Comparison of Reaction Pathways of Ethylene ... - ACS Publications

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Comparison of Reaction Pathways of Ethylene Glycol, Acetaldehyde, and Acetic Acid on Tungsten Carbide and Ni-Modified Tungsten Carbide Surfaces Weiting Yu,† Zachary J. Mellinger,‡ Mark A. Barteau,† and Jingguang G. Chen*,† †

Catalysis Center for Energy Innovation (CCEI), Department of Chemical and Bimolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ‡ Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States ABSTRACT: Selectively converting biomass-derived oxygenates to H2 or syngas (H2 and CO) is critical in the utilization of biomass to replace fossil fuels. In previous studies, monolayer (ML) Ni on a Pt substrate showed enhanced conversion and selectivity for oxygenate conversion. In the current work, tungsten monocarbide (WC) is used to support monolayer Ni, with the aim of replacing ML Ni−Pt with ML Ni−WC. C2 oxygenates with different functional groups, ethylene glycol, acetaldehyde, and acetic acid, are studied on clean WC and Ni-modified WC surfaces. For each C2 oxygenate, density functional theory (DFT) calculations reveal different binding energies on WC and Ni−WC surfaces. Parallel experimental measurements using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) confirm the different reaction pathways on the two types of surfaces, with the dominant decomposition pathway being C−O bond scission on clean WC and C−C bond cleavage on Ni-modified WC surfaces. Furthermore, using ethylene glycol decomposition as a probe reaction, the ML Ni−WC surface exhibits a similar net reaction pathway as that of ML Ni−Pt(111).

1. INTRODUCTION Biomass derivatives are promising alternative energy sources to replace fossil fuels. One useful pathway for biomass utilization is through the reforming reaction of biomass-derived oxygenates to produce H2 or syngas (H2 + CO). According to the literature,1 the content of alcohols, aldehydes, and acetic acid can be as high as 30% in the crude bio-oil from the pyrolysis of biomass. Therefore, it is important to understand the bond scission sequence of the O−H, CO, and COOH functional groups in the reforming reactions of these oxygenates. Previously, Dumesic and co-workers2 have identified Pt as a promising monometallic catalyst for aqueous-phase reforming of oxygenates with both high conversion and high H2 selectivity. Bimetallic catalysts are known to often exhibit unique properties different from either of the parent metals.3−7 For example, the reforming reaction of ethylene glycol has been studied on Ni−Pt(111) surfaces, with the surface monolayer (ML) Ni−Pt(111)8 exhibiting higher conversion than either of the parent metal surfaces. DFT calculations reveal that the higher conversion of ethylene glycol is related to its stronger binding on the ML Ni−Pt(111) surface. However, the ML Ni− Pt(111) bimetallic configuration favorable for ethylene glycol reforming is not stable at high temperatures9 because the surface monolayer Ni atoms undergo diffusion into the Pt bulk10 and lose the bimetallic activity to a significant extent. It is therefore important to identify an alternative catalyst with higher thermal stability than the ML Ni−Pt(111) surface while maintaining a similar reforming activity. Metal carbides11−16 © 2012 American Chemical Society

have been shown to possess some Pt-like properties but with lower cost than Pt. Furthermore, tungsten monocarbide (WC) is an effective barrier to prevent the diffusion of the metal overlayer,17 as confirmed by a previous study of the thermal behavior of ML Ni on a WC substrate.18 Therefore, replacing ML Ni−Pt(111) with ML Ni−WC can potentially enhance the stability of the monolayer Ni catalyst. However, it remains to be shown that ML Ni−WC exhibits similar activity as the ML Ni−Pt(111) surface for small oxygenate decomposition. In the current study, three C2 oxygenates with different functional groups, ethylene glycol, acetaldehyde, and acetic acid, were studied on Ni-modified WC surfaces for the production of syngas. The net reaction pathways of the three C2 oxygenates were compared on clean WC and Ni-modified WC surfaces. Since the decomposition activity of an oxygenate has been correlated to its binding energy on the surface in previous studies,8 DFT calculations of the binding energies of the C2 oxygenates were performed in the current study on WC and Ni−WC surfaces. Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) were utilized to experimentally determine the decomposition pathways of the C2 oxygenates. Received: November 8, 2011 Revised: February 12, 2012 Published: February 15, 2012 5720

