Oxidation State Of Gold Research Articles

Electroless deposition of gold into poly-3,4-ethylenedioxythiophene films and their characterization performed in chloride-containing solutions

Au-containing polymer films were obtained by electroless deposition of gold from diluted solutions of HAuCl4 into preliminarily reduced poly-3,4-ethylenedioxythiophene (PEDOT) films. Structural peculiarities of such pristine and composite films were characterized by scanning and transmission electron microscopy methods. It was established that the gold clusters forming under such deposition appear on the outer surface of polymer films and their pores. The clusters’ sizes ranged between 30 and 100 nm depending on the time of exposition of a PEDOT film in solutions of Au(III) ions and the concentration of these ions. It was also observed that in contrast to pristine PEDOT films, cyclic voltammograms (CVs) of composite films in the presence of chloride ions show additional redox peaks resulting from oxidation of gold with formation of an insoluble product and followed by the product reduction under reversal of the potential scan direction. As a result of parallel electrochemical quartz crystal microbalance (EQCM) and CV measurements, it was also established that the number of chloride ions per one transferring electron in the gold oxidation process is near to unity. To elucidate the oxidation degree of gold in the presence of chloride ions, a special procedure of changing the electrode potential was used. It consisted of clamping the high anodic potential in the region of gold oxidation (0.97 V, Ag/AgCl) and subsequent gradual decrease of the electrode potential with a constant scan rate. Under these conditions, it was possible to completely oxidize all the gold particles containing in a composite film and find out the maximum amount of electricity consumed for the product particles’ reduction. A comparison between such data and the results obtained in EQCM determinations of the gold content in the same film led to the conclusion that the oxidation state of gold in the complexes formed is Au(III). The effects of chloride ion concentration and scan rate of the electrode potential on current responses of PEDOT–Au films were investigated. Some primary conclusions on the kinetics of the studied processes are made.

Interaction between gold (III) chloride and potassium hexacyanoferrate (II/III)—Does it lead to gold analogue of Prussian blue?

Prussian blue analogues are a class of compounds formed by the reaction between metal salt and potassium hexacyanoferrate (II/III). In our earlier report, the formation of Au@Prussian blue nano-composite was noticed on potential cycling the glassy carbon electrode in a medium containing gold (III) chloride and potassium hexacyanoferrate (III). Hence in this work, the formation of gold hexacyanoferrate was attempted by a simple chemical reaction. The reaction of gold (III) chloride with potassium hexacyanoferrate (II/III) was examined by UV–Vis spectroscopy and found that there is no redox reaction between gold (III) chloride and potassium hexacyanoferrate (III). However, the redox reaction occurs between gold (III) chloride and potassium hexacyanoferrate (II) leading to the formation of charge transfer band and the conversion of hexacyanoferrate (II) to hexacyanoferrate (III) was evidenced by the emergence of new absorption peaks in UV–Vis spectra. The oxidation state of gold in Au–Fe complex was found to be +1 from X-ray photoelectron spectroscopy. The stability of the Au–Fe complex was also studied by cyclic voltammetry. Cyclic voltammetric results indicated the presence of high spin iron in Au–Fe complex. Hence ‘as formed’ Au complex may be KFe x [Au(CN) 2] y . The results revealed that the formation of gold hexacyanoferrate was not feasible by simple chemical or electrochemical reaction in contrast to other Prussian blue analogues.

Effect of gold oxidation state on the epoxidation and hydrogenation of propylene on Au/TS-1

Gold nanoparticles on titanium oxide and titania-silica supports are active for the formation of propylene oxide by the oxidation of propylene with hydrogen and oxygen mixtures. Cyanide treatment of gold supported on titanosilicate zeolite supports (Au/TS-1) yielded unexpected results. Catalysts treated with weak solutions of sodium cyanide resulted in preferential removal of small gold particles, while catalysts treated with strong solutions resulted in dissolution of the gold and re-precipitation as gold (+1) cyanide. X-ray absorption spectroscopy demonstrated that catalysts that produce propylene oxide in the presence of hydrogen and oxygen mixtures had supported gold (+3) oxide nanoparticles of 3 nm size after synthesis, which were reduced to gold metal at reaction conditions. Samples treated with strong solutions of sodium cyanide resulted in supported gold (+1) cyanide particles of large size, 9–11 nm. These particles did not produce propylene oxide but, surprisingly, showed high selectivity toward propylene hydrogenation. Increasing gold (+1) cyanide particle size resulted in a decrease in hydrogenation activity.

