Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “reduction”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 973 records · Page 54

Hydricity, electrochemistry, and excited-state chemistry of Ir complexes for CO 2 reduction

Here, we prepared electron-rich derivatives of [Ir(tpy)(ppy)Cl] + with modification of the bidentate (ppy) or tridentate (tpy) ligands in attempt to increase the reactivity for CO 2 reduction and the ability to transfer hydrides (hydricity). Density functional theory (DFT) calculations reveal that complexes with dimethyl-substituted ppy have similar hydricities to the non-substituted parent complex, and photocatalytic CO 2 reduction studies show selective CO formation. Substitution of tpy for bis(benzimidazole)-phenyl or -pyridine (L3 and L4, respectively) induces changes in the physical properties much more pronounced than addition of methyl groups to ppy. Theoretical data predict [Ir(L3)(ppy)(H)] is the strongest hydride donor among complexes studied in this work, but [Ir(L3)(ppy)(NCCH 3 )] + cannot be reduced photochemically because the excited state reduction potential is only 0.52 V due to the negative ground state potential of –1.91 V. The excited state [Ir(L4)(ppy)(NCCH 3 )] 2+ is the strongest oxidant among complexes studied in this work and the singly reduced species is formed readily upon photolysis in the presence of tertiary amines. Both [Ir(L3)(ppy)(NCCH 3 )] + and [Ir(L4)(ppy)(NCCH 3 )] 2+ exhibit electrocatalytic current for CO 2 reduction. While a significantly greater overpotential is needed for the L3 complex, a small amount of formate (5-10 %) generation in addition to CO was observed as predicted by the DFT calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Reaction mechanism of the selective reduction of CO 2 to CO by a tetraaza [Co II N 4 H] 2+ complex in the presence of protons

The tetraaza [Co II N 4 H] 2+ complex ( 1 ) is remarkable for its ability to selectively reduce CO 2 to CO with 45% Faradaic efficiency and a CO to H 2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO 2 reduction. The selectivity for CO 2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO 2 , and bicarbonate. After reduction of the catalyst by two electrons, formation of [Co I N 4 H] + –-CO 2 - is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [Co I N 4 H] + –CO 2 H, followed by reaction with CO 2 to form [Co II N 4 H] 2+ –CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO 2 as a Lewis acid to assist in C–O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1 . Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Spectroelectrochemical investigations of nickel cyclam indicate different reaction mechanisms for electrocatalytic CO 2 and H + reduction

Rising levels of atmospheric carbon dioxide continue to motivate the development of catalysts that can efficiently convert CO 2 to useful products in water without substantial amounts of H 2 formed as a byproduct. In addition to synthetic efforts, mechanistic investigations on existing catalysts are necessary to understand the molecular factors contributing to activity and selectivity, which can guide rational improvements and increase catalyst robustness. [Ni(cyclam)] 2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) is one such catalyst, known for decades to be capable of selective CO 2 reduction to CO in water, but with little mechanistic information experimentally established or catalytic intermediates characterized. To better understand the mechanisms of aqueous H + and CO 2 reduction by [Ni(cyclam)] 2+ , spectroelectrochemical investigations were performed in conjunction with activity assays. Both large surface area glassy carbon and amorphous graphite rod working electrodes were tested, with the latter found to be significantly more active and selective for CO production. Optical, resonance Raman, and EPR spectroelectrochemical experiments on [Ni(cyclam)] 2+ during catalysis under N 2 , CO 2 , and CO gases show the appearance of a single species, independent of electrode used. Identical signals are observed under oxidizing potentials. Spectroscopic and electrochemical analysis coupled with density functional theory calculations suggest that the signals observed originate from [Ni(cyclam)(H 2 PO4)] 2+ . The generation of a NiIII species under catalytic, reducing conditions suggests an ECCE mechanism for H + reduction by [Ni(cyclam)] 2+ , which differs from the proton-coupled, ECEC pathway proposed for CO 2 reduction. The divergent mechanisms seen for the two reactions may underlie the differential reactivity of [Ni(cyclam)] 2+ towards each substrate, with implications for the design of increasingly selective molecular catalysts. Finally, this observation also highlights the substantial impact of buffer and electrode choice when characterizing and benchmarking the catalytic properties of new compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Development of nickel-incorporated MCM-41–carbon composites and their application in nitrophenol reduction

