Electrooxidation of Glycerol on Gold in Acidic Medium: A Combined

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electrooxidation of Glycerol on Gold in Acidic Medium: A Combined Experimental and DFT Study Mikael Valter, Michael Busch, Bjorn Wickman, Henrik Grönbeck, Jonas Baltrusaitis, and Anders Hellman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02685 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Electrooxidation of Glycerol on Gold in Acidic Medium: a Combined Experimental and DFT Study Mikael Valter,† Michael Busch,‡ Björn Wickman,‡ Henrik Grönbeck,† Jonas Baltrusaitis,¶ and Anders Hellman∗,† †Department of Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ‡Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ¶Department of Chemical and Biomolecular Engineering, Lehigh University, B336 Iacocca Hall, 111 Research Drive, Bethlehem, PA 18015, USA E-mail: [email protected] Phone: +46 31 772 56 11

Abstract Glycerol is a byproduct of biodiesel production and an abundant feedstock for synthesis of high-value chemicals. A promising approach for valorization of glycerol is electrooxidation on gold. In this work, we investigate electrooxidation of glycerol on gold in acidic media using cyclic voltammetry and density functional theory calculations. Experimentally, we observe activity for electrooxidation above a potential of 0.5 V vs. the reversible hydrogen electrode (RHE). A Pourbaix diagram is calculated to evaluate the surface coverage under reaction conditions, indicating that the surface is free from adsorbates at the measured onset potential. Computationally, we find that the onset

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potentials for partial dehydrogenation of glycerol to dihydroxyacetone, 2,3-dihydroxy2-propenal and glyceraldehyde are 0.39, 0.39, and 0.60 V vs. RHE, respectively, while complete dehydrogenation to carbon monoxide requires 0.50 V vs. RHE. Our theoretical and experimental findings are in agreement and show the possibility of using gold as a catalyst for production of hydrogen and other valuable chemicals from glycerol.

Introduction Glycerol is a byproduct originating from transesterification of vegetable oils and animal fat into biodiesel. 1 The global production of biodiesel during 2015 was 26 million t and is projected to increase to 35 million t by 2025, 2,3 corresponding to 3 and 4 million t of glycerol, 4 respectively. Glycerol can be used as a feedstock in biological conversion systems for production of valuable chemicals, such as acrolein, butanol, and syngas. 5 Another possibility to utilize glycerol is in conventional catalytic processes, 6 for instance, dehydrogenation 7 or electrooxidation 8–10 to produce H2 and derivatives such as dihydroxyacetone (DHA), glyceraldehyde or glyceric acid. H2 has many applications, such as direct fuel applications, 11,12 or upgrading biodiesel via hydrogenation of unsaturated fatty methyl esters, 13 while other derivatives are of use in cosmetic industry, 10 and production of bioplastics. 14 Glycerol electrooxidation has been studied experimentally on transition metals, including platinum and gold. 15–19 One one hand, gold has been shown to be an excellent electrocatalyst for glycerol oxidation in alkaline solution. 15,17 It displays an order of magnitude higher activity as compared to platinum, which has been attributed to the more anodic potential of gold oxidation. 17,20 On the other hand, gold is reported to have no activity in acidic solution, 15–17 owing to the lack of proton acceptors such as hydroxyl ions in solution (OH− ), as well as surface-bonded hydroxo adsorbates (*OH). 17,20 The presence of Brønsted bases shifts the equilibrium of

− + R−OH − )− −* − R−O + H

2


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to the right, which promotes glycerol oxidation, as R−O – is more reactive than R−OH. 20 In this work, we show experimentally that glycerol electrooxidation on gold has a low, but detectable, activity in acidic solution. In particular, glycerol oxidation is detected on the negative going scan, which has previously been observed for glycerol oxidation on platinum and platinum-ruthenium alloys in acidic media. 17,21,22 The activity for glycerol oxidation on gold in acidic media raises the question of the underlying reaction mechanism without initial glycerol deprotonation in solution. In our computational approach, we model the surface as Au(111), since this facet has the lowest surface energy, 23 and thus should be abundant. Our computed Pourbaix diagram indicates that the reaction occurs without presence of hydroxo adsorbates. Using the theoretical normal hydrogen electrode 24 (NHE), we study the thermodynamics of glycerol dehydrogenation on bare Au(111) to find possible reaction paths and corresponding theoretical onset potentials.

