Article pubs.acs.org/JPCC
Improved Visible Photocatalytic Activity on Titania Modified with −O−Pd−Cl Species Assisted by Oxidative Addition Reaction of Pd0 Yajun Yang,† Yanlong Yu,† Chunling Zhang,† Jianhui Wang,‡ and Yaan Cao*,† †
MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, P. R. China ‡ Department of Chemistry, College of Science, Tianjin University, Tianjin 300387, P. R. China S Supporting Information *
ABSTRACT: We introduce the idea of Pd catalysis used in cross-coupling reactions into photocatalysis. The −O−Pd−Cl surface species modified on nanoscale TiO2 can remarkably enhance the photocatalytic activity under visible-light irradiation in the degradation of 4-bromophenol. It is revealed that the catalytically active Pd0 is in situ generated by the reduction of photogenerated electrons from TiO2. The −O−Pd0−Cl species can react with 4-bromophenol to form hydroxyphenyl radicals via an oxidative addition reaction. The photocatalytic mechanism assisted by the oxidative addition reaction on −O− Pd−Cl species is also demonstrated. We were hence able to rationally tailor the coordination environment of Pd on the TiO2 surface to obtain high photocatalytic activity and selectivity.
1. INTRODUCTION TiO2 has been investigated extensively for degradation of organic pollutants, photoreduction of CO2, and more, due to its high activity and stability.1−4 However, its large band gap (3.2 eV for anatase) and high electron−hole recombination rate limit its practical applications.5 To address these problems, many efforts have been devoted to develop visible-responsive TiO2-based photocatalysts with high electron−hole separation efficiency, such as morphology engineering, deposition of noble metals, doping of metal or nonmetal atoms, photosensitization, and coupling with a narrow band gap semiconductor or nanocluster.6−14 Modifying TiO2 with noble metals (e. g., Pd, Pt, and Ag) is usually considered an effective method to improve the photocatalytic activity.15−18 For these noble metalmodified TiO2 system, the formed Schottky barrier or the plasma adsorption of noble metals contributes to the enhancement in photocatalytic activity.19−21 However, the photocatalytic activities under visible light irradiation are still too low for practical applications, and the use of Pd catalysis to enhance the efficiency and selectivity of photocatalysis has been largely overlooked. Herein, two kinds of TiO2-based photocatalysts with different surface Pd species were prepared with a sol−gel method. The in situ photoreduced Pd0, the oxidative addition intermediates and the radicals coupling products were detected during the photodegradation process of 4-bromophbenol (4BrPh) by TiO2−Pd−Cl. Therefore, we experimentally confirmed the catalysis mechanism of −O−Pd−Cl species in promoting photocatalytic activity and selectivity for the first time. This work will contribute to understanding the photophysics- and photochemistry-related mechanisms in © XXXX American Chemical Society
detail, and provide feasible routes to the design of photocatalysts with high activity and selectivity.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. All chemicals used were of analytical grade and the water was deionized water. Pure TiO2 was prepared by a sol−gel method. In a typical procedure, a certain volume of deionized water was added into 40 mL of absolute ethanol. Then under continuous stirring, 12 mL of Ti(OC4H9)4 and 1 mL of HCl solution (12 mol L−1) were added dropwise into the mixture. After 24 h of aging, the gels were dried and calcined at 723 K in a muffle furnace for 2.5 h. The preparation procedure of TiO2−Pd is almost the same as pure TiO2 except that certain volume of PdCl2 (0.1169 mol L−1) solution instead of deionized water was added into 40 mL of absolute ethanol. Unless stated otherwise, the nominal molar ratio of Pd2+ to Ti4+ is fixed at 1.5% in the precursor. A volume of 4 mL TiCl4 aqueous solution (1 mol L−1) was added during the preparation process and this catalyst is denoted as TiO2− Pd−Cl (the nominal molar ratio of Pd2+ to Ti4+ is also 1.5%). For comparison, other molar ratios of Pd2+ to Ti4+ (0.5%, 1.0%, 2.0%, and 2.5%) were also used. 2.2. Characterization. The crystalline structures of the samples were characterized by Renishaw invia MV4000 Raman spectrometer and Rigaku D/max 2500 X-ray diffraction spectrometer (Cu Kα, λ = 1.54056 Å). XPS measurements were done with a Thermo ESCALAB 250 spectrometer with Al Received: October 12, 2016 Revised: December 2, 2016 Published: December 15, 2016 A
