Stoichiometric Chemistry Research Articles

Low Temperature and Rapid Synthesis of Li‐Rich Li(Li0.17Mn0.83)2O4 Spinel Cathodes Derived from Metal‐Organic Frameworks for Lithium‐Ion Batteries

AbstractLi‐rich spinel materials (Li1+xMn2−xO4) have shown promise for lithium‐ion batteries. Nevertheless, the preparation of Li1+xMn2−xO4 faces significant challenges due to the difficulty in achieving a balance between well‐crystallized phases and stoichiometric chemistry. Moreover, the synthesis process is highly sensitive to calcination temperature and time, making it susceptible to phase transformations. Therefore, the rational selection of precursors and corresponding calcination procedures is absolutely essential. Herein, we make full use of the nature of metal‐organic frameworks (MOFs) to achieve phase‐controlled synthesis of Li(Li0.17Mn0.83)2O4 (LMO−F) spinel cathodes in 8 minutes at 500 °C. The composition and structural evolution during the pyrolysis process were systematically investigated to clarify the relationship between precursors and derivatives. Notably, the LMO−F achieved good electrochemical performance with 100.4 mAh g−1 at 50 mA g−1 after 100 cycles.

Constructing a new irrigation model for desert riparian forests based on response of canopy EVI loss and tree rings growth to groundwater fluctuation

Vegetation in arid desert areas plays an extremely important role in maintaining biodiversity and inhibiting desertification. Quantitative understanding of vegetation response to groundwater is essential to priming drought prevention and mitigation, which is a topic that is attracting considerable attention. However, previous studies have rarely assessed the response of vegetation to groundwater from probabilistic or stoichiometric chemical perspective. To address this question, a highly sensitive copula joint distribution of the response of enhanced vegetation index (EVI) loss to groundwater was constructed and a stoichiometric chemistry indicator of the Nitrogen (N) and Phosphorus (P) concentrations in tree rings was innovatively put forward to verify its sensitivity to groundwater. The results obtained revealed that the correlation of EVI with groundwater was better than that with SPEI and TVDI; Furthermore, the critical groundwater thresholds of the EVI loss rate, described as “beginning”, “reaching the maximum”, and “the slowest” were 3.85 m, 6.18–6.75 m, and 7.89 m, respectively. At the inter-annual scale, by measuring the N and P contents of Populus euphratica tree rings, and constructing response model of element contents to groundwater/rainfall, the key thresholds were determined at 5.20–5.31 m and 60 mm, which challenges the traditional understanding that rainfall does not bring ecological benefits to desert riparian forests. To further promote vegetation restoration, based on the response rule of EVI loss and stoichiometric chemical elements of tree rings to groundwater and the intermediate overflow rotation irrigation method, a more advanced desert riparian forest irrigation mode, named “branch-infiltration-rotation-irrigation”, is here proposed.

The Effect of Computer Simulation on Grade 11 Learners’ Conceptualisation of Stoichiometric Chemistry

This study, titled <i>The Effect of Computer Simulation on Grade 11 Learners’ Conceptualisation of Stoichiometric Chemistry</i>, was carried out at a school in the Frances Baard District of the Northern Cape province of South Africa. Poor conceptualisation of stoichiometric chemistry by learners in Grades 10-12 in South Africa and, hence, their failure to apply the concepts to problem-solving in the same and other topics in chemistry, is a cause for concern. The study was conducted with the theoretical framework of activity theory. A mixed method design that consisted of a pre-test post-test experimental design, a questionnaire and interviews were used for the study. Two Grade 11 physical sciences classes which consisted of a control and experimental group were taught stoichiometric chemistry after the SCAT pre-test. The experimental group obtained an intervention comprising computer simulations during teaching, while the control group was taught using the lecture method. The ANOVA results showed that learners of the control and experimental groups were comparable in terms of prior knowledge of the topic of stoichiometric chemistry. The paired t-test showed that both groups improved their performance in stoichiometric chemistry. However, the ANCOVA results showed that the experimental group had a greater improvement in performance than the control group. The results of the SCAT post-test, observation of lessons, questionnaire and interviews showed that the experimental group conceptualised stoichiometric chemistry better than the control group. The quantitative and qualitative data was triangulated, and it also indicated that the experimental group conceptualised stoichiometric chemistry better than the control group. It is therefore suggested that using computer simulations for the teaching and learning of stoichiometric chemistry is a better method to improve its conceptualisation in the FET phase of schools in South Africa.

