Carbon-supported Gold Catalysts Research Articles

Cost-efficient electrosynthesis of hydrogen peroxide in acidic and neutral solutions

Oxygen reduction reaction is considered a green and low-carbon approach for H2O2 production. Low space-time yield and high energy consumption are serious challenges to the implementation of a commercial scale now. A cost-efficient H2O2 electrosynthesis using a recycled gold catalyst is developed here. The carbon-supported gold catalyst is synthesized using electroplating gold wastewater, which indicates good catalysis performances for both sulfite oxidation and H2O2 production. The sulfite/air fuel cell adopting the bifunctional gold catalyst can directly produce 8.70±0.53 g L−1 H2O2 in 0.5 mol L−1 Na2SO4 solution, with a good current efficiency of 71.56 %±2.13 % and a remarkable space-time yield of 27.20±1.17 mg cm−2 h−1. Adopting the desulfurization solution from flue gas as the anodic sacrificial agent, the electricity costs for acidic and neutral H2O2 electrosynthesis are only 2.09–3.82 kWh kg−1. The reported H2O2 electrosynthesis performances, especially for power consumption, are also summarized and compared.

Self-oxidation of Au-[Bmim][Cl3] catalyst with enhanced activity and stability for acetylene hydrochlorination

There is great potential for carbon-supported gold catalysts to replace toxic mercuric chloride-based catalysts in the acetylene hydrochlorination, which is a key process for producing PVC. However, the catalyst design is essentially hindered by the difficulties in enhancing the catalytic activity of cationic gold and suppressing the deactivation of active species. We report a strategy for self-oxidation of Au3+ by the Au-SILP system with the [Bmim][Cl3] ILs. This strategy can stabilize all gold species in the state of highly oxidized Au3+ instead of coexisting in the Au3+/Au+ site, which makes the catalyst exhibit excellent catalytic activity. High coordination numbers Au-Cl entities reduce the electron transfer between Au3+ and C2H2 and weaken the adsorption of C2H2 on the Au active sites. Furthermore, [Bmim][Cl3] ILs can self-oxidize gold nanoparticles on the support surface into Au3+. This study provides a new method for improving the stability and activity of gold-based acetylene hydrochlorination catalysts.

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.

Ordered Mesoporous Carbon‐supported Morphologically‐controlled Nano‐Gold: Role of Support as well as the Shape and Size of Gold Nanoparticles on the Selective Oxidation of Glycerol

Glyceric acid is one of the important intermediates of glycerol oxidation. However, it is extremely difficult to achieve high glyceric acid selectively as the reaction follows a complex pathway with the formation of a variety of undesired products. In this context, periodic mesoporous carbon-supported gold catalysts show promise for selective formation of the targeted product. Therefore, in this investigation, an effort has made to study the effect of particle size and shape of a series of carbon supported with varied sized/shaped gold catalysts on the liquid-phase oxidation of glycerol. Further, the role played by the surface of gold-nanoparticles has prompted us to investigate in detail the isotropic (nanocube/nanosphere) and anisotropic (nanoslabs/nanorods) gold supported onto both activated/amorphous carbon (Au/AC) and ordered mesoporous carbon (Au/CMK-3) towards glycerol oxidation. It was observed that the CMK-3 supported gold nanocube exhibited the highest yield towards glyceric acid with an excellent stability in terms of recyclability/reusability.

Efficient base-free oxidation of monosaccharide into sugar acid under mild conditions using hierarchical porous carbon supported gold catalysts

The highly selective synthesis of d-xylonic acid and d-gluconic acid from monosaccharide catalyzed by Au/NC-3 under base-free aerobic oxidation conditions.

Nanospherical mesoporous carbon-supported gold as an efficient heterogeneous catalyst in the elimination of mass transport limitations

This study proposes ordered mesoporous carbon nanosphere-supported gold catalysts to eliminate the diffusion limitation inside the long channels of bulk mesoporous materials. The nanospherical catalysts have a spherical morphology with a diameter of approximately 90 nm, uniform pore sizes of 2.5 nm, and high surface areas of approximately 600 m2/g; and the bulk catalysts have similar mesoporous textural properties with nanospherical catalysts but an irregular bulk shape. Metallic gold particles are both intercalated inside carbonaceous walls. The Madon-Boudart (MB) test shows that nanospherical catalysts exhibit no diffusion limitations in the reduction of nitroarenes, even for a large 2,6-dimethylnitrobenzene (2,6-DMNB) molecule. By comparison, the bulk-ordered mesoporous carbon-supported gold catalysts display distinct mass transfer inhibition. The nanospherical mesoporous carbon-supported gold catalysts are highly active and selective for the reduction of nitroarenes to corresponding amines, reaching turnover frequency (TOF) values of 13.7, 16.1, 7.6 and 26.8 min−1 for converting 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), 4-(tert-butyl)-2-nitrophenol (4-TB-2-NP), and 2,6-DMNB, respectively. The catalysts are stable and can be re-used more than 15 times.

Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural.

Oxidation of 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid is an important transformation for the production of bio-based polymers. Carbon-supported gold catalysts hold great promise for this transformation. Here we demonstrate that the activity, selectivity, and stability of the carbon-supported gold nanoparticles in the oxidation of 5-hydroxymethylfurfural strongly depend on the surface properties of the carbon support. Gold nanoparticles supported on basic carbon materials with a low density of functional groups demonstrate higher activity in 5-hydroxymethylfurfural oxidation (TOFAu up to 1195 h–1), higher selectivity to 2,5-furandicarboxylic acid, and better stability in comparison to gold nanoparticles supported on carbon materials with acidic surface groups. Surface groups of basic carbon supports that are positively charged under the reaction conditions result in a higher adsorption and local concentration of hydroxyl ions, which act as cocatalysts for gold and enhance gold-catalyzed dehydrogenation. Negatively charged surface groups of acidic carbons repel hydroxyls and the intermediate monoacid anions, which leads to lower reaction rates and a high selectivity toward 2,5-hydroxymethylfurancarboxylic acid. Understanding the role of support surface charge and local hydroxyl anion concentration provides a basis for the rational design of the optimal carbon support surface chemistry for highly active, selective, and stable catalysts for the oxidation of 5-hydroxymethylfurfural and related reactions.

Influence of Different Operating Parameters on the Partial Reduction of Oxygen for Hydrogen Peroxide Production

Hydrogen peroxide has gained increasing importance as green oxidiser in a number of chemical and other processes. This includes processes performed in smaller scale local facilities. In order to maintain a low environmental food print a local production is to be preferred. Thereby it would be particularly beneficial if the process could make use of excess renewable electricity. This is why the electrochemical production of hydrogen peroxide has recently attracted increasing interest. An energy efficient option for the electrochemical hydrogen peroxide production is by the partial reduction of oxygen. The challenge is to obtain a high hydrogen peroxide current efficiency (HPCE). Beside the possible parallel complete reduction of oxygen to water the subsequent electrochemical reduction of H2O2 to water and the heterogeneously catalysed disproportionation of H2O2 into water and oxygen are reactions which can reduce the HPCE. In this study different factors influencing the HPCE were studied. All tests were done using a rotating ring disk electrode set-up from PINE instruments. In all tests a platinum ring electrode set to a potential of 1.2 V vs. reversible hydrogen electrode (RHE) was used to detect the hydrogen peroxide leaving the disk working electrode. The reduction of potassium ferricyanide K3[Fe(CN)6] to potassium ferrocyanide K4[Fe(CN)6] at the disk electrode and the reversed oxidation of the formed K4[Fe(CN)6] at the ring electrode were used to calibrate the capture efficiency of the RRDE tip before each experiment. For a first set of experiments a polycrystalline platinum disk electrode was used as working electrode to study the effects of reaction conditions like temperature and pH on the HPCE. The effect of the rotation speed on the HPCE was used as indicator for the reduction of the HPCE by subsequent decomposition of the primary formed hydrogen peroxide via a further reduction step or disproportionation at the disk electrode. An increase of the HPCE with increasing rotation speed is expected if the subsequent decomposition of H2O2 is significant because the mass transport for the removal of formed H2O2 out of the reaction zone gets enhanced. Further tests were made by applying different supported catalyst onto a glassy carbon disk electrode to investigate the effect of the chemical composition of the electrode on the HPCE. Catalysts investigated were gold on carbon Au/C and palladium gold alloy on carbon PdAu/C. For the tests at the platinum electrode the trends known from literature could be confirmed. In an acidic environment (0.1 M HClO4) the hydrogen peroxide production could be strongly increased by reducing the cell temperature from 25 °C to 10 °C in accordance with the findings of Yamanaka (1). It also was found that the hydrogen peroxide production is strongly increased with rotating speed indicating that subsequent H2O2 decomposition proceeds fast at the platinum disk electrode in accordance with the findings of Seidel et al. (2). If the reaction is conducted in an alkaline environment (0.1 M KOH) similar trends with respect to the effect of temperature and rotating speed were observed. The major effect of the increased pH was that the onset of the H2O2 formation was observed at significant higher potentials than under acidic conditions and that high H2O2 production rates were achieved at much lower over potentials (cf. fig 1). With a carbon supported gold catalyst higher H2O2 –concentrations were achieved in the alkaline environment, though at slightly higher over potentials than for the Pt disk electrode. The H2O2 production is almost independent of the rotation speed and is higher at 25 °C than at 10 °C. This indicates that the gold catalyst does not catalyse the decomposition of H2O2. Under acidic conditions the oxygen reduction activity as well as the H2O2 formation rate are both strongly reduced and occur only at very significant over potential (cf. fig 2) By alloying the gold catalyst with palladium an enhancement of the catalytic performance in the acidic environment was achieved even at ambient temperature (cf. fig 3). As the acidic environment is more beneficial to the downstream processing of the H2O2 PdAu/C is currently the most suitable catalyst. In the contribution further tests on the effect of the kind of acid and of H2O2 stabilisation with phosphoric acid additions will be reported. The support of this work by the Fraunhofer-Gesellschaft as part of the project “Current as Raw Material” contract 007-600830 is gratefully acknowledged.

