Silica-Supported Au–CuOx Hybrid Nanocrystals as Active and Selective Catalysts for the Formation of Acetaldehyde from the Oxidation of Ethanol

Synergy in Au-CuO Janus Structure for Catalytic Isopropanol Oxidative Dehydrogenation to Acetone

The controlled oxidation of alcohols to the corresponding ketones or aldehydes via selective cleavage of the β-C-H bond of alcohols under mild conditions still remains a significant challenge. Although the metal/oxide interface is highly active and selective, the interfacial sites fall far behind the demand, due to the large and thick support. Herein, we successfully develop a unique Au-CuO Janus structure (average particle size=3.8 nm) with an ultrathin CuO layer (0.5 nm thickness) via a bimetal in situ activation and separation strategy. The resulting Au-CuO interfacial sites prominently enhance isopropanol adsorption and decrease the energy barrier of β-C-H bond scission from 1.44 to 0.01 eV due to the strong affinity between the O atom of CuO and the H atom of isopropanol, compared with Au sites alone, thereby achieving ultrahigh acetone selectivity (99.3 %) over 1.1 wt % AuCu_0.75 /Al₂ O₃ at 100 °C and atmospheric pressure with 97.5 % isopropanol conversion. Furthermore, Au-CuO Janus structures supported on SiO₂ , TiO₂ or CeO₂ exhibit remarkable catalytic performance, and great promotion in activity and acetone selectivity is achieved as well for other reducible oxides derived from Fe, Co, Ni and Mn. This study should help to develop strategies for maximized interfacial site construction and structure optimization for efficient β-C-H bond activation.

High-Density Formation of Metal/Oxide Interfacial Catalytic Active Sites through Hybrid Clustering

We developed a method for preparing catalysts based on hybrid clusters that formed high-density metal/oxide interfacial active sites. A Ru-V hybrid cluster, [{Ru(cym)}₄V₆O₁₉] (cym = p-cymene), was used as a precursor to prepare Ru-V catalysts. Transmission electron microscopy and X-ray absorption spectroscopy (XAS) analyses revealed that composite nanoparticles of Ru and V were formed through hybrid clustering, while conventional coimpregnation of Ru and V afforded separate nanoparticles. The activity of the Ru-V catalysts toward N-alkylation of amines with alcohols depended on the mixing method (hybrid clustering > coimpregnation > physical mixing ≈ pristine Ru). The formation mechanism of the composite nanoparticles from the hybrid cluster was revealed using in situ XAS analysis. Finally, we proposed a simple but efficient catalyst preparation method, based on in situ formation of hybrid cluster precursors combined with a conventional coimpregnation method.

Highly Efficient Ir-CoOx Hybrid Nanostructures for the Selective Hydrogenation of Furfural to Furfuryl Alcohol

Decoration of noble metals with transition-metal oxides has been intensively studied for heterogeneous catalysis. However, controllable syntheses of metal-metal oxide heterostructures are difficult, and elucidation of such interfaces is still challenging. In this work, supported IrCo alloy nanoparticles are transformed into supported Ir-CoO_x close-contact nanostructures by in situ calcination and following selective reduction. Relative to Ir/Al₂O₃, Ir-CoO_x/Al₂O₃ shows greatly enhanced activities for the hydrogenation of furfural derivatives to the corresponding furfuryl alcohol derivatives with more than 99% selectivity and demonstrates significantly improved activities and selectivity for hydrogenations of α,β-unsaturated aldehydes to α,β-unsaturated alcohols. The modification of Ir surfaces with CoO_x prevents Ir nanoparticles from growing, achieving high thermal and catalytic stabilities. Theoretic calculation suggests that the better catalytic performance of Ir-CoO_x/Al₂O₃ is ascribed to the Ir-CoO_x interaction, which promotes the absorption of furfural as well as desorption of furfuryl alcohol, resulting in enhanced catalytic activities.

