Solar driven CO2 hydrogenation to HCOOH on (TiO2)n (n = 1-6) atomic clusters

Artificial photosynthesis is a crucial reaction that addresses energy and environmental challenges by converting CO2 into fuels and value-added chemicals. However, efficient catalytic activity using earth-abundant materials can be challenging due to intrinsic limitations. Herein, we explore neutral (TiO2)n (n = 1-6) atomic clusters for CO2 hydrogenation via comprehensive ab initio calculations combined with time-dependent functional theory. Our results show that these (TiO2)n clusters exhibit outstanding thermodynamic stabilities and decent surficial activities for CO2 activation and H2 dissociation, both of which possess kinetic barriers down to 0-0.74 eV. We establish a relationship between the binding strength of *CO2 species and electron characterization for these (TiO2)n clusters. These clusters, which have a wide energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccpied molecular orbital (LUMO) that allows them to harvest the solar light in the ultraviolet regime, enabling efficient catalysis for driving the catalysis of CO2 conversion. They provide exclusive reaction channels and high selectivity for yielding HCOOH products via the carboxyl mechanism, involving the kinetic barrier of the limiting step of 0.74-1.25 eV. We also investigated the substrate effect on supported (TiO2)n clusters, with non-metallic substrates featuring inert surfaces serving as suitable options for anchoring (TiO2)n clusters while preserving their intrinsic activity and selectivity. These computational results have significant implications not only for meeting energy demands but also for mitigating carbon emissions by utilizing CO2 as an alternative feedstock rather than considering it solely as a greenhouse gas.

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.

Recognition of Water-Induced Double-Edged Sword Effects in Photocatalytic Selective Oxidation of Toluene on Titanium Dioxide Clusters with Density Functional Theory Calculations

Recently, water promotion effects in the selective oxidation of benzyl alcohol to benzaldehyde have been experimentally recognized and identified. However, the effects of water on the photocatalytic selective oxidation of toluene into benzaldehyde remain elusive. In this work, the Ti₃O₉H₆ clusters in different solvents (water and toluene solvent) are used to study the water-induced effects in photocatalytic oxidation reactions in kinetics and thermodynamics using density functional theory (DFT) calculations. In addition, the influences of the OH groups on catalysts (Ti-OH bonds) from photocatalytic water splitting are also considered. The results clearly demonstrate the water-induced double-edged sword effects in the photocatalytic selective oxidation of toluene. We expect that our work can not only shed light on the mechanisms of photocatalytic selective oxidation of toluene into benzaldehyde and other activation reactions of sp³ C-H bonds but also design and modulate highly efficient photocatalysts.

Water Gas Shift Reaction Catalyzed by Rhodium-Manganese Oxide Cluster Anions

Fundamental understanding of the nature of active sites in real-life water gas shift (WGS) catalysts that can convert CO and H₂O into CO₂ and H₂ is crucial to engineer related catalysts performing under ambient conditions. Herein, we identified that the WGS reaction can be, in principle, catalyzed by rhodium-manganese oxide clusters Rh₂MnO_1,2^- in the gas phase at room temperature. This is the first example of the construction of such a potential catalysis in cluster science because it is challenging to discover clusters that can abstract the oxygen from H₂O and then supply the anchored oxygen to oxidize CO. The WGS reaction was characterized by mass spectrometry, photoelectron spectroscopy, and quantum-chemical calculations. The coordinated oxygen in Rh₂MnO_1,2^- is paramount for the generation of an electron-rich Mn⁺-Rh^- bond that is critical to capture and reduce H₂O and giving rise to a polarized Rh⁺-Rh^- bond that functions as the real redox center to drive the WGS reaction.

Photooxidation of Isoprene by Titanium Oxide Cluster Anions with Dimensions up to a Nanosize

Titania (TiO₂) nanoparticles are active photocatalysts, and isoprene (C₅H₈) is a biogenic volatile organic compound that contributes crucially to global particulate matter generation. Herein, the direct photooxidation of isoprene by titanium oxide cluster anions with dimensions up to a nanosize by both ultraviolet (UV) and visible (Vis) light excitations has been successfully identified through mass spectrometric experiments combined with quantum chemistry calculations. The potential role of "dry" titania in atmospheric isoprene oxidation has been revealed, and a clear picture of the photooxidation mechanism on titanium oxide nanoparticles has been provided explicitly at the molecular level. The adsorption of isoprene on the atomic oxygen radicals (O^•-) of titanium oxide clusters leads to the formation of the crucial interfacial state (IS) within the band gap of titanium oxides. This IS is demonstrated to be the significant factor in delivering the electron from the π orbital of C₅H₈ to the Ti_3d orbital in the photooxidation process (C₅H₈ + Ti⁴⁺-O^•- → C₅H₈O + Ti³⁺) and creating photoactivity in the Vis region. It is revealed that after the photogeneration of the O^•- radicals by UV excitation on the TiO₂ particle surface, the subsequent reactions can be induced by Vis excitation through the IS. This multicolor strategy in both the UV and Vis regions can enhance the efficiency of solar energy harvesting and improve the product yield of the photocatalysis on TiO₂ nanoparticles. New insights have been provided into both the atmospheric chemistry of isoprene and the photochemistry of TiO₂ nanoparticles.

