Computational Evidence for Lewis Base-Promoted CO2

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Computational Evidence for Lewis Base-Promoted CO2 Hydrogenation to Formic Acid on Gold Surface Xiang-Ying Lv, Gang Lu, Zhi-Qiao Wang, Zhong-Ning Xu, and Guo-Cong Guo ACS Catal., Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Computational Evidence for Lewis BasePromoted CO2 Hydrogenation to Formic Acid on Gold Surface Xiangying Lv†,‡, Gang Lu†,*, Zhi-Qiao Wang†, Zhong-Ning Xu†,*, Guo-Cong Guo†,* †State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡Key Laboratory for Yellow River and Huai River Water Environment and Pollution Control, Ministry of Education, Henan Key Laboratory for Environmental Pollution Control, School of Environment, Henan Normal University, Xinxiang, Henan 453007, P. R. China

Abstract: The mechanism and the effect of Lewis bases in gold-catalyzed CO2 hydrogenation to formic acid were investigated using first principle calculations. The calculations indicate different types of gold surfaces are all capable of heterolytically splitting H2 by coupling with Lewis bases (e.g. NH3). The generated hydride and proton (in NH4) on gold surfaces can be concertedly transferred to CO2 with high reactivity. Further, instead of the disfavored hydride transfer to the formate (HCOO) intermediate, the effective proton transfer from NH4 to HCOO provides an alternative pathway for the formation of formic acid.

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Keywords: CO2 hydrogenation, heterogeneous gold nanocatalysis, concerted hydride and proton transfer, formic acid, first principle calculations

1. Introduction Hydrogenation of CO2 to HCOOH offers great potentials for storing renewable hydrogen energy in liquid fuels and has significant advantages by using abundant and inexpensive CO2 to produce chemicals with numerous applications.

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This process has been increasingly

investigated by using homogenous catalysts based on various transition metal complexes.6-11 In contrast, heterogeneous catalysts are studied less extensively to facilitate this reaction. 12 - 16 Recently, the gold catalyst was reported to catalyze CO2 hydrogenation to formic acid at mild condition in the presence of Lewis bases (e.g. NEt3, equation (1)).13 The work showed that the supported nanogold (Au/TiO2) was effective under employed conditions, and the Au black obtained via reduction of Au(OH)3 also afforded the product of HCOOH/NEt3 albeit with low conversion. However, the detailed reaction mechanism remains elusive. Although the role of Lewis bases is well accepted to stabilize the product via forming formate salts or adducts,17 it is still unexplored to reveal the effect of Lewis bases from the kinetic aspect.

CO2 + H2

Au/TiO2 or Au black NEt3, 40 °C

HCOOH/NEt3

(1)

Previously, Hoffmann18 pointed that metal surface is a nice reservoir of electrons and could accept electrons as a Lewis acid. Sakurai group 19 also reported that gold nanoclusters could behave as Lewis acid catalysts in aqueous media under aerobic conditions. Recently, Corma20 reviewed that gold clusters and nanoparticles can act as Lewis acids to interact with Lewis bases such as alkenes and alkynes. In addition, we have computationally and experimentally revealed


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that gold surface can serve as a Lewis acid coupling with Lewis bases (e.g. imine and nitrile) to construct Lewis acid/base pairs to activate H2 and achieve hydrogenations of small imine and nitrile.21

Scheme 1. Mechanism of CO2 Hydrogenation on Metal Surface.

The mechanism of CO2 hydrogenation on metal surface has been widely studied with computations.22-31 As summarized in Scheme 1a, H2 was firstly cleaved to generate two hydrides on metal surface. The subsequent hydride transfer can occur via three possible pathways: the concerted hydride transfer (I in Scheme 1a) and the stepwise additions of hydride to CO2 via the carboxyl (COOH, II) and formate (HCOO, III) intermediates. It is possible for CO2 hydrogenation on gold surface via these mechanisms. Nevertheless, based on the synergetic Lewis acid/base principle typically employed in the frustrated Lewis pair (FLP) chemistry,32-34 we propose that CO2 hydrogenation to HCOOH could be promoted by the cooperative effect of gold surface with Lewis base. As shown in Scheme 1b, the gold is considered to serve as a Lewis


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acid, generating a Lewis acid/base couple with NH3, which is active to cleave H2.21 The formed hydride and NH4 on gold surface could be transferred to CO2 via three possible pathways (IV, V, VI in Scheme 1b). Here, we perform first principle calculations to study the effect of Lewis bases in the reaction of CO2 + H2 → HCOOH on gold surface. The calculations show that the Au/NH3 couples enable the generation of surface hydride and proton, and the reaction is significantly facilitated via the concerted hydride and proton transfer mechanism.

