Preparation, Characterization, and Unusual Reactivity of Fe-MCM-41

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Highly anisotropic distribution of iron nanoparticles within MCM-41 Mesoporous Silica

Electron microscopy techniques are used to visualize the spatial distribution of iron nanoparticles inside a mesoporous MCM-41 molecular sieve. Direct observation of the iron oxide nanoparticles by STEM-HAADF imaging reveals a highly non-uniform spatial distribution inside the mesopores. These particles are retained in the pores after a reduction treatment unlike the behavior found in other similar systems. It is found that thermal treatments induce changes in its morphology, creating nanowires from particle strings.

Nanoporous silica-supported nanometric palladium: synthesis, characterization, and catalytic deep oxidation of benzene

In this present study, nanoporous silica SBA-15 supported palladium catalysts are prepared through two different methods. The catalysts are employed for catalytic deep oxidation reaction of benzene at a high gas hourly space velocity of 100,000 h(-1). It is found that the traditional aqueous impregnation method has some difficulties and disadvantages in obtaining highly dispersed palladium active phases. Whereas, when a grafting procedure is employed, palladium tends to be highly dispersed as nanoparticles due to the confinement of the nanosized pore channels of the support materials. The catalysts prepared via the grafting procedure catalyze the benzene oxidation far more effectively than those prepared via aqueous impregnation method, and complete conversion of benzene can be achieved below 190 degrees C over the most active catalyst. The nanoporous silica-supported palladium catalysts are promising materials for the control of some types of VOCs such as benzene.

Synthesis, Characterization, and Stability of Fe−MCM-41 for Production of Carbon Nanotubes by Acetylene Pyrolysis

Fe-substituted MCM-41 molecular sieves with ca. 1, 2, and 3 wt % Fe were synthesized hydrothermally using different sources of colloidal silica (HiSil and Cab-O-Sil) and characterized by ICP, XRD, N2 physisorption, UV-vis, EPR, TPR, and X-ray absorption. Catalysts synthesized from Cab-O-Sil showed higher structural order and stability than those from HiSil. The local environment of Fe in the mesoporous material as studied by UV-vis reveals the dominance of framework Fe in all the as-synthesized Fe-MCM-41 samples. Dislodgement of some Fe species to extraframework location occurs upon calcination, and this effect is more severe for Fe-MCM-41 (2 wt %) and Fe-MCM-41 (3 wt %), as confirmed by EPR and X-ray absorption. These materials have been used as catalytic templates for the production of carbon nanotubes (CNTs) by acetylene pyrolysis at atmospheric pressure. A relationship between the Fe loading in MCM-41 and the carbon species produced during this reaction has been established. Using our optimized conditions for this system, Fe-MCM-41 with ca. 2 wt % Fe showed the best results with particularly high selectivity for single-wall carbon nanotube (SWNT) production. This catalyst was selective for carbon nanotubes with a low amount of amorphous carbon for a narrow range of temperatures from 1073 to 1123 K. To account for the different selectivity of these catalysts for CNTs production, the local environment and chemical state of Fe in the used catalyst was further probed by X-band EPR.