Formation Of Surface Oxygen Vacancies Research Articles

Black TiO2@Ag core-shell nanoparticles combined with WO3 nanorods on rGO nanosheets for PEC analysis

Well-coated black TiO2 and Ag core-shell nanoparticles (NPs) were prepared using a simple wet-chemical approach, which is combined with the WO3 nanorods over rGO nanosheets. The inherent properties of the composite nanostructures were examined thoroughly by diffuse reflectance spectroscopy, X-ray diffraction, Raman scattering, X-ray photoelectron spectroscopy, transmission electron microscopy, and photoelectrochemical (PEC) analysis. The formation of surface oxygen vacancies in the black TiO2 NPs facilitated tuning the optical properties in the visible region, which was enhanced by the surface plasmon resonance behavior of the Ag NPs. While combining the black TiO2@Ag core-shell NPs with intermediate bandgap WO3 nanorods and rGO nanosheets promoted PEC performance in the visible light irradiation. The composite nanostructure exhibited enhanced PEC efficiency (Jp=0.97 mAcm−2 and ABPE=0.62%) compared to the bare NPs. This work discusses the possible PEC mechanism based on the change in optical and structural properties.

Atomic-Resolution In Situ Exploration of the Phase Transition Triggered Failure in a Single-Crystalline Ni-Rich Cathode.

Single-crystalline cathode materials LiNixCoyMn1-y-zO2 (x ≥ 0.6) are important candidates for obtaining better cyclic stability and achieving high energy densities of Li-ion batteries. However, it is liable to initiate phase transitions inside the grains during electrochemical cycling, and the processes and regions of these phase transitions have remained unknown. In this research, we conducted an intrinsic study, investigating the chemicals and microstructural evolution of single-crystalline LiNi0.83Co0.11Mn0.06O2 using in situ biasing transmission electron microscopy at an atomic scale. We observed that the layered structure on the surface of the single-crystalline material was degraded during the charging process, resulting in continuous phase transitions and the formation of surface oxygen vacancies, which can reduce both the structural and thermal stability of the material. Uneven delithiation led to the formation of high-density defects and discontinuous inactive electrochemical phases, such as local antiphase boundaries and the rock salt phase, in the bulk of the material. The non-uniformity of the structure and the coexistence of active and inactive phases introduce significant tensile stress, which can lead to intragranular cracks inside the grains. As the number of cycles increases, the structural degradation caused by the intragranular phase transition will further increase, ultimately affecting the cycling capacity and stability of the battery. This work has broad implications for creating lithium-ion batteries that are effective and long-lasting.

Crystalline phase transformation of Sr-Fe based oxygen carriers modulate the cyclic performance for syngas and CO coproduction in chemical looping ethanol reforming coupled with CO2 splitting

The chemical looping reforming coupled with CO2 splitting (CLR-CS) offers a promising solution for syngas production and simultaneous decomposition of CO2. In this study, the Sr-Fe based oxygen carriers (OCs) were evaluated in CLR-CS using ethanol as fuel. The presence of Sr significantly improved the oxidation performance of iron oxides. The Sr element in fresh Sr-Fe based OCs mainly formed two crystal phases, including SrFe12O19 and Sr4Fe6O13. Increasing the amount of Sr element made it easier to form Sr4Fe6O13 crystal phase. The Sr4Fe6O13 was favorable for syngas production, due to the higher surface oxygen vacancy formation energy with a range of 3.13–3.37 eV. However, the lower oxygen transfer capacity (OTC) of Sr4Fe6O13 resulted in a lower CO yield in the of stage of CO2 decomposition. Compared with Sr4Fe6O13, SrFe12O19 exhibited the higher OTC and the higher reactivity of lattice oxygen, which was preferable to produce more CO2 in the stage of ethanol reforming in fuel reactor. During the cycle tests, the crystalline phase of SrFe12O19 didn’t regenerate under the CO2 atmosphere but transformed into Sr4Fe6O13. It was attributed to the lower energy barrier of CO2 oxidation on the oxygen defect surface of Sr4Fe6O13. The phase transformation of SrFe12O19 accompanied the formation of Fe3O4. The excessive Fe3O4 had poor stability and inactivated easily due to sintering and agglomeration in Sr1Fe4 and Sr1Fe6 samples. The Sr1Fe2, namely the molar ratio of SrO to Fe2O3 of 1:2, exhibited the better performance for syngas (0.291 Nm3/kg OC) and CO (0.141 Nm3/kg OC) coproduction at 800 °C. Additionally, the Sr1Fe2 presented an excellent cyclic stability in 30 cycles due to the suitable proportion of Sr4Fe6O13 and iron oxides formed, which maintained a high syngas yield and avoided sintering of iron oxides. Overall, the Sr1Fe2 is a promising OC candidate in CLR-CS process.

