Supported Au catalysts for low temperature CO oxidation

Mechanistic Understanding of Oxygen Activation on Bulk Au(111) Surface Using Tip-Enhanced Raman Spectroscopy

Gaining mechanistic understanding of oxygen activation on metal surfaces is a topical area of research in surface science. However, direct investigation of on-surface oxidation processes at the nanoscale and the empirical validation of oxygen activation pathways remain challenging for the conventional analytical tools. In this study, we applied tip-enhanced Raman spectroscopy (TERS) to gain mechanistic insights into oxygen activation on bulk Au(111) surface. Specifically, oxidation of 4-aminothiophenol (4-ATP) to 4-nitrothiophenol (4-NTP) on Au(111) surface was investigated using hyperspectral TERS imaging. Nanoscale TERS images revealed a markedly higher oxidation efficiency in disordered 4-ATP adlayers compared to the ordered adlayers signifying that the oxidation of 4-ATP molecules proceeds via interaction with the on-surface oxidative species. These results were further validated via direct oxidation of the 4-ATP adlayers with H2O2 solution. Finally, TERS measurements of oxidized 4-ATP adlayers in the presence of H2O18 provided the first empirical evidence for the generation of oxidative species on bulk Au(111) surface via water-mediated activation of molecular oxygen. This study expands our mechanistic understanding of oxidation chemistry on bulk Au surface by elucidating the oxygen activation pathway.

A Water-Promoted Mars-van Krevelen Reaction Dominates Low-Temperature CO Oxidation over Au-Fe2O3 but Not over Au-TiO2

We provide experimental evidence that is inconsistent with often proposed Langmuir-Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au-Fe2O3. Passing CO and H2O, but no O2, over Au-γ-Fe2O3 at 25 °C, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with H218O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe2O3. Guided by density functional theory, we instead postulate a water-promoted Mars-van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO2 production in the absence of O2 and with CO oxidation in the presence of H218O and 16O2. In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.

Distinct Site Motifs Activate O2 and H2 on Supported Au Nanoparticles in Liquid Water

Au nanoparticles catalyze the activation and conversion of small molecules with rates and kinetic barriers that depend on the dimensions of the nanoparticle, composition of the support, and presence of catalytically culpable water molecules that solvate these interfaces. Here, molecular interpretations of steady-state rate measurements, kinetic isotope effects, and structural characterizations reveal how the interface of Au nanoparticles, liquid water, and metal oxide supports mediate the kinetically relevant activation of H2 and sequential reduction of O2-derived intermediates during the formation of H2O2 and H2O. Rates of H2 consumption are 10-100 fold greater on Au nanoparticles supported on metal oxides (e.g., titania) compared to more inert and hydrophobic materials (carbon, boron nitride). Similarly, Au nanoparticles on reducible and Lewis acidic supports (e.g., lanthana) bind dioxygen intermediates more strongly and present lower barriers (<22 kJ mol-1) for O-O bond dissociation than inert interfaces formed with silica (>70 kJ mol-1). Selectivities for H2O2 formation increase significantly as the diameters of the Au nanoparticles increase because differences in nanoparticle size change the relative fractions of exposed sites that exist at Au-support interfaces. In contrast, site-normalized rates and barriers for H2 activation depend weakly on the size of Au nanoparticles and the associated differences in active site motifs. These findings suggest that H2O aids the activation of H2 at sites present across all surface Au atoms when nanoparticles are solvated by water. However, molecular O2 preferentially binds and dissociates at Au-support interfaces, leading to greater structure sensitivity for barriers of O-O dissociation across different support identities and sizes of Au nanoparticles. These insights differ from prior knowledge from studies of gas-phase reactions of H2 and O2 upon Au nanoparticle catalysts within dilute vapor pressures of water (10-4 to 0.1 kPa H2O), in which catalysis occurs at the perimeter of the Au-support interface. In contrast, contacting Au catalysts with liquid water (55.5 M H2O) expands catalysis to all surface Au atoms and enables appreciable H2O2 formation.

