Engineering active sites in a controllable manner plays a critical role in developing catalysts with desired catalytic performance. A reliable and tunable molecular engineering strategy is presented to boost the oxygen reduction reaction (ORR) performance of single-atom catalysts (SACs) by manipulating the steric hindrance effect of metalloporphyrins. The results demonstrate that the metalloporphyrin molecules with four tert-butylphenyl groups as steric hindrance moieties can be used to prepare various SACs with targeted active sites on conductive carbon black (CB). It is found that the single iron catalyst on CB prepared using tert-butylphenyl iron-porphyrins as a precursor (denoted as t-Fe-800/CB) exhibits significantly enhanced ORR performance compared with the Fe-800/CB catalyst prepared from unsubstituted iron-porphyrin. Moreover, the t-Fe-800/CB catalyst exhibits superior ORR performance relative to t-Mn-800/CB and t-Co-800/CB with different metal centers, indicating that the intrinsic ORR activity originates from single Fe sites. The remarkable ORR properties are mainly attributed to the enhanced intrinsic activity and density of Fe active sites, as well as improved conductivity and mass transfer induced by the steric hindrance effect. The optimized t-Fe-800/CB catalyst also delivers impressive performance in both flexible and aqueous Zn–air batteries. This study offers a new perspective for the development of advanced SAC electrocatalysts for energy conversion applications.

The transition from a linear economy to a circular carbon economy urgently requires sustainable and efficient technologies for converting non-fossil biomass and waste plastics into fuels and high-value chemicals. Solar-driven photocatalytic technology has emerged as a promising strategy due to its mild reaction conditions and potential for selective transformation, which addresses the limitations of traditional recycling and conversion methods (e.g., high energy consumption, harsh conditions, and poor selectivity). However, current photocatalytic valorization systems still suffer from insufficient activity and selectivity, mainly due to the inability to precisely regulate reaction pathways. Considering that selective bond activation (especially C–H and C–C bond activation) is the key determinant, this review focuses on the photocatalytic valorization of biomass and plastics, classifies reaction pathways based on dominant bond selectivity, and mainly emphasizes the contrast between C–H and C–C bond activation. This classification approach overcomes the limitations of traditional substrate-based classification, providing new insights for the rational design of highly selective photocatalytic systems to realize the valorization of biomass and waste plastics.