金與銀及合金奈米粒子催化異相一氧化碳氧化與均相三環芳香烴氫化反應研究

Abstract

本研究利用反應動力學，原位漫反射紅外光譜與氧氣溫度程控脫附等實驗技術以及理論反應模型計算研究金銀合金附載於中孔洞MCM-41材料應用於異相催化一氧化碳氧化反應，並探討可能之催化反應行為與機制。 針對利用一鍋法合成具顆粒較小之金銀合金材料，在反應溫度為80度時，其高的反應活性與罕見的一級反應級數表現可能與在合金表面上，吸附態的一氧化碳與氧氣以極為鄰近的位置，進行非解離與非競爭吸附的Langmuir-Hinshelwood反應行為。相對小的反應活化能，幾乎可以忽略的一氧化碳與氧氣表面覆蓋率以及隨著反應溫度上昇，伴隨的氣體脫附現象，皆為催化活性隨著反應溫度呈現先上昇，下降再上昇異常行為的可能原因。而在高溫時，合金催化劑表現類似於單金屬催化劑，暗示隨著不同溫度，合金具有不同的反應行為與參與催化反應的活性位置。 針對利用兩步法合成具有顆粒較大之金銀合金材料，在反應溫度為80度時，其高的反應活性與一級反應級數表現被實驗證實在合金表面吸附態的一氧化碳與氧氣在極靠近的位置進行非解離與非競爭或競爭吸附的Langmuir-Hinshelwood 反應行為。隨著反應溫度增加，反應活性下降的原因可能與吸附態一氧化碳和氧氣進行脫附行為或氧氣進行解離式吸附有關。 在室溫下水溶液中，利用微胞保護之金與銀奈米顆粒，其在金屬與微胞界面中，三環芳香烴可進行均相催化的氫化反應，實驗證實催化活性具有明顯的粒子大小控制效應而氫化反應的可能反應行為為奈米顆粒扮演類似具有傳導電子的奈米電極角色。金與銀奈米顆粒催化活性的差異與金屬親近電子與釋放電子至周圍水溶液環境的能力有關。

The heterogeneously catalytic CO Oxidation with Au-Ag alloy deposited on inert and acidic mesoporous aliminosilicate MCM-41 support, prepared by either one-pot or two-step procedure, has been investigated in terms of the experimental kinetics, in-situ DRIFTS, O2 pulse adsorption, O2-TPD and theoretical reaction modeling. For one-pot/3:1 Au-Ag/MCM-41 alloy catalyst, the unexpectedly high catalytic activity at 80oC may be associated with the non-dissociative and non-competitive adsorption Langmuir-Hinshelwood model between CO and O2 species in intimate proximity on the alloy surface. The small activation energy, negligible surface coverage and desorption with raising temperature for both CO and O2 may give rise to the unusual behavior in reaction rate above 80oC. At higher temperature, the different reaction behavior and/or active site for CO oxidation could be altered, which may behave like supported monometallic metal catalyst. For two-step/5:1 Au-Ag/MCM-41 catalyst, the high catalytic activity at 80oC could be due to non-dissociative and non-competitive or competitive Langmuir-Hinshelwood model between adsorbed CO on Au and O2 on Ag in close proximity of Au-Ag alloy surface as the RDS. The decrease in CO conversion with the increasing temperature could be caused by either desorption of both CO and O2 or dissociative adsorption for O2 on the Au-Ag alloy surface. Anthracene hydrogenation in aqueous micellar solutions at room temperature is homogeneously catalyzed by ionic-surfactant-protected Au and Ag nanoparticles with well-controlled particle sizes. A remarkable size-dependence of catalytic activity is derived. The difference in the optical property of meal nanoparticles could be related to the charging of their surfaces, indicating that both the metal nanoparticles play a role as the nanoelectrode storing electrons from hydrides. The behavior about the electron transfer-relaying effects of metal nanoparticles is proposed for the hydrogenation reaction.