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Dilute palladium-in-gold alloys have potential as efficient oxidation catalysts; controlling the Pd surface distribution is critical.
We report Isolated Pd atoms supported on high surface area MnO 2 , prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O 2 , 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO 2 in facilitating redox turnovers.
Optoelectronic tweezers (OET) offer a means for parallel trapping and dynamic manipulation of micro-scale particles using low-intensity light. Such capabilities can facilitate the formation of bulk materials with a precisely tailored microstructure. Here, we report the use of OET to vertically align, trap, and reposition sheets of graphene oxide (GO) in liquids, paving the way for textured and patterned graphene macroassemblies that could offer superior performance for applications in energy storage, catalysis, and electronic devices. Trapping can be achieved with low-power light from inexpensive digital projectors and diode lasers, making it simple for users to create and apply patterns while avoiding undesirable photothermal heating effects. To give users a quantitative idea of trap stiffness, we also present a theoretical framework for predicting the maximum achievable speed of a GO platelet in an OET trap.
In this work, p-channel 4H-SiC MOSFETs were fabricated and analyzed at high temperature. It is demonstrated that nitridation of the gate oxide enables enhancement mode operation in these devices. Nitrogen incorporation at the 4H-SiC/SiO2 interface by nitric oxide annealing reduces the interface trap density energetically located in the lower half of the 4H-SiC bandgap, resulting in viable high temperature p-channel devices. In the 27–300 °C temperature range, the threshold voltage decreases with increasing temperature, consistent with the reduction of occupied interface traps at higher temperatures. The hole channel mobility is weakly temperature dependent under strong inversion conditions. Hall measurements support that above the threshold voltage, the mobility is limited by surface roughness scattering. In weaker inversion, the channel conductivity is limited by interface hole trapping and Coulomb scattering. In addition, high temperature bias stress measurements confirm a temperature activated hole trapping under negative gate bias, which requires further investigations.
Lytic polysaccharide monooxygenases (LPMOs) are a group of recently discovered enzymes that play important roles in the decomposition of recalcitrant polysaccharides. Here, we report the biochemical, structural, and computational characterization of an LPMO from the white-rot fungus Heterobasidion irregulare (HiLPMO9B). This enzyme oxidizes cellulose at the C1 carbon of glycosidic linkages. The crystal structure of HiLPMO9B was determined at 2.1 A resolution using X-ray crystallography. Unlike the majority of the currently available C1-specific LPMO structures, the HiLPMO9B structure contains an extended L2 loop, connecting ..beta..-strands ..beta..2 and ..beta..3 of the ..beta..-sandwich structure. Molecular dynamics (MD) simulations suggest roles for both aromatic and acidic residues in the substrate binding of HiLPMO9B, with the main contribution from the residues located on the extended region of the L2 loop (Tyr20) and the LC loop (Asp205, Tyr207, and Glu210). Asp205 and Glu210 were found to be involved in the hydrogen bonding with the hydroxyl group of the C6 carbon of glucose moieties directly or via a water molecule. Two different binding orientations were observed over the course of the MD simulations. In each orientation, the active-site copper of this LPMO preferentially skewed toward the pyranose C1 of the glycosidic linkage over the targeted glycosidic bond. This study provides additional insight into cellulose binding by C1-specific LPMOs, giving a molecular-level picture of active site substrate interactions.
Lanthanum strontium cobaltite (LSCo) is considered as a good candidate cathode contact material for solid oxide fuel cells, due to high electrical conductivity. However, LSCo has a very large coefficient of thermal expansion (CTE) than the cells and metallic interconnects. As a result, poor mechanical stability is expected during thermal cycling. To minimize the CTE mismatch, we investigate a composite approach involving mixing LSCo with an inert material of low CTE, such as mullite at volume fractions from 0.1 to 0.4. Composite’s CTE shows a decreasing trend with increasing mullite volume fractions, and is consistent with model predictions. X-ray powder diffraction analysis of sintered LSCo/mullite composites exhibits no presence of other phases for samples aged for 500h at 800oC, indicating chemical compatibility. Electrical conductivity by a 4-pt method shows a decreasing trend with increasing mullite content. Contact strength of as-sintered and thermally cycled samples show that only the composite with 0.4 volume fraction has a measurable strength; the other composites have no strength. Overall, the composite approach is demonstrated in the LSCo/mullite system to lower the CTE and hence achieve thermal cycle stability. The addition of the inert phase to the LSCo matrix; however, also reduces the electrical conductivity.