MCM-41–carbon composites were obtained by soft-templating strategy by adding resorcinol and formaldehyde as carbon precursors during the synthesis of MCM-41. The synthesis of these composites was optimized until mesoporous MCM-41 domains remained highly ordered and the overall specific surface area and pore volume remained relatively high. The one-pot synthetic procedure was further extended to obtain the Ni-containing MCM-41–carbon composites using nickel nitrate as an additive. A homogeneous distribution of nickel nanoparticles in the composites was achieved by adding histidine to the reaction mixture, which was able to form a complex with nickel ion. Carbonization of organic components in the composites produced carbon that was responsible for in situ reduction of nickel(II) to metallic nickel (Ni0) at elevated temperatures, eliminating the post-synthesis reduction step carried out in the presence of hydrogen gas or other reducing agent. Furthermore, the resulting composites showed enhanced catalytic activity toward nitrophenol reduction obeying the zero-order kinetics. Three unique features of the resulting composites, namely mesoporous channels of MCM-41, carbon-induced in situ reduction of Ni ions to metallic Ni nanoparticles, and abundance of highly active Ni sites due to high dispersion of Ni0, make this strategy applicable for the synthesis of other transition metal-based catalysts. XRD and STEM analyses revealed a homogeneous distribution of nickel nanoparticles in the composites without significant distortion of their ordered mesoporosity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Prussian blue as a co-catalyst for enhanced Cr (VI) photocatalytic reduction promoted by titania-based nanoparticles and aerogels

Hexavalent chromium (Cr(VI)) is an noxious and highly toxic heavy metal that presents a serious threat to human health if present even in low concentrations in drinking water. Photocatalytic reduction of Cr(VI) to its less toxic Cr(III) state is a potential strategy to combat Cr(VI) pollution, but the efficiency of the process is low, especially in the absence of hole scavenger organic reagents. To address this issue and prepare efficient photocatalysts for Cr(VI) removal from water, in this study, we explored Prussian blue (PB) as a co-catalyst for improving the photoreduction performance of different high surface area TiO 2 -based materials (titania or silica–titania nanoparticles and aerogels). Here, the photocatalyst nanomaterials were surface-modified with nanocrystalline PB using the photodeposition route. The PB layer acts as an effective electron acceptor/mediator between the semiconductor photocatalyst and Cr(VI) species. All the PB-modified photocatalysts exhibit higher photocatalytic activity (up to 9 times faster) as compared to the unmodified photocatalysts towards reduction of Cr(VI). Importantly, the PB-modified photocatalysts exhibited high photocatalytic performance (98–99% reduction in 40 min for pH = 5.6 and in 10 min for pH = 3) without addition of organic reagents. The simple approach reported herein can be followed to prepare new PB-photocatalyst systems with improved photocatalytic performance towards Cr(VI) reduction and other target applications.

36 MATERIALS SCIENCE↗

Mapping the effects of physical and chemical reduction parameters on local atomic distributions within bimetallic nanoparticles

Bimetallic nanoparticles prove advantageous over their monometallic counterparts due to the tunable, hybrid properties that result from combining different atomic species in a controlled way. The favorable optical and catalytic properties resulting from AgAu nanoparticle formation have been widely attributed to the existence of Ag–Au bonds, the maximization of which assumes the formation of a homogeneous alloy. Despite the importance of atomic scale structure in these systems, synthetic studies are typically not paired with structural characterization at the atomic scale. Herein, a comprehensive synthetic exploration of physical and chemical reduction parameters of resulting nanoparticle products is complemented with thorough X-ray characterization to probe how these parameters affect atomic scale alloy distributions within AgAu nanoparticles. Presented evidence shows Ag is substantially underincorporated into nanoparticle constructs compared with solution Ag : Au ratios regardless of precursor :reductant ratio or volume of reductant added. Both Ag and Au exhibit significant local clustering, with Ag distributed preferentially towards the nanoparticle surface. Most significantly, the results of this investigation suggest that reduction parameters alone can affect the local alloy distributions and homogeneity within bimetallic nanoparticles, even when the ratio of metallic precursors remains constant. Overall, this investigation presents the ability to control alloy distributions using kinetics and provides new considerations for optimizing synthetic methods to produce functional bimetallic nanoparticles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

