Au Nanoparticle Catalysts Research Articles

Insight into restructuring of Pd-Au nanoparticles using EXAFS

We acquired Pd K-edge and Au L3-edge EXAFS spectra of two libraries of titania-supported Pd-doped Au nanoparticle catalysts representing the as-prepared and post-catalysis state. The nanoparticles have narrow particle size distributions, in the range 2.8–6.6 nm, and were synthesized to preferentially locate the palladium atoms on the surface of a gold core, with Pd mole fractions of 1.4–38%. EXAFS spectra of both the as-prepared samples and post-catalysis Pd-Au particles were collected and modeled. EXAFS refinement of all samples showed contributions from both Pd-Au and Pd-Pd scattering paths, revealing that Pd and Au components alloyed together. Pd was in a partially oxidized state as evidenced by a Pd-O scattering contribution, unlike Au. Overlaying measured coordination numbers from the libraries with predicted values, using models of surface and bulk alloying, showed that Pd restructures among Au as a result of exposure to the reaction conditions.

Polydopamine modified Au/FAU catalytic membrane for CO preferential oxidation

A hollow-fiber-supported stable Au/FAU catalytic membrane was successfully synthesized through a polydopamine coating modification-removal strategy and used as a flow-through catalytic membrane reactor for preferential oxidation of CO. Small Au nanoparticles can be efficiently isolated by dopamine and the dopamine-derived carbon shells. The interactions between Au nanoparticles and zeolite layer support are enhanced during annealing at high temperature under an inert atmosphere. A zeolite membrane supported Au nanoparticle catalyst was obtained after the removal of carbon shells, which showed high catalytic activity and stability for the removal of CO from hydrogen.

Interaction-Induced Self-Assembly of Au@La2O3 Core–Shell Nanoparticles on La2O2CO3 Nanorods with Enhanced Catalytic Activity and Stability for Soot Oxidation

The sintering resistance of supported Au nanoparticle (NP) catalysts is crucial to their practical application in heterogeneous catalysis reactions. Herein, a series of Au@La2O3 core–shell nanoparticle catalysts supported on the surfaces of La2O2CO3 nanorods (Aun@La2O3/LOC-R) were successfully synthesized via the method of interaction-induced self-assembly. Supported Au NPs with uniform size are deposited on the surfaces of La2O2CO3 nanorods by the gas bubbling-assisted membrane reduction (GBMR) method. La2O3 shell layers spontaneously formed and then partially coated the surface of Au NPs for interaction-induced self-assembly of Au@La2O3 core–shell NPs during the process of calcination at 600 °C. The strong interaction between Au NPs and La2O3 oxides increases the density of active sites (oxygen vacancies) for enhancing the adsorption–activation properties of O2. Aun@La2O3/LOC-R catalysts show high catalytic activity and stability for soot oxidation under the conditions of loose contact between soot and ...

One-pot synthesized porphyrin-based polymer supported gold nanoparticles as efficient catalysts for alkyne hydration and alcohol oxidation in water

The construction of porous organic polymer from 5,10,15,20-tetraphenylporphyrin (TPP) was described using one-pot Friedel-Crafts alkylation reaction. Au was simultaneously loaded on the porphyrin-based polymer denoted as Au/KAPs(DCM-TPP). This polymer-supported Au was fully characterized by many physicochemical methods. It was found to possess BET surface area of 796 m2 g−1, good thermal stability above 250 °C and Au nanoparticles with an average size of 8 nm. It was used as an efficient heterogeneous catalyst for alkyne hydration and alcohol oxidation in water. This Au catalyst exhibited excellent catalytic efficiency and recycling performance without loss of activity at least five times. A new strategy to synthesize polymer-supported Au nanoparticle catalyst was finally provided.

Catalytic performance of Ag, Au and Ag-Au nanoparticles synthesized by lichen extract

AbstractIn the present study, the green chemistry approach for the biosynthesis of Ag, Au and Ag-Au bimetallic nanoparticles (NPs) was applied using lichen extract [Cetraria islandica(L.) Ach.]. The lichen extract acts both as a reducing and stabilizing agent. The monometallic and bimetallic NPs were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-Vis) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that NPs were successfully synthesized and the prepared structures were generally spherical. The synthesized nanostructures exhibited excellent catalytic activities towards reduction of nitrophenols (4-nitrophenol; 4-NP) to aminophenols (4-aminophenol; 4-AP) with sodium borohydride (NaBH4). It was determined that bimetallic NPs exhibit more effective catalytic activity than monometallic Ag and Au nanostructures. This is the first report on 4-NP reduction with Ag, Au and Au-Ag NP catalysts prepared by lichen extract.

