Adsorption and decomposition of formic acid on MgO(001) surface as investigated by temperature programmed desorption and sum-frequency generation spectroscopy: Recurrence induced defect sites

Abstract

The change of the MgO(001) surface during the adsorption and thermal desorption of formic acid was investigated by temperature programmed desorption (TPD) and infrared-visible sum-frequency generation SFG spectroscopy. The TPD results revealed that the feature of desorption signals changed as the adsorption/thermal desorption process was repeated, despite no significant change of the low energy electron diffraction (LEED) pattern. The surface formate, produced by dissociative adsorption of formic acid, decomposed at 560 K and 700 K, giving H2O+CO and H2+CO2 as primary and minor products, respectively, but the broad desorption peak of H2O started to appear at 300 K. The 560 K peak appeared from an early stage, while the 700 K peak gained intensity as the experiment was repeated and the density of defects increased. The adsorption site for the 700 K peak was ascribed to the catalytic reaction of magnesia powder. The SFG spectra gave three resonance peaks of CH stretching bands at 2850 cm−1, 2870 cm−1, and 2920 cm−1 with a different dependence on the dose time, history of surface, and polarization of incident visible beam. The former two bands were assigned to the formates with bridged configuration, and the third band to the formate with unidentate configuration. The polarization dependence suggested that the CH bonds of bridged formates stood perpendicular to the surface, and that of the unidentate formate tilted approximately 30° from the surface normal. A vibration nonresonant background signal was present on the SFG signal, showing characteristic dependence on the polarization of incident light beams, as well as the coverage and history of the surface, and was interpreted as originating from the defect sites on the surface.

Quantum chemical prediction of the adsorption conformations and dynamics at HCOOH‐covered ZnO(1010) surfaces

“…The ZnO(1010) surface is a mixed ion‐terminated surface with an equal abundance of reactive three‐fold coordinated Zn and O sites. There is a clear difference in morphology along the two principal surface (0001) and (1120) directions, which makes it more complicated than, e.g., the well‐known MgO(001) surface 25, 26. Further, the (0001) direction lacks symmetry, making the ZnO(1010) surface more complicated than, for example, the much studied TiO 2 (110) surface 19, 27, 28.…”

Temperature evolution of structure and bonding of formic acid and formate on fully oxidized and highly reduced CeO2(111)

“…9 One of the most intriguing possibilities for studying a reducible oxide is the role that oxygen defects play in the reaction of the oxide with reactants. The interaction between surface formates and oxygen vacancies has been studied on TiO 2 , 10,11 and the association of formates with oxygen vacancies has been proposed on MgO(001) 12 and ZrO 2 (100). 13 Dilara and Vohs used sputtering to induce oxygen defects in ZrO 2 and found that the CO : CO 2 product ratio increased with the occurrence of O vacancies.…”

Molecules at Surfaces and Interfaces Studied Using Vibrational Spectroscopies and Related Techniques

“…Studies of this kind can be extended to higher temperature investigation, to follow reaction intermediates in heterogeneous processes. 67 We note, in passing, that interesting vibrational studies of adsorption on a well-defined oxide surface [MgO(100) 68 and NiO (111) 69 ] have also been reported, underscoring the usefulness of the IR-visible SFG technique for a wide variety of surfaces.…”