The influence of H2O or/and O2 introduction during the low-temperature gas-phase sulfation of organic COS + CS2 on the conversion and deposition of sulfur-containing species in the sulfated CeO2-OS catalyst for NH3-SCR.

Herein, the typical components of blast furnace gas, including H2O and O2, were introduced to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst during the gas-phase sulfation of organic COS + CS2 at 50 °C. The characterization results demonstrate that the introduction of O2 or H2O during gas-phase sulfation enhances the conversion of organic COS + CS2 on a cubic fluorite CeO2 surface and reduces the formation of sulfur and sulfates in the catalyst, but decreases the BET surface area and pore volume of the sulfated CeO2-OS catalyst. However, the introduction of O2 or H2O during the gas-phase sulfation increases the molar ratios of Ce3+/(Ce3+ + Ce4+) and Oβ/(Oα + Oβ + Oγ) on the sulfated CeO2-OS catalyst surface, thus promoting the formation of surface oxygen vacancies and chemisorbed oxygen, and these properties of the catalyst are further enhanced by the co-existence of O2 and H2O. Furthermore, the reduction of sulfates formed under the action of O2 or H2O decreases the weak acid sites of the sulfated CeO2-OS catalyst, but the few and highly dispersive sulfates present stronger reducibility, and the proportion of medium-strong acid sites of the catalyst increases. These factors help to improve the NH3-SCR activity of the sulfated CeO2-OS catalyst. Thus, there exists a synergistic effect of H2O and O2 introduction during gas-phase sulfation on the physical-chemical properties and catalytic performance of the sulfated CeO2-OS catalyst by organic COS + CS2 at 50 °C.

Ti species regulated CuO/CeO2 catalyst for efficient simultaneously NH3-SCR denitration and CO oxidation under oxygen-rich conditions

Synergistic control of multiple pollutants has high benefits during the control of emissions from fuel utilization, and catalysis can contribute to efficient removal of multiple pollutants. In this work, Ti species regulated CuO/CeO2 (Cu/Ce) catalyst was constructed to efficient simultaneous NH3-SCR denitration and CO oxidation under oxygen-rich conditions. It is found that the introduction of Ti species into Cu/Ce catalyst induces the formation of three-dimensional pore structure and promotes the phase transition from crystalline to amorphous, which significantly enhances the specific surface area and the dispersion of CuO and CeO2. The well dispersed CuO and CeO2 species provide a larger number of adsorbed sites for CO and NO and enhance the Cu-Ce interaction, which can contribute to reducing the competitive adsorption between CO and NO and the formation of a richer surface oxygen vacancies. Owing to above improvements, the Ti species regulated Cu/Ce catalyst can achieve more than 90% removal efficiency of NOx and CO at 148–218 °C, and the active temperature windows widened by 36 °C in comparison with Cu/Ce catalyst. Through this work, we expect to shed some light on the synergistic control of CO and NOx under oxygen-rich conditions.

