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Catalyst Site Selection via Control over NonCovalent Interactions in Self-Assembled Monolayers Gaurav Kumar, Chih-Heng Lien, Michael J. Janik, and J. Will Medlin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01074 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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Catalyst Site Selection via Control Over Non-Covalent Interactions in Self-Assembled Monolayers Gaurav Kumar †§, Chih-Heng Lien ‡§, Michael J. Janik*†, J. Will Medlin*‡ †

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA. ‡ Department of Chemical and Biological Engineering, University of Colorado - Boulder, Boulder, CO 80309, USA § These authors contributed equally to this work ABSTRACT: One strategy for controlling selectivity in surface-catalyzed reactions is to precisely control the types of surface sites available for reaction. Here, we show that such control can be achieved on Pd/Al2O3 catalysts modified by alkanethiol selfassembled monolayers (SAMs) by changing the length of the modifier’s alkyl tail. Density functional theory (DFT) calculations show that thiolates with short alkyl chains preferentially bind to undercoordinated Pd step sites, but that adsorption on (111) terrace sites is more favorable at higher chain lengths due to greater stabilization by van der Waals interactions. Linear alkanethiol SAMs with chain lengths ranging from six to eighteen carbon atoms were deposited on Pd/Al2O3 catalysts to probe this predicted effect experimentally. Infrared spectroscopy measurements conducted after CO adsorption confirmed that increases in alkyl chain length resulted in increasing specificity in poisoning of terrace sites. The catalysts were also evaluated for furfuryl alcohol hydrogenation, a structure-sensitive probe reaction. Selectivity to the desired hydrodeoxygenation to methylfuran increased from 60% as chain length increased from six to eighteen carbons, consistent with increasing efficiency for thiolate blocking of Pd terrace sites. DFT models demonstrate that the presence of thiolates strongly suppressed decarbonylation reactions, and that step sites surrounded by thiolates could still be active for hydrodeoxygenation. This work demonstrates that site availability, and thus catalyst selectivity, can be tuned by changing the architecture of SAM precursors. KEYWORDS: thiolate self-assembled monolayers, hydrodeoxygenation, Palladium, van der Waals interaction, DFT+D, biomass INTRODUCTION Controlling reaction selectivity is one of the major objectives of heterogeneous catalysis. For example, as shown in Scheme 1, catalysts for the selective hydrodeoxygenation (HDO) of furfural and furfuryl alcohol must limit the rate of decarbonylation (DC) that produces furan and related products. The rates of C-O bond scission (via HDO) and C-C

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bond scission (via DC) over Pd catalysts are strongly related to the configurations that the reactants adopt on the surface; furfuryl oxygenates that adsorb in flat-lying configurations preferentially undergo DC, while more upright configurations have been associated with HDO1,2. It has also been proposed that DC and HDO are carried out on different types of Pd sites3,4. DC is rapid on particle terraces where the aromatic furan ring can coordinate to the surface, whereas HDO has been associated with corner and edge sites. Thus, one can potentially control selectivity by controlling the relative abundance of terrace and edge sites. Several groups have recently used self-assembled monolayers (SAMs) formed from organic thiolates as selectivity modifiers for supported metal catalysts2–11. For furfural and furfuryl alcohol hydrogenation over Pd/Al2O3 catalysts, application of octadecanethiol (C18) coatings resulted in a dramatic reduction in the DC rate with only a small change in the HDO rate, leading to high HDO selectivities3,4. Diffuse reflectance infrared spectroscopy (DRIFTS) measurements conducted after CO adsorption on the catalysts showed that the SAM selectively eliminated terrace rather than defect sites. This result was surprising, since one would expect thiolate adsorbates to form stronger metal-sulfur bonds at undercoordinated Pd atoms present at defects. The assembly of alkanethiolate monolayers has been studied on a wide variety of metal surfaces, including Pd. Most studies have focused on close-packed single crystals or on thin metal films with surfaces largely consisting of the (111) facet12. On Pd(111), linear alkanethiols are known to assemble in a √3 x √3 R30o structure13. Increases in alkyl tail length have been found to improve the crystallinity of the monolayer, and also to increase the tilt angle of the thiolate with respect to the surface normal13. Although thiolates are used extensively in functionalizing metal nanoparticles, the SAM structure has not been very extensively investigated on small nanoparticles relevant for catalysis. Most studies of thiolate structure have been conducted for Au nanoparticles, and it has been found that smaller Au nanoparticles have higher saturation thiolate coverages12. These higher coverages have been attributed to a combination of increased affinity of thiolates for undercoordinated Au sites and to curvature effects that decrease crowding at the outer surface of the organic layer. Density functional theory (DFT) methods have been previously applied to investigate SAM structures on low index facets of late transition metals14–19. Though these studies have helped resolve or corroborated SAM structures, difficulties in representing chain-to-chain dispersion interactions and in considering surface defects limit the ability of DFT to resolve SAM structures on metal nanoparticles. For example, DFT was used to investigate alkanethiol binding on Au (111) as a function of the alkyl chain length20. For this study, GGA and BLYP functionals were used, which underestimate dispersion interactions between chains. This underestimation resulted in an under-prediction of binding strength at higher chain lengths; in fact, little change in binding energy was observed on increasing the thiolate chain length. By comparing


