Catalyst For Reduction Research Articles

Electrochemical Reductive Cross-Coupling of Alkyl or Alkenyl Halides with gem-Difluoroalkenes.

Herein, we describe an electroreductive cross-electrophile coupling protocol for the construction of valuable monofluoroalkenes from easily accessible alkyl or alkenyl halides with gem-difluoroalkenes. The reaction can be conducted under sustainable and mild conditions delivering valuable and functionalized monofluoroalkenes with excellent Z-selectivity. The protocol's most notable advantage is the in situ release of nickel catalyst from the inexpensive electrodes without the addition of extra hazardous metal catalyst and superstoichiometric reductant.

Time‐Resolved Oxidation State Changes Are Key to Elucidating the Bifunctionality of Perovskite Catalysts for Oxygen Evolution and Reduction

In a unified regenerative fuel cell (URFC) or reversible fuel cell, the oxygen bifunctional catalyst must switch reversibly between the oxygen reduction reaction (ORR), fuel cell mode, and the oxygen evolution reaction (OER), electrolyzer mode. However, it is often unclear what effect alternating between ORR and OER has on the electrochemical behavior and physiochemical properties of the catalyst. Herein, operando X‐ray absorption spectroscopy (XAS) is utilized to monitor the continuous and dynamic evolution of the Co, Mn, and Fe oxidation states of perovskite catalysts Ba0.5Sr0.5Co0.8Fe0.2O3‐δ (BSCF) and La0.4Sr0.6MnO3‐δ (LSM), while the potential is oscillated between reducing and oxidizing potentials with cyclic voltammetry. The results reveal the importance of investigating bifunctional catalysts by alternating between fuel cell and electrolyzer operation and highlight the limitations and challenges of bifunctional catalysts. It is shown that the requirements for ORR and OER performance are divergent and that the oxidative potentials of OER are detrimental to ORR activity. These findings are used to give guidelines for future bifunctional catalyst design. Additionally, it is demonstrated how sunlight can be used to reactivate the ORR activity of LSM after rigorous cycling.

Testing functional anchor groups for the efficient immobilization of molecular catalysts on silver surfaces

Modifications of complexes by attachment of anchor groups are widely used to control molecule-surface interactions. This is of importance for the fabrication of (catalytically active) hybrid systems, viz. of surface immobilized molecular catalysts. In this study, the complex fac-Re(S-Sbpy)(CO)3Cl (S-Sbpy = 3,3′-disulfide-2,2′-bipyridine), a sulfurated derivative of the prominent Re(bpy)(CO)3Cl class of CO2 reduction catalysts, was deposited onto the clean Ag(001) surface at room temperature. The complex is thermostable upon sublimation as supported by infrared absorption and nuclear magnetic resonance spectroscopy. Its anchoring process has been analyzed using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The growth behavior was directly contrasted to the one of the parent complex fac-Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine). The sulfurated complex nucleates as single molecule at different surface sites and at molecule clusters. In contrast, for the parent complex nucleation only occurs in clusters of several molecules at specifically oriented surface steps. While this shows that surface immobilization of the sulfurated complex is more efficient as compared to the parent, symmetry analysis of the STM topographic data supported by DFT calculations indicates that more than 90% of the complexes adsorb in a geometric configuration very similar to the one of the parent complex.

Recent Advances in Carbon-Based Single-Atom Catalysts for Electrochemical Oxygen Reduction to Hydrogen Peroxide in Acidic Media

Electrochemical oxygen reduction reaction (ORR) via the 2e− pathway in an acidic media shows great techno-economic potential for the production of hydrogen peroxide. Currently, carbon-based single-atom catalysts (C-SACs) have attracted extensive attention due to their tunable electronic structures, low cost, and sufficient stability in acidic media. This review summarizes recent advances in metal centers and their coordination environment in C-SACs for 2e−-ORR. Firstly, the reaction mechanism of 2e−-ORR on the active sites of C-SACs is systematically presented. Secondly, the structural regulation strategies for the active sites of 2e−-ORR are further summarized, including the metal active center, its species and configurations of nitrogen coordination or heteroatom coordination, and their near functional groups or substitute groups, which would provide available and proper ideas for developing superior acidic 2e−-ORR electrocatalysts of C-SACs. Finally, we propose the current challenges and future opportunities regarding the acidic 2e−-ORR pathway on C-SACs, which will eventually accelerate the development of the distributed H2O2 electrosynthesis process.

