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Photocatalytic conversion of carbon dioxide by ABTaO (A= Sr, Ba; B= K, Na) using ammonia as an efficient sacrificial reagent Zeai Huang, Sumika Yoshizawa, Kentaro Teramura, Hiroyuki Asakura, Saburo Hosokawa, and Tsunehiro Tanaka ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00134 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

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Photocatalytic conversion of carbon dioxide by

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A2BTa5O15 (A= Sr, Ba; B= K, Na) using ammonia

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as an efficient sacrificial reagent

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Zeai Huang†, Sumika Yoshizawa†, Kentaro Teramura†,‡,*, Hiroyuki Asakura†,‡, Saburo

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Hosokawa†,‡, Tsunehiro Tanaka†,‡,*

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†

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Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.

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‡

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,

Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-

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Ohara, Nishikyo-ku, Kyoto 615-8245, Japan.

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Email addresses of corresponding authors:

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Kentaro Teramura: [email protected];

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Tsunehiro Tanaka: [email protected]

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ABSTRACT: The fabrication of phase-pure A2BTa5O15 (A= Sr, Ba; B= K, Na) was achieved

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using a facile, one-pot, flux method in chloride salts. The as-fabricated catalysts were employed

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for the photocatalytic conversion of CO2 using H2O as an electron donor. Among them,

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Ba2NaTa5O15 showed the highest activity with a selectivity of 97.6% toward CO evolution.

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Additionally, Ba2KTa5O15 showed the highest photocatalytic activity toward CO evolution when

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a filter was used to eliminate light of wavelengths shorter than 280 nm. The photocatalytic

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activity of Ba2KTa5O15 was further enhanced using NH3·H2O as a sacrificial reagent. With the

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exclusion of light of wavelengths shorter than 280 nm, Ba2KTa5O15 showed photocatalytic

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activity toward CO as high as 117 µmol h−1, with selectivity as high as 98.6% when 0.7 M

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NH3·H2O was used as a sacrificial reagent.

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KEYWORDS: Flux method, Tantalates, Ammonia solution, Photocatalysis, Conversion of CO2

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INTRODUCTION

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Artificial photosynthesis has been developed in recent decades to convert solar energy into

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chemical energy. In artificial photosynthesis, CO2 and H2O are converted into hydrocarbons and

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O2 using sunlight with catalysts inspired by natural plant photosynthesis. Since the pioneering

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work in the late 1970s, this field has garnered a large amount of attention due to the increasing

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urgency to address excessive CO2 emissions in the atmosphere.1-2 One-step photocatalytic

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conversion of CO2 by H2O using heterogenous catalysts is considered a promising method to

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directly produce chemical feedstocks and high-value chemicals using sunlight.3-4

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Various photocatalysts have been designed for the conversion of CO2 by H2O.5-9 Recently,

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KCaSrTa5O15,10-11 Sr2KTa5O15,12 and K2RETa5O15 (RE = rare-earth element)13 with tetragonal

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tungsten bronze (TTB) structures were reported to exhibit good photocatalytic activity and

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selectivity toward CO evolution from CO2 conversion in and by H2O. Alkali metal, alkaline-

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earth metal, and rare-earth metal cations on the surface are expected to show strong ability for

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CO2 adsorption.14-15 Additionally, these tantalates have relatively negative band potentials near

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the bottom of the conduction band formed by Ta 5d orbitals. Because the required reduction

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potential to form products from CO2, such as CO (E0 = −0.52 V at pH = 7 vs. NHE) or HCOOH

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(E0 = −0.61 V at pH = 7 vs. NHE), is relatively negative as compared as water splitting to H2 (E0

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= −0.41 V at pH = 7 vs. NHE). Therefore, these tantalates can provide sufficient energy for the

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formation of reduction products of CO2.8 For example, NaTaO3 shows very low activity for the

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photocatalytic conversion of CO2 in and by H2O; however, doping with Ca, Sr, Ba, and La shows

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remarkable enhancement of both photocatalytic activity and selectivity toward CO evolution.8 In

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addition to enhanced charge separation, the improved adsorption of CO2 by these doped alkaline-


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earth metal, and rare-earth metal cations is believed to enhance photocatalytic activity toward

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CO evolution.13

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The efficient utilization of light during photocatalytic conversion of CO2 in and by H2O, is an

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important issue, however, of great challenging. Having a narrower band gap is important for

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achieving more efficient light utilization. In a previous work, Sr2KTa5O15 fabricated using a one-

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pot flux method showed better photocatalytic activity and selectivity toward CO evolution from

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the conversion of CO2 in and by H2O, compared to a solid-state reaction (SSR) method used by

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our group. The band gap of Sr2KTa5O15 was determined to be ~3.89 eV, indicating that the

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absorption of light was limited to wavelengths shorter than ~320 nm. The bond angle of Ta–O–

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Ta in Ba2KTa5O15 should be closer to 180° compared to that of Sr2KTa5O15.16-18 It was found that

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the electron-hole pairs migrate more easily in the crystal and the band gap becomes narrower

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when the bond angle of Ta–O–Ta approaches 180°.19 Therefore, substitution of Sr cations with

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Ba cations in Sr2KTa5O15 could result in a narrower band gap, showing possibility to use visible

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light, which represent 45% of the solar light flux, for the photocatalytic conversion of CO2.

