• <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 ?

    Understanding the Role of Cu/ZnO lnteraction in CO2 Hydrogenation to Methanol

    2021-06-02 11:39:42CongmingLiKuoChenXiaoyueWangNanXueHengquanYang
    物理化學(xué)學(xué)報(bào) 2021年5期

    Congming Li , Kuo Chen Xiaoyue Wang Nan Xue , Hengquan Yang ,

    1 Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology,Taiyuan 030024, China.

    2 School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China.

    Abstract: Using renewable green hydrogen and carbon dioxide (CO2) to produce methanol is one of the fundamental ways to reduce CO2 emissions in the future, and research and development related to catalysts for efficient and stable methanol synthesis is one of the key factors in determining the entire synthesis process. Metal nanoparticles stabilized on a support are frequently employed to catalyze the methanol synthesis reaction. Metal-support interactions (MSIs) in these supported catalysts can play a significant role in catalysis. Tuning the MSI is an effective strategy to modulate the activity,selectivity, and stability of heterogeneous catalysts. Numerous studies have been conducted on this topic; however, a systematic understanding of the role of various strengths of MSI is lacking. Herein, three Cu/ZnO-SiO2 catalysts with different strengths of MSI, namely, normal precipitation Cu/ZnO-SiO2 (Nor-CZS), co-precipitation Cu/ZnO-SiO2 (Co-CZS), and reverse precipitation Cu/ZnO-SiO2(Re-CZS), were successfully prepared to determine the role of such interactions in the hydrogenation of CO2 to methanol.The results of temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) characterization illustrated that the MSI of the catalysts was considerably affected by the precipitation sequence. Fourier transform infrared reflection spectroscopy (FT-IR) results indicated that the Cu species existed as CuO in all cases and that copper phyllosilicate was absent (except for strong Cu-SiO2 interaction). Transmission electron microscopy (TEM), X-ray diffraction (XRD), and N2O chemical titration results revealed that strong interactions between the Cu and Zn species would promote the dispersion of Cu species, thereby leading to a higher CO2 conversion rate and improved catalytic stability. As expected, the Re-CZS catalyst exhibited the highest activity with 12.4% CO2 conversion, followed by the Co-CZS catalyst(12.1%), and the Nor-CZS catalyst (9.8%). After the same reaction time, the normalized CO2 conversion of the three catalysts decreased in the following order: Re-CZS (75%) > Co-CZS (70%) > Nor-CZS (65%). Notably, the methanol selectivity of the Re-CZS catalyst was found to level off after a prolonged period, in contrast to that of Co-CZS and Nor-CZS. Investigation of the structural evolution of the catalyst with time on stream revealed that the high methanol selectivity of the catalyst was caused by the reconstruction of the catalyst, which was induced by the strong MSI between the Cu and Zn species, and the migration of ZnO onto Cu species, which caused an enlargement of the Cu/ZnO interface. This work offers an alternative strategy for the rational and optimized design of efficient catalysts.

    Key Words: Cu/ZnO-SiO2 catalysts; Metal-support interaction; CO2 hydrogenation; Methanol

    1 lntroduction

    Methanol as an important C1building-block could be converted to various high value-added chemicals. Cu-based catalyst has attracted extensive attention in methanol synthesis because of its excellent redox property, low cost and high recyclability1,2. Among various Cu-based catalysts, Cu/ZnO based catalysts are appealing for the CO2hydrogenation to methanol process due to their unique catalytic properties3,4.

    As a structure-sensitive reaction, catalytic performance of CO2hydrogenation to methanol is significantly affected by catalyst morphology, particle size, crystalline size of support and especially metal-support interaction5. Moreover, depending on the complexity of real systems, dynamic behaviors of structure as well as surface composition observed during reaction process thus affect the nature of active sites and the catalytic performance of Cu-ZnO based catalysts6. Several studies found the directly proportional relationship between methanol synthesis activity and Cu0surface areas (SACu) determined with N2O adsorption7,8. However, taken separately, Cu and ZnO have a low activity for methanol synthesis as well as nonlinearity in correlation of SACuand catalytic activity is often emerged, which indicate the existence of metal-support interaction such as Cu-Zn interaction (CuZn alloy) and Cu-ZnO interaction9. Fujitani et al. observed that Zn is present in the metallic form and proposed that the metallic Cu-Zn alloy is a catalytic active species for the methanol synthesis from CO2and H210. Sebastian et al. supported the formation of a Cu-Zn surface alloy during pretreatment and further found a strong interdependency of methanol synthesis activity and the Zn coverage11.

    On the other hand, Cu-ZnO interaction already acts as a leading proposed model, which exerts a significantly impact on the catalytic performance, such as stability, activity and selectivity via a synergy of Cu and ZnO at the interface,electronic interactions and strong metal-support interactions(SMSI). It is acknowledged that the high-energy sites at the interface are generally accepted as the catalytic sites for methanol synthesis12. Recently, Chen and Rodriguez et al. found that surface Zn transforms into ZnO during CO2hydrogenation to methanol and allows the formation of active Cu-ZnO interface, accompanied with an increase in the catalytic activity for methanol synthesis13. Tsang et al. investigated that morphology effect of ZnO on its interaction with Cu. The results showed that the exposed polar (002) face in platelike ZnO had a much stronger electronic interaction at Cu-ZnO interface,leading to a higher methanol selectivity from CO2and H214. The formation of Cu-ZnO interface drive the transfer of charge from ZnO:Al metal oxide toward Cu metal particles and accumulate at the interface of the Cu-ZnO:Al, which decrease apparent activation energy and increase the reaction order of CO215.Willinger et al. reported that the formation of metastable“graphite-like” ZnO overlayer on Cu nanoparticles induced by strong metal-support interaction (SMSI) during reductive activation. The synergy between Cu and ZnO provide multiple functions in promoting active sites and stabilizing Cu phase mesostructure16. A surface enrichment of Zn in the Cu-ZnOAl2O3catalyst fabricate a unique synergistic Cu-ZnO interaction in boosting CO2hydrogenation and inhibiting RWGS reaction as well as the electronic stabilization of Cu species against reoxidation by CO2and/or H2O, leading to higher activity and methanol selectivity for CO2hydrogenation17. In addition, the dispersion of Cu species could be greatly promoted by the introduction of Zn species, and more synergetic Cu-ZnO sites,therefore, could be exposed to the feed gases leading to a higher reaction activity18. Although the Cu-ZnO interaction has been widely studied, the effect of tuning various strength of metalsupport interaction on catalyst structure and catalytic performance not yet is investigated.

    In the present work, three CuZnO/SiO2catalysts with different strength of metal-support interaction (MSI) were prepared in order to clarify the impact of the interaction between Cu and Zn species on the catalytic performance. The structural evolution of catalyst under the reaction conditions was investigated to establish the relationship between the catalyst structure and catalytic performance. Experimental results suggested that stronger MSI played a critical role in the CO2hydrogenation, and could facilitate the formation of well dispersed metal species thereby affecting the catalytic performance.