Catalytic activities and properties of Au(III)/Schiff-base complexes in methanol oxidative carbonylation

Several Au(III)/Schiff-base complexes were studied and proven to be effective catalysts for oxidative carbonylation of methanol to generate dimethyl carbonate (DMC). Effects of Schiff-base ligands, promoters, and promoter mole ratio to Au(III)/Schiff-base complexes on catalytic activity were studied. When the reaction was carried out at a methanol/Au(III) molar ratio of 5060:1, an [AuCl 2(phen)]Cl/KI (phen = 1,10-phenanthroline) ratio of 1:4, a CO/O 2 pressure of 2:1, and a temperature of 120 °C for 3 h, the conversion, selectivity, and TOF value of the model reaction were 10.8%, 98%, and 138.9 h −1, respectively. The catalyst was characterized by FTIR, UV–vis, 1H NMR, and cyclic voltammetry. The oxidation state of gold during the reaction and the role of KI were discussed using data on electrochemical experiments and ESI-MS. [AuI 2(phen)] + was considered an intermediate in the reaction. A plausible Au(III)/Au(I) catalytic cycle mechanism was proposed.

Photodeposition of gold on titanium dioxide for photocatalytic phenol oxidation

The influence of experimental conditions during the photodeposition in the preparation of supported Au on TiO 2 has been studied. Besides preparation pH, light intensity and deposition time showed to have a high influence on the final properties of gold deposits. Photodeposition using illumination with a high light intensity UV–vis lamp (140 W/m 2 UVA range) resulted to be an ineffective method for obtaining nanoparticles of gold on the titania, producing very large and poorly distributed gold deposits. Thus obtained materials did not show any important improvement of their photocatalytic activity tested for phenol oxidation. By contrast, photodeposition using a low light intensity of illumination (0.15 W/m 2 UVA range), produced materials with notably improved photocatalytic activity. The illumination with such a low light intensity allowed the control of the amount, aggregation and oxidation state of gold by changing deposition time, enabling a feasible method of tailoring Au–TiO 2 with the appropriate properties for a high photocatalytic activity. Best photocatalytic behaviour for phenol photodegradation was obtained for Au–TiO 2 samples prepared by photodeposition at low light intensity with 120 min photodeposition time for catalysts with a 0.5% and 1% nominal content of gold and with 60 min photodeposition time for catalyst with a 2% nominal content of gold.

Vital roles of hydroxyl groups and gold oxidation states in Au/ZrO2 catalysts for 1,3-butadiene hydrogenation

Hydrogenation of 1,3-butadiene over Au/ZrO2 catalysts with different gold loadings and calcination temperatures was reported. The catalysts were characterized in depth to understand their structure–property relationship. Gold oxidation states and surface hydroxyl groups, which were found to be sensitive to the gold loading, calcination temperature, and treatment with water, were shown to play vital roles in the hydrogenation activity of Au/ZrO2. Continued activity decrease was seen when the density of surface hydroxyl groups was lowered by elevating the pre-calcination temperature of ZrO2. Fully dehydroxylated Au/ZrO2 was essentially inactive, but became very active after partial regeneration of the hydroxyl groups by water treatment. Moreover, the activity of Au/ZrO2 increased with increasing Au3+/Au0 ratio. Isolated Au3+ ions at the support surface showed up to two orders of magnitude higher activity than Au0 atoms on Au particles. Several models are proposed to address the structural features of active sites for H2 activation in Au/ZrO2 catalysts.

