**1. Introduction**

The capture of CO2 and further conversion of this greenhouse gas into chemical fuels (CO, CH4, CH3OH, etc.) has been a hot topic during the past decades [1]. The reduction of CO2 is complicated by its inherent chemical stability [2,3]. Despite this issue, photocatalytic CO2 reduction is still seen as one of the promising ways to sustainably produce chemicals [4]. To date, a wide variety of photocatalysts such as TiO2, ZnO, CdS, perovskite oxides and their composites, and metal (complex) functionalized derivatives, have been tested. But the obtained performances under solar irradiation are still limited, mainly due to the low efficiency of using visible light.

Over the last decades, metal-organic frameworks (MOFs) have been one of the fastest developing materials, exhibiting unique properties such as structural flexibility, large specific surface area, tunable but uniform cavities, and easy ligand functionalization [5,6]. MOF-5 was the first MOF reported to have photocatalytic activity in UV-induced phenol photodegradation [7]. Fu et al. successfully employed an amine-functionalized Ti-based MOF, NH2-MIL-125(Ti), in CO2 photoreduction [8]. Ever since, MOFs are not only used as matrix for other semiconductors or noble metals but are also seen as promising photocatalysts on their own [9–11].

Recently, all-inorganic CsPbBr3 perovskites have emerged as promising photocatalysts in various applications ranging from dye degradation [12] to selective organic reactions [13] and renewable fuel generation from CO2 and water [14]. These materials combine the excellent optoelectronic properties of organic-inorganic hybrid perovskites, such as a bandgap largely overlapping with the visible part of the electromagnetic spectrum, a high absorption coefficient, and long-range charge transport, with good temperature stability [15,16]. However, the performances of MHPs are limited by their poor structural stability in a humid atmosphere. Heterojunction formation with materials such as graphene oxide, MOFs, TiO2, C3N4, etc., has been explored to further improve charge carrier separation and protect the

MHPs from a polar environment [17–21]. The use of MOFs in combination with MHPs is motivated by the composites improved stability as compared to the parent MHP material in photocatalytic reactions in the presence of water.

Herein, we report the excellent performance of CsPbBr3/MIL-100(Fe) composites for the photocatalytic reduction of CO2. Among the reported MOF photocatalysts, Fe-based MIL-100 was chosen due to its low cost, high chemical and water stability, and the intense and complementary visible light absorption to the far red [22,23]. We reasoned that the CsPbBr3/MIL-100(Fe) composite should show an extended and improved absorption and hence has the potential to be an efficient photocatalyst. The composite material was generated through the in situ synthesis of MIL-100(Fe) on perovskite particles, which tends to improve the stability of the composite and offers more efficient electron transfer [24]. MIL-100(Fe) introduction endows the composite with a high specific surface area and enhanced visible light response.
