Gold-based Catalysts Research Articles

Unravelling the Role of Oxygen Vacancies in the Mechanism of the Reverse Water–Gas Shift Reaction by Operando DRIFTS and Ultraviolet–Visible Spectroscopy

The reaction mechanism of the reverse water–gas shift (RWGS) reaction was investigated using two commercial gold-based catalysts supported on Al2O3 and TiO2. The surface species formed during the reaction and reaction mechanisms were elucidated by transient and steady-state operando DRIFTS studies. It was revealed that RWGS reaction over Au/Al2O3 proceeds through the formation of formate intermediates that are reduced to CO. In the case of the Au/TiO2 catalyst, the reaction goes through a redox mechanism with the suggested formation of hydroxycarbonyl intermediates, which further decompose to CO and water. The Ti3+ species, the surface hydroxyls, and oxygen vacancies jointly participate. The absence of carbonyl species adsorbed on gold particles during the reaction for both catalysts indicates that the reaction pathway involving dissociative adsorption of CO2 on Au particles can be discarded. To complete the study, operando ultraviolet–visible spectroscopy was successfully applied to confirm the presence ...

Bimetallic Au-Li/SAC catalysts for acetylene hydrochlorination

A series of Au-Li/SAC catalysts for acetylene hydrochlorination were prepared with HAuCl4⋅4H2O and LiCl as precursors by an incipient wetness impregnation method and characterized by XRD, BET, TEM, TG, XPS and TPR techniques. It is found that Au1Li0.5/SAC shows the highest activity and stability with the acetylene conversion of 91.3% after 48 h reaction running. The addition of Li can disperse the Au species well and inhibit coke formation on the catalyst surface, Li additives can also inhibit the reduction of active gold species during the reaction time, thereby improving the activity and the long-term stability of gold-based catalysts.

Carbon-Supported Au Nanoparticles: Catalytic Activity Ruled Out by Carbon Support

The catalytic oxidation of glycerol produces high added value chemical products. Gold-based catalysts showed activity and selectivity depending on particle size and specific preparation method. Moreover, the support plays a fundamental role in modulating the stability of the catalytic system. However, the literature is still lacking of a precise disclosure of these important relationships. Herein, we synthesized two series of gold catalysts on different carbon supports (Vulcan-XC72R, X40S and Norit GSX), the first synthesized by solvated metal atom deposition (SMAD) and the second by Sol Immobilization technique (SOL). First of all, the specific physico-chemical properties of the supports and the synthesis procedure influenced the dispersion and the size of Au NPs making a direct comparison among the different carbon difficult. In particular, on Vulcan-XC72R, AuNPs showed narrow size and good dispersion, whereas on Norit GSX and X40S-Camel a notably wider size distribution has been revealed. XPS analyses showed Au exposure changed accordingly to the presence of oxygen species, lower O content corresponding to lower Au exposure. Unexpectedly however this does not correspond to a lower activity, being the lowest Au/C (%at), the Vulcan ones, the most active catalysts. Comparing SOL and SMAD prepared samples on the same carbon, the SOL ones always result more active.

Plasma catalysis application of gold nanoparticles for acetaldehyde decomposition

The decomposition of acetaldehyde was investigated in a plasma-catalytic reactor. The gold-based catalysts were obtained by growing Au nanoparticles on silica beads or plates previously coated by a titanium oxo-alkoxy monolayer. The characterization of the synthesized catalysts was carried out by SEM, AFM and UV–Visible spectroscopy. Below an exposure time to UV of 20 min, a homogeneous surface coverage by the relatively small gold nanoparticle (R < 16 nm) was achieved. Compared to our previous study with silver catalysts, the same amount of the pollutant, acetaldehyde, can be decomposed with the Au-based nano-structured catalysts, under similar experimental conditions. However, using the gold catalyst enhanced the process selectivity towards CO2. Furthermore, we showed that the gold particles size and distribution strongly affect acetaldehyde degradation. Plasma treatment doesn’t influence the surface Au atoms density but change the oxidation state of gold. On the basis of the results of the by-products formation, a basic reaction mechanism involved plasma induced chemical reactions in the discharge region and catalytic reactions on the catalyst surface was proposed.

Mechanistic investigations of the Au catalysed C-H bond activations in on-surface synthesis.

