Adsorption and Reactivity of Cellulosic Aldoses on Transition Metals

Article pubs.acs.org/JPCC

Adsorption and Reactivity of Cellulosic Aldoses on Transition Metals Quang Thang Trinh, Bhadravathi Krishnamurthy Chethana, and Samir H. Mushrif* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore S Supporting Information *

ABSTRACT: The adsorption of cellulosic biomass derived aldoses on heterogeneous catalysts is the first and governing step in their conversion to platform chemicals. However, there is a discrepancy between theoretical and experimental observations in determining the most stable configuration of adsorbed aldoses on transition metal surfaces: conventional density functional theory (DFT) calculations with the Perdew, Burke, and Ernzerhof (PBE) functional (Perdew et al. Phys. Rev. Lett. 1996, 77, 3865) predicted η2(C,O) as the most stable structure, whereas experiments could only detect the η1(O) configuration. Our calculations reveal that the revised PBE (Hammer et al. Phys. Rev. B 1999, 59, 7134) functional can correctly predict the most stable adsorption configuration of aldoses on transition metals. Additionally, for C6 aldoses like glucose, which exist mostly in the ring form, the open-chain adsorption configuration is a result of the transition-metal-catalyzed ring-opening process. Adsorbed glucose in the cyclic form undergoes deprotonation and ring opening, and the resultant open-chain configuration closely resembles the η1(O) structure, thus explaining why the η1(O) configuration was detected in experiments. Entropy calculations also demonstrate that the transformation from η1(O) to η2(C,O) is thermodynamically not favorable even at higher temperatures. Finally, based on the most stable η1(O) adsorbed configuration, the catalytic activity of Pd and Pt surfaces toward the decomposition, oxidation, and hydrogenation reactions is evaluated.

1. INTRODUCTION There are strong incentives to produce chemicals and fuels from renewable biomass resources to reduce the reliance on diminishing fossil resources.1−3 Lignocellulosic biomass is expected to be the most promising renewable feedstock, owing to its wide abundance, high energy content, and sustainability.1,3 The transformation of cellulose, the main component of lignocellulosic biomass, into chemicals and fuels is essential for the utilization of lignocellulosic biomass as a feedstock. Under high-temperature conditions in liquid-phase catalytic processes, cellulose is usually hydrolyzed to glucose, which consequently can be transformed into many high-value added chemicals.3−6 Heterogeneous catalysts, in particularly transition metals, have proven to be active catalysts for the selective conversion of glucose. Additionally, they are easy to separate and recycle, have good stability, and are noncorrosive and highly selective.5−11 Understanding the adsorption and bonding of biomass molecules, typically of glucose, on the surface of these catalysts is crucial to predict and explain their catalytic activity and preferred reaction pathways.7,9 Due to the extremely low vapor pressure of solid glucose at room temperature,12,13 surface science studies of its adsorption and reaction on transition metal surfaces are challenging. Complications arise in using the molecular beam dosing system to introduce the sample into the ultrahigh vacuum (UHV) chamber.14,15 However, recently, Vohs and co-workers managed to overcome these problems and successfully conducted the surface science study of the bonding configurations and decomposition of glucose and smaller © XXXX American Chemical Society

aldoses (glyceraldehyde and glycolaldehyde) on various transition metal surfaces.14−17 The combination of temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) revealed that glucose and its surrogates (glycolaldehyde and glyceraldehyde) coordinate with Pd(111) and Pt(111) surfaces in the η1(O) configuration via the carbonyl group (cf. Scheme 1), where the aldose adsorbs on metal surfaces only via the oxygen lone pair electrons.14,15 However, contrary to experiments, density functional theory (DFT) calculations predicted that the η2(C,O) configuration (cf. Scheme 1), where both the carbonyl Scheme 1. Adsorption Structures of Aldoses on Transition Metal Surfaces

