Enhancement Of Catalytic Activity Research Articles

Enhancement of Catalytic Activity and Selectivity for the Gaseous Electroreduction of CO2 to CO: Guidelines for the Selection of Carbon Supports (Adv. Sustainable Syst. 12/2021)

CO2 Reduction Recently, as a way to address global warming, research on electrochemical carbon dioxide conversion to valuable chemicals has been being actively conducted. In article number 2100216, Won Suk Jung, Hyung-Suk Oh and co-workers report the development of silver nanoparticles supported on a carbon nano-cage as a catalyst for converting CO2 to CO. The catalyst demonstrates high catalytic activity and durability, and is easy to mass-produce by applying it to a stacked zero-gap electrolyzer.

Surface Electronic Structure Modulation of Cobalt Nitride Nanowire Arrays via Selenium Deposition for Efficient Hydrogen Evolution

AbstractNonmetal heteroatom incorporation into the lattice of host materials is a common way to regulate the surface electronic structure of electrocatalysts to boost their electrocatalytic performance. However, the heteroatom incorporation will inevitably trigger lattice strain and vacancy, which may lead to large changes in the structure of host materials. In this situation, the reconstruction of doped catalysts easily occurs during the catalytic process under harsh alkaline media, which hinders understanding the structure–activity correlation between catalysts and catalytic performance. Herein, taking cobalt nitride as an example, it is promulgated that a low‐temperature selenium sublimation strategy can effectively optimize the surface electronic structure in Co4N nanowire arrays with no significant change of bulk phase structure. Benefiting from surface selenium modification, the optimized SeCo4N nanowire arrays exhibit a 6.5 times enhancement of catalytic activity with structural phase stability for hydrogen evolution reaction in basic media. These findings may provide a new concept to design stable structured catalysts for energy‐related applications.

Carbon supported copper catalyst prepared in situ by one-pot pyrolysis of Bougainvillea glabra: An efficient and stable catalyst for selective oxidation of cyclohexane

So far, the poor stability of Cu2O is still the major challenge in heterogeneous catalysis. Herein, Bougainvillea glabra (B. glabra) derived carbon supported copper composite (Cu2O/BGC) was synthesized in situ by one-pot pyrolysis method. In the synthetic strategy, B. glabra was used as reductant and carbon source simultaneously, and copper nitrate as copper precursor. Cu2O/BGC exhibited a significant enhancement in both catalytic activity and stability for cyclohexane oxidation with tert-butyl hydroperoxide as oxidant under solvent-free condition. Cu2O/BGC with a Cu/C ratio of 0.8 calcined at 800 °C showed the highest catalytic cyclohexane conversion efficiency (77.1%) and cyclohexanone selectivity (68.5%). Its turnover number (TON) is 12.2- and 1.3-fold compared to pure Cu2O and Cu2O/BGC prepared by deposition-reduction method, respectively. Hot filtration experiment confirmed that the sample was a heterogeneous catalyst which could be reused five times without losing its activity.

Solution‐Processed Graphene Thin‐Film Enables Binder‐Free, Efficient Loading of Nanocatalysts for Electrochemical Water Splitting

AbstractElectrochemical water splitting is promising to produce high purity hydrogen that can serve as clean and renewable energy. Reducing the size of catalysts down to the nanometer regime is critical for enhancing the electrochemical performance because reactions mostly occur at the surface of materials. However, fully exploiting the advantage of small nanocatalysts is challenging because accessible catalytic surface area is reduced by particle aggregation and the use of binders. In addition, a significant fraction of nanocatalysts is lost through the electrode pores during catalyst loading. Here, a simple strategy to efficiently load nanocatalysts without using binders is reported. By introducing a thin layer of graphene nanosheets on a porous membrane, a uniform nanocatalyst film via vacuum filtration of the dispersion can be created. The nanocatalyst/graphene composite can be transferred to a carbon paper without fracturing to function as a catalyst layer. The method is compatible with various nanomaterials to realize electrodes for hydrogen and oxygen evolution reactions and even bifunctional electrodes that exhibit activities for both. Significantly, the resulting electrodes show better performance than those produced by directly filtrating through the carbon paper. Moreover, the binder‐free surface allows for further enhancement in catalytic activity via postchemical treatments.

