High Reaction Energy Research Articles

Principles of designing electrocatalysts to boost C–N coupling reactions for urea synthesis

AbstractThe electrocatalytic C–N coupling reaction can achieve green and sustainable urea synthesis as well as CO2 conversion and nitrogen fixation. However, the electrocatalytic C–N coupling reaction still faces challenges such as difficult adsorption and activation of reactive species, a large number of reactive intermediates, high reaction energy barriers, and inert reactive kinetics, resulting in the low urea yielding rate and Faradic efficiency. The development of efficient catalysts is key to improve the urea yielding rate and Faradic efficiency. This review covers the development history and basic principles of electrocatalytic C–N coupling for urea production, analyzes the nanostructure–catalytic activity relationship as well as the electronic structure–catalytic activity relationship, and discusses the main reaction mechanism of electrocatalytic C–N coupling for urea production. Based on these analyses, the concept of designing efficient C–N coupling catalysts is derived. Finally, the research status of electrocatalytic C–N coupling for urea synthesis is summarized, and the prospect for developing efficient electrocatalysts and C–N coupling mechanism are proposed.

Regulation of charge transfer direction and key steps via Y modification of redox-active sites on borocarbonitride for photocatalytic CO2 reduction

Hexagonal borocarbonitride (h-BCN) have garnered significant attention for their unique optoelectronic properties in solar-to-fuel conversions. However, the efficiency of h-BCN in the field of photocatalytic CO2 reduction is limited by high reaction energy barriers and serious charge recombination. Here, a Y-modified h-BCN were synthesized through an organic-inorganic hybrid precursor pyrolysis method. Both theoretical simulation and experimental analysis demonstrated that the Y-modified sample exhibits an inhomogeneous charge distribution that favours rapid charge transfer and can decrease the reaction barrier of the rate-control step. Further investigations indicated that adding Y changes the electronic structure of BCN, which leads to rapid charge transfer/separation and an increased lifespan for photoinduced electrons. As a result, 2.5%Y/BCN exhibited improved CO2 photoreduction activity with a fuel yield of 50.35 μmol g−1. Notably, this outstanding performance was achieved without the use of any cocatalyst or sacrificial agent. In the meanwhile, cycle test shows the photocatalyst's great stability. The findings provide a useful suggestions for BCN-based photocatalysts.

Novel coal-based carbon encapsulating Co-N-C derived from ZIF-67 composite as an efficient chainmail electrocatalyst for zinc-air batteries

Energy shortages and the global climate crisis have led researchers to develop renewable clean energy sources. Zinc-air batteries have attracted much attention due to their high theoretical energy density, safety, and conversion efficiency. However, the cathodic oxygen reduction reaction (ORR) exhibits slow reaction kinetics and high reaction energy barriers, which require catalysts with high activity, selectivity, and stability to improve the reaction rate. Chainmail catalysts have attracted attention in the field of energy conversion and storage due to their structural stability. In this paper, coal-based carbon encapsulating Co-N-C chainmail electrocatalysts are designed and constructed, which can effectively reduce the thermal crumpling phenomenon of the framework structure, enhance the defect content and provide active sites for ORR. The results show that G/Co-N-C-0.025 exhibits a higher half-wave potential than that of Pt/C, with a high ultimate current density of 4.6 mA cm−2, good cycling stability, and high methanol resistance. A primary zinc-air battery is assembled, with an open-circuit voltage of 1.417 V and a maximum power density of 82 mW cm−2. The specific capacity is 789.2 mAh g−1 at a current of 20 mA cm−2. This work provides new ideas for the design and application of high-performance chainmail electrocatalysts.