Received: April 13, 2015 Revised: July 3, 2015

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DOI: 10.1021/acs.jpcc.5b03534 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

thermodynamic and kinetic properties),33adsorption energy,33−35 and energy of spin complex systems.36 We limited our study on the adsorbed configurations of glycolaldehyde, glyceraldehyde, and glucose on Pd(111) and Pt(111) surfaces due to the availability of experimental data on these systems.14,15 For glycolaldehyde and glyceraldehyde, which are chain-form aldoses, their adsorptions are directly processed from the gas phase to the adsorbed configuration on metal surfaces. However, for glucose, which exits majorly in the cyclic form in the gas phase and for which the ring opening in the gas phase is very challenging,22 the adsorption of the chain form of glucose has to be processed via the metal-catalyzed ringopening of cyclic glucose. Therefore, the adsorption of ring glucose on transition metal surfaces and the ring-opening mechanism were also studied. Finally, we study the surface chemistry of glucose on these transition metal surfaces.

carbon and the oxygen atom of the aldose interact with the surface metal atoms, is the most stable adsorbed structure on both Pd(111) and Pt(111) surfaces (34 and 28 kJ/mol more stable for glucose adsorbed on Pd(111) and Pt(111), respectively).14,15 The inconsistency between theoretical and experimental observations in describing the adsorption structure of aldehydes on transition metal surfaces was also observed in saturated aldehydes, e.g., acetaldehyde adsorbed on the Pt(111) surface. DFT reported that adsorbed acetaldehyde prefers the η2(C,O) configuration on the Pt(111) surface,18,19 but experimental surface science studies revealed the dominance of the η1(O) configuration.20 The difference in the adsorbed conformations of the reactants is believed to affect activation barriers and selectivities. For example, adsorption configuration of ethanol governs the selectivity of Rh(111) toward C−H versus O−H activation.21 Hence, to be able to computationally screen and test different transition metals for a variety of biomass reactions, it is of the utmost importance to be able to predict the adsorption of glucose correctly using popular DFT methods. Additionally, most of the theoretical investigations of glucose on transition metal surfaces have ignored the adsorption of glucose in the ring form and the catalytic opening of the ring. In the gas phase and aqueous phase, glucose exists mainly in the cyclic form (called glucopyranose) (higher than 99%), while the proportion of open-chain form is very limited.13 The ring opening of gas-phase glucose can only occur at a very high temperature, e.g., at 500 °C with activation energy of 247 kJ/ mol.22 In water, the ring opening of glucose is also challenged by the high activation enthalpy of 193 kJ/mol.23 The ring opening of cyclic glucose can be facilitated by homogeneous catalysts, e.g., Lewis acid CrCl324 and Brønsted acid,25 or heterogeneous catalysts like Sn-β zeolite,26,27 with the ringopening activation barrier in the range of 42−126 kJ/mol. On Pd(111) and Pt(111) surfaces, HREELS studies have identified the intense signal of ν(CO) stretch around 1715 cm−1, corresponding to the formyl group for adsorbed glucose.14,15 This detection is also evidence for the ring opening of glucose upon adsorption since there is no such ν(CO) stretch in the IR spectrum of cyclic glucose.28,29 The strong adsorption of the chain form in the η1(O) configuration was suggested to be the driving force for the ring opening; however, its mechanism is not yet fully understood.14,15 It has to be noted that the openchain adsorbed configuration of glucose is a result of the ringopening process, and hence the ring-opening step could be a factor in deciding the dominant structure of adsorbed glucose. Therefore, investigations into the ring-opening process of glucose are important to comprehensively evaluate its subsequent surface chemistry. In this study, we revisit the theoretical calculations of the adsorbed configuration of glucose on transition metal surfaces and explain the discrepancy that exists in the present literature. We compare the stability of adsorbed η1(O) and η2(C,O) configurations of glucose on transition metal surfaces by using the revised Perdew−Burke−Enzerhof (RPBE) functional30 and the original PBE functional.31 The RPBE functional, developed by Hammer and Norskov,30 was reported to give superior chemisorption energies for atomic and molecular bonding to surfaces and has been very popular for catalysis applications. Multiple benchmark studies were conducted to show that RPBE is better than conventional GGA functionals (PBE, PW91, ...) in describing atomization energies of small molecules,32 chemical reactions on transition metals (both

