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Extra gold makes for slower reactions

Electron microscopy images showing gold (red) and silver nanoparticles (green)

university of MANCHESTER Electron microscopy images showing gold (red) and silver nanoparticles (green)university of MANCHESTER

Pedro Henrique Cury Camargo, chemist at the USP Chemistry Institute, has discovered that catalyst materials made of gold and silver nanoparticles become less efficient as the proportion of gold in their composition increases. Catalysts are substances that accelerate chemical reactions, and some of them have industrial applications. Taken separately, gold is a stronger catalyst than silver. “Because of this, it was believed that catalytic activity would increase proportionally to the amount of gold,” Camargo explains. “But surprisingly, we observed the opposite effect.” To arrive at this result, the chemist synthesized nanostructures with different compositions, gradually increasing the proportion of gold up to a 34% gold, 66% silver alloy. At this ratio, the bimetallic nanostructures exhibited the lowest catalytic activity. The highest was observed at 18% gold to 82% silver. To understand why, Camargo sought collaboration with researcher Sarah Haigh from the University of Manchester, in England. “Using spectroscopy techniques in association with high-resolution electron microscopy, we were able to 3D-map the relative distribution of elements in the bimetallic nanoparticles,” he explains. The two researchers discovered that when the proportion of gold increases, this element moves toward the center of the nanoparticles, leaving silver on the surface – the most important region for catalysis to occur. But silver is a weaker catalyst, and therefore the bimetallic compound becomes less efficient. “When gold content is lowered to 18%, the surface will be occupied by a mix of the two metals, with a slightly higher proportion of gold,” Camargo explains. “So the nanostructures with this exact proportion are the most efficient.”