Nanoframes of Co3O4–Mo2N Heterointerfaces Enable High‐Performance Bifunctionality toward Both Electrocatalytic HER and OER

AbstractInterfacial engineering of heterostructured catalysts has attracted great interest in enabling both hydrogen and oxygen evolution reactions (HER and OER), by fine tuning of the interfacial geometry and electronic structures. However, they are not well structured for high‐performing bifunctionalities, largely due to the confined single particle morphologies, where the exposed surfaces and interfaces are limited. Herein, a hollow nanoframing strategy is purposely devised for interconnected Co3O4–Mo2N heterostructures that are designed with interfacial charge transfer from Mo2N to Co3O4, as rationalized by theoretical calculations and confirmed by X‐ray photoelectron spectroscopy analyses. It is shown that by the controllable pyrolysis of bimetallic Mo–Co Prussian blue analogue nanoframes (NFs) with an optimal Mo/Co ratio, the desired nanoframes of Co3O4–Mo2N heterostructure are successfully formed. The as‐synthesized Co3O4–Mo2N NFs not only inherit the functionalities of individual components and the electrolyte‐accessible nanoframe structure, and they also give an ideal heterointerface with strong electron interaction and favorable H2O/H* adsorption energies, leading to a remarkable enhancement in bifunctional catalytic activities (i.e., 12.9‐fold and 20‐fold higher current density under the 300 mV overpotential, as compared to the single‐phased Co3O4 NFs alone toward HER and OER, respectively), while remaining a robust stability.

Controllable Synthesis of Hollow Microtubular Covalent Organic Frameworks as an Enzyme-Immobilized Platform for Enhancing Catalytic Activity.

Despite great achievement that has been made in the synthesis of covalent organic frameworks (COFs), precise construction of COFs with well-defined nano/microstructures poses a rigorous challenge. Herein, we introduce a simple template-free strategy for controllable synthesis of hollow microtubular COFs. The obtained COFs show a spontaneous morphology transformation from a microfiber to a hollow microtubular structure when the concentrations of catalytic acid are regulated elaborately. Furthermore, the as-prepared COFs exhibit high crystallinity, well-defined hollow tubular morphology, and high surface areas (∼2600 m2/g). Taking the advantages of the unique morphological structure, the hollow microtubular COFs can serve as an ideal host material for enzymes. The resultant biocomposites show high catalytic performance and can be successfully applied to rapid and high-efficiency proteolysis of proteins. This work blazes a trail for controllable synthesis of the hollow microtubular COFs through a template-free process and expands the application of COFs as a promising platform for enzyme immobilization.

Novel fluorine-doped cobalt molybdate nanosheets with enriched oxygen-vacancies for improved oxygen evolution reaction activity

Herein, we have synthesized fluorine-doped cobalt molybdate (F-CoMoO4) nanosheet arrays on graphite felt (GF) to efficiently promote the oxygen evolution reaction (OER) kinetics. Experimental results show that F-CoMoO4 has two significant effects: 1) inducing rich oxygen vacancies, and 2) optimizing the electronic configuration of CoMoO4, which is beneficial for exposure of active sites. The as-obtained F-CoMoO4-x-2@GF electrocatalyst exhibits lower OER overpotential of 256 mV at 10 mA cm−2 with a small Tafel slope of 64.4 mV dec−1 in alkaline solution, resulting in a nearly 100 mV of OER catalytic activity enhancement compared with that of pure CoMoO4. DFT results reveal that the change of Mo 4d state reduces the band-gap and increases the electrical conductivity of CoMoO4, thus optimizing its intrinsic activity. The synthesis strategy used in this work may provide some ideas for enhancing the electrical conductivity of other transition metal oxides (TMOs).

