Exploring the Catalytic Properties of Unsupported and TiO2-Supported Cu5 Clusters: CO2 Decomposition to CO and CO2 Photoactivation.

In this work, we explore the decomposition of CO2 on unsupported and TiO2-supported Cu5 clusters via computational modeling, using both finite cluster and periodic slab structures of the rutile TiO2(110) surface. While the energy needed for C=O bond breaking is already significantly reduced upon adsorption onto the unsupported metal catalyst (it drops from 7.8 to 1.3 eV), gas desorption before bond activation is still the inevitable outcome due to the remaining barrier height even at 0 K. However, when the Cu5 cluster itself is supported on TiO2, reactant and product adsorption is strongly enhanced, the barrier for bond breaking is further reduced, and a spontaneous decomposition of the molecule is predicted. This finding is linked to our previous work on charge-transfer processes in the Cu5-TiO2 system triggered by solar photons, since a combination of both phenomena at suitable temperatures would allow for a photoinduced activation of CO2 by sunlight.

Introduction

During the past few years, highly stable metal clusters of subnanometer size, as required in industrial applications, have emerged as a new generation of catalysts and photocatalysts with appealing properties arising from their molecule-like electronic structures. As opposed to metal nanoparticles in the visible region,[1] these “atomic” or subnanometer-sized clusters do not sustain their metallicity and do not show plasmonic behavior. Instead, the presence of a molecule-like HOMO–LUMO gap impacts their chemical and physical properties, making them innovative materials for applications including luminescence,[2] sensing,[3] therapeutics,[4] energy conversion,[5] catalysis,[6] and electrochemical applications.[7,8] In particular, Cu5 clusters have been shown to be less susceptible to oxidation than larger systems like Cu8 or Cu20 and have therefore been proposed as promising catalysts.[9,10] Moreover, it has been observed that Cu5 clusters are stable against oxidation up to a temperature as high as 423 K.[10] These clusters can be synthesized by kinetic control using electrochemical methods,[11] showing an exceptional chemical and thermodynamical stability in solution over the whole pH range.[11] As discussed in ref (10), high monodispersity of synthesized Cu5 clusters has been shown since the method of cluster synthesis was extremely size-selective. As compared with closed-shell cases, the outer unpaired electrons of open-shell clusters such as the Cu5 cluster are expected to be more active in chemical reactivity either by sharing or transferring them. For all these reasons, we have chosen the Cu5 as the potential catalysts in this work.

This article addresses the decomposition of CO2 over Cu5 into CO due to its potential relevance in the context of climate change and global warming (see ref (12) for a very recent and comprehensive review on heterogeneous CO2 reduction). The CO2 transformation onto copper clusters into methanol has recently attracted much attention.[13,14] A deterrent in the CO2 elimination is the high stability of the C=O bond, which necessitates an energy as high as 7.8 eV[15] in order for it to be broken in the gas phase. The catalytic properties of metal clusters can be optimized through suitable supporting materials, which affect their geometry and electronic structure as desired.[16] For instance, the electronic structure of Au8 clusters is strongly influenced by the MgO support which increases its CO oxidation reactivity.[17] For Cu4 clusters, it could be shown that the Al2O3 support is lowering the energetic barrier for C=O bond-breaking to less than 1 eV due to the strong interaction of the CO2 molecule with the copper cluster.[18] Hence, similar effects are expected for a metal-oxide support of Cu5 clusters.

In this work, the TiO2 surface has been selected as the support due to its abundance, nontoxicity, biological inertness, and chemical stability. In fact, it is one of the most popular materials for (photo)catalytic applications and solar energy conversion. Moreover, we have recently shown that the deposition of a single monolayer of Cu5 clusters on a TiO2 surface improves its optical properties significantly,[19] making it a visible-light photoactive material. More specifically, we demonstrate that, when deposited on the surface of titanium dioxide, the copper clusters are able to shift the adsorption from the high energy range, i.e. the UV spectrum, toward the visible light, where the sun has its maximum energy output. As a consequence, much more energy can be harvested from sunlight, and the coated titanium dioxide stores this energy temporarily in the form of charge pairs—electrons and holes—which is a perfect prerequisite for follow-up chemistry.

The CO2 activation and dissociation on TiO2-supported Cu (n < 5)[20] and Cu5 clusters[21] has been addressed in previous works,[20,21] with the specific surface being rutile TiO2(110) in ref (20) and anatase TiO2(101) in ref (21). Thus, Iyempeurumal and Deskins[20] found that clusters of 1–4 copper atoms supported on the rutile TiO2(110) surface stabilize a bent CO2 molecule (i.e., the precursors for CO2 decomposition), especially the Cu2 dimer. More recently, Jafarzadeh et al.[21] considered, besides the CO2 activation, its dissociation into CO and O fragments attached to Cu5- and Ni5-modified anatase TiO2 surfaces, addressing also the impact of plasma-induced surface charging. The authors found that adding plasma-induced excess electrons stabilize further bent CO2 structures.[21] Moreover, it was found that the dissociation of CO2 on charged clusters is energetically more favorable than that on neutral clusters.[21] However, actual reaction paths to CO2 dissociation were not considered.

