Density Functional Theory Study on the Cleavage Mechanism of the

Article pubs.acs.org/JPCC

Density Functional Theory Study on the Cleavage Mechanism of the Carbonyl Bond in Amides on Flat and Stepped Ru Surfaces: Hydrogen-Induced or Direct C−O Bond Breaking? Xiao-Ming Cao,†,‡ Robbie Burch,‡ Christopher Hardacre,‡ and P. Hu*,‡ †

State Key Laboratory of Chemical Engineering, Center for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, P. R. China ‡ School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG, United Kingdom ABSTRACT: We have performed density functional theory (DFT) calculations to investigate the reaction mechanism of the cleavage of the carbonyl bond in amides on both flat and stepped Ru surfaces. The simplest amide molecule, N,Ndimethylacetamide (DMA), was used as the exemplar model molecule. Through the calculations, the most stable transition states (TSs) in all the pathways on both flat and stepped Ru surfaces are identified. Comparing the energy profiles of different reaction pathways, we find that a direct cleavage mechanism is always energetically favored as compared with an alternative hydrogen-induced mechanism on either the flat or stepped Ru surface. It is easier for the dissociation process to occur on the stepped surface than on the flat surface. However, as compared with the terrace, the superiority of step sites boosting the C−O bond dissociation is not as evident as that on CO dissociation. acids to alcohols.10−12 The different combinations of group 8 to 10 late transition metal with group 6 or 7 early transition metal were reported to result in high yields at much milder conditions compared with those used previously. The first use of Re/Pd for the hydrogenation of amides in a 1988 BP patent effectively decreased the pressure of H2 to 130 atm at 200 °C.13 In addition, bimetallic catalysts (Rh/Re, Rh/Mo, Ru/Re, Ru/Mo, etc.) have been applied in the hydrogenation of secondary and tertiary amides reported by Hirosawa et al.9 The pressure of H2 was further reduced to 100 atm at 160−180 °C. It has lately been claimed that Rh/Mo, Ru/Re, and Rh/Re from metal− carbonyl precursors are effective bimetallic catalysts for the selective hydrogenation of primary amides cyclohexylcarboxamide to cyclohexanemethylamine.14,15 In this study 50−100 bar H2 and 130−160 °C temperature was used. More recently, under the guidance of density functional theory (DFT) calculations, Pt−Re/TiO2 catalysts have been developed and the pressures and temperatures have been reduced further to 20 bar H2 pressure and 120 °C for the selective reduction of hydrogenation of N-methylpyrrolidin-2-one to N-methylpyrrolidine.16 Despite these achievements, further improvements are needed. There is no doubt that the understanding of reaction mechanism would be helpful to find better catalysts to achieve a great conversion under the facile reaction conditions. However, to the best of our knowledge, there has been no report

1. INTRODUCTION Amines are commercially used in numerous chemical industries, such as corrosion inhibitors, plastics, pharmaceuticals, cosmetics, surfactants, agrochemicals, and dyes.1 Driven by the rapid growth of market demand, the manufacture of amines has led to an increasing number of studies. In addition, the synthesis of amines is of great importance to organic chemistry. The preparation of amines from amides is one possible approach. Metal hydrides are generally utilized to reduce amides to amines, among which lithium aluminum hydride is the most widely used.2 An alternative approach to yield amines is the heterogeneously catalytic hydrogenation of amides. Due to the low cost of separating solid heterogeneous catalyst from the gas or liquid reactants and products and using H2 instead of expensive hydride reagents, this method could become a potentially economical route if the high yield can be achieved at the proper temperature and under low hydrogen pressure. However, the selectively catalytic hydrogenation of amides is acknowledged as a challenging task in the hydrogenation of various classes of carbonyl compounds.3 In the vast majority of reported catalytic systems, harsh reaction conditions (very high hydrogen pressures and high temperatures) are used to enable sufficient activity to be achieved. The most intensively studied system is based on copper−chromite−oxide which requires 200−350 bar of H2 and 250−400 °C in dioxane medium.4−8 Monometallic catalysts, such as Rh, Ru, Re, Mo, and W,9 with metal carbonyls as precursor have also been used but have shown low conversions. In the past decades, bimetallic catalysts have been developed for the selective hydrogenation of carboxylic © 2012 American Chemical Society

