Effect Of Alkali Cations Research Articles

Synthesis parameters and formulation of metakaolin based geopolymer matrix composites for high‐temperature applications (1150°C)

AbstractGeopolymer matrix composites (GMCs) are actively being developed for high‐temperature applications. In this work, we investigated different composite preparation parameters, such as the elaboration conditions (time, temperature and pressure) and composite dimensions (number of folds and surface area). In addition, we studied the effect of alkali cations (NaK or K), on metakaolin based geopolymer matrix composition, and textile type (N312, N610) of the composites on fiber–matrix interactions in high‐temperature geopolymer composites and their flexural behavior. The process we ultimately selected was as follows: an 8.5‐fold composite of surface area 100 cm2 was pressed for 2 h at 120°C. After thermal treatment at 1150°C, composites containing the N312 textile with B2O3 showed greater heterogeneity, viscous flow and lower flexural strength (7 MPa) due to fiber–matrix interactions. In contrast, above 1150°C, composites with the N610 textile had no fiber–matrix interactions and showed greater flexural strength (51 MPa). The specific flexural strength correlated well with the overmodifying element ratio based on the composite composition. In fact, the presence of viscous flow suggested that boron and some aluminum atoms could act as network modifiers. Thus, the two ratios were or depending on the presence of viscous flow.

Element-dependent effects of alkali cations on nitrate reduction to ammonia

Catalytic conversion of nitrate (NO3−) pollutants into ammonia (NH3) offers a sustainable and promising route for both wastewater treatment and NH3 synthesis. Alkali cations are prevalent in nitrate solutions, but their roles beyond charge balance in catalytic NO3− conversion have been generally ignored. Herein, we report the promotion effect of K+ cations in KNO3 solution for NO3− reduction over a TiO2-supported Ni single-atom catalyst (Ni1/TiO2). For photocatalytic NO3− reduction reaction, Ni1/TiO2 exhibited a 1.9-fold NH3 yield rate with nearly 100% selectivity in KNO3 solution relative to that in NaNO3 solution. Mechanistic studies reveal that the K+ cations from KNO3 gradually bonded with the surface of Ni1/TiO2, in situ forming a K-O-Ni moiety during reaction, whereas the Na+ ions were unable to interact with the catalyst in NaNO3 solution. The charge accumulation on the Ni sites induced by the incorporation of K atom promoted the adsorption and activation of NO3−. Furthermore, the K-O-Ni moiety facilitated the multiple proton-electron coupling of NO3− into NH3 by stabilizing the intermediates.

Stabilization of FAPbI3 with multifunctional alkali-functionalized polymer.

The defects located at the interfaces and grain boundaries (GBs) of perovskite films are detrimental to the photovoltaic performance and stability of perovskite solar cells. Manipulating the perovskite crystallization process and tailoring the interfaces with molecular passivators are mainly effective strategies to mitigate performance loss and instability. Herein we report a new strategy to manipulate the crystallization process of FAPbI3 -rich perovskite by incorporating a small amount of alkali functionalized polymers into the antisolvent solution. The synergic effects of alkali cations and polyacrylic acid anion effectively passivate the defects on the surface and GBs of perovskite films. As a result, the rubidium (Rb)-functionalized polyacrylic acid significantly improves the power conversion efficiency of FAPbI3 perovskite solar cells to approaching 25% and reduces the risk of lead ion (Pb2+ ) leakage via the strong interaction between C = O bond and Pb2+ . In addition, the unencapsulated device shows enhanced operational stability, retaining 80% of its initial efficiency after 500h operation at maximum power point under one-sun illumination. This article is protected by copyright. All rights reserved.

