Reaction Mechanisms of Crotonaldehyde Hydrogenation on Pt(111

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Reaction Mechanisms of Crotonaldehyde Hydrogenation on Pt(111): Density Functional Theory and Microkinetic Modeling Xiao-Ming Cao, Robbie Burch, Christopher Hardacre, and P. Hu* School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Belfast, BT9 5AG, U.K. ABSTRACT: The microkinetics based on density function theory (DFT) calculations is utilized to investigate the reaction mechanism of crotonaldehyde hydrogenation on Pt(111) in the free energy landscape. The dominant reaction channel of each hydrogenation product is identified. Each of them begins with the first surface hydrogenation of the carbonyl oxygen of crotonaldehyde on the surface. A new mechanism, 1,4-addition mechanism generating enols (butenol), which readily tautomerize to saturated aldehydes (butanal), is identified as a primary mechanism to yield saturated aldehydes instead of the 3,4-addition via direct hydrogenation of the ethylenic bond. The calculation results also show that the full hydrogenation product, butylalcohol, mainly stems from the deep hydrogenation of surface open-shell dihydrogenation intermediates. It is found that the apparent barriers of the dominant pathways to yield three final products are similar on Pt(111), which makes it difficult to achieve a high selectivity to the desired crotyl alcohol (COL).

1. INTRODUCTION The chemo- and regioselective hydrogenation of α,β-unsaturated aldehydes to the corresponding unsaturated alcohols is of great importance in flavor and fragrance chemistry1,2 and in the pharmaceutical industry.3 The hydrogenation of α,β-unsaturated aldehydes on the conjugated function groups may proceed through a series of hydrogenation pathways and corresponding intermediates (see Scheme 1, where all the possible intermediates during the crotonaldehyde hydrogenation processes are described). Scientifically, it is fundamental and challenging to selectively hydrogenate the carbonyl group from the conjugated ethylenic and carbonyl function groups on the grounds that the saturated aldehydes instead of the desired unsaturated alcohols are the thermodynamically favored hydrogenation products. On one hand, a great number of experiments scanning the different metals such as Au,49 Os,10,11 Pt,1218 Rh,19 and PtSn2022 alloy have been performed for crotonaldehyde hydrogenation, aiming to achieve high selectivity to crotyl alcohol, amidst which Pt is the most commonly used metal. The role of the support has also been extensively investigated.2327 The saturated aldehyde (butanal) is the main product when the hydrogenation of α, β-unsaturated aldehydes occurs on Pt catalysts dispersed on silica or alumina, whereas the selectivity to crotyl alcohol could achieve 53% using a Pt/TiO2 catalyst.24 On the other hand, the reaction mechanism was not extensively discussed until recent density functional theory (DFT) calculations which have been dedicated to this issue. The DFT calculations began with the research of the chemisorption mode. The results showed that the favorable chemisorption modes of unsaturated aldehydes on the surface are not via the carbonyl bond,28 thus rejecting the preliminary results from semiempirical r 2011 American Chemical Society

extended H€uckel calculations with lower accuracy which showed that crotonaldehyde and cinnamaldehyde were adsorbed via the di-σCO mode.29 This appears to be in accordance with the prevailing wisdom that the carbonyl bond is more difficult to hydrogenate than the ethylenic bond.30 However, the recent DFT calculation work on the acrolein hydrogenation on Pt(111) from Loffreda et al.31,32 revealed that the existence of a so-called half EleyRideal mechanism for unsaturated aldehydes hydrogenation makes the carbonyl bond kinetically easier to hydrogenate than the ethylenic bond even though the carbonyl bond does not adsorb on the surface. In order to explain the low selectivity to unsaturated alcohol, they further proposed that the selectivity of the reaction depends on not only the surface hydrogenation steps but also the desorption steps of the partially hydrogenated products.32 In addition, the selectivity was recently investigated on different metal facets by DFT calculations. The work from Li et al.33 showed that the ethylenic bond is easier to hydrogenate on a Au20 cluster while the hydrogenation of the carbonyl bond is kinetically favored on Au(110). This result is in agreement with the experimental discovery23 that the selectivity is structure-sensitive. Although some mechanistic studies have been performed, it is worth noting that previous theoretical analysis mainly focused on the selective hydrogenation of either the carbonyl or the ethylenic group, corresponding to 1,2- and 3,4-addition mechanisms, respectively, whereas another reaction channel, 1,4-addition giving enol which can readily tautomerize into saturated aldehyde,3439 Received: July 10, 2011 Revised: August 16, 2011 Published: August 18, 2011 19819

dx.doi.org/10.1021/jp206520w | J. Phys. Chem. C 2011, 115, 19819–19827

The Journal of Physical Chemistry C

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Scheme 1. All the Possible Intermediates in the Process of trans-Crotonaldehyde (CAL, Unsaturated Aldehyde) Hydrogenation on Pt(111)a

a

The four available attack sites are numbered from 1 to 4, corresponding to O1, C2, C3, and C4, respectively. From the left to right, transcrotonaldehyde, mono-hydrogenation intermediates, di-hydrogenation intermediates, tri-hydrogenation intermediates, and full hydrogenation product n-butylalcohol (BOL), respectively.