Received: March 14, 2012 Revised: August 6, 2012 Published: August 6, 2012 18713

dx.doi.org/10.1021/jp3024793 | J. Phys. Chem. C 2012, 116, 18713−18721

The Journal of Physical Chemistry C

Article

Scheme 1. Possible Reaction Pathways of the Activation of the Carbonyl Bond in N,N-Dimethylacetamide (DMA) and the Intermediates in the Corresponding Pathways

regarding the reaction mechanism of the heterogeneous catalytic hydrogenation of amides so far. The production of corresponding amines via the reduction of amides must undergo the activation process of the carbonyl bond in amides, which could generally pass through one of two possible pathways: direct or hydrogen-induced cleavages. In the direct mechanism, the carbonyl bond of adsorbed amide first dissociates yielding aminocarbene and oxygen which are subsequently hydrogenated to amine and water, respectively. In the alternative mechanism, the adsorbed H first attacks the carbonyl bond, which assists the further cleavage of the C−O bond. It is still not clear which pathway is predominant. Moreover, the catalytic cleavage mechanism of the carbonyl bond in other classes of molecules over metal surfaces is still not confirmed.17−22 Hence, it is essential to explore the detailed mechanism, which may provide insights into the development of improved catalysts. In addition to the activity issue, the selectivity concerning the competition between C−O and C−N bond scissions is also significant in the amide hydrogenation, which has been reported in both experimental23−25 and theoretical work26 on the homogeneous hydrogenation process. However, it is worth mentioning that for the heterogeneous hydrogenation of amide, especially for tertiary amides, the reaction activity is of prime importance. As shown in the work of Beamson et al.,27 using different heterogeneous catalysts, the yields of alcohols and undesired amines are similar while the high selectivity to the desired amine is obtained only when its high conversion is achieved. This indicates that the difficulty of the carbonyl bond activation leads to the low yield of the desired amine, resulting in the low selectivity. Hence, in this paper we mainly focus on the understanding of the process of carbonyl bond activation in order to facilitate further reduction of hydrogen pressure. For primary or secondary amides, the activation process may start with the first dehydrogenation on N in addition to the common reaction pathways mentioned above which can be applicable for all the amides. However, we concentrated on the general reaction mechanisms in the presented work in order to

understand the common activation process of the carbonyl bond for all kinds of amides. N,N-Dimethylacetamide (DMA), the simplest tertiary amide, was chosen as a model compound. The work on DMA will provide a fundamental understanding on the catalytic hydrogenation processes of other amides. Aiming to locate the main reaction mechanism, we performed DFT calculations to provide the geometries and the energy barriers for the cleavage of the carbonyl bond in DMA via the direct route as well as the hydrogen-induced route (Scheme 1). In addition, it has been reported that the surface structure is important for the dissociation of CO28−30 and therefore, the calculations were carried out over both flat and stepped surfaces of Ru, which is one of the major catalyst components used for the hydrogenation of amides in the experimental reports. Ru was chosen mainly for two reasons. First, Ru is a good catalyst to activate the carbonyl bond, which is the key step in the reaction of amide hydrogenation. For example, Ru is an excellent catalyst for Fischer−Tropsch synthesis18,31,32 in which CO activation is an important step. Second, the major catalyst components used in the literature9−16 are mainly W, Re, Mo, Ru, Rh, Pd, and Pt, and Ru locates in the middle of the transition metals in the periodic table. Hence, the investigation of Ru for amide hydrogenation is a good starting point and is likely to provide a base to understand the reaction on the other metals.

2. METHODOLOGIES The DFT calculations were implemented with the Perdew− Burke−Ernzerhof (PBE) generalized gradient approximation (GGA) functional,33 using VASP code.34,35 The projectoraugmented-wave (PAW) pseudopotentials36 were utilized to describe the core electron interactions with a 400 eV cutoff energy of the plane-wave basis set. The Ru(0001) surface was modeled as a periodic 4-layer slab with a ∼12 Å vacuum region placed between periodically repeated slabs. For the study of the reactions occurring on the flat surface, a p(3 × 3) supercell was used with the corresponding γ-centered 3 × 3 × 1 Monkhorst− Pack k-point mesh sampling for the surface Brillouin zone 18714



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Table 1. The Adsorption Configurations and Corresponding Adsorption Energies of Intermediates, Which May Be Involved in the Reactions on the Flat and Stepped Ru Surfaces Ru(0001)

a

Ru(0001)-Step

speciesa

ads conf

ads energy (eV)

ads conf

ads energy (eV)

