ISSN / EISSN : 2155-5435 / 2155-5435
Published by: American Chemical Society (ACS) (10.1021)
Total articles ≅ 9,162
Latest articles in this journal
ACS Catalysis pp 9317-9332; doi:10.1021/acscatal.1c01868
The enhancement of the oxygen reduction reaction (ORR) activity of platinum nanoparticles (Pt NPs) using transition metal oxide (MOx, M = Ti, Nb, Ta, W, Y, and Zr) supports has been examined. To enable the use of transition metal oxides having low electric conductivity as supports, Pt NPs were formed on thin transition metal oxides formed on conducting cup-stacked carbon nanotubes (CSCNTs). Metal oxide composites (M1M2Ox) prepared from two types of transition metal (M1M2: TiNb, NbTa, and TaW) precursors were also used as supports. Pt NPs were photodeposited on MOx/CSCNTs and M1M2Ox/CSCNT supports, resulting in MOx/CSCNT- and M1M2Ox/CSCNT-supported Pt NP catalysts (abbreviated as Pt/MOx/CSCNTs and Pt/M1M2Ox/CSCNTs). Their ORR activities in 0.1 M HClO4 aqueous solution were found to significantly depend on the atomic ratio of M1 and M2 in M1M2Ox and the type of metal oxide support. A “volcano-type” dependence of the ORR activity (represented as the current density, mass activity, and specific activity at 0.9 V vs reversible hydrogen electrode (RHE)) on the Pt d-band center, relative to the Fermi level, was obtained in a series of the Pt/MOx/CSCNTs and Pt/M1M2Ox/CSCNT catalysts. It was found that the d-band center values (ranging from −3.83 to −3.42 eV) of Pt deposited on MOx/CSCNTs and M1M2Ox/CSCNT supports were lower than that (−3.39 eV) of the reference Pt/carbon black (CB) and that the Pt/TiNbOx (Ti/Nb = 1:6.6 in atomic ratio)/CSCNTs with a d-band center of −3.59 eV exhibited the maximum ORR activity, in agreement with the theoretical expectation that an ORR catalyst having a d-band center that is ca. 0.2 eV lower than that of Pt would have maximal ORR activity.
ACS Catalysis pp 9345-9354; doi:10.1021/acscatal.1c01088
Catalytic oxidation of methane (CH4) over nonprecious Ni/CeO2 catalysts has received a lot of attention due to the large natural gas reserves found in North America and the prohibitive cost of palladium-based catalysts, commonly used for CH4 oxidation. However, the catalytic mechanism of CH4 oxidation over Ni/CeO2 still remains unclear. Moreover, the parameters affecting the reaction rates, the interaction between nickel and CeO2, and the reaction intermediates are still not well understood. Herein, kinetic model fitting, CH4 temperature-programmed reduction-mass spectroscopy (CH4 TPR-MS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations were combined to elucidate the mechanism of complete oxidation of CH4 over Ni/CeO2. CH4 TPR-MS showed that the complete oxidation of CH4 over Ni/CeO2 requires 55–120 °C lower compared to bare CeO2 or Ni/quartz sand; complete oxidation of CH4 took place when the surface oxygen species were abundant, while partial oxidation products (CO, H2) were formed when the oxygen species were depleted. In situ DRIFTS showed that CH3, CH2, CO, and CO2 were formed after CH4 activation over Ni/CeO2, while CH3O species were not observed. Combining those findings with kinetic model fitting, a redox Mars–van Krevelen (MvK) mechanism showed the best description of the experimental observations. The MvK mechanism involves the reaction of dissociated oxygen species with gas-phase CH4 while water inhibits the reaction rate by adsorbing on the oxidized sites. Moreover, CH4 activation leads to the reduction of the active sites and oxygen vacancy formation followed by reoxidation of the active sites by gas-phase O2. A CH4 oxidation reaction pathway over Ni/CeO2 is proposed by DFT calculations. In summary, the findings shown here suggest that CH4 oxidation over Ni/CeO2 follows a redox MvK mechanism and provides guidance for the rational design of non-precious-metal catalysts for CH4 oxidation reactions.
ACS Catalysis pp 9366-9369; doi:10.1021/acscatal.1c02298
Enantioselective protonation is conceptually one of the most attractive methods to generate an α-chiral center. However, enantioselective protonation presents major challenges, especially in water. Herein, we report a tandem Michael addition/enantioselective protonation reaction catalyzed by an artificial enzyme employing two abiological catalytic sites in a synergistic fashion: a genetically encoded noncanonical p-aminophenylalanine residue and a Lewis acid Cu(II) complex. The exquisite stereocontrol achieved in the protonation of the transient enamine intermediate is illustrated by up to >20:1 dr and >99% ee of the product. These results illustrate the potential of exploiting synergistic catalysis in artificial enzymes for challenging reactions.
