Energy Barrier For Proton Transfer Research Articles

Facilitating proton-coupled electron transfer for energy-efficient CO2 desorption by C@ZrO2 hollow composites

Catalyst-aided CO2 desorption offers a promising reduction in the energy penalty of amine scrubbing, but faces the challenges of slow mass transfer, sluggish desorption kinetics, and susceptibility to deactivation. In this work, a hollow composite catalyst C@ZrO2 was developed to simultaneously realize rapid electron, proton and mass transfer for CO2 desorption via facilitating proton-coupled electron transfer (PCET) effect. The CO2 desorption rate of benchmark MEA solution increased by 126% and relative heat duty decreased by 69.9% in presence of C@ZrO2, and its catalytic performance didn’t noticeably decline after 10 cycles. Additionally, the desorption rate of biphasic solvents with a high viscosity increased by 112% and relative heat duty decreased by 57.3% with C@ZrO2. Based on theoretical calculation, the facilitated PCET efficiency of carbon layer increased the binding energy of carbamate to enhance its mass transfer rate from the liquid phase to the C@ZrO2 surface, and stretched the C-N bond to reduce the proton transfer energy barrier by 66.8%. This work provided a novel insight to design catalysts for energy-efficient and stable CO2 capture.

Experimental and theoretical investigation of an ionic liquid-based biphasic solvent for post-combustion CO2 Capture: Breaking through the “Trade-off” effect of viscosity and loading

To break through the “trade-off” effect of biphasic solvents between rich-phase viscosity and CO2 loading, we synthesized a novel ionic liquid (diethylenetriamine-3-hydroxypyridine, [DETA][3HPyr]) with multiple active sites and combined it with the organic solvent diethylene glycol monobutyl ether (DGME) and water to prepare a biphasic solvent for CO2 absorption, the best absorption condition was optimized. The results demonstrated that after absorption, the solvent transformed from homogeneous to biphasic with highly self-concentrated absorption products in the rich phase. The volume of the rich phase accounted for only 36.3 % of the total solvent, while its viscosity decreased significantly to 28.1 mPa·s, with a high blend absorption capacity of 2.67 mol·kg−1. The species of absorption product and absorption mechanism was investigated using 13C NMR. The stable presence of carbamic acid in the system was confirmed, contributing to enhanced absorption capacity. Density functional theory calculations revealed that DGME stabilized carbamic acid through intermolecular hydrogen bonding interactions, and highly polar ions generated during absorption and uneven charge distribution between ions dominated the phase-change process and increased the water content in the rich phase. Thermodynamic energy barrier calculation showed that the solvent effect of DGME reduced the Gibbs free energy barriers of proton transfer and facilitated reaction progress. 3IL4D3H exhibited excellent cyclic performance with low regeneration energy consumption at 1.77 GJ·t−1. This is the first work to revealing the intrinsic dynamics of carbamic acid formation, and provides a new perspective for the phase-change mechanism of ionic liquid-based biphasic solvent.

Design strategy of polyamine for CO2 absorption guided by a novel descriptor of hydrogen bond strength

Mixed amine solutions have been proposed as a promising method for improving the efficiency of CO2 adsorption. However, the process of experimentally screening the performance of different amine pairings can be labor-intensive. Here, a novel descriptor was proposed for the proton transfer energy barrier (Eb) to accelerate this process. The Eb of the proton transfer process was calculated using density functional theory (DFT) for 42 mixed systems. The independent gradient model based on Hirshfeld partition (IGMH) found the N–H⋯N type hydrogen bond (HB) is the dominant factor for the interactions. Based on the Atoms-In-Molecules (AIM) approach, the stronger the hydrogen bond, the lower the Eb of the proton transfer process. Three descriptors were identified (electron density (ρ-BCP), energy density (E(r)), and the electron localization function (ELF)) for linking the energy barrier to the strength of HBs. Surprisingly, this descriptor can also be applicable to tri-mixed amine systems, with a high square of the correlation coefficient (R2 > 0.8). Using this descriptor, the Eb of the proton transfer process for the quaternary mixed amines system was successfully predicted. We successfully build the relationship between the hydrogen bond strength and the reaction energy barrier, providing theoretical guidance for the design of mixed amines.

