Computational Insights into the Activity of Transition Metals for

Mar 1, 2016 - Computational Design of New Heterofullerene-Based Biomimetic α-Carbonic Anhydrase Analogues. Manju Verma , Parag A. Deshpande. ChemPhys...
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Computational Insights into the Activity of Transition Metals for Biomimetic CO2 Hydration Manju Verma, K. B. Sravan Kumar, and Parag A. Deshpande* Quantum and Molecular Engineering Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India S Supporting Information *

ABSTRACT: Density functional theory (DFT) calculations were carried out on five transition metals (Co, Ni, Pd, Rh, Ru) to test their activities toward the biomimetic carbon dioxide hydration reaction. Periodic plane-wave calculations demonstrated the formation of surface species in accordance with the mechanism of the reaction known for α-carbonic anhydrase action. To determine different activation barriers for the different elementary steps involved in the reaction, DFT calculations using a cluster model of transition metals with Gaussian-type orbitals were carried out. The periodic and cluster calculations were found to correspond to a mechanism of the reaction constitituting seven steps, namely, surface adsorption of H2O, deprotonation and surface OH formation, adsorption of CO2, OH attack on adsorbed CO2, proton transfer, H2O attack on surface HCO3− complex, and HCO3− displacement by H2O. The behaviors of the metals were found to be different in a vacuum and in the solvated state, with Co being the best potential candidate for the biomimetic CO2 hydration reaction in a vacuum and Ru being the best candidate for the reaction in solution.

1. INTRODUCTION The increasing rate of emissions of greenhouse gases generated by human activities has been extensively discussed in the recent past owing to environmental concerns, such as global warming.1−5 Solomon et al.1 predicted that the effects of irreversible dry-season rainfall reductions and global average sea-level rise because of increased CO2 concentrations in the environment would continue for at least 1000 years even after CO2 emissions are stopped. These adverse effects have made it extremely important to reduce CO2 emissions and to develop systems for its capture and storage. Several investigators have attempted to develop systems for the capture of CO2,6−11 and of the various approaches available, the conversion of CO2 to carbonic acid by hydration (CO2 + H2O ⇌ HCO3− + H+) and subsequent conversion to environmentally friendly calcium carbonate has been shown to be an environmentally and geologically safe route.9−11 The CO2 hydration reaction (CHR) is one of the slowest steps of this mineralization process,12 and therefore, it is desirable to design efficient catalytic systems for the CHR. The enzyme carbonic anhydrase, which is abundantly available in nature, has been reported to exhibit excellent catalytic activity for the CHR in living organisms.6−11,13 Attempts have been made to use carbonic anhydrase directly in CHR-based biomimetic CO2 capture systems for industrial applications.8,14,15 Carbonic anhydrase as a biocatalyst exhibits a high efficiency and selectivity toward the CHR.13 However, despite its high efficiency and selectivity, carbonic anhydrase has several limitations for direct industrial applications. Because carbonic anhydrase is soluble in water, reuse of the enzyme is © 2016 American Chemical Society

difficult unless it is immobilized on a heterogeneous surface, compromising its native activity. Immobilization using chitosan and alginate, metal-based nanoparticles, mesoporous silica, polyurethane foam, and thin liquid membranes has been explored to test the feasibility of its large-scale usage.7,8,14 The instability of carbonic anhydrase makes it difficult to store over a long period.7 These disadvantages of carbonic anhydrase, including short operational time, cost of extraction after use, susceptibility to inhibition by small anions,14 and specific operating conditions of pH values of 7−10 and temperatures of 4−30 °C,8,13 make the direct industrial application of carbonic anhydrase impossible. Because of these limitations, recent efforts have been directed toward the development of chemical catalysts to meet the conditions present in industrial reactors. The catalysts that have been reported to show activity toward the biomimetic CHR include organic and organometallic catalysts.15−17 Recent studies have reported halogens and their compounds and borate ion to show catalytic activity for the CHR.18−21 The conventional organometallic bioanalogues suffer from the disadvantage of ligand instability.22 The recent trend, therefore, is the exploration of small molecules capable of mimicking the CHR mechanism.23 Despite good catalytic activities and stabilities, the aforementioned bioanalogues still suffer from homogeneity of the reaction system and the difficulty associated with recovering the remaining catalyst. Therefore, Received: January 15, 2016 Revised: February 29, 2016 Published: March 1, 2016 5577

