Metal Catalysts Research Articles

Strong interaction between promoter and metal in Pd-Ba/TiO2 catalysts for formaldehyde oxidation

As our previous works found, alkali metals have a common promotion effect on supported noble metals catalysts for formaldehyde (HCHO) oxidation. As second-group elements, alkaline earth metals (AEMs) are neighbors to the first-group elements and share some properties in common. However, detailed investigations into the specific mechanisms underlying AEMs’ effects on HCHO oxidation remain limited. In this study, we found that Ba addition showed a similar promotion effect on HCHO oxidation for Pd/TiO2. Ba species stabilized Pd groups, improved the dispersion, and even caused a large number of monatomic-like Pd sites to appear, which may be attributed to the electronic interaction between promoter and metal (EIPM) between Ba and Pd. Besides, AEM loading had the important effect of increasing the electron density of metallic Pd nanoparticles, which further improved the ability for O2 activation and so enhanced the mobility of chemisorbed oxygen on the catalyst surface. For Pd/TiO2, the HCHO oxidation path is mainly HCHO→HCOOH→HCOO→H2O+CO2. By contrast, for Pd-Ba/TiO2, with more surface-active species, the formate intermediate was more likely to be directly oxidized into H2O and CO2, which is a more effective reaction pathway. The details of the EIPM between Pd and Ba were investigated by GPAW (DFT calculation module) in ASE (Atomic Simulation Environment). The AEM Ba acted as an electron donor and could interact with Pd d orbital electrons through BaO sp orbital electrons. Ba species were highly dispersed on the carrier due to the Ba-Ti interaction. Ba species dispersed over large areas stabilized the Pd particles and donated electrons to Pd. Therefore, adding an AEM is an efficacious strategy to improve the performance of the catalytic oxidation of HCHO.

A Review of Catalyst Integration in Hydrothermal Gasification

Industrial scale-up of hydrothermal supercritical water gasification process requires catalytic integration to reduce the high operational temperatures and pressures to enhance controlled chemical reaction pathways, product yields, and overall process economics. There is greater literature disparity in consensus on what is the best catalyst and reactor design for hydrothermal gasification. This arises from the limited research on catalysis in continuous flow hydrothermal systems and rudimentary lab-scale experimentation on simple biomasses. This review summarizes the literature status of catalytic hydrothermal processing, especially for continuous gasification and in situ catalyst handling. The rationale for using low and high temperatures during catalytic hydrothermal processing is highlighted. The role of homogeneous and heterogeneous catalysts in hydrothermal gasification is presented. In addition, the rationale behind certain designs and component selection for catalytic investigations in continuous hydrothermal conversion is highlighted. Furthermore, the effect of different classes of catalysts on the reactor and reactions are elaborated. Overall, design and infrastructural challenges such as plugging, corrosion, agglomeration of the catalysts, catalyst metal leaching, and practical assessment of catalyst integration towards enhancement of process economics still present open questions. Therefore, strategies for catalytic configuration in continuous hydrothermal process must be evaluated on a system-by-system basis depending on the feedstock and experimental goals.

Thiamine (Vitamin B1) Promoted Organic Transformations: A Recent Updates.

Due to the growing significance of sustainable and environmentally friendly organic transformations, there has been increasing interest in utilizing vitamins as catalysts owing to their green nature, biocompatibility, and ease of preparation. Among these, Vitamin B1, also known as thiamine stands out for its nonflammable, water-soluble, inexpensive, and non-toxic characteristics. This review summarized recent developments on the catalytic application of Vitamin B1 in organic transformations, particularly in facilitating C-C and C-X (N, O, S) bond formations, thus demonstrating its efficacy in synthesizing complex molecules. Vitamin B1 exhibits versatility in these reactions, functioning as both an organocatalyst as well as a co-catalyst or ligand with other metal catalysts. The review also delves into the application of thiamine diphosphate-dependent enzymes as catalysts in organic reactions, drawing inspiration from natural enzymatic processes. Additionally, the mechanistic intricacies of thiamine-catalyzed reactions and the roles of co-catalysts or additives are thoroughly examined, providing insights into reaction pathways and facilitating informed catalyst design strategies.