ACS Catalysis pp 9333-9344; doi:10.1021/acscatal.1c01997
Microkinetic analysis plays an important role in catalyst design. Although Langmuirian microkinetics is widely used, surface kinetics is actually non-Langmuirian, which depends on the surface nonuniformity caused by either the adsorbate–adsorbate interactions or various types of active sites exposed on the surfaces. Herein we proposed an approach based on the maximum rate analysis for an accurate and efficient analysis of surface kinetics, where the details of the surface nonuniformity were incorporated into the apparent rate coefficients. The present approach was verified by using the water-gas shift reaction on Cu(111) surfaces as a case study. Furthermore, a formulation of free energy landscape (FEL) was proposed to provide a general and intuitive picture of the catalytic reaction network, which was used to understand how surface coverages could affect the values of the macroscopic measurables. Our results demonstrated that even a mild coverage effect, which changed the overall rate only slightly, could significantly change the values of the macroscopic measurables, such as the reaction orders and the apparent activation energies. These were mainly due to the fact that even a mild coverage effect could change the rate-determining steps and weaken the binding strength of some key intermediates. Our results highlight the importance of the coverage-dependent microkinetic analysis aided by the present formulation FEL, which offers a useful tool to bridge theory and experiment and to reveal the in situ nature of the active sites, the rate-determining steps, and the key intermediates, which in turn are beneficial to the rational design of catalysts.
ACS Catalysis pp 9355-9365; doi:10.1021/acscatal.1c02434
Ordered Pt alloy electrocatalysts supported on carbon nanomaterials have attracted widespread attention, especially for the oxygen reduction reaction (ORR), due to the catalytic performance derived from their unique electronic and geometric structures. However, it is still urgent to fabricate uniform and structurally ordered Pt alloy electrocatalysts based on simple methods. Herein, a two-step direct annealing method was applied to synthesize uniform and ordered PtFe alloy nanoparticles loaded on single-wall carbon nanohorns (SWCNHs) under the protection of a thin N-doped carbon (NC) shell, which was in situ generated from the polymerization and pyrolysis of a small organic ligand, namely, aniline, during the first annealing treatment. After the second annealing treatment in a H2 atmosphere for 9 h, the obtained sample, denoted as [email protected]/SWCNHs(H2-9h), exhibited uniform and ordered PtFe nanoparticles with a face-centered tetragonal (fct) structure (ordered degree: >80%, mean size: ∼5.2 nm) on the graphitic SWCNH support. Without removing the NC shell, the [email protected]/SWCNHs(H2-9h) sample showed mass activity (1.53 A/mgPt at 0.9 V) and specific activity (3.61 mA/cm2 at 0.9 V) toward the ORR due to the enhanced electronic interaction derived from the ordered fct-PtFe structure. Importantly, it still retained high catalytic activity after a long-term stability test, mainly owing to the ordered fct-PtFe structure and the protection of the NC shell, which provides strong resistance toward the Fe leaching and nanoparticle aggregation, respectively. The presented strategy is generalized to fabricate different ordered PtM or Pt3M (M = Fe/Co) alloy electrocatalysts.
ACS Catalysis pp 9279-9292; doi:10.1021/acscatal.1c00331
Pt-based catalysts are widely used in propane dehydrogenation to meet the dramatically increased demand of propylene from an on-purpose catalytic process. Although the process has been commercialized with high selectivity for decades, the prevention of coke deposition is still a daunting challenge. Herein, in order to provide a full coverage of the impact of coke deposition, we critically analyze the process of coke formation on Pt-based catalyst. First, the intrinsic nature of coke, including composition, distribution, and effects, is presented. The developments of kinetics model of coke growth are discussed and compared to offer insight into mechanism of coke formation. Moreover, the focus is put on the ways to prevent coke, which included cofeeding reductant gas, promoter, and support engineering. The advantage and limitation of each method is well elaborated, and the unique working principle behind each prevention method is uncovered. The new developments of single atom/site and confined metal cluster strategies are indicated. The regeneration of the deactivated catalyst is also discussed, which has a direct influence on the coke elimination and metal dispersion. In the end, the possible optimization strategies are suggested for the future Pt catalyst rational design. The current work provides a comprehensive summary of the coke formation, which lays out a solid and essential base for the further developments of Pt catalysts in propane direct dehydrogenation.