N,N-dimethylacetamide (DMA) N,N-dimethylaminoethylol (DMAEOL) N,N-dimethylaminoethyloxy (DMAEO) N,N-dimethylaminoethylidene (DMAED) N,N-dimethylaminoethyl (DMAE) OH O H

(π-CN) (σ−O), bridge (π-CN), top (σ−N) (σ−O), bridge μ2(C) (σ−N), hcp (σ−C) (σ−N), bridge μ3(O), fcc μ3(O), hcp μ3(H), fcc

−0.09 −1.52 −2.68 −2.47 −1.70 −3.24 −5.83 −2.84

(π-CN) (σ−O), step-edge-bridge (π-CN) (σ−O), step-edge-bridge (σ−N) (σ−O), step-edge-bridge μ2(C) (σ−N), near-edge-hcp (σ−C) (σ−N), step-edge-bridge μ2(O), step-edge-bridge μ2(O), step-edge-bridge μ2(H), step-edge-bridge

−1.32 −2.44 −3.42 −2.54 −2.40 −3.97 −5.93 −2.91

The capital abbreviation in parentheses denotes the molecules and intermediates throughout the paper.

Figure 1. Top and side views (insert) of the intermediates that may be formed during the cleavage process of the carbonyl bond in DMA over Ru(0001). The dark green balls are Ru atoms. The white, gray, blue, and red balls are H, C, N, and O atoms, respectively: (a) DMA, (b) DMAEOL, (c) DMAEO, (d) DMAED, and (e) DMAE.

Figure 2. Top and side views (insert) of the intermediates that may be formed during the cleavage process of the carbonyl bond in DMA over stepped Ru(0001). The Ru atoms on the top terrace including the step edge are golden, the other Ru atoms including those in the lower terrace are dark green. The white, gray, blue, and red balls are H, C, N, and O atoms, respectively: (a) DMA, (b) DMAEOL, (c) DMAEO, (d) DMAED, and (e) DMAE.

integration. A p(5 × 3) unit cell was utilized for the stepped Ru surfaces. The monatomic stepped surface was modeled by

removing two neighboring rows of the Ru atoms on the top layer. The corresponding γ-centered Monkhorst−Pack mesh 18715



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with 2 × 3 × 1 k-point was sampled in the surface Brillouin zone. Geometry optimization was carried out by the BFGS algorithm. During all of the optimization process, the bottom two layers of Ru atoms were fixed in the slab while the top two layers and adsorbates were relaxed. Transition states (TSs) of all the catalytic reactions were determined in terms of the constrained-minimization approach whereby to locate the TSs along the reaction coordinate.37−39 The search is not stopped until (1) all the remaining forces on the relaxing atoms have been optimized to be less than 0.05 eV/ Å and (2) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest degrees of freedom.

Scheme 2. Two Possible Locations of Products Formed Following the Cleavage of Carbonyl Bond in DMA via the Transition States, TS1f and TS2f, with the Corresponding Energy Barriers (eV), Respectively, on Ru(0001)

3. RESULTS 3.1. Chemisorption. We started the study with the calculations on the DMA chemisorption and the intermediates involved in all the possible reaction pathways. The adsorption energies from DFT calculation (Ead) are defined in the following equation:

