Carbon-supported Pt during aqueous phenol hydrogenation with and without applied electrical potential: X-ray absorption and theoretical studies of structure and adsorbates

Nirala Singh,†,‡,§ Manh-Thuong Nguyen,† David C. Cantu,†, B. Layla Mehdi,†, ||| Nigel D. Browning,†, |||

John L. Fulton,† Jian Zheng,† Mahalingam Balasubramanian,║ Oliver Y. Gutiérrez,† Vassiliki-Alexandra Glezakou,† Roger Rousseau,† Niranjan Govind,† Donald M. Camaioni,† Charles T. Campbell,‡ Johannes A. Lercher†,*†Pacific Northwest National Laboratory, Richland, Washington 99354, United States‡Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.§Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States║Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States

|||University of Liverpool, School of Engineering, Liverpool, L69 3GQ, United Kingdom

KEYWORDS. Hydrogenation, Electrocatalysis, X-ray Absorption Spectroscopy.

ABSTRACT. Adsorbed hydrogen and phenol on Pt nanoparticles during (electro)catalytic hydrogenation are explored by combining X-ray absorption spectroscopy and ab initio simulations. Direct evidence for two types of Pt-C bonds at the surface of the metal particles detected by X-ray absorption spectroscopy suggest strong bonding between metal and the carbon support as well as adsorption of phenol nearly parallel to the surface. Hydrogen and phenol compete for accessible Pt sites. The surface concentrations are compatible with the proposal that atomic hydrogen and chemisorbed phenol are the species reacting in the rate-determining step of hydrogenation in the presence and absence of an external cathodic potential. During electrocatalytic hydrogenation the external electric potential controls the concentration of species on the surface, but does not impose structural or electronic property changes of the Pt compared to Pt particles in presence of hydrogen gas. Increasing reaction rates with increasing cathodic potential are attributed to the increase in chemical potential of adsorbed H.

1. INTRODUCTION

Understanding the elementary steps for the hydrogenation of functionalized aromatic and aliphatic organic molecules is key to the development of selective and highly active catalysts [1–3]. In contrast to hydrogenation catalyzed at a gas-solid interface, the presence of a condensed phase makes the characterization of the adsorbed species challenging. Here, we apply Pt-edge X-ray absorption combined with ab initio simulations to characterize the adsorbates on and the structure of carbon-supported Pt during aqueous hydrogenation of phenol both with and without applied electrochemical potential.

The emergence of on-purpose electrochemical hydrogenation stimulates interest to replace conventional hydrogenation with H2 (thermochemical hydrogenation, TCH) by direct electrocatalytic hydrogenation (ECH) [4–11][4–13]. Considering the possible elementary reaction steps, it is obvious that at least a fraction of them must be common for both pathways. Exploring these commonalities as well as the differences in the reaction is expected to guide synthesis of conductive catalysts with a higher catalytic vector.

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The reduction of phenol is used as model reaction, because the combination of arene ring hydrogenation and the hydrogenation cyclohexanone provides information about hydrogen addition to polar and apolar unsaturated bonds. A recent example shows that the rate of phenol electrocatalytic hydrogenation increases with increasing cathodic potential, and that the reaction is zero-order in phenol, implying high phenol coverages [12].[14]. The electrochemical hydrogenation of phenol is concluded to proceed via the reaction of adsorbed phenol and hydrogen generated on a catalytically active metal particle [13].[15]. One may deduce from this observation that a high rate of reaction requires an optimum balance between adsorbed phenol and hydrogen. However, it is unclear, which role the specific nature of the metal and its electronic state play in influencing reaction rates.

Understanding these elementary steps requires qualitative and quantitative information on the state of the catalyst and the interacting reacting species during catalysis. Here, we apply X-ray absorption spectroscopies, i.e., near edge (XANES) and the extended fine structures (EXAFS), supported by ab initio simulations, to clarify the state of and adsorbates on carbon-supported Pt in the aqueous hydrogenation of phenol by H2 or H+

aq without and with applied electrochemical potential. By using catalysts having a high fraction of surface Pt atoms, the X-ray spectra are weighted heavily by surface structures. The ab initio molecular dynamics (AIMD) simulations here use of large systems (100s-1000s of atoms), permitting statistical sampling of >104 of configurations (~1 nm particles on carbon support), to realistically mimic the experimental conditions [14–20][16–22].

2. RESULTS AND DISCUSSION

2.1 Structure of the Pt/C catalyst. Two catalysts were used to conduct the study. The “Pt/C 1.5 nm” sample, with Pt particles having an average diameter of 1.5 nm, was prepared on Vulcan carbon by strong electrostatic adsorption of tetraamonium platinum nitrate [21][23], followed by reduction at 375 °C in H2. The “Pt/C 2 nm” sample having an average particle diameter of 2 nm was supported on activated carbon.

The high-angle annular dark field (HAADF) STEM images, Figure 1, showed core regions with a well-ordered lattice as well as disordered parts of the particles which are speculated to result from beam damage. The Pt-Pt distances within the ordered domains were approximately 2.8 Å, in agreement with the distance of metallic Pt. It cannot be excluded that the amorphous region additionally contains two-dimensional patches of platinum oxide. Approximately 5-20% of Pt is concluded to exist as single atoms or aggregates of 1-4 Pt atoms based on the metal content and the size distributions shown in Figure 1.

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Figure 1. a) Particle size distribution of the Pt/C 2 nm sample and atomic resolution HAADF STEM images showing typical particle morphology including presence of facets. Frequency axis maximum is 70 particles. b) Particle size distribution of the Pt/C 1.5 nm sample and STEM images showing crystalline Pt nanoparticles coexisting with atomically-dispersed Pt species. Frequency axis maximum is 100 particles. In both cases, the catalyst is in the as-prepared condition. Arrows highlight single Pt atoms both in the nanocrystal and on the carbon support surface. Scale bar in all images is 2 nm.

Figure 2. a) k1-weighted Im[χ(R)] spectra of the ex situ Pt/C 1.5 nm, of Pt/C 1.5 nm reduced at 300 °C in H2 after cooling to 25°C in helium gas and of a Pt metal foil (scaled by 0.3). b) XANES spectra for the same sample conditions, showing that the ex situ sample contains some platinum oxide and that this was reduced to metal with H2.

