Understanding the Intrinsic Surface Reactivity of Single-Layer and

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Understanding the intrinsic surface reactivity of single and multilayer PdO(101) on Pd(100) Vikram Mehar, Minkyu Kim, Mikhail Shipilin, Maxime Van den Bossche, Johan Gustafson, Lindsay Richard Merte, Uta Hejral, Henrik Gronbeck, Edvin Lundgren, Aravind Asthagiri, and Jason F. Weaver ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02191 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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Understanding the intrinsic surface reactivity of single and multilayer PdO(101) on Pd(100) Vikram Mehar,1,† Minkyu Kim,2,† Mikhail Shipilin,3 Maxime van den Bossche,4 Johan Gustafson,3 Lindsay R. Merte,5 Uta Hejral,3 Henrik Grönbeck,4 Edvin Lundgren,3 Aravind Asthagiri2 and Jason F. Weaver1,* 1

2

Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA William G. Lowrie Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA 3 Division of Synchrotron Radiation Research, Lund University, SE-22100 Lund, Sweden 4 Department of Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden 5 Materials Science and Applied Mathematics, Malmö University, SE-205 06 Malmö, Sweden

†

Vikram Mehar and Minkyu Kim contributed equally to this work.

*To whom correspondence should be addressed, [email protected] Tel. 352-392-0869, Fax. 352-392-9513

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Abstract We investigated the intrinsic reactivity of CO on single and multilayer PdO(101) grown on Pd(100) using temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (~119 vs. 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101) whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge and atop-Pdcus sites (coordinatively-unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pdcus sites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101) as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity of multilayer PdO(101)/Pd(100). We show that the O-atom below the Pdcus site, present only on multi-layer PdO(101), modifies the electronic states of the Pdcus atom in a way that enhances the CO-Pdcus bonding. Lastly, we show that a precursor-mediated kinetic model, with energetics determined from the present study, predicts that the intrinsic CO oxidation rates achieved on both single and multilayer PdO(101)/Pd(100) can be expected to exceed the gaseous CO diffusion rate to the surface during steady-state CO oxidation at elevated pressures, even though the intrinsic reaction rates are 4-5 orders of magnitude lower on single layer than on multilayer PdO(101)/Pd(100).

Keywords: CO oxidation, Pd(100), PdO, palladium, infrared spectroscopy, RAIRS, DFT, surface oxide


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Introduction The oxidation of CO on transition-metal surfaces plays a central role in gas purification and exhaust gas remediation, and has served as a prototype system for uncovering fundamental aspects of gas-solid interactions, particularly the role of metal oxide formation in applications of oxidation catalysis. Investigations using in situ diagnostics demonstrate that CO oxidation over transition metals occurs in distinct kinetic regimes as a function of temperature.1-14 At low temperature, adsorbed CO hinders the dissociative chemisorption of O2 and limits CO oxidation, resulting in a so-called CO-inhibited regime. As the reaction temperature increases, CO partly desorbs and the CO oxidation rate exhibits an abrupt increase as oxygen adsorbs and the catalytic surface transforms from a CO-rich to an O-rich state. The nature of the catalytically-active, Orich surface has emerged as a central issue for understanding applications of oxidation catalysis and has not yet been fully resolved. The oxidation of CO on the Pd(100) surface has been investigated widely using in situ methods,2-5, 8-9, 15-21 in large part because the oxidation of Pd(100) is well-characterized and PdO surfaces can be highly active in promoting oxidation reactions.22 Prior studies show that chemisorbed oxygen atoms arrange into ordered p(2 × 2) and c(2 × 2) structures during the initial stages of Pd(100) oxidation, and that, with increasing oxygen chemical potential, an ordered (√5 × √5)R27° surface oxide initially replaces chemisorbed phases at temperatures above ~550 K,2325

followed by the development of an epitaxial, multilayer of PdO(101). The (√5 × √5)R27°

oxide (“√5 oxide”) may be regarded as a single layer of PdO(101) on the Pd(100) surface and serves as a template for formation of an epitaxial PdO(101) multilayer film .4, 26-28 A p(5 × 5) structure can also form during oxygen adsorption on Pd(100) at temperatures below 550 K, and 3 ACS Paragon Plus Environment

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has a lower oxygen coverage than the √5 oxide.29 Characterizing the intrinsic surface reactivity of the oxygen-rich phases that develop on Pd(100) is essential for developing atomic-scale models of oxidation catalysis on this surface. Indeed, several first-principles computational investigations address the possible role of the √5 oxide during CO oxidation over Pd(100), with this system serving as a benchmark for evaluating computational modeling of oxidation catalysis.19, 30-34 Early studies generated debate about whether the metallic surface or Pd oxide phases, especially the √5 oxide, are responsible for the abrupt transition from CO-inhibited to COuninhibited kinetic regimes during CO oxidation on Pd(100).6, 9 Recently, however, it has been shown that CO oxidation becomes limited by the rate of gaseous CO transport to the surface once the surface achieves high catalytic activity.15-16,

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A consequence of the mass-transfer

limitation (MTL) is that the measured CO oxidation rates can remain constant even as different O-rich phases develop on the surface. For example, a recent near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) study shows that metallic Pd(100) covered with chemisorbed O-atoms is dominant when the MTL is reached at total pressures less than 0.1 mbar,3 whereas surface X-ray diffraction (SXRD) shows that MTL rates are also maintained when either the single- or multilayer, epitaxial PdO(101) films form on Pd(100) during CO oxidation at higher O2 pressure (> 1 mbar).2, 4 These prior studies demonstrate that chemisorbed O-atoms on Pd(100) as well as both the single and multiple layer PdO(101) structures can have intrinsic CO oxidation rates that are higher than the CO mass transfer rate to the surface. As a result, the in situ measurements20-21 have been unable to characterize the intrinsic reactivity of the various surface oxygen phases that form on Pd(100).


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Studies performed in ultrahigh vacuum (UHV) reveal large differences in the reactivity of single and multilayer oxides on Pd surfaces. Several investigations combining UHV measurements with DFT calculations show that multilayer PdO(101) grown on Pd(111) is highly active for CO oxidation during both temperature programmed reaction spectroscopy (TPRS) and isothermal rate experiments.35-39 The high reactivity arises, in large part, from the presence of high-concentrations of coordinatively unsaturated (cus) Pd and O-atom pairs at the PdO(101) surface. In contrast to the high reactivity of multilayer PdO(101), an early experimental study shows that CO oxidation occurs very slowly on the √5 oxide on Pd(100) under isothermal reaction conditions performed at low CO pressures (< 10-8 Torr), and that chemisorbed O-atoms are substantially more reactive for the conditions studied.40 A recent study reveals that the √6 surface oxide on Pd(111) also exhibits a low CO oxidation activity at low CO pressures compared with both metallic Pd(111) and multilayer PdO(101).38-39 The findings of low CO oxidation activity of the √5 oxide in UHV are in stark contrast with the results of in situ studies which show that the √5 oxide is reactive enough to maintain the mass transfer limit during steady-state CO oxidation. Perhaps surprisingly, only the investigation by Zheng and Altman provides experimental data about the intrinsic CO oxidation activity of the √5 oxide on Pd(100),40 and surface science studies of multilayer PdO(101) on Pd(100) are absent due to difficulties in preparing this oxide for characterization in UHV. A dramatic difference in the chemical activity of single and multilayer PdO(101) on Pd(100) is interesting, given that both systems exhibit (nominally) identical surface structures, with each featuring the same concentration of cus-Pd/O pairs. A recent in situ investigation of methane oxidation provides insights for understanding differences in the intrinsic reactivity of single and multilayer PdO(101) on Pd(100).41 Those authors report that the much higher activity of


