Supported Gold Catalysts Research Articles

Cerium(III) phosphate as a promising support for gold catalysts addressed to selective oxidation of glucose under base-free conditions

Supported gold nanoparticles are widely used active oxidation catalysts, however, their efficiency and stability vastly depend on the type of supports utilized. This study aims to unravel the potential application of cerium(III) phosphate as a new support for gold catalysts for selective oxidation of glucose under base-free conditions. Particular interest was directed to: i) the identification of the key factors that promote high activity of supported gold species, ii) the optimization of reaction conditions to assess the impact of catalyst loading and reaction time/temperature on the efficiency of glucose oxidation, and iii) the elucidation of the stability of Au/CePO4 catalyst in several reaction cycles. The results show that Au/CePO4 exhibits unique surface properties that enable obtaining of much higher activity of gold species in glucose oxidation compared to other well-known metal oxide-based systems. Precisely, it was found that the TOF value calculated for Au/CePO4 was ca. 2.5 times higher than in the case of Au/CeO2 (TOF = 5846 vs. 2309 h-1, respectively), and ca. 1.5 times higher than that observed for Au/TiO2 (Mintek, TOF = 3839 h-1). The presented results facilitate the development of more active and highly selective supported gold catalysts for oxidation of biomass-derived platform molecules (e.g., glucose).

Generalized Principles for the Descriptor-Based Design of Supported Gold Catalysts

Generalized Principles for the Descriptor-Based Design of Supported Gold Catalysts

Optimization of Metal-Support Cooperation for Boosting the Performance of Supported Gold Catalysts for the Borylation of C-O and C-N Bonds.

The cooperation of multiple catalytic components is a powerful tool for intermolecular bond formation, specifically, cross-coupling reactions. Supported metal catalysts have interfacial sites between metal nanoparticles and their supports where multiple catalytic elements can work in cooperation to efficiently promote intermolecular reactions. Hence, the establishment of novel guidelines for designing active interfacial sites of supported metal catalysts is indispensable for heterogeneous catalysts which enable efficient cross-coupling reactions. In this article, we performed kinetic and theoretical studies to elucidate the effect of metal-support cooperation for the borylation of C-O bonds by supported gold catalysts and revealed that the Lewis acid density of the supports determined the number of active sites at which metal nanoparticles (NPs) and Lewis acid at the surface of the supports work in cooperation. Furthermore, DFT calculations revealed that strong adsorption of diborons at the interface between Au NPs and supports and a decrease in the LUMO level of adsorbed diboron were responsible for efficient C-O bond borylation. Supported Au catalysts with the optimized metal-metal oxide cooperation sites, namely, Au/α-Fe2O3 catalyst, showed excellent activity for C-O bond borylation, and also enabled the synthesis of organoboron compounds by using continuous-flow reactions. Furthermore, Au/α-Fe2O3 showed high activity for direct C-N bond borylation without the transformation of amino groups to ammonium cations. The results described herein suggest that the optimization of metal-metal oxide cooperation is beneficial for taking full advantage of the potential performance of supported metal catalysts for intermolecular reactions.

Supported Gold Catalysts for Base-Free Furfural Oxidation: The State of the Art and Machine-Learning-Enabled Optimization.

Supported gold nanoparticles have proven to be highly effective catalysts for the base-free oxidation of furfural, a compound derived from biomass. Their small size enables a high surface-area-to-volume ratio, providing abundant active sites for the reaction to take place. These gold nanoparticles serve as catalysts by providing surfaces for furfural molecules to adsorb onto and facilitating electron transfer between the substrate and the oxidizing agent. The role of the support in this reaction has been widely studied, and gold-support interactions have been found to be beneficial. However, the exact mechanism of furfural oxidation under base-free conditions remains an active area of research and is not yet fully understood. In this review, we delve into the essential factors that influence the selectivity of furfural oxidation. We present an optimization process that highlights the significant role of machine learning in identifying the best catalyst for this reaction. The principal objective of this study is to provide a comprehensive review of research conducted over the past five years concerning the catalytic oxidation of furfural under base-free conditions. By conducting tree decision making on experimental data from recent articles, a total of 93 gold-based catalysts are compared. The relative variable importance chart analysis reveals that the support preparation method and the pH of the solution are the most crucial factors determining the yield of furoic acid in this oxidation process.

