High Surface Concentration Research Articles

Estimation of surface-level NO2 and O3 concentrations using TROPOMI data and machine learning over East Asia

In East Asia, air quality has been recognized as an important public health problem. In particular, the surface concentrations of air pollutants are closely related to human life. This study aims to develop models for estimating high spatial resolution surface concentrations of NO2 and O3 from TROPOspheric Monitoring Instrument (TROPOMI) data in East Asia. The machine learning was adopted by fusion of various satellite-based variables, numerical model-based meteorological variables, and land-use variables. Four machine learning approaches—Support Vector Regression (SVR), Random Forest (RF), Extreme Gradient Boost (XGB), and Light Gradient Boosting Machine (LGBM)—were evaluated and compared with Multiple Linear Regression (MLR) as a base statistical method. This study also modeled the NO2 and O3 concentrations over the ocean surface (i.e., land model for scheme 1 and ocean model for scheme 2). The estimated surface concentrations were validated through three cross-validation approaches (i.e., random, temporal, and spatial). The results showed that the NO2 model produced R2 of 0.63–0.70 and normalized root-mean-square-error (nRMSE) of 38.3–42.2% and the O3 model resulted in R2 of 0.65–0.78 and nRMSE of 19.6–24.7% for scheme 1. The indirect validation based on the stations near the coastline for scheme 2 showed slight decrease (~0.3–2.4%) in nRMSE when compared to scheme 1. The contributions of input variables to the models were analyzed based on SHapely Additive exPlanations (SHAP) values. The NO2 vertical column density among the TROPOMI-derived variables showed the largest contribution in both the NO2 and O3 models.

Self-assembly of N-heterocyclic carbenes on Au(111)

Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

Sm-MnOx catalysts for low-temperature selective catalytic reduction of NOx with NH3: Effect of precipitation agent

A series of Sm–Mn mixed oxide catalysts were prepared via precipitation using various precipitants, namely Na2CO3 (NH4)2CO3, and NH3·H2O, and evaluated for the selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Various characterisation techniques were used to determine the physicochemical properties of the catalysts, and it is found that their catalytic performance is greatly influenced by the nature of the precipitation agent used. It is found that Sm0.1Mn–Na2CO3 and Sm0.1Mn-(NH4)2CO3 exhibit superior catalytic performance in the SCR reaction to that of Sm0.1Mn–NH3·H2O due to an abundance of surface acid sites, high surface concentration of Mn4+, and high NO oxidation capacity. From in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analysis, we conclude that the Sm–Mn catalysts follow both Eley-Rideal and Langmuir–Hinshelwood mechanisms, and that the Eley-Rideal mechanism is dominant at elevated temperatures.

Factors triggering germination in plasma-activated cotton seeds: water imbibition vs. reactive species’ formation

Plasma technology has emerged as an efficient, simple, and eco-friendly method for activating seed germination processes. Most studies on this subject have focused on the morphological and wetting effects on the surface state of seeds and attributed the improvement in germination occasionally found to the induced hydrophilicity and a higher water imbibition rate. Recently, the involvement of reactive oxygen and nitrogen species in the germination process has also been highlighted. In this work, we study the effect of a very low-power cold atmospheric air-plasma treatment on the germination and sterilization of cotton seeds in normal (i.e., water abundant) and simulated drought conditions. Optical emission spectroscopy of the plasma has revealed the formation of different excited species of molecular nitrogen and oxygen, as well as OH and NO radicals that are deemed responsible for the chemical functionalization and slight morphological changes observed on the surfaces of cotton seeds. The physicochemical changes of the seed surface were analyzed by scanning electron microscopy, energy-dispersive x-ray spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), and x-ray photoemission spectroscopy. This latter technique has shown the formation of a high surface concentration of oxygenated (i.e. –CO x , –C–OH, –O x ) and oxinitrided (i.e. –NO x ) functional groups which disappear from the surface upon exposure to vapor or liquid water. This process must entail the diffusion of these species (together with that of potassium ions that were plasma segregated to the surface) into the interior of seeds. The fact that the water uptake capacity of seeds is not significantly modified by the plasma treatment suggests that the observed in-diffusion of active oxygen and nitrogen species is an important factor in the enhancement of germination capacity induced by plasma activation, particularly under conditions of water scarcity, where germination differences of more than 60% could be found between pristine and plasma-treated seeds. The analysis by FT-IR of the adsorption of deuterated water (D2O) molecules is proposed as a useful procedure to directly monitor water uptake/release processes from seed surfaces.

