Catalytic Enhancement of CO Oxidation on LaFeO3 Regulated by

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Catalytic Enhancement of CO Oxidation on LaFeO3 Regulated by Ruddlesden-Popper Stacking Faults Reut Bornovski, Liang-Feng Huang, Eswaravara Prasadarao Komarala, James M. Rondinelli, and Brian A. Rosen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09404 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Catalytic Enhancement of CO Oxidation on LaFeO3 Regulated by Ruddlesden-Popper Stacking Faults Reut Bornovski1, Liang-Feng Huang2,3, Eswaravara Prasadarao Komarala1, James M. Rondinelli2, and Brian A. Rosen1* 1) Department of Materials Science and Engineering, Tel Aviv University, Ramat Aviv, 69987001, ISRAEL 2) Department of Materials Science and Engineering, Northwestern University, Evanston Illinois 60208-3108, USA 3) Key Laboratory of Marine Materials and Related Technologies, Key Laboratory of Marine Materials and Protective Technologies of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, CHINA *corresponding author: [email protected] Keywords: defect engineering, catalysis, perovskites, DFT, adsorption, fault structure ABSTRACT The influence of planar defects, in the form of stacking faults, within perovskite oxides on catalytic activity has received little attention because controlling stacking fault densities presents a major synthetic challenge. Furthermore, stacking faults in ceramics are not thought to appreciably impact surface chemistry, which partly explains why their direct effect on catalysis is generally ignored. Here, we show that Ruddlesden-Popper (RP) stacking faults in otherwise stoichiometric LaFeO3 can be broadly controlled by modulating the ceramic synthesis route. Electronic structure calculations along with electron microscopy and spectroscopy show that energetically favorable RP faults occur both near the surface and in bunches and enhance CO oxidation kinetics. DFT+U shows that subsurface RP faults strengthen the adsorption and co-adsorption of CO, O, and O2, which could lower the apparent activation energy of CO oxidation on faulted catalysts compared to their pristine counterparts. Our work suggests that planar defects should be considered a new and useful feature in hierarchal nanoscale design of future catalysts. INTRODUCTION CO oxidation catalysts are exceptionally important for regulating emissions from automotive and stationary power sources, and generally comprise of an active component made from noble metals such as platinum or palladium. Typical strategies for enhancing CO oxidation kinetics on metal oxide catalysts include modulating the orientation of exposed planes (for example, 1 ACS Paragon Plus Environment

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enhanced activity has been shown on CoO(110)), nanoparticle size, oxidation state of the active metal ion (e.g. Co3+/Co2+, Fe4+/Fe3+), or oxygen ion mobility1-4. While catalysts containing cobalt or nickel oxides show promising activity, concerns relating to their toxicity in CO-rich atmospheres may hinder their future applicability

1, 5-9.

In this work, we pursue an entirely

different approach focusing on planar defects as a strategy for modulating the CO oxidation activity, as opposed to zero-dimensional (0D) point defects which have received comparably more attention. Presently, defect engineering for the catalytic enhancement of perovskite-type oxides has largely been regulated to the study of 0D point defects such as vacancies, interstitials, and lattice substitutions

10-13.

These defects are intended to simultaneously modulate the chemistry of the

bulk and surface, and are therefore expected to tune catalytic activity. Stacking faults, a type of 2D planar defect, are not considered to significantly alter the catalytic activity of perovskite catalysts because they are thought to affect the bulk structure of the material rather than its surface. Nonetheless, stacking faults in metals have been shown to influence catalytic methanol production from syngas by stabilizing the active steps on a copper surface

14-15.

In ceramics, the

presence of extended defects is sometimes attributed to changes in surface area, but their direct connection to catalysis is rarely reported

16-18.

Compared to point defects, the role of extended

lattice imperfections on catalytic activity, often manifesting as Ruddlesden-Popper (RP, An+1BnO3n+1) faults, has largely gone unexplored. Despite this, there has been renewed interest in their formation and stability dielectrics

21,

17, 19-20

and recent exploitation to achieve highly tunable low-loss

indicating the importance of controlled atomic (2D defect) structures on

macroscopic responses. Here, we studied the effect of Ruddlesden-Popper faults in the perovskite LaFeO3 on the apparent activation energy of CO oxidation. We used the co-precipitation, sol-gel, and combustion synthesis techniques to broadly control the stacking fault density in LaFeO3, and subsequently correlated the planar defect density to CO oxidation activity. Our electronic structure calculations, based on density functional theory, were used to formulate a microscopic model and assess the relevant energetics regarding planar RP fault formation and their impact on adsorption strength. Our work demonstrates the power of planar defect control as a useful 2 ACS Paragon Plus Environment

