The adsorption and incorporation of oxygen on Cu(110) and its reaction with carbon monoxide

Abstract

The initial interaction of oxygen with a Cu(ll0) surface occurs in two stages, character&d by (2 X 1) and ~$6 X 2) overlayer structures and then a third stage where the cf6 X 2) structure remains but further oxygen uptake is registered only with e~psometry. The fust stage is nonactivated and is accornp~~~ by a work function increase of 370-420 meV, depending on sample temperature. The subsequent appearance of the extra features associated with the c(6 X 2) structure in the second stage is accompanied by a decrease in work function of -100 meV and is characterised by an apparent activation energy of -18 M mol-r . The adsorbed oxygen can be removed at all coverages by gas-phase CO. The reaction appears to foHOw Langmuir-Hiishelwood kinetics with an apparent activation energy of -25 M mol-' . fntroductionThe interaction of oxygen with the (110) surface of copper results in the initial formation of a (2 X 1) surface structure which is assumed to correspond to 0.5 monolayers [l--4]. Further oxygen exposure produces a ~(6 X 2) structure f5]. The first stage of the interaction is known to be accompanied by an increase in work function. Thus Ertl and Kiippers [6] have reported an increase of 0.2 eV after an exposure of 6 L and Delchar f7] a m~~urn increase of 0.67 eV after 2 X 10" L at room temperature. Delchar established that a change of 0.40 eV had already been reached after about 100 L. On the (100) surface a similar reaction sequence has been observed. The formation of the initial (u/z X 42) R45" structure is accompanied by an increase in the work function as might be expected, but the formation of the subsequent (d2 X 242) R4.5" structure by a decrease [8,9]. This has been 285

The adsorption and incorporation of oxygen on Cu(100) and its reaction with carbon monoxide; Comparison with Cu(111) and Cu(110)

“…If their results are extrapolated into our temperature range, it would appear, as with Cu( 110) [28], that in the present study the critical oxygen exposure at which the formation and growth of nuclei starts was not reached.…”

“…2). Both for Cu(ll1) [26] and for Cu(ll0) [28] we have strong arguments that a ratio of 2.3 holds for farbithe 6A per 0 atom in stage A to the 6A per 0 atom in stage B. If this ratio is assumed to hold for the Cu(100) system as well, then the oxygen coverage at 6A = 0.8" ((1/2 X 2d2)R4S" LEED pattern) is % of a monolayer, which is in excellent agreement with ref.…”

“…By analogy with ref. [28], a distinction can be made between different stages; these will be discussed in more detail below. A linear relationship between the oxygen Auger signal and 66 was found upon the initial interaction of 0s with Cu(100) up to 6A = 0.65" (fig.…”

“…Another reason may be the mechanism, proposed in ref. [28] to explain the reduced reaction probabilities for the same reaction on Cu(ll0) at higher initial oxygen coverages. According to this mechanism the growth of islands of bare copper surface is inhibited by small domains, which still contain incorporated oxygen.…”

Ellipsometry of clean surfaces, submonolayer and monolayer films

“…This has been applied to determine the kinetics of the adsorption of O2 and N20 and of the removal of adsorbed oxygen by CO. For Ni(lOO) the ellipsometric changes in the first, chemisorption stage were too small to derive a reliable coverage dependence [68]. After the first stage a further, generally much slower uptake (incorporation) of oxygen takes place on CU [67,69] and Ni [68], accompanied by reconstruction of the surface. The ellipsometric effects per incorporated atom are smaller for Cu and larger for Ni.…”