甲醇蒸汽重組產氫觸媒之設計

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题名:	甲醇蒸汽重組產氫觸媒之設計
作者:	張丞鈞;Chang,Cheng-chun
贡献者:	化學工程與材料工程學系
关键词:	甲醇蒸汽重組;甲醇複合式蒸汽重組;銅觸媒;氧化鋯;氧化鈰;貴金屬;Steam reforming of methanol;oxidative steam reforming of methanol;copper catalysts;ZrO2;CeO2;noble metal
日期:	2014-01-20
上传时间:	2014-04-02 14:53:26 (UTC+8)
出版者:	國立中央大學
摘要:	本研究宗旨在設計效能佳的SRM及OSRM產氫觸媒。本研究分三階段，第一階段為最佳觸媒成份比例探討，以商用觸媒G66B的成份比例30/60/10為起始參考，藉調變觸媒基礎成份比例，探討各成份對反應的影響，並確立其最適比例範圍。再於最適比例觸媒分別引入不同比例CeO2及ZrO2取代ZnO，有系統地探討其對觸媒於SRM及OSRM反應的真實影響及其扮演的角色，並確立其適當添加量。另於CuO/ZnO/Al2O3觸媒中分別以含浸法及共沉澱法引入少量貴金屬Pd、Pt、Ru或Rh作為促進劑，探討貴金屬對銅基觸媒於甲醇產氫反應的影響及其扮演的角色。第二階段藉由探討共沉澱pH值、熟化溫度及煅燒溫度對觸媒於SRM反應的影響，將最佳成份比例觸媒進一步最適化。第三階段以SRM反應進行觸媒穩定性測試，探討觸媒活性衰退的原因。最後以Power law及Langmuir-Hinshelwood模式進行最佳觸媒於SRM反應之動力分析。 CuO、ZnO及Al2O3均為觸媒不可或缺的成份，Cu0是觸媒主要活性位置，Cu0與ZnO間的協同效果是影響觸媒活性的重要因素，Al2O3扮演增強觸媒機械性質的角色，但添加Al2O3相對減少ZnO含量，使CuO分散性下降，大幅減少Cu0與ZnO之間有效接觸，對觸媒活性有明顯的負面效果，僅宜少量添加。觸媒基礎成份比例最適範圍為CuO/ZnO/Al2O3(30/60/10)~CuO/ZnO/Al2O3(40/50/10)。 CeO2能提升觸媒分散性、Cu0表面積及觸媒還原能力，但有抑制methoxy脫氫轉化成formate的負面效果，明顯不利於觸媒活性，此結果與文獻報導的差異來自於文獻探討觸媒成份Al2O3含量偏高，添加CeO2取代Al2O3而降低後者對觸媒活性的負面影響。 ZrO2可提升觸媒整體分散性、Cu0表面積及還原能力，還可提升甲醇吸附量，但ZrO2對CuO/ZnO/Al2O3觸媒活性促進效果受限於Cu0與ZnO之間的有效接觸，呈現效果所需CuO/ZnO臨界比值約為0.8，當CuO/ZnO比值大於0.8，ZrO2促進效果無法顯現，小於0.8，ZrO2促進效果隨CuO/ZnO比值愈低而越明顯，最適添加量約為20 wt%。 CuO/ZnO/Al2O3(30/60/10)觸媒以共沉澱法引入貴金屬不影響觸媒結構及分散性等物理性質，但可有效提升觸媒還原能力，貴金屬主要扮演幫助擷取H原子促使甲醇解離吸附及反應中間物脫氫轉化的角色，添加Pd、Pt、Ru或Rh皆可提升CuO/ZnO/Al2O3觸媒於SRM及OSRM反應活性，Pt促進效果最佳，Pd次之，最適添加量皆為1 wt%。Pt不影響觸媒CO選擇率，但Pd、Ru、Rh會增加產物中CO的濃度。若以臨濕含浸法引入貴金屬無法彰顯促進效果，甚至抑制觸媒活性，並大幅增加產物中CO濃度。共沉澱pH值8及熟化溫度80°C均有利於CuO/ZnO/Al2O3(30/60/10)、1%Pt-CuO/ZnO/Al2O3(30/60/10)及CuO/ZnO/ZrO2/Al2O3(30/40/20/10)觸媒前驅物形成aurichalcite結構，經煅燒後CuO及ZnO分散性較佳，可提升Cu0與ZnO之間的有效接觸。CuO/ZnO/Al2O3(30/60/10)及添加Pt之觸媒最適煅燒溫度為350°C，CuO/ZnO/ZrO2/Al2O3(30/40/20/10)觸媒最適煅燒溫度為450°C。提升CuO/ZnO/ZrO2/Al2O3(30/40/20/10)觸媒煅燒溫度不使觸媒燒結，還可強化Cu0與ZnO及ZrO2之間的協同作用而提升觸媒活性。反應達穩定前，ZnO燒結是觸媒衰退的主因。反應達穩定後，ZnO分散性變化不顯著，觸媒表面積碳是觸媒長時間反應活性衰退的主要因素。添加ZrO2可有效防止觸媒反應過程中燒結並減少積碳，增進觸媒穩定性，縮短觸媒達穩定所需時間。CuO/ZnO/ZrO2/Al2O3(30/40/20/10)觸媒是本研究活性及穩定性最佳之甲醇產氫觸媒。 Power Law及Langmuir-Hinshelwood模式表達之反應速率式均能準確預測SRM反應速率，且Langmuir-Hinshelwood動力模式所反映之SRM反應機制與本研究分析論述的機制一致。; The goal of our work was to design a highly active and selective catalyst for the steam reforming of methanol (SRM) and oxidative (or combined) steam reforming of methanol (OSRM). First of all, the composition (CuO/ZnO/Al2O3 = 30/60/10) of a commercial catalyst G66B was used as a reference for investigating not only the optimal composition of CuO/ZnO/Al2O3 catalysts but also the effect of each component of the catalyst on the SRM and OSRM. The effects of CeO2 and ZrO2 over a wide range of compositions of the catalysts and their optimal composition of the catalysts were also investigated. The effect of noble metal (Pd, Pt, Ru or Rh) on the SRM and OSRM reaction over CuO/ZnO/Al2O3 catalyst were also systematically studied. The noble metal was deposited on CuO/ZnO/Al2O3 by incipient impregnation and coprecipitation methods. The roles of noble metal and CuO/ZnO/Al2O3 were clarified. Since the appropriate composition of the catalysts were determined, the catalysts were further optimized through establishing the appropriate preparation conditions such as pH value during coprecipitation, aging temperature and calcination temperature. Ultimately, the stability of these catalysts in the SRM were also observed in order to clarify the optimal catalysts. Then the kinetic modeling of SRM over the catalyst was studied. Copper oxide and zinc oxide are the