Theoretical Studies of Acrolein Hydrogenation to Propenol and Propanal on Au 3 and Au 5

The stepwise hydrogenation of the C=C bond and C=O group of acrolein on Au3 and Au5 model systems is investigated using the density functional theory(DFT) PW91 functional. Our results show that the C=C hydrogenation is more favorable than that of C=O bond on Au3 with the barriers of the rate-determining step being 0.35 and 0.62 eV respectively. On the other hand, the C=O reduction is preferred over the hydrogenation of the C=C bond on Au5. The corresponding barriers of the rate-determining steps are 0.45 and 0.54 eV, respectively. This demonstrated that the second hydrogenation step controls the reaction on both Au3 and Au5 for C=O and C=C hydrogenation and the C=O hydrogenation on Au5 is preferred over the hydrogenation of the C=C bond, which is helpful to address the reactivity of small size-selected supported gold clusters.


Introduction
Unsaturated alcohols which are a kind of important chemical intermediates used to synthesize pharmaceuticals and flavoring material [1,2] are usually produced via selective hydrogenation of α, β-unsaturated aldehydes.Normally the C=C bond is more active than the C=O bond both thermodynamically and kinetically [3].Acrolein is the smallest α,β-unsaturated aldehyde molecule containing conjugated C=C and C=O double bond, and is used as a probe molecule to study the hydrogenation activity toward the C=C and C=O bonds on the Ag [4][5][6][7], Pt [8][9][10][11], Ni/Pt(111) [12,13] surface both experimentally and theoretically.However, the selective hydrogenation of acrolein to propenol is considered the most difficult one and the selectivity to unsaturated alcohols on conventional catalysts of group VIII metals is generally low (~2% on Pt) [9].It is a challenging task to increase the selectivity for the unsaturated alcohols with respect to the saturated aldehydes.
Recently gold exhibits unexpected activity for the selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols.[14][15][16][17] Experimental studies substantiated the crucial role of Au as an active component in the selective hydrogenation.But the periodic density functional calculation of our group showed that acrolein is very weakly adsorbed on the clean regular Au (111) or the flat Au(110) surface [18].A higher selectivity had been obtained on supported gold nano-particles [9].Gold supported on TiO2, Al2O3 or SiO2 was demonstrated to catalyze alkene hydrogenation, and it was suggested that small gold clusters were the catalytically active species.[14,15] Although there are already many studies in the literature about the selective hydrogenation of acrolein on the different metal surfaces and supported metal catalysts, little information is known about acrolein hydrogenation on the gold clusters.No doubt, a comprehensive knowledge of hydrogenation of acrolein on the gold cluster is very instructive for improving the performance of the known noble metal catalysts and for screening new and better candidates.In this paper, we address acrolein hydrogenation on Au clusters.The whole paper is arranged as follows.The favorable pathways of C=C and C=O hydrogenation on Au3 and Au5 are discussed in detail in Part II.
Computational methods are presented in Part III.Finally, the conclusions are presented in Part IV.

