Due to the powerful quantum confined space effects and multiple modes of small atomic sizes, metal nanoclusters (NCs) like thiolate-protected noble metals, such as silver (Ag) and gold (Au), which have a core sizes less than 3 nm, have developed a class of "metallic molecules" with multiple optical, magnetic, and electronic properties. To find a well-defined nanocatalysts, especially ligand-passivated metal NCs, great strides have been achieved in the efficient synthesis of atomically precise nanoparticles. Methods of synthesis such as bottom-up growth, top-down approach, ligand engineering, and interconversion system, are mentioned in this overview. Such clearly defined metal NCs have demonstrated considerable promise in catalysis research and have evolved into a distinct class of model catalysts. Focusing on the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER), this article attempts to outline current developments in NCs of molecular metals employed in electrocatalytic reactions. The paper highlights the relationship between the structure and performance of the catalytic mechanism and examines the potential effects of metal cluster sizes, metal core structures, charges, ligands, and metal–ligand binding patterns on their electrocatalytic activity. Future research opportunities and challenges are also proposed.

Heterojunction composites with intimate interfaces can shorten the diffusion distance, which leads to a shorter path for photogenerated carriers, thereby increasing photocatalytic activity. Herein, we report the fabrication of Ti₃C₂-Bi₂WO₆ (TC-BW) heterojunctions hinged by Bi₂Ti₂O₇ joints via an in situ hydrothermal reaction of Ti₃C₂ in the presence of Na₂WO₄ and Bi(NO₃)₃. The TC-BW was characterized using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and Raman spectroscopy. TC-BW showed superior photocatalytic activity (productivity over 15TC-WB reaches up to 5.0 mmol_{reacted BA}·g_cat.⁻¹·h⁻¹) in the oxidation of benzyl alcohol using light-emitting diode (LED) light, arising from the surface defects and intimate heterojunction interface between the Ti₃C₂ MXene and Bi₂WO₆ nanosheets. TC-BW heterojunctions provide an enhanced separation efficiency of photogenerated charges, which in turn yields superior photocatalytic activity. Furthermore, it is well substantiated by density functional theory (DFT) calculations. In summary, this study elucidates the preparation of heterojunction composites with intimate interfaces for highly efficient photooxidation.

Catalysts for chemoselective hydrogenation are of vital importance for the synthesis of various important chemicals and intermediates. Herein we developed a simple method for preparing a highly efficient Ni-MoC_x nanocomposite catalyst via temperature-programmed carburization of a polyoxometalate precursor. X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analyses indicate that the resulting mesoporous nanocomposite catalyst is made up of well-dispersed metallic nickel particles embedded in a MoC_x matrix. This catalyst exhibits high activity and selectivity (> 99%) in the hydrogenation of various substituted nitroaromatics to corresponding anilines. The high efficiency is attributed to the intimate contact of the constituents favoring electron transfer and hydrogen adsorption. Dihydrogen is physisorbed on the carbide support and dissociates on the nickel particles, as evidenced by Mo K-edge X-ray absorption near-edge structure (XANES) spectra, density functional theory (DFT), and hydrogen–deuterium exchange. The remarkable catalytic performance of the catalyst could be traced back to the synergistic interaction between the Ni particles and the carbide support. In-situ infrared spectroscopy and DFT simulations indicated that the adsorption/activation of the nitro group is favored compared to that of other substituents at the aromatic ring. In recyclability tests, the Ni-MoC_x nanocomposite showed no significant loss of catalytic performance in seven consecutive runs, indicating its robust nature.

Morphological effects of nanoparticles are crucial in many solid-catalyzed chemical transformations. We herein prepared two manganese-ceria solid solutions, well-defined MnCeO_x nanorods and MnCeO_x-nanocubes, exposing preferentially (111) and (100) facets of ceria, respectively. The incorporation of Mn dopant into ceria lattice strongly enhanced the catalytic performance in the NO reduction with CO. MnCeO_x (111) catalyst outperformed MnCeO_x (100) counterpart due to its higher population density of oxygen vacancy defects. In-situ infrared spectroscopy investigations indicated that the reaction pathway over MnCeO_x and pristine CeO₂ is similar and that besides the direct pathway, an indirect pathway via adsorbed hyponitrite as an intermediate cannot be ruled out. X-ray photoelectron and Raman spectroscopies as well as first-principles density functional theory (DFT) calculations indicate that the enhanced catalytic performance of MnCeO_x can be traced back to its “Mn–O_L(VÖ)–Mn–O_L(VÖ)–Ce” connectivities. The Mn dopant strongly facilitates the formation of surface oxygen vacancies (VÖ) by liberating surface lattice oxygen (O_L) via CO* + O_L → CO₂* + VÖ and promotes the reduction of NO, according to NO* + VÖ → N* + O_L and 2N* → N₂. The Mn dopant impact on both the adsorption of CO and activation of O_L reveals that a balance between these two effects is critical for facilitating all reaction steps.

