Competitive Adsorption of Nitrite and Hydrogen on Palladium during Nitrite Hydrogenation

Water Confinement on Polymer Coatings Dictates Proton-Electron Transfer on Metal-Catalyzed Hydrogenation of Nitrite

Enzymes can precisely control the speed and selectivity of chemical reactions by modifying locally the solvent-reactant interactions. To extrapolate these attributes to heterogeneous catalysts, we have employed thermoresponsive poly n-isopropylacrylamide (p-NIPAM) brushes bonded to silica spheres containing palladium. These polymers can form hydrogen bonds with water molecules at low temperatures (<32 °C) allowing the polymer to stay swollen. Detailed reaction kinetics of nitrite hydrogenation showed that p-NIPAM decreases the apparent activation barrier by a factor of 3 at low temperatures. Diffusion-ordered spectroscopy nuclear magnetic resonance and ab initio molecular dynamics simulations showed that when p-NIPAM is present, water molecules near the surface are less mobile. This confinement perturbs the water interaction with the metal, reducing the barrier for the proton-electron transfer reduction of nitrite. Notably, this enhancement vanishes at high temperature as the polymer collapses on itself exposing the Pd to unconfined water. The fully reversible nature of this process opens the door for creating homeostatic catalysts with controlled water-confinement.

Tuning the Selectivity of Nitrate Reduction via Fine Composition Control of RuPdNP Catalysts

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 N2 preferred 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 N2 formation. 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 N2 production, 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 N2 production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.

Highly Selective Nitrite Hydrogenation to Ammonia over Iridium Nanoclusters: Competitive Adsorption Mechanism

Wet denitrification is a promising approach to control nitrogen oxides (NOx) produced in fossil fuel combustion. Yet, the highly concentrated nitrite (NO2-) wastewater generated poses a major threat to the aqueous environment. Here, iridium nanoclusters (d = 1.63 nm) deposited on TiO2 were applied for NO2- reduction to ammonia (NRA), showing an exceptional NH4+ selectivity of 95% and a production rate of 20.51 mgN·L-1·h-1, which held significant potential for NO2- wastewater purification and ammonia resource recovery. Notably, an interesting non-first-order NO2- hydrogenation kinetics was observed, which was further confirmed to result from the competitive adsorption mechanism between H2 and NO2- over iridium. The NRA pathways on the Ir(111) surface were explored via density functional theory calculations with the NO2-* → NO* → HNO* → HNOH* → H2NOH* → NH2* → NH3* identified as the most energetically favorable pathway and the NO* → HNO* confirmed as the rate-determining step. In situ DRIFTS further experimentally verified the generation of HNO* intermediate during NO* hydrogenation on Ir(111). To verify NRA kinetics at varied NO2- concentrations or H2 pressures, a kinetic model was derived based on the Langmuir-Hinshelwood competitive adsorption mechanism. These findings provide mechanistic insights into the NRA pathways on Ir nanocatalysts, which will be beneficial for wet denitrification waste stream decontamination and valorization.

Formic acid generating in-situ H2 and CO2 for nitrite reduction in aqueous phase

The aim of this work is to explore and to understand the effect of pH, concentrations and presence of oxygen traces on reduction nitrite in drinking water with Pd/γ-Al2O3, using...

Enhanced adsorption and reduction performance of nitrate by Fe-Pd-Fe3O4 embedded multi-walled carbon nanotubes

Multi walled carbon nanotubes (MWCNTs) have attracted more and more attention as adsorbents due to their excellent adsorption properties. By loading metal particles on MWCNTs, the chemical reduction ability of adsorbed pollutants could be provided, so as to achieve the purpose of adsorption and degradation of pollutants. Therefore, the removal process of NO₃^--N by Fe-Pd-Fe₃O₄/MWCNTs was studied, including rapid adsorption of initial pollutants, gradual reduction of intermediate products and re-adsorption of final products. The results showed that Fe-Pd-Fe₃O₄/MWCNTs completely removed NO₃^--N within 2 h, 39% and 25% of which were converted into NO₂^--N and NH₄⁺-N. The adsorption efficiency, kinetics, capacity and adsorption energy all followed the order of NH₄⁺-N > NO₂^--N > NO₃^--N. With the recoverability and reusability of Fe-Pd-Fe₃O₄/MWCNTs having been confirmed in 5 consecutive cycles, the removal rate of NO₃^--N still reached 43%. It has been shown that MWCNTs prolonged the reducing power for NO₃^--N, due to avoiding the aggregation of metal particles. The rapid adsorption of initial pollutants, effective stepwise reduction and convenient recovery processes were of great value for the rehabilitation of polluted water.