2. Computational methods All calculations were performed using the Vienna ab initio Simulation Package (VASP) code. 35 , 36 The projector augmented wave (PAW) potentials 37 , 38 were used for electron-ion interactions, and the generalized gradient approximation (GGA-PW91)39 was used to describe the exchange-correlation functional. The electron wave function was expanded using plane waves with an energy cutoff of 400 eV. To better understand the reactivity of gold-catalyzed CO2 hydrogenation, we chose Au(111), Au(211), Au(111)-mono40 and Au38 to model the closepacked, step-edged, single-atom-arranged surfaces and the nano-sized clusters, respectively. Au(111), Au(211) and Au(111)-mono surfaces were modeled by a four-layer slab with a p(3×3) unit cell, a six-layer slab with a p(2×4) unit cell and a four-layer slab with a p(4×4) unit cell, respectively. A (3×3×1) k-point mesh was used for Au(111) and a (2×2×1) k-point mesh was used for Au(211) and Au(111)-mono. The atoms in the bottom two layers of Au(111) and Au(111)-mono and in the bottom three layers of Au(211) were fixed at the bulk positions that were taken from the calculated lattice constant of 4.17 Å (the experimental value is 4.08 Å). The rest of atoms were allowed to relax. The dipole correction was considered in the z direction of these slab models. The Au38 nanoparticle with a typical cuboctahedral shape and ~1 nm diameter


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was placed in a 20×20×20 Å3 cubic box. Isolated gas-phase molecules were optimized in a 15×15×15 Å3 unit cell. All atoms of Au38 and gas-phase molecules were always fully relaxed in optimization. Energies and forces were converged to within 10–4 eV and 0.02 eV/Å, respectively. The transition states were optimized by using the climbing-image nudged elastic band method (CI-NEB)41,42 at a reduced force threshold of 0.05 eV/Å. All transition states were confirmed by frequency analysis. Bader charges were calculated using the standard method developed by the Henkelman group.43

3. Results and Discussion Mechanism of Hydride Transfer on Clean Gold Surface. We first computed the energetics of CO2 hydrogenation on clean Au(111), Au(211), Au(111)-mono and Au38 via the three pathways shown in Scheme1a. The geometries and barriers for the key transition states are shown in Figure 1. The calculated barrier for H2 activation on Au(111) is 1.20 eV (1-TSa) with respect to the adsorption of H2 on Au(111) (donated as H2*, * represents gold surface, similarly hereinafter). The incapacity of Au(111) to split H2 to generate hydride indicates that it is disfavored for CO2 hydrogenation via the hydride transfer mechanism. Further, we calculated the hydride transfer steps on Au(111) and all three pathways have high barriers (2-TSa, 3-TSa, 4TSa). In contrast, Au(211), Au(111)-mono and Au38 are active for H2 cleavage (1-TSb, 1-TSc, 1-TSd) due to the lower coordination number of surface Au atoms. On these surfaces, in particular for Au(111)-mono and Au38, the hydride transfer via the formate pathway (4-TSc and 4-TSd) is significantly more favorable than the concerted hydride transfer (2-TSc and 2-TSd) and the carboxyl (3-TSc and 3-TSd) pathways. However, due to the high stability of the bidentate adsorption of HCOO*, the second hydride transfer from Au surface to the O atom of


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HCOO* to generate HCOOH is disfavored. The barriers are 1.17 eV on Au(111)-mono (5-TSc) and 1.00 eV on Au38 (5-TSd). Thus, on the clean surface of these models of gold catalysts, CO2 hydrogenation is suppressed either by H2 activation or by hydride transfer processes.

Figure 1. Structures of Au(111), Au(211), Au(111)-mono, Au38 and the key transition states with geometric parameters (Å) in the hydride transfer pathways for CO2 hydrogenation. Activation energies (∆E‡, eV) are with respect to the respective resting states.


