SEARCH · Search NASA

Results for “formate mechanistic studies”

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.

SourceResearch connectionFrom yearThrough year

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

At least 37 records · Page 2

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Carbon Nanotube Formation on Cr-Doped Ferrite Catalyst during Water Gas Shift Membrane Reaction: Mechanistic Implications and Extended Studies on Dry Gas Conversions

A nanocrystalline chromium-doped ferrite (FeCr) catalyst was shown to coproduce H2 and multiwalled carbon nanotubes (MWCNTs) during water gas shift (WGS) reaction in a H2-permselective zeolite membrane reactor (MR) at reaction pressures of ~20 bar. The FeCr catalyst was further demonstrated in the synthesis of highly crystalline and dimensionally uniform MWCNTs from a dry gas mixture of CO and CH4, which were the apparent sources for MWCNT growth in the WGS MR. In both the WGS MR and dry gas reactions, the operating temperature was 500 °C, which is significantly lower than those commonly used in MWCNT production by chemical vapor deposition (CVD) method from CO, CH4, or any other precursor gases. Extensive ex situ characterizations of the reaction products revealed that the FeCr catalyst remained in partially reduced states of Fe3+/Fe2+ and Cr6+/Cr3+ in WGS membrane reaction while further reduction of Fe2+ to Fe0 occurred in the CO/CH4 dry gas environments. The formation of the metallic Fe nanoparticles or catalyst surface dramatically improved the crystallinity and dimensional uniformity of the MWCNTs from dry gas reaction as compared to that from WGS reaction in the MR. Reaction of the CO/CH4 mixture containing 500 ppmv H2S also resulted in high-quality MWCNTs similar to those from the H2S-free feed gas, demonstrating excellent sulfur tolerance of the FeCr catalyst that is practically meaningful for utilization of biogas and cheap coal-derived syngas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Improving productivity and stability for CO 2 hydrogenation by using pincer-ligated Mn complexes with hemilabile ligands

Pincer ligands are widely used to support transition metal catalysts, but first-row systems often deactivate too quickly for widespread use. Here, pincer ligands bearing an additional hemilabile donor were designed to stabilize Mn catalysts for CO 2 hydrogenation to formate. Ligands of the type (RCH 2 CH 2 )N(CH 2 CH 2 P i Pr 2 ) 2 ( iPr PN R P; R = OMe, CH 2 OMe, NMe 2 , P i Pr 2 ) were synthesized and coordinated to Mn to form complexes of the type ( iPr PN R P)Mn(CO) 2 H. Their performance was compared to {MeN(CH 2 CH 2 P i Pr 2 ) 2 }Mn(CO) 2 H, which lacks a pendant donor. The pendant donor identity and arm length strongly influenced catalytic activity, but all pendant arm containing systems extended catalyst lifetime. Notably, ( iPr PN CH2OMe P)Mn(CO) 2 H achieved turnover frequencies of 158,000 h -1 and turnover numbers of 838,000, exceeding most state-of-the-art catalysts. Mechanistic studies revealed that hemilabile coordination enhances stability by avoiding deactivation, although excessive binding can lead to inactive states. Here, this work demonstrates hemilabile donors dramatically improve pincer-based Mn catalysts and provides design principles for broader applications.

CO2 hydrogenation↗

DOE OSTI2024

PCET‐Driven Reactivity of Neptunyl(VI) Yields Oxo‐Bridged Np(V) and Np(IV) Species

Two unconventional polynuclear complexes of neptunium (Np) featuring mono-mathematical equation -oxo motifs have been accessed by proton-coupled electron transfer (PCET) reactivity involving the dissolution of neptunyl(VI) diacetate dihydrate (NpO 2 (OAc) 2 (H 2 O) 2 ∙ HOAc) in methanol followed by addition of a pentadentate Schiff-base ligand. One complex is a mixed-valent [Np V ,Np IV , Np V ] trimer with two bridging mathematical equation μ 2 -oxos and the other is a [Np V , Np V ] dimer featuring a single mathematical equation μ 2 -oxo. In both complexes the outer Np centers are capped with terminal oxo ligands. Spectroscopic and spectrokinetic studies aimed at elucidating mechanistic details of complex formation in this system show that intermediate multinuclear [Np V O 2 (OAc)] n species form prior to metal chelation by the ligand; electrolysis experiments demonstrate that production of Np(V) gives rise to asynchronous proton transfer that does not occur otherwise (in the Np(VI) state) as well as condensation with loss of H 2 O and formation of the polynuclear complexes. We attribute the oxo-deficient nature of these products, with respect to conventional actinyl ([AnO 2 ] m+ ) species, to the reduction/condensation reaction sequence of PCET.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Investigating Kinetic Mechanisms of Soot Formation in Plasma Pyrolysis of Methane via Active Learning (Final Technical Report)