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The use of H2O as an electron donor also limits the activity and selectivity for the

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photocatalytic conversion of CO2. This is due to competitive reduction of H2O with the reduction

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of CO2 during photocatalytic conversion of CO2 in H2O without a sacrificial reagent. Pan and co-

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workers reported the use of triethanolamine (TEOA) as a sacrificial reagent for the

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photocatalytic conversion of CO2 by carbon-coated indium-oxide nanobelts, and the highest

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formation of CO and CH4 was 126.6 and 27.9 µmol h−1, respectively, with Pt as a cocatalyst.20

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Isotopic experiments demonstrated that CO and CH4 were generated as main products from the

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introduced

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group achieved photocatalytic conversion of CO2 in a chloride (Cl–) containing aqueous solution,

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CO2, and tiny amount of CH3OH, HCHO, and HCOOH were also detected. Our


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and the use of Cl– as a hole scavenger significantly enhanced the photocatalytic activity and

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selectivity toward CO evolution by Ni-Al layered double hydroxides (LDH).21 Very recently, our

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research group also designed a highly efficient process for the photocatalytic conversion of CO2

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to CO in an aqueous NH4HCO3 solution with a highest production rate of CO at 550.7 µmol h–1,

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which is a promising one-step system for carbon capture and utilization.22 These results

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motivated us to enhance the efficiency of photocatalytic conversion of CO2 using cheap and

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readily available sacrificial reagents.

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In the present study, A2BTa5O15 (A= Sr, Ba; B= K, Na) catalysts were fabricated using a

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facile, one-pot, flux method. The photocatalytic conversion of CO2 using H2O or ammonia (or

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ammonium ions) as electron donors with a Pyrex® filter suggested that the fabricated catalysts

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could use longer wavelengths of light, which is an important property for practical applications.

12 13

EXPERIMENTAL

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Fabrication of A2BTa5O15 (A= Sr, Ba; B= K, Na) and deposition of cocatalyst

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A2BTa5O15 (A= Sr, Ba; B= K, Na) was fabricated by a flux method. A stoichiometric mixture

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of Sr(NO3)2 (98%, Wako), or Ba(NO3)2 (99.9%, Wako) with Ta2O5 (99.9%, Kojundo) was

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ground with a specific amount of NaCl (99.5%, Wako), or KCl (99.9%, Wako) in an Al2O3

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crucible for 10 min. The mass ratio of flux to precursors, abbreviated as F/P (flux / precursors),

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was kept at 1.0. The mixture was then transferred to a 50 mL Al2O3 crucible. The resulting

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mixture was covered and calcined at 1423 K for 6 h in air. After cooling to room temperature,

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the mixture was thoroughly washed three times with 300 mL of distilled water at 358 K for 0.5 h

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each, and filtered with 750 mL of water, in order to remove any residual salts, then dried in air at

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80 °C. The Ag cocatalyst (1.0 wt%) was deposited on the as-fabricated catalyst by a chemical

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reduction method. Specifically, 1.8 mL of an aqueous NaPH2O2 solution (0.33 mol L−1) was


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added to 50 mL of a suspension of the photocatalyst (1.5 g) containing 1.4 mL of AgNO3 (0.1

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mol L−1). After stirring at 358 K for 1.5 h, the suspension was filtered and dried in air at room

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temperature.

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Characterization

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The structure and crystallinity of the samples were characterized by X-ray diffraction (XRD)

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using a Rigaku Multiflex powder X-ray diffractometer. SEM images were obtained from field

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emission scanning electron microscopy (FE-SEM, SU-8220, Hitachi High-Technologies) at an

8

acceleration voltage of 3.0 kV. The element mappings were performed on the SEM with an

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energy-dispersive X-ray spectroscope (EDS) at 10.0 kV. The Brunauer–Emmett–Teller (BET)

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surface area was measured by N2 adsorption at 77 K using a volumetric gas adsorption apparatus

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(BEL mini, Bel Japan, Inc.). The UV-Vis diffuse reflectance (UV-Vis DR) spectra were

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measured using a JASCO Corporation V-670 spectrometer equipped with an integrating sphere.

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Spectralon® filled with BaSO4, supplied by Labsphere Inc., was used as a reflection standard.

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The band gap of catalyst was calculated from the absorption spectra using the following

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equation established by Tauc, Davis, and Mott and comprising the absorption coefficient and

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photon energy(Eq. 1):23-24

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[F (R∞) hν] 1/n = A (hν − Eg)

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where an n of 1/2 in our case is assigned to direct band semiconductors. h is Planck's

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constant, ν is the vibration frequency, A is proportionality constant, and Eg represents the band

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gap.