Recently, Au-based nanostructures have attracted extensive interest due to their excellent activities in heterogeneous catalysis. The reaction mechanisms have been interpreted qualitatively by the quantum confinement effect due to the low-coordination of Au atoms in nanostructures. In this work, systematic first-principles calculations were carried out to obtain an in-depth understanding of the origin of C-H bond activations with Au-based catalysts in on-surface synthesis. Combining density functional theory (DFT) calculations and scanning tunneling microscopy (STM) studies, we reveal that the d-band centre and the d-band width of the Au-5dz2 orbital in an energy window of -6.80 to 0.00 eV may serve as theoretical descriptors for the prediction of the activity of Au catalysts in C-H bond activations. This work may therefore inspire further investigations on the design of new catalysts.

Insights into in situ one-step synthesis of carbon-supported nano-particulate gold-based catalysts for efficient electrocatalytic CO2 reduction

Carbon-supported gold/gold compound catalysts prepared by directly forming nanoparticles on carbon are easily available and efficient for CO2 electroreduction.

Advances in Base-Free Oxidation of Bio-Based Compounds on Supported Gold Catalysts

The oxidation of bio-based molecules in general, and of carbohydrates and furanics in particular, is a highly attractive process. The catalytic conversion of renewable compounds is of high importance. Acids and other chemical intermediates issued from oxidation processes have many applications related, especially, to food and detergents, as well as to pharmaceutics, cosmetics, and the chemical industry. Until now, the oxidation of sugars, furfural, or 5-hydroxymethylfurfural has been mainly conducted through biochemical processes or with strong inorganic oxidants. The use of these processes very often presents many disadvantages, especially regarding products separation and selectivity control. Generally, the oxidation is performed in batch conditions using an appropriate catalyst and a basic aqueous solution (pH 7–9), while bubbling oxygen or air through the slurry. However, there is a renewed interest in working in base-free conditions to avoid the production of salts. Actually, this gives direct access to different acids or diacids without laborious product purification steps. This review focuses on processes applying gold-based catalysts, and on the catalytic properties of these systems in the base-free oxidation of important compounds: C5–C6 sugars, furfural, and 5-hydroxymethylfurfural. A better understanding of the chemical and physical properties of the catalysts and of the operating conditions applied in the oxidation reactions is essential. For this reason, in this review we put emphasis on these most impacting factors.

Analysis of the propylene epoxidation mechanism on supported gold nanoparticles

The direct propylene epoxidation reaction has been investigated experimentally in the past by several different groups, and gold-based catalysts tend to provide high selectivity for propylene oxide, but the conversion is relatively low. Models that can connect the atomistic catalytic details to the observed experimental data are desired, in order to identify new catalyst structures and formulations. While electronic structure calculations have been used to quantify some of the key reaction steps in the direct propylene epoxidation reaction, atomistic models for translating this information into more experimentally-relevant data are needed. Here, kinetic Monte Carlo (KMC) simulations are used to bridge this gap in the modeling hierarchy. Relevant data from previous experiments and electronic structure calculations are used to parameterize a KMC model for predicting propylene oxide production from an Au/TiO2/SiO2 catalyst. The model connects the H2/O2-related reactions occurring on the Au sites with the epoxidation step on the isolated Ti surface sites. In addition, the composition in the bulk gas phase is synchronized with the dynamic reaction events occurring on the surface. The KMC model is able to adequately reproduce the experimental trends with respect to temperature and different reactant partial pressures. However, this is only achieved by considering the re-adsorption of trace amounts of the oxidant (H2O2) from the gas phase, versus merely assuming that desorbed species are immediately swept away in the gas stream.

Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies

Since the early discovery of the catalytic activity of gold at low temperature, there has been a growing interest in Au and Au-based catalysis for a new class of applications. The complexity of the catalysts currently used ranges from single crystal to 3D structured materials. To improve the efficiency of such catalysts, a better understanding of the catalytic process is required, from both the kinetic and material viewpoints. The understanding of such processes can be achieved using environmental imaging techniques allowing the observation of catalytic processes under reaction conditions, so as to study the systems in conditions as close as possible to industrial conditions. This review focuses on the description of catalytic processes occurring on Au-based catalysts with selected in situ imaging techniques, i.e., PEEM/LEEM, FIM/FEM and E-TEM, allowing a wide range of pressure and material complexity to be covered. These techniques, among others, are applied to unravel the presence of spatiotemporal behaviours, study mass transport and phase separation, determine activation energies of elementary steps, observe the morphological changes of supported nanoparticles, and finally correlate the surface composition with the catalytic reactivity.

