PNNL

Abstract

Understanding thermal transformations of formic acid (FA) on metal oxide surfaces is important for many catalytical reactions. In many reactions, FA serves as a reactant, product, or probe molecule, and formates represent critical reaction intermediates. Here we study thermally induced reactions of FA on a single-crystalline and nano-crystalline anatase TiO2(101). We employ a combination of scanning tunneling microscopy (STM), temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) to follow the FA surface intermediates and reaction products above room temperature. We find that the primary reaction products desorbing at about 300, 480, and 515 K are molecular water, carbon monoxide, and formaldehyde, respectively. Bidentate (BD) formate and bridging hydroxyl (HOb) are identified as central intermediates in the FA transformations. Bridging oxygen vacancies (VO) are also likely participants despite their low stability at the surface. Further, the parallel studies on single crystals and faceted TiO2(101) nanoparticles reveal the spectroscopic commonalities of surface species and of the thermal conversion of molecular and deprotonated forms of FAUnderstanding thermal transformations of formic acid (FA) on metal oxide surfaces is important for many catalytical reactions. In many reactions, FA serves as a reactant, product, or probe molecule, and formates represent critical reaction intermediates. Here we study thermally induced reactions of FA on a single-crystalline and nano-crystalline anatase TiO2(101). We employ a combination of scanning tunneling microscopy (STM), temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) to follow the FA surface intermediates and reaction products above room temperature. We find that the primary reaction products desorbing at about 300, 480, and 515 K are molecular water, carbon monoxide, and formaldehyde, respectively. Bidentate (BD) formate and bridging hydroxyl (HOb) are identified as central intermediates in the FA transformations. Bridging oxygen vacancies (VO) are also likely participants despite their low stability at the surface. Further, the parallel studies on single crystals and faceted TiO2(101) nanoparticles reveal the spectroscopic commonalities of surface species and of the thermal conversion of molecular and deprotonated forms of FA NGP, Yang W, Yiqing W, RM, AD, FG, RR, Yong W, GAK, RR, , and ZD were supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences and Biosciences (CSGB) under FWP 47319. BW and AS acknowledge the support of DOE BES, CSGB Division under Award DESC0007347. The experimental studies were performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. We also acknowledge computational resources from the TIGRESS high-performance computer center at Princeton University.