以錳鈰氧化物與石墨烯複合材於不足量臭氧條件進行一氧化氮觸媒氧化

Abstract

氮氧化物(NOx)為由工業鍋爐燃燒所產生之主要氣狀污染物之一，其對於環境的危害包含導致酸雨和參與光化學反應、形成光煙霧等等。現今，工業上大多以氨氣作為還原劑之選擇觸媒還原法(NH3-SCR)作為控制氮氧化物排放的主要方式。然而NH3¬-SCR其操作溫度不僅高、容許操作溫度範圍也相對侷限，這項限制使得操作溫度較低之工業製程並不適合使用NH3-SCR來控制NOx濃度。因此，作為可行之解決方案之一，以臭氧(O3)輔助之一氧化氮(NO)觸媒氧化法(catalytic oxidation)逐漸受到重視。若同時使用觸媒以及臭氧，即可實現於50‒300 °C之溫度區間中皆能達到高NO轉化率之NO氧化系統。 於本研究中，我們嘗試合成錳鈰氧化物與石墨烯之複合材(MnOx-CeOx-GO)作為觸媒，並調整鈰(Ce)和氧化石墨烯(graphene oxide, GO)於觸媒中的含量，以觀察Ce和GO之添加對觸媒活性的影響。我們於兩種不同的氧化環境測試觸媒於50‒350 °C之NO催化氧化活性。兩種氧化環境分別為：(1)含氧(O2)環境、(2)含O2和O3環境。NO催化氧化實驗之結果顯示Ce和GO之添加皆有助於提升NO催化氧化之活性，尤以150‒200 °C之活性提升最顯著。改質後的MnOx-CeOx-GO觸媒最高能於250 °C達成90%之NO轉化效率。 NO於本研究所建置之系統中透過兩種氧化機制進行反應，分別為：(1)以O3為氧化劑之NO氧化，以及(2)以O2為氧化劑之NO催化氧化。前者於溫度150 °C以下時有顯著NO轉化能力，後者則於溫度200‒250 °C時有較佳的NO轉化能力。於NO氧化實驗中，結果顯示出O3並不會和錳氧化物與氧化石墨烯之複合材(MnOx-GO)進行交互作用，兩者各自於系統中完成了NO之氧化反應，且O3有效地彌補了觸媒於150 °C下之低NO轉化率。使用MnOx-GO及O3輔助之NO觸媒氧化於50‒300 °C都展現出可觀的NO轉化能力。然而，本研究中所使用之MnOx-CeOx-GO則對於O3有一定的分解能力。Ce的添加將促使O3於觸媒表面發生之分解反應。此一分解反應對於同時使用O3和觸媒之氧化系統不利，進而導致150°C以下之NO轉化率低落。 針對上述兩種氧化機制之有效溫度，本研究分別於100°C和250°C進行二氧化硫(SO2)之抗性測試。結果顯示SO2雖對於由O3驅動之NO氧化並沒有負面影響，但對於觸媒卻會造成不可逆之失活(deactivation)反應。本研究所使用之錳氧化物與重量比例4%之氧化石墨烯複合材(MnOx-GO (4 wt%))在經過1.5小時之SO2毒化後，其250°C之NO轉化能力由80%降至10%，而錳鈰氧化物與重量比例10%之氧化石墨烯複合材(MnOx-CeOx-GO (10 wt%))在經過1.5小時之SO2毒化後，其250°C之NO轉化能力由90%降至40%，顯示Ce和GO的改質對於SO2抗性亦有相當程度的提升。

Nitrogen oxides (NOx), as one of the major gaseous pollutants resulting from industrial boiler combustion, may cause acid rain and take part in photochemical reactions in the ambient. Flue gas NOx emission has been widely controlled by selective catalytic reduction with ammonia (NH3-SCR); however, its relatively high and narrow operating temperature has limited its feasibility in low-temperature industries. To cope with the limitation, catalytic oxidation of nitric oxide (NO) with ozone (O3) was proposed as a possible solution. With the simultaneous use of O3 and catalysts, the catalytic oxidation may reach a significant NO conversion rate in a broad temperature range of 50‒300 °C. In this study, we synthesize the manganese and cerium oxide-supported graphene-based catalysts (MnOx-CeOx-GO) with different graphene oxide (GO) loadings. Besides, we evaluate the NO catalytic oxidation performance of each catalyst over 50‒350 °C under two oxidation modes: (1) oxygen (O2) only and (2) O2 with O3. Cerium and GO modification considerably enhanced NO oxidation efficiency around 150-200 °C. Two NO oxidation pathways were found in this research, which are: (1) NO homogeneous oxidation with O3, and (2) NO catalytic oxidation with O2 over catalysts. The first route with O3 is considered effective below 150 °C. The latter pathway with O2 over catalysts, as described by Mars-van Krevelen (MvK) mechanism, shows significant effectiveness around 200‒250 °C. No interaction between O3 and manganese oxide-supported graphene-based catalysts (MnOx-GO) was found during the NO oxidation tests. O3 successfully complements the NO oxidation efficiency below 150 °C. The NO oxidation with O3 over MnOx-GO catalysts exhibits excellent NO conversion over 50‒300 °C. However, O3 decomposition was observed over the MnOx-CeOx¬-GO catalysts. Ce doped on the catalysts’ surface may participate in decomposing O3, leading to poor NO oxidation with O3. Sulfur dioxide (SO2) tolerance tests were performed at 100 °C and 250 °C to survey the SO2 toxic effect on two oxidation mechanisms. SO2 showed no adverse effect on the NO oxidation with O3 but irreversibly deactivated the catalysts. The NO conversion at 250 °C over MnOx-GO (4 wt%) dropped from 80% to 10% after 1.5 h of SO2 exposure, while the NO conversion over MnOx-CeOx-GO (10 wt%) dropped from 90% to 40% under the same operating condition. The results indicated that Ce and GO modification could slightly enhance the SO2 resistance of the catalysts.