K atom promotion of O2 chemisorption on Au(111) Surface

applications. Here, we investigate by low-temperature scanning tunneling .... very low 0.001 Langmuir dose of O2, brighter dots appear among the preex...
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K atom promotion of O2 chemisorption on Au(111) Surface Jindong Ren, Yanan Wang, Jin Zhao, Shijing Tan, and Hrvoje Petek J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13843 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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Journal of the American Chemical Society

K atom promotion of O2 chemisorption on Au(111) Surface Jindong Ren,1 Yanan Wang,2,3 Jin Zhao,*1,2,3 Shijing Tan1 and Hrvoje Petek*1 Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA 1

Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 2

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei, Anhui 230026, P. R. China 3

ABSTRACT: Alkali atoms are known to promote or poison surface catalytic chemistry. To explore alkali promotion of catalysis

and to characterize discharge species in alkali-oxygen batteries, we examine coadsorption of K and O2 on Au(111) surface at the atomic scale by scanning tunneling microscopy (STM) and density functional theory (DFT). On a clean Au(111) surface, O2 molecules may weakly physisorb, but when Au(111) is decorated with K+ ions, they chemisorb into structures that depend on the adsorbate concentrations and substrate templating. At low K coverages, an ordered quantum lattice of K2O2 complexes forms through intramolecular attractive and intermolecule repulsive interactions. For higher K and O2 coverages, the K2O2 complexes condense first into triangular islands, which further coalesce into rhombohedral islands, and ultimately into incommensurate films. No structures display internal contrast possibly because of high structural mutability. DFT calculations explain the alkalipromoted coadsorption in terms of three center, cation-π interactions where pairs of K+ coordinate the π-orbitals on each side of O2 molecules, and in addition O2 forms a covalent bond to Au(111) surface. The K promoted adsorption of O2 is catalyzed by charge transfer from K atoms to Au(111) substrate and ultimately to O2 molecules, forming O𝛿_ 2 in a redox state between the peroxo and superoxo. Tunneling dI/dV spectra of K2O2 complexes exhibit inordinately intense inelastic progression involving excitation of the O−O stretching vibration, but absence of a Kondo effect suggests that the magnetic moment of O2 is quenched.

Introduction Alkali atoms on metal surfaces are known as strong promoters or sometimes poisons of catalytic reactions. They particularly promote catalytic dissociation of high electron density, weakly adsorbing molecules that possess multiple chemical bonds such as O2, N2, CO, and CO2, which accept electrons into their antibonding orbitals.1-7 Also, chemical and physical processes that turn on when alkali atoms interact with O2 molecules on metal surfaces are of practical interest because species like K2O2 are identified as energy extraction determining products of ultrahigh energy density alkali-O2 batteries.8-11 When K atoms and O2 molecules interact with metals they, respectively, donate and accept charge affecting work functions,12 and consequently interfacial charge transfer properties that affect catalytic processes. Notably, at submonolayer coverage, a charge transfer13 of nearly 1e- from alkali atoms to metals forms a strongly polarized surface dipole layer,14-16 a unique chemical environment with properties not available in homogeneous phases; such environment enables, for example, chemisorption of molecules such as N2, and activates its dissociation, to initiate the Haber-Bosch (HB) synthesis of NH3. Because N2 adsorbs weakly and is difficult to reduce, the HB process requires high temperatures and pressures making it highly energy intensive.1, 17-19 O2 molecule interactions with metal substrates are also weak: they range from dissociative chemisorption to weak physisorption. On Au(111) surface, O2 molecules weakly physisorb,20-21 but we find that the coadsorption with alkali atoms provokes their strong chemisorption.22 Of broader,