Highly efficient recycling of polyester wastes to diols using Ru and Mo dual-atom catalyst

The chemical recycling of polyester wastes is of great significance for sustainable development, which also provides an opportunity to access various oxygen-containing chemicals, but generally suffers from low efficiency or separation difficulty. Herein, we report anatase TiO2 supported Ru and Mo dual-atom catalysts, which achieve transformation of various polyesters into corresponding diols in 100% selectivity via hydrolysis and subsequent hydrogenation in water under mild conditions (e.g., 160 °C, 4 MPa). Compelling evidence is provided for the coexistence of Ru single-atom and O-bridged Ru and Mo dual-atom sites within this kind of catalysts. It is verified that the Ru single-atom sites activate H2 for hydrogenation of carboxylic acid derived from polyester hydrolysis, and the O-bridged Ru and Mo dual-atom sites suppress hydrodeoxygenation of the resultant alcohols due to a high reaction energy barrier. Notably, this kind of dual-atom catalysts can be regenerated with high activity and stability. This work presents an effective way to reconstruct polyester wastes into valuable diols, which may have promising application potential.

Tunable electronic coupling of Fe-doped CoS2/reduced graphene oxide composites for boosting bifunctional water splitting activity

The electrochemical water splitting process is always restrained for its sluggish kinetics and high reaction energy barrier. Therefore, developing outstanding bifunctional catalysts to enhance the reaction kinetics and electron transfer efficiency of water splitting is crucial. In this work, we reported a bifunctional nanohybrid electrocatalyst consisting of Fe-doped CoS2 nanocage-decorated reduced graphene oxide (FCSRGO) with superior water splitting activity. The Fe doping adjusts the density of state (DOS) to increase the conductivity and decreases the adsorption free energy of intermediates, contributing to optimized catalytic activity. Furthermore, the incorporation of RGO reduces the agglomeration of CoS2 and improves the conductivity, thereby amplifying the charge/species transfer efficiency. By optimizing the amount of RGO, FCSRGO shows exceptional electrocatalytic performance, achieving a benchmark current density of 10 mA·cm−2 at remarkably low overpotentials (ƞ10) of 107 mV and 239 mV in 1 M KOH solution for HER and OER, respectively. Moreover, the nanohybrid exhibits remarkable stability with the current density retention of 89.8% for HER and 85.8% for OER after 36000 s test. This work highlights the creating of high-performance electrocatalysts for efficient and sustainable hydrogen and oxygen evolution through heteroatom doping and carbon incorporation.

Modulating Fe/P ratios in Fe-P alloy through smelting reduction for long-term electrocatalytic overall water splitting

Owing to strong Fe-P interaction that differs the electron distribution beneath metal/phosphide interface of Fe-P alloy, the charge transfer of Fe-P alloy has been accelerated during the electrocatalytic oxidation process and improved the efficiency and durability of overall water splitting. In this work, a novel metallurgical technology in combination with smelting reduction and Single Roller Melting Spinning (SRMS) for the purpose of electrochemical overall water splitting where High-phosphorus Oolitic Iron Ore (HPOIO) has been directly used as the main raw material is developed for preparing amorphous Fe-P alloys strips. The rational modulation on the Fe/P ratio can alter the crystal structure and crystallinity of Fe-P alloy, favor electron transfer, and further trap the positively charged H+. The obtained FeP electrocatalyst exhibits 436 and 527 mV at 10 mA cm−1 with Tafel slopes of 102.3 and 77.2 mV dec−1 for HER and OER in 1.0 mol/L KOH solution, respectively, especially with long-term stability (∼207 h for HER and ∼42 h for OER). Specifically, the DFT calculation displaying structural advantages and componential superiorities exhibited that P in Fe-P amorphous alloy regulated by systematic P addition optimization might increase the energy density in the Fermi level. Furthermore, the phosphorus content brought about high active surface areas, low impedance, and variable reaction paths-caused low reaction energy barriers with the improved amorphicity and absorption and desorption of intermediates, thereby boosting the overall water splitting activity. This study displays a novel strategy to develop Fe-P amorphous alloy for the stable and efficient overall water splitting.

