Mechanistic Effects of Water on the Fe-Catalyzed Hydrodeoxygenation

Jan 10, 2018 - A mechanistic understanding of the roles of water is essential for developing highly active and selective catalysts for hydrodeoxygenat...
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Research Article Cite This: ACS Catal. 2018, 8, 2200−2208

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Mechanistic Effects of Water on the Fe-Catalyzed Hydrodeoxygenation of Phenol. The Role of Brønsted Acid Sites Alyssa J. R. Hensley,† Yong Wang,†,‡ Donghai Mei,*,§,‡ and Jean-Sabin McEwen*,†,‡,∥,⊥ †

The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, ∥Department of Physics and Astronomy, and Department of Chemistry, Washington State University, Pullman, Washington 99164, United States ‡ Institute for Integrated Catalysis and §Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ⊥

S Supporting Information *

ABSTRACT: A mechanistic understanding of the roles of water is essential for developing highly active and selective catalysts for hydrodeoxygenation (HDO) reactions because water is ubiquitous in such reaction systems. Here we present a study for phenol HDO on Fe catalysts using density functional theory which examines the effect of water on three elementary pathways for phenol HDO using an explicit solvation model. The presence of water is found to significantly decrease activation barriers required by hydrogenation reactions via two pathways. First, proton transfer in the hydrogen bonding network of the liquid water phase is nearly barrierless, which significantly promotes the direct tautomerization of phenol. Second, due to the high degree of oxophilicity on Fe, liquid water molecules are found to be easily dissociated into surface hydroxyl groups that can act as Brønsted acid sites. These sites dramatically promote hydrogenation reactions on the Fe surface. As a result, hydrogen-assisted dehydroxylation becomes the dominant phenol HDO pathway. This work provides fundamental insights into aqueous phase HDO of biomass-derived oxygenates over Fe-based catalysts; e.g., the activity of Fe-based catalysts can be optimized by tuning the surface coverage of Brønsted acid sites via surface doping. KEYWORDS: hydrodeoxygenation, mechanistic effect of liquid water, phenol, Fe catalyst, density functional theory, reaction pathways, Brønsted acid sites

1. INTRODUCTION The development of highly active and selective catalysts for the hydrodeoxygenation (HDO) of pyrolytic bio-oils is crucial to increasing the usability of such renewable resources. One key component to designing such catalysts is to gain knowledge of the reaction routes and kinetically relevant elementary steps over a range of promising catalysts. Thus far, theoretical studies have examined the HDO of different phenolic compounds as model bio-oil compounds on Pt,1−7 Ru,3,5,8−12 and Fe5,13,14 surfaces in the vapor phase. Overall, these studies have shown that the more oxophilic metals, e.g., Ru and Fe, preferentially directly cleave the C−O bond while the less oxophilic Pt prefers a HDO reaction route proceeding via a tautomerization pathway, in agreement with experimental kinetic studies on Pt/ SiO2 catalysts.3,9,10,15,16 While these studies provide useful insight into the elementary surface processes occurring during the HDO reaction of phenolics, the effect of water on these processes has been largely ignored except for in a few studies.9,17,18 Water is a significant component of bio-oils and is a byproduct of the HDO reaction,19 meaning that water will be present during HDO catalysis. Of particular importance for oxophilic HDO catalysts is the effect of surface hydroxyl groups from water © 2018 American Chemical Society

dissociation on the HDO pathway. The presence of oxygencontaining species on the HDO catalyst surface could deactivate oxophilic metal catalysts by blocking active sites, thereby oxidizing the surface. However, surface hydroxyls from water dissociation over oxophilic metal catalysts could potentially act as Brønsted acid sites which could affect the HDO reaction. The catalytic assistance of Brønsted acid sites has been shown to be key to many reactions.9,20−22 Therefore, it is of crucial importance to understand the effect of water and its decomposition products in the design of HDO catalysts for bio-oil upgrading. Density functional theory (DFT) calculations were used to investigate the effect of an aqueous environment on the three major HDO pathways for phenol on Fe(110): (1) direct dehydroxylation (DHOx), (2) hydrogen-assisted dehydroxylation (H-DHOx), and (3) tautomerization (Taut.).13 These three major pathways are shown in Scheme 1. The Fe surface was chosen, as this metal has been shown to be highly selective for cleaving C−O bonds.13,16,23−25 The present contribution Received: August 2, 2017 Revised: January 9, 2018 Published: January 10, 2018 2200

