Van Krevelen Mechanism Research Articles

Environmental water-mediated dual-mechanism regulation of PtCuδ-MnOx-catalyzed acetone oxidation: Enhancement of MvK and inhibition of L-H mechanism

Environmental H2O has a significant effect on the reaction mechanism and process of VOC oxidation, of which the intrinsic mechanism is hardly reported. Herein, a series of PtCuδ-MnOx catalysts with different strong metal-supported interactions were constructed by replacing some Pt species with Cu species. Among them, the PtCu3-MnOx catalyst exhibited superior catalytic performance (T90 = 170 ℃), high water resistance, and long-term stability. Furthermore, in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTs) showed that the acetone oxidation followed synergistic cooperation of the Langmuir-Hinshelwood (L-H) and Mars van Krevelen (MvK) mechanism. Acetone oxidation over PtCu3-MnOx occurred via the L-H mechanism producing surface acetate and formate species intermediate. Above 100 °C, adsorbed acetone (i.e., acetate and formate species) reacts with MnOx lattice oxygen according to the MvK mechanism to form CO2 and H2O. When water vapor was present, the L-H mechanism was impaired, inhibiting the deep oxidation of acetone. This research provides an in-depth exploration of how water vapor impacts the reaction mechanism of acetone oxidation. It offers valuable insights for creating dependable catalysts suitable for use in various industrial settings.

Room temperature thermocatalytic removal of formaldehyde in air using a copper manganite spinel-supported palladium catalyst with ultralow noble metal content

The room temperature (RT)-catalytic oxidation of gaseous formaldehyde (FA) in the dark is regarded as one of the most viable technical options for its removal from indoor air. Herein, the copper manganite spinel (CuMn2O4) supported palladium (Pd) catalysts are firstly synthesized for the thermocatalytic oxidation of FA in air. In particular, 0.05% Pd-CuMn2O4-R (‘R’ denotes high-temperature reduction pre-treatment using hydrogen) achieves 100% conversion (XFA) of 50 ppm FA at RT (gas hourly space velocity of 4342 h−1). The performance of 0.05% Pd-CuMn2O4-R against FA, if assessed in terms of the reaction kinetic rate (r at 10% XFA), is estimated as 3.01E−02 mmol g−1h−1. The lowering of XFA with the increases in FA concentration, flow rate, and relative humidity indicates the detrimental effects of such variables on the catalytic activity. FA oxidation kinetics predominantly follows the Mars van Krevelen mechanism. In-situ diffuse reflectance infrared Fourier transform spectroscopy shows that FA oxidation proceeds through multiple reaction intermediates (e.g., dioxymethylene and formate). The density functional theory simulations indicate that the catalytic oxidation efficacy of FA over CuMn2O4 increases by the synergy between Pd loading and surface reduction. The present study represents the first report on the synthesis of CuMn2O4-supported Pd catalysts and their application for the RT oxidative removal of FA in air under dark conditions. The findings of the present work offer valuable insights into the construction of efficient and cost-effective catalysts with ultra-low noble metal content for the RT oxidative removal of VOCs in indoor environment.

Promoting Effect of Pd Nanoparticles on SrTi0.8Mn0.2O3 in the Reverse Water‐Gas Shift Reaction via the Mars–van Krevelen Mechanism

The reverse water‐gas shift (RWGS) reaction serves as a critical pathway for converting CO2 into diverse chemicals. The Mars–van Krevelen (MvK) mechanism, which leverages lattice oxygen as the oxidant and oxygen vacancies as reductants, offers an alternative catalytic strategy for the selective RWGS reaction. While Mn‐substituted SrTiO3 (i.e., SrTi0.8Mn0.2O3) has been shown to promote the RWGS reaction selectively via the MvK mechanism, achieving a sufficient conversion of CO2 necessitates elevated temperatures. This study investigated the effect of Pd‐loaded SrTi0.8Mn0.2O3 on the activation of adsorbed H2 molecules, which generated oxygen vacancies and enhanced CO2 conversion. Notably, 1.0 wt% Pd‐loaded SrTi0.8Mn0.2O3 yielded 13.4% of CO at 673 K, whereas pristine SrTi0.8Mn0.2O3 and Pd‐loaded SrTiO3 yielded negligible or minimal amount of CO. Hydrogen temperature‐programmed reduction and X‐ray absorption spectroscopy measurements revealed that Pd promoted the formation of oxygen vacancies via both thermodynamic and kinetic mechanisms. Fourier transform infrared spectroscopy and kinetic studies revealed that the RWGS reaction over Pd‐loaded SrTi0.8Mn0.2O3 proceeded primarily via the MvK mechanism with a partial contribution from the Langmuir–Hinshelwood mechanism. This study underscores the effectiveness of combining metal and MvK‐type catalysts to enhance the efficiency of the RWGS reaction.

