- Email: [email protected]
Rational removal of stabilizer-ligands from platinum nanoparticles supported on photocatalysts by self-photocatalysis degradation
Catalysis Today 242 (2015) 372–380
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Rational removal of stabilizer-ligands from platinum nanoparticles supported on photocatalysts by self-photocatalysis degradation Zhi Jiang a,b , Wenfeng Shangguan a,b,∗ a b
Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2014 Received in revised form 17 July 2014 Accepted 21 July 2014 Available online 22 August 2014 Keywords: Removal of ligands Pt/TiO2 Photocatalysis Degradation
a b s t r a c t The emergence of precise synthesis supported catalysts based on colloidal method is of fundamental interest for catalysis community. Stabilizer-ligands are necessary to be removed after synthesis process to get “clean” catalyst. Here we reported that a procedure, self-photodegradation in vacuum, could effectively remove the ligands on the surface of photocatalyst oxide without affecting the morphology and electron state of particle and support, while calcination method was identified to influence both of them. We showed how that would result in quite a different performance in photocatalysis hydrogen evolution from aqueous methanol solution. These results thus outlined a rational and green strategy for “collaborating” with photocatalysis process to build up designed supported catalyst architectures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction To get improved understanding of structure–activity relationship via studies that systematically vary specific metal nanoparticles on the surface of metal oxide is of obvious fascination for two catalysis communities: to traditional heterogeneous catalysis chemists, whose objective is to improve the activity of metal nanoparticles (called primary catalysts) dispersed on support oxide, and to photochemists, who wish to maximize the photocatalysis activity under the assistance of metal nanoparticles (called cocatalysts) on semiconductor oxide. In order to do this, it is critical to learn to synthesis well defined and controllable metal on metal oxide. Pre-formed nanoparticles deposition method [1,2] and in situ polyol route [3] based on colloid chemistry was proven particularly suitable for producing tailored support catalyst systems. Exquisite control over metal nanoparticles morphologies in colloid synthesis was achieved by tuning the synthesis parameters, such as the nature and concentration of stabilizer and/or shape directing agent (polymer, surfactant). However, removal of the residue ligands to get clean catalyst remains to be a challenge. Traditional techniques, such as calcination and UV–ozone or VUV–ozone may result in irreversible modification of the nanoparticles or involved
∗ Corresponding author at: Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel.: +86 02134206020. E-mail address: [email protected] (W. Shangguan). http://dx.doi.org/10.1016/j.cattod.2014.07.037 0920-5861/© 2014 Elsevier B.V. All rights reserved.
environmental pollution ozone [1,4–9]. Hot water extraction was facile and efficient, but it was reported worked only for some stabilizing ligands [10]. Semiconductor physics teaches that, in the case of semiconductor oxide, such as TiO2 , ZrO2 , WO3 , as support, absorption of photons will produce electron-hole pairs and reactive oxygen species in the oxide. These species can participate in a host of redox reactions. By taking advantage of this principle, we proposed using photodegradation coupling with oxygen bubbling to removal the residue ligands on semiconductor surface by using UV light >300 nm as light source [3]. However, the precise atomic scale characteristics of its influence on both the nature of deposited metal and semiconductor metal oxide still remain unclear. That is crucial for subsequent catalysis study. Removal protecting ligands may result in change of the deposited colloidal nanoparticles. Haruta et al. found that increasing the calcination temperature to 873 K increased the size of deposited gold colloids from 5 nm to 12 nm and resulted in greater contact between the Au particles and TiO2 support [4]. Removal process may also result in change of support oxide. As support was usually considered either be inert or just partly participate in traditional heterogeneous catalytic reactions, there is little concern with such effect. However, this influence is especially compelling in photocatalysis, as change in semiconductor oxide may affect crucial processes in photocatalysis, i.e. photon absorption and charge form, separation and migration. This perspective thus not only highlights the emerging necessity on investigating the novel photo degradation method in detail, but also suggests that it is necessary to compare its influence in photocatalysis reaction with traditional
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
removal method, such as calcination, as it is often neglected in previous research. In this work, using poly(N-vinylpyrrolidone) (PVP) and tetramethylammonium bromide(TMABr) stabilized Pt supported on TiO2 as model catalyst, we probed how the response of change in both Pt and TiO2 structure after removal ligands by photodegradation (coupled with oxygen bubbling) or calcination (in air) toward photocatalysis hydrogen production activity. Our results demonstrated the influence on the Pt electron state and TiO2 surface state after removal of ligands controls the photocatalysis activity of Pt/TiO2 . We finally proposed to self-photodegradation in vacuum as a more convenient and green way to remove the stabilizing ligands without significantly changing the electron structure of Pt and TiO2 . We anticipate this result will help to develop a rational route to remove stabilizer-ligands especially for photocatalysis reaction.
373
area. In all photodegradation removal experiments, the 0.5 g photocatalyst powders were suspended in 100 ml water solution. For the UV/O2 process, the reaction cell was connected to a bubbling system with O2 flow rate at 100 ml/min during photo irradiation process. For the self-photodegradation process, the reaction cell was evacuated to pressure under 1 torr before photo irradiation. The temperature for all the photo degradation reactions was maintained at 25 ± 5 ◦ C. A 300-W Xenon lamp was used as the light source. The dark blue product after every 3 h irradiation was collected by discarding the colorless supernatant and suspended substance. The products were further washed by water (250 ml, filtration) and dried in vacuum oven in 100 ◦ C for 12 h. Calcination samples (0.5 g) were heated to 200 ◦ C and 400 ◦ C in air respectively at a heating rate of 2.5 ◦ C/min and kept at that temperature for 4 h.
2. Experimental 2.1. Catalyst preparation Ammonium tetrachloroplatinate (II) ((NH4 )2 PtCl4 , 99%; Sigma–Aldrich), poly vinylpyrrolidone (PVP, Mw = 29,000; Sigma–Aldrich), tetramethylammonium bromide (TMABr, 98%; Sinoreagent), ethylene glycol (>99%; Sinoreagent), TiO2 (P25, Degauss), solvents (analytical grade; Sinoreagent) including acetone, ethanol, and hexanes were used as supplied without further purification. Before in situ polyol synthesis, P25 powders were aged by heating to 500 ◦ C at 2.5 ◦ C/min and staying at 500 ◦ C for 4 h. This pretreatment was to minimize the influence of temperature history when photocatalysis degradation samples were compared with calcinations samples. This calcined P25 TiO2 sample was named as P25 500 ◦ C in the following paper. Ligands stabilizing platinum supported on TiO2 (2 wt%) was synthesized by in situ polyol process [3]. In a typical synthesis, 0.9 mmol of tetramethylammonium bromide (TMABr), 1.20 mmol of poly(vinylpyrrolidone) and calculated amounts of TiO2 powders (2 wt% Pt) were dissolved into 30 ml of ethylene glycol in a 100 ml round-bottom flask and ultrasound for 30 min. A total of 0.1 mmol of Pt ions using (NH4 )2 Pt(II)Cl4 as Pt source was then added to the mixture. The mixed solution was heated quickly to 180 ◦ C in oil bath and held at this temperature for 25 min under argon protection and magnetic stirring. After the solution was cooled to room temperature, the solution was centrifuged at 3000 rpm for 5 min. The dark blue product was then collected by discarding the colorless supernatant. The products were further washed two times by precipitation/dissolution (re-dispersed in ethanol and then precipitated by adding hexanes) and dried at 80 ◦ C for 6 h. The final products were named as “as-synthesis Pt/TiO2 ” in the following paper. Another group of ligands coating TiO2 without platinum loading was synthesized by the same in situ polyol process. This ligands coating TiO2 was named as “as-synthesis ligands/TiO2 ” in the following paper. Pt/TiO2 was also prepared by photodeposition method to be showed as reference samples for FTIR. Photodeposition samples were prepared in top-irradiation Pyrex reaction cell. 0.5 g P25 (500 ◦ C) powders were suspended in 100 ml water solution. Calculated amount of Pt ions using (NH4 )2 Pt(II)Cl4 as Pt source was added to the solution, mixed under ultrasound for 30 min, subsequently irradiated under UV light for 6 h and dried in 100 ◦ C for 12 h. This sample was named as Pt/TiO2 PD. 2.2. Ligands removal Photocatalysis degradation samples were prepared in topirradiation Pyrex reaction cell with 28.26 cm2 effective irradiation
2.3. Measurement of photocatalysis hydrogen evolution from aqueous methanol solution Top-irradiation Pyrex reaction cell with 28.26 cm2 effective irradiation area was used in photocatalytic hydrogen evolution. The reaction cell was connected to a vacuum system for vacuuming before photocatalysis reaction. After photocatalysis hydrogen evolution reaction from aqueous methanol solution under lamp, the evolved hydrogen was analyzed by a thermal conductivity detector (TCD) gas chromatograph (China; GC-9200, MS-5A zeolite column, argon as the carrier gas). In all experiments, the 0.1 g photocatalyst powder was suspended in water-methanol solution (water 45 ml, methanol 20 ml), in which methanol played a role as the sacrificial agent. The temperature for all the photocatalytic reactions was maintained at 25 ± 5 ◦ C. A 300-W Xenon lamp was used as the light source.
