Ab Initio Surface Phase Diagrams for Coadsorption of Aromatics and

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Ab Initio Surface Phase Diagrams for Coadsorption of Aromatics and Hydrogen on the Pt(111) Surface Glen Allen Ferguson,†,§ Vassili Vorotnikov,†,§ Nicholas Wunder,‡ Jared Clark,† Kenny Gruchalla,‡ Timothy Bartholomew,† David J. Robichaud,† and Gregg T. Beckham*,† †

National Bioenergy Center, and ‡Computational Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Supported metal catalysts are commonly used for the hydrogenation and deoxygenation of biomass-derived aromatic compounds in catalytic fast pyrolysis. To date, the substrate−adsorbate interactions under reaction conditions crucial to these processes remain poorly understood, yet understanding this is critical to constructing detailed mechanistic models of the reactions important to catalytic fast pyrolysis. Density functional theory (DFT) has been used in identifying mechanistic details, but many of these works assume surface models that are not representative of realistic conditions, for example, under which the surface is covered with some concentration of hydrogen and aromatic compounds. In this study, we investigate hydrogen-guaiacol coadsorption on Pt(111) using van der Waals-corrected DFT and ab initio thermodynamics over a range of temperatures and pressures relevant to bio-oil upgrading. We find that relative coverage of hydrogen and guaiacol is strongly dependent on the temperature and pressure of the system. Under conditions relevant to ex situ catalytic fast pyrolysis (CFP; 620−730 K, 1−10 bar), guaiacol and hydrogen chemisorb to the surface with a submonolayer hydrogen (∼0.44 ML H), while under conditions relevant to hydrotreating (470−580 K, 10−200 bar), the surface exhibits a full-monolayer hydrogen coverage with guaiacol physisorbed to the surface. These results correlate with experimentally observed selectivities, which show ring saturation to methoxycyclohexanol at hydrotreating conditions and deoxygenation to phenol at CFP-relevant conditions. Additionally, the vibrational energy of the adsorbates on the surface significantly contributes to surface energy at higher coverage. Ignoring this contribution results in not only quantitatively, but also qualitatively incorrect interpretation of coadsorption, shifting the phase boundaries by more than 200 K and ∼10−20 bar and predicting no guaiacol adsorption under CFP and hydrotreating conditions. The implications of this work are discussed in the context of modeling hydrogenation and deoxygenation reactions on Pt(111), and we find that only the models representative of equilibrium surface coverage can capture the hydrogenation kinetics correctly. Last, as a major outcome of this work, we introduce a freely available web-based tool, dubbed the Surface Phase Explorer (SPE), which allows researchers to conveniently determine surface composition for any one- or two-component system at thermodynamic equilibrium over a wide range of temperatures and pressures on any crystalline surface using standard DFT output.

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metal choice affecting both activity and product selectivity.1−8 The process conditions also can dramatically influence the product distribution. Hydrotreating conditions, defined by lower temperatures (470−580 K) and high H2 pressure (10− 200 bar), favor the saturation of the aromatic rings in compounds derived primarily from lignin during pyrolysis.2−4,9−14 CFP conditions, defined by elevated temperatures (620−730 K) and lower H2 pressure (1−10 bar), favor either saturated or unsaturated products depending on reactor residence time.1,5,11,15−24

INTRODUCTION Biomass pyrolysis is a promising conversion approach to produce renewable transportation fuels. A newly emerging pyrolysis approach, dubbed ex situ catalytic fast pyrolysis (CFP), involves rapidly heating dried biomass to produce pyrolysis vapors and then catalytically upgrading these vapors in a separate unit to produce stabilized bio-oil.1 The pyrolysis vapors from the heating unit have high oxygen content (10−40 wt % depending on the feedstock) and require deoxygenation to increase their fuel value. Among biomass constituents, lignin pyrolysis vapors are composed of particularly difficult-todeoxygenate aromatics, including phenols, guaiacols, and syringols.1 Supported noble metals and cheaper, non-noble metals are capable of acting as deoxygenation catalysts with © XXXX American Chemical Society

