The ever-increasing demand for metal catalysts on the global market has encouraged the development of new approaches for the synthesis of highly efficient catalysts. Metal and organometallic vapor-based synthesis methods allow exerting a high control over the morphological and structural features of the supported metal active phase leading to a significant contribution to design advanced catalysts and understand reaction mechanisms at atomic scale. In this mini-review, we describe recent advances on the synthesis of heterogeneous catalysts by three different promising methods using vapors of metal or organometallic complexes as precursors: atomic layer deposition (ALD), cluster beam deposition (CBD) and metal vapor synthesis (MVS). Catalytic applications over the last decade of heterogeneous catalysts containing single atoms, clusters or nanoparticles are reported and discussed pointing out the potentiality of these three approaches.

Photoelectrochemical (PEC) water splitting devices using semiconductors and electrocatalysts rely on heterogeneous interfaces that drive charge separation, thus determining potential gradients that dictate the reaction efficiency. The PEC potential of the electrocatalyst depends on the chemical oxidation state of forming elements, which may strongly vary under the photoinduced charge flow. However, element-sensitive, real-time measurements of the oxidation state of the electrocatalyst are not generally possible using conventional X-ray absorption techniques. Here, we show that fixed-energy X-ray absorption voltammetry and chronoamperometry, which measure the X-ray absorption coefficient variations along with photocurrent, can follow in real time the redox kinetics of electrocatalysts. To demonstrate the validity, we investigate hematite (alpha-Fe2O3) photoanodes covered with a nickel hydroxide electrocatalyst and show that it is fully oxidized by photogenerated holes to nickel oxyhydroxide with Ni reaching a higher oxidation state (Ni-IV) than that observed under electrocatalytic oxygen evolution in dark conditions. Highly oxidized Ni results from charge accumulation in the overlayer and can be observed only in the case of thick layers (with low PEC performance). On the other hand, the average oxidation state of Ni reaches lower values, under operative conditions, for very thin layers, resulting in high PEC activity. We complete our study by presenting PEC activity and impedance spectroscopy analysis using different thicknesses of the electrocatalyst, thus giving a detailed picture of the multiple and complex charge transfer processes occurring at a semiconductor/electrocatalyst junction.

Anisotropic gold nanoparticles (AuNPs), with their unique physical and optical properties, are emerging as smart and key nanomaterials and are being exploited in many crucial fields. To further improve their range of action, anisotropic AuNPs have been coupled with semiconductors, mainly TiO2 (titania), receiving great interest as powerful platforms both in biomedicine and in catalytic applications. Such hybrid nanoparticles show new properties that arise from the synergic action of the components and rely on NP size, morphology, and arrangement. Therefore, continuous advances in design and fabrication of new hybrid titania@gold NPs (TiO2@AuNPs) are urgent and highly desirable. Here, we propose an effective protocol to produce multibranched AuNPs covered by a controlled TiO2 thin layer, exploiting a one-pot microfluidic process. The proposed method allows the in-flow and reliable synthesis of titania-functionalized-anisotropic gold nanoparticles by avoiding the use of toxic surfactants and controlling the titania shell formation. TiO2@AuNPs have been fully characterized in terms of morphology, stability, and biocompatibility, and their activity in photocatalysis has been tested and verified.

Tungstenocene(IV) dichloride was successfully deposited and grafted on the surface of a commercially-available non-ordered silica support via either a liquid-phase or a dry impregnation approach. After a high-temperature calcination step, two W(VI)-grafted silica catalysts, with a metal loading around 1.2-1.7 wt.%, were obtained. They were fully characterized by physisorption and spectroscopic techniques, which evidenced well-dispersed tungsten oxide polyoxo cluster sites, for the catalyst prepared via liquid-phase grafting, and evenly dispersed larger monoclinic tungsten(VI) oxide aggregates, for the catalyst prepared via dry impregnation, respectively. Both W/SiO2 solids showed moderate to good conversion values in the epoxidation of (R)-( +)-limonene and methyl oleate (up to 68%), in the presence of aqueous hydrogen peroxide, with good selectivity to the desired epoxides (63% and 78%, respectively). The heterogeneous character of the most interesting W/SiO2 catalyst prepared via dry impregnation was confirmed by a hot centrifugation test and by an extended recovery and reuse of the solid in six catalytic runs.

The synthesis, characterization and catalytic performance of a conceptually simple, novel NbOx-SiO2 catalyst are here described. The niobium(V)-silica catalyst was prepared starting from cheap and viable reactants, by alkaline deposition of NH4Nb(C2O4)2·H2O in the presence of fructose as a stabilizer and subsequent calcination. The NbOx-SiO2 solid (0.95 Nb wt.%) was tested in the liquid-phase epoxidation with aqueous hydrogen peroxide of methyl oleate, as a model substrate. It was then tested in the epoxidation of a mixture of methyl esters (FAMEs) obtained by transesterification with methanol and purification of karanja oil, extracted from the autochthonous Indian variety of Millettia pinnata tree. The catalyst showed a promising performance in terms of methyl oleate conversion (up to 75%) and selectivity to epoxide (up to 82%). It was then tested on the FAME mixture from karanja oil, where interesting conversion values were attained (up to 70%), although with lower selectivities and yields to the mixture of desired epoxidized FAMEs. The solid withstood four catalytic cycles overall, during which a non-negligible surface reorganization of the Nb(V) sites was observed. However, this restructuring did not negatively affect the performance of the catalysts in terms of conversion or selectivity.