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PW91 density functional and MP2 energies with experimentally observed thiol binding energies on Au (111), the non-bonded vdW chain interactions have been shown to be responsible for the surface ordering of long chain SAMs14,21.

Scheme 1. Furfural alcohol reaction pathway on Palladium catalysts

In this contribution, we have used a combination of experimental and theoretical techniques to consider how thiolate chain length impacts furfuryl alcohol HDO/DC selectivity. Density functional theory (DFT) calculations, including dispersion corrections (DFT+D), were used to determine the variation in SAM adsorption strength between flat (111), and stepped (211) and (221) Pd surfaces, and suggest that longer alkyl chains will form well-ordered SAMs on terraces leaving steps open for selective HDO. Experimental characterization and evaluation of catalysts with different thiolate chain lengths were used to validate predicted trends, collectively providing illustration of the mechanism by which SAMs can alter catalytic selectivity. RESULTS AND DISCUSSION DFT Analysis of Thiolate Binding Energies and Facet Preferences. Figure 1a displays the computed binding strength of thiolates with various chain lengths for different coverages on Pd (111). For the highest coverage of 1/2 ML (Figure 1a, inset), the binding energy decreases with increasing chain length due to strong repulsion between chains at this coverage. For all lower coverages, the binding energy of the thiolate increases with increasing chain length due to higher van der Waals attraction between the alkyl tails. At the lowest coverage considered (1/9 ML), the binding strength only increases slightly with chain length. At intermediate (1/6 ML – 1/3 ML) coverages, however, the thiolates form ordered overlayers with a binding strength that substantially increases with chain


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length. For C4 or longer alkyl chains, there is a noticeable preference to form a (√3 × √3) R30o (1/3 ML) coverage. LEED and STM studies have also previously reported the 1/3 ML adsorption of alkanethiolates over the Pd (111) surface22,23, in agreement with these DFT+D results. Though Figure 1 shows adsorption energies per thiolate, normalization by surface area across the various surface coverages would only accentuate the preference of the high coverage (√3 × √3) R30o SAM.

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Figure 1. Alkanethiolate binding energy as a function of alkyl chain length on a) Pd (111) surfaces at various coverages ( 1/2 ML, 1/3 ML, 1/4 ML, 1/6 ML and 1/9 ML), and b) Pd (111) and Pd (211) surfaces at various coverages ( 111-1/3 ML, 1111/4 ML, 211-1/3 ML, 211-1/2 ML). For Pd (111) surfaces, binding energy is maximum at 1/3 ML coverage for long chains; further increase in coverage leads to less favorable binding (see inset in figure a). Figure 1b compares the adsorption energy of the thiolates at (211) steps and (111) terrace sites. For the (211) step, we calculated the binding energy of thiolates at 1/3 ML, 1/2 ML and 1 ML. For the (211) step, the binding per thiolate is strongest for 1/2 ML coverage, defined as 1 thiolate adsorbed per 2 Pd atoms along the step ridge. At 1/3 ML and ½ ML on the step, the binding strength increased with chain length; however, at 1 ML, similar to the 1/2 ML on (111) terrace, the binding strength at (211) steps decreased with chain length due to steric effects (Table S3). For thiolate adsorption on the (211) surface, we consider adsorption on the step region only. Co-adsorption on the terrace and step is considered for the (221) surface below. As expected, the S-surface interaction is stronger for under-coordinated Pd atoms present at step sites, as indicated by the stronger adsorption energy of short-chain thiolates on (211) versus (111) surfaces. However, as the thiolate chain length increases, the inter-chain vdW interactions begin to dictate the preferred adsorption configuration rather than the Pd-S interaction, leading to preferential binding of the thiolates over the terrace sites. For alkyl chains of C8 or longer, the binding energy per thiolate to the Pd


