a large spread of values is found for O atoms with the same coordination: δiso goes from a minimum of 463 ppm, to a maximum of 639 ppm, Fig. 5(a). Thus, the spread of values for O2c sites found for the stepped (4 3) surface is confirmed here. In general, it remains true that lowcoordinated oxygens give rise to much larger chemical shifts. This is confirmed by the analysis of the δiso values associated to O3c sites on the surface of Zr16O32: 384, 388 and 448 ppm, with an average of 407 ppm, Fig. 5(a). The oxygen anions in the inner part of the nanoparticle, all O4c sites, have δiso values in a range 267−356 ppm, with an average of 317 ppm. Thus, there is a clear dependence of δiso on the coordination of the O atoms, but clearly other factors related to longerrange interactions and local environment contribute to determine the final value of the chemical shift. Table 4.17O NMR average chemical shift (δisoav, ppm) for various oxygen atoms of the Zr16O32 nanoparticle.

O2C

O3C

O4C

O atoms

δiso

1-O2C (2 atoms)

639

2-O2C (2 atoms)

595

3-O2C (2 atoms)

568

4-O2C (2 atoms)

563

5-O2C (2 atoms)

556

6-O2C (2 atoms)

534

7-O2C (2 atoms)

521

8-O2C (2 atoms)

464

9-O2C (2 atoms)

463

10-O3C (2 atoms)

448

11-O3C (2 atoms)

388

12-O3C (2 atoms)

384

13-O4C (2 atoms)

356

14-O4C (2 atoms)

335

15-O4C (2 atoms)

311

16-O4C (2 atoms)

267

δisoav

Exp34

545

-

407

400, 374

317

322


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(a)

(b)

Fig 5. The 17O NMR chemical shift (δiso, ppm) of oxygen atoms. (a) Zr16O32 nanoparticle; (b) Zr40O80 nanoparticle.

The trends observed for the small Zr16O32 cluster are confirmed for the larger Zr40O80 particle. Here too one can identify on the surface O2c and O3c sites, while in the inner part (“bulk”) we have only O4c ions. The δiso values for the surface O2c sites are all at high values, in a relatively narrow range of 576-584 ppm (average value 580 ppm), Fig. 5(b) and Table 5. A wider range of values is found for the O3c sites, from 386 to 436 ppm, with an average value of 408 ppm. Once more, this shows that low-coordinated O sites should be distinguishable based on their chemical shift. In fact, δiso for the inner part of the nanoparticle (O4c sites) is in the range 294−354 ppm, with an average of 324 ppm, Fig. 5(b) and Table 5. Further details about the 17O NMR δiso of the Zr40O80 nanoparticle can be found in Table S6. So far, experiments on

17O

NMR chemical shifts have been reported for bulk zirconia or

zirconia nanoparticles of about 15 nm in size.34 In these experiments values of the chemical shift around 374-407 ppm have been measured and attributed to O3c atoms; this assignment is supported by the present results, (δisoav = 408 ppm); also the signal at 322 ppm, and attributed to O4c atoms, is consistent with the present calculations (δisoav = 324 ppm), Table 5. However, no chemical shifts above 410 ppm have been observed, while our results show that O2c sites should exhibit chemical shifts above 500 ppm. This result suggests that on the zirconia nanoparticles used in the experiments the number of O2c sites is close to zero or below the detection limit. This is not surprising if we think that these sites are highly reactive and that they can interact with ambient 11 ACS Paragon Plus Environment


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molecules such as H2O or CO2 to form new surface species, with passivation and elimination of O2c sites. This aspect of the problem will be discussed in the following Sections of the paper. Table 5. 17O NMR average chemical shift (δisoav, ppm) for various oxygen atoms of the Zr40O80 nanoparticle.

