Mechanisms of the WaterâGas Shift Reaction Catalyzed by Ruthenium

Article pubs.acs.org/JPCA

Mechanisms of the Water−Gas Shift Reaction Catalyzed by Ruthenium Carbonyl Complexes Naying Liu, Ling Guo,* Zhaoru Cao, Wenli Li, Xiaoli Zheng, Yayin Shi, Juan Guo, and Yaru Xi School of Chemistry and Material Science, Modern College of Arts and Sciences, Shanxi Normal University, Linfen 041004, China S Supporting Information *

ABSTRACT: Density functional theory (DFT) is employed to study the water−gas shift (WGS) reaction in the gas phase for two complexes, Ru3(CO)12 and Ru(CO)5. Here we report four mechanisms of ruthenium carbonyl complexes catalyzed for WGS reaction. The energetic span model is applied to evaluate efficiency of the four catalytic pathways. Our results indicate that mechanism C and D show a good catalytic behavior, which is in agreement with results from the literature. The mechanism C and D not only include the important intermediate Ru3(CO)11H− but also exclude the energy-demanding OH− desorption and revise an unfavorable factor of the previous mechanism. Two complexes along mechanisms B have the highest turnover frequency (TOF) values. The trinuclear carbonyl complexes-Ru3(CO)12 is preferred over mononuclear carbonyl Ru(CO)5 by comparing TOF due to the fact that metal−metal cooperativity can enhance activity to the WGS reaction. In this work, the nature of interaction between transition states and intermediates is also analyzed by the detailed electronic densities of states, and we further clarify high catalytic activity of ruthenium carbonyl complexes as well. Our conclusions provide a guide to design catalysts for the WGS reaction. barriers than the mononuclear mechanism. Chen et al.,11 by using density functional theory, analyzed the mechanism of metal carbonyl complexes catalyzed by WGS reaction; the result inferred that Fe and Ru species are more favorable than Os species. Ford12,13 and King et al.14,15 identified Ru3(CO)12 and the group 6 metal hexacarbonyl complexes, such as Cr(CO)6 and M(CO)6, as homogeneous WGS reaction catalysts in the early years. Since then, homogeneous transition metal catalysts, especially, ruthenium-based catalyst have been reported that testify WGS reaction catalytic efficiency.16−20 In contrast with the above-mentioned study, experimental evidence indicated ruthenium carbonyl complexes as active catalyst. For example, the Ru(CO)5 might be more effective than Fe(CO)5 for WGS reaction.21 Very recently, Werner et al.22,23 discovered that ruthenium-based catalyst by supported ionic liquid phase (SILP) proceed at lower temperature than usual solvents. Schulz et al.24 analyzed the mechanism of WGS reaction catalyzed by Ru(CO)5 in solution. However, the tendency of ruthenium prefers polynuclear carbonyl complexes,25 and the nature of polynuclear ruthenium complexes as catalyst is less clear. In order deeper understand this, we consider polynuclear ruthenium species for WGS reaction, in agreement with reported data. Hastings et al. determined that the equilibrium constant for trimerization was 3 × 106 mol/L at

1. INTRODUCTION The water−gas-shift (WGS) reaction, CO + H2O → CO2 + H2, have seen a considerable interest in the catalysis chemistry in the past few years. The current attention derives mainly from the necessity of H2 and reduction of CO gas. The energy consumption becomes very serious in the global, thus the search for renewable, sustainable, and clean sources of energy is still a significant challenge.1,2 The renewable hydrogen energy can be a potential alternative to fossil fuels and then readily to the fuel cells and other industrial application.3 In the meantime, the substitution of toxic carbon monoxide (CO) from various reactions by nontoxic carbon dioxide (CO2) may be one of the options for reducing content of CO. Industrially, the WGS reaction is catalyzed by two different operating temperatures, FeCr-based catalysts for high temperature (350−500 °C) and Cu/oxide-based catalysts for the low temperature (180−250 °C). But application of various transition-metal carbonyl species has aroused great attention due to its mild conditions in the homogeneous catalyzed WGS reaction. Many experimental and theoretical studies,4 such as, Fe(CO)5,5 Ru3(CO)12,6 and Rh6(CO)16,7 have been employed as homogeneous catalysis, and some investigations on the WGS reaction have also been published. As noted, the ruthenium carbonyl complexes for WGS reaction are carried out at a fairly low temperature, and the chemistry of this and related species attract considerable attention.8,9 Kuriakose et al.10 studied binuclear mechanism with Fe(CO)5 catalyst in WGS reaction and suggested that the binuclear mechanism lead to lower © XXXX American Chemical Society

Received: January 11, 2016 Revised: March 24, 2016

A

DOI: 10.1021/acs.jpca.6b00301 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Scheme 1. Mechanistic Proposal A Following Torrent et al. for Ru3(CO)12 Catalyzed WGS Reaction

Scheme 2. Mechanistic Proposal B Following Barrows for Ru3(CO)12 Catalyzed WGS Reaction

298 K and standard pressure in experiment.26 Laine et al. describes the reaction where a homogeneous condition prepared from Ru3(CO)12 is an active catalyst.12 The ruthenium-based catalysts have been used in industry traditionally, but the mechanism of Ru3(CO)12 catalyzed WGS reaction is still not clear because many investigations only focus on a single reaction. A thorough understanding of WGS reaction mechanism is crucial to improve catalyst design. The generally accepted homogeneous catalysis mechanism is shown in Schemes 1−4. The classical mechanism that is based on experimental observation has been investigated by Sunderlin et al.27 Subsequently, Torrent at al.28 substantiated this mechanism by theoretical approach. On account that the OH− desorption step was found to have a high energy barrier, three other alternatives were proposed from literature. Barrows29 proposed new reaction paths that avoided the OH− desorption step including the Fe(CO)4COOH− that directly converts to Fe(CO)4CHO− and back to Fe(CO)4COOH− (see Scheme

