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Ceria supported Pt/PtO-nanostructures: Efficient photocatalyst for sacrificial donor assisted hydrogen generation under Visible-NIR light irradiation Nilesh Rameshrao Manwar, Anushree A. Chilkalwar, Kamala Kanta Nanda, Yatendra S Chaudhary, Jan Subrt, Sadhana Suresh Rayalu, and Nitin K. Labhsetwar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01789 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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ACS Sustainable Chemistry & Engineering

Ceria supported Pt/PtO-nanostructures: Efficient photocatalyst for sacrificial donor assisted hydrogen generation under VisibleNIR light irradiation Nilesh R. Manwar,ab‡Anushree A.Chilkalwar,ab‡Kamala K. Nanda,acYatendra S. Chaudhary,ac Jan Subrt,d Sadhana S. Rayaluab and Nitin K. Labhsetwar*ab a

CSIR-Network Institute of Solar Energy (CSIR-NISE), New Delhi, India

b

Environmental Materials Division, CSIR-National Environmental Engineering Research Institute (CSIR-

NEERI), Nehru Marg, Nagpur-440020, (M.S.), India. c

Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology,

Bhubaneswar 751 013, Odisha India d

Institute of Inorganic Chemistry of the AS CR, Husinec-Rez, Czech Republic

* Corresponding author: Dr. Nitin Labhsetwar Sr. Principal Scientist, *e-mail: [email protected]; [email protected] Environmental Materials Division CSIR -National Environmental Engineering Research Institute (CSIR- NEERI), Nehru Marg, Nagpur-440020, (M.S.), India.

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ABSTRACT: In photocatalysis, imperative photoredox behavior and narrow band gap are important properties to exploit solar light for water splitting reaction. Nanostructured ceria (cerium dioxide/CeO2) with Ce3+/Ce4+ (photoredox couple) shows significant enhancement in photocatalytic activity, however, no significant activity for water splitting reaction. The present study mainly focuses on incorporation of Pt on nanostructured mesoporous ceria by wet impregnation method and its evaluations for donor assisted photocatalytic water splitting reaction. The BET analysis shows much higher surface area (119-131 m2g-1) for unmodified as well as Pt modified mesoceria samples as compared to commercial ceria (24.4 m2g-1), although structure was not ordered. The incorporation of Pt on mesoceria shows remarkable influence on photocatalytic hydrogen generation activity, and 1wt% Pt was found as optimized content, with broader light absorption. This photocatalyst was optimized with respect to photocatalyst dose, use of different sacrificial donors and their concentrations as well as other experimental parameters, with 34 h time course evaluation, yielding cumulative 1.52 mmols of hydrogen, under visible-NIR light irradiation and using ethanol as a sacrificial donor. The XPS, BET and photoluminescence studies implies that the enhanced photocatalytic hydrogen evolution in the case of mesoceria is due to the unison of high surface area, reduced recombination of photogenerated charge carrier and lower Ce3+concentration in the case of mesoceria.

KEYWORDS: Pt/Ceria; Photocatalysis; water splitting; visible-NIR light.

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INTRODUCTION Semiconductor photocatalysis has been a subject of extensive research including that for the development of efficient photocatalysts for water splitting reaction. A large number of water splitting systems being explored, are unable to harvest wide range of solar radiation spectrum and thus leading to significant loss in overall solar energy conversion efficiency.1 Inorganic semiconducting materials including wide band gap metal oxides TiO2,2 ZnO,3 show poor solar conversion efficiency because of predominant light absorption in UV region only.4 On the other hand, the low band gap materials (e.g. Cu2O, CdS,6 CdSe7 etc.) harvest only visible solar radiation and are unstable (undergo photo-corrosion). Therefore, utilizing wider solar irradiance is still a challenge to ensure significant advances in solar to hydrogen conversion efficiency. In order to achieve the wider solar radiation harvesting visible-near infrared (Vis-NIR), various strategies are being explored. One is the fabrication of hetero-structures based on wide band gap and narrow band gap semiconductor or incorporation of rare earth upconversion luminescent agents into the host semiconductor.5 Other approach is the sensitization of semiconductors with NIR responsive dye.6,7Furthermore, Vis-NIR wavelength active photoelectrochemical water splitting systems have been explored with upconversion phenomenon using nanoparticle based systems.8 However, near-infrared (NIR) and infrared (IR) light active materials promoted by sensitizers are not very effective because the NIR and IR active sensitizers frequently absorb sub-band gap energy. Such higher wavelength irradiation, with energy < 1.8 eV cannot provide sufficient energy potential for exciting electron–hole pairs with adequate energy to support the overall water splitting reaction.8-9 Widely studied efficient photocatalysts are metal oxides because of their high stability and 3 ACS Paragon Plus Environment


