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Abstract There is not a clear understanding of the extent by which naturally occurring reactions can attenuate trichloroethene (TCE) and its daughter products within low permeability zones (LPZs), and addressing this knowledge gap requires advancement of methods to accurately measure in situ volatile chemical concentrations. In this study, a soil coring method that freezes the soil in‐situ (a.k.a., cryogenic coring) was utilized to measure depth‐discrete distributions of TCE and its volatile reaction products through a TCE‐impacted silty clay aquitard, and results were compared with those from adjacent soil cores taken using a conventional coring approach. Vertical concentration profiles of TCE,cis‐1,2‐dichloroethylene (DCE), vinyl chloride (VC), ethane, and methane were all compared between the two coring methods, and results indicate the two coring methods recovered statistically equivalent concentrations of volatiles across most depths of the fine‐grained cohesive clayey soil at the study site. Biotic reductive dechlorination was the dominant TCE reaction pathway at the site; several reduced gasses that are possible markers for abiotic reduction were detected, but their concentrations and intervals of occurrence were not sufficiently consistent to indicate whether they were from abiotic TCE reduction or unrelated biological processes. Overall, cryogenic coring yielded improved recovery of sand lenses compared to conventional coring, but offered no apparent benefits for improved recovery of TCE and its volatile reaction products in the low permeability aquitard material at the site. 

Abstract Herein, aqueous nitrate (NO₃⁻) reduction is used to explore composition‐selectivity relationships of randomly alloyed ruthenium‐palladium nanoparticle catalysts to provide insights into the factors affecting selectivity during this and other industrially relevant catalytic reactions. NO₃⁻reduction proceeds through nitrite (NO₂⁻) and then nitric oxide (NO), before diverging to form either dinitrogen (N₂) or ammonium (NH₄⁺) as final products, with N₂preferred in potable water treatment but NH₄⁺preferred for nitrogen recovery. It is shown that the NO₃⁻and NO starting feedstocks favor NH₄⁺formation using Ru‐rich catalysts, while Pd‐rich catalysts favor N₂formation. Conversely, a NO₂⁻starting feedstock favors NH₄⁺at ≈50 atomic‐% Ru and selectivity decreases with higher Ru content. Mechanistic differences have been probed using density functional theory (DFT). Results show that, for NO₃⁻and NO feedstocks, the thermodynamics of the competing pathways for N–H and N–N formation lead to preferential NH₄⁺ or N₂production, respectively, while Ru‐rich surfaces are susceptible to poisoning by NO₂⁻feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N₂production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.