• <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 ?

    TiO2-Supported Single-Atom Catalysts for Photocatalytic Reactions

    2021-07-13 09:56:26XuemeiZhou
    物理化學(xué)學(xué)報 2021年6期

    Xuemei Zhou

    School of Chemical Engineering, Sichuan University, Chengdu 610065, China.

    Abstract: Titania (TiO2) has been among the most widely investigated and used metal oxides over the past years, as it has various functional applications. Extensive research into TiO2 and industrial interest in this material have been triggered by its high abundance, excellent corrosion resistance, and low cost. To improve the activity of TiO2 in heterogeneous catalytic reactions, noble metals are used to accelerate the reactions.However, in the case of nanoparticles supported on TiO2, the active sites are usually limited to the peripheral sites of the noble metal particles or at the interface between the particle and the support. Thus, highly dispersed single metal atoms are desired for the effective utilization of precious noble metals. The study of oxide-supported isolated atoms, the so-called single-atom catalysts (SACs), was pioneered by Zhang’s group.The high dispersion of precious noble metals results helps reduce the cost associated with catalyst preparation. Because of the presence of active centers as single atoms, the deactivation of metal atoms during the reaction, e.g., by coking for large agglomerates, is retarded. The unique coordination environment of the noble metal center provides special sites for the reaction, consequently increasing the selectivity of the reaction, including the enantioselectivity and stereoselectivity. Hence, supported SACs can bridge homogenous and heterogeneous reactions in solution as they provide selective reaction sites and are recyclable. Moreover, owing to the high site homogeneity of the isolated metal atoms, SACs are ideal models for establishing the structure-activity relationships. The present review provides an overview of recent works on the synthesis, characterization, and photocatalytic applications of SACs (Pt1, Pd1,Ir1, Rh1, Cu1, Ru1) supported on TiO2. The preparation of single atoms on TiO2 includes the creation of surface defective sites, surface modification, stabilization by high-temperature shockwave treatment, and metal-ligand self-assembly.Conventional characterization methods are categorized as microscopic imaging and spectroscopic methods, such as aberration-corrected scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), extended X-ray absorption fine structure analysis (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).We attempted to address the critical factors that lead to the stabilization of single-metal atoms on TiO2, and elucidate the mechanism underlying the photocatalytic hydrogen evolution and CO2 reduction. Although many fascinating applications of TiO2-supported SACs in photocatalysis could only be addressed superficially and in a referencing manner, we hope to provide interested readers with guidelines based on the wide literature, and more specifically, to provide a comprehensive overview of TiO2-supported SACs.

    Key Words: Titanium dioxide; Single atom catalyst; Stabilization; Characterization; Photocatalytic H2 evolution

    1 Introduction

    Titanium dioxide (TiO2) is among the most widely investigated metal oxides over the past years based on the various functional applications1, such as pigments in paints,dye-sensitized solar cells2,3, photocatalysts4,5, self-cleaning coatings6, biomedicine7,8, ion-insertion batteries9,10, and electrochromic devices11,12. The extensive research and industrial interest on titania is promoted by its high abundance,high resistance against corrosion, and low-cost13–16. The photocatalysis of titania is determined by its unique semiconductive properties. It shows a band-gap of 3.0 eV for titania rutile and 3.2 eV for titania anatase1, that suggests the electron at the valence band of titania can be excited to the conduction band by ultraviolet (UV) light (λ< 413 nm for rutile,andλ< 388 nm for anatase), leaving a hole at the valence band.The photogenerated electrons can diffuse in micrometer length range in anatase, enabling the photocatalytic reaction to occur in a short distance. The relative band-edge positions, valence band maximum (VBM) and conduction band minimum (CBM),determine the occurrence of a photocatalytic redox reaction,exploiting electrons in the CBM or holes in the VBM.

    A considerable amount of work has been focused on modifying the band gap and band-edge positions of titania to trigger a broad range of redox reactions and to harvest the full spectrum of solar light. One way to regulate the band-edge positions of titania is to form a junction with a secondary semiconductor17,18, such as a Z-scheme19–24or an S-scheme25junction. An alternative approach is to decorate TiO2with noble metal particles26, as a co-catalyst to accelerate the separation of photogenerated electron and holes27. Specific noble metals, e.g.,Ag or Au, work as plasmonic centers to generate hot electrons28.

    Upon the attachment of a nanoparticle on a support, the active sites are usually limited on the peripheral sites of noble metal particles or at the interface between the particle and the support29–31. The highly dispersed single metal atoms are thus desired to enhance the utilization efficiency of precious noble metals. The study of oxide-supported isolated atoms, so called single atom catalysts (SACs), has been pioneered in 2011 by Zhang’s group32, who has developed single atom dispersion of Pt on FeOxby a tuned co-precipitation method. Fig. 1 summarizes the advantages of supported single metal atoms in heterogenous reactions.

    Fig. 1 A summary of advantages using TiO2-supported single atoms in heterogenous catalysis.

    The high dispersion of precious noble metals results in a low cost upon catalyst preparation. Due to the presence of active centers as single atoms, the deactivation of metal atoms during the reaction, e.g., coking, is slowed down. The unique coordination surrounding the noble metal center provides a special reaction site for the reaction, which consequently increases the selectivity of the reaction, such as enantioselectivity and stereoselectivity33. The supported SACs are hence supposed to bridge the homogenous reactions in solution and heterogeneous reactions, by providing a selective reaction site and becoming recyclable. Moreover, owing the high site homogeneity of the isolated metal atoms, the SACs are ideal models to establish the structure-activity relationship. For example, for Pt SACs on nitrogen-doped graphene support, the partially unoccupied density of states of the Pt atoms’ 5dorbitals are responsible for the excellent performance in the electrochemical hydrogen evolution reaction34.

    Though excellent reviews and perspectives have discussed the history, current state and future of single atom catalysts in preparation and industrial applications35–38, in this mini review,the author tries to summarize the current progress on the titania supported single atoms and the representative applications for photocatalytic energy production. Other configurations of the single metal atoms are beyond the scope of this mini-review,such as SACs embedded in metal-organic frameworks39–42, in single atom alloys43–45, or supported on zeolites46,47, on other metal oxides32,48–51.

    The employment of an economic metal oxide support, TiO2,and highly dispersed noble metals, becomes a promising approach to design low-cost and efficient photocatalysts, that are feasible to be scaled up. The demands from fossil energy to sustainable clean energy, is promotional to the investigation of photocatalysts, that convert the solar light to chemical energy,e.g., the fuels for the future, hydrogen.

    2 Evaluation of the character of TiO2-supported single metal atoms

    In this section, the characterization techniques for titaniasupported single atoms will be assessed, to give a brief overview on the evaluation of the single atoms. Two approaches are included here, microscopic techniques to visualize the metal atoms and spectroscopic techniques to reveal the metal surrounding bond information. Table 1 gives an overview on these techniques and the key experimental conditions.

