Understanding the CO Oxidation on Pt Nanoparticles Supported on MOFs by Operando XPS

Exploring the effect of molecular size and framework functionalisation on transport in metal-organic frameworks using pulsed-field gradient nuclear magnetic resonance

Molecular transport is an important aspect in metal-organic frameworks (MOFs) as it affects many of their applications, such as adsorption/separation, drug delivery and catalysis. Yet probing the fundamental diffusion mechanisms in MOFs is challenging, and the interplay between the MOF's features (such as the pore structure and linker dynamics) and molecular transport remains mostly unexplored. Here, the pulsed-field gradient nuclear magnetic resonance (PFG NMR) technique is used to probe the diffusion of several probe molecules, i.e., water, xylenes and 1,3,5-triisopropylbenzene (TIPB), within the UiO-66 MOF and its derivatives (UiO-66NH2 and UiO-66Br). Exploiting differences in the size of probe molecules we were able to probe the diffusion rate selectively in the different pore environments of the MOFs. In particular, when relatively small molecules, such as water and small hydrocarbons, were used as probes, the PFG NMR log attenuation plots were non-linear with two distinctive diffusion regions, suggesting faster diffusion in the inter-crystalline space and slower diffusion within crystal aggregates, the latter occurring mostly inside the framework of the MOFs. Conversely, experiments with a larger probe molecule, i.e., TIPB, with a kinetic diameter of 0.95 nm, which makes it unable to access the framework windows of the MOF crystals, showed linear PFG NMR log attenuation plots, which indicates diffusion occurring in a single environment, most likely in the inter-crystalline space. Analysis of the apparent tortuosity values of the systems under investigation highlights the role of linker functionalisation in influencing the molecular diffusion of the probe molecules, which affects both intra-molecular interactions and pore accessibility within the MOF crystals. The findings of this work demonstrate that the diffusion behaviour of probe molecules within MOFs is influenced by the pore size, structure, functionalisation of the MOF linker and molecular interactions. Our study contributes to further advance the understanding of mass transport in MOFs by PFG NMR and provides insights that can inform the design and optimisation of MOF-based materials for various applications.

Ionic liquid post-modified carboxylate-rich MOFs for efficient catalytic CO2 cycloaddition under solvent-free conditions

The synthesis of cyclic carbonates through cycloaddition reactions between epoxides and carbon dioxide (CO2) is an important industrial process. Metal-Organic Frameworks (MOFs) have functional and ordered pore structures, making them attractive catalysts for converting gas molecules into valuable products. One approach to enhance the catalytic activity of MOFs in CO2 cycloaddition reactions is to create open metal sites within MOFs. In this study, the amino-functionalized rare earth Gd-MOF (Gd-TPTC-NH2) and its ionic liquid composite catalysts (Gd-TPTC-NH-[BMIM]Br) were synthesized using 2'-amino-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid (H4TPTC-NH2) as the ligand. The catalytic performance of these two catalysts was observed in the cycloaddition reaction of CO2 and epoxides. Under the optimized reaction conditions, Gd-TPTC-NH-[BMIM]Br can effectively catalyze the cycloaddition reaction of a variety of epoxide substrates with good to excellent yields of cyclic carbonate products. Comparatively, epichlorohydrin and epibromohydrin, which possess halogen substituents, promote higher yields of cyclic carbonates due to the electron-withdrawing nature of Cl and Br substituents. Additionally, the Gd-TPTC-NH-[BMIM]Br catalyst demonstrated good recyclability and reproducibility, maintaining its catalytic activity without any changes in its structure or properties after five reuse cycles.

A high activity mesoporous Pt@KIT-6 nanocomposite for selective hydrogenation of halogenated nitroarenes in a continuous-flow microreactor

In this study, we designed a Pt@KIT-6 nanocomposite prepared by impregnating platinum nanoparticles on the nanopores of the KIT-6 mesoporous material. This Pt@KIT-6 nanocomposite was used as a catalyst in a micro fixed bed reactor (MFBR) for the continuous-flow hydrogenation of halogenated nitroarenes, which demonstrates three advantages. First, the Pt@KIT-6 nanocomposite has a stable mesoporous nanostructure, which effectively enhances the active site and hydrogen adsorption capacity. The uniformly distributed pore structure and large specific surface area were confirmed by electron microscopy and N2 physisorption, respectively. In addition, the aggregation of the loaded metal was avoided, which facilitated the maintenance of high activity and selectivity. The conversion and selectivity reached 99% within 5.0 minutes at room temperature (20 °C). Furthermore, the continuous-flow microreactor allows precise control and timely transfer of the reaction system, reducing the impact of haloid acids. The activity and selectivity of the Pt@KIT-6 nanocomposite showed virtually no degradation after 24 hours of continuous operation of the entire continuous-flow system. Overall, the Pt@KIT-6 nanocomposite showed good catalysis for the hydrogenation of halogenated nitroarenes in the continuous-flow microreactor. This work provides insights into the rational design of a highly active and selective catalyst for selective hydrogenation systems.

