Photochemical Hydrogenation of CO2 to CH3OH and Pyridine to 1, 2

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Photochemical Hydrogenation of CO to CHOH and Pyridine to 1,2-Dihydropyridine Using Plasmon-Facilitated Chemisorbed Hydrogen on Au Surface: Theoretical Perspective Gaurab Ganguly, Munia Sultana, and Ankan Paul J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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The Journal of Physical Chemistry

Photochemical Hydrogenation of CO2 to CH3OH and Pyridine to 1,2-Dihydropyridine using PlasmonFacilitated Chemisorbed Hydrogen on Au Surface: Theoretical Perspective Gaurab Ganguly, Munia Sultana, and Ankan Paul*. Raman Center for Atomic, Molecular and Optical Science, Indian Association for the Cultivation of Science, Kolkata 700032, India. AUTHOR INFORMATION Corresponding Author [email protected]

ABSTRACT: CO2 hydrogenation to methanol, a renewable fuel, under benign conditions and without the use of sacrificial agents is a rarity and certainly a much sought after goal of present day research. Herein, we report using well calibrated computational tools the viability of hydrogenating CO2 to CH3OH and pyridine to 1,2-dihydropyridine, a renewable organohydride which can also reduce CO2 to CH3OH sustainably, using hydrogens chemisorbed on Au nanoparticle (AuNP) surface by Surface Plasmon Resonance (SPR). Our studies predict that these

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hydrogenations can occur at appreciable rates at room temperature. Thus, we reveal the untapped potential of the chemisorbed hydrogens on Au(111) surface, achieved by "hot electron" generation through SPR, in facilitating endoergic hydrogenations akin to those enabled by NADPH.

INTRODUCTION

Recently, the long held notion of inertness of metallic Gold1 was delivered an image shattering blow, when it was shown that endoergic hydrogenation of Au(111) surface can be achieved using incident photons.2-3 The incident photons of a particular frequency alter the electronic structure of gold in such a way that a collective excitation involving multiple e-/h+ pairs (known as surface plasmons resonance (SPR)) becomes possible, which ultimately decays to generate "hot electrons" and "hot holes".4-5 This phenomenon is a key step, which transforms light energy to chemical energy6-10 thus, facilitating endoergic chemisorption of H2 on the Au(111) surface.2-3 Evidently, this remarkable discovery has ushered in innumerable possibilities and has triggered a wave of experimental and theoretical studies.11-15 Of late, photocatalysis on metal surfaces has emerged as one of the most notable offshoots of such plasmonic processes.11-15 As chemisorption of H2 on Au(111) surface is intrinsically endoergic in nature,1 the chemisorbed hydrogens on AuNP (AuNP{H}2) are at a higher chemical potential than usual and in principle have the ability to drive hydrogenations, perhaps, most importantly, the difficult ones. A sustainable route to hydrogenation of CO2, the principle cause of global warming, has always been considered to be an insurmountable problem.16-17 Hydrogenation of CO2 to HCOOH is an integral component of CO2/HCOOH based chemical hydrogen storage acid cycle,18 whereas reduction of CO2 to CH3OH is of immense importance in methanol based fuel economy.19


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The 2e-/2H+ reduction of CO2 to HCOOH is endoergic in nature (H2+CO2 HCOOH; ∆G0rxn=+7.8 kcal/mol).18 Hence, it is usually driven by either the use of sacrificial reagents20-22 or the use of extremely high reduction potential of -0.61 V in a "base free" condition.23 Whereas the 6e-/6H+ reduction of CO2 to CH3OH has been attempted by electrochemical, photoelectrochemical and photochemical means.24-26 In 2008, Bocarsly and co-workers used catalytic amount of pyridine (Py) in photo-electrochemical reduction of CO2 to CH3OH on the p-GaP(111) semiconducting photocathode.27 In an elegant study Musgrave and co-workers showed that it is the Py which is first reduced to a dearomatized species, 1,2-dihydropyridine (1,2PyH2) via a coupled proton transfer-electron transfer mechanism and later the in situ generated 1,2PyH2 reduces CO2 through concerted hydride transfer proton transfer (HTPT) steps.28-29 Recently we have also shown 1,2PyH2 is capable to hydrogenate amino-boranes to form amine-boranes.30 It is to be noted the dearomatization of Py to produce renewable fuel 1,2PyH2 is itself a thermodynamically uphill process.28-29 As AuNP{H}2 can also be easily generated from AuNP by purely photolytic means;23

we hypothesized that Py/1,2PyH2 and AuNP/AuNP{H}2 systems would possess similar chemical

reactivity. Hence, one could devise a photocatalytic route which involves photochemical hydrogenation of AuNP to form AuNP{H}2 followed by reduction of CO2 to HCOOH and ultimately to CH3OH without the use of a sacrificial agent. Bare and hydrogen atom chemisorbed gold clusters has been traditionally used for Facilitating catalytic oxidation of CO to CO2.31-33 However, the endoergic hydrogenation of gold surface opens up the possibility of reduction of CO2 to HCOOH/methanol. In this communication we report computational investigations which


