Intercalating lithium into the lattice of silver nanoparticles boosts catalytic hydrogenation of carbon-oxygen bonds

Coinage metal nanoparticles with high dispersion can serve as highly efficient heterogeneous catalysts. However, owing to their low melting point, poor thermal stability remains a major obstacle towards their application under reaction conditions. It is a common practice to use porous inorganic templates such as mesoporous silica SBA-15 to disperse Ag nanoparticles (NPs) against aggregation but their stability is far from satisfactory. Here, we show that the catalytic activity for hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG) over Ag NPs dispersed on SBA-15 silica can be further promoted by incorporation of alkali metal ions at small loading, which follows the inverse order of their cationic size: Li+ > Na+ > K+ > Rb+. Among these, 5Ag1-Li0.05/SBA-15 can double the MG yield compared to pristine 5Ag/SBA-15 under identical conditions with superior thermal stability. Akin to the effect of an ionic surfactant on stabilization of a micro-emulsion, the cationic charge of an alkali metal ion can maintain dispersion and modulate the surface valence of Ag NPs. Interstitial Li in the octahedral holes of the face center packed Ag lattice is for the first time confirmed by X-ray pair distribution function and electron ptychography. It is believed that this interstitial-stabilization of coinage metal nanoparticles could be broadly applicable to multi-metallic nanomaterials for a broad range of C-O bond activating catalytic reactions of esters.

Recent Advances in the Engineering of Single-Atom Catalysts Through Metal-Organic Frameworks

This mini-review highlights some recent progress in the engineering of single-atom catalysts (SACs) through metal-organic frameworks (MOFs) and derivatives. The inherent molecular and chemical specificities within the MOFs and derivatives can offer stabilisation of the SACs with high atomic isolation and dispersion. As MOFs are often considered an infinite array of self-assembled molecular catalysts, specifically designed structures can provide further functionalities to suit the needs of different catalytic applications. In brief, we can divide the preparation approaches into three main categories: (1) fabrication onto functional groups of the ligands, (2) fabrication onto Lewis acid sites of nodal centres, and (3) synthesis via a pyrolysis-mediated technique. Through these approaches, strong metal-support interactions can be established to aid the fine-tuning of the catalytic properties. We also discuss how recent progress in the development of state-of-the-art microscopic, spectroscopic, and crystallographic techniques has enabled scientists to elucidate the structure-activity relationship.

A rational study on the geometric and electronic properties of single-atom catalysts for enhanced catalytic performance

We investigate the geometric and electronic properties of single-atom catalysts (SACs) within metal-organic frameworks (MOFs) with respect to electrocatalytic CO2 reduction as a model reaction. A series of mid-to-late 3d transition metals have been immobilised within the microporous cavity of UiO-66-NH2. By employing Rietveld refinement of new-generation synchrotron diffraction, we not only identified the crystallographic and atomic parameters of the SACs that are stabilised with a robust MN(MOF) bonding of ca. 2.0 Å, but also elucidated the end-on coordination geometry with CO2. A volcano trend in the FEs of CO has been observed. In particular, the confinement effect within the rigid MOF can greatly facilitate redox hopping between the Cu SACs, rendering high FEs of CH4 and C2H4 at a current density of -100 mA cm-2. Although only demonstrated in selected SACs within UiO-66-NH2, this study sheds light on the rational engineering of molecular interactions(s) with SACs for the sustainable provision of fine chemicals.

Cooperative catalytically active sites for methanol activation by single metal ion-doped H-ZSM-5

Catalytic conversion of methanol to aromatics and hydrocarbons is regarded as a key alternative technology to oil processing. Although the inclusion of foreign metal species in H-ZSM-5 containing Brønsted acid site (BAS) is commonly found to enhance product yields, the nature of catalytically active sites and the rationalization for catalytic performance still remain obscure. Herein, by acquiring comparable structural parameters by both X-ray and neutron powder diffractions over a number of metal-modified ZSM-5 zeolites, it is demonstrated for the first time that active pairs of metal site-BAS within molecular distance is created when single and isolated transition metal cation is ion-exchanged with the zeolites. According to our DFT model, this could lead to the initial heterolytic cleavage of small molecules such as water and methanol by the pair with subsequent reactions to form products at high selectivity as that observed experimentally. It may account for their active and selective catalytic routes of small molecule activations.

Diffraction studies and DFT calculations show the formation of frustrated Lewis pair (FLP) over M-ZSM-5 for heterolytic cleavage of CH₃OH.