Applying density functional theory (DFT), time-dependent DFT, and an approach combining DFT with reduced density matrix theory, we focus on exploring the following aspects: (1) possible reaction energy pathways to both CO2 activation and dissociation on unsupported as well as TiO2-supported Cu5 clusters; (2) the optical response of the system under solar irradiation. Thus, in section , the computational approach and the details of our calculations are presented. Section focuses on analyzing the reaction energy pathways as well as the UV–vis absorption spectra of unsupported and supporteed Cu5 clusters. Finally, section closes with the concluding remarks.

Methods

Density functional theory (DFT) is applied to shed light on the catalytic mechanism for CO2 decomposition to CO on unsupported and TiO2-supported Cu5 clusters. We employ a dispersion-corrected DFT-D3 ansatz,[22,23] given its excellent performance in describing the adsorption of small silver clusters on the same surface.[24] Structural optimizations and the calculation of interaction energies are performed with the Perdew–Burke–Ernzerhof (PBE) density functional and the Becke–Johnson (BJ) damping[22] for the D3 dispersion correction. We will refer to this combination as the PBE-D3(BJ) scheme. Both finite cluster and periodic slab models (see Figure ) have been used to account for TiO2(110) rutile surface effects. We first explore minimum energy pathways for both physisorption and chemisorption of the CO2 molecule on supported and unsupported Cu5 clusters. Next, we seek for possible reaction pathways leading to C=O bond breaking, starting with the lowest-energy chemisorption states found for the attached CO2 molecule. Additionally, time-dependent density functional calculations of the UV–vis spectra are carried out to explore the possibility that a photoinduced activation of physisorbed CO2 occurs via electron transfer from TiO2-supported Cu5 clusters to the attached CO2 molecule. Finally, as a second route to obtain the UV–vis spectra, a reduced density matrix (RDM) approach in the Redfield approximation[25] is employed, with the orbitals generated from periodic DFT calculations. In particular, we employed the HES06 hybrid functional of Heyd, Scuseria, and Ernzerhof,[26,27] a well-established treatment for the band gap analysis of semiconductors including TiO2.[28] This combined RDM-DFT treatment[29,30] has provided UV–vis absorption spectra in very good agreement with the experiment for the Cu5-decorated rutile TiO2(110) surface.[19]

Figure 1

Picture illustrating the cluster (left-hand panel) and slab (right-hand panel) models used to characterize the interaction of a CO2 molecule with a TiO2-supported Cu5 cluster. Red, brown, dark blue, cyan, and white balls indicate the positions of oxygen, carbon, copper, titanium, and hydrogen atoms, respectively.

If not explicitly mentioned otherwise, distances and energies are given in angström (1 = 1010 m) and electronvolt (1 eV = 1.602176565(35) 10–19 m2 kg s2) units, respectively.

Cluster Model Calculations

Cluster model calculations were performed by applying the PBE-D3(BJ) scheme with the ORCA[31] suite of programs (version 4.0.1.2). For this purpose, an atom-centered def2-TZVPP[32] basis set was used for copper and carbon atoms while the (augmented) polarized correlation-consistent triple-ζ (aug-cc-pVTZ) basis of Woon and Dunning, Jr.,[33] as reported in ref (34), was employed for oxygen and titanium atoms. As can be seen from Figure (left-hand panel), a hydrogen-saturated cluster model of stoichiometry (TiO2)13(H2O)14 was employed to model the rutile TiO2(110) surface, in which the number of hydrogen atoms are chosen so that the whole cluster remains electrically neutral. As mentioned in ref (19), this cluster model provides a very similar description of the Cu5–TiO2(110) system to that obtained via periodic calculations. For the sake of accuracy, we have also realized state-of-the-art periodic model calculations in this work (see section ). In fact, the periodic model provides a better account of the extended nature of the surface and, particularly, of (long-range) dispersion corrections. However, the cluster model has allowed a vis-a-vis comparison of CO2 adsorption properties on supported and unsupported Cu5 clusters, as well as the application of more expensive ab initio methods.

We assume the system to be in a doublet spin state since the quartet spin state is higher in energy for the free Cu5 cluster (by 0.64 eV at PBE-D3(BJ) level). PBE-D3(BJ) interaction energies were found to agree within 10% with reference values obtained with the domain-based pair natural orbital correlation approach DLPNO–CCSD(T)[35] as well as the symmetry-adapted perturbation theory [SAPT(DFT)] method[36,37] (see ref (24)) for the related Ag2/TiO2(110) system.