    2 Materials and methods

    2.1 Catalyst synthesis

    2.1.1 Normal precipitation method

    10.0 g silica gel (Sigma-Aldrich, 99.9%) was added into a 250 mL beaker containing 200 mL deionized water, which was denoted as suspension A. 6.21 g Cu(NO3)2·3H2O (AR) and 4.12 g Zn(NO3)2·6H2O (AR) were dissolved in deionized water to obtain metal salt solution B ([M2+] = 1 mol·L-1). Initially, the solution B was dropped into the suspension A. After being stirring for 0.5 h, the mixture was heated to 70 °C. Subsequently,a proper amount of Na2CO3(AR) solution (1 mol·L-1) was added into this mixture to adjust the pH to 7. Next, the resultant mixture was further stirred for 1 h and the generated solid was recovered by hot flirtation and washed with deionized water five times. The obtained solid was dried at 60 °C in air overnight and calcined at 350 °C for 4 h, eventually obtaining the catalyst that was denoted as Nor-CZS.

    2.1.2 Co-precipitation method

    10.0 g silica gel (Sigma-Aldrich, 99.9%) was suspended in 200 mL deionized water. After this suspension was heated to 70 °C. A mixture of nitrate solution (mentioned above) and Na2CO3(AR) solution (1 mol·L-1) was dropwise added into the above suspension. The suspension was stirred by a magnetic stirrer and its pH was always maintained at 7 during the preparation process. The following steps were similar with Section 2.1.1. The final sample was denoted as Co-CZS.

    2.1.3 Reverse precipitation method

    A given amount of Na2CO3(AR) solution (1 mol·L-1) was poured into a silica gel suspension (10.0 g silica gel (Sigma-Aldrich, 99.9%)) was suspended in 200 mL deionized water).The resultant mixture was heated to 70 °C. The mixed nitrite solution (mentioned in Section 2.1.1) was then dropped into this mixture. Afterwards, Na2CO3(1 mol·L-1) solution was added to the suspension quickly to adjust the pH to 7. The following steps were similar with Section 2.1.1 and the achieved sample was denoted as Re-CZS.

    2.2 Catalyst characterization

    N2physical adsorption/desorption was carried out on a 3H-2000PS2 instrument. Briefly, after being degassed for 4 h at 150 °C, the sample was characterized at liquid nitrogen temperature. The specific surface area, average pore size and total pore volume were obtained by the B.E.T method and Barrett-Joyner-Halenda (BJH) model, respectively.

    Wide range X-ray diffraction (XRD) pattern was collected from a Rigaku D/Max2500 V diffractometer equipped with Kαradiation of Cu source (λ = 0.154056 nm). All the XRD patterns were scanned from 10° to 80° at a speed of 8 (°)·min-1.

    Temperature-programming desorption (TPD) was performed on a Micromeritics AutoChem 2920 system. For example, 40 mg sample was reduced by 10% H2/Ar gas flow at 250 °C (heating rate 10 °C·min-1) for 2 h. After cooling down to 50 °C under Ar flow, the sample was treated with 10% H2/Ar (10% CO/He or 10% CO2/He) for 30 min. Then the sample was flushed with Ar again to sweep off the physically adsorbed H2(CO or CO2).Finally, the system was heated to 800 °C and the H2(CO or CO2)desorption signal was monitored using a TCD detector. The operation of CO-TPD and CO2-TPD was similar with H2-TPD.

    Temperature-programming reduction (TPR) was also carried out on Micromeritics AutoChem 2920 system. The sample(about 40 mg) was placed in a U-shape quartz reactor.Subsequently, the system was heated to 400 °C and kept for 0.5 h under Ar stream to remove adsorbed impurities. After cooling down to 50 °C, 10% H2/Ar (flow rate of 30 mL·min-1) was introduced into the system. Then, the sample was heated to 500 °C (10 °C·min-1) and H2consumption was calculated by TCD detector.

    The dispersion of Cu species was estimated by using the N2O chemisorption method as stated in our previous work19.

    X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger spectra (XAES) were recorded on an ESCALab220i-XL photoelectron spectrometer with monochromatic Al KαX-ray source (300 W). The C 1s spectral line (284.8 eV) was selected as the reference to correct the static charge accumulation.

    Fourier Transform Infrared Reflection spectroscopy (FT-IR)were recorded on a BRUKER TENSORII spectrometer,equipped with an electromagnetic source in the mid-IR region(4000-400 cm-1). The samples were diluted with 1% (w) KBr and then measured at room temperature.

    Transmission electron microscopy (TEM) was conducted by utilizing JEOL JEM-2100FMII microscope and operated at an accelerating voltage of 200 kV. Especially, at least 80 Cu particles were selected to estimate the average particle size and the surface average particle size was calculated by Eq. (1):

    dis the average particle size andDiis the diameter of the particle.

    2.3 Catalyst evaluation

    Catalytic reactions were tested on a fixed-bed stainless steel reactor (φ9 mm × l450 mm). Before reaction, about 0.5 g sample granules (20 to 40 mesh) was loaded into the middle of the tube reactor and reduced at 250 °C by 10% H2in N2for 4 h.Subsequently, the premixed reaction gas (consisted of 24% CO2,72% H2and 4% Ar where Ar was acted as internal standard) was flushed into reactor for 45 min and then the system pressure was raised to 3 MPa under 50 °C. Finally, the system was heated to 250 °C to evaluate its catalytic performance. The products were analyzed by an on-line gas chromatograph (Panna A91)equipped with a thermal conductivity detector (TCD) and a flame ionized detector (FID).

    The CO2conversion (denoted asX(CO2)) was calculated according to an internal standard method (4% Ar as an internal standard), assuming that the amount of Ar remained constant in the reaction process. The equation as follow:

    3 Results and discussion

    3.1 Metal-support interaction

    TPR analysis were employed to check the metal-support interaction and reducibility of Cu species on the three catalysts(Fig. 1). The main reduction peaks appeared between 240-280 °C illustrating well dispersed Cu species on all the samples20.It is generally considered that higher reduction temperature of CuO might originate from the stronger metal-support interaction and/or lager particle size (poor dispersion of Cu species).

    Fig. 1 TPR profiles of the CuZn/SiO2 catalysts prepared with different methods.

    As listed in Table 1, the H2consumption of Re-CZS, Co-CZS and Nor-CZS catalysts was 1.64, 1.57 and 1.46 mmol·g-1,respectively. The higher hydrogen consumption was related with the more reducible Cu species existing on the catalyst due to the higher dispersion of Cu species. On the other hand, higher reduction temperature was reasonably attributed to the stronger metal-support interaction rather than lager CuO particles. It is thus believed that the Re-CZS catalyst possessed the strongest metal-support interaction while Nor-CZS catalyst might possess the weakest metal-support interaction. Furthermore, the Co-CZS sample showed two reduction peaks suggesting higher heterogeneity of Cu species (various particles size) which would be testified later (Section 3.2).

