Fourier Transform Reflection−Absorption IR Spectroscopy Study of Formate Adsorption on TiO2(110)

Abstract

The adsorption of the formate species produced by dissociative adsorption of formic acid on TiO2(110) has been investigated using a combination of X-ray photoelectron spectroscopy (XPS) and Fourier transform reflection−absorption IR spectroscopy (FT-RAIRS). Adsorption at 300 K produces a formate coverage of 0.59 ML, in agreement with previous estimates and higher than that expected for a perfect (2 × 1) overlayer observed in low-energy electron diffraction (LEED). p-Polarized FT-RAIRS measurements carried out as a function of azimuthal angle (Φ) reveal the presence of two formate species through the coupling of both the tangential (It) and normal (In) components of the radiation to the symmetric (νsym(OCO)) and antisymmetric (νasym(OCO)) vibrational modes of the molecule. Species A has the OCO plane aligned in the 〈001〉 direction with C2v symmetry; νasym(OCO) = 1566 cm-1 is observed as an absorption band (coupled to It) at Φ = 90°, and νsym(OCO) = 1363 cm-1, observed as a transmission band (coupled to In). Species B has the OCO plane aligned in the 〈110〉 direction with Cs symmetry; νasym(OCO) = 1535 cm-1, observed as an absorption band (coupled to It) at Φ = 0°, and νsym(OCO) = 1393 cm-1, observed as a transmission band (coupled to In). Changes in the region of the νsym(OCO) at 1363 cm-1 for species A as a function of azimuthal angle (this band should be insensitive to Φ) are associated with an absorption band resulting from a coupling of νsym(OCO) of species B to It. This is a result of the reduction in symmetry from C2v to Cs and, as expected from the adsorption geometry, is observed at Φ = 0° and not at Φ = 90°. We estimate that the coverages of species A and species B at 300 K are 0.4 and 0.2 ML, respectively. Species A is the adsorbate responsible for the formation of the (2 × 1) ordered structure and adsorbed between the TiO2(110) oxygen rows. Species B we suggest is an adsorbed formate molecule with one of the oxygens incorporated in the TiO2(110) oxygen rows and is the species associated with the so-called disordered phase. It is clear from FT-RAIRS that the coverage of species B is increased by adsorbing formic acid at higher surface temperatures. We suggest that species B adsorbs at defects (oxygen vacancies) created through dissociative adsorption of formic acid to produce species A and the subsequent reaction of two hydroxyl groups to produce water. The existence and interdependence of the two adsorbed formate species provide an explanation for the complex activity observed for various catalytic reactions of formic acid over titania.