Scalable spray-dried high-capacity MoC1-x/NC-Li2C2O4 prelithiation composite for lithium-ion batteries

Li2C2O4 is recognized as a promising prelithiation reagent for compensating active lithium owing to its residue-free, high-capacity, air-stable and cost-effective advantages. However, the inherent challenges of low conductivity and high reaction energy barriers necessitate the incorporation of a catalyst to expedite charge conduction and enhance electrochemical activity. It is essential to achieve an efficient composite Li2C2O4 with the catalyst while maintaining substantial prelithiation capacity. Herein, a scalable spray-dried strategy is developed to fabricate MoC1-x/NC-Li2C2O4 composites. The defect-rich nano-MoC1-x/NC catalysis facilitates intimate contact with Li2C2O4, leading to the formation of homogeneous microspheres that promote efficient solid-solid catalysis. Remarkably, the specific capacity of MoC1-x/NC-Li2C2O4 can reach up to 1117 mAh g−1 (calculation based on the complete release of CO2) with a low ratio of catalysts, demonstrating significant application potential. Furthermore, the activation potential of homogeneous MoC1-x/NC-Li2C2O4 composite can be reduced to 4.12 V at 50 °C, while maintaining favorable electrode integrity and wettability. Upon implementation in Gr||NCM622 and SiC||NCM622 full cell, the initial discharge specific capacity is improved by 10.5 % and 11.6 %, respectively. Notably, sufficient active lithium post-prelithiation preserves the cathode and SEI structural stability, ensuring a stable cycling performance. This work presents a scalable prelithiation material with great promise for advanced lithium-ion battery applications.

Construction of covalent organic framework functionalized with carbene dual active sites for enhancing CO2 carboxylation conversion

It is challenging to drive the integral insertion of linear CO2 with thermodynamic stability and kinetic inertness as carboxylation reagents into C-H, O-H, and other chemical bonds. The known strategies mainly focus on unilaterally activating substrates as nucleophilic reagents, and the CO2 insertion process has to overcome high reaction energy barriers. Herein, this work reports copper carbene NHC-CuCl and CO2 adduct NHC-CO2 as dual active sites are introduced into the porous crystal covalent organic framework to effectively drive the simultaneous activation of substrate and CO2. The catalyst COF@NHC-CuCl/CO2 exhibited high space–time yield (STY = 71.9 h−1 at 40 °C and 36.7 h−1 at 25 °C, ambient pressure) in the carboxylative coupling of propargylic alcohols, which is superior to most of the cutting-edge heterogeneous catalysts. Experimental data and density functional theory calculation revealed that the carboxylation reaction involving NHC-CO2 and ordinary CO2 source follows different reaction mechanisms. The free energy barrier of the CO2 insertion process (TS:35.9 kcal/mol → 17.4 kcal/mol) is effectively reduced by NHC modulation of the CO2 bond energy parameters to facilitate the reaction. Due to the unique structural design, COF@NHC-CuCl/CO2 exhibited outstanding diffusion mass transfer, catalytic diversity and reusability. This work proposes a novel construction strategy of heterogeneous dual activation catalyst to provide a sustainable solution for efficiently producing value-added chemicals from CO2 under mild conditions.

Surface‐Redox Pseudocapacitance‐Dominated Charge Storage Mechanism Enabled by the Reconstructed Cathode/Electrolyte Interface for High‐Rate Magnesium Batteries

AbstractTh all phenyl complex (APC) electrolyte is generally accepted to be compatible with Mg metal anodes, offering excellent plating/stripping reversibility. However, the large Cl desolvation penalty of the MgCl+ solvation structure in APC electrolyte causes a high reaction energy barrier at the cathode/electrolyte interface, resulting in unsatisfactory rate performance. Herein, the interface reconstruction strategy of an anatase TiO2 cathode is proposed by the combination of ultrathin carbon coating and oxygen vacancies, which realizes the fast surface‐redox pseudocapacitance charge storage mechanism via MgCl+, circumventing the sluggish solid‐phase migration of Mg2+. Theoretical calculations verify that the introduction of oxygen vacancies in TiO2, not only increases the intrinsic electronic conductivity, but also improves the adsorption capability for MgCl+, which enhances the surface‐redox pseudocapacitance of TiO2. Moreover, in situ Raman measurements, ex situ XPS spectra and XRD patterns demonstrate the structural integrity of TiO2 without undergoing phase change and the rapid reversible storage of MgCl+. Furthermore, in situ electrochemical impedance spectra reveal that the reconstructed cathode/electrolyte interface promotes the kinetics of active cations and induces the less potential‐dependent charge storage process. Consequently, TiO2 exhibits a remarkable rate performance (discharge capacity of 68.9 mAh g−1 at 1 A g−1) and long‐lifespan over 3000 cycles at 0.5 A g−1.

