Gold-based Catalysts Research Articles

A systematic study on Au-PDA@Fe3O4 core-shell catalysts for enhanced catalytic reduction of 4-nitrophenol

The development of nanogold catalysts with high activity and high stability is of great significance for their industrial applications. In this paper, Au-PDA@Fe3O4 catalysts were prepared through the gold sol adsorption method. First, micron-sized magnetic Fe3O4 spheres were synthesized via a solvothermal method, serving as the core to endow the catalyst with excellent recyclability. Subsequently, a layer of polydopamine (PDA) shell was polymerized on the magnetic core surface. By controlling the timing of introducing the gold sol containing Au nanoparticles (NPs), the spatial position of Au NPs within the PDA layer was regulated, leading to a series of magnetic core-shell nano gold catalysts with varying spatial positions of Au. Using the 4-nitrophenol (4-NP) hydrogenation as the probe, the influence of Au spatial position, Au size, and other conditions on the catalytic performance was investigated. Results showed that, in the prepared Au-PDA@Fe3O4-16 catalyst, the Au NPs are encapsulated within the PDA shell and positioned close to the outer layer. This not only provides remarkable stability to the Au NPs but also maintains their proximity to the surface of the PDA shell, allowing easier access of the substrate 4-NP to the Au NPs and conferring good reactivity upon the catalyst. The strong electron metal-support interaction between PDA and Au enables PDA to effectively stabilize Au NPs. The prepared Au-PDA@Fe3O4-16 catalyst exhibits excellent stability, maintaining a high conversion rate of 91% even after 14 cycles of use. Our findings not only deepen the understanding of the catalytic mechanisms involved but also pave the path towards the creation of highly efficient and robust gold-based catalysts, facilitating their widespread utilization in industrial applications.

Harvesting low-cost gold-based catalysts from e-waste by a cationic covalent triazine organic framework for 4-nitrophenol reduction

Gold (Au)-based catalysts are commonly used for the hydrogenation of 4-nitrophenol, however, the high cost of Au has limited their widespread application. Therefore, developing affordable Au-based catalysts is highly desirable yet challenging. Herein, we synthesized a cationic covalent triazine framework to recover Au from e-waste, leading to the creation of cost-effective Au NPs@iCTF-TPM catalysts. Adsorption experiments demonstrate that cationic iCTF-TPM exhibits exceptional Au adsorption capabilities, with a maximum capacity of 1919 mg g−1. Notably, the iCTF-TPM selectively recovers Au from the e-waste leaching solution, achieving 99 % removal of Au within just 5 min. Moreover, the adsorbed AuCl4− can be self-reduced to form fine Au nanoparticles, resulting in the formation of Au NPs@iCTF-TPM catalysts. Remarkably, Au NPs@iCTF-TPM could convert 4-nitrophenol to 4-nitroaniline with a low activation energy of 26.15kJ mol−1 and a high rate constant of 1.73 × 10−2 s−1. Electron paramagnetic resonance spectroscopy indicates that the high catalytic activity is primarily due to the generation of active hydrogen radicals facilitated by Au NPs@iCTF-TPM. This study opens new avenues for developing low-cost Au-based catalysts from e-waste leaching solutions.

Gold oxide formation on Au(111) under CO oxidation conditions at room temperature.

Although gold-based catalysts are promising candidates for selective low-temperature CO oxidation, the reaction mechanism is not fully understood. On a Au(111) model catalyst, we observe the formation of gold oxide islands under exposure to atmospheric pressures of oxygen or CO oxidation reaction conditions in an in situ scanning tunneling microscope. The gold oxide formation is interpreted in line with the water-enabled dissociation of O2 on the step edges of Au(111). Contaminants on the gold surface can strongly promote the gold oxide formation even on the terraces. On the other hand, TiO2 nanoparticles on the Au(111) do not show any influence on the formation of the gold oxide and are thus not providing a significant amount of atomic oxygen to the gold at room temperature. Overall, the presence of gold oxide is likely under industrial conditions.

Selective vapor-phase formation of dimethylformamide via oxidative coupling of methanol and dimethylamine over bimetallic catalysts

Selective dimethylformamide formation occurs over PdAu; reactivity and selectivity are sensitive to Pd : Au ratio. Reaction kinetics suggest a crowded surface and that beneficial effects of surface hydroxyls are induced by co-feeding water.

