Oxygen atoms, concentration determination

To find out whether the given primary or secondary product, or the sum of these, is formed by a single step, or by a chain of conversions, the limiting primary product concentration should be compared with the initial oxygen atom concentration determined from CO. If these are equal, this is an indication that the primary product is formed by a nonchain mechanism. 

Thus, in admitting CO into the stream containing oxygen atoms, the initial oxygen atom concentration may be determined from the limiting rate of C02 accumulation. Certainly, another substance may be used instead of CO and determination of the oxygen atom concentration could then be made from the limiting rate of the primary product accumulation (or of the sum of primary products). But one must be certain that the formation of these products does not proceed by a chain mechanism. 

The rate constants for the disappearance of oxygen atoms were determined using the method described earlier.61 To obtain the value of k + k"ro (Af)(0)o the relative concentrations of atoms along the vessel length, i.e., the dependence of (0)o/(0)a should be determined. 

Relative concentrations of oxygen atoms were determined from the limiting amount of a primary or a secondary product (or the sum of these) in O atom reactions with the substance studied. The reactant was admitted in sufficient concentrations at various sites of the reaction vessel, and the limiting amount of reaction products was determined. A curve plotted as log (O)0/(O)r vs. X was used for determining the rate constant for the disappearance of atoms. 

The rate of CO2 formation due to reaction (V.33) is set proportional to Poofo where the surface concentration of adsorbed oxygen atoms is determined by a steady-state condition, i.e., the rate of formation of 0(ad) must be equal to the rate of consumption of 0(ad). Further analysis of the rate data shows that in reaction (V.38) predominantly O2 molecules adsorbed normal to the surface react with CO(g). Thus, one arrives at the rate law (40)... 

The most direct way to test the validity of a mechanism is to determine what intermediates are present during the reaction. If oxygen atoms were detected, we would know that Mechanism I is a reasonable description of NO2 decomposition. Likewise, the observation of NO3 molecules would suggest that Mechanism II is reasonable. In practice, the detection of intermediates is quite difficult because they are usually reactive enough to be consumed as rapidly as they are produced. As a result, the concentration of an intermediate in a reaction mixture is very low. Highly sensitive measuring techniques are required for the direct detection of chemical intermediates. 

Mechanism II begins with fast reversible ozone decomposition followed by a rate-determining bimolecular collision of an oxygen atom with a molecule of NO. The rate of the slow step is as follows Rate = 2[N0][0 This rate expression contains the concentration of an intermediate, atomic oxygen. To convert the rate expression into a form that can be compared with the experimental rate law, assume that the rate of the first step is equal to the rate of its reverse process. Then solve the equality for the concentration of the intermediate ... 

The method of semiconductor sensors allows one to determine the flux of atoms, to which the sensor was exposed, from electric conductivity measurements (provided coefficients of ionization and reflection of oxygen atoms from zinc oxide films are known). In other words, the sensor technique can be used in this case as an absolute method [21]. Indeed, variation of electric conductivity of a semiconductor film Acrpi due to adsorption is known to be caused by variation of carrier concentration An in the film, rather than by variation of their mobility / [21] ... 

It is obvious that during deformation of the sample due to mechanical loading the creation and annihilation defects will also take place. Similar to preceding experiments in this case the value of deformation would determine the concentration of defects. However, in case of mechanical loading the defects will be evenly spread over the whole volume of samples, whereas in case of silver oxidation they remain localized only in the surface-adjacent layers. Therefore, emission of oxygen atoms under conditions of mechanical deformation of samples in oxygen atmosphere has low probability due to intensive annihilation of defects in surface-adjacent layers. Special experiments confirmed this conclusion. 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

In order to ascertain that at a given CO flow rate the (C02) / concentration determined experimentally equals that of initial oxygen atoms, a curve was plotted for CO accumulation as a function of the CO flow rate (Fig. 2). This curve corresponded to a vessel with a cross section of s = 2.4 cm.2, to a pressure of molecular oxygen in the flow 4 mm. 

The relative 0 atom concentrations, for instance, that along the reaction vessel, may be determined from the limiting accumulation rate of one product (primary or secondary), since at a given temperature the fraction of oxygen atoms consumed by formation of this product is constant. 

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