Efficient recovery and recycling/upcycling of precious metals using hydrazide-functionalized star-shaped polymers

There is a growing demand for adsorption technologies for recovering and recycling precious metals (PMs) in various industries. Unfortunately, amine-functionalized polymers widely used as metal adsorbents are ineffective at recovering PMs owing to their unsatisfactory PM adsorption performance. Herein, a star-shaped, hydrazide-functionalized polymer (S-PAcH) is proposed as a readily recoverable standalone adsorbent with high PM adsorption performance. The compact chain structure of S-PAcH containing numerous hydrazide groups with strong reducibility promotes PM adsorption by enhancing PM reduction while forming large, collectable precipitates. Compared with previously reported PM adsorbents, commercial amine polymers, and reducing agents, S-PAcH exhibited significantly higher adsorption capacity, selectivity, and kinetics toward three PMs (gold, palladium, and platinum) with model, simulated, and real-world feed solutions. The superior PM recovery performance of S-PAcH was attributed to its strong reduction capability combined with its chemisorption mechanism. Moreover, PM-adsorbed S-PAcH could be refined into high-purity PMs via calcination, directly utilized (upcycled) as catalysts for dye reduction, or regenerated for reuse, demonstrating its high practical feasibility. Our proposed PM adsorbents would have a tremendous impact on various industrial sectors from the perspectives of environmental protection and sustainable development.

Modulating the Activity and SO2 Resistance of α-Fe2O3 Catalysts for NH3-SCR of NOx via Crystal Facet Engineering.

The development of Fe-based catalysts for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) has garnered significant attention due to their exceptional SO2 resistance. However, the influence of different sulfur-containing species (e.g., ferric sulfates and ammonium sulfates) on the NH3-SCR activity of Fe-based catalysts as well as its dependence on exposed crystal facets of Fe2O3 has not been revealed. This work disclosed that nanorod-like α-Fe2O3 (Fe2O3-NR) predominantly exposing (110) facet performed better than nanosheet-like α-Fe2O3 (Fe2O3-NS) predominantly exposing (001) facet in NH3-SCR reaction, due to the advantages of Fe2O3-NR in redox properties and surface acidity. Furthermore, the results of the SO2/H2O resistance test at a critical temperature of 250 °C, catalytic performance evaluations on Fe2O3-NR and Fe2O3-NS sulfated by SO2 + O2 or deposited with NH4HSO4 (ABS), and systematic characterization revealed that the reactivity of ammonium sulfates on Fe2O3 catalysts to NO(+O2) contributed to their improved catalytic performance, while ferric sulfates showed enhancing and inhibiting effects on NH3-SCR activity on Fe2O3-NR and Fe2O3-NS, respectively; despite this, Fe2O3-NR showed higher affinity for SO2 + O2. This work set a milestone in understanding the NH3-SCR reaction on Fe2O3 catalysts in the presence of SO2 from the aspect of crystal facet engineering.

Asymmetric Coordination Regulating D-orbital Spin-electron Filling in Single-atom Iron Catalyst for Efficient Oxygen Reduction.

The single-atom Fe-N-C catalyst has shown great promise for the oxygen reduction reaction (ORR), yet the intrinsic activity is not satisfactory. There is a pressing need to gain a deeper understanding of the charge configuration of the Fe-N-C catalyst and to develop rational modulation strategies. Herein, we have prepared a single-atom Fe catalyst with the co-coordination of N and O (denoted as Fe-N/O-C) and adjacent defect, proposing a strategy to optimize the d-orbital spin-electron filling of Fe sites by fine-tuning the first coordination shell. The Fe-N/O-C exhibits significantly better ORR activity compared to its Fe-N-C counterpart and commercial Pt/C, with a much more positive half-wave potential (0.927 V) and higher kinetic current density. Moreover, using the Fe-N/O-C catalyst, the Zn-air battery and proton exchange membrane fuel cell achieve peak power densities of up to 490 and 1179 mW cm-2, respectively. Theoretical studies and in situ electrochemical Raman spectroscopy reveal that Fe-N/O-C undergoes charge redistribution and negative shifting of the d-band center compared to Fe-N-C, thus optimizing the adsorption free energy of ORR intermediates. This work demonstrates the feasibility of introducing an asymmetric first coordination shell for single-atom catalysts and provides a new optimization direction for their practical application.

