Quantification of active sites for the determination of methanol oxidationturn-over frequencies using methanol chemisorption and in situ infrared techniques. 1. Supported metal oxide catalysts

Methanol oxidation over metal oxide catalysts is industrially important for the production of formaldehyde, but knowledge about the intrinsic catalysi s taking place is often obscured by a lack of knowledge as to the number of active sites present on the catalyst surface. In the present study, the nu mber of surface sites active in methanol oxidation has been determined over a wide range of supported metal oxide catalysts using quantitative methano l chemisorption and in-situ infrared titration techniques performed at an e xperimentally optimized temperature of 110 degreesC. It was found that a st eric limitation of about 0.3 methoxylated surface species (e.g., strongly L ewis-bound CH3OHads, and dissociatively adsorbed -OCH3,ads, which are the r eactive surface intermediates in methanol oxidation) exists per active depo sited metal oxide metal atom across all supported metal oxides. Hence, the use of methanol chemisorption for counting active surface sites is more rea listic than other site-counting methods for the kinetic modeling of methano l oxidation, where during steady-state reaction the departure of the actual coverage of methoxylated surface intermediates from the maximum saturation surface coverage is of critical importance. Methanol oxidation turn-over f requencies (TOF = methanol molecules converted per second per active surfac e site) calculated using these new methanol chemisorption surface site dens ities increased by a factor of similar to3 the TOFs estimated in previous s tudies using the total number of deposited metal oxide metal atoms. Neverth eless, the support effect observed previously (TOFs for MoO3 and V2O5 suppo rted on oxides of Zr similar to Ce > Ti > Al much greater than Si) remains virtually unchanged as a general trend in the present study and correlates with the support cation electronegativity via bridging M-O -Support bonds. The methanol chemisorption technique may now be used with confidence to sea rch for similar ligand effects in bulk metal oxides, where counting active sites has traditionally been very difficult (subject of part 2, Burcham, L. J.; Briand, L. E.; Wachs, 1. E. Langmuir 2001, 17, 6175, of the present tw o-paper series).