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basis sets. Ultrasoft pseudopotentials were used for all atoms. A kinetic energy cutoff of 35 Ry for wave functions and 420 Ry for charge densities was used. A k-mesh of 5 × 5 × 1 was used for sampling the Brillouin zone. The relaxation of the top layer and the reactant species was carried out until the residual forces on all atoms were less than 1 × 10−3 au and the change in energy in the subsequent optimization was less than 1 × 10−4 au. A tight convergence criterion of 1 × 10−6 au was imposed for self-consistent energy calculations. All energies were scaled with reference to the energies of the noninteracting metal slab, CO2, and H2O. Mechanistic insights were gained by employing DFT with Gaussian-type orbitals. The Gaussian 09 simulation package30 was used to carry out DFT calculations of seven-membered Co, Ni, Pd, Rh, and Ru clusters. The Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional was used for all calculations.31 The valence electrons of metal atoms were described with the LANL2DZ basis set with LANL2 as the effective core potential. H, C, and O atoms were described using 6-31+G(d,p) basis sets. Complete energy minimization was carried out by relaxing all of the atoms. Vibrational frequency analysis was carried out to ensure correct minima for intermediates and transition states with the absence of any imaginary frequencies and one imaginary frequency for intermediates and transition states. Calculations were also carried out with dispersion, cavitation, and the repulsioncorrected polarizable continuum model with water as the solvent.32 All energies were scaled with reference to those of the noninteracting metal cluster, CO2, and H2O. The visualization and representation of the structures was done using VMD.33

a truly heterogeneous bioanalogue is sought. This was achieved only recently by Bhaduri and Šiller, who studied the CHR catalyzed by nickel nanoparticles at room temperature and atmospheric pressure and described the system as active and biomimetic. The study by Bhaduri and Šiller9 caught the attention of investigators worldwide, as can be appreciated by the comments and rebuttals of their work.24,25 Although their study provided a phenomenal leap forward in the development of truly heterogeneous catalysts, disputes regarding the kinetics and mechanism of the reaction remain, primarily because of the complexity of the experimental procedures inherent in dealing with the system under concern. The aim of this study was two-fold. First, we provide conclusive mechanistic insights into the CHR action of Ni nanoparticles using density functional theory (DFT) calculations. Ours is the first study to provide details on each elementary step of the mechanism and the associated energetics involved during the CHR, which is helpful in mechanistic and kinetic discrimination. The second purpose of this study was to explore alternatives to Ni for the development of heterogeneous catalysts. The activity of Ni toward the CHR encouraged us to study the corresponding activities of other transition metals, namely, Co, Pd, Rh, and Ru. Among the various transition metals studied here, Ni is the most abundant,26 and when augmented with its magnetic properties, it becomes a potential candidate for the exploration of CHR activity. This is possibly the reason behind Bhaduri and Šiller selection of Ni as a CHR catalyst. Although relatively less prevalent than Ni, Co is also abundantly available27 and is a conventionally studied catalyst candidate. The other precious metals studied here, namely, Pd, Rh, and Ru, are not very abundant and are therefore costly. However, their exceptional catalytic activities for a large number of gas-phase reactions make an attempt to assess their CHR catalysis potentials worthwhile. The potential energy surfaces generated for the CHR demonstrated their biomimetic activities for catalyzing the reaction. Therefore, this study not only supports and provides vital mechanistic insights into Bhaduri and Šiller’s proposition of biomimesis in a Nibased heterogeneous system but also suggests a direction for the exploration of alternative transition-metal systems that could be employed for heterogeneous CHR systems.

3. RESULTS AND DISCUSSION 3.1. Identification of Surface Species. Bhaduri and Šiller speculated that the mechanism of the CHR over Ni nanoparticles is biomimetic, consisting of water dissociation, CO2 attack, and HCO3− formation steps.9,10 This mechanism, on a detailed analysis, seems to be a very simplistic version of the α-carbonic anhydrase action. In this study, we test a detailed mechanism for the CHR over a Ni(111) surface with the elementary steps borrowed from those on α-carbonic anhydrase. The mechanism is shown in Figure 1. The catalytic cycle involves seven steps leading to a reaction pathway consisting of seven surface species. The first step is the formation of a H2O−catalyst complex (intermediate 1) by adsorption of H2O on the surface of the catalyst. The proton released in this step is retained on the catalyst surface and might be released to the solvent depending on the operating pH. The second step involves deprotonation of this complex, giving a surface OH group with the metal (intermediate 2) that acts as the center for attack on CO2. In the third step, adsorption of CO2 takes place over the OH−catalyst complex through the O center of the complex and the C atom of CO2. This results in the formation of an M−OH−CO2 complex. In this complex, CO2 is complexed linearly with the catalyst (intermediate 3). The fourth step is the attack of the catalyst on CO2, resulting in the bending of CO2 to form an M−OH−CO2 complex in which CO2 is complexed in an angular configuration with the catalyst (intermediate 4). The fifth step involves the transfer of a proton from oxygen originally coming from OH to the nearby oxygen atom of CO2 in the M−OH−CO2 complex, giving intermediate 5. The product HCO3− can now be observed to be complexed with the catalyst. The sixth step is