was overlooked. Despite the calculations of the full competitive first and second hydrogenation pathways on acrolein31 being carried out, the probability of forming the saturated aldehyde through a 1,4-addition pathway was not discussed. It is commonly acknowledged that 1,4-addition is usually a mechanism of great importance on the hydrogenation of conjugate compounds in organic synthesis.4042 Hence it may be possible that the low selectivity to unsaturated alcohol of α,β-unsaturated aldehydes hydrogenation results from a high yield of saturated aldehyde via the 1,4-addition mechanism. It is essential to further investigate the influence of 1,4-addition mechanism on the selectivity from theoretical and kinetic aspects. Furthermore, the reaction channel to yield the full hydrogenation product, the saturated alcohol, is regarded as the further hydrogenation product from closedshell saturated aldehyde or unsaturated alcohol43 whereas the possibility of the full hydrogenation product only undergoing adsorbed open-shell species is not taken into account. Since the desorption process of the product is likely to be the key step to affect the reaction selectivity and the difficulty of the desorption step could be overestimated in the total energy landscape,44 it is preferred to comprehensively understand the reaction process in the free energy landscape. In addition, as the experimental work45 showed that flat Pt(111) is the preferentially oriented facet for the unsaturated alcohol, this paper focuses on achieving insight into the hydrogenation of crotonaldehyde on Pt(111). The microkinetics based on the Gibbs free energy landscape from DFT data and statistical thermodynamic treatments was carried out to simulate the TOF of each product. The contribution from each hydrogenation pathway is also described according to the DFT and kinetic data. The microkinetic modeling facilitates the quantitative definition of the dominant route to generate products. The microkinetic analysis sheds light on the fact that butanal (BAL) mainly originates from a 1,4-addition mechanism rather than from 3,4-addition, which decreases the selectivity of the crotonaldehyde hydrogenation to crotyl alcohol on Pt(111). The full hydrogenation product, butanol (BOL),

mostly comes from the deep hydrogenation of the adsorbed openshelled species instead of the partial hydrogenation products, crotyl alcohol or butanal.

2. THEORETICAL METHODS In this work, all periodic DFT calculations were performed with the Vienna ab initio simulation program (VASP),46,47 using PerdewBurkeErnzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation functional.48 The projector-augmented-wave (PAW) pseudopotentials49 were utilized to describe the core electron interactions. For the expansion of plane-wave basis set, the cutoff was set to 400 eV. The metal surface was modeled by a periodic 4-layer slab with a ∼12 Å vacuum region placed between periodically repeated slabs. A p(3 3) supercell (1/9 ML) was used with 3 3 1 MonkhorstPack k-point mesh sampling for Brillouin zone integration. Previous calculations31,32,44 have shown that the setup above is sufficient to obtain converged energy. During all the optimizations, the bottom two layers of metal atoms were fixed in the slab while the top two layers and adsorbates were relaxed. Transition states (TSs) of all the reactions were located with constrain minimization method.5052 The numerical calculations of the second derivatives of the potential energy surface based on the harmonic oscillator approximation provided the vibrational frequencies and corresponding normal modes. A geometrical displacement of 0.01 Å was utilized for all vibrational calculations. The vibrational calculations verified the geometric structures: no imaginary frequency for each adsorption intermediate and only one imaginary frequency for each TS, respectively. The statistical mechanics based on Boltzmann distribution was used to calculate the zero-point-energy (ZPE), entropy, internal energy and enthalpy derived from partition functions to achieve the free energy landscape. The free energy barriers of adsorption/desorption processes were calculated following the collision theory model and the microscopic reversibility principle. The detailed derivation can be found in our previous work.44 19820

dx.doi.org/10.1021/jp206520w |J. Phys. Chem. C 2011, 115, 19819–19827


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The microkinetic simulations were implemented on the basis of the free energy landscape. In our microkinetic model, the ideal continuous-stirred tank reactor (CSTR) was utilized to simulate the reactor. The model parameters follow the catalytic experimental work of Marinelli et al.53 The simulation was performed at 353 K with the mixture of crotonaldehyde (33 mbar) and hydrogen (978 mbar) pressure under the total pressure of 20 bar. Mathematica was used to solve the complete set of timedependent CSTR differential algebraic equations.44