The Application of Copper-Gold Catalysts in the Selective Oxidation of Glycerol at Acid and Basic Conditions

The crude glycerol is produced during the transesterification of animal fats and vegetable oils, but it is a by-product of this process. Currently, its elimination is a problem in the chemical industry. The main goal of this work was the preparation, characterization and application of mesoporous cerium-zirconium oxide as supports for copper and gold species and the comparison of selected factors on the properties of catalysts in glycerol oxidation in the liquid phase. The samples were characterized using adsorption and desorption of nitrogen, XRD, UV-vis, XPS, TEM, SEM, and STEM-EDXS. The obtained results of glycerol oxidation show that the bimetallic copper-gold catalysts are more active and selective to glyceric acid in this reaction than analogous monometallic gold catalysts. Additionally, bimetallic catalysts are also characterized by the catalytic stability, and their application leads to the increase of selectivity to glyceric acid during their reusing in glycerol oxidation in alkali media. In this work, the influence of selected factors, e.g., oxygen source and its pressure, solution pH, and base content on the catalytic activity of bimetallic catalysts is discussed.

Effect of ultra-low amount of gold in oxide-supported bimetallic Au–Fe and Au–Cu catalysts on liquid-phase aerobic glycerol oxidation in water

Low-loaded Au–Fe and Au–Cu supported bimetallic catalysts showed exceptional activity in liquid-phase glycerol oxidation. Strong synergetic effect of Au–Fe (Cu) interaction and Au content tuned the oxidation activity and selectivity of the catalysts.

Organochalcogen ligands in catalysis of oxidation of alcohols and transfer hydrogenation

Organochalcogen compounds have been used as the building blocks for the development of a variety of catalysts that have been studied comprehensively during the last two decades for several chemical transformations. Transfer hydrogenation (reduction of carbonyl compounds to alcohols) and oxidation of alcohols (conversion of alcohols to their respective ketones and aldehydes) are also among such chemical transformations. Some compilations are available in the literature on the development of catalysts, based on organochalcogen ligands, and their applications in Heck reaction, Suzuki reaction, and other related aspects. Some review articles have also been published on different aspects of oxidation of alcohols and transfer hydrogenation. However, no such article is available in the literature on the syntheses and use of organochalcogen ligated catalysts for these two reactions. In this perspective, a survey of developments pertaining to the synthetic aspects of such organochalcogen (S/Se/Te) based catalysts for the two reactions has been made. In addition to covering the syntheses of chalcogen ligands, their metal complexes and nanoparticles (NPs), emphasis has also been placed on the efficient conversion of different substrates during catalytic reactions, diversity in catalytic potential and mechanistic aspects of catalysis. It also includes the analysis of comparison (in terms of efficiency) between this unique class of catalysts and efficient catalysts without a chalcogen donor. The future scope of this area has also been highlighted.

Synergistic effect between gold nanoparticles and Fe-doped γ-MnO2 toward enhanced aerobic selective oxidation of ethanol

Gold nanoparticles supported on Fe-doped γ-MnO₂ were found to show a strong synergistic effect in the aerobic selective oxidation of ethanol to acetaldehyde.

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.

Atomic-Scale Observation of Bimetallic Au-CuOx Nanoparticles and Their Interfaces for Activation of CO Molecules

Supported gold nanoparticles with sizes below 5 nm display attractive catalytic activities for heterogeneous reactions, particularly those promoted by secondary metal (e.g., Cu) because of the well-defined synergy between metal compositions. However, the specific atomic structure at interfaces is less interpreted systematically. In this work, various bimetallic Au-CuO_x catalysts with specific surface structures were synthesized and explored by aberration-corrected scanning transmission electron microscopy (AC-STEM), temperature-programmed experiments and in situ DRIFT experiments. Results suggest that the atomic structure and interfaces between gold and CuO_x are determined by the nucleation behaviors of the nanoparticles and result in subsequently the distinctive ability for CO activation. Bimetallic CuO*/Au sample formatted by gold particles surrounded with CuO_x nanoclusters have rough surface with prominently exposed low-coordinated Au step defects. Whereas the bimetallic Au@CuO sample formatted by copper precursor in the presence of gold nanoparticles have core-shell structure with relatively smooth surface. The former structure of CuO*/Au displays much accelerated properties for CO adsorption and activation with 90% CO converted to CO₂ at 90 °C and nice stability with time on stream. The results clearly determine from atomic scale the significance of exposed gold step sites and intrinsic formation of defected surface by different nucleation. The above properties are directly responsible for the induced variation in chemical composition and the catalytic activity.

