Direct Observation of Heterogeneous Surface Reactivity and Reconstruction on Terminations of Grain Boundaries of Platinum

Materials defects are very important for enhancing the catalytic functions and applications. However, the surface defects of materials are usually diverse, and their catalytic activity is generally measured at the averaging level. How to directly measure/observe the catalytic activity of the single defective site is extremely important for the rational design of highly efficient catalysts; however, it remains a grand challenge. Herein, we directly observe the reactivity and simultaneously surface reconstructions of the single defective site by "storing"catalytic trajectories and collectively presenting reactivity profiles on solid surfaces via in situ transmission electron microscopy using a thermally catalyzed graphitic layer growth model reaction on terminations of grain boundaries (GBs) of platinum. The direct in situ observation results for single defective sites reveal that the surface reactivity decreases in the order of concave terminations of high-angle GBs > concave terminations of low-angle GBs > roughened edge boundaries > flat surfaces. In particular, we find that the heterogeneous reconstructions appear the surface-smoothening on high-angle GBs, while the surface-roughening on low-angle GBs and edge boundaries, which is rationalized by two competitive processes: the release of excessive strain energy and the adsorption-induced step formation. Comprehensively, the concave terminations of low-angle GBs and the roughened edge boundaries represent promising catalytic surface defects with a fine balance between reactivity and stability. The DFT calculations result reveals a novel rhombohedral Volcano-type Zebra-crossing plot for the structure-activity relation regarding the improved reactivity by strained defect sites, different from a conventional Volcano-type plot in catalysis studies. We expect the current in situ method, direct observation of catalytic roles of surface defects, and their in situ restructuring would assist the design and synthesis of more nanocatalysts in the future.