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Comparative Kinetic Monte Carlo Study of Acetic Acid Decomposition to Surface Carbon Species and Undesirable Byproducts on Pd(100) and Pd/Au(100) from Density Functional Theory Based Calculations Yanping Huang, Xiuqin Dong, Yingzhe Yu, and Minhua Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05072 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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The Journal of Physical Chemistry

Comparative Kinetic Monte Carlo Study of Acetic Acid Decomposition to Surface Carbon Species and Undesirable Byproducts on Pd(100) and Pd/Au(100) from Density Functional Theory Based Calculations Yanping Huangab, Xiuqin Donga, Yingzhe Yua,*1, Minhua Zhangab,*2 a

Key Laboratory for Green Chemical Technology of Ministry of Education,

R＆D Center for Petrochemical Technology, Tianjin University, P. R. China b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Tianjin 300072, P. R. China *1

Corresponding author. Tel.: +86-22-27405972; fax: +86-22-27406119. E-mail address: [email protected] (Yingzhe Yu)

*2

Corresponding author. Tel.: +86-22-27406119; fax: +86-22-27406119. E-mail address: [email protected] (Minhua Zhang)

ABSTRACT

Acetic acid decomposition on Pd(100) and Pd/Au(100) during vinyl acetate (VA) synthesis from ethylene acetoxylation is the primary source of such undesirable byproducts as methanol (CH3OH), acetaldehyde (CH3CHO), ketene (CH2CO), acetone (CH3COCH3), and methane (CH4), and also one possible source of surface carbon formation, which will lead to the deactivation of the catalyst. In this work, density functional theory (DFT) calculations and kinetic Monte Carlo (kMC)

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simulation were performed to probe the mechanism of acetic acid decomposition on Pd(100) and Pd/Au(100) at the molecular level. The corresponding adsorption of relevant species involved in acetic acid decomposition on Pd(100) and Pd/Au(100) was investigated and the transition states of the elementary reactions involved were identified. The results show that the most probable pathway of acetic acid decomposition on Pd/Au(100) is CH3COOH → CH3COO+H → CO2+CH3+H → CO2+CH4, followed by CH3COOH → CH3COO+H → CO2+CH3+H → CO2+CH2+2H → CO2+CH+3H → CO2+C+4H, which agrees nicely with the results of many other researchers that CO2, CH4, H2 and surface carbon are the main products of acetic acid decomposition. If researchers can find some way to suppress the decarboxylation of CH3COO at the beginning in the future, then the overall undesirable byproducts can be further greatly reduced, which can significantly improve the quality of VA. Our work can provide guidance for designing and developing novel catalysts with high efficiency for VA synthesis from ethylene acetoxylation.

1. INTRODUCTION

Recently, acetic acid decomposition is drawing the attention of increasing researchers. Acetic acid decomposition catalyzed by photocatalysts has been investigated as a typical eco-friendly representative of treatment for air pollution and wastewater1-3. Besides, steam reforming of acetic acid is studied as a model substance in bio-oil for biomass-derived hydrogen production which can be widely used in fuel

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cells and combustion engines as one of the promising clean and renewable energy resources4-8. Because of its wide application, acetic acid decomposition on many different catalysts have been examined, such as Ag9, Pt10, Rh11, Co12, Pt/ZrO28, CuO/Rutile13, TiO214, Pt/TiO214,15, Pt/CeO216, MgO5,17 and so on. Great efforts also have been devoted to acetic acid decomposition on palladium18-21, since acetic acid is one important reactant of vinyl acetate (VA) synthesis from ethylene acetoxylation and it is also one possible source of surface carbon formation and many sorts of undesirable byproducts generated during VA synthesis from ethylene acetoxylation, according to the following equation22-24. CH3COOH + C2H4 + 0.5O2 → CH3COOCHCH2 + H2O

(1)

Acetic acid can form surface acetate species through dehydrogenation or form acetyl through dehydroxylation. Subsequently, it is easy for acetate to undergo decarboxylation to produce CH3 and CO2. C2H2O2 generated through acetate decomposition can quickly decompose to form CO2 and CH2, which further decompose to CH and H18. Barteau et al.

