Surface Oxygen Vacancy Density Research Articles

P123-assisted synthesis of Co3O4 catalyst for efficiently catalyzing N2O decomposition under simulated real tail-gases

The 8.0P-Co3O4 catalyst was prepared by sol-gel method with the assistance of P123 and investigated for catalyzing N2O decomposition. Research indicated that although the specific surface area of 8.0P-Co3O4 catalyst was only 18 m2·g−1, it exposed more highly active crystal planes, such as (111), (331), (400), and (222), and possessed higher surface oxygen vacancy density (5.64 µmol·m−2) compared with that of 0P-Co3O4 and Co3O4(P) catalysts. Due to these reasons, the corresponding reaction activation energy of 8.0P-Co3O4 for the reaction was significantly reduced, and the catalytic activity of 8.0P-Co3O4 was enhanced compared with that of Co3O4(P). Most importantly, the N2O conversion can maintain about 73 % over the catalyst for at least 24 h when the three impurity gases of 5 vol% O2, 100 ppmv NO, and 2 vol% H2O were coexisted at 400 °C, GHSV = 20,000 h−1.

Role of oxygen vacancy in high-entropy Cu1Zn1Al0.5Ce5Zr0.5Ox for CO2 hydrogenation reaction

High-entropy oxides (HEOs) have garnered significant attention in catalysis due to their excellent redox properties and superior stability. In this study, we prepared and investigated a high-entropy oxide, Cu1Zn1Al0.5Ce5Zr0.5Ox, to elucidate the impact of oxygen vacancy density on the CO2 hydrogenation reaction. Comparisons were made with binary or ternary solid solutions composed of the same cations present in this HEO, and possessing the same phase structure. The HEO exhibits a higher surface oxygen vacancy density as evidenced by Raman spectroscopy and XPS. The increased number of oxygen vacancies significantly increases the active sites and enhances the strength for CO2 adsorption. Combined with kinetic analysis, it is suggested that the enhanced CO2 adsorption leads to improved CO2 conversion on the HEO. Moreover, the formation of oxygen vacancies facilitates H2 dissociation and supply, which is pivotal for methanol formation on the HEO. The stability of the HEO Cu1Zn1Al0.5Ce5Zr0.5Ox surpasses that of the medium entropy oxide, showing no significant deactivation after 100 hours of reaction.

Pt/CeCrO2−x: A multifunctional catalyst for tandem catalysis of lignocellulose hydro-liquefaction and Guerbet reaction

Catalytic transfer hydrogenation (CTH) of lignocellulose liquefaction using renewable ethanol presents a promising alternative to conventional hydrogenation techniques. However, the development of efficient and stable multifunctional catalysts remains a significant challenge. Herein, a Pt/Ce9Cr1O2−x catalyst was developed to facilitate the CTH reactions and achieve nearly complete utilization of lignocellulose. Isotope labeling experiments were conducted to elucidate the mechanism of CTH. The introduction of Cr3+ increased the density of surface oxygen vacancies, consequently enhancing the hydrodeoxygenation ability and metal anchoring. The solid solution structure promotes the electrons transfer from Cr atoms to Pt atoms, forming electron-rich Ptδ+ species that promote ethanol dehydrogenation and lignocellulose hydrodeoxygenation. Furthermore, Pt/Ce9Cr1O2−x catalyst provided optimal acid-base sites and hydroxyl groups, thereby improving ethanol upgrading efficiency and wettability.