Proton Relay for the Rate Enhancement of Electrochemical Hydrogen Reactions at Heterogeneous Interfaces.

Proton transfer is critically important to many electrocatalytic reactions, and directed proton delivery could open new avenues for the design of electrocatalysts. However, although this approach has been successful in molecular electrocatalysis, proton transfer has not received the same attention in heterogeneous electrocatalyst design. Here, we report that a metal oxide proton relay can be built within heterogeneous electrocatalyst architectures and improves the kinetics of electrochemical hydrogen evolution and oxidation reactions. The volcano-type relationship between activity enhancement and pKa of amine additives confirms this improvement; we observe maximum rate enhancement when the pKa of a proton relay matches the pH of the electrolyte solution. Density-functional-theory-based reactivity studies reveal a decreased proton transfer energy barrier with a metal oxide proton relay. These findings demonstrate the possibility of controlling the proton delivery and enhancing the reaction kinetics by tuning the chemical properties and structures at heterogeneous interfaces.

Design of a perovskite oxide cathode for a protonic ceramic fuel cell

High catalytic activity, low-cost and stable cathode in a temperature range 550–700 °C is essential for the development of protonic ceramic fuel cells (PCFCs). Doping nickel into perovskite La0.5Sr0.5MnO3-δ(LSM) is designed as a cobalt-free cathode based on theoretical calculations and experiments. La0.5Sr0.5Mn0.9Ni0.1O3-δ (LSMNi) as cathode shows higher proton conductivity and ORR activity than the undoped LSM. The PCFCs with LSMNi exhibit low polarization resistance and high peak power density 1.1 W cm−2 at 700 °C. The density functional theory simulations indicate that doping with nickel decreases the oxygen vacancy formation energy and promotes the formation of hydroxide defects. The decrease in proton transfer energy barriers and hydration energy improves the proton conductivity. The improved performance is attributed to fast proton transfer and rapid kinetics of oxygen reduction on the surface of LSMNi. This work provides a novel approach to design cobalt-free cathode for a protonic ceramic fuel cell.

Accurate SCC-DFTB Parametrization of Liquid Water with Improved Atomic Charges and Iterative Boltzmann Inversion.

This work presents improvements of the description of liquid water within the self-consistent-charge density-functional based tight-binding scheme combining the use of Weighted Mulliken (WMull) charges and optimized O-H repulsive potential through the iterative Boltzmann inversion (IBI) process. The quality of the newly developed models is validated considering pair radial distribution functions (RDFs), as well as other structural, energetic, thermodynamic, and dynamic properties. The use of WMull charges certainly improves the agreement with experimental data, however leading to over-structured RDFs at short distance, that can be further improved by considering an optimized O-H repulsive potential obtained by the IBI process. Three different schemes were used to optimize this potential: (i) optimization including short O-H distances. This led to accurate RDFs as well as improved self-diffusion coefficient and heat of vaporization, while the proton transfer energy barrier is severely deteriorated; (ii) optimization starting at long distance. The proton transfer energy barrier is recovered while the heat of vaporization is deteriorated and the O-H RDF is less accurate at short distance; (iii) optimization within the path-integral molecular dynamics scheme which allows us to exclude nuclear quantum effects from the repulsive potential. The latter potential, in conjunction with the WMull improved atomic charges, provides similar results as (i) for structural, dynamic, and thermodynamic properties while recovering a large part of the proton transfer energy barrier. It therefore offers a good compromise to study both dynamic properties and chemistry within liquid water at a quantum chemical level.

Improved proton-transfer barriers with van der Waals density functionals: Role of repulsive non-local correlation.