Identifying Drivers Behind Spatial Variability of Methane Concentrations in East Siberian Ponds

Waterbody methane emissions per area are negatively correlated with the size of the emitting waterbody. Thus, ponds, defined here as having an area smaller than 8 · 104m2, contribute out of proportion to the aquatic methane budget compared to the total area they cover and compared to other waterbodies. However, methane concentrations in and methane emissions from ponds show more spatial variability than larger waterbodies. We need to better understand this variability to improve upscaling estimates of freshwater methane emissions. In this regard, the Arctic permafrost landscape is an important region, which, besides carbon-rich soils, features a high pond density and is exposed to above-average climatic warming. We studied 41 polygonal-tundra ponds in the Lena River Delta, northeast Siberia. We collected water samples at different locations and depths in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the key properties of the ponds to identify drivers of surface water methane concentrations. The ponds can be categorized into three geomorphological types with distinct differences in drivers of methane concentrations: polygonal-center ponds, ice-wedge ponds and larger merged polygonal ponds. All ponds are supersaturated in methane, but ice-wedge ponds exhibit the highest surface water concentrations. We find that ice-wedge ponds feature a strong stratification due to consistently low bottom temperatures. This causes surface concentrations to mainly depend on wind speed and on the amount of methane that has accumulated in the hypolimnion. In polygonal-center ponds, high methane surface concentrations are mostly determined by a small water depth. Apart from the influence of water depth on mixing speed, water depth controls the overgrown fraction, the fraction of the pond covered by vascular plants. The plants provide labile substrate to the methane-producing microbes. This link can also be seen in merged polygonal ponds, which furthermore show the strongest dependence on area as well as an anticorrelation to energy input indicating that stratification influences the surface water methane concentrations in larger ponds. Overall, our findings underpin the strong variability of methane concentrations in ponds. No single driver could explain a significant part of the variability over all pond types suggesting that more complex upscaling methods such as process-based modeling are needed.

Ru-M (M = Fe or Co) Catalysts with high Ru surface concentration for Fischer-Tropsch synthesis

Ru-Co/Al2O3 and Ru-Fe/Al2O3 have been prepared by the reduction-deposition method. This approach favours the interaction between Ru and Fe or Co in the final catalysts, while increasing surface enrichment of Ru. Their performance for the Fischer-Tropsch Synthesis (FTS) has been evaluated in a fixed bed reactor. Ru-Co/Al2O3 displays the highest activity in the series, doubling the steady-state CO conversion rate of a Ru/Al2O3 catalyst with 3 times higher Ru loading. Ru-Fe/Al2O3 displays higher initial CO conversion and -CH2- productivity than Ru/Al2O3, but its activity declines during operation reaching lower conversion rates than Ru/Al2O3. In addition to CO2, Ru-Fe/Al2O3 produces oxygenated products, mainly alcohols. To a lesser extent, the formation of oxygenates over Ru-Co/Al2O3 and Ru/Al2O3 is also observed, and ascribed to the small crystallite size of the Ru particles. The evolution of CO2 during time on stream indicates that Fe is segregated to the surface of this catalyst during the FTS.

Methanol reforming by nanostructured Pd/Sm-doped ceria catalysts

Catalysts of 2 wt% Pd/Sm-doped Ceria (SDC) were prepared and evaluated for hydrogen production by methanol reforming. Three preparation methods were used: a one-step method in which formation of the SDC phase and Pd incorporation took place in a single citrate-complexation process (Method A); impregnation of pre-prepared SDC nanopowder supports using Pd(NO3)2 solution (Method B); and impregnation of pre-prepared SDC from H2PdCl4 solution (Method C). Both methanol conversion and hydrogen yield increased as: bare supports<<Method A < Method C < Method B, the latter achieving values of up to 97.4 % and 236.2 %, respectively. Pd was well dispersed as fine nanoparticles by Methods B and C, but larger particles were resolved for Method A. Method B resulted in a high surface concentration of Pd, further improving catalytic activity. Undesirable Cl was retained in samples of Method C. Method B resulted in catalysts with superior nanostructure, resulting in the best activity and CO2 selectivity.