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ACS Catalysis Scheme 1. Three Major Pathways for the Catalytic HDO of Phenola

a

The M represents bonding to a surface metal atom.

as the total energy of the system had reached equilibrium within 10 ps. The entire system was then further optimized using ab initio calculations at 0 K. Because it was observed that a portion of water molecules dissociated into surface oxygen and hydroxyl species on Fe(110) in the AIMD simulations (Figure S1), phenol hydrogenation reactions were also studied in the presence of both surface hydrogen and hydroxyl species on the Fe(110) surface. The Bader35 charge analyses were performed with an optimized grid for the fast Fourier transform (Table S1). The optimized water layer obtained from the phenol+water/Fe(110) AIMD simulations was used as the starting configuration for the solvent layer in all of the ground state optimization calculations on the tested reaction intermediates. No additional AIMD simulations were performed on the reaction intermediates. All DFT calculations were performed using a cutoff energy of 450 eV and a Monkhorst−Pack k-point34 mesh of (3 × 3 × 1). The water layer and all adspecies were allowed to fully relax during each ground state and minimum energy pathway (MEP) optimization. The transition states and MEPs were obtained using the Climbing Image Nudged Elastic Band (CINEB) method.36 One elementary reaction step, i.e., phenol’s direct tautomerization, was performed using an implicit solvation model, VASPsol,37,38 to provide a comparison between the explicit and implicit models. By calculating the vibrational frequencies of the initial, transition, and final state structures, including the vibrations of the solvent, the Gibbs activation and reaction energies at 623 K were estimated using standard statistical mechanics principles:39

shows that the major effect of water on the HDO of phenol is water dissociation to Brønsted acid sites on the Fe surface. Overall, our results here suggest that the performance of Febased catalysts for the HDO of phenolics could be improved by ensuring that Brønsted acid sites are present on the surface.

2. METHODS The results discussed here were obtained using the Vienna Ab Initio Simulation Package (VASP).26,27 The core electrons and electron−electron exchange-correlation effects were treated using the projector-augmented wave (PAW)28 method and the optB88-vdW functional,29,30 respectively. Methfessel−Paxton31 smearing (N = 1) of 0.1 eV was used, and the energy and force tolerances were set to 10−4 eV and 0.035 eV/Å, respectively. Spin polarization was used, but dipole corrections were found to be unnecessary. The lattice constant of Fe was 2.827 Å and the surface was modeled using a p(4 × 4) supercell with a fourlayer thick surface. In all of the work discussed here, the bottom two layers of the surface were fixed into their bulk positions and the top two layers were allowed to relax (Figure S1), leading to an asymmetric surface. The effect of the surface thickness on the reaction energies was previously tested and found to have a negligible effect on reaction energies.13 To setup the aqueous environment, Ab Initio Molecular Dynamics (AIMD) simulations were first performed on a pure water system above our Fe(110) surface model made up of 30 water molecules, corresponding to a density of 0.73 g/cm3 (the density of liquid water at 623 K).32 Liquid water was used here to ensure that there was interaction between water and surface species during our calculations. According to Herbert et al.,33 the energy drift is the slope of a line fit to the energy versus time and is acceptable when the drift multiplied by the total simulation time is similar in value to the energy noise, which is defined as the root-mean-square fluctuations in the energy after the energy drift has been removed. Using these definitions, we determined that the system was thermalized (drift and noise after 10 ps were 2.3 × 10−4 eV/atom/ps and 3.6 × 10−3 eV/ atom, respectively). This water/Fe(110) model was taken, and a phenol adsorbate was added by removing three water molecules to make room and to keep the liquid phase above the Fe(110) surface approximately constant, consistent with previous work.18 AIMD simulations were then run on the water+phenol/Fe(110) system. All AIMD simulations were run with a cutoff energy of 300 eV and with a Monkhorst−Pack34 k-point mesh of (1 × 1 × 1). The time step was set at 2 fs, and the mass of the hydrogen was set to 2 au to allow for the larger time step in the AIMD simulations. The phenol+water/ Fe(110) system was thermalized with AIMD simulations at 623 K in the canonical ensemble (NVT) for an additional 10 ps (drift and noise were 1.2 × 10−4 eV/atom/ps and 3.1 × 10−3 eV/atom, respectively). Longer simulation times were not used,

⎛q ⎞ ΔGact = Eact − kBT ln⎜⎜ TS ⎟⎟ ⎝ qIS ⎠

(1)