Unveiling the role of proton concentration in dinuclear metal complexes for boosting photocatalytic CO2 reduction

The reaction kinetics of photocatalytic CO2 reduction is highly dependent on the transfer rate of electrons and protons to the CO2 molecules adsorbed on catalytic centers. Studies on uncovering the proton effect in catalysts on photocatalytic activity of CO2 reduction are significant but rarely reported. In this paper, we, from the molecular level, revealed that the photocatalytic activity of CO2 reduction is closely related to the proton availability in catalysts. Specifically, four dinuclear Co(II) complexes based on Robson-type ligands with different number of carboxylic groups (-nCOOH; n = 0, 2, 4, 6) were designed and synthesized. All these complexes show photocatalytic activity for CO2 reduction to CO in a water-containing system upon visible-light illumination. Interestingly, the CO yields increase positively with the increase of the carboxylic-group number in dinuclear Co(II) complexes. The one containing -6COOH shows the best photocatalytic activity for CO2 reduction to CO, with the TON value reaching as high as 10,294. The value is 1.8, 3.4, and 7.8 times higher than those containing -4COOH, -2COOH, and -0COOH, respectively. The high TON value also makes the dinuclear Co(II) complex with -6COOH outstanding among reported homogeneous molecular catalysts for photocatalytic CO2 reduction. Control experiments and density functional theory calculation indicated that more carboxylic groups in the catalyst endow the catalyst with more proton relays, thus accelerating the proton transfer and boosting the photocatalytic CO2 reduction. This study, at a molecular level, elucidates that more carboxylic groups in catalysts are beneficial for boosting the reaction kinetics of photocatalytic CO2 reduction.

Poly(ionic liquid) Ionomers Help Prevent Active Site Aggregation, in Single-Site Oxygen Reduction Catalysts

Poly(ionic liquid) Ionomers Help Prevent Active Site Aggregation, in Single-Site Oxygen Reduction Catalysts

Mechanistic Insights into Electrocatalytic Hydrogen Evolution by an Exceptionally Stable Cobalt Complex.

Co(aPPy) is one of the most stable and active molecular first-row transition-metal catalysts for proton reduction reported to date. Understanding the origin of its high performance via mechanistic studies could aid in developing even better catalysts. In this work, the catalytic mechanism of Co(aPPy) was electrochemically probed, in both organic solvents and water. We found that different mechanisms can occur depending on the solvent and the acidity of the medium. In organic solvent with a strong acid as the proton source, catalysis initiates directly after a single-electron reduction of CoII to CoI, whereas in the presence of a weaker acid, the cobalt center needs to be reduced twice before catalysis occurs. In the aqueous phase, we found drastically different electrochemical behavior, where the Co(aPPy) complex was found to be a precatalyst to a different electrocatalytic species. We propose that in this active catalyst, the pyridine ring has dissociated and acts as a proton relay at pH ≤ 5, which opens up a fast protonation pathway of the CoI intermediate and results in a high catalytic activity. Furthermore, we determined with constant potential bulk electrolysis that the catalyst is most stable at pH 3. The catalyst thus functions optimally at low pH in an aqueous environment, where the pyridine acts as a proton shuttle and where the high acidity also prevents catalyst deactivation.

Regulating Cu Oxidation State for Electrocatalytic CO2 Conversion into CO with Near-Unity Selectivity via Oxygen Spillover.

Cu-based catalysts are the most intensively studied in the field of electrocatalytic CO2 reduction reaction (CO2RR), demonstrating the capacity to yield diverse C1 and C2+ products albeit with unsatisfactory selectivity. Manipulation of the oxidation state of Cu sites during CO2RR process proves advantageous in modulating the selectivity of productions, but poses a formidable challenge. Here, an oxygen spillover strategy is proposed to enhance the oxidation state of Cu during CO2RR by incorporating the oxygen donor Sb2O4. The Cu-Sb bimetallic oxide catalyst attains a remarkable CO2-to-CO selectivity approaching unity, in stark contrast to the diverse product distribution observed with bare CuO. The exceptional Faradaic efficiency of CO can be maintained across a wide range of potential windows of ≈700mV in 1m KOH, and remains independent of the Cu/Sb ratio (ranging from 0.1:1 to 10:1). Correlative calculations and experimental results reveal that oxygen spillover from Sb2O4 to Cu sites maintains the relatively high valence state of Cu during CO2RR, which diminishes the binding strength of *CO, thereby achieving heightened selectivity in CO production. These findings propose the role of oxygen spillover in CO2RR over Cu-based catalysts, and shed light on the rational design of highly selective CO2 reduction catalysts.

