Au Catalysts Research Articles

Progress in catalyst developments for CO<sub>2</sub> hydrogenation coupled with <italic>N</italic>-alkylation of arylamines and nitroarenes

N -alkyl aromatic amines are widely used as important intermediates in the fields of petrochemical industry, rubber industry, dye preparation, drug research and development and fine chemical synthesis. The traditional synthesis of N -alkylarylamines has many problems, such as high price of raw materials, corrosion of reaction equipment and easy to cause environmental pollution. In recent years, great progress has been made in the research of catalysts for coupling CO2 hydrogenation with N -alkylation of arylamines and nitroarenes to prepare N -alkyl aromatic amines. The use of abundant greenhouse gas CO2 as an alternative C1 resource for N -alkylation reaction gives water as the only by-product, which makes the reaction greener and environmentally friendly. In 2013, Beydoun and co-workers first reported that a homogeneous ruthenium complex ([Ru(triphos)(tmm)]) in trifluoroimide (HNTf2) can effectively catalyze CO2 hydrogenation coupled with N -alkylation of secondary amines and primary amines, giving good product yields. Subsequently, heterogeneous catalytic CO2 hydrogenation coupled with N -alkylation using supported Pd, Rh, Pt, Au, and Cu catalysts and those with Zn and Ga as promoters have been extensively studied. Several catalytic systems showed high selectivity in preparation of N -alkylation products under relatively mild conditions. In addition, hydrogen silane and borohydride were used as strong reducing agents to replace H2, achieving high efficiency in N -alkylation of arylamines or nitroarenes, although there exist disadvantages of high toxicity and non-recyclability in these systems. It was found that the factors affecting the selectivity of N -alkylation products mainly include active metals, carrier materials, and reaction conditions (temperature, pressure, CO2/H2 ratio, and CO2/H2 feed sequence). It mainly manifested that: (1) Active metals with moderate reducing properties can avoid side reactions of aromatic ring hydrogenation; (2) introduction of basic sites in the carrier material can achieve effective adsorption of CO2 and aniline; (3) when the reaction raw material is aniline or nitrobenzene, compared with MA, the reaction requires an increase in temperature, H2/CO2 ratio and reaction time; and (4) a catalyst with high selectivity to the target product should perform well in CO2 hydrogenation and N -alkylation but control the deep reduction of products. Thus, to improve the activity and selectivity of known catalytic system and design a new catalyst, it is of importance to elucidate the reaction mechanism. For instance, with the controlled experiments, in-situ infrared spectroscopic studies and other characterization techniques, the results indicated that the essential intermediates may include HCO*, HCOO–, and HCOOH when CO2 hydrogenation coupled with N -alkylation of aniline. Considering the kinetics, activating CO2 molecule or increasing the nucleophilicity of nucleophiles C- or N- will contribute to the formation of C–N or C–C bonds. Further, the obtained experimental results showed that the recycle stability of catalysts for CO2 hydrogenation coupled with N -alkylation is mainly affected by carbon deposition and sintering of active sites. However, the deactivated catalysts can be readily regenerated by calcination or reduction. Using bimetal catalysts can markedly improve the catalyst performance and stability. At present, improving the catalyst performance and selectivity to N -alkylation products with mild reaction conditions become the issues to focus on. In addition, selective conversion of CO2 into other functionalized N -alkyl compounds remains to be further deepened and improved. Nevertheless, the selective conversion of CO2 to C2 compounds for coupling with N -alkylation of arylamines or nitroarenes is a challenging topic. We also notice that use of MOF (metal-organic framework) materials as catalysts for CO2 hydrogenation to synthesize methanol and ethanol has achieved significant progress. MOF materials and photo-thermal synergistic catalysis may be applied to coupl CO2 hydrogenation with N -alkylation of aromatics. This paper reviews the research progress of N -alkylation of arylamines and nitroarenes coupled with CO2 hydrogenation, with emphasis on homogeneous and heterogeneous catalysts for the alkylation of aniline, alkylaniline and nitrobenzene coupled with CO2 hydrogenation. The structure-performance relationship, influencing factors and catalytic mechanism of the catalysts are discussed. The development prospect of catalysts in this field is also prospected.