2. COMPUTATIONAL DETAILS To establish the surface species formed during the CHR over a transition-metal surface, periodic DFT calculations were implemented using the Quantum Espresso package.28 For the transition metals Pd and Rh, the supercell consisted of three atomic layers of the (111) plane with a 3 × 3 surface supercell, whereas for the transition metals Ni, Co, and Ru, the supercell consisted of a four-layered 3 × 3 slab of the (111) plane. The choice of three versus four layers in the supercell was dictated by the symmetry of the crystal system. The supercells were developed from crystallographic information files. A vacuum of 10 Å was imposed to prevent interactions between the adsorbate and the periodic image of the slab. For all metals, the first layer of the metal and the reacting species were relaxed, and the remaining layers were fixed at their crystallographic coordinates. Periodic boundary conditions in all three directions were employed. Spin-polarized DFT calculations were performed using the PWscf (plane-wave self-consistent field) package of Quantum Espresso, with the Perdew−Burke− Ernzerhof exchange-correlation functional29 and plane-wave 5578

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Figure 1. Proposed mechanism of biomimetic CO2 hydration over transition metals.

the attack of H2O on the product complex, giving intermediate 6. Displacement of HCO3− ion by H2O takes place, resulting in intermediate 7. The catalytic cycle is completed by the desorption of product HCO3− ion and a proton, restoring intermediate 1. The presence of the intermediate surface species mentioned in the proposed mechanism was tested over the (111) surfaces of Co, Ni, Pd, Rh, and Ru. Representative DFT-optimized structures are shown in Figure 2. The energy landscapes for the reaction mechanism are shown in Figure 3. A detailed summary of the elementary reaction energies over the 111 planes of the investigated transtion metals is provided in Table 1. The activation energies and reaction energies over clusters of transtion metal atoms are summarized in Table 2. We provide a detailed description of all of the elementary steps of the proposed reaction mechanism in the following subsections. 3.1.1. Step 1: Surface Adsorption of H2O. Among the various sites available for the adsorption of H2O on the (111) plane of a metal, H2O adsorption was found to be favored at the atop sites of all of the metals (Figure 2a). Adsorption was energetically favored over all of the metals with adsorption energies of −10.88, −8.96, −8.04, −9.16, and −11.36 kcal/mol on Co, Ni, Pd, Rh, and Ru, respectively. Therefore, the adsorption complex was the most stable on Ru and the least stable on Pd, although the difference in adsorption energies was marginal. The H−O−H bond angle in adsorbed H2O was observed to increase from 104.5° to 105.7−106° in all H2O− metal complexes. The highest H−O−H bond angle was observed for the Ru−H 2O complex, whereas the lowest H− O−H bond angle was observed for the Rh−H 2O complex. Phatak et al.34 reported the adsorption energies of water on Ni(111) and Pd(111) to be −6.68 and −6.91 kcal/mol, respectively. The adsorption energies found in this study are in agreement with their observations (−8.96 and −8.04 kcal/mol, respectively). Yang and Whitten35 reported atop-site adsorption of H2O on Ni(111) with an adsorption energy of −11.53 kcal/

Figure 2. Different surface intermediates in the biomimetic mechanism obtained using peroidic plane-wave DFT calculations: (a) metal−H2O complex, (b) H2O split into proton and hydroxyl, (c) CO2 adsorbed linearly on intermediate 2, (d) bent OH−CO2 complex, (e) metal−HCO3− complex, (f) H2O attack on metal−HCO3− complex, (g) displacement of HCO3− by H2O. Color key: ochre, metal; black, carbon; red, oxygen; white, hydrogen.

