Au Catalysts Research Articles

Electron transfer effect from Au to Pt in Au-Pt/TiO2 towards efficient catalytic activity in CO oxidation at low temperature

The performances of supported noble metal catalysts are closely related to the size, composition, structure and metal-support interaction. In the current work, bimetallic Pt-Au nanoalloys are loaded in situ on titanium dioxide in one step by flame spray pyrolysis (FSP). The formation of alloyed Pt-Au nanocrystals without phase separation is systematic proven by various characterization methods. And in-situ DRIFTs and XPS reveal the electronic synergy effect between Au and Pt weakens the agglomeration of the noble metal and reduces the CO poisoning. This synergy effect makes the alloy catalyst's low-temperature catalytic activity increase by 20% compared to the single-metal Pt catalyst. The total conversion temperature (T100) increase by 100 °C relative to the Au catalyst. As a continuous and scalable industrial production process, FSP can produce alloys and other new structural materials with less time and cost, compared to other wet preparation methods.

Advances in Designing Au Nanoparticles for Catalytic Epoxidation of Propylene with H2 and O2

Au nanoparticles, which can be used in various industrial and environmental applications, have drawn substantial research interest. In this review, a comprehensive background and some insights are provided regarding recent studies concerning the use of Au nanoparticles for catalytic propylene epoxidation with H2 and O2. Over the last two decades, substantial progress has been made toward the efficient production of propylene oxide (PO); this includes the design of highly dispersed Au catalysts on Ti-modified mesoporous silica supports, the optimization of catalytic epoxidation, and the determination of the mechanisms and reaction pathways of epoxidation. Particularly, the critical roles of catalyst synthesis, the types of material support, Au nanoparticle sizes, and the dispersion amounts of Au nanoparticles are emphasized in this review. In future studies, novel, practical, robust, and highly PO-selective Au nanoparticle catalyst systems are expected to be continually designed for the enhanced catalytic epoxidation of propylene.

Facile synthesis of precious-metal single-site catalysts using organic solvents.

Single-site catalysts can demonstrate high activity and selectivity in many catalytic reactions. The synthesis of these materials by impregnation from strongly oxidizing aqueous solutions or pH-controlled deposition often leads to low metal loadings or a range of metal species. Here, we demonstrate that simple impregnation of the metal precursors onto activated carbon from a low-boiling-point, low-polarity solvent, such as acetone, results in catalysts with an atomic dispersion of cationic metal species. We show the generality of this method by producing single-site Au, Pd, Ru and Pt catalysts supported on carbon in a facile manner. Single-site Au/C catalysts have previously been validated commercially to produce vinyl chloride, and here we show that this facile synthesis method can produce effective catalysts for acetylene hydrochlorination in the absence of the highly oxidizing acidic solvents previously used.

Diamond nanocrystal thin films: Case study on surface texture and power spectral density properties

Analyzing diamond nanocrystal (DNC) thin film morphology produced by the HFCVD technique is the main objective of the present work. Stereometric analysis of three-dimensional surface microtextures was carried out based on data obtained through atomic force microscopy (AFM), while the ISO 25178-2:2012 standard was applied to characterize surface topography. The Abbott–Firestone curve, peak count histograms, and Cartesian graphs, which were extracted through AFM images, gave valuable statistical information. As can be seen, the most isotropic sample was the Au catalyst (etched) deposited by the hot filament chemical vapor deposition method. Moreover, by increasing the time of DNC growth from 15 min to 60 min, the surface roughness was increased. In addition, the average power spectral density was calculated and furrows were determined for all samples.

Selective Growth of Stacking Fault Free ⟨100⟩ Nanowires on a Polycrystalline Substrate for Energy Conversion Application.

Cubic semiconductor nanowires grown along ⟨100⟩ directions have been reported to be promising for optoelectronics and energy conversion applications, owing to their pure zinc-blende structure without any stacking fault. But, until date, only limited success has been achieved in growing ⟨100⟩ oriented nanowires. Here we report the selective growth of stacking fault free ⟨100⟩ nanowires on a commercial transparent conductive polycrystalline fluorine-doped SnO2 (FTO) glass substrate via a simple and cost-effective chemical vapor deposition (CVD) method. By means of crystallographic analysis and density functional theory calculation, we prove that the orientation relationship between the Au catalyst and the FTO substrate play a vital role in inducing the selective growth of ⟨100⟩ nanowires, which opens a new pathway for controlling the growth directions of nanowires via the elaborate selection of the catalyst and substrate couples during the vapor-solid-liquid (VLS) growth process. The ZnSe nanowires grown on the FTO substrate are further applied as a photoanode in photoelectrochemical (PEC) water splitting. It exhibits a higher photocurrent than the ZnSe nanowires do without preferential orientations on a Sn-doped In2O3 (ITO) glass substrate, which we believe to be correlated with the smooth transport of charge carriers in ZnSe ⟨100⟩ nanowires with pure zinc-blende structures, in distinct contrast with the severe electron scattering happened at the stacking faults in ZnSe nanowires on the ITO substrate, as well as the efficient charge transfer across the intensively interacting nanowire-substrate interfaces.