(1)

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Photocatalytic conversion of CO2

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The photocatalytic conversion of CO2 by H2O was carried out in a flow system using an inner-

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irradiation-type reaction vessel at room temperature and ambient pressure. The photocatalyst (1.0


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g) was dispersed in ultrapure water (1.0 L) containing a certain concentration of additives, and

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CO2 gas (99.999%) was bubbled into the solution at a flow rate of 30 mL min-1 for

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approximately 1 hour before photoirradiation. In the case of NH3·H2O as an additive, unless

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otherwise mentioned, CO2 was bubbled until there was no change of pH value to achieve an

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equilibrium of absorption of CO2 before the addition of photocatalyst. The suspension was

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irradiated with a 400 W high-pressure mercury lamp with a quartz jacket or a Pyrex® jacket as a

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filter connected to a water cooling system. The generated gaseous products, such as H2, O2, and

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N2, were analyzed by thermal conductivity detector-gas chromatography (TCD-GC) using a GC-

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8A chromatograph (Shimadzu Corp.) equipped with a Molecular Sieve 5Å column with Ar as

10

the carrier gas, and CO was analyzed by flame ionization detector-gas chromatography (FID-

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GC) with a methanizer using a ShinCarbon ST column with N2 as the carrier gas.

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The selectivity toward CO evolution compared to H2 evolution and the balance between

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consumed electrons (e−) and holes (h+) can generally be described using the formulae shown

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below (Eqs. 2 and 3):

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Selectivity toward CO evolution (%) = 100 × 2RCO / (2RCO + 2RH2)

(2)

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Consumed e−/h+ = (2RCO + 2RH2) / (4RO2 + 6RN2)

(3)

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where RCO, RH2, RO2, and RN2 represent the rate of formation of CO, H2, O2, and N2,

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respectively.

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RESULTS AND DISCUSSION

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(b)

2K

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(621) (002)

(531) (630) (601)

(610)

(321) (401) (411) (520) (421) (440) (530)

(211)

(320)

(310) (001)

(330) / (221)

(400)

(201)

(111)

(220)

(i)

(i)

1

(ii)

(210)

(ii)

(200)

Intensity (cps)

(410)

200

(420) (311)

(a)

Intensity (cps)

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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20

25

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35

40

45

50

10

15

2θ / degree

20

25

30

35

40

45

50

2θ / degree

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Figure 1 XRD patterns of (i) Sr2KTa5O15, and (ii) Ba2KTa5O15 catalysts calcined at (a) 1173 K

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for 3 h in KCl flux (F/P = 1.0) with K2CO3 added as a potassium precursor, and (b) 1423 K for 6

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h using KCl as both a potassium source and the flux (F/P = 1.0). Closed triangles: orthorhombic

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BaTa2O6.

6 7

Initially, we attempted to fabricate Ba2KTa5O15 using the same synthetic conditions as that of

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Sr2KTa5O15, which had been optimized at 1173 K for 3 h with a flux to precursor weight ratio of

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1.0.12 Figure 1(a) shows the XRD patterns of the fabricated catalysts. It can be clearly seen that

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only Sr2KTa5O15 showed a phase pure TTB structure consistent with the previous reports.12, 25 In

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the case of Ba2KTa5O15, the additional phase BaTa2O6 was found as an impurity phase in

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addition to the TTB structure (Figure 1(ii) in (a)). The formation of ANb2O6 (A= Sr, Ba) at

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moderate temperatures is thought to be the intermediate phase for the fabrication of the TTB

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structure of Sr2KNb5O15.26-28 This suggested that increasing the calcination temperature and time

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may yield a pure TTB phase. Therefore, we modified the flux method to that of the fabrication of

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K2RETa5O15 (RE = Rare-earth element), which is 1423 K for 6 h, using KCl as both the

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potassium source and flux.13 As shown in Figure 1(b), a pure TTB phase of A2KTa5O15 (A= Sr,


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Ba) was formed under this higher temperature.25, 29 In addition, the crystallinity of these two

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samples showed obvious enhancement under high calcination temperature conditions. It should

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also be noted that the relative peak intensities showed obvious differences between Sr2KTa5O15

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and Ba2KTa5O15. For instance, the intensity corresponding to the (310)/(001) planes is close to

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that of the (311) plane in Sr2KTa5O15, however, the intensity of the peak corresponding to

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(310)/(001) is much lower relative to (311) in the case of Ba2KTa5O15.

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Potassium cations in A2KTa5O15 (A= Sr, Ba) can be substituted with sodium cations.30 We

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succeeded in the fabrication of A2NaTa5O15 (A= Sr, Ba) using NaCl as the sodium source and

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flux. Figure 2 shows the XRD patterns of as-prepared A2NaTa5O15 (A= Sr, Ba) catalysts.

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Obvious peak splitting was observed, which is distinct from the A2KTa5O15 (A= Sr, Ba) catalysts

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fabricated using KCl flux. This splitting is due to the small ionic size of Na cations in the TTB

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structure, which increases the tetragonality of TTB. This effect is similar to that observed in

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Ba4R0.67□1.33Nb10O30 (R = Y, Dy, Gd, Sm, Nd and La; □ = vacancy).31

2K

Intensity (cps)

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


(b)

(a) 10

14

20

30

40

50

2θ / degree

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Figure 2 XRD patterns of (a) Sr2NaTa5O15, and (b) Ba2NaTa5O15 catalysts calcined at 1423 K

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for 6 h using NaCl as both sodium source and flux (F/P = 1.0).