Received: July 15, 2016 Revised: October 29, 2016 Published: November 2, 2016 A

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5.4.1.38−41 The ion−electron interactions were described using the PAW potentials.42,43 The electron−electron exchange and correlation energies were computed using the Perdew, Burke, and Ernzerhof (PBE) functional.44 The van der Waals (vdW) forces were calculated using the method of Tkatchenko and Scheffler (TS) as implemented in VASP.45 For bulk Pt fcc lattice calculations, we used a tetrahedron method with Blochl corrections, a 15 × 15 × 15 Monkhorst− Pack k-point mesh, and a cutoff energy of 500 eV.46,47 The lattice constant was obtained using the Murnaghan−Birch equation of state and confirming the minimum to within 0.0001 Å via single-point energy calculations.48,49 The Pt lattice constant was found to be 3.9338 Å, in excellent agreement with experimental value of 3.92 Å. A 5-layer 4 × 4 Pt(111) slab was built using the computed lattice constant with a 30 Å vacuum layer between so that the estimated energies are unaffected by the periodicity of the supercell. The bottom three layers were frozen to bulk Pt geometry, while the top two layers were allowed to relax. For all structural relaxations, we used the Methfessel−Paxton method with a smearing parameter of 0.2 eV, a 5 × 5 × 1 k-mesh, and a cutoff energy of 400 eV.50 The relaxation was performed until all forces were lower than 0.02 eV/Å using a conjugate-gradient algorithm. Dipole corrections were included after initial geometry optimization, but did not affect the total energies by more than 0.01 eV (and surface energies by more than 0.1 meV/Å2).51,52 Clean surface energy was computed separately, using a 20-layer 1 × 1 Pt(111) slab with a 30 Å vacuum layer and a 25 × 25 × 1 k-mesh, relaxing the outer 5 layers on each side as suggested by previous DFT convergence studies.53 Our clean surface energy was computed to be 138 meV/Å2, which is in good agreement with the experimental value54 of 155 meV/Å2. In contrast, the PBEcomputed Pt(111) clean surface energy was estimated at 104 meV/Å2.53 Gas-phase optimizations were performed in a 20 × 21 × 22 Å box using the Gaussian smearing method until all forces were lower than 0.01 eV/Å. Adsorbate vibrational frequencies were calculated using the finite difference method with a displacement of 0.015 Å. In these calculations, the surface atoms were fixed, and only adsorbate vibrations were assumed to have noncanceling contributions to surface energy. For computational efficiency, the k-point sampling was reduced to the Γ-point. The resulting values were used to estimate the zero-point energies (ZPE) and the temperature-dependent corrections using standard approximations. Even with strict optimization criteria, imaginary frequencies associated with translation of physisorbed guaiacol were unavoidable. In these cases, the imaginary frequencies were dropped from the calculations. For the models chosen in this study, the smaller k-mesh and ignoring imaginary frequencies were found to have a negligible effect on the ZPE and the temperature-dependent contributions to surface energy as shown in the Supporting Information, sections 1 and 8. Surface Energy Estimation. In this work, we use the atomistic thermodynamic framework to address the coadsorption of guaiacol and hydrogen on Pt(111).30,55 The thermodynamically preferred surface configurations are determined by minimizing the surface energy, γn(T, {Pi}). Each surface configuration, n, has the following dependence on temperature, T, and pressures of hydrogen and guaiacol, PH2 and PGua (represented by Pi):

One hypothesis to explain the differences in the product distribution as a function of reaction conditions is that the relative coverage of hydrogen and the aromatic oxygenate on the catalyst surface is sensitive to reaction conditions, which in turn causes differences in activity or selectivity. DFT modeling efforts hint at conditions playing a crucial role in resolving phenolic deoxygenation over Pt(111) and possibly other aromatics over metallic surfaces.11,25 Most of the recent work has focused on reactions occurring over bare metal surfaces, resulting in endothermic elementary hydrogenation reactions with relatively high barriers ranging from 0.94 to 1.35 eV.25−28 The corresponding effective barriers for benzyl hydrogenation reach as high as 2.42 eV, clearly too large for hydrogenation observed under hydrotreating conditions. In contrast, a DFT model surface assuming monolayer hydrogen coverage, H/ Pt(111), results in overall exothermic and facile hydrogenation of the aromatic ring with the effective barrier of just 0.69 eV.11 Even intermediate hydrogen coverage was shown to significantly affect aromatic hydrogenation kinetics.29 It is therefore imperative to understand the effect of the gas-phase reservoir on the surface state to develop DFT representations that reflect reaction conditions. An established approach in determining surface coverage is to compute and plot ab initio phase diagrams (AIPDs). Within this method, the coverage and configuration of the adsorbates are computed by minimizing surface free energy of the system.30−32 Numerous studies have successfully employed these techniques to estimate the surface composition involving small adsorbates, including CO, hydroxyl, water, and atomic species.30,32−36 Others have gained insight into the coadsorption of small alkenes and hydrogen on Pd surfaces.37 However, such analyses have not been performed systematically to our knowledge for the coadsorption of oxygenated aromatics and hydrogen. Assuming negligible vibrational energy and entropy contributions to surface fragments typical of such small species, past works were able to produce AIPDs as a function of chemical potential of the relevant gas-phase species. In such diagrams, the surface composition dependence on three variables (T, PA, and PB) is effectively reduced to two variables (μA and μB), and all relevant conditions can be visualized in two dimensions. However, without these assumptions, all three variables are required, resulting in more intricate T−PA−PB diagrams such as those presented in a study of hydrogen-alkene coadsorption on Pd surfaces.37 In this study, we investigate the coadsorption of guaiacol and hydrogen on Pt(111) relevant to hydrotreating and CFP conditions, evaluate the nature of guaiacol adsorption, and assess the vibrational energy and entropy contributions to surface energy. With these contributions being appreciable, we construct a three-dimensional phase diagram for the thermodynamic state as a function of partial pressure of guaiacol, partial pressure of hydrogen, and temperature. Last, we develop an easy-to-use compilation-free online application to interactively visualize coadsorption. This tool, called Surface Phase Explorer (SPE), allows one to determine surface composition for any one- or two-component system at the thermodynamic equilibrium over a wide range of temperatures and pressures, and requires only standard DFT output associated with each considered configuration.