structures of intermediate states (MSs) following the C−O bond breaking via the corresponding two TSs. In one case, oxygen atom adsorbs at the preferred hcp site and DMAED adsorbs on the metastable fcc site via the TS1f with a barrier of 0.78 eV. At the TS1f, the O−C distance elongates to 2.115 Å. The adsorbed oxygen is close to the bridge site and tilts toward the fcc site, while the DMAED fragment locates at the bridge site with a di-σ adsorption configuration in which both C and N atoms bind on an off-top site. In the other case, the oxygen atom migrates to the metal-stable fcc site and DMAED adsorbs on the hcp site (its most favored adsorption site) via the TS2f with a slightly lower barrier (0.74 eV) relative to the TS1f. At the TS2f, the oxygen is situated between the hcp and bridge sites, while the DMAED fragment is similar to that found at the TS1f. It is interesting that the divalent carbon atom at the TS is located at the top site instead of the bridge site, which is the general location of divalent adsorbate at TS.42 This indicates that the stronger electronegativity of neighboring N attracts electrons from the divalent C atom, which decreases the uncoordinated extent of divalent C atom and facilitates the stabilization of divalent C atom at the top site. 3.2.2. Cleavage Over Stepped Surfaces. On the stepped surface, as depicted at Scheme 3, starting with DMA adsorbing at the step-edge-bridge site, five possible TSs have to be taken into account for the direct cleavage of carbonyl bond. The calculated barrier is 1.81 eV at the TS1s. The oxygen and DMAED fragments are located at the bridge site on the upper terrace and the step-edge-bridge site, respectively, similar to the configuration of the TS1f on the flat surface. Likewise, the C−O distance of 2.095 Å is similar to that of the TS1f. The C−O bond length at the TS2s is 2.035 Å in which the adsorption sites of oxygen and DMAED fragment are simply exchanged as compared with those found at the TS1s. The calculated barrier of 1.76 eV is slightly lower than that of the TS1s. At the TS3s, both the O and DMAED fragments are located at the step-edge. The oxygen atom is close to the step-edgebridge site and the C atom in the DMAED fragment losing bonding with the step-edge-top site is 2.006 Å away from the O atom. Meanwhile the distance of the Ru−N bond increases to 2.284 Å, implying a decrease of the bonding between Ru and N. This barrier is 0.41 eV higher than that of the TS2s. At the TS4s, the oxygen atom migrates to the hcp site of the lower terrace. Although the DMAED fragment is located at the step-edge-bridge site as the TS1s, the DMAED is orientated toward the lower terrace and the adsorbed C atom is 1.895 Å away from the O atom. This is closer to the step-edge-bridge

Ead = E(ad sorbate/surface) − E(adsorbate) − E(surface) (1)

where E(adsorbate/surface), E(adsorbate), and E(surface) are the total energies of the adsorbate binding with metal surface, gaseous adsorbate, and clean surfaces, respectively. Table 1 lists the adsorption energies of the intermediates with the most stable adsorption configuration on both flat and stepped Ru surfaces and the corresponding geometry structures shown in Figures 1 and 2, respectively. As shown in Table 1, the adsorption energies of intermediates on the stepped surface generally exceed those on the flat surface. Except for N,Ndimethylaminoethylidene (DMAED), the variations of the adsorption energies between the intermediates on flat and stepped Ru surfaces are at least 0.70 eV. This results from the higher activity of the metal atoms at the step-edge because of the lower coordination number (CN) of the metal atoms at the step-edge (CN = 7) as compared with those on the flat surface (CN = 9). Furthermore, it is clear from Table 1 that each intermediate binds with the stepped surface via the same atoms as those with the flat surface. Most of the adsorbed intermediates are favorable at the bridge site on the flat surface and the step-edge-bridge site is preferred on the stepped surface.40 In contrast, the preferential adsorption site of DMAED alters from the hcp site on the flat surface to the near-edge-hcp site instead of the active step-edge-bridge site on the stepped surface. The similar DMAED adsorption structure on the stepped surface to that on the flat surface leads to similar binding energies on both surfaces. In addition, it is clear from Figures 1 and 2 that the steric hindrance between the side chains of adsorbate, namely methyl groups and the surface, can be effectively reduced from the close-pack flat surface to the step. The decrease in the 2-orbital-4-electron (steric) repulsion41 can further facilitate the adsorption occurring on the step compared to the flat surface. This also indicates that the complex amide molecule with larger side chains would prefer to adsorb on the step more than simpler molecules. 3.2. Direct Cleavage of Carbonyl Bond. In the direct cleavage pathway, initiating from adsorbed DMA, the carbonyl bond is broken by the surface Ru atom, which produces DMAED and oxygen. 3.2.1. Cleavage on Flat Surfaces. On the flat Ru surface, as illustrated at Scheme 2, there are two possible adsorption 18716