    The XPS spectrum of Cu 2p(Fig. S1, in Supporting Information, the same below) revealed the distinct chemical environments of the Cu species. The emergence of shake-up satellite peaks at 941.6-944.9 eV unveiled that the Cu species in the fresh samples mainly existed in the form of CuO21. All the Cu 2p3/2signals consisted of two overlapped peaks and their core level signals were deconvoluted into two individual peaks (Fig.2 and Table 2). The peak at 935.1-935.6 eV is usually attributed to the Cu species with stronger interaction with support matrix,and the latter peak (from 933.3 to 933.7 eV) might be ascribed to Cu species with the weaker metal-support interaction22. The characterization results convincingly validated that the metalsupport interaction was successfully altered by the precipitation sequence.

    Fig. 2 XPS spectra of Cu 2p3/2 region of various fresh CuZnO/SiO2catalysts.

    As listed in Table 2, Cu species in the form of both strong metal-support interaction and weaker metal-support interaction were also observed in three catalysts. However, the Re-CZS catalyst possessed the most Cu species (~78%) with strong metal-support interaction, closely followed by Co-CZS (~73%).In contrast, the Nor-CZS contained the most Cu species (~60%)with weaker metal-support interaction. The significant difference in the fraction of Cu species with a strong metalsupport interaction indicated that three catalysts possessed different strength of metal-support interaction. By combining theH2-TPR and XPS results, it could be proposed that the Re-CZS catalyst provided more Cu species with strong metal-support interaction than Nor-CZS catalyst.

    Table 1 Chemical properties of various CuZnO/SiO2 catalysts.

    Table 2 XPS results of Cu 2p3/2 region of various fresh CuZnO/SiO2 catalysts.

    3.2 Textural properties

    The N2-physisorption isotherms for all the catalysts are shown in Fig. 3, in which isotherm of silica gel was also included for comparison. Obviously, the isotherm was of type IV and exhibited a H1 hysteresis loop for silica gel support illustrating that the existence of abundant cylindrical channels on the support. Meanwhile, the high similarity of all isotherms for four samples (including silica gel) indicated that the textures of support were almost preserved after the loading of Cu species23.

    As listed in Table 3, after loading metal, the BET surface area and pore volume of all catalysts decreased in the following order:Nor-CZS > Co-CZS > Re-CZS. It was noted that the micropore of Co-CZS and Re-CZS samples was not detected, indicating the metal species were mainly distributed on the external-surface of SiO2support and then the micropore was blocked by metal particles. Interestingly, the Nor-CZS catalyst displayed the lowest BET surface area and the minimum pore volume and average pore size.

    XRD patterns are shown in Fig. 4. The presence of weak and broaden diffraction peaks at 35.6° and 38.9° implied that most of the Cu species on all the calcined samples were in the form of well dispersed CuO. The crystallite sizes of CuO in the three catalysts displayed in a descending order: Co-CZS (9.1 nm) >Nor-CZS (7.8 nm) > Re-CZS (n.d.) (n.d. meaning diffraction peaks on Re-CZS were too weak to calculate the CuO crystallite sizes according to the Scherrer formula). As a complementary characterization for XRD, N2O chemisorption results are also listed in Table 1. The dispersion of Cu was 18.5% (Re-CZS),16.1% (Co-CZS) and 12.2% (Nor-CZS), respectively. However,the dispersion of Cu particles determined from the N2O chemisorption result was not in full accord with the XRD results.This phenomenon might be attributed to the existence of trace amount of lager CuO (monitored by XRD) on the Co-CZS catalyst19,23, while the less dispersion of Cu determined by N2O chemisorptions on Nor-CZS might also originate from its relatively weak metal-support interaction.

    Fig. 3 N2-physisorption isotherms and pore size distribution of various CuZn/SiO2 catalysts.

    Fig. 4 XRD patterns of various CuZn/SiO2 catalysts.

    Table 3 Textural properties of silica gel and as-calcined catalysts prepared by different methods

    To further illustrate the above conjectures, the representative TEM micrographs and size distribution for three different samples are shown in Fig. 5 and Table 1. The crystallite sizes of CuO in these three catalysts displayed in a descending order: Co-CZS (10.5 nm) > Nor-CZS (9.8 nm) > Re-CZS (8.5 nm).Obviously, partial agglomeration of Cu particles occurred on the support surface for the Co-CZScatalyst. Taking XRD along with N2O chemisorption results into consideration, it could be concluded that the CuZnO/SiO2catalysts with different metalsupport interaction could be modulated through varying the precipitation subsequence24.

    Fig. 5 TEM micrographs and size distribution for fresh (a, d) Nor-CZS catalyst, (b, e) Co-CZS catalyst, (c, f) Re-CZS catalyst.

    To further exclude the formation of Cu phyllosilicate, FT-IR analysis was carried out (Fig. 6). The characteristic bands at 800 cm-1and 1118 cm-1are ascribed to υSiOof different vibration modes of SiO223,25-27. Complete decomposition of carbonates was confirmed by the absence of 1430-1450 cm-1bands in all calcined samples28,29. Particularly, the characteristic δOHband at 670 cm-1was not detected, indicating the absence of Cu phyllosilicate in all the samples27,30.

    Fig. 6 FT-IR spectra of various CuZn/SiO2 catalysts.

    Taken together, the Cu species were well dispersed on all fresh catalysts in the form of CuO regardless of the preparation method. The absence of copper phyllosilicate in all the catalysts elucidated that their notable difference in the metal-support interaction possibly originated from the distinction of the interaction between Cu and ZnO, instead of Cu and SiO2, which was significantly impacted by metal loading process. The stronger metal-support interaction between Cu and ZnO might lead to a higher dispersion of Cu species31,32.

    3.3 Catalysis

    3.3.1 Catalytic performance

    The steady-state activities of the catalysts were measured in a fixed-bed reactor to investigate the relationship between metalsupport interaction and activity of CO2hydrogenation to methanol. It is generally considered that catalytic activity of CO2hydrogenation to methanol is related to the dispersion of Cu species. As described above, the Re-CZS catalyst was supposed to have the best catalytic activity while the Co-CZS catalyst might possess secondary catalytic activity and the Nor-CZS catalyst ought to exhibit the poorest catalytic activity. As expected, the Re-CZS catalyst exhibited the highest activity with 12.4% CO2conversion, followed by the Co-CZS catalyst(12.1%) and then the Nor-CZS catalyst (9.8%) (Fig. S2A and Table 4). The turnover frequency (TOF) values were also calculated with the assumption that each surface Cu atom was an active site (Table 4). An opposite tendency between CO2conversion rate and TOF values was observed due to the small size of Cu particles33,34. The Nor-CZS catalyst exhibited slightly higher TOF value than the Co-CZS sample, demonstrating that the existence of small amount of large bulk CuO had negligible impact on the Cu intrinsic activity. Since, under the current synthesis condition, CO2was mainly involved in two parallel reactions as follows35-37:

    Table 4 The initial CO2 conversion rate, methanol selectivity and methanol productivity (PMeOH).

