Eco-Friendly Photochemical Production of H2O2 through O2

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Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Hetero-Elements Gun-hee Moon, Mamoru Fujitsuka, Sooyeon Kim, Tetsuro Majima, Xinchen Wang, and Wonyong Choi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03334 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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ACS Catalysis

Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Hetero-Elements

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Gun-hee Moon1, Mamoru Fujitsuka2, Sooyeon Kim2, Tetsuro Majima2, Xinchen Wang3, and Wonyong Choi1* 1

Department of Chemical Engineering & Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 37673 Korea 2

The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Osaka 567-0047, Japan 3

College of Chemistry, Fuzhou University, Fuzhou 350002, P. R. China

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Submitted to

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ACS Catalysis

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* To whom correspondence should be addressed (W. Choi)

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E-mail: [email protected]

Fax: +82-54-279-8299

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ACS Paragon Plus Environment

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Abstract

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We report that in-situ incorporation of both potassium and phosphate species into

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polymeric carbon nitride (CN) framework highly enhanced the photo-production of

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hydrogen peroxide (H2O2) without the use of any noble metal co-catalysts. The

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incorporation of the earth-abundant hetero-elements (K, P, and O) (i) introduced the

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negative surface charge over the entire pH through the surface functionalization by

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phosphate species, (ii) increased the lifetime of the transient species in picosecond

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timescale via the formation of charge separation states, (iii) facilitated the interfacial

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electron transfer to dioxygen, and (iv) inhibited the decomposition of in-situ generated

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H2O2. As a result, the modified CN showed the apparent quantum yields (Φ: for H2O2

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production) that are enhanced by about 25 and 17 times (Φ420 = 8.0%; Φ320 = 26.2%) from

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those of bare CN (Φ420 = 0.32%; Φ320 = 1.55%) under monochromatic irradiation of 420

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and 320 nm, respectively. This study clearly demonstrated a simple way to design multiple

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hetero-elements incorporated CN compounds that consist of earth-abundant elements only

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(C, N, K, P, O) for the development of practical and economical solar conversion

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photocatalytic materials.

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Keywords: Photocatalysis, Carbon nitride, Solar fuel, Hydrogen peroxide synthesis, Proton

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coupled electron transfer (PCET)

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2


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1. Introduction

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The

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oxide/sulfide/nitride should overcome many critical problems including a low solar

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light absorption, a low conversion/selectivity, a lack of long-term stability, a high

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operating cost, etc.1,2 In this regard, polymeric carbon nitride consisting of earth-

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abundant carbon and nitrogen elements only is an attractive material because of its

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visible light absorption capability and the superior stability against photo-corrosion.

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Furthermore, the conduction band (CB) and valence band (VB) potentials (ECB = -

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1.3 VNHE and EVB = 1.4 VNHE at pH 7) are thermodynamically suitable for various

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reactions such as water-splitting, partial oxidation, oxygen reduction reaction (ORR),

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and the degradation of organic pollutants.3 On the other hand, a still low photo-

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conversion efficiency needs to be overcome, so various strategies such as

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morphology control, doping, bandgap engineering, co-catalysts loading, and

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hybridization with other materials have been actively investigated.4

practical

application

of

photocatalysts

associated

with

metal

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Hydrogen peroxide (H2O2) is an eco-friendly oxidant that is widely utilized for

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chemical synthesis, disinfection, bleaching agent, liquid propellant, Fenton reaction,

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etc. H2O2 can be produced by the reaction of H2 and O2 via anthraquinone method5

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or the reduction of O2 through proton-coupled electron transfer (PCET) (O2 + 2H+ +

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2e- → H2O2 ; E0 = 0.695 VNHE).6 The photosynthetic production of H2O2 through

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PCET is safe and environmentally-benign since it needs only water, dioxygen, and

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solar light. Most of metal oxide semiconductors that are widely employed for

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photosynthetic and photocatalytic conversions are active under UV light region only,

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and H2O2 having a strong affinity to the oxide surface decomposes as soon as it is

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produced.7,8 Recently, a relatively high photo-conversion efficiency of H2O2

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production was observed over polymeric CN since tri-s-triazine moieties consisting

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of CN polymeric matrix accelerate a selective two electron transfer to O2 via the

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formation of superoxo radical and 1,4-endoperoxide species sequentially.9

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Moreover, the photoefficiency of bare CN for H2O2 production could be enhanced

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through the modification of bandgap (by incorporation of pyromellitic diimide into

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polymeric CN matrix)10 and surface properties (by amine rich mesoporous structure)

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of CN.11 In this work, potassium, phosphorus, and oxygen elements were simply

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introduced into polymeric CN matrix by solid-state thermal polymerization of

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melamine in the presence of potassium phosphate dibasic (KPD, K2HPO4). Not only

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the surface catalytic behaviour induced by the phosphate species but also the

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formation of charge trapping sites originated from hetero-elements existing over

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polymeric CN matrix, established a high apparent quantum efficiency (AQE) for

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H2O2 production without adding any noble metal cocatalysts.

