Silicon Nanocages for Selective Carbon Dioxide Conversion under

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Silicon Nanocages for Selective Carbon Dioxide Conversion under Visible Light Si Zhou, Xiaowei Yang, Wei Pei, Jijun Zhao, and Aijun Du J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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The Journal of Physical Chemistry

Silicon Nanocages for Selective Carbon Dioxide Conversion under Visible Light Si Zhou,†,‡ Xiaowei Yang,† Wei Pei,† Jijun Zhao,*,† and Aijun Du*,‡ †Key

Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Dalian University

of Technology), Ministry of Education, Dalian 116024, China ‡School

of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty,

Queensland University of Technology, Gardens Point Campus, Brisbane, QLD 4001, Australia

Abstract Artificial photosynthesis for CO2 conversion to fuels and value-added chemicals is a tactic to close the anthropogenic carbon cycle. To this end, developing efficient catalysts composed of earth-abundant, economic and eco-friendly elements is desirable but challenging. By comprehensive ab initio calculations, we show for the first time that caged silicon clusters doped by vanadium atom (VSin, n = 12-15) can catalyze CO2 hydrogenation to various C1 products (i.e., carbon monoxide, formic acid, formaldehyde, methanol and methane) with kinetic barriers down to 0.67~1.53 eV and selectivity uniquely determined by the cluster size and geometry. These clusters, which can be produced in laboratory with good stability, have suitable energy gap and can absorb sun light from visible to ultraviolet regime for driving the catalysis. Hence, these metal-doped Si clusters form a potential family of photocatalysts for selective CO2 hydrogenation. Their superior catalytic activity stems from the unsaturated states of Si cage, which are mediated by the sp-d hybridization and V-Si charge transfer. The CO2 adsorption strength is correlated to the coordination number and p orbital center of Si atoms. Such geometry-electronic structure-activity correlation should be applicable to atomically precise design of novel silicon-based nanocatalysts for various renewable energy applications.

*Corresponding *Corresponding

authors. Email: [email protected], Phone: 86-0411-84709748 (J. Zhao) authors. Email: [email protected], Phone: 61-7-3138-6980 (A. Du) 1

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1. Introduction With growing energy crisis and global warming, developing artificial photosynthesis that converts carbon dioxide (CO2) to fuels and value-added chemicals by solar energy has drawn increasing attentions. Semiconductors are usually exploited for CO2 photo-conversion.1 As the most abundant semiconductor on earth, silicon crystal has preferable bandgap for visible-light absorption, and is considered a key priority for future energy research. Early studies showed that p-type silicon crystal can photo-reduce CO2 to carbon monoxide and formic acid under large negative bias voltages.2-4 The catalytic activity and selectivity of silicon can be improved by chemical functionalization,5 modification with metal nanoparticles,6 and sculpturing into nanostructures.7 However, the performance of reported silicon-based catalysts remains ineligible for industrial applications yet. Clusters, composed of few to hundreds of atoms, exhibit extraordinary physicochemical properties for catalysis.8-10 Experiments showed that Cu4 clusters supported on Al2O3 substrate can catalyze CO2 hydrogenation to methanol under near-atmospheric reaction conditions.11 Doping a foreign atom (Au, Pd, Pt and Ag) into the center of Ag25 clusters promotes the catalytic carboxylation of CO2 with terminal alkyne to produce propiolic acid.12 Ab inito calculations predicted the unique selectivity of the ligand-protected Cu32H20 clusters for CO2 reduction to formic acid.13 Metal-modified Mo6S8 clusters and Pd4 clusters on In2O3 substrate were proposed to selectively catalyze CO2 hydrogenation to methanol.14-15 Apparently, adding, removing or substituting a single atom can substantially alter the geometrical and electronic structures of atomic clusters, which allows precise control of their catalytic behaviors.9, 16 Silicon clusters, with enhanced surface reactivity than their bulk counterpart, possess many merits to be catalysts such as high atomic utilization, nontoxicity, non-precious and earth-abundant elemental composition. However, bare silicon clusters have poor stability, making their structures and properties hard to control.17,18 Kumar et al. predicted that silicon cage clusters can be stabilized by encapsulating a transition metal atom in the cage.19 Very soon, various gas phase metal-doped silicon 2


