Dissociative Adsorption Of Oxygen Research Articles

A comparative density functional theory study of oxygen doping versus adsorption on graphene to tune its band gap

Graphene, having a perfect two-dimensional crystal structure, has many excellent features such as a high specific surface area, and extraordinary electrical, thermal and mechanical properties. However, its usage in electronic devices is possible only if band gap of desired value is induced in this gapless semi-metal. Therefore, first principle calculations have been carried out to investigate the role of oxygen (O) doping versus adsorption and, the impurity concentration and coverage to induce band gap in graphene employing PBE at GGA level. The band gap is induced owing to production of vacancies, dissociative adsorption of oxygen, subsituational doping and pre-dissociated oxygen adsorption. It is interesting to note that band gap is introduced by both the processes of doping and adsorption of O. The oxygen doping leads to induction of two energy gaps, smaller in value above and larger below the Fermi level; while adsorption irrespective of adsorption configuration produces single direct gap. Increase both in concentration and coverage leads to enhance band gap value maximum being 1.85 eV in case of hexagonal doping of 12.5% concentration with an exception in adsorption case. The results allow us to conclude that adsorption is as useful as doping to tune the band gap in graphene enabling its applications in designing high performance electronic devices.

CO Oxidation Efficiency and Hysteresis Behavior over Mesoporous Pd/SiO2 Catalyst

Carbon monoxide (CO) oxidation is considered an important reaction in heterogeneous industrial catalysis and has been extensively studied. Pd supported on SiO2 aerogel catalysts exhibit good catalytic activity toward this reaction owing to their CO bond activation capability and thermal stability. Pd/SiO2 catalysts were investigated using carbon monoxide (CO) oxidation as a model reaction. The catalyst becomes active, and the conversion increases after the temperature reaches the ignition temperature (Tig). A normal hysteresis in carbon monoxide (CO) oxidation has been observed, where the catalysts continue to exhibit high catalytic activity (CO conversion remains at 100%) during the extinction even at temperatures lower than Tig. The catalyst was characterized using BET, TEM, XPS, TGA-DSC, and FTIR. In this work, the influence of pretreatment conditions and stability of the active sites on the catalytic activity and hysteresis is presented. The CO oxidation on the Pd/SiO2 catalyst has been attributed to the dissociative adsorption of molecular oxygen and the activation of the C-O bond, followed by diffusion of adsorbates at Tig to form CO2. Whereas, the hysteresis has been explained by the enhanced stability of the active site caused by thermal effects, pretreatment conditions, Pd-SiO2 support interaction, and PdO formation and decomposition.

Effect of grain boundaries in La0.84Sr0.16CoO3-δ on oxygen diffusivity and surface exchange kinetics.

The single crystal and polycrystalline specimens of La0.84Sr0.16CoO3-δ oxide were synthesized and characterized by X-ray powder diffraction analysis, energy dispersive X-ray microanalysis, the electron backscatter diffraction technique, and X-ray photoelectron spectroscopy. A thin slab was prepared from the grown single crystal with its surface corresponding to the (110) plane. The kinetics of the oxygen exchange between the gas phase and a single crystal and a polycrystalline specimen was studied by means of 16O/18O oxygen isotope exchange at T = 750-850 °C and PO2 = 5.3 × 10-3-2.2 × 10-2 atm. Temperature dependencies of the oxygen heterogeneous exchange rate, the oxygen dissociative adsorption and incorporation rates, and oxygen diffusion coefficients were obtained. The relationship between the crystallographic orientation of oxides and the kinetic parameters of oxides has been established. Correlations between the surface state and the rates of individual stages of oxygen exchange as well as oxygen diffusion pathways in the single crystal compared with those in the polycrystalline specimen are considered.

