More than 95% (in volume) of all of today’s chemical products are manufactured through catalytic processes, making research into more efficient catalytic materials a thrilling and very dynamic ...research field. In this regard, metal–organic frameworks (MOFs) offer great opportunities for the rational design of new catalytic solids, as highlighted by the unprecedented number of publications appearing over the past decade. In this review, the recent advances in the application of MOFs in heterogeneous catalysis are discussed. MOFs with intrinsic thermocatalytic activity, as hosts for the incorporation of metal nanoparticles, as precursors for the manufacture of composite catalysts and those active in photo- and electrocatalytic processes are critically reviewed. The review is wrapped up with our personal view on future research directions.
Hydrogen storage in the form of intermediate artificial fuels such as methanol is important for future chemical and energy applications, and the electrochemical regeneration of hydrogen from methanol ...is thermodynamically favorable compared to direct water splitting. However, CO produced from methanol oxidation can adsorb to H2‐evolution catalysts and drastically reduce activity. In this study, we explore the origins of CO immunity in Mo‐containing H2‐evolution catalysts. Unlike conventional catalysts such as Pt or Ni, Mo‐based catalysts display remarkable immunity to CO poisoning. The origin of this behavior in NiMo appears to arise from the apparent inability of CO to bind Mo under electrocatalytic conditions, with mechanistic consequences for the H2‐evolution reaction (HER) in these systems. This specific property of Mo‐based HER catalysts makes them ideal in environments where poisons might be present.
Finding an antidote: Electrochemical methanol reforming to hydrogen is a simple, yet underdeveloped, approach for hydrogen storage; however, conventional metal catalysts are poisoned either by methanol crossover or by intermediate species (such as CO) from the corresponding methanol oxidation. Herein, the relative immunity of Mo‐based catalysts to CO is demonstrated, and the origins of this behavior are explored, with surprising implications for the general mechanism of H2 evolution on Mo‐based catalysts.
Semiconductor systems for photocatalytic overall water splitting into H2 and O2 gases typically require metal cocatalyst particles, such as Pt, to efficiently catalyze H2 evolution. However, such ...metal catalyst surfaces also serve as recombination sites for H2 and O2, forming H2O. We herein report the photon-induced fabrication of microporous SiO2 membranes that can selectively restrict passage of O2 and larger hydrated ions while allowing penetration of protons, water, and H2. The SiO2 layers were selectively photodeposited on Pt nanoparticles on SrTiO3 photocatalyst by using tetramethylammonium (TMA) as a structure-directing agent (SDA), resulting in the formation of core–shell Pt@SiO2 cocatalysts. The resulting photocatalyst exhibited both improved overall water splitting performance under irradiation and with no H2/O2 recombination in the dark. The function of the SiO2 layers was investigated electrochemically by fabricating the SiO2 layers on a Pt electrode via an analogous cathodic deposition protocol. The uniform, dense, yet amorphous layers possess microporosity originating from ring structures formed during the hydrolysis of the silicate precursor in the presence of TMA, suggesting a double-role for TMA in coordinating silicate to cathodic surfaces and in creating a microporous material. The resulting layers were able to function as a molecular sieve, allowing for exclusive H2 generation while excluding unwanted side reactions by O2 or ferricyanide. The SiO2 layer is stable for extended periods of time in photocatalytic conditions, demonstrating promise as a nontoxic material for selective H2 evolution.
A Ni–Mo composite functions as a promising non-noble metal electrocatalyst for the hydrogen evolution reaction (HER) in alkaline water. Despite its industrial relevance, the kinetic origin of the ...high catalytic activity remains under debate. The present report discusses a reaction mechanism of HER on Ni–Mo catalysts by combining experimental and theoretical studies. In contrast to a Ni catalyst, a Ni–Mo catalyst is insensitive to CO gas introduced during HER. In situ spectroscopic measurements including Raman spectroscopy and electron paramagnetic resonance (EPR) show that Mo3+ prevails during HER catalysis. Density functional theory (DFT) simulations corroborate the thermodynamic stability and HER activity of Mo3+-containing centers on Ni(111) at HER potentials. Notably, Ni is demonstrated to play no direct role as a catalytic site but to effectively disperse and activate the oxidized catalytic Mo species. The results illustrate how to improve the electrocatalytic activity for alkaline HER.
