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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effects of Low-temperature Pre-oxidation on the Titanomagnetite Ore Structure and Reduction Behaviors in a Fluidized Bed

With respect to high efficient utilization of low-grade iron ore resource, the behavior of low-temperature “973 K to 1123 K (700 °C to 850 °C)” oxidation, on the phase transition of SA TTM ore (South African titanomagnetite), and its effect on subsequent reduction was investigated. The results showed that hematite and rutile are the oxidation product below 1048 K (775 °C), while pseudobrookite is the stable phase above 1073 K (800 °C). With the increase in temperature and oxidation time, there is a competitive relationship between the amount of hematite and pseudobrookite generated. The reduction efficiency of SA TTM was significantly improved by oxidation pretreatment, primarily due to the dissociation of titania-ferrous oxides to more easily reducible hematite. But the generation of pseudobrookite phase decreases the amount of free hematite available for reduction, which weakens the improvement effect of pre-oxidation. The equilibrium relationship between the metallization degree and the gas reduction potential for TTM ore with pre-oxidation treatment has been built. Finally, the reduction metallization degree for the first and second step can be improved averagely by 16.67 and 3.45 pct, respectively, for sample pre-oxidized at 1098 K (825 °C) for 15 and 90 minutes, while 26.96 and 7.4 pct, improvement is achieved for sample pre-oxidized at a lower temperature of 1048 K (775 °C) for 120 minutes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Carbothermic Reduction Kinetics of Phosphorous Vaporization from Tri-calcium Phosphate (TCP) Under Microwave Rapid Heating With/Without the Presence of Fe{sub 3}O{sub 4}

The kinetics of vapor phase dephosphorization from tri-calcium phosphate (TCP) by carbothermic reduction was studied with and without the presence of Fe{sub 3}O{sub 4}. Microwave heating was utilized to obtain large variations in the heating rate (HR). In the reduction of TCP alone, the phosphorous removal fraction (RF; equal to ∆P{sub 2}O{sub 5}/P{sub 2}O{sub 5}{sup 0}, where ∆P{sub 2}O{sub 5} is the weight change and P{sub 2}O{sub 5}{sup 0} is the P{sub 2}O{sub 5} weight before heating) decreased as the HR increased. In other words, a shorter residence time at a high temperature resulted in a smaller reduction fraction of TCP. An apparently third-order reaction was postulated to account for the kinetics using a fitting simulation based on the additive law of the reaction progress. On the other hand, the phosphorous removal (dephosphorization) rate (RR; equal to ∆P{sub 2}O{sub 3}/t{sub MW}, where t{sub MW} is the microwave heating time period) increased as the HR increased above 1200 °C. The reduction ratio of Fe{sub 3}O{sub 4} above 1100 °C is higher than 97 pct regardless of the heating rate. The reduction of TCP in the presence of Fe{sub 3}O{sub 4} showed that RF increased slightly with increasing HR despite a shorter residence time at a high temperature. The RR also increased with the HR even though RF decreased to half of the values observed in the cases without Fe{sub 3}O{sub 4} for temperatures above 1200 °C. The practicality and optimal operation conditions of phosphorus vapor removal were discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Hydrogen Addition on Softening and Melting Reduction Behaviors of Ferrous Burden in Gas-Injection Blast Furnace

The softening and melting reduction behaviors of ferrous burden in a gas-injection blast furnace (BF) have been investigated experimentally with the assistance of H{sub 2}. The results indicate that the initial softening temperature of the burden in the BF is lower than that in the traditional BF, while the opposite trend is observed for its melting and dripping temperatures, thus widening the softening range and narrowing the melting zone. As a result, the permeability of the stock column is apparently improved, owing to the decreased amount of the produced melt. After H{sub 2} gas is added, the thickness of the iron shell of the burden pellet increases, and the quantity of its liquid wüstite core decreases due to the higher reduction degree. The reduction rate of iron oxides is much faster than the carburization rate with the H{sub 2} addition, and the dripping behavior of the ferrous burden is determined by the carburization with a high reduction potential. After taking into account the effects of H{sub 2} addition on the iron oxide reduction rate, melt quantity, burden microstructure, and energy consumed by the gas-injection BF, it has been concluded that the optimal H{sub 2} content lies in the range between 10 and 15 pct.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Lean NO x reduction over Ag/alumina catalysts via ethanol-SCR using ethanol/gasoline blends