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Figure 3 SEM images of (a) Sr2KTa5O15, (b) Sr2NaTa5O15, (c) Ba2KTa5O15, and (d)

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Ba2NaTa5O15 catalysts fabricated at 1423 K for 6 h using the flux method (F/P = 1.0).

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SEM images of pure TTB structures of A2BTa5O15 (A= Sr, Ba; B = K, Na) are shown in

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Figure 3. Rods with diameters up to a few hundred nanometers and lengths up to several

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micrometers were observed in Sr2KTa5O15 (Figure 3(a)). Alternatively, the substitution of K with

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Na cations in Sr2KTa5O15 had an obvious effect on the morphology. Smaller irregular particles

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were the main morphology with some longer rods in Sr2NaTa5O15 (Figure 3(b)). The smaller

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particles had sizes of ~100-300 hundred nanometers, which were much smaller than that of the

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rods. We have found that substitution of K with Na cations suppressed the aspect ratio

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(length/diameter); this is because the growth of [001] direction was inhibited in the NaCl flux. In

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the case of Ba2KTa5O15 and Ba2NaTa5O15, a similar phenomenon was found (Figure 3 (c) and

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(d)). Defined rods with diameters around 1 µm and lengths of 1~2 µm were formed in

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Ba2KTa5O15. A mixture of primarily rods with a few smaller irregular particles were formed in

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Ba2NaTa5O15. These results are consistent with XRD results; the use of NaCl as the sodium


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source and flux affected the growth of specific facets. Specifically, the growth in the [001]

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direction was inhibited (Figures 1(i) in (b), 2(a) and 3(d)), therefore shorter particles were

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formed preferentially over rods. The morphology change from Ba2KTa5O15 to Ba2NaTa5O15 is

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obviously different than that from Sr2KTa5O15 to Sr2NaTa5O15. This can be explained as follows;

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the ionic radius of Sr2+ (1.50 Å, CN 15) is smaller than that of Ba2+ (1.65 Å, CN 15), therefore

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the structural distortion in Sr2NaTa5O15 is much larger than in Ba2NaTa5O15. 6

4

(a)

3 2

4 3 2

(iv)

(ii)

2

(iii)

(iv) (ii)

1

(i)

1

(i)

(iii)

0

0 200

7

(b)

5

(F(R∞ )hν ν)

Kubelka-Munk Function (a.u.)

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


220

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400

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

Photon energy / eV

Wavelength / nm

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Figure 4 UV-Vis DR spectra (a) and the corresponding Tauc plot (b) of (i) Sr2KTa5O15, (ii)

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Sr2NaTa5O15, (iii) Ba2KTa5O15, and (iv) Ba2NaTa5O15 catalysts fabricated at 1423 K for 6 h

10

using the flux method (F/P = 1.0).

11 12

The UV-Vis DR spectra of the catalysts are shown in Figure 4(a). The absorption edges of

13

Sr2KTa5O15 and Sr2NaTa5O15 are close to each other. Sr2NaTa5O15 has an absorption edge of

14

~315 nm, and Sr2KTa5O15 has an absorption edge of ~320 nm (Figure 4(i) and (ii) in (a)).

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However, a larger difference was found in Ba2KTa5O15 and Ba2NaTa5O15. The Ba2KTa5O15

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shows absorption edge at ~330 nm, which is approximately 10 nm wider than that of

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Ba2NaTa5O15 (Figure 4(iii) and (iv) in (a)). A possible explanation for the red shift of

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Ba2KTa5O15 may be the enlarged ionic radius of potassium. Because the ionic radius of K+ is


11


1

larger than that of Na+, the distortion of the TaO6 framework is different in the two catalysts. The

2

distortion of the TaO6 framework resulting from the different ionic radii contributes to changes

3

in the energy structures.32 The band gaps of Sr2KTa5O15, Sr2NaTa5O15, Ba2KTa5O15, and

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Ba2NaTa5O15 were estimated to be 3.87, 3.89, 3.74, and 3.87 eV, respectively (Figure 4(b)).33

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Ba2KTa5O15 showed an obviously narrower band gap compared with others, indicating it can

6

absorb longer wavelengths of light (Figure 4(iii) in (b)).

100

80 70

80

60 60

50 40

40

30 20

20

10 0

(i)

(ii)

(iii)

(iv)

0

Selectivity toward CO (%)

90

100

(b)

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90

8

80

7

70

6

60

5

50

4

40

3

30

2

20

1

10

0

(i)

(ii)

(iii)

(iv)

Selectivity toward CO (%)

10 -1

100

(a)

Rate of evolved products / µmol h

-1

120 Rate of evolved products / µmol h

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

Page 12 of 31

0

7 8

Figure 5 Rates of CO (red filled), O2 (green dotted), and H2 (blue slashed) evolutions and

9

selectivity toward CO evolution using (a) quartz and (b) Pyrex® filters over (i) Sr2KTa5O15, (ii)

10

Sr2NaTa5O15, (iii) Ba2KTa5O15, and (iv) Ba2NaTa5O15 catalysts fabricated at 1423 K for 6 h

11

using the flux method (F/P = 1.0). Amount of catalyst: 1.0 g; cocatalyst loading: 1.0 wt.% Ag;

12

additive: 0.1 M NaHCO3; light source: 400 W high-pressure Hg lamp; water volume: 1.0 L; CO2

13

flow rate: 30 mL min−1.

