The thermal decomposition of methane. II. Secondary reactions, autocatalysis and carbon formation; non-Arrhenius behaviour in the reaction of CH3 with ethane

1976 ◽  
Vol 54 (20) ◽  
pp. 3175-3184 ◽  
Author(s):  
C.-J. Chen ◽  
M. H. Back ◽  
R. A. Back
Keyword(s):  
Thermal Decomposition ◽  
Steady State ◽  
Methyl Radicals ◽  
Carbon Formation ◽  
Radical Chain ◽  
Primary Products ◽  
Secondary Reactions ◽  
Decomposition Of Methane ◽  
State Concentration ◽  
Absorption Measurements

In the thermal decomposition of methane at temperatures from 880 to 1103 K, hydrogen and ethane are the only primary products. The rate of formation of ethane falls rapidly towards zero as the reaction progresses until ethane reaches a steady-state concentration. This behaviour is interpreted in terms of a radical chain mechanism,[Formula: see text]Values of k4 were obtained which confirm the non-Arrhenius behaviour of this reaction at these temperatures. Similar chain sequences propagated by addition or abstraction reactions of methyl radicals with ethylene, propylene, and acetylene can account for the formation and disappearance of these secondary products.At a later stage in the pyrolysis a marked autocatalysis is observed and the yield of ethane increases sharply above its steady-state value. It is concluded that this autocatalysis is largely a homogeneous process and is not caused by or associated with carbon formation. Deposition of carbon on the surface was observed at a still later stage of the decomposition, and was quantitatively estimated by light absorption measurements. Possible mechanisms for the auto catalysis are discussed.

The Oxidation of Formyl Radicals

1974 ◽  
Vol 29 (2) ◽  
pp. 251-255 ◽  
Author(s):  
N. Washida ◽  
Richard I. Martinez ◽  
Kyle D. Bayes
Keyword(s):  
Steady State ◽  
Rate Constant ◽  
Reaction Times ◽  
Initial Growth ◽  
Methyl Radicals ◽  
Absolute Rate ◽  
Atmospheric Reactions ◽  
Low Concentrations ◽  
Absolute Rate Constant ◽  
State Concentration

Steady state concentrations of formyl radicals were measured with a photoionization mass spectrometer. The reaction of ethylene with oxygen atoms in a system free of O2 was used to form CHO and CH3. Preliminary experiments showed that the reaction of methyl radicals did not interfere with the CHO measurements. By using low concentrations of O and short reaction times, it was possible to observe the initial growth of the CHO concentration. From the rate of approach of CHO to its steady state concentration, the absolute rate constant for the reaction O + CHO was determined to be (2.1+0.4)×10-10 cm3 molecule-1 sec-1. Addition of molecular oxygen to this system caused a decrease in the steady state CHO concentration, due to the reaction, CHO+O2→HO2+C0 as was suggested by Groth and coworkers in 1938. The rate constant for this reaction was calculated to be (5.7±1.2)×10-12 cm3 molecule-1 sec-1. The importance of these rate constants for combustion and atmospheric reactions are discussed briefly.

THE PHOTOLYSIS OF WATER VAPOR AND REACTIONS OF HYDROXYL RADICALS

1964 ◽  
Vol 42 (4) ◽  
pp. 753-763 ◽  
Author(s):  
A. Y.-M. Ung ◽  
R. A. Back
Keyword(s):  
Carbon Monoxide ◽  
Steady State ◽  
Water Vapor ◽  
Hydroxyl Radicals ◽  
Kinetic Studies ◽  
Steady State Concentration ◽  
Arrhenius Parameters ◽  
Secondary Reactions ◽  
State Concentration

The photolysis of water vapor at 1849 Å has been investigated as a possible source of hydroxyl radicals for kinetic studies. At temperatures from 23 to 350 °C and pressures from 1.3 to 28 mm, H2 and H2O2 were the only detectable products. Experiments with added oxygen indicated that O2 may have been present as an intermediate at a very low steady-state concentration, although this is not certain. Possible mechanisms are discussed.At temperatures from 200 to 350 °C, carbon monoxide appeared to react quantitatively with the hydroxyl radicals produced in the photolysis of water by the reaction, [Formula: see text] Rates of this reaction relative to those of the reactions, [Formula: see text] and [Formula: see text] were estimated from the decrement in the yield of CO2 when H2 or D2 was added to the H2O–CO system, and the following Arrhenius parameters were obtained:[Formula: see text]At temperatures below 200 °C, hydroxyl radicals were not completely converted to CO2, as the yield of CO2 increased to a maximum, then decreased again, with increasing pressure of CO. The mechanism of this system is complex, but probably involves secondary reactions of HCO or COOH radicals.

