Experimental and Theoretical Studies of Ethanol Synthesis from

Article pubs.acs.org/JPCC

Experimental and Theoretical Studies of Ethanol Synthesis from Syngas over CuZnAl Catalysts without Other Promoters Zhi-Jun Zuo,*,† Le Wang,† Lin-Mei Yu,† Pei-De Han,‡ and Wei Huang*,† †

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province and ‡College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi China S Supporting Information *

ABSTRACT: Ethanol synthesis from syngas over CuZnAl catalyst without other promoters is studied using theoretical and experimental methods. The possible reaction paths of the ethanol synthesis in thermodynamic and dynamic over Cu cluster and Cu−O species adsorbed on ZnO surface are systematically identified at the molecular level. Three possible paths involving the formation of CH3 as the key intermediate are proposed, which are COH, CHOH, CH2OH, and CH3; CHO, CH, CH2, and CH3; and CH3OH and CH3. CO insertion into the CH3 intermediate produces CH3CO, which is further hydrogenated to yield CH3CHO and CH3CHOH and finally obtain ethanol. The CuZnAl catalyst, which is prepared by complete liquid-phase technology, has high ethanol selectivity and stability because of the strong interaction between Cu species and ZnO. In summary, the coexistence of both Cu0 and Cu+ is necessary for ethanol synthesis from syngas over CuZnAl catalyst without other promoters.

1. INTRODUCTION The diminishing supply of fossil fuels, increasing concern on global climate change, and rising crude oil prices have stimulated intensive research and development in the field of hydrogen energy.1 Ethanol has become an attractive liquidhydrogen source because of its nontoxicity and producibility from renewable sources. Currently, ethanol is produced by two major processes: fermentation derived from corn or sugar cane and hydration of petroleum-based ethylene. Ethanol synthesis from CO or CO2 hydrogenation was developed because of the lower ethanol yield of fermentation.2−7 Catalysts for ethanol synthesis from CO or CO2 hydrogenation can be broadly categorized into four groups: Rh-based catalysts,8−10 alkali-promoted Mo-based catalysts,11−13 modified Fischer−Tropsch (F−T)-type catalysts,7,14−17 and modified methanol synthesis catalysts (Cu-based).4−6,18 Of these groups, the Rh-based catalysts provide the highest C2-oxygenate selectivity and have low methanol formation. However, the application of Rh-based catalysts is limited because Rh is a precious metal. Meanwhile, both modified F−T catalysts and alkali-promoted Mo-based catalysts provide moderate ethanol selectivity but produce a mass of hydrocarbons and a mixture of C1−C6 alcohols, and the separation of ethanol is difficult from the mixture. Cu-based catalysts with alkali-promoters provide extremely low selectivity for ethanol formation, and methanol remains the dominant product. Among these non-noble metal catalysts, CuFe- or CuCo-based catalysts are regarded as promising candidates for mixed alcohol synthesis from syngas.2,3,7,14−16,19−21 At present, Cu-based catalysts are typically alkali-promoted Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3 or alkali-promoted combinations of all or some of the following components: Mn, Cr, Th, and others. In these studies, the choice of promoter is vital in determining whether ethanol is formed. In alkali-promoted © 2014 American Chemical Society

Cu-based catalysts, the univalent alkali possibly serves as the active sites for C−C bond formation. In Cu-based catalysts containing alkali-promoted combinations of Mn, Cr, Th, and others, Hofstadt et al.22 suggested that Cu+ favors the formation of oxygen-containing species (e.g., CH3O), whereas metallic Cu (Cu0) favors the formation of a methylene structure (CH2). The oxygen-containing species and CH2 then combine to form a C2 precursor. In ethanol synthesis, Cu+ and Cu0 must be present, and the Cu+/Cu0 ratio is affected by promoters such as Mn, Cr, and Th. However, methanol also remains the dominant product. Furthermore, the mechanism of ethanol synthesis over Cu+/Cu0 sites has no direct evidence to date. Given the importance of ethanol, a number of theoretical studies have focused on ethanol synthesis, particularly on the use of Rh-based catalysts.23−25 Surface hydrocarbon species (CHx) produced by CHxO (4 > x ≥ 0) dissociation occurs through CO insertion to form C2 oxygenated species. The C2 oxygenated species is then hydrogenated to form ethanol. In general, these studies propose that CH3 or CH2 is the most favorable monomer on the Rh(111) surface among all CHx (x = 1 to 3) species from syngas and is predominantly responsible for ethanol formation.10,23−27 Recently, Wang et al.28−30 studied ethanol synthesis from syngas over Cu(hkl) surfaces using the density-functional theory (DFT). They found that CO dissociation is not energetically favored on a Cu(111) surface, and CH2 and CH3 formation is the rate-limiting step, and CH3OH is readily formed by CO hydrogenation rather than CH2 and CH3.30 CH2 is the most favorable monomer on the Cu(110) surface. However, CH2 hydrogenates to form CH3, and CH4 becomes the dominant reaction.29 On the Received: March 21, 2014 Revised: May 30, 2014 Published: May 30, 2014 12890