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(111) surface (at 1/3 ML) is stronger than to the Pd (211) surface (at 1/2 ML step atom coverage). As the Pd particles used experimentally are approximately 4 nm in diameter, there will be significantly more (111) surface exposure than step sites. The larger number of Pd (111) sites together with their stronger thiolate binding (for C8 or longer) suggest long-chain thiolates will preferentially form SAMs on the terrace sites. Shorter chains will not demonstrate such a clear preference. The structures of C6-thiolate covered Pd (111) and Pd (211) surface at various coverages are illustrated in Table S1. Though these results suggest particle coating with long-chain thiolates will block (111) terrace sites, further analysis is necessary to consider whether thiolates may simultaneously coat (111) terraces and step atoms. We modeled C6-thiolate binding on a Pd (221) surface, consisting of 9 terrace and 3 step atoms. Figure 2 illustrates three models of the (221) surfaces coated with C6 thiolate. In Figures 2a and 2b, the terrace is covered with a 1/3 ML thiolate layer, alternatively terminating with a thiolate at the step ridge (a) or step base (b). Both of these structures are stable minima, with (b) slightly energetically preferred by 0.15 eV. Here, the average binding energy to the structure in Figure 2b is 3.99 eV, which is stronger than the (111) 1/3 ML adsorption. The (221) step surface allows surface atom restructuring leading to the inter-atom distance varying between 2.73 Å to 2.89 Å instead of a constant 2.79 Å on a clean (111) terrace surface. This surface reconstruction leads to an increase in the binding energy for the (221) surface. The structures in Figures 2 (a) and (b) leave an open region on which catalytic furfuryl alcohol conversion may occur, as discussed below. In Figure 2(c), both the step ridge and base sites are occupied. Such a structure generates significant repulsion such that a DFT minimum could not be located. We conclude that a SAM layer would preferentially form on the (111) terraces, provided the alkyl chain is of significant length, and that open breaks in the SAM structure would occur in the region about step sites. Shorter chain length thiolates will show less preference to form ordered structures on terraces, and will likely contain a mixture of step and terrace adsorbed thiolates due to the stronger Pd-S binding at the steps.


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Figure 2. Side and top views of C6 alkane-thiolate self-assembled monolayers over the Pd (221) surface. Panels (a) and (b) show optimized structures with the terrace covered with a 1/3 ML thiolate layer terminating at the step ridge (a) or step base (b). Panel (c) shows both the step ridge and step base with adsorbed thiolates, leading to significant steric repulsion such that a local minimum energy structure could not be located. The black squares represent regions of open area, and black lines delineate the step ridge Pd atoms. Experimental Reaction Studies. Previous studies suggested C18-induced furfural/furfuryl alcohol HDO selectivity resulted from preferential blocking of terrace sites3,4. The DFT results of the previous section can be taken to predict that this preferential blocking will be more effective with longer-chain thiolates, and that shorter chains will clearly discriminating between step and terrace sites and therefore not having as strong an impact on selectivity. Figure 1b suggests a rough approximation of the ranges of chain length over which an impact should arise, as chain lengths C7 and shorter show stronger binding at the step edge whereas longer chains show stronger binding in a (√3 × √3) R30o 1/3 ML layer on terrace sites. We used C6, C10, C14, and C18 alkyl thiol coated particles to investigate the trends in selectivity with chain length. Reactor experiments were conducted at differential conversion in a continuous flow reactor. Conversions were held approximately constant by varying catalyst weights for the different types of coatings. Addition of a C6 thiol modifier was found to slightly improve methyl-furan (MF) selectivity from 5% to >15%. As the tail length was increased from C10 to C14 to C18, MF selectivity also improved monotonically, as shown in Figure 3. These selectivity increases were accompanied by a decrease in


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selectivity to the decarbonylation products tetrahydrofuran (THF) and furan as well as the overall reaction rate. These results are consistent with a mechanism by which sites active for decarbonylation are selectively blocked by the SAM and show that furfuryl alcohol HDO selectivity can be manipulated by thiolate chain length. 0.7

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Figure 3. Furfuryl alcohol selectivity and turnover frequency over uncoated and alkanethiolate-coated Pd/Al2O3 at 413K. The conversion of fed furfuryl alcohol was 4% ± 2% and the mass balance was above 92% in all the cases. To compare to previous studies, we note that the studies reported in Figure 3 were conducted at a temperature of 140 °C. This temperature was selected to decrease the possibility of SAM decomposition, particularly for the shorter-chain thiols. However, as reported previously, the activation energy for methyl furan production has been found to be much higher on the C18-coated catalyst than the uncoated catalyst; in fact, at a temperature of 190 °C, the rate of methyl furan production is nearly identical for a C18coated and uncoated catalyst3,4. Surface Coating Characterization. We conducted several types of characterization to elucidate reasons for the reactivity trends. Elemental analysis results (Figure S1) revealed that, prior to reaction, all samples had similar loadings of Pd on Al2O3 and that the sulfur coverage of C6 monolayers on Pd/Al2O3 was slightly higher than the others. After reaction, the ratios of sulfur to Pd had no obvious change except for the C6-coated catalyst, where the sulfur loading decreased by approximately 30%. This decrease is attributed to partial desorption of physisorbed or more weakly chemisorbed thiols. Due to


