Fe₃O₄@Carbon Microsphere Supported Ag–Au Bimetallic Nanocrystals with the Enhanced Catalytic Activity and Selectivity for the Reduction of Nitroaromatic Compounds

The heterostructure Ag–Au bimetallic nanocrystals supported on Fe 3 O 4 @carbon composite microspheres were synthesized by one facile and controllable approach, wherein the Ag nanocrystals attached on the Fe 3 O 4 @carbon microspheres were prepared first and served as reductant for the galvanic replacement reaction with the Au precursor (HAuCl 4 ). Upon varying the feeding amounts of the Au precursor, the bimetallic compositions on the Fe 3 O 4 @carbon microsphere could be readily tuned resulting in a series of composite microspheres with different Au-to-Ag molar ratios. Subsequently, we thus investigated the catalytic activity and selectivity of the magnetic composite catalysts from two sides. First, 4-nitrophenol (4-NP) was applied as a model molecule to study the effect of different Au-to-Ag molar ratios on catalytic capabilities of the resulting composite microspheres. It was found that upon the addition of NaBH 4 the catalytic capability was markedly enhanced when the Au content was increased. The maximum activity parameter value reached 1580 s –1 g –1, which is far higher than those of known monometallic composites. Also, they could give the equally high yields for other nitroaromatic compounds with various substituents, irrespective of the linked electron-donating or electron-withdrawing groups. Second, the synergistic effects of the carbon substrate in the catalysis reaction were demonstrated. When compared with colloidal SiO 2, TiO 2, and poly(styrene- co -acrylic acid) substrates, the carbon support not only facilitated the enhancement of the catalytic performance of the noble metal nanocrystals but was also more suitable for the in situ preparation of Au–Ag bimetallic nanocrystals using the GRR. Besides, the particles’ convenience in terms of their magnetic separability and outstanding reusability was validated through many successive reduction reaction cycles. In light of these unique characteristics, the Fe 3 O 4 @C@Ag–Au composite microspheres show promising and great potential for practical applications.