Catalytic processes support the chemical industry—they account for more than 85% of global chemical production. Heterogeneous catalysts operating at the liquid–solid or gas–solid interface prevail in modern industrial catalysis that converts raw feedstocks into valuable chemicals and fuels.
The surface–interface structure of heterogeneous catalysts dominates their catalytic performance. Therefore, any improvement requires a thorough understanding of active sites, or ensembles of atoms, at surfaces and interfaces. The rational design of active sites for heterogeneous catalysis remains a challenge, mainly because of the complex structure–activity relationship in most systems. In addition, activation and deactivation dynamics can modify active sites.
Recent advances in nanoscience have opened the way for synthesizing advanced nanomaterials with controllable morphologies and electronic properties and abundant surfaces or interfaces for adsorption and reaction. These tailored nanomaterials are an ideal platform for developing new heterogeneous substances for catalytic, photocatalytic, and electrochemical conversions.
Tierui Zhang at the Technical Institute of Physics and Chemistry of Chinese Academy of Sciences (Beijing) and his colleagues there and at several other institutions (see “Institutions Contributing to this Work”) explored this strategy to develop a series of heterogeneous catalysts with high specific surface areas and tailored active sites. The catalysts exhibit outstanding catalytic performance in CO2 photoreduction, the electrochemical oxygen-reduction reaction (ORR), and visible-light–driven water splitting.
Institutions Contributing to this Work
Technical Institute of Physics and Chemistry of Chinese Academy of Sciences (Beijing)
Northwest University (Xi’an, China)
University of Auckland (New Zealand)
University of Oxford (UK)
University of Chinese Academy of Sciences (Beijing)
Beijing Institute of Technology
Tailoring surface defects for CO2 photoreduction
Surface defects play an important role in catalysis and photocatalysis. They consist of surface reaction sites with highly unsaturated valences that promote reactant adsorption. Defects strongly influence the surface electronic structure and reactivity of catalytic materials; in photocatalytic materials, they affect the efficiency of charge separation and photocatalyst performance.
Layered materials, such as ultrathin nanosheets with thicknesses approaching atomic dimensions, possess abundant coordinatively unsaturated metal defect sites, which can serve as active sites for catalytic transformations. The large specific surface areas of ultrathin nanosheets also maximize the active sites’ availability to reactants bypassing traditional diffusion barriers that limit the performance of conventional 3-D catalysts and photocatalysts.
Zhang and his colleagues synthesized ultrathin zinc-containing layered double hydroxide (LDH) nanosheets with coordinatively unsaturated zinc defects (yellow circles in Figure 1). Under UV–vis irradiation, the high surface area of ultrathin ZnAl-LDH nanosheets showed excellent activity for the photoreduction of CO2 to CO in the presence of water vapor. The catalytic activity of ZnAl-LDH was ≈20 times greater than that of a commercial ZnO nanoparticle reference photocatalyst.
The researchers introduced coordinatively unsaturated zinc ions by increasing the density of oxygen-vacancy defects (Vo) surrounding the cations. To achieve the high Vo concentrations, they decreased the lateral dimension of ZnAl-LDH sheets from 5 μm to 40 nm and then further reduced the nanoplatelet thickness to ≈2 structural layers. Advanced characterization measurements provided clear evidence for the formation of Zn+-Vo complexes in these ultrathin ZnAl-LDH nanosheets. Density functional theory (DFT) calculations showed that the coordinatively unsaturated zinc centers serve as trapping sites to efficiently promote the adsorption of CO2 on the surface of LDH nanosheets. The sites also facilitate electron transfer to the adsorbate, thereby enhancing the rates of photocatalytic CO2 reduction to CO in the presence of water vapor.
This work provides a solid platform for development of defect-containing catalysts with high specific surface areas for efficient CO2 photoreduction and other applications. (Adv. Mater. DOI: 10.1002/adma.201503730; Adv. Energy Mater. DOI:10.1002/aenm201501974)
Tailored surfaces and interfaces for ORR
Metal- and nitrogen-doped carbon (M,N-C) catalysts are the most promising alternatives to Pt/C for fuel cell ORR catalysts. The catalytic performance of conventional M,N-C electrocatalysts is limited, however, by the aggregation and fusion of catalyst nanoparticles under the pyrolysis conditions used to synthesize them. Nanoparticle fusion severely reduces the concentration of active sites available for electrochemical reactions, thereby limiting the overall electrochemical activity.
To overcome that limitation, Zhang and his coauthors developed a mesoporous silica protection strategy to inhibit the fusion of catalyst nanoparticles during the pyrolysis of a metal–organic framework, the cobalt-substituted zeolite imidazolate framework ZIF-8. This protection strategy yielded high specific surface area Co,N-C products with outstanding ORR catalytic activity, superior to Pt/C at the same loading in alkaline media and comparable with Pt/C in acidic media. Figure 2 shows the Raman spectrum of Co,N-C; the peaks at 1580 and 1352 cm–1 confirm the presence of graphitic sp2 carbon and disordered sp3 carbon, respectively.
Rapid interfacial electron transfer between the highly dispersed Co,N-C catalyst and the supporting electrode was responsible for the high rates. The group’s strategy improves the specific surface area of M,N-C electrocatalysts for ORR and avoids the fusion of catalyst particles that typically plagues high-temperature pyrolysis syntheses. (Adv. Mater. DOI: 10.1002/adma.201505045)
What’s a Schottky Junction?
A p–n junction, defined as an interface between p- and n-type semiconductor materials, is the fundamental building block for semiconductor-based electronics. In cases in which a metal plays the role of the p-type semiconductor, it is known as a Schottky junction and shows nonlinear resistance. The junction was identified by German physicist Walter H. Schottky (1886–1976), but junctions of this type were known before he was born.
Optimized interfaces for superior electron transfer in water splitting
In heterogeneous catalyst systems, the metal–support interface controls the availability of active sites and the rate of electron transfer between the metal and its support. Zhang’s group recently developed photocatalytic systems with highly efficient photoelectron-generated charge transfer across the metal–support interface.
The team first prepared a near-perfect CdS-Cd Schottky junction by synthesizing cadmium metal nanosheets, then partially oxidizing the metal to CdO and converting the CdO to CdS (see “What’s a Schottky Junction?”). This CdS-Cd junction promoted superior visible-light–driven photocatalytic hydrogen production performance when compared with CdS, graphene-CdS, and carbon nanotube–CdS nanostructures.
The authors attribute this enhancement to the intimate CdS contact with cadmium, which allows improved photogenerated electron separation and transfer.
This strategy is not only applicable to Cd-based systems, but also could be widely applied to other photocatalyst or electrocatalyst systems that contain titanium, zinc, tantalum, or nickel as a way to achieve more efficient electron transfer. (Adv. Energy Mater. DOI: 10.1002/aenm.20151501241)
Pathways to practical industrial nanocatalysts?
The pioneering work of Zhang and his colleagues offers promising new directions in the structural design and straightforward synthesis of highly efficient catalysts for renewable energy applications. The strategy takes full advantage of surface and interfacial phenomena at the nanoscale for achieving improved reactant adsorption and charge transfer.
This work also provides important insights into complex catalytic phenomena at the atomic level; and it may eventually lead to the rational design of industrial heterogeneous catalysts. The need, however, remains to develop in situ characterization techniques that can be applied to catalysts and photocatalysts under real-world reaction conditions. Zhang acknowledged this when he told me, “We have come far recently, but there is still is a very long road ahead.”