Fe decorated CeO2 microsphere catalyst with surface oxygen defect for NO reduction by CO

The selective catalytic reduction of nitrogen oxides (NOx) by carbon monoxide (CO-SCR) is a promising way to deal with exhaust gasses. And Effective redox cycle and NO dissociation ability are crucial factors for catalytic reduction of NOx. Herein, Fe decorated CeO2 microsphere catalysts with surface oxygen defect were synthesized by solvothermal and calcination method. Among all catalysts, 0.5Fe-CeO2 sample showed excellent catalytic performance with efficiency near 96% for NO reduction under the condition of 550 ppmv NO, 1200 ppmv CO, and 40,000 h − 1 of GHSV. It was found that Fe doping could efficiently regulate Ce4+/Ce3+ redox couple and oxygen vacancy for catalytic activity. XRD, Raman, XPS, EPR and H2-TPR etc. were employed to explore the physicochemical properties of samples. The results showed that Fe had been incorporated into CeO2 by XRD, and the results of Raman spectrum and XPS indicated that Ce4+/Ce3+ redox couple existed in 0.5Fe-CeO2 catalyst. Furthermore, the H2-TPR spectrum displayed the formation of oxygen vacancy in the samples. N2 adsorption-desorption isotherms indicated that the doping of Fe could enlarge the specific surface area, pore volume and pore dimeter of the samples, and thus greatly enhanced the NO adsorption and further NO catalytic reduction on catalysts. In addition, other characterization of physicochemical properties and density functional theory (DFT) calculation were performed to reveal the mechanism of NO + CO reaction. As a result, a mechanism was proposed to elucidate highly efficient NO reduction over the catalysts of Fe decorated CeO2 microspheres. Finally, the synergistic effects of redox center and oxygen vacancy on catalytic activity enhancement were revealed. This work provides a promising way to the design of efficient catalysts for NO reduction.

Acid-activated layered δ-MnO2 promotes VOCs combustion

The layered δ-MnO2 nanomaterial is an inexpensive and environmentally friendly nanocatalyst for volatile organic compounds (VOCs) catalytic oxidation. However, easy agglomeration and sluggish kinetics lead to its poor catalytic activity. Herein, we develop an acid-activated method to prepare δ-MnO2 catalysts with weakened MnO bonds, enhanced specific surface area and increased active surface lattice oxygen species, which significantly enhances catalytic performance for toluene oxidation. The δ-MnO2 catalyst treated by 1 M hydrochloric acid for 3 h can completely convert toluene into CO2 and H2O at 280 °C under the GHSV of 72,000 mL g−1h−1, and it can maintain good catalytic stability for more than 50 h without obvious potential decay. Further, the acid-activated strategy can be extended to other phase structured MnO2 with varying degrees of catalytic activity enhancement. The results demonstrate that the acid-activated way has great potential in improving catalytic oxidation activity of nanocatalysts for eliminating VOCs.

Enhancement of Catalytic Activity and Stability of La0.6Ca0.4Fe0.7Ni0.3O2.9 Perovskite with ppm Concentration of Fe in the Electrolyte for the Oxygen Evolution Reaction

The catalytic activity and stability of an iron-nickel based oxygen-deficient perovskite for the oxygen evolution reaction (OER) are drastically improved with the ppm additive of Fe ions to the alkaline electrolyte. The enhancement is attributed to a 1–2 nm restructured Ni0.5Fe0.5Ox(OH)2-x (oxy)hydroxide layer, as demonstrated with scanning transmission electron microscopy. La0.6Ca0.4Fe0.7Ni0.3O2.9 shows almost a four-fold increase in OER activity after Fe addition relative to the as-prepared pristine electrolyte, which demonstrates the low Tafel slope of 44 ± 2.4 mV dec−1 and the superior intrinsic activity of 706 ± 71 A g−1oxide at 1.61 V vs. RHE.

Unlocking Asymmetric Michael Additions in an Archetypical Class I Aldolase by Directed Evolution.

Class I aldolases catalyze asymmetric aldol addition reactions and have found extensive application in the biocatalytic synthesis of chiral β-hydroxy-carbonyl compounds. However, the usefulness of these powerful enzymes for application in other C–C bond-forming reactions remains thus far unexplored. The redesign of class I aldolases to expand their catalytic repertoire to include non-native carboligation reactions therefore continues to be a major challenge. Here, we report the successful redesign of 2-deoxy-d-ribose-5-phosphate aldolase (DERA) from Escherichia coli, an archetypical class I aldolase, to proficiently catalyze enantioselective Michael additions of nitromethane to α,β-unsaturated aldehydes to yield various pharmaceutically relevant chiral synthons. After 11 rounds of directed evolution, the redesigned DERA enzyme (DERA-MA) carried 12 amino-acid substitutions and had an impressive 190-fold enhancement in catalytic activity compared to the wildtype enzyme. The high catalytic efficiency of DERA-MA for this abiological reaction makes it a proficient “Michaelase” with potential for biocatalytic application. Crystallographic analysis provides a structural context for the evolved activity. Whereas an aldolase acts naturally by activating the enzyme-bound substrate as a nucleophile (enamine-based mechanism), DERA-MA instead acts by activating the enzyme-bound substrate as an electrophile (iminium-based mechanism). This work demonstrates the power of directed evolution to expand the reaction scope of natural aldolases to include asymmetric Michael addition reactions and presents opportunities to explore iminium catalysis with DERA-derived catalysts inspired by developments in the organocatalysis field.

