Oxygen Chemisorption Rate Research Articles

Characterization of surface-oxides on char under periodically changing oxidation/desorption conditions

A novel experimental method based on periodic swinging of oxidation/desorption reaction stages is used to investigate the formation and release of surface oxides on carbon under mild oxidation conditions. The chemical nature of the carbon-oxygen complex and the key mechanistic features of the interaction between solid carbon and oxygen are assessed by continuous monitoring of CO/CO2 release during the experiments and by probing - via XPS - the chemical nature of oxygen moieties at different stages of the experiments. Sub-bituminous char is used as carbon substrate. The proposed technique turns out to be a simple though effective method to assess the nature and extent of surface oxides formed under different reaction conditions. Results show that oxygen is extensively chemisorbed on carbon as epoxy moieties below ~750 K. At higher temperatures, isomerization into “edge” oxides (ether-hydroxyl and carbonyl-carboxyl functionalities) takes place, followed by oxide decomposition and desorption as CO and CO2. Estimates of the oxygen chemisorption rate as a function of temperature are given. The study provides new insights into the extent and chemical nature of surface oxides on carbons, that represent one key to their successful application in several areas of energy conversion and storage. Oxides affect surface physico-chemical (polarity, wettability) and electrical (capacitance, resistivity) properties, relevant to application of carbons in batteries, supercapacitors and fuel cells. Moreover, understanding surface oxides on carbon is helpful in designing novel concepts of energy conversion from carbon through chemical looping, transient operation of stationary combustors, synthesis of carbon-based catalysts, beneficiation of ash.

Preparation of Tunable (BaSrMg)O for Oxygen Chemisorption: Formation Mechanism and Characterization.

Composition-tunable single-phase three-component alkaline earth oxide of (BaSrMg)O was prepared based on the consecutive precipitation and thermal decomposition of (BaSrMg)CO3. First, the single-phase (BaSrMg)CO3 was coprecipitated via ion self-assembly, phase-transformation, and agglomeration. The element composition of (BaSrMg)CO3 could be simply tuned by the composition of the reactants. Then, (BaSrMg)CO3 was converted to (BaSrMg)O under an H2 atmosphere at 750 °C. This (BaSrMg)O showed fast chemisorption-desorption responses with oxygen chemisorption rate: t80 = 3.9 min and desorption rate: t80 = 14 min and a high thermal stability for the redox reaction of BaO-BaO2. In addition, the chemisorption capacity of (BaSrMg)O (4.39% Sr composition) is ∼1.92 mmol/g, which is much higher than the chemisorption capacity of BaO/MgO at 1.75 mmol/g (Jin et al., Ind. Eng. Chem. Res., 2005, 44, 2942), while the transient oxygen pressure for the redox reaction of (BaSrMg)O (4.39% Sr composition) was significantly enhanced from 76 to 135 mmHg due to the inclusion of Sr in (BaSrMg)O. The transient oxygen pressure could be further improved via adjusting the Sr composition in (BaSrMg)O. Consequently, the tunable (BaSrMg)O has a high potential as a chemisorbent for the industrial application of oxygen separation.

Effect of potassium on physicochemical properties of CrO x/Al 2O 3 and CrO x/TiO 2 catalysts for oxidative dehydrogenation of isobutane: The role of oxygen chemisorption

Potassium addition has different influence on the selectivity to isobutene in oxidative dehydrogenation of isobutane for the two chromium based catalysts: the decrease for CrO x /Al 2O 3(CrAl) and the increase for CrO x /TiO 2(CrTi). This contrary effect of potassium was found to be associated with the increase in the rate of oxygen chemisorption (equivalent to the rate of reoxidation) and the increase in coverage with oxygen for CrAl, and with the decrease of these parameters for CrTi catalyst. Acid–base properties, determined by isopronanol decomposition and reducibility, estimated from TPR data, seem to affect less the catalytic properties, since they change on the potassium addition in the same way: acidity and reducibility decrease and basicity increases in the K-doped catalysts. Electron work function was found to decrease for potassium doped CrAl and did not change for CrTi catalyst.

Structure sensitivity of dimethylamine deep oxidation over Pt/Al2O3 catalysts

The deep oxidation of dimethylamine (DMA) was studied over Pt/Al2O3 catalysts with small (1nm) and large (7.8–15.5nm) Pt crystallite sizes. The turnover frequency (TOF) was higher for the large than for the small Pt crystallites, indicating that the reaction is structure sensitive. Two kinetic models were used to interpret the obtained results, i.e., the Mars van Krevelen and a mechanism based on the adsorption of oxygen and adsorption of dimethylamine on different active sites were employed. Both models showed that the activation energy for the oxygen chemisorption rate constant (ko) decreased with increasing of Pt crystallite size and that the activation energy for the surface reaction rate constant (ki) was independent of the Pt crystallite size. The structure sensitivity may be explained by differences in the reactivity of the oxygen adsorbed on these Pt crystallites.The Mars van Krevelen model fits the TOF values very well at concentrations of DMA higher than 1500ppm, while in the lower concentrations region, the model under predicts the experimental data. The model based on the adsorption of oxygen and DMA on different active sites fits the experimental data quite well over the whole temperature and concentration range. The fitted values of the Henry adsorption constant are independent of the Pt crystallite size.