⎛q ⎞ ΔGrxn = Erxn − kBT ln⎜⎜ FS ⎟⎟ ⎝ qIS ⎠

(2)

q=

∏ i

e−ℏυi /2kBT 1 − e−ℏυi / kBT

(3)

where Eact and Erxn are the DFT-calculated activation and reaction energies, qIS, qTS, and qFS are the initial, transition, and final state vibrational partition functions calculated according to eq 3, νi is the ith vibrational frequency, and ℏ, kB, and T are Planck’s constant, Boltzmann’s constant, and reaction temperature, respectively. Here the Gibbs activation and reaction energies were calculated for only surface reactions. Including the Gibbs adsorption and desorption energies for surface species would only result in a shift of the overall Gibbs energy scale of our studied pathways without affecting the results for the elementary surface reactions themselves. Therefore, we 2201

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Figure 1. Gibbs MEP for the direct tautomerization of phenol to cyclohexadienone over Fe(110) under the UHV (data taken from Hensley et al.13) and aqueous environments with either an implicit or explicit water model. The structures shown are for the aqueous environment using the implicit (red outline) and explicit water models (blue outline). The large gold, black, red, and white spheres represent the Fe, carbon, oxygen, and hydrogen directly participating in the reaction, while the small pink and gray spheres represent the oxygen and hydrogen not directly participating in the reaction. The DFT-calculated MEP for the explicit model is shown with top-down views of the structures in Figure S15.

interactions. Based on these results, an explicit solvation model is necessary to truly capture the H2O−adsorbate interactions. Therefore, all calculations performed here include explicitly modeled water molecules. 3.2. Effect of H2O on HDO Pathways. To fully elucidate the aqueous environment effect on the individual reaction steps for the DHOx, H-DHOx, and Taut. pathways, we examined every elementary step in each pathway under an aqueous environment. As noted in Methods, a portion of the H2O molecules were seen to dissociate during the AIMD simulations, which is consistent with the low dissociation barrier of water on Fe(110),13,43 as well as the strong adsorption energies for oxygen and hydroxyl on Fe(110).43−45 Due to the asymmetry of the model surface (i.e., whether the exposed surface layer was allowed to relax or was fixed), the coverages of oxygen, hydrogen, and hydroxyl differed between the two exposed surface layers (Figure S1). For the surface of interest, i.e., the layers allowed to relax during the AIMD simulations, only hydrogen and hydroxyl were present and their coverages were both 0.0625 monolayers (ML, where 1 ML = 1 adsorbate per surface Fe atom). Using a Bader charge analysis, we found that the oxygen and hydrogen in the surface hydroxyl species had a charge of +0.66 and −1.22 electrons, respectively, as shown in Figure S1 and Table S1. This suggests that the surface hydroxyl species could act as proton donors in hydrogenation reactions and are hereafter referred to as Brønsted acid sites. The effect of both the aqueous environment and the surface Brønsted acid sites on the elementary HDO pathways were examined, and the resulting Gibbs activation and reaction energies were compared with the previously calculated UHV results.13 3.2.1. Direct Dehydroxylation (DHOx). The DHOx pathway is the simplest of the three HDO pathways and is composed of only two major reaction steps, the cleavage of the C−O bond and the hydrogenation of surface phenyl to form benzene (Scheme 1). Of these two elementary reaction steps, the most crucial step is arguably the cleavage of the C−O bond, as the removal of oxygen functional groups is the goal of HDO. Due to the polar nature of the hydroxyl functional group on phenol, it is possible that the presence of water could weaken the C−O bond through hydrogen bonding interactions with the surrounding water network. A comparison of the Gibbs MEP for the cleavage of the C−O bond in phenol under the UHV

assumed that the necessary reactants, i.e., phenol and hydrogen, were already adsorbed onto the surface, which is supported by the work of Hong et al., who have shown that the adsorption and desorption of surface species are not likely to be rate limiting for the HDO of phenolic compounds.40 All calculated DFT activation energies, DFT reaction energies, Gibbs activation energies, Gibbs reaction energies, and MEPs are shown in Supporting Information (Tables S2 and S3 and Figures S4−S27). All structures were visualized using VESTA.41 The effects of liquid water on the phenol HDO reaction pathways are investigated by comparing the aqueous environment calculation results performed with the same pathways previously calculated under ultrahigh vacuum (UHV) conditions.13 However, the metal-assisted tautomerization pathway for phenol on Fe(110) investigated here (section 3.2.3) was not included in the original work under UHV conditions. Therefore, we have used the same settings discussed in the previous work to calculate this metal-assisted tautomerization pathway under UHV conditions for consistency.

