CO2 Reactivity on Cobalt-Based Perovskites - The Journal of Physical

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

CO Reactivity on Cobalt-Based Perovskites Jonathan Hwang, Reshma R. Rao, Yu Katayama, Dongkyu Lee, Xiao Renshaw Wang, Ethan J. Crumlin, Thirumalai Venkatesan, Ho Nyung Lee, and Yang Shao-Horn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06104 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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The Journal of Physical Chemistry

CO2 Reactivity on Cobalt-Based Perovskites Jonathan HWANG1*, Reshma R. RAO2, Yu KATAYAMA3, Dongkyu LEE4,10, Xiao Renshaw WANG5, Ethan CRUMLIN6, Thirumalai VENKATESAN7,8,9, Ho Nyung LEE10, Yang SHAO-HORN1,2,11*

1

Department of Materials Science and Engineering, 2Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

3

Department of Applied Chemistry, Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Yamaguchi, Japan 4

Mechanical Engineering Department College of Engineering and Computing, University of South Carolina, Columbia, SC, USA

5

School of Physical and Mathematical Sciences and School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

6

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 7 8

9

NUSNNI-Nanocore, Singapore

Department of Physics, National University of Singapore, Singapore

Department of Electrical and Computer Engineering, National University of Singapore, Singapore 10

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

11

Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA

Corresponding Authors* Email: [email protected], [email protected]

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Abstract Understanding the interaction of CO2 with perovskite metal oxide surfaces is crucial for the design of various perovskite (electro)chemical functionalities, such as solid oxide fuel cells, catalytic oxidation reactions, and gas sensing. In this study, we experimentally investigated the reactivity of CO2 with a series of cobalt-based perovskites (i.e., LaCoO3, La0.4Sr0.6CoO3, SrCoO2.5, and Pr0.5Ba0.5CoO3-δ) by a combined ambient-pressure XPS (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) approach. Isobaric measurements by AP-XPS on epitaxial pulsed laser deposition-grown (100) oriented thin films under 1 mTorr CO2 showed the formation and uptake of adsorbed adventitious-like C-C/C-H, -CO species, monodentate carbonate, and bidentate (b)carbonates. DRIFTS measurements on powder samples under CO2 atmosphere revealed the presence of multiple configurations of carbonate in the asymmetric O-C-O stretching region with peak splittings of ~100 cm-1 and ~300 cm-1 correlated to the monodentate and bidentate bound carbonate adsorbates, respectively. The synergy between chemical state identification by AP-XPS and vibrational state detection by DRIFTS allows both the carbonaceous species type and configuration to be identified. We further demonstrate that the surface chemistry of the A-site cation strongly influences CO2 reactivity; the La, Sr, and Ba cations in the LaCoO3, La0.4Sr0.6CoO3, SrCoO2.5, and Pr0.5Ba0.5CoO3 thin films showed significant carbon adsorbate speciation. Additionally, we link the La0.4Sr0.6CoO3 surface chemistry to its surface reactivity towards formation of bidentate (bi)carbonate species via exchange of lattice oxygen with carbonate oxygen. In conclusion, we show that the perovskite electronic structure ultimately dictates the driving force for formation of oxidized oxo-carbonaceous species (CO3) versus reduced species (C-C/C-H). A higher O 2p-band center relative to the Fermi level was correlated with a higher degree of (bi)carbonate formation relative to the other carbonaceous species observed (C-C/C-H and –CO) due to a more facile charge transfer from oxygen states at the Fermi level to free CO2 gas.

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Introduction The interaction and reactivity of molecular CO2 with metal oxides underlies numerous key chemical and electrochemical processes relevant for energy storage and conversion,1 gas sensing,2,3 gas separation,4 and toxicant removal. More fundamental understanding on how CO2 reacts with different surfaces can enhance design strategies for chemical transformation of CO2 through (electro)chemical means such as electrochemical reduction of CO2,5 gas-phase CO2 hydrogenation to produce fuels (e.g. methane),6 or the reverse water gas shift reaction.7 Further, CO2 is formed upon deep oxidation of highly toxic organic compounds.8 Of interest is to understand the reactivity between CO2 and perovskite oxides due to the significant influence of CO2 as reactants or products in these processes. Tuning the perovskite surface reactivity towards CO2(g) and other carbon-containing molecules through its electronic structure represents tremendous opportunities to further advance the performance of gas sensors,2,3 CO2-tolerant electrodes for solid oxide fuel cells (SOFCs) operating under air at intermediate temperatures,9 and electrochemical reduction of CO2 in solid oxide electrolyzer cells (SOEC) to make fuels at high temperatures.10,11 Unfortunately tuning surface CO2 reactivity for catalytic reactions12 or CO2 tolerance,13 is largely empirical. For example, the use of acid-base concepts in the choice of substituent cations in perovskites is wellknown,14 where CO2 and the metal oxides represent the acid and basic species, respectively, so incorporating less basic (higher acidity) cations minimize CO2-induced electrode degradation. Based on these concepts, doping perovskite chemistries with lower basicity cations like Ta5+ and Nb5+ were shown to exhibit improved tolerance to CO2 when utilized as oxygen permeating membranes or electrode materials in SOFCs.15,16

