Oxomanganese Research Articles

Synthesis of Mn/Co-MOF for effective removal of U(VI) from aqueous solution

Ultrasound-assisted synthesis of Mn/Co-MOF nanomaterial was used to capture uranium from aqueous solutions. Tests of Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectra (FT-IR), Zeta potential analysis, thermogravimetric analysis (TGA), and X-ray diffraction (XRD) suggest that cobalt ions were replaced partially by manganese ions to generate MOF during the synthesis process and form manganous oxide particles loaded on the surface of Mn/Co-MOF. The optimal immobilization conditions of U(VI) were systematically studied by solution pH, kinetic, contact time and preparatory uranium concentration. XPS spectroscopy analysis indicated that the chelation of imidazole ring to uranium and Mn3O4 possibly played a certain role in the adsorption process. The results indicate that the adsorption isotherms of the Mn/Co-MOF for uranium suit Langmuir isotherm model (maximum adsorption capacity were 763.36 mg/g). Furthermore, the adsorption kinetics of Mn/Co-MOF match comfortably with the pseudo-second-order kinetic model.

A DFT study on the reaction mechanisms of the oxidation of ethylene mediated by technetium and manganese oxo complexes.

The oxidation of ethylene catalyzed by manganese and technetium oxo complexes of the type MO3L (M = Tc, Mn, and L = O-, Cl-, F-, OH-, Br-, I-) on both singlet and triplet potential energy surfaces (PESs) have been studied. All molecular structures were stable on the singlet PES except for the formation of the dioxylate intermediate for the MnO3L (L = O-, Cl-, F-, OH-, Br-, I-) catalyzed pathway. Frontier molecular orbital calculations showed that electrons flow from the HOMO of ethylene into the LUMO of the metal-oxo complex for all complexes studied except for MO3L (M = Tc, Mn, and L = O-) where the vice versa occurs. In the reaction of both TcO3L and MnO3L (L = O-, Cl-, F-, OH-, Br-, I-) with ethylene, it was observed that the formation of the dioxylate intermediate along the [3 + 2] addition pathway on the singlet reaction surface is both kinetically and thermodynamically favorable over its formation via the [2 + 2] pathway. Furthermore, it was observed that TcO4- and MnO4- catalyzed pathways exclusively form diols on the singlet PES. The formation of epoxides on the singlet surface is kinetically favorable through the [2 + 1] and [2 + 2] channel for the MnO3L (L = F-, Cl-, Br-, I-, OH-) and TcO3L (L = F-, Cl-, Br-, I-, OH-) catalyzed surfaces respectively. In all cases, the TcO3L complexes were found to be polar compared to the MnO3L complexes. The MnO4- (singlet) and MnO3F (singlet) are the best catalysts for the exclusive formation of the diols and epoxides respectively.

Over-heating triggered thermal runaway characteristic of lithium ion hybrid supercapacitor based lithium nickel cobalt manganate oxide/activated carbon cathode and hard carbon anode

The energy density increase and the application of battery-type electrode materials and organic electrolyte for lithium ion hybrid supercapacitors (Li-HSCs) inevitably accompany the rising of thermal runaway. Herein, we fabricated a large-format pouch cell Li-HSC with activated carbon (AC) and lithium nickel cobalt manganate oxide (NCM) composite cathode, hard carbon (HC) anode, and ceramic polyethylene terephthalate (PET) nonwoven separator. The as-prepared Li-HSC delivers a capacitance of 34,476 F and a high energy density of 126 Wh kg−1 (based on the mass of device). The thermal runaway characteristic and gas release of this large-format Li-HSC are analyzed. Thermal runaway occurs for fully charged NCM/AC//HC Li-HSC at 249 °C, but not for fully charged AC//HC Li-HSC. The O2 releasing by NCM component accelerates the self-heating reactions, and the introduction of NCM into cathode is proved to contribute on triggering thermal runaway of NCM/AC//HC Li-HSC. The Li-HSC displays the highest runaway temperature (Tpeak) of 490.9 °C with smoke spurting out and does not catch fire or explode during thermal runaway. The main gas components at thermal runaway are C2H4, O2 C3H6 and C4H8, produced by the reaction between anode and electrolyte, and decomposition of NCM and electrolyte. The ratio of hydrocarbon gases and O2 decreases, while that of CO and CO2 increases at the Tpeak.