Гальваномагнитные свойства в анизотропных слоистых пленках на основе халькогенидов висмута

In anisotropic layered films of a multicomponent n-Bi1.92In0.02Te2.85Se0.15 solid solution in strong magnetic fields from 2 to 14 T at low temperatures, quantum oscillations of magnetoresistance associated with surface states of electrons in 3D topological insulators were studied. From the analysis of the spectral distribution of the amplitudes of quantum oscillations of magnetoresistance, the main parameters of the Dirac fermion surface states are determined. A comparison of the results with the data obtained by the method of scanning tunnel spectroscopy was carried out. It is shown that a high surface concentration determines the contribution of the Dirac fermion surface states to the thermoelectric properties of n-Bi1.92In0.02Te2.85Se0.15.

Mechanistic insight into propane dehydrogenation into propylene over chromium (III) oxide by cluster approach and Density Functional Theory calculations

A preliminary study to provides insight into the kinetic and thermodynamic assessment of the reaction mechanism involved in the non-oxidative dehydrogenation (NOD) of propane to propylene over Cr2O3, using a density functional theory (DFT) approach, has been undertaken. The result obtained from the study presents the number of steps involved in the reaction and their thermodynamic conditions across different routes. The rate-determining step (RDS) and a feasible reaction pathway to promote propylene production were also identified. The results obtained from the study of the 6-steps reaction mechanism for dehydrogenation of propane into propylene identified the first hydrogen abstraction and hydrogen desorption to be endothermic. In contrast, other steps that include propane’s adsorption, hydrogen diffusion, and the second stage of hydrogen abstraction were identified as exothermic. The study of different reaction routes presented in the energy profiles confirms the Cr-O (S1, that is, the reaction pathway that activates the propane across the Cr-O site at the alpha or the terminal carbon of the propane) pathway to be the thermodynamically feasible pathway for the production of propylene. The first hydrogen abstraction step was identified as the potential rate-determining step for defining the rate of the propane dehydrogenation process. This study also unveils that the significant participation of Cr sites in the propane dehydrogenation process and how the Cr high surface concentration would hinder the desorption of propylene and thereby promote the production of undesired products due to the stronger affinity that exists between the propylene and Cr-Cr site, which makes it more stable on the surface. These findings thereby result in Cr-site substitution suggestion to prevent deep dehydrogenation in propane conversion to propylene. This insight would aid in improving the catalyst performance.

Synergistically enhanced heterogeneous activation of persulfate for aqueous carbamazepine degradation using Fe3O4@SBA-15

The exploration of low-cost, high-performance and stable catalytic materials for sulfate radical-based advanced oxidation processes (SR-AOPs) is of great importance. This study presents Fe3O4-wrapped SBA-15 mesoporous silica catalyst (Fe3O4@SBA-15) for persulfate (PS) activation. The Fe3O4@SBA-15 with an Fe3O4 to SBA-15 weight ratio of 3:1 exhibited an impressive carbamazepine (CBZ) removal efficiency of ~100% after 30 min of SR-AOP at an initial pH of 3.0, a temperature of 25 °C, an initial PS concentration of 300 mg L−1 and a catalyst concentration of 0.50 g L−1. The primary oxidizing species produced in the system were identified as SO4− and HO by electron paramagnetic resonance spectra and radical quenching experiments. Benefiting from the synergetic effects of improved Fe3O4 dispersion and enhanced adsorption of CBZ and PS by SBA-15, the as-obtained heterogeneous Fe3O4@SBA-15 catalysts offer large numbers of active sites for free radical generation and high surface concentrations of CBZ and PS for SR-AOPs, as verified by physicochemical characterization and Langmuir−Hinshelwood model analysis. In addition, the activity of Fe3O4@SBA-15 was maintained throughout six successive cycling tests. Various inorganic anions, including Cl−, NO3−, HCO3−, and CO32−, as well as organic material in natural water, exert a negative impact on the Fe3O4@SBA-15 catalyzed SR-AOPs and deserve special attention.