Au Nanoparticles Confined in SBA-15 as a Highly Efficient and Stable Catalyst for Hydrogenation of Quinoline to 1,2,3,4-Tetrahydroquinoline

Au nanoparticles confined within the mesopores of the modified SBA-15 were obtained through adsorption-reduction method and firstly employed as chemoselective catalyst for quinoline hydrogenation. The effects of Au loadings, calcination temperature as well as structure of support on catalytic performances of Au catalysts were explored. The as-obtained 1.2%Au@SBA-15-500 catalyst exhibited high activity, excellent selectivity towards 1,2,3,4-tetrahydroquinoline and extraordinary sintering-resistant property as high as 800 °C, which is sharp contrast to the 1.3%Au/SiO2-500 catalyst. It also showed good recyclability and versatility for quinoline derivatives. The observed properties were assigned to small-sized Au nanoparticles and mesopores of SBA-15. Our work provides a facile and promising approach to construct metal nanocatalysts with high catalytic performance by the use of mesoporous materials.

Effects of Preparation Parameters of NiAl Oxide-Supported Au Catalysts on Nitro Compounds Chemoselective Hydrogenation.

Effects of preparation parameters of NiAl oxide-supported Au nanocatalysts on their performance in the chemoselective hydrogenation of nitro compounds were investigated. The deposition–precipitation method, low Au loading, and low Ni/Al molar ratio of the support contributed to the generation of small-sized Au nanoparticles. High catalytic properties were related to the small sizes of Au particles and appropriate basicity of supports. Accordingly, the 0.43% Au/NiAlO-2-500 (the Ni/Al molar ratio of the support = 2) showed high activity and excellent selectivity for nitro hydrogenation. It also showed good versatility for other nitro compounds and good recyclability. Interestingly, for the first time, this Au catalyst switched its selectivity to vinyl hydrogenation by mere regulation of the composition of the support (the Ni/Al molar ratio of the support = 4). The observed shift in selectivity was ascribed to the different adsorption behaviors of the nitro and vinyl group on Au nanocatalysts. It provides a novel and facile strategy to construct Au nanocatalysts with high activity and switchable selectivity for hydrogenation of nitro compounds by the fine tuning of preparation parameters.

Activation conditions of bentonite supports over gold-based catalysts for production of lactic acid from glycerol

It was investigated the lactic acid production by glycerol oxidation using % 1 Au/bentonite catalysts in alkaline medium. The reactions were performed in a 100 mL volume three-necked and round-bottomed flask placed in an oil bath over a period of 4 h at 90 °C reaction temperature and atmospheric pressure. 1 wt% Au based bentonite catalysts were prepared by incipient to wetness method. Gold metal is chosen for catalyst active material since it is active under mild conditions and activation conditions of support material were investigated. Bentonite supports, activated under different conditions (temperature and acidic medium) were used as supports. Results of bentonite supported studies showed that the use of heterogeneous catalysts was necessary and bentonite activation improved catalytic activity, but the application of chemical activation negatively affected lactic acid selectivity. The highest glycerol conversion (76%) has been obtained with 1 wt% Au catalyst that the support was activated by thermally and chemically at 700 °C and 1 M HCl. And also the maximum lactic acid amount (18%) was obtained with this catalyst. The chemical activation of the support material at high acid concentration has caused a high conversion and lactic acid selectivity.

Nucleation and growth of metal-catalyzed silicon nanowires under plasma

We report the results of a microscopic study of the nucleation and early growth stages of metal-catalyzed silicon nanowires in plasma-enhanced chemical vapor deposition. The nucleation of silicon nanowires is investigated as a function of different deposition conditions and metal catalysts (Sn, In and Au) using correlation of atomic force microscopy and scanning electron microscopy. This correlation method enabled us to visualize individual catalytic nanoparticles before and after the nanowire growth and identify the key parameters influencing the nanowire nucleation under plasma. The size and position of catalytic nanoparticles are found to play a significant role in the nucleation. We demonstrate that only small isolated nanoparticles in the range of 10–20 nm contribute to the nanowire growth under plasma, while larger nanoparticles are inactive because they get buried under a layer of a-Si:H before reaching supersaturation. Systematic analysis of different growth parameters reveals that the nanowire growth in plasma contradicts the vapor–liquid–solid mechanism at thermal equilibrium in many ways. The nanowire growth is much faster and proceeds even at negligible silicon solubility and bellow the eutectic temperature of the metal-silicon alloy. Based on the observations, we propose the nanowire growth under plasma to be characterized by the rapid solidification mechanism, where a crystalline silicon phase emerges from a metastable supersaturated liquid metal-silicon phase in local nonequilibrium.