25

examined acetic acid decomposition on

Au(110) by temperature-programmed reaction (TPR) and found CO2, CH4, H2, CO and surface carbon. Haley et al.19 used XPS, TPR and HREELS to study the adsorption and reactions of acetic acid on Pd(111) and they found CO2, H2O, CO, H2 and surface carbon were generated through thermal decomposition of acetic acid. Besides, they pointed out that methyl is the main source of surface carbon and the monodentate acetate is the source of CO. However, monodentate acetate decomposition yields atomic oxygen, which may react with CO to produce CO2. It is 3



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very likely that the well-known water-gas shift reaction (WGSR) is involved in this process, according to the following equation. CO + H2O ⇌ CO2 + H2

(2)

In addition, methanol, acetone, acetaldehyde and ketene were detected in experimental studies16. According to the intermediates detected the experimental studies, the reaction network of acetic acid decomposition is proposed, as is shown in Figure 1.

Figure 1 the reaction network of acetic acid decomposition (In the following text, Rx will stand for the corresponding reaction in this figure with the number x near the arrow.)

Meanwhile, researchers have been paying increasing attention to bimetallic catalysts for VA synthesis from ethylene acetoxylation because the combination of a reactive metal with a less reactive one can lead to the formation of particular ensembles of active sites that can increase the activity and the selectivity of supported 4


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catalysts26-28, which helps inhibit the undesired reactions which yield CO, CO2 and surface carbon. Alloying Au with Pd appears to be an effective way to prevent PdCx formation in Pd-based catalyst29. Kumar et al. have found that Pd(111), Pd(100), Pd/Au(111) and Pd/Au(100) single crystals all can catalyze the reaction, among which Pd/Au(100) is the most active30-32. They suggested that larger Pd ensembles are not required, that larger ensembles containing contiguous Pd atoms are much less efficient than a properly spaced pair of Pd monomers and that the critical reaction site for VA synthesis consists of two diagonal Pd atoms. Besides, they found that on both Pd/Au(100) surface and Pd/Au(111) surface when the Pd coverage is 0.1 monolayer (ML), the reaction rate reaches maximum. Thus, according to the above description, the following configurations of Pd(100) and Pd/Au(100) are adopted in this work, as is shown in Figure 2.

Figure 2 the configurations of Pd(100) and Pd/Au(100) with two diagonal Pd atoms Kinetic Monte Carlo (kMC) simulation method is a stochastic approach to examine the events occurring on the catalyst surface33. In principle, kMC allows us to keep track of the exact dynamic evolution of all surface species as a function of time and reaction conditions. Therefore, kMC can directly and explicitly exhibit the 5



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possible pathways of a complex reaction network. Combined with DFT calculations, it has been widely applied in heterogeneous catalysis34-39, as well as homogeneous processes in solutions40. In this work, with density functional theory method, the adsorption of the key intermediates, the energetics of the key reactions involved in acetic acid decomposition on Pd(100) and Pd/Au(100) were investigated and kMC simulation method was employed to explore the possible pathways of acetic acid decomposition on Pd(100) and Pd/Au(100) in order to provide insights of catalytic mechanism of acetic acid decomposition and the promoting effect of Au in Pd-based catalysts in VA synthesis at the molecular level and provide theoretical reference and guidance for further application and development of efficient commercial catalysts and the control of carbon deposit for VA synthesis from ethylene acetoxylation.

