Nitrite Reduction Mechanism on a Pd Surface
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- 作者：Hyeyoung Shin ; Sungyoon Jung ; Sungjun Bae ; Woojin Lee ; Hyungjun Kim
- 刊名：Environmental Science & Technology
- 出版年：2014
- 出版时间：November 4, 2014
- 年：2014
- 卷：48
- 期：21
- 页码：12768-12774
- 全文大小：415K
- ISSN：1520-5851
文摘
Nitrate (NO3鈥?/sup>) is one of the most harmful contaminants in the groundwater, and it causes various health problems. Bimetallic catalysts, usually palladium (Pd) coupled with secondary metallic catalyst, are found to properly treat nitrate-containing wastewaters; however, the selectivity toward N2 production over ammonia (NH3) production still requires further improvement. Because the N2 selectivity is determined at the nitrite (NO2鈥?/sup>) reduction step on the Pd surface, which occurs after NO3鈥?/sup> is decomposed into NO2鈥?/sup> on the secondary metallic catalyst, we here performed density functional theory (DFT) calculations and experiments to investigate the NO2鈥?/sup> reduction pathway on the Pd surface activated by hydrogen. Based on extensive DFT calculations on the relative energetics among 鈭?00 possible intermediates, we found that NO2鈥?/sup> is easily reduced to NO* on the Pd surface, followed by either sequential hydrogenation steps to yield NH3 or a decomposition step to N* and O* (an adsorbate on Pd is denoted using an asterisk). Based on the calculated high migration barrier of N*, we further discussed that the direct combination of two N* to yield N2 is kinetically less favorable than the combination of a highly mobile H* with N* to yield NH3. Instead, the reduction of NO2鈥?/sup> in the vicinity of the N* can yield N2O* that can be preferentially transformed into N2 via diverse reaction pathways. Our DFT results suggest that enhancing the likelihood of N* encountering NO2鈥?/sup> in the solution phase before combination with surface H* is important for maximizing the N2 selectivity. This is further supported by our experiments on NO2鈥?/sup> reduction by Pd/TiO2, showing that both a decreased H2 flow rate and an increased NO2鈥?/sup> concentration increased the N2 selectivity (78.6鈥?3.6% and 57.8鈥?0.9%, respectively).