Application of in-situ temperature programmed techniques to catalytic oxidation reactions on conducting mixed oxides

A test bed for development of catalysts in a temperature programmed reactor is described. The effluent species are monitored in real time. Such data are collected into spreadsheet arrays which can be interrogated to yield kinetic data. An appropriate reactor design can therefore proceed directly from laboratory measurements to generate whole plant simulation analysis and commercial evaluation. For the oxidative coupling of methane, the hydrocarbon to oxygen ratio in the feed is of particular importance since the state of oxidation plays a significant role in determining the selectivity of the catalyst to the optimum product distribution. Homogeneous gas phase reactions may also occur at high temperatures, hence the reactor volume both upstream and downstream of the catalyst must also be considered. The stability of the catalyst under reactor conditions can be further assessed by following temperature programmed, thermal gravimetric and differential thermal analyses in diverse oxidizing, reducing or reaction atmospheres. Temperature programmed AC electrical measurements also give further insight into changes in the catalyst both at the surface and in the bulk as chemical reactions proceed. Examples of these techniques on a variety of mixed oxides such as Li-Ni-Co-O, La-Sr-Co-Fe-O and K-β′' alumina are presented. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland     

Monte Carlo simulation of temperature programmed reaction and surface explosion during CO oxidation on a Pt catalyst

A Monte Carlo model of CO oxidation on a Pt(111) surface that includes finite rates of adsorption-desorption and reaction and the effect of the catalyst temperature is presented. The results show that, as expected from the reaction-adsorption probabilities, the surface coverage changes from being almost completely covered by CO at low temperature (60°C), to being completely covered by oxygen at high temperature (160°C). Furthermore, it was found that an unstable state occurs when cooling down the oxygen covered surface from 160°C to 60°C. It is shown that if a site for CO adsorption is created under this metastable state, a surface explosion that propagates spatially occurs. Thus the MC simulations provide a method to describe a catalytic reaction on surfaces with strongly non-linear spatio-temporal dynamics.     

Noninvasive monitoring of brain temperature during mild hypothermia

The main purpose of this study was to verify the feasibility of brain temperature mapping with high-spatial- and reduced-spectral-resolution magnetic resonance spectroscopic imaging (MRSI). A secondary goal was to determine the temperature coefficient of water chemical shift in the brain with and without internal spectral reference. The accuracy of the proposed MRSI method was verified using a water and vegetable oil phantom. Selective decrease of the brain temperature of pigs was induced by intranasal cooling. Temperature reductions between 2°C and 4°C were achieved within 20 min. The relative changes in temperature during the cooling process were monitored using MRSI. The reference temperature was measured with MR-compatible fiber-optic probes. Single-voxel ¹H MRS was used for measurement of absolute brain temperature at baseline and at the end of cooling. The temperature coefficient of the water chemical shift of brain tissue measured by MRSI without internal reference was −0.0192±0.0019 ppm/°C. The temperature coefficients of the water chemical shift relative to N-acetylaspartate, choline-containing compounds and creatine were −0.0096±0.0009, −0.0083±0.0007 and −0.0091±0.0011 ppm/°C, respectively. The results of this study indicate that MRSI with high spatial and reduced spectral resolutions is a reliable tool for monitoring long-term temperature changes in the brain.     

The temperature programmed desorption of hydrogen from platinum and platinum-gold films

The thermal desorption of hydrogen from Pt and PtAu films has been measured in an ultra-high vacuum system by means of a mass spectrometer. On the average, hydrogen is more loosely bound on the alloys than on pure Pt. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys while only a very small percentage is desorbed from Pt at this temperature. After maximum coverage of Pt films by hydrogen adsorption three desorption peaks have been observed: γ (120 K), β₁ (200 K) and β₂ (330 K). The same peaks have been found for the alloys as well but the relative population of the various adsorption types was different. The relative peak heights vary with the alloy composition.     

The F band of mixed ionic crystals as a function of temperature

Abstract

Experimental and theoretical study of the F absorption band were carried out in mixed ionic crystals as a function of temperature and molar composition. Both the point-ion and pseudopotential methods of Gourary–Adrian and Bartram–Stoneham–Gash, respectively, have been used to obtain the F band shift effect in KCl_1?x Br _x mixed crystals with molar fraction x and the temperature T of the crystal. The theoretical values were compared with the experimental ones, and a good agreement between experimental and theoretical calculations has been found.     

Electrochemical characterization of mixed conducting and composite SOFC cathodes

The electrochemical performance of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (L58SCF), La_0.9Sr_1.1FeO_4−δ (LS2F) and LSM (La_0.65Sr_0.3MnO_3−δ)/LSM–YSZ (50 wt.% LSM–50 wt.% ZrO₂ (8 mol% Y₂O₃)) cathode electrodes interfaced to a double layer Ce_0.8Gd_0.2O_2−δ (CGO)/YSZ electrolyte was studied in the temperature range of 600 to 850 °C and under flow of 21% O₂/He mixture, using impedance spectroscopy and current density–overpotential measurements. The L58SCF cathode exhibited the highest electrocatalytic activity for oxygen reduction, according to the order: LS2F/CGO/YSZ ≤ LSM/LSM–YSZ/CGO/YSZ < L58SCF/CGO/YSZ.     