The thermal decomposition of methane. III. Methyl radical exchange in CH4–CD4 mixtures

1977 ◽  
Vol 55 (10) ◽  
pp. 1624-1628 ◽  
Author(s):  
C.-J. Chen ◽  
M. H. Back ◽  
R. A. Back
Keyword(s):  
Thermal Decomposition ◽  
Shock Tube ◽  
Exchange Reaction ◽  
Rate Constant ◽  
Average Rate ◽  
Static System ◽  
Chain Mechanism ◽  
Methyl Radical ◽  
Radical Chain ◽  
Decomposition Of Methane

The thermal methyl-radical exchange reaction, CH4 + CD4 → CH3D + CD3H, has been studied in a static system at temperatures from 880 to 1103 K, with equimolar mixtures at a pressure of 440 Torr. The exchange occurs by a methyl-radical chain mechanism, propagated by the reactions CH3 + CD4 → CH3D + CD3, and CH3 + CD4 → CH3H + CD3. Values of an average rate constant for these reactions have been estimated; kx = 1.42 × 106 ℓ mol−1 s−1 at 995 K. Comparison with shock tube data and photochemical measurements, at higher and lower temperatures respectively, indicates pronounced non-Arrhenius behaviour.

Regulation of cytosolic free Ca2+ concentration in acinar cells of rat pancreas

1983 ◽  
Vol 245 (3) ◽  
pp. G347-G357 ◽  
Author(s):  
H. Streb ◽  
I. Schulz
Keyword(s):  
Steady State ◽  
Incubation Medium ◽  
Minimal Medium ◽  
Acinar Cells ◽  
Pancreatic Acinar Cells ◽  
Atpase Inhibitor ◽  
Rat Pancreas ◽  
Mitochondrial Inhibitors ◽  
Pancreatic Cells ◽  
State Concentration

Ca2+ uptake into isolated exocrine pancreatic cells with highly permeable plasma membrane was determined by measuring the decrease in free Ca2+ concentration of the surrounding incubation medium with a Ca2+-specific electrode. In the presence of Mg-ATP and respiratory substrates the free Ca2+ concentration of the incubation medium decreased rapidly after addition of leaky cells until a stable medium free Ca2+ concentration of 4.2 +/- 0.1 X 10(-7) mol/l was obtained. Changes in the medium free Ca2+ concentration at steady state by addition of Ca2+ or EGTA were buffered by cellular uptake or release, respectively, until the steady-state free Ca2+ concentration was reestablished. When nonmitochondrial Ca2+ uptake was determined in the presence of a combination of mitochondrial inhibitors (10(-5) mol/l antimycin, 5 X 10(-6) mol/l oligomycin, and 10(-2) mol/l azide), the rate of uptake was considerably reduced, while the steady-state concentration was unaltered. In contrast, mitochondrial uptake that could be observed in the presence of the ATPase inhibitor vanadate (2 X 10(-3) mol/l) proceeded at the same rate as the control, but the minimal medium free Ca2+ concentration reached was 2.4 +/- 0.1 X 10(-7) mol/l higher than the control. Addition of secretagogues at steady-state free Ca2+ concentration resulted in a Ca2+ release of 0.73 +/- 0.08 nmol/mg protein. The increase in medium free Ca2+ concentration was entirely transient and followed by reuptake to the prestimulation level. The data indicate that a cytosolic free Ca2+ concentration of 4 X 10(-7) mol/l can be regulated in pancreatic acinar cells by a nonmitochondrial Mg2+-dependent Ca2+ pool.

scholarly journals Carbons Formed in Methane Thermal and Thermocatalytic Decomposition Processes: Properties and Applications

2021 ◽  
Vol 7 (3) ◽  
pp. 50
Author(s):  
Emmi Välimäki ◽  
Lasse Yli-Varo ◽  
Henrik Romar ◽  
Ulla Lassi
Keyword(s):  
Thermal Decomposition ◽  
Hydrogen Production ◽  
Energy Systems ◽  
Hydrogen Gas ◽  
Solid Carbon ◽  
Hydrogen Economy ◽  
Decomposition Processes ◽  
Different Types ◽  
Decomposition Of Methane ◽  
Thermocatalytic Decomposition

The hydrogen economy will play a key role in future energy systems. Several thermal and catalytic methods for hydrogen production have been presented. In this review, methane thermocatalytic and thermal decomposition into hydrogen gas and solid carbon are considered. These processes, known as the thermal decomposition of methane (TDM) and thermocatalytic decomposition (TCD) of methane, respectively, appear to have the greatest potential for hydrogen production. In particular, the focus is on the different types and properties of carbons formed during the decomposition processes. The applications for carbons are also investigated.

