Soccer Ball-like Assembly of Edge-to-edge Oriented 2D-silica Nanosheets: A Promising Catalyst Support for High-Temperature Reforming.

Controlled assembly of nanoparticles into well-defined assembled architectures through precise manipulation of spatial arrangement and interactions allows the development of advanced mesoscale materials with tailored structures, hierarchical functionalities, and enhanced properties. Despite remarkable advancements, the controlled assembly of highly anisotropic 2Dnanosheets is significantly challenging, primarily due to the limited availability of selective edge-to-edge connectivity compared to the abundant large faces. Innovative strategies are needed to unlock the full potential of 2D-nanomaterialsin self-assembled structures with distinct and desirable properties. This research unveils the discovery of controlled self-assembly of 2D-silica nanosheets (2D-SiNSs) into hollow micron-sized soccer ball-like shells (SA-SiMS). The assembly is driven by the physical flexibility of the 2D-SiNSs and the differential electricdouble-layer charge gradient creating electrostatic bias on the edge and face regions. The resulting SA-SiMS structures exhibit high mechanical stability, even at high-temperatures, and exhibit excellent performance as catalyst support in the dry reforming of methane. The SA-SiMS structures facilitate improved mass transport, leading to enhanced reaction rates, while the thin silica shell prevents sintering of small catalyst nanocrystals, thereby preventing coke formation. This discovery sheds light on the controllable self-assembly of 2D nanomaterials and provides insights into the design and synthesis of advanced mesoscale materials with tailored properties.

Boosting Thermal Stability of Volatile Os Catalysts by Downsizing to Atomically Dispersed Species.

Os-based catalysts present remarkable catalytic activity; however, their use has been limited by the undesirable side reactions that generate highly toxic and volatile OsO4 even at room temperature. Herein, we demonstrate that the thermal stability of Os-based catalysts can be dramatically improved by downsizing Os nanoparticles (NPs) into atomically dispersed species. We observed that Os NPs were converted into OsO4 after calcination at 250 °C followed by sublimation, whereas single Os sites retained their structure after calcination. Temperature-programmed oxidation analysis confirmed that Os NPs started to undergo oxidation at 130 °C, whereas atomically dispersed Os preserved its state up to 300 °C. The CO oxidation activity of the atomically dispersed Os catalyst at 400 °C (100% conversion) was stably preserved over 30 h. By contrast, the activity of Os NP catalyst declined drastically. This study highlights the unique catalytic behavior of atomically dispersed catalysts, which is distinct from that of NP-based catalysts.

Synergistic effect of quinary molten salts and ruthenium catalyst for high-power-density lithium-carbon dioxide cell.

With a recent increase in interest in metal-gas batteries, the lithium-carbon dioxide cell has attracted considerable attention because of its extraordinary carbon dioxide-capture ability during the discharge process and its potential application as a power source for Mars exploration. However, owing to the stable lithium carbonate discharge product, the cell enables operation only at low current densities, which significantly limits the application of lithium-carbon dioxide batteries and effective carbon dioxide-capture cells. Here, we investigate a high-performance lithium-carbon dioxide cell using a quinary molten salt electrolyte and ruthenium nanoparticles on the carbon cathode. The nitrate-based molten salt electrolyte allows us to observe the enhanced carbon dioxide-capture rate and the reduced discharge-charge over-potential gap with that of conventional lithium-carbon dioxide cells. Furthermore, owing to the ruthernium catalyst, the cell sustains its performance over more than 300 cycles at a current density of 10.0 A g-1 and exhibits a peak power density of 33.4 mW cm-2.

Enhanced hot electron generation by inverse metal-oxide interfaces on catalytic nanodiode.