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In this paper, the application of the linear least squares (LLS) methodology allows for quantitative determination of variation in material composition with depth. LLS fits were applied to decompose and enhance the interpretation of spectra obtained by Auger electron spectroscopy during depth profiles of oxidized plutonium surfaces. By means of the LLS algorithm, chemical state assignments of the Pu P 1 VV/O 45 VV, O KLL, and C KLL Auger transitions were determined and the existence of a subsurface oxycarbide layer was identified, with confirmation of the oxide and metal components provided from comparison to previous measurements of standard samples.
Here, cuprous oxide on copper foil electrodes prepared via electrochemical deposition and thermal annealing are investigated towards CO 2 electrochemical reduction at low overpotential. The thickness of the electrochemical deposited Cu 2 O was controlled by varying the constant-current deposition time. The surface morphology and roughness were examined with SEM and CV respectively. The electrode fabricated by cuprous oxide deposited for 20 min demonstrated the best faradic efficiency (7.02%) and specific activity (0.123 mA/cm 2 ) towards format/formic acid formation at -0.5 V vs. RHE in CO 2 saturated 0.5 M K 2 CO 3 among studied samples.
Pt/TiO 2 as electrocatalyst was found to significantly increase the rate of the membrane chemical degradation in a polymer electrolyte membrane (PEM) fuel cell. The increased degradation was proved to be due to TiO x (TiO 2 or Ti 4 O 7 ), which is widely recognized as a promising corrosion-resistant electrocatalyst support. The membrane degradation (thinning) appears to be preferentially in the side facing anode electrode. Migrated Ti species as a result of TiO x dissolution during fuel cell operation in an acidic environment was quantified by electron probe microanalysis (EPMA). Total fluorine inventory loss of the membrane strongly depends on the quantity of migrated Ti species (likely Ti 3+ and/or Ti 4+ ). The membrane chemical degradation induced by TiO x is proposed to be due to dissolved ionic Ti species reacting with H 2 O 2 through a Fenton reaction. TiO x is determined to be inviable as electrocatalyst support for PEM fuel cells, and future research is suggested to be directed toward alternative oxides with similar corrosion-resistance.
Siloxane, a common contaminant present in biogas, is known for adverse effects on cogeneration prime movers. In this study, the siloxane deposition products and mechanism in the solid oxide fuel cell nickel-yttria stabilized zirconia (Ni-YSZ) anode are investigated analytically and experimentally. An SOFC with Ni-YSZ anode and pure Ni/YSZ pellets were exposed to a simulated biogas-reformate fuel with octamethylcyclotetrasiloxane (D4) contamination at 750 °C. The electrochemical characterization results show that the SOFCs performance degradation caused by D4 contamination is irreversible. Morphology and XRD results illustrate that silicon and carbon deposition can both be detected in the anode and pellets. Graphite, SiC and SiO2 are all possible products based on the results of XRD test. Here, according to the formation of graphite and SiC, the new mechanism suggests that carbon is also an essential factor in siloxane contamination of Ni-YSZ anodes besides silicon, which can be explained by the catalytic and electrochemical analysis.
When a metal oxide surface is immersed in aqueous solution, it has the ability to bind, orient, and order interfacial water, affecting both chemical and physical interactions with the surface. Structured interfacial water thus possesses time-averaged, spatially varying polarization charge and potential that are comparable to those arising due to ion accumulation. It is well established that interfacial water structure propagates from the surface into bulk solution. Here, we show that interfacial water structure also propagates laterally, with important consequences. The constant pH molecular dynamics was used to impose a pH difference between opposite faces of a model goethite (a-FeOOH) nanoparticle and quantify water polarization charge on intervening faces. We find that the structure of water on one face is strongly affected by the structure on nearby surfaces, revealing the importance of long-range lateral hydrogen bonding networks with implications for particle aggregation, oriented attachment, and processes such as dissolution and growth.