    Table 1 Techniques used to characterize the single atoms on TiO2 surfaces and respective experimental conditions.

    2.1 Microscopic techniques

    Aberration-corrected scanning transmission electron microscopy (STEM)52,53, that can minimize noise issues, has been the breakthrough in recent years to acquire images of heavy metal atoms on titania supports. To obtain atomic resolution images, the contrast between the supported heavy metals and the titania support should be sufficient. Noble metals, Pt54,55, Rh56,Ir57, Au58, and Pd59can be clearly resolved on titania (Fig. 2).In addition, elemental mapping enables to reveal the dispersion profile of the supported metal. Electron energy loss spectroscopy(EELS) offers information on the chemical oxidation states.

    Fig. 2 Representative high-angle annular dark-field (HAADF)STEM images of TiO2-supported Rh catalyst reduced at (a)100 °C and (b) 300 °C.

    For SACs on a flat surface, scanning tunneling microscopy(STM) is a frequently used technique. When a fine tip is rastered across the surface, species as small as atoms can be imaged.Therefore, STM offers a chance in the characterization of adsorbates and single metal atoms on single crystals43,44,60–63.For example, for the Pt1Cu alloy catalyst, STM reveals that Pt atoms exist as individual, and isolated species on the Cu surface at the substituted sites of Cu, with a low Pt coverage (0.02 monolayer)44.

    2.2 Spectroscopic techniques

    X-ray absorption spectroscopy (XAS)32,64, operated at a synchrotron beamline, is a powerful tool to characterize thestructural information of the catalyst. XAS simultaneously acquires the extended X-ray adsorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure spectroscopy (XANES). The electrons, ejected from the metal atoms, immediately interact with the atoms neighboring the absorbing atoms and get scattered. EXAFS region reveals the neighboring bond information of the absorbing metal atoms, that contains the coordination numbers and distances between atoms after Fourier transform (FT) of the spectrum. Besides conventional XANES, high-energy resolution fluorescence detection XANES (HERFD-XANES)64is a highly sensitive technique to investigate the surroundings of metal atoms on the support. It is determined with high-energy fluorescence detection and allows the resolution of spectral features that can be directly assigned to specific coordination of ligands to a metal atom.

    The CO molecule is sensitive to the changes in metal oxidation states and the local environment upon chemical adsorption54,56,59,65,66. Provided with featured vibrational frequency, CO is an excellent probe to check the nature of the catalyst, for example, to distinguish isolated, coordinativelyunsaturated metal single-atoms from larger clusters (Fig. 3)67–69.In the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), peaks with a vibrational frequency above 2100 cm?1are ascribed to the stretching band of linearly adsorbed CO on cationic Pt species (Pt coordinated with oxidizing ligands, oxidized Pt clusters, or isolated Pt species)65,67.The full-width at half maximum (FWHM) of CO stretching mode is an indication to discern the aggregation size of Pt. On TiO2, FWHM of CO band for homogeneously isolated Pt species is smaller than 15 cm?1(6–8 cm?1), by contrast, FWHM of CO band for metallic Pt particles is larger than 25 cm?1. This measure should be carefully applied to other oxide supports. For instance, the FWHM of CO band for isolated Pt on CeO2or FeOx(> 25 cm?1) is reported to be larger than that on titania29,32,70. In another work, NO is used as a probe molecule to verify the single metal species71. Solid-state magic-angle spinning NMR can provide bond information about the nature of the metal species as well. In a recent work, it reveals the information surrounding Al sites for an Al2O3-supported single Pt catalyst72. Nonetheless,such technique requires a high loading of supported metal that is sufficient to provide reliable spectroscopic information.

    3 Stabilization of the single metal atoms on TiO2

    In this section, we will introduce the methods to stabilize single metal atoms on the surface of titania, including anatase,rutile, TiO2(B), and their defective surfaces (see an overview in Fig. 4). The key factors to control the synthesis procedure will be discussed as well. Due to the high thermodynamic energy of isolated single atoms on titania, these dispersed atoms intend to move and aggregate during synthesis, also catalysis. One of the challenges, upon synthesis, is to stabilize these energetic single metal atoms on the surface of titania. The approaches include surface defective sites, surface modification, high temperature shockwave, metal-ligand self-assembly and others.

    3.1 Surface defects

    Defect engineering has been a successful technique to tune the optical and electric properties of titanium dioxide in devices and catalysis73,74. Recent work by Li’s group57,58has utilized the function of surface defects to stabilize noble metal atom (Au and Ir) on the oxygen vacancy (Ov) sites of titania. These vacancies are created by annealing titania anatase nanosheets in H2/Ar at 200 °C for 2 h. This annealing condition leads to the highest amount of Ti3+sites58. The Au atoms are deposited on the surface from HAuCl4precursor by deposition-precipitation method, with a loading of 0.3% (w). The loading of single metal atoms on the oxide surface is reported to be below 1.5% (w) to reach a homogeneous dispersion of metal atoms. The Au atoms are located at the oxygen vacancy sites and bond with Ti after post-calcination at 200 °C in Ar/H258. EXAFS spectrum shows the Au-Ti bond with a mean bond length of 2.79 ? (1 ? = 0.1 nm) and a coordination number of 1.8. This method can be extended to a TiO2(B) support57. A metal-support interaction, Ir-Ti bond, can be regulated, by the impregnation of H2IrCl6on defective TiO2(B) and the post-annealing. EXAFS spectrum shows the Ir is bonded with three surrounding Ti atoms and form an Ir1Ti3chemical bond (Fig. 5). Such coordination between metal atoms and the support Ti offers a minimized energy of supported isolated metal centers, thus establishes a firm structure for either isolated Au or Ir atoms.

    Fig. 3 Temperature-programmed desorption (TPD) of CO IR spectra on (a) single Pt, (b) metallic Pt and (c) oxidized Pt catalysts.

    Fig. 4 An overview of the preparation methods for TiO2-supported single metal atoms.

    Fig. 5 (a) FT at the Ir L3-edge of Ir powder, Ir on defective TiO2(B), Ir on C3N4 and IrO2. (b) Corresponding fitting of the EXAFS spectrum of Ir1/def-TiO2(B) at the R space. The inset of panel (b) shows the local structure of the Ir1Ti3 obtained from the DFT simulations.

