• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Construction of Pt-M (M = Co, Ni, Fe)/g-C3N4 Composites for Highly Efficient Photocatalytic H2 Generation

    2020-07-23 08:19:36LiangWangChengluZhuLishaYinWeiHuang
    物理化學(xué)學(xué)報 2020年7期

    Liang Wang , Chenglu Zhu , Lisha Yin ,2,*, Wei Huang

    1 Institute of Advanced Materials, Nanjing Tech University, Nanjing 211816, P.R.China.

    2 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798.

    Abstract:Platinum (Pt) is recognized as an excellent cocatalyst which not only suppresses the charge carrier recombination of the photocatalyst but also reduces the overpotential for photocatalytic H2 generation.Albeit of its good performance, the high cost and low abundance restricted the utilization of Pt in large-scale photocatalytic H2 generation.Pt based transition metal alloys are demonstrated to reveal enhanced activities towards various catalytic reactions,suggesting the possibility to substitute Pt as the cocatalyst.In the present work,Pt was partially substituted with Co, Ni, and Fe and Pt-M (M = Co, Ni, and Fe)/g-C3N4 composites were constructed through co-reduction of H2PtCl6 and transition metal salts by the reductant of ethylene glycol.The crystal structure and valence states were measured by X-ray diffractometer (XRD) and X-ray photoelectron spectrometer (XPS), respectively.The higher degree of XRD peaks and larger binding energies for Pt 4f5/2 and Pt 4f7/2 after incorporating Co2+ ions indicated that Co was successfully introduced into the lattice of Pt and Pt-Co bimetallic alloys was attained through the solvothermal treatment.The morphology was subsequently observed by transmission electron microscope (TEM), which showed a good dispersion of Pt-Co nanoparticles on the surface of g-C3N4.Meanwhile, the shrinkage of lattice fringe after introducing cobalt salt further confirmed the presence of Pt-Co bimetallic alloys.The UV-Vis absorption spectra of g-C3N4 and Pt, Pt-Co deposited g-C3N4 were subsequently performed.It was found that the absorption edges were all consistent for all three samples as anticipated, implying that the band gap energy was maintained after hybridizing with Pt or Pt-Co alloys.Furthermore, the photocatalytic H2 generation was carried out over the as-prepared composites with triethanolamine (TEOA) as sacrificial reagent.Under visible-light illumination, the1%(w) Pt2.5M/g-C3N4 (M = Co, Fe, Ni) composites all exhibited higher or comparable activity towards photocatalytic H2 generation when compared to 1% (w) Pt loaded counterpart.In addition, the atomic ratios of Pt/Co and the loading amount of Pt-Co cocatalyst were modified to optimize the photocatalytic performance, among which, 1% (w) Pt2.5Co/g-C3N4 composite revealed the highest activity with a 1.6-time enhancement.Electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra indicated that the enhancement might be attributed to improved charge transfer from g-C3N4 to Pt2.5Co cocatalyst and inhibited charge carrier recombination in the presence of Pt2.5Co cocatalyst.Therefore, the present study demonstrates the great potential to partially replace Pt with low-cost and abundant transition metals and to fabricate Pt based bimetallic alloys as promising cocatalysts for highly efficient photocatalytic H2 generation.

    Key Words:Photocatalytic H2 generation;Composite;Cocatalyst;g-C3N4;Pt-Co alloy

    1 Introduction

    Photocatalytic water splitting is a promising strategy to generate clean and sustainable hydrogen as an energy carrier.The core challenge in this process is the choice of a suitable photocatalyst.An ideal photocatalyst should possess appropriate band structure to respond to visible light as well as produce photoexcited electrons with sufficient reduction potential to split water into H21.Besides, the photocatalyst should be chemically stable, low-cost and non-toxic.From this point of view, g-C3N4is a potential candidate which could meet the requirements aforementioned2.Therefore, g-C3N4has attracted enormous attention since the first report of its application in photocatalytic water splitting3,4.Nevertheless, the photocatalytic performance of g-C3N4is moderate under visible light due to rapid recombination of photogenerated e-/h+pairs and high overpotential2,5,6.On the one hand, the lifetime of the photogenerated e-/h+pairs is only a few nanoseconds (ns)7.If they are not able to migrate to the surface of g-C3N4within this duration, they would recombine.On the other hand, the weak van der Waals’ forces between the layers and the hydrogen bonds between the domains of g-C3N4structure would inhibit the migration and transfer of photogenerated charge carriers as well.Consequently, only a small proportion of photogenerated e-/h+pairs would reach the surface reactive sites through tri-s-triazine unit connected planes and participate in the following redox reactions.

    Metal nanoparticles are of great significance in the applications such as catalysis, electronics, and biology, etc.which account for two thirds of all elements in the periodic table8-10.Loading of noble metals (e.g.Pt, Pd, Au and Ag) as cocatalysts is reported to be an efficient approach to enhance the photocatalytic performance of various photocatalysts11-13.Generally, Pt and other noble metals show much larger work functions (> 5.0 eV)as compared to the semiconductor photocatalysts.For noble metal/semiconductor composites, the mismatch of Fermi levels would drive the electrons to migrate from n-type semiconductor to the metal, resulting in band bending and formation of Schottky barrier which would then hinder the recombination of photogenerated e-/h+pairs14.Moreover, the presence of metal nanoparticles as cocatalysts could also reduce the overpotential from several hundred millivolts (mV) to tens of mV for the surface redox reaction, further enhancing the photocatalytic performance15.Meanwhile, cocatalysts would accelerate photocatalytic reactions by inhibiting backward reactions16.

