Surface Labile Oxygen Research Articles

Dual-element substitution induced integrated defect structure to suppress voltage decay and capacity fading of Li-rich Mn-based cathode

Li-rich manganese-based oxide (LRMO) is considered one of the most promising cathode materials for next-generation lithium-ion batteries due to its high energy density. However, many issues need to be addressed before its large-scale commercialization, including significant voltage decay and capacity fading. Herein, a Sn4+/Na+ co-doping induced integrated defect structure (oxygen vacancies, stacking faults, and surface spinel phase) strategy is proposed to suppress the voltage decay and enhance the cycling performance of LRMO. The integrated surface defect structures have significantly favorable effects on the LRMO, where the oxygen vacancies remove surface labile oxygen and suppress surface oxygen release, the induced stacking faults alleviate the stress accumulation during cycling, the surface spinel phase promotes the Li+ diffusion and prevents the outward migration of cations, and the co-doped Sn4+/Na+ stabilize the layered structure. As a result, the modified sample Na2SnO3-1 % (NSO-1) achieves excellent cycling performance (capacity of 207 mAh/g and capacity retention of 96.71 % after 100 cycles at 0.5C) and a smaller voltage decay (less than 1.5 mV per cycle) compared with the unmodified LRMO. This work provides a new valuable strategy to suppress capacity fading and voltage decay of LRMO through dual-element substitution induced surface defect engineering.

Promotional effect of WO3 in V2O5–WO3/TiO2 on low-temperature activity in the catalytic oxidation of 1,2-dichloroethane

Transition metal oxides are promising catalysts for the degradation of chlorinated volatile organic compounds (CVOCs). However, their practical applications are limited due to their poor low-temperature (<300 °C) activity. In this study, we introduced WOx into VOx/TiO2 to modulate its redox and acidic properties. We found that the V2O5(2 wt%)–WO3(10 wt%)/TiO2 (2V10W/TiO2) catalyst exhibited a 1,2-dichloroethane (DCE) conversion of 90 % at 256 °C, which is comparable to that of Ru-based catalysts (DCE conversion of 90 % at 257–340 °C) for the oxidation of DCE. The addition of WOx into VOx/TiO2 gave rise to the formation of labile surface oxygen species and promoted interactions between V and W, which improved the reducibility, oxygen mobility, and Brønsted acidity of the 2V10W/TiO2 catalyst. These properties of 2V10W/TiO2 lowered its activation barrier for DCE oxidation and mitigated coke formation and Cl deposition on the catalyst, enhancing the low-temperature activity and long-term stability. The 2V10W/TiO2 catalyst also exhibited superior resistance to water. This study provides insights into catalytic design for balancing the redox and acidic properties in the catalytic oxidation of CVOCs.

Co-existence of atomically dispersed Ru and Ce3+ sites is responsible for excellent low temperature N2O reduction activity of Ru/CeO2

Nitrous oxide N2O reduction is a big challenge due to high global warming potential of N2O. (∼300 times higher compared with CO2). The best known catalysts, such as Rh/ceria, require relatively high temperatures for N2O decomposition. Herein, we report that Ru/ceria catalysts with low Ru loading of ∼0.25 wt% efficiently catalyze low temperature N2O reduction by CO starting at 100 ℃ (full N2O conversion below 200 ℃) under industrially relevant flow rates and gas concentrations. This remarkable performance stems from maintaining isolated Ru cations even on reduced ceria surface and, simultaneously, the propensity of Ru to affect ceria surface to form labile surface oxygen thereby creating large number of oxygen vacancies (Ce3+ cations) in the presence of CO. In contrast, for Rh/CeO2 catalysts with equivalent metal loading, the activity is much lower because atomically dispersed Rh sinters into metallic clusters at the onset reaction temperature (∼200 °C): these clusters are much less effective than isolated single Ru ions, with lower Ce3+ concentration maintained on reduced Rh/CeO2 catalyst. Our study highlights the benefits of gaining molecular-level insight into the dynamic nature of catalytically active sites under reaction conditions for preparing catalysts containing low loading of precious metals with unsurpassed low temperature activity.