Effect of Elemental Substitution in (Ba,Sr)6RE2Co4O15−BaCe0.5Pr0.3Y0.2O3-δ for Cathodes of Proton-Conducting Ceramic Fuel Cells

Solid oxide fuel cells are promising energy conversion devices because of high efficiency and high fuel flexibility, but their high-operating temperature (700-900 ºC) causes problems such as fast degradation of components and long start-up/shut-down time. To overcome these problems, low- temperature operation is preferable. Proton-conducting ceramic fuel cells (PCFCs) are promising energy conversion devices for intermediate-temperature operation, especially below 600 ºC. This is because the activation energy of proton migration in proton-conducting oxides, such as rare earth-doped barium cerate, is lower than that of the oxide ion. At the current state, however, the performance of PCFCs is mainly limited by the cathode materials.Recently, we have reported that the composite of Ba4Sr2Sm2Co4O15 (BSSC4224)–BaCe0.5Pr0.3Y0.2O3 (BCPY) showed higher activity below 600 ºC in 3% humidified synthetic air, as compared with high-performance cathodes reported1). In this composite, BSSC4224 mainly served as an active site for the dissociative absorption of oxygen. In this study, then, the effect of the elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)6 RE 2Co4O15, on the ASR was studied. Symmetric cells consisting of the BaCe0.8Y0.2O3 (BCY) disk electrolyte and Ba5SrRE 2Co4O15 (RE =Nd, La, Eu, Sm, Pr, and Gd), Ba4Sr2Gd2Co4O15 or Ba5SrGd2Co4O15 (BSGC5124)–BCPY electrodes were fabricated. The electrode performance was measured in 3% humidified synthetic air (21% O2–79% N2) at 450–700 ºC by impedance spectroscopy.In impedance analysis, BSGC5124 had the lowest ASR below 550 ºC among Ba5SrRE 2Co4O15 electrodes. Then, the composite of BSGC5124–BCPY was prepared and its performance was compared with that of BSSC4224–BCPY composite. We found that BSGC5124–BCPY has comparable area-specific resistance and activation energy to BSSC4224–BCPY although BSGC5124 itself had much lower area-specific resistance and activation energy than BSSC4224. Therefore, we concluded that the dissociative adsorption of oxygen on the BSGC5124 phase is not the rate limiting step for the oxygen reduction reaction in this composite system. [1] T. Matsui, K. Manriki, K. Miyazaki, H. Muroyama, K. Eguchi, J. Mat er. Chem. A, 6, 14188 (2018).

Electrostatic spray deposited Ca3Co4O9+δ and Ca3Co4O9+δ/Ce0.9Gd0.1O1.95 cathodes for SOFC: A comparative impedance analysis study

Mixed conducting Ca3Co4O9+δ (CCO) is an interesting cathode material for application in Solid Oxide Fuel Cells (SOFC). In a previous study it has been shown that addition of Ce0.9Gd0.1O1.95 (CGO) significantly enhances the electrode properties, reducing the Area Specific Resistance (ASR) to ~0.5 Ω cm2 at 700°C for the CCO/CGO 50/50% composition. As the microstructure of a composite electrode has a significant influence on the frequency dispersion, it is of interest to prepare electrodes with quite different microstructures. Electrostatic Spray Deposition (ESD) is a technique that is able to produce a large variety of microstructures by modifying the process parameters. Very different microstructures can help in elucidating the major charge transport and transfer processes in an electrode.In this study pure CCO and a CCO/CGO composite (50/50%), both prepared with ESD, are compared and analyzed with Electrochemical Impedance Spectroscopy (EIS). The analysis of the ESD-CCO/CGO composition showed remarkable similarities with the screen-printed cathodes from the previous study, although a clear change in the magnitudes of the separate contributions (low-frequency redox, mid-frequency Gerischer and high-frequency diffusion) was observed. The ASR was close to the screen-printed one, but showed two apparent activation energies in the Arrhenius graph. A tentative model indicates that the Gerischer process is related to dissociative oxygen adsorption and (surface) diffusion at the CGO phase. The limiting factor is the density of the triple-phase boundaries (TPBs) between CCO, CGO and the ambient, which is for the ESD-CCO/CGO cathode apparently lower than for the microstructure of the screen-printed cathodes. It was noted that for the ESD-CCO/CGO cathodes the Distribution Function of Relaxation Times (DFRT) presented a more consistent image of the temperature dependence than the standard Complex Nonlinear Least Squares (CNLS) analysis.The pure ESD-CCO cathode showed a remarkable dispersion, which could be interpreted with a Finite Length Warburg (FLW) model. Although analysis with a simple Equivalent Circuit (EqC) was not feasible, partial CNLS-analysis of the high- and low-frequency regimes resulted in a parameter set that is consistent with the FLW-model. Considering the coral-like microstructure, the oxygen reaction at the electrode could be interpreted as slow oxygen dissociation and fast diffusion towards a more dense CCO layer at the electrolyte interface, followed by a fast oxygen exchange step at the CCO-layer/platelets + ambient interface. Combining FLW parameters with published chemical diffusion values, an apparent effective layer thickness of 2.2 μm could be estimated.

