Single-Sites and Nanoparticles at Tailored Interfaces Prepared via Surface Organometallic Chemistry from Thermolytic Molecular Precursors

Abstract

CONSPECTUS: Heterogeneous catalysts are complex by nature, making particularly difficult to assess the structure of their active sites. Such complexity is inherited in part from their mode of preparation, which typically involves coprecipitation or impregnation of metal salts in aqueous solution, and the associated complex surface chemistries. In this context, surface organometallic chemistry (SOMC) has emerged as a powerful approach to generate well-defined surface species, where the metal sites are introduced by grafting tailored molecular precursors. When combined with thermolytic molecular precursors (TMPs) that can lose their organic moieties quite readily upon thermal treatment, SOMC provides access to supported isolated metal sites with defined oxidation state and nuclearity inherited from the precursor. The resulting surface species bear unusual coordination imposed by the surface that provides them high reactivity in comparison with their molecular precursor. In addition, these molecularly defined species bare strong resemblance with the active sites of supported metal oxides. However, they typically contain a higher proportion of active sites making structure−activity relationship possible. They thus constitute ideal models for this important class of industrial catalysts that are used in numerous applications such as olefin epoxidation (Shell process), olefin metathesis (triolefin process), ethylene polymerization (Phillips catalysts), or propane dehydrogenation (Catofin and related processes). This SOMC/TMP approach can thus provide detailed information about the structure of active sites in industrial catalysts, their mode of initiation and deactivation, as well as the role of the support and specific thermal treatment on the final activity of the catalysts. Nonetheless, these structurally characterized surface sites still exhibit heterogeneous environments borrowed from the support itself, that explain the intrinsic complexity of heterogeneous catalysis. Furthermore, SOMC/TMP can also be used to generate and investigate supported metal nanoparticles. Starting from the well-defined isolated sites, that also contain adjacent surface OH groups, one can graft a second metal and then generate after treatment under hydrogen small and narrowly dispersed alloys or nanoparticles with tailored interfaces that can show improved catalytic performances and are amiable to detailed structure−activity relationships. This approach is illustrated by two case studies: (1) formation of supported copper nanoparticles at tailored interfaces that contain isolated metal sites for the selective hydrogenation of carbon dioxide to methanol, allowing for a detailed understanding of the role of dopants and supports in heterogeneous catalysis, and (2) preparation of highly selective and productive propane dehydrogenation catalysts based on silica-supported Pt x Ga y alloy. Overall, this Account shows how the combination of SOMC and TMP helps to generate catalysts, particularly suited for elucidating structural characterization o...

Silica-supported, narrowly distributed, subnanometric Pt–Zn particles from single sites with high propane dehydrogenation performance

“…9,11,12 While being simple catalyst preparation methods, impregnation techniques typically yield poorly dened systems with inhomogeneous distribution of the components due to complex dissolution/precipitation events that occur in aqueous conditions. In order to develop more controlled preparation methods, surface organometallic chemistry (SOMC) [13][14][15] in combination with the thermolytic molecular precursor approach (TMP) 16,17 has emerged as a powerful preparation technique. In particular, it has been shown that supported single-sites can be used as building blocks to generate supported nanoparticles with controlled interfaces, allowing the introduction of dopants at the interface between silica and the metal particles or alternatively yielding a bimetallic alloy supported on SiO 2 .…”

Enhanced CH3OH selectivity in CO2 hydrogenation using Cu-based catalysts generated via SOMC from GaIII single-sites

“…27,30 Recently, it has been shown that surface organometallic chemistry (SOMC) combined with thermolytic molecular precursors (TMP) constitutes a powerful synthetic strategy to generate supported metal nanoparticles surrounded by well-dened isolated promoter sites that allows investigating the role of interfacial sites via in situ spectroscopic methods. 31,32 This approach can also yield supported alloyed nanoparticles, depending on the selection of metal and promoters (vide infra). The SOMC/TMP approach can be summarized as follows: in a rst step, the support, SiO 2 , is dehydroxylated at 700 C to obtain isolated surface silanol (Si-OH) groups (1 OH, nm À2 ) that are used as anchoring groups to gra the TMP.…”

Small and Narrowly Distributed Copper Nanoparticles Supported on Carbon Prepared by Surface Organometallic Chemistry for Selective Hydrogenation and CO2 Electroconversion Processes

“…The resulting material contains an increased Cu loading, reaching 5.75 wt %, while preserving a high Cu dispersion and a rather small size for Cu NPs of 6.0±1.8 nm (Figure S4). The good retention of size for small supported (Cu) particles upon increasing metal loading was also observed with silica as a support and appears as an intrinsic property of the SOMC approach …”