B and Ga Co-Doping in Epitaxial SiGe: Challenges and Opportunities

Contact resistances are inherent to any device connected to the outside world. Lowering their contribution is essential for high-performance applications. This is the case for advanced Si CMOS technologies, for which series resistance parasitics become the main performance detractor from the 14 nm node onwards [1-2]. For p-type fin and gate-all-around field effect transistors, focus is placed on Ti contacts to p-Si1-xGex source/drain (S/D) [3].Several non-equilibrium doping techniques have been explored to enhance active doping in S/D regions and decrease contact resistivities (ρc) down to ≤ 1x10-9 Ω.cm2. In most cases, low temperature doping processes are used to favor the incorporation of dopants and their activation [4]. Alternative dopants, with higher solid solubilities and advantageous segregation behaviors favoring the formation of dopant-enriched films at the metal-semiconductor interface, are also evaluated. Very low ρc values have e.g. been reported by combining Ga ion implantation in Si0.4Ge0.6 and ns laser annealing [5]; whereas in situ doping was optimized in Si1-xGex:B [6-9] and Si0.5Ge0.5:B:Ga, grown by chemical vapor deposition using metalorganic (MO) Ga precursors [4,10]. In this contribution, we highlight opportunities and challenges offered by this approach.The maximum solubility of Ga in Ge is more than 100x higher than that of B [11-12]. However, controlling the incorporation and distribution of Ga dopants in Si0.5Ge0.5(:B) is not straightforward. This relates to the tendency for Ga to segregate towards the surface, as reported in [4,10] and shown in Fig. 1(a). We rely on Si0.5Ge0.5:B:Ga co-doped epi layers in which bulk resistivity is lowered thanks to high B-doping concentrations, while contact resistivity minimization is expected from Ga segregation towards the contact region. It appears, however, that Ga surface doping concentrations significantly higher than 3E20 cm-3 are required to efficiently reduce ρc [5,13]. Targeting such high surface concentrations comes with the risk of Ga agglomeration into Ga-rich defects on S/D surfaces. Another detractor lies in the possible consumption of superficial Ga during native oxide removal preceding contact processing. Results presented in Fig. 1(b) suggest such a ~ 10x decrease in Ga concentration close to the S/D surface after native oxide removal. Systematic investigations are underway to clarify this aspect, which could lead us to reconsider our contacting scheme.Activating high levels of Ga dopants in in situ doped epi layers is an additional challenge. Preliminary results indicate that, in the conditions explored in this work, only a small fraction of the dopants incorporated in SiGe:Ga are active. This limitation may be attributed to Ga dopants clustering or to interactions with unwanted C incorporated during growth. Fig. 1(c) illustrates our effort in reducing C incorporation in SiGe:B:Ga by utilizing 2 different Ga MO precursors, namely MO1 and MO2, while keeping all other growth parameters the same. For this test, an undoped SiGe cap was used to separate contributions from adventitious C and C really incorporated in the SiGe:B:Ga layer. Ga profiles appear to be affected by the capping, with part of the Ga dopants migrating towards the top part of the stack. C impurities are on the other hand assumed to be frozen in their original position [14]. While the level of incorporated C is not significantly affected by the choice of precursor, a shorter Ga incorporation delay, materialized by blue arrows in Fig. 1(c), and a larger [Ga] / [C] ratio are extracted from the layer grown with MO2.