2. COMPUTATIONAL METHODS DFT calculations have been performed using the ab initio totalenergy and molecular-dynamics program VASP (Vienna ab initio simulation program) developed at the Fakultät für Physik of the Universität Wien.37,38 All the DFT simulations implemented a plane-wave basis set with a cutoff kinetic energy of 450 eV and the projector-augmented wave (PAW) method. Two functionals were evaluated in the calculations, the conventional Perdew−Burke−Ernzerhof functional (PBE)31 and the revised Perdew−Burke−Ernzerhof functional (RPBE).30 Pd(111) and Pt(111) surfaces were modeled using periodic 4 × 4 unit cells (corresponding to 1/16 monolayer (ML) coverage of adsorbates) with five-layer slabs of the (111) facet of FCC crystal of Pd and Pt, respectively. A vacuum of 20 Å above the top layer was used to separate the repeated slab to minimize the interaction. Optimized lattice constants given by PBE and RPBE functional for Pd are 3.94 and 3.97 Å (experimentally 3.88 Å),39 respectively, and for Pt are 3.97 and 4.00 Å (experimentally 3.92 Å),39 respectively (Figure S1, Supporting Information). The bottom two layers were constrained at the bulk positions, while the top three layers and the adsorbates were fully relaxed and optimized. The detailed description of the modeled unit cell is presented in Figure S2 of the Supporting Information. Fixing the two bottom layers is a reasonable choice to save computational time since complete relaxation of all the atoms in the unit cell changes the calculated binding energy by less than 1 kJ/mol and the geometry by 7.5 × 10−4 Å (Tables T1 and T2, Supporting Information). The Brillouin zone was sampled with a 3 × 3 × 1Monkhorst−Pack grid for all calculations. Geometries were relaxed until the energy changes by less than 0.1 kJ/mol. Transition states were located using the nudged elastic band (NEB) method, and frequency calculations confirmed the nature of the transition states. NEB calculations were performed for all reactant and product configurations within 10 kJ/mol of the most stable coadsorbed configuration within the unit cell, and only the lowest-energy transition states are included. Convergence tests to optimally choose the structural model parameters include the number of layers and vacuum size and for simulation parameters include the cutoff energy and Monkhorst−Pack grid. These are presented in the Supporting Information (Table T3 and Figure S3). Increasing the slab thickness to 7 layers, the interslab spacing to 25 Å, or the grid to (5 × 5 × 1) only changes the calculated binding energies by less than 5 kJ/mol. Thermodynamic properties including entropy, enthalpy correction, and zero-point energy B


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Figure 1. η1(O) and η2(C,O) adsorption configurations on transition metal surfaces for glycolaldehyde (a,b), glyceraldehyde (c,d), and glucose (e,f). Big blue balls represent the metal atoms (Pd in this figure), and small gray, red, and white balls represent carbon, oxygen, and hydrogen atoms, respectively.

Table 1. Energy (kJ/mol) of the η1(O) Configuration of Glycolaldehyde, Glyceraldehyde, and Glucose on Pd(111) and Pt(111) Surfaces, Relative to the Energy of the η2(C,O) Configuration on the Same Metal Surface and the Equilibrium Distribution between These Two Configurations, Calculated Using PBE and RPBE Functionals metal

functional

structure

glycolaldeyde

glyceraldehyde

Pd

PBE

η2(C,O) η1(O) keq η2(C,O) η1(O) keq η2(C,O) η1(O) keq η2(C,O) η1(O) keq

0 +26.1 4.0 0 −7.1 4.4 0 +24.5 3.3 0 −4.2 3.6

0 +28.8 2.5 × 10−12 0 +4.8 0.19 0 +27.1 2.3 × 10−11 0 −0.9 61.9

RPBE

Pt

PBE

RPBE

Nη1(O) Nη2(C,O)

