Molecular approaches to catalysis

Titania-Carbon Nitride Interfaces in Gold-Catalyzed CO Oxidation

Gold-catalyzed CO oxidation is a reaction of both practical and fundamental interest. In particular, rate-determining oxygen activation pathways have attracted a lot of attention. They have been found to depend on the surface chemistry of the catalyst support, titania providing the most active catalysts and carbon nitride leading to inactive catalysts. Here, we show that C₃N₄-TiO₂ composites with rather similar surface chemistries can be engineered by using titania nanotubes as hard templates and by performing the polycondensation of melamine and dicyandiamide in air and in ammonia. By varying the C₃N₄ content from 2 to 75 wt %, the mesoporosity can be tuned from 8 to 40 nm. A systematic study of CO oxidation turnover numbers in the absence and in the presence of hydrogen over the composites loaded with well-calibrated 2-4 nm gold nanoparticles clearly shows that (1) the chemical composition of the support surface has much less impact on PROX (preferential oxidation of CO in excess hydrogen) than on dry CO oxidation, (2) NH₂-terminated supports are as active as OH-terminated supports in PROX, (3) hydrogen/water-mediated CO oxidation pathways are active on C₃N₄-based Au catalysts, and (4) PROX activity requires a rather large porosity (40 nm), which suggests the involvement of much larger intermediates than the usually postulated peroxo-type species.

Catalytic Oxidation of Benzyl Alcohol to Benzaldehyde on Au8 and Au6Pd2 Clusters: A DFT Study on the Reaction Mechanism

Density functional theory calculations were performed to investigate the reaction mechanism of the aerobic oxidation of benzyl alcohol to benzaldehyde catalyzed by Au and Au–Pd clusters. Two consecutive reaction mechanisms were examined with Au8 and Au6Pd2 clusters: (1) the oxidation of benzyl alcohol with dissociated O atoms on metal clusters generating benzaldehyde and H2O; and (2) oxidation with adsorbed oxygen molecules generating benzaldehyde and H2O2. The calculations show that the aerobic oxidation of benzyl alcohol energetically prefers to proceed in the former mechanism, which agrees with the experimental observation. We demonstrate that the role of Au centers around the activation of molecular oxygen to peroxide-like species, which are capable of the H–abstraction of benzyl alcohol. The roles of Pd in the Au6Pd2 cluster are: (1) increasing the electron distribution to neighboring Au atoms, which facilitates the activation of O2; and (2) stabilizing the adsorption complex and transition states by the interaction between positively charged Pd atoms and the π-bond of benzyl alcohol, both of which are the origin of the lower energy barriers than those of Au8.

Modulation of the Activity of Gold Clusters Immobilized on Functionalized Mesoporous Materials in the Oxidation of Cyclohexene via the Functional Group. The Case of Aminopropyl Moiety

Gold nanoclusters and isolated gold atoms have been produced in a two-liquid phase procedure that involves a solution of gold in aqua regia and rosemary essential oil as organic layer. These gold entities have been immobilized on the ordered mesoporous silica material SBA-15 functionalized with different amounts of aminopropyl groups. The resulting materials have been characterized by XRD, N₂ adsorption, chemical analysis, TGA, ²⁹Si MAS NMR, ¹³C CP/MAS NMR, UV-vis spectroscopy, XPS, and STEM. The Au-containing materials retain the ordering and porosity of the pristine support. Gold content varies in the range of 0.07–0.7 wt% as a function of the specific immobilization conditions, while STEM evidences the presence of isolated gold atoms. XPS shows a shift of the Au 4f BE toward values lower than those of metallic gold. The catalytic activity in the oxidation of cyclohexene with molecular oxygen at atmospheric pressure parallels the Au content of the aminopropyl-SBA-15 supports. This activity is higher than that of analogous Au entities immobilized on SBA-15 functionalized with thiol or sulfonate groups, the activity decreasing in the order Au-NH₂ > Au-SO₃⁻ > Au-SH. This behavior has been attributed to differences in the interaction strength between the functional group and the Au entities, which is optimum for the aminopropyl groups.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

Heterogeneous Cross-Coupling over Gold Nanoclusters

Au clusters with the precise numbers of gold atoms, a novel nanogold material, have recently attracted increasing interest in the nanoscience because of very unique and unexpected properties. The unique interaction and electron transfer between gold clusters and reactants make the clusters promising catalysts during organic transformations. The AunLm nanoclusters (where L represents organic ligands and n and m mean the number of gold atoms and ligands, respectively) have been well investigated and developed for selective oxidation, hydrogenation, photo-catalysis, and so on. These gold clusters possess unique frameworks, providing insights into the catalytic processes and an excellent arena to correlate the atomic frameworks with their intrinsic catalytic properties and to further investigate the tentative reaction mechanisms. This review comprehensively summarizes the very latest advances in the catalytic applications of the Au nanoclusters for the C-C cross-coupling reactions, e.g., Ullmann, Sonogashira, Suzuki cross-couplings, and A3-coupling reactions. It is found that the proposed catalytically active sites are associated with the exposure of gold atoms on the surface of the metal core when partial capping organic ligands are selectively detached under the reaction conditions. Finally, the tentative catalytic mechanisms over the ligand-capped Au nanoclusters and the relationship of structure and catalytic performances at the atomic level using computational methods are explored in detail.