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available surface reactive sites. When these surface reactive sites come in close contact with metal nanoparticle, they form nanocomposite semiconducting materials, which can enhance the optical properties and photocatalytic performance of these nanocomposite materials in visibleNIR region.10 As evident, while very large numbers of studies are reported on band gap engineering related efforts through doping of elements in semiconductor oxides, relatively less attention was paid to exploit structural modifications as well as redox properties of semiconductor oxide materials. The cerium dioxide (or ceria), which exhibit the coexistence of Ce3+/Ce4+ when its size reduced to nanoscale (typically < 10 nm), is a promising support for metal nanoparticle widely used in uphill reactions. Excellent optical and redox chemical properties comprising of reduced / oxidized states that simply alternate between Ce3+/Ce4+ and oxygen vacancy in the lattices of ceria are the key reasons for its potential as a support and composite material with photocatalytic properties. There are several reports on the application of ceria in photocatalysis, thermophotocatalysis, water gas shift reactions, fuel cell, auto-exhaust, oxygen storage support, oxygen sensoretc.11–16 The unison of the ceria redox (Ce3+/Ce4+) chemistry, high porosity, light harvesting in Vis-NIR region and its modification with co-catalysts may significantly influence the overall photocatalytic hydrogen generation activity.

Herein, the detailed investigation on the role of redox chemistry of Ce3+/ Ce4+ and co-catalyst in governing the recombination of photo generated charge carriers and thus the photocatalytic hydrogen generation activity has been undertaken. In the present study, we have systematically explored Pt/PtO incorporated nanostructured ceria for photocatalytic hydrogen generation through sacrificial donor assisted water splitting reaction. We used modified chemical synthesis 4 ACS Paragon Plus Environment

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methods to alter the band gap of ceria and created various defects, which narrowed the band energy to drive visible-NIR photocatalytic water splitting reaction. The nanostructured ceria formation and decrease in particle size were analyzed by powder X-ray diffraction (p-XRD). Furthermore, the influence of Pt concentration and electronic states of cerium on the nanostructured ceria surface was investigated by using X-ray photoelectron spectroscopy (XPS). Surface morphology and amount of impregnated Pt were estimated by HR-TEM with EDA analysis. With the decrease in particle size, there is an enhancement in the redox properties of nanostructured ceria. Optical (light absorption) properties were studied by diffuse reflectance and photoluminescence spectra. The photocatalytic water splitting performance of Pt/PtO assisted nanostructured ceria was evaluated for hydrogen generation using 400W tungsten lamp (visibleNIR light) with several different experimental conditions. On comparison with commercial ceria, the synthesized 1wt % Pt/mesoceria shows significantly improved photocatalytic activity with good stability and consistent hydrogen evolution up to 34h as investigated in the present study.

EXPERIMENTAL SECTION Materials High purity chemicals were used for the synthesis of mesoceria. Chitosan was used as a natural bio-template, which was procured from M/s. Panova Ltd., Chennai, India. Analytical grade acetic acid, platinum chloride and ammonia solutions were purchased from Merck India, whereas analytical grade cerium nitrate was procured from Hi-Media, India. Commercial ceria used for comparison was procured from M/s Loba Chemicals, India. Deionized water was used for all the synthesis and evaluation experiments.

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Synthesis Details Nanostructured ceria was synthesized by modified process of precipitation/combustion using chitosan as a template as described elsewhere,17 while Pt incorporation was done by using the wet impregnation method. Precipitation/combustion method: The typical procedure for synthesis of mesoceria by precipitation-combustion method was used from our previous work.17 9 g of chitosan was dissolved in 300 mL of 5% acetic acid under 1h of vigorous stirring. An aqueous cerium (III) nitrate hexahydrate solution was subsequently added to the chitosan slurry with stirring for 3h. Then the solution was precipitated with 12.5% of ammonia solution. The precipitate was allowed to stabilize at room temperature with continuous stirring for another 1 h. The catalyst was dried at 60oC in vacuum oven, and was then calcined at 400o C (ramping rate 10°C min-1) with a hold time at this temperature for a further 4h. The calcination step is highly exothermic and forms a foamy mass of mesoceria. Wet- impregnation method: 0.5, 1 and 3 wt%, Pt/mesoceria catalysts were synthesized by wet impregnation method, as follows: an aqueous solution of Pt precursor was wet impregnated onto the calcined ceria with continuous stirring at 60oC until sample was completely dried. After impregnation, catalysts were reduced with 10% hydrogen with flow of 5 ml min-1 at 350oc for 4h. Characterization studies p-XRD patterns of synthesized samples were acquired using Rigaku Miniflex-II Desktop X-ray diffractometer with Cu Kα radiations source at 30 kV and 15 mA with monochromator. All powder samples were scanned with scan speed of 20 min-1 with 2 theta scan range 10-1000. X-