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multilayer PdO(101) for CH4 oxidation originates from an electronic effect wherein underlying lattice O-atoms in the multilayer oxide modify the electronic structure of the Pdcus atoms in a way that is favorable for CH4 binding and activation; such underlying O-atoms are absent in single-layer PdO(101) and Pauli repulsion consequently dominates the CH4 interaction with single-layer PdO(101). Further exploration is needed to clarify this ligand effect42 and determine its influence on the intrinsic chemical properties of the single and multi-layer PdO(101) structures during CO oxidation. In the present study, we investigated the intrinsic CO oxidation activity of single and multilayer PdO(101) grown on Pd(100) in UHV using TPRS and reflection absorption infrared spectroscopy (RAIRS) as well as DFT calculations. We find that CO adsorbs weakly on singlelayer PdO(101) and oxidizes to a negligible extent during TPRS, whereas CO binds strongly and undergoes extensive oxidation on multilayer PdO(101). Using DFT, we show that the ligand effect induced by the underlying O-atoms is responsible for the dramatic enhancement in CO binding and reactivity on multilayer PdO(101), and present a detailed analysis clarifying the electronic origins of the effect. Finally, we show that a precursor-mediated kinetic model, using energetics determined from our results, predicts that the intrinsic CO oxidation rates achieved on both single and multilayer PdO(101)/Pd(100) can be expected to exceed the gaseous CO diffusion rate to the surface at elevated pressure, even though the intrinsic reaction rates at these oxide surfaces differ by 4-5 orders of magnitude. Our findings elucidate seemingly disparate conclusions about the CO oxidation activity of the single and multi-layer PdO(101) structures, and demonstrate how UHV studies performed in combination with DFT calculations can play a critical role in characterizing the intrinsic reactivity of catalytic surfaces.


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Experimental details The experiments reported in this study were conducted in an ultrahigh vacuum (UHV) chamber with a typical base pressure of 2 × 10-10 Torr.38, 43 The UHV chamber is equipped with a fourgrid retarding field analyzer for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES), a quadrupole mass spectrometer (QMS) used for TPD and TPRS experiments, and a Fourier transform infrared spectroscopy (FTIR) system for RAIRS measurements. A single-stage differentially pumped chamber44 is also attached to the main UHV chamber which houses an inductively coupled RF plasma source that is used to generate atomic oxygen beams. The FTIR system employed in this study consists of a MIR source (Bruker Tensor 27), a set of mirrors and lenses and an external liquid N2 cooled HgCdTe (MCT) detector. Outside of the UHV chamber, the MIR beam travels within a sealed box which is purged continuously with carbon dioxide and water free compressed air. In the purge box, a set of flat mirrors are used to direct the MIR beam onto a parabolic mirror that focuses the beam. The focused MIR beam enters the UHV chamber through a differentially pumped KBr window and then reflects from the sample surface at an angle of ∼80° relative to the surface normal. The reflected MIR beam exits the UHV chamber through another KBr window and is directed onto the MCT detector within a second purge box. We averaged 512 scans at a resolution of 4 cm-1 for all RAIR spectra reported here. The Pd(100) crystal used in the present study is a circular disk (8 mm × 2 mm) spot-welded to W wires and attached to a copper sample holder that is in thermal contact with a liquid nitrogen cooled reservoir. A type K thermocouple is spotwelded to the backside of the crystal to measure the sample temperature. The sample is resistively heated and the temperature is


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controlled using a PID controller that adjusts the output of a dc power supply. In this setup, we are able to maintain or linearly ramp the sample temperature from 88 to 1250 K. Initial sample cleaning consisted of cycles of sputtering with 1000 eV Ar+ ions at a surface temperature of 300 K, followed by annealing at 1000 K for 10 min and oxygen treatment in 5 × 10-7 Torr oxygen background while cycling the temperature from 500 to 1000 K. Subsequent cleaning involved routinely exposing the sample to an atomic oxygen beam for 30 min at 850 K, followed by flashing the sample to 1000 K to desorb oxygen and carbon oxides. We considered the Pd(100) sample to be clean when we could not detect contaminants with AES and did not observe CO and CO2 production during TPRS after oxygen adsorption. We grew single and multilayer PdO(101) by oxidizing the Pd(100) surface in UHV, following the procedures discussed below. Prior studies demonstrate that multilayer PdO(101) develops on low-index Pd facets under realistic reaction conditions and is thus relevant for understanding Pd-catalyzed oxidation reactions.41,

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The stoichiometric PdO(101) surface is

characterized by a rectangular unit cell and is comprised of alternating rows of 3-fold (3f) and 4fold (4f) coordinated Pd atoms and O atoms, where the 3f atoms are referred to as coordinatively unsaturated (cus) since these atoms are missing a bonding partner compared with the bulk atoms of PdO (Figure 1). The Pdcus and Ocus atoms of PdO(101) are highly reactive, with the Pdcus atoms serving as preferred adsorbate binding sites and the Ocus atoms readily oxidizing adsorbed species.22 We define 1 ML (monolayer) as equal to the Pdcus site density of PdO(101). After preparing the PdO(101) layer, we cooled the sample to 100 K and then exposed to 15 L (Langmuir) of CO which was found to be sufficient to saturate PdO(101). After the CO exposure, we collected TPRS spectra by positioning the CO saturated PdO(101) surface in front of a shielded quadrupole mass spectrometer (Hiden) at a distance of ∼5 mm. The sample


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temperature increases to 700 K at 1 K/s as the mass spectrometer monitors CO and CO2 desorption from the surface. We represent the CO2 TPRS trace using the measured m/z = 44 signal intensity and we represent the CO TPRS with the m/z = 28 intensity after correcting for a small contribution (~11%) from CO2 fragmentation in the ionizer. We report raw TPRS data in the Supporting Information (S1) and describe the background subtraction and smoothing that we employed. The CO desorption yield measured in the TPRS spectrum is estimated by scaling CO TPD spectra obtained after saturating Pd(100) with CO at 350 K, assuming that the saturation coverage is 50% of the Pd(100) atomic density.46-48 The CO2 yield is estimated by applying a factor of 1.3 to the calibration factor of CO, where we determined this scaling factor by comparison with recent experiments performed in the apparatus used in the present work.

Computational details The plane wave DFT calculations were performed using projector augmented wave pseudopotentials49 provided in the Vienna ab initio simulation package (VASP).50-51 The Perdew-Burke-Ernzerhof

(PBE)

exchange-correlation

functional52

and

Heyd−Scuseria−Ernzerhof (HSE06)53 hybrid functional were used with a plane wave cutoff of 450 eV. The multilayer PdO(101) was modeled by four layers with the bottom layer fixed. The PBE bulk lattice constant of PdO (a = 3.10 Å and c = 5.39 Å) was used to fix the lateral dimensions of the PdO(101) slab. For the single-layer and two-layer PdO(101) on Pd(100), we used 1-2 layers of PdO(101) in the (√5 × √5)R27° surface unit cell adsorbed on top of five layers of Pd(100) with the bottom two layers of the Pd(100) fixed. The resulting PdO(101) film on Pd(100) is compressively strained by less than 1% and 0.1 % in the a1 and a2 directions of the surface cell (Figure 1). We have confirmed that this small strain has a negligible effect on the CO


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binding energies. As discussed below, the majority of the PdO(101) domains grown experimentally show only small lattice strain so we expect that strain has a negligible influence on the energetics of CO on the surface. Lattice atoms not fixed to the bulk positions were relaxed until the forces were less than 0.01 eV/Å. A vacuum spacing of 18-20 Å was used between the periodic slabs in the surface normal direction. A 4 × 1 unit cell was used for multilayer PdO(101) with a corresponding 4 × 2 × 1 Monkhorst-Pack k-point mesh and a 2 × 1 unit cell was used for PdO(101) films on Pd(100) with a corresponding 5 × 5 × 1 Monkhorst-Pack k-point mesh. The calculations were performed with a single CO molecule adsorbed on the surface, resulting in a CO coverage of 25 and 50% of the total density of Pdcus atoms for the considered systems. In the present study, we define the binding energy, , of an adsorbed CO molecule on the surface using the expression, = + − / where / is the energy of the state containing the adsorbed CO molecule, is the energy of the bare surface, and is the energy of an isolated CO molecule in the gas phase. In this way, a positive number corresponds to exothermic adsorption. All reported binding energies are corrected for zero-point vibrational energy. For HSE06 values reported in the paper, we have used the PBE vibrational frequencies for zero-point vibrational energy since HSE06 is known to overestimate vibrational frequencies. We evaluated the barriers for CO2 formation on the singleand multilayer PdO(101) surfaces using the climbing nudged elastic band (cNEB) method.54 The vibrational modes for all minima (transition states) were evaluated to confirm that zero (one) imaginary frequencies were present. To understand CO adsorption on single layer versus multilayer PdO(101), we examined the partial density of states (pDOS) of the d orbitals of Pdcus atoms. In addition, we have used the