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COMPUTATIONAL DETAILS DFT Calculations. All DFT calculations were carried out using the Vienna Ab Initio Simulation Package (VASP), version B

DOI: 10.1021/acs.jpcc.6b07057 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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1 ⎛ surf ⎜G N ,{N } − NMg bulk − M A ⎜⎝ M i

⎞

function of temperature for H atom in the atop site, chemisorbed guaiacol, physisorbed guaiacol, and a representative high-coverage coadsorbed state. For a single H adsorbed on 2 4 × 4 Pt(111), γvib H is quite small, lower than 1 meV/Å over vib this temperature range. The guaiacol contribution, γG , whether chemisorbed or physisorbed, is greater than 10 meV/Å2 per molecule at higher temperatures, and thus should not be ignored in estimating surface energies. We have also considered computing vibrational energy of guaiacol either directly from vibrational frequencies or using the Campbell−Sellers (CS) correlation,56 which relates the adsorbate entropy to the gasphase entropy. Because DFT-estimated gas-phase entropies compare well with experimentally measured values, we can use the CS correlation directly with these values to compute γvib ads of physisorbed molecules, including guaiacol (see SI, section 5). A difference of up to 2 meV/Å2 can be seen at lower temperatures, but is not expected to change the trends in the phase diagrams (see SI, section 6). Despite single adsorbate contributions being relatively small, the overall contribution is additive and becomes significant at higher coverage. Vibrational surface energy can contribute up to 30 meV/Å2 at higher coverage as shown in Figure 1. Having substantial γvib ads contribution for coadsorbed aromatic species and hydrogen results in the need for expensive vibrational analysis using DFT, even using state-of-the-art computational resources. In previous works, it has been shown that the vibrational frequencies of small adsorbates do not change significantly given site similarity. Although collective modes and coverage may contribute to slight frequency shifts,57−59 we have estimated γvib ads by scaling single adsorbate’s vib vib (i.e., γ and γ ) with the number of each species, Ni, thus γvib i G H alleviating some of the computational burden. The resulting model can be found in SI, section 5. The advantage of this model is its ability to estimate γvib ads within acceptable error for any coadsorbed configuration by only computing γvib i for each adsorbate i. For the remainder of this work, however, we compute γvib ads directly from DFT. Hydrogen Adsorption. In building the AIPD of hydrogen and guaiacol on Pt(111), we first calculated hydrogen coverage effects in the absence of coadsorbed guaiacol. The number of surface hydrogens was increased from 1 to 28 on a 4 × 4 Pt(111) surface, systematically populating the atop sites first, followed by the 3-fold hollow sites. These sites were chosen for consistency with previous experimental and DFT results.59 The lowest energy state was determined by taking the minimum of two optimized structures at each coverage: (i) one with hydrogen occupying as many adjacent sites as possible to maximize hydrogen−hydrogen interactions and (ii) one with hydrogen occupying sites as distant as possible to minimize these interactions. The differences between the two types of configurations were found to be small; nonetheless, lowinteraction configurations were found to be lowest in energy. These results indicate the lack of strong hydrogen−hydrogen interactions even at high coverage, and hence no patterning is expected on Pt(111). It should be noted that this result is specific to Pt(111), and should not be assumed for other adsorbates or surfaces. Figure 2 shows the energies for hydrogen adsorbed on Pt(111). As can be seen in Figure 2A, there is an inflection after the first 16 atop sites are filled (recall there are 16 Pt atoms in the surface layer of the unit cell). The total energy rises after 20 H atoms adsorb to the surface. The first few hydrogen atoms adsorbed in the secondary hollow sites (beyond θH = 1.00 ML)

∑ Niμi (T , Pi)⎟⎟ i

⎠ (1)

Gsurf NM,{Ni}

Here, is the Gibbs free energy of the surface with {Ni} adsorbates of type i (here, atomic hydrogen and molecular guaiacol), NM is the number of bulk units making up the slab, gbulk M is the Gibbs free energy of a single unit of the bulk metal, μi (T, Pi) is the chemical potential of each adsorbate (see SI, section 2), and A is the surface area. This equation can be rearranged as the sum of surface energy contributions: tot ZPE vib γn(T , {Pi}) = γclean + γads + γads + γads (T ) + γref (T , {Pi})

(2)

Here, γclean is the clean surface energy and can be obtained either experimentally or using DFT as shown above. The term γref(T, {Pi}) is associated with the chemical potential of the reference gas-phase species and has been discussed by Reuter and co-workers and can be found in SI, section 1.30,33 The term γtot ads represents the total energy difference due to adsorption. vib The terms γZPE ads and γads(T) are associated with adsorbate vibrations. Within this formulation, we assume that the surface phonon density of the solid remains unchanged by the adsorbates, allowing us to focus solely on the adsorbate− vib surface interactions.30,37 Both γZPE ads and γads(T) require the calculation of vibrational frequencies and are typically ignored within the atomistic thermodynamic framework. However, Canduela-Rodriguez et al. have recently shown that on Pd(111), the benzene−hydrogen coadsorption trends can shift when considering surface adsorbate vibrations.55 In this work, we analyze the effect of adsorbate vibrations and include them in the resulting phase diagrams. The derivation of eq 2 can be found in SI, section 3.

