Article pubs.acs.org/IECR
Role of Keto−Enol Isomerization on Surface Chemistry and Hydrogenation of Acetone on Pt(111): A DFT study Min Xu,† Xiu-Lan Huai,*,† and Hui Liu‡ †
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
‡
ABSTRACT: The adsorption, tautomerization, and hydrogenation of acetone on Pt(111) surfaces were investigated by using density function theory calculations. The role of the keto−enol isomerization on selective hydrogenation and unselective decomposition of acetone was also studied. The results revealed that only a small amount of the adsorbed acetone could undergo enolization and subsequently decompose on clean Pt(111) surface at moderate condition. The enol tautomer is the origin of acetone decomposition on Pt(111) surface. The effect of the keto−enol isomerization on hydrogenation of acetone on Pt(111) under sufficient hydrogen pressure is not significant. These results suggested that the keto−enol isomerization could not enhance the selective hydrogenation of acetone on Pt(111) surface.
1. INTRODUCTION
influences the selective hydrogenation and unselective decomposition. To address the above issues, this work aims to study the keto−enol isomerization of acetone on Pt(111) surface, and then to understand the effect of keto−enol isomerization on catalytic hydrogenation and decomposition of acetone by using density function theory calculations.
Surface chemistry of acetone on metal surfaces is of great importance to understand the likely reaction pathways of acetone and keto-containing compounds; for example, enantioselective hydrogenation of α-ketoesters,1,2 aldol condensation,3,4 and high-temperature hydrogenation of acetone for chemical heat pump.5,6 The decomposition of acetone, which is an obstacle for the continuous operation of the isopropanol−acetone−hydrogen chemical heat pump, can occur at high temperature.6 The adsorption configurations of acetone on the surfaces of the catalysts should be responsible for unselective decomposition of acetone.7 Surface science studies of acetone adsorption on Pt(111)7 have suggested that the dominant adsorption form of acetone is a η1(O) end-on geometry in which the oxygen atom is bonded to the surfaces with the CO bond almost perpendicular to the surfaces, and the minority μ2(C, O) adsorbed structure in which the carbonyl group was parallel to the surface are also present. Some researchers obtained similar results on Ru(001)8 and Pd(111).9 However, on Cu(100), 10 Cu(110), 11 Cu(111), 12 and Au(111),13 only a weakly adsorbed η1(O) was observed and a μ2(C, O) adsorbed species was not found. Recent study14 from RAIRS experiments showed that a μ2(C, O) enolate species could be formed on Ni(111) via keto−enol isomerization and subsequently deprotonation of the enol. The DFT calculations by Jeffery et al.15 suggested that the minority μ2(C, O) adsorbed species of acetone found in surface science experiments was a μ2(C, O) enolate on Pt(111). These results indicated that keto−enol isomerization could occur on Pt(111) surface. Due to the fact that the CC bond is hydrogenated at a much higher rate than the CO bond, the enol formation may accelerate the hydrogenation of acetone. The parallel adsorption structure and strong interaction of enol also may lead to the decrease of the barrier of acetone decomposition. However, the activation barrier of keto−enol isomerization of acetone on metal surfaces has not been reported, and one cannot clearly state how the presence of the enol formation © 2014 American Chemical Society
2. COMPUTATIONAL METHODS The periodic slab calculations based on density functional theory with generalized gradient approximation (DFT− GGA)16,17 are carried out for Pt(111) slabs. A four-layer (3 × 3) unit cell, with 15 Å of vacuum gap between any two successive metal slabs, is used. All calculations are performed with the package CASTEP.18 Core states of all atoms in the simulation are represented using projector augmented-wave (PAW) pseudopotentials. The exchange-correlation energy and potential are described by the Perdew−Wang generalized gradient approximation (PW91). A plane wave cutoff of 400 eV is used in our calculations, and was found to be sufficient for all calculations. The surface Brillouin zone is sampled by (3 × 3 × 1) Monkhorst−Pack grid with convergence of the total energy with respect to k-point sampling accelerated using second-order Methfessel−Paxton smearing with a width of 0.1 eV. The calculated lattice constant of Pt is 3.98 Å, in good agreement with the experimental value of 3.93 Å.19 A (4 × 4) five-layer unit cell is used to test the level of error in using a four-layer (3 × 3) unit cell. The changes in the adsorption energy for acetone are found to be within 0.03 eV. During the optimizations, the uppermost two layers of Pt(111) surface, as well as the adsorbate atoms, are allowed to relax until the maximum value of the force is below 0.03 eV/Å, and other metal atoms are fixed in their bulk-terminated positions. Received: Revised: Accepted: Published: 5451