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indicate that the addition of the H atom does not play a significant role in activating the C−O bond in this step. Subsequently, the formed DMAEO dissociates into oxygen atom and N,N-dimethylaminoethyl (DMAE). This elementary step is exothermic by 0.37 eV whereas the cleavage process has to overcome an energy barrier of 1.66 eV, which is 0.63 eV higher than the first hydrogenation step. In the case when the reaction occurs on the stepped surface, the hydrogenation of the C end is an endothermic process (0.55 eV), which is different from the reaction occurring on the flat surface. In this elementary step, the calculated barrier is 0.86 eV. At the TS, the hydrogen shares one Ru atom with C and N atoms of DMA and the C−H distance is 1.537 Å. Similar to the direct cleavage process, the subsequent cleavage step to yield oxygen atom and DMAE occurs via one or more of the five different final states from the corresponding TSs. As Figure 3 displays, among these five TSs, it is interesting to note that the TS1s and TS4s share the lowest energy barrier (1.63 eV), which implies that DMAEO is most likely to dissociate via either the TS1s or the TS4s. 3.3.2. O−H Activation Pathway. In this pathway, the O end of the carbonyl bond is first hydrogenated to produce N,Ndimethylaminoethylol (DMAEOL), which activates the C−O bond to help with the subsequent C−O bond cleavage. On the flat surface, the hydrogenation of carbonyl O does not easily occur since it is an endothermic step (0.76 eV). Furthermore, the corresponding energy barrier rises to 1.23 eV. After the hydrogenation, the distance of the C−O bond increases from 1.330 Å in DMA to 1.434 Å in DMAEOL, implying that the C−O bond is activated by the H atom. This induces the facile decomposition of DMAEOL to yield hydroxyl and N,Ndimethylaminoethylidene (DMAED) via a lower energy barrier (0.62 eV). The lower barrier is accompanied by an obvious exothermic process (−0.72 eV) for the dissociation due to the stronger bonding interactions of hydroxyl and DMAED with Ru surfaces as shown in Table 1. On the stepped surface, the hydrogenation of the O end is difficult due to the large endothermic process (1.13 eV). A high energy barrier (1.78 eV) has to be overcome in order to activate the C−O bond. Similar to the C−O cleavage steps in the other cleavage pathways over the stepped surface, one of five location combinations of two final products (hydroxyl and DMAED) are possible starting from DMAEOL. As depicted in Figure 3, this elementary step is most likely to go through the TS3s, at which a mild 0.67 eV energy barrier needs to be overcome. Unlike the most stable TSs in the other pathways, both DMAED fragment and hydroxyl locate at the top sites of the step edge at the TS3s. The C−O distance is 2.101 Å, which is 0.304 and 0.175 Å longer than those of the corresponding TS3s in the direct cleavage and C−H activation pathways, respectively. This indicates that the reduction of bonding competition makes the TS3s the most stable TS for this step. The energy gain in this step is 0.76 eV, which is similar to that found over the flat surface.

Scheme 3. Five Possible Locations of Counterparts after the Cleavage of Carbonyl Bond in DMA via the TS1s-TS5s with the Corresponding Energy Barriers (eV), Respectively, over Ru(0001)-Step

site compared with its position at the TS1s. It is interesting that the calculated barrier (1.81 eV) is the same as that of the TS1s. At the TS5s, the oxygen sits on the step-edge-bridge site and the DMAED fragment moves to the lower terrace. This TS is unfavorable due to the high barrier (2.28 eV), which is the highest among these five TSs. Comparing the five TSs above, the direct cleavage process on the Ru step surface is most likely to undergo the TS2s. On account of the small differences of the energy barriers among the TS1s, TS2s, and TS4s, the possibility of the carbonyl bond dissociation via the TS1s and TS4s cannot be fully excluded under realistic experimental reaction conditions. 3.3. Hydrogen-Induced Cleavage. An alternative mechanism for the activation of the carbonyl bond is the C−O bond breaking with the assistance of hydrogen. We investigated two possible pathways: C−H activation and O−H activation pathways. 3.3.1. C−H Activation Pathway. In this pathway, the reaction initiates from the attack on the C end of the carbonyl bond from the adsorbed H atom to form N,N-dimethylaminoethyloxy (DMAEO), followed by the subsequent cleavage of the activated C−O bond. On the flat surface, the hydrogenation of carbonyl C, which has a tiny exothermic elementary step (−0.01 eV), requires a barrier of 1.03 eV to form DMAEO. At the TS of this elementary step, the hydrogen diffuses from the initial fcc site to the off-top site where the C and N atoms in DMA are originally bound. At the TS, the hydrogen is at a distance of 1.382 Å from the C atom. Moreover, the distance between the adsorbed C atom and the occupied Ru atom increases from 2.277 Å to 2.501 Å and the C atom is very weakly bound to the Ru atom. It is also found that the distance of the C−O bond of DMAEO only lengthens 0.032 Å relative to that found in DMA (1.330 Å). The trivial variation may