DOI: 10.1021/acs.jpcc.6b10312 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Kα as the excitation source. The binding energy was referenced to the C 1s peak at 284.8 eV. The BET technique was used to measure the surface area of all the samples with a Tristar 3000 surface area analyzer. The photoluminescence measurements were carried out with an Edinburgh FLS 920P instrument with an excitation wavelength of 420 nm. Diffuse reflectance UV− visible (UV−vis) absorption spectra were recorded on a UV− vis spectrometer (U-4100, Hitachi). The measurements of the ζ potential were carried out in a ZETAPALS/BI-200SM using 1 mg of catalyst particles ultrasonically suspended in 4 mL of aqueous 4-BrPh or phenol solution. GC−MS analysis was performed on an Agilent 7890A GC system with an Agilent 5975C mass spectrometer and a HP-5MS column. Total organic carbon (TOC) was determined by using a Jena multi N/C3100 analyzer. 2.3. Catalytic Activity Measurement. The photocatalytic reaction was carried out with 0.01 g of the sample suspended in 40 mL of 4-BrPh or phenol solution. The photocatalytic system included a 400 W sunlamp, and a 420 nm cutoff filter for the removal of UV light. A water jacket was used to keep the suspension temperature constant within the range of 25.0 ± 1.0 °C. Before the experiment, the suspension was magnetically stirred in dark for 30 min to ensure the adsorption/desorption equilibrium on the TiO2 surface. The suspension was sampled at regular intervals during the reaction and centrifuged to remove the particles to measure the concentration change of 4BrPh or phenol in the solution with a UV−vis spectrometer.
Figure 1. Photocatalytic degradation of 4-BrPh (5 × 10−5 mol L−1) (A) and phenol (5 × 10−5 mol L−1) (B) using pure TiO2, TiO2−Pd, and TiO2−Pd−Cl upon visible light irradiation (λ > 420 nm). Blank experiments were carried out without catalyst under otherwise identical conditions.
3. RESULTS AND DISCUSSION The photodegradation of 4-BrPh and phenol under visible light irradiation was applied to evaluate the photocatalytic activity of TiO2−Pd and TiO2−Pd−Cl (Figure 1). The adsorption curves in the dark indicate that the adsorption for all the samples can reach equilibrium after 30 min (Figure S1). As shown in Figure 1A and Table S1, the 4-BrPh can hardly be degraded without photocatalysts (blank experiment) or by pure TiO2 under visible light irradiation. TiO2−Pd (degradation ratio after 6h: 55.10%, specific photocatalytic activity: 3.67 × 10−5 mol g−1 h−1) exhibits a remarkably improved photocatalytic activity. Furthermore, the photocatalytic activity is further improved for TiO2−Pd−Cl (degradation ratio: 71.98%, specific photocatalytic activity: 4.73 × 10−5 mol g−1 h−1). However, the photodegradation of phenol was quite different from that of 4BrPh (Figure 1B and Table S2). Pure TiO2 still exhibits very low photocatalytic activity. TiO2−Pd (degradation ratio, 57.95%; specific photocatalytic activity, 3.86 × 10−5 mol g−1 h−1) exhibits a better photocatalytic activity than TiO2−Pd−Cl (degradation ratio: 46.85%, specific photocatalytic activity: 3.12 × 10−5 mol g−1 h−1), suggesting possible different mechanisms in degrading 4-BrPh and phenol for TiO2−Pd and TiO2−Pd− Cl photocatalysts. For TiO2−Pd and TiO2−Pd−Cl, the ln(C0/ C) values of 4-BrPh and phenol exhibit a nearly linear relationship with irradiation time (Figure S2) and the rates of photocatalytic decomposition of 4-BrPh and phenol were fitted with the Langmuir−Hinshelwood (L−H) kinetics model (Figure S3), suggesting a pseudo-first-order reaction.22 The disappearance of TOC confirmed that 4-BrPh and phenol had been not only degraded but also mineralized efficiently by TiO2−Pd and TiO2−Pd−Cl (Figure S4). Measurements of the photodegradation performance of the samples with different molar ratio of Pd2+ to Ti4+ showed that 1.5% is the optimal molar ratio (Figure S5). The detailed photocatalytic mechanism will be discussed in the following sections.
