Highly Selective and Durable Photochemical CO2 Reduction by

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Highly Selective and Durable Photochemical CO2 Reduction by Molecular Mn(I) Catalyst Fixed on Particular Dye-Sensitized TiO2 Platform Sung-Jun Woo, Sunghan Choi, So-Yoen Kim, Pil Soo Kim, Ju Hyoung Jo, Chul Hoon Kim, Ho-Jin Son, Chyongjin Pac, and Sang Ook Kang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03816 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Highly Selective and Durable Photochemical CO2 Reduction by Molecular Mn(I) Catalyst Fixed on Particular Dye-Sensitized TiO2 Platform Sung-Jun Woo, Sunghan Choi, So-Yoen Kim, Pil Soo Kim, Ju Hyoung Jo, Chul Hoon Kim, HoJin Son,* Chyongjin Pac,* and Sang Ook Kang* Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Korea KEYWORDS: CO2 to formate conversion, organic-inorganic hybrid systems, molecular catalyst TiO2 immobilization, heterogeneous catalysis, photocatalysis.

ABSTRACT. A Mn(I)-based hybrid system (OrgD-|TiO2|-MnP) for photocatalytic CO2 reduction is designed to be a co-assembly of Mn(4,4′-Y2-bpy)(CO)3Br (MnP; Y = CH2PO(OH)2) and (E)-3[5-(4-(diphenylamino)phenyl)-2,2′-bithiophen-2′-yl]-2-cyanoacrylic

acid

(OrgD)

on

TiO2

semiconductor particles. The OrgD-|TiO2|-MnP hybrid reveals persistent photocatalytic behavior, giving high turnover numbers and good product selectivity (HCOO– versus CO). As a typical run, visible-light irradiation of the hybrid catalyst in the presence of 0.1 M electron donor (ED) and 0.001 M LiClO4 persistently produced HCOO– with a >99% selectivity accompanied by a trace amount of CO; the turnover number (TONformate) reached ~250 after 23 h irradiation. The product selectivity (HCOO–/CO) was found to be controlled by changing the loading amount of MnP on

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the TiO2 surface. In-situ FTIR analysis of the hybrid during photocatalysis revealed that at low Mn concentration, the Mn‒H monomeric mechanism associated with HCOO– formation is dominant, whereas at high Mn concentration, CO is formed via an Mn‒Mn dimer mechanism.

INTRODUCTION The combustion of fossil fuels has resulted in the rapid rise of CO2 concentration (the most abundant greenhouse gas) in the atmosphere, consequently causing global warming. Thus, the photocatalytic reduction of CO2 to useful C1 feedstock (i.e., CO, formate, CH3OH, and CH4) is considered to be the best option to resolve the on-going environmental hazard caused by carbonbased fuels.[1-8] Strenuous efforts have been made to design an efficient catalyst for carbon dioxide conversion. Among several strategies, synthetic homogeneous[9,10] and heterogeneous[10,11] catalytic systems based on molecular transition metal complexes have received much attention as a potential candidate for photo- and electro-catalytic CO2 reduction, because of the easy control of their redox potentials through ligand modifications, facile coordination of CO2 to the metal center in the reduction state(s), and oxidation-state jump of the metal center allowing multi-electron reduction processes for CO2 reductions.[12-18] In many cases reported so far, Ru[19-21]- and Re[10,2124]

-based complexes with 2,2′-bipyridine (bpy) or related ligands have been extensively studied for

the electrochemical and photocatalytic reductions of CO2, mainly to CO and HCOO–. However, the major drawbacks of Ru- and Re-catalysts have very often been pointed out concerning the cost of such noble metals, which should limit their practical usage at large scale. In this regard, firstrow transition metal complexes based on Mn(I), Fe(II), Co(II), Ni(II), and Cu(II) have been proposed as potential candidates for the practical catalysis of CO2 reduction.[9,10,17,18,25-46] Among them, a molecular complex incorporating Mn, (bpy-R)MnICO3Br (bpy-R = 2,2′-bipyridine-based