Controlled Synthesis and Enhanced Catalytic Activity of Well-Defined Close-Contact Pd-ZnO Nanostructures

In this study, PdZn-ordered intermetallic nanoparticles (NPs) were prepared in liquid phase by butyllithium co-reduction of their precursors at 240 °C. Through calcination and subsequent reduction with H₂, the synthesized PdZn NPs were then in situ transformed into Pd-ZnO heteroaggregate nanocatalysts on alumina supports. Various characterization techniques, such as diffuse reflectance Fourier transform infrared with CO probes, transmission electron microscopy, X-ray diffraction, H₂ temperature-programmed reduction, and X-ray photoelectron spectra, reveal that PdZn NPs are ordered intermetallic compounds, and in situ transformation of PdZn alloy NPs results in close-contact Pd-ZnO heteroaggregates, where the interfaces are highly active and the interaction between Pd and ZnO prevents the active particles from agglomeration. The catalytic hydrogenations of nitrophenols over Pd/Al₂O₃ and Pd-ZnO/Al₂O₃ were investigated. The results show that Pd-ZnO/Al₂O₃ illustrates an enhanced catalytic activity relative to Pd/Al₂O₃, and no obvious activity degradation was observed in the recycle catalytic experiments over such nanostructures. It is concluded that the Pd-ZnO interaction not only enhances the catalytic hydrogenation activity but also promotes the thermal and catalytic stability.

Pt-Embedded CuO x-CeO2 Multicore-Shell Composites: Interfacial Redox Reaction-Directed Synthesis and Composition-Dependent Performance for CO Oxidation

Exploring the state-of-the-art heterogeneous catalysts has been a general concern for sustainable and clean energy. Here, Pt-embedded CuO _x-CeO₂ multicore-shell (Pt/CuO _x-CeO₂ MS) composites are fabricated at room temperature via a one-pot and template-free procedure for catalyzing CO oxidation, a classical probe reaction, showing a volcano-shaped relationship between the composition and catalytic activity. We experimentally unravel that the Pt/CuO _x-CeO₂ MS composites are derived from an interfacial autoredox process, where Pt nanoparticles (NPs) are in situ encapsulated by self-assembled ceria nanospheres with CuO _x clusters adhered through deposition/precipitation-calcination process. Only Cu-O and Pt-Pt coordination structures are determined for CuO _x clusters and Pt NPs in Pt/CuO _x-CeO₂ MS, respectively. Importantly, the close vicinity between Pt and CeO₂ benefits to more oxygen vacancies in CeO₂ counterparts and results in thin oxide layers on Pt NPs. Meanwhile, the introduction of CuO _x clusters is crucial for triggering synergistic catalysis, which leads to high resistance to aggregation of Pt NPs and improvement of catalytic performance. In CO oxidation reaction, both Pt^δ+-CO and Cu⁺-CO can act as active sites during CO adsorption and activation. Nonetheless, redundant content of Pt or Cu will induce a strongly bound Pt-O-Ce or Cu-[O _x]-Ce structures in air-calcinated Pt/CuO _x-CeO₂ MS composites, respectively, which are both deleterious to catalytic reactivity. As a result, the composition-dependent catalytic activity and superior durability of Pt/CuO _x-CeO₂ MS composites toward CO oxidation reaction are achieved. This work should be instructive for fabricating desirable multicomponent catalysts composed of noble metal and bimetallic oxide composites for diverse heterogeneous catalysis.

Visible light-driven methanol dehydrogenation and conversion into 1,1-dimethoxymethane over a non-noble metal photocatalyst under acidic conditions

The dehydrogenation and conversion of methanol into 1,1-dimethoxymethane (DMM) was achieved over noble metal free photocatalyst CdS/Ni₂P under visible light.