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RESULTS Vibrational Surface Energy of Adsorbates. The vibrational surface energies, γvib i , for each adsorbate configuration were first calculated. In Figure 1, these energies are shown as a

Figure 1. Vibrational contributions to surface energy as a function of temperature for species adsorbed on a 4 × 4 Pt(111): H atom in the atop site (blue), chemisorbed guaiacol (green), physisorbed guaiacol (red), physisorbed guaiacol estimated using the Campbell−Sellers correlation56 (black dashed), and coadsorbed guaiacol and 16 hydrogen atoms (magenta). C

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Figure 2. (A) Total energy of dissociated hydrogen relative to the gas-phase hydrogen and (B) average adsorption energy per hydrogen atom as a function of hydrogen coverage on Pt(111) at 0 K. Total energies (●) and the ZPE-corrected values (green ■) are shown.

conditions, while submonolayer coverage as low as 0.375 ML (6 H in the plot) may be possible under CFP conditions. Coadsorption of Hydrogen and Guaiacol. As with hydrogen, we investigated guaiacol adsorption on the 4 × 4 Pt(111) model surface. Preliminary screening of coadsorption was used to identify the likely binding modes of guaiacol. Figure 4 shows four main adsorption modes found in this

still have slightly negative adsorption energy until the coverage of 20 H atoms (θH = 1.25 ML). Above 1.25 ML, the repulsive interactions become strong enough to cause the relative energy to rise. These changes are also reflected in the average adsorption energy per hydrogen in Figure 2B, where the energy on a per-hydrogen basis rises slowly until 16 hydrogen atoms are adsorbed, then rises more steeply with additional hydrogens. These results are consistent with the generalized gradient approximation (GGA) functional calculations of Hamada et al.59Figure 2 indicates that the maximum hydrogen coverage limit is 20 atoms (θH = 1.25 ML) at 0 K; however, when temperature and pressure effects are included, this high level of coverage is unlikely at any relevant conditions. The above results were then used to construct AIPD for hydrogen, shown in Figure 3. The largest stable regions under hydrotreating or CFP conditions correspond to a coverage of 1 ML (16 H in the plot). Our results indicate that hydrogen coverage of at least 1 ML is expected under hydrotreating

Figure 4. Guaiacol adsorption modes considered with varying hydrogen coverage. The numbers indicate the number of unsaturated carbons covalently interacting with the surface. The sphere colors represent atom types: Pt (blue), C (gray), H (white), and O (red).

screening. At low coverage, guaiacol was found to adsorb with all six unsaturated carbons covalently interacting with the surface, in line with previous experimental and computational studies.60−65 However, physisorbed and tilted conformations with two and four unsaturated carbons interacting with the surface were also found to be stable at higher hydrogen coverage. Therefore, in the search for the minimum-energy conformation, we considered each of these four binding modes and varied the number of hydrogens from 0 to 28. The results for guaiacol−hydrogen coadsorption at 0 K are presented in Figure 5. The guaiacol adsorption mode changes from chemisorbed at low hydrogen coverage to physisorbed at high hydrogen coverage. At low hydrogen coverage up to 8 H atoms (θH < 0.50 ML), chemisorbed guaiacol with six carbon− metal bonds was found to be energetically favorable. However, chemisorbed guaiacol is energetically favored only as long as the C−Pt covalent interactions are greater than the repulsion introduced by additional hydrogen. This competition between guaiacol chemisorption (i.e., C−Pt bonding) and hydrogen repulsive interactions becomes evident at higher coverage. After 10 H atoms (θH = 0.63 ML) are introduced to Pt(111), hydrogen displaces the covalent C−Pt bonds, and a tilted (2C’s) conformation is preferred. Increasing the hydrogen coverage beyond 14 H atoms (θH > 0.88 ML) results in

Figure 3. Surface hydrogen (H) phase diagram assuming no coadsorbed guaiacol on Pt(111). Legend colors correspond to surface configurations that minimize surface energy. The associated number of hydrogen atoms assumes a 4 × 4 (16-Pt atom) surface. The black box represents conditions typical to catalytic fast pyrolysis (CFP), and the white box represents conditions typical to hydrotreating (HT). Zeropoint energies and vibrational contributions are included and computed directly from DFT results. D

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Figure 5. Guaiacol adsorption energy at 0 K as a function of hydrogen coverage for four conformations: chemisorbed with all 6 unsaturated carbons (●), tilted chemisorbed with 4 unsaturated carbons (red ■), tilted chemisorbed with 2 unsaturated carbons (green ▲), and physisorbed (blue ◆). The dotted black line represents the minimum-energy conformations. Selected minimum-energy coadsorbed states are shown on the sides.

Figure 6. Phase diagrams for guaiacol-hydrogen (G-H) coadsorption on Pt(111) with varying guaiacol pressure: (A) PG = 10−6 bar, (B) PG = 10−3 bar, and (C) PG = 102 bar. Legend colors correspond to surface configurations that minimize surface energy. The associated number of hydrogen atoms and guaiacol molecules assumes a 4 × 4 (16 Pt-atom) surface. The black box represents conditions typical to catalytic fast pyrolysis (CFP), and the white box represents conditions typical to hydrotreating (HT). Zero-point energies and vibrational contributions are included and computed directly from DFT results. The reference pressure is P° = 1 bar.