O2C

O3C

O4C

O atoms

δiso

1-O2C (8 atoms)

584

2-O2C (4 atoms)

579

3-O2C (4 atoms)

578

4-O2C (8 atoms)

576

5-O3C (8 atoms)

436

6-O3C (4 atoms)

404

7-O3C (4 atoms)

403

8-O3C (4 atoms)

400

9-O3C (4 atoms)

388

10-O3C (4 atoms)

386

11-O4C (4 atoms)

354

12-O4C (4 atoms)

339

13-O4C (4 atoms)

338

14-O4C (8 atoms)

321

15-O4C (4 atoms)

302

16-O4C (4 atoms)

294

δisoav

Exp34

580

-

408

400, 374

324

322

3.3.2. Oxygen-deficient nanoparticles So far, we have considered only stoichiometric surfaces or nanoparticles. However, the synthesis of non-stoichiometric nanoparticles has been reported, in particular of oxygen deficient, reduced zirconia nanoparticles.57 It has been shown that these systems can exhibit interesting magnetic properties due to the formation of stable Zr3+ ions at low-coordinated sites.40 Here the focus is not on the appearance of a magnetic ordering in sub-stoichiometric ZrO2 nanoparticles, but rather the shifts induced in the 17O NMR chemical shift by chemical reduction. The

17O

chemical shift associated to an O vacancy clearly depends on the nature of the

vacancy. In ionic materials, such as MgO and bulk ZrO2, an O vacancy results in excess electrons localized in the cavity left by the missing O atom.58 In reducible materials, such as TiO2 or ZrO2 nanoparticles, the excess electrons are localized on empty d states of the transition metal atom, with formation of Ti3+ or Zr3+ ions (the present case). In covalent-polar solids, such as SiO2, the missing O atom results in new direct Si-Si covalent bonds. Different chemical shits of the neighboring O 12 ACS Paragon Plus Environment

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atoms are expected for different kinds of O vacancies. Here we observe relatively small chemical shifts for the reduced compared to the stoichiometric particles, due to the fact that the excess of charge is localized on low-coordinated Zr ions; in other cases, such as the ionic apatite compounds, more sizable 17O chemical shifts have been reported in correspondence of the formation of this kind of defect.59 In Zr19O32 6 oxygen atoms are missing with respect to a stoichiometric aggregate, resulting in 12 excess electrons that are accommodated in the 4d orbitals of the low-coordinated Zr ions. In this particle all the oxygen ions are either O3c on the surface (24 atoms), or O4c in the inner part (8 atoms). The results of 17O NMR δiso are summarized in Fig. 6(a) (details in Table S7). δiso for O3c is 420 ppm. Compared to the stoichiometric nanoparticle of similar size there is only a small shift, from 407 ppm, Fig. 5(a), to 420 ppm. The O4c sites have chemical shifts at 271 ppm. This is a bit lower than δisoav = 317 ppm for the stoichiometric counterpart, Fig. 5(a). Thus, the presence of excess electrons due to the missing O atoms has some moderate effect on the chemical shift of O3c and O4c atoms which, however, could also be due to the structural changes that follow the removal of the six O2c atoms.

(a)

(b)

Fig 6. The 17O NMR chemical shift (δiso, ppm) of oxygen atoms. (a)Zr19O32 nanoparticle; (b): Zr44O80 nanoparticle.

As for the non-stoichiometric Zr19O32nanopartcle, in Zr44O80 the O2c atoms have been removed, resulting in 16 excess electrons (8 missing O atoms). All the oxygen anions on the surface are O3c and the inner ones are O4c. The 17O chemical shift of the oxygen anions are summarized in Fig. 6(b) 13 ACS Paragon Plus Environment


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(and Table S8). There are two non-equivalent O3c ions on the surface, characterized by δiso 363 and 427 ppm, with δisoav = 395 ppm. This is not too far from the 420 ppm value found the O3c sites of the smaller oxygen-deficient nanoparticle, Fig. 6(a). Also in the inner region of the Zr44O80 nanoparticle there are two non-equivalent O4c sites, with δiso 315 and 293 ppm, and δisoav = 310 ppm, Fig. 6(b). Also in this case, the changes with respect to the stoichiometric Zr40O80 nanoparticle are small, of the order of 20 ppm. In conclusion, a major change in the electronic structure (the reduced, oxygen deficient nanoparticles are magnetic and electron-rich) does not result in major changes in the values of the 17O

chemical shift. Since the oxygen-deficient nanoparticles have been obtained by removing all the

O2c atoms, the main effect is that the signals associated to these sites are no longer present.