2). Because the mechanism in Scheme 2 did not contain experimentally observed Fe(CO)4H−, Rozanska et al.30 also raised the new reaction mechanism, which included Fe(CO)4H− as well as avoided the OH− desorption step (see Scheme 2). Then Zhang et al.31 reported a more favorable reaction mechanism that it has lower barriers based on the previous mechanism. In this work, on the one hand we seriously analyze the geometry of Ru 3 (CO) 12 and Ru(CO) 5 , possible four mechanisms, and corresponding relative energy profiles of WGS reaction catalyzed by Ru3(CO)12 and Ru(CO)5. On the other hand, we employ the ESM to estimate catalytic efficiency of ruthenium carbonyl complexes. Finally, to gain insight into catalytic activity of two catalysts, our work is checked by the detailed electronic states of d-band center. Further theoretical study will help to understand ruthenium carbonyl catalyst for WGS reaction, as well as provide a theoretical guide. B


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The Journal of Physical Chemistry A Scheme 3. Mechanistic Proposal C Following Rozanska et al. for Ru3(CO)12 Catalyzed WGS Reaction

Scheme 4. Mechanistic Proposal D Following Zhang et al. for Ru3(CO)12 Catalyzed WGS Reaction

2. COMPUTATIONAL METHOD AND DETAILS All of the optimized structure of WGS reaction, including reactants, intermediates, transition states, and products, are computed by using spin-polarized density functional theory (DFT) in the Gaussian 09 programs32 level. The standard 631++g** basis set was used for hydrogen, oxygen, and carbon. Considering the strong relativistic effect of Ru, the LANL2DZ pseudo potential is adopted for the valence electrons, and its core electrons are represented by the LANL2DZ effective core potential (ECP).33,34 This method of many systems have been proven that it is very valid for many system including transition atoms such as Fe(CO)5, Ru(CO)5, and Mo(CO)6. Single point energy calculations were carried out at the Perdew−Burke− Ernzerh (PBE) of DFT functional35 for all reaction steps involved in the reaction mechanism. In the present study, we explore the structure of Ru3(CO)12 and Ru(CO)5 with D3h symmetry, Indeed, for Ru3(CO)12 the energy of the optimized structure calculated with D3h symmetry

dropped by 5.2 kJ/mol when no symmetry constraints were kept. We carried out improved energy calculations with the PBE/6-31++G** method optimized geometries by using DFT, and the optimized geometries catalyzed for WGS reaction are listed in Figure 1 and Figure S2 (Supporting Information). We focus on the catalytic performance of the Ru3(CO)12 in this work. Therefore, the transition states (TS) on different surfaces are searched with the complete LST/QST method, and the subsequent frequency calculation can find only a minus value, verifying the nature of the saddle point. At the same time, the next calculation has proved that saddle point links reactant with product. At this level of theory, we have also calculated the harmonic vibrational frequencies to verify the nature of the corresponding stationary points (minima or transition state) to provide the zero point vibrational energy (ZPE) and the thermodynamic contributions to the enthalpy and free energy. Moreover, to ensure that the transition states connect the C


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The Journal of Physical Chemistry A

Figure 1. continued

D


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Figure 1. Potential energy surfaces for WGS reaction promoted by Ru3(CO)12 through four mechanistic pathways in gas phase. The corresponding intermediates and transition states are also presented. All values are Gibbs free energy, which is in kcal/mol.

and Ru(CO)5 with D3h symmetry, which is in agreement with the previous works.41,42 The detail of the structures catalyzed WGS reaction are put in Figure 1 and Figure.S1, respectively. For the Ru(CO)5 and Fe(CO)5, their structures are very similar. Also, the Fe(CO)5 have been discussed in previous literature,10 and they show great agreement with the literature. Hence, we will mainly discuss Ru3(CO)12. The calculated bond lengths and bond angles of Ru3(CO)12 are listed in Table 2, and

desired reactants and products, we have performed intrinsic reaction coordinate calculations (IRC).36 The accuracy and reliability of chosen functional have also been confirmed by comparing the theoretical results with experimental values38,39 illustrated in Table 1. The calculated Table 1. Calculated Reaction Energy of WGSR-Related Step in Comparison with Experiment Results in kcal/mol

this work

experimentala a

BP86/6-31+ +G** B3LYP/631++G** PBE/6-31+ +G** ΔH (kJ/ mol)

Ru(CO)5→ Ru(CO)4+CO

HCOOH→ CO2+H2

CO +H2O→ CO2+H2

109.3

−23.9

−70.2

80.8

−26.2

−43.7

115.2

−21.6

−43.9

116

−22.0

−41.2

Table 2. Calculated and Experimental Bond Lengths and Bond Angles in Ru3(CO)12 Complexesa RuRu RuCa RuCb RuRuCa RuRuCb CaRuCa′ CbRuCb′ CaRuCb

Reference 39.