    The reaction conditions for methanol synthesis were then assessed. As shown in Fig. 7, the Co-CZS catalyst displayed thebest reaction activity under low reaction pressure, while the best reaction activity was obtained over the Re-CZS catalyst with increasing the pressure. However, the former possessed the better methanol selectivity compared with later consistently.

    Fig. 7 Effect of reaction temperature on CO2 conversion (A) and MeOH selectivity (B) and effect of reaction pressure on CO2 conversion (C) and MeOH selectivity (D).

    On the other hand, methanol synthesis was always accompanied by intense exothermic process and accordingly restrained when increasing temperature, while RWGS reaction would be facilitated due to its endothermic process. For this reason, the enhancement of CO2conversion rate accompanied with a decline of methanol selectivity was observed throughout the temperature-rising process. Similar with the former results,the Nor-CZS catalyst also exhibited the highest methanol selectivity and the Re-CZS catalyst possessed the poorest methanol selectivity when the reaction temperature was lower than 270 °C. However, an interesting phenomenon was found when the reaction temperature increased to 290 °C, where the highest methanol selectivity over the Re-CZS catalyst, Nor-CZS and Co-CZS catalysts displayed similar methanol selectivity.These experimental results illustrated that reaction temperature showed great effect on catalytic behavior in comparison to the effect of the reaction pressure. Namely, strong metal-support interaction would improve the dispersion of Cu species and further promote catalytic activity, however, it led to a negative influence on methanol synthesis.

    3.3.2 Catalytic stability

    It was recognized that the catalytic deactivation could be significantly suppressed by the enhancement of metal-support interaction. To compare the relative rate of deactivation, catalytic stability for CO2hydrogenation are evaluated (Fig. 8). Clearly,catalytic activity dropped rapidly during the initial stage of reaction for all catalysts. Especially for the Nor-CZS and Co-CZS catalysts, they suffered from the most severe catalytic deactivation and the reduction of CO2conversion rate(normalized) was more than 20% (22.3%) in the initial 400 min.In contrast, the Re-CZS catalyst displayed better catalytic stability in the initial 400 min and its CO2conversion(normalized) reduced by 15.0%. The stability of three catalysts decreased in the following sequence: Re-CZS > Co-CZS > Nor-CZS. As mentioned above, CO2was involved in two parallel reactions under the methanol synthesis conditions. Therefore,hot spots might be formed on the catalyst due to the high methanol synthesis rate. The higher methanol selectivity would give rise to the severe sintering of Cu species through particle migration and coalescence leading to a dramatic particle growth and significant decrease of Cu surface area, although the reaction temperature was far below the Tammann temperature of Cu species38. Moreover, as mentioned in our previous work19, the existence of large CuO particles could also accelerate particles growth through Ostwald ripening process, in which the growth of larger particles was at the expense of smaller particles. It was reasonable to propose that the similar catalytic stability of Nor-CZS and Co-CZS catalyst at the initial reaction period resulted from higher methanol synthesis rate and the existence of bulk CuO particles for Co-CZS catalyst. With time on stream, the Ostwald ripening was restrained by the vanishing Cu particles meanwhile particle migration and coalescence was also retarded by the decline of methanol yield and relatively stronger metalsupport interaction. Accordingly, the Co-CZS catalyst exhibited the similar deactivation rate with Nor-CZS catalyst at the initial stage and the higher catalytic stability at the medium-late reaction period.

    3.3.3 Structural evolution of catalyst

    As shown in Fig. 8D, the decline trend of CO2conversion was not exactly in line with the decrease process of methanol yield.Especially for the Re-CZS catalyst, in contrast to the maintenance of CO2conversion, the maintenance of the methanol yield was better. After 40 h, the CO2conversion rate decreased by 27.1% (normalized) while its reduction of methanol yield was only about 18.2% (normalized). Obviously,according to Fig. 8C, the enhancement of methanol selectivity over the Re-CZS catalyst during the reaction process was responsible for this phenomenon, while methanol selectivity for the other catalysts exhibited little variation. The Re-CZS catalyst was selected to investigate the cause for this phenomenon. The samples that were calcined, reduced, reacted for 8 h and 40 h,marked as Re-CZS-C, Re-CZS-0, Re-CZS-8 and Re-CZS-40,were characterized with XPS, H2-TPD, CO-TPD and CO2-TPD in detail.

    Fig. 8 Catalytic performance for all catalysts, (A) normalized CO2 conversion, (B) normalized methanol yield, (C) methanol selectivity,(D) deactivation ratio of CO2 conversion and methanol yield.

    As revealed by the XPS results (Fig. S3 and Table S1), the reduction process led to a dramatic decline of Cu content and Cu/(Cu + Zn) ratio (Table S1) on the catalyst’s surface,meanwhile the Zn content on its surface increased slightly. This finding was in good accordance with the results reported by other groups: a part of Zn species migrated from catalyst’s bulk phase to Cu species’ surface owing to strong metal-support interaction32. As a result, partial Cu species on catalyst surface was overlapped by ZnO leading to sample structural restruction after activation process39.

    On the other hand, the H2-TPD results are shown in Fig. S4C and Table S2, reminding that twice reduction process generated insignificant impact on H2adsorption property. Generally, the peak 1, locating at lower temperature, was assigned to desorption of hydrogen adsorbing on Cu surface40, while the broader signal(peak 2), ranging from 300 °C to 650 °C, corresponded to the desorption of strongly adsorbing hydrogen from the surface of ZnO sites deriving from Cu surface through hydrogen spillover41,42. At the initial reaction stage (500 min), the H2desorption originating from Cu surface sharply decreased suggesting the decline of approachable Cu sites on the catalyst surface. Meanwhile, the amounts of H2desorbing from ZnO sites dramatically enhanced which strongly indicated the increase of ZnO surface area due to the further migration of Zn species from mixed bulk phase as reaction proceeds16. Particularly, the decline of Zn/(Cu + Zn + Si) ratio on the catalyst surface (Table S1) manifested that the excessive Zn species would coalesce on the surface of catalyst forming larger ZnO particles. As reaction time prolonged, the H2-TPD results indicated that the desorbing H2from the ZnO surface significantly decreased with the slight enhancement of desorbing H2quantity from Cu sites (still lower than fresh reduced catalyst). This finding further confirmed that leaching and growth of Zn species coexisted on the catalyst although the former process might be limited with time on stream and the slight increase of desorbing H2quantity from Cu surface might be attributed to migration of Zn species on the surface. Moreover, the Cu/(Cu + Zn) ratio on catalyst’s surface showed limited variation while both of the Cu and Zn content on the surface of Re-CZS catalyst declined indicating particle growth for both of zinc and copper species. Herein, the structure evolution of catalyst was proposed and displayed in Scheme 1.

