Decrease In Active Surface Area Research Articles

Investigating the effects of deformation-induced densification on the constitutive behavior and corrosion resistance of Al–Zn–Mg powder metallurgy alloy during hot deformation

The electrochemical behavior is most strongly influenced by deformation, which may either accelerate or decelerate failure during service. Therefore, in the present study, the Al–5.6Zn–2Mg alloy fabricated by powder metallurgy was subjected to hot deformation at various temperatures (300–500 °C) and strain rates (0.5–0.005 s−1) on a hydraulic press, and then, the effect of deformation-induced densification on corrosion behavior was studied using electrochemical corrosion tests. Flow stress is mathematically modeled using the strain-compensated Arrhenius model. The model developed was reliable to predict, with an average absolute relative error (AARE) of 3.16% and an R2 value of 0.98. The potentiodynamic polarization results showed that as the degree of porosity in a sintered sample is reduced, there was a decrease in the corrosion current density and an increase in the pitting potential. Al–5.6Zn–2Mg corrosion resistance significantly improved with deformation temperature, whereas strain rates decrease it, due to a decrease in pore content. Icorr of 2.4982 µA/cm2 and Epit of 0.876 V have been found for the highly deformed preforms under 500 °C and 0.005 s−1 strain rate condition. EIS results showed a high contact resistance of 2659 Ohm·cm2 and high hardness value of 113.12 Hv for the preform deformed under 500 °C and 0.005 s−1 strain rate. The reduction in porosity, decrease in active surface area after the deformation process, densification behavior, and hardness are all related to the electrochemical behavior. Pitting corrosion was observed in deformed preforms as a corrosion mechanism; the dynamic recovery feature is observed from the flow behavior.

Multimodal Synchrotron X-Ray Tools for Studying Spinel Oxide Nanocrystals-Based Mg Cathodes

Magnesium battery has huge potential to meet the future needs of large-scale energy storage due to the high abundance of Mg element, high volumetric and gravimetric capacities, and improved safety compared to Li metal battery. The complex solid-electrolyte interface (SEI) formed on Mg anode surface and slow diffusion rate of magnesium ion in cathode bulk are two critical challenges to achieve high capacity and energy density of Mg battery. Nanostructured spinel oxides with well-defined morphology and crystalline structures are promising candidates for Mg battery cathodes, but their dynamic evolutions of chemical composition and electronic/geometric structure during charging and discharging are still on the early stage, which hinders understanding of the interaction and degradation mechanisms on spinel oxide-based cathodes. Herein, in-situ/or ex-situ soft X-ray absorption spectroscopy (sXAS) and small/wide-angle X-ray scattering (SAXS/WAXS) to track the changes of structure and surface/bulk chemistry of spinel oxide nanocrystals during Mg intercalation/deintercalation processes at both local and macroscopical levels. The Mg0.5Fe2.5O4 (MFO) nanocrystals were tested in a Swagelok cell with an electrolyte of 0.3 M Mg(TFSI)2 and 0.15 M MgCl2 at room temperature. An initial discharge capacity of 187 mAh g-1 was achieved and 83% capacity was retained after 20 cycles. sXAS results reveal that Fe3+/Fe3+ redox is highly reversible during charge/discharge process, and MgF2 slowly formed on the surface as well. In-situ WAXS shows that rocksalt phases of FeO and MgO were both observed during discharge process, and the former one quickly disappeared and the latter one slowly converted to pristine phase during charge process. Moreover, MFO with small size (~5 nm) delivered double specific capacity but poorer cyclability compared to large size (~18 nm), which is ascribed to the nanocrystal aggregation and decrease of active surface area and formation of interfacial crystal boundary based on in-situ SAXS and ex-situ STEM. More structural features of shape and crystallinity degree of MFO were also investigated and correlated to their battery performance. This work highlight the powerful role of multimodal in-situ synchrotron X-ray techniques in understanding the structure-performance of spinel oxide nanocrystals-based Mg cathodes. Figure 1

Catalytic Stability Studies Employing Dedicated Model Catalysts.

