Gold Particle Size Distribution Research Articles

SAXS Application for the Determination of the Gold Nanoparticle Sizes in Au/C Catalysts: Advantages over Other Methods

In the case of Au/C catalysts have been shown advantages of application SAXS with masking liquid technique for determining supported metal particle sizes compared with TEM and XRD as standard methods. According to SAXS data particle size distributions of gold in a wide range (1–50 nm) were obtained with full consideration of all size fractions of particles present in the samples under study. Also values of the mass fraction of “X-ray amorphous” gold particles with size less than 4 nm (WSAXS) were determined. It has been found that the oxidative treatment of the carbon support before deposition of metallic gold precursor complexes has a significant effect on the size distribution of gold particles in the final catalyst. Comparison of the results of measuring the rate of CO oxidation by an excess of moist air at 40°C on Au/C catalysts with the (WSAXS) values found for these catalysts showed that the catalytic activity increases exponentially as (WSAXS) increases. High activity in CO oxidation reaction was demonstrated by Au/C catalysts with (WSAXS) ≥ 80%.

Monolithic stirrer reactor: The selective lactose oxidation in liquid phase over Au/Al2O3 nanostructured catalysts

The performance of rotating metallic monolith stirrer reactor was studied for selective lactose oxidation in liquid phase at 65 °C, atmospheric pressure and with air as oxidant agent. The Au/Al2O3deposition on metallic substrates was performed by wash-coating, producing catalyst coating thicknesses between 5 and 20 μm. Monoliths with different configuration (channel size between 0.36 and 1.06 mm) were used as stirrer blades in a batch reactor. Internal and external mass transfer limitations were observed during liquid phase lactose oxidation. For stirring rates equal or higher than 600 rpm there were no important external diffusional restrictions and this was also independent of the monolith configuration. Coating with thickness higher than 15 μm presents loss of catalyst effectiveness due to internal diffusional restrictions. Excellent stability in the catalytic tests was obtained after three regeneration-reaction cycles. Regeneration was carried out at 400 °C in air flow. Gold particle size distribution in the monolith washcoat, determined by TEM before and after reaction, was homogeneous with a medium size of around 5 nm. This is in agreement with the very good reproducibility and stability obtained in the catalytic tests. After calcination at 500 °C, some sintering and a heterogeneous distribution of metal particle size was observed, accompanied by a slight loss in catalyst activity. It is concluded that metallic monolith stirrer reactors are a promising application for selective lactose oxidation in liquid phase.

Enhanced X-RAYS degradation of methylene blue in the presence of gold microspheres

Promising alternatives to increase the degradation of dyes in polluted water are radiation and the addition of particulate materials as catalysts. Specifically, gold particles have demonstrated an enhancement of dye degradation in the presence of UV light and/or reducing agents by increasing the rate at which reactive oxygen species are produced. Herein, the enhanced decomposition of methylene blue (MB) under ionizing radiation, X-rays, in the presence of gold particles was investigated. The influence of X-ray operating voltage, gold particle size distribution, and gold concentration in the degradation of MB was studied. Irradiation of MB was performed at a dose rate of ~35 Gy/min and X-ray irradiator operating voltages of 50 kV, 100 kV, and 225 kV. The presence of spherical gold microparticles enhanced the reaction rate of MB up to 61.7%. Results showed that dye degradation combining ionizing radiation and a catalyst such as gold particles is a promising methodology to accelerate the clean-up process of polluted water resources.