Hydrogenation of the C=C bond
Acrolein hydrogenation is believed to follow the Houriti-Polanyi mechanism [19].We have investigated adsorption and dissociation of H2 molecule on gold clusters and found that H2 dissociation can take place on Au3 and Au5, [20] and acrolein binds to Au3 and Au5 via the most strongly in di-σ-(C, O) mode with the binding energy of 1.52 eV and 1.43 eV respectively [21].Thus, we investigated acrolein hydrogenation on gold clusters using Au3 and Au5 as representative models.Theoretically there are various hydrogenation products on the gold clusters.Now we use klmn to denote these species.Here k, l, m and n are integers which represent the number of H atom bonding to C1, C2, C3 and O atoms of acrolein, respectively.The atom numbering of acrolein is C1-C2-C3-O4 (scheme1).For example, acrolein (2110) means that there are two,one and one H atoms bonded to C1, C2 and C3 atoms, respectively, and the O atom has no H atom [22].We confined our study to stepwise hydrogenation that produce the desired product propenol (2121) and the main undesired side product propanal (3210) through route 43 (R43) and route 12 (R12) (Route nm means that the first H atom attacks at the nth atom and the second H at mth atom.For the ordering of atoms, please see Scheme 1), respectively.In principle, there are other pathways to propenol such as R34(R21).However, according to the well-accepted mechanism and our located co-adsorption structures, these routes are excluded because the co-adsorbed H atoms sit quite far away from the attacked atom (C3 or C2 respectively).It should be pointed out there are various initial states with different stability as shown in Figure A1-A4 in supporting information.However, only those from the most stable initial states are investigated here.and the Au, which results in a elongated C1-Au bond distance of 3.00 Å, compared to 2.34 Å in 3IS1.
Correspondingly the C1-H bond reduces from 2.83 Å in 3IS1 to 1.80 Å in 3TS1.Generally speaking, the framework bonds change little in 3TS1 with respect to the corresponding values in 3IS1 (Figure 1).
After the 3TS1, the reaction results in an intermediate product 3IM1A.In this structure, the framework changes a lot.For example, the C1-C2 bond is extended to A lot of calculations demonstrate species with unpaired electrons interact with the substrates favorably through the atom with its unpaired electron.However, 3IM1A is not the most stable adsorption complex and it adsorbs through C1 and O4, not C2 which has an unpaired electron.We found that 3IM1B is the most stable adsorption configuration.Conversion 3IM1A to the most stable complex of 3IM1B releases the heat of 0.34 eV (Table 1).In 3IM1B, the H-Au bond is broken and the C2-Au bond is formed with a bond distance of 2.23 Å which is shorter by 0.11 Å than C1-Au in 3IS1.
Because of stronger interaction of C2 and Au, C2-C3 bond is weakened and increases from 1.39 Å to   As in the C=C hydrogenation, an intermediate product, 3IM4A, is also formed firstly (Figure 1).
In this case, the O-Au bond length is 2.46 Å, indicating that O-Au bond is formed.The H-Au bond is broken.3IM4A is also not stable enough to be the final product because the saturated O atom produces an unpaired electron on C3, which makes C3 possess strong bonding ability.
Transformation of 3IM4A into 3IM4B releases the heat of 0.32 eV.
Based on the 3IM4B, we studied the second step of C=O hydrogenation.3IS43 shown in Figure 1 illustrates the initial structure (co-adsorptions structure) of H with 2111, the product of the first reaction heat of 0.38 eV, which implies that C=O hydrogenation on Au3 is favorable thermodynamically.However, the energy barrier of 43 pathway (0.94 eV) is higher than that of 12 pathway (0.35 eV), which indicates that on Au3 C=C hydrogenation is more favorable than C=O hydrogenation.

Acrolein hydrogenation on Au5
The most stable coadsorption structures of 2110 and H atom on Au5 were chosen as initial states (5IS1) for C=C and C=O hydrogenation (Figure 2).In the 5IS1, acrolein interacts directly with Au5 via

