Complexity in the Decomposition of Formic Acid on the TiO2(110) Surface

Abstract

The decomposition of formic acid on the TiO2(110) surface was examined with temperature-programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS), and high-resolution electron energy loss spectroscopy (HREELS). Formic acid decomposed upon adsorption on TiO2(110) at 110 K to formate and a proton. The formate species was identified in HREELS by the symmetric O−C−O stretching mode at 1365 cm-1 and the C−H stretching mode at 2920 cm-1. The deposited acid proton was difficult to detect in HREELS but presumably formed a hydroxyl group at a bridging two-coordinate O2- site. TPD indicated that the major formic acid decomposition products were CO and H2O, suggestive of a dehydration process. However, a dehydration mechanism is unsuitable for describing the decomposition of formic acid on TiO2(110) because the CO and H2O products were formed from independent sources and by independent surface processes. CO desorbed during decomposition of formate between 400 and 700 K, but the majority of the desorbing H2O resulted from condensation of bridging hydroxyl groups below 500 K with virtually no water desorbed during formate decomposition. Condensation of bridging hydroxyl groups left a disordered surface consisting of oxygen vacancies and adsorbed formate species. The decomposition of formate was not a straightforward process. Although CO was the major formate decomposition product, formaldehyde desorbed at 548 K as the major hydrogen-containing product, with formic acid (from formate disproportionation), water, and acetylene as additional hydrogen-containing products. Carbon dioxide desorption was also detected and coincided with formaldehyde desorption. The production of formaldehyde probably involved the oxygen vacancies produced below 500 K during water desorption. TPD results from HC16O16OH decomposition on the 18O-enriched TiO2(110) surface indicated that oxygen exchange between the surface and the formic acid adlayer resulted in significant amounts 18O-containing TPD products (carbon monoxide, water, and formaldehyde). Analysis of the desorption rates for the two carbon monoxide species revealed that C18O production occurred with a higher activation barrier (75 kJ/mol) than did C16O (52 kJ/mol). SSIMS results indicated that oxygen exchange occurred between the surface and some adsorbed formate species at temperatures as low as 400 K. These findings reveal that the decomposition of formic acid on TiO2(110) is complex and cannot adequately be described by a simple dehydration mechanism.