2.2 State of Pt from catalyst precursor to acting catalyst. The white line peak of the X-ray absorption spectrum of Pt at ~11566 eV was significantly more intense for the fresh Pt/C 1.5 nm than for the reduced catalyst (inset Figure 2). This indicates that a significant fraction of Pt oxide [22][24] is present in the starting material. Correspondingly, the EXAFS peak at approximately R = 1.6 Å for the as prepared sample is at least partially attributed to Pt-O bonds. However, the peaks for Pt-Pt scattering at 3-4 Å expected for PtO2 were not detected, pointing to the possibility that a large fraction of bonds are

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related to the interaction between the metal particle and the C support. Correspondingly, there is an EXAFS peak at approximately R = 1.6 Å due to Pt-O scattering. However, there is no Pt-Pt scattering detected around 3-4 Å (uncorrected for phase shift) that is expected for crystalline PtO2, suggesting that the PtO2 is either in such an amorphous state that it has no well-defined Pt-Pt distance, or that the regions of PtO2 contain only a tiny fraction of the total number of Pt atoms, as for the surface oxide.

Upon treatment in H2 at 300 °C, the amplitude of the XANES decreased to nearly the value for metallic Pt (Figure 2 inset). This was also observed when cathode potentials below -0.5 V vs. Ag/AgCl were applied. The Pt foil spectrum is included to approximately represent the scattering from only different types of Pt-Pt interactions. For the nanoparticle, the number of nearest-neighbor Pt’s is only about 6, thus the foil spectra needs to be scaled to approximately compare the overall set of Pt-Pt features. A scaling of 0.3 approximately matches the amplitudes around 2.7 Å in Figure 2. This process demonstrates that there is excess amplitude at 1.6 Å that is not represented by the foil. The significant scattering around 1.6 Å in the EXAFS spectra in excess to the signal from that of the Pt metal foil is tentatively assigned to the interaction of Pt with the carbon support. The relatively high concentration of 1-4 Pt atom clusters (based on STEM images in Figure 1) leads to strong contributions of Pt-support interactions.

For samples under reducing conditions (H2 treatment or applied potential), we believe that the absence of Pt oxide or surface oxide is shown by both EXAFS and XANES (specifically, the lack of white line intensity (for PtO2) under reducing conditions). When we let the Pt/C sit in electrolyte partially saturated with air (i.e., without applying a potential and in the absence of any reductant, not shown), we see an increase in white line intensity which we attribute to formation of this surface oxide species (similar to the ‘As prepared’ sample in the inset). However, even when this white line XANES peak is fully converted to metallic Pt by applying a reducing potential or reducing with H2, there is still EXAFS scattering from a light element, most probably Pt-C, since there is no longer surface oxide, based on the electronic structure from XANES.

Normalized Pt L3-edge XANES spectra recorded during hydrogenation at 80 °C and 30 bar H2 and electrocatalytic hydrogenation at -0.55 V vs. Ag/AgCl are shown in Figure 3. In all cases, the white line intensity at 11566 eV was nearly identical to that of Pt metal (reference Pt foil), indicating full reduction of Pt [22].[24]. The XANES in presence and absence of the electrical potential does not show an influence of the cathodic potential on the spectra, and in turn on the electronic properties of the Pt nanoparticles.

After reduction in 30 bar H2 at pH 5, the catalysts exhibited a broad shoulder (from the white line peak to ~9 eV higher energy), which has been assigned to splitting of the Pt density of states due to adsorbed hydrogen atoms (Had) [23–25][25–27]. Thus, a significant concentration of Had is concluded to exist after this reduction. The addition of phenol reduced the XANES amplitude of H ad for all reaction conditions, more pronounced for Pt/C 2 nm than for Pt/C 1.5 nm. As phenol has been shown to block Pt sites for Had in water [26–30][28–32], this lower intensity for Had is attributed to the competitive adsorption.

Decreasing the pH from 5 to 1 under these same conditions slightly decreased the Pt-H ad signal. A similar effect was observed under electrochemical conditions upon increasing the potential from -0.55 to -0.3 V vs. Ag/AgCl. Decreasing the H2 pressure from 30 bar to 2 bar in the presence of phenol, however, hardly affected the XANES spectra.

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Figure 3. In situ Pt L3-edge XANES spectra (normalized µ(E)) for: a) Pt/C 2 nm in water at 80 °C and 30 bar H2 without and with 0.3 M phenol. Also shown is the effect of changing to pH 1 using a 0.1 M perchloric acid electrolyte including 0.3 M phenol. For comparison, the reference spectrum from a Pt(0) foil is also included. b) Pt/C 1.5 nm in water at 80 °C and 30 bar H2 without and with 0.3 M phenol. Also shown is a reduced pressure condition of 2 bar H2 also at 80 °C and with 0.3 M phenol. c) Pt/C 2 nm in 100 mM degassed aqueous acetate buffer at -0.55 V vs. Ag/AgCl without phenol and with 0.1 M

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phenol, and at -0.3V vs. Ag/AgCl with 0.1 M phenol. d) Pt/C 1.5 nm in aqueous acetate buffer at -0.55V without and with 0.1 M phenol.

2.3 XANES differences induced by hydrogen and phenol adsorption. The differences between the XANES spectra of Figure 3a and that of a Pt foil are shown in Figure 4a. Two broad bands at 11569 and 11572 eV are assigned to the presence of C and H atoms, respectively, directly bonded to the Pt surface. According to this assignment, the dominant Pt-Had feature is mostly replaced by the Pt-C feature upon introduction of phenol. The residual Pt-Had signal after phenol addition indicates that adsorbed phenol reduces but does not completely block formation of Had. This is consistent with the partial blocking of the cyclic voltammetry peaks due to Had on Pt upon phenol addition [9]. The relatively larger Pt-Had

signal (in absence of phenol) for the 1.5 nm Pt compared to the 2 nm Pt is consistent with the larger fraction of surface Pt atoms on these smaller particles.

Because the carbon support may in fact be partially populated with oxygen, the Pt catalyst may be interacting with a combination of both C and O terminated surfaces, but to avoid confusion between a surface Pt oxide interaction (rather than Pt adsorbed onto an O functional group on carbon), we refer to these support interactions as Pt-C. Since mainly our conclusions are in the changes from the base carbon support as we vary experimental conditions, we do not believe the exact nature of the carbon support will greatly influence our observations here (for example, the appearance of interactions and electronic structure changes with addition of phenol).

