Surface Au-H Species as Self-Generated Prosthetic Groups of a Formate Dehydrogenase-like Au Nanozyme to Engineer Multienzymatic Activities

Although the past decade has witnessed a rapid development of oxidoreductase-mimicking nanozymes, the mimicry of cofactors that play key roles in mediating electron and proton transfer remains limited. This study explores how surface Au-H species conjugated to Au nanoparticles (NPs) that imitate formate dehydrogenase (FDH) can serve as cofactors, analogous to NADH in natural enzymes, offering diverse possibilities for FDH-mimicking Au nanozymes to mimic various enzymes. Once O2 is present, Au-H species assist Au NPs to complete the on-demand H2O2 generation for cascade reactions. Alternatively, when oxidizing organic molecules are introduced as substrates, Au-H species confer nitro reductase- and aldehyde reductase-like activities on Au NPs under anaerobic conditions. Furthermore, similar to the dehydrogenase-NADH complex, Au NPs possessing Au-H species are gifted with esterase-like activity for ester hydrolysis. By revealing that Au-H species are prosthetic groups for FDH-mimicking Au nanozymes, this work may inspire explorations into future self-generated cofactor mimics for nanozymes, thereby circumventing the need for exogenous cofactors.

Dioxygen Activation by Gold(I)-Distorted Porphyrin Dinuclear Complexes

Interactions between gold-based materials and dioxygen (O2) have motivated researchers to understand reaction mechanisms for O2 activation by homo- and heterogeneous gold catalysts. In this work, gold(I) porphyrin dinuclear complexes were synthesized with a saddle-distorted porphyrin ligand. The gold(I) porphyrin complexes showed unprecedented O2 activation in the presence of protic solvents to form gold(III) tetradentate porphyrin complexes. Mechanistic insights into the O2 activation by the gold(I) center were elucidated by spectroscopic measurements and theoretical calculations, revealing that dissociation of halides on the gold(I) center by alcohol solvents and hydrogen bonding of an N-H proton in the distorted porphyrin with dioxygen played important roles in establishing the unique reactivities of gold(I) complexes.

Insights into the Effect of Metal Ratio on Cooperative Redox Enhancement Effects over Au- and Pd-Mediated Alcohol Oxidation

The aerobic oxidation of alcohols and aldehydes over supported heterogeneous catalysts can be considered as comprising two complementary and linked processes: dehydrogenation and oxygen reduction. Significant rate enhancements can be observed when these processes are catalyzed by independent active sites, coupled by electron transport between the two catalysts. This effect, termed cooperative redox enhancement (CORE), could significantly influence how researchers approach catalyst design, but a greater understanding of the factors which influence it is required. Herein, we demonstrate that the Au/Pd ratio used in physical mixtures of monometallic catalysts and phase-separated Au and Pd bimetallic catalysts dramatically influences the degree to which CORE effects can promote alcohol oxidation. Perhaps more interestingly, the roles of Au and Pd in this coupled system are determined to be interchangeable. Preliminarily, we hypothesize that this is attributed to the relative rates of the coupled reactions and demonstrate how physical properties can influence this. This deeper understanding of the factors which influence CORE is an important development in bimetallic catalysis.

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Oxidation of Alcohols into Carbonyl Compounds Using a CuO@GO Nano Catalyst in Oxygen Atmospheres

In this article, the oxidation of alcohols into carbonyl compounds was studied in oxygen atmospheres using a copper oxide on graphene oxide (CuO@GO) nano composites catalyst, synthesized by the wet chemistry method. CuO@GO nano composites were prepared from GO, and CuO NPs by the sol-gel method. The transformation of aromatic alcohols into corresponding carbonyl compounds in good-to-high yields were observed using the CuO@GO catalyst under an oxygen atmosphere. Synthesized CuO@GO was confirmed by FT-IR, XRD, XPS, TEM, FE-SEM, TEM, and SEM analyses, and revealed intercalation of CuO-NPs on/in GO nano sheets through the chelation of Cu+2 ions with CO, COOH, and OH groups presenting on the GO nano sheets. The catalytic activity of CuO@GO nano composites for the conversion of alcohols into carbonyl compounds were evaluated through TOF (2.56 × 10−3 mol g−1 min−1). The use of CuO@GO has shown catalytic activity and recyclability with a high conversion of alcohols to ketones. We assume that the proposed CuO@GO catalyst can be used for other key organic transformations and will be evaluated in the future.