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projected crystal orbital Hamilton population (pCOHP) method to analyze and quantify CO-Pdcus orbital interactions in these systems. The pCOHP provides a measure of the overlap between specific atomic orbitals, which indicates bonding, antibonding, and nonbonding contributions, and therefore can provide a relative quantification of the bonding.55-57 The LOBSTER software was used to obtain the COHP from the VASP output.58 This analysis requires the proper identification of axes for the Pdcus d orbitals. Pd adopts a square planar configuration in PdO with the xy plane defined by the four O anion ligands (see Figure S2, Supporting Information).59 Based on this configuration the Pdcus rows on the PdO(101) surface are aligned with the z-axis and thus along the axis of the dz2 orbital, while the missing O ligand is oriented in the direction of the dx2-y2 orbital. Therefore, the CO molecule is aligned towards the dx2-y2 orbital when it adsorbs on top of the Pdcus site. We note that this orbital was misidentified as dz2 in earlier work on CH4 and O2 adsorption on PdO(101).41, 60 Section S2 describes the details of the rotation that we apply to our surface unit cell to align the simulation cell axis to the Pdcus d orbitals before we generate pDOS and pCOHP data.

Results and Discussion Structure of single and multilayer PdO(101) on Pd(100) Oxide formation on Pd(100) was observed more than 30 years ago,61 and recent studies show that a single layer of PdO(101) can form during Pd(100) oxidation, with this surface oxide adopting an orientation in which the PdO(101) lattice vectors are rotated by 26.57° relative to the high-symmetry [001] and [010] directions of the Pd(100) substrate, resulting in a (√5 × √5)R27° coincident structure with a square unit cell (Figure 1a).28 The ideal (√5 × √5)R27° structure may be represented in matrix notation as

2 1 in the Pd(100) basis. Bulk-terminated PdO(101) is −1 2 11

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characterized by a rectangular unit cell with lattice vectors a1 and a2 that align with the [010] and [101] crystallographic directions of PdO and have magnitudes of 3.043 Å and 6.143 Å, respectively. The square unit cell of the √5 oxide has ideal dimensions of 2a1 × a2, with 2a1 = a2 (Figure 1a), where the single-layer PdO(101) lattice is elongated by 1.0% and 0.1% along the a1 and a2 directions compared with bulk PdO(101). Recently, Shipilin et al. have reported that the coincident unit cell distorts slightly from the ideal (√5 × √5)R27° structure to improve the longrange commensurability between the single-layer PdO(101) and the Pd(100) substrate.27


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a)

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Figure 1: a) Model structure, b) measured and c) simulated LEED patterns of single-layer PdO(101)/Pd(100) in the (√5 × √5)R27° structure. d) Model structure of bulk PdO(101), e) measured LEED pattern of multi-layer


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PdO(101) grown on Pd(100) and f) simulated LEED pattern of epitaxial multi-layer PdO(101)/Pd(100). Open red circles in c) highlight the extra LEED spots expected for single-layer PdO(101)/Pd(100) due to diffraction from the metal-oxide coincidence lattice, and the open blue circles are the Pd(100) substrate spots. Basis vectors for the rotational domains of the √5 oxide and epitaxial PdO(101) are also shown in c) and f), respectively. Red and blue arrows in b) and e) mark the locations of these spots in the measured LEED patterns, each obtained at 110 eV beam energy.

Following reported procedures, we prepared the √5 oxide by exposing Pd(100) to 5 × 10-7 Torr of O2 at a surface temperature of 570 K until reaching saturation uptake of oxygen.24, 28 The resulting oxide structure contains about 0.80 ML of O-atoms according to O2 TPD and exhibits a sharp (√5 × √5)R27° LEED pattern (Figure 1b). The LEED pattern obtained from the √5 oxide results from two rotational domains of the square unit cell (Figure 1c). Consistent with previous studies, we find that the √5 oxide only forms at temperatures above ~550 K.24, 28 At temperatures from 400 to 500 K, we find that saturation O2 exposure to Pd(100) produces a sharp p(5 × 5) LEED pattern that transforms to a superposition of (√5 × √5)R27° and p(2 × 2) patterns upon heating the surface to 550 K (not shown), in good agreement with previous findings.24 We grew multilayer PdO by exposing the Pd(100) surface to an O-atom beam at a surface temperature of 500 K. From O2 TPD, we estimate that the resulting oxide contains 6.6 ML of Oatoms, corresponding to about eight layers of PdO(101). Figure 1d shows a representation of bulk PdO(101). LEED exhibits only a diffuse background after forming the PdO multilayer at 500 K (not shown), but diffraction spots become evident after heating to 650 K. The resulting LEED pattern (Figure 1e) is consistent with domains of an epitaxial PdO(101) structure with the same orientation as the √5 oxide on Pd(100). Shipilin et al. have reported that a similar epitaxial PdO(101) multilayer growing in a Stranski–Krastanov growth mode develops during the oxidation of Pd(100) at sufficiently high O2 partial pressures.4 The LEED pattern obtained from the multilayer PdO(101) exhibits higher background intensity and dimmer spots compared with


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LEED from the √5 oxide, suggesting that the multilayer PdO(101) forms smaller domains and possibly a rougher morphology than the √5 oxide on Pd(100) for the conditions studied. For a sufficiently thick PdO(101) layer, diffraction spots from the PdO(101)/Pd(100) coincidence lattice should no longer be visible and the LEED pattern would then arise from four orientations of the rectangular PdO(101) surface unit cell. The epitaxial, multilayer PdO(101) structure may be represented in matrix notation as

1 1/2 in the Pd(100) basis. Simulated −1 2

LEED patterns show that the epitaxial PdO(101) structure will generate diffraction spots at the same locations as seen for the √5 oxide (Figure 1c,f). However, LEED from the √5 oxide produces additional spots at the (0.2, 0.6), (0.6, 0.2) and (1, 1) positions (open red circles, Figure 1c) that should not be seen for epitaxial PdO(101), where these reciprocal space points are given in the Pd(100) basis. We find that the intensities of the extra spots are significantly diminished in the LEED pattern obtained from multilayer PdO(101) compared with the other spots (Figure 1b,e). This diminution is more pronounced for the (0.2, 0.6) and (0.6, 0.2) spots (red arrows), compared with the (1, 1) spots from the Pd(100) substrate (blue arrows). The low intensity of the extra spots supports the conclusion that multilayer PdO(101) grows as an epitaxial structure under the conditions studied. However, the dimness of the LEED spots as well as the visibility of the (1, 1) spots suggests that epitaxial PdO(101) grows by the Stranski-Krastanov mechanism and forms islands of varying thickness. A fraction of the PdO(101) domains appears to be thin enough to allow detection of diffracted electrons from the Pd(100) substrate. Notably, TPRS and RAIRS measurements presented below show that single-layer PdO(101)/Pd(100) is exposed in only small quantities within the multilayer PdO(101), and is mainly present at the perimeter of the sample (see S3).


