Single-atom catalysts offer a pathway to cost-efficient catalysis using the minimal amount of precious metals. However, preparing and keeping them stable during operation remains a challenge. Here we report the synthesis of double transition metal MXene nanosheets—Mo2TiC2Tx, with abundant exposed basal planes and Mo vacancies in the outer layers—by electrochemical exfoliation, enabled by the interaction between protons and the surface functional groups of Mo2TiC2Tx. The as-formed Mo vacancies are used to immobilize single Pt atoms, enhancing the MXene’s catalytic activity for the hydrogen evolution reaction. The developed catalyst exhibits a high catalytic ability with low overpotentials of 30 and 77 mV to achieve 10 and 100 mA cm−2 and a mass activity about 40 times greater than the commercial platinum-on-carbon catalyst. The strong covalent interactions between positively charged Pt single atoms and the MXene contribute to the exceptional catalytic performance and stability. Single-atom catalysts are very attractive due to their ability to maintain high activities at the lowest possible precious metal loading. Here, a double transition metal MXene that effectively anchors single Pt atoms is reported, and exhibits superior performance and stability towards the hydrogen evolution reaction.

Single platinum atoms immobilized on an MXene as an efficient catalyst for the hydrogen evolution reaction

Recent advance in MXenes: A promising 2D material for catalysis, sensor and chemical adsorption

MXene, an important and increasingly popular category of postgraphene 2D nanomaterials, has been rigorously investigated since early 2011 because of advantages including flexible tunability in element composition, hydrophobicity, metallic nature, unique in-plane anisotropic structure, high charge-carrier mobility, tunable band gap, and favorable optical and mechanical properties. To fully exploit these potentials and further expand beyond the existing boundaries, novel functional nanostructures spanning monolayer, multilayer, nanoparticles, and composites have been developed by means of intercalation, delamination, functionalization, hybridization, among others. Undeniably, the cutting-edge developments and applications of clay-inspired 2D MXene platform as electrochemical electrode or photo-electrocatalyst have conferred superior performance and have made significant impact in the field of energy and advanced catalysis. This review provides an overview of the fundamental properties and synthesis routes of pure MXene, functionalized MXene and their hybrids, highlights the state-of-the-art progresses of MXene-based applications with respect to supercapacitors, batteries, electrocatalysis and photocatalysis, and presents the challenges and prospects in the burgeoning field.

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Clay-Inspired MXene-Based Electrochemical Devices and Photo-Electrocatalyst: State-of-the-Art Progresses and Challenges.

Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir–Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir–Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an in...

Molecular insight in structure and activity of highly efficient, low-Ir Ir-Ni oxide catalysts for electrochemical water splitting (OER)

The support plays an important role for supported metal catalysts by positioning itself as a macromolecular ligand, which conditions the nature of the active site and contributes indirectly but also sometimes directly to the reactivity. Metal species such as nanoparticles, clusters, or single atoms can be deposited on carbon materials for various catalytic reactions. All the carbon materials used as catalyst support constitute a large family of compounds that can vary both at textural and at structural levels. Today, the recent developments of well-controlled synthesis methodologies, advanced characterization techniques, and modeling tools allow one to correlate the relationships between metal/support/reactant at the molecular level. Based on these considerations, in this Review article, we wish to provide some answers to the question "How and why anchoring metal nanoparticles, clusters, or single atoms on carbon materials for catalysis?". To do this, we will rely on both experimental and theoretical studies. We will first analyze what sites are available on the surface of a carbon support for the anchoring of the active phase. Then, we will describe some important effects in catalysis inherent to the presence of a carbon-type support (metal-support interaction, confinement, spillover, and surface functional group effects). These effects will be commented on by putting into perspective catalytic results obtained in numerous reactions of thermal catalysis, electrocatalysis, or photocatalysis.

A Theory/Experience Description of Support Effects in Carbon-Supported Catalysts.