Functionalized α-Al2O3 supported gold catalyst for photocatalytic oxidative esterification of benzyl alcohol with methanol to corresponding esters

Alpha-alumina (α-Al2O3) is traditionally considered to be unsuitable as a support for high performance gold-based catalysts because of its small specific surface area. Herein, a highly active α-Al2O3-supported nearly sub-nanometer Au particles photocatalyst for oxidative esterification of benzyl alcohol under visible-light irradiation in a mild condition was synthesized via the electronic interaction between gold precursor and a layer of 3-aminopropyltriethoxysilane (APTES) pre-grafted on the α-Al2O3 called as functionalized α-Al2O3. APTES could serve as a linker to strongly anchor Au nanoparticles (NPs) to α-Al2O3 surface, leading to nearly sub-nanometer Au particles (1.8 ± 0.1 nm). The enhanced metal-support interaction reinforces the interaction of plasmonic hot holes of Au NPs and intermediate molecules, which promote the rate-limiting step of C-H bond cleavage and facilitate the activation of oxygen molecules due to the increased electron cloud density around Au NPs. Meanwhile, α-Al2O3 support can play a synergistic role because of the reactant adsorption can be improved by the acid-base pairs contained.

Defective Au nanoparticles manipulated by Au-MgxAl-LDH interplay for alkali-free oxidation of benzyl alcohol

Supported gold catalyst, especially in terms of nanosized particles with defective surface and low-coordinated structure, plays a vital role in the heterogeneous catalysis. However, such defective Au nanoparticles are hardly acquired when supporting on non-reducible metal oxides/hydroxides due to insufficient Au-support interaction. In this work, we found that the defective Au NPs could be fabricated via the strong interplay between Au and crystal lattices of layered double hydroxides-MgxAl-LDH. Experimental and theoretical evidences revealed that the formation of a preferential-oriented LDH(015) plane enabled the surface configuration for defective Au centers with the maximized number of step sites at Mg/Al ratio of 3.0–3.5. The optimized Au/Mg3.5Al-LDH catalyst shows a boosting catalytic activity for benzyl alcohol oxidation (TOF, 1008 h−1) at only 80 °C with the lowest activation energy in the absence of alkali. The work provides a new insight for manipulating the defective-enriched Au particles via the fine-tuning Au-LDH interplay for alkali-free selective oxidations.

Harnessing Supported Gold Nanoparticle as a Single‐Electron Transfer Catalyst for Decarboxylative Cross‐Coupling

AbstractWe found that gold nanoparticles function as single‐electron transfer catalysts to promote decarboxylative cross‐coupling between N‐hydroxy phthalimide esters and disilanes or diborons. A variety of disilanes coupled with alkyl radicals could be generated from N‐hydroxy phthalimide esters via single‐electron reduction by active carbon‐supported gold nanoparticle catalysts to afford diverse alkylsilanes without the need for external bases. Furthermore, the present gold catalytic system was also effective for decarboxylative carbon‐boron coupling to afford alkyl boronates. A detailed mechanistic investigation revealed that single‐electron transfer mediated by supported gold nanoparticles enabled the generation of alkyl radical and silyl radical, thereby enabling radical cross‐coupling. The reusability and environmentally‐friendly nature of supported gold catalyst as well as the scalability of the reaction system enable the practical transformation of carboxylic acid derivatives into value‐added organosilicon and organoboron compounds.

