Adsorption of Oxygenates on Alkanethiol-Functionalized Pd(111) Surfaces: Mechanistic Insights into the Role of Self-Assembled Monolayers on Catalysis

Abstract

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.

Self-assembled monolayers of thiolates on metals: a review article on sulfur-metal chemistry and surface structures

“…This model exhibits a larger stability than those involving simple RSc adsorbates on unreconstructed Pd(111) surfaces (Fig. 10b,b 0 ) 179,180 as concluded from the analysis of the g values in Table 6. Also, DFT calculations reveal that thiol adsorption on the PdS x surface is able to extract Pd atoms from the rst layers forming RS-Pd ad -SR moieties (Fig.…”

New Insights into the Chemistry of Thiolate-Protected Palladium Nanoparticles

“…Certainly, even today it is simply described as a √3×√3 R30° n-alkanethiolate SAM on Pd(111) surfaces. [9][10][11] Also, when considering the surface of Pd NPs, the same lack of precision or incomplete description of their chemistry is observed in the literature. 10,[12][13][14] Recently, Pd particles (3.0 nm in size) prepared by the two-phase Brust-Schiffrin method 3 in the presence of hexanethiol, −for the application in hydrogen sensing and catalysis− where simply described as hexanethiolate-coated Pd monolayer-protected clusters.…”

Increased Catalytic Activity of Surface‐Immobilized Palladium Complexes in the Fluorogenic Deprotection of an Alloc‐Derivatized Coumarin

“…[17] This effect was attributed to selective binding of the alkene moiety on the coated Pd surface at the expense of the epoxide ring. [18] It was also demonstrated that the activity of the modified catalyst was dependent on the molecular arrangement of the SAMs on the surface, less well-ordered SAMs restricting access to active sites.These studies suggest that an intricate association of factors governs the activity of surface-modified catalysts. SAMs of transition-metal complexes can possibly afford efficient alternatives to homogeneous catalysts regarding activity and selectivity, allowing the design of tailored heterogeneous catalysts.…”