3. RESULTS AND DISCUSSION 3.1. Comparison between Implicit and Explicit Models. When modeling the effects of water on reaction pathways, it is important to determine whether an implicit model is sufficient or if explicit water molecules are necessary. The comparison performed here between the implicit and explicit water models used the tautomerization of phenol to cyclohexadienone as a test case. Under UHV conditions, the direct tautomerization of phenol to cyclohexadienone on Fe(110) was found to have a prohibitively high barrier of ∼2 eV (Figure 1), similar to that seen for phenol on Pt(111).18 However, performing this reaction in an aqueous environment with explicitly modeled water molecules resulted in hydrogen being transferred through the hydrogen bonding network, reducing the Gibbs activation energy for the direct tautomerization of phenol to ∼1 eV. As is shown in Figure 1, this reduction in the Gibbs activation energy due to the presence of water is not captured using the implicit model because the reduction comes purely from the physical presence of water molecules in the explicit model. This difference between the explicit and implicit models is similar to that noted by Bodenschatz et al.42 who showed that the implicit solvation model did not accurately reproduce the H2O−adsorbate 2202

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on the order of 0.02 to 0.23 eV. This suggests that elementary reaction steps where water acts as a spectator species are relatively unaffected by the aqueous environment. A similar effect was found in the aqueous phase CO hydrogenation on Ru.46 Overall, these results show that the effect of an aqueous environment on the Gibbs activation and reaction energies is minimal when water acts as a spectator species. For the second reaction step in the DHOx pathway, two different scenarios were examined to determine the effect of water on the hydrogenation of phenyl: (1) hydrogenation via a surface hydrogen atom and (2) hydrogenation via a proton from an adjacent Brønsted acid site. These Gibbs MEPs are shown in Figure 3 along with the UHV Gibbs MEP for comparison. In the first scenario, water was treated as a spectator as the distance between the nearest water molecules and the surface was larger than ∼3.5 Å, preventing the surface hydrogen and aqueous water molecules from interacting. Similar to the results presented for the cleavage of the C−O bond in Figure 2 where water acted as a spectator, the addition of the aqueous environment did not significantly change the Gibbs activation and reaction energies for phenyl hydrogenation (Figure 3). In the second scenario, the hydrogenation of phenyl occurred via the proton transfer from a Brønsted acid site (surface hydroxyl group), as shown in Figure 3. Phenyl hydrogenation via the proton transfer from these Brønsted acid sites was found to have lower Gibbs activation and reaction energies by ∼0.5 and ∼0.7 eV, respectively, relative to the first hydrogenation scenario (via surface hydrogen). In fact, the presence of Brønsted acid sites lowered both the Gibbs activation and reaction energies for all the elementary hydrogenation reaction steps studied here, i.e., all benzene formation and ketone hydrogenation reactions (Tables S2 and S3). Overall, these results show that a major effect of water and Brønsted acid sites on the HDO pathways is to assist hydrogenation reactions via proton transfer. Overall, for the DHOx pathway, the most significant change due to the aqueous environment is seen in the hydrogenation of phenyl to benzene when Brønsted acid sites are present (step 2 in Figure 4a), with the Gibbs activation energy decreasing from 0.87 eV in UHV to 0.26 eV in the presence of Brønsted

and aqueous environment (see Figure 2) shows that the Gibbs activation and reaction energies are slightly increased in the

Figure 2. Gibbs MEP for the direct cleavage of the C−O bond in phenol over Fe(110) under the UHV (data taken from Hensley et al.13) and aqueous environments. The structures shown are for the aqueous environment (red outline), and the sphere colors are identical to those in Figure 1. The DFT-calculated MEP for the aqueous environment is shown with top-down views of the structures in Figure S4.

aqueous environment. The possibility that the phenyl species present after the cleavage of the C−O bond would switch to a vertical adsorption geometry in the aqueous environment was tested, as shown in Figure S2. However, we found that a coadsorbed water molecule from the aqueous layer was necessary to keep the phenyl vertically adsorbed during optimization and that this process increased the energy of the system by 0.88 eV relative to the flat-lying phenyl geometry. Similar changes due to the aqueous environment are observed in the Gibbs MEP for the cleavage of the C−O bond in the partially hydrogenated phenol (see Tables S2 and S3). These differences in Gibbs activation and reaction energies are small,

