Au-based Catalysts Research Articles

Fabrication of gold nanoparticles within hierarchically ZSM-5-based micro-/mesostructures (MMZ) with enhanced stability for catalytic reduction of p-nitrophenol and methylene blue

Gold nanoparticles (Au NPs) have received great attention in diverse catalytic reactions due to their excellent activity. However, the activity is severely relying on dispersion and size of Au NPs. Therefore, regulation of Au NPs for degradation of organic pollutants in water is of vital significance for environmental remediation. Herein, we developed microporous/mesoporous silica, MMZ, using ZSM-5 (SiO2/Al2O3 = 28) as structural constructing precursor and cetyltrimethylamoniumbromide (CTAB) as template via sequential dissolution-self-assembly strategy. The Au precursor is introduced into MMZ and ZSM-5 by ultrasound assisted impregnation and subsequent reduced for Au0 formation. The obtained materials were characterized by XRD, TEM, XPS, SEM, UV-DRS, N2 adsorption and TG/DTG techniques. XRD and TEM study revealed that up to1.0 wt% of Au can be well dispersed with smaller size of 2.8 nm in the material from MMZ (denoted as 1.0Au/MMZ), while serious aggregation and formation of large Au NPs occurs over ZSM-5 with the same Au loading (denoted as 1.0Au/ZSM-5). Catalytic studies also demonstrated that Au/MMZ has much higher catalytic activity for reduction of p-nitrophenol (P-NP) and methylene blue (MB) to p-aminophenol (P-AP) and leuco methylene blue compared with Au/ZSM-5 analogue and other reported Au-based catalysts previously. Moreover, Au/MMZ has good reusability and showed no change in its original catalytic performance till 7th cycles in reduction of P-NP and MB, which is attributed to stable structure and well dispersion of Au NPs over MMZ. Therefore, compared with other Au based catalysts used for catalytic reaction, this Au/MMZ is cheap and promising for industrial applications.

Hollow mesoporous CeO2 microspheres for efficient loading of Au single-atoms to catalyze the water-gas shift reaction

Abstract The atomically dispersed catalysts have aroused great attention in the recent decade. However, the preparation of atomically dispersed Au catalysts presents more challenge than other atomically dispersed catalysts such as Pt, Pd, etc. From the aspect of support, this work contributes a novel hollow mesoporous CeO2 microspheres (CeO2-hm) for efficient dispersion of single Au atoms (Au1). Contributed by the high specific surface area and mesoporous structure, CeO2-hm possesses much higher capacity for undertaking Au1, than commercial CeO2 particles (CeO2-c). Typically, with Au loading of 0.2 at%, CeO2-hm supported Au (Au1/CeO2-hm) presents atomic dispersion while CeO2-c supported Au (Au/CeO2-c) contains large amount of Au nanoparticles. Confirmed by catalyzing the water-gas shift (WGS) reaction, the obtained Au1/CeO2 catalysts exhibit notably higher performance than the Au-based catalysts supported by CeO2-c and CeO2 synthesized through co-precipitation and sol-gel strategies. For example, 0.2 at% Au1/CeO2-hm exhibits 9.1 times reactivity of equal Au loaded Au/CeO2-c catalyst. Besides, the CeO2-hm supported single-atom Au catalyst presents good tolerance under long-time WGS running.

Enhancement of anode performance for alkaline-acid direct glycerol fuel cells

Abstract An alkaline-acid direct glycerol fuel cell (AA-DGFC) was proposed for a portable power generating device that requires high power density. A good design of cell components and optimum cell operating conditions are the key factors to yield high performance of fuel cells. In this work, AA-DGFC with Pt/C anode catalyst showed outstanding performance with open-circuit voltage as high as 1.72V and the peak power density of 330 mWcm−2 which is about 2.7 times higher than the performance of a typical anion exchange membrane direct glycerol fuel cell (AEDGFC). The operating conditions providing the best performance were at glycerol to NaOH mole ratio of 1:5, 1.0M glycerol concentration, an anolyte volumetric flow rate of 1mLmin-1 and cell temperature of 80 °C. For the design of the cell component, the optimum Nafion ionomer content in the anode microporous layer was 20 wt%. Among the Au-based catalysts at the anode studied Au/C, Au–Ni/C and Au–Ag/C, the Au–Ni/C outperformed the others with the power density of 142 mWcm−2.

