Dissociation Of Molecular Oxygen Research Articles

Synergistic effects of bimetallic PtRu/MCM-41 nanocatalysts for glycerol oxidation in base-free medium: Structure and electronic coupling dependent activity

Bimetallic PtRu catalyst was first reported for oxidation of glycerol to glyceric acid in base-free medium using O2 as the oxidant. A combination of density functional theory (DFT) calculations and multi-characterizations (e.g., H2-TPR, HAADF-STEM and XPS) revealed that the strong interaction between Pt and Ru could promote the dispersion of PtRu alloy nanoparticles and enhance the electronic coupling effect on the metal surface. Meanwhile, compared to the monometallic Pt catalyst, the introduction of Ru contributes to the direct dissociation of molecular oxygen and water to hydroxyl group, leading to the excellent catalytic activity. A volcanic-shaped relationship between Ru/Pt ratio, catalytic performance, structure-sensitivity and electronic coupling effect was systematically established. Furthermore, the role of structure-sensitivity and electronic coupling effect for the enhanced catalytic activity of PtRu catalysts with different Ru/Pt ratios are distinguished in detail. Finally, the Pt0.8Ru0.8/MCM-41 catalyst showed excellent catalytic activity (TOF: 823.9 h−1), glyceric acid selectivity (80.1%) and stability (recycling for 5th) under the optimized conditions (80 °C, 1 MPa O2 and 12 h). The insights and methodology reported here may pave the way to the rational design of bimetallic catalysts for efficient conversion of bio-derived substrates under mild conditions.

Molecular and Dissociative Adsorption of Oxygen on Au-Pd Bimetallic Clusters: Role of Composition and Spin State of the Cluster.

Utilization of molecular oxygen as an oxidizing agent in industrially important reactions is the ultimate goal to design environmentally benign processes under ambient conditions. However, the high thermal stability and a large O–O dissociation barrier in O2 molecule pose a great challenge toward its successful application in the oxidative chemistry. To achieve this goal, different catalysts based on monometallic and bimetallic clusters have been developed over the years to promote binding and dissociation of molecular oxygen. The successful design of efficient metal cluster catalysis needs an in-depth knowledge of synergistic effects between different metal atoms and intrinsic catalytic mechanisms for O2 adsorption and dissociation. Here, we present a systematic theoretical investigation of reaction pathways for O2 adsorption and dissociation on Au8, Pd8, and Au8–nPdn (n = 1–7) nanoclusters in different spin states. The density functional calculations point out that the O2 dissociation barriers can be significantly reduced with the help of certain bimetallic clusters along specific spin channels. Our results particularly indicate that Au5Pd3 and Au1Pd7 show very large O2 binding energies of 1.76 and 1.69 eV, respectively. The enhanced O2 binding subsequently leads to low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without the involvement of any spin flip-over for O2 dissociation. Furthermore, the computed O2 dissociation barriers are significantly low as compared to the already reported barriers (1.95–3.65 eV) on monometallic and bimetallic Au–Ag clusters. The results provide key mechanistic insights into the interaction and dissociation of molecular oxygen with Au–Pd clusters, which can prove informative for the design of efficient catalysts for oxidative chemistry involving molecular oxygen as a reactant.

A Model of Fluxes of Solar Extreme Ultraviolet Irradiance

We develop an approach to create a model of the solar extreme ultraviolet spectrum in the wavelength range responsible for dissociation of molecular oxygen (115−242 nm). The model is based on the concept of the linear dependence of irradiance fluxes in intervals 1 nm in width on the intensity in the Lyman-alpha hydrogen line (this line is supposed to be measured by photometers of the extreme ultraviolet irradiance onboard space probes). The linear dependence coefficients were obtained for each of these intervals. Comparison of the model calculation results with observations showed that the model inaccuracy does not exceed 1−2%, which is sufficient for calculations of the thermospheric state.