    Pt is the model cocatalyst that works efficiently on a majority of photocatalysts for photocatalytic hydrogen generation due to its high work function (5.6-6.1 eV)17.For example, Yuet al.reported that photodeposited Pt on TiO2nanosheets with exposed (001) facet could exhibit a rate as high as 333.5μmol?h-1for photocatalytic H2generation at 2% (w) loading amount, albeit TiO2alone is inactive under UV light illumination ascribed to rapid charge carrier recombination18.Xu and coworkers found that platinum were loaded onto the surface of CdS in the form of Pt0and Pt2+, respectively, after photochemical reduction from H2PtCl6in alkaline solution and acidic/neutral solution19.Madea and co-workers further demonstrated that the nonionic complex (bis(1,5-cyclooctadiene)platinum complex)led to a better dispersion of Pt on g-C3N4than ionic H2PtCl6and hence an enhanced activity was attained20.Despite of its high efficiency, the rareness and high cost limit the implementation of Pt in large-scale photocatalytic H2generation in the future.Therefore, low-cost and highly efficient cocatalysts composed of abundant elements should be explored.

    Recently, Pt based transition metal alloys have been demonstrated to show excellent performances in a variety of catalytic reactions21.For instance, Pt3Co, Pt3Ni and Pt3Fe alloys revealed higher activity than pure Pt for oxygen reduction reaction (ORR)22-25.PtCo-Cu2ZnGeS4hetero-structured nanoparticles could facilitate the reduction of I-3to I-in dyesensitized solar cells and exhibited an impressive power conversion efficiency of 8.12% as a counter electrode, which was higher than the Pt counterpart (7.69%)26.In case of photocatalytic H2generation, Yu and co-authors achieved almost doubled photocatalytic activity upon loading Pt3Co on CdS as compared to the Pt-loaded counterpart27.However, the study of Pt3Co and other Pt based transition metal alloys as cocatalysts in photocatalysis is still on the way.Herein, we report the substitution of Pt with Co, Ni or Fe and preparation of Pt-M(M = Co, Ni, Fe) bimetallic alloys, which arein situanchored on the surface of g-C3N4with high dispersion and good stability through simultaneous reduction of Pt4+and Co2+/Ni2+/Fe3+by ethylene glycol.Results indicated that Pt-M (M = Co, Ni, Fe)cocatalysts could remarkably enhance the photocatalytic performance of g-C3N4towards hydrogen generation.Among the three composites, Pt-Co/g-C3N4sample with Pt/Co ratio of 2.5/1 and loading amount of 1% (w) exhibited 1.6-fold enhancement in comparison with Pt/g-C3N4counterpart under the same conditions, providing a good example for the development of low-cost and highly efficient photocatalytic systems.

    2 Experimental

    2.1 Materials

    Melamine (99%), dihydrogen hexachloroplatinate (IV)hexahydrate (H2PtCl6?6H2O, 8% in water), cobalt nitrate hexahydrate (Co(NO3)2?6H2O, 98%), ethylene glycol(≥ 99%),ferric chloride hexahydrate (FeCl3?6H2O, 97%), nickel acetate tetrahydrate (Ni(CH3COO)2?4H2O, 98%), and triethanolamine(TEOA, ≥ 98%) were purchased from Sigma-Aldrich and used directly without further purification.

    2.2 Samples preparation

    2.2.1 Preparation of g-C3N4

    g-C3N4(denoted as CN from hereafter) was preparedviapolycondensation method using melamine as the precursor according to our previous work28.

    2.2.2 Preparation of Pt/g-C3N4, Co/g-C3N4and Pt-M/g-C3N4(M = Co, Fe, Ni) composites

    Pt-Co alloys were anchored onto the surface of CNviaa polyol process in which H2PtCl6?6H2O, and Co(NO3)2?6H2O were utilized as Pt and Co precursor, respectively.In a typical synthesis of 1% (w) PtxCo/g-C3N4composite (wherexdonated as the atomic ratio of Pt/Co and the sample was labelled as PtxCoCN), a desired amount of H2PtCl6?6H2O and Co(NO3)2?6H2O (the resulting PtxCo alloys were all 3 mg), and 297 mg CN were subsequently added into 20 mL ethylene glycol at the pH of 11 (adjusted by 1 mol?L-1NaOH aqueous solution).Then the mixture was sonicated and stirred to get a homogeneous solution before transferred into a 40-mL Teflonlined stainless steel autoclave.Afterwards, the autoclave was kept at 473 K for 10 h.Finally, the obtained sample was washed with distilled water and absolute ethanol for three times, and dried in the vacuum oven.

    For other loading amount of PtxCoCN samples, the amount of PtxCo alloy was maintained at 3 mg whereas the amount of CN varied accordingly.

    For 1% (w) Pt (or Co) loaded sample, only Pt precursor (or Co precursor) was added with a final metal product of 3 mg.

    In the case of 1% (w) Pt2.5Fe/g-C3N4 and Pt2.5Ni/g-C3N4(donated as Pt2.5FeCN and Pt2.5NiCN, respectively), FeCl3?6H2O and Ni(CH3COO)2?4H2O were used as the transition metal precursors, respectively, while the other conditions were kept the same.

    2.3 Characterizations

    The crystallographic structures of the resulting samples were characterized by a Shimadzu XRD-6000 X-ray diffractometer(Japan) with CuKαsource under 2-theta mode from 10° to 80°at a scan rate of 1 (°)?min-1.The morphologies were examined by JEOL JEM-2010 and JEOL JEM-2100F transmission electron microscope (TEM, Oxford, UK).The UV-Vis diffuse reflectance spectra were recorded by Lambda 750UV/Vis/NIR spectrophotometer (Perkin-Elmer, USA).The photoluminescence(PL) spectra were measured by using 325 nm excitation on the Shimazu RF-5310PC fluorometer (Japan).

