Detailed Reaction Mechanism Research Articles

The Reaction Mechanism and Rates at Ru Single-Atom Catalysts for Hydrogenation of Biomass BHMF to Produce BHMTHF for Renewable Polymers.

Realizing high selectivity for producing biodegradable 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) for renewable polymers from 5-hydroxymethylfurfural (HMF) biomass through ring hydrogenation on single-atom catalysts poses a considerable challenge due to the complexity of HMF functional groups and the difficulty of H2 dissociation. We developed a detailed reaction mechanism based on ab initio molecular dynamics (AIMD) and quantum mechanics (QM) to find that Ru single-atom catalysts can simultaneously dissociate H2 and perform the ring hydrogenation of biomass-derived 2,5-bis(hydroxymethyl)furan (BHMF) to produce biodegradable BHMTHF, with a free energy barrier of 0.82 eV. The unique property of Ru single-atom sites enables H2 to dissociate easily on a single active site of Ru to participate directly in the reaction without diffusion. Furthermore, our predicted reaction rate from microkinetic analysis indicates that ring hydrogenation and side-chain hydrogenolysis are much faster than ring-opening hydrogenation over the range of 300-550 K. The product BHMTHF dominates with a selectivity of 98.9% at 300 and 78.4% at 550 K (the second product is 5-methylfurfural (5-MFA)). This study underscores the unique effectiveness of Ru single atoms in ring hydrogenation reactions using H2 as the hydrogen source, offering insights for the design of single-atom catalysts for other biomass reactions.

Oxygen and Electron Storage Effects in W-Modified Ceria Catalysts for Ammonia Conversion.

Tungsten-modified CeO2 is an excellent catalyst for the catalytic conversion of ammonia. However, the geometric and electronic properties of this catalyst and the detailed reaction mechanisms are not well understood. In this work, the potential configurations of various monomer tungsten oxides supported on the CeO2(111) surface (WOX(x=0-4)/CeO2(111)) are systematically studied and their relative stabilities are evaluated by using on-site Coulomb interaction corrected density functional theory calculations. Their performances are also investigated in enhancing the catalytic efficiency of NH3 adsorption and activation. It is found that the WOx clusters can always form tetrahedron-like structures on the CeO2(111) surface, and the CeO2(111) can exhibit both oxygen- and electron-storage roles to help the WOX maintain such tetrahedron-like WO4 structures and keep the W species at the highest 6+ state. Moreover, the flexibility of the tetrahedral WO4 structure leads to the preferential heterolytic NH3 dissociation at the WO sites, forming stable WNH2 and OH species. This study deepens the understanding of the unique oxygen- and electron-storage effects of the CeO2 support, it also provides valuable insights into the extraordinary catalytic properties of the W-modified CeO2 in NH3 conversion, paving the way for the rational design of more efficient CeO2-based NH3 treatment catalysts.

Reaction Mechanism of the Terminal Plastoquinone QB in Photosystem II as Revealed by Time-Resolved Infrared Spectroscopy.

The secondary plastoquinone (PQ) electron acceptor QB in photosystem II (PSII) undergoes a two-step photoreaction through electron transfer from the primary PQ electron acceptor QA, converting into plastoquinol (PQH2). However, the detailed mechanism of the QB reactions remains elusive. Here, we investigated the reaction mechanism of QB in cyanobacterial PSII core complexes using two time-revolved infrared (TRIR) methods: dispersive-type TRIR spectroscopy and rapid-scan Fourier transform infrared spectroscopy. Upon the first flash, the ∼140 μs phase is attributed to electron transfer from QA•- to QB, while the ∼2.2 and ∼440 ms phases are assigned to the binding of an internal PQ in a nearby cavity to the vacant QB site and an external PQ traveling to the QB site through channels, respectively, followed by immediate electron transfer. The resultant QB•- is suggested to be in equilibrium with QBH•, which is protonated at the distal oxygen. Upon the second flash, the ∼130 μs and ∼3.3 ms phases are attributed to electron transfer to QBH• and the protonation of QB•- followed by electron transfer, respectively, forming QBH-, which then immediately accepts a proton from D1-H215 at the proximal oxygen to become QBH2. The resultant D1-H215 anion is reprotonated in ∼22 ms via a pathway involving the bicarbonate ligand. The final ∼490 ms phase may reflect the release of PQH2 and its replacement with PQ. The present results highlight the importance of time-resolved infrared spectroscopy in elucidating the mechanism of QB reactions in PSII.