    Except for the powder morphology of titania, recent work from Schmuki’s group has shown that, on a thin sputtered TiO2layer, an atomic-scale defect engineering approach is able to form and control traps for single atom platinum75. The density of single Pt atoms on the TiO2layer is precisely controlled by the degree of reduction treatment, that is by annealing the catalyst composite (Pt decorated TiO2) in H2/Ar at different temperatures. Such atomic-scale defective sites pin single Pt atoms from a dilute aqueous Pt solution. Using a concentration of 120 μmol·L?1of H2PtCl6precursor, the density of single Pt atoms is the highest provided annealing at 500 °C. The anchored single Pt is supposed to be bonded with surface oxygen, giving a Ptδ+oxidation state. In the work by Leeet al.76, the Cu atoms are loaded on TiO2anatase (101) by a modified wrap-bake-peel process, in which the tetraethyl orthosilicate is employed to confine the Cu atoms during synthesis and has been removed at the last step. These single Cu atoms were site-specifically distributed at the Ti vacancy sites76.

    It is worth noting that the formation path of oxygen vacancy varies on different titania crystal surfaces (anatase, rutile). The migration and diffuse of Ov on different titania crystal surfaces are impacted by the energy difference between the oxygen atoms neighboring the Ov, and oxygen at perfect sites. Therefore, the distribution profiles of Ov, which further determines the density of supported single metal atoms, should be considered on different surfaces. Additionally, the stability of these defects in the reaction environment is vital to be explored, particularly under an oxygen-containing reaction condition. Under the oxidative reaction atmosphere, the consumption of Ov may lead to the reconstruction of the titania surface. The defect engineering is applicable to other reducible metal oxide, like CeO2, FeO3, V2O5, or metal sulfide.

    3.2 Surface modification

    Here, external surface modifications are discussed (defect engineering, in this review, is considered to be an internal modification), which include introducing binding sites, e.g.,nitrogen77, changing the surface electrostatics56,65, or ion exchange55.

    The strong electrostatic adsorption (SEA)78is among the impregnation method to prepare highly dispersed metals on silica. Various cationic metal ammine complexes are used as precursors, e.g., [Pd(NH3)4]+2, [Cu(NH3)4]+2, [Co(NH3)6]+3,[Ru(NH3)6]+2, [Ru(NH3)6]+3and [Ni(NH3)6]+2, that interact strongly with a negatively-charged surface78. Christopher’s group has reported the preparation of single noble metal atom per titania particle (~5 nm diameter), employing the SEA interactions (Fig. 6)65,67. The weight loading of Pt is tuned between 0.025% (w) to 0.15% (w). The large synthesis volume is beneficial for the homogeneous deposition of Pt on the titania.The pH of titania suspension, before adding of Pt (II) complexes,is regulated above 8 by adding NH4OH solution. The catalysts were then calcined in air to ensure the removal of remaining ammine ligands79.

    Fig. 6 Illustration of isolated Pt on TiO2 surface by the strong electrostatic adsorption (SEA), and a linearly adsorbed CO molecule on an isolated Pt atom Pt, C, O and Ti atoms are shown in blue, gray, red and white, respectively.

    The advantage of such a preparation method is assuring a high uniformity of isolated metal atoms on the surface. The isolated Pt is substituting into a six-fold coordinated Ti atom position and shows a formal oxidation state of 4+. EXAFS demonstrates the coordination number (CN) of Pt to be six for the first shell Pt―O54.The total CN of Pt―O drops to 2 after mild reduction and shows a formal oxidation state of 2+. The mild reduction pulls Pt atom out from the lattice, giving an adsorbed PtO2structure that is sitting on top of the Ti cation column. For both above-mentioned structures, the isolated Pt is locked along the axis without lateral motion. However, a harsh reduction leads to an adsorbed PtOH structure that shows a formal oxidation state of 1+. These PtOH species are attached to the defective sites of titania (steps and terrace) and are mobile on the surface. In a modified SEA approach, the ammonium aqueous solution, or ammonium carbonate solution is applied to adjust the pH to 8–9. With the addition of metal chloride precursor, homogeneously dispersed Pt80or Rh56on the titania support is prepared.

    Alternatively, the spatial confinement on the titania surface is employed to stabilize the single metal atoms. The titania nanotubes with lamellar structures have been reported to trap cationic Pt in between the layers on the wall in the work by Liet al.55. In their experiments, through ion exchange with Na+ion,the cationic Pt complex is intercalated into interlayers of titanate sheets. The following calcination induces phase transformation from titanate to anatase TiO2, and the reduction treatment enhances the Pt-titania interaction. In this tubular layered Pt1δ+/TiO2, the isolated Pt is bond with surface oxygen, showing aδ+ oxidation state. During the reductive annealing treatment in H2/Ar, the hydrogen spillover effect generates surface defects on titania, forming a Pt―O―Ti3+structure81. Zhou’s group77has reported, in theory, a strategy to prepare Ru doped titania in the presence of nitrogen dopants. The nitrogen dopants promote the substitution of Ti by Ru1on anatase, which is interesting to be further explored experimentally.

    3.3 High-temperature shockwave

    Metal nanoparticles become oxidatively fragmented with oxygen at high temperature, e.g., platinum forms volatile PtO2at 1100 K. These Pt species in the gas phase are transported through the carrier gas phase and react with the support to form isolated cationic Pt on the surface82. In 2014, Datye and coworkers have reported the volatile mobile Pt from Pt particles is captured by PdO, forming bimetallic Pt-Pd nanoparticles. This is ascribed to the slowed Ostwald Ripening process of Pt during the high temperature annealing (650 °C) in air83. Such phenomena has been explored in the following work to synthesize ceriasupported isolated Pt with a high loading, named as hightemperature vapor-phase synthesis84–86. High temperature preparation promotes the bond formation between the Pt and CeO2with improved stability. But this method is not compatible with temperature-sensitive substrates.

    Recently Yao and coworkers87have reported the preparation of Pt single atom on TiO2by applying a high temperature shockwave. The TiO2support is deposited as a thin layer (2.5 nm) on the carbon nanofibers by atomic layer deposition due to the low thermal conductivity of titania. A periodic on-and-off high temperature (1500–2000 K) heating treatment is applied,with a feature of 50 ms on-state and 600 ms off-state (Fig. 7).The on-state activates the TiO2by creating Pt-surface defect bonds, and the off-state assures the stability of such structure.Theoretical calculation shows that, on defective titania, the energy barrier for a single Pt escaping from Pt6cluster is thermodynamically favorable (ΔG= ?1.43 eV). However, on perfect TiO2surfaces, an energy barrier of +2.14 eV needs to be overcome to disperse Pt6cluster into Pt5+ Pt1. Datye88has mentioned that the key process is the short heating, followed by a rapid quenching. The multiple cycles of pulse allow the full dispersion of atomic volatile Pt into the mobile lattice defects that are created at high temperature on-state88. This method has been generalized to prepare isolated Ru and Co on titania.

