Synthesis,Structure, Properties,and Applications of Bimetallic Nanoparticles of Noble Metals

Bimetallic nanoparticles of noble metals are of high interest in imaging, biomedical devices, including nanomedicine, and heterogeneous catalysis. Synthesis, properties, characterization, biological properties, and practical applicability of nanoparticles on the basis of platinum group metals and the coin metals Ag and Au are discussed, also in comparison with the corresponding monometallic nanoparticles. In addition to the parameters that are required to characterize monometallic nanoparticles (mainly size, size distribution, shape, crystallographic nature, surface functionalization, charge), further information is required for a full characterization of bimetallic nanoparticles. This concerns the overall elemental composition of a bimetallic nanoparticle population (ratio of the two metals) and the internal distribution of the elements in individual nanoparticles (e.g., the presence of homogeneous alloys, core–shell systems, and possible intermediate stages). It is also important to ensure that all particles are identical in terms of elemental composition, that is, that the homogeneity of the particle population is given. Macroscopic properties like light absorption, antibacterial effects, and catalytic activity depend on these properties. The currently available methods for a full characterization of bimetallic nanoparticles are discussed, and future developments in this field are outlined.     

Metal Nanocrystal‐Embedded Hollow Mesoporous TiO2 and ZrO2 Microspheres Prepared with Polystyrene Nanospheres as Carriers and Templates

Noble metal nanocrystals with different shapes and compositions are embedded in hollow mesoporous metal oxide microspheres through an ultrasonic aerosol spray. Polystyrene (PS) nanospheres are employed simultaneously as a hard template to create hollow interiors inside the oxide microspheres and as the carrier to bring pregrown metal nanocrystals, including Pd nanocubes, Au nanorods, and Au core/Pd shell nanorods, into the oxide microspheres. Calcination removes the PS template and causes the metal nanocrystals to adsorb on the inner surface of the hollow oxide microspheres. The catalytic performances of the Pd nanocube‐embedded TiO₂ and ZrO₂ microspheres are investigated using the reduction of 4‐nitrophenol as a model reaction. The presence of the mesopores in the oxide microspheres allows the reactant molecules to diffuse into the hollow interiors and subsequently interact with the Pd nanocubes. The embedding of the metal nanocrystals in the hollow oxide microspheres prevents the aggregation of the metal nanocrystals and reduces the loss of the catalyst during recycling. The Pd nanocube‐embedded ZrO₂ microspheres are found to exhibit a much higher catalytic activity, a much larger catalytic reaction rate, and a superior recyclability in comparison with a commercial Pd/C catalyst. This preparation approach could potentially be utilized to incorporate various types of mono‐ and multimetallic nanocrystals with different sizes, shapes, and compositions into hollow mesoporous oxide microspheres. Such a capability can facilitate the studies of the catalytic properties of various combinations of metal nanocrystals and metal oxide supports and therefore guide the design and creation of high‐performance catalysts.     

Pt Nanoparticles Sensitized Ordered Mesoporous WO3 Semiconductor: Gas Sensing Performance and Mechanism Study

In this study, a straightforward coassembly strategy is demonstrated to synthesize Pt sensitized mesoporous WO₃ with crystalline framework through the simultaneous coassembly of amphiphilic poly(ethylene oxide)‐b‐polystyrene, hydrophobic platinum precursors, and hydrophilic tungsten precursors. The obtained WO₃/Pt nanocomposites possess large pore size (≈13 nm), high surface area (128 m² g^?1), large pore volume (0.32 cm³ g^?1), and Pt nanoparticles (≈4 nm) in situ homogeneously distributed in mesopores, and they exhibit excellent catalytic sensing response to CO of low concentration at low working temperature with good sensitivity, ultrashort response‐recovery time (16 s/1 s), and high selectivity. In‐depth study reveals that besides the contribution from the fast diffusion of gaseous molecules and rich interfaces in mesoporous WO₃/Pt nanocomposites, the partially oxidized Pt nanoparticles that chemically and electronically sensitize the crystalline WO₃ matrix, dramatically enhance the sensitivity and selectivity.     