N-Octanol oxidation on Au/TiO2 catalysts promoted with La and Ce oxides

This study aims to improve gold-based catalyst effectiveness in n-octanol oxidation by adjusting the structural and electronic properties of gold through incorporation of additives to support. Au/La/TiO2, Au/Ce/TiO2, and Au/TiO2 catalysts, prepared by deposition-precipitation with urea, were tested in the base-free oxidation of octanol under mild conditions in liquid phase. The influence of additives on structural properties of the catalyst and the active phase, and on the electronic and redox properties of the active gold species was studied by TEM, SR-XRD, EXAFS, FTIR of adsorbed CO, and XPS. Support modifiers affected gold particle size distribution, but no direct dependence of the catalyst activity on Au NPs average size was observed. The activity of as-prepared catalysts was limited, and even lower in the presence of modifiers at that stage. Reductive pretreatment boosted the activity of every catalyst, and support modification with La and Ce increased four- and two-fold, respectively, the activity of Au/TiO2 under the same reaction conditions. The nature of modifiers also affected the product distribution where the outstanding performance of Au/La/TiO2 gives rise to the highest ester selectivity at isoconversion with no acid formation. Overall, the studied catalysts, and especially Au/Ce/TiO2, and Au/La/TiO2, hold promise for n-octanol oxidation under mild conditions.

Population and hierarchy of active species in gold iron oxide catalysts for carbon monoxide oxidation.

The identity of active species in supported gold catalysts for low temperature carbon monoxide oxidation remains an unsettled debate. With large amounts of experimental evidence supporting theories of either gold nanoparticles or sub-nm gold species being active, it was recently proposed that a size-dependent activity hierarchy should exist. Here we study the diverging catalytic behaviours after heat treatment of Au/FeOx materials prepared via co-precipitation and deposition precipitation methods. After ruling out any support effects, the gold particle size distributions in different catalysts are quantitatively studied using aberration corrected scanning transmission electron microscopy (STEM). A counting protocol is developed to reveal the true particle size distribution from HAADF-STEM images, which reliably includes all the gold species present. Correlation of the populations of the various gold species present with catalysis results demonstrate that a size-dependent activity hierarchy must exist in the Au/FeOx catalyst.

Synthesis and characterization of novel Au/θ-Al2-xFexO3 nanomaterials with high thermal stability in catalytic oxidation of carbon monoxide

Applications as a catalyst in engine exhaust systems require a uniform distribution of gold nanoparticles on a thermally stable alumina support. The novel thermally stable nanomaterials comprised of gold nanoparticles on the iron-modified theta-alumina support have been synthesized by the method of deposition-precipitation using urea. The material precursor was proved to involve both a gold precursor and adsorbed urea. The generation of gold nanoparticles from the gold precursor occurred below 300°C, whereas the complete elimination of organic compounds proceeded only at 500°C in air or even at 650–700°C in a reducing gas flow. The gold particle size distribution was narrow with a mean size about 2nm and 3.4nm after calcination at 300°C and 500°C respectively. The prepared materials are highly active and stable as the catalysts of carbon monoxide oxidation in the presence of ammonia, and maintain the catalytic activity even after prolong thermal treatment at 750°C.

Structural and Optical Characteristics for Gold Thin Films Deposited on Polymer Substrates

Structural and Optical Characteristics for Gold Thin Films Deposited on Polymer Substrates Gold nanostructures are receiving great interest due to their wide applications in many fields, biomedical, nonlinear optical device, aerospace and space science technology. In this research, fabrication technique for gold thin films deposited onto polypropylene, as polymer substrates is discussed. Synthesis of gold nanoparticles and thin film deposition technique are carried out in the experimental setup initiated at Nano- NRIAG Unit (NNU). For gold nanoparticles, phase and particle size distributions have been determined by transmission electron microscopy (TEM). Optical properties have been characterized by UV-Vis spectroscopy. For gold/polypropylene (film/substrate) systems, immersion method is used to localize gold nanoparticles on polymer substrate. Thin film growth is performed with different intensities and particle sizes. The spectral analysis and properties of the formed nano films are performed using the CCD image processing to characterize the nanocomposite and surface structures. Two samples with low and heavy particle concentrations are considered in these analyses. The obtained results are morphologically analyzed.