dx.doi.org/10.1021/jp502828c | J. Phys. Chem. C 2014, 118, 12890−12898

The Journal of Physical Chemistry C

Article

Meshes of 3 × 2 × 1 k-points were used for the ZnO (1010̅ ) surface. Without counting the adsorbates, the vacuum between the slabs was set to span a range of 15 Å to ensure that the slabs do not exhibit any significant interaction. 2.2. Experimental Studies. 2.2.1. Catalyst Preparation. The CuZnAl slurry catalyst was prepared by complete liquidphase technology.40 The innovation core of the technology is based on the thought that catalytic materials can grow in a similar surrounding as its final reaction surrounding, where the catalysts are prepared directly from solution to slurry without calcination process in conventional preparation methods. Aluminum isopropylate [(C3H7O)3Al] (1.6 mol/L), which was bought in 2009, was dissolved in a mixture of distilled water and the surfactant polyvinylpyrrolidone (0.0003 mol/L) and kept at 353 K for 1.5 h in a water bath. Afterward, 0.05 mol/L of nitric acid was added to the solution at 368 K under vigorous stirring for 1 h. Another solution, which was prepared by dissolving Cu(NO3)2 (0.8 mol/L) and Zn(NO3)2 (0.65 mol/L) salts in ethanol, was added to the mixture. The mixture was stirred under reflux at 368 K for 10 h and then placed in an oven at 343 K for 12 h to obtain a gel. The gel was treated in a mixture of liquid paraffin (mixture of alkanes in the range of C12 to C17) (300 mL) and sorbitan monooleate (Span 80, C24H44O6) (0.5 mL) under mechanical stirring and then heated under a temperature program in a N2 atmosphere. The CuZnAl slurry catalyst (Cu/Zn/Al = 2:1:0.8 mol ratio) was then obtained. 2.2.2. Catalyst Characterization. The catalysts were suspended in the slurry bed during the reaction and then solid catalysts were obtained using the Soxhlet extraction method. The solid catalysts were subsequently characterized by XPS analysis. If high- or ultrahigh-vacuum conditions were not required in the analysis [such as in powder X-ray diffraction (XRD)], the slurry catalysts were directly tested without Soxhlet extraction. XPS was conducted using an ESCALAB 250 spectrometer (VG Scientific Ltd., UK) equipped with monochromated Al Kα (hν = 1486.6 eV, 150 W). The binding energies were calibrated by the C 1s peak at 284.6 eV. TPR was performed in a fixedbed reactor. For each TPR experiment, 50 mg of the samples were loaded into the reactor and heated to 783 K at a rate of 10 K/min using a temperature controller under a H2/N2 (5/95) reducing gas with a flow rate of 30 mL/min. Hydrogen consumption was recorded using a thermal conductivity detector (TCD). XRD patterns were recorded using a Rigaku D/max 2500 X-ray powder diffractometer with monochromated CuKα (40 kV/100 mA) radiation at a 4°/min scanning rate. 2.2.3. Catalytic Activity Test. Activity tests on the Cu/Zn/Al catalysts were performed in a 0.5 L slurry reactor with continuous mechanical agitation. Prior to the reaction, the catalyst was reduced with 20 vol % H2 and 80 vol % N2 at 553 K for 10 h under atmospheric pressure in liquid paraffin. Ethanol was synthesized at 523 K and 4.5 MPa. A mixed gas of H2 and CO (H2/CO = 2) was fed into the reactor at a rate of 150 mL/min. The products were analyzed by a gas chromatograph equipped with a flame ionization detector and TCD using GDX-502 and TDX-01 columns, respectively.32,41 The conversion of CO was calculated based on the fraction of CO that formed carbon-containing products according to CO conversion: XCO =[∑viyi/(yCO+∑viyi)]·100%. The selectivity of a certain product was calculated based on carbon efficiency by using the following formula: Product selectivity: Si = (viyi

