Ti‐Substituted Keggin‐Type Polyoxotungstate as Proton and Electron Reservoir Encaged into Metal–Organic Framework for Carbon Dioxide Photoreduction

Abstract

multiple electrons and protons, and the long-lived electron-proton coupled intermediates. This is of particular importance for the gas-solid reactions involving only water and CO 2 systems. Metal-organic frameworks (MOFs) are promising photocatalysts in CO 2 reduction due to the high CO 2 adsorption, large surface areas, open and orderly pores and channels, and remarkable structural and functional designability. [6][7][8][9] Nevertheless, MOFs are typically poor electron and proton conductors owing to the low overlap between the π orbits of organic ligand and the d orbitals of metal ion, as well as lacking of sufficient proton-active sites to receive and give protons in the pores of MOFs. To enhance electron activities of MOF photocatalysts, varieties of semiconductor or metal nanoparticles are encaged into or deposited on MOFs. [10][11][12][13] In contrast, a few attentions are put on improving proton activity in the CO 2 reduction process.Keggin-type polyoxometalates (POMs) are consist of well-defined metal-oxygen clusters with reversible multiple electron redox activities. [14][15][16][17] Therefore, they generally act as an electron reservoir in composite photocatalysts to promote electron-hole separation. [18][19][20] Keggin-type POMs, being as strong Brønsted acid and having quasispherical structure with abundant exterior oxygen atoms, are incorporated into the pores of MOFs to enhance proton transfer. [21][22][23] These suggest that integrating POMs into MOF hybrids might be of great potential in the proton-coupled electron CO 2 reduction reaction. In this vein, we choose keggin-type POM, [PTi 2 W 10 O 40 ] 7− (PTiW), [24] and MOF, HKUST-1, namely Cu 3 (BTC) 2 (BTC: benzene-1,3,5-tricarboxylate), the excellent CO 2 storage material, [25,26] as promising candidates. To improve visible-light absorption, Au nanoparticles (Au NPs) are loaded in PTiW-encaged HKUST-1. For clarity, we denote the hybrid material of PTiW encaged into HKUST-1 as NENU-10 and Auloaded NENU-10 as Au@NENU-10. Compared to the unsubstituted counterpart of [PW 12 O 40 ] 3− (PW12), Ti-substituted PTiW shows higher reduction activity and protonation ability due to the more negative charges and the stronger alkaline both on terminal oxygen of TiO and bridge oxygen of TiOW (Figure 1), [27,28] which provide driving force and active protons and electrons necessary for CO 2 photoreduction. Remarkably, A key challenge for photocatalystic CO 2 reduction is the design and synthesis of photocatalyst with remarkable performance in visible-light absorption, CO 2 adsorption, and electron-coupled proton transfer. Here a visible lightdriven hybrid photocatalyst Au@NENU-10, consisting of Au nanoparticles (NPs), Ti-substituted keggin-type polyoxometalate [PTi 2 W 10 O 40 ] 7− (PTiW), and HKUST-1, is synthesized by the one-pot method at atmosphere condition where PTiW acts as both electrons' and protons' reservoir, and a reactive active center is encaged into HKUST-1 to boost CO 2 reduction, HKUST-1 as a microreactor to concentrate CO 2 molecules, and Au ...

Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal–Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction

“…However, the use of PDF and d-PDF methods is still limited in the field of guest@MOF materials, with only a few reports, most of which concerning the adsorption of small molecules. 30,[23][24][25][26][27][28][29][30][31][32][33][34][35] To the best of our knowledge, the investigation of POM@MOF and (POM,cat)@MOF composites by differential PDF analysis is the first of the kind.…”

POM@MOF Hybrids: Synthesis and Applications

“…The authors observed an enhanced chemical and thermal stability after the embedding of POM and no POM leaching was noted during catalysis in several studies [40,41]. In one of these studies, Shuxia Liu's group prepared a series of Keggin-type POMs in Cu-BTC, denoted as NENU-n, n = 1~10, and formulated as [Cu 2 (BTC) 4 2 (X = Si, Ge, P, As, V, Ti; M = W, Mo), by using a one-pot hydrothermal synthesis [42,43]. The templating effect of the POMs resulted in highly crystalline composite materials which showed an enhanced thermal stability.…”

Polyoxometalate-Based Catalysts for CO2 Conversion

“…The Au NPs were in-situ deposited during the assembly of NENU-10 and NENU-3 as shown in Figure 7.) was recently achieved [45]. Table 2 illustrated the catalytic performance of as-prepared hybrid in the reductive CO 2 transformation to CO and CH 4 under visible-light irradiation.…”

“…The SEM images for Au@NENU-3 and Au@NENU-10, respectively (h, i). Reprinted with permission from [45]. Copyright © 2018 Wiley-VCH Verlag GmbH and Co. KGaA.…”