Surface ozone exceedances in Melbourne, Australia are shown to be under NOx control, as demonstrated using formaldehyde:NO2 and glyoxal:formaldehyde ratios

Two and a half years of multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of nitrogen dioxide (NO2), formaldehyde (HCHO) and glyoxal (CHOCHO) are presented alongside in-situ ozone (O3) measurements in Melbourne, Australia. Seasonal and diurnal cycles, vertical profiles and relationships with key meteorological variables are provided. NO2 and CHOCHO were found at highest concentration for low wind speeds implying that their sources were predominantly localised and anthropogenic. HCHO showed an exponential relationship with temperature and a strong wind direction dependence from the northern and eastern sectors, and therefore most likely originated from oxidation of biogenic volatile organic compounds (VOCs) from surrounding forested and rural areas. The glyoxal:formaldehyde ratio (Rgf), reported for the first time in Australia, was consistently high compared to values elsewhere in the world with a mean of 0.105 ± 0.0503 and tended to increase with increasing anthropogenic influence. The HCHO:NO2 ratio (Rfn) was used to characterise tropospheric ozone formation conditions. A strong relationship was found between high temperature, low Rgf, high Rfn and high ozone surface concentrations. Therefore, we propose that both Rgf and Rfn may be useful indicators of tropospheric ozone production regimes and concentrations. The Rfn showed that the vast majority of high ozone production episodes occurred under NOx-limited conditions, suggesting that surface ozone pollution events in Melbourne could be curtailed using NOx emission controls.

In situ exsolution of Rh nanoparticles on a perovskite oxide surface: Efficient Rh catalysts for Dry reforming

The catalytic activity of the Rh-exsolved Sr0.92Y0.08Ti2O3−δ- perovskite catalyst (SYTRh5) was examined for dry reforming of methane. The exsolution of the Rh nanoparticles over the SYT perovskite oxide surface was carried out under various reducing environments where the extent of Rh exsolution was significantly determined by the reduction time (4, 12, 24 h) and temperature (800, 900, 1,000 °C). STYRh5 catalysts treated at a longer reduction time and a higher reduction temperature revealed formation of larger metallic Rh nanoparticles on the perovskite oxide with higher surface concentration. For dry reforming activity, the SYTRh5 catalysts reduced at 900 and 1,000 °C for 24 h showed significantly higher methane conversion compared to others. The high catalytic performance of the SYTRh5 (900 and 1,000 °C, 24 h) catalysts was attributed to the high coke-resistance of the larger Rh-exsolved nanoparticles and stronger anchoring sites resulted from the exsolution process. Post-analysis TEM images exhibited limited carbon deposition and particle agglomeration of Rh over the SYTRh5 (900 and 1,000 °C, 24 h) catalysts. Lastly, in-situ H2S poisoning was conducted to examine the regeneration ability of SYTRh5. Although catalyst deactivation was observed, the catalytic activity of SYTRh5 (900 and 1,000 °C, 24 h) was completely recovered to the original level once the H2S flow was interrupted, indicating facile desorption of sulfur species from the Rh-exsolved nanoparticles.

Mesoscale Modeling of Exploiting Methane Hydrate by CO2 Replacement in Homogeneous Porous Media

In this study, the objective is to establish a general mesoscale model for replacement so as to precisely estimate the flux of methane hydrate dissociation and CO2 hydrate formation in the cage of hydrate. If homogeneous porous media is assumed, porosity (void fraction) of methane hydrate sediment can be obtained from silica packing in experiment. Based on considering the driving force of free energy in dissociation and formation processes, a new modeling of replacement process is established by considering the individual flux of hydrate surface at stable and unstable regions of CH4 hydrate. At stable CH4 and CO2 hydrates region, it provides a simple situation of discussing the recovery process which CO2 guest particle replaces CH4 in hydrate due to free energy difference. Here, the formation rate of CO2 hydrate is dominated by the dissociation rate of CH4. However, large amount of CH4 dissociated at the surface of hydrate at unstable region. The formation rate become dominated by the formation flux of CO2 hydrate itself. In addition, the high-pressure effect of accelerating the guest gas trapped in hydrate is also considered by applying kinetic theory of gases. By applying CFD method and unstructured grid, it is possible to consider momentum, concentration and thermal distributions in non-equilibrium state simultaneously. To compare with experimental results, flux of methane hydrate dissociation at high surface concentration in simulation are consistent with experiment.