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2. Results and Discussion

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2.1. Hetero-Elements (K, P, and O) Incorporated CN Polymeric Structure

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The X-ray diffraction (XRD) patterns in Figure 1a show that the in-situ

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incorporation of hetero-elements into CN framework induced a significant reduction

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of the typical peaks of CN (at 27.5o and 13.0o), ascribed to the interlayer (002)

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stacking and the interplanar structural packing of tri-s-triazine units forming 1-

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dimensional melon strands, respectively.12 According to the KPD content added (x

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mmol; x = 1, 3, 5, 7.5, and 10), the samples are noted as KPD-CN-1, 3, 5, 7.5, and

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10, respectively. When the KPD content increased from x = 1 to 10 at 550 oC, the

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peak intensity at 13.0o and 27.5o gradually decreased (Figure S1a), which indicates

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that the polymeric structure of KPD-CN-x is possibly derived from the simultaneous

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condensation of melamine and KPD. Basically, prior to reaching the sublimation

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temperature of melamine (350 oC), the condensed melamine phosphates consisting

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of various phosphate species such as orthophosphate, pyrophosphate, polyphosphate,

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and polymethaphosphate can be generated by the step-by-step condensation

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reaction,13 which are known as a halogen-free flame retardant with an excellent

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thermal resistance.14 In the case of sole melamine, melamine-based products are

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dominant up to 350 oC, but tri-s-triazine (melem) starts to be generated as a result of

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melamine rearrangement over 350 oC.15 With further increase of the temperature, the

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condensation of tri-s-triazine to polymeric networks induces the formation of CN

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around 520 oC, and the decomposition is started to be proceeded above 600 oC. The

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XRD pattern of KPD-CN-7.5 exhibits that the melamine and KPD peaks disappeared

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with increasing the calcination temperature (Figure S1b), and any trace peaks of

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original melamine and KPD were not observed at all (Figure S1c and S1d). The

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result supports that the hetero-elements were well incorporated into the CN

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framework without leaving melamine or KPD compound unreacted.

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Diffuse reflectance UV-visible spectra (DRS) demonstrates that the incorporation

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of KPD into CN increased the absorbance over UV light region, whilst the change of

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visible light absorption was almost negligible (Figure 1b and Figure S2a). As a result,

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the difference of bandgap size between CN and KPD-CN-7.5 was estimated to be

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only about 0.08 eV. To confirm the shift of the CB and VB potential affected by

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KPD, the X-ray photoelectron spectroscopy (XPS) VB analysis and the

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electrochemical Mott-Schottky analysis were carried out. The VB maximum edges

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of CN and KPD-CN-7.5 are determined to be about 1.75 and 1.86 eV, respectively

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(Figure 1c), and the Mott-Schottky plots showing n-type characteristics (positive

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slope) of CN and KPD-CN-7.5 are almost the same (Figure 1d). Contrary to the

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previous studies on bandgap-engineered CN which clearly showed the bandgap size

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decrease/increase and the shift of the CB/VB potential as well,16 the hetero-elements

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did not markedly change their electronic band structure.

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As presented in Figure 2a and Table S1, the typical C-N peaks of CN between

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1200 and 1600 cm-1 were little changed in KPD-CN-7.5. The incorporation of

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potassium ions into CN framework shifted the stretching modes of tri-s-triazine ring

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at 1393 and 1538 cm-1 to 1404 and 1571 cm-1, respectively, due to a strong ion-

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dipole interaction at the nitride pots.4,17 Similar with chelation, metal ions including

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cobalt, manganese, potassium, iron, etc. can be coordinated with nonbonding

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electrons of nitrogen in the nitride pots (pore induced by trigonal bonding of tri-s-

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triazines).4, 17 The P-relevant peak of phosphate at 847 and 984 cm-1 and the P-O-C

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stretching at 903 and 1146 cm-1 were newly formed and these peaks became

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predominant with increasing KPD content (Figure S2b).18-20 On the other hand, the

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out-of-plane bending mode (also assigned as the deformation mode of N-H) at 888

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cm-1 completely disappeared.21,22 The result indicates that potassium ions and

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phosphate species are well-assembled inside polymeric CN matrix.

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The surface property of CN and KPD-CN-7.5 was also evaluated by zeta-potential

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analysis since the primary (C-NH2), secondary (C-NH-C), and tertiary (C3N) amines

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present in CN can be protonated and deprotonated depending on pH.23 The CN

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exhibits the point of zero charge (PZC) at around pH 3 (Figure 2b). However, KPD-

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CN-7.5 predominantly carries negative charge over the entire pH range from 1 to 11

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without showing PZC in terms of phosphate species.

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To analyze the chemical states for each carbon, nitrogen, potassium, phosphorus,

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and oxygen element consisting of CN and KPD-CN-7.5, a high-resolution XPS

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analysis was carried out. As shown in Figure 2c and Figure S3b, the K2p peaks were

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clearly observed at 292.6 and 295.4 eV in KPD-CN-7.5 since potassium ions were

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coordinated with nonbonding electrons of nitrogen via ion-dipole interaction, which

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is analogous to the molecular structure of metalloporphyrin and KMnO4•18-crown-

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6-ether.24,25 In Figure S3a and S3b, the C1s peaks were deconvoluted into six

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components in both CN and KPD-CN-7.5. Each deconvoluted peak (C1-6, N1-5,

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and O1-3) is denoted in Table S2. The relative intensity of the C2, C5, and C6 peaks

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increased along with the decrease of C4 intensity, which revealed that more carbon

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sites were interacted or replaced by the oxygen atoms present in phosphate species.

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Furthermore, the binding energy of C4 was positively shifted (0.2 eV) due to higher

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electronegativity of oxygen than nitrogen, and the formation of cyano group

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increased the C3 peak intensity. In the case of N1s spectra (Figure S3c and S3d), the

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ratio of N2 to (N3 + N4) was calculated as 2.12 in CN and 2.43 in KPD-CN-7.5,

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respectively. The N2 mainly participates in the build-up of bandgap structure but the

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N3 and N4 are in charge of bulk and surface properties so it can be concluded that

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the surface properties induced by protonated N3 and N4 are less accessible.26 The

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elemental XPS C/N ratio (Table S3) and elemental C/N ratio obtained by elemental

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analysis (Table S4) of CN steadily decreased with increasing the atomic percent of

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phosphorus and oxygen. Although the C/N ratio of bulk CN is not consistent

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between the elemental XPS and elemental C/N analysis (the real C/N ratio can be

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changed by the experiment conditions27,28), the decrease trend is the same in both

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cases. The P2p band was absent in CN and the single symmetric peak at 132.9 eV

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was observed in KPD-CN-7.5 (Figure S3e and S3f), which indicates that P in KPD-

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CN-7.5 is mainly combined with O instead of C and N (since P2p binding energy of

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P-C, P-O, and P-N is about 131.0, 132.9, and 133.5 eV, respectively29-31). As a result,

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the phosphate incorporation in the CN matrix induced the appearance of the O1s

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peak centered at 531.0 eV that is absent in CN (Figure S3g and S3h).