Electrosynthesis of Transition Metal Coordinated Polymers for Active and Stable Oxygen Evolution

AbstractTransition metal coordination polymers (TM‐CP) are promising inexpensive and flexible electrocatalysts for oxygen evolution reaction in water electrolysis, while their facile synthesis and controllable regulation remain challenging. Here we report an anodic oxidation‐electrodeposition strategy for the growth of TM‐CP (TM=Fe, Co, Ni, Cr, Mn; CP=polyaniline, polypyrrole) films on a variety of metal substrates that act as both catalyst supports and metal ion sources. An exemplified bimetallic NiFe‐polypyrrole (NiFe‐PPy) features superior mechanical stability in friction and exhibits high activity with long‐term durability in alkaline seawater (over 2000 h) and anion exchange membrane electrolyzer devices at current density of 500 mA cm−2. Spectroscopic and microscopic analysis unravels the configurations with atomically distributed metal sites induced by d‐π conjugation, which transforms into a mosaic structure with NiFe (oxy)hydroxides embedded in PPy matrix during oxygen evolution. The superior catalytic performance is ascribed to the anchoring effect of PPy that inhibits metal dissolution, the strong substrate‐to‐catalyst interaction that ensures good adhesion, and the Fe/Ni−N coordination that modulates the electronic structures to facilitate the deprotonation of *OOH intermediate. This work provides a general strategy and mechanistic insight into building robust inorganic/polymer composite electrodes for oxygen electrocatalysis.

Electrosynthesis of Transition Metal Coordinated Polymers for Active and Stable Oxygen Evolution.

Transition metal coordination polymers (TM-CP) are promising inexpensive and flexible electrocatalysts for oxygen evolution reaction in water electrolysis, while their facile synthesis and controllable regulation remain challenging. Here we report an anodic oxidation-electrodeposition strategy for the growth of TM-CP (TM=Fe, Co, Ni, Cr, Mn; CP=polyaniline, polypyrrole) films on a variety of metal substrates that act as both catalyst supports and metal ion sources. An exemplified bimetallic NiFe-polypyrrole (NiFe-PPy) features superior mechanical stability in friction and exhibits high activity with long-term durability in alkaline seawater (over 2000 h) and anion exchange membrane electrolyzer devices at current density of 500 mA cm-2. Spectroscopic and microscopic analysis unravels the configurations with atomically distributed metal sites induced by d-π conjugation, which transforms into a mosaic structure with NiFe (oxy)hydroxides embedded in PPy matrix during oxygen evolution. The superior catalytic performance is ascribed to the anchoring effect of PPy that inhibits metal dissolution, the strong substrate-to-catalyst interaction that ensures good adhesion, and the Fe/Ni-N coordination that modulates the electronic structures to facilitate the deprotonation of *OOH intermediate. This work provides a general strategy and mechanistic insight into building robust inorganic/polymer composite electrodes for oxygen electrocatalysis.

Synthesis of High-Molecular-Weight Polypropylene Elastomer by Propylene Polymerization Using α-Diimine Nickel Catalysts.

The α-diimine late transition metal catalyst represents a new strategy for the synthesis of atactic polypropylene elastomer. Taking into account the properties of the material, enhancing the molecular weight of polypropylene at an elevated temperature through modifying the catalyst structure, and further increasing the activity of α-diimine catalyst for propylene polymerization, are urgent problems to be solved. In this work, two α-diimine nickel(II) catalysts with multiple hydroxymethyl phenyl substituents were synthesized and used for propylene homopolymerization. The maximum catalytic activity was 5.40 × 105 gPP/molNi·h, and the activity was still maintained above 105 gPP/molNi·h at 50 °C. The large steric hindrance of catalysts inhibited the chain-walking and chain-transfer reactions, resulting in polypropylene with high molecular weights (407~1101 kg/mol) and low 1,3-enchainment content (3.57~16.96%) in toluene. The low tensile strength (0.3~1.0 MPa), high elongation at break (218~403%) and strain recovery properties (S.R. ~50%, 10 tension cycles) of the resulting polypropylenes, as well as the visible light transmittance of approximately 90%, indicate the characteristics of the transparent elastomer.