Composite membranes from quaternized chitosan reinforced with surface-functionalized PVDF electrospun nanofibers for alkaline direct methanol fuel cells

In order to obtain a dual-functional nanofiber framework that could simultaneously serve as an ultra-strong reinforcing substrate and a fast ion transport medium, herein, silica coated poly(vinylidene fluoride) (SiO2@PVDF) electrospun nanofiber mat was firstly prepared using a universal polydopamine-assisted sol-gel method. After grafting with dense quaternary ammonium groups, quaternized SiO2@PVDF (QSiO2@PVDF) was yield. This mat with densely cationized surface can not only provide a reliable mechanical support for super-hydrophilic quaternized chitosan (QCS), but also significantly improve the ionic conductivity of the whole system. The resulting QCS impregnated membrane exhibits satisfactory wet strength of as high as 11.9 MPa while pure QCS is hardly self-supporting. Moreover, the surface high-concentration cationic groups can establish channel-like ion transport pathways with the aid of 1D continuous nanofibers, thus obtaining high ionic conductivity of 0.041 S cm−1 (80 °C), which is about 1.3 times higher than that of pure QCS. The single alkaline fuel cell using this membrane demonstrates a maximum power density of 98.7 mW cm−2. Therefore, this relatively independent design of mechanically support fiber core and high-concentration ion conduction surface provides a new methodology for fabricating high-performance anion exchange membranes.

Ni/La2O3 and Ni/MgO–La2O3 catalysts for the decomposition of NH3 into hydrogen

The decomposition of NH3 for hydrogen production was studied using Ni/La2O3 catalysts at varying compositions and temperatures prepared via surfactant-templated synthesis to elucidate the influence of catalyst active metal content, support composition and calcination temperature on the catalytic activity. The catalytic performance of all samples was studied between 300 and 600 °C under atmospheric pressure. The catalytic activity of the sample were as follows: 10Ni/La2O3-450 > 10Ni/La2O3-550 > 10Ni/La2O3-650 ≈ 10Ni/La2O3-750 ≈ 10Ni/La2O3-850. The excellent activity (100%) of 10Ni/La2O3-450 could be due to the high surface area, basicity strength and concentration of surface oxygen species of the catalyst as evidenced by BET, CO2-TPD and XPS. In addition, to adjust the activity of the catalyst support, the molar ratios of Mg and La were varied (1:1, 3:1, 5:1, 7:1 and 9:1). The 5Ni/5MgLa (5:1 M ratio) was found to be the most active (100%) relative to other Ni/MgLa formulations. Furthermore, the Ni content in the Ni/5MgLa sample was adjusted between 10 and 40 wt%. Increasing the Ni content of the catalysts increased NH3 conversion with the 40 wt% Ni formulation demonstrating complete NH3 conversion at 600 °C and a high gas hourly space velocities (GHSV) (30,000 mL∙h−1∙gcat−1).

Development of high surface area bulk W2N catalysts for hydrogen production from ammonia decomposition

High surface area tungsten nitride catalysts synthesized from ammonium meta-tungstate and employed as catalysts for ecofriendly H2 production from NH3. A series of tungsten nitride catalysts synthesized by using CiA (citric acid) as chelating agent with different molar ratio of W and CiA. The synthesized materials characterized using BET-surface area, X-ray diffraction, X-ray photoelectron spectroscopy and SEM techniques. The BET value of as-synthesized tungsten nitride was raised from 25 to 80 m2 g−1. The influence of amount of CiA in preparation on the catalyst's surface area was investigated. The catalyst performance measured within the desired range of temperature 300–600 °C. A pure phase of tungsten nitride was formed by this preparation method. The catalyst with the ratio of CiA/W = 3 exhibited the best catalytic performance. The increased activity of WN-31 catalyst was mainly due to increased surface area, decreased particle size and high surface concentration. The WN-31 catalyst showed stable performance during time on study for 25 h. These bulk tungsten-based materials are easy to synthesize and highly stable material in the reaction atmosphere.