To support these XANES peak assignments, four experimental standards are included in Figure 4b. Two of the experimental standards in Figure 4b, Pt/zeolite in water at 40 bar H2 at either 25 or 180 °C, are known to be nearly free of C-Pt contributions, and represent Pt-Had, with its peak at 11572 eV [23–25]. [25–27]. The shape of these Pt-Had peaks strongly resemble spectra after adsorption of H2 from the gas phase onto Pt [31].[33]. The other two experimental standards in Figure 4b labeled “Pt/zeolite 25°C 1 bar H2

” (no water) and “Pt/C H2O” (reduced in water), are samples that are known to contain significant concentration of adventitious carbonThe other two experimental standards in Figure 4b labeled “Pt/zeolite 25°C 1 bar H2

” (no water) and “Pt/C H2O” (reduced in water), are samples that are known to contain significant concentration of adventitious carbon from exposure to the atmosphere, detected by XPS [34], that leads to a large number of Pt-C bonds, and show a XANES difference peaks at 11569 eV. Details of the preparation of these standards are included in the Supporting Information.

Time-dependent density functional theory (TD-DFT) based XANES calculations [32,33][35,36] of a model Pt6 cluster are shown in Figure 4c and 4d, and support the assignment of the features at 11569 and 11572 eV to Pt-Had and Pt-C, respectively. The simulated difference XANES spectra, created by subtraction from a bare Pt6 cluster reference, are shown in Figure 4 for a Pt6 cluster with single adsorbed H in different sites (atop, bridge, hollow) (Figure 4c), a single carbon adsorbed onto different sites (atop, bridge, hollow) and for an adsorbed phenol molecule (Figure 4d).

The differences between the Pt6 and Pt-Had spectra stem primarily from the splitting of the Pt6 density of states due to H bonding. For the Pt-Had species (Figure 4c), a weak doublet was found at 11572 eV whose shape is similar to the experimental standards for the Pt-Had species in Figure 4b. The location of the H atom in on top, hollow or bridge sites has a relatively minor effect on the spectra. However, calculations indicate bridge and top sites are by far the most preferred adsorption sites on a Pt50

nanoparticle more similar to the experimental ones used in this work (see Figure S12S14, SI). This is in good agreement with the observation that the computed XANES from TD-DFT from H on a bridge site Thus, the most relevant spectra in Figure 4c most closely matches the experimental standards is that of clean the bridge H. The TD-DFT spectra show splitting of about 2 eV between the Pt-HadH (bridging)

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and Pt-C top site (Figure 4c,d) whereas the experiment shows splitting of about 3.5 eV (Figure 4b). While the theory splitting is not as great as for the experiment, this is primarily ascribed to the approximation of the limited sized of the TD-DFT Pt6 cluster. Due to the high density of unoccupied states for large metallic clusters, a Pt6 cluster is still tractable with all electron relativistic TD-DFT calculations while larger clusters are not. Qualitatively, the experiment and theory are in good agreement. For a more detailed plot of the difference between the TD-DFT XANES calculation for bridge adsorbed H on Pt6 and the bare Pt6, see Figure S3 in the Supporting Information.

The differences between the Pt6 and Pt-C species are more complex than between Pt6 and Pt-H due to the manifestation of new Pt-C hybridized unoccupied states in the latter. For the Pt-C species in Figure 4d, there is a primary peak at about 11570 eV whose shape agrees well with the experimental Pt-C species in Figure 4b. The difference in the peak energy between the Pt-Had and Pt-C features is captured, but not quite as prominent as in the experimental system.

Figure 5 presents XANES difference spectra like those in Figure 4a (from Figure 3a) but now also including the spectra from parts b-d of Figure 3, to show the effects of changing particle size and TCH versus ECH conditions. Since the XANES spectrum for a bare metallic nanoparticle can be different from that of the bulk foil, we also include in our analysis the difference between nanoparticle spectra with and without phenol. This alternative representation of the difference spectra, also shown in Figure 5, does not change the interpretation of assignments of the Pt-H and Pt-C XANES peaks. For all cases, the addition of phenol reduced the Pt-Had signal, while the Pt-C grew. The impact of adding phenol was slightly larger for the XANES of Pt/C 2 nm than of Pt/C 1.5 nm. The results indicate that phenol displaces Pt-Had both in presence and absence of an applied potential. The extent of displacement depends on the experimental conditions, i.e., it was especially pronounced under TCH conditions, Figure 5a-b. Phenol addition under TCH conditions depleted the concentration of Pt-Had species, leaving primarily Pt-C species.

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Figure 4. XANES difference spectra (relative to a bulk Pt foil in air) for: a) the Pt/C samples in Figure 3a with and without phenol. b) a Pt on zeolite reference reduced in water at 25 °C and 180 °C, as well as reduced in H2 gas, along with a Pt/C catalyst (1 wt% loading) reduced in H2 gas. c) TD-DFT XANES difference spectra for a Pt6 cluster having a single hydrogen atom adsorbed on the top, hollow or bridge site minus the spectrum for a bare Pt6 cluster d) TD-DFT XANES difference spectra for a carbon atom at top, hollow or bridge sites on the Pt6 cluster, and a phenol molecule adsorbed onto the cluster.

In presence of applied potential (Figure 5c), the signal was mostly dominated by Pt-C although there was still significant reduction in Pt-Had upon phenol addition. Figure 5c also shows the effect of changing the potential from -0.3 V to -0.55. The subtle change (see light gray line in Figure 5c) indicates an increase in the Pt-Had species at more negative potentials. The change is attributed to the higher H+

aq electrochemical potential, which is consistent with the observed higher rates of hydrogenation at more negative potentials on Pt [34,35].[37,38]. The increasing coverage of Had is concluded to increase the hydrogenation rate of adsorbed phenol. The small change in the magnitude of the XANES signal from -0.3 V to -0.55 V does not match the larger increase in reaction rate (an order of magnitude increase in rate over this potential range). Since Had probably becomes much less stable at very high coverages (due to Had-Had repulsion and the entropic cost of competitive site filling), the activation barrier for its addition to phenol may also decrease and this would explain the dramatic rate

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increase. Also, the XANES signal here may not be linear in H coverage. For Pt/C 1.5 nm under ECH conditions (Figure 5d), the effect of adding phenol was less obvious.

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Figure 5. XANES Pt L3-edge difference spectra for the conditions listed in Figure 3a-d. The “- change” spectra after phenol addition were smoothed and show the spectra associated with adsorbed H that is replaced by phenol. Electrochemical measurements are at -0.55 V vs. Ag/AgCl unless otherwise specified.