Regium-π bonds: An Unexplored Link between Noble Metal Nanoparticles and Aromatic Surfaces

The ability of metal clusters involving elements from group 11 (Ag, Cu, Au) to favorably interact with π systems of different size and electronic nature was evaluated at the PBE0-D3/def2-TZVPP//PBE0-D3/def2-TZVP level of theory. The M₉ clusters (M=Cu, Ag, Au) were used as σ-hole and σ-lump donors, and benzene, trifluorobenzene, and hexafluorobenzene as aromatic rings. In addition, the study was expanded to the analysis of extended π systems by using naphthalene and anthracene as well as their corresponding perfluorinated derivatives. Furthermore, Bader's theory of Atoms in Molecules as well as natural bonding orbital and spin-density calculations were used to further investigate and characterize the regium-π and σ-lump complexes described herein. Apparently, regium-π bonds have not previously been described in the literature and may be of great importance in the understanding of organocatalytic processes involving aromatic substrates as well as in the design of new materials based on this novel subclass of σ-hole bonding.

Sub-nanometre metal clusters for catalytic carbon–carbon and carbon–heteroatom cross-coupling reactions

Potential new catalysts for C–C and C–Het cross-coupling bond-forming reactions.

Mechanism of the Aerobic Homocoupling of Phenylboronic Acid on Au₂₀⁻: A DFT Study

The mechanism of the gold nanocluster-catalyzed aerobic homocoupling of arylboronic acids has been elucidated by means of DFT calculations with Au20(-) as a model cluster for the Au:[poly(N-vinylpyrrolidin-2-one)] catalyst. We found that oxygen affects the adsorption of phenylboronic acid and, by lowering the energy barrier, a water molecule enhances dissociation of the C-B bond, which is probably the rate-determining step. The key role of oxygen is in activating the surface of the gold cluster by generating Lewis acidic sites for adsorption and activation of the phenylboronic acid, leading to the formation of biphenyl through a superoxo-like species. Moreover, the oxygen adsorbed on the Au nanocluster can act as an oxidant for phenylboronic acid, giving phenol as a byproduct. As shown by NBO analysis, the basic aqueous reaction medium facilitates the reductive elimination process by weakening the Au-C bond, thereby enhancing the formation of biphenyl. The coupling of phenyl and reductive elimination of biphenyl occur at the top or facet site with low-energy-barrier through spillover of phenyl group on Au NC. The present findings are useful for the interpretation or design of other coupling reactions with Au NC.

Plasmon induced self-assembly of gold nanorods in polymer films

Photoactivated orientation of gold nanorods (AuNRs) incorporated into polymethylmethacrylate and SU-8 photoresist films occurs under red light activation. Excitation of the longitudinal surface plasmon resonance band leads to selective orientation of the nanostructures, revealed by a change in the absorption spectra.

Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters: a computational study

Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au13, d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ mol(-1). At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ mol(-1) relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au38, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and gold-palladium alloys.

Highly effective catalysis of the double-icosahedral Ru(19) cluster for dinitrogen dissociation - a first-principles investigation

The N2 bond cleavage is the rate-limiting step in the synthesis of ammonia, and ruthenium is a catalyst well known for this reaction. The double-icosahedral (D5h) Ru19 cluster is famous as an active catalyst, and has a remarkable stability towards the adsorption of H2, N2 and CO. Using first-principles calculations, we have investigated the adsorption and dissociation of dinitrogen on a double-icosahedral Ru19 cluster. Our results show that the hollow site in the rhombus region (BHB site) of the Ru19 cluster possesses the greatest catalytic activity to dissociate N2, with the reaction barrier of 0.89 eV and an exothermicity of -1.45 eV. Multiple coadsorption of N2 on the cluster (i.e. coadsorption of 2N2 and 3N2 on a single Ru19 cluster) causes the barrier to dissociate N2 to be less on a BHB site than for adsorption of a single N2. To understand the catalytic properties of a Ru19 cluster towards N2 bond cleavage, we calculated the electron population, vibrational wavenumbers and local densities of states; the results are explicable.

Theoretical and experimental insights into the origin of the catalytic activity of subnanometric gold clusters: attempts to predict reactivity with clusters and nanoparticles of gold