Synergy of adsorption-photocatalysis and enriched surface oxygen vacancies over lignin-biochar/Bi2MoO6 nanocomposites for organic pollutant removal and bamboo ECF bleaching effluent treatment

LC/Bi 2 MoO 6 composites were successfully fabricated and applied to the effective degradation of methylene blue (MB) under visible light illumination. The composition, morphology and photoelectrochemical properties of prepared samples were characterized by various techniques. EPR analysis reveals that the density of surface oxygen vacancies was enhanced after lignin-biochar addition. Under optimal conditions, the degradation efficiency of MB over LC/Bi 2 MoO 6 was 95.34 % at 1 h under visible light irradiation, and the reaction rate constant ( k ) was 18.96 folds higher than that of pristine Bi 2 MoO 6 . The enhanced photocatalytic performance of LC/Bi 2 MoO 6 was due to the synergistic effect between adsorption-photocatalysis and enriched surface oxygen vacancies, which greatly promotes the separation of photogenerated charge carriers. Moreover, the MB degradation removal with LC/Bi 2 MoO 6 decreased by 3.84 % at 1 h under visible light irradiation after four cycles, illustrating excellent stability and reusability in the photocatalytic recycling process. Notably, the bandgap of Bi 2 MoO 6 decreased from 2.66 eV to 2.26 eV with the addition of lignin-biochar. Radical capture experiment and ESR characterization demonstrate that •OH radicals were the main active radical in the photocatalytic process, which increases the radical concentration during the combination of lignin-biochar. Furthermore, the LC/Bi 2 MoO 6 composites were also applied to the degradation of bamboo ECF bleaching effluent, with a higher COD Cr and AOX removal of 75.72 % and 69.64 %, respectively. Herein, this work provides a novel insight for constructing economic and efficient photocatalysts for wastewater remediation and biomass utilization. • A novel LC/Bi 2 MoO 6 composites with enriched oxygen vacancies was prepared. • LC/Bi 2 MoO 6 composites exhibits superior adsorption and visible-light photocatalytic activity for MB and bamboo ECF bleaching effluent degradation. • The photocatalytic mechanism of LC/Bi 2 MoO 6 was proposed.

CuO decorated vacancy-rich CeO2 nanopencils for highly efficient catalytic NO reduction by CO at low temperature.

With the rapid development of transportation and vehicles, the elimination of NOx and CO has highly attracted public attention. In this work, vacancy-rich CeO2 nanopencil supported CuO catalysts (CuO/CeO2-NPC) were successfully prepared for NO reduction by CO. Importantly, CeO2 with nanopencil-like shape (CeO2-NPC) have been synthesis by solvothermal method for the first time. The physicochemical properties of all samples were studied in detail by combining the means of X-ray diffraction (XRD), Raman spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), H2-temperature-programmed reduction (H2-TPR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 physisorption (Brunauer-Emmett-Teller), and NO and CO temperature-programmed desorption (NO-TPD and CO-TPD) techniques. Compared with CeO2 nanorods and nanoparticles supported CuO catalysts (CuO/CeO2-NR and CuO/CeO2-NP), the CuO/CeO2-NPC catalysts showed the highest catalytic activity, affording more than 90% NO conversion at 69°C as well as excellent H2O tolerance at 150°C, which is superior to catalysts previously reported. Characterization results indicated that the synergistic effect between the well-dispersed CuO and the CeO2 nanopencil support enables a favorable electron transfer between these components and enhances the density of surface oxygen vacancies and Cu+ species, which consequently accelerating the redox cycle. The results indicated that the morphology control of CeO2 support could be an efficient way to evidently enhance the catalytic performance for NO + CO reaction.

Rationally designed state-of-the-art approach for enhancing the ultraviolet photon detection performance of ZnO thin film

Here, we demonstrate a simple and effective state-of-the-art (surface passivation) approach for enhancing the UV photo detection performance of ZnO thin films synthesized via spray pyrolysis. The surface passivated Ag/ZnO/Ag device (using polyvinyl alcohol layer) has low dark current of 1.1 nA as compared with the device without surface passivation (0.04 μA). The significant decay in the dark current after surface passivation is due the reduction in the density of surface oxygen vacancies confirmed by PL measurements. The UV photon detection capability of the surface passivated ZnO thin film exhibits a significantly high photo response (3675), high photosensitivity (3.6 × 105), photo-responsivity (0.6 A/W) and fast response time (0.6 ms) under UV light (340 nm, 140 μW/cm2). This straightforward approach superimposes surface defects, making the ZnO thin film more suitable for a wide range of electronic device applications while preserving the material's intrinsic properties.