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ligand), has recently received much attention, because of the structural/mechanistic similarity to related Re(I) complexes, which have been intensively studied due to the selective and efficient formation of CO by the photo- and electro-reduction of CO2.[28,47-50] However, compared with the Re(I) complex analogues, the Mn(I) complexes show lower product selectivity (formation of both CO and HCOO–) and lower durability under uncontrolled conditions. It is suggested that HCOO– is formed via an Mn–H intermediate after one-electron reduction of the Mn complex in competition with CO formation via the so-called dimeric mechanism involving the formation of (R-bpy)(CO)3Mn0‒Mn0(CO)3(bpy-R). It has been also known that the Mn–Mn dimer formation causes a lowering of the catalytic activity of the Mn catalyst due to the participation of two competitive catalytic pathways, as well as an increase of overpotential for the two electron reduction to [(bpy-R)Mn(CO)3]–.[28,47,51,52] For this reason, possible strategies to avoid the dimeric catalytic pathway should be required for the selective and efficient CO2 reduction by the Mn(I)based complexes. Cohen and Kubiak have demonstrated that the incorporation of Mn catalyst in metal-organic frameworks (MOFs) prevents dimerization of the singly-reduced Mn complex to result in the selective photocatalytic formation of formate.[53] In a similar strategy, Fontecave and Mougel have also reported that immobilization of Mn catalyst on bipyridine functionalities of periodic mesoporous organosilicas (PMO) greatly enhances the photoreduction activities of CO2 into CO and HCOO– with inhibited bimolecular processes by site isolation of Mn catalysts.[54] In electrochemical CO2 reduction, Reisner and co-workers have reported that the adsorption of a Mn catalyst via the pyrene anchoring group on a multiwall carbon nanotube (MWCNT) electrode enhances the electrocatalytic activity and durability with variable product selectivity that is sensitive to the concentration of the Mn complex: at low Mn catalyst loading, HCOO– was the major product (efficiently alleviating the Mn‒Mn dimeric mechanism with a spatial isolation of


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Mn catalyst), whereas at high Mn catalyst loadings, CO was the main CO2 reduction product.[55] Kubiak and co-workers have also reported that the dimer formation is effectively suppressed through incorporating a bulky moiety to the coordinating bipyridine ligand.[47] While recent advances for tuning the catalytic pathway in Mn(I)-based CO2 reduction continue to emerge (in the field of electrochemistry), limited reports appear on the photosensitizer-driven reaction of Mn(I) complex with controlled product selectivity. Therefore, a new strategy that focuses on improving product selectivity with efficient/durable activity is highly needed in photocatalytic CO2 reduction based on Mn(I) complex. Recently, the immobilization of a molecular catalyst on a semiconductor (typically TiO2) for photocatalytic CO2 reduction has emerged as an effective tool; 1) to improve catalytic durability by suppressing unwanted interactions between long-lived reactive species of the catalyst and 2) to enhance the catalytic efficiency by smooth electron supply to the anchored catalyst through TiO2 after electron injection from an excited-state photosensitizer.[56-61] This strategy can be expected to apply to spatial isolation of Mn catalyst on the surface of TiO2 that might allow the dominant participation of the monomeric mechanism associated with high product selectivity in the photocatalytic CO2 reduction. Scheme 1 illustrates the present reaction system with the key components and overall photochemical reaction pathways. Herein, we present the selective and sustainable photocatalytic behavior of the Mn-based hybrid catalyst prepared by the coimmobilization of the Mn(I) catalyst (MnP) and the visible-light photosensitizer (OrgD) on TiO2 particles

(OrgD-|TiO2|-MnP);

OrgD

is

(E)-2-cyano-3-(5′-(5″-(p-

(diphenylamino)phenyl)thiophen-2″-yl)thiophen-2′-yl)-acrylic acid, MnP is fac-[Mn(4,4′-Y2bpy)(CO)3Br] (Y = CH2PO(OH)2), and the sacrificial electron donor (ED) is 1,3-dimethyl-2phenyl-1,3-dihydrobenzimidazole (BIH). This approach offers details of the photocatalytic CO2


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reduction activities of the Mn complex-based ternary hybrid system. It was found that different transient intermediates can be probed by in-situ FTIR analysis of the hybrid system. This analysis showed that low surface loading of MnP leads to high selectivity toward HCOO– production.

Scheme 1. Conceptual representation for the reaction system and components used in this study.

EXPERIMENTAL SECTION Synthesis

and

characterization.

The

organic

sensitizer

(OrgD),[62]

4,4′-

Bis(diethoxyphosphorylmethyl)-2,2’-bipyridine,[59,60] and sacrificial electron donor (BIH)[63] were prepared according to the method reported in previous work. The Mn complex (MnP) was synthesized by modifying synthetic protocols described by previous reports, details of which are described below (see Scheme 2 and the experimental section).