Cu(211) surface, CH3 is the most favorable monomer both thermodynamically and dynamically compared with CH and CH2. However, the activation energy of CH3OH formation is lower than that of CH3 formation, indicating that the Cu(211) surface has a higher catalytic performance for CH3OH formation than CH3. As a result, the productivity and selectivity of ethanol is low because of the smaller number of CH3 sources as well as increased CH3OH formation.28 Eventually, the use of promoters for Cu catalysts was proposed to achieve high productivity and increase the selectivity for C2 oxygenates.28 DFT studies showed that Rh can facilitate CH3 formation and CO insertion into CH3, effectively limiting methanol formation to achieve high productivity and selectivity for ethanol.28,31 In our previous study,32 CuZnAl catalysts prepared by complete liquid-phase technology through the addition of (C3H7O)3Al into Cu(NO3)2 and Zn(NO3)2 solutions exhibited excellent ethanol selectivity (approximately 40%) at the initial stages of the reaction, and the total selectivity for methanol and ethanol was approximately 80%. However, the ethanol selectivity sharply decreased to 5% after 5 d of reaction. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) results showed that ethanol synthesis requires strong interactions between the Cu species and zinc or aluminum oxide. Based on the aforementioned experimental and theoretical results, researchers hypothesized that ethanol synthesis from syngas requires two active sites on the Cu-based catalysts as well as a promoter. Our previous studies showed that ethanol can be synthesized from syngas over CuZnAl catalysts without the use of other promoters. If two active sites are present on the CuZnAl surface, what are the reaction mechanisms involved? What are the differences between CuZnAl catalysts with and without another promoter? Improving the stability of the Cu species is also vital in preparing high-selectivity Cu-based catalysts for ethanol synthesis from syngas. To answer these questions, the detailed, molecular-level mechanism of ethanol formation from syngas on CuZnAl catalysts with two Cu active sites is investigated.

2. THEORETICAL AND EXPERIMENTAL METHODS 2.1. Computational Methods and Models. DFT calculations were performed using the Dmol3 Materials Studio software package.33,34 The electronic structures were obtained by solving the Kohn−Sham equation self-consistently under spin-unrestricted conditions.35,36 DFT was also used for core electrons by applying the PW91 generalized-gradient approximation to the exchange-correlation energy.37 A doublenumeric quality basis set with polarization functions was used. The transition state (TS) was identified using the complete linear/quadratic synchronous transit method.38 The equilibrium lattice constants of ZnO are a = 3.295 Å and c = 5.294 Å, whereas the experimental values are a = 3.250 Å and c = 5.207 Å.39 The ZnO (101̅0) surface was modeled from our optimized bulk ZnO structure using eight layers of metals with p(2 × 2) super cells. During the geometry optimization of Cu3 cluster and Cu−O species adsorption on ZnO (1010̅ ) surface, the six uppermost substrate layers of ZnO (101̅0) surface, Cu3 cluster and Cu−O species were allowed to relax, the bottom two layers of ZnO (101̅0) surface were constrained at the bulk position. During the geometry optimization of ethanol synthesis from syngas hydrogenation including adsorption and reaction, the Cu−O species and the bottom two layers were constrained, and others were allowed to relax. 12891



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/∑viyi)·100%, where vi is the number of carbon atoms of product i in tail gas; yi is the mole number of product i detected in both liquid phase and gas phase; yCO is the mole number of CO detected in tail gas.

