Metal Particle Size Effects on the Photocatalytic Hydrogen Ion Reduction

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Metal particle size effects on the photocatalytic hydrogen ion reduction Zakiya Al Azri, Maher Al-Oufi, Andrew Chan, Geoffrey I.N. Waterhouse, and Hicham Idriss ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Metal particle size effects on the photocatalytic hydrogen ion reduction. Z.H.N. Al-Azri* a,c,e, M. AlOufib, A. Chana,e, G. I.N. Waterhousea,e, H. Idriss*b,d a School of Chemical Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand b Corporate

Research and Development (CRD), Saudi Basic Industries Corporation (SABIC) at KAUST, Thuwal 23955-6900, Saudi Arabia c Department of Chemistry, College of Science, Sultan Qaboos University, PO Box 36, Al-Khod 123, Oman d Department of Chemistry, University College London, London, UK e The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6140, New Zealand Corresponding authors: Email: [email protected] Email: [email protected] Telephone number: 966-12-2852444

ABSTRACT The main characteristics of noble metals on a semiconductor photocatalysts within the context of hydrogen ions reduction are particles’ size, electronic structure and dispersion. In this work, we have systematically studied Au, Pd and Pt particles on TiO2, with mean diameter of 5.2-5.8, ca. 2 and 1.2-1.5 nm, respectively at different coverages. Ethanol photo-reforming with water was used as the example from which extraction of reaction rates per mass, per particle and per atom has been analyzed. Irrespective of the metal nature, a narrow range for maximum catalytic performance was observed when the rate is normalized per unit mass or unit mole. Starting from a very low metal content (0.25 wt. % for each metal), the H2 production rates decreased with increasing the number of Pd, Pt or Au particles. Yet, the highest rate per particle is that of gold at any metal coverage. This rate exceeded by two orders of magnitudes that of Pt and by one order of magnitude that of Pd. This results indicate that unlike the case of thermal catalytic reaction, large particles perform better than small particles. Extraction of reaction rates from this study and from previous studies on Ni and Ag deposited on TiO2 indicated a direct relationship with the work-function of the metals and a volcano-shape relationship with their dband center position. Key words: Photocatalysis; hydrogen production; metal particle size; work-function; d-band center; reaction rate.

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1. INTRODUCTION The transition from a fossil fuel economy to a hydrogen-based economy for transportation, chemical production and electricity generation would occur when H2 from renewables is produced on a largescale and in an environmentally responsible manner1. While photocatalytic pure water splitting using powder materials is at present elusive that of alcohol photo-reforming is possible, even though it has its own limitations. While many semiconductors catalysts are found active most are not stable in their working environment making extraction of reliable results needed to understand the reaction mechanism difficult. Because of the stability and relative reproducibility of TiO2, it became the prototype for fundamental research at both the powder and single crystal levels.2-5 In the case of TiO2 (and most oxide semiconductors), however, fast electron-hole recombination follows photo-excitation which in turn limits the overall efficiency. Increasing charge separation6, mobility7-9 and the lifetime of photogenerated charge carriers10-11 is therefore a much thought after objective. Adding an alcohol (or other oxygenates) as a hole scavenger to water is one way to enhance photocatalytic H2 production rates.12-15 Transient absorption spectroscopy (TAS) studies have shown that that anatase polymorph drives holes more efficiently that the rutile phase16-17. In addition, single crystal anatase (101) have also shown far longer excited electrons life time when compared to the rutile single crystal (110).18 Ethanol is an example of a promising hole scavenger, due to its ease of production from biomass feedstock, its high H:C ratio of 3 and its high hydrogen content per unit volume in the liquid state. Further, as a liquid fuel ethanol is safe to handle and distribute.19 A common approach for enhancing the activity of TiO2 semiconductor for water splitting and alcohol photo-reforming is to deposit metals and/or metal oxides to enhance the reduction and oxidation steps of the reaction.9-13 Metal nanoparticles (or clusters) with high work functions such as Pd, Pt, Au or Ni are the most effective in promoting H2 production over TiO2 surfaces under UV excitation.20-24 Evidences suggest that during photoreaction, the photogenerated electrons in M/TiO2 system (M for metal) have shorter lifetime when compared to TiO2 alone10, 11, in line with numerous EPR studies where the g-tensor signal attributed to Ti3+ cations largely disappears in the presence of a noble metal12. Both observations, the disappearance of EPR signal attributed to Ti3+ cations and the decrease in the excited electron life time can be linked to a charge transfer from the semiconductor to the metal particle causing enhancement in the catalytic activity. While high H2 production rates have been reported by different research groups for Pd/TiO2, Pt/TiO2 and Au/TiO2 photocatalysts in ethanol-water mixtures under UV, only few detailed comparative studies have been studied.25-27 Most of the studies have focused on pure ethanol14,