Identifying the electronic behavior of metal-oxide interfaces is essential for understanding the origin of catalytic properties and for engineering catalyst structures with the desired reactivity. For a mechanistic understanding of hot electron dynamics at inverse oxide/metal interfaces, we employed a new catalytic nanodiode by combining Co3O4 nanocubes (NCs) with a Pt/TiO2 nanodiode that exhibits nanoscale metal-oxide interfaces. We show that the chemicurrent, which is well correlated with the catalytic activity, is enhanced at the inverse oxide/metal (CoO/Pt) interfaces during H2 oxidation. Based on quantitative visualization of the electronic transfer efficiency with chemicurrent yield, we show that electronic perturbation of oxide/metal interfacial sites not only promotes the generation of hot electrons, but improves catalytic activity.

Boosting hot electron flux and catalytic activity at metal-oxide interfaces of PtCo bimetallic nanoparticles.

Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interface.

High-performance hybrid oxide catalyst of manganese and cobalt for low-pressure methanol synthesis.

Carbon dioxide capture and use as a carbon feedstock presents both environmental and industrial benefits. Here we report the discovery of a hybrid oxide catalyst comprising manganese oxide nanoparticles supported on mesoporous spinel cobalt oxide, which catalyses the conversion of carbon dioxide to methanol at high yields. In addition, carbon-carbon bond formation is observed through the production of ethylene. We document the existence of an active interface between cobalt oxide surface layers and manganese oxide nanoparticles by using X-ray absorption spectroscopy and electron energy-loss spectroscopy in the scanning transmission electron microscopy mode. Through control experiments, we find that the catalyst's chemical nature and architecture are the key factors in enabling the enhanced methanol synthesis and ethylene production. To demonstrate the industrial applicability, the catalyst is also run under high conversion regimes, showing its potential as a substitute for current methanol synthesis technologies.

Hollow MnOxPy and Pt/MnOxPy yolk/shell nanoparticles as a T1 MRI contrast agent.

Hollow MnOxPy and Pt/MnOxPy yolk/shell nanoparticles were fabricated via the nanoscale acid-etching process from the solid MnO and Pt/MnO core/shell nanoparticles, respectively. In the synthesis, alkylphosphonic acid impurity in trioctylphosphine oxide was a key component, resulting in amorphous hollow metal phosphate nanoparticles. Hollow nanoparticles containing Mn(2+) ions showed positively enhanced T1 relaxation, in which the r1 values of the hollow MnOxPy and Pt/MnOxPy yolk/shell were greater than that of the original MnO nanocrystals. The increased surface area of the hollow nanoparticles enhanced interaction between Mn(2+) ions of the nanoparticle surface and water molecules. Cytotoxicity experiment revealed that Mn ions released from hollow walls of the MnOxPy nanoparticles were responsible for the cytotoxicity, while Pt ions from the Pt/MnOxPy yolk/shell were not released in the cells. These nanoparticles provide potential insights into an anticancer drug, enabling simultaneous T1 magnetic resonance imaging (MRI) and therapy.

Comparing the catalytic oxidation of ethanol at the solid-gas and solid-liquid interfaces over size-controlled Pt nanoparticles: striking differences in kinetics and mechanism.

Pt nanoparticles with controlled size (2, 4, and 6 nm) are synthesized and tested in ethanol oxidation by molecular oxygen at 60 °C to acetaldehyde and carbon dioxide both in the gas and liquid phases. The turnover frequency of the reaction is ∼80 times faster, and the activation energy is ∼5 times higher at the gas-solid interface compared to the liquid-solid interface. The catalytic activity is highly dependent on the size of the Pt nanoparticles; however, the selectivity is not size sensitive. Acetaldehyde is the main product in both media, while twice as much carbon dioxide was observed in the gas phase compared to the liquid phase. Added water boosts the reaction in the liquid phase; however, it acts as an inhibitor in the gas phase. The more water vapor was added, the more carbon dioxide was formed in the gas phase, while the selectivity was not affected by the concentration of the water in the liquid phase. The differences in the reaction kinetics of the solid-gas and solid-liquid interfaces can be attributed to the molecular orientation deviation of the ethanol molecules on the Pt surface in the gas and liquid phases as evidenced by sum frequency generation vibrational spectroscopy.