Oxidant-free and metal-free highly-selective catalytic production of vanillic aldehyde through activated carbon fiber-mediated aerobic oxidation

Typically, oxidizing agents (such as H2O2) are necessitated for vanillic alcohol (VLA) conversion; nevertheless, intensive consumption of H2O2 is required due to the stoichiometric chemistry, and metal catalysts are usually required to improve conversion of VLA by H2O2. Nevertheless, for pursuing the goal of sustainability, it would be highly desired to develop an oxidizing agent-free and metal-free process which can selectively oxidize VLA to vanillic aldehyde (VAE). The aim of this study is to propose activated carbon fiber (ACF) as a highly effective catalyst for VLA conversion to VAE without using metals and oxidizing agents. At 120 °C within 30 min, ACF could enable CVLA = 100% with SVAE = 100% without using metals/oxidants. The effect of radical scavengers and ESR analyses further reveal that the oxidation of VLA to VAE by ACF could be attributed to reactive oxygen species derived from ACF-mediated aerobic reactions.

Different Kinetic Reactivity of Electrons in Distinct TiO2 Nanoparticle Trap States.

Electrons added to TiO2 and other semiconductors often occupy trap states, whose reactivity can determine the catalytic and stoichiometric chemistry of the material. We previously showed that reduced aqueous colloidal TiO2 nanoparticles have two distinct classes of thermally-equilibrated trapped electrons, termed Red/e - and Blue/e -. Presented here are parallel optical and electron paramagnetic resonance (EPR) kinetic studies of the reactivity of these electrons with solution-based oxidants. Optical stopped-flow measurements monitoring reactions of TiO2/e - with sub-stoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO2/e - absorbance followed by a secondary, slow (minutes) increase in the broad TiO2/e - optical feature. Analysis revealed that the fast decrease is due to the preferential oxidation of the Red/e - trap states, and the slow increase results from re-equilibration of electrons from Blue to Red states. This kinetic model was confirmed by freeze-quench EPR measurements. Quantitative analysis of the kinetic data demonstrated that Red/e - react ~5 times faster than Blue/e - with the nitroxyl radical oxidant, 4-MeO-TEMPO. Similar reactivity patterns were also observed in oxidations of TiO2/e - by O2, which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant KI3. This suggests that the faster intrinsic reactivity of one trap state over another on the seconds-minutes timescale is likely a general feature of reduced TiO2 reactivity. This differential trap state reactivity is likely to influence the performance of TiO2 in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.

Redox-Neutral Propargylic C–H Functionalization by Using Iron Catalysis

AbstractIn spite of their rich stoichiometric chemistry, cyclopentadienyliron(II) dicarbonyl complexes are rarely used as catalysts in organic synthesis. Inspired by precedents in the chemistry of cationic olefin complexes and neutral allylmetal species, our group has developed a coupling of alkynes or alkenes with aldehydes and other carbonyl electrophiles to give homopropargylic and homoallylic alcohols, respectively, by using a substituted cyclopentadienyliron(II) dicarbonyl complex as the catalyst. In this article, we first contextualize this development within the conceptual background of C–H functionalization chemistry and relative to key stoichiometric precedents. We then give an account of our group’s discovery and development of the catalytic α-functionalization of alkenes and alkynes with electrophilic reagents.IntroductionPreliminary Stoichiometric WorkHydroxyalkylation Development and ScopeConclusions and Future Directions

The impact of plasma-enhanced atomic layer deposited ZrSiOx insulators on low voltage operated In-Sn-Zn-O thin film transistors

Abstract We deposited Zr silicate by plasma-enhanced atomic layer deposition (PEALD) and investigated its gate insulator performance in indium-tin-zinc oxide (ITZO) thin film transistors (TFTs). Deposited Zr silicate had the desired characteristics of both ZrO 2 , a high-k dielectric, and SiO 2 . The drawbacks of ZrO 2 thin film can be compensated by employing Zr silicate thin film because the latter has a moderate dielectric constant of 8.4, stoichiometric chemistry, smooth surface, and a low leakage current. Zr silicates as gate insulators may play important roles to reduce coulomb scattering and decrease charge trap density, resulting in high electrical performance and appropriate device stability when used in ITZO TFTs.