Oxygen vacancy-enhanced magnesium-doped ferrous molybdate photocatalytic-peroxymonosulfate synergistic degradation of pollutants

Mg-doped FeMoO4 composites (labeled Mg/FeMoO4) enriched with oxygen vacancies were prepared and applied in the photocatalytic-peroxymonosulfate synergistic degradation of phenol. The results showed that the 5% Mg/FeMoO4 composites could completely degrade phenol (20 ppm) after 20 min, with a 2.7-fold increase in the synergistic degradation rate constant compared to that of FeMoO4. Experimental results show that Mg doping effectively improved the charge separation efficiency while favoring the formation of surface oxygen vacancies. The presence of oxygen vacancies can promote photogenerated electron-hole separation, accelerate the valence transition of Fe2+/Fe3+, and improve the activation efficiency of PMS. On the other hand, oxygen vacancies can also act as PMS adsorption activation sites to decompose and generate more active species, which is conducive to promoting synergistic degradation activity. Quenching experiment and Electron Paramagnetic Resonance (EPR) results demonstrate that active free radicals (·SO4−, ·OH and ·O2−) and non-free radicals (1O2) play significant roles in the reaction process. Based on this, an oxygen vacancy-enhanced synergistic degradation mechanism is proposed, which provides ideas for the coupling of advanced oxidation technologies as well as the design of catalytic materials to improve pollutant degradation performance.

Propane Dehydrogenation over Cobalt Aluminates: Evaluation of Potential Catalytic Active Sites

Non-oxidative propane dehydrogenation (PDH) is becoming an increasingly important approach to propylene production, while cobalt-containing catalysts have recently demonstrated great potential for use in this reaction, providing efficiencies comparable to those of industrially employed Pt- and Cr-based catalytic systems. It is therefore essential to clarify the nature of their active sites, especially since contradictory opinions on this issue are expressed in the literature. In this study, efforts were made to determine the state of Co in cobalt aluminates (CoAl2O4-Al2O3) responsible for PDH under typical operating conditions (600 °C, 1 atm). It is shown that the catalyst with a low cobalt content (Co/Al = 0.1) ensured the highest selectivity to propylene, ca. 95%, while maintaining significant propylene conversion. The structural motifs such as cobalt oxide and metallic cobalt nanoparticles, in addition to tetrahedral Co2+ species in the CoAl2O4 spinel system, were evaluated as potential active-site ensembles based on the obtained catalytic performance data in combination with the XRD, H2-TPR, TEM and XPS characteristics of as-synthesized, spent and spent–regenerated catalysts. It is revealed that the most likely catalytic sites linked to PDH are the Co-oxide forms tightly covering alumina or embedded in the spinel structure. However, additional in situ tuning is certainly needed, probably through the formation of surface oxygen vacancies rather than through a deeper reduction in Co0 as previously thought.

Unraveling the synergy between MnOx and CeO2 in MnOx-CeO2 SCR catalysts based on experimental and DFT studies

MnOx-CeO2 catalysts have shown great potential as selective catalytic reduction of NOx by NH3 (NH3-SCR) at temperature below 300 °C. While the improved catalytic activity due to the synergy between MnOx and CeO2 in MnOx-CeO2 catalysts is widely acknowledged, the synergistic mechanism in NH3-SCR reaction remains elusive due to the lack of direct evidences. In this study, we report a comprehensive study of how MnOx and CeO2 interacted during NH3-SCR in two designed MnOx-CeO2 catalysts with different Ce4+ concentration through both experimental and theoretical approaches. Our results reveal that electron transfer from Mn2+ to Ce4+ induced the formation of surface oxygen vacancy on the CeO2, leading to the synergistic promotion of low-temperature catalytic performance by establishing Mn-redox and Ce-redox cycles to activate NH3 and O2, respectively. This work provides a distinct mechanism diagram of the synergy between MnOx and CeO2, which sheds light on the rational design of low-temperature NH3-SCR catalysts.

