Supported Gold Nanocatalysts Research Articles

Atomic Scale Observation of the Structural Dynamics of Supported Gold Nanocatalysts under 1,3‐Butadiene byin situEnvironmental Transmission Electron Microscopy

AbstractSupported gold catalysts have been reported to exhibit exceptional selectivity towards the desirable product, an alkene, in chemoselective hydrogenation of highly unsaturated alkynes and alkadienes. However, insights on the interactions between the gold nanoparticles and the 1,3‐butadiene adsorbates are lacking. In this work, using atomic scalein situETEM with the support of DFT‐based calculations, the reactivity of TiO2supported gold nanoparticles below 7 nm towards 1,3‐butadiene is investigated by monitoring their structure and dynamics under 105 Pa of 0.46 % C4H6/He during cooling at 300 °C and 200 °C. For sizes larger than 5 nm, the initial truncated octahedron morphology of gold nanoparticles is found to evolve towards a rounded morphology, with the growth of more open (110) surfaces and higher index facets at the expense of the initially present (100) and (111) facets, reflecting the adsorption of butadiene on low coordination sites at 300 °C. The morphology changes are shown to be accompanied by nucleation and glide of dislocations. At 200 °C, 1,3‐butadiene adsorption is shown to predominateon (111) facets, with the nanoparticles reverting to a faceted morphology while maintaining their fcc structure. For smaller particle size, Au fcc structure is also maintained under 1,3‐butadiene.

Effect of Polyvinyl Alcohol Ligands on Supported Gold Nano-Catalysts: Morphological and Kinetics Studies.

The effect of polyvinyl alcohol (PVA) stabilizers and gold nanoparticles supported on active carbon (AuNPs/AC) was investigated in this article. Polymers with different molecular weights and hydrolysis degrees have been synthesized and used, like the stabilizing agent of Au nano-catalysts obtained by the sol-immobilization method. The reduction of 4-nitrophenol with NaBH4 has been used as a model reaction to investigate the catalytic activity of synthesized Au/AC catalysts. In addition, we report several characterization techniques such as ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) in order to correlate the properties of the polymer with the metal nanoparticle size and the catalytic activity. A volcano plot was observed linking the catalytic performance with hydrolysis degree and the maximum of the curve was identified at a value of 60%. The Au:PVA-60 weight ratio was changed in order to explain how the amount of the polymer can influence catalytic properties. The effect of nitroaromatic ring substituents on the catalytic mechanism was examined by the Hammett theory. Moreover, the reusability of the catalyst was investigated, with little to no decrease in activity observed over five catalytic cycles. Morphological and kinetic studies reported in this paper reveal the effect of the PVA polymeric stabilizer properties on the size and catalytic activity of supported gold nanoparticles.

Sintering-resistant Au/iron oxide-hydroxyapatite nanocatalysts achieved by tuning strong metal-support interactions

Supported gold nanocatalysts have been treated as extraordinarily effective catalysts in heterogeneous catalysis field. However, their poor thermodynamic stability in reactions at elevated temperature remains to be resolved for practical applications. The strong metal-support interaction (SMSI), a vital concept in chemical industry, can be applied to design catalysts and enhance their catalytic performances. Herein, we reported a sintering-resistant gold catalyst by anchoring Au nanoparticles at the interface between Fe2O3 and hydroxyapatite, in which one side of the Au nanoparticles was partially encapsulated while the other side was exposed. Owing to the strong metal-support interaction between Au and support, the resultant gold nanocatalyst was anti-sintering and presented remarkable activity and durability after calcination at 600 °C compared with commercial gold catalyst denoted as RR2Fe, providing a promising strategy to stabilize gold catalysts, especially for oxidation reactions at high temperature.

Catalytic Vapor Phase Oxidation of Glycerol to Glyceric Acid Over Activated Carbon Supported Gold Nanocatalysts

A series of activated carbon (AC) supported Au nanocatalysts with different loadings of Au were prepared by using the homogeneous deposition–precipitation (HDP) method. The samples were characterised with myriad techniques such as X-ray diffraction (XRD), CO-chemisorption, N2 adsorption–desorption measurements, transmission electron microscopy (TEM), inductively coupled plasma-optical emission spectrometer (ICP-OES) and X-ray photoelectron spectroscopy (XPS) to understand the structural and textural properties in detail. The catalysts were tested for the vapour phase oxidation of glycerol to glyceric acid under base-free medium in an aerobic condition at normal atmospheric pressure. The Au/AC nanocatalysts with smaller size Au particles ([Formula: see text][Formula: see text]nm) showed higher glycerol conversion and selectivity for glyceric acid, and also a longer catalyst life. While the larger Au particles ([Formula: see text][Formula: see text]nm) showed less activity and selectivity. Among all the nanocatalysts tested, the 1.0[Formula: see text]wt.% Au/AC sample having smaller particle size of Au showed the best catalytic performance in terms of glycerol conversion and glyceric acid selectivity. These results suggest that the oxidation activities of Au/AC nanocatalysts are strongly influenced by the size of Au nanoparticle, nature of the support material and through a metal-support interaction.