    Fig. 7 Transformation of metal nanoparticles into embedded SACs by shockwave (upper row), into large particles by slow heating (lower row).

    3.4 Metal-ligand coordination

    The application of external ligands, coordinating with the metal centers, has been realized in the metal-organic frameworks39,40,89–93. Here we will particularly focus on the use of metal oxide as a support to load single metal atoms. The inorganic ligands, like ―O ligand oxo94, and organic ligands(e.g., bipyridyl groups)50,95have been reported. In our group, the metal ligand self-assembly has been observed and evidenced on a flat surface96,97. For example, Pt coordinates with bipyridyl tetrazine and forms planar chain structure on Au(100) surfaces98.Our following work shows that the metal-ligand coordination takes place on titania surface99, where the Pt is bond with surface oxygen and organic ligand bipyridyl tetrazine. Alternatively, the organometallic precursors react directly with functional groups(e.g., ―OH groups) on the metal oxide surface and form metal―O bond. Gateset al. have reported the loading of M(C2H4)2(acac) (M, Rh, Ir; acac, acetylacetonato) on titania surface100. For inorganic ligand, Flytzani-Stephanopoulos’s group has reported inorganometallic isolated gold complex on TiO2surface in alkaline solution94. The Au species are stabilized on the surface by forming a Au1―Ox―Na9―(OH)ycluster. For the metal-ligand approach, a strong coordination between metal and ligand is crucial to the stabilization.

    3.5 Other approaches

    Atomic layer deposition (ALD)101,102has attracted increasing attention and has been reported to conformally prepare single atoms on the surface. Lei’s group has shown that Pd1is decorated in the nanocavity of TiO2by ALD101. The precursor Pd(hfac)2(Pd hexafluoroacetylacetonate) is chemically adsorbed,following with the selective deposition of TiO2by ALD on the substrate, not on the Pd(hfac)2sites. The ligand hfac templates the formation of the nanocavity and isolates the Pd1sites. After annealing, the Pd1/TiO2shows a strong metal support interaction(SMSI) to stabilize the Pd atoms.

    In another work, the photochemical reduction of frozen chloroplatinic acid solution by UV light is reported to generate single Pt atoms. This single Pt atom solution is then anchored on a substrate like TiO2, porous carbonetc103. On a porous substate,a high loading of the supported metal is reported, e.g., using a mesoporous titania allows a loading of Cu up to 12.33%. The Cu exists as mononuclear Cu (I) at low loadings and as CuO at high loadings104.

    4 Use of TiO2-supported SACs in photocatalysis

    Excellent reviews have summarized the applications of SACs in the heterogeneous catalysis105–108, electrochemical catalysis35,and photocatalysis109–111. Here we will focus on the application of titania-supported single atoms for photocatalysis and try to address the underlying mechanism for the improved photoactivity112. Different from nanoparticles, single atoms have unique electronic properties, e.g., the lack of the plasmonic effect under illumination36. The discussion will emphasize the photocatalytic H2production and cover the other photocatalytic reactions, like CO2reduction. These reactions are important applications for titania-based catalysts, that aim on solving the environmental and energy issues.

    4.1 Photocatalytic H2 evolution

    High purity hydrogen is used in various industrial applications, e.g., the fuel cell. H2evolution by photocatalysis is the most low-cost method to produce green hydrogen113, green hydrogen is the hydrogen generated in a green chemical process.Abundant metal oxides with tunable bandgap are ideal models to be employed to fabricate the catalysts114–116. Considering the promotional effect of noble metals to the reaction rate, the metal oxides are usually decorated with noble metals to reach a plausible hydrogen generation rate.

    Due to the high dispersion of single atoms, the atom utilization efficiency of the noble metals is generally higher than the nanoparticles. Additionally, the single Pt atom is functionalized as a H atom recombination center, for the production of H2molecules75. Coupling with a surface defect state of titania, that serves as an electron transfer mediator74,117,the single Pt decorated titania shows the superior effectiveness on the activity, compared to the Pt nanoparticles (Fig. 8a). These isolated Pt species are anchored firmly on the surface of defective titania, by showing a high retention of the activity stability over the reaction time75.

    Fig. 8 (a) An illustration of electron transfer from TiO2 to single Pt,mediated by the defective states. (b) The hydrogen evolution rate of single Pt compared to supported Pt NPs.

    Alternatively, Chenet al.81have reported that a Pt-O-Ti3+active center is responsible for the high hydrogen generation rate. During the photocatalytic reaction, the photogenerated electrons from Ti3+efficiently transfer to single Pt atoms through the Pt―O―Ti3+active centers. The electron-hole recombination rate is thus suppressed, and the overall charge transfer efficiency is enhanced. The atomic utilization efficiency of Pt, in this work,is reported to be ~591 fold higher than that for the supported Pt nanoparticles (Fig. 8b)81. The DFT calculation further pointed out that substituted Pt on a six-coordinated Ti site is the active Pt for the hydrogen production reaction due to the capability to dissociate H2O and form adsorbed H*81. By contrast, due to the formation of a depletion layer between TiO2and Pt NPs, the electrons transfer directly from the TiO2to the Pt sites and react with H* to form H2molecules. It has been reported in the hydrogen evolution reaction, the activation battier for the H*atom adsorption on Pt SACs is small than that on the Pt[111]34.

    Moreover, the facet of titania impacts the activity of SACs in the photocatalytic hydrogen production. Weiet al.80have reported that the Pt single atoms on TiO2[001] shows 3 times higher activity in the H2evolution rate than that of Pt decorated on TiO2[101] facet. The improved activity is ascribed to the higher CBM and higher surface energy of TiO2[001], compared to TiO2[101] surface. On anatase [001] surface, Basset’s group95has reported the single Pt atoms coordinate with organic ligand (1,5-cyclooctadiene), forming a dimethylplatinum(II)[(CH3)2Pt(COD)] structure. In this configuration, the single Pt is significantly more active in the photocatalytic hydrogen evolution, compared to the conventional Pt particles. They attribute the enhanced reaction rate for single Pt to the strong suppression of the backwards reaction of H2and O2under dark conditions95.

    In addition to the Pt single atom, titania-supported single Cu has shown to be active for the photocatalytic hydrogen generation76,104. The substitution of Pt by Cu atoms allows for the preparation of a low-cost, and eco-friendly photocatalyst.The catalyst experiences a different reaction path during the photocatalysis, in comparison to Pt. The single Cu atom needs to be activated by light and can be deactivated after the reaction by oxidation (Fig. 9)76.

    Fig. 9 Photoactivation cycle of Cu1/TiO2.