2.4. Characterization methods Thermogravimetric (TG) and differential scanning calorimeter (DSC) analysis were carried out using STA449F3 (NETZSCH, Germany). IR spectra were measured on a FTIR spectrometer (A Nicolet 6700 from Thermo Nicolet Corp., Madison, WI). Elemental analyses by inductively coupled plasma atomic emission spectroscopy (ICP-AES) were conducted at iCAP 6000 Radial (Thermo). Morphology of the Pt/TiO2 samples was analyzed using TEM (JEOL 2100HT). X-ray diffraction patterns were measured on a Rigaku D/Max-2200/PC X-ray diffractometer. The UV–vis diffuse reflection spectra (DRS) were determined by a UV–vis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the Kulbelka–Munk method. The surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis UltraDLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C 1s = 284.8 eV) as a reference. Pt L3-edge absorption spectra (XAFS) were performed on the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics (SINAP), China, operated at 3.5 GeV with injection currents of 140–210 mA. Si(1 1 1) double-crystal monochromator was used. The harmonic rejection mirror was used to reduce high order harmonic component of the monochrome beam. Pt foil was used as reference sample and measured in the transmission mode, and all other samples were measured in mode by Lytle detector. We used IFEFFIT software to calibrate the energy scale, to correct the background signal and to normalize the intensity on analyzing the XAFS data at the Pt L3 edge.
374
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
3.2. The effectiveness of different ligands removal methods: FTIR and ICP analysis
Fig. 1. TG and DSC curves of as-synthesis ligands stabilizing Pt/TiO2 samples (ramping at 2.5 ◦ C/min under 20 ml/min flowing air): thermal decomposition of ligands occurs in the 200–400 ◦ C.
3. Results and discussion 3.1. Simultaneously TG/DSC characterization of the as-synthesis Pt/TiO2 in air The thermal degradation behavior of PVP or PVP/protected noble metal, such as Pt or Rh has been extensively studied [6,11,12]. However, as new synthesis route, the in situ polyol process introduced PVP and TMABr together with solution residues on the surface of Pt and TiO2 . Thus there was very few information about the status of the stabilizing ligands when the as-synthesis Pt/TiO2 was subjected to calcination treatment. Fig. 1 showed the TG/DSC curves to estimate the exact temperature for thermal removal of ligands on the as-synthesis Pt/TiO2 samples. There were three main mass loss stages for the as-synthesis Pt/TiO2 in air:
• About 4% of mass was lost during the first stage in the temperature range of 80 ◦ C to 125 ◦ C. This mass loss stage was accompanied with one small endothermic peak at 100 ◦ C, which could be ascribed to the loss of water and violate solutions. • At the second stage, about 4% of mass was lost in the temperature range of 125 ◦ C to 400 ◦ C. The second mass loss stage was corresponding to three exothermic events with peak at 245.2 ◦ C, 288.9 ◦ C and 364.3 ◦ C respectively as shown in the DSC curves. This stage was the main stage for the thermal degradation of ligands on Pt/TiO2. • At the third stage, only about 0.2% of volatile fractions were lost in the temperature range from 400 ◦ C to 800 ◦ C. There was also one exothermic peak at 724.5 ◦ C. However it did not overlap the tiny mass loss at the third stage as shown in TG curves. Thus this exothermic peak could only be ascribed to the TiO2 phase change of anatase to rutile [13].
As the change of the TiO2 phase and state of specific Pt nanocrystals (NCs) was inevitable with the increase of temperature, the calcination temperature was chosen at 400 ◦ C to get a good tradeoff between the removals of most fraction of ligands on the surface while keeping the state of Pt/TiO2 almost unaffected with the calcination process.
The covering of stabilizing ligands on the metal NCs and the support were harmful for most catalysis application and catalysis mechanism study as they may block access to light and immigration of charges to the reactants (photocatalysis application) or adsorption of chemicals on active sites (gas–solid heterogeneous catalysis). Thus removal of the capping ligands was important for the catalysis applications of colloidal metal NCs once the metal NCs were immobilized on support oxide. In our previous study, we proposed to remove the ligands on the surface of Pt/TiO2 by using UV(>300 nm) coupling with O2 bubbling. This UV(>300 nm)/O2 method was different from the traditional UV/O3 method (UV <257 nm) [5] as the latter is primarily the result of photosensitized oxidation, a process in which the short wavelength UV light and atomic oxygen and/or ozone directly attacked the contaminant molecule to dissociate the contaminate molecules. UV(>300 nm)/O2 , on contrast, mainly took advantage of the oxidative radicals originated from the excitation of semiconductor in water by UV light. Strong oxidative radicals, such as superoxide anions (O2 − ) or hydroxyl radicals (• OH) were produced respectively when the oxygen encountering with electrons or the holes encountering with water [14,15]. The interaction between the contaminate molecules with the O2 − and • OH will lead to the removal of the stabilizing ligands to form simpler volatile molecules [14,15]. Inspired by the fact that the hole was strong oxidizing agent itself [14,15], we further proposed in this paper that just the holes generated when TiO2 was excited by UV light(>300 nm) may be capable of directly oxidize the stabilizing molecules covered on the surface of Pt NCs and TiO2 . As the whole cleaning process was operated in vacuum and no oxygen aid was applied, this method was named as “self photodegradation method” or “UV/vacuum method”. If UV/vacuum was capable of removing ligands, this route should be more facile (no bubbling system involved) and mild (no superoxide anions involved) as compared with the UV/O2 or calcination method. The convenience and mildness were crucial for ligands removal as the former was important for catalysis application while the latter helped to preserve the state of supported NCs to the maximum extent. The cleaning effects of different treatment methods were followed and compared by IR spectroscopy. The FTIR bands of the as-synthesis Pt/TiO2 or ligands/P25 were a typical mixture of PVP and TMABr. The detailed IR bands can be found elsewhere [6,16]. The FT-IR spectra of Pt/TiO2 (PD) and P25 were also shown in Fig. 2.1 and 2.2 as reference. The evolution of IR bands in the treatment process was complex but not the key interesting of this paper. Three important points were noted here. First, all three methods were effective on the removal of the ligands. The significantly decrease in the IR intensity or even completely disappear of the PVP and TMABr vibration peaks (1000–2000 cm−1 ) with the increase of irradiation time (Fig. 2.1, from 3 h to 6 h) or calcination temperature (Fig. 2.2, from 200 ◦ C to 400 ◦ C) were direct evidences indicating a gradual removal of PVP and TMABr in the as-synthesis Pt/TiO2 or ligands/TiO2 . The second point was that the removal effectiveness of photodegradation method was comparable to the calcination method. The quickly removal of the IR band related to PVP and TMABr at the range of 1000–2000 cm−1 for the ligands protecting Pt/P25 or P25 irradiated in O2 for the first three hours was the direct evidence to prove the effectiveness of photodegradation. The third point should be clarified was that 400 ◦ C was rational temperature point to remove the ligands in the calcination method. The significant IR bands related to the residual ligands or pyrolysis products in the Pt/TiO2 or TiO2 calcined at 200 ◦ C (Fig. 2.2) and the change of the color of ligands/TiO2 from white to yellow when temperature increased to 200 ◦ C (Fig. 2.3) was consistent with the
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
375
Fig. 2. (2.1) FT-IR spectroscopy analysis of the as synthesis ligands/Pt/TiO2 catalysts, the calcination series (200 ◦ C, 400 ◦ C), the photodegradation series (O2 and vacuum) and certain reference material. (2.2) FT-IR spectroscopy analysis of the as synthesis ligands/TiO2 catalysts, the calcination series (200 ◦ C, 400 ◦ C), the photodegradation series (O2 and vacuum) and certain reference material. (2.3) Sample photos of as synthesis ligands/TiO2 and the calcination series: calcined at 200 ◦ C, calcined at 400 ◦ C.
TG/DSC results, which indicating that 200 ◦ C was not high enough to remove the ligands. Furthermore, the quick diminish of the ligands IR bands for the Pt/TiO2 or TiO2 (O2 , 3 h) (Fig. 2.1 and 2.2) and the white color of the samples after photodegradation treatment (not shown here) suggested that the formed degradation products with lower molecular weight could be quickly removed from the surface of Pt/TiO2 as the powders were suspended in solution during photodegradation process. While for the calcination method, crosslinking or ring open reaction may left non-volatile products, such as amorphous carbon or other products on the surface of Pt/TiO2 , which was suggested by the FT-IR results for Pt/TiO2 or ligands/TiO2 (200 ◦ C). Similar UV–Raman spectra results were also reported for PVP/Pt oxidation by O2 in 350 ◦ C [16]. The possible loss of active components in the removal process was checked by ICP. ICP results showed that almost no platinum was lost for all treated samples, shown in Fig. 2.1, as compared with the as-synthesis of Pt/TiO2 . Thus, FTIR and ICP results suggested that the self-photocatalysis degradation method was a promising facile method to remove the remaining capping agent on the surface compared with other methods. Then new questions arise. How about the influence of the removal process on the status of the supported NCs and support? Will such influence lead to different photocatalysis activity for hydrogen evolution from aqueous methanol solution? 3.3. XRD and UV–vis characterization of Pt/TiO2 under different removal procedure Removal of the stabilizing ligands could lead to the change of supported NCs and the support itself. This is one key issue that could emerge after the ligands removal treatment. Figs. 3 and 4 showed the powder X-ray diffraction (XRD) patterns and the UV–vis absorption spectra of Pt/TiO2 after different ligands removal procedures. As the UV–vis absorption spectra for the Pt/TiO2 treated under irradiation in oxygen almost coincide with the samples irradiated in vacuum, only the UV–vis curve for the former Pt/TiO2 (irradiation in oxygen) was showed in Fig. 4. As shown in Fig. 3, all samples exhibited distinctive diffractions which could be assigned to the mixture of anatase and rutile
Fig. 3. XRD patterns of Pt/TiO2 treated under different ligands removal procedures. The labeled was based on standard patterns for anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276).