Two Isostructural URJC-4 Materials: From Hydrogen Physisorption to Heterogeneous Reductive Amination through Hydrogen Molecule Activation at Low Pressure

Herein, two novel isostructural metal-organic frameworks (MOFs) M-URJC-4 (M = Co, Ni; URJC = "Universidad Rey Juan Carlos") with open metal sites, permanent microposity, and large surface areas and pore volumes have been developed. These novel MOFs, with polyhedral morphology, crystallize in the monoclinic P2₁/c space group, exhibiting a three-dimensional structure with microporous channels along the c axis. Initially, they were fully characterized and tested in hydrogen (H₂) adsorption at different conditions of temperature and pressure. The physisorption capacities of both materials surpassed the gravimetric H₂ uptake shown by most MOF materials under the same conditions. On the basis of the outstanding adsorption properties, the Ni-URJC-4 material was used as a catalyst in a one-pot reductive amination reaction using various carbonyl compounds and primary amines. A possible chemical pathway to obtain secondary amines was proposed via imine formation, and remarkable performances were accomplished. This work evidences the dual ability of M-URJC-4 materials to be used as a H₂ adsorbent and a catalyst in reductive amination reactions, activating molecular H₂ at low pressures for the reduction of C═N double bonds and providing reference structural features for the design of new versatile heterogeneous materials for industrial application.

A computational model for the catalytic hydrogel membrane reactor

The catalytic hydrogel membrane reactor (CHMR) is a promising new technology for hydrogenation of aqueous contaminants in drinking water. It offers numerous benefits over conventional three-phase reactors, including immobilization of nano-catalysts, high reactivity, and control over the hydrogen (H₂) supply concentration. In this study, a computational model of the CHMR was developed using AQUASIM and calibrated with 32 experimental datasets for a nitrite (NO₂^-)-reducing CHMR using palladium (Pd) nano-catalysts (~4.6 nm). The model was then used to identify key factors impacting the behavior of the CHMR, including hydrogel catalyst density, H₂ supply pressure, influent and bulk NO₂^- concentrations, and hydrogel thickness. Based on the model calibration, the reaction rate constants for the NO₂^- steady-state adsorption Hinshelwood reaction equation, k₁ and k₂, were 0.0039 m³ mole-Pd^-1 s^-1 and 0.027 (mole-H₂ m³)^1/2 mole-Pd^-1 s^-1, respectively. The reactant flux, which is the overall NO₂^- removal rate for the CHMR, is affected by the NO₂^- reduction rate at each catalyst site, which is in turn controlled by the available NO₂^- and H₂ concentrations that are regulated by their mass transport behavior. Reactant transport in the CHMR is counter-diffusional. So for thick hydrogels, the concurrent concentrations of NO₂^- and H₂ are limiting in the middle region along the x-y plane of the hydrogel, which results in a low overall NO₂^- removal rate (i.e., flux). Thinner hydrogels provide higher concurrent reactant concentrations throughout the hydrogel, resulting in higher fluxes. However, if the hydrogel is too thin, the flux becomes limited by the amount of Pd that can be loaded, and unused H₂ can diffuse into the bulk and promote biofilm growth. The hydrogel thickness that maximized the NO₂^- flux ranged between 30 and 150 μm for the conditions tested. The computational model is the first to describe CHMR behavior, and it is an important tool for the further development of the CHMR. It also can be adapted to assess CHMR behavior for other contaminants or catalysts or used for other types of interfacial catalytic membrane reactors.

Activity and stability of the catalytic hydrogel membrane reactor for treating oxidized contaminants

The catalytic hydrogel membrane reactor (CHMR) is an interfacial membrane process that uses nano-sized catalysts for the hydrogenation of oxidized contaminants in drinking water. In this study, the CHMR was operated as a continuous-flow reactor using nitrite (NO₂^-) as a model contaminant and palladium (Pd) as a model catalyst. Using the overall bulk reaction rate for NO₂^- reduction as a metric for catalytic activity, we evaluated the effect of the hydrogen gas (H₂) delivery method to the CHMR, the initial H₂ and NO₂^- concentrations, Pd density in the hydrogel, and the presence of Pd-deactivating species. The chemical stability of the catalytic hydrogel was evaluated in the presence of aqueous cations (H⁺, Na⁺, Ca²⁺) and a mixture of ions in a hard groundwater. Delivering H₂ to the CHMR lumens using a vented operation mode, where the reactor is sealed and the lumens are periodically flushed to the atmosphere, allowed for a combination of a high H₂ consumption efficiency and catalytic activity. The overall reaction rate of NO₂^- was dependent on relative concentrations of H₂ and NO₂^- at catalytic sites, which was governed by both the chemical reaction and mass transport rates. The intrinsic catalytic reaction rate was combined with a counter-diffusional mass transport component in a 1-D computational model to describe the CHMR. Common Pd-deactivating species [sulfite, bisulfide, natural organic matter] hindered the reaction rate, but the hydrogel afforded some protection from deactivation compared to a batch suspension. No chemical degradation of the hydrogel structure was observed for a model water (pH > 4, Na⁺, Ca²⁺) and a hard groundwater after 21 days of exposure, attesting to its stability under natural water conditions.