    2.4 Photocatalytic H2 generation

    A 45-mL closed glass tube reactor was applied in this experiment.In a typical photocatalytic H2generation test, 10 mg of the obtained sample was dispersed into 10 mL of 15% (vol.)TEOA aqueous solution in the reactor.Subsequently, the reactor was sealed with a rubber seal, and the system was purged with nitrogen gas under stirring to drive away the residual air.Afterwards, the reactor was illuminated under 300-W xenon lamp (MAX-302, Asahi Spectra, USA) coupled with a UV cutoff filter (λ> 420 nm).In this process, the suspension was stirred at 750 r?min-1to maintain a homogeneous illumination environment.The generated H2in this process was quantified by a gas chromatograph (GC-7890A, Agilent) with a TCD detector at an interval of 1 h.

    2.5 Photoelectrochemical analyses

    The photoelectrochemical (PEC) analyses were carried out in an electrochemical cell with a standard three-electrode apparatus on an electrochemical analyzer (Solartron Instruments SI287).A Pt wire and an Ag/AgCl electrode were employed as the counter electrode and the reference electrode, respectively.Prior to the fabrication of the working electrode, a stock solution contained a mixture of 50 μL Nafion (Sigma-Aldrich), 1 mL ethanol and 4 mL DI water was prepared.Then 8 mg of the as-prepared samples were added into 2 mL of the stock solution and sonicated for 2 h to obtain a homogeneous suspension. Afterwards, the desired working electrode was obtained by dispersing the suspension onto the fluorine-doped tin oxide(FTO) glass piece with an exposed area of 0.25 cm2.During the measurement, Xe arc lamp was employed as light source whereas potassium phosphate buffer solution (KPi, 1 mol?L-1,pH = 7) was used as electrolyte.The Nyquist plots of electrochemical impedance spectra (denoted as EIS) were measured by employing the same setup.During the measurement, the perturbation signal was set at 20 mV and the frequency varied from 10 mHz to 100 kHz.

    3 Results and discussion

    3.1 Structural characterizations

    The polyol reduction method has been extensively used to prepare various alloys and alloy composites, for instance Pt-Co and Pt3Co loaded CdS and TiO227,29-32.In this work, the deposition of Pt and Pt2.5Co nanoparticles onto CN were also achieved by co-reduction of H2PtCl6and Co(NO3)2in ethylene glycol under high-temperature and high-pressure treatment.XRD analyses were performed for the as-prepared 1% (w) PtCN and 10% (w) Pt2.5CoCN to confirm the crystallographic phase of Pt and Pt2.5Co in the composites and to evaluate the influence of Co-substitution in the Pt crystal structure (Fig.1).Two characteristic peaks for CN (13.0° and 27.4°) were observed for both samples as expected.Besides, a small peak at 39.8° was observed for Pt loaded sample, which is assigned to (111) plane of standard Pt (JCPDS 70-2431).However, this peak shifted to 40.5° for the Pt2.5Co loaded sample as indicated in the inset of Fig.1, in good accordance with that reported by Yuet al.27.The Pt 4f5/2and Pt 4f7/2peaks in the XPS measurements (Fig.S1,Supporting Information (SI)) also shifted to higher binding energies after the introduction of Co, in consistent with that reported by Choiet al.33.The shifts both imply the successful preparation of Pt-Co alloy since the smaller size of Co atom could lead to a contraction of lattice fringe and consequently a higher diffraction peak and binding energy.In addition, similar peak shifts towards higher degree were observed for Pt2.5FeCN and Pt2.5NiCN composites (Fig.S2 (SI)), suggesting the successful deposition of Pt2.5Fe and Pt2.5Ni on the surface of g-C3N4.

    Fig.1 XRD patterns of 1% (w) PtCN (black) and 10% (w) Pt2.5CoCN (blue and inset).

    The morphology of as-prepared 1% (w) PtCN and 1% (w)Pt2.5CoCN were examined under TEM and high resolution TEM(HRTEM) as displayed in Fig.2.The bright field TEM image of PtCN (Fig.2a) and dark field TEM image of Pt2.5CoCN (Fig.2b)indicated that the metal cocatalyst nanoparticles were highly dispersed on the surface of CN.The HRTEM image further revealed the lattice fringe of Pt and Pt2.5Co were respectively 0.234 and 0.222 nm, corresponding to (111) facet of Pt.The shrinkage of the lattice fringe after partially replacing Pt with Co further confirmed the successfully preparation of Pt2.5Co alloy,consistent with the XRD patterns.Besides, the energy dispersive spectrum (EDS) (Fig.S3 (SI)) also revealed the presence of C,N, Pt, Co, providing another evidence for the successful deposition of Pt2.5Co alloy on the surface of g-C3N4.

    The UV-Vis diffuse reflectance spectra of pristine CN, 1% (w)PtCN and 1% (w) Pt2.5CoCN were shown in Fig.3.In comparison to pristine CN, enhanced light absorption between 460 and 800 nm was observed for the Pt and Pt2.5Co loaded sample, which might be ascribed to light absorption and scattering by the cocatalyst particles since both Pt and Pt2.5Co are black.

    Fig.2 TEM images of 1% (w) PtCN (a, b) and 1% (w) Pt2.5CoCN (c, d).

    Fig.3 UV-Vis diffuse reflectance spectra of pristine g-C3N4 (green),1% (w) Pt/g-C3N4 (black), 1% (w) Pt2.5Co/g-C3N4 (blue).