The NO reduction by CO over NiO x /CeO 2 catalysts with a fixed Ni surface density: pretreatment effects on the catalyst structure and catalytic activity

NiO x /CeO 2 catalysts have shown promising results in various catalytic reactions, including the NO reduction by CO. Here, in this study, we investigated the effect of oxidation and reduction treatments on the NO reduction by CO over pretreated NiO x /CeO 2 catalysts. A series of oxidized and reduced supported NiO x catalysts were synthesized in two steps: (1) the pretreatment of CeO 2 supports at different temperatures (400, 500, and 700 °C) under oxidizing and reducing conditions and (2) the synthesis of NiO x /CeO 2 catalysts by an incipient wetness impregnation method (IWI) with a fixed surface density of 5.3 Ni per nm 2 under specific conditions (oxidation/reduction) applied during the CeO 2 treatment. The prepared catalysts were characterized via BET, ICP/OES, Raman, XPS, and XRD to understand their physicochemical properties. The results showed that as the pretreatment temperature increased, the physicochemical properties of NiO x /CeO 2 catalysts were changed: a decreased specific surface area (SSA), decreased oxygen vacancy/defect sites, and an increased crystallite size. The oxidized NiO x /CeO 2 catalyst at a lower pretreatment temperature displayed a better catalytic activity, indicating that the physicochemical properties of the catalysts were key factors in enhancing the catalytic activity. To understand the intermediate species and reaction mechanism during the NO reduction by CO, an in situ DRIFTS study was performed and a possible reaction mechanism was also discussed.

36 MATERIALS SCIENCE↗

Electrocatalytic properties of a novel ruthenium( ii ) terpyridine-based complex towards CO 2 reduction

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO 2 reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2"-terpyridine, acpy - = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc + , attributed to consecutive 1 e - reduction. Under CO 2 atmosphere, a catalytic wave is observed (E onset = 2.1 V vs. Fc/Fc + ), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (k obs = 1.5 s -1 ). In the presence of 1% water, an improvement in the catalytic activity is observed (TON CO = 21 and k obs = 2.0 s -1 ) and, additionally, formate was also detected (TON HCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru–COO - ) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. Here the results establish important relationships between selectivity, ligand structure and reaction conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO 2

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl 2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO 2 and H 2 O reached as high as ∼10 to 50 mA cm −2 , and remained stable for >2 days. Selectivity for the reduction of CO 2 to CO vs. H 2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO 2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO 2 reduction at sizable current densities.

Chemistry↗

Substituents effects on the electrocatalytic CO 2 reduction by cobalt corroles in solution

Electrochemical CO 2 reduction catalysis with cobalt corrole complexes in solution is reported. Corroles have attracted attention as contracted and trianionic tetrapyrrolic macrocycles that can be compared to leading porphyrin catalysts for CO 2 reduction, but most studies focus on heterogenized systems with poorly defined electrochemical responses. Electrochemical studies of cobalt corroles bearing axial triphenylphosphine ligands to ensure solubility are reported. The voltammetry provides mechanistic insights supporting CO 2 activation after the formal Co II /Co I reduction. The series of cobalt complexes, including a newly designed corrole with mixed perfluorophenyl/ortho-dimethoxyphenyl substituent pattern, provide evidence for electron-rich catalysts having stronger interactions with CO 2 . The primary product of CO 2 reduction is CO, formed at a rate of ca. 90 s −1 .