Proton-transfer (PT) between organic complexes is a common and important biochemical process. Unfortunately, PT energy barriers are difficult to accurately predict using density functional theory (DFT); in particular, using the generalized gradient approximation (GGA) tends to underestimate PT barriers. Moreover, PT typically occurs in environments where dispersion forces contribute to the cohesion of the system; thus, a suitable exchange-correlation functional should accurately describe both dispersion forces and PT barriers. This paper provides benchmark results for the PT barriers of several density functionals, including several variants of the van der Waals density functional (vdW-DF). The benchmark set comprises small organic molecules with inter- and intra-molecular PT. The results show that replacing GGA correlation with a fully non-local vdW-DF correlation increases the PT barriers, making it closer to the quantum chemical reference values. In contrast, including non-local correlations with the Vydrov-Voorhis method or dispersion-corrections at the DFT-D3 or the Tkatchenko-Scheffler level has barely any impact on the PT barriers. Hybrid functionals also increase and improve the energies, resulting in an excellent performance of hybrid versions of vdW-DF-cx and vdW-DF2-B86R. For the formic acid dimer PT system, we analyzed the GGA exchange and non-local correlation contributions. The analysis shows that the repulsive part of the non-local correlation kernel plays a key role in the PT energy barriers predicted with vdW-DF.

Exploring the influence of intermolecular hydrogen bonding on the fluorescence properties of HQCT and HQPH fluorescent chemosensors

Recently, two trace water detection probes, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone(HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine(HQPH) have been successfully designed in the experiment. The original intramolecular proton transfer can be prevented by the water molecules, leading to fluorescence quenching. In order to investigate the fluorescence quenching mechanism, the effect of water molecules on the excited state proton transfer process will be studied in detail. In this contribution, the six models have been optimized and the related analysis have been carried out. When water molecules are involved in the proton transfer process, the energy barrier decreases significantly and the conversion of the enol structure to the keto structure is accelerated. Moreover, the intermolecular hydrogen bonding, not participating in the proton transfer process, can facilitate the proton transfer process by affecting the distribution of the electrostatic potential within the molecule, which in turn lowers the energy barrier for proton transfer.

Potential energy barrier for proton transfer in compressed benzoic acid.

Benzoic acid (BA) is a model system for studying proton transfer (PT) reactions. The properties of solid BA subject to high pressure (exceeding 1 kbar = 0.1 GPa) are of particular interest due to the possibility of compression-tuning of the PT barrier. Here we present simulations aimed at evaluating the value of this barrier in solid BA in the 1 atm – 15 GPa pressure range. We find that pressure-induced shortening of O⋯O contacts within the BA dimers leads to a decrease in the PT barrier, and subsequent symmetrization of the hydrogen bond. However, this effect is obtained only after taking into account zero-point energy (ZPE) differences between BA tautomers and the transition state. The obtained results shed light on previous experiments on compressed benzoic acid, and indicate that a common scaling behavior with respect to the O⋯O distance might be applicable for hydrogen-bond symmetrization in both organic and inorganic systems.

Unveiling the influence of atomic electronegativity on the double ESIPT processes of uralenol: A theoretical study

In this work, the effects of atomic electronegativity (O, S, and Se atoms) on the competitive double excited-state intramolecular proton transfer (ESIPT) reactions and photophysical characteristics of uralenol (URA) were systematically explored by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated hydrogen bond parameters, infrared (IR) vibrational spectra, reduced density gradient (RDG) scatter plots, interaction region indicator (IRI) isosurface and topology parameters have confirmed the six-membered intramolecular hydrogen bond (IHB) O4H5…O3 is the stronger one in all the three studied compounds. Subsequently, frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population analysis essentially uncover that the electron redistribution has induced the ESIPT process. Besides, the constructed potential energy curves (PECs) have indicated that the ESIPT process prefers to occur along the O4H5…O3 rather than the O1H2…O3 and the proton-transfer energy barrier is gradually decreased with the weakening of atomic electronegativity from URA to URA-S and URA-Se. In a conclusion, the attenuating of atomic electronegativity has enhanced the IHBs of URA and thereby promoting the ESIPT reaction, which is helpful for further developing novel fluorophores based on ESIPT behavior in the future.

