Partial pressure | EPFL Graph Search

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture (Dalton's Law). The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures but not according to their concentrations in gas mixtures or liquids. This general property of gases is also true in chemical reactions of gases in biology. For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood; consequently, mixture ratios, like that of breathab

Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCI3

Matthieu Bondil

In this study, the mechanism and kinetics of C3H8 dehydrogenation and cracking are examined over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl3. The present study demonstrates that GaH cations are the active centers for C3H8 dehydrogenation and cracking, independent of the Ga/A1 ratio. For identical reaction conditions, GaH cations in Ga/H-MFI exhibit a turnover frequency for C3H8 dehydrogenation that is 2 orders of magnitude higher and for C3H8 cracking, that is 1 order of magnitude higher than the corresponding turnover frequencies over H-MFI. C3H8 dehydrogenation and cracking exhibit first-order kinetics with respect to C3H8 over H-MFI, but both reactions exhibit first-order kinetics over Ga/H-MFI only at very low C3H8 partial pressures and zero-order kinetics at higher C3H8 partial pressures. H-2 inhibits both reactions over Ga/H-MFI. It is also found that the ratio of the rate of dehydrogenation to the rate of cracking over Ga/H-MFI is independent of C3H8 and H-2 partial pressures but weakly dependent on temperature. Measured activation enthalpies together with theoretical analysis are consistent with a mechanism in which both the dehydrogenation and cracking of C3H8 proceed over Ga/H-MFI via reversible, heterolytic dissociation of C3H8 at GaH sites to form C3H7-GaH-H+ cation pairs. The rate-determining step for dehydrogenation is the beta-hydride elimination of C3H6 and H-2 from the C3H7 fragment. The rate-determining step for cracking is C-C bond attack of the same propyl fragment by the proximal Bronsted acid O-H group. H-2 inhibits both dehydrogenation and cracking over Ga/H-MFI via reaction with GaH cations to form GaH2-H+ cation pairs.