Cationic AuIII versus AuI : Catalyst-Controlled Divergent Reactivity of Alkyne-Tethered Lactams.

Switchable reactivity through cationic gold-based catalyst control built on the oxidation state, namely cationic AuIII versus AuI , has been achieved in the direct functionalization of 2-azetidinone-tethered alkynyl indoles.

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.

Thoughts on the use of gold-based catalysts in environmental protection catalysis

Gold catalysts can exhibit a very high low-temperature activity for the oxidation of carbon monoxide and can be selective in the presence of hydrogen. This paper aims to discuss the prospects for gold-based catalysts to operate in autocatalyst exhaust treatment systems by reviewing very recent results which suggest that the long-sought-after high thermal stabilization of nanogold particles may be within reach. It discusses the CO oxidation reaction at high temperature together with reflections on hydrocarbon total oxidation and the reduction of nitric oxide by carbon monoxide and hydrocarbons. The practical means to achieve high-temperature stability are discussed together with options for enhancing the catalytic activity of gold, based on the extensive amount of work which has now been recorded for low-temperature operations. The potential for thrifting gold is also discussed together with a rough assessment of economic factors which appear to make gold a more attractive metal for this application than the established platinum group elements.

Au/Cu–Fe–La–Al2O3: A highly active, selective and stable catalysts for preferential oxidation of carbon monooxide

A series of gold-based catalysts supported by Fe–La–Al2O3 composite doped with various amounts of copper were prepared using a modified deposition-precipitation method. Investigations were carried out based on the reaction of the preferential carbon monoxide oxidation (CO-PROX) in H2-rich stream and CO oxidation at low temperatures in an O2-rich stream as a reference to examine the effects of copper on the catalytic performance. The high dispersion of gold nanoparticles in alumina-based support, the formation of Au–Cu alloy, and the close contacts of these neat or alloyed nanoparticles with CuO species, making Au partially positively charged, were examined with H2-TPR, XPS, and HRTEM-FFT techniques. Gold catalysts were obtained with very promising performance for polymer electrolyte membrane fuel cell with positive results in activity, CO2 selectivity, and thermal stability and long-term storage and on-line stability. Although H2 addition decreased the rate of CO oxidation over the studied gold catalysts, the activity and selectivity in CO2 formation were continuously improved with an increase in copper loading.

Chemical Preparation of Supported Bimetallic Catalysts. Gold-Based Bimetallic, a Case Study

This review focuses on the chemical methods used to prepare supported bimetallic heterogeneous catalysts, i.e., bimetallic nanoparticles deposited on a support. The review is limited to the preparation of gold-based bimetallic catalysts and moreover to bimetallic nanoparticles supported on powder inorganic supports, i.e., on the surface or in the porosity, and not on model supports such as single crystals.

Comparative Analysis of Reactant and Product Adsorption Energies in the Selective Oxidative Coupling of Alcohols to Esters on Au(111)

Gold-based heterogeneous catalysts have attracted significant attention due to their selective partial oxidation capabilities, providing promising alternatives for the traditional industrial homogeneous catalysts. In the current study, the energetics of adsorption/desorption of alcohols (CH3OH/methanol, CH3CH2OH/ethanol, CH3CH2CH2OH/n-propanol) and esters (HCOOCH3/methyl formate, CH3COOCH3/methyl acetate, and CH3COOCH2CH3/ethyl acetate) on a planar Au(111) surface was investigated in conjunction with oxidative coupling reactions by means of temperature programmed desorption (TPD) and dispersion-corrected density functional theory (DFT) calculations. The results reveal a complex interplay between inter-molecular and surface-molecule interactions, both mediated by weak van der Waals forces, which dictates their relative stability on the gold surface. Both experimental and theoretical adsorption/desorption energies of the investigated esters are lower than those of the alcohols from which they originate through oxidative coupling reactions. This result can be interpreted as an important indication in favor of the selectivity of Au surfaces in alcohol oxidative coupling/partial oxidation reactions, allowing facile removal of partial oxidation products immediately after their generation preventing their complete oxidation to higher oxygenates.