Recent Advances of Precise Cu Nanoclusters in Microporous Materials

This minireview highlights some recent advances in the rational design of precise Cu nanoclusters supported on microporous materials, including zeolites and metal-organic frameworks. The development of comprehensive characterisation techniques enables scientists to elucidate the structure-activity relationship of these catalysts, which aids the subsequent engineering of more superior catalytic systems at an atomistic perspective.

Chemical state tuning of surface Ce species on pristine CeO2 with 2400% boosting in peroxidase-like activity for glucose detection

2400% increase in CeO₂ peroxidase-like activity can be easily achieved on the (100) surface due to the promoted H₂O₂ adsorption/activation.

Interstitial Boron Atoms in the Palladium Lattice of an Industrial Type of Nanocatalyst: Properties and Structural Modifications

It is well-established that the inclusion of small atomic species such as boron (B) in powder metal catalysts can subtly modify catalytic properties, and the associated changes in the metal lattice imply that the B atoms are located in the interstitial sites. However, there is no compelling evidence for the occurrence of interstitial B atoms, and there is a concomitant lack of detailed structural information describing the nature of this occupancy and its effects on the metal host. In this work, we use an innovative combination of high-resolution ¹¹B magic-angle-spinning (MAS) and ¹⁰⁵Pd static solid-state NMR nuclear magnetic resonance (NMR), synchrotron X-ray diffraction (SXRD), in situ X-ray pair distribution function (XPDF), scanning transmission electron microscopy-annular dark field imaging (STEM-ADF), electron ptychography, and electron energy loss spectroscopy (EELS) to investigate the B atom positions, properties, and structural modifications to the palladium lattice of an industrial type interstitial boron doped palladium nanoparticle catalyst system (Pd-^intB/C NPs). In this study, we report that upon B incorporation into the Pd lattice, the overall face centered cubic (FCC) lattice is maintained; however, short-range disorder is introduced. The ¹⁰⁵Pd static solid-state NMR illustrates how different types (and levels) of structural strain and disorder are introduced in the nanoparticle history. These structural distortions can lead to the appearance of small amounts of local hexagonal close packed (HCP) structured material in localized regions. The short-range lattice tailoring of the Pd framework to accommodate interstitial B dopants in the octahedral sites of the distorted FCC structure can be imaged by electron ptychography. This study describes new toolsets that enable the characterization of industrial metal nanocatalysts across length scales from macro- to microanalysis, which gives important guidance to the structure-activity relationship of the system.

Evaluation of the molecular poisoning phenomenon of W sites in ZSM-5 via synchrotron X-ray powder diffraction

The traditional investigation of complex catalyst poisoning phenomena is in the operation level: poisonings commonly attributed to macroscopic coke deposition and particle size change, etc. Here, we demonstrate that high-resolution SXRD can reveal the structure of the organic molecule-active site complex in a 3-D environment, leading to an understanding of the poisoning mechanism at the molecular level.

Entrapped Single Tungstate Site in Zeolite for Cooperative Catalysis of Olefin Metathesis with Brønsted Acid Site

Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Brønsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS-MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and trans-2-butene to propene, under mild reaction conditions, the propene production rate over WO _x/USY is ca. 7300 times that over the industrial WO₃/SiO₂-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO _x/USY for a wide range of reaction conditions.

Decarboxylation of Lactones over Zn/ZSM-5: Elucidation of the Structure of the Active Site and Molecular Interactions

Herein, we report the catalytic decarboxylation of γ-valerolactone (GVL) over Zn/ZSM-5 to butene, followed by aromatization at high yield with co-feeding of water. An evaluation of the catalytic performance after prolonged periods of time showed that a water molecule is essential to maintain the decarboxylation and aromatization activities and avoid rapid catalyst deactivation. Synchrotron X-ray powder diffraction and Rietveld refinement were then used to elucidate the structures of adsorbed GVL and immobilized Zn species in combination with EXAFS and NMR spectroscopy. A new route for the cooperative hydrolysis of GVL by framework Zn-OH and Brønsted acidic sites to butene and then to aromatic compounds has thus been demonstrated. The structures and fundamental pathways for the nucleophilic attack of terminal Zn-OH sites are comparable to those of Zn-containing enzymes in biological systems.

Spatial differentiation of Brønsted acid sites by probe molecule in zeolite USY using synchrotron X-ray powder diffraction

A direct correlation of extra-framework Al³⁺ in a sodalite cage (HY) with the enhanced Brønsted acid site evaluated by synchrotron X-ray powder diffraction, Rietveld refinement and the use of a pyridine probe molecule.