crystallites based on standard patterns for anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276). The intensity ratio of anatase and rutile, which could reflect the ratio of anatase and rutile phase in the P25 support, almost kept unchanged for samples under different after treatment conditions. Metallic platinum peaks (JCPDS 652868) with 2 values of 39.8◦ , 46.2◦ and 67.4◦ were not observed in XRD pattern for all samples. The lack of Pt diffraction peaks indicated that the Pt NCs were highly dispersed on the surface of TiO2 . Thus it was difficult to generate observable XRD peaks. The XRD results suggested that the removal procedure, even for the calcination treatment, did not cause serve agglomeration of Pt NCs on the TiO2 surface. The UV–vis spectra showed that the absorption edges of Pt/TiO2 after removal of the ligands only shifted very slightly as compared with the as-synthesis Pt/TiO2 (Fig. 4). This was consistent
376
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
shown in Fig. 6, the H2 evolution amount for the three samples was stable since the first 1 h and during the 7 h test time. H2 evolution amount for the Pt/TiO2 (UV–vacuum) was 35% and 90% higher than that of Pt/TiO2 (UV–oxygen) and Pt/TiO2 (calcination 400 ◦ C), which may indicate the influence of the different treatment methods on the following three key process in photocatalysis process, including [17]: (i) absorption of photons by the semiconductor particles (ii) charge separation and migration in the semiconductor particles (iii) the surface chemical reactions to generate H2 . However, the characterization analysis based on ICP, XRD, UV–vis, TEM already showed that the crystal phase of TiO2 and distribution of Pt were not significantly influenced by the different treatment method. Then the question arises that what factor contributes to the significant difference catalysis activity as shown in Fig. 6. 3.6. XPS spectra of Pt/TiO2 under different removal procedure
Fig. 4. UV–vis DRS spectroscopy analysis of Pt/TiO2 treated under different ligands removal procedures.
with the XRD results that the procedure to remove the ligands did not change the crystal phase of TiO2 even to a lesser extent. 3.4. TEM characterization of Pt/TiO2 under different removal procedure Transmission electron microscopy (TEM) characterization was performed on the Pt/TiO2 samples under different ligands removal procedures and at different stages of treatment in order to observe the possible changes in the morphology or the occurrence of possible aggregation of Pt/TiO2 . Fig. 5a–c showed the TEM images of the freshly prepared Pt/TiO2 , after 200 ◦ C and 400 ◦ C calcination treatment, respectively. There were significantly ligands residues left on the freshly prepared Pt/TiO2 and 200 ◦ C treatment samples, which was auxiliary evidence suggesting the calcination treatment at 200 ◦ C was not high enough to remove the ligands on the TiO2 surface. The ligands on the surface of Pt/TiO2 calcined at 400 ◦ C decreased significantly, suggesting a substantial reduction of the amount of capping agent, which was consistent with the DSC/TG and FTIR results. However the higher temperature (400 ◦ C) resulted in the denser of the TiO2 particles due to the shrink of bulkier PVP on TiO2 surface during the heating process. While the photodegradation method resulted in better dispersion of the Pt/TiO2 nanoparticles compared with the fresh prepared Pt/TiO2 . This suggested that the photodegradation method was mild after treatment method to conserve the shape and size of the Pt/TiO2 particles to the maximum extent. Furthermore, TEM analysis on the Pt/TiO2 exposure to different treatment conditions revealed little changes in the average size distribution of the Pt NCs compared with the assynthesis Pt/TiO2 , which may be due to the high dispersion of Pt NCs on the TiO2 support. Based on TEM and XRD results, it seemed likely that the highly dispersed platinum particles on the surface of TiO2 were not significantly influenced by the different after treatment methods. Since the aim of the removal treatment was to eliminate stabilizer ligands while keeping the morphology of the active phase intact, the photodegradation methods were seemed to be most applicable for further study. 3.5. Photocatalytic activity for hydrogen evolution The photocatalytic activities for hydrogen evolution from water using methanol as holes sacrifice are summarized in Fig. 6. As
To further understand the influence of ligands removal treatment on the surface chemistry properties of Pt/TiO2 , an XPS investigation was conducted to study the local chemical environments and oxidation states of Pt/TiO2 before and after different treatment methods and to probe the interaction of the capping ligands with Pt/TiO2 . The C 1s line of alkyl carbon in PVP molecules at 284.8 eV is used as the reference line. As shown in Fig. 7.1a, the C 1s spectrum of the Pt/TiO2 , contained at least three components at binding energies of 284.8 eV (C C), 286.5 eV (COC O), and 288.6 eV (O C O) [18]. The intensity of the C 1s peak decreased under the calcination or photo irradiation treatment, suggesting the gradual removal of capping agent, which was consistent with the FTIR results. The C 1s spectrum of the Pt/TiO2 treated under calcination showed one additional shoulder at a binding energy of 281.9 eV. The intensity of this peak increased with the increase of calcination temperature, while it was not observed for the as-synthesis Pt/TiO2 or treated under photo irradiation. This peak could be ascribed to Ti C bond [19], however, accidental hydrocarbon may also contribute to this shoulder peak. Ti 2p spectrum provided additional support for the formation of Ti C bond in the calcined samples. As shown in Fig. 7.2b, all samples showed a characteristic peak of TiO2 at binding energy of 458.4 eV [20]. This indicated that the TiO2 , although treated differently, was almost in the same chemical environment as the as-synthesis Pt/TiO2 . However, one addition shoulder peak appeared at 455.6 eV for the calcined Pt/TiO2 sample, and the intensity of that peak increased with the calcination temperature from 200 ◦ C to 400 ◦ C, which also indicated the possible formation of Ti C bond. The Ti 2p spectra collaborated with the C 1s results thus strongly suggested the formation of Ti C bonds between the residual nonvolatile graphitic even amorphous carbon formed during the process of thermal degradation of ligands material and the titanium of the TiO2 . The lack of the shift of absorbance edge of TiO2 to the higher wavelength region for calcined Pt/TiO2 compared with as synthesis Pt/TiO2 (Fig. 4) may be due to the absence of Ti C bond in bulk TiO2 . Pt 4f spectrum was showed in Fig. 7.3. It should be noted that a wide range of binding energy was reported in Refs. [21]. Thus to distinguish the difference of Pt oxide is challenge. We have chosen the BE of as-synthesis Pt/TiO2 as Pt0 and refer to the highest BE species detected in the calcination (400 ◦ C) samples as PtO2 , which will be further supported by XAFS results. After irradiated 6 h in vacuum, no PtO or PtO2 species were detected on the Pt/TiO2 while small amount of PtOx appeared on the Pt/TiO2 (UV/oxygen) as indicated by the shoulder peak appeared after 74 eV. The calcination process leaded to a gradual oxidation of Pt NCs with the increase of the calcination temperature. Calcination at 400 ◦ C has resulted in almost completely oxidation of Pt NCs. It is quite possible that oxygen atoms formed during O2 spillover on the platinum surface can
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
377
Fig. 5. (5.1) Representative TEM images of the Pt/TiO2 at magnification of 50 K: as-synthesis (a) calcined at 200 ◦ C (b) calcined at 400 ◦ C (c) photo irradiated for 3 h in oxygen (d) photo irradiated for 6 h in oxygen (e) photo irradiated for 6 h in vacuum (f). (5.2) Representative TEM images of the Pt/TiO2 at magnification of 100 K: as-synthesis (a) calcined at 200 ◦ C (b) calcined at 400 ◦ C (c) photo irradiated for 3 h in oxygen (d) photo irradiated for 6 h in oxygen (e) photo irradiated for 6 h in vacuum (f). (5.3) Representative TEM images of the Pt/TiO2 at magnification of 50 K: as synthesis (a) calcined at 200 ◦ C (b) showing the significant ligands residue.
378
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
photodegradation method could reserve the state of Pt and TiO2 to the maximum extent. Such modification in the state of the Pt and TiO2 will substantially influence the light harvest and charge separation and migration in the semiconductor particles during the photocatalysis process as shown in Fig. 6. 3.7. X-ray absorption spectra of Pt/TiO2 under different removal procedure
Fig. 6. Photocatalysis hydrogen evolution amount from water/methanol for Pt/TiO2 calcined at 400 ◦ C, photo irradiated with oxygen bubbling for 6 h, photo irradiated in vacuum for 6 h.
speed the oxidation the Pt NCs itself. The presence of residual ethylene glycol or ascorbic acid oxidation products may also influence the oxidation of Pt NCs in the calcination process. Based on the XPS results, one important point could be noted here. Both the surface chemistry and electron state of Pt NCs and TiO2 was influenced by the calcination method while
X-ray absorption fine structure (XAFS) study for Pt NCs supported on TiO2 was further carried out to elucidate the relationship between the state of platinum and the photocatalysis activity. The samples investigated included the Pt/TiO2 calcined in 400 ◦ C, photo irradiated in vacuum and in oxygen before and after photocatalysis hydrogen evolution reaction from aqueous methanol solution to provide additional information about the dispersion, particle size, oxidation state, and coordination situation of platinum supported on TiO2 . Fig. 8.1 showed the normalized X-ray absorption nearedge structure (XANES) spectra of as-synthesized Pt/TiO2 and the reference spectra of Pt foil and PtO2 . The while line intensities in the spectra reflected the oxidation state of Pt in different samples [22]. The white line intensity of the as-synthesis Pt/TiO2 is clearly lower than that of all other Pt/TiO2 samples, and is similar to that of the Pt foil, which indicates the dominance of Pt0 NCs in the samples. The Pt/TiO2 calcined in 400 ◦ C before photocatalysis hydrogen evolution reaction exhibited a sharpest peak between PtO2 and Pt foil, indicating it was is highly positive charge. The XPS results already indicated the creation of PtOx , mainly in the state of PtO2 , in the samples calcined in 400 ◦ C while the as-synthesis
Fig. 7. (7.1) C 1s photoelectron emission spectra of Pt/TiO2 : as synthesis, the calcination series (200 ◦ C, 400 ◦ C), photodegradation series (vacuum, oxygen). (7.2) Ti2p photoelectron emission spectra of Pt/TiO2 : as synthesis, the calcination series (200 ◦ C, 400 ◦ C), photodegradation series (vacuum, oxygen). (7.3) Pt 4f photoelectron emission spectra of Pt/TiO2 : as synthesis, the calcination series (200 ◦ C, 400 ◦ C), photodegradation series (vacuum, oxygen).