Low-temperature catalytic methane deep oxidation over sol-gel derived mesoporous hausmannite (Mn3O4) spherical particles

In this study, Mn3O4 spherical particles (SPs) were synthesized by the sol-gel process, after which they were thermally annealed at 400 °C, and comprehensively characterized. X-ray Diffraction (XRD) revealed that Mn3O4 exhibited a tetragonal spinel structure, and Fourier transformed infrared (FTIR) spectroscopy identified surface-adsorbed functional groups. Scanning electron microscopy (SEM) and the specific surface area analyses by Brunauer−Emmett−Teller (BET) revealed a porous, homogeneous surface composed of strongly agglomerated spherical grains with an estimated average particle size of ∼35 nm, which corresponded to a large specific surface area of ∼81.5 m2/g. X-ray photoelectron spectroscopy (XPS) analysis indicated that Mn3O4 was composed of metallic cations (Mn4+, Mn3+, and Mn2+) and oxygen species (O2−, OH− and CO32−). The optical bandgap energy is ∼2.55 eV. Assessment of the catalytic performance of the Mn3O4 SPs indicated T90 conversion of CH4 to CO2 and H2O at 398 °C for gas hourly space velocity (GHSV) of 72000 mL3 g−1 h−1. This observed performance can be attributed to the cooperative effects of the smallest spherical grain size with a mesoporous structure, which is responsible for the larger specific surface area and available surface-active oxygenated species. The cooperative effect of the good reducibility, higher ratio of active species (OLat/OAds), and results of density functional theory (DFT) calculations suggested that the total oxidation of CH4 over the mesoporous Mn3O4 SPs might take place via a two-term process in which both the Langmuir−Hinshelwood and Mars−van Krevelen mechanisms are cooperatively involved.

Kinetic modeling of the oxidative dehydrogenation of propane with CO2 over a CrOx/SiO2 catalyst and assessment of CO2 utilization

Oxidative dehydrogenation of propane with carbon dioxide (ODPC) has gained attention as a sustainable process capable of simultaneously producing propylene and reducing carbon dioxide (CO2) emissions. In this study, we developed a kinetic model of the ODPC over a silica-supported chromium oxide catalyst based on the Mars–van Krevelen (MvK) mechanism using the experimental data obtained under controlled conditions. The constructed model estimated both reactant conversions and propylene yield within an absolute error of 3% of the experimental data, confirming the integrity of the kinetic model. The kinetic analysis provides insight into the activation of propane and CO2 via several reactions occurring under ODPC conditions, emphasizing that the ODPC cycle is limited by the high energy barrier for CO2 activation. In addition, it allows comparison of the contributions of non-oxidative and oxidative dehydrogenation reactions, the two main reactions producing propylene under ODPC conditions. The estimation results show a significant increase in the reaction rate of ODPC with increasing CO2 partial pressure, in contrast to a nearly constant reaction rate of non-oxidative dehydrogenation, highlighting the promoting effect of CO2 in enhancing the ODPC reaction. Furthermore, the potential of the ODPC process is assessed by considering propylene productivity, net CO2 emissions, including the energy consumption for the separation process, and the global warming index of the energy grid. A comprehensive investigation of the ODPC process through controlled experiments, kinetic modeling and evaluation of net CO2 utilization provides insight into future strategies for using the ODPC as an efficient and eco-friendly process.

A Comparison of the Mechanisms and Activation Barriers for Ammonia Synthesis on Metal Nitrides (Ta3N5, Mn6N5, Fe3Mo3N, Co3Mo3N)

In this study we perform a comparison of the reaction mechanism and the activation barrier for the rate-determining step in various metal nitrides (Ta3N5, Mn6N5, Fe3Mo3N, Co3Mo3N) for the ammonia synthesis reaction. The reactions are explained with simplified schematics and the energy profiles for the various reaction mechanisms are given in order to screen the catalytic activity of the catalysts for the ammonia synthesis reaction. We find that the catalytic activity ranks in the following order: Co3Mo3N > Fe3Mo3N > Ta3N5 > Mn6N5. We also find that the reaction mechanism proceeds either by a Langmuir–Hinshelwood and an Eley–Rideal/Mars–van Krevelen mechanism. This is an overview of about 10 years of computational research conducted to provide an overview of the progress established in this field of study.