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this difference between pre- and post-reaction samples, catalysts were characterized before and after reaction throughout this work. Sachtler et al. have reported that bridged CO prefers to occupy high coordination terrace sites, while linear CO adsorption is relatively stable on the lower coordinated Pd atoms (vertices and edges)24. Therefore, although part of bridging CO is still observed on Pd edges, the absorbance intensity ratio of linear CO to multisite CO in IR spectra has been broadly used to study the relative abundance of various surface sites25–29. This approach has previously demonstrated that the intensity ratio increased with smaller Pd particle size and higher Pd particle dispersion. In this study, CO DRIFTS was used to determine the relative availability of linear and bridging/hollow sites on the catalyst’s surface. The CO stretching peaks on the catalyst surface under different CO pressures, both before and after reaction, are shown in Figures S2 and S3. Figure 4 shows the spectra at a single CO pressure of 3000 mTorr before and after furfuryl alcohol exposure. Also shown in Figure 4 is the ratio of the integrated peak area for linear to multisite CO adsorption. Linearly adsorbed CO is assigned to the peak between 2000 and 2100 cm-1, whereas bridging and hollow sites are associated with the broad peak below 2000 cm-1. The bridging (~1950 cm-1) and hollow sites (~1840 cm-1) are predominantly located on particle terraces, whereas linear sites are occupied on edge/defect sites (~2040 cm-1), particle corners and/or Pd(111) at pressures above 1.2 torr (~2090 cm-1)29–32. Our CO vibrational calculations using DFT resulted in similar stretching frequencies on uncoated Pd for atop (2053 cm-1 on terrace, 2040 cm-1 on step), bridge (1888 cm-1 on step) and hollow (1809 cm-1) binding. On the uncoated catalyst, CO primarily adsorbed in bridging and hollow sites, with a smaller contribution from linear sites. Modification of the catalyst with C6 monolayers resulted in a sharp change in the spectrum, where the ratio of bridging/hollow versus linear sites became more equivalent. Increases in the tail length lowered the proportion of bridging and three-fold adsorption and shifted the linear CO adsorption peak. It was noticeable that the difference between C18 and the other coatings was larger at low CO pressures (Fig. S4). We attribute this behavior to the fact that the linear peak can be due to a combination of adsorption on terrace and edge sites, with the former tending to be filled at higher pressures. This is consistent with a high proportion of available sites on the C18-coated catalyst being at particle edges, as discussed above. Additionally, a redshift of linear CO stretching with chain length was observed on both pre- and post-reaction catalysts, shown in Figure 4(a) and (b). The redshift in linear CO stretching could be consistent with the longer-chain thiolates preferentially blocking CO linear adsorption on terraces and exposing edge/defect sites, though this could also be partly due to fewer dipole coupling interactions30. For the uncoated surface, the linear CO stretching peak redshifted from 2071 to 2056 cm-1 after reaction. We assign this redshift to the presence of coadsorbates produced during reaction on the catalyst terraces and less linear adsorption on catalyst terraces. The C-H stretching modes detected after exposure


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to furfuryl alcohol in Figure S5 (b) indicates the presence of carbonaceous deposits formed on the catalyst surface.

Figure 4. (a) CO features for uncoated and SAM-coated Pd/Al2O3 in DRIFTS spectra after CO adsorption (a) before and (b) after reaction at 3000 mtorr CO pressure. (c) The area ratio of two CO adsorption features in DRIFTS for uncoated and SAM-coated Pd/Al2O3 after reaction. Error bars for all cases indicate the standard deviation from 4 repeated measurements.