Table 1. Adsorption Energies (Ead), Adsorption Free Energies (Gad) at 353 K under the Standard Pressure and Geometry Parameters for Intermediates on Pt(111)a adsorption energy (eV) adsorption free species without ZPE

with ZPE

energy (eV)

bond

distance (Å)

b

2.63

2.48

2.15

HPt

1.877

CALb

0.79

0.77

0.04

O1Pt

2.117

3. RESULTS AND DISCUSSION

C2Pt

2.200

3.1. Thermochemical Properties of Adsorbed Intermediates. We started to understand the reaction processes with the

C3Pt C4Pt

2.150 2.146

C2Pt

2.126

C3Pt

2.180

investigation of the thermochemical properties of surface intermediates. x* is defined as the adsorbed intermediate x in this paper. Ead(x), the adsorption energy of reaction intermediate x, is calculated as follows: EadðxÞ ¼ EðxÞ-EðxÞ-Eðsurf Þ

MS1b

MS2b

1.84

2.14

1.81

2.12

0.98

1.28

ð1Þ

Here, E(x) and E(x*) are the total energies of the intermediate x and the corresponding adsorbed intermediate, respectively. The molar Gibbs free adsorption energy Gad(x, T) at reaction temperature of 353 K and under the standard pressure is as follows: Gadðx, TÞ ¼ EadðxÞ þ ΔZPEðxÞ þ ΔUðx, tÞ þ RT TΔSðx, TÞ

H

C4Pt

2.134

O1Pt

2.049

C3Pt

2.109

C4Pt

2.164 2.114 2.154

MS3

1.61

1.48

0.65

O1Pt C2Pt C4Pt

2.123

MS4

1.18

1.12

0.54

O1Pt

2.093

C2Pt

2.279

C3Pt

2.141

C3Pt

2.138

ð2Þ

COLb

Here, ΔZPE(x), ΔU(x, t), and ΔS(x, T) are the changes of zeropoint-energy, internal energy and entropy during the adsorption process, respectively. In this paper, MSi, MSij, MSijl, MSijlk (i, j, l, k = 1, 2, 3, or 4) denote the mono-, di-, tri-, and tetrahydrogenated intermediates, respectively, where the number 1, 2, 3, and 4 corresponds to the hydrogenation on O1, C2, C3, and C4 marked in Scheme 1, respectively. TSi, TSij, TSijl, TSijlk are the corresponding TSs. The hydrogenation elementary steps are defined as +i, +ij, +ijl, and +ijlk. The sequence of hydrogenation is arranged from the left to the right. For instance, the +12 pathway means the first hydrogenation on O1 via TS1 and the second hydrogenation on C2 via TS12. The whole reaction starts from the gaseous crotonaldehyde (CAL) with favored E-(s)-trans configuration adsorbing on Pt(111) via the most stable η4-trans-tetra-σ configuration.44 In fact, in terms of its Gibbs free adsorption energy, CAL does not easily adsorb on Pt(111), being even 0.04 eV less stable on Pt(111) than the gaseous state under reaction conditions. This is attributed to a great entropy loss of the adsorbate with respect to gaseous CAL. As described in Scheme 1 for the reaction network, four first hydrogenation elementary steps from CAL* can yield four corresponding monohydrogenation intermediates: hydrogenation on O1 to the 1-hydroxycrotyl (MS1*), on C2 to the crotyloxy (MS2*), on C3 to the 1-formyl-1-propyl (MS3*), and on C4 to the 1-formyl-2-propyl (MS4*), respectively. All the monohydrogenation intermediates chemisorb on the surface in the form of η3-trans-tri-σ via the remaining double bond and radical atom next to the hydrogenated atom. The adsorbed partial hydrogenation products are followed by the subsequent second hydrogenation step from the corresponding intermediates states. As Scheme 1 shows, adsorbed crotonyl alcohol (COL*) can be produced via the corresponding MS1* or MS2*, and analogously,

ENOL

0.84 0.78

0.84 0.79

0.13 0.08

C4Pt

2.141

C2Pt C3Pt

2.141 2.130

BAL

0.24

0.20

0.39

O1Pt

2.326

MS13

3.28

3.07

2.18

C2Pt

2.118

C4Pt

2.117 2.068

MS23

2.38

2.25

1.39

O1Pt C4Pt

2.142

MS24

2.90

2.74

1.92

O1Pt

2.017

MS123

1.59

1.49

0.69

C3Pt C4Pt

2.144 2.138

MS124

1.75

1.61

0.79

O1Pt

2.325

C3Pt

2.120

MS134

1.66

1.55

0.82

C2Pt

2.140

MS234

1.50

1.33

0.57

O1Pt

2.178

BOL

0.21

0.18

0.43

O1Pt

2.362

2.156 a Adsorption energies with and without the zero-point-energy (ZPE) correction are given. b The adsorption data for these species come from our previous work.44

