Catalytic reduction of pyrolyzed biomass is required to remove oxygen and produce transportation fuels, but limited knowledge of how hydrodeoxygenation (HDO) catalysts work stymies the rational design of more efficient and stable catalysts, which in turn limits deployment of biofuels. This work reports results from a novel study utilizing both isotopically labeled phenol (which models the most recalcitrant components of biofuels) with D 2 O and DFT calculations to provide insight into the mechanism of the highly efficient HDO catalyst, Ru/TiO 2 . The data point to the importance of interface sites between Ru nanoparticles and the TiO 2 support and suggest that water acts as a cocatalyst favoring a direct deoxygenation pathway in which the phenolic OH is replaced directly with H to form benzene. Rather than its reducibility, we propose that the amphoteric nature of TiO 2 facilitates H 2 heterolysis to generate an active site water molecule that promotes the catalytic C−O bond scission of phenol. This work has clear implications for efforts to scale-up the hydrogen-efficient conversion of wood waste into transportation fuels and biochemicals.
The selective cleavage of C−O bonds in phenolic species is perhaps the most difficult transformation required for converting biomass‐derived monomers to aromatic fuels and chemicals. Metals supported on reducible oxides, such as Ru/TiO2, have demonstrated considerable promise for a variety of selective C−O cleavage reactions, but the active site has been subject of a great deal of speculation. This paper employs a combination of theory and experiments to investigate the nature of the active site for the selective transformation of m‐cresol to toluene. Through variation of metal loading, particle size and support phase, we show that sites responsible for direct C−O cleavage of m‐cresol lie at the perimeter of the metal particle. The activation barrier for C−O cleavage is reduced from 1.4 eV on the Ru surface to 0.7 eV at an interfacial site. The introduction of water facilitates a further reduction to 0.3 eV via a proton‐assisted C‐O cleavage. These results answer a longstanding question regarding the nature of these important active sites, with broad implications for biomass upgrading.
A new
atomic-scale anisotropy in the photoreaction of surface carboxylates
on rutile TiO2(110) induced by gold clusters is found.
STM and DFT+U are used to study this phenomenon by monitoring the
photoreaction of a prototype hole-scavenger molecule, benzoic acid,
over stoichiometric (s) s-TiO2, Au9/s-TiO2, and reduced (r) Au9/r-TiO2. STM results
show that benzoic acid adsorption displaces a large fraction of Au
clusters from the terraces toward their edges. DFT calculations explain
that Au9 clusters on stoichiometric TiO2 are
distorted by benzoic acid adsorption. The influence of sub-monolayers
of Au on the UV/visible photoreaction of benzoic acid was explored
at room temperature, with adsorbate depletion taken as a measure of
activity. The empty sites, observed upon photoexcitation, occurred
in elongated chains (2 to 6 molecules long) in the [11̅0] and
[001] directions. A roughly 3-fold higher depletion rate is observed
in the [001] direction. This is linked to the anisotropic conduction
of excited electrons along [001], with subsequent trapping by Au clusters
leaving a higher concentration of holes and thus an increased decomposition
rate. To our knowledge this is the first time that atomic-scale directionality
of a chemical reaction is reported upon photoexcitation of the semiconductor.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.