Hydrogenation of the C=C bond
At the beginning of this reaction, the H atom moves toward the C1 atom, accompanied by elongation of C1-Au bond.This motion eventually allows the H atom to insert into C1-Au bond of 5IM1A (Figure 2).In 5TS1, the C1-Au, C1 In the final state, 5IM4A, the O-Au bond is 2.27 Å, indicating that the O-Au interaction is still significant.Unlikely 5IM1A, 5IM4A can interact directly with Au5 via O-Au bond.Because hydrogenation at the O atom, the C3 atom of C=O bond becomes a radical.Thus, a stable adsorption structure should involve C3.Our computation indicates that 5IM4B is 0.39 eV more stable than 5IM4A.
In 5IM4B, the C3-Au bond distance is 2.28 Å in the 5IM4B, and the product 2111 is stabilized by both C1-Au and C3-Au bonding.The second step of C=O hydrogenation begins with coadsorption state (5IS43) of H atom and 2111.In the initial state (5IS43) the H atom sits at the top site of a Au atom with H-Au distance of 1.62 Å. Due to the H-Au bond, the C3-Au bond no longer exists.In the TS structure 5TS43，the H-C3 is 1.80 Å and the H-Au bond is elongated from 1.62 to 1.66 Å, showing that the H-Au interaction is only slightly weakened.The final product is 2121 (5FS43).It adsorbs on Au5 via the π-C=Cmode.The rate-determining step of the C=O hydrogenation reaction on Au5 is an exothermic reaction with a reaction heat of -0.25 eV.The energy barrier is 0.45 eV.Compared to the barrier of C=C hydrogenation on Au5, we can conclude that on Au5 C=O hydrogenation is more favorable than C=C hydrogenation kinetically.more stable than the precursor 3IS12.Therefore, thermodynamically the second step is more difficult and thus, control the production of 3210, the product of the C=C bond hydrogenation.On the other hand, the barrier for the C=O hydrogenation are 0.94 (3TS4) and 0.62 (3TS43) eV, which are also similar to the step hydrogenation of C=C bond on Au3 (Figure 3).The first step to 3IM4A is endothermic whereas conversion to the most stable structure releases heat of 0.48 eV.The second hydrogenation step lowers the energy of the system by 0.46 eV.

Competition of the C=C and C=O hydrogenation
Compared with the energy barrier of 12, we found that C=C hydrogenation with the highest energy barrier of 0.35 eV, is more favorable than C=O hydrogenation with the highest barrier of 0.94 eV.This result is in agreement with the experimental observation that the yield of C=C reduction is higher than that of the C=O hydrogenation.
The stepwise hydrogenation barriers of C=C bond on Au5 are 0.30 (5TS1) and 0.54 eV (Figure 4) respectively.The most stable intermediate 5IM1B is almost 1 eV lower in energy than the precursor 5IS12, indicating that formation of 5IS12 is thermodynamically less likely.Thus, the second step is the rate-limiting step.Both reactions are exothermic, > 0.3 eV (Figure 4).For the C=O hydrogenation (43 pathway), the corresponding energy barriers are 0.17 and 0.45 eV, which also shows that the second step is rate-determining step.The calculated results show that C=O hydrogenation barriers is slightly lower than the corresponding values for the C=C hydrogenation, indicating that C=O reduction is preferred on Au5.

Comparison of acrolein hydrogenation on Au3 and Au5
Above we discussed the kinetics of C=C and C=O hydrogenation on Au3 and Au5 clusters respectively.Now we compare the C=C hydrogenation on Au3 and Au5.The energy barrier of 3TS1 (0.29 eV on Au3) for the first hydrogenation step is almost equal to that of 5TS1 (0.30 eV on Au5).
However, the energy barrier of 5TS12 (0.54 eV on Au5) is 0.19 eV higher than that of 3TS12 (0.35 eV on Au3), indicating that the energy barrier of C=C hydrogenation increases from Au3 to Au5.
For C=O hydrogenation on Au3 and Au5, the calculated energy barrier of 3TS4 (0.94 eV) in the first step is 0.77 eV higher than that of 5TS4 (0.17 eV).Moreover, in the second step, the energy barrier of 3TS43 (0.62 eV) is 0.17 eV higher than that of 5TS43 (0.45 eV).These results indicate that the energy barrier of C=O hydrogenation decreases from Au3 to Au5.
To recap, C=C hydrogenation is favored on Au3 (the energy barrier of the rate-limiting step: 0.35 eV for C=C vs 0.94 eV for C=O) while the C=O hydrogenation is preferred on Au5 (0.45 eV for C=O vs. 0.54 eV for C=C).These facts indicate that the selectivity of acrolein hydrogenation has close relationship with cluster size, which is consistent with the experiments observation that the activity and selectivity to the desired unsaturated alcohol depends on particle size and increases with increasing particle size [17,[23][24].
In addition, we also compare the comparison of acrolein hydrogenation on Au(110) and Au20 clusters.