Kumar, Sachin [Technion–Israel Institute of Techno↗

Dynamics of inverse metal oxides on metal catalysts using spectro-kinetics: reversible Brønsted acid site formation and irreversible reduction

Brønsted acid sites (BASs) in inverse catalysts are vital for the selective hydrogenolysis of polyols, specifically cleaving secondary C–O bonds. These BASs form dynamically in situ in an H 2 environment. While H 2 enables rapid BAS generation on short timescales, it reduces the catalyst at prolonged exposures. The active center for BAS generation, the kinetics of BAS formation, its reverse decomposition, and the irreversible oxide reduction have lacked direct experimental evidence. Here, aided by advanced spectro-kinetic studies, we identify trimeric W 3 O x sites on Pt as the active centers for BAS generation, whereas isolated WO x species on SiO 2 act merely as spectator species, demonstrated using an inverse WO x /Pt catalyst as a representative system. A detailed kinetic profile capturing the dynamics of W 3 O x sites on Pt is also established. The rate constant for BAS formation is two orders of magnitude higher than for its decomposition, which is one order of magnitude faster than the irreversible site reduction. Co-fed H 2 O suppresses the site reduction by ∼50%. Furthermore, the H 2 partial pressure plays an important role. While lower gas-phase H 2 partial pressure does not influence the reversible BAS formation, it can significantly (∼3×) suppress catalyst reduction. Finally, these findings offer critical insights into optimizing reaction conditions through periodic H 2 pulsing, enhancing catalyst stability and performance in hydrogenolysis reactions.

Sourav, Sagar [Indian Inst. of Technology (IIT), M↗

Revealing EDL-driven reduction mechanisms in binary, ternary, and quaternary fluorinated electrolytes via an integrated MD–DFT–ML framework

Accurately predicting solid electrolyte interphase (SEI) formation requires explicitly resolving the electric double layer (EDL) structure, which deviates significantly from that of the bulk electrolyte. Although an established molecular dynamics (MD) and Density Functional Theory (DFT) framework can model SEI formation by evaluating reduction reactions of local clusters in the EDL, it suffers from a combinatorial computational bottleneck. To overcome this limitation, we introduce a machine-learning-accelerated simulation workflow (MD–DFT–ML), integrating a gradient-boosted regression model trained on EDL composition data to efficiently predict reduction potentials. We apply this framework to seven fluorinated electrolytes comprising fluorinated anions, a fluorinated ester solvent, two types of diluent (ion-solvating ester vs. non-solvating ether), and an FEC additive. The analysis shows that the EDL selectively accumulates cation-binding species; consequently, the non–cation-binding ether diluent rarely enters the EDL and makes minimal contributions to SEI formation. DFT calculations on statistically representative EDL clusters provide reduction potentials and fluorine-release pathways, while the ML model, which substantially reduces the DFT workload, predicts cluster reduction energies with a mean absolute error of 0.1 eV. The combined MD–DFT–ML approach also quantifies contributions from different sources to LiF formation in the SEI. This methodology establishes a generalizable route for multiscale modeling electrolyte and interphase design for next-generation electrochemical energy-storage systems.

DFT-MD-ML workflow↗

Acid-catalyzed C–H reductive elimination from Au (III)

An unusual mode of C–H reductive elimination from a Au III complex catalyzed by acid is reported. The addition of triflic acid (HOTf) to [( tBu PCP)Au III –H]OTf (1-OTf, tBu PCP = 2,6-C 6 H 3 (CH 2 P t Bu 2 ) 2 promotes formal C–H reductive elimination. The Au I product [( tBu P(C–H)P)Au I ]OTf (2-OTf, tBu P(C–H)P = κ 2 (1,3-C 6 H 4 (CH 2 P t Bu 2 ) 2 )) was characterized by NMR spectroscopy and X-ray diffraction. It was shown that the reductive elimination of 1-OTf has a dependence on the identity of the acid catalyst which cannot be explained by acid strength. Mechanistic experiments are consistent with a concerted mechanism for C–H reductive elimination involving participation of basic moieties on the acid.