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Solid-state

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P magic angle spinning (MAS) nuclear magnetic resonance (NMR)

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spectra of CN and KPD-CN-7.5 are compared in Figure 2d. While no peak is

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observed at all in CN, a broad peak from 17 to -50 ppm (δmax = -11.6 ppm) and

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spinning side bands are clearly shown in KPD-CN-7.5. In general, the chemical

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shifts on phosphate monoester, orthophosphate, phosphate diester, pyrophosphate,

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and polyphosphate are normally observed around at 0 ≤ δ ≤ 10, 0 ≤ δ ≤ 5, 0 ≤ δ ≤ -5,

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-5 ≤ δ ≤ -10, and -10 ≤ δ ≤ -30 ppm, respectively.32,33 Furthermore, there is no

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prominent chemical shift induced by the phosphonate group (C-P) usually observed

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over 20 ppm. The result supports that the phosphate species do not exist as a sole

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compound, but various species are incorporated into the complex CN matrix. Solid-

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state proton-decoupled

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magnetic resonance (CP-MAS NMR) spectrum of CN exhibited two evident signals

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(Figure S4). The strong peak recorded at 164.7 ppm belongs to the terminal carbon

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(N2-C-NH2 groups), and the second peak at 157.0 ppm is ascribed to the internal

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carbon (C-(N)3 groups).34 The

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revealed two signals centered at 163.9 and 156.6 ppm. The quantitative intensity

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analysis of CP-MAS NMR signals is not reliable but the number of signals and their

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relative ratio provide the useful information. The ratio of the terminal carbon to the

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internal one of CN and KPD-CN-7.5 was measured as 1.55 and 2.88, respectively.

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Considering the decrease of C/N ratio in KPD-CN-7.5, the replacement of carbon

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atoms could occur on the internal carbon sites and the formation of C-O-P bond

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might take place on the terminal sites.

13

C cross polarization and magic angle spinning nuclear

13

C CP-MAS NMR spectrum of KPD-CN-7.5 also

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The morphologies and the chemical composition of CN and KPD-CN-7.5 were

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confirmed by high resolution transmission electron microscope (HR-TEM) and

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energy-dispersive X-ray spectroscopy (EDS) analysis, respectively. The EDS

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mapping confirmed the presence of predominant carbon and nitrogen, and the small

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amount of oxygen as well, but any potassium and phosphorus was not measured at

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all in CN (Figure 3b-3f). However, in the case of KPD-CN-7.5, carbon, nitrogen,

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potassium, phosphorus, and oxygen were homogeneously dispersed over the

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materials (Figure 3h-3l). To further characterize the properties of CN and KPD-CN-

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7.5, the BET surface area and pore volume measurement (Figure S5a),

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thermogravimetric analysis (Figure S5b), and field-emission scanning electron

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microscopy (Figure S6) were carried out (see Supporting Information regarding the

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detailed result discussion).

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2.2. Photocatalytic Production of H2O2

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First of all, it should be mentioned that the present study is not an ideal

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photosynthesis of H2O2 that utilizes water and O2 only since an excess of ethanol

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was used as an electron donor instead of water. Considering the standard reduction

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potentials of O2 reduction (E0(O2/H2O2) = 0.695 VNHE) and ethanol oxidation

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(E0(CO2/C2H5OH) = 0.085 VNHE), the overall process of H2O2 production in the

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present study is thermochemically spontaneous (that is, photocatalytic, not

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photosynthetic). The main focus of this work is to investigate the effects of hetero-

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elements on the half reaction of H2O2 production via O2 reduction, not to establish

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the ideal photosynthesis. On the other hand, the ethanol oxidation coupled with the

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production of H2O2 was not analysed in this work because the photoreaction was

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carried out under continuous O2 purging condition, which should volatilize the

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products of ethanol oxidation immediately from the solvent. However, in a previous

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study which utilized pure CN for the photochemical production of H2O2 in

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ethanol/water solvent in a closed reactor,9 most of ethanol was oxidized to

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acetaldehyde with almost stoichiometric production of H2O2 with a molar ratio of

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1:1 (i.e., O2 + CH3CH2OH → H2O2 + CH3CHO). The ethanol oxidation part should

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be similar in this study as well.

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The photoactivity was tested by monitoring the visible light-induced production

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of H2O2. The photo-production of H2O2 was compared among the CN and KPD-CN-

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x (x=1, 3, 5, 7.5, and 10) samples in the presence of ethanol as an electron donor in

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the O2-saturated catalyst suspension under visible light irradiation (λ ≥ 420 nm)

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(Figure S7a). All KPD-CN-x samples exhibited markedly enhanced activities

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compared with CN although the specific surface areas of all KPD-CN-x samples are

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significantly lower than bare CN (see Figure S5a). This indicates that the surface

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area of the catalyst samples is not an important factor and that the enhancement of

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the photocatalytic activity should be ascribed to other modified properties (e.g.,

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charge recombination, separation and interfacial transfer, nature of catalytic active

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sites) induced by the incorporation of potassium and phosphate species. The

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photoactivity for H2O2 production increased with increasing KPD content and

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reached a maximum at x=7.5. Therefore, the KPD-CN-7.5 was chosen as an optimal

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sample for further experiments. To investigate the wavelength dependence of the

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H2O2 production, the AQY was measured using a monochromatic light (λ = 320, 370,

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420, 470, and 520 nm) irradiation. As shown in Figure 4a, AQY for the production

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of H2O2 at 320 nm and 420 nm in KPD-CN-7.5 was 17 and 25 times higher than that

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of CN, respectively. In addition, the turnover frequency (TOF) of the KPD-CN-7.5

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at 320 nm and 420 nm was 49.8 and 73.1 times higher than that of CN, respectively.