Insight into alkali cation effect on CO2 electroreduction catalyzed by monodispersed Cu-N-C under applied potential

Atomically dispersed copper and nitrogen-doped carbon (Cu-N-C) materials are promising electrodriven CO2 reduction (CO2RR) catalysts. A comprehensive mechanistic understanding of Cu-N-C towards systematic improvement, however, is hampered by the complexity of electrode-electrolyte interface around Cu. Here, we adopted an electric double layer model to investigate the impact of alkali metal cations on the two-electron CO2RR catalyzed by Cu-N4-C under applied potential. The grand canonical density functional theory calculations show that, at U=−1.2 V vs. SHE, hydrated Na+ ions near the surface facilitate formation of bent CO2− bonding with Cu; with an increasingly negative potential, the electrosorption of CO2 (Cu+CO2+e−→Cu-CO2−) instead of the formation of COOH becomes the presumable rate determining step for Na+-aided CO formation. Further, a possible Cu(I) may be vital for the adsorption of anionic COOH. Our study demonstrates the crucial role of alkali metal ion in the early stage of CO2RR on Cu-N4-C and the importance of explicit consideration of the applied potential in simulation for a better understanding of the reaction mechanism.

Effect of Alkali Cation on Performance of Alkali-Activated Slag Mortar in Cold Environments

Alkali-activated slag (AAS) is a promising cementing material for winter construction due to its continuous hydration at sub-zero temperature. In order to obtain a higher mechanical strength of AAS mortar in winter construction, the most efficient alkali activator should be selected. Potassium silicate and sodium silicate as alkaline activators of AAS mortar possess a high strength and hydration rate at ordinary temperature. Potassium silicate and sodium silicate as alkali activators showed different properties because of different alkali cations, so the effect of alkali cation on the performance of AAS mortar was studied at sub-zero temperature. The mechanical properties of potassium-silicate-activated AAS (K-AAS) mortar and sodium-silicate-activated AAS (N-AAS) mortar were thoroughly compared at the ambient temperatures of −10 °C, 0 °C, and 20 °C. The compressive and flexural strength of K-AAS mortar was increased by 130.4% and 72.3% at the age of 1 day and increased by 49% and 33.7% at the age of 28 days at the ambient temperature of −10 °C compared with N-AAS mortar. In order to reveal the mechanism behind the influence of different alkali cations on the mechanical properties of AAS mortar, the hydration heat, hydration products, and pore structures of AAS were characterized. The hydration heat exhibited that the rate of heat release and the cumulative heat release of K-AAS are higher than N-AAS, indicating the higher reaction degree of K-AAS. DTG/TG and ATR-FTIR spectra showed that K-AAS generated more gel product C(-A)-S-H compared with N-AAS at the same curing temperature. The MIP results exhibited that the porosity of K-AAS was lower than N-AAS. Finally, the mechanism explanation of the effect of alkali cation on the performance of AAS mortar was proposed. To sum up, potassium silicate should be selected as an alkali activator of AAS in winter construction.

Enhanced water-induced effects enabled by alkali-stabilized Pd-OHx species for oxidation of benzyl alcohol

The water promotion effects, where water can provide a solution-mediated reaction pathway in various heterogeneous chemical catalysis, have been presented and attracted wide attention recently, yet, the rational design of catalysts with a certain ability of enhancing water-induced reaction process is full of challenges and difficulties. Here, we show that by incorporating alkali (Na, K) cations as an electronic and/or structural promoter into Pd/rGO-ZnCr2O4 (rGO, reduced graphene oxide), the obtained Pd(Na)/rGO-ZnCr2O4 as a representative example demonstrates an outstanding benzyl alcohol oxidation activity in the Pickering emulsion system in comparison to the alkali-free counterpart. The response experiments of water injection confirm the enhanced activity, and the Na-modified catalyst can further enhance the promotion effects of water on the reaction. The effects of alkali cations for Pd nanoparticles are identified and deciphered by a series of experimental characterizations (XPS, in situ CO-DRIFTS, and CO-TPR coupled with MS), showing that there is abundant −OH on the surface of the catalyst, which is stabilized by the formation of Pd−OHx. The alkali-stabilized Pd−OHx is helpful to enhance the water-induced reaction process. According to the results of in situ Raman as well as UV-vis absorption spectra, the Na-modulated Pd(Na)/rGO-ZnCr2O4 enables the beneficial characteristics for distorting the benzyl alcohol structure and enhancing the adsorption of benzyl alcohol. Further, the mechanism for enhanced water promotion effects is rationally proposed. The strategy of alkali cations-modified catalysts can provide a new direction to effectively enhance the chemical reaction involving small molecule water.

