Structure Determination of Formic Acid Reaction Products on TiO₂(110)

Abstract

Using chemical-state-specific scanned-energy-mode photoelectron diffraction (PhD) from O 1s and C 1s photoemission, we have determined the local structure of the surface species produced on the rutile TiO 2 (110) surface as a result of room temperature exposure to formic acid. The results show clear evidence for the coexistence of formate, HCOO, and hydroxyl, OH, surface species. The formate species is aligned along [001], bridging an adjacent pair of surface 5-fold-coordinated Ti atoms with the formate O atoms nearly atop the Ti atoms with a Ti-O bond length of 2.08 ( 0.03 Å, consistent with scanning tunneling microscopy observations, a number of theoretical calculations, and an earlier very restricted PhD study. The hydroxyl species are formed by H attachment to the surface bridging O atoms and have a Ti-O bond length of 2.02 ( 0.05 Å, significantly longer than for the bridging oxygen atoms on a bulk-terminated surface or as previously reported for the clean surface. Our results exclude the possibility of a large (1/3) fractional occupation by the formate species of a second site azimuthally rotated by 90°and bonded to a surface oxygen vacancy site, as proposed in some earlier infrared and X-ray absorption spectroscopic studies. A much smaller concentration of such a second species cannot be excluded.

Adsorption structure of glycine on TiO2(1 1 0): A photoelectron diffraction determination

“…The magnitude and sign of near-surface layer relaxations of clean TiO 2 (110) have been studied extensively in the last few years, and a PhD study has contributed to this topic [34], but the local character of the structural information intrinsic to this technique means PhD data from an adsorbate has limited sensitivity to these parameters. We might remark, though, that the earlier study of coadsorbed formate and hydroxyl on this surface [20] showed smaller relaxations than those of the clean surface, and this may also be inferred in the present study, consistent with the behaviour of metal surfaces that adsorbates commonly reduce 13 the magnitude of any clean surface relaxation.…”

“…This chemical shift is essentially the same as that seen for formate on TiO 2 (110), but also for hydroxyl species on this surface [20].…”

“…The Cu-Cu nearest-neighbour distance on both Cu surfaces is 2.55 Å, quite similar to the O-O distance in carboxylates of ~2.27 Å; the spacing of the undercoordinated (five-fold coordinated) Ti atoms on the TiO 2 (110)(1x1) surface is somewhat larger (2.96 Å), but it is well-established that the simplest carboxylate species, formate (HCOO), does bond to adjacent pairs of these Ti atoms on this surface in a symmetric fashion ( Fig. 1) with the O atoms some 0.3 Å off-3 atop and the O-O distance equal to 2.39±0.08 Å [20]. Perpendicular to this carboxylate O-O alignment, however, the metal-metal atomic distances on the three surfaces are very different, namely, 2.55 Å (Cu(100)), 3.61 Å (Cu(110)) and 6.50 Å (TiO 2 (110)).…”

“…An important implication of assuming deprotonation to the glycinate species, of course, is that we need to consider the fate of the H atoms that are removed. In the case of formic acid reaction with TiO 2 (110), there is clear evidence from the associated O 1s PhD structural study that the H atoms are adsorbed on the surface on the bridging oxygen atoms to produce local hydroxyl species [20]. Because the O 1s chemical shifts of the adsorbed carboxylate and the hydroxyl species, relative to the emission from the underlying oxide, are essentially identical, this means that the O 1s shoulder in the photoemission spectra contains contributions from both of these coadsorbed species.…”

Structure of a Model Dye/Titania Interface: Geometry of Benzoate on Rutile-TiO2 (110)(1 × 1)

“…[1][2][3][4] ). To date, most progress has been achieved for the simplest -COOH containing molecule, namely formic acid (HCOOH), on the prototypical rutile TiO 2 (110)(1×1) surface 2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] . On this substrate, it is well known that at room temperature formic acid adsorbs dissociatively, i.e.…”