Catalytic combustion of propane on Pd-modified Al–La–Ce catalyst – from reaction kinetics and mechanism to monolithic reactor tests and scale-up

Abstract A propane combustion catalyst was prepared by supporting of Pd on optimized multiphase composition, containing Al2O3, La2O3 and CeO2 aiming for possible application in catalytic converters for abatement of propane in waste gases. The catalyst characterization has been made by N2- physisorption, XRD, SEM/EDX, TEM and XPS. The obtained values for reaction order towards propane and oxygen are 0.57 and 0.14, respectively. The negative reaction order towards the water vapour (−0.26) shows an inhibition effect of the water molecules. According to the kinetics model calculations, the reaction pathway over Pd-modified La–Ce catalyst proceeds most probably through Langmuir–Hinshelwood mechanism with adsorption of propane and oxygen on different types of sites, dissociative adsorption of oxygen, whereupon water molecules compete with propane molecules for one and the same type of adsorption sites. For practical evaluation of the synthesized material, a sample of Pd/Al2O3–La2O3–CeO2, supported on rolled Al-containing stainless steel (Aluchrom VDM®) to form a single monolithic channel was prepared and tested. Two-dimensional heterogeneous models were used to simulate the propane combustion from laboratory reactor to full-scale adiabatic monolithic converter for ensuring an effective abatement of propane emissions.

Improved electrocatalytic properties of La0.3Ca0.7Fe0.7Cr0.3O3-δ electrodes for oxygen reduction reaction: The effect of electrode thickness

Three-electrode half-cells with La0.3Ca0.7Fe0.7Cr0.3O3-δ porous electrodes were prepared by screen-printing technology. The electrodes with thicknesses ranging from ∼12 to ∼62 μm exhibited similar microstructures. The electrocatalytic properties of the electrodes for oxygen reduction reaction were investigated with respect to electrode thickness. The contributions of various electrode processes to the overall electrode polarization were resolved, among which the dissociative adsorption of molecular oxygen on the surface made the most substantial contribution. A significant dependence of the electrocatalytic properties on electrode thickness was detected. Tuning electrode thickness was demonstrated to be effective in optimizing the electrocatalytic properties. The electrode with a thickness of ∼43 μm achieved the optimal electrocatalytic properties among the electrodes investigated. At 800 °C in air, this electrode showed a polarization resistance of ∼0.06 Ω⋅ cm2, an exchange current density of 797 mA⋅ cm−2 and an overpotential of 9.3 mV under an applied cathodic current density of 200 mA⋅ cm−2.

Rate-Limiting Steps Elucidated By Electrochemical Impedance Spectroscopy of a Yttria-Stabilized Zirconia 2-in-1 Temperature and Oxygen Gas Sensor Measured at Extreme Temperatures