Recyclable TPA-Modified MIL-88-Supported Ionic Pt as a Highly Efficient Catalyst for Alkene Hydrosilylation

The hydrosilylation reaction driven by a homogeneous catalyst has been widely used in the industrial synthesis of functionalized silicone compounds. However, the homogeneous catalyst for hydrosilylation has the shortcomings of nonrecyclability, undesirable side reactions, and high cost. In this work, a highly efficient heterogeneous catalyst was prepared by loading Pt ions on MIL-88 modified with trimethoxy[3-(phenylamino)propyl]silane. In comparison with previous research studies, the resulting catalyst can exhibit high catalytic activity and excellent stability during the hydrosilylation reaction, which was attributed to the presence of a pyrrolic nitrogen structure between TPA-MIL-88 and the Pt ion. Besides them, 1.2%Pt/TPA-MIL-88 showed the highest catalytic activity and can be reused five times without significant deactivation. Importantly, 1.2%Pt/TPA-MIL-88 also achieved satisfactory results when it was used to catalyze the hydrosilylation reaction for other olefins, implying great potential for application in the silicone industry.

Active site identification and CO oxidation in UiO-66-XX thin films

Metal-organic frameworks (MOFs) offer an intrinsically porous and chemically tunable platform for gas adsorption, separation, and catalysis. We investigate thin film derivatives of the well-studied Zr-O based MOF powders to understand their adsorption properties and reactivity with their adaption to thin films, involving diverse functionality with the incorporation of different linker groups and the inclusion of embedded metal nanoparticles: UiO-66, UiO-66-NH₂, and Pt@UiO-66-NH₂. Using transflectance IR spectroscopy, we determine the active sites in each film upon consideration of the acid-base properties of the adsorption sites and guest species, and perform metal-based catalysis with CO oxidation of a Pt@UiO-66-NH₂film. Our study shows how surface science characterization techniques can be used to characterize the reactivity and the chemical and electronic structure of MOFs.

Cobalt Phthalocyanine Supported on Mesoporous CeO2 as an Active Molecular Catalyst for CO Oxidation

Heterogenization of biomolecules by immobilizing on a metal oxide support could greatly enhance their catalytic activity and stability, but their interactions are generally weak. Herein, cobalt phthalocyanine (CoPc) molecules were firmly anchored on a Ce-based metal-organic framework (Ce-BTC) due to π-π stacking interaction between CoPc and aromatic frameworks of the BTC linker, which was followed by a calcination treatment to convert Ce-BTC to mesoporous CeO₂ and realize a molecular-level dispersion of CoPc on the surface of CeO₂. Various characterization results confirm the successful fabrication of molecular-based CoPc/CeO₂ catalysts which exhibited good CO oxidation performance. Importantly, we found that the mixing manner of Ce-BTC and CoPc remarkably affects the physicochemical properties which then determined the catalytic performance of the resultant CoPc/CeO₂ catalysts. In contrast, the direct physical mixing of CoPc and CeO₂ led to poor performance toward CO oxidation, manifesting that the Ce-BTC-mediated CoPc loading strategy is promising for the heterogenization of catalytic biomolecules.

An X-ray Photoelectron Spectroscopy Study of Postsynthetic Exchange in UiO-66

Postsynthetic exchange (PSE) is a method that is widely used to change the composition of metal-organic frameworks (MOFs) by replacing connecting linkers or metal nodes after the framework has been synthesized. However, few techniques can probe the nature and distribution of exchanged species following PSE. Herein, we show that X-ray photoelectron spectroscopy can be used to compare the relative concentrations of exchanged ligands at the surface and interior regions of MOF particles. Specifically, PSE of iodobenzene dicarboxylate ligands results in a gradient distribution from surface to bulk in UiO-66 nanoparticles that depends on PSE time. X-ray photoelectron spectroscopy also reveals differences between the surface chemistry of the PSE product and that of the direct synthesis product.

Controlled growth of ultrafine metal nanoparticles mediated by solid supports

As a unique class of nanomaterials with a high surface-area-to-volume ratio and narrow size distribution, ultrafine metal nanoparticles (UMNPs) have shown exciting properties in many applications, particularly in the field of catalysis. Growing UMNPs in situ on solid supports enables precise control of the UMNP size, and the supports can effectively prevent the aggregation of UMNPs and maintain their high catalytic activity. In this review, we summarize the recent research progress in controlled growth of UMNPs using various solid supports and their applications in catalysis.

