Highly Efficient Transition Metal Nanoparticle Catalysts in Aqueous Solutions

Trace Thiol Moieties in the Ligand Layer Induce Superior Catalytic Gold Nanoclusters

We here show that the typical poison of thiols, if below a certain level, promotes rather than suppresses the catalytic activity of gold nanoclusters (AuNCs). A few thiol groups functionalized hyperbranched polyethylenimine (PEI, Mn = 2000 Da) patched on a mesoporous polymeric bead aid the direct synthesis of AuNCs. The nucleation efficiency of AuNC is 93-fold favored at a level of 2 thiols per PEI (0.04 equiv of the amino units) than that by neat PEI, and AuNCs (1.3 nm) are obtained up to a gold load of 6.3% on the support. Unexpectedly, the catalytic activity of AuNCs is favored by the thiol up to 2 thiols per PEI, as evaluated from the surface-normalized rate constant of the model reaction of 4-nitrophenol-reduction. The catalytic promotion by thiols probably stems from optimized electron density on AuNC. If the residual NH groups of PEI were further fully treated with glycidyltrimethylammonium chloride, the catalytic activity is again enhanced, where the accelerated mass transfer is responsible for the promotion. Overall, the catalytic activity reaches an unprecedented value (metal-normalized rate constant kc = 29.4 L mmol-1 s-1 and turnover frequency = 1623 h-1, as evaluated with the model reaction of 4-nitrophenol reduction) ever reported for supported AuNCs. Our results suggest that orthogonal ligand optimization is an effective manner of triggering the release of the catalytic potential of AuNCs, among which thiol is unique.

Electronic properties and adsorption mechanism of Ru-doped copper clusters towards CH3OH molecule: A DFT investigation

In this study, we have investigated the stability and electronic properties of the Cu_nRu (n = 2-10) nanoclusters and their interaction with the CH₃OH molecule without and with the presence of O₂ molecule by using DFT calculations with TPSS/SDD/6-311g(d,p) level of theory. Based on the second energy difference (Δ²E), the results reveal that the Cu_nRu (n = 4, 6 and 8) clusters are relatively more stable than their neighboring clusters. The values obtained for the Fukui function (f^-) proves that the Ru atom in the Cu_nRu clusters is an excellent adsorption site for the molecules. The interaction of the Cu_nRu clusters with CH₃OH molecule exhibits that the Ru atom is the preferred adsorption site for the CH₃OH molecule, where the O atom of the CH₃OH molecule is strongly chemisorbed onto the Ru site of the clusters, forming a strong bond between the Ru and O atoms. The copper sites of the clusters were found less preferred for the adsorption of CH₃OH, and the complexes formed between both species are less stable than those obtained from the CH₃OH chemisorption over the Ru site of the clusters. The interaction of CH₃OH with the clusters was also evaluated in an oxidizing environment, and the results obtained reveal that the molecule is greatly chemisorbed over the ruthenium site with adsorption energies which vary from - 1.18 to - 2.05 eV. In the presence of the oxygen, the gap energy of the clusters was sharply changed after their interactions with the CH₃OH molecule, suggesting that these clusters can easily detect the above molecule with great sensitivity. Therefore, the presence of the oxygen not only does not prevent the adsorption process, but it considerably promotes the CH₃OH chemisorption onto the ruthenium site of the clusters and therefore significantly rises their sensitivity performance. In conclusion, the Cu_nRu clusters could be employed as effective nanosensors for the CH₃OH molecule detection.

Ultrafast Ambient-Air Exsolution on Metal Oxide via Momentary Photothermal Effect

The process of exsolution for the synthesis of strongly anchored metal nanoparticles (NPs) on host oxide lattices has been proposed as a promising strategy for designing robust catalyst-support composite systems. However, because conventional exsolution processes occur in harsh reducing environments at high temperatures for long periods of time, the choice of support materials and dopant metals are limited to those with inherently high thermal and chemical stability. Herein, we report the exsolution of a series of noble metal catalysts (Pt, Rh, and Ir) from metal oxide nanofibers (WO₃ NFs) supports in an entirely ambient environment induced by intense pulsed light (IPL)-derived momentary photothermal treatment (>1000 °C). Since the exsolution process spans an extremely short period of time (<20 ms), unwanted structural artifacts such as decreased surface area and phase transition of the support materials are effectively suppressed. At the same time, exsolved NPs (<5 nm) with uniform size distributions could successfully be formed. To prove the practical utility of exsolved catalytic NPs functionalized on WO₃ NFs, the chemiresistive gas sensing characteristics of exsolved Pt-decorated WO₃ NFs were analyzed, exhibiting high durability (>200 cyclic exposures), enhanced response (R_air/R_gas > 800 @ 1 ppm/350 °C), and selectivity toward H₂S target gas. Altogether, we successfully demonstrated that ultrafast exsolution within a few milliseconds could be induced in ambient conditions using the IPL-derived momentary photothermal treatment and contributed to expanding the practical viability of the exsolution-based synthetic approaches for the production of highly stable catalyst systems.

Self-Assembly of Poly(Janus particle)s into Unimolecular and Oligomeric Spherical Micelles

Using shape-persistent Janus particles to construct poly(Janus particle)s and studying their self-assembly behaviors are of great interest, but remain largely unexplored. In this work, we reported a type of amphiphiles constructed by the ring-opening metathesis polymerization of nonspherical molecular Janus particles (APOSS-BPOSS), called poly(Janus particle)s (poly(APOSS-BPOSS)_n, n = 12, 17, 22, and 35, and M_n = 35-100 kg/mol). Unlike traditional bottlebrush polymers consisting of flexible side chains, these poly(Janus particles) consist of rigid hydrophilic and hydrophobic polyhedral oligomeric silsesquioxane (POSS) cages as side chains. Interestingly, instead of maintaining an expected extended chain conformation, they could also collapse and then self-assemble to form unconventional unimolecular or oligomeric spherical micelles in solutions with a feature size smaller than 7 nm. More importantly, unlike traditional amphiphilic polymer brushes that could form unimolecular micelles at a relatively high degree of polymerization by self-assembly, these poly(Janus particles)s could accomplish self-assembly at a quite low degree of polymerization because of their unique chemical structure and molecular topology. The formation of unimolecular and oligomeric micelles was also further confirmed by dissipative particle dynamics simulations. This study of introducing the POSS-based poly(Janus particle)s as a class of shape amphiphiles will provide a model system for generating unimolecular and oligomeric micellar nanostructures through solution self-assembly.

Regulating the size and spatial distribution of Pd nanoparticles supported by the defect engineered metal–organic framework HKUST-1 and applied in the aerobic oxidation of cinnamyl alcohol

A series of novel Pd@DE-HKUST-1(Cu/Pd) catalysts with different pydc feeding ratios were successfully synthesized. The size regime and the spatial distribution of the Pd NPs can be controlled by the amount of framework incorporated pydc.