Supported Au Catalysts Research Articles

Heterogeneous Gold Nanoparticle-Based Catalysts for the Synthesis of Click-Derived Triazoles via the Azide-Alkyne Cycloaddition Reaction

A supported gold nanoparticle-catalyzed strategy has been utilized to promote a click chemistry reaction for the synthesis of 1,2,3-triazoles via the azide-alkyne cycloaddition (AAC) reaction. While the advent of effective non-copper catalysts (i.e., Ru, Ag, Ir) has demonstrated the catalysis of the AAC reaction, additional robust catalytic systems complementary to the copper catalyzed AAC remain in high demand. Herein, Au nanoparticles supported on Al2O3, Fe2O3, TiO2 and ZnO, along with gold reference catalysts (gold on carbon and gold on titania supplied by the World Gold Council) were used as catalysts for the AAC reaction. The supported Au nanoparticles with metal loadings of 0.7–1.6% (w/w relative to support) were able to selectively obtain 1,4-disubstituted-1,2,3-triazoles in moderate yields up to 79% after 15 min, under microwave irradiation at 150 °C using a 0.5–1.0 mol% catalyst loading through a one-pot three-component (terminal alkyne, organohalide and sodium azide) procedure according to the “click” rules. Among the supported Au catalysts, Au/TiO2 gave the best results.

3D printing of recombinant Escherichia coli/Au nanocomposites as agitating paddles towards robust catalytic reduction of 4-nitrophenol

Three-dimensional (3D) printing technology has received remarkable attention in manufacturing catalysts with tailored shapes and high precision, particularly facilitating catalyst recovery, maximizing heat/mass transfer, as well as enhancing catalytic performance. Herein, an engineered recombinant Escherichia coli strain (denoted as e-E. coli) with overexpressing metallothionein (a metal-binding protein) was explored to synthesize Au nanoparticles serving as both reducing and stabilizing agents. Then, the mixed inks containing e-E. coli/Au composite and biocompatible polymers (sodium alginate and gelatin) were extruded based on a direct ink writing method followed by chemical crosslinking to form robust 3D grids with square symmetry. To boost the mass transfer and minimize pressure drop, the monolith catalysts were assembled into agitating paddles and used for liquid-phase batch reactions (volume: 1 L). As such, the reaction solutions were mixed internally via the powered “catalytic paddles” with high mechanical strength, excellent reactivity, and easy recyclability, which could be reused at least 7 cycles without performance loss. Our work provides a novel strategy for the fabrication of supported Au catalysts, and the proof-of-concept “catalytic paddles” by 3D printing technology can be applied to other industrial solution-based reactions.

Oxidative Coupling of Methanol with Molecularly Adsorbed Oxygen on Au Surface to Methyl Formate.

Supported Au catalysts efficiently catalyze the oxidative coupling of methanol with O2 to methyl formate, in which the atomic O species (O(a)) formed via O2 dissociation on the Au surface has been considered as the active oxygen species. Herein we report for the first time that the oxidative coupling of methanol can occur facilely with molecularly adsorbed O2 species (O2(a)) on a Au(997) surface at temperatures as low as around 125 K, while that with O(a) occurs at around 175 K. Both experimental and theoretical calculation results demonstrate a novel reaction mechanism of oxidative coupling of CH3OH with O2(a) via a dioxymethylene (H2COO) intermediate, resulting in the production of both HCOOCH3 and HCOOCH3. These results reveal the unique reactivity of molecularly adsorbed O2 species on Au surfaces for low-temperature oxidation reactions.

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies.

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO2 proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO2 reduction. Here, by employing findings from operando X‐ray absorption spectroscopy at the near and extended Au and Ce LIII energy edges, we focus on the fundamental aspects of highly active Au/CeO2 catalysts, mainly in the CO oxidation for understanding their complex structure‐reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O2 activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO2 support, and the dynamics of O2 activation.

The Effect of Hydrogenated TiO2 to the Au/TiO2 Catalyst in Catalyzing CO Oxidation.