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In addition to the CO DRIFTS experiments, our DFT calculations also show that, on an uncoated Pd surface, CO binding is favored on the bridge and hollow sites. CO binds strongly on Pd (111) hollow sites and Pd (211) bridge sites with binding energies of 2.22 eV and 2.17 eV, respectively. In contrast, CO binds weakly on atop sites (1.67 eV for (111) and 1.75 eV for (211)). This agrees with the smaller linear (atop) CO stretching peak in the DRIFTS spectra on the uncoated Pd surface. However, on a C6 √3 × √3 SAM coated Pd (111) surface, CO binding on the hollow site is diminished to a binding energy of 0.46 eV. The bridge and atop binding on the (211) step is favored with binding energies of 1.86 eV and 1.76 eV, respectively. This is consistent with the results of prior DFT calculations33–37, and with the relative increase in the linear CO stretching peak with respect to the bridge/three-fold peak, and the observed area ratio as defined in Figure 4c. We also used DRIFTS to analyze the structure of the monolayers. The C-H stretching region in DRIFTS before and after reaction is shown in Figure S5. Before exposure, increases in chain length resulted in an increased intensity in the symmetric and asymmetric CH2 stretching regions near 2854 and 2924 cm-1, respectively. After exposure, peaks associated with reaction intermediates and spectator species appeared, making analysis of changes in monolayer density difficult. However, the C18-coated catalyst was clearly still dominated by the thiolate coating, whereas the shortest chain lengths indicate considerable coverage of other organic species. Additionally, the CH2 asymmetric stretching peak had an obvious shift from 2924 to 2918 cm-1, which indicated the thiol monolayers on the Pd surface formed a more ordered structure through reorganization. The rearrangement of SAM during thermal annealing has been previously studied on gold and platinum surfaces38,39. In addition to thermal activation, the presence of hydrogen also facilitates thiolate mobility and SAMs reorganization on Pt and Au surfaces, presumably through formation of more weakly adsorbed transient species such as thiols39,40. In this study, for investigating the reorganization of thiolate monolayers on the Pd surface, in situ DRIFT spectra for C6, C10, C14 and C18-modified catalysts were collected after annealing at different temperatures under hydrogen exposure, as shown in Figure 5 and S6. These figures indicate that the asymmetric CH2 stretching peak shifted to lower frequency after heating in hydrogen, especially for long-chain thiolate monolayers. The C6 asymmetric CH2 stretching at 2923 cm-1 shifted to 2920 cm-1 at 60°C, while the same peak from C18 shifted from 2923 to 2915 cm-1 at 140°C, indicating extensive reorganization of thiolate SAMs on Pd surface in the reducing environment to produce a well-organized structure. This reorganization effect was clearly greater for the longer alkyl tail, as observed previously for Pt catalysts39, and consistent with the reorganization being driven largely by the thiol tail. However, some role of metal atom rearrangement to facilitate SAM organization cannot be ruled out.


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Figure 5. Solid and dash lines are the spectra before and after cooling down (a) C6 and (b) C18-coated Pd/Al2O3. DFT Studies of Reaction Pathways. To get mechanistic insights into the hydrodeoxygenation/decarbonylation of furfuryl alcohol (FOL) to methyl-furan (MF) / furan (FN), we used DFT to study the thermodynamic stabilities of the various reactive surface intermediates on (111) terrace, (221) step and SAM-coated (221) surfaces. Unlike previous mechanistic studies on Pd (111) surfaces that mapped all possible reaction pathways1,41, herein, our goal is to demonstrate the significant differences in the reaction scheme on Pd terrace and steps with a SAM coating. The presence of the SAM significantly alters the binding strength of FOL and other intermediates to the surface. On Pd (111), FOL binds with the aromatic ring nearly parallel to the surface. The furan ring occupies a hollow site with its H atoms and O atom pointing away from the surface showing loss of sp2 character. The calculated relative energy of bound FOL is -1.80 eV, which is consistent with a previous DFT study on furanic chemistry on Pd(111)41. On Pd (221), the furanic ring sits flat at the bridge site with a binding energy of -2.12 eV. In order to mimic the surface coverage of SAMs while retaining computational tractability, we crowded the (221) surface with C1 thiolates in the configuration of Figure 2b, leaving regions about the steps open for FOL conversion. In addition to providing site selectivity, the SAM coating prevents flat binding of the furanic