Gold nanoparticle supported on mesoporous vanadium oxide for photo-oxidation of 2-naphthol with hydrogen peroxide and aerobic oxidation of benzyl alcohols

Vanadium oxide supported gold nanocatalysts designated as AuVOx was found to be highly efficient for photocatalytic oxidation of benzyl alcohol and 2-naphthol. The photocatalytic ability of the AuVOx catalyst towards the selective oxidation of 2-naphthol to BINOL was found to be dependent on the nature of the support. The selectivity of the reaction was found to be significantly altered on hybridization with multi-walled carbon nanotubes (AuVOx-MWCNTs). The well-resolved surface plasmon resonance (SPR) band originated from the gold nanoparticles (Au-NPs) on the vanadium oxides support helped in stimulating the photo-oxidation of benzyl alcohol to benzaldehyde and 2-naphthol to BINOL with almost 100% selectivity. The most notable achievement using the AuVOx and AuVOx-MWCNTs catalyst was in the successful photo-oxidation of 2-naphthol to BINOL in presence of hydrogen peroxide (H2O2). The photo-activity of the material and the creation of oxygen defect sites participated in the photochemical processes were estimated from photoluminescence analysis and ESR study. The photochemical reaction was supposed to proceed via superoxide radical generation favored by the oxygen defect sites.

Development of Easily Separable ZnO‐Supported Au Nanocatalyst for the Oxidative Esterification of Alcohols and Reduction of Nitroarenes

AbstractA simple and practical protocol is described for the reduction of aromatic nitro compounds and the oxidative esterification of alcohols by using a recyclable 3D assembled porous rectangular ZnO nanoplates supported gold nanocatalysts (Au/ZnO) under mild conditions. The support 3D assembled ZnO plate was synthesized simple strategy without employing any toxic reductants or capping agents and Au was loaded through simple impregnation method. The synthesized catalyst was characterized by powder X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), electron microscopic study (TEM & SEM), diffuse reflectance spectrum (DRS). The developed catalyst showed excellent catalytic activity for both the reaction and ended with good to excellent yield, up to 97% yield for nitro reduction and 90% yield for oxidative esterification. The catalyst is recyclable for a minimum five times without any significant loss in catalytic activity. The catalytic activities are superior or sometime comparable to that of the reported catalyst. The use of a developed catalyst is beneficial for of low cost and availability of good support ZnO and advantageous with respect to that of separation, easily separable by simple filtration due to the bigger size of support ZnO.

Effects of divalent metal ions of hydrotalcites on catalytic behavior of supported gold nanocatalysts for chemoselective hydrogenation of 3-nitrostyrene

The effect of the divalent metal ions on the hydrotalcite (HT) (MAl-HT; M = Mg, Zn, Ni)-supported thiolated Au25 nanoclusters (NCs) as the precatalysts for the chemoselective hydrogenation of 3-nitrostyrene to 3-vinylaniline was investigated. The highest chemoselectivity was obtained over the Au25/ZnAl-HT-300 (calcined at 300 °C) catalyst, with a maintained selectivity of desired product above 98%. The Au25/NiAl-HT-300 catalyst exhibited the highest activity, although the particle size of gold (3.2 nm) was greater than those of the Au25/MgAl-HT-300 (2.2 nm) and Au25/ZnAl-HT-300 (1.7 nm) catalysts. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results of CO adsorption revealed that Ni interacting intimately with gold could be reduced easily, which affected the catalytic behavior of the Au25/NiAl-HT-300 catalyst. Furthermore, the results of the in situ DRIFTS of the adsorption of nitrostyrene at 10 bar of hydrogen suggested that, besides the condensation route, it also followed the direct route to produce aniline on Au25/NiAl-HT-300, which was different from the other two catalysts. This work provides new insight into the support effect over the gold catalysts for selective hydrogenation reactions.