dx.doi.org/10.1021/jp210756f | J. Phys. Chem. C 2012, 116, 5720−5729

The Journal of Physical Chemistry C

Article

2. THEORETICAL AND EXPERIMENTAL METHODS 2.1. DFT Calculations. All DFT calculations in this work were performed with the Vienna ab initio simulation package (VASP)19−21 and the PW91 functional22 in the generalized gradient approximation (GGA).23 A kinetic cutoff energy of 396 eV for the plane wave truncation and a 3 × 3 × 1 k-point grid were chosen for the calculations. The WC surface was modeled by a periodic 3 × 3 tungsten-terminated WC(0001) slab with a carbon layer inserted below each of the three tungsten layers. The WC(0001) configuration was derived by adding seven equivalent vacuum layers onto the three tungsten−carbon layers. The bottom two tungsten−carbon layers were frozen at the lattice constants of bulk WC (a = b = 2.92 Å, c = 2.84 Å), while the top layers were allowed to relax to reach the lowest energy configuration. The ML Ni−WC surface was modeled by replacing one vacuum layer by Ni atoms on the optimized WC(0001) surface. Ethylene glycol was adsorbed through the two oxygen atoms at an atop site on adjacent metal atoms.24 Acetaldehyde was adsorbed on the surface in a di-σ configuration. Acetic acid bonded on the surface through a η1 μ1(O) configuration25 with the oxygen atom in the carbonyl group binding on the surface. The optimized adsorption geometries of ethylene glycol, acetaldehyde, and acetic acid on ML Ni−WC surface are shown in Figure 1. The binding energy value was obtained with

and carbon monoxide, were of research purity and were used without further purification. The purity of all the reagents was verified using mass spectrometry before usage. Acetaldehyde, acetic acid, and ethylene glycol were dosed to the surface through a 0.7 cm diameter stainless steel tube about 10 cm away facing the center of the surface with the exposure indicated in Langmuirs (L; 1 L = 1 × 10−6 Torr·s). The TPD experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10−10 Torr, as described previously.18 A polycrystalline tungsten foil (Alfa Aesar, 99.95%) was attached to two tantalum posts through which it could be heated resistively and cooled with liquid nitrogen. The temperature of the W foil was measured with a chromelalumel K type thermocouple welded onto the back of the sample. The WC surface was prepared by sputtering a clean W surface at 300 K with ethylene at 3 × 10−5 Torr, followed by annealing to 1200 K, as described previously.17 The composition and morphology of the WC surface have been previously characterized using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).26 The growth mechanism of Ni on the WC substrate was also characterized using Auger electron spectroscopy (AES) as a function of Ni deposition time, revealing a layer-by-layer growth mechanism of Ni over the WC substrate.18 In the current study, different Ni coverages on the WC surface were achieved through physical vapor deposition (PVD) by controlling the current of a Ni evaporation source (Alfa Aesar, 99.95%) and deposition time. AES measurements were performed to verify the desirable Ni coverages.17,27 TPD experiments were performed at a linear heating rate of 3 K/s. The products were monitored using a quadrupole mass spectrometer (UTI 100C). The HREELS measurements were carried out in a separate UHV chamber, equipped with AES, Ni source, sputter gun, and HREELS, as described previously.28 HREELS measurements were performed to identify intermediates to better understand the reaction pathways of C2 oxygenates on WC and Ni−WC surfaces. Each C2 molecule was dosed to the surface at 100 K. Following the initial scan, the surface was flashed to higher temperatures at 3 K/s and cooled down to 100 K before each HREEL spectrum was recorded.

Figure 1. Top and side views of optimized adsorption geometries of (a) ethylene glycol, (b) acetaldehyde, and (c) acetic acid on ML Ni− WC surface (W, gray; Ni, blue; C, aqua; O, red; H, white).

3. RESULTS AND DISCUSSION 3.1. DFT Calculations of Binding Energies of C2 Oxygenates. According to a previous study of ethanol and ethylene glycol on Ni/Pt(111) surfaces,8 adsorbate binding energy is a useful descriptor to predict the extent of reaction of C2 oxygenates. For example, the high reforming and total decomposition activities of ethanol and ethylene glycol on the ML Ni−Pt(111) surface are attributed to their strong binding energies. In the current study, the binding energies of C2 oxygenates on ML Ni−WC and WC surfaces were calculated and compared with those on the ML Ni−Pt(111) surface. As shown in Table 1, the binding energies of ethylene glycol, acetaldehdye, and acetic acid on ML Ni−WC are similar to the corresponding values on ML Ni−Pt(111). Therefore, the ML Ni−WC surface is predicted to show a similar catalytic property as the ML Ni−Pt(111) surface toward the decomposition of the three C2 oxygenate molecules. In comparison, the binding energies on WC are noticeably higher than those on ML Ni−Pt(111) and ML Ni−WC surfaces, suggesting that different reaction pathways may be favored on the WC surface.

