Bimetallic Au-Pd Nanoparticles Confined in Tubular Mesoporous Carbon as Highly Selective and Reusable Benzyl Alcohol Oxidation Catalysts

摘要

Size-uniform and highly dispersed bimetallic AuPd nanoparticles were formed in situ and confined in tubular mesoporous carbon by successive incipient wetness impregnation and a thermal annealing method. The bimodal mesoporous carbon (CMK-5) encapsulated AuPd nanoparticles with 1 wt?% metal loading enables superior activity compared with mono-modal mesoporous carbon for benzyl alcohol oxidation. A conversion of >99?% and selectivity of >99?% can be reached within 3 h under mild conditions, for example, at 80?degrees C and at atmospheric pressure. It is found that during the catalyst preparation, the Au and Pd precursor impregnation sequence is a key factor to the formation of AuPd nanoparticles under identical conditions. A relatively high activity is realized by the first impregnation of the Au precursor, followed by the second impregnation of the Pd precursor. The crystalline structure and distribution of AuPd nanoparticles are characterized by high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), and XRD. It is found that a larger proportion of surface-exposed Pd atoms in AuPd nanoparticles have a positive effect on catalytic activity. The AuPd nanoparticles have a narrow size distribution that is concentrated at approximately 4 nm. TEM and N2 adsorption results reveal that the catalyst has a large surface area and well-developed bimodal pore interconnectivity, which contribute to its excellent activity. The used catalyst retained a high selectivity but conversion decreased with recycling. The deactivation mechanism was attributed to the tiny amount of incompletely removed benzaldehyde adsorbed onto the active surface, which blocked access from the active sites to benzyl alcohol. Importantly, the used catalyst can be recovered by a simple heat treatment at 200?degrees C in air after the catalytic cycles have completed; the adsorbed benzaldehyde is removed from the active surface. Hence, AuPd nanoparticles confined in tubular mesoporous carbon can be used as an alternative method to develop highly dispersed nanocatalysts to improve catalytic efficiency.