Date of Award
Doctor of Philosophy (PhD)
Chemical & Biochemical Engineering
J. Will Medlin
John L. Falconer
Jennifer N. Cha
Jesse E. Hensley
In this thesis we investigate the role of oxophilic metal modifiers in deoxygenation catalysts, starting with surface science experiments and extending to supported catalyst studies.
Carbon-oxygen bond scission is critical for biomass upgrading applications, but these bonds tend to be strong in the aromatic oxygenates that make up a large portion of pyrolysis oil. Bimetallic catalysts containing a noble metal and an oxophilic metal have shown promising activity and selectivity for deoxygenation, but the role of each metal in the overall reaction is unclear. Gaining a fundamental understanding of the roles of oxophilic sites will facilitate a systematic approach to catalyst design.
Structure-property relations were investigated under ultra-high vacuum (UHV) using a Pt(111) single crystal modified with sub-monolayer quantities of molybdenum. X-ray photoelectron spectroscopy suggested that when the surface is pretreated in hydrogen the Pt and Mo sites interact, resulting in a significant electronic effect on the Pt atoms. Low energy electron diffraction indicated that the hydrogen-reduced surface is well ordered, with Mo atoms inserted into the Pt lattice. The electronic effect was apparent using temperature-programmed desorption (TPD) experiments; as the coverage of Mo increased the desorption temperatures of carbon monoxide and hydrogen decreased by 10 and 30 K, respectively. In addition, these reduced Pt-Mo sites were shown to allow dissociation of water into hydrogen and surface hydroxyls, a process that does not occur on unmodified Pt(111). This may be important for deoxygenation because surface hydroxyl groups may act as acid sites. Alternatively, when the surface was treated in oxygen, the oxidized Mo formed an inert, disordered surface layer that only served to block active sites.
To extend this approach to more complex reactants representative of pyrolysis oil, the surface chemistry of benzyl alcohol was studied using TPD. Pt(111) catalyzed both decarbonylation to form benzene and carbon monoxide as well as complete decomposition to hydrogen and surface carbon. Incorporation of Mo improved the selectivity to hydrogenolysis, forming toluene as the major organic product. Toluene TPD and density functional theory (DFT) calculations suggested that the selectivity improvement may be in part due to reduced adsorption strength of the aromatic ring. These results were extended to alumina-supported Pt and PtMo catalysts. When Mo was incorporated into particles with a large fraction of terrace (111) sites the same shift in reaction pathway was observed as in the UHV results, with increased hydrogenolysis activity to produce toluene and lower activity for decarbonylation.
Supported Pt and PtMo catalysts were also studied for deoxygenation of m-cresol, an aromatic oxygenate containing a very strong C-O bond. Incorporation of Mo was found to increase selectivity to the deoxygenated alkane product. DFT calculations suggest this may be due in part to strong binding of the oxygen-containing functional group to the Mo site, resulting in an altered adsorption orientation that allows an additional deoxygenation pathway to be accessible by facilitating tautomerization.
Finally, preliminary studies have been performed to design an improved deoxygenation catalyst. The previous results suggest that two key influences of an oxophilic metal modifier are to reduce adsorption strength of carbon and to increase the strength of the interaction between the surface and the oxygen-containing moiety. DFT calculations were performed to calculate the difference in carbon versus oxygen adsorption strength on various bimetallic surfaces. Using this approach, several new bimetallic catalysts were identified as promising candidates, including PtW and PdFe. Preliminary evaluations of these catalysts suggest that by tuning this difference in adsorption strengths an improved deoxygenation catalyst may be synthesized.
Robinson, Allison M., "The Role of Oxophilic Metal Promoters in Bimetallic Hydrodeoxygenation Catalysts" (2016). Chemical Engineering Graduate Theses & Dissertations. 5.