Concentration Of Surface Oxygen Species Research Articles

Functionalization of Graphite with Oxidative Plasma.

Surface-modified graphite is studied as an electrode material, an adsorbent, and a membrane component, among other applications. Modifying the graphite with plasma can be used to create relevant surface functionalities, in particular, various oxygen groups. The application of surface-oxidized graphite often requires its use in an aqueous environment. The application in an aqueous environment is not an issue for acid-oxidized carbons, but a discrepancy in the structure–activity relationship may arise because plasma-oxidized carbons show a time-dependent decrease in the degree of functionalization and related properties. Moreover, plasma-oxidized materials are often characterized in terms of their chemical and physical properties, most notably their degree of functionalization after plasma treatment, without contact with water. In this study, we used low-temperature plasma oxidation with pure oxygen and carbon dioxide and sample-washing with concentrated nitric and sulfuric acids. To evaluate the electronic properties of modified graphite, the work function changes and surface oxygen content were measured just after plasma modification and after water immersion. We show that water immersion drastically decreases the work function of plasma-treated samples, which is accompanied by a decrease in the number of radicals introduced by plasma. Our results demonstrate that the increase in stable work function as a result of plasma treatment, brought about by an increase in the surface oxygen species concentration, can be realized most effectively for the acid-washed graphite.

The Key Role of Active Sites in the Development of Selective Metal Oxide Sensor Materials.

Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In2O3, SnO2, WO3; mixed-metal oxides BaSnO3, Bi2WO6), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH3, SO2, VOC, NO2) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO3 and Bi2WO6 to SO2 and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

Ni/La2O3 and Ni/MgO–La2O3 catalysts for the decomposition of NH3 into hydrogen

The decomposition of NH3 for hydrogen production was studied using Ni/La2O3 catalysts at varying compositions and temperatures prepared via surfactant-templated synthesis to elucidate the influence of catalyst active metal content, support composition and calcination temperature on the catalytic activity. The catalytic performance of all samples was studied between 300 and 600 °C under atmospheric pressure. The catalytic activity of the sample were as follows: 10Ni/La2O3-450 > 10Ni/La2O3-550 > 10Ni/La2O3-650 ≈ 10Ni/La2O3-750 ≈ 10Ni/La2O3-850. The excellent activity (100%) of 10Ni/La2O3-450 could be due to the high surface area, basicity strength and concentration of surface oxygen species of the catalyst as evidenced by BET, CO2-TPD and XPS. In addition, to adjust the activity of the catalyst support, the molar ratios of Mg and La were varied (1:1, 3:1, 5:1, 7:1 and 9:1). The 5Ni/5MgLa (5:1 M ratio) was found to be the most active (100%) relative to other Ni/MgLa formulations. Furthermore, the Ni content in the Ni/5MgLa sample was adjusted between 10 and 40 wt%. Increasing the Ni content of the catalysts increased NH3 conversion with the 40 wt% Ni formulation demonstrating complete NH3 conversion at 600 °C and a high gas hourly space velocities (GHSV) (30,000 mL∙h−1∙gcat−1).

Highly efficient base-free aerobic oxidation of alcohols over gold nanoparticles supported on ZnO-CuO mixed oxides

AbstractThe design and preparation of suitable supports are of great importance for gold catalysts to attain excellent catalytic performance for alcohol oxidation. In this work, we found that ZnO-CuO mixed oxides supported gold catalysts showed much better catalytic activity for base-free aerobic oxidation of benzyl alcohol than Au/ZnO and Au/CuO catalysts, and among them Au/Zn0.7Cu0.3O displayed the best catalytic performance. In addition, the Au/Zn0.7Cu0.3O catalyst could selectively catalyze the aerobic oxidation of a wide range of alcohols to produce the corresponding carbonyl compounds with high yields under mild conditions without base. Further characterizations indicated that the outstanding catalytic performance of Au/Zn0.7Cu0.3O was correlated with the small size of Au nanoparticles (NPs), good low-temperature reducibility, high concentration of surface oxygen species, and collaborative interaction between Au NPs and mixed oxide.

Advanced Electrochemical Properties of PrBa0.5Sr0.5Co1.9Ni0.1O5+δ as a Bifunctional Catalyst for Rechargeable Zinc‐Air Batteries