28-32

or methanol photo-


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reforming.33-35 Uncertainty still exists as to the relative activities and the role of these three metal for H2 production. The following three points summarize them: (i)

What is the optimal conditions for best performance (i.e. metal loading, particle size or dispersion, and alcohol concentrations),

(ii)

With respect to point (i) what is the effect of metal electronic properties, especially the d-band center position and work function, on performance; and

(iii)

Does the mechanistic pathway for ethanol photo-reforming in the presence of each metal change?

Recent studies have provided further insights on the effect of metals on the TiO2 performance, for H2 production in alcohol-water mixtures. Jovic et al. evaluated the activity of a series of Au/TiO2 photocatalysts (Au loadings = 0–10 wt.%) for H2 production in 80 vol.% ethanol under UV irradiation and observed that the optimal Au loadings was in the range 0.5–2 wt.% with a non-optimized quantum yield of ~25 %.36 Chen et al. compared the catalytic activity of Ni/TiO2 and 2 wt.% Au/TiO2 (as a reference photocatalyst) for H2 production from ethanol-water mixtures and reported that the 0.5 wt.% Ni/TiO2 photocatalyst displayed comparable activity to Au/TiO2; actually superior at low ethanol content in water.37 Bowker et al. reported that H2 production rates for methanol photo-reforming over 1 wt.% metal loaded/P25 TiO2 photocatalysts followed the order Pd/TiO2 > Pt/TiO2 > Ir/TiO2 > Au/TiO2.38 Strataki et al. compared the relative activity of various metal catalysts (Pt, Au, Rh and Ru) deposited on nanocrystalline TiO2 thin films under UV irradiation in 10 vol. % ethanol and concluded that Pt afforded the highest hydrogen production rate.39 The same group studied the effect of ethanol concentration on the activity of Pt/TiO2 and observed that increasing alcohol concentration initially increased the rate, until a plateau was reached where H2 production rate was largely independent of ethanol concentration. It is obvious that in these studies metal dispersion, particle size and relative surface coverage with the alcohol would affect the reaction rates and therefore hinder direct extraction of fundamental results. The electronic structure of noble metals is important to their activity in the photocatalytic H2 production. Some work related to thermal reactions involving hydrogen has been conducted before. From example, Skoplyak et al. found a linear correlation between the d-band centre of Pt(111) and bimetallic Ni/Pt(111), with their steam reforming activities for H2 production from oxygenates (ethylene glycol and ethanol); with the activity increasing as the surface d-band centre moves closer to the Fermi level.40 Tedsree et al. examined the rate of formic acid (HCOOH) decomposition over various metals and concluded that the closer the d-band centre is to the Fermi level, the higher the activity due to the stronger adsorption energy of formic acid until reaching an optimum at Pd.41 To date, only a few studies ACS Paragon Plus Environment


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attempt to correlate H2 production rates photo-catalytically to the d-band centre position of various metal co-catalysts, a point that is examined in this current study.40 The mechanistic pathways of ethanol photo-reforming over M/TiO2 photocatalysts in ethanol-water mixtures has been the subject of a few investigations. Bamwenda et al. reported that Pt/TiO2 and Au/TiO2 photocatalysts released H2 as the main product under UV illumination in aqueous ethanol (50 vol.%) solution, observing a high selectivity to CH4 over CO2, with Pt/TiO2 being the most active photocatalyst.25 A recent study by Puga et al. examined ethanol photodecomposition over Au/P25 TiO2 (Au particle size = 1.5-6.5 nm) under UV-rich or simulated solar light and found that the main products of this reaction are H2 and acetaldehyde in stoichiometric amounts.30 The gas phase product concentrations followed the order H2 > CH4 > CO > C2H4 > CO2 > C2H6 > C3H8. Nadeem and coworkers reported that the sum of CH3CHO and CH4 (or CO) in the gas phase was almost the same as the amount of H2 produced from pure ethanol over 2 wt.% Au/TiO2 anatase nanoparticles under UV excitation.29 Ampelli et al. examined the H2 evolution activity of different CuOx/TiO2 catalysts in 50 vol.% aqueous ethanol and determined that acetaldehyde was the main side product and only small amounts of CO2 initially formed, later disappearing, while small amounts of other hydrocarbons were detected.42 Waterhouse et al. examined ethanol photoreforming over a 2 wt. % Au/TiO2 inverse opal photocatalyst under UV excitation. In 0.5 vol.% ethanol under direct sunlight excitation the products formed were H2 > CO2 >> C2H4> CH3CHO ≈ CH4.43 A large number of reactions are involved including water gas shift, ethanol dehydrogenation and de-carbonylation reactions as well as the Photo-Kolbe reaction. Here, the effect of metal loading, metal particle size, and their intrinsic electronic properties as well as the concentration of ethanol on the photocatalytic activity of the semiconductor TiO2 (with M=Pd, Pt, Au) for H2 production in ethanol-water mixtures (at different ratios) are systematically explored. M/TiO2 photocatalysts with metal loadings of 0-4 wt. % were synthesized by the deposition-precipitation method using the conventional semiconductor photocatalyst. Degussa P25 (anatase + rutile) was used as the support to allow meaningful comparison with the work of other groups. 2. EXPERIMENTAL 2.1 Materials All chemicals were obtained Sigma-Aldrich and used without further purification. These were Palladium chloride (99%), chloroplatinic acid hexahydrate (98%), tetrachloroauric acid trihydrate (99%), urea (99.5%), hydrochloric acid (37 wt. %) and ethanol (99.5%). Degussa P25 TiO2 (85 wt.