This paper focuses on the activity for lean NO x reduction over sol-gel synthesized silver alumina (Ag/Al 2 O 3 ) catalysts, with and without platinum doping, using ethanol (EtOH), EtOH/C 3 H 6 and EtOH/gasoline blends as reducing agents. The effect of ethanol concentration, both by varying the hydrocarbon-to-NO x ratio and by alternating the gasoline concentration in the EtOH/gasoline mixture, is investigated. High activity for NO x reduction is demonstrated for powder catalysts for EtOH and EtOH/C 3 H 6 as well as for monolith coated catalysts (EtOH and EtOH/gasoline). The results show that pure Ag/Al 2 O 3 catalysts display higher NO x reduction and lower light-off temperature as compared to the platinum doped samples. The 4 wt.% Ag/Al 2 O 3 catalyst displays 100% reduction in the range 340–425 °C, with up to 37% selectivity towards NH 3 . These results are also supported by DRIFTS (Diffuse reflection infrared Fourier transform spectroscopy) experiments. Finally, the high ammonia formation could, in combination with an NH 3 -SCR catalyst, be utilized to construct a NO x reduction system with lower fuel penalty cf. stand alone HC-SCR. In addition, it would result in an overall decrease in CO 2 emissions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Comparison of Pt-BaO/Al 2 O 3 and Pt-CeO 2 /Al 2 O 3 for NOx storage and reduction: Impact of cycling frequency

NOx reduction under net lean and near-stoichiometric conditions was carried out on Pt/Al 2 O 3 , Pt/CeO 2 /Al 2 O 3 and Pt/BaO/Al 2 O 3 washcoated monoliths to compare performance features and identify reaction pathways. The impact of the storage components (BaO, CeO 2 ) on the NOx conversion and byproduct (NH 3 and N 2 O) yields was quantified for a range of feed temperatures, reductant types (H 2 , and C 3 H 6 ), O 2 feed concentrations, and cycle times. The NOx storage functionality is essential for NOx reduction under net lean conditions while the oxygen storage functionality promotes NOx reduction for near-stoichiometric conditions. NOx conversion by H 2 under lean conditions is dependent on the NOx storage capacity of the catalyst, with Pt/CeO 2 /Al 2 O 3 and Pt/BaO/Al 2 O 3 exhibiting the highest NOx conversion below and above 300 °C, respectively. High NOx conversion is achieved over Pt/CeO 2 /Al 2 O 3 for anaerobic rich feeds at temperatures above 400 °C. Increasing the O 2 feed concentration enhances NOx conversion over Pt/CeO 2 /Al 2 O 3 below 400 °C but inhibits NOx conversion above 400 °C. The former is attributed to promotion of NO oxidation leading to NOx storage while the latter is attributed to O 2 inhibition of NO decomposition/reduction. Shorter cycle times increase the NOx conversion with C 3 H 6 as reductant over Pt/BaO/Al 2 O 3 under lean conditions and over PCA for the near-stoichiometric feed. The findings confirm that improved NOx storage utilization is mainly responsible for NOx conversion enhancement. A ceria redox pathway has only a secondary effect on NOx conversion under excess O 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Long-term carbon emission reduction potential of building retrofits with dynamically changing electricity emission factors

Buildings account for approximately 36% of the United States' total carbon emissions and building retrofits have great potential to reduce carbon emissions. Current research adopts a constant electricity emission factor although it changes over time due to the increase of renewable energy generation. Here, to accurately predict emission reduction potential of building retrofits, this study develops a novel method by using dynamically changing electricity emission factors. Using medium office buildings as an example, we predicted emission reduction of eight building retrofit measures from 2020 to 2050 in five locations in the U.S. with distinct climates and renewable adoption rates. To evaluate emission reduction potential sensitivity to the compositions of electricity generation, five scenarios for renewable energy adoptions are investigated. The results reveal several new phenomena on emission reduction potential of building retrofits for medium offices in the U.S.: (1) it decreases from 2026 to 2050; (2) it has the same trend with coal usage; and (3) it reaches the maximum under the high renewable cost scenario. Based on the results, it is recommended that building retrofits should focus on 1) improving lighting and equipment efficiency; 2) locations with higher coal usage rate, and 3) buildings under the high renewable cost scenario. The new method can also be used for predicting emission reduction potential of the building sector in the U.S. by applying to other building types and regions.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Promoting electrochemical reduction of CO 2 to ethanol by B/N-doped sp 3 /sp 2 nanocarbon electrode