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However, fundamental understanding of CO2 reactivity on the complex perovskite surface and the physical origin of their reactivity is not well understood. Previous understanding of the CO2 interaction with perovskites include studies of LaMO3 perovskites (M = transition metal)17,18 for catalyzing gas-phase oxidation reactions of CO and higher-order hydrocarbons like propene and isobutene, where CO2 is the favored oxidation product.19,20 As the M cation was varied across the first-row transition metals, the reactivity and adsorption followed a M-shaped trend, with maximum in reactivity in the d4 and d6 cations (Mn and Co, respectively). This was attributed to the change in crystal field stabilization energy (∆Ec) upon adsorption of chemisorbed species, and so demonstrated the relationship between cation electron occupancy and surface reactivity to gaseous species. In this study, we examine the class of perovskite-based ACoO3-δ oxides to understand how the bulk perovskite chemistry and electronic structure influences the surface reactivity of CO2. We have chosen four perovskites with increasing metal-oxygen covalency

from

LaCoO3 (LCO),

La0.4Sr0.6CoO3 (LSC),

SrCoO2.5

(SCO),

to

Pr0.5Ba0.5CoO3-δ (PBCO), where the substitution of lower valency A-site cations results in electronic compensation by higher valency B-site Co states, effectively energetically lowering the B-site metal density of states.21,22 This metal-covalency can be quantified by the oxygen 2p-band center, which is defined as the difference in energy between the Fermi level and the centroid of all O 2p states.23 Ambient pressure XPS (AP-XPS)24,25 experiments on well-defined thin film surfaces of LCO, LSC, SCO and PBCO were combined with Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

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experiments of corresponding powder samples to quantitatively define carbon surface speciation and elucidate the mechanisms of CO2 reaction with metal oxides.

Experimental Methods Thin Film Fabrication and Characterization Epitaxial thin films of the (A,A’)CoO3 (A = La, Sr, Pr and A’ = Sr and Ba) were fabricated by pulsed laser deposition (PLD) on single crystal (001)-oriented 0.5 wt% Nb:SrTiO3 (Princeton Scientific). PLD targets of LCO, LSC, SCO, and PBCO were synthesized through solid-state reaction from stoichiometric amounts of La2O3, Co3O4, SrCO3, BaCO3, and Pr2O.22,26 PLD was performed using a KrF excimer laser (λ = 248 nm) at a pulse frequency of 10 Hz and laser fluence of ~1.6 J cm-2. The number of pulses were calibrated by reflection high-energy electron diffraction (RHEED) during growth to give ~25 nm film thickness. After completing the film deposition, the samples were cooled down to room temperature in the PLD chamber for ≈1 hour under a pO2 of 200 mTorr. Film surface morphologies were examined by atomic force microscopy (AFM, Bruker Dimension Icon). AFM indicates flat, conformal film deposition by PLD (Fig. S1). Thin film analysis was conducted by High-resolution X-ray Diffraction (HRXRD, Panalytical) to determine the structure and crystallographic orientation of the perovskite thin films. The long- range out-of-plane scans (Fig. S2) indicate the thin films were grown in the desired perovskite phase. The SCO film showed additional half-order peaks corresponding to the ordered oxygen vacancies of the brownmillerite structure. Highresolution spectra taken about the substrate (002) peak were also measured to verify

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epitaxy, with the calculated out-of-plane lattice parameters consistent with determined literature values (Fig. S3).

Ambient Pressure XPS AP-XPS measurements were conducted on Beamline 9.3.2 at the Advanced Light Source in Lawrence Berkeley National Laboratory. Each 5×10 cm2 sample was loaded onto a ceramic heater, and a thermocouple was secured by an Al2O3 piece. A piece of Au foil was also loaded onto the sample to ground the sample and provide a calibration by the Au 4f peak. Because the C-C/C-H adventitious-like species provides a natural internal calibration due to its ubiquity as a carbon source and by the fact that their binding energy values differ by less than 0.1 eV – less than the spectral resolution for peak identification (Table S1), internal calibration of the C 1s 490 eV spectra by the adventitious binding energy of 284.8 eV were consistent with those calibration by Au 4f. Isobars were collected for each sample by cooling from 300 °C to room temperature under 1 mTorr CO2. Prior to measurements, each sample was cleaned by heating the sample to 300 °C under 100 mTorr O2 to oxidize residual species on the surface. Quantification of beam damage under the conditions of this study were done by comparing the O 1s 735 eV spectrum on the clean LCO at the same spot (Fig. S4), indicating that the effect of beam damage is minimal to the analysis of this study. Isobars of each sample were conducted under 1 mTorr CO2; the sample was allowed to equilibrate for 15 minutes prior to measurement at each temperature. C 1s and O 1s spectra were measured every 25 °C, while the metal core level spectra were measured every 100 °C. The incident mean free path (IMFP) of the C 1s 490 eV and the metal core 350 eV spectrum was estimated to be ~6 Angstroms.27

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Analysis of the XPS spectra was done by CasaXPS to quantify the reactivity of CO2 on the surface and investigate the surface chemistries of the perovskite thin films. All spectra were fit using Gaussian-Lorentzian peaks after a Shirley-type background subtraction. The peak fitting parameters are summarized in Table S1.

DRIFTS Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectra were obtained on an FT-IR NICOLET8700 (Thermo Fisher Scientific) equipped with deuterated triglycine sulfate (DTGS) detector. DRIFT measurements were conducted on the chemistries of the corresponding powders. Experimental protocols were designed to be as comparable to the AP-XPS measurements as possible. 5% by weight KBr in perovskite were prepared by mechanical mixing. Prior to measurements, residual surface species were removed by heating the sample up to 300 °C in under a pure O2 flow at 200 mL/min. Then the gas flow was switched to pure CO2, with a flow rate of 20 mL/min controlled by a mass flow controller (MKS). Under constant flow, the temperature of the sample was subsequently cooled from 300 °C while collecting DRIFTS spectra every 25 °C until room temperature was reached. DRIFT measurements were done with a 4 cm-1 resolution in the 4000–500 cm-1 spectral range; 512 scans were averaged. The spectra were also deconvoluted using CasaXPS with a pure Gaussian peak for bicarbonate peaks and a pure Lorentzian lineshape for carbonate peaks.