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Figure 5(a) presents the formation rates of evolved gas products over A2BTa5O15 (A= Sr, Ba;

16

B = K, Na) catalysts fabricated using the flux method. CO as the reduction product of CO2, H2 as

17

the reduction product of H2O, and O2 as the only oxidation product were detected from all four

18

catalysts. Other reduction products, such as CH4 or HCOOH, HCHO, and CH3OH, were not


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1

detected. As expected, all four catalysts showed photocatalytic activity and high selectivity

2

toward CO evolution, with an e−/h+ close to 1.0, which indicated H2O as an electron donor.

3

Specifically, photocatalytic activity of Sr2KTa5O15 toward CO was 49.3 µmol h−1, and

4

Sr2NaTa5O15 showed an obvious decrease of about half activity toward CO evolution (26.8 µmol

5

h−1) with a slight decrease in the selectivity toward CO from 94.1% to 90.6%. Ba2KTa5O15 and

6

Ba2NaTa5O15 showed a significant enhancement in rates of CO evolution, which were 69.5 and

7

100 µmol h−1 with selectivities as high as 96.9 and 97.6%, respectively (Figure 5(iii) and (iv) in

8

(a)).

9

The BET surface areas of Sr2KTa5O15, Sr2NaTa5O15, Ba2KTa5O15, and Ba2NaTa5O15 were

10

measured to be 2.60, 2.23, 1.00, and 1.45 m2 g−1, respectively. Among them, Ba2NaTa5O15 and

11

Ba2KTa5O15 showed the highest activity using a quartz filter and a Pyrex® filter, respectively,

12

although both photocatalysts had lower surface areas than those of Sr2NaTa5O15 and

13

Sr2NaTa5O15. Therefore, the surface areas were not the reason for the enhanced activity of CO

14

evolution. The better activities may be due to better anisotropic properties, which are beneficial

15

for charge transfer, in Ba containing compounds than in Sr containing compounds. An evidence

16

could be found with the change of Ag cocatalysts before and after reaction. Taking

17

Ba2NaTa5O15, which showed the highest activity of CO, and Sr2NaTa5O15, which showed the

18

lowest activity of CO, as examples, Ag cocatalysts with a few nanometers were well dispersed

19

on the surface of both catalysts before reaction (Figure S1(a) and S1(c)). After 5 h of

20

photoirradition, most of Ag cocatalysts were well dispersed on some specific facets and the sizes

21

of Ag cocatalysts were slightly increased on Ba2NaTa5O15 (Figure S1(b)). Similar phenomena

22

were also found on Ag loaded BaLa4Ti4O15 and SrNb2O6.

23

Sr2NaTa5O15 were randomly dispersed and the sizes were significantly enlarged (Figure S1(d)).


5, 34

However, Ag cocatalysts on

13


Page 14 of 31

1

We expect that Ag cocatalysts are dissolved by photogenerated holes and redeposited onto some

2

specific facets of catalysts during photoirradiation due to the anisotropic property, which could

3

be beneficial for electron-hole separation.5 Anisotropic facets could be beneficial for the charge

4

separation, which has been studied in some photocatalysts of TiO2, BiVO4, and SrTiO3.35-37

5

The absorption edges of A2BTa5O15 (A= Sr, Ba; B = K, Na) are above 300 nm with narrower

6

band gaps than Ga2O3 and ZnGa2O4.38-39 It is therefore possible to use longer wavelengths of

7

light for the photocatalytic conversion of CO2, which is desirable for practical applications. We

8

thus carried out experiments under the same conditions as above, except with a Pyrex® filter,

9

which filters out wavelengths of light shorter than 280 nm, in place of a quartz filter. Figure S2

10

shows the absorption spectra of the quartz and Pyrex® glass used in our system. It can be clearly

11

seen that quartz allows the transmission of almost all wavelengths whereas Pyrex® only allow the

12

transmission of wavelengths shorter than 280 nm. We carried out the photocatalytic conversion

13

of CO2 by H2O over ZnGa2O4/Ga2O3 with the Pyrex® filter, which has been reported to show

14

high activity and selectivity toward CO evolution with a quartz filter. Tiny amount of CO (~0.2

15

µmol) was detected after 5-hour irradiation (Figure S3(a)). This is because the absorption edge of

16

ZnGa2O4/Ga2O3 is limited at ~270 nm (Figure S3(b)). Consequently, it cannot use the light

17

wavelength longer than 280 nm when the Pyrex® glass was used as a filter.