Strontium promoted activated carbon-supported gold catalysts for non-mercury catalytic acetylene hydrochlorination

Gold–strontium catalysts were prepared to assess the effect of a Sr(ii) additive on the catalytic performance of gold-based catalysts for acetylene hydrochlorination, using activated carbon as the support.

Investigation of the active species in the carbon-supported gold catalyst for acetylene hydrochlorination

The active site for a Au/C catalysts for acetylene hydrochlorination involves a Au+–Au3+ couple.

One-pot oxidation of cellobiose to gluconic acid. Unprecedented high selectivity on bifunctional gold catalysts over mesoporous carbon by integrated texture and surface chemistry optimization

Bifunctional catalysts coupling acid sites for activation of glycosidic bond via hydrolysis with metallic sites for further oxidation of glucose intermediate offer an advanced technological solution toward direct conversion of cellulose to platform chemicals. Gold (metallic functionality) was supported on pristine and functionalized mesoporous carbons including; carbon xerogels (CXs) with distinct morphologies and ordered mesoporous carbons (OMCs). Phenolic groups (acidic sites) were introduced on the surface of these carbons by air treatment, which was confirmed by XPS, TPD and IR results. The bifunctional Au catalysts were applied in the direct tandem oxidative conversion of cellobiose to gluconic acid. A remarkably high selectivity of nearly 80% to gluconic acid was obtained in a short reaction time of only 75min using Au catalyst supported on functionalized CX presenting larger average mesopores size. The adsorption of the substrate (cellobiose), intermediate (glucose) and product (gluconic acid), influenced by the polarity of the carbon support, was found to have a significant effect on the selectivity to gluconic acid. The combined effect of the adequate texture and surface chemistry of the support played a vital role in the performance of the bifunctional catalyst in this process. It was proposed that a conformational change in cellobiose exposing glycosidic bond for hydrolysis, was induced upon adsorption in the larger mesopores of CX, leading to higher selectivity to glucose. On the other hand, the phenolic groups on the carbon surface played a double role as binding sites for cellobiose and as catalytic sites for the selective hydrolysis of cellobiose to glucose. The tandem reaction and its consecutive steps (cellobiose hydrolysis and glucose oxidation) were modeled, and the rate constants were derived and compared. The oxidation of glucose by Au nanoparticles was found to take place directly on the surface of the bifunctional catalysts without desorbing to the reaction medium. The present findings contribute to the advancement in understanding of the role of carbon acidic groups in the catalytic performance of bifunctional carbon supported gold catalysts in one-pot tandem reactions of biomass valorization.

Performance improvement of activated nanoporous carbon supported gold catalyst as an anode for direct borohydride–hydrogen peroxide fuel cells

A new carbon carrier A-NPC was firstly loaded with Au nanoparticles used for direct borohydride–hydrogen peroxide fuel cells.

Influence of particle size on the electrocatalytic oxidation of glycerol over carbon-supported gold nanoparticles

The influence of gold particle size on glycerol oxidation was investigated using carbon-supported gold catalysts. Small gold particles of average diameter ≤ 4.7nm had higher activities (on a mass basis) than medium-sized (14.7nm) particles and were at least twice as active as catalysts containing large (≥ 43nm) gold particles. The small gold particles were also activated at lower potentials, resulting in lower onset potentials for glycerol oxidation. On the other hand, large gold particles had much higher onset potentials and were less stable during glycerol oxidation. While small gold particles were more active on a mass basis (primarily due to surface area effects), it was found that the specific activity (per unit real surface area) increased with increasing gold particle size. Based on evidence from Pb underpotential deposition experiments, it is proposed that this increase in specific activity can be related to an changes in the proportion of Au(111) and Au(110) surfaces as a function of particle size, with the Au(111) plane (dominant on large particles) having higher intrinsic activity compared to the Au(110) surface. This analysis also suggests that the lower onset potentials for glycerol oxidation on small nanoparticles can be attributed to the larger fraction of Au(110) facets at the surface of the small particles, and that the deactivation of larger particles is related to the high proportion of Au(111) facets on the surface of these particles. This analysis provides an important structural rationale for understanding the electrocatalytic behaviour of Au nanoparticles towards glycerol oxidation–independent of inherent size effects. As the important requirements for fuel cell catalysts are high mass activity, low overpotentials and high stability, our investigation demonstrates that all these conditions are met by the catalysts containing small gold particles defined by predominantly Au(110) facets.