Electrochemical reduction of CO 2 to value-added chemicals holds promise for carbon utilization and renewable electricity storage. However, selective CO 2 reduction to multi-carbon fuels remains a significant challenge. Here, we report that B/N-doped sp 3 /sp 2 hybridized nanocarbon (BNHC), consisting of ultra-small nanoparticles with a sp 3 carbon core covered by a sp 2 carbon shell, is an efficient electrocatalyst for electrochemical reduction of CO 2 to ethanol at relatively low overpotentials. CO 2 reduction occurs with a Faradaic efficiency of 58.8%-69.1% for ethanol and acetate production at –0.5 ~ –0.6 V (vs. RHE), among which 51.6%-56.0% is for ethanol. The high selectivity for ethanol is due to the integrated effect of sp 3 /sp 2 carbon and B/N doping. Finally, both sp 3 carbon and B/N doping contribute to enhanced ethanol production with sp 2 carbon reducing the overpotential for CO 2 reduction to ethanol.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

An investigation of the reduction of TiO 2 by Mg in H 2 atmosphere

Hydrogen assisted magnesiothermic reduction (HAMR) of TiO 2 is a promising new process developed recently to produce Ti metal with low cost and less energy consumption. Here this work focuses on the understanding and optimization of the reduction process to control the quality of the reduced product. TiO 2 precursor particles with different relative densities were prepared, and the effects of the relative densities on reduction by Mg in H 2 were studied. The changes of oxygen contents vs. reduction time at various temperatures were investigated. The phase transformations during the reduction were analyzed both theoretically and experimentally. The relationship between the morphology of the reduced products and the relative densities of the TiO 2 precursors was analyzed by SEM characterization and the likely reason for the morphology evolution was discussed.

42 ENGINEERING↗
DOE OSTI2019

Fischer-Tropsch synthesis: Direct cobalt nitrate reduction of promoted Co/TiO 2 catalysts

The effect of the direct reduction of cobalt nitrate versus the more conventional calcination/reduction treatment has been investigated. Porosity properties of the catalysts are not significantly modified by avoiding the calcination step, as similar BET surface area, pore volume and pore diameter are obtained for the activated catalysts. In contrast, the cobalt reducibility decreases, but smaller cobalt particles size and higher dispersion are obtained. The reduction phenomena occurring for the uncalcined catalysts are more complex because of the additional nitrate decomposition steps. TPR-MS and TPR-XANES point out that CoO x intermediate species are formed during the reductive nitrate decomposition. However, these species are oxidized by NO X (formed by nitrate decomposition) to spinel type Co 3 O 4 , which is then converted to CoO prior to the final reduction step to Co 0 . The addition of promoters (Pt, Re, Ru, Ag) improves the cobalt reducibility, especially by shifting the final reduction step (i.e., CoO to Co 0 ) to lower temperature. FT activity testing data show that activated uncalcined catalysts have higher CO conversion following the initial decline and leveling off period relative to the activated calcined catalyst. We find that the best performance is achieved with uncalcined Pt-12%Co/TiO 2 . This catalyst has the highest CO steady state conversion, which is 1.2 times higher than the Pt-promoted calcined catalyst. Moreover, its deactivation rate is 0.13%/h compared to 0.2%/h for the corresponding calcined catalyst. The difference in the catalytic activity is even higher for the un-promoted samples, where the activated uncalcined catalyst has almost double the CO conversion as compared to its calcined counterpart. Finally, the addition of other promoters such as Ru, Re and Ag has no significant effect on catalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Modelling and analysis of salt-convection effect on oxide reduction process for uranium oxides using smoothed particle hydrodynamics