18

The results obtained with the Pyrex® filter over A2BTa5O15 (A= Sr, Ba; B = K, Na) are shown

19

in Figure 5(b), among them, Ba2KTa5O15 showed the highest photocatalytic activity for the

20

formation of CO (6.50 µmol h−1) with a selectivity of 94.2%. It is interesting that Ba2NaTa5O15

21

showed the highest photocatalytic activity when a quartz filter was used for the reaction (Figure

22

5(iv) in (a)). This discrepancy is reasonable because Ba2KTa5O15 has the narrowest band gap.

23

We also noted that the selectivity toward CO evolution decreased compared to when using the


14

Page 15 of 31 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


1

quartz filter. This is because water splitting cannot be completely suppressed, and a small

2

amount of H2 contributes significantly to the selectivity when CO evolution rate is low.

3

Additionally, the electron balance was not close to 1.0, which might be due to adsorbed O2

4

and/or oxygen vacancies on the surface of the catalysts.40-41 Some of O2 gases might be adsorbed

5

on the surface of catalyst and/or dissolved in an aqueous solution since only a few micromolar of

6

O2 was formed after 5 h of irradiation.41 Additionally, the formed oxygen may be used for the

7

oxidation of Ag cocatalysts since the Ag was partly oxidized after the photocatalytic reaction on

8

Sr2KTa5O15.12 Nevertheless, we succeeded in fabricating photocatalysts with high formation

9

rates of CO that can absorb longer wavelengths of light for the conversion of CO2 in and by H2O,

10

which has been rarely reported until now.

11

It has been reported that basic solutions (buffers) are essential for achieving high

12

photocatalytic activity and/or selectivity toward CO evolution from the photocatalytic conversion

13

of CO2 over many heterogeneous catalysts in flowing systems.8,

14

interested in studying the effects of additive type and concentration on the photocatalytic activity

15

of Ba2KTa5O15 with a Pyrex® filter, which showed the highest activity (Figure 5(iii) in (b)).

16

Table 1 summarizes the results; Ba2KTa5O15 without any additive showed the lowest activity and

17

selectivity toward CO (Entry 1). NaHCO3 was reported to be important for the enhancement of

18

overall water splitting.43-44 CO also can be formed in an aqueous solution of NaHCO3 over ZrO2

19

under photoirradiation due to the reduction of dissolved CO2 in bicarbonate species. The

20

formation rate of CO was increased with increasing the concentration of NaHCO3 over ZrO2.43

21

The concentration of NaHCO3 also showed a notable effect on photocatalytic conversion of CO2

22

using H2O as an electron donor over ZnGa2O4/Ga2O3 when using the quartz filter.42 Interestingly,

23

the concentration of NaHCO3 showed no obvious effect on the photocatalytic activity and


22, 42

We were therefore

15


Page 16 of 31

1

selectivity over Ba2KTa5O15 when using the Pyrex® filter (Entry 2 and 3). The reason for this

2

effect might be that the rate determination using a Pyrex® filter is different than with the quartz

3

filter. The emission spectrum of High Pressure Hg lamp is shown in Figure S4(a). When the

4

quartz filter is used for the photocatalytic reaction, almost all the wavelengths of light can pass

5

through the filter, however, the Pyrex® jacket filter cuts off the wavelength of light at shorter

6

than 280 nm (Figure S2). The wavelength of light at 313.2 nm can be mainly used due to the

7

limitation of the band gap of Ba2KTa5O15 (Figure S4(b)), which might be the rate determination

8

factor rather than the concentration of NaHCO3.

9 10

Table 1 Effect of additive type and concentration on the photocatalytic conversion of CO2 using

11

irradiation with light of wavelengths longer than 280 nm.

Additive Entry

Type of additive

Rate of evolved gas / µmol h−1[a] Selectivity toward CO e− / h+ [b]

concentration (mol L−1)

H2

O2

N2

CO

pH[c]

(%)

1

None

-

24.3

trace

trace

2.60

9.67

-

3.91

2

NaHCO3

0.1

0.39

1.93

trace

6.50

94.3

1.8

6.77

3

NaHCO3

0.5

0.37

0.80

trace

5.80

94.0

3.9

7.30

4

NH3·H2O

0.1

0.70

trace

0.41

17.0

96.0

14

6.71

5

NH3·H2O

0.3

0.72

trace

0.58

38.2

98.2

22

7.14

6

NH3·H2O

0.5

0.97

trace

1.62

47.5

98.0

10

7.34

7

NH3·H2O

0.7

1.74

trace

6.66

117.1

98.5

5.9

7.40

8

NH3·H2O

1.0

1.62

trace

4.41

85.1

98.1

6.6

7.48


16

Page 17 of 31 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

1 2 3 4


9

NH4HCO3

0.1

0.49

trace

0.23

19.2

97.5

29

6.68

10

NH4HCO3

0.5

0.80

trace

1.38

47.3

98.3

12

7.29

[a]

Formation rate after 1 h of irradiation. [b]Ratio between consumed electrons and holes after 1 h of irradiation. [c]pH value was recorded after 1 h of irradiation. Conditions: catalyst: 1.0 wt.% Ag loaded 1.0 g Ba2KTa5O15; light source: 400 W high-pressure Hg lamp; water volume: 1.0 L; CO2 flow rate: 30 mL min−1.