3.4. Adsorption of the H2O and CO2 molecules on the t-ZrO2 (101) surface Oxide surfaces exposed to air react and give rise to new surface species. Two molecules are particularly relevant in this context, water and carbon dioxide, as they are present in sizable amounts in the environment and react preferentially with low-coordinated and exposed sites at oxide surfaces. In this section, we will explore the reactivity of the t-ZrO2 (101) surface using 17O NMR as a technique that can provide specific information on the reaction products. 3.4.1. H2O adsorption (hydroxylated surface) During the optimization, starting from molecular adsorption, and considering a single water molecule, water dissociative adsorption occurs spontaneously the t-ZrO2 (101) surface, see Fig. 7. A OH- unit binds to a surface Zr ion, and a proton binds to a surface O ion, but a hydrogen bond is formed, with the proton at 1.606 Å from the oxygen atom of the OH group, Fig. 7. The dissociative adsorption occurs with an energy gain of -1.38 eV. The results of 17O NMR are summarized in Table 6 and Fig. 7 (see also Table S9). δiso for the OH- groups of the hydroxylated surface is 146 and 200 ppm, respectively. Thus, the presence of hydroxyl groups results in a distinct signature in the NMR spectrum. On the other hand, the other O atoms of the surface, and of the inner layers of the zirconia support, are basically unperturbed and exhibit chemical shifts similar to those of the clean surface. These results for the

17O

NMR signal are consistent with those reported based on experimental

measurements and previous DFT calculations,9 see Table 6.


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


Fig 7. Optimal structure of a H2O molecule adsorbed on the t-ZrO2 (101) surface. The values of the chemical shifts for the O atoms are given in the figure.

Table 6.17O NMR average chemical shift (δisoav, ppm) of surface and bulk O atoms for the dissociative adsorption of H2O on the t-ZrO2 (101) slab. Previous work9

this work

OH- groups and layers

δisoav layers

δisoav bulk

δisoav calc

OH- water

146

-

145

OH- surface

200

1 (O3C)

456

2 (O4C)

374

3 (O4C)

373

4 (O4C)

378

5 (O4C)

376

6 (O4C)

376

7 (O4C)

378

8 (O4C)

371

9 (O4C)

382

10 (O3C)

454

204

δiso Exp 100

-

413

440

376

378

378

-

410

440

3.4.2. CO2 adsorption On the basis of our previous work on the adsorption of CO2 on the tetragonal zirconia surface,60 we have considered both physisorbed or linearly adsorbed CO2, Fig. 8(a), and chemisorbed species, with formation of various forms of carbonates, CO32- (I) and CO32- (II), Fig. 8(b) and 8(c), respectively. As for the case of water adsorption, the variation of the average 17O chemical shift of



Page 16 of 23

surface and bulk ions with respect to the clean surface is not significant, see for comparison Tables 1 and 7. Not surprisingly, completely different values are found for the O atoms of the carbonate species and the O ions of the surface directly involved in the interaction. When physisorption takes place, linear CO2, Fig. 8(a), two similar values are found of δiso(17O), 64 and 73 ppm. When surface carbonates are formed, Fig. 8(b) and 8(c), values of chemical shift from 258 and 274 ppm are computed, see also Table 7. These chemical shifts are clearly distinct from the rest of the O atoms of the surface, and should allow to identify the presence of carbonate species on the surface. More complex could be the distinction among the various kinds of carbonate species that can form. For completeness, we report also in Table 8 the

13C

chemical shifts of adsorbed CO2 and

CO32- moieties on the t-ZrO2 (101) surface. For these species experimental measurements are available, which allows a direct comparison with the present calculations. In particular, for surface carbonate species values of δiso of 158-162 ppm and 167-171 ppm have been observed and attributed to different carbonate species. These values are fully consistent with our calculations that report a δiso of 163 and 177 ppm for 13C of carbonate species I and II, respectively. On this basis, we can propose a tentative structural assignment to the measured values. Furthermore, this represents an additional validation of the present computational setup for the study of NMR chemical shifts.

Figure 8. Optimal structures of adsorbed CO2 and CO32-units on the t-ZrO2 surface. The values of the chemical shifts

for the O atoms are given in the figure.