Ru calcd[1]

Ru calcd[2]

Ru calcd[3]

2.854 1.942 1.921 89.5 90.3 178.3 104.1 90.3

2.948 1.970 1.941 89.07 98.29 177.8 103.4 90.7

2.929 1.963 1.927 89.06 98.66 177.8 102.7 90.6

2.920 1.960 1.921 89.03 95.38 177.8 104.2 90.4

a The Ru calcd[1], Ru calcd[2], and Ru calcd[3] are represent as the B3LYP, BP86, and PBE method, respectively. bReference 38.

bond lengths of CO, H2, and CO2 at 6-31++g** level are 1.15, 0.75, and 1.18 Å, and their binding energy values are 11.48, 4.26, and 5.90 eV, respectively. The corresponding experimental values37 are 1.14, 0.73, and 1.16 Å and 11.23, 4.48, and 5.50 eV. It shows further the reliability of the functional used in this calculation. Furthermore, we also calculate turnover frequency (TOF) values of mechanism to analyze efficiency of catalyst. The ESM, which is derived from the Arrhenius−Eyring transition state theory, developed further and proposed by Kozuch et al.40 to calculate TOF TOF =

Ru exptlb

the rationality of the geometry is confirmed by comparing the theoretical result with experimental one.38 The trinuclear ruthenium complexes-Ru3(CO)12 is prepared by reported literature. Its composition and molecular structure has been known for a long time.22,23,43 The Ru3(CO)12 molecule adopts D3h symmetry with the ruthenium atom taking a pseudooctahedral geometry composed by two Ru atoms and two carbonyls in the equatorial and two axial carbonyls (see the Figure.S1) In the past decade, many studies indicated that Ru3(CO)12 existed as the most hindered structure, corresponding to the D3, C2v, and D3h isomer. Nevertheless, the experimental analysis of the derived charge density by X-ray methods has been reported. The result showed that the Ru3(CO)12 favors pseudo D3h symmetry. In other words, the Ru3(CO)12 actually takes the geometry of the sterically less favored isomer, while the

kBT −δE /RT e h

where δE is the energy span; see ref 40 for further details.

3. RESULTS AND DISCUSSION 3.1. Structures of the Ru3(CO)12 and Ru(CO)5 Complexes. In this work, we explore the structure of Ru3(CO)12 E


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Mechanism A. Mechanism A following Torrent28 is shown in Scheme 1. The complexes-Ru3(CO)12 reacts with OH− to form metallocarboxyl acid anion 2-Ru3(CO)11COOH− as first intermediate, which is a highly exothermic and barrierless process. For Ru 3 (CO) 12 , the length of the RuCO (equatorial) distance (1.921 Å) is shorter than the RuCO (axial) distance (1.960 Å) (see Figure S1), which is in agreement with the experimental value (RuCO (equatorial) (1.921 Å), RuCO (axial) (1.942 Å)). For the intermediate 2, it is the most stable geometry, where the COOH− group takes the axial position, and the formation of COOH− group bring out the changes of RuCO bond lengths in 2: all of the Ru CO bonds become shorter, while all of the CO bonds elongate slightly. The RuCOOH− bond distance shows an increase, which is 0.243 Å. All computed ΔG1/2 of this step is −85.62 kcal/mol along four mechanisms in the gas phase. We suggest that the direction of attack of OH− is nearly equidistant from the axial and the equatorial carbonyl ligands for Ru3(CO)12, because the initial approach is controlled by forces of electrostatic nature as a result of both ligands. Because the axial carbonyl ligands are attacked in the final stage, which is controlled by molecular orbital type interactions, it can be attributed to no empty d orbital pointing to the equatorial ligands in Ru3(CO)12, or the empty ∏*CO orbital is lower energy than the axial carbonyl ligands. Thus, the reaction allows the attack of hydroxyl at about 120° of the RuC bond. The next step, 2 decomposes into CO2 forming the anion 3HRu3(CO)11− through a energy barrier (TTS2/3)of 35.67 kcal/mol with an exothermicity of −9.97 kcal/mol. The negatively charged H− in the intermediate 3 also occupies the axial position. The structure of transition state (TTS2/3) in the process is the four-centered structure (Figure 1); the MC and OH bonds are gradually broken, whereas the MH bonds are gradually formed. The distances of axial CO ligands are 1.170 Å; one cause may be the influence of hydride because the hydride is a better σ-donor than CO. In the last step, a H2O molecule breaks its HH bond, and the proton transfers from water to metal hydride 3 to yield the dihydride intermediate 4Ru(CO)11H2OH−. The computed ΔG3/4 of this step is −3.63 kcal/mol. However, the OH− desorption step to form intermediate 5 is found to have a high barrier; the step is the rate-determining step in the process of the WGS reaction. Proceeding to the next step, 5 isomerizes into the intermediate 6. As shown in Figure 1, we can see that isomers 5 and 6 are close in energy, and the energy barrier to reach TS5/6 is also low, only 3.81 kcal/mol. From Figure 1, we can see that the optimized species 6 is different from 5 due to the short HH bond length (0.75 Å). The H2 molecule still takes the axial plane in 6, which is linked to the Ru atom through a weak σbond; hydride is a strong σ-donor as well. In the 6, similarly, the relative order of the MCax/MCeq distances is opposite to those in 5. The back-donation in 6 moves from Ru atom to CO ligands and then shifts to H2, leading to a shorter MCax bond length. Finally, the assumed reaction pathway from 6 to initial reactant 1-Ru3(CO)12 included reductive elimination of H2 and addition CO. All reaction pathways are nearly identical for both Ru(CO)5 (see Figure S2) and Ru3(CO)12, including the change in bond length and structure. For example, the COOH− group causes the change of RuCO bond lengths in the case of Ru(CO)5 in which all the RuCO bonds become shorter, whereas all of CO bonds elongate slightly, indicating that the trend is nt the ultimate difference between Ru(CO)5 and Ru3(CO)12. Let