one adsorbed C2 oxygenate molecule in the 3 × 3 unit cell, corresponding to a surface coverage of 1/9 monolayer. The binding energy (BE) was calculated using the following equation: BEmolecule/slab = Emolecule/slab − Eslab − Emolecule

where Emolecule/slab is the total energy of the slab plus the adsorbed C2 oxygenate, Eslab is the energy of the bare slab, and Emolecule is the energy of the free C2 oxygenate in the gas phase. The vibrational frequencies of ethylene glycol, acetaldehyde, and acetic acid were calculated on WC(0001) surface from numerical derivatives of the atomic forces. The calculation of the force constant matrix included the two opposite displacements from the equilibrium position of each coordinate. 2.2. Experimental Techniques. Ethylene glycol (SigmaAldrich, 99.8%), acetaldehyde (Sigma-Aldrich, 99.85%), and acetic acid (Sigma-Aldrich, 99.7%) samples were transferred into glass sample cylinders and purified using repeated freeze− pump−thaw cycles. All other gases, hydrogen, neon, ethylene, 5721



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On the basis of the TPD results, possible net reaction pathways of ethylene glycol can be written as follows:

Table 1. Binding Energies of Ethylene Glycol, Acetaldehyde, Acetic Acid, Hydrogen, and Carbon Monoxide on ML Ni− Pt(111), ML Ni−WC, and WC Surfaces

aC2H6O2 → aC2H 4 + aH2 + 2aO(ad)

(1)

bC2H6O2 → 2bCO + 3bH2

(2)

cC2H6O2 → 2cC(ad) + 3c H2 + 2cO(ad)

(3)

surfaces

binding energies (kcal/mol)

ethylene glycol acetaldehyde acetic acid H215 CO

ML Ni−Pt(111)

ML Ni−WC

WC

−19.0

−19.6

−23.9

−28.4 −18.6 −69.0 −51.1

−27.0 −19.2 −68.1 −46.4

−38.6 −24.7 −77.6

Here, reaction 1 represents the deoxygenation reaction pathway to produce ethylene and hydrogen, reaction 2 is the syngas formation pathway, and reaction 3 is the complete decomposition. The values of a, b, and c represent the amount of ethylene glycol undergoing each reaction pathway. Reaction 2 represents the desired reaction in which H2 and CO (syngas) are produced without any carbon or oxygen atom deposition on the surface. The values of a and b are calculated from the desorption peak areas of ethylene and CO, respectively. The value of c is calculated by the hydrogen peak area after subtracting H2 contributions from reactions 1 and 2. After subtracting the amount of H2 from the other two reaction pathways, the value of c is zero, indicating that the complete decomposition pathway is negligible on the WC and Ni−WC surfaces. The values of a and b, representing the fraction of ethylene glycol undergoing deoxygenation and syngas formation pathways, respectively, are summarized in Table 2. On the WC surface, around 0.112 molecule of ethylene glycol per metal atom undergoes the deoxygenation reaction. When the surface is modified with Ni atoms, the deoxygenation reaction no longer occurs, and syngas formation becomes the dominant pathway, with the 1 ML Ni−WC surface showing the highest syngas formation activity, close to the value of ethylene glycol on the ML Ni−Pt(111) surface.8 HREELS experiments were carried out to monitor the surface reaction intermediates from ethylene glycol decomposition, as shown in Figure 2. The HREEL spectrum on clean WC at 100 K exhibits the following characteristic vibrational modes of molecularly adsorbed ethylene glycol, as assigned in Table 3:24,29 δ(CCO), 510 cm−1; τ(OH), 704 cm−1; ν(CC), ρr(CH2), 870 cm−1; νs(CO), 1080 cm−1; ρt(CH2), 1245 cm−1; ρw(CH2), 1350 cm−1; δ(CH2), 1440 cm−1; νas(CH), 2920 cm−1; ν(OH), 3301 cm−1. No change is observed in the HREEL spectrum upon heating to 200 K, indicating that ethylene glycol is still molecularly adsorbed on the WC surface with the O−H bond intact. After heating to 300 K, the