2019

Heterogeneous kinetics of some atmospherically relevant reactions

A low pressure reactor (Knudsen cell) coupled to molecular beam modulated mass spectrometry was used to study the heterogeneous kinetics of the following three systems: (1) NO2 on amorphous carbon, (2) H2O vapor on ice and (3) gaseous HCl on ice. The first system was investigated at ambient temperature while the two last ones were investigated at temperatures typical of the stratosphere, that is roughly between 160 and 220K. The solid phase samples correspond to model surfaces whose physicochemical properties come very close to the ones of atmospheric particulates. Firstly, we studied the interaction of gaseous NO2 with three different commercial samples of amorphous carbon having widely differing physicochemical properties. Using in situ laser-induced fluorescence, the only major product was unambiguously determined to be NO with the oxidation product of carbon apparently remaining on the surface. Probing the gas phase with mass spectrometry (MS), both pulsed valve dosing and steady state experiments were performed and revealed a complex reaction mechanism for both the uptake as well as product formation. The initial uptake coefficient γo for NO2 was (6.4±2.0)·10-2 and proved to be identical within experimental uncertainty for all three types of amorphous carbon. The initial NO2 uptake rate was independent of the mass of the carbon sample and scaled with its external geometrical surface. Applying a simple chemical kinetic model to both pulsed dosing and steady state experiments resulted in an effective surface for uptake on amorphous carbon ranging between a factor of 2.8 to 8.4 larger than the geometrical surface area, but inversely proportional to the measured BET surface area of the three types of amorphous carbon. This means that the amorphous carbon with the highest BET surface (FW2) had the lowest external surface for NO2 adsorption. The chemical kinetic model included competing processes between Langmuir-type adsorption and inhibition controlling the adsorption kinetics of NO2 and revealed that the NO generation rate differed greatly between the three carbon samples examined. All samples showed saturation effects of differing degree that were partially reversible through prolonged pumping at 10-4 Torr and/or heating. Virgin amorphous carbon samples did not take up H2O vapor at 20 mTorr, and no HONO and/or HNO3 was detected in simultaneous NO2/H2O exposure experiments. CO and CO2 were detected when a sample previously exposed to NO2 was heated by an incandescent lamp. Moreover, upon heating such a sample, a MS signal m/e 62 originating from NO3 and/or N2O5 was detected. In addition to the experiments conducted on the three commercial carbon samples, we carried out uptake experiments of NO2 on acetylene soot. Originating from the flame of an acetylene burner, this soot was freshly deposited on glass dishes before each experiment. The initial uptake coefficient γo for NO2 was measured to be (3.0±1.1)·10-2 is smaller by a factor of two compared to γo observed in the uptake on commercial samples whose mass was a factor of 10 higher. In addition to NO, a net formation of HONO (m/e 47) was observed. This HONO formation is strongly dependent on the instantaneous water desorption from the sample as well as on the NO2 partial pressure in the reactor. On the other hand, it is not enhanced by an external water flow which did not show any interaction with the carbon sample. Upon heating the sample using an incandescent lamp, significant amounts of water desorbed, suggesting that the water vapor had condensed into the micropores of the soot during combustion. No quantitative results are presented yet. However, as no correlation between HONO formation and NO partial pressure has been observed so far, we suggest a reaction mechanism that only involves NO2 and H2O but no NO: 2 NO2 (g) + H2O HONO + HNO3 In a second study, the kinetics of condensation and evaporation of water on ice was determined in the temperature range of 170 to 220K. The rate of evaporation Fev corresponds to the evaporation of 70±10 monolayers of H2O per second at 200K. The condensation rate constant kc has a negative activation energy of -3.l±1.5kcal/mol which is significantly larger than the one recently measured by Haynes et al. We report the first real time kinetic measurements of water condensation on ice at stratospherically relevant temperatures. At temperatures above 160K, pulsed dosing experiments show a dose dependence of the condensation rate coefficient. For example, the condensation rate coefficient increases by a factor of five with a 1000 fold increase of the H2O dose from 1·1015 to 1·1018 molecules per pulse at 180K. This pressure dependence of the condensation rate constant may be explained by an autocatalytic mechanism involving the competition of two condensation channels. These two channels feed two different surface species, one of which leads to autocatalytic condensation. This mechanism gives insight into the manner in which a gas phase molecule is stepwise incorporated into tetrahedrally coordinated bulk ice. In a third and ongoing study, the uptake kinetics of HCl on ice has been investigated by pulsed valve experiments in the temperature range of 180 to 210K and are discussed as a function of the state of the surface. In fact, some of the pulsed valve experiments present a relaxation to a steady state level. It is shown that this level originates from the evaporation of a liquid solution corresponding to a temperature specific HCl concentration. The threshold of the injected dose leading to this solution layer is estimated to be 8·1014 molecules. This threshold corresponding to a fraction of a monolayer may be explained by the formation of small domains along grain boundaries or other lattice imperfections. Within detection limit, no difference in keff has been observed for pulses on the solution or on a previously exposed sample without formation of the solution. In the case of pulses forming a solution layer, a fraction of 20±20% of the injected molecules remains permanently adsorbed on the surface. In the temperature range from 210 to 180K, the pulsed valve experiments yield values of keff roughly between 5 and 25s-1 and an activation energy of -1.6±0.7kcal/mol which are consistent with the values observed by Flueckiger in steady state experiments using the 15mm escape aperture (kuni varying from 12 to 23s-1 and an activation energy of -1.9±0.25 kcal/mol).

EPFL1996

Carbon dioxide hydrogenation to methanol at low pressure and temperature

The measurements obtained were used to investigates the methanol synthesis reaction from pure carbon dioxide and hydrogen over binary (CuO/ZrO2) and ternary supported catalysts (CuO/ZnO/Al2O3). A comparison of different catalysts is presented. The influence of the most important reaction parameters, i.e. temperature, pressure, space velocity and feed gas composition is examined at moderate temperatures (≤ 300 °C) and pressures (≤ 20 bar). The influence of the partial pressures of hydrogen, carbon dioxide, carbon monoxide, water and methanol are studied in detail with the most efficient catalyst. According to the results, water has a decisive effect on the catalyst activity and performance. The measured results were used to derive a possible Langmuir-Hinshelwood kinetic model of the methanol synthesis reaction. The catalyst surface is analyzed by in situ diffuse reflectance Fourier transform infrared spectroscopy. The identification and evolution of intermediates on CuO/ZnO/Al2O3 catalysts confirm that the reaction proceeds by prior formation of the carbonate on the copper, followed by hydrogenation of the carbonate to the formate and thereafter to methoxy and methanol. For CuO/ZrO2 catalysts results demonstrate that the methanol formation occurs via π – bound formaldehyde and methoxy. Responses of sine shape variation applied to the reactants are also examined. The conversion of CO2 with hydrogen to methanol is investigated in dielectric-barrier discharges with and without catalysts at low temperatures (≤ 100 °C) and pressures (≤ 10 bar). The combination of discharges and catalysts lower the activation energy of the reaction resulting in a decrease in the catalyst optimum temperature. The presence of the catalyst in the discharge increases the methanol yield and selectivity by more than a factor of 10. Electrochemical reduction of carbon dioxide is briefly investigated by using TiO2/Ni complex and TiO2/Ru complex thin film electrodes. A considerable decrease in overvoltage is achieved together with respectable current densities.

EPFL1998