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Considering that the production of H2O2 requires a PCET process, KPD-CN-7.5 is

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advantageous since it has phosphate functionalized polymeric structure, which can

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facilitate a proton transfer process.16,35-37 Since the production of H2O2 via PCET

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should be highly dependent on the proton concentration, it decreased with increasing

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pH (Figure S7b). The production of H2O2 at pH 1.0 was more than doubled from that

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at pH 3.0 for CN while the enhancement factor ([H2O2]pH=1/[H2O2]pH=3) was only 1.1

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for KPD-CN-7.5. The proton transfer in KPD-CN-7.5 was facilitated enough by the

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presence of the phosphate species, and therefore, a higher concentration of proton (at

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lower pH) has a relatively lower enhancement effect on the production of H2O2 than

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in the case of CN. Moreover, KPD-CN-7.5 exhibited much higher photoactivities

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than CN up to pH 9.0. To check the product selectivity for H2O2 compared to H2, the

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H2 evolution test was also carried out in the presence of ethanol in a tightly-sealed

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reactor under Ar(g) atmosphere under UV light irradiation for 12 h. Both CN and

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KPD-CN-7.5 evolved a very tiny amount of H2 (below 0.1 µmol), which supports

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that two-electron transfer to O2 coupled with protons (Table S5, eq. 5: O2 + 2H+ +

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2e- → H2O2) is much more favored than the direct two-electron transfer to protons

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(Table S5, eq. 18: 2H+ + 2e- → H2).

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The photoactivity tests with CN and KPD-CN-7.5 were performed in air-, Ar-,

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and O2-saturated suspensions to verify the role of dioxygen for the photoproduction

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of H2O2 (Figure 4b). In general, the photo-excited CB electrons can be scavenged by

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dissolved O2 in air- or O2-saturated condition but the protons should uptake the

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electrons in the absence of O2 (Ar-atmosphere). The rate of H2O2 formation on KPD-

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CN-7.5 was markedly enhanced in the O2-saturated suspension from that in the air-

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saturated one. While the [H2O2]O2/[H2O2]air at 3 h was measured to be 1.7 for KPD-

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CN-7.5, it was 1.2 for CN. Under Ar-saturated condition, no H2O2 was formed not

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only with CN but also with KPD-CN-7.5, which confirms that H2O2 is generated

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through the reduction of O2. The KPD-CN-7.5 was stable enough to maintain the

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photocatalytic activity during the repeated cycles of H2O2 production as shown in

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Figure 4c. On the other hand, it should be noted that the overall production of H2O2

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is determined by the photocatalytic decomposition of H2O2 as well as its production

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since the photogenerated H2O2 can be subsequently decomposed on the

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photocatalyst surface. Therefore, the overall photoconversion efficiency can be

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enhanced not only by facilitating the photoreduction of O2 but also by inhibiting the

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decomposition of H2O2 on the photocatalyst surface. For example, our recent study

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demonstrated that the modification of TiO2 surface by phosphate loading inhibited

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the adsorption of H2O2 with significantly retarding the photodecomposition of H2O2,

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which resulted in a highly enhanced production of H2O2.6 In this respect, KPD-CN-

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7.5 is much better than TiO2 for H2O2 production because the adsorption of H2O2 on

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the former is negligible but the latter shows some adsorption affinity for H2O2

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[Adsorption of 100 µM H2O2 in the dark under continuous stirring for 0.5 h:

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[H2O2]ad = 0 µM for KPD-CN-7.5 (surface area: 3.4 m2/g) vs. [H2O2]ad = 12.1 µM

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for TiO2 (surface area: 47 m2/g); Although the surface area of the tested TiO2 is

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higher than KPD-CN-7.5, the higher adsorption of H2O2 on TiO2 should be ascribed

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mainly to the strong surface complexation of H2O2 on TiO2 (>Ti(IV)-OH)surf + H2O2

287

→ (>Ti(IV)–OOH)surf + H2O)] As a result, CN and KPD-CN-7.5 did not degrade

288

H2O2 at all under visible light (λ ≥ 420 nm) (Figure S7c). Under UV irradiation (λ ≥

289

320 nm), on the other hand, CN exhibited some activity for H2O2 decomposition but

290

KPD-CN-7.5 did not degrade H2O2 at all. The inactivity of KPD-CN-7.5 for the

291

decomposition of H2O2 should also contribute to the higher activity of KPD-CN-7.5

292

than CN. In addition, the production of H2O2 was highly affected by not only the

293

type of the electron donor (2-propanol > ethanol > methanol >> water) (Figure S8a

294

and S8b) but also the ratio of alcohol to water (Figure S8c). With CN, the

295

photocatalytic production of H2O2 was dramatically inhibited by increasing the water

296

content (from 10% to 90%) whereas that of KPD-CN-7.5 was less affected by

297

increasing the water content. That is, KPD-CN-7.5 should be much more favoured

298

than CN for the production of H2O2 in water-rich environment.

299 300

2.3. Charge Transfer Dynamics

301

The charge separation behaviour of carbon nitrides was investigated by steady-state

302

photoluminescence (PL), time-resolved PL (TRPL), slurry-type photocurrent

303

measurement, and time-resolved diffuse reflectance spectroscopy using femtosecond

304

laser. As shown in Figure 5a, the radiative recombination of excited charge pairs was

305

clearly observed in CN while the PL intensity was markedly reduced with increasing

306

KPD content, which indicates that the radiative recombination is much retarded in

307

KPD-CN-7.5 compared with CN. The wavelength dependent PL emission intensity

14


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308

for CN, KPD-CN-3, KPD-CN-7.5 reveals that the PL intensity linearly enhanced

309

with increasing the excitation wavelength for all samples since the excitation light

310

intensity has a linear correlation with the excitation wavelength as well (Figure S9).