Report of the use of topical ozone together with professional oral hygiene procedures in patient with gingival mucous membrane pemphigoid

Event Abstract Back to Event Report of the use of topical ozone together with professional oral hygiene procedures in patient with gingival mucous membrane pemphigoid Paola Carcieri1*, Roberto Broccoletti1, Alessio Gambino1, Davide Conrotto1, Mario Carbone1 and Paolo Giacomo Arduino1 1 University of Turin, Department of Surgical Sciences, Oral Medicine Section, Italy Aim. Mucous membrane pemphigoid (MMP) is a heterogeneous group of autoimmune chronic inflammatory, subepithelial blistering disorder, predominantly involving the mucous membranes. It has a female predilection and commonly occurring after the fifth decade of life. The oral mucosa is affected in more than 90% of cases. Even if this condition has been reported as non-plaque induced, effective dento-gingival plaque control sometimes could resolve the gingival inflammation. Ozone is naturally produced by the photo dissociation of molecular oxygen into activated oxygen atoms which then react with further oxygen molecules. This transient radical anion rapidly becomes protonated, which in turn decomposes to an even powerful oxidant, the hydroxyl radical OH. Application of ozone in periodontics comes as a result of its physic-chemical properties. Ozone therapy has a wide range of applications in treating various diseases owing to its unique properties including antimicrobial, immunostimulant, analgesic, detoxicating, bioenergetic and biosynthetic actions. Materials and Methods. A 70-year-old female patient presented with 1-year history of intact blisters in her mouth, which ruptured leaving throbbing ulcers and peeling of the gingiva; she also reported pain and burning sensation while taking hot and spicy foods. A biopsy was performed with MMP evidence. A professional oral hygiene regimen was started to deal with the gingival status associated together with the use of local ozone applications. Severity of pain was detailed using a Visual Analogue Scale (VAS). Patient received non-surgical periodontal therapy, including personalized oral hygiene instructions, and thorough supragingival scaling and polishing with the removal of all deposits and staining. Clinical outcome variables were recorded at baseline and after the intervention and included full-mouth plaque scores (FMPS), full-mouth bleeding scores (FMBS), and the clinical extension of gingival involvement. Results. After finishing the proposed protocol, a significant and statistical reduction was observed for FMPS, FMBS, and clinical gingival involvement, without using any topical or systemic immunosuppressant. Discussion. Professional oral hygiene procedures and non-surgical periodontal therapy, including topical use of ozone therapy, were associated with a prominent improvement of gingival clinical outcomes in this female with exclusive MMP related gingivitis.

Catalytic conversions of atmospheric sulfur dioxide and formation of acid rain over mineral dusts: Molecular oxygen as the oxygen source

Sulfur dioxide (SO2) ranks as a major air pollutant and is likely to generate acid rain. When molecular oxygen is the oxygen source, the regular surfaces of gibbsite (one of the most abundant mineral dusts) show no reactivity for SO2 conversions to H2SO4, while the partially dehydrated (100) surface with coordination-unsaturated Al sites becomes catalytically effective. Because of the easy availability of molecular oxygen, results manifest that acid rain can form under all atmospheric conditions and may account for the high conversion ratio of atmospheric SO2. The (100) and (001) surfaces show divergent catalytic effects, and hydrolysis is always the rate-limiting step. Path A (hydrolysis and then oxidation) is preferred for (100) surface, whereas a third path with obviously lower activation barriers is presented for (001) surface, which is non-existent for (100) surface. Atomic oxygen originating from the dissociation of molecular oxygen is catalytically active for (100) surface, while the active site of (001) surface fails to be recovered, suggesting that SO2 conversions over gibbsite surfaces are facet-controlled. This work also offers an environmentally friendly route for production of H2SO4 (one of the essential compounds in chemical industry), directly using molecular oxygen as the oxygen source.

