FTIR Spectroscopy of Alcohol and Formate Interactions with Mesoporous TiO₂ Surfaces

Abstract

The effects of pH and ultraviolet (UV) light with ligated formic acid on mesoporous TiO2 were characterized by transmission Fourier transform infrared (FTIR) spectroscopy and compared with adsorbed formate complexes. Surface-modified anatase thin films were prepared from acidic aqueous nanoparticulate anatase suspensions diluted with methanol and ethanol. Bands assigned to carboxylic acid groups displayed unique bonding character in the ligated formic acid on the anatase surface. For increased proton concentrations in the films, separation in -COO stretching bands (delta nu) for formic acid increased (increase in frequency for nuC=O and decrease in frequency for nuC-O). With UV exposure, surface-bound organics were rapidly removed by photocatalytic oxidation at 40 degrees C and 40% relative humidity (RH). In addition, the delta nu of the formic acid bands decreased as organics were mineralized to carbonates and CO2 with UV light. Aqueous formic acid adsorption experiments showed a distinctly different bonding environment lacking carbonate, and the delta nu for the carboxylic groups indicated a bridging bidentate coordination. The delta nu of the bands increased with increasing proton concentration, with both bands shifting to higher wavenumbers. The shifts may be ascribed to the influence of protonation on surface charge and the effect of that charge on the electronegativity of carboxylate groups bound to the surface. As alcohols are used in the mesoporous TiO2 solar cell preparation, implications of these surface modifications to dye-sensitized photovoltaics are discussed.

A surface science perspective on TiO2 photocatalysis

“…Formic acid: The photooxidation of formate (from formic acid) on TiO 2 surface does not appear to pass through any detectable surface intermediates before resulting in CO 2 (or CO − 2 in some cases) [715,733,769,790,[961][962][963]. There is disagreement in the literature regarding whether formate photodecomposes directly by a hole-mediated pathway (Reaction 7),…”

Fourier-Transform Infrared and Raman Spectroscopy of Pure and Doped TiO2 Photocatalysts

“…Evidence for coordinated carbonate formation has been reported by several authors [18,20,132,148]. Reported band frequencies and assignments of (bi)carbonate on various TiO 2 nanoparticles are compiled in Table 8.2.…”

“…In contrast, adsorption and thermal reactions of carboxylic acids, and formic acid in particular, have been extensively studied on both single crystals [22,25,101,102,[116][117][118][119][120][121][122][123][124][125][126][127][128] (primarily rutile TiO 2 (110)) and TiO 2 nanoparticles (see Refs. [1,4,8,9,89,110,[129][130][131][132][133] and references therein). Guided by the schematic reaction scheme outlines in Figure 8.11, which highlight the importance of intermediate adsorbate structures, such as simple carboxylates and acetone, we will explore in greater detail in Section 8.5.1 the formic acid/formate and acetone adsorption systems on the various polymorphs of TiO 2 and doped TiO 2 nanoparticles.…”

“…For example, it has been shown that the charge of the CH hydrogen atom in HCOOH correlates strongly to adsorption to Ti 4 þ Lewis acid sites on rutile TiO 2 (110) [106]. A word of Adsorption of formic acid (HCOOH) on oxides from the gas phase has been extensively studied by a wide range of methods [1,4,17,20,22,25,101,102,110,114,[117][118][119][120][121][122][123][124][125][126][127][128][129][130][131][132][133]135]. Formic acid, the simplest organic acid, is a suitable probe molecule to study the important carboxylate-TiO 2 surface interactions discussed in the previous section.…”

“…Indeed vibrational spectroscopy has been widely employed to characterize the HCOOH/TiO 2 and HCOO/TiO 2 adsorption systems in gaseous [4,9,17,20,102,114,119,130,133,138] and aqueous phases [131,132,139,140]. The general features of these studies are outlined below.…”