Fe3(CO)12 does not. Slebodnick and co-worker44 considered smaller metal carbonyl complexes-Fe3(CO)12, fit inside the compact arrangement of carbonyl groups, such as Ru3(CO)12 and Os3(CO)12, which require the larger space provided by the pseudo-octahedral complexes. Therefore, the ruthenium carbonyl complexes adopt D3h symmetry because there is more repulsion between ruthenium atoms than the iron atoms. Meanwhile, the reason can be explained by utilizing metal valence or electronic factor that prefers to the D3h geometry. The Ru atom has a pseudo octahedral geometry in the D3h structure and is supposedly well set up for the composition of six strong bonds; then it can also be elucidated by using the Hoffmann’s fragment molecular orbital theory. The fragment of M(CO)4 group in the Ru3(CO)12 geometry has two frontier orbitals and is isolobal to a methylene group, thus the three M(CO)4 groups would prefer to the D3h symmetry. In addition, the reaction of bare cluster-Ru3(CO)12 with other compound is of interest. For example, the reaction of a N-heterocyclic carbene (NHC) with Ru3(CO)12 generates new compounds.45 Because the NHCs have attracted numerous attention, the newly synthesized compounds proved to be effective in homogeneous condition. 3.2. Four Different Reaction Mechanisms Proposed for Ruthenium Carbonyl Complexes-Ru3(CO)12 and Ru(CO)5 Catalyzed for WGS Reaction. We describe four possible reaction mechanisms labeled A−D in detail; in particular, we discuss complexes-Ru3(CO)12. The free energy profiles are shown in Figure 1. Also, the corresponding reaction energy (ΔG) and activation barrier (Ea) are listed in Table 3. Table 3. Reaction Energies (ΔG) and Activation energy Barriers (Ea), in kcal/mol, of the Elementary Steps on Ru3(CO)12 and Ru(CO)5 Catalysts in the Gas Phase along Mechanism A, B, C, and D

mechanism A 2 + CO + H2O → TTS1 → 3 + CO2 + H2O + CO 5 + OH− + CO2+ CO → TTS2 → 6 + OH− + CO2 + CO mechanism B 2 + CO + H2O → BTS1 → 3+CO2 + H2O 3 + CO2 + H2O → BTS2 → 2 + H2 + CO2 mechanism C 2 + CO + H2O → RTS1 → 3 + CO2 + CO + H2O 3 + CO2 + CO + H2O→RTS2→4 + CO2 + CO 5 + H2 + CO2 + CO→RST3→2 + H2 + CO2 mechanism D 2 + CO + H2O → ZTS1 → 3 + CO2 + CO + H2O 3 + CO2 + CO + H2O → ZTS2 → 4 + CO2 + CO 5 + H2 + CO2 + CO → ZST3 → 6 + H2 + CO2 6 + H2 + CO2 → ZTS4 → 2 + H2 + CO2