Introduction Yttria-stabilized zirconia (YSZ) is known for its good ionic conductivity, essentially being synonymous with the term “lambda sensor”. Automotive lambda sensors have been fabricated with YSZ as the electrolyte for 30+ years and have operated in a temperature region of 600oC to 800oC. There is an increasing demand for higher operating temperature sensors, specifically a 2-in-1 temperature and oxygen sensor which is stable, selective, and sensitive in extreme environments. However, there remains a disagreement among scholars about the effects on oxygen vacancy activation energy at temperatures higher than 800oC. Also, the rate-limiting step of oxygen reduction on the platinum electrode is not inherently clear. A 2-in-1 temperature and oxygen sensor was fabricated and characterized with the analysis of electrochemical impedance spectroscopy (EIS), DC resistance, and open circuit potential measurements at temperatures between 600oC and 1200oC. The temperature dependence on individual activation energies within the sensor was elucidated directly through these techniques. Methods Yttria-stabilized zirconia (MTI Corp) was employed as the solid-state electrolyte in sensor fabrication, with the primary electrode material as porous platinum (ESL 5570). Platinum paste was screen printed onto planar YSZ substrates in an interdigitated electrode configuration, then fired at 1500oC for 2 hours in nitrogen atmosphere (1 atm). The sensor was placed in a ThermoScientific tube furnace capable of reaching 1500oC, with mass flow controllers varying oxygen gas concentration throughout electrical measurements. Platinum foil acted as a Faraday cage around the sensor to shield from furnace inductive fluctuations and minimize noise during low frequency EIS measurements. A thermocouple placed next to the sensor validated furnace operating temperature and cooling endcaps on the furnace tube created a natural temperature gradient between sensor placement and the electrical connections outside of the furnace. Figure 1 displays the environmental testing setup. Results and Conclusions Figure 2a displays an impedance Nyquist spectrum that contains real and imaginary components of impedance measured at 0.21 atm PO2, where electrolyte resistance decreases with an increase in temperature. Figure 2b displays the Arrhenius plot of isolated YSZ electrolytic resistance extracted from figure 2a, where there are distinct activation regions at moderate and high temperatures. In agreement with Badwal [1], the shift to lower EA at higher temperatures was due to a gradual transition between an association region where oxygen vacancies are bound to dopant cations, and a dissociation region where vacancies are free and mobile.At a known temperature, EIS also elucidates oxygen concentration dependence on sensor electrode resistance. Figure 3a shows sensor electrode resistance, fitted from figure 2a, as a function of oxygen concentration. Investigating the oxygen reduction reaction (ORR) variation on platinum at various temperature regions, it is suggested that ORR on platinum is co-limited by oxygen adsorption and by oxygen transport along the surface of the electrode, in agreement with Adler [2]. Figure 3b shows that electrode 2 is dominant at temperatures below 1050oC, indicative of oxygen diffusion limitation on the surface and through platinum. Electrode 1 is dominant at temperatures above 1050oC, where dissociative adsorption of oxygen on the electrode surface is predominant.

Deposition of nanostructured LSM perovskite thin film on dense YSZ substrate by airbrushed solution combustion (ASC) for application in SOFC cathodes

To make SOFC high efficiency energy generation devices, thin ceramic films are proposed as their main components. The rate of the oxygen reduction reaction is relevant for the overall performance of the SOFC, hence a lot of attention is given to the cathodes and their interfaces. The airbrushed solution combustion (ASC) method was used to fabricate an LSM thin film on a dense YSZ substrate. A single phase LSM perovskite was obtained with very thin and interconnected porosity, and a small average grain size (55 nm). The nanostructured LSM thin film electrode showed a low total activation energy (1.27 eV) at high temperatures, but a high area specific resistance at 850°C (55 Ω.cm2). The activation energy for the dissociative adsorption and diffusion of oxygen was significantly low (1.27 eV), while the charge transfer and oxygen ion migration activation energy at the LSM/YSZ interface (1.28 eV) was closer to those usually reported.

Rate-Determining Steps of Oxygen Surface Exchange Kinetics on Sr2Fe1.5Mo0.5O6−δ

The oxygen surface kinetics of Sr2Fe1.5Mo0.5O6−δ was determined using the 16O2/18O2 isotope exchange method with gas phase analysis at 600–800 °C. The heterogeneous exchange rates (rH) and the oxygen diffusion coefficients (D) were calculated by processing the concentration dependences of the 18O fraction using Ezin’s model. The rates of oxygen dissociative adsorption (ra) and incorporation (ri) were calculated based on a model using the three exchange type rates. It has been established that the rates ra and ri were comparable in this temperature range. Assumptions were made about the effect of the chemical composition of the surface on the rate of oxygen adsorption. It was found that the oxygen exchange coefficient (k) of Sr2Fe1.5Mo0.5O6−δ is comparable to that of La0.6Sr0.4MnO3±δ oxide. High values of the oxygen diffusion coefficient were found for Sr2Fe1.5Mo0.5O6−δ. The values were comparable to those of the double cobaltite praseodymium-barium and exceed by more than an order those of lanthanum-strontium manganite.

Catalytic activity for dissociative oxygen adsorption of Co-based oxides at high temperature evaluated by a modified pulse isotopic exchange technique

A modified pulse isotopic exchange technique is proposed to quantitatively evaluate the dissociative adsorption rate of Co-based oxides at high temperature. Their catalytic activity is significantly enhanced by the formation of Co-ion clusters.

