Dehui Deng

Iron Encapsulated within Pod-like Carbon Nanotubes for Oxygen Reduction Reaction

Chainmail for catalysts: a catalyst with iron nanoparticles confined inside pea-pod-like carbon nanotubes exhibits a high activity and remarkable stability as a cathode catalyst in polymer electrolyte membrane fuel cells (PEMFC), even in presence of SO(2). The approach offers a new route to electro- and heterogeneous catalysts for harsh conditions.

Catalysis with two-dimensional materials and their heterostructures

Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.

Enhanced Electron Penetration through an Ultrathin Graphene Layer for Highly Efficient Catalysis of the Hydrogen Evolution Reaction

Major challenges encountered when trying to replace precious-metal-based electrocatalysts of the hydrogen evolution reaction (HER) in acidic media are related to the low efficiency and stability of non-precious-metal compounds. Therefore, new concepts and strategies have to be devised to develop electrocatalysts that are based on earth-abundant materials. Herein, we report a hierarchical architecture that consists of ultrathin graphene shells (only 1-3 layers) that encapsulate a uniform CoNi nanoalloy to enhance its HER performance in acidic media. The optimized catalyst exhibits high stability and activity with an onset overpotential of almost zero versus the reversible hydrogen electrode (RHE) and an overpotential of only 142 mV at 10 mA cm(-2) , which is quite close to that of commercial 40 % Pt/C catalysts. Density functional theory (DFT) calculations indicate that the ultrathin graphene shells strongly promote electron penetration from the CoNi nanoalloy to the graphene surface. With nitrogen dopants, they synergistically increase the electron density on the graphene surface, which results in superior HER activity on the graphene shells.

Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen

Upgrading Methane Sans Oxygen Direct routes to converting methane to higher hydrocarbons can allow natural gas to be used to provide chemical feedstocks. However, the reaction conditions needed to activate the strong C-H bond tend to overoxidize the products. Guo et al. (p. 616) report a high-temperature nonoxidative route that exposes methane to isolated iron sites on a silica catalyst. Methyl radicals were generated and coupled in the gas phase to form ethylene and aromatics along with hydrogen. The isolation of the active sites avoided surface reactions between the radicals that would deposit solid carbon. Methyl radicals that form at isolated iron sites in a silica matrix form gas-phase products and do not deposit solid carbon. The efficient use of natural gas will require catalysts that can activate the first C–H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test.

Highly active and durable non-precious-metal catalysts encapsulated in carbon nanotubes for hydrogen evolution reaction

Employing a low-cost and highly efficient electrocatalyst to replace Pt-based catalysts for hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Earth-abundant transition metals such as Fe, Co and Ni have been investigated as promising alternatives in alkaline electrolytes. However, these non-precious-metal catalysts are not stable in acids, excluding their application in the acidic solid polymer electrolyte (SPE). Herein, we report a strategy to encapsulate 3d transition metals Fe, Co and the FeCo alloy into nitrogen-doped carbon nanotubes (CNTs) and investigated their HER activity in acidic electrolytes. The optimized catalysts exhibited long-term durability and high activity with only an ∼70 mV onset overpotential vs. RHE which is quite close to that of the commercial 40% Pt/C catalyst, demonstrating the potential for the replacement of Pt-based catalysts. Density functional theory (DFT) calculations indicated that the introduction of metal and nitrogen dopants can synergistically optimize the electronic structure of the CNTs and the adsorption free energy of H atoms on CNTs, and therefore promote the HER with a Volmer–Heyrovsky mechanism.

Triggering the electrocatalytic hydrogen evolution activity of the inert two-dimensional MoS2 surface via single-atom metal doping

Electrocatalytic splitting of water is one of the most efficient technologies for hydrogen production, and two-dimensional (2D) MoS2 has been considered as a potential alternative to Pt-based catalysts in the hydrogen evolution reaction (HER). However, the catalytic activity of 2D MoS2 is always contributed from its edge sites, leaving a large number of in-plane domains useless. Herein, we for the first time demonstrated that the catalytic activity of in-plane S atoms of MoS2 can be triggered via single-atom metal doping in HER. In experiments, single Pt atom-doped, few-layer MoS2 nanosheets (Pt–MoS2) showed a significantly enhanced HER activity compared with pure MoS2, originating from the tuned adsorption behavior of H atoms on the in-plane S sites neighboring the doped Pt atoms, according to the density functional theory (DFT) calculations. Furthermore, the HER activity of MoS2 doped with a number of transition metals was screened by virtue of DFT calculations, resulting in a volcano curve along the adsorption free energy of H atoms , which was further confirmed in experiment by using non-precious metals such as Co and Ni atoms doping 2D MoS2 as the catalysts.

