3.1. Catalytic CO Oxidation
CO oxidation has been extensively investigated in the field of heterogeneous catalysis due to the fundamental interest and its close relevance in practical applications, such as gas sensors for the detection of trace amounts of CO, automotive exhaust gas treatment, and polymer electrolyte fuel cells [
37,
38,
39,
40]. Herein, we addressed the CO oxidation reaction based on metal or metal oxide NPs/MOF composites.
Table 1 summarizes the catalytic activities for CO oxidation of several MOF-supported NPs catalysts [
28,
41,
42,
43,
44,
45,
46,
47,
48,
49]. Generally, MOF-supported NPs catalysts have shown good catalytic performance for CO oxidation at relatively high temperatures.
Xu et al. [
41] reported a pioneering study that described MOF-supported noble metal NPs as an efficient catalyst for CO oxidation. The catalytic activity over
[email protected] for CO oxidation increases with increasing Au loading from 0.5 to 5 wt %, accompanied by the decrease of the temperature of 50% conversion of CO from 225 °C to 170 °C. The 5 wt %
[email protected] achieves a complete conversion of CO at approximately 210 °C. Afterwards, it was reported that the total conversion of CO was achieved at around 200 °C by the Pt and Au NPs, supported on NH
2-MIL-101(Al) and UIO-66, respectively [
42,
43]. Importantly, the reaction temperature of complete conversion of CO can be further reduced to below 150 °C by incorporating Pt, Pd, and Ag NPs with apposite MOF supports [
28,
44,
45,
46,
47,
48]. Wang and co-workers proposed that both the size of the metal NPs and the nature of the support play an important role on the catalytic performance of
[email protected] for CO oxidation through a combination study of experiment and DFT calculation [
45]. EL-Shall et al. [
46] reported high CO oxidation activities over Pd NPs supported on Ce-MOF. The
[email protected] catalyst with 5 wt % Pd loading shows surprisingly high catalytic activity, with a complete conversion at 96 °C. The authors proposed that the high activity was mostly attributed to the interaction of the Pd NPs and the Ce sites within Ce-MOF.
In addition to MOF-supported noble metal NPs, MOF-supported metal oxide NPs as an active catalyst for CO oxidation was reported. Wang et al. [
49] firstly employed ZIF-8 as host to prepare hexagonal Co
3O
4 NPs via the thermolysis of cobalt nitrate that is accommodated in the pores of the MOF host at a low temperature of 200 °C. The Co
3O
4@ZIF-8 composite exhibited excellent catalytic activity for CO oxidation, which was related to the highly dispersed Co
3O
4 NPs in the well-retained MOF networks. Complete conversion of CO was achieved at 80 °C by the resulting composite catalyst with good cycling stability and long-term stability. Furthermore, this synthesis method can be easily extended to the preparation of other metal oxide NPs.
3.2. Catalytic CO2 Conversion
Catalytic conversion of CO
2 into valuable chemicals, such as CO, CH
4, CH
3OH, HCOOH, cyclic carbonates, and so on, has consistently drawn significant attention [
16,
18,
50,
51,
52,
53]. The MOF based composites have been developed as active catalysts for the conversion of CO
2.
Recently, metal or metal oxide NPs incorporated in MOFs have been proved to be effective catalysts for converting CO
2 to valuable chemicals, including CO, CH
4, CH
3OH, and light olefins [
33,
53,
54,
55,
56,
57]. The Materials of Institute Lavoisior (MIL) and University of Oslo (UiO) families, and their surface modified MOFs were mainly used as supports due to their high thermal stability and high chemical stability in water. An efficient catalyst, which was prepared by encapsulating single Cu nanocrystal (18 nm) into UiO-66, was reported recently for CO
2 hydrogenation to methanol. It shows a steady eight-fold yield over the benchmark Cu/ZnO/Al
2O
3 catalyst, with a 100% selectivity to methanol [
54]. Interestingly, Wang and co-workers recently extended this approach with the use of a UiO-bpy MOF, which anchored ultrafine Cu/ZnO
x NPs within the pores to restrain the agglomeration of Cu NPs and phase separation between Cu and ZnO
x [
55]. As shown in
Figure 2, the resulting Cu/ZnO
x@MOF catalysts exhibit remarkably higher activity (space-time yield of 2.59 g
MeOH kg
Cu−1 h
−1), higher selectivity (100%), and higher stability (>100 h) for methanol synthesis from CO
2 hydrogenation, when compared to the commercial Cu/ZnO/Al
2O
3 catalyst. Similarly, Lu et al. [
33] prepared Ni NPs encapsulated in MIL-101(Cr) composites by double solvent method (DSM) and multiple impregnation method (IM) for CO
2 methanation. The
[email protected](DSM) catalyst with Ni loading of 20 wt % exhibited surprisingly higher activity for CO
2 methanation than
[email protected](IM), giving a CH
4 turnover frequency (TOF) value of 1.63 × 10
−3·s
−1 at 300 °C. The author contributed the higher activity of
[email protected](DSM) to the more exposed Ni(111) facet, which was demonstrated by the result of DFT calculations that the Ni(111) plane has lower potential energy barrier (10.0 kcal/mol) for CO
2 dissociation into CO
ads and O
ads than Ni(200) facet (20.3 kcal/mol).