2. COMPUTATIONAL MODELS AND METHODS

2.1 COMPUTATIONAL SURFACE MODELS

Based on the optimized Pd bulk structure, the five-layered Pd(100) surface model was built. The calculated value of the lattice constant of Pd bulk was 3.96 Å, which is close to the experimental value of 3.89 Å, , as is shown in Figure 2. On the basis of the optimized Au bulk structure, the Pd/Au(100) surface model was built through replacing two diagonal Au atoms on Au(100) with two Pd atoms, as is shown in Figure 2. The Pd(100) and Pd/Au(100) surface were modeled, using a 4×4 unit cell consisting of five layers of atoms with 15 Å of vacuum. The two upmost surface 6


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layers and the adsorbates have been optimized, while the lowest layers are kept fixed. The calculated distance of the surface neighboring Au atoms is 2.887 Å. The calculated distance of the surface Pd and its first neighboring Au is 2.870 Å. The distance of the diagonal Pd atoms is 4.094 Å, which is close to the value of 4.080 Å reported by Chen31.

2.2 COMPUTATIONAL DETAILS

Based on spin unrestricted DFT calculations conducted with DMol3 in Materials Studio. The double numerical plus polarization (DNP) basis set is used in the calculation. The generalized gradient approximation (GGA) with the revised Perdew– Burke–Ernzerhoff (rPBE) functional41 was used as the exchange-correlation functional. A smearing of 0.005 Ha was adopted to accelerate the convergence. The k-point sampling consists of 2×2×1 Monkhorst–Packpoints42. The convergence criteria for geometry optimization and energy calculations were set as 1.0×10-5Ha, 2.0×10-5Ha, 0.004 Ha/Å, and 0.005 Å for the tolerance of self-consistent field (SCF), energy, maximum force, and maximum displacement, respectively. Complete LST/QST43 was used to identify the possible transition state. Then, the possible transition state was refined. Finally, whether it was the real transition state (TS) was determined by the frequency analysis, and all the transition states had a single imaginary frequency. The convergence criterion is the same as that of geometry optimization. The adsorption energy of different reactants on Pd/Au(100) surfaces is defined 7



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as: Eads(A)= EA*-slab- Eslab- EAgas

(3)

where EA*-slab, Eslab and EAgas are the total energies of different reactants adsorbed on the corresponding Pd/Au(100) surface slabs, the bare Pd/Au(100) surface slabs and the free reactants, respectively. The activation barriers were calculated as the total electronic energy difference between the transition state (TS) and the reactant state (or initial state, IS). The elementary reaction energies were calculated as the total electronic energy difference between the product state (or final state, FS) and IS. A negative value for the reaction energy indicates an exothermic reaction, while a positive value shows an endothermic reaction. The activation barriers, ∆𝐸, and reaction energies, ∆𝐻, are calculated as follows. ∆𝐸=ETS-EIS

(4)

∆𝐻=EFS-EIS

(5)

where EIS, ETS and EFS represent the total energy of IS, TS, and FS. The detailed kMC approach has been reported in some previous publications 35-39

. The primary information can be provided as follows. KMCLib developed by Leetmaa et al.44 was used, and variable step size method

(VSSM) was adopted for kinetic Monte Carlo algorithm, the detailed procedure of VSSM is shown below. 1. Build a collection of lists, one per configuration, containing all the enabled reactions of that configuration. 8


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2. Select a time the based on the sum of the rate constants of all these stored reactions. 3. Select a configuration, with a probability proportional to the sum of the rate constants of the stored reactions of each configuration. 4. Select a reaction of this configuration, uniformly. 5. If it is stillpossible, execute it. 6. Update time and the lists. 7. Continue from 2. In the kMC models, the Pd(100) and Pd/Au(100) surface are represented by a two-dimensional square periodic grid of 64 × 64 lattices containing three types of surface sites: top sites, bridge sites, and hollow sites. The preferred adsorption site of each species was determined by the DFT calculations. Periodic boundary conditions were employed to provide an adequate representation of the periodicity exhibited by the Pd(100) and Pd/Au(100) surface. In this way, surface species reappear at the opposite side of the lattice after jumping across the boundary. The initial state of present kMC simulations corresponded to an acetic acid atmosphere with relatively low pressure which can continuously impinge on the Pd(100) and Pd/Au(100) surface with a reasonable rate. The rates of such elementary events as surface reactions and desorption are calculated through transition state theory (TST), where the rates can be calculated as follows.