Separation of chiral molecules by temperature programmed desorption

The Monte Carlo simulation method was used to model thermal desorption of a pair of enantiomers from a solid surface with a chiral periodic pattern of active sites. The main objective of the study was to determine the optimal number of the active sites and their spatial distribution within the unit cell of the surface to achieve the most efficient separation of the enantiomers. For that purpose we tested the series of chiral patterns which were found previously for the equilibrium adsorption. Temperature programmed desorption spectra were calculated using a square lattice of adsorption sites in which the active sites were distributed spatially according to the candidate patterns. Additionally, influence of relaxation of the adsorbed layer on the relative shift of the TPD peaks of the enantiomers was assessed and the key factors affecting the chiral separation were identified.     

The use of thermal imaging to monitoring skin temperature during cryotherapy: A systematic review

Cryotherapy has been applied on clinical injuries and as a method for exercise recovery. It is aimed to reduce edema, nervous conduction velocity, and tissue metabolism, as well as to accelerate the recovery process of the muscle injury induced by exercise. Objective: This review aim to investigate the applicability of thermal imaging as a method for monitoring skin temperature during cryotherapy. Method: Search the Web of Science database using the terms “Cryotherapy”, “Thermography”, “Thermal Image” and “Cooling”. Results: Nineteen studies met the inclusion criteria and pass the PEDro scale quality evaluation. Evidence support the use of thermal imaging as a method for monitoring the skin temperature during cryotherapy, and it is superior to other contact methods and subjective methods of assessing skin temperature. Conclusion: Thermography seems to be an efficient, trustworthy and secure method in order to monitoring skin temperature during cryotherapy application. Evidence supports the use of thermography in detriment of contact methods as well as other subjective ones.     

Transport properties and phase stability of mixed conducting oxide membranes

At high strontium doping levels, perovskite oxides containing iron have suitable stability and transport properties for use as oxide ion transport membranes. In our studies of these materials, we have investigated the pO₂ and temperature dependence of the conductivity and non-stoichiometry of La_1−xSr_xFe_1−yM_yO_3−δ (M = Cr, Ti) by using electrochemical cells and the thermal expansion by dilatometry. Non-equilibrium behavior is observed in both the chemical expansion data and also in the conductivity and stoichiometry and suggests the occurrence of microscopic phase segregation on reduction. Analysis of the microstructure of quenched samples confirms the occurrence of local phase separation. Bulk diffusion and surface exchange coefficients under near-gradientless conditions have been determined by the electrical conductivity relaxation (ECR) technique and by isotope exchange depth profiling (IEDP). Evaluation of transport under a chemical gradient was accomplished by transient isotopic tracing of operating membranes. The isotope transients (¹⁶O₂–¹⁸O₂) were performed on tubular membranes operating at steady state at temperatures between 1023 K and 1173 K and allow an unambiguous separation of surface and bulk resistances to oxygen permeation under steady state conditions, a separation not possible by permeation measurements alone.     

Distinguishing the impact of temperature on iron catalyst during the catalytic-pyrolysis of waste polypropylene

The catalytic pyrolysis of waste plastics with iron-based catalyst can produce H₂ rich gas, liquid oil and carbon nanotube (CNTs) together. While the catalytic pyrolysis mechanism is still unclear, in this study, the catalytic pyrolysis of polypropylene (PP) was explored in depth, and the influence of catalyst and temperature was distinguished. The results indicated that a lower temperature led to the generation of waxes, while a higher temperature promoted the formation of aromatic hydrocarbons when plastic pyrolysis was performed without a catalyst. In addition, a large number of carbon deposits, mainly in the form of spheres, were collected when the temperature was over 800 ℃. These carbon spheres originated from the agglomeration of aromatic hydrocarbons. Once catalysts were introduced, a large amount of liquid oil was transferred into carbon deposits at both lower and higher catalytic temperatures, simultaneously, leading to more light gases releasing, like hydrogen. At a lower temperature (≤ 800 ℃), it was mainly CNTs while carbon spheres are the main solid product at higher temperatures (> 800 ℃). In addition, two different mechanisms of CNTs formation were also concluded that the base-growth model dominated the of generation CNTs at 600 °C whereas the CNTs followed the tip-growth model at 800 ℃. The results show that the optimized temperature for the catalytic process should be around 800 ^o℃ where approximately 35 mmol/g_plastic hydrogen, 50% hydrogen efficiency and over 320 mg/g_plastic carbon nanotubes (CNTs) were obtained.     

Evolution of the attractor of a macrosystem with conducting phase concentration and temperature

Model experiments are performed to study temperature evolution of the attractors of insulator-conductor and insulator-semiconductor macrosystems with different concentrations of the filler. A model based on the percolation theory and synergetics statements is proposed to explain the observed processes. The system’s instability intervals are experimentally determined.     

