4-Nitrophenol reduction catalysed by Au-Ag bimetallic nanoparticles supported on LDH: Homogeneous vs. heterogeneous catalysis

Bimetallic nanoparticles, which are composed of two metal elements in a particle, exhibit much higher catalytic activity than respective monometallic ones due to new bi-functional or synergistic effects, so-called a ligand and/or an ensemble effect. In this work, a novel homogeneous as well as heterogeneous Au-Ag bimetallic nano catalyst have been synthesized and supported on layer double hydroxide (LDH) by a simple wet chemical process. The support of LDH has reduced the size as well as capped the bimetallic nanoparticles and hence prevents the agglomeration. The surface morphology and chemical composition of these bimetallic nanoparticles were examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. It was found that alloy (40-60 nm) and core-shell (15-30 nm) Au-Ag bimetallic nanoparticles were formed for heterogeneous and homogeneous composites, respectively. Moreover, the different compositions of as-synthesized Au-Ag bimetallic nanoparticles were utilized for the catalytic reduction of 4-nitrophenol. The highest activity was observed for the Au-Ag nanoparticles prepared with Au-Ag ratio at 1:3 and the activity became 10 to 45 times higher than that of original monometallic Au or Ag nanoparticles. The rate kinetics was studied for both homogeneous and heterogeneous system on the reduction of 4-nitrophenol and observed that the rate of reduction was greater in case of homogeneous catalysts as compared to heterogeneous catalysts. Also, the heterogeneous catalysts were effortlessly recovered and reused (up to 5 cycles) after completion of the reduction reaction.