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fac-[Mn(4,4′-bis-(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO)3Br] (MnPE). A Et2O solution (50 mL) containing a mixture of 4,4′-Bis(diethoxyphosphorylmethyl)-2,2′-bipyridine (0.524 g, 1.148 mmol) and Mn(CO)5Br (0.292 g, 1.043 mmol) was refluxed for 4 h and then cooled to room temperature. The reaction mixture was further maintained at –20 °C using ice/NaCl bath for 30 min to give a yellow solid powder. The residue was purified by silica gel chromatography using dichloromethane/methanol (20:1 v/v) as eluent to give MnPE as a light yellow solid. Yield 45% (0.701 g). 1H NMR (300 MHz, DMF-d7) δ 9.27 (d, J = 5.4 Hz, 2H), 8.65 (s, 2H), 7.81 (d, J = 5.4 Hz, 2H), 4.08 (m, 8H) 3.62 (d, J = 22.8 Hz, 4H), 1.22 (t, J = 6.9 Hz, 12H). ESI-MS (m/z): calcd. for C23H30BrMnN2O9P2 [M]: 673.9990, found [M–H]–: 672.8930. Anal. Calcd (%) for C23H30BrMnN2O9P2: C, 40.91; H, 4.48; N, 4.15. Found (%): C, 40.79; H, 4.45; N, 4.17. fac-[Mn(4,4′-bis-(dihydroxyphosphorylmethyl)-2,2′-bipyridine)(CO)3Br]

(MnP).

Bromotrimethylsilane (TMSBr) (6.9 mL, 52.6 mmol) was added dropwise to a dry CH3Cl solution (30 mL) containing a mixture of MnPE (0.500 g, 0.74 mmol). The reaction mixture was refluxed for 24 h. To this solution were added methanol (17 mL) and stirred for 3 h, and then yellow solid precipitated was collected by filtration. After evaporation under reduced pressure, the remaining solid was recrystallized from CH2Cl2/MeOH/n-Hexane mixture solvent to give the product (MnP) as a light yellow solid. Yield 67% (0.300 g). 1H NMR (300 MHz, DMSO-d6) δ 9.07 (d, J = 5.4 Hz, 2H), 8.42 (s, 2H), 7.60 (d, J = 5.4 Hz, 2H), 3.28 (d, J = 22.2 Hz, 4H). ESI-MS (m/z): calcd. for C15H14BrMnN2O9P2 [M]: 561.8738, found [M–H]–: 560.9206. Anal. Calcd (%) for C15H14BrMnN2O9P2: C, 32.00; H, 2.51; N, 4.98. Found (%): C, 31.85; H, 2.48; N, 5.01. Crystal Structure Determination. Fine crystal of MnPE obtained from a CH2Cl2/n-hexane solution was sealed in glass capillaries under argon and mounted on a diffractometer. The preliminary examination and data collection were performed by Bruker SMART APEX CCD X-


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ray diffractometer at the Korea Basic Science Institute (KBSI Seoul center) equipped with a sealedtube X-ray source (50 kV × 1 mA) using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The preliminary unit-cell constants were determined using a set of 50 narrow frame (0.30° in ω) scans. The double pass method of scanning was used to exclude noise. The collected frames were integrated using an orientation matrix determined from the narrow-frame scans. The SMART software package was used for data collection, and SAINT was used for frame integration.[64] The final cell constants were determined through global refinement of the xyz centroids of the reflections harvested from the entire data set. Structure solution and refinement were performed using the SHELXTL-PLUS software package.[64] Crystallographic data was deposited with the Cambridge Crystallographic Data Centre as supplementary publications (CCDC-1824333 (MnPE)). Additional crystallographic data are available in the Tables S1–S3 of the Supporting Information (SI). Preparation of Mn-immobilized OrgD-|TiO2|-MnP Hybrid Catalyst: Commercially available TiO2 particles (Hombikat UV-100) were thoroughly washed with distilled water, ultrasonically treated in water, separated by centrifugation, and then dried in an oven under N2. The specific Brunauer-Emmett-Teller (BET) surface areas were determined to be >250 m2/g. The TiO2 particles (0.1 g) were stirred overnight in an acetonitrile/tert-butanol solution of OrgD (15 μmol) in the dark and then subjected to centrifugation. The collected solids were washed with the solvent and then dried in an oven under N2. For the preparation of OrgD-|TiO2|-MnP sample (Figure S6 in the SI), the Dye-deposited TiO2 powders (0.1 g) were dispersed into an acetonitrile/tert-butanol solution of MnP (1 to 20 μmol) and allowed to stand overnight under stirring. The collected solids were separated by centrifugation, washed with the solvent and then dried in an oven (70 °C) and stored under N2 in the dark. The complete loading of MnP was confirmed by absorbance comparison