Ultrasensitive Detection of VOCs Using a High‐Resolution CuO/Cu2O/Ag Nanopattern Sensor

The p‐type semiconducting copper oxides (CuO and Cu₂O) are promising materials for gas sensors, owing to their characteristic oxygen adsorption properties and low operation temperature. In this study, the sensing performance of a CuO‐based chemiresistor is significantly enhanced by incorporating Ag nanoparticles on high‐resolution p‐type CuO/Cu₂O nanopattern channels. The high‐resolution CuO/Cu₂O/Ag nanochannel is fabricated using a unique top‐down nanolithographic approach. The gas response (ΔR/R_a) of the CuO/Cu₂O/Ag gas sensor increases by a maximum factor of 7.3 for various volatile organic compounds compared with a pristine CuO/Cu₂O gas sensor. The sensors exhibit remarkable sensitivity (ΔR/R_a = 8.04) at 125 parts per billion (ppb) for acetone analytes. As far as it is known, this is the highest sensitivity achieved for p‐type metal oxide semiconductor (MOS)‐based gas sensors compared to previous studies. Furthermore, the outstanding gas responses observed in this study are superior to the most of n‐type MOS‐based gas sensors. The high sensitivity of the sensor is attributed to i) the high resolution (≈30 nm), high aspect ratio (≈12), and ultrasmall grain boundaries (≈10 nm) of the CuO/Cu₂O nanopatterns and ii) the electronic sensitization and chemical sensitization effects induced by incorporating Ag nanoparticles on the CuO/Cu₂O channels.     

具有不凝结气体存在的凝结换热研究

介绍了不凝结气体对凝结换热的影响,提出了在自行复叠制冷系统中消除不凝结气体对凝结换热的影响的一种新方法,然后进行了有不凝性气体存在时的换热系数的计算及相关的试验研究。试验结果证明了新的方法使得自行复叠制冷系统中有不凝性气体存在时的凝结换热大大加强,换热系数已经接近或达到纯工质在相同状况下的换热系数。     

Deoxidation of Weld Metal When Welding with Self-Shielding Cored Wires

With self-shielding cored wires, high-quality and high-productivity welding can be carried out without an external shielding. In this case, porosity of welds is usually prevented by the major weakening of the oxygen activity in the weld metal by means of a strong deoxidation by titanium and aluminum. The weakening of the oxygen activity in the weld metal prevents the generation of carbon monoxide in the weld pool in the course of welding.

Alloying of the weld metal with titanium and aluminum, however, has an unfavorable influence on the quality of welded joints. An increased titanium content, i.e., above 0.3 wt%, produces a primarily martensitic microstructure with a toughness level below 20 J at 20°C. Also, the deoxidation by aluminum provides a little lower toughness at the expense of a less favorable microstructure. Welds with around 0.75% of Al and 0.1% of Ti show a ferritic/pearlitic microstructure with toughness of around 100 J at 20°C.

With some types of cored wires for surfacing, surfacing without an external shielding can produce high-quality surfacing welds. These are cored wires for surfacing of casting metals containing strong deoxidants (Ti, Al, etc.) and also cored wires for surfacing of wear-resistant materials with a high carbon content, i.e., above 2 wt%. The occurrence of porosity in wear-resistant surfacing welds is prevented by a strong deoxidation of the weld metal by carbon and by an increased partial pressure of carbon monoxide generated above the weld pool due to carbon combustion at elevated arc temperatures.     

Switchgrass as an alternate feedstock for power generation: an integrated environmental, energy and economic life-cycle assessment

An environmental biocomplexity analysis is done on the environmental, energy, economic and technological implications of using switchgrass (Panicum virgatum) to replace coal in power generation. We evaluate cost, environmental impact and net greenhouse gas emissions. In the analysis, alternatives for production and transport are considered. The analysis shows that the most effective technologies for switchgrass preparation are harvesting loose material for hauling and chopping and then compressing it into modules and transporting. The GHG emission mitigation is found to be substantial with the mitigation contribution under cofiring found to be greater per ton of switchgrass than for switchgrass fired alone. This paper also analyzes the implications of switchgrass use under alternative cofiring ratios, coal prices, hauling distances and per acre yields.     

Spontaneous Formation of Noble‐ and Heavy‐Metal‐Free Alloyed Semiconductor Quantum Rods for Efficient Photocatalysis

Quasi‐1D cadmium chalcogenide quantum rods (QRs) are benchmark semiconductor materials that are combined with noble metals to constitute QR heterostructures for efficient photocatalysis. However, the high toxicity of cadmium and cost of noble metals are the main obstacles to their widespread use. Herein, a facile colloidal synthetic approach is reported that leads to the spontaneous formation of cadmium‐free alloyed ZnS_xSe_1?x QRs from polydisperse ZnSe nanowires by alkylthiol etching. The obtained non‐noble‐metal ZnS_xSe_1?x QRs can not only be directly adopted as efficient photocatalysts for water oxidation, showing a striking oxygen evolution capability of 3000 µmol g^?1 h^?1, but also be utilized to prepare QR‐sensitized TiO₂ photoanodes which present enhanced photo‐electrochemical (PEC) activity. Density functional theory (DFT) simulations reveal that alloyed ZnS_xSe_1?x QRs have highly active Zn sites on the (100) surface and reduced energy barrier for oxygen evolution, which in turn, are beneficial to their outstanding photocatalytic and PEC activities.