Ru3(CO)12

Ru(CO)5

ΔGT1 EaT1 ΔGT2 EaT2

−9.97 35.67 −13.81 3.75

−3.41 22.48 10.44 10.87

ΔGB1 EaB1 ΔGB2 EaB2

−15.33 35.49 −5.21 36.81

−11.02 23.31 −9.53 38.51

ΔGR1 EaR1 ΔGR2 EaR2 ΔGR3 EaR3

−9.97 35.67 5.94 21.84 −16.48 9.33

−3.41 22.48 12.47 33.34 −32.80 12.88

ΔGZ1 EaZ1 ΔGZ2 EaZ2 ΔGZ3 EaZ3 ΔGZ4 EaZ4

−9.97 35.67 5.94 21.84 −12.84 6.72 0.39 7.61

−3.41 22.48 12.47 33.34 −31.01 7.46 −1.47 10.49 F


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Ru3(CO)11CHO−. The step 1−3 in mechanism C is same as in mechanism A; 2 transfers 3 by releasing a molecular CO2. Like the COOH− group in the 2-Ru3(CO)11COOH− is same as that discussed above in mechanism A, the H− in 3-Ru3(CO)11H− lies in the axial plane because the coordination of H− to Ru3(CO)12 and the M-CO bonds in 3 become clearly shorter than those in the 1; however, the axial MCO bonds are longer than the equatorial ones. For mechanism B, the pathway proceeds with the addition of CO by 2 to form the intermediate 3-Ru3(CO)11CHO−. The computed energy barrier of the BTS2/3 is 34.83 lower than the RTS2/3(35.67 kcal/mol); it is kinetically feasible with a relatively low activation energy. In the next step of mechanism C, 3-Ru3(CO)11H− can react with H2O through RTS2 that allows the conversion without OH− desorption and calculates the transition state that links the 3 and H 2 O to produce an crucial intermediate 4Ru3(CO)10H2COOH−, which is different from those of mechanism A and B. The HH bond length is computed to be 2.180 Å in the 4, which is much longer than that in the free H2 molecule (0.750 Å) (see Figure S1). The computed energy barriers of RTS3/4 is 21.48 kcal/mol lower than TTS2/3 with respect to that step (Ru3(CO)11H− + H2O) in mechanism A. The 4 has the highest energy over all intermediate structure for the entire process but 4 transforms to the intermediate 5Ru3(CO)10COOH− through desorption of H2. As shown in Figure 1, the computed energy of 4 and 5 is nearly approximate (−89.65 and −89.78 kcal/mol, respectively); in other words, the reaction energy barrier of step 4 → 5 is negligible. From the process we can get that mechanism C is reasonable compared to the energy of 4 and 5 in mechanism A (−102.62 and 15.03 kcal/mol, respectively); mechanism C is an alternative to mechanism A in which OH− desorption is not required. The final step, 5, reacts with CO to form 2; the conversion of this process with an energy barrier (RTS5/2) of 9.33 kcal/mol occurs rather easily. That is, mechanism C is more reactive than mechanism A, and mechanism C is appropriate in the WGS reaction for the metal carbonyl complexes as well. Importantly, mechanism C not only includes the important intermediate Ru3(CO)11H− but also excludes the energy-demanding OH− desorption. The reaction step 1−3 of the mechanism A from Ru(CO)4COOH− to the species 3 -Ru(CO)4H− via TTS2/3 (RTS2/3) is identical with the mechanism C. From Figure 1 and Table 3, the process in the Ru(CO)5 has been found to have a higher energy barrier than that in Ru3(CO)12 by about 11.86 kcal/mol. The cause may be that trinuclear systemRu3(CO)12 has great space advantages in the process of forming transition states structures. H2O reacts with CO ligand of other Ru(CO)4 unit to form COOH group, and the presence of two ruthenium center in TS3/4 for Ru3(CO)12 makes a sixmembered ring, which is less sterically crowed and therefore more favorable than the more highly strained five-memebered ring formed in the case of Ru(CO)5. Proceeding to the next reaction process, from 5-Ru(CO)4 to 2-Ru(CO)4COOH− involves the approach of a new CO to intermediate 2; a 9.33 kcal/mol energy barrier for Ru3(CO)12 is easier to overcome for the catalytic cycle than 12.88 kcal/mol for Ru(CO)5. Mechanism D. Mechanism D following Zhang31 is shown in Scheme 4. Zhang et al. revised the reaction pathways for WGS reaction. From Scheme 4 and Figure 1, we can see that the first half of the detailed reaction cycle, the steps 1−4, are consistent with mechanism C. The next step is different, namely, two H atoms dissociate from 4-Ru3(CO)10H2COOH− to form the

us take a look at the transition state. The energy barrier of step 2 → 3 (TTS2/3) for the Ru3(CO)12 is 35.67 kca/mol, higher than the Ru(CO)5 (22.48 kca/mol); it should be able to proceed because the exothermic process of 1 → 2 is very strong and the energy is more than 80 kcal/mol (330 kJ/mol). The CO2 producing reaction barrier of the iron carboxyl complexes come from the study by Kumar Vanka. Our results show great agreement with the literature. The computed barrier of TTS5/6 in Ru3(CO)12 (3.75 kcal/mol) and in Ru(CO)5 (10.87 kcal/ mol) suggests that the step is easier to process. In conclusion, the high barrier of the step 3 → 4 + OH− makes mechanism A following Torrent not suitable for WGS reaction. To improve the above-mentioned mechanism, a new mechanism is proposed. Mechanism B. Mechanism B following Barrows29 is shown in Scheme 2, and the corresponding reaction profile is shown in Figure 1. In the course of the investigation of the Torrent mechanism, Barrows discovered the new one. Mechanism B carries out through three steps in the entire catalytic cycle. The corresponding ΔG and Ea are listed in Table 3. Similar to the case in mechanism A, 1-Ru3(CO)12 also catalyzes by OH− to form the intermediate 2-Ru3(CO)11COOH−. Then, the pathway proceeds with the addition of CO by species 2 to form the intermediate 3-Ru3(CO)11CHO−. For the transition state (BTS2/3) structure in this step of mechanism B, the incoming CO approaches from the leaving CO2 ligand at a 56.8° angle. The BTS2/3 lies 34.83 kcal/mol slightly lower than the value of TTS2/3 (35.67 kcal/mol in Figure 1). The BTS2/3 involves a less strained five-membered ring structure in comparison to the four-membered ring structure in TTS2/3 (Figure 1); this leads to the reaction between 2 and 3 being easy. The 3 is predicted to complex with H2O to produce 4 in mechanism A. However, from a thermochemical point of view reaction 3 → 4 + OH− in mechanism A is favorable. The same account was put forward by Chen and co-workers46 to illustrate the unfavorable energies for the reaction of CO and OH* to form HCOO*. The break of the OH bond needs larger energy, which is in good agreement with the experimental observations of Rodriguez.17 The final step involves dissociation of H2O and release of H2 to return 2, indicating the accomplishment of the reaction. As a result of the strong O−H bond energy, the computed energy barrier is 36.81 kcal/mol for BTS3/2. Also, this step 3 → 2 is the rate-determining step for WGS reaction. For Ru(CO)5, the reaction mechanism and those intermediates in mechanism B are same as the Ru3(CO)12. The corresponding energy barrier of the step 2 → 3 for this mechanism is 23.31 kcal/mol, lower than those of the Ru3(CO)12, however, this mechanism avoids the experimentally observed intermediate Ru3(CO)11H− in the catalytic cycle. Despite this, theory predicts that Scheme 2 is a feasible pathway in energy and that the step of no OH− desorption is required in the mechanism. To revise the above-mentioned disadvantage, a new mechanism is proposed again. Mechanism C. Mechanism C following Rozanska and Vuilleumier30 is shown in Scheme 3. The corresponding reaction profile is shown in Figure 1. The first reaction between OH− and the 1-Ru3(CO)12 is highly exothermic and forms the 2-Ru3(CO)11COOH−, which is in great agreement of the mechanism A and B. Then mechanism A and B are along two different pathways: For the mechanism A, 2 releases CO2 and forms the hydride intermediate 3-Ru3(CO)11H−. For the mechanism B, the oxidation of CO by 2 forms the 3G