The Double-Edged Sword Effect of Water in the Low-Temperature CO Oxidation on Pt(111) Surface

The positive and negative effects of water on low-temperature CO oxidation were comprehensively investigated on a Pt(111) surface by thermal desorption spectroscopy, X-ray photoelectron spectroscopy, and polarization-modulated infrared reflection absorption spectroscopy. At 110 K, the interaction between H2O and preadsorbed O2 forms the {O2(a)·(H2O)n} complex on the Pt(111) surface through hydrogen bonding. Upon heating the surface to 170 K, the O2 in the {O2(a)·(H2O)n} complex is subject to dissociation. On the O2-saturated Pt(111) surface (50 L O2/Pt(111)), part of the chemisorbed O2 on Pt(111) can be displaced by H2O and forms {O2(g)·(H2O)n} trapped by H2O molecules at high H2O coverage (>0.44 ML H2O). At this point, high H2O coverage also weakens the molecular oxygen dissociation. Regarding the CO oxidation, a new CO2 production channel by the interaction of CO with the {O2(a)·(H2O)n} complex at ∼153 K has been discovered by exposing H2O to the CO/O2/Pt(111) surface at 110 K, which dominates the low-t...

Molecular oxygen adsorption and dissociation on Au12M clusters with M = Cu, Ag or Ir

In this work, we present a density functional theory study of the structural and electronic properties of isolated neutral clusters of the type Au12M, with M = Cu, Ag, or Ir. On the other hand, there is experimental evidence that gold-silver, gold-copper and gold-iridium nanoparticles have an enhanced catalytic activity for the CO oxidation reaction. In order to address these phenomena, we also performed density functional calculations of the adsorption and dissociation of O2 on these nanoparticles. Moreover, to understand the effects of Cu, Ag, and Ir impurity atoms on the dissociation of O2, we also analyze this reaction in the corresponding pure gold cluster. The results indicate that the substitution of one gold atom in a Au13 cluster by Ag, Cu or Ir diminishes the activation energy barrier for the O2 dissociation by nearly 1 eV. This energy barrier is similar for Au12Ag and Au12Cu, whereas for Au12Ir is even lower. These results suggest that the addition of other transition metal atoms to gold nanoclusters can enhance their catalytic activity towards the CO oxidation reaction, independently of the effect that the substrate could have on supported nanoclusters.

Study of electron beam induced ion-pair dissociation dynamics of O2 using velocity slice imaging spectrometer

The ion-pair dissociation (IPD) of molecular oxygen due to 21–35 eV energy electron collision has been studied using the time sliced velocity map imaging technique. The threshold of the process and the kinetic energy and angular distribution of the fragment negative ions are measured. The IPD is found to be occurring due to pre-dissociation of a Rydberg state via ion-pair state for lower incident electron energies as well as from direct excitation to the ion-pair states for higher primary beam energy. The location and symmetry of the excited states are determined from the kinetic energy and angular distribution data respectively.

Energy dispersive spectrometry and first principles studies on the oxidation of pentlandite.

Experimental and computational studies were carried out to investigate the oxidation of pentlandite (Fe4.5Ni4.5S8). The oxidation product was first analyzed by energy dispersive spectrometry to reveal the elemental distribution at the cross section. Our experimental study shows that the Fe atoms in pentlandite migrated to the surface and were preferentially oxidized to form a thin layer of Fe2O3, whereas the Ni atoms remained at the center of the grain. Furthermore, density functional theory calculations were performed to investigate the adsorption and diffusion of atomic oxygen as well as the adsorption and dissociation of molecular oxygen on the (001) and (010) surfaces of pentlandite. From the calculated adsorption energies of atomic oxygen at the different sites of the (001) and (010) surfaces, we found that oxidation of the Fe sites was preferable to that of the Ni sites when exposed to an oxidizing atmosphere. For molecular oxygen adsorption on the surfaces of pentlandite, the bridge sites (Fe-Ni and Fe-Fe) were found to be the most favorable adsorption sites. The dissociative adsorption of O2 is thermodynamically more favorable than the molecular adsorption. Calculated dissociation barriers show that the oxidation is feasible during high temperature roasting.

