Quantification of active sites for the determination of methanol oxidation turn-over frequencies using methanol chemisorption and in situ infrared techniques. 2 - Bulk metal oxide catalysts

Abstract

Bulk metal oxide catalysts, especially bulk mixed-metal molybdates such as Fe2(MoO4)3, often exhibit high methanol oxidation activity and selectivity. However, the difficulties involved in determining active surface site densities on these catalysts have, in the past, generally prevented side-by-side comparisons of their intrinsic activities, or turn-over frequencies (TOFs). In the present study, high temperature (110 °C) methanol chemisorption and in-situ infrared spectroscopy have been employed to directly and quantitatively determine the number of active metal oxide surface sites available for methanol oxidation. The IR spectra indicate that methanol chemisorption on these catalysts produces both associatively adsorbed, intact Lewis-bound surface methanol species (CH3OHads, species I) on acidic sites, as well as dissociatively adsorbed surface methoxy species (-OCH3, species II) on less acidic or basic sites. In fact, the Lewis acidity of bulk mixed-metal molybdates relative to the methanol probe molecule was found to decrease as follows: Fe2(MoO4)3, NiMoO4 (species I predominates) > MnMoO4, CoMoO4, ZnMoO4, Al2(MoO4)3 > Ce(MoO4)2 > Bi2Mo3O12 > Zr(MoO4)2 (species II predominates). It also appears that Mo cations are the primary methanol chemisorption sites in many of the bulk mixed-metal molybdates, including commercially important Fe2(MoO4)3. By quantifying the surface concentrations of the adsorbed methoxylated reaction intermediates from the IR spectra, it was then possible to normalize the catalytic methanol oxidation activities for the calculation of TOFs. The methanol oxidation TOFs of bulk molybdates were shown to be relatively similar to those of model supported catalysts with the same co-cation (e.g., MoO3/NiO vs NiMoO4)-possibly due to the formation of a "monolayer" of surface molybdenum oxide species on the surfaces of the bulk metal molybdates. In addition, the bulk mixed-metal molybdates were found to exhibit the same ligand effect as that discovered previously in supported metal oxide catalysts, in which the TOF generally decreases with increasing ligand cation electronegativity due to electronic variations in localized M-O-Ligand bonds.