Theoretical Analysis of Metals Supported on Tungsten Oxide Nanowires (W18O49) for Water Dissociation Reaction

AbstractPt is the cause of the high total cost of fuel cells and electrolyzers, leading to difficult commercialization. Here, various types of metal atoms, i.e., Pt, Pd, Ni, Ir, Ag, and Rh, suitable for catalysts are used and supported by W18O49 nanowires for oxygen evolution reaction (OER) by the density functional theory (DFT) method. Four adsorbate molecules involved in OER were tested on adsorption energy: OH, O, OOH, and OO. Although the adsorption energy of these adsorbate molecules indicates that W18O49 has low adsorption energy, the Gibbs free energy diagram demonstrates that W18O49 has high OER reaction energy. Pt, Pd, Ni, and Rh have the lowest Gibbs energy to initiate the reaction and reasonable Gibbs free energy for other OER reactions. Bimetallic or trimetallic active sites can be developed along with selecting other metals with Pt, Pd, Rh, and Ni to reduce the Gibbs free energy difference for the decomposition of OH to O and OOH to H2O. Ag metal can also be applied as a second or third metal because Ag exhibits a relatively low Gibbs free energy difference in the O to OOH step. A selectivity study of each step on bimetallic and trimetallic active sites needs to be performed.

A MoS2/Cu1.8S/NiS@MoSX heterostructured electro-catalyst for high-efficiency hydrogen evolution in alkaline solution

For the electrolysis of aquatic hydrogen process efficiently in alkaline solution, the synthesis of high-activity and low-cost electrocatalyst are needed to reduce the high reaction energy barrier of HER. In this paper, a cost-effective MoS2/Cu1.8S/NiS@MoSX heterostructure catalyst was synthesized simply by a one-step hydrothermal method. In the preparation process, Cu mesh was used instead of traditional copper salts as a Cu source, enabling the spontaneous formation of Cu1.8S and NiS together on a large specific surface area MoSx substrate, which helps to accelerate the dissociation of H2O. The catalyst had an overpotential as low as 87 mV at 10 mA/cm2 and only 31 mV was increased after 1000 cycles of CV test in 1 M KOH solution. The characterization analysis showed that the heterogeneous structure of the catalyst contained a large amount of bridging S22− and vacancy defects, which greatly promoted the catalyst stability and catalytic effect.

Synergetic Oxidation of the Hydroxyl Radical and Superoxide Anion Lowers the Benzoquinone Intermediate Conversion Barrier and Potentiates Effective Aromatic Pollutant Mineralization.

Regulation of the free radical types is crucial but challenging in the ubiquitous heterogeneous catalytic oxidation for chemosynthesis, biotherapy, and environmental remediation. Here, using aromatic pollutant (AP) removal as a prototype, we identify the massive accumulation of the benzoquinone (BQ) intermediate in the hydroxyl radical (•OH)-mediated AP degradation process. Theoretical prediction and experiments demonstrate that BQ is both a Lewis acid and base because of its unique molecular and electronic structure caused by the existence of symmetrical carbonyl groups; therefore, it is hard to be electrophilically added by oxidizing •OH as a result of the high reaction energy barrier (ΔG = 1.74 eV). Fortunately, the introduction of the superoxide anion (•O2-) significantly lowers the conversion barrier (ΔG = 0.91 eV) of BQ because •O2- can act as the electron donor and acceptor simultaneously, electrophilically and nucleophilically add to BQ synchronously, and break it down. Subsequently, the breakdown products can then be further oxidized by •OH until completely mineralized. Such synergistic oxidation based on •OH and •O2- timely eliminates BQ, potentiates AP mineralization, and inhibits electrode fouling caused by high-resistance polymeric BQ; more importantly, it effectively reduces toxicity, saves energy and costs, and decreases the environmental footprint, evidenced by the life cycle assessment.