Kinetics of deep oxidation of n-hexane and toluene over Pt/Al2O3 catalysts: Platinum crystallite size effect

The kinetics of n-hexane and toluene complete oxidation has been studied over 0.12% Pt/Al2O3 catalysts. Measurements were performed in an external recycle reactor. Reaction rates were measured as a function of temperature and reactants concentrations for two platinum crystallite sizes, 1.0 and 15.5nm. The turnover frequency (TOF) for the oxidation of both hydrocarbons is about 10 times higher for the 15.5nm than for the 1.0nm Pt crystallite size. The reaction constants are calculated according to Mars van Krevelen (MVK) mechanism. The oxygen chemisorption rate constant (kO) is the same during the n-hexane and the toluene oxidation for each sizes of platinum crystallite. For both hydrocarbons the oxygen chemisorption rate constant and the surface reaction rate constant (ki) are higher for 15.5nm than for 1.0nm crystallite size. The activation energy for kO decreases with the increase of the Pt crystallite size. The activation energy for ki is independent on the Pt crystallite size for both hydrocarbons. The values of kinetic constants reveal that the increase of the reaction rate for large Pt crystallites could be related to the lower activation energy for kO. This implies that on large Pt crystallites oxygen is held on Pt atoms with a lower bond strength that on small Pt crystallites, which explains the structure sensitivity found for n-hexane and toluene oxidation.

Structural transformations in thin oxide layers on the (211) surface of an Al–Li alloy

Optical second harmonic generation is used in conjunction with Auger electron spectroscopy to study the evolution with temperature T of oxygen adsorbed at 320 K on the clean, annealed (211) surface of an Al–Li alloy with a bulk Li concentration of 6.5 at. %. The oxygen is chemically bonded to form an oxide. The presence of Li at the clean alloy surface increases the rate of oxygen adsorption by a factor of ∼60 relative to pure Al. The oxygen chemisorption rate is consistent with a sticking coefficient of 1 for O2 on the clean unoxidized surface. For all oxygen exposures, the surface oxide film that is formed undergoes two distinct transformations. A structural transformation occurs between T=450 K and T=860 K, depending on oxygen exposure. This transformation exposes bare Al. Between 860 and 905 K, Li diffuses rapidly to the surface, thus increasing the surface concentration of elemental Li, and the remaining surface oxide becomes predominantly Li2O.

Modeling of oxygen chemisorption kinetics on coal char

It has been demonstrated that the char active surface area (ASA) is a relevant parameter for evaluating the reactivity of low- and high-temperature coal chars. The kinetics of oxygen chemisorption by coal chars is important (1) for evaluating the autoignition potential of pyrolysis chars during temporary storage or transport and (2) to investigate the mechanisms involved in oxygen chemisorption, as heterogeneous combustion is influenced by the chemisorption of oxygen on char surface. Rates of oxygen uptake by char at a range of chemisorption temperatures were monitored using a thermogravimetric balance. Oxygen chemisorption rate data on low-temperature (500°C) coal char at various temperatures have been tested with three kinetic models and one diffusion model. Furthermore, chemisorption data from a low-temperature (500°C) char have been compared with data from a high-temperature (950°C) char. Chemisorption between 100° and 215°C was investigated. Possible controlling mechanisms suggested by each model are presented.

Factors influencing chemisorption and ignition of wood chars

Solid discs of cottonwood have been pyrolyzed under nitrogen to an appropriate temperature and then cooled under nitrogen to chemisorption temperature for study of oxygen chemisorption activity. The following variables were studied, with the effects indicated. Presorption of nitrogen slowed the initial rate of oxygen chemisorption but had no effect on long-term (4 hours) chemisorption. Variation of pyrolysis temperature led to a pronounced maximum chemisorption for 450–500°C chars. The causes of this effect are assumed to be chemical in origin and are being studied further. Removal of indigenous metal ions from the wood and replacement with individual metal ions resulted in small but significant changes in chemisorption in the chars. Ignition of the char samples prepared by prepared by pyrolysis in nitrogen has been studied without removal of the char from the thermal balance. Ignition was dramatically influenced by the nature of the cations present. The complete removal of cations was most effective in raising ignition temperature or preventing ignition. The physical form of the wood samples (i.e., fine and coarse powders and solid discs) also had considerable influence on ignition, either in oxygen or in air.