Role of Surface Oxygen Vacancies and Oxygen Species on CuO Nanostructured Surfaces in Model Catalytic Oxidation and Reductions: Insight into the Structure-Activity Relationship Toward the Performance

Defect engineering, such as modification of oxygen vacancy density, has been considered as an effective approach to tailor the catalytic performance on transition-metal oxide nanostructured surfaces. The role of oxygen vacancies (O_V) on the surface of the as-prepared, zinnia-shaped morphology of CuO nanostructures and their marigold forms on calcination at 800 °C has been investigated through the study of model catalytic reactions of reduction of 4-nitrophenol and aerobic oxidation of benzyl alcohol. The O_V on the surfaces of different morphologies of CuO have been identified and quantified through Rietveld analysis and HRTEM, EPR, and XPS studies. The structure-activity relationships between surface oxygen vacancies (O_V) and catalytic performance have been systematically investigated. The enhanced catalytic performance of the cubic CuO nanostructures compared to their as-prepared forms has been attributed to the formation of surface oxygen species on the reactive and dominant (110) surface that has low oxygen vacancy formation energy. The mechanistic role of surface oxygen species in the studied reactions has been quantitatively correlated with the catalytic activity of the different morphological forms of the CuO nanostructures.

Synthesis of active, robust and cationic Au25 cluster catalysts on double metal hydroxide by long-term oxidative aging of Au25(SR)18

Synthesis of an atomically precise Au₂₅ cluster catalyst was attempted by long-term, low-temperature aging of Au₂₅(BaET)₁₈ (BaET-H = 2-(Boc-amino)ethanethiol) on various double metal hydroxide (DMH) supports. X-ray absorption fine structure analysis revealed that bare Au₂₅ clusters with high loading (1 wt%) were successfully generated on the DMH containing Co and Ce (Co₃Ce) by oxidative aging in air at 150 °C for >12 h. X-ray absorption near-edge structure and X-ray photoelectron spectroscopies showed that the Au₂₅ clusters on Co₃Ce were positively charged. The Au₂₅/Co₃Ce catalyst thus synthesized exhibited superior catalytic performance in the aerobic oxidation of benzyl alcohol under ambient conditions (TOF = 1097 h^-1 with >97% selectivity to benzoic acid) and high durability owing to a strong anchoring effect. Based on kinetic experiments, we propose that abstraction of hydride from α-carbon of benzyl alkoxide by Au₂₅ is the rate-determining step of benzyl alcohol oxidation by Au₂₅/Co₃Ce.

Advance in Selective Alcohol and Polyol Oxidation Catalysis

The aerobic oxidation of organic molecules and in particular alcohols and bio-derived poly alcohols to value-added commodity molecules is under continuous investigation, due to the importance of oxidation products (aldehydes, ketones carboxylic acids and esters) and the challenging nature of this chemical transformation, since rather harsh reaction conditions (T > 100 °C) are needed to gain a significant substrate conversion [...]

Boosting the catalytic behavior and stability of a gold catalyst with structure regulated by ceria

In this work, a series of colloidal gold nanoparticles with controllable sizes were anchored on carbon nanotubes (CNT) for the aerobic oxidation of benzyl alcohol. The intrinsic influence of Au particles on the catalytic behavior was unraveled based on different nanoscale-gold systems. The Au/CNT-A sample with smaller Au sizes deserved a faster reaction rate, mainly resulting from the higher dispersion degree (23.5%) of Au with the available exposed sites contributed by small gold particles. However, monometallic Au/CNT samples lacked long-term stability. CeO2 was herein decorated to regulate the chemical and surface structure of the Au/CNT. An appropriate CeO2 content tuned the sizes and chemical states of Au by electron delivery with better metal dispersion. Small CeO2 crystals that were preferentially neighboring the Au particles facilitated the generation of Au-CeO2 interfaces, and benefited the continuous supplementation of oxygen species. The collaborative functions between the size effect and surface chemistry accounted for the higher benzaldehyde yield and sustainably stepped-up reaction rates by Au-Ce5/CNT with 5 wt% CeO2.

Copper/TEMPO-catalyzed continuous aerobic alcohol oxidation in a micro-packed bed reactor

A micro-packed bed reactor has been developed to demonstrate aerobic oxidation of alcohols to aldehydes, utilizing green oxidant O2 and effective Cu/TEMPO-catalyzed system.

Support-Activity Relationship in Heterogeneous Catalysis for Biomass Valorization and Fine-Chemicals Production

Heterogeneous catalysts are progressively expanding their field of application, from high-throughput reactions for traditional industrial chemistry with production volumes reaching millions of tons per year, a sector in which they are key players, to more niche applications for the production of fine chemicals. These novel applications require a progressive utilization reduction of fossil feedstocks, in favor of renewable ones. Biomasses are the most accessible source of organic precursors, having as advantage their low cost and even distribution across the globe. Unfortunately, they are intrinsically inhomogeneous in nature and their efficient exploitation requires novel catalysts. In this process, an accurate design of the active phase performing the reaction is important; nevertheless, we are often neglecting the importance of the support in guaranteeing stable performances and improving catalytic activity. This review has the goal of gathering and highlighting the cases in which the supports (either derived or not from biomass wastes) share the worth of performing the catalysis with the active phase, for those reactions involving the synthesis of fine chemicals starting from biomasses as feedstocks.