Enhanced stability and catalytic activity of subnanometric platinum cluster by surface doping of zirconium in CeO2

There has been a continuous effort to improve the thermal stability of subnanometric platinum (Pt) cluster (<2 nm) catalyst because Pt cluster on CeO2 support can be mobile and aggregated into nanoparticle on heating at elevated temperatures, yet this great challenge remains. In this study, a strategy is reported to improve the thermal stability of subnanometric Pt cluster by hydrothermal deposition method. Based on this method, zirconium (Zr) was precisely doped on surface of Ce0.95Zr0.05O2 by accurate control of Pt subnanometric cluster size. The surface doping of Zr is favorable for forming the Zr–O–Ce site and activating surface lattice oxygen atoms, which results in strong electronic interactions to stabilize the Pt subnanometric cluster. After high-temperature aging treatment at 1000 °C/4 h, the single atom Pt supported on CeO2 is aggregated into larger sized (>3 nm) nanoparticle. In contrast, the single atom Pt supported on Ce0.95Zr0.05O2 displays less agglomeration into subnanometric cluster with size of (1.4 ± 0.3) nm. Moreover, the CO oxide catalytic performance of Ce0.95Zr0.05O2–Pt is 26 % and 31 % higher than that of CeO2–Pt and commercial Al2O3–Pt catalysts, respectively. The experimental and density functional theory (DFT) calculations indicate that the Zr–O–Ce site and Pt subnanometric cluster interface have more defect sites and active oxygen species than CeO2–Pt interface, which activate the Mars van Krevelen (MvK) mechanism, facilitating the catalytic performance.

Optimizing the CeO2-MnO2 interface via H2O2 etching for surface lattice oxygen activation in the plasma catalytic degradation of trimethylamine

Trimethylamine (TMA) is a nitrogen-containing malodorous gas that is toxic and hazardous to humans. Plasma catalysis is a promising technique for removing low-concentration malodorous gases. CeO2/MnO2 nanowires with different Ce–Mn contact interfaces prepared by impregnation, physical mixing, and H2O2 etching were investigated for TMA degradation in a plasma catalytic system. The CMO-H sample prepared by H2O2 etching achieved a removal efficiency of 100% at 98 J/L, demonstrating the highest activity. Characterization revealed that H2O2 etching led to the uniform formation of CeO2 nanoparticles on MnO2, promoting the transfer of electrons between Ce and Mn species. Surface lattice oxygen and high-valent manganese were confirmed to be the main active species, consistent with the Mars van Krevelen mechanism. Density functional theory calculations confirmed that the CMO-H sample exhibited the lowest oxygen vacancy formation energy. In situ plasma diffuse reflection infrared transform spectroscopy analysis and gas chromatography-mass spectrometry results showed the possible degradation pathway of TMA. In summary, the H2O2 etching process optimized CeO2-MnO2 interactions and improved the utilization of lattice oxygen active sites, enhancing the catalytic degradation of TMA and providing new insights into the design of an efficient plasma catalytic system for environmental remediation.

Unraveling the H2S selective oxidation in cobalt phosphate cluster loaded polymeric carbon nitride

H2S selective catalytic oxidation via photocatalysis is a low-cost and efficient approach. Herein, we modeled a (CoP)4 cluster loaded melon-based carbon nitride to represent experimental cobalt phosphate cocatalyst adsorption on polymeric carbon nitride for revealing the mechanism of H2S selective catalytic oxidation to SO2 via the density functional theory (DFT) calculations. Due to the hydrogen bond effect, the calculated results show that H2S selective oxidation prefers to occur through the quasi Mars–van Krevelen (quasi-MvK) mechanism rather than Langmuir-Hinshelwood (LH) mechanism according to the thermodynamic perspective, in which quasi-MvK mechanism exhibits a favorable limiting potential of 0.26 V. Overall, this work offers a basic understanding for microscopic mechanism of H2S catalytic oxidation.

Catalytic Oxidation of Chlorobenzene over HSiW/CeO2 as a Co-Benefit of NOx Reduction: Remarkable Inhibition of Chlorobenzene Oxidation by NH3.