Method Cyclic voltammetry was performed on a polycrystalline gold wire in a solution of 0.1 M HClO4 (Sigma-Aldrich 70 %, ACS reagent grade, diluted with milli-Q water) and 0.5 M H2 SO4 (Sigma-Aldrich 95–98 %, ACS reagent grade, diluted with milli-Q water) and 0.5 M glycerol (Sigma-Aldrich, 99.5 % purity). An Ag/AgCl reference electrode (B3420+) from SI Analytics was used, and a graphite rod (Sigma-Aldrich 99.995 % purity) was used as counter electrode. The scan rate was 100 mV/s. Nitrogen was bubbled to purge atmospheric oxygen 20 minutes before and during the experiments. The experiments were conducted with an SP-300 potentiostat from BioLogic Instruments. Underpotential deposition of copper (UPD) was carried out in in 1 mM CuSO4 (SigmaAldrich, 99.995 % purity) and 0.5 M H2 SO4 . The potential was scanned down to 0.2 V vs. the reversible hydrogen electrode (RHE) in order to deposit one monolayer of Cu and the obtained charge was used to calculate the electrochemical area, as described by Rouya et

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al. 25 Density functional calculations were performed using VASP, 26–28 with the optB86b-vdW exchange-correlation functional. 29–31 This functional was chosen as it gives accurate adsorption energies on gold. 32 The projector augmented wave method 33 was used to model the interaction between the valence electrons and the core. The Kohn-Sham orbitals were obtained using a plane wave basis set with 450 eV as cutoff energy and a Gaussian smearing of 0.05 eV was applied to the Fermi level discontinuity. Calculations on gas phase radicals were performed using spin polarization. The Au(111) surface was modelled as a 4-layer p(3 × 3) supercell, for the dehydrogenation reaction, and a 4-layer p(2 × 2) supercell, for the Pourbaix diagram. Solvent effects were ignored owing to the complexity and lack of fundamental understanding of the double layer. 34 This is a common approximation that has previously been used successfully to understand electrochemical reactions. 35,36 The periodic surface slabs were separated by a vacuum of 20 Å. The p(3 × 3) and p(2 × 2) supercells were sampled in a Monkhorst–Pack grid 37 with (6,6,1) and (8,8,1) k-points, respectively. The gas phase species were computed in a (20 × 20 × 20) Å cell using only the gamma point. The Quasi-Newton method was used for structural relaxations with total residual force of 0.02 eV/Å as convergence criterion. Vibrational modes were calculated by diagonalization of the partial Hessian matrix. Only adsorbates were considered and the forces were computed by means of the central difference approximation with a displacement of 0.05 Å. Gibbs free energy was calculated at standard temperature and pressure. The adsorbates were treated in the harmonic approximation with only vibrational degrees of freedom. As the pV -term was considered negligible, Gibbs free energy of the adsorbate was approximated as Helmholtz free energy, which was calculated at 298 K with vibrational entropy and zeropoint correction. Gibbs free energy was calculated for gas phase species by treating them as ideal gases. Electrochemical reactions were modelled using the theoretical NHE by Rossmeisl and

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Nørskov, which assumes coupled electron-proton transfer. 24 Gibbs free energy is calculated as

∆G = GC3 Hx O∗3 − G∗ − Gglycerol(g) +

8−x GH2 (g) − (8 − x)kB T ln 10 × pH + (8 − x)eUNHE (1) 2

where a star indicates an adsorption site, kB is the Boltzmann constant, x the number of hydrogen atoms in the species, e the elementary charge and UNHE the potential vs. NHE. Using RHE, the pH dependence is included in the potential term for coupled electron-proton transfer, leaving us with

∆G = GC3 Hx O∗3 − G∗ − Gglycerol(g) +

8−x GH2 (g) + (8 − x)eURHE 2

(2)

Adsorption energy for a species is defined as the energy of the adsorbed species relative to the species in gas phase.

