Confinement Effects in Well-Defined Metal-Organic Frameworks (MOFs) for Selective CO2 Hydrogenation: A Review

Decarbonization has become an urgent affair to restrain global warming. CO₂ hydrogenation coupled with H₂ derived from water electrolysis is considered a promising route to mitigate the negative impact of carbon emission and also promote the application of hydrogen. It is of great significance to develop catalysts with excellent performance and large-scale implementation. In the past decades, metal-organic frameworks (MOFs) have been widely involved in the rational design of catalysts for CO₂ hydrogenation due to their high surface areas, tunable porosities, well-ordered pore structures, and diversities in metals and functional groups. Confinement effects in MOFs or MOF-derived materials have been reported to promote the stability of CO₂ hydrogenation catalysts, such as molecular complexes of immobilization effect, active sites in size effect, stabilization in the encapsulation effect, and electron transfer and interfacial catalysis in the synergistic effect. This review attempts to summarize the progress of MOF-based CO₂ hydrogenation catalysts up to now, and demonstrate the synthetic strategies, unique features, and enhancement mechanisms compared with traditionally supported catalysts. Great emphasis will be placed on various confinement effects in CO₂ hydrogenation. The challenges and opportunities in precise design, synthesis, and applications of MOF-confined catalysis for CO₂ hydrogenation are also summarized.

Functionalization of Mesoporous Semiconductor Metal Oxides for Gas Sensing: Recent Advances and Emerging Challenges

With the emerging of the Internet of Things, chemiresistive gas sensors have been extensively applied in industrial production, food safety, medical diagnosis, and environment detection, etc. Considerable efforts have been devoted to improving the gas-sensing performance through tailoring the structure, functions, defects and electrical conductivity of sensitive materials. Among the numerous sensitive materials, mesoporous semiconductor metal oxides possess unparalleled properties, including tunable pore size, high specific surface area, abundant metal-oxygen bonds, and rapid mass transfer/diffusion behavior (Knudsen diffusion), which have been regarded as the most potential sensitive materials. Herein, the synthesis strategies for mesoporous metal oxides are overviewed, the classical functionalization techniques of sensitive materials are also systemically summarized as a highlight, including construction of mesoporous structure, regulation of micro-nano structure (i.e., heterojunctions), noble metal sensitization (e.g., Au, Pt, Ag, Pd) and heteroatomic doping (e.g., C, N, Si, S). In addition, the structure-function relationship of sensitive materials has been discussed at molecular-atomic level, especially for the chemical sensitization effect, elucidating the interface adsorption/catalytic mechanism. Moreover, the challenges and perspectives are proposed, which will open a new door for the development of intelligent gas sensor in various applications.

Ni-Foam-Structured Ni-Al2O3 Ensemble as an Efficient Catalyst for Gas-Phase Acetone Hydrogenation to Isopropanol

The free-standing Ni-Al₂O₃ ensemble derived from NiAl-layered double hydroxides (NiAl-LDHs) grown onto a Ni-foam has been developed for the exothermic gas-phase acetone hydrogenation to isopropanol. This approach works effectively and efficiently to achieve a unique combination of high activity/selectivity and enhanced heat/mass transfer stemmed from the Ni-foam. The outstanding catalyst is obtained by direct reduction of the un-calcined NiAl-LDH/Ni-foam, with a high turnover frequency of 0.90 s^-1, being capable of converting 90.8% acetone into isopropanol with almost 100% selectivity under stoichiometric H₂/acetone molar ratio, atmospheric pressure at 80 °C, and a WHSV_acetone of 10 h^-1. The catalyst derivation using the un-calcined NiAl-LDH/Ni-foam enables the Ni nanoparticles to be intertwined with Al₂O₃ to form a large Ni-Al₂O₃ interface, without interruption of impurities such as irreducible NiO (in the case of calcined NiAl-LDH/Ni-foam samples), which markedly improves the strong acetone adsorption next to the Ni⁰ hydrogenation sites, thereby leading to a dramatic improvement of catalyst activity.

