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Photocatalytic Formaldehyde Oxidation over Plasmonic Au/TiO2 under Visible Light: Moisture Indispensability and Light Enhancement Xiaobing Zhu, Can Jin, Xiao-Song Li, Jing-Lin Liu, Zhi-Guang Sun, Chuan Shi, Xingguo Li, and Ai-Min Zhu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01658 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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Photocatalytic Formaldehyde Oxidation over Plasmonic Au/TiO2 under Visible Light: Moisture Indispensability and Light Enhancement Xiaobing Zhu†, Can Jin†, Xiao-Song Li†, Jing-Lin Liu†, Zhi-Guang Sun†, Chuan Shi†, Xingguo Li‡, Ai-Min Zhu†,* †

Laboratory of Plasma Physical Chemistry, Center for Hydrogen Energy and Environmental Catalysis, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China. ‡

Beijing National Laboratory for Molecular Science (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

ABSTRACT: With an extension of the absorption band towards visible light, plasmonic photocatalysts hence directly harvest energy from solar light but not compromising the activity, offering a desirable way addressing energy and environmental issues. Here we demonstrate photocatalytic oxidation of formaldehyde in air over plasmonic Au/TiO2 catalyst under visible light in a single-pass continuous flow reactor. Compared to that under dark, a significant enhancement of up to five times reaction rate at 13% RH under visible light is achieved. Au/TiO2 catalyst exhibits very high activity, a complete conversion of formaldehyde of 83.3% under visible light at 44% RH, but is completely inactive in dry air even under visible light. Also, the plasmonic Au/TiO2 is efficient for photocatalytic oxidation of formaldehyde under visible light,

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which is evidenced by a slight difference of conversion between UV light and visible light. To disclose the underlying mechanism, in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectra studies are conducted. The contributions of TiO2 and Au (supported on TiO2), moisture and visible light are identified. It is ascertained that moisture is indispensable to carbonate decomposition and also accelerates dioxymethylene (DOM) oxidation to formate. Visible light enhances the rate-determining steps of formate oxidation to carbonate and carbonate decomposition. It appropriately illustrates the remarkable difference in activities. Based on the spectra experiments, hence such a pathway of formaldehyde oxidation is proposed, which undergoes four sequential reaction steps (k1, k2, k3, k4) with reaction conditions dependent on moisture and visible light over Au/TiO2 catalyst. Based on the approximately identical spectra which indicate the same reaction pathways under visible light or under dark, the insights into the mechanism for photocatalytic oxidation of formaldehyde in air under visible light over Au/TiO2, are obtained. KEYWORDS: Au/TiO2 catalyst, formaldehyde oxidation, photocatalysis, surface plasmonic resonance, visible light, reaction mechanism

1. INTRODUCTION Formaldehyde became known as a typical indoor air pollutant of volatile organic compounds (VOCs) for decades, which causes the “sick building” syndrome. It is required to be eliminated efficiently under the ambient conditions of pressure, temperature and humidity 1-6. Photocatalysis is an appropriate and promising technology for energy and environmental applications 7, of which the absorption of photon energy from solar light by photocatalysts can potentially drive redox reactions in particular under ambient conditions. Semiconductor titania (TiO2) as the


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benchmark photocatalysts due to high activity, chemical stability and the abundance has been extensively studied on the formaldehyde removal 1-2, 8-10.

However, as restricted by the wide

bandgap (3.2 eV for anatase and 3.0 eV for rutile) it can only absorb ultraviolet (UV) light which constitutes unfavorably less than 5% of the solar spectrum 11. To more efficiently utilize the solar light, state of the art photocatalysts such as nanocomposites 12-16, Z scheme 17-18 and visible light driven photocatalysts12-13, 15, 17-18, aim at the shifting the absorption band towards the visible light region. TiO2 supported Au catalyst (Au/TiO2) features great potentials due to the broad and strong absorption of visible light via surface plasmon resonance (SPR) of Au nanoparticles

11-12, 19-23

. Plasmonic photocatalysis takes

advantages of the synergy of noble metals (e.g. Au, Ag) and semiconductor photocatalysts (e.g. TiO2), and brings in significant changes to many aspects of the photocatalysis. It features the SPR effects on the promoted absorption, local electric field and photo-excitation of electrons and holes, and the formation of Schottky barrier junction to suppress the recombination of electrons and holes. The activity over plasmonic Au/TiO2 photocatalysts under visible light can be rationalized by the hot-electron transfer mechanism, of which hot electrons are excited by absorption of visible light on Au nanoparticles through SPR effect and injected to the conduction band of TiO2. However, very few studies

24-26

focused on visible light driven photocatalytic removal of

formaldehyde from air over plasmonic Au nanoparticles. Remarkable activities over plasmonic Au nanoparticles supported on non-photoactive ZrO2 or photoactive Fe2O3, TiO2 etc.

24-25

in a

batch reactor under blue light irradiation (λ, 400-500 nm) were achieved, but no activity was shown under dark 24. In our recent work 26, the kinetic study of the reaction over Au/TiO2 under irradiation by a Xe lamp (λ ≥ 420 nm) was performed. Moreover, the adsorption of formaldehyde


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on single crystal TiO2 (e.g. rutile (110)) has been studied in good details both computationally2730

and fundamentally (by experiments)28-31. The adsorption of formaldehyde exists in the

configuration of tilted adsorption (more stable) and straight adsorption27. First-principles DFT calculation elucidates the preferred adsorption to form dioxymethylene (DOM)27-28. So far, the reaction mechanism for visible light driven photocatalytic oxidation of formaldehyde over plasmonic Au/TiO2 is still far from being understood. In the present work, photocatalytic formaldehyde oxidation in air over plasmonic Au/TiO2 under visible light (green light) is performed. Activities for the reaction under various conditions of moisture and light (representing visible light thereafter, unless otherwise specified) in a singlepass continuous flow reactor are evaluated. Subsequently, in-situ diffuse reflectance infrared Fourier transform (DRIFT) spectra studies on the reaction mechanism in the DRIFT cell are conducted. With respect to the intermediate identification, and the course of intermediate conversion and CO2 production, four experiments are conducted, (i) the overall reaction of formaldehyde oxidation under dark, or (ii) the overall reaction under subsequent conditions of dry and light, wet and light followed by wet and dark, (iii) the conversion of formate or carbonate intermediate that is preconditioned by the overall reaction at wet under light, and (iv) the decomposition of carbonate species that is formed from CO2 adsorption, respectively. As a result, surface intermediates of DOM, formate and carbonate are identified, and the roles of TiO2 and Au nanoparticles (supported on TiO2), moisture and light are disclosed. Hence, the reaction pathway of formaldehyde oxidation is proposed. New insights into the reaction mechanism for photocatalytic formaldehyde oxidation under visible light over Au/TiO2 are obtained.

