Hot adatoms

That carbon monoxide could be oxidised in a facile reaction at cryogenic temperature (100 K) was first established in 1987 by XPS at an aluminium surface.21 The participation of reactive oxygen transients O 1 (s) was central to the mechanism proposed, whereas the chemisorbed oxide O2 state present at 295 K was unreactive. This provided a further impetus for the transient concept that was suggested for the mechanism of the oxidation of ammonia at a magnesium surface (see Chapter 2). Of particular relevance, and of crucial significance, was Ertl s observation by STM in 1992 that oxygen chemisorption at Al(lll) resulted in kinetically hot adatoms (Figures 4.1 and 4.7). 

A quite dramatic effect of this kind (which is still not yet fully understood theoretically) has been observed with the dissociative adsorption of oxygen on an Al(l 11) surface where the "hot" adatoms travel up to 10 nm across the surface before they come to rest [30]. An alternative explanation consists in the assumption of an abstraction process, where only one O atom remains on... 

Particles created by photodissodation may also move across the surface with enhanced reactivity. The formation of CO2 upon irradiation of a CO + O2 adlayer on Pt(l 11) was indeed the first hint for a "hot" adatom mechanism of this reaction, as discussed in Chapter 3 [70]. Subsequent detailed studies of this system [71] showed that the CO2 molecules formed in this way at 25K substrate temperature are strongly peaked along the surface normal and exhibit translational energies up to 1.35 eV. 

Reactions involving hot adatoms, such as discussed earlier, clearly represent an intermediate situation between the two limiting cases LH and ER, since the reactants are not in complete thermal equilibrium with the surface (as for LH). Neither does the reaction take place by direct collision from the gas phase (as for ER). Such a kind of intermediate mechanism ("precursor" mechanism) has been proposed [16], whereafter one of the reactants is not fully accommodated with the surface but reacts from a state that is vibrationally excited with respect to motion normal to the surface. 

However, experiments with the H + Dad reaction on Ni(l 10) revealed, apart from HD, also D2 as product [24]. In addition, the rate of HD formation was not found to be simply proportional to the Dad coverage as expected for a pure ER mechanism [25,26]. As a consequence, a more complex sequence of elementary steps was proposed [25] Apart from the pure ER chaimel, a fraction of the incident H atoms may become adsorbed or trapped (without full accommodation) as hot adatoms. The latter may then react with or transfer their energy to adsorbed D atoms that thus become excited and may even react further to D2. Computer simulations were able to qualitatively rationalize the observed experimental observations [27,28]. 

Wahnstrdm G, Lee A B and Strdmquist J 1996 Motion of hot oxygen adatoms on eorrugated metal surfaees J. Chem. Phys. 105 326... 

Watanabe, K., Takagi, N. and Matsumoto, Y. (2004) Direct time-domain observation of ultrafast dephasing in adsorbate-substrate vibration rmder the influence of a hot electron bafh Cs adatoms on Pt(lll). Phys. Rev. Lett., 92, 057401. 

Support for the special reactivity of hot oxygen adatoms also came from Matsushima s temperature-programmed desorption study4 of CO oxidation at Pt(lll), when CO and 02 were coadsorbed at low temperature with C02 desorbed at 150 K, the temperature at which 02 dissociates. This temperature is some 150 K lower than that for C02 formation when oxygen is preadsorbed (thermally accommodated) at the Pt(lll) surface. 

The existence of hot O adatoms was also believed to be responsible for the desorption of CO2 at 140 K from the Pt(lOO) hex surface partly covered with COads and 02,ads- Fadeev et al (58) used TPR and high-resolution EELS (HREELS) to investigate CO oxidation on Pt(lOO). Some of the results are summarized in Fig. 13. 

Photochemistry of adsorbed O2 molecules results presumably from transient capture of an excited metal electron by the 3cr state of O2. After decay to the ground state, the excited molecules may rapidly diffuse, desorb, or dissociate [72]. By monitoring the coUision-induced desorption of coadsorbed noble gas atoms, a value of 0.7 eV for the kinetic energy of the fastest "hot" O adatoms was determined [73]. 

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