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predict that hydrogenation of CO2 to HCOOH and to CH3OH and Py to 1,2PyH2, can be achieved using the aforementioned photocatalytic route (see Scheme 1).

Scheme 1. Proposed Catalytic Reduction of CO2 to Methanol and Py to 1,2PyH2 by AuNP/ AuNP{H}2 redox couple. COMPUTATIONAL MODEL AND METHODS For our computational investigations we chose the Td Au20 cluster and its phosphine ligated analogue. The Td Au20 cluster is unique among the smaller gold clusters as it not only possesses a Au(111) surface,34-35 but also is known to exhibit SPR properties which leads to the formation of "hot electrons".14,36 Additionally, the phosphinated analogue was chosen as in recent times significant success have been achieved in synthesizing Td Au20(PPh3)4 in condensed phase,37 whose four triangular faces also inherently resemble the bulk fcc Au(111) surface.37 All the ground state geometries were optimized in vacuum with GGA exchange-correlation PW91 density functional, which is found to be the best among several functionals after systematic calibration for Au based systems. Whereas, the GGA functional PBE, which provides a good description of the excited states of Au nano-particles,38-39 was chosen for TD-DFT computations. The plasmonic excitations are actually multiple single electron excitations and can be well treated by TD-DFT.4041

All non-metal atoms have been given all electron 6-31++G(d,p) basis set, while the effective


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core potentials of the LANL2 group were employed for the core electrons of the gold atoms (Au), and the corresponding LANL2DZ basis set was used for their valence electrons. The CPCM model which gives fairly reliable result for solid-solvent interaction has been chosen as the solvent model with the Universal Force Field (UFF) radii. The dielectric constant in the CPCM calculations was set to 7.4257 to simulate tetrahydrofuran (THF). Harmonic vibrational frequencies at the same level of theory were computed to characterize the structures as minimum (all real frequencies) and transition state (one imaginary mode) and also to extract thermo-chemical information. All the computations were done with Gaussian 09 package.15 Detailed justification for the choice of model systems and the functionals along with the other computational details for free energy calculation are provided in the section S1-4 in the Supporting Information, SI.

Figure 1. Optimized geometry of (a) Au20, (b) Au20(PH3)4, (c) Au20{H}2 and (d) Au20(PH3)4{H}2 along with the NBO charges and natural population. Bond parameters are given in Å.


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RESULT AND DISCUSSION i)

H-H σ-Bond Activation Mediated by Au20 cluster

Chemisorption of H2 on Au(111) surface is known to be endoergic which is associated with an energy penalty of ~0.5 eV (11.5 kcal/mol).1 Our computations predicted a similar trend for Au20 and Au20(PH3)4 clusters. The associated total energy cost for chemisorbing H2 on Au20 and Au20(PH3)4 is 16.3 and 11.1 kcal/mol and the free energy penalty turned out to be 22.9 and 17.4 kcal/mol respectively. The Optimized geometries of the most stable structures of the H2 chemisorbed gold-clusters, Au20{H}2 and Au20(PH3)4{H}2 and the corresponding bare goldclusters, Au20 and Au20(PH3)4 are shown in Figure 1 along with the NBO charges and natural population on the chemisorbed hydrogens. Hence, a characteristic endoergic trait was observed for chemisorption of H2 for model gold clusters, which is similar to the one reported for Au(111) surface.1 Moreover, theoretical studies have shown that bond activation of physisorbed molecules can be achieved through SPR activity of Au20 cluster.14,15 Motivated by these findings we investigated the possibility of H2 dissociation on Au20(PH3)4 cluster. Potential Energy Surface (PES) plots for chemisorption of H2 on the Au20(PH3)4 in the ground electronic state (S0) (reaction coordinate approximately equivalent to H-H σ-bond distance) as shown in Figure 2a, shows very similar characteristics as H2 dissociation over Au(111) surface.1 Experimentally "hot electron" assisted H2 dissociation on AuNP/TiO2 system occurs typically at ~543 nm radiation.2 The TDDFT generated absorption spectra of bare Au20(PH3)4 showed a strong triply degenerate excitation at 2.35 eV (~527 nm) as shown in section S5(i) in the SI. The subsequent analysis of the orbitals involved in this transition revealed that the excitation indeed involves multiple e-/h+ pairs, thus suggesting that the incident radiation gives rise to plasmon generation. Then we studied the excitations involved in the H2 physisorbed Au20(PH3)4 system. In the ground state optimized