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Role of H2O in Catalytic Conversion of C1 Molecules

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

Reducibility Studies of Ceria, Ce0.85Zr0.15O2 (CZ) and Au/CZ Catalysts after Alkali Ion Doping: Impact on Activity in Oxidation of NO and CO

The aim of these studies was to perform thorough research on the influence of alkali metal ions (Li, Na, K and Cs) on the properties of nanogold catalysts supported on ceria–zirconia. The addition of alkali metal ions onto CeO2 further affected the reducibility, which was not noted for the Zr-doped support (Ce0.85Zr0.15O2). Despite the substantial impact of alkali metal ions on the reducibility of ceria, the activity in CO oxidation did not change much. In contrast, they do not have a large effect on the reducibility of Au/CZ but suppressed the activity of this system in CO oxidation. The results show that for CO oxidation, the negative effect of potassium ions is greater than that of sodium, which corresponds to the shift in the Tmax of the reduction peak towards higher temperatures. The negative effect of Li+ and Cs+ spans 50% CO conversion. The negative effect was visible for CO oxidation in both the model stream and the complex stream, which also contained hydrocarbons and NO. In the case of NO oxidation to NO2, two temperature regimes were observed for Au + 0.3 at% K/CZ, namely in the temperature range below 350 °C; the effect of potassium ions was beneficial for NO oxidation, whereas at higher temperatures, the undoped gold catalyst produced more NO2.

Recent Advances in the Applications of Mesoporous Silica in Heterogenous Catalysis

Mesoporous silica is a class of silica material with a large specific surface area, high specific pore volume and meso-sized pores. These properties make mesoporous silica a good choice of...

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

A novel Zn-Al spinel-alumina composite supported gold catalyst for efficient CO oxidation

A spinel-alumina inert oxide supported gold catalyst with high Au dispersion and excellent CO oxidation activity was developed by a deposition-precipitation method. The activation atmosphere could tune the reaction pathway by adjusting the amount of surface adsorbed water species, thus transforming the reaction intermediates from HCO₃^- or CO₃^2- to COOH.

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO₂ proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO₂ reduction. Here, by employing findings from operando X‐ray absorption spectroscopy at the near and extended Au and Ce L_III energy edges, we focus on the fundamental aspects of highly active Au/CeO₂ catalysts, mainly in the CO oxidation for understanding their complex structure‐reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O₂ activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO₂ support, and the dynamics of O₂ activation.

Mechanochemical Synthesis of Catalytic Materials

The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route-in contrast to more conventional synthesis-in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance.

Continuous 2-Methyl-3-butyn-2-ol Selective Hydrogenation on Pd/γ-Al2O3 as a Green Pathway of Vitamin A Precursor Synthesis

In this work, the effect of pretreatment conditions (10% H2/Ar flow rate 25 mL/min and 400 °C, 3 h or 600 °C, 17 h) on the catalytic performance of 1 wt.% Pd/γ-Al2O3 has been evaluated for hydrogenation of 2-methyl-3-butyn-2-ol in continuous-flow mode. Two palladium catalysts have been tested under different conditions of pressure and temperature and characterized using various physicochemical techniques. The catalytic performance of red(400 °C)-Pd/γ-Al2O3 and red(600 °C)-Pd/γ-Al2O3 are affected by the coexistence of several related factors like the competition between PdH and PdCx formation during the reaction, structure sensitivity, hydrogen spillover to the alumina support and presence or absence of Pd–Al species. High-temperature reduction leads to formation of Pd–Al species in addition to pure Pd. The Pd–Al species which reveal unique electronic properties by decreasing the Pdδ− surface concentration via electron transfer from Pd to Al, leading to a weaker Pd–Alkyl bonding, additionally assisted by the hydrogen spillover, are the sites of improved semi-hydrogenation of 2-methyl-3-butyn-2-ol towards 2-methyl-3-buten-2-ol (97%)—an important intermediate for vitamin A synthesis.

