Thursday, January 17, 2019

SERS, XPS and DFT investigation on palladium surfaces coated with 2,2′-bipyridine monolayers

M. Muniz-Miranda, F. Miranda-Muniz, S. Caporali, N. Calisi, P. Alfonso, Applied Surface Science, 457, 98-103 2018
Highlighted by Michaella Raglione, Sajeewani Kumarage, and Glorianne Dorce

Palladium (II) chloride complexes containing diimine ligands like what is shown in Figure 1 have been widely used as a reaction catalyst. One application of a palladium catalyst is the Heck reaction, which utilizes a Palladium (II) complex intermediate to activate the reaction of an unsaturated halide with an alkene in the presence of a base. Often, palladium is used in a suspension, but it lacks the colloidal stability to maintain a homogeneous mixture, which can lower its efficiency. Furthermore, heterogenous catalysis provides high yield, and facilitates the reusability of catalysts compared to homogeneous catalysis. Thus, Muniz-Miranda et al. recently investigated the use of Palladium coated with 2,2’-bipyridine monolayers for heterogenous catalysis.

Figure 1. Crystal structure of byp-PdCl2 from Table S1 supplementary material.

Their experiments centered around Palladium plate which was wetted with 2,2’-bipyridine (bpy). In order to test their work, Muniz-Miranda et al. utilized the surface plasmon effect of Ag nanoparticles (nps) to enhance their surface enhance Raman signal (SERS). The Ag nps were obtained through laser ablation in the bpy solution without free chloride anions. To determine the resulting complex, DFT calculations were performed using GAUSSIAN 09 software with a B3LYP/6-311++G(d,p) for non-metal atoms, and Lanl2DZ for palladium basis sets. To ensure their experimental methods would work, byp-PdCl2 was collected and the Raman spectra was compared to the calculated spectra.

Muniz-Miranda et al.  have made bpy-PdX, and by comparing the resulting calculated Raman active modes to the experimental bpy-PdR (R=O,O2,(OH)2)  revealed that they created bpy-Pd(OH)2.

Due to similar structural and spectroscopic characteristics of the bpy-Pd(OH)2 and bpy-PdCl2, the catalysts are expected to function similarly. The similarities in these catalysts opens the possibility of utilization of the much simpler heterogenous nucleation of the bpy-Pd(OH)2 complex for reaction mechanisms. This suggests that combined benefits of both heterogenous and homogenous nucleation can be achieved: improved yield and reusability as well as selectivity control.

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