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% anatase, 15 wt. % rutile, 99.5%) was obtained from a local supplier. Milli-Q water (18.2 M∙cm resistivity) was used for catalysts preparation and the photocatalytic H2 production tests. 2.1 Preparation of M/TiO2 photo-catalysts All metals (Pd, Pt or Au) on top of TiO2 were deposited by precipitation with urea following the procedure described by Bamwenda et al. and Zanella et al..25, 44 In brief, HAuCl4.3H2O (1.654 g), PdCl2 (0.745 g) or H2PtCl6.6H2O (2.175 g) were separately dissolved in 1 L Milli-Q water to a give metal stock solution; 4.2 × 10-3 mol L-1. PdCl2 was dissolved in HCl (37 wt. %, 1 mL), with a final volume of 1 L with Milli-Q water. For the preparation of the 0.25- 4 wt. % Pd/TiO2 catalysts series, the Pd2+ stock solutions (with volumes of 16.8, 33.7, 67.8, 137.0, 279.7 mL) were diluted with Milli-Q water; total volume of 300 mL. 7.50 g of urea and 3 g of P25 TiO2 were then added to the solutions under mechanical stirring. The solutions were then heated to 80 oC and held at this temperature for 8 h. After vacuum filtration and washing with Milli-Q the resulting powders were air dried at 50 oC overnight. The same procedure was used for the preparation of the 0.25-4 wt. % Pt/TiO2 and Au/TiO2 catalysts series using the corresponding Pt4+ or Au3+ stock solutions. The catalysts were then calcined at 350 oC in static air for 2 h. This resulted in the reduction of Au3+ to Au0. Pd/TiO2 and Pt/TiO2 catalysts obtained by were subsequently heated to 500 oC for 2 h in a H2/N2 flow (5 vol.% H2, with a flow rate of 100 mLmin-1) to further reduce any cationic species to their metallic form.45

2.3 M/TiO2 photo-catalyst characterization TEM studies were conducted using a JEOL 2012F TEM operating at 200 kV. Powders samples were dispersed in ethanol and deposited on holey carbon coated copper grids. XRD patterns were acquired using a Philips PW-1130 diffractometer. Data were taken from 2θ = 2-100° (0.02°, 2° min-1) using Cu Kα X-rays (λ = 1.5418 Å). UV-Vis absorbance spectra were collected with a Shimadzu UV-2600PC spectrophotometer fitted with a Shimadzu ISR-260 integrating sphere. The base line reference was obtained using BaSO4. XPS data were acquired at the soft X-ray beamline of the Australian Synchrotron, equipped with a hemispherical electron energy analyzer and an analysis chamber of base pressure ~1×10-10 Torr. Spectra were excited at a photon energy of 1486.7 eV, and calibrated against the C 1s signal of adventitious hydrocarbons at 285.0 eV. M/TiO2 powders were pressed into thin pellets (~0.1 mm thickness) for the analyses. A pass energy of 40 eV was used for the survey scans; binding energy range 1200-0 eV. Narrow scans were collected with a pass energy of 20 eV. N2 physisorption isotherms were acquired using the Micromeritics Tristar 3000 instrument. Pore volumes and diameters were calculated from the adsorption isotherms by the Barrett-Joyner-Halenda (BJH) method. All samples were degassed at 100 oC in vacuum for 1 h prior to measurements. ACS Paragon Plus Environment


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Photoluminescence data were collected on a Perkin–Elmer LS-55 Luminescence Spectrometer. The excitation energy used was 310 nm with a 290 nm cut-off filter.