Linear-Structure Single-Atom Gold(I) Catalyst for Dehydrogenative Coupling of Organosilanes with Alcohols.

A strategy for the synthesis of a gold-based single-atom catalyst (SAC) via a one-step room temperature reduction of Au(III) salt and stabilization of Au(I) ions on nitrile-functionalized graphene (cyanographene; G-CN) is described. The graphene-supported G(CN)-Au catalyst exhibits a unique linear structure of the Au(I) active sites promoting a multistep mode of action in dehydrogenative coupling of organosilanes with alcohols under mild reaction conditions as proven by advanced XPS, XAFS, XANES, and EPR techniques along with DFT calculations. The linear structure being perfectly accessible toward the reactant molecules and the cyanographene-induced charge transfer resulting in the exclusive Au(I) valence state contribute to the superior efficiency of the emerging two-dimensional SAC. The developed G(CN)-Au SAC, despite its low metal loading (ca. 0.6 wt %), appear to be the most efficient catalyst for Si-H bond activation with a turnover frequency of up to 139,494 h-1 and high selectivities, significantly overcoming all reported homogeneous gold catalysts. Moreover, it can be easily prepared in a multigram batch scale, is recyclable, and works well toward more than 40 organosilanes. This work opens the door for applications of SACs with a linear structure of the active site for advanced catalytic applications.

Liquid Phase Oxidation of Glycerol over Gold-based Catalysts in Flow and Batch Reactor

Liquid Phase Oxidation of Glycerol over Gold-based Catalysts in Flow and Batch Reactor

Ultra-Low Loading of Gold on Nickel Foam for Nitrogen Electrochemistry

Ammonia (NH3) is widely used in various fields, and it is also considered a promising carbon free energy carrier, due to its high hydrogen content. The nitrogen reduction reaction (NRR), which converts nitrogen into ammonia by using protons from water as the hydrogen source, is receiving a lot of attention, since effective process optimization would make it possible to overcome the Haber–Bosch method. In this study, we used a solution-based approach to obtain functionalized porous Ni foam substrates with a small amount of gold (<0.1 mg cm−1). We investigated several deposition conditions and obtained different morphologies. The electrochemical performance of various catalysts on the hydrogen evolution reaction (HER) and NRR has been characterized. The ammonia production yield was determined by chronoamperometry experiments at several potentials, and the results showed a maximum ammonia yield rate of 20 µg h−1 mgcat−1 and a Faradaic efficiency of 5.22%. This study demonstrates the potential of gold-based catalysts for sustainable ammonia production and highlights the importance of optimizing deposition conditions to improve the selectivity toward HER.

Supported Gold Catalysts for Base-Free Furfural Oxidation: The State of the Art and Machine-Learning-Enabled Optimization.

Supported gold nanoparticles have proven to be highly effective catalysts for the base-free oxidation of furfural, a compound derived from biomass. Their small size enables a high surface-area-to-volume ratio, providing abundant active sites for the reaction to take place. These gold nanoparticles serve as catalysts by providing surfaces for furfural molecules to adsorb onto and facilitating electron transfer between the substrate and the oxidizing agent. The role of the support in this reaction has been widely studied, and gold-support interactions have been found to be beneficial. However, the exact mechanism of furfural oxidation under base-free conditions remains an active area of research and is not yet fully understood. In this review, we delve into the essential factors that influence the selectivity of furfural oxidation. We present an optimization process that highlights the significant role of machine learning in identifying the best catalyst for this reaction. The principal objective of this study is to provide a comprehensive review of research conducted over the past five years concerning the catalytic oxidation of furfural under base-free conditions. By conducting tree decision making on experimental data from recent articles, a total of 93 gold-based catalysts are compared. The relative variable importance chart analysis reveals that the support preparation method and the pH of the solution are the most crucial factors determining the yield of furoic acid in this oxidation process.

