雙金屬觸媒Pt-Sn/BN在丙烷脫氫之研究

Deydrogenation of Propane Using Bimetallic Pt-Sn/BN Catalyst

林上傑(Shang-Jie Lin)

指導教授 ： 吳紀聖

國立臺灣大學/工學院/化學工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.00102

摘要

摘要

本研究以BN支撐的雙金屬觸媒在丙烷的脫氫及氧化脫氫反應，我們利用共臨界沾濕法製備不同Sn 含量的Pt-Sn/BN 觸媒，並另外製備單金屬觸媒1.1wt%Pt/BN及1.0wt%Sn/BN，且也選用商業觸媒載體γ-Al2O3以做為比較。觸媒的檢測有N2吸附比表面積測定、氫氣化學吸附測定、X-ray Diffraction（XRD）、X-ray Photoelectron Spectroscopy（XPS）及Transmission Electron Microscope（TEM）。從XPS上，發現大部分Sn 的價態在400℃氫氣還原後仍維持在氧化態，而Pt-Sn/BN 則有部分的氧化錫被還原為金屬態而形成Pt-Sn合金。另外，在XRD結果也顯示，Pt-Sn/BN上有SnPt3及PtSn晶相存在。而Pt-Sn/γ-Al2O3則沒有任何Pt-Sn 合金在XRD上被偵測到。在600℃下的反應結果顯示，錫的加入有效地增加了Pt/BN的活性及觸媒壽命，錫的添加量越多，丙烯的選擇率與產率越高。當反應進行六小時後，在以BN支撐的雙金屬觸媒中，擁有最低Pt分散度（0.67%）的Pt-Sn(0.75wt%)/BN，保有最高的丙烯產率（25.3%），相較於整體表現最佳丙烯產率（29.5%）的Pt-Sn(0.50wt%)/γ-Al2O3（Pt分散度為40%）在分散度上有極大的差異。推測PtxSn晶相的形成使Pt-Sn /BN中的Sn能將部份電子傳遞給鉑金屬而增加其電子密度，繼而降低對C=C鍵(alkene)的束縛能，進而促進了反應活性、丙烯選擇率及觸媒壽命。此外，在丙烷氧化脫氫的反應中，以γ-Al2O3所支撐的觸媒發生明顯的失活現象，而在BN上並無此現象，推測此為反應中所生成的水氣遮蔽了活性基，降低了觸媒活性。因此，觸媒在丙烷脫氫及氧化脫氫的活性及壽命的表現上，除了與本身的性質與Pt分散度有關外，表面所形成的Pt-Sn合金及其晶相也具有決定性的影響。

關鍵字

丙烷 ； 脫氫 ； 雙金屬 ； 丙烯

並列摘要

Abstract

In the present study we focused on the dehydrogenation and oxidative dehydrogenation of propane over Pt-Sn/BN catalysts with various Sn loadings which were prepared by co-incipient wetness impregnation. We also prepared monometallic catalysts 1.1wt%Pt/BN and 1.0wt%Sn/BN for comparison. A commercial catalyst support γ-Al2O3 was chosen to compare with BN. Several characterization techniques were used including BET surface area measurement by N2 adsorption, hydrogen selective chemisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). From the XPS analysis, most of tin remained Sn2+ after H2 reduction at 400oC. PtxSn alloys were formed due to partially reduced Sn in Pt-Sn/BN. Furthermore, the crystalline phases of PtSn and SnPt3 alloys were also observed from the XRD spectra of Pt-Sn/BN catalysts. However PtxSn alloys were not detected in Pt-Sn/γ-Al2O3 by XRD. The reaction results at 600℃ revealed that the tin addition clearly improved the activity and lifetime of Pt/BN. The more tin in Pt-Sn/BN, the higher the selectivity and yield to propylene were. After six-hour reaction, a maximum yield of propylene (25.3%) was maintained using Pt-Sn(0.75wt%)/BN catalyst with the poorest Pt dispersion (0.67%) in all of Pt-Sn/BN catalysts. The Pt dispersion of Pt-Sn(0.75wt%)/BN was much smaller than that of Pt-Sn(0.50wt%)/γ-Al2O3. However, the yield of propylene on Pt-Sn(0.75wt%)/BN was almost equal to Pt-Sn (0.50wt%)/ γ-Al2O3 (propylene yield 29.5%). We suggest that the electropositive metal B (Sn) on Pt-Sn/BN acts as an electron-donor increasing the electron density on metal A (Pt), thus depresses the C=C (alkene) binding energy, then enhances the catalytic activity, propylene selectivity and lifetime. Furthermore, the catalysts of γ-Al2O3 were deactivated rapidly in the oxidative dehydrogenation of propane. We were speculating that active sites were blocked by the formation of H2O during reaction, thus decreasing activity. Consequently, not only the nature of supports and Pt dispersion, but also PtxSn alloys and crystalline phases of PtSn on the BN-supported affected the performance of catalysts for propane
dehydrogenation and oxidative dehydrogenation.

並列關鍵字

BN ； Pt-Sn ； dehydrogenation ； propane ； bimetallic

參考文獻

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