2.4 Structural characterization by EXAFS. The k1-weighted Im[χ(R)] plots for Pt/C 1.5 nm under thermochemical conditions are shown in Figure 6a. (See Supporting Information Figure S1 for χ(k) plot.) In Figure 6a, the Im[χ(R)] spectra for the catalyst with and without 0.3 M phenol are very similar except for the region around 1.6 Å (distance without phase-shift correction), where a small increase in signal assigned to Pt-C was observed upon phenol addition. Although EXAFS cannot discriminate between C and O neighbors, comparison of XANES to metallic Pt standards indicates that oxidized Pt species are not present under these conditions. Thus, we conclude the signal around 1.6 Å does not originate from Pt-O scattering. Most aromatics compounds such as phenol adsorb with the ring parallel to the platinum surface via binding through carbon atoms, while the oxygen group points slight away from the surface [20,36,37].[22,39,40]. Thus, the increase in scattering signal around 1.6 Å is attributed to Pt-C from adsorbed phenol. The initial signal detected in the absence of phenol is assigned to interactions of the platinum nanoparticle surface atoms or the atomically dispersed Pt atoms on the carbon support.

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Figure 6. a) EXAFS k1-weighted Im[χ(R)] plots of Pt/C 1.5 nm at 80 °C and 30 bar H2 in water without and with 0.3 M phenol. For comparison, the reference spectrum from a Pt(0) foil (scaled 0.3) is also included. b) The k1-weighted Im[χ(R)] plots for the theoretical fits to the experimental spectrum with structural parameters listed in Table 2. For corresponding |χ(R)| and χ(k) plots, see Supporting Information.

The structural parameters from fitting the data are shownsummarized in Table 1., with a complete listing of the fitting parameters reported in Tables S1-S2 of the Supporting Information. The coordination numbers of Pt-Pt match those expected from STEM (~6 corresponding to 1-1.5 nm [38,39][41,42] for Pt/C 1.5 nm and ~9.3-10 corresponding to 2.5-5 nm [38,39] for Pt/C 2 nm).[41,42] for Pt/C 2 nm). Discrepancies for the Pt/C 2 nm particle between STEM and EXAFS may arise due to the fact that the EXAFS data are from a weighted average (per atom), whereas the STEM histogram (Figure 1) is the median particle size (per particle). Adding phenol to the solution increased the Pt-C coordination number from 0.8 to 1.2 and from 1.8 to 2.1 for the Pt/C 2 nm and Pt/C 1.5 nm, respectively (Table 1). Two different Pt-C distances are found at approximately 2.15 and 2.85 Å. The greater (fractional) increase in the Pt-C coordination number for the Pt/C 2 nm particle than the Pt/C 1.5 nm particle is consistent with the larger decrease in the Pt-Had XANES signal upon phenol addition for the larger particles (Figure 3). This suggests that the larger particles have a larger fraction of Pt atoms that are active for phenol adsorption in water. This is reasonable, since phenol occupies an ensemble of ~7 surface Pt atoms at saturation on Pt(111) [40].[43]. The smaller change with addition of phenol (in both XANES and EXAFS) for the Pt/C 1.5 nm compared with the Pt/C 2 nm catalyst may be due to the much higher fraction of Pt in 1-4 atom clusters for Pt/C 1.5 nm, as well as smaller Pt particles on average. Since phenol binds to a large ensemble of Pt surface atoms, some of the Pt surface atoms may not be in large enough ensembles to adsorb phenol strongly enough. Similar results are seen in ECH conditions (Figure 5c versus 5d).

When the pressure was lowered from 30 to 2 bar for Pt/C 1.5 nm in TCH, neither the Pt-C nor Pt-Pt coordination numbers changed (Table 1). This constancy of Pt-Pt or Pt-C coordination numbers matches the relatively unchanged XANES at the lower pressure (Figure 5b). For Pt/C 2 nm, changing from water with phenol to 0.1 M perchloric acid with phenol had no effect on the Pt-Pt coordination numbers and

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distances. The Pt-C coordination number increased slightly (from 1.2 to 1.4) indicating slightly higher concentrations of adsorbed phenol. This higher concentrations of adsorbed phenol at pH 1 than at pH 5 agree well with the slight decrease of the Pt-Had concentration as the pH decreases (Figure 3a and Figure 5a), and align well with the hypothesis that phenol and hydrogen compete for accessible metal sites.

The Pt-C distance measured in the presence of phenol was always ~2.15 Å, both in presence and absence of applied potential (Table 1). The value of 2.15 Å matches the reported calculated distance between adsorbed phenol and a Pt surface in the presence of water [20][22]. In the absence of phenol, a shorter Pt-C was measured (2.05 Å), suggesting that other carbon species were present and were forming a different bond to Pt than phenol (attributed to the C support, see below). In the presence of phenol, the Pt-C distance represents a weighted average of Pt-phenol and Pt-support interaction.

Table 1. Fitting parameters for EXAFS of the Pt/C under different thermal (TCH) catalytic and electrochemical conditions. The electrochemical results, are at room temperature and ambient pressure. Supporting electrolyte is 100 mM sodium acetate/100 mM acetic acid for all solutions. A complete tabulation of the higher scattering shells and various reference compounds is given in Table S1.

Thermal Pt-Pt interactions Pt-C interactions

Catalyst/Temp. Aqueous phase reactants Pt-Pt, ŧ

CN Pt-C, Å #

CN

Pt/C 2 nm

30 bar H2‡ 80°C

None 2.776 Å 10.1 ± 1.5 2.03 Å 0.8 ± 0.4

0.3 M phenol 2.756 10.1 ± 1.5

2.13 1.2 ± 0.5

0.3 M phenol, 0.1 M HClO4

2.756 10.1 2.14 1.4 ± 0.5

Pt/C 1.5 nm

30 bar H2‡ 80°C

None 2.747 6.3 ± 1.3 2.14 1.8 ± 0.4

0.3 M phenol 2.747 6.3 2.14 2.1 ± 0.4

0.3 M phenol, 2 bar H2 2.747 6.3 2.14 2.0 ± 0.3

0.3 M phenol, 2 bar H2 + 6 hrs

2.750 7.6 ± 1.4 2.14 2.0 ± 0.5

Electrochemical Pt-Pt interactions Pt-C interactions

Catalyst/Potential*

Aqueous phase reactant Pt-Pt, ŧ

CN Pt-C, Å #

CN

Pt/C 2 nm 2.763 Å 9.3 ± 1.3 2.08 Å 0.6 ± 0.3

-0.55 V 0.1 M phenol 2.763 9.3 2.08 0.9 ± 0.4

-0.30 V 0.1 M phenol 2.765 9.4 ± 0.5 2.09 0.9 ± 0.5

Pt/C 1.5 nm 2.745 5.4 ± 0.6 2.15 1.9 ± 0.4

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-0.55 V 0.1 M phenol 2.745 5.4 2.16 1.9 ± 0.4

†Reduced at 300 °C in H2, cooled to 45°C in helium gas for measurement. ‡Water is saturated with H2

partial pressure indicated. * V vs Ag/AgCl. § Typical uncertainties, R ± 0.007. σ2 varies slightly around 0.008 Å2. Third cumulant (C3) is equal to 0.0001 Å3 for Pt foil and ranges from approximately 0.0003-0.0004 Å3 for the nanoparticle spectra. #Typical uncertainties, R ± 0.015. σ2 varies slightly around ~0.010 Å2.