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TPRS from single and multilayer PdO(101) after CO saturation at 100 K We performed TPRS experiments that reveal dramatic differences in the reactivity of the singlelayer and multilayer PdO(101) structures on Pd(100) toward the binding and oxidation of CO. Figures 2a and b compare CO and CO2 TPRS traces obtained from the single and multilayer PdO(101) structures after saturating the surfaces with CO at 100 K. The CO TPRS trace obtained from single-layer PdO(101) exhibits two sharp peaks at 118 and 151 K, while the CO2 trace exhibits negligible intensity (Figure 2a). This finding demonstrates that CO binds weakly on the single-layer PdO(101) structure and desorbs during TPRS without reacting. An implication is that the energy barrier for CO oxidation on the single-layer PdO(101) is higher than that for desorption. We used Redhead analysis to estimate the CO binding energies from the TPRS peaks, with desorption pre-factors (near 1014 s-1) computed using the Campbell-Sellers (CS) correlation.62-63 We discuss the computation of desorption pre-factors in Section S4, and note that the CS correlation accurately predicts desorption pre-factors for several gas-surface systems and that the computed pre-factors are intermediate to those determined under limiting conditions, i.e., lattice gas vs. 2D ideal gas.62, 64 Our analysis predicts that the CO TPRS peaks at 118 and 151 K correspond to binding energies of 33 and 43 kJ/mol, respectively, where these values agree reasonably well with the range suggested previously by Gao et al.65 We are unable to estimate an energy barrier for CO oxidation on single-layer PdO(101) due to the lack of CO2 production during TPRS. However, we performed TPRS simulations with a model that considers


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competitive desorption and oxidation (“reaction”) of adsorbed CO and predict that CO2 production becomes measurable when Er is set lower than about 45 kJ/mol (Section S5), suggesting that this value is a lower-bound for the CO oxidation barrier on single-layer PdO(101). Lastly, we estimate a saturation coverage of 0.79 ML for CO adsorbed on the singlelayer PdO(101)/Pd(100) structure at 100 K. This coverage may, however, represent a lower bound because the CO desorption rate becomes appreciable just above 100 K and the attainable CO coverage could depend sensitively on small variations in the adsorption temperature in this case. a)

b) 2.0

CO and CO2 TPRS Multilayer PdO(101) TS= 100 K

CO and CO2 TPRS Single layer PdO(101) Ts= 100 K

10

Desorption rate (a.u.)

1.5

Desorption rate (a.u.)

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

ACS Catalysis

[CO]o = 0.79 ML

CO

Rxn yield = 0%

5

[CO]o = 0.72 ML

CO

Rxn yield = 89%

CO2

1.0

0.5

CO2 0 200

300

400

500

600

200

700

Temperature (K)

300

400

500

600

700

Temperature (K)

Figure 2: CO (red) and CO2 (black) TPRS traces obtained from a) single-layer PdO(101) and b) multilayer PdO(101) (~6.6 ML) after CO saturation at 100 K.

Our TPRS results show that CO binds relatively strongly on the multilayer PdO(101) structure and that nearly 90% of the adsorbed CO oxidizes to CO2 during TPRS (Figure 2b), in contrast with the negligible reactivity of CO on the PdO(101) single layer. We show data obtained from a multilayer PdO(101) structure grown at 500 K, without heating to 650 K, since desorption from under-oxidized areas at the perimeter of the sample are minimized for the asgrown structure (see S3). The CO and CO2 TPRS traces obtained from the multilayer PdO(101)/Pd(100) structure agree well with our previous TPRS results obtained from CO 17 ACS Paragon Plus Environment

ACS Catalysis 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

adsorbed on multilayer PdO(101) grown on Pd(111),36, 38 demonstrating that the underlying Pd substrate has little influence on the reactivity of multilayer PdO(101) toward CO. The CO TPRS trace exhibits broad features centered at ~390 and 475 K (Figure 2b) that are consistent with CO desorbing from multilayer PdO(101), and arise from CO adsorbed on Pdcus sites located next to Ocus atoms vs. next to Ocus-vacancies (Ov) that are generated during CO oxidation on initially pristine PdO(101).38 Using a CS desorption pre-factor of 7 × 1014 s-1, we estimate that the CO TPRS peak at ~390 K corresponds to a binding energy of 119 kJ/mol. The CO TPRS trace also exhibits small peaks at ~118 and 150 K that arise from CO desorption from under-oxidized areas near the perimeter of the sample (see S3). The CO2 TPRS trace obtained from the PdO(101) multilayer is broad and exhibits two main features centered at ~360 and 490 K (Figure 2b). We have previously shown that the CO2 features near 360 and 490 K originate from the oxidation of CO-Pdcus/Ocus and CO-Pdcus/Ov species, respectively.38 Simulations of the TPRS spectra predict that the CO oxidation barrier on multilayer PdO(101) is about 90 ± 5 kJ/mol. The sharp CO2 TPRS peak near 120 K likely arises from the sample heating wires, and corresponds to a very small amount of CO2. Our TPRS results indeed reveal a dramatic difference in the reactivity of single vs. multi-layer PdO(101)/Pd(100) toward CO; multilayer PdO(101) is highly reactive toward CO whereas single-layer PdO(101) is completely inactive during the TPRS experiments. This behavior is consistent with a previous study which reports that a ligand effect is responsible for the low vs. high activity of single vs. multilayer PdO(101) on Pd(100) toward methane oxidation.41

RAIRS from single and multilayer PdO(101) during CO adsorption at 100 K


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RAIR spectra demonstrate that CO populates different sites on the single and multilayer PdO(101) films, respectively. Figures 3a and b show RAIR spectra obtained from the films as a function of the CO exposure for an adsorption temperature of 100 K. The RAIR spectra obtained from the PdO(101) single layer exhibit two features that we attribute to CO adsorbed on bridge and atop Pdcus sites, respectively (Figure 3a). The bridge and atop-CO features are initially located at 1957 and 2134 cm-1 and intensify with increasing CO exposure. The bridge-CO feature is initially more intense than the atop-CO feature, and blueshifts slightly to 1960 cm-1 for CO exposures up to about 0.5 L, while the atop-CO feature remains at 2134 cm-1. We observe only the bridge-CO feature after heating a saturated layer to ~125 K, and thus conclude that desorption of bridge-CO species produces the higher temperature CO TPRS peak (~151 K) obtained from single-layer PdO(101). The bridge-CO peak exhibits an abrupt redshift to 1950 cm-1 as the CO layer saturates after an exposure of about 1 L. These RAIRS results demonstrate that adsorbed CO molecules occupy both atop and bridge-Pdcus sites on the single-layer PdO(101) surface. We speculate that a change in the distribution of the adsorbed CO among binding sites may be responsible for the sudden redshift in the bridge-CO stretch band as the CO layer approaches saturation. a)

b) 0.008

0.008

CO RAIRS Single layer PdO(101) Ts= 100 K

CO RAIRS Multilayer PdO(101) Ts= 100 K

0.006

0.006

atop-CO -1 2134 cm

CO exposure 0.2 L 0.3 L 0.4 L 0.5 L 0.7 L 1L

0.004 bridge-CO

Absorbance (-∆R/R)

Absorbance (-∆R/R)

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

ACS Catalysis

-1

1960 cm

-1

1950 cm 0.002

0.000 2200

atop-CO -1 2144 cm

CO exposure 0.3 L 0.6 L 0.9 L 1.2 L 1.5 L

0.004

0.002

0.000 2100

2000

1900

1800

2200

-1

2100

2000

1900 -1

Wavenumber (cm )

Wavenumber (cm )

Figure 3: RAIR spectra obtained from a) single-layer PdO(101) and b) multilayer PdO(101) (~6.6 ML) as a function of the CO exposure at 100 K.


1800

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The CO RAIR spectra obtained from the multilayer PdO(101)/Pd(100) film exhibit a dominant peak at 2144 cm-1 that arises from CO adsorbed on atop-Pdcus sites. The atop-CO peak shifts negligibly as the CO exposure increases to saturation at 100 K (Figure 3b). The spectra also exhibit a weak feature near 1970 cm-1 that we attribute to CO adsorbed on under-oxidized domains, mainly single-layer PdO(101), located near the perimeter of the sample (S3). Consistent with the TPRS data, the RAIR spectra from the multilayer PdO(101)/Pd(100) structure agree closely with those reported previously for CO adsorbed on multilayer PdO(101) grown on Pd(111),27, 29 further demonstrating that the adsorption of CO on multilayer PdO(101) is insensitive to the structure of the metallic Pd substrate. Our results reveal a marked difference in the CO binding site preference on the PdO(101) structures grown on Pd(100), with CO occupying both bridge and atop Pdcus sites on the single-layer oxide but occupying only atopPdcus sites on the multilayer oxide. The C-O stretch bands from atop-CO species on single vs. multi-layer PdO(101) occur at similar frequencies, 2134 vs. 2144 cm-1, respectively, even though the atop-CO binding energies differ significantly on these systems (33 vs. 119 kJ/mol). This result underlines the two contributions to the bonding of CO to metal sites and stresses that the CO stretch vibration depends on both the 5σ-donation and the 2π* backdonation. Below, we show that DFT-PBE calculations reproduce the similar C-O stretch frequencies for atop-CO on single and multiple layer PdO(101), and include a discussion in Section S6 about the effects that cause this similarity, such as an offsetting influence from forward vs. back-donation contributions in the CO-Pdcus bonding.