Precise mono-doping of metal atom into metal particles at a specific particle position (e.g., the central site) in a highly controllable manner is still a challenge. In this work, we develop a highly controllable strategy for exchanging a single Ag atom into the central gold site of Au₁₃Ag₁₂(PPh₃)₁₀Cl₈ (Ph = phenyl) nanoclusters. Interestingly, a “pigeon-pair” cluster of {[Au₁₃Ag₁₂(PPh₃)₁₀Cl₈]·[Au₁₂Ag₁₃(PPh₃)₁₀Cl₈]}²⁺ is obtained and confirmed by electrospray ionization mass spectrometry (ESI-MS), thermogravimetric analysis (TGA) and single crystal X-ray diffraction (SCXRD) analysis. The experimental results and density functional theory (DFT) calculations suggest that the single-metal-atom exchanging from [Au₁₃Ag₁₂(PPh₃)₁₀Cl₈]⁺ to [Au₁₂Ag₁₃(PPh₃)₁₀Cl₈]⁺ occurs at the central position through the side entry of the μ₃-bridging Cl atoms. Finally, the effects on the electronic structure and properties caused by the single-atom exchange at the central site are shown by the enhancement of fluorescence and catalytic activity in the photocatalytic oxidation of ethanol.

We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. Bi_xCe_1-xO_δ nanocomposites of various compositions (x = 0.06-0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO₂ lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the Bi_xCe_1-xO_δ catalysts was found to be strongly enhanced with increasing O_V concentration. The concentration of oxygen vacancies exhibited a maximum for Bi_0.12Ce_0.88O_δ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 ℃ and a gas-hourly space velocity of 2,880 mL·g_cat^-1·h^-1. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (-OCH₃) species. The activation of CO₂ to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.

We report experimental and mechanistic understanding of methanol oxidation to produce methyl formate using CuO/TiO₂-spindle composite as a promising photocatalyst under mild conditions with over 97% conversion and 83% selectivity. The catalysts are obtained via precise depositing of CuO nanoclusters (size: ~ 3.5 nm) at the {101} facet of the TiO₂ to optimally tune exciton recombination through oxygen vacancies generation, evidenced by photoluminescence and Raman spectroscopy measurements. The turnover frequency (TOF) and the apparent quantum efficiency (AQE) of the 7%CuO/TiO₂-spindle composites reach up to 23.8 mol_methanol·g_cat^-1·h^-1 and 55.2% at 25 °C, respectively, which are substantially higher than these previously reported photocatalysts. Further, the in-situ attenuated total reflection infrared spectroscopy analysis reveals that the methanol oxidation most likely takes place through the conversion of adsorbed methoxy (CH₃O*) to formaldehyde (CHO*) intermediate, a subject of major debate for a long time. The adsorbed formaldehyde (CHO*) thus produced reacts with another CH₃O* species in its close proximity to form the final product of methyl formate. Results of this study provide insights into the reaction mechanism, and offer guidelines to systematically develop and apply photocatalysts for methanol conversion and related reactions via surface engineering.