Figure 3. Potential energy surface for the biomimetic reaction mechanism obtained using periodic plane-wave DFT calculations.

mol, and Stulen et al.36 reported a binding energy of −9.91 kcal/mol based on thermal desorption. Thus, the energies for H2O adsorption calculated in this work were in agreement with previous experimental and computational reports. Similarly, the binding energy of H2O on Rh(111) of −8.07 kcal/mol observed by Pozzo et al.37 is in good agreement with our calculated value of −9.16 kcal/mol. Michaelides et al.38 reported the adsorption energy of a water molecule on the Ru(0001) plane as −8.76 kcal/mol and the enthalpy change for 5579

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3.1.3. Step 3: Adsorption of CO2. Adsorption of CO2 giving intermediate 3 (Figure 2c) was found to be energetically favored over all of the metals considered. From the energy landscape in Figure 3, it can be seen that, for the 2 → 3 transition, the adsorption energies over different metals were in the range from −1.90 to −6.47 kcal/mol. The adsorption energy of CO2 was the highest for the case of Co and lowest for Ru. No direct coordination of CO2 with any of the surface metal atoms was observed. CO2 interacted with the surface through the surface OH groups. CO2 was almost linear in structure, with O−C−O bond angles very close to 180° on different metals. 3.1.4. Step 4: OH Attack on Adsorbed CO2. The transition of linear CO2 to a bent OH−CO2 complex (Figure 2d) was found to be an energy-intensive step over all of the catalysts except Pd, with intermediate 4 being less stable than intermediate 3. A large range of energy changes was observed over different metals for this step, with the largest being 8.1 kcal/mol over Co and the lowest being 0.63 kcal/mol over Ru (see Figure 3). Over Pd, the net energy release was 0.19 kcal/ mol. An interaction between the metal and an oxygen atom of CO2 was observed, along with the interaction of the carbon atom of CO2 and the oxygen atom of an OH group. The O−M (metal) distances were in the range of 2.04−2.24 Å, with the greatest O−M distance observed for Ru and lowest for Ni. The O−C distances were in the range of 1.41−1.44 Å. The O−C− O bond angles significantly decreased to nearly 130° from 180° in all of the OH−CO2 complexes. 3.1.5. Step 5: Proton Transfer. This step involved the transfer of a proton between the oxygen atom of OH and an oxygen atom of CO2, forming a metal−HCO3− complex (Figure 2e). This step was found to be exothermic with a net release of energy. As can be seen from Figure 3, the net release of energy for this step was in the range of 9.77−14.29 kcal/mol, with the highest energy release over Ru and the lowest over Pd. Interactions were observed to develop between two oxygens of newly formed HCO3− species and the atoms of the metal surface. Bidentate HCO3− species were hence formed with M− O bond distances in the range of 1.97−2.15 Å. 3.1.6. Step 6: H2O Attack on Surface HCO3− Complex. Although the surface HCO3− complex is the complex in which the desired product HCO3− can be observed to appear, its

Table 1. Reaction Energies of the Elementary Steps of the CO2 Hydration Reaction over the 111 Plane of Transition Metals (kcal/mol) elementary step

Co(111)

Ni(111)

Pd(111)

Rh(111)

Ru(111)