December 2, 2013 March 5, 2014 March 7, 2014 March 7, 2014 dx.doi.org/10.1021/ie404080x | Ind. Eng. Chem. Res. 2014, 53, 5451−5454
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the results of Jeffery et al.15 and Alcalá et al.24 also demonstrate that the μ2(C, O)-acetone is destabilized. The optimized structure of enol isomer on Pt(111) is the μ2(C, C1) mode with the CC bond almost perpendicular to the surfaces with adsorption energy of −-0.84 eV. The optimized adsorption configuration of the enolate on Pt(111) is the μ2(C, O) mode with adsorption energy of −0.64 eV. These results are consistent with the calculations of Jeffery et al.15 Other surface adsorption structures of enol and enolate isomer have been proved to be unfavorable via analyzing the adsorption energy and steric interaction by Jeffery et al.15 Figure 2 shows the configurations and structure parameters of the transient state and final state for keto−enol isomerization and decomposition of acetone. The potential energy diagram for surface reaction of acetone on Pt(111) is also obtained as shown in Figure 3. The results indicate that the energy barrier
To calculate the adsorption energy of molecular species we use the following formula: Eads = Ead/slab − Ead − Eslab
(1)
where Ead/slab is the total calculated energy for the adsorbate on the surface, Ead is a reference calculation on the isolated molecule, and Eslab is the calculated energy for the surface without the adsorbate present. Positive adsorption energy indicates that the adsorption is endothermic, and negative value of Ead indicates the exothermic characteristic of the adsorption. For the enol isomer, the reference gas phase state is also taken as acetone. The nudged elastic band (NEB)21 method was used to determine the minimum energy paths (MEPs) for all the steps.
3. RESULTS AND DISCUSSION 3.1. Adsorption, Tautomerization, and Decomposition of Acetone on Pt(111) Surface. The adsorption of the species for tautomerization of acetone on Pt(111) surface are investigated first. The optimized configurations and corresponding structure parameters are shown in Figure 1. The most
Figure 1. Optimized adsorption configuration for (a) acetone, (b) enol-isomer, and (c) enolate adsorbed on Pt(111) (unit: Å). Figure 3. Potential energy diagram for surface chemistry of acetone on Pt(111).
stable adsorption structure of acetone on Pt(111) is the η1(O) end-on geometry with adsorption energy of −0.40 eV. Our calculations show that the μ2(C, O) adsorbed species of acetone is unfavorable from the view of adsorption energy, and
of keto−enol isomerization of acetone (1.06 eV) is lower than the activation barrier of C−C bond scission that leads to
Figure 2. Configurations and structure parameters of the transition state (TS) and finial state (FS) for (a) isomerization of acetone, (b) C−C cleavage of acetone, (c) C−H cleavage of acetone, (d) deprotonation of enol-isomer, (e) C−C cleavage of enol-isomer, and (f) C−C cleavage of enolate on Pt(111) (unit: Å). 5452
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decomposition, implying the direct decomposition of acetone is impossible at room temperature. The enolization via intramolecular hydrogen transfer from the methyl group to the carbonyl oxygen atom has a lower barrier than the methyl deprotonation of acetone on Pt(111). The reason can be interpreted by the structures of the transition state for keto− enol isomerization and methyl deprotonation of acetone shown in Figure 2a and c. For the transition state for keto−enol isomerization (Figure 2a), the C−H bond distance increases to 2.16 Å and the O−H decreases to 1.87 Å. At the same time, the C−O carbonyl bond increases from 1.25 to 1.38 Å. Due to the formation of O−H bond, the geometry shown in Figure 2a is more stable than that of the