Selectivity Regulation of Au/Titanate by Biochar Modification for Selective Oxidation of Benzyl Alcohol

In organic synthesis, it is important to control the selectivity target product with high purity and reduce the cost of energy and equipment for separation. This study investigated supported gold catalysts on biochar-modified titanate-based nanofibers in order to regulate the catalytic performances by biochar content and surface properties. The catalysts were characterized by SEM, TEM, XRD, XPS, ICP-OES, UV-Vis to confirm their morphology, particle size distribution of Au NPs, crystal structures, oxidation state of Au and other key elements, real Au loading, and optical properties. In the test of selective oxidation of benzyl alcohol to benzaldehyde, the biochar modification could improve the selectivity toward benzaldehyde. Moreover, the influence of catalyst calcination conditions, reaction time, reaction atmospheres, reaction temperatures and solvent were systematically investigated. These results are useful for peer researchers in rational catalyst design.

One-pot synthesis of stable cationic gold species highly active in the CO oxidation confined into mordenite-like zeolite

The presence of cationic gold species is a key factor for the CO oxidation over the supported gold catalysts based on non-reducible metal oxides. The deactivation of such catalysts is mainly associated with the Au nanoparticles (AuNPs) sintering and the concomitant reduction of cationic Au species. Here we report the high catalytic stability at the CO oxidation of the cationic Au species confined into a mordenite-like zeolite matrix prepared by a one-pot synthesis method. The sample with lowest Au loading (0.32 % wt.) exhibited the higher catalytic performance and stability during CO oxidation. Detailed FTIR analysis showed that the CO oxidation on sample with low Au content (0.32 wt%) occurred preferentially via CO interaction with the cationic interface Au-hydroxyl (Au1+–OH−) species to produce CO2(g) via decarboxylation. In contrast, the sample with a high Au content (1.56 wt%) displayed lower CO oxidation ability, which was associated with a low content and less stability of cationic gold species. Proposed synthesis method permits to obtain in a one-pot way stable cationic gold species highly effectively for the CO oxidation.

Systematic Investigation on Supported Gold Catalysts Prepared by Fluorine-Free Basic Etching Ti3AlC2 in Selective Oxidation of Aromatic Alcohols to Aldehydes.

Conventional methods to prepare supported metal catalysts are chemical reduction and wet impregnation. This study developed and systematically investigated a novel reduction method based on simultaneous Ti3AlC2 fluorine-free etching and metal deposition to prepare gold catalysts. The new series of Aupre/Ti3AlxC2Ty catalysts were characterized by XRD, XPS, TEM, and SEM and were tested in the selective oxidation of representative aromatic alcohols to aldehydes. The catalytic results demonstrate the effectiveness of the preparation method and better catalytic performances of Aupre/Ti3AlxC2Ty, compared with those of catalysts prepared by traditional methods. Moreover, this work presents a comprehensive study on the influence of calcination in air, H2, and Ar, and we found that the catalyst of Aupre/Ti3AlxC2Ty-Air600 obtained by calcination in air at 600 °C performed the best, owing to the synergistic effect between tiny surface TiO2 species and Au NPs. The tests of reusability and hot filtration confirmed the catalyst stability.

Influencia del pretratamiento en los nanocompuestos de CeO2 y Au/CeO2 para mejorar la creación de defectos superficiales que permitan modificar la interbanda óptica

Cerium oxide as a material within heterogeneous catalysis remains a relevant research topic due to its redox behavior, directly related to the presence of oxygen vacancies (Vo-) which are responsible for ceria’s high oxygen storage capacity. CeO2 also have been employed as a support for active noble metal particles; in particular, supported gold catalyst has been proposed as a candidate to be used in different reactions such as CO oxidation, catalytic combustion of hydrocarbons, WGS reaction, among others. Despite its relevance, few works have a detailed understanding of the pretreatment effect on these catalysts. For this reason, in this work, we study the in situ behavior of CeO2 and Au/CeO2 during different pretreatments in temperature and atmosphere by Raman and Diffuse reflectance UV-Vis. These techniques allow us to follow in real-time the surface changes of Au nanoparticles and CeO2. We demonstrate a direct correlation lattice structural defects of CeO2 with modifications formed by electronic states in the optical interband and, the deposition of Au nanoparticles on the surface of CeO2 allows to improve the properties formed by the electronic states between the valence band and the conduction band by increasing more than twice the structural defects compared to CeO2 alone.