Supported Au catalysts are widely used for CO oxidation due to their extremely high activity, and the modification of a support structure is a crucial method to improve catalytic performance. Herein, we prepared gold catalysts supported on flaky TiO2 and on TiO2 hydrogenated at different temperatures (200, 400, and 600 °C). We found that the sample with the support pretreated in hydrogen at 600 °C (0.5Au/TiO2-H600) showed the greatest advantages in activity and stability over the sample with as-prepared TiO2 nanosheets (0.5Au/TiO2-UC). First, calcination at 600 °C changed the exposed surface of TiO2 from {001} to {101}, and gold nanoparticles (2-3 nm) were observed as highly reactive species on 0.5Au/TiO2-H600. Moreover, the increase of oxygen vacancies on the surface of 0.5Au/TiO2-H600 was conducive to oxygen activation and promoted the catalytic activity. Therefore, we emphasized the important role of the support and gave an effective method to improve the catalytic performance by regulating the support structure.

Gold Nanoparticles on Nanosheets Derived from Layered Rare-Earth Hydroxides for Catalytic Glycerol-to-Lactic Acid Conversion.

Layered rare-earth hydroxides (LREHs), as a series of special lamellar compounds having a similar structure to layered double hydroxides (LDHs), are becoming a new type of catalyst materials. In this study, we have prepared a series of uniform LREH (RE = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) nanosheets through a reverse-microemulsion method. After deposition-precipitation of HAuCl4 and calcination, supported Au catalysts (denoted as Au/LREO) were subsequently obtained. The catalytic properties of all the derived Au/LREO catalysts were evaluated by aerobic conversion of glycerol to lactic acid under mild conditions (90 °C, 1 atm). Among these catalysts, Au/LPrO displays the best performances, including the highest glycerol conversion, lactic acid, and C3 product selectivity. Both the catalytic activities and the characterizations of the structure of Au/LREO indicate that the kind of rare-earth ions plays a key role in determining the Au particle size and its valence state and reducibility, which are the important factors correlated with the catalytic activities in glycerol conversion. In fact, the three features of gold particles, the extra-small size (∼3 nm), high content of Au0 species, and high reducibility, are the essential prerequisites for achieving the superior catalytic performance of Au/LPrO.

Practical Synthesis of Allyl, Allenyl, and Benzyl Boronates through SN1′-Type Borylation under Heterogeneous Gold Catalysis

Efficient borylation of sp3 C–O bonds by supported Au catalysts is described. Au nanoparticles supported on TiO2 showed high activity under mild conditions employing low catalyst loading conditions...

Mechanistic study of C–H bond activation by O2 on negatively charged Au clusters: α,β-dehydrogenation of 1-methyl-4-piperidone by supported Au catalysts

Aerobic C–H activation by Au/OMS-2 catalyst is driven by charge transfer from OMS-2 to adsorbed O2via Au cluster.

Dehydrogenation of ethane assisted by CO2 over Y-doped ceria supported Au catalysts

Dehydrogenation of ethane assisted by CO2 has been investigated over Au catalyst supported on Y-doped CeO2 nanorod. The catalysts were characterized by XRD, TEM, N2 adsorption, XPS, TG, H2-TPR and Raman spectroscopy. Enhanced activity can be obtained after the proper Y doping. The optimal catalyst Au/Ce0.9Y0.1 exhibits an ethane conversion of 20.9% and nearly 100% selectivity to ethylene without any evident tendency of deactivation in 50 h. The reducibility of surface oxygen species is found to play a key role in the dehydrogenation assisted by CO2, which can be improved by proper Y doping into CeO2 nanorod. The promoting effect of CO2 on activity is evident over Au/Ce0.9Y0.1 catalyst, due to the quick transformation of H2 product into H2O by the reverse water gas shift reaction. The catalyst stability is also improved by the addition of CO2, attributed to the elimination of coke by Bouduoard reaction.

