Interspersing CeOx Clusters to the Pt-TiO2 Interfaces for Catalytic Promotion of TiO2-Supported Pt Nanoparticles.

We propose an interface-engineered oxide-supported Pt nanoparticle-based catalyst with improved low-temperature activity toward CO oxidation. By wet-impregnating 1 wt % Ce on TiO2, we synthesized hybrid oxide support of CeOx-TiO2, in which dense CeOx clusters formed on the surface of TiO2. Then, the Pt/CeOx-TiO2 catalyst was synthesized by impregnating 2 wt % Pt on the CeOx-TiO2 supporting oxide. Pt-CeOx-TiO2 triphase interfaces were eventually formed upon impregnation of Pt on CeOx-TiO2. The Pt-CeOx-TiO2 interfaces open up the interface-mediated Mars-van Krevelen CO oxidation pathway, thus providing additional interfacial reaction sites for CO oxidation. Consequently, the specific reaction rate of Pt/CeOx-TiO2 for CO oxidation was increased by 3.2 times compared with that of Pt/TiO2 at 140 °C. Our results demonstrate a widely applicable and straightforward method of catalytic activation of the interfaces between metal nanoparticles and supporting oxides, which enabled fine-tuning of the catalytic performance of oxide-supported metal nanoparticle classes of heterogeneous catalysts.

Sub-micro droplet reactors for green synthesis of Li3VO4 anode materials in lithium ion batteries.

The conventional solid-state reaction suffers from low diffusivity, high energy consumption, and uncontrolled morphology. These limitations are competed by the presence of water in solution route reaction. Herein, based on concept of combining above methods, we report a facile solid-state reaction conducted in water vapor at low temperature along with calcium doping for modifying lithium vanadate as anode material for lithium-ion batteries. The optimized material, delivers a superior specific capacity of 543.1, 477.1, and 337.2 mAh g-1 after 200 and 1000 cycles at current densities of 100, 1000 and 4000 mA g-1, respectively, which is attributed to the contribution of pseudocapacitance. In this work, we also use experimental and theoretical calculation to demonstrate that the enhancement of doped lithium vanadate is attributed to particles confinement of droplets in water vapor along with the surface and structure variation of calcium doping effect.

Direct Growth of Highly Conductive Large-Area Stretchable Graphene.

The direct synthesis of inherently defect-free, large-area graphene on flexible substrates is a key technology for soft electronic devices. In the present work, in situ plasma-assisted thermal chemical vapor deposition is implemented in order to synthesize 4 in. diameter high-quality graphene directly on 10 nm thick Ti-buffered substrates at 100 °C. The in situ synthesized monolayer graphene displays outstanding stretching properties coupled with low sheet resistance. Further improved mechanical and electronic performances are achieved by the in situ multi-stacking of graphene. The four-layered graphene multi-stack is shown to display an ultralow resistance of ≈6 Ω sq-1, which is consistently maintained during the harsh repeat stretching tests and is assisted by self-p-doping under ambient conditions. Graphene-field effect transistors fabricated on polydimethylsiloxane substrates reveal an unprecedented hole mobility of ≈21 000 cm2 V-1 s-1 at a gate voltage of -4 V, irrespective of the channel length, which is consistently maintained during the repeat stretching test of 5000 cycles at 140% parallel strain.

How Rh surface breaks CO2 molecules under ambient pressure.

Utilization of carbon dioxide (CO2) molecules leads to increased interest in the sustainable synthesis of methane (CH4) or methanol (CH3OH). The representative reaction intermediate consisting of a carbonyl or formate group determines yields of the fuel source during catalytic reactions. However, their selective initial surface reaction processes have been assumed without a fundamental understanding at the molecular level. Here, we report direct observations of spontaneous CO2 dissociation over the model rhodium (Rh) catalyst at 0.1 mbar CO2. The linear geometry of CO2 gas molecules turns into a chemically active bent-structure at the interface, which allows non-uniform charge transfers between chemisorbed CO2 and surface Rh atoms. By combining scanning tunneling microscopy, X-ray photoelectron spectroscopy at near-ambient pressure, and computational calculations, we reveal strong evidence for chemical bond cleavage of O‒CO* with ordered intermediates structure formation of (2 × 2)-CO on an atomically flat Rh(111) surface at room temperature.

In Situ Engineering of Pd Nanosponge Armored with Graphene Dots Using Br- toward High-Performance and Stable Electrocatalyst for the Hydrogen Evolution Reaction.