Effect of preparation method on active sites variation for catalytic oxidation of toluene over Pt/MCM-48

Toluene oxidation behavior has been investigated on a series of catalysts with Pt supported on MCM-48 that has a large specific surface area & easy-transported three-dimensional channels prepared by four different methods. It was found that various introduction ways of Pt have a great influence on both Pt content and state, resulting in a difference in catalytic oxidation performance. Pt/M48-PG prepared via the post-grafting method possesses both the highest Pt content (0.83 wt%) and active species of Pt0 & surface labile oxygen (accounting for 46% and 55%, respectively), resulting in the lowest T90 (155 °C) and the highest TOF (7.92 ×10−3·s−1). Characterization results showed that the amine functional groups could be successfully grafted onto the support's surface, which is beneficial for the binding of platinum precursor with support, leading to the high actual loading efficiency and dispersion of Pt, both of which contribute to retaining more Pt0 and surface labile oxygen active species.

Phosphorylation of Li-Rich Mn-Based Layered Oxides for Anion Redox and Structural Stability.

Li-rich Mn-based layered oxides are proposed to be candidates for high-energy Li-ion batteries. However, their large-scale production is still hampered by poor rate capability and severe voltage decay. It was mainly attributed to the irreversible oxygen loss, which induces transition metal ion migration, electrolyte consumption, and structural evolution. Herein, we propose an effective strategy of phosphorylation, in which the phosphate ion is induced to remove the surface labile oxygen. It urges the Li2MnO3 component to transform to the spinel-like structure and promotes the anionic redox process, thus facilitating lithium-ion diffusion and improving structural stability. As a result, the Li2MnO3 component is more prone to be activated, with the capacity increased by 18% in comparison with the pristine one. It also exhibits a superior capacity retention of 86.1% after 150 extended cycles and better rate performance delivering a capacity of 148.1 mA h g-1 even at 10 C. The effective phosphorylation opens a new way to tune anion redox chemistry and obtain structurally stable materials.

Constructing a robust integrated surface structure for enhancing the performance of Li-rich Mn-based oxides cathodes

Li-rich Mn-based oxides (LRMO) have emerged as the next-generation cathode materials for the advanced lithium-ion batteries due to its high capacity and low cost. However, its practical application has been impeded by the irreversible oxygen loss and structural deterioration of LRMO surface, resulting in low initial coulombic efficiency (ICE), capacity, and voltage degradation as well as sluggish kinetic reaction. Herein, we present a surface engineering strategy that uses NH4BF4 treatment to simultaneously integrate spinel phase, oxygen vacancies, and dual-element (B and F) doping on the LRMO surface. As a consequence, the treated LRMO, namely NHBF-2, shows much higher ICE of 89.41% with an increased capacity retention of 88.9% at 1 C after 100 cycles than the untreated LRMO (76.87% and 79.6% for ICE and capacity retention, respectively). Such improved performance of LRMO can be attributed to the robust integrated surface, where oxygen vacancies remove surface labile oxygen and suppress irreversible oxygen releasing, spinel phase promotes the Li+ diffusion kinetic, and B and F doping aids in stabilizing the surface structure. This study provides guidance for designing high-energy cathode materials with a stable surface by using surface modification on LRMO.

Magnetic-field-assisted catalytic oxidation of arsine over Fe/HZSM-5 catalyst: Synergistic effect of Fe species and activated surface oxygen