Functionalized α-Al2O3 supported gold catalyst for photocatalytic oxidative esterification of benzyl alcohol with methanol to corresponding esters

Alpha-alumina (α-Al2O3) is traditionally considered to be unsuitable as a support for high performance gold-based catalysts because of its small specific surface area. Herein, a highly active α-Al2O3-supported nearly sub-nanometer Au particles photocatalyst for oxidative esterification of benzyl alcohol under visible-light irradiation in a mild condition was synthesized via the electronic interaction between gold precursor and a layer of 3-aminopropyltriethoxysilane (APTES) pre-grafted on the α-Al2O3 called as functionalized α-Al2O3. APTES could serve as a linker to strongly anchor Au nanoparticles (NPs) to α-Al2O3 surface, leading to nearly sub-nanometer Au particles (1.8 ± 0.1 nm). The enhanced metal-support interaction reinforces the interaction of plasmonic hot holes of Au NPs and intermediate molecules, which promote the rate-limiting step of C-H bond cleavage and facilitate the activation of oxygen molecules due to the increased electron cloud density around Au NPs. Meanwhile, α-Al2O3 support can play a synergistic role because of the reactant adsorption can be improved by the acid-base pairs contained.

Turning copper into gold — Tuning a gas diffusion electrode catalyst for the efficient conversion of CO[formula omitted] to C[formula omitted]H[formula omitted] or CO

Copper is regarded as a unique electrocatalyst for the CO2 reduction reaction as it is able to produce products that require more than two electrons, such as C2H4 and C2H5OH. When combined with the high activity of a gas diffusion electrode, copper becomes a great choice of catalyst for the CO2 reduction reaction, however, it has a history of poor selectivity for a given product. In this work, we have synthesised a CuO electrocatalyst that has typical copper-like selectivity initially, however, as we tune the Nafion loading, we are able to shift the selectivity towards that of a gold-based catalyst. We propose that this is due to the Nafion providing an in-penetrable barrier to the electrolyte that prevents the catalyst from being flooded, which is supported by electrochemical, XAS, and TEM analysis.

Boosting the catalysis of gold supported on perovskites by strong metal-support interaction

Gold catalysts show excellent selectivity in chemoselective hydrogenation reactions. However, their reactivity and stability are relatively inferior to Pt-group metal catalysts due to the moderate hydrogen activation and carbonaceous deposits. Herein, we reported that the strong metal-support interaction (SMSI) between gold nanoparticles and perovskites can increase the catalytic performance of gold-based catalysts in 3-nitrostyrene chemoselective hydrogenation reaction. HAADF-STEM elemental mapping images and line scan profiles confirmed that the oxide support can partially cover the surface of gold NPs, increasing the interfacial area between gold NPs and perovskites support. H2-D2 exchange reaction, reaction kinetic analysis and density functional theory calculations revealed that the apparent activation energy of 3-NS chemoselective hydrogenation reaction over gold supported on perovskites with SMSI effect was significantly decreased by the more facile H2 dissociation, 3-NS and its’ intermediate activation at the increased interface. This discovery of SMSI effect in Au and perovskites offers new insights into tuning performance of gold-based catalysts via changing the types of metal ions in perovskites.

Effect of R-SO2-R′ ligands on the gold-based catalysts for acetylene hydrochlorination: A DFT study

Gold-based mercury-free catalysts have great potential in the industrial production of vinyl chloride. In order to further understand the factors affecting the catalytic activity, the catalytic reaction mechanisms of five kinds of sulfur-containing ligand gold-based catalysts are studied by DFT calculation at the PBE0/def2-TZVP level, and dispersion-corrected DFT-D3 is performed. The results show that the rate control step in the reaction path is the transfer of proton hydrogen. With the increase of Mulliken charges of Au and S atoms in the catalyst, the energy barrier of acetylene hydrochlorination decreases. The catalyst formed by methanesulfonic acid and AuCl3 has the lowest energy barrier of acetylene hydrochlorination (23.18 kcal/mol). Compared to AuCl3 catalyst, adding ligands can significantly reduce the products desorption energy. When the ligand has an electron-withdrawing group, the corresponding acetylene self-polymerization barrier is lower. These results provide valuable theoretical guidance for preparation of efficient sulfur-containing ligand gold-based catalysts in future.