311

However, the slope linearly decreased with increasing KPD content, which implies

312

that the hetero-elements effectively prohibit the radiative recombination through the

313

formation of charge trapping sites. The PL excitation spectrum detected at 470 nm

314

for CN shows three distinct PL bands at 271, 367, and 429 nm together with a broad

315

background signal (Figure 5b). Although the PL band at 271 nm was not observed

316

and the background signal was weak in KPD-CN-7.5, the other two PL bands were

317

still visible. The result supports that the hetero-elements generate the trapping sites

318

for charge carriers rather than the change of band structure. The mobile electrons and

319

holes produced by the excitons can freely diffuse or can be shallowly trapped over

320

the polymeric CN lattice in CN, whereas the charge carriers can be deeply trapped

321

and then the non-radiative recombination markedly takes place in KPD-CN-7.5.

322

Furthermore, it is quite probable that the recombination can be facilitated by the

323

decrease of CN wt% with increasing KPD content (Table S4) since the radiative

324

recombination occurs in polymeric CN matrix. Therefore, the average lifetime of

325

charge carriers in CN, KPD-CN-3, and KPD-CN-7.5 was estimated by fitting the

326

decay curve with three exponential terms to yield 2.51, 1.68, 0.83 ns, respectively

327

(Figure 5c). The interfacial electron transfer was also investigated by measuring the

328

PW12O303-/4- redox couple (Eo = +0.22 VNHE)-mediated photocurrent collected in the

329

visible light-irradiated suspension of catalysts.38 Figure 5d shows that the time

15


ACS Catalysis

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

330

profiles of photocurrent generation were enhanced with KPD-CN-7.5 in comparison

331

with CN, which is consistent with the higher visible light activity of KPD-CN-7.5.

332

To directly observe the presence of charge trapping sites on CN, KPD-CN-3, and

333

KPD-CN-7.5, time-resolved diffuse reflectance spectra were measured. To avoid the

334

precipitation problem occurring in aqueous suspension, all samples were

335

homogeneously coated on glass slide and reflectance change induced by laser

336

excitation was measured. Until now, few transient absorption spectroscopic studies

337

were done for CN (protonated CN,28 colloidal CN,27 mesoporous CN,39 and

338

polymeric CN40), so more depth investigation is highly required. In this study, we

339

focused on the lifetime of transient species formed by trapped charge carriers to

340

confirm the role of hetero-elements incorporated in CN framework. As shown in

341

Figure 6, the broad positive absorption band was similarly observed for all samples

342

after laser pulse at 400 nm under air. Recently, Zhang et al. reported that the positive

343

absorption was attributed to the formation of charge separation states by

344

photogenerated electron/hole pairs in colloidal CN system.27 Kuriki et al. also

345

defined that the broad positive absorption was originated from deeply trapped

346

electron/hole pairs.39 The transient absorption decay for charge separation states was

347

well fitted by the biexponential equation for all samples, and the average lifetime of

348

CN, KPD-CN-3, and KPD-CN-7.5 was calculated as 353.1, 477.7, and 1194.0 ps,

349

respectively. Although the transient absorption intensity was a little reduced in KPD-

350

CN-7.5 relative to CN due to less wt% of CN content, the charge separation process

351

is improved by the hetero-elements. The results support that the incorporation of

16


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ACS Catalysis

352

hetero-elements into polymeric CN matrix could generate not only the extra charge

353

trapping sites but also the catalytic sites favourable to the PCET process, which

354

should influence the recombination kinetics and the interfacial charge transfer

355

characteristics to facilitate the photoproduction of H2O2.

356 357

2.4. Other Hetero-Elements Incorporated CN Materials

358

Other chemicals instead of KPD were employed as alternative reagents to investigate

359

the cationic and anionic effects on the photocatalytic production of H2O2

360

systematically and their XPS elemental analysis was also carried out (Figure 7 and

361

Table S6). When the ratio of proton to potassium was changed using H3PO4,

362

KH2PO4, K2HPO4, and K3PO4, the relative percentage of potassium, phosphorus,

363

and oxygen consisting of the samples increased in the order K3PO4 > K2HPO4 >

364

KH2PO4 > H3PO4. However, the highest photocatalytic activity was obtained with

365

K2HPO4 instead of K3PO4 because the polymeric CN matrix was decomposed in the

366

presence of K3PO4 (the C/N ratio = 0.32) (Figure 7a). No potassium was found in the

367

sample when H3PO4 was used. On the other hand, when H2SO4, KHSO4, and K2SO4

368

were employed as alternative reagents, the potassium content in the CN matrix was

369

in the order of K2SO4 > KHSO4 > H2SO4 but no sulfur element was found in the

370

samples. The highest photocatalytic activity was obtained with KHSO4, but the

371

amount of H2O2 produced was lower than that of KPD-CN-7.5 (Figure 7b). The

372

result demonstrated that KPD is a good source to incorporate both phosphate species

373

and potassium ions into the CN matrix. As for the cation effect, different dihydrogen

17


ACS Catalysis

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

374

phosphate reagents containing various cations were also tested and compared for the

375

photocatalytic activities for H2O2 production, which exhibited the reactivity order of

376

KH2PO4 > NaH2PO4 > LiH2PO4 > H3PO4 > (NH4)H2PO4 (Figure 7c). When different

377

anions (HCO3-, HSO4-, and H2PO42-) combined with potassium ion were tested, the

378

best photocatalytic activity was obtained with H2PO42- (Figure 7d). Among various

379

combinations of cationic and anionic species for the incorporation into the CN

380

matrix, the combination of potassium and dihydrogen phosphate in KPD-CN-7.5

381

showed the best photocatalytic performance.

