Quantitative Differences in Sulfur Poisoning Phenomena over Ruthenium and Palladium: An Attempt To Deconvolute Geometric and Electronic Poisoning Effects Using Model Catalysts

Sulfur poisoning over noble-metal catalysts has traditionally been regarded as very complex and precluding from easy rational understanding, because of the problems of interference from using different supports, inability of controlling coverage due to nonuniform metal particle size, intrinsic size/shape effect of metal component, etc. Here, high-quality polyvinylpyrrolidone (PVP) polymer-supported ruthenium and palladium model nanocatalysts using without solid support are equivalently modified with preadsorbed mercaptoethanol over a range of surface concentrations in order to compare sulfur poisoning effects on the two important noble metals commonly used in industry. A typical consecutive hydrogenation reactions of alkyne to alkene and then to alkane is studied under mild reaction conditions in the liquid phase. The first stage alkyne hydrogenation is well-known to be surface insensitive, because of strong adsorption of alkyne on both metals. However, the second stage, surface-sensitive hydrogenation/isomerization of weakly adsorbed alkenes, is highly influenced by perturbations in metal surface electronic states induced by sulfur adsorbates. Using a combination of 13C NMR, Fourier transform infrared (FTIR) measurements of chemisorbed CO, kinetic products analysis and density functional theory (DFT) calculations, the electronic and geometric components of sulfur poisoning can be assigned in an almost-quantitative manner for the first time, over these two metal nanocatalysts. It is found that this sulfur adsorbate dwells preferentially on terrace sites for both metals at high coverage, causing deactivation by surface site blockage for the alkyne hydrogenation. The adsorbate can also deplete electron density from the metal surface (mixing with higher vacant band states of sulfur). As a result, reduction in adsorption strength for alkenes in the second-stage hydrogenation, leading to deactivation by electronic effects, is observed. This component is shown to contribute more significantly to the total deactivation for palladium (electron-rich metal) than ruthenium (electron-poor metal). At 60% sulfur coverage on Pd, the electronic contribution to surface adsorption can be totally cancelled out. This work clearly shows that the differing nature of metals can result in very different degrees of geometric and electronic deactivation upon sulfur adsorption over a size range of 2–3 nm without any interference from solid support, particle size/shape variations, giving important insights to developing more sulfur-tolerant catalysts in the future.