Unravelling the effect of alkali cations and halide anions on the de-hydrogenation properties of ammine zinc borohydrides

This work sheds light on the role of the alkali cations, halide anion substitution as well as borohydride anion vacancies on the de-hydrogenation properties of ammine zinc borohydrides. A new liquid ammonia-based synthesis approach employed in this study enabled direct solvothermal synthesis of several novel and previously reported ammine zinc borohydride compounds such as AxZn(BH4)2+x(NH3)2 for x = 0; 1 and M = Li; Na; K. The experimental results are supported by first principle calculations and molecular dynamics techniques for the interpretation of inherently complex hydrogen release mechanisms. We demonstrate that the salt metathesis reaction heavily relied upon in literature for synthesis of these compounds can result in preferential halide anion substitution of certain borohydride sites in the structure which have significant consequences for hydrogen storage properties. Conclusions of this work is applicable not only to zinc system but to any ammine metal borohydride system.

Influence of Alkalis on Natural Carbonation of Limestone Calcined Clay Cement Pastes

Vulnerability to atmospheric carbonation is one of the major durability concerns for limestone calcined clay cement (LC3) concrete due to its relatively low overall alkalinity. In this study, the natural carbonation behaviors of ternary ordinary Portland cement-metakaolin-limestone (OPC-MK-LS) blends containing various sulfate salts (i.e., anhydrous CaSO4, Na2SO4, and K2SO4) are studied, with the aim of revealing the influence of alkali cations (Na+, K+). Detailed analyses on the hydrated phase assemblage, composition, microstructure, and pore structure of LC3 pastes prior to and post indoor carbonation are conducted. The results show that the incorporation of sulfate salts accelerates the setting and strength gain of LC3 pastes, likely through enhancement of ettringite formation, but undermines its later age strength achievement due to the deleterious effect of alkali cations (Na+, K+) on late age OPC hydration. The carbonation resistance of LC3 systems is considerably undermined, particularly with the incorporation of Na2SO4 or K2SO4 salts, due to the simultaneous pore coarsening effect and reduced CO2-binding capacity. The carbonation-induced phase and microstructural alterations of LC3 pastes are discussed and compared with those of reference OPC pastes.

Effect of Electrolyte Cations on Organic Electrosynthesis: The Case of Adiponitrile Electrochemical Production

Electrolyte ions have a profound impact on the reaction environment of electrochemical systems and can be key drivers in determining the reaction rate and selectivity of electro-organic reactions. We combine experimental and computational approaches to understand the individual effect of the size and concentration of supporting alkali cations, as well as their synergies with other electrolyte ions on the electrosynthesis of adiponitrile (ADN). The size of supporting alkali cations influences the surface charge density, availability of water molecules, and stability of reaction intermediates. Larger alkali cations can help limit hydrogen evolution and the early protonation of intermediates by lowering the availability of water molecules in the near electrode region. A selectivity of 93% towards ADN was achieved at −20 mA cm−2 in electrolytes containing cesium phosphate salts, ethylenediaminetetraacetic acid, and tetraalkylammonium ions (TAA ions). Electrolytes containing only supporting phosphate salts promote the early hydrogenation of intermediate species leading to low ADN selectivities (i.e., <10%). However, the combined effect of alkali cations and selectivity-directing ions (i.e., TAA ions) is essential in the enhancement of ADN synthesis. The insights gained in this study provide guidelines for the design of aqueous electrolytes that improve selectivity and limit hydrogen evolution in organic electrosynthesis.