When optimizing the geometries in the cluster model, the atoms of both CO2 and Cu5 subsystems were allowed to relax, while the atoms of the support were kept fixed to experimentally determined values of the TiO2(110)-(1 × 1) surface.[38] Using this computational protocol, the adsorption energies were found to agree rather well with those obtained using the periodic slab model (see below), in which all the atoms were allowed to relax. Moreover, the employment of the ORCA suite of programs allowed us to obtain relaxed surface scans in constrained optimizations for which specific internal coordinates are kept frozen (i.e., the value of the C–O bond length). The modeling through a finite cluster was also used to test the performance of the PBE-D3(BJ) approach against higher levels of ab initio theory such as second-order Möller–Plesset perturbation theory (MP2) level. This way, additional calculations on the physisorption interaction energies of CO2 on TiO2-supported Cu5 clusters showed that the PBE-D3(BJ) approach provides values agreeing to within 10% with those obtained at MP2 level with the same basis set, and within 4% with those calculated using the larger def2-QZVPP[32] basis set and the same PBE-D3(BJ) scheme.

Time-dependent DFT (TDDFT) calculations of the UV–vis spectra were also performed using the PBE-D3(BJ) scheme and the cluster model. The number of roots were limited to 110 for the TiO2-supported Cu5 cluster, with a focus on the first transition involving the “jump” of an electron from the highest-energy “doubled-occupied” molecular orbital (referred to as HOMO) of the complete system to an unoccupied molecular orbital with high density around the attached CO2 molecule.

Periodic Calculations

Periodic electronic structure calculations are performed with the Vienna ab initio simulation package (VASP 5.4.4),[39,40] following a similar computational approach to that reported in previous work on He-, Ag5-, and Cu5–TiO2(110) interactions[19,24,41] as well as a systematic analysis of noble-gas atoms on the same surface.[42] Electron–ion interactions are described by the projector augmented-wave method,[40,43] using PAW–PBE pseudopotentials as implemented in the program. The electrons of the O(2s, 2p), C(2s, 2p), Ti(3s, 4s, 3p, 3d) and Cu(3d, 4s) orbitals are treated explicitly as valence electrons. A plane wave basis set with a kinetic energy cutoff of 700 eV is used. A Gaussian smearing of 0.05 eV is employed to account for partial occupancies, and the Brillouin zone is sampled at the Γ point. Test calculations showed that interaction energies at the potential minimum, using a 5 × 5 × 1 Monkhorst–Pack[44]k-point mesh, are similar (within ca. 0.01 eV) to those calculated at the Γ point. By shifting the kinetic energy cutoff from 700 to 1000 eV, the interaction energies were found to vary by less than 1 meV. The convergence criterion was 10–4 eV for the self-consistent electronic minimization. Geometries were relaxed with a force threshold of 0.02 eV/Å.

The Cu5-decorated surface was modeled via periodic slabs, using a 4 × 2 supercell (four TiO2 trilayers giving ca. 13 Å slab width). Adsorption was modeled on one side of the slab, with 38 Å of vacuum above it. This large vacuum region allowed the description of long-range tails of the interaction potentials while avoiding unphysical overlaps of electronic densities. Interaction energies are derived viawith ECu as the total energy of the system, ECu as the energy of the supported-TiO2 Cu5 cluster, and ECO denoting the energy of the free (gas-phase) CO2 molecule, all calculated in the same supercell slab for the sake of consistency.

Adsorption energies are calculated with the PBE-D3(BJ) scheme with the Hubbard term (DFT+U) added and including spin-polarization. The values of U reported in previous studies of Cu clusters (n ≤ 5) on the (101) and (100) surfaces of anatase[45] and rutile[19] were used (4.2 eV for titanium and 5.2 eV for copper). Due to the known underestimation of the band gap with the PBE functional, the photoabsorption spectra are calculated with the HSE06 exchange-correlation functional instead, which uses a screened Coulomb potential for increased efficiency on metallic systems.[26,27] This approach was applied using a HF/GGA mixing ratio of 25:75 with the screening parameter of 0.11 bohr,–1 as recommended in ref (27). All the surface ions and atoms from both the Cu5 cluster and the attached CO2 molecule were relaxed using the PBE-D3(BJ) method but with the Hubbard term (DFT+U) added. Finally, the optimized geometries, obtained at the PBE+U/D3 level, were used in final HSE06 calculations of the electronic structures. This computational protocol is the same as in our previous calculations of the UV–vis spectra for the Cu5–TiO2(110) system.[19]

Reduced Density Matrix Treatment

Photoabsorption spectra are calculated using the computational approach previously applied to the Ag5/TiO2 and Cu5/TiO2 systems in refs (19 and 24). The relaxation processes involved are described by the reduced density matrix (RDM) approach in the Redfield approximation,[25] based on orbitals taken from calculations employing the HSE06 hybrid functional. This combination of RDM and DFT, proposed by Micha and collaborators,[29,30] has been successfully applied to silver[24,46−48] and copper[19] clusters on semiconductor surfaces.[49]

Very briefly, in the presence of a monochromatic electromagnetic field of frequency Ω, the evolution equation for the reduced density ρ in the Schrödinger picture takes the formwith F̂ denoting the effective Kohn–Sham Hamiltonian (the indices refer to its representation in the Kohn–Sham basis set), D̂ as the electric dipole moment operator, and R as the Redfield coefficients, i.e., the Kohn–Sham components of the relaxation tensor. The latter are defined as in ref (25) and are implemented as described in ref (29).