    Obviously, during the sample activation process, restructure of catalyst occurred due to the strong metal-support interaction.At the initial reaction stage, the instability of the catalyst structure led to the growth of Cu particles and the dramatic decline of catalytic activity was accordingly observed. However,the Zn species migrating from bulk phase due to the strong metal-support interaction prevented this process to some extent16,43. With the reaction proceeding, the growth of Cu species was further suppressed and hence the deactivation of catalyst was retarded. Remarkably, leaching and coalescence of Zn species occurred throughout the whole reaction process,which exerted a crucial impact on the methanol synthesis process. From these reaction results (Fig. S2 and Fig. 8C), it was convincible to propose that the catalytic activity (CO2conversion) was principally determined by the exposing Cu species, while the methanol selectivity and productivity was significantly influenced by the leaching of Zn species.

    It was widely acknowledged that there was an intimate synergy between Cu and Zn species, where ZnO acted as direct promoter for C―O bond activation and hydrogen reservoir. In consequence, similar variation trend between the CO2and H2chemisorption results was expected44. The CO2-TPD profiles were shown in Fig. S4A, with their details summarized in Table S2. As expected, similar to the H2-TPR result, the twice reduction process also generated the slight impact on CO2adsorption property and its variation trend further confirmed our hypothesis as proposed in Scheme 1.

    Scheme 1 Catalyst structure evolution process.

    Scheme 2 The reaction mechanism of methanol synthesis process.

    To further investigate the relationship between structure evolution of the catalyst and formation of methanol, CO2-TPD results (ranging from 230 °C to 700 °C) at various reaction times were deconvoluted into three Gaussian peaks, as shown in Fig.9, and the relative content and the number of each basic sites were listed in Table 5. Notably, because of relatively high reaction temperature (250 °C), the CO2desorption peak at the lower temperature (about 100 °C) was considered to have a limited contribution to the CO2activation and therefore it was convincible to consider that the catalytic performance corresponded to later peak. Generally, different desorption temperature of the three peaks indicated the existence of different CO2adsorbing species on the catalyst. In other words,it’s logical to deduce that there should be three different CO2adsorption sites existed on the catalyst surface.

    Fig. 9 CO2-TPD results for Re-CZS catalyst with different reaction time.

    Table 5 Relative contents of deconvoluted peaks from CO2-TPD.

    The results illustrated that migration of ZnO could not only improve the adsorption capacity of H2but also alter the number and property of CO2adsorption sites which played a crucial role in the CO2hydrogenation. As reported45, the weakly and medially basic sites (peak 2) were corresponded to metal-oxygen pairs (such as isolated ZnO) with different adsorption manners in which the RWGS reaction mainly occurred, while the strongly basic sites (peak 3) were assigned to the low coordination oxygen atoms which greatly facilitated formation of methanol.The results indicated that the relative content and the number of weakly and medially basic site decreased with time on stream,while strongly basic sites displayed a contrary tendency indicating the transformation of basic sites during the reaction process. As a result, the improvement of methanol selectivity was obtained with time on stream. It was also noted that the enhancement of strongly basic sties owing to the migration of Zn species suggested that more Cu/ZnO interface could facilitate formation of low coordination oxygen atoms.

    Eventually, CO-TPD has also been carried out to further clarify the impact of ZnO migration process. Different from the H2-TPD and CO2-TPD results, the CO-TPD results (Fig. S4B)clearly manifested that the capacity for the CO adsorption in the temperature range of 200 to 700 °C was significantly enhanced as the reaction progress. Therefore, it was plausible to propose that the adsorption sites for CO2and CO was definitely different and the CO conversion (deriving from RWGS) to methanol might be further facilitated owing to the improvement of CO adsorption capacity.

    Conclusively, the migration of ZnO from the bulk catalyst owing to the strong-metal support interaction could not only improve the adsorption capacity of H2which could facilitate methanol synthesis process, but alter the property of CO2adsorption sites which was considered to greatly influence the CO2hydrogenation performance of catalyst46. However, the excessive Zn species would accelerate the formation of slightly lager ZnO particles resulting in a negative impact on CO2hydrogenation process and a positive impact on CO hydrogenation process. Therefore, it’s rational to infer that there should exist a balance between leaching and migration of Zn species, which was greatly affected by the metal-support interaction.

    Based on the above investigation, a reaction mechanism was proposed (Scheme 2). Initially, the H2dissociates on the Cuδ+site and then spillovers to the Cu/ZnO interface and the ZnO surface through route Haand Hb, respectively. Then, the H* (generated by spillover from Cuδ+site) would react with the different adsorbed CO2. Subsequently, methanol and CO were produced on different active sites. Particularly, the majority of CO generated from RWGS reaction would desorb from catalyst surface while the others might furtherreact with the H* to form methanol.

    4 Conclusions

    In summary, three Cu/ZnO-SiO2catalysts with different metal-support interaction could be modulated through varying the precipitation sequence. The role of Cu-ZnO interaction on the catalytic activity of CO2hydrogenation to methanol was investigated by a combination of characterization and reaction.It is revealed that intimate interaction between the Cu and Zn species promotes the dispersion of Cu species, which was beneficial to the CO2activation. Stronger metal-support interaction (MSI) was conducive to improve the activity and stability of the catalyst. The migration of ZnO from bulk catalyst induced by the strong metal-support interaction (MSI) not only promoted the adsorption and activation of H2and CO2, thereby facilitating the activation efficiency of the reactants, but also changed the reaction pathway of the CO2hydrogenation to some extent. We believe that this work is helpful for understanding of the metal-support interaction in Cu-based catalysts, and also provide guidance for the rational and optimized design of efficient catalysts.

    Supporting information: available free of charge via the internet at http://www.whxb.pku.edu.cn.

