Key Takeaways & Executive Findings
- •• • Zn SAs@GO achieves an 82.5% reduction in Zn dosage while increasing crosslinking density by 6.6% compared to commercial ZnO, directly addressing EU Aquatic Acute 1 hazard classification and raw material cost pressures. • • The Zn–O bond length of 2.03 Å in Zn SAs@GO confers moderate binding energy, enabling controlled release of Zn2+ ions that accelerate vulcanization kinetics and improve crosslink network formation. • • Homogeneous atomic dispersion of Zn on GO yields near-100% atom utilization efficiency, eliminating the aggregation and surface-energy limitations of nano-ZnO and ensuring consistent vulcanization performance. • • The Zn SAs@GO/NR composites exhibit faster vulcanization rates and enhanced mechanical properties, demonstrating a scalable route for industrial rubber production with reduced environmental footprint.
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Abstract
Reducing zinc oxide (ZnO) consumption in rubber vulcanization is imperative due to cost and environmental toxicity, yet lowering ZnO inevitably compromises crosslinking density and mechanical properties. This study introduces Zn single-atoms anchored on graphene oxide (Zn SAs@GO) as controlled-release catalysts for efficient natural rubber (NR) vulcanization. X-ray absorption spectroscopy (XAS) reveals a Zn–O bond length of 2.03 Å with moderate binding energy, enabling controlled release of Zn2+ ions that participate in vulcanization with enhanced activity. The homogeneous dispersion of Zn atoms ensures near-complete utilization, resulting in a 6.6% increase in crosslinking density compared to commercial ZnO while reducing Zn dosage by 82.5%. The Zn SAs@GO/NR composites exhibit faster vulcanization rates and superior mechanical and physical properties relative to ZnO nanoparticles and crystalline ZnO controls. This work demonstrates a scalable preparation of single-atom catalysts and provides a viable pathway for industrial rubber manufacturing with significantly reduced zinc content.
1. Introduction
Commercial rubber vulcanization relies on ZnO as the primary activator, but its low affinity for rubber and the fact that only surface Zn2+ participates in the reaction necessitate high loadings. This inefficiency, combined with ZnO's classification as Aquatic Acute 1 (very toxic to aquatic life with long-lasting effects), imposes significant environmental and economic burdens. Regulatory pressures and raw material costs demand a drastic reduction in zinc usage, yet conventional approaches such as nano-ZnO suffer from aggregation due to high surface energy, and organic modification adds process complexity without resolving the fundamental underutilization of zinc.
Single-atom catalysts (SACs) offer a transformative solution by maximizing metal utilization and tailoring coordination environments. This study presents Zn single-atoms supported on graphene oxide (Zn SAs@GO) synthesized via a facile room-temperature hydrolysis method. The Zn–O–C anchoring creates a moderate Zn–O bond (2.03 Å) that facilitates controlled release of Zn2+ during vulcanization. By homogeneously dispersing Zn atoms, the catalyst achieves an 82.5% reduction in Zn dosage while increasing crosslinking density by 6.6% relative to commercial ZnO, thereby overcoming the trade-off between zinc reduction and mechanical performance.
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ZHANG Qi, YU Ge, GAO Xiaoping, LI Peng, LIN Ze, LI Ruilong, WU Yuen (2025). Single-atom zinc controlled-release catalysts for efficient vulcanization of rubber. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3472-2
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Frequently Asked Questions
What is the exact Zn–O bond length in Zn SAs@GO and how does it influence vulcanization kinetics?
XAS analysis determines a Zn–O bond length of 2.03 Å, which is longer than that in crystalline ZnO, resulting in weaker Zn–O binding. This moderate binding energy allows controlled release of Zn2+ ions, which readily react with accelerators to form active Zn(II) complexes, thereby accelerating vulcanization and increasing crosslinking density by 6.6% compared to commercial ZnO.
How does the crosslinking density of Zn SAs@GO/NR compare to that of ZnO NPs@GO and C-ZnO/GO composites?
Zn SAs@GO/NR exhibits a 6.6% higher crosslinking density than C-ZnO/GO at 82.5% lower Zn dosage. Compared to ZnO NPs@GO, the single-atom catalyst also shows superior crosslinking density due to homogeneous atomic dispersion and near-100% atom utilization, which eliminates aggregation and ensures full participation of Zn in the vulcanization network formation.
What are the mechanical property improvements observed in Zn SAs@GO/NR composites?
The mechanical and physical properties of Zn SAs@GO/NR composites are improved relative to ZnO-based composites, as stated in the conclusions. Although specific numerical values for tensile strength or elongation at break are not provided in the extracted text, the increase in crosslinking density directly correlates with enhanced mechanical performance, validating the efficacy of the single-atom approach.
Can Zn SAs@GO be produced at industrial scale, and what are the cost implications?
The synthesis employs a facile room-temperature hydrolysis method, which is amenable to large-scale preparation. The 82.5% reduction in Zn dosage significantly lowers raw material costs and mitigates environmental pollution, potentially offsetting the cost of graphene oxide support. This work contributes to the development of scalable SAC production for industrial rubber manufacturing.
What is the mechanism by which Zn SAs@GO achieves controlled release of Zn2+ during vulcanization?
The Zn atoms are anchored to GO via Zn–O–C bonds with a Zn–O bond length of 2.03 Å, which is weaker than in crystalline ZnO. This moderate binding energy enables the Zn2+ ions to leach out under vulcanization conditions and participate in the reaction, providing a controlled-release profile that maintains high catalytic activity throughout the crosslinking process.
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