VOC氧化鉑觸媒的XPS/SIMS研究(2/2)

Abstract

本研究使用兩種不同結晶度的h-BN(hexagonal-boron nitride)及Al2 O3 做為觸媒載體，高結晶度的標為a-BN 、較低結晶度的標為b-BN ，以H2 PtCl6 做為觸媒前軀物使用臨界沾濕法製備白金觸媒，通入不同溫度空氣或氮氫混合氣對觸媒做前處理，對benzene 進行深度氧化反應，發現以高於400 ℃的空氣進行前處理或是以300 ℃以上氮氫混合氣前處理過的Pt/BN觸媒擁有低於150 ℃的點火點，經300 ℃空氣前處理後的0.37%Pt/a-BN 觸媒也具有低於150 ℃的點火點，但是3.7%Pt/a-BN、0.37%Pt/b-BN 或3.7%Pt/b-BN 經300℃空氣前處理後點火點仍約在170 ℃附近，但所有Pt/BN 觸媒對Benzene 反應的點火點皆遠低於Pt/Al2 O3 的250 ℃以上。以XPS 測量Pt 的4f7/2 束縛能發現對benzene 深度氧化反應點火點低於150 ℃的觸媒表面Pt 較接近於還原金屬態，而以reflective UV-vis 作檢測，可在點火點高於150 ℃的Pt/BN 上發現以Cl-Pt 電子轉移(charge transfer band)的UV 吸收峰，以TPR測試發現在BN 表面上的Pt 能在較低溫度被還原，推測Pt/BN 觸媒擁有高氧化活性是因為BN 與Pt 之間不發生電子轉移的現象，使得Pt 表面維持還原金屬態的電子構造較易活化氧原子所致。而在BN 上較低比表面積及載體對貴金屬的弱固定力對活性及觸媒壽命的影響並沒有被觀察到。

Two crystalline h-BN’s(hexagonal boron nitride) and Al2O3 were used as supports. High-crystallinity BN was assigned as a-BN, and low-crystallinity BN was assigned as b-BN. Pt catalysts were prepared by incipient wetness using H2 PtCl6 as precursor. Pretreatment was performed under air or H2 /N2 mixture. The light-off temperature (LOT) of benzene deep oxidation was found below 150 C when Pt/BN was pretreated under air at 400 C, or H2 /N2 under 300 C. The 0.37 wt% Pt/a-BN with air pretreatment at 300 C also gave LOT below 150 C. However, the LOT’s were near 170 C for 3.7 wt% Pt/a-BN, 0.37 wt% Pt/b-BN and 3.7 wt% Pt/b-BN after pretreatment of air at 300 C. All LOT’s of Pt/BN were still below that of Pt/Al2 O3 which was above 250 C. From the XPS, the binding energy of Pt 4f7/2 indicated the reduced status of Pt, which is closely related to the LOT below 150 C. The reflective UV-vis observed the charge transfer band of Cl-Pt on the catalysts which gave LOT above 150 C. From the H2 TPR, the reduction temperature of Pt/BN was low because of negligible electron interaction between Pt and BN support. Thus Pt was maintained in reduced status and could activate oxygen molecule effectively. Therefore Pt/BN gave the high activity of oxidation. The activity and life-time of Pt/BN were not influenced by the small surface area and the weak anchorage of Pt under our experimental conditions.