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strategy for catalytic enhancement that can be utilized in known materials and novel digitally designed ceramics. RESULTS Ruddlesden-Popper (RP) faults in LaFeO3 LaFeO3 (LF) was synthesized using three different synthetic methods to modulate the planar stacking-fault densities in the ferrate ceramic. These methods included co-precipitation (cp-LF), solgel (sg-LF), and combustion synthesis (cs-LF). SEM images in Figure S1 show that for all phases, porous particles were formed between 10-20 microns in diameter. Rietveld fitting of Xray diffraction (XRD) patterns show that all synthetic variations yielded orthorhombic LaFeO3, where the unit cell size volume within 0.1% of each other (Figure S2). BET of the three samples yielded surface areas of 8.2, 6.5, and 9.1 m2/g for cs-LF, sg-LF, and cp-LF respectively. We hypothesized that synthesis factors such as temperature, heating and cooling rates, rate of pH change, and crystallization kinetics would enable us the ability to broadly control the density of stacking faults. In particular, factors which drive rapid crystallization, such as a large stepchange in the pH during co-precipitation, have been shown to lead to stacking fault formation in LaNiO3 17. Figure 1a-c shows characteristic transmission electron microscopy (TEM) images of LF obtained from the aforementioned processing methods. Ruddlesden-Popper (RP) stacking faults, when present, are seen in the [001] direction. Along this direction, the stacking sequence alternates between LaO and FeO2 layers (Figure 1d), where an RP fault (n=1) is indicated by two adjacent LaO layers. Each image in Figure 1 was taken along the [010] zone-axis with the [001] and [100] directions marked for clarity. Figure 1e shows a bright-field TEM simulation of a pristine LaFeO3 crystal, an RP fault (n=1), and a La-vacancy fault where there are two adjacent FeO2 layers. We observed that the density of RP stacking faults varied considerably as a function of the synthetic route. While the cs-LF phase appeared pristine and free of faults, many of the sg-LF particles showed a faulted structure near the surface. By comparison, nearly all the cp-LF particles showed a high density of RP faults traversing through the bulk from one free surface of

3 ACS Paragon Plus Environment


the particle to the other. This observation is supported by the fact that stacking faults must be bounded either by interfaces or by partial dislocations.

Figure 1: Bright-field TEM images of LaFeO3 particles looking down the [010] zone axis synthesized by (a) combustion synthesis, cs-LF (b) solgel synthesis, sg-LF and (c) coprecipitation synthesis cp-LF. Yellow arrows indicate the region in the materials where stacking faults are observed. Dashed white lines indicate the particle surface whereas dash-dot lines indicate domain boundaries (d) atomistic space-filling model of a pristine orthorhombic LaFeO3 crystal in the [010] zone-axis showing the alternate FeO2-LaO stacking sequence in the [001] direction. (e) TEM image simulation of a pristine LaFeO3 and the same with an n=1 RuddlesdenPopper (RP) fault and La vacancy fault. Scale bar 10nm. Microscopically, the average stacking fault density in cp-LF was estimated to be ≈0.2 faults/nm, whereas the density in sg-LF was estimated to be an order of magnitude smaller (≈0.05 faults/nm). In this calculation, hundreds of particles characteristic of each LF sample were analyzed and both the RP faults and La-vacancy faults were considered, although RP faults were far more prevalent. No faulting was found in any of the imaged cs-LF particles. This estimate considered how many faults were found per nanometer by measuring the particle size in the [001] direction. While the measurement of stacking fault density can be estimated using several 4 ACS Paragon Plus Environment

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techniques, we believed that this analysis provided a semi-quantitative value sufficient to enable a basis for comparison among the otherwise identical variations of LaFeO3. To ensure that the RP faults were not mistaken for Moiré fringes or the result of a tilted grain boundary, a highangle annular dark-field (HAADF) image from the cp-LF phase is shown in Figure S3. Owing to the fact that HAADF is sensitive to atomic number, RP faults show up as dark-contrast layers owing to the difference in atomic number (Z) between iron and lanthanum. An indexed fast Fourier-transformed image of the pristine and faulted regions of sg-LF is shown in Figure S4. Faulted regions within LaFeO3 also displayed the characteristic streaking in the direction of the stacking faults

22.

Macroscopically the presence RP faults also serve to reduce the average

domain size of the ideal perovskite. Reitveld analysis of the XRD pattern for each sample gave a domain size of 131 nm for the pristine cs-LF, 86.1 nm for sg-LF, and 40.3 nm for the highly faulted cp-LF.

Figure 2: Periodic supercells of LaFeO3 used during stability and catalytic computations containing surface (left) and bulk (right) Ruddlesden-Popper faults. The index, n, in the RP-n notation describes the depth of the surface fault or the number of perovskite unit cells between adjacent faults in the bulk.