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.

Bovine serum albumin templated porous CeO2 to support Au catalyst for benzene oxidation

Bovine serum albumin (BSA)-templated CeO2 was prepared by citrate sol-gel method and used as a support to synthesize Au catalyst for benzene oxidation. It was found that the Au/BSA-CeO2 with 3.2 nm Au nanoparticles (NPs) gave rise to superior catalytic performance for benzene oxidation in which the temperature for 90 % benzene conversion was only 210 °C. In addition, the catalytic activity of Au/BSA-CeO2 remained stable for 140 h of on-stream reaction. Besides, density functional theory calculation demonstrated how the porous structure of CeO2 influenced the oxygen vacancy generation, and the synergetic effects of Au NPs and CeO2 support facilitated the activation of benzene which was adsorbed on Au NPs prior to benzene oxidation. The catalysis of benzene oxidation over Au/CeO2 catalyst follows the dual-site mechanism. BAS-templated CeO2 support possessed hierarchical structure, abundant adsorbed oxygen species and synergic interaction with Au that determined the performance of Au/BSA-CeO2.

Identification of single-atom active sites in CO oxidation over oxide-supported Au catalysts

Here we present a combined operando infrared spectroscopic and theoretical analysis of ceria-supported gold catalysts during room temperature CO oxidation that identifies an active site for the reaction as a single gold site on the ceria support forming an Olattice–Au+–CO species. As monitored by operando infrared spectroscopy, the isolated Au+ gold site is either present as a result of the catalyst synthesis or formed under reaction conditions after CO adsorption at the perimeter of the Au–ceria interface. Our results provide new insights into the chemical nature of the active gold site and the reaction mechanism by detecting the formation of active and inhibiting species simultaneously.

Complete Benzene Oxidation over Mono and Bimetallic Pd—Au Catalysts on Alumina-Supported Y-Doped Ceria

The protection of environment and human health stimulates intensive research for abatement of volatile organic compounds (VOCs) in the atmosphere. Complete catalytic oxidation is an efficient, environmentally friendly and economically feasible method for elimination of VOCs. This study aims to design high performing and cost-effective catalytic formulations by exploration of appropriate and economically viable supports. Alumina-supported ceria (30 wt.%) and Y2O3 (1 wt.%)-doped ceria were prepared by mechanical mixing and were used as support of mono Au (2 wt.%) and Pd (1 wt.%) and bimetallic Pd-Au catalysts. The characterization by textural measurements, X-ray powder diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), EPR (electron paramagnetic resonance) and temperature-programmed reduction (TPR) was carried out in order to clarify the relationship between catalyst composition, textural, structural and surface properties, reducibility and catalytic performance for complete benzene oxidation. Among all studied catalysts, Pd-based catalysts exhibited the best combustion activity. In particular, monometallic Pd on alumina supported Y-doped ceria attained 100% of complete benzene conversion at 180 °C. These catalytic materials have potential to meet stringent emission regulations in an economical and effective way.

Stereodivergent Metal-Catalyzed Allene Cycloisomerizations.

Metal-catalyzed allene cycloisomerizations provide rapid entry into five-membered carbocyclic frameworks, a common motif in natural products and pharmaceuticals. While both Au(I) and Pd(0)-catalyzed allene cycloisomerizations give 5-endo-dig cyclization, Pd prefers the syn diastereomer in contrast to the anti isomer observed with Au. The change in stereoselectivity is proposed to arise from buildup of A1,3 strain during the key carbopalladation step to furnish the cycloisomerized products in moderate to good dr with yields comparable to Au(I) catalysts.

Phenoxylation of Alkynes through Mono‐ and Dual Activation Using Group 11 (Cu, Ag, Au) Catalysts

NHC‐gold(I) based catalysts have displayed outstanding results toward hydroalkoxylation of terminal and internal alkynes in solvent‐free conditions and using low catalyst loadings. It has been demonstrated that, in the hydrophenoxylation reaction the gold complex is composed by two moieties that determine the rate of the reaction by activating both substrates synergistically, i.e. [Au(OR)(NHC)] and [Au(η2‐alkyne)(NHC)]+. Then, these bimetallic systems act cooperatively toward the hydroalkoxylation reaction. Herein, density functional theory studies were carried out to get insights on the mechanism of hydrophenoxylation. The rate‐determining step, which corresponds to the formation of the C(alkyne)–O(alcohol) bond between [Au(OR)(NHC)] and [Au(η2‐alkyne)(NHC)]+, was studied using energy decomposition analyses (EDA). It was found that the C–O bond shows strong electrostatic and orbital interactions between both fragments in the homobimetallic, heterobimetallic and monogold mechanisms. Moreover, the analyses were expanded to copper and argentum, and the steric sensibility was also studied through the use of different NHC ligands, including IMes, IMe, SIMes, IPr, and IPr*, that differ on their steric demand.