High Sensitivity and Selectivity of Array Gas Sensor through Glancing Angle Deposition Method

In this study, we propose an array-type gas sensor with high selectivity and response using multiple oxide semiconductors. The sensor array was composed of SnO2 and In2O3, and the detection characteristics were improved by using Pt, Au, and Pd catalysts. All samples were deposited directly on the Pt interdigitated electrode (IDE) through the e-beam evaporator glancing angle deposition (GAD) method. They grew in the form of well-aligned nanorods at off-axis angles. The prepared SnO2 and In2O3 nanorod samples were exposed to CH3COCH3, C7H8, and NO2 gases in a 300℃ dry condition. Au-decorated SnO2, Au-decorated In2O3, and Pd-decorated In2O3 exhibited high selectivity for CH3COCH3, C7H8, and NO2, respectively. They demonstrated a high detection limit of the sub ppb level computationally. In addition, measurements from each sensor were executed in the 40% relative humidity condition. Although there was a slight reduction in detection response, high selectivity and distinguishable detection characteristics were confirmed

Dehydrogenation of ethane assisted by CO2 over Y-doped ceria supported Au catalysts

Dehydrogenation of ethane assisted by CO2 has been investigated over Au catalyst supported on Y-doped CeO2 nanorod. The catalysts were characterized by XRD, TEM, N2 adsorption, XPS, TG, H2-TPR and Raman spectroscopy. Enhanced activity can be obtained after the proper Y doping. The optimal catalyst Au/Ce0.9Y0.1 exhibits an ethane conversion of 20.9% and nearly 100% selectivity to ethylene without any evident tendency of deactivation in 50 h. The reducibility of surface oxygen species is found to play a key role in the dehydrogenation assisted by CO2, which can be improved by proper Y doping into CeO2 nanorod. The promoting effect of CO2 on activity is evident over Au/Ce0.9Y0.1 catalyst, due to the quick transformation of H2 product into H2O by the reverse water gas shift reaction. The catalyst stability is also improved by the addition of CO2, attributed to the elimination of coke by Bouduoard reaction.

Methanol oxidation over rutile Au1@TiO2 catalyst: Importance of facets and oxygen vacancy

Single-atom catalysts have attracted exceptional interest of researchers. However, the arguing whether tuning the interactions between the dispersed single metals and the support can enhance the activity, selectivity and stability of single atom catalysts is still open. Herein, single atom Au catalyst (rutile Au1@TiO2) is presented via density functional theory calculations, which exhibits high efficiency for methanol oxidation to formate, especially the exposed (110) facet. Rutile Au1@TiO2 (110) facet prefers to localize at 6-fold Ti site under rich-oxygen conditions; methanol goes through adsorption, OH dissociation, CH dissociation, O2 molecule adsorption, and withdraws H2O and formate to gas phase separately. In particular, a significant step on the way to methanol oxidation is the oxygen vacancy formation at the reduced rutile Au1@TiO2 (110) surface after CH scission and H2O desorption. The subsequent adsorbed O2 molecule does only provide oxygen species for methanol oxidation, but not fill surface oxygen vacancy, which promotes formate formation and desorption to develop very well.