AbstractVarious simple and layered perovskites have been extensively investigated as promising cathode materials for metal‐air batteries and fuel cells, because of their tunable structural, electronic, and chemical properties. However, the electrocatalytic activity of perovskite oxides towards the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) must be improved by increasing the slow kinetics and by decreasing the overpotential associated with those reactions. Doping with other transition metals such as Ni, Mn, Fe and Cu represents one of the effective chemical methods to improve the catalytic activity of perovskite oxides. Herein, we investigate Ni‐doped cobalt‐based double perovskites, PrBa0.5Sr0.5Co2‐xNixO5+δ (x=0, 0.1, 0.2 and 0.3), as promising cathode materials for rechargeable alkaline Zn‐air batteries. PrBa0.5Sr0.5Co1.9Ni0.1O5+δ (PBSCN1) shows low Tafel slopes (OER: 83 mV dec−1 and ORR: 67 mV dec−1), favorable onset potentials (OER: 1.513 V vs. RHE at 1 mA cm−2 and ORR: 0.720 V vs. RHE at −1 mA cm−2), and high limiting currents (OER: 25.20 mA cm−2 and ORR: −5.67 mA cm−2). In addition, it shows improved discharge‐charge performances for a full‐cell Zn‐air battery. The enhanced electrochemical properties of PBSCN1 could be achieved by the high concentration of surface oxygen species, and the coexistence of different chemical states of cobalt cations caused by the presence of nickel cations in the lattice. Based on these results, PrBa0.5Sr0.5Co1.9Ni0.1O5+δ could be considered a promising cathode material for Zn‐air battery systems.

Template‐free Scalable Synthesis of Flower‐like Co3‐xMnxO4 Spinel Catalysts for Toluene Oxidation

AbstractThe rational design of low‐cost transition metal catalysts that exhibit high activity and selectivity may be the most significantly investigated in heterogeneous catalysis. In this study, Co3‐xMnxO4 (x=0.75, 1.0, and 1.5) mixed metal oxides were successfully synthesized by a controlled template‐free autoclave strategy and studied for toluene oxidation. It is found that the Co‐rich sample showed markedly enhanced activity and the 3D dandelion‐like Co2.25Mn0.75O4 catalyst exhibited in the highest toluene oxidation rate (8.9 μmol/(gcats)) and a 100 % toluene conversion at 239 °C. In situ DRIFTS study indicates that toluene was sequentially oxidized to benzyl radical, benzaldehyde, benzene, oxalic acid, and finally to CO2 and H2O. The interaction between Co and Mn, in conjunction with the high concentration of surface oxygen species and rich surface oxygen vacancies, reasonably explains the elevated catalytic activity and thermal stability for toluene oxidation over 3D flower‐like Co3‐xMnxO4 spinel catalysts.

K–Mn supported on three-dimensionally ordered macroporous La0.8Ce0.2FeO3 catalysts for the catalytic combustion of soot

Three-dimensionally ordered macroporous (3DOM) La0.8Ce0.2FeO3 perovskite was successfully constructed through a colloidal crystal template way. And the K–Mn/3DOM La0.8Ce0.2FeO3 catalyst was obtained by an incipient-wetness impregnation method. The physicochemical characterizations of the material were conducted by means of XRD, FESEM, TEM, N2 adsorption–desorption, H2-TPR, XPS and O2-TPD techniques, and the catalytic performance was evaluated for soot removal. The intact well-ordered macroporous skeleton was remained after K and Mn were loaded, which offered an ideal environment for soot combustion. The generated K2Mn4O8 was well decorated on the skeleton of the 3DOM perovskite, which could increase the concentration of surface oxygen species and enhance the reducibility of catalyst. The as-prepared catalyst exhibited good catalytic performance for soot removal, and could lower the T50 of soot to 377°C.

Growth and surface modification of LaFeO3 thin films induced by reductive annealing

The mixed electronic and ionic conductivity of perovskite oxides has enabled their use in diverse applications such as automotive exhaust catalysts, solid oxide fuel cell cathodes, and visible light photocatalysts. The redox chemistry at the surface of perovskite oxides is largely dependent on the oxidation state of the metal cations as well as the oxide surface stoichiometry. In this study, LaFeO3 (LFO) thin films grown on yttria-stabilized zirconia (YSZ) was characterized using both bulk and surface sensitive techniques. A combination of in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) demonstrated that the film is primarily textured in the [100] direction and is stoichiometric. High-resolution transmission electron microscopy measurements show regions that are dominated by [100] oriented LFO grains that are oriented with respect to the substrates lattice. However, selected regions of the film show multiple domains of grains that are not [100] oriented. The film was annealed in an ultra-high vacuum chamber to simulate reducing conditions and studied by angle-resolved X-ray photoelectron spectroscopy (XPS). Iron was found to exist as Fe(0), Fe(II), and Fe(III) depending on the annealing conditions and the depth within the film. A decrease in the concentration of surface oxygen species was correlated with iron reduction. These results should help guide and enhance the design of LFO materials for catalytic applications.