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.

Carbon Grain Supported Nano-Sized Au for Low Temperature Removal of VOCs in Humid Condition: Effect of Catalyst’s Synthesis Conditions

This study reports effects of pH and PVA/Au mass ratio on properties and catalytic activity of carbon grain supported nano Au catalysts for treatment of VOC pollutants at low temperatures. The catalysts were synthesized at various pH of the sol solution and different PVA/Au mass ratios. They were characterized by specified techniques to evaluate crystallinity, surface properties, and morphology. The catalytic activity for VOCs removal has been conducted in the temperature range 100-200 C and in the presence of water vapor. The relationship between catalyst’s characterization and the low-temperature catalytic activity was evaluated, and the synergy effect in the dual functional adsorbent/catalyst was reported. Nano Au supported on granular carbon, which was synthesized with mass ratio PVA/Au = 4 at pH = 4, can removal toluene about 50.3 % from initial concentration of 314 ppmv after 60 min reaction at the reaction temperature 150 C and high humidity (RH = 60 %). Furthermore, moisture reduces the activity only slightly due to the hydrophobicity of activated carbon, and the durability of the catalyst was also reported.

Photothermal reduction of 4-nitrophenol using rod-shaped core–shell structured catalysts

Metal nanoparticles can be used in the advanced applications in catalysis due to their multi-functionality and easy tunable of catalyst size. Especially, nanoparticles composed of various metallic species exhibit valuable properties such as localized surface plasmon resonance (LSPR), which was the result of absorption of resonant light. Gold nanoparticles (AuNPs) absorb light strongly and readily convert photon energy into heat quickly and efficiently via localized photothermal effect. Therefore, herein, we tried to improve the catalytic performance through the photothermal effects via LSPR of NPs. To demonstrate this effect, a 4-NP reduction test was performed with representative materials Au and Pd were used. In addition, core–shell structure was utilized to easy recover the metal catalysts. To induce magnetic properties in structured catalysts and prevent the oxidization of NPs, iron oxide and silica was used as core and shell materials, respectively. The photothermal conversion of the rod-shaped core–shell structured catalysts decorated with PdNP and AuNP was 8.12% and 57.55%, respectively. The photothermal effect of the catalyst was evaluated by performing catalytic reduction of 4-NP based on the prepared particles, and the primary rate constant for Pd and Au catalysts was enhanced as 18% and 41% through infra-red irradiation, respectively.

Boosting the catalysis of gold by O2 activation at Au-SiO2 interface

Supported gold (Au) nanocatalysts have attracted extensive interests in the past decades because of their unique catalytic properties for a number of key chemical reactions, especially in (selective) oxidations. The activation of O2 on Au nanocatalysts is crucial and remains a challenge because only small Au nanoparticles (NPs) can effectively activate O2. This severely limits their practical application because Au NPs inevitably sinter into larger ones during reaction due to their low Taman temperature. Here we construct a Au-SiO2 interface by depositing thin SiO2 layer onto Au/TiO2 and calcination at high temperatures and demonstrate that the interface can be not only highly sintering resistant but also extremely active for O2 activation. This work provides insights into the catalysis of Au nanocatalysts and paves a way for the design and development of highly active supported Au catalysts with excellent thermal stability.

Continuous Flow Synthesis of Bimetallic AuPd Catalysts for the Selective Oxidation of 5‐Hydroxymethylfurfural to 2,5‐Furandicarboxylic Acid

AbstractThe production of 2,5‐furandicarboxylic acid (FDCA) from the selective oxidation of 5‐hydroxymethylfurfural (HMF) is a critical step in the production of biopolymers from biomass‐derived materials. In this study, we report the catalytic performance of monometallic Au and Pd, and bimetallic AuPd nanoparticles with different Au : Pd molar ratios synthesised under continuous flow conditions using a millifluidic set‐up and subsequently deposited onto titanium dioxide as the chosen support. This synthetic technique provided a better control over mean particle size and metal alloy composition, that resulted in higher FDCA yield when the catalysts were compared to similar batch‐synthesised materials. A 99% FDCA yield was obtained with the millifluidic‐prepared AuPd/TiO2 catalyst (Au : Pd molar composition of 75 : 25) after being calcined and reduced at 200 °C. The heat treatment caused a partial removal of the protective ligand (polyvinyl alcohol) encapsulating the nanoparticles and so induced stronger metal‐support interactions. The catalyst reusability was also tested, and showed limited particle sintering after five reaction cycles.