Within the Redfield approximation, the relaxation tensor incorporates not only fast electronic dissipation due to electronic fluctuations in the medium but also the relatively slow relaxation due to vibrations of the atomic lattice. It is convenient to perform a coordinate transformation into a rotating frame accounting for the electromagnetic field oscillation. This is described by the equationswhere ε is the energy of the ith Kohn–Sham orbital. Time averaging over the fast terms in the equation of motion for the RDM yieldsas stationary-state solutions for the diagonal elements.[29] In it, HOMO and LUMO denote the lowest-energy unoccupied and the highest-energy occupied molecular orbital, respectively. Γ is a depopulation rate, and the sum terms g are given bywith γ denoting the decoherence rate, Ω as the Rabi frequencies given by , and Δ(Ω) = Ω – (ε – ε) as detunings. The diagonal elements provide the populations of the KS orbitals. The population relaxation rate ℏΓ and the decoherence rate ℏγ are kept fixed to values of 0.15 and 150 meV (27 ps and 27 fs). These values have been chosen according to known rates for phonon decay and electronic density excitations in semiconductors (see, e.g., ref (50)).

In terms of the stationary populations, the absorbance is given by[18,24,47,48,51]where f̅ is an oscillator strength per active electron.[52] The solar flux absorption spectrum is then expressed aswhere the solar flux is approximated by the blackbody flux distribution, normalized to an incident photon flux of 1 kW/m2,with C the flux normalization constant and the temperature T set to 5800 K.

Results and Discussion

Reaction Pathways: CO2 Interaction with Unsupported Cu5 Clusters

Let us first analyze the interaction of CO2 with unsupported Cu5 clusters. Our results have indicated that a planar trapezoidal structure of Cu5 is only slightly energetically favored (by 0.13 eV when the energy difference is calculated with the PBE-D3(BJ) scheme) over a trigonal bipyramidal structure so that we have considered both. By relaxing the geometries of the Cu5 and CO2 reactants at each intermolecular Cu5–CO2 distance, defined here as the distance between the carbon atom and the central atom of the Cu5 cluster, we obtain the interaction energies shown in the upper panel of Figure . Zero energy is set to having CO2 at infinite distance from the cluster. The energy pathway is characterized by a very shallow minimum of about −0.15 eV at a long Cu5–CO2 distance (about 5 Å) and a relatively deep potential minimum of about −0.6 eV at a shorter distance (∼3.9 Å), with a very low energy barrier in between. The shallow energy minimum emerges from a weak dispersion-dominated interaction between the two reactant species. Note that the barrier is appearing only if a bending of the CO2 molecule is allowed. Preliminary calculations of the same reaction pathway at the MP2 level of theory yield a slightly higher energy barrier (about 0.2 eV). Work is in progress to get a better estimate of the barrier to chemisorption using multireference perturbation theory, allowing us to better characterize the mixing between covalent and ionic contributions (see, for example, ref (53)).An analysis of Löwdin reduced orbital charges reveals no net charge transfer between the Cu5 and CO2 species but a strong polarization of both reactant species at the energy minimum configuration. The CO2 bending gives rise to the formation of a dipole moment that interacts attractively with induced dipole and quadrupole moments formed in the polarized Cu5 cluster. From the Gibbs energies at the right-hand panel, it can be seen that the energy minimum is deep enough to “survive” at room temperature but not at temperatures higher than 100 °C.

Figure 2

Surface scans characterizing the CO2/Cu5 interaction. Left-hand upper panel: relaxed surface scan (RSS) as a function of the distance between the carbon atom and the central atom of the (planar trapezoidal) Cu5 cluster. Middle panel: RSS as a function of one C=O distance for the (planar trapezoidal) Cu5 cluster. Bottom panel: RSS as a function of one C=O distance for the (bipyramidal) Cu5 cluster. Right-hand panels: Gibbs energies as a function of temperature at the energy minimum (blue and red lines) and transition state (green lines) configurations as well as the asymptote for CO desorption from the Cu5–O product species (dotted orange line). The nonperiodic cluster model has been used.