382

383

3. Conclusions

384

Despite the simple synthetic procedure for CN, its polymeric structure is still

385

controversial (even s-heptazine has been recently considered as a basic building

386

block) and the synthetic mechanism is not fully understood.41 Therefore, it does not

387

make much sense to conclude the exact structure and synthetic mechanism in the

388

present case of modified CN. However, the analyses using various spectroscopic

389

methods revealed that the earth-abundant hetero-elements (K, P, and O) could be

390

incorporated into the carbon nitride framework via thermal polymerization to modify

391

the surface and charge transfer properties significantly and consequently to enhance

392

the photoactivities for the production of H2O2. According to DRS, XPS, and Mott-

393

Schottky plots, the incorporation of the multiple hetero-elements does not affect the

394

bandgap size and the band potential shift. The presence and chemical states of the

18


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ACS Catalysis

395

hetero-elements were confirmed by FT-IR, Zeta-potential analysis, XPS, solid state

396

NMR, and EDX mapping, which comes to conclusions that 1) potassium is captured

397

as an ion by non-pair electrons of nitrogen present in CN matrix, 2) phosphorus and

398

oxygen elements exist as various phosphate species, and 3) carbon sites are attacked

399

by the oxygen present in the phosphate species relative to nitrogen sites. The high

400

photoactivity of KPD-CN-7.5 and particularly high selectivity toward H2O2 over H2

401

are attributed to 1) the enhanced light absorption (UV light region), 2) the increased

402

lifetime of the transient species, 3) the effective interfacial charge transfer to

403

dioxygen via PCET, and 4) the inhibited decomposition of in-situ generated H2O2. It

404

should be noted that such unique activity and selectivity for H2O2 production cannot

405

be an additive property of K-doped CN and P-doped CN at all (see Figure 7). The

406

KPD-modified CN is uniquely suited for the photochemical production of H2O2. The

407

incorporation of various earth-abundant elements into CN could be also possible for

408

a variety of applications. This study clearly demonstrated that the hetero-elements

409

incorporated CN can be exploited for cost-effective and environment-friendly

410

photocatalysts without utilizing any noble metals, which should be important for the

411

development of practical and economical solar conversion photocatalytic materials.

412

413

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ACS Catalysis

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

414

4. Experimental Section

415

Materials preparation: To synthesize CN, 4 g of melamine (99%, Aldrich) in a

416

porcelain cup covered with a cap was calcined at 550 oC for 4 h (heating and cooling rate

417

was 2.2 oC/min, respectively.). After grinding the CN sample homogeneously, the aqueous

418

solution containing CN (1 g/L) was sonicated in an ultrasonic bath (JAC 4020, 400 W,

419

Sonic) for 3 h, filtered, washed, and finally dried at room temperature. Modified CN

420

compounds were prepared through a similar synthetic procedure of CN. 4 g of melamine

421

and each 1, 3, 5, 7.5, and 10 mmol of KPD (K2HPO4 99.95%, Aldrich) were

422

homogeneously ground, respectively. On the basis of 7.5 mmol, 4 g of melamine was

423

mixed and ground with phosphoric acid (85 wt.% in water, Aldrich), potassium phosphate

424

monobasic (99.99%, Aldrich), potassium phosphate tribasic (98%, Aldrich), sulfuric acid

425

(95.0-98.0%, Aldrich), potassium bisulfate (99%, Aldrich), potassium sulfate (99.99%,

426

Aldrich), ammonium phosphate monobasic (99.999%, Aldrich), lithium phosphate

427

monobasic (99.99%, Aldrich), sodium phosphate monobasic (99.0%, Aldrich), and

428

potassium bicarbonate (99.95%, Aldrich), respectively. Phosphoric acid or sulfuric acid

429

mixed melamine powder was dried at 80 oC for 3 h to remove water and then was ground

430

again. The other procedure is the same with CN synthetic procedure.

431

Photocatalytic activity tests: 20 mg of each sample was dispersed in the solution

432

containing 36 mL of purified water and 4 mL of ethanol by ultra-sonication. The pH of the

433

suspension was controlled by HClO4 or KOH. Before the irradiation, the suspension was

434

stirred under O2 purging for 30 min to make it O2-saturated. The reactor was irradiated by a

20


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ACS Catalysis

435

300-W Xe arc lamp (Oriel) under continuous stirring and purging. Light was passed

436

through a 10-cm IR filter and a cutoff filter (λ ≥ 420 nm or λ ≥ 320 nm), and then was

437

focused onto the reactor. The intensity of the incident light filtered through cutoff filters (λ

438

≥ 420 nm and λ ≥ 320 nm) was determined to be about 726.8 mW/cm2 and 833 mW/cm2,

439

respectively, which was measured by a power meter (Newport 1830-C).

440

To calculate the apparent quantum yield (AQY), the incident light was passed through a

441

SAP301 grating monochromator (Newport, Oriel 77250). The light intensity was measured

442

using an optical power meter (1815-C, Newport) with a photodiode detector (818-UV,

443

Newport). The light intensity at 320, 370, 420, 470, and 520 nm was measured as 0.16,

444

0.40, 0.74, 1.06, and 0.75 mW, respectively. AQY was estimated from the formula of

445

ΦAQY(%) = (Number of H2O2 formed x 2)/ (Number of incident photons) x 100. The

446

turnover number was calculated after 6 h irradiation. The number of C6N8 units on the

447

surface was assumed as 7.55 x 10-5 mol/g in both CN and KPD-CN-7.5. The C6N8 unit per

448

area was 0.22 nm-2, and the surface area of CN and KPD-CN-7.5 was 10 and 3.4 m2/g,

449

respectively.

