The Ni(I) hydrogen oxidation catalyst Ni(PCy 2N tBu 2)2+ (1 +; PCy 2N tBu 2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane) has been studied using a combination of ...electron paramagnetic resonance (EPR) techniques (X-, Q-, and D-band, electron–nuclear double resonance, hyperfine sublevel correlation spectroscopy), X-ray crystallography, and density functional theory (DFT) calculations. Crystallographic and DFT studies indicate that the molecular structure of 1 + is highly symmetrical. EPR spectroscopy has allowed determination of the electronic g tensor and the spin density distribution on the ligands, and revealed that the Ni(I) center does not interact strongly with the potentially coordinating solvents acetonitrile and butyronitrile. The EPR spectra and magnetic parameters of 1 + are found to be distinctly different from those for the related compound Ni(PPh 2NPh 2)2+ (4 +). One significant contributor to these differences is that the molecular structure of 4 + is unsymmetrical, unlike that of 1 +. DFT calculations on derivatives in which the R and R′ groups are systematically varied have allowed elucidation of structure/substituent relationships and their corresponding influence on the magnetic resonance parameters.
Catalyst synthesis with precise control over the structure of catalytic active sites at the atomic level is of essential importance for the scientific understanding of reaction mechanisms and for ...rational design of advanced catalysts with high performance. Such precise control is achievable using atomic layer deposition (ALD). ALD is similar to chemical vapor deposition (CVD), except that the deposition is split into a sequence of two self-limiting surface reactions between gaseous precursor molecules and a substrate. The unique self-limiting feature of ALD allows conformal deposition of catalytic materials on a high surface area catalyst support at the atomic level. The deposited catalytic materials can be precisely constructed on the support by varying the number and type of ALD cycles. As an alternative to the wet-chemistry based conventional methods, ALD provides a cycle-by-cycle “bottom-up” approach for nanostructuring supported catalysts with near atomic precision.
In this review, we summarize recent attempts to synthesize supported catalysts with ALD. Nucleation and growth of metals by ALD on oxides and carbon materials for precise synthesis of supported monometallic catalyst are reviewed. The capability of achieving precise control over the particle size of monometallic nanoparticles by ALD is emphasized. The resulting metal catalysts with high dispersions and uniformity often show comparable or remarkably higher activity than those prepared by conventional methods. For supported bimetallic catalyst synthesis, we summarize the strategies for controlling the deposition of the secondary metal selectively on the primary metal nanoparticle but not on the support to exclude monometallic formation. As a review of the surface chemistry and growth behavior of metal ALD on metal surfaces, we demonstrate the ways to precisely tune size, composition and structure of bimetallic metal nanoparticles. The cycle-by-cycle “bottom up” construction of bimetallic (or multiple components) nanoparticles with near atomic precision on supports by ALD is illustrated. Applying metal oxide ALD over metal nanoparticles can be used to precisely synthesize nanostructured metal catalysts. In this part, the surface chemistry of Al2O3 ALD on metals is specifically reviewed. Next, we discuss the methods of tailoring the catalytic performance of metal catalysts including activity, selectivity and stability, through selective blocking of the low-coordination sites of metal nanoparticles, the confinement effect, and the formation of new metal-oxide interfaces. Synthesis of supported metal oxide catalysts with high dispersions and “bottom up” nanostructured photocatalytic architectures are also included. Therein, the surface chemistry and morphology of oxide ALD on oxides and carbon materials as well as their catalytic performance are summarized.
The in‐depth mechanism on the simultaneous activation of O2 and surface lattice O2− on one active metallic site has not been elucidated yet. Herein, we report a strategy for the construction of ...abundant oxygen activation sites by rational design of Cu1/TiO2 single atom catalysts (SACs). The charge transfer between isolated Cu and TiO2 support generates abundant CuI and 2‐coordinated Olat sites in Cu1−O−Ti hybridization structure, which facilitates the chemisorption and activation of O2 molecules. Simultaneously, the Cu1−O−Ti induced TiO2 lattice distortion activate the adjacent surface lattice O2−, achieving the dual activation of O2 and surface lattice O2−. The Cu1−O−Ti active site switches the CO oxidation mechanism from Eley‐Rideal (80 °C) to Mars–van Krevelen route (200 °C) with the increase of reaction temperature. The dual activation of O2 and surface lattice O2− can by modulating the electron properties of SACs can boost the heterogeneous catalytic oxidation activity.