There is an urgent need to develop novel and high-performance catalysts for chlorinated volatile organic compound oxidation as a co-benefit of NOx. In this work, HSiW/CeO2 was used for chlorobenzene (CB) oxidation as a co-benefit of NOx reduction and the inhibition mechanism of NH3 was explored. CB oxidation over HSiW/CeO2 primarily followed the Mars-van-Krevelen mechanism and the Eley-Rideal mechanism, and the CB oxidation rate was influenced by the concentrations of surface adsorbed CB, Ce4+ ions, lattice oxygen species, gaseous CB, and surface adsorbed oxygen species. NH3 not only strongly inhibited CB adsorption onto HSiW/CeO2, but also noticeably decreased the amount of lattice oxygen species; hence, NH3 had a detrimental effect on the Mars-van-Krevelen mechanism. Meanwhile, NH3 caused a decrease in the amount of oxygen species adsorbed on HSiW/CeO2, which hindered the Eley-Rideal mechanism of CB oxidation. Hence, NH3 significantly hindered CB oxidation over HSiW/CeO2. This suggests that the removal of NOx and CB over this catalyst operated more like a two-stage process rather than a synergistic one. Therefore, to achieve simultaneous NOx and CB removal, it would be more meaningful to focus on improving the performances of HSiW/CeO2 for NOx reduction and CB oxidation separately.

Chitosan –NH2 derived efficient Co3O4 catalyst for styrene catalytic oxidation: Simultaneously regulating particle size and Co valence

In this study, a Co3O4 catalyst is synthesised using the chitosan-assisted sol–gel method, which simultaneously regulates the grain size, Co valence and surface acidity of the catalyst through a chitosan functional group. The complexation of the free –NH2 complex inhibits particle agglomeration; thus, the average particle size of the catalyst decreases from 82 to 31 nm. Concurrently, Raman spectroscopy, hydrogen temperature-programmed reduction, electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy experiments demonstrate that doping with chitosan N sources effectively modulates Co2+ to promote the formation of oxygen vacancies. In addition, water washing after catalyst preparation can considerably improve the low-temperature (below 250 °C) activity of the catalyst and eliminate the side effects of alkali metal on catalyst activity. Moreover, the presence of Brønsted and Lewis acid sites promotes the adsorption of C8H8. Consequently, CS/Co3O4-W presents the highest catalytic oxidation activity for C8H8 at low temperatures (R250 °C = 8.33 μmol g−1 s−1, WHSV = 120,000 mL hr−1∙g−1). In situ DRIFTS and 18O2 isotope experiments demonstrate that the oxidation of the C8H8 reaction is primarily dominated by the Mars–van Krevelen mechanism. Furthermore, CS/Co3O4-W exhibits superior water resistance (1- and 2- vol% H2O), which has the potential to be implemented in industrial applications.

Degradation of 2-naphthol in water and antibacterial/antiviral activity by Zn2SnO4, ZnSn(OH)6 and Y2Sn2O7

White single-phase powders of Zn2SnO4, ZnSn(OH)6 and Y2Sn2O7 were prepared using hydrothermal method. In the 2-naphthol aqueous solutions in which these powders were immersed, ZnSn(OH)6 and Y2Sn2O7 reduced the 2-naphthol concentration in the dark. The concentration-decreasing rate temperature dependence, the activation energy, and the binding energy change suggested that the oxidative decomposition of 2-naphthol resulted from the Mars – van Krevelen (MvK) mechanism of Sn. Near non-occurrence of ion elution in water was confirmed for these materials, except for Zn in Zn2SnO4. Among these samples, Y2Sn2O7 exhibited the highest antibacterial and antiviral activities against Escherichia coli, Staphylococcus aureus, bacteriophage Qβ, and bacteriophage Φ6 in the dark. Adsorption attributable to electrostatic interactions between cationic samples and bacteria/viruses and the MvK mechanism were expected to lead to protein denaturation and consequently to antibacterial activity and antiviral activity. For Y2Sn2O7, the specific interaction between Y and phosphate ions possibly improved their antibacterial and antiviral activities. This white, non-eluting material was found to have degrees of high antibacterial and antiviral activity.