adsorbed but-1-en-1-ol (ENOL*) can be formed via MS1* or MS4* and adsorbed n-butyl aldehyde (BAL*) via MS3* or MS4*, respectively. As displayed in Table 1, the adsorption energies of COL and ENOL are close to each other owing to the similar most stable adsorption configuration (di-σ-CC), while the adsorption energy of BAL which adsorbs atop Pt(111) via aldehyde oxygen (on-top configuration) is weaker with respect to those two molecules above. Moreover, in the free energy landscape, the adsorbed BAL is 0.39 eV less stable than the gaseous one. Thus, it is very likely that generated BAL directly desorbs to the gas phase. Since the entropies of the final products in the gas phase are close 19821



Figure 1. Most stable adsorption structures of all the possible intermediates in the process of trans-crotonaldehyde (CAL) hydrogenation on Pt(111). Pt is in blue, O in red, and H in white.

to each other, their stability order in terms of the free adsorption energy is identical to that in terms of the adsorption energy data. Except for the desorption channel, these three closed-shell species and the other four open-shell dihydrogenation intermediates (the so-called closed-shell or open-shell structure only denotes the structure of the intermediate in the gas phase in this paper) could be further hydrogenated to the full hydrogenation product BOL via four possible trihydrogenation intermediate states. Similar to BAL, BOL is 0.43 eV more stable in the gas phase than adsorbing on the surface under the reaction conditions in the free energy landscape. This indicates that BOL tends to readily desorb to the gas phase after the surface hydrogenation. All the most stable adsorption structures of intermediates are summerized in Figure 1. 3.2. Reaction Pathway. 3.2.1. Partial Hydrogenation Pathways. 1,2-Addition to Yield COL. The hydrogenation of the carbonyl bond of CAL* can produce COL via one of two possible pathways: +12 or +21 hydrogenation. Since our previous work44 has reported the results of these two pathways, here we just briefly summarize the related results as follows. In the +12 hydrogenation pathway, the first hydrogenation on O1 of crotonaldehyde yields MS1* which is the most stable monohydrogenation intermediate on Pt(111). The free energy barrier for producing the MS1* via the TS1* is only 0.52 eV, which is the lowest free energy barrier among all the possible first hydrogenation steps. The second hydrogenation on C2 forming COL* undergoes the TS12*, which is 0.15 eV higher than the TS1* in the free energy profile. Consequently, the TS12* affects the reaction rate more significantly than the TS1* in this pathway. In the +21 hydrogenation pathway, H first attacks the C2 via the TS2* which is 0.17 eV higher than the TS1*. Since the TS21* is 0.38 eV less stable than the TS2* in the free energy profile due to the low

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stability of MS2*, the TS21* is the key TS of this pathway. Because the TS21* is 0.39 eV less stable than the TS12*, the +21 addition is a less favorable pathway to yield COL. Finally, the generated COL* desorbs to the gas phase with a 0.49 eV desorption free energy barrier, which is lower than the free energy barrier of the hydrogenation of MS1* to COL* (0.84 eV). However, as the TSdes-COL is slightly higher (0.06 eV) than the TS12* in the free energy profile, either the TS12* or the TSdes-COL is likely to influence the turnover frequency (TOF). 3,4-Addition to Yield BAL. For 3,4-addition pathways, the hydrogenation occurs on the ethylenic bond. It is a direct approach to yield BAL. This can be formed via two symmetric hydrogenation pathways: +34 or +43 pathways. We found that both the TS structures and the free energy barriers in the two surface hydrogenation pathways of 3,4-addition are similar on Pt(111). In contrast to all the TSs in the surface hydrogenation process of +12 addition pathways, where all the attacked atoms almost lose the bonding with the surface, it is intriguing that the attacked C atom coadsorbs on the same Pt atom with the hydrogen in all the TSs of 3,4-addition pathways. The CH distances are 1.638, 1.616, 1.641, and 1.635 Å for TS3*, TS4*, TS34*, and TS43*, respectively, and the PtC distances of the corresponding TSs are 2.395, 2.340, 2.318, and 2.385 Å, respectively. In addition, the hydrogenation of the ethylenic bond generally requires higher free energy barriers than that of the carbonyl bond: The free energy barriers are 0.94, 0.87, 0.92, and 0.83 eV via TS3*, TS4*, TS34*, and TS43*, respectively, much higher than those in the hydrogenation of the O atom in the 1, 2-addition mechanism (only 0.52 and 0.37 eV for TS1* and TS21*, respectively). As shown in Figure 2, in both +34 and +43 pathways the first hydrogenation is evidently an endothermic step followed by an exothermic process for the second hydrogenation step. In spite of the close stability of intermediates and TSs (∼0.1 eV difference in each corresponding hydrogenation step), the intermediates and TSs in the +43 pathway are all more stable than the corresponding ones in the +34 pathway. As a result, the +43 pathway becomes a preferential pathway of the 3,4-addition mechanism. Because BAL in the gas phase is far more stable than its corresponding adsorbed state, the desorption free energy barrier is negligible. Accordingly, it is can be inferred that we may even ignore the desorption step of BAL*. 1,4-Addition to Yield BAL. 1,4-Addition is an alternative feasible mechanism for the yield of BAL. It is an indirect way to yield BAL as compared to the 3,4-addition. The ENOL is produced via the surface hydrogenation, and then it tautomerizes readily to BAL. The tautomerization process is not presented here because it is so rapid that the formation of short-lived ENOL is usually hard to detect by the regular experimental techniques.54 The recent theoretical work also demonstrates its rapid conversion.3439 Thus, it is so swift relative to the surface hydrogenation and desorption process that it hardly affects the TOF of the whole reaction. The +14 pathway is the same as the +12 pathway in that the hydrogenation of CAL* first undergoes TS1* with a low free energy barrier and then forms the most stable intermediate, MS1*. Then, H migrates toward the MS1* and attacks on the C4 atom. At TS14*, the associating CH bond with a distance of 1.477 Å shares one Pt atom. The produced ENOL* has to pass through TS14* with a high second hydrogenation free energy barrier (0.92 eV), which is 0.15 eV higher than that of TS12*. Furthermore, the desorbed ENOL will readily tautomerize to the 19822