Figure 3. Gibbs MEP for the hydrogenation of phenyl over Fe(110) under the UHV (data taken from Hensley et al.13) and aqueous environments. The structures shown are for the reactions involving a surface hydrogen (red outline) and a Brønsted acid site (blue outline) in an aqueous environment. The sphere colors are identical to those in Figure 1. The DFT calculated MEPs for the aqueous environment are shown with top-down views of the structures in Figures S5 and S6. 2203

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phenol with an activation barrier of 0.70 eV under aqueous conditions with Brønsted acid sites (step 1 in Figure 4b). Overall, the H-DHOx pathway becomes significantly more favorable on Fe(110) in the presence of an aqueous environment and Brønsted acid sites. 3.2.3. Tautomerization (Taut.). The Taut. pathway is similar to the H-DHOx pathway for all reaction steps except the initial hydrogenation of phenol, which occurs in this pathway by the tautomerization of phenol to cyclohexadienone followed by the hydrogenation of cyclohexadienone (Scheme 1). On a metal surface, tautomerization can occur either directly, as shown in Figure 1, or through a metal-assisted pathway, shown in Figure 5.3,13,47 Here, we investigate the effect of an aqueous environment on both types of tautomerization pathways for adsorbed phenol on Fe(110).

Figure 4. Gibbs MEP for the HDO of phenol through the (a) DHOx, (b) H-DHOx, and (c) Taut. pathways for the UHV and aqueous with Brønsted acid sites environments. The last steps in each pathway are the regeneration of the Brønsted acid sites, i.e., step 3 in panel a, steps 5 and 6 in panel b, and steps 7 and 8 in panel c.

acid sites. Neither the aqueous environment nor the presence of Brønsted acid sites has a significant effect on the most activated step for this pathway, namely the cleavage of the C− O bond which has a Gibbs activation energy of 1.08 and 1.11 eV in the UHV and aqueous environments, respectively. 3.2.2. Hydrogen-Assisted Dehydroxylation (H-DHOx). The H-DHOx pathway involves the partial hydrogenation of adsorbed phenol prior to the cleavage of the C−O bond, which is then followed by benzene formation through a set of hydrogenation and dehydrogenation reaction steps (Scheme 1). As shown in Figure 3, the aqueous environment has a significant effect in promoting hydrogenation reactions, which leads to a greater energetic favorability of the H-DHOx pathway in the aqueous environment as compared to the UHV environment (Figure 4b). In the H-DHOx pathway, the presence of Brønsted acid sites make the initial hydrogenation of the adsorbed phenol significantly more favorable (step 1 in Figure 4b), with the Gibbs reaction energy changing from 0.66 eV under UHV conditions to −0.11 eV under an aqueous environment with Brønsted acid sites. In fact, the presence of Brønsted acid sites lowers the Gibbs activation and reaction energies for all hydrogenation reaction steps in the H-DHOx pathway by 0.1−0.9 eV (steps 1 and 3 in Figure 4b). The most activated step for this pathway also changes in the presence of Brønsted acid sites, from the formation of partially hydrogenated benzene with an activation barrier of 1.00 eV under UHV conditions (step 3 in Figure 4b) to the hydrogenation of

Figure 5. Gibbs MEP for the metal-assisted tautomerization of phenol over Fe(110) under the UHV and aqueous environments. The structures shown are for the UHV environment (black outline), aqueous environment with the reaction involving a surface hydrogen (red outline), and aqueous environment with the reaction involving a Brønsted acid site (blue outline). The sphere colors are identical to those in Figure 1. The DFT-calculated MEPs for all pathways are shown with top-down views of the structures in Figures S16−S19, S23, and S24.

The effect of an aqueous environment on the direct tautomerization of phenol was examined and the results are shown in Figure 1. These results show that the direct tautomerization pathway under UHV conditions is highly unfavorable, with an activation Gibbs energy of 1.92 eV.13 However, under aqueous conditions, the hydrogen bonding network significantly lowers the activation barrier for the direct tautomerization pathway to 1.09 eV. Comparing these results to the work of Yoon et al.,18 the decrease in the tautomerization barrier for phenol on Fe(110) in the presence of water is greater than that seen on Ni(111) (activation barrier ∼1.10 eV in UHV and ∼0.65 eV in water) but smaller than that seen on 2204

