Comparative DFT study on selective hydrogenation of acrolein catalyzed by pure Mo2C(001) and Pt/Mo2C(001)

In this paper, Density Functional Theory (DFT) calculations were conducted to study the adsorption and stepwise hydrogenation of acrolein (CH 2 =CHCH=O) on pure Mo 2 C(001) and Pt/Mo 2 C(001). The electronic properties were investigated by Mulliken population analysis. The results showed that Mo atoms obtained some electrons from surrounding Pt and C atoms, thereby enhancing the hydrogenation activity of Mo atoms around Pt atoms and forming local active sites dominated by Mo atoms around Pt atoms. As a result, the adsorption energy of the species on Pt/Mo 2 C(001) is generally higher than that on Mo 2 C(001), and the activation energies of the elementary reactions involved in stepwise hydrogenation of acrolein on Pt/Mo 2 C(001) are lower than those on Mo 2 C(001). Moreover, Pt/Mo 2 C(001) exhibits higher selectivity for C=O bond hydrogenation than Mo 2 C(001) and produces more allyl alcohol.


Introduction
Recently, the selective hydrogenation of α, β-unsaturated aldehydes to produce corresponding unsaturated alcohols has attracted wide attention, because these unsaturated alcohols can be employed to produce valuable ne chemical and pharmaceutical intermediates and products [1,2,3,4,5,6]. α, βunsaturated aldehydes contain C=C bonds and C=O bonds. Generally, it is easier for C=C bonds to be hydrogenated than C=O bonds in thermodynamics and kinetics [7,8,9,10,11]. Therefore, we need to nd a suitable catalyst to control the selectivity of hydrogenation, i.e., to improve the selectivity for C=O hydrogenation, so as to obtain the unsaturated alcohol required for production [12,13,14,15].
Acrolein is the simplest α, β-unsaturated aldehyde with abundant sources. It can be formed by the decomposition of certain pollutants in the air or the incomplete combustion of organic matters, or as a metabolite in the human body. The electrophilic properties of acrolein play an important role in the reaction mechanism [16,17]. Figure 1 is a reaction network of the selective hydrogenation of acrolein to propanol. There are four steps in total: the rst step is to selectively hydrogenate C=O and C=C bonds to generate four intermediates; the second step is to add another hydrogen atom into the intermediates generated in the rst step, forming propanal, enol and allyl alcohol, respectively; in the third step, four intermediates are generated via six routes; and in the fourth step, propanol is generated as the nal hydrogenation product.
In the past research, Ir, Co, Pt, Ag and Ru are often used as catalysts to catalyze the hydrogenation of acrolein to unsaturated alcohols [18,19,20,21,22]. Luo et al. [23] studied the selective hydrogenation of acrolein to propanal on Ni(111). The selectivity of acrolein hydrogenation on Au(110), In/Au(110), Au 20 , Au(211), Pt(211), Pt(111), Ag(110) and O sub /Ag(111) was discussed. The results show that Pt is bene cial to the hydrogenation of C=O bond. In order to improve selectivity of the desired product, a second component can also be added to form a bimetallic catalyst. For example, such bimetallic catalysts as Ni/Pt(111), Pt/M(M=Ni, Co, Cu), Pt/Ni/Pt(111) and Pd/Ag(111) [24,25,26,27] exhibit better stability and activity. Such supported catalysts as Ag/TiO 2 , Ag/SiO 2 and Au/ZnO [28,29,30] can be used to promote the interaction between components of the catalysts and fully expose the active centers.
Although precious metal materials exhibit great catalytic performance, they are expensive and short of resources. Transition metal carbides have received extensive attention because of their excellent physical and chemical properties, such as extremely high hardness, high melting point, excellent electrical conductivity and thermal conductivity. Among various transition metal carbides, Mo 2 C not only has excellent catalytic activity, but also exhibits great resistance to CO and S poisoning and resistance to sintering [31]. It can be used as a low-cost catalyst to partially replace precious metal materials such as Pd and Pt. Rocha et al. [32] reported the theoretical and experimental studies on the continuous gas phase hydrogenation of benzene with bulk Mo 2 C at 363 K. The results showed that the initial conversion rate of benzene to cyclohexane was 100%. However, the strong adsorption of benzene on the surface of Mo 2 C will cause deactivation of the catalyst later. Shi et al. [33] reported the mechanism of the selective hydrogenation of furfural catalyzed by Mo 2 C(101) to methyl furan. Two schemes were proposed to promote the formation of methyl furan and inhibit the formation of furan. Frauwallner et al. [34] tested the catalytic activity of Mo 2 C catalysts for toluene hydrogenation and the results showed that the catalytic activity of Mo 2 C is equivalent to that of precious metals. Hollak et al. [35] compared activity, selectivity and stability of W 2 C and Mo 2 C catalysts supported by carbon nano bers in oleic acid hydrodeoxygenation. The results showed that Mo 2 C supported by carbon nano bers showed higher activity and stability. Burueva et al. [36] reported that the phase composition of Mo 2 C has a signi cant effect on the selectivity of stepwise hydrogenation. By changing the gas hourly space velocity of the carburizing gas mixture, a defective phase can be generated. The results showed that β-Mo 2 C exhibited the best hydrogenation performance. Oliveira et al. [37] reported the mechanism of hydrodeoxygenation of acrylic acid on Mo 2 C. The overall mechanism of the four-step reaction of acrylic acid to propane is proposed, and the main product is propane.
However, the mechanism of selective hydrogenation of acrolein on Mo 2 C is still unclear. Wang et al. [38] used Density Functional Theory calculations to study the stability of the β-Mo 2 C surface, and con rmed that the Mo 2 C(001) is the most stable at low temperatures and the surface is easily exposed. Under low carburizing ability, the surface with more molybdenum atoms is more stable. The Mo 2 C(001) is selected in this research because its structure is simple and it is the most densely lled surface, and it is expected to exist in a large amount on the nanostructured Mo 2 C [39]. Therefore, in this study, the molybdenumterminated Mo 2 C(001) surface was adopted as the catalyst model. The mechanism of selective hydrogenation of acrolein on Mo 2 C(001) and Pt/Mo 2 C(001) surface was studied by Density Functional Theory calculations.