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Figure 2. Combined free energy profile along mechanisms A−D for the WGS reaction on Ru3(CO)12 complexes in gas phase.

intermediate 5-Ru3(CO)10COOH−. The 5 have different structure with respect to the COOH group in the mechanism C; corresponding structures of 4 and 5 are depicted in Figure 1. In the COOH group of the RTS5/2, the combination of H atom and CO is far away from the Ru atom. Similarly, in the 5 this H atom is anti to CO and adjacent to the ruthenium carbonyl species. The process 5 → 2 has to undergo two structural changes through H atom migration or rotation. Therefore, it is found that there should be other intermediates and pathways. We put forward new structures 5, 6, and transition states ZTS5/6 and ZTS6/2 (see Figure 1 or Figure S1). Scheme 4 shows that 4 can directly turn to new intermediate 5 through desorption of H2, followed by 6 is generated by addition of CO with the energy barrier of 6.72 kcal/mol (ZTS5/6). Finally, 6 evolves return to 2 by H rotation of the COOH group, indicating accomplishment of the reaction cycle. The last step is endothermic by 0.39 kcal/mol and the corresponding the energy barrier is 7.61 kcal/mol; we find less barrier than RTS5/2 in mechanism C by about 1.72 kcal/mol. Therefore, it contributes to WGS reaction to be effective than those of mechanism C. In addition, ZTS6/2 is obviously similar to that of both 6 and 2 in geometry, and the bond length of the CO single bond in the COOH group (1.398 Å) is appropriately just between those of 2(1.397 Å) and 6(1.400 Å) (see Figure S1). So the step occurs rather easily. Similar to results for the discussion above, for Ru(CO)5, the RTS5/2 lies 12.88 kcal/mol higher than ZTS6/2(10.49 kcal/ mol). In addition, the values of Ru3(CO)12 are found to be 9.33 kcal/mol (RTS5/2), 7.61 kcal/mol (ZTS6/2) lower than that of the Ru(CO)5, manifesting Ru3(CO)12 should be favorable to proceed along mechanism D. As mechanisms A−D indicate, the OH− desorption from Ru3(CO)12 proceed through the energy-demanding pathway, so mechanism A is inappropriate in WGS reaction. However, the step of no OH− desorption is required in mechanism B; the reaction mechanism is still not perfect because the important intermadiate Ru3(CO)11H− observed in experiment is not included in the WGS reaction process. The mechanism C overcomes the above-mentioned pros and cons. However, mechanism D revises mechanism C to get more detailed results; ultimately we can see that the transition state ZTS3

connects new intermediate 5 and 6 directly and the energy barrier is lower than RTS5/2 in mechanism C. We suggest that mechanism D is thus the favorable pathway for improved WGS reaction. 3.3. Comparison of the Efficiency Ru3(CO)12 and Ru(CO)5 Following Different Mechanism Using ESM. The Ru3(CO)12 and Ru(CO)5 catalysts have similar pathways following four mechanisms A−D. To further understand efficiency of the two catalysts, the ESM is used, which has proven to be a fundamental and available tool. According to the equations (ref 40) one must calculate the XTOF to judge TOFdetermining transition state (TDTS) and TOF-determining intermediate (TDI). Also, the TOF is computed by TDTS and TDI in the equation. The combined energy profile along four mechanisms is shown in Figure 2 and Figure S3, respectively. We receive the following results: 1. As shown in Figure. 2 and Figure S3, we can see that the four mechanisms have shared intermediates and transition states. Also, all of the mechanistic pathways start from a same species, Ru3(CO)12 and Ru(CO)5,respectively. According to the concept of ESM, if one accessible state has very low energy, it will be the TDI for the four mechanisms. It is similar when it comes to estimating the TDTS, which both are the key states affecting catalytic cycle. Thus, it is necessary to compare the energy profiles for the four mechanisms to find the TDI and TDTS in this case. Figure 2 indicates that those shown in pink are the lowest lying intermediate and transition state for our calculations, which would be the TDI and TDTS for WGS reaction when using the Ru3(CO)12 catalyst. We have found that both TDI and TDTS correspond to the mechanism B. This is also same when it comes to estimating the Ru(CO)5 catalyst. 2. The ESM shows that in the correlations between δE and the TOF values, the lower the computed δE is, the larger the TOF values is. From the TOF calculation (see Table 4), the Ru3(CO)12 shows lower δE than the Ru(CO)5 (33.15 and 38.51 kcal/mol, respectively). The Ru3(CO)12 has the highest TOF value, 3.11 × 10−12 s−1 (for Ru(CO)5, 3.66 × 10−16 s−1), which thus indicates that catalytic activity of Ru3(CO)12 is higher than that of Ru(CO)5 for WGS reaction, verifying that it H


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The Journal of Physical Chemistry A Table 4. TOF Values (in s−1), Key State Energies (in kcal/ mol) at 298.15 K, and Energetic Span Values (δE) Obtained for the Trinuclear Ru3(CO)12 and Mononuclear Ru(CO)5, Fe(CO)5 Catalysts in the Gas Phase along the Four Mechanisms complexes

