Synthesis of titanate nanotubes via microwave hydrothermal treatment: Study on the characterization and photocatalytic potential
|關鍵字:||二氧化鈦;氧化鈦奈米管;微波水熱法;光催化;水中氨氮;氣相三氯乙烯;TiO2;Titanate nanotubes;Microwave hydrothermal;Photocatalysis;Aqueous ammonia;Trichloroethylene||公開日期:||2008||摘要:||儘管以TiO2為主之光催化程序多年來已被廣泛地討論與探究，然仍因其高度穩定性與低耗費等優點而不失為一引人入勝之綠色科技。近年來，利用TiO2所衍生出之氧化鈦奈米管(titanate nanotubes, TNTs)由於其高比表面積而持續受到多方關注。但利用傳統水熱法合成TNTs常需20小時以上之合成時間才可以得到完整之管狀結構。因此，本研究旨在利用微波水熱法嘗試增加其合成動力，並探究微波能於其物化特性之影響。而微波型TNTs之光催化潛勢亦以液相氨氮與氣相三氯乙烯作為模擬污染物進而評估其光催化程序之可行性。130℃與90分鐘之水熱合成條件下，傳統水熱法與微波水熱法所得到之比表面積分別為76 與 256 m2g-1。此結果顯示所輸入之微波能可大幅增加TNTs之合成動力。再者，微波型TNTs之結構傾向於NaxH2-xTi3O7，其中之Na/H比則由所輸入之微波能所主導。此現象可經由多種分析技術所證明，諸如XRD晶相圖、氨氣程溫脫附實驗、X射線光電子電譜等。而對於後熱程序對於TNTs之結晶行為而言，結構內具較多氫離子之TNTs會經由[TiO6]之重組堆疊而形成anatase晶相；而當TNTs具有較多之鈉離子時，經700℃燒結後會形成柱狀結構之Na2Ti6O13。Na2Ti6O13內之(Ti6O13)2-為TNTs結構內之(Ti3O7)2-層經由拓樸聯結沿著方向所結合而成。於TNTs之光催化潛勢而言，TNTs晶相對水中氨氮並無強大之光催化能力，然結構內若有rutile晶相存在時，則可提升其光催化效能。而經酸洗後之TNTs則因為其離子交換能力之增強而可提升其光催化效能。此因素為除酸洗可促進TNTs結構內之rutile晶相成長外，藉由離子交換而置入TNTs結構內之氨氮則可避免因過度負載觸媒量所造成之遮蔽效應。而當利用TNTs作為TCE降解之測試觸媒時，TCE之降解速率會隨著TNTs之燒結溫度而提升(100~500℃)。此為因經燒結後之TNTs具有強度明顯之anatase晶相，再者多種晶相的呈現可造就電子傳遞效應而提升光催化效率。而當利用Pt,Pd修飾經TNTs燒結所得TiO2顆粒時，實驗結果說明Pt與Pd之參與皆會造成TCE光降解之效率變低，尤以Pd顆粒之影響為深。此為因Pd顆粒與TiO2之氧原子半徑相似而鍵入TiO2結構內而導致內部氧原子移動率不佳。再者，Pt對TCE光降解之中間產物DCAC與phosgene的生成並無影響，Pd則傾向選擇phosogene之生成與具有較高礦化率之特性。綜言之，微波型TNTs雖無強勢之光催化潛勢與離子交換能力，然其仍可藉由此兩功能並存之特性應用於多種領域中。
Despite TiO2-based photocatalysis has been extensively investigated and examined over the past decades, it is still a highly engrossing technology owing to the stability and low cost. Recently, TiO2-induced titanate nanotubes (TNTs) have received much attention as a result of high specific surface area. Traditional method in fabricating TNTs, however, needs at least 20 hr reaction time to achieve a perfect tube structure. Therefore, this research aimed to the rapid formation kinetics of TNTs with the aid of microwave irradiation and attempted to investigate the effect of microwave irradiation on the characterization of titanate nanotubes (microwave-induced TNTs). Photocatalytic behavior of microwave-induced TNTs towards the degradation of gaseous trichloroethylene (TCE) and aqueous ammonia (NH3/NH4+) were also examined to survey the photocatalytic potential of microwave-induced TNTs.ased on the performance of BET surface area (SBET), TNTs synthesized at 130℃ for 1.5 hr with and without 400W irradiation presented the SBET of 256 and 76 m2g-1, respectively. The result indicates that the formation kinetics of TNTs is significantly enhanced via microwave hydrothermal treatment. The microwave-induced TNTs are preferentially assigned for NaxH2-xTi3O7 whose Na/H ratio is dominated by the applied lever of microwave irradiation during fabrication process. This phenomenon can be evidenced by various determinations including powder X-ray diffraction, NH3-temperature programmable desorption, X-ray photoelectron spectroscopic, and ionic coupled plasma-atomic emission spectrometry. Regarding the behavior of TNTs after thermal treatment, TNTs with abounding H atoms presented anatase phase at 500℃ through rearrangement and restacking of [TiO6]. The sintered TNTs synthesized under high irradiation power presented the rod shape at 700℃ which mainly comprise of Na2Ti6O13. The (Ti6O13)2- unit within Na2Ti6O13 is constructed by two (Ti3O7)2- layers within TNTs via the topotactical connection along the  direction during thermal process.s for the photocatalytic potential of TNTs, a pure TNTs phase presents no powerful ability towards photocatalytic NH3/NH4+ while the photocatalytic efficiency can be enhanced with the presence of rutile phase within TNTs. Regarding the effect of acid-washing treatment on TNTs, the acid-treated TNTs with enhanced ion exchangeability considerably improve the NH3/NH4+ degradation and NO2-/NO3- yields. This result is likely ascribed to the easy intercalation of NH3/NH4+ into the structure of acid-washing TNTs so that the photocatalytic oxidation of intercalated NH3/NH4+ is not limited to the shielding effect resulting from the overload of TNTs. In the case of photocatalytic TCE over TNTs, the efficiency of TCE degradation enhances with increasing sintering temperature until 700℃。This phenomenon is attributed to the recrystallization of anatase phase and the construction of inter-particle electron transfer effect. Photocatalytic TCE over Pt/Pd doped TNTs-induced TiO2 was also investigated in terms of the effect of Pt and Pd on the TCE degradation and on the yields of dichloroacetyl chloride (DCAC) and phosgene. In the presence of Pt and Pd, the degradation of TCE was retarded; especially Pd had a significantly negative effect on TCE degradation, which was ascribed to the intercalation of Pd into the lattice of TiO2. Moreover, Pt had no influence on the selectivity toward DCAC and phosgene while the selectivity toward phosgene in the presence of Pd was enhanced. As for the behavior of Pt and Pd in TCE degradation, Pt doped TiO2 exhibited the same photocatalytic behavior as P25 TiO2 whereas Pd doped TiO2 led to a different photocatalytic mechanism. Although microwave-induced TNTs have no powerful ability in photocatalysis and ion exchange, they can still be considered as a potential material in some applications owing to the corresponding bi-functions.
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