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before and after each adsorption step of MnP: the supernatant, which is separated after centrifugation of the MnP–treated suspensions, showed the negligible absorption intensity of MnP. Photocatalyzed CO2 Reduction. Suspensions of OrgD-|TiO2|-MnP particles (10 mg with photosensitizer (OrgD) and reduction catalyst (MnP) in 3 mL distilled DMF containing BIH (0.1 M) were placed in a Pyrex cell (1 cm pass length; 6.0 mL total volume), bubbled with CO2 for 30 min, and sealed with a septum. A series of samples were set on a homemade merry-go-round apparatus and then irradiated under magnetic stirring with a LED lamp (λ >400 nm, 60 W, model Fc-6051, Cree Inc.). Homogeneous-solution photocatalysis was carried out in 3 mL of DMF solutions containing Ru(bpy)32+ (0.05 mM), MnPE (0.05 mM), and BIH (0.1 M). The gaseous products (CO and H2) accumulated in the overhead space of the cell were determined by gas chromatography (HP6890A GC equipped with a TCD detector) using a SUPELCO CarboxenTM 1010 PLOT fused silica capillary column (5 Å). The liquid phase of the irradiated samples was subjected to high-performance liquid chromatography (HPLC) analysis using a Waters 515 pump, a Waters 486 UV detector operated at UV 210 nm, a RSpak KC-811 Column (Shodex), and 0.05 M H3PO4 aqueous solution eluent. Apparent Quantum Yield Measurement. The apparent quantum yield Φ(CO) for HCOO– production was determined for the OrgD-|TiO2|-MnP suspensions, a band-pass filter (420−450 nm) was used to isolate the 436 nm light from the emission light of a high-pressure mercury lamp (1000 W, model 6171, Newport Corporation), and the incident light flux was determined using a 0.2 M ferrioxalate actinometer solution.[65] The apparent quantum yield (AQY) of HCOO– formation for the hybrid system in the presence of 0.001 M LiClO4 was determined in a linear time–conversion region. As defined in Eq. 1, the measured AQY is not the real quantum yield with


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scientific definition, but represents a relative estimate for the utilization efficiency of photogenerated electrons into HCOO– formation with respect to the incident light intensity, because the number of photons absorbed by the sensitizer cannot be exactly determined. The relatively low AQY appears to arise, at least in part, from poor light harvesting by the dye, due to extensive light scattering in the particle dispersion system. AQY(%) =

amount of formate generated per unit time number of incident photons per unit time

x 100

(Eq. 1)

In-situ Infrared Spectroscopy. A homemade in-situ FTIR spectroscopy tool is designed and made for analyzing a chemical intermediate of Mn(I) molecular catalyst immobilized on OrgD|TiO2 particles during photocatalysis. Figure S19 and S20 of the SI show the detailed description of the accessory parts designed for this measurement. Three mirror reflectance accessories were installed in the Nicolet 6700 FTIR spectrometer (from Thermo Electron Corporation). After PTFE gasket type ring was placed in the cell base body, CaF2 window was inserted, and Teflon spacer was placed on the CaF2 window. In order to conduct the IR study under an environment similar to the homogeneous-solution photocatalytic reaction, a CO2-saturated DMF solution containing MnPE (0.05 mM), Ru(bpy)32+ (0.05 mM), and BIH (0.1 M) was injected through a silicon tube into the IR beam-penetrating active cell equipped with a CaF2 window. In the case of IR measurements for heterogeneous ternary system (OrgD-|TiO2|-MnP), a mesoporous TiO2 film was prepared on a CaF2 plate and then sequentially treated with solutions of OrgD and MnP for the immobilization of the components on the TiO2 film. The composite plate was set at the center of active cell filled with CO2-saturated DMF solution containing 0.1 M BIH. In this FTIR study, the loading amount of MnP was set at (~0.67, 3.3, or ~6.6) μmol, ~(7, 33, or 66) times higher than that (0.1 μmol per 10 mg TiO2 particles) employed in the photocatalyzed CO2 reduction experiment.