Acetylene hydrochlorination over supported ionic liquid phase (SILP) gold-based catalyst: Stabilization of cationic Au species via chemical activation of hydrogen chloride and corresponding mechanisms

The activation of HCl by cationic Au in the presence of C2H2 is important for the construction of active Au sites and in acetylene hydrochlorination. Here, we report a strategy for activating HCl by the Au-based supported ionic liquid phase (Au–SILP) technology with the [N(CN)2−] anion. This strategy enables HCl to accept electrons from [N(CN)2−] anions in Au–[N(CN)2−] complexes rather than from pure [Bmim][N(CN)2], leading to notable improvement in both the reaction path and the stability of the catalyst without changing the reaction triggered by acetylene adsorption. Furthermore, the induction period of the Au–SILP catalyst was shown to be absent in the reaction process due to the high Au(III) content in the Au(III)/Au(I) site and the high substrate diffusion rate in the ionic liquid layer. This work provides a facile method to improve the stability of Au-based catalysts for acetylene hydrochlorination.

On the importance of the structure in the catalytic reactivity of Au-based catalysts

Au-based materials are remarkably efficient catalysts in the domain of partial oxidation reactions. Nonetheless, questions remain about the physico-chemical phenomena involved at the molecular level. In this work, the catalytic properties of Au-Ag samples, in the form of ultrathin Ag layers on a Au(1 1 1) surface and Au-based model nanoparticles, have been investigated with different microscopy techniques. Using photoemission electron microscopy (PEEM), the exposure of a Au(1 1 1) single crystal doped with various amounts of Ag (0 to 3 monolayers) to O2/H2 and O2/CH3OH gas mixtures did not lead to any specific spatiotemporal pattern formation. In contrast, the use of curved nanoscopic Au and Au-8.8 at.% Ag tip-samples analysed by field emission microscopy (FEM) under similar experimental conditions indicates the presence of catalytic activity. The influence of the silver concentration and of the morphology on the reactivity is discussed. This work highlights the necessity of different experimental approaches aimed at bridging the materials gap often encountered between surface science studies and applied catalysis.

Charge-Mediated Auδ+−Oxygen Vacancy towards Glycerol Oxidation with Largely Improved Catalytic Performance

Catalytic conversion of biomass-derived glycerol (GLY) to high value-added products (e.g., glyceric acid, GLYA) has attracted considerable attention. Herein, three Au nano-catalysts supported on mixed metal oxide (MMO) (denoted as Au/Ni3Al-MMO, Au/Co3Al-MMO and Au/Mg3Al-MMO) were prepared via a two-step process: Au supported on layered double hydroxides precursors (Au/LDHs) was synthesized using a deposition-precipitation approach, followed by a further reduction treatment to obtain Au/MMO samples. The optimized Au/Ni3Al-MMO catalyst exhibits prominent catalytic behavior towards selective oxidation of GLY to produce GLYA (conversion: 89.9%; yield: 57.8%; TOF: 5914 h−1), which stands at the highest level compared with previously reported Au-based catalysts. XPS, in situ DRIFTS, and in situ Raman verify the formation of charge-mediated Auδ+−oxygen vacancy (Auδ+−Ov) originating from electron transfer from Au atom to oxygen vacancy at the interface, whose concentration is dependent on the strength of metal-support interaction. Researches on the structure-property relationship on the basis of in situ FT-IR and kinetic isotope effect (KIE) further verify that the charge-mediated interfacial sites (Auδ+−Ov) play a crucial role in determining catalytic behavior, in which Auδ+accelerates the bond cleavage of the rate-determining step (α-C–H) while Ov actives oxygen to generate superoxide anion (O2−). This work gives a systematic knowledge of structure-property correlation and reaction mechanism, which would pave the way for rational design and preparation of efficient heterogeneous catalysts towards the chemical transformation of biomass-based polyols.