topical chemical significance and interest, however, alkali-O2 bonding is a paradigm for cation-π interactions fundamental to molecular recognition and receptor functions.23 Thus, the strongly ionic two-dimensional (2D) environment of chemisorbed alkali atoms is uniquely interesting for interactions it fosters with electron rich molecules like O2. Direct spectroscopic investigations of alkali-oxygen interactions on metal surfaces have been performed by metastable quenching spectroscopy,3 electron energy loss spectroscopy (EELS),24 and X-ray photoelectron 25-26 spectroscopy. These experimental methods inform on the bond formation, force constants, or breaking, and charge transfer at surfaces, which may induce nonadiabatic charged particle transfer and chemistry.27 For example, formation of KOx bond has been discussed based on vibrational and photoemission spectroscopy, and theory.26, 28 Molecular species like K2O2 form solids29 and have also been trapped in rare gas matrices to be examined by vibrational spectroscopy.30-31 Although such volume or area averaged measurements have characterized the interplay between ionic cluster size, structure and shape, it has been difficult to extract quantitative, atomic scale insights into the structures, charge distributions, and local densities of states of alkali-oxygen complexes.32 Also, whether O2 is captured in peroxide and superoxide states is key to understanding alkali promoted processes and energy extraction in high energy redox reactions such as alkali-O2 batteries, but specific redox states have been invoked based on spectroscopic, thermodynamic, and electrochemical arguments.3, 11, 33 Thus, an STM

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experiment and DFT theory study of the molecular and electronic structures of alkali-O2 co-adsorption complexes can greatly advance our understanding of their roles in heterogeneous catalysis and energy conversion/storage applications. Here, we investigate by low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) and theory, the coverage dependent structure, electronic properties, and the spin state of K2O2 complexes formed by adsorption of O2 molecules onto K+ ion-decorated Au(111) surfaces. We find that at low O2 coverage, pairs of K+ ions sandwich each O2 molecule by bilateral, orthogonal coordination of its π*-orbitals. According to DFT calculations, thus formed K2O2 complexes have a distorted rhombic structure, like the matrix isolated species, but also partly because of covalent bond formation between the O2 moiety and the substrate.31 Although these complexes do not reveal internal structure by STM imaging on Au(111) surface even at 4.5 K, by DFT calculations and counting of the number of K+ ions that complex each O2 molecule, we confirm stability of such co-chemisorbed species. Differential conductance (dI/dV) inelastic electron tunneling (IET) measurements of individual K2O2 molecules manifest symmetric nearly equally spaced peaks, and conductance thresholds with respect to the Fermi level, which we attribute to excitation of the O−O stretching vibration of the complex,.34-35 Unlike, the two dimensional (2D) O2 lattice that has been investigated as physisorbed structure on Au(110)-1x2 surface,36 the dI/dV measurements of K2O2 shows no evidence of the Kondo effect suggesting that the magnetic moment of O2 is quenched. At low coverages, K2O2 forms a hexagonal liquid structure similar though less dense than that of K+ ions, expressing a dipoledipole repulsion.22 Increasing the K and O2 coverage forms equilateral triangular K-O islands, which coalesce into trapezoids, and eventually into a monolayer on Au(111) surface.

EXPERIMENT Experiments are performed in an LT-STM system with a base pressure of 99.95%) to a precise coverage by controlling the source current and deposition time. After deposition, the sample is transferred into the STM chamber for subsequent measurements with a tungsten tip, in the constant-current mode. Next, O2 molecules are adsorbed onto K/Au(111) surface at 4.5 K. Spectroscopic measurements are recorded by a lock-in technique with a sinusoidal modulation voltage of Vmod=8 mV rms at 773 Hz.

THEORY Plane-wave pseudopotential DFT calculations using the “Vienna ab initio simulation package” (VASP) code37-39 are used to calculate K2O2 on Au(111) surface. The geometry optimization and the electronic structure calculations are

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performed with the Perdew−Burke–Ernzerhof (PBE) functional with generalized gradient approximation (GGA).40 The core electrons are described by the projector-augmented wave method.41 The structures are relaxed using a conjugate gradient scheme without symmetry restrictions until the maximum force on each atom is less than 0.02 eV Å-1. The periodically repeated slabs are decoupled by 15 Å vacuum gaps, and an energy cutoff is 500 eV. The K2O2 complex formation is calculated starting with the initial K atom coverage as determined by the experimentally measured K-K distances: on a (6×3) Au(111) supercell, with the lattice parameters (a = 1.730 and b = 0.865 nm), two K atoms are placed onto the surface to form a K+ lattice with an average separation of r=0.865 nm. To explore the structure of K2O2 complex on Au(111), one O2 molecule is introduced between the K+ ions on the Au(111) surface; the structure is next optimized by calculating the K2O2 complex energy minimum (Figure 1). The slab contains three Au layers, where the lowest one is fixed to the bulk structure, while the upper two are relaxed together with the K2O2 complex. Otherwise, the Au(111) surface is unreconstructed. Only the Γ point is used is to simulate the supercell.