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clusters (MSin) were prepared in experiment, with selectable cluster size n up to 20 and dopant elements covering most of 3d, 4d and 5d transition metal.20-24 Recently, Nakajima’s group developed a large-scale synthesis method and produced TiSi16 and TaSi16 clusters in 100 mg scale.25 Moreover, these clusters can be deposited and immobilized on certain substrates and exhibit excellent thermal stability — keeping their original structures without aggregation at over 500 K and showing high oxidation resistance.26-28 The geometries and electronic structures of MSin clusters highly depend on the doping element and cluster size, endowing them tunable electronic gap,29 magnetic moment30-31 and hyperpolarizability.32 Hence, the family of MSin clusters with rich chemical compositions and controllable physicochemical properties provide an ideal platform for designing novel silicon-based catalysts. To our knowledge, however, the catalytic properties of MSin clusters have not been explored yet. Here we exploit vanadium-doped Si clusters (VSin, n = 12-16) for catalytic CO2 hydrogenation by first-principles calculations. These clusters in the gas phase have been produced in laboratory, with confirmed endohedral caged structures.22-24,

31, 33

We show that the VSin clusters with n = 12-15 are effective catalysts for CO2 hydrogenation with distinct product selectivity and can utilize the solar energy from visible to ultraviolet regime. The reaction pathways, free energies and kinetic barriers are systematically calculated to elucidate the dependence of catalytic performance on the cluster size and geometry. A correlation between activity and electronic structures is further established, providing the key descriptors for precisely tailoring the catalytic properties of the VSin clusters.

2. Computational methods The geometries, energetic and electronic properties, and optical absorption of neutral VSin (n = 12-16) clusters were investigated by density functional theory (DFT) calculations implemented in the Gaussian09 package34 with aug-cc-pVDZ basis set and the Heyd-Scuseria-Ernzerhof (HSE06) functional.35 The ground state structures of VSin clusters were searched independently by our homemade 3



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comprehensive genetic algorithm (CGA) code36 combined with DFT optimization (see S1 of Supporting Information (SI) for the details of CGA-DFT search). Our previous studies demonstrated that the HSE06 functional can accurately describe the structural and electronic properties of pristine and metal doped Si clusters.37-38 The optical absorption spectra were computed by the time-dependent density functional theory (TDDFT). The natural population analysis (NPA) was conducted using the Natural Bond Orbital (NBO) 3.1 program39 implemented in the Gaussian09 package. To describe the interactions of molecules or reaction intermediates with the VSin clusters, the London dispersion interaction has to be properly included, which however is not implemented under the HSE06 scheme in the Gaussian09 program. To investigate the catalytic properties of VSin clusters, here we employed the Vienna ab initio simulation package (VASP) with the planewave basis set (energy cutoff of 500 eV),40 projector augmented wave (PAW) potentials,41 HSE06 functional, and Grimme’s DFT-D3 scheme for dispersion correction.42 A cubic supercell with dimension of 20 Å was adopted, and its Brillouin zone was sampled by the  point. According to our test calculations (Table S1-S3 in SI), VASP and Gaussian09 both using the HSE06 hybrid functional give nearly the same electronic structures of the VSin clusters, while the former with D3 correction yields more reliable results on adsorption properties. The Gibbs free energy of a molecule or reaction intermediate adsorbed on the VSin cluster was computed as: G = EDFT + ZPE – TS

(1)

where EDFT is DFT total energy, ZPE is zero-point energy, S is entropy, T is temperature and set to be 300 K here. The values of ZPE and S of gaseous molecules can be acquired from the NIST-JANAF thermochemical table.43 For reaction intermediates, we calculated their vibrational frequencies and then obtained ZPE and S under the thermodynamics model (Table S4 in SI).44 To determine the transition states and kinetic barriers for each elemental step of CO2 hydrogenation reactions, we used the climbing-image nudged elastic band (CI-NEB) method45 implemented in VASP with five images to mimic the reaction 4


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path. The intermediate images were relaxed until the perpendicular forces were smaller than 0.02 eV/Å.