Abstract Kroupaite (IMA 2017-031), ideally KPb0.5[(UO2)8O4(OH)10]·10H2O, is a new uranyl-oxide hydroxylhydrate mineral found underground in the Svornost mine, Jáchymov, Czechia. Electron-probe micro-analysis (WDS) provided the empirical formula (K1.28Na0.07)Σ1.35(Pb0.23Cu0.14Ca0.05Bi0.03Co0.02Al0.01)Σ0.48 [(UO2)7.90(SO4)0.04O4.04(OH)10.00]·10H2O, on the basis of 40 O atoms apfu. Sheets in the crystal structure of kroupaite adopt the fourmarierite anion topology, and therefore kroupaite belongs to the schoepite-family of minerals with related structures differing in the interlayer composition and arrangement, and charge of the sheets. Uptake of dangerous radionuclides (90Sr or 135Cs) into the structure of kroupaite and other uranyl-oxide hydroxy-hydrate is evaluated based on crystal-chemical considerations and Voronoi-Dirichlet polyhedra measures. These calculations show the importance of these phases for the safe disposal of nuclear waste.
Solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC) have received great interest due to their highly effective reversibility as power generation and H2 production system without releasing any greenhouse gases into environment. The LSCF electrode exhibits a higher structural and performance stability under both SOFC and SOEC operation due to its mixed ionic and electronic conductivity, and there is no immediate delamination taking place during the initial several hundred hours operation. However, the LSCF based air electrode still presents significant performance degradation (with the increased resistance) over the prolonged operation, such as over 1000 hours of operation under SOFC and SOEC. The influence factors for the cell’s performance and stability need to be optimized to improve the power generation for SOFC and H2 production for SOEC. The effects of operational temperature on the performance and durability for both SOFC and SOEC are electrochemical operation dependent. The performance and performance durability for the first 1500h were currently studied under optimized operational temperature for reversible SOFC/SOEC.
The generation of structural defects in metal oxide catalysts offers a potential pathway to improve performance. Herein, we investigated the effect of thermal hydrogenation and low-temperature plasma treatments on mixed SiO 2 /TiO 2 materials. Hydrogenation at 500 °C resulted in the reduction of the material to produce Ti 3+ in the bulk TiO 2 . In contrast, low temperature plasma treatment for 10 or 20 min generated surface Ti 3+ species via the removal of oxygen on both the neat and hydrogenated material. Assessing the photocatalytic activity of the materials demonstrated a 40–130% increase in the rate of formic acid oxidation after plasma treatment. A strong relationship between the Ti 3+ content and catalyst activity was established, although a change in the Si–Ti interaction after plasma treating of the neat SiO 2 /TiO 2 material was found to limit performance, and suggests that performance is not determined solely by the presence of Ti 3+ .
It has been widely observed around the world that the frequency and intensity of new particle formation (NPF) events are reduced during periods of high relative humidity (RH). The current study focuses on how RH affects the formation of highly oxidized molecules (HOMs), which are key components of NPF and initial growth caused by oxidized organics. The ozonolysis of α -pinene, limonene, and Δ 3 -carene, with and without OH scavengers, were carried out under low NO x conditions under a range of RH (from ~3 % to ~92 %) in a temperature-controlled flow tube to generate secondary organic aerosol (SOA). A Scanning Mobility Particle Sizer (SMPS) was used to measure the size distribution of generated particles, and a novel transverse ionization chemical ionization inlet with a high-resolution time-of-fight mass spectrometer detected HOMs. A major finding from this work is that neither the detected HOMs nor their abundance changed significantly with RH, which indicates that the detected HOMs must be formed from water-independent pathways. In fact, the distinguished OH - and O 3 -derived peroxy radicals ( RO 2 ), HOM monomers, and HOM dimers could mostly be explained by the autoxidation of RO 2 followed by bimolecular reactions with other RO 2 or hydroperoxy radicals ( HO 2 ), rather than from a water-influenced pathway like through the formation of a stabilized Criegee intermediate (sCI). However, as RH increased from ~3 % to ~92 %, the total SOA number concentrations decreased by a factor of 2–3 while SOA mass concentrations remained relatively constant. These observations show that, while high RH appears to inhibit NPF as evident by the decreasing number concentration, this reduction is not caused by a decrease in RO 2 -derived HOM formation. Possible explanations for these phenomena were discussed.