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.

Using a combination of first-principles modelling, X-ray absorption spectroscopy, and diffuse reflectance spectroscopy measurements, we explore the properties of Ag 5 -modified TiO 2 surfaces. A general electron polarization phenomenon associated with surface polarons on TiO 2 has been revealed theoretically and confirmed experimentally. First, the Ag 5 cluster donates an electron to TiO 2 , leading to the formation of polaronic Ti 3+ 3d 1 states on the rutile TiO 2 (110) surface. The analysis of polarization effects in the nearby electronic structure accompanying the polaron formation is confirmed with X-ray absorption spectroscopy measurements at the Ti K-edge of TiO 2 nanoparticles. Next, the UV-Vis optical absorption spectrum of the polaronic state is also computationally modelled and an enlargement of the polaron wavefunction is predicted. Moreover, we find an overall improvement of the UV-Vis optical response of the material through diffuse reflectance spectroscopy measurements. Finally, we predict that charge-transfer processes at the Ag 5 -TiO 2 interface triggered by solar photons might allow for a photoinduced activation of CO 2 by sunlight. † Electronic supplementary information (ESI) available: Complementary results from the rst-principles modelling: polaronic states in reduced TiO 2 (110) surfaces, and characterization of the most relevant orbitals in the photoexcitation of the Ag 5 /TiO 2 (110) system. See

An ab initio study of the interaction of O 2 , the most abundant radical and oxidant species in the atmosphere, with a Cu 5 cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for by using the multireference perturbation theory, allowing the experimentally confirmed resistivity of Cu 5 clusters toward oxidation to be investigated. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O 2 with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multireference approach, the transition barrier for O 2 activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu 5 arranged in a bipyramidal structure, the O–O bond cleavage is confirmed as the rate-determining step. However, for planar Cu 5 , the high energy barrier for O 2 activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case.

The recent advent of cutting-edge experimental techniques allows for a precise synthesis of subnanometer metal clusters composed by just a few atoms, opening new possibilities for subnanometer science. In this work, via first principles modeling, we show how the decoration of perfect and reduced TiO 2 surfaces with Ag 5 atomic clusters enables the stabilization of multiple surface polarons. Moreover, we predict that Ag 5 clusters are capable to promote defect-induced polarons transfer from subsurface to surface sites of reduced TiO 2 samples. For both planar and pyramidal Ag 5 clusters, and considering four different positions of bridging oxygen vacancies, we model up to fourteen polaronic structures, leading to one hundred and thirty four polaronic states. About 71% of these configurations encompasses coexisting surface polarons. The most stable states are associated with large inter-polarons distances (> 7.5 Å on average), not only due to the repulsive interaction between trapped Ti 3+ 3d 1 electrons, but also to the interference between their corresponding electronic polarizations clouds [López-Caballero et al., J. Mat. Chem. A 8 (2020) 6842-6853]. As a result, the most stable ferromagnetic and anti-ferromagnetic arrangements are energetically quasi-degenerate. However, as the average inter-polarons distance decreases, most (≥ 70%) of the polaronic configurations become ferromagnetic. The optical excitation of the midgap polaronic states with photon energy at the end of the visible causes the enlargement of the polaronic wave function over the surface layer. The ability of Ag 5 atomic clusters to stabilize multiple surface polarons and extend the optical response of TiO 2 surfaces towards the visible bears importance in improving their (photo-)catalytic properties, and illustrates the potential of this new generation of subnanometer-sized materials.

Subnanometer-sized metal clusters often feature a molecule-like electronic structure, which makes their physical and chemical properties significantly different from those of nanoparticles and bulk material. Considering potential applications, there is a major concern about their thermal stability and susceptibility towards oxidation. Cu clusters of only 5 atoms (Cu₅ clusters) are first synthesized in high concentration using a new-generation wet chemical method. Next, it is shown that, contrary to what is currently assumed, Cu₅ clusters display nobility, beyond resistance to irreversible oxidation, at a broad range of temperatures and oxygen pressures. The outstanding nobility arises from an unusual reversible oxidation which is observed by <i>in situ</i> X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy on Cu₅ clusters deposited onto highly oriented pyrolitic graphite at different oxygen pressures and up to 773 K. This atypical property is explained by a theoretical approach combining different state-of-the-art first principles theories. It reveals the essential role of collective quantum effects in the physical mechanism responsible for the nobility of Cu₅ clusters, encompassing a structural ‘breathing’ through concerted Cu–Cu elongations/contractions upon O₂ uptake/release, and collective charge transfer as well. A predictive phase diagram of their reversible oxidation states is also delivered, agreeing with the experimental observations. The collective quantum effects responsible of the observed nobility are expected to be general in subnanometer-sized metal clusters, pushing this new generation of materials to an upper level.