Ceria is a support known for its influence on reaction rates of supported metals mostly through providing reactive oxygen species or by influencing the oxidation state of the metals. For this reason, the performance and associated mechanisms of ceria-supported metals have been intensively studied. Yet, structural changes caused by ceria and reaction conditions to supported metal have been recognized only recently, and their effects on catalytic performance need to be assessed using appropriate tools and catalytic studies. In this talk, we demonstrate how novel information can be obtained using preformed colloidal metals in combination with ceria support for the hydrogenation of CO2. Catalytic CO2 reduction to fuels and chemicals is one of the major pursuits in reducing greenhouse gas emissions. The reaction at atmospheric pressure mainly yields CH4 via methanation and CO via the reverse water gas shift (RWGS) reaction. Historically, it has been known that ruthenium (Ru) is an active and stable methanation catalyst, and that CeO2-supported Ru is more active for this reaction compared to Ru/Al2O3 since ceria provides a lower-energy pathway for activating CO2. However, we recently showed that ceria-supported Ru may become very selective for the RWGS reaction instead when the catalyst is pretreated in oxygen. We will discuss how this catalytic behavior can be rationalized by understanding structural changes occurring to the supported Ru particles, which convert into atomic species that are no more active for methanation. We will also explain how this behavior is associated with the peculiar properties of ceria as a support. In additional work, we also studied in detail the effect of ceria structure and Ru weight loading on the structural changes of these catalysts. We demonstrate that ceria structure is important to stabilize atomically dispersed species, and how the presence of mixed phases (atomic species and particles) leads to exclusive methanation performance, thus partially answering questions related to the reactivity of materials containing a mixture of atomic and particle species. Overall, this talk will highlight how the use of colloidal phase can help describe and characterize catalysts where structural changes happening under reaction conditions determine their final catalytic activity, and in particular how ceria can determine important changes that make it such a unique catalytic support.

Low temperature restructuring of CeO2 supported Ru nanoparticles determines selectivity in CO2 catalytic reduction

2019 Fall National Meeting

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