Surface modification through chemisorption of hydrocarbons

Surface modification of a 90Cu10Ni alloy through chemisorption of hydrocarbon to the clean ion-bombarded surface was investigated by means of Auger electron spectroscopy. The amount of volatile hydrocarbon chemisorbed increased with the molecular weight and pressure of the hydrocarbon, but not necessarily with the temperature. The presence of prechemisorbed hydrocarbon on the surface, as determined by the carbon Auger peak, greatly slowed down the rate of oxygen chemisorption. Values calculated for the Elovich constants for oxygen chemisorption yield inferences regarding the mechanism of oxidation inhibition by preadsorbed hydrocarbon.

Rates and heats of oxygen chemisorption on Saran chars at 100°C

Chemisorption of oxygen on samples of Saran char was followed at 100°C by differential scanning calorimetry (over a 28-min period) and thermal gravimetric analysis. The three samples used were: a Saran char prepared at 900°C; this char containing about 100 ppm of iron; and this char activated in air at 425°C to 63.8% carbon burn-off. A chemisorption temperature of 100°C was chosen since physical adsorption of O 2 and carbon gasification were negligible. Both the addition of iron to Saran char and its activation increase the rate and enthalpy ( ΔH) of oxygen chemisorption. The ΔH and/or the rate of chemisorption on the activated char is also enhanced by increasing the degassing (desorption) temperature from 500 to 700°C, increasing the rate of heating to a desorption temperature of 600°C, and decreasing outgassing time at 600°C. With three cycles of desorption-chemisorption of oxygen on the activated char, the rate of oxygen chemisorption decreases for outgassing temperatures of 500, 600, and 700°C, but ΔH only changes following outgassing the char at 600 and 700°C. In all of these studies ΔH varied from a low of −42 to a high of −81 kcal/mole O 2. For the original and activated Saran chars, the rate of heat produced as a result of oxygen chemisorption was first order in O 2 pressure at constant coverages.

The Reduction of Oxygen on Teflon‐Bonded Perovskite Oxide Electrodes

The electrochemical reduction of oxygen on Teflon‐bonded electrodes in 45% was studied as a function of temperature and oxygen partial pressure. The activation energy was 10.75 kcal/mole and the relationship between and obeys the relationship , suggesting that the rate of oxygen chemisorption is the rate‐controlling step, Galvanostatic oxygen stripping showed that the surface coverage of oxygen was only 1% for both Sr‐doped and , and that the coverage was independent of temperature in the range 25°–80°C.

Kinetic Studies of Oxygen Chemisorption on the Rutile Single-crystal Surface by Means of Electrical Conductivity

Abstract The reaction of oxygen with the surface of a rutile single crystal was studied in the temperature range from 212 to 362°C and in the pressure range from 10−1 to 102 Torr. The process of the surface oxidation was examined by monitoring the electrical conductance change due to oxygen chemisorption. Two types of chemisorption kinetics were found for the rutile which had been reduced to different extents in vacuo before the introduction of oxygen. For the low-reduced rutile, the kinetics obeyed the parabolic rate law; this was interpreted by the diffusion-controlled reaction theory. On the other hand, it was found for the high-reduced rutile that the kinetics obeyed the Elovich equation; this could be explained by the boundary layer theory. The apparent activation energy for the former was 14.2 kcal/mol. For the latter, it was 4 kcal/mol and increased with an increase in the chemisorbed surface-charge density. In addition, the rate of oxygen chemisorption in the latter case was proportional to the pressure at a constant density of the chemisorbed surface charge.

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

The initial stages of the interaction of oxygen gas with a clean Ni (100) surface have been studied by a combination of LEED, AES, work function change and ion bombardment sectioning techniques. The reaction could be divided into three reaction regions: a fast dissociative chemisorption leading to surface structures based on the initial nickel interatomic spacing and resulting in an oxygen coverage of approximately 0.4 monolayers; a rapid oxidation leading to epitaxial NiO, two layers thick ; and a final slow thickening of bulk NiO. The first two regions were dependent only upon oxygen exposure. The third region was observed only at high gas-phase oxygen pressures or very low surface temperatures. Kinetics analyses are developed to explain the rate of oxygen chemisorption and the rate of oxide nucleation and growth.

Oxidation of propylene on cobalt-manganese spinels with lithium, titanium, and copper oxides added

1. Adding oxides of lithium, titanium, and copper to samples of straight and reverse cobalt-manganese spinels does not change the direction of deep propylene oxidation nor does it change the form of the kinetic law. 2. With spinels containing additives as well as with pure samples, the oxidation rate is observed to vary in the same way as the oxygen chemisorption rate. 3. Adding lithium in high concentrations poisons the propylene oxidation.