× 104

× 10−9

× 104

0 +26.9 2.4 0 −18.8 1.4 0 +13.3 1.3 0 −27.2 3.2

× 10−11

× 1010

× 10−2

× 1014

ΔG is the computed free energy difference between the two configurations.

of different adsorbed configurations are calculated from statistical thermodynamics.40,41 To calculate the harmonic vibrational frequencies, the higher break condition for the electronic self-consistency loop of 10−6 eV was used. Since the binding energies of aldoses on transition metals are within the range of 30−70 kJ/mol and the experimental HREELS study on the adsorption of glucose on transition metals was conducted only up to 350 K,14,15 we treat the adsorbed aldoses as harmonic oscillators and neglect the contribution of adsorbate translation and/or rotation, relative to the surface in the entropy calculations.41 More justification is provided at the end of Section 3.2. The equilibrium distributions of the η2(C,O) and η1(O) configurations at 115 K (similar to that in the experimental HREELs study) are calculated using the Boltzmann distribution keq =

× 10−11

glucose

3. RESULTS AND DISCUSSION 3.1. Adsorption of Glucose and Its Surrogates on Transition Metals. We examined the adsorption of glucose and smaller aldoses (glycolaldehyde, glyceraldehyde) on Pd(111) and Pt(111) surfaces. Figure 1 shows different adsorption configurations (η1(O) and η2(C,O)) of glycolaldehyde, glyceraldehyde, and glucose on Pd(111) surfaces (calculations were also performed on Pt(111) surfaces). It should be noted that not all the possible structural conformations of a large molecule, such as glucose, could be investigated due to the complexity of the structure and huge computation cost. To compare the relative stability of η1(O) and η2(C,O) configurations, the energies of the adsorbed structures are reported in Table 1, using the RPBE and PBE functionals. As can be seen from Table 1, the most stable structure described by the PBE functional is the η2(C,O) configuration, where the aldoses adsorb on the surface via the carbon and the

= e−ΔG / RT

where Nη2(C,O) and Nη1(O) are the number of moles of adsorbed aldoses in the η2(C,O) and η1(O) configuration, respectively. C


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Figure 2. Different adsorption configurations of the cyclic-form glucose (glucopyranose) on Pd(111). Glucose coordinates via the hydroxyl groups of C1 (a), C2 (b), both C2 +C3 (c), both C3 + C4 (d), C4 (e), and C6 (f). Inserted numbers are energies in kJ/mol, relative to the energy of the structure in (a). Carbon atoms are numbered in (a). Energies for glucose on Pt(111) are given in in parentheses. Color coding for glucose atoms is the same as in Figure 1.

adding an additional molecule of glyceraldehyde close to the existing preadsorbed molecule in the η1(O) configuration is −56.2 kJ/mol (highly favorable) (refer to Table T4, Supporting Information). Consequently, two molecules of glyceraldehyde coadsorbed close to each other in the η1(O) configuration are 59.4 kJ/mol more stable than those in the η 2 (C,O) configuration (Figure S4, Supporting Information). The detailed discussion on intermolecular interaction of the adsorbed molecules and their structures is presented in the Supporting Information. These results demonstrate the importance of the exchangecorrelation functional in describing the adsorption of glucose and its surrogates on transition metal surfaces and further reveal that the RPBE functional predicts configurations which are in agreement with the experimental data.14,15 The RPBE functional only differs from the PBE functional in the mathematical form of the exchange energy enhancement factor but helps to optimize the value of the reduced density gradient. This was stated as the reason that gave rise to its improvement in predicting the chemisorption energy.30 3.2. Adsorption of Closed-Ring Glucose and the RingOpening Mechanism of Glucose on Transition Metals. For reasons cited before, the ring opening of glucose on Pd(111) and Pt(111), using the RPBE functional, was also investigated. The issue of ring opening does not arise for C2 and C3 aldoses but is important for C6 sugars like glucose, which exist mainly in its ring form.13 The adsorption of the cyclic-form glucose on Pd(111) is shown in Figure 2. Note that, similar to the adsorption of the open-chain structure, not all possible structural conformers of glucose in the cyclic form could be explored. Due to the saturated structure of the ring, cyclic glucose can only coordinate with the metal surface via its hydroxyl groups. Glucose adsorption on the metal surface, via different hydroxyl groups, and the relative energies are shown in Figure 2(a−f). The most stable adsorption configuration is the one where glucose coordinates via its hydroxyl group attached to carbon C1 in the ring (Figure 2a). This structure is around 10 kJ/mol more stable than other configurations, where glucose coordinates via a single hydroxyl group of other carbon atoms in the ring (Figure 2b, 2e, and 2f). There are two additional structures where glucose coordinates with the surface

