(589g) Catalytic Requirements for Alkanol, Alkane, Alkene, and CO Oxidation on Dispersed Metal Clusters

Catalytic
Requirements for Alkanol, Alkane, Alkene, and CO Oxidation on Dispersed Metal
Clusters

Weifeng Tu, Petar Lachkov, Yifei Yang, and
Ya-Huei (Cathy) Chin*

Department of Chemical Engineering
and Applied Chemistry, University of Toronto, Toronto, Canada

  *cathy.chin@utoronto.ca

Reactions of alcohol, alkane, alkene, or
CO with oxygen on dispersed metal clusters are important pathways for
synthesizing value-added chemicals and fuels, producing energy, or eliminating
pollutants. These reactions occur via kinetically coupled reductant and oxygen activation
steps on metal cluster surfaces. Here, we utilize kinetic and isotopic studies in
combination with surface titration techniques to probe the elementary steps and
their kinetic relevance, report the catalytic involvement of reactive oxidant
species, and then discuss the mechanistic synergies across these oxidation
reactions. We report a direct relation connecting the oxidant and reductant
pressures to the bulk chemical state of the metal clusters, coverages of
reactive oxygen species, and their reactivities in these oxidation reactions,
demonstrated here for the case of methanol oxidative dehydrogenation and
methane oxidation reactions. The oxidant and reductant pressure ratio determines
the relative rates of oxygen activation and oxygen removal on cluster surfaces and
thus the bulk chemical state, instantaneous oxygen coverages, and the relative
abundance of oxygen atom pair, oxygen atom-oxygen vacancy pair, and metal atom
pair at cluster surfaces. These site-pairs activate methanol via kinetically
distinct routes, thus leading to different rate dependencies. For the case of CO
oxidation, turnover rates vary with not only O* but also O₂* coverages
and for propene epoxidation, the rates vary with reactive oxygen (O* and O₂*)
and also with OOH* intermediates formed from hydrogen abstraction by O₂*.
The presence of the diverse oxygen species and their distinct reactivities in
activating the reductants lead to the marked difference in rates,
selectivities, and the apparent complex kinetic dependence on reactant
pressures.