Insight into the chemical adsorption properties of CO molecules supported on Au or Cu and hybridized Au-CuO nanoparticles

Although nanosized Au clusters have been well developed for many applications, fundamental understanding of their adsorption/activation behaviors in catalytic applications is still lacking, especially when other elements provide promotion or hybridization functions. Au hybridized with Cu element is a highly investigated system; Cu is in the same element group as Au and thus displays similar physicochemical properties. However, their hybrids are not well understood in terms of their chemical states and adsorption/activation properties. In this work, typical γ-Al₂O₃-supported Au and CuO as well as Au-CuO nanoparticles were prepared and characterized to explore their adsorption/activation properties in depth using CO as a probe molecule using advanced techniques, such as XPS, HR-TEM, temperature programmed experiments and operando DRIFT combined with mass spectra. It was found that gold and copper can both act as active sites during CO adsorption and activation. The CO-TPD and operando DRIFT results also revealed that CO molecules were able to react with surface oxygenated species, resulting in the direct formation of CO₂ over the three samples in the absence of gaseous O₂. The gold step sites (Au^step) participated more readily in the reaction, especially under gaseous O₂-free conditions. During adsorption, CO molecules were more preferentially adsorbed on Au⁰ sites at lower temperature comparing with those on the Cu⁰ sites. However, competitive adsorption occurred between CO adsorbed on Au⁰ and Cu⁰ with increased reaction temperature, and the synergy between the Au and Cu compositions was too strong to suppress the adsorption and activation of the CO molecules. The dynamic adsorption equilibrium over 120 °C to 200 °C resulted in the appearance of a hysteresis performance platform.

A Rational Solid-State Synthesis of Supported Au-Ni Bimetallic Nanoparticles with Enhanced Activity for Gas-Phase Selective Oxidation of Alcohols

A facile confined solid-state seed-mediated alloying strategy is applied for the rational synthesis of supported Au-Ni bimetallic nanoparticles (BMNPs). The method sequentially deposits nickel salts and AuNP seeds into the ordered array of extra-large mesopores (EP-FDU-12 support) followed by a high-temperature annealing process. The size, structure, and composition of the AuNi BMNPs can be well tuned by varying the AuNP seeds, annealing temperature, and feeding ratio of metal precursors. Kinetic studies and DFT calculations suggest that the introduction of the Ni component can significantly prompt the O₂ activation on AuNPs, which is critical for the selective alcohol oxidation using molecular O₂ as the oxidant. The optimal Au-Ni BMNP catalyst showed the highest turnover frequency (TOF) (59 000 h^-1, 240 °C) and highest space-time yield (STY) of benzyl aldehyde (BAD) productivity (9.23 kg·g_Au^-1·h^-1) in the gas-phase oxidation of benzyl alcohol (BA), which is at least about 5-fold higher than that of other supported Au catalysts.

Hot Hole Enhanced Synergistic Catalytic Oxidation on Pt-Cu Alloy Clusters

Hot holes in Pt-Cu alloy clusters can act as catalyst to accelerate the intrinsic aerobic oxidation reactions. It is described that under visible light irradiation the synergistic alcohol catalytic oxidation on Pt-Cu alloy clusters (≈1.1 nm)/TiO₂ nanobelts could be significant promoted by interband-excitation-generated long-lifetime hot holes in the clusters.

Reaction-Induced Self-Assembly of CoO@Cu2 O Nanocomposites In Situ onto SiC-Foam for Gas-Phase Oxidation of Bioethanol to Acetaldehyde

A high-performance SiC-foam-structured nanocomposite catalyst of CoO@Cu₂ O (i.e., 50-100 nm CoO partially covered with ca. 10 nm Cu₂ O) was engineered from nano- to macro-scales in one step for the high-throughput gas-phase aerobic oxidation of bioethanol to acetaldehyde. This special CoO@Cu₂ O nanostructure shows much higher activity/selectivity than other binary metal-oxide assemblies such as CuO_x &CoO nano-mixtures or inverse Cu₂ O@CoO nanostructures. The catalyst was facilely but exclusively obtainable by in situ reaction-induced transformation of the respective metal nitrates supported on SiC-foam into the CoO@Cu₂ O nanostructure in the reaction stream. It achieved 95 % conversion with 98 % selectivity under mild conditions and was stable for at least 150 h for a feed of 20 vol % ethanol (much higher than in the literature: 1-6 vol %) at a high EtOH weight hourly space velocity of 8.5 h^-1 . Abundant Cu₂ O-CoO interfaces and high stability of the CoO@Cu₂ O nanostructure were responsible for the high activity/selectivity and promising stability in this reaction.