Ardon, Yotam [University of Pennsylvania, Philadel↗

The application scope of the reductive perturbation method and the upper limit of the dust acoustic solitary waves in a dusty plasma

The dust acoustic solitary waves have been numerically investigated by using one dimensional electrostatic particle-in-cell method. By comparing the numerical results with those obtained from the traditional reductive perturbation method, it is found that there exist the maximum dimensionless amplitude and propagation speed of the dust acoustic solitary wave. And these limitations of the solitary wave are explained by using the Sagdeev potential technique. Furthermore, it is noticed that although ϵ ≪ 1 is required in the reductive perturbation method generally, the reductive perturbation method is also valid for ϵ < 1 in a dusty plasma, which may be extended to branches where the reductive perturbation method is used.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

On the reduction criterion for random quantum states

In this paper, we study the reduction criterion for detecting entanglement of large dimensional bipartite quantum systems. We first obtain an explicit formula for the moments of a random quantum state to which the reduction criterion has been applied. We show that the empirical eigenvalue distribution of this random matrix converges strongly to a limit that we compute, in three different asymptotic regimes. We then employ tools from free probability theory to study the asymptotic positivity of the reduction operators. Finally, we compare the reduction criterion with other entanglement criteria, via thresholds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Rapid data-driven model reduction of nonlinear dynamical systems including chemical reaction networks using ℓ 1 -regularization

We develop a new data-driven paradigm for efficient model reduction of a broad class of nonlinear dynamical systems. Our model reduction method directly enables the interpretation of key components of the dynamical system, unlike traditional projection-based model reduction methods that focus on reducing computational complexity more than interpretability. Our method is not application specific and is simple to implement on nonlinear dynamical systems arising from a variety of different fields. It requires minimal parameterization using a single parameter to trade-off between model complexity and estimation error. We use a data-driven paradigm to formulate model reduction as an efficient convex optimization problem that scales polynomially in the original size of the complex system, enabling systems with as many as thousands of components to be reduced in a matter of minutes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

The role of sub-surface hydrogen on CO 2 reduction and dynamics on Ni(110): An ab initio molecular dynamics study

The catalytic reduction of carbon dioxide is a crucial step in many chemical industrial reactions, such as methanol synthesis, the reverse water-gas shift reaction (rWGS) and formic acid synthesis. Here, we investigate the role of bulk hydrogen, where hydrogen atoms are found deep inside a metal surface as opposed to subsurface ones, upon CO 2 reduction over a Ni(110) surface using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. While it has previously been shown that subsurface hydrogen stabilizes CO 2 and can aid in overcoming reaction barriers, the role of bulk hydrogen is less studied and thus unknown with regard to CO 2 reduction. Furthermore, we find that the presence of bulk hydrogen can significantly alter the electronic structure of the Ni(110) surface, particularly the work function and d-band center, such that CO 2 adsorbs more strongly to the surface and is more easily reduced. Our results show an enhanced CO 2 dissociation in the presence of bulk hydrogen, shedding light on a hitherto underappreciated mechanistic pathway for CO 2 reduction on metal surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO 2 reduction catalyst

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS’s excited state. Upon oxidation, TEA and TEOA form TEA •+ and TEOA •+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA • and TEOA • , respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA • and TEOA • radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA • and TEOA • radicals in CH 3 CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO 2 reduction photocatalyst, fac-ReCl(bpy)(CO) 3 (bpy = 2,2'-bipyridine), by these radicals [k TEA• = (4.4 ± 0.3) × 10 9 M –1 s –1 and k TEOA• = (9.3 ± 0.6) × 10 7 M –1 s –1 ]. The ~50× smaller rate constant for TEOA • indicates, that in contrast to a previous assumption, TEA • is a more potent reductant than TEOA • (by ~0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. Furthermore, we also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH 3 CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA • radicals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