Rapid Preparation of Manganese Monoxide by Microwave-Enhanced Selective Carbothermal Reduction

Most of the manganese resources in China have existed in the form of low-grade pyrolusite which is not utilized efficiently because of the high energy consumption and environmental pollution during the reduction process. Applying microwave heating to minerals reduction endows improved production efficiency and reduced production costs. In the present work, rapid preparation of manganese monoxide (MnO) was attempted through reducing low-grade pyrolusite with coal reducing agent by microwave heating, with the samples characterized by XRD, scanning electron microscopy, XPS as well as TG/DSC. The influences of the reduction reaction parameters on the reduction process of Mn in pyrolusite were comprehensively studied. The results indicated that higher temperatures and longer holding times facilitated the reduction roasting of pyrolusite, and manganese monoxide can be fabricated with a reduction ratio of 97.7% obtained at 650°C for 50 min. The mechanism of the gradual transformation of MnO2 to MnO from the macroscopic to the molecular level was also revealed in the order of MnO2 → Mn2O3 → Mn3O4 → MnO. Compared to traditional roasting, the proposed microwave-enhanced roasting process benefits from the superior kinetic conditions provided by the synergy between microwave enhancement and compact pellets, and thus reduced the roasting temperature and roasting time.

High-performance zinc-ion battery cathode enabled by deficient manganese monoxide/graphene heterostructures

Rechargeable aqueous zinc-ion battery (ZIB) with manganese-based cathode has attracted much attention in recent years. However, the electrochemical performance of ZIBs are limited by cathode materials. Herein, an accordion-like heterostructure of manganese monoxide uniformly grown between graphene layers (A-MnO/G) is developed for ZIB cathode. The covalent bonded graphene-MnO interphase facilitates the formation of vacancies/defects that result in a disordered phase of MnO, contributing to easy access for Zn2+ to abundant active sites. As a result, A-MnO/G-based ZIB delivers a high capacity of 398.5 mAh g−1 at 0.1 A g−1. Meanwhile, the battery exhibits 70% capacity retention after 2000 cycles at 3 A g−1. First principles calculations reveal that the heterostructures significantly lower the formation energy of oxygen vacancies which further lower the adsorption energy of Zn2+ to boost capacity. Furthermore, flexible transparent batteries assembled with A-MnO/G cathode demonstrate great potential for serving as power sources for practical use.

Homogeneous intercalated graphene/manganic oxide hybrid fiber electrode assembly by non-liquid-crystal spinning for wearable energy storage

• Well-dispersed MnO nanoparticle within NLC rGO dispersions were successfully obtained. • Continual rGO/MnO hybrid fiber can be fabricated by using a homemade apparatus and followed by a chemical reduction. • The sample of rGO/MnO-20 fiber possesses a high capacitance of 123.3 f g −1 (at 0.2 a g −1 ). • A fiber-shaped all-solid-state supercapacitor with energy density of 2.67 mWh cm −3 was assembled from optimized hybrid fibers, which is flexible and could be weaved into a textile. Reduced graphene oxide (rGO)-based fibers with high electrochemical performance have recently showed great potential in the field of flexible energy storage devices. However, they still suffer from low capacitance due to the severe stacking of graphene sheets. Hybrids with nanofillers are an efficient way to improve the electrochemical performance of rGO fibers. Nevertheless, controlling the distribution of nanoparticles in the matrix is still an enormous challenge due to the strong attraction among these nanoparticles which results into agglomeration. Here, we continually prepared rGO hybrid fibers via non-liquid-crystal spinning, accompanied by chemical reduction. Manganic oxide (MnO X ) nanoparticles remained well-dispersed in GO dispersion during the continuous spinning of rGO/MnO X hybrid fibers. Results showed that rGO/MnO X -20 hybrid fibers possessed the best capacitance of 123.3 F g −1 (87.6 F cm −3 ) and 97.1 F g −1 (68.9 F cm −3 ) at the current density of 0.2 A g −1 , and 0.5 A g −1 respectively. Furthermore, a fiber-shaped all-solid-state supercapacitor assembly from the optimized hybrid fibers demonstrated an energy density of 2.67 mWh cm −3 (3.76 mWh g −1 ) at the power density of 24.76 mWh cm −3 (34.89 mWh g −1 ). These fiber-based devices show great potential for application in the fields of wearable electronics and energy storage devices.

