CO chemisorption on TiO2(110): Oxygen vacancy site influence on CO adsorption

Abstract

CO chemisorption has been studied on TiO2(110) under surface conditions where oxygen anion vacancy sites are not present (oxidized surface), compared to conditions where the vacancy sites are present (annealed surface). The binding energy of CO on the nondefective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage. CO...CO repulsive interactions have been observed at higher coverages. When anion vacancy sites are produced under controlled annealing conditions in vacuum at 900 K, a significant increase in the desorption temperature of a portion of the chemisorbed CO is observed. This observation, coupled with measurements showing that defective TiO2(110) does not have enhanced CO chemisorption capacity, suggests that CO adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites. It is postulated that enhanced CO bonding occurs via the interaction of the O moiety of CO with the anion vacancy site while primary adsorbate bonding occurs via the C moiety to Ti lattice sites. Neither CO2 production nor oxygen exchange in CO occurs when CO desorbs from defective TiO2(110).

An experimental setup combining a highly sensitive detector for reaction products with a mass-selected cluster source and a low-temperature STM for advanced nanocatalysis measurements

“…In both TD spectra, a desorption peak at about 150 K is seen. In agreement with the literature, this peak is attributed to CO desorbing from the TiO 2 substrate [39,52]. The peak intensity is much lower before annealing since the substrate area covered by Pt is higher.…”

IR spectroscopy applied to metal oxide surfaces: adsorbate vibrations and beyond

“…According to Yates and co-workers [80], the photocatalytic formation of CO 2 is brought about by activation of O 2 molecules through electron-hole (e-h) pairs generated within the TiO 2 by absorption of UV photons [80,82]. The fact that characteristic vibrational features of formate species (HCOO ad ) or carbonates (CO 3ad ) could not be observed in the IRRAS data indicates the absence of a longlived, chemically distinct intermediate species.…”

“…The capability of IRRAS to detect formate species, a possible reaction intermediate, has been demonstrated by the observation of characteristic IR-bands after exposure of rutile-TiO 2 (1 1 0) to gas phase formic acid [21]. In earlier work, the products of this photocatalytic process could be studied only in the gas phase [82], thus prohibiting a direct observation of surface intermediates. These IRRAS observation therefore provide the first direct evidence that the reaction of adsorbed CO proceeds most likely with either O-adatoms or activated O 2 species, leading directly to CO 2 -formation.…”

Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy

“…This band is the most prominent one, which is easy to rationalize since Ti 4+ sites are the majority surface sites for CO adsorption on the catalyst. A shift to lower frequencies of the vibration of a probe molecule is often explained with an electron density donating substrate where a stronger back donation of the substrate into the empty π * orbital of CO takes place [60]. In single crystal experiments on anatase TiO 2 the band has been assigned to CO adsorbed onto Ti 4+ Lewis sites in (101) and (112) facets in anatase [36].…”