Characterizations of Bimetallic Pt-Fe Supported Catalysts and Selective Hydrogenation of Crotonaldehyde
|關鍵字:||巴豆醛;選持性氫化;Pt-Fe/BN catalyst;crotonaldehyde;selective hydrogenation;Pt-Fe/BN catalyst||公開日期:||2005||摘要:||本研究主要以BN支撐雙金屬觸媒在巴豆醛的選擇性氣相氫化反應，利用共臨濕含浸法將Pt與Fe的前軀物負載於BN上，Pt含量為1.1wt%，而鐵含量則從0.1至0.6wt%。此外，也製備單金屬觸媒Pt1.1wt%/BN及Fe0.8wt%/BN並選用以BN結構類似之石墨載體與商用觸媒載體γ-Al2O3作為比較。以三種載體支撐之雙金屬觸媒巴豆醇的選擇性皆隨著鐵含量的增加而增加，但活性卻反而降低。以BN及石墨支撐Pt-Fe雙金屬觸媒其鐵含量為0.2wt%時有巴豆醇產率最適值，其巴豆醇的選擇性而提升不儘是陽電性的Fe與高還原電位Pt之間彼此電子傳遞Pt電子密度提高而抑制C=C雙鍵的氫化，Fe以氧化態形式（Fen+）存在於表面更能吸附而活化C=O。此外，BN與石墨也提供其電子至Pt使得C=C更難活化。觸媒檢測包括N2吸附BET比表面積測定、氫氣與一氧化碳化學吸附測定、XRD、XPS、TEM、SEM-EDAX、ICP-AES、TPR進行鑑定。綜合TPR、XPS、XRD、TEM結果除了證明以BN支撐之雙金屬觸媒Pt-Fe/BN有PtFe合金相的產生外，覆蓋於其上之Fen+極化了C=O使巴豆醇選擇性提升許多。以石墨支撐之雙金屬反應結果與BN類似，但更能抑制C=C雙鍵氫化而得到最高的巴豆醇選擇性0.82（Fe/Pt=1.91），但在相同鐵含量下活性略低於BN觸媒。石墨亦為惰性物質，負載於其上之金屬也會出現聚集，TPR得知不同金屬含量的還原溫度也與BN觸媒相近，XRD也顯示純BN與石墨載體結構相似，XPS亦顯示Fen+存在，但Pt訊號較不明顯。以γ-Al2O3支撐之雙金屬觸媒對巴豆醇選擇性不佳，雖然XPS得知觸媒表面有Fen+存在，其選擇性的微幅增加是由於丁醛的生成量大幅減少，可能與Pt分散度降低有關，並推論Fen+所存在的環境可能缺乏氫原子以致於無法促進C=O氫化的速率外，Fen+易與γ-Al2O3作用有關。
The selective hydrogenation of crotonaldehyde was studied in gas phase over bimetallic BN-supported catalysts which were prepared by co-incipient wetness impregnation. The compositions of these catalysts were platinum 1.1wt% and iron varied from 0.1 to 0.6 wt%. Monometallic catalysts Pt1.1wt%/BN and Fe0.8wt%/BN were prepared for comparison. Some commercial catalyst supports, graphite and γ-Al2O3, were chosen to compare with BN. The reaction results revealed that the higher iron content on three bimetallic supported catalysts, the higher selectivity to crotyl alcohol was, but the activity was depressed at the same time. On BN and graphite bimetallic Pt-Fe supported catalysts, there was an optimum yield of crotyl alcohol at 0.2wt% Fe (Fe/Pt=0.635).The improvement of selectivity to crotyl alcohol was not only attributed to the electropositive metal B (Fe) acting as an electron-donor that increased the electron density on metal A (Pt), thus depressing the C=C hydrogenation, but also the electropositive metal (oxidized metal species) on the surface acting as electrophilic or Lewis sites for the adsorption and activation of the C=O. In addition, the BN and graphite also donated their pi electron to Pt, thus the C=C hydrogenation was hardly to activate. The catalysts were characterized by BET surface area measurement by N2 adsorption , H2 and CO chemisorption, X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) , transmission electron microscope (TEM), scanning electron microscope/energy dispersive spectrometer (SEM/EDX), inductively coupled plasma-atomic emission spectrometer (ICP-AES) , and temperature programming reduction (TPR).Summarized XRD,XPS,TEM and TPR results, Pt-Fe alloy were formed on Pt-Fe/BN and the Fen+ above (or next to ) the alloy polarized the C=O to enhance the selectivity to crotyl alcohol appreciably. The reaction results on Pt-Fe/graphite were similar to that of Pt-Fe/BN. But graphite provided more pi electrons to Pt which raised the electron density to depress the C=C hydrogenation, resulting in the highest selectivity to crotyl alcohol－0.82 (Fe/Pt=1.91),but the yield of crotyl alcohol on Pt-Fe/graphite was lower than Pt-Fe/BN at the same iron content. Graphite is also a inert species, we found the reduction temperature of various metal compositions loaded on graphite were almost the same with loaded on BN ,and the structure of pure support graphite and BN（graphitic-like）were identical to each other revealed by XRD pattern. The selectivity to crotyl alcohol on γ-Al2O3 supported catalysts was not increased efficiently. However, the Fen+ presented at the surface which was detected by XPS. The slightly improvement of selectivity to crotyl alcohol was attributed to the yield of butyraldehyde decreasing appreciably and it might caused by the decrease of Pt dispersion. We suggested that there were not sufficient hydrogen atoms surrounding with Fen+ to enhance the rate of C=O hydrogenation and also related to Fen+ easily interacting with γ-Al2O3.
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