Free-radical triggered formation of dioxins from 2-chlorophenol pyrolysis investigated by synchrotron radiation photoionization mass spectrometry

Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and polychlorinated naphthalenes (PCNs) are a category of highly toxic and environmentally persistent pollutants released particularly via thermal processes of chlorine-containing materials. However, the detailed reaction mechanism, especially the evolution of related radicals remains elusive for decades. Herein we have for the first time characterized the radicals and intermediates during pyrolysis of 2-chlorophenol resulting in PCDD/Fs and PCNs, using a flow tube reactor coupled with in-situ synchrotron radiation photoionization mass spectrometry (SR-PIMS). Transient species including 2-chlorophenoxy (C6H4ClO•), phenoxy (C6H5O•), chloro-cyclopentadienyl (•C5H4Cl), chloro-cyclopentadiene (C5H5Cl), fulvenone ketene (C6H4O) and o-benzyne (o-C6H4), were identified via m/z and photoionization efficiency profile. Potential energy surfaces of the early-stage mechanism and the associated rate constants and branching ratios were elucidated. Successively, the formation mechanisms of PCDD/Fs and PCNs from these transient intermediates at high temperatures were proposed which have experimentally validated and refined the previous mechanism. The results suggested that the combination of 2-chlorophenoxy radicals with another 2-chlorophenoxy, phenoxy, phenyl, or o-benzyne leads to the formation of PCDD/Fs, while PCNs are generated from the self-coupling of chloro-cyclopentadienyl.

A Detailed Reaction Mechanism for Thiosulfate Oxidation by Ozone in Aqueous Environments.

The ozone oxidation, or ozonation, of thiosulfate is an important reaction for wastewater processing, where it is used for remediation of mining effluents, and for studying aerosol chemistry, where its fast reaction rate makes it an excellent model reaction. Although thiosulfate ozonation has been studied since the 1950s, challenges remain in developing a realistic reaction mechanism that can satisfactorily account for all observed products with a sequence of elementary reaction steps. Here, we present novel measurements using trapped microdroplets to study the pH-dependent thiosulfate ozonation kinetics. We detect known products and intermediates, including SO32-, SO42-, S3O62-, and S4O62-, establishing agreement with the literature. However, we identify S2O42- as a new reaction intermediate and find that the currently accepted mechanism does not directly explain observed pH effects. Thus, we develop a new mechanism, which incorporates S2O42- as an intermediate and uses elementary steps to explain the pH dependence of thiosulfate ozonation. The proposed mechanism is tested using a kinetic model benchmarked to the experiments presented here, then compared to literature data. We demonstrate good agreement between the proposed thiosulfate ozonation mechanism and experiments, suggesting that the insights in this paper can be leveraged in wastewater treatment and in understanding potential climate impacts.

Bifunctional iron-catalyzed alkyne Z-selective hydroalkylation and tandem Z-E inversion via radical molding and flipping

The challenging synthesis of thermodynamic-unfavored cis-olefins through catalytic cross-coupling reactions requires the synergistic interaction of substrate-activating units and configuration-regulating catalysts. Successfully hitting these two birds with one stone, we herein develop a convenient photoredox access to Z-alkenes from alkynes and light alkanes with a bifunctional iron-catalyzed system possessing both C(sp3)−H activation and configuration-controlling abilities. The protocol exhibits 100% atom utilization, mild conditions, a broad substrate scope, and compatibility with multitudinous functional groups. The detailed reaction mechanism and the origin of geometry regulation are well investigated by experimental and computational studies. Progressively, a catalytic amount of diaryl disulfides is introduced for consecutive photoinduced Z−E isomerization via reversible radical addition and flipping. Big steric hindrance substituents assembled on the disulfide emerge necessity for suppressing double-bond migration. This tandem strategy paves a promising way for stereoselective alkene construction and will bring significant inspiration for the development of transition metal photocatalysis.