The fabrication of hollow ZrO2 nanoreactors encapsulating Au-Fe2O3 dumbbell nanoparticles for CO oxidation

Nanosized Au catalysts suffer from serious sintering problems during synthesis or catalytic reactions at high temperatures. In this work, we integrate dumbbell-shaped Au-Fe₃O₄ heterostructures into hollow ZrO₂ nanocages to make Au-Fe₂O₃@ZrO₂ yolk-shell nanoreactors with high activity as well as ultra-high sintering resistance for high-temperature CO oxidation. The synthesis starts with the fabrication of a (Au-Fe₃O₄)@SiO₂@ZrO₂ core-shell nanostructure with a Au-Fe₃O₄ dumbbell nanoparticle (DB) core and SiO₂/ZrO₂ double shells, followed by calcination and the selective removal of the inner SiO₂ shell with alkaline solution to obtain Au-Fe₂O₃@ZrO₂ nanoreactors. The retained ZrO₂ hollow (outer) shells protect the Au NPs from aggregation at temperatures up to 900 °C and show excellent long-term stability. Compared to Au@ZrO₂ yolk-shell nanoreactors, Au-Fe₂O₃@ZrO₂ shows improved activity in CO oxidation due to the active Au-Fe₂O₃ interface. This strategy can be extended to other yolk-shell nanoreactors with various nanocomposites and for different catalytic reactions.

CO Oxidation on Planar Au/TiO2 Model Catalysts under Realistic Conditions: A Combined Kinetic and IR Study

The oxidation of CO on planar Au/TiO2 model catalysts was investigated under pressure and temperature conditions similar to those for experiments with more realistic Au/TiO2 powder catalysts. The effects of a change of temperature, pressure, and gold coverage on the CO oxidation activity were studied. Additionally, the reasons for the deactivation of the catalysts were examined in long-term experiments. From kinetic measurements, the activation energy and the reaction order for the CO oxidation reaction were derived and a close correspondence with results of powder catalysts was found, although the overall turnover frequency (TOF) measured in our experiments was around one order of magnitude lower compared to results of powder catalysts under similar conditions. Furthermore, long-term experiments at 80 °C showed a decrease of the activity of the model catalysts after some hours. Simultaneous in-situ IR experiments revealed a decrease of the signal intensity of the CO vibration band, while the tendency for the build-up of side products (e. g. carbonates, carboxylates) of the CO oxidation reaction on the surface of the planar model catalysts was rather low.

Highly selective gas sensing enabled by filters

Portable and inexpensive gas sensors are essential for the next generation of non-invasive medical diagnostics, smart air quality monitoring & control, human search & rescue and food quality assessment to name a few of their immediate applications. Therein, analyte selectivity in complex gas mixtures like breath or indoor air remains the major challenge. Filters are an effective and versatile, though often unrecognized, route to overcome selectivity issues by exploiting additional properties of target analytes (e.g., molecular size and surface affinity) besides reactivity with the sensing material. This review provides a tutorial for the material engineering of sorption, size-selective and catalytic filters. Of specific interest are high surface area sorbents (e.g., activated carbon, silica gels and porous polymers) with tunable properties, microporous materials (e.g., zeolites and metal-organic frameworks) and heterogeneous catalysts, respectively. Emphasis is placed on material design for targeted gas separation, portable device integration and performance. Finally, research frontiers and opportunities for low-cost gas sensing systems in emerging applications are highlighted.