ACS Catalysis pp 9262-9270; doi:10.1021/acscatal.1c02187
The inherently unfavorable thermodynamics for the direct partial oxidation of CH4 with O2 limits the system to high selectivities only at low conversions. We demonstrate a tandem strategy capable of circumventing this selectivity-conversion limit by performing sequential oxidation of CH4 to CH3OH over a selective Cu-exchanged zeolite followed by C-alkylation of CH3OH with benzene over an acidic zeolite. Using a small-pore zeolite (SSZ-13, CHA topology) to host the Cu species is essential to achieve increased yields by maximizing CH4-to-CH3OH selectivities while also protecting the final alkylate product from overoxidation via size-exclusion. Cofeeding CH4, oxygen, water, and benzene over a mixture of Cu-SSZ-13 and H-ZSM-5 resulted in 77% toluene selectivity at 663 K and 1 bar compared to only 2% CH3OH selectivity in the absence of benzene under identical conditions at isoconversion. A record productivity of 1.7 μmol min–1 gcat–1 was achieved at 11 bar and 603 K (80% toluene selectivity at 0.37% CH4 conversion), which represents a 30-fold improvement over current continuous processes over Cu-based zeolites. Our findings demonstrate the importance of protecting the methanol product to achieve high selectivities and help close the gap to realize more efficient small-scale CH4 conversion processes.
ACS Catalysis pp 9300-9316; doi:10.1021/acscatal.1c01682
The increasing scarcity of iridium (Ir) and its rutile-type oxide (IrO2), the current state-of-the-art oxygen evolution reaction (OER) catalysts, is driving the transition toward the use of mixed Ir oxides with a highly active yet inexpensive metal (IrxM1–xO2). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in Ir-Ru mixed oxide nanoparticles (IrxRu1–xO2 NPs), especially at high relative contents, is rarely evaluated for long-term application as water electrolyzers. In this work, we bridge the knowledge gap by performing a thorough study on the composition- and phase-dependent stability of well-defined IrxRu1–xO2 NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (IrxRu1–xOy) present an amorphous coral-like structure with a hydrous Ir-Ru oxide phase, which upon post-synthetic thermal treatment fully converts to a rutile-type structure followed by a selective Ir enrichment at the NP topmost surface. It was demonstrated that Ir incorporation into a RuO2 matrix drastically reduced Ru dissolution by ca. 10-fold at the expense of worsening Ir inherent stability, regardless of the oxide phase present. Hydrous IrxRu1–xOy NPs, however, were shown to be 1000-fold less stable than rutile-type IrxRu1–xO2, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrOy. For rutile-type IrxRu1–xO2, the sequential start-up/shut-down OER protocol employed revealed a steady-state dissolution for both Ir and Ru, as well as the key role of surface Ru species in OER activity: minimal Ru surface losses (<1 at. %) yielded OER activities for tested Ir0.2Ru0.8O2 equivalent to those of untested Ir0.8Ru0.2O2. Ir enrichment at the NP topmost surface, which mitigates selective subsurface Ru dissolution, is identified as the origin of the NP stabilization. These results suggest Ru-rich IrxRu1–xO2 NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction.
ACS Catalysis pp 9252-9261; doi:10.1021/acscatal.1c01929
A bulky and electron-rich N-heterocyclic carbene–palladium complex (SIPr)Ph2Pd(cin)Cl was synthesized and characterized. It was found to be highly efficient and versatile for the coupling of different (hetero)aryl chlorides with various (hetero)aryl amines at room temperature, especially for the challenging amination of five- or six-membered ring heteroaryl chlorides with five- or six-membered ring heteroaryl amines. It was also successfully applied with high yields to the synthesis of various commercial pharmaceuticals and candidate drugs or compounds with potential pharmacological activities. All of these demonstrate its excellent catalytic efficacy in Buchwald–Hartwig amination and broad application prospects in relevant pharmaceutical preparations. DFT calculations suggest that the steric-induced electronic interaction makes the ligand more electron-donating, and the steric effect effectively regulates the rotation of the iPr-Ph-iPr group in the catalyzed system due to the introduction of the diphenyl skeleton. Considering the electronic effect and the steric effect together, the oxidative addition activation barriers of the (SIPr)Ph2 and (SIPr) ligands are close to each other. Reductive elimination is the rate-determining step of the (SIPr)Ph2Pd(cin)Cl-catalyzed system in the catalytic cycle, and the appropriate steric hindrance of the (SIPr)Ph2 ligand greatly reduces the energy barrier of this step. The perfect combination of the electron-donating and steric hindrance abilities of the ligand significantly improves the catalytic activity.
ACS Catalysis pp 9271-9278; doi:10.1021/acscatal.1c01328
To date, numerous methods have been successfully developed to functionalize N-heteroaryl C–H bonds. In contrast, dearomative tandem functionalization of N-heteroarenes is still a subject to be explored. Reported herein is an example on reductive dearomatization-induced tandem functionalization of N-heteroarenes by ruthenium catalysis, which offers a general method for diastereoselective construction of fused heterocycles featuring a cyclic syn-N, O-acetal motif from N-heteroarenes, phenols, and paraformaldehyde. Mechanistic study reveals that the products are formed via a tandem sequence of pyridyl C3-benzylation and hydroxymethylation followed by C2-aryloxylation of N-heteroarenium salts, proceeding with broad substrate scope, good functional group tolerance, high atom efficiency, and applicability for postfunctionalization of some biomedical molecules.