Soft X-ray signatures of cationic manganese–oxo systems, including a high-spin manganese(v) complex

Manganese-oxo species catalyze key reactions, including C-H bond activation or dioxygen formation in natural photosynthesis. To better understand relevant reaction intermediates, we characterize electronic states and geometric structures of [MnOn]+ manganese-oxo complexes that represent a wide range of manganese oxidation states. To this end, we apply soft X-ray spectroscopy in a cryogenic ion trap, combined with multiconfigurational wavefunction calculations. We identify [MnO2]+ as a rare high-spin manganese(V) oxo complex with key similarities to six-coordinated manganese(V) oxo systems that are proposed as reaction intermediates in catalytic dioxygen bond formation.

Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR-Lewis Acid)] Cores.

A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe4)[MnV(HMPAB)(O)] [2; H4HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn-Oterm distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the Mn≡O formulation. The reaction of redox-inactive metal ions (Mn+ = Li+, Ca2+, Mg2+, Y3+, and Sc3+) with 2 resulted in the formation of 2-Mn+ species, which were characterized by UV-vis, 1H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the Mn+ ions in 2. Complex 2 revealed one-electron-reduction potential at -0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of Mn+ ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2-Mn+ species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of Mn+ ions, and a plot of log k2 versus Lewis acidity of Mn+ ions (ΔE) followed a linear relationship. It was observed that 2-Sc3+ was ca. 3200 times more reactive toward the OAT reaction compared to 2. Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar3P substrates. In contrast, 2-Ca2+ and 2-Sc3+ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2-Mn+ adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of Mn+ ions. On the basis of Eyring analysis of 2 and 2-Mn+ adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2-Sc3+, which is different from the reaction mechanism of 2 and 2-Ca2+.

Synthetic melanin facilitates MnO supercapacitors with high specific capacitance and wide operation potential window

Although tremendous efforts have been devoted to manganous oxide (MnO)-based hybrid electrode materials, the rational design and facile preparation of high-performance MnO hybrid supercapacitors are still full of challenges, due to the inevitable agglomeration and limited stability of MnO within carbon matrixes. In this work, we developed a direct one-pot polymerization method to synthesize a series of Mn ion doped polylevodopa [P(L-DOPA)] synthetic melanin nanoparticles (Mn@SMNPs) for further transforming into small-sized hybrid carbon nanoparticles (MnO@CNPs, 20–80 nm) consisting of many stable Mn-related chemical bindings (C–N–Mn). The obtained MnO@CNPs electrode materials with physically spatial protection and well distribution of MnO could display high specific capacitance values (e.g., 545 F g−1 at 0.5 A g−1 in 6 M KOH electrolyte) within a wide operation voltage of 1.3 V. This work will continue to provide an inspiration towards the fabrication of the next-generation of high-performance synthetic melanin-based energy storage materials and devices.