Study of chemiluminescence of methane–air flame stabilized on a flat porous burner

In this work, the spatial distribution and spectral characteristics of the chemiluminescence of chemically excited species, OH∗ and CH∗, are experimentally and numerically studied by using a stationary premixed methane–air flame stabilized on the surface of a flat porous burner for various equivalence ratio and normal pressure. Numerical simulations are carried out using detailed reaction mechanisms, and the experimental study includes high-resolution spatial and spectral optical measurements. Despite the data reported in the literature, it is found that (i) the rotational degrees of freedom of OH∗ and CH∗ are not in thermal equilibrium with the surrounding gas and therefore cannot be used to measure flame temperature; (ii) there is no direct correlation between the heat release rate and the distribution of OH∗ and CH∗; (iii) the detailed reaction mechanisms not only quantitatively, and also qualitatively differ in description of the OH∗ and CH∗ concentrations. Since the chemically excited species are well localized in a direction normal to the flame surface, they are demonstrated to be a very accurate markers of flame location. The shape of the combustion front can be reconstructed and resolved up to the accuracy of tens of microns, which is very important for estimation of blow-off critical parameters and measurement of the laminar burning velocity.Novelty and significance statementCurrently, there is a growing interest in the development of sensors for combustion control systems, including active control and suppression of instabilities, in combustion chambers of various devices and engines based on chemiluminescence of excited reaction species. The possibility of non-invasive determination of parameters such as flame temperature, stoichiometry, heat release rate location, etc. using this technique is discussed. We have found that most of these parameters cannot be estimated either due to fundamental limitations or insufficient knowledge of the reaction kinetics involved in the production of these species. Nevertheless, since OH* and CH* are well localized in the direction normal to the flame surface, they can be used as very accurate markers of flame shape and position, allowing us to reconstruct the flame surface to within tens of microns resolution, which is very important for estimating blow-off critical parameters and measuring laminar burning velocity.

Physics-informed neural networks coupled with flamelet/progress variable model for solving combustion physics considering detailed reaction mechanism

In recent years, physics-informed neural networks (PINNs) have shown potential as a method for solving combustion physics. However, current efforts using PINNs for the direct predictions of multi-dimensional flames only use global reaction mechanisms. Considering detailed chemistry is crucial for understanding detailed combustion physics, and how to accurately and efficiently consider detailed mechanisms under the framework of PINNs has not been explored yet and is still an open question. To this end, this paper proposes a PINN/flamelet/progress variable (FPV) approach to accurately and efficiently solve combustion physics, considering detailed chemistry. Specifically, the combustion thermophysical properties are tabulated using several control variables, with the FPV model considering detailed chemistry. Then, PINNs are used to solve the governing equations of continuity, momentum, and control variables with the thermophysical properties extracted from the FPV library. The performance of the proposed PINN/FPV approach is assessed for diffusion flames in a two-dimensional laminar mixing layer by comparing it with the computational fluid dynamics (CFD) results. It has been found that the PINN/FPV model can accurately reproduce the flow and combustion fields, regardless of the presence or absence of observation points. The quantitative statistics demonstrated that the mean relative error was less than 10%, and R2 values were all higher than 0.94. The applicability and stability of this model were further verified on other unseen cases with variable parameters. This study provides an efficient and accurate method to consider detailed reaction mechanisms in solving combustion physics using PINNs.

Insight and assessment of CO2 capture using diethylenetriamine/2-(diethylamino)ethanol/n-propanol water-lean biphasic solvent: Experimental and quantum chemical calculation

Biphasic solvents hold immense promise for CO2 capture, but often struggle to maintain stable performance over long-term cyclic use. Alcohols possess low dielectric constants, which can provide protons for amine-CO2 reaction products, enhancing the regeneration performance of absorbents while maintaining stability. In this study, diethylenetriamine (DETA) + 2-(diethylamino)ethanol (DEEA) + H2O was used as the biphasic solvent, with n-propanol (PrOH) serving as the regulator to develop the DETA + DEEA + PrOH water-lean biphasic solvent. Replacing half of the water with PrOH improved the cyclic stability of the absorbent, reduced the rich phase ratio (46 %) and energy consumption (2.05 GJ·ton−1 CO2), while effectively enhanced corrosion resistance and thermal stability, confirming the feasibility of PrOH as the regulator. The detailed reaction mechanism of the absorbent for CO2 absorption was elucidated through 13C NMR and DFT calculations. Quantum chemical calculations have provided the first compelling evidence for PrOH’s involvement in the absorption reaction: PrOH stores the reaction products in the form of PrOCOOH/PrOCOO−. By regenerating DETA, it restores its CO2 absorption capacity. Through polarity and hydrogen bonding, the reaction products aggregate in one phase, enhancing CO2 loading. In summary the PrOH regulated water-lean biphasic solvent presents a highly promising absorbent for industrial use.