Results and Discussion

CO2 Speciation on Cobalt Perovskites from APXPS and DRIFTS

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Figure 1: C 1s @ 490 eV isobars of ACoO3-δ cooling from 300 °C to 25 °C under 1 mTorr of CO2, where the A-site is occupied by A) La B) La0.4Sr0.6 C) Sr and D) Pr0.5Ba0.5. Spectrum (IMFP ~ 6A°) were collected every 25 °C after 15 minutes of equilibration at each condition. Prior to introducing CO2 to the AP-XPS chamber (blue), the sample was exposed to 100 mTorr O2 at 300 °C, removing residual carbonaceous species at the surface (clean, grey).

After removal of surface contaminants on the thin films under 100 mTorr O2 at 300°C, insignificant amounts of carbon species and surface impurities were observed in the C 1s spectra collected at 490 eV incident energy (Fig. 1A-D, grey) and a survey spectra collected at 735 eV incident energy (Fig. S5), indicating a clean surface. O2 was removed and 1 mTorr CO2 was immediately dosed at 300 °C. During cooling to 25 °C under a constant pressure of 1 mTorr CO2, significant changes in the C 1s 490 eV spectra across all four chemistries were observed (Fig. 1A-D, blue). The peak intensities observed in the ~284.8 eV and ~289 eV regions are indicative of adventitious-like carbon (C-C, C-H) species and (bi)carbonate (-CO3) species, respectively. These distinct peak features have been observed on lanthanide perovskite surfaces previously under water vapor and had been ascribed to contributions from trace carbon impurities under ultra-

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high vacuum conditions.28 Based on the C 1s spectrum isobar, qualitative trends in the carbon speciation can be observed. While the adventitious-like species is the majority for LCO (Fig. 1A) at these ranges of pressure and temperature conditions, -CO3 carbonatetype species are instead the majority species for SCO (Fig. 1C) and PBCO (Fig. 1D). On LSC (Fig. 1B), significant contribution from both the C-C/C-H adventitious-like carbon species and (bi)carbonate species are observed in observable amounts.

Figure 2: Deconvolution of C 1s spectrum collected at 490 eV incident photon energy. XPS spectrum under 1 mTorr of CO2 at A) 300 °C and B) 25 °C for the four chemistries described. Raw data in counts

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per second are represented as circles and the fitted components are shown as solid colors. Four distinct peaks were identified with binding energy values at 284.8 eV, 286.5 eV, 288.8 eV and 290.6 eV, corresponding to C-C/C-H, -CO, and two states of –CO3, respectively. Fitting parameters for the XPS spectrum are described in the SI (Table S1). Peaks were normalized to Shirley background intensities (~283 eV BE) for comparison across chemistries.

To further quantify and identify oxo-carbonaceous formed as a function of CO2 uptake, the C 1s spectra were deconvoluted to four components: the C-C/C-H bonding species at 284.8 eV, -CO bonding at 286.5 eV, monodentate carbonate species at 288.8 eV and (bi)carbonate species at 290.6 eV, as shown in Fig. 2. The spectral features were identified in this manner because the binding energy values of the deconvoluted peaks fall within the ranges of reported literature binding energy values (Fig. S6) for the peak assignments observed here. The C 1s spectra demonstrate little evidence of protonated species observed previously for CO2 adsorption such as formate (287.3-287.7 eV),29 methoxy (285.2 eV),30 or carboxylate (288.5-285.8 eV),29 suggesting the formation of carbon-oxygen bonding –CO species (i.e. C=O or C≡O species) adsorbed on the surface. As shown in the isobar C 1s spectrum (Fig. 1) qualitatively, the quantitative deconvolution of the C 1s show that significant amounts of the C-C/C-H – like species at 284.8 eV was found on LCO and LSCO while little was found for SCO and PBCO at 300 °C.

On the other hand, -CO3 carbonate species at 288.8 eV and at 290.6 eV were instead

the majority species for SCO and PBCO. At 25 °C, the C-C/C-H species at 284.8 eV grew for all oxides and the -CO component at 286.5 eV increased for LSC only relative to other components upon cooling. The identified carbonaceous surface species formed from CO2 adsorption on these cobalt-based perovskite thin films from APXPS measurements were supported largely by

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DRIFTS measurements on corresponding powders in pure CO2 under a 20 mL min-1 flow rate at room temperature (Fig. 3).

Figure 3: DRIFTS spectrum of the symmetric carbonate O-C-O stretching region and the carbon monoxide C-O stretching region at room temperature under flow rate of 20 mL min-1 of pure CO2 on perovskite polycrystalline powders. A) Monodentate carbonate (light blue, ∆~100 cm-1), bidentate carbonate (dark blue, ∆~300 cm-1), and (bi)carbonate species (black dashed) were observed. B) A sharp peak corresponding to physisorbed carbon monoxide (FWHM = 5 cm-1) and chemisorbed carbon monoxide (FWHM = 20 cm-1) were centered at 2076 and 2060 cm-1, respectively.