Results and discussion Cyclic voltammetry was carried out to investigate the activity of glycerol oxidation in acidic media, shown in Figure 1. In order to avoid oxygen evolution, which is reported to start at 1.9 V, 38 the potential was scanned between 0.1 V and 1.65 V vs. RHE. The surface oxidation occurs from 1.3–1.4 V on the anodic (positive going) scan, and the corresponding surface reduction can be seen clearly at 1.2–1.3 V on the cathodic (negative going) scan. All currents and charges below are defined as the difference of the data with and without glycerol. Glycerol oxidation in HClO4 (Figure 1a) takes place in three regions: On the anodic scan (i) before the surface reaction, starting at ∼0.5 V, (ii) after surface oxidation, and (iii) on the cathodic scan around 0.6 V. The maximum current in the metallic surface region is 0.08 mA/cm2 and in the oxidized surface region 0.34 mA/cm2 . The total oxidative charge over a cycle is 1.10 mC/cm2 . Based on these observations, we conclude that gold has an

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a)

ii

iii

i

b)

ii

i iii

Figure 1: Cyclic voltammogram of a polycrystalline gold electrode in (a) 0.1 M HClO4 and (b) 0.1 M H2 SO4 with and without 0.5 M glycerol (blue solid and red dash-dot, respectively). Scan rate 100 mV/s. Regions of interest for glycerol oxidation are marked with i, ii, and iii, respectively.

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measurable activity for glycerol oxidation in HClO4 . In contrast to HClO4 , no activity for glycerol oxidation is observed in 0.5 M H2 SO4 (Figure 1b) before surface oxidation on the anodic scan (i). After surface oxidation (ii), as well as on the negative going scan (iii), some activity is observed. The activity is lower in H2 SO4 compared to HClO4 ; the maximum current in the metallic surface region is 0.03 mA/cm2 and in the oxidized surface region 0.27 mA/cm2 . The total oxidative charge over a cycle was 0.16 mC/cm2 , implying that glycerol oxidation also has activity in H2 SO4 . A possible explanation for the different electrolyte activities is the competition between the electrolyte and glycerol. Both perchlorate and sulfate ions are expected to compete with the relatively weakly bound electrically neutral glycerol for free adsorption sites. In the case of perchorate, still sufficiently many free sites are available to facilitate the experimentally observed glycerol oxidation. In contrast, sulfate is known to bind stronger than perchlorate to the anode, 39,40 resulting in blocking of the surface in agreement with the lack of observed activity. Kwon et al. 17 and Beden et al. 15 investigated glycerol oxidation on gold in H2 SO4 and reported no activity. A possible explanation for this is that a lower glycerol concentration (0.1 M) was used in these studies. 15,17 Additionally, in Ref. [ 17 ], the glycerol oxidation on the cathodic scan would not have been detected since anodic linear sweep voltammetry was used. Our voltammograms are qualitatively similar to reported results in HClO4 and H2 SO4 on platinum and Pt/Ru, 16,21,22 with respect to the glycerol oxidation on the cathodic scan at 0.6 V vs. RHE (Figure 1, iii). Prior to this peak, we observe a small reduction pre-peak at 0.8 V, similar to reports from Kahyaoglu et al., 16 which might be of importance for the main peak. The underlying reason for the oxidation on the cathodic scan is not known. Possible explanations include removal of electrolyte anions from the surface, temporary formation of active sites owing to reduction of the surface, or access to active sites with higher reactivity, e.g. kinks, steps, and edges. However, these speculations are outside the scope of this study. Knowing that there is activity for glycerol oxidation on gold, we decided to study the

