STM and XPS Studies of the Oxidation of Aniline at Cu(110) Surfaces

Abstract

Aniline chemisorption at clean and partially oxidized Cu(110) surfaces at 293 K is compared with the reaction of a 300:1 aniline/dioxygen mixture at the same surface using STM and XPS. Limited dissociation occurs at the clean surface but in the presence of chemisorbed oxygen efficient oxy-dehydrogenation takes place with water desorption and the formation of chemisorbed phenyl imide (C6H5N(a)) with a reaction stoichiometry that changes with coverage. The adsorption site of the phenyl group is identified by STM to be the 2-fold hollow and it is proposed that the nitrogen is situated over the short bridge site. Chemisorptive replacement of oxygen gives a maximum phenyl imide concentration of 2.8 × 1014 molecules cm-2 at which coverage the surface is dominated by a mixture of three ordered domains with structures described by , , and unit meshes. Adsorption of aniline and dioxygen mixtures however results in phenyl imide concentrations up to 3.4 × 1014 cm-2 molecules cm-2 and a highly ordered biphasic structure characterized by and domains. A discrepancy between the concentrations measured by XPS and those calculated from the STM structures is discussed in terms of π-stacking of the phenyl rings in the adsorbed monolayer. Finally the chemistry of aniline is compared with that of ammonia and the importance of the NH bond strength and the basicity of the amine discussed.

Molecularly Resolved Studies of the Reaction of Pyridine and Dimethylamine with Oxygen at a Cu(110) Surface

“…2) or are a first step in the reaction. In the case of ammonia and aniline, there was little evidence for complex formation during reaction [10,42] but in these cases the N-H bond is relatively weak and the gas phase basicity relatively low; any complex would therefore be expected to be very short lived. We are pursuing this point in further experimental and theoretical work.…”

“…Similarly, the kinetics of ammonia oxidation at Cu(110) surfaces were explained with a Monte Carlo model [7] of the atomic structure of the partial oxygen adlayer based upon evidence provided by STM. We have been studying the role of local atomic structure in surface reaction mechanisms by STM, and in this paper we continue our studies of the interaction of amines with oxygen [8][9][10][11][12][13].…”

“…The images in figure 7 were recorded after the coadsorption of oxygen and dimethylamine in a $1:1 mixture at a Cu(110) surface at 293 K. The advantage of the coadsorption approach is that transient oxygen states can be intercepted before the formation of less reactive oxygen islands and this can lead to high concentrations of well ordered surface species [2,10,45]. In the present case, coadsorption results in a well ordered monolayer, characterised in the main by a (3 · 1) unit cell but with a limited number of (2 · 1) features also present.…”

“…This resulted, on Cu(110), in a change in the local structure of the chemisorbed oxygen. To explore this mechanism further we have recently investigated [9,10] the adsorption of aniline at Cu(110) surfaces. Aniline is a significantly weaker base than pyridine but unlike pyridine is still capable of hydrogen bonding interactions.…”

Molecularly resolved studies of the role of basicity in the reaction of amines with oxygen at a Cu(110) surface

“…The product of the reaction is characterised by structures orientated at 35°to the [1 0 0] direction, but cannot be identified unambiguously from the present data. From the reaction stoichiometries determined from the XPS data, amides are the initial products of both ammonia [40] and aniline [41] but these dehydrogenate rapidly at room temperature to imides (NR(a)); it is likely that methylamine follows a similar pattern resulting in a chemisorbed methyl imide but vibrational spectroscopy is needed for a definitive assignment. At high concentrations, the postulated methyl imide forms a c(2 · 2) structure which is much closer packed than any of the other imide and amides studied so far, NH(a) for example forms a c(2 · 4) structure on Cu(1 1 0) [42] whereas the densest packing achieved for phenyl imide is a 3 0 1 2 structure [41].…”