Oxygen ion transport via molten salt continuum is a key kinetic feature of electrochemical reduction process for uranium oxides in molten salt. During the electrolysis, the evolution of oxygen ion concentration field in molten salt continuum appears from porous metallic layer to open bulk salt. Despite the important role affecting process throughput, its effect coupled with controlled salt movement was not adequately investigated. To capture the reactive oxygen ion transport phenomena, a multidimensional reduction model based on smoothed particle hydrodynamics (SPH) was combined with the advection-diffusion model. A pelletized oxide fuel bed capturing interconnected salt continuum is considered as a reactant material geometry, which allows forced convective delivery of molten salt through the reactant materials with a relatively low pressure head. Here, in this study, reduction time of a pelletized-fuel bed was quantified with respect to key design parameters of the reactant material configuration. The simulation results demonstrated the advantage of salt convection which effectively removes oxygen ion products from the reactant materials and consequently accelerates the reduction process. Moreover, the reduction-time correlation of a single pelletized fuel was developed as a function of relevant dimensionless parameters and was extended to the application for a pelletized-fuel bed. Finally, several insights and implications in the use of a pelletized oxide fuel were presented. The developed SPH-based framework enabled multi-physics analysis of governing dynamics in appreciating and screening candidate reactant material configurations in a cost-effective manner.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

A bioinspired cobalt catalyst based on a tripodal imidazole/pyridine platform capable of water reduction and oxidation

The prototypical drug carrier [Co II (L 1 )Cl]PF 6 (1), where L 1 is a tripodal amine bound to pyridine and methyl-imidazoles, had its electrocatalytic water splitting activity studied under different pH conditions. This species contains a high-spin 3d 7 Co II metal center, and is capable of generating both H 2 from water reduction and O 2 from water oxidation. Turnover numbers reach 390 after 3 h for water reduction. Initial water oxidation activity is molecular, with TONs of 71 at pH 7 and 103 at pH 11.5. The results reveal that species 1 can undergo several redox transformations, including reduction to the 3d 8 Co I species that precedes a LS 3d 6 hydride for water reduction, as well as nominal Co IV $=$O and Co III -OOH species required for water oxidation. Finally, post-catalytic analyses confirm the molecular nature of reduction and support initial molecular activity for oxidation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Hierarchical porous silver metal using Pluronic F-127 and graphene oxide as reinforcing agents for the reduction of o-nitroaniline to 1, 2-benzenediamine

An elegant method is used to prepare silver monoliths with Pluronic F-127(F-127) as sacrificial template by modified sol-gel method. Si nanoparticles (SiNPs) and graphene oxide (GO) are added in situ to Ag/F-127 hydrogel for the reduction of ο-nitroaniline (ο-NA) to 1, 2-benzenediamine. Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Thermogravimetric analysis (TGA), Raman Spectroscopy, Powder X-Ray Diffraction (PXRD) analysis and Brunauer-Emmett-Teller (BET) Nitrogen adsorption techniques were used for characterization of monoliths. An epoch-making catalytic activity of Ag/F-127/GO monoliths is observed in the reduction of ο-NA to 1, 2-benzenediamine in presence of NaBH{sub 4} in aqueous media. The catalyst Ag/F-127/GO took only 2 min which is the minimum time reported so far with significant rate constant claimed itself a leading catalyst for the reduction of ο-NA to 1,2-benzenediamine. Pseudo first order rate constant (k) and Turn over frequency (TOF) values are 0.231 min{sup −1} and 30.053×10{sup 19} molecules min{sup −1} respectively suggest that the catalyst has industrial importance. Recyclability and stability of Ag/F-127/GO catalyst are studied successfully up to 10 cycles. Energy of activation (E{sub a}), and thermodynamic parameters viz. activation enthalpy (ΔH{sup ≠}), activation Gibbs free energy (ΔG{sup ≠}), and entropy of activation (ΔS{sup ≠}) were also ascertained. Catalytic activities of Ag/F-127, Ag/F-127/Dextran, Ag/F-127/Trimethylbenzene (TMB), Ag/F-127/SiNPs, and Ag/F-127/Si/GO monoliths were also studied. - Graphical abstract: Significant catalytic activities of silver monoliths against the reduction of ο-NA to 1,2 benzenediamine. - Highlights: • A new catalyst synthesized Ag/F-127/GO for the reduction of ο- NA to 1, 2- benzenediamine took only 2 min. • Turn over frequency of as synthesized catalyst was 30.053×10{sup 19} molecules min{sup −1} claimed itself a leading catalyst. • Recyclability of the catalyst was up to 10 cycles. • The synthesis is non toxic, economically viable and environmentally benign.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Tracking Dendritic Growth in Hydrogen-Based Hematite Reduction via Computer Vision