IM1−IM2 IM2−IM3 IM3−IM4 IM4−IM5 IM5−IM6 IM6−IM7

−11.75 −6.47 8.10 −11.00 −12.81 −3.90

−11.12 −3.01 6.50 −11.49 −7.55 −4.73

2.03 −3.58 −0.23 −9.77 −6.35 −7.83

0.97 −2.19 2.24 −11.82 −6.61 −6.87

−6.13 −1.97 0.65 −14.29 −6.62 −12.41

H2O dissociation as −6.22 kcal/mol, and the calculated values for these steps on Ru(111) were found to be similar to the reported energies. 3.1.2. Step 2: Deprotonation and Surface OH Formation. The deprotonation of adsorbed H2O resulted in intermediate 2 (Figure 2b), in which an active hydroxyl group (OH) is present on the surface of the metal. Migration of a OH group and a proton was observed from the atop site of H2O to adjacent hexagonal close-packed (hcp) sites to be occupied by OH and the proton. From the energy landscapes shown in Figure 3, the net energy release during this step was found to be the highest over Co, at 11.75 kcal/mol. The net energy requirement was highest over Pd, requiring an energy of 2.07 kcal/mol. Hammer and Nørskov39 studied the dissociation of water over transitionmetal surfaces in detail and correlated the binding energies with the d-band center of the metals. This results in differences in the behavior of the endergonic and exergonic natures of H− OH species formation over the metals, as was also observed in this study and in those by Wang et al.40 and Michaelides et al.38 The water dissociation energies over Co, Ni, Pd, Rh, and Ru obtained in this study were −11.53, −11.06, 2.02, 0.96, and −5.99 kcal/mol, respectively, and these values are in agreement with the water dissociation energies on these transition metals reported by Michel et al.41 The reaction energy of H2O dissociation over Pd(111) was obtained as 2.02 kcal/mol, which is also similar to 1.38 kcal/mol reaction energy reported by Phatak et al.34 Wang et al.40 reported the dissociation energy of H2O as −12.91 kcal/mol over Ni(111) and −8.07 kcal/mol over Ru(0001), which are also in good agreement with our calculated results.

Table 2. Activation Energies and Reaction Energies of the Elementary Steps of the CO2 Hydration Reaction over TransitionMetal Clusters in a Vacuum and in the Solvated State (kcal/mol) Co7 elementary step

ΔGact

Ni7 ΔGr

ΔGact

Pd7 ΔGr

IM1−IM2 IM2−IM3 IM3−IM4 IM4−IM5 IM5−IM6 IM6−IM7

14.78 − 6.55 −27.07 − 3.24

−0.43 3.80 −7.13 17.20 −29.93 −0.98

17.60 − 10.65 −33.11 − 11.96

−18.85 4.58 2.49 22.38 −35.45 8.30

IM1−IM2 IM2−IM3 IM3−IM4 IM4−IM5 IM5−IM6 IM6−IM7

2.34 − 20.29 34.44 − 30.60

−52.44 13.14 −11.63 −5.55 3.76 7.71

4.48 − 6.05 21.68 − 7.38

−33.76 6.50 −6.74 −5.81 −27.49 1.90

ΔGact

Rh7 ΔGr

In a Vacuum 22.62 −1.01 − 5.40 7.92 −0.88 −26.57 14.94 − −29.59 10.53 4.34 In the Solvated State 11.70 −11.53 − 8.72 0.74 −12.01 18.43 −6.08 − 6.29 6.29 0.25 5580

Ru7

ΔGact

ΔGr

ΔGact

ΔGr

10.66 − 6.90 −23.14 − 1.13

−23.24 2.86 2.98 13.68 −19.44 −2.20

5.57 − 8.71 −29.25 − 11.20

−16.80 5.30 −4.31 17.76 −31.33 6.96

6.30 − 2.31 20.06 − 0.74

−17.67 8.52 −15.57 −4.92 4.56 −4.08

2.27 − 1.28 20.85 − 5.38

−31.01 6.77 −13.61 −5.54 4.18 1.46

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Figure 4. Different intermediates in the biomimetic mechanism obtained from DFT calculations using Gaussian-type orbitals: (a) metal−H2O complex, (b) H2O split into proton and hydroxyl, (c) CO2 adsorbed linearly on intermediate 2, (d) bent OH−CO2 complex, (e) metal−HCO3− complex, (f) H2O attack on metal−HCO3− complex, (g) displacement of HCO3− by H2O. Color key: ochre, metal; black, carbon; red, oxygen; white, hydrogen.