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Although the Pt-C distance measured here by EXAFS in the presence of phenol is similar to the theoretically calculated distance of phenol on a Pt(111) slab [20][22], calculated adsorption of phenol on a Pt nanoparticle can be a helpful comparison. For this comparison, molecular dynamics (MD)-EXAFS of a graphene-supported Pt50 cluster with 50 adsorbed hydrogens at 30°C is shown in Figure 7a, compared to the experimental result for Pt/C 1.5 nm. The Pt-Pt signal for 2 to 3.5 Å appears to match distances in the experimental nanoparticles indicating that the current model adequately accounts for the distribution of Pt sites and their dynamic motions, and validates the structural similarity of the Pt/C 1.5 nm and theoretical Pt50 cluster. The difference between experiment and theory in the signal strength at 2.15 Å is caused by the fact that the Pt-C path was not included in this MD-EXAFS simulation. This is seen in Figure 7b.

Figure 7b provides two different types of MD-EXAFS simulations, in this case for the phenol-saturated Pt50 cluster (see inset of Figure 7b or Figure 8d). One spectrum shows only the Pt-Pt scattering contribution, while the other is a full representation of both Pt-Pt and Pt-C contributions (from phenol). The results in Figure 7b thus shows the contributions in the EXAFS spectra arising from Pt-C scattering paths. The surface-saturated phenol leads to increased amplitudes around 1.6 Å (distance not corrected for EXAFS phase shift), in agreement with experimentally observed changes (Figure 6a and results from Table 1). This clearly shows that excess signal at 1.6 Å is due to Pt-C interaction (from phenol). To compare this scattering to other possible species, we have explored Pt-C interactions computationally from Pt-graphene, single Pt atoms on graphene and effects of graphene defects, listed in Table 2. First and second shell Pt-C fitting that matches between the experiment and computational work supports the assignment of Pt-C interactions to phenol. However, with EXAFS we are unable to discriminate phenol binding from binding of other types of aromatic species. Also, as noted in the discussion associated with Figure S9, the peak at 2.15 Å may include some Pt-O contributions from water, which decrease upon phenol adsorption.”

It is notable that the experimental spectrum has a stronger Pt-C signal than the simulated Pt50 cluster that is saturated with phenol. Although Pt50 clusters are only weakly interacting with the graphene support, the experimental samples have a significant fraction of atomically dispersed Pt on the carbon surface (from STEM, Figure 1). These dispersed Pt would have a high degree of carbon coordination, especially with edges and corners of the support. Defect sites in the carbon support, which are not considered in the MD-EXAFS, may also cause increased Pt-C coordination. The radial distribution functions of these species are, however, considered below.

Finally, the standard EXAFS fitting routine is applied to the MD-EXAFS spectra and the resultant fitted structural parameters are shown in Table S1. These parameters are in good agreement with the experimental Pt-Pt and Pt-C distances.

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Figure 7. a) EXAFS k2 weighting of imaginary chi. The MD-EXAFS is for a 50 atom Pt cluster with adsorbed hydrogen (Pt:H = 1, corresponding to Figure 6c) where the spectra was calculated using FEFF9 over 40 MD frames taken from 4-ps trajectories. The experimental EXAFS is at 30 bar H2, 80 °C with phenol in water. b) MD-EXAFS generated for 50 atom Pt cluster with 15 phenols (corresponding to Figure 6d). MD-EXAFS is shown considering only Pt-Pt paths and considering Pt-C (C of phenol molecules).

2.5 AIMD Simulation of Pt50 interactions with hydrogen and phenol. Because of the agreement between the simulated spectra for the Pt50 cluster and the experimental spectra, further computational analysis are used to analyze the interactions in a way that is inaccessible solely by experiments. For instance, AIMD simulations coupled to spectroscopic simulations yield information on cluster shape and dynamics, binding modes/sites, binding energies, and contributions from specific configurations, such as defects on the carbon support or single Pt atoms, as well as information about the electronic structure that is not distinguishable from XANES. Figure 8a, b, c, d show snapshots of Pt 50 with three different Pt:H ratios of 0, 0.5 and 1. Figures 8e, f show the bonding charge density between phenol and the Pt particle for the cases in 8a and 8b.

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Figure 8. Snapshots of Pt50 nanoparticles supported on graphene at different H:Pt ratios with phenol bound, a) H:Pt = 0, b) H:Pt = 0.5, c) H:Pt = 1 and d) saturated phenol with no hydrogen. Bonding charge density e) between the phenol molecule and the H:Pt=0 Pt particle and f) between the phenol molecule and the H:Pt=0.5 Pt particle computed as ∆ ρ=ρ ( Pt−phenol )− ρ ( Pt )−ρ ( phenol ), where Pt is the supported particle with or without H atoms, and the iso surface value is set at 0.005 au; the depletion charge density is green and the accumulation charge density is in red. Blue spheres are Pt atoms.

Moment of inertia analyses (see SI Figure S6S7) show that the nanoparticle without H is contracted, while the Pt atoms relax from the center when H is present. The shape is less affected by the presence of phenol. This is further evidenced in Figure 9a showing the Pt-Pt radial distribution functions. At 1:1 Pt:H loading the Pt-Pt radial distribution changes slightly. Pt-Pt distances (1st peak in Figure 9a) in the absence of H and with an H:Pt ratio of 0.5 are 2.65 Å apart, and when the H:Pt ratio is 1, Pt-Pt distances increase to 2.75 Å. The relaxed, longer Pt-Pt distances from simulation are in close agreement with those observed experimentally (see Table 1) from Pt/C 1.5 nm and Pt/C 2 nm. The negligible changes in Pt particle shape with addition of phenol also match what was seen experimentally. The effects of H and/or phenol adsorption on the dynamics and shape of the nanoparticle are discussed in the SI. The possibility of a scattering signal from water was also explored and was not found to be significant, as discussed in the SI, especially as associated with Figure S9.