5 6

Very recently, we developed the use of ammonia and/or ammonium ions as efficient sacrificial

7

additives for the photocatalytic conversion of CO2.22 Although H2O is the best choice as an

8

electron donor for the photocatalytic conversion of CO2, an ammonia solution, as relatively

9

inexpensive additive, can significantly enhance the formation of CO. In this work, we studied the

10

effect of differing ammonia solution concentrations on the photocatalytic activity for the

11

conversion of CO2 using Ba2KTa5O15 with a Pyrex® filter.

12

The formation rate of CO in 0.1 M NH3·H2O was 17.0 µmol h-1 (Entry 4), which was 2.6 times

13

than that in 0.1 M NaHCO3 (Entry 2). The selectivity toward CO was also enhanced to 96.0%.

14

N2 was the only detected oxidation product instead of O2, which was produced from the

15

decomposition of ammonia and/or ammonium ions, indicating the use of these species as

16

electron donors. Increasing the NH3·H2O concentration from 0.1 to 0.7 M increased the

17

formation rate of CO to 117 µmol h−1 with a selectivity toward CO evolution as high as 98.6%

18

(Entry 4 to 7). Further increasing the concentration of NH3·H2O to 1.0 M led to a decrease in

19

photocatalytic activity toward CO evolution (Entry 8). In all cases, the selectivity toward CO

20

evolution was higher than 95.0%. We also used NH4HCO3 as an additive to confirm the function

21

of ammonia and/or ammonium ions. Both 0.1 and 0.5 M concentrations of NH4HCO3 as an

22

additive showed similar results for the formation rate and selectivity toward CO as compared to

23

that of NH3·H2O (Entry 9 and 10). Thus, the efficiency of photocatalytic conversion of CO2 can


17


1

be further enhanced under photoirradiation with wavelengths of light longer than 280 nm using

2

Ba2KTa5O15 and an ammonia solution as an additive.

160

(a)

140 120 100 80 60 40 20 0 0

1

2

3

4

160 (b) 140 120 100 80 60 40 20 0

5

0

1

Amount of evolved products / µmol

160

(c)

140 120 100 80 60 40 20 0 0

1

2

3

4

Photoirradiation time / h

2

3

4

5



4

5



3


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

Page 18 of 31

5

160

(d)

140 120 100 80 60 40 20 0 0

1

2

3

4

5


6

Figure 6 The evolution of CO (red circle), O2 (green square), N2 (yellow diamond), and H2 (blue

7

triangle) over time (5 h) using Ba2KTa5O15 with a Pyrex® filter under (a) typical conditions, (b)

8

without a Ag cocatalyst, (c) with an Ar flow rate of 30 mL min−1, and (d) without NH3·H2O

9

addition. Amount of catalyst: 1.0 g; cocatalyst loading: 1.0 wt.% Ag; additive: 0.1 M NH3·H2O;

10

light source: 400 W high-pressure Hg lamp; water volume: 1.0 L; CO2 flow rate: 30 mL min−1.

11 12

Figure 6(a) shows formed gases over time by Ba2KTa5O15 with a Pyrex® filter during a 5-hour

13

reaction. CO was steadily formed under photoirradiation. The total amounts of CO, N2, and H2

14

after 5 hours were 99.7, 3.91, and 4.11 µmol, respectively. No product was obtained in the


18

Page 19 of 31 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


1

absence of photoirradiation or photocatalyst (not shown). Without Ag cocatalyst, H2 is evolved

2

as the main product with tiny amount of CO. N2 was not detected during the reaction time

3

(Figure 6 (b)). This result indicated that the Ag cocatalyst was involved in both oxidation and

4

reduction reactions. This is reasonable because Ag is also used as the oxidation cocatalyst of

5

ammonia solutions.45-46 The Ag cocatalysts were randomly deposited on the surface of

6

Ba2KTa5O15 by using the chemical reduction method (Figure S5), which was thought to function

7

as the active sites for the reduction of CO2.5, 47 We have also carried out the reaction without

8

catalysts but with AgNO3 or Ag powder, however no product was detected (not shown).Without

9

CO2 flowing (i.e. with only ammonia solution) no CO was detected during the reaction (Figure 6

10

(c)). H2 and N2 evolved in the amounts of 134 and 46.0 µmol, respectively. The ratio of H2 to N2

11

was close to 3.0 indicating the complete decomposition of NH3 by Ba2KTa5O15 with a Ag

12

cocatalyst. Without the addition of an ammonia solution, 79.6 µmol of H2 and only 5.38 µmol of

13

CO were detected after 5 hours (Figure 6 (d)), indicating that water splitting, rather than the

14

conversion of CO2, was the main reaction without ammonia additive. We have concluded that an

15

ammonia solution functions as both the buffer for increasing the solubility of CO2 in water, and

16

as the sacrificial reagent for more efficient photocatalytic conversion of CO2.

