In Situ Formation of Au-Pd Bimetallic Active Sites Promoting the Physically Mixed Monometallic Catalysts in the Liquid-Phase Oxidation of Alcohols

Selective oxidation of crotyl alcohol by AuxPd bimetallic pseudo-single-atom catalysts

Pseudo single-atom Pd catalysts dispersed in gold nanoparticle matrices show high selectivity and activity for room temperature crotyl alcohol oxidation.

Optimising surface d charge of AuPd nanoalloy catalysts for enhanced catalytic activity

Understanding the catalytic mechanism of bimetallic nanocatalysts remains challenging. Here, we adopt an adsorbate mediated thermal reduction approach to yield monodispersed AuPd catalysts with continuous change of the Pd-Au coordination numbers embedded in a mesoporous carbonaceous matrix. The structure of nanoalloys is well-defined, allowing for a direct determination of the structure-property relationship. The results show that the Pd single atom and dimer are the active sites for the base-free oxidation of primary alcohols. Remarkably, the d-orbital charge on the surface of Pd serves as a descriptor to the adsorbate states and hence the catalytic performance. The maximum d-charge gain occurred in a composition with 33–50 at% Pd corresponds to up to 9 times enhancement in the reaction rate compared to the neat Pd. The findings not only open an avenue towards the rational design of catalysts but also enable the identification of key steps involved in the catalytic reactions.

Understanding the catalytic mechanism of bimetallic nanocatalysts remains challenging. Here the authors demonstrate that the d-orbital charge on the surface of Pd in a well-defined AuPd nanoalloy serves as a descriptor to the adsorbate states and hence the catalytic performance.

A Cooperative Effect in a Novel Bimetallic Mo-V Nanocomplex Catalyzed Selective Aerobic C-H Oxidation

In this study, a new heterobimetallic Mo(VI)-V(V) organosilicon Schiff base complex has been prepared and characterized by different techniques, such as FTIR, Raman, MS, ICP-AES, TGA, and XPS. The bimetallic nanocomplex, revealed by TEM images, showed high oxidation stability and desired activity in the aerobic oxidation of a structurally diverse set of benzylic alcohols in ethanol as a safe solvent. Further, oxidation of benzylic hydrocarbons successfully occurred, producing the target compounds in high yields and excellent selectivities. Our results demonstrated a cooperative effect between Mo(VI) and V(V) as redox active sites in an organosilicon Schiff base framework. A facile and practical reusability of the solid catalyst at the end of the reaction was observed.

Metal-Support Cooperative Effects in Au/VPO for the Aerobic Oxidation of Benzyl Alcohol to Benzyl Benzoate

This paper studies the cooperative effect of Au nanoparticles deposited on vanadyl pyrophosphate oxide (VPO) in the liquid phase oxidation of benzyl alcohol. VPO was prepared using the classical method by thermally treating VOHPO₄·0.5H₂O precursor in reacting atmosphere at 420 °C for a period of 72 h. Au nanoparticles were deposited by incipient wetness method. The catalysts were characterized by means of XRD, TEM, XPS and Raman. The bulk VPO catalyst contains vanadyl pyrophosphate phase ((VO)₂P₂O₇), and a small amount of VOPO₄. The catalytic system exhibits a high activity in the base-free liquid phase oxidation of alcohols compared to Au on activated carbon, classic catalyst used for this type of reaction. Au/VPO showed a high peculiar selectivity to benzyl benzoate (76%), an important product used in the pharmaceutical and perfume industries. This behavior might be ascribed to the presence of strong acid sites of VPO, as determined by liquid phase titration. Stability tests performed on Au/VPO showed a deactivation of 10% after the first run, but a constant conversion along the following five cycles. This phenomenon can be attributed to the increase of mean Au particle size (from 19.1 to 23.4 nm) after recycling tests as well as the partial leaching of Au and V in the reaction media. Moreover, XRD evidenced a modification in the VPO structure with the partial formation of VOHPO₄·0.5H₂O phase.

Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support

Highly active and stable bimetallic Au-Pd catalysts have been extensively studied for several liquid-phase oxidation reactions in recent years, but there are far fewer reports on the use of these catalysts for low-temperature gas-phase reactions. Here we initially established the presence of a synergistic effect in a range of bimetallic Au-Pd/CeZrO₄ catalysts, by measuring their activity for selective oxidation of benzyl alcohol. The catalysts were then evaluated for low-temperature WGS, CO oxidation, and formic acid decomposition, all of which are believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the introduction of Pd to a monometallic Au catalyst resulted in a significant decrease in catalytic activity. Furthermore, monometallic Pd was more active than Pd-rich bimetallic catalysts. The nature of the anti-synergy was probed by several ex situ techniques, which all indicated a growth in metal nanoparticle size with Pd addition. However, the most definitive information was provided by in situ CO-DRIFTS, in which CO adsorption associated with interfacial sites was found to vary with the molar ratio of the metals and could be correlated with the catalytic activity of each reaction. As a similar correlation was observed between activity and the presence of Au⁰* (as detected by XPS), it is proposed that peripheral Au⁰* species form part of the active centers in the most active catalysts for the three gas-phase reactions. In contrast, the active sites for the selective oxidation of benzyl alcohol are generally thought to be electronically modified gold atoms at the surface of the nanoparticles.