The middle panel of Figure illustrates how the adsorbed CO2 molecule, starting from its energy minimum configuration as shown in the upper panel, becomes decomposed by increasing one of the C=O distances. The planar Cu5 cluster catalyzes the CO2 decomposition, but the energetic barrier to break the CO bond is still too high (∼1.3 eV) to provide a reasonable reaction rate at room temperature. The final configuration with the CO fragment attached to Cu5 is rather unstable: At about 200 °C, the asymptote for CO desorption from Cu5–O lies approximately at the same energy as the transition state for C=O bond breaking and reduces significantly at higher temperatures due to increasing entropy.

This picture changes remarkably when considering the bipyramidal trigonal structure of Cu5 (bottom panel). Not only the energetic barrier of the rate-limiting step (C=O breaking) is clearly lower (∼0.8 eV) but also the complex formed upon C=O breaking are very stable, as both fragments remain adsorbed at ambient temperature. Also, in contrast with the planar Cu5 counterpart, the entrance channel is characterized by a very weak interaction of the CO2 molecule with the bipyramidal-shaped Cu5 cluster (about −0.3 eV). This finding once again illustrates the extreme sensitivity of atomic cluster properties with respect to structural reconfigurations, and it brings us straight to a final but crucial extension of our model with respect to the cluster support.

Reaction Pathways: CO2 Interaction with TiO2–Supported Cu5 Clusters

In this section, we focus on how the CO2–Cu5 interaction is modified when the Cu5 atomic cluster is supported on the rutile TiO2(110) surface. Figure summarizes the main adsorption geometries found using both the nonperiodic (left-hand panel) and periodic (right-hand panel) approaches, with the corresponding adsorption energies and main geometrical parameters summarized in Table . It can be seen that, with the exception of the adsorption energy for the most attractive chemisorption configuration (labeled as “4” in Table and Figure ), nonperiodic and periodic calculations provide rather similar results. The larger discrepancies in the latter case might be ascribed to the fact that the copper cluster is lying flat on the surface and therefore too close to the boundaries of the actual cluster model. This is also reflected in the larger d(Cu–Cumiddle) distance obtained in the periodic calculation for the physisorption configuration labeled as “2” (see Table ) since the Ti atom becomes located close to the cluster model boundaries (see Figure ). The discrepancies in structural parameters should be reduced upon enlargement of the cluster model. However, the next cluster size was too large for a TDDFT treatment.

Figure 3

Main CO2 adsorption geometries on the TiO2-supported Cu2 cluster, using finite cluster (left-hand panel) and periodic slab (right-hand panel) models. Isodensity surfaces of the HOMOs are also shown.

Table 1

Geometry Parameters (Distances between Carbon and Oxygen Atoms, d, and O–C–O Angle of CO2, α) and Adsorption Energies Eads (in eV) (See Figure for the Labeling of the Oxygen and Copper Atoms) Corresponding to the Adsorption Configurations Presented in Figure for the Non-Periodic Cluster and Periodic Slab Models Shown in Figure

	d(O1–C) (Å)		d(O2–C) (Å)		d(C–Cumiddle) (Å)		α(O1–C–O2) (deg)		Eads(eV)
label	cluster	slab	cluster	slab	cluster	slab	cluster	slab	cluster	slab
1	1.18	1.26	1.19	1.25	4.41	4.46	173	180	–0.18	–0.22
2	1.17	1.18	1.17	1.18	3.42	3.87	179	177	–0.33	–0.28
3	1.25	1.25	1.23	1.24	3.76	3.76	140	139	–0.50	–0.39
4	1.21	1.22	1.40	1.35	2.35	2.34	121	125	–1.54	–0.82

When considering chemisorption configurations (labeled as “3” and “4” in Figure ), a Bader decomposition[53] shows that the Cu5 cluster donates about 0.5 and 0.7 |e| of electronic charge to the attached CO2 molecule, while the charge donation is almost negligible (below 0.02 |e|) when physisorption configurations are analyzed instead (labeled as “1” and “2” in Figure ). There is a direct correlation between how much the CO2 molecule becomes bent and how much electronic charge it accumulates from the Cu5 cluster. In fact, upon bending, the energy of the antibonding LUMO orbital of the CO2 orbital becomes lower and thus closer to that of the HOMO of the Cu5 cluster, enhancing the probability of electron-transfer.

Using the finite cluster model, the upper panel of Figure shows the interaction energies as a function of the intermolecular distance between the carbon atom and the central Cu atom. It can be readily observed that the interaction is strongly influenced by the support: the potential energy minimum from the surface scan is now located at a configuration with the CO2 molecule physisorbed on top of one 5-fold Ti atom. As expected, with a well-depth of −0.33 eV, the physisorption minimum is dominated by the dispersion component of the interaction (−0.24 eV). There is almost zero net charge transfer to the CO2 molecule (less than 0.02 |e|) but a slight polarization is occurring at the C atom. At this physisorption configuration, there is almost no bending of the CO2 molecule (see Table ). Also, a very good agreement is found between the adsorption energies and adsorption geometries obtained for the cluster and the periodic slab models of the rutile TiO2(110) surface (see Table and Figure ). As can be seen in Figure , the physisorption nature of this configuration is also reflected in the shape of the HOMO. It is very similar to that obtained without the attached CO2 molecule (see ref (19)), dominated by 4s orbitals centered on the Cu atoms, bearing also important 3p and 3d contributions.