Active Site Identification for Glycerol Hydrodeoxygenation over the Oxygen Modified Molybdenum Carbide Surface

Density functional theory and microkinetic reactor modeling were used to investigate the hydrodeoxygenation (HDO) mechanism of glycerol on the oxygenated Mo2C catalyst surface to understand the activity and product selectivity under practically relevant reaction conditions. Reactor simulations with multiple active site models predicted that a fully oxygenated surface with acid–base (OH,O) pairs is active for glycerol dehydration and it can selectively cleave one C–O bond to produce 3-hydroxypropanal (HPA). The acid sites are not directly involved in the C–O bond scission process; however, surface oxygen vacancy formation is promoted in the presence of acid sites. The rate-limiting C–O bond cleavage process occurs on the exposed Mo sites with a concerted β-hydrogen transfer to the nearest conjugate base (Mo–O) via an E2 elimination mechanism. Dehydration of HPA to acrolein was observed at longer residence times. Our analysis revealed that reaction conditions, such as temperature and partial pressure of H2, can be tuned to promote further deoxygenation of HPA and acrolein to produce propanal and propylene.

Effect of different valence metals doping on methane activation over La2O3(001) surface

La2O3 as a catalyst is used for oxidative coupling of methane (OCM) reactions due to its excellent stability and high C2 selectivity, but poor activity on methane dissociation limits its wide application. Different valence metals are doped on the La2O3(001) surface to improve the methane conversion activity, and the activation of methane on metal-doped La2O3(001) surfaces has been investigated via the density functional theory (DFT) calculations. The relationship between the valence states of doped metals and the methane conversion activities shows that doping low valence metals (Li, Na, K, Mg, Ca, Sr and Ba) and equivalent metals (Al, Ga, In) can significantly improve the conversion activity of methane. Among them, the activation energy of methane on the Li-La2O3(001) surface is the lowest, which is only 13.0 kJ/mol. However, doping of high valence metals (Zr, Nb, Re and W) cannot improve the CH4 dissociation activity. Furthermore, the relationships between surface oxygen vacancy formation energies, acid-base properties and the activation energies of CH4 have also been investigated. The results show that with the increase of metal valence state, the oxygen vacancy formation energy increases, while the dissociation activity of CH4 decreases. The introduction of alkali and alkaline earth metals increases the alkalinity of La2O3(001) surface, and the alkalinity of La2O3(001) doped with the alkali metal is stronger than that with the alkaline earth metal, exhibiting higher dissociation activity of CH4. Our research may provide a guide for improving methane conversion activity on La2O3 catalysts.

Plasma-catalytic CO2 methanation over NiFen/(Mg, Al)Ox catalysts: Catalyst development and process optimisation

Plasma-assisted catalytic carbon dioxide (CO2) methanation is a promising approach to valorise CO2 under mild conditions, and the system requires rational design regarding both catalysts and processes to advance it towards practical applications. Herein, the bimetallic NiFen/(Mg, Al)Ox catalysts were developed for plasma-catalytic CO2 methanation, and the effect of atomic ratio of Ni and Fe and reduction temperature on the catalysts was investigated systematically. The findings show that a higher reduction temperature was beneficial to the formation of Ni3Fe alloy and abundant surface oxygen vacancies, and increased CO2 adsorption ability. As a result, the NiFe0.1/(Mg, Al)Ox-800 catalyst led to a good catalytic performance in the plasma-assisted catalysis with about 84.7% CO2 conversion and 100% methane (CH4) selectivity. In addition, this study also employed response surface methodology (RSM) to explore the optimisation of plasma-catalytic CO2 methanation for the first time. Results from RSM show that in the plasma-catalytic system the selective CO2 hydrogenation to CH4 was favoured by higher applied voltage, lower gas flow rate and higher gas ratio (H2: CO2).

2,3-Dihydroxynaphthalene invoked surface oxygen vacancy effect on Fe2O3 nanorods for photoanodic signal transduction tactic

