應用銀/二氧化錳觸媒於氨氣選擇性觸媒氧化反應: 銀負載以及載體結構對於反應活性之影響

Abstract

以氨氣作為還原劑之選擇性觸媒還原反應(NH3-SCR)，已被證實是一個有效去除氮氧化物(NOx)的方法，由於NH3-SCR反應中所添加的氨氣無法完全地與NOx進行反應，因此會造成氨溢漏(NH3-slip)的問題，過量的氨氣排放至大氣中，將造成霧霾以及PM2.5等環境危害。為了解決氨溢漏的問題，氨氣選擇性氧化反應(NH3-SCO)藉由氨氣與過量的氧氣進行反應，將其轉化成無毒的氮氣，被視為一個有效的解決方案。其中Ag/MnO2觸媒已被報導可於較低的反應溫度下，達成優異的氨氣轉化效率及氮氣選擇率[1]。 於本研究中我們嘗試以二氧化錳(MnO2)作為載體，並擔載銀奈米顆粒形成Ag/MnO2觸媒，藉以探討NH3及O2分子於Ag/MnO2觸媒表面的活化，由一氧化碳-氧氣脈衝式表面反應(CO-O2 pulse surface reaction)結果得知，擔載銀奈米顆粒後，表面活性氧數量提升了約3.7倍，且表面活性氧位點可以透過吸附氣相O2分子，達到重新的活化以及再生。X光吸收光譜(XAS)、X光電子能譜(XPS)及氫氣程序升溫還原(H2-TPR)結果顯示MnO2載體與Ag奈米顆粒間存在電荷傳遞的現象，此電荷轉移的現象將會大幅提升MnO2載體的還原性質，同時增加MnO2與Ag奈米顆粒接觸介面之晶格氧活動度以及反應性。氨氣脈衝式表面反應(NH3-pulse surface reaction)結果顯示，擔載銀奈米顆粒後，氨氣與表面活性氧間的反應活性提升，N2生成量提升約3.2倍。 除此之外，我們更將Ag奈米顆粒擔載於四種不同結構之MnO2載體，分別為α-、β-、γ-以及δ-MnO2，藉此比較不同MnO2載體結構對於反應的影響，由XAS以及H2-TPR結果得知: Ag奈米顆粒與不同結構MnO2之間的電荷傳遞量不盡相同，經迴歸分析後證實，金屬與載體之間的交互作用力，與NH3-SCO催化活性呈現高度的正相關。

Selective catalytic reduction with ammonia (or NH3-SCR) has been identified as one of the most effective techniques to eliminate the nitric oxides (NOx) pollutant. Nevertheless, the NH3 slip during the SCR process (i.e., NH3 is not completely consumed by reacting with NOx) should be avoided as NH3 is a precursor of haze and PM2.5. The selective catalytic oxidation (SCO) of NH3 into N2 and H2O is a promising approach to remove NH3. Ag/MnO2 catalysts are reported to be active for SCO of NH3 at low reaction temperatures with high N2 selectivity[1]. In this study, the activations of NH3 and O2 over the Ag/MnO2 catalyst were investigated to explore the associated mechanism of SCO of NH3. Pulse reactions with CO-O2 titration show that a small amount of active oxygen species is present on the pristine MnO2 surface, and its amount is increased by 3.7 times when the surface is decorated with Ag nanoparticles. X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction with H2 show that there is an electron transfer from the MnO2 support to the Ag nanoparticles of Ag/MnO2. Such electron transfer may enhance the redox ability of MnO2, by which increases the mobility of lattice oxygen at the perimeter of the Ag-MnO2 interface, thus resulting in more active oxygen species on Ag/MnO2 that are responsible for SCO of NH3. NH3-pulse surface reactions show that the reaction between NH3 and active oxygen can be enhanced by Ag nanoparticles decoration. The amount of N2 is increased by 3.2 times after loading Ag nanoparticles. Moreover, we also synthesized MnO2 support with distinct structures (i.e., α-, β-, γ-, and δ-MnO2) followed by impregnation of Ag nanoparticles to yield MnO2-supported Ag catalysts. It is found that the structure of MnO2 support has profound influences on the catalytic performance of Ag/MnO2 on low-temperature SCO of NH3; the temperatures for 100% NH3 conversion of α-, β-, γ-, and δ-MnO2 supported Ag catalysts are 126, 146, 90 and 244 ℃ respectively. XAS and H2-TPR results showed that Ag/γ-MnO2 possesses the highest amount of electron transfer. There is a linear correlation between metal-support electronic interaction and NH3-SCO performance.