Nitrite Reduction Mechanism on a Pd Surface

Abstract

Nitrate (NO3-) 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-) reduction step on the Pd surface, which occurs after NO3- is decomposed into NO2- on the secondary metallic catalyst, we here performed density functional theory (DFT) calculations and experiments to investigate the NO2- reduction pathway on the Pd surface activated by hydrogen. Based on extensive DFT calculations on the relative energetics among ∼100 possible intermediates, we found that NO2- 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- 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- 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- reduction by Pd/TiO2, showing that both a decreased H2 flow rate and an increased NO2- concentration increased the N2 selectivity (78.6-93.6% and 57.8-90.9%, respectively).

Hydrogenation of aqueous nitrate and nitrite with ruthenium catalysts

“…For Ru/C, the distribution of N 2 :ammonium shifts increasingly towards N 2 with increasing initial nitrite concentration (Fig. 8b), similar to trends reported for Pd-based catalysts [85]. Scheme 1 depicts the generally accepted mechanism of nitrate reduction on Pd-based catalysts.…”

State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates

“…More recently, Shin et al analysed the role of NO as an intermediate in NH 4 + evolution[43]. In this report, two different pathways for the reduction of NO to NH 4…”

Exploring beyond palladium: Catalytic reduction of aqueous oxyanion pollutants with alternative platinum group metals and new mechanistic implications

“…Thus, lower cost and more sustainable reductive catalytic technologies [15,16] Various factors such as size and morphology of both the metal nanoparticles and the support materials can also influence the reactivity of platinum group metals [19,57], but the effects are generally less pronounced compared to the effects of metal identity (e.g., Pd versus Rh in ClO 3 − reduction, or Pd versus Ru in NO 3 − reduction) as demonstrated in this study. Computational studies [19,58] are also needed to provide detailed theoretical insights into these metal-specific effects and metal-substrate interactions. We emphasize that research priority should be given to further examining novel activity of these previously overlooked metals in water treatment applications, such as (1) exploring multi-functional activities in treating various emerging and challenging contaminants (e.g., nitro [59], fluoro [40][41][42][43] and polychloro [60,61]…”