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
379
Fig. 8. (8.1) The normalized XANES spectra at the Pt L3 edge of Pt/TiO2 before and after photocatalysis hydrogen evolution reaction from aqueous methanol solution: the calcination series (400 ◦ C), the photodegradation series (6 h in vacuum, 6 h in oxygen). (8.2) The radial distribution functions (RDF) of Pt/TiO2 after removal ligands and before photocatalysis hydrogen evolution reaction from aqueous methanol solution: the calcination series(400 ◦ C), the photodegradation series (6 h in vacuum, 6 h in oxygen). (8.3) The radial distribution functions (RDF) of Pt/TiO2 after photocatalysis hydrogen evolution reaction from aqueous methanol solution: the calcination series (400 ◦ C), the photodegradation series (6 h in vacuum, 6 h in oxygen).
samples was mainly in metallic state, in good agreement with the XANES data. It also need to mention that the white line intensity of all samples has decreased after photocatalysis water decomposition reaction accompanied by the shift forward of the white line peak as compared with the samples before reaction suggesting negative charged effect in the photocatalysis water decomposition reactions. This phenomenon is more prominent in the samples calcined in 400 ◦ C as compared with the treatment such as irradiation in oxygen or vacuum. In the Fourier transforms (r space, Fig. 8.2) of the EXAFS data, for all samples except Pt/TiO2 calcined in 400 ◦ C, there was one prominent peak at 2.42 A˚ (no phase correction) from the Pt–Pt contribution and a very weak peak at about 1.5 A˚ (no phase correction) from the Pt–O contributions. In the Fourier transforms (r space, Fig. 8.3) of the EXAFS data, all samples after photocatalysis water decomposition showed increase in the Pt–Pt contribution and decrease in the Pt–O contribution as compared with the specific samples before reaction. As shown in Fig. 8.3, the coordination state of the Pt in the Pt/TiO2 (UV/vacuum) after photocatalysis water splitting reaction showed a Pt–Pt contribution most similar to the as-synthesis Pt/TiO2. This suggested that the self-photodegradation in vacuum could remove the capping agent while keep the state of supported Pt NCs and TiO2 almost intact. That was the key aim in removal capping ligands. Thus self-photodegradation method was proved as a promising alternative technique to the traditional calcination method or the UV/O2 method proposed in our previous study [3].
The above results could also explain the different treatment methods on the final photocatalysis water decomposition activity as shown in Fig. 7. Photoexcited TiO2 undergoes charge equilibration when they are in contact with Pt NCs. The charge equilibration depending on the nature of the metallic nanoparticles has direct influence in dictating the energetic of the composite. Higher metallic state favor the formation of Schottky barrier between Pt and TiO2 to trap electron immigrated to the surface [17]. Thus the final products treated using self-photodegradation method have the highest Pt metallic state corresponding to the highest photocatalysis activity. The formation of amorphous carbon and Ti C bond in the TiO2 surface supported by IR and XPS results may also hurt the photocatalysis activity by hindering the light harvest and block the charges immigration to the TiO2 surface. Thus the Pt/TiO2 samples (calcined in 400 ◦ C) with highest ratio of PtO2 and strongest interaction with the carbon residue in the surface demonstrated the lowest hydrogen production activity. 4. Conclusion As a proof of concept, we demonstrated that the selfphotodegradation method is an available strategy to remove the ligands on the surface of Pt/TiO2 while keeping the engineered controlled state or morphological characteristics to the maximum extent as compared with the as-synthesis Pt/TiO2 . This treatment method can lead to improved performance of cocatalystsemiconductor heterogeneous photocatalyst for photocatalysis
380
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
water decomposition activity as it has highest metallic state of Pt. This study also demonstrate that the traditional calcination method was identified to influence both the state of Pt and TiO2 , leading to lowest photocatalysis water decomposing activity as the coupling effect of the highest ratio of positive charged Pt and Ti C bond coexist in the system. Acknowledgements This work was supported by the financial support from Shanghai Natural Science Foundation (14ZR1421900), National Science Foundation of China (50906050), the National High Technology Research and Development Program of China (2012AA051501). References [1] J.-D. Grunwaldt, et al., Preparation of supported gold catalysts for lowtemperature CO oxidation via size-controlled gold colloids, J. Catal. 181 (2) (1999) 223–232. [2] (a) H. Einaga, et al., Generation of active sites for CO photooxidation on TiO2 by platinum deposition, J. Phys. Chem. B 107 (35) (2003) 9290–9297; (b) M. Miyake, K. Miyabayashi, Shape and size controlled Pt nanocrystals as novel model catalysts, Catal. Surv. Asia 16 (1) (2012) 1–13. [3] Z. Jiang, et al., In situ controllable synthesis platinum nanocrystals on TiO2 by novel polyol-process combined with light induced photocatalysis oxidation, Chem. Commun. 48 (77) (2012) 9598–9600. [4] S. Tsubota, et al., Effect of calcination temperature on the catalytic activity of Au colloids mechanically mixed with TiO2 powder for CO oxidation, Catal. Lett. 56 (2–3) (1998) 131–135. [5] L.D. Menard, et al., Preparation of TiO2 -supported Au nanoparticle catalysts from a Au13 cluster precursor: ligand removal using ozone exposure versus a rapid thermal treatment, J. Catal. 243 (1) (2006) 64–73. [6] M. Loría-Bastarrachea, et al., A TG/FTIR study on the thermal degradation of poly (vinyl pyrrolidone), J. Therm. Anal. Calorim. 104 (2) (2011) 737–742. [7] M. Crespo-Quesada, et al., UV–ozone cleaning of supported poly (vinylpyrrolidone)-stabilized palladium nanocubes: effect of stabilizer removal on morphology and catalytic behavior, Langmuir 27 (12) (2011) 7909–7916.
[8] M. Comotti, et al., Support effect in high activity gold catalysts for CO oxidation, J. Am. Chem. Soc. 128 (3) (2006) 917–924. [9] S. Pang, et al., Decomposition of monolayer coverage on gold nanoparticles by UV/ozone treatment, Chem. Lett. 34 (4) (2005) 544–545. [10] J.A. Lopez-Sanchez, et al., Facile removal of stabilizer-ligands from supported gold nanoparticles, Nat. Chem. 3 (7) (2011) 551–556. [11] Y. Borodko, et al., Spectroscopic study of the thermal degradation of PVPcapped Rh and Pt nanoparticles in H2 and O2 environments, J. Phys. Chem. C 114 (2) (2009) 1117–1126. [12] C. Peniche, et al., Study of the thermal degradation of poly (N-vinyl-2pyrrolidone) by thermogravimetry–FTIR, J. Appl. Polym. Sci. 50 (3) (1993) 485–493. [13] K.J.A. Raj, B. Viswanathan, Effect of surface area, pore volume and particle size of P25 titania on the phase transformation of anatase to rutile, Indian J. Chem. A 48 (2009) 1378. [14] R. Andreozzi, et al., Advanced oxidation processes (AOP) for water purification and recovery, Catal. Today 53 (1) (1999) 51–59. [15] (a) W. Choi, Pure and modified TiO2 photocatalysts and their environmental applications, Catal. Surv. Asia 10 (1) (2006) 16–28; (b) J. Zhang, Y. Nosaka, Mechanism of the OH radical generation in photocatalysis with TiO2 of different crystalline types, J. Phys. Chem. C 118 (20) (2014) 10824–10832. [16] Y. Borodko, et al., Probing the interaction of poly (vinylpyrrolidone) with platinum nanocrystals by UV–Raman and FTIR, J. Phys. Chem. B 110 (46) (2006) 23052–23059. [17] A. Kudo, Photocatalyst materials for water splitting, Catal. Surv. Asia 7 (1) (2003) 31–38. [18] D. Briggs, G. Beamson, Primary and secondary oxygen-induced C1s binding energy shifts in X-ray photoelectron spectroscopy of polymers, Anal. Chem. 64 (15) (1992) 1729–1736. [19] H. Irie, Y. Watanabe, K. Hashimoto, Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst, Chem. Lett. 8 (2003) 772–773. [20] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corporation, Eden Prairie, MN, 1992. [21] L.K. Ono, et al., Formation and thermal stability of platinum oxides on sizeselected platinum nanoparticles: support effects, J. Phys. Chem. C 114 (50) (2010) 22119–22133. [22] (a) Z. Jiang, et al., Influence of support and metal precursor on the state and CO catalytic oxidation activity of platinum supported on TiO2 , J. Phys. Chem. C 116 (36) (2012) 19396–19404; (b) N. Ichikuni, Y. Iwasawa, In situ d electron density of Pt particles on supports by XANES, Catalysis letters 20 (1–2) (1993) 87–95.