Influences of CeO2 morphology on enhanced performance of electro-Fenton for wastewater treatment

As a kind of rare-earth oxide, CeO2 has been considered as a great potential material for its abundant oxygen vacancies and catalysis activity in wastewater treatment. In this study, three ceria samples with different morphological structure were prepared, and their effect on contaminates degradation in electro-Fenton (E-Fenton) system was investigated. It is found that the morphology of CeO2 has great influence on promoting the performance of E-Fenton process. The rod-like CeO2 (R–CeO2) induced E-Fenton system shows higher azo dye X3B removal and mineralization rates than cubs (C–CeO2) and octahedrons (O–CeO2), and the degradation kinetic rate constants are 0.28, 0.169 and 0.181 min−1 respectively, already surpass that of the blank one (0.120 min−1). The H2O2 generation capacity of R–CeO2 induced E-Fenton system is also superior to the others, and the corresponding Faraday current efficiency even increases to 166.2% at 2.5 min. Characterizations by scanning-transmission electron microscopy (STEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), Raman spectroscopy were conducted to disclose the mystery behind this phenomenon. The results indicate that the difference of surface oxygen vacancy density in different shaped CeO2 acts as a magic driving force for the accelerated oxygen reduction, and then leads to the enhanced degradation efficiency of E-Fenton system. This work provides a new insight into the development and application of rare-earth elements based catalyst in E-Fenton technology.

Oxygen vacancies promoted heterogeneous catalytic ozonation of atrazine by defective 4A zeolite

The ozone activation reaction is becoming more popular with the treatment of organic pollutants, while the key to improving the efficiency heavily lies in the selection of active catalysts. In this study, we demonstrate that synthetic 4A zeolite with a high surface oxygen vacancy density and acidity exhibits excellent activity toward the ozonation process, where the removal rates of atrazine (ATZ) reached 87.5% at 6 min, and the removal rates of Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) reached 66.7% and 35.8% at 15 min, respectively. Compared with commercial 4A zeolite and synthetic 4A zeolite with acid treatment, the ozonation activities of 4A zeolites positively correlate with the surface acidity. Mechanistic investigations have identified that hydroxyl radicals are the main reactive oxygen species, and thus, the degradation pathway of ATZ is provided. Moreover, the influences of catalyst dosage, reaction temperature, pH and cycling stability on ozonation performance are systematically evaluated.

Oxygen Vacancy Engineering of Molybdenum Oxide Nanobelts by Fe Ion Intercalation for Aerobic Oxidative Desulfurization

Aerobic oxidative desulfurization (AODS) represents a sustainable way to deeply desulfurize transportation fuels through a high-performance catalyst to convert thiophenes into sulfones in an efficient way. Here, we report that iron (Fe) intercalated molybdenum oxides (MoOx) nanobelts can be a durable and robust catalyst for AODS, catalyzing the reaction efficiently at 80 °C with air as a sustainable oxygen source. By combining systematic characterization, kinetic analysis, and density functional theory calculation, we demonstrate that the guest Fe ions could precisely modulate the electronic structure of Mo sites, leading to a significant increase in surface oxygen vacancy density. An optimized doping amount enables our catalyst with excellent catalytic performance via thorough conversion of various sulfides. The catalyst maintained almost unchanged activity in repeated uses, qualifying it as a candidate for the sustainable AODS of fuel. We anticipate that the strategy demonstrated here will provide practical guidance on the rational design and optimization of Mo-based catalysts to promote aerobic oxidation reactions.

Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution

Perovskites of strontium cobalt oxyhalides having the chemical formulae Sr2CoO4-xHx (H = F, Cl, and Br; x = 0 and 1) were prepared using a solid-phase synthesis approach and comparatively evaluated as electrocatalysts for oxygen evolution in an alkaline solution. The perovskite electrocatalyst crystal phase, surface morphology, and composition were examined by X-ray diffraction, a scanning electron microscope, and energy-dispersive X-ray (EDX) mapping. The electrochemical investigations of the oxyhalides catalysts showed that the doping of F, Cl, or Br into the Sr2CoO4 parent oxide enhances the electrocatalytic activity for the oxygen evolution reaction (OER) with the onset potential as well as the potential required to achieve a current density of 10 mA/cm2 shifting to lower potential values in the order of Sr2CoO4 (1.64, 1.73) > Sr2CoO3Br (1.61, 1.65) > Sr2CoO3Cl (1.53, 1.60) > Sr2CoO3F (1.50, 1.56) V vs. HRE which indicates that Sr2CoO3F is the most active electrode among the studied catalysts under static and steady-state conditions. Moreover, Sr2CoO3F demonstrates long-term stability and remarkably less charge transfer resistance (Rct = 36.8 ohm) than the other oxyhalide counterparts during the OER. The doping of the perovskites with halide ions particularly the fluoride-ion enhances the surface oxygen vacancy density due to electron withdrawal away from the Co-atom which improves the ionic and electronic conductivity as well as the electrochemical activity of the oxygen evolution in alkaline solution.

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction.

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO3 perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO3 induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO3 ; the former exhibit a low onset overpotential of 380 mV at 10 mA cm-2 and a small Tafel slope of 70.8 mV dec-1 . Oxygen vacancy formation is accompanied by mixed Ni2+ /Ni3+ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO3 in accordance with the enhanced OER activity that is observed.

The effect of oxygen annealing on characteristics of β-Ga2O3 solar-blind photodetectors on SiC substrate by ion-cutting process

In this work, a high quality (2̅01) β-Ga2O3 single-crystalline film was transferred to the 4H-SiC substrate by ion-cutting technique, on which the metal-semiconductor-metal (MSM) solar-blind photodetectors (PDs) were fabricated. The effect of the oxygen annealing on the performance of PDs has been systematically studied. The PD fabricated on the as-transferred film exhibits a high responsivity (R) of 2.24 A/W with a large external quantum efficiency of 1.21 × 103%, and an ultrahigh responsivity rejection ratio (R240nm/R350nm) of 1.02 × 105. In comparison, the PD based on the β-Ga2O3 film annealed in oxygen atmosphere exhibits a reduced dark current of 0.6 nA at the bias of 5 V, a fast decay time of 41 ms but a relatively reduced internal gain. It is resulted from the increased Schottky barrier height and the suppression of persistent photoconductivity effect due to the reduced density of surface oxygen vacancies and defects upon annealing. These results would promote a better understanding of dark current and responsivity of PDs modulated by oxygen annealing.

Nanotubular OMS-2 Supported Single-Atom Platinum Catalysts Highly Active for Benzene Oxidation

The cryptomelane-type octahedral molecular sieve (OMS-2) exhibits excellent catalytic activity for the oxidation of volatile organic compounds (VOCs) due to its unique structure, and single-atom catalysts (SACs) have recently attracted much attention in heterogeneous catalysis. Herein, we adopted the hydrothermal redox reaction of KMnO4 and HCl aqueous solution at 120 °C for 12 h to first synthesize the nanotubular OMS-2 support and used the vitamin C and NaBH4 reduction methods to prepare the OMS-2-supported single-atom Pt and PtNP catalysts for benzene oxidation, respectively. It was found that the OMS-2-supported single-atom Pt catalyst with a Pt loading of 0.0383 wt % (0.0383Pt1/OMS-2) exhibited the best catalytic performance for benzene oxidation (a 90% benzene conversion was achieved at 189 °C and 20 000 mL g–1 h–1 (space velocity)), which was associated with its high surface oxygen vacancy density, good low-temperature reducibility, and strong benzene adsorption ability. It was also shown that benzene could be dissociated more readily over the supported single-atom Pt1 catalyst than over the supported PtNP catalyst, and the phenolates or benzoquinone as well as methyl groups, lipid, and vinyl species were the main intermediates in the oxidation of benzene over 0.0383Pt1/OMS-2. The results of this study are useful in developing the high-performance single-atom catalysts that can be applied for the oxidative removal of VOCs.