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intermediates. In this regard, a C1 thiolate coating on the Pd (221) terrace sites is sufficient to mimic the SAM effect of inhibiting flat binding on the (221) step. The C1 thiolate matches the height of the vertically adsorbed reaction intermediates, and little thiolate restructuring occurs due to the adsorbates, such that using longer alkyl chains would have little impact on the calculated reaction energetics. On the C1 SAM coated Pd (221) surface, the inability of the furanic ring to bind flat on the surface leads to a significantly weakened binding strength of -0.72 eV. Adsorption of FOL on SAM coated Pd (111) was not considered, as the (√3 × √3) R30o 1/3 ML coating does not leave any openings large enough to place a furyl ring structure. Figure 6 represents the reaction energy diagram for FOL conversion to a) FN, and b) MF. FOL conversion to FN proceeds via O-H and C-H scissions to an adsorbed furfural intermediate. On Pd (111), initial C-H scission is thermodynamically favored over O-H scission, whereas, on Pd (221) step and SAM coated (221), O-H scission yields a relatively stable intermediate. This intermediate is then decarbonylated by a C-C scission to form co-adsorbed furyl and CO. The furyl ring hydrogenates to form FN, which desorbs along with CO. Alternatively, FOL can undergo hydrodeoxygenation (Figure 6(b)) by hydrogenation and removal of the alcoholic oxygen in the form of water. The remaining (C4H3O)CH2- hydrogenates to form MF. The structures of all adsorbed intermediates are illustrated in Table S2.


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Figure 6. Reaction energy diagram for furfuryl alcohol a) decarbonylation to furan, and b) hydrodeoxygenation to methyl furan, on ( ) Pd (111) terrace, ( ) Pd (221) step, and a C1 thiolate SAM coated ( ) Pd (221) surface. The image shows the lack of binding sites for decarbonylated CO. The furyl ring and the C1 thiolates cover all possible binding sites for CO. The black line represents the reconstructed step edge. On uncoated Pd (111) and (221), the FOL conversion energetic path is similar, with the (221) steps exhibiting higher stability of surface intermediates. Following previous results1,41, on Pd (111), we assume that FOL decomposition proceeds via C-H scission followed by the O-H scission. Although experimentally alcohol decomposition has been found to proceed through initial O-H scission on Pd(111), this has been attributed to Hbonding interactions between alcohol dimers that are not probed here42. On Pd (221) surfaces, however, FOL decomposition involves O-H scission followed by C-H scission. Other than this, there are no distinct conformational or reaction thermodynamic differences that could lead to major differences in selectivity between the two types of sites. A quick examination of Figure 6 (a) and (b) might suggest both uncoated surfaces


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are selective to HDO due to a more favorable reaction path; however, DC can produce gas phase H2 in most steps that would entropically drive the forward reaction. Others have considered the selectivity differences between the facets for furfural conversion on Pd43, and we point the reader to these studies for more complete consideration, while also noting that the lack of consideration of coverage effects is a potential factor. The noncoated surfaces could have a high coverage of H* or hydrocarbon surface intermediates that alter the binding strength of the ring structures. The SAM-coated Pd (221) surface shows a significant difference in the stability of surface intermediates compared to the uncoated surfaces ((111) and (221)). All surface intermediates, with the exception of C4H3O*, are less stable due to the presence of the SAM and the less favorable, more vertical, adsorption configurations it induces. As shown in Figure 6(b), the HDO reaction path remains favorable on the SAM coated surface, with a cascade of downhill reaction energies. When compared to the uncoated Pd (111) and (221) surfaces, C-C scission (decarbonylation) is highly unfavorable on the SAM coated Pd (221) surface due to the inability to provide binding sites to both furyl and CO species. As shown in Figure 6(a), the reaction energy for decarbonylation is very high; in fact, furyl was observed to undergo ring-opening during optimization to allow room for co-adsorption. The FOL to MF reaction path does not experience such an effect, which leads to higher predicted selectivity towards the formation of MF. The difference in the reaction energetics of the decarbonylation pathway between uncoated and thiolate-coated Pd surface is so significant that it allows for a mechanistic explanation for the SAM-induced selectivity away from furan without the need for computationally expensive activation barrier calculations. The reaction rate on the C18 thiol coated surface is quite low (< 5 ms-1), probably too low to be of commercial relevance. However, the decreased overall rate is almost entirely due to a dramatic reduction in the rates of formation of all non-selective reactions; as shown in Figure 7, the MF rate decreases by a much smaller amount. Thus, optimization of the underlying catalyst material or reaction conditions to achieve higher inherent MF rates could yield a viable catalyst.


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Turnover Frequency ( 1 / s)