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Figure 9. Radial distributions, g(R), for Pt50 nanoparticle from AIMD for a) Pt-Pt, b) Pt-C, and c) Pt-H. See SI for full definition and discussion of normalization of g(R). The coordination numbers of Pt-Pt and Pt-C are also shown. For comparison, the first Pt-Pt distance in Pt metal is indicated by the vertical red line, the experimental value for the bulk is 2.77 Å. The various plots represent different loading of H as indicated and also a condition in which the surface is saturated by phenol. For the Pt-H g(R), phenol refers to a single adsorbed phenol, saturated phenol refers to 15 phenol molecules.

As shown in Figure 8c, Table 2, and the Pt-C radial distribution function in Figure 9b, phenol binds to the Pt50 nanoparticle, with the 6 carbon atoms coordinating with 4 Pt atoms at an average distance of 2.15 Å (identical to the experimental Pt-C distance in Table 1). However, the distance depends on the Had coverage. When the H:Pt ratio is increased to 0.5, phenol binds less strongly, resulting in an increase in the average Pt-Cphenol distance to 2.25 Å. A second Pt-C distance was also observed at 2.8 Å in Figure 9b, that matches the distance seen experimentally. This contribution stems from the second closest carbon in the aromatic ring of the adsorbed phenol.

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The state of the Pt carbon support is also important as shown in Table 2. Upon introduction of a defect (removing a C atom), a significant reduction in the Pt-C distance from 2.31 to 2.01 Å was observed. (Also see the Pt-graphene g(R) in the Supporting Information). The interaction of a single Pt atom with the graphene support, representing the atomically dispersed Pt observed in STEM images, also leads to this shorter Pt-C distance of 2.10 Å, in agreement with the experimental measurements. Thus, we attribute the high coordination number of Pt-C even in the absence of phenol to the interaction with the C support from both single Pt atoms with the support and Pt nanoparticles with defect sites in the C support.

Table 2. Average Pt-C bond distances and the corresponding coordination numbers (CN) from DFT cluster optimizations for Pt50 cluster and for single Pt atoms interacting with various supports (graphene (Gr) or defect graphene) or with phenol.

System Pt50/Gr

Pt50/Gr with defect §

Pt1/Gr

Phenol1 on Pt50/Gr

rPt-C (Å)‡ 2.31

2.01 2.10 2.15

Pt-C CN†

6 3 2 6

‡ Average distance for first peak in the g(R)Pt-C. †Number of Pt-C bonds at the reported distance. §A single C atom is removed from the graphene sheet in the region below the nanoparticle.

In Table 3 we list the binding energies of H and phenol on the Pt particle at different concentrations of Had. Hydrogen weakens the strength of the adsorption of phenol by about 190 kJ/mol when 25 H atoms were added to the Pt50 particle. With or without the presence of phenol, the adsorption energy per H atom changes by about 10 kJ/mol, when 25 more H atoms were added to the particle, and the adsorption was still exothermic. This suggests that the surface of the particle is not saturated by H, i.e., available adsorption sites for H atoms still exist.

Table 3. Phenol and 2 Had binding energies calculated with respect to phenol or H2 gas on a Pt50 cluster. Zero-point energy corrections are not included.

H:Pt ratio

Phenol Binding Energy (kJ/mol)

Hydrogen Binding Energy: H2 2 Had

(kJ/mol)

With Phenol

Without Phenol

0 -290 N/A N/A

0.5 -96 -68 -58

1 -110 -58 -48

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Figure 8e illustrates how binding of one phenol causes charge transfer extending through most of the other 50 Pt atoms, which will affect binding of subsequent phenols at distant Pt sites. The changes in the charge density are also instructive in understanding the XANES response: one sees that the directly-bonding Pt atoms (3 or 4) most strongly influence the electronic state.

Figures 8 e,f show that the bonding charge density between phenol and 50 Pt atoms is significantly reduced by hydrogen co-adsorption. Hydrogen blocks Pt sites of phenol, leaving phenol with only two carbon atoms directly binding to Pt atoms. Consistently, the binding energy of phenol on the nanoparticle is reduced by ~190 kJ/mol by the presence of H atoms, see Table 3.

Furthermore, while the Pt-C interaction from phenol (without Had) is strong, producing a narrow, ordered peaks in the g(R)Pt-C, the interaction with graphene support is relatively weak. The g(R)Pt-C

between the Pt nanoparticle and the graphene support is almost without structure (see Supporting Information).

Finally, to understand effects of an applied bias on the binding strength of hydrogen and phenol on Pt, we calculated the adsorption energy of these species under charged conditions on a Pt(111) slab, in the absence of solvent. In Table 4 we list the excess surface charge density applied in our calculations, the corresponding voltage (V vs. SHE, see SI, Figure S10S12 and associated discussion), and the adsorption energy of a dissociative hydrogen molecule and a phenol molecule on the Pt surface, at a coverage of 1 adsorbate per 30 Pt surface atoms. Note that the value for phenol adsorption depends on the long distance cutoff radius used in the DFT-D3 VdW correction. In the current work Rcut=10 Å as opposed to the 6 Å employed by Yoon et al. [20][22] which results in an asymptotically converged binding energy that is higher by ~72 kJ/mol. The values at 0 V are within ~30% of experimental energies [40,41].[43,44]. Moreover, the trend in binding energy with respect to charge does not depend on this parameter, indicating the robustness of the current finding independent of the computational approach. The energies are different in Table 3 due to the use of a cluster there and different coverages. The adsorption energy of Had is unchanged by applied voltage, but phenol’s adsorption is weakened by ~50 kJ/mol per V applied, as expected from its downward dipole.

Table 4. Applied electron density, corresponding voltage, and the binding energy of a dissociatively adsorbed H2 molecule and a phenol molecule on a Pt(111) slab, relative to gas.