Injection Metal-Assisted Catalytic Etching (MACE) of Si Powder: Discovery of Low-Load MACE and Pore Distribution Tunability Using Ag, Au, Pd, Pt and Cu Catalysts

Injection of H2O2 to control both the rate and extent of etching allowed us to discover a new regime of MACE with extremely small quantities of deposited metal as a catalyst: low-load MACE (LL-MACE).1 The structure of particles subjected to LL-MACE is completely different compared to conventional MACE. Si nanowires and mesoporous micro- or nano-particles are produced with high yield, low cost and controlled properties suitable for applications in e.g. lithium-ion batteries, drug delivery, as well as biomedical imaging and contrast enhancement. Different metal catalysts behave in distinct manners than can be traced to differences in nucleation and reaction kinetics.2 Not only Ag, Au, Pd and Pt but also inexpensive Cu can be used to perform LL-MACE.Previously, controlled reactant injection and acetic acid were used to transform stain etching into the improved process known as regenerative electroless etching (ReEtching).3 Here injection and acetic acid are exploited to enhance MACE and extend it to the low-load regime. Ag, Au, Pd, Pt and Cu can be galvanically deposited onto Si powder out of solutions of HF(aq). Nucleation of deposited metal nanoparticles is made more uniform by dispersing the Si powder in acetic acid and injecting the dissolved metal ions at a controlled rate. The use of a syringe pump to deliver not only metal ions during deposition but also the oxidant (H2O2) during MACE is essential for increased product uniformity and yield. Temperatures of ~0–50 °C and grades of Si (i.e. electronics grade Si with various doping levels or metallurgical grade Si) produced significantly different pore size distributions. Doping levels and the associated changes in band bending and space charge layer width4 are linked to changes in the pore size distribution.As shown in Fig. 1(A), high loading of Ag resulted in deposition of dendrites and Ag particles. These broke into ~50–100 nm Ag nanoparticles that etched in a correlated fashion to generate parallel etch track pores. The uniform size distribution of the Ag nanoparticles was generated dynamically during etching. This conventional high-load MACE (HL-MACE) did not produce Si nanowires (Si NWs) directly. However, etch track pore walls were cleaved readily by ultrasonic agitation to form Si NWs. Etching far removed from the metal nanoparticles (remote etching) also occurred for highly doped Si.As shown in Fig. 1(B), reducing the loading of Ag by a factor of 100 to 1000 compared to HL-MACE created 10–30 nm nanoparticles that etched in an uncorrelated fashion and generated randomly directed etch track pores. Significant etching far removed from the Ag catalyst also occurred. The combination of localized and remote etching resulted in a bimodal distribution of mesoporosity peaking at ~4 nm from remote etching and 10–30 nm from localized etching. The bimodal distribution was confirmed by the BJH method of nitrogen adsorption porosimetry. The pore size distribution could be controlled by changing the process temperature, the grade of Si and the metal composition. Whereas Ag, Au, Pd and Pt performed well in both the HL and LL limits, Cu generated porosification only in the LL limit.Funded by grants #314552 and #314412 from Academy of Finland and the National Science Foundation award #1825331. The microscopy studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA). 1 K. Tamarov, J. D. Swanson, B. A. Unger, K. W. Kolasinski, A. T. Ernst, M. Aindow, V.-P. Lehto, and J. Riikonen, ACS Appl. Mater. Interfaces 12, 4787 (2020). 2 K. W. Kolasinski, W. B. Barclay, Y. Sun, and M. Aindow, Electrochim. Acta 158, 219 (2015). 3 K. W. Kolasinski, N. J. Gimbar, H. Yu, M. Aindow, E. Mäkilä, and J. Salonen, Angew. Chem., Int. Ed. Engl. 56, 624 (2017). 4 E. Torralba, S. Le Gall, R. Lachaume, V. Magnin, J. Harari, M. Halbwax, J.-P. Vilcot, C. Cachet-Vivier, and S. Bastide, ACS Appl. Mater. Interfaces 8, 31375 (2016). Figure 1