Enhancing the CO2 methanation activity of Ni/CeO2 via activation treatment-determined metal-support interaction

The metal-support interaction is of critical importance to enhance the catalytic activity and selectivity. However, it is still challenging to construct an appropriate interaction starting from the catalyst fabrication and/or activation. We herein established low-temperature treatment of Ni2+ ions impregnated on ceria in reductive atmosphere and reduction–oxidation cycles as effective approachs to regulate the metal-support interaction and raise the catalytic performance in the CO2 methanation. The proposed construction approach yielded Ni/CeO2 that displayed highly dispersed Ni nanoparticles in contact with CeO2 (111) and (100) facet, higher density of surface oxygen vacancies and larger amounts of weak basic sites relative to the reference samples, which increased the capacity for H2 and CO2 adsorption/activation. The interaction resulted in appreciably (2–3 fold) higher activity in the CO2 methanation with maintaining almost full selectivity to CH4 and high stability. Coverage of Ni surface by CeO2−x thin layer as a typical structure of strong metal-support interaction resulting from high-temperature reduction, can be alleviated via reduction–oxidation cycles. We also demonstrate the activation treatment-determined metal-support interaction effect can generally extend to (TiO2 and ZrO2) supported Ni catalysts.

Atomically dispersed copper species on ceria for the low-temperature water-gas shift reaction

The structure of copper species, dispersed on nanostructured ceria (particles, rods and cubes), was analyzed by scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). It was interestingly found that the density of surface oxygen vacancies (or defect sites), induced by the shape of ceria, determined the geometrical structure and the chemical state of copper species. Atomically dispersed species and monolayers containing few to tens of atoms were formed on ceria particles and rods owing to the enriched anchoring sites, but copper clusters/particles co-existed, together with the highly dispersed atoms and monolayers, on cubic ceria. The atomically dispersed copper sites and monolayers interacted strongly with ceria, involving a remarkable charge transfer from copper to ceria at their interfaces. The activity for the low-temperature water-gas shift reaction of the Cu/CeO2 catalysts was associated with the fraction of the positively-charged copper atoms, demonstrating that the active sites could be tuned by dispersing Cu species on shape-controlled ceria particles.

The promotional effect of surface Ru decoration on the catalytic performance of Co-based nanocatalysts for guaiacol hydrodeoxygenation

In the present work, carbon-supported Ru-decorated Co-based nanocatalysts were fabricated via a layered double hydroxide/carbon composite precursor approach and applied to the efficient hydrodeoxygenation (HDO) of guaiacol to produce cyclohexanol. It was demonstrated that uniform and highly dispersed Co nanoparticles could be formed on the carbon matrix, and the decoration of a small amount of Ru on the surface of Co nanoparticles could introduce stronger hydrogenolysis active sites. Furthermore, the reduction temperature for catalyst precursors could tune the size of Co-containing nanoparticles and regulate the density of surface oxygen vacancies originating from CoOx species. Under the mild reaction conditions (200 ℃ and 1.0 MPa hydrogen pressure), as-fabricated Ru-Co/C catalyst obtained at the reduction temperature of 600 °C showed excellent catalytic activity in the HDO of guaiacol, with a high cyclohexanol yield of ∼94 %, which was attributable to surface exposure of highly dispersive Ru° sites and the formation of abundant defective oxygen vacancies. The present results provide a new approach for designing high-performance Co-based HDO nanocatalysts by both the surface decoration of small amounts of precious metals and the introduction of surface defective structures.

