Dissociative Adsorption of Benzoic Acid on Well-Ordered Cobalt Oxide Surfaces: Role of the Protons

We performed a surface science model study to reveal the role of the protons which are released upon linking of organic molecules to oxide surfaces via carboxylic acid anchor groups. Specifically, we studied the adsorption, dissociation, and thermal stability of deuterated benzoic acid (C6H5COOD, dl-BA) on three different atomically defined cobalt oxide surfaces, namely, (i) Co3O4(111), (ii) CoO(111), and (iii) CoO(100). All surfaces were prepared in the form of thin films grown on Ir(100). dl-BA was deposited at 300 K via physical vapor deposition (PVD). The interfacial chemistry and film formation was monitored in situ by isothermal time-resolved infrared reflection absorption spectroscopy (TR-IRAS) under ultrahigh vacuum (UHV) conditions. For all three surfaces, we monitored the surface carboxylate and the surface hydroxyl groups as a function of coverage. The thermal stability of the films was probed by temperature-programmed IRAS (TP-IRAS). The comparison between the three surfaces reveals pronounced structure sensitivity. (i) On Co3O4(111), dl-BA binds via a chelating and symmetric carboxylate, strongly tilted with respect to the substrate. The surface hydroxyl groups give rise to a broad vibrational band indicating their involvement in hydrogen bonds. The coadsorbate layer is stable up to 400 K. Above this temperature, hydroxyl desorbs as water, leading to oxygen depletion most likely, followed by decomposition of the benzoate between 420 and 560 K, leaving behind aromatic residues on the surface. (ii) On the oxygen-terminated CoO(111) surface, dl-BA forms of a slightly distorted and slightly tilted bridging carboxylate. Similar as on Co3O4(111), the surface hydroxyl groups form hydrogen bonds. The film is stable up to 420 K. At higher temperature, the surface benzoates decompose slowly over a large temperature range partly, most likely via CO, release and formation of aromatic residues. The surface hydroxyl groups are stable up to higher temperatures (480 K) as compared to Co3O4(111). (iii) On CoO(100) a completely different behavior is observed. The surface benzoate forms a well-defined mixed coadsorbate layer with free surface hydroxyl groups, which are not involved in hydrogen bonds. The orientation of the carboxylate is strongly coverage dependent. At low coverage, the benzoate is tilted with respect to the surface, whereas a fully perpendicular orientation is adopted at high coverage. This film shows the lowest thermal stability. Above 345 K the surface benzoate and protons of the nearby hydroxyl group recombine, leading to desorption of intact dl-BA. Only a small amount of surface benzoate remains, an effect that we mostly attribute to defect sites.