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: Oxidative coupling of methane (OCM) is a chemical reaction that directly converts methane and oxygen, the main components of natural gas, into higher-value hydrocarbons such as ethane and ethylene. However, the reaction presents a fundamental selectivity challenge.
The C–H bonds in methane are highly stable and difficult to activate at low temperatures. Furthermore, as ethane and ethylene are more reactive than methane, they readily overoxidize to carbon oxides (COx). Conventional catalysts used for OCM require operating temperatures near 800°C (1,472°F) and can undergo substantial deactivation during prolonged operation, hindering widespread industrial adoption.
While scientists have tested a wide range of catalysts, simultaneously achieving low-temperature activity, high product yields and long-term stability remains a critical hurdle. To address these challenges, researchers at Institute of Science Tokyo (Science Tokyo), Japan, have developed lanthanoid-based high-entropy oxide (HEO) catalysts, materials in which five or more metallic elements are incorporated into a common crystal structure. The research team, led by Professor Keigo Kamata from the Materials and Structures Laboratory, Institute of Integrated Research, Science Tokyo, and Assistant Professor Keiju Wachi from the Department of Applied Chemistry, School of Engineering, The University of Tokyo, Japan, demonstrated that HEOs combining lanthanoid elements exhibit exceptional low-temperature activity and long-term stability during OCM.
Their study was published online in the Journal of the American Chemical Society. The researchers screened 55 HEO compositions spanning five crystal structures and 24 elements. "As a result of the screening, lanthanoid-containing systems emerged as promising candidates, with C-type rare-earth HEOs combining relatively high C2 yields with structural stability," says Kamata.
Seven HEOs were synthesized as nanoparticle catalysts using a sol–gel method developed in-house. HEOs featuring five homogeneously integrated elements were synthesized by calcining amorphous precursors prepared from metal acetates and aspartic acid at 750–800°C (1,382–1,472°F). Further screening led to the selection of HEO-2, composed of lanthanum (La), samarium (Sm), europium (Eu), gadolinium (Gd) and dysprosium (Dy), (LaSmEuGdDy)0.4O3, as a representative catalyst for detailed investigation.
In catalytic tests, HEO-2 initiated C2 hydrocarbon formation at 525°C (977°F), a temperature significantly lower than those required by previously investigated catalysts. At 600°C (1,112°F), HEO-2 reached a 12.3% C2 yield. The researchers linked this low-temperature activity to the catalyst's surface basicity.
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