Optimising sampling protocols via the heterogeneity test: challenges in coarse gold mineralisation

Determination of the sampling constant is an important step when applying the Fundamental Sampling Error equation to optimise sampling protocols. The classic method for its determination is based on the heterogeneity test (HT). In coarse gold-dominated mineralisation, the HT sometimes provides an evaluation of the fine-gold background grade population heterogeneity, but understates that of the important high-grade coarse-gold component. This is because the total mass of fragments is too small to represent the full gold particle size distribution of the mineralisation. Despite this, single heterogeneity tests (SHT) are often undertaken on deposits where the presence of coarse gold is ignored, not realised or understated. Resultant sampling and assaying protocols are frequently of poor quality and do not match the mineralisation characteristics. For coarse gold-dominated mineralisation, an empirical approach for sampling constant determination is recommended via direct estimation of the liberation diameter.

Placer gold sampling—The overall measurement error using gravity concentration on particle size ranges during sample treatment

Author Summary: Placer deposits are generally characterized by low grade of free gold. This is the case in French Guiana where the main placer deposits are in the river bed. Most have already been exploited by very small mining companies using sluices. If this technology is efficient for coarse gold, it releases fine gold in the tailings. During the last years, studies have been performed on various sites and recoveries have been estimated between 40 and 60% depending on the size distribution of gold particles and of the quality of the sluice configuration. Many recent or ancient tailings are available with a non-negligible quantity of remaining gold, offering retreatment opportunities. They are generally found in the form of sand heaps with the shape of an alluvial fan originating at the sluice discharge. Due to the resulting large distribution heterogeneity it is necessary to take many samples at many strategically deployed locations. These samples have to be large enough to be representative of the local material. As gold is mainly liberated in this type of lot, traditional sample treatment with successive size reductions and sub-samplings is not efficient and can be very expensive. Another approach using sieving and gravity concentration per particle range is preferred and presented here. After presentation of the sampling and measurement protocol used, this paper focuses on estimation of the overall sampling error. Various tailing cases are presented for which retreatment decision depends on the level of confidence obtained for the estimate of the quantity of recoverable gold.

Gold nanoparticles supported on conventional silica as catalysts for the low-temperature CO oxidation

A catalytic system where gold nanoparticles are dispersed on the surface of a conventional mesoporous silica was investigated. The sample (2.25wt% Au) was prepared by functionalizing the silica surface with 3-mercaptopropyltrimethoxysilane, anchoring gold from an HAuCl4 solution and then reducing it with sodium citrate. Prior to the catalytic runs, the Au/SiO2–SH system was submitted to different thermal treatments. The catalyst, either as-made or after each thermal treatment, was characterized by N2 adsorption–desorption at −196°C, CHS elemental analysis, solid state 29Si CPMAS NMR spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy of CO at −196°C. The bare and functionalized supports were also characterized. Large Au nanoparticles (ca. 30nm) form readily by adding the HAuCl4 solution to the functionalized support and, besides such particles, a second family of small ones originates upon thermal treatment (2–5nm, depending on the temperature and the atmosphere), leading to a bimodal gold particle size distribution. Remarkable CO conversion is obtained when the catalyst is calcined in air at 560°C and subsequently treated in H2/He at 600°C or directly treated in H2/He at 600°C. Based on the treatment-induced modifications in the physico-chemical properties of both the carrier and the supported phase, a reaction mechanism is outlined.

High-Temperature Stable Gold Nanoparticle Catalysts for Application under Severe Conditions: The Role of TiO2 Nanodomains in Structure and Activity

Metal nanoparticles with precisely controlled size are highly attractive for heterogeneous catalysis. However, their poor thermal stability remains a major concern in their application at realistic operating conditions. This paper demonstrates the possibility of synthesizing gold nanoparticles with exceptional thermal stability. This has been achieved by using a simple conventional deposition–precipitation technique. The material employed as catalyst consists of gold supported on a TiO2-impregnated SiO2 bimodal mesoporous support. The resulting material shows gold nanoparticles with a narrow size distribution around 3.0 nm, homogeneously dispersed over the TiO2/SiO2 material. Most interestingly, the gold nanoparticles show exceptional thermal stability; calcination temperatures as high as 800 °C have been employed, and negligible changes in the gold particle size distribution are apparent. Additionally, the presence of an amorphous titanium silicate phase is partially preserved, and these factors lead to remarkable activity to catalyze a range of oxidation reactions.