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H2 Activation by Au/NH3 Couples. To study the effect of Lewis bases on CO2 hydrogenations on gold surface, we take the simple Lewis base NH3 as an example. Previously, we have found that H2 activation on Au(111) can be promoted by Lewis bases, such as NH3, Me2C=NH, and HCN.21 The reactivity can be attributed to the synergetic Lewis acid/base effect of Au(111) with Lewis bases. Here, we further investigate whether the same Lewis acid/base effect can be applied to the couples of Au(211)/NH3, Au(111)-mono/NH3, and Au38/NH3 for H2 activation. As shown in Figure 2, the adsorptions of NH3 on Au(211), Au(111)-mono and Au38 lead to the formation of Au/NH3 couples, which are exothermic by –0.53 eV (6b), –0.63 eV (6c) and –0.90 eV (6d), respectively. H2 activations by these Au/NH3 couples also have low barriers, 0.57, 0.61 and 0.56 eV for Au(211)/NH3 (8-TSb), Au(111)-mono/NH3 (8-TSc) and Au38/NH3 (8-TSd) with respect to 7b–7d, respectively, which are comparable to that of Au(111)/NH3 (8TSa, 0.67 eV). The barriers of H2 activation by these Au/NH3 couples are lower than 0.75 eV, a typical limit for the occurrence of surface reactions at room temperature.44 Similar to the H2mediated charge transfer from Lewis bases to Lewis acids in FLP systems, the NH3 moiety donates electrons (i.e., Lewis basic effects) and the gold surfaces can effectively accept electrons (i.e., Lewis acidic effects) in the transition states of H2 activation. The simultaneous cooperation of these two effects leads to the high reactivity of H2 cleavage (see detailed charge analyses in Figure S1 of supporting information). In addition, the comparable barriers of H2 activation on different types of Au surfaces suggest that this synergetic Lewis acid/base effect is slightly influenced by the surface structures and sizes of gold catalysts.


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Figure 2. Optimized structures with key geometric parameters (Å) for H2 activation by Au(111)/NH3 (black), Au(211)/NH3 (red), Au(111)-mono/NH3 (green) and Au38/NH3 (blue). Energies (∆E, eV) are with respect to the separated Au surface and NH3.

Mechanism of Hydride and Proton Transfer to CO2. After H2 activation by Au/NH3 couples, the hydride (H*) and proton (NH4*) are generated on Au surfaces. Although the formations of these species (9a–9d in Figure 2) are uphill in energy with respect to the complexes of H2 with Au/NH3 (7a–7d), it is possible for transferring the hydride and proton in 9a–9d to CO2. Three pathways shown in Scheme 1b may occur: (i) The hydride (H*) and proton (NH4*) can be concertedly transferred to CO2. (ii) The formation of COOH* via hydride transfer is unreachable due to the higher barriers (3-TSa–3-TSd in Figure 1). Nevertheless, the COOH* would be accessible via the proton transfer from NH4* to one of the oxygen atoms in CO2. (iii) After the formation of HCOO* with relatively low barriers on Au(111)-mono and Au38, instead of the direct second hydride transfer to the bidentate HCOO* that requires high barriers (5-TSc and 5-TSd), the proton transfer from NH4* to HCOO* would offer an alternative route to generate formic acid. These three pathways will be discussed separately below. Concerted Hydride and Proton Transfer Pathway. As shown in Figure 3, the interactions of CO2 with the co-adsorbed H* and NH4* (9a–9d) are favored by the interactions of the O… H−N hydrogen bond in 10a–10d. Subsequently, the hydride on Au surface and the proton in NH4* concertedly attack the carbon and oxygen atoms in CO2, respectively. This process requires quite low barriers on all four gold surfaces, 0.33 (11-TSa), 0.33 (11-TSb), 0.27 (11TSc) and 0.24 (11-TSd) eV with respect to 10a–10d on Au(111), Au(211), Au(111)-mono and Au38, respectively. The resulting ammonium formate 13 can dissociate from the Au surfaces and


