DFT Analysis of NO Oxidation Intermediates on Undoped and Doped

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DFT Analysis of NO Oxidation Intermediates on Undoped and Doped LaCoO Perovskite 3

Michael W Penninger, Chang Hwan Kim, Levi T. Thompson, and William F. Schneider J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06351 • Publication Date (Web): 20 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015

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The Journal of Physical Chemistry

DFT Analysis of NO Oxidation Intermediates on Undoped and Doped LaCoO3 Perovskite Michael W. Penninger,† Chang Hwan Kim,‡,§ Levi T. Thomson,¶ and William F. Schneider∗,† Department of Chemical and Biomolecular Engineering, 182 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN 46556, General Motors Global R&D, Warren, MI 48090, and Department of Chemical Engineering and Hydrogen Energy Technology Laboratory, University of Michigan, Ann Arbor, MI 48109-2136 E-mail: [email protected]

∗

To whom correspondence should be addressed Department of Chemical and Biomolecular Engineering, 182 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN 46556 ‡ General Motors Global R&D, Warren, MI 48090 ¶ Department of Chemical Engineering and Hydrogen Energy Technology Laboratory, University of Michigan, Ann Arbor, MI 48109-2136 § Current: Advanced Catalysts and Emission-control Research Lab, Powertrain Center 2, R&D Division, 150, HyundaiYeonguso-ro, Hwaseong-si, Gyeonggi-do, 445-706, Korea †

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Abstract Perovskites are of interest as low-cost replacements for Pt-based NO oxidation catalysts. While the mechanism of Pt-catalyzed NO oxidation is fairly well understood, such is not the case for the oxides. The perovskite LaCoO3 itself has been shown to have good NO oxidation activity, and Sr substitution improves NO oxidation rates and reduces NO2 inhibition. In this work, we report density functional theory (DFT) results for the adsorption of NOx (x = 1, 2) to the undoped and Sr doped (100) LaO and CoO2 terminated LaCoO3 . Further, we used first-principles thermodynamic models to determine the most common surface species under NO oxidation conditions. Nitrates and adsorbed NO are most stable on the LaO and CoO2 terminations respectively. We explored the relative free energies of surface and vacancy-mediated pathways for NO oxidation. The vacancy-mediated pathways suffer from energetically costly removal of surface oxygen, while the surface pathway is most feasible for NO oxidation to occur. The perovskite surface free energy reaction pathway is compared to RuO2 , MgO, and Pt. The CoO2 termination surface pathway is energetically most similar to that of Pt and is considered to be the most plausible for NO oxidation.

Introduction A molecular-level understanding of the catalytic chemistry of the nitrogen oxides (NOx , x = 1, 2) is important for developing materials and strategies to efficiently remove NOx from combustion exhaust. Metal oxides are of interest as NOx sorbents for application in so-called − lean-NOx traps (LNTs). 1–3 NOx generally adsorb to form nitrites (NO− 2 ) and nitrates (NO3 )

through oxide-mediated electron transfers. 4–6 Recently there has been renewed interest in the development of metal oxide catalysts for the catalytic oxidation of NO with O2 to NO2 , in the hopes of developing a more cost-effective alternative to Pt-based NO oxidation catalysts. 7 Unlike the sorbents, these catalysts are generally oxides of redox-active metals. 2 ACS Paragon Plus Environment

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The oxides of a number of single transition metals are observed to be active for catalytic NO oxidation, including PdO, 8 RhO2 , 9 and Co3 O4 . 9,10 Weiss et al. found that NO oxidation follows the same rate law on PdO as that observed on Pt 11 and inferred a similar mechanism on the two, in which O2 dissociation is rate limiting and the active surface is O-covered. Difference in rate were observed to correlate with the reduction potential of the oxide. Density functional theory (DFT) calculations have probed the adsorption and reactions of NO over the (110) facets of the rutile oxides RuO2 , IrO2 , and PtO2 , 12,13 as well as a PdO overlayer on Pd(100). 14 Mixed metal oxides promise a wider range of chemical tunability than is possible with a monometal oxide. A variety of mixed metal materials have been shown to have NO oxidation activity. 15 In particular, Kim et al. 16 found that supported perovskite catalysts LaMO3 (M = Co, Mn) exhibited NO to NO2 conversions comparable to to greater than that of a supported Pt catalyst at temperature ranges between 200-400◦ C. Sr-, 16,17 Ag-, 18 and Cu-doping 19 are all found to have promoting effects on oxidation activity. Choi et al. 17 showed that the doped material retained the perovskite structure and related its promoting effect to observed decreases in NO, NO2 , and O2 desorption temperatures in doped samples. Say et al. 20 used vibrational spectroscopy to observe features attributed to adsorbed nitrite and nitrate at 323K after saturating the surface with 5 Torr NO2 for 10 minutes. Various aspects of NO oxidation chemistry on the perovskite oxides have been explored with DFT calculations. One of the complexities of this modeling is the potential for the oxide to expose different facets and surface terminations. In an attempt to address this issue, Chen et al. 21 used an LDA+U model and first-principles thermodynamics to determine the relative stability of various surface terminations of LaCoO3 , demonstrating that the LaOterminated (100) and oxygen-rich (111) LaO3 facets are preferred under strongly oxidizing conditions like those found during NO oxidation. Zhou et al. 19 modeled a lower-symmetry surface of the same oxide at the GGA level and identified a vacancy-mediated NO oxida3 ACS Paragon Plus Environment