Figure 4

Surface scans characterizing the interaction between the CO2 molecule and the TiO2-supported Cu5 cluster. Upper panel: relaxed surface scan (RSS) as a function of the distance between the carbon atom and the central atom of the Cu5 cluster (vertical approach of the CO2 molecule). Middle panel: RSS as a function of the distance between the carbon atom and the center-of-mass of the Cu5 cluster (lateral approach of the CO2 molecule). Bottom panel: RSS as a function of one C=O distance. The nonperiodic cluster model shown in Figure (left-hand panel) has been used.

It is interesting to analyze the reasons for the adsorption energy differences which occur for the CO2 molecule adsorbed on top of unsupported and supported Cu5 clusters (−0.61 vs −0.22 eV). The free Cu5 cluster is highly polarizable and the electronic charge becomes pushed toward the two terminal copper atoms upon the approach of the CO2 molecule so that the electrostatic interaction is optimized. This polarization effect is somewhat constrained in the TiO2-supported Cu5 cluster since the support causes a marked redistribution of the charge (see ref (19)), with the two terminal copper atoms already being negatively charged. This redistribution is only slightly modified when the CO2 molecule approaches the cluster in the symmetric “on top”-configuration. A very different picture emerges if the CO2 molecule is approaching from a lateral side of the Cu5 cluster (see middle panel of Figure and configuration labeled as “3” in Figure and Table ): in this scenario, the charge distribution on copper atoms becomes polarized toward the opposite side from the attachment of the CO2 molecule. This redistribution eases the bending of the CO2 molecule (even without a barrier), with the carbon atom becoming negatively charged by about −0.5 |e| according to a Bader decomposition applied to the periodic model (configuration labeled as “3” in a right-hand panel of Figure ). As a result, the adsorption energy becomes significantly lower (−0.5 eV). This adsorption energy is slightly below to that obtained considering a periodic slab model of the TiO2(110) surface (−0.4 eV, see Table ). As can be seen in Figure , the HOMO isodensity profile is very different from those obtained in physisorption scenarios: there is a clear mixing of orbitals of the Cu5 cluster with the lowest-energy unoccupied molecular orbital (LUMO) of the free CO2 molecule (i.e., the antibonding π* orbital). It is dominated by 3d components from the copper atoms, bearing also important 4s-type contributions, while carbon and oxygen atoms provide 2s- and 3d-type (carbon) and 2p-type (oxygen) contributions.

Interestingly, the charge transfer to CO2 in chemisorption configurations comes from the HOMO of TiO2-supported Cu5 and not the lowest-energy single-occupied molecular orbital (referred to as SOMO). This holds true for nonperiodic as well as periodic calculations. In fact, as analyzed in ref (19)., the energy of the HOMO is very close to the bottom of the conduction band while the energy of the SOMO is about 1 eV lower (i.e., too far away from the LUMO orbital of the CO2 molecule). When the system is modeled by a periodic slab, the unpaired electron from the SOMO orbital becomes localized at a 5-fold Ti ion (i.e., characterizing a small polaron Ti3+ state). In fact, the SOMO is localized in a Ti(3d) orbital lying in the surface plane, showing no overlap with frontier orbitals of the approaching CO2 molecule. This is illustrated in Figure , presenting the electronic density of states (EDOS) together with isodensity profiles of the SOMO and HOMO. As mentioned in the introduction, as compared with closed-shell cases, the outer unpaired electrons of open-shell clusters such as the bare Cu5 cluster are expected to be the ones shared and/or transferred to a molecular adsorbate such as CO2. Our results clearly show that the open-shell TiO2-supported Cu5 cluster is a different case since the unpaired electron is localized at the small polaron Ti3+ state so that one of the paired electrons occupying HOMO orbitals is mainly responsible for the chemical bonding with CO2.

Figure 5

Electronic density of states (EDOS) corresponding to the CO2/Cu5/TiO2(110) system in the chemisorption configuration labeled as “3” in Figure . The insets present the SOMO and HOMO. The periodic cluster model shown in Figure (right-hand panel) has been used.