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ACS Catalysis

DFT results for CO adsorption and reaction on PdO(101) CO binding sites and energies on single and multilayer PdO(101) CO adsorption and reaction have previously been investigated on multilayer PdO(101) using different DFT implementations.38,

43, 45, 66

Consistent with experimental findings, DFT-PBE

predicts that CO binds strongly on multilayer PdO(101) (~137 kJ/mol) and oxidizes to CO2 via a highly facile pathway in which the barrier for CO oxidation is about 75 kJ/mol lower than the activation energy for CO desorption.38 Here, we compare DFT results for multilayer PdO(101) to similar calculations for CO adsorption on one and two layers of PdO(101) on Pd(100). Table 1 summarizes the stable CO adsorption states along with the associated C-O stretch frequencies and relevant bond lengths for all three systems. Prior studies show that the PBE functional predicts similar binding energies for CO on the atop and bridge-Pdcus sites of multilayer PdO(101), and that CO should thus populate both types of sites.38, 43 However, RAIR spectra demonstrate that CO only occupies atop-Pdcus sites on multilayer PdO(101) at temperatures down to 90 K and coverages up to monolayer saturation (Figure 3b).36, 38 Compared with the PBE results, calculations using the HSE06 functional predict a similar binding energy of the atop-CO species but a dramatic destabilization (~70 kJ/mol) of the bridge-CO species on multilayer PdO(101) (Table 1). As a result, the HSE06 calculations predict that CO binds much more strongly on atop vs. bridge Pdcus sites (131 vs. 66 kJ/mol), and thus that CO would occupy only atop-Pdcus sites on multilayer PdO(101), in good agreement with the experimental findings (Figure 3b).38 The HSE06 calculations overestimate the binding energy of atop-CO on multilayer PdO(101) by about 12 kJ/mol (131 vs. 119 kJ/mol), whereas the computed PBE C-O stretch frequency of the atop-CO species agrees well with the value measured experimentally (2133 vs. 2144 cm-1). We note that we report PBE C-O stretch frequencies in our discussion because the


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HSE06 functional overestimates the experimental frequencies for molecules while PBE gives values in better agreement with experiments.38, 67

Table 1. DFT-PBE binding energy, C-O stretch frequency, and bond lengths for CO stable minima on PdO(101). Single-point HSE06 binding energies are reported in parentheses. The C-O stretch frequencies have been scaled by 1.024 based on the DFT PBE gas phase CO stretch frequency of 2120 cm-1 versus the experimental harmonic stretch frequency value of 2170 cm-1.68

νC-O (cm-1)

Eb (kJ/mol)

C-O (Å)

Pd-C (Å)

Multilayer PdO(101) Atop

139.0 (130.8)

2133

1.15

1.87

Bridge

136.6 (66.4)

1956

1.17

2.03

2 ML PdO(101)/Pd(100) Atop

128.2 (128.1)

2134

1.15

1.89

Bridge

129.0 (78.5)

1967

1.17

2.04

1 ML PdO(101)/Pd(100) Atop

44.6 (52.0)

2124

1.15

1.93

Bridge

76.9 (66.1)

1953

1.17

2.06

The large destabilization of CO adsorbed on the bridge-Pdcus site of multilayer PdO(101) predicted by the HSE06 functional can be understood by examining how the different functionals influence the d-band center of the Pdcus sites relative to the HOMO-LUMO of CO (see S7). Table S4 shows that the HSE06 functional increases the HOMO-LUMO gap of CO by 2.24 eV 22 ACS Paragon Plus Environment

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ACS Catalysis

with the LUMO moving up by ~1 eV. In addition, HSE06 shifts the d-band center for Pdcus on multilayer PdO(101) to lower energies by nearly 1.3 eV. Together, the upshift in the CO LUMO and the downshift of the d-band center weaken the CO-Pdcus π-bonding that results from backdonation from Pdcus d-states to the LUMO 2π* CO orbital. The contribution from π-bonding has a much larger effect on CO binding on the bridge-Pdcus site and thereby causes large destabilization of the bridge-CO species on multilayer PdO(101). We note that a similar effect due to HSE03 was reported for CO on Cu(111) but an effect on the d-band center was not observed for CO on Pd(111) and Pt(111), resulting in a much larger destabilization of CO adsorbed on the hollow site of Cu(111) compared with Pd(111) and Pt(111).69-70 Our DFT calculations predict that CO adsorption on single-layer PdO(101)/Pd(100) is markedly different from multilayer PdO(101) (Table 1). First, our DFT calculations predict that CO achieves significantly stronger binding on atop-Pdcus sites of multilayer PdO(101) compared with Pdcus sites of single-layer PdO(101)/Pd(100). The predicted enhancement in CO binding strength on multilayer vs. single-layer PdO(101) agrees well with our experimental estimates (~75 kJ/mol) and is the primary cause of the lower reactivity of the single-layer oxide. Our calculations also predict that CO can bind on both atop and bridge Pdcus sites of single-layer PdO(101)/Pd(100), with a slight preference for the bridge site at low coverage. This finding is also consistent with our experimental results as RAIRS reveals the formation of both atop and bridge-CO species on single-layer PdO(101) (Figure 3a). A key difference from multilayer PdO(101) is that the DFT calculations predict only a small decrease in the binding energy of the bridge CO-species on the single-layer PdO(101) structure (~11 kJ/mol) using HSE06 vs. PBE. This smaller destabilization of the bridge-CO species on single-layer PdO(101) can be understood by noting that the downward shift in the d-band center


ACS Catalysis 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

is only ~0.7 eV compared with the larger 1.3 eV shift observed for multilayer PdO(101). As a result, HSE06 weakens the CO-Pdcus π-bonding to a lesser extent for single-layer vs. multilayer PdO(101). Moreover, the downward shift of both the HOMO of CO and the d-band center enhances the CO-Pdcus σ-bonding, causing the binding energy of atop-CO on single-layer PdO(101) to increase by ~8 kJ/mol in the HSE06 vs. PBE calculations. This enhancement, in combination with the downward shift in the bridge-CO binding energy, leads to a HSE06computed binding energy difference of only 14 kJ/mol between the atop and bridge CO species on single-layer PdO(101). The computed binding-energy difference is sufficiently small to expect that CO would occupy both atop and bridge-Pdcus sites on single-layer PdO(101), and is close in value to the energy difference (~10 kJ/mol) determined from the CO TPRS peaks at 118 and 151 K (Figure 2a). We note that the HSE06 binding energies for atop and bridge-CO species, respectively, are greater than the TPRS values by ~20 kJ/mol, where this overbinding is similar in magnitude to that predicted for CO on multilayer PdO(101). We have evaluated the CO binding energies at higher coverages on single-layer PdO(101)/Pd(100) (Table S5) and find only a small coverage dependence for atop-CO, whereas the bridge-CO species becomes more destabilized at higher coverages, resulting in nearly the same binding energies for both atop and bridge-CO species. Overall, coverage effects alone are unable to account for the overbinding predicted by HSE06 compared with the experimental results. We expect that some of this error is due to the HSE06 functional which is known to be less accurate for metal surfaces,69-70 but appears to be the best current choice of functionals for the mixed system of PdO(101) and Pd(100). Although the DFT calculations overestimate the binding strength of CO, both PBE and HSE06 calculations predict that CO will bind on both the atop and bridge-Pdcus sites of single layer PdO(101)/Pd(100).