Rigidly Axial O Coordination-Induced Spin Polarization on Single Ni-N4-C Site by MXene Coupling for Boosting Electrochemical CO2 Reduction to CO.

Regulating the spin states in transition-metal (TM)-based single-atom catalysts (SACs), such as the TM-Nx-C configurations, is crucial for improving the catalytic activity. However, the role of spin in single Ni atoms facilitating the electrochemical CO2 reduction reaction (CO2RR) has been largely overlooked. Using first-principles simulations, we investigated the electrocatalytic performance of Ni-N4-C SACs vertically stacked on the O-terminated MXene nanosheets for the CO2RR. The terminated O atoms on MXene axially interact with the Ni atom due to significant charge transfer between them. Unlike the pure Ni-N4 site, which lacks spin polarization, the newly formed Ni-N4O configuration breaks the spin degeneracy of Ni d orbitals, dramatically lifting the energy level of spin-down d orbitals relative to that of spin-up d orbitals. As a result, the d electrons of Ni in the two spin channels are rearranged, leading to large net spin moments of 1.4 μB. Compared to the Ni-N4 site, the partially filled minority-spin dz2 orbitals of Ni on Ni-N4O weaken the occupied d-π* orbitals between Ni and *COOH, significantly stabilizing the key intermediate. The detailed reaction mechanisms and energetics show that four MXenes, namely, Hf3C2, Zr3C2, Hf2C, and Zr2C, can induce a large spin on the Ni site, thereby improving catalytic activity for CO2 reduction to CO, with a lower onset potential of about -0.75 V vs SHE compared to pure Ni SACs (-1.17 V) according to the potential-constant model with an explicit solvent environment.

Modelling the ejection of primary aerosols during the fast pyrolysis of biomass anisotropic particles

A model for the fast pyrolysis of anisotropic biomass particles is presented which considers bubbling dynamics within the liquid intermediate phase (metaplast) and aerosol ejection from this phase. The model employs the population balance equation and the method of moments to estimate the production rate and resultant size distribution of aerosol ejections, incorporating a detailed CRECK reaction mechanism, and considers the effect of anisotropic biomass microstructure on the intraparticle transport of mass and energy. This study investigates the impact of particle size, heating rate (heat transfer coefficient), and lignocellulosic composition on aerosol ejection. The model predicts that, at high heating rates (convective heat transfer coefficient of 359 W/m2.K), aerosols can contribute over 20% to the heavy fraction yield in bio-oil for small particles (1 mm diameter, 4 mm length). The model can predict aerosol size distribution and surface area, indicating an average size of 20 μm for bubbles and 5 μm for aerosols during increased bubble production and aerosol ejection rates. These findings are consistent with prior experimental results and provide essential information for future modeling of extra-particle reactions of the aerosols as they progress through the reactor.

Theoretical and experimental study on the pyrolysis of N-methylpyrrolidone

As a widely used organic solvent, N-methylpyrrolidone (NMP) may suffer thermal decomposition during large-scale industrial usage. The complex potential energy surface and corresponding pressure-dependent rate coefficients of NMP and NMP radicals were obtained at a high level of theory. The intermediates and products of NMP pyrolysis within 900–1200 K were identified and measured by synchrotron vacuum ultraviolet photoionization mass spectrometry. Theoretical calculations show that the Keto-enol tautomerization and direct ring-opening channel yielding C2H4, CO, and CH3NCH2 exhibit the highest rate coefficient among NMP unimolecular reactions at low and high temperatures, respectively. However, the fuel decomposition process cannot be exclusively dominated by single low-barrier channel. The ring-opening reaction forming imino aldehyde radical and following decarbonylation reactions are favored channels during the NMP radical decomposition, while the formation of the dihydropyrrolidone via H or CH3 loss competes with them at higher temperatures. Various smaller heterocycle radicals can be produced and act as intermediates for the multi-step isomerization of the ring-opening products with a relatively low energy barrier, changing the position of the functional group and radical site. A variety of nitrogenous and oxygenated species were observed in the experiment, such as ketene, methyl isocyanate, pyrrole, benzene, and different kinds of imines. Among them, C2H4, HCN, and CO are the major products for NMP decomposition. Potential reaction pathways from NMP to the major experimentally observed products were inferred based on the calculated results and existing knowledge on the pyrolysis of nitrogenous and oxygenated compounds. These results shed light on the complex C/H/O/N reaction system. The calculated rate coefficients and extensive mole fractions data could provide good support for the modeling of a detailed reaction mechanism in the future.