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Rational removal of stabilizer-ligands from platinum nanoparticles supported on photocatalysts by self-photocatalysis degradation Zhi Jiang a,b , Wenfeng Shangguan a,b,∗ a b
Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2014 Received in revised form 17 July 2014 Accepted 21 July 2014 Available online 22 August 2014 Keywords: Removal of ligands Pt/TiO2 Photocatalysis Degradation
a b s t r a c t The emergence of precise synthesis supported catalysts based on colloidal method is of fundamental interest for catalysis community. Stabilizer-ligands are necessary to be removed after synthesis process to get “clean” catalyst. Here we reported that a procedure, self-photodegradation in vacuum, could effectively remove the ligands on the surface of photocatalyst oxide without affecting the morphology and electron state of particle and support, while calcination method was identified to influence both of them. We showed how that would result in quite a different performance in photocatalysis hydrogen evolution from aqueous methanol solution. These results thus outlined a rational and green strategy for “collaborating” with photocatalysis process to build up designed supported catalyst architectures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction To get improved understanding of structure–activity relationship via studies that systematically vary specific metal nanoparticles on the surface of metal oxide is of obvious fascination for two catalysis communities: to traditional heterogeneous catalysis chemists, whose objective is to improve the activity of metal nanoparticles (called primary catalysts) dispersed on support oxide, and to photochemists, who wish to maximize the photocatalysis activity under the assistance of metal nanoparticles (called cocatalysts) on semiconductor oxide. In order to do this, it is critical to learn to synthesis well defined and controllable metal on metal oxide. Pre-formed nanoparticles deposition method [1,2] and in situ polyol route [3] based on colloid chemistry was proven particularly suitable for producing tailored support catalyst systems. Exquisite control over metal nanoparticles morphologies in colloid synthesis was achieved by tuning the synthesis parameters, such as the nature and concentration of stabilizer and/or shape directing agent (polymer, surfactant). However, removal of the residue ligands to get clean catalyst remains to be a challenge. Traditional techniques, such as calcination and UV–ozone or VUV–ozone may result in irreversible modification of the nanoparticles or involved
∗ Corresponding author at: Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel.: +86 02134206020. E-mail address: [email protected] (W. Shangguan). http://dx.doi.org/10.1016/j.cattod.2014.07.037 0920-5861/© 2014 Elsevier B.V. All rights reserved.
environmental pollution ozone [1,4–9]. Hot water extraction was facile and efficient, but it was reported worked only for some stabilizing ligands [10]. Semiconductor physics teaches that, in the case of semiconductor oxide, such as TiO2 , ZrO2 , WO3 , as support, absorption of photons will produce electron-hole pairs and reactive oxygen species in the oxide. These species can participate in a host of redox reactions. By taking advantage of this principle, we proposed using photodegradation coupling with oxygen bubbling to removal the residue ligands on semiconductor surface by using UV light >300 nm as light source [3]. However, the precise atomic scale characteristics of its influence on both the nature of deposited metal and semiconductor metal oxide still remain unclear. That is crucial for subsequent catalysis study. Removal protecting ligands may result in change of the deposited colloidal nanoparticles. Haruta et al. found that increasing the calcination temperature to 873 K increased the size of deposited gold colloids from 5 nm to 12 nm and resulted in greater contact between the Au particles and TiO2 support [4]. Removal process may also result in change of support oxide. As support was usually considered either be inert or just partly participate in traditional heterogeneous catalytic reactions, there is little concern with such effect. However, this influence is especially compelling in photocatalysis, as change in semiconductor oxide may affect crucial processes in photocatalysis, i.e. photon absorption and charge form, separation and migration. This perspective thus not only highlights the emerging necessity on investigating the novel photo degradation method in detail, but also suggests that it is necessary to compare its influence in photocatalysis reaction with traditional
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
removal method, such as calcination, as it is often neglected in previous research. In this work, using poly(N-vinylpyrrolidone) (PVP) and tetramethylammonium bromide(TMABr) stabilized Pt supported on TiO2 as model catalyst, we probed how the response of change in both Pt and TiO2 structure after removal ligands by photodegradation (coupled with oxygen bubbling) or calcination (in air) toward photocatalysis hydrogen production activity. Our results demonstrated the influence on the Pt electron state and TiO2 surface state after removal of ligands controls the photocatalysis activity of Pt/TiO2 . We finally proposed to self-photodegradation in vacuum as a more convenient and green way to remove the stabilizing ligands without significantly changing the electron structure of Pt and TiO2 . We anticipate this result will help to develop a rational route to remove stabilizer-ligands especially for photocatalysis reaction.
373
area. In all photodegradation removal experiments, the 0.5 g photocatalyst powders were suspended in 100 ml water solution. For the UV/O2 process, the reaction cell was connected to a bubbling system with O2 flow rate at 100 ml/min during photo irradiation process. For the self-photodegradation process, the reaction cell was evacuated to pressure under 1 torr before photo irradiation. The temperature for all the photo degradation reactions was maintained at 25 ± 5 ◦ C. A 300-W Xenon lamp was used as the light source. The dark blue product after every 3 h irradiation was collected by discarding the colorless supernatant and suspended substance. The products were further washed by water (250 ml, filtration) and dried in vacuum oven in 100 ◦ C for 12 h. Calcination samples (0.5 g) were heated to 200 ◦ C and 400 ◦ C in air respectively at a heating rate of 2.5 ◦ C/min and kept at that temperature for 4 h.
2. Experimental 2.1. Catalyst preparation Ammonium tetrachloroplatinate (II) ((NH4 )2 PtCl4 , 99%; Sigma–Aldrich), poly vinylpyrrolidone (PVP, Mw = 29,000; Sigma–Aldrich), tetramethylammonium bromide (TMABr, 98%; Sinoreagent), ethylene glycol (>99%; Sinoreagent), TiO2 (P25, Degauss), solvents (analytical grade; Sinoreagent) including acetone, ethanol, and hexanes were used as supplied without further purification. Before in situ polyol synthesis, P25 powders were aged by heating to 500 ◦ C at 2.5 ◦ C/min and staying at 500 ◦ C for 4 h. This pretreatment was to minimize the influence of temperature history when photocatalysis degradation samples were compared with calcinations samples. This calcined P25 TiO2 sample was named as P25 500 ◦ C in the following paper. Ligands stabilizing platinum supported on TiO2 (2 wt%) was synthesized by in situ polyol process [3]. In a typical synthesis, 0.9 mmol of tetramethylammonium bromide (TMABr), 1.20 mmol of poly(vinylpyrrolidone) and calculated amounts of TiO2 powders (2 wt% Pt) were dissolved into 30 ml of ethylene glycol in a 100 ml round-bottom flask and ultrasound for 30 min. A total of 0.1 mmol of Pt ions using (NH4 )2 Pt(II)Cl4 as Pt source was then added to the mixture. The mixed solution was heated quickly to 180 ◦ C in oil bath and held at this temperature for 25 min under argon protection and magnetic stirring. After the solution was cooled to room temperature, the solution was centrifuged at 3000 rpm for 5 min. The dark blue product was then collected by discarding the colorless supernatant. The products were further washed two times by precipitation/dissolution (re-dispersed in ethanol and then precipitated by adding hexanes) and dried at 80 ◦ C for 6 h. The final products were named as “as-synthesis Pt/TiO2 ” in the following paper. Another group of ligands coating TiO2 without platinum loading was synthesized by the same in situ polyol process. This ligands coating TiO2 was named as “as-synthesis ligands/TiO2 ” in the following paper. Pt/TiO2 was also prepared by photodeposition method to be showed as reference samples for FTIR. Photodeposition samples were prepared in top-irradiation Pyrex reaction cell. 0.5 g P25 (500 ◦ C) powders were suspended in 100 ml water solution. Calculated amount of Pt ions using (NH4 )2 Pt(II)Cl4 as Pt source was added to the solution, mixed under ultrasound for 30 min, subsequently irradiated under UV light for 6 h and dried in 100 ◦ C for 12 h. This sample was named as Pt/TiO2 PD. 2.2. Ligands removal Photocatalysis degradation samples were prepared in topirradiation Pyrex reaction cell with 28.26 cm2 effective irradiation
2.3. Measurement of photocatalysis hydrogen evolution from aqueous methanol solution Top-irradiation Pyrex reaction cell with 28.26 cm2 effective irradiation area was used in photocatalytic hydrogen evolution. The reaction cell was connected to a vacuum system for vacuuming before photocatalysis reaction. After photocatalysis hydrogen evolution reaction from aqueous methanol solution under lamp, the evolved hydrogen was analyzed by a thermal conductivity detector (TCD) gas chromatograph (China; GC-9200, MS-5A zeolite column, argon as the carrier gas). In all experiments, the 0.1 g photocatalyst powder was suspended in water-methanol solution (water 45 ml, methanol 20 ml), in which methanol played a role as the sacrificial agent. The temperature for all the photocatalytic reactions was maintained at 25 ± 5 ◦ C. A 300-W Xenon lamp was used as the light source.