Metal/oxide interfacial effects on the selective oxidation of primary alcohols

A main obstacle in the rational development of heterogeneous catalysts is the difficulty in identifying active sites. Here we show metal/oxide interfacial sites are highly active for the oxidation of benzyl alcohol and other industrially important primary alcohols on a range of metals and oxides combinations. Scanning tunnelling microscopy together with density functional theory calculations on FeO/Pt(111) reveals that benzyl alcohol enriches preferentially at the oxygen-terminated FeO/Pt(111) interface and undergoes readily O–H and C–H dissociations with the aid of interfacial oxygen, which is also validated in the model study of Cu₂O/Ag(111). We demonstrate that the interfacial effects are independent of metal or oxide sizes and the way by which the interfaces were constructed. It inspires us to inversely support nano-oxides on micro-metals to make the structure more stable against sintering while the number of active sites is not sacrificed. The catalyst lifetime, by taking the inverse design, is thereby significantly prolonged.

Metals supported on metal oxides are common heterogeneous catalysts. Here the authors show that metal/oxide interfacial sites are highly active for alcohol oxidation—independent of the particle sizes—and use this information to design inverse oxide-on-metal particles with high activity and stability.

High efficiency and stability of Au–Cu/hydroxyapatite catalyst for the oxidation of carbon monoxide

Synergistic effect between gold and CuO_x phase for high efficiency and stability.

A unique opportunity for the utilization of glass wastes as a resource for catalytic applications: toward a cleaner environment

n-glass-waste-SO₃H at the service of catalytic application.

A magnetically recyclable iron oxide-supported copper oxide nanocatalyst (Fe3O4–CuO) for one-pot synthesis of S-aryl dithiocarbamates under solvent-free conditions

A convenient preparation of S-aryl dithiocarbamates from amine, carbon disulfide and aryl iodide was developed by using the Fe₃O₄–CuO nanocatalyst under solvent free conditions.

Non-Oxidative Dehydrogenation Pathways for the Conversion of C2 -C4 Alcohols to Carbonyl Compounds

Gold nanoparticles (NPs) supported on hydrotalcite (Au/HT) are highly active and selective catalysts for the continuous, gas-phase, non-oxidative dehydrogenation of bioderived C2 -C4 alcohols. A sharp increase in turn over frequency (TOF) is noted when the size of Au NPs is less than 5 nm relating to the strong synergy between metallic Au NPs and the acid-base groups on the support surface. It is shown that catalytic activity depends critically on Au NP size, support composition, and support pretreatments. A reaction pathway elucidated from kinetic isotope effects suggests that the abstraction of β-H by Au NPs (C-H activation) is the rate-determining step in the dehydrogenation of bioderived C2 -C4 alcohols.

Solvent-Free Synthesis of Zeolite Crystals Encapsulating Gold-Palladium Nanoparticles for the Selective Oxidation of Bioethanol

The conversion of bioethanol into valuable products is an important area in the conversion of biomass. We demonstrate the successful synthesis of bimetallic gold-palladium (Au-Pd) nanoparticles encapsulated within S-1 zeolite crystals (AuPd@S-1) by a solvent-free strategy. This strategy allows highly efficient use of the noble metals, with more than 96 % of the gold and palladium being loaded into the final samples. Electron microscopy characterization and investigations with probe molecules confirm that the Au-Pd nanoparticles are encapsulated inside the S-1 crystals. The AuPd@S-1 catalyst is very active for the aerobic oxidation of bioethanol, giving 100 % conversion and 99 % selectivity to acetic acid. Even in the presence of 90 % water, the catalyst still gives a conversion higher than 80 % and a selectivity of 95 %. More importantly, the AuPd@S-1 catalyst exhibits excellent stability in the oxidation of bioethanol. These features are important for future practical applications of the AuPd@S-1 catalyst.