oxygen in the formyl group (Figure 1a,c,e). This is consistent with the computational results reported by McManus et al.14,15 but contrary to the experimental report,14,15 where only the η1(O) configuration was detected (Figure 1b,d,f). The equilibrium distribution calculated using the PBE functional shows that, at 115 K, as in the HREELs experiments,14 adsorbed aldoses exist majorly in the η2(C,O) configuration (higher than 99.999%), as presented in Table 1. However, upon implementing the RPBE functional, the energetics and the stability trend reversed for Pt (cf. Table 1). The most stable structure predicted by RPBE was the η1(O) configuration for all three aldoses on both metals, except for glyceraldehyde on Pd(111). The RPBE predictions are consistent with the experimental observation that only the η1(O) configuration was detected during the adsorption of aldoses on Pd(111) and Pt(111) surfaces.14−16 The calculated equilibrium distribution using the RPBE functional showed that the η1(O) configuration is dominant, and although both configurations are present to at least some extent, the quantity of the bidentate sigma η2(C,O) configuration might be below the experimental detection limits and could not be identified by the equipment (for the adsorption of glycoaldehyde and glucose). Although the η1(O) configuration of the adsorbed glyceraldehyde on Pd(111) is less stable than the η2(C,O) configuration, with the implementation of RPBE, the energy difference has dropped from +28.8 to +4.8 kJ/mol and significantly changes the equilibrium distribution between the η1(O) and η2(C,O) configuration from 2.5 × 10−12 to 0.19 (Table 1). It has to be noted that, in our study, DFT calculations for the adsorption of glyceraldehyde on Pd(111) were only done at a very low coverage of 1/16 ML. Actually saturated adsorption coverages were used in all HREELs experiments conducted by Vohs and collarborators.14 At higher coverages, the interaction between the adsorbed molecules and lesser availability of adsorption sites (the η2(C,O) configuration requires three surface sites, while the η1(O) configuration only occupies one surface site) could possibly make the η1(O) configuration even more favorable.42,43 Indeed, the computed differential adsorption energy, using the RPBE functional, for adding one more molecule of glyceraldehyde close to the existing preadsorbed molecule in the η2(C,O) configuration is +13.4 kJ/mol (not favorable), while the adsorption energy for D