Energy Barrier of Proton Transfer in 3d Transition Metal-Doped Boehmite

The crystal structure of boehmite contains a large number of hydrogen-bond chains, which are deemed proton-transfer channels that are essential for proton-exchange-membrane materials. The efficienc...

Tuning proton conductivity and energy barriers for proton transfer.

Proton transport is critical for many technologies and for a variety of biochemical and biophysical processes. Proton transfer between molecules (via structural diffusion) is considered to be an efficient mechanism in highly proton conducting materials. Yet, the mechanism and what controls energy barriers for this process remain poorly understood. It was shown that mixing phosphoric acid (PA) with lidocaine leads to an increase in proton conductivity at the same liquid viscosity. However, recent simulations of mixtures of PA with various bases, including lidocaine, suggested no decrease in the proton transfer energy barrier. To elucidate this surprising result, we have performed broadband dielectric spectroscopy to verify the predictions of the simulations for mixtures of PA with several bases. Our results reveal that adding bases to PA increases the energy barriers for proton transfer, and the observed increase in proton conductivity at a similar viscosity appears to be related to the increase in the glass transition temperature (Tg) of the mixture. Moreover, the energy barrier seems to increase with Tg of the mixtures, emphasizing the importance of molecular mobility or interactions in the proton transfer mechanism.

Theoretical study on the ESIPT of fluorescent probe molecules N-(2-(4-(dimethylamino)phenyl)-3-hydroxy-4-oxo-4h -chromen-6-yl) butyramide in different solvents

The solvatochromic effect and excited-state intramolecular proton transfer (ESIPT) of N-(2-(4-(dimethylamino)phenyl)-3-hydroxy-4-oxo-4H-chromen-6-yl)butyramide (NDHB) molecules in different kinds of solvents were studied using density functional theory and time-dependent density functional theory. Hydrogen bonding is the driving force in proton transfer (PT). Functional analysis of reduced density gradient confirms that the intramolecular hydrogen bond is strongly reinforced and thus has facilitated the PT in the first excited state (S1). In addition, the dielectric constant of the solvent can influence this process. These results are also confirmed by the comparison of relevant structural parameters and analysis of infrared vibration spectrum. Frontier molecular orbitals are analyzed in different kinds of solvents, illuminating the solvatochromic effect on spectral properties. Quantitative analyses of NBO and Mulliken's charge reveal that charge redistribution upon photoexcitation enhances the hydrogen bond. Potential energy curves are also scanned to clarify the effect of solvents on ESIPT. Compared with the energy barriers of PT in different kinds of solvents, ESIPT becomes highly favorable when the solvent dielectric constant decreases.

Modulating O-H-based excited-state intramolecular proton transfer by alkyl-substitutions at various positions of 1-hydroxy-11H-benzo[b]fluoren-11-one

The excited-state intramolecular proton transfer (ESIPT) reaction in 1-hydroxy-11H-benzo[b]fluoren-11-one (HBF) is found to be highly sensitive to the presence of specific substituents in positions C2, C4 and C8 with respect to the hydroxyl group, leading to different fluorescent behaviors of HBF with different substituents (J. Phys. Chem. C, 2018, 122, 21833). However, a systematic and comprehensive computational study of the influence of alkyl-substitutions at various positions of HBF on the intramolecular hydrogen bond strength, adsorption and fluorescence spectra and finally the proton transfer energy barriers is scare up to now. Therefore, in this work, the excited-state overall perspective of the proton transfer process of HBF and its seven tert-butyl substituted derivatives (Scheme 1) have been theoretically studied at TD-PBE0/TZVP theory level with the DFT and TDDFT methods. It is concluded that alkyl-substitution at positions C2 and C4 of the parent molecule should increase the strength of intramolecular hydrogen bonding O1-H⋯O, which decrease both the S1-state forward proton transfer energy barriers and the energy differences between forms S1-PT and S1 and eventually facilitate the ESIPT process, whereas alkyl-substitution at position C8 of the parent molecule should have slight opposite effect.