2. EXPERIMENTAL SECTION


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2.1 Catalyst Preparation and Characterization Au/TiO2 catalyst is prepared by the modified impregnation method

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using TiO2 (P25,

Deggusa) and HAuCl4 solution as catalyst support and Au precursor, respectively. The Au loading is determined as 1.07% in weight by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 2000DV, USA). The catalyst is dispersed in deionized (D.I.) water under stirring for 15 min. Such a slurry is coated uniformly onto a glass substrate, followed by drying at 80 oC for 1 h and calcination at 200 oC for 2h in air with a ramp rate of 10 oC/min. Samples of 15±1 mg Au/TiO2 on a 25 mm × 25 mm substrate are used for activity testing. Observations on the morphology of Au/TiO2 sample are conducted by a transmission electron microscopy (TEM) (HT7700, HITACHI, Japan) operating at 100 kV. The average size and the distribution are given on the basis of 310 counts of Au nanoparticles on various TEM images. UV-Vis diffuse reflectance spectra measurements on Au/TiO2 sample are conducted in air by lambda 750s spectrometer (Perkin-Elmer, USA) at wavelengths ranging from 200 to 850 nm. 2.2 Photocatalytic Activity Evaluation Photocatalytic oxidation of formaldehyde in a single-pass continuous flow reactor is conducted, as shown in Figure S1 and described elsewhere 10, 26. The sample locates underneath a quartz window that allows visible light irradiation vertically onto catalyst. The reactor is maintained at 20±1 oC by water circulation and operated under ambient pressure. Formaldehyde is produced via the vaporization of solid 1,3,5-trioxane (>99%, ALDRICH) with a carrier gas of N2, followed by the catalytic depolymerization of trioxane vapor at 433 K over 85% phosphoric acid coated glass pellets. Prior to any reaction, the measurement on formaldehyde concentration is conducted by the conversion of formaldehyde to CO2 in a homemade VOC-to-CO2 converter at 673 K3. A gaseous mixture of 200 mL/min synthetic air (20% O2 balance 80% N2) containing


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50±2 ppm formaldehyde with a designated humidity is fed in the reactor. The reaction proceeds under dark or under visible light with time on stream (donated as TOS). Relative humidity (donated as RH) in reactant gas is measured by a dew point hygrometer (635-2, Testo, Germany). The product gas is analyzed by an online mass spectrometer (HPR 20 QIC, HIDEN), and no organic product is identified. CO and CO2 concentrations are determined online by a COx analyzer (S710, Sick-Maihak, Germany). A green LED provides a source of visible light (green light) of low flux (also called light intensity elsewhere). The emission spectra of the green LED are measured by a monochromator (SR-750-B1, Andor, UK). During the photocatalytic oxidation reaction, unless otherwise specified, the flux of green light is maintained at 38.5 mW/cm2 that is measured by an optical power and energy meter (PM100USB, Thorlabs, USA). The conversion of formaldehyde XHCHO, residence time τ, reaction rate r, and apparent rate constant ka obeying first-order kinetics10, 26, are calculated according to equations below: ஼಴ೀమ

ܺு஼ுை = ஼ ௏

߬=ி= ‫=ݎ‬

× 100%

(1)

ಹ಴ಹೀ

ௌ×ఋ ி

ி×௑ಹ಴ಹೀ ×஼ಹ಴ಹೀ ௠೎ೌ೟

− ݈݊ሺ1 − ܺு஼ுை ሻ = ݇௔ ߬

(2) (3) (4)

where ‫ܥ‬௖௢మ and CHCHO, represent the concentrations of CO2 in outlet gas and formaldehyde in inlet gas, and symbols of V, F, S, mcat, δ for effective volume of the reactor, flow rate, effective area of catalyst sample, mass of Au/TiO2 catalyst, the gap distance of 1 mm between the sample and the quartz window, respectively.


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As mentioned above, CO2 is the only product. Hence, Eq. 1 is defined using CO2 concentration in outlet gas instead of the difference of input and output of formaldehyde concentration. The carbon balance before and after reactions is around 95%. 2.3 In-situ DRIFT Spectra Analysis In-situ DRIFT spectra are measured by FT-IR spectrometer (Nicolet 6700, Thermo Fisher, USA) equipped with a MCT detector, in the range of wavenumber from 4000 to 1000 cm-1 with a resolution of 4 cm-1. The catalyst sample in powder is loaded in a DRIFT cell (HVC-DRM-5, Harrick, USA) equipped with two ZnSe windows and one UV-quartz window under irradiation by a green LED. DRIFT samples are pretreated in synthetic air at 150 oC for 30 min followed by cooling down to 20±1 oC, subsequently the spectra are collected for background. After the pretreatment, a gas mixture of 200mL/min synthetic air containing 50±2 ppm formaldehyde with a designated humidity is fed in the DRIFT cell, and the reaction is maintained at 20±1 oC. The outlet gas is online monitored simultaneously by the COx analyzer. Four experiments are conducted, e.g. (i) the overall reaction of formaldehyde oxidation over Au/TiO2 under dark at dry or wet (35% RH) condition, compared with TiO2, (ii) the overall reaction over Au/TiO2 under subsequent conditions of dry and light (at 0 - 90 min), wet and light (90 - 180 min) followed by wet and dark (180 - 210 min), for which RH is 48%, (iii) the disintegrated reaction of formaldehyde oxidation on Au/TiO2, consisting of the preconditioned formate or carbonate by the overall reaction at wet (9% RH) under light for 50 min, followed by the conversion of formate or carbonate with discontinued formaldehyde under light for 70 min or under dark for 80 min, and (iv) the decomposition of carbonate species that is formed from CO2 adsorption on Au/TiO2, consisting of the preconditioned carbonate species arising from CO2


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adsorption for 50 min using around 510 ppm CO2 in synthetic air at dry condition, followed by the decomposition of carbonate species with discontinued CO2 gas line at wet (13% RH) under dark or under light. A commercial cylinder gas mixture of CO2 and N2 provides the CO2. In consistence with each experimental design, 35% or 48% RH is set for (i, ii) the overall reaction, while 9% or 13% RH for (iii) the disintegrated reaction and (iv) the carbonate species decomposition, respectively.

