Investigating the Surface Chemistry of Hydrogenation Reactions on Supported Metal Catalysts

Open Access
Author:
Ingalls, Megan
Area of Honors:
Chemical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Michael John Janik, Thesis Supervisor
  • Darrell Velegol, Faculty Reader
  • Michael John Janik, Honors Advisor
Keywords:
  • Self-Assembled Monolayer
  • Single Atom Catalyst
  • Hydrogenation
  • Catalysis
  • DFT
Abstract:
Organic modifiers can impact heterogeneous catalyst selectivity and activity by altering the adsorption preferences and surface reaction paths. Dually coated supported nanoparticle metal catalysts are prepared, with organic modifiers on both metal particles and their oxide support. Thiols are used to modify metals, with the sulfur head group binding to the surface, while phosphonic acids are used to modify the support. Both thiol and phosphonic acid modifiers form self-assembled monolayers (SAM) on the catalyst surface. Cinnamaldehyde hydrogenation was used to probe the performance of the as-prepared catalysts; the selectivity to cinnamyl alcohol could be potentially improved without significant decreases in activity by coating two self-assembled monolayers on the surface of the catalyst. For the dually coated catalyst, the thiol modifier mainly increased selectivity as was expected from previous work, while the phosphonate coating mainly increased the activity. When adding both thiols and phosphonates to the catalysts in different orders, it was found that the selectivity resembled the second SAM that was added to the catalyst. These changes in selectivity from the second SAM added to the surface were less drastic than the selectivity changes from a single SAM on the surface. For the activity of the catalyst, all of the catalysts with phosphonate coating showed increased activity for cinnamyl alcohol production in comparison to the uncoated Pt/Al2O3. The largest increase was seen when phosphonate was added after the thiol, likely from the thiol leaving open the selective active sites that favor cinnamyl alcohol production while the larger amount of phosphonate effected the electronic properties on the surface to increase the rate. A next potential step in this research is to use organic modifiers to tune the performance of single metal atom catalysts on oxide supports. As an initial step towards this goal, a computational model was developed to investigate hydrogenation reactions on a supported single metal atom catalyst. The catalyst modeled was a single Pt atom catalyst on the 101 surface of anatase TiO2. Acetaldehyde and trans-2-butene were used to model the functional groups in cinnamaldehyde that can be selectively hydrogenated. Multiple models were made to observe how the oxidation state of Pt0, Pt2+, and Pt4+ impacts hydrogenation reaction energies. In the Pt0 models, the adsorption of hydrogen to the surface for the reaction was unfavorable. The absence of hydrogen adsorbed to the surface would cause this catalyst to be unable to hydrogenate trans-2-butene or acetaldehyde. Both the Pt2+ and Pt4+ catalysts adsorbed hydrogen very strongly; however, this very low energy state that was created made it unfavorable for the hydrogen to leave the surface to hydrogenate the trans-2-butene and acetaldehyde.