Does the Encapsulation Strategy of Pt Nanoparticles with Carbon Layers Really Ensure Both Highly Active and Durable Electrocatalysis in Fuel Cells?

Platinum is a key component of commercialized proton exchange membrane fuel cells (PEMFCs) to lower the energy cost of the sluggish oxygen reduction reaction (ORR) at the cathode. Beyond the significant advances in improving its initial activity, securing catalytic durability is the next challenge for the successful implementation of PEMFCs. Encapsulation of Pt nanoparticles (NPs) with thin carbon or silica layers has recently been highlighted as a promising strategy for alleviating Pt degradation. However, unexpectedly similar or occasionally even better catalytic activity on site-blocked Pt NPs has raised fundamental interest about the nature of their catalytic sites and the origin of the prolonged durability. Herein, to answer these questions, we investigate the ORR and methanol oxidation reaction activities of carbon-encapsulated Pt NP (C@Pt/C) catalysts. By controlling the robustness of the carbon shells synthetically and electrochemically, directly exposed Pt sites, at which facile transport of reactant/product molecules occurs via the loosely packed or highly defective carbon layers, are identified as the main catalytic sites. Interestingly, online differential electrochemical mass spectroscopy and inductively coupled plasma-mass spectrometry coupled to electrochemical flow cells verify a trade-off relationship between the activity and stability of the catalysts. In addition to their role as a physical barrier for prohibiting the dissolution and agglomeration of the Pt NPs, the carbon shell acts as a sacrificial agent, questioning the practical legitimacy of the strategy for achieving both high activity and durability

Catalytic Interplay of Ga, Pt, and Ce on the Alumina Surface Enabling High Activity, Selectivity, and Stability in Propane Dehydrogenation

Pt-based bimetallic catalysts have been widely investigated in propane dehydrogenation (PDH) owing to their high activity in C–H cleavage and propylene selectivity. However, upon repeated coke oxidation for catalyst regeneration, they suffer from significant metal sintering and dealloying. Recently, γ-Al2O3 doped with Ga, Pt, and Ce was reported to exhibit superior catalytic activity, selectivity, and stability in PDH, but the catalytic role of each element has not been clearly understood because of the complexity of this system. In this study, we rigorously investigated the reaction mechanism and catalytic interplay of each component (Ga, Pt, and Ce). Selective poisoning, in situ diffuse reflectance infrared Fourier transform spectroscopy, and H2–D2 exchange revealed that Ga3+ is responsible for the heterolytic dissociation of the C–H bond of propane, while Pt0 facilitates the sluggish H recombination into H2 via reverse spillover. Catalyst deactivation during repeated reaction–regeneration cycles is mainly due to the irreversible sintering of Pt0. Notably, optimal Ce doping (∼2 wt %) selectively generated atomically dispersed Ce3+ sites on the γ-Al2O3 surface, which greatly suppressed the sintering of Pt0 particles by increasing the metal–support interactions. In contrast, excessive Ce loading generated discrete CeO2 domains, which stabilized the Pt species in the form of Pt2+ inactive for H recombination. Thus, excessive Ce loading led to an even more severe loss of catalytic activity and selectivity. The present results demonstrate that the selective generation of atomically dispersed Ce3+ on the γ-Al2O3 surface is important for stabilizing Pt0 species, which is essential for simultaneously achieving high catalytic activity, selectivity, and longevity in PDH

Significant Roles of Carbon Pore and Surface Structure in AuPd/C Catalyst for Achieving High Chemoselectivity in Direct Hydrogen Peroxide Synthesis

Direct synthesis of hydrogen peroxide (H₂O₂) from hydrogen (H₂) and oxygen (O₂) has been widely investigated as an attractive way for small-scale/on-site H₂O₂ production. Among various catalysts, carbon-supported AuPd catalysts have been reported to exhibit the most promising H₂O₂ productivity and selectivity. In this work, to better understand the catalytic role of the surface properties and porous structures of the carbon supports, we systematically investigated AuPd catalysts supported on various nanostructured carbons including activated carbon, carbon nanotube, carbon black, and ordered mesoporous carbons. The results showed that a high density of oxygen functional groups on the carbon surface was essential for synthesizing highly dispersed bimetallic catalysts with effective AuPd alloying, which is a prerequisite for achieving high H₂O₂ selectivity. Regarding porous structure, a solely mesoporous carbon support was superior to microporous ones. Microporous carbons such as activated carbon suffered from diffusion limitation, leading to significantly slower H₂ conversion than mesoporous catalysts. Furthermore, H₂O₂ produced from AuPd catalyst in the micropores was more prone to subsequent disproportionation/hydrogenation into H₂O due to retarded diffusion of the H₂O₂ out of the microporous structure, which led to decreased H₂O₂ selectivity. The present study showed that solely mesoporous carbons with high surface oxygen content are most desirable as a support for AuPd catalyst in order to achieve high H₂O₂ productivity and selectivity

Hydrogen Peroxide Synthesis via Enhanced Two-Electron Oxygen Reduction Pathway on Carbon-Coated Pt Surface

Continuous on-site electrochemical production of hydrogen peroxide (H₂O₂) can provide an attractive alternative to the present anthraquinone-based H₂O₂ production technology. A major challenge in the electrocatalyst design for H₂O₂ production is that O₂ adsorption on the Pt surface thermodynamically favors “side-on” configuration over “end-on” configuration, which leads to a dissociation of O–O bond via dominant 4-electron pathway. This prefers H₂O production rather than H₂O₂ production during the electrochemical oxygen reduction reaction (ORR). In the present work, we demonstrate that controlled coating of Pt catalysts with amorphous carbon layers can induce selective end-on adsorption of O₂ on the Pt surface by eliminating accessible Pt ensemble sites, which allows significantly enhanced H₂O₂ production selectivity in the ORR. Experimental results and theoretical modeling reveal that 4-electron pathway is strongly suppressed in the course of ORR due to a thermodynamically unfavored end-on adsorption of O₂ (the first electron transfer step) with 0.54 V overpotential. As a result, the carbon-coated Pt catalysts show an onset potential of ∼0.7 V for ORR and remarkably enhanced H₂O₂ selectivity up to 41%. Notably, the produced H₂O₂ cannot access the Pt surface due to the steric hindrance of the coated carbon layers, and thus no significant H₂O₂ decomposition via disproportionation/reduction reactions is observed. Furthermore, the catalyst shows superior stability without considerable performance degradation because the amorphous carbon layers protect Pt catalysts against the leaching and ripening in acidic operating conditions