4. DISCUSSION 4.1. The Main Reaction Pathway. As displayed in Figure 4, comparing the overall barriers (with reference to the adsorbed DMA and H) of three pathways, i.e., the direct cleavage (0.74 eV), C−H activation (1.62 eV), and O−H activation (1.35 eV) on the Ru flat surface, it is obvious that the direct cleavage pathway is preferred. In contrast to the C−H activation pathway, the carbonyl bond is more difficult to 18717



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Figure 3. The top and side view (inserted) of all the possible dissociation TSs and their corresponding energy barriers (eV) in three different dissociation pathways. Flata: Only the most stable TS over flat is shown. TS2f is favored in the direct dissociation and the O−H activation pathway while TS1f is favored in the C−H activation pathway.

cleavage barrier (0.67 eV), the overall barrier (1.80 eV) in the O−H activation pathway is still 0.04 eV higher than that of the direct cleavage pathway resulting from the instability of DMAEOL. In the C−H activation pathway, the hydrogenation barrier over the stepped surface (0.86 eV) is lower than the overall barrier in the direct cleavage pathway. However, this is different from the process over the flat surface: Since the hydrogenation step is endothermic and because of the high barrier of the subsequent cleavage step (1.66 eV), the overall barrier up to 2.18 eV is still too high to overcome. It is clear that the main cleavage pathway of the carbonyl bond in DMA is almost independent of the surface structures of Ru; the direct cleavage mechanism is energetically favored on both Ru flat and stepped surfaces.

hydrogenate at the O end but much easier to break in the O−H activation pathway. On the whole, due to the lower overall barrier, the O−H activation is considered to be favorable compared with the C−H activation. Although the energy barrier of the C−O bond cleavage in the O−H activation pathway is only 0.62 eV, lower than that in the direct cleavage one due to the lower stability of the adsorbed DMAEOL, the overall barrier of this step increases to 1.35 eV, which is much higher than that in the direct cleavage process. Hence, the O− H activation pathway is less important than the direct cleavage pathway. On the stepped surface, as shown in Figure 5, the direct cleavage pathway is still the preferential pathway requiring the lowest overall barrier with respect to the adsorbed DMA and H. Similar to the process over the flat surface, in spite of a lower 18718



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eV) sites. The overall dissociative adsorption rate r can be estimated in terms of the Arrhenius equation r ≈ A e−Ea / RT C

(2)

where T is the temperature, A is the pre-exponential factor, and C is the concentration of surface sites. At 500 K, a typical reaction temperature of the amide hydrogenation, the 0.28 eV reduction of apparent barrier mentioned above corresponds to the increment of the reaction rates by ∼103. According to eq 2, in addition to the activation energy barrier, the overall rate is proportional to the number of surface sites. Considering the number of flat sites is usually 10−100 times larger than that of the step sites, the rate for this reaction over the stepped surface will be around 10−100 times larger than that over the flat surface. This result is consistent with the general rule that the dissociation reaction tends to occur over stepped rather than flat surfaces.30 However, compared with the dissociation of CO in which the energy barrier decreases by 1.05 eV18 from the flat to the stepped Ru surface, the change of energy barrier for the cleavage of the CO bond in amide is much smaller. This indicates that the carbonyl bond dissociation of amide is less structure sensitive than the dissociation of CO. To provide insights into the cleavage process of the carbonyl bond, we used the decomposition scheme proposed by Liu and Hu 43 (Scheme 4) to investigate the reaction barrier quantitatively. The scheme is briefly summarized as follows:

Figure 4. The reaction pathways over Ru(0001). Black line: The direct cleavage pathway. Blue line: O−H activation pathway. Red line: C−H activation pathway. The stabilities of the TSs with respect to absorbed DMA and H are listed (eV) and the energy barriers of the corresponding elementary steps are listed in parentheses (eV).