Computational surface models
Based on the optimized Mo 2 C bulk structure, a Mo 2 C(001) surface model was built , using a 22 unit cell consisting of ve layers of atoms with 15 Å of vacuum, as is shown in Figure 2a. The distance between adjacent C and Mo atoms in Figure 2a is between 2.08Å and 2.21Å, the distance between two adjacent C atoms and Mo atoms is 2.88Å. As shown in Figure 2b, the distance between the Pt atom and the adjacent Mo atom is 2.88 Å, and the distance between the Pt atom and the adjacent C atom is 2.05 Å. The two upmost surface layers and the adsorbates have been optimized, while the lowest three layers are kept xed.

Computational details
Based on spin unrestricted DFT calculations conducted with DMol 3 in Materials Studio, the double numerical plus polarization (DNP) basis set was used in the calculations. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhoff (PBE) functional [40] was used as the exchangecorrelation functional. A smearing of 0.005 Ha was adopted to accelerate the convergence. The k-point sampling consists of 221 Monkhorst-Pack points [41]. The convergence criteria for geometry optimization and energy calculations were set as 1.0×10 -5 Ha, 2.0×10 -5 Ha, 0.004 Ha/Å, and 0.005 Å for the tolerance of self-consistent eld (SCF), energy, maximum force, and maximum displacement, respectively. Complete LST/QST[42] was used to identify the possible transition state(TS). The convergence criterion is the same as that of geometry optimization.
The adsorption energies of different species on Mo 2 C(001) and Pt/Mo 2 C(001) are de ned as follows:

E ads(A) =E A/slab -E slab -Egas A
Where E A/slab , E slab and Egas Arepresent the total energy of species A adsorbed on Mo 2 C(001) or Pt/Mo 2 C(001), the total energy of bare Mo 2 C(001) or Pt/Mo 2 C(001), and the total energy of species A in the vacuum.
The co-adsorption energies on Mo 2 C(001) and Pt/Mo 2 C(001) are de ned as: Where E (A+B)/slab , Egas A, Egas Band E slab represent the total energy of species A and B co-adsorbed on the surface of Mo 2 C(001) or Pt/Mo 2 C(001), the total energy of species A in the vacuum, and the total energy of species B in the vacuum, the total energy of bare Mo 2 C(001) or Pt/Mo 2 C(001).
The activation energy of all elementary reactions is the total energy difference between the TS and the initial state (IS), as shown below: The reaction energy is the total energy difference between the nal state (FS) and the IS, as shown below:

