| 摘要: | 本研究製備與改質鈀銀薄膜以應用於氫氣分離。研究中利用多種無電鍍法在改質後之多孔銀基板上鍍覆鈀銀膜，包含傳統無電鍍共鍍法(conventional electroless co-plating (C-ELCP))，連續無電鍍法(successive electroless plating (S-ELP))和銀離子控制無電鍍共鍍法(Ag-controlled electroless co-plating (Ag-ELCP))。鈀銀薄膜和銀基板之形貌、表面官能基和結構分別藉由掃描電子顯微鏡(scanning electron microscope, SEM)，傅立葉轉換紅外光譜(Fourier transform infrared spectroscopy, FTIR)和X-光繞射儀(X-ray diffraction, XRD)來進行鑑定。
本研究中選用經由Pd70Ag30奈米顆粒改質之多孔銀基板作為鍍覆鈀銀薄膜的基材，先將銀顆粒鍍覆一層聚乙烯吡咯烷酮(PVP)或硬脂酸(SA)進行改質，並由傅立葉轉換紅外光譜(Fourier transform infrared spectroscopy, FTIR) 確認銀顆粒外圍成功包覆SA或PVP後，再經油壓機在1500 psi壓力下持力3分鐘，再經由350 oC持溫1小時的熱處理即可得到多孔銀基材，再填充Pd70Ag30奈米顆粒後，銀基板表面孔徑減少且變得平滑。
根據SEM的觀察可發現，在400 rpm鍍液的轉速下，可利用C-ELP法在已經由 Pd70Ag30奈米顆粒表面改質後的銀基板上得到緻密與均勻的鈀銀薄膜。然而，即使將無電鍍液中鈀/銀比例提高至90/10，成功鍍覆在Ag-PVP基板上的薄膜中鈀的含量僅只有37.6 %。而利用S-ELP法可製備出鈀/銀比例為70/30之薄膜，但其分別由銀、鈀和鈀銀相所組成，經由不同的氣氛熱處理可發現，空氣熱處理後的鈀銀薄膜會產生氧化現象，而經由氫氣與氮氣熱處理後，薄膜可成功形成合金相。以Ag-ELCP法製備出的鈀銀薄膜擁有最佳且緻密的表面結構，經由不同熱處理時間發現，薄膜緻密性隨著熱處理時間增長而提升，經由15小時氫氣熱處理的薄膜中鈀含量由原本60-80 %降至20-30 %，其原因為在長時間的熱處理下，會造成銀從基板大量偏析至薄膜中。
在氫氣滲透實驗可發現，氫氣在不同壓力下對應的滲透通量遵循線性關係，偏離了Sievert’s law，表示氫氣滲透鈀銀薄膜之速率決定步驟為表面反應。在本研究中，利用Ag-ELCP法製備出的鈀銀薄膜氫/氮選擇性為最高，約等於3，此乃因鈀銀薄膜的表面氧化造成一些微孔級的缺陷而降低選擇性。
;In this study, the preparation and modification of PdAg membrane used for H2 separation has been studied. The PdAg membranes are deposited on the modified Ag substrate by various electroless plating methods, including conventional electroless co-plating (C-ELCP), successive electroless plating (S-ELP), and Ag-controlled electroless co-plating (Ag-ELCP). The morphologies, surface function groups, and structures of the prepared Ag substrates and PdAg membranes are analyzed by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD), respectively.
The porous Ag substrates with surface modification of Pd70Ag30 nanoparticles are used to support the membrane. Ag powders are coated by SA or PVP, which are confirmed by Fourier transform infrared spectroscopy (FTIR). Afterwards, Ag-SA and Ag-PVP powders are mechanically pressed at 1500 psi for 3 min, and heat-treated at 350 oC for 1 h in air to get porous Ag substrates. By filling with Pd70Ag30 nanoparticles, the pore size has decreased and the surface becomes smooth.
SEM observation reveals that a uniform and dense PdAg membrane can be prepared by C-ELP at 400 rpm of stirring rate on Ag-PVP substrate. However, even the ratio of Pd/Ag in plating bath is up to 90/10, the Pd composition on the Ag-PVP substrate is only 37.6 %. On the other hand, when prepared by S-ELP method, it is found that desired Pd/Ag ratio of 70/30 with separated Ag, Pd, and PdAg phases of the membrane can be obtained. After heat treatment at various atmospheres, it is noted that air heat treatment results in the oxidation of Pd. However, N2 and H2 treatments can promote the formation of PdAg alloy phase. For the membrane prepared by Ag-CLCP, a membrane with the most uniform and dense structure can be prepared. During heat treatment, Pd composition decrease and the membrane become denser with increasing heat treatment time. The SEM line-scan results reveal that the Pd content is in the range of 60 and 80 at% for as-prepared C-Pd70. After heat treatment for 15 h, the Pd composition drops to 20~30 at % due to significant Ag segregation from the substrate during long-term heat treatment.
The corresponding permeation fluxes of H2 at various pressures follow a linear relationship, suggesting that Sievert’s law is not obeyed in the PdAg membranes and the permeation rate is dominant by surface-reaction-controlled process. The H2/N2 selectivity is about 3 for C-Pd70 sample prepared by Ag-ELCP method because of the formation of micrometer-scaled hillocks and pinholes which declines the selectively. |