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a ½ ML *OH

b ¼ ML *O

c ½ ML *O

d ¼ ML *O ½ ML *OH

Au(111) a or b

c or d

Figure 2: Surface Pourbaix diagram of Au(111). The surface is bare until 1.0 V vs. RHE, where (a) 1/2 ML *OH and (b) 1/4 ML *O are formed. At 1.5 V, (c) 1/2 ML *O and (d) a mixture of 1/4 ML *O and 1/2 ML *OH are formed.

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mechanism computationally. Choosing Au(111) as model surface, we constructed a surface Pourbaix diagram 41 (Figure 2) to evaluate the surface coverage of adsorbed hydroxo (*OH) and oxo (*O) at reaction conditions. The configuration space was mapped by calculating 1/4, 1/2, 3/4, and 1 monolayer (ML) of *OH and *O at fcc, bcc, bridge and top sites. The surface starts being covered by *OH and *O at 1.0 V vs. RHE. In this region, (a) 1/2 ML *OH (bridge) and (b) 1/4 ML *O (fcc) differ energetically by only 0.002 eV/Å2 and are thus expected to coexist. At 1.5 V, (c) 1/2 ML *O (fcc) and (d) a mixture of 1/4 ML *O (fcc) and 1/2 ML *OH (top, bridge) are formed with an energy difference of 0.003 eV/Å2 . The other configurations are less stable and are therefore not included in the diagram. The initial gold oxidation at 1.0 V is in agreement with previous computational results. 38 At higher potentials, thin bulk hydroxides and oxides are reported to form. 38 According to our experiments as well as in literature, gold oxidation starts at 1.2–1.3 V on the anodic scan, while reduction ends at 1.0–1.1 V on the cathodic scan. 38,42,43 This is in good agreement with our theoretical results. It has been reported that adsorbed CO enhances co-adsorption of OH groups at as low potentials as 0.6 V vs. RHE 44 and that it enhances electrooxidation of methanol. 45 Formation of CO from glycerol oxidation could thus potentially have an autocatalytic effect, enhancing reaction kinetics. However, such an effect lies outside the scope of the present study. We conclude that the Au(111) surface is water covered below 1.0 V vs. RHE. Neglecting solvent effects, glycerol oxidation is, therefore, modeled assuming a bare Au(111) surface. There is a large number of possible ways of oxidizing glycerol. As the Pourbaix diagram shows that there is no oxygen available for glycerol oxidation, we restrict ourselves to study thermodynamics of electrochemical dehydrogenation, with the addition of allowing concerted C-C bond breaking. As the number of possible intermediates still remain large, glycerol dehydrogenation paths were explored through the following scheme: From glycerol, all possible ways of single deprotonation were tested. Then, we continue with the two most thermodynamically favorable steps by deprotonating them in turn, and so on. Based on this,

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a)