Влияние механического растяжения на адсорбционные свойства легированного азотом графена

AbstractThis paper presents the results of quantum chemical modeling of chemisorption of atomic hydrogen and epoxy, carboxyl, and hydroxyl functional groups on nitrogen-doped graphene. It is shown that the substitutional nitrogen atom does not bind to adsorbing groups directly, but significantly increases the adsorption activity of neighboring carbon atoms. Mechanical stretching of doped graphene reduces the adsorption energy of all the aforementioned radicals. This reduction is significantly greater for the epoxy group than for the other functional groups. The results obtained confirm that, upon a sufficient stretching of a nitrogen-doped graphene sheet, the dissociation of molecular hydrogen and oxygen with subsequent precipitation of the resulting radicals onto graphene can be energetically favorable.

Concave Cubes as Experimental Models of Catalytic Active Sites for the Oxygen-Assisted Coupling of Alcohols by Dilute (Ag)Au Alloys

A major challenge in understanding structure–function relationships in heterogeneous catalysis is bridging the materials complexity gap between the well-ordered surfaces used in fundamental experimental and computational studies and the more complex and dynamic materials that exist under catalytic operating conditions. In this work, we utilized (Ag)Au concave cube nanoparticles as experimental models to test a prediction made by theory regarding a potential bimetallic active site for the dissociation of molecular oxygen, which is a key initiating step in the selective oxygen-assisted coupling of alcohols. As a consequence of their method of synthesis, the concave cubes have surfaces that are rich in Ag-stabilized Au step edges, which is the active site proposed by theory, and thus we predicted that they would have high activity for the methanol coupling reaction. Indeed, in addition to 99% selectivity toward the desired coupling product, methyl formate, the concave cubes show a major increase in activity compared to ozone-activated nanoporous gold, a comparable dilute (Ag)Au alloy catalyst for the same reaction, even without an activating ozone pretreament. Further, the well-defined surfaces of these concave cubes open up opportunities for in-situ microscopy and spectroscopy experiments that can provide a better understanding of (Ag)Au active sites. More broadly, this work highlights how nanoparticles with controlled shapes and well-defined surfaces can be rationally tailored to experimentally validate predictions from theory.

Influence of Mechanical Stretching on Adsorption Properties of Nitrogen-Doped Graphene

This paper presents the results of quantum chemical modeling of chemisorption of atomic hydrogen and epoxy, carboxyl, and hydroxyl functional groups on nitrogen-doped graphene. It is shown that the substitutional nitrogen atom does not bind to adsorbing groups directly, but significantly increases the adsorption activity of neighboring carbon atoms. Mechanical stretching of doped graphene reduces the adsorption energy of all the aforementioned radicals. This reduction is significantly greater for the epoxy group than for the other functional groups. The results obtained confirm that, upon a sufficient stretching of a nitrogen-doped graphene sheet, the dissociation of molecular hydrogen and oxygen with subsequent precipitation of the resulting radicals onto graphene can be energetically favorable.

Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO2 Catalyst for Selective Oxidation of Methanol to Methyl Formate.

Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents an energy efficient and sustainable route for the synthesis of esters. Specifically, the bimetallic PdAu/TiO2 system was found to be highly active and selective toward photocatalytic production of methyl formate (MF) from gas-phase methanol. In the current paper, we applied the electronic structure density functional theory method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over the PdAu/TiO2 catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanoclusters to the interface of PdAu/TiO2 and subsequent formation of a unique structure, resembling a core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron-hole separation capability of PdAu nanoparticles deposited onto the anatase surface and hence the remarkably enhanced photocatalytic activity, in comparison to their monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple-point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO2 is responsible for total oxidation of methanol to CO2 (no MF formation) in the gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles-TiO2 catalysts for the photocatalytic selective oxidation of methanol to MF.