TOF

intermediate/ ΔG

transition state/ ΔG

δE

Ru3(CO)12 Ru(CO)5 Fe(CO)5

3.11 × 10−12 3.66 × 10−16 4.42 × 10−19

−100.95 −88.54 −87.27

−50.13 −50.03 −44.78

33.15 38.51 42.49

Table 5. Reaction Energies (ΔG) and Activation Energy Barriers (Ea) in kcal/mol of the Elementary Steps on Ru3(CO)12 Catalysts along the Four Mechanisms

mechanism A 2 + CO + H2O → TTS1 → 3 + CO2 + H2O + CO 5 + OH− + CO2 + CO → TTS2 → 6 + OH− + CO2 + CO mechanism B 2 + CO + H2O → BTS1 → 3 + CO2 + H2O

is a promising candidate for an improved WGS reaction catalyst. To verify that the above results are reasonable, we compared it with the species-Fe(CO)5 reported in literature using the same method in this work. The Fe(CO)5 has attracted increasing interest in the homogeneously catalyzed WGS reaction, which led to a lot of theoretical studies. The Fe(CO)5 along four mechanisms have been further compared with the help of TOF calculations. Table 4 shows the computed TOF values and δE. We can get the following results: (1) For the complexes Fe(CO)5, the TDTS and TDI for the four mechanisms for the WGS reaction are consistent with the Ru(CO)5. (2) The δE of the mechanism B is 42.49 kcal/mol, corresponding to higher TOF values (4.42 × 10−19s−1), which is in good agreement of the reported data. But our proposed Ru3(CO)12 (3.11 × 10−12s−1) has a maximum value of TOF values. Thus, Ru3(CO)12 is an high-efficiency catalyst for WGS reaction as discussed earlier. In a word, trinulcear ruthenium carboxyl complexes maybe improve the yield and show active catalytic property than that of mononuclear catalyst in homogeneous WGS reaction. 3.4. Solvation Effect in the WGS Reaction. In addition to the gas phase calculations based on the discussed results, we study also the energy profiles of Ru3(CO)12 catalyst for the four mechanisms after containing solvent effect because such differences of reaction energy may change the energetics of the four mechanisms. The computed reaction energy and activation barrier are given in Table 5. To confirm the catalytic efficiency of the Ru3(CO)12 catalyst, the ESM has also been employed with the results in the solvent phase calculations. As shown in Figure 3, shown in orange are the lowest lying intermediate and transition states for the four mechanisms, which would be the TDI and TDTS for WGS reaction for the Ru3(CO)12 catalyst in solvent phase. Importantly, both the TDTS and TDI correspond to the mechanism B, which are in agreement with those of gas phase. The corresponded TOF values are given in Table 6. Comparison of TOF values for the gas phase and the solvent phase, the TOF value of Ru3(CO)12 catalyst in solvent phase (1.94 × 10−8 s−1) is higher than that in gas phase (3.11 × 10−12 s−1), So the catalytic efficiency of Ru3(CO)12 is high in the solvent phase, and the efficiency increases with the addition of methanol, which is in good agreement with the experimental observations. The reason can be that the methanol plays a role not only as solvent but also as reactant for the WGS reaction, providing the OH− ions that are crucial for the formation of the intermediate Ru3(CO)11COOH−. 3.5. Comparison of d-Band Center Position with Catalytic Activity. We further understand catalytic activity of ruthenium carbonyl complexes by using the Hammer−

3 + CO2 + H2O → BTS2 → 2 + H2 + CO2 mechanism C 2 + CO + H2O→RTS1→3 + CO2 + CO + H2O 3 + CO2 + CO + H2O → RTS2 → 4 + CO2 + CO 5 + H2 + CO2 + CO → RST3 → 2 + H2 + CO2 mechanism D 2 + CO + H2O → ZTS1 → 3 + CO2 + CO + H2O 3 + CO2 + CO + H2O → ZTS2 → 4 + CO2 + CO 5 + H2 + CO2 + CO → ZST3 → 6 + H2 + CO2 6 + H2 + CO2 → ZTS4 → 2 + H2 + CO2

gas phase

solvent phase

ΔGT1 EaT1 ΔGT2 EaT2

−9.97 35.67 −13.81 3.75

−11.06 36.67 −15.20 4.07

ΔGB1 EaB1 ΔGB2 EaB2

−15.33 35.49 −5.21 36.81

−13.74 28.10 −3.01 42.35

ΔGR1 EaR1 ΔGR2 EaR2 ΔGR3 EaR3

−9.97 35.67 5.94 21.84 −16.48 9.33

−11.06 36.67 10.89 27.18 −17.70 13.01

ΔGZ1 EaZ1 ΔGZ2 EaZ2 ΔGZ3 EaZ3 ΔGZ4 EaZ4

−9.97 35.67 5.94 21.84 −12.84 6.72 0.39 7.61

−11.06 36.67 10.89 27.18 −17.24 7.52 3.89 12.17

Nørskov model.47,48 The model has systematically studied which parameters influence the reactivity of the metal surface. It suggests that the molecular adsorption energy is dependent primarily on the occupancy of the bonding and antibonding states formed by hybridization of the wave functions of the adsorbate and metal’s d-state electrons. If the antibonding state lies below the Fermi level (EF), the interaction between the adsorbate and metal surface becomes repulsive, thereby giving rise to a weak chemisorption. This situation can be well explained by the position of the d-band center of a metal. The position of d-band center (εd) can be described by the energy of the d-band center relative to the Femi level (EF). The position of d-band center (εd) moving toward Femi level (EF) affects the activity of the catalyst and leads to a stronger chemisorption. The relevance is applicable descriptor to explain catalytic reactivity of WGS reaction. Figure 4 and Table S2 present a summary of our analysis, corresponding key intermediates 1, 2, 3 + H2O, 1, ′2′, 3′ + H2O respectively. We get the following conclusions: 1. The d-band center value of Ru3(CO)12 is closer to EF than that of Ru(CO)5; the result indicates that the catalytic activity of Ru3(CO)12 is better. 2. From Figure 4, we can see that intermediate 3 adsorbed a H2O molecule and the d-center value is −3.00 eV. At the same time, the corresponding d-center value, 3′ with a H2O molecule, is −3.03 eV. Compared to the intermediate 3 and 3′, the d-band center value of the 3 is closer to Fermi energy; H2O chemisorbs on the 3 surface with the appropriately strong effect, predicting reaction of 3 with H2O proceed easily, implying that water dissociation is easier to occur. It in turn I


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Figure 3. Combined free energy profile along mechanisms A−D for the WGS reaction on Ru3(CO)12 complexes in solvent phase.

