Date Published: May 04, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Jingshan S. Du, Ting Bian, Junjie Yu, Yingying Jiang, Xiaowei Wang, Yucong Yan, Yi Jiang, Chuanhong Jin, Hui Zhang, Deren Yang.
Ultrafine Pt nanoparticles loaded on ceria (CeO2) are promising nanostructured catalysts for many important reactions. However, such catalysts often suffer from thermal instability due to coarsening of Pt nanoparticles at elevated temperatures, especially for those with high Pt loading, which leads to severe deterioration of catalytic performances. Here, a facile strategy is developed to improve the thermal stability of ultrafine (1–2 nm)‐Pt/CeO2 catalysts with high Pt content (≈14 wt%) by partially embedding Pt nanoparticles at the surface of CeO2 through the redox reaction at the solid–solution interface. Ex situ heating studies demonstrate the significant increase in thermal stability of such embedded nanostructures compared to the conventional loaded catalysts. The microscopic pathways for interparticle coarsening of Pt embedded or loaded on CeO2 are further investigated by in situ electron microscopy at elevated temperatures. Their morphology and size evolution with heating temperature indicate that migration and coalescence of Pt nanoparticles are remarkably suppressed in the embedded structure up to about 450 °C, which may account for the improved thermal stability compared to the conventional loaded structure.
Noble‐metal catalysts, such as Pt‐based nanostructures, play a crucial role in many important industrial processes due to their fascinating catalytic properties. However, the whole industry and research efforts have been fighting with the sintering effect of the highly active nanoparticles (NPs) for more than decades because aggregation and coarsening significantly lowers the active area of catalysts. It is therefore necessary to load or anchor metal NPs on thermally stable supports in an aim to physically separate and stabilize the active sites.1 Among the various types of materials, ceria (CeO2) has been particularly recognized as a very important support for Pt due to its high oxygen storage capacity2 and the strong metal–support interaction (SMSI) effect between them.3, 4 As such, loading Pt NPs to nanostructured CeO2 has attracted considerable research interests in the past decades, including the approaches in the organic phase,5 aqueous solution routes,6, 7, 8 and vapor depositions.9, 10
The surface‐embedded Pt/CeO2 hybrid nanostructures were synthesized by titrating an aqueous solution containing Ce(NO3)3 and cetyltrimethylammonium bromide (CTAB) with NaOH solution using a syringe pump at 70 °C under Ar protection, with K2PtCl4 solution being injected halfway. The hybrid nanostructure shows a rod‐like shape with dense and small NPs of 1–2 nm in size anchored on the surface, as revealed by transmission electron microscopy (TEM) (Figures S1 and S3A, Supporting Information) and aberration‐corrected high‐angle annular dark‐field scanning TEM (HAADF‐STEM) images (Figure1A). These nanorods are composed of fluorite‐type CeO2, as revealed by selected area electron diffraction (SAED) in Figure S2 (Supporting Information). Energy dispersive X‐ray spectroscopy (EDX) mapping and high‐resolution transmission electron microscopy (HRTEM) images indicate that Pt NPs are dispersed uniformly on the surface of CeO2 nanorods (Figure 1B), with the lattice fringes of Pt and CeO2 being clearly revealed (Figure 1C). Inductively coupled plasma mass spectrometry (ICP‐MS) shows that the Pt loading reaches ≈14 wt%, which is higher than many thermally stable Pt/CeO2 and other related catalysts in the literatures (see Table S1 in the Supporting Information for the comparison, most of their Pt loading values are lower than 10 wt%).
In summary, we have demonstrated the synthesis of ultrafine and high content Pt NPs partially embedded at the surface of CeO2 nanorods through an S–S interfacial redox reaction. Such embedded nanostructures showed much higher thermal stability against the sintering process relative to the conventional ones with ultrafine Pt NPs loaded on CeO2, thereby exhibiting the significantly enhanced properties toward hydrogenation of p‐nitrophenol after heat treatment. The key to endow the embedded nanostructures with high thermal stability in a broad temperature range is the suppression of particle migration and coalescence due to the unique architecture, which was clearly revealed by in situ heating experiments with electron microscopy. This work offers new insights to the design and understanding of sinter‐resistant catalysts based on metal NPs embedded at oxide surfaces. One may further apply this synthesis strategy to other materials systems involving similar valence‐changing oxide supports and noble‐metal NPs,25, 26, 27, 45, 46 depending on the specific target reactions. Taken together, this report will lead to an enhanced understanding of both the design rules and the mechanistic pathways for sinter‐resistant hybrid nanocatalysts.
Chemicals: Cerium nitrate (Ce(NO3)3·6H2O, 99.95% metals basis), cetyl trimethylammonium bromide (CTAB, 99%), and sodium borohydride (NaBH4, 98%) were supplied by Aladdin Chemicals. Potassium tetrachloroplatinate (K2PtCl4, 99.99% trace metals basis) was supplied by Aldrich. PVP (MW ≈ 40 000) was supplied by Sigma‐Aldrich. Sodium hydroxide (NaOH, analytical reagent) was supplied by Sinopharm Chemical Reagent. These chemicals were used without further purification. Ultrapure water was produced by a Millipore Synergy water purification system with a resistivity of 18.2 MΩ cm (25 °C).
The authors declare no conflict of interest.