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geometry the adsorbed H2 shows almost no interaction with the Au cluster (~3.6 Å away from the Au(111) face) and the interaction is purely of dispersion origin. The interaction is so weak that even the inclusion of D3 version of Grimme,s empirical dispersion correction to the density functional (PW91-D3) does not have any significant effect on the interaction distance of the physisorbed H2-Au20(PH3)4. Two possible charge transfer (CT) mechanisms may operate at the adsorbate-metal interface i.e. metal valence band to molecule unoccupied σ*-orbital of H2 and

E (kcal/mol)

S0 Au20(PH3)4{H}2

E (kcal/mol)

S1 Au20(PH3)4{H}2

S0

Au20 (PH3)4 +H2

Au20 (PH3)4 +H2

H-H σ bond dissociation coordinate on Au 20(PH3)4 (Å)


(a)

(b)

S0 S1

S2 E (Hartree)

Natural Population Change (in e) compared to free H2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S1 S0



(d)

(c)

Figure 2. (a) Minimum energy relaxed scan of H-H σ-bond distance on Au20(PH3)4 in the ground electronic state (S0); (b) Minimum energy relaxed scan of H-H σ-bond distance on Au20(PH3)4 in the first excited state electronic state (S1 :red) and the corresponding ground electronic state energy (S0 :blue) at S1 state relaxed geometry; (c) Natural population change (in e) in the ground (S0) and first excited (S1) state compared to free H2 along the relaxed scan of H-H σ-bond distance on Au20(PH3)4 in the first excited electronic state (S1); and (d) SA-CASSCF(2,2) single point energy of the three low lying states (S0, S1 and S2) on the S1 state DFT optimized geometry


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from occupied σ-orbital of H2 to metal valence band CT. The adsorption showed a maximum at 2.4 eV (~517 nm) (please see section S5(ii) in the SI). This indicated that the absorption maximum does not change much in the presence of H2 i.e. no signature of metal-to-molecule CT. During the free movement of H2 over Au(111) surface of the Au20(PH3)4 the H-H bond dissociation mode may couple with the H2-Au(111) approach mode which may induce CT from AuNP to σ* of H2 leading to H-H bond dissociation. A PES as a function of reaction coordinate (approximately equivalent to H-H σ-bond distance) in the first excited state (S1) using TD-DFT was constructed to explore this possibility. The relaxed S1 state optimized energy (reaction coordinate approximately equivalent to H-H σ-bond distance) along with the ground state (S0) energy at the relaxed S1 optimized geometry was plotted in Figure 2b. We found the H-H σ-bond dissociation barrier in the S1 state is 10.1 kcal/mol (see Figure 2b) which is significantly low compared to the H-H dissociation barrier of ~35 kcal/mol in the ground state (see Figure 2a). As the Figure 2a represents the

Figure 3. Orbital correlation diagram of the frontier Kohn-Sham orbitals (HOMO and LUMO) around the CI.


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relaxed H-H σ-bond scan along the S0 electronic state and the Figure 2b represents the relaxed HH σ-bond scan along the S1 electronic state, the plot for the H2 dissociation curve for the S0 state in Figure 2a is a bit different in shape compared to the curve for the S0 state in Figure 2b. The photochemical H-H σ-bond dissociation on Au(111) is kinetically viable and thermodynamically downhill. The plot of natural population change in both the states (S0 and S1) along the reaction coordinate clearly shows that CT occurs from the Au cluster to H-H σ*-orbital in agreement with the findings of Halas et al.2 (see Figure 2c). The two curves (S0 and S1) nearly touch each other at about 1.2 Å indicating the existence of a Conical Intersection (CI) between the two states. Analyzing the coefficients of TD-DFT around the CI clearly suggests that it is a HOMO-LUMO transition or in other words it reduces to a CAS(2,2) problem from a plasmonic excitation when H2 is close to the Au surface and the H-H σ-bond distance is ≥ 1.0 Å. The SA-CASSCF(2,2)/631++G(d,p)/LANL2DZ(Au) single point calculations also predicted an avoided crossing between S0 and S1 at about 1.2 Å (see Figure 2d). It is to be noted that all the energy units reported in Figure 2(a&b) are full reaction coordinate of H-H σ-bond distance and the relative energy is reported in kcal/mol unit. While the SA-CASSCF plot is reported only for a small portion of the H-H σ-bond distance near the CI and reported in the absolute energy unit (Eh). The marked difference in the frontier Kohn-Sham orbitals in the two geometries can be observed from the change in nodal surface between the chemisorbed hydrogens (H1 and H2). Essentially, the swapping of the frontier orbitals i.e. HOMO and LUMO around the CI further suggests the crossing/avoided crossing of S0 and S1 states (for orbital correlation diagram see Figure 3). After crossing the CI the system is likely to fall on the S0 surface which would essentially lead to chemisoprtion of H2 i.e. Au20(PH3)4{H}2.