(Invited) Electric-Field Driven Selective Electrocatalytic CO2 Conversion

The efficient electrochemical conversion of CO2 to fuels or stock chemicals with high-energy density would be a major step forward in the introduction of a carbon neutral energy cycle. Especially, understanding the role of electrocatalysts, supports, and electrolytes that can efficiently reduce CO2 to fuels with high selectivity is a subject of significant interest.One of the achievements in electrochemical CO2 reduction is the clarification of the catalysts toward two-electron involved products such as CO and formate. For instance, CO is selectively generated on Au, Ag, and single-atom catalysts and formate has been selective on Hg, Pb, Sn or alloy catalysts. Interestingly, from our recent work, CO/formate ratio can be selectively controlled on formate selective catalyst by tuning the interfacial electric field.[1] On copper catalyst, the selection of alkali cations has direct influence on activity and product selectivity; increasing the size of mono-valent cations can increase the activity and selectivity toward C-C coupled products by modulating the interaction energy between adsorbates and electric fields at the interface.[2] In addition, copper catalyst with a specific atomic-scale gap accelerates the reaction kinetics and selectivity to C2+ products.[3] Therefore, understanding the roles of catalyst, support and electrolyte offers the design of efficient, yet cheap electrochemical CO2 reduction systems.[1] Y. Kwon et al., ACS Sustain. Chem. Eng. 2020, 8, 2117.[2] Y. Kwon et al., J. Am. Chem. Soc. 2016, 138, 13006.[3] Y. Kwon et al., Adv. Energy Mater. 2020, 1903423.

Effect of Polarity of Surfactant on Formation of through-Silicon Via Using Metal-Assisted Chemical Etching

In this study, we demonstrated formation of high aspect vertical holes in Si substrate using wet chemical approach, MacEtch process, for the application of through silicon via (TSV) holes. It is important that the addition of the surfactants in the etching solution to obtain the well- defined straight holes in the Si substrate using MacEtch, regardless the polarity of the surfactants. MacEtch is expected to be used for formation of TSV instead of Deep reactive ion etching (Deep-RIE) because of lower costs than Deep-RIE [1,2,3]. However, it is difficult to obtain high aspect vertical holes using MacEtch process due to its instability of etching direction for prolonged process as shown in Fig. 1(a). Addition of surfactant is usefulness to improve the morphology of etched holes. Micro scale holes were prepared in N-type Si (100) substrate using MacEtch process. The disc shape Au deposited by sputtering were used as catalyst. Before deposition of the Au catalyst, a Ti interlayer with a thickness of 10 nm was deposited directly on the Si substrate with patterned resist. The patterned Au discs (10 μm in diameter), which were deposited on a Ti / Si substrate using a photolithography lift-off process. The thicknesses of deposited Au were 10 nm for Ti/Si substrate. The base pressure of the deposition chamber was maintained at 10-5 Pa, and purity of Au and Ti target used for the deposition were 99.99 %, respectively. For the MacEtch process, a mixture of 1.0 M hydrofluoric acid (HF), 1.3 M hydrogen peroxide (H2O2) and deionized water was used as the base etching solution. Benzalkonium chloride (BKC), and polyethylene glycol (PEG), which were cationic and non-polar surfactant, respectively, were used as the additive for the etching solutions. The MacEtch of the Si substrate with patterned Au were performed for 120 min at 40 oC in the etching solution. Fig.1 shows cross sectional SEM images of typical shape of etched Si holes using MacEtch. The direction of etched Si hole was changed during MacEtch without surfactants, as shown Fig.1 (a), and these changes of direction occurred more than 60% Si holes. By contrast, we observed drastic improvement on direction of etched holes in almost all holes in addition of PEG, as indicated Fig.1 (b). Similarly, adding BKC in MacEtch process was also observed improvement. The catalyst at the bottom of substrate without surfactant was deformed and partially detached from the Si substrate, as shown Fig.1 (d). However, with PEG and BKC, the catalyst contacted to the bottom and was suppressed deformation, as indicated Fig.1 (e), (f). In addition, BKC suppressed at 25 times low concentration compared with PEG. This phenomenon was is considered due to influence of polarity of hydrogen part or structure of surfactants. Particularly, the shape of catalyst film with BKC kept flat during MacEtch process compared with PEG. Therefore, defamation of catalyst has a strong correlation with formation of bended holes. Moreover, the suppression effect of catalyst is stronger in cation, BKC, than in non-ion, PEG.