Dual Cocatalysts Synergistically Promote Perylene Diimide Polymer Charge Transfer for Enhanced Photocatalytic Water Oxidation

Water oxidation is a critical reaction in artificial photosynthesis which is limited by a high reaction energy barrier and often requires matching cocatalysts. Dual cocatalysts Co3O4 and Pt are combined with a perylene diimide (PDI) polymer, accomplishing a photocatalytic O2 production rate of 24.4 mmol g–1 h–1 under visible light irradiation, which is a 5.4-fold enhancement compared with PDI alone. Moreover, the apparent quantum yield of the O2 evolution reaction reaches 6.9% at 420 nm and remains 1.2% at 590 nm. The dual cocatalysts-constructed interfacial electric fields provide an anisotropic driving force for photogenerated holes to Co3O4 and electrons to Pt, synergistically improving the spatial charge separation efficiency for water oxidation. Co3O4 molecules serve as active sites for the water oxidation reaction to improve the utilization of photogenerated holes on the surface. This work provides a valuable demonstration of the interaction process and mechanism of cocatalysts, guiding the selection of cocatalysts for high-efficiency solar energy conversion.

Effects of synthetic maturation on phenanthrenes and dibenzothiophenes over a maturity range of 0.6 to 4.7% EASY%Ro

As the exploration target of the shale gas industry is shifting to more deeply buried strata, it is of great application value to study the distribution of molecular maturity ratios in the late dry gas zone and the mechanisms behind them for a robust thermal maturity evaluation. This study investigates the continuous molecular evolution of parent and alkylated phenanthrenes (P, A-Ps), anthracenes (An, A-An), and dibenzothiophenes (DBT, A-DBTs) during the synthetic maturation (Gold tube hydrous pyrolysis) from 0.6% to 4.7% EASY%Ro of two marine shales. The two untreated samples show different elemental composition, bulk geochemistry, and molecular geochemistry characteristics. The distribution sequence of classes with different degrees of alkylation indicates that higher reaction energy is needed for the demethylation of compounds with a lower degree of alkylation. However, the occurrence of trimethyl- and dimethyl-classes of A-Ps and A-DBTs at 550 °C (EASY%Ro: 4.4%) and 600 °C (EASY%Ro: 4.7%) is proposed to result from the breakdown of pyrobitumen at high pyrolysis temperature. The inclusion of An with P when calculating maturity ratios has an influence on their correlation with EASY%Ro at very high maturity. Common molecular thermal ratios including MPI-1, MPI-3, MPDF, MPR, and DMPR were calculated and compared with previous works. All proxies show general trends of firstly increasing and then decreasing along with thermal evolution. The separate trends in the molecular thermal evolution of the two source rocks could be attributed to the difference in organic matter composition and sedimentary facies. Two new thermal molecular maturity ratios applicable in the late gas window, i.e. 2+3-MDBT/4-MDBT and (2,4,6-TMDBT + 1,4,6-TMDBT)/(3,4,6-TMDBT + 2,6,7-TMDBT) are proposed based on the sensitivity of 2+3-MDBT, 2,4,6-TMDBT and 1,4,6-TMDBT isomers towards high maturity.

Non-noble-metal CuSe promotes charge separation and photocatalytic CO2 reduction on porous g-C3N4 nanosheets

Photocatalytic CO2 reduction based on g-C3N4 with inexpensive, efficient, stable, and suitable energy band structure, serve as a promising candidate for alleviating the greenhouse effect. However, g-C3N4 still suffers from low CO2 photoreduction performance owing to severe charge recombination and high surface reaction energy barriers. Herein, porous g-C3N4/CuSe (PCN/CS) Schottky heterojunctions were fabricated by loading non-precious CS nanosheets on porous PCN nanosheets, contributing to a decreased the reaction energy barrier and elevated charge separation. The improved charge separation by Schottky heterojunction were unveiled by the transient photocurrents as well as photoluminescence (PL) spectra. The photogenerated electrons transfer from PCN to CS was demonstrated by density functional theory (DFT) calculations and XPS analysis. Meanwhile, CS lowers the energy berries and serves as an active site for CO2 reduction, thus accelerating the CO2 photoreduction processes. The experimental results manifest that the CO2 photoreduction efficiency of PCN/CS was significantly higher than that of pure PCN and CS. The efficiency of CO2 photoreduction to CO in the composite sample was 25.03 μmol·g−1·h−1, which was 6.53 times and 2.45 times higher than that of pure CS and PCN, respectively. This work provides a new insight on the design of Schottky junctions by anchoring non-precious metal on PCN for ameliorated CO2 photoreduction performance.

