For example, in 2017, Wu et al. demonstrated a novel spectroscopic approach to observe chemical transformations at nanoscale resolution up to 25 nm, enabling scientists to identify the role of surface defects in activating industrially relevant catalytic routes. However, such high-spatialresolution mapping of catalytic reactions requires specialized expertise and dedicated equipment, which is certainly non-trivial and not readily available elsewhere. In general, defects are ubiquitous in solid catalysts, and this phenomenon is more prominent in materials at the nanoscale owing to the high surface energy, leading to defect structures to be more easily formed during materials' synthesis or modification. Such defects of the crystal structure can be readily classified based on their spatial dimensions (D), i.e., point defects (0D), line defects (1D), interfacial or planar defects (2D), and bulk defects (3D). 0D defects (e.g., vacancy, substitutional, interstitial, and antisite defects, depending on their characteristics) are generally referred to as point defects as they only found in every direction in atomic scale throughout the catalysts. 1D defects or line defects represent defects in which the atom arrangement within the crystal structure is periodically deviating from the ideal lattice structure in a line area, which can be classified as edge or spiral dislocations, kinks, steps, and so forth. 2D defects are commonly found when "mistakes" in the stacking of subsequent planes with respect to the former formed planes occur during crystal growth. These 2D defects form a transitional region that breaks the periodicity of ordered atoms arrangement, including grain boundary, stacking fault, and crystal twinning. Bulk defects are 3D macroscopic defects with relatively large scale, indicating voids, pores, impurities, and precipitations in the crystal. On an interesting note, defects can also be classified (other than the spatial dimension classification) into stoichiometric defects (usually of thermal causes such as Frenkel and Schottky defects), non-stoichiometric defects, electronic defects, and doping defects depending on the formation causes.