3. RESULTS 3.1 Catalyst Morphologies and the UV-Vis Absorption Figure S2 shows the morphology of Au/TiO2 catalyst and the size distribution of Au nanoparticles by TEM images. Au/TiO2 catalyst features around 4.5 nm Au nanoparticles in average diameter. Such a small size of Au nanoparticles supported on TiO2 accounts for an observation of sample in purple in Figure S1 (bottom right). UV-Vis diffuse reflectance spectra on Au/TiO2 catalyst is shown in Figure 1a. Clearly, a strong absorption peak centered at around 560 nm of wavelength due to the SPR effect of Au nanoparticles

32

is observed. The absorption in the UV region is attributed to the electron

transition from valence band to conduction band of TiO2. Figure 1b shows the emission spectrum of LED lamp centered at 521 nm of wavelength with 30 nm of full width at half maximum (FWHM) representing green light. It can be strongly absorbed by Au/TiO2 catalyst. Accordingly, green light emitted by LED provides visible light source for the photocatalytic activity and insitu DRIFT spectra studies on Au/TiO2. 3.2 Photocatalytic Activity Evaluation


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Effects of moisture and/or light on the activities for formaldehyde oxidation over Au/TiO2 catalyst in the single-pass continuous flow reactor are investigated. The reaction is conducted under conditions of visible light, 20 oC, and 0.19 s of residence time. Here, 0% RH represents the dry condition of inlet gas, which might contain trace moisture33-34. Figure 2 shows (a) reaction rate of formaldehyde oxidation under dark or under light over Au/TiO2 catalyst as a function of RH, and formaldehyde conversion as a function of TOS (b) at various conditions of RH and light, or (c) under various irradiations of UV 365 nm LED or green LED with a light flux of 19 mW/cm2 at 44% RH. The reaction rate is calculated based on the formaldehyde conversion at 25 min of TOS. In Figure 2b, in term of at dry condition, formaldehyde conversion approximately approaches zero through entire TOS under dark (curve 2) and slightly rises and quickly drops down to zero again under light (curve 1), which manifests that at dry condition the contribution of light to the reaction is negligible. At the beginning 10 min of TOS, a few percentage of formaldehyde conversion “slightly rises”, which may be ascribed to the existence of trace moisture in dry inlet gas. However, with the accumulation of carbonate species (see Section 3.3.2), the active sites are occupied and blocked due to the lack of water at dry condition, and the overall reaction is stopped. As a result, a subsequent “quickly drops drown to zero again” is observed. On the contrary, in terms of at wet condition, the conversions (curves 4, 6) approach steady even if they are under dark. Concerning the negligible activities in dry air under dark (curve 2) and under light (curve 1), it indicates that moisture is indispensible for formaldehyde oxidation over Au/TiO2 catalyst. Apparently, the conversions at wet condition are enhanced under light (curves 3, 5) relative to that under dark (curves 4, 6), respectively. The conversion of 83.3% for formaldehyde oxidation at 44% RH under light over Au/TiO2 catalyst is achieved.


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Moreover, a significant enhancement in reaction rate under wet and light is observed in Figure 2a. In particular, at 13% RH the reaction rate (also the activity) under light is up to five times that under dark. With an increase in RH from 13% to 44%, the reaction rate under dark increases over threefold, while that under light only increases by 11%. Hence, the reaction rate under light weakly depends on humidity at above 13% RH, compared to that under dark. In Figure 2c, at 44% RH with a light flux of 19 mW/cm2, formaldehyde conversion of 87.9% under irradiation of UV 365 nm LED is achieved, relative to 72.8% for visible light by green LED. The slightly lower formaldehyde conversion of visible light than UV light, elucidates that the photocatalytic formaldehyde oxidation under visible light over plasmonic Au/TiO2 is efficient. In terms of effect of humidity on conversion, it was observed in literatures35-36 that the rate of oxidation increases then decreases with increasing gas phase H2O concentration. However, in the present work it is not the case. Formaldehyde conversion of 83.2% under light is still achieved even if RH increases up to 76% (the data not shown in Figure 2a). It manifests that formaldehyde conversion keeps approximately steady at high RH, which may be attributed to a “zero-sum effect”10. An increase in RH might be detrimental to formaldehyde adsorption due to the occupation of adsorption sites by water molecules, but the rate-determining step of carbonate decomposition is promoted in presence of more water (see Section 4.1). Two opposites of the weakened formaldehyde adsorption and the promoted carbonate decomposition, result in the “zero-sum effect” of the rate at high RH in the present work. Apparent rate constant ka for formaldehyde conversion (74.3%, 83.3%) at 13% or 44% RH under light is calculated 7.15 s-1, 9.42 s-1, compared with that (15.4%, 49.7%) under dark for 0.88 s-1, 3.62 s-1, respectively. Moreover, under the same conditions (at 44% RH under light), formaldehyde conversion over 0.2% Au/TiO2 decreases to 22.3%, compared with that of 83.3%


