Comprehensive Mechanisms of Electrocatalytic CO2 Reduction by [Ir(bip)(ppy)(CH3CN)](PF6)2
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99 GENERAL AND MISCELLANEOUS↗
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Electrocatalysis in CO 2 -Binding Organic Liquids with an Iron Porphyrin
Direct electrochemical upgrading of CO 2 in capture media is an attractive approach to carbon capture that can bypass the energy requirement for the thermal release of pure CO 2 . Here we investigate the electrocatalytic activity of iron(tetraphenylporphyrin) in the presence of organic solvents that convert into ionic liquids upon exposure to CO 2 . Four different solvent systems were tested, all of which capture CO 2 in the form of an alkyl carbonate (or carbamate) anion and an acidic ammonium cation. The electrocatalytic selectivity exhibited a strong dependence on the acidity of the capture medium, with the most basic solvent affording a high selectivity for production of CO instead of H 2 . Experimental and computational studies support a canonical mechanism in which the catalyst reacts with free CO 2 in solution, as opposed to a reaction with the alkyl carbonate that is present in high concentration. Kinetic analysis indicates that the rate-limiting step is changed from C–O protonolysis in traditional solvents to the binding of CO 2 in the capture media. Quantitative 13 C– 13 C EXSY revealed that the dissociation of the alkyl carbonate into free, solvated CO 2 is very rapid (~15 s –1 ) compared to the interconversion of HCO 3 – /CO 2 in aqueous solution. These results underscore the need to understand the mechanism and kinetics for both the release of captured CO 2 and its electrocatalytic conversion.
Carbon Dioxide↗
ZeroCAL: Eliminating Carbon Dioxide Emissions from Limestone’s Decomposition to Decarbonize Cement Production
Limestone (calcite, CaCO 3 ) is an abundant and cost-effective source of calcium oxide (CaO) for cement and lime production. However, the thermochemical decomposition of limestone (~800 °C, 1 bar) to produce lime (CaO) results in substantial carbon dioxide (CO 2(g) ) emissions and energy use, i.e., ~1 tonne [t] of CO 2 and ~1.4 MWh per t of CaO produced. Here, we describe a new pathway to use CaCO 3 as a Ca source to make hydrated lime (portlandite, Ca(OH) 2 ) at ambient conditions (p, T) while nearly eliminating process CO 2(g) emissions (as low as 1.5 mol. % of the CO 2 in the precursor CaCO 3 , equivalent to 9 kg of CO 2(g) per t of Ca(OH) 2 ) within an aqueous flowelectrolysis/ pH-swing process that coproduces hydrogen (H 2(g) ) and oxygen (O 2(g) ). Because Ca(OH) 2 is a zero-carbon precursor for cement and lime production, this approach represents a significant advancement in the production of zero-carbon cement. The Zero CArbon Lime (ZeroCAL) process includes dissolution, separation/recovery, and electrolysis stages according to the following steps: (Step 1) chelator (e.g., ethylenediaminetetraacetic acid, EDTA)-promoted dissolution of CaCO 3 and complexation of Ca 2+ under basic (>pH 9) conditions, (Step 2a) Ca enrichment and separation using nanofiltration (NF), which allows separation of the Ca-EDTA complex from the accompanying bicarbonate (HCO 3 – ) species, (Step 2b) acidity-promoted decomplexation of Ca from EDTA, which allows near-complete chelator recovery and the formation of a Ca-enriched stream, and (Step 3) rapid precipitation of Ca(OH) 2 from the Ca-enriched stream using electrolytically produced alkalinity. These reactions can be conducted in a seawater matrix yielding coproducts including hydrochloric acid (HCl) and sodium bicarbonate (NaHCO 3 ), resulting from electrolysis and limestone dissolution, respectively. Careful analysis of the reaction stoichiometries and energy balances indicates that approximately 1.35 t of CaCO 3 , 1.09 t of water, 0.79 t of sodium chloride (NaCl), and ~2 MWh of electrical energy are required to produce 1 t of Ca(OH) 2 , with significant opportunity for process intensification. This approach has major implications for decarbonizing cement production within a paradigm that emphasizes the use of existing cement plants and electrification of industrial operations, while also creating approaches for alkalinity production that enable cost-effective and scalable CO 2 mineralization via Ca(OH) 2 carbonation.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Thermodynamic and Kinetic Activity Descriptors for the Catalytic Hydrogenation of Ketones