Carrier-phase direct numerical simulation and flamelet modeling of NOx formation in a pulverized coal/ammonia co-firing flame

A carrier-phase direct numerical simulation (CP-DNS) of a coal/ammonia co-firing flame in a turbulent mixing layer is performed with a detailed chemical reaction mechanism including NOx formation. The combustion characteristics and NOx formation mechanism of the coal/ammonia co-firing flame are investigated in detail through a conditional reaction pathway analysis. A new four-fuel-stream flamelet model is proposed to adapt to the piloted pulverized coal/ammonia co-combustion system, in which the complex mixing among the volatiles, char off-gases, ammonia, and the pilot stream is characterized with four mixture fractions. The flamelet solutions in the fourfold mixture fraction space are transformed into a uniform space to facilitate the access to the flamelet table. The performance of the flamelet model is evaluated through an a priori analysis, a combustion mode analysis, and a chemical timescale analysis. For the turbulent coal/ammonia co-firing flame studied, two distinct flame fronts within the shear layer are observed with the upper part being dominated by ammonia combustion and the lower part being governed by coal combustion. The conditional analyses show that NO species is mainly generated by the lower layer governed by pulverized coal combustion where the nitrogen-containing species from volatile combustion are highly involved in the NO formation. The NO concentration first increases with decreasing the fraction of ammonia in the local computational cell, and then decreases towards pure pulverized coal combustion, which is characterized with a newly introduced manifold coordinate. While the temperature and the major and intermediate species mass fractions in the coal/ammonia co-firing flame can be reasonably predicted by the proposed flamelet model, the NOx species mass fractions are not well predicted in the region where pulverized coal combustion dominates. It is clarified that the inaccurate prediction of the NOx species is not caused by interpolation errors, multiple combustion modes or the kinetics of NOx formation, but mainly attributed to the definition of the progress variable.Novelty and significance statementThe novelty of this research is the first DNS of a coal/ammonia co-firing flame in a turbulent mixing layer, and the proposal of a new flamelet tabulation method for predicting NOx formation in the coal/ammonia co-firing flame. It is significant because• Coal/ammonia co-firing promises to provide continuous, secure power supply at a much reduced carbon footprint compared to pure coal combustion;• The underlying physics governing the coal/ammonia co-firing are analyzed using carrier-phase DNS;• The performance of the newly proposed flamelet model in predicting the NOx species is evaluated based on the DNS dataset.

Construction of “ant-like tentacle” structure for ultra-sensitive detection of low-concentration ammonia through colorimetric fluorescent dual-signal gas-sensitive cotton fabric

Detection and monitoring of ammonia (NH3) are crucial in various industries, including plant safety management, food freshness testing, and water pollution control. Nevertheless, creating portable, low-cost, highly sensitive, and easily regenerated ppm-level NH3 sensors poses a significant challenge. In this investigation, an innovative “ant-like tentacle” fabrication strategy was proposed, and a colorimetric fluorescent dual-signal gas-sensitive cotton fabric (PAH-fabric) for NH3 detection was successfully prepared by conventional dyeing using suitable molecular-level photoacid (PAH) sensitive units. The visual recognition lower detection limit of the ultra-low is 1.09 ppm-level. PAH-fabric is not only straightforward, convenient, and cost-effective to prepare, but it can also be efficiently regenerated and recycled multiple times (maintaining excellent gas-sensitive performance even after 100 cycles) by strategically leveraging volatile acid fumigation. Detailed molecular reaction mechanisms involved in the NH3 response and PAH-fabric regeneration are elucidated. PAH-fabric, available either as a portable kit or an alarm system, offers a promising approach for ultra-low NH3 detection. The demonstrated “ant-like tentacle” fabrication strategy introduces numerous possibilities for designing and developing sensors with adjustable response thresholds, particularly those requiring high sensitivity.

Predicting and understanding photocatalytic CO2 reduction reaction with IR spectroscopy-based interpretable machine learning framework.

