Photocatalytic Hydrogen Evolution Using Ni–Pd/TiO2 - ACS Publications

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Photocatalytic Hydrogen Evolution Using Ni−Pd/TiO2: Correlation of Light Absorption, Charge-Carrier Dynamics, and Quantum Efficiency Ana L. Luna,† Diana Dragoe,‡ Kunlei Wang,§ Patricia Beaunier,∥ Ewa Kowalska,§ Bunsho Ohtani,§ Daniel Bahena Uribe,⊥ Miguel A. Valenzuela,# Hynd Remita,*,†,○ and Christophe Colbeau-Justin*,† †

Laboratoire de Chimie Physique, CNRS UMR 8000, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay, France Institut de Chimie Moléculaire et des Matériaux, UMR 8182 CNRS, Université Paris-Sud- Université Paris-Saclay, 91405 Orsay, France § Institute for Catalysis, Hokkaido University, North 21, West 10, Sapporo 001-0021, Japan ∥ Sorbonne Universités, UPMC, UMR 7197-CNRS, Laboratoire de Réactivité de Surface, F-75005 Paris, France ⊥ Advanced Laboratory of Electron Nanoscopy, Cinvestav, Ave. IPN 2508, 07360 Mexico City, Mexico # Laboratorio de Catálisis y Materiales, ESIQIE−Instituto Politécnico Nacional, Zacatenco, 07738 Mexico City, Mexico ○ CNRS, Laboratoire de Chimie Physique, UMR 8000, 91405 Orsay, France ‡

S Supporting Information *

ABSTRACT: TiO2 surface modification with bimetallic nanoparticles (NPs) has demonstrated to be a strategy to enhance the hydrogen generation via photocatalysis and to minimize the use of expensive noble metals. A better understanding of the role of bimetallic NPs is of crucial importance to design efficient photocatalysts. Here, we show a systematic study of surface modification of commercial TiO2 (P25) with monoand bimetallic (Ni, Pd, and Ni−Pd) NPs synthesized by radiolysis. The photocatalysts were characterized by High Resolution Transmission Microscopy (HRTEM), Scanning Transmission Electron Microscope (STEM), X-ray Diffraction (XRD), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and UV−vis Diffuse Reflectance Spectroscopy (DRS). The charge-carrier dynamics was studied by Time Resolved Microwave Conductivity (TRMC). The photocatalytic activity was evaluated for hydrogen generation under UV−vis irradiation using polychromatic and monochromatic lights (action spectra analysis of apparent quantum efficiency). TiO2 modified with Pd−Ni bimetallic NPs exhibits a high activity for H2 generation, and a synergetic effect of the two metals was obtained. The study of light absorption, charge-carrier dynamics, and photocatalytic activity revealed that the main role of the metal NPs is to act as catalytic sites for recombination of atomic hydrogen. them to participate in reduction reactions.9−11 More recently, it has also been proposed that the main role of the metal NPs is to promote the recombination of the atomic hydrogen.12−14 Among different noble metals, Pt is the most effective for hydrogen generation via photocatalysis.15 However, due to high cost and limited reserves of Pt, it is important to find efficient and less expensive catalysts. A potential substitute of Pt is Pd because it is cheaper and 50 times more abundant on the Earth than Pt. Pd-based catalysts are very efficient in many catalytic reactions (including Suzuki, Heck reactions,16 and hydrogenation reactions), 17 and in fuel cells (for ethanol oxidation).18 Additionally, Pd/TiO2 has shown a high-perform-

1. INTRODUCTION Since the reports published by Fujishima and Honda in the early 1970s,1 heterogeneous photocatalysis has become a subject of great interest with important potential applications in environment, solar energy conversion, and solar fuel production among others.2−5 Particularly for hydrogen generation, photocatalysis is considered as a promising technology toward a sustainable energy economy.6−8 TiO2 photocatalysts have shown to be capable to produce H2 under UV irradiation, however the reaction efficiency is very low. It has been proven that the photocatalytic activity of TiO2 is significantly improved when its surface is modified with metal nanoparticles (NPs). The enhancement of the photocatalytic performance has been generally explained by the spatial separation of charge-carriers due to electron transfer from TiO2 to the metal NPs, prolonging their lifetime and enabling © 2017 American Chemical Society

