Metalloporphyrin-Based Metal–Organic Frameworks for Photocatalytic Carbon Dioxide Reduction: The Influence of Metal Centers

Abstract

Photocatalysis is one of the most promising technologies to achieve efficient carbon dioxide reduction reaction (CO2RR) under mild conditions. Herein, metalloporphyrin-based metal–organic frameworks (MOFs) with different metal centers, denoted as PCN-222, were utilized as visible-light photocatalysts for CO2 reduction. Due to the combination of the conjugated planar macrocyclic structures of metalloporphyrins and the stable porous structures of MOFs, all PCN-222 materials exhibited excellent light-harvesting and CO2-adsorbing abilities. Among the studied MOFs of varied metal centers (M = Pt, Fe, Cu, Zn, Mn), PCN-222(2H&Zn) exhibited the highest photocatalytic CO2RR performance, with an average CO yield of 3.92 μmol g−1 h−1 without any organic solvent or sacrificial agent. Furthermore, this was three and seven times higher than that of PCN-222(Zn) (1.36 μmol g−1 h−1) and PCN-222(2H) (0.557 μmol g−1 h−1). The superior photocatalytic activity of PCN-222(2H&Zn) was attributed to its effective photoexcited electron–hole separation and transportation compared with other PCN-222(2H&M) materials. The obtained results indicate that Zn ions in the porphyrin’s center played an important role in the reaction of active sites for the adsorption–activation of CO2. In addition, PCN-222(2H&Zn) showed the highest CO2 selectivity (almost 100%) and stability. This work provides a clear guide for the design of efficient photocatalysts.

Cu-Metalated Porphyrin-Based MOFs Coupled with Anatase as Photocatalysts for CO2 Reduction: The Effect of Metalation Proportion

“…Formation of a heterojunction in such a type of composites may improve charge handling properties [9,32,50,51]. The conduction band (CB) and valence band of both PCN-224 and PCN 222 have been reported to be less positive than that of anatase [18,23,32]. Therefore, taking into account that both TiO 2 and MOF are excited under artificial solar light irradiation and the tight interaction of the two parts in the composite as revealed by XPS, efficient charge separation may take place due to their matched band structure [18,23,32].…”

“…Among the different MOFs studied, of particular importance are stable structures that contain porphyrins as organic linkers. These structures offer the advantage of visible light absorption capacity [19,20], the ability for efficient and fast exciton migration and transportation from the photoexcited porphyrin moiety to suitable catalysts [21,22] and the presence of a metalation center [23] that may affect the electronic properties and control catalytic activity.…”

“…Porphyrinic zirconium MOFs have been studied in CO 2 photocatalytic reduction [14,[23][24][25]. The Zr ions in the metal clusters may have multiple redox states (i.e., 4+ or 3+) [23,25] and act as semiconductors while the photoexcited porphyrin linker can activate these clusters [25][26][27].…”

“…Porphyrinic zirconium MOFs have been studied in CO 2 photocatalytic reduction [14,[23][24][25]. The Zr ions in the metal clusters may have multiple redox states (i.e., 4+ or 3+) [23,25] and act as semiconductors while the photoexcited porphyrin linker can activate these clusters [25][26][27]. Heterojunctions have been developed via the coupling of such MOF structures with other photocatalysts [18,27], improving charge recombination phenomena.…”

“…Heterojunctions have been developed via the coupling of such MOF structures with other photocatalysts [18,27], improving charge recombination phenomena. In addition, metalation of the porphyrin linker as well as the degree of metalation (partial metalated porphyrins) may also affect the overall photocatalytic activity [19,23,[28][29][30].…”