Promotional effect of cerium on nickel-containing mesoporous silica for carbon dioxide reforming of methane

A series of Ce-promoted 6.7% Ni-containing mesoporous silica (Ce-Ni-SiO2) with varying Ce content (0.5%–4.8%) was prepared using the evaporation-induced self-assembly method. The characterization results showed that Ce and Ni species were homogeneously incorporated into the mesoporous silica matrix. The catalytic properties of these samples in the dry reforming of methane reaction revealed that the catalysts (e.g., 1.2% Ce-Ni-SiO2) containing highly dispersed small Ni particles exhibited excellent catalytic activity and long-term stability, which is attributed to the anchoring effect of the Ce and its ability to increase surface oxygen species concentration.

Steam reforming of iso-octane toward hydrogen production over mono- and bi-metallic Cu–Co/CeO2 catalysts: Structure-activity correlations

Τhe feasibility of tailoring the iso-octane steam reforming activity of Cu/CeO2 catalysts through the use of Co as a second active metal (Cu20−xCox, where x = 0, 5, 10, 15, 20 wt%), is investigated. Characterization studies, involving N2 adsorption–desorption at −196 °C (BET), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Reduction (H2-TPR), were carried out to reveal the impact of the morphological, structural and surface properties of the catalysts on the reforming performance. The results showed that reforming activity was monotonically increased upon increasing cobalt loading. The Co/CeO2 catalyst demonstrated the optimum performance with a H2 yield of 70–80% in the 600–800 °C temperature interval. The Co/CeO2 catalyst exhibited also excellent stability at temperatures above 700 °C, while Cu-based catalysts rapidly deactivated in long term stability tests. A close correlation between surface/redox properties and steam reforming efficiency was established. The lower reducibility of Co/CeO2 catalysts, associated with the formation of Co3+ species, in Co3O4-like phase, can be accounted for the enhanced carbon tolerance of Co-based catalysts. Furthermore, the high concentration of surface oxygen species on Co/CeO2 catalysts can be considered for their enhanced performance. On the other hand, the Cu-induced easier reducibility of bimetallic catalysts, in conjunction with carbon deposition and active phase sintering can be accounted for their inferior steam reforming performance. Irreversible changes in the redox properties of Cu-based catalysts, taking place under reaction conditions, could be resulted to ceria deactivation thus hindering the redox process to keep on.

Catalytic toluene oxidation over the three-dimensionally ordered macroporous EuFeO3 catalysts fabricated by the sucrose-assisted polymethyl methacrylate-templating method

Three-dimensionally macroporous perovskite-type oxides EuFeO3 (EFO-3DOM, EFO-sucrose-1, EFO-sucrose-2, and EFO-sucrose-3, respectively) have been prepared using the polymethyl methacrylate-templating method in the absence or presence of sucrose. Physicochemical properties of the materials were characterized by means of a number of analytical techniques, and their catalytic activities were evaluated for the total oxidation of toluene. It is shown that all of the EFO samples were of single-phase orthorhombic crystal structure with a 3DOM architecture. The sucrose addition during the preparation process had a great effect on the surface area and porous structure of the final product. A clear correlation of surface area, surface oxygen species concentration, and low-temperature reducibility with the catalytic performance was observed. The EFO-sucrose-1 catalyst performed the best, giving the T50% and T90% of 312 and 347 °C at space velocity = 20,000 mL/(g h), respectively. The apparent activation energies of the 3DOM-structured EFO samples were in the range of 82–97 kJ/mol. It is concluded that the higher surface area and oxygen adspecies concentration and better low-temperature reducibility account for the good catalytic activity of EFO-sucrose-1.

Effect of sulfur doping on the photocatalytic performance of BiVO4 under visible light illumination

Porous monoclinic bismuth vanadate (BiVO 4− δ ) and sulfur-doped bismuth vanadates (BiVO 4− δ S 0.05 , BiVO 4− δ S 0.08 , and BiVO 4− δ S 0.12 ) were synthesized by a dodecylamine-assisted alcohol-hydrothermal route in the absence and presence of thiourea or Na 2 S. The physicochemical properties of the materials were characterized and their photocatalytic performance for the degradation of methylene blue and formaldehyde under visible light was evaluated. The samples have a single phase monoclinic scheetlite crystal structure with a porous olive-like morphology, surface areas of 8.4–12.5 m 2 /g, and bandgap energies of 2.40–2.48 eV. Surface Bi 5+ , Bi 3+ , V 5+ , and V 4+ species were present on the S-doped BiVO 4− δ samples. Sulfur doping influenced the surface Bi 5+ /Bi 3+ , V 5+ /V 4+ , and O ads /O latt molar ratios, and the amount of sulfur doped had an important effect on the photocatalytic performance. Under visible light, BiVO 4− δ S 0.08 performed the best in the photodegradation of methylene blue and formaldehyde. A higher surface oxygen species concentration and a lower bandgap energy were responsible for the excellent visible light photocatalytic performance of BiVO 4− δ S 0.08 . Porous BiVO 4− δ and BiVO 4− δ S σ are fabricated using dodecylamine-assisted alcohol-hydrothermal strategy. The higher O ads concentration and lower bandgap energy account for excellent photocatalytic performance of BiVO 4− δ S 0.08 for methylene blue and formaldehyde degradation.