Activity descriptors are a powerful tool for the design of catalysts than can efficiently utilize H 2 with minimal energy losses. In this study, we develop the use of hydricity and H - self-exchange rates as thermodynamic and kinetic descriptors for the hydrogenation of ketones by molecular catalysts. Two complexes with known hydricity, HRh(dmpe) 2 and HCo(dmpe) 2 , were investigated for the catalytic hydrogenation of ketones under mild conditions (1.5 atm, 25 °C). The rhodium catalyst proved to be an efficient catalyst for a wide range of ketones, whereas the cobalt catalyst could only hydrogenate electron-deficient ketones. Using a combination of experiment and electronic structure theory, thermodynamic hydricity values were established for 46 alkoxide/ketone pairs in both MeCN and THF solvent. Through comparison of the hydricities of the catalysts and substrates, it was determined that catalysis was only observed for catalyst/ketone pairs with an exergonic H - transfer step. Mechanistic studies revealed that H - transfer was rate-limiting step for catalysis, allowing for the experimental and computation construction of linear free-energy relationships (LFERs) for H - transfer. Further analysis revealed the LFERs could be reproduced using Marcus theory, in which the H - self-exchange rates for the HRh/Rh + and ketone/alkoxide pairs were used to predict the experimentally measured catalytic barriers within 2 kcal mol -1 . Finally, these studies significantly expand the scope of catalytic reactions that can be analyzed with a thermodynamic hydricity descriptor and firmly establish Marcus theory as a valid approach to develop kinetic descriptors for designing catalysts for H - transfer reactions.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
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↗
A Post-translational Histidine–Histidine Cross-Link Enhances Enzymatic Oxygen Reduction Activity with Greater pH Adaptability
Cross-linked protein residues exist as enzyme cofactors to enable or enhance catalytic activities. Despite their importance in nature, the chemical identity of the cross-links is limited to certain amino acid combinations, whose function and the formation mechanism remain insufficiently understood due to the difficulty in isolating native enzymes without the cross-links. Herein, we report the formation and characterization of both His-Tyr and His-His cross-links under oxidative enzymatic turnover conditions in L29H/F33Y/F43H Mb, a structural and functional model of heme-copper oxidase (HCO). The connectivity of the cross-link was characterized as Ne ε2 (His29)-Cd δ2 (His43) by mass spectrometry (LC-MS/MS) and nuclear magnetic resonance (NMR). Interestingly, formation of the cross-link significantly enhances the oxygen reduction activity of the enzyme at neutral or basic pH with higher product specificity. X-ray crystallography has identified a novel Tyr-His cross-link through a Tyr-O-His linkage. Our mechanistic studies indicate the involvement of high-valent heme-iron and the neighboring tyrosine in an oxidative self-processing pathway to generate the cross-link. In conclusion, this work serves as a new example while providing insights into the enzyme cross-link formation, allowing the design of artificial biocatalysts containing these novel cross-links with higher activity and pH adaptability.