Au-Pd alloy nanoparticles supported on layered double hydroxide for heterogeneously catalyzed aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones to phenols

Phenol, an important industrial chemical, is widely produced using the well-developed cumene process. However, demand for the development of a novel alternative method for synthesizing phenol from benzene has been increasing. Herein, we report a novel system for the synthesis of phenols through aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones, including ketone-alcohol (KA) oil, catalyzed by Mg-Al-layered double hydroxide (LDH)-supported Au-Pd alloy nanoparticles (Au-Pd/LDH). Alloying of Au and Pd and basicity of LDH are key factors in achieving the present transformation. Although monometallic Au/LDH, Pd/LDH, and their physical mixture showed almost no catalytic activity, Au-Pd/LDH exhibited markedly high catalytic activity for the dehydrogenative phenol production. Mechanistic studies showed that β-H elimination from Pd-enolate species is accelerated by Au species, likely via electronic ligand effects. Moreover, the effect of supports was critical; despite the high catalytic performance of Au-Pd/LDH, Au-Pd bimetallic nanoparticles supported on Al2O3, TiO2, MgO, and CeO2 were ineffective. Thus, the basicity of LDH plays a deterministic role in the present dehydrogenation possibly through its assistance in the deprotonation steps. The synthetic scope of the Au-Pd/LDH-catalyzed system was very broad; various substituted cyclohexanols and cyclohexanones were efficiently converted into the corresponding phenols, and N-substituted anilines were synthesized from cyclohexanones and amines. In addition, the observed catalysis was truly heterogeneous, and Au-Pd/LDH could be reused without substantial loss of its high performance. The present transformation is scalable, utilizes O2 in air as the terminal oxidant, and generates water as the only by-product, highlighting the potential practical utility and environmentally benign nature of the present transformation. Dehydrogenative aromatization of cyclohexanols proceeds through (1) oxidation of cyclohexanols to cyclohexanones; (2) dehydrogenation of cyclohexanones to cyclohexenones; and (3) disproportionation of cyclohexenones to afford the desired phenols. In the present Au-Pd/LDH-catalyzed transformation, the oxidation of the Pd-H species is included in the rate-determining step.

Characterisation of gold catalysts

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions.

New challenges in gold catalysis: bimetallic systems

Since the discovery of the peculiar catalytic activity of gold catalysts, it became clear that gold could play a fundamental role also as a modifier.

Dynamics of palladium on nanocarbon in the direct synthesis of H2O2

This work aims to clarify the nanostructural transformation accompanying the loss of activity and selectivity for the hydrogen peroxide synthesis of palladium and gold-palladium nanoparticles supported on N-functionalized carbon nanotubes. High-resolution X-ray photoemission spectroscopy (XPS) allows the discrimination of metallic palladium, electronically modified metallic palladium hosting impurities, and cationic palladium. This is paralleled by the morphological heterogeneity observed by high-resolution TEM, in which nanoparticles with an average size of 2 nm coexisted with very small palladium clusters. The morphological distribution of palladium is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent to which these processes occur as a result of the instability of the particle at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to palladium clusters with a modified electronic structure compared with palladium metal or palladium oxides. These beneficial species are formed and stabilized on carbons modified with nitrogen atoms in substitutional positions. The formation of larger metallic palladium particles not only reduces the number of active sites for the synthesis, but also enhances the activity for deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of palladium clusters by all means is thus the key to better performing catalysts.

Potential of Gold Nanoparticles for Oxidation in Fine Chemical Synthesis

In recent years supported gold nanoparticles have emerged as efficient catalysts with considerable synthetic potential for liquid-phase oxidation reactions based on molecular oxygen as oxidant. Here we critically review the most attractive applications related to the selective oxidation of functional groups containing O, N, or Si heteroatoms. The reactions include the oxidation of alcohols, aldehydes, and organosilanes; the diverse transformations of amines; benzylic oxidations; and some one-pot multistep reactions starting with alcohol or amine oxidation. In complex liquid-phase transformations relying on bifunctional catalysis, appropriate choice of the support is frequently more important than the size of the gold particles. In some oxidation reactions gold nanoparticles outperform the traditional platinum-group metal catalysts, but the latest results indicate the superiority of bimetallic particles containing gold and platinum, palladium, or rhodium. The environmentally benign nature of the transformations is discussed.

Effect of size of catalytically active phases in the dehydrogenation of alcohols and the challenging selective oxidation of hydrocarbons

The size of the active phase is one of the most important factors in determining the catalytic behaviour of a heterogeneous catalyst. This Feature Article focuses on the size effects in two types of reactions, i.e., the metal nanoparticle-catalysed dehydrogenation of alcohols and the metal oxide nanocluster-catalysed selective oxidation of hydrocarbons (including the selective oxidation of methane and ethane and the epoxidation of propylene). For Pd or Au nanoparticle-catalysed oxidative or non-oxidative dehydrogenation of alcohols, the size of metal nanoparticles mainly controls the catalytic activity by affecting the activation of reactants (either alcohol or O(2)). The size of oxidic molybdenum species loaded on SBA-15 determines not only the activity but also the selectivity of oxygenates in the selective oxidation of ethane; highly dispersed molybdenum species are suitable for acetaldehyde formation, while molybdenum oxide nanoparticles exhibit higher formaldehyde selectivity. Cu(II) and Fe(III) isolated on mesoporous silica are highly efficient for the selective oxidation of methane to formaldehyde, while the corresponding oxide clusters mainly catalyse the complete oxidation of methane. The lattice oxygen in iron or copper oxide clusters is responsible for the complete oxidation, while the isolated Cu(I) or Fe(II) generated during the reaction can activate molecular oxygen forming active oxygen species for the selective oxidation of methane. Highly dispersed Cu(I) and Fe(II) species also function for the epoxidation of propylene by O(2) and N(2)O, respectively. Alkali metal ions work as promoters for the epoxidation of propylene by enhancing the dispersion of copper or iron species and weakening the acidity.