3.2. Comparison of the Activities of Different Metals. The discussion in the previous section on the structure and energetics of different surface species on transition metals proposed on the basis of α-carbonic anhydrase action conclusively demonstrated the plausibility of CHR biomimesis. However, to compare the activities of different metals, one must determine the activation energies of different elementary steps over different metals. Determination of the activation barriers with the molecular model used in this study with periodic plane-wave DFT proved to be prohibitively timeconsuming. Use of DFT with Gaussian-type orbitals to describe the reaction over a metal cluster could prove to be beneficial in such a case. Further, frequency analysis using Gaussian-type orbitals makes calculations of the Gibbs free energy feasible. The Gibbs free energy change can be used as a measure to assess the spontaneity and feasibility of a process and, therefore, we develop the free energy landscape for the mechanism of the reaction over stable seven-membered clusters of different transition metals tested in the previous section. All of the metal clusters were found to stabilize in a pentagonal bipyramidal structure. Structures of the intermediates of the reaction mechanism over the metal clusters are shown in Figure 4. Because the reaction has traditionally been carried out experimentally in water as the solvent, we carried out DFT analysis both in the gas phase and in the solvated state with water as the solvent. The potential energy surfaces for the reaction mechanism on these five metals (Co, Ni, Pd, Rh, and Ru) in a vacuum and in the solvated state are given in panels a and b, respectively, of Figure 5. We next describe in detail the potential energy surfaces in the two media over the five metals. 3.2.1. Step 1: Surface Adsorption of H2O. H2O adsorbed at the atop position of the metal atoms present on the edges of the pentagon constituting the metal cluster. The adsorption complex is shown as intermediate 1 in Figure 4a. The H2O adsorption step was energetically favored over all of the clusters, except for the Co cluster solvated in water, for which adsorption was mildly endergonic with a free energy change of 4.17 kcal/mol. The H2O adsorption complex was found to be most stable over Ni, with free energy decreases of 9.95 kcal/ mol in a vacuum and 5.40 kcal/mol in the solvated state. Adsorption was favored more in a vacuum than in the solvated

release is proposed to take place through H2O attack and HCO3− displacement in accordance with the α-carbonic action. A metal−HCO3−−H2O complex is formed (Figure 2f), which results in further stabilization of the surface species. This step is most favored over Co with an energy decrease of 12.81 kcal/ mol. No significant changes in the O−H distances and H−O− H bond angles of H2O were observed over different metal complexes. 3.1.7. Step 7: HCO3− Displacement by H2O. This step involves the displacement of HCO3− by H2O. A water molecule displaces the HCO3− ion and is assumed to undergo direct coordination with the metal surface, with the adsorption site being the same atop site that was originally observed for H2O adsorption in step 1. A net release of energy was observed for this step, with intermediate 7 (Figure 2g) being more stable than intermediate 6 for all of the catalysts. The energy landscape in Figure 3 shows a net release of energy in a range of 3.9−12.41 kcal/mol, with the highest release of energy over Ru and the lowest release of energy over Co. Of the two metal− oxygen interactions with two oxygens of HCO3− present until the previous step, only one interaction remained in this step. An interaction between the metal and an oxygen atom of H2O was observed, which resulted in the simultaneous displacement of HCO3− from that metal atom. An analysis of the magnitudes of the energy changes for the different elementary steps proposed for the biomimetic CHR over transition metals, shown in Figure 3, supports the plausibility of Bhaduri and Šiller’s proposition of Ni being an active α-carbonic anhydrase analogue. H2O attack (step 6) and HCO3− displacement (step 7) were proposed in accordance with the reported α-carbonic anhydrase action, although the product HCO3− seems to go downhill. Therefore, it can be commented that the release of the product can take place directly or through H2O displacement depending on the operating pH. Further, HCO3− ion removal might seem to be difficult through H2O displacement step on the basis of energy. However, an analysis of the free energy changes can be used to gain better insight into this step. Therefore, we carried out an analysis of the free energy changes using DFT calculations with Gaussian-type orbitals, as detailed in the section. 5581