The highly selective conversion of carbon dioxide into value-added products is extremely valuable. However, even with the aid of in situ characterization techniques, it remains challenging to directly correlate extensive spectral data carrying microscopic information with macroscopic performance. Herein, we adopted advanced machine learning (ML) approaches to establish an accurate and interpretable relationship between vibrational spectral signals and catalytic performances to uncover hidden physical insights. Focusing on photocatalytic CO2 reduction, our model is shown to effectively and accurately predict the CO production activity and selectivity based solely on the infrared (IR) spectral signals, the generalizability of which is additionally demonstrated with a new Bi5O7I photocatalytic system. More importantly, further model analysis has revealed a novel strategy to steer CO selectivity, the physical sanity of which is verified by a detailed reaction mechanism analysis. This work demonstrates the tremendous potential of machine-learned spectroscopy to efficiently identify reaction control factors, which can further lay the foundation for targeted optimization and reverse design.

Synergistic promotion strategy of “dual-site” and “dual-path” to enhance the C–C coupling between CO2 and HCHO driven by photoelectrocatalysis

Photoelectrocatalytic coupling CO2 and volatile organic compounds (VOCs) is a promising green strategy for the synergistic conversion of the two carbon-containing resources to C2 products. The catalytic efficiency is always at the mercy of chemical inertness of CO2 and the competitive hydrogen evolution of H2O. Herein, a modified g-C3N4/ZnAl-LDH Z-scheme heterojunction catalyst with dual reaction site was rationally designed and precisely constructed. The Faraday efficiency of ethanol reached 68.67% with a corresponding formation rate of 227.3 μmol g−1 h−1. As revealed by in-situ characterizations and density functional theory calculations, CO2 and HCHO were absorbed at Zn site and N site, respectively. Then, *CO generated from CO2 and HCHO was converted to *CH3O and *CHO on the dual-active-site heterojunction. The detailed reaction mechanism experiments indicated that C–C coupling only occurred between *CO and *CH3O in electrocatalysis process. Apart from the “*CO + *CH3O” path, another “*CO + *CHO” coupling path was also detected in photoelectrocatalytic process. The selectivity of ethanol was significantly enhanced due to the synthesis of dual-site catalyst and the dual-path coupling path between CO2 and HCHO simultaneously driven by light and electricity.

Microscopic insights into the reaction mechanism of Fe-based oxygen carrier with CH4 in chemical-looping combustion

Fe-based oxygen carriers are regarded as a promising class of oxygen carriers in the field of chemical looping combustion. However, the elucidation of the detailed microscopic reaction mechanism of oxygen carriers remains a significant challenge. In this work, we combine in situ spectroscopy, density-functional theory (DFT) calculations, and micro-kinetic modelling to reveal the microscopic reaction mechanism of CH4 combustion on α-Fe2O3. The macroscopic reaction characterization of hematite with CH4 was carried out on a fixed bed, and gas chromatography (GC) was utilized to detect the combustion products (CO2, H2O and H2). Subsequently, in situ spectroscopy and DFT were combined to reveal the detailed pathways for the CH₄ oxidation process. The results indicated that the reaction intermediates evolve from methoxy (CH3O*) to the bridged bidentate formate (b-HCOO*) to the monodentate formate (m-HCOO*), which ultimately leads to the formation of CO2 through a combination of H transfer and lattice oxygen oxidation. A micro-kinetic model was developed which incorporates surface reaction with the oxygen transport. This revealed that CH3O* and CH3O* → CH2OO* are the key intermediates and radical reactions, respectively, for CO2 production. The elucidation of the reaction mechanism may prove beneficial for the future development of novel Fe-based oxygen carriers.

Two Different Pathways from CO2 to HCO3− in Aqueous Solutions of Arginine Sodium Salt

AbstractAmino acids and their salts are amine compounds with great potential for the chemical absorption of CO2 via the amine method. The CO2 absorption properties of various amino acids and their salts have been previously reported. However, the detailed reaction mechanism of CO2 with basic amino acids in aqueous solutions has not yet been clarified. In this study, we focused on arginine (Arg), which has the highest basicity among the standard amino acids, and its monohydrochloride (ArgCl) and sodium (NaArg) salts for the dissolution of CO2 in the aqueous solutions of Arg, ArgCl, and NaArg. CO2 was chemically dissolved in aqueous Arg and NaArg solutions but not in the aqueous ArgCl solution. One‐dimensional 1H and 13C and two‐dimensional 1H‐13C heteronuclear multiple bond correlation nuclear magnetic resonance spectroscopy indicated that the final products in the aqueous Arg and NaArg solutions were protonated Arg and HCO3−. HCO3− was formed from both CO2‐bonded Arg/Arg− and CO32− intermediates in the aqueous NaArg solution but only from the CO2‐bonded Arg/Arg− intermediate in the aqueous Arg solution. In addition to their high environmental and biological compatibility, Arg and NaArg exhibit high CO2 absorption capacity, demonstrating their potential for practical use as naturally derived CO2 chemical absorbents.