Influence of gold nanoparticle loading in Au/C on the activity towards electrocatalytic glycerol oxidation

The influence of gold loading on the electrocatalytic oxidation of glycerol over carbon supported gold nanoparticles has been investigated. Over the range of 1–20wt% Au/C, the gold particle size distributions were similar with an overall average particle diameter of 4.6nm and an average specific gold surface area of 43±4m2g−1. As expected, cyclic voltammetry measurements in 1M KOH revealed that the number of electrochemically active gold sites were directly proportional to the gold loading. This observation, together with the particle size analysis, strongly suggests that the inherent electrocatalytic behaviour of the gold nanoparticles should be independent of gold loading. Despite this, very clear evidence was found to suggest that the electrocatalytic glycerol oxidation behaviour is influenced by gold loading: both the drop-off and onset potential are functions of gold loading. These findings are explained by the differences in mass transfer and subsequent accumulation of oxidation products within the electrocatalytic layers, with layers containing higher gold nanoparticle loadings having higher levels of accumulation. This increased accumulation is speculated to inhibit gold oxide formation thus shifting the characteristic drop-off in anodic current during the forward sweep of the voltammograms to higher potentials. While this accumulation allows glycerol oxidation to occur over a wider potential range, the specific activity for glycerol oxidation is reduced, presumably due to partial blocking the glycerol oxidation reaction by the adsorbed oxidation products. These findings have important implications not only for evaluating the specific activity electrocatalytic materials but also understanding the electrocatalytic oxidation of glycerol over gold nanoparticles.

Effects of sample mass on gravity recoverable gold test results in low-grade ores

Gravity recoverable gold characterisation methodologies are the single- or three-stage tests, which generally use a standard feed sample mass. A case study of low-grade coarse gold-dominated mineralisation is presented that demonstrates the high variability of test results using different sample masses. Samples were processed using the single-stage test and subsequently entire development rounds were batched through a plant for comparison. Unsurprisingly the results indicate that the small (≤50 kg) test samples grossly understate plant gravity recoverable gold and display poor precision. Larger samples display improved precision, but still understate plant gravity recoverable gold. The small mass samples are unrepresentative as they do not contain the full gold particle size distribution. Poor representivity is enhanced by gold particle clustering. Small samples generally capture finer more abundant and disseminated gold particles, but rarely contain clustered gold. The use of standard GRG test sample masses is challenged. Test work should be based on spatially distributed representative field samples, that if required are split to representative sub-samples for testing. An early stage gold particle size characterisation programme is required to optimise sample mass and improve representivity.

Pt/Au Alloys as Reduction Promoters for Co/TiO<sub>2</sub> Fischer-Tropsch Catalysts

Noble metals, such as platinum or gold, may promote the reduction of cobalt in supported cobalt catalysts either by direct contact or indirectly via hydrogen spill-over. The synthesis go gold nano-particles using THPC in the presence of a calcined Co/TiO2-catalyst precursor results in a broadening of the gold particle size distribution possibly due to the association of gold with Co3O4. This was not observed for platinum and platinum-gold alloys synthesized in the same manner. Platinum-gold alloys are less effective than platinum as a reduction promoter, possibly due to the presence of gold on the surface of the alloy particles.