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over 1.07% Au/TiO2. The decrease in Au loading (the less interface of Au and TiO2) accounts for the decrease in formaldehyde conversion. The control experiment of the reaction at wet under light but without Au/TiO2, is performed, and no any product is detected. As indicated by moisture indispensability and light enhancement for the activity over Au/TiO2, it is of interest to gain insights in-depth into the intermediates, the reaction pathway and the reaction mechanism by in-situ DRIFT spectra 37-39. 3.3 In-situ DRIFT Spectra Studies With respect to the intermediates identification, and the course of intermediates conversion and CO2 production, four experiments by in-situ DRIFT spectra are conducted in the DRIFT cell. Also, it is correlated with the contributions of TiO2 and Au (supported on TiO2), moisture and light. For instance, the intermediates identification can be achieved by the spectra at specified TOS, the course of intermediates conversion and CO2 production represented by the intensity of featured peaks and CO2 concentration as a function of TOS, respectively. Moreover, CO2 production is a signal for the overall reaction of formaldehyde oxidation, otherwise no CO2 production for the incomplete reactions. In detail, in order to identify the contributions of TiO2 and Au, (i) the overall reaction of formaldehyde oxidation under dark is conducted (Figures 3A, 4). In terms of the contributions of moisture and light, (ii) the overall reaction under subsequent conditions of dry and light, wet and light followed by wet and dark (Figures 3B, 5), (iii) the conversion of formate or carbonate intermediate that is preconditioned by the overall reaction at wet under light (donated as the disintegrated reaction) (Figures 6, S4), and (iv) the decomposition of carbonate species that is formed from CO2 adsorption (Figures 7, S5), are conducted. 3.3.1 Contributions of TiO2 and Au


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In order to identify the contributions of TiO2 and Au, the overall reaction of formaldehyde oxidation only under dark (excluding under light) over TiO2 or Au/TiO2 is performed, since semiconductor TiO2 (P25, Deggusa) is not responsive to visible light. With the consistent TOS, Figure 3A and Figure 4 show the spectra at 120 min of TOS, and peaks intensity and CO2 production vs. TOS, respectively. In general, due to the existence of moisture the peaks at wet condition may have a slight shift compared to that at dry condition. In terms of bare TiO2, the bands for DOM species are clearly observed at dry condition or overlapped and slightly red-shifted at wet condition, while the bands for formate and carbonate intermediates are nearly not observed. No production of CO2 in gaseous phase at both dry and wet conditions during the whole 120 min of TOS is detected (the data not shown here). Table 1 shows infrared bands of surface species on TiO2 or Au/TiO2. At dry condition, featured bands of

ν(CO) at 1176 and 1117 cm-1 in Figure 3A(a), as well as a weak band of νs(CH2) at 2864 cm-1 in Figure S3A(a) that are ascribed to DOM 40-42 appear. Figure S3 is a full scan version of Figure 3. Adsorption of formaldehyde molecules on TiO2 occurs through the donating a lone pair of oxygen (of carbonyl group) to Lewis acid site of surface Ti4+ cation. Accordingly, the carbon (of carbonyl group) turns into more electrophilic and is attacked by nucleophilic surface oxygen ion hence to form DOM

40

. It is consistent with First-principles DFT calculation result of the

preferred adsorption to form DOM27-28, arising from the surface oxygen of TiO2. In consistence with literature

43

, slightly negative bands at 3676 and 3633 cm-1 in Figure S3A(a) that are

ascribed to the stretching modes of OH group on Ti4+ cation are observed. It indicates that the decrease in the amount of OH groups corresponds to the modification of hydrogen bonding by interactions with formaldehyde molecules or the intermediates at dry condition. However, at wet condition, the bands of DOM are overlapped and slightly red-shifted (ascribed to the interactions


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with water), and a broad and strong band of ν(OH) at 3334 cm-1 in Figure S3A(b) and a narrow band of δ(OH) at 1651 cm-1 in Figure 3A(b) representing adsorbed water appear. It suggests that water molecules occupy most of adsorption sites on TiO2 due to its much higher concentration than formaldehyde at wet condition. It turns out that, over TiO2, the formaldehyde oxidation at dry or wet condition is incomplete because of no observed production of CO2 and stops at the step of DOM formation. For Au/TiO2, DOM appears at dry condition but completely disappears at wet condition, and formate or carbonate is observed at both dry and wet conditions. Continuous production of CO2 in gaseous phase at wet condition during the whole 120 min of TOS is observed, in contrast to no production of CO2 at dry condition in Figure 4. As shown in Figure 3A(c, d), featured band of

νas(COO) at 1572 cm-1 representing formate cm-1 for monodentate carbonate

4

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and the bands of ν(COO) at 1365 cm-1 or 1304

or bidentate carbonate

44

, respectively, are clearly observed.

The peak intensity of formate or carbonate approaches steady more quickly at wet condition than that at dry condition in Figure 4. At dry condition in Figure 4a, DOM intermediate appears, rises quickly and keeps nearly steady, and formate or carbonate intermediate increases relatively slowly. On the contrary, at wet condition in Figure 4b, DOM completely disappears, formate or carbonate goes up rapidly and maintains steady. So far, the sequential steps of formaldehyde conversion to DOM, formate, carbonate and CO2, are introduced and shown in Scheme 1, to which k1, k2, k3 and k4 of kinetic parameter correspond, respectively. As far as the contribution of bare TiO2 is concerned, TiO2 promotes formaldehyde oxidation to DOM quickly even at dry condition, as indicated by the DOM symbol (adsorbed on Lewis acid site of surface Ti4+ cation) at step k1 in Scheme 1. However, the reaction is incomplete and stops at the step of DOM formation due to no observed production of CO2, hence DOM can not


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be oxidized further over bare TiO2 at dry or wet condition. Hence, a “STOP @ no Au” sign is assigned at the step of DOM formation in Scheme 1. Concerning the contribution of Au nanoparticles (supported on TiO2), Au promotes (1) the DOM oxidation to formate or carbonate even at dry condition, compared to that not observed over bare TiO2 in Figure 3A (c vs. a), or (2) the disappearance of DOM and the formation of formate or carbonate at wet condition, relative to that over bare TiO2 in Figure 3A (d vs. b). Due to the existence of Au the featured bands of DOM exhibit a slight redshift (2 cm-1) to 1174 and 115 cm-1 at dry condition, compared to bare TiO2. Herein, in terms of conventional catalysis for the reaction under dark, it is generally accepted that the reaction (i.e. DOM oxidation) occurs at the perimeter interface of Au and TiO234, 45-47, where oxygen molecules are activated by water to form active oxygen species. As a result, DOM can be oxidized rapidly over Au/TiO2 to formate, subsequently to carbonate (in dry air) or further to CO2 (in wet air). Hence, a “STOP @ no H2O” sign is assigned at the step of carbonate formation in Scheme 1. It is deduced that the steps of k1, k2 must be very faster since they can proceed even under the most unfavorable conditions of at dry under dark in Figures 3A(a, c), 4a. Due to the complete disappearance of DOM and the formation (representing the accumulation) of formate or carbonate at wet under dark in Figure 4b, steps k3 and k4 are much slower than step k2. Herein, the contributions of TiO2 and Au to the reaction under dark are discussed, and the contribution of Au under light will be discussed in DISCUSSION section. 3.3.2 Moisture Contribution In terms of the overall reaction under dark, the moisture contribution over Au/TiO2 is distinctive from that over bare TiO2 in section 3.3.1. Over bare TiO2, moisture has a slight or negligible influence on formaldehyde oxidation to DOM. The bands for DOM species are


