(744d) Identification of Active Sites for Selective C-O Cleavage Reactions on Metals Supported on Reducible Oxides

Conversion of biomass to higher value products relies on catalysts that have the ability to selectively cleave C-O bonds while avoiding side reactions such as the hydrogenation and C-C hydrogenolysis. A promising family of catalysts that have shown promise for these selective transformations are metal nanoparticles (such as Pd, Ru, or Ni) on reducible supports such as TiO₂. In many of these cases, the active site responsible for the selective conversion of phenolic and furanic species is not well understood. Common proposals that an active site is created at the perimeter of the metal particle/support interface vs. a promoter effect of the metal nanoparticle to generate defects on the support that selectively convert these functional groups. Here we present contrasting cases of cleavage of CO bonds that occur via new interfacially active sites at the metal/support interface [1] vs. through promoter effects [2-3] with furanic and phenolic compounds. We contrast these two cases under conditions severe enough (>350°C) to generate oxygen vacancies on the TiO₂ support at kinetically significant rates. We identify the active site responsible for the enhanced activity through a combination of varying nanoparticle size and support polymorph, and provide contrasting examples of each using anisole and cresol. We then demonstrate a new technique to identify the active site responsible for several selective C-O scission reactions by manipulating the distance between a metal and a reducible oxide over a conductive hydrogen highway. This approach allows for the identification and quantification of turnover frequencies over the various prospective active sites.

1. Experimental and First-Principles Evidence for Interfacial Activity of Ru/TiO₂ for the Direct Conversion of m-Cresol to Toluene. Taiwo Omotoso, Byeongjin Baek, Lars C. Grabow, Steven Crossley, ChemCatChem 9(14), 2642-2651, doi:10.1002/cctc.201700157, 2017

2. Understanding the role of TiO₂ crystal structure on the enhanced activity and stability of Ru/TiO₂ catalysts for the conversion of lignin-derived oxygenates Taiwo Omotoso, Sunya Boonyasuwat, Steven Crossley, Green Chemistry, 16, 645-652 2014.

3. Conversion of Guaiacol over Supported Ru Catalysts, Sunya Boonyasuwat, Taiwo Omotoso, Daniel E. Resasco, Steven P. Crossley, Catalysis Letters 143 (8), 783, 2013.