Liu, Yiwei [Univ. of Texas, Austin, TX (United Sta↗
Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces
N-heterocyclic carbenes (NHCs) have been widely utilized for the formation of self-assembled monolayers (SAMs) on various surfaces. The main methodologies for preparation of NHCs-based SAMs either requires inert atmosphere and strong base for deprotonation of imidazolium precursors or the use of specifically-synthesized precursors such as NHC(H)[HCO 3 ] salts or NHC–CO 2 adducts. Herein, we demonstrate an electrochemical approach for surface-anchoring of NHCs which overcomes the need for dry environment, addition of exogenous strong base or restricting synthetic steps. In the electrochemical deposition, water reduction reaction is used to generate high concentration of hydroxide ions in proximity to a metal electrode. Imidazolium cations were deprotonated by hydroxide ions, leading to carbenes formation that self-assembled on the electrode’s surface. SAMs of NO 2 -functionalized NHCs and dimethyl-benzimidazole were electrochemically deposited on Au films. SAMs of NHCs were also electrochemically deposited on Pt, Pd and Ag films, demonstrating the wide metal scope of this deposition technique.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Solar-driven selective conversion of millimolar dissolved carbon to fuels with molecular flux generation
Abstract The direct utilization of dissolved inorganic carbon in seawater for CO 2 conversion promises chemical production on-demand and with zero carbon footprint. Photoelectrochemical (PEC) CO 2 reduction (CO 2 R) devices promise the sustainable conversion of dissolved carbon in seawater to carbon products using sunlight as the only energy input. However, the diffusion-dominant transport mechanism and the near-zero concentration of CO 2 (aq) (CO 2 dissolved in aqueous solution) in static seawater has made it extremely challenging to achieve high solar-to-fuel (STF) efficiency and high carbon-product selectivity. Here, where CO 2 (aq) as a reactant generated in situ by acidification of HCO 3 - flows continuously from BiVO 4 photoanodes to Si photocathodes, enabling a single-step conversion of dissolved carbon into products. Our PEC device significantly increases the CO selectivity from 3% to 21%, which approaches the 30% theoretical limit according to multi-physics modeling. Meanwhile, the Si/BiVO 4 PEC CO 2 R device achieved a STF efficiency of 0.71%. Such flow engineering achieves flow-dependent selectivity, rate, and stability in simulated seawater, thus promising practical solar fuel production at scale.
Liu, Bin↗
Formation, chemical evolution and solidification of the dense liquid phase of calcium (bi)carbonate
Metal carbonates, which are ubiquitous in the near-surface mineral record, are a major product of biomineralizing organisms and serve as important targets for capturing anthropogenic CO 2 emissions. However, pathways of carbonate mineralization typically diverge from classical predictions due to the involvement of disordered precursors, such as the dense liquid phase (DLP), yet little is known about DLP formation or solidification processes. Using in situ methods we report that a highly hydrated bicarbonate DLP forms via liquid–liquid phase separation and transforms into hollow hydrated amorphous CaCO 3 particles. Acidic proteins and polymers extend DLP lifetimes while leaving the pathway and chemistry unchanged. Molecular simulations suggest that the DLP forms via direct condensation of solvated Ca 2+ •(HCO 3 – ) 2 complexes that react due to proximity effects in the confined DLP droplets. Furthermore, our findings provide insight into CaCO 3 nucleation that is mediated by liquid–liquid phase separation, advancing the ability to direct carbonate mineralization and elucidating an often-proposed complex pathway of biomineralization.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Photodissociation of iso-propoxy (i-C 3 H 7 O) radical at 248 nm
Photodissociation of the i-C 3 H 7 O radical is investigated using fast beam photofragment translational spectroscopy. Neutral i-C 3 H 7 O radicals are produced through the photodetachment of a fast beam of i-C 3 H 7 O - anions and are subsequently dissociated at 248 nm (5.0 eV). Furthermore, the dominant product channels are CH 3 + CH 3 CHO and OH + C 3 H 6 with some contribution from H + C 3 H 6 O. The CH 3 and H loss channels are attributed to dissociation on the ground electronic state of i-C 3 H 7 O, but in a nonstatistical manner because calculated RRKM dissociation rates exceed the rate of energy randomization. Translational energy and angular distributions for OH loss are consistent with ground state dissociation, but the branching ratio for this channel is considerably higher than predicted from RRKM rate calculations. Additionally, i-C 3 H 7 O undergoes three-body fragmentation to CH 3 + CH 3 + HCO and CH 3 + CH 4 + CO. These three-body channels are attributed to dissociation of i-C 3 H 7 O to CH 3 + CH 3 CHO, followed by secondary dissociation of CH 3 CHO on its ground electronic state.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Translating aqueous CO 2 hydrogenation activity to electrocatalytic reduction with a homogeneous cobalt catalyst
