A density functional theory study of the oxidation of methanol to formaldehyde over vanadia supported on silica, titania, and zirconia
Density functional theory was used to investigate the mechanism and kinetics of methanol oxidation to formaldehyde over vanadia supported on silica, titania, and zirconia. The catalytically active site was modeled as an isolated VO{sub 4} unit attached to the support. The calculated geometry and vibrational frequencies of the active site are in good agreement with experimental measurements both for model compounds and oxide-supported vanadia. Methanol adsorption is found to occur preferentially with the rupture of a V-O-M bond (M = Si, Ti, Zr) and with preferential attachment of a methoxy group to V. The vibrational frequencies of the methoxy group are in good agreement with those observed experimentally as are the calculated isobars. The formation of formaldehyde is assumed to occur via the transfer of an H atom of a methoxy group to the O atom of the V=O group. The activation energy for this process is found to be in the range of 199-214 kJ/mol and apparent activation energies for the overall oxidation of methanol to formaldehyde are predicted to lie in the range of 112-123 kJ/mol, which is significantly higher than that found experimentally. Moreover, the predicted turnover frequency (TOF) for methanol oxidation is found to be essentially independent of support composition, whereas experiments show that the TOF is 10{sup 3} greater for titania- and zirconia-supported vanadia than for silica-supported vanadia. Based on these findings, it is proposed that the formation of formaldehyde from methoxy groups may require pairs of adjacent VO{sub 4} groups or V{sub 2}O{sub 7} dimer structures.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director. Office of Science. Office of Basic Energy Sciences. Chemical Sciences Division; Civilian Research and Development Foundation RC1-2204 (US)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 836790
- Report Number(s):
- LBNL-49412; R&D Project: 402001; TRN: US200504%%301
- Journal Information:
- Journal of Physical Chemistry B, Vol. 106, Issue 32; Other Information: Submitted to Journal of Physical Chemistry B: Volume 106, No.32; Journal Publication Date: 8/15/2002; PBD: 5 Sep 2002
- Country of Publication:
- United States
- Language:
- English
Similar Records
Vanadia-titania aerogels: I. Preparation, morphological properties, and activity for the selective catalytic reduction of NO by NH{sub 3}
Structural characteristics and reactivity/reducibility properties of dispersed and bilayered V{sub 2}O{sub 5}/TiO{sub 2}/SiO{sub 2} catalysts