Long-term stability of heterogeneous catalysts is an omnipresent and pressing concern in industrial processes. Catalysts with high activity and selectivity can be searched for by high-throughput screening methods based maybe on educated guesses provided by ab initio thermodynamics or scaling relations. However, high-throughput screening is not feasible and is hardly able to identify long-term stable catalyst so that a rational and knowledge-driven approach is called for to identify potentially stable and active catalysts. Unfortunately, our current microscopic understanding on stability issues is quite poor. We propose that this gap in knowledge can be at least partly closed by investigating dedicated model catalyst materials with well-defined morphology that allow for a tight link to theory and the application of standard characterization methods. This topic is highly interdisciplinary, combining sophisticated inorganic synthesis with catalysis research, surface chemistry, and powerful theoretical modeling. In this Account, we focus on the stability issues of Deacon catalysts (RuO2 and CeO2-based materials) for recovering Cl2 from HCl by aerobic oxidation and how to deepen our microscopic insight into the underlying processes. The main stability problems under harsh Deacon reaction conditions concomitant with a substantial loss in activity arise from deep chlorination of the catalyst, leaching of volatile chlorides and oxychlorides, and decrease in active surface area by particle sintering. In general, powder materials with undefined particle shape are not well suited for examining catalyst stability, because changes in the morphology are difficult to recognize, for instance, by electron microscopy. Rather, we focus here on model materials with well-defined starting morphologies, including electrospun nanofibers, shape-controlled nanoparticles, and well-defined ultrathin crystalline layers. CeO2 is able to stabilize shape-controlled particles, exposing a single facet orientation so that comparing activity and stability studies can reveal structure sensitive properties. We develop a quasi-steady-state kinetic approach that allows us to model the catalyst chlorination as a function of temperature and gas feed composition. For the case of pure CeO2 nanocubes, this simple approach predicts chlorination to be efficiently suppressed by addition of little amounts of water in the reaction feed or by keeping the catalyst at higher temperature. Both process parameters have great impact on the actual reactor design. Thermal stabilization of CeO2 by intermixing Zr has been known in automotive exhaust catalysis for decades, but this does not necessarily imply also chemical stabilization of CeO2 against bulk-chlorination since Zr can readily form volatile ZrCl4 and may quickly lose its stabilizing effect. Nevertheless, with model experiments the stabilizing effect of Zr in the Deacon process over mixed CexZr1-xO2 nanorods is clearly evidenced. Even higher stability can be accomplished with ultrathin CeO2 coatings on preformed ZrO2 particles, demonstrating the great promise of atomic layer deposition (ALD) in catalysis synthesis.

Influence of Phosphoric Acid Modification on Catalytic Properties of γ-χ Al2O3 Catalysts for Dehydration of Ethanol to Diethyl Ether

In this present work, diethyl ether, which is currently served as promising alternative fuel for diesel engines, was produced via catalytic dehydration of ethanol over H3PO4-modified g-c Al2O3 catalysts. The impact of H3PO4 addition on catalytic performance and characteristics of catalysts was investigated. While catalytic dehydration of ethanol was performed in a fixed-bed microreactor at the temperature ranging from 200ºC to 400ºC under atmospheric pressure, catalyst characterization was conducted by inductively coupled plasma (ICP), X-ray diffraction (XRD), N2 physisorption, temperature-programmed desorption of ammonia (NH3-TPD) and thermogravimetric (TG) analysis. The results showed that although the H3PO4 addition tended to decrease surface area of catalyst resulting in the reduction of ethanol conversion, the Al2O3 containing 5 wt% of phosphorus (5P/Al2O3) was the most suitable catalyst for the catalytic dehydration of ethanol to diethyl ether since it exhibited the highest catalytic ability regarding diethyl ether yield and the quantity of coke formation as well as it had similar long-term stability to conventional Al2O3 catalyst. The NH3-TPD profiles of catalysts revealed that catalysts containing more weak acidity sites were preferred for dehydration of ethanol into diethyl ether and the adequate promotion of H3PO4 would lower the amount of medium surface acidity with increasing catalyst weak surface acidity. Nevertheless, when the excessive amount of H3PO4 was introduced, it caused the destruction of catalysts structure, which resulted in the catalyst incapability due to the decrease in active surface area and pore enlargement.