critical components of these catalysts on SRM and OSRM. Metallic copper is the active site of the CuO-based catalysts. The synergestic effect between Cu and ZnO plays an important role in SRM and OSRM reactions. Typically, Al2O3 is introduced into the binary catalyst CuO/ZnO to improve its dispersion, stability and mechanical strength. However, the interface of Cu and ZnO were decreased due to the replacement of ZnO by Al2O3 and SRM and OSRM reaction were weakened. The optimal composition of the catalysts were CuO/ZnO/Al2O3 (30/60/10) to CuO/ZnO/Al2O3(40/50/10). Cerium oxide increased the reducibility of the catalyst, but inhibited the transformation of methoxy to formate thus weakened the SRM and OSRM reaction. Zirconium oxide improved the dispersion and reducibility of the CuO/ZnO/Al2O3 catalysts and promoted the adsorption of methanol. The promoting effect of ZrO2 was observed only on the CuO/ZnO/Al2O3 with the CuO/ZnO ratio less than 0.8. A lower CuO/ZnO ratio in the CuO/ZnO/Al2O3 catalysts was associated with greater promoting effect of ZrO2 in the SRM and OSRM reactions, which depend on appropriate interactions through the interface between CuO and ZnO. No obvious effect of Pd and even a negative effect of Pt were observed by incipient impregnation method. With co-precipitation, noble metals were homogeneously dispersed in CuO/ZnO/Al2O3(30/60/10). They improved the reducibility of the catalysts and enhanced the dissociative adsorption of methanol. Introducing Pd, Rh or Ru promoted the conversion of methanol, but enhanced the formation of CO. Depositing Pt exhibited a high conversion of methanol and a low selectivity of CO in the SRM and OSRM reaction. The promoting effect of noble metals involved facilitating the split and adsorption of H atoms during the dehydrogenation of the intermediates in the reaction. The formation of aurichalcite structure of the precursor of CuO/ZnO/Al2O3 (30/60/10)、1%Pt-CuO/ZnO/Al2O3(30/60/10) and CuO/ZnO/ZrO2/Al2O3(30/40/20/10) catalysts were benifited under optimal preparation conditions which the pH value of 8 during coprecipitation and the aging temperature of 80°C. The structure of aurichalcite was adventageous to the dispersion of CuO and ZnO as well the interface between them. The opitmal caicination temperature of 350°C for CuO/ZnO/Al2O3(30/60/10) and 1%Pt-CuO/ZnO/Al2O3(30/60/10) and 450°C for CuO/ZnO/ZrO2/Al2O3(30/40/20/10). The synergestic effect among Cu0, ZnO and ZrO2 was strenthen via increasing the calcination temperature of the CuO/ZnO/ZrO2/Al2O3(30/40/20/10) catalyst thus promote the catalytic activity. The sintering of ZnO leaded to deactivation of the catalysts before a steady convertion was obtained. Since the catalytic activity reached to a steady state, the decreasing of the dispersion of ZnO could not be observed therefore the deactivation of the catalysts was caused by the coking formation on the surface of the catalysts. ZrO2 could prevent sintering and coking of the catalysts and promote the stability of the catalysts effectively. CuO/ZnO/ZrO2/Al2O3(30/40/20/10) was the optimal catalyst for SRM and OSRM reaction. The rate equation of SRM over CuO/ZnO/ZrO2/Al2O3 (30/40/20/10) obtained by Power Law and Langmuir-Hinshelwood model could predicte the reaction rate precicly and the result of kinetic study confirmed that the mechanism of SRM which was speculated and used in this study was quite reasonable.
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