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tion pathway but did not consider the impact of potential competing processes. Recently, Zhang et al. 22 utilizes the GGA+U formalism and first-principle thermodynamics to explore low energy surfaces of LaCoO3 and Sr doped LaCoO3 showing that the (100) CoO2 and LaO terminations are most preferred at high temperature in oxidizing conditions. We previously reported GGA-based models of lattice and extra-lattice oxygen-exchange on the LaOand CoO2 -terminated (100) facets of undoped and Sr-doped LaCoO3 . 17 The results reveal different O2 chemistries on the two terminations that correspond well with experimentally observed O2 temperature programmed desorption (TPD). The prior work notwithstanding, there remains a need for a computational characterization of NOx chemistry on the perovskite oxides. Following our prior approach, we consider the high symmetry (100) facets of LaCoO3 including both the LaO- and CoO2 -terminations in an effort to illustrate the range of possible NOx adsorption behaviors on these materials. We report computed adsorption structures, preferred binding sites, binding energies, vibrational spectra, and electronic analysis on both surfaces without and with Sr dopant. To relate the results to NO oxidation conditions, we compute surface phase stability diagrams and free energy pathways for NO oxidation to NO2 . We find the LaO-termination to strongly bind nitrate and to likely be inactive towards NO oxidation; a CoO2 -termination presents more favorable adsorption energies and is more likely relevant to observed NO oxidation.

Computational Details Computational procedures follow those previously reported for oxygen adsorbates on the (100) terminations of LaCoO3 , which provide good agreement with experimentally observed oxygen thermal desorption behavior. 17 First principle periodic supercell plane-wave DFT calculations were performed using the Vienna ab initio Software Package (VASP). 23 The core electronic states were treated with projector augmented wave (PAW) method 24,25 and


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exchange and correlation treating within the PW91 generalized gradient approximation (GGA). 26 The formally Co3+ ions are d6 and paramagnetic in bulk LaCoO3 . To achieve internally consistent results, all supercells were initialized and converged to ferromagnetic states. Adsorption energies are insensitive to total magnetization. The plane wave cut-off energy was 400 eV with a 2 × 2 × 1 k-point sampling of the first Brillouin zone. All atoms were relaxed until forces were < 0.05 eV/Å. Vibrational frequencies were calculated for optimized adsorbate structures including the nearest-neighbor surface atoms using a harmonic model with 0.025 Å displacements. Bader charges 27–29 are reported as the difference between valence electrons of the species and computed Bader charge. VESTA 3 software was used to visualize all molecular structures. 30 Undoped LaCoO3 adopts the rhombohedral R3m space group at ambient temperature, but exchange of Sr (≈ 8% Sr:La) for La ions converts the structure to cubic Pm3m. For convenience we report results for the undoped and doped systems using the cubic structure. Test calculations show that this choice has a small impact on the adsorption energies of interest here. The LaCoO3 cubic bulk lattice constant is computed to be 3.824 Å, in agreement with the experimental value of 3.805. 31 As shown in Figure 1A, the LaCoO3 slab model consists of a 2 × 2 cubic supercell with six alternating layers of LaO and CoO2 planes. The model contains approximately 15 Å of vacuum spacing between slabs to minimize interactions between periodic images. Figure 1A shows Jahn-Teller distortions and rotation of the CoO6 octahedra upon relaxation, similar to the behavior noted by Lee et al . 32

Results We used a stoichiometric, asymmetric slab LaCoO3 containing four LaO or CoO2 formula units per layer to represent the LaO- and CoO2 -terminated (100) facets, as illustrated in Figure 1A. Figure 1B shows a top view of the LaO(100) termination; the surface exposes



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both La and O ions as well as hollow sites labeled as “La-La bridge.” Slight asymmetries resulting from twisting of the underlying CoO6 octahedra are apparent; adsorption energies are found to be insensitive to these slight geometric variations. Figure 1C similarly shows the CoO2 termination and the corresponding Co-atop, O-atop, and hollow sites. The effects of CoO6 tilting are much less evident on this termination. We placed adsorbates NO, NO2 , and NO3 in all sites and in various orientations and relaxed to identify lowest energy structures. All reported structures are minima as verified by analysis of the computed harmonic force constants, reported in the Supplementary Information (SI). Computed NO, NO2 , and NO3 binding energies were referenced to the corresponding gas-phase molecules. We previously showed that a Sr dopant prefers to segregate to the LaO termination. 17 We consider two dopant models, including one with one Sr ion substituted for La in the top LaO layer and a second with a Sr ion substituted for La subsurface to the CoO2 termination. Both reduce the symmetry of the surface and introduce new surface sites. The dopant structures are reported in the SI.