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overlapped and slightly red-shifted at wet condition compared with that at dry condition in Figure 3A(b vs. a). The reaction is incomplete and stops at the step of DOM formation even at wet condition. However, over Au/TiO2, moisture contributes to the complete disappearance of bands of DOM, as well as the blue-shifted band of formate and the red-shifted band(s) of carbonate, compared with that at dry in Figures 3A(d vs. c). Also, moisture promotes the DOM oxidization rapidly to formate, subsequently to carbonate and to CO2 compared with no CO2 production at dry in Figures 4(b vs. a). In terms of the overall reaction under light, the moisture contribution only over Au/TiO2 is investigated. For the overall reaction under subsequent conditions of dry and light, wet and light followed by wet and dark, with the consistent TOS, Figure 3B and Figure 5 show the spectra at 90, 180, 210 min of TOS, and the peaks intensity and CO2 production vs. TOS, respectively. The moisture contributes to the complete disappearance of bands of DOM under light, as well as the blue-shifted band of formate and the red-shifted band(s) of carbonate, compared with that at dry in Figures 3B(b vs. a). Similarly, in Figure 5(regions a, b), under light, introducing moisture results in a complete drop down to zero in the peak intensity of DOM, and a sharp increase followed by a dramatic decrease till steady of formate, and a remarkable decrease till steady of carbonate. Continuous production of CO2 at wet is observed, in contrast to no production of CO2 at dry. No production of CO2 at dry under light manifests that carbonate can not be decomposed on, thus blocks catalyst surface. The sharp increase in intensity of formate means its fast accumulation, indicating the production rate (from DOM oxidation, step k2) higher than the consumption rate (to form carbonate, step k3). Herein, step k3 is relatively slower than steps k1 and k2.


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Consequently, over Au/TiO2, it is ascertained that moisture is indispensable to carbonate decomposition and also accelerates DOM oxidation to formate, as represented by the “STOP @ no H2O” sign (also water molecule symbol at step k4), and the symbol marked with dashed square of water molecule at step k2 in Scheme 1, respectively. It suggests that both under dark and under light, moisture definitely contributes to the complete disappearance of bands of DOM, as well as the blue-shifted band of formate and the red-shifted band(s) of carbonate, and promotes the complete oxidation of DOM to formate, subsequently to carbonate and to CO2. On the contrary, over bare TiO2 only under dark, moisture has a slight or negligible influence on to the overlapped and slightly red-shifted bands of DOM, and the reaction is incomplete and stops at the step of DOM formation. 3.3.3 Light Contribution In terms of the overall reaction, light contribution over Au/TiO2 at wet is distinctive from that at dry. At wet condition, light contributes to the enhancement in the conversion of formate or carbonate. In Figure 5(regions b, c), the absence of light leads to an increase in the intensity of formate or carbonate and a decrease in CO2 production, which in turn, manifests that the existence of light enhances the oxidation of formate or carbonate decomposition to CO2 production. On the contrary, at dry condition, light contributes to no obvious changes in the peaks intensity of DOM, formate and carbonate, and CO2 concentration (no CO2 production) as a function of TOS in Figure 5(region a) vs. Figure 4a. For the conversion of formate or carbonate that is preconditioned by the overall reaction at wet under light (the disintegrated reaction) over Au/TiO2, light contributes to the accelerated conversion of formate or carbonate. Figure 6 shows the conversion of formate or carbonate of the disintegrated formaldehyde oxidation as a function of TOS. It consists of the accumulation


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(precondition) of formate or carbonate by the overall reaction at wet under light, followed by the conversion of formate or carbonate with discontinued formaldehyde under dark or under light. Here, two transitions of switching on/off formaldehyde between regions (from a to b, and from c to d) occur, assuming that the basically same states of regions a, c arising from the same preconditioning. Apparently, the conversion of formate or carbonate under light is accelerated relative to that under dark in Figure 6(regions d vs. b). Furthermore, in order to observe the change in the intensities of specified peaks, the spectra at 50, 60, 70, 80, 120 min of TOS are presented in Figure S4. The conversion of formate or carbonate under light is nearly completed at 80 min of TOS in Figure S4b and Figure 6(region d), relative to that under dark in Figure S4a and Figure 6(region b). Moreover, in quite consistence with that in Figures 3A(d vs. c) or 3B(b vs. a) in section 3.3.2, the blue-shifted band of formate and the red-shifted band(s) of carbonate with the conversion (decomposition) under dark or under light is also observed in Figures S4a or S4b. It may be illustrated that in consistence with its variation from dry to wet condition (the former), the amount of adsorbed water molecules per intermediate (the latter) increases with the decomposition at fixed RH. Light promotes the decomposition of carbonate species over Au/TiO2. Figure 7 shows the decomposition of carbonate species that is formed from CO2 adsorption as a function of TOS. Figure S5 shows DRIFT spectrum of saturated adsorption of CO2 on Au/TiO2, with featured bands representing carbonate. After the saturated adsorption of CO2 at dry condition, the decomposition of carbonate occurs in presence of moisture, and is accelerated under light relative to that under dark in Figure 7a. Also the profile of CO2 production as a function of TOS (Figure 7b) is consistent with the profile of the normalized intensity of carbonate during the decomposition.