Selective oxidation of glycerol to dihydroxyacetone over N-doped porous carbon stabilized CuxO supported Au catalysts

In this work, N-doped porous carbon stabilized CuxO (Cu-NPCs) were synthesized by a one-pot method using copper nitrate as copper precursor with the aid of Pluronic F127 under acidic condition. The characterization results confirmed that CuxO species (CuO and Cu2O) existed in NPC in a highly dispersed state due to the strong interaction between CuxO and the doped N atoms. The obtained Cu-NPCs were used as supports to prepare Au catalysts for glycerol (GLY) oxidation to dihydroxyacetone (DHA) under base-free conditions. During the reaction, GLY preferentially adsorbed onto Au nanoparticles via the secondary OH groups due to the porous characteristics of the supports, the enhanced surface basicity from the N functionalities accelerates the alkoxide formation by abstracting proton from GLY, and the surface Cu+ sites instead of Au nanoparticles activate O2 molecules to produce active oxygen species, as a result, Au/Cu-NPC-15 %-H, which is the best catalyst in Au/Cu-NPCs, exhibited much higher catalytic activity and selectivity than those of Au/CuO. Moreover, due to the stabilization of CuxO species by NPC, relatively stable surface Cu+ content, and the strong interaction between Au nanoparticles and the supports, Au/Cu-NPC-15 %-H also possessed much better catalytic stability than that of Au/CuO during four consecutive catalytic runs.

Engineering three-layer core–shell S-1/TS-1@dendritic-SiO2 supported Au catalysts towards improved performance for propene epoxidation with H2 and O2

The advocacy of green chemical industry has led to the development of highly efficient catalysts for direct gas-phase propene epoxidation with green, sustainable and simple essence. The S-1/TS-1@dendritic-SiO2 material with three-layer core–shell structure was developed and used as the support for Au catalysts, which showed simultaneously fantastic PO formation rate, PO selectivity and stability (over 100 h) for propene epoxidation with H2 and O2. It is found that silicalite-1 (S-1) core and the middle thin layer of TS-1 offer great mass transfer ability, which could be responsible for the excellent stability. The designed dendritic SiO2 shell covers part of the acid sites on the external surface of TS-1, inhibiting the side reactions and improving the PO selectivity. Furthermore, three kinds of SiO2 shell morphologies (i.e., dendritic, net, mesoporous shell) were designed, and relationship between shell morphology and catalytic performance was elucidated. The results in this paper harbour tremendous guiding significance for the design of highly efficient epoxidation catalysts.

Preparation of Polyaniline Microtubes as the Gold Catalyst Support with Improved Catalytic Performances for the Reduction of Nitrophenols

Polyaniline (PANI) microtubes supported Au catalysts were prepared and evaluated in the catalytic reduction of nitrophenols to aminophenols with NaBH4 as the reductant. PANI microtubes were prepared through a template-free method with salicylic acid as the dopant. HAuCl4 was used as a gold precursor. Au nanoparticles were loaded on PANI microtubes by an in situ redox reaction between PANI and HAuCl4. The morphology and structure of the as-prepared PANI/Au composite catalyst were investigated using SEM, TEM, FT-IR, XRD and Raman spectra. We found that the metallic Au nanoparticles were loaded on PANI through the in situ redox reaction and located closer to the imine nitrogens of PANI. And the interaction between PANI and Au nanoparticles led the metallic Au negatively charged and favored the improvement of catalytic activities in the reduction of 4-nitrophenol (4-NP). A comparative study for the catalytic reduction of 3-nitrophenol (3-NP) and 2-nitrophenol (2-NP) showed that the activity sequence of PANI/Au was 3-NP > 2-NP > 4-NP.