Stabilizer effects on the synthesis of gold-containing microparticles. Application to the liquid phase oxidation of glycerol

Gold-containing poly(urea-formaldehyde) microparticles were prepared by the in situ polymerization method using a series of stabilization agents with different chemical nature. The effects of cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA) and tetrakis(hydroxymethyl)phosphonium chloride (THPC) on the morphology, the particle size of encapsulated gold, the gold particle size distribution and the encapsulation efficiency were investigated by using scanning electron microscopy, X-ray diffraction and atomic absorption analyses. The chemical nature of stabilizer had a marked influence on both the encapsulated gold particle size and the encapsulation efficiency. Both gold particle size and gold encapsulation efficiency increased when decreasing the stabilizer polarity number. All the microparticles here prepared were tested in the liquid phase selective oxidation of glycerol. The glycerol conversion increased and the glyceric acid selectivity decreased when decreasing gold particle sizes. Results showed that use of stabilizers with hydrophobic surfaces enhanced the selectivity to C3 products in the resulting catalysts. On the other hand, the use of stabilizers with hydrophilic surfaces increased the C–C bond cleavage products in the resulting catalysts.

The Influence of Different Preparation Methods on the Activity for CO Oxidation on the LDHs-Supported Gold Catalysts

Au nanoparticles supported on MgAl-LDHs has been synthesized by coprecipitation method, gold colloid routs as well as intercalation and assembly method respectively. The influence of the above different preparation methods on the activity of the gold catalysts for CO oxidation was investigated, and the phase and gold particle size distribution of the catalysts were analyzed by XRD and TEM respectively. The results show that 100% conversion of CO on the fresh prepared gold catalysts synthesized by gold colloid routs and intercalation assembly method can be achieved at room temperature when the catalysts were activated at 473K for 2h in the air. The gold particle size distribution was range from 2nm to 7nm.

Preferential oxidation of CO in H 2 rich stream (PROX) over gold catalysts supported on doped ceria: Effect of water and CO 2

Gold catalysts on mechanochemically prepared mixed ceria supports doped with FeO x and MnO x were studied in the reaction of preferential oxidation of CO in H 2 rich stream (PROX) in the presence of 20% CO 2 and 10% H 2O. The average size and the size distribution of gold particles were estimated by HRTEM. By means of XPS the Au XPS states at various binding energies, assigned to differently charged Au nanoparticles, were distinguished. The catalytic activity and selectivity in the oxidation of CO in dry H 2-reach stream was high, however the PROX activity was significantly suppressed by the addition of high concentrations of water and CO 2. The two step calcinations of catalysts, first at 400 °C and then at 550 °C, showed no better performance in PROX reaction carried out at realistic experimental conditions. However, after the two-step calcination procedure, nanosized gold particles of high thermal stability were obtained over MnO x doped ceria.

Deposition–precipitation with Urea to prepare Au/Mg(OH)2 catalysts: Influence of the preparation conditions on metal size and load

Abstract Deposition–precipitation with Urea method to prepare Au/Mg(OH) 2 was investigated. The preparation was carried out at 80 °C using an aqueous solution of HAuCl 4 and Urea in presence of MgO as support precursor. The pH of the solution was 9.5–10 owing to the rapid hydration of MgO to Mg(OH) 2 . The influence of the preparation conditions (Urea concentration, preparation procedure, addition of magnesium citrate) on the gold load and particle size distribution was evaluated. The structural characterization of preparations was carried out by means of X-ray diffractometry and transmission electron microscopy. Among the parameters investigated, the preparation procedure, that is the control of the interaction between gold species and the support, is the main step for the deposition of the highest amount of gold (> 80% of the theoretical amount) having small size and a narrow distribution ( d = 4.1 nm, σ = ±1.1 nm). The synthesized catalysts were tested in the CO oxidation reaction.