Effect of morphology on the performance of MnOx catalysts for selective catalytic reduction of NO with NH3

Modulating the physicochemical properties of catalysts through the construction of special crystal structures or microscopic morphology provides new insights for developing high-performance low-temperature denitrification catalysts. This paper explores the effect of morphology structure on catalyst performance and identifies the dominant driving factors. The catalyst activity sequence was found to be MnOx spiny nanospheres (Mn-SNS) > MnOx nanowires (Mn-N) > MnOx nanospheres (Mn-NS) > MnOx nanoparticles (Mn-P). Among these, the Mn-SNS catalyst prepared by the hydrothermal method exhibited the best catalytic activity and the good resistance to water and sulfur. It achieved over 70% NOx conversion at 150 °C, and maintained 100% at 200–350 °C. In the presence of SO2 and H2O, the NOx conversion was maintained at 87%. The spherical structure consisting of nanospikes provides the Mn-SNS catalysts with a large specific surface area, more (Mn4++Mn3+), abundant acid sites, and stronger reducibility, thus exhibiting excellent performance through the "fast-SCR" pathway. It also inhibits oxide crystallization and provides more sites for the adsorption activation of NOx and NH3. The nanowire structures can provide a large specific surface area and a high concentration of Mn3+ and Mn4+ for MnOx catalyst. Characterization analyses revealed that the specific surface area, the ratio of (Mn4++Mn3+)/Mn, and the number of Lewis acid sites (L-NH3) are the most important factors affecting the catalytic activity. This present empirical analysis provides new ideas for designing high-performance catalysts with a large specific surface area.

NHC-CDI Ligands Boost Multicarbon Production in Electrocatalytic CO2 Reduction by Increasing Accumulated Charged Intermediates and Promoting *CO Dimerization on Cu.

Copper-based materials exhibit significant potential as catalysts for electrochemical CO2 reduction, owing to their capacity to generate multicarbon hydrocarbons. The molecular functionalization of Cu electrodes represents a simple yet powerful strategy for improving the intrinsic activity of these materials by favoring specific reaction pathways through the creation of tailored microenvironments around the surface active sites. However, despite its success, comprehensive mechanistic insights derived from experimental techniques are often limited, leaving the active role of surface modifiers inconclusive. In this work, we show that N-heterocyclic carbene-carbodiimide-functionalized Cu catalysts display a remarkable activity for multicarbon product formation, surpassing bare Cu electrodes by more than an order of magnitude. These hybrid catalysts operate efficiently using an electrolyzer equipped with a gas diffusion electrode, achieving a multicarbon product selectivity of 58% with a partial current density of ca. -80 mA cm-2. We found that the activity for multicarbon product formation is closely linked to the surface charge that accumulates during electrocatalysis, stemming from surface intermediate buildup. Through X-ray photoelectron spectroscopy, we elucidated the role of the molecular additives in altering the electronic structure of the Cu electrodes, promoting the stabilization of surface CO. Additionally, in situ Raman measurements established the identity of the reaction intermediates that accumulate during electrocatalysis, indicating preferential CO binding on Cu step sites, known for facilitating C-C coupling. This study underscores the significant potential of molecular surface modifications in developing efficient electrocatalysts for CO2 reduction, highlighting surface charge as a pivotal descriptor of multicarbon product activity.