A Roadmap for Production of Cement and Concrete with Low-CO2 Emissions

This review will show that low-CO2 cements can be produced to give superior durability, based on a sound understanding of their microstructure and how it impacts macro-engineering properties. For example, it is essential that aluminium is available in calcium-rich alkali-activated systems to offset the depolymerisation effect of alkali cations on C-(N-)A-S-H gel. The upper limit on alkali cation incorporation into a gel greatly affects mix design and source material selection. A high substitution of cement clinker in low-CO2 cements may result in a reduction of pH buffering capacity, hence susceptibility to carbonation and corrosion of steel reinforcement. With careful mix design, a more refined pore structure and associated lower permeability can still give a highly durable concrete. It is essential to expand thermodynamic databases for current and prospective cementitious materials so that concrete performance and durability can be predicted when using low-CO2 binders. Cationic copolymer and amphoteric plasticisers, when available commercially, will enhance the development of alkali-activated materials. The development of supersonic shockwave reactors will enable the conversion of a wide range of virgin and secondary source materials into cementitious materials, replacing blast furnace slag and coal fly ash that have dwindling supply. A major obstacle to the commercial adoption of low-CO2 concrete is the prescriptive nature of existing standards and design codes, so there is an urgent need to shift towards performance-based standards. The roadmap presented here is not an extension of current cement practice, but a new way of integrating fundamental research, equipment innovation, and commercial opportunity.

Structural response of alkali metal borates at Fe2O3 sliding interface: The effect of alkali cations

Structural transformation and chemical reaction induced by sliding motion is an interesting topic in material science and tribochemistry. In this paper, we used density functional theory (DFT) to study the adaptation of molecular and electronic structures at the iron oxide–alkali borate interface. Sodium borates with different sodium concentrations and two other common alkali metal borates of lithium and potassium have been confined between two Fe2O3 surfaces and subjected to the sliding at 1 and 38GPa. The results suggest a significant impact of alkali elements on the energy shape and bonding behavior in the systems. The low energy variation was obtained in the systems with sodium borate at 33% and 50% concentration of Na2O. Meanwhile, high variation energy was encountered with the sodium borate with low Na2O concentration (25% Na2O), lithium borate and potassium borates at 3 GPa. The formation of an “easy-shear” pattern was found in the lowest energy variation system with 50% Na2O at 1 GPa. The high variation energy was closely related to the dissociation of iron atoms from the oxide surface and the subsequent rupture of Fe−O bonds of the surface. Based on the current simulations, the possible interfacial layering has been proposed to rationalize experimental findings on the friction and wear of borate lubricated iron oxide surfaces.

Oxygen evolution on gold: The effects of alkali-metal cations and iron impurities from alkaline electrolytes

Effects of alkali cations and iron impurities in alkaline electrolytes on the activity of gold surface for the oxygen evolution reaction (OER) are studied. Cyclic voltammograms obtained for bare gold evidenced no activity for OER in the potential window bellow 2.0 VRHE, while for gold electrodes multi-cycled in the hydr(oxide) region, i.e., 1.25–1.50 VRHE, the OER activity is high and present an onset potential of 1.55 VRHE. Linear sweep voltammetry shows that the OER activity depends on the nature of alkali cation in the solution and also on the content of iron incorporated onto gold oxide electrodes. In situ surface-enhanced Raman spectroscopy (SERS) detected a band at 790 cm−1 suggesting the presence of OO− species on Au close to 1.90 VRHE and a broader band at a relatively lower potential (1.55 VRHE) that could be related to Fe-like species OO−, OOH and/or FeO that were adsorbed on gold electrode at a relatively low potential (∼1.55 VRHE), such species are likely to be the reactive centers participating in the electrocatalysis of the OER in alkaline media.

(Invited) Effects of the Electrochemical Double Layer on CO Reduction and Hydrogen Evolution on Cu Electrodes