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The observation of –CO species is supported by observations of spectral peaks observed in the ~2125 to ~2025 cm-1 range across all four perovskite chemistries (Fig 3B), which correspond to the C≡O-like stretching mode. Both a broader peak (FWHM ~ 20 cm-1) at 2060 cm-1 and a sharper peak (FWHM ~ 5 cm-1) at 2076 cm-1 in the carbon monoxide region of the spectra, corresponding to linearly chemisorbed CO and physisorbed CO,17 were identified (Fig. 3B). These are likely the products of CO2 dissociation on the surface, which has been broadly observed on transition metals31,32 and binary transition metal oxides,33 indicating that the –CO peaks observed in AP-XPS could likely result from adsorbed carbon monoxide species. The formation of carbonate-like species observed in AP-XPS is also supported by the DRIFTS measurements; spectrum occurring in the 1700 - 1100 cm-1 region are fingerprints of the vibrational O-C-O stretching mode in adsorbed (bi)carbonate species which can be used to elucidate bonding configuration details of surface adsorbates. Broad peak signals centered at 1290 cm-1 on LCO to 1595 cm-1 PBCO were observed (Fig. 3A), which can be attributed to a convolution of the rich array of chemical environments present in the adsorbed (bi)carbonate species such as adsorbate coverage, configuration, and adsorbate-adsorbate interactions.34,35 The spectra of LCO spectra at room temperature under pure CO2 gas shows two broad peaks (FWHM ~ 70 cm-1) centered at 1290 and 1386 cm-1. The peak splitting (∆) of 96 cm-1 in this study compared to ∆ =110 cm-1 (1355 cm-1, 1465 cm-1) observed by Tascón et al.18 indicate the presence of the O-CO component of the monodentate carbonate configuration, which has been extensively shown empirically to exhibit ∆~100 cm-1 on oxide surfaces due lowering of vibrational symmetry when carbonate is bonded to the surface.34 The monodentate carbonate

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configuration was similarly observed for higher covalency perovskite chemistries (LSC, SCO, PBCO), with ∆ values of 65, 66, and 73 cm-1, respectively, in agreement with the AP-XPS observation of a monodentate carbonate configuration in the low-temperature range. On LCO, the lack of vibrational signal in the 1200 and 1500 cm-1 region for LaCoO3 suggests the absence of carbonate species with a higher degree of coordination to the surface such as bidentate or bridged carbonates, as these configurations typically exhibit a ∆ of 300 and 400 cm-1, respectively,34 due to the higher polarizing power when two of the carbonate oxygen atoms are bound to the surface cations.36 In contrast, strong peak signals centered at 1600 cm-1 and 1310 cm-1 on LSC and 1280 cm-1 and 1595 cm-1 on PBCO were observed, having ∆ values of 290 cm-1 and 315 cm-1, respectively, indicative of the bidentate carbonate configuration while SCO has much attenuated signals at 1344 cm-1 and 1622 cm-1 having a ∆ of 282 cm-1. The bidentate carbonate configuration found for LSC, PBCO and SCO powder was not observed for LCO, which is in agreement with higher binding energy peaks observed in AP-XPS C 1s spectra of corresponding thin films (Fig. 2). We cannot exclude the presence of polydentate carbonates such as the flat-lying carbonate configuration observed in metals37 or other three-fold or four-fold coordinated carbonates in the DRIFTS as these species exhibit small vibrational splittings.34 The smaller ∆ observed in literature for these specific configurations results from the full or partial recovery of vibrational symmetry because all three oxygen atoms in the carbonate species are bonded in some manner to a surface site.34 Additionally, a clear sharp 1384 cm-1 peak was observed for LCO, LSC, SCO, and PBCO, which could likely result from bicarbonate formation when CO2 reacts with

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H2O and hydroxyl species on oxide surfaces,35 since the examined powder oxides were previously exposed to ambient H2O and CO2. No comparable feature across the four oxide chemistries were observed in the AP-XPS measurements, indicating that the formation of these bicarbonate-like species observed in DRIFTS likely originates from the nature of sample powder and storage in the DRIFT measurements, and not intrinsically due to CO2 reaction with a clean oxide surface as measured by AP-XPS. In summary, the DRIFT measurements demonstrate that the 286.5 eV, 288.8 eV, and 290.6 eV peaks observed in the C 1s spectrum of the AP-XPS measurements likely originate from carbon monoxide-like stretching in the C≡O region, monodentate carbonate stretching in the O-C-O region, and bidentate carbonate and/or bicarbonate stretching in the O-C-O region, respectively. This agreement in both the vibrational state and chemical state energies of CO2 speciation in the perovskite chemistries lends support to the current AP-XPS peak assignments and the complementary nature of AP-XPS and DRIFTS measurements on thin films and powders.

Temperature-Dependent Evolution of Carbonaceous Species on Perovskites from APXPS

Figure 4: Quantification of carbonaceous species of the C 1s 490 eV spectrum as a function of temperature under 1 mTorr CO2 from 300 oC to room temperature for A) LCO B) LSC C) SCO D) PBCO. The

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intensities of C-C/C-H (grey circle), -CO (yellow triangle), monodentate carbonate (blue open square), and bidentate (bi)carbonate (blue filled square) peak areas were normalized by the Shirley-type background intensities in the C 1s 490 eV spectrum. E) Total carbonate species intensities relative to the adventitiouslike C-C/C-H species.