Stabilizing supported gold catalysts in acetylene hydrochlorination by constructing an acetylene–deficient reaction phase

In the process of acetylene hydrochlorination, the rapid deactivation of supported gold (Au) catalysts by acetylene is still a huge challenge. Here, we provide an innovative strategy for constructing an acetylene–deficient reaction phase on the active site by coating an ionic liquid film on the Au(H2O)/C surface. The reactant ratio of C2H2 to HCl in this acetylene–deficient reaction phase is 1:132, in contrast to the 1:1 M ratio in the gas phase, thus boosting the catalytic stability of Au(H2O)/C catalysts. The kinetic and theoretical analysis showed that the reduction of cationic gold by C2H2 and the generation of carbon deposition can be inhibited in this constructed reaction phase during reaction. The current work not only broadens the scope of supported Au catalysts in acetylene hydrochlorination, but also verifies the perspective of the tunability of stoichiometric balance, which can be used in other catalytic applications.

Carbon Monoxide Oxidation over rGO-Mediated Gold/Cobalt Oxide Catalysts with Strong Metal-Support Interaction.

The strong interaction between Au nanoparticles and support (Au-metal oxide interface) usually governs the performance of a supported Au catalyst in heterogeneous catalysis. In this study, a series of Au/reduced graphene oxide (rGO)/three-dimensionally ordered macroporous (3DOM) Co3O4 catalysts with similar textural properties were prepared using the poly(methyl methacrylate)-templating and poly(vinyl alcohol)-protected reduction strategies. It was found that introducing reduced graphene oxide (rGO) as an electron-transfer bridge between Au and 3DOM Co3O4 could significantly strengthen the strong metal-support interaction (SMSI), thus enhancing the catalytic activity for CO oxidation. Among all of the catalysts, 1.86 wt % Au/2 wt % rGO/3DOM Co3O4 (1.86Au/2rGO/3DOM Co3O4) showed the highest catalytic activity: the CO reaction rate at 40 °C (432.8 μmol/(gAu s)) was 2 times higher than that (208.2 μmol/(gAu s)) over 1.87Au/3DOM Co3O4. The introduction of rGO could improve the activation of oxygen molecules and hence increase the low-temperature catalytic activity. The strategy for strengthening the SMSI via rGO mediation would guide the designing of highly efficient supported metal catalysts for low-temperature oxidation of CO.

Conversion of glycerol to lactic acid over Au/bentonite catalysts in alkaline solution

The oxidation of glycerol into lactic acid using 1 wt% Au/bentonite as heterogeneous catalyst was studied. Several reaction parameters were evaluated: catalyst reduction, oxygen presence, catalyst amount, hydroxide type and NaOH/glycerol molar ratio and reaction time. The textural and structural properties of the bentonite supported Au catalysts were evaluated by XRD, SEM and ICP. The reactions were performed in a 100 mL volume three-necked and round-bottomed flask placed in an oil bath at 90 °C reaction temperature and atmospheric pressure. It was possible to achieve a glycerol conversion of about 82.4% with selectivity to lactic acid of 92.3% and 76.1% yield, at 90 °C of reaction temperature, NaOH/glycerol = 8 mol ratio by using 0.25 g catalyst after 240 min reaction.

Oxidation of 1,2-Propanediol to Lactic Acid under Novel Supported Au Catalysts and Reaction Kinetics

This work demonstrates that 5wt% gold catalysts supported on Al2O3 and TiO2 nanopowders as well as on TiO2 nanofibers are active in the 1,2-propanediol (1,2-PDO) oxidation to lactic acid. The influence of catalysts different parameters and reaction conditions on 1,2-PDO conversion and oxidation process selectivity was studied. The best result was achieved using 5wt%Au/TiO2 catalyst over the following oxidation parameters: c0(1,2-PDO) = 0.3 mol/L, n (1,2-PDO)/n (Au) = 4000 mol/mol, p (O2) = 6 bar, c0(NaOH) = 2 mol/L, t = 60 °C: 1,2-propanediol conversion was 98 % and lactic acid selectivity 89 %.

Infrared Thermography as a Powerful, Versatile, and Elegant Research Tool in Chemistry: Principles and Application to Catalysis and Adsorption.