The isolated Cu1+ on Cu1/TiO2 single atom catalysts (SACs) facilitated the simultaneous activation of chemisorbed O2 and 2‐coordinated surface lattice O2−, which triggered the CO oxidation through E‐R (80 °C) and MvK (200 °C) routes synergistically with elevated temperature. This finding provides insights and strategy for the rational design of efficient oxidation catalysts with sufficient reactive oxygen species for practical application.
Catalyst deactivation is ultimately inevitable, and one of the processes known to cause deactivation is sintering of metal particles. Consequently, numerous methods to reverse the sintering process ...by redispersing metal nanoparticles have been developed. These methods are discussed in this perspective, and the reported mechanisms of redispersion are summarized. Additionally, the longer-term practical use of such treatments and the benefits this can bring are briefly disclosed.
Atomically dispersed Pd (Pd1) catalysts supported on annealed nanodiamond were prepared through a deposition–precipitation method toward propane direct dehydrogenation (PDDH). The Pd1 catalyst is ...superior to Pd cluster/particle catalysts in activity and stability. Combining experimental characterizations and DFT calculations revealed that atomically dispersed Pd species have strong interactions with the hybrid nanodiamond/graphene support, leading to better resistance to coke formation. More importantly, Pd sintering is inevitable in the cluster/particle catalysts, while the high dispersion of Pd species in Pd1 catalyst is well-preserved during the reaction, which is caused in part by the redistribution or migration of Pd single atoms onto the carbonaceous compound (coke). As a result, the Pd1 catalyst shows significantly better activity and stability in high-reaction temperatures than Pd cluster/particle catalysts. This work reveals deeper insights on designing highly dispersed metal catalysts with the ability of in situ regeneration of active sites in high-temperature catalytic reactions.
Ammonia (NH3) synthesis is an important industrial chemical process. Recently, electrochemically converting the earth-abundant dinitrogen (N2) in the aqueous phase to NH3 at ambient conditions has ...been proposed as an alternative to the well-established Haber–Bosch process. Catalysts for the electrochemical N2 reduction to NH3 play crucial roles in realizing this NH3 synthesis route. Electrochemical N2 reduction has been studied for decades, and many studies have emerged in the past few years. Herein, we provide a comprehensive review to summarize various catalysts used for achieving electrochemical N2 reduction to NH3, including homogeneous, heterogeneous and biological catalysts, as well as relevant computational studies to understand their reaction mechanisms. We compare the advantages and shortcomings of these catalytic systems. Future research directions for realizing catalysts with low overpotentials, high energy efficiency, good scalability, and stability modularity are also proposed. This review provides an overview of this fast-growing research field and encourages more studies toward the rational design of catalysts for electrochemical N2 reduction to NH3 under ambient conditions.
Ammonia synthesis through electrochemical reduction of nitrogen molecules is a promising strategy to significantly reduce the energy consumption in traditional industrial process. Detailed mechanism ...study of multistep complex nitrogen reduction reaction is prerequisite for the design of highly efficient catalyst. Stable atomically dispersed catalyst with unique geometric and electronic structure is suitable for the mechanism clarification of such a complex reaction. In this study, d‐block transition‐metal (TM) anchored C2N single layer catalyst is investigated by the density functional theory (DFT) calculation. Both single TM‐anchored single atom catalyst (SAC) and double TM‐anchored double atom catalyst (DAC) exhibit good thermodynamic stability in atomically dispersed catalyst. In the case of SACs, IVB metals (Ti, Zr, Hf) exhibit the highest reactivity and lowest overpotential. While in the case of DACs, Cr─Cr system leads to the NH3 formation, but V─V system leads to the N2H4 formation. The SACs show much lower overpotential and stronger activation of N2 molecule than the DACs due to the different activation mechanisms: traditional σ‐donation/π‐backdonation N2 activation mechanism is found in SACs, while a new π‐donation/π‐backdonation N2 activation mechanism is found in the DACs. The present work demonstrates that the different catalytic effect for NRR between SAC and DAC and their corresponding electronic structure origin, which gives more insight into the single atom catalyst.