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The loaded Mn concentrations are determined based on 1.5 mg mesoporous TiO2 deposited onto CaF2 window. Note that the 0.1 μmol loading amount of MnP (similar to actual photocatalytic condition) is too low to allow the detection of the relevant IR absorption bands. After purging the DMF solution containing 0.1 M BIH with CO2 gas in a dark condition for 30 min, the 0.05 mL of purged BIH solution was injected into the active spacer (0.1 mm pass length; 0.03 mL total volume) of the in-situ FTIR tool, through which the IR-beam can pass with simultaneous LED irradiation (see Figure S19 of the SI). The change of IR peaks was traced over time ((0 to 60) min), under LED irradiation (>400 nm).

RESULTS AND DISCUSSION Preparation and characterization of MnP Scheme 2 describes the synthetic procedure of MnP. Refluxing of 4,4′-Y2-bipyridine (Y = CH2PO(OEt)2) with [MnBr(CO)5] in diethyl ether gave MnPE in 45% yield, which was subsequently treated with bromotrimethylsilane (TMSBr) to produce the desired MnP in 67% yield. The prepared Mn complexes (MnPE and MnP) were characterized by 1H NMR and highresolution ESI-MS spectroscopy (see the Experimental Section, and Figure S1–S4 of the SI). The solid-state structure of MnPE was confirmed by X-ray crystallography (Figure S5 in the SI). Table S1 of the SI summarizes the crystal data. Table S2 and S3 of the SI list the selected bond distance and angles. MnPE showed an Mn1‒N1 bond length of 2.042(2) Å, an Mn1‒Br1 bond length of 2.515(6) Å, and a torsion angle for N1‒C5‒C6‒N2 in the 2,2′-bipyridine of ‒10.00(3)°. The values are within the ranges of normal metal-carbon/nitrogen bond and torsion angle found in other bipyridyl Re analogues.[66] In comparison, [ReBr(4,4′-diethylphosphonate-2,2′-bipyridine)(CO)3]


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displayed a Re1‒N1 bond length of 2.613(7) Å, Re1‒Br1 bond length of 2.629(10) Å, and a torsion angle (N1‒C1‒C2‒N2) in the 2,2′-bipyridine of 6.3(11)°. Overall, MnPE has metal‒N and metal‒ C bond lengths that are shorter than those of bipyridyl Re complex due to the relatively small radius of first row (3d) Mn metal ion compared to third row (5d) Re metal.

Scheme

2.

Synthetic

Route

to

fac-[Mn(4,4′-bis-(dihydroxyphosphorylmethyl)-2,2′-

bipyridine)(CO)3Br] (MnP)[a] O

Cl

O N

Cl

N

(i)

O O

P O

[a]

O P

O O N N

(ii)

O P

O O

P O

HO

CO N

Mn

N

Br

HO

CO

(iii)

CO

MnPE

O HO

P

HO

O P

CO N N

Mn Br

CO CO

MnP

Conditions: (i) triethylphosphite (10 equiv), 24 h, 140 C ̊ , N2 (88% yield); (ii) [Mn(CO)5Br] (0.9 equiv), Et2O, 4 h,

reflux, N2 (45% yield); (iii) TMSBr, CHCl3, 24 h, reflux, N2 (67% yield).

Integration and characterization of Dye-|TiO2|-MnP ternary hybrid The successful anchoring of the functional components (OrgD and MnP) on TiO2 nanoparticles (Hombikat UV-100, Huntsman with Brunauer-Emmett-Teller (BET) surface areas of greater than 250 m2/g) was confirmed by the IR absorption peaks characteristic of the CO ligands of MnP ((2031, 1936, and 1919) cm‒1) (Figure 1) and by diffuse-reflectance spectra (DRS) showing the absorption maximum of OrgD at 450 nm (Figure S8 of the SI). The supernatant obtained by centrifugation after each adsorption step of OrgD and MnP was transparent, a result confirming a high-efficiency loading of the components supplied.