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diffusion barrier for this process was reported to be only 5 kJ/ mol for Pt(111)44,45 and 14 kJ/mol for Pd(111).46 The chemisorbed hydrogen needs to come in the vicinity of the ring oxygen atom of the glucose molecule for the ring opening to be initiated. Figures 4a−c and 4d−f show the initial, transition, and final states of the glucose ring opening on Pt(111) and Pd(111), respectively. The activation energy barrier for the ring opening on Pt(111) is only 6 kJ/mol, whereas on Pd(111), it is 61 kJ/mol (Figure 4b,e). The most stable configuration for the chemisorbed hydrogen on Pd(111) is a hollow site, and it needs to come to the top site in the transition state (Figure 4e) which requires additional energy. Hence, the relative energy of the TS on Pd (111) is much higher than that on Pt(111) (on Pt the most stable site for the chemisorbed hydrogen is the top site only, as needed to form the transition state). It is important to note that the product, i.e., the open-chain glucose, is in the η1(O) configuration on both metals. However, it can be argued that the η1(O) configuration could be transformed into the η2(C,O) configuration at higher temperatures. To investigate the possibility of the transformation between the η1(O) and η2(C,O) configurations of glucose, we calculated the entropy of both the structures on the Pd(111) surface. Since the numbers of atoms in those structures are the same, the entropy difference can provide insights into the driving force for the transformation. The entropy computed at 298 K for the η1(O) configuration is 301 J/K mol, while that for the η2(C,O) configuration is 262 J/K mol. The lower entropy of the η2(C,O) configuration is expected since it is a more constrained structure with both C and O in the carbonyl group and the hydroxyl group attached to the α-carbon in the backbone bound to the surface (Figure 1e), whereas only the formyl oxygen is bound to the surface in the η1(O) configuration (Figure 1f). Since the change in the entropy for the η1(O) to η2(C,O) transformation is negative, the net entropic contribution (−TΔS) to the free energy will be positive and will increase with the increase in temperature. Thus, it can be said that the η1(O) to η2(C,O) transformation is not thermodynamically favorable, even at higher temperatures. Hence, the product from the glucose ring opening, i.e., the η1(O) configuration, is unlikely to go to the η2(C,O) configuration, citing another possible reason for the dominance of the experimentally detected η1(O) configuration of adsorbed glucose on transition metal surfaces. It is also worthy to note

via two hydroxyl groups (Figure 2c and 2d), and their stability is almost the same as the structure in Figure 2a. This trend in the relative energies of different adsorption configurations is the same for the Pt(111) surface (Figure 2). It is interesting to note that the most stable adsorption configuration of the ring glucose (via C1 hydroxyl group) is the one that also initiates the ring-opening process, as will be seen later. For the ring opening of glucose, the hydrogen from the hydroxyl group attached to the C1 atom in the ring needs to be transferred to the ring oxygen.23−26 Therefore, the first step to be studied is the deprotonation of the C1 hydroxyl coordinated to the metal surface. The transition state (TS) and activation energies for this step are shown in Figure 3. The activation

Figure 3. Transition states for the activation of the O−H bond in ring glucose by Pt(111) surface and activation energies (a) for the hydroxyl group attached to C1 and (b) for the hydroxyl group attached to C2. Activation energies for Pd(111) are given in parentheses. Color coding for glucose atoms is the same as in Figure 1.

energy barrier for the deprotonation of the hydroxyl group attached to C1 on Pt(111) is 78 kJ/mol. On Pd(111), the activation barrier for this reaction is 95 kJ/mol. We have also evaluated the deprotonation of other hydroxyl groups attached to different carbon atoms in the glucose ring, and the corresponding activation barriers are higher by at least 10 kJ/ mol than that of the deprotonation of the C1 hydroxyl group. This is not surprising since the adsorption of glucose via the C1 hydroxyl group is the strongest. Figure 3b show the TS of O− H dissociation on Pt(111) from the hydroxyl group attached to the C2 carbon in the glucose ring (structure in Figure 2c), with the activation barrier of 89 kJ/mol. After the deprotonation, the generated chemisorbed hydrogen atom can easily diffuse on transition metal surfaces. The

Figure 4. (a) Initial state, (b) transition state, and (c) final state of the glucose ring opening on the Pt(111) surface. (d) Initial state, (e) transition state, and (f) final state of the glucose ring opening on the Pd(111) surface. Activation energy barriers and bond distances are also shown. Color coding of glucose atoms is the same as in Figure 1. E