17

Figure 7 shows the evolution of H2, N2, and CO over time from the photocatalytic conversion

18

of CO2 using an ammonia solution as an additive with Ba2KTa5O15 while excluding light of

19

wavelengths shorter than 280 nm for 20 hours of photoirradiation. CO was evolved steadily as

20

the main reduction product, and only a tiny amount of H2 was generated through the 20 hours.

21

The formation rate of CO reduced by 25% after 20 h of photoirradiation (Figure S6), this should

22

be due to the aggregation of Ag cocatalyst on the photocatalyst, which was studied in previous

23

works.7, 12 After 20 hours of irradiation, the total amount of formed H2, N2, and CO were 14.4,


19


Page 20 of 31

1

158, and 718 µmol, respectively. It should be noted that the e−/h+ balance decreased from 7.1 at 1

2

hour to 1.0 at 20 hours. This result means that stoichiometric formation of H2, N2, and CO was

3

achieved in our system under long periods of irradiation, which strongly indicates the use of

4

ammonia or ammonium ions as electron donors during the reaction when filtering out light of

5

wavelengths shorter than 280 nm. We propose that intermediate species such as N2H4 were

6

formed at the beginning of reaction,22, 48 however, no detectable oxidation products, such as

7

NO3− and NO2−, were found. Nevertheless, Ba2KTa5O15 is a promising catalyst for the

8

photocatalytic conversion of CO2 using both H2O and ammonia (or ammonium ions) as electron

9

donors with irradiation by longer wavelengths of light.

10 11

Figure 7 The evolution of CO (red circle), N2 (yellow diamond), and H2 (blue triangle) over time

12

(20 h), and the consumed electron-hole balance over Ba2KTa5O15 using a Pyrex® filter. Amount

13

of catalyst: 1.0 g; cocatalyst loading: 1.0 wt.% Ag; additive: 0.5 M NH3·H2O; light source: 400

14

W high-pressure Hg lamp; water volume: 1.0 L; CO2 flow rate: 30 mL min−1.


20

Page 21 of 31 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


1 2 3 4

CONCLUSION

5

A series of materials of TTB structure, A2BTa5O15 (A= Sr, Ba; B = K, Na), were fabricated

6

using a facile flux method. These catalysts showed photocatalytic activity for the conversion of

7

CO2 in and by H2O with excellent selectivity toward CO (higher than 95%). Among them,

8

Ba2KTa5O15, with rod-like morphology, showed the highest photocatalytic activity toward CO

9

evolution when photo-irradiated with light of wavelengths longer than 280 nm. The

10

photocatalytic activity toward CO evolution was further enhanced using a solution of ammonia

11

or ammonium bicarbonate as additives without decreasing the selectivity toward CO evolution.

12

This catalyst system is therefore promising as a platform for more efficient utilization of light.

13

ASSOCIATED CONTENT

14

Supporting Information

15

The Supporting Information is available free of charge on the ACS Publications website at DOI:

16

XXX.

17

SEM images and EDS mapping of 1.0 wt% Ag loaded Ba2NaTa5O15; UV-Vis absorption spectra

18

of quartz and Pyrex® glass; The formation amounts of gases over ZnGa2O4/Ga2O3 with a quartz

19

filter and its UV-Vis absorption spectra; The emission spectrum of High Pressure Hg lamp and

20

the comparation of active absorption wavelength using Pyrex® filter under Hg lamp irradiation

21

and the absorption spectra of Ba2KTa5O15; The formation rates of gases during 20 h

22

photoirradiation and the CO selectivity over Ba2KTa5O15 using a Pyrex® filter.

23

AUTHOR INFORMATION

24

Corresponding Authors


21


1

(K.T.) E-mail: [email protected].

2

(T.T.) E-mail: [email protected].

3

ORCID

4

Zeai Huang: 0000-0002-7079-2504

5

Kentaro Teramura: 0000-0003-2916-4597

6

Hiroyuki Asakura: 0000-0001-6451-4738

7

Saburo Hosokawa: 0000-0003-1251-3543

8

Tsunehiro Tanaka: 0000-0002-1371-5836

9

Page 22 of 31

Notes

10

The authors declare no competing financial interest.

11 12

ACKNOWLEDGMENT

13

This study was partially supported by a Grant-in-Aid for Scientific Research on Innovative

14

Areas "All Nippon Artificial Photosynthesis Project for Living Earth" (No. 2406) of the Ministry

15

of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, the Precursory

16

Research for Embryonic Science and Technology (PRESTO), supported by the Japan Science

17

and Technology Agency (JST), and the Program for Elements Strategy Initiative for Catalysts &

18

Batteries (ESICB), commissioned by the MEXT of Japan. Zeai Huang thanks the State

19

Scholarship of China Scholarship Council, affiliated with the Ministry of Education of the P. R.

20

China.

21 22

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23

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Page 30 of 31

For Table of Contents Use Only

1

2 3

Synopsis

4

CO2 was captured and stored in ammonia solution, further converted to CO over A2BTa5O15 (A=

5

Sr, Ba; B= K, Na) photocatalysts.


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218x102mm (96 x 96 DPI)


Photocatalytic Conversion of Carbon Dioxide over ... - ACS Publications

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