    The as-prepared Cu1/TiO2catalyst is regarded as a non-active catalyst at the resting state (CT0)76, which gets photoactivated and generates electrons and holes (CT1). The electrons transfer from the CBM of TiO2to thedorbitals of the single Cu atoms(CT2), which results in a polarization field between the TiO2and the Cu atom. That is, around the single Cu atoms, a local lattice distortion occurs on titania (CT3). Such a state is completely different from the original resting state (CT1) and becomes significantly active for the hydrogen generation. The theoretical calculation shows that at a Ti vacancy site, a Cu atom provides mid-gap states with Cudx2?y2anddz2character. At state CT3, the electron is localized into the Cudz2anti-bonding state, that leads to the lattice distortion by elongating the backside oxygen coordination. As a result, the hydrogen evolution rate is enhanced by H2O reacting at the electron localized Cu sites.

    However, Zhouet al.77have reported that it is difficult to accurately determine the single metal atom induced localized gap state in TiO2. They employed co-dopants, nitrogen, and Ru,to calculate the electronic structure and water splitting activity.It is suggested that the pre-doping with nitrogen stabilizes the thermodynamic and kinetic solubility of single Ru atoms in anatase TiO2. The chemical energy of H bonding changes and the light adsorption edge redshifts to the visible light range. This theoretical finding is interesting to be further explored experimentally using a photoelectrochemical or photocatalytic catalyst77.

    4.2 Photocatalytic CO2 reduction

    Reduction of CO2into value-added fuels by solar light-driven catalysis is supposed to be an economic approach118. Not only the products are valuable, e.g., methane, methanol, or long-chain hydrocarbons, also the vast amount of greenhouse gas CO2is consumed. Although p-type semiconductors, e.g., ZnTe, Cu2O,and GaP, are first studied and used in CO2photoreduction due to their relatively negative CBM, they often suffer photocorrosion problems. Therefore titania, as a highly stable, low-cost, and safe semiconductor, is currently widely investigated to reduce CO2when suitable modifications. In a photoelectrochemical configuration, the titania, modified with a p-type semiconductor or with dopants, has been applied as a photocathode119–125. The oxygen evolution from H2O by reacting with holes occurs on the counter electrode. Various single metal atoms have been applied,e.g., Co1126, Er1127, Ir189, Ru121,128, Cu1129, decorated on metal oxide, organic semiconductor or in a metal-organic-framework structure. The single atoms are usually immobilized in a metalligand complex, for example, [Ru(byp)3]2+21. Titania-supported single Ir appears to be active for the selectively thermal reduction of CO2to CO with IrO2active sites. The single Ir active site effectively blocks the over-reduction of CO2to methane128.

    Liet al.129have reported that the defective titania supported Cu1species are active for the CO2photoreduction to CO under open circuit conditions. The presence of surface defects (Ti3+)provides active sites for the adsorption of the oxygen atoms from CO2molecules. Photogenerated electrons transfer from bulk TiO2to surface Ti3+sites and migrate to theπorbitals of adsorbed CO2. The surface adsorbed H2O also participates in the cycle as an electron donor and form HCO3?129.

    4.3 Other reactions

    One of the important applications of photogenerated holes is to oxidize methane to produce fuels, e.g., methanol130,131. The use of single atoms for the methane oxidation has been studied in heterogeneous thermal catalysis132,133, but has not been fully studied in photocatalysis. However, the photocatalytic reactions are worthy to be developed due to the high selectivity and utilization of single atoms. Pratsiniset al. have reported the removal of NOxusing titania-supported isolated Pd by photocatalysis71. The isolated Pd is resistant to nitrate poisoning and highly selective to the conversion of NO to nitrate.

    5 Concluding remarks

    While in the present overview many aspects of the fascinating application in photocatalysis for TiO2-supported SACs could only be addressed on the surface and in a referencing manner,we hope to provide interested readers with a guideline through the wide literature to the topic and, more specifically, to give a comprehensive overview of titania-supported SACs. However,several following issues need to be addressed on the application of single atoms (Fig. 10).

    Fig. 10 Questions that are relevant to the TiO2-supported SACs in photocatalysis.

    First, the Schottky junction is used to describe the junction between macroscopic metal/titania junction or the small nanoparticle/titania junction, which does not apply when much smaller nanoparticles (few nanometers, clusters) are placed on highly doped titania. These single metal atoms are considered as dopants. However, it is difficult to accurately determining the localized gap state of a single metal atom. As mentioned above,Zhouet al.77propose to use co-dopants in TiO2powders when performing the density functional theory calculation. The later work by Donget al. has shown that the Au SACs couple on the TiO2[110] surface by interfacial electronic interactions134,applying a combined first-principle calculation with STM study,the Au SACs introduce intermediate gap states that are located at ?0.85 eV (Au SA at the Ti 5c site) and ?1.3 eV (Au SA at the Ov site), below the Fermi level. These states provide a channel for the transfer of a photogenerated hole under UV illumination.The hole weakens the Ti―Au bonding and activates the diffusion of Au SACs.

    Second, the kinetic analysis of supported single metal borrows the concepts and ideas from electrochemical theory. However,this may not work for a complicated photocatalytic system under open circuit conditions, e.g., with a Z-scheme, S-scheme junction, or with antenna dye molecules.

    Third, under reaction conditions, the stability of the single atoms needs to be further explored135. For example, the holes weaken the Au―Ti bond that destabilizes the single Au atoms,134resulting in aggregation95or leaching during the photocatalysis in solution. Current characterization techniques are under development to meet the demands forin situ/operando photocatalytic measurements.

    Last, for specific reactions, an active center as a dimer136, a trimer, or in a paired structure136,137may outperform the single metal center. For example, the neighboring noble metal facilitates the dissociation of hydrogen molecules in a hydrogenation reaction. It is worthy to consider whether a specific reaction benefits from the use of an isolated single metal center.

    Nevertheless, the high demand in the industry (sustainable chemical processes) of low-cost catalysts and robust supports is promotional to the development of research on SACs. In photocatalysis, there are several aspects that are exciting to be further investigated, e.g., the establishment of the theory, the methodology on the synthesis, and the exploit of single atoms on other solar light-driven reactions.

    Conflict of Interest:The author declares no competing interests.