2.4. Characterization methods Thermogravimetric (TG) and differential scanning calorimeter (DSC) analysis were carried out using STA449F3 (NETZSCH, Germany). IR spectra were measured on a FTIR spectrometer (A Nicolet 6700 from Thermo Nicolet Corp., Madison, WI). Elemental analyses by inductively coupled plasma atomic emission spectroscopy (ICP-AES) were conducted at iCAP 6000 Radial (Thermo). Morphology of the Pt/TiO2 samples was analyzed using TEM (JEOL 2100HT). X-ray diffraction patterns were measured on a Rigaku D/Max-2200/PC X-ray diffractometer. The UV–vis diffuse reflection spectra (DRS) were determined by a UV–vis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the Kulbelka–Munk method. The surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis UltraDLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C 1s = 284.8 eV) as a reference. Pt L3-edge absorption spectra (XAFS) were performed on the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics (SINAP), China, operated at 3.5 GeV with injection currents of 140–210 mA. Si(1 1 1) double-crystal monochromator was used. The harmonic rejection mirror was used to reduce high order harmonic component of the monochrome beam. Pt foil was used as reference sample and measured in the transmission mode, and all other samples were measured in mode by Lytle detector. We used IFEFFIT software to calibrate the energy scale, to correct the background signal and to normalize the intensity on analyzing the XAFS data at the Pt L3 edge.
374
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
3.2. The effectiveness of different ligands removal methods: FTIR and ICP analysis
Fig. 1. TG and DSC curves of as-synthesis ligands stabilizing Pt/TiO2 samples (ramping at 2.5 ◦ C/min under 20 ml/min flowing air): thermal decomposition of ligands occurs in the 200–400 ◦ C.
3. Results and discussion 3.1. Simultaneously TG/DSC characterization of the as-synthesis Pt/TiO2 in air The thermal degradation behavior of PVP or PVP/protected noble metal, such as Pt or Rh has been extensively studied [6,11,12]. However, as new synthesis route, the in situ polyol process introduced PVP and TMABr together with solution residues on the surface of Pt and TiO2 . Thus there was very few information about the status of the stabilizing ligands when the as-synthesis Pt/TiO2 was subjected to calcination treatment. Fig. 1 showed the TG/DSC curves to estimate the exact temperature for thermal removal of ligands on the as-synthesis Pt/TiO2 samples. There were three main mass loss stages for the as-synthesis Pt/TiO2 in air:
• About 4% of mass was lost during the first stage in the temperature range of 80 ◦ C to 125 ◦ C. This mass loss stage was accompanied with one small endothermic peak at 100 ◦ C, which could be ascribed to the loss of water and violate solutions. • At the second stage, about 4% of mass was lost in the temperature range of 125 ◦ C to 400 ◦ C. The second mass loss stage was corresponding to three exothermic events with peak at 245.2 ◦ C, 288.9 ◦ C and 364.3 ◦ C respectively as shown in the DSC curves. This stage was the main stage for the thermal degradation of ligands on Pt/TiO2. • At the third stage, only about 0.2% of volatile fractions were lost in the temperature range from 400 ◦ C to 800 ◦ C. There was also one exothermic peak at 724.5 ◦ C. However it did not overlap the tiny mass loss at the third stage as shown in TG curves. Thus this exothermic peak could only be ascribed to the TiO2 phase change of anatase to rutile [13].
As the change of the TiO2 phase and state of specific Pt nanocrystals (NCs) was inevitable with the increase of temperature, the calcination temperature was chosen at 400 ◦ C to get a good tradeoff between the removals of most fraction of ligands on the surface while keeping the state of Pt/TiO2 almost unaffected with the calcination process.
The covering of stabilizing ligands on the metal NCs and the support were harmful for most catalysis application and catalysis mechanism study as they may block access to light and immigration of charges to the reactants (photocatalysis application) or adsorption of chemicals on active sites (gas–solid heterogeneous catalysis). Thus removal of the capping ligands was important for the catalysis applications of colloidal metal NCs once the metal NCs were immobilized on support oxide. In our previous study, we proposed to remove the ligands on the surface of Pt/TiO2 by using UV(>300 nm) coupling with O2 bubbling. This UV(>300 nm)/O2 method was different from the traditional UV/O3 method (UV <257 nm) [5] as the latter is primarily the result of photosensitized oxidation, a process in which the short wavelength UV light and atomic oxygen and/or ozone directly attacked the contaminant molecule to dissociate the contaminate molecules. UV(>300 nm)/O2 , on contrast, mainly took advantage of the oxidative radicals originated from the excitation of semiconductor in water by UV light. Strong oxidative radicals, such as superoxide anions (O2 − ) or hydroxyl radicals (• OH) were produced respectively when the oxygen encountering with electrons or the holes encountering with water [14,15]. The interaction between the contaminate molecules with the O2 − and • OH will lead to the removal of the stabilizing ligands to form simpler volatile molecules [14,15]. Inspired by the fact that the hole was strong oxidizing agent itself [14,15], we further proposed in this paper that just the holes generated when TiO2 was excited by UV light(>300 nm) may be capable of directly oxidize the stabilizing molecules covered on the surface of Pt NCs and TiO2 . As the whole cleaning process was operated in vacuum and no oxygen aid was applied, this method was named as “self photodegradation method” or “UV/vacuum method”. If UV/vacuum was capable of removing ligands, this route should be more facile (no bubbling system involved) and mild (no superoxide anions involved) as compared with the UV/O2 or calcination method. The convenience and mildness were crucial for ligands removal as the former was important for catalysis application while the latter helped to preserve the state of supported NCs to the maximum extent. The cleaning effects of different treatment methods were followed and compared by IR spectroscopy. The FTIR bands of the as-synthesis Pt/TiO2 or ligands/P25 were a typical mixture of PVP and TMABr. The detailed IR bands can be found elsewhere [6,16]. The FT-IR spectra of Pt/TiO2 (PD) and P25 were also shown in Fig. 2.1 and 2.2 as reference. The evolution of IR bands in the treatment process was complex but not the key interesting of this paper. Three important points were noted here. First, all three methods were effective on the removal of the ligands. The significantly decrease in the IR intensity or even completely disappear of the PVP and TMABr vibration peaks (1000–2000 cm−1 ) with the increase of irradiation time (Fig. 2.1, from 3 h to 6 h) or calcination temperature (Fig. 2.2, from 200 ◦ C to 400 ◦ C) were direct evidences indicating a gradual removal of PVP and TMABr in the as-synthesis Pt/TiO2 or ligands/TiO2 . The second point was that the removal effectiveness of photodegradation method was comparable to the calcination method. The quickly removal of the IR band related to PVP and TMABr at the range of 1000–2000 cm−1 for the ligands protecting Pt/P25 or P25 irradiated in O2 for the first three hours was the direct evidence to prove the effectiveness of photodegradation. The third point should be clarified was that 400 ◦ C was rational temperature point to remove the ligands in the calcination method. The significant IR bands related to the residual ligands or pyrolysis products in the Pt/TiO2 or TiO2 calcined at 200 ◦ C (Fig. 2.2) and the change of the color of ligands/TiO2 from white to yellow when temperature increased to 200 ◦ C (Fig. 2.3) was consistent with the
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
375
Fig. 2. (2.1) FT-IR spectroscopy analysis of the as synthesis ligands/Pt/TiO2 catalysts, the calcination series (200 ◦ C, 400 ◦ C), the photodegradation series (O2 and vacuum) and certain reference material. (2.2) FT-IR spectroscopy analysis of the as synthesis ligands/TiO2 catalysts, the calcination series (200 ◦ C, 400 ◦ C), the photodegradation series (O2 and vacuum) and certain reference material. (2.3) Sample photos of as synthesis ligands/TiO2 and the calcination series: calcined at 200 ◦ C, calcined at 400 ◦ C.
TG/DSC results, which indicating that 200 ◦ C was not high enough to remove the ligands. Furthermore, the quick diminish of the ligands IR bands for the Pt/TiO2 or TiO2 (O2 , 3 h) (Fig. 2.1 and 2.2) and the white color of the samples after photodegradation treatment (not shown here) suggested that the formed degradation products with lower molecular weight could be quickly removed from the surface of Pt/TiO2 as the powders were suspended in solution during photodegradation process. While for the calcination method, crosslinking or ring open reaction may left non-volatile products, such as amorphous carbon or other products on the surface of Pt/TiO2 , which was suggested by the FT-IR results for Pt/TiO2 or ligands/TiO2 (200 ◦ C). Similar UV–Raman spectra results were also reported for PVP/Pt oxidation by O2 in 350 ◦ C [16]. The possible loss of active components in the removal process was checked by ICP. ICP results showed that almost no platinum was lost for all treated samples, shown in Fig. 2.1, as compared with the as-synthesis of Pt/TiO2 . Thus, FTIR and ICP results suggested that the self-photocatalysis degradation method was a promising facile method to remove the remaining capping agent on the surface compared with other methods. Then new questions arise. How about the influence of the removal process on the status of the supported NCs and support? Will such influence lead to different photocatalysis activity for hydrogen evolution from aqueous methanol solution? 3.3. XRD and UV–vis characterization of Pt/TiO2 under different removal procedure Removal of the stabilizing ligands could lead to the change of supported NCs and the support itself. This is one key issue that could emerge after the ligands removal treatment. Figs. 3 and 4 showed the powder X-ray diffraction (XRD) patterns and the UV–vis absorption spectra of Pt/TiO2 after different ligands removal procedures. As the UV–vis absorption spectra for the Pt/TiO2 treated under irradiation in oxygen almost coincide with the samples irradiated in vacuum, only the UV–vis curve for the former Pt/TiO2 (irradiation in oxygen) was showed in Fig. 4. As shown in Fig. 3, all samples exhibited distinctive diffractions which could be assigned to the mixture of anatase and rutile
Fig. 3. XRD patterns of Pt/TiO2 treated under different ligands removal procedures. The labeled was based on standard patterns for anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276).