where Eslab+adsorbate, Eslab, and Eadsorbate are the energies of the slab with the adsorbate on the surface, clean slab, and free adsorbate, respectively. A negative Eads signifies exothermic adsorption, which indicates strong interaction between the adsorbate and the surface. In this section, all possible reactants, intermediates, and products involved in the possible pathways of ethanol formation from syngas are discussed. The most stable configurations of these intermediates as determined by our DFT calculations are shown in Figure 2, and the corresponding adsorption energy and key geometric parameters are listed in Table 1. CO has two different adsorptive models. One is as a bridge site for Cu3 cluster, which bind with the O atoms on the ZnO(101̅0) surface. The C−Cu bond lengths are 1.866 and 2.017 Å, and the corresponding adsorption energy is −1.65 eV. The other model involves the adsorption on adjacent Cu3 cluster and Cu−O species, in which C and O bind with O and Cu, and the adsorption energy is −1.31 eV. At this stage, the adsorption configuration is similar to that of CO2 with the help of O from the Cu−O species. However, the C−O bond lengths are 1.344 and 1.334 Å, which are longer than that of free CO2 (1.176 Å). C2H5OH preferentially adsorbs on the surface of the Cu3 cluster that is adjacent to the Cu−O species at an adsorption energy of −0.60 eV. For CH4, the surface becomes too far after optimization, and the adsorption energy is −0.09 eV. C and CH2 preferentially adsorb onto the Cu−O species via the C atoms at adsorption energies of −4.97 and −3.62 eV, respectively. H and OH tend to coordinate to the Cu3 cluster bridge site, which bind with the O atoms of the ZnO(101̅0) surface at adsorption energies of −2.83 and −4.02 eV, respectively. Meanwhile, O preferentially adsorbs at the hollow site of the Cu3 cluster, and the adsorption energy is −5.10 eV. CHO, COH, CHOH, CH2O, CH2OH, CHCO, CH3CO, CH3CHO, and CH3COH are bound to the adjacent Cu of the Cu3 cluster and to O of the Cu−O species through O and C; the corresponding adsorption energies are −3.96, −2.74, −3.05, −2.03, −2.77, −2.96, −3.89, −1.83, and −2.69 eV, respectively. CH3 is adsorbed onto the O sites, and the adsorption energy is −2.64 eV. CH3CHOH has two adsorption configurations with corresponding adsorption energies of −3.22 and −3.34 eV. CH3O preferentially adsorbs on the surface of the Cu3 cluster, which bind with the O atoms of ZnO(101̅0) surface at an adsorption energy of −2.61 eV. CH is bound to the adjacent Cu of the Cu3 cluster and to O of the Cu−O species through C, and the adsorption energy is −5.59 eV. CH2CO is bound to the adjacent Cu of the Cu3 cluster and to O of the Cu−O species through two C atoms, and the adsorption energy is −1.90 eV. 3.1.2. Reaction Path of Ethanol Synthesis from Syngas. The overall ethanol synthesis from CO hydrogenation is exothermic (ΔH298 K = −2.63 eV),7 indicating that the synthesis is thermodynamically favorable. However, the reaction is clearly inhibited by kinetics.25 Three possible mechanisms underlie the CO reaction: first is CO hydrogenation to form CHO, second is CO hydrogenation to form COH, and third is direct CO dissociation to form C and O atoms. The activation and reaction energies are shown in Figure 3. For CO hydrogenation, the activation energies of CHO and COH formation are 1.12 and 0.96 eV, and the corresponding reaction energies are −1.89 and 0.15 eV, respectively. The activation energy for CO dissociation is high at 3.54 eV. These results indicate that the CO dissociation pathway is highly

3. RESULTS AND DISCUSSION To obtain a detailed understanding of ethanol synthesis from CO hydrogenation, the process was simulated using DFT calculations. Neither ZnO nor bulk Cu0 alone possesses high catalytic activity. Thus, a strong synergetic interaction between the metal and the support is proposed.22,42,43 The most widely accepted scheme is that Cu forms a monolayer or small clusters over the ZnO surface such that the surface properties of the species formed are different from those of bulk Cu.44−48 Rodriguez et al.49,50 proposed that the Cu cluster exhibits high activation for water−gas shift and methanol synthesis. Recently, Hu et al.42 investigated Cu deposition and growth over ZnO (101̅0) and found that the structure containing three Cu atoms has the strongest interaction with the ZnO (101̅0) surface, forming a two-dimensional (2D) cluster. At the same time, the atoms form a three-dimensional (3D) cluster on the surface with increasing Cu deposition. In our experiment,32 oxidized Cu was also observed. Thus, the Cu3 cluster and Cu−O species adsorbed on the ZnO (101̅0) surface were constructed in our model. It must be pointed out that the Cu3 cluster and sole Cu−O species do not exist on a real catalyst, but according to our understanding, we think the reaction of ethanol synthesis occurs on the interface of Cu/Cu2O on ZnO surface. However, the main difficulty encountered in the work is lack of information about the geometrical structure of the particular Cu−Cu2O interface.51 For this reason the Cu−O and Cu3 cluster adsorption on ZnO surface is chosen. It is considered that the model can reflect how the reaction occur on the CuZnAl catalyst to a certain extent, and at the same time, it is allowed to bring the effect of dimension and structure to the reaction which has been proved structure-sensitive. The configurations of the Cu3 cluster and Cu−O species adsorbed on the ZnO (101̅0) surface after optimization are shown in Figure 1. The Cu−O species insert into the Zn−O bond and form Zn−O−Cu−O, thus indicating the synergistic effect between the Cu−O species and the ZnO surface. 3.1. Adsorption of Reactants and Possible Intermediates. The adsorption energy, Eads, is defined as follows: Eads = Eslab + adsorbate − Eslab − Eadsorbate