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ACS Catalysis

Furthermore, the computed PBE C-O stretch frequencies of the atop and bridge-CO species on single-layer PdO(101)/Pd(100) sites (2124 and 1953 cm-1) agree well with the C-O stretch bands observed in RAIRS at low CO coverages (2134 and 1957 cm-1).

Ligand effect: Bonding analysis of CO on single and multilayer PdO(101) The absence or presence of an O-atom below the Pdcus site is responsible for the different binding characteristics of CO on single and multilayer PdO(101). The influence of these O-atoms is analogous to the trans-ligand effect known from organometallic chemistry, wherein an electron withdrawing ligand lowers the electron density at the Pdcus atom and can thereby modify the bonding of another ligand at the trans-position.71-72 Similar to our results with CO, Martin et al. have shown that the trans-ligand effect causes multilayer PdO(101) to be highly active for promoting CH4 oxidation, whereas single-layer PdO(101)/Pd(100) is inactive for this reaction.41 We also performed DFT calculations which show that the binding strength and preferred binding site of CO adsorbed on two layers of PdO(101) on Pd(100) are similar to that determined for CO on multilayer PdO(101) (Table 1). This finding provides further evidence that the trans-ligand effect plays a dominant role in determining the different CO binding characteristics on single vs. multi-layer PdO(101). The ligand effect can also be clearly seen in a charge density difference plot for CO adsorption on single and multi-layer PdO(101) (Figure 4). The charge density difference (ρdiff) is defined by ρdiff(r) = ρCO-slab – (ρCO + ρslab). Here, ρCO-slab is the electron density associated with CO adsorbed on the PdO(101) slab and ρCO and ρslab are obtained by deleting the PdO(101) slab and CO molecule, respectively, from the CO-adsorbed configuration and performing a single-point calculation (i.e. no relaxation). Such a procedure provides a fingerprint of the bonding interactions between CO and the PdO(101) surface. The analysis reveals larger


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charge accumulation along the OC-Pdcus bond of CO adsorbed on multilayer PdO(101), reflecting more efficient 5σ-donation and stronger adsorption. Furthermore, the O lattice atom bonded to the underside of the Pdcus atom exhibits an increase in electron density suggesting charge transfer to the O lattice atom; such an interaction is absent for the single-layer PdO(101) structure.

Figure 4. Charge-density difference plot (electrons/bohr3) for CO on an atop site of (a) singe-layer PdO(101)/Pd(100) and (b) multilayer PdO(101) computed using DFT-PBE and viewed along the [010] direction generated using VESTA.73 The insets show representations of the corresponding CO/PdO(101) structures. The plane for the charge density consists of the Pdcus atom and its two neighboring surface O atoms

To understand the ligand effect in more detail, we evaluated the pCOHP between the CO molecule and the d-orbitals of the Pdcus atom in the atop configuration for both single layer


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ACS Catalysis

PdO(101)/Pd(100) and multilayer PdO(101). We note that our analysis is based on the pDOS determined from DFT-PBE calculations, however we find that analysis of the HSE results is qualitatively similar as both functionals predict that atop-CO binding is substantially stronger on multilayer than on single layer PdO(101). The integrated pCOHP (IpCOHP), obtained by integrating the pCOHP up to the Fermi level, provides a quantitative approach for evaluating the strength of specific orbital interactions.74-76 Table 2 shows our results for the interactions involving σ and π bonding of adsorbed CO on the single and multilayer PdO(101) structures, and Figure 5 compares the pDOS of the d-states that are most involved in the bonding. Our analysis shows that both σ and π bonding interactions are weaker for atop-CO on the single layer PdO(101) as compared to the multilayer PdO(101). Since the σ bonding involves donation from the CO 5σ orbital into the Pd 4 orbital, this interaction is strengthened when there is an enhancement of empty states near the Fermi level, and leads to reduced Pauli repulsion as charge transfer from the CO molecule to the d orbital occurs. Figure 5a shows a considerable enhancement of the empty states near the Fermi level when going from single to multilayer PdO(101).

Table 2. IpCOHP values of atop sites on the single-layer PdO(101) (1ML PdO(101)) and multilayer PdO(101) (Bulk PdO(101)) with their respective Eb (kJ/mol). The σ interaction includes Pd 4 with C 2py and 2s orbitals. π interaction involves Pd 4dyz with C 2pz and Pd 4dxy with C 2px orbital. Note that a more negative (positive) value for the IpCOHP is indicative of stronger bonding (antibonding) interactions.

Surfaces Bulk PdO(101) 1ML PdO(101)

σ interaction Bonding Antibonding

π interaction Bonding Antibonding

Eb

Total

139.0

-1.19

-2.10

0.91

-1.26

-1.37

0.11

44.6

-0.90

-1.91

1.01

-0.96

-1.01

0.05


Total

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a)

b)

Figure 5. Projected density of states of (a) and (b) dyz+dxy for single-layer PdO(101)/Pd(100) and multilayer PdO(101) with respect to Fermi level. The Fermi level is located at 0 eV and indicated by the dashed line.

The pCOHP analysis shows that the π bonding interaction between CO and a Pdcus atom is also stronger on multi- vs. single-layer PdO(101). In agreement with this observation, the partial density of states of multilayer PdO(101) in Figure 5b exhibits a higher density of filled dxy and dyz states near the Fermi level compared with single-layer PdO(101). Because the π-bonding interaction involves back donation from the filled dxy and dyz orbitals of the Pdcus atom into the CO 2π* orbitals, the increase in density of filled dxy and dyz states strengthens the CO-Pdcus πbonding on multilayer PdO(101). Future photoemission measurements of the valence band region would provide a useful comparison with the computed pDOS of CO adsorbed on single and multilayer PdO(101). The computed differences in the density of d states at the Pdcus atom for single and multi-layer PdO(101) can be associated with the absence or presence of an Oligand below the Pdcus site. The interaction between the Pdcus atom and the O-ligand predominantly involves the same d-states as the CO-Pdcus interaction due to symmetry. Our results, thus, demonstrate that the O-atom below the Pdcus site in the multilayer PdO(101) structure removes electrons from the Pdcus atom, leading to an increase in density of


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ACS Catalysis

empty states just above the Fermi level, while also donating electrons into the dxy and dyz orbitals, leading to an increase in the density of filled dxy and dyz states (Figure 5b). These changes reinforce both the σ and π bonding interactions between CO and a Pdcus atom, resulting in significantly stronger CO binding on multilayer vs. single layer PdO(101). We can support this idea by examining CO adsorption on a free standing single PdO(101) layer. For the atop CO species, the PBE binding energy decreases from 139 to 75 kJ/mol for multilayer PdO(101) versus the free standing single layer PdO(101) which is close to that of single-layer PdO(101) on Pd(100) (62 kJ/mol). While the metal substrate clearly introduces additional effects that further reduce the CO binding energy, our result with the free-standing single-layer demonstrates that the O-atom trans-ligand plays the dominant role in enhancing the bonding of atop CO on the multilayer PdO(101) structure.