Electrocatalytic Study of the Oxygen Reduction Reaction at Gold Nanoparticles in the Absence and Presence of Interactions with SnO x Supports.

Here we report that density functional theory (DFT) can be used to accurately predict how Au nanoparticle (NP) catalysts cooperate with SnO x ( x = 1.9 or 2.0) supports to carry out the oxygen reduction reaction (ORR). Specifically, dendrimers were used to encapsulate AuNPs and prevent their interactions with the underlying SnO x supports. After removal of the dendrimers, however, the AuNPs are brought into direct contact with the support and hence feel its effect. The results show that both SnO1.9 and SnO2.0 supports strongly enhance the electrocatalytic properties of AuNPs for the ORR. In the case of AuNP interaction with a SnO1.9 support, the number of electrons involved in the ORR ( neff) increases from 2.1 ± 0.2 to 2.9 ± 0.1 following removal of the dendrimers, indicating an increased preference for the desired four-electron product (water), while the overpotential decreases by 0.32 V. Similarly, direct interactions between AuNPs and a SnO2.0 support result in an increase in neff from 2.2 ± 0.1 to 3.1 ± 0.1 and a reduction of the overpotential by 0.28 V. These experimental results are in excellent agreement with the theoretically predicted onset potential shift of 0.30 V. According to the DFT calculations, the observed activity enhancements are attributed to the existence of anionic Au resulting from electron transfer from surface oxygen vacancies within the SnO x supports to the AuNPs. This theoretical finding was confirmed experimentally using X-ray photoelectron spectroscopy. Importantly, the calculations reported here were performed prior to the experiments. In other words, this study represents an unusual case of theory accurately predicting the electrocatalytic manifestation of strong metal support interactions.

H2 Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H2 Activation at the Metal-Support Interface.

Water adsorbed at the metal-support interface (MSI) plays an important role in multiple reactions. Due to its importance in CO preferential oxidation (PrOx), we examined H2 oxidation kinetics in the presence of water over Au/TiO2 and Au/Al2O3 catalysts, reaching the following mechanistic conclusions: (i) O2 activation follows a similar mechanism to that proposed in CO oxidation catalysis; (ii) weakly adsorbed H2O is a strong reaction inhibitor; (iii) fast H2 activation occurs at the MSI, and (iv) H2 activation kinetics are inconsistent with traditional dissociative H2 chemisorption on metals. Density functional theory (DFT) calculations using a supported Au nanorod model suggest H2 activation proceeds through a heterolytic dissociation mechanism, resulting in a formal hydride residing on the Au and a proton bound to a surface TiOH group. This potential mechanism was supported by infrared spectroscopy experiments during H2 adsorption on a deuterated Au/TiO2 surface, which showed rapid H-D scrambling with surface hydroxyl groups. DFT calculations suggest that the reaction proceeds largely through proton-mediated pathways and that typical Brønsted-Evans Polanyi behavior is broken by introducing weak acid/base sites at the MSI. The kinetics data were successfully reinterpreted in the context of the heterolytic H2 activation mechanism, tying together the experimental and computational evidence and rationalizing the observed inhibition by physiorbed water on the support as blocking the MSI sites required for heterolytic H2 activation. In addition to providing evidence for this unusual H2 activation mechanism, these results offer additional insight into why water dramatically improves CO PrOx catalysis over Au.

Enhancement of the activity and durability in CO oxidation over silica-supported Au nanoparticle catalyst via CeOx modification

The supported Au nanoparticles have been regarded as promising catalysts for CO oxidation but still suffer from unsatisfactory catalytic activity and durability. Herein, we show a simple and efficient strategy to simultaneously enhance the catalytic activity and durability for CO oxidation. Key to the success is modification of the supported Au nanoparticle catalyst with nanosized CeOx to construct abundant Au-CeOx interface. Owing to the maximized interfacial effect on the CeOx-modified Au nanoparticles, the concentration of positively-charged Au species (Auδ+) was remarkably improved, leading to enhanced catalytic activities in the oxidation of CO. Importantly, the stability of Au nanoparticles is remarkably increased by CeOx modification, exhibiting good durability in a continuous test of CO oxidation at higher temperatures.