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ii)

Reduction of CO2 to Methanol with the Chemisorbed Hydrogens on Au20

Following this we checked the reactivity of the chemisorbed H2 on Au20(PH3)4 and Au20. For CO2 hydrogenation to methanol, the most favorable pathway is shown in Figure 4. For optimized structures of the transition states please see section S6 in the SI. The first step for CO 2 conversion to formic acid HCOOH from chemisorbed hydrogen on Au20(PH3)4 was predicted to be exoergic by 5.0 kcal/mol. We found that this exoergic transformation (Au20(PH3)4{H}2 + CO2(g)  Au20(PH3)4 +HCOOH) occurs through a single concerted HTPT transition state

(H1) (H2)

(H2)

(H1) (H1) (H2)

14.0 (17.4)

20

10

0

G (kcal/mol)

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O=C=O

TS(CO2)

10.2 (-2.1)

HCOOH

0.0 (0.0)

TS(HCOOH)

-5.0 (-9.8)

3Au20(PH3)4{H}2

2Au20(PH3)4{H}2 +Au20(PH3)4

-10

CH2(OH)2

⸗

-16.1 (-26.4)

1.6 (-8.7)

TS(CH2(OH)2)

2CH2(OH)2 Au20(PH3)4{H}2 +2Au20(PH3)4

-20

-30

H2C=O + H2O -11.7 (-22.1)

CH2(OH)2 +H2C=O Au20(PH3)4{H}2 +2Au20(PH3)4

-7.5 (-17.1)

TS(H2C=O) CH3OH

-43.4 (-59.4)

-40

Au

C

O

P

H

3Au20(PH3)4

-50

CO2 reduction to HCOOH

HCOOH reduction to CH2(OH)2

H2O elimination from CH2(OH)2 to form H2C=O

H2C=O reduction to CH3OH

Figure 4. Gibbs free-energy profile of the conversion of CO2 to CH3OH and H2O with three concerted HTPT steps by Au20(PH3)4{H}2/Au20(PH3)4 redox couple. The values in the parenthesis correspond to the free energy changes for the conversion of CO2 to CH3OH and H2O with three concerted HTPT steps by Au20{H}2/Au20 redox couple


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Figure 5. Natural Population Change of H1 and H2 (Chemisorbed hydrogens on Au20(PH3)4 ) along the IRC path of Transition state TS(CO2) involved in the reduc-tion of CO2 to HCOOH.

(TS(CO2)) with a free energy activation barrier, ∆G0‡ of 14.0 kcal/mol. Intriguingly, unlike the bipolar hydrogens in ammonia-borane (AB) and 1,2PyH2 both the chemisorbed hydrogen atoms on gold surface are negatively charged (for natural population and NBO charges please see Figure 1c,d). It was befuddling how these hydridic hydrogens facilitated a HTPT reaction in reduction of CO2. So, we investigated the change of natural population along the intrinsic reaction coordinate (IRC) for the reduction step of CO2 to HCOOH, which revealed that going from the reactant to the product the natural population of the H2 atom decreases drastically indicating that it gains protic character along the reaction path and the H1 atom retains its hydridic nature along the IRC path (see Figure 5). Incidentally, this reduction mode is reminiscent of the simultenious/concerted twohydrogen transfer reduction of CO2 by AB as proposed by Zimmerman et al.43 but with a comperatively lower kinetic barrier. The CO2 reduction to HCOOH through concerted HTPT by AB (in THF) involves a free-energy barrier of 24.2 kcal/mol whereas with Au20(PH3)4{H}2 (in