Lowering the Operating Temperature of Gold Acetylene Hydrochlorination Catalysts Using Oxidized Carbon Supports

The commercialization of gold for acetylene hydrochlorination represents a major scientific landmark. The development of second-generation gold catalysts continues with a focus on derivatives and drop-in replacements with higher activity and stability. Here, we show the influence that the support surface oxygen has on the activity of carbon supported gold catalysts. Variation in the surface oxygen content of carbon is achieved through careful modification of the Hummers chemical oxidation method prior to the deposition of gold. All oxidized carbon-based catalysts resulted in a marked increase in activity at 200 °C when compared to the standard nontreated carbon, with an optimum oxygen content of ca. 18 at % being observed. Increasing oxygen and relative concentration of C–O functionality yields catalysts with light-off temperatures 30–50 °C below the standard catalyst. This understanding opens a promising avenue to produce high activity acetylene hydrochlorination catalysts that can operate at lower temperatures.

Role of interfacial oxygen vacancies in low-loaded Au-based catalysts for the low-temperature reverse water gas shift reaction

Gold-based catalysts have shown high catalytic activity for reverse water gas shift (RWGS) reactions at low temperatures. Despite extensive studies, the RWGS reaction on Au-based catalysts with very low Au content (< 0.1 wt%) has not yet been investigated. In this study, TiO2 and ZrO2 supported gold catalysts with such low gold loading have been synthesized and tested for RWGS. The catalysts were investigated by a series of in/ex-situ characterization techniques, including ICP-OES, XRD, BET, XPS, STEM, STEM-EELS, TAP, in-situ DRIFTS and in-situ EPR. At 250 oC, the Au/TiO2 catalyst showed almost 10 times higher activity than Au/ZrO2. In-situ DRIFTS results suggest that the formate mechanism is the predominant mechanism over Au/ZrO2, while over Au/TiO2 the reaction proceeds via the formation of hydroxycarbonyl intermediates. A combined study including STEM, STEM-EELS, XPS, and in-situ EPR suggests that the interfacial Au–Ov–Ti3+ sites are responsible for the superior activity of Au/TiO2.

Operando X‐ray Powder Diffraction Study of Mechanochemical Activation Tested for the CO Oxidation over Au@Fe2O3 as Model Reaction

AbstractMechanochemistry has proven to be an excellent green synthesis method for preparing organic, pharmaceutical, and inorganic materials. Mechanocatalysis, inducing a catalytic reaction by mechanical forces, is an emerging field because neither external temperature nor pressure inputs are required. Previous studies reported enhanced catalytic activity during the mechanical treatment of supported gold catalysts for CO oxidation. So far, the processes inside the milling vessel during mechanocatalysis could not be monitored. In this work, the results of high‐energy operando X‐ray powder diffraction experiments and online gas analysis will be reported. A specific milling setup with a custom‐made vessel and gas dosing system was developed. To prove the feasibility of the experimental setup for operando diffraction studies during mechanocatalysis, the CO oxidation with Au@Fe2O3 as a catalyst was selected as a well‐known model reaction. The operando studies enabled monitoring morphology changes of the support as well as changes in the crystallite size of the gold catalyst. The change of the crystal size is directly correlated to changes in the active surface area and thus to the CO2 yield. The studies confirm the successful implementation of the operando setup, and its potential to be applied to other catalytic reactions.