Ultrastable bimetallic catalyst with tuned surface electronic properties for highly selective oxidation of cyclohexane

Well-dispersed Au-Pt (2–5 nm) and Au-Pd (1–5 nm) nanoparticles (NPs) supported on mesoporous silica (meso-SiO2) were prepared by a facile one-pot synthesis method using 3-mercapto(propyl)trimethoxysilane (MPTMS) to anchor metal NPs to the support. The obtained catalysts were characterized and evaluated in cyclohexane oxidation with molecular oxygen. The bimetallic NPs catalysts prepared in this work exhibited a high stability with enhanced dispersion because of the intercalation of the metal alloy NPs in the meso-SiO2 channels via the anchoring effect of MPTMS. The generation of surface Auδ− species in Au-Pt and Au-Pd catalysts was identified due to the alloying of Au with Pt and Pd. Although the electronic modification of Au atoms weakened the capability of Au for O2 activation and leads to lower cyclohexane conversions on bimetallic catalysts, the deep oxidation reactions were reduced and a higher selectivity of desired products, K/A oil (cyclohexanol and cyclohexanone) was achieved. This work demonstrates an effective preparation method to improve the stability and surface chemical modification of Au NP catalysts, which provides general implications for other noble metal NPs catalysts.

Enhanced CO2 reduction activity of polyethylene glycol-modified Au nanoparticles prepared via liquid medium sputtering

The electrochemical conversion of CO2 into useful chemicals such as CO is a promising strategy to reduce CO2 emissions from fossil fuel consumption and to mitigate the impacts of global warming. Although tremendous effort has been devoted to the practical use of CO2conversion techniques, these techniques still suffer from deficient catalytic activity toward CO2 reduction as well as a complex catalyst synthesis procedure. In this study, an effective strategy to enhance the catalytic CO2 reduction activity with a unique synthesis method is proposed. Polyethylene glycol (PEG)-coated Au nanoparticles supported on a porous carbon support are prepared by a facile, cost-effective, and biocompatible one-step sputtering deposition method, termed liquid medium sputtering. The use of PEG as a liquid medium is advantageous in terms of catalytic activity and stability by producing PEG layers on the Au surface. The prepared PEG-coated Au nanoparticle catalyst exhibits a CO Faradaic efficiency of 100% at −0.57 VRHE and excellent stability during 10 h of operation due to the high solubility of PEG for CO2.

HCHO oxidation over highly dispersed Au nanoparticles supported on mesoporous silica with superior activity and stability

We firstly reported highly active Au nanoparticles catalyst supported on mesoporous silica for low-temperature HCHO oxidation. Amine as the positively charged groups was post grafted onto the surface of mesoporous silica before gold loading (PG), and it showed 15% HCHO conversion to CO2 at room temperature and complete conversion was obtained at 55 °C. No activity decrease was observed within 8 h. As comparison, mesoporous silica supported Au catalysts was also prepared by straightforward chemical reduction (CR) and impregnation (IM), which behaved poor HCHO catalytic activity with complete conversions at 200 °C and 90 °C respectively. The activation energy (Ea) for HCHO oxidation over PG was 17.6 kJ/mol, which was much lower than those (22.2–50.2 kJ/mol) over IM and CR samples. It was found that amine-functionalized mesoporous silica contributed to the well-dispersed and uniform Au nanoparticles with the optimal size distribution about 2–4 nm, and large amounts of low-coordinated Au atoms and electronic structure like bulk Au were observed on these Au nanoparticles, which accelerated HCHO adsorption and transformation into intermediate species like DOM, formate and CO(a). Particularly, the interaction between amine and Au effectively inhibited the reduction of Auδ+ to Au0, which positively contributed to the best catalytic performance. Moreover, Au/SiO2 system prepared by amine post-grafting method showed poor activity of complete combustion at 210 °C. Comparatively, it was proposed that MCM-41 with well-ordered and open ends of pore structure showed better HCHO adsorption-diffusion capacity and the ability of controlling Au particles in high dispersion, which was great benefit to the high catalytic activity of HCHO oxidation.

In situ synthesis of easily separable Au nanoparticles catalysts based on cellulose hydrogels

Cellulose hydrogels are “green” reductants and supports for Au nanoparticles (Au NPs). Herein, we report a facile method for synthesizing Au NPs based on alkali/urea regenerated cellulose hydrogels. By controlling the precursor concentration and the reaction temperature, the size of the Au NPs can be adjusted. We successfully demonstrate that the Au NP/cellulose hydrogels can be used as efficient heterogeneous catalysts in the reduction of 4-nitrophenol (4-NP). Au NPs with smaller sizes have higher catalytic activity. The highest turnover frequency (TOF) of the Au NPs/cellulose hydrogels is 19.4 h−1. The Au NP/cellulose hydrogels can be easily isolated after the catalytic reaction. We report a facile method for synthesizing Au NPs based on alkali/urea regenerated cellulose hydrogels. By controlling the precursor concentration and the reaction temperature, the size of the Au NPs can be adjusted. The Au NP/cellulose hydrogels can be used as efficient heterogeneous catalysts in the reduction of 4-nitrophenol.