Particle size is one of the key parameters determining the unexpected catalytic activity of gold, with reactivity improving as the particle gets smaller. While this is valid in the 1-5 nm range, chemists are now investigating the influence of particle size in the subnanometer regime. This is due to recent advances in both characterization techniques and synthetic routes capable of stabilizing these size-controlled gold clusters. Researchers reported in early studies that small clusters or aggregates of a few atoms can be extremely active in some reactions, while 1-2 nm nanoparticles are catalytically more efficient for other reactions. Furthermore, the possibility that small gold clusters generated in situ from gold salts or complexes could be the real active species in homogeneous gold-catalyzed organic reactions should be considered. In this Account, we address two questions. First, what is the origin of the enhanced reactivity of gold clusters on the subnanometer scale? And second, how can we predict the reactions where small clusters should work better than larger nanoparticles? Both geometric factors and electronic or quantum size effects become important in the subnanometer regime. Geometric reasons play a key role in hydrogenation reactions, where only accessible low coordinated neutral Au atoms are needed to dissociate H2. The quantum size effects of gold clusters are important as well, as clusters formed by only a few atoms have discrete molecule-like electronic states and their chemical reactivity is related to interactions between the cluster's frontier molecular orbitals and those of the reactant molecules. From first principles calculations, we predict an enhanced reactivity of small planar clusters for reactions involving activation of CC multiple bonds in alkenes and alkynes through Lewis acid-base interactions, and a better catalytic performance of 3D gold nanoparticles in redox reactions involving bond dissociation by oxidative addition and new bond formation by reductive elimination. In oxidation reactions with molecular O2, initial dissociation of O2 into basic oxygen atoms would be more effectively catalyzed by gold nanoparticles of ∼1 nm diameter. In contrast, small planar clusters should be more active for reactions following a radical pathway involving peroxo or hydroperoxo intermediates. We have experimentally confirmed these predictions for a series of Lewis acid and oxidation reactions catalyzed by gold clusters and nanoparticles either in solution or supported on solid carriers.

The chemical origin and catalytic activity of coinage metals: from oxidation to dehydrogenation

Electronegative adspecies on inactive coinage metals can dramatically enhance their catalytic activity for oxidation as well as dehydrogenation reactions.

New multicomponent catalysts for the selective aerobic oxidative condensation of benzylamine to N-benzylidenebenzylamine

Nano-oxide domains partially embedded inside the UVM-7 silica walls act as excellent support for Au in the oxidative condensation of benzylamine.

Design of an ultrasmall Au nanocluster-CeO2 mesoporous nanocomposite catalyst for nitrobenzene reduction

In this work we are inspired to explore gold nanoclusters supported on mesoporous CeO2 nanospheres as nanocatalysts for the reduction of nitrobenzene. Ultrasmall Au nanoclusters (NCs) and mesoporous CeO2 nanospheres were readily synthesized and well characterized. Due to their ultrasmall size, the as-prepared Au clusters can be easily absorbed into the mesopores of the mesoporous CeO2 nanospheres. Owing to the unique mesoporous structure of the CeO2 support, Au nanoclusters in the Au@CeO2 may effectively prevent the aggregation which usually results in a rapid decay of the catalytic activity. It is notable that the ultrasmall gold nanoclusters possess uniform size distribution and good dispersibility on the mesoporous CeO2 supports. Compared to other catalyst systems with different oxide supports, the as-prepared Au nanocluster-CeO2 nanocomposite nanocatalysts showed efficient catalytic performance in transforming nitrobenzene into azoxybenzene. In addition, a plausible mechanism was deeply investigated to explain the transforming process. Au@CeO2 exhibited efficient catalytic activity for reduction of nitrobenzene. This strategy may be easily extended to fabricate many other heterogeneous catalysts including ultrasmall metal nanoclusters and mesoporous oxides.

Can Surface Plasmon Fields Provide a New Way to Photosensitize Organic Photoreactions? From Designer Nanoparticles to Custom Applications

In this Perspective, we explore the opportunities that plasmon excitation may offer for the practitioners in organic chemistry. Beyond the interesting physical properties and lively colors of colloidal solutions of noble metal nanostructures, excitation of plasmon transitions can trigger a variety of processes, from the simple heat delivery with pinpoint precision, to the enhanced generation of excited states in the immediate vicinity of the nanoparticle, to electron- and hole-transfer processes that can readily participate in photoredox processes. In understanding how particles are produced, what properties they have, and the diversity of nanostructures and environments in which they can be produced, we aim at providing the small steps toward a paradigm that will allow organic chemists to take advantage of the opportunities that await in the area of plasmon-assisted processes.

Theoretical tools for studying gold nanoparticles as catalysts for oxidation and hydrogenation reactions

In this contribution, the ability of small isolated gold NP to dissociate O2 and generate a reactive surface oxide layer, the nature of the new gold active sites generated, and their implication in the mechanism of alcohol oxidation to aldehydes has been analyzed from a theoretical point of view. The nature of the active sites involved in H2 dissociation and the possible ways in which Au/TiO2 catalysts can be modified in order to increase their activity toward hydrogenation of nitroaromatics without modifying their high chemoselectivity is also explored.

Small gold clusters formed in solution give reaction turnover numbers of 10(7) at room temperature

Very small gold clusters (3 to 10 atoms) formed from conventional gold salts and complexes can catalyze various organic reactions at room temperature, even when present at concentrations of parts per billion. Absorption and emission ultraviolet-visible spectroscopy and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry revealed that, for example, the ester-assisted hydration of alkynes began only when clusters of three to five gold atoms were formed. The turnover numbers and turnover frequencies associated with these catalyzed reactions can be as high as 10(7) and 10(5) per hour, respectively.