2.4 H2 production tests on the M/TiO2 photocatalysts Hydrogen production experiments were carried out in a 105 mL tubular Pyrex reactor. Each catalyst (0.0065 g) was placed in the reactor then evacuated with a nitrogen flow for 20 min to remove molecular oxygen. Water (20 mL) or alcohol-water mixture (20 mL) was then injected into the reactor. After sonication for 5 min the suspension was then stirred in the dark for 30 min. The reactor was exposed to UV light, supplied from a Spectraline model SB-1000P/F lamp (200 W, 365 nm) at a distance of 8 cm. The photon flux at the sample was measured to be about 6.0 mW cm-2. The reaction temperature was 30-35 oC. Gas head space samples of 1 mL were collected at 20 min intervals for quantitative analysis using a Shimadzu GC 20 14 equipped with a TCD detector and Carboxen-1010 plot capillary column (L × I.D. 30 m × 0.53 mm, average thickness 30 μm). Photocatalytic data are the mean of 3 replicate runs. For the analysis of the other reaction products, the reactor set up used was different. A Pyrex reactor with a volume of 128 mL with photon flux of ~6.0-6.5 mW cm-2 at the reactor. The flux was focused around the reactor using a light funnel. An external fan was used to maintain the reaction temperature at about 35 oC. A similar protocol as described above was employed. Products evolution was examined by taking gas head space samples (1 mL) at 1 hour intervals and injecting these into Agilent GC-TCD equipped with a Porapak Q packed column (3 feet × 1/8 inch in external diameter × 2 m long) at 45 oC and N2 was used as a carrier gas (ca. 20 mL/min). Products observed in this study were eluted in the following order: hydrogen, methane, carbon dioxide, ethylene, ethane, acetaldehyde and ethanol.

3. RESULTS AND DISCUSSION 3.1 Photocatalysts Characterization Figure. 1 shows TEM images for 0.5 and 2 wt. % M/TiO2 (M = Pd, Pt and Au). All photocatalysts exhibited similar topographies under TEM apart from the increase in the number and size of separate metal nanoparticles (appears as small dark spots). Pd/TiO2 and Pt/TiO2 photocatalysts exhibit average particle size of ~2 nm and ~1.5 nm, respectively (Table 1 and Figure S1A). In contrast Au nanoparticles are of average size ~5.5 nm (Table 1 and Figure S1B.) with metal-support contact angles >90o (the total number of metal particles are listed in Table S1). The Pd and Pt nanoparticles seem to scatter on both anatase and rutile TiO2 support particles, however, Au nanoparticles preferentially develop at the interface between two TiO2 crystallites. The average particle size of metal nanoparticle in the present study followed the trend: Pt < Pd < Au, in a good accord to previous studies.25, 46-47 This trend can be understood by the total surface free energy minimization which play a crucial part in size, shape and uniformity control of these metals. The zero valent metal atoms of Pd, Pt and Au nanoparticles are ACS Paragon Plus Environment


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formed during nucleation and growth processes under a reducing atmosphere (H2 reduction and during photoreaction in the case of Pd and Pt, while only calcination in air for Au). Upon heating, these metal atoms attain thermal kinetic energy and become mobile and coalesce with each other to produce small clusters. The resulting metal clusters are thermodynamically unstable and can dissolve before they reach a critical size or overcome a critical free energy barrier and become thermodynamically stable nuclei.48 Free atoms, or unstable small metal clusters, become consumed by these stable nuclei to eventually grow into nanoparticles. The experimental values of the surface free energies () for Pt, Pd and Au metals are: 2.48, 1.88 and 1.50 J m-2, respectively49, following the same trend of their average particle sizes determined here. Large surface free energies require higher minimization of the surface free energy, resulting in difficulty in nucleation.48 In the case of Pt, for example, only small number of clusters can reach a critical size to minimize the excess free energy by partial wetting of TiO2 support phases.36 Hence, Pt will be more favorable to display higher wetting due to its high free surface energy and hence spread through the surface support, evidence for the strong-metal support interaction (contact angle

Metal Particle Size Effects on the Photocatalytic Hydrogen Ion Reduction

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