New π‐donation/π‐backdonation N2 activation mechanism is found on DAC, while traditional σ‐donation/π‐backdonation N2 activation mechanism is found on SAC.
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•Desilication with NaOH treatment produces a mesoporous ZSM-5 catalyst support.•Desilicated ZrO2/ZSM-5 catalyst gives a more efficient solid acidic catalyst.•Optimal conversion of ...97.84% was obtained with desilicated ZrO2/ZSM-5.•Desilicated ZrO2/ZSM-5 gives excellent stability and reusability.
Bio-based fuels, such as biodiesel, have recently gained attention due to concern about ever-increasing energy consumption and global warming. Solid catalysts are key materials in the production of green biodiesel. The use of solid catalysts in the production of biodiesel can lower the cost of the separation and thus reduce biodiesel costs. In the present study, zeolite-supported ZrO2 catalyst (ZrO2/ZSM-5) and a series of desilicated zeolite-supported ZrO2 catalyst ZrO2/ZSM-5(0.1–0.3 M) were successfully synthesized by impregnation method and used as new catalysts for the production of soybean oil biodiesel through transesterification. Desilication at various sodium hydroxide concentrations enhances the external surface area and mesoporous volume, without jeopardizing crystallinity or microporosity of the catalysts. The synthesized catalysts were characterized via X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), pyridine-FTIR, scanning electron microscopy, TEM, XRF, TGA and nitrogen adsorption-desorption. Nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC–MS) were used to characterize the biodiesel that was produced. The as-prepared ZrO2/ZSM-5(0.2 M) catalyst exhibited better catalytic activity in the transesterification of soybean oil with methanol, compared to ZrO2/ZSM-5 catalyst, due to large pore volume, high surface area, and high dispersion of ZrO2 on the desilicated zeolite support with improved acidity and high number of active sites. A biodiesel conversion of 90.1% and 97.84% were obtained for zeolite-supported ZrO2 catalyst and desilicated zeolite-supported ZrO2 catalyst, respectively, under optimum reaction conditions of 1.0 wt% catalyst loading, 1:16 soybean oil/methanol molar ratio at 200 °C in 4 h reaction time. Moreover, the desilicated zeolite-supported ZrO2 catalyst exhibited excellent stability and re-usability giving a biodiesel conversion of 93.8% after three cycles of reuse.
Transition metal carbide compound has been extensively investigated as a catalyst for hydrogenation, for example, due to its noble metal-like properties. Herein a facile synthetic strategy is applied ...to control the thickness of atomic-layer Pt clusters strongly anchored on N-doped Mo
C nanorods (Pt/N-Mo
C) and it is found that the Pt atomic layers modify Mo
C function as a high-performance and robust catalyst for hydrogen evolution. The optimized 1.08 wt% Pt/N-Mo
C exhibits 25-fold, 10-fold, and 15-fold better mass activity than the benchmark 20 wt% Pt/C in neutral, acidic, and alkaline media, respectively. This catalyst also represents an extremely low overpotential of -8.3 mV at current density of 10 mA cm
, much better than the majority of reported electrocatalysts and even the commercial reference catalyst (20 wt%) Pt/C. Furthermore, it exhibits an outstanding long-term operational durability of 120 h. Theoretical calculation predicts that the ultrathin layer of Pt clusters on Mo-Mo
C yields the lowest absolute value of ΔG
. Experimental results demonstrate that the atomic layer of Pt clusters anchored on Mo
C substrate greatly enhances electron and mass transportation efficiency and structural stability. These findings could provide the foundation for developing highly effective and scalable hydrogen evolution catalysts.
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