In order to understand the evolution of CO2 uptake as a function of temperature for each chemistry, the normalized peak intensities of the deconvoluted peaks in the C 1s spectra are shown in Fig. 4A-D. Across all four chemistries, the total peak area intensities in the C 1s 490 eV spectrum increase as temperature is decreased (Fig. S7), with the largest magnitude of increase observed for LCO, indicating the reaction and uptake of oxo-carbonaceous species from CO2 on the perovskite surface. The carbon species originating from adventitious carbon (284.8 eV) was found to increase significantly with decreasing temperature for LCO (~7 times in Fig. 4A), SCO (~ 5 times in Fig. 4C) and PBCO (> 10 times, Fig. 4D) while that for LSC remained nearly unchanged (Fig. 4B). In addition, the -CO component (286.5 eV) in the LCO, SCO and PBCO spectrum increases slightly upon cooling while that for LSC increases sharply until its peak intensity is approximately the same as that of the C-C/C-H peak at room temperature. Moreover, little growth in the total (bi)carbonate species (sum of 288.8 eV and 290.6 eV peaks) upon cooling was found for LCO while more significant growth was found for SCO, PBCO and LSCO. To quantify and compare the speciation of the most oxidized carbonaceous species – total (bi)carbonates – and the most reduced products – C-C/C-H – , the 288.8 and 290.6 eV peak areas were normalized to the 284.8 eV peak area in the C 1s spectrum (Fig. 4E). This value can be described as the relative fraction of oxidized adsorbates ((bi)carbonates) formed on the surface as identified from the APXPS C 1s spectrum deconvolution (Fig. 2) and DRIFTS measurements (Fig. 3), which increased as the chemistry was varied from LCO, LSC, SCO to PBCO at 300 °C. Upon cooling, the

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relative fraction of surface (bi)carbonates monotonically decreases for LCO, SCO, and PBCO, but a considerable increase was found on LSC due to the significant increase of bidentate (bi)carbonates and relatively constant C-C/C-H species (Fig 4E). At 25 °C, the order of the fraction of total carbonates relative to the C-C/C-H species detected by APXPS goes from LCO < SCO < PBCO < LSCO.

Temperature-Dependent Perovskite Surface Chemistry

Figure 5: Surface chemistry changes of the A-site cation of the four perovskite chemistries between the clean conditions (grey, 300 °C, pO2 = 100 mTorr) and CO2 - exposed conditions (black, 25 °C, pCO2 = 1 mTorr). Background subtracted spectra for the A) LCO La 4d B) LSC La 4d C) LSC Sr 3d D) SCO Sr 3d and E) PBCO Ba 4d. Spectra shown were measured with an incident photon energy of 350 eV (~6 Å IMFP) F) The surface contribution the A-site cation of the corresponding perovskite chemistry. The surface contribution of the La species was quantified by the area percentage of the main non-satellite peaks – where higher percentage indicates higher contribution of ionic adsorbed species. Sr and Ba species surface contribution were quantified by the peak area ratio surface: lattice. G) Qualitative schematic of the changes in LSC surface chemistry and CO2 reactivity as a function of temperature. Sr segregation at the surface (SrLa) was hypothesized to originate from oxygen vacancies formed by exposure to the oxygen-deficient CO2 atmosphere.

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Core levels of A-site metal and B-site cobalt ions were measured in AP-XPS measurements to understand the relationship between perovskite surface chemistry and their surface reactivity to CO2, namely, the changes in the oxidation of the B-site cobalt ions and segregation of A-site metal ions under the CO2-rich conditions studied here. The Co 3p spectrum of the perovskite films after surface cleaning at 300 °C prior to CO2 exposure consists of contributions from the 3p1/2 and 3p3/2 peaks at binding energies of ~60 and ~62 eV, respectively. Upon dosing to 1 mTorr CO2 and subsequently cooling from 300 °C to room temperature, no discernable changes were found in the Co 3p spectrum for LCO, LSCO, SCO, and PBCO (Fig. S8). The observation suggests that there was no significant reduction in the cobalt oxidation upon reacting with CO2 on the surface within the depth of APXPS detection (~6 Angstroms). It should be noted, however, that the lack of changes in the Co 3p spectra does not preclude the Co B-site termination involvement in the CO2 reaction with the perovskite surface, as the large spectral peak width of Co 3p precludes deconvolution of “surface” and “lattice” components, and detection of subtle changes in the surface cobalt oxidation associated with CO2 reactivity. The chemical state of A-site species in the perovskite films were also conducted, as A-site ions can play a critical role in surface reactivity of perovskites as shown previously for the hydroxylation of H2O on (001) LaCoO3 epitaxial thin films,38 and perovskites can have thermodynamically stable39,40 AO-terminated surfaces such as the LaO-terminated (001) surface of LaCoO3. Analysis of the La 4d spectra (Fig. 5A) suggests the formation of species like La-CO3, La-CO, and La-adventitious carbon upon exposing LCO to CO2. On the LCO surface, the La 4d spectra consists of contributions

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from both the 4d5/2 and 4d3/2 peaks, in addition to strong contributions from satellite peaks. The satellite peaks, which originate from charge transfer between the ligand valence states and the lanthanum 4f states, have been shown to correlate with the degree of ionicity, so they serve as a useful indicator of adsorbate bonding to the surface.38,41 The satellite peaks can be deconvoluted into antibonding and bonding peaks for each of the main 4d5/2 and 4d3/2 peaks, resulting in six distinct peaks total in the La 4d spectrum (Fig. S9). On the clean LaCoO3 surface, the percent peak area of the La 4d 735 eV spectra (~12 Å IMFP) constituting the satellite peaks (49%) match well with those previously observed on the clean surface of LCO thin films (49%).38 Depth-resolved measurements on the more surface-sensitive La 4d 350 eV (~6 Å IMFP) on the same clean LaCoO3 surface show that the satellite contributions are decreased (32%), indicating that more ionic species dominate the sampling depth closer to the surface, since it has been shown for the La 4d spectra that more ionic species like La(OH)3 have smaller satellite contribution (29%) than more covalent species like La2O3 (58%).41 The measured satellite contributions of the LCO La 4d in this study (49%) fall between that of La2O3 and La(OH)3 as expected because of its mixed ionic-covalent character. Upon introducing the clean surface to 1 mTorr CO2 and incrementally decreasing the sample temperature to 25 °C, the satellite contributions at the 735 eV and 350 eV incident photon energies decrease from 49% to 38% and from 32% to 28%, respectively, changes that are consistent with the formation of more ionic La-surface species (La-CO3, La-CO, and Laadventitious carbon) during reaction with CO2.41 Similar changes were found for lanthanum in LSC, where the satellite contribution in the La 4d 350 eV spectra decreased from 33% on the clean surface to 25%