3.2. Ethylene Glycol Reaction Pathways on WC and Ni−WC Surfaces. TPD experiments of ethylene glycol were performed on clean WC and 0.5 ML and 1 ML Ni−WC surfaces. Figure 1 displays the TPD spectra of the reaction products after exposing the surfaces to 6 L of ethylene glycol at 200 K. A peak around 225 K is observed in all the TPD spectra in Figure 1, corresponding to the cracking fragments of molecular desorption of ethylene glycol. Figure 1a compares the desorption of the hydrogen product from the three surfaces, showing a broad peak from WC and relatively sharp peaks from the 0.5 and 1 ML Ni−WC surfaces. Figure 1b,c shows the desorption profiles corresponding to the cracking patterns of ethylene, and Figure 1d is from either ethylene or CO. On clean WC, the simultaneous detection of peaks at m/e = 26, 27, and 28 indicates that ethylene is produced from the decomposition of ethylene glycol through the C−O bond scission while keeping the C−C bond intact. In contrast, in the TPD spectra of the 0.5 ML and 1 ML Ni−WC surfaces, sharp desorption peaks are observed at m/e = 2 and m/e = 28, indicating that H2 and CO are produced from ethylene glycol decomposition on Ni-modified WC surfaces. When the WC surface is modified with Ni atoms, the bond scission mechanism of ethylene glycol follows the syngas formation pathway with the C−C, C−H, and O−H bond cleavage while keeping the C−O bond intact. Comparing to the desorption temperatures of the H2 and CO products on Ni−WC surfaces in Figure 1 to those from the adsorption of H2 or CO (spectra not shown), the desorption of the H2 product appears to be reaction-limited, while that of the CO product is desorption-limited.

Table 2. Product Yields of C2 Oxygenates on WC and Ni−WC Surfaces from TPD Measurements conversion (molecule per metal atom) species

surfaces

deoxygenation

syngas production

HO(CH2)2OH

WC 0.5 ML Ni−WC 1 ML Ni−WC ML Ni−Pt8 WC 0.5 ML Ni−WC 1 ML Ni−WC WC 0.5 ML Ni−WC 1 ML Ni−WC WC 0.5 ML Ni−WC 1 ML Ni−WC Ni film

0.112 0.000 0.000 0.023 0.098 0.000 0.000 0.016 0.000 0.000 0.042 0.000 0.000 0.000

0.000 0.155 0.183 0.144 0.000 0.009 0.105 0.000 0.116 0.187 0.000 0.082 0.049 0.047

CH3CHO

HOCH2CHO38

CH3COOH

5722

methane production

carbon dioxide production

total

0.000 0.022 0.122 0.152

0.112 0.155 0.183 0.167 0.098 0.055 0.177 0.016 0.116 0.187 0.042 0.104 0.171 0.199

0.000 0.046 0.072



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Figure 2. TPD spectra of (a) H2, (b) C2H4 (m/e = 26), (c) C2H4 (m/e = 27), and (d) C2H4/CO (m/e = 28) following 6 L exposure of ethylene glycol on WC and Ni−WC surfaces.

intensity, consistent with the TPD result that the ethylene product is produced between 300 and 600 K. The HREEL spectra of ethylene glycol on 0.5 and 1 ML Ni− WC are very similar, with the latter surface shown in Figure 2b. All of the ethylene glycol vibrational modes are observed in the HREEL spectrum on 1 ML Ni−WC at 100 K, indicating that ethylene glycol is molecularly adsorbed on the 1 ML Ni−WC surface. After heating to 300 K, the τ(OH) mode at 704 cm−1 and ν(OH) mode 3301 cm−1 disappear, indicating the O−H bond scission. Different from the clean WC surface, the δ(CCO) mode at 530 cm−1 also disappears at 300 K on the 1 ML Ni−WC surface, indicative of the C−C bond cleavage. In comparison to the corresponding spectra on clean WC, the intensity of the remaining modes at 300 K decrease very quickly by 400 K, and the peaks disappear at 500 K on the 1 ML Ni− WC surface. Both differences are consistent with the TPD results showing C−C bond scission of ethylene glycol to produce H2 and CO between 300 and 500 K on the 1 ML Ni− WC surface. 3.3. Acetaldehyde Reaction Pathways on WC and Ni− WC Surfaces. Figure 3 displays the TPD spectra of (a) H2, (b) CH4, (c) C2H4, and (d) C2H4/CO after dosing 4 L

Table 3. Vibrational Assignment for Adsorbed Ethylene Glycol vibrational frequency (cm−1) for ethylene glycol modes

liquid29

Pt(111)24

WC

δ(CCO) τ(OH) ν(CC), ρr(CH2) νs(CO) ρt(CH2) ρw(CH2) δ(CH2) νas(CH) ν(OH)