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The Journal of Physical Chemistry C that at higher temperatures adsorbed molecules need to be treated as a 2D fluid on the surface.41 The onset temperature for this phenomenon correlates with the binding energy, and it would be lower for weaker binding species. For example, Baylock et al. have studied various adsorbates on Ni(111), during the steam reforming of methane at 1073 K.41 They divided the adsorbates into two categories: strongly bound adsorbates and weakly bound adsorbates. It is suggested that for the strongly bound adsorbates the diffusion barrier is large relative to the thermal energy of the adsorbate (kBT), and the entropy calculations could be performed using the standard harmonic oscillator partition function. For the weakly bound adsorbates, the translation and/or rotation barrier is much smaller than kBT, and hence it is necessary to treat the adsorbates using the free translation and/or free rotation statistical thermodynamic model.41 However, the calculated binding energies of glucose on the Pd(111) surface in our study are 69.2 and 43.4 kJ/mol for the η2(C,O) and η1(O) configurations, respectively. These values are higher than the binding energy of 20 kJ/mol for a smaller adsorbed aldehyde, studied by Baylock et al.41 The computed diffusion barrier of HCHO in that study is 10 kJ/mol, relatively larger than kBT (at T = 1073 K), and the adsorption of HCHO was not considered falling within the weakly bound cases. Besides, the temperature in the experimental study of glucose adsorption on transitional metals was only up to 350 K;14,15 therefore, treating the adsorbed glucose as a harmonic oscillator in our procedure is a reasonable assumption. 3.3. Surface Chemistry of the Adsorbed Glucose Molecule on Transition Metals. With an appropriate functional and correct adsorption configuration, to evaluate the surface chemistry of glucose on transition metals, we investigate the activation of glucose on Pt(111) and Pd(111) surfaces via the deprotonation of an α-hydroxyl group (Figure 5a) and the formyl C−H activation (Figure 5b). Formyl C−H activation has been reported to be the initial and key step in glucose oxidation toward gluconic acid on the transition metal catalyst.47−50 We did not consider the C−C and C−O scissions since they were reported to occur at much higher temperatures.14,15 Besides that, since adsorbed glucose on the transition metal exposes the carbonyl functional group, we also investigate the additive reaction into the carbonyl group via the hydrogenation reaction (Figure 5c,d). Activation barriers for the deprotonation of the hydroxyl group attached to an α-carbon in the backbone are 181 and 174 kJ/mol on Pt(111) and Pd(111), respectively. The transition state is shown in Figure 5a. Due to the high energy barrier, the activation of the α-hydroxyl group will only occur at very high temperatures. This is consistent with the proposed formation of the α-oxo-η2 surface intermediates during the HREELS study of glucose on transition metals at high temperatures.14,15 In contrast to the deprotonation of an α-hydroxyl group, the activation of the glucose formyl C−H bond is feasible with the activation barrier of 79 kJ/mol on Pt(111) (Figure 5b). This is consistent with the reported barrier of 64 kJ/mol to activate the formyl C−H bond in adsorbed glycolaldehyde on the Pt(111) surface (used as a probe molecule to study surface chemistry for more complex biomass derivatives) by Salciccioli et al.51 The formyl C−H dissociation is even easier on Pd(111) with an activation barrier of only 51 kJ/mol. This is in excellent agreement with the experimentally detected ν(Pd−C) stretch at 302 cm−1 in the HREEL spectrum of glucose on Pd(111) at 290 K.14 Glycolaldehyde has been used as a probe molecule to

Figure 5. Transition states and activation barriers for reactions on the Pt(111) surface: (a) glucose C−H activation; (b) glucose O−H activation; (c) addition of H into carbonyl C atom, and (d) addition of H into carbonyl O atom (d). The activation energies for similar reactions on Pd(111) are in parentheses. Color coding of glucose atoms is the same as in Figure 1.