CO oxidation pathways and reaction probability Figure 6 shows pathways for CO oxidation on single-layer PdO(101)/Pd(100) and multilayer PdO(101) determined from PBE calculations. We examined the oxidation of CO initially adsorbed on the bridge-Pdcus site of single-layer PdO(101) as this binding site is preferred at low coverage. The computed CO oxidation pathways are similar on each PdO(101) surface and involve an adsorbed CO molecule first abstracting a nearby Ocus atom to produce an adsorbed CO2 molecule and an Ov site. The predicted energetics indicate that CO2 will readily desorb because the energy required for CO to oxidize to adsorbed CO2 is significantly greater than the CO2 binding energy. This behavior is consistent with prior work showing that CO2 binds relatively weakly on PdO(101)77 and that CO2 desorbs in reaction-limited features during TPRS. The PBE calculations predict that the energy barriers for CO abstraction of an Ocus atom, i.e., the


ACS Catalysis

reaction barriers, are Er = 66.4 and 74.8 kJ/mol for single and multi-layer PdO(101), respectively. The PBE value is about 17% lower than our experimental estimate of the CO oxidation barrier on multilayer PdO(101), while the PBE value is about 32% higher than the lower-bound of the CO oxidation barrier on single-layer PdO(101) that we estimate from TPRS simulations (Er ~ 45 kJ/mol).

a)

b) 50

50

CO (g) + PdO(101) 0

Energy (kJ/mol)

0

Energy (kJ/mol)

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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-50 -64

CO2(g) + PdO(101)-1Ov

-100

CO (g) + 1ML PdO(101) -10.5

-50

-76.9 -100

CO2(g) + 1ML PdO(101)-1Ov

-106 -136

-150

-139

-135

-134

-150

Figure 6: Energy diagrams computed using PBE for CO oxidation on a) multilayer PdO(101) and b) single-layer PdO(101)/Pd(100). Energies are defined relative to an isolated CO molecule and surface.

The major difference in the computed CO reactivity of the two oxide structures is that the CO binding energy is significantly lower on single as compared to multilayer PdO(101), with values of Ed = 77 and 139 kJ/mol, respectively, where Ed represents the energy barrier for CO 30 ACS Paragon Plus Environment

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desorption. The apparent activation energy for reaction (Eapp = Er – Ed) represents the energy barrier for reaction measured relative to the gaseous CO reactant, and controls the kinetic behavior at temperatures that are relevant to practical reaction conditions (see S9). The computed, apparent activation energies are equal to Eapp = -10.5 and -64.2 kJ/mol for CO oxidation on single and multilayer PdO(101), respectively, based on the PBE energies. The more negative value of Eapp means that the kinetic barriers are more favorable for CO oxidation on multilayer PdO(101) compared with the single-layer oxide. We thus conclude that the stronger binding of CO, and hence the much lower apparent activation energy for reaction, is responsible for the higher CO oxidation activity of multi- vs. single layer PdO(101). However, PBE predicts a negative Eapp and thus suggests facile CO oxidation on single-layer PdO(101) in UHV, in conflict with our finding of negligible CO oxidation activity of the single-layer structure during TPRS experiments. We assert that this discrepancy results from the overbinding predicted by the PBE calculations, and show below that correcting for this overbinding leads to rate predictions that are consistent with the observed activity of single and multi-layer PdO(101) toward CO oxidation. We estimated probabilities and rates of CO oxidation on single and multi-layer PdO(101) using a precursor-mediated kinetic model with rate parameters based on both experimental and computational results. The model considers CO oxidation on the pristine oxide surfaces at low CO coverage via the Mars van Krevelen mechanisms illustrated in Figure 6 in which adsorbed CO acts as the precursor for reaction. In the model, we define the intrinsic reaction probability Pr as the probability that an adsorbed CO molecule will react rather than desorb and express Pr as a branching probability written in the familiar form, Pr = kr / (kr + kd) where kr and kd represent rate coefficients for reaction and desorption, respectively. Here, the term "intrinsic reactivity" refers


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to the reactivity of CO on the pristine oxides containing negligible concentrations of Ov sites. We described the rate coefficients using the Arrhenius equation with apparent reaction pre-factors of

νr/νd = 0.008 and 0.006 for CO oxidation on single and multi-layer PdO(101), respectively, where the values of νr/νd were determined from an analysis of our DFT results (see S9) and lie in the typical range reported for such reactions.78 We note that apparent pre-factors for precursormediated surface reactions are less than unity in most cases, with values typically between 10-3 and 10-1 for small molecules, because the transition state for desorption will generally have a higher entropy than the transition state for reaction. Using the energy barriers determined from PBE calculations, the model predicts CO reaction probabilities near unity at temperatures below that for the onset of CO desorption from both the single and multi-layer PdO(101) structures, i.e., below ~150 and 350 K, respectively, meaning that a large fraction of the adsorbed CO should oxidize to CO2 during TPRS on both oxide structures. This prediction conflicts with our experimental finding that a negligible quantity of CO adsorbed on the single-layer PdO(101) oxidizes during TPRS. The kinetic model reproduces the qualitative TPRS result of low and high CO oxidation activity on single and multilayer PdO(101) when using the experimental estimates of the CO binding energies and the PBE results of the reaction barriers. In this case, the CO binding energy used in the model decreases from 139 to 119 kJ/mol for multilayer PdO(101) and from 77 to 43 kJ/mol for single-layer PdO(101), resulting in apparent reaction barriers of -44 and +23 kJ/mol, respectively. The relatively large, positive value of the apparent barrier causes the CO reaction probability on single-layer PdO(101) to become negligible (Pr < 10-8) at temperatures near that for CO desorption (< 200 K), whereas the reaction probability remains close to unity for CO oxidation on multilayer PdO(101) up to at least 400 K. These results are consistent with our


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findings from TPRS and reinforce our conclusion that PBE significantly overestimates the CO oxidation activity of single-layer PdO(101)/Pd(100) due to the predicted overbinding of CO on the Pdcus sites of this surface. Matera et al. have also recently noted that the PBE energetics cause the CO oxidation activity of single-layer PdO(101)/Pd(100) to be overestimated.19 For consistency, we elected to use the PBE values of Er in our kinetic simulations for both oxides. This approach assumes that the PBE calculations overestimate the initial and transition state energy for CO oxidation by the same amount for a given oxide. This assumption is not rigorously accurate as confirmed by the difference between the experimental and PBE values of Er for the multilayer PdO(101) (~90 vs. 75 kJ/mol). Importantly, however, the main conclusions of our reaction probability and rate calculations are unaffected by reasonable changes in the Er values, including calculations performed using the lower-bound of Er determined from our experiments. We further illustrate this finding below. Our findings demonstrate that single-layer PdO(101)/Pd(100) is inactive in promoting CO oxidation during TPRS experiments performed in UHV. At first glance, our results appear to contradict prior investigations which report that the single-layer PdO(101)/Pd(100) structure is highly active in promoting CO oxidation during reaction at semi-realistic conditions (e.g., pressures above ~1 mbar).2,

4

However, we show below that, while multilayer PdO(101) is

indeed overwhelmingly more active than single-layer PdO(101)/Pd(100), both oxides can achieve CO oxidation rates that exceed the diffusion rate of gaseous CO to the surface and thus that both oxides can exhibit high activity under the conditions studied in typical in situ experiments.


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Intrinsic reaction rates vs. CO diffusion rate Figure 7 shows initial CO oxidation rates on single and multi-layer PdO(101) computed as a function of temperature using the precursor-mediated kinetic model with binding energies estimated from TPRS experiments and reaction barriers determined from PBE calculations. We performed the calculations for a CO partial pressure of 5 mbar, in consideration of conditions that have recently been employed in an in situ study that reports measurements of the gas-phase concentration profile during CO oxidation on Pd(100).19 Figure 7 also shows an estimate of the diffusion rate of CO to the catalytic surface, where our calculation assumes that CO must diffuse through a 5 mm thick region to reach the surface and neglects convective terms (see S10). Recent measurements show that catalytic ignition produces a region above the Pd(100) sample that is highly depleted of CO, with this region being ~5 mm in thickness for the operating conditions reported by Matera et al.19 Figure 7 shows diffusion rates computed for conditions that we deemed most reasonable for approximating the experimental conditions of Matera et al.19 (S10).


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9

10

8

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Rate (molecules/site/s)

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Rate of CO oxidation PCO = 5 mbar Adjusted Ed values

6

Er (PBE) - Ed (exp)

10

Multilayer PdO(101)

5

10

4

10

Er (min) - Ed (exp)

3

Single layer PdO(101)/Pd(100)

10

2

10

Er (PBE)

1

10

Diffusion rate of CO(g)

- Ed (exp)

0

10

-1

10

400

500

600

700

Tsample (K) Figure 7: Initial CO oxidation rates on pristine single (blue) and multi-layer (red) PdO(101) computed as a function of temperature using the precursor-mediated kinetic model described in the text. The blue dashed curve was calculated using the lower-bound of Er (“Er (min)”) for CO oxidation on single-layer PdO(101) as determined from TPRS simulations. Also shown is an estimate of the diffusion rate of gaseous CO to the surface, assuming that diffusion occurs through a CO-depleted region above the sample of 5 mm thickness (see S10). Rates are given in units of molecule⋅(site⋅s)-1, where a site represents a Pdcus site.

