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.

Contrasting the Behaviour of MoO3 and MoO2 for the Oxidation of Methanol

“…It is quite clear that MoO 3 is a highly successful material for the selective oxidation of formaldehyde and that Mo 6+ is not only involved in the catalysis, it is the crucial component for selective performance and high formaldehyde yield. As proposed by ourselves [16] and others (see, for example, [19][20][21]) it is likely that the first step in the catalysis is methanol adsorption, followed by hydrogen abstraction by surface oxygen anions to form surface methoxy species, as also identified by IR measurements [22]. We can write the latter step as follows; based on a two Mo site mechanismMo 6þ O 2À Mo 6þ + CH 3 OH !…”

Towards a green enantiomeric esterification of R/S-ketoprofen: A theoretical and experimental investigation

“…The chemisorption of the alcohols generates intermediate methoxy and propoxy species that further dehydrate over active sites of acidic nature [40][41][42].…”

On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study

“…1-7 Titanium dioxide, for example is a photocatalyst used in very large-scale water purification applications 8 , for hydrogen production from photocatalytic water splitting 9,10 , and for selective photooxidation reactions in organic chemistry 11 . 1 , [13][14][15][16] Methanol photochemistry on TiO 2 has often been the model for studying reaction mechanims. Methanol represents a model for many organic compounds and is an appropriate molecular probe to explore oxide surface properties.…”