Defining the loading of single-atom catalysts: weight fraction or atomic fraction?

Single-atom catalysts (SACs) with the utmost atom-utilization capability are recently considered as the new frontier in catalysis. Theoretically, each isolated metal atom in SACs constitutes an active site during the catalysis. The single metal loading in SACs plays a significant role in tuning the catalytic activity as each isolated atom in SACs can function as a catalytically active point. Many studies calculated SA in weight fraction (wt%), however, defining SACs loading in wt% is less accurate to represent the number of active sites, due to the difference in the atomic mass between the SA and support, especially for heavy metal atoms and light elements. Compared to the wt%, atomic percentage (at%) can be more representative of the loading of SAs.