Selective Hydrogenation of Cinnamaldehyde Catalyzed by ZnO-Fe2O3 Mixed Oxide Supported Gold Nanocatalysts

ZnO-Fe2O3 mixed oxides and supported gold nanocatalysts were prepared by using coprecipitation and deposition–precipitation methods, respectively. Cinnamaldehyde hydrogenation over various ZnO-Fe2O3 mixed oxides supported gold nanocatalysts have been investigated at 140 °C and a hydrogen pressure of 1.0 MPa. The molar ratio of Fe to Zn was found to greatly affect the selective hydrogenation catalytic activity of ZnO-Fe2O3 mixed oxide supported gold nanocatalysts. Among these supported gold nanocatalysts in this work, Au/Zn0.7Fe0.3Ox (Au loading of 1.74 wt %) exhibited the highest conversion of cinnamaldehyde and high selectivity to cinnamal alcohol. The excellent catalytic activity of Au/Zn0.7Fe0.3Ox was tightly associated with a large surface area, small gold nanoparticles, and good H2 dissociation ability at low temperature.

Synthesis of Supported Gold Nanocatalysts for the Oxidation of Alkyl Benzenes

Selective oxidation of alkyl benzene, to its corresponding oxygen containing products, is a crucial reaction in organic researches and industrial manufacturing. In particular, the direct oxidation of lavish and cheapest aromatics, having carbon-hydrogen bonds, into corresponding ketones are the key transformations as the oxidation products are very important platform compounds for the production of useful prime, special chemicals and high economic valued fine chemicals, agrochemicals, pharmaceutical and perfumes in large scale. However, the oxidations of aromatic hydrocarbons have been remaining a challenging task due to the limitations of a suitable catalyst and requirement of chemical treatments (potassium permanganate/dichromate and ammonium cerium nitrate) which have been proven to be corrosive, not reusable, hazardous, and environmentally unfriendly and, also, have no selectivity at all. Instead of, scientists are paying more attention to use heterogeneous green catalysts along with support as well as novel oxidants e.g. hydrogen peroxide, tert-butyl peroxide and so on, due to its ecofriendly nature and availability. Silica/carbon/metal oxide supported nanoporous gold is a favorable catalyst due to its three dimensional open pore network structures, high surface to volume ratio, high reusability, distinct optolectronic and physio-chemical properties. Mesoporous carbon/silica/metal oxide supports provide well scattering of metal nanocatalysts and facilitate the transportation of molecules through the nanopores/nanochannels, thus increase the product with lowest cost and time. This paper has reviewed various gold-skeleton green catalysts and their synthetic method and mechanistic schemes for the selective oxidation of alkyl substituted aromatics.

Design of novel indium oxide supported gold nanocatalysts and their application in homocoupling of arylboronic acids

Gold (Au) nanoparticles stabilized on metal oxide supports offer superior catalytic activity and recyclability in organic catalysis. We report for the first time synthesis of indium oxide stabilized gold (Au@In2O3) nanocatalysts using an electrochemical procedure and their application in homocoupling of arylboronic acids. In2O3 nanoparticles prepared via sol-gel process are subjected to sacrificial anode electrolysis (SAE) under inert condition for electrodeposition of nano Au on In2O3. Thus Au@In2O3 nanoparticles obtained are thermally annealed at high temperature to partially oxidize Au and to remove any surfactants. XPS results show the existence of both elemental (nano Au0) and cationic (Au3+) species in Au@In2O3 nanocatalysts, while SEM images confirm the presence of nanoscale Au (<10nm) particles on In2O3 surface. Au@In2O3 nanocatalysts are tested for arylboronic acids homocoupling under different conditions and it is found that they are highly active in organic medium with K2CO3 base and demonstrate excellent conversion (>97%) and selectivity (>98%). The catalyst recyclability and performance towards differently substituted arylboronic acids is also studied and a plausible mechanism of action is proposed.

PMMA-templating generation and high catalytic performance of chain-like ordered macroporous LaMnO3 supported gold nanocatalysts for the oxidation of carbon monoxide and toluene

Rhombohedrally crystallized chain-like LaMnO3 and its supported gold (xAu/LaMnO3; x=1.4, 3.1, and 4.9wt%) catalysts have been prepared using the poly(ethylene glycol)-assisted polymethyl methacrylate-templating and gas bubble-assisted polyvinyl alcohol-protected reduction methods, respectively. It is shown that there were good correlations of surface adsorbed oxygen species concentration and low-temperature reducibility with catalytic activity of the samples for the oxidation of CO and toluene. Among the LaMnO3 and xAu/LaMnO3 samples, 4.9Au/LaMnO3 performed the best, giving the T50% and T90% of 61 and 91°C for CO oxidation, and of 201 and 226°C for toluene combustion, respectively. The apparent activation energies (29–50 and 47–62kJ/mol) of the chain-like LaMnO3 and xAu/LaMnO3 samples were much smaller than those (63 and 97kJ/mol) of the bulk LaMnO3 sample for the oxidation of CO and toluene, respectively. We believe that the higher surface area and oxygen adspecies concentration and better low-temperature reducibility as well as the strong interaction between Au nanoparticles and chain-like LaMnO3 support might account for the high catalytic performance of 4.9Au/LaMnO3.

