Fundamental Understanding of Palladium-Based Catalysts: From Pyrene-Based COFs to Size-Controlled Bimetallic Nanostructures

Abstract

The catalytic properties of single atoms (SAs) have attracted growing attention due to their unique electronic characteristics and their ability to maximize atomic efficiency compared to nanoparticles. As particle size decreases, the coordination environment around the metal atoms becomes increasingly unsaturated, which raises the surface free energy and enhances the reactivity of the metal species toward supports and adsorbates—this underpins the well-known size effects in metal nano-catalysts. In the extreme case of single-atom catalysts (SACs), the combination of highly active valence electrons, quantum confinement effects, and discrete energy levels leads to maximized surface free energy and distinct chemical behavior. Covalent organic frameworks (COFs), a class of porous and crystalline materials composed of light elements (H, B, C, N, and O), are attractive supports for SACs due to their designable periodic structures and stable covalent bonding. These frameworks offer uniform and tunable binding sites ideal for anchoring metal atoms. Therefore, Single metal atoms anchored on COFs offer a powerful platform for tailoring active sites, enabling the optimization of catalytic activity, selectivity, and stability, with promising potential for use in a wide range of industrial chemical processes. Ethylene hydrogenation has long been used as a model reaction in heterogeneous catalysis to better understand the mechanism of selective acetylene hydrogenation over metal catalysts. Improving selectivity often depends on how strongly the reactants adsorb onto the metal surface. When ethylene binds more weakly to the active sites, selectivity improves— an insight that has led to growing interest in designing catalysts with isolated single metal atoms. Pd single-atom catalysts are highly valued for their activity in ethylene and acetylene hydrogenation under ambient conditions. In this work, we employed PdCl2-functionalized, pyrene-based COFs that provide uniform binding sites ideal for stabilizing isolated palladium atoms. In this study, we used X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a probe molecule along with catalytic performance evaluation through ethylene hydrogenation kinetics. However, the pyrene-based COF inherently incorporates palladium impurities during synthesis, and the observed catalytic activity of pristine COF can vary significantly depending on the level of these residual Pd species. This makes it difficult to distinguish between the intrinsic activity of the pristine COF and that of atomically dispersed Pd. To reduce these impurities, we implemented two monomer purification methods prior to COF synthesis: triphenylphosphine (PPh3) treatment and acid column purification. These approaches lowered the Pd content from 0.35% to 0.23% and 0.04%, respectively. PPh3 purification was not optimal, as it introduced structural defects in the COF, leading to higher catalytic activity compared to the non-purified COF. Conversely, acid column purification preserved the Pd and N coordination environment, revealing distinct activity differences between the 4% Pd-loaded sample and the pristine COF. Notably, even at a reduced 1% Pd loading— critical for stability under reaction conditions—a significantly higher activity than that of the pristine COF was observed. This work highlights the critical role of purification and its underlying mechanisms in shaping the coordination environment and structural integrity of COFs, which in turn significantly impacts their catalytic activity and selectivity in ethylene and acetylene hydrogenation. Bimetallic catalysts often outperform their monometallic counterparts and are widely used in essential chemical transformations such as selective hydrogenation, reforming, coupling, and oxidation. Among them, PdAu alloy catalysts have garnered particular interest due to their ability to undergo dynamic surface restructuring, which plays a critical role in tuning catalytic activity and selectivity. The extent of this restructuring is influenced by factors such as the Pd-to-Au ratio, nanoparticle size, and the nature of adsorbates under reaction conditions. This study investigates how surface structures in Au, Pd, and PdAu nanoparticles evolve with variations in composition, particle size, and temperature. Results show that larger and smaller nanoparticles tend to form Au rich surface. Notably, increasing the temperature from cryogenic levels induces Pd migration from the core to the surface, facilitating the formation of Pd trimers. These structural changes are reversible, with Pd atoms re-segregating back into the bulk upon cooling, highlighting the temperature-sensitive and reversible nature of surface restructuring in PdAu catalysts.