ri  vi exp(

Ei ) RT

(6)

Where vi is the pre-exponential factor, R is the gas constant, T is the reaction temperature and Ei is the activation barrier of the elementary event i. A theoretical 9



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value of 1013 s-1 was chosen for the pre-exponential factors of all the surface reactions and desorption steps45. Since acetic acid is the only source considered in the adsorption step on Pd(100) and Pd/Au(100) surface, the adsorption rate of ethylene is directly given with a moderate value that is not too large nor too small instead of calculating it from a given temperature and pressure. The temperature (450 K) of VA synthesis from ethylene acetoxylation was adopted. Given the fact that the main effect of diffusion is to bring the adlayer to steady state or equilibrium, and the diffusion rate calculated from Equation 1 does this at a much shorter time scale than the time scale of all the other elementary events. Thus, in principle, we can reduce the diffusion rate to the extent that the adlayer is still brought to steady state or equilibrium without changing the kinetics. When the surface coverage of all the species involved in the system reached a relatively steady value with only slight fluctuations which is caused by the stochastic nature of kMC simulations, the simulation was stopped.

3. RESULTS AND DISCUSSION

3.1 Adsorption and Elementary Reactions Involved in Acetic Acid Decomposition on Pd/Au(100)

The adsorption of all the species involved in acetic acid decomposition on Pd(100) and Pd/Au(100) shown in Figure 1 was examined, and the stable adsorption configuration of each species is shown in Figure 3, and the adsorption energy and the geometric parameters of each species involved in acetic acid decomposition are listed in Table 1. 10


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(A) CH3COOH

(E) CH3CHO

(B) CH3COO

(C) CH2COO

(F) CH2CO

(G) CH3OH

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(D) CH3CO

(H) CH3COCH3


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(I) CH4

(J) CH3

(M) C

(N) CO2

(K) CH2

(O)H2O

12


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(L) CH

(P) OH

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(a) CH3COOH

(e) CH3CHO

(b) CH3COO

(f) CH2CO

(c) CH2COO

(g) CH3OH

13


(d) CH3CO

(h) CH3COCH3


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(i) CH4

(j) CH3

(k) CH2

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(l) CH

(m) C (n) CO2 (o) H2O (p) OH Figure 3 the stable configuration of the species involved in acetic acid decomposition on Pd(100) and Pd/Au(100)

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Table 1 the adsorption energy (Eads, kcal/mol) and the geometric parameters (Å) of each species involved in acetic acid decomposition on Pd(100) and Pd/Au(100) Species

Pd(100)

Pd/Au(100)

Eads

Distance

Eads

Distance

CH3COOH

-17.06

Pd-Oc(2.241)

-13.10

CH3COO

-54.42

Pd-Oh(2.131;2.131)

-45.17

CH2COO

-79.79

Pd-Oh(2.262;2.213)

-19.59

CH3CO

-26.55

Pd-O(2.287)

-37.36

CH3CHO CH2CO CH3OH CH3COCH3 CH4 CH3 CH2

21.43 24.60 23.54 20.30 29.59 -42.89 -90.86

Pd-O(2.336) Pd-O(3.272) Pd-O(2.354) Pd-O(2.309) Pd-H(2.369) Pd-C(2.053) Pd-C(2.012;2.012)

-11.10 -10.44 -9.86 -11.48 -7.97 -37.79 -76.39

CH

-146.20

-121.21

C

-166.73

Pd-C(2.091, 2.091, 2.091, 2.091) Pd-C(2.002,2.011, 2.011,2.011,2.052)