3. Results and discussion 3.1. Comparison of activity of MSi12 clusters For most transition metal doped silicon clusters (MSin), complete encapsulation of the guest metal atom usually occurs for n ≥ 12.29, 46 Therefore, we first examine the activity of Si12 clusters doped by various 3d transition metal atoms (M = Ti, V, Cr, Mn, Fe, Co, Ni). Their ground state structures were reported by previous studies31, 47 and confirmed by our DFT based CGA search. As shown in Figure 1a, all these MSi12 clusters have cage structures with metal atoms embedded in the center. Among them, the Ti, V, Cr, Mn and Fe doped cages are (modified) hexagonal prisms, while the Co and Ni doped clusters adopt bicapped pentagonal prisms. To preliminarily assess the activity of various MSi12 clusters, we calculate the adsorption energy and kinetic barrier for chemisorption of a CO2 molecule, as well as the barrier for dissociation of a H2 molecule on the clusters, both of which are important steps for CO2 hydrogenation. A CO2 molecule has to overcome barriers of 0.59 ~ 1.14 eV to transit from physisorption to chemisorption, then it can chemisorb on the MSi12 clusters with adsorption energies of −0.67 ~ +0.06 eV and (Figure 1b). Meanwhile, dissociation of a H2 molecule involves barriers of 0.66 ~ 1.69 eV. Among these MSi12 clusters explored, VSi12 has suitable adsorption strength with CO2 (−0.43 eV), and the lowest barriers for CO2 chemisorption and H2 dissociation (0.60 and 0.66 eV, respectively). TiSi12 cluster exhibits similar activity as that of VSi12, with slightly higher barrier for H2 dissociation (0.86 eV). The other MSi12 clusters have slow kinetics to decompose H2 (barriers ≥ 1.40 eV). Therefore, we infer that V doped silicon clusters are potential catalysts for CO2 hydrogenation; thus their size-dependent catalytic behavior is further explored hereafter.

5



Figure 1. (a) Ground state structures of various transition metal doped Si12 clusters (MSi12). The Si and metal atoms are shown in yellow and silver (or cyan, purple, green) colors, respectively. (b) Kinetic barrier (Ea) for chemisorption of CO2 and dissociation of H2 molecule (top panel) and adsorption energy of CO2 molecule (bottom panel) on various MSi12 clusters.

3.2. Geometrical and electronic structures of VSin clusters The ground state structures of VSin (n = 12-16) clusters from CGA search are presented in Figure 2. As cluster grows bigger, the cage geometry transforms from hexagonal prism to Frank-Kasper-like polyhedron, resembling those of TiSin and CrSin clusters reported previously.29, 46 The equilibrium Si–Si and Si–V bond lengths are in the range of 2.34 ~ 2.60 Å and 2.60 ~ 2.78 Å, respectively. With increasing cluster size, the VSin clusters comprise more triangular facets, and the averaged coordination number of surface Si atoms rises from 4.0 to 5.2, indicating that the Si atoms become more saturated and might be more inert. For n ≥ 17, an apical Si bud starts to evolve on the VSin cage.48 Intuitively, such structures may be over-reactive and have poor stability during catalytic reactions, and thus are not considered in this work. The key electronic properties of the VSin clusters are summarized in Table 1. The binding energies of these clusters are lower than those of pure Sin clusters by about 0.2 eV/atom, indicating enhanced stability of silicon clusters upon V doping. 6


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Moreover, all these clusters have open electronic shells and carry a small magnetic moment of 0.65 ~ 1.10 μB. The gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) ranges from 1.45 to 2.54 eV and 1.03 to 2.57 eV for the spin-up and spin-down states, respectively. These gap amplitudes are suitable for absorption of visible light energy, which may be utilized for driving the catalysis of CO2 hydrogenation as will be discussed later. Table 1. Structural, energetic and electronic properties of VSin (n = 12-16) clusters, including binding energy per atom (EB), total magnetic moment (M), electron transfer from Si atoms to V atom (CT), average coordination number of Si atoms (CN), average NAO bond order (BO) of V–Si bonds, p orbital center of Si atoms (εp), and HOMO-LUMO gap of spin-up and spin-down states (EHL↑ and EHL↓, respectively). The binding energy is the energy of VSin cluster relative to the energies of individual Si and V atoms, and the numbers in parentheses are the EB values of pure Sin clusters with the same n Property