102 bar. Besides the hydrogen-only adsorption configurations (purple regions), guaiacol−hydrogen coadsorption structures (red regions) are now pertinent. The lightest red region indicates a single guaiacol adsorbed on Pt(111). Increasingly darker shades of red indicate the regions of coadsorption with increasing hydrogen coverage. At low guaiacol pressures (Figure 6A and B), hydrotreating- and CFP-relevant surface thermodynamics are shown to be identical to those in the absence of guaiacol (Figure 3). As guaiacol pressure increases, guaiacol−hydrogen coadsorption begins to dominate under hydrotreating and CFP conditions. Interestingly, chemisorbed guaiacol appears to be dominant under CFP conditions (1 guaiacol + 4−12 H), while physisorbed guaiacol is likely under hydrotreating conditions (1 guaiacol + 15−16H). Because of the fundamental limitation of constrained thermodynamics, the precise transitions between hydrogen-only and coadsorbed regions (purple and red) are subject to the inclusion of other guaiacol coverages. Because we only consider one guaiacol coverage in this work, the exact pressure of guaiacol needed for coadsorption is not likely to be accurate. However, because we considered a larger range of hydrogen coverages, the transition

physisorbed conformations being energetically favorable. The tilted (4C’s) conformation was not favored at any coverage. As shown in Figure 5, the trend in guaiacol adsorption energy (ΔEguaiacol) with increasing hydrogen coverage is related to the associated physical phenomena. At low coverage (θH < 0.50 ML), the adsorbate−adsorbate interactions are rather small, accompanied by an energy increase of ∼0.3 eV. There is a steep energetic increase of ∼0.3 eV between 8 and 10 H (0.50 ML < θH < 0.63 ML) due to stronger repulsive interactions. These are followed by a conformational change to partially chemisorbed (2C’s) guaiacol and another small ∼0.1 eV energetic increase between 10 and 13 H (0.63 ML < θH < 0.81 ML) where H is able to occupy the sites previously occupied by C−Pt bonds. Above 14 H coverage (θH > 0.88 ML), guaiacol is fully displaced by hydrogen, and so its binding strength is no longer affected by hydrogen coverage. The above energies were used to construct AIPD for the coadsorbed guaiacol and hydrogen. The inclusion of a second surface component introduces an extra dimension as demonstrated in Figure 6, where a two-dimensional phase diagram is shown at three guaiacol pressures of 10−6, 10−3, and E

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of vibrational energy was found to be larger for water and hydroxyl adsorbates on the RuO2 surface.32 Here, we found that the lower-frequency vibrations of hydrogen, the large number of adsorbates, and larger aromatic adsorbates contribute more significantly to γvib ads under hydrotreating- and CFP-relevant conditions. Guaiacol coadsorbed with a monolayer of hydrogen on Pt(111) contributes over 20 meV/Å2 at higher temperatures (Figure 1), which is greater than the desired accuracy of ∼10 meV/Å2 suggested by Scheffler and coworkers.30,32 The importance of vibrational energy can be further assessed in terms of its effect on the constructed phase diagrams. Exclusion of γvib ads from surface energy estimates can shift the phase boundaries of physisorbed or chemisorbed guaiacol by more than 200 K, depending on guaiacol pressure (see SI, section 6). For physisorbed guaiacol, we have demonstrated how the CS correlation56 can be used in conjunction with DFT-estimated gas-phase entropies, which we show to be accurate within the PBE-TS method used here (see SI, section 7). Despite underestimating the entropy of physisorbed molecules (see SI, section 7), we found no considerable differences in γvib ads, whether computed directly from vibrational frequencies or from the CS correlation (see SI, section 6). For chemisorbed or strongly bound species, no such correlation exists due to the lack of experimental calorimetric data. Until such data become available, the only option is to estimate vibrational energy from the DFT-obtained vibrational frequencies, as done here. The typical plots for surface energy as a function of chemical potential of reference species no longer apply because the vibrational energy, γvib ads(T), of the adsorbates has to be taken into account as in eq 2. Instead, we resort directly to experimentally more intuitive T−P phase diagrams. The single-component results (Figure 3) are similar to those previously reported and show comparable transitions from clean to 1 ML surface coverage.66 Guaiacol adsorption can affect these thermodynamic minima. Under higher guaiacol pressures, chemisorbed guaiacol coadsorbed with ∼0.5 ML H becomes preferred over 1 ML H (Figure 6), implying that hydrogen competes with chemisorbed guaiacol for Pt sites. However, physisorbed guaiacol does not compete with hydrogen for Pt sites. Therefore, we expect that guaiacol pressure may influence hydrogen coverage under CFP conditions, but not under hydrotreating conditions. In recent years, adsorbates’ binding on metal surfaces has been tied to reactivity.60,67−71 Of particular interest to this work is the idea that surface crowding can affect preferential binding and, by extension, product selectivity.60,70 Assuming that guaiacol’s aryl−OH and aryl−OCH3 bond breaking is more likely when easily accessible, we propose that chemisorbed guaiacol favors deoxygenation while physisorbed state favors hydrogenation. On the basis of these assumptions, thermodynamically preferred adsorption modes of guaiacol predicted in Figure 6 are consistent with selectivity trends at various experimental conditions.12,13,16,23,24 At conditions resembling CFP, guaiacol undergoes significant deoxygenation, forming phenol as the main product.16,23,24 In contrast, lower temperatures and higher H2 pressures almost exclusively favor benzyl ring saturation to methoxycyclohexanol.12,13 As a result, coadsorption phase diagrams can provide first-hand insight into product selectivity. We have analyzed guaiacol coadsorption with hydrogen; yet, this work has more general implications for hydrogenation and deoxygenation of other aromatic compounds interacting with