    国产精品久久久久久精品电影| 久久97久久精品| 午夜日本视频在线| 人人妻人人看人人澡| 男人狂女人下面高潮的视频| 午夜亚洲福利在线播放| 亚洲伊人久久精品综合| av在线蜜桃| 特级一级黄色大片| 国产 一区 欧美 日韩| 日本熟妇午夜| av在线观看视频网站免费| 国产精品一区二区三区四区免费观看| av在线app专区| 又爽又黄无遮挡网站| 2021少妇久久久久久久久久久| 亚洲,一卡二卡三卡| 欧美性猛交╳xxx乱大交人| 2021天堂中文幕一二区在线观| 亚洲性久久影院| 国产欧美日韩一区二区三区在线 | 久久精品综合一区二区三区| 亚洲精品久久久久久婷婷小说| 一个人看视频在线观看www免费| 亚洲不卡免费看| 亚洲欧洲国产日韩| 久热这里只有精品99| 麻豆精品久久久久久蜜桃| 欧美xxxx黑人xx丫x性爽| 久久ye,这里只有精品| 精品久久久久久电影网| 国产精品国产三级专区第一集| 国产av码专区亚洲av| 国产又色又爽无遮挡免| 青春草视频在线免费观看| 成人二区视频| 亚洲综合精品二区| 在线观看美女被高潮喷水网站| 99久久精品热视频| 日日啪夜夜撸| 高清午夜精品一区二区三区| 久久亚洲国产成人精品v| 天堂网av新在线| 狂野欧美白嫩少妇大欣赏| 成人无遮挡网站| 亚洲国产欧美人成| 久久国产乱子免费精品| 成年女人看的毛片在线观看| 成人毛片60女人毛片免费| 欧美成人精品欧美一级黄| 日韩欧美精品免费久久| 久久久精品欧美日韩精品| 日本色播在线视频| 日本-黄色视频高清免费观看| 看黄色毛片网站| 一级av片app| 国内精品宾馆在线| 91久久精品国产一区二区三区| 日本午夜av视频| 久久精品人妻少妇| 一级毛片黄色毛片免费观看视频| 麻豆成人av视频| 一级爰片在线观看| 久久精品久久久久久久性| 久久久精品欧美日韩精品| 爱豆传媒免费全集在线观看| 日本午夜av视频| 国内少妇人妻偷人精品xxx网站| 色综合色国产| 97超碰精品成人国产| 又黄又爽又刺激的免费视频.| 免费看av在线观看网站| 男人和女人高潮做爰伦理| 久久97久久精品| 国产男女内射视频| 成人黄色视频免费在线看| 天天躁夜夜躁狠狠久久av| 国产精品一区二区在线观看99| 午夜视频国产福利| 日本三级黄在线观看| a级毛色黄片| 美女cb高潮喷水在线观看| 午夜亚洲福利在线播放| 免费不卡的大黄色大毛片视频在线观看| freevideosex欧美| 少妇高潮的动态图| 人体艺术视频欧美日本| 亚洲激情五月婷婷啪啪| 夫妻性生交免费视频一级片| 久久这里有精品视频免费| 香蕉精品网在线| 亚洲成人久久爱视频| 18禁在线无遮挡免费观看视频| 狂野欧美激情性xxxx在线观看| 精品午夜福利在线看| 亚洲欧美成人综合另类久久久| 国产精品国产三级专区第一集| 麻豆久久精品国产亚洲av| 久久鲁丝午夜福利片| 看黄色毛片网站| 秋霞伦理黄片| 国产精品不卡视频一区二区| 国产乱人偷精品视频| 狂野欧美白嫩少妇大欣赏| 国产老妇伦熟女老妇高清| 久久午夜福利片| 欧美xxxx黑人xx丫x性爽| 一二三四中文在线观看免费高清| 精品久久国产蜜桃| 久久久国产一区二区| 国产成年人精品一区二区| 日韩不卡一区二区三区视频在线| av网站免费在线观看视频| 精品一区二区三卡| 国产精品伦人一区二区| 在线免费观看不下载黄p国产| 亚洲av免费高清在线观看| 毛片一级片免费看久久久久| 国产免费又黄又爽又色| 国产成人精品久久久久久| 91久久精品电影网| 婷婷色麻豆天堂久久| 哪个播放器可以免费观看大片| 18禁裸乳无遮挡动漫免费视频 | 极品教师在线视频| 久久久久久久亚洲中文字幕| 久久久精品免费免费高清| 伦精品一区二区三区| 夫妻性生交免费视频一级片| av网站免费在线观看视频| eeuss影院久久| 日韩av免费高清视频| 校园人妻丝袜中文字幕| 日韩欧美精品免费久久| 欧美xxxx性猛交bbbb| 国产精品一二三区在线看| 成人亚洲精品av一区二区| a级一级毛片免费在线观看| 日韩一区二区视频免费看| 精品久久久噜噜| 人妻 亚洲 视频| 成人午夜精彩视频在线观看| 亚洲欧美成人精品一区二区| 色综合色国产| av网站免费在线观看视频| 三级国产精品欧美在线观看| 国内精品美女久久久久久| 国产高潮美女av| 免费av不卡在线播放| 亚洲人与动物交配视频| 男人狂女人下面高潮的视频| 精品99又大又爽又粗少妇毛片| 99热这里只有是精品50| 午夜免费观看性视频| 国产午夜精品一二区理论片| 亚洲在久久综合| 国产黄片美女视频| 插逼视频在线观看| 亚洲av欧美aⅴ国产| 亚洲av欧美aⅴ国产| 午夜日本视频在线| 午夜福利在线在线| 日本午夜av视频| 美女主播在线视频| 精品国产三级普通话版| 国产男人的电影天堂91| 青春草亚洲视频在线观看| 色视频在线一区二区三区| 91精品伊人久久大香线蕉| 大片免费播放器 马上看| 国产 精品1| 91精品国产九色| 中文资源天堂在线| 看免费成人av毛片| 国产老妇女一区| 制服丝袜香蕉在线| 2022亚洲国产成人精品| 国产午夜精品久久久久久一区二区三区| 亚洲熟女精品中文字幕| 亚洲欧美日韩无卡精品| 日韩中字成人| 成人国产av品久久久| 免费大片黄手机在线观看| 99久久九九国产精品国产免费| 国产探花极品一区二区| 九九在线视频观看精品| 亚洲精品aⅴ在线观看| 亚洲av中文av极速乱| 久久久久久久久大av| 一级毛片 在线播放| 国产视频首页在线观看| 少妇熟女欧美另类| 女人久久www免费人成看片| 欧美激情国产日韩精品一区| 免费av不卡在线播放| 欧美日韩视频精品一区| 美女主播在线视频| 日韩大片免费观看网站| 精品久久久久久久久av| 亚洲色图综合在线观看| 2021少妇久久久久久久久久久| 51国产日韩欧美| 丝瓜视频免费看黄片| 尤物成人国产欧美一区二区三区| 