RESULTS AND DISCUSSION We investigate the combined adsorption K atoms and O2 molecules on Au(111) surface. We find that O2 molecules do not adsorb on Au(111) surface from the background gas at 4.5 K, consistent with a report of weak physisorption.21 Therefore, we first modify Au(111) surface by chemisorption of K atoms, and subsequently, dose O2 molecules onto K/Au(111). Upon chemisorption, the low ionization potential K atoms transfer their 4s valence electron to the high electron affinity Au(111) substrate,15 and therefore exist as a 2D array of cations; each K+ ion and its screening negative image charge form a dipole with a strength p.13, 15 K+ ions on Au(111) experience very little corrugation and can assume any registry with the substrate with little energetic cost. The main, driver for ordering of alkali atoms at 120 K with respect to O2 molecule dosing, desorption, or dissociation. High coverage K−O2 film growth At higher initial K+ ion coverages, we find that the density of K+−O2 triangles increases, and at the highest densities we observe their condensation into larger structures (Figure 4a4c). As their density increases, pairs of islands coalesce into rhombohedral structures, which maintain the 1.6 nm edge length (see Figure 4b). As the yellow dashed lines in Figure 4b indicate, the rhombohedral islands also preferentially nucleate along the Au(111) hcp domains of the herringbone reconstruction. Finally, at the highest coverage, a structureless K−O monolayer (see Figure 4c) forms. In none of these structures can we image the component ions, possibly because they are intrinsically fluctional and because imperfect epitaxial relationship with the substrate causes facile internal ion motion. Also, the lack of atomic/molecule contrast could also be an electronic effect. The K+−O2 film does not lift the substrate herringbone reconstruction as is evident in Figure 4b-4c; its formation could be important for alkali-O2 batteries, because it could affect the mass transport of K2O2 under applied potentials or the energy that can be extracted at high alkali atom densities.

Figure 4. Topography of K+−O2 surface for increasing K coverage: (c)>(b)>(a). (c) inset, dI/dV spectrum of the K+−O2 monolayer. (T = 4.5 K; tunneling parameters: I=-0.1 nA, Vb=0.4 V). Another aspect to consider in K+−O2 film imaging is the spin state of O2. Isolated O2 has a spin triplet ground state, but if it receives charge upon chemisorption it can be reduced to a doublet (superoxo) or singlet (peroxo) valence. Therefore, to establish the spin state of O2―δ interacting with K+ ions and Au(111) surface, we performed STS measurements, such as in Figures 3a and 4c, on single K2O2 dots and K+−O2 film. Our measurements are in part motivated by the work of Jiang et al.36 who reported a Kondo resonance for an O2 lattice

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physisorbed on the Au(110) surface. In the case of the O2/K/Au(111) system, the STS spectra show no evidence of Kondo resonance near the Fermi level, consistent with O2―δ being in the peroxo-singlet form. Quenching of the magnetic moment of O2 is consistent with the lack of spin-polarization in the DOS of K2O2 on Au surface in Figure 2a. To reconcile why the magnetic moment is quenched consistent with peroxo, but the calculated charge transfer is consistent with the superoxo, and yet the O−O bond length and stretching frequency are intermediate state between the two states, we investigate the O−O bonding in more detail. The paradox is reconciled by the calculating the charge redistribution within O2, as shown in Figure S4. This analysis shows that besides charge transfer from 2K and Au(111), there is also charge donation from a lower σ bonding orbital to the upper nonbonding π* orbital; this explains quenching of the magnetic moment, and how a large change in O−O bonding occurs for relatively small charge transfer. Such charge donation has been implicated in promotion of O−O bond dissociation on a more catalytically active Pd(111) surface.60 Remarkably, K atom adsorption on the inert Au(111) surface with respect to O2 chemisorption changes the surface catalytic properties to much more chemically active Pd(111) surface. These results have significant implications on the HB synthesis of NH3, by showing how the alkali coadsorption with O2 can weaken the π bonding in an electron rich molecule, increase the chemisorption strength of a reactant, and enhance the catalytic properties of a metal surface.