Accessing 2‐(Chalcogen)benzothiazoles via Electrochemical Intermolecular Cyclization of Isocyanides with Thiols or Diselenides

Abstract2‐(chalcogen)benzothiazoles are frequently used as core structures for the development of pharmaceutical agents. A strategy for the synthesis of 2‐(chalcogen)benzothiazoles under electrooxidative conditions using isocyanides, thiols or diselenides as substrates was developed. This protocol allows access to structurally diverse 2‐(chalcogen)benzothiazoles in good yields and with good functional group tolerance, which does not require heavy metal catalysts and exogenous oxidants. Furthermore, gram‐scale synthesis was performed to demonstrate the scale‐up applicability of this reaction.

Electrochemically Induced Desulfurization of Thioureas for the Synthesis of Substituted 5‐Aminotetrazoles

AbstractA one‐pot method has been developed for synthesizing diverse substituted 5‐aminotetrazoles via the electrochemically induced desulfurizative addition–cyclization of thioureas with azidotrimethylsilane. This methodology enables the construction of C−N and N−N bonds under mild reaction conditions without transition metal catalysts or external oxidizing agents. The applicability of this approach is demonstrated by its broad substrate scope, compatibility with various functional groups, scalability to gram‐scale synthesis, and practicality evidenced by the synthesis of an antibacterial agent.

Sabatier principle based design of high performance FeCoNiMnMoP high entropy electrocatalysis for alkaline water splitting

FeCoNi-based high entropy intermetallic compounds are promising substitutes for noble metal catalysts for hydrogen evolution reaction (HER) and oxygen evolution (OER), while their activity is restricted by the synergistic effect between the elements. In this paper, the theoretical model of FeCoNiMnMoP bifunctional catalyst has been reasonably designed given the volcanic curve of M−H bond energy as a valuable guide for candidate metal elements. Theoretical calculations show that the synergistic effect between multiple sites not only accelerates the electron transfer but also reduces the rate-determining step (RDS) barrier. At the same time, the introduction of Mo also enhances the total density of states and reduces the d-band center, thus optimizing the adsorption and dissociation of intermediates. The experimental results confirm that the theoretical model has excellent catalytic performance. In the alkaline solution, FeCoNiMnMoP‖FeCoNiMnMoP requires only an ultra-low cell voltage of 1.49 V for overall water splitting when the current density is 10 mA cm−2. In general, novel insights on the effects of intermetallic synergies on water decomposition are provided, which is helpful for the design of bifunctional catalysts based on volcanic diagrams.

Facile synthesis of NiCo2O4 nanosheets efficient for electrocatalysis of water

Effective regulation of transition metal spinel morphology is crucial for stable bifunctional catalysts in the electrocatalysis of water. In this work, we crafted a fine NiCo2O4 (F-NCO) microflower structure assembled by nanosheet arrays, which exhibited highly efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), outperforming most of the previous transition metal catalysts. Electronic microscope analysis revealed that the thickness of the F-NCO catalyst was only 1.6 nm, much smaller than that (58.8 nm) of bulk NiCo2O4 (B-NCO), and the ultrathin lamellar structure facilitated the exposure of active sites and improved the reaction kinetics. Compared with the B-NCO catalyst, the overpotential of the F-NCO catalyst in HER and OER was reduced by 36 and 82 mV, the active surface area increased by 2.68 and 4.16 times, and the charge transfer resistance values were reduced by 0.42 and 0.61 times, respectively. In the assembled cell, F-NCO/CFP(+)||F-NCO/CFP(−) electrolyzer was able to achieve 10 mA cm−2 at a low cell voltage of 1.74 V, which was much lower than B-NCO/CFP(+)||B-NCO/CFP(−) electrolyzer. In the 12-hour stability test, the potentiometric time dropped by only 30 mV, lower than 70 mV of the B-NCO catalyst. This work presents a facile strategy for developing a bifunctional catalyst for the electrocatalysis of water.

Preparation of Non-Noble Metal Catalyst FeCo2O4/MoS2 for Production of Hydrogen and Oxygen by Electrochemical Decomposition of Water

Preparation of Non-Noble Metal Catalyst FeCo2O4/MoS2 for Production of Hydrogen and Oxygen by Electrochemical Decomposition of Water

Directional Construction of Low-Coordination Fe-N3 Coupled with Intrinsic Carbon Defects for High-Efficiency Oxygen Reduction.