Received: February 6, 2017 Revised: May 24, 2017 Published: June 6, 2017 14302

DOI: 10.1021/acs.jpcc.7b01167 J. Phys. Chem. C 2017, 121, 14302−14311

Article

The Journal of Physical Chemistry C ance in photocatalysis for water depollution,19 water splitting,20 and biomass reforming (methanol,21 glucose,22 and glycerol23). Bimetallic NPs deposited on titania have demonstrated to be an excellent option to minimize the cost and to increase the activity for hydrogen evolution.24−26 TiO2 modified with Pdbased NPs has been explored showing interesting results, for example, Pd−Pt/TiO2 have shown a higher hydrogen evolution than Pt/TiO2.27,28 On the other hand, titania modified with Pd−Au NPs exhibits a photocatalytic reaction rate for H2 similar than that of Pt NPs.11 TiO2 modified with bimetallic Pd−Cu NPs shows enhanced photocatalytic activity compared to Pd/TiO2 and Cu/TiO2.29,30 In particular, Ni−Pd bimetallic NPs as alloys or core−shell nanostructures, have shown a higher catalytic activity in selective hydrogenation reactions than their monometallic counterparts. 31 However, this bimetallic system has not yet been investigated for photocatalytic applications. Although such works have made great strides about bimetallic NPs, many questions related to their structure and composition on the optical and photocatalytic properties still remain unanswered.32 Understanding the role of the bimetallic NPs is inherent to develop new, more efficient, and inexpensive photocatalysts. Radiolysis is a powerful technique to synthesize mono- and bimetallic nanoparticles of controlled size, structure, and composition in solutions and on supports.18,19,33−37 Here, we present surface modification of TiO2 surface with Pd, Ni and Pd−Ni NPs induced by radiolysis. The photocatalytic activity of the system Pd−Ni/TiO2 is reported for the first time. We show that Pd−Ni/TiO2 is very efficient for photocatalytic hydrogen production, and this activity is higher than that of Pd/TiO2 and Ni/TiO2. The aim of this work is to explain the enhancement of the photocatalytic performance of modified TiO2 through an integral study of their structural characterization, charge-carrier dynamics, and photocatalytic activity.

determined using an atomic ratio of Ni/Pd = 10 or 0.1. The samples were labeled as x-NiyPdz/TiO2, where x corresponds to the metal loading of the abundant metal and y and z is the atomic ratio used, for example, 0.5-Ni10Pd1/TiO2 sample indicates that it contains 0.5 wt % of Ni (abundant metal in this case) and the atomic ratio Ni:Pd is 10:1. Also, the corresponding monometallic counterparts (Ni/TiO2 and Pd/ TiO2) of each bimetallic sample were synthesized. 2.3. Characterization. UV−vis Diffuse Reflectance Spectra (DRS) measurements were carried out on an Agilent Technologies Cary 500 spectrophotometer with an integrating sphere, and using PTFG as reference. High Resolution Transmission Electron Microscopy (HRTEM) measurements were performed on a JEOL JEM 2010 transmission electron microscope equipped with a LaB6 filament and operated at 200 kV. The chemical analyses were obtained by Energy-Dispersive X-ray Spectroscopy (EDS) microanalyser (PGT-IMIX PC) mounted to the microscope. The samples were also analyzed by Scanning Transmission Electron Microscopy (STEM) using a JEOL ARM (200F): 200 kV FEGSTEM/TEM equipped with a CEOS Cs corrector on the illumination system. HAADF-STEM images were acquired with a camera length of 8 cm/6 cm and a collection angle of 68−280 mrad/90−270 mrad. The BF-STEM images were obtained using a 3 mm/1 mm aperture and a collection angle of 17 mrad/5.6 mrad (camera length in this case was 8 cm). The HAADF as well as the BF images were acquired using a digiscan GATAN camera. EDS measurements for line scan profiles as well as chemical maps were obtained with a solidstate detector and software for two-dimensional mapping from Oxford. EELS spectra were acquired by Enfina 1000 gatan system. XPS measurements were performed on a K-Alpha X-ray Photoelectron Spectrometer (Thermo Fisher Scientific) under ultrahigh vacuum (base pressure in the low 10−9 mbar), equipped with a monochromatic Al source using a spot size of 400 μm. The hemispherical analyzer was operated at 0° take off angle in the Constant Analyzer Energy (CAE) mode, with a pass energy of 200 eV and a step of 1 eV for the acquisition of survey scans and a pass energy of 50 eV and a step of 0.1 eV for the acquisition of narrow windows. Charge compensation was done by means of a “dual beam” flood gun. The C 1s signal at 284.8 eV was used as reference for the bonding energy scale. X-ray Diffraction (XRD) analyses were performed on a Rigaku Smart Lab instrument with Cu Kα radiation (λ = 0.154 nm) at 40 kV and 30 mA. The XRD patterns were recorded for 2 theta range from 10 to 90° with a scan rate of 1°/min and a step of 0.008°. The peak identification was done using Rigaku PDXL. The dynamic of charge-carriers in the photocatalysts was studied by Time Resolved Microwave Conductivity (TRMC) method.39,40 The samples were put in an aluminum sample holder, which was connected to a microwave guide. The amount of sample was controlled by a PTFE support placed in the cavity of sample holder. The incident microwaves were generated by a Gunn diode of the Kα band at 30 GHz. A laser (EKSPLA, NT342B) tunable in the range between 220 and 2000 nm, equipped with an optical parametric oscillator (OPO), was used as pulsed light source. It delivered 8 ns FWMH pulses with a frequency of 10 Hz. To minimize the noise, a TRMC signal was obtained averaging measurements during 200 laser pulses. The range of the examined monochromatic wavelengths was from 310 to 505 nm. The

