Origin of the Boosting Effect of Polyoxometalates in Photocatalysis: The Case of CO₂ Reduction by a Rh-Containing Metal–Organic Framework

Abstract

The immobilization of polyoxometalates (POMs) near catalytic centers of metal–organic frameworks (MOFs) has been reported as an advantageous strategy to boost their photocatalytic activity toward strategic reactions such as CO2 reduction (CO2RR) or hydrogen evolution (HER), although the reasons for such enhancement are still poorly understood. Unveiling the role of POM guests in the reaction mechanisms is therefore a key step toward the development of the next generation of multicomponent catalytic materials with optimal photocatalytic performances. Here, we elucidate the remarkable role of encapsulated [PW12O40]3– (PW 12 ) polyoxometalates in boosting the photocatalytic activity of the Rh-functionalized UiO-67 MOF toward CO2RR and HER by combining theoretical density functional theory and microkinetic modeling approaches with experimental photophysical and spectroscopic techniques. First, we characterized in detail the reaction mechanism for CO2RR and HER catalyzed by the PW 12 -containing Rh-functionalized MOF, using [Ru­(bpy)3]2+ as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial electron donor in acetonitrile. Our results reveal that the encapsulated POMs act as efficient electron reservoirs, which quench [Ru­(bpy)3]+the photogenerated reduced form of the PSand transfer the electrons to the Rh catalytic sites of the MOF. Notably, this is shown to favor the regeneration of the oxidized PS over its unproductive degradation, boosting the turnover numbers of the photocatalytic system. Such a mechanism can explain not only the higher formate and H2 product yields in the POM-containing catalyst but also the experimentally observed higher impact on the HER pathway than that on the CO2RR one, as the source of protons is generated in the reductive quenching of the photoexcited PS by TEOA. Finally, our computational exploration was extended to a whole variety of POMs, which allowed establishing relationships between their redox potentials and the activity of the related POM-containing catalytic materials. The optimal activity is reached when both the ability of the POM to accept electrons and that of its reduced form to reduce the Rh catalyst are simultaneously maximized, leading to a volcano plot whereby POMs with a moderate redox potential display the highest impact on photocatalytic performances.

Electronic Structure Manipulation via Site-Selective Atomically Dispersed Ni for Efficient Photocatalytic CO2 Reduction

“…On B-Ni 1 /WO 2.72 , the free energies (Δ G ) of *CO 2 , *COOH, and *CO intermediates are integrally decreased compared to those on S-Ni 1 /WO 2.72 (Figure c). The conversion of *CO 2 to *COOH is usually considered to be the RDS in CO 2 reduction to CO product. , As can be seen from Figure c, in contrast to pristine WO 2.72 with a RDS energy barrier as high as 1.45 eV, the RDS energy barriers for B-Ni 1 /WO 2.72 and S-Ni 1 /WO 2.72 are reduced to 1.35 and 0.97 eV, respectively, which suggests that atomically dispersed Ni sites in B-Ni 1 /WO 2.72 are more conducive for *CO 2 hydrogenation to yield *COOH. − A reasonable explanation due to the electron gain from the subsurface Ni atoms, the bulk-doped Ni single atoms would lose fewer electrons (0.07 e or 0.25 e) than the surface-anchored Ni atoms. This not only indicates that the higher electron density of Ni atoms in B-Ni 1 /WO 2.72 enhances the binding strength to *CO 2 and activates the protonation of CO 2 molecules to form *COOH but indirectly verifies the existence of an electron-transport channel. − In brief, the bulk-doped Ni atoms are endowed with higher electron concentration and an upshifted d -band center in contrast to the surface-anchored ones, thereby enhancing the adsorption of reaction intermediates and decreasing the energy barriers for CO 2 reduction to targeted CO product.…”

Polyoxometalate-based frameworks for photocatalysis and photothermal catalysis

“…(PW 12 , RhCp*)@UiO-67 as was used a model catalyst to investigate its photocatalytic activity for CO 2 RR and HER in acetonitrile with [Ru(bpy) 3 ] 2+ as the photosensitizer (PS) and triethanolamine (TEOA) as a sacrificial electron donor by combining theoretical density functional theory and microscopic kinetic modeling approaches with experimental photophysics and spectroscopic techniques. 72 The results show that PW 12 encapsulated in MOFs acts as an electron reservoir, accepting an electron from the photogenerated reduced form [Ru(bpy) 3 ] + and transferring the electron to the catalytic site Rh (Fig. 3a).…”

Photoelectroactive metal–organic frameworks

“…3d). 58 The encapsulation of guest species, such as metal NPs/clusters, 98,109,154,156,157 metal oxide/sulde/selenide, 56,[158][159][160] QDs, 57,161 g-C 3 N 4 , 94,162,163 graphene, 59 and POMs, 164 in host MOFs to prepare hybrid composites for photocatalytic CO 2 reduction reactions has also been widely explored. For instance, the encapsulation of Pt NPs inside NH 2 -UiO-68, which was constructed from ZrCl 4 and amino-terphenyl-4,4 ′′ -dicarboxylic acid (NH 2 -tpdc), produced CO from CO 2 with a rate of 400.2 mmol g −1 in the presence of TEOA under visible light irradiation (400-780 nm).…”