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then isomerize to the more stable conformer 14. This indicates that formic acid can be easily delivered from CO2 hydrogenation via the concerted hydride and proton transfer pathway. Notably, in the transition states 11-TSa–11-TSd, CO2 interacts with the hydride and proton in an outer-sphere manner, where CO2 does not bond with the Au surface. This is significantly different from common stepwise hydride transfer mechanism that requires the direct binding of CO2 on metal surface, such as II and III in Scheme 1a. Although the outer-sphere hydride and proton transfer mechanism has been well studied in homogeneous hydrogenations of CO245-47 and other unsaturated compounds, 48 - 52 this mechanism is revealed for the first time in hydrogenation of CO2 on metal surface. By comparison, as shown in the transition states of CO2 hydrogenation in Au/NH3, Ir-N45 and B/N53 FLP systems (Scheme 2), the Au surface plays the same roles as iridium and borane centers, where the bound hydrogen bears negative charges, favoring the nucleophilic attack to the carbon center of CO2. The nitrogen moieties in these three systems also share a similar feature that offers a proton reacting with the oxygen of CO2. These similarities represent a feasibility that employs a homogeneous mechanism to understand reactions on metal surface for mechanistically bridging heterogeneous and homogeneous catalysis.54


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Figure 3. Pathways of the concerted hydride and proton transfer on Au(111) (black), Au(211) (red), Au(111)-mono (green) and Au38 (blue). Key geometric parameters (Å) are shown in the structures. Energies (∆E, eV) are with respect to the separated Au surface and NH3.

Scheme 2. Outer-sphere Hydrogen Transfer Mechanism.


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We further investigate the origin of differences in reactivity of CO2 hydrogenation via various hydrogen transfer mechanisms. Compared to the adsorbed hydride (H*, Figure 4a) with stable geometries on the four Au surfaces, the charges and geometries of the H* in the transition states would have significant changes. Thus, we calculated the Bader charges and geometric distortion energies55,56 of H* and studied their relationships with the barriers of hydrogen transfer steps. As shown in Figure 4b, a good linear correlation was observed between the barrier and the charge change of H* (R2 = 0.95). In contrast, the barrier poorly correlates with the geometric distortion energy of H* (R2 = 0.30, see Figure S2 for details). This result indicates the H* in the transition state bearing more negative charges leads to higher reactivity of attacking the carbon in CO2, which is clearly demonstrated in the transition states of concerted hydride and proton transfer (11-TSa–11-TSd shown in the blue cycle in Figure 4b). This is because the Au surface gains more electrons from NH3 in the H2 activation step, thus offering more negative H* and leading to high reactivity of nucleophilic attack. In the meantime, the interactions of NH4* with CO2 in 11-TSa–11-TSd can enhance the electrophilicity of the carbon (bearing more positive charges than those without NH4*, Figure 4a) and thus contribute to the reactivity. The deviations of 2-TSa, 4-TSa and 4-TSb from the correlation are due to the contributions from the geometric distortion of H* to the reactivity, as evidenced by the relatively large distortion energies of H* in the transition states (Figure 4a).


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Figure 4. (a) Bader charges and geometric distortion energies (eV) of adsorbed hydrogen (H*) on Au surfaces. The ∆Edist(H*) is the energy to distort the H* with stable geometries into the transition state geometries of H*. (b) Relationship between the barrier of hydrogen transfer and the charge change of H*. The charge change is the difference between the charge of H* in the transition state and that of the H* with stable geometries.

Proton Transfer Followed by Hydride Transfer Pathway. Next, we consider whether the proton in NH4* can be transferred to one oxygen atom of CO2 to form COOH*, which then is followed by hydride transfer from H* to the carbon atom in COOH*. As shown in Figure 5, the co-adsorptions of NH4* and CO2 on Au(111) and Au38 are also favored due to the O…H−N hydrogen bonding, which are exothermic by 0.20 eV (16a) and 0.13 eV (16d), respectively. After co-adsorption, the proton and hydride transfer steps occur successively. On Au(111), both the proton (17-TSa, ∆E‡16a→17-TSa = 0.61 eV) and hydride (20-TSa, ∆E‡16a→20-TSa = 1.25 eV) transfers are significantly higher than the concerted hydride and proton transfer (11-TSa, ∆E‡10a→11-TSa = 0.33 eV, Figure 3). Although the proton transfer from NH4* to CO2 on Au38 is barrierless (16d→18d), the ensuing hydride transfer has a higher barrier (20-TSd, ∆E‡18d→20-TSd = 0.55 eV) than that of the concerted hydride and proton transfer (11-TSd, ∆E‡10d→11-TSd = 0.24 eV, Figure 3). It is noted that in the geometry of 20-TSd the proton tends to bond with NH3 again, and this makes 20-TSd share the feature of the transition state of concerted hydride and proton transfer (11-TSd). In addition, the reactions on Au(211) and Au(111)-mono also show that the hydride transfer step is disfavored (see the energy profiles in Figure S3).


