Surface Sulfur Species Influence Hydrogenation Performance of Palladium-Sulfur Nanosheets

Abstract

Olefins play a crucial role as fundamental raw materials in organic synthesis, particularly in the production of polyolefins and synthetic rubber. The conversion of alkynes to olefins is pivotal in both the polymer and fine chemical industries. However, this process faces significant challenges in terms of equilibrium selectivity and activity. The inherent low solubility of hydrogen, coupled with the thermodynamic ease of hydrogenating intermediate olefins compared to alkynes, contributes to a decline in olefin selectivity due to further hydrogenation leading to alkanes. Palladium-based catalysts, widely used for hydrogenation, exhibit robust hydrogen adsorption but lack selectivity. Researchers commonly modify catalyst structures by introducing other metals or nonmetals to create intermetallic compounds, aiming to enhance olefin selectivity. This study focuses on synthesizing palladium-sulfur nanosheets (Pd-S NSs) using various sulfur sources to explore the impact of surface S species on the catalytic efficiency of selectively hydrogenating alkynes. Among these, Pd-S-PT NSs/C, utilizing 1,4-benzenedithiol (PT) as the sulfur source, demonstrated high styrene selectivity (92.3%–96.7%) following phenylacetylene hydrogenation for 2 h, showing notable selectivity for different alkynes’ end-groups. Contrastingly, Pd-S-TU NSs/C, with thiourea (TU) as the sulfur source, exhibited poor olefin selectivity (72.4%). X-ray photoelectron spectroscopy (XPS) revealed that the improved olefin selectivity in Pd-S-PT NSs/C was attributed to hindered electron transfer from Pd to S, as well as the presence of surface S⁰ species, maintaining high hydrogenation activity while avoiding over-hydrogenation induced by oxidized S species (S⁴⁺). In situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) demonstrated weak styrene adsorption on Pd-S-PT NSs, inhibiting further hydrogenation to ethylbenzene. The ease of styrene desorption on Pd-S-PT NSs, indicated by reduced adsorption strength with increasing desorption temperature, highlighted high olefin selectivity. Conversely, stronger styrene adsorption on Pd-S-TU NSs facilitated additional hydrogenation to produce ethylbenzene, suggesting that the presence of additional S⁴⁺ species hindered improved styrene selectivity. This study not only introduces efficient catalysts for olefin hydrogenation but also advances fundamental research on precisely controlling catalytic processes, particularly focusing on the nuanced control of catalytic surfaces.

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