H+ + e-

7b OH

O O

O

PDS 0.50 V

Au

O

H+ + e-

Au

Au

Au

O CO + Au

O

H+ + e-

OH

CO Au

3 CO

Most favorable route

OH HO

HO

HO

HO

3a 2a HO

H+ + e-

Au

O

OH

Dissolution O

O

HO

4b

Au Au

Au

OH HO Au Au

H+ + eHO

OH

H+ + e-

OH

OH HO

HO

O

OH Au Au

Au H+ + e-

1ab O

H+ + e-

PDS 0.62 V H+ + e-

Au

2c OH

H+ + e-

Au Au

O

OH

1c

Au

OH

HO

HO PDS 0.60 V

5b

OH

OH

0 OH

4a

O

H+ + e-

Glyceraldehyde route H+ + e-

H+ + e-

Second most favorable route

O Au Au

OH

O Au Au

6b

HO

8

5a O

OH

O

Au Au

3x

HO

H+ + e-

C•

7a

6a

HO

HO

2b

Dissolution

O

Au Au OH

OH

3b

H+ + e-

Au

O HO

OH

b) Gibbs Free Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.5

PDS 0.50

7b 6a

0.03 0.27

1.0

6b

Most favorable route Second most favorable route Glyceraldehyde route

PDS 0.60

0.5 OH HO

-0.18

2b

0.39 -0.21

5a

3 CO

8 0.03

0.40

0.40

3b

-0.66

-0.31

0.0

0.08

-0.31

5b 1c 1ab

OH

4a

-0.40

7a

0.38

3a 0.42

-0.26

4b

PDS 0.62

2a

0 2c

-0.5

Glycerol

0

- (H+, e-) - 2(H+, e-) - 3(H+,e-) - 4(H+,e-) - 5(H+,e-) - 6(H+,e-) - 7(H+,e-) - 8(H+,e-)

CO

Figure 3: Catalytic routes of glycerol dehydrogenation on Au(111), presented as (a) catalytic cycles and (b) energy landscape. The two most favorable complete dehydrogenation routes to CO are marked in red with squares and blue with circles, respectively, while a path ending in adsorbed glyceraldehyde is shown in green with triangles. Potential determining steps (PDS) are marked for each route.

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we find the two most favorable complete dehydrogenation paths to CO, shown in Figure 3. These paths require 0.50 V and 0.62 V vs. RHE, respectively, in order to be thermodynamically favorable. A path to glyceraldehyde is included inspired by experimental observation on platinum in acidic media and on gold at higher pH by Kwon et al. 17 Starting from adsorbed glycerol (0), the most favorable route begins with dehydrogenation of the secondary carbon (1ab, requiring 0.39 V vs. RHE), and the secondary hydroxyl group, forming DHA (2a, -0.31 V). DHA is weakly bound to the surface, with an adsorption energy of -0.15 eV, and may desorb. The next steps are removal of hydrogen from the primary carbons, forming a six-membered ring (3a, 0.42 V) and a five-membered ring (4a, 0.40 V) with surface gold, respectively. Continuing from 4a, a di-aldehyde is formed by deprotonization of the primary hydroxyl groups (5a, 0.08 V; 6a, 0.50 V). The transition from 5a to 6a is the potential determining step (PDS). The last two steps are the removal of hydrogens on the primary carbons. Assuming concerted C-C bond breaking, this leads to formation of CO (7a, 0.03 V; 8, -0.31 V), which readily desorbs. C-C bond breaking is exergonic by -0.31 eV and -0.99 eV for 7a and 8, respectively. Thus, the last dehydrogenation steps with decoupled C-C bond breaking requires potentials of 0.41 V and 0.31 V, respectively, which does not change the PDS. The second most favorable route starts also with steps 0 and 1ab, followed by dehydrogenation of a primary carbon. Surface gold binds into the C-C double bond, forming a π-complex (2b, -0.18 V). Next, a five-membered ring is formed by dehydrogenation of a primary hydroxyl group (3b, 0.38 V). The deprotonation of a primary carbon leads to formation of 2,3-dihydroxy-2-propenal (4b, -0.26 V). This species is stable, forming a weak van der Waals bond to the surface, with an adsorption energy of -0.19 eV, and may thus desorb. Removal of hydrogen from the aldehyde group (5b, 0.62 V) is the PDS. Deprotonation of the second primary carbon results in the formation of a five-membered ring with surface gold (6b, 0.40 V). Deprotonation of the secondary carbon (7b, 0.27 V) is followed by concerted C-C bond breaking and CO formation (8, -0.38 V). With decoupled C-C bond breaking, step