The Raman spectra and XRD patterns for TiO2, TiO2−Pd, and TiO2−Pd−Cl are shown in Figure 2 and Figure S6,
Figure 2. Raman spectra of as-prepared pure TiO2, TiO2−Pd, and TiO2−Pd−Cl.
respectively. Anatase is the major phase for all the samples and a trace amount of rutile is also detected.23 The lattice parameters and cell volumes for TiO2−Pd and TiO2−Pd−Cl are almost the same as those for pure TiO2 (Table S3), implying Pd2+ ions did not enter into the lattice of TiO2 and may exist on the surface. Additionally, no formation of Pd nanoparticles was observed in the TEM images of TiO2−Pd and TiO2−Pd−Cl (Figure S7), indicating that Pd species were evenly dispersed on the sample surface. The detailed chemical states of the Pd species were investigated with XPS. The XPS Pd 3d spectra for TiO2−Pd and TiO2−Pd−Cl are shown in Figure 3. The Pd 3d spectrum can be deconvolved into two pairs of doublet peaks. The strong peak of Pd 3d5/2 at B
DOI: 10.1021/acs.jpcc.6b10312 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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−O−Pd−Cl and −O−Pd−O− species. It is revealed that the absorption at visible region for TiO2−Pd is stronger than TiO2−Pd−Cl due to the different surface species. The photographs of the photocatalyst are shown in Figure S9. Photoluminescence (PL) measurements were performed to investigate the effect of surface Pd species on the separation of photogenerated charge carriers.27 A decrease in PL intensity usually suggests an efficient separation of charge carriers.12 As shown in Figure 4B, it is found the PL intensity of TiO2−Pd is lower compared with TiO2−Pd−Cl, suggesting a more efficient separation of charge carriers for TiO2−Pd than that for TiO2− Pd−Cl. In addition, the ζ potential of TiO2−Pd−Cl remained nearly unchanged, and that for TiO2−Pd changed obviously before and after adsorption of phenol or 4-BrPh, indicating a stronger adsorption ability for TiO2−Pd than TiO2−Pd−Cl (Table S4). According to the UV−vis absorption spectra, PL spectra, ζ potentials measurements and the photocatalytic mechanism, it is expected that TiO2−Pd would exhibit better photocatalytic activity than TiO2−Pd−Cl in degradation of phenol or 4-BrPh. However, TiO2−Pd−Cl exhibited higher photocatalytic activity in degradation of 4-BrPh than TiO2−Pd. These results imply a new photocatalytic mechanism for TiO2− Pd−Cl in degradation of 4-BrPh. A new catalysis mechanism is proposed during the photocatalytic process for TiO2−Pd−Cl, as shown in Scheme 1. In photodegradation of 4-BrPh, the active −O−Pd0−Cl species is formed via the reduction of −O−PdII−Cl on the surface of TiO2 by photogenerated electrons under irradiation. The −O−Pd0−Cl species react with the 4-BrPh adsorbed on the surface of the photocatalyst to form oxidative addition intermediates and further produce the short-lived hydroxyphenyl radicals (HPR). The HPR can be photodegraded into CO2 and H2O by the reactive oxygen species generated from photogenerated electrons and holes.28 It is believed that the oxidative addition reaction on −O−Pd0−Cl species plays an important role during the photocatalytic procedure. To demonstrate the reaction scheme discussed above, TiO2−Pd− Cl and TiO2−Pd upon visible-light irradiation and after reaction are characterized using XPS analysis, Raman spectroscopy and GC−MS analysis, shown in Figure 5, 6, 7 and Figure S10. Pd 3d XPS spectra of TiO2−Pd−Cl and TiO2−Pd after visible light irradiation without 4-BrPh are shown in Figure 5. After visible-light irradiation for 4 h, the peaks of metallic Pd0 are observed (at around 334.6 eV for Pd 3d5/2) for TiO2−Pd− Cl.29 However, hardly any metallic Pd0 is detected for TiO2− Pd. It is demonstrated that −O−PdII−Cl species can be reduced by the photogenerated electrons to form −O−Pd0−Cl species, while −O−Pd−O− species cannot be reduced by the photogenerated electrons. The Raman spectra of the photocatalysts before and after photocatalytic reaction are shown in Figure 6. An additional peak at about 230 cm−1 is observed both for the TiO2−Pd−Cl before and after reaction, ascribed to the Pd−Cl stretching vibration peak,30 confirming the existence of −O−Pd−Cl surface species. A new peak at about 276.8 cm−1 is observed for TiO2−Pd−Cl after the reaction, assigned to Br−Pd stretching vibration.31 This verifies that the Pd atom in −O−Pd−Cl species coordinate with Br, implying the formation of oxidative addition intermediates (Scheme 1). In addition, another peak at about 719.6 cm−1 is found for TiO2−Pd−Cl after the reaction, ascribed to the out-of-plane vibration of p-hydroxydiphenyl.32 This molecule is the self-coupling product of HPR,28 suggesting
Figure 3. XPS spectra of Pd 3d for the as-prepared TiO2−Pd and TiO2−Pd−Cl.