Catalytic activity enhancement in pillared zeolites produced from exfoliated MWW monolayers in solution

Layered zeolites, especially MWW, have been used to synthesize pillared micro/mesoporous hybrids with the goal to enhance catalytic activity towards larger molecules. The critical preparation step is expansion of the interlayer space by swelling with cationic surfactants at high pH. Recently reported solutions of MWW monolayers, obtained by exfoliation of the zeolite MCM-56, can be used to prepare analogous materials by flocculation (precipitation) with surfactant solutions. This previously unavailable approach is studied in this work with both high pH (0.2 M, pH > 13.3) and lower pH (0.01 M, pH~12) solutions, and resulted in finding conditions for preparation of catalysts that can be more active than pure microporous layers (zeolite) despite large content of unreactive silica pillars. Both the high and low pH conditions afforded similar expanded surfactant-MWW composites, but their behavior differed during pillaring with TEOS. Well-defined expanded interlayer distances indicated by a low angle reflection in XRD above 3 nm d-spacing was observed only with the high alkalinity preparations. An additional benefit was observed with TEOS treatment in the presence of isopropyl alcohol leading to a product showing high catalytic activity in model alkylation reaction (mesitylene with benzyl alcohol) proceeding faster than with the pure zeolite. The products obtained under different conditions were characterized by textural methods, nitrogen adsorption and QE-TPDA, and FT-IR to investigate acid site parameters. Notably, the MCM-56 used in this work was prepared with addition of aniline as the structure-promoting agent and is the second formulation affording exfoliable MWW materials.

Photo-assistant electrocatalytic activity improvement towards oxygen evolution

As one of the most important reactions involved in water splitting, oxygen evolution reaction (OER) suffers from a slow kinetic. Herein, with a composite catalyst of Au/Co-FeOOH, it was shown that the photo-irradiation could greatly activate the catalytic performance for OER due to the plasmon effect of Au nanoparticles. With 532 nm of photo-irradiation, the overpotential at 10 mA cm−2 for OER decreased from 386 to 309 mV with the optimized composite. It is believed that the two factors, hot electron excitation promoting the formation of active species and the charge transfer modulating the electronic structure of active sites, are responsible for the enhancement of catalytic activity. These findings highlighted the possibility of activation of OER catalysis by photo-irradiation and would open up a new route for the design of advanced electrocatalysts.

Catalytic Activity Enhancement of Cu-Zn-Based Catalyst for Methanol Steam Reforming with Magnetic Inducement

Magnetic inducement was applied during metal loading to enhance Cu-Zn catalysts for methanol steam reforming in the temperature range of 200–300 °C. The supports used in this study were the γ-Al2O3 support and CeO2-Al2O3 supports prepared under different magnetic environments. Cu-Zn loading between the north and south poles (N-S) on the CeO2-Al2O3 support, prepared between two north poles (N-N), led to the highest H2 production at 300 °C (2796 ± 76 µmol/min), which is triple that of Cu-Zn/CeO2-Al2O3 prepared without magnetic inducement and ~11-fold the activity of the Cu-Zn/Al2O3 reference catalyst. The N-S magnetic environment during metal loading leads to lower reduction temperatures and larger Cu(1+):Cu(2+) ratio. These results showed that the pole arrangement of magnets during metal loading could affect the catalytic activity of the Cu-Zn catalyst owing to its influence on the reducibility and the oxidation state of Cu active metal.