Promotional effect of stannum on Pd/CeO2 catalysts for CO and C3H6 co-oxidation

The successful control of hydrocarbon and CO emissions from low-temperature diesel exhausts requires the use of highly active co-oxidation catalysts. In this study, Sn was used to enhance the catalytic performance of Pd/CeO2 in CO and C3H6 co-oxidation conditions. CeO2 with added stannum (Sn) was prepared as a support using the co-precipitation method, and Pd was loaded onto the support using the impregnation method. After Sn addition (the optimal Ce/Sn ratio is 0.75:0.25), the T50 values of CO and C3H6 are reduced by 20 and 32 °C, respectively. A series of characterization methods indicates that the addition of Sn to the support greatly enhances its lattice oxygen mobility and increases the proportion of PdO. During the co-oxidation process, stronger lattice oxygen mobility allows CO to react faster through the Mars–van Krevelen mechanism, weakening the competition with C3H6 for O2. A higher PdO content enhances the C3H6 oxidation capability. Moreover, CO can more readily reduce PdO than Pd2+ in solid solution with the support, which consequently further enhances co-oxidation activity. Therefore, the addition of Sn is a simple and effective strategy for enhancing the performance of Pd/CeO2 catalysts in CO and C3H6 co-oxidation reactions. Furthermore, the promotional effect of CO achieved in this study contributes to a deeper understanding of the interactions that occur during the co-oxidation of C3H6 and CO.

Catalytic Oxidation of CO over LaNi1/3Sb5/3O6 Synthesized by Different Methods

Methods for the synthesis of LaNi1/3Sb5/3O6 with a rosiaite structure have been developed using citrate method and coprecipitation followed by annealing. The influence of synthesis conditions on the morphology of the samples has been demonstrated. A comparative analysis of the catalytic properties of LaNi1/3Sb5/3O6 synthesized by various methods, in the reaction of CO oxidation has been carried out. The catalyst synthesized by the citrate method demonstrated the greatest efficiency and stability (the temperature of 90% CO conversion was T90 = 336°C). The LaNi1/3Sb5/3O6 surface was studied before and after catalysis by in situ diffuse reflectance IR spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed O2 desorption. It has been shown that the catalytic oxidation of CO on the LaNi1/3Sb5/3O6 surface proceeds according to the Mars–van Krevelen mechanism and is accompanied by redox Sb3+ ↔ Sb5+ processes. It has been established that no contamination of the sample surface occurs during the catalysis process.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8H0.2 Catalyst for CO2 Hydrogenation to Methanol

AbstractTuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal‐support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3‐fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High‐pressure steady‐state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3. An operando high‐pressure diffuse reflectance infrared spectroscopy (DRIFTS)‐SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate‐limiting. However, the catalytic reactivity of Cu/BaTiO2.8H0.2 towards the formate species is much higher than Cu/BaTiO3, likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8 H0.2 Catalyst for CO2 Hydrogenation to Methanol.

Tuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal-support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3-fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8 H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8 H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High-pressure steady-state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8 H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3 . An operando high-pressure diffuse reflectance infrared spectroscopy (DRIFTS)-SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate-limiting. However, the catalytic reactivity of Cu/BaTiO2.8 H0.2 towards the formate species is much higher than Cu/BaTiO3 , likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Oxygen-vacancy-type Mars–van Krevelen mechanism drives ultrafast dioxygen electroreduction to hydrogen peroxide

The electrochemical oxygen reduction reaction (ORR) along a two-electron transfer pathway has been considered as an eco-friendly route for producing hydrogen peroxide (H2O2). However, large-scale industrial application of this ORR technology calls for ultrafast and effective generation of H2O2 under operating conditions (current densities >1 A/cm2 and Faradaic efficiency ≈ 100%). This imposes strict criteria for exploring innovative strategies for enhancing the adsorption and activation of O2 under vigorous reaction condition, which represents a significant challenge thus far. Here, we report an ‘oxygen-vacancy-type’ Mars–van Krevelen mechanism for promoting ORR. Our theoretical calculations show that the structural oxygen vacancies of zinc oxide catalysts effectively alter the electron densities of nearby metal active sites, producing a more electron-deficient Zn center, which, in turn, assists the adsorption and activation of O2. A catalyst electrode designed as that exhibits superior ORR activities with a Faradaic efficiency of 98.1% at a current density of 1 A/cm2 (H2O2 yield rate of 621.88 mg/h/cm2). Further mechanism study has been performed through in situ Raman spectroscopy to monitor the adsorption and activation of oxygen intermediate (∗O2) of ORR, providing additional experimental evidence for the Mars–van Krevelen mechanism.