First-principles investigation of adsorption and dissociation of molecular oxygen on pure Pd, Ni-doped Pd and NiPd alloy clusters

A theoretical study on the adsorption of the molecular oxygen on the ground state structures of pure Pdn, Ni-doped Pdn−1 and Pdn/2Nin/2 (n = 4, 6, 8 and 10) clusters was performed. The dissociation pathways of the molecular oxygen on the Pd4, Pd3Ni and Pd2Ni2 clusters were calculated. All calculations were performed using the auxiliary density functional theory (ADFT) approach as it is implemented in the deMon2k code. The objective of this work was to evaluate how the adsorption energy of the molecular oxygen is modified when the pure Pd clusters are doped with a Ni atom (Ni-doped Pdn−1) and when the composition of the Pd:Ni is 1:1 (Pdn/2Nin/2). Besides, the activation energy for the dissociation of the O2 on the Pd4, Pd3Ni and Pd2Ni2 clusters was calculated, too. Higher adsorption energies are found when the molecular oxygen is bound to two Ni atoms. Finally, the Ni-doped Pd3 and Pd2Ni2 clusters present lower activation energy compared to the one calculated for the pure Pd4 cluster.

Mechanistic insight into the facet-dependent selectivity of ethylene epoxidation on Ag nanocatalysts

Ethylene epoxidation reaction to selectively produce the ethylene oxide (EO) is a crucial heterogeneous catalytic chemical process in industry. However, the intrinsic catalytic selectivity depending on the facet of commonly used Ag catalysts is still far from understanding. In this work, we used the periodical density functional theory (DFT) calculations to gain an atomistic insight into the facet-dependent reaction selectivity of the ethylene epoxidation on Ag catalysts. We investigated the adsorption of atomic or molecular oxygen and oxametallacycle (OMC) intermediate, and further study the dissociation of molecular oxygen and the reaction mechanism of ethylene epoxidation on (111), (110), (100), and (211) surfaces of Ag catalysts. Our results indicate that the adsorption of the O atom is not facet-dependent under low oxygen coverage. Moreover, although the adsorption of molecular oxygen is facet-dependent, the dissociation of molecular oxygen would not have the essential influence on selectively producing EO in the ethylene epoxidation reaction. In addition, the catalytic selectivity of ethylene epoxidation reaction is facet-dependent, following the order of (110)>(100)>(111)>(211), which is consistent with the order of the adsorption strength of the OMC intermediate. Our results show that the stability of the OMC intermediate has an essential effect on the selectivity to EO in the ethylene epoxidation reaction. We have faith in that our results can provide useful information which would guide the intellectual design of the catalysts for ethylene epoxidation by controlling their morphologies.

Electrochemical Stripping of Atomic Oxygen on Single-Crystalline Platinum: Bridging Gas-Phase and Electrochemical Oxidation.

To understand the interaction between Pt and surface oxygenated species in electrocatalysis, this paper correlates the electrochemistry of atomic oxygen on Pt formed in the gas phase with electrochemically generated oxygen species on a variety of single-crystal platinum surfaces. The atomic oxygen adsorbed on single-crystalline Pt electrodes, made by thermal dissociation of molecular oxygen, is used for voltammetry measurements in acidic electrolytes (HClO4 and H2SO4). The essential knowledge of coverage, binding energy, and surface construction of atomic oxygen is correlated with the charge, potential, and shape of voltammograms, respectively. The differences of the voltammograms between the oxide made by thermal dissociation of molecular oxygen and electrochemical oxidation imply that atomic oxygen is not an intermediate of the electrochemical oxidation of Pt(111). The reconstruction of (100) terrace and step and the low-potential stripping of atomic oxygen on (111) step site provide insight into the first stages of degradation of Pt-based electrocatalysts.

Neodymium nickelate Nd2-xSrxNi1-yCoyO4 ± d (x and y = 0 or 0.05) as cathode materials for the oxygen reduction reaction

