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

Abstract

Pyridine and dimethylamine have been studied at clean and oxidised Cu(110) surfaces as model systems for the interaction of amines with heterogeneous catalysts using scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Both molecules interact strongly with sub monolayer concentrations of chemisorbed oxygen causing a change from the well known p(2 · 1)O(a) islands to a (3 · 1) structure. XPS shows a 1:1 correspondence between the concentration of surface oxygen and that of chemisorbed pyridine but the stoichiometry of the dimethylamine/oxygen system could not be directly measured because of a slow reaction which results in the desorption of oxygen as water and the formation of a chemisorbed amide. The amide also decomposes at room temperature and desorbs leaving a clean surface. However, the 2:1 stoichiometry of the dimethylamine/oxygen reaction suggests a 1:1 dimethylamine:oxygen ratio in the (3 · 1) structure. The results of the study are interpreted in terms of an amineoxygen complex, which may provide a general model for the interaction of amines with oxygen at metal surfaces.

XPS and STM studies of the oxidation of hydrogen chloride at Cu(100) surfaces

“…They showed that ammonia oxida-48 tion at the b100N ends of oxygen islands was at least 100 times faster 49 than oxidation at the b110 N sides, confirming earlier predictions 50 based on Monte Carlo modeling of XPS data [13]. Extending this work, 51 we have since demonstrated the influence of oxygen islands at surfaces 52 on the pathways, kinetics, and products of a number of different reac-53 tions [16,17] but also the way in which reactants such as the amines 54 can change the local structure of the oxygen islands and thus influence 55 the reaction rate [18][19][20]. A particular interest has been to explore the 56 effect of intercepting oxygen in a transient state, i.e.…”

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

“…These structures are not observed in the case of ammonia or aniline. The (3 · 1) oxygen/dimethylamine structure closely resembles that observed between pyridine and surface oxygen at Cu(1 1 0) surfaces [29], with evidence for a 1:1 oxygen:nitrogen ratio in both cases [32]. A similar pyridine/ oxygen complex at a Cu(1 1 1) surface was attributed [33], on the basis of HREELS data, to a pyridine oxide, but in the case of Cu(1 1 0) a direct interaction between the nitrogen and the added copper atoms of the oxygen-copper chains seems more plausible.…”

“…The formation of an intermediate complex between the amines and the surface oxygen is supported by evidence that the (3 · 1) restructured surfaces involve a 1:1 ratio of amine to surface oxygen in the cases of both dimethylamine and pyridine [32]. Vibrational data [33] from pyridine adsorption at a Cu(1 1 1) surface suggested the formation of a pyridine-oxide type compound, but on Cu(1 1 0) the added row structure results in positively charged copper adatoms closely associated with the oxygen and a more likely scenario is for an intermediate complex between Unwrapping via unstable (3 · 1) complex product À864.5 Aniline [41] No intermediates observed À850.6 Ammonia [28,51] Chain unzipping À819.0 2,2,2-Trifluoro-ethylamine Chain unzipping À813.0 a The gas phase basicity is the Gibbs free energy associated with the protonation of the neutral molecule.…”

“…We have, on the contrary, studied reactions with other adsorbed species, particularly carbon dioxide [25,26] and oxygen [25,[27][28][29][30]. In the latter case our studies of the oxidation of a number of amines including ammonia, dimethylamine and aniline at Cu(1 1 0) surfaces [31,32] have shown that they all react readily with low surface coverages of oxygen at room temperature to form water and an adsorbed imide or amide (Eq. (1)):…”

“…Whereas the complete (2 · 1)O(a) adlayer is unreactive towards ammonia and aniline at room temperature [28], dimethylamine reacts readily removing all the oxygen [32]. Secondly, at lower oxygen concentrations, STM images show a structural change in the (2 · 1) oxygen islands to a well defined (3 · 1) structure in the presence of dimethylamine [32]; the [1 0 0] directed copper-oxygen chains separating from each other by an extra lattice spacing in the [1 1 0] direction but retaining the ·1 periodicity in the [1 0 0] direction. These structures are not observed in the case of ammonia or aniline.…”