High-temperature catalytic reforming of n-hexane over supported and core-shell Pt nanoparticle catalysts: role of oxide-metal interface and thermal stability.

Designing catalysts with high thermal stability and resistance to deactivation while simultaneously maintaining their catalytic activity and selectivity is of key importance in high-temperature reforming reactions. We prepared Pt nanoparticle catalysts supported on either mesoporous SiO2 or TiO2. Sandwich-type Pt core@shell catalysts (SiO2@Pt@SiO2 and SiO2@Pt@TiO2) were also synthesized from Pt nanoparticles deposited on SiO2 spheres, which were encapsulated by either mesoporous SiO2 or TiO2 shells. n-Hexane reforming was carried out over these four catalysts at 240-500 °C with a hexane/H2 ratio of 1:5 to investigate thermal stability and the role of the support. For the production of high-octane gasoline, branched C6 isomers are more highly desired than other cyclic, aromatic, and cracking products. Over Pt/TiO2 catalyst, production of 2-methylpentane and 3-methylpentane via isomerization was increased selectively up to 420 °C by charge transfer at Pt-TiO2 interfaces, as compared to Pt/SiO2. When thermal stability was compared between supported catalysts and sandwich-type core@shell catalysts, the Pt/SiO2 catalyst suffered sintering above 400 °C, whereas the SiO2@Pt@SiO2 catalyst preserved the Pt nanoparticle size and shape up to 500 °C. The SiO2@Pt@TiO2 catalyst led to Pt nanoparticle sintering due to incomplete protection of the TiO2 shells during the reaction at 500 °C. Interestingly, over the Pt/TiO2 catalyst, the average size of Pt nanoparticles was maintained even after 500 °C without sintering. In situ ambient pressure X-ray photoelectron spectroscopy demonstrated that the Pt/TiO2 catalyst did not exhibit TiO2 overgrowth on the Pt surface or deactivation by Pt sintering up to 600 °C. The extraordinarily high stability of the Pt/TiO2 catalyst promoted high reaction rates (2.0 µmol · g(-1) · s(-1)), which was 8 times greater than other catalysts and high isomer selectivity (53.0% of C6 isomers at 440 °C). By the strong metal-support interaction, the Pt/TiO2 was turned out as the best catalyst with great thermal stability as well as high reaction rate and product selectivity in high-temperature reforming reaction.

Designed catalysts from Pt nanoparticles supported on macroporous oxides for selective isomerization of n-hexane.

Selective isomerization toward branched hydrocarbons is an important catalytic process in oil refining to obtain high-octane gasoline with minimal content of aromatic compounds. Colloidal Pt nanoparticles with controlled sizes of 1.7, 2.7, and 5.5 nm were deposited onto ordered macroporous oxides of SiO2, Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2 to investigate Pt size- and support-dependent catalytic selectivity in n-hexane isomerization. Among the macroporous oxides, Nb2O5 and Ta2O5 exhibited the highest product selectivity, yielding predominantly branched C6 isomers, including 2- or 3-methylpentane, as desired products of n-hexane isomerization (140 Torr n-hexane and 620 Torr H2 at 360 °C). In situ characterizations including X-ray diffraction and ambient-pressure X-ray photoelectron spectroscopy showed that the crystal structures of the oxides in Pt/oxide catalysts were not changed during the reaction and oxidation states of Nb2O5 were maintained under both H2 and O2 conditions. Fourier transform infrared spectra of pyridine adsorbed on the oxides showed that Lewis sites were the dominant acidic site of the oxides. Macroporous Nb2O5 and Ta2O5 were identified to play key roles in the selective isomerization by charge transfer at Pt-oxide interfaces. The selectivity was revealed to be Pt size-dependent, with improved isomer production as Pt sizes increased from 1.7 to 5.5 nm. When 5.5 nm Pt nanoparticles were supported on Nb2O5 or Ta2O5, the selectivity toward branched C6 isomers was further increased, reaching ca. 97% with a minimum content of benzene, due to the combined effects of the Pt size and the strong metal-support interaction.

