2 Chemical Science

PF Activity Report 2013 #31

Monitoring of Electron Flow in Layered Double Hydroxides
to Photoreduce Carbon Dioxide into Fuels

P

hotocatalytic conversion of CO2 into fuel would simultaneously solve the problems of global warming and shortage of sustainable energy. Layered double hydroxides (LDHs), especially comprising CuII sites, are advantageous to photoreduce CO2 selectively into methanol using H2. By monitoring the pre-edge peak due to 1s-3dtransition for Cu sites in LDHs, electron diffusion rates to inlayer Cu sites (580 mmol h−1 gcat−1) and to interlayer Cu sites(36 mmol h−1 gcat−1) were evaluated and was sufficiently fast compared to photocatalysis of CO2. The preferable interaction of CO2 with Cu sites in LDHs was also demonstrated and suggested to control the photoreactivity with CO2 basedon multiple scattering peak intensity for Zn, Cu, and Ga in X-ray absorption near-edge structures.
Carbon dioxide is one of the major greenhousegases and methods to reduce its concentration in theatmosphere and its emission to the environment require urgent attention [1]. It would be advantageous tocapture CO2 from the atmosphere or factories/power
stations and convert it to fuel using natural light, as thisapproach would simultaneously solve the problems ofglobal warming and shortage of sustainable energy. Recently, many researchers have investigated this topic.
A study of selective CO2 photoconversion to methanol (68 mol%) was reported using H2 and layered doublehydroxide (LDH) of [Zn1.5Cu1.5GaIII(OH)8]+2CO32−·mH2O (1)[2]. LDH is one of the families of clays, but is unusual inthat the cationic layers intercalate anionic species, e.g.
carbonates, whereas in the majority of clays the layers
are anionic and intercalate cationic species. The inclusion of Cu ions in the LDH cationic layers improves theselectivity for methanol formation compared to CO formation. Methanol is more favorable as it can be directly
used in the existing infrastructure as fuel whereas CO

needs to be converted to hydrocarbons or alcohols foruse as an energy. The substitution of interlayer carbonate anions of LDH for [Cu(OH)4]2− anions further boosted the methanol selectivity to 88 mol% [3].
In principle, photocatalysis takes place starting fromthe charge separation in semiconductor/chemical bonds
due to light irradiation. For compound 1, ultraviolet (UV)light of energy greater than 3.5 eV excites electrons (e−’s)
at the valence band (major O 2p character) to the conduction band (metal d, s, p character) in the LDH. If thee−’s on metal and holes on oxygen diffuse to the surfaceof photocatalysts, reduction and oxidation reactions takeplace independently, e.g. CO2 reduction and H2 oxidation [2, 3] or H2O oxidation [4]. Thus, to understand thephotoreduction mechanism of CO2 into fuels (methanol,CO), it is necessary to monitor the e− flow in LDH uponlight irradiation and the interaction of CO2 with LDH surface. In this study, these steps were monitored using Xray absorption near-edge structure (XANES) spectroscopy [5].
(II)
(II)

0.10

6 min
light off
51 min46 min40 min34 min29 min23 min17 min
12 min6 minlight on(a)0.08Normalized intensity

0.06

(I)
(I)
Normalized intensity

1.6(b)

1.2
0.8

0.05
0.04

1s-3d0.030.40.02(b)

0.01

8970

8980

(III)
(III)
0.015

0.020(b)

0.08
0.06
0.04

(a)

0.02

00

Pre-edge peak intensity

(a)

0.04

9000

89809020

9040+2–9060

Energy (eV)

08970

8975

8980

Energy (eV)

0.013

0.012
0LightOff[1] Y. Izumi, Coord. Chem. Rev. 257, 171 (2013).
[2] N. Ahmed, Y. Shibata, T. Taniguchi and Y. Izumi, J. Catal.
279, 123 (2011).
[3] N. Ahmed, M. Morikawa and Y. Izumi, Catal. Today 185, 263(2012).
[4] M. Morikawa, Y. Ogura, N. Ahmed, S. Kawamura, G. Mikami,
S. Okamoto and Y. Izumi, Catal. Sci. Technol. 4, 1644 (2014).
[5] M. Morikawa, N. Ahmed, Y. Yoshida and Y. Izumi, Appl.
Catal. B 144, 561 (2014).
[6] Y. Izumi and H. Nagamori, Bull. Chem. Soc. Jpn. 73, 1581
(2000).