Figure 1. Optimized configuration of Cu cluster and Cu−O species adsorption on a ZnO(101̅0) surface. 12892



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Figure 2. Most stable adsorption configuration of intermediates involved in ethanol synthesis from syngas. C, O, H, Zn and Cu atoms are shown in the gray, red, white, royal blue, and orange balls, respectively.

unlikely, which is consistent with previous studies.25,28−31 The stability of CHO formation is clearly higher than that of COH formation by 2.04 eV, but the activation energy for CHO formation is higher than that for COH by 0.16 eV. Ethanol synthesis from syngas is inhibited by kinetics and DFT usually has an uncertainty of ±0.2 eV, thus, both reactions are considered in our study. CHO formation is more favorable than COH formation over Cu(hkl) active sites.28−30,52 For example, Zhang et al.28 found that the activation energy for CHO formation is lower than that for COH formation by approximately 0.79 eV. The difference in the activation energies for CHO and COH shows that the presence of two active sites affects the CO hydrogenation pathway. COH can further react in two ways: CHOH formation from COH hydrogenation or C formation from COH dissociation (Figure 4). CHOH formation is preferred over C formation because of the lower activation energy of the former (by 0.85 eV) as well as its higher stability (by 0.54 eV). In a similar manner, CHOH is preferentially hydrogenated rather than dissociated into CH and O (Figure 4), with corresponding activation energies of 0.33 eV vs 0.82 eV. Meanwhile, CH2OH can further react in two ways: one is dissociation into CH2, and the other is hydrogenation to form CH3OH. The activation energy and reaction energy of CH2 formation are 1.04 and −0.59 eV, respectively. Two adsorption models represent CH3OH adsorption on two active sites: one is vertical adsorption, and the other is parallel adsorption. The O−H bond in the vertical adsorption model spontaneously breaks during geometric optimization, in which the H atoms binds with the O atoms of the Cu−O species and CH3O is adsorbed onto the bridge Cu3 cluster sites. In the parallel adsorption model, the C−O bond also spontaneously breaks during geometric optimization, in which CH3 and O are adsorbed onto the O of the Cu−O species and the hollow sites of the Cu3 cluster. The results indicate that CH3OH cannot be synthesized from CH2OH hydrogenation. Moreover, the probability of CH3OH parallel adsorption is higher than that of CH3OH vertical adsorption because of CH2OH parallel adsorption, and the CH3 content is higher than that of CH3O from CH2OH hydrogenation. In a similar manner, CH3O on the Cu3 cluster

prefers to be hydrogenated rather than be dissociated into CH3 and O (Figure S1). An activation energy of 1.96 eV is obtained for CH2O dissociation, whereas an activation energy of 0.62 eV is calculated for CH3OH formation. Therefore, CH3 formation from CH3O appears unlikely on the single active site of the Cu3 cluster. Three possible products can be obtained from the further reaction of CHO: CHOH and CH2O from CHO hydrogenation and CH from CHO dissociation (Figure 5). The activation energy of CHO dissociation (1.09 eV) is clearly lower than that of CH2O (1.83 eV) or CHOH formation (2.05 eV) from CHO hydrogenation. These results indicate that CHO dissociation is preferred over CHO hydrogenation. In previous studies,25,28,29,31 CHO hydrogenation was found to be more favored over CHO dissociation on Cu, Rh, or Cu−Rh alloys. The results indicate that the presence of two active sites affect the path of the CHO reaction. Further CH reaction has three possible products, i.e., C, CH2, and CHCO. As shown in Figure 5, the CH intermediate can be more easily hydrogenated by adding one hydrogen atom to carbon and forming CH2 than by its dissociation or by CO insertion. The corresponding activation energies are 0.65, 1.32, and 2.10 eV, respectively. Meanwhile, CH2CO and CH3 intermediates are likely formed in the further reaction of CH 2 (Figure 5). The CH 3 intermediate from CH2 hydrogenation is preferred over CH2CO formation by CO insertion because of its lower formation energy (by 1.54 eV) and higher stability (by 0.65 eV). Figure 6 shows that the activation energy for CH3CO formation from CO hydrogenation is slightly lower than that for CH4 formation from CH3 hydrogenation (0.96 eV vs 1.12 eV), indicating that the formation of CH3CO is slightly easier than that of CH4. The CH4 formed does not attach to the surface but rather desorbs immediately. Two possible intermediates are formed from the further reaction of CH3CO: CH3CHO and CH3COH. The CH3CO intermediate can more readily form CH3CHO than CH3COH (0.81 eV vs 1.15 eV). Meanwhile, only one product, CH3CHOH, is formed when CH3CHO is further hydrogenated. The activation energy for this reaction is 1.02 eV. For CH3CH2O adsorption onto the 12893