Characteristic Study of Silicon Nitride Films Deposited by LPCVD and PECVD

This paper analyzes and compares the characteristics of silicon nitride films deposited by low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), with special attention to the hydrogenation and chemical composition of silicon nitride films. Three different LPCVD processes at various DCS and NH3 gas flow rates and deposition temperatures, together with PECVD using SiH4 and NH3 and ICP CVD using SiH4 and N2, were compared. The silicon nitride film deposition rate decreases with an increasing NH3/DCS ratio in LPCVD, which also leads to an increase in the refractive index and a decrease in the residual stress in the film. There is nearly no hydrogen incorporated in the LPCVD films, which differs from PECVD and ICP CVD that show significant Si-H and N-H bonds. The chemical composition of silicon nitride films is mostly Si-rich, except for the LPCVD process at high NH3/DCS ratio with near stoichiometric chemistry.

Li-Rich Layered Cathode Materials: Transition Metals in Transit

In the most recent issue of Nature Communications , William Gent, William Chueh, and co-authors examine in depth the electronic structure and redox processes that occur during cycling of Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 and reveal an important new insight that suggests oxygen redox is coupled with local transition metal migration. Aided by a suite of cutting-edge characterization methods, the authors formulate a clear and unprecedented view of both local and averaged structure evolution during cycling.

Amorphous Boron Nitride: A Universal, Ultrathin Dielectric For 2D Nanoelectronics

Next‐generation nanoelectronics based on 2D materials ideally will require reliable, flexible, transparent, and versatile dielectrics for transistor gate barriers, environmental passivation layers, capacitor spacers, and other device elements. Ultrathin amorphous boron nitride of thicknesses from 2 to 17 nm, described in this work, may offer these attributes, as the material is demonstrated to be universal in structure and stoichiometric chemistry on numerous substrates including flexible polydimethylsiloxane, amorphous silicon dioxide, crystalline Al2O3, other 2D materials including graphene, 2D MoS2, and conducting metals and metal foils. The versatile, large area pulsed laser deposition growth technique is performed at temperatures less than 200 °C and without modifying processing conditions, allowing for seamless integration into 2D device architectures. A device‐scale dielectric constant of 5.9 ± 0.65 at 1 kHz, breakdown voltage of 9.8 ± 1.0 MV cm−1, and bandgap of 4.5 eV were measured for various thicknesses of the ultrathin a‐BN material, representing values higher than previously reported chemical vapor deposited h‐BN and nearing single crystal h‐BN.

Alkyne reactions with trimethylphosphine complexes of iridium: Lessons for the catalysis of vinyl ester formation and alkyne dimerization

The combination of iridium with trimethylphosphine ligands yields very electron rich iridium compounds that are active for terminal alkyne dimerization chemistry as well as the addition of carboxylic acids to alkynes. The structures, catalytic and stoichiometric chemistry of some iridium tris-trimethlyphosphine compounds with benzoate ligands are detailed in this paper. The roles that these compounds may play in the catalytic cycles involved in the dimerization of terminal alkynes and the addition of benzoic acid to terminal alkynes are examined and possible mechanisms for these catalytic reactions are proposed.

Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido and β-diketiminato ligands

The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(N(t)Bu)][X] (BDI = [Ar]NC(CH(3))CHC(CH(3))N[Ar], Ar = 2,6-(i)Pr(2)C(6)H(3); M = Nb, Ta; X = MeB(C(6)F(5))(3), B(C(6)F(5))(4)] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R(3)PC(O)Me)M(N(t)Bu)][B(C(6)F(5))(4)] (R = Et, Cy) in the presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb complex with XylNC (Xyl = 2,6-Me(2)-C(6)H(3)) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylN[double bond, length as m-dash]C(Me))Nb(N(t)Bu)][B(C(6)F(5))(4)], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe(2))Nb(N(t)Bu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph(3)SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex.

