Ambient-condition acetylene hydrogenation to ethylene over WS2-confined atomic Pd sites

Ambient-condition acetylene hydrogenation to ethylene (AC-AHE), integrated with acetylene production from coal and natural gas, presents a promising non-oil route for ethylene production with low energy input, contrasting the high-temperature (above 750 °C) classical naphtha steam cracking method. Pd-based catalysts are widely used for AHE, but suffer from low room-temperature conversion (<50%) and over-hydrogenation at higher temperatures. High reaction temperatures, typically required for Pd, Cu, and Ni-based catalysts, lead to high energy consumption and undesired byproducts like ethane, green oil, and coke, reducing catalyst lifetime. Various strategies have been explored to mitigate over-hydrogenation, including creating atomically dispersed Pd sites, forming bimetallic alloys, site poisoning/covering (as in Lindlar catalysts), and weakening ethylene adsorption through doping with p-block elements. However, improvements in selectivity often come at the cost of activity, creating a trade-off. While catalysts like Pd1Au1@MOF and Ni-S/C have shown promise, they still face challenges related to high reaction temperatures, low reaction rates, or poor stability. Therefore, developing a catalyst with high activity, selectivity, and stability for AC-AHE remains a significant challenge due to the complex interplay between these factors.

The existing literature extensively explores various catalytic systems for acetylene hydrogenation to ethylene. Studies have focused on modifying Pd catalysts to enhance selectivity and activity. Bimetallic alloys incorporating Pd with metals like Ag, Cu, In, Au, and Ga have been investigated to tune the electronic properties and surface interactions, thereby influencing the selectivity towards ethylene. The use of supports such as metal oxides (e.g., Al2O3) and carbon materials has also been explored to influence catalyst dispersion and activity. Furthermore, the creation of single-atom catalysts (SACs) with atomically dispersed Pd sites has emerged as a promising strategy for enhancing selectivity by controlling the coordination environment of the active Pd sites. However, the challenge of achieving high activity and stability at ambient conditions while maintaining high ethylene selectivity remains a focus of ongoing research. The existing literature reveals a complex interplay between activity, selectivity, and stability, making the development of efficient and robust catalysts for ambient-condition acetylene hydrogenation a significant challenge. This work aims to address this challenge by exploring a novel catalytic system.

The study employed a combination of experimental and theoretical methods. The Pd/WS2 catalysts were synthesized via wet impregnation with varying Pd loadings (0.2 wt%, 0.8 wt%, 1.1 wt%, and 2.2 wt%). Reference catalysts, including Pd loaded on WO3 (0.7 Pd/WO3) and a conventional Pd1Ag3/Al2O3 catalyst, were also prepared. Inductively coupled plasma optical emission spectrometry (ICP-OES) determined the actual Pd content. Catalytic performance was evaluated in a fixed-bed flow reactor under atmospheric pressure and room temperature (25 °C). The feed gas consisted of 1% acetylene, 20% hydrogen, balanced with helium. Product analysis was performed using gas chromatography. Extensive characterization techniques were utilized to investigate the catalysts' structure and electronic properties. These included X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) mapping, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic-resolution electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray absorption near-edge structure (XANES) spectroscopy. H-D exchange experiments were conducted to assess H2 dissociation activity. Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) to investigate the reaction mechanisms and adsorption properties on different Pd sites on the WS2 surface. Three models were built representing single-atom Pd on W, S, and hollow sites. The calculations focused on adsorption free energies of H2 and C2H2, activation energies for rate-limiting steps, and the reaction pathways for ethylene formation and over-hydrogenation to ethane.

The 0.8 Pd/WS2 catalyst demonstrated superior performance, achieving over 99% acetylene conversion and 70% ethylene selectivity at 25 °C with a GHSV of 360,000 mL gcat⁻¹ h⁻¹. The space-time yield of ethylene (1123 molC2H4 molPd⁻¹ h⁻¹) was nearly four times higher than that of the Pd1Ag3/Al2O3 catalyst. Remarkable stability was observed, with no significant change in performance after 500 h at a GHSV of 480,000 mL gcat⁻¹ h⁻¹. Characterizations revealed that Pd atoms were atomically dispersed on WS2, predominantly occupying W sites coordinated by S atoms. EXAFS and XPS data confirmed the dominance of Pd-S coordination and the presence of electron-deficient Pdδ+ species. The in-situ XPS demonstrated a redshift of 1.0 eV for 0.8 Pd/WS2, indicating that the presence of the neighboring S atom is more difficult to reduce due to the confinement effect of S atoms around Pd atoms. The post-reduction XPS spectra reveal three species: Pd⁺ (0.82) at 336.3 eV, Pd²⁺ at 337.3 eV and Pd⁰ at 335.5 eV. The 0.8 Pd/WS2 and 0.2 Pd/WS2 catalysts contain over 50% Pd⁺ species, whereas the 2.2 Pd/WS2 and 0.7 Pd/WO3 catalyst are composed of over 76% Pd⁰ species and less than 6% Pd⁺ species. This indicates that the 0.8 Pd/WS2 is composed of a larger amount of positively charged Pd than the 2.2 Pd/WS2 catalyst. H-D exchange experiments showed higher H2 dissociation activity for Pd/WS2 catalysts compared to WO3. DFT calculations supported the experimental findings, showing that the Pd/W site exhibited lower activation energies for C2H2 hydrogenation and favored C2H4 desorption over over-hydrogenation. The Pd-S coordination created electron-deficient Pdδ+ sites, enhancing C2H2 adsorption and weakening C2H4 adsorption, leading to the high selectivity for C2H4 production.

The results demonstrate the exceptional performance of the S-confined atomic Pd species on WS2 for AC-AHE. The high activity and selectivity are attributed to the unique electronic structure of the Pdδ+ species, which arises from the Pd-S coordination. This configuration promotes C2H2 adsorption and H2 dissociation while weakening C2H4 adsorption, thus favoring ethylene formation and inhibiting over-hydrogenation. The superior stability is likely due to the strong interaction between Pd and the WS2 support, preventing Pd aggregation and deactivation. The findings offer valuable insights into catalyst design for AC-AHE, highlighting the importance of controlling the atomic-level structure and electronic properties of the active sites. The achieved performance significantly advances the potential for sustainable and energy-efficient ethylene production.

This study presents a highly efficient catalyst for ambient-condition acetylene hydrogenation to ethylene. The 0.8 Pd/WS2 catalyst exhibits superior activity, selectivity, and stability, outperforming other Pd-based catalysts. The unique Pdδ+ sites, arising from Pd-S coordination, are responsible for the enhanced performance. This work offers a promising pathway for developing sustainable and energy-efficient ethylene production technologies. Future studies could focus on optimizing the catalyst synthesis and exploring the potential for scaling up the process for industrial applications. Investigating the long-term stability under industrial relevant conditions, including the presence of impurities or higher reactant concentrations, would also be beneficial.

While the catalyst shows excellent performance under the tested conditions, further studies are needed to evaluate its robustness under more diverse and challenging conditions, such as those encountered in industrial settings. The study focused on a specific set of reaction parameters, and investigating the impact of variations in temperature, pressure, and gas composition on catalytic performance could provide a more comprehensive understanding. The long-term stability test was conducted at a higher GHSV, but real-world industrial applications may require even more stringent testing over longer durations. The DFT calculations provided valuable mechanistic insights, but the accuracy of these simulations depends on the chosen models and approximations, and the experimental observations could provide more detailed insights.