To model the surface of LaFeO3, stoichiometric slabs with lateral areas of 5.57×5.65 Å2 and thicknesses of 16~18 Å were constructed in periodic unit cells for the pristine surface and 5 ACS Paragon Plus Environment


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surfaces with RP defects at different depths (Figure 2). The formation energy of each RP fault configuration was then calculated in the bulk and surface geometries as: La4Fe4O12 + La + ½ O2 → La4Fe4O12 ۰LaO

(surface)

n۰LaFeO3 + La + ½ O2 → Lan+1FenO3n+1

(bulk)

Figure 3 shows the variation in the formation energy of an RP-n fault, using both the bulk and surface geometries, as a function of index, n. For surface formation, this index represents the number of perovskite unit cells beneath the surface whereas for bulk formation, this index is used to describe the number of perovskite unit cells separating two RP faults. Formation energies are calculated according to the reactions above, where the metal La and O2 gas are used as the reference reactants to form the RP fault. Owing to the reference states (metal La and O2 gas), the RP formation energies are shown here as having large negative absolute energies. The bulk RP and the surface geometries were constructed to capture the complexity of the experiments in the simplest, yet still complex models. Experimentally, we were not able to place isolated stacking faults in LF nanoparticles in the precise positions that we modeled (e.g. RP-1/2), but rather, we were able to control the density of such faults over a broad range by modulating the synthetic route. This means that the real particles contained more complexities than can be fully captured by the calculations. We found that when considering RP formation, there is an energetic advantage of about 0.3 eV for their formation using the surface-mechanism over the bulk-mechanism. In the case of the bulk, there is an energetic advantage of about 0.1 eV for a RP fault to be near to another RP fault (n=1 compared to n≥2). Both of these conclusions are supported experimentally by TEM in Figure 1 since the RP faults were found both in the vicinity of the surface, and those faults found in the bulk accumulated in bunches.


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Figure 3: Formation energy of Ruddlesden-Popper faults on the surface and in the bulk as a function of index calculated at the DFT level. It is well known that lanthanum-containing perovskites (LaBO3) show lanthanum enrichment at their surface. While the La:B ratio at the surface should nominally be 1, XPS has shown that experimentally the surface La:B ratio is closer to 2

23-25.

It has been proposed that surface

lanthanum enrichment is energetically favorable and can be supported either by islands of lanthanum oxide (La2O3), or RP faults near the surface

26.

XPS showed that the pristine cs-LF

sample had an La:Fe ratio of 2.10. We suggest that this enrichment is attributed only to the formation of La2O3 islands, since no RP faults were seen in TEM. The sg-LF sample, which showed a low concentration of RP faults near the surface, gave a lower ratio of 1.96, whereas the highly faulted cp-LF phase gave the lowest ratio of 1.78. This suggests that the amount of La2O3 islands on the surface decreased as the amount of La-rich RP stacking faults in the material increased. This scenario is also supported by our formation energy calculations, which show that the presence of sub-surface RP faults can also serve to lower the surface energy. CO oxidation on pristine and faulted LaFeO3 catalysts Owing to the fact that the surface mechanism for RP fault formation is energetically favored, and that transition metal B cations are largely agreed to be the active-site for CO oxidation on LaBO3 materials

27,

we next calculated adsorption energies on FeO2-terminated slabs where stacking

faults were placed 1/2 or 3/2 perovskite layers beneath the surface (RP-1/2 and RP-3/2 geometries, respectively). The modeling of CO oxidation on LaFeO3 was considered by using two different cycling pathways based on participation of surface lattice oxygen, shown in Figure 4.



In Path 1 (Path 2), surface lattice oxygen does (not) participate in the oxidation mechanism. Path 1 is preferred based on the fact that the calculated adsorption energy of CO on pristine LaFeO3 (-0.258 eV) is much larger than that of O2 (-0.107 eV); however, it may be difficult for adsorbed O2 to find a surface vacancy, which would reduce the preference for CO oxidation via Path 1. In contrast, Path 2 may be favorable from the perspective that in most experiments, the partial pressure of O2 far exceeds that of CO (here 20:1), and therefore diatomic oxygen would be adsorbed first on the surface. Figure 4 shows the reaction-energy change between each elementary step of the CO oxidation cycle on the surface of pristine, RP-1/2 and RP-3/2 structures. We use blue and red surfaces in this schematic to highlight the primary differences between the two pathways. CYCLE PATH #1 CO

(a)

CO

CO2

CO2

O

O2

O CO

CO2

6

7

O CO

CO2

5

6

CO2

 (eV)

0 -1 -2 -4 -5

Pristine 0

1

RP-1/2 Fault

RP-3/2 Fault

2

4

3

5

8

Reaction-Step Index

CYCLE PATH #2 O2

(b)

O2

O2 CO

O CO2

O

CO2

CO2

0

 (eV)