VSi12

VSi13

VSi14

VSi15

VSi16

–4.04

–4.07

–4.09

–4.15

–4.17

(–3.85)

(–3.92)

(–3.98)

(–3.99)

(–3.97)

M (μB)

0.70

1.10

0.89

0.65

0.94

CT (e)

3.08

3.83

4.03

4.26

4.33

CN

4.00

4.69

5.00

5.07

5.19

BO

0.61

0.59

0.55

0.53

0.51

εp (eV)

–7.18

–7.32

–7.33

–7.40

–7.58

EHL↑ (eV)

2.54

1.70

2.44

1.70

1.45

EHL↓ (eV)

1.03

2.25

0.90

1.45

2.57

EB (eV/atom)

7



Figure 2. Atomic structures of VSin (n = 12-16) clusters in their ground states. The Si and V atoms are shown in yellow and cyan colors, respectively.

3.3. CO2 chemisorption and H2 dissociation For all the considered VSin clusters, a CO2 molecule can chemisorb on the cluster with adsorption energy of −0.43 ~ +0.22 eV (Figure 3a). The adsorption strength weakens as the cluster size increases. The chemisorbed CO2 molecule is bended with O-C-O angle of 122 ~ 125° since the C atom and one O atom form covalent bonds with Si atoms. The CO2 molecule gains about 0.82 ~ 0.87 electrons from the VSin cluster. From physi- to chemi-sorption, it has to overcome a moderate barrier of 0.53 ~ 0.79 eV (Figure 3b). The most active sites for CO2 adsorption are the Si atoms on the square (VSi15, VSi16), pentagon (VSi14) or hexagon (VSi12, VSi13) faces of the clusters. On the contrary, the Si atoms on the vertex of triangular facets have higher coordination number and thus inert for CO2 adsorption. Dissociation of H2 molecule on the VSin clusters involves a barrier of 0.64 ~ 1.20 eV, leading to the formation of two separated H adatoms (Figure 3b, Figure S3 in SI). The adsorption energy of an H atom on the cluster is −0.27 ~ +0.06 eV (Figure 3a). Overall speaking, VSi12 and VSi14 have low barriers ≤ 0.69 eV for CO2 chemisorption and H2 dissociation. VSi13 and VSi15 have to overcome higher barriers of 0.74 and 0.79 eV to activate CO2, and 0.90 and 0.64 eV to dissociate a H2 molecule, respectively. VSi13 is relatively inert to H2 and weakly binds with H* species, which would affect the kinetics of CO2 hydrogenation reaction as will be shown later. The VSi16 cage with high coordination number comprises only triangular facets and 8


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weakly adsorbs CO2 and H2 molecules, and thus will not be considered in the following.

Figure 3. (a) Adsorption energy (ΔEX*) of CO2, CO, HCOOH and H2 on VSin (n = 12-16) clusters (ΔEX* = EX* – E* – EX, where EX* and E* are the energies of a catalyst cluster with and without chemisorption of molecule X, respectively; EX is the energy of a free molecule. The chemisorbed H2 dissociates into two H atoms). The black line is a linear fitting of ΔECO2*. The structures of VSin clusters chemisorbed with a CO2 molecule are shown on the right. The C, O, Si and V atoms are represented in grey, red, yellow and cyan colors, respectively. (b) Kinetic barrier (Ea) for chemisorption of CO2 and dissociation of H2 molecule, and the barrier of rate-limit step for CO2 hydrogenation to various products on the VSin clusters.

3.4. Mechanism of CO2 hydrogenation Figure 4 and Figure S1, S2 in SI display the free energy diagrams of the most efficient pathway of CO2 hydrogenation on the VSin clusters to C1 products — carbon monoxide (CO), formic acid (HCOOH), formaldehyde (CH2O), methanol (CH3OH) and methane (CH4) (we use * to indicate reaction intermediates adsorbed on the clusters hereafter, while the formula without * represents a free molecule). Generally, 9