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ray difractograms were compared with a standard JCPDS cards and Sherrer’s equation (d= 0.89λ/βcosθ) was used to calculate the crystallite size using the diffraction peaks of (111) plane. Optical adsorption properties of samples were measured by UV-visible diffuse reflectance spectra obtained on an UV−vis/NIR Agilent spectrophotometer and these absorption spectra were scanned in the range of 850nm to 200nm and converted to Kubleka-Munk function for band gap calculations. Photoluminescence (PL) spectra were recorded using Edinburgh, FLS 980 Fluorescence spectrometer at room temperature. The textural property of mesoceria was investigated by N2 adsorption–desorption isotherms to calculate BET-SA using a Micromeritics ASAP-2000 instrument. The morphological observations and structural details were analyzed by high resolution transmission electron microscopy (HR-TEM) through an instrument (JEOL2100F, Japan) and TEM with EDS was analyzed to estimate elemental analysis over nanostructured ceria. X-ray photoelectron spectroscopy (XPS) measurements were carried out to examine electronic / chemical states of Ce 3d and Pt 4f using a custom-built ambient pressure XPS system from Prevac and equipped with a VG Scienta SAX 100 emission controller monochromator using an Al Kα anode (1486.6 eV) in transmission lens mode. The photoelectron energy was analyzed using VG Scienta’s R3000 differential pump analyzer.18-19 Photocatalytic hydrogen evolution study: Initial screening experiments of photocatalytic hydrogen evaluation were carried out in 20mL vials for optimizing various parameters including photocatalyst dose, different sacrificial donors etc. With these optimized conditions, photocatalytic water splitting reactions were further carried out in a 270mL tubular borosilicate photoreactor with an inlet port for inert gas purging and an 7 ACS Paragon Plus Environment


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outlet for automatic injection to GC. In this typical experimental set-up, photocatalytic hydrogen evaluation study was performed using 100mL distilled water, optimized amount of Pt/mesoceria photocatalyst and sacrificial donor. The reactor was then flushed out with nitrogen, which removes headspace air. Above photocatalytic reaction system was illuminated externally with two 200W tungsten filament lamps. (The distribution of light irradiance spectra of W filament lamp is listed in Figure S1 depicted in the supporting information). The evolved gas was injected directly to 1mL sample loop controlled by gas-sampling valve (GSV) equipped with 5A0 molecular sieve column of GC Shimadzu-2014. The amount of H2 was analyzed by thermal conductivity detector (TCD) and N2 was used as carrier gas.

RESULTS AND DISCUSSION Crystal structure and surface morphology analysis: The XRD patterns of all the samples as prepared, mesoceria, x% Pt/mesoceria (x=0.5, 1 & 3) and commercial CeO2 were examined for their crystal structure and phase composition, as given in Figure 1. The diffraction peaks [(111), (200), (220), (311), (222), (400), (331), (420), (422), (511)] exhibit the formation of fluorite crystal structure, with space group Fm-3m (225) as analyzed with JCPDS database card no. 34-0394-Cerianite-(Ce)-CeO2. The commercial ceria shows sharp diffraction peaks with high intensity. Whereas, the as-synthesized ceria and Pt loaded ceria samples show broadened diffraction peaks. It implies the formation of ceria nanocrystals. The crystallite size is of the order of 6 nm for the as-synthesized ceria samples, as calculated using Debye Sherrer’s equation. The crystallite size for the commercial ceria is of the order of 77 nm. The decrease in size also suggests the increased surface area of the assynthesized-ceria. No significant change in the diffraction peaks were observed upon 8 ACS Paragon Plus Environment

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incorporation of Pt on ceria.

(e) (d)

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(a)

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CeO2(220)

CeO2(200)

(b)

CeO2(111)

(c)

Intensity (a.u.)

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100

2 Theta (deg.)

Figure 1.X-Ray diffraction patterns of (a) Commercial ceria (b) Mesoceria(c) 0.5wt % Pt/Mesoceria (d) 1 wt % Pt/Mesoceria (e) 3wt % Pt/Mesoceria

In order to examine the surface area and porosity of photocatalyst samples, BET measurements were performed. The crystallite size, pore size, surface are values are summarized in Table-1. The BET surface area of as-synthesized ceria is 131.7 m2g-1 with the pore size of approximately 3.36 nm, revealing the fact, that the synthesized ceria samples are highly porous in nature with pores in mesopore range. Whereas, BET surface area of commercial ceria was found to be 24.4 m2g-1.17These pore characteristics of mesoceria is mainly due to the template (chitosan) used for synthesizing it. This method offers easy and low cost synthesis of ceria with substantial increase in its surface area and with meso size pores without ordering. The crystallite size of mesoceria was within 6-8nm range suggesting its nanocrystalline nature.