Preparation of Oxygen Reduction Catalyst Electrodes by Electrochemical Acidification and Synergistic Electrodeposition

A proton exchange membrane fuel cell (PEMFC) is an efficient and environmentally friendly power production technology that uses hydrogen energy. The cathodic oxygen reduction electrode is a critical component in the development of PEMFC. Most techniques deposit catalyst nanoparticles in areas that are inaccessible for catalytic processes, reducing platinum utilization. The substrate used in this study was carbon paper (CP) with a self-supporting structure. First, electrochemical acidification technology was employed to modify the CP’s surface, followed by nanoparticle manufacturing and fixation on the CP in a single step by electrodeposition. The Pt/C0.5V2.24CP catalyst electrode demonstrated high-quality activity in the oxygen reduction reaction (ORR), with a homogeneous particle dispersion and particle size of around 50 nm. The mass activity and electrochemical active surface area (ECSA) of the Pt/C0.5V2.24CP catalyst electrode were 1.74 and 3.98 times higher than those of the Pt/C/CP-1 electrodes made with commercial catalysts, respectively. After 5000 cycles of accelerated durability testing (ADT), the mass activity and ECSA were 1.28 times and 6.16 times more than Pt/C/CP-1. This paper successfully proved the viability of electrodepositing Pt nanoparticles on CP following acidification, and that the electrochemical acidification methods have a positive influence on improving electrode ORR activity.

Cu-BTC-derived Cu@C as nano-heater embedded in 3D g-C3N4 for photothermal-coupled photocatalytic CO2 reduction under full-spectrum solar irradiation

This study developed a novel Cu@C/NCN-T composite for efficient photothermal catalytic CO2 reduction. The composite, consisting of carbon-coated Cu (Cu@C) derived from Cu-BTC embedded in three-dimensional graphitic carbon nitride (NCN-T), was immobilized on the surface of carbonized wood (C-wood) floating on water, creating a triphase system (CO2, H2O, and catalyst). The optimized composite demonstrated remarkable CO2 photothermal reduction performance, achieving a rate of 31.3 μmol·g-1·h-1. This performance surpassed that of pure NCN-T and Cu@C by 1.6 and 3.8 times, respectively. The combination of Cu and carbon materials endowed Cu@C/NCN-T with excellent light-absorption properties, resulting in a photothermal effect that elevated the surface temperature and facilitated the catalytic reaction. Furthermore, the Schottky junction formed between Cu@C and NCN-T promoted the separation of electron-hole pairs, with Cu@C acting as charge channels. A charge transfer mechanism was proposed to explain these findings. This catalyst engineering approach holds promise for the development of efficient non-noble metal catalysts for rapid CO2 photothermal reduction.

Systematic analysis of dual-functional catalysts for simultaneous CO-NOx reduction: Toward an effective catalyst design strategy

The development of a multifunctional catalyst suitable for application in existing facilities with limited space, with specific emphasis on its capability to concurrently remove CO and NOx is vital for meeting stringent environmental regulations. This is particularly pertinent to industries such as liquefied natural gas-based power plants and multipollutant-emitting industrial facilities. Another important consideration is that the catalyst should operate over a wide temperature range to facilitate its application across various industrial sites. In this study, we synthesized vanadium-based catalysts with a series of noble metals (Pt, Pd, and Au). The V–W–Pd/TiO2 catalyst achieved a simultaneous CO–NOx reduction efficiency greater than 90% in the widest temperature range, 228–321 °C. The excellent catalytic activities resulted from the Pd active sites, which oxidated CO and adsorbed NOx. This enables the catalyst to easily participate in the NH3-SCR reaction and facilitates NOx reduction. Moreover, we systematically analyzed the reaction properties of each catalyst using diffuse reflectance infrared Fourier transform and temperature programmed desorption methods. This enabled the elucidation of the reaction characteristics and competitive adsorption properties of the catalysts.

Reinforcing the efficiency of photocatalytic CO2 conversion with H2O vapor through the integration of photothermal Cu/C@Bi/C supported on 3D g-C3N4 under full-spectrum solar irradiation