X-ray absorption spectroscopy studies of the adducts formed between cytotoxic gold compounds and two major serum proteins

Gold metallodrugs form a class of promising antiproliferative agents showing a high propensity to react with proteins. We exploit here X-ray absorption spectroscopy (XAS) methods [both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)] to gain insight into the nature of the adducts formed between three representative gold(I, III) metallodrugs (i.e., auranofin, [Au(2,2'-bipyridine)(OH)(2)](PF(6)), Aubipy, and dinuclear [Au(2)(6,6'-dimethyl-2,2'-bipyridine)(2)(μ-O)(2)](PF(6))(2), Auoxo6) and two major plasma proteins, namely, bovine serum albumin (BSA) and human serum apotransferrin (apoTf). The following metallodrug-protein systems were investigated in depth: auranofin/apoTf, Aubipy/BSA, and Auoxo6/apoTf. XANES spectra revealed that auranofin, upon protein binding, conserves its gold(I) oxidation state. Protein binding most probably takes place through release of the thiosugar ligand and its subsequent replacement by a thiol (or a thioether) from the protein. This hypothesis is independently supported by EXAFS results. In contrast, the reactions of Aubipy with serum albumin and of Auoxo6 with serum apoTf invariantly result in gold(III) to gold(I) reduction. Gold(III) reduction, clearly documented by XANES, is accompanied, in both cases, by release of the bipyridyl ligands; for Auoxo6 cleavage of the gold-gold dioxo bridge is also observed. Gold(III) reduction leads to formation of protein-bound gold(I) species, with deeply modified metal coordination environments, as evidenced by EXAFS. In these adducts, the gold(I) centers are probably anchored to the protein through nitrogen donors. In general, these two XAS methods, i.e., XANES and EXAFS, used here jointly, allowed us to gain independent structural information on metallodrug/protein systems; detailed insight into the gold oxidation state and the local environment of protein-bound metal atoms was achieved in the various cases.

Effect of the oxidation state of gold on the complete oxidation of isobutane on Au/CeO2 catalysts

Catalysts of highly dispersed gold supported on ceria were prepared by deposition precipitation method. Au is dispersed as Au0, Au+ and Au3+ species on ceria. The content of Au+ and Au3+ was highest on catalyst prepared on uncalcined ceria, which possess least ordered surface. It is inferred that oxygen vacancy on disordered ceria surface is essential for the preparation of highly dispersed gold catalysts and in stabilizing monolayer surface Au+ clusters while cationic vacancies are sites for substitutional Au3+. Au/CeO2 catalysts showed low-temperature isobutane oxidation activity with maximum conversion at 150–180°C. Ex-situ XPS results demonstrated that the low temperature isobutane oxidation activity was closely related to the content of Au+ which we interpreted as surface gold oxide formed under reaction conditions. Isobutane oxidation activity associated with ceria at temperature above 300°C was enhanced by substitutional Au3+.

CO-free hydrogen production over Au/CeO2–Fe2O3 catalysts: Part 1. Impact of the support composition on the performance for the preferential CO oxidation reaction

Ce–Fe mixed oxides were prepared by urea gelation coprecipitation method and used as supports of gold catalysts. The impact of the support composition on the catalytic performance for the preferential CO oxidation (PROX) was studied by varying the Ce/(Ce+Fe) ratio. A deep characterization study by different tools such as XRD, HRTEM, TPR and FTIR spectroscopy was undertaken in order to correlate the structural characteristics of the catalysts and the gold oxidation state and dispersion with the catalytic properties. The results revealed that the variation of the support composition led to significant differences in the gold particles size (in the range 1–25nm), which affected strongly the CO oxidation activity of Au/CeO2–Fe2O3 catalysts under PROX conditions. The following activity order was observed: Au/CeO2≈Au/Ce50Fe50>Au/Ce75Fe25>Au/Ce25Fe75>Au/Fe2O3. The support with composition 50wt.% CeO2–50wt.% Fe2O3 appeared beneficial not only for nucleation and growth of highly dispersed gold particles (1–1.8nm), but also for activation of oxygen and its mobility. Moreover, the presence of Fe2O3 in the supports composition improved the resistance towards deactivation by CO2. The CeO2–Fe2O3 supports comprised different amount of two phases: cubic CeO2-like solid solution and hematite. The analysis of the characterization data suggested that the solid solution formation probably proceeded via a dopant interstitial compensation mechanism.