    3.2 Photocatalytic performance

    The photocatalytic activities for H2production over the CN samples loaded with 1% (w) cocatalyst (Pt or Pt-Co alloys of different atomic ratios) were evaluated under visible light illumination in an aqueous solution of 15% (vol.) TEOA as hole scavenger.As indicated in Fig.4a, b, the PtCN showed a medium H2evolution rate of 2.95 μmol?h-1(equal to 295 μmol?h-1?g-1).The introduction of Co into Pt and formation of Pt-Co alloys resulted in a significant improvement in photocatalytic H2evolution rate, demonstrating a vital influence of the Co content in the alloys on the photocatalytic activity.In the presence of a small amount of Co (Pt3Co), the activity over Pt3Co loaded CN was enhanced to 3.91 μmol?h-1(equal to 391 μmol?h-1?g-1).When the Co content further increased to Pt2.5Co, the rate over the composite could ramp up to 4.63 μmol?h-1(equal to 463μmol?h-1?g-1), which is 1.6 times in comparison to pure PtCN.Further increasing Co content in the Pt-Co alloy led to gradual decrease in photocatalytic H2evolution rate.Pt2CoCN exhibited a rate of 3.27 μmol?h-1(equal to 327 μmol?h-1?g-1), slightly higher than that of PtCN.In the case of large ratio of Co, the rate dropped dramatically to 0.49 μmol?h-1(equal to 49μmol?h-1?g-1) for PtCo6CN.Pure Co loaded CN showed a negligible activity of 0.05 μmol?h-1(equal to 5 μmol?h-1?g-1),much lower than that of pure Pt loaded counterpart, indicating Co is a less effective cocatalyst compared with Pt.Among all the samples, 1% (w) Pt2.5CoCN showed the highest activity for H2generation.

    Subsequently, the photocatalytic H2production activities over Pt2.5CoCN of various loading amounts were investigated under the same conditions, as shown in Fig.4c, d.Compared to the 1%(w) counterpart, the 0.5% (w) loaded sample showed a slightly lower activity, whereas the activities of 3% (w) and 10% (w)loaded samples were much lower.At high loading amount, the excessive cocatalyst nanoparticles on the CN surface would block the visible light absorption of CN and lead to decreased photocatalytic activities.These results indicated that 1% (w)Pt2.5CoCN exhibited the highest photocatalytic H2production rate under current conditions.Furthermore, the stability of 1%(w) Pt2.5CoCN sample for H2generation was verified through continuous visible-light irradiation for 25 h (Fig.4e).A linear plot of photocatalytic H2evolution rateversustime was obtained, indicating a constant H2production rate.Hence, the asprepared sample exhibited very good stability under the present test conditions.

    Fig.4 Photocatalytic H2 evolution performance over Pt, Co and Pt-Co alloys of different atomic ratios loaded CN at 1% (w) loading amount (a, b)and over Pt2.5Co CN at various loading amounts (c, d) and stability test over 1% (w) Pt2.5CoCN (e) and photocatalytic H2 evolution rate over Pt based bimetallic cocatalysts loaded CN at 1% (w) loading amount (f).

    In addition, other Pt-M (M = Ni, Fe) bimetallic alloys were also synthesized through similar approaches and applied for photocatalytic H2evolution.Fig.4f showed that 1% (w)Pt2.5FeCN exhibited a higher rate than the 1% (w) Pt loaded sample, while 1% (w) Pt2.5NiCN exhibited a similar rate compared to the Pt counterpart.The better performance of Pt2.5Co cocatalyst might be ascribed to the smaller particle size and better dispersion on g-C3N4surface.As indicated in Fig.S4 and S5 (SI), the average size for Pt2.5Co is 1.72 nm, whereas the mean sizes are 2.64 and 2.59 nm for Pt2.5Fe and Pt2.5Ni,respectively.Moreover, aggregation is observed for both Pt2.5FeCN and Pt2.5NiCN composites, but not for Pt2.5CoCN composite.Nevertheless, our studies demonstrate that these Pt based bimetallic alloys (Pt2.5Co, Pt2.5Fe, Pt2.5Ni) are promising Pt substitute as cocatalysts for photocatalytic H2evolution for the consideration of cost and efficiency.

    3.3 Proposed mechanism

    A tentative mechanism is proposed for the high photocatalytic H2generation activity over 1% (w) Pt2.5CoCN, as illustrated in Fig.5.Upon illumination of visible light, the valence band (VB)electrons of CN are excited and transported to the conduction band (CB).In the presence of Pt2.5Co, the electrons on CB of CN could rapidly migrate to Pt2.5Co from CN attributed to lower Fermi level.These electrons would then participate in the proton reduction to produce H2.

    Fig.5 A tentative mechanism proposed for enhanced photocatalytic performance of Pt2.5CoCN.

    Fig.6 (a) PL spectra of pristine CN, Pt and Pt2.5Co loaded CN.(b) Nyquist plots of EIS with pristine CN (red, solid: in the dark,hollow: under light), Pt (black, solid: in the dark, hollow: under light)and Pt2.5Co (blue, solid: in the dark, hollow: under light) loaded CN working electrodes at voltage of 0.3 V (vs RHE).

    PL spectra were performed to verify this charge transfer process (Fig.6a).A quenched PL intensity was observed from pristine CN to PtCN, and subsequently Pt2.5CoCN, suggesting that the electrons transfer from CN to Pt2.5Co is more effective than that from CN to Pt.Moreover, Nyquist plots of EIS measurement were conducted to study the interfacial charge transfer properties of FTO/CN, FTO/CN/Pt, FTO/CN/Pt2.5Co.It was found that the charge transfer resistance decreased in the order of CN > PtCN > Pt2.5CoCN, indicating that the electrons transfer are more efficient at CN/Pt2.5Co interfaces and the charge recombination is effectively suppressed in the presence of Pt2.5Co cocatalyst.