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Therefore, it is ascertained that visible light enhances the rate-determining steps of formate oxidation to carbonate (k3) and carbonate decomposition (k4), as represented by the two symbols marked with dashed square of visible light at steps k3, k4 in Scheme 1. It suggests that light exhibits, (1) the contribution to the enhanced conversion of formate or carbonate at wet, and no obvious contribution at dry for the overall reaction, (2) the accelerated conversion of formate or carbonate for the disintegrated reaction, and (3) the promoted decomposition of carbonate species that is formed from CO2 adsorption. As a consequence, it is ascertained that moisture is indispensable to carbonate decomposition and also accelerates DOM oxidation to formate (in section 3.3.2), and visible light enhances the rate-determining steps of formate oxidation to carbonate and carbonate decomposition (in section 3.3.3), which appropriately illustrate the above-mentioned remarkable difference in the reaction rate (also the activity) (in section 3.2). 3.3.4 Identical Spectra under Light or under Dark More interestingly, in-situ DRIFT spectra over Au/TiO2 under dark are basically identical to that under light in terms of the peaks positions (species information). No more unique peak under light appears compared to that at dry under dark, or at wet condition. For instance, at dry condition, the in-situ DRIFT spectra under light are approximately identical to the spectra under dark in terms of the peaks assignment (variety and position) in Figures 3B(a) vs. 3A(c) (also Figures S3B(a) vs. S3A(c)). In a similar way, at wet condition, the spectra under light are also approximately identical to the spectra under dark in Figures 3B(b) vs. 3A(d) or 3B(c) (also Figures S3B(b) vs. S3A(d) or S3B(c)).

4. DISCUSSION


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From above in-situ DRIFT spectra experiments in section 3.3, we show that formaldehyde oxidation undergoes four sequential reaction steps (k1, k2, k3, k4) with reaction conditions dependent on moisture and light over Au/TiO2, hence such a reaction pathway of formaldehyde oxidation is proposed in Scheme 1. Based on the approximately identical spectra we observed in section 3.3.4, indicating identical reaction pathways, we discuss the reaction mechanisms under light or under dark conditions. 4.1 Reaction Pathway Scheme 1 shows the proposed pathway for photocatalytic oxidation of formaldehyde over Au/TiO2 under visible light. As mentioned in section 3.3.1, steps of k1, k2 are much faster than k3, k4. Rate enhancement of the overall reaction is attributed to the acceleration of rate-determining steps such as formate oxidation to carbonate (k3) and carbonate decomposition (k4). At step k1, formaldehyde oxidation to DOM in air occurs quickly, since DOM can be formed even on bare TiO2 (in absence of Au) at dry under dark in Figure 3A(a). It is represented by the DOM symbol that is adsorbed on Lewis acid site of surface Ti4+ cation (dark gray atom ball) in Scheme 1, and other intermediates adsorbed are shown thereafter. At step k2, DOM oxidation to formate at dry condition in the presence of Au and moisture (herein, trace moisture) occurs quickly, since formate can be formed over Au/TiO2 at dry under dark in Figure 3A(c, d) or under light in Figure 3B(a, b). As mentioned in section 3.2, at the dry condition of inlet gas, the trace moisture may be presented and DOM oxidation to formate may proceed over Au/TiO2. Interestingly, it is emphasized that the Au is a prerequisite (represented by the “STOP @ no Au” sign in Scheme 1) for step k2, since the further oxidation of DOM does not occur over bare TiO2 (no Au) at all conditions in Figure 3A(a, b), but does over Au/TiO2 (in presence of Au) at all conditions in Figures 3A(c, d), 3B(a, b). Moreover, over Au/TiO2, the


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moisture accelerates DOM oxidation to formate under dark in Figure 4b or under light Figure 5 (region b), as represented by the symbol marked with dashed square of water molecule at step k2 in Scheme 1. At step k3, formate is further oxidized to carbonate which can be enhanced by light, as represented by the symbol marked with dashed square of visible light at step k3 in Scheme 1. The conversion of formate or carbonate intermediate is accelerated under light relative to that under dark in Figure 6, which confirms the enhancement of light on step k3. Moreover, step k3 is much slower than step k2 because the sharp increase in the peak intensity of formate by the introducing moisture at 90 min of TOS is observed in Figure 5 (regions a, b). At step k4, carbonate is decomposed to CO2 in presence of moisture which can be enhanced by visible light, as represented by the symbol marked with dashed square of visible light at step k4 in Scheme 1. The moisture is indispensable to carbonate decomposition, which is evidenced by continuous CO2 production at wet compared to no CO2 at dry under dark in Figures (4b vs. 4a), or under light in Figure 5 (regions b vs. a). More interestingly, it has to be emphasized that the moisture is a prerequisite for step k4, as represented by the “STOP @ no H2O” sign (also water molecule symbol at step k4) at step k4 in Scheme 1. In dry air, the decomposition of carbonate does not proceed, hence the formed blockage of carbonate species on catalyst surface stops the reaction either under light in Figure 5 (region a) or under dark in Figure 4a. The enhancement of visible light on carbonate decomposition to CO2 is observed based on the carbonate intermediate intensity of the overall reaction in Figure 5 (regions b vs. c), the decomposition of carbonate of disintegrated reaction of formaldehyde oxidation in Figure 6 (regions d vs. b), and the decomposition of carbonate that is formed from CO2 adsorption in Figure 7a. 4.2 Mechanism under Light or under Dark


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Worth noting is that in-situ DRIFT spectra under light or under dark are approximately identical in terms of peak positions (species information) and only have variations in the intensity, as shown at dry condition in Figures S3B(a) vs. S3A(c), and at wet condition in Figures S3B(b) vs. S3A(d) or S3B(c) in section 3.3.4. It suggests that the reaction features the same surface intermediates and proceeds via the same pathway under light or under dark. Therefore, for photocatalytic reaction under visible light over catalysts that are also active under dark, the reaction pathway for the photocatalytic reaction is the same as that for the reaction under dark. However, their mechanisms are in a way of photocatalysis coupled with conventional catalysis vs. conventional catalysis. For plasmonic photocatalysis over Au/TiO2 (central, Scheme 1), the most profound feature is the SPR to promote the absorption of visible light, the intense local electric field and the photoexcitation of electrons and holes. Another important feature is the formation of Schottky barrier junction (ΦSB, central, Scheme 1) at the interface between Au nanoparticles and TiO2, which considerably suppresses the recombination of the photo-excited electrons and holes. For Au/TiO2, the height of Schottky barrier junction is approximately 0.9 eV, while that of hot electrons excited by Au plasmon excitation may increase up to ~2 eV above the Au Fermi level (EF) 20, 4851