the reaction proceeds through either the formate pathway (HCOO*) to form HCOOH, or the carboxyl pathway (COOH*) to produce CO, with the former path being lower in energy by 0.32 ~ 0.72 eV than the latter one. For VSi12 and VSi14, the binding strengths of CO* and HCOOH* species are relatively strong, i.e. the adsorption energies are −0.76 and −0.81 eV for CO*, −0.68 and −0.58 eV for HCOOH*, respectively (Figure 3a). As a result, further hydrogenation of CO* and HCOOH* species is possible,49 thus forming HCO* species and then leading to CH2O, CH3OH and CH4 products. On the other hand, VSi13 and VSi15 weakly bind with CO* and HCOOH* species with adsorption energies of −0.31 ~ 0.09 eV, indicating their easy desorption into gaseous molecules. Therefore, VSi13 and VSi15 may selectively catalyze CO2 hydrogenation to CO and HCOOH products, while formation of CH2O, CH3OH and CH4 on VSi12 or VSi14 catalysts is also possible. Upon chemisorption of molecules and reaction intermediates, all the considered clusters maintain their geometrical structures without notable deformation of the cage framework, showing the robustness of metal-doped silicon cages as catalysts.

Figure 4. Free energy diagram of CO2 hydrogenation on the VSi12 cluster. The colored line segments indicate the formation of various products. The atomic structures of reaction intermediates are presented for each step. The H, C, O, Si and V atoms are shown in white, grey, red, yellow and cyan colors, respectively.

We calculate the kinetic barrier of each elemental step by adsorbing a H* species 10


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in adjacent to the reaction intermediate on the VSin clusters (see Table 2 and 3, Figure 5, Figure S3 in SI). As revealed by Figure 3b, the considered four clusters exhibit distinct product selectivity. VSi12 is highly selective toward CH2O, by going through COOH*, HCOOH* and HCO* intermediates. The rate-limit step is hydrogenation of HCO* to form CH2O with a barrier of 0.67 eV. Formation of CO, HCOOH, CH3OH and CH4 involves higher barriers of 1.03, 1.39, 1.28 and 1.53 eV, respectively. HCOOH production is limited by the reaction of CO2* and H* species to form HCOO* with a barrier of 1.39 eV. In comparison, formation of COOH* and its further hydrogenation to CO*, HCOOH*, HCO*, HCOH* and H2COH* intermediates are kinetically readily with barriers of 0.41 ~ 0.96 eV, which favorably produce CO and CH3OH in the end. Formation of CH4 by proceeding through CH2* and CH3* intermediates is relatively more difficult and involves higher barriers of 1.19 ~ 1.53 eV. VSi14 follows similar reaction pathways as those of VSi12 (Figure S1 in SI), but has higher selectivity toward HCOOH with a barrier of 0.75 eV. Formation of CO and CH2O has to overcome higher barriers of 1.38 and 1.04 eV, respectively, while CH3OH and CH4 productions suffer from slow kinetics with high barriers up to 1.66 and 1.87 eV, respectively. For VSi13 and VSi15 with weak binding capability, CO* and HCOOH* prefer to desorb as gaseous molecules at ambient conditions rather than further hydrogenation (Table 2, Figure S2 in SI). VSi15 can promote formations of both CO and HCOOH with barriers of 1.00 and 1.18 eV, while VSi13 has slow kinetics due to the unfavorable interaction with H* species and higher barriers of 1.83 and 1.44 eV, respectively. In short, VSi12 and VSi14 possess excellent activity and selectivity for catalysis of CO2 hydrogenation to CH2O and HCOOH products, respectively, which could be operated at room temperature. At elevated temperatures, other C1 products are also possible. VSi13 and VSi15 have relatively lower activity, and are selective for HCOOH and CO products, respectively. The barriers of rate-limit steps to various C1 products for the current ViSin clusters are quite competitive to the reported theoretical values of metal catalysts used in experiment, e.g., 1.60 ~ 1.80 eV and 0.88 ~ 1.20 eV for CH3OH formation on Cu(111) and Zn modified Cu surface, respectively,49-52 0.83 eV 11



for Ni(111) to produce HCOOH,53 1.08, 1.18 and 1.69 eV for CO, CH3OH and CH4 formation on the Cu4 cluster supported by Al2O3, respectively,11 and are also comparable to the theoretically proposed Ti2CO2 MXene catalyst with 0.53, 0.75, 1.32 and 1.92 eV barriers for HCOOH, CH2O, CH3OH and CH4 formation, respectively.54

Figure 5. Atomic structures of reactants and products for the elemental steps of CO2 hydrogenation to form CH2O, CH3OH and CH4 on the VSi12 cluster. The red numbers and the structure next to them show the kinetic barrier (in eV) and transition state for each step, respectively. The H, C, O, Si and V atoms are shown in white, grey, red, yellow and cyan colors, respectively.