Reductive amination of 5-acetoxymethylfurfural over Pt/Al2O3 catalyst in a flow reactor

Aminomethylhydroxymethylfuran derivatives are well known for their pharmaceutical activities. In this work, the two-step one-pot procedure for the synthesis of N-substituted 5-(acetoxymethyl)-2-furfuryl amines was proposed, which includes two successive reactions: the condensation of 5-acetoxymethylfurfural (AMF) with primary amines and the reduction of obtained imines with hydrogen over supported metal catalysts in a flow reactor. The comparison of the properties of Au, Pd and Pt catalysts in the hydrogenation of imine obtained by reaction of AMF with aniline showed that Pt-based catalysts (both Pt/Al2O3 and Pt/C) are highly active and provide high selectivity and stability, while Pd and Au nanoparticles supported on γ-alumina and carbon show low selectivity. A wide range of N-substituted 5-(acetoxymethyl)-2-furfuryl amines was synthesized in good to excellent yields using Pt/Al2O3 catalyst and methanol as a solvent.

Hydrogenation of Carbon Dioxide to Methanol over Non-Cu-based Heterogeneous Catalysts.

The increasing atmospheric CO2 level makes CO2 reduction an urgent challenge facing the world. Catalytic transformation of CO2 into chemicals and fuels utilizing renewable energy is one of the promising approaches toward alleviating CO2 emissions. In particular, the selective hydrogenation of CO2 to methanol utilizing renewable hydrogen potentially enables large scale transformation of CO2 . The Cu-based catalysts have been extensively investigated in CO2 hydrogenation. However, it is not only limited by long-term instability but also displays unsatisfactory catalytic performance. The supported metal-based catalysts (Pd, Pt, Au, and Ag) can achieve high methanol selectivity at low temperatures. The mixed oxide catalysts represented by Ma ZrOx (Ma =Zn, Ga, and Cd) solid solution catalysts present high methanol selectivity and catalytic activity as well as excellent stability. This Review focuses on the recent advances in developing Non-Cu-based heterogeneous catalysts and current understandings of catalyst design and catalytic performance. First, the thermodynamics of CO2 hydrogenation to methanol is discussed. Then, the progress in supported metal-based catalysts, bimetallic alloys or intermetallic compounds catalysts, and mixed oxide catalysts is discussed. Finally, a summary and a perspective are presented.

Theoretical Insights into Ester-Directed Reactions between Propiolates with 1,2-Benzisoxazoles by Au(I) Catalyst: [4 + 2]-Annulation versus Michael-Type Products

Au(I)-catalyzed selective reactions of Et- and tBu-substituted propiolates (1a and 1a′) with 1,2-benzisoxazole(2a) provide a new strategy for purposefully access to desired bioactive heterocycles. ...

Potassium Titanate Nanobelts: A Unique Support for Au and AuRh Nanoparticles in the Catalytic Reduction of NO with CO

AbstractThe catalytic reduction of NO with CO was examined over monometallic Au and bimetallic AuRh nanoparticles supported on potassium titanate (K2Ti8O17) nanobelts (KTN) and TiO2. The highly crystalline KTN, prepared from TiO2 by a hydrothermal process using KOH, were several μm long and ca. 8 nm wide. Both Au and bimetallic AuRh catalysts showed a striking enhancement of the catalytic performance with KTN as support, compared to corresponding TiO2‐supported catalysts. The reasons for this behavior could be traced back using in situ diffuse reflectance infrared Fourier transform spectroscopy in tandem with modulation excitation spectroscopy, which proved that the KTN support promotes the formation of surface nitrate (−NO3−), carbonate (−CO32−), and isocyanide (−NCO) species. In particular, the combination of KTN with bimetallic AuRh nanoparticles results in the facile formation of isocyanide, which is a pivotal intermediate for producing dinitrogen molecules via the reaction with NO; NCO(a)+NO(a)→N2+CO2.