. Hence, a considerable fraction of hot electrons (at SPR state) of Au can be directly injected

into the conduction band of TiO2 and initiate the photocatalytic reaction on the TiO2 surface 52-53. As depicted in the central of Scheme 1, plasmon activation of Au by visible light generates positive charges on the Au particles (Au δ+) and conduction-band electrons on TiO2. In general, it is accepted that a substrate (electron donor, e.g. VOCs) oxidation reaction occurs at the sites of holes for semiconductor (titania) photocatalysis by UV light

54

. Such a similar mechanism of

“oxidation via holes (positive charge)” is taken to illustrate the visible-light photocatalysis,


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namely, the plasmonic Au/TiO2 photocatalyst might oxidize VOCs on the Au surface (via positive charges)

55

. Herein, if it follows above mechanism of “oxidation via holes” indicating

that formaldehyde can be oxidized by Au δ+ cations (on Au surface) 55, the DRIFT spectra under light must show the featured peak(s) in response to the intrinsic chemical property of Auδ+ cations (arising from the SPR), which should be different from the spectra under dark. As mentioned in section 3.3.4, however, no more unique peak under light appears compared to that under dark (at both dry and wet conditions). Hence the very identical DRIFT spectra with only variations in intensity on Au/TiO2 under light or under dark, accounts for our reaction mechanism distinguished from the mechanism of “oxidation via holes”. Consequently, in an appropriate interpretation way, formaldehyde oxidation over plasmonic Au/TiO2 at wet under light occurs via conventional catalysis coupled with photocatalysis. For conventional catalysis as mentioned in section 3.3.1, formaldehyde is adsorbed on TiO2 surface (via Ti4+) and attacked by nucleophilic active oxygen species at the interface between Au and TiO2 to form DOM, subsequent formate, carbonate or CO2. At the perimeter interface of Au and TiO2, oxygen molecules are activated by water to form active oxygen species. In terms of plasmonic photocatalysis, due to the SPR effect hot electrons (at SPR state) of Au directly injected into the conduction band of TiO2, are consumed by the reduction of O2 (electron acceptor) to form superoxide radical species at the interface of Au and TiO2, as evidenced by electron paramagnetic resonance (EPR) spectra 12, simultaneously, Auδ+ cations (arising from the SPR) might migrate to the interface of Au and TiO2 to promote formaldehyde oxidation (in particular formate oxidation and carbonate decomposition) and the formation of OH radicals via water oxidation as an oxidant. As a result, it initiates the photocatalytic reaction over plasmonic Au/TiO2. Therefore, for both conventional catalysis and photocatalysis, formaldehyde oxidation


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reaction proceeds via the same reaction pathway, represented by the approximately identical DRIFT spectra under light or under dark, since the reaction occurs at the interface of Au and TiO2 (the same reaction sites). Visible light drives the SPR effect over plasmonic Au/TiO2, which accounts for the accelerated formaldehyde oxidation, in particular rate-determining steps of formate oxidation to carbonate and carbonate decomposition.

5. CONCLUSIONS Photocatalytic oxidation of formaldehyde in air over plasmonic Au/TiO2 under visible light in the single-pass continuous flow reactor is conducted. In wet air, Au/TiO2 catalyst exhibits very high activity, a complete conversion of formaldehyde of 83.3% at 44% RH under visible light, and the remarkably enhanced reaction rate (of up to five times under visible light at 13% RH compared to under dark), while in dry air, it is not active even under visible light. The plasmonic Au/TiO2 is efficient for photocatalytic oxidation of formaldehyde under visible light, which is evidenced by a slight difference of conversion between UV light and visible light. The in-situ DRIFT studies on formaldehyde oxidation under dark over Au/TiO2 compared to bare TiO2, show that the formaldehyde oxidation stops at DOM for TiO2 (in absence of Au). For Au/TiO2 in dry air, the reaction terminates at formate and carbonate without CO2 production, while in wet air, DOM disappears, formate and carbonate approach (then keep) steady rapidly, and the reaction proceeds with a continuous production of CO2. In wet air, visible light contributes to the enhanced formate oxidation and carbonate decomposition. Also the enhancement of visible light is evidenced by the promoted decomposition of carbonate species that is formed from CO2 adsorption.


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Interestingly, in-situ DRIFT spectra under dark or under visible light are approximately identical in terms of peak position and only have variations in the intensity. It suggests that the reaction features the same surface intermediates and proceeds via the same pathway under dark and under visible light. Consequently, it is ascertained that moisture is indispensable to carbonate decomposition and also accelerates DOM oxidation to formate, and visible light enhances the rate-determining steps such as formate oxidation to carbonate and carbonate decomposition. It appropriately illustrates the above-mentioned remarkable difference in the reaction rate (the activity). The pathway of formaldehyde oxidation is proposed. New insights into the mechanism for photocatalytic oxidation of formaldehyde under visible light over plasmonic Au/TiO2, which is related to the contributions of moisture and visible light, are obtained.

ASSOCIATED CONTENT Supporting Information Reactor and Au/TiO2 morphologies (Figures S1, S2), full scan version of in-situ DRIFT spectra of Figure 3 (Figure S3), in-situ DRIFT spectra at various TOS representing the changes in the intensity of specified peaks (Figure S4), DRIFT spectrum of saturated adsorption of CO2 on Au/TiO2 (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected], Tel & Fax: +86-411-84706094


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Notes - The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (21673030) and the Fundamental Research Funds for the Central Universities (DUT16QY49). REFERENCES 1.

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54. Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735-758. 55. Tsukamoto, D.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. J. Am. Chem. Soc. 2012, 134, 6309-6315.