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Table 1.Physicochemical properties Crystallite size

BET surface area

Pore diameter

(nm)

(m2g-1)

(nm)

Commercial ceria

77.4

24.4

-

2.82

Mesoceria

6.56

131.7

3.36

2.63

0.5wt %Pt / Mesoceria

7.11

-

-

2.60

1wt %Pt/ Mesoceria

7.0

119.4

3.52

2.52

3wt %Pt/ Mesoceria

6.95

-

-

2.53

Catalyst

Band Gap (eV)

Further, to get insight in to the structural details, HRTEM with EDS analysis (Figure 3) were done for a representative sample of 1wt%Pt/mesoceria. To rule out any ambiguity, 1wt%Pt containing commercial ceria sample was also prepared. Figure 2shows TEM images of 1wt%Pt/commercial ceria, consisting of cubic, triangular and tetragonal shaped particles, which are not uniform. The particle size observed was in the range of 20 to 90 nm with consistent lattice space distance (interplanar distance) of 0.31nm, which correspond to (111) plane of CeO2. Whereas 1wt%Pt/mesoceria predominantly shows cubic shaped particles with particle size below 10nm, and is in agreement with XRD results. HRTEM images as shown in Figure 3(a)also 10 ACS Paragon Plus Environment

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inferred the highly crystalline nature of mesoceria. EDS analyses results showing atomic percentage of cerium and platinum is represented in Figure 3 (b).

Figure 2.TEM images of 1wt % Pt/commercial ceria

Figure 3. (a) HRTEM images of 1wt % Pt/mesoceria.



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Figure 3.(b) EDS analysis of 1wt% Pt/mesoceria

XPS Analysis: The XPS analysis was carried out to study electronic states and chemical composition. XPS survey spectra of the mesoceria and Pt/mesoceria in the binding energy range from 0 to 1100 eV. The peaks corresponding to Ce, Pt, O and C were clearly observed. The highly resolved XPS spectra of Ce 3d with different chemical states of mesoceria and Pt/mesoceria is shown in Figure 4. The Ce 3d spectrum is composed of two multiplets (v and u) corresponding to the spin–orbit split of 3d5/2 and 3d3/2 core electrons, respectively. The peaks labeled with v0, v', u0 and u' imply the presence of Ce3+. Whereas unmarked peaks in the figure exhibit the presence of Ce4+.20-21 The shifting, broadening and asymmetrical nature of Ce 3d peaks is due to mixing of oxidation states of CeO2 and Ce2O3 and also due to the coupling of the spin and angular momentum of Ce 3d levels.22 A semi quantitative analysis of the integrated peak area can provide the concentration of Ce3+ 12 ACS Paragon Plus Environment

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ions in the synthesized ceria photocatalysts.23 The concentration of Ce3+ in mesoceria is 0.26 which is lower than that of commercial ceria (0.33). Upon Pt modification, the concentration of Ce3+ increases due to reduction environment while modifying ceria with Pt. It is important to note the Ce3+ concentration of 1 wt% Pt-mesoceria (0.43) is little higher than that of 1wt% Ptcommercial ceria (0.40) due to former’s mesoporous nature exposing more surface area towards the reduction environment creating more oxygen vacancies.

v'

uo

u'

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Binding Energy (eV)

Binding Energy (eV) 5

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d

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Intensity (cps)

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5

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Binding Energy (eV)

895

900

905

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915

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Binding Energy (eV)

Figure 4: XPS spectra of (a) commercial ceria (b) 1 wt% Pt-Commercial Ceria (c) mesoceria (d) 1 wt% Pt-mesoceria

XPS spectrum observed for platinum component is shown in Figure 5. The Pt 4f7/2 has binding 13 ACS Paragon Plus Environment


energy between 79 to 70eV and thus signify the presence of platinum where observed in Pt(0) and Pt(II) states has an binding energy of 71.1 and 72eV respectively. Peaks at 73.1, 75.8 and 76.7 correspond to Pt (IV) state. The platinum present in Pt(II), Pt(IV) may be due to incomplete reduction at 350oC and also due to the presence of surface –OH groups. This suggests stronger interaction of platinum with ceria.

Raw data

73.1

72

Intensity (a.u.)