In this Review, diverse chemical problems that have been approached by means of infrared thermography (IRT) are covered in depth. Moreover, some novel steps forward in this field are made, described and discussed. Namely, the latest-generation IRT performance capabilities are harnessed in full; the initial phase of catalytic CO oxidation (called "fast ignition") is presented at the 0.01 s temporal resolution; at the same resolution, the thermal manifestation of the adsorption-desorption wave propagation after the gaseous reactant pulsed (0.6 s) wetting is exhibited. Furthermore, a radical difference in the thermal behavior of differently calcined γ-Al2 O3 supported Au catalysts, which underwent successive H2 O and CO attacks, is demonstrated, and the generally accepted fact that the catalyst temperature reflects the catalytic activity is validated experimentally. It is shown that latest-generation IRT may serve as unique and highly informative research tool in chemistry.

Enhanced Hydrogen Production from Ethanol Photoreforming by Site-Specific Deposition of Au on Cu2O/TiO2 p-n Junction

Hydrogen production by photoreforming of biomass-derived ethanol is a renewable way of obtaining clean fuel. We developed a site-specific deposition strategy to construct supported Au catalysts by rationally constructing Ti3+ defects inTiO2 nanorods and Cu2O-TiO2 p-n junction across the interface of two components. The Au nanoparticles (~2.5 nm) were selectively anchored onto either TiO2 nanorods (Au@TiO2/Cu2O) or Cu2O nanocubes (Au@Cu2O/TiO2) or both TiO2 and Cu2O (Au@TiO2/Cu2O@Au) with the same Au loading. The electronic structure of supported Au species was changed by forming Au@TiO2 interface due to the adjacent Ti3+ defects and the associated oxygen vacancies while unchanged in Au@Cu2O/TiO2 catalyst. The p-n junction of TiO2/Cu2O promoted charge separation and transfer across the junction. During ethanol photoreforming, Au@TiO2/Cu2O catalyst possessing both the Au@TiO2 interface and the p-n junction showed the highest H2 production rate of 8548 μmol gcat−1 h−1 under simulated solar light, apparently superior to both Au@TiO2 and Au@Cu2O/TiO2 catalyst. The acetaldehyde was produced in liquid phase at an almost stoichiometric rate, and C−C cleavage of ethanol molecules to form CH4 or CO2 was greatly inhibited. Extensive spectroscopic results support the claim that Au adjacent to surface Ti3+ defects could be active sites for H2 production and p-n junction of TiO2/Cu2O facilitates photo-generated charge transfer and further dehydrogenation of ethanol to acetaldehyde during the photoreforming.

The Catalytic Stability of Au/FeLaO3/Al2O3 Catalyst for Low Temperature CO Oxidation

Alumina modified by LaFeO3 was used as the support of gold (Au) catalysts for low temperature CO oxidation. In dry reactant feed, the catalyst showed catalytic stability similar to that found for the Al2O3 supported Au catalyst. However, the modification of FeLaO3 perovskite significantly improved the catalytic stability of the Au catalyst in the co-presence of gaseous reactants and water vapor. The modification gives the catalyst a double advantage: it enables the presence of moisture that improves the catalytic activity and promotes the formation of larger pores in the modified catalyst that facilitate a timely removal of water vapor from the catalyst. Furthermore, it also prevents the aggregation of gold nanoparticles as well as the deactivation caused by excessive water vapor.

Au/TiO2 for Ethane Dehydrogenation: Effect of Silica Doping

Dehydrogenation of ethane to ethylene over silica-doped TiO2 supported Au catalysts was investigated. The silica doping of TiO2 support greatly improved the dehydrogenation activity of Au supported catalysts, which can be ascribed to the reduced Au particle size. An ethane conversion of 15.8% and ethylene selectivity of 94.8% can be obtained over the catalyst with SiO2 doping of 20 wt%. However, the activity would begin to decrease with further increasing of SiO2 content, since the proportion of Auδ+ species decreased monotonically with SiO2 amount, which may also play an important role in ethane dehydrogenation.