The reduction of hematite to metallic iron using hydrogen (H2) as a reducing agent presents a promising pathway for decarbonizing steel production. In this study, we employ a combination of in situ confocal scanning laser microscopy (CSLM) and advanced computer vision techniques to quantitatively analyze dendritic growth of ferrite during H2-based reduction of iron oxide at high temperatures. A workflow integrating Watershed Image Segmentation (WIS) and Lucas-Kanade Optical Flow (LKOF) is developed to extract both global and local kinetic information from time-resolved micrograph sequences. H2 reduction experiments conducted at 1400 degrees C and 1500 degrees C demonstrate a clear correlation between temperature and reduction rate, as evidenced by accuracy of fitted Johnson-Mehl-Avrami-Kolmogorov (JMAK) parameters. Optical flow analysis further elucidates the anisotropic and branched nature of dendritic growth, providing spatially resolved velocity fields that correlate well with global transformation kinetics. The proposed methodology demonstrates strong agreement with experimental measurements and literature values, offering a robust framework for automated image-based analysis to study kinetics through microstructural evolution in the reduction of iron ore, and likely other reaction-diffusion phenomena.

08 HYDROGEN↗
DOE OSTI2018

Photocatalytic reduction of CO2 using CdS nanorods on porous anodic alumina support

Highlights: • Commercial grade quality of aluminum foil was used to fabricate uniformly ordered PAA. • A more compassionate method of SILAR was adopted for synthesis on CdS on PAA. • Photocatalytic reduction of CO{sub 2} was carried on flat sheet reactor with mirror reflectors, which proves to be a potential candidate for large scale application. • Enhanced photocatalytic reduction of CO{sub 2} was obtained, when CdS was retained along the pore wall of the support. - Abstract: Solar reduction of CO{sub 2} to valuable chemicals has captured attention of several researchers due to its cumulative effect on reduction of global warming and also as an alternative to fossil fuels. Hexagonally ordered porous anodic alumina (PAA), prepared from commercial aluminium foil, was used as a support for CdS photocatalyst due to its unique honeycomb structure. The CdS nanorods were vertically grown on the PAA template by successive ion layer adsorption reaction (SILAR) method. The photocatalytic reduction of CO{sub 2} by CdS to CH{sub 3}OH was investigated using a flat sheet reactor under 10 W LED lamp. A peak CH{sub 3}OH formation at 144.5 μmol/g/h with 45.4% of photocatalytic efficiency and 1.97% of CO{sub 2} conversion efficiency in a single pass were obtained.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Chemical identity of carbon substrates drives differences in denitrification and N 2 O reduction within agricultural soils

Rates of nitrous oxide (N 2 O) production from agricultural soils are highly variable across space and time. Improving predictions of N 2 O emissions will require improving our understanding of the drivers of denitrification and the sources of variability in the rates of N 2 O production between soils and over time. While the amount of available carbon (C) is a known control on denitrification and N 2 O reduction, relatively little attention has been paid to the effect of the chemical identity of C substrates on rates of denitrification and N 2 O reduction. We investigated the effects of twelve different C-substrate additions on the production and reduction of N 2 O in five soils taken from two distinct agricultural locations in Michigan under multiple land uses. We provided additions of glucose, cellulose, N-acetyl-glucosamine, chitin, amino acids, protein, vanillyl alcohol, lignin, citrate, succinate, methanol, and water in laboratory denitrification potential assays to determine the effects of denitrifier C preference on denitrification rates. We found that amino acids, protein, and organic acids stimulated the greatest rates of denitrification potential across all land uses. Similarly, we found these same substrates caused the most N 2 O reduction, resulting in the lowest net concentrations of N 2 O. Soils from agricultural rotations without cover crops had overall lower rates of denitrifier activity, leading to less net N 2 O production compared to soils from other land uses. In general, C-utilization patterns were similar among all soils, and C-substrate identity had a much stronger effect than land use. Here, we demonstrate that the chemical identity of available C gives rise to wide variability in rates of denitrification and N 2 O reduction.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Trajectory prediction dimensionality reduction for low-cost connected automated vehicle systems