Plasma pyrolysis of methane is an effective route for zero-carbon hydrogen production. Yet, soot generated from pyrolysis of hydrocarbons is detrimental to the climate and human health. There is ample experimental and theoretical evidence that suggests polycyclic aromatic hydrocarbons (PAHs) are the molecular precursors to soot particles. The reaction pathways of PAH formation are intricately dependent on a multitude of process parameters, whose kinetic mechanisms are not well-understood in plasma pyrolysis. This project aims to leverage advances in the kinetic modeling of soot formation in combustion, as well as in surrogate modeling and active learning, to systematically investigate the effects of process parameter on the kinetics of PAH formation in plasma pyrolysis of methane. To this end, we propose to use the PAH formation kinetics model developed by the PPPL/PU group based on the well-established ABF and HACA mechanisms, coupled with low-temperature plasma models. We will develop an active learning (AL) framework based on Bayesian optimization to systematically and data-efficiently explore the complex and multivariable parameter space of plasma pyrolysis in order to quantify the effects of plasma and feed parameters on the ABF and HACA kinetic pathways. AL is the branch of machine learning concerned with systematically querying samples from a system (experimental or computational) to train a data-driven model that maps design parameters to a performance criterion. We will use the data generated via AL to perform global sensitivity analysis, combined with uncertainty quantification, to elucidate the impact of different reaction pathways on minimizing formation of soot precursors. This study will result in an improved understanding of kinetics of PAH formation in plasma pyrolysis and can pave the way for more advanced mechanistic studies (e.g., soot nucleation mechanisms). Additionally, the findings will be useful for establishing practical strategies for increasing the pyrolysis efficiency and producing high-grade carbon for synthesis of nanomaterials.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Photocatalytic Transfer Hydrogenation in Water: Insight into Mechanism and Catalyst Speciation

Organometallic catalysts with strong metal-to-ligand charge-transfer (MLCT) optical transitions and reactive metal-hydride bonds provide opportunities for new excited-state reactivity. Toward this end, we have investigated photoinduced transfer hydrogenation reactions for a series of Cp*IrH complexes with functionalized bipyridyl ligands. Overcoming the baseline reactivity of this class of catalyst toward hydrogen evolution in aqueous media presents a major challenge for focusing this photochemical reactivity into desired transformations of more complex organic substrates. In this work, a photochemically induced aqueous ketone reduction catalyzed by [Cp*Ir(bpy)H][Cl] is reported and benchmarked to the competing hydrogen evolution reaction (HER). A detailed mechanistic study is conducted of the reduction of cyclohexanone with sodium formate delivering the reducing equivalent. Examining the kinetic profile of the cyclohexanone reduction revealed an unexpected induction period, suggesting a more complex underlying reaction mechanism where product selectivity was not simply a function of direct competition from a common excited-state reactive intermediate. The reaction kinetics were analyzed by variable time normalization analysis (VTNA), and the catalyst resting state was interrogated through operando light-emitting diode (LED) illumination H-1 NMR spectroscopy. Overall, these and other mechanistic studies provide insight into a photochemical reaction pathway to ketone reduction that is distinct from that previously reported for HER.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1986