Pd-Oc(3.021) Au-Hh(2.312) Pd-Oh(2.185) Au-Oc(2.324) Pd-Oh(2.168) Au-Oc(2.282) Pd-O(2.925) Au-C(2.115) Pd-O(2.918) Pd-O(3.882) Au-O(3.227) Pd-O(2.804) Pd-H(2.595) Pd-C(2.083) Pd-C(2.038) Au-C(2.092) Pd-C(2.069, 2.056) Au-C(2.263, 2.262) Pd-C(1.955, 1.958) Au-C(2.190, 2.190)

-130.38

Note: Oc represents O of carboxyl, Oh represents O of hydroxyl and Hh represents H of hydroxyl.

In the calculations, for the most stable co-adsorption configuration on Pd(100) and Pd/Au(100), it is considered that the relevant species are placed at the adjacent preferable adsorption sites. Finally, the elementary reactions involved in acetic acid decomposition were investigated on the most active Pd(100) and Pd/Au(100) surface. The calculated 15



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activation barriers and reaction energies of the elementary reactions involved in acetic acid decomposition on Pd(100) and Pd/Au(100) are listed in Figure 4, the distances of the atoms of the broken bonds and the newly formed bonds during acetic acid decomposition on Pd/Au(100) are listed in Table 2 and the corresponding TS of each elementary reaction is shown in Figure 5.

(a) energetics on Pd(100)

(b) energetics on Pd/Au(100) Figure 4 The activation barriers, ∆𝐸(kcal/mol), and reaction energies, ∆𝐻(kcal/mol), 16


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of the elementary reactions involved in acetic acid decomposition on Pd(100) and Pd/Au(100) (the red numbers and the blue numbers represent the activation barrier and the reaction energy of each elementary reaction, respectively)

TS1

TS5

TS2

TS3

TS6

TS7

17


TS4

TS8


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TS9

TS10

TS11

TS13 (a) TS on Pd (100)

18


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TS12

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TS1

TS5

TS2

TS3

TS6

TS7

19


TS4

TS8


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TS9

TS10

TS11

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TS12

TS13 (b) TS on Pd/Au(100) Figure 5 the configurations of the transition states involved in acetic acid decomposition on Pd(100) and Pd/Au(100) (The No. on the right side of TS corresponds to the Reaction with the same No. in Table 2)

20


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Table 2 the distances (Å) of the atoms of the broken bonds and the newly formed bonds during acetic acid decomposition on Pd(100) and Pd/Au(100) No. Reaction 1 2 3 4 5 6 7 8 9 10 11 12 13

bond

CH3COOH → CH3COO+H CH3COOH → CH3CO+OH CH3COO → CH2COO+H CH3COO → CO2+CH3 CH3+OH → CH3OH CH3CO → CH2CO+H CH2COO → CO2+CH2 CH3 → CH2+H CH3+H → CH4 CH3+CH3CO → CH3COCH3 CH3CO+H → CH3CHO CH2 → CH+H CH → C+H

O-H C-O C-H C-C C-O C-H C-C C-H C-H C-C C-H C-H C-H

IS 1.013 1.329 1.102 1.513 3.301 1.098 1.462 1.104 3.413 3.844 3.868 1.103 1.111

Pd(100) TS 1.542 3.223 2.053 2.641 2.384 2.274 2.423 3.162 1.769 2.869 2.600 1.192 2.063

FS 3.502 3.775 3.622 3.940 1.449 5.910 4.433 4.401 1.109 1.506 1.117 3.171 3.400

Pd/Au(100) IS TS FS 0.993 1.794 3.156 1.362 2.466 3.409 1.099 1.893 2.435 1.529 2.635 4.608 3.656 2.603 2.603 1.099 1.886 5.686 1.430 2.383 5.074 1.097 1.872 2.517 3.273 2.761 1.099 3.684 2.937 1.519 3.143 2.130 1.119 1.101 1.946 3.104 1.110 1.716 3.167