Here, to facilitate low-cost connected automated vehicle (CAV) system development, this study proposes two interpretable dimensionality reduction techniques in vehicle trajectory prediction, i.e., the piecewise Taylor series approximation (PTA) and the piecewise Fourier series approximation (PFA), to lower computation complexity, reduce device investment, and decrease computation energy consumption. Two benchmarks are developed, the long short-term memory (LSTM)-based model without dimensionality reduction and the LSTM-based model with encoder-decoder (a widely used dimensionality reduction technique). Results show that the four predictions have similar accuracy, and the training time (proportional to computation energy consumption) of models with dimensionality reduction techniques is greatly reduced. The reduction is even more significant when PTA/PFA is used. Sensitivity analysis advises PFA/PTA parameter selections to reduce computation complexity without significant loss of prediction accuracy. Further, the robustness of the LSTM PTA/PFA is proven by the investigation of data noises.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2022

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO 2 Reduction by Manganese Bipyridyl Complexes

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph) 2 bpy vs mes 2 bpy ligands) and Lewis basic ((Ph) 2 bpy vs (MeOPh) 2 bpy vs (MeSPh) 2 bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO 2 reduction mechanisms with [Mn I (R 2 bpy)(CO) 3 (CH 3 CN)] + precatalysts, and on their respective transition-state geometries/energies for rate-determining C–OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph) 2 bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO 2 binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes2bpy-based precatalysts. Cyclic voltammetry conducted in CO 2 -saturated electrolyte for the (Ph) 2 bpy-based precatalyst [2-CH 3 CN] + confirms that CO 2 binding occurs at the two-electron-reduced activated catalyst [2] – in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO 2 binding. This observation is supported by computed free energies of the parent–child reaction for [Mn–Mn] o dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO 2 binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH 3 CN] + maintains a lower overpotential than [2-CH 3 CN] + at the protonation-first pathway with comparable kinetic performance, whereas an ~2-fold greater TOF max is observed at its reduction-first pathway with an almost identical overpotential as [2-CH 3 CN] + . Notably, excessive steric bulk in the mes2bpy-based precatalyst [3-CH 3 CN] + results in increased activation free energies of the C–OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH 3 CN] + and [2-CH 3 CN] + . In fact, [3-CH 3 CN] + requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Furthermore, voltammetry recorded under 1 atm of CO 2 with 2.8 M (5%) H 2 O establishes [2-CH 3 CN] + to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie “on the fence” when providing sufficient steric bulk to hinder (but not prevent) [Mn–Mn] o dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C–OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH 3 CN] + possesses an increased steric presence relative to [2-CH 3 CN] + , this is offset by its capacity to stabilize the C–OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Directed Assembly of Nanoparticle Catalysts on Nanowire Photoelectrodes for Photoelectrochemical CO 2 Reduction

Reducing carbon dioxide with a multicomponent artificial photosynthetic system, closely mimicking nature, represents a promising approach for energy storage. Previous works have focused on exploiting light-harvesting semiconductor nanowires (NW) for photoelectrochemical water splitting. With the newly developed CO 2 reduction nanoparticle (NP) catalysts, direct interfacing of these nanocatalysts with NW light absorbers for photoelectrochemical reduction of CO 2 becomes feasible. Here, we demonstrate a directed assembly of NP catalysts on vertical NW substrates for CO 2 -to-CO conversion under illumination. Guided by the one-dimensional geometry, well-dispersed assembly of Au 3 Cu NPs on the surface of Si NW arrays was achieved with facile coverage tunability. Such Au 3 Cu NP decorated Si NW arrays can readily serve as effective CO 2 reduction photoelectrodes, exhibiting high CO 2 -to-CO selectivity close to 80% at -0.20 V vs RHE with suppressed hydrogen evolution. A reduction of 120 mV overpotential compared to the planar (PL) counterpart was observed resulting from the optimized spatial arrangement of NP catalysts on the high surface area NW arrays. In addition, this system showed consistent photoelectrochemical CO 2 reduction capability up to 18 h. Furthermore, this simple photoelectrode assembly process will lead to further progress in artificial photosynthesis, by allowing the combination of developments in each subfield to create an efficient light-driven system generating carbon-based fuels.

77 NANOSCIENCE AND NANOTECHNOLOGY↗