Synthesis and characterization of highly dispersed bimetallic Au-Rh nanoparticles supported on titanate nanotubes for CO oxidation reaction at low temperature

Low-temperature CO oxidation was carried out by using rhodium incorporated into titanate nanotubes (Rh/NTs) prepared by the sol-gel and hydrothermal methods; otherwise, gold nanoparticles were deposited homogeneously onto the Rh/NT surface through the deposition-precipitation with urea (DPU) method. The Au-Rh/NT sample exhibited high metal dispersion (55%), outstanding CO oxidation at low temperature, and better resistance to deactivation than the monometallic Rh/NT and Au/NT samples. The characterization of bimetallic samples, with particle sizes from 1 to 3 nm, revealed the remarkable presence of interacting Au and Rh species in metallic state. In this way, Au⁰ and Rh⁰ were answerable for the higher catalytic activity observed in the bimetallic samples. The interaction between Au and Rh in the nanoparticles of Au-Rh/NT promoted a synergistic effect on the CO oxidation reaction, explained by the creation of new CO adsorption sites.

Infrared Thermography as a Powerful, Versatile, and Elegant Research Tool in Chemistry: Principles and Application to Catalysis and Adsorption

In this Review, diverse chemical problems that have been approached by means of infrared thermography (IRT) are covered in depth. Moreover, some novel steps forward in this field are made, described and discussed. Namely, the latest-generation IRT performance capabilities are harnessed in full; the initial phase of catalytic CO oxidation (called "fast ignition") is presented at the 0.01 s temporal resolution; at the same resolution, the thermal manifestation of the adsorption-desorption wave propagation after the gaseous reactant pulsed (0.6 s) wetting is exhibited. Furthermore, a radical difference in the thermal behavior of differently calcined γ-Al₂ O₃ supported Au catalysts, which underwent successive H₂ O and CO attacks, is demonstrated, and the generally accepted fact that the catalyst temperature reflects the catalytic activity is validated experimentally. It is shown that latest-generation IRT may serve as unique and highly informative research tool in chemistry.

Efficient metal overlayer catalysts on the Nb2C monolayer for CO oxidation from first-principles screening

Based on the first-principles calculation, the configurations of different metal overlayers on the monolayer Nb₂C (MXene) (M_ML/Nb₂C) (M  =  Rh, Ir, Pd, Pt, Ag, Au) were studied aiming to find a kind of complex system with high CO-tolerance and high CO conversion efficiency. Combined with the stability of the composite systems and their adsorption properties on small gas molecules, Ag_ML/Nb₂C was screened out and further tested for CO oxidation reaction. By comparing the energy barriers of different reaction pathways, we concluded that CO oxidation reaction could be carried out on Ag_ML/Nb₂C var the LH mechanism with a small energy barrier of 0.35 eV. The rate-determining step was the oxidation of CO by the adsorbed oxygen atom. The Ag_ML/Nb₂C showed good activity for CO oxidation, which would provide a theoretical basis for designing the electrode material for the proton exchange membrane fuel cells (PEMFCs).

One-pot synthesis of Au–Fe2O3@SiO2 core–shell nanoreactors for CO oxidation

The combination of the Au–Fe₂O₃ phase and core–shell structure helps in achieving high activity and good thermal stability.

Interaction of carbon monoxide with doped metal clusters

Highlight of experimental and computational studies about the interaction of CO with transition and coinage metal clusters, particularly discussing the influence of dopant atoms.

Efficient Ce–Co composite oxide decorated Au nanoparticles for catalytic oxidation of CO in the simulated atmosphere of a CO2 laser

Gold nanoparticles have a high activity for CO oxidation, making them suitable to be used in a CO₂ laser which maintains its efficiency and stability via the recombination of CO and O₂ produced by the CO₂ decomposition. However, the high concentration of CO₂ in the working environment greatly reduces the activity of the catalyst and makes the already unstable gold nanoparticles even more so. A novel Au/Ce-Co-O_x/Al₂O₃ gold catalyst, prepared by a deposition precipitation method in this study, displays high activity and good stability for CO oxidation in a simulated atmosphere of a CO₂ laser with the feed gases containing a high concentration of CO₂ up to 60 vol% but a low concentration of O₂ for the stoichiometric reaction with CO. An excellent performance for CO oxidation under CO₂-rich conditions could be achieved by decorating the surface of the Al₂O₃ support with Ce–Co composite oxides. The strong interaction between gold and the composite support, accompanied by the increase of labile lattice oxygen species and the decrease of surface basicity, led to a high CO oxidation rate and resistance towards CO₂ poisoning.