    Why do electrons and holes migrate more effectively from CN to Pt2.5Co? According to Wakisaka and co-workers, the 5delectron density of Pt decreased when alloying with a second component34.This might allow the photogenerated electrons to be trapped for a longer time on Pt-Co alloys.On the other hand,Pt could catalyse the back reaction of generated hydrogen due to its robust adsorption ability35.As such, comparing to pure Pt,the Pt-Co alloys exhibit weaker binding with the produced hydrogen36, which reduces the chance of back reaction (H2to H2O) on the metal surface.Therefore, the Pt-Co alloy allow for higher activity of photocatalytic H2generation owing to the promoted charge transfer from excited CN and the reduced rate of back reaction (H2 oxidation).Certainly, excessive Co in the Pt-Co alloy would play negative roles since Pt is the major part to lower down the energy barrier of H+reduction into H atoms.As such, there is an optimal ratio of Pt/Co.In our studies, the optimal ratio (Pt2.5Co) is based on the experimental observation.

    4 Conclusions

    To summarize, Pt-MCN (M = Co, Fe, Ni) composites were developed by a polyol reduction method with ethylene glycol as the reducing agent.The Pt based bimetallic alloys were all demonstrated to be lower-cost and efficient cocatalysts on the surface of g-C3N4.Remarkably, the 1% (w) Pt2.5CoCN exhibited the highest photocatalytic H2evolution rate, which was even higher than that of 1% (w) PtCN under the same conditions.To explain the enhancement in photocatalytic activity, PL spectra and EIS spectra were conducted.The results indicated that more effective electrons/holes transfer was achieved for Pt2.5CoCN,which was the main reason for the enhancement.Thus, this work provides a universal approach for Pt based transition metal alloys as a class of promising cocatalysts superior to Pt for photocatalytic H2 generation in the future.

    Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.

