Hexagonal boron nitride (hBN) has emerged as a promising inert support material for metal nanoparticles in oxidative catalysis due to its unique structural and electronic properties. Unlike traditional oxide supports such as alumina (Al₂O₃) or ceria (CeO₂), hBN offers exceptional thermal stability, chemical inertness, and resistance to sintering, making it an ideal candidate for high-temperature catalytic applications. This article explores the role of hBN in stabilizing metal nanoparticles, its electronic interactions with metals like gold (Au) and palladium (Pd), and its performance in oxidative reactions such as CO oxidation and selective alcohol oxidation. A comparison with conventional oxide supports highlights the advantages and limitations of hBN in industrial catalysis.
hBN is a layered material with a structure analogous to graphite, consisting of alternating boron and nitrogen atoms arranged in a hexagonal lattice. The strong covalent bonding within the layers and weak van der Waals interactions between them contribute to its high thermal stability, with decomposition temperatures exceeding 1000°C in inert atmospheres. This property is critical for catalytic applications where thermal degradation of the support can lead to deactivation. Unlike oxide supports, which may undergo phase transitions or sintering at elevated temperatures, hBN maintains its structural integrity, preventing the aggregation of metal nanoparticles. Studies have shown that Au and Pd nanoparticles supported on hBN exhibit minimal sintering even after prolonged exposure to temperatures above 500°C, whereas the same metals on Al₂O₃ or CeO₂ show significant particle growth under similar conditions.
The inert nature of hBN also plays a crucial role in preserving the catalytic activity of metal nanoparticles. Oxide supports often participate in redox reactions, leading to strong metal-support interactions (SMSI) that can alter the electronic state of the metal. While SMSI can enhance activity in some cases, it may also lead to undesirable side reactions or deactivation. In contrast, hBN’s lack of reactive surface sites minimizes electronic perturbations of the metal nanoparticles, allowing them to retain their intrinsic catalytic properties. For example, Pd nanoparticles on hBN demonstrate higher selectivity in alcohol oxidation reactions compared to those on CeO₂, where over-oxidation to undesired byproducts is more prevalent. The absence of acidic or basic sites on hBN further reduces the likelihood of side reactions, making it particularly suitable for selective oxidation processes.
Electronic interactions between hBN and metal nanoparticles, though weaker than those with oxide supports, still influence catalytic performance. The work function of hBN (approximately 4.5 eV) is lower than that of metals like Au (5.1 eV) and Pd (5.3 eV), leading to electron transfer from the support to the metal. This electron donation can slightly modify the d-band center of the metal, affecting its adsorption properties and reactivity. In CO oxidation, for instance, Au/hBN catalysts exhibit higher activity than Au/Al₂O₃ due to the weaker CO adsorption strength on electron-rich Au nanoparticles, facilitating oxygen activation. However, the effect is less pronounced than with reducible oxides like CeO₂, where oxygen vacancies play a significant role in catalytic cycles.
Industrial applicability of hBN-supported catalysts depends on their performance under realistic conditions. The high thermal stability and sintering resistance of hBN make it attractive for processes requiring long-term operation at elevated temperatures, such as automotive exhaust treatment or chemical manufacturing. However, challenges remain in scaling up hBN synthesis and achieving uniform dispersion of metal nanoparticles. While solution-phase exfoliation and chemical vapor deposition (CVD) methods can produce high-quality hBN, cost-effective large-scale production is still under development. In contrast, oxide supports like Al₂O₃ are widely available and inexpensive, but their lower stability limits their use in high-temperature applications.
Comparisons between hBN and oxide supports reveal trade-offs in catalytic design. Al₂O₃ provides strong metal anchoring sites and tunable acidity, beneficial for reactions requiring Lewis acid catalysis. CeO₂’s oxygen storage capacity enhances redox reactions, making it ideal for CO oxidation and water-gas shift reactions. However, both oxides suffer from sintering and phase instability under harsh conditions. hBN, while inert and stable, lacks the synergistic effects seen with reducible oxides, necessitating careful optimization of metal nanoparticle size and composition to achieve comparable activity. Recent advances in hybrid supports, such as hBN-coated oxides, aim to combine the benefits of both materials.
In summary, hBN’s exceptional thermal stability, chemical inertness, and resistance to sintering make it a compelling support for metal nanoparticles in oxidative catalysis. Its weak electronic interactions with metals preserve their intrinsic activity and selectivity, offering advantages over traditional oxide supports in high-temperature applications. While challenges in large-scale production and nanoparticle dispersion remain, ongoing research into hBN-based catalysts holds promise for industrial adoption, particularly in processes requiring durability and selectivity. The development of hybrid supports may further bridge the gap between the inertness of hBN and the reactivity of oxides, unlocking new possibilities in catalytic design.