電鍍法在不□鋼載體製備鈀膜與其在氫氣純化及滲透的應用

Abstract

有關鈀膜之製備，目前已知技術係以無電鍍法（electroless）、真空濺鍍（sputtering）、或冷軋法（cold-rolled）來進行。其中，有關在多孔性金屬載體上的鈀膜覆鍍，目前皆採用無電鍍法製程，然而，無電鍍法係依賴化學還原後金屬微粒在基材上的物理吸附而附著鍍膜，鍍膜與多孔性金屬載體間的附著力不強，故所製備的鈀膜常在使用一段時間後，因鈀膜與載體熱膨脹係的差異，鈀膜會從多孔性金屬載體上剝離。本研究成功以電鍍的方式於多孔性金屬載體上提供高附著性之鈀膜，可用於提供氫氣純化或合成用之觸媒反應器的鈀膜管件。

本論文係欲研究於多孔性不□鋼載體上電鍍鈀膜，並探討氫氣滲透電鍍鈀膜之行為，論文架構包含三個主題，分別為 1. 多孔性不□鋼表面前處理。 2. 電鍍鈀膜之備製。 3. 氫氣滲透電鍍鈀膜行為研究。

首先針對AISI 316L多孔不□鋼於60∼80℃的1：1、2：1以及3：1磷硫酸混合液進行電解拋光研究。電解拋光的拋光電位選定在陽極極化曲線中平坦的極限電流區，並搭配旋轉圓柱電極系統（RCE）進行測試。由結果可以得知混合液的比例與拋光溫度對於多孔不□鋼有顯著的影響。最佳的表面整平與亮化效果的條件是在70℃的2：1的混合液。然而1：1和2：1混合液的溫度若是超過75℃，則PSS表面的孔洞就會明顯的擴大，推測其原因可能是在孔洞內快速的解離所致。此外在3：1的混合液中隨著溫度的變化（60∼80℃），PSS表面會有不同的形貌產生。但PSS形貌均具有圓角化的情況發生。所有的混合液比例與拋光溫度對於PSS的影響均利用電化學測試設備進行測試，而PSS表面形貌的變化則利用掃瞄式電子顯微鏡來進行觀察與輔證。

接著，已經成功地利用旋轉圓柱電極系統在PSS表面以電鍍的方式備製鈀膜， 在電鍍的過程中，使用高轉速以及低電流密度可以有效的減少氫氣的共沈積，得到表面平整且無缺陷的鈀膜。藉由電鍍所備製的鈀膜對於氫氣滲透特性，如氫氣流量與選擇率(H2/He >100,000)等均有不錯的表現。在甲醇重組反應中，電鍍鈀膜由混和氫氣中粹取之氫氣純度亦相當優異（4N8）。此外由結果發現電鍍鈀膜在250-350℃的相變化溫度區間中，當氫氣壓力為9kgf/cm2時電鍍鈀膜並無產生氫脆的現象，因此推測電鍍鈀膜對於氫脆有較佳的抵抗性。

最後針對H2＋Y的混和氫氣透過鈀膜的滲透行為，利用新式的定濃度方式來進行研究。定濃度法乃為解決氫氣混合氣體進行氫氣滲透鈀膜實驗時，引發氫氣濃度遞減效應所創建氫氣滲透鈀膜的實驗方式。由實驗結果得知，混和的氫氣的滲透性與純氫的滲透行為截然不同。由Sievert's equation Q = JH / [(PRH)1/2–(PPH)1/2]，即使調整了氫氣的分壓也無法得到正確的氫氣流量與滲透量，而混和氫氣中的氫氣濃度也會影響到氫氣的流量與滲透量。

雖然鈀膜只容許氫氣滲透，不使其他氣體滲透通過(如氮氣、氬氣或甲烷)，但在氫氣滲透現象存在下少量的其他氣體都會與氫氣一同滲透過鈀膜造成滲透端氫氣純度的低落，即使鈀膜並無缺陷存在此一現象依舊存在，此為一種新現象，文獻中尚無記錄。造成這種滲透的原因歸咎於鈀晶格在吸附氫氣後的伸縮性膨脹造成晶格間孔隙的放大。

研究結果將以掃瞄式電子顯鏡、穿透式電子顯鏡、X-ray能譜分析儀、電化學檢測技術以及氣相層析儀，來觀察並探討電鍍鈀膜與氫氣滲透之影響。

In this thesis, a novel palladium membrane electroplated on porous stainless steel support was developed for hydrogen separation process. Also the performance of the as-developed Pd- membrane was studied. The thesis is reported sequentially in three sections as follows:

First, the electrochemical polishing of porous stainless steel (PSS), AISI 316L, in the phosphoric-sulfuric mixed acid with a volume-ratios of 1 : 1, 2 : 1 and 3 : 1 at temperatures ranging from 60°C to 80°C was studied. Electrochemical polishing of PSS was performed in the potential located in the limiting-current plateau of its anodic polarization curve using a rotating cylinder electrode (RCE). The results show that the electrochemical polishing of PSS is strongly affected by the volume ratio of the mixed acid and the polishing temperature. An optimal brightening and leveling surface of PSS could be achieved by polishing in 2 : 1 v/v mixed acid at 70°C. Whereas, polishing in 1 : 1 and 2 : 1 v/v ratios at and above 75°C would result in formation of enlarged pores on the PASS surface due to high dissolution rate within the pores. There is difference in the surface morphology of PSS when polishing in 3 : 1 v/v mixed acid at temperature range from 60°C to 80°C, but with rounded edges around the surface pores of PSS. The effect of the acid volume-ratio and the polishing temperatures on polishing PSS was discussed based on the results of electrochemical test and the observed polishing surface morphology using scanning electron microscope (SEM).

Secondly, a novel electroplating method of preparing Pd membrane on an AISI 316L porous stainless steel support is successfully developed by proper control of rotation speed of the support. The use of low current density with high rotation speed enables us to avoid hydrogen absorption during electroplating and to prepare smooth thin membrane with defect-free. The electroplated-Pd membrane shows well property in the hydrogen permeation, flux and excellent permselectivity(H2/He >100,000), the Pd membrane is used in a steam reformer of methanol for the production of high purity hydrogen. It was also found that the membrane spontaneously exhibited resistance to hydrogen embrittlement around the phase transition temperature of 280oC, when operated in the temperature range of 250oC-350oC under the hydrogen pressure of 9kgf/cm2.

Finally, the newly developed constant concentration method was adopted to study the hydrogen permeation of a hydrogen mixture of H2 + Y through a palladium membrane tube. The hydrogen permeation of the hydrogen mixture differs from that of a single hydrogen feed. For the hydrogen mixture, the well known Sieverts equation, Q = JH / [(PRH)1/2–(PPH)1/2], fails to yield the correct hydrogen flux or permeance even after the pressure terms are adjusted to the partial pressure of hydrogen. The hydrogen concentration in the mixture affects both the flux and the permeance.

Significant abnormal permeation of the non-hydrogen gas, Y, in the hydrogen permeates is detected during the hydrogen permeation of the mixture, H2 + Y, even though Y-gas alone does not permeate through the defect-free palladium membrane. This Y-gas slippage in the presence of hydrogen in the mixture is tentatively attributed to the expansion of the palladium atomic lattice, enlarging inter-cluster openings Y-gas permeates through the enlarged structure or grain boundary of the palladium atoms in the membrane.