Carboxylated Chitosan Nanocrystals: A Synthetic Route and Application as Superior Support for Gold-Catalyzed Reactions.

In this study, we demonstrate for the first time the fabrication of carboxylated chitosan nanocrystals (ChsNC) with high degree of deacetylation (DDA) at >80% and narrow size distribution. We also studied its application as a sustainable support material for metal-based catalysts. Carboxylated chitin nanocrystals (ChNCs) were initially prepared through partial cleavage of glycosidic bonds in chitin by ammonium persulfate, with concurrent oxidation of chitin C6 primary alcohols to produce carboxylate groups on the surface of the ChNCs. ChsNCs were subsequently prepared using an alkaline deacetylation procedure in the presence of NaBH4 to preserve the nanorod structure of the biomaterial. The resulting nanocrystals feature both carboxyl and amino functional groups. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy were used to determine the morphology and composition of these carboxylated ChNCs and ChsNCs. Subsequently, we tested the ability of the as-made ChsNCs as a biomass-based catalyst support for Au nanoparticles (NPs) using the 4-nitrophenol reduction and the aldehyde-amine-alkyne (A3) coupling reactions to demonstrate its capabilities in regard to the ones of cellulose nanocrystals (CNCs). In particular, Au NPs over ChsNCs featured the highest turnover frequency (TOF) value for the 4-nitrophenol reduction reported for all Au-based catalysts supported on carbon-based systems. Spectroscopic and imaging techniques confirmed the importance of precisely controlling the redox state of Au as it is being deposited to afford a highly disperse active site on the bionano-support.

Single-Site Au/Carbon Catalysts with Single-Atom and Au Nanoparticles for Acetylene Hydrochlorination

Single-site cationic Au supported on carbon (Au SAs/AC) has been identified as the active site for acetylene hydrochlorination, and single-site cationic Au tends to sinter into devitalized particles during a reaction. Many efforts have been devoted to the preparation and stabilization of the Au SAs. Herein, we report an unexpected phenomenon that the Au SAs can be dynamically created from Au nanoparticles during the acetylene hydrochlorination reaction, and the single-site cationic Au sites can be stabilized by the defective carbon support. The dynamic formation and stabilization of single gold atoms under realistic conditions provide a new strategy for the design of stable Au-based acetylene hydrochlorination catalysts.

A review of recent progress on electrocatalysts toward efficient glycerol electrooxidation

Abstract Glycerol electrooxidation has attracted immense attention due to the economic advantage it could add to biodiesel production. One of the significant challenges for the industrial development of glycerol electrooxidation process is the search for a suitable electrocatalyst that is sustainable, cost effective, and tolerant to carbonaceous species, results in high performance, and is capable of replacing the conventional Pt/C catalyst. We review suitable, sustainable, and inexpensive alternative electrocatalysts with enhanced activity, selectivity, and durability, ensuring the economic viability of the glycerol electrooxidation process. The alternatives discussed here include Pd-based, Au-based, Ni-based, and Ag-based catalysts, as well as the combination of two or three of these metals. Also discussed here are the prospective materials that are yet to be explored for glycerol oxidation but are reported to be bifunctional (being capable of both anodic and cathodic reaction). These include heteroatom-doped metal-free electrocatalysts, which are carbon materials doped with one or two heteroatoms (N, B, S, P, F, I, Br, Cl), and heteroatom-doped nonprecious transition metals. Rational design of these materials can produce electrocatalysts with activity comparable to that of Pt/C catalysts. The takeaway from this review is that it provides an insight into further study and engineering applications on the efficient and cost-effective conversion of glycerol to value-added chemicals.