Effect of High Current Density on the Performance in a Molten Carbonate Fuel Cell

The effect of current density on the cell performance was investigated in a coin type molten carbonate fuel cell. The cell temperature was 700oC under atmospheric condition. The three different of current densities, 150, 300, and 450 mA/cm2 were used. The cell performance with respect to the current density was analysed via steady-state polarization, step-chronopotentiometry, and impedance methods. The electrochemical analyses revealed that cell performance rapidly degraded by increasing current density. This may result from decrease of active surface area caused by electrolyte loss. This work reports that life of MCFC strongly depends on current density.

Mercury(II) Trace Detection Using a Glassy Carbon Electrode Functionalized by Chemically Prepared Gold Nanoparticles. Influence of Coating Process on Surface Reactivity and Analytical Performances

An electrochemical sensor dedicated to Hg(II) trace detection was elaborated based on a gold nanoparticles (AuNPs) modified glassy carbon (GC) electrode. AuNPs were prepared using the Turkevich method and deposited on GC by drop casting. Different protocols including suspension filtration and evaporation temperature control were tested and their influence both on coating morphology and electrochemical activity assessed. From structural characterization, it can be concluded that neither the filtration step nor the drop evaporating temperature have a significant impact on coating morphology at the mesoscale level. However, regarding to the electrochemical activity of the functionalized electrodes, results showed that when some heterogeneities due to (AuNPs) aggregation were present in the coating, the electrochemical activity was reduced. Contrary to what was observed in our previous studies dealing with electrodeposited AuNPs, cycling an AuNPs-GC electrode in H2SO4 lead in all cases to a decrease in active surface area and in a higher density of surface defects, thus revealing a higher surface reactivity of chemically-synthesized AuNPs. The electrochemical activation procedure was found to influence the analytical performances of the functionalized electrode with respect to Hg(II) assay in the picomolar range, but not in the nanomolar range, as a consequence of a saturated concentration effect. A linear concentration range was obtained between 2 and 12 pM with a normalized sensitivity of 0.296 μA pM-1 min-1 using square wave anodic stripping voltammetry (SWASV) as the detection mode. A limit of detection (LOD) down to 1 pM was reached.

Kinetic study on in-situ and cooling char combustion in a two-step reaction analyzer

Cooling char, which is prepared in an inert atmosphere and then reacted with oxygen after cooling, has been widely used in the kinetic study of char combustion. However, there is no cooling process in commercial combustion systems and thus the impacts of the cooling process on char reactivity remain unclear. To illustrate this question, a two-step reaction analyzer was developed to study the combustion kinetics of in-situ char, which was produced at a constant high temperature in an inert atmosphere and then directly reacted with oxygen at different temperatures. Two types of Chinese coal, Shenhua sub-bituminous coal and Yangquan anthracite, were selected as samples. The results showed that the reaction rate of in-situ char was about 1.1–1.25 times of that of rapid cooling char, suggesting that the cooling process has a significant effect on char reactivity. Then the combustion rate and structure of chars cooled at different cooling rates were investigated to determine the reason for char deactivation during the cooling process. The cooling char reactivity decreased with the decreasing cooling rate, but the specific surface area did not decreased considerably. This result revealed that char deactivation during the cooling process was independent of specific surface area. As the cooling rate decreased, the oxygen chemisorption capacity (active surface area) of cooling char decreased for Shenhua coal, indicating that the observed char deactivation during the cooling process might be caused by the decrease of active surface area. In addition, Yangquan anthracite, as a high rank coal, was less sensitive to the cooling process due to its higher carbon crystallites and provision of fewer active sites for oxidation.