From the discussion above, the position of the d-band center is well correlated with the small molecular adsorption energy. The closer of the d-band center is from the Fermi level, the greater is the adsorption energy, and the smaller molecular dissociation barrier, which is more favorable for catalytic reaction.

Table 6. TOF Values (in s−1), Key State Energies (in kcal/ mol) at 298.15 K and Energetic Span Values (δE, Obtained for the Trinuclear Ru3(CO)12 Catalysts in the Gas and Solvent Phase along the Four Mechanisms TOF gas phase solvent phase

−12

3.11 × 10 1.94 × 10−8

intermediate/ ΔG

transition state/ ΔG

δE

−100.95 −47.33

−50.13 −5.49

33.15 27.97

4. CONCLUSION In summary, trinuclear carbonyl complexes-Ru3(CO)12 and mononuclear carbonyl complexes-Ru(CO)5 that catalyze for WGS reaction mechanism have been studied by density functional theory calculations with the aim to shed light on

indicates that the metal−metal cooperativity plays a key role in the catalytic cycle. This is also analogical when it comes to intermediate 2 and 2′.

Figure 4. The d-band density of states for Ru3(CO)12 (left panel) and Ru(CO)5 (right panel). The dashed line represents the Fermi level. The vertical red line corresponds to the position of the d-band centers. J


The Journal of Physical Chemistry A the reaction mechanism and catalytic activity. Several results drawn from these calculations include the following: 1. The study of ruthenium carbonyl complexes (Ru3(CO)12 and Ru(CO)5) catalyzed for WGS reaction where structure is D3h symmetry, and four mechanisms also are discussed in detail. Also, the Gibbs free energy profiles of the four mechanisms are constructed. We infer that the mechanism D plays an important role in WGS reaction. In mechanism A, high barrier of the OH− desorption makes mechanism A unlikely to proceed. No the step of OH− desorption is required in mechanism B, C, and D. However, because mechanism B avoided the important intermediate Ru3(CO)11H− observed experimentally, it is not suitable for WGS reaction. Both mechanism C and D overcome the above-mentioned condition, but the last proposalmechanism D is favorable in energy. 2. From the energy profiles (Figure 1), we know that the key step of H2O dissociation (ZTS3/4) for Ru3(CO)12 in mechanism D is 21.48 kcal/mol, lower than the corresponding energy barriers of Ru(CO)5 and Fe(CO)5 (33.34 and 37.72 kcal/mol, respectively), implying that trinulcear carbonyl complexes-Ru3(CO)12 is more effective than mononuclear complexes as catalyst for WGS reaction. 3. Both Ru3(CO)12 and Ru(CO)5 catalysts are further compared with the efficiency along different mechanism by using ESM instead of the rate-determining step. The latter has been criticized as not necessary, misleading, and outmoded. The result shows that the TOF value of Ru3(CO)12 in mechanism B shows (3.11 × 10−12 s−1, for Ru3(CO)12; 3.66 × 10−1 6s−1, for Ru(CO)5) higher catalytic performance, which thus indicates catalytic activity of Ru3(CO)12 is higher than that of Ru(CO)5 for WGS reaction. The value of TOF is similar to those of Ru(CO)5 (3.66 × 10−16 s−1, Table 3) and Fe(CO)5 (4.42 × 10−19 s−1), which have been used in almost all experimental and computational studies and proved WGS reaction catalysts with high performance. Also, we find that the catalytic efficiency of Ru3(CO)12 is much more favored in solvent phase by comparing the TOF values. Compared to two catalysts, a crucial effect has been observed on the basis of the computed density of states, that is, the position of d-band center (εd) moving toward EF, which is more favorable for catalytic reaction.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (Grant 20603021), the Natural Science Foundation of Shanxi (Grant 2013011009-6), the High School 131 Leading Talent Project of Shanxi, Graduate Project for Education and Innovation of Shanxi Province, Undergraduate Training Programs for Innovation and Entrepreneurship of Shanxi Province (Grants 105088, 2015537, and WL2015CXCY-SJ-01) and Shanxi Normal University (SD2015CXXM-80, WL2015CXCY-YJ-18), and Teaching Reform Project of Shanxi Normal University (WL2015JGXM-YJ-13).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b00301. Mechanistic proposals A−D for Ru(CO)5 catalyzed WGS reaction (Schemes 1−4), the optimized geometries of Ru3(CO)12 and Ru(CO)5 by following Scheme 4 (Figure S1), the potential energy surfaces for WGS reaction promoted by Ru(CO)5 through four mechanisms (Figure S2), combined free energy profile along mechanisms A−D for WGS reaction on Ru(CO)5 (Figure S3), the position of d-band centers (εd) (Table S1), optimized Cartesian coordinates of all structures in standard xyz format, full citation ref 32. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-357-2398380. E-mail: [email protected]. Notes

The authors declare no competing financial interest. K


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