General Strategy for Designing Highly Selective Gas-Sensing Nanoreactors: Morphological Control of SnO2 Hollow Spheres and Configurational Tuning of Au Catalysts.

Catalyst-loaded hollow spheres are effective at detecting ethanol with high chemical reactivity. However, this has limited the widespread use of catalyst-loaded hollow spheres in designing highly selective gas sensors to less-reactive gases such as aromatics (e.g., xylene). Herein, we report the preparation of xylene-selective Au-SnO2 nanoreactors by loading Au nanoclusters on the inner surface of SnO2 hollow shells using the layer-by-layer assembly technique. The results revealed that the sensor based on SnO2 hollow spheres loaded with Au nanoclusters on the inner surface exhibited unprecedentedly high xylene selectivity and an ultrahigh xylene response, high enough to be used for indoor air quality monitoring, whereas the sensor based on SnO2 hollow spheres loaded with Au nanoclusters on the outer surface exhibited the typical ethanol-sensitive sensing behaviors as frequently reported in the literature. In addition, the xylene selectivity and response were optimized when the hollow shell was sufficiently thin (∼25 nm) and semipermeable (pore size = ∼3.5 nm), while the selectivity and response decreased when the shell was thick or highly gas permeable with large mesopores (∼30 nm). Accordingly, the underlying mechanism responsible for the unprecedentedly high xylene sensing performance is discussed in relation to the configuration of the loaded Au nanoclusters and the morphological characteristics including shell thickness and pore size distribution. This novel nanoreactor concept can be widely used to design highly selective gas sensors especially to less-reactive gases such as aromatics, aldehydes, and ketones.

Hydrochlorination of Acetylene Using the Novel SiC Foam with Hollow Structure Supported Au/C Catalysts

Acetylene hydrochlorination using Au/C as catalyst, is an important process to produce vinyl chloride. It is a highly exothermic and diffusion-controlled reaction with local hot spots and poor diffusion process in the fixed-bed reactor, weakening the catalytic performance of Au catalyst. The structuring of catalyst using solid SiC foam ceramic can solve the above problems through optimizing spatial distribution of Au/C, reducing the pressure drop and temperature rise. In order to further disperse Au/C, the novel SiC foam with hollow structure was manufactured to compare with solid SiC foam. The hollow foam structured catalyst has inside and outside surface, thinner catalyst coating and smaller Au particles. Thus it provides more active site accessible to the reactants and further shortens the diffusion distance, then improving the catalytic efficiency. The catalytic lifetime (acetylene conversion above 90%) of hollow SiC foam structured catalyst is 205 hours while that of solid foam is 126 hours at acetylene gaseous hourly space velocity (GHSV) of 120 h−1. In conclusion, the better dispersion of Au/C on hollow SiC foam supported structured catalyst is the main reason for activity and stability enhancement.

Efficient Conversion of Glycerol into High Value‐Added Chemicals by Partial Oxidation

AbstractGlycerol can be effectively converted to glyceric acid, a high value‐added pharmaceutical raw material, through its partial oxidation over an Au/Al2O3 catalyst under strongly basic conditions. The factors important for the highly selective production of glyceric acid were investigated experimentally. It was clarified that NaOH was involved in the glycerol activation step to a glycerol alkoxide intermediate (2, 3‐dihydroxypropoxide) in the liquid phase, then glyceric acid was formed by OOH species derived from O2 on an Au catalyst in the partial oxidation step. We have newly discovered the concerted effect of NaOH and O2 in different reaction steps.