The state-of-art signal transduction mechanism of anodic photoelectrochemistry is constrained to the hole oxidation reaction, which greatly hinders its application for prospective biosensing applications. Herein, we present an innovative strategy for signal transduction by exploiting the in situ formation of surface oxygen vacancies (VOs) on Fe2O3 nanorods (NRs) through the self-coordination of 2,3-dihydroxynaphthalene (2,3-DHN) on their surfaces. The 2,3-DHN was connected with Fe(Ⅲ) on the surface of Fe2O3 NRs vis the formation of the five-membered ring structures accompanied by the generation of VOs. And the generated VOs introduced a new defect energy level for trapping the photogenerated holes, which enhanced the charge separation and realized the enhancement of photocurrent signal. The developed signal transduction strategy was validated by the first photoelectrochemical (PEC) sensing platform for β-glucoside (β-Glu) and lipase (LPS), which can catalyze the hydrolysis of 3-hydroxy-2-naphthalenyl-β-D-glucoside and naphthalene-2,3-diol diacetate, respectively, to produce 2,3-DHN for signal stimuli. The β-Glu and LPS were detected with linear ranges of 0.01–10.0 U/mL and 0.001–5.0 mg/mL, respectively. Detection limits of 3.3 × 10−3 U/mL and 0.32 μg/mL (S/N = 3) were achieved, for β-Glu and LPS, respectively. The present study not only provides a new strategy for spontaneous induction of VOs in situ for n-type semiconductors, but also innovates the anodic PEC signal transduction strategy with broadened biosensing applications.

Mechanistic Insight into the Promotion of the Low-Temperature NH3-Selective Catalytic Reduction Activity over MnxCe1-xOy Catalysts: A Combined Experimental and Density Functional Theory Study.

CeO2 has attracted much attention in the field of selective catalytic reduction of NO with NH3 (NH3-SCR). However, poor low-temperature activity and a narrow operation window restrict the industrial application of Ce-based oxide catalysts. Herein, the low-temperature NH3-SCR activity of Ce-based oxide catalysts was dramatically improved by Mn doping, and the mechanism was elucidated at the atomic level by experimental measurements and density functional theory calculations. We found that the addition of Mn significantly promoted the formation of surface oxygen vacancies. The oxygen vacancies easily captured O2 in air and formed active oxygen species (superoxide and peroxide) on the surface. The surface active oxygen species efficiently oxidized NO into NO2 and then facilitated the "fast SCR" reaction. This study provides atomic-level insights into the promotion of the NH3-SCR activity over Mn-Ce composite oxides and is beneficial for the development of low-temperature Ce-based catalysts.

Synergistic Effect of Pt and Dual Ni/Co Cations in Hydrotalcite-Derived Pt/Ni1.5Co0.5AlO Catalysts for Promoting Soot Combustion

In this article, the catalysts of hydrotalcite-derived Ni1.5Co0.5AlO nanosheet-supported highly dispersed Pt nanoparticles (Ptn/Ni1.5Co0.5AlO, where n% is the weigh percentage of the Pt element in the catalysts) were elaborately fabricated by the gas-bubble-assisted membrane--reduction method. The specific porous structure formed by the stack of hydrotalcite-derived Ni1.5Co0.5AlO nanosheets can increase the transfer mass efficiency of the reactants (O2, NO, and soot) and the strong Pt-Ni1.5Co0.5AlO interaction can weaken the Ni/Co-O bond for promoting the mobility of lattice oxygen and the formation of surface-oxygen vacancies. The Ptn/Ni1.5Co0.5AlO catalysts exhibited excellent catalytic activity and stability during diesel soot combustion under the loose contact mode between soot particles and catalysts. Among all the catalysts, the Pt2/Ni1.5Co0.5AlO catalyst showed the highest catalytic activities for soot combustion (T50 = 350 °C, TOF = 6.63 × 10-3 s-1). Based on the characterization results, the catalytic mechanism for soot combustion is proposed: the synergistic effect of Pt and dual Ni/Co cations in the Pt/Ni1.5Co0.5AlO catalysts can promote the vital step of catalyzing NO oxidation to NO2 in the NO-assisted soot oxidation mechanism. This insight into the synergistic effect of Pt and dual Ni/Co cations for soot combustion provides new strategies for reducing the amounts of noble metals in high-efficient catalysts.

Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces

Surface terminations and defects play a central role in determining how water interacts with metal oxides, thereby setting important properties of the interface that govern reactivity such as the type and distribution of hydroxyl groups. However, the interconnections between facets and defects remain poorly understood. This limits the usefulness of conventional notions such as that hydroxylation is controlled by metal cation exposure at the surface. Here, using hematite (α-Fe2O3) as a model system, we show how oxygen vacancies overwhelm surface cation-dependent hydroxylation behavior. Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy was used to monitor the adsorption of molecular water and its dissociation to form hydroxyl groups in situ on (001), (012), or (104) facet-engineered hematite nanoparticles. Supported by density functional theory calculations of the respective surface energies and oxygen vacancy formation energies, the findings show how oxygen vacancies are more prone to form on higher energy facets and induce surface hydroxylation at extremely low relative humidity values of 5 × 10-5%. When these vacancies are eliminated, the extent of surface hydroxylation across the facets is as expected from the areal density of exposed iron cations at the surface. These findings help answer fundamental questions about the nature of reducible metal oxide-water interfaces in natural and technological settings and lay the groundwork for rational design of improved oxide-based catalysts.

MOF-Derived CeO2 and CeZrOx Solid Solutions: Exploring Ce Reduction through FTIR and NEXAFS Spectroscopy.

The development of Ce-based materials is directly dependent on the catalyst surface defects, which is caused by the calcination steps required to increase structural stability. At the same time, the evaluation of cerium's redox properties under reaction conditions is of increasing relevant importance. The synthesis of Ce-UiO-66 and CeZr-UiO-66 and their subsequent calcination are presented here as a simple and inexpensive approach for achieving homogeneous and stable CeO2 and CeZrOx nanocrystals. The resulting materials constitute an ideal case study to thoroughly understand cerium redox properties. The Ce3+/Ce4+ redox properties are investigated by H2-TPR experiments exploited by in situ FT-IR and Ce M5-edge AP-NEXAFS spectroscopy. In the latter case, Ce3+ formation is quantified using the MCR-ALS protocol. FT-IR is then presented as a high potential/easily accessible technique for extracting valuable information about the cerium oxidation state under operating conditions. The dependence of the OH stretching vibration frequency on temperature and Ce reduction is described, providing a novel tool for qualitative monitoring of surface oxygen vacancy formation. Based on the reported results, the molecular absorption coefficient of the Ce3+ characteristic IR transition is tentatively evaluated, thus providing a basis for future Ce3+ quantification through FT-IR spectroscopy. Finally, the FT-IR limitations for Ce3+ quantification are discussed.

Catechol exerts an in situ surface oxygen vacancy effect on BiOI: an innovative signal transduction mode for cathodic photoelectrochemistry.

Cathodic photoelectrochemistry, a research hotspot in state-of-art bioassays, is generally circumscribed by its monotonous signal transduction tactic of photoinduced electron transfer (PET) mechanism, which significantly narrows the scope of its applications. In this study, we reveal the surface oxygen vacancy (VO) formation elicited by the spontaneous coordination of catechol (CA) onto the surface of BiOI nanoplates for the innovative operation of the cathodic PEC signal transduction tactic. The in situ-generated VO functions as a carrier separation center to efficiently promote photocurrent generation. Taking tyrosinase (TYR) and Escherichia coli O157:H7 (E. coli O157:H7) as model targets, the established signal transduction tactic was validated as efficient and sensitive for the detection of the two targets with linear ranges from 1.0 × 10-4 to 1.0 U mL-1 and 5.0 to 1.0 × 106 CFU mL-1, respectively. Low-detection limits of 1.0 × 10-4 U mL-1 and 3.0 CFU mL-1 were achieved for TYR and E. coli O157:H7, respectively. This study opens up a new perspective of in situ generated surface VO on semiconductors, which underlies an innovative PEC signal transduction mechanism with convincing analytical performance. Hopefully, it might encourage more explorations of new methodologies for introducing surface vacancies with exquisite applications.