Synthesis of aurocyanide and auricyanide complexes of phosphines, phosphine sulfides and phosphine selenides, and their characterization by IR, far-IR, UV solution, and solid-state NMR spectroscopic methods

A series of aurocyanide and auricyanide complexes of phosphines, phosphine sulfides, and phosphine selenides were synthesized. These new complexes have the general formula [L n Au(CN) m ], where L could be Cy3P, (2-CN-Et)3P, Me3PS, Et3PS, Ph3PS, Me3PSe, or Ph3PSe. Auricyanide was reacted with L at 1 : 2 ratio. Products were characterized using elemental analysis, melting point, UV, IR, far-IR solution, and solid-state NMR spectroscopy. Phosphine ligands cause gold(III) reduction to gold(I); less redox tendency was found for phosphine sulfides and phosphine selenides. Tri-coordinate complexes [L2AuCN] were produced from phosphine ligands with gold-tetracyanide. IR and UV spectroscopic methods were used to identify gold oxidation state in the synthesized complexes.

Influence of Monomer Feeding on a Fast Gold Nanoparticles Synthesis: Time-Resolved XANES and SAXS Experiments

A comprehensive study of the mechanism of gold nanoparticle formation has been conducted using third-generation synchrotrons. The particles were obained by reduction of AuCl(3) by BH(4)(-) in toluene. Gold oxidation state was monitored by X-ray absorption near edge spectroscopy (XANES), while the size and concentration of the nanoparticles were assessed by small-angle X-ray scattering (SAXS). A time-resolution of 100 ms has been achieved for a total formation time of a few seconds. The change with time of the total amount of Au(0) present in the solution is obtained by XANES. The change of the amount of Au(0) inside the nanoparticles is obtained from the SAXS signal. The comparison between these two quantities shows that a measurable amount of Au(0) exists transiently as monomers (or very small entities) in solution and this quantity is linked to an observed burst of nucleation of nanoparticles. The reduction kinetics is strongly influenced by the presence of ligands and a change in temperature. A model coupling the observed reduction kinetics and nucleation and growth laws is able to recover the final size and number densities of the explored experimental conditions.

The Formation of Gold Clusters Supported on Mesoporous Silica Material Surfaces: A Molecular Picture

The interaction of gold particles with a silica surface is investigated using a combination of theoretical and experimental techniques. It is found that the physisorption strength of gold on silica decreases with increasing chloride concentration at the surface, with increasing cluster size, and with decreasing oxidation state of gold. However, the gold precursor is adsorbed more strongly than the corresponding precursor fragments and this via an H-bond network. It is also found that chemisorbed, grafted species may coexist on a silica surface, depending on the experimental conditions. From the vibrational frequencies and especially by the presence of a band at 3650 cm−1, it is found that the gold species after calcination are probably present under the form of digrafted chemisorbed species. This information enables us to propose a scenario for the formation of gold clusters at the silica surface.

Mechanism Study of the Gold-Catalyzed Cycloisomerization of α-Aminoallenes: Oxidation State of Active Species and Influence of Counterion

A computational study with the B3LYP density functional theory was carried out to study the reaction mechanism for the cycloisomerization of allenes catalyzed by Au(I) and Au(III) complexes. The catalytic performance of Au complexes in different oxidation states as well as the effects of the counterion on the catalytic activities has been studied in detail. Our calculations show that the catalytic reaction is initiated by coordination of the Au(I) or Au(III) catalyst to the distal double bond of allene and activation of allene toward facile nucleophilic attack, then 3-pyrroline obtained via two-step proton shift, followed by demetalation. On the basis of our calculations, H shifts are key steps of the catalytic cycle, which are significantly affected by the gold oxidation state, counterion, ligands, and assistant catalyst. AuCl is found to be more reactive than AuCl(3); however, the Au(III)-catalyzed path does not involve an oxidation state change from Au(III) to Au(I). Our calculated results rationalize the experimental findings well and overthrow the previous conjecture about Au(I) serving as the catalytically active species for Au(III)-catalyzed cycloisomerization.