Hybrid Au/CuO NPs supported on carbon (Vulcan-XC72) with different Au/Cu molar ratios (i.e., 13/1, 4/1, 1/1, and 1/17) were synthesized by the solvated metal atom dispersion approach. High-resolution transmission electron microscopy, scanning transmission electron microscopy electron energy loss spectroscopy maps, and extended X-ray absorption fine structure measurements showed the presence of a Au/CuO core shell heterostructure having narrow size distributions (mean diameter < 4.7 nm) regardless of their composition. The role of their structure and composition was evaluated for catalytic liquid-phase selective benzyl alcohol oxidation with respect to their monometallic counterparts. As a result, a strong synergistic effect of Au/CuO heterostructures was revealed, which is strictly dependent on the CuO coverage degree of the Au-rich core and thus to the Au/Cu molar ratio. Highest catalytic activity was observed when the CuO shell only partially covers the Au-NP surface (i.e., at high Au/Cu molar ratios: 13/1, 4/1). On the other hand, at high Cu loadings (i.e., Au/Cu molar ratios: 1/1 and 1/17), the CuO shell wraps the Au core completely, inhibiting the catalytic activity.

The activity of two Cu/SiO2 catalysts prepared by the chemisorption hydrolysis technique has been tested in the hydrogenation reaction of 3-methyl-cyclohexanone. Both catalysts were found to be very active at 60 °C and 1 atm of H2. Characterization of the materials by FT-IR of adsorbed CO and TEM put in light the presence of well formed Cu cristallites. By assuming a cuboctahedral model we could show that the hydrogenation activity is linked to high coordination sites on the metal particle. A comparison is also reported with a sample prepared by ammonia evaporation that was found to be inactive in the hydrogenation reaction under the same experimental conditions.

The synthesis of N-heterocyclic carbene (NHC)-stabilized palladium nanoparticles (PdNPs) by an entirely new strategy comprising the NHC functionalization of ligand-free PdNPs obtained by metal vapor synthesis is described. Detailed characterization confirms the formation of very small monodisperse PdNPs (2.3 nm) and the presence of the NHC ligand on the Pd surface. The stable NHC-functionalized PdNPs dispersed onto a carbon support showed high activity in the hydrogenation of limonene with enhanced regioselectivity in comparison to bare PdNPs on carbon.

The broad challenges of energy and environment have become a main focus of research efforts to develop more active and selective catalytic systems for key chemical transformations. Surface organometallic chemistry (SOMC) is an established concept, associated with specific tools, for the design, preparation and characterization of well-defined single-site catalysts. The objective is to enter a catalytic cycle through a presumed catalytic intermediate prepared from organometallic or coordination compounds to generate well defined surface organometallic fragments (SOMFs) or surface coordination fragments (SCFs). These notions are the basis of the "catalysis by design" strategy ("structure-activity" relationship) in which a better understanding of the mechanistic aspects of the catalytic process led to the improvement of catalyst performances. In this review the application of SOMC strategy for the design and preparation of catalysts for industrially relevant processes that are crucial to the energy and environment is discussed. In particular, the focus will be on the conversion of energy-related feedstocks, such as methane and higher alkanes that are primary products of the oil and gas industry, and of their product of combustion, CO2, whose efficient capture and conversion is currently indicated as a top priority for the environment. Among the main topics related to energy and environment, catalytic oxidation is also considered as a key subject of this review.

A class of heterogeneous catalysts based on commercial bentonite from natural origin, containing at least 80 wt% of montmorillonite clay, was designed to transform selectively and under mild conditions toxic organosulfur and organophosphorus chemical warfare agents into non-noxious products with a reduced impact on health and environment. The bentonite from the natural origin was modified by introducing iron species and acid sites in the interlayer space, aiming to obtain a sorbent with strong catalytic oxidising and hydrolytic properties. The catalytic performance of these materials was evaluated in the oxidative abatement of (2-chloroethyl)ethyl sulfide (CEES), a simulant of sulfur mustard, in the presence of aqueous hydrogen peroxide as an oxidant. A new decontamination formulation was, moreover, proposed and obtained by mixing sodium perborate, as a solid oxidant, to iron-bentonite catalysts. Solid-phase decontamination tests, performed on a cotton textile support contaminated with organosulfide and organophosphonate simulant agents revealed the good activity of the solid formulation, especially in the in situ detoxification of blistering agents. Tests carried out on the real blistering warfare agent, sulfur mustard (HD agent), showed that, thanks to the co-presence of the iron-based clay together with the solid oxidant component, a good decontamination of the test surface from the real warfare agent could be achieved (80% contaminant degradation, under ambient conditions, in 24 h).