Results And Discussion
For the convenience of description, as shown in Figure 1, the atoms of acrolein (CH 2 =CHCH=O) are marked from left to right respectively as C1, C2, C3, O0, and the black dots indicate the lone electrons. On Mo 2 C(001) and Pt/Mo 2 C(001), acrolein is hydrogenated to allyl alcohol. In order to consider the formation of byproducts of the hydrogenation reaction, the reaction is divided into four steps, and the activation energy barriers each step need to overcome as well as the stable adsorption con guration and adsorption energy are analyzed.

Electronic properties
By studying the surface electronic properties of Mo 2 C(001) and Pt/Mo 2 C(001), we can better explain the electron transfer between Pt atoms and Mo 2 C, and understand the synergetic effect between Pt and  Table 1. Table 1 summarizes the corresponding adsorption energies of the aforementioned species on Mo 2 C(001) and Pt/Mo 2 C(001). Figure 4 shows the stable adsorption con gurations of adsorbates on Mo 2 C(001) and Pt/Mo 2 C(001). It can be seen from Table 1 that the adsorption energies of almost all the species on Pt/Mo 2 C(001) are larger than that on Mo 2 C(001), which proves that the addition of Pt atoms onto Mo 2 C(001) will enhance the interaction between the metal and most species. Binding strength of the adsorbate may be related to the transfer of electrons from Pt and C atoms to Mo atoms. It can be found from Figure 4 that when the adsorbate is on the surface of pure Mo 2 C, the surface adsorption site is mainly the top site of Mo atoms. C atoms only play a role of donating electrons and do not participate in the reaction. On Pt/Mo 2 C(001), most adsorbates interact with the Pt atoms, and the active sites become the bridge sites of Pt and Mo. For example, on Pt/Mo 2 C(001), H atoms tend to be adsorbed on the bridge sites between Pt atoms and Mo atoms. On Mo 2 C(001), the most stable H adsorption site is the bridge site between two Mo atoms on the rst layer. The stable adsorption sites of other adsorbates on Pt/Mo 2 C(001) also involve Pt atoms and Mo atoms on the rst layer. This may also explain the increase of the absolute value of the adsorption energy, because there is an extra active Pt atom to interact with the adsorbate, which also reduces the adsorption range of the adsorbed material and facilitates the selective hydrogenation.

Co-adsorption
In this study, the co-adsorption of H atoms and various intermediates on Mo 2 C(001) and Pt/Mo 2 C(001) was studied. The comparisons of speci c co-adsorption energies are shown in Table 2. The adsorption con guration is shown in Figure 5 (top view and side view). It can be seen that the addition of Pt onto Mo 2 C(001) increases the co-adsorption energy of almost all adsorbates, because the added Pt atoms have a strong binding strength with most species.  Table 3 summarizes the changes in bond lengths of each species on Mo 2 C(001) and Pt/Mo 2 C(001) during acrolein hydrogenation. It can be seen from Table 3 that the process of acrolein hydrogenation to propanol is a stretching process, and it can be seen from the table that the stretching degree of C=C bond is greater than that of C=O bond. The bond length of this site is obviously greater than that of other bonds at a larger distance. When the bond length of Mo 2 C(001) and Pt/Mo 2 C(001) changes, in general, the bond length on Pt/Mo 2 C(001) is smaller than the corresponding bond length on Mo 2 C(001). The bond length stretch on the surface is smaller, probably because Mo seizes the electrons of the surrounding Pt and C atoms, which makes the binding strength of the adsorbate and Mo atoms stronger, so the bond length of the adsorption site on the Mo atom changes greatly, and the bond length change on the Pt site becomes smaller.    Figure 6 that the hydrogenation activity of acrolein on Mo 2 C(001) is lower than that on Pt/Mo 2 C(001). Next, the hydrogenation on the two catalyst surfaces will be compared.