Models and computational details
Calculations were performed using generalized gradient functional PW91 [25,26] implemented in Gaussian 03 program package.[27] We used the effective core potential (ECP) basis set, cep-121G (6D, 10F) for Au, C and O, [28,29] and a triple zeta basis set for H. [30] Frequency calculations were performed to characterize all the optimized structures to be local minima or transition states on the potential energy surfaces.The intrinsic reaction coordinate calculations were further carried out to ensure that the obtained transition states connect the right reactants and products.

Conclusion
The selective hydrogenation acrolein on the small gold cluster Au3 and Au5 is investigated using density functional theory PW91 functional.The mechanism is explored along two pathways: hydrogenation of C=C bond and C=O group.We show that on Au3 C=C hydrogenation is more favorable than that of C=O bond with the corresponding barrier of rate-determining step of 0.35 eV vs 0.94 eV.On the other hand, on Au5, C=O reduction is favored than hydrogenation of C=C bond.The corresponding barriers of the rate-determining steps are 0.45 and 0.54 eV, respectively.

Scheme 1 .
Scheme 1. Reaction pathway of acrolein hydrogenation forming the propanal and propenol.The optimal mechanism is marked in red on the Au3 and Au5.

Figure 1
Figure 1 shows the most stable precursor structure for the hydrogenation of the C=C bond in which the attacking H atom and the attacked C1 atom are sitting at the same gold atom (Figure 1, 3IS1) with H-Au bond length of 1.61 Å.Compared to di-σ-(C, O)/Au3[21], the C1-Au bond length of 3IS1 is elongated from 2.17 Å to 2.34 Å and C1-C2 bond is stretched by 0.03 Å.The O-Au, C3-O and
hydrogenation.The H-Au bond length is 1.62 Å.The C2-Au distance is elongated from 2.84 Å of 3IM4B to 3.56 Å.In the 3TS43 the H atom links C3 and the Au with H-Au and H-C3 being 1.68 and 1.76 Å respectively.As shown in Figure1, the C3-O bond is elongated by 0.03 Å due to H addition.The hydrogenation product 3FS43 is 2121.Because the C=O bond is hydrogenated, the C2-C3 bond length (1.53 Å) and C3-O bond length (1.47 Å) in 3FS43 are close to the C-C single bond length in ethane (1.55 Å) and C-O bond length in methanol.C1-C2 bond length is 0.07 Å longer than C=C double bond length (1.36 Å) of the isolated acrolein.As shown in Figure 1, 2121 interacts with gold atom via π-C=C mode.The C=O hydrogenation reaction on Au3 is an exothermic reaction with a

Figure 3 and
Figure 3 and Figure 4 summaries the PES for the C=C and C=O hydrogenation on Au3 and Au5.The calculated barrier for the first and second steps of the C=C bond hydrogenation of acrolein (R12 pathway) are 0.29 (3TS1) and 0.35 (3TS12) eV.These two barriers are close to each others, though the latter is slightly higher than the former.Both reactions are essentially neutral with the endothermicity of less than 0.05 eV.Note the co-adsorbed intermediate 3IM1B with H (0.56 eV) is

Figure 3 .
Figure 3. Pathways of acrolein(AC) hydrogenation on Au3.The energies of the IS plus half of the energy of an isolated H2 molecule is taken as the zero energy reference point (AC/Au3+1/2H2).All the energies of IMs and TSs for the first step are referred to the energies of the adsorbed structure together with half of the energy of the free H2 molecule.