Aerobic oxidation of 1,6-hexanediol to adipic acid over Au-based catalysts: the role of basic supports

1,6-Hexanediol is a relevant building blocks that could be obtained from biomass and transformed under base free conditions into adipic acid used for polymer synthesis.

Ag@Au Core-Shell Nanowires for Nearly 100 % CO2 -to-CO Electroreduction.

Electrochemical reduction of carbon dioxide (CO2 ) to CO is regarded as an efficient method to utilize the greenhouse gas CO2 , because the CO product can be further converted into high value-added chemicals via the Fisher-Tropsch process. Among all electrocatalysts used for CO2 -to-CO reduction, Au-based catalysts have been demonstrated to possess high selectivity, but their precious price limits their future large-scale applications. Thus, simultaneously achieving high selectivity and reasonable price is of great importance for the development of Au-based catalysts. Here, we report Ag@Au core-shell nanowires as electrocatalyst for CO2 reduction, in which a nanometer-thick Au film is uniformly deposited on the core Ag nanowire. Importantly, the Ag@Au catalyst with a relative low Au content can drive CO generation with nearly 100 % Faraday efficiency in 0.1 m KCl electrolyte at an overpotential of ca. -1.0 V. This high selectivity of CO2 reduction could be attributed to a suitable adsorption strength for the key intermediate on Au film together with the synergistic effects between the Au shell and Ag core and the strong interaction between CO2 and Cl- ions in the electrolyte, which may further pave the way for the development of high-efficiency electrocatalysts for CO2 reduction.

Core@shell structured Au@SnO2 nanoparticles with improved N2 adsorption/activation and electrical conductivity for efficient N2 fixation

The design of electrocatalysts with enhanced adsorption and activation of nitrogen (N2) is critical for boosting the electrochemical N2 reduction (ENR). Herein, we developed an efficient strategy to facilitate N2 adsorption and activation for N2 electroreduction into ammonia (NH3) by vacancy engineering of core@shell structured Au@SnO2 nanoparticles (NPs). We found that the ultrathin amorphous SnO2 shell with enriched oxygen vacancies was conducive to adsorb N2 as well as promoted the N2 activation, meanwhile the metallic Au core ensured the good electrical conductivity for accelerating electrons transport during the electrochemical N2 reduction reaction, synergistically boosting the N2 electroreduction catalysis. As confirmed by the 15N-labeling and controlled experiments, the core@shell Au@amorphous SnO2 NPs with abundant oxygen vacancies show the best performance for N2 electroreduction with the NH3 yield rate of 21.9 μg h−1 mg−1cat and faradaic efficiency of 15.2% at −0.2 VRHE, which surpass the Au@crystalline SnO2 NPs, individual Au NPs and all reported Au-based catalysts for ENR.

Oxidative esterification of acetol with methanol to methyl pyruvate over hydroxyapatite supported gold catalyst: Essential roles of acid-base properties

Acetol is a major light oxygenate and readily produced from staged or fast pyrolysis of lignocellulose biomass. Herein we report that acetol can be selectively converted to methyl pyruvate, an important fine chemical, through oxidative esterification over Au-based catalysts. Detailed experimental studies showed that Au on amphoteric supports with appropriate strength and balanced ratio of acid and base sites can facilitate the desired oxidative-esterification pathway without accelerating undesired aldol-condensation or Cannizzaro reactions. In particular, hydroxyapatite (with a Ca/P ratio of 1.62) supported Au achieved 87% selectivity to methyl pyruvate at an acetol conversion of 62%. Au/hydroxyapatite (ratio of Ca/P = 1.62) catalyst exhibits superior performance toward the production of methyl pyruvate from biomass-derived acetol, where the featured acid-base properties in hydroxyapatite play an indispensable role.

