Strategies for Selective Catalyst Design Via Controlled Synthesis of Surface Ligands on Pd Nanoparticles

The controlled synthesis of metal nanoparticles (NPs) capped with surface-bound ligands has been shown as an approach for tuning catalytic activity and selectivity of supported metal catalysts for heterogeneous reactions [1]. One example is the Pd-catalyzed direct synthesis of hydrogen peroxide (H₂O₂) where ligands that possess H-bonding functionality were suggested to stabilize critical oxygenated intermediates of the reaction and enhance proton transfer, thereby improving H₂O₂ selectivity compared to unmodified catalysts [1]. Although promising, there exists a knowledge gap on how to rationally design organic ligand modifiers on metal surfaces to achieve targeted catalytic functionality.

To address this knowledge gap, in this work, we focus on the synthesis of Pd NPs with surface bound ligands that provides control over both the metal NP size and the nature of the surface ligands. The goal is to use these ligand capped Pd NPs to develop structure-catalytic function relations that can guide our understanding of the role of ligands in catalytic reactions. We initially focused on the synthesis of Pd NPs using a one-pot colloidal approach by directly capping the NPs with trioctylphosphine (TOP) and polyethylene glycol (PEG) ligands [2, 3]. As a reference, we employ a phosphate-modified variant of cetyltrimethylammonium bromide (CTAB), denoted as “CTAP”, as an alternate ligand system where CTAP can undergo facile ligand exchanges. CTAP is formed from CTAB by strategically exchanging the bromide with a phosphate group to create a unique functionality that adds a buffering effect during the reduction of the Pd precursor in solution and maintains CTAB’s favorable binding to stabilize the Pd NPs. Exchange of CTAP with hydrophobic (TOP) and hydrophilic ligands (PEG) was successfully achieved using phase exchange and concentration gradient approaches, respectively. The resulting ligand capped (CTAB, TOP, and PEG) Pd NPs were characterized to assess the impact of synthesis approach (ligand exchange vs. direct capping) and ligand type on Pd NP shape and size. Pd NPs were supported on silica and carbon to evaluate the effects of ligand identity, binding mode, and support environment on direct H₂O₂ synthesis and other reactions. This work sets the stage for tailoring Pd catalyst synthesis through controlled ligand design for tuning catalytic function of metal NPs.

References

[1] de L. e Freitas, L., Puértolas, B., Zhang, J., et al., J. ACS Catal. 10, 5202-5207 (2020)

[2] Wang, B., Zhang, J., Herrera, L. P., et al., J. Indust. Engin. Chem. Research. 58, 43032-4041 (2019)

[3] Paz Herrera, L., Freitas De Lima E Freitas, L., Hong, J., et al., J. Catal. Technol. 12, 1476-1486 (2022)