Oxygen vacancy engineering with flame heating approach towards enhanced photoelectrochemical water oxidation on WO3 photoanode

Abstract Oxygen vacancies are a double-edged sword for photoelectrochemical (PEC) devices and a comprehensive understanding on their role in PEC processes is important. Although there is existing work that illustrates their effect on PEC processes, clarifying the influence of surface and bulk oxygen vacancies, respectively, on the PEC performance is challenging. Herein, we present the fabrication of WO3 photoanodes with tunable bulk and surface oxygen vacancy with a novel two-step flame heating approach that is proving to be a powerful platform for this purpose. We found that both the conductivity and trapping state density of the WO3 photoanode increase with bulk oxygen vacancy density, leading to a volcano-like relationship between the kinetics of charge separation/transport and bulk oxygen vacancy density. Moreover, both the interfacial charge transfer rate constant and the charge recombination rate constant on the surface of the WO3 photoanode increase with surface oxygen vacancy density, which also leads to a volcano-like relationship between the charge injection efficiency and surface oxygen vacancy density. By tuning the surface and bulk oxygen vacancy density simultaneously, the PEC performance of the WO3 photoanode was increased by ca. 10 times, which illustrates the strength of delicate oxygen vacancy engineering in optimizing PEC devices.

Improved hydrogen sensing of (004) oriented anatase TiO2 thin films through post annealing

Safety is the most important concern for hydrogen production, transportation and storage. A hydrogen sensor based on (004) oriented anatase TiO2 thin film is fabricated with high density and specific orientation by introducing a seed layer and an annealing process for the TiO2 thin film. The sensor has a detection limit as low as 1 ppm with a remarkable sensitivity of 17% and fast response and recovery times as short as 5 s and 64 s respectively. Modelling results based on density functional theory indicate that the energy barrier for H2 dissociation on the (004) surface is about 0.458 eV. Both Ti and O atoms at the TiO2 (004) surface take part in the H2 dissociation process. The experimental and theoretical results indicate that surface oxygen has played an important role in hydrogen sensing; lowering the density of surface oxygen vacancies is beneficial in promoting hydrogen sensing.

Preferential Oxidation of CO in H2 on Pure Co3O4−x and Pt/Co3O4−x

AbstractDriven by the development of a catalyst made of earth‐abundant elements for on‐board purification of H2 of this energy conversion technology, preferential oxidation (PROX) on pure Co3O4 nanorods and Co3O4 nanorods with supported Pt nanoparticles was explored with the aid of in situ studies. This catalyst remains its 100 % conversion of CO in H2 at a gas hourly space velocity of 42 857 mL h−1 g−1 at 120 °C for at least 96 h. In situ studies showed that the active surface phase during PROX is nonstoichiometric Co3O4−x. A correlation between density of surface oxygen vacancies and conversion of CO to CO2 suggest that oxygen vacancy is a necessary component of a catalytic site for PROX on Co3O4−x. Compared to pure Co3O4 nanorods, anchoring Pt nanoparticles on Co3O4 nanorods unfortunately increases selectivity for oxidation of H2 owing to the low dissociation barrier of molecular H2 on Pt. Co3O4−x exhibits much higher selectivity for CO oxidation in PROX than Pt/Co3O4−x at a temperature lower than 140 °C.

Controllable Synthesis and Photocatalytic Properties Study of Na2Ti3O7 and H2Ti3O7 Nanotubes with High Exposed Facet (010).

The role of replacing Na+ with H+ of titanate in promoting photocatalytic performance was investigated. The experimental results showed that H2Ti3O7 and Na2Ti3O7 catalysts with the same high exposed (010) facet had the similar light absorption capacity, TiO6 octahedral structure, and specific surface area. By comparing to Na2Ti3O7, H2Ti3O7 had longer lifetime and higher separation efficiency of the photo-generated electron-hole pairs, and also had higher density of surface oxygen vacancies, which resulted in the excellent performances for photocatalytic hydrogen production and dye degradation reactions.