Surface charge

(# e-s/nm2)

Voltage (V)

Binding Energy (kJ/mol)

H2 2 Had

Phenol

0 0 -110 -240

0.25 -0.5 -110 -220

0.50 -0.8 -110 -200

2.6 Implications for Catalytic Activity. The degree of adsorption of phenol and H from X-ray absorption spectroscopy (XAFS) measurements are in agreement with reported kinetic models for both TCH and ECH processes. The prevalent hypothesis is that the hydrogenation from both TCH and ECH

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proceeds through surface hydrogenation of adsorbed H and phenol, where the coverage of each species controls the rate [34,35].[37,38]. Both of these species are observed in all cases by XAFS (with phenol present), with the reaction conditions controlling the coverages of both (e.g., more cathodic applied potential increasing the coverage of Had). The competition of phenol and hydrogen is evident from XANES in this work, where the introduction of phenol into the solution decreased the amount of adsorbed hydrogen (Figures 3 and 5), indicative of a competitive adsorption mechanism (and less than complete coverage of hydrogen on the surface in the presence of phenol, even at 30 bar H2). The reaction is reported to be zero-order in phenol at the conditions studied [12,34].[14,37]. The XAFS shows that hydrogen is still present on the catalyst surface, although at lower amounts than without phenol present. This is fully consistent with a Langmuir-Hinshelwood type reaction mechanism with competitive adsorption of Had and phenol (or partially hydrogenated phenol), where the phenol coverage is high enough that we reach a maximum in rate vs. phenol concentration.

In addition to the competitive adsorption observed from XAFS, our calculations indicate that phenol’s adsorption is weakened by the presence of H, similar to what is experimentally observed for adsorption of, e.g., benzene, pentane, olefins on Pt [42–44].[45–47].

3. CONCLUSIONS

We provide here direct spectroscopic evidence of the adsorbed species involved in the rate-determining step of aqueous-phase catalytic phenol hydrogenation under thermochemical and electrochemical conditions. In the presence and absence of applied potential, addition of phenol partially displaces adsorbed H on the Pt surface, implying competitive adsorption of the two species. The results did not show evidence of changes in the electronic state of Pt under an applied potential. The adsorption of phenol was also unaffected by the applied potential based on the identical Pt-C distances and coordination numbers measured with and without applied potential. Theory suggests that phenol binds to the nanoparticle mainly through 4 Pt atoms with the aromatic ring parallel to the surface and Pt-C distances averaging 2.15 Å, matching the experimentally measured Pt-C distance both with and without applied potential. The H binds mostly at the bridging sites that also binding phenol or at corner and edge sites that are regions that do not optimally bind phenol. The binding energy of phenol weakens significantly by increasing Had coverage.

Calculations indicate that phenol binding is weakened upon applying more negative potential, but still binds in a similar manner as without potential. This change is mostly related to the correspondingly higher coverage of adsorbed H.

The high coordination number of Pt to C from EXAFS even in the absence of phenol is associated with interactions with the carbon support. For smaller Pt particles, this contribution from bonds to the support increases. These Pt-C bonds are present even under hydrogenation conditions in water at high temperature. The high coordination is attributed to the presence of defects in the carbon support and the presence of single Pt atoms, both of which have high coordination numbers and smaller Pt-C distances (2.0 to 2.1 Å) than Pt on graphene sheets, as matched by the experiments. These Pt-C coordinations are consistently detected using HAADF STEM, XANES, EXAFS, and AIMD.

4. EXPERIMENTAL AND THEORETICAL METHODS

4.1 Catalyst Preparation and Characterization. Two different Pt/C catalysts were used in these experiments. The first, designated as Pt/C 2 nm, was a 5 wt% Pt on activated carbon purchased from Sigma Aldrich, having an average particle size of ~2 nm. The second catalyst, designated Pt/C 1.5 nm,

20

was synthesized according to the electrostatic adsorption procedure by Regalbuto et al. that was shown to result in small Pt particle sizes (~1.5 nm) [21][23]. The carbon support may be partially covered in oxygen, as graphenic compounds in air or water can form graphene oxide.

The scanning transmission electron microscope (STEM) experiments were performed on a JEOL Cs corrected ARM 200kV equipped with a cold field-emission microscope – the instrument has a nominal 0.1 nm probe size under standard operating conditions. All images were acquired in the high angle annular dark field (HAADF) or Z-contrast imaging mode, making them extremely sensitive to the presence of Pt atoms. A summary of the catalyst characterization is shown in Table 5. Further experimental details are included in the Supporting Information. H2 chemisorption on Pt/C 2 nm indicated a particle size of 2.5 nm.

Table 5. Catalyst characterization

Catalyst

Particle size by STEM

Particle size by EXAFS

Pt loading by ICP-AES

Pt/C 1.5 nm

~1-1.5 nm

1-1.5 nm

1.45 ± 0.10 wt%

Pt/C 2 nm

~1.5-2 nm

2.5-5 nm

4.23 ± 0.02 wt%

4.2 XAFS Measurements under Hydrothermal and Gas-phase Conditions. The hydrothermal batch reactor for operando XAFS measurements has been previously described [45].[48]. Briefly, two 0.5 mm thick × 3 mm diameter glassy carbon windows were used as the X-ray window in the reactor cell. A 1.5 mm thick × 2 mm diameter catalyst pellet of the Pt/C or Pt/zeolite were formed by die-pressing and then loaded into a batch reactor with water.

4.3 Preparation of the Pt/C Electrodes and Electrolyte. The Pt/C catalyst was loaded onto the carbon felt (Alfa Aesar, 6.35 mm thick, 99.0%) electrode without the use of a binder (e.g. Nafion or Teflon) that would impede the transport of organic reactants to the catalyst surface. In the first step, the Pt/C catalysts were mixed in water to form a dilute suspension. The carbon felt (6.35 mm thick) serving as the electrode was pre-soaked in electrolyte for 1 hr prior to catalyst loading. This felt was split in half (parallel to its surface) in order to coat only the inner surface with Pt/C particles. The Pt/C suspension was loaded into a syringe and then the Pt/C was deposited onto the felt by passing the dispersion through the felt that was held in a standard syringe holder. In this process the particles were physisorbed or electrostatically deposited onto the felt surface. For Pt/C 2 nm, 15 mg of catalyst dispersed in 20 mL of water was syringed through the felt six times in order to capture greater than 95% of the suspended catalyst particles. For the Pt/C 1.5 nm material, 20 mg of catalyst was dispersed into 20 mL of water. The two halves of the felt were joined so that the Pt/C particles were sandwiched between the two pieces of felt. The carbon felt was cut to a diameter suitable for the electrochemical cell.