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be seen that adsorption of both H2O and CO2 was greatly favored over Ni in the solvated state. This is possibly the reason behind the observations of Bhaduri and Šiller of a good activity of Ni nanoparticles when they carried out experiments in water. They also observed an enhanced CO2 uptake upon the introduction of Ni nanoparticles.25 Our results from theory are in agreement with these experimental observations. An additional interaction between the metal and an oxygen atom of CO2 was observed, along with the interaction of the carbon of CO2 and the oxygen of the OH group. The distances between the oxygen of CO2 and the metal were in the ranges of 2.32−2.53 Å in a vacuum and 2.28−2.93 Å in the solvated state. In a vacuum, the greatest O−M distance was observed for Rh, and the lowest was observed for Co, whereas in the solvated case, the O−M distance was the greatest for Rh and the lowest for Ni. CO2 interacted with the metal−OH complex through two centers. O−C interactions invoving the O atom of OH and the C atom of CO2 were observed. Apart from this, an O atom of CO2 was also found to interact with the metal surface. In a vacuum, the highest O−C distance was observed over Ni, and the lowest was observed over Co and Rh. In the solvated state, the O−C distance was greatest over Ni and lowest over Rh. The O−C and O−M distances were higher in the solvated state than in a vacuum over all metal clusters. CO2 was nearly linear with bond angles in the range of 175−179.5°. Bending was observed to be greater in a vacuum than in the solvated state. 3.2.4. Step 4: OH Attack on Adsorbed CO2. Formation of a bent OH−CO2 complex from the linear OH−CO2 complex was observed as an activated process. Free energy barriers in the range of 2.16−4.10 kcal/mol in a vacuum and 0.16−9.07 kcal/mol in the solvated state were observed for bent OH− CO2 complex formation. In a vacuum, the highest activation barrier for this step was observed over Ni, and lowest was observed over Rh, whereas in the solvated state, the highest barrier was over Pd, and the lowest was over Ru. An increase in the activation barrier was observed for Co and Pd, whereas a decrease in the activation barrier was observed for Ni, Rh, and Ru upon solvation in water. In intermediate 4, upon bending of CO2, the C−O bond distances mentioned in step 3 decreased to the range of 1.43−1.48 Å in a vacuum and 1.42−1.43 Å in the solvated state. The M−O bond distances for CO2 also decreased to the range of 1.87−2.1 Å in a vacuum and 1.99− 2.21 Å in the solvated state. The O−C−O bond angles decreased significantly to nearly 130° in both the vacuum and solvated cases. This is consistent with the observation over metal surfaces. The O−C−O bond angles were slightly smaller in the solvated state than in a vacuum over all metal clusters. 3.2.5. Step 5: Proton Transfer. Deprotonation and proton transfer are the two energy-consuming steps involved in the biological CHR. Therefore, careful analysis and comparison of this step with the deprotonation step is required. The protontransfer step involves proton transfer between the oxygen of OH and an oxygen of CO2, forming a metal−HCO3− complex (Figure 4e). From the potential energy surfaces in Figure 5, it can be seen that the activation barrier for this step was the highest when compared to the other steps of the reaction. The activation energies were found to be in the range of 8.22−10.32 kcal/mol in a vacuum and 10.36−17.94 kcal/mol in the solvated state. In a vacuum, the highest activation barrier was observed for Ru, and the lowest was observed for Co, whereas in the solvated state, the highest was observed for Co, and the lowest was observed for Pd. The activation barriers were higher in the solvated state than in a vacuum for all metal clusters

Figure 5. Free energy landscapes for the biomimetic reaction mechanism obtained by DFT calculations using Gaussian-type orbitals (a) in a vacuum and (b) solvated in water.

state consistently for all of the metals. A slight increase in the O−H bond length of H2O from 0.97 to 0.98 Å was observed for all of the complexes in the vacuum and solvated cases because of the interaction of oxygen with the metal site. In case of the complexes in a vacuum, the bond H−O−H increased from 104.9° in H2O to the range of 105.4−106.4° in the metal−H2O complexes. Similar increments in H−O−H bond angles were observed in the other solvated cluster−H2O complexes. The H−O−H bond angles were higher in the case of complexes in a vacuum than those in the solvated state. 3.2.2. Step 2: Deprotonation and Surface OH Formation. H2O, which was observed to adsorb at the atop site over all metals, was found to undergo dissociation resulting in surface OH and H groups. OH was observed to interact with the metal surface through the O center. Both OH and H groups adsorbed on bridge sites. The activation barrier was found to decrease to the range of 2.90−8.28 kcal/mol in the solvated state, with the trend of the highest barrier being for Pd and the lowest barrier being for Ru continued here as well. The activation barriers were consistently lower for all of the metal clusters in the solvated state than in a vacuum. M−O bond lengths were nearly 2 Å in intermediate 2, and these bond lengths were higher in the solvated state than in a vacuum over all metal clusters. 3.2.3. Step 3: Adsorption of CO2. Adsorption of CO2 to give intermediate 3 (Figure 4c) was found to be energetically favored over all metal clusters in both cases, except for Co in the solvated state. The metal clusters were found to show adsorption energies in the s range from −2.35 to −3.34 kcal/ mol in a vacuum and from −1.48 to 1.89 kcal/mol in the solvated state. Whereas Co showed the highest adsorption energy in a vacuum, it was least favored in the solvated state. The adsorption energy was the lowest over Pd in a vacuum. Ni exhibited the strongest adsorption in the solvated state. It can 5582

DOI: 10.1021/acs.jpcc.6b00464 J. Phys. Chem. C 2016, 120, 5577−5584

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The Journal of Physical Chemistry C