The electrochemical reduction of carbon dioxide to fuels and high-value commodity chemicals is of great technological interest. Cu is the only pure electrocatalyst that facilitates the conversion of carbon dioxide to hydrocarbons at significant reaction rates. Cu is therefore an important prototypical catalyst for the reduction of carbon dioxide. However, the product selectivity of this process for specific desired products is poor. This product selectivity is not only controlled by the electrode material and its morphology, but also by the cations of the supporting electrolyte. While it has been shown that the identity and concentration of cations can greatly change the reaction selectivity of the process, a fundamental understanding of these effects is mostly lacking to date. Herein, we systematically explored the effects of quaternary alkyl ammonium ions on the reduction of CO to ethylene on Cu electrodes. With differential electrochemical mass spectrometry (DEMS), we determined that ethylene is produced in the presence of methyl4N+ and ethyl4N+. By contrast, we did not observe the formation of this product in the presence of propyl4N+ and butyl4N+. Using surface-enhanced infrared absorption spectroscopy (SEIRAS), we determined that the changes in selectivity are likely not due to blocking of catalytic sites or changes in the interfacial electric field. However, SEIRAS showed that the interaction of surface-adsorbed CO and water is disrupted in the presence of the two larger cations. This finding indicates that the interaction of interfacial water with surface-adsorbed CO is critical for the formation of ethylene. We will discuss the mechanistic implications of these findings. Furthermore, we will discuss the effects of alkali cations on the hydrogen evolution reaction, which is an undesired side-reaction in electrochemical carbon dioxide reduction. Using electroanalytical techniques, we showed that the effects are strongly dependent on the pH of the electrolyte. We rationalize these observation with dependence of the reaction mechanism of the hydrogen evolution reaction on pH, and the resulting differential interactions of cations with the intermediates in the different pH regimes.

Effect of alkali cations on the photomagnetic behavior of CoFe Prussian blue analogue nanoparticles embedded in ordered mesoporous silica

The size of CoFe Prussian blue analogue nanoparticles containing either rubidium or cesium cations was controlled by their formation through a nucleation process inside the calibrated pores of an ordered mesoporous silica matrix. The corresponding references were synthesized in powder form using the same ratio between the metal species in the reaction solution. The obtained samples were characterized via infrared (IR) spectroscopy, X-ray diffraction, and magnetometry to understand the differences arising from the size reduction and the nature of the inserted alkali cation. We were able to show that the nature of the cation has a significant influence on the photoswitching properties at the macroscale in the powders. However, on the nanoscale, the influence of the surface contribution becomes more prominent and cooperative effects disappear, resulting in a new role of the alkali cation in the photomagnetic properties.

Effect of alkali cations on Pt based catalyst towards methanol oxidation reaction in acidic medium

Abstract This communication demonstrates the effect of electrostatically adsorbed alkali cations at electrochemical interface towards methanol oxidation reaction (MOR) in acidic medium. The well-known electrostatic interaction between adsorbed OH/(OHad) species of catalyst surface and hydrated metal cations (M+(H2O)x) at the interface could be accountable for the observed different electrocatalytic activity of Pt/C and f-MWCNT/Pt-Co alloy NPs towards MOR. This cation effect has been found predominantly in weekly adsorbed ClO4− ion at Pt/C and f-MWCNT/Pt-Co alloy NPs surface than that of strongly adsorbed SO42− ion. Cyclic voltammetry results reveal that the strongly adsorbed cation at the interface shifts the oxidation potential more positively than that of weekly adsorbed cation.

Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments

Stainless steel is a ubiquitous structural material and one that finds extensive use in core-internal components in nuclear power plants. Stainless steel features superior corrosion resistance (e.g., as compared to ordinary steel) due to the formation of passivating iron and/or chromium oxides on its surfaces. However, the breakdown of such passivating oxide films, e.g., due to localized deformation and slip line formation following exposure to radiation, or aggressive ions renders stainless steel susceptible to corrosion-related degradation. Herein, the effects of alkali cations (i.e., K+, Li+) and the interactions between the passivated steel surface and the solution are examined using 304L stainless steel. Scanning electrochemical microscopy and atomic force microscopy are used to examine the inert-to-reactive transition of the steel surface both in the native state and in the presence of applied potentials. Careful analysis of interaction forces, in solution, within ≤10 nm of the steel surface, reveals that the interaction between the hydrated alkali cations and the substrate affects the structure of the electrical double layer (EDL). As a result, a higher surface reactivity is indicated in the presence of Li+ relative to K+ due to the effects of the former species in disrupting the EDL. These findings provide new insights into the role of the water chemistry not only on affecting metallic corrosion but also in other applications, such as batteries and electrochemical devices.