Selective oxidation of glycerol to dihydroxyacetone over N-doped porous carbon stabilized CuxO supported Au catalysts

In this work, N-doped porous carbon stabilized CuxO (Cu-NPCs) were synthesized by a one-pot method using copper nitrate as copper precursor with the aid of Pluronic F127 under acidic condition. The characterization results confirmed that CuxO species (CuO and Cu2O) existed in NPC in a highly dispersed state due to the strong interaction between CuxO and the doped N atoms. The obtained Cu-NPCs were used as supports to prepare Au catalysts for glycerol (GLY) oxidation to dihydroxyacetone (DHA) under base-free conditions. During the reaction, GLY preferentially adsorbed onto Au nanoparticles via the secondary OH groups due to the porous characteristics of the supports, the enhanced surface basicity from the N functionalities accelerates the alkoxide formation by abstracting proton from GLY, and the surface Cu+ sites instead of Au nanoparticles activate O2 molecules to produce active oxygen species, as a result, Au/Cu-NPC-15 %-H, which is the best catalyst in Au/Cu-NPCs, exhibited much higher catalytic activity and selectivity than those of Au/CuO. Moreover, due to the stabilization of CuxO species by NPC, relatively stable surface Cu+ content, and the strong interaction between Au nanoparticles and the supports, Au/Cu-NPC-15 %-H also possessed much better catalytic stability than that of Au/CuO during four consecutive catalytic runs.

Investigation of stability of gold nanoparticles modified zinc–cobalt coating in an alkaline sodium borohydride solution

The electrochemical stability and durability of ZnCo alloy thin layers deposited on the titanium surface (denoted as ZnCo/Ti) and those modified by small amounts of Au nanoparticles (denoted as AuZnCo/Ti) prepared via the electrochemical metal deposition technique and a simple galvanic displacement have been investigated in alkaline sodium borohydride (NaBH4) solutions. The physical properties of the fabricated AuZnCo/Ti catalysts have been examined using field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The electrocatalytic activity of the ZnCo/Ti and AuZnCo/Ti catalysts toward the oxidation of NaBH4 has been evaluated in an alkaline medium using cyclic voltammetry (CV) and chronoamperometry (CA), whereas the catalytic efficiency of the catalysts for the hydrolysis reaction of NaBH4 has been also examined by measuring the amount of generated hydrogen via the classic water-displacement method. It has been determined that the modification of the ZnCo alloy coating by Au nanoparticles apparently improves not only the morphology and structure of the catalyst, but also the activity and stability of the one for the oxidation of NaBH4 in an alkaline medium as compared to those of ZnCo/Ti and bare Au. The AuZnCo/Ti catalysts that have Au loadings of 63 and 306 µg cm–2 give ca. 12 and 11, respectively, times higher NaBH4 oxidation current densities as compared to those of the bare Au catalyst. Moreover, the AuZnCo/Ti catalysts catalyze the hydrolysis reaction of NaBH4 in alkaline solutions.

Ultrastable titania-supported Au nanoparticles covered by chromia via photo-induced strategy for low-temperature CO oxidation

Here we report the design and preparation of an ultrastable Au nanocatalyst through a photo-deposition strategy at room temperature, in which Au nanoparticles were covered by an island-like dispersed Cr2O3 overlayer. The latter was induced by the excited electrons in the TiO2 conduction band transferred to the Au nanoparticle. The resulting titania-supported Au catalyst promoted by chromia is resistant to sintering and exhibits excellent activity and stability in the low-temperature CO oxidation reaction and air atmosphere.

Oxidation of Dichloromethane over Au, Pt, and Pt-Au Containing Catalysts Supported on γ-Al2O3 and CeO2-Al2O3.