Changes in bond lengths
The rst step is that the H atom is added to acrolein to form four intermediates, i.e., CH 3   Propanal (CH 3 CH 2 CH=O) is mainly produced by hydrogenation on the C=C bond. The hydrogenation of unsaturated aldehydes is most likely to produce saturated aldehydes. Therefore, when hydrogenating acrolein to produce allyl alcohol, the formation of saturated aldehyde (propanal) must be suppressed. The hydrogenation sites on acrolein are C1 and C2. When the C2 position is used as a hydrogenation site, the activation energy on Mo 2 C(001) is 19.78 kcal/mol and 67.78 kcal/mol, respectively, while the former is endothermic with the reaction energy of 1.16 kcal/mol, and the latter is exothermic by -6.94 kcal/mol. The energy barriers of two reactions on Pt/Mo 2 C(001) become 30.84 kcal/mol and 14.36 kcal/mol, respectively. The former is an endothermic reaction with an endothermic value of 3.47 kcal/mol, and the latter is an exothermic reaction with a heat release of -0.39 kcal/mol. The reaction occurs more easily on Pt/Mo 2 C(001) than on Mo 2 C(001).
The formation of enol (CH 3 CH=CHOH) also has two hydrogenation sites. When H atom is added to the Since the enol (CH 3 CH=CHOH) itself is unstable, the calculation is only to make the data more complete.
To sum up, among the three products generated from acrolein, i.e., allyl alcohol (CH 2 =CHCH 2 OH), propanal (CH 3 CH 2 CH=O) and enol (CH 3 CH=CHOH), hydrogenation on Pt/Mo 2 C(001) is easier than that on Mo 2 C(001), and on Pt/Mo 2 C(001), the selectivity for allyl alcohol is obviously greater than that of propanal. The most likely route for producing allyl alcohol is CH 2 =CHCH=O→CH 2 =CHCH 2 O→CH 2 =CHCH 2 OH, and the most probable route for generating propanal (CH 3 CH 2 CH=O) is CH 2 =CHCH=O→CH 2 CH 2 CH=O→CH 3 CH 2 CH=O. On the surface of Pt/Mo 2 C(001) and Mo 2 C(001), a higher activation energy barrier needs to be overcome to generate enol (CH 3 CH=CHOH), so the reaction is not easy to occur. The C=C hydrogenation of acrolein on Mo 2 C(001) has high selectivity, but it is only conducive to the production of C2 hydrogenation product CH 2 CH 2 CH=O, which is not conducive to the production of the desired target product allyl alcohol. Figure 9 shows the energy barrier diagram of the second step reaction. The results show that the upper energy barrier of Pt/Mo 2 C (001) is signi cantly reduced.
The third step was investigated in order to examine the stability of several products of acrolein hydrogenation, i.e., allyl alcohol and propanal. To make the data more complete, enol hydrogenation was also further investigated. The third step hydrogenation formed four intermediates, CH 3  kcal/mol and -6.62 kcal/mol. The results show that enols are more prone to hydrogenation reaction on Mo 2 C(001) than on Pt/Mo 2 C(001). Figure 10 shows the energy change of each path during the third step of hydrogenation. Since CH 3 CH 2 CHOH is di cult to obtain, propanol mainly derives from C=C bond hydrogenation. In general, the main products of acrolein hydrogenation are allyl alcohol and propanol. And the selectivity for allyl alcohol is higher than that of propanol. Figure 11 shows energy pro les of the forth hydrogenation step on Pt/Mo 2 C(001) (left) and Mo 2 C(001) (right) surfaces.

Conclusions
The adsorption energy and co-adsorption energy of most species on Pt/Mo 2 C(001) are greater than that bonds. And with further calculations, it is found that the target product, allyl alcohol, is not prone to further hydrogenation, while propanal is prone to further hydrogenation to propanol. Therefore, the most likely products produced by acrolein hydrogenation on Pt/Mo 2 C(001) are allyl alcohol and propanol, and the allyl alcohol selectivity is higher than that of propanol.

Declarations
Funding: This research was supported by the National Natural Science Foundation of China (No.  The energy distribution of the second step hydrogenation reaction on the surface of Pt/Mo2C (001) (left) and Mo2C (001) (right) Figure 10 Energy pro les of the third hydrogenation step on Pt/Mo2C(001) (left) and Mo2C(001) (right) surfaces