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Table 1. Adsorption Energies (Eads, eV) and Geometrical Parameters (Å) of Intermediates Eads

configuration

C O

−4.97 −5.10

C−Cu−O species O-hollow Cu cluster

H

−2.83

OH CO

−4.02 −1.31

intermediates

−1.65 CHO

−3.96

COH

−2.74

CHOH

−3.05

CH

−5.59

CH2

−3.62

CH3 CH4 CH2O

−2.64 −0.09 −2.03

CH2OH

−2.77

CH3O CHCO

−2.61 −2.96

CH2CO

−1.90

CH3CO

−3.89

CH3CHO

−1.83

CH3COH

−2.69

CH3CHOH

−3.22 −3.34

C2H5OH

−0.60

dCu−O

Cu−C:2.017 1.908, 1.920, 1.883 H-bridge Cu cluster Cu− H:1.713, 1.661 O-bridge Cu cluster 1.948, 1.959 O−Cu of Cu cluster, C−O 1.973 of Cu−O species C-bridge Cu cluster Cu−C: 1.866, 2.017 O−Cu of Cu cluster, C−O 1.991 of Cu−O species O−Cu of Cu cluster, C−O 2.074 of Cu−O species O−Cu of Cu cluster, C−O 2.127 of Cu−O species C−Cu−O species Cu−C: 2.249 C−Cu−O species Cu−C: 2.008 C−O of Cu−O species no bond O−Cu of Cu cluster, C−O 1.863 of Cu−O species O−Cu of Cu cluster, C−O 2.055 of Cu−O species O-bridge of Cu cluster 1.951, 1.939 O−Cu of Cu cluster, C−O 1.965 of Cu−O species C−Cu of Cu cluster, C−O Cu−C:1.917 of Cu−O species O−Cu of Cu cluster, C−O 1.966 of Cu−O species O−Cu of Cu cluster, C−O 1.973 of Cu−O species O−Cu of Cu cluster, C−O 1.321 of Cu−O species O−Cu of Cu cluster, C−O 2.051 of Cu−O species O−Cu of Cu cluster, C−O 2.043 of Cu−O species O-top of Cu cluster 2.008

dC−O 1.237

1.344

1.319 1.293

Figure 4. Hydrogenation COH on copper species adsorption on ZnO (101̅0) surface (unit: eV). See Figure 2 for color coding.

1.323 1.350 1.352 1.416 1.483 1.380

1.363 1.431 1.327 1.323

Figure 5. Hydrogenation CHO on copper species adsorption on ZnO (101̅0) surface (unit: eV). See Figure 2 for color coding.

1.990 1.417 1.399

Figure 6. CH3 insertion by CO or hydrogenation, then further hydrogenation on copper species adsorption on ZnO (101̅0) surface (unit: eV). See Figure 2 for color coding.

adjacent Cu surface of the Cu3 cluster and onto O of the Cu−O species, one C−H bond of the CH2 group spontaneously breaks during geometric optimization, and the H atom bonds with the O atoms to form CH3CHOH. C2H5OH is then

Figure 3. Hydrogenation or dissociation of CO on copper species adsorption on ZnO (101̅0) surface (unit: eV). 12894