The oxygen reduction reaction (ORR) was investigated using two new cathode materials based on neodymium nickelate with formula Nd2-xSrxNi1-yCoyO4±δ (with x and y=0 or 0.05) prepared by the citrate method. The ORR was measured in 0.5M NaOH with a rotating disk electrode (RDE) at different rotation speeds after purging with pure oxygen. The synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), whereas the electrochemical analysis was obtained by linear voltammetry and impedance spectroscopy.The results show that before the ORR, the grains of the undoped material are spherical in shape with a size around 400nm, while after the oxygen reduction reaction, the grain size increases. In the doped material, on the contrary, the results are reversed and the grain size becomes smaller. In addition, we show that the ORR mechanism proceeds in a single step in the undoped material while it is carried out in two successive steps in the case of the doped one. XPS analysis confirms that the sites responsible for the ORR are the OH− groups. Doping with Sr or Co does not improve the electrochemical properties: this dopant adversely affects the formation of the sites responsible for the dissociation of molecular oxygen. For larger hydroxide groups content (2.30, Nd2NiO4±δ) the electrochemical currents are much greater relative to that of the other Nd1.95Sr0.05Ni0.95Co0.05O4±δ (0.98) material. This work helps to understand the role of OH− groups, and the reduction mechanisms with 2 and 4 electrons.

Oxygen-Molecule Adsorption and Dissociation on BCN Graphene: A First-Principles Study.

Boron and nitrogen co-doped (BCN) graphene is an attractive material for use as a metal-free oxygen reduction reaction electrocatalyst and as other catalysts due to its unique structure and electronic properties. Reported here is the structure, determined by using density functional theory, of the active O2 -dissociation site of BCN graphene containing different types of BN cluster. The results show that the edge termination and shape of substitutional BN clusters are two important factors that determine the catalytic activity of BCN graphene for the dissociation of molecular oxygen. N-Terminated triangular BN (t-BN) cluster doping can reduce the energy barrier more effectively compared to a t-BN with a B edge or quadrangular BN cluster. Interestingly, the B atom neighboring the N edge, only in the case of N-terminated t-BN doping, is determined to be the most active site for O2 dissociation due to the barrier being as low as 0.08 eV. The electronic structure calculations reveal that in addition to the large positive charge densities, the catalytic activity of graphene enhanced by B,N doping is also attributed to the increased density of states of the π* states of the active site around the Fermi level.

Methanol Partial Oxidation on Ag(1 1 1) from First Principles

AbstractIn this work, we examine the thermochemistry and kinetics of the partial oxidation of methanol to formaldehyde on silver surfaces. Periodic density functional theory calculations employing the BEEF‐vdW functional are used to identify the most stable phases of the silver surface under relevant reaction conditions and the reaction energetics are obtained on these surfaces. The calculated binding energies and transition state energies are used as input in a mean‐field microkinetic model providing the reaction kinetics on silver surfaces under different reaction conditions. Our results show that, under conditions pertaining to methanol partial oxidation, oxygen is present at low concentrations and it plays a critical role in the catalytic reaction. Surface oxygen promotes the reaction by activating the OH bond in methanol, thus forming a methoxy intermediate, which can react further to form formaldehyde. The dissociation of molecular oxygen is identified as the most critical step.

Electrochemical Performance of La1.5Sr0.5Ni1-xFexO4+δ Cathode for IT-SOFCs

La1.5Sr0.5Ni1-xFexO4+δ (LSNF, x=0, 0.3, 0.5) series materials are synthesized as potential cathode materials for intermediate-temperature solid fuel cells by an EDTA-citrate complexation process. The effects of Fe substitution for Ni on the structure, transport property and electrochemical performances are investigated. X-ray diffraction shows that all samples with Fe-doping are identified as single phase materials with a tetragonal I4/mmm space group. Fe-doping decreases electrical conductivity of La1.5Sr0.5Ni1-xFexO4+δ. The study of symmetrical cells with La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) as an electrolyte shows a decreasing tendency of polarization resistance (Rp) of La1.5Sr0.5Ni1-xFexO4+δ with increasing Fe content. The impedance data at different oxygen partial pressures reveal that the electrode reaction is controlled by the molecular oxygen dissociation and charge transfer processes. The substitution of Fe improves the electrode kinetics and enhances the electrochemical performance of LSNF electrode significantly. The LSGM electrolyte (400μm) supported single cell with La1.5Sr0.5Ni0.5Fe0.5O4+δ as cathode and Ni-Gd0.1Ce0.9O2-δ as anode delivers relatively high power density of 282mWcm−2 at 800°C.