from chemisorbed to physisorbed guaiacol with increasing hydrogen pressure is likely to be meaningful. Surface Phase Explorer (SPE). As discussed above, the standard phase diagrams with the chemical potential being the independent variable are no longer adequate when the adsorbate vibrations become significant. Instead, surface energy is minimized as a function of three variables: T, PH2, and PG. The resulting two-dimensional phase plots can only be realized by fixing one of the variables, as shown in Figure 6. In turn, this requires a further need for interactive visualization. With the desire to extend this work to any pressure, temperature, coadsorbates, and surfaces of interest, we developed a webbased tool dubbed Surface Phase Explorer (SPE) to provide interactive visualization to a user investigating a multiparameter system. SPE is open-source, compilation-free, and is capable of computing energetically stable surface regions provided standard DFT output, such as total energies. The code is versatile with respect to user input, not necessitating additional information, such as adsorbate vibrational energies or zeropoint energies. The user is thus left to “explore” surface composition as a function of operating conditions and the governing assumptions in surface energy calculations. To facilitate exploration, this tool creates interactive and downloadable phase diagrams from user-supplied ab initio data. It is possible to create surface AIPDs for single species adsorbing to a surface as well as coadsorption of two species to a surface. The entire temperature range can be explored for all adsorbates using either chemical potential or temperature−pressure charts that are interactive and downloadable. The surface composition of coadsorbed hydrogen and guaiacol on Pt(111) reported in the sections above is provided in the tool as demonstration input set. A typical graphic user interface is shown in Figure 7.

Figure 7. A three-dimensional part of the Surface Phase Explorer (SPE) as it typically appears in a web browser.

For personal use, a demo with hydrogen−guaiacol coadsorption and more can be found at https://spe.nrel.gov. The complete description of the SPE workflow is provided in SI, section 4.

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DISCUSSION Among contributions to surface energy, γvib ads(T) contains the entropic effects associated with the adsorbates, and is relatively expensive to calculate. For cases with high-frequency vibrational modes, such as oxygen on the Pd(100) surface, this term can be neglected with only a small loss of accuracy.33 The contribution F

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The Journal of Physical Chemistry C Table 1. DFT Model Effect on the Energetics in Aromatic Ring Hydrogenation on Pt(111)a +4H molecule

θH (ML)

ΔEmolec ads

benzene73 m-cresol tautomer28 toluene25 benzene - path 129 benzene - path 174 benzene - path 229 benzene - path 229 benzene - path 229 m-cresol11

0 0 0 0 0 0 0.44 0.44 1

−0.9 −2.2 −2.3 −0.7 −0.7 −0.7 −0.2 −0.8

ΔEHads2

ΔErxn

−0.8 −1.1 −0.8 −1.0 −0.8 −0.6 −0.4

0.8 0.5 1.3 0.4 0.2 0.3 −0.8 −1.2 0.3

+6H EA 1.5 2.0 1.1 1.4 1.1 0.7 0.7 0.7

ΔErxn

1.6 0.2 0.3 0.2 −1.1 −1.5 −0.2

EA

method

2.4 1.4 1.5 1.2 0.9 0.9 0.7

VASP; PW91 VASP; optPBE-vdW VASP; PBE-D3 VASP; PW91 Pt22 cluster; BP86 VASP; PW91 VASP; PW91 VASP; optPBE-vdW VASP; PBE-TS

Hydrogen coverage, θH, represents the model surface used in DFT calculations. The columns labeled “+4H” and “+6H” represent reaction energetics for the first 4 H additions to the benzyl ring and full ring saturation, respectively. ΔErxn is the surface reaction energy, and EA is the effective barrier defined as the difference between the maximum transition state energy and the lowest energy preceding it. All energies are electronic energies, in eV. a

the high-coverage model (θH = 1 ML). Instead, the calculations revealed concerted steps, in which neighboring surface hydrogen atoms migrate and hydrogenate one of the C−C double bonds.11 Therefore, having a good guess at the hydrogen-aromatic coadsorption can improve the DFT model choice and result in mechanistic details consistent with experimental conditions of interest. It is clear that determining the equilibrium surface composition with relatively high accuracy can be a useful tool in finding a proper DFT model surface and possibly even inferring associated activity and selectivity. However, we must caution that this analysis serves as a starting point for understanding surface composition and depends upon what is assumed to be the steady state of the reaction. In this study, we considered the coadsorption of guaiacol and hydrogen. However, other species, reaction intermediates, or coproducts may occupy the surface. For instance, decomposition products, such as CO or water, may influence surface composition depending on external conditions or metal choice. Furthermore, reaction intermediates may experience kinetic trapping, in which case the reaction is not thermodynamically controlled and the surface composition never reaches equilibrium with gas reservoir. Last, functional choice can play a significant role in coadsorption (see SI, section 8). While the hydrogen-only phase diagram is practically unaffected, neglecting vdW contributions in this work lead to no guaiacol−hydrogen coadsorption at guaiacol pressures below 109 bar. Even at higher pressures, no physisorbed guaiacol regions show up in the phase diagrams.