The key goal of these calculations is to compare initial CO oxidation rates for single and multilayer PdO(101) with the rate of gaseous CO transport to the surface. We computed the CO oxidation rates using the expression Rr = F*Pr, where Pr is the reaction probability defined above and F is the incident flux of gaseous CO computed using the Hertz-Knudsen equation for a CO partial pressure of 5 mbar and varying sample temperature (see S10). The computed rates approximate initial CO oxidation rates at the idealized oxide surfaces rather than steady-state reaction rates since the model computes the rate of CO oxidation on pristine PdO(101) surfaces and omits the regeneration of Ocus atoms by gaseous O2. Microkinetic or kinetic Monte Carlo modeling is needed to examine the steady-state CO oxidation kinetics.19, 32, 34, 79-80 Our model also excludes any influence of Ov sites and metallic domains on the CO oxidation rate. We 35 ACS Paragon Plus Environment

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emphasize that our intention is not to compute CO oxidation rates for quantitative comparison with steady-state experimental results but instead to provide an approximate assessment of the intrinsic CO reaction rates on these oxides for comparison with the CO diffusion rate to the surface. As expected, the precursor-mediated model predicts that the intrinsic CO oxidation rate is four to five orders of magnitude higher on multilayer PdO(101) compared with single-layer PdO(101)/Pd(100) for temperatures between 400 and 700 K (Figure 7). The computed CO oxidation rates on multilayer PdO(101) decrease from ~2.5 × 106 to 1.5 × 106 molecule⋅(site⋅s)-1 with increasing temperature over this range, while the rates on single-layer PdO(101)/Pd(100) increase from about 10 to 250 molecule⋅(site⋅s)-1. We also estimate that the CO diffusion rate lies in the same range as the intrinsic CO oxidation rate on single-layer PdO(101) for the temperature range considered. Work by Matera et al.19 shows that ignition of CO oxidation over Pd(100) occurs just above 500 K for the conditions considered here, with the rate plateauing at temperatures above ~575 K and becoming limited by the rate of gaseous CO transport to the surface. Our estimates support the idea that CO oxidation will become mass transfer limited on both single and multilayer PdO(101) on Pd(100) after reaction ignition. This interpretation is clearly supported for CO oxidation on multilayer PdO(101) given that the initial reaction rate far exceeds the estimated CO diffusion rate. Our estimates also support this view for CO oxidation on single-layer PdO(101) as the computed reaction rate is higher than the CO diffusion rate at temperatures above 575 K. To illustrate the sensitivity of the predicted rates on the energetics, we also show the CO oxidation rate on single-layer PdO(101) computed using the lower-bound of the reaction barrier (Er = 45 kJ/mol) that we determined from TPRS simulations. In this case,


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the computed CO oxidation rate is significantly higher than the CO diffusion rate, but remains several orders of magnitude lower than the intrinsic reaction rate on the multilayer oxide. Overall, our calculations demonstrate that the intrinsic CO oxidation activity of multilayer PdO(101) is exceedingly greater than that of single-layer PdO(101)/Pd(100), but that both oxides can be expected to sustain CO oxidation rates that exceed the mass-transfer limit. These systems represent an example in which the intrinsic reactivity measured under UHV conditions differs dramatically from that observed at elevated pressure, steady-state conditions. We have presented a scenario where the two studies can be reconciled due to mass-transfer limitations but we note that the details of the full Mars-van Krevelen reaction sequence on the oxide film needs to be modeled in more detail to establish the complex interplay between mass transfer, reaction conditions at or near the surface, and surface reaction kinetics. The present study highlights the important role of UHV measurements for testing DFT calculations and ultimately developing accurate atomic-level descriptions of surface catalysis as well as the need to consider external mass transport in modeling CO oxidation catalysis.

Summary We investigated the adsorption and oxidation of CO on single and multi-layer PdO(101) on Pd(100) using TPRS and RAIRS experiments as well as DFT calculations. We find that CO binds weakly and oxidizes to a negligible extent on single-layer PdO(101)/Pd(100) during TPRS, with the CO desorbing in sharp TPRS peaks at 118 and 151 K. In contrast, CO binds strongly on multilayer PdO(101), desorbing in a broad peak at ~390 K, and nearly 90% of the adsorbed CO oxidizes to CO2 during TPRS at saturation of the initial CO layer. Our results, thus, reveal that the CO binding affinity and intrinsic oxidation activity is significantly higher for multi-layer


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compared with single layer PdO(101) on Pd(100). RAIRS further shows that CO molecules adsorb on both bridge and atop-Pdcus sites of single-layer PdO(101)/Pd(100), whereas CO binds exclusively on atop-Pdcus sites of multilayer PdO(101), thus demonstrating a marked difference in the CO binding site preference on these PdO(101) films. Our DFT calculations predict much stronger CO binding on multilayer than on single layer PdO(101)/Pd(100) and reproduce the CO binding site preferences observed with RAIRS. Bonding analysis shows that the O-atom below the Pdcus site changes the electronic structure of the Pdcus atoms of multilayer PdO(101), resulting in enhancements in both σ donation and π back-donation in the CO-Pdcus interaction and, thus, stronger CO bonding on multi- as compared to single layer PdO(101). Our calculations further show that the stronger binding of CO is entirely responsible for the higher CO oxidation activity of the multi-layer PdO(101) structure. Both PBE and HSE06 overestimate the CO binding energy on single and multi-layer PdO(101)/Pd(100), causing predicted CO oxidation rates to also be too high. We show that a precursor-mediated kinetic model, using experimentally-determined CO binding energies and DFT-derived reaction barriers, predicts reaction probabilities that are qualitatively consistent with the finding that CO oxidation occurs negligibly on single-layer PdO(101) but extensively on multilayer PdO(101) during TPRS. Further, the kinetic model predicts that, even though multilayer PdO(101) is exceedingly more reactive, both the single and multi-layer PdO(101)/Pd(100) structures can yield CO oxidation rates that exceed the mass transfer limit during steady-state CO oxidation at realistic reaction conditions. Overall, the present study reports intrinsic reactivities of single and multi-layer PdO(101) on Pd(100) that provide a consistent description of the CO oxidation activity of these surfaces under both UHV and highpressure conditions.


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Supporting Information Procedure for processing of TPRS traces; Procedure for generating orbital-decomposed partial DOS for the PdO(101) surface; Contribution of sample perimeter in measurements with multilayer PdO(101); Pre-factors for CO desorption; TPRS kinetic model; Origin of atop-CO stretch frequencies on single and multiple layer PdO(101)/Pd(100); Coverage effect in CO adsorption on single-layer PdO(101)/Pd(100); Destabilization of CO binding energy by DFTHSE; Precursor-mediated kinetic model; Calculation of gaseous CO diffusion rates. This information is available free of charge on the ACS Publications website.

Acknowledgements We thank Rachel Farber and Aravind Kadiri for assistance with experiments. We acknowledge the Ohio Supercomputing Center for providing computational resources. We gratefully acknowledge financial support for this work provided by the Department of Energy, Office of Basic Energy Sciences, Catalysis Science Division through Grant DE-FG02-03ER15478 and the Swedish Research Council. The calculations were in part performed at C3SE (Gothenburg) via a SNIC grant. We gratefully acknowledge support of the Röntgen-Ångström cluster “Catalysis on the atomic scale” (Project No. 349-2011-6491) by the Swedish Research Council, as well as the Swedish Foundation for International Cooperation in Research and Higher Education (STINT).

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ToC Graphic CO on multilayer PdO(101)

Ligand Effect CO2(g)

CO on single layer PdO(101)

CO2(g)


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Understanding the Intrinsic Surface Reactivity of Single-Layer and

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