around 336.3 eV can be ascribed to Pd2+ bound to two undercoordinated oxygen atoms on the surface of TiO2, forming an −O−Pd−O− structure. The other peak of Pd 3d5/2 at 337.6 eV was between that of PdCl2 (337.9 eV) and PdO (336.3 eV).24 Moreover, the peak of Cl 2p3/2 (198.3 eV) for TiO2−Pd and TiO2−Pd−Cl falls between that of PdCl2 (198.9 eV) and TiCl4 (198.2 eV), shown in Figure S8.25 These indicate the introduced Pd ion links with one undercoordinated O atom and one Cl atom to form an −O−Pd−Cl structure on the surface of TiO2. In addition, compared with TiO2−Pd, the peak intensity of −O−Pd−Cl is much larger than that of −O− Pd−O− for TiO2−Pd−Cl. These results suggest −O−Pd−O− is the main surface species for TiO2−Pd and −O−Pd−Cl is the main species for TiO2−Pd−Cl. UV−vis absorption spectroscopy was used to investigate the optical absorption properties of the samples, shown in Figure 4A. The strong absorption at around 340 nm is ascribed to the band−band transition of TiO2.26 Compared with pure TiO2, the absorption in the visible region is enhanced for TiO2−Pd− Cl and TiO2−Pd owing to the electron transition related to
Figure 4. UV−vis absorption spectra of pure TiO2, TiO2−Pd, and TiO2−Pd−Cl (A). Photoluminescence spectra of pure TiO2, TiO2− Pd, and TiO2−Pd−Cl (B). C
DOI: 10.1021/acs.jpcc.6b10312 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Scheme 1. Schematic View of Proposed Photodegradation Pathways of 4-BrPh by TiO2−Pd−Cl
using GC−MS analysis. p-Hydroxydiphenyl was detected at a retention time of ∼12.7 min (Figure S10). Furthermore, no Raman peaks related to Br−Pd stretching vibration and phydroxydiphenyl are found for TiO2 and TiO2−Pd after the reaction, suggesting that the oxidative addition reaction did not happen on the surface of TiO2−Pd and TiO2. The Pd 3d XPS spectra of TiO2−Pd−Cl and TiO2−Pd after 6 h photodegradation reaction are plotted in Figure 7. For TiO2−Pd, there is almost no Pd0 detected. For TiO2−Pd−Cl, the peaks related to −O−Pd0−Cl and −O−PdII−Cl species are observed. The peak intensity of −O−Pd0−Cl is weaker than that without 4-BrPh upon irradiation for 4 h (Figure 5) and that for −O−PdII−Cl becomes stronger. This implies the −O− PdII−Cl was reduced to −O−Pd0−Cl upon irradiation and thereafter reoxidized into −O−PdII−Cl through the oxidative addition reaction via pathway described in Scheme 1. Figure 8 shows the absorption spectra of the 4-BrPh solution during photodegradation for TiO2−Pd (Figure 8A) and TiO2− Pd−Cl (Figure 8B). The two peaks at around 225 and 280 nm correspond to the absorption maximum of 4-BrPh. The shapes of the absorption peaks remain unchanged and the peak
Figure 5. Pd 3d XPS spectra of TiO2−Pd and TiO2−Pd−Cl after visible irradiation for 4 h without the addition of 4-BrPh.