12


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Table 2. Heat of reaction (ΔH, defined as the difference in DFT total energies between the initial and final states) and kinetic barrier (Ea) of the elemental steps of CO2 hydrogenation to CO and HCOOH products on VSin (n = 12-15) clusters. The energies are given in unit of eV VSi12

Reaction

VSi13

VSi14

VSi15

ΔH

Ea

ΔH

Ea

ΔH

Ea

ΔH

Ea

H2 → 2H*

–0.55

0.66

0.06

0.90

–0.32

0.69

0.09

0.64

CO2 → CO2*

–0.43

0.60

–0.31

0.74

–0.21

0.53

0.04

0.79

CO2* + H* → HCOO*

–0.08

1.39

–0.31

1.44

–1.21

0.73

–0.87

0.74

HCOO* + H* → HCOOH (g)

0.49

0.97

0.20

1.10

0.75

0.75

0

1.18

CO2* + H* → COOH*

0.24

0.49

0.41

1.83

–0.77

0.30

–0.43

0.72

COOH* + H* → CO* + H2O

0.44

1.03

0.33

1.29

0.33

1.38

0.38

1.00

CO* → CO (g)

0.76

0.76

0.31

0.31

0.81

0.81

0.06

0.06

Table 3. Heat of reaction (ΔH) and kinetic barrier (Ea) of elemental steps of CO2 hydrogenation to CH2O, CH3OH and CH4 products on the VSi12 and VSi14 clusters. The energies are given in unit of eV VSi12

Reaction

VSi14

ΔH

Ea

ΔH

Ea

CO* + H* → HCO*

–0.33

0.32

–1.03

0.78

COOH* + H* → HCOOH*

–0.68

0.41

–0.13

1.04

HCOOH* + H* → HCO + H2O

0.49

0.60

–0.28

0.81

HCO* + H* → CH2O (g)

0.10

0.67

0.56

0.72

HCO* + H* → HCOH*

–0.53

0.53

0.10

0.98

HCOH* + H* → H2COH*

–0.42

0.96

–1.18

1.40

H2COH* + H* → CH3OH (g)

0.16

1.28

0.38

1.66

H2COH* + H* → CH2* + H2O

–0.74

1.19

0.01

1.18

CH2* + H* → CH3*

–0.56

1.44

–1.11

0.20

CH3* + H* → CH4 (g)

0

1.53

0.36

1.87

13



For the energetically demanding catalytic processes, harvesting sun light to drive the reactions through photocatalysis or photo-thermal catalysis can reduce heat input and improve reaction rate.55-57 As displayed in Figure 6, our considered VSin clusters show high intensities of optical absorption in visible and ultraviolet regimes. This effect is beneficial from their moderate HOMO-LUMO gaps ranging from 1.03 to 2.57 eV. Therefore, these VSin clusters can capture a large portion of sunlight, which provides opportunities for photocatalysis or photo-thermal catalysis of CO2 hydrogenation.

Figure 6. Optical absorption spectra of VSin (n = 12-15) clusters.

3.5. Activity-electronic structure relationship The activity of the VSin clusters is ascribed to the unsaturated states of Si atoms. Hollow Si cages are sp2 hybridized with dangling pz orbitals.58 Doping V atom into the cage help saturate these dangling states by forming V−Si bonds. Indeed, hybridization between the s, p orbitals of Si atoms and the d orbital of V atom is revealed by the density of states (DOS) and frontier molecular orbitals in Figure 7a, b. According to natural population analysis, there is a total charge transfer of 3.08 ~ 4.33 electrons from Si to V. The V−Si bond order is 0.51 ~ 0.61 (Table 1), indicating a mixed covalent-metallic bond nature.19 The sp-d hybridization and charge transfer between Si and V atoms significantly stabilize the VSin clusters, as demonstrated by the enhanced binding energies of VSin with regard to pristine Sin clusters (Table 1). 14