Diverse coordination behaviors of the silyl-linked bis(amidinate) ligand [SiMe 2{NC(Ph)N(Ph)} 2] 2− to zirconium center

The reactions between the silyl-linked bis(amidinate) ligand [SiMe 2{NC(Ph)N(Ph)Li(thf) 2} 2] and ZrCl 4 in different stoichiometric chemistry, 2:1, 1:1, and 1:2, were explored. The resulting zirconium compounds, 1, 2, and 3, showed different molecular structural features which were confirmed by X-ray crystallographic analysis. The metal center in 1 was located in a highly saturated coordination environment with four amidinate moieties. Compound 2 exhibited an unprecedented linked tris(amidinate) N–C–N–Si–N–C–N–Si–N–C–N skeleton formed after the intermolecular rearrangement. In compound 3, a novel silyl-linked imino-amidinate ligand via the intramolecular rearrangement was involved. The mechanism for the formation of compounds 2 and 3 was proposed. Compound 2 was found to be a highly active catalyst for ethylene polymerization in the presence of methylalumoxane (MAO).

Exploring the Innate Immune System: Using Complement-Mediated Cell Lysis in the Classroom

When teaching biology, instructors often face the challenge of teaching various subdisciplines. One of the subdisciplines that often receives very little attention is the immune system. Part of the reason for this oversight is the perception that the immune system is complex and the appreciation that few typical high school or undergraduate college laboratories have the resources, e.g., tissue culture hoods and incubators, to examine the immune system extensively. When the immune system is addressed, the lessons often focus on the adaptive immune system and the charismatic T cells, B cells, and macrophage cells that comprise it. This article explains how complement-mediated bacterial cell lysis provides an opportunity to introduce students to the wonders of the innate immune system and specifically highlight the often-overlooked importance and properties of the complement system. The results of complement-mediated bacterial destruction are dramatic and, while the actual pathway is a rather complex cascade of protein activation, conceptually the process is both easily explained by instructors and understood by students. Additionally, the exercise presented here can be modified to examine a number of different variables of immune system function. An examination of complement-mediated cell lysis links the phenomena of microbiological growth, enzymatic and stoichiometric chemistry, protein-heat instability (lability), immune pattern recognition, protein activation, protein cascades, and the cooperation between the innate and adaptive immune systems. The lab presented here can act as a springboard into any of these topics. Background The Adaptive Immune Response The more familiar understanding of the immune response involves the activity of B and T cells. The capacity for certain B or T cells to be selected and expanded based on surface receptor/antigen engagement drives the concept of adaptive immunity. B and T cells have surface receptors that are created by a similar mechanism and which recognize single, unique foreign antigens. When B cell and T cell surface receptors encounter an appropriate foreign antigen, the individual cell (clone) undergoes rapid We term this phenomenon expansion. The clonal expansion of appropriate B and T cell occurs in the lymph nodes and other lymphoid tissues and is characterized by the familiar lymph node swelling observed during infection. This antigen-specific cell expansion and subsequent immune attack clears the body of pathogens. Antigen-specific memory B and T cells survive and provide immunologic memory. The rapid re-activation and expansion of these cells in future antigen encounters explains why certain pathogens do not infect us more than once and underlies the concept of vaccinations. The Innate Immune Response The adaptive immune system discussed above is fundamentally a reactive, second line defense against pathogens. The first line of defense against disease is the innate immune system. Innate immunity is composed of skin secretions, barriers to pathogen entry, phagocytic cells, and blood proteins that neutralize pathogens before or soon after they enter the body. One part of the innate immune response is the protein complement system discussed in this article. The complement system is a set of nine proteins found in the blood of all mammals and is thought to have existed in animals as far back as 300 million years ago. (Wood, 2006; Sunyer et al., 2005). It is responsible for the recognition and destruction of foreign pathogens including viruses, bacteria (von Lackum et al., 2005), fungi (Speth et al., 2004), and other single-celled organisms (Inal, 2004). The complement system acts through a protein activation cascade in which individual complement proteins bind to the targeted pathogen and subsequently recruit additional complement pathway proteins. Proteins involved in the complement process (referred to as C1 through C9) are inactive until they enter the cascade where they join the complex and become activated themselves. …

The formation mechanism of aluminum oxide tunnel barriers: Three-dimensional atom probe analysis