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ACS Catalysis

(a)

(b)

Bare TiO2 TiO2|-MnP

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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21.5 µM MnP Supernatant after adsorption of MnP on TiO2

0.1

+ TiO2

+ MnP

2400

2200

2000

1800

1600

1400

0.0

400

−1

Wavenumber / cm

500

600

Wavelength / nm

Figure 1. IR spectra of bare TiO2 and TiO2|-MnP in KBr discs (sample: KBr ≈ 1:100) (a, left) and comparable absorption spectra of DMF solution of MnP before (black line) and after (blue line) adsorption process of MnP with TiO2 particles (b, right).

Photocatalytic activity Dispersed solution of OrgD-|TiO2|-MnP (10 mg) in 3 mL of CO2-saturated DMF containing 0.1 M BIH was irradiated at ≥400 nm using an LED lamp (Cree Inc., 60 W). Table 1 summarizes the compositions of HCOO–, CO, and H2 observed under various conditions, including turnover numbers (TONs, mol of the products/mol of MnP) and net formation amounts in μmol. First, the effects of loaded amounts of OrgD and MnP on the photocatalytic behavior were investigated to identify the optimal conditions. First of all, it should be noted that HCOO– is the exclusive product when the loaded amount of MnP is lower than 0.1 μmol. The photocatalytic activity for the HCOO– formation increased with the increase in the loaded amount of MnP on 10 mg OrgD-|TiO2| particles from (0.025 to 0.1) μmol (entries 1 to 3) to give a maximum at 0.1 µmol and then decreased with further increase of loaded MnP amount (entry 4 to 8). This is reminiscent of similar


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observations in the case of a Re(I)-based ternary analogue (OrgD-|TiO2|-Re(I) catalyst).[59] On the other hand, further increase in the amount of the MnP on TiO2 surface of (0.1 to 3.2) μmol results in a substantial decrease of HCOO– production accompanied by significant CO production (Figure 2 and Figure S14 of the SI). In turn, the optimum loading amount of OrgD was determined to be 1.484 µmol on 10 mg TiO2|-MnP(0.1 µmol) (entries 9 to 11). Figure 3 shows typical plots of TON versus irradiation time for the formation of HCOO–, CO, and H2 under optimum conditions.

Table 1. Results of Visible-Light-Driven CO2 Reduction with Mn Complex-Based Hybrid TiO2 Catalyst in Different Conditions[a] entry

OrgD(μmol)-|TiO2|-MnP(μmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

OrgD(1.48)-|TiO2|-MnP(0.025) OrgD(1.48)-|TiO2|-MnP(0.05) OrgD(1.48)-|TiO2|-MnP(0.10) OrgD(1.48)-|TiO2|-MnP(0.20) OrgD(1.48)-|TiO2|-MnP(0.40) OrgD(1.48)-|TiO2|-MnP(0.80) OrgD(1.48)-|TiO2|-MnP(1.60) OrgD(1.48)-|TiO2|-MnP(3.20) OrgD(2.97)-|TiO2|-MnP(0.1) OrgD(1.48)-|TiO2|-MnP(0.1) OrgD(0.74)-|TiO2|-MnP(0.1) OrgD(1.48)-|TiO2 OrgD(1.48)-|TiO2|-MnP(0.1)[c] OrgD(1.48)-|TiO2|-MnP(0.1)[d] OrgD(1.48)-|TiO2|-MnP(0.1)[e] OrgD(1.48)-|ZrO2|-MnP(0.1) OrgD(1.48)-|TiO2|-MnP(0.1)[f] Ru(bpy)32+ + MnPE[g]

[a]The

tirr. [h] 26 26 28 28 28 28 28 28 31 31 30 33 10 10 10 10 20 10

HCOO– TON μmol 60.0 ± 4.0 1.5 ± 0.1 50.0 ± 2.0 2.5 ± 0.1 143 ± 11 14.3 ± 1.1 55.0 ± 6.0 11.0 ± 1.2 26.3 ± 3.0 10.5 ± 1.2 10.4 ± 2.0 8.3 ± 1.6 6.3 ± 0.6 10.0 ± 1.0 4.0 ± 0.3 12.8 ± 0.9 92.5 9.25 151 15.1 42 4.2 -[b] -[b] [b] -[b] [b] -[b] [b] -[b] 7.4 0.74 165 ± 20 16.5 ± 2.0 86 12.9

CO TON μmol

Highly Selective and Durable Photochemical CO2 Reduction by

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