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Figure 2. Free energy profiles of 3,4-addition pathways for the hydrogenation of crotonaldehyde (CAL) to butyl-aldehyde (BAL) on Pt(111). The solid and dashed lines represent +43 and +34 pathways, respectively. Some key intermediates and transition states are shown in the inserts. Pt is in blue, O in red, and H in white.

Figure 3. Free energy profiles of 1,4-addition pathways for the hydrogenation of crotonaldehyde (CAL) to but-1-en-1-ol (ENOL) on Pt(111). The solid and dashed lines represent +14 and +41 pathways, respectively. Some key intermediates and transition states are shown in the inserts. Pt is in blue, O in red, and H in white.

most stable partial hydrogenation product, BAL. This indicates that the generation of ENOL* is kinetically slightly inferior to, but thermodynamically superior to, that of COL*. This could account for the reaction selectivity. It is clear from Figure 3 that TS14* is 0.29 eV less stable than TS1*. Consequently, the TOF of this pathway would mainly depend on the second hydrogenation step. In the +41 pathway, the first hydrogenation on C4 generates MS4* through TS4* with a 0.87 eV free energy barrier as it does in the +43 pathway. Subsequently, the second hydrogenation yielding ENOL* occurs easily through TS41*, where the atop H atom is 1.442 Å away from the O and the CdO moiety does not directly bond with the Pt atom. The tiny free energy barrier (only 0.06 eV) results from this strongly exothermic elementary step,

implying a transient intermediate of MS4*. This result also shows that the first hydrogenation may determine the TOF of this pathway. In the free energy profile displayed in Figure 3, the TS of ENOL* desorption possessing a small free energy barrier (0.44 eV) locates below the rate-determining TSs, TS14* and TS4*, in the surface hydrogenation steps of the +14 and +41 pathways, respectively. Hence, the desorption step would not control the TOF of the 1,4-addition pathways. The desorbed ENOL will subsequently readily convert to BAL. 3.2.2. Pathways of the Full Hydrogenation to BOL. As depicted in Scheme 1, starting from CAL, in total there are 24 possible pathways to arrive at BOL. However, as discussed above, the first hydrogenation always tends to occur on oxygen on Pt(111) both 19823



Figure 4. Free energy profiles for the hydrogenation of crotonaldehyde (CAL) to butylalcohol (BOL) on Pt(111). The free energy profile of +1234 pathway was reported in our previous work44 to address the difference between the free energy and total energy profiles.

