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, d1-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). d1-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), d1-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, d1-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 CO₂ 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 d1-BA. Only a small amount of surface benzoate remains, an effect that we mostly attribute to defect sites

Benzoic Acid and Phthalic Acid on Atomically Well-Defined MgO(100) Thin Films: Adsorption, Interface Reaction, and Thin Film Growth

To better understand the interaction and the growth of thin films of functionalized organic molecules on oxide surfaces, we have studied the adsorption, reaction, and desorption of benzoic acid (BA) and phthalic acid (PA) on a well-ordered MgO(100) thin film grown on a Ag(100) single crystal surface. We have applied isothermal time-resolved infrared reflection–absorption spectroscopy (TR-IRAS) and temperature-programmed IRAS (TP-IRAS) under ultrahigh-vacuum (UHV) conditions. BA is dosed using a supersonic molecular beam (SSMB) source while PA is deposited by physical vapor deposition (PVD). For both molecules we have explored the film growth as a function of temperature, both in the monolayer and in the multilayer regime. We have also investigated structural transitions and desorption by temperature-programmed experiments in the range from 100 to 400 K. In addition, we carried out density-functional (DF) calculations. We find that both molecules BA and PA bind through the carboxyl groups to the MgO(100) surface. Upon adsorption at 100 K BA binds in an asymmetric bidentate geometry which exhibits a small tilting angle between the aromatic plane and the surface. Beyond the monolayer, a disordered multilayer film grows, which crystallizes under formation of dimers at around 180 K as indicated by a characteristic splitting of the IR bands. The BA multilayer desorbs at 240 K. Upon adsorption at 300 K, only a BA monolayer forms. Again, BA forms an asymmetric bidentate but with a larger tilting angle compared to low-temperature adsorption. For PA adsorption at 100 K, the adsorption mechanism is observed to change with coverage. At low coverage, both carboxyl groups are deprotonated, and the molecule forms an asymmetric bis-bidentate carboxylate with the aromatic plane nearly perpendicular to the surface. At high coverage, only one carboxyl group binds to the surface and forms an asymmetric bidentate carboxylate while the molecules maintain an upright standing orientation. During PVD of PA, a small fraction of phthalic anhydride (PAA) is formed which coadsorbs at low temperature. Upon annealing, the PAA desorbs around 250 K, triggering a structural transformation of the PA multilayer during which the PA adopts a more flat lying orientation. The PA multilayer itself desorbs around 310 K. Therefore, only monolayer of PA is stable around 300 K. Again, the adsorption mechanism is coverage dependent, changing from a bis-bidentate carboxylate at lower coverage to a monobidentate carboxylate at higher coverage

Functionalized Porphyrins on an Atomically Defined Oxide Surface: Anchoring and Coverage-Dependent Reorientation of MCTPP on Co₃O₄(111)

We have studied the adsorption of tetraphenylporphyrin (2HTPP) and its carboxylated counterpart mono-<i>para</i>-carboxyphenyltriphenylporphyrin (MCTPP) on an atomically defined Co₃O₄(111) film under ultrahigh vacuum (UHV) conditions. Using time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we show that 2HTPP adsorbs molecularly in a flat-lying orientation, whereas MCTPP binds to the surface via formation of a chelating bidentate carboxylate upon deposition at 400 K. Combining TR-IRAS and density-functional theory (DFT), we determine the molecular tilting angle as a function of coverage. We show that the MCTPP adsorption geometry changes from a nearly flat-lying orientation (tilting angle <30°) at low coverage to a nearly perfectly upright-standing orientation (tilting angle of approximately 80°) in the full monolayer

Coverage-Dependent Anchoring of 4,4′-Biphenyl Dicarboxylic Acid to CoO(111) Thin Films

We investigated the adsorption behavior of 4,4′-biphenhyl dicarboxylic acid (BDA) on well-ordered CoO(111) films grown on Ir(100) as a function of coverage and temperature using time-resolved and temperature-programmed infrared reflection absorption spectroscopy (TR-IRAS, TP-IRAS) in combination with density functional theory (DFT) and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. To compare the binding behavior of BDA as a function of the oxide film thickness, three different CoO(111) film thicknesses were explored: films of about 20 bilayers (BLs) (approximately 5 nm), 2 BLs, and 1 BL. The two carboxylic acid groups of BDA offer two potential anchoring points to the oxide surface. At 150 K, intact BDA adsorbs on 20 BL thick oxide films in planar geometry with the phenyl rings aligned parallel to the surface. With decreasing oxide film thickness, we observe an increasing tendency for deprotonation and the formation of flat-lying BDA molecules anchored as dicarboxylates. After saturation of the first monolayer, intact BDA multilayers grow with molecules aligned parallel to the surface. The BDA multilayer desorbs at around 360 K. Completely different growth behavior is observed if BDA is deposited above the multilayer desorption temperature. Initially, doubly deprotonated dicarboxylates are formed by adopting a flat-lying orientation. With increasing exposure, however, the adsorbate layer transforms into upright standing monocarboxylates. A sharp OH stretching band (3584 cm^–1) and a blue-shifted CO stretching band (1759 cm^–1) indicate weakly interacting apical carboxylic acid groups at the vacuum interface. The anchored monocarboxylate phase slowly desorbs in a temperature range of up to 470 K. At higher temperature, a flat-lying doubly deprotonated BDA is formed, which desorbs and decomposes in a temperature range of up to 600 K