476 700 864 1087 1212 1332 1459 2935 3275

530 700 865 1080 1250 1360 1450 2920 3270

510 704 870 1080 1245 1350 1440 2920 3301

WC from DFT 502 818,852 1001,1008,1026 1153,1178,1215,1267 1355,1399 1417,1439 2959,2994,3035,3044 3446,3521

intensities of all vibrational modes decrease, resulting from the molecular desorption of ethylene glycol at 225 K in the TPD result in Figure 1. The disappearance of the τ(OH) and ν(OH) modes indicates that the O−H bond cleavage occurs between 200 and 300 K to produce an ethylenedioxy (OCH2CH2O) intermediate. After heating the surface to 500 K, all of the vibrational modes are still present but with a slight decrease in 5723



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Figure 3. HREEL spectra of 4 L ethylene glycol on (a) WC and (b) 1 ML Ni−WC.

On clean WC, all of the acetaldehyde molecules follow the deoxygenation reaction with a conversion of 0.098 molecule per metal atom. The two Ni-modified WC surfaces do not show any deoxygenation but produces syngas and methane, with the 1 ML Ni−WC surface exhibiting the highest syngas production and total activities. Figure 4 displays the HREELS results on clean WC and 1 ML Ni−WC surfaces following 1 L acetaldehyde exposure. In Figure 4a, the HREEL spectrum at 100 K exhibits the characteristic vibrational modes of molecularly adsorbed acetaldehyde, as assigned in Table 4:30−32 δ(CCO), 548 cm−1; ρ(CH3), 866 cm−1; ν(CC), 1116 cm−1; δ(CH3), 1434 cm−1; ν(CO), 1650 cm−1; ν(CH3), 2949 cm−1. Upon heating to 300 K, the ν(CO) mode disappears, although the δ(CCO) mode at 548 cm−1 is still present, indicating that the CO bond in acetaldehyde does not break but changes to bind with the surface in a di-σ configuration. At the same temperature, the ν(CC) mode shifts from 1116 cm−1 to 1042 cm−1. After heating the surface to 400 K, the intensities of all the vibrational modes decrease, consistent with the TPD result that the ethylene product starts to desorb from WC around 350 K. The vibrational spectrum of acetaldehyde on 1 ML Ni−WC at 100 K is very similar to that on the clean WC surface. The peak intensities start to decrease after the surface is heated to 200 K, due to acetaldehyde molecular desorption at 160 K (shown in TPD in Figure 3). Upon heating to 300 K, the peak intensities of the δ(CCO) mode at 528 cm−1 and the ν(CC) mode at 1109 cm−1 decrease significantly, indicating that the C−C bond starts to cleave between 200 and 300 K. The C−C bond cleavage temperature is consistent with the TPD observation that the methane product desorbs from 1 ML Ni−WC at 285 K. The disappearance of the ν(CO) mode at 300 K suggests that acetaldehyde binds with the surface through a di-σ configuration via the C−O moiety. After heating to 400 K, the ν(CC) mode at 1109 cm−1 completely disappears, and the remaining peaks are described to hydrocarbon fragments. In summary, on the basis of the HREELS results of acetaldehyde on clean WC, the C−C bond remains intact,

acetaldehyde on WC and Ni−WC surfaces. Figure 3a shows that H2 is produced from the two Ni-modified WC surfaces but not from clean WC. Figure 3b reveals that methane desorbs from the 0.5 ML Ni−WC surface at 335 K and 1 ML Ni−WC surface at 285 K, while the desorption of methane is not observed from clean WC. Similar to that described in ethylene glycol TPD, simultaneous detection of m/e = 26 (not shown), m/e = 27 (Figure 3c), and m/e = 28 (Figure 3d) is indicative of ethylene production at 380 K from the clean WC surface. Figure 3d shows the desorption of the CO product from the 0.5 and 1 ML Ni−WC surfaces at 400 K. Overall, the TPD results reveal that acetaldehyde reacts on clean WC to produce ethylene. However, on Ni-modified WC surfaces, H2, CH4, and CO are the reaction products from acetaldehyde. On the basis of the desorption temperatures of molecularly adsorbed H2 and CO on Ni−WC surfaces (spectra not shown), the desorption of H2 from acetaldehyde decomposition is reaction-limited. The desorption temperature of the CO product, at ∼400 K on 1 ML Ni−WC, is slightly higher than that of molecularly adsorbed CO (∼375 K), indicating that the desorption of the CO product occurs as soon as it is formed, i.e., via a reaction-limited process. On the basis of the TPD results, possible net reaction pathways of acetaldehyde can be written as follows: aCH3CHO → aC2H 4 + aO(ad)