The traditional way for arsine (AsH3) removal from the reducing exhaust gas by catalytic oxidation undershot the expect goal in pollutant emission control. This laboratory scale work used magnetic field to improve the chemical environment of AsH3 catalytic oxidation over magnetic response catalyst Fe/HZSM-5. After introducing external magnetic field at a magnetic field intensity of 0.1 T, the catalytic performance of Fe/HZSM-5 for AsH3 removal was improved by 52%, and the turnover frequency (TOF) value increased from 4.65 × 10−3 s−1 to 5.73 × 10−3 s−1. The structure–activity relationship of the catalyst in the presence of magnetic field was investigated. The characterization results showed that Fe is mainly anchored onto the surface of Fe/HZSM-5 catalyst through an oxygen bridge, while a small proportion of Fe exists in the form of iron oxides. The Fe species and surface labile oxygen (O−) in the aluminum tetrahedra are the magnetic active sites of Fe/HZSM-5, magnetic field promotes the reaction by increasing the activity of them. It was discovered that paramagnetic O− increased by 50% in the presence of magnetic field compared with that in its absence. Therefore, a magnetic-field-assisted oxygen cycling process is proposed. In terms of the stability of catalyst, formation of more amorphous oxidized arsenic after reaction in the presence of magnetic field reduced the toxicity to the strong acid sites of the catalyst, and the holding time of excellent AsH3 removal efficiency was prolonged from 23 h to 35 h. These results provide new insights into magnetic-field-assisted zeolite catalysis.

Amino-acid modulated hierarchical In/H-Beta zeolites for selective catalytic reduction of NO with CH4 in the presence of H2O and SO2.

Selective catalytic reduction of NO with CH4 (CH4-SCR) has been studied over a series of amino-acid mediated hierarchical beta zeolites with indium exchange. Amino acid mesoporogens greatly affect the NO reduction (DeNOx) efficiency of In/H-Beta catalysts. Mesoporous In/H-Beta-P synthesized using proline exhibits the highest NOx removal efficiency of 40% in excess oxygen and poisonous SO2 and H2O, 10% higher than our previously optimized In/H-Beta catalyst using commercial beta zeolites with a similar Si/Al ratio. Analyses using XRD, N2 adsorption-desorption, EPR, SEM, TEM, EDX, ICP, 27Al and 29Si MAS NMR, XPS, H2-TPR, NH3-TPD, and Py-IR reveal that amino acids promote beta crystallization, modulate zeolite acid sites and surface oxygen species, and generate hierarchical pore architectures without affecting the Si/Al ratio, indium content, and percentage of the active InO+ species. The mosaic-structured In/H-Beta-P exhibits the strongest Brønsted acidity and surface labile oxygen which enhance the oxyindium interaction with the zeolite framework, promoting CH4-SCR activity. The strong acidity, surface active oxygen species, and mesopores lead to excellent stability of the In/H-Beta-P catalyst in the presence of SO2 and H2O, withstanding several catalytic DeNOx cycles under harsh reaction conditions.

Elucidating the role of La3+/Sm3+ in the carbon paths of dry reforming of methane over Ni/Ce-La(Sm)-Cu-O using transient kinetics and isotopic techniques

The different effects of the presence of La3+ and Sm3+ heteroatoms in the 5 wt% Ni/45Ce-45(Sm or La)-10Cu-O catalytic system on the carbon deposition and removal reaction paths in the dry reforming of methane (DRM) at 750 °C were investigated using transient kinetic and isotopic experiments. The relative initial rates of carbon oxidation by lattice oxygen of support and that by oxygen derived from CO2 dissociation under DRM reaction conditions were quantified. Ni nanoparticles (23-nm) supported on La 3+-doped ceria exhibited at least 3 times higher initial rates of carbon oxidation to CO by lattice oxygen, and ~ 13 times lower rates of carbon accumulation than Ni (18-nm) supported on Sm3+-doped ceria. The concentration and mobility of labile surface oxygen at the Ni-support interface region seems to correlate with carbon accumulation. Ni/Ce-La(or Sm)-–10Cu-O formed NiCu alloy nanoparticles, partly responsible for lowering carbon deposition and increasing carbon oxidation rates to CO.

