Pits confined in ultrathin cerium (IV) oxide for understanding catalytic centers in carbon monoxide oxidation
 2015-01-12  Font Size:[ Large Medium Small ]

Prof. Yi Xie’s group from Hefei National Laboratory for Physical Sciences at the Microscale (HFNL) in the University of Science and Technology of China (USTC) has recently achieved new progress in the field of atomically-thin two-dimensional nanosheets. The researchers reported the synthesis of freestanding 3-atomically-thin CeO2 sheets with ca. 20% surface pits occupancy via an “ultrafast open space transformation” strategy and also investigated the semi-quantifying relationship of model-structure-performance in CO catalytic oxidation, opening the door for designing catalytic centers. This work has been published in Nature Communications with the title of "Pits confined in ultrathin cerium (IV) oxide for understanding catalytic centers in carbon monoxide oxidation".
As is well-known, catalysis can strongly accelerate the speed of chemical reactions through changing the reaction pathway and lowering the activation energy, which enables its wide applications in many fields such as the industrial ammonia production in Haber process, the catalytic converters in automobiles and the enzymatic catalytic processes in living systems. Although many catalytic kinetics such as Langmuir-Hinshelwood mechanism, the Eley-Rideal mechanism and the Mars-Van Krevelen mechanism have been proposed to investigate the catalytic reaction processes, the atomic-level insights into the underlying mechanism of catalysis is still an open question. This is mainly due to the large differences between idealized models and real catalysts, and hence the knowledge gained from these models could not be directly applicable to the real catalysts. To deeply evaluate where catalysis occurs and which site has the highest activity, simplifying and bridging the catalyst model with the real catalyst is of paramount importance. In this work, taking the typical CO oxidation catalyst of CeO2 as an example, the researchers initially develop an ideal model consisting of intact atomically-thin CeO2 sheets to maximize the number of coordinately-unsaturated active sites. To further promote the activity of the catalytic sites, the researchers further make lots of artificial pits with lower coordination numbers on the surface of atomically-thin CeO2 sheets. Collaborated with Prof. Shiqiang Wei in National Synchrotron Radiation Laboratory, they applied synchrotron radiation XAFS to resolve the fine structure of the-obtained pits-confined atomically-thin CeO2 sheets, intact atomically-thin CeO2 sheets and bulk CeO2, respectively. XAFS results directly evidence the lower average coordination number of 4.6 for the abundant pit-surrounding Ce sites, much smaller than the 6.5 and 8 for the Ce atoms in the intact atomically-thin CeO2 sheets and bulk CeO2. Density-functional calculations demonstrate that the 4- and 5-coordinated pit-surrounding Ce sites perform their respective functions in CO adsorption and O2 activation, thus reducing the activation barrier and decreasing the catalyst poisoning chance. Also, the highly coordination-unsaturated pit-surrounding Ce sites bring about the increased hole carrier density and hence assure fast CO diffusion along the two-dimensional conducting channel of surface pits. As a result, the pits-confined atomically-thin CeO2 sheets lowered the apparent activation energy from 122.9 to 61.7 kJ mol-1 and definitely decreased the CO conversion temperatures. This work provides crucial insights into the role of active centers in catalysis through first semi-quantifying the model-structure-performance relationship, opening the door for designing catalytic centers.

Schematic illustration for deeply evaluating where catalysis occurs and which site has the highest activity. (a) bulk CeO2; (b) intact and (c) pits-confined atomically-thin CeO2 sheets; P1 and P2 stand for the representative pit-surrounding surface Ce sites, while P3 represents the bottom Ce site of the pit; S stands for the surface Ce site of atomically-thin sheet and around the defects of bulk material; A, V and B represent the arris, vertex and surface Ce sites of bulk material, respectively.



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