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Figure 5. Pathways of proton transfer followed by hydride transfer on Au(111) (black) and Au38 (blue). Key geometric parameters (Å) are shown in the structures. Energies (∆E, eV) are with respect to the separated Au surface and NH3.

Hydride Transfer Followed by Proton Transfer Pathway. Since the formation of HCOO* on Au38 requires relatively low barriers, we thus study the reactivity of proton transfer from NH4* to HCOO*. Figure 6 shows that the proton transfer step (24-TSd) has a low barrier of 0.40 eV with respect to the bidentate HCOO* (22d). This process leads to the formation of coadsorbed NH3* and HCOOH* (25d). The additional free NH3 can interact with the adsorbed HCOOH and promote the dissociation of ammonium formate 14 from Au38 surface. The accompanied Au38/NH3 couple (6d) is readily for the next round of H2 activation. Thus, even for


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the Au surfaces that are active to cleave H2 and form the formate intermediate, NH4* derived from the H2 activation by Au/NH3 couples is critical for the protonation of the stable bidentate HCOO* intermediate to deliver formic acid.

Figure 6. Key steps in the pathway of hydride transfer followed by proton transfer on Au38. Key geometric parameters (Å) are shown in the structures. Energies (∆E, eV) are with respect to the separated Au38 and H2.

4. Conclusion In summary, first principle calculations were performed to investigate the mechanism and the effect of Lewis bases in gold-catalyzed CO2 hydrogenation to formic acid. Based on the four models of gold catalysts, close-packed Au(111), step-edged Au(211), single-atom-arranged Au(111)-mono and nano-sized Au38, the calculations show that CO2 hydrogenation on the clean Au surfaces of these models is hampered either by H2 activation (e.g. on Au(111) surface ) or by the hydride transfer step (e.g. on Au(211), Au(111)-mono and Au38 surfaces). However, in the presence of Lewis bases, such as NH3, the reaction can be significantly facilitated via Lewis


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base-assisted H2 activation and subsequent hydride and proton transfer mechanism. The role of Lewis bases is twofold: (i) Based on the synergetic Lewis acid/base principle, Lewis bases can couple with the four different models of gold surfaces, leading to heterolytic H2 cleavage and the generation of hydride (H*) and proton (in NH4*) on Au surface. (ii) The concerted hydride and proton transfer to CO2 is favored on all four gold surfaces. The surface hydride derived from the H2 cleavage by the Au/NH3 couples bears more negative charges due to the electron donation from NH3 to Au surfaces. Also, the interaction of NH4* with CO2 can enhance the electrophilicity of the carbon of CO2. These two effects cooperatively promote the nucleophilic attack of surface hydride to CO2. In addition, compared to the disfavored transfer of surface hydride to formate, the proton transfer from NH4* to HCOO* is feasible to form formic acid. This Lewis base-assisted CO2 hydrogenation on gold surface may have useful implications for the design of new catalysts in CO2 transformation.

Supporting Information. Charge analysis of H2 activation transition states, relationship of the barrier with the distortion energy of H*, and energy profiles for the proton transfer followed by hydride transfer pathway on Au(211) and Au(111)-mono surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author * [email protected] * [email protected] * [email protected]


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Present Addresses GL: Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States

Acknowledgement This work was supported by the National Natural Science Foundation of China (91545201, 91645116, 21507025, 21403237, 21303203), the Key Research Project of Henan Province (15A610001), and the Scientific Research Starting Foundation of Henan Normal University (5101219170104).

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Computational Evidence for Lewis Base-Promoted CO2

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