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-1

Furan THF MF 10

10

10

-2

-3

-4

Uncoat ed

C6

C1 0

C1 4

C1 8

Figure 7: Product turnover frequency over uncoated and alkanethiolate-coated Pd/Al2O3. On SAM deposition, while the turnover frequency of the non-selective furan decreases by orders of magnitude, the ToF of the selective product Methyl-Furan only decreases to a quarter of the ToF on uncoated catalyst. Discussion. By increasing the length of the alkyl tail on thiolate modifiers, the preference for blocking of terrace sites by the SAM was increased. Selective poisoning of particular types of sites is often used in catalyst design. Most other approaches to specific site blocking, including those involving Pd catalysts, have employed additional metals that will hypothetically have different affinities for different surface facets44. Monte-Carlo and DFT calculations have demonstrated that the second metal with lower surface energy (Cu, Ag and Au) preferentially occupies the low coordination sites on Pd particle surface, such as high index planes, corners and edges45–47. In addition to simulation results, the strong tendency of Cu atoms to locate at low-coordinated surface sites was supported by low energy ion scattering spectroscopy and FTIR spectroscopy after CO adsorption on the surface; Cu addition was found to cause Pd catalyst selectivity improvement for the hydrogenation of 1,3-butadiene to butene and the dehydrogenation of ethanol to ethanal at the cost of activity48,49. However, the strong occupancy preference of co-metal atoms on Pd surface is often limited to low coordination sites. Compared with bimetallic catalysts, C18 monolayers deposited on Pd surfaces have the opposite tendency in selective site occupancy, with preference for terrace sites. In this study, we see that this selective poisoning of terraces was enabled by the presence of alkyl groups on the surface-bound atoms. Dispersion interactions between the long alkyl chains provide a driving force needed to form an ordered structure on terrace sites, whereas short chains preferred to occupy low-


ACS Catalysis

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coordinated site due to the strong covalent bond. In this way, we see that self-assembling organic modifiers offer a unique platform for modification of specific reaction sites. It is useful to briefly discuss the relation of this work to prior studies of SAM-coated catalysts and of furfuryl surface chemistry. Although previous studies have shown that C18 thiols appeared to selectively poison Pd terrace sites4,50, the reasons for this were not clear, since there is a general expectation that thiols will bind more strongly to undercoordinated atoms. Here, by varying the tail length, we have shown that the long alkyl tail is essential to selective terrace site poisoning, since it provides the van der Waals forces that drive assembly. Moreover, previous single crystal studies of furfuryl alcohol surface chemistry have suggested decarbonylation requires flat-lying adsorbates, suggesting that selective terrace poisoning via site blocking could improve selectivity1,3. Here, we show for the first time, with DFT calculations, that this intuitive understanding can be rigorously demonstrated. Our results may have implications beyond thiol SAMs, because they suggest that non-covalent interactions between adsorbates can change the distribution of available sites. These interactions could be particularly important wherever adsorbates have signigicant van der Waals interactions, or where the non-covalent interactions have a stronger nature (like H-bonding). CONCLUSIONS The selectivity of Pd/Al2O3 catalysts for furfural hydrodeoxygenation was controlled by adjusting the pre-adsorbed thiol chain length. DFT calculations indicated that short thiols prefer to occupy Pd edge sites because of the stronger surface-sulfur bond at undercoordinated Pd atoms. However, longer thiolates prefer to adsorb on terrace sites due to non-covalent interactions between neighboring chains that become increasingly important with longer chain lengths. The calculation results were supported by DRIFT spectra collected after CO adsorption on the catalyst surface. With longer thiolate chain lengths, the number of contiguous Pd terrace sites was decreased, which shut down decarbonylation and ring hydrogenation of furfuryl alcohol and improved the selectivity to methyl furan. DFT calculations showed that while thiol adsorption near terraces poisoned the decarbonylation reaction, the favorability for hydrodeoxygenation was not strongly affected by the presence of sulfur. Thus, the tail function of organic monolayers can serve as an effective handle to block specific non-selective sites on supported metal catalysts. EXPERIMENTAL SECTION Materials Preparation. 1-Hexanethiol (95%), 1-dodecanethiol (98%), 1tetradecanethiol (98%), 1-hexdecanethiol (95%), 1-octadecanethiol (98%) and 5 wt% Pd/Al2O3 were obtained from Sigma-Aldrich. No CH stretching was observed on the uncoated catalyst surface by DRIFTS and there was no performance difference after