Water–gas shift on gold catalysts: catalyst systems and fundamental studies

Since the pioneering finding by Haruta et al. that small gold nanoparticles on reducible supports can be highly active for low-temperature CO oxidation, the synthesis, characterization, and application of supported gold catalysts have attracted much attention. The water-gas shift reaction (WGSR: CO + H2O = CO2 + H2) is important for removing CO and upgrading the purity of H2 for fuel cell applications, ammonia synthesis, and selective hydrogenation processes. In recent years, much attention has been paid to exploration the possibility of using supported gold nanocatalysts for WGSR and understanding the fundamental aspects related to catalyst deactivation mechanisms, nature of active sites, and reaction mechanisms. Here we summarize recent advances in the development of supported gold catalysts for this reaction and fundamental insights that can be gained, and furnish our assessment on the status of research progress.

Selective hydrogenation of acetylene in excess ethylene over SiO2 supported Au–Ag bimetallic catalyst

Supported gold nanocatalysts have been reported to be active in selective hydrogenation of acetylene. In this work, SiO2 supported Au–Ag bimetallic catalyst is studied in the selective hydrogenation of acetylene in excess ethylene. Au and Ag were reductively deposited on a silica surface functionalized by APTES (3-aminopropyltriethoxysilane). They form Au–Ag alloy nanoparticles of very small size. The catalytic activity of Au–Ag bimetallic system showed better catalytic activity at high temperature than that of monometallic gold catalyst. According to the TEM and XRD results, Ag stabilized the nanoparticles against sintering during high temperature calcinations. Non-thermal O2 plasma was applied to remove the APTES under mild conditions instead of high temperature calcination. The results showed that the conversion of acetylene was much higher over Au–Ag/SiO2 catalyst pretreated by O2 plasma than that of pretreated by calcination at 500°C, although the latter catalyst had similar particle size.

Solvent-free selective oxidation of alcohols by molecular oxygen over gold nanoparticles supported on β-MnO2 nanorods

Abstract The performances of a series of MnO 2 -supported gold nanocatalysts with varying gold loading were studied in the solvent-free selective oxidation of alcohols by molecular oxygen. The gold was deposited onto the β-MnO 2 support, featuring one dimensional rod-like morphology, by means of homogeneous deposition–precipitation. The catalytic activity of the catalyst for the solvent-free aerobic oxidation of benzyl alcohol was found to increase with increasing gold loading, with the highest specific activity in terms of turnover frequencies (TOF) being achieved over the catalyst with 5 wt.% gold loading. Both XRD and TEM results showed that the majority of Au particles of various catalysts have diameters in the range of 2–6 nm. Temperature-programmed reduction (TPR) results indicated that the reducibility of MnO 2 was significantly enhanced with the surface-deposited gold nanoparticles. X-ray photoelectron spectroscopy (XPS) revealed the coexistence of both oxidized and metallic gold species on the surface of MnO 2 , the composition of which depends critically on the gold loading. The superior specific activity of the catalyst with 5% gold loading was attributed to the presence of higher amounts of positive gold species and more surface oxygen vacancy sites resulting from stronger metal–support interactions as compared to other catalysts.

Colloidal deposition synthesis of supported gold nanocatalysts based on Au–Fe3O4 dumbbell nanoparticles

Highly active Au catalysts with a dumbbell-like heterostructure for CO oxidation were prepared through colloidal deposition method; both activities and stabilities were investigated under different experimental conditions.

Preparation and Comparison of Supported Gold Nanocatalysts on Anatase, Brookite, Rutile, and P25 Polymorphs of TiO2 for Catalytic Oxidation of CO

Nanosized anatase (< or = 10 nm), rutile (< or = 10 nm), and brookite (approximately 70 nm) titania particles have been successfully synthesized via sonication and hydrothermal methods. Gold was deposited with high dispersion onto the surfaces of anatase, rutile, brookite, and commercial titania (P25) supports through a deposition-precipitation (D-P) process. All catalysts were exposed to an identical sequence of treatment and measurements of catalytic CO oxidation activity. The as-synthesized catalysts have high activity with concomitant Au reduction upon exposure to the reactant stream. Mild reduction at 423 K produces comparably high activity catalysts for every support. Deactivation of the four catalysts was observed following a sequence of treatments at temperatures up to 573 K. The brookite-supported gold catalyst sustains the highest catalytic activity after all treatments. XRD and TEM results indicate that the gold particles supported on brookite are smaller than those on the other supports following the reaction and pretreatment sequences.