不同粒徑大小水鈉錳礦之熱行為研究

Abstract

水鈉錳礦（Birnessite，(Na, Ca)0.5(Mn4+, Mn3+)2O4 · 1.5 H2O）是自然界中常見的黏土礦物之一，以錳氧八面體組成主要的片狀結構，層間可填入多樣陽離子（如Na+、Mg2+、K+等）及水分子，廣泛分布於深海錳核、表層土壤及氧化錳礦床等地，因特殊的層狀構造及氧化特性，水鈉錳礦為具潛力的新興應用材料。水鈉錳礦常以非常細小的晶體狀態存在，合成時亦為奈米尺度大小的晶體，因此在升溫時「尺寸效應」將劇烈影響脫水行為及相變反應，且不同於其他黏土礦物，水鈉錳礦之金屬氧化態多元，可變化性大，然而有關粒徑大小對熱行為影響之研究卻相對缺乏，因此本研究以水鈉錳礦作為研究材料，進一步了解粒徑對其熱行為的影響，包含：脫水及相變反應，並使用主導動力學曲線模型（Master Kinetics Curve model，MKC）推測反應機制。 本研究使用氧化法（Oxidation method）搭配老化法（Aging），成功合成不同粒徑大小之水鈉錳礦（平均長軸 200 nm及 >1 µm），接著分別使用熱重分析儀（TGA）、熱膨脹儀（DIL）升溫至900°C，測量其連續的重量變化及體積變化，藉以推測反應溫度；並搭配X光粉末繞射儀（XRD）及掃描式電子顯微鏡（SEM）分別進行晶相及表面形貌分析，以確認階段性變化；除此之外，也使用主導動力學曲線模型（MKC），以電腦數值模擬的方式進行擬合，分析不同階段的反應動力學。 研究成果之主要貢獻可分為三個部分： 第一部分釐清了水鈉錳礦在大氣中升溫之階段性反應，過去文獻因分析儀器限制及環境因素，對水鈉錳礦相變之中間相眾說紛紜，而本研究透過使用不同熱分析儀器進行交叉比對後，發現其熱行為具有五個階段：(1) 100~400°C 脫水反應、(2) 500°C相變反應，產生2×2的管狀礦物Na0.2MnO2；(3) 約550~600°C時，相變為2×3之管狀礦物Mn0.4MnO2及Mn2O3；(4) 790~810°C時，變為複雜之管狀礦物Na0.44MnO2及Mn2O3；最後 (5) 在860~880°C時轉變成最為穩定的黑錳礦（Hausmannite，Mn3O4）。大氣條件下，水鈉錳礦隨著溫度上升形成鈉錳比（Na/Mn）逐漸上升的高氧化態管狀礦物，並伴隨著脫氧，產生氧化態較低的Mn2O3及Mn3O4。本研究發現在大氣中加熱，水鈉錳礦第一個相變產物為2×2的管狀礦物Na0.2MnO2，其因重量變化不明顯而常被前人研究忽略。 第二部分為發現水鈉錳礦之二步驟脫水反應，第一步驟脫水反應發生於110~130°C，造成重量變化大而體積收縮小，為脫去「層間水」所造成，且在MKC動力學擬合分析中視活化能39.6 kJ/mol，而第二步驟在170~190°C時發生，其重量變化小、體積收縮卻較劇烈，可能為「晶格水」散失所造成，視活化能相對較大，為42.3 kJ/mol。 第三部分整合不同分析儀器的結果，並提出「粒徑」對水鈉錳礦熱行為的影響，不同粒徑之水鈉錳礦在二步驟脫水行為中表現不同，且相變溫度亦有差異。粒徑較小之水鈉錳礦可能含有較多「晶格水」，而「層間水」較少，因此第一步驟脫層間水的反應較粒徑大之樣本緩和，而第二步驟脫晶格水反應則較明顯，且相變反應的溫度皆提早約20°C。因為在進行合成時，水鈉錳礦由水錳礦（Pyrochroite）經順構轉變（Topotactical Conversion）而來，其單位晶格由Mn(OH)6逐漸脫水形成MnO6，而粒徑小之樣本長晶時間較短，因此殘存較多（OH-），而粒徑大之樣本經老化過程，其晶格水（OH-）轉變為層間水。 以上為本研究之研究成果，確立了水鈉錳礦加溫後脫水及相變行為的反應歷程，並提出粒徑的影響範圍，期望以上研究結果能增進我們對於水鈉錳礦熱行為之瞭解，並對學術界及應用端有所幫助。

Birnessite [(Na,Ca)0.5(Mn4+,Mn3+)2O4 · 1.5 H2O] is one of the most common clay minerals, and it is a layer-structured manganese oxide consisting of MnO6 octahedral sheets with the interlayer space filled in cations and water molecules. Different from other clay minerals, the oxidation state of birnessite is variability, and due to its special layered structure and oxidation characteristics, birnessite becomes a potential application material. No matter in the nature form or synthesis sample, birnessite is often in very fine crystal or amorphous state, so the “small scale effect” will seriously affect the thermal behaviors. however, the research on the influence of particle size on thermal behavior of birnessite is relatively lacking. Thus, this study uses Birnessite as a research material to understand the relationship of particle size and thermal behavior. In this study, we first use an oxidation method to synthesize birnessite powder of about 200 nm in diameter, and then aged it for 5 days to derive the larger sized sample which is about 1 µm. In order to understand the thermal behavior of the two different sized birnessite, thermogravimetric analysis (TGA) and a dilatometer were used to measure their weight and volume changes at various temperatures, and X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to determine their phases and morphologies. In addition, a computer simulation program of Master Kinetics Curve model (MKC) was used to analyze our data and to describe the kinetics of the phase transformation behaviors of birnessite. Generally, there are three main contributions of this research results. The first part: establishes the general thermal reaction of birnessite in the atmosphere. This study uses three analysis instruments to compare the result, and speculated that the thermal behavior under atmospheric conditions has 5 stage: (1) dehydrate before 400°C; (2) transform to 2x2 tunnel-structured Na0.2MnO2 at 500°C; (3) transform to 2x3 tunnel-structured Mn0.4MnO2 + Mn2O3 at around 550-600°C; (4) transform to Na0.44MnO2 + Mn2O3 at around 790-810°C; and finally (5) transform to Mn3O4 at 860-880°C. Therefore, this study found a 2x2 tunnel tunnel-structured Na0.2MnO2, which is often ignored due to insignificant weight change. The second part: founds the two-step dehydration reaction of birnessite. The first step of dehydration occurs at 110-130°C, resulting in large weight change and small volume shrinkage, which may be caused by removing the interlayer water. In the MKC dynamics fitting analysis, the apparent activation energy (Qa) is 39.6 kJ/mol; On the other hand, the second step occurs at higher temperature, around 170-190°C, the weight loss is small, which may be caused by lossing lattice water. The Qa is 42.3 kJ/mol, which means it needs more energy to removel the lattice water. The third part: integrated the influence of particle size on the thermal behavior of birnessite. The different sizes of birnessite have different performances in the two-step dehydration behavior, and the phase transition temperature also differs. Because of the Topotactical Conversion, there are more time to change lattice water to interlayer water in aging sample. So compare to the normal sample without aging, the aged birnessite has more interlayer water and less lattice water. Therefore, the reaction temperatures of the aged birnessite is about 20°C higher than that of the normal one. The above is the research results of the influence of particle size on the thermal behavior of birnessite. These results enhance our understanding of the thermal behaviors of birnessite, and will be useful in many practical applications.