Gold nanoparticle as a Lewis acid catalyst in 1,3-dipolar cycloaddition reaction

Fe3O4@SiO2@Au nanoparticles were synthesized and characterized by FT-IR, XRD, SEM, AAS, VSM, EDS and XPS measurements. In this study we revealed that gold nanoparticles can act as an efficient catalyst in the regio- and diastereoselective one-pot reactions of aldehydes, dibromoformaldoxime and CH-acids like oxindole or 1,3-indandione. To the best of our knowledge, employing Au-based heterogeneous catalyst for the 1,3-dipolar cycloaddition reaction has not been previously reported.

Structure and reactivity of single site Ti catalysts for propylene epoxidation

Propylene epoxidation using molecular oxygen and hydrogen mixture on Au-based catalysts has attracted attention because of high propylene oxide selectivity and the use of an inexpensive and environmental friendly oxidant. Single-site titanium on metal oxide supports plays an important role in achieving high reactivity and selectivity in propylene epoxidation. Here we used TiO2 atomic layer deposition (ALD) to synthesize single-site titanium imbedded in the SiO2 framework for propylene epoxidation. High temperature calcination was used as post-treatment to control the titania structure and TiO coordination number. Using UV–vis spectroscopy and X-ray absorption spectroscopy, we successfully established that under similar propylene conversion the selectivity to propylene oxide (PO) is strongly correlated to the TiO coordination number and bond length. Using a cluster model, density functional theory (DFT) calculations indicate that the partial charges of single TiSiO2 sites scale linearly as a function of the coordination number. Also, the predicted TiO bond lengths follow the same trend as found in the experiments, providing additional support for the observed experimental activity relationships.

Core-Shell Au@SnO2 Nanostructures Supported on Na2Ti4O9 Nanobelts as a Highly Active and Deactivation-Resistant Catalyst toward Selective Nitroaromatics Reduction.

Catalysis using gold (Au) nanoparticles has become an important field of chemistry. However, activity loss caused by aggregation or leaching of Au nanoparticles greatly limits their application in catalytic reaction. Herein, we report a facile and green synthesis of a core-shell Au@SnO2 nanocomposite, exhibiting excellent activity toward selective nitroaromatics reduction under mild conditions. The core-shell Au@SnO2 nanocomposite (Au size = ∼50 nm; shell thickness = ca. 16 nm) is conceived and validated by a direct redox reaction between HAuCl4 and SnF2. Optimization of the core size, shell thickness, and dispersion of Au@SnO2 has been introduced by an alkaline surface supported by negatively charged metal oxide Na2Ti4O9. The as-obtained Au-Sn-Na2Ti4O9 catalyst with much smaller Au cores (ca. 5 nm) and thinner SnO2 nondensed shells (ca. 4 nm) exhibits highly improved catalytic activities for nitro reduction compared to most of the known Au-based catalysts. Moreover, the core-shell Au@SnO2 structure inhibits the leaching and agglomeration of Au nanoparticles and thus leads to superior catalytic durability.

Efficient imine synthesis from oxidative coupling of alcohols and amines under air atmosphere catalysed by Zn-doped Al2O3 supported Au nanoparticles

Direct oxidative coupling of alcohols and amines is regarded as an effective and green approach for imine synthesis under mild conditions. In this work, Zn-doped γ-Al2O3 supported Au nanoparticles was demonstrated as highly active and selective heterogeneous catalyst for a series of imine productions with good to excellent yields, from alcohols and amines via direct oxidative coupling under air atmosphere without extra base additives. Various physicochemical techniques, including ICP-MS, XRD, N2 physisorption, TEM, XPS and CO2-/NH3-TPD, were used to study the properties of the catalysts. Well-dispersed Au0 nanoparticles with a mean size of ca. 2.9 nm were found highly effective in activating alcohols in the presence of reactive amines. The amount of Zn2+ dopant and the calcination temperature of support during catalyst preparation showed crucial impact on tuning the intrinsic activity for oxidation of benzyl alcohol to benzaldehyde (i.e., the rate-determining step for the model reaction), which was disclosed to be related with the active surface oxygen species and the acidic-basic property of support. The 0.4% Au/Zn0.02Al2O3 catalyst calcined at 400 °C exhibited the highest TOF (39.1 h−1) at 60 °C based on a >99% yield to benzylideneaniline among all the ever-reported Au-based catalysts. Moreover, this catalyst could afford 98% yield to benzylideneaniline at only 30 °C and work effectively for the gram-scale synthesis. It also showed considerable stability after five consecutive recycling.