    国产精品乱码一区二三区的特点| 最近视频中文字幕2019在线8| 少妇人妻一区二区三区视频| 欧美激情久久久久久爽电影| 成人美女网站在线观看视频| 97超级碰碰碰精品色视频在线观看| 免费观看人在逋| 久久久久久久久大av| 日本黄色视频三级网站网址| 日本精品一区二区三区蜜桃| 日日摸夜夜添夜夜添小说| 国产91av在线免费观看| 国产黄色视频一区二区在线观看 | 亚洲人与动物交配视频| 亚洲av成人av| 最好的美女福利视频网| 亚洲第一区二区三区不卡| 精品日产1卡2卡| 毛片女人毛片| 深爱激情五月婷婷| 长腿黑丝高跟| 欧美又色又爽又黄视频| 亚洲成人av在线免费| 免费av观看视频| 精品久久久噜噜| 99热这里只有精品一区| 国产中年淑女户外野战色| 99久国产av精品| 国产色婷婷99| 搡老熟女国产l中国老女人| 国产在线男女| 日韩在线高清观看一区二区三区| 免费观看精品视频网站| 色5月婷婷丁香| 女同久久另类99精品国产91| 男人舔奶头视频| 嫩草影视91久久| av天堂在线播放| 五月伊人婷婷丁香| 少妇丰满av| 久久精品综合一区二区三区| 国内精品宾馆在线| 夜夜夜夜夜久久久久| 特大巨黑吊av在线直播| 午夜爱爱视频在线播放| 亚洲aⅴ乱码一区二区在线播放| 国产欧美日韩精品亚洲av| 男女之事视频高清在线观看| 国产一区亚洲一区在线观看| 亚洲国产精品sss在线观看| 成人av一区二区三区在线看| 自拍偷自拍亚洲精品老妇| av免费在线看不卡| 综合色丁香网| 色播亚洲综合网| 国产欧美日韩一区二区精品| 欧美高清性xxxxhd video| 国产欧美日韩精品一区二区| 国产午夜精品论理片| 嫩草影视91久久| 免费av观看视频| 日本熟妇午夜| 日韩在线高清观看一区二区三区| av天堂中文字幕网| 成人亚洲欧美一区二区av| 高清午夜精品一区二区三区 | 18+在线观看网站| 十八禁国产超污无遮挡网站| 如何舔出高潮| 桃色一区二区三区在线观看| 国产亚洲精品久久久com| 亚洲人与动物交配视频| 最后的刺客免费高清国语| 三级毛片av免费| 国产国拍精品亚洲av在线观看| 免费看a级黄色片| 午夜福利18| 美女大奶头视频| 精华霜和精华液先用哪个| 日产精品乱码卡一卡2卡三| 日韩欧美一区二区三区在线观看| 亚洲内射少妇av| 听说在线观看完整版免费高清| 偷拍熟女少妇极品色| 91在线精品国自产拍蜜月| 男插女下体视频免费在线播放| 99九九线精品视频在线观看视频| 99久久中文字幕三级久久日本| 日本五十路高清| 久久精品91蜜桃| 天天一区二区日本电影三级| 中文字幕久久专区| 亚洲av不卡在线观看| 国产麻豆成人av免费视频| 亚洲精品久久国产高清桃花| 国产精品精品国产色婷婷| 美女黄网站色视频| 亚洲自拍偷在线| 国产一级毛片七仙女欲春2| 久久鲁丝午夜福利片| 午夜视频国产福利| 美女免费视频网站| 精品少妇黑人巨大在线播放 | 亚洲综合色惰| 国产一区二区三区av在线 | 你懂的网址亚洲精品在线观看 | 国产一区二区激情短视频| 久久精品国产清高在天天线| 一级a爱片免费观看的视频| 国产三级中文精品| 久久精品91蜜桃| 人人妻人人澡人人爽人人夜夜 | 国产亚洲欧美98| 成人性生交大片免费视频hd| 搡老妇女老女人老熟妇| 男女边吃奶边做爰视频| 99热网站在线观看| 美女大奶头视频| 亚洲精品在线观看二区| 国内精品美女久久久久久| 国产精品1区2区在线观看.| 国产免费男女视频| 午夜日韩欧美国产| 久久久久久久亚洲中文字幕| 国产精品久久电影中文字幕| 欧美3d第一页| 麻豆成人午夜福利视频| 男插女下体视频免费在线播放| 99久久无色码亚洲精品果冻| 亚洲av一区综合| a级毛片a级免费在线| 亚洲自偷自拍三级| 免费av毛片视频| 亚洲欧美成人综合另类久久久 | 99热这里只有是精品在线观看| 亚洲国产色片| 精品欧美国产一区二区三| 在线观看午夜福利视频| 91在线精品国自产拍蜜月| 成人漫画全彩无遮挡| 欧美性猛交黑人性爽| 欧美精品国产亚洲| 久久久久久久久久黄片| 99久久精品热视频| 热99在线观看视频| 在线观看一区二区三区| 无遮挡黄片免费观看| 蜜臀久久99精品久久宅男| 国产爱豆传媒在线观看| 亚洲欧美精品自产自拍| 在线观看免费视频日本深夜| 老熟妇乱子伦视频在线观看| 久久鲁丝午夜福利片| 内射极品少妇av片p| 搡老熟女国产l中国老女人| 97超碰精品成人国产| 日日摸夜夜添夜夜添小说| 99热精品在线国产| 免费人成在线观看视频色| 日本一本二区三区精品| av在线老鸭窝| 99热这里只有是精品50| 淫妇啪啪啪对白视频| 淫妇啪啪啪对白视频| 精品一区二区三区av网在线观看| 久久人妻av系列| 亚洲天堂国产精品一区在线| 亚洲第一电影网av| 日韩欧美国产在线观看| 精品久久久久久久久久久久久| 欧洲精品卡2卡3卡4卡5卡区| 内地一区二区视频在线| 内地一区二区视频在线| 高清毛片免费看| 亚洲国产精品国产精品| 日韩欧美国产在线观看| 国产高清三级在线| 精品一区二区三区av网在线观看| 亚洲在线观看片| 99热精品在线国产| a级一级毛片免费在线观看| 国产伦精品一区二区三区四那| 日日摸夜夜添夜夜添小说| 成人精品一区二区免费| 久久久久久久久中文| 欧美日本视频| av在线亚洲专区| 伊人久久精品亚洲午夜| 国产毛片a区久久久久| 人人妻人人澡人人爽人人夜夜 | 在线播放无遮挡| 亚洲三级黄色毛片| 亚洲三级黄色毛片| a级一级毛片免费在线观看| 免费人成在线观看视频色| 啦啦啦韩国在线观看视频| 午夜视频国产福利| 小蜜桃在线观看免费完整版高清| 免费人成在线观看视频色| 级片在线观看| 黄色欧美视频在线观看| 可以在线观看毛片的网站| 久久久久久久久中文| 国产午夜精品论理片| 国内精品一区二区在线观看| 免费无遮挡裸体视频| 