Oxygen adsorption rate

Ohta and Tezuka (5) studied the reaction in the pressure range 0.8 - 2.4 MN/m2 and temperatures of 423 K. They found that the oxygen adsorption rate was proportional to the partial pressure of oxygen and increased rapidly with increase in temperature. The rate of oxidation was not affected by the presence of cobalt and manganese salts, but the length of the induction period was a function of these catalysts. 

Above 750 °C, oxygen adsorption rates decrease rapidly so that the rate of reaction (9) drops because of decreasing oxygen coverage until above 1200 °C the unimolecular ammonia decomposition is the only reaction fast enough to produce a significant yield. Loffler and Schmidt have reported separately that NH3 decomposition rates at 1000 °C are of the order of 1.7 x 10 moles cm s , similar to the rate of N2 formation from NH3/O2 mixtures. 

SO2, while Cl was removed with Hg as HCl. Meisenheimer and Wilson (74) showed that reaction with C2H4CI2 to cover 25% of a silver surface with Cl lowered the oxygen adsorption rate to less than one-tenth the value observed with clean silver. 

Small changes in impurity content did not affect this rate but the presence of water vapor and changes in partial pressure of oxygen were critical (61,62). Steam and various impurities and binders also affect the oxidation of siUcon carbide (63). Differences have been observed in oxygen adsorption on the different SiC crystal faces (64). 

The latter acts as a sacrificial promoter. It is a promoter, as it forces oxygen to populate the weakly bonded (and highly reactive) oxygen adsorption state. It is also sacrificed as it is consumed by C2H4 at a rate I/2F, equal to its rate of supply. 

Figure 5.23. Thermal desorption spectra of oxygen on a Pt film deposited on YSZ4,7 after gaseous oxygen adsorption at 673 K and an 02 pressure of 4x1 O 6 Torr for 1800 s (7.2 kL) followed by electrochemical 02 supply (I=+15 pA) for various time periods. Desorption was performed with linear heating rate, p=l K/s, 2FNo/I=2570 s. Reprinted from ref. 7 with permission from Academic Press.

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

In case of weaker dependence of the rate of oxygen adsorption on ZnO Kds on the value s of solvent, compared to that of Kads as a function of e (which is supported by experimental results), we can deduce the following equation on the basis of the expressions (3.28) and (3.29) ... 

Second, the energy of emission activation is close to the energy of activation of adsorption of oxygen by silver - 146 kj/mol [49]. This implies that emission of O-atoms occurs due to energy of defect annihilation in the surface-adjacent layers of catalyst due to adsorption of oxygen. From the stand-point of such assumption it is obvious that depletion of emission is linked with sloroing down of the oxygen absorption rate (as shown, for instance, in Fig. 6.12). 

Figure 1. Solid lines - cyclic voltammetric curves for hydrogen and oxygen adsorption at the three Pt single crystal planes investigated determined from the pure (CO free) 0.1 M HC10, solution. Dotted lines - cyclic voltammetric curves of adsorbed CO oxidation (note the sensitivity for the CO oxidation curves has been ten fold attenuated. Sweep rate 50 mv/s.

Figures. (A)VoltammogramsofPt(lI0)cooledinH2 + Arand0.1Af HCIO4. (B) Voltammogram of Pt( 110) cooled in H2 + Ar and 0.5M H2SO4, where the dotted curve was observed after the first oxygen adsorption-desorption cycle. The sweep rate was 50 mV s. (From Ref. 26.)...

It is also agreed that the slope becomes much steeper because of a change in the regime, when the oxygen adsorption becomes the rate determining reaction instead of the electrochemical step (Fig. 6) [70, 73-76]. This has also been suggested from experiments by scanning electrochemical microscopy [77]. This... 

Che and Naccache (199) have studied the kinetics of 02 formed on slightly reduced anatase using EPR. They found that the adsorption could be explained on the basis of different formation rates for 02 adsorbed at different sites, with zero- and first-order kinetics for the oxygen and Ti3+ concentrations, respectively. Using the same approach, Hauser (200) has extended this work and proposed different models to explain the kinetics based on the formation of 02, O2-, and 02 ions for which activation energies around 1 kcal/mol were obtained. Nikisha et al. (201) have studied the oxygen adsorption kinetics using EPR, conductivity, and volumetric measurements. 

Results were modelled on the basis of CO adsorption/desorption equilibrium with oxygen adsorption as the rate determining step and an irreversible Langmuir-Hinshelwood surface reaction. It was assumed that adsorption and... 

It is clear from the kinetics that both ethylene and oxygen adsorption are important since both compounds appear in the rate equations with non-zero orders. Moreover, it is well known that ethylene is not adsorbed on pure silver, but that it does adsorb on a surface that is partially covered with oxygen. This implies that ethylene is either adsorbed on top of pre-adsorbed oxygen or on silver sites that are activated by the presence of oxygen (i.e. by formation of surface oxides, or another form of electron transfer or polarization). Consequently, two different mechanisms arise for the formation of ethylene oxide. The (direct) combustion of ethylene is another point of discussion. Although many favour the idea that different oxygen species are involved, others assume the same oxygen species, but different forms of ethylene adsorption. 

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