Novel Au/Ce-Co-O_x/Al₂O₃ showed a very excellent performance for CO oxidation in the simulated atmosphere of CO₂-laser mainly due to SMSI and labile lattice oxygen.

Supported Gold Nanoparticles as Catalysts for the Oxidation of Alcohols and Alkanes

Supporting gold nanoparticles have shown to be extremely active for many industrially important reactions, including oxidations. Two representative examples are the oxidation of alcohols and alkanes, that are substrates of industrial interest, but whose oxidation is still challenging. This review deals with these reactions, giving an insight of the first studies performed by gold based catalysts in these reactions and the most recent developments in the field.

Plasmonic-enhanced catalytic activity of methanol oxidation on Au-graphene-Cu nanosandwiches

The plasmonic-enhanced catalytic activity of methanol oxidation on Au-based catalysts provides a promising strategy for direct methanol fuel cells (DMFCs) to avoid the CO poisoning of traditional Pt-based catalysts. However, the effect of surface plasmon resonance on the light-enhanced methanol oxidation activity of Au or Au-based catalysts has not been fully understood. The mechanism by which hot plasmonic carriers participate in the methanol oxidation reaction (MOR) has not been elucidated. Herein, Au nanoparticles (Au NPs) are loaded on a support of single-layer graphene-Cu contacts (SG/Cu) to construct a nanosandwich structure of a Au-graphene-Cu catalytic electrode (Au-n/SG/Cu). The Au-6T/SG/Cu catalytic electrode exhibits an MOR catalytic activity of approximately 288 μA μg-1 under simulated solar light irradiation, which is approximately 1.7 times higher than that without irradiation. The chemisorption capacity of OH- anions is enhanced on the Au-6T/SG/Cu catalytic electrode compared with the pure Au NP surface. The adsorbed OH- anions are oxidised into ˙OH radicals by the trapped positive holes on the Au NP surface. These OH radicals possessed a high oxidation capacity for the direct oxidation of HCOO- intermediates and promoted the complete methanol oxidation on Au NPs, which is beneficial for improving the fuel efficiency of DMFCs.

Effect of gold addition by the recharge method on silver supported catalysts in the catalytic wet air oxidation (CWAO) of phenol

Catalysts Ag/ZrO₂–CeO₂ and Au/ZrO₂–CeO₂ were synthesized by a deposition–precipitation method and Ag–Au/ZrO₂–CeO₂ was prepared using a recharge method for the second metal (Au). The materials were characterized by physisorption of N₂, XRD, ICP, UV-vis RDS, H2-TPR, XPS and TEM. The results obtained show that the specific areas for monometallic materials were 29–37 m² g⁻¹ and 27–74 m² g⁻¹ for bimetallics. The tetragonal crystal phase of ZrO₂ stabilizes when CeO₂ quantity increases. Using XPS an increment in Ce³⁺ species abundance was determined for bimetallic catalysts in contrast to the monometallic ones; according to the Ag 3d region, this metal oxidation was observed when augmenting the content of CeO₂ in the materials, and with Au the opposite effect was produced. It was determined by TEM, that the average size of the metallic particles was smaller at bimetallic catalysts due the preparation method. Catalytic activity was evaluated by CWAO of phenol, the Ag–Au/ZrO₂–CeO₂ catalyst with 20% wt of cerium reached a degradation of 100% within an hour, being the most active catalyst. Maleic, formic and oxalic acid were identified as reaction intermediates; and at the end of the reaction acetic acid was identified as the main by-product, because it is the most refractory and the conditions for oxidation must be more severe.