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THF) the barrier is 14.0 kcal/mol which. This type of concerted HTPT is also observed in first and second row TM catalyzed dehydrogenation of ammonia-borane/amine-borane.44-45 On the other hand the reduction of CO2 by 1,2PyH2, which is reminiscent of the biological reduction of CO2 by NADPH/NADP+ redox couple, involves two sequential HTPT steps mediated by proton relay (formate ion HCOO(-) production followed by formic acid generation) with a single TS with the free-energy barrier of 14.3 kcal/mol.28 After formic acid generation we checked the possibility of a further reduction i.e. HCOOH to methanediol (CH2(OH)2): Au20(PH3)4{H}2 + HCOOH  Au20(PH3)4 + CH2(OH)2, which is known to be more challenging than that of CO2 due to its high negative electron affinity (EA) (-1.22 eV) compared to -0.60 eV of CO2.46-47 This reduction is 11.1 kcal/mol exoergic with respect to HCOOH and occurs through a concerted HTPT, TS(HCOOH) with a free energy activation barrier, ∆G0‡ of 15.2 kcal/mol (see Figure 4). Natural population fluctuation for the hydrogens (H1 and H2) along the IRC path of TS(HCOOH) revealed similar trends to those found for the first reduction step (Please see section S7 in the SI). This concerted HTPT step is analogous to the reduction of HCOOH by both AB and 1,2PyH2. For the last reduction step to happen H2C(OH)2 must convert to H2C=O. H2C(OH)2 can be considered as hydrated formaldehyde (CH2=O). This dehydration is predicted to occur via proton shuttle mechanism through the participation of -OH group of another molecule of H2C(OH)2 (TS(H2C(OH)2) at a ∆G0‡ of 14.5 kcal/mol. As expected, we found that H2C=O is finally reduced to CH3OH via another concerted HTPT step (TS(H2C=O)) with ∆G0‡ of merely 4.2 kcal/mol and the reduction (Au20(PH3)4{H}2 + CH2=O  Au20(PH3)4 +CH3OH) is highly exoergic (∆G0rxn=32.8 kcal/mol), which eventually drives the formation of CH3OH (see Figure 4). Just like the previous reduction steps, H1 and H2 are transferred as hydridic and protic hydrogens in the methanol, CH3OH and the change in natural population in H1 and H2 are shown in Figure S8 in


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the SI. Similar reactivity was also found for bare Au20{H}2 entity and the corresponding barriers also suggested that the reduction of CO2 to CH3OH would be facile. The conversion of CO2 to CH3OH is a multi-step process which involves four TSs. The Rate Determining Barrier (RDB) for this transformation is predicted to have a free energy barrier of 17.7 kcal/mol (TS(H2C(OH)2),which is the dehydration of H2C(OH)2 to give H2C=O).The highest barrier which involves the participation of Au20(PH3)4{H}2, is the second reduction step of CO2 i.e. TS(HCOOH) (∆G0‡ = 15.2 kcal/mol). On the other hand the free energy barrier for Au20NP{H}2 auto-destruction route via thermal desorption of H2 from Au20(PH3)4{H}2 (i.e. Au20(PH3)4{H}2  Au20(PH3)4 + H2) is predicted to be 18.5 kcal/mol (please see section S8 in the SI). So, one may argue that these two processes might compete. However, it must be noted that the photochemical route always populates only the hydrogenated product Au20(PH3)4{H}2 in presence of H2 under radiation and once CH3OH is formed huge thermodynamic stability (∆G0rxn=43.4 kcal/mol) would be gained, which is expected to drive the overall reduction at room tempeature. As it it has been shown that -OH bond of water can be activated by plasmonic Au20, one may anticipate similar reactivity for CH3OH under incident photon. To allay doubts if such reactions would prevent CH3OH formation we checked the barrier of reformation of methanol from the chemisorbed state on Au surface. Highly exoergic reformation of CH3OH from the chemisorbed state was found to occur through a paltry barrier of 4.5 kcal/mol (please see section S9 in the SI).


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iii)

Reduction of Pyridine to 1,2-dihydropyridine Mediated by Chemisorbed Hydrogens on Au20 1,2PyH2 has recently been identified as a potential renewable hydride which can

hydrogenate CO2 to CH3OH effectively.11 We investigated the prospect of hydrogenating Py to 1,2PyH2 using Au20(PH3)4{H}2 , which can be a potential sustainable route for the organohydride production. Unlike CO2 reduction steps Py reduction proceeds through step-wise HTPT steps (see

(H2) (H1)

8

5.6 (1.6)

6

TS(N-H) G (kcal/mol)

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4

(Py) 2

0.0 (0.0)

-0.7 (-7.2)

0

-1.9 (-11.7)

Au20(PH3)4{H}2 -2

TS(C-H)

-4

Au -6

(PyH2)

[Au20(PH3)4(H)](-)[PyH](+)

C

N

Proton transfer (H2)

P

H

-3.8 (-8.2)

Au20(PH3)4

Hydride transfer (H1)

Figure 6. Gibbs free-energy profile of the conversion of Py to 1,2PyH2 with step-wise HTPT by Au20(PH3)4{H}2/Au20(PH3)4 redox couple. The values in the parenthesis correspond to the conversion of Py to 1,2PyH2 Au20{H}2/Au20 redox couple.


