Investigation and Development of Advanced Pd-Based and Non-Precious Metal-Based Catalysts for Acetylene Semi-Hydrogenation

The semi-hydrogenation of acetylene plays a crucial role in industrial polymer-grade ethylene production. This is due to the fact that even minute traces of acetylene in the crude ethylene stream can lead to the poisoning of downstream Ziegler-Natta polymerization catalysts. While effective, its sluggish kinetics require operation under harsh conditions (100 – 250 ℃, > 5 bar) and the utilization of costly precious metal (predominantly Pd) catalysts, leading to considerable capital and energy expenses. Furthermore, state-of-the-art Pd-based catalysts encounter issues such as over-hydrogenation and oligomerization of hydrogenated products, resulting in the formation of undesired ethane and green oil. In response to these challenges and to align with the demands of the polymer industry, three distinct approaches were proposed and explored to develop advanced catalysts for acetylene semi-hydrogenation, aiming for improved activity and selectivity.

Our initial approach involves the meticulous design and synthesis of a series of metal/metal oxide core/shell structures. After treatment in hydrogen at elevated temperatures, yolk/shell structures, denoted as M/FeOx-H (M = Pd/Pt/Au), exhibited strong metal-metal oxide interactions. The oxide shell is hypothesized to restrict the expansion of the Pd core and inhibit subsurface hydride formation, thereby mitigating ethane generation. Simultaneously, the porous FeOx shell could enable the exposure of active Pd sites to reactants. A series of in situ and ex situ characterizations were conducted, including X-ray absorption spectroscopy and scanning transmission electron microscopy, to examine the dynamic behavior of metal-oxide interactions during hydrogen treatment. The characterization results revealed that after high-temperature reductive treatment, Pd exhibits strong interaction with FeOx, characterized by the formation of Pd single atoms within the FeOx matrix and enhanced Pd-Fe bonding. Conversely, Pt undergoes transformation into ordered PtFe intermetallics and Pt single atoms upon immediate coating with FeOx. In contrast, Au does not manifest strong bonding with FeOx. Among the M/FeOx-H catalysts, Pd/FeOx-H demonstrated the most superior catalytic performance in acetylene semi-hydrogenation tests. Notably, under a 50 sccm flow of 0.5% acetylene and 3% H2 in argon, it achieved complete acetylene conversion and an 86.5% ethylene selectivity at 60 ℃.

The second strategy involves optimizing the adsorption strength of C2Hx species on the catalyst surface. To achieve this, Pd-Cu nanocubes with a Cu core and an ordered B2 intermetallic CuPd shell, featuring controllable atomic layers on the surface (Cu/B2 CuPd), were designed and synthesized. This structural motif, resembling single-atom alloys, facilitates the creation of isolated Pd sites on the nanocube surface by Cu. In the catalytic tests, the Cu/B2 CuPd catalyst achieved complete acetylene conversion and 95.2% ethylene selectivity at 90 ℃ under a 50 sccm flow of 0.5% acetylene and 3% H2 in argon. Even under more demanding ethylene-rich conditions (acetylene : ethylene = 1 : 20), the Cu/B2 CuPd catalyst maintained an impressive selectivity of almost 90% at 80 ℃. The integrated experimental and computational investigations further elucidated that the single-atom alloy motif on the surface of Cu/B2 CuPd catalysts induces a mild π-bonding of C2H4, promoting its desorption rather than over-hydrogenation.

Motivated by the demand for green hydrogen, we also investigated the electrocatalytic pathways of acetylene semi-hydrogenation, enabling the reaction between acetylene and water. We conducted a screening test involving five representative metal nanoparticle catalysts, encompassing coinage metals, precious metals, and post-transition metals, namely, Cu, Ag, Au, Pd, and Bi. These catalysts were deliberately synthesized within the size range of 7 to 14 nm, and testing was conducted in a flow-cell setup with a 10 sccm feed of 5% acetylene in argon. Our analysis unveiled significant selectivity towards ethylene for both Cu and Ag nanoparticles, with Cu displaying a Faradaic efficiency of 84.4% and Ag showing a Faradaic efficiency of 84.9% for ethylene at -0.6 V vs. reversible hydrogen electrode (RHE). Particularly, Cu nanoparticles exhibited higher intrinsic activity, with an electrochemically active surface area-corrected partial current density of -0.23 mA cm-2 compared to -0.16 mA cm-2 for Ag nanoparticles. Subsequently, we compared the performance of various Cu electrocatalysts, including 7 nm, 11 nm, and 40 nm Cu nanoparticles, alongside 45 nm Cu nanocubes. An intriguing finding was the positive correlation observed between the size of Cu electrocatalysts and their activity. Notably, the 45 nm Cu nanocubes exhibited outstanding performance among all the investigated Cu electrocatalysts, achieving a Faradaic efficiency of 82.0% and an acetylene conversion of 51.1% at -0.7 V vs. RHE. Stability evaluations were conducted on Cu nanocubes via 12-hour chronoamperometry measurements. In general, a 20.0% decline in current density and a 2.4% decrease in Faradaic efficiency were observed for Cu nanocubes. Transmission electron microscopy results revealed structural deformation in the Cu nanocubes. Future research efforts will prioritize enhancing the stability of Cu-based electrocatalysts while concurrently improving the activity of non-Cu-based counterparts, such as Ag.