Prediction Descriptor for Catalytic Activity of Platinum Nanoparticles/Metal-Organic Framework Composites

Supported metal nanoparticles (MNPs) have exhibited superior catalytic performance in various heterogeneous catalysis applications, which is usually influenced or even determined by the physicochemical properties of their porous supports. It is well acknowledged that understanding the regulation mechanism of supports is an important prerequisite to predict the catalytic performance of supported MNPs as well as the development of advanced catalysts. Here, we demonstrated that different transition-metal clusters (from Group IIIB to Group IIB) within metal-organic frameworks (MOFs) could accurately regulate the surface electronic status of supported platinum nanoparticles (Pt NPs), and the Pt/MOF composites showed a periodic activity trend in hydrogenation of 1-hexene. A strong correlation was found between the catalytic activity of Pt/MOF composites and the number of electrons in their outmost d orbitals of the transition-metal species, suggesting that the latter could play the role of prediction descriptor. Furthermore, this descriptor can be extended to predict the hydrogenation activity of more Pt/MOF composites and provide an important guiding principle for the design of supported MNPs catalysts.

Trends in Sustainable Synthesis of Organics by Gold Nanoparticles Embedded in Polymer Matrices

Gold nanoparticles (AuNPs) have emerged in recent decades as attractive and selective catalysts for sustainable organic synthesis. Nanostructured gold is indeed environmentally friendly and benign for human health; at the same time, it is active, under different morphologies, in a large variety of oxidation and reduction reactions of interest for the chemical industry. To stabilize the AuNPs and optimize the chemical environment of the catalytic sites, a wide library of natural and synthetic polymers has been proposed. This review describes the main routes for the preparation of AuNPs supported/embedded in synthetic organic polymers and compares the performances of these catalysts with those of the most popular AuNPs supported onto inorganic materials applied in hydrogenation and oxidation reactions. Some examples of cascade coupling reactions are also discussed where the polymer-supported AuNPs allow for the attainment of remarkable activity and selectivity.

New Magic Au24 Cluster Stabilized by PVP: Selective Formation, Atomic Structure, and Oxidation Catalysis

An unprecedented magic number cluster, Au24Cl x (x = 0-3), was selectively synthesized by the kinetically controlled reduction of the Au precursor ions in a microfluidic mixer in the presence of a large excess of poly(N-vinyl-2-pyrrolidone) (PVP). The atomic structure of the PVP-stabilized Au24Cl x was investigated by means of aberration-corrected transmission electron microscopy (ACTEM) and density functional theory (DFT) calculations. ACTEM video imaging revealed that the Au24Cl x clusters were stable against dissociation but fluctuated during the observation period. Some of the high-resolution ACTEM snapshots were explained by DFT-optimized isomeric structures in which all the constituent atoms were located on the surface. This observation suggests that the featureless optical spectrum of Au24Cl x is associated with the coexistence of distinctive isomers. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy of CO adsorbates revealed the electron-rich nature of Au24Cl x clusters due to the interaction with PVP. The Au24Cl x :PVP clusters catalyzed the aerobic oxidation of benzyl alcohol derivatives without degradation. Hammett analysis and the kinetic isotope effect indicated that the hydride elimination by Au24Cl x was the rate-limiting step with an apparent activation energy of 56 ± 3 kJ/mol, whereas the oxygen pressure dependence of the reaction kinetics suggested the involvement of hydrogen abstraction by coadsorbed O2 as a faster process.

Au/TiO2-Catalyzed Benzyl Alcohol Oxidation on Morphologically Precise Anatase Nanoparticles

Au nanoparticles (NP) on TiO₂ have been shown to be effective catalysts for selective oxidation reactions by using molecular oxygen. In this work, we have studied the influence of support morphology on the catalytic activity of Au/TiO₂ catalysts. Two TiO₂ anatase supports, a nanoplatelet-shaped material with predominantly the {001} facet exposed and a truncated bipyramidal-shaped nanoparticle with predominantly the {101} facet exposed, were prepared by using a nonaqueous solvothermal method and characterized by using DRIFTS, XPS, and TEM. Au nanoparticles were deposited on the supports by using the deposition-precipitation method, and particle sizes were determined by using STEM. Au nanoparticles were smaller on the support with the majority of the {101} facet exposed. The resulting materials were used to catalyze the aerobic oxidation of benzyl alcohol and trifluoromethylbenzyl alcohol. Support morphology impacts the catalytic activity of Au/TiO₂; reaction rates for reactions catalyzed by the predominantly {101} material were higher. Much of the increased reactivity can be explained by the presence of smaller Au particles on the predominantly {101} material, providing more Au/TiO₂ interface area, which is where catalysis occurs. The remaining modest differences between the two catalysts are likely due to geometric effects as Hammett slopes show no evidence for electronic differences between the Au particles on the different materials.

Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids

Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal-organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.

Deposition of highly dispersed gold nanoparticles onto metal phosphates by deposition–precipitation with aqueous ammonia

Deposition–precipitation with aqueous ammonia enabled small gold nanoparticles to be deposited onto a series of metal phosphates with high dispersity and density.

Solvent and additive-free efficient aerobic oxidation of alcohols by a perovskite oxide-based heterogeneous catalyst

A perovskite oxide has been utilized for the solvent and additive-free heterogeneous oxidation of various alcohols.

Covalent structured catalytic materials containing single-atom metal sites with controllable spatial and chemical properties: concept and application

Keep your distance! A simple and effective protocol for connecting macrocycle polymers creates a new and versatile class of highly stable single-site catalytic materials.

Atomic-scale engineering of metal–oxide interfaces for advanced catalysis using atomic layer deposition

Two main routes to optimization of metal–oxide interfaces: reducing metal particle size and oxide overcoating.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

Acceptorless Alcohol Dehydrogenation Catalysed by Pd/C

Although the selective oxidation of alcohols to carbonyl compounds is a critical reaction, it is often plagued by several challenges related to sustainability. Here, the continuous, acceptorless dehydrogenation of alcohols to carbonyl compounds over heterogeneous catalysts was demonstrated, in the absence of oxidants, bases or acceptor molecules. In addition to improving selectivity and atom efficiency, the absence of an acceptor resulted in the co-production of molecular H₂ , a clean energy source, and permitted dehydrogenation to proceed at >98 % selectivity at turnover frequency values amongst the highest in the literature. Moreover, excellent durability was observed during continuous operation over 48 h, reaching space-time yields of 0.683 g_(product)  mL^-1  h^-1 , better than the state of the art by over two orders of magnitude. Alongside these breakthroughs, the basic kinetic parameters of the reaction were also determined, allowing some of the elementary reaction steps to be identified.

Supported Gold Nanoparticles as Catalysts for the Oxidation of Alcohols and Alkanes

Supporting gold nanoparticles have shown to be extremely active for many industrially important reactions, including oxidations. Two representative examples are the oxidation of alcohols and alkanes, that are substrates of industrial interest, but whose oxidation is still challenging. This review deals with these reactions, giving an insight of the first studies performed by gold based catalysts in these reactions and the most recent developments in the field.

Heterogeneous Gold Catalysis: From Discovery to Applications

In the present review, the important and current developments of gold catalysts for a wide range of applications are comprehensively summarized. This review also provides a detailed study of the literature data concerning the preparation, characterization, and catalytic applications of gold catalysts. Additionally, the main aspects of using supported gold nanoparticles (AuNPs) as catalysts for oxidation reactions are considered. In particular, the oxidation of benzyl alcohol to benzaldehyde and the production of adipic acid from cyclohexane are discussed in detail. Lastly, the key properties of gold catalysts are described, and an outlook on the application of gold catalysts is presented.

Reshaping Dynamics of Gold Nanoparticles under H2 and O2 at Atmospheric Pressure

Despite intensive research efforts, the nature of the active sites for O₂ and H₂ adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue in heterogeneous catalysis. This stems from the absence of a clear picture of the structural evolution of Au NPs at near reaction conditions, i. e., at high pressures and high temperatures. We hereby report real-space observations of the equilibrium shapes of titania-supported Au NPs under O₂ and H₂ at atmospheric pressure using gas transmission electron microscopy. In situ TEM observations show instantaneous changes in the equilibrium shape of Au NPs during cooling under O₂ from 400 °C to room temperature. In comparison, no instant change in equilibrium shape is observed under a H₂ environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O₂, atomic oxygen, and H₂ is predicted using a multiscale structure reconstruction model. Excellent agreement between TEM observations and theoretical modeling of Au NPs under O₂ provides strong evidence for the molecular adsorption of oxygen on the Au NPs below 120 °C on specific Au facets, which are identified in this work. In the case of H₂, theoretical modeling predicts no interaction with gold atoms that explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O₂ and H₂ adsorption properties of Au NPs under real working conditions and shows a way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.

Bulk gold catalyzes hydride transfer in the Cannizzaro and related reactions

Disproportionation reactions of benzaldehyde and benzyl alcohol are catalyzed by bulk gold with hot water as the only other reagent.