Podlike N‐Doped Carbon Nanotubes Encapsulating FeNi Alloy Nanoparticles: High‐Performance Counter Electrode Materials for Dye‐Sensitized Solar Cells

Podlike nitrogen-doped carbon nanotubes encapsulating FeNi alloy nanoparticles (Pod(N)-FeNi) were prepared by the direct pyrolysis of organometallic precursors. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization measurements revealed their excellent electrocatalytic activities in the I(-)/I3(-) redox reaction of dye-sensitized solar cells (DSSCs). This is suggested to arise from the modification of the surface electronic properties of the carbon by the encapsulated metal alloy nanoparticles (NPs). Sequential scanning with EIS and CV further showed the high electrochemical stability of the Pod(N)-FeNi composite. DSSCs with Pod(N)-FeNi as the counter electrode (CE) presented a power conversion efficiency of 8.82%, which is superior to that of the control device with sputtered Pt as the CE. The Pod(N)-FeNi composite thus shows promise as an environmentally friendly, low-cost, and highly efficient CE material for DSSCs.

Single layer graphene encapsulating non-precious metals as high-performance electrocatalysts for water oxidation

The oxygen evolution reaction (OER) is recognized as a key half-reaction in water electrolysis for clean hydrogen energy, which is kinetically not favored and usually requires precious metal catalysts such as IrO2 and RuO2 to reduce the overpotential. The major challenge in using non-precious metals in place of these precious metal catalysts for OER is their low efficiency and poor stability, which urgently demand new concepts and strategies to tackle this issue. Herein, we report a universal strategy to directly synthesize single layer graphene encapsulating uniform earth-abundant 3d transition-metal nanoparticles such as Fe, Co, Ni and their alloys in a confined channel of mesoporous silica. The single atomic thickness of the graphene shell immensely promotes the electron transfer from the encapsulated metals to the graphene surface, which efficiently optimizes the electronic structure of the graphene surface and thereby triggers the OER activity of the inert graphene surface. We investigated a series of non-precious 3d metals encapsulated within single layer graphene, and found that the encapsulated FeNi alloy showed the best OER activity in alkaline solutions with only 280 mV overpotential at 10 mA cm−2, and also possessed a high durability after 10 000 cycles. Both the activity and durability of the non-precious catalyst even exceed that of the commercial IrO2 catalyst, showing great potential to replace precious metal catalysts in the OER.

A single iron site confined in a graphene matrix for the catalytic oxidation of benzene at room temperature.

A coordinatively unsaturated single iron site confined in a graphene matrix shows an ultrahigh activity for catalytic oxidation. Coordinatively unsaturated (CUS) iron sites are highly active in catalytic oxidation reactions; however, maintaining the CUS structure of iron during heterogeneous catalytic reactions is a great challenge. Here, we report a strategy to stabilize single-atom CUS iron sites by embedding highly dispersed FeN4 centers in the graphene matrix. The atomic structure of FeN4 centers in graphene was revealed for the first time by combining high-resolution transmission electron microscopy/high-angle annular dark-field scanning transmission electron microscopy with low-temperature scanning tunneling microscopy. These confined single-atom iron sites exhibit high performance in the direct catalytic oxidation of benzene to phenol at room temperature, with a conversion of 23.4% and a yield of 18.7%, and can even proceed efficiently at 0°C with a phenol yield of 8.3% after 24 hours. Both experimental measurements and density functional theory calculations indicate that the formation of the Fe═O intermediate structure is a key step to promoting the conversion of benzene to phenol. These findings could pave the way toward highly efficient nonprecious catalysts for low-temperature oxidation reactions in heterogeneous catalysis and electrocatalysis.

Size effect of graphene on electrocatalytic activation of oxygen

Crystals of graphite nanosheets, achieved via a simple ball milling approach, show a significant size effect in electrocatalytic activation of oxygen.