Pt. For instance, Griffin et al. studied hydrogenation and deoxygenation of m-cresol over Pt/C, showing that the catalyst is more active under hydrotreating conditions and favors saturated products. In contrast, Pt/C is not as active under CFP conditions and favors deoxygenation products.11 Considering similarities between guaiacol and m-cresol adsorption11 and attributing deoxygenation activity to decreased surface crowding and chemisorbed oxygenates, CFP conditions are indeed more favorable for deoxygenation on Pt according to our results. The results of this work can be influential in determining the correct DFT model surface when investigating reactions of aromatics in the presence of hydrogen on Pt. Few DFT studies have addressed the kinetics for the complete hydrogenation of the benzyl ring over Pt(111), and the results are vastly different depending on the model choice and assumptions.11,25,29,72−74 To compare these DFT results, we computed the surface reaction energies, ΔErxn, and the effective reaction barriers, EA (difference between the highest transition state energy and the lowest energy intermediate preceding it), for the first four hydrogen additions as well as full saturation. Table 1 shows these quantities for benzene, toluene, m-cresol, and an m-cresol ketone tautomer ring hydrogenation over Pt(111) using various models and functionals. The low-coverage limit models (θH = 0 ML) resulted in endothermic reaction energies with effective barriers ranging from 1.1 to 2.4 eV, depending on the functional choice. Intermediate coverage models, closely resembling the CFP conditions (θH = 0.44 ML), result in exothermic reaction energies and effective surface barriers of 0.9 eV. Last, the high surface coverage model closely resembling the hydrotreating conditions (θH = 1 ML) also shows the surface reaction being exothermic with the lowest effective barrier of 0.7 eV. Experiments at lower temperatures of 345− 500 K, representative of high hydrogen coverage, result in a hydrogenation activation energy of 0.25−0.5 eV.75−77 On the basis of the DFT-computed energetics, it is evident that only the models representative of equilibrium surface coverage can capture the hydrogenation kinetics correctly. Besides the energetic differences highlighted in Table 1, the resulting hydrogenation mechanisms are also model-dependent. The low-coverage models for chemisorbed aromatics show single H additions as elementary steps, resulting in six hydrogenation reactions that do not necessarily favor hydrogenation of one C−C double bond at a time.25,29,72 On the other hand, such elementary steps were not found plausible for

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CONCLUSIONS In this work, we used DFT with van der Waals corrections to identify thermodynamically stable surface configurations of coadsorbed hydrogen and guaiacol on Pt(111) under a wide range of operating conditions, including hydrotreating and catalytic fast pyrolysis (CFP). We find that in the absence of guaiacol, a full monolayer of hydrogen is expected under hydrotreating conditions, while submonolayer coverage is possible under CFP conditions. These results are also consistent with the previous hydrogen adsorption reports.59 At finite guaiacol pressures, coadsorbed guaiacol and hydrogen are present on the surface. Under CFP conditions, guaiacol is chemisorbed with submonolayer hydrogen. Under hydrotreating conditions, guaiacol is physisorbed with a full monolayer of hydrogen. Considering these differences, we G