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ACS Catalysis Pt(111) (activation barrier of ∼1.91 eV in UHV and ∼0.61 eV in water). In addition to water-mediated hydrogen transfer, phenol tautomerization can occur via a metal-assisted pathway. In this pathway, the phenol’s hydroxyl hydrogen is first transferred to the surface before forming a C−H bond with the ortho ring carbon and consequently cyclohexadienone (Figure 5).3,47 Under UHV conditions, the metal-assisted tautomerization pathway is significantly more favorable than the direct pathway, with the most-activated step being the ketone formation step with a Gibbs activation energy of 0.76 eV. This result is similar to that noted by Tan et al. for the tautomerization of m-cresol over Pt(111) and Ru(0001).3 Under aqueous conditions with the hydrogenation occurring via a surface hydrogen, the Gibbs activation energy for the most activated step decreases to ∼0.6 eV, with the most activated step being the formation of the ketone. The metal-assisted tautomerization pathways for the UHV environment and aqueous environment with a surface hydrogen results both show structural and energetic similarities, with the major exception being the Gibbs reaction energy as the aqueous environment seems to stabilize the cyclohexadienone product as compared to the UHV environment. This is consistent with the other hydrogenation reactions from surface hydrogen studied here (Tables S2 and S3), suggesting that the aqueous environment has a stabilization effect on partially hydrogenated ring species. Altering the aqueous environment pathway such that hydrogen transfer for this pathway occurs to and from a Brønsted acid site significantly alters the Gibbs energies of phenol dehydrogenation (Figure 5A−C) as the Gibbs activation energy decreases by ∼0.2 eV and the Gibbs reaction energy increases by ∼1.0 eV relative to the previous pathways, the latter of which is likely due to the greater favorability of surface oxygen as compared to surface hydroxyl. Additionally, for the formation of cyclohexadienone (Figure 5C−E), the presence of the Brønsted acid site did not lower the energy cost of the hydrogenation reaction. Examining the final state structure for this pathway shows that the surface oxygen is not hydrogen bonding in any way to the water layer above it, as the closest water is at a distance of ∼3.6 Å, while the initial state does indeed have hydrogen bonding occurring between the Brønsted acid site and the water layer above as the closest water is at a distance of ∼1.6 Å. This suggests that it is the loss of hydrogen bonding between the surface species and the water layer that accounts for the Gibbs energy changes between the hydrogenation pathways under an aqueous environment using a surface hydrogen and a Brønsted acid site. For the overall Taut. pathway, the aqueous environment significantly assists in the hydrogenation of the oxygen group in cyclohexadienone (step 3 in Figure 4c), decreasing the activation barrier from 1.4 eV under UHV to 0.2 eV in the aqueous environment. This is due to the shuttling of protons through the water network (Figure S21). The most activated step for the Taut. pathway changes from the hydrogenation of the cyclohexadienone’s oxygen group under UHV (step 3 in Figure 4c) to a competition between the formation of cyclohexadienone and C−O cleavage steps under an aqueous environment with Brønsted acid sites (steps 2 and 4 in Figure 4c), with the most activated steps’ barriers being reduced by ∼0.8 eV relative to the Taut. pathway under UHV. Overall, the Taut. pathway becomes significantly more favorable in the presence of an aqueous environment and Brønsted acid sites. 3.2.4. Regenerating the Brønsted Acid Sites. As shown above, Brønsted acid sites on Fe(110) have a significant effect

on the hydrogenation reactions involved in the HDO of phenol, suggesting that the reformation of the Brønsted acid sites will be crucial to this reaction. This prompts the question of how difficult it is to regenerate these sites, as pathways in Figure 4 require either one or two Brønsted acid sites. As shown in Figure 6, the regeneration of a Brønsted acid site from

Figure 6. Gibbs MEP for the reformation of the Brønsted acid site over Fe(110), Pd/Fe(110), and Pt/Fe(110) under the aqueous environment. The structures shown are for the Fe(110) (red outline) and Pd/Fe(110) (blue outline) surfaces. The Pt/Fe(110) side views are not shown here, as they are identical to Pd/Fe(110). The sphere colors are identical to those in Figure 1 with the silver sphere representing Pd. The DFT-calculated MEPs for all pathways are shown with top-down views of the structures in Figures S22, S26, and S27.

