Haobo Li

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.

Selective conversion of syngas to light olefins.

Small olefins from syngas The conversion of coal or natural gas to liquid fuels or chemicals often proceeds through the production of CO and H2. This mixture, known as syngas, is then converted to hydrocarbons with Fischer-Tropsch catalysts. For the light olefins (ethylene to butylenes) needed for chemical and polymer synthesis, conventional catalysts are mechanistically limited to <60% conversion and deactivate through carbon buildup. Jiao et al. developed a bifunctional catalyst that achieves higher conversions and avoids deactivation (see the Perspective by de Jong). A zinc-chromium oxide creates ketene intermediates that are then coupled over a zeolite. Science, this issue p. 1065, see also p. 1030 A composite catalyst circumvents conventional limitations on the Fischer-Tropsch synthesis of light olefins from syngas. [Also see Perspective by de Jong] Although considerable progress has been made in direct synthesis gas (syngas) conversion to light olefins (C2=–C4=) via Fischer-Tropsch synthesis (FTS), the wide product distribution remains a challenge, with a theoretical limit of only 58% for C2–C4 hydrocarbons. We present a process that reaches C2=–C4= selectivity as high as 80% and C2–C4 94% at carbon monoxide (CO) conversion of 17%. This is enabled by a bifunctional catalyst affording two types of active sites with complementary properties. The partially reduced oxide surface (ZnCrOx) activates CO and H2, and C−C coupling is subsequently manipulated within the confined acidic pores of zeolites. No obvious deactivation is observed within 110 hours. Furthermore, this composite catalyst and the process may allow use of coal- and biomass-derived syngas with a low H2/CO ratio.

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.

Multiscale structural and electronic control of molybdenum disulfide foam for highly efficient hydrogen production

Hydrogen production through water splitting has been considered as a green, pure and high-efficient technique. As an important half-reaction involved, hydrogen evolution reaction is a complex electrochemical process involving liquid-solid-gas three-phase interface behaviour. Therefore, new concepts and strategies of material design are needed to smooth each pivotal step. Here we report a multiscale structural and electronic control of molybdenum disulfide foam to synergistically promote the hydrogen evolution process. The optimized three-dimensional molybdenum disulfide foam with uniform mesopores, vertically aligned two-dimensional layers and cobalt atoms doping demonstrated a high hydrogen evolution activity and stability. In addition, density functional theory calculations indicate that molybdenum disulfide with moderate cobalt doping content possesses the optimal activity. This study demonstrates the validity of multiscale control in molybdenum disulfide via overall consideration of the mass transport, and the accessibility, quantity and capability of active sites towards electrocatalytic hydrogen evolution, which may also be extended to other energy-related processes.

Hexagonal Boron Nitride Cover on Pt(111): A New Route to Tune Molecule-Metal Interaction and Metal-Catalyzed Reactions

In heterogeneous catalysis molecule-metal interaction is often modulated through structural modifications at the surface or under the surface of the metal catalyst. Here, we suggest an alternative way toward this modulation by placing a two-dimensional (2D) cover on the metal surface. As an illustration, CO adsorption on Pt(111) surface has been studied under 2D hexagonal boron nitride (h-BN) overlayer. Dynamic imaging data from surface electron microscopy and in situ surface spectroscopic results under near ambient pressure conditions confirm that CO molecules readily intercalate monolayer h-BN sheets on Pt(111) in CO atmosphere but desorb from the h-BN/Pt(111) interface even around room temperature in ultrahigh vacuum. The interaction of CO with Pt has been strongly weakened due to the confinement effect of the h-BN cover, and consequently, CO oxidation at the h-BN/Pt(111) interface was enhanced thanks to the alleviated CO poisoning effect.

Highly doped and exposed Cu(I)–N active sites within graphene towards efficient oxygen reduction for zinc–air batteries

A coordinatively unsaturated copper–nitrogen architecture in copper metalloenzymes is essential for its capability to catalyze the oxygen reduction reaction (ORR). However, the stabilization of analogous active sites in realistic catalysts remains a key challenge. Herein, we report a facile route to synthesize highly doped and exposed copper(I)–nitrogen (Cu(I)–N) active sites within graphene (Cu–N©C) by pyrolysis of coordinatively saturated copper phthalocyanine, which is inert for the ORR, together with dicyandiamide. Cu(I)–N is identified as the active site for catalyzing the ORR by combining physicochemical and electrochemical studies, as well as density function theory calculations. The graphene matrix could stabilize the high density of Cu(I)–N active sites with a copper loading higher than 8.5 wt%, while acting as the electron-conducting path. The ORR activity increases with the specific surface area of the Cu–N©C catalysts due to more exposed Cu(I)–N active sites. The optimum Cu–N©C catalyst demonstrates a high ORR activity and stability, as well as an excellent performance and stability in zinc–air batteries with ultralow catalyst loading.