Total oxidation of propene over Au/xCeO2-Al2O3 catalysts: Influence of the CeO2 loading and the activation treatment

This study explores the reaction of oxidation of propene (1200ppm) under excess oxygen (9%) over Au/xCeO2/Al2O3 catalysts (x=1.5, 3, 5, and 10wt% CeO2) to mimic the conditions of VOC decomposition. The present study reveals the influence of several factors that affect the oxidative performance of the Au/xCeO2/Al2O3 catalysts, such as the ceria loading, the catalyst activation method, i.e., the gold oxidation state, the gold particle size and the distribution of gold on ceria and alumina. The Au/xCeO2/Al2O3 catalysts were found 100% selective to CO2. Their activity was intermediate between that of Au/Al2O3 (poorly active) and Au/CeO2 (active). Interestingly, the activity increased with the ceria loading when the samples were activated under H2 at 300°C, but decreased when they were activated under O2 at 500°C. Characterization of the Au/xCeO2/Al2O3 catalysts after both types of activation was performed by TEM, EFTEM, XPS and CO oxidation reaction. The results show that whatever the mode of activation, as the CeO2 loading increased, the proportion of gold particles on alumina decreased while that on ceria increased, but gold remained unreduced on ceria after calcination under O2 whereas it was metallic after reduction under H2. Gold on ceria being more active when it is metallic than when it is unreduced, this explained the evolution of the catalytic results of propene oxidation as the ceria loading varied.

Characterization of gold nanorods in vivo by integrated analytical techniques: their uptake, retention, and chemical forms

Integrated analytical techniques were used to study the tissue distribution and structural information of gold nanorods (Au NRs) in Sprague-Dawley rats through tail intravenous injection. Before in vivo experiments were conducted, careful characterization of Au NRs was performed. The zeta potential proved that adsorption of bovine serum albumin on Au NRs turned the surface charges from positive to negative as in an in vitro simulation. The biodistribution of Au NRs was investigated quantitatively by inductively coupled plasma mass spectrometry at different time points after injection. As target tissues, both liver and spleen were chosen to further demonstrate the intracellular localization of Au NRs by the combination of transmission electron microscopy and energy-dispersive X-ray spectroscopy. Moreover, synchrotron-radiation-based X-ray absorption spectroscopy was employed and it was observed that long-term retention of Au NRs in liver and spleen did not induce obvious changes in the oxidation states of gold. Therefore, the present systematic method can provide important information about the fates of Au NRs in vivo and can also be extended to study the biological effects of other metallic nanomaterials in the future.

Supported gold catalysts for the decomposition of VOC: Total oxidation of propene in low concentration as model reaction