Evidence of highly active cobalt oxide catalyst for the Fischer-Tropsch synthesis and CO2 hydrogenation.

Hydrogenations of CO or CO2 are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H2 (i.e., Fischer-Tropsch synthesis) and CO2/H2 reactions. Exploiting synchrotron based in situ characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 °C while supported on TiO2 or SiO2 and that the complete reduction of cobalt can only be achieved at 450 °C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO2. In fact, the catalytic results indicate an enhancement of 10-fold for the CO2/H2 reaction rate and 2-fold for the CO/H2 reaction rate for the Co/TiO2 treated at 250 °C in H2 versus Co/TiO2 treated at 450 °C. Inversely, the activity of cobalt supported on SiO2 has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO2, we observed an increase of methane production for the CO2/H2 reaction whereas it is more selective to unsaturated products for the CO/H2 reaction. In situ investigation of the catalysts indicated wetting of the TiO2 support by CoO(x) and partial encapsulation of metallic Co by TiO(2-x).

Enhanced CO oxidation rates at the interface of mesoporous oxides and Pt nanoparticles.

The interaction of the metal and support in oxide-supported transition-metal catalysts has been proven to have extremely favorable effects on catalytic performance. Herein, mesoporous Co3O4, NiO, MnO2, Fe2O3, and CeO2 were synthesized and utilized in CO oxidation reactions to compare the catalytic activities before and after loading of 2.5 nm Pt nanoparticles. Turnover frequencies (TOFs) of pure mesoporous oxides were 0.0002­0.015 s(­1), while mesoporous silica was catalytically inactive in CO oxidation. When Pt nanoparticles were loaded onto the oxides, the TOFs of the Pt/metal oxide systems (0.1­500 s(­1)) were orders of magnitude greater than those of the pure oxides or the silica-supported Pt nanoparticles. The catalytic activities of various Pt/oxide systems were further influenced by varying the ratio of CO and O2 in the reactant gas feed, which provided insight into the mechanism of the observed support effect. In situ characterization using near-edge X-ray absorption fine structure (NEXAFS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) under catalytically relevant reaction conditions demonstrated a strong correlation between the oxidation state of the oxide support and the catalytic activity at the oxide­metal interface. Through catalytic activity measurements and in situ X-ray spectroscopic probes, CoO, Mn3O4, and CeO2 have been identified as the active surface phases of the oxide at the interface with Pt nanoparticles.

Influence of size-induced oxidation state of platinum nanoparticles on selectivity and activity in catalytic methanol oxidation in the gas phase.

Pt nanoparticles with various sizes of 1, 2, 4, and 6 nm were synthesized and studied as catalysts for gas-phase methanol oxidation reaction toward formaldehyde and carbon dioxide under ambient pressure (10 Torr of methanol, 50 Torr of oxygen, and 710 Torr of helium) at a low temperature of 60 °C. While the 2, 4, and 6 nm nanoparticles exhibited similar catalytic activity and selectivity, the 1 nm nanoparticles showed a significantly higher selectivity toward partial oxidation of methanol to formaldehyde, but a lower total turnover frequency. The observed size effect in catalysis was correlated to the size-dependent structure and oxidation state of the Pt nanoparticles. X-ray photoelectron spectroscopy and infrared vibrational spectroscopy using adsorbed CO as molecular probes revealed that the 1 nm nanoparticles were predominantly oxidized while the 2, 4, and 6 nm nanoparticles were largely metallic. Transmission electron microscopy imaging witnessed the transition from crystalline to quasicrystalline structure as the size of the Pt nanoparticles was reduced to 1 nm. The results highlighted the important impact of size-induced oxidation state of Pt nanoparticles on catalytic selectivity as well as activity in gas-phase methanol oxidation reactions.