In this study, 1, 2-bis(diphenylphosphino)ethane (dppe) ligands are used to synthesize gold nanoclusters with an icosahedral Au₁₃ core. The nanoclusters are characterized and formulated as [Au₁₃(dppe)5Cl₂]Cl₃ using synchrotron radiation X-ray diffraction, UV/Vis absorption spectroscopy, electrospray ionization mass spectrometry, and density functional theory (DFT) calculations. The bidentate feature of dppe ligands and the positions of coordinating surface gold atoms induce a helical arrangement that forms a propeller-like structure, which reduces the symmetry of the gold nanocluster to C₁. Therefore, dppe ligands perform as a directing agent to create chiral an ansa metallamacrocycle [Au₁₃(dppe)₅Cl₂]³⁺ nanocluster, as confirmed by simulated electronic circular dichroism spectrum. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the [Au₁₃(dppe)₅Cl₂]³⁺ cluster is determined as approx. 1.9 eV, and further confirmed by ultraviolet photoemission spectroscopy analysis and DFT simulation. Furthermore, the photoactivity of [Au₁₃(dppe)₅Cl₂]³⁺ is investigated, with the nanocluster shown to possess near-infrared photoluminescence properties, which can be employed for ¹O₂ photogeneration. The quantum yield of ¹O₂ photogeneration using the [Au₁₃(dppe)₅Cl₂]³⁺ nanocluster is up to 0.71, which is considerably higher than those of anthracene (an organic dye), and Au₂₅ and Au₃₈ nanoclusters.

We developed a general and effective strategy to afford rod-like [Au₂₅(SPh)₅(PPh₃)₁₀X₂]X₂ (X = Cl/Br) nanoclusters, capped by conjugated delocalized pπ electron mediated ligands. The detailed atomic structure of these materials was resolved by synchrotron radiation X-ray diffraction (SRXRD) combined with electrospray ionization mass spectrometry (ESI-MS) and UV–vis analyses. The Au₁₇(SR)₃(PPh₃)₆X₂ minimum asymmetric unit, with exposed Au atoms at the center, can serve as an important model to understand the transformation of homogold nanoclusters into alloy nanoclusters. The conjugated delocalized pπ electrons of the thiolate ligands can effectively tune the electronic properties of the Au₂₅ kernel, as qualitatively evidenced by the energy gaps measured by UV–vis experiments and density functional theory (DFT) calculations. The delocalized electrons distinctly flow to the orbitals of the Au₂₅ kernel via the S atoms of the aromatic thiolates. The ESI-MS analysis indicates that Au₃ clusters are formed during the etching reactions, which provide an opportunity to gain insight into the intriguing conversion pathway of the Au_n(PPh₃)_mX_y precursor to the final Au₂₅ nanorods. Finally, the thiophenol-protected Au₂₅ nanorods, immobilized on activated carbon, show good catalytic activity in the aerobic oxidation of glucose to gluconic acid (74% glucose conversion and 100% selectivity for gluconic acid), much higher than that of the aliphatic Au₂₅ analogue. The Au₂₅(SPh)₅(PPh₃)₁₀X₂ catalyst yields a turnover frequency (TOF) of 13.5 s^–1, higher than that of commercial catalysts such as Pd/activated carbon (AC) and Pd-Bi/AC. The insight obtained from this study will support the development and design of efficient nanogold catalysts for special oxidation reactions.

Heteroatom dopants can greatly modify the electronic and physical properties and catalytic performance of gold nanoclusters. In this study, we investigate the catalytic activity of [Au_25-x(PET)_18-xM]NH₃ (PET = 2-phenylethanethiolate, and M = Cu, Co, Ni, and Zn) nanoclusters in aerobic alcohol oxidation. The [Au_25-x(PET)_18-xM]NH₃ nanoclusters are thoroughly characterized by matrix assisted laser desorption ionization (MALDI) mass spectrometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and inductively coupled plasma–mass spectrometry (ICP-MS). The XPS analyses suggest that the transition metals strongly interact with the gold atoms of the nanoclusters. The CeO₂-supported nanoclusters show catalytic activity, based on the conversion of benzyl alcohol, in the order, [Au_25-x(PET)_18-xNi] > [Au_25-x(PET)_18-xCu] > [Au_25-x(PET)_18-xZn] > [Au_25-x(PET)_18-xCo]. Regarding product selectivity, the [Au_25-x(PET)_18-xZn] and [Au_25-x(PET)_18-xCo] catalysts preferably yield benzaldehyde, [Au_25-x(PET)_18-xCu] yields benzaldehyde and benzyl acid, and [Au_25-x(PET)_18-xNi] yields benzyl acid. The exposed metal atoms are considered as the catalytic active sites. Also, the catalytic performance (including activity and selectivity) of the [Au_25-x(PET)_18-xM] catalysts is greatly turned and mediated by the transition metal type.