Bridging the Gap between CO Adsorption Studies on Gold Model Surfaces and Supported Nanoparticles

An in-depth understanding of CO adsorption on highly dispersed gold nanoparticles (AuNPs) is critically important to fully interpret the catalytic behavior of supported gold systems in processes such as CO oxidation, PROX (selective oxidation of CO in presence of a large excess of H2), [5–7] or LT-WGS (low-temperature water gas-shift) reactions. Despite its relevance, the quantitative data and fundamental information presently available on the CO–Au interaction mainly comes from studies carried out on model single-crystal and thin-film surfaces under experimental conditions far from those at which catalytic assays on supported gold systems are typically run. Probably because of the very weak and singular nature of the CO–Au interaction, which on the basis of both theoretical and experimental studies is generally acknowledged to take place on low-coordination surface sites, and the additional contribution of the support, a few studies have been carried out that were aimed at estimating the amount of CO adsorbed at low temperature on powdered or model supported AuNPs. To our knowledge, however, none of these have arrived at a detailed quantitative description of this process under conditions close to those occurring in real catalytic reactions. To bridge this gap, we have developed an approach in which AuNP size distributions, as determined from HAADFSTEM (high-angle annular dark-field scanning transmission electron microscopy) and quantitative CO adsorption data, as determined from volumetric adsorption at 308 K, under CO partial pressures ranging from 6.65 10 Pa to 3.99 10 Pa, are jointly analyzed with the help of a nanostructural model for the gold particles. This model could be deduced from the analysis of images recorded in a parallel HRTEM (highresolution transmission electron microscopy) study. As discussed herein, this approach gives a substantial experimental support to the extension to supported gold catalysts of the chemical principles governing the CO adsorption on model surfaces. We investigated two catalyst samples, 2.5 wt% Au/ Ce0.62Zr0.38O2 (Au/CZ) and 1.5 wt% Au/Ce0.50Tb0.12Zr0.38O2 x (Au/CTZ), which have significantly different gold particle size distributions. Two consecutive CO volumetric adsorption isotherms were recorded on the gold catalysts and the corresponding supports. Prior to running the second isotherms, samples were evacuated (residual pressure Pres< 1.33 10 4 Pa) for 30 min at 308 K. By processing the volumetric data in a similar way to earlier studies (for details, see the Supporting Information), the amounts of CO adsorbed on the AuNPs, on the surface cations of the supports (weak adsorption), and on the surface anions of the supports, which mainly consist of strongly chemisorbed carbonate species, could be determined from the difference of the two isotherms. The amount of CO adsorbed on the AuNPs at PCO= 1.33 10 4 Pa (100 Torr) was used as a measurement of the saturation coverage. The corresponding data are reported in Table 1 and the Supporting Information, Figure S1. Gold particle size distributions were determined for each of the investigated catalysts from the analysis of series of experimental ultra-high-resolution HAADF-STEM images (Supporting Information, Figure S2). In accordance with the physical principles lying behind the mechanism of image formation, this technique is particularly suitable for obtaining reliable metal particle size distributions in oxide-supported metal catalysts. Moreover, as recently shown, this technique can be fruitfully applied to a very fine characterization of AuNPs dispersed on mixed oxides of heavy elements, as is the case of those investigated herein. (Size distributions for Au/CZ and Au/CTZ catalysts are shown in the Supporting Information, Figure S2). [*] M. L pez-Haro, Dr. J. J. Delgado, J. M. Cies, E. del Rio, S. Bernal, Dr. M. A. Cauqui, Dr. S. Trasobares, Dr. J. A. P rez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016-288 E-mail: jose.calvino@uca.es

Synthesis and characterization of gold nanoparticles supported on zinc oxide via the deposition-precipitation method

Gold nanoparticles supported on zinc oxide nanoparticles were synthesized at several pH levels via the deposition-precipitation (DP) method. The effects of pH on gold loading, particle size, and particle size distribution on the support were studied at the iso-electric point (IEP) as well as below and above the IEP of ZnO. The addition of the support significantly changed the pH of the solution. The effects of adjusting the pH before and after the addition of the support into the gold chloride solution were also investigated. Gold particles with diameters of less than 5 nm were obtained. The results revealed that gold loading depends on the pH, while gold particle size and distribution are independent of pH adjustment. Structural and elemental characterizations of the gold nanorods were carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-rays (EDX), atomic absorption spectrometry (AAS), and ultraviolet-visible spectrophotometry (UV-Vis).