Kinetics of wet air oxidation of sodium sulfide over heterogeneous iron catalyst

AbstractWet air oxidation (WAO) is an established technique for reducing the chemical oxygen demand (COD) of refinery sulfidic spent caustic waste. In the present work, the heterogeneous form of the cheap and abundant catalyst ferrous sulfate (FeSO4) was employed for WAO of sodium sulfide. The performance of this catalyst in the oxidative destruction of this model compound is thus far unfamiliar. Kinetic data for the non‐catalytic and catalytic oxidation processes was collected in a batch reactor. For the catalytic process, temperature (T), oxygen partial pressure () and catalyst concentration (ω) were varied in the ranges 80‐150°C, 0.69‐2.06 MPa and 0.8‐2.4 g/L respectively. Around 94% COD was destroyed within 1 h when feed containing 8 g/L of sulfide was oxidized at T = 100°C, = 0.69 MPa, and ω = 0.8 g/L. First, the data on disappearance of COD were fitted to a power law model and reaction rate constants were determined. The activation energy for the non‐catalytic (91 kJ/mol) and catalytic (50 kJ/mol) oxidation process was found from the temperature dependence of the rate constants. Second, hyperbolic models based on Langmuir‐Hinshelwood (L‐H) and Eley‐Rideal (E‐L) kinetics were used for fitting kinetic data. It was found that the L‐H model suggesting dissociative adsorption of oxygen provided the best fit. In this way, a deep insight into oxidation kinetics of sodium sulfide was provided.

Oxygen isotope exchange in proton-conducting oxides based on lanthanum scandates

A method of the oxygen isotope exchange with equilibration of the gas phase was used in order to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients of the proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600–900 °C and oxygen pressure of 1.01 kPa. The diffusion and oxygen surface exchange coefficients increase with the strontium content in oxides. The rates of the individual stages of the oxygen exchange process on the surface of the oxides were determined. The oxygen incorporation was revealed to be rate-determining for the undoped LaScO3 oxide. However, for the strontium-doped oxides La1–xSrxScO3–δ (x = 0.04; 0.09), the difference between the rates of oxygen dissociative adsorption and oxygen incorporation decreases with increasing the strontium concentration, in a way that for the La0.91Sr0.09ScO3−δ oxide these stages become competing. The possible reasons for these differences in oxygen surface exchange kinetics were analyzed in this paper. Using the obtained oxygen diffusion coefficients, the oxygen-ionic conductivities in accordance with the Nernst-Einstein equation, and the contributions of the oxygen-ion and proton components of the total conductivity of the La1–xSrxScO3–δ (x = 0.04; 0.09) oxides in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa) were calculated. The proton transference numbers were found to be close to unit at the wet reducing atmospheres in the temperature range of 500–600°С.

Effect of Cobalt on the Catalytic Properties of Platinum during the Oxidation of CO: Experimental Data and Quantum-Chemical Simulation

The oxidation of CO over Pt and Pt–Co catalysts was studied both experimentally and by quantum-chemistry calculations. Density functional theory simulation shows that CO is oxidized on Pt13 via dissociative oxygen adsorption. The calculated activation energy for the dissociation of O2 on Pt13 is 56 kJ/mol. It is found that when a cobalt atom is introduced into the cluster, the activation energy falls for the stages of CO oxidation and the dissociation of O2. Catalytic tests performed on Pt and Pt0.5Co0.5 nanopowders confirm the Pt–Co system displays high activity in the oxidation of CO.

Influence of strontium content on the oxygen surface exchange kinetics and oxygen diffusion in La1–xSrxCoO3–δ oxides

Oxygen surface exchange kinetics between the gas phase oxygen and La1–xSrxCoO3–δ (x = 0, 0.3, 0.4, 0.6) oxides at 0.67 kPa oxygen pressure in the 600–850°°С range were investigated. The mechanism within the two-step model including two consecutive stages, oxygen dissociative adsorption and oxygen adatom incorporation into the oxide lattice, was analysed. The oxygen diffusion coefficient (D), the rate of heterogeneous oxygen exchange (rH) and the elementary rates of this process — dissociative adsorption (ra) and incorporation (ri) — were determined. The effect of oxygen vacancy concentration, strontium segregation on the surface, temperature and oxygen pressure on rH, ra and ri are discussed.

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.