Fabrication of mesoporous SiO2/Au/Co3O4 hollow spheres catalysts with core-shell structure for liquid phase oxidation of benzyl alcohol to benzaldehyde

Abstract A core-shell structure Au-based catalysts mSiO2/Au/Co3O4 HS were successfully derived from in-situ growth ZIF-67 hollow spheres, which were synthesized by a one-step soft template method. The samples were characterized by SEM, TEM, EDX, XPS, XRD and BET to confirm its successful preparation and structural features. The catalytic performance of the catalysts were evaluated by liquid phase oxidation of benzyl alcohol to benzaldehyde with O2 as the oxidant under alkaline conditions. The experimental results illustrated that reaction conditions, including effect of catalyst amount, oxygen flow rate, reaction temperature and reaction time, were optimized to be 40 mg, 50 ml/min, 140 °C and 5 h, respectively. Meanwhile, 55% conversion of benzyl alcohol and 84% selectivitiy to benzaldehyde together with the excellent reusability and stability were achieved under optimal reaction conditions, which was attributed to the core-shell structure leading to the encapsulation of Au NPs and provision of multiple active reaction interface. Based on the intermediates formed from the de-protonation of substrate, a plausible oxidation reaction mechanism with mSiO2/Au/Co3O4 HS catalysts was tentatively proposed to study the relationship between structure and catalytic performance.

Gold nanoparticles supported on MgOx-Al2O3 composite oxide: An efficient catalyst for selective hydrogenation of acetylene

Supported gold nanoparticles are highly selective catalysts for hydrogenation of acetylene, but low activity and heavy coke effect have been major concern for the Au-based catalysts. Based on a modified impregnation-calcination strategy, a series of MgOx-Al2O3 composite oxides with different crystal structures from mixed phase to pure spinel was prepared. Due to its crystal defect and synergetic effect, Au nanoparticles on MgOx-Al2O3 components treated at 550 °C (abbreviated as Au/3%MgOx-Al2O3-550) with well controlled smaller particle size gave an enhanced activity for the selective acetylene hydrogenation. The conversion rate was nearly twice as high as that of all other samples at 250 °C. The H2-TPR and XPS results indicate the partially positive charged Au species (Auδ+) played a crucial role in facilitating the adsorption of the CC group. Both the temperature programmed oxidation and Raman scattering also indicated that the Au/3%MgOx-Al2O3-550 showed better decoking effect than Au/Al2O3. This report opens up promising possibilities on engineering the crystal phase of support for the highly efficient heterogeneous gold catalysts.

In situ fabrication of nanostructured Au–Bi as high-efficient electrocatalyst for water splitting

It is essential that the development of efficient and excellent stability electrocatalysts for water oxidation, which could covert effectively renewable electricity to chemical energy. Herein, it is fabricated firstly in situ that a novel Au–Bi anode oxygen-evolving catalyst with an outstanding catalytic performance for water splitting by electrolytic deposition from dilute AuCl4− solutions in potassium tetraborate electrolyte at pH 11. The H2 generated for water splitting rate of the Au–Bi is 116.38 μmol cm−2 h−1 and the Faradaic efficiency reaches 90.43% in 0.1 M tripotassium phosphate solution. Moreover, the main active component of the Au–Bi is gold oxide (Au2O3). What is more, this work expands the research on Au-based oxygen-evolving catalysts and provides increased insight into water splitting.