Highly Durable Shell Formation on Rh for Increased Amount of Metal-Support Interfaces from Enhanced Surface Defect Sites by Fe Doping on CeO₂

Catalyst architecture by control of structure, size, composition, and dispersion can tune the interaction between metal and support, which highly differentiate the activation and reaction route of the catalyst. As support defect sites play a key role in structure manipulation, several strategies for site control successfully improved catalytic activity and selectivity. However, defective surfaces triggered catalyst deactivation from metal aggregation, leaching, or surface area loss. Therefore, surface defect control accompanied with high durability is essential. One of the most striking methods to improve catalytic durability is strong metal-support interaction (SMSI). SMSI was mainly reported for noble metal nanoparticles covered with partially reduced oxide shells after the reduction process. The main feature of SMSI is a migration of unstable oxide with substoichiometric surface oxygen concentration to metal nanoparticles, forming thin overlayers. Accordingly, surface defect manipulation can largely affect SMSI. The effects of SMSI include suppressing aggregation of particles by spatial isolation in shell and generation of new reaction sites like metal-support interfaces. The classic route to trigger SMSI is reduction treatment as flowing H₂ at a high temperature can eject surface oxygen, forming surface defect sites. However, some of the classic shells after reduction pretreatment were formed with high crystallinity and impermeable to gas species, leading to damaged catalytic activity. Also, such overlayers had durability issues from partial elimination in a humid atmosphere and pre-sintering of metal particles before shell formation. Therefore, the ideal features of the covering shell are (1) oxide shell needs to be thin and permeable to gaseous species not to suppress the catalytic activity, (2) overlayer should fully encapsulate the metal particles to prevent aggregation, and (3) have high durability in the reaction environment.<br/>Herein, I tuned the typical CeO₂ support surface to induce SMSI. Fe doping into CeO₂ (FC) highly facilitated surface oxygen transfer and increased the amount of surface oxygen defect sites. Well dispersed Fe sites on CeO₂ were examined with high-resolution XRD and TEM images. Enhanced surface properties and defect sites after Fe doping were proved with H₂-TPR, Raman spectra, and XPS results. 1 wt % of Rh was deposited on FC support and Rh particles were encapsulated under overlayer after reduction as SMSI was facilitated on FC. Shell formation with changed physical properties was analyzed with XAFS results. Highly metallic Rh particles were formed after encapsulation, confirmed with XANES result. The oxidation state of these particles was even more metallic than Rh particles on CeO₂. New type bonding between oxygen sites on shell and Rh particle was shown on EXAFS result, further confirming the encapsulation. Shell structure generated lots of Rh-FC interfaces, where the adsorption strength of CO on Rh was highly weakened. Thermal CO₂ reduction to CO with high activity and selectivity was conducted using the catalyst. As FC shell had gas permeability, reactant gases came through the shell and reached the Rh-FC interfaces. Both at low and high temperatures, 100 % CO selectivity was achieved. The FC shell showed generality, as only CO was produced even using other metal species(Pt, Ru, Ir, Pd). The final structure showed high durability, showing no degradation after 150 hours reaction and 3 times of recycling tests. This investigation proposes that surface defect control of catalyst support is another promising strategy for highly durable shell construction with a large amount of metal-support interfaces, and the catalyst showed a broad potential for the application of green gases.