study the reactivity of glucose on Pt(111) in a comprehensive study by Chen and co-workers.52 In that study, HREELs data also showed that glycolaldehyde adsorbed initially on the Pt(111) surface in the η1(O) configuration, and after heating to 175 K, the characteristic signal for ν(CO) becomes broader and shifted lower. It might be argued that instead of the conversion from η1(O) to the η2(C,O) configuration upon increase in temperature (which is entropically not favorable) the disappearance of the ν(CO) mode could be a result of the activation of C−H formyl, and the acetyl products are bound to the surface via the di-σ η(C,O) bond (Figure 5b). The observations that (i) the activation of the formyl C−H bond is the key step in the oxidation of glucose to gluconic acid,47−50 (ii) Pd and Pt can catalyze the formyl C−H dissociation in glucose, and (iii) both Pt and Pd are also good catalysts for O2 activation53,54 could collectively explain why Pd and Pt are very good catalysts for the selective oxidation of glucose to gluconic acid using O2 as oxidizing agent.55−61 Finally, to evaluate the additive reaction on the carbonyl group of glucose, hydrogenation reaction is studied. Hydrogenation of carbon and oxygen atoms in the carbonyl group is considered, and the transition states are shown in Figures 5c and 5d. In general, both the hydrogenation activation barriers into glucose carbonyl on Pd(111) are higher than those on the Pt(111) surface. Hydrogenation at carbon and oxygen atoms of the carbonyl group is certainly feasible on the Pt(111) surface, with the hydrogenation at the oxygen atom slightly more preferred (Figure 5c). It is also consistent with the study by F


The Journal of Physical Chemistry C Salciccioli et al.,51 wherein the insertion of hydrogen into the adsorbed glycolaldehyde on Pt(111) is reported to be favorable. On Pd(111), the hydrogenation onto the oxygen atom is easier with an activation barrier of 78 kJ/mol (Figure 5d), but the hydrogenation on the carbon atom is more challenging with a higher activation barrier of 108 kJ/mol. These results reflect the good catalytic hydrogenation behavior of Pt and are also consistent with the high activity of Pt in the conversion of glucose to sorbitol.5,6,61−65

ACKNOWLEDGMENTS

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REFERENCES

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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.5b03534. Detail of slab modeling, convergence test, influence of coverage toward the adsorption configuration of glyceraldehyde, and equilibrium distribution of η2(C,O) and η1(O) configuration for adsorbed aldoses (PDF)

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Financial support from Nanyang Technological University (NTU), Singapore, is acknowledged. BKC acknowledges the financial support provided by The Ministry of Education (MOE), Singapore, under the Academic Research Fund (AcRF) Tier-1 grant (Grant No. RGT36/13). We also thank the anonymous reviewers for their comments and suggestions, which helped us improve the manuscript.

4. CONCLUSIONS In conclusion, we demonstrate that in order to correctly predict stable adsorption configurations of glucose and its surrogates on transition metal surfaces the revised Perdew−Burke− Enzerhof (RPBE) functional should be used over the conventional PBE functional. In the present study, the RPBE functional predicted the η1(O) configuration to be the most stable one on Pd (111) and Pt (111), in perfect agreement with experimental results. Since glucose and similar 5/6 carbon containing aldoses exist in closed-ring form in solutions and in the gas phase, the transition-metal-catalyzed ring opening of glucose was investigated. Ring glucose coordinated with the transition metal surface via the hydroxyl group attached to carbon C1 in the ring and underwent the deprotonation prior to the ring opening. The generated chemisorbed H diffused toward the ring oxygen and assisted the C−O bond scission in the ring with an activation barrier of 6 kJ/mol on Pt(111) and 61 kJ/mol on Pd(111). The resultant open-chain adsorbed configuration after the ring opening is η1(O), further supporting the dominance of this configuration, as observed in the experiments. The transformation from η1(O) to η2(C,O) is not thermodynamically favorable even at higher temperatures. Finally, the surface catalytic activities of Pd(111) and Pt(111) toward different glucose conversion reactions are evaluated. The dissociation of the α-hydroxyl group of glucose is challenging and can only occur at high temperatures. However, the activation of the formyl C−H bond is easy on both Pd and Pt, suggesting that Pd and Pt are potential catalysts for the oxidation of glucose toward gluconic acid. The hydrogenation of both carbon and oxygen atoms in the glucose’ carbonyl group on Pt(111) is also feasible, while it is more difficult on Pd(111). This can explain the higher activity of Pt in the hydrogenation of glucose to sorbitol.

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AUTHOR INFORMATION

Corresponding Author

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

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. G


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Adsorption and Reactivity of Cellulosic Aldoses on Transition Metals

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