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

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-1 -2 -4 -5

Pristine 0

RP-1/2 Fault 1

2

RP-3/2 Fault 3

4

Reaction-Step Index


7

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(c) Reaction Path

RP-1/2

RP-3/2

Pristine

CO + O2 → CO2 + O

-2.702

-2.009

-1.587

CO + O → CO2

-3.627

-4.163

-4.578

CO → CO2 + VO

+0.273

-0.050

-0.999

O2 + VO → O

-2.975

-1.959

-0.588

O2 → 2×O

+0.924

+2.154

+2.991

Figure 4: Schematic diagram and thermodynamic reaction-energy change for two possible CO oxidation cycling paths for the pristine, RP-1/2, and RP-3/2 surfaces. (a) Cycle Path 1 which exploits surface lattice oxygen for CO oxidation (b) Cycle Path 2 which allows for reactions only with adsorbed species. Empty red circles represent an oxygen vacancy on the surface, downarrow signifies adsorption on the surface, up-arrow signifies desorption (c) DFT calculated reaction energies (in eV) for each elementary step on pristine and faulted LaFeO3. Although the faulted structure appears to have little impact on the thermodynamics for most of the elementary steps, we find a key difference. Carbon monoxide and oxygen adsorption, which is often believed to be the rate-limiting step for CO oxidation on metal oxides

28-29,

is more

downhill with both RP-1/2 and RP-3/2 structures compared to the pristine LF (here shown path 1, index 5). By contrast, the formation of the surface vacancy by CO oxidation to CO2 is more downhill on the pristine surface. Calculating the kinetic barriers of this system was not possible in this study since owing to the contributions of the surface and strong electronic correlation in LaFeO3. Nudged elastic band (NEB) calculations were prohibitive due to the unavoidable numerical divergence. The electronic system therein had a very low numerical stability, and the problem was highly nonconvex. Fortunately, since the rate-limiting step for the oxidation process is the adsorption of CO or O2, the overall oxidation rate should therefore increase with increasing the adsorbate stability, shown in Figure 5. The thermodynamic profiles of these two cycling paths also serve to illustrate the adsorption steps that precede the formation of CO2. As such, the adsorption strength of CO (path 1, index 1), the co-adsorption of CO and O (path 1, index 6, and path 2 index 5) and the co-adsorption of CO and O2 (path 2, index 2) are compared on pristine, RP-1/2, and RP-3/2 surfaces. The configuration of CO2, CO, and O adsorption onto the surface is shown in Figure S5.



1.5

Adsorption Energy (eV)

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

1.0

CO+O 0.5

0.0

CO

-0.5

CO+O2 -1.0

RP-1/2

RP-3/2

Pristine

Surface Type

Figure 5: DFT calculated (co-)adsorption energy for CO, CO+O2, and CO+O on a pristine, RP1/2, and RP-3/2 defective surfaces.

The co-adsorption of CO with molecular or atomic oxygen showed a significant strengthening (more negative adsorption energy) on the surface of the RP-1/2 structure compared to pristine LaFeO3, whereas CO adsorption showed a small strengthening. Thus, the considerable enhancement effect of RP-1/2 on the adsorption stabilities of molecular and atomic oxygen plays a key role, which is an especially important mechanism for the catalytic cycles taking place in an atmosphere with a relatively high O2 ratio. By comparison, the adsorption strength of CO and its co-adsorption with O2 was found to decrease on the surface of the RP-3/2 structure compared to pristine; however, the RP-3/2 structure is energetically far less favorable than RP-1/2. Such mechanism can be further understood in-depth from the calculated projected electronic densities of states (DOS), together with the differential electronic densities (Δρ) driven by the adsorbatesurface interaction (Figure 6).


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Figure 6: The projected electronic densities of states (DOS, left column) and differential electronic densities (Δρ, right column, isovalue is 0.04 eÅ-3) of pristine and RP-1/2 surfaces. The projected DOS of the adsorbates (O and O2) and the surface Fe atom beneath are shown to reveal the adsorbate-surface bonding effects. The positive (in red) and negative (in blue) parts of Δρ indicate spatial regions of electron depletion and accumulation, respectively.

When the projected DOSs of an adsorbate and a surface atom have a concomitant overlapping peak position, there should exist considerable electronic hybridization through covalent interactions at the corresponding energy level. Figure 6 clearly shows that the RP-1/2 fault increases the number of overlapping orbital states between the adsorbate O2/O and surface Fe, indicating strengthened adsorbate-surface bonding. This is further supported by the change in 11 ACS Paragon Plus Environment


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electron density, Δρ, where we find that the adsorbate-Fe bonds are occupied by more electrons and enhance the covalent bond. It is the reduced effective atomic coordination of Fe that makes it easier to form more/stronger covalent bonds with adsorbates

30.

