CO oxidation mechanism

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

Given the results obtained on platinum electrodes discussed in some detail in the previous section, it is clearly of fundamental interest to study the mechanism of CO oxidation on other transition metal electrodes, and to compare the results with platinum. Rhodium has been the electrode material that has been studied in greatest detail after platinum, and results obtained with rhodium have provided some very significant insights into some of the general issues about the CO oxidation mechanism. 

Even though there have been appreciably more studies of CS2, COS is known to exist as an intermediate in CS2 flames. Thus it appears logical to analyze the COS oxidation mechanism first. Both substances show explosion limit curves that indicate that branched-chain mechanisms exist. Most of the reaction studies used flash photolysis hence very little information exists on what the chain-initiating mechanism for thermal conditions would be. 

In remote tropospheric air, where NO concentrations can be quite low (17), the HO + CO oxidation mechanism can follow other pathways, leading to net ozone destruction rather than formation (18, 19). Reactions 1 through 5 typify the more complex catalytic reactivity of HO with hydrocarbons, which produce a complex array of oxidation products while generating ozone pho-tochemically (11-13). 

The evolution of the concepts of the CO oxidation mechanism is shown in Table 1. Surveys of modern viewpoints on the separate steps of this complex reaction (1) can be found in the literature [6-35],... 

There appears to be a close link between the actual CO oxidation mechanism and the nature of Pt-CO bonds the route involving RU-H2O is associated with a CO molecule bridge bonded to two adjacent Pt sites,whereas the route involving Ru(OH) is linked to a CO molecule linearly bonded to a single Pt site. ... 

The size effect is probably the most proposed interpretation for the CO oxidation mechanism. For example, this effect on the adsorption of CO was investigated by Rice et al. using in situ infrared reflectance spectroscopy from five different samples of platinum particles with mean diameters of 2.0, 2.5, 3.2, and 3.9 nm, respectively. These authors found that the infrared COl stret-... 

I ig- 4.17. Schematic illustration of the structural properties and the CO oxidation mechanism of palladium, zirconia catalyst derived by in situ activation from glassy PdjjZr, precursor... 

Some authors favoured the CO oxidation mechanism over Ce02 via an adsorbed superoxide species (Sass et al. 1986, Tarasov et al. 1989). Superoxide and peroxide species can be considered as intermediates formed during O2 adsorption/dissociation according to the following scheme (Zhang and Klabunde 1992) ... 

The essential role of NOx in ozone formation is evident in the CO oxidation mechanism (Section 6.4). For example, in the low NOx limit (NOx-limited), the rate of O3 formation increases linearly as [NO] increases and the rate is independent of [CO]. In the high NOx limit (NOx-saturated), the rate of O3 formation increases with [CO], but decreases as [NOx] increases. The explanation for the behavior in the high NOx limit is that, with ample NOx available, as NOx increases, the rate of the OH + N02 termination reaction increases, removing both HOx and NOx from the system, limiting OH - H02 cycling, and thereby decreasing the rate of 03 formation. 

A CO oxidation mechanism involving gold particles only is proposed especially when the support is non-reducible. For instance, Schubert et al. [54] deduced from experiments of CO-O2 time resolved titration and of isotopic exchange, that when gold is supported on magnesia, silica or alumina, the O2 adsorption takes place on low coordinated sites of the gold surface, but leads to a lower activity than when oxygen can be activated on a reducible support. 

With this background, the CO oxidation mechanism on palladium in the presence of sub-surface hydrogen is assumed to be... 

Figure 13. Estimated CO/Pt coverages in 0.3 methanol for the catalysts depicted in Fig. 10, and the indicated regions where each CO oxidation mechanism dominates, as determined by the binding site of the active OH in the CO oxidation. The brackets indicate the net amount of CO stripped via either the BF or Dsl mechanism due to mobile CO moving toward the Ru islands.

In conclusion, the CO oxidation mechanism on Ft, and other d-metals is well understood and serves as a benchmark reaction to characterize reactivity. However, with respect to behavior for supported metal particles and small clusters under ambient conditions, there is still the need for studies in order to fully understand the role of the size, particularly with respect to the electronic structure. 

Fig. 4.14 Graph for Langmuir-Hinshelwood CO oxidation mechanism with dissociative oxygen adsorption.

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