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80

71.1 76.7

75.8 75.3

78

76

74

72

70

68

Binding Energy (eV)

Figure 5.XPS spectra of Pt 4f of 1wt % Pt/Mesoceria As regards the O 1s peaks ( see FigureS2 supplied in the supporting information), these were centered at ~530 eV and showed a shoulder at ~531.1eV. The peaks at 530.1 eV can be attributed to Ce2O3 and CeO224,while that at 531.1eV is due to OH− ions.25 However, in case of mesoceria sample, these peaks appeared at slightly lower binding energy. The slight increase in binding energy values thus suggests strong interaction of platinum with cerium via oxygen of mesoceria. The XPS spectrum for the C1s (see Figure S3 supplied in the supporting information), The peak at 285.2eV can be separated into two components centered at binding energies of 285.2 and 286.7eV. These were observed only in case of Pt incorporated ceria sample, while in case of 14 ACS Paragon Plus Environment

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mesoceria this peak was observed at 285.8eV. The peak at 286.7eV can be assigned to the carbon contributions from the R-C- group used during catalyst synthesis.17 The higher binding energy suggests that small amount of carbon may got incorporated into the interstitial positions of the CeO2 or Ce2O3 lattice.

Optical Absorption properties: To study the optical properties (bang gap, Eg), diffuse reflectance UV-Vis spectra (DRS) were recorded for all the ceria based samples. Synthesized mesoceria and Pt impregnated mesoceria materials show UV-visible absorbance spectra with broad band absorption from 800 to 300 nm, as shown in Figure 6 (a). This improved visible light absorption can be attributed to defect sites initiation in oxygen anion vacancies in the lattices of ceria and presence of Ce3+. Hence, with increase in Ce3+ concentration, band gap decreases and formation of localized gap states in the band gap can be expected. Overall, nanostructured ceria has narrow band gap because of oxygen vacancies, presence of defects and an increased Ce3+ concentration. Furthermore, diffuse reflectance measurement was exercised and converted into Kubelka-Munk function (f(R)) as given in following equation to extract absorption coefficient (α): (1)

f ( R ) = (1 − R ) / 2 R = α / s

Where, f(R) is the Kubelka-Munk function, R is reflectance and s is scattering coefficient. For direct band gap energy (Eg) calculation, the tauc plot were analyzed by extrapolating (αhv)2vs photon energy using following equation:

(αhυ)2 = A(hυ − Εg)

(2)



Where, hv is the energy of incident photon and A is the absorbance constant. The value of Eg was obtained from Tauc plot by linear extrapolation with hv axis, as shown in Figure 6 (b). Tauc plot analysis of all the samples clearly shows decrease in band gap energy from ~2.8 eV to ~2.6 eV. The details of Eg values for all the prepared photocatalyst is given in table-1. The shift in the band gap (/absorption edge) to the visible region upon Pt loading is due to the electronic transitions from the band edge of ceria to the redox state of Pt nanocrystals. Although, some contribution of the scattering factor by the increase in the Pt loading concentration can’t be ruled out.26

1.0 Comercial CeO2 Mesoceria 0.5wt% Pt/mesoceria 1wt% Pt/mesoceria 3wt% Pt/mesoceria

0.8

0.6

Comercial CeO2

200

Mesoceria 0.5wt% Pt/mesoceria 1wt% Pt/mesoceria 3wt% Pt/mesoceria

150

(alfa*hv)2

Absorbance (a.u.)

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100

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(b)

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400

500

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800

2.0

2.5

3.0

3.5

4.0

4.5

hv (eV)

Wavelength (nm)

Figure 6.(a) Diffuse reflectance UV-visspectra;(b) Tauc Plot

Photocatalytic hydrogen evolution study: Photocatalytic activity of synthesized photocatalysts was examined towards hydrogen generation from a dilute aqueous ethanol solution under Vis-NIR irradiation. Initial photocatalytic water splitting experiments were carried out in a 20ml photoreactor to optimize the photocatalyst dose, use of different sacrificial donors and amount of sacrificial donor. The hydrogen evolution rate


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for commercial ceria, synthesized mesoceria and Pt impregnated mesoceria samples are shown in Figure 7. The 1wt% Pt/mesoceria sample shows highest hydrogen evolution rate among all the ceria based photocatalysts. There is significant increase HER in mesoceria as compared with commercial CeO2. Noble metal Pt doping increase this HER activity because of increased reduction sites. The results are given (see Figure S2 supplied in the supporting information), which convincingly infer the hydrogen generation impact under illumination using present mesoporous ceria as well as under dark condition but at 800C to study the effect of temperature as the temperature did raise to 800C during illumination. Furthermore, the optimized 1wt% Pt/mesoceria photocatalyst was subjected to scale-up hydrogen generation evaluation study, using a bench scale 270ml photoreactor. The time coarse performance and stability of photocatalyst was also studied using this reactor connected to an online GC to avoid any hydrogen losses during experiments.