CONCLUSIONS We have reported the real-space observation of K2O2 complexes by low-temperature scanning tunneling microscopy/spectroscopy on Au(111) surface. Decoration of the Au(111) surface with a K+ ion dipole superstructure remarkably increases the O2 chemisorption and leads to formation of O2―δ species; this is enabled by decreasing of the work function of the weakly physisorbing Au(111) surface, which facilitates the reduction of the gas phase O2, and thereby introducing anions to interact with the existing K+ cation lattice. The ability of the K+ ion dipole superstructure to capture O2 molecules through ionic interactions with the π* orbitals in K2O2 complexes is a dramatic manifestation of the alkali atom promotion of catalytic processes. The charge transfer to and concomitant intramolecular charge donation within O2 weakens to O−O bond, but does not dissociate it on the Au(111) surface. It would be interesting to examine other metals, for which it is known that charge transfer promotes the O2 dissociation.61 The K2O2 complexes are representative of structures that could form through coadsorption with other electron rich, highly stable molecules such as N2 and CO2. Thus, the observed coadsorption system is of interest for studying processes such as Haber-Bosch synthesis of NH3, and as a model system for CO2 capture and potentially reduction to carbon-based fuels. The K+−O2 interactions on metal surfaces are also of interest for alkali-air batteries where K2O2 like complexes are thought to form and possibly aggregate into polymers. In combination with experimental statistical analysis and DFT calculations, the atomic scale structures of different coadsorption motifs are determined. The DFT

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Journal of the American Chemical Society calculations indicate that the O2―δ charge on the in K2O2 complexes is close to the superoxo species, but the bond length and O−O stretching vibrational frequency from DFT, spin-polarized electronic structure calculation, and dI/dV spectra (no Kondo effect) suggest that it is closer to the peroxo species. Further analysis of the nature of the complex could be deduced by examining whether K2O2 complexes undergo peroxo or superoxo chemistry.21, 52, 62 The ambivalence of O2―δ appears to be caused by its additional covalent bonding to the Au(111) substrate, which is highly surprising considering that O2 weakly physisorbs without K coadsorption; such interactions could be stronger and used on other metal substrates, to drive otherwise difficult to activate chemistry. For example, the peroxo or superoxo species are predicted to have different affinity for CO2 capture,63 which could be tested by LT-STM imaging. We found that LT-STM dI/dV spectroscopy of ionic species such as K2O2 complexes produces unusually strong IETS of the O−O stretching vibration and that interactions, like charge transfer and chemical bonding with K+ ions and the Au(111) surface.

AUTHOR INFORMATION * [email protected], *[email protected]

Supporting Information dI/dV spectra of the clean Au(111) and K/Au(111) surfaces, estimation of the number of K atoms within in each intermediate density triangular island, O2 dosing and temperature dependence measurements, and chemisorptioninduced charge redistribution analysis. The Supporting Information is available free of charge on the ACS Publications website at Notes The authors declare no competing financial interest.

Acknowledgements This research was supported by DOE-BES Division of Chemical Sciences, Geosciences, and Biosciences Grant No. DE-SC0002313. J. Z. acknowledges the support of National Key R&D Program of China (2016YFA0200604 and 2017YFA0204904), National Natural Science Foundation of China, Grant No. 11620101003, 21421063; the Fundamental Research Funds for the Central Universities WK3510000005. Calculations are performed at Environmental Molecular Sciences Laboratory at the PNNL, a user facility sponsored by the DOE Office of Biological and Environmental Research.

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