In this study, we report a facile synthetic pathway to three-dimensional (3D) Pd nanosponge-shaped networks wrapped by graphene dots (Pd@G-NSs), which show superior electrocatalytic activity toward the hydrogen evolution reaction (HER) and exhibited excellent long-term stability in acidic media. Pd@G-NSs were synthesized by simply mixing Pd precursors, reducing agent, carbon dots (Cdots), and Br- ion at 30 °C. Experimental results and density functional theory (DFT) calculations suggested that the Br- ions played an essential role in accelerating the exfoliation of Cdot, supplying graphene layers, which could wrap the nanosponge-shaped Pd and finally form Pd@G-NS. In the absence of the Br- ions, only aggregated Pd nanoparticles (NPs) were formed and randomly mixed with Cdots. The resultant Pd@G-NS exhibited a high electrochemically active surface area and accelerated charge transport characteristics, leading to its superior electrocatalytic activity toward the HER in acidic media. The HER overpotential of Pd@G-NS was 32 mV at 10 mA cm-2, and the Tafel slope was 33 mV dec-1. Furthermore, the unique Pd@G-NS catalyst showed long-term stability for over 3000 cycles in acidic media as well, owing to the protection of Pd nanosponges by graphene dot wrapping. The overall HER performance of the Pd@G-NS catalyst exceeded that of commercial Pt/C.

Oxygen activation on the interface between Pt nanoparticles and mesoporous defective TiO2 during CO oxidation.

Platinum-based heterogeneous catalysts are mostly used in various commercial chemical processes because of their high catalytic activity, influenced by the metal/oxide interaction. To design rational catalysts with high performance, it is crucial to understand the relationship between the metal-oxide interface and the reaction pathway. Here, we investigate the role of oxygen defect sites in the reaction mechanism for CO oxidation using Pt nanoparticles supported on mesoporous TiO2 catalysts with oxygen defects. We show an intrinsic correlation between the catalytic reactivity and the local properties of titania with oxygen defects (i.e., Ti3+ sites). In situ infrared spectroscopy observations of the Pt/mesoporous TiO2-x catalyst indicate that an oxygen molecule bond can be activated at the perimeter between the Pt and an oxygen vacancy in TiO2 by neighboring CO molecules on the Pt surface before CO oxidation begins. The proposed reaction pathways for O2 activation at the Pt/TiO2-x interface based on density functional theory confirm our experimental findings. We suggest that this provides valuable insight into the intrinsic origin of the metal/support interaction influenced by the presence of oxygen vacancies, which clarifies the pivotal role played by the support.

Efficient Sn Recovery from SnO2 by Alkane (CxHy=2x+2, 0 ≤ x ≤ 4) Reduction.

We study the mechanism of alkane reduction of SnO2 for efficient low-temperature recovery of Sn from SnO2. Based on thermodynamic simulation results, we comparatively analyze the reduction behavior and the efficiency of SnO2 reduction by H2 and alkanes (CxHy=2x+2, 0 ≤ x ≤ 4). We found that alkanes (n·CxHy) with the higher nx generally complete the reduction of SnO2 (T100) at the lower temperature. Moreover, the T100 of the SnO2 reduction by alkanes (n·CxHy) was decreased from the T100 of pure hydrogen with the same amount of hydrogen atoms (n·Hy). We found that the concentration of a gas phase product mixture, the amount of the produced solid carbon, and the T100 complementary vary as a function of the nx and ny, the total amount of carbon and hydrogen atoms in the reducing gas phase molecules. Our results demonstrate a viability of the low temperature reduction method of SnO2 by alkanes for efficient recovery of Sn from SnO2, which can be applied for Sn recovery from Sn containing industrial wastes or Sn ores with economic value added that is held by the co-produced H2.

Unravelling inherent electrocatalysis of mixed-conducting oxide activated by metal nanoparticle for fuel cell electrodes.

Highly active metal nanoparticles are desired to serve in high-temperature electrocatalysis, for example, in solid oxide electrochemical cells. Unfortunately, the low thermal stability of nanosized particles and the sophisticated interface requirement for electrode structures to support concurrent ionic and electronic transport make it hard to identify the exact catalytic role of nanoparticles embedded within complex electrode architectures. Here we present an accurate analysis of the reactivity of oxide electrodes boosted by metal nanoparticles, where all particles participate in the reaction. Monodisperse particles (Pt, Pd, Au and Co), 10 nm in size and stable at high temperature (more than 600 °C), are uniformly distributed onto mixed-conducting oxide electrodes as a model electrochemical cell via self-assembled nanopatterning. We identify how the metal catalysts activate hydrogen electrooxidation on the ceria-based electrode surface and quantify how rapidly the reaction rate increases with proper choice of metal. These results suggest an ideal electrode design for high-temperature electrochemical applications.

Defect-Free Graphene Synthesized Directly at 150 °C via Chemical Vapor Deposition with No Transfer.