100

93.43 78.93

80

Rate (µmols h-1)

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60

37.98

40

20

12.3 0

1.18 Comercial CeO2 Mesoceria

0.5%Pt/Ceria 1% Pt/Ceria

3% Pt/Ceria

Photocatalysts

Figure 7.Photocatalytic hydrogen generation on different photocatalysts (Photocatalyst dose100mg/100mL; Time- 2h; Water: Ethanol- 100:5ml; mode of analysis- Online GC)



Photocatalyst dose study: In order to optimize the photocatalytic hydrogen evolution rate, the photocatalyst doses were varied from 1 to 20 mg in a 10 ml water-ethanol mixture. The effect of photocatalyst dose was explored at a fixed amount of water to ethanol ratio and optimized illumination intensity27. There is continuous enhancement in the hydrogen evolution rate with the increase in the photocatalyst dose up to 10 mg. However, there is no further enhancement with the increase in the photocatalyst dose to 20 mg. These results clearly show that the optimized dose of 1wt% Pt/mesoceria is 10mg in 10ml for the hydrogen evolution rate observed as 197 µmolh-1. Further experiments on photocatalytic hydrogen generation were carried out by using this optimized dose of 1wt% Pt/mesoceria.

197

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120

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80

67

40

0 1mg

5mg

10mg

15mg

20mg

Photocatalyst Dose (mg/10ml)

Figure 8.Effect of photocatalyst dose on hydrogen evolution rate. (Photocatalyst- 1wt%Pt/mesoceria; Time- 2h; Water: Ethanol- 10:0.5 ml; mode of analysis-offline GC)

Use of different sacrificial donors: The recombination of the photogenerated charge carriers (e- and h+) via oxygen vacancies and


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crystal defects is of a serious concern for the photocatalyst and governs overall efficiency of the hydrogen generation. Such recombination may be reduced to a great extent by using suitable sacrificial donors, which facilitate the photoredox reactions (water splitting). The effect of different donors on the hydrogen evolution rate was analyzed under the optimized conditions, Figure 9. Among different sacrificial donors used, ethanol shows better hydrogen evolution. The concentration of ethanol as a sacrificial donor was varied from 0.1 to 1 ml to further optimize the hydrogen evolution rate, Figure 10. The optimal concentration of 0.5 ml ethanol shows the best hydrogen evolution rate.

200

197 164

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Rate (µmols h-1)

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153

120

80

40

0.789

0 Ethanol

Methanol

Glycerol

EDTA

Sacrifacial donors

Figure 9.Effect of different sacrificial donors on photocatalytic hydrogen generation. (Photocatalyst1wt% Pt/mesoceria; photocatalyst dose- 10mg; Time- 2h; Water: Ethanol- 10:0.5 ml; mode of analysisoffline GC)



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150

100

50

0 0.1ml

0.3ml

0.5ml

1ml

Amount of Ethanol

Figure 10.Effect of sacrificial donor concentration on hydrogen evolution rate. (Photocatalyst1wt% Pt/mesoceria; photocatalyst dose- 10mg; Time- 2h; Water: Ethanol- 10:0.5 ml; mode of analysisoffline GC) Online photocatalytic hydrogen evaluations in batch mode were also carried out to rule out any analytical errors, by using a specially designed evaluation set-up. The photocatalyst stability was also tested upto 34h to establish that the hydrogen generation is not due to some temporary phenomenon, and to know whether the photocatalyst is stable under the operating conditions. The almost linear increase in cumulative hydrogen generation with time was observed, which implies the sustainable photocatalytic hydrogen generation activity of the Pt/mesoceria for water splitting reaction and good stability of the photocatalyst. As shown in Figure 11,the average hydrogen evolution rate of 0.045 mmolh-1was observed for the present Pt/mesoceria. The present photocatalytic system was replenished with ethanol after 12 and 26h and the total hydrogen generation realized during this experiment was 1.52 mmols after 34h of visible-NIR light illumination Attempt has been to estimate apparent quantum efficiency (AQE) for this reaction


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(see Table S1 and related content supplied in the supporting information) which shows a value of AQE is 1.57% under the present experimental conditions. The other photocatalysts reported for visible-NIR region activity reports value of 0.0293 and 0.048 mmolh-1 for g‑C3N4 sensitized by zinc phthalocyanine derivative and added chenodeoxycholic acid respectively. However, clearly those photocatalyst systems cannot be easily compared with present simple Pt-ceria photocatalyst due to complexity of sensitized photocatalyst compositions as well as the evaluation conditions used.7,28