N-doped carbon layer-coated Au nanocatalyst for H2-free conversion of 5-hydroxymethylfurfural to 5-methylfurfural

Deoxygenative upgrading of 5-hydromethylfurfural (HMF) into valuable chemicals has attracted intensive research interest in recent years, with product selectivity control remaining an important topic. Herein, TiO2 supported gold catalysts coated with a thin N-doped porous carbon (NPC) layer were developed via a polydopamine-coating-carbonization strategy and utilized for pathway-specific conversion of HMF into 5-methylfurfural (5-MF) with the use of renewable formic acid (FA) as the deoxygenation reagent. The as-fabricated Au/TiO2@NPC exhibited excellent catalytic performance with a high yield of 5-MF (>95%). The catalytic behavior of [email protected] catalysts was shown to be correlated with the suitable combination of highly dispersed Au nanoparticles and favorable interfacial interactions in the [email protected] core-shell hetero-nanoarchitectures, thereby facilitating the preferential esterification of HMF with FA and suppressing unproductive FA dehydrogenation, which promoted the selective formylation/decarboxylation of hydroxy-methyl group in HMF in a pathway-specific manner. The present NPC/metal interfacial engineering strategy may provide a potential guide for the rational design of advanced catalysts for a wide variety of heterogeneous catalysis processes in terms of the conversion of biomass source.

High efficient and stable thiol-modified dendritic mesoporous silica nanospheres supported gold catalysts for gas-phase selective oxidation of benzyl alcohol with ultra-long lifetime

Surface protective ligands play a pivotal role to dominate the physical and chemical performances of metal nanoparticles (NPs) at nanoscale interface. Herein, thiol-modified dendritic mesoporous silica nanospheres (DMSNs) supported Au NPs catalysts with tunable loading weight of thiol groups were prepared and used in gas-phase selective oxidation of benzyl alcohol with air as oxidant. The chemical reactivity and stability of the obtained Au NPs@DMSNs catalysts was strongly dependent on the loading weight of thiol groups grafted on the framework of DMSNs. Strikingly, the Au/DMSNs-4.4%SH catalyst with low loading weight of Au (∼2%) and thiol ligand (∼4.4%) achieved high conversion of benzyl alcohol (Conv. 91%) and selectivity for benzaldehyde (Sel. 98%) and unprecedented long lifetime up to 820 h at 250 °C, which surpassed all the reported Au based nanocatalysts. Through the study of the deactivation mechanism of Au NPs catalysts, the excellent catalytic performance and ultra-long lifetime of Au NPs catalyst was originated from the adsorption and packing of thiol groups on the surface of Au NPs, which limited the growth of Au NPs and concomitantly constructed a unique nanoconfined microenvironment at nanoscale interface for the synergistic adsorption and activation of reaction substrate and oxygen. Our results show the coverage-dependent ligand-induced surface effects on regulating the electronic structures, surface activity, optical properties, and chemisorption strength of metal NPs.

Enhancement effect of strong metal-support interaction (SMSI) on the catalytic activity of substituted-hydroxyapatite supported Au clusters

Cation- and anion-substituted hydroxyapatites (sHAPs) supported gold catalysts showed strong metal-support interaction (SMSI) under oxidative atmosphere. Mg- and Ce-substituted sHAPs stabilized Au clusters and imparted positively charged Au. The Au on these sHAPs became more positively charged upon formation of the oxidative SMSI. The catalytic activities for transvinylation of benzoic acid with vinyl acetate and isomerization of 3,4-diacetoxybut-1-ene (34DABE) correlated well with the cationic properties of Au; Au/Mg0.1SrHAP and Au/Ce0.1SrHAP with SMSI exhibited higher catalytic activity than other Au/sHAPs and the same catalysts without SMSI. The enhanced cationic properties of Au particles induced by oxidative SMSI surpassed the disadvantages of the reduction of the exposed surface Au atoms caused by SMSI. This work reveals that controlling the degree of SMSI is an efficient methodology to tune an optimal electronic state of metal NPs for target reactions and to provide highly active and durable catalysts.