Regulating the coordination environment of Fe-Nx sites is an efficient but challenging approach for promoting the intrinsic catalytic activity of single-atom Fe/N-codoped carbon (Fe-N-C) toward the oxygen reduction reaction (ORR). Herein, low-coordination Fe-N3 sites coupled with carbon vacancies (Fe-N3/CV) are directionally constructed in Fe-N-C via pyrolysis of a metal-organic framework (MOF) precursor with N3-Zn-O-Fe moieties, which are delicately prefabricated by chemically anchoring Fe3+ onto a H2O-etching induced linker-missing Zn-N3 site in the MOF precursor. The optimized Fe-N-C with the Fe-N3/CV sites displays a high ORR half-wave potential of 0.92 V (vs RHE), which is attributed to the optimized electronic structure and binding strengths of the active Fe center toward the ORR intermediates stemming from the synergy of the asymmetric configuration of Fe-N3 as well as the adjacent carbon vacancies. This work could be enlightening for the design and construction of high-activity coupling sites in metal and nitrogen-codoped carbon catalysts.

High Temperature Solid Oxide Electrolysis for Green Hydrogen Production.

Global warming and energy crises have motivated the development of renewable energy and its energy carriers. Green hydrogen is the most promising renewable energy carrier and will be fundamental to future energy conversion and storage systems. Solid Oxide Electrolysis Cells (SOECs) are a promising green hydrogen production technology featuring high electrical efficiency, no noble metal catalyst usage, and reversible operation. This review provides a timely summary of the latest SOEC progress, covering developments at various levels, from cells to stacks to systems. Cell/stack components, configurations, advanced electrode material/fabrication, and novel characterization methods are discussed. Electrochemical and durable performance for each cell/stack configuration is reviewed, focusing on degradation mechanisms and associated mitigation strategies. SOEC system integration with renewable energy and downstream users is outlined, showing flexibility, robustness, scalability, viability, and energy efficiency. Challenges of cost and durability are expected to be overcome by innovation in material, fabrication, production, integration, and operation. Overall, this comprehensive review identifies the SOEC commercialization bottleneck, encourages further technology development, and envisions a future green hydrogen society with net-zero carbon emissions.

Free radical mechanisms of ammonium sulfate as intensively used industrial materials on suppressing organic pollutants

Organic free radicals are critical intermediates for the generation and inhibition of organic pollutants during industrial processes. Clarifying the free radical mechanism of pollutant inhibition is significant for their efficient control. Ammonium sulfate is intensively used in industrial materials to suppress organic pollutants. In this study, organic free radical intermediate species in metal-catalyzed reactions inhibited by ammonium sulfate were identified using continuous-wave electron paramagnetic resonance (EPR) spectroscopy, providing direct evidence for the free radical mechanisms of organic pollutants inhibition. The transverse (T2) and longitudinal (T1) relaxation time variations catalyzed by different metal catalysts in the presence of ammonium sulfate were compared using pulsed-wave EPR. Consequently, after the addition of ammonium sulfate, the observed increase in T2 suggests that ammonium sulfate leads to radical concentration reduction. A decrease in the T1 relaxation time suggests the enhanced interaction between organic radicals and metals, which is an obstacle to subsequent radical reactions. Therefore, ammonium sulfate dominantly changed the free radical intermediates species, concentrations, and their reactivity, and then inhibited the organic pollutants formations. The inhibition mechanisms of ammonium sulfate on metal-catalyzed pollutants were then proposed combining EPR analysis, X-ray characterization, and high-resolution mass spectrometry screening. As a result, (1) occupying the active sites of metal catalysis and (2) inhibiting free radical intermediates are the two main intrinsic inhibition mechanisms of ammonium sulfate. The findings provide new perspectives on the efficient inhibition of organic pollutants in industrial processes involving various metal catalysts.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation.

The dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130-140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF-mediated dehydrogenative synthesis. The TTT-DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT-DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well-suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates' functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Theoretical insight into the NO* Coverage-Dependent selectivity of Pd and Cu electrocatalysts for nitrate reduction

Nitrate (NO3−) is a concerning contaminant in groundwater that is harmful to human health. Electrocatalytic reduction of NO3− can be achieved with transition metals such as Pd and Cu. In this work, we apply density functional theory (DFT) to discern how the electronic properties of the metal catalyst affect the selectivity of the nitrate reduction mechanism toward either N2 products (e.g., N2 and N2O) or N1 products (e.g., NH3 and NH4+). We find that the greater d-band filling of Cu results in more electron accumulation on the N atom of adsorbed NO*, which drives N–H bond formation favoring the production of HNO* and eventually N1 products (NH3). Conversely, the more delocalized d orbitals of Pd lead to a strong adsorbate–adsorbate coverage effect that lowers key N–N coupling barriers at high NO* coverage favoring selectivity toward N2 products (N2 and N2O). This work elucidates why, at the electronic-structure level, nitrate reduction is highly sensitive to NO* coverage on some metals (such as Pd) and less so on others (such as Cu).