crystallites based on standard patterns for anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276). The intensity ratio of anatase and rutile, which could reflect the ratio of anatase and rutile phase in the P25 support, almost kept unchanged for samples under different after treatment conditions. Metallic platinum peaks (JCPDS 652868) with 2 values of 39.8◦ , 46.2◦ and 67.4◦ were not observed in XRD pattern for all samples. The lack of Pt diffraction peaks indicated that the Pt NCs were highly dispersed on the surface of TiO2 . Thus it was difficult to generate observable XRD peaks. The XRD results suggested that the removal procedure, even for the calcination treatment, did not cause serve agglomeration of Pt NCs on the TiO2 surface. The UV–vis spectra showed that the absorption edges of Pt/TiO2 after removal of the ligands only shifted very slightly as compared with the as-synthesis Pt/TiO2 (Fig. 4). This was consistent
376
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
shown in Fig. 6, the H2 evolution amount for the three samples was stable since the first 1 h and during the 7 h test time. H2 evolution amount for the Pt/TiO2 (UV–vacuum) was 35% and 90% higher than that of Pt/TiO2 (UV–oxygen) and Pt/TiO2 (calcination 400 ◦ C), which may indicate the influence of the different treatment methods on the following three key process in photocatalysis process, including [17]: (i) absorption of photons by the semiconductor particles (ii) charge separation and migration in the semiconductor particles (iii) the surface chemical reactions to generate H2 . However, the characterization analysis based on ICP, XRD, UV–vis, TEM already showed that the crystal phase of TiO2 and distribution of Pt were not significantly influenced by the different treatment method. Then the question arises that what factor contributes to the significant difference catalysis activity as shown in Fig. 6. 3.6. XPS spectra of Pt/TiO2 under different removal procedure
Fig. 4. UV–vis DRS spectroscopy analysis of Pt/TiO2 treated under different ligands removal procedures.
with the XRD results that the procedure to remove the ligands did not change the crystal phase of TiO2 even to a lesser extent. 3.4. TEM characterization of Pt/TiO2 under different removal procedure Transmission electron microscopy (TEM) characterization was performed on the Pt/TiO2 samples under different ligands removal procedures and at different stages of treatment in order to observe the possible changes in the morphology or the occurrence of possible aggregation of Pt/TiO2 . Fig. 5a–c showed the TEM images of the freshly prepared Pt/TiO2 , after 200 ◦ C and 400 ◦ C calcination treatment, respectively. There were significantly ligands residues left on the freshly prepared Pt/TiO2 and 200 ◦ C treatment samples, which was auxiliary evidence suggesting the calcination treatment at 200 ◦ C was not high enough to remove the ligands on the TiO2 surface. The ligands on the surface of Pt/TiO2 calcined at 400 ◦ C decreased significantly, suggesting a substantial reduction of the amount of capping agent, which was consistent with the DSC/TG and FTIR results. However the higher temperature (400 ◦ C) resulted in the denser of the TiO2 particles due to the shrink of bulkier PVP on TiO2 surface during the heating process. While the photodegradation method resulted in better dispersion of the Pt/TiO2 nanoparticles compared with the fresh prepared Pt/TiO2 . This suggested that the photodegradation method was mild after treatment method to conserve the shape and size of the Pt/TiO2 particles to the maximum extent. Furthermore, TEM analysis on the Pt/TiO2 exposure to different treatment conditions revealed little changes in the average size distribution of the Pt NCs compared with the assynthesis Pt/TiO2 , which may be due to the high dispersion of Pt NCs on the TiO2 support. Based on TEM and XRD results, it seemed likely that the highly dispersed platinum particles on the surface of TiO2 were not significantly influenced by the different after treatment methods. Since the aim of the removal treatment was to eliminate stabilizer ligands while keeping the morphology of the active phase intact, the photodegradation methods were seemed to be most applicable for further study. 3.5. Photocatalytic activity for hydrogen evolution The photocatalytic activities for hydrogen evolution from water using methanol as holes sacrifice are summarized in Fig. 6. As
To further understand the influence of ligands removal treatment on the surface chemistry properties of Pt/TiO2 , an XPS investigation was conducted to study the local chemical environments and oxidation states of Pt/TiO2 before and after different treatment methods and to probe the interaction of the capping ligands with Pt/TiO2 . The C 1s line of alkyl carbon in PVP molecules at 284.8 eV is used as the reference line. As shown in Fig. 7.1a, the C 1s spectrum of the Pt/TiO2 , contained at least three components at binding energies of 284.8 eV (C C), 286.5 eV (COC O), and 288.6 eV (O C O) [18]. The intensity of the C 1s peak decreased under the calcination or photo irradiation treatment, suggesting the gradual removal of capping agent, which was consistent with the FTIR results. The C 1s spectrum of the Pt/TiO2 treated under calcination showed one additional shoulder at a binding energy of 281.9 eV. The intensity of this peak increased with the increase of calcination temperature, while it was not observed for the as-synthesis Pt/TiO2 or treated under photo irradiation. This peak could be ascribed to Ti C bond [19], however, accidental hydrocarbon may also contribute to this shoulder peak. Ti 2p spectrum provided additional support for the formation of Ti C bond in the calcined samples. As shown in Fig. 7.2b, all samples showed a characteristic peak of TiO2 at binding energy of 458.4 eV [20]. This indicated that the TiO2 , although treated differently, was almost in the same chemical environment as the as-synthesis Pt/TiO2 . However, one addition shoulder peak appeared at 455.6 eV for the calcined Pt/TiO2 sample, and the intensity of that peak increased with the calcination temperature from 200 ◦ C to 400 ◦ C, which also indicated the possible formation of Ti C bond. The Ti 2p spectra collaborated with the C 1s results thus strongly suggested the formation of Ti C bonds between the residual nonvolatile graphitic even amorphous carbon formed during the process of thermal degradation of ligands material and the titanium of the TiO2 . The lack of the shift of absorbance edge of TiO2 to the higher wavelength region for calcined Pt/TiO2 compared with as synthesis Pt/TiO2 (Fig. 4) may be due to the absence of Ti C bond in bulk TiO2 . Pt 4f spectrum was showed in Fig. 7.3. It should be noted that a wide range of binding energy was reported in Refs. [21]. Thus to distinguish the difference of Pt oxide is challenge. We have chosen the BE of as-synthesis Pt/TiO2 as Pt0 and refer to the highest BE species detected in the calcination (400 ◦ C) samples as PtO2 , which will be further supported by XAFS results. After irradiated 6 h in vacuum, no PtO or PtO2 species were detected on the Pt/TiO2 while small amount of PtOx appeared on the Pt/TiO2 (UV/oxygen) as indicated by the shoulder peak appeared after 74 eV. The calcination process leaded to a gradual oxidation of Pt NCs with the increase of the calcination temperature. Calcination at 400 ◦ C has resulted in almost completely oxidation of Pt NCs. It is quite possible that oxygen atoms formed during O2 spillover on the platinum surface can
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
377
Fig. 5. (5.1) Representative TEM images of the Pt/TiO2 at magnification of 50 K: as-synthesis (a) calcined at 200 ◦ C (b) calcined at 400 ◦ C (c) photo irradiated for 3 h in oxygen (d) photo irradiated for 6 h in oxygen (e) photo irradiated for 6 h in vacuum (f). (5.2) Representative TEM images of the Pt/TiO2 at magnification of 100 K: as-synthesis (a) calcined at 200 ◦ C (b) calcined at 400 ◦ C (c) photo irradiated for 3 h in oxygen (d) photo irradiated for 6 h in oxygen (e) photo irradiated for 6 h in vacuum (f). (5.3) Representative TEM images of the Pt/TiO2 at magnification of 50 K: as synthesis (a) calcined at 200 ◦ C (b) showing the significant ligands residue.
378
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
photodegradation method could reserve the state of Pt and TiO2 to the maximum extent. Such modification in the state of the Pt and TiO2 will substantially influence the light harvest and charge separation and migration in the semiconductor particles during the photocatalysis process as shown in Fig. 6. 3.7. X-ray absorption spectra of Pt/TiO2 under different removal procedure
Fig. 6. Photocatalysis hydrogen evolution amount from water/methanol for Pt/TiO2 calcined at 400 ◦ C, photo irradiated with oxygen bubbling for 6 h, photo irradiated in vacuum for 6 h.
speed the oxidation the Pt NCs itself. The presence of residual ethylene glycol or ascorbic acid oxidation products may also influence the oxidation of Pt NCs in the calcination process. Based on the XPS results, one important point could be noted here. Both the surface chemistry and electron state of Pt NCs and TiO2 was influenced by the calcination method while
X-ray absorption fine structure (XAFS) study for Pt NCs supported on TiO2 was further carried out to elucidate the relationship between the state of platinum and the photocatalysis activity. The samples investigated included the Pt/TiO2 calcined in 400 ◦ C, photo irradiated in vacuum and in oxygen before and after photocatalysis hydrogen evolution reaction from aqueous methanol solution to provide additional information about the dispersion, particle size, oxidation state, and coordination situation of platinum supported on TiO2 . Fig. 8.1 showed the normalized X-ray absorption nearedge structure (XANES) spectra of as-synthesized Pt/TiO2 and the reference spectra of Pt foil and PtO2 . The while line intensities in the spectra reflected the oxidation state of Pt in different samples [22]. The white line intensity of the as-synthesis Pt/TiO2 is clearly lower than that of all other Pt/TiO2 samples, and is similar to that of the Pt foil, which indicates the dominance of Pt0 NCs in the samples. The Pt/TiO2 calcined in 400 ◦ C before photocatalysis hydrogen evolution reaction exhibited a sharpest peak between PtO2 and Pt foil, indicating it was is highly positive charge. The XPS results already indicated the creation of PtOx , mainly in the state of PtO2 , in the samples calcined in 400 ◦ C while the as-synthesis
Fig. 7. (7.1) C 1s photoelectron emission spectra of Pt/TiO2 : as synthesis, the calcination series (200 ◦ C, 400 ◦ C), photodegradation series (vacuum, oxygen). (7.2) Ti2p photoelectron emission spectra of Pt/TiO2 : as synthesis, the calcination series (200 ◦ C, 400 ◦ C), photodegradation series (vacuum, oxygen). (7.3) Pt 4f photoelectron emission spectra of Pt/TiO2 : as synthesis, the calcination series (200 ◦ C, 400 ◦ C), photodegradation series (vacuum, oxygen).