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under 1 mTorr CO2 at 25 °C as a result of the ionicity of adsorbed carbon species (Fig. 5B). This finding is in agreement with A-site surface termination of LSC as shown by DFT calculations, where Sr substitution of La in LCO stabilizes the La1-xSrxO termination40 relative to CoO2 termination, and low-energy ion scattering evidence of a La1-xSrxO termination for La0.6Sr0.4CoO3.42 In addition, the surface Sr contribution was quantified by the area ratios of the “surface” to the “lattice” Sr 3d3/2 peaks at binding energies of ~134.8 and 133.6 eV, respectively (Fig. S10). The identified “surface” strontium peak binding energies falls within reported values for the surface strontium chemical environment43–45; these “surface” Sr species not only captures undercoordinated Sr terminations, but also includes segregated Sr phases and strontiumcarbonates observed previously on La1-xSrxCoO3 system in AP-XPS.43,46 “Lattice” peaks capture Sr species residing in the near-surface lattice at AP-XPS-detectable depths (~6 Angstroms). A significant decrease in the peak intensity in the higher binding energy “surface” Sr 3d peak (hν= 350 eV) was observed between the initially clean surface and under 1 mTorr CO2 (Fig. 5C). Quantitatively, the surface: lattice area ratio decreased from 2.1 for the clean surface upon cooling to 1.18 at room temperature under 1 mTorr CO2. This observation contrasts with that observed previously on epitaxial La0.8Sr0.2CoO3 thin films between 220 °C and 500 °C under an oxygen pressure of 10-3 atm, where increasing (decreasing) temperature led to observations of decreased (increased) Sr “surface” species and increased Sr “lattice” contribution43. This difference can be attributed to the favorability of surface oxygen vacancy formation in the presence of a CO2 atmosphere. The formation of oxygen vacancies at the surface (within ~6 Angstrom) is supported by the slight increase (0.3 eV) in the binding energy of the oxygen lattice

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peak in the O 1s 735 eV spectra, accompanied by decrease in the lattice oxygen species intensities (~16%) upon cooling under 1 mTorr CO2 (Fig. S11). The ~531.9 eV peak consisting of the surface oxygen and oxo-carbonaceous species is relatively unchanged in peak intensity, which may be due to the conversion of surface oxygen species to adsorbate carbonates. The oxygen vacancy formation is electronically compensated by the B-site (Co) valency, as observed by the reduction of the Co4+ peak in the Co L-edge X-ray absorption peak upon decreasing temperature from 300 to 25 °C under 1 mTorr CO2 (Fig. S12). Additionally, increasing surface oxygen vacancies in the perovskite structure can be coupled to increased surface Sr segregation in the perovskite structure upon exposure to 1 mTorr CO2, which is associated with electrostatic interactions between the positively charged oxygen vacancies and negatively charged Sr2+ relative to La3+ (Kröger-Vink notation). This manifests as a decrease in the surface: lattice as Sr2+ migrate towards the surface, increasing the concentration of Sr species as “lattice” peaks at the near-surface. The coupling of oxygen vacancies and Sr segregation to the surface at relatively low oxygen pressure47 has been observed in as-grown and surface-modified La0.8Sr0.2CoO348,49 and La0.6Sr0.4CoO3 thin films by coherent Bragg rod analysis (COBRA). This indicates that the near-surface stoichiometry of Sr could be higher than that of the nominal bulk, resulting in formation of Sr-enriched SrCoOx-like species near the surface. The changes in the Sr 3d spectra in SCO were different from those in LSC (Fig. S10); in SCO, the surface: lattice area ratio of the Sr 3d 350 eV spectra increased from 0.67 to 1.28 between the clean surface and 1 mTorr CO2 at room temperature (Fig. S10). This supports the formation of adsorbed carbonaceous species on the SrO-terminated

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surface, especially SrCO3-like species as suggested by the C 1s 490 eV spectrum (Fig. 5D). As observed in both the AP-XPS C 1s 490 eV spectrum and DRIFTS measurements, this species likely adopts the monodentate configuration. Similarly, PBCO was found to have A-site ions reacting with CO2, where Ba2+ ions are more reactive than Pr3+. The Ba 4d spectrum in the PBCO thin film was deconvoluted with both “surface” and “lattice” peaks for the Ba 4d5/2 and 4d3/2 peaks (Fig. S10). The Ba 4d 350 eV spectrum on the clean sample initially showed a surface: lattice area ratio of 1.8 which increases to 2.1 at the completion of the CO2 isobar at room temperature in 1 mTorr CO2. The distinct growth of the higher binding energy peaks of the Ba “surface” species indicates the participation of Ba species in CO2 reactivity at the perovskite surface (Fig. 5E). Unlike the Ba 4d spectrum, the oxidation state-sensitive Pr 4d spectrum50 is constant across all conditions measured under CO2, which suggests that Pr species plays a lesser role than Ba in the CO2 surface reactivity for this chemistry. This hypothesis is supported by the following: 1) experimental51 and density functional theory calculations22 indicate preferential termination by the Ba-O surface in the polycrystalline and (001) orientation of the PBCO double perovskite structure and 2) Ba2+ is more basic than Pr3+.52,53 In summary, the evolution of perovskite surface chemistry upon exposure to CO2 was observed through the bonding of surface adsorbates by La in LCO and LSC, Sr in SCO, and Ba in PBCO (Fig. 5F), in addition to Sr mobility and segregation in LSC (Fig. 5G). These observations provide means to understand CO2 reactivity trends in the context of both bulk and surface chemistry.