Preparation of mesoporous oxides and their support effects on Pt nanoparticle catalysts in catalytic hydrogenation of furfural.

Mesoporous SiO(2), Al(2)O(3), TiO(2), Nb(2)O(5), and Ta(2)O(5) were synthesized through a soft-templating approach by a self-assembled framework of Pluronic P123 and utilized for the preparation of 3-dimensional catalysts as supports. Colloidal Pt nanoparticles with an average diameter of 1.9 nm were incorporated into the mesoporous oxides by sonication-induced capillary inclusion. The Pt nanoparticles supported on mesoporous oxides were evaluated in the hydrogenation reaction of furfural (70 torr furfural and 700 torr H(2) with a balance of He) to study the effect of catalyst supports on selectivity. In the temperature ranges of 170-240°C, the major products of this reaction were furan, furfuryl alcohol, and 2-methyl furan through a main reaction pathway of either decarbonylation or carbonyl group hydrogenation. While Pt nanoparticles with the size ranges of 1.5-7.1 exhibited strong structure-dependent selectivity, various supports loaded with only 1.9 nm Pt nanoparticles produced dominantly furan as a major product. Compared to the inert silica support, TiO(2) and Nb(2)O(5) facilitated an increase in the production of furfuryl alcohol via carbonyl group hydrogenation as a result of a charge transfer interaction between the Pt and the acidic surface of the oxides. The same trend was confirmed on 2-dimensional type catalysts, in which thin films of SiO(2), Al(2)O(3), TiO(2), Nb(2)O(5), and ZrO(2) were prepared as supports. When furfural hydrogenation was conducted (1 torr furfural, 100 torr H(2), and 659 torr He) over Pt nanoparticle monolayers deposited on oxide substrates, only TiO(2) was shown to increase the production of furfuryl alcohol, while other oxides produced furan.

High structure sensitivity of vapor-phase furfural decarbonylation/hydrogenation reaction network as a function of size and shape of Pt nanoparticles.

Vapor-phase transformations of furfural in H(2) over a series of Pt nanoparticles (NPs) with various particle sizes (1.5-7.1 nm size range) and shapes (rounded, cubes, octahedra) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on MCF-17 mesoporous silica were investigated at ambient pressure in the 443-513 K temperature range. Furan and furfuryl alcohol (FFA) were two primary products as a result of furfural decarbonylation and hydrogenation reactions, respectively. Under conditions of the study both reactions exhibited structure sensitivity evidenced by changes in product selectivities, turnover rates (TORs), and apparent activation energies (E(A)'s) with Pt particle size and shape. For instance, upon an increase in Pt particle size from 1.5 to 7.1 nm, the selectivity toward FFA increases from 1% to 66%, the TOR of FFA production increases from 1 × 10(-3) s(-1) to 7.6 × 10(-2) s(-1), and E(A) decreases from 104 kJ mol(-1) to 15 kJ mol(-1) (9.3 kPa furfural, 93 kPa H(2), 473 K). Conversely, under the same experimental conditions the decarbonylation reaction path is enhanced over smaller nanoparticles. The smallest NPs (1.5 nm) produced the highest selectivity (96%) and highest TOR values (8.8 × 10(-2) s(-1)) toward furan formation. The E(A) values for decarbonylation (∼62 kJ mol(-1)) was Pt particle size independent. Furan was further converted to propylene via a decarbonylation reaction, but also to dihydrofuran, tetrahydrofuran, and n-butanol in secondary reactions. Furfuryl alcohol was converted to mostly to 2-methylfuran.

Colloid chemistry of nanocatalysts: a molecular view.