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Figure 6). At first the bridged hydrogen H2 is transferred to the N atom of Py and the free energy barrier (∆G0‡) is predicted to be 6.4 kcal/mol (for optimized transition state structures see please section S10 in the SI). Natural population analysis on each points of the IRC path clearly suggests that the H2 attains a protic character as it moves towards N and when it is completely transferred to N-H it becomes truly protic (please see section S11 in the SI). The product [Au20(PH3)4{H}()[PyH](+)

is slightly stable compared to the starting reactants. Then the 2nd hydrogen (H1) is

transferred to the ortho carbon of PyH(+) as a hydride via an almost barrier less transition state (∆G0‡ = 1.2 kcal/mol) and 1,2PyH2 is formed. Note, the RDB for this reduction process is significantly lower than the barrier for desorption of H2 from the Au cluster. As shown in Figure 6, the predicted barriers for Py reduction is much lesser than that of auto-destruction of Au20(PH3)4{H}2 which clearly shows that Py reduction could be much faster than CO2 reduction by Au20(PH3)4{H}2/Au20(PH3)4 redox couple. So, one can also assume that the presence of Py in the reaction mixture can catalyze the CO2 reduction as in situ generated 1,2PyH2 can reduces CO2 via a concerted HTPT mechanism as suggested by Musgrave and co-workers.11 CONCLUSION In summary, our quantum chemical computational studies reveal that H2 activation is attainable by Au20 cluster under UV radiation. Moreover, our results affirm that the chemisorption of H2 happen through a conical intersection (CI) between S0 and S1 electronic states. These chemisorbed hydrogens are at higher chemical potential and can be used to drive several endoergic hydrogenation reactions, such as Py reduction and CO2 hydrogenation to CH3OH, without the use of sacrificial reagents. The associated activation barriers are quite low for these processes to operate at appreciable rates at room temperature.


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ASSOCIATED CONTENT Supporting Information. Methods for calculating reaction free energies, benchmarking different density functionals, further justification for model system, TDDFT spectra and figures depicting intermediates and transition states with bond parameters are presented in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT G.G. and M.S. acknowledge CSIR-SRF and CSIR-JRF fellowship respectively. A.P. acknowledges funding from BRNS. REFERENCES (1) Hammer, B.; Norskov, J. K. Why Gold is the Noblest of all the Metals. Nature 1995, 376, 238-240. (2) Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Hot Electrons Do the Impossible: PlasmonInduced Dissociation of H2 on Au. Nano Lett. 2013, 13, 240−247. (3) Mukherjee, S.; Zhou, L.; Goodman, A. M.; Large, N.; Orozco, C. A.; Zhang, Y.; Nordlander, P.; Halas, N. J. Hot-Electron-Induced Dissociation of H2 on Gold Nanoparticles Supported on SiO2. J. Am. Chem. Soc. 2014, 136, 64–67. (4) Jain, P. K.; El-Sayed, M. A. Plasmonic Coupling in Noble Metal Nanostructures. Chem. Phys. Lett. 2010, 487, 153−164. (5) Krauter, C. M.; Schirmer, J.; Jacob, C. R.; Pernpointner, M.; Dreuw, A. Plasmons in Molecules: Microscopic Characterization Based on Orbital Transitions and Momentum Conservation. J. Chem. Phys. 2014, 141, 104101.