Alkali Cation Effect During the Oscillatory Electroreduction of H2O2 on Pt

AbstractHydrogen Peroxide Reduction Reaction (HPRR) catalyzed by Pt was studied in alkaline media and the effect of alkali cations were analyzed by the changes on dynamic instabilities presented by this system under high ohmic drop conditions. The origin of these instabilities is the competition between OH2− electroreduction and hydrogen adsorption on Pt surfaces, the latter being the first step for hydrogen evolution reaction (HER). Both HER and hydrogen adsorption are cation‐dependent which suggests that noncovalent interactions are able to stabilize the water molecules favoring the Pt−H formation. Thus, in an interphase, in which the noncovalent interactions are more effective, HPRR will proceed as an indirect poisoned system, reflecting on slow frequency oscillations (ω), for instance ωLiOH &lt; ωNaOH &lt; ωKOH for several H2O2 concentrations.

Probing promoting effects of alkali cations on the reduction of CO at the aqueous electrolyte/copper interface

The catalytic selectivity and reactivity of an electrocatalytic interface can profoundly depend on the identity of the supporting electrolyte's cation. In the case of CO2 reduction on copper electrodes, these cation effects have been utilized to suppress undesired hydrogen evolution and to promote the formation of C2 reduction products. However, to more effectively steer the catalytic selectivity of the electrolyte/copper interface by cations, it is crucial to reveal the various physical mechanisms by which cations impact the catalytic properties of this prototypical interface for CO2 reduction. Herein, we employ surface-sensitive infrared spectroscopy to probe how alkali cations (Li+, K+, and Cs+) control the coverage of CO, a key intermediate in CO2 reduction, on a polycrystalline copper electrode. We find that surface-adsorbed CO experiences an increasingly larger interfacial electric field with increasing cation size. The reduction of CO is further promoted by the two larger cations, leading to a significant drop of the CO coverage at high cathodic potential around -1 V vs. RHE. Our results demonstrate for the first time that the coverage of CO on the electrode is very sensitive to the identity of the cation. Since the relative coverage of CO and hydrogen on the copper surface affects the catalytic rates of CO2 reduction and hydrogen evolution, our results represent an essential step towards a better understanding of how cation effects control the product distribution.

Effects of Alkali Cations and Halide Anions on the Self-Assembly of Phosphatidylcholine in Oils.

The interactions between ions and phospholipids are closely associated with the structures and functions of cell membrane. Instead of conventional aqueous systems, we systematically investigated the effects of inorganic ions on the self-assembly of lecithin, a zwitterionic phosphatidylcholine, in cyclohexane. Previous studies have shown that addition of inorganic salts with specific divalent and trivalent cations can transform lecithin organosols into organogels. In this study, we focused on the effect of monovalent alkali halides. Fourier transform infrared spectroscopy was used to demonstrate that the binding strength of the alkali cations with the phosphate of lecithin is in the order Li+ > Na+ > K+. More importantly, the cation-phosphate interaction is affected by the paired halide anions, and the effect follows the series I- > Br- > Cl-. The salts of stronger interactions with lecithin, including LiCl, LiBr, LiI, and NaI, were found to induce cylindrical micelles sufficiently long to form organogels, while others remain organosols. A mechanism based on the charge density of ions and the enthalpy change of the ion exchange between alkali halides and lecithin headgroup is provided to explain the contrasting interactions and the effectiveness of the salts to induce organogelation.

ChemInform Abstract: Effect of Alkali Cations on Two‐Dimensional Networks of Two New Quaternary Thioarsenates (III) Prepared by a Facile Surfactant‐Thermal Method.

AbstractNaAg2AsS3·H2O (I) and KAg2AsS3 (II) are prepared by surfactant‐thermal (solvothermal) reaction of a 2:1:0.5:2 molar mixture of NaOH (KOH), Ag, As2S3, and S in N2H4·H2O (or en for (II)) and PEG‐400 (autoclave, 130 °C, 7 d, 95% and 98% yield of (I) and (II), resp.).