Noradrenaline-induced calorigenesis in warm- or cold-acclimated rats. In vivo estimation of adrenoceptor concentration of noradrenaline effecting half-maximal response

1980 ◽  
Vol 58 (9) ◽  
pp. 1072-1077 ◽  
Author(s):  
Florent Depocas ◽  
Gloria Zaror-Behrens ◽  
Suzanne Lacelle
Keyword(s):  
Adipose Tissue ◽  
Steady State ◽  
Brown Adipose Tissue ◽  
Maximal Response ◽  
Brown Adipose ◽  
Acclimation Group ◽  
Arterial Plasma ◽  
State Concentration ◽  
Na Uptake

Desmethylimipramine (DMI, 1 mg DMI∙HCl kg−1) and normetanephrine (NMN, 1 μg min−1 g−0.74) were used to inhibit, respectively, neuronal and extraneuronal uptakes of noradrenaline (NA) during calorigenesis induced in barbital-sedated warm-acclimated (WA) or cold-acclimated (CA) rats by infusion of NA, a procedure which mimics the effects of NA released within calorigenic tissues in response to cold exposure. The doses of the inhibitors were selected for maximal effectiveness in potentiating calorigenic response and for minimal side effects. For rats of either acclimation group treated with DMI and NMN, with DMI only, or with neither inhibitor the doses of NA required to evoke approximately half-maximal calorigenic responses were, respectively, 0.5, 1.0, and 3.5 ng min−1 g−0.74. The corresponding steady-state concentrations of NA in arterial plasma averaged 14.3, 21.7, and 43.2 nM in the three groups of WA rats and 10.0, 14.8, and 31.9 nM in the three groups of CA rats. Reduction by NA uptake inhibitors of the circulating levels of NA necessary to stimulate calorigenesis, half-maximally, presumably in brown adipose tissue, indicates a reduction in the steepness of the NA concentration gradient between capillary plasma and synaptic clefts in that tissue. The steady-state concentration of NA in blood plasma of rats treated with DMI and NMN and infused with NA at a dose of 0.5 ng min−1 g−0.74 (~1 × 10−8 M) is a good estimate of the NA concentration required at calorigenic adrenoceptors to effect half-maximal activation. Presumably, this concentration is also an estimate of that resulting from NA released at nerve endings during cold-induced activation of nonshivering thermogenesis at half-maximal rates in brown adipose tissue.

Divergent effects of intravenous GSH and cysteine on renal and hepatic GSH

1992 ◽  
Vol 263 (2) ◽  
pp. R348-R352 ◽  
Author(s):  
S. Aebi ◽  
B. H. Lauterburg
Keyword(s):  
Steady State ◽  
De Novo ◽  
Feedback Inhibition ◽  
Therapeutic Use ◽  
Steady State Concentration ◽  
De Novo Synthesis ◽  
State Concentration ◽  
Cellular Thiol

There is a growing interest in the therapeutic use of sulfhydryls. To assess the effect of glutathione (GSH) and cysteine on the cellular thiol status, thiols were administered intravenously to rats in doses ranging from 1.67 to 8.35 mmol/kg with and without pretreatment with 4 mmol/kg buthionine-[S,R]-sulfoximine (BSO), an inhibitor of GSH synthesis. One hour after administration of 1.67 mmol/kg GSH, the concentration of GSH rose from 5.2 +/- 1.0 to 8.4 +/- 0.9 mumol/g and from 2.5 +/- 0.5 to 3.7 +/- 0.7 mumol/g in liver and kidneys, respectively. After 8.35 mmol/kg, hepatic GSH did not increase further, but renal GSH rose to 6.7 +/- 1.8 mumol/g. Infusion of cysteine increased hepatic GSH to the same extent as intravenous GSH, but renal GSH did not increase after 1.67 mmol/kg and even significantly decreased to 0.6 +/- 0.2 mumol/g after 8.35 mmol/kg. In the presence of BSO, GSH resulted in a significant increase in renal but not hepatic GSH, suggesting that the kidneys take up intact GSH and indicating that the increment in hepatic GSH was due to de novo synthesis. The present data show that hepatic GSH can be markedly increased in vivo by increasing the supply of cysteine. Measurements of hepatic cysteine indicate that up to a concentration of approximately 0.5 mumol/g cysteine is a key determinant of hepatic GSH, such that the physiological steady-state concentration of GSH in the liver appears to be mainly determined by the availability of cysteine. At higher concentrations GSH does not increase further, possibly due to feedback inhibition of GSH synthesis or increased efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Sign in / Sign up

Export Citation Format

Share Document