O 2p-Band Center-Dependent Oxide Reactivity with CO2

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Figure 6: Mechanistic driving forces for CO2 reactivity on perovskite surfaces. A) Correlation of O 2pband center with the percentage of carbonaceous observed as total carbonates, which was quantified by the area intensity ratio: total (bi)carbonates/ (total (bi)carbonates + -CO + C-C/C-H). B) Charge transfer from the oxide to CO2 acceptor orbitals of covalent (right) compared to less covalent (left) chemistries in a simple band schematic. Charge transfer from the lesser covalent oxide consists primarily from the metal 3d states while that from the more covalent oxide consists some additionally from the oxygen 2p states. C) Proposed mechanism for CO2 reactivity on Co-based perovskite surfaces where red = oxygen, green = carbon, grey = surface cation. The driving force for the charge transfer branch (right) relative to the dissociation branch (left) is dictated by the O 2p band-center and relative metal/oxygen states at the Fermi level.

The relative fraction of the oxidized carbonates – (H)CO3 species – formed compared to the total oxo-carbonaceous species formed by reaction with CO2 – the cumulative amount of (H)CO3, -CO, C-C/C-H species - was found to increase with greater Co-O covalency, calculated by quantifying the distance of the O 2p-band center to the Fermi level (Fig. 6A). The trend in the calculated O 2p-band center can be reasonably expected to correlate with the experimental O 2p-band center of first-row transition metal perovskite oxides, as has been demonstrated previously by spectroscopic methods.21 Fig.

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6A demonstrates a strong linear correlation coefficient of r2 = 0.95 at 300 °C but a noticeably weaker one of r2 = 0.83 at 25 °C after the sample has been cooled. Overall, this positive correlation is likely due to relationship between the O 2pband center (Fig. S13) and the ability of the surface oxygen sites to donate charge to oxygen-bound carbonaceous adsorbates (Fig. 6B). The monodentate-configured carbonate species can be formed by direct adsorption of the slightly electropositive carbon on the electronegative surface oxygen sites (Fig. 6C, right branch). This reaction scheme was demonstrated previously in quantum cluster calculations that showed carbonate formation from CO2(g) on alkaline earth oxides – MgO and CaO – is primarily determined by charge transfer from the surface to adsorbed carbonate. This model of a slightly-bent CO2 adsorbed on O2- surface ions can be similarly applied to perovskite surfaces54 and also agrees experimentally with Tascón and Tejuca’s observations of CO2 adsorption on LaCoO3 polycrystalline powders as observed in Fourier-transform infrared spectroscopy (FTIR).18 Therefore, it is proposed that the electronic structure of the O 2pband center captures the ease of oxygen electron transfer from the surface to CO2 π* acceptor orbitals,55 where a smaller distance between the O 2p-band center and Fermi level facilitates this localized charge transfer. Tuning the Co-O covalency in these ACoO3 perovskites and decreasing the oxide charge-transfer energy between metal 3d and oxygen 2p states in the bulk electronic structure21,56 increases the affinity towards (bi) carbonate formation from CO2. These observations experimentally support the role of the electronic structure in defining the acid-base characteristics of surfaces, where high electronegativity and band gap are linked to strong oxide acidity for binary oxides.57 In this classical framework, increasing the Co3+ to Co4+ valency from LCO to PBCO drives

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the increasing basicity of these perovskite oxide chemistries through decreasing the oxide charge-transfer energy in the electronic structure. While this framework describes the CO2 speciation on these perovskites quite well at the initial CO2 dosing at 300 °C, a much poorer correlation is found between CO2 carbonate formation and electronic structure after cooling the sample to 25 °C (r2 = 0.83). However, when peak area contributions from the bidentate (bi)carbonate species is excluded, the correlation between O 2p-band center and the relative monodentate carbonate formation – calculated by the percentage of monodentate carbonate formed among monodentate carbonate, C-C/C-H, and –CO species – holds well from 300 °C to 25 °C (r2 = 0.9 and 0.97, respectively) in Fig. S14. This indicates that the source of this discrepancy between the nominal bulk electronic structure and total (bi)carbonate speciation on the surface must originate from the formation of bidentate species under temperature cooling under CO2, which occur most noticeably in the LSC and PBCO species. Indeed, this is the case at room temperature as LSC exhibits a significant higher percentage of bidentate (bi)carbonate formation (~33% of total carbonaceous species formed) compared to PBCO (~6%) of the total carbonaceous species. This phenomenon may be attributed to the surface chemistry observed in the dual A-site LSC and PBCO. Bidentate carbonate formation on oxides has been posited to originate from “oxygen scrambling” between adsorbed CO2 and near-surface lattice oxygen species (Fig. 6C, right branch). This process links our observations of oxygen vacancy formation and Sr segregation on LSC to the formation of bidentate configured carbonates, where increasing favorability of oxygen vacancy formation at the surface of LSC results in enhanced oxygen exchange between lattice oxygen and (bi)dentate carbonate formation