Accessing Frustrated Lewis Pair Chemistry through Robust Gold@N-Doped Carbon for Selective Hydrogenation of Alkynes

Pyrolysis of Au(OAc)3 in the presence of 1,10-phenanthroline over TiO2 furnishes a highly active and selective Au nanoparticle (NP) catalyst embedded in a nitrogen-doped carbon support, Au@N-doped carbon/TiO2 catalyst. Parameters such as pyrolysis temperature, type of support, and nitrogen ligands as well as Au/ligand molar ratios were systematically investigated. Highly selective hydrogenation of numerous structurally diverse alkynes proceeded in moderate to excellent yield under mild conditions. The high selectivity toward the industrially important alkene substrates, functional group tolerance, and the high recyclability makes the catalytic system unique. Both high activity and selectivity are correlated with a frustrated Lewis pairs interface formed by the combination of gold and nitrogen atoms of N-doped carbon that, according to density functional theory calculations, can serve as a basic site to promote the heterolytic activation of H2 under very mild conditions. This “fully heterogeneous” and recy...

Supported Au Nanoparticles with N-Heterocyclic Carbene Ligands as Active and Stable Heterogeneous Catalysts for Lactonization

Attachment of N-heterocyclic carbenes (NHCs) on the surface of metal nanoparticle (NP) catalysts permits fine-tuning of catalytic activity and product selectivity. Yet, NHC-coated Au NPs have been seldom used in catalysis beyond hydrogenation chemistry. One challenge in this field has been to develop a platform that permits arbitrary ligand modification without having to compromise NP stability toward aggregation or leaching. Herein, we exploit the strategy of supported dendrimer-encapsulated metal clusters (DEMCs) to achieve aggregation-stable yet active heterogeneous Au NP catalysts with NHC ligands. Dendrimers function as aggregation-inhibitors during the NP synthesis, and NHCs, well-known for their strong attachment to the gold surface, provide a handle to modify the stereochemistry, stereoelectronics, and chemical functionality of the NP surface. Indeed, compared to "ligandless" Au NPs which are virtually inactive below 80 °C, the NHC-ligated Au NP catalysts enable a model lactonization reaction to proceed at 20 °C on the same time scale (hours). Based on Eyring analysis, proto-deauration is the turnover-limiting step accelerated by the NHC ligands. Furthermore, the use of chiral NHCs led to asymmetric induction (up to 16% enantiomeric excess) in the lactonization transformations, which demonstrates the potential of supported DEMCs with ancillary ligands in enantioselective catalysis.

Reaction Pathway Dependence in Plasmonic Catalysis: Hydrogenation as a Model Molecular Transformation.

The localized surface plasmon resonance (LSPR) excitation in plasmonic nanoparticles can enhance or mediate chemical transformations. Increased reaction rates for several reactions have been reported due to this phenomenon; however, the fundamental understanding of mechanisms and factors that affect activities remains limited. Here, by investigating hydrogenation reactions as a model transformation and employing different reducing agents, H2 and NaBH4 , which led to different hydrogenation reaction pathways, we observed that plasmonic excitation of Au nanoparticle catalysts can lead to negative effects over the activities. The underlying physical reason was explored using density functional theory calculations. We observed that positive versus negative effects on the plasmonic catalytic activity is reaction-pathway dependent. These results shed important insights on our current understanding of plasmonic catalysis, demonstrating reaction pathways must be taken into account for the design of plasmonic nanocatalysts.

Plasmonic Control of Multi-Electron Transfer and C-C Coupling in Visible-Light-Driven CO2 Reduction on Au Nanoparticles.