surface O and H is significantly unfavorable with Gibbs activation and reaction energies of 1.36 and 1.13 eV, respectively. There is a shift in the oxygen species’ adsorption site which is attributed to the change in hydrogen bonding between the oxygen and hydroxyl adspecies and the water layer. This shift should have a minimal effect on the energetics, as previous work has found a barrier of ∼0.06 eV for a similar oxygen diffusion pathway under UHV conditions.45 This suggests that the Fe surface will oxidize quickly under an aqueous environment and become HDO inactive, which is consistent with the results for the vapor phase HDO of phenolics over Fe alone.5,16,25,40,48 Previous work has shown that the addition of a noble metal promoter significantly improves the HDO performance of Fe catalysts and that such an improvement could be connected to the improved oxidation resistance of the doped Fe catalysts.16,25,40,48,49 On the basis of this previous work, we hypothesize that the addition of a noble metal promoter to Fe(110) would stabilize the Brønsted acid sites. This hypothesis was tested by replacing a surface Fe atom adjacent to the Brønsted acid site with either a Pd or Pt dopant (Figure 6). These results show that the addition of the Pd and Pt dopant lowers the Gibbs activation/reaction energies for the regeneration of the Brønsted acid site by ∼0.1/∼0.5 and ∼0.1/ ∼0.4 eV, respectively. In addition to these energetic changes due to the addition of the dopant, the initial-state structure for both pathways was altered as well with the reacting hydrogen diffusing across the surface to a 3-fold hollow site ∼0.8 Å further from the dopant (see Figures S22, S26, and S27). For comparison, the dopant effect on the DFT-calculated reaction energies for the partial hydrogenation of phenol and C−O cleavage in the H-DHOx pathway (steps 1 and 2 in Figure 4b) were calculated and found to differ from the Fe(110) results by less than 0.05 eV (structures shown in Figure S3). While these results are qualitatively consistent with vaporphase experiments and show that a potential synergy might 2205

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coverage of surface Brønsted acid sites. However, the main problem with using metallic Fe as a catalyst material experimentally is that in water it will easily oxidize and deactivate. This makes testing the predictions made here difficult. Here we address this difficulty by proposing a series of experimental tests that could be performed which would determine the validity of our predictions. We suggest generating a series of light-off curves for the vapor phase HDO of a model phenolic compound over unsupported Fe catalysts that have been modified so as to be resistant to complete oxidation. The modification could be the addition of a noble metal dopant, such as Pd, which has been previously shown to enhance the oxidation resistance of metallic Fe.40,48,49 Another modification to the system could be the application of a positive external electric field which has been shown to increase the barrier for H2O and OH dissociation over Ni surfaces.50 Such a positive electric field could be generated locally by doping the surface with alkali metals, such as K+, which will generate a positive electric field in its vicinity.51−53 Unsupported Fe catalysts could be synthesized in a manner similar to the work of Hong et al. and then systematically exposed to water.16 Using a surface-sensitive technique such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), vibrational sum frequency generation (SFG) spectroscopy, or X-ray photoelectron spectroscopy (XPS), the coverage of surface hydroxyls resulting from the dosing of water would be carefully determined. Next, a model phenolic compound would be introduced to the Fe catalyst and we would directly observe the fate of the adsorbed phenolic compound as a function of temperature. On the basis of our predictions in this manuscript, there should be a correlation between the coverage of Brønsted acid sites on the Fe catalyst and the light-off temperature of the HDO reaction, i.e., we should expect more facile conversion of the adsorbed phenolic compound in the presence of OH. This would provide significant evidence for our predicted change in pathway for the HDO of phenolic compounds.

