Oxygen states

Since A expresses the ratio of the rates of consumption of the two oxygen states by C2H4 one has ... 

The selective ionization of EEPs is attained due to an ionization potential difference between the excited and ground states of an atom or a molecule, a difference that is equal to the excitation energy. Thus, the ionization potentials of oxygen states and Zj are 1.0 and 1.6 eV,... 

Figure 2.1 Real-time photoemission study (hv = 6.2 eV) of the interaction of oxygen (Po2 = 10- Torr) with a nickel surface at 300 K. The photocurrent decreases initially (A B), then recovers (B-C), before finally decreasing (CD). Surface reconstruction occurs (B-C) with further support from studies of the work function. The work function measured by the capacitor method15 increases by 1.5 eV with oxygen exposure at 80 K followed by a rapid decrease on warming to 295 K and an increase on further oxygen exposure at 295 K. These observations suggest that three different oxygen states are involved in the formation of the chemisorbed overlayer. (Reproduced from Refs. 15, 42).

Figure 2.2 Reactivity of oxygen states chemisorbed at Ni(210) (a) at 295 K and (b) at 77 K to water adsorbed at 77 K. The oxygen concentration ct is calculated from the 0(1 s) spectra. The oxygen state preadsorbed at 295 K is unreactive with water desorption complete at 160K whereas that at 77 K is reactive, resulting in surface hydroxylation.37 (Reproduced from Refs. 37, 42).

Oxygen chemisorption at cryogenic temperatures provided the clue for the presence of metastable reactive oxygen states at metal surfaces, with XPS... 

In 1999, we reported21 low-temperature studies of oxygen states at Cu(110). At 110K the oxygen state present at high coverage is largely disordered but... 

Figure 4.11 STM images of oxygen chemisorption at Cu(110) (a) at low coverage at 120 K (b) at high coverage at 110 K (c) (2x1) and c(6 x 2) oxygen states present after warming from 110 to 290 K (d) (2 x 1)0 strings present when oxygen is chemisorbed at 290 K. These distinct oxygen states would be expected to exhibit variations in chemical reactivity. (Reproduced from Ref. 21).

Spectroscopic studies during the period 1986-1990 drew attention through coadsorption studies to transient oxygen states existing when a dioxygen... 

Spectroscopic studies (XPS and HREELS) established first in 1980 that the activity of oxygen states in the oxidation of ammonia at copper-O surfaces was... 

Figure 5.1 XPS evidence for oxygen states active in the oxidation of ammonia at Cu(110) at 290 K, for oxygen coverages of 0 = 1.0 and 0.5 and for an ammonia-rich NH3-02 mixture. Note the high activity for NH formation with the 30 1 mixture.

At Cu(l 10) surfaces, a number of different oxygen states have been investigated by STM (a) Cu(110)-O where the oxygen coverage is close to unity (b) Cu(110)-O where the oxygen coverage is < 1.0 and (c) Cu(110) exposed to an oxygen ammonia mixture. 

Figure 5.2 Oxygen states present at the ends of -Cu-O-Cu-O- chains are established as the active sites in ammonia oxidation at Cu(110) from a Monte Carlo simulation of the growth of the oxygen adlayer. The reactivity (the experimental curve) is best fitted to the atoms present at chain ends. (Reproduced from Ref. 7).

Although it was not possible to distinguish between the specificity of oxygen states responsible for methoxy and formate formation, they were to be associated with isolated oxygen adatoms and oxygen states present at the periphery of the (2 x 1)0 islands. 

The experimental evidence, first based on spectroscopic studies of coadsorption and later by STM, indicated that there was a good case to be made for transient oxygen states being able to open up a non-activated route for the oxidation of ammonia at Cu(110) and Cu(lll) surfaces. The theory group at the Technische Universiteit Eindhoven considered5 the energies associated with various elementary steps in ammonia oxidation using density functional calculations with a Cu(8,3) cluster as a computational model of the Cu(lll) surface. At a Cu(lll) surface, the barrier for activation is + 344 k.I mol 1, which is insurmountable copper has a nearly full d-band, which makes it difficult for it to accept electrons or to carry out N-H activation. Four steps were considered as possible pathways for the initial activation (dissociation) of ammonia (Table 5.1). 

A molecular oxygen state is the most likely to be involved, it would require a barrier of only 67 k.f mol 1 and is exothermic a hydroperoxide state is formed together with NH2(a). When the heats of adsorption of ammonia and oxygen are taken account of, then according to Neurock44,45 there is no apparent activation barrier to N-H activation. 

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