BEAMLINES
BL-7C, BL-9C and BL-01B1 (SPring-8, Priority Program forDisaster-Affected Quantum Beam Facilities)

Y. Izumi, S. Kawamura, M. Morikawa and A. Naveed(Chiba Univ.)

(B)

Light
Off0.010(b)(a)

Light On+50

REFERENCES

8 minlight off
180 min
164 min
142 min
125 min
105 min89 min72 min56 min39 min17 min13 min8985 light on

y = 0.014347 - 4.1324e-5x R= 0.97459

Light On(A)monitored using Cu K-edge XANES spectroscopy [2].
For compound 1, a post-edge peak intensity at 9016 eV
decreased to 76% by heating in vacuum at 383 K. Upon
introduction of CO2, the peak intensity increased back to87% of the fresh sample at 290 K and 91% of the fresh
one at 423 K. As a hydroxy group shares three metal
atoms in LDHs (Fig. 2), the post-edge peak intensity at
9689 and 10398 eV above Zn and Ga K-edge, respectively, also exhibited a similar semi-reversible trend. Thereversible in-situ peak intensity changes were not observed for the post-edge peaks at 9688 and 10397 eV
for Cu-free [Zn3Ga(OH)8]+2CO32−·mH2O. Thus, Cu sites
adsorbed CO2 and transferred the trapped e−’s to CO2
during the photoconversion to fuel.

y = 0.010472 - 5.8004e-6x R= 0.97817[Zn1.5Cu1.5Ga(OH)8] 2(CO3) ·mH2O0.014

The e − excitation and flow in compound 1 and[Zn3Ga(OH)8]+2[Cu(OH)4]2−·mH2O (2) under CO2 + H2
and UV-visible light irradiation was monitored byCu K-edge XANES spectroscopy (Fig. 1). A pre-edge
peak at 8979 eV because of a 1s-3d transition wasused to evaluate the population of CuII sites in the LDH
compounds [6]. When the CuII sites trap diffused e−’s,
they are reduced to CuI. The pre-edge peak does notappear for the CuI sites with d10 configuration.
The pre-edge peak intensity for incipient compound1 monotonously decreased by 15% within 50 min of irradiation [Fig. 1(II)-(a) & (III)-(a)]. As 0.55 mmol of Cusites were included in 170 mg of LDH disk, the e− trap
rate was 580 mmol h−1 gcat−1. The e−’s flow was within thecationic layer [Fig. 2(A)].
The 1s-3d pre-edge peak intensity of incipientcompound 2 also monotonously decreased by 11%
within 180 min of irradiation [Fig. 1(II)-(b) & (III)-(b)].
Based on Cu K-edge extended X-ray absorption finestructure analysis, the [Cu(OH)4]2− anions dehydrated
with hydroxy groups of the cationic layer to form(m–O)3Cu(OH)(H2O)2 [Fig. 2(B)] [5]. “m–O” means an O
atom bound to the cationic layer. The reduction rate of
0.17 mmol of out-of-layer CuII sites in 170 mg of LDH
disk was 36 mmol h−1 gcat−1. The rate was essentiallyslower diffusion in the perpendicular direction to interlayer Cu sites later than the faster e− diffusion in the cationic layer (580 mmol h−1 gcat−1).
Monitored e − trap (36–580 mmol h −1 g cat−1) was
fast enough compared to photocatalytic CO2 conversion into methanol & CO using compounds 1 and 2(2.1–2.2 mmol- e−’s_used h−1 gcat−1) irradiated under light.
The interaction of CO2 with Cu sites of LDHs was also

0.009

Figure 2: Structure of compound 1 (A) and structure of compound 2 (B). Interlayer carbonates are not drawn in (A) for clarity.
2–[Zn3Ga(OH)8] 2[Cu(OH)4] ·mH2O

050

100150200

Time of photo-irradiation (min)Figure 1: (I) Normalized Cu K-edge XANES spectra of compound 1 preheated at 423 K (a) and fresh compound 2 (b) and (II) under CO2 + H2and UV–visible light irradiation and subsequent change after the light was turned off, and (III) time course of the 1s–3d peak intensity for thetwo LDH photocatalysts under CO2 + H2 and UV–visible light irradiation and subsequent change after the light was turned off.

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Highlights

Highlights

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