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8 would have required 0.58 V, not changing the PDS. The glyceraldehyde route starts from glycerol (0), followed by dehydrogenation of a primary carbon (1c, 0.60 V), which is the PDS. Glyceraldehyde (2c, -0.66 V) is formed by dehydrogenation the corresponding primary hydroxyl group. Further dehydrogenation would require 0.70 V vs. RHE. From these results we conclude that activity, owing to the first deprotonation, should start at 0.39 V vs. RHE. This is in good agreement with our experimental onset potential of ∼0.5 V vs. RHE, considering that kinetic effects are not treated in our theoretical study. There are, to the best of our knowledge, no reports on oxidation products on gold in acidic media. In neutral media, glyceraldehyde is reported to be produced from 0.8 V vs. RHE, 17 which, if the computational model surface is valid, agrees with our results. Our reaction mechanism can be compared to Liu and Greeley’s computational work on glycerol dehydrogenation on late transition metals, in particular on Pt(111). 46,47 The study is conducted in the framework of heterogeneous catalysis, but as H2 is used as reference for the removed hydrogen, we can interpret their results using the theoretical NHE. 24 Using the approach above, one finds that the two first electrochemical dehydrogenation steps on platinum are identical to the second most favorable route (1ab and 2b in Figure 3a). Then, the second primary carbon is deprotonated, deviating from our routes on gold. The primary hydroxyl groups are deprotonated (0.38 V), which is most likely the PDS if concerted C-C bond breaking is allowed. Liu and Greeley consider complete dehydrogenation with decoupled C-C bond breaking. 46,47 Thus, their route ends with the formation of a surface-bound (C=O)-(C=O)-(C=O), which is then the PDS (0.73 V). Considering our calculated thermodynamic C-C bond breaking gain on gold above, and considering a reported experimental onset potential on platinum of ∼0.4 V vs. RHE, 16,17,22 we conclude that concerted C-C bond breaking occurs in electrochemical dehydrogenation on platinum, similar to our result on gold.

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Conclusions We have shown that glycerol oxidation on gold occurs to a measurable extent in acidic media, in particular on the bare metallic surface < 1.0 V, in contrast to previous reports. 15,17 The choice of electrolyte is important, as the activity in HClO4 is significantly higher than in H2 SO4 . We speculate that the difference in activity is due to competitive adsorption, as it is known that sulfate ions adsorb stronger than perchlorate ions. 39,40 In order to investigate the glycerol oxidation reaction mechanism, we have validated the surface coverage at reaction conditions by constructing a theoretical surface Pourbaix diagram for Au(111). It was found that hydroxy groups and oxo groups appear on the surface above 1.0 V vs. RHE. We have explored the most thermodynamically favorable glycerol dehydrogenation paths on Au(111). From the energy landscape (Figure 3b), we determine which products should be observed at different potentials. At 0.39 V vs. RHE, DHA and 2,3-dihydroxy-2-propenal (2a and 4b) start to form. This is in reasonable agreement with the experimental onset at ∼0.5 V.

At 0.50 V, CO is produced and at 0.60 V, glyceraldehyde (2c) is formed but expected

to stay adsorbed on the surface. All of these reactions occur well below the adsorption of *O and *OH at 1.0 V. As biodiesel production is increasing globally, so is the production of glycerol. Our results show the potential of glycerol electrooxidation on gold as a means of production of valuable chemicals, such as hydrogen gas, glyceraldehyde and dihydroxyacetone.

Acknowledgement The authors gratefully acknowledge support from Formas and the Swedish Research Council and the Röntgen-Ångström project HEXCHEM. The electronic structure calculations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and UPPMAX. This work used the Extreme Science and Engineering Dis13



covery Environment (XSEDE), 48 which is supported by National Science Foundation grant number ACI-1053575.

Supporting Information Available The following file is available free of charge. • Supporting information for: Electrooxidation of glycerol on gold in acidic medium: a combined experimental and DFT study This material is available free of charge via the Internet at http://pubs.acs.org/.

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