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
379
Fig. 8. (8.1) The normalized XANES spectra at the Pt L3 edge of Pt/TiO2 before and after photocatalysis hydrogen evolution reaction from aqueous methanol solution: the calcination series (400 ◦ C), the photodegradation series (6 h in vacuum, 6 h in oxygen). (8.2) The radial distribution functions (RDF) of Pt/TiO2 after removal ligands and before photocatalysis hydrogen evolution reaction from aqueous methanol solution: the calcination series(400 ◦ C), the photodegradation series (6 h in vacuum, 6 h in oxygen). (8.3) The radial distribution functions (RDF) of Pt/TiO2 after photocatalysis hydrogen evolution reaction from aqueous methanol solution: the calcination series (400 ◦ C), the photodegradation series (6 h in vacuum, 6 h in oxygen).
samples was mainly in metallic state, in good agreement with the XANES data. It also need to mention that the white line intensity of all samples has decreased after photocatalysis water decomposition reaction accompanied by the shift forward of the white line peak as compared with the samples before reaction suggesting negative charged effect in the photocatalysis water decomposition reactions. This phenomenon is more prominent in the samples calcined in 400 ◦ C as compared with the treatment such as irradiation in oxygen or vacuum. In the Fourier transforms (r space, Fig. 8.2) of the EXAFS data, for all samples except Pt/TiO2 calcined in 400 ◦ C, there was one prominent peak at 2.42 A˚ (no phase correction) from the Pt–Pt contribution and a very weak peak at about 1.5 A˚ (no phase correction) from the Pt–O contributions. In the Fourier transforms (r space, Fig. 8.3) of the EXAFS data, all samples after photocatalysis water decomposition showed increase in the Pt–Pt contribution and decrease in the Pt–O contribution as compared with the specific samples before reaction. As shown in Fig. 8.3, the coordination state of the Pt in the Pt/TiO2 (UV/vacuum) after photocatalysis water splitting reaction showed a Pt–Pt contribution most similar to the as-synthesis Pt/TiO2. This suggested that the self-photodegradation in vacuum could remove the capping agent while keep the state of supported Pt NCs and TiO2 almost intact. That was the key aim in removal capping ligands. Thus self-photodegradation method was proved as a promising alternative technique to the traditional calcination method or the UV/O2 method proposed in our previous study [3].
The above results could also explain the different treatment methods on the final photocatalysis water decomposition activity as shown in Fig. 7. Photoexcited TiO2 undergoes charge equilibration when they are in contact with Pt NCs. The charge equilibration depending on the nature of the metallic nanoparticles has direct influence in dictating the energetic of the composite. Higher metallic state favor the formation of Schottky barrier between Pt and TiO2 to trap electron immigrated to the surface [17]. Thus the final products treated using self-photodegradation method have the highest Pt metallic state corresponding to the highest photocatalysis activity. The formation of amorphous carbon and Ti C bond in the TiO2 surface supported by IR and XPS results may also hurt the photocatalysis activity by hindering the light harvest and block the charges immigration to the TiO2 surface. Thus the Pt/TiO2 samples (calcined in 400 ◦ C) with highest ratio of PtO2 and strongest interaction with the carbon residue in the surface demonstrated the lowest hydrogen production activity. 4. Conclusion As a proof of concept, we demonstrated that the selfphotodegradation method is an available strategy to remove the ligands on the surface of Pt/TiO2 while keeping the engineered controlled state or morphological characteristics to the maximum extent as compared with the as-synthesis Pt/TiO2 . This treatment method can lead to improved performance of cocatalystsemiconductor heterogeneous photocatalyst for photocatalysis
380
Z. Jiang, W. Shangguan / Catalysis Today 242 (2015) 372–380
water decomposition activity as it has highest metallic state of Pt. This study also demonstrate that the traditional calcination method was identified to influence both the state of Pt and TiO2 , leading to lowest photocatalysis water decomposing activity as the coupling effect of the highest ratio of positive charged Pt and Ti C bond coexist in the system. Acknowledgements This work was supported by the financial support from Shanghai Natural Science Foundation (14ZR1421900), National Science Foundation of China (50906050), the National High Technology Research and Development Program of China (2012AA051501). References [1] J.-D. Grunwaldt, et al., Preparation of supported gold catalysts for lowtemperature CO oxidation via size-controlled gold colloids, J. Catal. 181 (2) (1999) 223–232. [2] (a) H. Einaga, et al., Generation of active sites for CO photooxidation on TiO2 by platinum deposition, J. Phys. Chem. B 107 (35) (2003) 9290–9297; (b) M. Miyake, K. Miyabayashi, Shape and size controlled Pt nanocrystals as novel model catalysts, Catal. Surv. Asia 16 (1) (2012) 1–13. [3] Z. Jiang, et al., In situ controllable synthesis platinum nanocrystals on TiO2 by novel polyol-process combined with light induced photocatalysis oxidation, Chem. Commun. 48 (77) (2012) 9598–9600. [4] S. Tsubota, et al., Effect of calcination temperature on the catalytic activity of Au colloids mechanically mixed with TiO2 powder for CO oxidation, Catal. Lett. 56 (2–3) (1998) 131–135. [5] L.D. Menard, et al., Preparation of TiO2 -supported Au nanoparticle catalysts from a Au13 cluster precursor: ligand removal using ozone exposure versus a rapid thermal treatment, J. Catal. 243 (1) (2006) 64–73. [6] M. Loría-Bastarrachea, et al., A TG/FTIR study on the thermal degradation of poly (vinyl pyrrolidone), J. Therm. Anal. Calorim. 104 (2) (2011) 737–742. [7] M. Crespo-Quesada, et al., UV–ozone cleaning of supported poly (vinylpyrrolidone)-stabilized palladium nanocubes: effect of stabilizer removal on morphology and catalytic behavior, Langmuir 27 (12) (2011) 7909–7916.
[8] M. Comotti, et al., Support effect in high activity gold catalysts for CO oxidation, J. Am. Chem. Soc. 128 (3) (2006) 917–924. [9] S. Pang, et al., Decomposition of monolayer coverage on gold nanoparticles by UV/ozone treatment, Chem. Lett. 34 (4) (2005) 544–545. [10] J.A. Lopez-Sanchez, et al., Facile removal of stabilizer-ligands from supported gold nanoparticles, Nat. Chem. 3 (7) (2011) 551–556. [11] Y. Borodko, et al., Spectroscopic study of the thermal degradation of PVPcapped Rh and Pt nanoparticles in H2 and O2 environments, J. Phys. Chem. C 114 (2) (2009) 1117–1126. [12] C. Peniche, et al., Study of the thermal degradation of poly (N-vinyl-2pyrrolidone) by thermogravimetry–FTIR, J. Appl. Polym. Sci. 50 (3) (1993) 485–493. [13] K.J.A. Raj, B. Viswanathan, Effect of surface area, pore volume and particle size of P25 titania on the phase transformation of anatase to rutile, Indian J. Chem. A 48 (2009) 1378. [14] R. Andreozzi, et al., Advanced oxidation processes (AOP) for water purification and recovery, Catal. Today 53 (1) (1999) 51–59. [15] (a) W. Choi, Pure and modified TiO2 photocatalysts and their environmental applications, Catal. Surv. Asia 10 (1) (2006) 16–28; (b) J. Zhang, Y. Nosaka, Mechanism of the OH radical generation in photocatalysis with TiO2 of different crystalline types, J. Phys. Chem. C 118 (20) (2014) 10824–10832. [16] Y. Borodko, et al., Probing the interaction of poly (vinylpyrrolidone) with platinum nanocrystals by UV–Raman and FTIR, J. Phys. Chem. B 110 (46) (2006) 23052–23059. [17] A. Kudo, Photocatalyst materials for water splitting, Catal. Surv. Asia 7 (1) (2003) 31–38. [18] D. Briggs, G. Beamson, Primary and secondary oxygen-induced C1s binding energy shifts in X-ray photoelectron spectroscopy of polymers, Anal. Chem. 64 (15) (1992) 1729–1736. [19] H. Irie, Y. Watanabe, K. Hashimoto, Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst, Chem. Lett. 8 (2003) 772–773. [20] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corporation, Eden Prairie, MN, 1992. [21] L.K. Ono, et al., Formation and thermal stability of platinum oxides on sizeselected platinum nanoparticles: support effects, J. Phys. Chem. C 114 (50) (2010) 22119–22133. [22] (a) Z. Jiang, et al., Influence of support and metal precursor on the state and CO catalytic oxidation activity of platinum supported on TiO2 , J. Phys. Chem. C 116 (36) (2012) 19396–19404; (b) N. Ichikuni, Y. Iwasawa, In situ d electron density of Pt particles on supports by XANES, Catalysis letters 20 (1–2) (1993) 87–95.