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ACS Catalysis

oxidizing and reducing the catalyst, so the commercial catalyst was directly used without pretreatment in our study. 1-Decanethiol (96%) and the ultra-high-purity CO were respectively obtained from Alfa Aesar and Specialty Gases of America. The ultra-highpurity H2 and He for the reactor system were obtained from Airgas. SAM Deposition. For preparing the SAMs-modified Pd/Al2O3, 75 mg of Pd/Al2O3 was immersed in a 40 mL, 2 mM thiolate ethanol solution for at least 12 h. Due to low solubility in ethanol, the same amount of Pd/Al2O3 was added in 80 mL, 0.5 mM C18 ethanol solution. After decanting the supernatant solution, the sample was rinsed with ethanol for more than 2 h and dried under air at 318 K for 20 min before use. The catalyst was directly used without reduction prior to coating due to no obvious difference found after reduction. Catalytic Experiments. To reach the same approximate conversion for comparison between different catalysts, the amount of catalyst was varied. Catalysts were evaluated under atmospheric pressure and 413 K with 3.8 mmol/min mixed furfuryl alcohol stream (furfuryl alcohol : H2 : He = 0.0015 : 0.29 : 0.71). An Agilent Technologies 7890A gas chromatograph equipped with a flame ionization detector and a 30 m x 0.320 mm Agilent HP-5 capillary column were used to analyze feed and product samples (split ratio 25:1, column flow rate 2 mL/min; the oven temperature was 313 K for 7 min, and then ramped to 388 K at 30 K/min). Infrared Spectroscopy. Fourier transform infrared analysis (Thermal Scientific Nicolet 6700 FT-IR with a Harrick reaction chamber) was performed with 100 scans at 4 cm-1 resolution. In CO DRIFTS experiments, samples were placed in a closed and gastight chamber that was evacuated to 100 mtorr for 1 h. Subsequently CO was introduced into the cell and the measurement was taken after 2 min to allow equilibration. Computational Details. DFT calculations were carried out using the Vienna Ab Initio Simulation Package (VASP), an ab initio total energy and molecular dynamics program51,52. The projector augmented-wave (PAW) method was used to represent the ion-core electron interactions53. A Pd FCC bulk lattice constant of 3.953 Å was used. Plane wave basis sets were used with an energy cutoff of 400 eV. To minimize spurious inter-slab interactions, a vacuum space of 30 Å was used. Surfaces were relaxed until forces on all the atoms were minimized to 0.05 eV Å-1. Relaxation to 0.02 eV Å-1 showed no significant structural or energy change in selected surface structures. Thiolate adsorption on both flat and stepped surfaces was considered. The terrace surface was modeled with a Pd (111) metal slab of five atomic layers with the bottom two layers frozen. Various unit cell sizes were used to model different thiolate coverages. The Brillouin zone was sampled with a 4 × 4 × 1 , 6 × 6 × 1 , 6 × 6 × 1 , 6 × 6 × 1 , and 6 × 12 × 1 k-point grid for 3 × 3 , 2 × 3 , 2 × 2 , √3 × √3 , and 2 × 1 unit cells respectively54. Step surfaces were modeled with Pd (211) and Pd (221) metal slabs of five and four atomic layers, respectively, with the bottom two layers frozen. The Brillouin


ACS Catalysis

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zone was sampled with a 4 × 4 × 1, 6 × 6 × 1, and 4 × 4 × 1 k-point grid for Pd (211) 3 × 3, 3 × 2, 2 × 2, and Pd (221) 3 × 4 unit cells, respectively. The effect of non-local dispersion interactions in the adsorption of large organic structures is not well captured by typical GGA functionals. vdW density functional (vdW-DF) calculations demonstrated the importance of van der Waals interactions for thiophene adsorption on Cu (110)55. Using experimental results on benzene, thiophene and pyridine adsorption on Au (111) and Cu (111), Tonigold and Gross56 compared standard GGA-DFT functionals with DFT+D methods. Their results showed that standard DFT underestimates the adsorption energy significantly, while the DFT+D method showed good agreement with experimental observations. Herein, we perform all the surface calculations using the Perdew, Burke, and Ernzerhof exchange correlation functional with the dispersion correction, PBE-D3 with Becke-Jonson damping57–59. The binding energy for thiolate species i was calculated as ∆ = + − , where Eslab+i, Eslab and Ei are the total electronic energies of the metal-adsorbate system, the metal slab and the adsorbed species in the gas phase respectively. With this definition, a more positive ΔEads indicates stronger adsorbate binding. For small chain thiolates, at all coverages, we calculate the binding energy at atop, bridge and hollow sites on the Pd (111) terrace, and atop and bridge sites on the Pd (211) surface. The site having the strongest binding for short chain thiolates was then used to calculate the binding strength for long chain thiolates. Reaction energetics for furfuryl alcohol HDO and DC are determined by calculating the energy of all surface intermediates and gas phase species relative to gasphase furfuryl alcohol and hydrogen. The gas phase energies of all molecular species are calculated in VASP using a large unit cell, and the reported relative energies are DFT energies without entropic or temperature corrections applied.

ASSOCIATED CONTENT Supporting Information Additional DFT calculation results, elemental analysis results, and DRIFTS figures described in the text are shown in Supporting Information. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation for funding this research through grants CBET-1160040 and DMREF Grant #1436206. This material is available free of charge via the Internet at http://pubs.acs.org.


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