免费av不卡在线播放| 你懂的网址亚洲精品在线观看 | 午夜免费激情av| 国产淫片久久久久久久久| 热99re8久久精品国产| 干丝袜人妻中文字幕| 免费黄网站久久成人精品| 又爽又黄a免费视频| 欧美一级a爱片免费观看看| 亚洲av中文字字幕乱码综合| 国产成人一区二区在线| 亚洲欧美日韩东京热| 小说图片视频综合网站| 色哟哟哟哟哟哟| 日日啪夜夜撸| 国产精品久久久久久精品电影| a级毛片a级免费在线| 激情 狠狠 欧美| videossex国产| 国产在线精品亚洲第一网站| 亚洲,欧美,日韩| 亚洲精品亚洲一区二区| 日韩强制内射视频| 亚洲国产精品成人久久小说 | a级毛片免费高清观看在线播放| 亚洲va在线va天堂va国产| 久久久国产成人精品二区| 蜜桃久久精品国产亚洲av| 淫妇啪啪啪对白视频| 亚洲国产日韩欧美精品在线观看| 黄色一级大片看看| 日本黄色视频三级网站网址| 亚洲精品亚洲一区二区| 欧美日韩精品成人综合77777| 久久久成人免费电影| 麻豆国产97在线/欧美| 一个人免费在线观看电影| 亚洲五月天丁香| 国产真实乱freesex| 久久久国产成人精品二区| 国产乱人偷精品视频| 大又大粗又爽又黄少妇毛片口| 久久久久久久久中文| 亚洲av五月六月丁香网| 欧美三级亚洲精品| 亚洲激情五月婷婷啪啪| 国产一区二区三区av在线 | 国产一区二区在线观看日韩| 日韩欧美三级三区| 嫩草影院精品99| 六月丁香七月| 熟女人妻精品中文字幕| 欧美丝袜亚洲另类| 一本一本综合久久| 一本一本综合久久| 中国美女看黄片| 久久九九热精品免费| 日本三级黄在线观看| 日韩欧美在线乱码| 亚洲人成网站在线播放欧美日韩| 99热只有精品国产| 99热这里只有是精品在线观看| 亚洲欧美成人精品一区二区| 精品人妻视频免费看| av在线天堂中文字幕| 最后的刺客免费高清国语| 老司机午夜福利在线观看视频| 十八禁国产超污无遮挡网站| 91狼人影院| 中文亚洲av片在线观看爽| 国产av在哪里看| 久久中文看片网| 一级毛片久久久久久久久女| 亚洲欧美清纯卡通| 欧美3d第一页| 人妻丰满熟妇av一区二区三区| 亚洲最大成人中文| 国产高清视频在线播放一区| 男人舔女人下体高潮全视频| 天堂√8在线中文| 色播亚洲综合网| 热99re8久久精品国产| 亚洲不卡免费看| 亚洲成av人片在线播放无| 国产熟女欧美一区二区| 无遮挡黄片免费观看| 欧美精品国产亚洲| 亚洲av熟女| 精品无人区乱码1区二区| 99久久成人亚洲精品观看| 成年女人毛片免费观看观看9| 女的被弄到高潮叫床怎么办| 国产综合懂色| 国产精品野战在线观看| 两个人的视频大全免费| 日本 av在线| 男女边吃奶边做爰视频| 亚洲av一区综合| 性色avwww在线观看| 亚洲人与动物交配视频| 亚洲不卡免费看| 欧美潮喷喷水| 欧美不卡视频在线免费观看| 久久久久久九九精品二区国产| 少妇被粗大猛烈的视频| 国产高清视频在线观看网站| 偷拍熟女少妇极品色| 男人的好看免费观看在线视频| 久久久久性生活片| 日本三级黄在线观看| 亚洲国产日韩欧美精品在线观看| 夜夜看夜夜爽夜夜摸| 国产 一区精品| 久久人人爽人人爽人人片va| 国产精品久久久久久av不卡| 国产69精品久久久久777片| 日韩欧美国产在线观看| 尾随美女入室| 久久久久久九九精品二区国产| 久久中文看片网| 毛片一级片免费看久久久久| 色视频www国产| 日韩一区二区视频免费看| 日本黄色视频三级网站网址| 日本 av在线| 日本爱情动作片www.在线观看 | 亚洲av二区三区四区| 国产午夜精品久久久久久一区二区三区 | 黄色配什么色好看| 99国产极品粉嫩在线观看| 露出奶头的视频| 能在线免费观看的黄片| 国内精品久久久久精免费| 中文亚洲av片在线观看爽| 丰满的人妻完整版| 欧美成人一区二区免费高清观看| 中出人妻视频一区二区| 久久精品国产亚洲av涩爱 | 国产精品福利在线免费观看| 中文字幕熟女人妻在线| 免费在线观看成人毛片| 欧美激情国产日韩精品一区| 国产男靠女视频免费网站| 日本爱情动作片www.在线观看 | 插逼视频在线观看| 亚洲精品粉嫩美女一区| 成人鲁丝片一二三区免费| 欧美区成人在线视频| 久久久久久久久大av| 狠狠狠狠99中文字幕| a级毛色黄片| 国产在线男女| 国产爱豆传媒在线观看| 久久人人爽人人爽人人片va| 国产精华一区二区三区| 中文字幕人妻熟人妻熟丝袜美| 欧美不卡视频在线免费观看| 成人三级黄色视频| 观看美女的网站| 女人被狂操c到高潮| 91狼人影院| 人人妻人人澡欧美一区二区| 淫秽高清视频在线观看| 可以在线观看的亚洲视频| 精品国内亚洲2022精品成人| 国内精品美女久久久久久| 99热只有精品国产| 久久久久九九精品影院| 级片在线观看| 熟女电影av网| 97超视频在线观看视频| 禁无遮挡网站| 精品一区二区三区av网在线观看| 夜夜看夜夜爽夜夜摸| 国产精品免费一区二区三区在线| 日韩欧美精品免费久久| 国产精品永久免费网站| 国产精品1区2区在线观看.| 男女下面进入的视频免费午夜| 欧美激情在线99| 蜜桃久久精品国产亚洲av| 国产白丝娇喘喷水9色精品| 欧美日韩在线观看h| 天美传媒精品一区二区| 极品教师在线视频| 欧美激情久久久久久爽电影| 日韩中字成人| 国产成人a∨麻豆精品| 久久久久久久久久久丰满| 99视频精品全部免费 在线| 色吧在线观看| 免费看av在线观看网站| 在线a可以看的网站| 亚洲内射少妇av| 久久久久精品国产欧美久久久| 亚洲国产精品sss在线观看| 成人av在线播放网站| 免费av观看视频| АⅤ资源中文在线天堂| 国产综合懂色| 久久热精品热| 在线观看美女被高潮喷水网站| 99热这里只有精品一区| 国产精品久久久久久久电影| 麻豆国产97在线/欧美| 国产69精品久久久久777片| 亚洲人成网站在线播放欧美日韩| 男女做爰动态图高潮gif福利片| 日本爱情动作片www.