A molecular cobalt CO 2 hydrogenation catalyst was explored for electrocatalytic CO 2 reduction under aqueous conditions. Here, the resulting pH-dependent selectivity between H 2 and HCO 2 - is rationalized with thermodynamic analysis and stoichiometric experiments.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Nanocapsules of unprecedented internal volume seamed by calcium ions
The inception of an unprecedented class of voluminous Platonic solids displaying hierarchical geometry based on pyrogallol[4]arene moieties seamed by divalent calcium ion is described. Single-crystal X-ray structural determination has established the highly conserved geometry of two original Ca 2+ -seamed nanocapsules to be essentially cubic in shape with C-ethylpyrogallol[4]arene units located along the twelve edges of the cube which are then bridged by metallic polyatomic cations ([Ca 4 Cl] 7+ or [Ca(HCO 2 )Na 4 ] 5+ ) at the six cube faces. The accessible volume of the nanocapsules is ca. 3500 Å 3 and 2500 Å 3 and is completely isolated from the exterior of the capsules. These remarkable nanocapsule discoveries cast a spotlight on a marginalized area of synthetic materials chemistry and encourage future exploration of diversiform supramolecular assemblies, networks, and capsules built on calcium, with clear benefits deriving from the intrinsic biocompatibility of calcium. Finally, a proof-of-concept is demonstrated for fluorescent reporter encapsulation and sustained release from the calcium-seamed nanocapsules, suggesting their potential as delivery vehicles for drugs, nutrients, preservatives, or antioxidants.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Product branching in the photodissociation of oxazole detected by broadband rotational spectroscopy
The products of the UV photodissociation of oxazole were measured via broadband rotational spectroscopy in a uniform supersonic flow. Seven products were detected associated with five channels, with HCN and HCO as the dominant species observed.
Downes-Ward, Briony↗
The impact of second coordination sphere functional group extension on product selectivity for manganese bipyridyl CO 2 reduction electrocatalysts
Utilizing the well established manganese bipyridyl class of homogeneous electrocatalyst, four new ligands are studied to probe the influence of distal, outer coordination sphere, H-bonding and steric effects on product selectivity for proton-coupled electrocatalytic CO 2 reduction. The presence of a simple acetate functional group in the second coordination sphere provides a high selectivy for CO 2 -to-CO conversion irrespective of proton source (H 2 O vs. PhOH) or applied potential. The o-(methoxybenzoate)phenyl second/outer coordination sphere at the bipyridyl 6,6′-positions imparts poor product selectivity. In contrast, upon conjugation of the acetate functional group with the N-Boc-alanine moiety, a CO : HCO 2 − product selectivity of ∼1 : 1 is observed at the high overpotential catalytic wave (for both H 2 O and PhOH acids). Computed enthalpy and free energy of activation parameters suggest that selective CO 2 insertion at the manganese hydride transition state is favored, over protonation, consistent with negligible hydrogen production during controlled potential electrolysis studies.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Time-resolved x-ray absorption spectroscopy probe in ultrafast surface chemistry
To celebrate the scientific achievement of Jo Stöhr, I present here a personal account of the use of x-ray absorption spectroscopy to probe dynamics on surfaces using x-ray lasers. In particular, I will review the investigation of ultrafast processes in adsorbates on surfaces using an optical pump and an x-ray absorption spectroscopy probe. Here, it is shown that it is possible to gain insight into the effects of electronic excitations in metals on adsorbates as well as laser-induced vibrational motions. Furthermore, the ultrafast optical pump allows the detection of the CO precursor state in the desorption channel, species close to the transition state in CO oxidation, and the transient HCO intermediate during CO hydrogenation on Ru(0001).