Exploring MXenes as supports for synthesis of highly efficient catalysts are extremely important for hydrodechlorination of chlorophenols because of the acute toxicity and strong potential bioaccumulation of chlorophenols. We reported a facile strategy to synthesize Rh nanoparticles deposited on 2D alk-Ti3C2X2 (X = O, F) (alk-Ti3C2X2 for short) MXene, which was prepared through the removal of the Al layers of Ti3AlC2 and post-treatment by alkalization. Modifying the Ti3C2X2 support by alkalization triggered the substitution of surface fluorine groups with surface oxygen species. The changes in the surface chemistry of alk-Ti3C2X2 effectively prevented particle aggregation to form well-dispersed Rh nanoparticles in small sizes with relative high metal dispersion, thereafter improving the hydrogen uptake capacity of the catalyst. Consequently, the Rh/alk-Ti3C2X2 catalyst significantly enhanced the catalytic performance for the HDC reaction, which could be assigned to the large number of active components from small Rh nanoparticles. This strategy provides a basis for investigating the use of MXenes as supports to synthesize well-defined metal nanoparticles in 2D layered structures for potential applications.

Promoted effect of alkalization on the catalytic performance of Rh/alk-Ti3C2X2 (XO, F) for the hydrodechlorination of chlorophenols in base-free aqueous medium

Hydrogen evolution from hydrolysis of ammonia borane catalyzed by Rh/g-C3N4 under mild conditions

Carbon-supported small Rh nanoparticles prepared with sodium citrate: Toward high catalytic activity for hydrogen evolution from ammonia borane hydrolysis

Metal nanoparticles (NPs) have wide applications in hydrogen evolution from ammonia-borane (AB) hydrolysis because they can provide large surface active areas for reactants, and thus produce high catalytic activity. Here, a hyper-cross-linked polymer (HCP-PPh3), which was synthesized through the Friedel-Crafts reaction of benzene and triphenylphosphine, was employed as a support to stabilize Rh NPs. The characterization results revealed that the Rh NPs were uniformly dispersed on the surface of HCP-PPh3, and that they had an average particle size of 2.1 nm. The as-prepared HCP-PPh3-Rh was used as an active catalyst for hydrogen generation from AB hydrolysis. This catalyst exhibited a high turnover frequency of 481 mol H2 (molRh min)-1 for AB hydrolysis under mild conditions. The high catalytic performance of HCP-PPh3-Rh can be attributed to the small size of Rh NPs and the strong interaction between the metal and HCP-PPh3. This work highlights a potentially powerful strategy for preparing highly active HCP stabilized metal NPs for AB hydrolysis to generate hydrogen.

Hyper-cross-linked polymer supported rhodium: an effective catalyst for hydrogen evolution from ammonia borane.

Rh-based materials have emerged as potential candidates for hydrogen revolution from electrolyzing water or ammonia borane (AB) hydrolysis. Nevertheless, most of the catalysts still suffer from the complex synthetic procedures combined with limited catalytic activity. Additionally, the facile syntheses of Rh catalysts with high efficiencies for both electrochemical water splitting and AB hydrolysis are still challenging. Herein, we develop a simple, green, and mass-producible ion-adsorption strategy to produce a Rh/C pre-catalyst (pre-Rh/C). The ultrafine and clean Rh nanoclusters immobilized on carbon are achieved via the in situ reduction of the pre-Rh/C during the hydrogen-evolution process. The resulting in situ Rh/C catalyst presents an outstanding electrocatalytic performance with low overpotentials of 8 and 30 mV at 10 mA cm-2 in 1.0 m KOH and 0.5 m H2 SO4 , respectively, outperforming the state-of-the-art Pt catalysts. Furthermore, the in situ Rh/C is also highly active for AB hydrolysis to produce hydrogen with a high turnover frequency of 1246 mol H 2 molRh-1 min-1 at 25 °C. The in situ-formed ultrafine Rh nanoclusters are responsible for the observed superior catalytic performance. This facile in situ strategy to realize a highly active catalyst shows promise for practical applications.

Min Hu

Papers

Promoted effect of alkalization on the catalytic performance of Rh/alk-Ti3C2X2 (XO, F) for the hydrodechlorination of chlorophenols in base-free aqueous medium

Hydrogen evolution from hydrolysis of ammonia borane catalyzed by Rh/g-C3N4 under mild conditions

Carbon-supported small Rh nanoparticles prepared with sodium citrate: Toward high catalytic activity for hydrogen evolution from ammonia borane hydrolysis

Hyper-cross-linked polymer supported rhodium: an effective catalyst for hydrogen evolution from ammonia borane.

Towards High-Efficiency Hydrogen Production through in situ Formation of Well-Dispersed Rhodium Nanoclusters