Addition of gold changed the properties of silver monometallic catalysts by inhibiting the low formation of intermediates and changed of reaction route by formic acid to CO₂ and water. Furthermore, the bimetallic catalyst showed in the reuse cycles the better stability in CWAO of phenol.

A High Capacity, Room Temperature, Hybrid Flow Battery Consisting of Liquid Na-Cs Anode and Aqueous NaI Catholyte

In this study, we have proposed a novel concept of hybrid flow batteries consisting of a molten Na-Cs anode and an aqueous NaI catholyte separated by a NaSICON membrane. A number of carbonaceous electrodes are studied using cyclic voltammetry (CV) for their potentials as the positive electrode of the aqueous NaI catholyte. The charge transfer impedance, interfacial impedance and NaSICON membrane impedance of the Na-Cs ‖ NaI hybrid flow battery are analyzed using electrochemical impedance spectroscopy. The performance of the Na-Cs ‖ NaI hybrid flow battery is evaluated through galvanostatic charge/discharge cycles. This study demonstrates, for the first time, the feasibility of the Na-Cs ‖ NaI hybrid flow battery and shows that the Na-Cs ‖ NaI hybrid flow battery has the potential to achieve the following properties simultaneously: (i) An aqueous NaI catholyte with good cycle stability, (ii) a durable and low impedance NaSICON membrane for a large number of cycles, (iii) stable interfaces at both anode/membrane and cathode/membrane interfaces, (iv) a molten Na-Cs anode capable of repeated Na plating and stripping, and (v) a flow battery with high Coulombic efficiency, high voltaic efficiency, and high energy efficiency.

Synthetic Methodologies to Gold Nanoshells: An Overview

Gold nanostructures that can be synthetically articulated to adapt diverse morphologies, offer a versatile platform and tunable properties for applications in a variety of areas, including biomedicine and diagnostics. Among several conformational architectures, gold nanoshells provide a highly advantageous combination of properties that can be fine-tuned in designing single or multi-purpose nanomaterials, especially for applications in biology. One of the important parameters for evaluating the efficacy of gold nano-architectures is their reproducible synthesis and surface functionalization with desired moieties. A variety of methods now exist that allow fabrication and chemical manipulation of their structure and resulting properties. This review article provides an overview and a discussion of synthetic methodologies to a diverse range of gold nanoshells, and a brief summary of surface functionalization and characterization methods employed to evaluate their overall composition.

Atmospheric-Pressure Cold Plasma Activating Au/P25 for CO Oxidation: Effect of Working Gas

Commercial TiO₂ (P25) supported gold (Au/P25) attracts increasing attention. In this work, atmospheric-pressure (AP) cold plasma was employed to activate the Au/P25-As catalyst prepared by a modified impregnation method. The influence of cold plasma working gas (oxygen, argon, hydrogen, and air) on the structure and performance of the obtained Au/P25 catalysts was investigated. X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), and X-ray spectroscopy (XPS) were adopted to characterize the Au/P25 catalysts. CO oxidation was used as model reaction probe to test the Au/P25 catalyst. XRD results reveal that supporting gold and AP cold plasma activation have little effect on the P25 support. CO oxidation activity over the Au/P25 catalysts follows the order: Au/P25-O₂P > Au/P25-As > Au/P25-ArP ≈ Au/P25-H₂P > Au/P25-AirP. Au/P25-AirP presents the poorest CO oxidation catalytic activity among the Au/P25 catalysts, which may be ascribed to the larger size of gold nanoparticles, low concentration of active [O]_s, as well as the poisoning [NO_x]_s. The poor catalytic performance of Au/P25-ArP and Au/P25-H₂P is ascribed to the lower concentration of [O]_s species. 100% CO conversion temperatures for Au/P25-O₂P is 40 °C, which is 30 °C lower than that over the as-prepared Au/P25-As catalyst. The excellent CO oxidation activity over Au/P25-O₂P is mainly attributed to the efficient decomposition of gold precursor species, small size of gold nanoparticles, and the high concentration of [O]_s species.