Effect of poly(N-vinylpyrrolidone) ligand on catalytic activities of Au nanoparticles supported on Nb2O5 for CO oxidation and furfural oxidation

The development of green preparation methods for acidic metal oxide supported gold catalysts is of great importance for the extensive use of gold catalysts as well as for a comprehensive understanding of gold catalysts. In this study, poly(N-vinylpyrrolidone) (PVP) was used as the stabilizer of gold colloids to deposit gold nanoparticles on an acidic metal oxide, Nb2O5. The effects of the PVP ligand on the preparation of Au(-PVP)/Nb2O5 and on the catalytic activities for gas-phase CO oxidation and liquid-phase furfural oxidation were investigated. As removing more ligand PVP of Au(-PVP)/Nb2O5, the catalytic activities for both CO oxidation and furfural oxidation increased. The optimal calcination temperature for Au(-PVP)/Nb2O5 was determined to be 400 °C, and calcination at that temperature gave gold nanoparticles with an average size of 6.4 ± 2.5 nm and of the support of Nb2O5 remained as the original crystalline structure. The temperature for 50% CO conversion (T50%) of Au(-PVP)/Nb2O5 (calcined at 400 °C) was 81 °C. In addition, Au(-PVP)/Nb2O5 (calcined at 400 °C) presented 43% furfural conversion with 100% selectivity of furoic acid under a mild conditions (0.5 MPa O2, 40 °C) in the batch system. These results suggested that Au(-PVP)/Nb2O5 is a good catalyst for both gas-phase and liquid-phase reactions.

Influence of the Support on Propene Oxidation over Gold Catalysts

The epoxidation of propene without forming a substantial amount of byproducts is one of the holy grails of catalysis. Supported Cu, Ag and Au catalysts are studied for this reaction and the activity of the supported metals is generally well understood. On the contrary, limited information is available on the influence of the support on the epoxide selectivity. The reaction of propene with equal amounts of hydrogen and oxygen was tested over gold nanoparticles deposited onto CeO2, TiO2, WO3, γ-Al2O3, SiO2, TiO2-SiO2 and titanosilicate-1. Several metal oxide supports caused further conversion of the synthesized propene oxide. Strongly acidic supports, such as WO3 and titanosilicate-1, catalyzed the isomerization of propene oxide towards propanal and acetone. Key factors for achieving high PO selectivity are having inert or neutralized surface sites, a low specific surface and/or a low density of surface -OH groups. This work provides insights and practical guidelines to which metal oxide support properties lead to which products in the reaction of propene in the presence of oxygen and hydrogen over supported gold catalysts.

Synergic effect between gold and vanadate substituted hydroxyapatite support for synthesis of methyl methacrylate by one-step oxidative esterification

• Au/HVP catalyst displays excellent performance for oxidative esterification. • The high selectivity derives from the good capacity of activation of oxygen. • O 2 was adsorbed and activated with the synergic effect of gold with the support. • The active sites are located at the interfacial sites between gold and the support. Synthesis of methyl methacrylate (MMA) via direct oxidative esterification from methacrolein (MAL) and methanol (MeOH) is of great significance in chemical industry. Supported gold catalysts are considered as one of the most potential candidates for this reaction but suffer from low activity and deactivation issue. Herein, by modulating the phase structure of hydroxyapatite (HAP) with different cations and anions, a synergic effect between gold and vanadate substituted hydroxyapatite (HVP) support for the synthesis of MMA was discovered. In direct oxidative esterification, among the evaluated catalyst/support systems, Au/HVP catalyst displays the best performance and unprecedented stability, which acts as one of the best-performing gold catalysts for oxidative esterification. Various characterizations such as X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), transmission electron microscope (TEM) and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO were conducted to disclose the structural and electronic property of the catalysts. Through in-situ FT-IR spectra of MAL and MeOH, the substrate was proved to be easily transformed on Au/HVP catalyst to ester. Besides, the presence of O 2 was found to facilitate the adsorption of MAL. The results of temperature programmed desorption (TPD) of O 2 implied that gold particles together with HVP support is promotional for activation and dissociation of oxygen, leading to good activity and high selectivity.