One-pot hydrothermal preparation and catalytic performance of porous strontium ferrite hollow spheres for the combustion of toluene

Strontium ferrite (SFO) hollow spheres with or without porous shells were fabricated via a novel one-pot hydrothermal route with the assistance of glucose and/or ethylenediamine. Physicochemical properties of the materials were characterized by means of a number of analytical techniques, and their catalytic performance was evaluated for the combustion of toluene. It is shown that the SFO samples possessed an orthorhombic structure and displayed a porous hollow spherical morphology with a surface area of 18–27m2/g. The SFO sample (SFO-3) derived hydrothermally at 170°C with a glucose/ethylenediamine volumetric ratio of 0.3/1.0 exhibited the highest surface area and oxygen adspecies concentration and the best low-temperature reducibility. Among the SFO samples, the SFO-3 sample showed the best catalytic activity for toluene combustion, and the T10%, T50%, and T90% were ca. 145, 255, and 298°C at a space velocity of 20,000mL/(gh), respectively. It is concluded that the good catalytic performance of SFO-3 was associated with its surface oxygen species concentration and better low-temperature reducibility.

Catalytic removal of toluene over three-dimensionally ordered macroporous Eu1–xSrxFeO3

Three-dimensionally ordered macroporous (3DOM) perovskite-type oxides EuFeO3 (EFO-3DOM) and Eu0.6Sr0.4FeO3 (ESFO-3DOM) were prepared by the citric acid-assisted polymethyl methacrylate-templating method. The physicochemical properties of the materials were characterized by means of numerous techniques. Catalytic activities of these porous samples were evaluated for the combustion of toluene. It is shown that the EFO-3DOM and ESFO-3DOM catalysts were of high-quality 3DOM architecture and single-phase orthorhombic crystal structure with a surface area of 16–31m2/g. The sequence in surface oxygen species concentration and low-temperature reducibility decreased in terms of ESFO-3DOM>EFO-3DOM>EFO-bulk, in good agreement with the order in catalytic activity. The ESFO-3DOM catalyst exhibited the best performance, giving the T10%, T50%, and T90% of 233, 278, and 305°C at a space velocity of 20,000mL/(gh), respectively. Apparent activation energies of the ESFO-3DOM, EFO-3DOM, and EFO-bulk catalysts were ca. 82, 96, and 104kJ/mol, respectively. The excellent catalytic activity of ESFO-3DOM might be associated with its higher surface area and surface oxygen species concentration and better low-temperature reducibility as well as high-quality 3DOM structure.

Porous Co3O4 nanowires and nanorods: Highly active catalysts for the combustion of toluene

Porous Co3O4 nanowires and nanorods (Co3O4-HT, Co3O4-HT-PEG, Co3O4-HT-CTAB, and Co3O4-ME-CTAB, respectively) have been fabricated via the hydrothermal or microemulsion route in the absence and presence of polyethylene glycol (PEG) or cetyltrimethylammonium bromide (CTAB), respectively. Physicochemical properties of the materials were characterized by means of numerous techniques, and their catalytic activities for toluene combustion were evaluated. It is shown that Co3O4-HT-PEG and Co3O4-HT-CTAB displayed a porous nanowire-like morphology, whereas Co3O4-ME-CTAB exhibited a porous nanorod-like shape. The porous Co3O4 samples (surface area=47–52m2/g) possessed much higher surface oxygen adspecies concentrations and much better low-temperature reducibility than the nonporous counterpart. The Co3O4-HT-CTAB sample showed the highest catalytic performance (T50%=195 and T90%=215°C at a space velocity of 20,000mL/(gh)). It is concluded that the excellent catalytic performance of Co3O4-HT-CTAB was associated with its higher surface area and surface oxygen species concentration, and better low-temperature reducibility.

Enhanced reactivity of metal-impregnated peat char and semi-coke during gas phase hydrogenolysis

Methane formation in the reaction between peat-derived chars and hydrogen was studied using peat semi-coke and doped chars. The latter were prepared by ion exchange of the initial material prior to pyrolysis. Impregnation with relatively large amounts of finely dispersed transition metals (up to ≈15 wt%) results in significant enhancement of methane production. Devolatilization prior to hydrogenolysis exerts a marked influence on the behaviour of the char due to its effect on the concentration of surface oxygen species. The lower the devolatilization temperature, the higher the methane yield. Two distinct peaks are present during the methane formation: a low-temperature peak involving the presence of CO and a high-temperature peak related to the direct reaction between H 2 and the char. The conversion profiles could be well approximated by a two-parameter model.