Effect of Ti-doping and Octahedral Morphology on Electrochemical Performance of Lithium Manganate Oxide as Cathode Materials for LIBs

Effect of Ti-doping and Octahedral Morphology on Electrochemical Performance of Lithium Manganate Oxide as Cathode Materials for LIBs

Facile and low-cost synthesis of carbon-supported manganese monoxide nanocomposites and evaluation of their superior catalytic effect toward magnesium hydride

A carbon-supported manganese monoxide nanocomposite (MnO@C) was synthesized using a low-cost D113 cation exchange resin and manganese acetate tetrahydrate to improve the dehydrogenation/hydrogenation properties of magnesium hydride (MgH2). It was found that the dehydrogenated MnO@C-doped MgH2 composite could uptake 6.0 wt% of hydrogen within 60 min at 100 °C, and it could even uptake hydrogen at ambient temperature. Moreover, the synthesized composite could release approximately 5.0 wt% of hydrogen within 6 min at 300 °C. The dehydrogenation and hydrogenation apparent activation energies of MnO@C-doped MgH2 were calculated as 94.6 kJ mol−1 and 22.5 kJ mol−1, respectively, which were reduced by 38.8% and 67.2% when compared to the undoped sample. The density functional theory revealed that Mn and Mg0.9Mn0.1O facilitated the fracture of MgH bonds, and thus improved the reversible hydrogen storage performance of MgH2.

Surface modification of manganese monoxide through chemical vapor deposition to attain high energy storage performance for aqueous zinc–ion batteries

Surface modification of the manganese-based oxide electrode is considered to be a viable strategy to improve electrochemical property in aqueous zinc-ion batteries (ZIBs). However, the modification method through traditional wet-chemical technology can hardly to satisfy high rate capability for aqueous ZIBs due to unhomogeneous and nonconformal coating originates from surface energy mismatch. Herein, a surface modification strategy based on chemical vapor deposition is developed to enhance the electrochemical property of the inactive MnO in aqueous ZIBs. The constructed carbon coating modified MnO electrode shows excellent reversible capacity and superior rate capability with remarkable energy density of 351 Wh kg−1 at 625 W kg−1. The energy storage mechanism of the electrode during the charge and discharge processes is elucidated according to the ex-situ measurements of X-ray diffraction and photoelectron spectroscopy, Fourier transform infrared spectra, and galvanostatic intermittent titration techniques. Moreover, soft-packaged batteries are fabricated with the carbon coating modified MnO, which shows great promises for the practical application of the material. The work paves the way for the exploitation of high performance surface-modified electrode through chemical vapor deposition for aqueous ZIBs.

Metal organic frameworks derived active functional groups decorated manganese monoxide for aqueous zinc ion battery

Engineering the elemental composition and structure of manganese-based cathode materials offer an effective strategy for the development of aqueous zinc ion batteries (AZIBs) with greatly enhanced specific capacity and good stability. In this work, a manganese monoxide (MnO) composite with MnO as the core and some organic intermediate with the active functional group as the outer layer was derived from Mn metal–organic framework (Mn-MOF). Benefiting from the unique structure, active functional groups, and the synergetic effect of core–shell, the obtained MnO-C display superior electrochemical performance, demonstrating specific capacity (727.7 mAh g−1 at 0.1 A g−1) in the (CF3O3S)2Zn aqueous electrolyte and good rate performance (413.8 mAh g−1 at 1.0 A g−1), which is higher than that of MnO2 (331.7 mAh g−1) and commercial MnO (234.5 mAh g−1). Furthermore, the Zn-storage mechanism in MnO-C is systematically studied and discussed via multiple analytical methods. This work provides an idea for the development of electrochemical activation strategies for advanced cathodes of high-performance zinc ion batteries.

A Force Field for a Manganese-Vanadium Water Oxidation Catalyst: Redox Potentials in Solution as Showcase

We present a molecular mechanics force field in AMBER format for the mixed-valence manganese vanadium oxide cluster [Mn4V4O17(OAc)3]3−—a synthetic analogue of the oxygen-evolving complex that catalyzes the water oxidation reaction in photosystem II—with parameter sets for two different oxidation states. Most force field parameters involving metal atoms have been newly parametrized and the harmonic terms refined using hybrid quantum mechanics/molecular mechanics reference simulations, although some parameters were adapted from pre-existing force fields of vanadate cages and manganese oxo dimers. The characteristic Jahn–Teller distortions of d4 MnIII ions in octahedral environments are recovered by the force field. As an application, the developed parameters have been used to calculate the redox potential of the [MnIIIMn3IV] ⇌ [Mn4IV]+e− half-reaction in acetonitrile by means of Marcus theory.