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found a correlation between the catalytic activity and the Cu0 and Cu+ site densities for ethanol synthesis from hydrogenation of DMO.56,57 This conclusion is consistent with those of previous studies, which suggested that ethanol synthesis is a structure-sensitive reaction.58,59 Compared with the findings of Wang et al.,28−30 our results show that the Cu−O species has a similar role of the promoter Rh for ethanol synthesis from syngas. To probe into the effect of temperature on ethanol synthesis from syngas, we have calculated the rate constant by using Eyring’s transition state theory (TST).28,60 Previous studies also show that C−O dissociation or C−C formation is the key factor for the ethanol synthesis over Rh- or Cu-based catalysts.24,25,28,29,31 Therefore, the rate constant of C−O dissociation, CHx hydrogenation, C−C formation, and ethanol formation are studied at typical catalytic conditions T = 523, 573, and 623 K, and the corresponding rate constants are shown in Table 2. It is found that the rate constant k increases

formed through the further hydrogenation of CH3CHOH. The activation energy for this reaction is 1.19 eV. The potential energy diagrams and the corresponding TS for the ethanol synthesis reaction from the CO hydrogenation pathway on copper species adsorption on the ZnO (1010̅ ) surface are shown in Figure 7. The possible routes for ethanol

Table 2. Rate Constant k(s−1) of C−O Dissociation, CHx Hydrogenation and C−C Formation Invoving in the Process of Ethanol Synthesis from Syngas rate constant k(s−1) elementary reaction C−O Dissociation CHO→CH+O CH2OH+H→CH3+OH CHx Hydrogenation CH+H→CH2 CH2+H→CH3 CH3+H→CH4 C−C Formation CH+CO→CHCO CH2+CO→CH2CO CH3+CO→CH3CO C2H5OH Formation CH3CHOH+H→C2H5OH

Figure 7. Potential energy diagrams and the corresponding TS for the ethanol synthesis reaction from the CO hydrogenation pathway on copper species adsorption on the ZnO (101̅0) surface.

523

573

623

3.42 × 102 1.01 × 106

3.09 × 103 4.53 × 106

1.98 × 104 1.61 × 107

5.94 × 106 1.61 × 1011 1.76 × 102

2.29 × 107 2.55 × 1011 1.68 × 103

7.16 × 107 3.77 × 1011 1.12 × 104

6.33 × 10−8 2.33 × 10−4 6.12 × 103

4.04 × 10−6 7.26 × 10−3 4.30 × 104

1.33 × 10−4 1.31 × 10−1 2.23 × 105

3.72 × 101

4.08 × 102

3.07 × 103

with the increasing of temperature on these reactions. For C− O dissociation, the rate constant of CH3 formation from CH2OH hydrogenation is obviously larger than that of CH formation from CHO dissociation at the same temperature, showing that the route of C−O dissociation is mainly from the CH2OH hydrogenation. For CH and CH2 hydrogenation and C−C formation, the rate constants of CH 2 and CH 3 hydrogenation are also obviously larger than that of the corresponding C−C formation either by CO insertion into CH and CH2, indicating that CH3 formation is preferable than that of CHCO and CH2CO fromation at the same temperature. For CH3CO, CH4 and C2H5OH formation, the rate constants are in the order k(CH3CO formation) > k(CH3 hydrogenation) > k(CH3CHOH hydrogenation) at the same reaction tempeature. It should be pointed out that the ratio of k(CH3CO formation)/k(CH3 hydrogenation) decreases from 34.77 to 19.91 with the reaction tempeature increasing from 523 to 623 K, and the ratio of k(CH3CHOH hydrogenation)/k(CH3 hydrogenation) increases from 0.21 to 0.27 with the reaction tempeature increasing from 523 to 623 K. The result shows that the increase of the reaction temperature is slightly favorable to the CH3 CHOH hydrogenation, but it is unfavorable to the CH3CO formation comparing with CH3 hydrogenation, indicating that a proper reaction temperature is