Phosphoric acid-loaded covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane

The development of proton exchange membranes (PEMs) with high loading and stable electrolytes is currently critical and challenging for applications in new energy related devices such as proton exchange membrane fuel cells (PEMFC). In this study, a novel porous organic skeleton (Covalent triazine framework, recorded as CTFp) is synthesized as a material for immobilized guest molecules via a simple nucleophilic substitution reaction. The phosphoric acid molecule (H3PO4) is extruded into the CTFp porous organic framework by vacuum assisted method (VAM). Since the molecular size of H3PO4 is smaller than the window size of the micropores in CTFp, a high loading of H3PO4 is achieved. The large amounts of basic groups distributed in CTFp can form a strong electrostatic interaction with H3PO4, which ensures the low dynamic leakage of H3PO4. PEMs with high proton conductivity are developed by embedding phosphoric acid-loaded CTFp (H3PO4@CTFp) in a SPEEK matrix. The acid-base pair formed between H3PO4@CTFp network and SPEEK optimizes the interfacial interaction and enhances the dispersion of H3PO4@CTFp in the composite membrane. H3PO4 stored in CTFp provides rich proton hopping sites for proton transport. The hydrogen bond network formed by self-dissociation of high concentration H3PO4 molecules constructs a proton transfer channel with low energy barrier for proton transfer, thereby significantly enhancing the proton conductivity of the membrane. The results show that the proton conductivity of the composite membrane at 80 °C is 0.313 S/cm when the filler content is 15%. It is worth noting that the phosphoric acid leakage rate of H3PO4@CTFp is only 15.3% after the filler is immersed in water at 60 °C for 30 days. Therefore, the SPEEK/H3PO4@CTFp composite membranes are promising to develop new PEMs with low acid loss and high proton conductivity.

Transparent proton transport through a two-dimensional nanomesh material

Molecular sieving is of great importance to proton exchange in fuel cells, water desalination, and gas separation. Two-dimensional crystals emerge as superior materials showing desirable molecular permeability and selectivity. Here we demonstrate that a graphdiyne membrane, an experimentally fabricated member in the graphyne family, shows superior proton conductivity and perfect selectivity thanks to its intrinsic nanomesh structure. The trans-membrane hydrogen bonds across graphdiyne serve as ideal channels for proton transport in Grotthuss mechanism. The free energy barrier for proton transfer across graphdiyne is ~2.4 kJ mol−1, nearly identical to that in bulk water (2.1 kJ mol−1), enabling “transparent” proton transport at room temperature. This results in a proton conductivity of 0.6 S cm−1 for graphdiyne, four orders of magnitude greater than graphene. Considering its ultimate pore size of 0.55 nm, graphdiyne membrane blocks soluble fuel molecules and exhibits superior proton selectivity. These advantages endow graphdiyne a great potential as proton exchange material.

Evidence of proton-coupled mixed-valency by electrochemical behavior on transition metal complex dimers bridged by two Ag+ ions.

H-Bonded metal complex dimers with reversible redox behaviour, which are connected by a low-barrier hydrogen bond (LBHB) with a very low energy barrier for proton transfer, can provide a unique mixed-valency state stabilized by the proton-coupled electron transfer (PCET) phenomenon. Using cyclic voltammetry measurements, newly prepared [ReIIICl2(PnPr3)2(Hbim)]2 (2) and [OsIIICl2(PnPr3)2(Hbim)]2 (3) existing as H-bonded dimers in a CH2Cl2 solution showed a four-step and four-electron transfer containing two mixed-valency states of ReIIReIII and ReIIIReIV, and OsIIOsIII and OsIIIOsVI, respectively. Furthermore, [ReIIICl2(PnPr3)2(Agbim)]2 (4) and [OsIIICl2(PnPr3)2(Agbim)]2 (5), bridged by two Ag+ ions instead of two H-bonding protons, were prepared, and their electrochemical behaviours changed to a two-step and four-electron transfer. It is clear that the H-bonded complex dimers 2 and 3, connected by an LBHB, can be electrochemically stabilized into unique pairs of mixed-valency states by PCET, and the H-bonding proton transfer also controls the electrochemical redox behaviour.