450

The recycle test was performed as follows. 20 mg of the sample was dispersed in the

451

solution containing 36 mL of purified water and 4 mL of ethanol (pHi = 3), which was

452

irradiated at λ ≥ 420 nm under continuous O2 purging condition. After 1st cycle, the

453

suspension was filtered, washed, and then re-dispersed in the solution (0.5 g/L of the

454

photocatalyst and 10 vol.% of ethanol) by ultra-sonication. The same procedure was

455

repeated up to 4th cycle. The photocatalytic decomposition of H2O2 with using bare CN and

456

KPD-CN-7.5 was carried out as follows: 0.5 g/L of the photocatalyst was dispersed in the

21


ACS Catalysis

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

457

solution containing 5 mM of H2O2 at pH 3.0. Before the irradiation, the suspension was

458

kept in dark for 30 min and then visible (λ ≥ 420 nm) or UV light (λ ≥ 320 nm) was

459

sequentially irradiated under continuous O2 purging condition. Methanol and 2-propanol

460

were also tested as an alternative electron donor. The photocatalytic H2 evolution was

461

tested as well to estimate the selectivity toward H2O2 by a gas chromatograph (GC,

462

HP6890N) with a thermal conductivity detector.

463

464

Supporting Information

465

Experimental details, characterizations of KPD-CN-x (x = 0, 1, 3, 5, 7.5, and 10) (XRD,

466

DRUVS, FT-IR, high resolution XPS,

467

volume, TGA, FE-SEM, steady-state photoluminescence, and elemental analysis), the

468

photocatalytic generation of H2O2 depending on KPD content, pH, and electron donors, and

469

the photocatalytic decomposition of H2O2 under UV and visible irradiation. This material is

470

available free of charge via the Internet at http://pubs.acs.org.

13

C CP-MAS NMR, BET surface area & pore

471

472

Acknowledgements

473

This work was financially supported by the Global Research Laboratory (GRL)

474

Program (No. NRF-2014K1A1A2041044) and KCAP (Sogang Univ.) (No. 2009-

475

0093880), which were funded by the Korea Government (MSIP) through the

476

National Research Foundation (NRF).

22


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ACS Catalysis

477

478

References

479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523

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24


Page 24 of 33

Page 25 of 33

558 559

560

563

* CN

KPD-CN-7.5 564 20

40

565

568

2.0 1.5

1 0

CN

2.0

2.5

3.0

3.5

4.0

hν ν (eV)

0.5

KPD-CN-7.5 300

400

500

600

700

Wavelength (nm)

2θ θ

(c)

2

1.0

0.0 200

80

CN KPD-CN-7.5

3

(d)

CN

4

3 10 -2

CN KPD-CN-7.5

KPD-CN-7.5

2

-2

567

2.5

C /10 F

566

60

4

(hν ν F(R))1/2

Intensity (a.u.)

561 562

(b) 3.0

*

Absorbance (K.M)

(a)

Intensity (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

ACS Catalysis

1

569 570

-2

0

2

4

6

8

10

12

0 -0.5

14

0.0

0.5

1.0

Potential (V) vs. Ag/AgCl

Binding Energy (eV) 571 572

Figure 1. (a) XRD spectra, (b) DRUVS spectra, (c) XPS valence band analysis, (d) Mott-

573

Schottky plots for bare CN and KPD-CN-7.5. Inset in (b): Tauc plot for bare CN and KPD-

574

CN-7.5.

575 576 577 578

25


ACS Catalysis

579

(b)

(a)

581 582 583

CN

C≡N

KPD-CN-7.5

584

*

*

586

20

CN KPD-CN-7.5

0

-20

-40

-60

1000

585

ζ potential (mV)

580

Transmittance (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

Page 26 of 33

1500

2000

2500

0

2

4

-1

P2p

8

10

12

pH

Wavenumber (cm )

(c)

6

(d)

C1s

O1s

K2p

CN

587 588

KPD-CN-7.5 KPD-CN-7.5

589

CN 130

590

135

* 285

290

295

530

535

100

* 50

0

-50

-100

ppm

Binding Energy (eV) 591 592

Figure 2. (a) ATR FT-IR spectra (Asterisks indicate the bonding, P-O-C, and the arrows

593

represent the peak shift of typical peaks of CN) of bare CN and KPD-CN-7.5. (b) zeta-

594

potential analysis of bare CN and KPD-CN-7.5. (c) XPS P2p, C1s, K2p, and O1s core-level

595

spectra of bare CN and KPD-CN-7.5. (d) 31P MAS NMR spectra of bare CN and KPD-CN-

596

7.5.

597 598 599

26


Page 27 of 33

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

ACS Catalysis

600

601 602

Figure 3. (a) TEM image of bare CN. (b-f) EDX mapping corresponding to panel (a). (g)

603

TEM image of KPD-CN-7.5. (h-l) EDX mapping corresponding to panel (g). Red, green,

604

blue, purple, and sky-blue color represent C (b and h), N (c and i), K (d and j), P (e and k),

605

and O (f and l) elements, respectively.

606 607 608 609 610 611 612 613 614 615

27


ACS Catalysis

616

(a) CN KPD-CN-7.5

617 618 619 620 621

(b) 2.0

CN (O2)

623 624

H2O2 [mM]

622

CN (Air) CN (Ar) KPD-CN-7.5 (O2)

1.5

KPD-CN-7.5 (Air) KPD-CN-7.5 (Ar)

1.0

0.5

625 0.0 0

626

(c) 627

2

628 629 630

4

6

8

Irradiation time (h) 3.5

KPD-CN-7.5 CN

3.0

[H2O2] (mM)

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 28 of 33

2.5

1st

2.0

3rd

2nd

4th

1.5 1.0 0.5 0.0

631

0

12

24

36

48

Irradiation time (h) 632 633

Figure 4. (a) Apparent quantum yield (AQY, Left axis) and turnover frequency (Right axis)

634

of bare CN and KPD-CN-7.5 for H2O2 production under monochromatic light irradiation.