Multiscale Approach to the Dissociative Adsorption of Oxygen on a Highly Dispersed Platinum Supported on γ-Al2O3

The understanding of the reactivity of supported platinum with oxygen is of paramount relevance in heterogeneous catalysis, inter alia. We present here a multiscale investigation of the interaction of highly dispersed Pt/γ-Al2O3 catalysts with O2, through the combination of temperature-programmed desorption (TPD) experiments, ab initio simulations on a model Pt13/γ-Al2O3 cluster, and kinetic simulations of the TPD, thanks to data obtained from ab initio calculations. The specific behavior of the sub-nanometric platinum particles is benchmarked against the one of the ideal Pt(111) surface, as predicted by similar ab initio-based kinetic modeling. This approach reveals a fully different reactivity of highly dispersed Pt nanoparticles with respect to Pt(111), with a much higher capacity of oxygen storage for given temperature and P(O2) conditions. In a large operating conditions interval, the Pt13 clusters are converted into an oxide, whose stoichiometry is close to PtO2, but with a very specific hemispheric...

Synergy between Ag nanoparticles and yttria-stabilized zirconia for soot oxidation

This study investigates a potential synergy between Ag nanoparticles and Yttria-Stabilized Zirconia (YSZ), an oxygen ionically conducting support, for developing an efficient and stable catalyst for Diesel Particulate Filter regeneration only with oxygen. The catalytic performances as well as the mechanism for soot combustion were thoroughly analyzed through Temperature-Programmed Oxidation and isotopic experiments. We found that Ag/YSZ catalysts reach the highest activity for low contents of Ag, i.e. 2 wt%. Isotopic experiments were carefully performed to prove that active oxygen species originate from YSZ lattice. Silver was shown to promote soot oxidation activity by activating the dissociative adsorption of oxygen that can replenish the YSZ support.

Oxygen Isotope Exchange in Proton-Conducting Oxides Based on Lanthanum Scandates

The method of oxygen isotope exchange with the gas phase equilibration have been used to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients with proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600−900°C at oxygen pressure 1.01 kPa. The paper determines that the diffusion and oxygen surface exchange coefficients increase with the increasing of the strontium content in the oxides. We have found out the rates of the individual stages of the oxygen exchange process on the surface of the oxides. It is shown that oxygen incorporation is rate-determining stage of the oxygen exchange on the surface of the undoped oxide, whereas for the strontium-doped oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) with increasing of strontium concentration, the difference between the rates of dissociative adsorption and oxygen incorporation decreases so that for the oxide La0,91Sr0,09ScO3–δ the stage of dissociative adsorption of oxygen becomes rate-determining stage. The paper analyzes the possible reasons of these differences in oxygen surface exchange kinetics. Moreover, the paper using the obtained oxygen diffusion coefficients that have been recalculated in the oxygen-ionic conductivities according to the Nernst-Einstein equation performs the contributions of the oxygen-ion and proton components of the total conductivity of oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa). Proton transference numbers are shown to be close to unit in the temperature range of 500–600 °С at the wet hydrogen-containing reducing atmosphere.

Catalytic oxidation of NO to NO2 for nitric acid production over a Pt/Al2O3 catalyst

The oxidation of nitric oxide (NO) to nitrogen dioxide (NO2) is a key step both in NOx abatement technologies as well as in the Ostwald process for nitric acid production. A 1 wt.% Pt/Al2O3 catalyst was used to study oxidation of nitric oxide at two different concentrations of NO; 400 ppm NO (representative of engine exhaust treatment) and 10% NO (nitric acid plant). The catalyst was characterised using N2 adsorption and CO chemisorption. The effect of temperature and feed concentration on catalytic activity was investigated.For a feed comprising of 10% NO and 6% O2, Pt/Al2O3 exhibits significant catalytic activity above 300 °C. Addition of 15% H2O in the feed had an insignificant effect on activity of the catalyst. We report for the first time the kinetics for oxidation of NO to NO2 under nitric acid plant conditions. An apparent activation energy of 33 kJ/mol was observed. The rate equation for the overall reaction was determined to be r=kfKG(PO2)0.5, where kf is the forward rate constant. The reaction is independent of NO concentration while it has half order dependency on oxygen. The reaction mechanism which fits our experimental observation consists of dissociative adsorption of oxygen, associative adsorption of nitric oxide with desorption of nitrogen dioxide as the rate limiting step.