A comparative study of prebiotic and present day translational models

It is generally recognized that the understanding of the molecular basis of primitive translation is a fundamental step in developing a theory of the origin of life. However, even in modern molecular biology, the mechanism for the decoding of messenger RNA triplet codons into an amino acid sequence of a protein on the ribosome is understood incompletely. Most of the proposed models for prebiotic translation lack, not only experimental support, but also a careful theoretical scrutiny of their compatibility with well understood stereochemical and energetic principles of nucleic acid structure, molecular recognition principles, and the chemistry of peptide bond formation. Present studies are concerned with comparative structural modelling and mechanistic simulation of the decoding apparatus ranging from those proposed for prebiotic conditions to the ones involved in modern biology. Any primitive decoding machinery based on nucleic acids and proteins, and most likely the modern day system, has to satisfy certain geometrical constraints. The charged amino acyl and the peptidyl termini of successive adaptors have to be adjacent in space in order to satisfy the stereochemical requirements for amide bond formation. Simultaneously, the same adaptors have to recognize successive codons on the messenger. This translational complex has to be realized by components that obey nucleic acid conformational principles, stabilities, and specificities. This generalized condition greatly restricts the number of acceptable adaptor structures.

Rein, R.↗

DOE OSTI2023

Aldehyde and Ketone Hydroboration Mediated by a Heterogeneous Single–Site Molybdenum–Dioxo Catalyst: Scope and Mechanistic Implications

Efficient hydroboration of aldehydes and ketones is demonstrated using a single-site MoO 2 catalyst supported on activated carbon. Under mild conditions with low catalytic loadings, the reaction exhibits chemoselectivity towards carbonyl reduction over halides, alkene, alkyne, nitrile, and ester groups, affording high yields of desired products. Furthermore, competition studies between aldehyde and ketone reduction using HBpin and HBcat indicate that HBpin preferably functionalizes aldehydes, while HBcat can functionalize either substrate under specific conditions. Mechanistic studies suggest that the reaction proceeds via the initial formation of a molybdenum-hydride species upon B–H activation and subsequent molybdenum-alkoxide species upon carbonyl activation by the Mo center. The experimental activation energy of 14.3 kcal/mol alignssatisfactorily with the DFT computed ΔH ≠ of 18.7 kcal/mol, further supporting the proposed mechanism. Combined experimental/computational analyses reveal that excess HBpin can possibly deactivate the catalyst. The presence of excess benzaldehyde efficiently mitigates deactivation by HBpin, providing significantly higher catalyst recyclability. Altogether, this non-toxic, air- and moisture-stable, active, and selective catalyst demonstrates significant potential for green processes.

08 HYDROGEN↗

DOE OSTI2025

CO Reduction to Ethylene and Cyclopropane via a Trappable Ruthenium Methylidene

Ruthenium complexes based on cis-[Ru(bpy) 2 (CO) 2 ] 2+ (bpy is 2,2′-bipyridine) can reduce CO 2 and CO to C 1 products including methanol, but access to products containing C–C bonds has been elusive. A reaction pathway to convert CO into multicarbon products ethylene and cyclopropane is presented here, along with mechanistic studies elucidating the key intermediates in C–C bond formation. The ruthenium hydroxymethyl complex [Ru(bpy′) 2 (CO)(CH 2 OH)] + (bpy′ = 5,5′-dimethyl-2,2′-bipyridine) undergoes protonolysis to generate the ethylene complex [Ru(bpy′) 2 (CO)(C 2 H 4 )] 2+ even at −80 °C, with free ethylene released upon warming to room temperature. Experimental evidence implicates a highly electrophilic methylidene complex [Ru(bpy′) 2 (CO)(CH 2 )] 2+ as the key intermediate. The methylidene was successfully trapped with nitriles and pyridine, forming adducts (ylide complexes) that each have a unique reactivity profile. With an appropriate nitrile, the adduct can be characterized at low temperature before warming generates ethylene. A more stable pyridine adduct [Ru(bpy′) 2 (CO)(CH 2 pyridine)] 2+ was crystallographically characterized. Even ethylene itself is sufficiently nucleophilic to react with the electrophilic methylidene, revealing a route from CO to the C 3 hydrocarbon cyclopropane. Furthermore, the methods for controlling the reactivity of hydroxymethyl and methylidene complexes toward C–C bond formation can inform the development of CO and CO 2 reduction catalysts.