Au, Pt, and Pt-Au catalysts supported on Al2O3 and CeO2-Al2O3 were studied in the oxidation of dichloromethane (DCM, CH2Cl2). High DCM oxidation activities and HCl selectivities were seen with all the catalysts. With the addition of Au, remarkably lower light-off temperatures were observed as they were reduced by 70 and 85 degrees with the Al2O3-supported and by 35 and 40 degrees with the CeO2-Al2O3-supported catalysts. Excellent HCl selectivities close to 100% were achieved with the Au/Al2O3 and Pt-Au/Al2O3 catalysts. The addition of ceria on alumina decreased the total acidity of these catalysts, resulting in lower performance. The 100-h stability test showed that the Pt-Au/Al2O3 catalyst was active and durable, but the selectivity towards the total oxidation products needs improvement. The results suggest that, with the Au-containing Al2O3-supported catalysts, DCM decomposition mainly occurs via direct DCM hydrolysis into formaldehyde and HCl followed by the oxidation of formaldehyde into CO and CO2.

In Situ Anchoring Ultra Small Au Nanoparticles by Conformal Coating of Carbon Layer within the Micro-Environments of Mesoporous Silica for Highly Efficient Catalytic Reduction of 4-nitrophenol

For the noble-metal based catalysts, the metal dispersion and sinter resistance of the metal nanoparticles (NPs) are the most vital factors for their application in series catalytic reactions. However, in the most noble-metal based catalysts, the weak interaction between metal NPs and support would be negative for these factors, and the poor catalytic performance would be obtained relatively. Committed to overcome these, in this work, the catalyst of Au@C/AMS would be designed and fabricated by the hydrothermal and calcinations process, in which the ultra small Au NPs are formed and stabilized in situ by the conformal coating of carbon layer in the micro-environment of mesoporous silica. The results demonstrate that the carbon layer would be directly formed in the mesoporous channels by calcinations in nitrogen atmosphere, and the ultra small Au NPs would be in situ anchored in the carbon layer. The average particle size of Au in Au@C/AMS is as small as 1.8 nm from TEM results, which is much smaller than those in the sample of Au/AMS without the protection of carbon layer. Besides, compared with the samples that the Au NPs are deposited on the outer and inner surface of mesoporous silica, a strong interaction would be built between Au NPs and support in the target Au@C/AMS sample. As expected, the catalyst of Au@C/AMS displays superior catalytic activity and stability in the liquid-phase reduction of 4-nitrophenol. The construction of this special architecture for Au-based catalysts could provide new sights for the design and fabrication of noble metal-based mesoporous materials with desirable catalytic activity and stability. In this work, the catalyst of Au@C/AMS would be designed and fabricated by the hydrothermal and calcinations process, in which the ultra small Au NPs are formed and stabilized in situ by the conformal coating of carbon layer in the micro-environment of mesoporous silica. Compared with the samples that the Au NPs are deposited on the outer and inner surface of mesoporous silica, a strong interaction would be built between Au NPs and support in the target Au@C/AMS sample. As expected, the catalyst of Au@C/AMS displays superior catalytic activity and stability in the liquid-phase reduction of 4-nitrophenol. The construction of this special architecture for Au-based catalysts could provide new sights for the design and fabrication of noble metal-based mesoporous materials with desirable catalytic activity and stability.

Construction of Medium-Sized Rings by Gold Catalysis.

Compounds having cyclic molecular frameworks are highly regarded for their abundance and diverse utilities. In particular, medium-sized carbocycles and heterocycles exist in a broad spectrum of natural products, bioactive therapeutics, and medicinally significant synthetic molecules. Metal-mediated methods have been developed for the preparation of compounds containing a medium-sized ring (MSR) through cyclization of different classes of substrates and acyclic precursors. This review focuses on the methodologies for construction of MSRs via gold catalysis. Given the challenges in enabling the assembly of different ring sizes, we present here accounts on Au-mediated cyclization giving notable 7-membered and medium-sized (8-11-membered ring) structures. Emphasis on the pathway and mode of cyclization and the selection of precursors ranging from structurally biased compounds were outlined. Reactivity patterns and the choice of efficient Au catalysts for controlling reaction performance and selectivity in addition to mechanistic attributes are examined.