Table 7. 17O NMR average chemical shift (δisoav, ppm) of surface and bulk O atoms for the dissociative adsorption of CO2 on the t-ZrO2 (101) surface. The 17O δiso for free CO2 is 43 ppm.


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


Layers

Linear CO2

CO32- (I)

δisoav layers δisoav bulk

CO32- (II)

δisoav layers

δisoav bulk

δisoav layers

δisoav bulk

CO2 or CO32-

69

-

267

-

264

-

1 (O3C)

453

-

457

-

453

-

2 (O4C)

383

373

367

3 (O4C)

370

368

367

4 (O4C)

377

375

374

5 (O4C)

375

6 (O4C)

375

7 (O4C)

378

377

377

8 (O4C)

371

370

370

9 (O4C)

382

382

382

10 (O3C)

454

374

376

374

-

375

374

455

374

-

454

373

-

Table 8. 13C chemical shift (δiso, ppm) of CO2, CO32-, and HCO3- units on the t-ZrO2 (101) surface. δiso

Exp53

Free CO2

135

125

Linear CO2

137

-

CO32- (I)

163

158-162

CO32- (II)

177

167-171

4. Conclusions We have studied by means of DFT calculations the

17O

NMR chemical shift in zirconia surfaces

and nanostructures, also in interaction with small molecules such as CO2 and H2O. We found, in agreement with literature data, that the computed NMR chemical shifts for O3c and O4c sites are very close to the measured values for surface and bulk O ions of ZrO2 nanoparticles. This opens in principle the perspective to use the 17O NMR chemical shift as a sensitive probe of the coordination, electronic nature and possibly even the basicity of oxygen atoms at various positions of zirconia surfaces or nanostructures. To this end, we have considered, besides the regular (101) surface of tetragonal ZrO2, also stepped surfaces and small zirconia nanoparticles exposing significant number of low-coordinated O atoms. Indeed, the

17O

NMR chemical shift for O2c sites is significantly

different from that of O3c and O4c one, providing a fingerprint of their existence. A comparison with existing experimental NMR spectra of ZrO2 nanoparticles does not show the presence of the large chemical shift associated to the O2c sites, suggesting that under the experimental conditions used, 17 ACS Paragon Plus Environment


Page 18 of 23

and for the size of nanoparticles considered, the number of this sites is very low and below the detection limit. We also found, however, that O2c atoms in various positions of a zirconia surface or nanoparticle exhibit quite different chemical shifts, depending on the environment and electrostatic potential around the atom. We also considered the effect of chemical reduction of zirconia nanoparticles on the

17O

NMR properties. To this end, highly reduced, oxygen-deficient nanoparticles have been studied. In these systems the O2c atoms have been removed, and excess electrons are left on the oxide where they occupy the Zr 4d empty states.40 The results, however, do not indicate a significant effect on the

17O

NMR chemical shift. The variations computed with respect to the stoichiometric

counterparts, are partly due to structural changes, and cannot be clearly attributed to the electronic effects associated to the excess of charge (chemical reduction). Finally, we have considered the adsorption of H2O and CO2 molecules, with formation of hydroxyl groups or of carbonate species on the surface of zirconia. Not surprisingly, this results in sizable shifts of the

17O

NMR chemical shifts associated to the O atoms of the OH or [CO32-]

surface groups which can be identified based on their NMR signals. Less relevant, or more difficult to interpret, are the changes in

17O

NMR chemical shift of the O atoms of the surface near the

adsorbed species. The results have also shown that the computed

13C

NMR chemical shift of

carbonaceous species is very close to the measured ones in MAS-NMR experiments, and a tentative assignment of the observed features to specific forms of carbonate ions on the surface of zirconia is proposed. Supporting Information Supporting Information including calculated NMR and EFG parameters of oxygen anions for all structures in this study Acknowledgments This work has been supported by the Italian MIUR through the PRIN Project 2015K7FZLH SMARTNESS "Solar driven chemistry: new materials for photo– and electro–catalysis" and the grant Dipartimenti di Eccellenza - 2017 "Materials For Energy". References 1. Bastow, T.; Stuart, S., 17O NMR in Simple Oxides. Chem. Phys. 1990, 143, 459-467.


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TOC Graphic


DFT Study of 17O NMR Spectroscopy Applied to Zirconia Surfaces

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