    国产高清不卡午夜福利| 亚洲一级一片aⅴ在线观看| 男女啪啪激烈高潮av片| 国产精品国产高清国产av| 免费看光身美女| 黄色日韩在线| 日本色播在线视频| 两个人视频免费观看高清| 国产中年淑女户外野战色| 级片在线观看| 成年版毛片免费区| 日韩中字成人| 亚洲国产精品成人综合色| 色视频www国产| 99热精品在线国产| 三级国产精品欧美在线观看| 亚洲18禁久久av| 欧美xxxx黑人xx丫x性爽| 女的被弄到高潮叫床怎么办| 国产精品国产高清国产av| 久久国产乱子免费精品| 天美传媒精品一区二区| 欧美成人免费av一区二区三区| 日韩在线高清观看一区二区三区| 搡老熟女国产l中国老女人| 国模一区二区三区四区视频| 高清毛片免费观看视频网站| 可以在线观看的亚洲视频| 99热这里只有精品一区| 午夜日韩欧美国产| 欧美日韩一区二区视频在线观看视频在线 | 99热这里只有是精品50| 亚洲久久久久久中文字幕| 色尼玛亚洲综合影院| 国产激情偷乱视频一区二区| 日日摸夜夜添夜夜添av毛片| 婷婷精品国产亚洲av| 在线观看av片永久免费下载| 波多野结衣高清无吗| 日韩精品有码人妻一区| 免费高清视频大片| 在线观看美女被高潮喷水网站| 亚洲精品成人久久久久久| 一区二区三区高清视频在线| 亚洲自偷自拍三级| 亚洲国产精品合色在线| 一本一本综合久久| 久久精品国产清高在天天线| 女同久久另类99精品国产91| 成人一区二区视频在线观看| 精品一区二区三区视频在线| 午夜亚洲福利在线播放| 亚洲av美国av| 村上凉子中文字幕在线| 在线观看66精品国产| 亚洲在线观看片| 非洲黑人性xxxx精品又粗又长| 国产黄色小视频在线观看| 丰满的人妻完整版| 国产三级中文精品| 日韩国内少妇激情av| 色尼玛亚洲综合影院| 97人妻精品一区二区三区麻豆| 91在线精品国自产拍蜜月| 男人舔女人下体高潮全视频| 97碰自拍视频| 成人精品一区二区免费| 日本一二三区视频观看| 中国国产av一级| 日日撸夜夜添| 村上凉子中文字幕在线| 寂寞人妻少妇视频99o| 亚洲,欧美,日韩| 精品久久久噜噜| 国产单亲对白刺激| 插阴视频在线观看视频| 久久综合国产亚洲精品| 99久久中文字幕三级久久日本| 亚洲激情五月婷婷啪啪| 色综合色国产| 一级a爱片免费观看的视频| 亚洲中文字幕日韩| 中文资源天堂在线| 老师上课跳d突然被开到最大视频| 狂野欧美白嫩少妇大欣赏| 久久午夜福利片| 亚洲最大成人av| 一进一出好大好爽视频| 又粗又爽又猛毛片免费看| 亚洲av二区三区四区| 国产免费男女视频| 黄色欧美视频在线观看| 两个人视频免费观看高清| 男人舔女人下体高潮全视频| 亚洲欧美日韩东京热| 99热精品在线国产| 国产乱人偷精品视频| 干丝袜人妻中文字幕| 国产成人影院久久av| 日日干狠狠操夜夜爽| 亚洲无线在线观看| www.色视频.com| 97碰自拍视频| 中文亚洲av片在线观看爽| 国内精品一区二区在线观看| 欧美日韩国产亚洲二区| av免费在线看不卡| 99视频精品全部免费 在线| 网址你懂的国产日韩在线| 又黄又爽又免费观看的视频| 午夜日韩欧美国产| 日韩一区二区视频免费看| 国产单亲对白刺激| 99热这里只有是精品在线观看| 熟女人妻精品中文字幕| 久久久久九九精品影院| 亚洲av熟女| 搡老妇女老女人老熟妇| 九九热线精品视视频播放| 久久国产乱子免费精品| 中文字幕av成人在线电影| 国产成人精品久久久久久| 国产精品国产高清国产av| 欧美人与善性xxx| 亚洲三级黄色毛片| 亚洲精品乱码久久久v下载方式| 啦啦啦啦在线视频资源| 麻豆成人午夜福利视频| 亚洲高清免费不卡视频| 最近视频中文字幕2019在线8| 亚洲av.av天堂| 老熟妇仑乱视频hdxx| 亚洲色图av天堂| 中国美白少妇内射xxxbb| 青春草视频在线免费观看| 久久久久精品国产欧美久久久| 美女 人体艺术 gogo| 日韩av在线大香蕉| 日韩欧美免费精品| 国国产精品蜜臀av免费| 香蕉av资源在线| 亚洲成人中文字幕在线播放| 一区福利在线观看| 欧美日韩精品成人综合77777| 日本色播在线视频| 2021天堂中文幕一二区在线观| 久久欧美精品欧美久久欧美| 男人舔奶头视频| av天堂在线播放| 97人妻精品一区二区三区麻豆| 丰满乱子伦码专区| 露出奶头的视频| 亚洲精品色激情综合| 久久久久久伊人网av| 99久久中文字幕三级久久日本| 亚洲精品色激情综合| 十八禁网站免费在线| 97超视频在线观看视频| 亚洲国产欧洲综合997久久,| 免费高清视频大片| 露出奶头的视频| 成人漫画全彩无遮挡| 亚洲最大成人av| 久久综合国产亚洲精品| 噜噜噜噜噜久久久久久91| 亚洲最大成人av| 亚洲最大成人av| 欧美性猛交╳xxx乱大交人| av在线天堂中文字幕| 观看美女的网站| 日韩成人av中文字幕在线观看 | 国产精品永久免费网站| 麻豆av噜噜一区二区三区| 最近的中文字幕免费完整| 能在线免费观看的黄片| 春色校园在线视频观看| 亚洲aⅴ乱码一区二区在线播放| 麻豆国产97在线/欧美| 亚洲18禁久久av| 五月玫瑰六月丁香| 亚州av有码| 我的女老师完整版在线观看| 国产精品一及| 国产私拍福利视频在线观看| 长腿黑丝高跟| 中文字幕熟女人妻在线| 久久久久久久久大av| 人人妻,人人澡人人爽秒播| 国产黄片美女视频| 99久久九九国产精品国产免费| 一级黄色大片毛片| 三级国产精品欧美在线观看| 又粗又爽又猛毛片免费看| 精品人妻视频免费看| 国产成人91sexporn| 免费不卡的大黄色大毛片视频在线观看 | 亚洲va在线va天堂va国产| 男人和女人高潮做爰伦理| 乱系列少妇在线播放| 又爽又黄无遮挡网站| 亚洲天堂国产精品一区在线| 此物有八面人人有两片| 久久久午夜欧美精品| 国产乱人视频| 亚洲自拍偷在线| 久久久成人免费电影| 少妇的逼好多水| 欧美一区二区亚洲| 美女内射精品一级片tv| 麻豆国产av国片精品| 亚洲成a人片在线一区二区| 精品99又大又爽又粗少妇毛片| 99久国产av精品| 精品久久久久久久久久久久久| 啦啦啦观看免费观看视频高清| 亚洲精华国产精华液的使用体验 | 性欧美人与动物交配| 亚洲欧美精品综合久久99| 尤物成人国产欧美一区二区三区| 又爽又黄a免费视频| 亚洲中文日韩欧美视频| 国产精品1区2区在线观看.