Direct electron transfer process-based peroxymonosulfate activation via surface labile oxygen over mullite oxide YMn2O5 for effective removal of bisphenol A

The development of efficient and eco-friendly Mn-based catalysts for peroxymonosulfate (PMS) activation and revealing the mechanism are highly desired for pollutants removal in wastewater. In this study, a ternary mullite oxide YMn2O5 (YMO) was synthesized for bisphenol A (BPA) degradation in the presence of PMS for the first time. Combined with XPS, EPR and TG, abundant surface labile oxygen species (Olab) with a ratio of 39.7% were identified in the mullite materials, which was higher than that for α-, β-, γ- and δ-MnO2. YMO also exhibited higher interfacial conductivity and stronger interactions among the catalyst, PMS and BPA than other MnO2. Superior performance in BPA degradation was observed with YMO, and Olab was suggested as the main active sites, for the normalized first-order rate constant was linearly dependent on the ratio of the oxygen species. Chemical quenching experiments, EPR and open circuit potential suggested that the degradation of BPA was mainly caused by the direct electron transfer process through the reactive Olab-PMS complexes. The influence of YMO concentration, PMS concentration, solution pH, different anions and reaction temperature was thoroughly studied. Moreover, the intermediates were identified and the possible degradation pathways were proposed. This work demonstrated that mullite oxide YMO with enriched Olab species can be used as eco-friendly catalysts for efficient PMS activation to remove recalcitrant organic compounds in wastewater.

Low-temperature removal of aromatics pollutants via surface labile oxygen over Mn-based mullite catalyst SmMn2O5

The development of high efficient oxide catalyst is crucial to remove the aromatics pollutants. Herein, we propose a ternary mullite SmMn2O5 to catalytically oxidize benzene and toluene at a low temperature. Through a joint exploration of experimental characterizations and density functional theory calculations, the active species on the surface of the material are accessed. In the presence of the two-coordinated surface labile oxygen (Olab), the hydrothermal-synthesized mullite achieves a remarkable deep oxidation performance with T90 at 223 °C (benzene) and 228 °C (toluene), superior to most reported oxide catalysts. Simultaneously, SmMn2O5 displays no deactivation after 150 h reactions with repeating water vapor. Combining in situ DRIFTS and DFT calculations, the dissociation of maleic anhydride into acetic anhydride species on Olab active sites turns out to be the rate-controlled step with a calculated kinetic barrier of 1.253 eV. These findings of Olab allow to understand the catalytic oxidation of metal oxides at the atomic level and are thus imperative for the catalyst development.

Accurate Control of Initial Coulombic Efficiency for Lithium-rich Manganese-based Layered Oxides by Surface Multicomponent Integration.

Low initial Coulombic efficiency (ICE) is an obstacle for practical application of Li-rich Mn-based layered oxides (LLOs), which is closely related with the irreversible oxygen evolution owing to the overoxidized reaction of surface labile oxygen. Here we report a NH4 F-assisted surface multicomponent integration technology to accurately control the ICE, by which oxygen vacancies, spinel-layered coherent structure, and F-doping are skillfully integrated on the surface of treated LLOs microspheres. Though the regulation on the removed amount of labile oxygen by surface integrated structure, the ICE of LLOs cathodes can adjust from starting value to 100 %. X-ray absorption spectroscopy, refined X-ray diffraction, and scanning transmission electron microscopy show that the removed labile oxygen mainly comes from Li2 MnO3 -like structure. Even operating at a high cut-off voltage of 5 V, the capacity retention of integrated sample at 200 mA g-1 is still larger than 98 % after 100 cycles.

Accurate Control of Initial Coulombic Efficiency for Lithium‐rich Manganese‐based Layered Oxides by Surface Multicomponent Integration

AbstractLow initial Coulombic efficiency (ICE) is an obstacle for practical application of Li‐rich Mn‐based layered oxides (LLOs), which is closely related with the irreversible oxygen evolution owing to the overoxidized reaction of surface labile oxygen. Here we report a NH4F‐assisted surface multicomponent integration technology to accurately control the ICE, by which oxygen vacancies, spinel‐layered coherent structure, and F‐doping are skillfully integrated on the surface of treated LLOs microspheres. Though the regulation on the removed amount of labile oxygen by surface integrated structure, the ICE of LLOs cathodes can adjust from starting value to 100 %. X‐ray absorption spectroscopy, refined X‐ray diffraction, and scanning transmission electron microscopy show that the removed labile oxygen mainly comes from Li2MnO3‐like structure. Even operating at a high cut‐off voltage of 5 V, the capacity retention of integrated sample at 200 mA g−1 is still larger than 98 % after 100 cycles.