3.2 Analysis of the Reaction Pathway of Acetic Acid Decomposition Based on DFT Calculations

With the reaction network of acetic acid decomposition on Pd(100) and Pd/Au(100) built, the reaction pathways are to be assessed. First of all, let’s consider the case on Pd(100). CH3COOH can take part in two different reactions: dehydrogenation to form CH3COO (R1); dehydroxylation to generate CH3CO (R2). Our calculations found that it is easier for acetic acid dehydroxylation to occur on Pd(100) because of relatively low activation barrier of 10.22 kcal/mol compared with that of acetic acid dehydrogenation with the value of 2kcal/mol. Starting from CH3COO, two reactions may happen: dehydrogenation to yield 21



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CH2COO (R3), or C-C cleavage to form CO2 and CH3(R4). It was found that the latter is favorable on Pd(100) with the activation barrier of 67.11 kcal/mol, compared with that of the former with the value of 108.61 kcal/mol. The activation barrier of C-C cleavage of CH2COO (R7) is 40.59 kcal/mol. CH3 may participate in four reactions as one reactant: coupling with OH to yield CH3OH(R5), dehydrogenation to form CH2 (R8), hydrogenation to produce CH4 (R9) and coupling with CH3CO to generate CH3COCH3(R10). The activation barrier of R8 is the lowest and its occurrence doesn’t rely on the supply a second kind of species, demonstrating that R8 is most likely to take place, compared with the other three reactions. Similarly, CH3CO can act as the reactant of three reactions: dehydrogenation to form CH2CO(R6), hydrogenation to generate CH3CHO(R11) and coupling with CH3 to produce CH3COCH3. Among the three reactions, the activation barrier of R6 is the lowest, showing that it is kinetically most favorable for CH2CO dehydrogenation to occur. Besides, the hydrogenation of CH2CO relies on the supply of H on Pd (100). Therefore, it may compete for surface H atoms with R9. Surface carbon deposit is the main cause of the deactivation of the catalyst. Surface carbon can be generated via consecutive dehydrogenation of CH2 (R12 and R13). From what has been discussed above, the following most possible routes of acetic acid decomposition on Pd(100) can be obtained: 1) CH3COOH → CH3CO →CH3CHO, followed by: 22


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2) CH3COOH → CH3COO → CH3 → CH2 → CH → C; 3) CH3COOH → CH3COO → CH3 → CH4; and 4) CH3COOH → CH3COO → CH3 → CH3OH. Through similar analysis of acetic acid decomposition on Pd/Au(100), the following most possible route of acetic acid decomposition on Pd/Au(100) can be obtained: CH3COOH → CH3COO+H → CO2+CH3+ H → CO2+CH4, followed by the second most possible route of acetic acid decomposition on Pd/Au(100): CH3COOH → CH3COO+H → CO2+CH3+ H → CO2+CH2+2H → CO2+CH+3H → CO2+C+4H, which agrees nicely with the results of many other researchers that CO2, CH4, H2 and surface carbon are the main products of acetic acid decomposition.

3.3 Analysis of the Reaction Pathway of Acetic Acid Decomposition Based on kMC Simulation

A total of 26 elementary events have been taken into consideration in the kMC simulation, including 1 adsorption step (CH3COOH adsorpiton), 15 elementary reaction (13 primary reactions listed in Table 1 together with H+H→ H2 and H+OH → H2O), 9 diffusion steps (the diffusion of H, OH, CH3, CH2, CH, CH3COOH, CH3COO, CH3CO and CH2COO) and 1 desorption step (CH3COOH desorption). Enough kMC simulation runs have been conducted so that the system can reach 23



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steady state where the coverage of each species and the rate of each elementary event exhibit slight fluctuation which actually is caused by the stochastic nature of kMC simulation and represents the dynamic equilibrium of the system. During the kMC simulation, the frequency that each elementary reaction occurs has been recorded, as is shown in Figure 6.