exist between noble metal promoters and Brønsted acid sites for the HDO of phenolics, a more thorough investigation is needed.16,40,48 Specifically, rigorous model studies of doped Fe surfaces must be performed that account for the surface distribution and interaction effects of dopants, such as Pd and Pt, and adspecies, such as oxygen, hydrogen, and hydroxyl. With such studies, realistic, catalytically relevant models of dopants and adspecies on Fe surfaces can be developed and used to more fully determine the dopant effect on the pathways for the HDO of phenolics. Such studies are currently beyond the scope of this work. 3.3. Dominant HDO Pathway under Aqueous Conditions. To determine the dominant HDO pathway for phenol over Fe(110) in an aqueous environment, it is useful now to turn to a comparison of the Gibbs energies for each pathway discussed, as shown in Figure 4. For the DHOx pathway, it is clear that this pathway is unlikely to dominate in an aqueous environment despite this being the simplest of the three pathways as the Gibbs energy for the most activated step (step 1 in Figure 4a) is ∼1.6 times larger than that seen for the most activated steps in the H-DHOx and Taut. pathways (step 1 in Figure 4b and steps 2 and 4 in Figure 4c, respectively). This leaves either the H-DHOx or the Taut. pathways as likely candidates for the dominant HDO pathway in an aqueous environment. As noted previously, the H-DHOx and Taut. pathways are identical for all reaction steps besides the initial hydrogenation of phenol. For the H-DHOx pathway, the hydrogenation of phenol (step 1 in Figure 4b) occurs in one step with an overall Gibbs activation and reaction energy of 0.70 and −0.11 eV, respectively. For the Taut. pathway, the metal-assisted tautomerization mechanism was determined to be the most favorable, complicating phenol hydrogenation, as this reaction now requires three elementary steps (steps 1−3 in Figure 4c), where steps 1 and 3 are not activated with Gibbs reaction energies of 0.12 and 0.22 eV and step 2 has a Gibbs activation and reaction energy of 0.62 and 0.16 eV, respectively. To deconvolute the comparison between the H-DHOx and Taut. pathways, the overall Gibbs activation and reaction energy was calculated for the first three steps in the Taut. pathway, with the resulting values being 0.74 and 0.50 eV, respectively. Comparing the overall Gibbs activation and reaction energy for the Taut. pathway to those for the first reaction step in the H-DHOx pathway, it is clear that both the Taut. and H-DHOx pathways have a similar overall Gibbs activation energy for phenol hydrogenation. However, a comparison of the Gibbs reaction energy for these pathways (−0.11 and 0.50 eV for the H-DHOx and Taut. pathways, respectively) shows that the HDHOx pathway is more likely to result in the HDO of phenol, while the Taut. pathway likely favors surface phenol over the HDO products. Therefore, the dominant pathway for phenol HDO over Fe(110) is likely the H-DHOx pathway. This differs significantly from that observed under UHV conditions due to the increased hydrogenation capabilities of Brønsted acid sites as compared to surface hydrogen atoms on Fe(110). 3.4. Perspective. Overall, we have determined that the dominant effect of the aqueous environment on the HDO of phenol on Fe(110) is found in the ability of the water network to shuttle protons to and from the adsorbed phenol and the increased hydrogenation capabilities of Brønsted acid sites as compared to surface hydrogen atoms. These results show that the dominant deoxygenation pathway for phenol on Fe(110) can be altered by changing the surface composition, i.e.,

4. CONCLUSIONS In summary, the major effect of an aqueous phase on the elementary HDO pathway for phenol on Fe(110) is in its ability to enhance hydrogenation reactions. Most notably, the dissociation of water on the oxophilic Fe surface produced Brønsted acid sites, which promotes hydrogenation steps by easily transferring hydrogen to the adsorbed aromatic compound via a proton transfer route. While the dominant HDO pathway was DHOx under UHV on the Fe catalyst surface, the presence of Brønsted acid sites significantly lowered the Gibbs activation and reaction energies for hydrogenation reactions. This resulted in a change in the dominant deoxygenation pathway to H-DHOx in an aqueous environment. Overall, these results suggest that the HDO of phenolics can be enhanced by designing Fe-based catalysts that favor the formation of Brønsted acid sites, i.e., tuning the surface coverage of Brønsted acid sites on the Fe-based catalyst to maximize the HDO of phenolics via surface promoters.

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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/acscatal.7b02576. 2206

DOI: 10.1021/acscatal.7b02576 ACS Catal. 2018, 8, 2200−2208

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DFT and Gibbs activation and reaction energies for each reaction studied; initial, transition, and final state structures for each reaction studied; the initial and final structures for the pure water layer’s AIMD simulation; the Bader charge results for the DFT optimized pure water structure; and the MEPs for each reaction studied here (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone: 509-335-8580. Fax: 509-335-4806. E-mail: js. [email protected]. *E-mail: [email protected]. ORCID

Alyssa J. R. Hensley: 0000-0002-7382-1286 Yong Wang: 0000-0002-8460-7410 Donghai Mei: 0000-0002-0286-4182 Jean-Sabin McEwen: 0000-0003-0931-4869 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE-AC0506OR23100. Once the DOE SCGSR program was completed, the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences under award number DE-SC0014560 primarily supported this work. Additional support from DOE Office of Science BES under DE-FG02-05ER15712 (Y.W.) and DEAC05- RL01830 and FWP-47319 (D.M.) is acknowledged. Our thanks also go to the donors of The American Chemical Society Petroleum Research Fund for partial support of this research. A portion of the computer time for the computational work was spent in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s office of Biological and Environmental Research and located at PNNL. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. DOE.

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DOI: 10.1021/acscatal.7b02576 ACS Catal. 2018, 8, 2200−2208