4. CONCLUSIONS The analyses performed using periodic plane-wave surface DFT calculations and Gaussian-type cluster calculations were in agreement, indicating the biomimesis in transition metals for catalyzing the CHR. The proposition of Bhaduri and Šiller was found to be correct, with a definite role of Ni nanoparticles in suspension as being responsible for the enhancement of CO2 hydration. Physically acceptable energy requirements and energy barriers during the elementary steps of the reaction on periodic systems confirmed the possibility of the reaction on these metals. Cluster studies further confirmed the activities of the tested metals, with Ru and Co being good potential candidates for the development of heterogeneous catalytic systems for the CHR. The computational insights gained and the screening of metals done in this study on the basis of activation barriers are expected to serve as a guide to experimentalists in exploring novel transition-metal-based CHR catalysts. Detailed activity measurements by either experimental techniques or theoretical methods need to be done to reach an optimal catalyst composition.

except for that involving Ni. There were interactions between two oxygens with the metal atoms, both of them at similar distances in the range of 2−2.25 Å in a vacuum and in the solvated state. The M−O distances were observed to be higher in the solvated state than in a vacuum. 3.2.6. H2O Attack on Surface HCO3− Complex. H2O attack on the metal−HCO3− complex giving intermediate 6 (Figure 4f) resulted in a decrease in the free energy of the system. As discussed previously for the observation of a decrease in energy during H2O attack on the metal surface, this step seems energetically favored even though it makes the release of the product difficult. This step was most favored over Pd, with a free energy decrease of 6.91 kcal/mol in a vacuum, and most favored over Ni, with a free energy decrease of 3.76 kcal/mol in the solvated state. Therefore, it can be seen that, even though the H2O attack step accompanied a decrease in the energy and the Gibbs free energy of the system, the magnitude of free energy change was smaller, making the release of the product not as difficult as was thought from the analysis of surface products. The adsorption energy of H2O was lower in the solvated state than in a vacuum. An increase in the H−O−H bond angle of H2O and a slight increase in the O−H bond length of H2O were observed in intermediate 6 for all of the metal clusters. As for the initial adsorption of H2O, the H−O− H bond angle was higher in the case of complexes in a vacuum than in the solvated state. The largest H−O−H bond angle was observed for Co in a vacuum and for Ni in the solvated state, whereas the smallest H−O−H bond angle was observed for Ni in a vacuum and for Rh in the solvated state. 3.2.7. HCO3− Displacement by H2O. Analysis of this step is important to assess whether the release of HCO3− ion will take place through the H2O attack pathway as observed in αcarbonic anhydrase action. This step of the displacement of HCO3− giving intermediate 7 (Figure 4g) was found to be an energy-intensive step requiring activation energies in the range of 1.29−5.91 kcal/mol in a vacuum and 0.08−14.34 kcal/mol in the solvated state. In a vacuum, the activation barrier was observed to be highest for Ru and lowest for Co, whereas in the solvated state, it was highest for Co and lowest for Rh. The displacement of HCO3− by H2O resulted in the formation of a product complex with a free energy similar to that of the metalbound HCO3− complex. Therefore, it can be concluded that the release of the product HCO3− complex following the biomimetic H2O attack pathway is as feasible as its direct desorption into the solution. Of the two M−O interactions with two oxygens of HCO3− present until the previous step, only one O−M interaction remained in this step. This is an important observation, as the product in this step assumes a unidentate configuration, observed consistently over all of the metal surfaces as well. Desorption of a unidentate HCO3− complex is easier than desorption of a bidentate HCO3− complex (intermediate 5). This also shows the feasibility of the biomimetic H2O attack pathway for the HCO3− ion release step. A decrease in the distance between the metal and the oxygen atom of H2O was observed compared to previous intermediate 6, which resulted in the simultaneous displacement of HCO3− from the metal atom. The new metal−oxygen (of H2O) distances were found to be smaller in a vacuum than in the solvated state. Completion of the catalytic cycle then requires just the desorption of unidenate HCO3− from the catalyst.

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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.jpcc.6b00464. Absolute DFT energies and important bond lengths and bond angles involved during the CHR with different transition metals in a vacuum (Table S1) and with water as the solvent (Table S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+91) 3222 283916. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Biotechnology of Ministry of Science and Technology, Government of India (Bioinformatics, Computational and Systems Biology Programme BT/PR7054/BID/7/422/2012).

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REFERENCES

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