A hierarchical flower-like hollow alumina supported bimetallic AuPd nanoparticle catalyst for enhanced solvent-free ethylbenzene oxidation

Currently, oxidation of alkylaromatics is considered as one of the most crucial chemical technologies to produce high added-value alcohols, ketones and carboxylic acids, due to its significant importance both in fine synthetic chemistry and in the academic field. In this work, a novel hierarchical marigold-like hollow alumina supported bimetallic AuPd nanoparticle catalyst was successfully fabricated and employed for highly efficient solvent-free ethylbenzene oxidation to produce acetophenone with the coexistence of both molecular oxygen and tert-butyl hydroperoxide as the oxidant and the initiator. The as-fabricated bimetallic AuPd nanocatalyst conferred a superior catalytic performance to the corresponding monometallic counterparts and commercial Al2O3 or solid Al2O3 microsphere supported AuPd ones, along with a high acetophenone selectivity of 88.2% at a conversion of 50.9% under mild reaction conditions (120 °C and oxygen pressure of 1.0 MPa), as well as an unprecedentedly high turnover frequency value of 46 768 h-1. Such exceptional efficiency of the catalyst was related to both the significant synergy between the Au-Pd atoms and strong metal-support interactions, and the unique hierarchical micro/nanostructure of the support being beneficial to the close contact of reactants with surface adsorption and reaction sites and easy product diffusion. Moreover, the present bimetallic AuPd catalyst was recyclable and stable. The developed approach is expected to offer exciting opportunities for designing other supported monometallic or bimetallic catalysts with various active components applied in heterogeneous catalysis.

CO Oxidation at 20 °C on Au Catalysts Supported on Mesoporous Silica: Effects of Support Structural Properties and Modifiers

In this work we report the effects of support structural properties and its modification with some metal oxides modifiers on the catalytic behavior of Au catalysts in the total CO oxidation at 20 °C. Au catalysts were supported on mesoporous silica materials (MSM) having different structural properties: Channel-like (SBA-15), cage-like (SBA-16), hexagonal (HMS), and disordered (DMS-1) structures. The effect of the modifier was evaluated by comparison of the catalytic response of the SBA-15-based catalysts modified with MgO, Fe₂O₃, TiO₂, and CeO₂. The chemical, structural, and electronic properties of the catalysts were investigated by a variety of techniques (metal content analysis by ICP-OES, N₂ physisorption, XRD, UV-vis DRS, DRIFTS of adsorbed CO and OH regions, oxygen storage capacity (OSC), HR-TEM, and XPS). The activity of calcined catalysts in the CO oxidation reaction were evaluated at steady state conditions, at 20 °C, atmospheric pressure, and when using, as feed, a 1%CO/1%O₂/98% gas mixture. The work clearly demonstrated that all Au catalysts supported on the mesoporous silicas modified with metal oxides were more active than the Au/SBA-15 and Au/MgO reference ones. The support structural properties and type of dopant were important factors influencing on the catalyst behavior. Concerning the support textural properties, it was found that the HMS substrate with the wormhole-structure offers better porosity and specific surface area than their silica counterparts having channel-like (SBA-15), cage-like (SBA-16), and disordered (DMS-1) mesoporous structures. Concerning the effect of modifier, the best catalytic response was achieved with the catalysts modified with MgO. After activation by calcination at 200 °C for 4 h, the Au/MgO/HMS catalyst exhibited the best catalytic performance, which was ascribed to the combined effects of the best structural properties, a large support oxygen storage capacity and homogeneous distribution of gold particles on the support (external and inner). Implications of the type of active sites (Au¹⁺ or Au⁰), support structural properties and role of modifier on the catalytic activity are discussed.

CO oxidation on SnO2 surfaces enhanced by metal doping

Doping metal atoms into a host metal oxide lattice can enhance its catalytic activity by modulating the properties of surface oxygen.