A structural analysis of W-Sb mixed oxide catalyst

An investigation on the structure of W-Sb mixed oxide catalyst, W₁₂Sb_xO_y (x = 1, 3, 5), is proposed. The W-Sb mixed oxide powders were prepared by the calcination of aqueous precursors, antimony tartrate and ammoniummetatungstate, and characterized with scanning electron microscope, X-ray diffractometer, and transmission electron microscope. At low content of Sb (x = 1), the W-Sb mixed oxide powder consisted of polyhedral particles, and their crystal structure was triclinic WO₃. At higher content (x = 3, 5), majority of the oxide powders were bar-shaped particles, consisting of triclinic WO₃ and tetragonal WO₃. With electron diffraction pattern and simulation, Sb incorporation into the cuboctahedral sites of perovskite-like WO₃ was proved and its effect on the phase transition from triclinic to tetragonal was discussed.     

Phase-difference and spectroscopic imaging for monitoring of human brain temperature during cooling

Decrease of the human brain temperature was induced by intranasal cooling. The main purpose of this study was to compare the two magnetic resonance methods for monitoring brain temperature changes during cooling: phase-difference and magnetic resonance spectroscopic imaging (MRSI) with high spatial resolution. Ten healthy volunteers were measured. Selective brain cooling was performed through nasal cavities using saline-cooled balloon catheters. MRSI was based on a radiofrequency spoiled gradient echo sequence. The spectral information was encoded by incrementing the echo time of the subsequent eight image records. Reconstructed voxel size was 1×1×5 mm³. Relative brain temperature was computed from the positions of water spectral lines. Phase maps were obtained from the first image record of the MRSI sequence. Mild hypothermia was achieved in 15–20 min. Mean brain temperature reduction varied in the interval <−3.0; − 0.6>°C and <−2.7; − 0.7>°C as measured by the MRSI and phase-difference methods, respectively. Very good correlation was found in all locations between the temperatures measured by both techniques except in the frontal lobe. Measurements in the transversal slices were more robust to the movement artifacts than those in the sagittal planes. Good agreement was found between the MRSI and phase-difference techniques.     

Temperature programmed desorption investigations of hydrogen and ammonia reactions on GaN

We report data for chemisorption and reaction of deuterium and isotopically labeled ammonia on single-crystalline GaN films grown on sapphire substrates. Temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) studies, following exposure of the clean GaN film at room temperature to the probe reactant species, were conducted under UHV conditions. Deuterium desorption took place over a wide temperature range, 525–;800 K, with molecular deuterium as the only product. At low exposures, two distinct deuterium desorption peaks at ∼ 660 and 770 K were observed. The deuterium desorption peak at 660 K shifted to lower temperatures with increasing D adatom coverages. TPD experiments after ammonia adsorption on GaN revealed small amounts of hydrogen desorbed at ∼ 600 K and over a range 660–;770 K, suggesting partial decomposition of ammonia. Molecular ammonia desorption was observed at ∼ 560 and 600 K, with the low temperature desorption state growing with increasing ammonia exposures. Further studies on deuterium-precovered GaN films indicated that ammonia production resulted from recombination of NH_x species and hydrogen adatoms on the surface.     

Subdiffusion of mixed origin with chemical reactions

We derive a reaction-subdiffusion equation that takes into account two microscopic mechanisms responsible for subdiffusion in real media. We show that the concentration profiles in media with identical subdiffusion exponents but with different microscopic structures can differ significantly at the same chemical kinetics.     

AC loss of low temperature superconducting AC wire caused by longitudinal and azimuthal AC magnetic field components

We investigated the AC loss characteristics of a low temperature NbTi AC wire by measuring the AC transport current losses in the external AC magnetic field whose components are the longitudinal and transverse ones. The measurement results showed that the AC losses were significantly dependent on the directions and magnitudes of the external longitudinal field component. The AC losses caused by the longitudinal and azimuthal field components were estimated by our previously derived model. The theoretical results well explained the dependence of the AC losses on the longitudinal field components. It was also shown that the AC losses can be substantially reduced by the proper choice of the twisting way.     

Differential-conversion temperature programmed desorption: a new method for obtaining bimolecular surface rate constants

For bimolecular surface reactions, temperature programmed desorption (TPD) cannot be used efficiently to detect the product if either of the reactants desorbs before the product. A modification of TPD (differential-conversion TPD, or DCTPD) that circumvents this problem is described in this paper. The reactant desorbing at high temperature is first adsorbed as in normal TPD. The surface is then exposed to a continuous flux of the other reactant, and the rate of product desorption is monitored at the same time. The flux is kept so low that the coverage of the reactant first adsorbed scarcely changes. The rate constant is then determined using this measured coverage and that calculated for the impinging species from its previously-determined adsorption/desorption kinetics. A formalism is also developed for cases in which a continuum of related pathways with a distribution of activation energies is present. Experimental application of DCTPD is demonstrated in the particular case of HCl production from SiH₄ and TiCl₄ on TiSi₂.