1.6

Cumulative mmole of Hydrogen

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1.4 Replenish with 5ml Ethanol

1.2 1.0 0.8

Replenish with 5ml Ethanol

0.6 0.4 0.2 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Time (hr)

Figure 11. Time course study on 1wt % Pt/mesoceria (Photocatalyst dose- 100mg; Time- 34h; Water: Ethanol- 100:5 ml; mode of analysis- online GC)

Correlation of photoluminescence and photocatalytic hydrogen generation activity: In order to get insight in to recombination of photogenerated charge carriers with the oxygen vacancies/defects, coexistence of Ce3+/Ce4+ species and its correlation with the overall photocatalytic hydrogen generation activity, detailed photoluminescence (PL) studies were



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undertaken.

Figure 12. Room-temperature photoluminescence spectra of the Pt modified and unmodified CeO2 samples, under 240 nm light excitation.

Figure 12 shows the PL spectra of all the CeO2 based samples, as measured at room temperature at an excitation wavelength of240 nm. Numerous emission peaks located at 397, 468, 482, 492, 552 and a shoulder at 590 nm were observed. The sharp emission peaks located at 397 nm (UV emission band) can be attributed to excitonic recombination corresponding to the near-band-edge emission of CeO2.29 This emission occurs from the 5d–4f transitions of Ce3+, between the 2D (5d1) ground state and the 2F5/2 (4f1) state.30The decrease in the intensity of UV emission band follows the order: commercial ceria>meso CeO2> 0.5 wt% Pt- CeO2> 3 wt% Pt- CeO2> 1 wt% Pt- CeO2, suggesting that the commercial CeO2 sample has the highest Ce3+ density as compared to that of other samples. It also corroborates with the XPS analysis, showing higher Ce3+ density in the case of commercial ceria than that of mesoceria. The Ce3+ acts as recombination center for photo generated charge carriers and thus it appears partially responsible, in addition to the lower surface area, for the lower photocatalytic hydrogen 22 ACS Paragon Plus Environment

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generation activity in the case of commercial CeO2. The PL data along with quantitative XPS analysis clearly suggest higher density of the Ce4+ in the case of mesoceria than that of commercial ceria. It further increases upon modification of mesoceria by incorporation of Pt metal on it. These observations corroborate with the improved photocatalytic hydrogen generation activity observed in the case of 1 wt% Pt loaded mesoceria. The stronger emission in the blue-green region (468 nm-495 nm) may be attributed to the transitions from different defect levels to O 2p band. These defects enhance charge carrier recombination losses in the semiconductor. The intensity of this peak decreases in the case Pt impregnated mesoceria. This further suggests better charge separation in the case of Pt impregnated mesoceria than that of commercial ceria and the unmodified mesoceria. Although the 3wt% Pt incorporatedCeO2 sample has the lowest emission in the blue-green region even as compared to 1wt% Pt-CeO2, there is higher density of Ce3+ as revealed by the PL peak area at 397 nm, in the case of 3 wt% Pt CeO2. Such variation of the density of Ce3+ seems to govern the overall photocatalytic activity and thus the hydrogen generation rate for 3 wt% Pt-CeO2 lies in-between 1wt% and 0.5 wt% Ptmodified CeO2. The emission peaks at 550 and 590 nm as shown in Figure 7 may be induced by the oxygen vacancies in the crystal with electronic energy levels below the Ce 4f band or by the transition from some localized states within the band gap to the valence band.31 Mechanistic aspects: The defects (Ce3+ and oxygen vacancies) associated with Pt/PtO incorporated nanostructured ceria along with use of sacrificial donor (ethanol) and N-IR -visible irradiation play significant role in the photocatalytic water splitting reaction mechanism. For better understanding of electronic interactions between small metal clusters (Pt) and reducible oxides (e.g. CeO2), we