| 日韩人妻高清精品专区| 少妇人妻一区二区三区视频| www.色视频.com| 国产一区二区亚洲精品在线观看| 深爱激情五月婷婷| 成人无遮挡网站| 免费观看的影片在线观看| 春色校园在线视频观看| 精品人妻熟女av久视频| 亚洲欧美精品综合久久99| 午夜精品一区二区三区免费看| 成人精品一区二区免费| 黄色一级大片看看| 国产成人精品久久久久久| 最好的美女福利视频网| 一区福利在线观看| 国产蜜桃级精品一区二区三区| 五月玫瑰六月丁香| 岛国在线免费视频观看| 好男人在线观看高清免费视频| 夜夜夜夜夜久久久久| 一本精品99久久精品77| 免费一级毛片在线播放高清视频| 搞女人的毛片| 尤物成人国产欧美一区二区三区| 观看美女的网站| 欧美激情在线99| 狂野欧美白嫩少妇大欣赏| 99久久成人亚洲精品观看| 中文字幕av在线有码专区| 小说图片视频综合网站| 午夜福利在线在线| 日日撸夜夜添| 国产黄色小视频在线观看| 国产欧美日韩一区二区精品| 欧美另类亚洲清纯唯美| 午夜视频国产福利| 婷婷精品国产亚洲av| 蜜臀久久99精品久久宅男| 久久久久久伊人网av| 一级毛片久久久久久久久女| 久久九九热精品免费| 精品午夜福利视频在线观看一区| 亚洲三级黄色毛片| 网址你懂的国产日韩在线| 99国产极品粉嫩在线观看| 亚洲av免费在线观看| 久久6这里有精品| 熟女人妻精品中文字幕| 蜜桃久久精品国产亚洲av| 亚洲精华国产精华液的使用体验 | 色5月婷婷丁香| 婷婷亚洲欧美| 国产精品久久久久久亚洲av鲁大| 久久精品国产自在天天线| or卡值多少钱| 美女cb高潮喷水在线观看| 老司机福利观看| 赤兔流量卡办理| 亚洲人成网站在线播放欧美日韩| 国产精品福利在线免费观看| 99在线人妻在线中文字幕| 一边摸一边抽搐一进一小说| 一级av片app| 国产精品野战在线观看| 小说图片视频综合网站| av天堂在线播放| 亚洲,欧美,日韩| 日本色播在线视频| 亚洲内射少妇av| 亚洲av.av天堂| 深夜a级毛片| 91麻豆精品激情在线观看国产| 亚洲七黄色美女视频| 精品无人区乱码1区二区| 亚洲久久久久久中文字幕| 搡老熟女国产l中国老女人| 91久久精品电影网| 日韩制服骚丝袜av| 露出奶头的视频| 春色校园在线视频观看| 欧美高清性xxxxhd video| 国产精品亚洲美女久久久| 午夜精品在线福利| aaaaa片日本免费| 亚洲第一电影网av| 特级一级黄色大片| 精品人妻偷拍中文字幕| av国产免费在线观看| 中国国产av一级| 国产亚洲av嫩草精品影院| 一卡2卡三卡四卡精品乱码亚洲| 精品国内亚洲2022精品成人| 伦理电影大哥的女人| 日韩人妻高清精品专区| 丰满人妻一区二区三区视频av| 嫩草影院新地址| 免费在线观看成人毛片| 真人做人爱边吃奶动态| 免费在线观看影片大全网站| 精品一区二区免费观看| 国产高清有码在线观看视频| 99热全是精品| 天堂av国产一区二区熟女人妻| 日本黄色视频三级网站网址| 日韩欧美一区二区三区在线观看| 69av精品久久久久久| 国产v大片淫在线免费观看| 亚洲,欧美,日韩| 自拍偷自拍亚洲精品老妇| 欧美性感艳星| 午夜激情欧美在线| 成人精品一区二区免费| 午夜福利高清视频| 国产精品三级大全| 国产欧美日韩一区二区精品| 黄色欧美视频在线观看| 国产视频一区二区在线看| 亚洲欧美成人综合另类久久久 | 国产欧美日韩精品亚洲av| 搡老熟女国产l中国老女人| 1000部很黄的大片| 我的女老师完整版在线观看| 日韩高清综合在线| 别揉我奶头 嗯啊视频| 亚洲经典国产精华液单| 3wmmmm亚洲av在线观看| 亚洲av一区综合| 蜜桃亚洲精品一区二区三区| 亚洲性久久影院| 亚洲精品日韩av片在线观看| 国产一区二区在线av高清观看| 一本久久中文字幕| 啦啦啦韩国在线观看视频| 欧美日本亚洲视频在线播放| 亚洲无线在线观看| 久久精品夜色国产| 亚洲国产日韩欧美精品在线观看| 国内揄拍国产精品人妻在线| 久久久成人免费电影| 一边摸一边抽搐一进一小说| 国产在视频线在精品| 免费无遮挡裸体视频| 久久99热这里只有精品18| 在线观看午夜福利视频| 性欧美人与动物交配| 国产精品乱码一区二三区的特点| 久久精品久久久久久噜噜老黄 | 成年版毛片免费区| 精华霜和精华液先用哪个| 日日摸夜夜添夜夜爱| 成年女人毛片免费观看观看9| 白带黄色成豆腐渣| 日本色播在线视频| 男女视频在线观看网站免费| 精品人妻一区二区三区麻豆 | 久久精品久久久久久噜噜老黄 | 免费无遮挡裸体视频| www.