Anchoring of a Carboxyl-Functionalized Norbornadiene Derivative to an Atomically Defined Cobalt Oxide Surface

We have investigated the anchoring of the molecular energy carrier norbornadiene (NBD) to an atomically defined oxide surface. To this end, we synthesized a carboxyl-functionalized NBD derivative, namely 1-(2′-norbornadienyl)­pentanoic acid (NBDA), and deposited it by physical vapor deposition (PVD) under ultrahigh vacuum (UHV) conditions onto a well-ordered Co₃O₄(111) film grown on Ir(100). In addition, we performed a comparative growth study with benzoic acid (BA) under identical conditions which was used as a reference. The interaction and orientation of NBDA and BA with the oxide surface were monitored in situ during film growth by isothermal time-resolved infrared reflection–absorption spectroscopy (TR-IRAS), both below and above the multilayer desorption temperature. The thermal behavior and stability of the films were investigated by temperature-programmed IRAS (TP-IRAS), with help of density functional (DF) calculations. BA binds to Co₃O₄(111) under formation of a symmetric chelating carboxylate with the molecular plane oriented nearly perpendicular to the surface. At low temperature (130 K), intact BA physisorbs in form of dimers on top of the saturated monolayer. Upon annealing to 155 K, a reordering transition is observed, in which BA in the multilayer adopts a more flat-lying orientation. The BA multilayer desorbs at 220 K, whereas the surface-anchored BA monolayer is stable up to 400 K. At higher temperature (400–550 K), desorption and decomposition are observed. Very similar to BA, NBDA binds to Co₃O₄(111) by formation of a symmetric chelating carboxylate. In the multilayer, which desorbs at 240 K, hydrogen-bonded NBDA dimers are formed. Upon PVD of NBDA at 300 K, only a surface anchored carboxylate is stable. The anchored NBDA film shows a characteristic restructuring behavior as a function of coverage. At low coverage the NBDA, adopts a flat-lying structure in which the norbornadiene unit interacts with the Co₃O₄ surface. With increasing coverage, the norbornadiene units detach from the oxide and the NBDA adopts an upright-standing orientation. Similar to BA, the anchored film is stable up to 400 K and decomposes in the temperature region between 400 and 550 K, leaving behind hydrocarbon residues on the oxide surface

Structure-Dependent Dissociation of Water on Cobalt Oxide

Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We studied H₂O on two atomically defined cobalt oxide surfaces, CoO(100) and Co₃O₄(111). Both surfaces are terminated by O^2– and Co²⁺ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H₂O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H₂O)_<i>n</i>(OH)_<i>m</i> clusters on Co₃O₄(111). We rationalize this structure dependence by the coordination number of surface Co²⁺. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H₂O whereas others do not. We propose that this structure dependence plays a key role in catalysis with cobalt oxide nanomaterials

6‑Alkoxy-5-aryl-3-pyridinecarboxamides, a New Series of Bioavailable Cannabinoid Receptor Type 1 (CB1) Antagonists Including Peripherally Selective Compounds

We identified 6-alkoxy-5-aryl-3-pyridinecarboxamides as potent CB1 receptor antagonists with high selectivity over CB2 receptors. The series was optimized to reduce lipophilicity compared to rimonabant to achieve peripherally active molecules with minimal central effects. Several compounds that showed high plasma exposures in rats were evaluated in vivo to probe the contribution of central vs peripheral CB1 agonism to metabolic improvement. Both rimonabant and <b>14g</b>, a potent brain penetrant CB1 receptor antagonist, significantly reduced the rate of body weight gain. However, <b>14h</b>, a molecule with markedly reduced brain exposure, had no significant effect on body weight. PK studies confirmed similarly high exposure of both <b>14h</b> and <b>14g</b> in the periphery but 10-fold lower exposure in the brain for <b>14h</b>. On the basis of these data, which are consistent with reported effects in tissue-specific CB1 receptor KO mice, we conclude that the metabolic benefits of CB1 receptor antagonists are primarily centrally mediated as originally believed