Life cycle assessment and economic analysis of a low concentrating photovoltaic system

ABSTRACTMany new photovoltaic (PV) applications, such as the concentrating PV (CPV) systems, are appearing on the market. The main characteristic of CPV systems is to concentrate sunlight on a receiver by means of optical devices and to decrease the solar cells area required. A low CPV (LCPV) system allows optimizing the PV effect with high increase of generated electric power as well as decrease of active surface area. In this paper, an economic analysis and a life cycle assessment (LCA) study of a particular LCPV scheme is presented and its environmental impacts are compared with those of a PV traditional system. The LCA study was performed with the software tool SimaPro 8.0.2, using the Econinvent 3.1 database. A functional unit of 1 kWh of electricity produced was chosen. Carbon Footprint, Ecological Footprint and ReCiPe 2008 were the methods used to assess the environmental impacts of the LCPV plant compared with a corresponding traditional system. All the methods demonstrated the environmental convenience of the LCPV system. The innovative system allowed saving 16.9% of CO2 equivalent in comparison with the traditional PV plant. The environmental impacts saving was 17% in terms of Ecological Footprint, and, finally, 15.8% with the ReCiPe method.

Electrophoretic deposition improves catalytic performance of Co3O4 nanoparticles for oxygen reduction/oxygen evolution reactions

Electrophoretic deposition was found to improve the activity of cobalt oxide nanoparticulate thin films for oxygen reduction/evolution in spite of an apparent decrease in active surface area.

Electrocatalytic Oxygen Evolution on Electrochemically Deposited Cobalt Oxide Films: Comparison with Thermally Deposited Films and Effect of Thermal Treatment

Electrocatalytic cobalt oxide layers have been prepared on nickel substrates using thermal decomposition and electrochemical deposition methods. Importantly, it was confirmed that the electrochemical deposition method could be applied to nickel foam substrates for use in zero-gap alkaline water electrolysis cells. The oxide layers produced were then investigated for their activity towards the oxygen evolution reaction in 30 wt % KOH solution and found to be superior compared with the uncoated nickel substrate. Layers produced by both methods had similar electrochemical behaviour, provided that the layers were annealed at temperatures ≥350 ∘C. This thermal treatment was required to mechanically stabilise the electrochemically deposited cobalt oxide layer. Due to this finding, the effect of annealing temperature was investigated for the electrochemically deposited layer, and it was found that the overpotential for oxygen evolution increased with increasing annealing temperature. Using cyclic voltammetry and impedance spectroscopy, it is concluded that the decrease in performance with increasing annealing temperature is largely caused by the corresponding decrease in active surface area. However, for annealing temperatures ≥400 ∘C, additional resistances are introduced that cause lower performance. The impedance data suggest that these additional resistances are caused by either a decrease in the conductivity of the cobalt oxide layer itself, or the formation of a passivating-like nickel oxide layer between the active cobalt oxide and the nickel substrate, or both. The resistances’ dependence on potential suggests that they originate from a semi-conducting material and these additional resistances ultimately give rise to non-linear Tafel behaviour.

Physical, Electrochemical, and Thermal Properties of Granulated Natural Graphite as Anodes for Li-Ion Batteries

Two different types of granulated graphites were synthesized by blending and kneading of natural graphite with pitch followed by sintering methods. The electrochemical performances of granulated graphites were investigated as anode materials for use in Li-ion batteries. The blending type granulated graphite possesses a large amount of cavities and voids, while the kneading type granulated graphite has a relatively compact microstructure, which is responsible for a high tap density. Both granulated graphites show improved the initial coulombic efficiencies as a result of decrease of surface area by the granulations. In particular, the kneading type granulated graphite exhibits an excellent rate-capability without significant capacity loss. In addition, the thermal stabilities of both granulated graphites were also improved, which could be attributed to the decrease of active surface area due to pitch coating.