Self-oxidation of Au-[Bmim][Cl3] catalyst with enhanced activity and stability for acetylene hydrochlorination

There is great potential for carbon-supported gold catalysts to replace toxic mercuric chloride-based catalysts in the acetylene hydrochlorination, which is a key process for producing PVC. However, the catalyst design is essentially hindered by the difficulties in enhancing the catalytic activity of cationic gold and suppressing the deactivation of active species. We report a strategy for self-oxidation of Au3+ by the Au-SILP system with the [Bmim][Cl3] ILs. This strategy can stabilize all gold species in the state of highly oxidized Au3+ instead of coexisting in the Au3+/Au+ site, which makes the catalyst exhibit excellent catalytic activity. High coordination numbers Au-Cl entities reduce the electron transfer between Au3+ and C2H2 and weaken the adsorption of C2H2 on the Au active sites. Furthermore, [Bmim][Cl3] ILs can self-oxidize gold nanoparticles on the support surface into Au3+. This study provides a new method for improving the stability and activity of gold-based acetylene hydrochlorination catalysts.

Sintering-resistant and highly active Au/SBA-15 catalyst for carbon monoxide oxidation

The low thermal stability of gold-based catalysts is a significant obstacle to their practical application, especially under demanding conditions, such as those prevailing in automotive exhaust gas converters. In this work, we demonstrate that it is possible to strongly anchor 2–3 nm size gold nanoparticles inside the pores of SBA-15 mesoporous silica. The Au/SBA-15 catalyst showed high thermal stability, even at a temperature of 700–800 °C. The gold was evenly distributed inside the SBA-15 mesopores using the adsorption of the [Au(en)2]Cl3 complex (en = ethylenediamine) in an alkaline environment. Due to the very small size of gold nanoparticles, the obtained catalyst was highly active in low-temperature CO oxidation. The high resistance of Au particles to sintering can be explained by the confinement effect and the strong metal-support interaction, induced during the catalyst precursor decomposition under an oxygen-free atmosphere at 400 °C. The carbonaceous layer, formed from ethylenediamine ligands decomposition, plays an essential role in stabilizing gold nanoparticles inside the SBA-15 pores. We believe that our findings may contribute to solving the problem of carbon monoxide emission during engine start-up.

A Facile Strategy to Obtain Low-Cost and High-Performance Gold-Based Catalysts from Artificial Electronic Waste by [Zr48 Ni6 ] Nano-Cages in MOFs for CO2 Electroreduction to CO.

Expensive gold-based catalysts are frequently used for electrochemical CO2 reduction into CO. A feasible approach to obtain low-cost Au-based catalysts is needed. Herein, a novel framework 1 assembled from [Zr48 Ni6 ] nano-cages is prepared. It exhibits a high BET surface area of 1569 m2 g-1 and high solvents/pH stability. 1 can not only selectively extract AuCl4 - from artificial electronic waste, but can then be transformed into low-cost catalyst Au nanoparticle@1-x (Au NPs@1-x, x=1, 2, 3, 4) with tuneable Au NPs sizes. The CO2 RR investigations revealed that the Au NPs@1-3 displayed an excellent FECO of 95.2 % with a current density of 102.9 mA cm-2 at -1.1 V, and such high catalytic activity can be maintained for at least 15 h without obvious decrease because the confinement effect of [Zr48 Ni6 ] nano-cages prevents Au NPs agglomeration. This work offers a facile strategy to obtain low-cost and high-performance Au-based catalysts for various reactions activated by Au.

Size, composition, and surface capping-dependent catalytic activity of spherical gold nanoparticles

Gold nanostructures are used as catalysts in heterogeneous catalytic processes and have intrigued chemists and materials scientists. Isotropic spherical gold nanoparticles (AuNPs) are ideal for catalysis due to their simple preparation process, controllable surface-active site, tunable size, and composition-dependent catalytic activity. In this study, spherical AuNPs with different size, composition, and surface capping agents have been prepared, and their catalytic activity in reduction of 4-nitrophenol (4-NP) is evaluated. The catalytic activity of AuNPs decreases as their size increases. Meanwhile, the catalytic activity of AuNPs with tartrate as the reducing agent show no evident changes because of containing anisotropic AuNPs. Moreover, silver not only improves monodisperse and spherical AuNPs, but also increases the catalytic activity of small AuNPs. Since the molecular structures of tartrate and citrate are similar, there is no remarkable difference in the catalytic activity of AuNPs using tartrate and citrate as capping agents. These results demonstrate the influence of size, composition, and surface capping on the catalytic activity of AuNPs. Overall, this study facilitates the applicability of gold-based catalyst and AuNPs in plasmonics, nanophotonics, biomedical photonics, and photocatalysis.