Oxygen Vacancies in Oxide Nanoclusters: When Silica Is More Reducible Than Titania

Oxygen vacancies are related to specific optical, conductivity and magnetic properties in macroscopic SiO2 and TiO2 compounds. As such, the ease with which oxygen vacancies form often determines the application potential of these materials in many technological fields. However, little is known about the role of oxygen vacancies in nanosized materials. In this work we compute the energies to create oxygen vacancies in highly stable nanoclusters of (TiO2)N, (SiO2)N, and mixed (TixSi1-xO2)N for sizes between N = 2 and N = 24 units. Contrary to the results for bulk and surfaces, we predict that removing an oxygen atom from global minima silica clusters is energetically more favorable than from the respective titania species. This unexpected chemical behavior is clearly linked to the inherent presence of terminal unsaturated oxygens at these nanoscale systems. In order to fully characterize our findings, we provide an extensive set of descriptors (oxygen vacancy formation energy, electron localization, density of states, relaxation energy, and geometry) that can be used to compare our results with those for other compositions and sizes. Our results will help in the search of novel nanomaterials for technological and scientific applications such as heterogeneous catalysis, electronics, and cluster chemistry.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

A DFT study on the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde in different solvents employing an OH-defected TiO2 cluster model

Density functional theory calculations are employed to study the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde (4-BBA) in acetonitrile and in ethanol solvents. A totally relaxed Ti₃O₉H₆ cluster model is proposed to represent titanium dioxide (TiO₂) surfaces. The reduction selectivity of an adsorbed 4-BBA molecule on Ti₃O₉H₆ has been investigated. Owing to the difference in the proton and H atom donating capabilities between explicit CH₃CN and C₂H₅OH solvent molecules, the photocatalytic reduction of 4-BBA is the debromination process in acetonitrile, whereas in ethanol it is the carbonyl reduction process. Therefore 4-BBA can be selectively reduced to benzaldehyde in acetonitrile and 4-bromobenzyl alcohol in ethanol, respectively. Our computational results have verified the reaction mechanism proposed by experiments and show that the debromination of 4-BBA would be efficient if both 4-BBA and Ti₃O₉H₆ have an extra photoelectron. The Ti₃O₉H₆ cluster, playing a role as a hydrogen source and a bridge to transfer photoelectrons from bulk TiO₂, would have potential to be an ideal molecular model for understanding photocatalytic reactions on the TiO₂ surface.

Excess charge driven dissociative hydrogen adsorption on Ti2O4−

The mechanism of dissociative D₂ adsorption on Ti₂O₄⁻ is studied using infrared photodissociation spectroscopy in combination with density functional theory calculations.

Ethylene C-H Bond Activation by Neutral Mn2O5 Clusters under Visible Light Irradiation

A photo excitation fast flow reactor coupled with a single-photon ionization (118 nm, 10.5 eV) time-of-flight mass spectrometry (TOFMS) instrument is used to investigate reactions of neutral MnmOn clusters with C2H4 under visible (532 nm) light irradiation. Association products Mn2O5(C2H4) and Mn3O6,7(C2H4) are observed without irradiation. Under light irradiation, the Mn2O5(C2H4) TOFMS feature decreases, and a new species, Mn2O5H2, is observed. This light-activated reaction suggests that the visible radiation can induce the chemistry, Mn2O5 + C2H4 + hv(532 nm) → Mn2O5*(C2H4) → Mn2O5H2 + C2H2. High barriers (0.67 and 0.59 eV) are obtained on the ground-state potential energy surface (PES); the reaction is barrierless and thermodynamically favorable on the first excited-state PES, as performed by time-dependent density functional theory calculations. The calculational and experimental results suggest that Mn2O5-like structures on manganese oxide surfaces are the appropriate active catalytic sites for visible light photocatalysis of ethylene dehydrogenation.