TS ETS = EATS + E BTS + E int

(3) TS

The chemisorption energy of TS, E , can be split into three terms as shown in eq 3: where ETS A is the chemisorption energy of A at the TS geometry without B and EBTS is the chemisorption energy of A at the TS geometry without A. These two terms are related to the surface electronic effect. ETS int is a quantitative measure of the interaction between A and B at the TS including (i) the bonding competition resulting from A and B sharing bonding with the same surface atoms, (ii) electrostatic interaction, mainly from dipole−dipole interaction, and (iii) the Pauli repulsion between A and B. The positive value of ETS int denotes that the repulsive interaction is prevailing while the negative value means the attractive interaction is dominant. The geometry effect of the surface plays an important role in this component. In this reaction, the DMAED fragment (denoted as CN as follows) and the O atom correspond to A and B, respectively. As listed in Table 2, when the cleavage process occurs over the flat surface, although a strong repulsion interaction exists between the counterparts at the TS, the electronic effect plays a predominant role. The values of ETS CN are very close between the TS1f and the TS2f. As the oxygen is close to the most stable adsorption site (fcc) at the TS2f, the greater value of ETS O makes the TS2f more stable than the TS1f. However, the sequence of stability of the five TSs over the stepped Ru surface depends on the overall effect of all components rather than only the electronic effect. At the TS1s, the DMAED fragment locates at the active step-edge-bridge site and O sits at the upper terrace. On the one hand, it has the most stable chemisorption energy of the DMAED fragment, which is 0.33 eV more stable than that over the flat surface due to the upshift of the d-band center along the step edge,40 which leads to the stronger electronic effect of the step relative to the flat surface. On the other hand, O migrates to the least stable position among those at the five TSs. Due to the similar configuration and C−O distance to that of the TS2f over the flat surface, a repulsive interaction of 0.22

Figure 5. The reaction pathways over Ru(0001)-step. Black line: The direct cleavage pathway. Blue line: O−H activation pathway. Red line: C−H activation pathway. The stabilities of the TSs with respect to absorbed DMA and H are listed (eV) and the energy barriers of the corresponding elementary steps are listed in parentheses (eV).

4.2. Structure Sensitivity. In general, the geometry of the surface structure plays a significant role in the dissociation process. We investigated its contribution to address this issue on the amide hydrogenation. Since the direct cleavage pathway has been demonstrated as the dominant pathway in Section 4.1, here we will focus the issue of structure sensitivity on the direct dissociation process. Since the C−O bond cleavage in CO is the simplest one and has been extensively researched and the structure sensitivity of CO activation has been shown to be important,17−22 we herein compare the C−O bond cleavage of CO with that of DMA in order to provide a base for understanding the C−O bond activation processes. As shown in Section 4.1, the overall energy barrier with respect to adsorbed DMA and H over the stepped Ru surface is 1.02 eV higher than that over the flat surface. However, the apparent barrier of the catalytic reaction from the experiment is usually referred to the gas phase, a reasonable reference to analyze where the reaction largely occurs over different structures rather than the adsorbed state. Comparing the calculated results, the dissociative barrier of the direct cleavage with respect to the gas phase declines by 0.28 eV when the reaction transfers from the flat (0.08 eV) to the step (−0.20 18719



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Scheme 4. Energy Diagram for the General AB → A + B Reaction over Metal Surfacesa

a

The figure is utilized to explain each term in eq 3.

For the TS4s and the TS5s, because of the long distances (2.907 and 3.209 Å, respectively) between the O and the Ru atom adsorbed by the C atom, the repulsion interactions from bonding competition are very weak, as is the Pauli repulsion. Such a low bonding competition and Pauli repulsion are manifested in the geometry advantage of the step structure.25 The weak chemisorption energies of counterparts lower the stability of TS5s and, especially for DMAED, its instability makes TS5s become the least stable TS due to the steric hindrance between the fragment and the surface. This indicates that this type of TS will become more unfavorable with the complication of the amide molecule. Comparing the electronic effect components of all the TSs on flat and stepped surfaces, it is interesting that the TS2f on the flat surface enjoys the strongest adsorption. However, the great repulsion interaction between the counterparts on the flat surface makes it less stable than the TSs on the step. Because of (i) a stronger bonding competition between CN and O fragments resulting from the activation of the C−O bond at the TS and (ii) the geometric limitation of one-dimensional sites of the step edge at the stepped surface, it is very difficult for both CN and O fragments in DMA to adsorb at the active step edge with the most stable configurations. Thus the electronic advantage of the active step edge compared to the flat surface, i.e., the lower coordination number of step edge atoms giving rise to the higher adsorption energy,44 to stabilize the reactants is weakened. This leads to the similar apparent barriers. This also explains why the advantage of step in the cleavage process of the carbonyl bond of amides is not as evident as that found in CO activation. However, at the IS on the stepped surface, the bonding competition and geometric limitation are much less effective due to the fact that the C−O bond is not activated and all the atoms in DMA are chemically saturated. Hence, both the carbonyl bond and N of DMA can locate at the active step edge with the most stable configuration. As a result, the adsorption state of DMA on the stepped surface in the IS is much more stable than that on the flat surface. This leads to the higher energy barrier on the stepped surface than that on the flat surface. However, it is worth mentioning that the rate of the activation process is mainly determined by the apparent barrier.