Fly ash waste-derived Fe@Fe3O4 core-shell nanoparticles for acetic acid ketonization

Iron oxide is a cost-effective catalyst for the ketonization of carboxylic acid components of bio-oil crude derived from the pyrolysis of lignocellulosic biomass. Fly ash (FA) waste from coal-fired power plants is an abundant source of iron oxide precursors. Here we report the reductive thermal processing of hematite derived from acid-leached fly ash. Annealing of the hydroxide precipitated leachate produces nanoparticulate α-Fe2O3 containing ∼10 wt% metal impurities (principally Al, Mg, Ca and Ti). Subsequent extended (5 h) reduction under H2 at 400 °C generates a Fe@Fe3O4 core-shell structure, and is accompanied by surface segregation of Al, Mg, Ca and Ti as their oxides, a decrease in particle size, and the concomitant formation of surface oxygen vacancies. These physicochemical changes increase the surface area and introduce Brønsted basicity alongside the intrinsic Lewis acidity of the non-stoichiometric magnetite, promoting ketonization through synergistic activation of two acetic acid molecules. The specific activity of the 5 h reduced FA derived catalyst (FA-5 h) was 3.9 mmol·g−1·min−1 at 400 °C, with a corresponding turnover frequency per acid site of 14.3 min−1. In-situ DRIFTS reveals acetic acid dissociatively adsorbs over FA-5 h, adopting a bidentate acetate binding mode identical to that for commercial magnetite, albeit the fly ash derived catalyst stabilises acetate to a higher temperature (∼300 °C) facilitating C-C coupling versus desorption. Successive doping of commercial magnetite by low concentrations of Al, Mg, Ca and Ti, and subsequent reduction, generated a model catalyst whose acid-base properties and ketonization activity reproduced those of FA-5 h. Surface segregated dopants, notably Ti, are largely responsible for the high activity of FA-5 h through the introduction of additional acid and base sites for acetic acid ketonization.

Synthesis of ultra-thin 2D MnO2 nanosheets rich in oxygen vacancy defects and the catalytic oxidation of n-heptane

N-heptane (C7H16) is a kind of VOCs from a wide range of sources, which harm to the atmosphere and human health. So far, most of the researches on C7H16 have focused on catalytic cracking rather than its low temperature degradation. As a kind of long straight carbon chain alkanes, the study of catalytic oxidation of C7H16 has a certain guiding value for the degradation and mechanism analysis of complex organic compounds. In our study, 2D birnessite-MnO2 nanosheet with the thickness of 1.7–2.5 nm was successfully prepared and it can achieve complete conversion of C7H16 at 252.31 °C, which is better than the bulk catalyst. The characterization (such as TEM, XRD, etc.) and kinetic experiments results showed that it has good catalytic oxidation performance due to its ultra-thin, large specific surface, abundant oxygen vacancies and excellent electron transfer efficiency, which not only exposes more surface atoms, but also absorbs more gaseous O2 and rapidly transforms into reactive oxygen species (ROS) that can participate in catalytic oxidation. Surface reactive oxygen species play an important role in catalytic oxidation due to the formation of surface oxygen vacancies.

Perovskite oxide LaCr0.25Fe0.25Co0.5O3-δ as an efficient non-noble cathode for direct ammonia fuel cells

LaCoO3-δ (LCO) based perovskites including LaCr0.5Co0.5O3-δ (LCCO), LaFe0.5Co0.5O3-δ (LFCO) and LaCr0.25Fe0.25Co0.5O3-δ (LCFCO) were prepared and their oxygen reduction reaction (ORR) activity was studied. The presence of both Cr and Fe played vital roles in enhancing activity, affecting structural and electronic properties respectively. Cr doping was associated with surface oxygen vacancy formation whilst Fe doping enhanced the onset and half-wave potential. Furthermore, the effects of calcinating temperatures were explored and LCFCO fired at 700 oC (LCFCO-700) gave superior activity. Remarkably, when LCFCO-700 was employed in a direct ammonia fuel cell (DAFC), an OCV of 0.72 V and a maximum current density of ∼320 mAcm-2 was achieved when using air as the oxidant. Under similar conditions, DAFCs with LaCr0.25Fe0.25Co0.5O3-δ/C cathode, show comparable performance to the DAFC using Pt/C cathode, indicating that perovskite oxides, such as LaCr0.25Fe0.25Co0.5O3-δ, are promising non-noble cathodes for DAFCs.