thermodynamically and kinetically. Hence, it is not necessary to enumerate all the pathways, and it stands to reason that only the pathways through the first hydrogenation on oxygen deserve to be taken into account. The other pathways must play a minor role in the yield of BOL. Then, the possible pathways can be reduced to six: +1234, +1243, +1324, +1342, +1342, +1423, and +1432. Among them, four pathways come from the deep hydrogenation of closed-shell dihydrogenation products COL* and ENOL*. This probably reduces the TOF of COL or BAL. The fourth hydrogenation step will directly generate gaseous BOL without passing through BOL* since BOL* with the most stable adsorption configuration (on-top) is still 0.43 eV less stable than the corresponding gaseous BOL under the reaction conditions. The further hydrogenation of COL* to BOL can go through +1234 or +1243 pathways. In both pathways, the third hydrogenation step has to overcome a very high free energy barrier, 0.95 eV via TS123* and 0.97 eV via TS124*, respectively. Although the free energy barriers of the subsequent fourth hydrogenation are not so high as the third hydrogenation step (0.67 eV via TS1234* and 0.78 eV via TS1243*, respectively) in the free energy profiles displayed in Figure 4, TS1234* and TS1243* are even less stable than TS123* and TS124* resulting from the fact that the reaction free energies of the third hydrogenation step absorb a great deal of heat (0.38 and 0.31 eV in +1234 and +1243 pathways, respectively). Starting from COL*, since the overall free energy barriers of +1234 and +1243 pathways are as high as 1.05 and 1.09 eV with respect to COL*, respectively, which are significantly higher than the desorption free energy barrier of COL* (0.49 eV), it is clear that COL* tends to desorb rather than undergo deep hydrogenation. ENOL* could be converted to BOL via the +1423 or the +1432 pathway. In the +1423 pathway, the third hydrogenation yields MS124* via TS142* with a 0.82 eV free energy barrier. At TS142*, the attacked C2 which is 2.559 Å away from the nearest Pt atom almost loses the bonding with the surface. Since ENOL* is the most stable dihydrogenation intermediate in Figure 4, TS142* becomes the most stable trihydrogenation TS as shown in Figure 4. However, similar to the +1243 pathway, the +1423 pathway has to undergo TS1243* which is the least stable TS in Figure 4 from MS124* to BOL. This implies that the +1423 pathway makes little contribution to the production of BOL. In the +1432 pathway, although the free energy barrier of the third

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hydrogenation via TS143* is 0.09 eV higher than that via TS142* and the free energy barrier from MS134* to BOL is the same as that from MS124* to BOL, the stronger stability of MS134* relative to MS124* leads to the fact that TS1432* is 0.20 eV lower than TS1423* in the free energy profiles. The overall free energy barriers of the +1432 and +1423 pathways with respect to ENOL* are 1.04 and 1.24 eV, respectively, significantly higher than the desorption free energy barrier of ENOL* (0.44 eV), implying that ENOL* can hardly transfer to BOL. The intermediates on the other two pathways, +1324 and +1342, are all open-shell species until the yield of BOL. Because of its open-shell radical structure, MS13* is much less stable than COL* and ENOL* as shown in Figure 4. However, because the distance of CPt at TS13* (2.564 Å) is longer than that at TS14* (2.312 Å), which reduces the bonding share with H, the free energy barrier via TS13* is even slightly lower than that via TS14* where C and H share one Pt atom. Starting from MS13*, there are two branches to yield BOL. In the +1324 pathway, the free energy barriers for the third hydrogenation (0.84 eV) and the fourth hydrogenation steps (0.67 eV) are low in comparison to the other pathways. However, this pathway passes the least stable dihydrogenation (MS13*) and trihydrogenation (MS123*) intermediates, as shown in Figure 4. This gives rise to the difficulty to produce BOL via this pathway. In the +1342 pathway, from MS13* to MS134* the free energy barrier is low (0.66 eV) and the reaction free energy is only 0.03 eV. The free energy barrier of the subsequent fourth hydrogenation step is still not high (0.78 eV). As Figure 4 depicted, the +1342 pathway and the +1432 pathway share the lowest overall free energy barrier with reference to CAL*+4H*. Hence, these two pathways are most likely to be the dominant pathways to yield BOL. Microkinetics analysis in the next section will show the detailed value of each pathway contribution. 3.3. Microkinetic Simulations. Aiming to quantitatively identify the dominant pathway to each product and the selectivity of the reaction network, the microkinetic model including 25 elementary steps described above was built on the basis of the presented free energy data obtained from DFT and statistical treatments in previous sections, which are listed in Table 2. The reaction rate constants were derived from transition state theory. All the kinetics results are presented when the reaction system arrives at the steady state. As shown in Table 3, in which all the surface coverages from microkinetic simulation are listed, H* is the dominant surface species. It is natural that the high coverage of H* is obtained because of the weak interaction between CAL and the surface. Second to H*, MS1* makes up a certain proportion of the surface. This is reasonable for three reasons: First, MS1* is the most stable monohydrogenation species, and from CAL* to MS1* it is a thermodynamically favored reaction. Second, it is kinetically easy to generate. The free energy barrier from CAL* to MS1* is facile, being lower than the other free energy barrier for the first hydrogenation step. Third, starting from MS1*, all the possible second hydrogenation steps need to absorb heat, and in fact, MS1* is the most stable surface species in the whole reaction network. As shown in Table 3, the other monohydrogenation species all have very low coverages, less than 1011. This indicates that the TOFs of the final products hardly come from these three species. Except for MS1* and H*, COL* and MS134* are the other two surface species with a higher coverage relative 19824