Exploring Various Catalysts for Nitrogen Reduction to Ammonia

Electrochemical nitrogen reduction reaction (eNRR), one of the most studied problems in catalysis, has the potential to replace the century-old Haber-Bosch process for ammonia synthesis. However, a low-cost, highly selective and efficient catalyst for eNRR is yet to be discovered. Metal based catalysts have been at the heart of numerous studies for eNRR, however there has been a shift towards metal-free catalysts due to their environmentally benign nature and low cost. In this study, we analyze 2D boron-based electrocatalysts for nitrogen reduction and explore the effect of carbon substitution on their activity. Using first-principles calculations, we demonstrate that pristine and carbon (C) substituted 𝛼 borophene catalyze eNRR at exceptionally low limiting potentials of -0.33 V and -0.25 V, respectively. These 𝛼 borophene based catalysts also show high selectivity towards eNRR over the competing hydrogen evolution reaction (HER). Our results show that C-substituted 𝛼 borophene exhibits outstanding catalytic activity towards eNRR via the distal pathway with a small activation barrier of 0.56 eV, almost half the value reported for Ru (0001). Finally, we identify two novel descriptors, adsorption energy of *NNH and charge transfer to *N2, which can be used in high-throughput screening of catalysts for eNRR. These findings demonstrate the strong potential of metal-free borophene based catalysts for efficient electrochemical reduction of N2, enabling low-cost and sustainable NH3 synthesis.We revisit the nitrogenase FeM cofactor (M=Fe/Mo/V) based catalysts for eNRR to decipher the key mechanism at play. Further, catalysts based on transition metal nitrides (TMNs) and transition metal dichalcogenides (TMDCs) are also being explored for the study and compared with the state-of-the-art materials available. The dominance of one NRR mechanism like the Mars Van Krevelen (MvK) mechanism on TMNs over others like the dissociative NRR is also examined. These findings will provide a more comprehensive and elaborate view of the ammonia synthesis for N2 reduction reaction space.

Fundamental Insights into Reaction Centers and Catalytic Mechanisms of Ni- and Co-Based Layered Oxyhydroxides for the Oxygen Evolution Reaction

Water splitting to generate green H2 fuel and O2 is essential for global CO2 reductions toward net-zero emissions. In this process, however, O2 generation at the anode through the oxygen evolution reaction (OER) is inherently slower by orders of magnitude than H2 generation at the cathode. Thus, improving OER efficiency has been a major effort in electrolysis. Ni-based and Co-based layered hydroxides are among the most active and studied non-previous catalysts in alkaline electrolytes. Although tremendous advances have been made toward improving the activity and the stability of the catalysts over the past decade, there is still a lack of fundamental insights into active sites and reaction mechanisms, which has hindered the establishment of structure-property relationships.This talk will first provide fundamental insights into reaction centers and catalytic mechanisms of classic Ni oxyhydroxides and Co oxyhydroxides with and without ligand intercalation. We will further discuss the multiple-site synergy in Ni- and Co-based layered double hydroxides (LDH) and ternary hydroxides. We will show that the OER proceeds via a Mars van Krevelen mechanism, starting with the oxidation of bridge OH at the reaction centers with dual metal sites, i.e., M1-OH-M2. We will demonstrate that, to approach the minimum overpotential dictated by a specific OH-OOH scaling relationship, the key is to break the OH-O scaling relationship. A possible route is to form binary metal oxyhydroxides with dual metal sites at the reaction centers or introduce a third element into NiFe LDH or CoFe LDH. If time permits, We will further discuss the role of the non-covalent interaction and polaron-like electronic states in promoting OER at M1-OH-M2 centers.

Transition metal enhanced chromium nitride as composite nitrogen carrier for sustainable chemical looping ammonia synthesis

Chemical looping ammonia synthesis (CLAS) is promising to achieve decentralized ammonia synthesis under ambient pressure. Here, we develop a highly selective composite nitrogen carrier for efficient CLAS based on transition metals (TMs=Co, Ni, Fe) decorated chromium nitride (CrN). Systematic studies indicate that the pristine CrN is extremely inert: only 4.5% lattice nitrogen can be consumed in reacting with H2 (700 °C, 1 bar). Upon loading cobalt, the composite nitrogen carrier achieves lattice nitrogen conversion of 50.7% and ammonia selectivity up to 98.1%. Furthermore, Co-CrN exhibits excellent CLAS performance, attaining an average ammonia production rate of 466.1 μmol g−1 h−1 in 12 chemical loopings, which is ∼10 times that of the pristine CrN. Theoretical calculations reveal that the nitrogen vacancies generated in hydrogenation play a crucial role as activation centers for N2 fixation through a Mars–van Krevelen mechanism. This work provides a novel strategy to optimize nitrogen carriers for enhanced CLAS.