Confined catalysis under two-dimensional materials

Significance Small spaces in nanoreactors may have big implications in chemistry, because the chemical nature of molecules and reactions within the nanospaces can be changed significantly due to the nanoconfinement effect. Two-dimensional (2D) nanoreactor formed under 2D materials can provide a well-defined model system to explore the confined catalysis. We demonstrate a general tendency for weakened surface adsorption under the confinement of graphene overlayer, illustrating the feasible modulation of surface reactions by placing a 2D cover on top of the surface. The developed concept “catalysis under cover” can be applied to reactions between two opposite 2D walls interacting with each other through van der Waals force, which helps to design high-performance nanocatalysts interfacing with 2D material overlayers. Confined microenvironments formed in heterogeneous catalysts have recently been recognized as equally important as catalytically active sites. Understanding the fundamentals of confined catalysis has become an important topic in heterogeneous catalysis. Well-defined 2D space between a catalyst surface and a 2D material overlayer provides an ideal microenvironment to explore the confined catalysis experimentally and theoretically. Using density functional theory calculations, we reveal that adsorption of atoms and molecules on a Pt(111) surface always has been weakened under monolayer graphene, which is attributed to the geometric constraint and confinement field in the 2D space between the graphene overlayer and the Pt(111) surface. A similar result has been found on Pt(110) and Pt(100) surfaces covered with graphene. The microenvironment created by coating a catalyst surface with 2D material overlayer can be used to modulate surface reactivity, which has been illustrated by optimizing oxygen reduction reaction activity on Pt(111) covered by various 2D materials. We demonstrate a concept of confined catalysis under 2D cover based on a weak van der Waals interaction between 2D material overlayers and underlying catalyst surfaces.

Creating a Nanospace under an h-BN Cover for Adlayer Growth on Nickel(111)

Heterostructures of two-dimensional (2D) atomic crystals have attracted increasing attention, while fabrication of the 2D stacking structures remains a challenge. In this work, we present a route toward formation of 2D heterostructures via confined growth of a 2D adlayer underneath the other 2D overlayer. Taking a hexagonal boron nitride (h-BN) monolayer on Ni(111) as a model system, both epitaxial and nonepitaxial h-BN islands have been identified on the Ni surface. Surface science studies combined with density functional theory calculations reveal that the nonepitaxial h-BN islands interact weakly with the Ni(111) surface, which creates a 2D nanospace underneath the h-BN islands. An additional h-BN or graphene layer can be grown in the space between the nonepitaxial h-BN islands and Ni(111) surface, forming h-BN/h-BN bilayer structures and h-BN/graphene heterostructures. These results suggest that confined growth under 2D covers may provide an effective route to obtain stacks of 2D atomic crystals.

Coordinatively unsaturated nickel–nitrogen sites towards selective and high-rate CO2 electroreduction

High Faradaic efficiency and appreciable current density are essential for future applications of the electrochemical CO2 reduction reaction (CO2RR). However, these goals are difficult to achieve simultaneously due to the severe side reaction – the hydrogen evolution reaction (HER). Herein, we successfully synthesized coordinatively unsaturated nickel–nitrogen (Ni–N) sites doped within porous carbon with a nickel loading as high as 5.44 wt% by pyrolysis of Zn/Ni bimetallic zeolitic imidazolate framework-8. Over the Ni–N composite catalysts, the CO current density increases with the overpotential and reaches 71.5 ± 2.9 mA cm−2 at −1.03 V (vs. a reversible hydrogen electrode, RHE), while maintaining a high CO Faradaic efficiency of 92.0–98.0% over a wide potential range of −0.53 to −1.03 V (vs. the RHE). Density functional theory calculations suggest that the CO2RR occurs more easily than the HER over the coordinatively unsaturated Ni–N site. Therefore, highly doped and coordinatively unsaturated Ni–N sites achieve high current density and Faradaic efficiency of the CO2RR simultaneously, breaking current limits in metal–nitrogen composite catalysts.

Highly efficient catalytic scavenging of oxygen free radicals with graphene-encapsulated metal nanoshields

Normal levels of oxygen free radicals play an important role in cellular signal transduction, redox homeostasis, regulatory pathways, and metabolic processes. However, radiolysis of water induced by high-energy radiation can produce excessive amounts of exogenous oxygen free radicals, which cause severe oxidative damages to all cellular components, disrupt cellular structures and signaling pathways, and eventually lead to death. Herein, we show that hybrid nanoshields based on single-layer graphene encapsulating metal nanoparticles exhibit high catalytic activity in scavenging oxygen superoxide(·