Finally, the bottom panel of Figure shows how the adsorption complex with a bent CO2 molecule attached to the lateral side of the Cu5 cluster (middle panel) evolves upon increasing of one of the C=O distances. Very remarkably, the Cu5 cluster then prefers to lie flat on the TiO2 support, and a rather stable adsorption CO2/Cu5 complex is obtained for an elongated C–O distance of about 1.4 Å. This is also clearly reflected in the mixing between Cu5 and CO2 orbitals, as can be observed in the HOMO isodensity profile (see the bottom panel of Figure ). At a variance with the chemisorption state labeled as “3” in Figure , the carbon atom provides mostly 2s- and 2p-type orbital contributions for CO2/Cu5 bond formation rather than 2s- and 3d-type contributions (see above). Essentially, when the Cu5 cluster is lying flat on the surface, the two terminal Cu atoms become bonded to in-plane oxygen ions. This feature favors the charge-transfer from the Cu5 cluster to the attached CO2 molecule so that the net donation becomes significantly larger (0.7 |e|). In turn, the CO2 molecule becomes more bent than when attached to the Cu5 cluster at the “raised” configuration (labeled as “3” in Figure ). Notice also that the HOMO expands around the carbon atom and both copper and titanium atoms and not only the former, resulting in a stronger CO2–Cu5 interaction. It should be noticed that the adsorption energies calculated for the CO2 molecule on the supported Cu5–TiO2 cluster are consistent with those reported by Afarzadeh et al. but considering the anatase TiO2(010) surface and, as large as −0.64 eV[20] with the CO2 molecule becoming also strongly bent (O–C–O angle of 129.5 deg).

Upon further increase of the C–O distance by about 2.0 Å, an energy barrier of about 0.4 eV has to be overcome. As a consequence, CO2 decomposes into an adsorbed CO fragment, which, at longer C–O distances, eventually desorbs from the catalyst, leaving behind a single oxygen atom which remains attached to the Cu5 cluster.

Starting with the structures of the complex in the bottom panel of Figure , we have realized a reoptimization using the periodic slab model of the rutile TiO2(110) surface, allowing the atoms from the support also to relax. This way, we obtain a reaction pathway for CO2 decomposition as shown in Figure . Although the values of the adsorption energies are below those obtained for the finite cluster model, the energy necessary for breaking the C=O bond is very similar (0.42 eV). The reaction pathway shown in Figure highlights the occurrence of a spontaneous activation and decomposition of CO2 on Cu5–TiO2. This outcome, along with the lower energy penalty (by about a factor of 3) in the rate-limiting step (C=O bond breaking), are in fact the most relevant differences when compared to the case of the unsupported Cu5 cluster scenario.

Figure 6

One possible reaction pathway for CO2 decomposition to CO onto the Cu5-modified TiO2(110) rutile surface. The periodic cluster model shown in Figure (right-hand panel) has been used.

A reaction pathway leading to CO2 dissociation to CO has also been found for the anatase TiO2-supported Pt8 cluster with an energy barrier of 1.01 eV.[54] The enhanced catalytic activity of the Pt8-modified TiO2 support was also rationalized in terms of the fluxional nature of the subnanometer-sized cluster. Similarly, a reconstruction of the Pt8 cluster was found upon CO2 adsorption. In our case, the C–O bond is even weaker if Cu5 cluster reconstruction is allowed. Another, very recent study investigated the nature of CO2 adsorption on Pt– atomic clusters (n = 4–7) as a function of cluster size.[55] The authors found the molecule to be highly activated yet still molecularly bound, but assume dissociative adsorption for larger cluster species.

UV–Vis Absorption Spectra

Cluster Model Calculations of the UV–Vis Absorption Spectra

Having analyzed the CO2/Cu5 and CO2/Cu5–TiO2 systems in the ground electronic state we focus now on its optical excitation. To this end, we have chosen the global minimum configuration of the unsupported CO2/Cu5 system (see section ), with the CO2 molecule adsorbed on top of the Cu5 cluster (adsorption energy of about 0.6 eV). Using the cluster model of the TiO2 surface shown in Figure (left-hand panel), we first compare the TDDFT spectra for CO2 adsorbed on unsupported and supported Cu5 clusters.

Figure (upper panel) illustrates how the irradiation of UV light onto the Cu5 cluster (photon energies from 3.5 to 4.3 eV) is driving an electron transfer from orbitals having the higher densities on copper atoms to orbitals bearing the higher densities centered on carbon atoms. Specifically, the electron transfer process gives rise to the formation of a complex that is better characterized as the CO2•– radical ion attached to the copper cluster. Preliminary calculations using the multistate complete-active-space second-order perturbation theory (CASPT2) method indicate that the well-depth of the PES in the corresponding excited ionic state is larger than 0.5 eV. As expected from the population of an orbital correlating to the SOMO antibonding orbital of the CO2– fragment at the asymptotic region, the C=O bond becomes weaker than in the ground electronic state. Therefore, a higher activity for CO2 reduction is expected upon photoexcitation.