16

Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Linic, S.; Christopher, P.; Ingram, D. B.; Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mat. 2011, 10, 911-921; (7) Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Photochemical Transformations on Plasmonic Metal Nanoparticles. Nat. Mat. 2015, 14, 567-576; (8) Lee, J.; Mubeen, S.; Ji, X.; Stucky, G. D.; Moskovits, M. Plasmonic Photoanodes for Solar Water Splitting with Visible Light. Nano Lett. 2012, 12, 5014-5019; (9) Silva, C. G.; Juarez, R.; Marino, T.; Molinari, R.; Garcia, H. Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water. J. Am.Chem. Soc. 2011, 133, 595−602; (10)

Christopher, P.; Xin, H.; Linic, S. Visible-Light-Enhanced Catalytic Oxidation

Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3, 467−472. (11)

Zhang, Z.; Kinzel, D.; Deckert, V. Photo-Induced or Plasmon-Induced Reaction:

Investigation of the Light-Induced Azo-Coupling of Amino Groups. J. Phys. Chem. C 2016, 120, 20978–20983. (12)

Zhao, L. B.; Zhang, M.; Huang, Y. F.; Williams, C. T.; Wu, D. Y.; Ren, B.; Tian,

Z. Q. Theoretical Study of Plasmon-Enhanced Surface Catalytic Coupling Reactions of Aromatic Amines and Nitro Compounds. J. Phys. Chem. Lett. 2014, 5, 1259−1266. (13)

Zhou, L.; Zhang, C.; McClain, M. J.; Manjavacas, A.; Krauter, C. M.; Tian, S.;

Berg, F.; Everitt, H. O.; Carter, E. A.; Nordlander, P.; Halas, N. J. Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen Dissociation. Nano Lett. 2016, 16, 1478–1484. (14)

Yan, L.; Wang, F.; Meng, S. Quantum Mode Selectivity of PlasmonInduced Water

Splitting on Gold Nanoparticles. ACS Nano 2016, 10, 5452–5458. (15)

Huang, Y. F.; Zhang, M.; Zhao, L. B.; Feng, J. M.; Wu, D. Y.; Ren, B.; Tian, Z. Q.

Activation of Oxygen on Gold and Silver Nanoparticles Assisted by Surface Plasmon Resonances. Angew. Chem. Int. Ed. 2014, 53, 2353 –2357.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16)

Page 18 of 21

Costentin, C.; Robert, M.; Saveant, J. M. Catalysis of the Electrochemical

Reduction of Carbon Dioxide. Chem. Soc. Rev. 2013, 42, 2423-2436. (17)

Wang, W.–H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E. CO2

Hydrogenation to Formate and Methanol as an Alternative to Photo- and Electrochemical CO2 Reduction. Chem. Rev. 2015, 115, 12936-12973. (18)

Grasemann, M.; Laurenczy, G. Formic Acid as a Hydrogen Source – Recent

Developments and Future Trends. Energy Environ. Sci. 2012, 5, 8171-8181. (19)

Olah, G. A. Beyond Oil and Gas: The Methanol Economy. Angew. Chem. Int. Ed.

2005, 44, 2636-2639. (20)

Menard, G.; Stephan, D. W. Room Temperature Reduction of CO2 to Methanol by

Al-Based Frustrated Lewis Pairs and Ammonia Borane J. Am. Chem. Soc. 2010, 132, 1796 –1797. (21)

Roy, L.; Zimmerman, P. M.; Paul. A. Changing Lanes from Concerted to Stepwise

Hydrogenation: The Reduction Mechanism of Frustrated Lewis Acid-Base Pair Trapped CO2 to Methanol by Ammonia-Borane. Chem. Eur. J. 2011, 17, 435 – 439. (22)

Leitner, W. Carbon Dioxide as a Raw Material: The Synthesis of Formic Acid and

Its Derivatives from CO2. Angew. Chem. Int. Ed. 1995, 34, 2207-2221. (23)

Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.; Kubiak, C. P.

Photochemical and Photoelectrochemical Reduction of CO2. Annual. Rev. Phys. Chem. 2012, 63, 541-569. (24)

Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and

Homogeneous Approaches to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 89−99. (25)

Morris, A. J.; Meyer, G. J.; Fujita, E. Molecular Approaches to the Photocatalytic

Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42, 1983-1994.


18

Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26)

Oh, Y.; Hu, X. Organic Molecules as Mediators and Catalysts for Photocatalytic

and Electrocatalytic CO2 Reduction. Chem. Soc. Rev. 2013, 42, 2253-2261 (27)

Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B. Selective Solar-Driven Reduction

of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell. J. Am. Chem. Soc. 2008, 130, 6342-6344. (28)

Lim, C.–H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B. Reduction of CO2 to

Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine. J. Am. Chem. Soc. 2014, 136, 16081-16095. (29)

Lim, C.–H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B. Catalytic Reduction of

CO2 by Renewable Organohydrides. J. Phys. Chem. Lett. 2015, 6, 5078-5092. (30)

Ganguly, G.; Malakar, T.; Paul, A. In Pursuit of Sustainable Hydrogen Storage with