Developing an efficient photocatalyst for CO2 conversion to chemical fuels that maximizes the utilization of solar energy, particularly near-infrared light, remains a persistent challenge. Meanwhile, it is difficult to suppress the H2 evolution during the photocatalytic process. Herein, Cu/C@Bi/C grafted on the surface of 3D g-C3N4 (NCN) can serve as a highly durable and active catalyst for photothermal synergistic catalytic CO2 reduction under full-spectrum solar irradiation in solid-liquid-gas three-phase interfaces, wherein the Cu/C@Bi/C serves as a harvesting-near-infrared light nanoreactor and an electron transport bridge for the enhancement of the interface local temperature and the efficient transfer of photoinduced electrons. As a result, the hybrid catalyst shows excellent photothermal catalytic activity of CO2 to CO (169.26 μmol·g-1·h-1) and CO2 to CH4 (1.94 μmol·g-1·h-1), outperforming the majority of the conventional photocatalysts, whereas the competitive reaction of H2 evolution can be inhibited. This study presents a feasible approach for the efficient utilization of solar energy by harnessing near-infrared light through nano-reactors, thereby providing a viable strategy for photothermal catalytic CO2 reduction.

Anchored silver-palladium aerogel on carbon cloth via diazonium chemistry for electrochemical reduction of CO2

Electrochemical CO2 reduction reaction (eCO2RR) is a promising route for removal of CO2 from the atmosphere and transforming it into value-added products. The development of new and efficient catalysts plays a vital role in this regard. Here a combination of carbon grafting and bimetallic metal aerogel is presented as a catalyst for eCO2RR. The grafted surface plays an essential function in assembling the catalyst on the carbon cloth (CC) surface as a supported catalyst. Herein, we describe the use of para-aminothiophenol (p-TP) as a grafting agent combined with diazonium chemistry to fix a metallic aerogel catalyst on CC as support. The silver, palladium and bi-metallic silver-palladium aerogels (Ag-p-TP, Pd-p-TP, and AgPd-p-TP) anchored onto the surface of CC yielding the corresponding surface-bound materials were used for testing the electrochemical reduction of CO2. The mono-metallic Ag carbon cloth (Ag-p-TP/CC) exhibited the ability in reducing CO2 to CO with moderate Faradaic efficiency (FE) of 74 % in 0.1 M, NaHCO3 electrolyte at pH = 7.5. The bimetallic AgPd-p-TP-CC further enhanced the Faradaic efficiency for CO formation to 88.7 %, with an unexpectedly low onset potential of −0.09 V vs. RHE. Surprisingly, formate was identified to be the liquid product when Pd-p-TP-CC and AgPd-p-TP-CC were used, with maximal FEformate of 19.0 % and 25.7 % at −1.0 V vs. RHE, respectively. In contrast, CC lacking the chemical grafting that allows immobilization of the metal aerogel (Ag-Pd/CC) exhibits lower FEco% for eCO2RR, highlighting the importance of this chemical modification of the carbon cloth surface. Diazonium chemistry coupled with metal aerogels is a new approach to generating surface-bound catalysts for the effective reduction of CO2.

Investigation of Co–Mn–Ce Ternary Composite Oxide Catalyst for Low-Temperature Selective Catalytic Reduction of NOx with NH3

Investigation of Co–Mn–Ce Ternary Composite Oxide Catalyst for Low-Temperature Selective Catalytic Reduction of NOx with NH3

Preparation of Mn/TiO2 catalysts using recovered manganese from spent alkaline batteries for low-temperature NH3-SCR

Black mass (BM) from spent alkaline Zn-MnO2 batteries was used for the first time as a Mn source in the preparation of Mn/TiO2 catalysts for low-temperature NH3-selective catalytic reduction (SCR) of NOx. To recover Mn species and eliminate alkali and Zn species, BM powder underwent DI-water washing, followed by carbothermal reduction. The resulting slags were further dissolved in HNO3, loaded onto TiO2 particles with ball milling, and then subjected to calcination. Nearly 100% of Zn species were removed from the BM via carbothermal reduction at 950 °C for 4 h with 5.0 wt% activated carbon. The resulting catalyst, derived from the treated BM, achieved similar NOx conversion (97%) as the catalyst prepared using a reagent-grade Mn chemical at 160 °C but a higher NOx-to-N2 conversion rate at 78%. The promoted N2 selectivity was attributed to a high Mn4+/Ti ratio and the presence of impurities from BM, such as Fe3+ ions, which enhanced oxidation ability of the catalyst. Conversely, insufficient removal of Zn or carbon additives in the slags led to a decreased Mn concentration, an increased proportion of Mn2+/Mn3+ species, increased surface OH groups, and reduced oxidation ability on the surface, thus reducing NOx conversion and N2 selectivity.