Role of interfacial oxygen vacancies in low-loaded Au-based catalysts for the low-temperature reverse water gas shift reaction

Gold-based catalysts have shown high catalytic activity for reverse water gas shift (RWGS) reactions at low temperatures. Despite extensive studies, the RWGS reaction on Au-based catalysts with very low Au content (< 0.1 wt%) has not yet been investigated. In this study, TiO2 and ZrO2 supported gold catalysts with such low gold loading have been synthesized and tested for RWGS. The catalysts were investigated by a series of in/ex-situ characterization techniques, including ICP-OES, XRD, BET, XPS, STEM, STEM-EELS, TAP, in-situ DRIFTS and in-situ EPR. At 250 oC, the Au/TiO2 catalyst showed almost 10 times higher activity than Au/ZrO2. In-situ DRIFTS results suggest that the formate mechanism is the predominant mechanism over Au/ZrO2, while over Au/TiO2 the reaction proceeds via the formation of hydroxycarbonyl intermediates. A combined study including STEM, STEM-EELS, XPS, and in-situ EPR suggests that the interfacial Au–Ov–Ti3+ sites are responsible for the superior activity of Au/TiO2.

Pincer and Macrocyclic Pyridylidene Amide (PYA) AuIII Complexes.

Gold-based homogeneous catalysis is dominated by redox neutral AuI systems. Redox-active gold-based catalysts are less common, principally because of redox cycles between AuI and AuIII being hampered by unfavorable potentials. We report gold(III) complexes containing pincer-based, donor-flexible pyridylidene amide (PYA) ligands to address these issues. These complexes act as electron reservoirs through two limiting resonance structures consisting of either soft, imine coordination sites or harder, zwitterionic amide donors. We further alter the donor properties by using the ortho-, meta-, and para-pyridylidene amide variants of the PYA pincer arms. These bis-PYA pincer ligands exhibited a high contribution of amide coordination in the solid-state of the gold(III) complexes; however, the solution data suggests a high contribution from the neutral L-type resonance forms. This L-type contribution, primarily shown through cyclic voltammetry studies, prevents reversible gold(III) reduction and also disfavors abstraction of the ancillary chloride ligand. Furthermore, a novel macrocyclic-PYA ligand is introduced, which shows secondary metal-ligand interactions.

Catalytic Conversion of CO2 over Atomically Precise Gold-Based Cluster Catalysts

Catalytic Conversion of CO₂ over Atomically Precise Gold-Based Cluster Catalysts

Adsorption and Activation of O2 on Small Gold Oxide Clusters: the Reactivity Dominated by Site-Specific Factors.

We experimentally explored adsorption and activation of O2 on small anionic clusters AuxOy- containing one to five gold atoms and between one and three oxygen atoms using an instrument including a magnetron sputtering cluster source, a micro flow reactor running at low temperature, and a time-of-flight mass spectrometer. Some species, including AuO-, one isomer of Au2O2-, Au3O-, one isomer of Au3O3-, and Au5O2-, can adsorb an O2 molecule. We theoretically explored the structures of these active species and the inert ones appearing in the experiment by combining a structure search strategy based on the genetic algorithm and the density functional theory (DFT) calculations. Impressively, all active species observed in the experiment have a -O-Au site, in which the gold atom is a dangling or a vertex atom. Each -O-Au site can strongly adsorb one O2 with its Au atom to form a straight-line structure -O-Au-O-, and the adsorbed O2 is significantly activated by accepting one electron with one of its π2p* orbitals. With no exception, all oxygen sites and the -O-Au-Au sites in AuxOy- are inert. Analyses on the density of states (DOS) of representative species well interpret the physical origins of the activity of -O-Au and the inertness of -O-Au-Au. The observations that site-specific factors dominate the reactivity of gold oxide clusters with O2 are in contrast to what happens in the reactions of Aun- with O2, where clusters' reactivity is completely determined by their global spins and electron detachment energies. The new conclusions in this work offer a reference to understand the crucial O2 activation processes in gold-based catalysts, since various gold oxide structures are commonly observed in these systems.