Artificial photosynthesis relies on the availability of synthetic photocatalysts that can drive CO2 reduction in the presence of water and light. From the standpoint of solar fuel production, it is desirable that these photocatalysts perform under visible light and produce energy-rich hydrocarbons from CO2 reduction. However, the multistep nature of CO2-to-hydrocarbon conversion poses a significant kinetic bottleneck when compared to CO production and H2 evolution. Here, we show that plasmonic Au nanoparticle photocatalysts can harvest visible light for multielectron, multiproton reduction of CO2 to yield C1 (methane) and C2 (ethane) hydrocarbons. The light-excitation attributes influence the C2 and C1 selectivity. The observed trends in activity and selectivity follow Poisson statistics of electron harvesting. Higher photon energies and flux favor simultaneous harvesting of more than one electron from the photocharged Au nanoparticle catalyst, inducing the C-C coupling required for C2 production. These findings elucidate the nature of plasmonic photocatalysis, which involves strong light-matter coupling, and set the stage for the controlled chemical bond formation by light excitation.

Performance of Supported Au-Pd Alloy Nano Particles Catalyst for Base-free Synthesis of Imines by Self-coupling of Amine

Abstract A series of supported nano particles catalysts, Au/γ-Al2O3, Pd/γ-Al2O3 and Au-Pd/γ-Al2O3, were prepared by the impregnation reduction method. The catalytic performance of these catalysts for direct base-free synthesis of imines by self-coupling of benzylamine was investigated. The results show that the yield of imine could reaches 93% over bimetallic alloy Au-Pd/γ-Al2O3 catalyst, much higher than monometallic Au or Pd nano particle catalyst. The used catalyst is recovered and 44% yield of imine could be obtained after 5 recycling use of Au-Pd/γ-Al2O3.

Evaluating differences in the active-site electronics of supported Au nanoparticle catalysts using Hammett and DFT studies

Supported metal catalysts, which are composed of metal nanoparticles dispersed on metal oxides or other high-surface-area materials, are ubiquitous in industrially catalysed reactions. Identifying and characterizing the catalytic active sites on these materials still remains a substantial challenge, even though it is required to guide rational design of practical heterogeneous catalysts. Metal-support interactions have an enormous impact on the chemistry of the catalytic active site and can determine the optimum support for a reaction; however, few direct probes of these interactions are available. Here we show how benzyl alcohol oxidation Hammett studies can be used to characterize differences in the catalytic activity of Au nanoparticles hosted on various metal-oxide supports. We combine reactivity analysis with density functional theory calculations to demonstrate that the slope of experimental Hammett plots is affected by electron donation from the underlying oxide support to the Au particles.

Nanoporous Catalysts for Biomass Conversion

The supported Au nanoparticles have been regarded as promising catalysts for CO oxidation but still suffer from unsatisfactory catalytic activity and durability. Herein, we show a simple and efficient strategy to simultaneously enhance the catalytic activity and durability for CO oxidation. Key to the success is modification of the supported Au nanoparticle catalyst with nanosized CeOx to construct abundant Au-CeOx interface. Owing to the maximized interfacial effect on the CeOx-modified Au nanoparticles, the concentration of positively-charged Au species (Auδ+) was remarkably improved, leading to enhanced catalytic activities in the oxidation of CO. Importantly, the stability of Au nanoparticles is remarkably increased by CeOx modification, exhibiting good durability in a continuous test of CO oxidation at higher temperatures.

Three-dimensionally ordered macroporous CeO2/Al2O3-supported Au nanoparticle catalysts: Effects of CeO2 nanolayers on catalytic activity in soot oxidation

A series of catalysts consisting of three-dimensionally ordered macroporous (3DOM) x-CeO2/Al2O3-supported Au nanoparticles (x = 2, 10, 20, and 40 wt%) were successfully synthesized using a reduction-deposition method. These catalysts were characterized using scanning electron microscopy, the Brunauer-Emmett-Teller method, X-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, and temperature-programmed reduction by H2. Au nanoparticles of mean particle size 5 nm were well dispersed and supported on the inner walls of uniform macropores. The 3DOM structure improved the contact efficiency between soot and the catalyst. An Al-Ce-O solid solution was formed in the multilayer support, i.e., x-CeO2/Al2O3, by the incorporation of Al3+ ions into the CeO2 lattice, which resulted in the creation of extrinsic oxygen vacancies. Strong interactions between the metal (Au) and the support (Ce) increased the amount of active oxygen species, and this promoted soot oxidation. The catalytic performance in soot combustion was evaluated using a temperature-programmed oxidation technique. The presence of CeO2 nanolayers in the 3DOM Au/x-CeO2/Al2O3 catalysts clearly improved the catalytic activities in soot oxidation. Among the prepared catalysts, 3DOM Au/20%CeO2/Al2O3 showed high catalytic activity and stability in diesel soot oxidation.