NOx Adsorption In a series of calculations, we placed NO, NO2 , and NO3 adsorbates in all possible sites and several possible orientations above the LaO(100) and CoO2 (100) sides of the LaCoO3 slab. In all cases the calculations converged to an adsorbed structure, and in many cases several calculations converged to the same final structure. Lowest energy structures on the LaO(100) and CoO2 (100) terminations are shown in Figures 2 and 3, respectively. We computed Bader charges for all adsorbates relative to their molecular values; results are summarized in Figure 4. For comparison, we include binding energies and Bader results for both adsorbed O and O2 . 17 Several general features are evident in these results. Adsorption is always associated with charge transfer to the adsorbate. The extent of charge transfer follows the general trend in 6 ACS Paragon Plus Environment

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La Atop

O

B)

O Atop

La La La Bridge

Co

O

C)

Co Atop

Hollow

O Atop

A) Figure 1: 6-layer LaCoO3 slab model (A) and exposed (100) LaO (B) and (100) CoO2 (C) terminations are shown. The slab model consists of alternating layers of CoO2 and LaO. The unique binding sites for the LaO (B) and CoO2 (C) termination are shown as a combination of atop, bridge, and hollow sites. The addition of the dopant (not shown) will introduce Sr-atop and Sr-La bridge sites to the LaO termination.



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O

O N

La

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1.30 1.30 1.20

1.27 1.22 2.84

2.84

NO

2.53

122°

1.28

115° 2.52

NO2

2.77

2.70

NO3

Figure 2: Optimized NOx adsorption geometry and positions on the LaO termination are shown. Atoms are represented by the following: green = La, orange=Sr, blue=Co, purple=N, red=Osurf , maroon=Oads

Co

1.23 La

O 1.24

N O

1.36 126°

128° 1.20 1.76

NO

2.23

2.23 1.90

NO2

1.92

NO3

Figure 3: Optimized NOx adsorption geometry and positions on the CoO2 termination. Atoms are represented by the following: green=La, orange=Sr, blue=Co, purple=N, red=Osurf , maroon=Oads 8 ACS Paragon Plus Environment

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-1.4 LaO -1.2

CoO2

-1.0 Bader Charge (e-)

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-0.8

-0.6

-0.4

-0.2

0.0

NO

NO2

NO3

O

O2

Figure 4: Net molecular Bader charge upon adsorption to LaO (circles) and CoO2 (triangles) terminated (100) facets of LaCoO3 . Adsorbates are roughly ordered from least to most strongly reduced. electron affinity, NO < NO2 < NO3 , and is greater for adsorbates at the LaO than at the CoO2 termination. Adsorbates tend to prefer to be associated with the cations over the oxygen anions on both terminations. Further, as shown in Table 1, adsorption energies are generally greater on the LaO- than CoO2 terminations. Table 1: GGA-computed bind energies (eV) from this work and Ref. 17 Undoped LaCoO3

Sr Doped LaCoO3

Adsorbates

LaO

CoO2

LaO

CoO2

Vacancy O O2 NO NO2 NO3

4.07 -1.66 -2.04 -0.93 -2.55 -3.83

1.74 0.13 -0.39 -1.48 -0.93 -0.97

3.87 -1.03 -1.40 -0.78 -2.29 -3.59

2.02 0.36 0.47 -1.36 -0.76 -0.81

The computed gas-phase bond length of NO is 1.17 Å; adsorption on the CoO2 and LaO terminations lengthens it 0.03 and 0.05 Å, respectively. These differences are consistent with charge transfer into the NO 2π ∗ state, and with the relative extent of charge transfer. NO 9 ACS Paragon Plus Environment


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trite and nitrate on LaCoO3 ; rather the redox active oxide itself reduces the adsorbates. As a result, adsorption energies tend to decrease with increasing coverage.

Sr doping effect We substituted a single Sr ion for La at the LaO termination and relaxed NOx species at atop Sr and in the Sr-La bridge locations in addition to those shown in Figure 1B (structures available in the SI). NOx adsorbates consistently migrate away from the Sr dopant. Binding site preferences remain the same for each adsorbate. As shown in Table 1, dopants destabilize binding by 0.1-0.2 eV. Consistent with this, adsorbate Bader charges are diminished by up to 0.2 e. We find minimal changes in N−O bonds (

DFT Analysis of NO Oxidation Intermediates on Undoped and Doped

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