Figure 6. Raman spectra of the photocatalyst before and after visible irradiation for 6 h with 4-BrPh.
Figure 7. Pd 3d XPS spectra of TiO2−Pd and TiO2−Pd−Cl after visible irradiation for 6 h with the addition of 4-BrPh.
the formation of much HPR during the photodegradation process for TiO2−Pd−Cl. Moreover, the photodegradation intermediates of 4-BrPh by TiO2−Pd−Cl at 6 h were identified
Figure 8. Absorption spectra of 4-BrPh solution during the photodegradation with the presence of TiO2−Pd (A) and TiO2− Pd−Cl (B). D
DOI: 10.1021/acs.jpcc.6b10312 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C intensities decrease with time during the photodegradation process of TiO2−Pd. This implies that direct cleavage of the phenolic chromophore predominated without much accumulated intermediates during photodegradation, indicating the degradation of 4-BrPh for TiO2−Pd follows the conventional photodegradation mechanism.28 Moreover, compared with TiO2−Pd, tailed absorption peaks appeared at around 325 nm for TiO2−Pd−Cl during the photodegradation process, ascribed to the p-hydroxydiphenyl formed via self-coupling reaction of HPR.28 This implies that HPR is formed through the oxidative addition reaction during the photodegradation process, as shown in Scheme 1. Additionally, reusability tests were performed by repeating the 4-BrPh photocatalytic degradation tests on TiO2−Pd−Cl to evaluate the reversibility and stability of −O−Pd−Cl species. Figure S11 shows that no significant loss in activity was observed after four cycles, indicating the excellent long-term reversibility and stability of −O−Pd−Cl species. On the basis of the discussion above, the photocatalytic mechanism for photodegradation of phenol and 4-BrPh can be explained as follows. For pure TiO2, the photocatalytic activity is poor because pure TiO2 has almost no visible light absorption. Compared with TiO2, the photocatalytic activity for TiO2−Pd is improved remarkably, owing to the existence of −O−Pd−O− species, which could extend the absorption into visible region and suppress the recombination of charge carriers. For TiO2−Pd−Cl, the visible-light response is weaker than that for TiO2−Pd; hence, the photocatalytic activity in degradation of phenol is not as good as that for TiO2−Pd. However, with the assistance of Pd catalysis, TiO2−Pd−Cl shows higher photocatalytic activity than TiO2−Pd on photodegradation of 4-BrPh. The 4-BrPh molecules adsorbed on the surface of TiO2−Pd−Cl can undergo oxidative addition reactions via photoreduced −O−Pd0−Cl species. As a result, more HPR can be generated, because less activation energy is required to cleave the Ph−Br bond by Pd catalysis.33 The formed HPR can eventually be oxidized to H2O and CO2 under visible irradiation.28 Hence, TiO2−Pd (without Pd catalysis) is less active than TiO2−Pd−Cl (with Pd catalysis) for photodegradation of 4-BrPh.
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AUTHOR INFORMATION
Corresponding Author
*(Y.C.) E-mail:
[email protected]. ORCID
Yaan Cao: 0000-0001-8921-4651 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51372120). We also thank the Ph.D. Candidate Research Innovation Fund of Nankai University.
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4. CONCLUSION In summary, TiO2 modified with −O−Pd−O− (TiO2−Pd) and −O−Pd−Cl species (TiO2−Pd−Cl) were prepared with a sol−gel method. Despite the weaker visible response, higher recombination rate of charge carriers and worse adsorption ability, TiO2−Pd−Cl exhibits better photocatalytic activity in photodegrading 4-BrPh than TiO2−Pd. It is revealed that the oxidative addition reaction of −O−Pd−Cl species played a predominant role in degradation of 4-BrPh for TiO2−Pd−Cl. Under irradiation, −O−PdII−Cl species are reduced to −O− Pd0−Cl species during photocatalytic reaction, promoting the formation of HPR with less activation energy via an oxidative addition reaction. The HPR is eventually photodegraded by the reactive oxygen species. This work is helpful for the design and preparation of highly active photocatalysts for practical applications.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10312. E
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