The utilization of solar energy for the conversion of CO
2 into valuable products is one of the best solutions to reduce carbon emission. Thus far, a range of photocatalysts, including TiO
2, CdS, Zn
2GeO
4, graphite-like carbon nitride (g-C
3N
4), and other compounds have been successfully combined with MOFs to photocatalytically reduce CO
2 [
58,
59,
60,
61,
62,
63,
64,
65]. For example, a metal-free semiconductor-composite (g-C
3N
4-ZIF-8) by growing ZIF-8 on the surface of g-C
3N
4 nanotubes for photocatalytic CO
2 conversion into CH
3OH was recently reported [
63]. The ZIF-8 on the surface increases CO
2 capture capacity, but impairs the surface charge transfer within the photocatalytic system due to the weaker electrical conductivity. The optimized ZIF-8 modified tubular g-C
3N
4 photocatalysts show the superior catalytic performance, giving a >3-fold yield of CH
3OH, relative to the bulk g-C
3N
4.
The separation efficiency of the photoinduced charge carriers plays an important role in photocatalysis [
18,
66]. The introduction of metal atoms into MOF photocatalysts may suppress the recombination of photoinduced electrons and holes and significantly increase their photocatalytic activity. Yaghi et al. [
64] reported the construction of Ag⊂Re
n-MOF with enhanced photocatalytic activity for CO
2 reduction to form CO, which resulted from the cooperation of the spatially confined photoactive Re centers and the intensified near-surface electric fields at the surface of Ag nanocubes (
Figure 3a). A fine balance of proximity between photoactive centers is needed for cooperatively enhanced photocatalytic activity in Re
n-MOFs. The optimal Re
3-MOF structure with the highest turnover on silver nanocubes shows a 7-fold enhancement in CO evolution rate over Re
3-MOF under visible light (
Figure 3b). Furthermore, Ag⊂Re
3-MOF structure exhibits long-term stability of up to 48 h when compared to molecular H
2ReTC, and the CO produced from Ag⊂Re
3-MOF almost doubles from that of H
2ReTC after 48 h (
Figure 3c).
Besides, catalytic processes that convert CO
2 into cyclic carbonates have been widely investigated due to their high atom efficiency and high value products [
67,
68]. A growing number of MOFs have been employed as catalysts for the formation of cyclic organic carbonate. Meanwhile, a strategy for combining MOFs with functional species, like ILs, together to form heterogeneous catalysts has been developed to enhance the catalytic activity for conversion of CO
2 into cyclic carbonates [
69,
70,
71]. Shi and co-workers [
72] reported two IL functionalized bifunctional catalyst, MIL-101-N(
n-Bu)
3Br and MIL-101-P(
n-Bu)
3Br, as prepared by the covalent post functionalization of MOFs (
Figure 4a). Due to the synergy of two functional sites including Lewis acid sites in the MOF framework and nucleophilic anion in the ILs, the MIL-101-N(
n-Bu)
3Br, and MIL-101-P(
n-Bu)
3Br catalysts showed the highest yield to propylene carbonate (PC) (>98%) for the cycloaddition reaction of CO
2 and propylene oxide (PO), when compared to other MOFs under mild and co-catalyst free conditions (
Figure 4b). Similarly, the catalytic activity of ILs supported ZIF-90 for the PO-CO
2 cycloaddition reaction is remarkably enhanced as compared to ZIF-90 [
73].