色视频.com| 99久国产av精品国产电影| 亚洲丝袜综合中文字幕| av中文乱码字幕在线| 欧美性猛交╳xxx乱大交人| 色视频www国产| 99久久九九国产精品国产免费| 日日撸夜夜添| 99久国产av精品国产电影| 人妻丰满熟妇av一区二区三区| 日本黄大片高清| 麻豆国产97在线/欧美| 久久精品夜色国产| 日韩成人伦理影院| 亚洲精品一区av在线观看| 99九九线精品视频在线观看视频| 美女高潮的动态| 日韩,欧美,国产一区二区三区 | 毛片一级片免费看久久久久| 一级毛片我不卡| 女人被狂操c到高潮| 久久久国产成人精品二区| 久久精品国产亚洲av涩爱 | 国产免费一级a男人的天堂| 亚洲一级一片aⅴ在线观看| 午夜精品在线福利| 真实男女啪啪啪动态图| 日本黄大片高清| 九九热线精品视视频播放| 久久久久性生活片| 久久久久久久久中文| 2021天堂中文幕一二区在线观| 成人午夜高清在线视频| 久久久久精品国产欧美久久久| 精品熟女少妇av免费看| 色综合站精品国产| 成人二区视频| 久久精品影院6| 精品一区二区三区视频在线| 在线国产一区二区在线| 午夜a级毛片| 日韩亚洲欧美综合| 亚洲中文日韩欧美视频| 成人三级黄色视频| 在线观看午夜福利视频| 国产av麻豆久久久久久久| 亚洲欧美日韩无卡精品| 午夜免费激情av| 真人做人爱边吃奶动态| 亚洲成人精品中文字幕电影| 亚洲aⅴ乱码一区二区在线播放| 久久久久久久亚洲中文字幕| 日日啪夜夜撸| 免费人成视频x8x8入口观看| 欧洲精品卡2卡3卡4卡5卡区| 我的老师免费观看完整版| 日韩成人av中文字幕在线观看 | 久久久成人免费电影| 亚洲国产色片| 亚洲国产精品合色在线| 天堂√8在线中文| 欧美中文日本在线观看视频| 在线天堂最新版资源| 欧美性猛交╳xxx乱大交人| 尤物成人国产欧美一区二区三区| 亚洲精品亚洲一区二区| 亚洲高清免费不卡视频| 欧美高清成人免费视频www| 日本 av在线| 麻豆成人午夜福利视频| 天美传媒精品一区二区| 国产单亲对白刺激| 免费观看在线日韩| 欧美日韩乱码在线| 久久久精品大字幕| 一级a爱片免费观看的视频| 男插女下体视频免费在线播放| 亚洲精华国产精华液的使用体验 | 国产三级中文精品| 久久6这里有精品| 大又大粗又爽又黄少妇毛片口| 最好的美女福利视频网| 亚洲va在线va天堂va国产| 亚洲精华国产精华液的使用体验 | 看非洲黑人一级黄片| 毛片女人毛片| 特级一级黄色大片| 日韩强制内射视频| 最近最新中文字幕大全电影3| 男人舔奶头视频| 18禁在线无遮挡免费观看视频 | 精品午夜福利在线看| 免费看日本二区| 一区二区三区高清视频在线| 色播亚洲综合网| 欧美一级a爱片免费观看看| 日本色播在线视频| 老司机影院成人| 婷婷亚洲欧美| 国产精品三级大全| 中文字幕人妻熟人妻熟丝袜美| 一区二区三区高清视频在线| 国产精品久久久久久av不卡| 国产激情偷乱视频一区二区| 久久99热这里只有精品18| 久久精品夜色国产| 熟女电影av网| 69人妻影院| 午夜视频国产福利| 菩萨蛮人人尽说江南好唐韦庄 | 日日摸夜夜添夜夜添小说| 久久精品国产自在天天线| 亚洲成人久久爱视频| 精品福利观看| 中国国产av一级| 久久久精品大字幕| 久久精品国产自在天天线| 日韩 亚洲 欧美在线| 亚洲成人av在线免费| or卡值多少钱| 丝袜喷水一区| 成人三级黄色视频| 熟女人妻精品中文字幕| 三级男女做爰猛烈吃奶摸视频| av在线天堂中文字幕| 欧美一级a爱片免费观看看| 国产高清激情床上av| 成年免费大片在线观看| 日韩欧美 国产精品| 亚洲精品影视一区二区三区av| 赤兔流量卡办理| 日韩三级伦理在线观看| 晚上一个人看的免费电影| 悠悠久久av| 亚洲精品久久国产高清桃花| 搞女人的毛片| 国产精品日韩av在线免费观看| 国产精品永久免费网站| 国产午夜精品久久久久久一区二区三区 | a级毛片a级免费在线| 日本三级黄在线观看| 国产三级在线视频| 能在线免费观看的黄片| 国产精品久久久久久久电影| 91久久精品国产一区二区成人| 日韩在线高清观看一区二区三区| 国产男人的电影天堂91| 久久精品国产鲁丝片午夜精品| 久久精品国产自在天天线| 成年免费大片在线观看| 欧美+亚洲+日韩+国产| 一本一本综合久久| 小说图片视频综合网站| 夜夜爽天天搞| 国产精品乱码一区二三区的特点| 亚洲精品国产成人久久av| 精品久久久噜噜| 亚洲av美国av| 观看免费一级毛片| 久久久久久伊人网av| 欧美极品一区二区三区四区| 国产一区二区三区av在线 | 久久九九热精品免费| av福利片在线观看| 成人午夜高清在线视频| 久久久久久久久久成人| 激情 狠狠 欧美| 欧美最黄视频在线播放免费| 色吧在线观看| 国产乱人偷精品视频| avwww免费| 成熟少妇高潮喷水视频| 欧美日本亚洲视频在线播放| 热99re8久久精品国产| 寂寞人妻少妇视频99o| 超碰av人人做人人爽久久| 成人精品一区二区免费| 国产黄a三级三级三级人| 亚洲欧美日韩高清在线视频| 国产成人91sexporn| 亚洲精品456在线播放app| 亚洲乱码一区二区免费版| 99热6这里只有精品| 又粗又爽又猛毛片免费看| 国产真实乱freesex| 国产女主播在线喷水免费视频网站 | 色在线成人网| 在线看三级毛片| 久久久久久九九精品二区国产| 国产精品嫩草影院av在线观看| 亚洲专区国产一区二区| 免费人成视频x8x8入口观看| 99久久九九国产精品国产免费| 欧美激情在线99| 婷婷亚洲欧美| 亚洲av熟女| 色综合色国产| 69人妻影院| 搞女人的毛片| 欧美一区二区亚洲| 中文资源天堂在线| 欧美精品国产亚洲| 精品99又大又爽又粗少妇毛片| 亚洲中文字幕日韩| 香蕉av资源在线| 免费观看的影片在线观看| 欧美高清成人免费视频www| 99久久久亚洲精品蜜臀av| 欧美一区二区亚洲| 搡老熟女国产l中国老女人| 久久久久久久午夜电影| 国内精品久久久久精免费| 国产一区二区在线av高清观看| 欧美最黄视频在线播放免费| 精品久久久久久久久av| 国产激情偷乱视频一区二区| 久久久精品94久久精品| 麻豆一二三区av精品| 女生性感内裤真人,穿戴方法视频| 亚洲人成网站高清观看| 亚洲av免费在线观看| 国内精品久久久久精免费| 长腿黑丝高跟| 成人精品一区二区免费| 97在线视频观看| 欧美日本视频| 欧美日韩乱码在线| 少妇高潮的动态图| 国产精品久久视频播放| 国产精品亚洲一级av第二区| 亚洲va在线va天堂va国产| 亚洲电影在线观看av| 午夜精品在线福利| 麻豆久久精品国产亚洲av| 亚洲成人久久爱视频|