Furthermore, the strong O2-Fe

interaction re-orients the O2 molecule, making it more parallel to the surface with RP-1/2 fault, whereas it remains tilted with a high angle on the pristine LF surface. To exactly explain the temperature-dependent oxidation rates measured experimentally, the kinetic barrier and reaction rates need to be calculated using, e.g., NEB plus transition-state theory. Owing to the highly correlated electrons and extremely low electronic stability, physically fundamental work to formulate a DFT numerical optimization method that can account for atomic-bond forming/breaking on such correlated surfaces will be highly desirable for future work in this field. However, the rate-limiting step for the oxidation process should be the adsorption of CO or O2, and the overall oxidation rate should therefore increase with increasing the adsorbate probability (stability); therefore, thermodynamic trends can still provide qualitative explanations for catalytic enhancement. Experimentally, temperature programmed desorption (CO-TPD) was used in order to evaluate the CO adsorption strength on each material using a procedure described previously by others.

31

Figure 7a shows the peak CO desorption temperature was highest (412°C) for the

highly faulted cp-LF and lowest (339°C) for the pristine cs-LF sample, consistent with DFT calculations on the energetically favored RP-1/2 structure. A second high-temperature CO-TPD peak (476°C), which is particularly dominant in the cs-LF sample, is due to CO desorption from La2O3, consistent with the suggestion that La-enrichment in samples free of RP faults is supported by 2D La2O3 islands on the surface 32. Temperature programmed desorption of oxygen and carbon dioxide is shown in Figure S6. The cs-LF and sg-LF, showed weakly chemisorbed oxygen in the range of 200-350°C, strongly chemisorbed oxygen between 500-600°C, and the release of lattice oxygen above 600°C. The release of weakly chemisorbed oxygen occurs at slightly higher temperatures in the material with some stacking faults (sg-LF) compared to the pristine material (cs-LF). The highly faulted material shows only strongly chemisorbed oxygen and the release of lattice oxygen, consistent with the results from our DOS calculations in Figure 6. CO2 desorption for all three samples were largely similar and mostly indicative of the


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relative thermal stability of the different carbonates which formed due to the acidic character of CO2 31. CO oxidation experiments were carried out between 300-400°C in a plug-flow reactor using 15 mg of catalyst at a gas-hourly space velocity (GHSV) of 40L/gcat∙h where the feed composition was 1% CO, 20% O2, and 79% Ar by volume. We confirmed using TEM that no significant change in general structure of faults occurs when the samples are heated to the maximum reaction temperature (Figure S7). A key finding of this study is that the apparent activation energy for CO oxidation on LaFeO3 is linked to the concentration of RP stacking faults in otherwise identical materials. The relative abundance of RP stacking faults (0.2, 0.05, and 0 faults/nm for cp-, sg- and cs-LF respectively) remained the only significant structural or chemical variant between the three samples. XPS of all three variants confirms that the oxidation state of iron was 3+, and did not appreciably vary between the samples (Figure S8). Thermographic analysis in air showed that none of the samples had meaningful differences in oxygen deficiency at the reaction temperature (Figure S9), confirming also that there are no significant changes in La stoichiometry

33.

In XPS however,

small changes in surface stoichiometry from 0D defects are indistinguishable from small changes in surface stoichiometry expected from sub-surface 2D RP faults. It is therefore possible that 0D surface defects combined with the RP faults observed in TEM contribute simultaneously to the catalytic enhancement. The Arrhenius curves in Figure 7b-d enable the comparison of CO oxidation kinetics on the three LaFeO3 variants, where it was found that the apparent activation energy decreased as the RP stacking fault density increased. The apparent activation energy of the defect-free cs-LF, slightly defective sg-LF (0.05 faults/nm), and highly defective cp-LF(0.2 faults/nm) was 49.6, 40.6, and 32.0 kJ/mol respectively. The activation energy experiments were repeated to 50 L/gcat∙h and 60 L/gcat∙h and identical values were obtained, confirming that the values reported here for the apparent activation energy are not influenced by the effect of mass transfer.



sg-LF cs-LF cp-LF

Signal (a.u)

(a)

0

200

400

600

Temperature (°C)

-10.0

(b)

(c)

EA, apparent = 49.6 kJ/mol

(d)



-10.5

ln(rate [mols/min])

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

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-11.0

-11.5

-12.0

Sample: sg-LF RP concentration: LOW

Sample: cs-LF RP concentration: NONE (PRISTINE) 1.5

1.6

1.7

1.5

Sample: cp-LF RP concentration: HIGH 1.6

1000/T [1000/K]

1000/T [1000/K]

1.7

1.5

1.6 1000/T [1000/K]

Figure 7: (a) temperature programmed desorption of CO from all three variants of LaFeO3 after adsorption at 50⁰C. Ahrrenius plots for CO oxidation between 300-400⁰C for (b) pristine cs-LF (c) slightly faulted sg-LF (d) highly faulted cp-LF.