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For the electrolyte, we used an acetate buffer (sodium acetate and acetic acid) to compare to previous electrocatalytic hydrogenation experiments [12,34,35][14,37,38], Acetate in aqueous solvent does not adsorb strongly onto Pt/C at cathodic potentials [46][49], making aqueous acetate buffer an ideal electrolytic solution for spectroscopic investigation. The electrolyte was prepare using 100 mM sodium acetate and acetic acid, with and without 0.1 M phenol (Honeywell 99.999% sodium acetate, Sigma Aldrich 99.99% acetic acid, Sigma Aldrich >99% phenol) Prior to measurement the electrolyte was first sparged with argon (99.999% purity) for 20 minutes, then degassed under vacuum to reduce the residual dissolved argon. This degassing was done to increase the electrolyte capacity for evolved hydrogen and oxygen in order to avoid bubble formation (which interferes with XAFS transmission spectra).

4.4 Electrochemical XAFS Cell. The electrochemical XAFS cell (shown in Scheme 1) was formed from a modified PMMA material using 3D printing. The cell is capable of operation to a maximum temperature of 95 °C and pressure of 10 bar. (Measurements in this report were done at ambient temperature and pressure.) The cell design allows for operation in either fluorescence or transmission modes. The XANES and EXAFS measurements were taken at the Sector 20 bending-magnet beamline of the Advanced Photon Source at Argonne National Laboratory. The experiments were performed in either transmission mode for the thermal reactions or fluorescence mode for the electrochemical reactions

Scheme 1. a) The electrochemical XAFS cell used in this work. Inset, upper right shows the carbon felt with blue arrows indicating direction of the electrolyte flow. Inset at the lower right is a microscope image of the Pt/C catalyst that has been incorporated into the carbon felt. b) Side view of the electrochemical cell, showing X-ray transmission or fluorescence beampaths (purple arrows) and the separation of the working and counter electrodes by use of a Nafion film.

4.5 DFT Calculations and Molecular Dynamics Simulations. All density functional theory (DFT) calculations and AIMD simulations were performed with the PBE density functional [47][50] as implemented in CP2K [48,49].[51,52]. Valence electrons were expanded in the MOLOPT short range double zeta basis sets [50][53], core electrons were represented with GTH pseudopotentials [51][54], and an additional plane wave basis of 420 Ry cutoff for electrostatic terms. Van der Waals interactions were considered with Grimme's DFT-D3 [52][55] corrections.

To test the assignments of the EXAFS and XANES signals described, AIMD simulations of Pt clusters on graphene in the presence of H and phenol (but not in presence of water) were conducted. A graphene-supported Pt50 cluster was selected as representation of the nanoparticles in the experimental Pt/C 1.5 nm system. A simulated particle of this size allows a direct comparison of various factors affecting the surface composition of the Pt particle. Pt50 nanoparticles with hydrogen to platinum ratios (H:Pt) of 0, 0.5, and 1 were placed over a graphene sheet (39.36 x 34.08 Å2, and 30 Å of vacuum space for periodic conditions) with and without a phenol molecule over the nanoparticles, giving six independent cases. Figure 8a, b, c, d shows snapshots of Pt50 after about 5 ps of equilibrated molecular

22

dynamics simulations in the presence of three different Pt:H ratios of 0, 0.5 and 1. Figure 8 e, f show the bonding charge density between phenol and the Pt particle for the cases in 8a and 8b.

Additionally, Pt50, with an H:Pt ratio of 0, was completely covered with phenol molecules to give the saturated phenol nanoparticle. The following protocol was followed to obtain optimized structures: after an initial optimization, 0.5 ps of AIMD at 1000 K were performed, followed by 0.5 ps at 300 K followed by 2 ps of temperature annealing to ~ 0 K. A final geometry optimization was performed on the annealed structure. Starting from the final optimized structures, 5 ps of simulation in the canonical ensemble (NVT) at 300 K were performed to obtain statistics of the nanoparticles. Finally, the NVT (T=300 K) simulations were followed by 2 ps of temperature annealing to ~0 K to obtain optimized structures to compute binding energies.

4.6 Computing EXAFS Spectra from Simulated Pt50 Structures. Theoretical EXAFS were calculated from the MD trajectories generated from the clusters for both the Pt50 H50 and Pt50 –phenol saturated structures using the method of MD-EXAFS [53–55][56–58]. Details are provided in the SI.

4.7 Computing XANES Spectra from Pt6 Clusters. The XANES calculations for the Pt6 clusters (octahedral) were performed at the L3-edge using the restricted excitation window time-dependent density functional theory (or REW-TDDFT) approach [56][59] as implemented in the NWChem quantum chemistry program [57][60]. Details are given in the SI. Due to the high density of unoccupied states, a Pt6 cluster is still tractable with all electron relativistic TD-DFT calculations. Larger clusters, for example, the Pt50 clusters that are optimized using AIMD, are computationally too expensive for the TD-DFT approach due to the large number of excited states that would need to be considered.

AUTHOR INFORMATION

Corresponding Author

*johannes.lercher@pnnl.gov

Present Addresses§N. Singh’s present address is Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States. D. Cantu’s present address is Chemical and Materials Engineering, University of Nevada, Reno, Nevada 89557, United States.

Author Contributions

All authors have given approval to the final version of the manuscript.

Supporting Information

Word document with k2 χ(k) plot, full EXAFS fitting parameters, details of chemical standards, calculated Pt-graphene bond distances, adsorption site information, moment of inertia, center of mass, and adsorption energies.

ACKNOWLEDGMENT

N.S. is funded by the WRF Innovation Fellowship in Clean Energy Institute. The research described in this paper is part of the Chemical Transformation Initiative at Pacific Northwest National Laboratory (PNNL), conducted under the Laboratory Directed Research and Development Program at PNNL, a

23

multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. Computational resources used by M.-T.N., D.C., V.-A.G., and R.R., were provided by DOE’s National Energy Research Scientific Computing Center located at Lawrence Berkeley National Laboratory and PNNL institutional computing. N.G. acknowledges computational resources for the XANES calculations provided by the Environmental Molecular Sciences Laboratory (EMSL), which is a DOE Office of Science User Facility located at PNNL. This research used resources of the Advanced Photon Source Sector 20, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357, and the Canadian Light Source and its funding partners. The authors would like to thank Kamlesh Suthar and Scott Russell for help in designing the cell.

ABBREVIATIONS

Thermal catalytic hydrogenation (TCH); Electrocatalytic hydrogenation (ECH); Extended X-ray Absorption Fine Structure (EXAFS); X-ray Absorption Near Edge Spectroscopy (XANES); High Angle Annular Dark Field (HAADF); Scanning Transmission Electron Microscope (STEM); Ab initio molecular dynamics (AIMD); Time-dependent density functional theory (TD-DFT); X-ray Absorption Spectroscopy (XAFS);

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