transition state for methyl deprotonation as shown in Figure 2c. The tautomerization of acetone on Pt(111) proceeds through a much lower activation barrier than that in the isolated gas phase (2.78−3.0 eV),22 but a higher barrier than that in zeolite (0.69−0.98 eV).23 Considering the low energy barrier for desorption of acetone on Pt(111), we suggest that only a small amount of the adsorbed acetone can transform into the enol isomer, and C−C bond scission as well as C−H bond scission of acetone might not occur at room temperature. The energy barriers of C−C and O−H bond scission of the enol-isomer are also shown in Figure 3. The results indicate that the hydroxyl proton of the enol isomer might be abstracted by Pt atom to yield enolate and H with the energy barrier of 0.77 eV. The produced enolate can undergo C−C bond scission to decompose with the barrier of 0.69 eV. It is concluded that acetone can undergo tautomerization on Pt(111) surface at moderate condition, and the keto−enol isomerization is the origin of the decomposition production of acetone. These findings can explain the surface experimental results on Pt(111) surface that the η1(O) acetone is the majority species with weak adsorption and that a small amount of μ2(C, O) species might decompose to CO and methane.7 3.1. Hydrogenation of Acetone on Pt(111) Surface. The results of Jeffery et al.15 suggest that the enol form of ketocontaining molecules can be formed and this transformation can accelerate the hydrogenation due to the fact that the CC bond was hydrogenated at a much higher rate than the CO bond. In our work, activation barrier for hydrogenation of acetone and enol isomer on Pt(111) is calculated and a potential energy diagram is shown in Figure 4. Initially, acetone and dihydrogen are adsorbed on the surface. The first hydrogenation of the adsorbed acetone has been verified to proceed via the addition of the hydrogen to the carbonyl carbon by Sinha et al.20 and Alcalá et al.24 Therefore, the possibility of the initiation step of protonating the carbonyl oxygen is not discussed in this work. The first step of acetone hydrogenation is to form an adsorbed alkoxy species with the activation energy of 0.62 eV, which is consistent with the DFT result of Alcalá et al.24 The activation barrier of the first step of enol hydrogenation is 0.34 eV, and that of the second step in acetone and enol hydrogenation sequence is 0.12 and 0.32 eV, respectively. These results indicate that the hydrogenation of the enol isomer is faster than that of acetone on Pt(111) surface at moderate condition (< 473 K, for gas/liquid-phase hydrogenation of acetone). However, we cannot obtain the conclusion that the presence of the enol-isomer increases the hydrogenation rate of acetone on Pt(111) according to the fact that the acetone enolization is not facile on Pt(111) surface as shown in the previous section. On the other hand, the tautomerization step can be inhibited under a sufficient
Figure 4. Potential energy diagram for hydrogenation of acetone and enol isomer on Pt(111).
coverage of hydrogen on Pt(111) as shown in the observations of Lavoie et al.25 It is concluded that the effect of keto−enol isomerization on hydrogenation of both acetone and ketocontaining molecules on Pt(111) surface is insignificant, especially under a sufficient H2 pressure.
4. CONCLUSION We investigated the adsorption, tautomerization, and hydrogenation of acetone on Pt(111) surfaces. More attention was paid to the role of the keto−enol isomerization on selective hydrogenation and unselective decomposition of acetone. In conclusion, we suggest that only a small amount of the adsorbed acetone can undergo enolization and subsequently decompose on clean Pt(111) surface. So the enol tautomer is the origin of acetone decomposition on Pt(111) surface. The effect of the keto−enol isomerization on hydrogenation of acetone on Pt(111) under sufficient hydrogen pressure is not significant due to the high activation barrier of enol formation. Hence, it is unlikely that the rate enhancement of acetone hydrogenation is due to the presence of the enol isomer on Pt(111) surface.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86-10-82543108. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (Grants 21306192 and 51276181) and the National Basic Research Program of China (Grant 2011CB710705).
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