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ment through the Study of Model Compounds. Green Chem. 2014, 16, 454−490. (2) Dwiatmoko, A. A.; Zhou, L.; Kim, I.; Choi, J.-W.; Suh, D. J.; Ha, J.-M. Hydrodeoxygenation of Lignin-Derived Monomers and Lignocellulose Pyrolysis Oil on the Carbon-Supported Ru Catalysts. Catal. Today 2016, 265, 192−198. (3) Lee, C. R.; Yoon, J. S.; Suh, Y.-W.; Choi, J.-W.; Ha, J.-M.; Suh, D. J.; Park, Y.-K. Catalytic Roles of Metals and Supports on Hydrodeoxygenation of Lignin Monomer Guaiacol. Catal. Commun. 2012, 17, 54−58. (4) Elliott, D. C.; Hart, T. R. Catalytic Hydroprocessing of Chemical Models for Bio-Oil. Energy Fuels 2009, 23, 631−637. (5) Nie, L.; Resasco, D. E. Kinetics and Mechanism of M-Cresol Hydrodeoxygenation on a Pt/SiO2 Catalyst. J. Catal. 2014, 317, 22− 29. (6) Nie, L.; de Souza, P. M.; Noronha, F. B.; An, W.; Sooknoi, T.; Resasco, D. E. Selective Conversion of M-Cresol to Toluene over Bimetallic Ni−Fe Catalysts. J. Mol. Catal. A: Chem. 2014, 388−389, 47−55. (7) Hong, Y.; Zhang, H.; Sun, J.; Ayman, K. M.; Hensley, A. J. R.; Gu, M.; Engelhard, M. H.; McEwen, J.-S.; Wang, Y. Synergistic Catalysis between Pd and Fe in Gas Phase Hydrodeoxygenation of M-Cresol. ACS Catal. 2014, 4, 3335−3345. (8) Mortensen, P. M.; Grunwaldt, J.-D.; Jensen, P. A.; Jensen, A. D. Screening of Catalysts for Hydrodeoxygenation of Phenol as a Model Compound for Bio-Oil. ACS Catal. 2013, 3, 1774−1785. (9) Ohta, H.; Kobayashi, H.; Hara, K.; Fukuoka, A. Hydrodeoxygenation of Phenols as Lignin Models under Acid-Free Conditions with Carbon-Supported Platinum Catalysts. Chem. Commun. 2011, 47, 12209−12211. (10) Wan, H.; Chaudhari, R. V.; Subramaniam, B. Catalytic Hydroprocessing of P-Cresol: Metal, Solvent and Mass-Transfer Effects. Top. Catal. 2012, 55, 129−139. (11) Griffin, M. B.; Ferguson, G. A.; Ruddy, D. A.; Biddy, M. J.; Beckham, G. T.; Schaidle, J. A. Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of M-Cresol over Pt/ C and Pt/Tio2 Catalysts. ACS Catal. 2016, 6, 2715−2727. (12) Hellinger, M.; Baier, S.; Mortensen, P.; Kleist, W.; Jensen, A.; Grunwaldt, J.-D. Continuous Catalytic Hydrodeoxygenation of Guaiacol over Pt/Sio2 and Pt/H-Mfi-90. Catalysts 2015, 5, 1152. (13) Hellinger, M.; Carvalho, H. W. P.; Baier, S.; Wang, D.; Kleist, W.; Grunwaldt, J.-D. Catalytic Hydrodeoxygenation of Guaiacol over Platinum Supported on Metal Oxides and Zeolites. Appl. Catal., A 2015, 490, 181−192. (14) He, Z.; Wang, X. Highly Selective Catalytic Hydrodeoxygenation of Guaiacol to Cyclohexane over Pt/TiO2 and Nimo/Al2O3 Catalysts. Front. Chem. Sci. Eng. 2014, 8, 369−377. (15) Foster, A. J.; Do, P. T. M.; Lobo, R. F. The Synergy of the Support Acid Function and the Metal Function in the Catalytic Hydrodeoxygenation of M-Cresol. Top. Catal. 2012, 55, 118−128. (16) Gao, D.; Xiao, Y.; Varma, A. Guaiacol Hydrodeoxygenation over Platinum Catalyst: Reaction Pathways and Kinetics. Ind. Eng. Chem. Res. 2015, 54, 10638−10644. (17) Gutierrez, A.; Kaila, R. K.; Honkela, M. L.; Slioor, R.; Krause, A. O. I. Hydrodeoxygenation of Guaiacol on Noble Metal Catalysts. Catal. Today 2009, 147, 239−246. (18) Zhao, H. Y.; Li, D.; Bui, P.; Oyama, S. T. Hydrodeoxygenation of Guaiacol as Model Compound for Pyrolysis Oil on Transition Metal Phosphide Hydroprocessing Catalysts. Appl. Catal., A 2011, 391, 305−310. (19) Echeandia, S.; Arias, P. L.; Barrio, V. L.; Pawelec, B.; Fierro, J. L. G. Synergy Effect in the Hdo of Phenol over Ni−W Catalysts Supported on Active Carbon: Effect of Tungsten Precursors. Appl. Catal., B 2010, 101, 1−12. (20) Olcese, R. N.; Bettahar, M.; Petitjean, D.; Malaman, B.; Giovanella, F.; Dufour, A. Gas-Phase Hydrodeoxygenation of Guaiacol over Fe/SiO2 Catalyst. Appl. Catal., B 2012, 115−116, 63−73.

expect that guaiacol pressure may influence hydrogen coverage change under CFP conditions, but not under hydrotreating conditions. CFP conditions were found to be more favorable for deoxygenation based on accessibility of the aryl−OH and aryl− OCH3 bonds for chemisorbed as compared to physisorbed guaiacol. These results are consistent with general trends in available experimental data for guaiacol reactions on supported Pt catalysts12,13,16,23,24 and are extendable to other model lignin oxygenates, such as m-cresol.11 As part of this study, we find that the vibrational contribution of the adsorbates is substantial for guaiacol coadsorbed with hydrogen and has to be included in the estimation of surface energy. A simple empirical model is proposed to estimate this contribution; it relates high-coverage configuration to its lowcoverage constituents with acceptable accuracy. With significant adsorbate vibrational contributions, we find it necessary to investigate the stability of coadsorbed structures as a function of three variables, T, PH2, and PG. To do so, we developed Surface Phase Explorer (SPE), which is an opensource, compile-free, web-based tool to interactively compute surface phase diagrams from DFT. This tool is freely accessible through nrel.gov and is readily equipped to produce fully interactive ab initio phase diagrams with one or two gas-phase components.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07057. Spreadsheet of adsorption energies and most stable (XLSX) Gas-phase chemical potential formulas, SPE workflow, semiempirical estimation of vibrational contribution to surface energy, importance of vibrational energy contribution in phase diagrams, and entropy estimation of gas-phase and physisorbed species (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

G.A.F. and V.V. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted as part of the Computational Pyrolysis Consortium supported by the U.S. Department of Energy’s Bioenergy Technologies Office (DOE-BETO) contract no. DE-AC36-08GO28308 with the National Renewable Energy Laboratory. Computer time was provided by Extreme Science and Engineering Discovery Environment (XSEDE) allocation MCB-090159 at the Texas Advanced Computing Center and by the National Renewable Energy Laboratory Computational Sciences Center supported by the DOE Office of EERE under contract number DE-AC36-08GO28308.

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