Liquid phase acetonitrile hydrogenation to ethylamine over a highly active and selective Ni–Co–B amorphous alloy catalyst

The ultrafine Ni–Co–B amorphous alloys with Co/(Co + Ni) molar ratio ( χ Co) varying from 0 to 1 was prepared by chemical reduction of mixed Ni 2+ and Co 2+ ions with BH 4 − in aqueous solution. During liquid phase acetonitrile hydrogenation to ethylamine, the specific activity ( R m) and the intrinsic activity (TON) of the Ni–Co–B catalyst first increased and then decreased with the increase of χ Co from 0 to 1. The maximum activity was obtained at χ Co = 0.5; the value of the activity was nearly twice as that of the Ni–B or the Co–B catalyst. Treatment of the Ni–Co–B catalyst at 873 K resulted in an abrupt decrease in the activity due both to a decrease in active surface area and, especially, to the crystallization and the decomposition of the Ni–Co–B amorphous alloy. The selectivity to ethylamine increased rapidly with χ Co and then remained constant at χ Co ≥ 0.5. The maximum yield of ethylamine could reach 93%, showing a good potential for industrial applications. According to kinetic studies and results of various characterization methods, such as ICP, XRD, EXAFS, XPS, SAED, TEM, DSC, TPD, and hydrogen chemisorption, the correlation of the catalytic performance to both the structural and the electronic characteristics was discussed briefly. The activation of the C N and/or C N bonds, the promotion on the hydrogen adsorption, and the inhibition on the ethylamine adsorption were the decisive factors responsible for the excellent activity and selectivity of the Ni–Co–B catalyst.

Activity of PtRuMeO x (Me = W, Mo or V) catalysts towards methanol oxidation and their characterization

The activity of unsupported ternary PtRuMeO x (Me=W, Mo, V) high surface area catalysts towards methanol oxidation under fuel cell relevant conditions, at constant potential and elevated temperature of 60 °C, was investigated by the thin-film electrode method. CO-stripping experiments for actual surface area detection, using differential electrochemical mass spectrometry (DEMS) to evaluate the amount of adsorbed CO, show that the electrochemically active area of the particle surface is significantly smaller than the total particle surface area determined by N 2 BET measurements, indicating that part of their surface consists of MeO x . The specific surface activity of the catalysts towards methanol oxidation at 60 °C, normalized to the current density per square centimeter of electrochemically active noble metal surface, decreases in the order PtRuVO x >PtRuMoO x >PtRu>PtRuWO x =PtRu (E-TEK). Apparently, the surface oxide leads to an acceleration of the methanol oxidation reaction, which is, however, partly compensated by a decrease in active surface area.

Thermal analysis of the oxidation of natural graphite: isothermal kinetic studies

The oxidation kinetics of natural graphite particles (2–40μm average particle size) with a flake-like morphology were investigated at 933, 982 and 1033K. The reaction rate in air increased with increase in temperature and with a decrease in particle size of natural graphite. The activation energy, derived from the classical Arrhenius relationship, was 188±2.2kJ/mol, in good agreement with published results. The activation energies of the natural graphite did not show any systematic trend with particle size. The rate constant, normalized for the area of active sites, is independent of the particle size and Brunauer–Emmet–Teller (BET) surface area, which strongly suggests that the edge sites play a significant role in the oxidation kinetics. This observation is consistent with conclusions reported in the literature that the oxidation kinetics of carbonaceous materials decreases with a decrease in active surface area.

Effect of carbon deposition on porosity and reactivity of a lignite char

Carbon deposition from the cracking of methane into the pores of a lignite char occurs at a significant rate at temperatures between 815 and 855 °C. The maximum amount of carbon deposited is much less than the amount of open pore volume within the char which is potentially available to accommodate carbon. Removal of the inorganic impurities from the char by acid washing significantly reduces the extent of carbon deposition. Carbon deposition reduces surface area and open pore volume of the char and accessibility of methane into the pore structure. Deposition of carbon has a large and lasting adverse effect on the subsequent reactivity of the char to air. This has been attributed both to a decrease in active surface area, and deactivation of catalytic inorganic impurities due to coating with carbon. Deposited carbon is much less reactive to air than the lignite char. Results indicate that in order to maximize subsequent char reactivity to oxidizing gases, carbon deposition from volatiles during the conversion of coal to char should be kept to a minimum.