Figure 7

UV–vis absorption spectra of unsupported (upper panel) and TiO2-supported (bottom panel) Cu5 clusters, as obtained at TDDFT level with the PBE-D3(BJ) scheme. The orbitals responsible of the most relevant transitions involving electron transfer from the Cu5 cluster to the physisorbed CO2 molecule are also shown. The insets present density isosurfaces of these orbitals. The nonperiodic cluster model shown in Figure (left-hand panel) has been used.

The bottom panel of Figure shows the absorption spectra of CO2 adsorbed on the Cu5-modified TiO2 surface. The electron “jump” from the HOMO to an orbital with density projection on the carbon atom is evident, with the responsible peaks located at about 0.8 eV, i.e., in the infrared spectral region. As discussed in ref (19), the HOMO of the Cu5–TiO2 system is dominated by 4s contributions from the copper atoms, bearing also 3p and 3d components. Essentially, the Cu5 cluster donates electronic charge so that a CO2•– radical attached to the Cu5–TiO2 composite is formed, similar to the unsupported case (see upper panel). However, as the main effect of the support, the photon energy necessary for the electronic transition is reduced by approximately 3 eV.

Periodic Calculations of the UV–Vis Absorption Spectra

In order to obtain a most accurate UV–vis absorption spectrum, we have used the periodic slab model of the rutile TiO2(110) surface shown in Figure (right-hand panel) and the RDM-DFT method as outlined in section , which employs the hybrid HSE06 functional. The accuracy of this methodological protocol was assessed in ref (19) for the Cu5–TiO2(110) system, where the theoretical photoabsorption spectra agreed very well with the experimental spectra recorded using diffuse reflectance measurements.

As shown in Figure , after depositing the Cu5 cluster on the TiO2(110) surface, the composite system presents absorption in the visible region. Furthermore, a strong enhancement of the absorption in the UV region is observed as compared with the unmodified material.[19] The physisorption of the CO2 molecule on top of the Cu5 clusters modifies the spectrum profile only slightly. The major modification is observed at about 2.1 eV (blue arrows in Figure ). As already described using the cluster approach, the transition responsible for the two additional peaks involves an electron “jump” from the HOMO so that the final state can be characterized as the CO2•– radical attached to the Cu5-modified surface.

Figure 8

Photoabsorption spectra of the rutile TiO2(110) surface, without adsorbates (dotted red lines), with the adsorbed Cu5 cluster (dotted green lines), and with the CO2 molecule physisorbed on top of the Cu5 cluster (blue lines). The blue arrows indicate the position of the most intense peaks involving a transition to orbitals with high density on the carbon atom. The inset presents the orbitals involved in the photoexcitation process associated with the indicated peaks. The periodic cluster model shown in Figure (right-hand panel) has been used.

Conclusions

In this article, we have explored the energy landscape characterizing the interaction of a CO2 molecule with an unsupported or TiO2-supported Cu5 cluster. We further investigated the occurrence of a photoinduced charge-transfer process between the cluster and the CO2 molecule. Thus, via computational modeling, we have shown how Cu5 clusters catalyze the CO2 decomposition by C=O bond activation and a reduction of the barrier for bond breaking. Dissociation often represents the rate-determining step in reactions involving metallic nanoparticles.[56] When supported on TiO2, C=O splitting becomes more favorable than spontaneous desorption of CO2. Moreover, time-dependent DFT and RDM-DFT calculations of the UV–vis spectra indicate that the TiO2-supported Cu5 cluster donates electron charge to a physisorbed CO2 molecule when illuminated with visible light, which is further beneficial for CO2 activation.

We point out two important findings: (1) CO2 can be trapped in a dispersion-dominated physisorption state and, when irradiated with visible light, is transformed into a radical CO2•–. This radical is a clear precursor-state for dissociation due to its weakened C=O bond. (2) The fluxionality of the subnanometric Cu5 cluster makes it an efficient functional environment (isomer lying flat to the surface) for the bending of the CO2 molecule. It is this enforced deformation that makes the adsorbed molecule more prone to accept electronic charge from the cluster, which in turn leads to a weaker C=O bond.

Altogether, our results, along with those presented in our previous work,[19] point out that TiO2-supported Cu5 clusters are not only innovative visible-light photoactive materials but also potential catalysts for CO2 reduction, highlighting how new catalytic and optical properties are acquired by subnanometer-sized metal clusters when deposited on technologically relevant materials. More generally, our work shows how the first-principles modeling of this new generation of ångström-sized catalysts and photocatalysts allows to understand them and, then, better control their properties. In particular, when both reactants, the metal cluster and the gas-phase molecule attached to it, are open-shell species, characterizations at a higher level of theory will become necessary, such as, e.g., those recently established by Aoiz and collaborators.[57−59]

According to our results, experimental measurements capable of detecting CO desorption from Cu5–TiO2 supported clusters as a function of temperature, with and without visible-light, would provide very useful insights regarding the conditions under which Cu5 clusters could become efficient catalysts for the removal of CO2 from the atmosphere.