2. EXPERIMENTAL SECTION 2.1. Chemicals. All reagents were pure and used as received. Commercial titanium dioxide P25, from Evonik (ca. 50 m2g−1, ca. 80% anatase with rutile and amorphous titania38), was used as photocatalyst. Nickel formate (Alfa Aesor) and tetraamminepalladium(II) dichloride (Sigma-Aldrich) were used as nickel and palladium precursors, respectively. Methanol and 2-propanol were purchased from Sigma-Aldrich. Deionized water (Milli-Q with 18.6 MΩ) was used throughout all experiments. 2.2. Sample Preparation. The samples were prepared using the following methodology. For each catalyst, 35 mL of an aqueous suspension was prepared. Such suspensions contained the metal precursors, 1.5 g of TiO2, and 2-propanol (0.2 M) as HO• radical scavenger. The amounts of metal precursors used in this study are shown in Table S1. The suspensions were alkalized with ammonium hydroxide until pH 11 and stirred for 5 h in the dark. Before irradiation, the suspensions were degassed by bubbling N2 for 15 min. The irradiation was carried out using a panoramic 60Co γ-source at a dose rate of 4.5 kGy h−1, the total dose used for each photocatalyst is reported in Table S1. The total dose was calculated to ensure the complete reduction of Ni2+ and Pd2+ to their zerovalent state. After irradiation, the samples were centrifuged. Then, the powders were dried at 30 °C for 16 h. The supported bimetallic NPs were prepared with a metal loading of 0.5 or 1 wt % corresponding to the predominant metal in the sample. The content of the second metal was 14303

DOI: 10.1021/acs.jpcc.7b01167 J. Phys. Chem. C 2017, 121, 14302−14311

Article

The Journal of Physical Chemistry C

Figure 1. HRTEM and size distribution of modified titania: (a) 1-Pd/TiO2; (b) 1-Ni1Pd10/TiO2; and (c) 1-Ni10Pd1/TiO2.

a function of monochromatic wavelength was calculated as the ratio of the number of electrons used in hydrogen generation to the flux of incident photons on the system, according to the stoichiometry of the reaction where two electrons were required.

wavelengths and their corresponding excitation energies are shown in Table S2. The laser energy delivered in a single pulse was measured using a pyroelectric energy sensor (ES111C, Thorlabs) connected to a power meter (PM100D, Thorlabs). The TRMC technique is based on the measurement of the change of the microwave power induced by a laser pulsed irradiation and reflected by a sample. Its principle has been widely described in a previous paper.39 The main data provided by TRMC are given by the maximum value of the signal (Imax), which is the number of the excess charge-carriers created by the laser pulse, and the decay, due to the decrease of the excess electrons (free electrons).39,40 A complete explanation of the analysis of TRMC signals is provided in Supporting Information. 2.4. Hydrogen Evolution under UV−Visible Light. The catalyst photoactivity was evaluated by measuring the amount of hydrogen gas produced as a function of reaction time. Photocatalytic reactions were performed in a 35 mL glass reactor equipped with a stir bar. 50 mg of photocatalyst was suspended in 5 mL of 50 vol % aqueous methanol. Before irradiation, the reactor was degassed with nitrogen and sealed with a rubber septum. A 400 W mercury arc (Eiko-sha 400) was used as UV−vis irradiation source. A gas sample of 0.2 mL from the headspace was taken every 15 min during 1 h. The amount of hydrogen was determined using a gas chromatograph (Shimadzu GC-8A). Thermodynamically, methanol oxidation is easier to carry out than water oxidation. Therefore, H2 is generated from water splitting and methanol reforming. 2.5. Action Spectra Measurements. The photocatalytic test was very similar to the latter described in section 2.4. The main difference was the irradiation source. In this case, the reactor was illuminated by a monochromatic light at 320, 350, 380, 410, and 440 nm (±5 nm) obtained from a diffraction grating-type illuminator (JASCO CRD-FD) equipped with a 300 W xenon lamp (Hamamatsu Photonics C2578−02). A quartz cuvette of 10.5 mL was used as reactor, in which 30 mg of sample and 3 mL of 50 vol % aqueous methanol were added and stirred. The total irradiation time was 60 min, and each 20 min a gas sample of 0.2 mL was taken and analyzed by gas chromatography. The Apparent Quantum Efficiency (AQE) as