CO2 Hydrogenation at Atmospheric Pressure and Low Temperature Using Plasma-Enhanced Catalysis over Supported Cobalt Oxide Catalysts

CO₂ is a promising renewable, cheap, and abundant C1 feedstock for producing valuable chemicals, such as CO and methanol. In conventional reactors, because of thermodynamic constraints, converting CO₂ to methanol requires high temperature and pressure, typically 250 °C and 20 bar. Nonthermal plasma is a better option, as it can convert CO₂ at near-ambient temperature and pressure. Adding a catalyst to such plasma setups can enhance conversion and selectivity. However, we know little about the effects of catalysts in such systems. Here, we study CO₂ hydrogenation in a dielectric barrier discharge plasma-catalysis setup under ambient conditions using MgO, γ-Al₂O₃, and a series of Co _x O _y /MgO catalysts. While all three catalyst types enhanced CO₂ conversion, Co _x O _y /MgO gave the best results, converting up to 35% of CO₂ and reaching the highest methanol yield (10%). Control experiments showed that the basic MgO support is more active than the acidic γ-Al₂O₃, and that MgO-supported cobalt oxide catalysts improve the selectivity toward methanol. The methanol yield can be tuned by changing the metal loading. Overall, our study shows the utility of plasma catalysis for CO₂ conversion under mild conditions, with the potential to reduce the energy footprint of CO₂-recycling processes.

An experimental approach for controlling confinement effects at catalyst interfaces

Catalysts are conventionally designed with a focus on enthalpic effects, manipulating the Arrhenius activation energy. This approach ignores the possibility of designing materials to control the entropic factors that determine the pre-exponential factor. Here we investigate a new method of designing supported Pt catalysts with varying degrees of molecular confinement at the active site. Combining these with fast and precise online measurements, we analyse the kinetics of a model reaction, the platinum-catalysed hydrolysis of ammonia borane. We control the environment around the Pt particles by erecting organophosphonic acid barriers of different heights and at different distances. This is done by first coating the particles with organothiols, then coating the surface with organophosphonic acids, and finally removing the thiols. The result is a set of catalysts with well-defined "empty areas" surrounding the active sites. Generating Arrhenius plots with >300 points each, we then compare the effects of each confinement scenario. We show experimentally that confining the reaction influences mainly the entropy part of the enthalpy/entropy trade-off, leaving the enthalpy unchanged. Furthermore, we find this entropy contribution is only relevant at very small distances (<3 Å for ammonia borane), where the "empty space" is of a similar size to the reactant molecule. This suggests that confinement effects observed over larger distances must be enthalpic in nature.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

Highly Dispersed Pt Nanoparticles on N-Doped Ordered Mesoporous Carbon as Effective Catalysts for Selective Hydrogenation of Nitroarenes

Highly-dispersed Pt nanoparticles supported on nitrogen-modified CMK-3 mesoporous carbon (Pt/N-CMK-3) were first fabricated by a two-step impregnation route. The influences of N content on the catalyst porous structure, Pt nanoparticle size, surface properties, and interaction between Pt species and the support were investigated in detail using N2 sorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectra (XPS). The N species acted as anchoring sites for the stabilization of Pt particles. Benefiting from the formation of ultrafine metal nanoparticles, the Pt/N-CMK-3 exhibited excellent catalytic activity and selectivity for the selective hydrogenation of nitro aromatics to the corresponding anilines with hydrogen. The Pt/N-CMK-3 catalyst could be reused eight times and keep its catalytic performance.

Promoting Pt catalysis for CO oxidation via the Mott-Schottky effect

CO oxidation is an important reaction both experimentally and industrially, and its performance is usually dominated by the charge states of catalysts. For example, CO oxidation on the platinum (Pt) surface requires a properly charged state for the balance of adsorption and activation of CO and O₂. Here, we present "Mott-Schottky modulated catalysis" on Pt nanoparticles (NPs) via an electron-donating carbon nitride (CN) support with a tunable Fermi level. We demonstrate that properly-charged Pt presents an excellent catalytic CO oxidation activity with an initial conversion temperature as low as 25 °C and total CO conversion below 85 °C. The tunable electronic structure of Pt NPs, which is regulated by the Fermi level of CN, is a key factor in dominating the catalytic performance. This "Mott-Schottky modulated catalysis" concept may be extended to maneuver the charge state on other metal catalysts for targeted catalytic reactions.

Coupling non-thermal plasma with Ni catalysts supported on BETA zeolite for catalytic CO2 methanation

Non-thermal plasma activation promotes CO₂ conversion over Ni catalysts supported on BETA zeolite via multiple reaction mechanisms.