Kinetics of the catalytic oxidation of morin on γ-Al2O3 supported gold nanoparticles and determination of gold nanoparticles surface area and sizes by quantitative ligand adsorption

We report on the kinetic analysis of catalytic activity of γ-Al2O3 supported gold nanoparticles (AuNPs) on the rate of oxidation of morin by hydrogen peroxide as model reaction. Gold-based catalysts were revealed to be very active in the oxidation of morin under mild conditions, especially when Li2O is used as additive, and cationic imidazolium ionic liquids are used as AuNPs stabilizer. All kinetic data could be modeled in terms of the Langmuir–Hinshelwood model; that is both reactants are assumed to be adsorbed on the surface of the nanoparticles. The apparent reaction rate could therefore be related to the surface of the nanoparticles, to the kinetics constant, related to the rate determining steps, and to the adsorption constant of the reactants. The organothiol adsorption-based technique for the determination of specific surface area and particle sizes of AuNPs on γ-Al2O3 supports is also reported. A fair agreement was found between particle sizes obtained from ligand adsorption and TEM methods. Atomic absorption spectrometry (AAS), X-ray diffraction spectrometry (XRD), N2-physisorption (BET), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) are used for the characterization of the catalysts, while the oxidation reaction and the ligand adsorption was followed by ultraviolet-visible spectrometry (UV–vis).

Theoretical mechanistic insights into propylene epoxidation on Au-based catalysts: Surface O versus OOH as oxidizing agents

Propylene oxide (PO) is an important bulk chemical used for synthesis of many value-added products. Much effort has been devoted to finding a simple and “green” process for PO production. Promising results in terms of activity and selectivity have been achieved for propylene oxidation on supported gold catalysts with a mixture of O2 and H2 or with O2 and H2O. In this work a detailed transformation network of competitive reaction pathways following the initial steps of oxidation has been studied theoretically using density functional theory (DFT). The results of calculations question some of the earlier assumptions regarding the mechanism of PO formation. Surface hydroperoxo species (OOH) formed in situ are shown to be responsible for the high selectivity of propylene epoxidation with O2 on gold-based catalysts using hydrogen or water as co-reactants.

Activity and Selectivity in Nitroarene Hydrogenation over Au Nanoparticles on the Edge/Corner of Anatase

Highly selective hydrogenation of molecules containing more than one reducible group is always challenging. Here we report an efficient strategy for rational preparation of highly selective gold-based catalysts in the hydrogenation of substituted nitroarenes by positioning gold nanoparticles on the edge/corner sites of anatase. Mechanistic studies reveal that the catalyst with gold nanoparticles on the edge/corner sites of anatase could form unique sites for selective adsorption and activation of nitro groups, thus leading to high activity and selectivity. This strategy for preparation of supported gold catalysts opens a new door for the design of highly efficient heterogeneous catalysts in the future.

π Activation of Alkynes in Homogeneous and Heterogeneous Gold Catalysis.

The activation of alkynes toward nucleophilic attack upon coordination to gold-based catalysts (neutral and positively charged gold clusters and gold complexes commonly used in homogeneous catalysis) is investigated to elucidate the role of the σ donation and π back-donation components of the Au-C bond (where we consider ethyne as prototype substrate). Charge displacement (CD) analysis is used to obtain a well-defined measure of σ donation and π back-donation and to find out how the corresponding charge flows affect the electron density at the electrophilic carbon undergoing the nucleophilic attack. This information is used to rationalize the activity of a series of catalysts in the nucleophilic attack step of a model hydroamination reaction. For the first time, the components of the Dewar-Chatt-Duncanson model, donation and back-donation, are put in quantitative correlation with the kinetic parameters of a chemical reaction.

Self-assembled nanospheres with multiple endohedral binding sites pre-organize catalysts and substrates for highly efficient reactions.

Tuning reagent and catalyst concentrations is crucial in the development of efficient catalytic transformations. In enzyme-catalysed reactions the substrate is bound-often by multiple non-covalent interactions-in a well-defined pocket close to the active site of the enzyme; this pre-organization facilitates highly efficient transformations. Here we report an artificial system that co-encapsulates multiple catalysts and substrates within the confined space defined by an M12L24 nanosphere that contains 24 endohedral guanidinium-binding sites. Cooperative binding means that sulfonate guests are bound much more strongly than carboxylates. This difference has been used to fix gold-based catalysts firmly, with the remaining binding sites left to pre-organize substrates. This strategy was applied to a Au(I)-catalysed cyclization of acetylenic acid to enol lactone in which the pre-organization resulted in much higher reaction rates. We also found that the encapsulated sulfonate-containing Au(I) catalysts did not convert neutral (acid) substrates, and so could have potential in the development of substrate-selective catalysis and base-triggered on/off switching of catalysis.