Figure 6 the frequency of each elementary reaction during the kMC simulation

From Figure 6, the kMC simulation also predicts that almost a various byproducts can be produced from acetic acid decomposition on Pd(100), such as CO2, CH2CO, CH3OH, CH4, CH3CHO, C, and CH3COCH3 (in descending order). However, the kMC simulation predicts that the same most possible route of acetic acid decomposition on Pd/Au(100) is CH3COOH → CH3COO → CH3 → CH4, followed by CH3COOH → CH3COO → CH3 → CH2 → CH → C, which agrees with the DFT calculation prediction. However, through kMC simulation, more detailed 24


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insights can be obtained. The great difference of activation barrier between CH3COOH → CH3COO+H and CH3COOH → CH3CO+OH makes almost all of the adsorbed CH3COOH molecules undergo dehydrogenation, while only a trace amount of them are deprived of hydroxyl. All of the CH3COO generated from CH3COOH dehydrogenation produce CO2 and CH4 through decarboxylation. It is obviously that two dehydrogenation reactions, CH3COO → CH2COO+H and CH3CO → CH2CO+H, actually just don’t happen because of their relatively high activation barrier and the lack of available surface sites to settle the H atoms to be produced. Although the activation barrier of CH3 → CH2+H is just slightly higher than that of CH3+H → CH4. An overwhelming amount of CH3 yields CH4 through hydrogenation, while only a small amount generates CH2 via dehydrogenation, the probable reason for which is that the CH4 can easily desorb from the surface which can make the surface less crowded, while the dehydrogenation process will add to the crowd of the surface, which is of great disadvantage on the crowded surface. Meanwhile, only a trace amount of CH3OH, CH3COCH3 and CH3CHO can be produced. The kMC simulation results show that the Pd/Au(100) can effectively prohibit the formation of many kinds of undesirable byproducts during VA synthesis from ethylene acetoxylation. However, a large amount of CH4 is still inevitable. If researchers can find some way to suppress the decarboxylation of CH3COO at the beginning in the future, then the overall undesirable byproducts can be further greatly reduced, which can significantly improve the quality of VA. The comparison of the kMC simulations of acetic acid decomposition on Pd(100) 25



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and Pd/Au(100) suggests that the addition of Au into the Pd catalyst can significantly suppress the production of many kinds of undesirable byproducts, such as CH2CO, CH3OH, CH3CHO, and CH3COCH3, from acetic acid decomposition through inhibiting acetic acid dehydroxylation.

4. CONCLUSIONS

With DFT calculations, a relatively comprehensive study on acetic acid decomposition on Pd/Au(100) was conducted. A reaction network of acetic acid decomposition on Pd/Au(100) including the pathways that lead to surface carbon formation and the routes that result in several possible undesirable byproducts during VA synthesis from ethylene acetoxylation on Pd/Au(100) was proposed. The adsorption of the key intermediates and the energetics of the key reactions involved in acetic acid decomposition on Pd/Au(100) were investigated in order to provide insights of catalytic mechanism of acetic acid decomposition at the molecular level. Our results show that the most probable pathway of acetic acid decomposition on Pd/Au(100) is CH3COOH → CH3COO+H → CO2+CH3+ H → CO2+CH4, followed by CH3COOH → CH3COO+H → CO2+CH3+ H → CO2+CH2+2H → CO2+CH+3H → CO2+C+4H, which agrees nicely with the results of many other researchers that CO2, CH4, H2 and surface carbon are the main products of acetic acid decomposition. If researchers can find some way to suppress the decarboxylation of CH3COO at the beginning in the future, then the overall undesirable byproducts can be further greatly reduced, which can significantly improve the quality of VA.

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ACKNOWLEDGMENTS

The authors would like to thank the key laboratory for green chemical technology of ministry of education of P. R. China for technical assistance. These calculations have been possible thanks to National Supercomputing Center in Tianjin, P. R. China.

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