Magnetic tunnel junctions have applications in a range of spin-electronic devices. The functional properties of such devices are critically dependant on the nanoscale morphology of the insulating barrier (usually only a few atomic layers thick) that separates two ferromagnetic layers. Here we report atomic-resolution three-dimensional visualization of magnetic tunnel junctions with an aluminum oxide barrier, using three-dimensional atom probe analysis and cross-sectional high resolution electron microscopy. Our results on barriers with a range of degrees of oxidation have enabled a mechanism for the oxidation to be proposed. Low oxidation times result in discrete oxide islands and further oxidation leads to a more continuous, but nonstoichiometric, barrier. Post-deposition annealing leads to an increase in the barrier area and near stoichiometric chemistry with evidence that oxidation proceeds along the top of grain boundaries in the underlying CoFe. Our three-dimensional visualization enables us to show that even perforated barriers (formed at shorter oxidation times) can have reasonable transport properties.

Non‐Conventional Oxidation Catalysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

Non-conventional oxidation catalysis

This short review provides an overview as well as background of ongoing work in our laboratory on unconventional catalysts in selective oxidation reactions with remarkable selectivity. The three cases reviewed here have in common that the catalysts are oxides that do not posses any formal redox capacity. The use of doped defective oxides gives rise to surface-redox chemistry in the case of partial oxidation of methane over yttrium-modified zirconia, where actual removal of oxygen from the surface sites indeed takes place. Surprisingly, this oxygen species is not able to activate hydrogen to a significant extent. In contrast, Li-modified MgO catalyst does not allow removal of oxygen; O − sites generated by Li are now able to abstract a H-radical from, e.g., propane. Radical gas-phase chemistry of the propyl radicals is then responsible for selective formation of olefins. The significant fact in this case is that olefins are activated to a lesser extent then alkanes. The remarkable selectivity obtained in propane oxygenation on earth-alkali-modified zeolite Y at unusually low temperatures is due to the confinement of radicals in the cages of the zeolite, turning radical chemistry into stoichiometric chemistry. The second significant effect is the very effective stabilization of the resulting oxygenate, acetone, in the cage. However, the stabilization thus also implies extremely strong adsorption of the acetone, thus preventing desorption of the product. Our work indicates that desorption can be assisted by offering water as a competitive adsorbing species, thus opening the possibility to close the catalytic cycle.

The Power of High‐Throughput Experimentation in Homogeneous Catalysis Research for Fine Chemicals

AbstractFor Abstract see ChemInform Abstract in Full Text.

Assessment of expert-novice chemistry problem solving using HyperCard: Early findings

Results of a HyperCard method for assessing the performance of expert and novice high school chemistry students solving stoichiometric chemistry problems (balancing chemical equations) have been reported. The assessment involved the use of a HyperCard instrument (“Hyperequation”) developed on a Macintosh platform to administer a set of five chemistry problems, register student responses, and collect data related to student performance. The chemistry problems have been previously tested and validated in traditional (pen-paper) assessment settings by other researchers. MANOVA results indicate a significant difference between the performance of expert and novice students solving the five stoichiometric chemistry problems using the Hyperequation. The study shows promise in that a HyperCard assessment method could differentiate between the performance of experts and novices in problem solving. The implication is that HyperCard might be a suitable technology for developing performance assessment methods not only in chemistry but also in other science disciplines.

High Oxidation State Organometallic Chemistry, A Challenge—the Example of Rhenium

AbstractHomogeneous catalysis as the major industrial outlet of organometallic basic research has been enjoying great benefit from organotransition metal species that promote bond forming between hydrocarbon fragments. Most of the commercially important processes that serve to produce large‐volume organic feedstock chemicals such as linear α‐olefins (Shell Higher Olefins Process), linear aldehydes (hydroformylation), acetaldehyde (Wacker‐Hoechst), acetic acid (Monsanto), adiponitrile (DuPont hydrocyanation of butadiene) operate at low‐valent metal centers. It is thus hardly surprising that by far the most part of organometallic research during the past few decades has been directed towards an understanding and the improvement of these catalytic reactions as well as towards the related stoichiometric chemistry. As a matter of consequence, our present knowledge on high‐valent organotransition metal compound is comparatively shallow, nor do we know much about the chemical relationship and interconvertability of high and low oxidation states within a given class of compounds. In this article I want to point out some ostensibly challenging perspectives of future organometallic research by describing a novel class of high oxidation state organorhenium compounds as well as by speculating on possible generalizations for other transition metals.