在线观看 | av在线蜜桃| 久久人人爽人人爽人人片va| 午夜激情福利司机影院| 亚洲国产日韩欧美精品在线观看| 欧美一级a爱片免费观看看| 欧美绝顶高潮抽搐喷水| av国产免费在线观看| 亚洲va在线va天堂va国产| 精品久久久噜噜| 一区二区三区高清视频在线| 男女边吃奶边做爰视频| 三级毛片av免费| 中文资源天堂在线| 国产三级在线视频| 91在线精品国自产拍蜜月| 亚洲久久久久久中文字幕| 亚洲经典国产精华液单| 免费高清视频大片| 欧美xxxx性猛交bbbb| 人人妻人人看人人澡| 又粗又爽又猛毛片免费看| 亚洲性夜色夜夜综合| 亚洲国产精品久久男人天堂| 99久久久亚洲精品蜜臀av| 精品久久久久久久末码| 啦啦啦韩国在线观看视频| 3wmmmm亚洲av在线观看| 日本欧美国产在线视频| 青春草视频在线免费观看| 婷婷色综合大香蕉| 午夜精品在线福利| 久久久久久久久久成人| 深夜a级毛片| 乱系列少妇在线播放| 日韩欧美在线乱码| 菩萨蛮人人尽说江南好唐韦庄 | 人妻丰满熟妇av一区二区三区| 亚洲精品日韩在线中文字幕 | 日本a在线网址| 国产精品电影一区二区三区| 99久久精品热视频| 亚洲第一电影网av| 一进一出好大好爽视频| 免费黄网站久久成人精品| 国内揄拍国产精品人妻在线| 热99re8久久精品国产| 两个人的视频大全免费| 女人十人毛片免费观看3o分钟| 久久久久免费精品人妻一区二区| 国产精品日韩av在线免费观看| 老女人水多毛片| 最好的美女福利视频网| 久久人人爽人人爽人人片va| 99热这里只有是精品在线观看| 国产极品精品免费视频能看的| 黄色视频,在线免费观看| 波多野结衣巨乳人妻| 男女做爰动态图高潮gif福利片| 蜜桃亚洲精品一区二区三区| 日韩人妻高清精品专区| 日本欧美国产在线视频| av在线亚洲专区| 欧美性感艳星| 亚洲在线观看片| 男人舔女人下体高潮全视频| 国产单亲对白刺激| 婷婷精品国产亚洲av在线| 六月丁香七月| 波多野结衣巨乳人妻| 婷婷六月久久综合丁香| 国产成人aa在线观看| 老熟妇乱子伦视频在线观看| 精品久久国产蜜桃| 久久久久久久久久久丰满| 蜜臀久久99精品久久宅男| 久久久色成人| 日本黄大片高清| 天美传媒精品一区二区| 久久久久久久久久成人| 久久久久久大精品| 老师上课跳d突然被开到最大视频| 国产伦精品一区二区三区视频9| 久久精品国产亚洲网站| 欧美+亚洲+日韩+国产| 禁无遮挡网站| 老熟妇乱子伦视频在线观看| 日本a在线网址| 伊人久久精品亚洲午夜| 有码 亚洲区| 日韩一区二区视频免费看| 床上黄色一级片| 国产精品永久免费网站| 婷婷精品国产亚洲av在线| 老司机影院成人| 日本熟妇午夜| 亚洲精品粉嫩美女一区| 在线播放国产精品三级| 在线观看av片永久免费下载| 日韩高清综合在线| 日韩在线高清观看一区二区三区| 国产伦精品一区二区三区视频9| 高清毛片免费看| 国产精品av视频在线免费观看| 成年av动漫网址| 国产精品,欧美在线| av在线老鸭窝| 黑人高潮一二区| 又黄又爽又刺激的免费视频.| 日日撸夜夜添| 久久久久精品国产欧美久久久| 色综合亚洲欧美另类图片| 九九爱精品视频在线观看| 国产精品,欧美在线| 国产大屁股一区二区在线视频| 久久久久久伊人网av| 日韩欧美国产在线观看| 国产欧美日韩精品亚洲av| av专区在线播放| 听说在线观看完整版免费高清| 国产在线男女| 狂野欧美激情性xxxx在线观看| 搞女人的毛片| 免费搜索国产男女视频| 亚洲专区国产一区二区| 午夜福利在线观看吧| 婷婷精品国产亚洲av| 女人十人毛片免费观看3o分钟| 久久久久国产精品人妻aⅴ院| 国产成人a∨麻豆精品| 白带黄色成豆腐渣| 日本免费a在线| 国产午夜精品论理片| 91久久精品国产一区二区三区| 中国美白少妇内射xxxbb| a级毛片免费高清观看在线播放| 国产激情偷乱视频一区二区| 香蕉av资源在线| 国产精品久久电影中文字幕| 午夜影院日韩av| 免费在线观看成人毛片| av在线蜜桃| 国产精华一区二区三区| 国语自产精品视频在线第100页| 欧美性猛交╳xxx乱大交人| 真人做人爱边吃奶动态| 天堂动漫精品| 啦啦啦观看免费观看视频高清| www.色视频.com| 国产伦一二天堂av在线观看| 欧美精品国产亚洲| 国产精品一区二区免费欧美| 国内揄拍国产精品人妻在线| 女的被弄到高潮叫床怎么办| 国产乱人视频| 能在线免费观看的黄片| 成人毛片a级毛片在线播放| 最后的刺客免费高清国语| 免费无遮挡裸体视频| 超碰av人人做人人爽久久| 午夜免费男女啪啪视频观看 | 亚洲欧美中文字幕日韩二区| 在线观看免费视频日本深夜| 一区二区三区免费毛片| 变态另类成人亚洲欧美熟女| 亚洲精品久久国产高清桃花| 亚洲国产高清在线一区二区三| 亚洲国产精品成人久久小说 | 少妇丰满av| 男女那种视频在线观看| 国产精品三级大全| 中文字幕av在线有码专区| 亚洲最大成人av| 亚洲性久久影院| 69av精品久久久久久| 国产白丝娇喘喷水9色精品| 18禁裸乳无遮挡免费网站照片| 亚洲专区国产一区二区| 中国美女看黄片| 亚洲精品一卡2卡三卡4卡5卡| 夜夜爽天天搞| 亚洲一区二区三区色噜噜| 久久人人爽人人片av| 久久精品夜夜夜夜夜久久蜜豆| 丝袜喷水一区| 最近视频中文字幕2019在线8| 国语自产精品视频在线第100页| 亚洲最大成人手机在线| 国产av在哪里看| 九九爱精品视频在线观看| 国产国拍精品亚洲av在线观看| 中文资源天堂在线| 午夜福利在线观看吧| 中文字幕av成人在线电影| 国产v大片淫在线免费观看| 日韩欧美三级三区| 人人妻人人看人人澡| 国产成人a∨麻豆精品| 色哟哟哟哟哟哟| 国产精品伦人一区二区| 99热网站在线观看|