Enhancement of Catalytic Activity and Selectivity for the Gaseous Electroreduction of CO2 to CO: Guidelines for the Selection of Carbon Supports

AbstractTo secure the economic feasibility of electrochemical carbon dioxide reduction reaction (CO2RR), a method of using support materials to reduce the amount of catalyst loading and increase catalytic activity for CO2RR can be a solution. Herein, guidelines for selecting carbon support materials suitable for CO2RR are reported. In a gaseous CO2‐fed CO2RR electrolyzer, an Ag nanoparticle‐composited carbon nanocage (denoted as Ag/CNC) electrode exhibits an optimal performance for CO2RR with a partial current density of ≈400 mA cm−2. Ag/CNC showcases a uniformly dispersed morphology with Ag nanoparticles and sufficient hydrophobicity after the reaction to prevent flooding, which is known to inhibit the mass transfer of CO2. Furthermore, the CNCs possess small amounts of carbon defects; this suppresses the catalytic activity for the hydrogen evolution reaction (HER), which is a side reaction of CO2RR. Results of a three‐stack cell experiment with the Ag/CNC electrode reveal the possibility of a support‐based strategy for achieving an industrial‐level scaling of CO2RR. This study suggests that the support material for CO2RR should be considered not only on the basis of the morphology of carbon, uniform dispersion of Ag nanoparticles, and hydrophobicity of the electrode but also factors influencing the HER, such as carbon defects.

An in-situ strategy to analyze multi-effect catalysis in iron-copper bimetals catalyzed Fenton-like processes

Iron-copper bimetals for Fenton-like catalysis were developed, due to efficient enhancement of catalytic activity by iron-copper synergy. However, deviation exists in mechanism analysis of multi-effect catalysis. In this study, a new in-situ strategy of shielding Fe(II) and Cu(I) from further reaction by trapping agents was developed, for analysis of iron catalysis alone, copper catalysis alone and synergistic effect in bimetallic catalyst systems. Inhibition efficiency and influence of trapping agents on Fe(II)/Cu(I) catalysis were quantitatively evaluated for reliability confirmation and method correction. The possible application scope was identified as pH of 4–6, and this strategy was available for analysis of five different groups of catalysts. The potential superiority and difference of the proposed strategy compared with current ex-situ strategy were also analyzed. The overestimation (12.1%–35.4%) of copper catalysis and biased analysis (0.1%–12.6%) of iron catalysis by ex-situ strategy can be alleviated, thereby improving accuracy for synergistic effect. In summary, this study proposed a new strategy to improve the understanding of iron-copper bimetallic catalysis, which can provide reference for other polymetallic catalysis analysis development.

Methane dehydroaromatization over Mo and W catalysts supported on ZSM-5

Methane aromatization reaction to produce benzene using tungsten and molybdenum catalysts supported on ZSM-5 was investigated at 800 oC. Catalysts were prepared by impregnation of tungsten and molybdenum salts on ZSM-5 zeolite with various metal loadings in the range of 2-10 wt. %. In order to obtain the catalytic structures before and after the reaction, catalysts were characterized by XRD and FTIR analysis. It was indicated from reactor tests that increase of metal loading on the catalyst surface leads to increase methane conversion (1.1% and 3.2% for 2W/ZSM-5 and 6 W/ZSM-5, and 2.4% and 4.8% for 2W/ZSM-5 and 6 W/ZSM-5, respectively, at time of stream equals 120 min). It was also concluded that Mo catalysts show higher activity and stability than W (methane conversion of 3.2 and 9 % using 10Mo/ ZSM-5 and 10 W/ZSM-5 catalysts respectively, at time of stream equals 100 min), and increase of Mo loading leads to enhancement of catalytic activity and methane conversion, which indicate initial activation of methane is occurred on metal sites of catalysts. This activation leads to occur the later reactions and production of final benzene. These conditions confirm the two-factor mechanism which include two stages: i) hemolysis break of C-H bond and CH3 radical formation and then ethylene formation, and ii) cyclization of ethylene species in the presence of acidic sites within the zeolite channels. Investigations on mesoporous HMS support showed no aromatic production, which show increase of support channel diameter leads to reduce the possibility of ring formation.