Three-dimensionally ordered macroporous MnSmOx composite oxides for propane combustion: Modification effect of Sm dopant

Three-dimensionally macroporous (3DOM) MnSmOx composite oxides were prepared via a hard-template method and utilized as catalytic materials for propane combustion. The modification effect of samarium dopant on both their physicochemical and catalytic properties were systematically investigated by means of characterization techniques and experimental approaches. Characterization results revealed that the increasing addition of samarium into MnOx favored the formation of 3DOM structure, and the doping of samarium induced the crystalline phase transformation of Mn2O3 into Mn3O4, concurrently leading to the remarkable improvement of redox ability and surface oxygen mobility as well as higher abundance of surface labile oxygen species. All above-mentioned properties were demonstrated to be the major determinants for boosting the propane combustion performances of Sm-doped catalysts. Among all, the 3DOM Mn0.85Sm0.15Ox (denoted as M0.85S0.15) exhibited the optimum catalytic activity and good regeneration capacity under humid conditions. Additionally, the comparative in situ DRIFTs analysis over M0.85S0.15 and the bulk counterpart (denoted as B-M0.85S0.15) emphasized the superiority of 3DOM architecture on promoting the surface adsorption and oxidation of propane molecules.

Labile oxygen promotion of the catalytic oxidation of acetone over a robust ternary Mn-based mullite GdMn2O5

Mn-based mullite catalyst GdMn2O5 is developed via the hydrothermal method to efficiently remove volatile organic compound acetone with a high efficiency and stability in total oxidation. The T90 over GdMn2O5 approaches to 160 s°C in contrast to 191 °C over 1 wt % Pt/Al2O3. The catalytic performance maintains after reaction at 200 °C for 260 h with 5.0 % H2O. The surface labile oxygen is the key to superior catalytic performance. According to the in situ DRIFTS and the oxidation pathway proposed by DFT calculations, the acetate species decomposition is assigned as the rate-limiting step. And the calculated kinetic barrier for the rate-limiting step, 151.7 kJ/mol (1.58 eV), is in line with the apparent activation energy from acetone oxidation over GdMn2O5 of 150.0 kJ/mol (1.56 eV). This work provides insights into developing highly efficient and hydrothermally stable volatile organic compounds removal catalysts via surface oxygen activation.

The superior performance of CoMnOx catalyst with ball-flowerlike structure for low-temperature selective catalytic reduction of NOx by NH3

A novel ball-flowerlike catalyst (CoMnOx-BF) was developed by hydrothermal method for the selective catalytic reduction (SCR) of NOx by NH3. Besides, a counterpart sample (CoMnOx) was prepared by coprecipitation way for comparison. The activity test results demonstrated that CoMnOx-BF catalyst exhibited admirable SCR performance and N2 selectivity in a broad temperature range of 150–350 °C, along with an excellent resistance to SO2 and high durability. Based on characterization experiments, it could be found that the superior surface area resulted from suppressed crystallization, the enrichment of surface labile oxygen and Mn4+, the stronger surface acidity and redox ability were all beneficial to the NH3-SCR performance of CoMnOx-BF catalyst. Additionally, the enhanced NO oxidation and more activated ad-reactants on catalyst surface could also conduce to the outstanding SCR activity of CoMnOx-BF at low temperature.

Comparative study on transition element doped Mn–Zr–Ti-oxides catalysts for the low-temperature selective catalytic reduction of NO with NH3

The effect of Cu, Fe, Co, Ni, Cr and Zn on Mn–Zr–Ti mixed oxides catalysts introduced by co-precipitation was investigated. The Mn–Co catalyst showed the highest NO conversion near 100% and a good N2 selectivity > 90% at 200–300 °C. Comparing with the Mn–non catalyst, the Mn–Co catalyst presented a higher reaction rate constant at 120 °C with 23.3 ml s−1 g−1. The Mn–Co catalyst possesses a high concentration of Mn4+ and surface labile oxygen, which should improve the redox property and increase catalytic activity. Additionally, the Mn–Co showed the highest ratio of Lewis acid sites. The resistance to SO2 was improved by incorporation of Co. In summary, the Mn–Zr–Ti mixed oxides catalyst have a better N2 selectivity than other Mn-based catalysts and could be improved by doping with Co.