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referred the recent excellent brief prospective on electronic interactions and charge transfers of metal atoms and clusters on oxide surfaces by Gianfranco Pacchioni.32 This work systematically describes small metal particles interaction with reducible (CeO2) and non reducible oxide (MgO, Al2O3). The inherent photoredox chemistry,33 defects behaviour of ceria,34 undesirable hydrogen oxidation reaction on oxidized platinum cluster35 are among the prominent factors governing this sacrificial donor assisted photocatalytic water splitting reaction mechanism on present Pt/mesoceria photocatalyst (Scheme 1.). It is evident that the Pt/PtO incorporated nanostructured ceria has a narrow band gap. On visible-NIR light irradiation, CeO2 absorbs photons of energy equal to or greater than the band gap value (≥ 2.6), thereby generating photoelectrons and holes in the conduction band and valence band respectively. These photogenerated electrons then finds their way to the Pt and PtO sites, present mostly on ceria surface, and reduces H+ generated from water to produce H2. The photogenerated holes simultaneously produced at valance band are probably used by Ce3+ species, resulting in photooxidation to Ce4+.The Ce3+ can be efficiently regenerated through oxidation of sacrificial donor by Ce4+ produced. In this way, the entire complex process is expected to become more efficient due to above discussed properties of nanostructured ceria, especially the efficient redox couple. Yet another facilitator of photocatalytic activity should be higher surface area of present nanostructured ceria adding to efficient light absorption and surface reactions. The optimum pore size of mesoceria is also expected to facilitate mass transfer of reactant molecules.


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Scheme 1. Proposed mechanism for photocatalytic hydrogen generation using Pt/PtOnanostructured ceria.

Based on the above explained mechanism for Pt/PtO on mesoceria with sacrificial donor, the following reactions are possible during photocatalytic hydrogen evolution with back reaction getting suppressed because of the presence of PtO species35 as expressed below; hυ CeO2  →e− + h+

CeO2−x + h+ → CeO2

CeO2 + EtOH→CeO2−x Pt/ PtO 2H +  →H2

Pt H2 ←→ 2H +



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PtO 2H+ →  H2

In brief, the higher photocatalytic hydrogen generation activity of mesoceria over commercial ceria is due to the unison of lower Ce3+ concentration, high surface area and small particle size. Further increase in the photocatalytic activity of Pt modified ceria than their unmodified counter parts is due to presence of co-catalyst Pt which traps the photogenerated electron before recombination, facilitating effective electron transfer at the photocatalyst-electrolyte interface. The photocatalytic activity of Pt modified mesoceria photocatalysts are dominated by the availability of higher number of photoactive sites than that of Pt modified commercial ceria. CONCLUSIONS We have synthesized nano sized Pt/PtO clusters supported on nanostructured ceria to promote photocatalytic hydrogen evolution in a sacrificial donor assisted water splitting reaction under visible-NIR radiations. Nanostructured ceria prepared by simple template based chemical method shows significant decrease in the band gap, due to the enhanced oxygen defects resulting in narrowed band energy to drive visible-NIR light activity for hydrogen evolution. Formation of nanostructured ceria with decrease in particle size and improved morphology as analyzed by pXRD and HRTEM analyses suggest the presence of oxygen defects in the lattices. Optical properties as examined by DRS and PL analysis show the narrow band gap and available lesser recombination sites, which may also be due to increased oxygen defects of present nanostructured ceria. Further, the influence of Pt concentration and its electronic states on the nanostructured ceria surface were investigated by using the XPS, which clearly shows the presence of Ce3+ and Ce4+ redox couple along with multivalent states of platinum. Presence of Pt/PtO clusters expected to enhance the photocatalytic H2 evolution performance and plays a 26 ACS Paragon Plus Environment

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crucial role in H2generationand suppress the back reaction. Based on all the above studies and hydrogen evolution analysis, 1wt% Pt/mesoceria is the optimized photocatalyst for this composition and shows average hydrogen evolution rate of 0.045 mmolh-1 with cumulative H2 evolution of 1.52 mmols after 34h of exposure under visible-NIR light. As the present Pt/PtO-ceria photocatalyst appears to be a simple and robust system to generate hydrogen through sacrificial donor assisted water splitting reaction, the mechanistic aspects are discussed based on photoredox nature of ceria, role of multivalent states (Pt/PtO) of platinum and available literature. Estimation of Pt (0) and PtO contents on nanostructured ceria as well as optimization of their contents to further suppress back reactions are under process. AUTHOR INFORMATION Corresponding Author * Fax: +91-712-2249900 Tel:+91-712-2249753. Email: [email protected] Notes The authors declare no competing financial interest. Author Contributions ‡These authors contributed equally. ACKNOWLEDGMENT The authors would sincerely like to acknowledge CSIR Network Project (NWP-56) for financial assistance. Characterization studies and interpretations have been carried out under the CSIR/NEERI-IIC

(Czech

Republic)

and

CSIR/NEERI-Kyushu

University

research

collaborations. Thanks are also due to Dr.CS Gopinath of CSIR-NCL for XPS analysis. REFERENCES 27 ACS Paragon Plus Environment


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For Table of Contents Use Only



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.

Synopsis:

Photocatalytic water splitting mechanism and time course study results on Pt/PtO promoted nanostructured ceria for sustainable energy generation.


PtO-Nanostructures: Efficient ... - ACS Publications

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