Boron-Nitride Fullerene as the Storage Medium. ChemSusChem 2016, 9, 1386 –1391. (31)

Li, L.; Gao, Y.; Li, H.; Zhao, Y.; Pei, Y.; Chen, Z.; Zheng, X. C. CO Oxidation on

TiO2 (110) Supported Subnanometer Gold Clusters: Size and Shape Effects. J. Am. Chem. Soc. 2013, 135, 19336−19346. (32)

Li, Z-Y.; Yuan, Z.; Li, X-N.; Zhao, Y-X.; He, S-G. CO Oxidation Catalyzed by

Single Gold Atoms Supported on Aluminum Oxide Clusters. J. Am. Chem. Soc. 2014, 136, 14307−14313. (33)

Manzoor, D.; Pal, S. Hydrogen Atom Chemisorbed Gold Clusters as Highly Active

Catalysts for Oxygen Activation and CO Oxidation. J. Phys. Chem. C 2014, 118, 30057−30062. (34)

Li, J.; Li, X.; Zhai, H.-J. Wang, L.-S. Au20: A Tetrahedral Cluster. Science 2003,

299, 864-867. (35)

Kryachko, E. S.; Remacle, F. The Magic Gold Cluster Au20. Int. J. Quant. Chem.

2007, 107, 2922-2934.


19


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Page 20 of 21

Long, R.; Prezhdo, O. V. Instantaneous Generation of Charge-Separated State on

TiO2 Surface Sensitized with Plasmonic Nanoparticles. J. Am. Chem. Soc. 2014, 136, 4343–4354. (37) Zhang, H. F.; Stender, M.; Zhang, R.; Wang, C.; Li, J.; Wang, L.S. Toward the Solution Synthesis of the Tetrahedral Au20 Cluster. J. Phys. Chem. B 2004, 108, 1125911263. (38) Idrobo, J. C.; Walkosz, W.; Yip, S. F.; Öğüt, S.; Wang, J.; Jellinek, J. Static Polarizabilities and Optical Absorption Spectra of Gold Clusters (Aun, n=2–14 and 20) from first principles Phys. Rev. B. 2007, 76, 205422. (39) Lisinetskaya, P. G.; Braun, C.; Proch, S.; Kim, Y. D.; Ganteför, G.; Mitrić, R. Excited State Nonadiabatic Dynamics of Bare and Hydrated Anionic Gold Clusters Au3 [H2O]n (n = 0–2). Phys. Chem. Chem. Phys. 2016, 18, 6411-6419. (40)

Burgess, R. W.; Keast, V. J. TDDFT Study of the Optical Absorption Spectra of

Bare Gold Clusters. J. Phys. Chem. C 2014, 118, 3194−3201. (41)

Guidez, E. B.; Aikens, C. M.; Origin and TDDFT Benchmarking of the Plasmon

Resonance in Acenes J. Phys. Chem. C 2013, 117, 21466−21475. (42)

Gaussian 09, Revision A.1; see Supporting Information for full reference.

(43)

Zimmerman, P. M.; Zhang, Z.; Musgrave, C. B. Simultaneous Two-Hydrogen

Transfer as a Mechanism for Efficient CO2 Reduction. Inorg. Chem. 2010, 49, 8724-8728. (44)

Paul, A.; Musgrave, C. B. Catalyzed Dehydrogenation of Ammonia–Borane by

Iridium Dihydrogen Pincer Complex Differs from Ethane Dehydrogenation. Angew. Chem. 2007, 119, 8301 –8304. (45)

Ganguly, G.; Malakar, T.; Paul, A. Theoretical Studies on the Mechanism of

Homogeneous Catalytic Olefin Hydrogenation and Amine−Borane Dehydrogenation by a Versatile Boryl-Ligand-Based Cobalt Catalyst. ACS Catal. 2015, 5, 2754−2769. (46)

Yoshioka, Y.; Schaefer, H. F.; Jordan, K. D. Theoretical Investigation of the

Electron Affinity of CO2. J. Chem. Phys. 1981, 75, 1040. (47)

Valadbeigi, Y.; Farrokhpour, H. DFT, CBS-Q, W1BD and G4MP2 Calculation of

the Proton and Electron Affinities, Gas Phase Basicities and Ionization Energies of


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Saturated and Unsaturated Carboxylic Acids (C1–C4). Int. J. Quantum Chem. 2013, 113, 1717-1721.

TOC GRAPHICS

H

H H

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+ H

+++ SPR

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Photochemical Hydrogenation of CO2 to CH3OH and Pyridine to 1, 2

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