Controlling Activity of Heterogeneous Cu Single‐Atom Catalysts by Fine‐Tuning the Redox Properties of CeO₂‐TiO₂ Supports

Abstract

Understanding and enhancing the activity of heterogeneous single-atom catalysts (SACs) are indispensable aspects to use them in industrial applications. Among these, low-temperature CO oxidation is considered as a model reaction for testing a wide-range of oxidation catalysts, and as an essential process for the exhaust aftertreatment in modern combustion engines. This study demonstrates that the activity of heterogeneous Cu/ CeO 2 À TiO 2 SAC catalysts can be fine-tuned by controlling the Ce/Ti ratio in CeO 2 À TiO 2 supports, with the highest catalytic activity achieved for a Ce/Ti molar ratio of 0.18. Based on DRIFT, EEL, and EPR spectroscopies, together with high-resolution electron microscopy and H 2 -TPR measurements, it is inferred that the optimized Ce/Ti ratio correlates with i) highest dispersion of separate CeO 2 particles on TiO 2 surface, ii) more facile reduction of Ce 4 + to Ce 3 + , and iii) enhanced formation of À Cu 2 + À OÀ Ce 4 + À .À Cu + À &À Ce 3 + À redox shuttles, which play a dual role for CO adsorption and O 2 -activation. These results extend the understanding of heterogeneous metal single-atom catalysts and constitute a robust approach for the rational control of their catalytic performance in technically relevant applications.

Mechanism and selectivity of MOF-supported Cu single-atom catalysts for preferential CO oxidation

“…11,12 Additionally, the use of MOFs as supports allows for loading higher density of isolated metal single sites compared to what can be achieved with typical catalyst supports, where limited loading in the latter case is essential to prevent sintering during reactions. [12][13][14][15] As an example, between 1.6 and 2 wt% of atomically dispersed Cu can be loaded into the UiO-66 framework, 16,17 compared to #0.05 wt% on CeO 2 -TiO 2 supports needed to ensure atomic dispersion of the loaded metal. 11,12,16 This enhances the gravimetric and volumetric density of these catalysts which is necessary for their practical applications, where size and mass of the reactor are decisive.…”

“…[12][13][14][15] As an example, between 1.6 and 2 wt% of atomically dispersed Cu can be loaded into the UiO-66 framework, 16,17 compared to #0.05 wt% on CeO 2 -TiO 2 supports needed to ensure atomic dispersion of the loaded metal. 11,12,16 This enhances the gravimetric and volumetric density of these catalysts which is necessary for their practical applications, where size and mass of the reactor are decisive. The tradeoff between the total catalyst-mass based activity and gravimetric efficiency in both cases should be optimized for possible technical applications.…”