Accelerating the design of catalysts for CO2 electroreduction to HCOOH: A data-driven DFT-ML screening of dual atom catalysts

Dual-atom catalysts (DACs) have emerged as potential catalysts for effective electroreduction of CO2 due to their high atom utilization efficiency and multiple active sites. However, the screening of DACs remains a challenge due to the large number of possible combinations, making exhaustive experimental or computational screening a daunting task. In this study, a density functional theory (DFT)-based machine learning (ML)-accelerated (DFT-ML) hybrid approach was developed to test a set of 406 dual transition metal catalysts on N-doped graphene (NG) for the electroreduction of CO2 to HCOOH. The results showed that the ML algorithms can successfully capture the relationship between the descriptors of the DACs (inputs) and the limiting potential for HCOOH generation (output). Of the four ML algorithms studied in this work, the feedforward neural network model achieved the highest prediction accuracy (the highest correlation coefficient (R2) of 0.960 and the lowest root mean square error (RMSE) of 0.319 eV on the test set) and the predicted results were verified by DFT calculations with an average absolute error of 0.14 eV. The DFT-ML approach identified Co-Co-NG and Ir-Fe-NG as the most active and stable electrocatalysts for the electrochemical reduction of CO2 to HCOOH. The DFT-ML hybrid approach exhibits exceptional prediction accuracy while enabling a significant reduction in screening time by an impressive 64% compared to conventional DFT-only calculations. These results demonstrate the immense potential of using ML methods to accelerate the screening and rational design of efficient catalysts for various energy and environmental applications.

Self-assembled formation of platinum single atoms and nanoclusters stabilized on MXene as an efficient catalyst for boosting electrocatalytic hydrogen evolution

Atomic isolated and dispersed single atom catalyst is ideal for high-performance industrial catalyst. However, the contribution from small amounts of nanoclusters coexisting with single atoms to the catalytic reaction activity is often neglected. In this work, a MXene-supported uniformly dispersed Pt single atoms and nanoclusters (Pt1+Ptn/MXene) were obtained by self-assembly through an impregnation method. The catalyst exhibits high electrochemical performance in the hydrogen evolution reaction with an overpotential of only 61.3 mV and a Tafel slope of 32.5 mV dec−1 at a current density of 10 mA cm−2 in an acidic environment. The XAFS analysis and further DFT calculation indicate that the synergistic effect between single atoms and nanoclusters can facilitate adsorption of H* and promote the HER reaction. This work provides an efficient method for integrating multiple active sites in noble metal catalysts.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation

AbstractThe dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130–140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF‐mediated dehydrogenative synthesis. The TTT‐DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT‐DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well‐suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates’ functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Sub-nanometric ruthenium clusters prepared by solid-state dispersion for catalytic selective aerobic oxidation of aromatic alcohols

The selective aerobic oxidation of alcohols to corresponding aldehydes is one of the most important challenges in the green chemical industry. In this work, Ru/Al2O3-SSD catalyst with highly dispersed metallic ruthenium clusters (average particle size of 0.97 nm) was prepared by solid-state dispersion (SSD) for efficient selective aerobic oxidation of aromatic alcohols to aldehydes. Compared with the counterpart catalyst Ru/Al2O3-IM prepared by wet impregnation (IM), this catalyst has smaller metal particles (0.97 nm vs. 3.4 nm), resulting in a special electronic structure, and exhibits much higher TOF (234 h−1 vs. 98 h−1) in the aerobic oxidation reaction of benzyl alcohol at 80 °C. The catalyst also shows excellent catalytic activity in the preparation of vanillin by selective aerobic oxidation of vanillyl alcohol at 100 °C, delivering a vanillin yield of 99%. The solid-state dispersion strategy provides an efficient and facile way for the fabrication of high-performance sub-nanometric metal catalysts.