The manner in which RP faults modulate CO oxidation kinetics may be explained by two mechanisms. The first is through Sabatier’s principle where an increase in bonding energy between the reactants and the catalytic surface, up to a certain optimum value, increases the reaction rate per site (here, manifested as a lower apparent activation energy when catalysts are tested under identical conditions). While we are were unable to calculate the optimum binding energy using DFT+U, Figure 5 shows that subsurface RP faults could be a unique tool to modulate the surface adsorption strength towards its optimum value.34


1.7

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An alternative mechanism for RP modulated catalytic enhancement is through bulk oxygen ion diffusion to the surface via the RP structure. While the CO oxidation mechanism on perovskites is widely agreed to be suprafacial (not influenced by bulk oxygen ion diffusion from the bulk to the surface), reports have shown that under limited circumstances, B-site substituted LaFeO3 have shown enhanced CO oxidation kinetics due to bulk oxygen ion diffusion

35-36.

In this case,

we consider unsubstituted LaFeO3 perovskites, but with varying levels of RP faults. The anisotropic structure of the RP faults, together with their ability to be either oxygen-deficient or oxygen-excess may enable the faults to facilitate ionic oxygen transfer from the bulk to the surface. Lee et al.

37

report that this may be even more effective in RP structure where n≥2. We

are inclined to believe that this mechanism is relevant owing to fact that cp-LF, with a significantly lower apparent activation energy, showed RP fault “bunches” in the bulk structure. CONCLUSIONS Surface lanthanum enrichment previously reported in La-containing perovskite oxides is explained here by DFT and TEM as La-rich Ruddlesden-Popper (RP) faults. These faults are energetically favored to be near to the free surface or occur in bunches within the bulk. The effect of RP faults on CO oxidation kinetics on LaFeO3 was studied by modulating the synthetic route until three appreciably different samples were received with a variable degree of RP faulting. The apparent activation energy of CO oxidation decreased as the degree of faulting in the catalyst increased. This drop in apparent activation energy is supported by DFT calculations where the (co-)adsorption energies of CO, O, and O2 were found to be stronger on the RP-1/2 faulted surface compared to the pristine material. The increase in reaction rate with binding energy suggests that faulted LF materials are on the increasing side of a volcano curve where the binding energy can be effectively modified by stacking faults, a type of multi-dimensional defect. The RP faults were found to be stable up to the maximum operating temperature, and may therefore serve as a mechanism for lowering the activation barriers for other energy-related reactions. While not being able to fully capture the full complexity of the nanoparticle system, our computational results from the most stable faulted configurations shed insight into the structures that we found in the TEM (i.e. the bunches) and the variation in the catalytic performance.This study opens the gateway for further investigating strategies which exploit multi-dimensional defect engineering for enhancing ceramic catalysis 15 ACS Paragon Plus Environment


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METHODS Computational methods The structures and energies are calculated using density functional theory (DFT) as implemented in the VASP code package 38. The projector augmented-wave method

39

is used to pseudize the

electronic wave functions and Hamiltonians, and the valence configurations of 5s25p65d16s2, 3p63d74s1, and 2p42s2 are considered to generate the La, Fe, and O pseudopotentials. To accurately expand the electronic wave functions and charge densities, a high cutoff energy of 800 eV is used. An energetic threshold of 10-7 and 10-6 eV/atom are used for the self-consistent electronic iteration and structural optimization, respectively. There exists strong electronic correlation between the 3d electrons at Fe atom, due to the high spatial localization of the 3d orbitals. To efficiently capture such correlation effects when using a regular density functional (e.g., localized-density approximation or generalized gradient approximation), a Hubbard U correction should be used with the conventional DFT electronic potential 40. The PBE functional in the generalized gradient approximation 41 is used here as the conventional DFT potential, and the rotationally invariant approach by Dudarev for DFT+U 42 is used. In experiment, bulk LaFeO3 exhibits G-type antiferromagnetism (G-AFM) with a magnetic moment of 3.8~4.6 μB on the Fe atom

43-44.

This can be reproduced in our calculation using an

effective Dudarev U of 4.2 eV, and the Fe magnetic moment is calculated to be 4.2 μB here. The fitted U value of 4.2 eV here is closely consistent with those (4~5 eV) obtained for Fe oxides using more sophisticated methods, e.g., linear response approach

45

and self-consistent Hubbard

U approach 46 with PBE functional. To model the surface of LaFeO3, stoichiometric slabs with the lateral areas of 5.57×5.65 Å2 and the thicknesses of 16~18 Å are constructed in the periodic unit cells for the pristine surface and surfaces with RP defects at different depths. There are two possible surfaces (i.e., FeO2 and LaO terminated) in contact with the environment. The LaO-terminated surfaces has a much higher reactivity and may also be spontaneously destroyed by many physical/chemical adsorbates

47-48.

