Oxygen coadsorbed

Figure 1.18 compares the activation energies of direct activation of NH3 and its activation by coadsorbed O on Pt(lll) [21]. As can be observed in this figure, reaction with coadsorbed O only lowers the barrier for NH3 activation by coadsorbed O. The other NH, intermediates have similar activation energies in the absence of coadsorbed oxygen. 

The reactions of ethylene, water, and methanol with coadsorbed oxygen on Pdf 100) were studied with temperature programmed reaction spectroscopy (TPRS) and high resolution electron energy loss spectroscopy (EELS). 

Jones and Barteau [275] have studied the cyclization and related reactions of iodoethanol on Ag(llO). The mechanism of diethyl ether formation on Ag(llO) and its dependence on the coadsorbed oxygen species has been studied by Jones etal. [276]. 

The removal of oxygen moieties from the surface by reaction with CO is also an essential step in the steam reforming reaction catalyzed by nickel or rhodium (47—52). On these metals, the oxygen removal most likely occurs via reaction of OHads ndth COads this reaction has a low activation barrier, probably less than 60 kj/mol. Coadsorbed oxygen (Oads) can lower the barrier further by accepting the hydrogen atom from OHads-... 

Ormerod RM, Lambert RM (1992) Critical ensemble required for acetylene cyclization on Pd(lll) - A study of steric inhibition by coadsorbed oxygen. J Phys Chem 96 8111... 

Both elementary reaction steps are endothermic, but the hydrogen transfer reaction with coadsorbed oxygen is considerably less endothermic than without coadsorbed oxygen. 

Figure 2.6 Impact of coadsorbed oxygen and surface-alloyed Au on the decomposition of crotonaldehyde and propene over a Pd(l 1 1) model catalyst determined by temperature-programmed reaction spectrometry. (Adapted from Ref [87] by permission of the PCCP Owner Societies.)...

Ru(OOl) under UHV conditions [67] show a decrease of CO desorption energy from = 140 kj mol for the pure metal to EfX = 107 kJ mol in the presence of coadsorbed oxygen (Oq = 0.5)) [62, 67]. Such a behavior is expected in the presence of electronegative coadsorbates, which cause a withdrawal of metal electrons and diminish the metal-jr back donation. The consequences of this effect are... 

The apparent activation energy for the dissociative adsorption of NH3 with preadsorbed O is significantly lower. It then follows that the effect of coadsorbed oxygen is much larger than that of the activation by steps. 

The low-temperature oxidation reaction of NH3 is extensively discussed in Chapter 6. The experimental evidence indicates that ammonia will only dissociatively adsorb when coadsorbed oxygen is present on the Pt surface. N2 is the initial product that forms in the presence of oxygen in a flow system operating under mild conditions. N2O is formed as a co-product as the catalyst begins to deactivate. 

Figure 3.50. Coadsorbed oxygen atoms are not always promoterst 1...

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