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Microkinetic models based on parameters obtained from density functional theory and transition state theory have been developed for the hydrodeoxygenation (HDO) of propanoic acid, a model lignocellulosic biomass-derived organic acid, over the flat Pd(100) and Pd(111) surfaces in both vapor and liquid phase reaction conditions. The more open Pd(100) surface was found to be 3–7 orders of magnitude more active than the Pd(111) surface in all reaction environments, indicating that the (111) surface is not catalytically active for the HDO of propanoic acid. Over Pd(100) and in vapor phase, liquid water, and liquid 1,4-dioxane, propanoic acid hydrodeoxygenation follows a decarbonylation (DCN) mechanism that is facilitated by initial α- and β-carbon dehydrogenation steps, prior to the rate controlling C–OH and (partially rate controlling) C–CO bond dissociations. Only over Pd(111) and aqueous reaction environments is the decarboxylation (DCX) preferred over the DCN with the C–CO 2 step being rate controlling. 

Abstract Nitrous acid (HONO) plays pivotal roles in various metal‐free as well as metal‐mediated routes relevant to biogeochemistry, atmospheric chemistry, and mammalian physiology. While the metastable nature of HONO hinders the detailed investigations into its reactivity at a transition metal site, this report herein utilizes a heteroditopic copper(II) cryptate [oC]Cu^IIfeaturing a proton‐responsive second‐coordination‐sphere located at a suitable distance from a [Cu^II](ONO) core, thereby enabling isolation of a [Cu^II](κ¹‐ONO⋅⋅⋅H⁺) complex (2H‐NO₂). A set of complementary analytical studies (UV‐vis,¹⁴N/¹⁵N FTIR,¹⁵N NMR, HRMS, EPR, and CHN) on2H‐NO₂and its¹⁵N‐isotopomer (2H‐¹⁵NO₂) reveals the formulation of2H‐NO₂as {[oCH]Cu^II(κ¹‐ONO)}(ClO₄)₂. Non‐covalent interaction index (NCI) based on reduced density gradient (RDG) analysis on {[oCH]Cu^II(κ¹‐ONO)}²⁺discloses a H‐bonding interaction between the apical 3° ammonium site and the nitrite anion bound to the copper(II) site. The FTIR spectra of [Cu^II](κ¹‐ONO⋅⋅⋅H⁺) species (2H‐NO₂) shows a shift of ammonium NH vibrational feature to a lower wavenumber due to the H‐bonding interaction with nitrite. The reactivity profile of [Cu^II](κ¹‐ONO⋅⋅⋅H⁺) species (2H‐NO₂) towards anaerobic nitration of substituted phenol (2,4‐DTBP) is distinctly different relative to that of the closely related tripodal [Cu^II]‐nitrite complexes (1‐NO₂/3‐NO₂/4‐NO₂).