Preparing Supported Metal Catalysts Coated with Thiols and Phosphonates

In the current study, dually coated supported metal catalysts are prepared; 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 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 opening 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.