Table 2. Decomposition of the TS of Direct CO Cleavage over Flat and Step Ru Surfaces (eV) flat step

TS1f TS2f TS1s TS2s TS3s TS4s TS5s

ETS

ETS CN

ETS O

TS ETS CN + EO

ETS int

−6.53 −6.68 −6.70 −6.87 −6.46 −6.83 −6.35

−1.72 −1.73 −2.06 −1.30 −1.45 −1.66 −0.95

−5.26 −5.59 −4.86 −5.35 −5.59 −5.14 −5.25

−6.98 −7.32 −6.92 −6.65 −7.03 −6.79 −6.20

0.45 0.64 0.22 −0.22 0.58 −0.03 −0.15

eV between counterparts is inevitable, mainly due to the bond competition. Interestingly, the distance between the O atom and the Ru atom adsorbed by the C atom is 0.203 Å longer compared with that found for the TS2f over the flat surface. At the TS2s, the chemisorption energy of the DMAE fragment is significantly weaker than those at the TS1s and TS3s. However, a large attractive interaction between counterparts at the TS2s makes up for the loss of chemisorption energy of the DMAE fragment, which leads to it being the most stable TS among these five. When both counterparts locate at the step edge like the TS3s, an unfavorable configuration of adsorbed DMAE fragment leads to a low chemisorption energy despite the location being on the active step edge. On the other hand, with the most stable O atom, the maximum adsorption energy of TS ETS CN + EO among the five TSs over the step is achieved. However, the repulsion interaction in the TS3s reaches maximum (0.58 eV) among all the possible TSs over the step for the following reactions. First, the fact that N, C, and O share one Ru atom gives rise to a severe bonding competition. Second, the shortest C−O distance (1.797 Å) implies a stronger Pauli repulsion as well. The overall effect including both electronic and geometric components leads to the fact that the dissociation involving the step edge only is not an energetically favored reaction channel. Since the fragment at step edge sites can decrease the steric hindrance compared with that staying at the terrace, this type of TS for the complex amide molecule with a large side chain may be favorable. 18720



Article

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5. CONCLUSIONS In the paper, extensive DFT calculations were used to explore the cleavage mechanism of the carbonyl bond in amides on both flat and stepped Ru surfaces based on the model molecule, DMA. Through the calculations, the most stable TS configuration in each pathway on both flat and stepped Ru surfaces was identified. On the basis of thorough analysis, the following conclusions are reached: (i) Comparing the energy profiles of different reaction pathways, the activation of the carbonyl group in amides tends to occur via the direct cleavage pathway instead of the alternative hydrogen-induced pathways on both flat and stepped Ru surfaces. This suggests that the activation pathway of the carbonyl bond in amides on the Ru surface is not related to the surface structure. (ii) Similar to CO dissociation, the amide carbonyl bond dissociation tends to occur on the step. However, when compared with the variation of energy barriers of CO dissociation on Ru(0001) from flat to stepped surfaces, the barrier change of carbonyl group dissociation in amide from flat to stepped surfaces is evidently smaller. This indicates that the dissociation of the carbonyl group in amide is significantly less structure sensitive as compared with CO dissociation. (iii) The energy analysis based on the decomposition scheme sheds light on the fact that the dissociation of amide on the step cannot achieve greater electronic and geometry effects as compared to that on the flat surface at the same time, which is different from CO dissociation. This gives rise to the result that the dissociation of the C−O bond in amide is less structure sensitive relative to CO dissociation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge East China University of Science and Technology and The Queen’s University of Belfast for computing time and the CASTech grant from the EPSRC. P.H. thanks Chinese government for the “Thousand Talents” program.

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REFERENCES

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Density Functional Theory Study on the Cleavage Mechanism of the

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