久久久a久久爽久久v久久| av网站免费在线观看视频| freevideosex欧美| 免费不卡的大黄色大毛片视频在线观看| 日本猛色少妇xxxxx猛交久久| 午夜亚洲福利在线播放| 日本一二三区视频观看| 建设人人有责人人尽责人人享有的 | av黄色大香蕉| 91精品一卡2卡3卡4卡| 一级毛片黄色毛片免费观看视频| 18禁裸乳无遮挡动漫免费视频 | 简卡轻食公司| 高清毛片免费看| 欧美人与善性xxx| 国内少妇人妻偷人精品xxx网站| av免费在线看不卡| 日本av手机在线免费观看| 亚洲精品成人av观看孕妇| 精品一区二区三卡| 九九在线视频观看精品| 亚洲av中文av极速乱| 国产精品国产三级国产av玫瑰| av在线天堂中文字幕| 婷婷色综合www| 最近最新中文字幕大全电影3| 伊人久久精品亚洲午夜| 国产精品久久久久久久电影| 深夜a级毛片| 日韩av免费高清视频| 男插女下体视频免费在线播放| 丝瓜视频免费看黄片| 人妻一区二区av| 国产永久视频网站| 日本wwww免费看| 校园人妻丝袜中文字幕| 国产精品99久久99久久久不卡 | a级一级毛片免费在线观看| www.av在线官网国产| 久久久久久九九精品二区国产| 精品一区二区免费观看| 午夜亚洲福利在线播放| 亚洲欧美中文字幕日韩二区| 精品亚洲乱码少妇综合久久| 看黄色毛片网站| 国产午夜福利久久久久久| 又大又黄又爽视频免费| 久久久精品94久久精品| 国语对白做爰xxxⅹ性视频网站| 久久久久性生活片| 国产大屁股一区二区在线视频| 亚洲国产精品999| 国产免费一级a男人的天堂| 丰满乱子伦码专区| 一本一本综合久久| 亚洲人成网站在线观看播放| 国产精品福利在线免费观看| 国产精品女同一区二区软件| 一本一本综合久久| 亚洲国产成人一精品久久久| 大陆偷拍与自拍| 亚洲天堂av无毛| 嫩草影院精品99| 午夜激情福利司机影院| 国产精品麻豆人妻色哟哟久久| 久久午夜福利片| 国产一级毛片在线| 亚洲精品一二三| 纵有疾风起免费观看全集完整版| 少妇人妻 视频| 久久影院123| 中文乱码字字幕精品一区二区三区| 欧美成人午夜免费资源| 91精品一卡2卡3卡4卡| 日本黄大片高清| 亚洲欧美成人综合另类久久久| 18禁在线播放成人免费| 下体分泌物呈黄色| 亚洲图色成人| 亚洲性久久影院| 男女啪啪激烈高潮av片| 免费观看性生交大片5| 国产一级毛片在线| 国产探花极品一区二区| 色吧在线观看| av女优亚洲男人天堂| 少妇的逼水好多| 只有这里有精品99| 91久久精品国产一区二区三区| 乱系列少妇在线播放| 久久精品国产亚洲av涩爱| 久久99精品国语久久久| 日韩强制内射视频| 成年人午夜在线观看视频| 久久精品国产a三级三级三级| 国产成人精品福利久久| 搡女人真爽免费视频火全软件| 大片免费播放器 马上看| 精品亚洲乱码少妇综合久久| 老司机影院毛片| 国产精品秋霞免费鲁丝片| 亚洲婷婷狠狠爱综合网| 丝瓜视频免费看黄片| 三级男女做爰猛烈吃奶摸视频| 搡女人真爽免费视频火全软件| 国精品久久久久久国模美| 久热久热在线精品观看| 国产探花极品一区二区| 黄片wwwwww| 色视频www国产| 午夜免费观看性视频| 国产女主播在线喷水免费视频网站| 国产成人精品婷婷| 在现免费观看毛片| 国产精品人妻久久久久久| 成人国产麻豆网| 欧美日韩国产mv在线观看视频 | 大片免费播放器 马上看| 91精品国产九色| 国产精品不卡视频一区二区| 国产免费一级a男人的天堂| 午夜激情久久久久久久| 成人无遮挡网站| 国产在线一区二区三区精| 欧美激情久久久久久爽电影| 神马国产精品三级电影在线观看| 亚洲国产精品专区欧美| 又大又黄又爽视频免费| 2022亚洲国产成人精品| 欧美bdsm另类| 在线观看美女被高潮喷水网站| 一级毛片我不卡| 亚洲在线观看片| 国产精品.久久久| 国内揄拍国产精品人妻在线| 久久6这里有精品| 晚上一个人看的免费电影| 国产欧美亚洲国产| 九草在线视频观看| av在线蜜桃| 97超视频在线观看视频| 美女内射精品一级片tv| 一级毛片我不卡| 亚洲国产精品成人久久小说| 久久精品国产a三级三级三级| 欧美三级亚洲精品| 一区二区av电影网| 免费观看性生交大片5| 免费看av在线观看网站| 看黄色毛片网站| 亚洲av中文av极速乱| 噜噜噜噜噜久久久久久91| 51国产日韩欧美| 18+在线观看网站| 国产免费一级a男人的天堂| 免费不卡的大黄色大毛片视频在线观看| 91在线精品国自产拍蜜月| 国产69精品久久久久777片| 日韩在线高清观看一区二区三区| 亚洲精品视频女| 欧美少妇被猛烈插入视频| 少妇 在线观看| 亚洲欧美成人精品一区二区| 神马国产精品三级电影在线观看| 大码成人一级视频| 麻豆成人午夜福利视频| 亚洲人成网站在线观看播放| 只有这里有精品99| 国产精品爽爽va在线观看网站| 国产精品99久久99久久久不卡 | 成人国产av品久久久| 麻豆国产97在线/欧美| 能在线免费看毛片的网站| 熟妇人妻不卡中文字幕| 白带黄色成豆腐渣| 日韩制服骚丝袜av| 免费观看无遮挡的男女| 美女视频免费永久观看网站| 91狼人影院| 国产一区二区在线观看日韩| 在线观看美女被高潮喷水网站| 亚洲精品成人av观看孕妇| 亚洲av中文字字幕乱码综合| 国产永久视频网站| 少妇的逼好多水| 日韩一区二区三区影片| 国产探花在线观看一区二区| 三级男女做爰猛烈吃奶摸视频| 在线观看一区二区三区| 欧美激情在线99| 精品亚洲乱码少妇综合久久| 天堂中文最新版在线下载 | 91精品伊人久久大香线蕉| 亚洲av在线观看美女高潮| 欧美最新免费一区二区三区| 国产成人免费观看mmmm| 日韩免费高清中文字幕av| 97在线人人人人妻| 人体艺术视频欧美日本| 亚洲国产欧美在线一区| 大香蕉97超碰在线| 99久久精品一区二区三区| 亚洲欧美一区二区三区黑人 | 日韩av在线免费看完整版不卡| 亚洲精品国产成人久久av| 亚洲精品456在线播放app| 亚洲最大成人中文| 一区二区av电影网| 三级国产精品片| 一区二区三区免费毛片| 晚上一个人看的免费电影| 网址你懂的国产日韩在线| 日韩大片免费观看网站| 简卡轻食公司| 亚洲无线观看免费| 成人国产麻豆网| 亚洲国产最新在线播放| 中文在线观看免费www的网站| 亚洲国产高清在线一区二区三| 亚洲综合色惰| 在线 av 中文字幕| 国产精品不卡视频一区二区| 国产精品无大码| 欧美性感艳星| 在线免费观看不下载黄p国产| 欧美人与善性xxx| 久久ye,这里只有精品| 最近最新中文字幕大全电影3| 男女边摸边吃奶| 日本爱情动作片www.