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Table 2. Kinetic Parameters for All Elementary Steps Included in the Microkinetic Model for the Hydrogenations of Crotonaldehyde at 353 Ka step i

reaction

ΔG°

G°

0.04

0.36

Table 3. Surface Coverages of Adsorbates at Steady State under the Reaction Conditions

CAL*

6.22 108

0.38

H* MS1*

9.60 101 3.30 102

0.52

MS2*

7.84 1014

0.68

0.69

MS3*

8.09 1012

CAL* + H* h MS3* + / CAL* + H* h MS4* + /

0.54 0.48

0.94 0.87

MS4*

3.45 1019

COL*

1.71 104

b

MS1* + H* h COL* + /

0.33

0.77

ENOL*

5.88 1010

b

8

MS2* + H* h COL* + /

0.46

0.37

BAL*

4.57 1021

9

MS3* + H* h BAL* + /

0.33

0.92

10

MS4* + H* h BAL* + /

0.28

0.83

MS13* MS123*

3.76 106 5.35 108

11

MS1* + H* h ENOL* + /

0.18

0.92

MS124*

7.11 109

12

MS4* + H* h ENOL* + /

0.40

0.06

MS134*

1.88 104

13 14b

MS1* + H* h MS13* + / COL* h COL + /

0.42 0.13

0.86 0.49

/

6.59 103

15

ENOL* h ENOL + /

0.08

0.44

16

BAL* h BAL + /

0.39

0.00

17

COL* + H* h MS123* + /

0.38

0.95

18

COL* + h MS124* + /

0.31

0.97

19

ENOL* H* h MS124* + /

0.45

0.82

20

ENOL* H* h MS134* + /

0.26

0.91

b

CAL + * h CAL*

b

2

H2 + 2* h 2H*

0.54

3b

CAL* + H* h MS1* + /

0.10

b

4

CAL* + H* h MS2* + /

5 6

1

7

Table 4. Turnover Frequency (TOF) of Each Hydrogenation Pathway and the Selectivity of Products TOF (s1) product

21 22

MS13* + H* h MS123* + / MS13* + H* h MS134* + /

0.29 0.03

0.84 0.66

COL

23

MS123* + H* h BOL* + /

0.65

0.67

BAL

24 25

final

pathway 3

+12

6.78 10

+21

2.36 107

+14

1.12 103

0.57

0.78

+41

1.13 10

MS134* + H* h BOL* + /

0.38

0.78

+34

2.87 1012

+43 +1234

3.95 1010 5.15 105

,q

to the other ones. Their coverages associate directly with the final products COL and BOL, respectively. The TOF of each hydrogenation pathway and the reaction selectivity are further investigated (Table 4). It is clear from Table 4 that the predominant pathway for each product can be easily identified: For COL, the main reaction pathway is +12 hydrogenation; the rate of +12 pathway is about 4 orders of magnitude larger than that of +21 pathway. For BAL, it is intriguing that an overwhelming majority of BAL is produced through the 1,4-addition mechanism while it is hardly produced via the traditional 3,4-addition mechanism. +14 and +43 hydrogenation pathways are the main channel for producing 1,4-addition and 3,4-addition, respectively. Comparing these two pathways, as depicted in Figure 5, it is clear that TS43* (the key TS in +43 pathway) is far less stable than TS14* (the key TS in +14 pathway). This difference leads to the difference of final reaction rates. Hence, 1,4-addition is the kinetically favored mechanism rather than 3,4-addition mechanism. For the full hydrogenation product BOL, the most significant pathway is the +1342 pathway, in which all the hydrogenation intermediates are open-shelled species. The other five pathways including all the pathways passing through COL* or ENOL* play a minor role in the final TOF. These results show that only a very small part of BOL stems from the deep hydrogenation of partial hydrogenation products. It indicates that the deep hydrogenation

BOL

selectivity 3

6.78 10

46.33%

1.12 103

7.67%

6.73 103

46.00%

7

MS124* + H* h BOL* + /

ΔG° and G° denote the reaction free energy and the free energy barrier of the forward reaction, respectively. The unit is eV. b The data for these reactions come from our previous work (ref 44). a