Defect Formation Thermodynamics of (A,A′)(B,B′)O3 (A = Mg,Ca,Sr and B = Ti,Mn,Cr,Fe,Mo) Perovskites

In a previous theoretical study [B. Grimm and T. Bredow, Oxygen Defect Formation Thermodynamics of CaMnO3: A Closer Look. Phys. Status Solidi B 2022, https://doi.org/10.1002/pssb.202200427], the structural, thermodynamic, and electronic properties of have been studied. The present study increases the range of compounds to ternary perovskites in a search for suitable anode materials for high‐temperature water electrolysis. compounds containing the abundant elements A = Mg,Ca,Sr and B = Ti,Mn,Cr,Fe,Mo are studied theoretically on hybrid density functional theory level. Criteria for a suitable electrode material are low‐oxygen defect formation energy and high reaction energy for decomposition into other phases. Based on these criteria, the most suitable perovskites are chosen as starting points to create stable mixed compounds. The compound δ is found to be the optimal choice as electrode material, with an oxygen defect concentration of at least δ present at 520 K and highly endothermic decomposition reactions.

Theoretical Insights into the Luminescence and Sensing Mechanisms of N,N′-Bis(salicylidene)-[2-(3′,4′-diaminophenyl)benzthiazole] for Copper(II)

The intramolecular proton transfer (IPT) reaction potential energy surfaces (PESs) of N,N'-bis(salicylidene)-[2-(3',4'-diaminophenyl)benzthiazole] (BTS) in the S0 state and S1 state are constructed. It is found that the IPT reactions in the ground state hardly take place due to the high reaction energy barrier for single-proton (6.3 kcal/mol) and double-proton transfer (14.1 kcal/mol) reactions and low backward reaction energy barriers for single-proton (1.9 kcal/mol) and double-proton transfer (1.2 kcal/mol) reactions. In comparison, an excited-state intramolecular single-proton transfer reaction is a barrierless and exothermic process, and thus, single-proton transfer tautomer T1H contributes most to the fluorescence emission. Based on the analysis of PESs, the experimental absorption and emission spectra are reproduced well by the calculated vertical excitation energies of BTS and its photoisomerization products, and the triple fluorescence emission profile in the experiment is reassigned unequivocally. Furthermore, thermodynamic analysis of the BTS-Cu(II) complex shows that the dinuclear complex (C1) with Cu(II) coordinating with O and N atoms of the hydrogen bonds is the most thermodynamically stable structure, and the intramolecular hydrogen bonding structure in BTS is destroyed due to the chelation of Cu(II) and BTS; as a result, the IPT reaction of C1 in S0 and S1 states is significantly inhibited. The inhibitor of Cu(II) in the IPT reaction plays a major role in fluorescence quenching.

Breaking the temperature limit of hydrothermal carbonization of lignocellulosic biomass by decoupling temperature and pressure

Hydrothermal carbonization (HTC) of lignocellulosic biomass is a promising technology for the production of carbon materials with negative carbon emissions. However, the high reaction temperature and energy consumption have limited the development of HTC technology. In conventional batch reactors, the temperature and pressure are typically coupled at saturated states. In this study, a decoupled temperature and pressure hydrothermal (DTPH) reaction system was developed to decrease the temperature of the HTC reaction of lignocellulosic biomass (rice straw and poplar leaves). The properties of hydrochars were analyzed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analyzer (TGA), etc. to propose the reaction mechanism. The results showed that the HTC reaction of lignocellulosic biomass could be realized at a low temperature of 200 °C in the DTPH process, breaking the temperature limit (230 °C) in the conventional process. The DTPH method could break the barrier of the crystalline structure of cellulose in the lignocellulosic biomass with high cellulose content, realizing the carbonization of cellulose and hemicellulose with the dehydration, unsaturated bond formation, and aromatization. The produced hydrochar had an appearance of carbon microspheres, with high calorific values, abundant oxygen-containing functional groups, a certain degree of graphitization, and good thermal stability. Cellulose acts not only as a barrier to protect itself and hemicellulose from decomposition, but also as a key precursor for the formation of carbon microspheres. This study shows a promising method for synthesizing carbon materials from lignocellulosic biomass with a carbon-negative effect.