Direct graphene synthesis on substrates via chemical vapor deposition (CVD) is an attractive approach for manufacturing flexible electronic devices. The temperature for graphene synthesis must be below ∼200 °C to prevent substrate deformation while fabricating flexible devices on plastic substrates. Herein, we report a process whereby defect-free graphene is directly synthesized on a variety of substrates via the introduction of an ultrathin Ti catalytic layer, due to the strong affinity of Ti to carbon. Ti with a thickness of 10 nm was naturally oxidized by exposure to air before and after the graphene synthesis, and the various functions of neither the substrates nor the graphene were influenced. This report offers experimental evidence of high-quality graphene synthesis on Ti-coated substrates at 150 °C via CVD. The proposed methodology was applied to the fabrication of flexible and transparent thin-film capacitors with top electrodes of high-quality graphene.

Design of Reduction Process of SnO2 by CH4 for Efficient Sn Recovery.

We design a novel method for the CH4 reduction of SnO2 for the efficient recovery of Sn from SnO2 through a study combining theory and experiment. The atomic-level process of CH4-SnO2 interaction and temperature-dependent reduction behavior of SnO2 were studied with a combination of a multi-scale computational method of thermodynamic simulations and density functional theory (DFT) calculations. We found that CH4 was a highly efficient and a versatile reducing agent, as the total reducing power of CH4 originates from the carbon and hydrogen of CH4, which sequentially reduce SnO2. Moreover, as a result of the CH4 reduction of SnO2, a mixture of CO and H2 was produced as a gas-phase product (syngas). The relative molar ratio of the produced gas-phase product was controllable by the reduction temperature and the amount of supplied CH4. The laboratory-scale experimental study confirmed that CH4 actively reduces SnO2, producing 99.34% high-purity Sn and H2 and CO. Our results present a novel method for an efficient, green, and economical recycling strategy for Sn with economic value added that is held by the co-produced clean energy source (syngas).

Understanding the atomic-level process of CO-adsorption-driven surface segregation of Pd in (AuPd)147 bimetallic nanoparticles.

When the elements that compose bimetallic catalysts interact asymmetrically with reaction feedstock, the surface concentration of the bimetallic catalysts and the morphology of the reaction center evolve dynamically as a function of environmental factors such as the partial pressure of the triggering molecule. Relevant experimental and theoretical findings of the dynamic structural evolution of bimetallic catalysts under the reaction conditions are emerging, thus enabling the design of more consistent, reliable, and efficient bimetallic catalysts. In an initial attempt to provide an atomic-level understanding of the adsorption-induced structural evolution of bimetallic nanoparticles (NPs) under CO oxidation conditions, we used density functional theory to study the details of CO-adsorption-driven Pd surface segregation in (AuPd)147 bimetallic NPs. The strong CO affinity of Pd provides a driving force for Pd surface segregation. We found that the vertex site of the NP becomes a gateway for the initial Pd-Au swapping and the subsequent formation of an internal vacancy. This self-generated internal vacancy easily diffuses inside the NP and activates Pd-Au swapping pathways in the (100) NP facet. Our results reveal how the surface and internal concentrations of bimetallic NPs respond immediately to changes in the reaction conditions. Our findings should aid in the rational design of highly active and versatile bimetallic catalysts by considering the environmental factors that systematically affect the structure of bimetallic catalysts under the reaction conditions.

Interface engineering for a rational design of poison-free bimetallic CO oxidation catalysts.

We use density functional theory calculations of Pt@Cu core@shell nanoparticles (NPs) to design bifunctional poison-free CO oxidation catalysts. By calculating the adsorption chemistry under CO oxidation conditions, we find that the Pt@Cu NPs will be active for CO oxidation with resistance to CO-poisoning. The CO oxidation pathway at the Pt-Cu interface is determined on the Pt NP covered with a full- and partial-shell of Cu. The exposed portion of the Pt core preferentially binds CO and the Cu shell binds O2, supplying oxygen for the reaction. The Pt-Cu interface provides CO-oxidation sites that are not poisoned by either CO or O2. Additional computational screening shows that this separation of reactant binding sites is possible for several other core@shell NPs. Our results indicate that the metal-metal interface within a single NP can be optimized for design of bifunctional catalytic systems with improved performance.

Evenly transferred single-layered graphene membrane assisted by strong substrate adhesion.

We explored the transfer of a single-layered graphene membrane assisted by substrate adhesion. A relatively larger adhesion force was measured on the SiO2 substrate compared with its van der Waals contribution, which is expected to result from the additional contribution of the chemical bonding force. Density functional theory calculations verified that the strong adhesion force was indeed accompanied by chemical bonding. The transfer of single-layered graphene and subsequent deposition of the dielectric layer were best performed on the SiO2 substrate exhibiting a larger adhesion force. This study suggests the selection and/or modification of the underlying substrate for proper transfer of graphene as well as other 2D materials similar to graphene.