Thus, only the FeO2-terminated surface is considered for the catalyzed oxidation of CO here due to its much favored stability in reality. The interaction between neighboring periodic slabs are excluded by separating them using a vacuum of 15 Å, together with an inter-slab dipolar 16 ACS Paragon Plus Environment

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correction 49. For the integration of electronic eigenstates and eigenvalues in the Brillouin zone, a reciprocal grid with a grid density of ≥

22 𝑎

×

22 𝑏

22

×1 (𝑎 ×

22 𝑏

×

22 𝑐

) for a surface system (bulk RP

Lan+1FenO3n+1) is used, where a, b, and c are the lattice constants of the unit cell scaled by the unit of angstrom. Synthesis Three samples were prepared using three well-established different synthesis routes 40, 50: Combustion synthesis of LaFeO3 (cs-LF) – A solution of La(NO3)3۰6H2O (0.433g, 1mmol), Fe(NO3)3۰9H2O (0.404gr, 1mmol) and Glycine (0.3g,4mmol) were mixed. The solution was heated until self-ignition. The sample was calcined in air at 800°C for 4hr. Solgel synthesis of LaFeO3 (sgLF) – La(NO3)3۰6H2O (0.433g, 1mmol), Fe(NO3)3۰9H2O (0.404gr, 1mmol) and citric acid (0.384g, 2mmol) were mixed in di-ionized water (5mL). The sample was heated under stirring until gel formed and left to dry over-night. The sample was calcined in air in two stages – 600°C for 2hr and 800°C for 4hr. Co-precipitation synthesis of LaFeO3 (cp-LF) – La(NO3)3 ۰6H2O (0.6495g, 1.5mmol), Fe(NO3)3 ۰9H2O (0.606gr, 1.5mmol) and PVP (0.15g) were mixed in di-ionized water (100mL). The solution was sonicated and pH corrected to 9 using NaOH solution (2.5M). The solution was refluxed over-night at 70°C and cooled to RT. The precipitate was washed in water and ethanol, dried and the calcined in air in three stages – 150°C for 1hr, 400°C for 4hr and 800°C for 12hr. Catalytic CO oxidation Catalysts (15 mg) were placed at the center of a quartz tube between two pieces of quartz wool. The reaction was initiated by feeding a mixture of 1%CO, 20%O2 and Ar as balance with the space velocity (GHSV) of 40 L/gcat۰h. The product stream was analyzed using SRI gas chromatograph fitted with MS-13X and Haysep-C packed columns. CO and CO2 were detected by an FID detector with a methanizer placed directly upstream. The carbon balance was closed within 5%.



XRD, XRD in-situ and data fitting The powder X-ray diffraction (P-XRD) measurements were taken in Bruker AXSA D8 Advance with Cu Kα (λ = 1.5406 Å) radiation source and analyzed using Diffrac.EVA software by Bruker. Signals were collected in 2 range of 10˚ to 100˚ with step-size of 0.005˚, counting times of 0.5 sec and sample rotation rate 15 rpm. Rietveld Refinement was done using DIFFRAC.TOPAS in order to analyze phase composition, determine cell parameters and atomic positions. In order to determine no new phases are formed during reaction the sample was scanned at RT, 573K and 673K under the flow condition specified above. TPD, BET, and TGA BET and TPD analysis was performed using a ChemBET-3000 Quantachrome instruments equipped with a TCD.Single point BET specific surface areas of the catalysts were determined by nitrogen adsorption at -196°C. Prior to BET measurement, catalyst was grounded and degassed in He flow for 1hr at 200°C to remove any adsorbed gas and then cooled down to room temperature. CO and CO2 TPD analysis was done on a 15 mg sample in a quartz tube. Prior to the test, the catalyst was degassed in He for 1 hr at 200°C. The adsorption of CO and CO2 was performed at RT, then He for was ran for 15 min to remove physisorbed gas. The sample was heated from RT to 873K under He and signal was recorded on the TCD. For O2-TPD, the sample was heated up to 550 ⁰C in an Ar atmosphere. The sample was then exposed to a 5%O2/Ar mixture and cooled to 50 ⁰C. Once cooled, the gas was switched back to Ar and the desorption curve was measured while heating at a rate of 5 K/min. TGA of the catalysts was measured by Netzsch STA 449 F5 Jupiter in Al2O3 crucibles in the range of 298-1273K at a heating rate of 5 K∙min-1 under O2 /N2 atmosphere .