3. RESULTS AND DISCUSSION Titania surface was modified by mono- and bimetallic nanoparticles (Pd, Ni, and Pd−Ni) induced by radiolysis. High-energy radiation (γ-rays, X-rays, electrons, or ions beams) of deoxygenated water leads to the formation of free radicals such as hydrated electrons (e−s) and hydrogen radicals (H•), which are very strong reducing species with the respective redox potentials: E(H2O/e−s) = −2.87 VNHE and E0(H+/H•) = −2.3 VNHE. These free radicals can reduce dissolved metal ions down to the zerovalent state. No chemical reducing agents are added to the solution. Energy deposition throughout the solution ensures an initial homogeneous distribution of the radiolytic radicals and consequently a homogeneous reduction.36 Water radiolysis leads also to formation of hydroxyl radicals (HO•), which are very strong oxidative species (E0(HO•/H2O) = +2.8 VNHE). To avoid competitive oxidation reactions, propan-2-ol is added to scavenge hydroxyl radicals: This reaction leads to the formation of reducing ((CH 3 ) 2 C • OH) alcohol radicals E 0 ((CH 3 ) 2 CHOH/ (CH3)2C•OH) = +1.8 VNHE at pH 7.41 Radiolysis leads to synthesis of metal nanoparticles of controlled size and composition.34,36,37,42 The morphology, size, and dispersion of the metal NPs were examined by HRTEM, see Figure 1. Pd NPs on the titania surface (Pd/TiO2) showed a spherical shape and an average size of 12 nm (Figure 1a). In the samples Ni/TiO2, Ni NPs were not observed even though the elemental analysis showed Ni signals. The difficulty to observe Ni NPs on the TiO2 surface is probably due to the very small size of the metal clusters and the proximity between Ni and Ti atomic numbers, thus, making hard to distinguish them by contrast. Particularly in bimetallic samples, a remarkable difference in size and distribution of metal NPs was observed; such differences seem to depend on the metal atomic ratio. The 14304

DOI: 10.1021/acs.jpcc.7b01167 J. Phys. Chem. C 2017, 121, 14302−14311

Article

The Journal of Physical Chemistry C

Figure 2. (a) EDS analysis and (b) elemental mapping performed at a selected large metal NP of 1-Ni10Pd1/TiO2 sample. (c) EELS composition elemental mapping of small NPs observed on TiO2 support in the 1-Ni1Pd10/TiO2 sample.

Figure 3. XPS spectra of the Pd 3d core level of (a) 1-Ni1Pd10/TiO2 and (b) 1-Ni10Pd1/TiO2 samples. (c) Ni 2p spectra of sample 1-Ni10Pd1/TiO2.

spots on TiO2 were distinguished, as shown Figure S2a. The EDS mapping of such spots showed only the Ni−K signal, which was weak because of the small size of the supported NPs. In the same sample, although few, large aggregates were also observed (Figure 2a). Their EDS analysis revealed that the large NPs are mainly formed by Pd, a small amount of Ni was also detected. EDS line profiles (Figure S2b) confirmed the latter. As illustrated in 2D mapping images (Figure 2b), Ni clusters are distributed on the surface of Pd NPs. In the 1-Ni1Pd10/TiO2 sample, many large Pd aggregates (Figure S2c) were observed in accordance with the HRTEM

samples Ni1Pd10/TiO2 exhibited large metallic aggregates with an average size of 30 nm. Such aggregates were composed of numerous individual nanoparticles with an average size