Smart confinement of MnO enabling highly reversible Mn(II)/Mn(III) redox for asymmetric supercapacitors

The energy density of the intensively-studied aqueous asymmetric supercapacitors (ASCs) is limited by the conventional carbon anodes with low specific capacitance. Low-valence manganese oxides with high pseudocapacitive capacitance that can work at low potential windows show great promise to substitute the capacitive carbon anodes, and yet is severely challenged by the dissolution of Mn(II) in aqueous electrolyte. In this work, we develop a high-performance anode that is free from Mn(II) dissolution through the smart confinement of manganese monoxide into nitrogen-rich carbon nanosheets (MnO@NCN, 12.28 at.% N)with both physically spatial protection and interfacial chemical binding (C–N–Mn and C–O–Mn). The strongly coupled MnO–NCN interface empowers this nanocomposite highly reversible Mn(II)/Mn(III) redox even at a negative potential of −1.2 V (vs. SCE). The MnO@NCN anode attains a high specific capacitance of 562.3 C g−1 and extremely long-term 10,000 cycles with tiny capacitance decay. An ASC consisted of NiCo-layered double hydroxides (NiCo-LDH) and the MnO@NCN anode delivers specific energy as high as 94 Wh kg−1, remarkably outperforming its counterparts built with the commonly-used carbon anodes and the vast majority of ASCs. This work establishes a viable strategy to stabilize manganese-oxide-based electrodes in aqueous energy storage systems.

Enhancing the performance of manganous oxide nanoparticles for lithium storage by in-situ construction of porous carbon embedment

Although having high theoretical capacity, manganous oxide (MnO) still faces challenges for application in anodes of lithium-ion batteries because of its notorious shortcomings such as enormous volume change and poor conductivity. Herein, a facile and relatively green strategy has been demonstrated to efficiently mitigate above issues, that is, in-situ embedding MnO nanoparticles into a porous carbon matrix by a NaCl template method. The well-designed porous carbon matrix enhances not only conductivity but also structural stability of the as-prepared MnO/C composite electrode during cyclic charge/discharge process. As a consequence, this MnO/C composite, evaluated as an anode of lithium-ion batteries, shows improved performance with 1501 and 1277 mAh g−1 after 180 and 250 cycles at 200 and even 1000 mA g−1, respectively. Moreover, this effective strategy may increase the margin of facile and green preparation of other high-performance metal oxide-based anodes for lithium-ion batteries.

Aromatic C−H Hydroxylation Reactions with Hydrogen Peroxide Catalyzed by Bulky Manganese Complexes

AbstractThe oxidation of aromatic substrates to phenols with H2O2 as a benign oxidant remains an ongoing challenge in synthetic chemistry. Herein, we successfully achieved to catalyze aromatic C−H bond oxidations using a series of biologically inspired manganese catalysts in fluorinated alcohol solvents. While introduction of bulky substituents into the ligand structure of the catalyst favors aromatic C−H oxidations in alkylbenzenes, oxidation occurs at the benzylic position with ligands bearing electron‐rich substituents. Therefore, the nature of the ligand is key in controlling the chemoselectivity of these Mn‐catalyzed C−H oxidations. We show that introduction of bulky groups into the ligand prevents catalyst inhibition through phenolate‐binding, consequently providing higher catalytic turnover numbers for phenol formation. Furthermore, employing halogenated carboxylic acids in the presence of bulky catalysts provides enhanced catalytic activities, which can be attributed to their low pKa values that reduces catalyst inhibition by phenolate protonation as well as to their electron‐withdrawing character that makes the manganese oxo species a more electrophilic oxidant. Moreover, to the best of our knowledge, the new system can accomplish the oxidation of alkylbenzenes with the highest yields so far reported for homogeneous arene hydroxylation catalysts. Overall our data provide a proof‐of‐concept of how Mn(II)/H2O2/RCO2H oxidation systems are easily tunable by means of the solvent, carboxylic acid additive, and steric demand of the ligand. The chemo‐ and site‐selectivity patterns of the current system, a negligible KIE, the observation of an NIH‐shift, and the effectiveness of using tBuOOH as oxidant overall suggest that hydroxylation of aromatic C−H bonds proceeds through a metal‐based mechanism, with no significant involvement of hydroxyl radicals, and via an arene oxide intermediate.magnified image