Figure 4 .
Figure 4. Pathways of acrolein(AC) hydrogenation on Au5.The energies of the IS plus half of the energy of an isolated H2 molecule is taken as the zero energy reference point (AC/Au5+1/2H2)

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 21 December 2016 doi:10.20944/preprints201612.0109.v1 C2
-C3 bond lengths vary within 0.01 Å.In this precursor structure, the H atom can only attack C1 atom.Therefore the hydrogenation route of C=C bond follows route 12. Attacking of the H atom to C1 leads to the 3TS1 transition state structure.In the 3TS1, the H atom is basically located between C1

Table 1 .
1.51 Å while the C2-C3 bond is shortened by 0.05 Å, compared with 3TS1.The H-C1 bond is formed with the bond length of 1.11 Å.The produced intermediate adsorbs on Au3 mainly through O interaction with the substrate by the O-Au bond reduced from 2.13 Å to 2.04 Å.Energy barriers and reaction energies of the first and second step of acrolein hydrogenation on Au3 and Au5.The energy of each initial state (IS) is taken as zero energy reference.

21 December 2016 doi:10.20944/preprints201612.0109.v1 C2
[21]elatively local.Product 3210 is saturated aldehyde and interacts with Au3 via C2-Au (2.69 Å) and C3-Au (2.59 Å) interaction.Note the corresponding C1-C2 bond length and C2-C3 bond length are equal and is close to the C-C bond length of ethane (1.55 Å) at the same level.This is understandable because C1 and C2 are "saturated".Kinetically, the energy barrier of 12 pathway is 0.35 eV, which shows that C=C hydrogenation on Au3 is likely (Table1).2.1.2.Hydrogenation of the C=C bond3IS4 is the initial state used for the first step of C=O hydrogenation.To attack the O atom of 3IS4, the H atom first rotates which weakens the O-Au bonding in the di-σ-(C, O)-cis-AC mode[21].In the 3TS4, the H atom is between the O atom and the Au with the O-H and H-Au bond lengths of 1.45 and 1.70 Å respectively (Figure1).The variation of C1-C2, C2-C3, C3-O and C1-Au is within 0.02 Å.Moreover, the Au-Au bond varies from 2.68 Å to 2.69 Å, indicating a slightly stabilizing deformation of Au3.In addition, O-Au bond is decrease from 5.95 Å in 3IS4 to 3.06 Å in 3TS4, which shows that the O-Au bond is also broken and the O atom does not interact with Au3 directly.
. The C2-Au distance, 2.18 Å, is significantly shorter than 2.34 Å in 3IS1.In the 3TS12, the H atom, C2 and the Au form a triangle structure.In the transition state 3TS12, the H-Au bond length is 1.63 Å and the C2-H bond distance decreases from 2.59 Å to 1.66 Å.However, C1-C2, C2-C3 and C=O bond lengths are almost unchanged.This shows that the influence of H attacking at Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted:
.21 Å to 2.56 Å in the 5TS1.The C1-C2, C2-C3, C3-O and O-Au vary slightly (within 0.01 Å), indicating that the influence of H attacking at C1 atom is relatively local in the first step of C=C bond hydrogenation.In the intermediate of 5IM1A (3110), the Au-H bond and the C1-H bonds are 1.92 Å and 1.20 Å respectively.The C1 atom cannot interact directly with Au5.Hydrogenation of C1 produces a radical centered on C2 atom.However, in 5IM1A C2 does no interact with Au5 directly.
-H and the H-Au length are 2.56, 1.70 and 1.70 Å, respectively.The triangle containing C1, the H and Au atoms are formed.The C1-Au bond increases from 2starts with the H atom approaching toward to the O-Au bond, which increases the O-Au distance from 2.17 Å in 5IS1 to 2.25 Å in 5TS4, indicating that the O-Au bond is weakened.Similar to the transition state 3TS4, the H-Au bond length is 2.04 Å and the O-H distance is 1.65 Å in the 5TS4.The C-O distance increases from 1.34 Å in 5IS1 to 1.37 Å in 5TS4.However, C1-Au, C1-C2 and C2-C3 bond lengths are almost unchanged (within 0.01 Å).This shows that the influence of H attacking at O is relatively local.