The field of heterogeneous photoredox catalysis has grown substantially and impacted organic synthesis because of the affordability and reusability of catalysts. This study reports radical ...trifluoromethylation with Cd–chalcogenide semiconductors. Cd semiconductors, particularly CdSe, are readily available, commercial, visible-light-responsive, heterogeneous photocatalysts. The potential of readily available Cd semiconductors, particularly CdSe, is confirmed by their increased photocatalytic activity toward trifluoromethylation with various substrates, such as (hetero)arenes and vinylic amides/acids, via addition, cyclization, and decarboxylation under visible light. The economic significance of this strategy is also highlighted through the scalable synthesis of biologically active molecules followed by catalyst reuse. Moreover, these catalysts are relatively inexpensive compared with transition metal-based homogeneous photocatalysts, presently used in organic synthesis.
The CO2 (dry) reforming of hydrocarbons offers an opportunity to convert greenhouse gases into synthesis gas, which can further transform to various valued products. Here we explore the influence of ...Rh particle size and support on the reforming of propane and methane. To that end, Rh nanoparticles with controlled sizes varying from 1.6‐8.0 nm were synthesized following a polyol reduction method and then dispersed on three different solids: CeZrO2, ZrO2, and CeO2. Catalytic turnover rates along with advanced characterization of fresh and spent catalysts reveal a linear correlation of turnover rates with Rh particle size for both methane and propane reforming. The nature and rate of coke deposition are highly dependent on the support used and its interaction with the metallic phase.
The influence of Rh particle size when supported on three different solids: CeZrO2, ZrO2, and CeO2 has been explored for the reforming of propane and methane. Catalytic turnover rates along with advanced characterization of fresh and spent catalysts reveal a linear correlation of turnover rates with Rh particle size for both methane and propane reforming. The nature and speed of coke deposition are highly dependent on the support used and its interaction with the metallic phase.
The use of spectroscopy to characterize electrocatalytic processes is vital to the understanding and continuing the development of new catalysts for clean energy transformations. Electron ...paramagnetic resonance spectroscopy (EPR), which allows for the study of unpaired electron spins, shows great fundamental promise for the study of electrocatalysts, but was previously hindered by design limitations. Recently, several groups have demonstrated that these limitations can be overcome, providing valuable understandings of electrocatalyst function that other techniques are less suitable for. In this review, we summarize these findings across a range of experimental approaches and systems and describe the importance of EPR to each of these studies. By providing outlines for how these studies were able to overcome experimental design challenges, we hope to provide insight into potentially interested users.
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Chemistry; Materials science; Materials chemistry
Mixed nickel iron oxide nanomaterials have great potential for use in fields as wide-ranging as optics, water oxidation catalysis, and biomedical applications; large-scale deployment for any of these ...applications is possible due to the abundance of iron and nickel in the earth’s crust. Here, we describe single-crystal mixed nickel iron oxide nanoparticles with a rocksalt crystal structure and an iron content greater than >10 at%. The nanoparticles were synthesized via thermal decomposition of nickel and iron oleates, yielding monodisperse nanoparticles <12 nm in size. A range of compositions were accessible, ranging from iron-poor to iron-rich. Despite the predicted poor solubility of iron in the NiO lattice, as based upon the equilibrium phase diagram, and the general tendency of pure Fe x O to oxidize, the single-crystal nanoparticles are nonetheless kinetically persistent even at elevated temperatures (200 °C). Furthermore, by controlling the drying conditions and decomposition time of the oleate precursors as well as the initial ratio of nickel to iron in the precursor oleate mixture a variety of nanoparticle shapes–namely stars, cubes, and spheres–were formed. This control of shape and composition affords tunability of physical properties, exemplified by magnetic properties herein.
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