Selective catalytic reduction of NOx with NH3 over Mn–Zr–Ti mixed oxide catalysts

Mixed oxide catalysts yMn–Zr–Ti (y/10/10 with y = 1, 3, 5, 7 and 9) for the selective catalytic reduction (SCR) of NOx with NH3 in the presence of oxygen were synthesized by co-precipitation. The catalyst 5Mn–Zr–Ti exhibited a catalytic activity higher than 87% above 160 °C and N2 selectivity above 92% at 100–300 °C. The SCR performance was quite stable at 180 and 200 °C for 15 h. Mn species are shown to improve the catalytic activity by providing more surface labile oxygen, while a synergistic effect among Mn, Zr and Ti species suppressed the generation of N2O. According to in situ DRIFT analysis, NOx was primarily reduced by the reaction of bidentate nitrate and monodentate nitrito species with adsorbed NH3. Subsequently, a comprehensive reaction mechanism was proposed. Accordingly, the formation of NH species was inhibited, and hence, the N2O formation by the reaction of NH and bidentate nitrate was suppressed. Furthermore, the investigation of the deactivation mechanism by SO2 indicated that both the deposition of ammonium sulfates and the sulfation of Mn active sites resulted in the deactivation of the catalyst 5Mn–Zr–Ti. The deposition of ammonium sulfates at 180 °C was more pronounced than that at 220 °C.

Direct Decomposition of NOx over TiO2 Supported Transition Metal Oxides at Low Temperatures

TiO2 supported transition metal oxides were investigated for the direct decomposition of nitrogen oxides (NOx) at lower temperatures. The results showed that the catalytic performance strongly depends on the kind of transition metal oxide deposited on the TiO2. Among the various catalysts examined, Mn/TiO2 and Co/TiO2 exhibited relatively high NOx conversion at lower temperatures in the presence of 3 vol % O2. The oxygen in the reaction stream had a positive impact on the NOx decomposition over the Mn/TiO2 catalyst. We have not observed any TiO2 phase conversion from anatase to rutile in the Mn/TiO2 during the NOx decomposition reaction at different temperatures (100–350 °C). NOx decomposition activity was shown to be governed by the surface labile oxygen rather than the gas phase oxygen. The Mn/TiO2 catalyst exhibited a good resistance to 10 vol % H2O and 100 ppm of SO2.

Mn/beta and Mn/ZSM-5 for the low-temperature selective catalytic reduction of NO with ammonia: Effect of manganese precursors

Two series of Mn/beta and Mn/ZSM-5 catalysts were prepared to study the influence of how different Mn precursors, introduced to the respective parent zeolites by wet impregnation, affected the selective catalytic reduction (SCR) of NO by NH3 across a low reaction temperature window of 50–350 °C. In this study, the catalysts were characterized using N2 adsorption/desorption, X-ray diffraction, X-ray fluorescence, H2 temperature-programmed reduction, NH3 temperature-programmed desorption and X-ray photoelectron spectroscopy. As the manganese chloride precursor only partially decomposed this primarily resulted in the formation of MnCl2 in addition to the presence of low levels of crystalline Mn3O4, which resulted in poor catalytic performance. However, the manganese nitrate precursor formed crystalline MnO2 as the major phase in addition to a minor presence of unconverted Mn-nitrate. Furthermore, manganese acetate resulted principally in a mixture of amorphous Mn2O3 and MnO2, and crystalline Mn3O4. From all the catalysts screened, the test performance data showed Mn/beta-Ac to exhibit the highest NO conversion (97.5%) at 240 °C, which remained >90% across a temperature window of 220–350 °C. The excellent catalytic performance was ascribed to the enrichment of highly dispersed MnOx (Mn2O3 and MnO2) species that act as the active phase in the NH3-SCR process. Furthermore, together with a suitable amount of weakly acidic centers, higher concentration of surface manganese and a greater presence of surface labile oxygen groups, SCR performance was collectively enhanced at low temperature.