Rational Design and Improvement of Cu-based catalysts in CO Oxidation

Carbon monoxide is one of the most hazardous pollutants in automotive gas exhaust emissions due to its severe impact on the human body and environment. There are many methods for CO removal, including adsorption, methanation, and catalytic oxidation. Catalyst oxidation has been considered the most efficient technique for CO removal. Although CO oxidation has received extensive attention in past decades, achieving high activity and stability at both engine working and cold starting temperatures is still challenging. Noble metal catalysts generally exhibit excellent catalytic activity in high-temperature regions. However, it still suffers from several obstacles, such as over-absorption of CO in low-temperature regions for Pt-based catalysts. Therefore, researchers still focus on seeking alternative candidates for noble metals due to their high cost and low availability, promising non-noble metals including manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) receive increasing attention due to their high catalytic activity and stability. Many forms of catalysts have been studied exclusively, such as metal catalysts, metal oxide catalysts, supported catalysts, zeolite, and carbon-based catalysts. Supported catalysts with available metal surface area and unique metal-support interfacial perimeter play pivotal roles in heterogeneous catalysis across various industrial applications. Depending on the size of supported active metal, supported metal catalysts can be categorized into particle, cluster, and single-atom catalysts. Among these, single-atom catalysts (SACs) with relatively specific active structures offer prominent advantages in optimizing catalytic activity and product selectivity, leading to an increasing interest in this research area. In recent years, the catalytic performance of SACs has been largely improved through some reported methods including adjusting coordination number, doping heterogeneous atoms, modulating support anchoring sites, and so on. Despite these advancements, it has always been ignored that with the change of the catalyst synthetic process as well as the metal-support interaction (MSI), supported active sites may appear at different positions in catalyst supports, especially at surface or subsurface, thus exhibiting distinct different catalytic behaviour with surrounding molecules. However, the isolated metal site-related location effect is very difficult to deeply explore, because the complexity of catalyst synthesis, combined with the absence of a metal atom location descriptor, poses significant obstacles to achieving precise control over the location of active metal. Herein, we first proposed an electronic metal-support-carbon interaction (EMSCI), which provides a complete picture of the mass and electron flow and expands on the traditional electronic metal-support interactions (EMSI) concept. Furthermore, we reported an exception of EMSI where the interaction between support and metal is not necessary to achieve a high catalytic activity in the CO oxidation reaction, especially in low-temperature regions. The reducibility of CeO2 is investigated by Ce L3 and M4,5 edge NEXAFS, it is confirmed that CeO2 cannot be reduced even under the reductive conditions. Moreover, the location-dependent Cu species have been investigated which are formed during the hydrothermal process using both ex situ and in situ X-ray techniques. The CO oxidation activity shows a positive relation to the percentage of Cu(CO)+ species detected during the reaction. Such behaviour resembles the intrinsic catalytic activity of a true Cu(CO)+ single site, in which the support is completely inactive. This unique phenomenon provides a new scope of understanding metal support interaction and a pathway to optimizing single-atom catalyst performance and catalyst design.