Proton Transfer in 1,2,4-Triazolium Dinitramide: Effect of Aqueous Microsolvation.

The gas phase proton transfer process in 1,2,4-triazolium dinitramide (TD) was studied using second-order perturbation theory to determine how the presence of one and two water molecules modifies the potential energy surface that connects the ion pair to the neutral pair. The presence of one water molecule can introduce small proton transfer energy barriers that separate the ion pair from the lower-energy neutral pair. These energy barriers are easily surmounted. Reaction paths were determined for single proton transfers and double proton transfers via one water molecule. In the presence of two water molecules, the global minimum is an ion pair, as are most of the lower-energy local minima. Energy barriers for single, double, and triple proton transfers were also found for TD in the presence of two water molecules. One TD ion pair structure with two water molecules has no corresponding neutral pair energy minimum. A quasi-atomic orbital analysis is used to understand the nature of the bonding in the various species studied in this work.

PROTON TRANSFER IN PHOSPHORUS-CONTAINING ACID–N,N-DIMETHYLFORMAMIDE SYSTEM WITH GLANCE OF ENVIRONMENT

Proton transfer processes in the molecular and ion-molecular complexes of phosphorus acids (phosphoric H3PO4, phosphorous H3PO3, methylphosphonic СН3Н2РО3) with N,N-dimethylformamide (DMF) was studied. The potential energy surface (PES) for proton transfer was studied using the B3LYP/6-31++G(d,p) level of theory, and the solvent effect (here DMF) on the PES was included using the conductor polarized continuum model (CPCM). For all cases, the energy profile for proton transfer represents a double well curve, if intermolecular O…Odistance for the hydrogen bond considered has a fixed length equal to 2.7 Å. The solvent effect favors a proton transfer in the molecular complexes, but no shift of the equilibrium towards ionic pairs is observed. As a result, the energy values associated with proton transfer are significantly reduced in comparison with those found for the gas phase. The proton transfer in the complexes of H3PO4 with DMF is more favored than this process for the cases with H3PO3 and СН3Н2РО3. The probability of proton transfer in the Н3РО4–DMF and (Н3РО4)2–DMF is nearly identical. On the contrary, the barrier height for transfer in Н3РО4–(DMF)n for n=1÷3 increases with increasing number of DMF molecules. The energy barrier for proton transfer in the DMFH+–DMF and H3PO4–H2PO4– is lower than the ones for the molecular complexes.

A quantum chemistry study for 1-ethyl-3-Methylimidazolium ion liquids with aprotic heterocyclic anions applied to carbon dioxide absorption

Flue gas is largely responsible for acid rain and global warming. A special class of imidazolium-based ionic liquids (ILs) has been given a great deal of attention as holding a high absorptivity for the acid gas component, such as CO2. We simulate a class of novel ionic liquids (ILs) formed between the 1-ethyl-3-methylimidazolium cation ([emim]+) and the aprotic heterocyclic anions (AHA), and present a molecular-level description of the interactions of ILs with CO2. The covalent bonds are found between anions and CO2 in the calculated minimum energy structures at B3LYP/6-311++G (d,p) level. And also the basicity of anion is found to have a promotional effect on the solubility of CO2 in ILs. The energy barriers of proton transfer show that [emim][Ind], [emim][4-Triaz] and [emim][3-Triaz] will produce imidazolium carbonates during the proton transfer process because of their high basicities. Both [emim][Tetz] and [emim][Bentri], however, can not prompt the formation of the intermolecular carbene because their basicities are too weak to do so. This implies that the formation of N-heterocyclic carbine complexes can be effectively tuned by varying the basifies of counteractions. Additionally, the infrared spectra are presented to investigate the ion pair vibration characteristics. And the further natural bond orbital (NBO) method is applied to reveal the mechanism of hydrogen bonding interactions.