Supported gold catalysts prepared by deposition–precipitation with urea were studied in the reaction of oxidation of propene in low concentration in a large excess of oxygen, so as to mimic the conditions of catalytic decomposition of a volatile organic compound of hydrocarbon-type (1200ppm C3H6, 9% O2 in He). Several parameters were investigated: the nature of the oxide support (alumina, titania, ceria), the gold loading, the conditions of catalyst activation (oxygen or hydrogen). Titania and alumina alone did not show any conversion in C3H6 oxidation up to 500°C, but when gold was added (1wt%), active catalysts were obtained with a higher activity for titania than for alumina. Ceria was the only support showing activity, and gold on ceria (1wt%) led to the most active catalyst. For the Au/CeO2 system, activation under H2 at 300°C leads to more active catalysts than activation in O2/He at 500°C, especially for gold loadings lower than 1wt%. XPS and CO oxidation performed at RT showed that gold on CeO2 was fully reduced to Au0 after activation under H2 whatever the gold loading. In contrast, after calcination, most of the gold remained under the initial AuIII state for the low loaded samples (≤1wt%) whereas part of it was reduced for the 4wt% Au/CeO2. Thus, ceria seems to be able to stabilise gold as AuIII up to a limited loading. Change in the gold oxidation state was detected for the calcined Au/CeO2 (1wt%) during C3H6 oxidation performed at increasing temperature, using CO oxidation and DRIFTS combined to CO adsorption. Indeed, gold, initially AuIII, starts reducing at 100°C to form metallic gold Au0, which was the active species for the reaction. Above 300°C, when 100% conversion was achieved, reoxidation of metallic gold species was observed.

Remarkable support crystal phase effect in Au/FeO x catalyzed oxidation of 1,4-butanediol to γ-butyrolactone

Iron oxides in different crystal forms with identical particle size and morphology are employed as supports for gold catalysts. By using the oxidative dehydrogenation of 1,4-butanediol to γ-butyrolactone as the probe reaction, combined with some characterizations such as XRD, TEM, XPS, and H 2–TPR, support effect of Au/FeO x catalysts is established in this study. Turnover frequency value follows the order Au/Fe 3O 4 > Au/ α-Fe 2O 3 > Au/ γ-Fe 2O 3. Size effect of Au/Fe 3O 4 catalysts with different gold particle size distributions obtained by treating under different atmospheres is also studied for comparison, and the results imply the slight effect of gold particle size. Variation in catalytic behavior of gold catalysts on different iron-oxide supports originates from the discrepancy in gold geometry, gold oxidation state, and gold-support contact interface structures which are caused by difference in support intrinsic properties such as microcrystalline structure and oxidation state.

Gold/hydroxyapatite catalysts: Synthesis, characterization and catalytic activity to CO oxidation

This work reports the synthesis, characterization and catalytic activity for CO oxidation of gold catalysts supported on calcium hydroxyapatite. On both, the hydroxyapatite support and the gold-supported hydroxyapatite catalyst, the CO conversion shows a peak near 100% of conversion at room temperature. The generation of structural vacancies by interaction of CO with the solid provokes the formation of peroxide species in the presence of gaseous oxygen, which seems to be responsible of this high conversion of CO at room temperature. Moreover, the influence of the pre-treatment temperature on the activity has been observed and related with the elimination of carbonate species and the generation of structural defects in the apatite structure, which are able to modify the gold oxidation state.

Formation of negative oxidation states of platinum and gold in redox ionic liquid: Electrochemical evidence

Formation of negative oxidation states of platinum and gold in redox ionic liquid: Electrochemical evidence

Formation of negative oxidation states of platinum and gold in redox ionic liquid: Electrochemical evidence

The electrochemical reduction of noble metal electrodes in the presence of redox ionic liquid, 1-ferrocenylethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [FcEMIM][TFSI], was investigated by cyclic voltammetry. Our experiments suggest the formation of metal with negative oxidation states, in the cases of platinum and gold electrodes [M n −, FcEMIM +]. By analogy with the previous work, the formation of these phases is concomitant with the insertion of the supporting electrolyte; which correspond in our experimental condition to the redox cation of the ionic liquid. As an exciting result, the electrochemical investigations of the reduced electrode in electrolytic solution, containing solvent and supporting electrolyte, evidence the presence of the ferrocene groups at the electrode surface. Moreover, the reduced electrode exhibits the presence of the ferrocene even after, contact with air, after ultrasound, and after physical polishing, highlighting the large stability of this organo-metallic phases formed in this media. The AFM investigations demonstrate the morphological change of the platinum surface after the reduction process. Finally, our works bring a formal electrochemical proof of the presence of the ionic liquid cation inside the electrode material after the cathodic treatment in this media.