(560ax) Mechanisms of C-O and C-C Bond Activation of Acetic Acid Hydrodeoxygenation over Pt-Mo Catalysts

Promising new technologies for biomass conversion involve production of bio-oils, which need to be upgraded to remove oxygen-containing hydrocarbons. Reactions of carboxylic acids are some of the slowest steps in the hydrodeoxygenation of biomass-derived feedstocks. Therefore, acetic acid, the simplest carboxylic acid, can serve as a useful model compound in the development of improved hydrodeoxygenation (HDO) catalysts. Furthermore, acetic acid itself is a significant component of bio-oils. Although supported Pt catalysts are known to be efficient in HDO, they exhibit insufficient activities and low selectivities with undesirable C-C bond breaking reactions.

In this study, more active and selective HDO catalysts were developed by adding Mo to Pt. While Mo catalysts do not have any activity, Pt-Mo bimetallic catalysts exhibit significantly improved activities and selectivities even at a small concentration of added Mo. The nature of the promotion effects was studied by characterizing a series of Pt, Mo and Pt-Mo catalysts with multiple techniques, including H₂ temperature programmed desorption, transmission electron microscopy (TEM), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM/EDS) and in situ X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS). In addition, reaction kinetic results were obtained in a flow reactor, and the reaction mechanism was analyzed with dispersion-corrected density functional theory (DFT-D) calculations.

Addition of Mo to Pt improved the reaction activity by more than an order of magnitude. More importantly, the selectivities were also improved. The formation of C₁ products (CO and CH₄) went down to less than 10 mol % for the Pt-Mo bimetallics. The formation of C₂ products, mostly CH₃CH₃, increased to more than 90 mol %. Furthermore, Pt-Mo catalysts were stable under the reaction conditions for at least 10 hours. The STEM/EDS and XAS results demonstrate that Mo formed subnanometer clusters that were fairly uniformly distributed on the support, including the surface of Pt nanoparticles. The dispersion of Pt nanoparticles was not affected by the Mo addition. With increasing Mo concentration, the reaction activity of the Pt-Mo bimetallics increased, went through a maximum and then declined. The DFT-D calculations show that although Mo atoms themselves are catalytically inactive, they promote neighboring Pt atoms by changing the stability of adsorbates and surface reaction energies. Mo atoms are oxophilic and, therefore, serve as preferential adsorption sites for acetic acid and other oxygenates. Due to the presence of Mo on Pt, C-O bond splitting reactions become more thermodynamically preferable, and C-C bond splitting reactions become suppressed.

While Mo catalysts are inactive and Pt catalysts have low activities and selectivities, Pt-Mo catalysts exhibit high activities and selectivities in acetic acid HDO. In addition, the Pt-Mo catalysts are stable. Although Mo atoms are catalytically inactive, they serve as preferential adsorption sites for oxygenates and, thus, promote neighboring Pt atoms by changing adsorbate properties and reaction energies. Beyond biomass processing, Pt-Mo formulations are promising catalysts for a wide variety of reactions that require a transformation of molecules with an oxygen atom due to oxophilicity of Mo. Beyond catalysis, Pt-Mo nanoparticles offer an exciting potential in sensing and biomedical applications as well as in numerous other science and technology fields that require tuning of surface-oxygen interactions.