XPS X-ray Photoelectron Spectroscopy (XPS) measurements were performed in UHV (2.5x10 -10 Torr base pressure) using 5600 Multi-Technique System (PHI, USA). The sample was irradiated with an Al Kα monochromated source (1486.6 eV) and the electrons were analyzed by a Spherical Capacitor Analyzer using the slit aperture of 0.8 mm. Sample charging during


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measurements was compensated with a charge Neutralizer (C1s at 285 eV was taken as an energy reference). Samples were analyzed only at the surface. SEM and TEM Microscopy Transmission electron microscopy (TEM) images in Figure 2 were taken by Field Emission TEM (Tecnai@ F20, Philips) operated at 200 kV. Analytical TEM images in Figure 3 were taken using a JEOL JEM-2100F microscope operating at accelerating voltage of 200 kV equipped with GATAN 894 US1000 camera. Energy dispersive spectroscopy (EDS) point analysis and mapping were performed using a JEOL JED-2300T energy dispersive X-ray spectrometer. Probe tracking (drift correction) allowed chemical analysis at the nanometer scale. JEOL Analytical Station software (v. 3.8.0.21) was used for the EDS data analysis. The quantitative analysis was performed by the standard less Cliff−Lorimer method. All scanning TEM (STEM) images were taken in high-angle annular darkfield (HAADF) mode using GATAN 806 HAADF STEM detector. Since some of the catalysts exhibited large particle size (10-20μm) a thinning process was required for TEM analysis. Catalyst powder was dispersed in embond 610 epoxy resin and deposited on an aluminum foil. The foil was dried on hot plate at 150⁰C. A hole 3mm in diameter was punched and using a Gatan PIPS 691 (Precision ion polishing system) at 4-5kV the hole surrounding was polished and thinned. TEM simulations were done by considering an original slab of LaFeO3 that was 9.1nm thick, a spherical aberration of 10μm, chromatic aberration of 1nm, a de-focus of 6nm, a convergence angle of 8-15 mrad and a focal spread of 3nm. Each atomic potential slice was kept of 0.44 angstroms. SEM images were taken by Quanta 200 Schottky emitter based field emission gun (FEG) environmental SEM (ESEM) with 20 kV source beam voltage at high vacuum mode (typically 10-5 mbar). Since the catalysts exhibited large particle size (10-20μm) a thinning process was required for preparing our samples for TEM analysis. Catalyst powder was dispersed in Embond 610 epoxy resin and deposited on an aluminum foil. The foil was dried on hot plate at 150⁰C. A hole 3mm in diameter was punched and using a Gatan PIPS 691 (Precision ion polishing system) at 4-5kV the hole surrounding was polished and thinned. The thinning procedure does not impact the structure or density of stacking faults observed



SUPPORTING INFORMATION Includes SEM images, XRD patterns and Reitveld fitting of cell parameters, HAADF imaging, indexed FFT images, adsorption configuration of all adsorbates, temperature programmed desorption of oxygen and carbon dioxide, high-temperature TEM, XPS, and TGA analysis. DATA REPRODUCIBILITY All catalytic experimental data presented represents experiments that were performed three times. In all cases, the variation did not exceed the size/thickness of the dot/line of the plot itself. CODE AVAILABILITY The Vienna Ab Initio Simulation Package (VASP) is a proprietary software available for purchase at https://www.vasp.at/. Data processing scripts written to process output files and create figures are available upon request. DATA AVAILABILITY The simulation output files are available upon reasonable request. They are not publicly available due to the very large file sizes. Parameters of the input files are described in computational methods. Atomic structures for the bulk and slab geometries are available upon request. ACKNOWLEDGEMENTS This research was supported by a grant from the United States – Israel Binational Science Foundation (BSF), Jerusalem, Israel. This grant was also supported by the Israeli Ministry of Energy (grant #217-11-027). This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number ACI-1548562. REFERENCES (1) Lin, H.-K.; Chiu, H.-C.; Tsai, H.-C.; Chien, S.-H.; Wang, C.-B. Synthesis, Characterization and Catalytic Oxidation of Carbon Monoxide Over Cobalt Oxide. Catalysis letters 2003, 88 (3-4), 169-174. (2) Taguchi, H.; Yamasaki, S.; Itadani, A.; Yosinaga, M.; Hirota, K. CO Oxidation on Perovskite-Type LaCoO3 Synthesized Using Ethylene Glycol And Citric Acid. Catalysis Communications 2008, 9 (9), 19131915. (3) Singh, S. A.; Madras, G. Detailed Mechanism and Kinetic Study of CO Oxidation n Cobalt Oxide Surfaces. Applied Catalysis A: General 2015, 504, 463-475. (4) Mankidy, B.; Balakrishnan, N.; Joseph, B.; Gupta, V. CO Oxidation by Cobalt Oxide: An Experimental Study on the Relationship Between Nanoparticle Size and Reaction Kinetics. Austin Journal of Chemical Engineering 2014, 1 (2), 1-6.


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Catalytic Enhancement of CO Oxidation on LaFeO3 Regulated by

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