Ni/CeO2 catalysts for low‐temperature CO2 methanation: Identifying effect of support morphology and oxygen vacancy

AbstractIn order to investigate the effect of support morphology and oxygen vacancy on the low temperature catalytic activity of Ni‐based catalyst for CO2 methanation, three Ni/CeO2 catalysts were prepared by impregnation method, in which CeO2 materials with different morphology of flowerlike, nanowire, and bulk were used. The characterization results showed that the morphology of CeO2 exhibited a significant effect on the preferential exposed crystal planes, which led to the change of oxygen vacancy concentration and metal dispersion, resulting in the enhancement of catalytic activity at low temperature. Among all catalysts, the nanowire‐like Ni/CeO2 catalyst (Ni/W) showed the highest CO2 conversion due to its enhanced oxygen vacancy concentration and CO2 adsorption capacity of the catalyst. In situ DRIFTS experiment showed that the intermediate product of Ni/W catalyst reaction route was formate. In a 100 h–400°C–lifetime test, Ni/W catalyst also showed excellent long‐term stability. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Ultrasound-activated Au/ZnO-based Trojan nanogenerators for combined targeted electro-stimulation and enhanced catalytic therapy of tumor

Integrating the advantages of multiple therapies into one therapeutic platform is highly desired for augmented treatment effects. Herein, by modifying MLS and PEG on ZnO-nanorods/AuNPs nanoassemblies and coating with C6 cell membrane (MP-Au/ZnO@CCM), we construct a novel ultrasound-activatable Au/ZnO-based tumor therapeutic platform (Trojan nanogenerators) and exploit it for effective tumor therapy. The developed MP-Au/ZnO@CCM nanogenerators worked well in multiple ways to achieve targeted electrostimulation and enhanced catalytic therapy of tumor, under ultrasound excitation. Firstly, the ZnO-nanogenerators will generate a piezoelectric potential difference of about 140 mV by ultrasonic excitation when targeted (by MLS moiety) to the mitochondria, disrupting the mitochondrial membrane potential for electrostimulation. Secondly, the nanogenerators enhances greatly multiple enzyme-like activities of Au nano-components for enhanced tumor catalytic therapy. Among them, the affinity and maximum reaction rate of peroxidase-like are increased by 34.99% and 53.36% respectively through kinetic analysis. Simulations and calculations revealed the roles of piezoelectric polarization electric field and displacement current in enhancing catalytic activity. The superior tumor suppressive effect of the MP-Au/ZnO@CCM was confirmed by in-vitro cell study and in-vivo C6 tumor-bearing mouse models. The developed ultrasound-activated nanogenerators will provide a unique electrostimulation tumor therapy strategy and opens a new way for improving the therapeutic efficacy of nanozymes. By combined use of nanogenerators and nanozymes, a novel ultrasound-activated Au/ZnO-based tumor therapeutic platform (Trojan nanogenerators) was constructed for effective tumor therapy. The developed Trojan nanogenerator can simultaneously perform mitochondrial targeted electrostimulation and enhanced catalytic therapy of tumor, under ultrasound excitation. The superior antitumor effect was confirmed through in vitro and in vivo experiments. • A novel ultrasound-activatable Au/ZnO-based Trojan nanogenerators was developed and employed for effective tumor therapy. • Targeted electrostimulation at the subcellular level using ZnO nanogenerators was realized for the first time. • Multiple enzyme-like activities of Au nano-components was boosted greatly for enhanced tumor catalytic therapy. • The vital roles of polarization electric field and displacement current on catalytic activity enhancement were revealed.

Syngas Production via CO2 Reforming of Methane over Aluminum-Promoted NiO-10Al2O3-ZrO2 Catalyst.

CO2 reforming of methane was studied at medium temperature (700 °C) using a GSHV of 48,000 h–1 over nickel catalysts supported on ZrO2 promoted by alumina. The catalysts were prepared by a one-step synthesis method and characterized by BET, H2-TPR, XRD, XPS, TEM, Raman spectroscopy, and TGA. The NiO–10Al2O3–ZrO2 catalyst exhibited higher catalytic performance in comparison with the NiO–ZrO2 catalyst. The enhancement of catalytic activity in dry reforming could be associated with the alterations in surface properties due to Al promotion. First, the Al promoter could modify the structure of ZrO2, leading to an increase of its pore volume and pore diameter. Second, the NiO–10Al2O3–ZrO2 catalyst exhibited high resistance to sintering. Third, the NiO–10Al2O3–ZrO2 catalyst showed high suppression to the loss of nickel during a long-term catalytic test. Finally, the addition of Al could inhibit the reduction of ZrO2 during the reduction and reaction, endowing further the stability.