Gas phase models of hydride transfer from divalent alkaline earth metals to CO2 and CH2O.

The reactivity of HMg+, HMgCl, and HMgCl2- in hydride transfer reactions with CO2 and CH2O were studied by means of the reverse reactions-decarboxylation of HCO2MgCln+/0/- and deformylation of CH3OMgCln+/0/- (n = 0-2)-by a combination of quantum chemical computations and mass spectrometry experiments. HCO2Mg+, HCO2MgCl and HCO2MgCl2- display similar energetics for unimolecular carbon dioxide loss; for CH3OMg+, CH3OMgCl and CH3OMgCl2-, formaldehyde loss is more favourable for the cationic species than for the anionic one, with the neutral species found in-between. Despite very similar overall thermochemistry for each of the charge states of the CO2 and CH2O systems, the intermediate reaction barriers are higher for the CO2 eliminations due to a more complex and demanding reaction mechanism. Exothermic ligand exchange is observed for CH3OMg+ reacting with CO2, forming HCO2Mg+ and CH2O. Both the thermochemistry and the presence of intermediate energy barriers slightly disfavour this type of ligand exchange for CH3OMgCl and CH3OMgCl2-. It was also found that CH3OMg+ reacts readily with H2O to eliminate H2, whereas quantum chemical computations predict that the corresponding neutral and anionic species suffer unfavourable reaction thermochemistry. A corresponding reaction for the magnesium formate compounds was not observed. Quantum chemical computations were performed to investigate periodic trends in reactivity. The energetic requirements for decarboxylation of HCO2M+, HCO2MCl and HCO2MCl2- increase in the order M = Be, Mg, Ca, Sr and Ba; the only exception is the cationic Be species for which the reaction is more endothermic than for the corresponding Mg species. For deformylation of CH3OM+, CH3OMCl and CH3OMCl2- the trends are more irregular and less pronounced than for the decarboxylation reactions; however, the Be species was found to have higher reaction energies than the Mg species for all the methoxy-compounds irrespective of charge state. The effect on decarboxylation and deformylation upon replacing Cl with F, Br or I was found to be minimal for all aforementioned species. The consequences of these observations on the reverse reactions, hydride transfer to CO2 and CH2O, are discussed, and the effects of systematic structural changes on reactivity are rationalized on the basis of thermochemical cycles and well-known periodic trends.

DFT studies of CO reaction behavior on α-Fe2O3(001) oxygen-vacancy surface in chemical looping reforming

Chemical looping reforming of methane to syngas (CO and H2) is one of the most promising routes for methane utilization, where the further reaction of CO on oxygen carrier surfaces is a primary determinant of CO selectivity. In this work, the effects of oxygen vacancy (Vo) on CO desorption, CO oxidation, and CO dissociation are systematically studied by using density functional theory calculattions. Our calculated results reveal that increasing Vo concentration can weaken CO desorption at Fe sites due to the enhanced localization of electrons in the Fe atoms. Also, the increase in Vo concentration from 1/12 ML to 1/6 ML leads to a dramatic increase of activation energy in the CO oxidation from 0.64 eV to 1.10 eV. Moreover, the increase in Vo concentration is conducive to CO dissociation, but the dissociation is still almost impossible due to the high reaction energies (large than 3.00 eV). Considering these three reaction paths, CO desorption can proceed spontaneously at reaction temperatures above 900 K. Increasing Vo concentration can improve the selectivity of syngas production due to the less favorable CO oxidation compared with CO desorption at high Vo concentrations (1/6 ML). This work reveals the microscopic mechanism that CO selectivity rises in the CLRM as the degree of Fe2O3 reduction increases.