This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Dilute alloy catalysts could improve fuel, plastic and pharmaceutical production, but their instability under heat holds back widespread use. Researchers at University of Michigan Engineering built a stability rulebook for this class of materials, which are made of a tiny amount of an active dopant metal dispersed in an inert host metal.
A study outlining the rulebook was published in the Journal of the American Chemical Society. Dilute alloy catalysts could help industrial chemical reactions use less precious metal, extend catalyst lifetime and reduce unwanted byproducts. For conventional single-metal catalysts, strengthening bonds for one reaction step interferes with bonds in the next step, slowing the reaction.
Dilute alloy catalysts can break this constraint, known as linear scaling relationships, by distributing the reaction steps among different active sites. The active dopant atoms—making up about 1% or less of the metal content—help initiate the reaction by activating the reactants. The resulting intermediate products then spill over to the host metal, where the reaction continues.
While highly active and selective, surface dopant atoms are prone to dissolving into the interior of the host metal at high temperatures, deactivating the catalyst. "Industrial application of dilute alloy catalysts requires both good activity and good stability. While other works have mostly focused on the activity part, we tackle the stability aspect by identifying the variables that govern catalyst stability," said Suljo Linic, the Martin Lewis Perl Collegiate Professor of Chemical Engineering and corresponding author of the study.
The research team began its investigation by synthesizing a dilute alloy catalyst made of approximately 18-nanometer-wide gold nanoparticles dotted with platinum atoms. They tested the alloy in two reactions—ethylene hydrogenation, used in plastics manufacturing, and carbon monoxide oxidation, used in automobile emissions control. Each reaction was operated at temperatures ranging from 50°C (122°F) to 250°C (482°F) while a spectroscopy technique tracked surface platinum atoms in real time.
During ethylene hydrogenation, the reaction rate sharply dropped when temperatures rose past 100°C (212°F) but continued to increase with temperature during carbon monoxide oxidation. This suggested that adsorbates—molecules that stick to a solid surface—that bind more strongly to active dopant metals would improve stability. Weak-binding ethylene could not hold platinum at the surface, allowing entropy to drive platinum atoms inside the gold particle.
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