Samantha Wilkerson
Joshua McNeely
Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCoII Unit Coupled to
Ir Oxide Nanocluster Catalyst
In the past several years, light-driven water oxidation and carbon dioxide reduction have been
developed through the use of photocatalysts for energy production. In this article, a closed
photosynthetic cycle was described that employed a photocatalyst that combined an oxo-bridged
binuclear ZrOCoII group with an iridium oxide nanocluster on an SBA-15 wafer.1 The overall
reaction catalyzed by this system is the reduction of CO2 to CO. Interestingly, labeling
experiments show that the oxygen produced does not come from the CO2 substrate but is the
product of water oxidation.
CO2  CO + 0.5 O2
ZrOCoII: CO2 + 2 H+ + 2 e-  CO + H2O
IrOx: H2O  2 H+ + 2 e- + 0.5 O2
Scheme 1 illustrates plausible steps of the photocatalytic process. Light excites the
ZrOCoII moiety to induce a single metal-metal charge transfer (MMCT) from Co(II) to Zr(IV).
The intermittent Zr(III) species reduced CO2 to CO2-. The regeneration of Co(II) requires an
electron supplied by the oxidation of water on the IrOX moiety. Water is oxidized through a
single electron transfer to Ir(IV), and Co(III) is reduced back to Co(II) through MMCT from
Ir(III) to Co(III). The ZrOCoII half-reaction was determined by loading the SBA-15 pores with
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CO2 and water vapor and observing 13CO and O2 as a result. The reaction with 18O labeled
water resulted in 18O2 formation, and this result shows that the released oxygen does not come
from CO2 but is generated by water oxidation at IrOx.
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Samantha Wilkerson
Joshua McNeely
The deposition of Ir(acac)3 proceeds with partial oxidation and forms an IrOx with
iridium ion oxidation states III and IV. The FT-IR spectra recorded during photodeposition
(Figure 1) shows the disappearance of [IrIII(acac)3] bands and an increase in [IrIV(acac)2]2+ bands
as the result of partial oxidation of Ir(III) to Ir(IV). Figure 2 shows the progress of
photodeposition over 60 minutes until the complete conversion of all Ir(acac)3.
(1) Kim, W.; Yuan, G.; McClure, B. A.; and Frei, H. Light Induced Carbon Dioxide Reduction
by Water at Binuclear ZrOCoII Unit Coupled to Ir Oxide Nanocluster Catalyst. J. Am. Chem.
Soc. 2014, 136, 11034-11042.
2
Samantha Wilkerson
Joshua McNeely
Scheme 1. Plausible Reaction Chemistry of the ZrOCo-IrOx Photocatalyst
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Samantha Wilkerson
Joshua McNeely
0.35
Absorbance
0.25
0.15
Function 1
Function 2
Function 3
Sum of Functions
0.05
-0.05
1900
1800
1700
1600
1500
1400
1300
Wavenumber (cm-1)
Figure 2. Simulation of the FT-IR spectrum recorded after calcination, trace 3.
𝑖=3
̅ −𝜈
̅𝑚𝑎𝑥 )2
−(𝜈
ℎ 1
2𝜎2
𝑓(𝜈̅ , 𝟑) = ∑ 𝑓𝑖 (𝜈̅ ), 𝑤ℎ𝑒𝑟𝑒 𝑓(𝜈̅ ) =
𝑒
𝑛 √2𝜋𝜎 2
𝑖=1
Function i:
Position
Width
Height
Normation
Function 1:
𝜈̅ max,1 = 1860 cm-1
1 = 25 cm-1
h1 = 0.025
n1 = 0.0160
Function 2:
𝜈̅ max,2 = 1630 cm-1
2 = 35 cm-1
h2 = 0.06
n2 = 0.0114
4
Function 3:
𝜈̅ max,3 = 1250 cm-1
3 = 90 cm-1
h3 = 0.30
n3 = 0.0380
Samantha Wilkerson
Joshua McNeely
0.03
0.02
0.01
Absorbance
0
Orange (trace 4)
Red (trace 5)
-0.01
Green (trace 6)
-0.02
-0.03
-0.04
-0.05
1700
1600
1500
1400
1300
Wavenumber (cm-1)
Figure 2. Simulation of the difference FT-IR spectra recorded in various stages of the
photodeposition, traces 4, 5, and 6.
𝑖=𝑚
̅ −𝜈
̅𝑚𝑎𝑥 )2
−(𝜈
ℎ 1
2
2𝜎
𝑓(𝜈̅ , 𝑵) = ∑ 𝑓𝑖 (𝜈̅ ), 𝑤ℎ𝑒𝑟𝑒 𝑓(𝜈̅ ) =
𝑒
𝑛 √2𝜋𝜎 2
𝑖=1
For N = 4 (𝜈̅ max,m [cm-1], m [cm-1], hm, nm): 1680, 15, 0.0008, 0.0266 (m = 1); 1630, 20,
-0.005, 0.0199 (m = 2); 1580, 10, 0.004, 0.0399 (m = 3); 1550, 6, -0.012, 0.0665 (m =
4); 1530, 6, -0.01, 0.0665 (m = 5); 1510, 8, 0.005, 0.0499 (m = 6); 1490, 3, 0.0009,
0.133 (m = 7); 1430, 10, -0.0007, 0.0399 (m = 8); 1385, 9, -0.004, 0.0443 (m = 9);
1362, 8, 0.003, 0.0483 (m = 10). For N = 5 (𝜈̅ max,m [cm-1], m [cm-1], hm, nm): 1680, 18,
0.0017, 0.0222 (m = 1); 1625, 15, -0.009, 0.0266 (m = 2); 1580, 25, 0.012, 0.0160 (m =
3); 1552, 8, -0.029, 0.0483 (m = 4); 1527, 5.5, -0.024, 0.0679 (m = 5); 1510, 8, 0.009,
0.0499 (m = 6); 1490, 2, 0.002, 0.199 (m = 7); 1470, 8, 0.0015, 0.0499 (m = 8); 1415,
8, 0.0022, 0.0499 (m=9); 1385, 6, -0.013, 0.0665 (m=10); 1360, 7, 0.012, 0.0570 (m =
5
Samantha Wilkerson
Joshua McNeely
11). For N = 6 (𝜈̅ max,m [cm-1], m [cm-1], hm, nm): 1690, 20, 0.0013, 0.199 (m = 1);
1627, 12, -0.009, 0.0328 (m = 2); 1585, 17, 0.019, 0.0235 (m = 3); 1552, 6, -0.045,
0.0629 (m = 4); 1527, 5, -0.047, 0.0737 (m = 5); 1510, 6, 0.007, 0.0665 (m = 6); 1490,
2, 0.0015, 0.199 (m = 7); 1465, 5, 0.0017, 0.0798 (m = 8); 1415, 6, 0.0022, 0.0665 (m =
9); 1385, 8, -0.028, 0.0499 (m = 10); 1360, 7, 0.022, 0.0570 (m = 11).
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