Plasma Treatment As an Effective Way of Modifying Carbon-Based Electrocatalysts for an Enhanced Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is pivotal in renewable energy technologies such as in fuel cells and metal-air batteries. Precious metal-free electrochemical ORR is a critical component in designing cost-effective electrochemical energy conversion devices. In this report we demonstrate an efficient surface modification of carbon nanotubes (CNT) through one-step oxygen plasma irradiation that induces doping and charge redistribution around the doped heteroatom oxygen that promotes ORR activity. The defect sites generated owing to oxygen dopant in CNTs were confirmed by Raman spectra, X-ray photoelectron spectroscopy (XPS) surface composition and CHNO elemental analysis. The O-doped CNTs were thoroughly characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) measurements. Our research shows that plasma-treated single-walled carbon nanotubes (SWCNT) are more effective ORR catalyst compared to multi-walled carbon nanotubes (MWCNT) due to the inherent structure of SWCNT that can access more defects and surface functional groups than MWCNT.In the following study, plasma modification approach was implemented in the synthesis of a highly active robust Metal-Nitrogen-Carbon (M-N-C) catalysts through NH3-plasma pretreatment on high surface area black pearl carbon. Pre-treated FeNC/NH3-C contains Fe3C and Fe-Nx as catalytically active sites. XPS confirms the presence of a high surface concentration of pyrrolic to pyridinic nitrogen and ferrous sites on FeNC/NH3-C that promotes low oxygen reduction overpotential (0.80 V vs RHE in 0.5 M H2SO4 and 0.96 V vs RHE in 0.1 M KOH). Hence plasma activates the surface and incorporates nitrogen which forms stable Fe-N coordination for accelerating oxygen reduction. Importantly, FeNC/NH3-C shows lower peroxide yield at 0.6V/RHE (<20% in 0.5 M H2SO4 and <2 % in 0.1 M KOH) that can reduce the detrimental effect of H2O2 on the catalysts support and exhibits high stability, seen from thousands of potential cycling stability tests of FeNC/NH3-C in acid solutions. Thereby this plasma-based new synthetic approach can promote more active and stable non-precious metal catalysts.

Field study of atmospheric boundary layer observation in a hilly Gobi Desert region and comparison with the CALMET/CALPUFF model

Abstract A field campaign consisting of atmospheric boundary layer (ABL) observations and tracer experiments was conducted in September 2016 in a hilly region of the Gobi Desert in northwest China. High-frequency wind shears were observed between 70 and 200 m based on both 100-meter tower data and wind profiles. The ABL observations indicated that sub-layers of different atmospheric stability present within the ABL weakened the vertical transfer of momentum and tracer gas, resulting in low wind speed and high surface concentration. The CALMET model showed thermal heterogeneity to some extent in the temperature fields and mixing height distributions, and large deviations in the friction velocity between the model and observations were noted. The comparison of the CALPUFF model with the tracer measurements indicated a general tendency toward under-prediction of the concentration in the far distance because of wind shears and vertical thermal discontinuity. The results of the SCIPUFF model showed the same trend as those of CALPUFF, and comparisons demonstrated a good consistency of the two models.

Conformal doping strategy for fin structures: tailoring of dopant profile through multiple monolayer doping and capping layer control

Monolayer doping (MLD) has attracted attention as a next-generation doping method because of its great potential for conformal doping of non-planar nanostructures. However, fundamental limitations are identified in conformal doping of fin structures using conventional MLD process, and fin structures doped with conventional MLD presents a poor dose conformality (surface concentration ratio between the side-wall and the top of the fin) of 0.32. The poor dose conformality is mainly caused by the hydrogen-terminated silicon surface formed differently depending on the orientation of each face of the fin structure and the capping layer deposited with low step coverage on the fin structure. As a strategy to overcome the limitations without increasing process complexity, multiple-MLD for dose compensation and capping layer thickness control for adjusting the dopant diffusivity are proposed. These methods are found to be effective in tailoring doping profiles on each face of the fin structure with a fine dose controllability from about 70% to 200%. Finally, it is demonstrated that the proposed approach allows conformal doping of the fin structure with a high surface concentration of 5 × 1020 atoms cm−3 and an excellent dose conformality of 0.98.