synthesis from CO hydrogenation are as follows: COH, CHOH, CH2OH, CH3, CH3CO, CH3CHO, and CH3CHOH or CHO, CH, CH 2 , CH 3 , CH 3 CO, CH 3 CHO, and CH3CHOH. The coexistence of Cu−O species and Cu3 cluster sites can result in the spontaneous dissociation of CH3OH because of the breaking of the CH3−OH or CH3O−H bond during geometric optimization. Therefore, another possible route of synthesis from methanol is CH3OH → CH3 → CH3CO → CH3CHO → CH3CHOH → C2H5OH. The activation energies of CHx−O (x = 0, 1, 2, 3) and CH3−OH bond breaking are higher than those of the corresponding CHxO (x = 0, 1, 2, 3) hydrogenation and CH3OH desorption over only Cu−O (or Cu3 cluster) species adsorbed on the ZnO (101̅0) surface. Methanol is then synthesized over the Cu−O species or Cu3 cluster sites. Previous studies28,29,53−55 also showed that methanol can be synthesized from CO hydrogenation over the Cu/ZrO2 interface or Cu sites via CHO, CH2O, and CH3O intermediates. The results showed that the coexistence of Cu3 cluster and Cu−O sites affect the synthesis routes of CO hydrogenation. For this reason, we think that ethanol synthesis from CO hydrogenation can occur in the synergism of two active sites such as Cu0 and Cu+. Ma et al. also 12895


The Journal of Physical Chemistry C required. In addition, it can be seen that the overall process of ethanol formation is controlled by CH3 hydrogenation and CO insertion into CH3, and the selectivity of ethanol is controlled by CH3 and methanol formations since methanol is synthesized over sole Cu3 cluster or Cu−O on ZnO surface. Recently, a CuZnAl slurry catalyst (Cu/Zn/Al =2:1:0.8 mol ratio) was obtained. It is found that the catalyst selectivity for ethanol is higher than that for methanol, and ethanol selectivity is similar to those of Rh-based catalysts varied at 32.7− 47.0%.61−64 The ethanol selectivity showed no observable change after 5 days of reaction (Figure S2). The TPR patterns show two reduction peaks (569 and 663 K) in the CuZnAl catalyst before reaction, and the two peaks are slightly shifted to low reduction temperature (559 and 613 K) (Figure S3). All results of XRD (Figure S4) and XPS measurements (Figure S5 and S6) indicate the existence of metallic and oxidized Cu species over the catalyst, and the ZnO species interact with the Cu species. Comparing with our previous result, the ethanol selectivity sharply decreased to 5% when the higher reduction temperature peak disappears. Therefore, the metallic and oxidized Cu species are the active sites of ethanol synthesis as suggested by Hofstadt et al.22 who concluded that Cu+ and Cu0 must be present in the ethanol synthesis of alkali-, Mn-, Cr-, and Th-promoted Cu-based catalysts. Gao et al.2 also found that the synergistic effect between Cu and Fe species improve the selectivity of mixed alcohols.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the procedure of Soxhlet method, CH3O hydrogenation or dissociation on copper species adsorption on ZnO surface, catalytic performance and catalyst characterization of CuZnAl catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of this study by the National Basic Research Program of China (2011CB211709), the key project of the National Natural Science Foundation of China (20336006), the National Natural Science Foundation of China (20676087 and 21306125), the Doctoral Program of Higher Education Priority Development Areas (20111402130002), China Postdoctoral Science Foundation Funded Project (2012M510784), Natural Science Foundation of Shanxi (012021005-1).

4. CONCLUSIONS Theoretical study was performed to investigate ethanol synthesis from syngas over CuZnAl catalyst. DFT result shows that the reaction over Cu3 cluster and Cu−O species adsorbed on the ZnO (101̅0) surface has three possible paths. The first possible route is COH → CHOH → CH2OH → CH3 → CH3CO → CH3CHO → CH3CHOH → C2H5OH; the second is CHO → CH → CH2→ CH3 → CH3CO → CH3CHO → CH3CHOH→ C2H5OH; and the third is CH3OH → CH3 → CH3CO → CH3CHO → CH3CHOH→ C2H5OH when parallel adsorption of CH3OH occurs on two active sites of the Cu3 cluster and Cu−O species. Experimental results show that when the higher reduction temperature peak disappears, the selectivity of ethanol sharply decreases. The higher the difficulty of catalyst reduction, the higher the stability of the CuZnAl catalyst. In a word, experimental and theoretical results show that ethanol synthesis from syngas over CuZnAl catalysts requires coexistence Cu0 and Cu+.

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AUTHOR INFORMATION

Corresponding Authors

*Fax/Tel.: +086 351 6018073; E-mail address: huangwei@tyut. edu.cn (W.H.). *E-mail: [email protected] (Z.-J.Z.). Notes

The authors declare no competing financial interest. 12896



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