635

(b) Comparison of the photoproduction of H2O2 under air-, N2-, and O2-saturated

636

conditions for bare CN and KPD-CN-7.5. (c) Repeated runs of H2O2 generation in the

28


Page 29 of 33

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

ACS Catalysis

637

visible light-irradiated suspension of bare CN and KPD-CN-7.5. (b, c) The experimental

638

conditions were: 0.5 g/L of photocatalyst, 10 vol% of ethanol, pHi = 3.0, and λ ≥ 420 nm

639

(polychromatic).

640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656

29


ACS Catalysis

657

(a)

CN

660 661

λex = 400 nm

4

KPD-CN-3

4

5

Intensity (10 CPS)

5

659

(b)

6

Intensity (10 CPS)

658

2

KPD-CN-7.5

CN

4 3

KPD-CN-3

2

x2 1 0 250

0 450

500

550

600

650

300

665 666 667 668

(d)

1

400

25

POM*

CN KPD-CN-3 KPD-CN-7.5

Photocurrent (µ µA)

664

(c)

350

Waverlength (nm)

Wavelength (nm) 663

KPD-CN-7.5

λem = 470 nm

662

PL Intensity (Norm.)

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 30 of 33

0.1

0.01

20

e

15 10

POM

-

KPD-CN-7.5 CN

5 0

0

10

20

30

40

50

0

500

1000

1500

2000

Time (s)

Time (ns)

669

Figure 5. (a) PL emission spectra (λex = 400 nm) and (b) PL excitation spectra at 470 nm of

670

bare CN, KPD-CN-3, and KPD-CN-7.5. (c) TRPL spectra monitored at 480-490 nm of bare

671

CN, KPD-CN-3, and KPD-CN-7.5 coated on glass slide after the excitation with 400 nm

672

laser pulse. The curves were fitted to triexponential equation: y = y0 + A1exp(-t/τ1) +

673

A2exp(-t/τ2) + A3exp(-t/τ3). (d) Time-profiles of POM-mediated photocurrent collected on a

674

Pt electrode in the irradiated suspension of bare CN and KPD-CN-7.5. The experimental

675

conditions were: 0.5 g/L catalyst, 2 mM PW12O303-, pHi = 3, λ ≥ 420 nm, 0.1 M NaClO4, Pt

676

electrode held at 0.7 V (vs Ag/AgCl), and continuously Ar-purged.

677 678

30


(a) %Abs

680 681

ps

0.20

10 52 102 202 522 1022 1522 2022

0.15 0.10

682 0.05

683

CN

0.00

684

700

ps 0.20

(d)

10 52 102 202 522 1022 1522 2022

0.15 0.10 0.05

KPD-CN-3

688 600

689

692

700

τ2 666.82 ps

A2

0.285

: 353.1 ps

0.2

500

1000

1500

2000

2500

3000

Delay Time (ps) 1.0 0.8

τ1

45.17 ps

A1

0.415

τ2

1103.5 ps

A2

0.287

: 477.7 ps

0.6 0.4 0.2

0

500

(e)

ps 0.20

(f)

10 52 102 202 522 1022 1522 2022

0.15 0.10 0.05

1000

1500

2000

2500

3000

Delay Time (ps)

Wavelength (nm)

KPD-CN-7.5

693 0.00

694

0.329

0.4

800

Normalized % Abs750nm

691

A1

0.0

0.00

690

τ1 81.48 ps

0.6

0

Wavelength (nm)

% Abs

687

0.8

800

(c) 685

1.0

0.0 600

686

(b)


679

% Abs

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

ACS Catalysis


Page 31 of 33

1.0 0.8

τ1 130.73 ps

A1

0.459

τ2 3057.0 ps

A2

0.262

: 1194.0 ps

0.6 0.4 0.2 0.0

600

700

800

0

500

Wavelength (nm)

1000

1500

2000

2500

3000

Delay Time (ps)

695

Figure 6. Time-resolved diffuse reflectance spectra for (a) bare CN, (c) KPD-CN-3, and (e)

696

KPD-CN-7.5 recorded by 400 nm femtosecond laser pulse excitation under air. Decay

697

curves of transient absorption intensity at 750 nm for (b) bare CN, (d) KPD-CN-3, and (f)

698

KPD-CN-7.5. The dotted line curves are fitted to the biexponential equation: y = y0 +

699

A1exp(-t/τ1) + A2exp(-t/τ2).

31


ACS Catalysis

700

703 704

None H2SO4

2.5

None H3PO4

2.0

K2HPO4

KHSO4

KH2PO4

H2O2 [mM]

702

(b) 1.5

(a) 3.0 H2O2 [mM]

701

K3PO4

1.5 1.0

1.0

K2SO4

0.5

0.5 0.0

0.0

705

0

4

8

0

12

4

Irradiation Time (h)

708 709 710

None H3PO4

None KHCO3

(NH4)H2PO4

1.5

LiH2PO4 NaH2PO4

1.0

KH2PO4

0.5

KHSO4

1.5

KH2PO4

1.0

0.5

0.0

0.0 0

711

12

(d) 2.0

(c) 2.0

707

8


H2O2 [mM]

706

H2O2 [mM]

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 32 of 33

4

8

12

0


4

8

12


712

Figure 7. (a) Different types of potassium phosphate. (b) Different kinds of potassium

713

sulfate. (c) Cationic effect based on dihydrogen phosphate species. (d) Anionic effect based

714

on potassium. The experimental conditions were as follows: 0.5 g/L of photocatalyst, 10

715

vol.% of ethanol, pHi = 3.0, and continuous O2-purged atmosphere.

716 717 718 719 720 721

32


Page 33 of 33

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

722

ACS Catalysis

Table of content (TOC)

723

33


Eco-Friendly Photochemical Production of H2O2 through O2

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