Adsorbed atomic oxygen

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

Figure 2,40. Schematic of the two extreme conformations of adsorbed atomic oxygen on Ag covalently bonded electrophilic oxygen (a-) and ionically bonded oxygen (P-).98 Reprinted with permission from Academic Press.

In the presence of adsorbed atomic oxygen, the activation barrier is substantially lowered to 132 kJ mol-1, the reaction is endothermic at 48kJmol 1, but the high activation energy suggests that the N-H bond would not be broken. However, at high temperatures it might be achieved. 

The amount of adsorbed atomic oxygen also changes considerably. Maximum quantity of atomic oxygen was observed temperature 500°C. It is established that as atomic oxygen is connected to a diamond surface.The atomic oxygen are formed at dissociations of chemical connected OH groups. 

Muhler M, et al. On the role of adsorbed atomic oxygen and C02 in copper-based methanol synthesis catalysts. Catal Lett. 1994 25(1—2) 1—10. 

Two other theories of passivity have been suggested by Reichenstein.1 namely, that in some cases the passivity is due to a high concentration of adsorbed molecular oxygen, whilst in others it is due to adsorbed atomic oxygen. As in the case of iron, however, it seems improbable that any one theory will account for all the known cases of passivity. It may well be that in certain eases each of the above-named theories holds, and that under the general term of passivity we are dealing with a variety of different phenomena.2... 

Then, in early 2006, Klust and Madix (55,56) reported, in two papers, results showing that the case of styrene epoxidation was not simple as once thought. Since adsorbed atomic oxygen is generally agreed to be the active species in the... 

A more recent investigation [373] - [375] proposed multiplying the rate equation by a correction factor 1-0, where 0 = a+ bT+cT n XH Q and is the molar fraction of H20. Some authors assume a different route (Eq. 34) for the formation of adsorbed atomic oxygen [376], [377] ... 

OOff -formation reactions shonld occur with comparable proba-bihties. Furthermore, although these two reaction pathways pro-dnce adsorbed atomic oxygen, the Of -dissociation pathway pro-... 

Concerted CH3-CH2-CH3 chemisorption with the formation of (OH)ad seems to be more plausible compared to chemisorption resulting in the formation of Had, because the 0-H bond is probably much stronger than the H-Pt bond. Subsequent reaction steps (after breaking the first C-H bond) are assumed to be rapid. Thus, adsorbed atomic oxygen is the dominant species under reaction conditions. 

Liu et al. used GGA calculations with the PBE functional to probe adsorption and reaction of 02 and CO on Au supported on defect-free TiO2(110).185 These calculations used a bilayer of Au forming a continuous strip across the support surface. 02 adsorption was found to be favorable only for adsorption on Au adjacent to the support surface. An analysis of the bonding character of these states indicated that the support enhances charge transfer from Au to 02 for Au atoms in close proximity to the support. CO oxidation via a reaction between adsorbed CO and adsorbed 02 at the Au/support interface was found to have a small activation barrier (0.1 eV). CO oxidation pathways involving adsorbed atomic oxygen were not examined, although adsorbed O is created by the pathway mentioned above. Also, dissociation of adsorbed 02 was found to occur with a barrier of 0.5 eV. 

Mechanism of the Epoxidation Reaction - Involvement of Adsorbed Oxygen Species. Herzog has reported that the use of nitrous oxide rather than oxygen as the oxidant for ethylene epoxidation results in a considerably reduced selectivity. Since nitrous oxide decomposition leads primarily to adsorbed atomic oxygen, Herzog concluded that molecular oxygen species are required for the partial oxidation reaction. More recently this experiment has been repeated in a closed recirculatory system the same result was obtained. When the reaction was carried out in the presence of both 02 and N2 0 the ethylene oxide contained 0 exclusively while was incorporated into the carbon dioxide. 

The purpose of calculating Henry s Law constants is usually to determine the parameters of the adsorption potential. This was the approach in Ref. [17], where the Henry s Law constant was calculated for a spherically symmetric model of CH4 molecules in a model microporous (specific surface area ca. 800 m /g) silica gel. The porous structure of this silica was taken to be the interstitial space between spherical particles (diameter ca. 2.7 nm ) arranged in two different ways as an equilibrium system that had the structure of a hard sphere fluid, and as a cluster consisting of spheres in contact. The atomic structure of the silica spheres was also modeled in two ways as a continuous medium (CM) and as an amorphous oxide (AO). The CM model considered each microsphere of silica gel to be a continuous density of oxide ions. The interaction of an adsorbed atom with such a sphere was then calculated by integration over the volume of the sphere. The CM model was also employed in Refs. [36] where an analytic expression for the atom - microsphere potential was obtained. In Ref. [37], the Henry s Law constants for spherically symmetric atoms in the CM model of silica gel were calculated for different temperatures and compared with the experimental data for Ar and CH4. This made it possible to determine the well-depth parameter of the LJ-potential e for the adsorbed atom - oxygen ion. This proved to be 339 K for CH4 and 305 K for Ar [37]. On the other hand, the summation over ions in the more realistic AO model yielded efk = 184A" for the CH4 - oxide ion LJ-potential [17]. Thus, the value of e for the CH4 - oxide ion interaction for a continuous model of the adsorbent is 1.8 times larger than for the atomic model. 

The surface dissociation of COj into CO(ads) and O(ads) has been proposed in the case of Na-modified Pd(lLl) and K-modified Pt(lll) 5 at submonolayer coverage on the basis of HREELS , and angle resolved ultraviolet photoemission spectroscopy (ARUPS). 59 pyjg presence of adsorbed atomic oxygen (a species capable of migrating into the bulk metal) for Na/Pd(lll) however, could not be detected with either of these techniques. 

From previous studies and the qualitative nature of the rate data a likely combination appeared to be a controlling surface reaction between adsorbed atomic oxygen and unadsorbed sulfur dioxide. In order to determine ail the constants in the rate equation for this mechanism, it is necessary to vary each partial pressure independently in the experimental work. Thus measuring the rate of reaction at different total pressures but at constant composition is not sufficient to determine all the adsorption equilibrium constants. Similarly, if the data are obtained at constant composition of initial reactants but varying conversions, the partial pressures of the individual components do not vary independently. However, in these cases it is possible to verify the validity of the rate equation even though values of the separate adsorption equilibrium constants cannot be ascertained. Olson and Schuler studied the effect of conversion alone and obtained the data in Table 9-1 at 480°C. 

Solution To develop an expression for the rate of reaction we must postulate the method of obtaining adsorbed atomic oxygen. If it is supposed that molecular oxygen is first adsorbed on a pair of vacant centers and that this product then dissociates into two adsorbed atoms, the process may be written... 

---