Enhanced Activity of Hydrochlorination of Acetylene Using Melamine-Modified Activated Carbon Supported Gold Catalyst

Enhanced Activity of Hydrochlorination of Acetylene Using Melamine-Modified Activated Carbon Supported Gold Catalyst

Enhancement of the selectivity to dihydroxyacetone in glycerol oxidation using gold nanoparticles supported on carbon nanotubes

Abstract The use of multi-walled carbon nanotubes as support for Au nanoparticles results in the preferential chemoselective orientation of glycerol oxidation towards the reaction of the secondary alcohol function; a dihydroxyacetone selectivity of 60% was obtained in combination with a high activity, which is an interesting result reported here for the first time. On the other hand, an activated carbon supported gold catalyst, prepared under the same conditions, with similar metal loading and average particle size, promotes the formation of glyceric acid. This suggests the importance of support textural properties on the gold catalytic performance in glycerol selective oxidation.

Carbon-Supported Gold Catalyst Modified by Doping with Ag for Cyclohexene Oxidation

Carbon-supported gold catalysts Au/C were prepared by an impregnation-reduction method and modified by AgNO3to obtain bi-metallic catalysts Au-Ag/C, which were characterized by X-ray-diffraction (XRD) and Transmission Electron Microscope (TEM). Their catalytic performance was tested in the oxidation of cyclohexene in an autoclave without any solvent. The results showed that Ag doping can significantly enhance the catalytic performance of carbon-supported gold catalyst. Au(1.0 wt.%)-Ag(1.0 wt.%)/C has been found to be an efficient catalyst for the cyclohexene oxidation with a conversion of 27.6% at 80 °C and 0.4 MPa for 12 h while selectivity for ∑C6products (including cyclohexene oxide, 2-cyclohexene-1-ol, 2-cyclohexene-1-one and cyclohexane-1,2-diol) exceeding 88.9%, especially the selectivity of cyclohexane-1,2-diol up to 47.6%. Moreover, the effects of Au, Ag content on catalytic performance were also reported.

Electroreduction of oxygen on carbon-supported gold catalysts

The electrochemical reduction of oxygen was studied on Au/C catalysts (20 and 30 wt%) in 0.5 M H 2SO 4 and 0.1 M KOH solutions using the rotating disk electrode (RDE) method. The thickness of the Au/C–Nafion layers was varied between 1.5 and 10 μm. The specific activity of Au was independent of catalyst loading in both solutions, indicating that the transport of reactants through the catalyst layer does not limit the process of oxygen reduction under these conditions. The mass activity of 20 wt% Au/C catalysts was higher due to smaller particle size. The number of electrons involved in the reaction and the Tafel slopes were found; the values of these parameters are similar to that of bulk polycrystalline gold and indicate that the mechanism of O 2 reduction is not affected by carbon support or the catalyst configuration.

Linoleic acid isomerization over mesoporous carbon supported gold catalysts

Gold catalyst supported on mesoporous carbon and silica were synthesized, characterized by TEM, XRD, XPS and tested in linoleic acid isomerization. Nature of the support affects the selectivity towards isomerization in relation to unwanted hydrogenation. In particular carbon support allowed much higher selectivity in double bond migration compared to silica. Effect of carbon surface oxidative pre-treatment on selectivity of catalyst was investigated.

Selective deactivation of gold catalyst

The progressive poisoning effect of different molecules on carbon supported gold catalysts has been evaluated during the aerobic oxidation of glucose. A geometrical model has been derived for describing the morphological properties of two catalysts made of carbon supported gold particles having a known size distribution centered at 3.30 and 7.89 nm respectively. The observed deactivation trend follows the order thiocyanate > cyanide ≈ cysteine > thiourea and it obeys an exponential law. The kinetics of catalyst deactivation has been interpreted by considering the important contribute of electronic factors which overlap the space shielding of active sites, due to long range poison–catalyst interaction influencing the entire metal particle. Considering the nature of the molecules showing a high poisoning effect and the promoting effect of OH −, a molecular model for electronic interactions in gold nanoparticles during the aerobic oxidation of glucose has been proposed where the dioxygen reduction step is differently influenced by soft and hard-nucleophiles.

Reactivation of a Carbon-supported Gold Catalyst for the Hydrochlorination of Acetylene

Acetylene hydrochlorination using a carbon-supported gold catalyst is studied. Reactivation of the catalyst is demonstrated using a brief treatment of the spent catalyst with boiling aqua regia and the process of reactivation and deactivation is characterised using X-ray photoelectron spectroscopy. Deactivation is considered to be due to loss of Au3+ which is restored by the aqua regia treatment.