一氧化氮在鈀/二氧化鈦觸媒上的表面化學

Abstract

選擇性催化還原反應(selective catalytic reduction, SCR)被認為是一個有效降低氮氧化物(NOx)的技術，在以氫氣(H¬2)及為還原劑的反應中，科學家們紛紛提出一氧化氮(NO)的解離為反應之速率決定步驟(rate determining step)。在過去的研究中，大多利用分子束(molecular beam)研究NO於金屬上的吸脫附與解離現象，因此我們希望利用脈衝實驗模擬分子束實驗，以探討NO於觸媒的表面化學。 本研究中我們利用二氧化鈦(TiO2)作為擔體，以沉積沉澱法(deposition-precipitation method)擔載0.5%的鈀(Pd)合成0.5Pd/TiO2觸媒，探討NO在TiO2和0.5Pd/TiO2觸媒表面上的吸脫附及解離現象。由一氧化氮程序升溫表面實驗(NO-TPSR)結果得知，擔載Pd有助於NO解離，且NO於兩種觸媒上會經由不同反應路徑產生N2O。由NO脈衝實驗結果可以得知，在100oC時，NO會以分子型態吸附於0.5Pd/TiO2表面上；在250oC時，NO會吸附於0.5Pd/TiO2表面上並產生解離，但沒有產生其他氣體；在400oC時，NO會吸附於觸媒表面上並產生N2O及N2等氣體，NO解離所產生的氧原子會進入觸媒表層底下，因此由CO脈衝產生的CO2所計算出的氧原子較NO解離所產生的氧原子少；在100oC時，NO會以分子形式吸附於觸媒表面上。透過原位擴散反射式紅外光光譜儀(in-situ DRIFTS)進行NO-TPSR結果得知，室溫下，NO會以nitrate (NO3-)的形式存在於TiO2觸媒表面上，升溫過程中熱穩定性較差的monodentate nitrate會逐漸轉變成熱穩定性較佳的bidentate nitrate，或分解形成NO或NO2並脫附。對於0.5Pd/TiO2觸媒，NO除了以nitrate的形式存在，還會吸附於Pd2+上，在升溫的過程中，在250oC時，NO會氧化且nitrate會還原形成nitrite (NO2-)，而nitrite可以進一步反應生成N2O。

Selective catalytic reduction (SCR) has been widely considered as an effective technology to reduce nitrogen oxides (NO and NO2). For H2-SCR, the dissociation of NO on the catalysts is rate-determining step for the formation of N2. In previous studies, molecular beams have been predominantly used to investigate the adsorption, desorption, and dissociation of NO on metals. Therefore, we aim to use pulse experiments to simulate molecular beam experiments in order to explore the surface chemistry of NO on catalysts. In this study, 0.5Pd/TiO2 was prepared by deposition-precipitation method. Temperature-programmed surface reaction with NO (NO-TPSR) results reveal that Pd facilitates NO dissociation, and N2O is formed through different reaction pathways on TiO2 and 0.5Pd/TiO2. NO pulse reaction experiments show that NO molecularly adsorbs on the surface of 0.5Pd/TiO₂ at 100°C. At 250°C, NO adsorbs on the catalyst surface and dissociates without other products. At 400°C, NO dissociates on the catalyst surface and produces N2O and N2. The oxygen atoms from NO dissociation dissolve into the bulk of catalyst, resulting in fewer oxygen atoms calculated from the CO2 produced by the CO pulse than the oxygen atoms generated by NO dissociation. In-situ DRIFTS study indicates that IR band assigned to nitrate (NO3-) are predominant in the spectra for TiO2 catalyst. During heating, monodentate nitrates with low thermal stability gradually convert to bidentate nitrates which have higher thermal stability, or decompose to NO and NO2. For the 0.5Pd/TiO2 catalyst, NO not only exists as NO3- but also adsorbs on Pd2+. During heating, the nitrates reduce to nitrites (NO2-), which can further react to produce N2O.