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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

Figure Captions Table 1. Infrared bands of surface species on TiO2 or Au/TiO2 catalyst, derivative from Figures 3, S3. Figure 1. (a) UV-Vis diffuse reflectance spectrum of Au/TiO2, and (b) emission spectrum of LED green light. Figure 2. (a) Reaction rate as a function of RH for formaldehyde oxidation over Au/TiO2 catalyst under light or under dark, and formaldehyde conversion as a function of TOS (b) at various conditions of RH and light, or (c) under various irradiations of UV 365 nm light or visible light at a light flux of 19 mW/cm2 at 44% RH. Figure 3. In-situ DRIFT spectra at wavenumbers between 2500 and 1000 cm-1 of formaldehyde oxidation (A) at 120 min of TOS under dark at (a) dry, (b) wet over TiO2, and (c) dry, (d) wet over Au/TiO2; (B) at 90, 180, and 210 min of TOS under subsequent conditions of (a) light and dry, (b) light and wet, and (c) dark and wet over Au/TiO2, respectively. TOS of (A), (B) is consistent with that of Figures 4, 5, respectively. RH of (A), (B) is 35%, 48%, respectively. A full scan version of Figure 3 is shown in Figure S3. Figure 4. Peak intensities of surface species and the produced CO2 concentration (red curves) for formaldehyde oxidation under dark over Au/TiO2 as a function of TOS under (a) dry or (b) wet condition (35% RH). TOS is consistent with Figure 3A. Figure 5. Peak intensities of surface species and the produced CO2 concentration (red curves) for formaldehyde oxidation over Au/TiO2 under subsequent conditions of (region a) light and dry, (region b) light and wet, and (region c) dark and wet, as a function of TOS. RH is 48%. TOS is consistent with that of Figure 3B.


31

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

Page 32 of 42

Figure 6. Peak intensities of surface species for the disintegrated reaction of formaldehyde oxidation on Au/TiO2 as a function of TOS. The disintegrated reaction consists of (regions a and c) the preconditioned surface species by the overall reaction at wet (9% RH) under light for 50 min, followed by the conversion of surface species with discontinued formaldehyde (region d) under light for 70 min or (region b) under dark for 80 min. The spectra at 50, 60, 70, 80, 120 min of TOS are shown in Figure S4. Figure 7. (a) Normalized intensity of carbonate species at around 1560 cm-1 of wavenumber, and (b) the produced CO2 concentration over Au/TiO2 at wet (13% RH) under dark or under light as a function of TOS, during the decomposition of carbonate species that is formed from CO2 adsorption. Scheme 1. Schematic diagram of the proposed pathway for photocatalytic oxidation of formaldehyde over Au/TiO2 under visible light. Symbols of white, gray, red and dark gray atom balls represent atoms of hydrogen, carbon, oxygen and titanium. The reaction proceeds from start of formaldehyde to end of CO2, in which the kinetic parameters k1, k2, k3, k4 represent four conversion steps involving formaldehyde, intermediates (DOM, formate and carbonate) and CO2. Symbols marked with dashed square of visible light and water molecule indicate the enhancement by visible light, and the acceleration by moisture onto the reaction steps.


32

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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

Table 1.

wavenumber (cm-1)

reference

ν(CO)

1176, 1117

40-42

νs(CH2)

2864

40-42

νas(COO)

1572

42

1365

4

1304

44

ν(OH)

3676, 3633, 3334 (broad)

43

δ(OH)

1651

43

species and assignment dioxymethylene (DOM)

formate

monodentate carbonate ν(COO)

bidentate carbonate ν(COO)

water


33

ACS Catalysis

Normalized intensity

Figure 1

Absorbance / 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

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b

1.0

0.5

0.0

a 1.2

0.8 200

300

400

500

600

700

800

Wavelength / nm


34

Page 35 of 42

Figure 2

-7

a visible light dark

100

-7

RH = 0%, visible light RH = 0%, dark RH = 13%, visible light RH = 13%, dark RH = 44%, visible light RH = 44%, dark

5

-1

3.0x10

-7

2.0x10

-7

1.0x10

-7

0.0

Conversion / %

-1

4.0x10

c

b 1 2 3 4 5 6

3

6

100

7 8

visible light UV light

8

Conversion / %

5.0x10

Rate / mol·s ·gcat

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

7

4 1 2

0 0 13

RH / %

44

0

10

20

30

0 0

TOS / min

10

20

30

TOS / min


35

ACS Catalysis

Figure 3

1585

d

0.5

0.5

1358 1641

c

1587 1645 1356

0.0 1.0

c

0.0 1115

1365

0.0

b

0.2

1587

b K-M

1304 1174

1572

0.5

1645

0.2

1358

1651

0.0 1572

1113

1304

1365

1117

a 0.5

a

1173

0.5

0.0

1176

K-M

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 36 of 42

0.0

0.0 2500

2000

1500

1000

2500

-1

2000

1500

1000 -1

Wavenumber / cm

Wavenumber / cm

A

B


36

Page 37 of 42

Figure 4

0.8

b

30

0.6

20

0.4 10

0.2 0.0 1.0 0.8

0 DOM Formate Carbonate

a

0.6

30 20

0.4

CO2 concentration / ppm

1.0

Peak intensity

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

10

0.2 0.0

0 0

30

60

90

120

TOS / min


37

ACS Catalysis

Figure 5

a

1.4

c

b

50

1.0 0.8 0

0.6 0.4 0.2 0.0 0

DOM Formate Carbonate

30

60

90

120

150

180


1.2

Peak intensity

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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-50 210

TOS / min


38

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Figure 6

1.0 0.8

c

d

a

b

0.6 0.4

Peak intensity

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

0.2 0.0

1.0 0.8 0.6 0.4 0.2 0.0 0

Formate Carbonate 50

100

TOS / min


39

ACS Catalysis

Figure 7


15

Normalized intensity

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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b

dark light

10

5

0 1.0

a

0.8 0.6 0.4 -5

0

5

10

15

20

TOS / min


40

Page 41 of 42

Scheme 1

START END

k1 k4

visible light

Au

SPR state

hot e-

TiO2 e-

STOP

conduction band

ΦSB

no Au

EF

h+

STOP

valence band

no H2O

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

k2

k3


41

ACS Catalysis

For TOC

k1 k4

visible light

Au TiO 2

STOP

STOP

eCB

no Au

SPR hot e- ΦSB EF h+

VB

no H2O

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 42 of 42

k3

k2


1

TiO2 under

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