(4)

bCH3CHO → bCO + bC(ad) + 2bH2

(5)

cCH3CHO → cCH 4 + cCO

(6)

dCH3CHO → 2dC(ad) + dO(ad) + 2d H2

(7)

Here, reaction 4 represents the deoxygenation reaction pathway, reaction 5 the syngas formation pathway, reaction 6 the methane production pathway and reaction 7 the complete decomposition. The values of a, b, c, and d represent the amount of acetaldehyde following each reaction pathway and are calculated by the area of each product desorption peak, as summarized in Table 2. The value of d is almost zero after subtracting the hydrogen peak area contributed by the syngas production reaction pathway and background H2 adsorption. 5724



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Figure 4. TPD spectra of (a) H2, (b) CH4, (c) C2H4 (m/e = 27), and (d) C2H4/CO (m/e = 28) with an exposure of 4 L acetaldehyde on WC and Ni−WC surfaces.

Table 4. Vibrational Assignment for Adsorbed Acetaldehyde vibrational frequency (cm−1) for acetaldehyde modes δ(CCO) ρ(CH3) ν(CC) δ(CH3) ν(CO) ν(CH3)

crystalline

30

522 882 1118 1431/1389 1722 2964/2918

Rh(111)

31

610 950 1135 1380 1460 2980

Pt(111)32

WC

607 913 1130 1430 1667 2984

548 866 1116 1434 1650 2949

WC from DFT 510,676 917,948 1071,1138,1294 1338,1411,1431 2956,2962,3031,3045

3.4. Acetic Acid Reaction Pathways on WC and Ni− WC Surfaces. Acetic acid TPD experiments were performed on clean WC and 0.5 and 1 ML Ni−WC surfaces, as shown in Figure 5. To highlight the difference between ML Ni−WC and a Ni film, a thick Ni overlayer (>5 ML) was deposited on the WC substrate, and the corresponding TPD results are also included in Figure 5. Figure 5a shows that no H2 desorption peak is detected from clean WC, but H2 desorption peaks are observed from 0.5 ML Ni−WC surface at 580 K, 1 ML Ni− WC surface at 470 K, and the Ni film at 435 K. Comparing Figures 5b (m/e = 27) and 5c (m/e = 28), ethylene is produced from acetic acid on the clean WC surface. On the other three Ni-modified WC surfaces, CO desorption peaks are observed at

consistent with the TPD result that hydrogen, methane, or carbon monoxide is not produced from acetaldehyde decomposition on WC. The only product observed in TPD is ethylene from the WC surface, revealing that acetaldehyde follows the deoxygenation reaction pathway with the C−C bond intact and the scission of the CO bond. When the WC surface is modified with Ni, the TPD spectra show that hydrogen, methane, and carbon monoxide are produced from acetaldehyde. This is consistent with the HREELS results that the vibrational peaks of the C−C bond decrease significantly between 200 and 300 K and completely disappear at 400 K on the 1 ML Ni−WC surface. 5725



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Figure 5. HREEL spectra of (a) WC and (b) 1 ML Ni−WC following 1 L acetaldehyde exposure.

intermediate. After heating the surface to 200 K, the ν(CO) mode at 1711 cm−1 disappears, demonstrating that the CO bond interacts with the WC surface, likely producing the bidentate bonding configuration of the COO group with the surface. The ν(CC) mode at 1028 cm−1 is still present at 600 K, suggesting that there is no C−C bond breaking on the clean WC surface. After heating to 700 K, the vibrational modes almost disappear completely, consistent with the formation of the ethylene product at 640 K from the clean WC surface. Figure 6b displays the thermal decomposition of acetic acid on the 1 ML Ni−WC surface. The acetic acid undergoes O−H bond scission at 100 K to produce a CH3COO intermediate, similar with that observed on clean WC. At 200 K, CH3COO binds on the surface through a bidentate configuration, as suggested by the disappearance of the ν(CO) mode at 1711 cm−1. No change is observed in the HREEL spectra after heating the surface to 500 K. The ν(CC) mode at 1028 cm−1 disappears at 600 K, indicating the C−C bond scission between 500 and 600 K, consistent with the desorption of the CO and CO2 reaction products between at 500 and 600 K (Figure 7). 3.5. Comparison of C2 Oxygenates on WC and Ni−WC surfaces. The activities of ethylene glycol, acetaldehyde, acetic acid, as well as the previously studied glycolaldehyde38 molecules are listed in Table 2 to compare the activity of C2 oxygenates with different combinations of −OH, −CO, and −COOH functional groups. On clean WC, all of the four C2 oxygenates follow the deoxygenation reaction pathway of C−O bond cleavage to produce ethylene. As compared in Table 2, among the three molecules, ethylene glycol shows the highest conversion (0.112 molecule per surface metal), followed by acetaldehyde (0.098), acetic acid (0.042), and glycolaldehyde38 (0.016), which can be qualitatively related to the extent of C−H bond scission and subsequent formation during the reaction. In the reaction of ethylene glycol to produce ethylene, the ethylenedioxy (OCH2CH2O) intermediate undergoes the C−O bond scission with the C−H bond intact, leading to a relatively facile production of ethylene. For acetaldehyde, the production of ethylene requires sequential C−H cleavage and formation. For

