Unveiling the lattice distortion and electron-donating effects in methoxy-functionalized MOF photocatalysts for H₂O₂ production

Highlights

• Substitution of electron-donating groups induces lattice distortion in MOF.

• The lattice distortion and electron-donating effect in photocatalysis are unveiled.

• Lattice distortion optimizes the protonation pathway of intermediate OOH*.

• Electron-donating effect promotes light harvesting and charge separation.

• Enhanced performance is achieved with a remarkable H₂O₂ yield of 312.9 mM g⁻¹ h⁻¹.

Abstract

Semiconducting metal-organic frameworks (MOFs) are emerging photocatalysts for H₂O₂ production via the two-electron oxygen reduction (2e-ORR) process. Whether and how the functional groups on MOF nodes affect the 2e-ORR mechanism and performance remain largely unexplored. Herein, we report that partial replacement of formate (-COO^-) by methoxy (-OMe) coordinated Zr-oxo cluster in UIO-66-NH₂ significantly enhances the H₂O₂ production. The introduction of -OMe groups induces lattice distortion in UIO-66-NH₂, resulting in the change of proton donor for the protonation of OOH* intermediate from -COOH to -NH₃⁺ with a lowered kinetic energy barrier. The -OMe groups with electron-donating effect contribute to promoted light harvesting and charge separation. The 2e-ORR selectivity of -OMe functionalized UIO-66-NH₂ is also enhanced, evidenced by rotating ring-disk electrode test and theoretical calculation. As a result, an improved photocatalytic ability is achieved with an H₂O₂ yield of 312.9 mM g⁻¹ h⁻¹, ~4.7 times higher than that of the original UIO-66-NH₂.

Introduction

Hydrogen peroxide (H₂O₂) is an oxidant with widespread applications in diverse fields [1]. The industrial production of H₂O₂ relies on the anthraquinone process, which is environmentally unfriendly and suitable mainly for centralized production. As a green approach [2], photocatalytic two-electron oxygen reduction reaction (2e-ORR) offers an on-site and sustainable route for H₂O₂ generation by using oxygen as a raw material, sunlight as energy source and semiconductors as photocatalysts [3]. Obviously, the rational design of photocatalysts with high performance is the key for efficient H₂O₂ production.

Over the past decade, numerous semiconductors such as graphitic carbon nitride (g-C₃N₄) [4], titanium dioxide [5], bismuth vanadate [6], resorcinol-formaldehyde resins [3] and metal-organic frameworks (MOFs) [7] have been reported for photosynthesis of H₂O₂ through the 2e-ORR process. Among them, MOFs have recently received great interest due to their unique merits of high specific surface area, tunable composition and structure, and easy functionalization [8]. For example, Zr-MOF and Ti-MOF have been applied as stable photocatalysts for reduction of O₂ into H₂O₂ [7], [9], [10], however their photocatalytic performance is moderate due to wide band gaps and limited light adsorption [11]. To promote the photocatalytic performance of MOF-based photocatalysts for H₂O₂ production, strategies such as metal node/linker modification [9], [10], heterojunction construction [12] and post-modification (e.g., Fe-O-Zr modified on metal nodes) [13] have been developed. These reports are mainly focused on optimizing the electronic structures of MOFs toward to enhance light harvesting and charge separation. For the 2e-ORR process, the reduction and protonation of O₂ to an intermediate OOH* and then OOH* to H₂O₂ * mainly determine the reaction activity and selectivity [14]. However, there are few reports on adjusting an important aspect in the reaction mechanism, i.e., the protonation pathway of OOH* , for effective H₂O₂ generation.[15].

Recently, defective MOFs have attracted much attention [16]. The structural defects may modulate the electronic and band structures in semiconductive materials [17] and provide more active sites for further functionalization and applications [17], [18], [19]. It is noted that lattice distortion is frequently observed in defective MOFs, which can be created by using linkers or metals with different sizes [20], and treated by ultraviolet-light [21] or high temperature[22]. The induced lattice distortion is beneficial for carbon capture [23] and oxygen electrocatalysis applications [21]. Moreover, the functionalization of MOFs with electron donating groups have been reported as a promising strategy to enhance the photocatalytic performance for visible light driven H₂ evolution [24]. However, the impact of lattice distortion or electron donating group modification on 2e-ORR is rarely reported for MOF photocatalysts.

Herein, methoxy (OMe) groups are incorporated into semiconducting UIO-66-NH₂ by partially replacing the formate on Zr-oxo nodes, offering significant performance enhancement for photocatalytic H₂O₂ production via 2e-ORR (Scheme 1a). The introduction of OMe group induces lattice distortion in UIO-66-NH₂, changing the proton donor for the protonation of OOH* intermediate from -COOH to -NH₃⁺ (Scheme 1b) with a lowered kinetic energy barrier. Moreover, the 2e-ORR is more favorable than the competitive 4e-ORR process. In addition, the electron-donating OMe groups endow UIO-66-NH₂ with reinforced light harvesting and facilitate charge separation. Thus, the OMe functionalized UIO-66-NH₂ exhibits an excellent H₂O₂ yield of 312.9 mM g⁻¹ h⁻¹, ~4.7 times higher than that of the original UIO-66-NH₂ and superior to reported MOF-based photocatalysts in similar reaction systems. To our knowledge, the contribution of lattice distortion and electron-donating effect of MOF-based photocatalysts in modulating the 2e-ORR reaction mechanism, especially the protonation pathway, is rarely reported.