Shaping Nanoparticles for Interface Catalysis: Concave Hollow Spheres via Deflation-Inflation Asymmetric Growth

Hollow spheres are charming objects in nature. In this work, an unexpected deflation-inflation asymmetric growth (DIAG) strategy is reported, generating hollow nanoparticles with tailored concave geometry for interface catalysis. Starting from aminophenol-formaldehyde (APF) nanospheres where the interior crosslinking degree is low, fully deflated nanobowls are obtained after etching by acetone. Due to APF etching and repolymerization reactions occuring asymmetrically within a single particle, an autonomous inflation process is observed similar to a deflated basketball that inflates back to a "normal" ball, which is rare at the nanoscale. A nucleophilic addition reaction between acetone and APF is elucidated to explain the chemistry origin of the DIAG process. Interestingly, the deflated APF hollow spheres enable preferential immobilization of lipase in the concave domain, which facilitates the stabilization of Pickering emulsion droplets for enhanced enzymatic catalysis at the oil-water interface. The study provides new understandings in the designable synthesis of hollow nanoparticles and paves the way toward a wide range of applications of asymmetric architectures.

Oxygen Vacancies Dominated NiS2 /CoS2 Interface Porous Nanowires for Portable Zn-Air Batteries Driven Water Splitting Devices

The development of highly active and stable oxygen evolution reaction (OER) electrocatalysts is crucial for improving the efficiency of water splitting and metal-air battery devices. Herein, an efficient strategy is demonstrated for making the oxygen vacancies dominated cobalt-nickel sulfide interface porous nanowires (NiS₂ /CoS₂ -O NWs) for boosting OER catalysis through in situ electrochemical reaction of NiS₂ /CoS₂ interface NWs. Because of the abundant oxygen vacancies and interface porous nanowires structure, they can catalyze the OER efficiently with a low overpotential of 235 mV at j = 10 mA cm^-2 and remarkable long-term stability in 1.0 m KOH. The home-made rechargeable portable Zn-air batteries by using NiS₂ /CoS₂ -O NWs as the air-cathode display a very high open-circuit voltage of 1.49 V, which can maintain for more than 30 h. Most importantly, a highly efficient self-driven water splitting device is designed with NiS₂ /CoS₂ -O NWs as both anode and cathode, powered by two-series-connected NiS₂ /CoS₂ -O NWs-based portable Zn-air batteries. The present work opens a new way for designing oxygen vacancies dominated interface nanowires as highly efficient multifunctional electrocatalysts for electrochemical reactions and renewable energy devices.

Reaction-Induced Self-Assembly of CoO@Cu2 O Nanocomposites In Situ onto SiC-Foam for Gas-Phase Oxidation of Bioethanol to Acetaldehyde

A high-performance SiC-foam-structured nanocomposite catalyst of CoO@Cu₂ O (i.e., 50-100 nm CoO partially covered with ca. 10 nm Cu₂ O) was engineered from nano- to macro-scales in one step for the high-throughput gas-phase aerobic oxidation of bioethanol to acetaldehyde. This special CoO@Cu₂ O nanostructure shows much higher activity/selectivity than other binary metal-oxide assemblies such as CuO_x &CoO nano-mixtures or inverse Cu₂ O@CoO nanostructures. The catalyst was facilely but exclusively obtainable by in situ reaction-induced transformation of the respective metal nitrates supported on SiC-foam into the CoO@Cu₂ O nanostructure in the reaction stream. It achieved 95 % conversion with 98 % selectivity under mild conditions and was stable for at least 150 h for a feed of 20 vol % ethanol (much higher than in the literature: 1-6 vol %) at a high EtOH weight hourly space velocity of 8.5 h^-1 . Abundant Cu₂ O-CoO interfaces and high stability of the CoO@Cu₂ O nanostructure were responsible for the high activity/selectivity and promising stability in this reaction.

Graphene cover-promoted metal-catalyzed reactions

Graphitic overlayers on metals have commonly been considered as inhibitors for surface reactions due to their chemical inertness and physical blockage of surface active sites. In this work, however, we find that surface reactions, for instance, CO adsorption/desorption and CO oxidation, can take place on Pt(111) surface covered by monolayer graphene sheets. Surface science measurements combined with density functional calculations show that the graphene overlayer weakens the strong interaction between CO and Pt and, consequently, facilitates the CO oxidation with lower apparent activation energy. These results suggest that interfaces between graphitic overlayers and metal surfaces act as 2D confined nanoreactors, in which catalytic reactions are promoted. The finding contrasts with the conventional knowledge that graphitic carbon poisons a catalyst surface but opens up an avenue to enhance catalytic performance through coating of metal catalysts with controlled graphitic covers.