在线观看| av黄色大香蕉| 人妻夜夜爽99麻豆av| 久久久久久久午夜电影| 国产v大片淫在线免费观看| 国产女主播在线喷水免费视频网站| 我要看日韩黄色一级片| 极品少妇高潮喷水抽搐| 在线观看av片永久免费下载| av在线天堂中文字幕| 国产在线一区二区三区精| 国产女主播在线喷水免费视频网站| 美女xxoo啪啪120秒动态图| 欧美成人精品欧美一级黄| 国产午夜福利久久久久久| 嫩草影院精品99| 精品久久久久久久人妻蜜臀av| 亚洲欧美日韩无卡精品| 一区二区三区四区激情视频| 精品久久久噜噜| 日韩人妻高清精品专区| 亚洲欧美日韩卡通动漫| 一级毛片aaaaaa免费看小| 黄片wwwwww| 麻豆成人午夜福利视频| 成人国产麻豆网| 三级国产精品欧美在线观看| 久久精品综合一区二区三区| 亚洲国产成人一精品久久久| 国产精品久久久久久久电影| 在线观看国产h片| 亚洲精品乱码久久久v下载方式| 永久免费av网站大全| 在线观看美女被高潮喷水网站| 国产成人精品一,二区| 日本与韩国留学比较| 久久久久九九精品影院| 免费看不卡的av| 亚洲一级一片aⅴ在线观看| 中文字幕制服av| 国产一区二区三区av在线| 国产免费一区二区三区四区乱码| 美女被艹到高潮喷水动态| 看非洲黑人一级黄片| 热99国产精品久久久久久7| 99久久九九国产精品国产免费| 日本猛色少妇xxxxx猛交久久| 中文字幕免费在线视频6| 日本-黄色视频高清免费观看| 亚洲精品第二区| 在线免费十八禁| 日韩国内少妇激情av| 国产成人精品婷婷| av线在线观看网站| 91午夜精品亚洲一区二区三区| 久久6这里有精品| 可以在线观看毛片的网站| 美女主播在线视频| 丝袜脚勾引网站| 免费av不卡在线播放| 听说在线观看完整版免费高清| 日本一本二区三区精品| av免费观看日本| 一级黄片播放器| 亚洲国产高清在线一区二区三| 免费大片18禁| 欧美97在线视频| 男女下面进入的视频免费午夜| 亚洲欧美一区二区三区国产| 国产男女超爽视频在线观看| 青青草视频在线视频观看| 中文字幕久久专区| 七月丁香在线播放| 九九爱精品视频在线观看| eeuss影院久久| 美女视频免费永久观看网站| 久久99蜜桃精品久久| 少妇熟女欧美另类| 国产女主播在线喷水免费视频网站| 欧美日韩国产mv在线观看视频 | 人妻 亚洲 视频| 国产av不卡久久| av黄色大香蕉| 亚洲欧洲国产日韩| 成年av动漫网址| 国产精品福利在线免费观看| 乱系列少妇在线播放| av在线蜜桃| 久久久色成人| 国产av码专区亚洲av| 欧美成人a在线观看| 九草在线视频观看| 视频中文字幕在线观看| 国产在线男女| 麻豆国产97在线/欧美| 久久久久久久亚洲中文字幕| 在线观看一区二区三区| 一级av片app| 国产极品天堂在线| 日韩 亚洲 欧美在线| 大话2 男鬼变身卡| videossex国产| 女人久久www免费人成看片| 视频中文字幕在线观看| 国产成人午夜福利电影在线观看| 国产亚洲91精品色在线| 国产人妻一区二区三区在| 十八禁网站网址无遮挡 | 亚洲av成人精品一区久久| 成人国产麻豆网| 别揉我奶头 嗯啊视频| 亚洲熟女精品中文字幕| 熟女人妻精品中文字幕| 久久精品人妻少妇| 最近的中文字幕免费完整| 交换朋友夫妻互换小说| 在线 av 中文字幕| 日本免费在线观看一区| 久久午夜福利片| 三级经典国产精品| 性插视频无遮挡在线免费观看| 亚洲在久久综合| 视频中文字幕在线观看| 欧美人与善性xxx| 欧美一区二区亚洲| 亚洲精华国产精华液的使用体验| 亚洲内射少妇av| 天堂网av新在线| 高清日韩中文字幕在线| 精品99又大又爽又粗少妇毛片| 亚洲av成人精品一二三区| 国产精品蜜桃在线观看| 国产精品无大码| 免费看av在线观看网站| 国产精品偷伦视频观看了| 黄色视频在线播放观看不卡| 国产成人精品久久久久久| 国产在视频线精品| 久久久久久久久久人人人人人人| 国产成人免费无遮挡视频| 国产欧美亚洲国产| av在线播放精品| 内射极品少妇av片p| 偷拍熟女少妇极品色| 国产精品不卡视频一区二区| 一级毛片电影观看| 亚洲av中文字字幕乱码综合| 高清av免费在线| 午夜精品一区二区三区免费看| 黑人高潮一二区| 亚洲在线观看片| 777米奇影视久久| 国产色爽女视频免费观看| 中国三级夫妇交换| 真实男女啪啪啪动态图| 免费av毛片视频| 中文字幕人妻熟人妻熟丝袜美| 亚洲国产精品专区欧美| 国产乱人偷精品视频| 国产精品秋霞免费鲁丝片| 亚洲精品乱码久久久v下载方式| 精品国产三级普通话版| 蜜臀久久99精品久久宅男| 日产精品乱码卡一卡2卡三| 免费在线观看成人毛片| 汤姆久久久久久久影院中文字幕| 人妻少妇偷人精品九色| 亚洲激情五月婷婷啪啪| 97精品久久久久久久久久精品| 男女边摸边吃奶| 少妇熟女欧美另类| 蜜臀久久99精品久久宅男| 建设人人有责人人尽责人人享有的 | 午夜激情久久久久久久| 成人一区二区视频在线观看| 国产成人精品婷婷| 午夜免费观看性视频| 一级毛片电影观看| 在线播放无遮挡| 午夜免费鲁丝| 国产毛片a区久久久久| 久久午夜福利片|