Carbene Compounds↗

DOE OSTI2021

Fatty acid binding protein 3 deficiency limits atherosclerosis development via macrophage foam cell formation inhibition

Highlights: • We firstly indicated that FABP3 was highly expressed in the aorta of HFD-fed ApoE{sup −/-} mice. • Silencing FABP3 attenuated atherosclerosis due to inhibited macrophage foam cell formation. • FABP3 knockdown decreased foamy macrophage formation partly by inhibiting the PPARγ signal pathway. Atherosclerosis is the underlying contributing factor of cardiovascular disease, which is a process of inflammation and lipid-rich lesion. Macrophage-derived foam cell is a key hallmark of atherosclerosis and connected with various factors of lipid metabolism. Here, we showed that fatty acid binding protein 3 (FABP3) was upregulated in the aorta of ApoE{sup −/-} mice with high-fat-diet (HFD) feeding. Knockdown of FABP3 in HFD-fed ApoE{sup −/-} mice notably facilitated cholesterol efflux, inhibited macrophage foam cell formation, and thus prevented atherogenesis. Furthermore, FABP3 silencing decreased the expression of peroxisome proliferator-activated receptor γ (PPARγ). Mechanistic studies had disclosed the involvement of PPARγ signaling in balancing cholesterol uptake and efflux and diminishing foam cell formation. These findings firstly revealed an anti-atherogenic role of FABP3 silencing in preventing foamy macrophage formation partly through PPARγ, which might be a beneficial approach for therapying atherosclerosis.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2022

An experimental and kinetic modeling study of the pyrolysis of isoprene, a significant biogenic hydrocarbon in naturally occurring vegetation fires

Isoprene dominates the carbon flux emitted by vegetation and constitutes 40% of non-methane biogenic emissions worldwide. Despite pyrolysis experiments at temperatures above 1000 K showing a link between isoprene combustion and aromatic species formation, comprehensive mechanistic research on isoprene is scarce in the literature. Here, in this work, we carry out an experimental and theoretical study to build, for the first time, a chemical kinetic model describing isoprene pyrolysis. The formation of polycyclic aromatic hydrocarbon (PAH) precursor species, often observed in vegetation fire plumes, is partially explained by isoprene pyrolysis experiments and theoretical modeling. Molecular dynamics (MD) simulations unveil reaction pathways from allylic isoprenyl radicals to allene and cyclopentadiene (CPD) intermediates, two relevant species detected in the experiments. Rate constants for these identified pathways are calculated using variational transition state theory to update the kinetic model, which is validated against single-pulse shock tube (SPST), and jet-stirred reactor (JSR) experimental data in the temperature range of 850–1690 K. The kinetic model presents satisfactory agreement with the SPST experimental data, and a reaction pathway analysis shows that association of propargyl radicals results in benzene formation. The JSR pathway analysis also identifies the prominent reactions for CPD, benzene, styrene, and toluene formation. Our model does not reproduce the CPD experimental profiles, indicating that additional studies are necessary. Overall, our findings advance the understanding of isoprene pyrolysis and its related atmospheric pollutants in naturally occurring vegetation fires where smoldering and oxygen-deficient combustion processes are present.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Future changes in isoprene-epoxydiol-derived secondary organic aerosol (IEPOX SOA) under the Shared Socioeconomic Pathways: the importance of physicochemical dependency

Abstract. Secondary organic aerosol (SOA) is a dominant contributor of fine particulate matter in the atmosphere, but the complexity of SOA formation chemistry hinders the accurate representation of SOA in models. Volatility-based SOA parameterizations have been adopted in many recent chemistry modeling studies and have shown a reasonable performance compared to observations. However, assumptions made in these empirical parameterizations can lead to substantial errors when applied to future climatic conditions as they do not include the mechanistic understanding of processes but are rather fitted to laboratory studies of SOA formation. This is particularly the case for SOA derived from isoprene epoxydiols (IEPOX SOA), for which we have a higher level of understanding of the fundamental processes than is currently parameterized in most models. We predict future SOA concentrations using an explicit mechanism and compare the predictions with the empirical parameterization based on the volatility basis set (VBS) approach. We then use the Community Earth System Model 2 (CESM2.1.0) with detailed isoprene chemistry and reactive uptake processes for the middle and end of the 21st century under four Shared Socioeconomic Pathways (SSPs): SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5. With the explicit chemical mechanism, we find that IEPOX SOA is predicted to increase on average under all future SSP scenarios but with some variability in the results depending on regions and the scenario chosen. Isoprene emissions are the main driver of IEPOX SOA changes in the future climate, but the IEPOX SOA yield from isoprene emissions also changes by up to 50 % depending on the SSP scenario, in particular due to different sulfur emissions. We conduct sensitivity simulations with and without CO2 inhibition of isoprene emissions that is highly uncertain, which results in factor of 2 differences in the predicted IEPOX SOA global burden, especially for the high-CO2 scenarios (SSP3–7.0 and SSP5–8.5). Aerosol pH also plays a critical role in the IEPOX SOA formation rate, requiring accurate calculation of aerosol pH in chemistry models. On the other hand, isoprene SOA calculated with the VBS scheme predicts a nearly constant SOA yield from isoprene emissions across all SSP scenarios; as a result, it mostly follows isoprene emissions regardless of region and scenario. This is because the VBS scheme does not consider heterogeneous chemistry; in other words, there is no dependency on aerosol properties. The discrepancy between the explicit mechanism and VBS parameterization in this study is likely to occur for other SOA components as well, which may also have dependencies that cannot be captured by VBS parameterizations. This study highlights the need for more explicit chemistry or for parameterizations that capture the dependence on key physicochemical drivers when predicting SOA concentrations for climate studies.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2023