Recent advances of a colloidal chemistry can offer great opportunities to fabricate and design nanocatalysts. Comprehensive understanding of a basic concept and theory of the colloidal synthetic chemistry facilitates to engineer elaborate nano-architectures such as bi- or multi-metallic, heterodimers, and core/shell. This colloidal solution technique not only enables to synthesize high surface mesoporous materials, but also provides a versatile tool to incorporate nanoparticles into mesoporous materials or onto substrates. For green chemistry, catalysis research has been pursued to design and fabricate a catalyst system that produces only one desired product (100% selectivity) at high turnover rates to reduce the production of undesirable wastes. Recent studies have shown that several molecular factors such as the surface structures, composition, and oxidation states affect the turnover frequency and reaction selectivity depending on the size, morphology, and composition of metal nanoparticles. Multipath reactions have been utilized to study the reaction selectivity as a function of size and shape of platinum nanoparticles. In the past, catalysts were evaluated and compared with characterizations before and after catalytic reaction. Much progress on in situ surface characterization techniques has permitted real-time monitoring of working catalysts under various conditions and provides molecular information during the reaction.

Large-scale synthesis of uniform and extremely small-sized iron oxide nanoparticles for high-resolution T1 magnetic resonance imaging contrast agents.

Uniform and extremely small-sized iron oxide nanoparticles (ESIONs) of < 4 nm were synthesized via the thermal decomposition of iron-oleate complex in the presence of oleyl alcohol. Oleyl alcohol lowered the reaction temperature by reducing iron-oleate complex, resulting in the production of small-sized nanoparticles. XRD pattern of 3 nm-sized nanoparticles revealed maghemite crystal structure. These nanoparticles exhibited very low magnetization derived from the spin-canting effect. The hydrophobic nanoparticles can be easily transformed to water-dispersible and biocompatible nanoparticles by capping with the poly(ethylene glycol)-derivatized phosphine oxide (PO-PEG) ligands. Toxic response was not observed with Fe concentration up to 100 µg/mL in MTT cell proliferation assay of POPEG-capped 3 nm-sized iron oxide nanoparticles. The 3 nm-sized nanoparticles exhibited a high r(1) relaxivity of 4.78 mM(-1) s(-1) and low r(2)/r(1) ratio of 6.12, demonstrating that ESIONs can be efficient T(1) contrast agents. The high r(1) relaxivities of ESIONs can be attributed to the large number of surface Fe(3+) ions with 5 unpaired valence electrons. In the in vivo T(1)-weighted magnetic resonance imaging (MRI), ESIONs showed longer circulation time than the clinically used gadolinium complex-based contrast agent, enabling high-resolution imaging. High-resolution blood pool MR imaging using ESIONs enabled clear observation of various blood vessels with sizes down to 0.2 mm. These results demonstrate the potential of ESIONs as T(1) MRI contrast agents in clinical settings.

In vitro cytotoxicity screening of water-dispersible metal oxide nanoparticles in human cell lines.

In this study, we present in vitro cytotoxicity of iron oxide (Fe(3)O(4)) and manganese oxide (MnO) using live/dead cell assay, lactate dehydrogenase assay, and reactive oxygen species detection with variation of the concentration of nanoparticles (5-500 microg/ml), incubation time (18-96 h), and different human cell lines (lung adenocarcinoma, breast cancer cells, and glioblastoma cells). The surface of nanoparticles is modified with polyethyleneglycol-derivatized phospholipid to enhance the biocompatibility, water-solubility, and stability under an aqueous media. While the cytotoxic effect was negligible for 18 h incubation even at highest concentration of 500 microg/ml, MnO nanoparticle represented higher level of toxicity than those of Fe(3)O(4) and the commercial medical contrast reagent, Feridex after 2 and 4 day incubation time. However, the cytotoxicity of Fe(3)O(4) is equivalent or better than Feridex based on the live/dead cell viability assay. The engineered MnO and Fe(3)O(4) exhibited excellent stability compared with Feridex for a prolonged incubation time.

Synthesis of uniform ferrimagnetic magnetite nanocubes.

We synthesized uniform ferrimagnetic magnetite nanocubes in the size range from 20 to 160 nm. The magnetic property of the nanocubes was characterized, and magnetic separation of the histidine-tagged protein was demonstrated.