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Hierarchical Ion Interactions in the Direct Air Capture of CO 2 at Air/Aqueous Interfaces
The direct air capture (DAC) of CO 2 using aqueous solvents is plagued by slow kinetics and interfacial barriers that limit effectiveness in combating climate change. Functionalizing air/aqueous surfaces with charged amphiphiles shows promise in accelerating DAC; however, insight into these interfaces and how they evolve in time remains poorly understood. Specifically, competitive ion interactions between DAC reagents and reaction products feedback onto the interfacial structure, thereby modulating interfacial chemical composition and overall function. In this work, we probe the role of glycine amino acid anions (Gly - ), an effective CO 2 capture reagent, that promotes the organization of cationic oligomers at air/aqueous interfaces. These surfaces are probed with vibrational sum frequency generation spectroscopy and molecular dynamics simulations. Our findings demonstrate that the competition for surface sites between Gly - and captured carbonaceous anions (HCO 3 - , CO 3 2 -, carbamates) drives changes in surface hydration, which in turn tunes oligomer ordering. This phenomenon is related to a hierarchical ordering of anions at the surface that are electrostatically attracted to the surface and their ability to compete for interfacial water. These results point to new ways to tune interfaces for DAC via stratification of ions based on relative surface propensities and specific ion effects.
54 ENVIRONMENTAL SCIENCES↗
Isomers and band assignments in the cryogenic vibrational spectrum of the binary complex between water and protonated formic acid using two color, IR–IR photobleaching and H/D isotopic substitution
Protonated formic acid (PFA) is purported to be the active species in the catalytic activation of condensation reactions at the acidic interface of microdroplets. Here, we investigate the fundamental interaction between PFA and water with cryogenic ion vibrational spectroscopy of the binary PFA–H 2 O complexes generated via electrospray ionization followed by buffer gas cooling to about 20 K. The patterns displayed by the isomer-specific IR spectra of D 2 -tagged PFA–H 2 O indicate that two distinct, non-interconverting rotamers are present at low temperatures based on the cis and trans structures of the HCO 2 $H$$^{+}_{2}$ core ion. Both of these occur with the water molecule attached to the OH that is in a cis-configuration relative to the CH group (denoted E), but differ in the E vs Z (cis vs trans relative to the CH group) orientation of the spectator OH. This assignment scheme corrects a previous theoretical analysis that invoked a scenario in which structures with E- and Z-bound water molecules interconvert at low (20 K) temperatures. Isomer-specific bands arising from the OH stretches and water bending modes are deconvoluted using isotopomer-specific spectroscopy of the complexes with partial H/D exchange. The dependence of the nominal shared proton OH stretch frequency on the deuteration of the tethered water confirms strong coupling between this mode and the water bending fundamental.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
State-to-state rate coefficients for HCS+ in rotationally inelastic collisions with H2 at low temperatures
ABSTRACT HCS+ ions have been detected in several regions of the interstellar medium (ISM), but an accurate determination of the chemical-physical conditions in the molecular clouds where this molecule is observed requires detailed knowledge of the collisional rate coefficients with the most common colliders in those environments. In this work, we study the dynamics of rotationally inelastic collisions of HCS+ + H2 at low temperature, and report, for the first time, a set of rate coefficients for this system. We used a recently developed potential energy surface for the HCS+–H2 van der Waals complex and computed state-to-state rotational rate coefficients for the lower rotational states of HCS+ in collision with both para- and ortho-H2, analysing the influence of the computed rate coefficients on the determination of critical densities. Additionally, the computed rate coefficients are compared with those obtained by scaling the ones from HCS+ in collision with He (an approximation that is sometimes used when data is lacking), and large differences are found. Furthermore, the approximation of using the rates for the HCO+ + H2 collision as a rough approximation for those of the HCS+ + H2 system is also evaluated. Finally, the complete set of de-excitation rate coefficients for the lowest 30 rotational states of HCS+ by collision with H2 is reported from 5 to 100 K.
79 ASTRONOMY AND ASTROPHYSICS↗