Cobalt‐Catalyzed Electroreductive Alkylation of Unactivated Alkyl Chlorides with Conjugated Olefins

Abstract Reactions of unactivated alkyl chlorides under mild and sustainable conditions are rare compared to those of alkyl bromides or iodides. As a result, synthetic methods capable of modifying the vast chemical space of chloroalkane reagents, wastes, and materials are limited. We report the cobalt‐catalyzed reductive addition of unactivated alkyl chlorides to conjugated alkenes. Co‐catalyzed activation of alkyl chlorides is performed under electroreductive conditions, and the resulting reactions constitute formal alkyl‐alkyl bond formation. In addition to developing an operationally simple methodology, detailed mechanistic studies provide insights into the elementary steps of a proposed catalytic cycle. In particular, we propose a switch in the mechanism of C−Cl bond activation from nucleophilic substitution to halogen atom abstraction, which is critical for efficiently generating alkyl radicals. These mechanistic insights were leveraged in designing ligands that enable couplings of primary, secondary, and tertiary alkyl chlorides.

Al Zubaydi, Samir↗

DOE OSTI2023

Cobalt‐Catalyzed Electroreductive Alkylation of Unactivated Alkyl Chlorides with Conjugated Olefins

Al Zubaydi, Samir↗

DOE OSTI2026

The role of transition metal formyl intermediates in the reduction of CO and CO 2

Transition-metal formyl (metalloformyl) complexes occupy a central position in the activation of small molecules, particularly in the reduction of carbon monoxide (CO) and carbon dioxide (CO 2 ). This review examines five decades of progress in the synthesis of metalloformyl complexes and investigations into their structure and reactivity. The bonding in the M–CHO unit is best described as a resonance hybrid between a classical σ-bound formyl ligand and an oxycarbene-like electronic structure, which governs their distinctive spectroscopic signatures and versatile reactivity. Established synthetic routes are summarized, including pathways involving hydride addition and CO insertion, alongside a discussion of the thermodynamic and kinetic factors that control formyl stability. Decomposition pathways and Lewis-acid stabilization strategies are analyzed as key design principles for extending metalloformyl lifetimes under catalytic conditions. Particular attention is given to hydride transfer processes and the role of metalloformyl intermediates as both reactive substrates and hydride sources in reduction chemistry. Lastly, emerging catalytic strategies that exploit metalloformyl intermediates in CO 2 and CO reduction are evaluated, highlighting how control of hydricity, redox potential, and secondary-sphere interactions enables selective C—H bond formation under comparatively mild conditions. Collectively, these studies establish metalloformyl complexes as mechanistically informative and functionally relevant intermediates that bridge fundamental organometallic chemistry with modern approaches to small-molecule activation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Organic superbases in Lewis pair catalyzed hydrogenations of imines