(Invited) Thermal and Plasmon-Mediated Catalytic Transfer Hydrogenation Reactions on Nanostructured Metal Surfaces

Catalytic hydrogenation reactions play crucial roles in fine chemical production and pharmaceutical synthesis. Compared to direct hydrogenation of organic compounds with pressurized hydrogen gas, catalytic transfer hydrogenation reactions using inexpensive and readily accessible small molecules as hydrogen-donors has been considered as a more versatile, scalable, and sustainable pathway toward enhanced chemoselectivity under mild reaction conditions. Noble metal nanoparticles, especially those within the sub-5 nm size regime, can efficiently catalyze the dehydrogenation of a series of hydrogen-storing molecules, such as ammonia borane, hydrazine, formaldehyde, formic acid, and isopropanol, to produce surface-adsorbed hydrogen species that actively hydrogenate a variety of organic substrate molecules. The transfer hydrogenation/hydrogenolysis reactions are mechanistically complex, and may occur selectively along multiple distinct pathways, exhibiting kinetic features signifying the Langmuir–Hinshelwood, Eley–Rideal, autocatalysis, and reversible reaction mechanisms, respectively, depending on the compositions of the metal catalysts and the chemical nature of the hydrogen donors. In this talk, I will share with the audience some new insights concerning the detailed mechanisms of transfer hydrogenation reactions over metal nanocatalyst surfaces. We employ surface-enhanced Raman scattering (SERS) as an in situ fingerprinting spectroscopic tool to precisely resolve the detailed structural evolution of molecular adsorbates during catalytic reactions, based upon which the key intermediates along different reaction pathways are unambiguously identified. The results of deliberately designed in situ SERS measurements show that the chemoselective transfer hydrogenation of nitrophenyl isocyanide by ammonia borane may proceed on noble metal nanocatalyst surfaces selectively through either a unimolecular or a bimolecular pathway, depending on how the nitrophenyl isocyanide adsorbates interact with the metal nanoparticle surfaces. The experimental observations are corroborated by density functional theory calculations, which shed light on the underlying relationships between catalyst-adsorbate interactions and reaction pathway selection. We have further demonstrated that the photoexcited plasmonic hot carriers in the metal nanoparticles can be effectively harnessed to fine-regulate the activation energy barriers associated with the rate-limiting steps for the transfer hydrogenation of nitrophenyl isocyanide on Pd nanocatalyst surfaces when ammonium formate serves as the hydrogen donor. Our results clearly demonstrate the feasibility of using plasmonic hot carriers to kinetically modulate catalytic molecule-transforming processes.

Solvent polarity dependent photo-induced excited state intramolecular proton transfer for triphenylamine centered bis unsymmetrical azine (TPOV): A computational study

The exquisite, regulated characteristics of triphenylamine (TPA) derivatives have served as a profound inspiration in the photochemical field. Herein, the novel TPA-centered bis unsymmetrical azine (i.e., 3-methoxysalicylaldehyde substituted 4-((E)-hydrazonomethyl)-N-(4-((E)-hydrazonomethyl)phenyl)-N-phenylaniline (TPOV)) is investigated regarding its photo-induced behaviors and excited state intramolecular proton transfer (ESIPT) mechanisms. Solvent polar-dependent hydrogen bonding interactions and charge recombination, which is induced by photoexcitation, can greatly promote the ESIPT reactions of the TPOV chemosensor. In order to unveil the detailed excited state reaction mechanism, we theoretically construct the S0-state and S1-state of potential energy surfaces (PESs). Based on the S1-state PESs for TPOV in toluene, chloroform, and acetonitrile solvents, we unveil the single ESIPT reaction of TPOV that happens along with alternative dual intramolecular hydrogen bonds (O1-H2···N3 and O4-H5···N6). The solvent polarity-regulated excited state might clear the path for the creation of innovative luminescent materials.