610, 470, and 435 K. Figure 5d (m/e = 44) represents the CO2 product, which is detected at temperatures very similar to those of H2 desorption from the corresponding surfaces. The desorption of the H2 and CO products from the decomposition of acetic acid are reaction-limited by comparing to the adsorption temperatures of H2 or CO on Ni−WC surfaces (spectra not shown). The desorption of the CO2 product should also be reaction-limited due to the weak binding of CO2 on surfaces. On the basis of the TPD results, possible net reaction pathways of acetic acid can be summarized as follows: aCH3COOH → aC2H 4 + 2aO(ad)

(8)

bCH3COOH → 2bCO + 2bH2

(9)

cCH3COOH → cCO2 + cC(ad) + 2c H2

(10)

dCH3COOH → 2dC(ad) + 2dO(ad) + 2d H2

(11)

Reactions 8−11 represent the deoxygenation reaction, syngas production, carbon dioxide production, and complete decomposition pathways of acetic acid, respectively. The values of a, b, c, and d represent the amount of acetic acid following each reaction pathway, as summarized in Table 2. The amount of acetic acid undergoing complete decomposition is negligible from all surfaces. On the clean WC surface, all of the reacted acetic acid molecules follow the deoxygenation reaction pathway. When the surface is modified with Ni, acetic acid follows the syngas and CO2 production reaction pathways. Among the three Ni-modified WC surfaces, the 0.5 ML Ni− WC surface shows the highest syngas production pathway, and the thick Ni film shows the highest total conversion. In Figure 6a, the HREEL spectrum following the adsorption of acetic acid on clean WC at 100 K exhibits the following vibrational modes, as summarized in Table 5:33−35 ν(M−O), 474 cm−1; δ(OCO), 670 cm−1; ν(CC), 1028 cm−1; ν(COO), 1427 cm−1; ν(CO), 1711 cm−1; ν(CH), 2976 cm−1. According to previous studies,36,37 the absence of the δ(OH) mode at 1186 cm−1 and the ν(OH) mode at 3125 cm−1 indicates the O−H bond scission to produce a CH3COO 5726



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Figure 6. TPD spectra of (a) H2, (b) C2H4 (m/e = 27), (c) C2H4/CO (m/e = 28), and (d) CO2 with 6 L acetic acid exposure on WC and Ni−WC surfaces.

Table 5. Vibrational Assignment for Adsorbed Acetic Acid vibrational frequency (cm−1) for acetic acid modes ν(M−O) δ(OCO) ν(CC) νs(COO) ν(CH)

CH3COO−34

Cu(100)35

Al(111)33

WC

WC from DFT

650 926 1413 2935

339 677 1041 1434 3000

425 675 1055 1470 3025

474 670 1028 1427 2976

460 578,582 853,891,994,1018,1143 1330,1378,1416,1421,1576 2544,3008,3064,3143

carboxylic COOH group. Furthermore, on the basis of the desorption temperatures of the CO product from the 1 ML Ni−WC surface, ∼370 K for glycolaldehyde, ∼375 K for ethylene glycol, ∼400 K for acetaldehyde, and ∼470 K for acetic acid, the latter is the least reactive molecule among the four oxygenates for syngas production. As shown in Table 2, the total conversion of ethylene glycol on ML Ni−WC (0.183) is similar to that on ML Ni−Pt(111) (0.167). Furthermore, since the evolution of CO is desorption limited in our TPD experiments, it is likely that product desorption may be rate-determining under steady-state catalytic reaction conditions. Thus, it is important to compare the H2

acetic acid, the production of ethylene requires two C−H cleavage and formation steps; this complexity is most likely responsible for the lowest ethylene yield among the three C2 oxygenates. On Ni-modified WC surfaces, the ML Ni−WC surface shows the highest reforming pathway and total conversion for all three oxygenates. It is also found that on ML Ni−WC, the overall conversion of the C2 oxygenates is very similar: 0.183 for ethylene glycol, 0.177 for acetaldehyde, 0.171 for acetic acid, and 0.187 for glycolaldehyde.38 Different from ethylene glycol, acetaldehyde, and glycolaldehyde, the decomposition of acetic acid produces both CO and CO2 due to the presence of the 5727



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Figure 7. HREEL spectra of 8 L acetic acid exposed on (a) WC and (b) 1 ML Ni−WC.