Organic superbases have recently emerged as powerful components in metal-free catalytic systems, yet their role in frustrated Lewis pair (FLP) catalyzed hydrogenation reactions remain underexplored. In this article, we investigate the diphosphazene superbases P 2 - t Bu (1) and P 2 -Et (2) as base components combined with electronically tunable boranes of formula 9-(X n -C 6 H 5-n )BBN, where X n = 4-NMe₂, 4-OMe, 4-Me, 4-H, 4-F, 4-OCF 3 , 4-CF 3 , 3,5-(CF3), in FLP-catalyzed hydrogenations of imines. While the more sterically encumbered base 1 forms persistent FLPs whose activity correlates with the hydride affinity of the borane, less hindered base 2 generates Lewis acid–base adducts and displays superior catalytic performance, especially with boranes of lower hydride affinity. A systematic screening identified the adduct 2/9-(4-MeO-C 6 H 4 )BBN as the most active catalytic system, achieving quantitative hydrogenation of N-benzylidene aniline at 80 °C and H 2 pressures as low as 40 bar in THF. Multinuclear NMR spectroscopic studies and DFT calculations support a mechanistic scenario in which reversible adduct formation and subsequent dissociation to the free acid–base pair precede heterolytic H₂ cleavage, with adduct stability and borane electrophilicity concurrently dictating catalytic activity. Under optimized conditions, the most active catalyst system, 2/9-(4-MeO-C 6 H 4 )BBN, hydrogenates a broad range of aldimines, including electron-rich 4-NMe₂ and push-pull substrates, mono- and bis-ortho-substituted imines, and selected heteroaryl imines, with excellent yields and selectivity. Collectively, these findings establish design principles for tuning BBN-derived FLP systems and expand the scope of metal-free hydrogenation catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Formic Acid: A Hydrogen-Bonding Cocatalyst for Formate Decomposition

Hydrogen bonding accelerates many catalytic reactions by orienting intermediates, stabilizing transition states, and even opening reaction pathways. However, most mechanistic studies regarding the decomposition of formic acid (FA), a promising hydrogen storage material, neglect hydrogen-bonding interactions even though FA is a strong hydrogen-bond donor and acceptor. Here, we probe the formation of bimolecular hydrogen-bonded complexes between FA and formate (FA–HCOO complexes) adsorbed on metal surfaces and how these complexes affect HCOO* decomposition. Using first-principles density functional theory (DFT) calculations on 12 close-packed (111)/(0001) and 8 open (100) surfaces of 12 transition metals—Ag, Au, Co, Cu, Ir, Ni, Os, Re, Pd, Pt, Rh, and Ru, we—show that FA–HCOO complexes are generally thermodynamically stable, even at elevated temperatures and pressures. We then illustrate that these complexes produce infrared spectroscopic signatures consistent with as yet unassigned experimental peaks. We last demonstrate that by stabilizing the dangling bond of monodentate HCOO*, these complexes significantly lower the barriers for rotation of HCOO* from a bidentate to a monodentate configuration, the rate-limiting step for HCOO* decomposition on many surfaces. FA thus acts as a cocatalyst for HCOO* decomposition. Our results may guide the community toward improved catalysts for reactions involving HCOO* such as FA decomposition, methanol steam reforming, and the water gas shift reaction. More broadly, our work highlights the ability of hydrogen bonding to modify the adsorbed structures of intermediates and lower the barriers for their reaction on heterogeneous catalysts. Lastly, this phenomenon can be relevant for other reactions involving ammonia, alcohols, and carboxylic acids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Circumventing Kinetic Barriers to Metal Hydride Formation with Metal–Ligand Cooperativity

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [Co III Cp(P Ph 2 N Bn 2 )(CH 3 CN)] 2+ to [HCo III Cp(P Ph 2 NBn 2 )] + . This complex catalytically converts CO 2 to formate under CO 2 reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the P Ph 2 N Bn 2 ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the P Ph 2 N Bn 2 ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pK a values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. Furthermore, this work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Understanding Pore Formation in ALD Alumina Overcoats

AlO X thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Furthermore, understanding the factors that affect ALD thin film porosity would enable future design of layered catalytic structures with tunable nanoscale features on industrially relevant high surface area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2 - 7 nm AlO X by ALD deposited at temperatures of 100 °C, 200 °C and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlO x deposition temperature and presence of Pd. Crystallization of the overcoat to γ-Al 2 O 3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlO X ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlO X ALD coatings.

36 MATERIALS SCIENCE↗