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through rotational-like mechanisms.58,59 The rotational driving force also originates from surface heterogeneity, as adsorbates switch from weak to strong basic sites.59 While significant formation of bidentate (b)carbonate was observed on both LSC and PBCO thin films at room temperature under 1 mTorr CO2, the higher degree of bidentate (bi)carbonate formation in LSC relative to PBCO and the other chemistries studied here may be attributed to the known formation of SrCoOx-like species on La1-xSrxCoO3 surfaces, as observed on La0.8Sr0.2CoO343,46,51 and also on LSC in the current study at room temperature under 1 mTorr CO2, increasing the surface O 2p-band center and decreasing surface oxygen vacancy formation energy as temperature is decreased.60 As such, the nominal bulk O 2p-band center may not be sufficient to capture the surface reactivity of LSC, as the surface adopts a higher oxygen 2p-band center through phenomena distinct to LSC compared to the rest of the cobalt chemistries studied here. Additionally, the role of oxygen vacancy formation in driving “oxygen scrambling” could also account for the observed formation of bidentate carbonate in the polycrystalline SCO samples shown in the DRIFTS measurements. Strong anisotropy in oxygen vacancy formation and mobility have been observed in SCO previously,61 such that exposed (114) facets with out-of-plane oxygen vacancy channels favor oxygen exchange.44 Polycrystalline powders exposing such facets could exhibit a higher driving force for isotopic “oxygen scrambling” in contrast to the (001)-oriented thin films measured by AP-XPS. The preferred orientation of the (114) facet in SCO powders has been reported previously62 and is similarly observed in the SCO powders of this study (Fig. S15). These dynamic observations of La1-xSrxCoO3 surface chemistry and reactivity and SCO anisotropy merit further investigation on their CO2 and small molecule reactivity.

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Lastly, the total carbon speciation on the perovskite surfaces was also investigated as a function of the bulk electronic structure (Fig. S16). The overall trend at both 300 °C and 25 °C was that LCO exhibited the largest degree of carbonaceous species formation, followed by PBCO and SCO, while LSC exhibited the least amount of CO2 speciation. These observations might result from the formation of SrOx and BaOx – related insulating phases, rendering these species unreactive towards the CO2 reactivity processes described thus far. Nevertheless, these differences in the film surfaces do not affect our understanding of the distribution of species formed upon CO2 exposure to perovskite chemistries. The described model for CO2 reactivity on perovskite surfaces for formation of oxo-carbonaceous species is summarized in Fig. 6C. The formation of reduced species can result following a series of dissociation steps upon CO2 adsorption via CO species to adventitious-like (C-C/C-H) species (Fig. 6C, left branch). On ultra-high vacuum (UHV) studies conducted on single crystal metals, it is postulated that the work function strongly dictates the formation of chemisorbed CO2-, where a clean metal with a work function lower than 5 eV can activate CO2, as a precursor to dissociation.31 These effects have been widely observed on oriented Ni single crystals32 and extended to oxides such as Fe2O3 for conversion of CO2 to reduced CO and C species by dissociation at high temperatures (500 °C) and high rates.33 This merits further investigation of dissociative pathway intermediates like CO, which previous vibrational studies have observed to adsorb linearly on metal sites of first-row transition metal perovskites.17 This framework also differentiates the role of the cation and ligand on the heterogeneous metal oxide surface, where metal sites are likely to promote the dissociation of CO2 upon adsorption

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whereas oxygen sites promote the formation of (bi)carbonates, providing a simple framework of metal-based activity and oxygen-based activity on metal oxide surfaces. As a result, we observe that highly covalent cobalt perovskite chemistries with higher O 2p-band centers preferentially form (bi)carbonate species over reduced C-C/C-H and/or – CO species due to the promotion of their oxygen surface activity.

Conclusions In this study, we demonstrate CO2 reactivity trends as a function of perovskite cobaltite bulk chemistry for LCO, LSC, SCO, and PBCO. We identify key CO2 speciation products on the (100)-oriented thin films and powder samples by AP-XPS and DRIFTS, respectively: adventitious carbon, -CO species, monodentate carbonate, and bidentate (bi)carbonate. Additionally, we identify relevant A-site species actively involved in CO2 surface reactivity on epitaxial perovskite thin films through the formation of higher binding energy species in AP-XPS on La, Sr, and Ba in LCO, LSC, SCO, and PBCO corresponding to surface A-site bonding to oxo-carbonaceous species. We show that the chemical reactivity trends of CO2 can be explained by a band description of electronic structure; a higher O 2p-band center relative to the Fermi level results in greater amounts of oxidized carbonate species formed relative to reduced adventitious-like carbon on the surface. These findings present new insights into the design of oxides for electrochemical and chemical functionalities in CO2-containing atmospheres.

Supporting Information

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Additional sample characterization details. Peak fitting parameters and deconvolution of XPS spectra. Acknowledgments This work was supported in part by Eni. The ALS beamline 9.3.2 is supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences and Materials Sciences Division of the US DOE at the Lawrence Berkeley National Laboratory under Contract DE-AC02-05CH11231. Some of the PLD film growth was conducted at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. X.R.W. acknowledges supports from the Nanyang Assistant Professorship grant from Nanyang Technological University and Academic Research Fund Tier 1 (RG108/17S) from Singapore Ministry of Education.

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