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Note that the dangling states of Si atoms are not fully saturated by V doping. Differential charge densities show that there is still electron accumulation on the Si cage, and the dangling states are weakened as the cluster size increases (Figure 7c). Consequently, the adsorption strength of CO2 molecule follows a linear relation with the cluster size and the coordination number of Si atoms (Figure 3a). To gain deeper insights, we examine the electronic interaction between a CO2 molecule and VSi12 cluster as depicted in Figure 7a. For a gaseous CO2 molecule, its 2π* orbital as LUMO locates at –0.61 eV relative to the vacuum level. Bending CO2 to O–C–O angle of 123° leads to substantial lowering of the 2π* orbital to –5.36 eV, which can accept the electrons from the HOMO of VSi12 cluster at –6.08 eV. The projected density of states shows that the Si p orbitals strongly interact with the 2π* orbital of CO2 molecule. Interestingly, the adsorption strength of CO2 is correlated with the center of Si p orbital (Figure 7c, Figure S4 in SI), which is defined as59-60 εp = ∫ ED(E)dE / ∫ D(E)dE

(2)

where D(E) is the DOS of Si p orbitals at a given energy E; the integral is performed for all the occupied states. Roughly speaking, the VSin cluster with smaller size has more unsaturated states, and its Si p orbitals are located at higher energy levels, which in turn would favorably interact with the 2π* orbital of CO2 molecule61 and lead to stronger binding with CO2. Therefore, the p orbital level reflecting the degree of saturation of Si dangling bonds can serve as a descriptor for modulating the CO2 adsorption strength. Such correlation between activity and electronic structure may be exploited for design of other metal-doped silicon clusters as catalysts of various chemical processes by taking advantage of their tunable electronic structures with proper selection of cluster size and doping element.

4. Conclusion In summary, our first-principle calculations show that vanadium-doped silicon clusters, i.e., VSin (n = 12-15), are promising photocatalysts for CO2 hydrogenation. These cage clusters possess satisfactory stability and high activity mediated by the sp-d hybridization and charge transfer between Si and V atoms. The selectivity to 15



various C1 products is uniquely determined by the cluster size and geometry. As cluster size increases, the dangling states of Si cage gradually diminish and the adsorption strength of CO2 molecule is weakened, which can be correlated to the coordination number and p orbital level of Si atoms. Moreover, the VSin clusters can capture a large portion of sunlight from visible to ultraviolet regime for driving the catalytic reactions, and may achieve high conversion efficiency at low temperatures. These theoretical results illuminate the great potential of metal-doped silicon clusters as an unprecedented family of efficient photocatalysts for CO2 conversion, and provide vital guidelines for tailoring their catalytic behaviors at atomistic precision.

Figure 7. (a) From left to right: density of states (DOS) of a free CO2 molecule, bended CO2 molecule (with C–O–C angle of 123°), CO2 chemisorbed on the VSi12 cluster, and a standalone VSi12 cluster, respectively. The energy is relative to vacuum. The occupied states are shadowed. The colored lines show the projected DOS from different atoms. The insets 16


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display the charge density distributions of HOMO and LUMO for the CO2 molecule. (b) HOMO and LUMO charge density distributions of the VSi12 cluster. (c) The p orbital center (εp, black symbols) and coordination number (CN, red symbols) of Si atoms as a function of the size of VSin (n = 12-16) clusters. The insets display the differential charge density distributions between the V atom and Si cage. The electron accumulation and depletion regions are represented by red and green colors, respectively, using an isosurface value of 2 × 10–3 e/Å3.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available. Computational details for searching global minimum structures, energetic and electronic properties calculated by different methods, zero-point energy and entropic correction, free energy diagrams of CO2 hydrogenation on VSin (n = 13-15) clusters, transition state structures, density of states.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (J. Z.). *Email: [email protected] (A. D.).

Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (11574040), the Fundamental Research Funds for the Central Universities of China (DUT17LAB19). A.D. also acknowledges Australian Research Council under Discovery Project (DP170103598). The authors acknowledge the computer resources provided by Queensland University of Technology, NCI National Facility, the Pawsey Supercomputing Centre through the National Computational Merit Allocation Scheme, and the Supercomputing Center of Dalian University of Technology.

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745x298mm (72 x 72 DPI)


Silicon Nanocages for Selective Carbon Dioxide Conversion under

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