θ (ML1)

species

,q

5

+1243

2.68 10

+1423

4.59 1011

+1432

2.47 1010

+1324

1.77 105

+1342

6.63 103

hardly influences the yield of partial hydrogenation products in the current reaction system. In addition, it is clear from Table 4 that two deep hydrogenation pathways undergoing ENOL* (+1432 and +1423 pathways) make the least contribution to the final TOF of BOL. It is especially intriguing that the +1432 pathway, sharing the same lowest overall barrier of the deep hydrogenation with the +1342 pathway (Figure 4), makes a small contribution to the final TOF of BOL, even smaller than the +1234,+1243, and +1342 pathways. This implies that the rapid desorption of ENOL* could further suppress the rates of those slower deep hydrogenation pathways passing through ENOL*. Furthermore, all these three dominant pathways to three different products undergo MS1*. We found that all the reaction rates of the pathways through MS2*, MS3*, or MS4* are negligible relative to the main reaction channels through MS1* (Table 4). This demonstrates that the first hydrogenation must occur on O1. The subsequent hydrogenation processes on different C atoms determine the selectivity of the final products. Therefore, the deep hydrogenation processes via MS2*, MS3*, or MS4* could be ignored. 19825



ARTICLE

Figure 5. Comparison of dominant pathways of the hydrogenation of crotonaldehyde (CAL) to different products on Pt(111).

On the basis of microkinetics data, the selectivity of a certain product Si can be calculated in terms of the following formula Si ¼

TOFi 3

∑i TOFi

100%

ð3Þ

where TOFi is the overall rate of the product i (i = 1, COL; i = 2, BAL; i = 3, BOL). As shown in Table 4, among the products desorbing from the Pt(111) surface, COL and BOL are the main products, 46.33% and 46.00%, respectively. The selectivity of the desired COL is almost identical to that of BOL. The selectivity to BAL is not high (7.67%). The selectivity from our calculations is slightly different from the experimental result of Marinelli et al. (13.0% COL, 50.0% BAL, and 33.6% BOL).53 However, it is worth noting that many Pt facets rather than only the Pt(111) facet may catalyze the reaction in the experimental work.53 It is very likely that the higher selectivity to BAL in the experimental results comes from the contribution of the other facets. In fact, this is actually in good agreement with the experimental results45 on the study of the influence of the selectivity from different Pt facets which demonstrates that different Pt facets lead to different selectivities and Pt(111) is the preferentially oriented facet for unsaturated alcohol formation. In addition, it is worth mentioning that the differences of the selectivities among these three products are very subtle considering DFT errors (the ∼0.1 eV error of energy corresponds to the 12 orders of magnitude error of TOF at the temperature of 353 K). As depicted in Figure 5, comparing these three products, the full hydrogenation product BOL is thermodynamically the most favored product, then BAL, and COL is the last one. However, the order of kinetic preference is different. BOL and COL are kinetically favored products. As Figure 5 displayed, the overall free energy barriers with respect to the gaseous crotonaldehyde and H2, i.e., the apparent barriers of the +12, +14, and +1342 pathways, are 0.37, 0.46, and 0.37 eV, respectively. The similar apparent barriers of these three pathways may account for the close selectivities of these three products.

4. CONCLUSIONS In this work, the microkinetics based on DFT calculations and statistical treatments were utilized to investigate the reaction mechanism of crotonaldehyde hydrogenation on Pt(111) in the free energy landscape. Through the detailed calculations, we identified the dominant reaction channel of each hydrogenation product. The desired product COL is mainly obtained via the +12 pathway. The 1,4-addition mechanism generating ENOL (butenol), which readily tautomerize to saturated aldehydes (BAL), is identified to be a primary mechanism to yield saturated aldehydes instead of the traditional view, the 3,4-addition via direct hydrogenation on the ethylenic bond. BAL is hardly produced from the 3,4-addition mechanism and comes mainly from the +14 pathway. The existence of a 1,4-addition channel could lower the selectivity to the unsaturated alcohol. Another undesired product, BOL, mainly comes from the deep hydrogenation of surface open-shell hydrogenation species while it rarely originates from the further hydrogenation of the partial hydrogenation products. Its dominant channel is the +1342 pathway. The calculation results also shed light on the fact that the dominant reaction channel of each product begins with the first surface hydrogenation on oxygen over Pt(111). It indicates that the selectivity depends on the subsequent hydrogenation on different carbon atoms. The similar apparent barriers of the dominant pathways to yield three final products on Pt(111) make it difficult to achieve a high selectivity to the desired COL. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +44 (0) 28 9097 4687.

’ ACKNOWLEDGMENT We gratefully acknowledge The Queen’s University of Belfast for computing time and the CASTech grant from the EPSRC. X.M.C. thanks The Queen’s University of Belfast for a Ph.D. studentship. 19826



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Reaction Mechanisms of Crotonaldehyde Hydrogenation on Pt(111

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