Niobium-containing mesoporous silica (Nb–SiO2) catalysts, which showed previously promising performances in the liquid-phase epoxidation of alkenes, were here applied to the epoxidation of unsaturated fatty acid methyl esters (FAMEs), selectively hydrogenated FAMEs and rapeseed oil triglyceride with aqueous hydrogen peroxide. Niobium-silica solids were prepared via grafting of niobocene dichloride onto the surface of different porous silica-based supports (non-ordered SiO2, ordered mesoporous MCM-41 and SBA-15 molecular sieves) via a dry impregnation procedure. Promising performance with very good selectivity towards desired monoepoxides were obtained, in particular over a Nb-grafted silica catalyst prepared from a commercial non-ordered silica support.

A one-pot one-step transformation of butanol into butyl butanoate takes place with excellent yield on a Cu/ZrO2 catalyst.

A computational study on the molecular mechanism of formic acid dehydrogenation in the presence of Cu-amine complex has been carried out in order to shed light on the role of the amine in determining the catalyst activity. The migration of H from the formate ligand to the metal center has been investigated in detail due to its endothermic nature. The relevance of both basicity and steric hindrance of the amine, as well as its binding ability to the metal center, in promoting hydrogen evolution has been confirmed.

Star-shaped gold nanoparticles (StarAuNPs) are extremely attractive nanomaterials, characterized by localized surface plasmon resonance which could be potentially employed in a large number of applications. However, the lack of a reliable and reproducible synthetic protocols for the production of StarAuNPs is the major limitation to their spreading. For the first time, here we present a robust protocol to manufacture reproducible StarAuNPs by exploiting a fluidic approach. Star-shaped AuNPs have been synthesized by means of a seed-less protocol, employing ascorbic acid as reducing agent at room temperature. Moreover, the versatility of the bench-top microfluidic protocol has been exploited to afford hydrophilic, hydrophobic and solid-supported engineered StarAuNPs, by avoiding intermediate NP purifications.

Lactose, a residue from the separation of high value-added proteins from whey, was converted into an equimolar mixture of sorbitol and dulcitol through a one-step cascade hydrolysis plus hydrogenation process.

CuPd bimetallic solvated metal atoms (SMA) synthesized by metal vapor synthesis (MVS) technique and supported on poly-4-vinylpyridine (PVPy) resin, showed significantly higher catalytic activity in Sonogashira-type carbon-carbon coupling reactions than the corresponding monometallic Cu and Pd systems as well as their physical mixture. The analysis of the bimetallic catalyst combining transmission electron microscopy techniques and X-ray absorption fine structure (XAFS) spectroscopy revealed the presence of small Pd nanoparticles (dm =2.5 nm) while the analysis of the X-ray absorption data, at the Cu K-edge suggests the formation of thin and incomplete Cu oxide layers around the Pd-rich cores.

A class of titanium-grafted mesoporous silica catalysts has been designed and prepared starting from molecularly defined metal clusters. The organosol mixture of zerovalent Ti13 clusters was impregnated onto the surface of ordered mesoporous silica molecular sieves (MCM-41 and MMM-2) and, after high-temperature calcination, an evenly dispersed non-single-site Ti(IV)nOx-like silica-supported catalyst was obtained. The catalytic solids, fully characterized by microscopic, spectroscopic and porosimetric techniques, showed standard performance in the liquid-phase epoxidation of a cyclic alkene, as limonene, but remarkably high selectivity values in the oxidative carboxylation of styrene, with tert-butylhydroperoxide and carbon dioxide in the presence of tetrabutylammonium bromide as a cocatalyst. Unprecedented high yields, up to 67%, in styrene carbonate were achieved after 24h, under solvent-free conditions. The catalysts displayed also a noteworthy stability of the performance to repeated recovery and reuse cycles.

Ultrafine Pd nanoparticles (dm=2.3nm), obtained by metal vapor synthesis technique, were immobilized into a poly(4-vinylpyridine)-based porous monolith by means of a new synthetic approach. The synthesis involves stabilization of Pd nanoparticles with 4-vinylpyridine ligand and their subsequent immobilization into the monolith by radical co-polymerization of the resulting metal-embedding monomer with ethylene glycol dimethacrylate in presence of porogenic agents (i.e. DMF and PEG-400) inside stainless-steel columns (HPLC type). The hybrid monolithic reactors containing highly dispersed Pd nanoparticles were effectively used as catalyst for Mizoroki–Heck cross-coupling reactions carried out under continuous-flow conditions. The devices showed long life-time (>65h) and very low Pd leaching (<2ppm).