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ACKNOWLEDGMENTS This material was based upon work supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004. Additionally, the DFT calculations in this work were performed using computational resources at the Center for Functional Nanomaterials, Brookhaven National Laboratory, supported by the U.S. DOE/BES, under Contract No. DE-AC02-98CH10886.

and CO desorption temperatures between ML Ni−Pt(111) and 1 ML Ni−WC surfaces. As shown in a previous study of ethylene glycol,8 the desorption of the H2 product occurred at 360 and 385 K on ML Ni−Pt(111), similar to the H2 desorption temperature (375 K) on the ML Ni−WC surface. The CO product desorbed from the ML Ni−Pt(111) surface between 300 and 500 K, similar to that from the ML Ni−WC surface (Figure 1). These similarities are consistent with the DFT calculations of the similar binding energies of reactants and products (Table 1) between the ML Ni−Pt(111) and ML Ni−WC surfaces, suggesting that WC can be potentially used as a replacement of Pt to support ML Ni for reforming reactions.

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(1) Gayubo, A. G.; Valle, B.; Aguayo, A. T.; Olazar, M.; Bilbao, J. J. Chem. Technol. Biotechnol. 2010, 85, 132−144. (2) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2003, 43, 13−26. (3) Berlowitz, P. J.; Goodman, D. W. Surf. Sci. 1987, 187, 463−480. (4) Berlowitz, P. J.; Houston, J. E.; White, J. M.; Goodman, D. W. Surf. Sci. 1988, 205, 1−11. (5) Chen, J. G.; Menning, C. A.; Zellner, M. B. Surf. Sci. Rep. 2008, 63, 201−254. (6) Goodman, D. W. Ultramicroscopy 1990, 34, 1−9. (7) Rodriguez, J. A. Surf. Sci. Rep. 1996, 24, 225−287. (8) Skoplyak, O.; Barteau, M. A.; Chen, J. G. J. Phys. Chem. B 2006, 110, 1686−1694. (9) Menning, C. A.; Chen, J. G. J. Chem. Phys. 2009, 130, 174709. (10) Kitchin, J. R.; Khan, N. A.; Barteau, M. A.; Chen, J. G.; Yakshinksiy, B.; Madey, T. E. Surf. Sci. 2003, 544, 295−308. (11) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides; Blackie Academic & Professional: Glasgow, U.K., 1996. (12) Hwu, H. H.; Chen, J. G. Chem. Rev. 2005, 105, 185−212. (13) Levy, R. B.; Boudart, M. Science 1973, 181, 547−549. (14) Esposito, D. V.; Hunt, S. T.; Stottlemyer, A. L.; Dobson, K. D.; McCandless, B. E.; Birkmire, R. W.; Chen, J. G. Angew. Chem., Int. Ed. 2010, 49, 9859−9862. (15) Ren, H.; Hansgen, D. A.; Stottlemyer, A. L.; Kelly, T. G.; Chen, J. G. ACS Catal. 2011, 1, 390−398. (16) Flaherty, D. W.; Berglund, S. P.; Mullins, C. B. J. Catal. 2010, 269, 33−43. (17) Gouypailler, P.; Pauleau, Y. J. Vac. Sci. Technol., A 1993, 11, 96−102.

4. CONCLUSIONS The decomposition of C2 oxygenates, ethylene glycol, acetaldehyde, and acetic acid as well as glycolaldehyde are compared on WC and Ni−WC surfaces. All the four C2 oxygenates react on the clean WC surface via the deoxygenation pathway to produce ethylene, resulting from the C−O bond scission with the C−C bond remaining intact. On Nimodified WC surfaces, C2 oxygenates follow the reaction pathways involving C−C bond cleavage. The Ni−WC surfaces show promising conversion to selectively convert C2 oxygenates to syngas. In addition, the similarities in the reaction of ethylene glycol on ML Ni−WC and ML Ni−Pt(111) suggest the possibility of using ML Ni−WC to replace ML Ni−Pt(111) with similar conversion but higher thermal stability and lower cost.

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REFERENCES

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