A novel process on the recovery of zinc and manganese from spent alkaline and zinc-carbon batteries

Spent alkaline and zinc-carbon batteries contain valuable elements (notably, Zn and Mn), which need to be recovered to keep a circular economy. In this study, the black mass materials from those spent batteries are pyrometallurgically treated via a series of process steps in a pilot-scale KALDO furnace to produce an Mn–Zn product, a ZnO product, and an MnO (manganese monoxide) product, toward applications of Mn–Zn micronutrient fertilizer, zinc metal, and manganese alloy, respectively. After an oxidative roasting step, an Mn–Zn product, containing 43% Mn, 22% Zn, and negligible amounts of toxic elements (notably, Cd, Hg, and Pb), could be produced, being suitable for the micronutrient fertilizer application. After a reductive roasting step, a ZnO product and an MnO product are produced. The attained ZnO product, containing up to 84.6% ZnO, is suitable for zinc metal production when the leaching steps are taken to remove most of the Cl and F in the product. The attained MnO product, containing up to 91.7% MnO, is of premium quality for manganese alloy production, preferably for SiMn alloy production due to its low phosphorus content. The proposed application scenarios could substantially improve the recovery efficiency of those spent batteries.

Electrochemical sensor based on the Mn3O4/CeO2 nanocomposite with abundant oxygen vacancies for highly sensitive detection of hydrogen peroxide released from living cells.

Based on the strategy of increasing the number of oxygen vacancies to improve the catalytic performance, we have developed a novel electrochemical sensor based on the multivalent metal oxides cerium dioxide and manganous oxide (Mn3O4/CeO2) for reliable determination of extracellular hydrogen peroxide (H2O2) released from living cells. The Mn3O4/CeO2 nanocomposite was characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The electrochemical performance of the Mn3O4/CeO2 nanocomposite modified glassy carbon electrode (Mn3O4/CeO2/GCE) was investigated. Owing to the abundant oxygen vacancies and strong synergistic effect between the multivalent Ce and Mn, the sensor exhibited excellent catalytic activity and selectivity for the electrochemical detection of H2O2 with a low quantitation limit of 2 nM. Moreover, Mn3O4/CeO2/GCE exhibited excellent reproducibility, repeatability, and long-term storage stability. Because of these remarkable analytical advantages, the constructed sensor was able to determine H2O2 released from living cells with satisfactory results. The results showed that the Mn3O4/CeO2 sensor is a promising candidate for a nanoenzymatic H2O2 sensor with the possibility of applications in physiology and diagnosis.

Sub-nanometric Manganous Oxide Clusters in Nitrogen Doped Mesoporous Carbon Nanosheets for High-Performance Lithium-Sulfur Batteries.

The greatest challenge for lithium-sulfur (Li-S) batteries application is the development of cathode hosts to address the low conductivity, huge volume change, and shuttling effect of sulfur or lithium polysulfides (LiPs). Herein, we demonstrate a composite host to circumvent these problems by confining sub-nanometric manganous oxide clusters (MOCs) in nitrogen doped mesoporous carbon nanosheets. The atomic structure of MOCs is well-characterized and optimized via the extended X-ray absorption fine structure analysis and density functional theory (DFT) calculations. Benefiting from the unique design, the assembled Li-S battery displays remarkable electrochemical performances including a high reversible capacity (990 mAh g-1 after 100 cycles at 0.2 A g-1) and a superior cycle life (60% retention over 250 cycles at 2 A g-1). Both the experimental results and DFT calculations demonstrate that the well-dispersed MOCs could significantly promote the chemisorption of LiPs, thus greatly improving the capacity and rate performance.